Table of Contents Author Guidelines Submit a Manuscript
PPAR Research
Volume 2008, Article ID 906542, 8 pages
http://dx.doi.org/10.1155/2008/906542
Review Article

PPAR Ligands as Antitumorigenic and Antiangiogenic Agents

Department of Medicine, Division of Nephrology and Hypertension, S-3223 Medical Center North, Vanderbilt University, Nashville, TN 37232, USA

Received 28 May 2008; Accepted 1 July 2008

Academic Editor: Dipak Panigrahy

Copyright © 2008 Ambra Pozzi and Jorge H. Capdevila. 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.

Abstract

Peroxisome proliferator-activated receptors (PPARs) belong to the nuclear receptor family of ligand-activated transcription factors. This subfamily is composed of three members—PPAR , PPAR , and PPAR —that differ in their cell and tissue distribution as well as in their target genes. PPAR is abundantly expressed in liver, brown adipose tissue, kidney, intestine, heart, and skeletal muscle; and its ligands have been used to treat diseases such as obesity and diabetes. The recent finding that members of the PPAR family, including the PPAR , are expressed by tumor and endothelial cells together with the observation that PPAR ligands regulate cell growth, survival, migration, and invasion, suggested that PPARs also play a role in cancer. In this review, we focus on the contribution of PPAR to tumor and endothelial cell functions and provide compelling evidence that PPAR can be viewed as a new class of ligand activated tumor “suppressor” gene with antiangiogenic and antitumorigenic activities. Given that PPAR ligands are currently used in medicine as hypolipidemic drugs with excellent tolerance and limited toxicity, PPAR activation might offer a novel and potentially low-toxic approach for the treatment of tumor-associated angiogenesis and cancer.

1. The Peroxisomal Proliferator-Activated Receptors (PPARs)

PPARs nuclear receptors that regulate many physiological processes, including lipid and glucose homeostasis, inflammation, and wound healing [1]. Three PPAR isotypes have been identified: α, δ (or β), and γ. Upon ligand binding, PPARs form heterodimers with the retinoic acid receptor and interact with specific response elements in the promoter region of target genes [2]. Although PPARs share extensive structural homology, each isotype appears to possess distinct functions. PPARγ is expressed mainly in adipose tissue and at lower levels in intestine and immune cells [35]. It controls adipocyte differentiation, glucose and lipid homeostasis [57] and has been implicated in the pathophysiology of insulin resistance and atherosclerosis [1, 8]. PPARγ ligands include long-chain fatty acids, prostaglandins, and other eicosanoids [4]. Among the synthetic PPARγ ligands, the thiazolidinediones are currently used as insulin sensitizers in patients with type-2 diabetes [9]. PPARδ is ubiquitously expressed and it is most abundant in brain, colon, and skin [10, 11], and binds molecules such as fatty acids and prostaglandins [4].

PPARα is primarily expressed in liver, brown adipose tissue, kidney, intestine, heart, and skeletal muscle. This receptor controls fatty acid metabolism and transport, peroxisomal and mitochondrial β-oxidation [3, 4]. Moreover, this receptor has been implicated in the pathophysiology of inflammation and cardiovascular diseases [12]. Several compounds bind PPARα, including fatty and phytanic acids [4], as well as the fibric acid derivatives used in medicine for the treatment of hyperlipidemias [1].

2. PPARs and Cancer

The observation that members of the PPAR family are expressed by tumor and endothelial cells [13, 14] together with the finding that PPAR ligands regulate cell growth, survival, migration, and invasion [15, 16] prompted investigators to determine whether these receptors play a role in the pathophysiology of tumorigenesis and angiogenesis [17, 18].

The anticancer effects of PPARγ agonists have been extensively studied because of their antiproliferative, proapoptotic, antiapoptotic, and differentiation-promoting activities [19]. In this context, activation of PPARγ has been reported to reduce tumor cell proliferation and invasion [20] and to enhance apoptosis [21]. PPARγ ligands also regulate endothelial cell growth, migration, and angiogenesis [2225], and influence the progression of vascular inflammation and tumorigenesis [26, 27]. Moreover, disruption of the PPARγ gene in the intestine enhances tumorigenesis in mice [28]. Although these studies suggest that PPARγ functions as a tumor suppressor factor and its activation might be beneficial for patients with tumors, PPARγ agonists have been shown also to increase the frequency of colon tumors [29] and to promote edema [30].

In contrast to PPARγ, PPARδ has been described as protumorigenic as its ligand-mediated activation increases tumor-associated angiogenesis [31]. Moreover, treatment of mice with PPARδ antagonists or crossing these mice with PPARδ-null mice prevents tumor growth and angiogenesis [31]. However, a recent study showed that activation of this receptor attenuates chemically-induced colon carcinogenesis, and that PPARδ-null mice exhibit increased colon polyp multiplicity, suggesting that ligand activation of this receptor can also inhibit carcinogenesis [32].

The analysis of the antitumorigenic properties of PPARα ligands has been less studied mostly due to the observation that long-term administration of certain PPARα agonists (Clofibrateand WY14643) induces hepatocarcinogenesis in rodents [3335], despite the fact that PPARα ligands are widely used in medicine as antilipidemic drugs with excellent tolerance and little or no reported side effects. The finding that fenofibrate decreases VEGF levels in patients with hyperlipidemiaand atherosclerosis [36] provided a rationale for analyzing PPARα and its ligands as a molecular target for cancer therapy. In this review, we highlight some of the key functions attributed to PPARα in the context of endothelial and tumor cell biology.

3. PPARα Targets in Angiogenesis

PPARα controls the transcription of many genes involved in cell functions such as lipid metabolisms, inflammation, cell cycle progression, and angiogenesis. Among the angiogenic targets, PPARα has been shown to regulate the expression of the vascular endothelial growth factor (VEGF), fibroblast growth factors (FGFs), members of the arachidonic acid P450 monooxygenases, thrombospondin and endostatin to name few (see also Figure 1 and Table 1). Biscetti et al. have recently shown that the selective PPARα agonist WY14643 promotes cornea angiogenesis in vivo and enhances endothelial tubulogenesis in vitro [37]. Interestingly, WY14643 can enhance endothelial cell tubulogenesis in vitro only when endothelial cells are cocultured with interstitial cells and this effect is accompanied by upregulation of interstitial-derived VEGF synthesis [37]. However, WY14643 does not directly promote endothelial cell migration or proliferation, and when used at 10–20 μM range it reduces both endothelial cell proliferation and migration [37]. Thus, this study indicates that while WY14643 might directly prevent endothelial cell functions, it might also promote angiogenesis by stimulating the production of nonendothelial VEGF. The observation that activation of PPARα prevents endothelial cell proliferation/migration parallels our findings that WY14643 prevents—in a PPARα-dependent fashion—endothelial cell proliferation in vitro and tumorigenesis in vivo [38]. The antiangiogenic properties of WY14643 are associated with a PPARα-dependent downregulation of the epoxygenase branch of the cytochrome P450 arachidonic acid monooxygenases [38]. The arachidonic acid epoxygenases are expressed by endothelial cells both in vitro and in vivo [3941] and catalyze the oxidation of arachidonic acid to four regioisomeric epoxyeicosatrienoic acids (EETs) [42, 43]. EETs have been shown to possess proangiogenic activities [39, 4447] and we have demonstrated that WY14643-mediated PPARα activation directly prevents endothelial cell migration and proliferation by downregulating endothelial arachidonate epoxygenase expression and EET biosynthesis [38]. Most importantly, in vivo treatment with WY14643 prevents primary tumor growth and tumor-associated angiogenesis by downregulating the levels of circulating EETs [38].

tab1
Table 1: Effect of PPARα activation on angiogenesis and tumorigenesis.
906542.fig.001
Figure 1: Schematic representation of the antiangiogenic and antitumorigenic properties of PPARα. PPARα ligands reduce tumor growth by direct inhibition of tumor cell functions (black pathway). In addition, they prevent tumor-associated angiogenesis via direct (red pathway) as well as indirect (green pathway) inhibition of endothelial cell functions.

Consistent with the observation that PPARα ligands might act as potent direct and/or indirect antiangiogenic factors, Panigrahy et al. have recently shown that fenofibrate suppresses VEGF-mediated endothelial cell proliferation as well as tumor cell-derived VEGF and FGF2 synthesis with concomitant stimulation of tumor-cells derived thrombospondin and endostatin [48]. Moreover, fenofibrate and WY14643 prevent VEGF-mediated endothelial cell migration by inhibiting Akt phosphorylation [24] and fenofibrate prevents endothelial cell proliferation by inhibiting cyclooxygenase-2 expression [25]. Finally, PPARα agonists were found to inhibit endothelial VEGFR2 expression by preventing Sp1-dependent promoter binding and transactivation [23]. Some of the major PPARα targets known to control endothelial cell functions and the effects of PPARα ligands on angiogenesis are summarized in Figure 1 and Table 1.

In conclusions these studies strongly suggest that by preventing endothelial cell functions PPARα ligands may protect the vasculature from pathological alterations associated with either metabolic disorders (i.e., atherosclerosis, diabetes) or cancer. Thus, PPARα can be considered as a new class of “antiangiogenic” gene, and suggest that its ligands may function as effective antiangiogenic drugs.

4. PPARα Targets in Cancer

The observation that PPARα is expressed by tumor cells [5961] started studies of the role of this nuclear receptor and its ligands on the prevention of tumor cell proliferation in vitro and in vivo. In this context it has been shown that PPARα ligands suppress the growth of several cancer lines—including colon, liver, breast, endometrial, and skin—in vitro [6266], as we all inhibit the metastatic potential of melanoma cells in vitro and in vivo [67, 68]. Furthermore, PPARα ligands decrease colon carcinogenesis [62] and the growth of human ovarian cancer in mice [49]. Although the mechanisms whereby PPARα directly prevents tumor cell functions have not been investigated in details, potential targets have been identified. Clofibrate, a PPARα ligand, significantly suppressed the growth of OVCAR-3 xenotransplanted tumors and inhibited ovarian tumor cell proliferation by increasing the expression of carbonyl reductase, an enzyme that promotes the conversion of protumorigenic prostaglandin E2 to inactive PGF2α [49]. Moreover, clofibrate reduced the levels of circulating VEGF in tumor-bearing mice [49], while bezafibrate, another PPARα ligand, decreased the number of intestinal polyps in mice possibly by lowering serum level of triglycerides and upregulating lipoprotein lipase synthesis [27, 51]. Finally, PPARα activation has been shown to inhibit vascular smooth muscle cell proliferation underlying intimal hyperplasia by inducing the expression of the tumor suppressor p16INK4a [56].

Whereas these studies clearly suggest that PPARα activation might be beneficial in reducing cancer growth, studies from the Gonzales laboratory demonstrate that long-term administration of certain PPARα agonists (clofibrate and WY14643) induces liver adenoma and carcinomas in rats and mice [35, 52, 69, 70]. The ability of PPARα ligands to induce hepatocarcinoma is PPARα-dependent and mediated by the novel microRNA let-7C/c-myc axis [52]. Let-7C is a micro RNA that controls cell growth by directly downregulating c-myc expression [52]. Upon treatment of mice with WY14643, the hepatic expression of let-7C decreases with the concomitant induction of c-myc and the increased expression of the oncogenic mir-17-92 cluster [52]. Thus, this novel rodent specific PPARα-regulated pathway might be responsible for increased hepatocellular proliferation and tumorigenesis.

All together, these findings indicate that, with few exceptions, PPARα ligands can be viewed as antitumorigenic agents either by directly preventing tumor cell functions or by preventing tumor-derived production of proangiogenic molecules. Some of the potential PPARα targets that control tumor cell functions and the effects of PPARα ligands on tumorigenesis are summarized in Figure 1 and Table 1.

5. PPARα Ligands and Tumorigenesis: Lessons from Mice

The generation of PPARα null mice has provided an excellent tool not only to determine whether the effects exerted by PPARα ligand are indeed PPARα-dependent, but also for discerning between host versus tumor-mediated PPARα responses (see Table 2 for details). In this regard, we have shown that wild-type mice injected with isogenic PPARα expressing tumor cells respond to WY14643 treatment and develop fewer and smaller tumors than untreated wild-type mice [38]. In contrast, the growth of the same tumor cells is not prevented in WY14643-treated PPARα null mice [38]. In agreement with our finding, absence of PPARα in the host animals abrogated the potent antitumor effect of fenofibrate [48]. Finally whereas in vivo activation of PPARα prevents vascular smooth muscle cell proliferation underlying intimal hyperplasia, PPARα deficiency leads to hyperplasia [56]. Taken together, these results strongly suggest that activation of PPARα in the host is a key element in preventing unwanted pathological cell growth.

tab2
Table 2: PPARα and tumorigenesis: lessons from the PPARα-null mice.

Although rodents are the only species in which activation of PPARα promotes liver cancer, for a long time it was thought that Di(2-ethylhexyl)phthalate (DEHP), a commonly used industrial plasticizer, might cause liver tumorigenesis presumably via activation of PPARα [55, 71]. The use of PPARα null mice has disproved this idea, as this plasticizer is able to induce tumorigenesis in both wild-type and PPARα-null mice [55, 71]. These results suggest the existence of pathways for DEHP-induced hepatic tumorigenesis that are independent of PPARα, but most likely dependent on DEHP-mediated oxidative stress [55].

PPARα null mice have been also instrumental to determine the role of rodent versus human PPARα in the promotion of liver carcinogenesis. Morimura et al. have generated a PPARα-humanized mouse in which the human PPARα is expressed in liver under control of the Tet-OFF system. Interestingly, prolonged exposure to WY14643 in these mice only led to a 5% incidence of liver tumors—including hepatocellular carcinoma—compared to the 71% observed in mice expressing the mouse PPARα [53]. More recently, Yanget al. generated a PPARα-humanized transgenic mouse where the complete human PPARα gene was introduced onto a PPARα-null background [54]. These PPARα-humanized mice express the human PPARα in liver as well as other tissues and respond to fenofibrate treatment by lowering serum triglycerides and by inducing the expression of enzymes involved in fatty acid metabolism [54]. However, in contrast to wild-type mice, treatment with fenofibrate did not cause significant hepatomegaly, hepatocyte proliferation, and most importantly hepatocarcinoma [54]. Thus, this study shows that the protumorigenic let-7C/c-myc pathway is activated only by the rodent, but not the human PPARα receptor. Most importantly, this work highlights the possibility that PPARα ligands might be used as safe drugs for the treatment of cancer in humans.

Although activation of PPARα in either endothelial or tumor cells has been proven to be beneficial in inhibiting cancer growth, it has also been shown that loss of host-derived PPARα can be advantageous as it prevents tumor growth and development [57]. The host cells responsible for this protection, however, are granulocytes rather than endothelial cells. Loss of PPARα leads to an increased infiltration to the side of injury of granulocytes that suppress tumor-associated angiogenesis via excess production of the endogenous angiogenesis inhibitor thrombospondin [57]. This study clearly indicates that both activation of PPARα in specific host cells (i.e., endothelia cells) and concomitant inhibition of PPARα in immuno cells (i.e., granulocytes) might lead to the same effects, namely protection from tumor growth.

6. Conclusions

The studies summarized in this review identify PPARα as a potential host-based target for the development of new antiangiogenic approaches to inhibit and/or prevent tumor growth. As an established modulator of gene transcription, PPARα regulates the expression of genes known to be involved in energy metabolism, cellular proliferation, and angiogenesis and to have positive effects on the control of dyslipidemia, inflammation, and cardiovascular diseases. Furthermore, several fibric acid derivatives bind to and activate human PPARα with limited or no documented unwanted consequences and have proven to be safe and effective hypolipidemic drugs. In this context, gemfibrozil safely reduced the risk of death from coronary heart disease, nonfatal myocardial infarction, or stroke by raising HDL cholesterol levels and lowering levels of triglycerides [72, 73].

The effects of PPARα ligands in animal models of tumor angiogenesis should help not only to stimulate further research of their usefulness as antitumorigenic agents, but also to facilitate their evaluation as valid tools for the treatment and/or prevention of human cancers. In this context, it is our hope that these studies will serve to encourage epidemiological studies of cancer incidence in patients using hypolipidemic drugs, and help to identify their potential beneficial effects as agents for tumor prevention and/or treatment. The urgency of new approaches for cancer treatment are indicated by the fact that most current antitumorigenic therapies are oriented towards a general inhibition of tumor cell growth and, as such, they suffer from lacking target selectivity and, in most cases, causing severe side effects and overall systemic toxicity. Thus, targeting PPARα may prove to be a potential therapeutic strategy—either alone or in combination with conventional chemotherapy—to inhibit and ideally prevent cancer with excellent tolerance and limited toxicity.

Acknowledgment

This work was supported in part by DK074359 (AP), GM37922 (JHC), and DK38226 (JHC).

References

  1. S. Kersten, B. Desvergne, and W. Wahli, “Roles of PPARS in health and disease,” Nature, vol. 405, no. 6785, pp. 421–424, 2000. View at Publisher · View at Google Scholar
  2. S. A. Kliewer, K. Umesono, D. J. Noonan, R. A. Heyman, and R. M. Evans, “Convergence of 9-cis retinoic acid and peroxisome proliferator signalling pathways through heterodimer formation of their receptors,” Nature, vol. 358, no. 6389, pp. 771–774, 1992. View at Publisher · View at Google Scholar
  3. A. Aranda and A. Pascual, “Nuclear hormone receptors and gene expression,” Physiological Reviews, vol. 81, no. 3, pp. 1269–1304, 2001. View at Google Scholar
  4. B. Desvergne and W. Wahli, “Peroxisome proliferator-activated receptors: nuclear control of metabolism,” Endocrine Reviews, vol. 20, no. 5, pp. 649–688, 1999. View at Publisher · View at Google Scholar
  5. R. K. Semple, V. K. K. Chatterjee, and S. O'Rahilly, “PPARγ and human metabolic disease,” Journal of Clinical Investigation, vol. 116, no. 3, pp. 581–589, 2006. View at Publisher · View at Google Scholar
  6. P. Tontonoz, E. Hu, and B. M. Spiegelman, “Stimulation of adipogenesis in fibroblasts by PPARγ2, a lipid-activated transcription factor,” Cell, vol. 79, no. 7, pp. 1147–1156, 1994. View at Publisher · View at Google Scholar
  7. M. Ricote, A. C. Li, T. M. Willson, C. J. Kelly, and C. K. Glass, “The peroxisome proliferator-activated receptor-γ is a negative regulator of macrophage activation,” Nature, vol. 391, no. 6662, pp. 79–82, 1998. View at Publisher · View at Google Scholar
  8. N. Kubota, Y. Terauchi, H. Miki et al., “PPAR? mediates high-fat diet-induced adipocyte hypertrophy and insulin resistance,” Molecular Cell, vol. 4, no. 4, pp. 597–609, 1999. View at Publisher · View at Google Scholar
  9. S. Schwartz, P. Raskin, V. Fonseca, and J. F. Graveline, “Effect of troglitazone in insulin-treated patients with type II diabetes mellitus,” The New England Journal of Medicine, vol. 338, no. 13, pp. 861–866, 1998. View at Publisher · View at Google Scholar
  10. S. A. Kliewer, B. M. Forman, B. Blumberg et al., “Differential expression and activation of a family of murine peroxisome proliferator-activated receptors,” Proceedings of the National Academy of Sciences of the United States of America, vol. 91, no. 15, pp. 7355–7359, 1994. View at Publisher · View at Google Scholar
  11. O. Braissant, F. Foufelle, C. Scotto, M. Dauça, and W. Wahli, “Differential expression of peroxisome proliferator-activated receptors (PPARs): tissue distribution of PPAR-α, -β, and -γ in the adult rat,” Endocrinology, vol. 137, no. 1, pp. 354–366, 1996. View at Publisher · View at Google Scholar
  12. G. Rizzo and S. Fiorucci, “PPARs and other nuclear receptors in inflammation,” Current Opinion in Pharmacology, vol. 6, no. 4, pp. 421–427, 2006. View at Publisher · View at Google Scholar
  13. R. M. Touyz and E. L. Schiffrin, “Peroxisome proliferator-activated receptors in vascular biology-molecular mechanisms and clinical implications,” Vascular Pharmacology, vol. 45, no. 1, pp. 19–28, 2006. View at Publisher · View at Google Scholar
  14. S. Z. Duan, M. G. Usher, and R. M. Mortensen, “Peroxisome proliferator-activated receptor-γ-mediated effects in the vasculature,” Circulation Research, vol. 102, no. 3, pp. 283–294, 2008. View at Publisher · View at Google Scholar
  15. S. Theocharis, A. Margeli, P. Vielh, and G. Kouraklis, “Peroxisome proliferator-activated receptor-γ ligands as cell-cycle modulators,” Cancer Treatment Reviews, vol. 30, no. 6, pp. 545–554, 2004. View at Publisher · View at Google Scholar
  16. R. A. Roberts, S. Chevalier, S. C. Hasmall, N. H. James, S. C. Cosulich, and N. Macdonald, “PPARα and the regulation of cell division and apoptosis,” Toxicology, vol. 181-182, pp. 167–170, 2002. View at Publisher · View at Google Scholar
  17. C. Giaginis, A. Tsantili-Kakoulidou, and S. Theocharis, “Peroxisome proliferator-activated receptor-γ ligands: potential pharmacological agents for targeting the angiogenesis signaling cascade in cancer,” PPAR Research, vol. 2008, Article ID 431763, 12 pages, 2008. View at Publisher · View at Google Scholar
  18. Y. Yasui, M. Kim, and T. Tanaka, “PPAR ligands for cancer chemoprevention,” PPAR Research, vol. 2008, Article ID 548919, 10 pages, 2008. View at Publisher · View at Google Scholar
  19. L. Michalik, B. Desvergne, and W. Wahli, “Peroxisome-proliferator-activated receptors and cancers: complex stories,” Nature Reviews Cancer, vol. 4, no. 1, pp. 61–70, 2004. View at Publisher · View at Google Scholar
  20. J.-S. Annicotte, I. Iankova, S. Miard et al., “Peroxisome proliferator-activated receptor ? regulates E-cadherin expression and inhibits growth and invasion of prostate cancer,” Molecular and Cellular Biology, vol. 26, no. 20, pp. 7561–7574, 2006. View at Publisher · View at Google Scholar
  21. M. Li, T. W. Lee, A. P. C. Yim, T. S. K. Mok, and G. G. Chen, “Apoptosis induced by troglitazone is both peroxisome proliterator-activated receptor-γ- and ERK-dependent in human non-small lung cancer cells,” Journal of Cellular Physiology, vol. 209, no. 2, pp. 428–438, 2006. View at Publisher · View at Google Scholar
  22. X. Xin, S. Yang, J. Kowalski, and M. E. Gerritsen, “Peroxisome proliferator-activated receptor γ ligands are potent inhibitors of angiogenesis in vitro and in vivo,” Journal of Biological Chemistry, vol. 274, no. 13, pp. 9116–9121, 1999. View at Publisher · View at Google Scholar
  23. M. Meissner, M. Stein, C. Urbich et al., “PPARa activators inhibit vascular endothelial growth factor receptor-2 expression by repressing Sp1-dependent DNA binding and transactivation,” Circulation Research, vol. 94, no. 3, pp. 324–332, 2004. View at Publisher · View at Google Scholar
  24. S. Goetze, F. Eilers, A. Bungenstock et al., “PPAR activators inhibit endothelial cell migration by targeting Akt,” Biochemical and Biophysical Research Communications, vol. 293, no. 5, pp. 1431–1437, 2002. View at Publisher · View at Google Scholar
  25. J. Varet, L. Vincent, P. Mirshahi et al., “Fenofibrate inhibits angiogenesis in vitro and in vivo,” Cellular and Molecular Life Sciences, vol. 60, no. 4, pp. 810–819, 2003. View at Publisher · View at Google Scholar
  26. C. Blanquart, O. Barbier, J. C. Fruchart, B. Staels, and C. Glineur, “Peroxisome proliferator-activated receptors: regulation of transcriptional activities and roles in inflammation,” The Journal of Steroid Biochemistry and Molecular Biology, vol. 85, no. 2–5, pp. 267–273, 2003. View at Publisher · View at Google Scholar
  27. N. Niho, M. Takahashi, T. Kitamura et al., “Concomitant suppression of hyperlipidemia and intestinal polyp formation in Apc-deficient mice by peroxisome proliferator-activated receptor ligands,” Cancer Research, vol. 63, no. 18, pp. 6090–6095, 2003. View at Google Scholar
  28. C. A. McAlpine, Y. Barak, I. Matise, and R. T. Cormier, “Intestinal-specific PPARγ deficiency enhances tumorigenesis in ApcMin/+ mice,” International Journal of Cancer, vol. 119, no. 10, pp. 2339–2346, 2006. View at Publisher · View at Google Scholar
  29. A.-M. Lefebvre, I. Chen, P. Desreumaux et al., “Activation of the peroxisome proliferator-activated receptor ? promotes the development of colon tumors in C57BL/6J-ApcMin/+ mice,” Nature Medicine, vol. 4, no. 9, pp. 1053–1057, 1998. View at Publisher · View at Google Scholar
  30. Y. Guan, C. Hao, D. R. Cha et al., “Thiazolidinediones expand body fluid volume through PPAR? stimulation of ENaC-mediated renal salt absorption,” Nature Medicine, vol. 11, no. 8, pp. 861–866, 2005. View at Publisher · View at Google Scholar
  31. D. Wang, H. Wang, Y. Guo et al., “Crosstalk between peroxisome proliferator-activated receptor d and VEGF stimulates cancer progression,” Proceedings of the National Academy of Sciences of the United States of America, vol. 103, no. 50, pp. 19069–19074, 2006. View at Publisher · View at Google Scholar
  32. H. E. Marin, M. A. Peraza, A. N. Billin et al., “Ligand activation of peroxisome proliferator-activated receptor ß inhibits colon carcinogenesis,” Cancer Research, vol. 66, no. 8, pp. 4394–4401, 2006. View at Publisher · View at Google Scholar
  33. F. J. Gonzalez and Y. M. Shah, “PPARα: mechanism of species differences and hepatocarcinogenesis of peroxisome proliferators,” Toxicology, vol. 246, no. 1, pp. 2–8, 2008. View at Publisher · View at Google Scholar
  34. J. M. Peters, C. Cheung, and F. J. Gonzalez, “Peroxisome proliferator-activated receptor-α and liver cancer: where do we stand?” Journal of Molecular Medicine, vol. 83, no. 10, pp. 774–785, 2005. View at Publisher · View at Google Scholar
  35. J. C. Corton, P. J. Lapinskas, and F. J. Gonzalez, “Central role of PPARα in the mechanism of action of hepatocarcinogenic peroxisome proliferators,” Mutation Research, vol. 448, no. 2, pp. 139–151, 2000. View at Publisher · View at Google Scholar
  36. A. D. Blann, F. M. Belgore, J. Constans, C. Conri, and G. Y. H. Lip, “Plasma vascular endothelial growth factor and its receptor Flt-1 in patients with hyperlipidemia and atherosclerosis and the effects of fluvastatin or fenofibrate,” The American Journal of Cardiology, vol. 87, no. 10, pp. 1160–1163, 2001. View at Publisher · View at Google Scholar
  37. F. Biscetti, E. Gaetani, A. Flex et al., “Selective activation of peroxisome proliferator-activated receptor (PPAR)a and PPAR? induces neoangiogenesis through a vascular endothelial growth factor-dependent mechanism,” Diabetes, vol. 57, no. 5, pp. 1394–1404, 2008. View at Publisher · View at Google Scholar
  38. A. Pozzi, M. R. Ibanez, A. E. Gatica et al., “Peroxisomal proliferator-activated receptor-a-dependent inhibition of endothelial cell proliferation and tumorigenesis,” Journal of Biological Chemistry, vol. 282, no. 24, pp. 17685–17695, 2007. View at Publisher · View at Google Scholar
  39. A. Pozzi, I. Macias-Perez, T. Abair et al., “Characterization of 5,6- and 8,9-epoxyeicosatrienoic acids (5,6- and 8,9-EET) as potent in vivo angiogenic lipids,” Journal of Biological Chemistry, vol. 280, no. 29, pp. 27138–27146, 2005. View at Publisher · View at Google Scholar
  40. U. R. Michaelis and I. Fleming, “From endothelium-derived hyperpolarizing factor (EDHF) to angiogenesis: epoxyeicosatrienoic acids (EETs) and cell signaling,” Pharmacology & Therapeutics, vol. 111, no. 3, pp. 584–595, 2006. View at Publisher · View at Google Scholar
  41. I. Fleming, “Epoxyeicosatrienoic acids, cell signaling and angiogenesis,” Prostaglandins & Other Lipid Mediators, vol. 82, no. 1–4, pp. 60–67, 2007. View at Publisher · View at Google Scholar
  42. J. H. Capdevila, J. R. Falck, and R. C. Harris, “Cytochrome P450 and arachidonic acid bioactivation: molecular and functional properties of the arachidonate monooxygenase,” Journal of Lipid Research, vol. 41, no. 2, pp. 163–181, 2000. View at Google Scholar
  43. J. H. Capdevila, J. R. Falck, E. Dishman, and A. Karara, “Cytochrome P-450 arachidonate oxygenase,” Methods in Enzymology, vol. 187, pp. 385–394, 1990. View at Publisher · View at Google Scholar
  44. M. Potente, U. R. Michaelis, B. Fisslthaler, R. Busse, and I. Fleming, “Cytochrome P450 2C9-induced endothelial cell proliferation involves induction of mitogen-activated protein (MAP) kinase phosphatase-1, inhibition of the c-Jun N-terminal kinase, and up-regulation of cyclin D1,” Journal of Biological Chemistry, vol. 277, no. 18, pp. 15671–15676, 2002. View at Publisher · View at Google Scholar
  45. M. Potente, B. Fisslthaler, R. Busse, and I. Fleming, “11,12-epoxyeicosatrienoic acid-induced inhibition of FOXO factors promotes endothelial proliferation by down-regulating p27Kip1,” Journal of Biological Chemistry, vol. 278, no. 32, pp. 29619–29625, 2003. View at Publisher · View at Google Scholar
  46. U. R. Michaelis, J. R. Falck, R. Schmidt, R. Busse, and I. Fleming, “Cytochrome P4502C9-derived epoxyeicosatrienoic acids induce the expression of cyclooxygenase-2 in endothelial cells,” Arteriosclerosis, Thrombosis, and Vascular Biology, vol. 25, no. 2, pp. 321–326, 2005. View at Publisher · View at Google Scholar
  47. U. R. Michaelis, B. Fisslthaler, M. Medhora, D. Harder, I. Fleming, and R. Busse, “Cytochrome P450 2C9-derived epoxyeicosatrienoic acids induce angiogenesis via cross-talk with the epidermal growth factor receptor (EGFR),” The FASEB Journal, vol. 17, no. 6, pp. 770–772, 2003. View at Publisher · View at Google Scholar
  48. D. Panigrahy, A. Kaipainen, S. Huang et al., “PPARa agonist fenofibrate suppresses tumor growth through direct and indirect angiogenesis inhibition,” Proceedings of the National Academy of Sciences of the United States of America, vol. 105, no. 3, pp. 985–990, 2008. View at Publisher · View at Google Scholar
  49. Y. Yokoyama, B. Xin, T. Shigeto et al., “Clofibric acid, a peroxisome proliferator-activated receptor a ligand, inhibits growth of human ovarian cancer,” Molecular Cancer Therapeutics, vol. 6, no. 4, pp. 1379–1386, 2007. View at Publisher · View at Google Scholar
  50. L. Jackson, W. Wahli, L. Michalik et al., “Potential role for peroxisome proliferator activated receptor (PPAR) in preventing colon cancer,” Gut, vol. 52, no. 9, pp. 1317–1322, 2003. View at Publisher · View at Google Scholar
  51. N. Niho, M. Mutoh, M. Takahashi, K. Tsutsumi, T. Sugimura, and K. Wakabayashi, “Concurrent suppression of hyperlipidemia and intestinal polyp formation by NO-1886, increasing lipoprotein lipase activity in Min mice,” Proceedings of the National Academy of Sciences of the United States of America, vol. 102, no. 8, pp. 2970–2974, 2005. View at Publisher · View at Google Scholar
  52. Y. M. Shah, K. Morimura, Q. Yang, T. Tanabe, M. Takagi, and F. J. Gonzalez, “Peroxisome proliferator-activated receptor α regulates a microRNA-mediated signaling cascade responsible for hepatocellular proliferation,” Molecular and Cellular Biology, vol. 27, no. 12, pp. 4238–4247, 2007. View at Publisher · View at Google Scholar
  53. K. Morimura, C. Cheung, J. M. Ward, J. K. Reddy, and F. J. Gonzalez, “Differential susceptibility of mice humanized for peroxisome proliferator-activated receptor α to Wy-14,643-induced liver tumorigenesis,” Carcinogenesis, vol. 27, no. 5, pp. 1074–1080, 2006. View at Publisher · View at Google Scholar
  54. Q. Yang, T. Nagano, Y. Shah, C. Cheung, S. Ito, and F. J. Gonzalez, “The PPARα-humanized mouse: a model to investigate species differences in liver toxicity mediated by PPARα,” Toxicological Sciences, vol. 101, no. 1, pp. 132–139, 2008. View at Publisher · View at Google Scholar
  55. Y. Ito, O. Yamanoshita, N. Asaeda et al., “Di(2-ethylhexyl)phthalate induces hepatic tumorigenesis through a peroxisome proliferator-activated receptor a-independent pathway,” Journal of Occupational Health, vol. 49, no. 3, pp. 172–182, 2007. View at Publisher · View at Google Scholar
  56. F. Gizard, C. Amant, O. Barbier et al., “PPARa inhibits vascular smooth muscle cell proliferation underlying intimal hyperplasia by inducing the tumor suppressor p16INK4a,” The Journal of Clinical Investigation, vol. 115, no. 11, pp. 3228–3238, 2005. View at Publisher · View at Google Scholar
  57. A. Kaipainen, M. W. Kieran, S. Huang et al., “PPARa deficiency in inflammatory cells suppresses tumor growth,” PLoS ONE, vol. 2, no. 2, p. e260, 2007. View at Publisher · View at Google Scholar
  58. P. Howroyd, C. Swanson, C. Dunn, R. C. Cattley, and J. C. Corton, “Decreased longevity and enhancement of age-dependent lesions in mice lacking the nuclear receptor peroxisome proliferator-activated receptor α (PPARα),” Toxicologic Pathology, vol. 32, no. 5, pp. 591–599, 2004. View at Publisher · View at Google Scholar
  59. G. P. Collett, A. M. Betts, M. I. Johnson et al., “Peroxisome proliferator-activated receptor a is an androgen-responsive gene in human prostate and is highly expressed in prostatic adenocarcinoma,” Clinical Cancer Research, vol. 6, no. 8, pp. 3241–3248, 2000. View at Google Scholar
  60. K. M. Suchanek, F. J. May, J. A. Robinson et al., “Peroxisome proliferator-activated receptor a in the human breast cancer cell lines MCF-7 and MDA-MB-231,” Molecular Carcinogenesis, vol. 34, no. 4, pp. 165–171, 2002. View at Publisher · View at Google Scholar
  61. K. M. Suchanek, F. J. May, W. J. Lee, N. A. Holman, and S. J. Roberts-Thomson, “Peroxisome proliferator-activated receptor β expression in human breast epithelial cell lines of tumorigenic and non-tumorigenic origin,” The International Journal of Biochemistry & Cell Biology, vol. 34, no. 9, pp. 1051–1058, 2002. View at Publisher · View at Google Scholar
  62. T. Tanaka, H. Kohno, S.-I. Yoshitani et al., “Ligands for peroxisome proliferator-activated receptors a and ? inhibit chemically induced colitis and formation of aberrant crypt foci in rats,” Cancer Research, vol. 61, no. 6, pp. 2424–2428, 2001. View at Google Scholar
  63. G. Muzio, M. Maggiora, M. Oraldi, A. Trombetta, and R. A. Canuto, “PPARα and PP2A are involved in the proapoptotic effect of conjugated linoleic acid on human hepatoma cell line SK-HEP-1,” International Journal of Cancer, vol. 121, no. 11, pp. 2395–2401, 2007. View at Publisher · View at Google Scholar
  64. M. Maggiora, M. Bologna, M. P. Cerù et al., “An overview of the effect of linoleic and conjugated-linoleic acids on the growth of several human tumor cell lines,” International Journal of Cancer, vol. 112, no. 6, pp. 909–919, 2004. View at Publisher · View at Google Scholar
  65. S. A. Saidi, C. M. Holland, D. S. Charnock-Jones, and S. K. Smith, “In vitro and in vivo effects of the PPAR-alpha agonists fenofibrate and retinoic acid in endometrial cancer,” Molecular Cancer, vol. 5, article 13, 2006. View at Publisher · View at Google Scholar
  66. P. Thuillier, G. J. Anchiraico, K. P. Nickel et al., “Activators of peroxisome proliferator-activated receptor-a partially inhibit mouse skin tumor promotion,” Molecular Carcinogenesis, vol. 29, no. 3, pp. 134–142, 2000. View at Publisher · View at Google Scholar
  67. M. Grabacka, P. M. Plonka, K. Urbanska, and K. Reiss, “Peroxisome proliferator-activated receptor α activation decreases metastatic potential of melanoma cells in vitro via down-regulation of Akt,” Clinical Cancer Research, vol. 12, no. 10, pp. 3028–3036, 2006. View at Publisher · View at Google Scholar
  68. M. Grabacka, W. Placha, P. M. Plonka et al., “Inhibition of melanoma metastases by fenofibrate,” Archives of Dermatological Research, vol. 296, no. 2, pp. 54–58, 2004. View at Publisher · View at Google Scholar
  69. T. Hays, I. Rusyn, A. M. Burns et al., “Role of peroxisome proliferator-activated receptor-a (PPARa) in bezafibrate-induced hepatocarcinogenesis and cholestasis,” Carcinogenesis, vol. 26, no. 1, pp. 219–227, 2005. View at Publisher · View at Google Scholar
  70. F. J. Gonzalez, “The peroxisome proliferator-activated receptor α (PPARα): role in hepatocarcinogenesis,” Molecular and Cellular Endocrinology, vol. 193, no. 1-2, pp. 71–79, 2002. View at Publisher · View at Google Scholar
  71. K. Takashima, Y. Ito, F. J. Gonzalez, and T. Nakajima, “Different mechanisms of DEHP-induced hepatocellular adenoma tumorigenesis in wild-type and Pparα-null mice,” Journal of Occupational Health, vol. 50, no. 2, pp. 169–180, 2008. View at Publisher · View at Google Scholar
  72. H. B. Rubins, S. J. Robins, D. Collins et al., “Gemfibrozil for the secondary prevention of coronary heart disease in men with low levels of high-density lipoprotein cholesterol,” The New England Journal of Medicine, vol. 341, no. 6, pp. 410–418, 1999. View at Publisher · View at Google Scholar
  73. M. H. Frick, O. Elo, K. Haapa et al., “Helsinki Heart Study: primary-prevention trial with gemfibrozil in middle-aged men with dyslipidemia. Safety of treatment, changes in risk factors, and incidence of coronary heart disease,” The New England Journal of Medicine, vol. 317, no. 20, pp. 1237–1245, 1987. View at Google Scholar