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Pathology Research International
Volume 2011 (2011), Article ID 605042, 12 pages
Tumor Suppressors and Cell-Cycle Proteins in Lung Cancer
1Section of Pathology, Department of Biochemistry, Second University of Naples, 80138 Naples, Italy
2Department of Public Health, Second University of Naples, 80138 Naples, Italy
3Third Division Cotugno Hospital, 80100 Naples, Italy
4SAFU Department, Regina Elena Cancer Institute, Via delle Messi d'Oro 156, 00158 Rome, Italy
Received 31 December 2010; Accepted 8 August 2011
Academic Editor: Ka F. To
Copyright © 2011 Alfonso Baldi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The cell cycle is the cascade of events that allows a growing cell to duplicate all its components and split into two daughter cells. Cell cycle progression is mediated by the activation of a highly conserved family of protein kinases, the cyclin-dependent kinases (CDKs). CDKs are also regulated by related proteins called cdk inhibitors grouped into two families: the INK4 inhibitors (p16, p15, p19, and p18) and the Cip/Kip inhibitors (p21, p27, and p53). Several studies report the importance of cell-cycle proteins in the pathogenesis and the prognosis of lung cancer. This paper will review the most recent data from the literature about the regulation of cell cycle. Finally, based essentially on the data generated in our laboratory, the expression, the diagnostic, and prognostic significance of cell-cycle molecules in lung cancer will be examined.
Lung cancer remains a major health challenge in the world. Despite improvements in staging and the integrated application of surgery, radiotherapy, and chemotherapy, the 5-year survival rate for individuals with lung cancer is only about 15% . Histologically, 80% of the lung cancers are diagnosed as non-small-cell lung cancer (NSCLC), whereas the remaining 20% of cases are diagnosed as small-cell lung cancer (SCLC). On the basis of cell morphology, adenocarcinoma and squamous cell carcinoma are the most common types of NSCLC. The current staging system for NSCLC is based upon the size and location of the primary tumor (T), the involvement of regional lymph nodes (N), and the presence of distant metastases (M) . The standard treatment of patients with stage I NSCLC (T1-2, N0, M0) is resection of the primary tumor alone (no adjuvant therapy) . Survival for patients with stage I disease ranges between 40 and 70%, and the failure is due to distant recurrences . This suggests that a significant proportion of patients with stage I NSCLC may actually be understaged. Therefore, if correctly identified, these patients may benefit from adjuvant therapy in addition to resection, with a predictable improvement in the survival rates. Indeed, to identify patients with stage I NSCLC who might benefit from adjuvant therapy, investigators have attempted to identify factors predicting poor prognosis. These studies included analysis of performance status, histologic subtype, size of the primary tumor, the degree of tumor differentiation, mitotic rate, and evidence of lymphatic or vascular invasion [4–8]. However, all of these factors have failed, to date, to precisely identify a group of stage I patients who would benefit from adjuvant therapy. Cigarette smoking remains the main risk factor for lung cancer, accounting for about 90% of the cases in men and 70% of the cases in women .
Our research group has investigated in the last years the possible involvement of several molecular mechanisms, such as cell cycle and apoptosis regulators, oncogenes and tumor suppressor genes, cell adhesion molecules, in the pathogenesis and progression of lung cancer [10–20]. Goal of this paper is to summarize some of the most recent findings about the regulation of cell cycle and about the role of cell-cycle proteins in lung-cancer pathogenesis and progression.
2. Cell Cycle
The cell cycle is the cascade of events that allows a growing cell to duplicate all its components and split into two daughter cells; it consists of four distinct phases: G1 phase, S phase, G2 phase (collectively known as interphase), and M phase. High fidelity duplication of DNA in each cell during the S phase and the proper migration of the duplicated chromosomes in mitosis are highly regulated processes.
Cyclins and cyclin-dependent kinases (CDKs) are the two critical classes of molecules involved in the regulation of cell-cycle progression. These proteins form an heterodimer in which cyclins are the regulatory subunits and CDKs are the catalytic subunits; when the complex is activated from external signals, CDKs activate or inactivate downstream target proteins to orchestrate coordinated entry into the next phase of the cell cycle .
In normal cells, CDKs are expressed throughout all the cycle; however, each cyclin protein has a restricted period of expression, and this limited expression of each cyclin protein is due to cell-cycle-dependent regulation of both cyclin gene transcription and protein degradation.
The cyclin/CDK complex is regulated by a number of phosphorylation and dephosphorylation events, resulting either in activation or inhibition of kinase activity : phosphorylation is carried out by cyclin-activating kinase (CAK), and dephosphorylation is mediated by members of the Cell Division Cycle 25 family (Cdc25) of dual-specificity protein phosphatases. The mammalian Cdc25 family consists of three members: Cdc25A, Cdc25B, and Cdc25C. Cdc25A promotes entry into S phase by acting on cyclin A/CDK2 and cyclin E/CDK2 and is required for DNA replication [23–26]. Cdc25B activation occurs during S phase and peaks during the G2 phase [27, 28]. Both Cdc25B and Cdc25C play roles in the G2/M transition. Cdc25C dephosphorylates cyclin B1/cdc2 and is essential for progression through the G2/M phase of the cell cycle .
Decision to enter S phase is made in mid-to-late G1 and is called the restriction point. Molecular switch from G1 to S phase and targets of the G1 phase cyclin/CDK complexes are members of the retinoblastoma protein (RB) family: RB, p107, and p130 . RB can function as either a transcriptional repressor or a transcriptional activator depending on its phosphorylation state and the proteins with which it binds: when hypophosphorylated and bound to the E2F family of transcription factors, it works as a transcriptional repressor . The E2F family mediates transcription of genes required for DNA synthesis, including cyclin E, cyclin A, cyclin B, dihydrofolate reductase, and thymidine kinase . Sequential phosphorylation of RB by cyclin D/CDK4/6 and cyclin E/CDK2 complexes inhibits the repressor activity of RB, as it results in the dissociation of E2F and RB, and S phase entry. As cells progress into S phase, maintenance of RB hyperphosphorylation is necessary for the successful completion of DNA replication . Mutations that affect the RB signaling pathway have been identified in the majority of human cancers .
CDKs are also regulated by a group of functionally related proteins called CDK inhibitors. The CDK inhibitors are grouped into two families: the INK4 inhibitors (p16, p15, p19, and p18) and the Cip/Kip inhibitors (p21, p27, and p53). The INK4 family specifically inhibits CDK4 and CDK6 activity during the G1 phase of the cell cycle, while the Cip/Kip family inhibits CDK activity during all phases of the cell cycle. Both families of these inhibitors can arrest cells in the G1 phase of the cell cycle by inhibiting the activities of CDKs and preventing their ability to phosphorylate and inactive RB and other RB-family proteins .
The S phase of the cell cycle is the stage in which occurs the DNA replication. At the beginning of the S phase, each chromosome is composed of one coiled DNA double helix molecule; at the end of this stage, each chromosome has two identical DNA double helix molecules and therefore is composed of two sister chromatids. During this phase, the centrosome is also duplicated [35, 36]. It is proposed that replication origins are in two different states during the cell cycle: one during G1 phase, before DNA replication begins, when a multiprotein prereplicative complex (pre-RC) assembles on the origin. The second state exists from the initiation of S phase to the end of M phase, when a postreplicative complex (post-RC) is present at the replication origins.
Initiator proteins required for pre-RC formation include the origin recognition complex (ORC), MCM proteins (that forms the replicative helicase), Cdc6, and Cdt1 (required to load MCM proteins on chromatin) [37–40]. This multiprotein complex is activated at the G1/S transition; subsequently, DNA replicating proteins such as DNA polymerase and are recruited to initiation sites . An increase in CDK2 and hcdc7 activity at the G1/S transition triggers initiation and converts the origin to the post-RC state. The CDK cycle controls the two states at replication origins, couples the initiation of S phase to the completion of M phase, and prevents rereplication events from occurring during a single round of the cell cycle [41, 42].
As cells enter into G2 phase, the cyclin B/cdc2 complexes are kept inactive by phosphorylation. At the end of this phase, the cells are ready to enter into mitosis, and cyclin B/cdc2 complexes are activated by dephosphorylation . The enzyme that dephosphorylates and activates cdc2 at the end of G2 and initiates mitosis is Cdc25C . This transition point is one of the most important during the progression of cell cycle, and different mechanisms control this stage. Cdc25C is localized in the cytoplasm during interphase and enters the nucleus just before mitosis. Also cyclin B1 is in the cytoplasm during S phase and G2 phase and is translocated to the nucleus at the beginning of mitosis . It is thought that the precise regulation of cyclin B1 localization prevents premature mitosis during interphase, while allowing regulated access of cyclin B1/cdc2 complexes to their nuclear substrates at the onset of mitosis. Although less well understood, Cdc25B and A/Cdc2 also play a role at the mitotic transition.
As cells enter mitosis, phosphorylation of key components causes significant changes in the architecture of the cell, and this phosphorylation is due mainly to cyclin B/cdc2 activity . This complex induces changes in the microtubule network, in the actin microfilaments, and in the nuclear lamina . Other cyclin B/cdc2 substrates include histone H1 and microtubule-associated proteins such as MAP4, MAP2, and stathmin . The family of polo-like protein kinases (Plks) also plays a critical role in several mitotic events . They are critical for the formation of a bipolar spindle. It is proposed that Plks initiate the onset of mitosis by activating Cdc25C. Plks are also important regulators of mitotic exit.
Mitotic exit requires sister chromatid separation, spindle disassembly, and cytokinesis. The initiation and coordination of these processes are controlled by degradation of key regulatory proteins. The mediator of this protein destruction is a multisubunit protein called the anaphase-promoting complex (APC) or cyclosome . Key APC substrates are the mitotic A- and B-type cyclins. Cyclin A is degraded in metaphase, whereas B-type cyclins are degraded when cells enter anaphase . Cyclin B1 destruction starts as soon as the last chromosomes are aligned on the metaphase plate and is complete by the end of metaphase . Another group of APC substrates are proteins that function as anaphase inhibitors. During G2, sister chromatids are held together by proteins called cohesins, which require inactivation by APC for anaphase initiation . Overall, the APC regulates two different steps in mitosis. First, sister chromatid separation is triggered by destruction of the anaphase inhibitors, after which spindle disassembly and mitotic exit are initiated by the degradation of mitotic cyclins. These two steps allow the cell to couple the exit from mitosis to the prior completion of anaphase.
3. Cell-Cycle Checkpoint
Cell-cycle checkpoints are signal transduction pathways monitoring the successful completion of events in one phase of the cell cycle before proceeding to the next phase. Cell-cycle checkpoints contain sensor proteins that scan chromatin for partially replicated DNA, DNA breaks, or other abnormalities. Sensor proteins are thought to translate DNA-derived stimuli into biochemical signals that modulate specific downstream target proteins and activate signaling pathways involved in DNA repair and cell-cycle arrest . Further, when cellular damage is irreparable, checkpoint signaling could eliminate potentially hazardous cells by permanent cell-cycle arrest or apoptosis.
The physiological relevance of these signaling pathways is supported by their evolutionary conservation and the finding that the major consequence of their alteration in humans is tumorigenesis ; in fact, the loss of cell-cycle checkpoints is a universal alteration identified in human cancer. Although numerous genetic alterations can result in loss of normal checkpoints, the hope is that common strategies will be developed against a wide variety of cancers.
3.1. G1/S Phase Checkpoint
The G1 cell-cycle checkpoint prevents damaged DNA from being replicated and is the best understood checkpoint in mammalian cells. Progression of cells through early G1, across the restriction point into late G1 and then into S phase, requires the coordinated regulation of multiple positive and negative factors . Cyclin D-CDK4/6 complexes promote early G1 progression, but cyclin E (or cyclin A)-CDK2 (or CDK1) activity is required to inactive RB by hyperphosphorylation to transit the restriction point into late G1 phase . RB inactivation results in release of E2F transcription factors and induction of late-G1-specific genes, including dihydrofolate reductase (DHFR), Emi1, and cyclin A . Cyclin A-associated kinase activity is required to initiate DNA synthesis, prevent rereplication, and enter mitosis.
Although cyclin A is transcriptionally induced by E2Fs at the restriction point, cyclin A protein does not accumulate until the late G1/S phase transition due to ubiquitination by the anaphase promoting complex (APC) and subsequent proteolysis by the 26S proteasome. APC is active throughout G1 phase by association with Cdh1 (APCCdh1), an activator that confers substrate specificity . Prior to initiation of S phase, APCCdh1 is inactivated by the binding of Emi1 to Cdh1, resulting in stabilization of cyclin A, activation of cyclin A-associated kinase activity, and subsequent inactivation of Cdh1 by phosphorylation . Thus, tight regulation of cyclin E- and A-associated kinase activity results in a coordinated G1 cell-cycle progression .
If DNA is damaged, the G1/S checkpoints prevent transition of cells into S phase. Due to its essential and rate-limiting role in G1/S transition, cyclin E/CDK2 is a key target for the DNA damage checkpoint . Damage to DNA inhibits the action of CDK2 thus stopping the progression of the cell cycle until the damage can be repaired. If the damage is so severe that it cannot be repaired, the cell self-destructs by apoptosis. It is proposed that after exposure of cells to UV or IR, the level of Cdc25A phosphatase rapidly decreases. Cdc25A removes the inhibitory phosphorylation on CDK2 that is required for G1/S transition. After IR or UV exposure, Cdc25A is rapidly phosphorylated by Chk2 and Chk1, respectively. Chk-mediated phosphorylation triggers accelerated turnover of Cdc25A and thus inhibition of CDK2 . An endpoint of this checkpoint signaling is inhibition of CDK2-dependent loading of Cdc45 onto the DNA prereplication complexes and thus inhibition of S phase .
Another important target for checkpoint signaling in cells that are in G1/S transition before the restriction point is the p53 tumor suppressor protein . In normal, non stressed cells, p53 protein has short half-life as result of the rapid MDM2-mediated degradation of the protein after synthesis [64, 65].
After exposure of cells to stress, p53 phosphorylation changes and protein levels increase significantly. Transducers that are required for p53-mediated maintenance of G1 checkpoint arrest are the same as those required for activation of the checkpoint, namely, the ATM/ATR and Chk2/Chk1kinases. Among the genes regulated by p53, the CDK-inhibitor p21WAF1/Cip1 plays a central role in G1 checkpoint by inhibiting CDKs that are essential for entry into S phase [66–68]. Thus, although ATM/ATR-mediated signaling can phosphorylate key targets Cdc25A and p53 within minutes after DNA damage, the impact of the signaling pathways regulated by Cdc25A and p53 on CDK2 activity and G1/S blockage are separated in time, due to the dependence of p53 signaling on transcription and protein synthesis.
Human cells also have evolved additional mechanisms to prolong a G1 cell-cycle checkpoint arrest. For example, after exposure of keratinocytes and melanocytes to physiological doses of UV radiation, there is an increase of the CDK-inhibitor p16INK4a . This factor acts as tumor suppressor which inhibits the activities of cyclin D-dependent kinases, CDK4 and CDK6, and regulates the activities of RB . Given the direct role that CDK-inhibitors play in regulation of the G1/S transition, it is not surprising that CDK-inhibitor function is often compromised in human tumors. The p16INK4A gene is the frequent target of mutations that ablate its function, including point mutations, promoter methylation, or homozygous deletions . Likewise, many human breast cancers have reduced p27Kip1 protein expression or aberrant subcellular localization of the protein that has been correlated with more aggressive tumors [72, 73].
3.2. S Phase Checkpoint
The S phase checkpoint monitors cell-cycle progression and decreases the rate of DNA synthesis following DNA damage. The responses of cells that are already in S phase at the time of the DNA damage will be critical for optimal outcome of the cell. These cells must respond virtually instantaneously to halt initiation of new replication forks throughout S phase, and the first action to prevent wrong DNA synthesis is the activation of the ATM (ataxia telangiectasia mutated) and/or ATR (ATM and Rad3-related) protein kinase. Both of these proteins belong to a structurally unique family of serine-threonine kinases, but they generally respond to distinct types of DNA damage.
ATM is the primary mediator of the response to DNA double strand breaks (DSBs) that can arise by exposure to ionizing radiation (IR) . For responses to other types of DNA damage, such as base damage caused by exposure to ultraviolet light or alkylating agents, the ATR kinase appears to be important for initiating the relevant signal transduction pathways .
Once ATM or ATR has been activated by the introduction of DNA damage, these protein kinases begin to phosphorylate substrates to help the cell arrest cell-cycle progression or repair DNA. As discussed previously, the phosphorylation of p53, MDM2, and Chk2 by ATM following DNA damage contributes to the arrest of cells in G1 before the restriction point. Among the proteins phosphorylated by ATM that contribute to arrest of cells in S phase are Nbs1, Brca1, SMC1, and FAncD2 [76–79]. The importance of this process in cancer formation in humans is suggested by the fact that many of these genes are mutated in familial cancer syndromes. For example, the cancer susceptibility syndromes Ataxia-telangiectasia, Nijmegen breakage syndrome, Fanconi’s anemia, and familial breast/ovarian carcinoma syndrome are caused by inherited mutations in ATM, Nbs1, FAncD2, and Brca1, respectively.
3.3. G2 Checkpoint
In addition to activation of the G1/S and S phase checkpoints, DNA damage also activates checkpoint arrest in G2 to prevent the passage of DNA lesions to two daughter cells during mitosis. At this stage, entry into mitosis is controlled by the activity of the cyclin-dependent kinase Cdc2. Maintenance of the inhibitory phosphorylations on Cdc2 is essential for G2 checkpoint activation. ATM and ATR indirectly modulate the phosphorylation status of these sites in response to DNA damage. These DNA damage checkpoint pathways all share common upstream signaling pathways made up of the ATM/ATR transducer and Chk2/Chk1 effector kinases .
Activation of the G2 checkpoint after genotoxic stress involves ATM-mediated phosphorylation and activation of the Chk1 and Chk2 kinases [81, 82]. It is proposed that direct inhibition of Cdc25 activity by Chk1 is sufficient for proficient checkpoint regulation of Cdc25 and that Cdc25C might be inhibited by another upstream kinase, Plk1 . The activity of Plk1 is inhibited in the G2 phase of human tumor cells exposed to ionizing radiation, camptothecin, and doxorubicin. Further, expression of a mutant Plk1 in which residues necessary for Plk1 activation are altered, prevents Plk1 inactivation, and leads to G2 override in cells treated with doxorubicin .
In addition to a role in G1/S checkpoint function, p53-mediated signaling plays an integral role in maintenance of the G2 checkpoint delay after activation of the checkpoint. p53 is believed to exert G2 checkpoint responses through transcriptional upregulation of the downstream target genes p21, 14-3-3, and GADD45. Similar to its regulation of the cyclin D1/cdk4,6 or cyclin E/cdk2 complexes at the G1/S checkpoint, p21 can bind to and inhibit the cyclin B1/cdc2 complex and inhibit cyclin-activated kinase-mediated cdc2 activation . The p53-dependent increase in 14-3-3-modulates the subcellular localization of the cyclin B1/Cdc2 complex, as the binding of 14-3-3 to cdc2 results in retention of the kinase in the cytoplasm . The p53-mediated GADD45-dependent G2 arrest is induced only after specific types of DNA damage, as lymphocytes from GADD45 knockout mice failed to arrest after exposure to UV radiation but retained the G2 checkpoint initiated by ionizing radiation .
3.4. Spindle Checkpoint
The mitotic spindle checkpoint monitors spindle microtubule structure, chromosome alignment on the spindle, and chromosome attachment to kinetochores during mitosis . The spindle checkpoint delays the onset of chromosome segregation during anaphase until any defects in the mitotic spindle are corrected. Unattached kinetochores are thought to be the source of the checkpoint signal, and mechanical tension at the kinetochore dictates whether the checkpoint is initiated or not . Activation of the spindle checkpoint prevents mitotic progression through inhibition of the anaphase-promoting complex activator, Cdc20 . Mediators of the spindle checkpoint pathway include the Mad2, Bub1, and Bub3 proteins . Mad2 localizes to the kinetochores during prometaphase until alignment of the chromosomes occurs in metaphase and regulates mitotic exit by interaction with components of the APC machinery (such as Cdc20) that mediate anaphase entry. Bub1 and Bub3 also localize to kinetochores and regulate chromosome/kinetochore interactions, and both are required for cell-cycle arrest after disruption of microtubule dynamics during mitosis. Inactivating mutations in Bub1 have been identified in human colon carcinoma cell lines, suggesting that disruption of the spindle checkpoint could occur during tumor progression.
Integral to cell-cycle regulation is the proper coordination of mitotic exit and subsequent S phase entry. After DNA synthesis, cells have a tetraploid (4N) DNA content that is reduced to a diploid (2N) DNA content in each daughter cell after successful completion of mitosis. Intact checkpoint pathways are needed to prevent the S phase entry of cells that have failed to properly segregate their chromosomes during mitosis. Cells with defective spindle checkpoint function can exit from mitosis with a 4N DNA content. These cells can inappropriately continue to the next cell-cycle division and, in the absence of a functional G1/S checkpoint, enter S phase with a 4N DNA content; this process is known as endoreduplication. Endoreduplication results in the generation of polyploid cells, that is, cells with a 4N or greater DNA content after mitotic exit. Cells that are RB-, p53-, p21-, or p16-deficient can endoreduplicate after microtubule inhibitor treatment [91, 92].
The G1 cell-cycle regulators, however, do not directly regulate the mitotic arrest induced by microtubule inhibitors; rather, absence of these proteins allows deregulated CDK2 activity, the precise control of which is required for normal cells to maintain proper coupling of mitotic exit and S phase entry [92, 93]. Thus, in addition to playing a role in checkpoint function after DNA damage, proteins that mediate the G1/S checkpoint through regulation of cdk2 activity also prevent inappropriate S phase entry after an abnormal mitotic exit and are critical to proper coordination of S phase and mitosis.
In Figure 1, the most important cell-cycle molecules involved in cancer pathogenesis and progression are depicted.
4. Cell-Cycle Proteins and Lung Cancer
4.1. Components of the G1 to S Phase Transition in Lung Cancers
Concerning lung cancer, most of the studies about the cell-cycle regulation in this neoplasm have been performed on the G1/S phase. The retinoblastoma gene family consists of three members, the product of the retinoblastoma gene (pRb), which is one of the most studied tumor suppressor genes, and two related proteins, p130 and p107, which have been shown to be structurally and functionally similar to pRb . Sequence analysis of these two proteins shows they share large regions of homology with pRb, especially in two discontinuous domains which make up the “pocket region” [95, 96]. The pocket domain is required for binding the three members of the Rb-family with several viral transforming oncoproteins, as well as with members of the E2F family . Both p130 and p107, like pRb, display growth suppressive properties, although the growth arrests mediated by the three pocket proteins are not identical. This suggests that, although the different members of the retinoblastoma gene family may complement each other, they are not fully redundant functionally [98, 99]. The Rb pocket proteins (pRb, p107, and p130) play a critical role in G1/S progression, at least in part, through binding and inactivation of factors (e.g., E2F) that promote transcription of genes required for DNA replication. Although p130, p107, similarly to pRb, interact with members of the E2F transcription family and have similar functional consequences, each pocket protein has a different temporal profile of interaction with different E2F/DP1 complexes. The binding of p130 to these complexes is detected predominantly during GO, 12–15 while that of p107 is detected during the G1 and S phases [98–104]. Thus, it is possible to propose a simple model in which the three members of the retinoblastoma gene family bind and modulate the activity of the E2F/DP complexes, as well as other transcription factors. In this model, the binding is regulated by different upstream signals such as cyclin/cdk complexes or viral oncoproteins. The flexibility of this pathway could explain the distinct activities of the three pocket proteins in the regulation of cellular division and cellular differentiation. Active (underphosphorylated) pRb can be inactivated and induced to release transcription factors when it is hyperphosphorylated (in mid-late G1) by cyclin/cdk4,6 complexes. In turn, the cyclin/cdk complexes are negatively regulated by two sets of inhibitors, the p21 family (p21Cip1/Waf1, p27Kip1, and p57Kip2) which interacts with all cyclin/cdks and the p16 family (INK4) which selectively inhibits the cdk4,6-mediated phosphorylation of pRb .
Disruption of this pathway is a prominent abnormality in both NSCLC and SCLC, albeit through different mechanisms. Variances in RB mRNA or protein expression in terms of absence, reduction in quantity, or alteration in function have been observed in this neoplasm [106–109]. The function of pRb is inactivated in more than 90% of SCLCs  as a result of different mechanisms including point mutations and abnormal mRNA expression . Changes in the other pocket proteins (p107 and p130) have been detected in a minority of cases . In contrast to SCLC, the majority of NSCLC cases exhibit abnormalities in the upstream regulators of the pRb pathway, including inactivation of p16 [113, 114] through different mechanisms , reduced levels of p27Kip [116, 117], and enhanced expression of cyclin D1 . It is likely that inactivation of cdk4,6 inhibitors (p16) and overexpression of cyclin D1 bypass the pRb checkpoint allowing progress through G1 into DNA synthesis [119, 120]. Immunohistochemical analyses of the RB gene product expression have been performed in malignant tissues from this human neoplasm . Because the three members of the retinoblastoma protein family exhibit different growth suppressive properties, suggesting that they are not fully functionally redundant, our research group investigates their pattern of expression in large group of specimens of lung cancer, using an immunohistochemical approach. These Rb-family members displayed distinctive patterns when compared and contrasted with the different parameters. The highest percentage of undetectable levels in all the specimens examined and the tightest inverse correlation (P value) with the histological grading and with PCNA expression in the most aggressive tumor types were found for pRb2/p130, which may suggest an important role for this protein in the pathogenesis and progression of lung cancer .
The tumor suppressor protein p53 also regulates progression through the G1 checkpoint of the cell-cycle. In particular, p53 is activated in response to DNA damage and serves to arrest cell-cycle progression in G1 and hence allow time for DNA repair. The fundamental importance of p53 in lung cancer is highlighted by the frequency of its mutations, 80% in SCLC and 50% in NSCLC . It is recognized that p53 is a point of convergence of a complex network of signaling pathways that regulate its level in the cell. In turn, p53 binds to specific DNA sequences and transactivates a group of target genes (including the cell-cycle inhibitor p21Waf1/Cip1), thereby inhibiting cell proliferation and promoting apoptosis. Recent developments in this area have focused on the identification of p53-related genes such as p73  and in the elucidation of their role in lung cancer [125, 126]. In Figure 2, exemplificative immunostaining in non-small-cell lung cancers for the retinoblastoma proteins family and for p53 are depicted.
While several of the factors involved in regulating cell-cycle control have been investigated in lung cancer, few studies have examined multiple factors in the same tumor series. Our research group recently sets up a study to evaluate the expression of p53, p21, p16, and PCNA proteins in a large series of non-small-cell lung cancers (NSCLCs) to assess the integrity of cell-cycle checkpoints in these tumors, to evaluate the coexpression of these proteins, and, finally, to examine the relationship between these cell-cycle regulators and the clinicopathological features of NSCLCs, including their ability to predict survival in NSCLC patients . When we looked at the correlation between clinicopathological data and expression of cell-cycle proteins, we found a negative correlation between lymph nodes status and p21, and p16 expression, suggesting a possible role for these two proteins in the progression of the disease. Interestingly, no correlation has been identified between p16, p21, and p53 expression. When we investigated by univariate analysis the correlation between different protein expressions and survival, we found that all the cell-cycle markers analyzed except for PCNA had a statistically significant correlation with survival. This result is in agreement with numerous data published about the cell-cycle checkpoints investigated in this paper and lung cancer [128–132]. Surprisingly, when we performed multivariate analysis, the only immunohistochemical parameter that resulted to influence overall survival was p16. This result is in agreement with the proposed hypothesis that the great majority of lung cancer samples have inactivated the RB/p16 tumor suppressor pathway. Among the clinical parameters, tumor staging was the only factor to influence survival in multivariate analysis. Finally, we grouped the lung cancer specimens based on p21 and p16 status. Interestingly, we found that the group of lung cancer specimens having both p21 and p16 negative displayed a significant shorter overall survival. Numerous data from the literature suggest the existence of a functional collaboration between distinct CDK inhibitor genes . Indeed, it has been recently demonstrated that cell-cycle inhibition by p16 is associated with a posttranscriptional induction of p21 and a strong inhibition of cyclin E-cdk2 kinase activity . Moreover, it has been shown that members of the p21 family of proteins promote the association of D-type cyclins with CDKs by counteracting the effects of p16 molecules . It has been, therefore, proposed that functional cooperation between different cell-cycle inhibitor proteins constitutes another level of regulation in cell growth control and tumor suppression. Taking into account the complicated functional network constituted by the cell-cycle regulator proteins, it appears evident that knowledge of the level of expression of these factors, and their coregulation, may be important in predicting patient clinical response to therapy.
In a different study, we determined the prognostic role of PCNA, p53, p27, pRb/p105, pRb2/p130, Cyclin D1, and p16 expression in a well-defined set of patients who underwent radical surgical treatment for non-small-cell lung cancer and had long-term followup . Moreover, we explored the association of molecular markers with pathologic and clinical characteristics of this lung cancer population. The availability of the expression status of all tumor markers in the same set of patients provided a unique opportunity to determine whether alterations in p53, p27, pRb/p105, pRb2/p130, Cyclin D1, and p16 expression exert a cooperative or synergistic effect on lung cancer progression, metastasis, and survival. Surprisingly, when we performed multivariate analysis, the only immunohistochemical parameters that resulted to influence overall survival were p16, Cyclin D1, and pRb2/p130. Moreover, we showed that simultaneous loss of expression of three of these factors, the cyclin D1, the cyclin-dependent kinase inhibitor p16, and the tumor suppressor pRb2/p130, identified a group of patients with worse prognosis. This result is in agreement with the proposed hypothesis that the great majority of lung cancer samples have inactivated the p16/Cyclin D1/retinoblastoma tumor suppressor pathway . Our results demonstrate detection of an aberrant p53 in a discrete number of the specimens, which, however, does not correlate with patient survival in multivariate analysis. This finding contrasts with a previous study from our group which reported on 61 non-small-cell lung cancers and does not clarify the still debated prognostic role of p53 in lung cancer patients . A recent study aimed to qualitatively review the association between p53 alterations and patient outcome by analyzing data from published papers, through a meta-analysis, showed that p53 mutation is a significant marker of poor prognosis in patients with lung adenocarcinoma . Finally, we grouped the lung cancer specimens based on Cyclin D1, pRb2/p130, and p16 status. Interestingly, we found that the group of lung cancer specimens having three adverse prognostic factors displayed a significant shorter overall survival. Numerous data from the literature suggest the existence of a functional collaboration between distinct CDK inhibitor genes . It has been therefore proposed that functional cooperation between different cell-cycle regulator proteins constitutes another level of regulation in cell growth control and tumor suppression .
4.2. Components of G2 and M Phases in Lung Tumors
High levels of cyclin B1 are observed in NSCLC . Cyclin B1 has also been reported as a significant prognostic factor in NSCLC in multivariate analysis, suggesting that cyclin B1 expression may be a prognostic marker for these patients. CDK1 activity is controlled by phosphorylation, and this process is regulated by the WEE1 and PLK1 kinases. Indeed, downregulation of WEE1 expression has been reported in lung tumors . By contrast, elevated levels of PLK1 are observed in NSCLC and overexpression of PLK1 is a negative prognostic factor in NSCLC patients . Overexpression of Aurora A transcript and protein has been reported in NSCLC and was correlated with poor differentiation . Although less extensively studied than Aurora genes, other mitotic genes display lung cancer-associated altered expression. They include microtubule-associated proteins such as TPX2 and TACC3 whose overexpression has been associated with poor clinical outcome .
4.3. Components of Cell-Cycle Checkpoints in Lung Tumors
CHFR, a mitotic checkpoint gene that delays chromosome condensation in response to microtubule poisons, has been described mutated and methylated in NSCLC . Somatic mutations of ATM that correlate with smoking history and the presence of DNA repair defects are also detected in NSCLC . Consistently, downregulation or absence of CHK2 expression has been reported in NSCLC, mainly due to hypermethylation of the CHK2 gene promoter . Defects in the SAC can lead to premature separation of sister chromatids and could facilitate chromosomal instability, which may favor tumor progression. Somatic mutations of several SAC regulators, such as Bub1 and Mad1, have been reported in lung tumors, but the effect of these mutations on mitotic checkpoint signaling has not been examined [148, 149]. Finally, RASSF1A, a key negative regulator for mitosis progression and well-known tumor suppressor, undergoes frequent tumor-specific epigenetic inactivation in a wide range of tumors, and especially in lung cancer .
In conclusion, all the works mentioned here provide useful information on the prognosis of newly diagnosed cases of lung cancer and would allow researchers to recognize a subgroup of patients with significantly improved survival, in which it could be possible to achieve better response to therapy. Nevertheless, targeting multiple checkpoint proteins may represent a good therapeutic strategy for the development of new molecular treatments for lung cancer. The data presented in this paper support this hypothesis and strongly suggest further works aimed at investigating the simultaneous expression of numerous cell-cycle regulators in lung cancer.
- S. Zöchbauer-Müller, A. F. Gazdar, and J. D. Minna, “Molecular pathogenesis of lung cancer,” Annual Review of Physiology, vol. 64, pp. 681–708, 2002.
- T. A. D'Amico, T. A. Aloia, M. B. Moore et al., “Predicting the sites of metastases from lung cancer using molecular biologic markers,” Annals of Thoracic Surgery, vol. 72, no. 4, pp. 1144–1148, 2001.
- C. Lu, J. C. Soria, X. Tang et al., “Prognostic factors in resected stage I non-small-cell lung cancer: a multivariate analysis of six molecular markers,” Journal of Clinical Oncology, vol. 22, no. 22, pp. 4575–4583, 2004.
- R. Feld, L. V. Rubinstein, and T. H. Weisenberger, “Sites of recurrence in resected stage I non-small-cell lung cancer: a guide for future studies,” Journal of Clinical Oncology, vol. 2, no. 12, pp. 1352–1358, 1984.
- T. A. D'Amico, M. Massey, J. E. Herndon II, M. B. Moore, and D. H. Harpole, “A biologic risk model for stage I lung cancer: immunohistochemical analysis of 408 patients with the use of ten molecular markers,” Journal of Thoracic and Cardiovascular Surgery, vol. 117, no. 4, pp. 736–743, 1999.
- R. Bordoni, “Consensus conference: multimodality management of early- and intermediate-stage non-small cell lung cancer,” The Oncologist, vol. 13, no. 9, pp. 945–953, 2008.
- D. Liu, C. L. Huang, K. Kameyama et al., “E-cadherin expression associated with differentiation and prognosis in patients with non-small cell lung cancer,” Annals of Thoracic Surgery, vol. 71, no. 3, pp. 949–954, 2001.
- T. A. D'Amico, T. A. Aloia, M. B. H. Moore et al., “Molecular biologic substaging of stage I lung cancer according to gender and histology,” Annals of Thoracic Surgery, vol. 69, no. 3, pp. 882–886, 2000.
- D. R. Shopland, “Tobacco use and its contribution to early cancer mortality with a special emphasis on cigarette smoking,” Environmental Health Perspectives, vol. 103, no. 8, pp. 131–142, 1995.
- V. Esposito, A. Baldi, A. De Luca et al., “Prognostic value of p53 in non-small cell lung cancer: relationship with proliferating cell nuclear antigen and cigarette smoking,” Human Pathology, vol. 28, no. 2, pp. 233–237, 1997.
- V. Esposito, A. Baldi, A. De Luca et al., “Prognostic role of the cyclin-dependent kinase inhibitor p27 in non-small cell lung cancer,” Cancer Research, vol. 57, no. 16, pp. 3381–3385, 1997.
- M. Caputi, V. Esposito, A. Baldi et al., “P21 expression in non-small cell lung cancer: relationship to survival,” The American Journal of Respiratory Cell and Molecular Biology, vol. 18, no. 2, pp. 213–217, 1998.
- A. M. Groeger, M. Caputi, V. Esposito et al., “Expression of p21 in non-small cell lung cancer relationship with PCNA,” Anticancer Research, vol. 20, no. 5, pp. 3301–3306, 2000.
- A. Baldi, V. Esposito, A. De Luca et al., “Differential expression of Rb2/p130 and p107 in normal human tissues and in primary lung cancer,” Clinical Cancer Research, vol. 3, no. 10, pp. 1691–1697, 1997.
- V. Esposito, A. M. Groeger, L. De et al., “Expression of surface protein receptors in lung cancer,” Anticancer Research, vol. 22, no. 6 C, pp. 4039–4044, 2002.
- A. M. Groeger, V. Esposito, A. De Luca et al., “Prognostic value of immunohistochemical expression of p53, BAX, BCL-2 and BCL-Xl in resected non-small cell lung cancer,” Histopathology, vol. 44, no. 1, pp. 54–63, 2004.
- B. Vincenzi, G. Schiavon, M. Silletta et al., “Cell cycle alterations and lung cancer,” Histology and Histopathology, vol. 21, pp. 423–435, 2006.
- V. Esposito, M. Campioni, A. De Luca et al., “Analysis of HtrA1 serine protease expression in human lung cancer,” Anticancer Research, vol. 26, no. 5, pp. 3455–3460, 2006.
- M. Campioni, V. Ambrogi, E. Pompeo et al., “Identification of genes down-regulated during lung cancer progression: a cDNA array study,” Journal of Experimental and Clinical Cancer Research, vol. 27, no. 1, article 38, 2008.
- T. C. Mineo, V. Ambrogi, A. Baldi et al., “Prognostic impact of VEGF, CD31, CD34, and CD105 expression and tumor vessel invasion after radical surgery for IB-IIA non-small cell lung cancer,” Journal of Clinical Pathology, vol. 57, no. 6, pp. 591–597, 2004.
- H. Matsushime, M. E. Ewen, D. K. Strom et al., “Identification and properties of an atypical catalytic subunit (p34PSK-J3/cdk4) for mammalian D type G1 cyclins,” Cell, vol. 71, no. 2, pp. 323–334, 1992.
- C. J. Sherr and J. M. Roberts, “CDK inhibitors: positive and negative regulators of G1-phase progression,” Genes and Development, vol. 13, no. 12, pp. 1501–1512, 1999.
- S. Jinno, K. Suto, A. Nagata et al., “Cdc25A is a novel phosphatase functioning early in the cell cycle,” The EMBO Journal, vol. 13, no. 7, pp. 1549–1556, 1994.
- P. Saha, Q. Eichbaum, E. D. Silberman, B. J. Mayer, and A. Dutta, “p21CIP1 and Cdc25A: competition between an inhibitor and an activator of cyclin-dependent kinases,” Molecular and Cellular Biology, vol. 17, no. 8, pp. 4338–4345, 1997.
- I. Hoffmann, G. Draetta, and E. Karsenti, “Activation of the phosphatase activity of human cdc25A by a cdk2-cyclin E dependent phosphorylation at the G1/S transition,” The EMBO Journal, vol. 13, no. 18, pp. 4302–4310, 1994.
- I. Blomberg and I. Hoffmann, “Ectopic expression of Cdc25A accelerates the G1/S transition and leads to premature activation of cyclin E- and cyclin A-dependent kinases,” Molecular and Cellular Biology, vol. 19, no. 9, pp. 6183–6194, 1999.
- C. Lammer, S. Wagerer, R. Saffrich, D. Mertens, W. Ansorge, and I. Hoffmann, “The cdc25B phosphatase is essential for the G2/M phase transition in human cells,” Journal of Cell Science, vol. 111, no. 16, pp. 2445–2453, 1998.
- P. A. Garner-Hamrick and C. Fisher, “Antisense phosphorothioate oligonucleotides specifically down-regulate cdc25B causing S-phase delay and persistent antiproliferative effects,” International Journal of Cancer, vol. 76, no. 5, pp. 720–728, 1998.
- U. Strausfeld, A. Fernandez, J. P. Capony et al., “Activation of p34(cdc2) protein kinase by microinjection of human cdc25C into mammalian cells. Requirement for prior phosphorylation of cdc25C by p34(cdc2) on sites phosphorylated at mitosis,” Journal of Biological Chemistry, vol. 269, no. 8, pp. 5989–6000, 1994.
- P. D. Adams, “Regulation of the retinoblastoma tumor suppressor protein by cyclin/cdks,” Biochimica et Biophysica Acta, vol. 1471, no. 3, pp. M123–M133, 2001.
- W. R. Sellers and W. G. Kaelin, “pRB as a modulator of transcription,” Biochimica et Biophysica Acta, vol. 1288, pp. M1–M5, 1996.
- T. L. Sladek, “E2F transcription factor action, regulation and possible role in human cancer,” Cell Proliferation, vol. 30, no. 3-4, pp. 97–105, 1997.
- E. S. Knudsen, C. Buckmaster, T. T. Chen, J. R. Feramisco, and J. Y. J. Wang, “Inhibition of DNA synthesis by RB: effects on G1/S transition and S-phase progression,” Genes and Development, vol. 12, no. 15, pp. 2278–2292, 1998.
- W. R. Sellers and W. G. Kaelin, “Role of the retinoblastoma protein in the pathogenesis of human cancer,” Journal of Clinical Oncology, vol. 15, no. 11, pp. 3301–3312, 1997.
- J. F. Diffley, “Eukaryotic DNA replication,” Current Opinion in Cell Biology, vol. 6, no. 3, pp. 368–372, 1994.
- A. Rowley, S. J. Dowell, and J. F. Diffley, “Recent developments in the initiation of chromosomal DNA replication: a complex picture emerges,” Biochimica et Biophysica Acta, vol. 1217, no. 3, pp. 239–256, 1994.
- S. P. Bell and B. Stillman, “ATP-dependent recognition of eukaryotic origins of DNA replication by a multiprotein complex,” Nature, vol. 357, no. 6374, pp. 128–134, 1992.
- B. Stillman, S. P. Bell, A. Dutta, and Y. Marahrens, “DNA replication and the cell cycle,” Ciba Foundation Symposium, vol. 170, pp. 147–156, 1992.
- A. Dutta and S. P. Bell, “Initiation of DNA replication in eukaryotic cells,” Annual Review of Cell and Developmental Biology, vol. 13, pp. 293–332, 1997.
- G. T. Maine, P. Sinha, and B. K. Tye, “Mutants of S. cerevisiae defective in the maintenance of minichromosomes,” Genetics, vol. 106, no. 3, pp. 365–385, 1984.
- H. Takisawa, S. Mimura, and Y. Kubota, “Eukaryotic DNA replication: from pre-replication complex to initiation complex,” Current Opinion in Cell Biology, vol. 12, no. 6, pp. 690–696, 2000.
- J. Wuarin and P. Nurse, “Regulating S phase: CDKs, licensing and proteolysis,” Cell, vol. 85, no. 6, pp. 785–787, 1996.
- B. Stillman, “Cell cycle control of DNA replication,” Science, vol. 274, no. 5293, pp. 1659–1664, 1996.
- W. Krek and E. A. Nigg, “Differential phosphorylation of vertebrate p34(cdc2) kinase at the G1/S and G2/M transitions of the cell cycle: identification of major phosphorylation sites,” The EMBO Journal, vol. 10, no. 2, pp. 305–316, 1991.
- T. R. Coleman and W. G. Dunphy, “Cdc2 regulatory factors,” Current Opinion in Cell Biology, vol. 6, no. 6, pp. 877–882, 1994.
- A. Hagting, C. Karlsson, P. Clute, M. Jackman, and J. Pines, “MPF localization is controlled by nuclear export,” The EMBO Journal, vol. 17, no. 14, pp. 4127–4138, 1998.
- E. A. Nigg, “Targets of cyclin-dependent protein kinases,” Current Opinion in Cell Biology, vol. 5, no. 2, pp. 187–193, 1993.
- A. Blangy, H. A. Lane, P. d'Hérin, M. Harper, M. Kress, and E. A. Nigg, “Phosphorylation by p34cdc2 regulates spindle association of human Eg5, a kinesin-related motor essential for bipolar spindle formation in vivo,” Cell, vol. 83, no. 7, pp. 1159–1169, 1995.
- P. K. Sorger, M. Dobles, R. Tournebize, and A. A. Hyman, “Coupling cell division and cell death to microtubule dynamics,” Current Opinion in Cell Biology, vol. 9, no. 6, pp. 807–814, 1997.
- S. Llamazares, A. Moreira, A. Tavares et al., “Polo encodes a protein kinase homolog required for mitosis in Drosophila,” Genes and Development, vol. 5, no. 12, pp. 2153–2165, 1991.
- D. O. Morgan, “Regulation of the APC and the exit from mitosis,” Nature Cell Biology, vol. 1, no. 2, pp. E47–E53, 1999.
- P. Gallant and E. A. Nigg, “Cyclin B2 undergoes cell cycle-dependent nuclear translocation and, when expressed as a non-destructible mutant, causes mitotic arrest in HeLa cells,” Journal of Cell Biology, vol. 117, no. 1, pp. 213–224, 1992.
- P. Clute and J. Pines, “Temporal and spatial control of cyclin B1 destruction in metaphase,” Nature Cell Biology, vol. 1, no. 2, pp. 82–87, 1999.
- H. Funabiki, H. Yamano, K. Kumada, K. Nagao, T. Hunt, and M. Yanagida, “Cut2 proteolysis required for sister-chromatid separation in fission yeast,” Nature, vol. 381, no. 6581, pp. 438–441, 1996.
- M. N. Boddy and P. Russell, “DNA replication checkpoint,” Current Biology, vol. 11, no. 23, pp. R953–R956, 2001.
- A. Ho and S. F. Dowdy, “Regulation of G1 cell-cycle progression by oncogenes and tumor suppressor genes,” Current Opinion in Genetics and Development, vol. 12, no. 1, pp. 47–52, 2002.
- E. Aleem, H. Kiyokawa, and P. Kaldis, “Cdc2-cyclin E complexes regulate the G1/S phase transition,” Nature Cell Biology, vol. 7, no. 8, pp. 831–836, 2005.
- S. A. Ezhevsky, A. Ho, M. Becker-Hapak, P. K. Davis, and S. F. Dowdy, “Differential regulation of retinoblastoma tumor suppressor protein by G1 cyclin-dependent kinase complexes in vivo,” Molecular and Cellular Biology, vol. 21, no. 14, pp. 4773–4784, 2001.
- G. Fang, H. Yu, and M. W. Kirschner, “Direct binding of CDC20 protein family members activates the anaphase-promoting complex in mitosis and G1,” Molecular Cell, vol. 2, no. 2, pp. 163–171, 1998.
- J. Y. Hsu, J. D. Reimann, C. S. Sørensen, J. Lukas, and P. K. Jackson, “E2F-dependent accumulation of hEmi1 regulates S phase entry by inhibiting APC(Cdh1),” Nature Cell Biology, vol. 4, no. 5, pp. 358–366, 2002.
- C. Lukas, C. S. Sørensen, E. Kramer et al., “Accumulation of cyclin B1 requires E2F and cyclin-A-dependent rearrangement of the anaphase-promoting complex,” Nature, vol. 401, no. 6755, pp. 815–818, 1999.
- J. Falck, N. Mailand, R. G. Syljuåsen, J. Bartek, and J. Lukas, “The ATM-Chk2-Cdc25A checkpoint pathway guards against radioresistant DNA synthesis,” Nature, vol. 410, no. 6830, pp. 842–847, 2001.
- V. Costanzo, K. Robertson, C. Y. Ying et al., “Reconstitution of an ATM-dependent checkpoint that inhibits chromosomal DNA replication following DNA damage,” Molecular Cell, vol. 6, no. 3, pp. 649–659, 2000.
- M. B. Kastan, O. Onyekwere, D. Sidransky, B. Vogelstein, and R. W. Craig, “Participation of p53 protein in the cellular response to DNA damage,” Cancer Research, vol. 51, no. 23, pp. 6304–6311, 1991.
- N. C. Reich and A. J. Levine, “Growth regulation of a cellular tumour antigen, p53, in nontransformed cells,” Nature, vol. 308, no. 5955, pp. 199–201, 1984.
- E. Reihsaus, M. Kohler, S. Kraiss, M. Oren, and M. Montenarh, “Regulation of the level of the oncoprotein p53 in non-transformed and transformed cells,” Oncogene, vol. 5, no. 1, pp. 137–145, 1990.
- J. W. Harper, G. R. Adami, N. Wei, K. Keyomarsi, and S. J. Elledge, “The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclin-dependent kinases,” Cell, vol. 75, no. 4, pp. 805–816, 1993.
- C. X Deng, P. M. Zhang, J. W. Herper, S. J. Elledge, and P. Leder, “Mice lacking p21(CIP1/WAF1) undergo normal development, but are defective in G1 checkpoint control,” Cell, vol. 82, no. 4, pp. 675–684, 1995.
- T. Waldman, K. W. Kinzler, and B. Vogelstein, “p21 is necessary for the p53-mediated G1 arrest in human cancer cells,” Cancer Research, vol. 55, no. 22, pp. 5187–5190, 1995.
- S. Pavey, S. Conroy, T. Russell, and B. Gabrielli, “Ultraviolet radiation induces p16(CDKN2A) expression in human skin,” Cancer Research, vol. 59, no. 17, pp. 4185–4189, 1999.
- D. E. Quelle, F. Zindy, R. A. Ashmun, and C. J. Sherr, “Alternative reading frames of the INK4a tumor suppressor gene encode two unrelated proteins capable of inducing cell cycle arrest,” Cell, vol. 83, no. 6, pp. 993–1000, 1995.
- N. E. Sharpless and R. A. DePinho, “The INK4A/ARF locus and its two gene products,” Current Opinion in Genetics and Development, vol. 9, no. 1, pp. 22–30, 1999.
- P. L. Porter, K. E. Malone, P. J. Heagerty et al., “Expression of cell-cycle regulators p27(Kip1) and cyclin E, alone and in combination, correlate with survival in young breast cancer patients,” Nature Medicine, vol. 3, no. 2, pp. 222–225, 1997.
- G. Viglietto, M. L. Motti, P. Bruni et al., “Cytoplasmic relocalization and inhibition of the cyclin-dependent kinase inhibitor p27kip1 by PKB/Akt-mediated phosphorylation in breast cancer,” Nature Medicine, vol. 8, no. 10, pp. 1136–1144, 2002.
- C. J. Bakkenist and M. B. Kastan, “DNA damage activates ATM through intermolecular autophosphorylation and dimer dissociation,” Nature, vol. 421, no. 6922, pp. 499–506, 2003.
- Y. Shiloh and M. B. Kastan, “ATM: genome stability, neuronal development, and cancer cross paths,” Advances in Cancer Research, vol. 83, pp. 209–254, 2001.
- D. S. Lim, S. T. Kim, B. Xu et al., “ATM phosphorylates p95/nbs1 in an S-phase checkpoint pathway,” Nature, vol. 404, no. 6778, pp. 613–614, 2000.
- T. Taniguchi, I. Garcia-Higuera, B. Xu et al., “Convergence of the fanconi anemia and ataxia telangiectasia signaling pathways,” Cell, vol. 109, no. 4, pp. 459–472, 2002.
- P. T. Yazdi, Y. Wang, S. Zhao, N. Patel, E. Y. Lee, and J. Qin, “SMC1 is a downstream effector in the ATM/NBS1 branch of the human S-phase checkpoint,” Genes and Development, vol. 16, no. 5, pp. 571–582, 2002.
- B. Xu, A. H. O'Donnell, S. T. Kim, and M. B. Kastan, “Phosphorylation of serine 1387 in Brca1 is specifically required for the Atm-mediated S-phase checkpoint after ionizing irradiation,” Cancer Research, vol. 62, no. 16, pp. 4588–4591, 2002.
- A. Hwang and R. J. Muschel, “Radiation and the G2 phase of the cell cycle,” Radiation Research, vol. 150, no. 5, pp. S52–S59, 1998.
- Y. Sanchez, S. Wong, R. S. Thoma et al., “Conservation of the Chk1 checkpoint pathway in mammals: linkage of DNA damage to Cdk regulation through Cdc25,” Science, vol. 277, no. 5331, pp. 1497–1501, 1997.
- B. Furnari, N. Rhind, and P. Russell, “Cdc25 mitotic inducer targeted by Chk1 DNA damage checkpoint kinase,” Science, vol. 277, no. 5331, pp. 1495–1497, 1997.
- V. A. J. Smits and R. H. Medema, “Checking out the G2/M transition,” Biochimica et Biophysica Acta—Gene Structure and Expression, vol. 1519, no. 1-2, pp. 1–12, 2001.
- V. A. J. Smits, R. Klompmaker, L. Arnaud, G. Rijksen, E. A. Nigg, and R. H. Medema, “Polo-like kinase-1 is a target of the DNA damage checkpoint,” Nature Cell Biology, vol. 2, no. 9, pp. 672–676, 2000.
- S. A. Innocente, J. L. A. Abrahamson, J. P. Cogswell, and J. M. Lee, “p53 regulates a G2 checkpoint through cyclin B1,” Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 5, pp. 2147–2152, 1999.
- T. A. Chan, H. Hermeking, C. Lengauer, K. W. Kinzler, and B. Vogelstein, “14-3-3Sigma is required to prevent mitotic catastrophe after DNA damage,” Nature, vol. 401, no. 6753, pp. 616–620, 1999.
- X. W. Wang, Q. M. Zhan, J. D. Coursen et al., “GADD45 induction of a G2/M cell cycle checkpoint,” Proceedings of the National Academy of Sciences of the United States of America, vol. 96, no. 7, pp. 3706–3711, 1999.
- D. J. Burke, “Complexity in the spindle checkpoint,” Current Opinion in Genetics and Development, vol. 10, no. 1, pp. 26–31, 2000.
- G. J. Gorbsky, “The mitotic spindle checkpoint,” Current Biology, vol. 11, no. 24, pp. R1001–R1004, 2001.
- S. H. Khan and G. M. Wahl, “p53 and pRb prevent rereplication in response to microtubule inhibitors by mediating a reversible G1 arrest,” Cancer Research, vol. 58, no. 3, pp. 396–401, 1998.
- Z. A. Stewart, S. D. Leach, and J. A. Pietenpol, “p21(Waf1/Cip1) inhibition of cyclin E/Cdk2 activity prevents endoreduplication after mitotic spindle disruption,” Molecular and Cellular Biology, vol. 19, no. 1, pp. 205–215, 1999.
- J. S. Lanni and T. Jacks, “Characterization of the p53-dependent postmitotic checkpoint following spindle disruption,” Molecular and Cellular Biology, vol. 18, no. 2, pp. 1055–1064, 1998.
- M. G. Paggi, A. Baldi, F. Bonetto, and A. Giordano, “The retinoblastoma protein family incell cycle and cancer,” Journal of Cellular Biochemistry, vol. 62, no. 3, pp. 418–430, 1996.
- M. E. Ewen, Y. Xing, J. B. Lawrence, and D. M. Livingston, “Molecular cloning, chromosomal mapping, and expression of the cDNA for p107, a retinoblastoma gene product-related protein,” Cell, vol. 66, no. 6, pp. 1155–1164, 1991.
- X. Mayol, X. Grana, A. Baldi, N. Sang, Q. Hu, and A. Giordano, “Cloning of a new member of the retinoblastoma gene family (pRb2) which binds to the E1A transforming domain,” Oncogene, vol. 8, no. 9, pp. 2561–2566, 1993.
- E. W. Lam and N. B. La Thangue, “DP and E2F proteins: coordinating transcription with cell cycle progression,” Current Opinion in Cell Biology, vol. 6, no. 6, pp. 859–866, 1994.
- L. Zhu, S. Van den Heuvel, K. Helin et al., “Inhibition of cell proliferation by p107, a relative of the retinoblastoma protein,” Genes and Development, vol. 7, no. 7, pp. 1111–1125, 1993.
- P. P. Claudio, C. M. Howard, A. Baldi et al., “p130/pRb2 has growth suppressive properties similar to yet distinctive from those of retinoblastoma family members pRb and p107,” Cancer Research, vol. 54, no. 21, pp. 5556–5560, 1994.
- D. Cobrinik, P. Whyte, D. S. Peeper, T. Jacks, and R. A. Weinberg, “Cell cycle-specific association of E2F with the p130 E1A-binding protein,” Genes and Development, vol. 7, no. 12 A, pp. 2392–2404, 1993.
- E. M. Hijmans, P. M. Voorhoeve, R. L. Beijersbergen, L. J. Van't Veer, and R. Bernards, “E2F-5, a new E2F family member that interacts with p130 in vivo,” Molecular and Cellular Biology, vol. 15, no. 6, pp. 3082–3089, 1995.
- G. Vairo, D. M. Livingston, and D. Ginsberg, “Functional interaction between E2F-4 and p130: evidence for distinct mechanisms underlying growth suppression by different retinoblastoma protein family members,” Genes and Development, vol. 9, no. 7, pp. 869–881, 1995.
- H. Jiang, J. Lin, S. M. Young et al., “Cell cycle gene expression and E2F transcription factor complexes in human melanoma cells induced to terminally differentiate,” Oncogene, vol. 11, no. 6, pp. 1179–1189, 1995.
- S. Shirodkar, M. Ewen, J. A. DeCaprio, J. Morgan, D. M. Livingston, and T. Chittenden, “The transcription factor E2F interacts with the retinoblastoma product and a p107-cyclin A complex in a cell cycle-regulated manner,” Cell, vol. 68, no. 1, pp. 157–166, 1992.
- C. Cordon-Cardo, “Mutation of cell cycle regulators: biological and clinical implications for human neoplasia,” The American Journal of Pathology, vol. 147, no. 3, pp. 545–560, 1995.
- J. Yokota, T. Akiyama, Y. K. T. Fung et al., “Altered expression of the retinoblastoma (RB) gene in small-cell carcinoma of the lung,” Oncogene, vol. 3, no. 4, pp. 471–475, 1988.
- J. W. Harbour, S. L. Lai, J. Whang-Peng, A. F. Gazdar, J. D. Minna, and F. J. Kaye, “Abnormalities in structure and expression of the human retinoblastoma gene in SCLC,” Science, vol. 241, no. 4863, pp. 353–357, 1988.
- H. J. Xu, S. X. Hu, P. T. Cagle, G. E. Moore, and W. F. Benedict, “Absence of retinoblastoma protein expression in primary non-small cell lung carcinomas,” Cancer Research, vol. 51, no. 10, pp. 2735–2739, 1991.
- H. J. Xu, D. C. Quinlan, A. G. Davidson et al., “Altered retinoblastoma protein expression and prognosis in early-stage non-small-cell lung carcinoma,” Journal of the National Cancer Institute, vol. 86, no. 9, pp. 695–699, 1994.
- R. Salgia and A. T. Skarin, “Molecular abnormalities in lung cancer,” Journal of Clinical Oncology, vol. 16, no. 3, pp. 1207–1217, 1998.
- V. Gouyer, S. Gazzéri, I. Bolon, C. Drevet, C. Brambilla, and E. Brambilla, “Mechanism of retinoblastoma gene inactivation in the spectrum of neuroendocrine lung tumors,” The American Journal of Respiratory Cell and Molecular Biology, vol. 18, no. 2, pp. 188–196, 1998.
- K. Helin, K. Holm, A. Niebuhr et al., “Loss of the retinoblastoma protein-related p130 protein in small cell lung carcinoma,” Proceedings of the National Academy of Sciences of the United States of America, vol. 94, no. 13, pp. 6933–6938, 1997.
- H. Tanaka, Y. Fujii, H. Hirabayashi et al., “Disruption of the RB pathway and cell-proliferative activity in non-small-cell lung cancers,” International Journal of Cancer, vol. 79, no. 2, pp. 111–115, 1998.
- K. Kashiwabara, T. Oyama, T. Sano, T. Fukuda, and T. Nakajima, “Correlation between methylation status of the p16/CDKN2 gene and the expression of p16 and Rb proteins in primary non-small cell lung cancers,” International Journal of Cancer, vol. 79, no. 3, pp. 215–220, 1998.
- S. Gazzeri, V. Gouyer, C. Vour'ch, C. Brambilla, and E. Brambilla, “Mechanisms of p16(INK4A) inactivation in non small-cell lung cancers,” Oncogene, vol. 16, no. 4, pp. 497–504, 1998.
- H. Kawana, J. I. Tamaru, T. Tanaka et al., “Role of p27(Kip1) and cyclin-dependent kinase 2 in the proliferation of non-small cell lung cancer,” The American Journal of Pathology, vol. 153, no. 2, pp. 505–513, 1998.
- Y. Yatabe, A. Masuda, T. Koshikawa et al., “p27(KIP1) in human lung cancers: differential changes in small cell and non-small cell carcinomas,” Cancer Research, vol. 58, no. 5, pp. 1042–1047, 1998.
- A. Marchetti, C. Doglioni, M. Barbareschi et al., “Cyclin D1 and retinoblastoma susceptibility gene alterations in non-small cell lung cancer,” International Journal of Cancer, vol. 75, no. 2, pp. 187–192, 1998.
- G. I. Shapiro, C. D. Edwards, M. E. Ewen, and B. J. Rollins, “p16(INK4a) participates in a G1 arrest checkpoint in response to DNA damage,” Molecular and Cellular Biology, vol. 18, no. 1, pp. 378–387, 1998.
- B. Driscoll, L. Wu, S. Buckley, F. L. Hall, K. D. Anderson, and D. Warburton, “Cyclin D1 antisense RNA destabilizes pRb and retards lung cancer cell growth,” The American Journal of Physiology, vol. 273, no. 5, pp. L941–L949, 1997.
- M. Higashiyama, O. Doi, K. Kodama, H. Yokouchi, and R. Tateishi, “Retinoblastoma protein expression in lung cancer: an immunohistochemical analysis,” Oncology, vol. 51, no. 6, pp. 544–551, 1994.
- A. Baldi, V. Esposito, A. De Luca et al., “Differential expression of the retinoblastoma gene family members pRb/p105, p107, and pRb2/p130 in lung cancer,” Clinical Cancer Research, vol. 2, no. 7, pp. 1239–1245, 1996.
- M. L. Agarwal, W. R. Taylor, M. V. Chernov, O. B. Chernova, and G. R. Stark, “The p53 network,” Journal of Biological Chemistry, vol. 273, no. 1, pp. 1–4, 1998.
- C. A. Jost, M. C. Marin, and W. G. Kaelin Jr., “p73 is a human p53-related protein that can induce apoptosis,” Nature, vol. 389, no. 6647, pp. 191–194, 1997.
- M. Mai, A. Yokomizo, C. Qian et al., “Activation of p73 silent allele in lung cancer,” Cancer Research, vol. 58, no. 11, pp. 2347–2349, 1998.
- S. Nomoto, N. Haruki, M. Kondo et al., “Search for mutations and examination of allelic expression imbalance of the p73 gene at 1p36.33 in human lung cancers,” Cancer Research, vol. 58, no. 7, pp. 1380–1383, 1998.
- V. Esposito, A. Baldi, B. Vincenzi et al., “Analysis of cell cycle regulator proteins in non-small cell lung cancer,” Journal of Clinical Pathology, vol. 57, no. 1, pp. 58–63, 2004.
- T. Mitsudomi, N. Hamajima, M. Ogawa, and T. Takahashi, “Prognostic significance of p53 alterations in patients with non-small cell lung cancer: a meta-analysis,” Clinical Cancer Research, vol. 6, no. 10, pp. 4055–4063, 2000.
- T. Shoji, F. Tanaka, T. Takata et al., “Clinical significance of p21 expression in non-small-cell lung cancer,” Journal of Clinical Oncology, vol. 20, no. 18, pp. 3865–3871, 2002.
- J. X. Zhou, G. A. Niehans, A. Shar, J. B. Rubins, S. P. Frizelle, and R. A. Kratzke, “Mechanisms of G1 checkpoint loss in resected early stage non-small cell lung cancer,” Lung Cancer, vol. 32, no. 1, pp. 27–38, 2001.
- F. J. Kaye, “Rb and cyclin dependent kinase pathways: defining a distinction between RB and p16 loss in lung cancer,” Oncogene, vol. 21, no. 45, pp. 6908–6914, 2002.
- A. M. Groeger, M. Caputi, V. Esposito et al., “Independent prognostic role of p16 expression in lung cancer,” Journal of Thoracic and Cardiovascular Surgery, vol. 118, no. 3, pp. 529–535, 1999.
- D. S. Franklin, V. L. Godfrey, D. A. O'Brien, C. Deng, and Y. Xiong, “Functional collaboration between different cyclin-dependent kinase inhibitors suppresses tumor growth with distinct tissue specificity,” Molecular and Cellular Biology, vol. 20, no. 16, pp. 6147–6158, 2000.
- J. Mitra, C. Y. Dai, K. Somasundaram et al., “Induction of p21 and inhibition of cdk2 mediated by the tumor suppressor p16,” Molecular and Cellular Biology, vol. 19, no. 5, pp. 3916–3928, 1999.
- D. Parry, D. Mahony, K. Wills, and E. Lees, “Cyclin D-CDK subunit arrangement is dependent on the availability of competing INK4 and p21 class inhibitors,” Molecular and Cellular Biology, vol. 19, no. 3, pp. 1775–1783, 1999.
- V. Esposito, A. Baldi, A. De Luca et al., “Cell cycle related proteins as prognostic parameters in radically resected non small cell lung cancer (NSCLC),” Journal of Clinical Pathology, vol. 58, no. 7, pp. 734–739, 2005.
- F. J. Kaye, “RB and cyclin dependent kinase pathways: defining a distinction between RB and p16 loss in lung cancer,” Oncogene, vol. 21, no. 45, pp. 6908–6914, 2002.
- V. Esposito, A. De Luca, A. Baldi et al., “Altered expression of p53 and Rb tumor suppressor genes in lung cancer: Relationship with survival,” International Journal of Oncology, vol. 9, no. 3, pp. 439–443, 1996.
- A. De Luca, V. Esposito, A. Baldi, and A. Giordano, “The retinoblastoma gene family and its role in proliferation, differentiation and development,” Histology and Histopathology, vol. 11, no. 4, pp. 1029–1034, 1996.
- J. Geradts, K. M. Fong, P. V. Zimmerman, R. Maynard, and J. D. Minna, “Correlation of abnormal RB, p16(ink4a), and p53 expression with 3p loss of heterozygosity, other genetic abnormalities, and clinical features in 103 primary non-small cell lung cancers,” Clinical Cancer Research, vol. 5, no. 4, pp. 791–800, 1999.
- T. Yoshida, S. Tanaka, A. Mogi, Y. Shitara, and H. Kuwano, “The clinical significance of Cyclin B1 and Wee1 expression in non-small-cell lung cancer,” Annals of Oncology, vol. 15, no. 2, pp. 252–256, 2004.
- G. Wolf, R. Elez, A. Doermer et al., “Prognostic significance of polo-like kinase (PLK) expression in non-small cell lung cancer,” Oncogene, vol. 14, no. 5, pp. 543–549, 1997.
- H. T. Xu, L. Ma, F. J. Qi et al., “Expression of serine threonine kinase 15 is associated with poor differentiation in lung squamous cell carcinoma and adenocarcinoma,” Pathology International, vol. 56, no. 7, pp. 375–380, 2006.
- C. K. Jung, J. H. Jung, G. S. Park, A. Lee, C. S. Kang, and K. Y. Lee, “Expression of transforming acidic coiled-coil containing protein 3 is a novel independent prognostic marker in non-small cell lung cancer,” Pathology International, vol. 56, no. 9, pp. 503–509, 2006.
- G. Mariatos, J. Bothos, P. Zacharatos et al., “Inactivating mutations targeting the chfr mitotic checkpoint gene in human lung cancer,” Cancer Research, vol. 63, no. 21, pp. 7185–7189, 2003.
- L. Ding, G. Getz, D. A. Wheeler et al., “Somatic mutations affect key pathways in lung adenocarcinoma,” Nature, vol. 455, no. 7216, pp. 1069–1075, 2008.
- P. Zhang, J. Wang, W. Gao, B. Z. Yuan, J. Rogers, and E. Reed, “CHK2 kinase expression is down-regulated due to promoter methylation in non-small cell lung cancer,” Molecular Cancer, vol. 3, article 14, 2004.
- A. Gemma, M. Seike, Y. Seike et al., “Somatic mutation of the hBUB1 mitotic checkpoint gene in primary lung cancer,” Genes Chromosomes and Cancer, vol. 29, no. 3, pp. 213–218, 2000.
- S. Nomoto, N. Haruki, T. Takahashi et al., “Search for in vivo somatic mutations in the mitotic checkpoint gene, hMAD1, in human lung cancers,” Oncogene, vol. 18, no. 50, pp. 7180–7183, 1999.
- A. Agathanggelou, W. N. Cooper, and F. Latif, “Role of the Ras-association domain family 1 tumor suppressor gene in human cancers,” Cancer Research, vol. 65, no. 9, pp. 3497–3508, 2005.