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The Scientific World Journal
Volume 2014 (2014), Article ID 235941, 12 pages
http://dx.doi.org/10.1155/2014/235941
Research Article

Diverse Effects of Lead Nitrate on the Proliferation, Differentiation, and Gene Expression of Stem Cells Isolated from a Dental Origin

1Department of Conservative Dentistry, Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia
2Hygieia Innovation Sdn. Bhd, Lot 1G-2G, Komplex Lanai, No. 2, Persiaran Seri Perdana, Persint 10, 62250 Putrajaya, Malaysia
3Department of Children’s Dentistry and Orthodontics, Faculty of Dentistry, University of Malaya, 50603 Kuala Lumpur, Malaysia

Received 7 August 2013; Accepted 27 October 2013; Published 27 January 2014

Academic Editors: C. Garcia, C. Montoliu, J.-M. Sabatier, and A. Yasutake

Copyright © 2014 Mariam Abdullah 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.

Abstract

Lead (Pb2+) exposure continues to be a significant public health problem. Therefore, it is vital to have a continuous epidemiological dataset for a better understanding of Pb2+ toxicity. In the present study, we have exposed stem cells isolated from deciduous and permanent teeth, periodontal ligament, and bone marrow to five different types of Pb2+ concentrations (160, 80, 40, 20, and 10 µM) for 24 hours to identify the adverse effects of Pb2+ on the proliferation, differentiation, and gene expression on these cell lines. We found that Pb2+ treatment altered the morphology and adhesion of the cells in a dose-dependent manner. There were no significant changes in terms of cell surface phenotypes. Cells exposed to Pb2+ continued to differentiate into chondrogenesis and adipogenesis, and a severe downregulation was observed in osteogenesis. Gene expression studies revealed a constant expression of key markers associated with stemness (Oct 4, Rex 1) and DNA repair enzyme markers, but downregulation occurred with some ectoderm and endoderm markers, demonstrating an irregular and untimely differentiation trail. Our study revealed for the first time that Pb2+ exposure not only affects the phenotypic characteristics but also induces significant alteration in the differentiation and gene expression in the cells.

1. Introduction

Lead (Pb2+) is regarded as one of the most toxic substances among heavy metals. It is derived from the environment as well as an industrial pollutant. Pb2+ causes serious illness which covers not only physiological and biochemical dysfunctions but also behavioural dysfunctions in humans [1, 2]. The impact is more prevalent in children [3] who have a high tendency to accumulate Pb2+ in their circumpulpal dentin [4]. The common route of exposure of Pb2+ is in the blood and eventually it is deposited in the hard tissues such as bone and teeth. The deposition in the latter tissue is permanent [5]. Therefore, teeth can be a useful long-term record of Pb2+ accumulation [6] and have been used as biological markers to environmental pollution [715].

Several factors are identified which influence the Pb2+ deposition in teeth. These include types of teeth and the presence of caries. The deposited Pb2+ in teeth during mineralization is largely retained since the teeth have hard stable tissues [16]. Pb2+ is distributed with the highest concentration in the circumpulpal dentin which is located in the innermost layer of dentin, adjacent to the dental pulp. The mechanism by which Pb2+ is deposited within the matrix of the primary and circumpulpal dentin is not clear although both primary and circumpulpal dentins accumulate Pb2+ with increased exposure and with postnatal age [17]. Nevertheless, it has been suggested that the deposition of Pb2+ in teeth is probably due to the similar oxidation number of both Ca2+ and Pb2+ ions. Studies also have shown that the toxicity effect of Pb2+ becomes apparent due to the ability of the metabolic cation Pb2+ to bind with specific ligands of biomolecular substances that play a vital role in various physiological functions [18, 19]. It also competes with ion transport and interferes with normal physiology [20]. There are also studies on the effect of Pb2+ on cells derived from human periodontal ligament [21]. This is due to availability of material and because of the incorporation of elements into the mineral phase of dental tissues [22].

Stem cells play an important role in maintaining the homeostasis and function of tissues and play a pivotal role in dealing with invaders such as Pb2+. This effect has been documented in previous work that described the effects of Pb2+ on dental cells [23].

We have studied the effects of various types of stem cells from a dental origin, namely, from deciduous teeth (SCDs), permanent teeth (DPSCs), and periodontal ligament (PDLs) as well as from bone marrow (BM-MSCs) against various Pb2+ concentrations with respect to their proliferation, multilineage differentiation capacity, and gene expression level. Interestingly, though all stem cells are almost similar to each other, we discovered significant differences in terms of proliferation and differentiation of stem cells derived from SCD, DP, PDL, and BM.

2. Materials and Methods

2.1. Tissue Collection and Isolation of Cells

This work was approved by the Medical Ethics Committee, Faculty of Dentistry, University of Malaya (medical ethics clearance number: DF CO1107/0066 (L)). All the subjects consented to the study. All subjects were nonsmokers, nonalcoholics, and free from any infectious diseases such as HIV, HBV, and HCV.

Bone Marrow (BM) stem cells or mesenchymal stem cells (MSCs) cultures were established from three donors (age: 24–35) as previously described [24]. Briefly, 60 mL BM was aspirated aseptically from the iliac crest of each patient under deep sedation. All processings of samples were done inside a class 100 biosafety hood. The BM was diluted (1 : 1) with knockout Dulbecco’s modified Eagle’s medium (KO-DMEM; Invitrogen, Carlsbad, CA, USA; http://www.invitrogen.com/) and centrifuged at 1800 rpm for 10 minutes to remove anticoagulants and other debris. The supernatant was discarded and the BM washed once with the culture medium. Mononuclear cells (MNCs) were isolated by layering onto a lymphoprep density gradient (1 : 2; Axis-Shield PoCAS). The MNCs present in the buffy coat were washed again with the culture medium. The mononuclear fractions that also contained MSCs were plated into culture flasks.

Dental pulp stem cells (DPSCs) cultures were established from sound intact third molars from adults (age: 24–35) and deciduous (SCDs) were established from molars from children (age: 3–12) as previously described [25]. Periodontal ligament (PDLs) tissues were separated from the root surface of deciduous teeth using a sterilized scalpel. Briefly, root surfaces were cleaned with Povidone-iodine (Sigma Aldrich, St. Louis, MO, USA; http://www.sigmaaldrich.com) and the pulp was extirpated within two hours of postextraction and then processed as follows. The pulp tissues as well as PDL tissues were minced into small fragments prior to digestion in a solution of three mg/mL collagenase type I (Gibco, Grand Island, NY, http://www.invitrogen.com) for 40 minutes at 37°C. After neutralization with a 10% Fetal Bovine Serum ( FBS, Fetal Bovine Serum, (FBS) (Hyclone; Thermo Scientific Inc, Waltham, MA; http://www.thermoscientific.com), the cells were centrifuged and were seeded in a culture flask.

All cells were cultured in an identical culture condition videlicet, in T75 cm2 culture flasks (BD Pharmingen, San Diego CA, USA; http://www.bdbiosciences.com) with a culture medium containing KO-DMEM, 0.5% and 10000 μg/mL of penicillin/streptomycin (Invitrogen), 1% 1x Glutamax (Invitrogen) and 10% FBS with a humidified atmosphere of 95% of air, and 5% of CO2 at 37°C. Nonadherent cells were removed 48 h after initial plating. The medium was replaced every three days until the cells reached 80–90% confluency.

2.2. Preparation of Pb2+

The stock concentration of Pb2+ was prepared by dissolving Pb2+ nitrate (Pb [NO3]2) (Cat No 203580, Sigma Aldrich) in distilled water at a concentration of 10-4 M. A total of 5 different Pb2+ concentrations (160, 80, 40, 20, and 10 μM) were used in this study with 50% ratio between each concentration. The concentration of Pb2+ was designed based on previous study [26].

2.3. Exposure of Cell Lines to Pb2+

The BM-MSCs, SCDs, DPSCs, and PDLs were seeded at 1000 cells/cm2 in the 6-well plates (BD Bioscience) and allowed to grow for 96 hours. Then freshly prepared Pb2+ solution was added into the culture media to obtain the aforesaid concentration. Exposed cells were cultured for 24 hours before they were used for the subsequent experiments.

2.4. Growth Kinetics

Exposed cell lines with Pb2+ were counted and assessed for viability using a trypan blue dye exclusion technique. The population doubling time (PDT) of Pb2+ exposed BM-MSCs, PDLs, SCDs, and DPSCs were analyzed using the formula: where NI is the inoculum cell number, NH is the cell harvest number, and is the time of the culture (in hours).

2.5. MTT Assay

MTT assay was performed as described previously [27]. Each experiment was done in triplicate. The relative cell viability (%) related to control wells containing cell culture medium without nanoparticles or PBS as a vehicle was calculated by test/control × 100, where test is the absorbance of the test sample and control is the absorbance of control sample.

2.6. LDH Assay

LDH assay was performed as described previously [27]. Each experiment was done in triplicate. The spectrophotometer was calibrated to zero absorbance using culture medium without cells. The relative LDH leakage (%) related to control wells containing cell culture medium without nanoparticles or PBS as a vehicle was calculated by test/control × 100, where test is the absorbance of the test sample and control is the absorbance of the control sample.

2.7. Senescence Associated with β-Galactosidase Assay

The Pb2+ exposed BM-MSCs, PDLs, SCDs, and DPSCs were tested for senescence-associated β-galactosidase (SA-βgal) staining with the SA-βgal staining kit (Sigma Aldrich) and used according to the manufacture’s instructions. Briefly, all cell lines were washed twice with DPBS (–Ca2+, –Mg2+, Invitrogen) and incubated with 1x fixative solution for 15 minutes at room temperature. Subsequently, the fixed cells were rewashed using 2 mL of DPBS (–Ca2+, –Mg2+, Invitrogen) and stained with 1 mL of the staining solution mixture overnight at 37°C in a dry incubator. The development of the blue colour was observed under a phase contrast microscope and the quantitative analyses of the SA-βgal staining were done by counting the percentage of blue-stained cells that represent senescence cells in the selected field of each sample.

2.8. Immunophenotyping Analysis

Pb2+ exposed cell lines were subjected to immunophenotyping analysis by using a flow cytometry. A total volume of 200 μL of a cell suspension (1 × 105 cells) was incubated with the labeled antibodies in the dark for 1 hour at 37°C. The following antibodies were used to mark the cell surface: epitopes-CD90-phycoerythrin (PE), CD44-PE, CD73-PE, CD166-PE and CD34-PE, and CD45-Fluoro-isothyocyanate (FITC), and HLA-DR-FITC (all from BD Pharmingen). All the analyses were standardized against a negative control of cells incubated with isotype-specific IgG1-PE and IgG1-FITC (BD Pharmingen). At least 10,000 events were acquired on Guava Technologies flow cytometer and the results were analyzed using Cytosoft, Version 5.2, from Guava Technologies.

2.9. Differentiation Assay

The Pb2+ exposed cell lines were replated at a density of 1000 cells/cm2 in 6-well plates and were grown to confluence and subjected to differentiation into adipogenic, chondrogenic, and osteogenic lineages according to the method described earlier [24]. The adipogenic lineage was initiated in a 3-week culture period by inducing the cells with 10% FBS, 200 μM indomethacin, 0.5 mM 3-Isobutyl-1-methylxanthine (IMBX), 10 μg/mL insulin, and 1 μM dexamethasone (all reagents from Sigma Aldrich). Lipid droplets were visualized by staining with Oil Red O staining (Sigma Aldrich). For the chondrogenesis differentiation, the cells were cultured in a media supplemented with ITS+1 (Sigma Aldrich), 50 μM of L-ascorbic acid-2 phosphates, 55 μM of sodium pyruvate (Invitrogen), 25 μM of L-proline (Sigma Aldrich), and 10 ng/mL of the transformation growth factor-beta (TGF-β) (Sigma Aldrich). Assessment of the proteoglycans accumulation was visualized by the Alizarin Blue staining (Sigma Aldrich). The osteogenic differentiation was stimulated in a 3-week culture period in a media supplemented with 10% FBS, 10−7 M dexamethasone, 10 mM-glycerol phosphate (Fluka, Buchs, Switzerland), and 100 μM of L-ascorbic acid-2 phosphate. The assessment of calcium accumulation was visualized by using the von Kossa staining technique (Sigma Aldrich).

2.10. Reverse Transcription Polymerase Chain Reaction (RT-PCR)

Total RNA from Pb2+ exposed BM-MSCs, PDLs, SCDs, and DPSCs was isolated with Trizol (Invitrogen) and used according to the manufacturer’s instructions. The RNA of the Pb2+ exposed cells was converted to cDNA with Superscript II to reverse transcriptase (Invitrogen) and used according to the manufacturer’s instructions. cDNA amplification was performed in a thermocycler at 94°C/1 min, a 58°C/30 sec, and 72°C/1 min. The expressions of ectoderm, endoderm, and pluripotent genes were quantified in duplicate with SYBR green master mix (Applied Biosystem, Foster City, CA, USA). For data analysis, the comparative CT method was used. For RT-PCR, the products were resolved on 1.5% agarose (Invitrogen) gel and was run in 1x Tris borate-EDTA buffer. The primer sequences are tabulated in Supplementary Table (see the Supplementary Materials available online at http://dx.doi.org/10.1155/2014/235941).

2.11. Soft Agar Colony Formation Assay

BM-MSCs, PDLs, SCDs, and DPSCs cells exposed to Pb2+ were tested for soft agar assay colony formation assay (Cells Biolab, INC) and used according to the manufacture’s instructions. Basically the Pb2+ exposed cell lines (160 μM) and control (HeLa cells) were seeding in a layer of 0.35% agar DMEM/FBS over a layer of 0.5% agar/DMEM/FBS. Cultures were maintained in a humidified 37°C incubator. On day 7 and day 30 after seeding, cells were fixed with pure ethanol containing 0.05% crystal violet and colony forming efficiency quantified by phase contrast microscope [28].

2.12. Statistical Analysis

The descriptive statistical analyses were performed using the software SPSS for Windows (Version 18.0, SPSS Predictive Analytics, Chicago). Data is presented as mean ± standard deviation (SD). Statistical comparisons were made using Student’s -test and values which were considered to be significant.

3. Results

3.1. Morphology of Cells Exposed to Pb2+

BM-MSCs, SCDs, DPSCs, and PDLs cultured in controlled conditions maintained a small and spindle-shape morphology and a similar observation was seen in cells exposed to 10 μM of Pb2+ (Figure 1). However, BM-MSCs began to canalize and loose its cell texture and shape as well as displaying granule-like structures as the concentration of Pb2+ increased to 160 μM. SCDs, DPSCs, and PDLs exhibited similar appearances but with a lesser degree.

235941.fig.001
Figure 1: Cytotoxic effect of Pb2+ to bone marrow mesenchymal stem cells (BM-MSCs), deciduous stem cells (SCDs), periodontal ligament stem cells (PDLs), and permanent stem cells (DPSCs); phase contrast microscope; 10x magnification of BM-MSCs, SCDs, PDLs, and DPSCs in presence of various concentrations of Pb2+. In all experiments, the results represent average of five culture replicates with standard deviation and a representative photomicrograph was given for each experiment.
3.2. Pb2+ Reduces the Proliferation Rate of BM-MSCs as Compared to Other Cell Lines

The 10 μM Pb2+ treatment did not significantly () inhibit the proliferation rate of any of the cell lines. However, a drastic inhibition of cell growth was observed in BM-MSCs exposed to 20 μM Pb2+ (cell count: 1.3 × 106 cells; MTT: 0.98 absorbance; LDH: 136%; ) and up to 160 μM Pb2+ treatment (cell count: 0.6 × 106 cells; MTT: 0.24 absorbance; LDH: 164%; ). While SCDs and DPSCs have constant rates of inhibition of cell growth, PDLs seem to be resistant to Pb2+ exposure. A small variation was observed in the control versus cells exposed to 160 μM (cell count: 7.10 × 105 cells; MTT: 0.78 absorbance; LDH: 146%; ) (Table 1). This result was reflected in the aging of cells. The percentage of SA-βgal activity was increased in a concentration-dependent manner in all cell lines with the highest in BM-MSCs (ratio) followed by SCDs, DPSCs, and PDLs (Figure 2).

tab1
Table 1: Changes in trypan blue dye exclusion assay, 3-(4,5-dimethylthiazol-2-yl)-2-,5-diphenyltetrazolium bromide (MTT) assay, and lactate dehydrogenase leakage (LDH) rate at various Pb 2+ concentrations (mean ± SD).
fig2
Figure 2: (a) Conventional qualitative SA-β-gal assay by X-gal staining of bone marrow mesenchymal stem cells (BM-MSCs), deciduous (SCDs), permanent (DPSCs), and periodontal ligament (PDLs) upon exposure to various concentrations of Pb2+. A locally region of senescence cell is shown; (b) quantification of percentage (%) of SA-β-gal positive cells exposed to various concentrations of Pb2+. In all experiments, the results represent average of five culture replicates with standard deviation and a representative photomicrograph was given for each experiment.
3.3. Pb2+ Alters the Expression of CD166 Marker Although Other Cell Surface Markers Remain Unchanged

Table 2 summarizes the immunophenotyping analysis of eight cell surface markers, namely, CD166, CD90, CD73, CD45, CD44, CD34, HLA-DR, and 7-AAD, from all sources of MSCs with the expression pattern being consistent throughout the Pb2+ concentrations. All four sources of MSCs in the control and Pb2+ treatments were found to be negative for the hematopoietic markers (CD34, CD45, and HLA-DR). On the other hand, a similar result was reported for the positive for cell surface markers (CD44, CD73, and CD90) among all the MSCs sources. Astonishingly, while the expressions of CD166 remain unchanged in BM-MSCs, DPSCs, and PDLs, it was lower in SCDs in Pb2+ treatments as compared to the control (see Supplementary Figures  1–4).

tab2
Table 2: Immunophenotype analysis of bone marrow mesenchymal stem cells (BM-MSCs), dental permanent stem cells (DPSCs), dental deciduous stem cells (SCDs), and periodontal ligament stem cells (PDLs), respectively, cultured before and after treatment of Pb2+. BM-MSCs were tested against human antigens CD34, CD44, CD45, CD73, CD166, and HLA-DR. 7-AAD was used to check the viability of the cells. The results represent average of 3 independent culture replicates.
3.4. Multilineage Capability of All MSCs Cell Lines after Exposure to Pb2+

We investigated the potential of BM-MSCs, SCDs, DPSCs, and PDLs to differentiate into osteogenic, chondrogenic, and adipogenic lineages at various concentrations of Pb2+ (Figure 3). Osteogenic differentiation was confirmed by the detection of the silver stained mineralized matrix in the control samples. While SCDs, DPSCs, and PDLs showed a weak deposition of calcium in the mineralized matrix only at the pretreatment with 160 μM of Pb2+, BM-MSCs preexposed to Pb2+ concentration of 40 μM have started to show a similar observation (Figure 3). Chondrogenic differentiation was detected by the presence of proteoglycans stained with alcian blue at day 21 in BM-MSCs, SCDs, PDLs, and DPSCs.

235941.fig.003
Figure 3: In vitro differential potentiality of bone marrow stem cells (BM-MSCs), deciduous stem cells (SCDs), periodontal ligament stem cells (PDLs), and permanent stem cells (DPSCs) in presence of various concentrations of Pb2+. Osteogenesis was confirmed by mineralized matrix deposition stained with von Kossa staining at day 21. Adipogenesis was detected by neutral oil droplet formation stained with Oil Red O at day 21. Chondogenesis was detected by the presence of proteoglycans stained with alcian blue dye at day 21. The results represent average of 3 independent culture replicates. A representative photomicrograph was given for each experiment.

No significant differences were noticed between the control and pretreatment with Pb2+ in all MSC sources. Adipogenic differentiation was confirmed in BM-MSCs, SCDs, DPSCs, and PDLs by the accumulation of neutral lipid vacuoles indicated by the Oil Red O stain as shown in control (Figure 3). We discovered that for dental derived stem cells, the lipid vacuoles were observed until the highest concentration, although for BM-MSCs the neutral lipid droplet showed less accumulation of lipid vacuoles of cells exposed to 80 up to 160 μM.

3.5. Gene Expression Profile of Stem Cells upon Exposure to Pb2+

We further analysed the effect of Pb2+ on MSCs through gene expression analysis. The gene analysis was only carried out with cell lines treated with 160 μM Pb2+ after taking into consideration that other concentrations of Pb2+ did not significantly impact the proliferation and differentiation of MSCs. Firstly, the effect of Pb2+ on the stemness and self-renewal (markers such as Nanog, Rex 1and Oct 4) were evaluated. Rex 1 and Oct 4 were uniformly maintained among the control and treated samples for all four sources of MSCs except for Nanog which was downregulated in PDLs compared to the other cell lines. Next, the effect of Pb2+ on MSCs in terms of lineage specificities was assessed. We observed a sharp decrease in expression of HNF-4α and SOX 17, early endoderm genes in the cells treated with Pb2+ from BM-MSCs. Nevertheless, the expression of HNF-4α and SOX 17 were similar in control and Pb2+ treated SCDs, DPSCs, and PDLs. In Pb2+ treated samples, the expression of SOX 1 and NURR1, in early ectoderm markers, was considerably downregulated in BM-MSCs, while the expression of KRT-15 was relatively stable among all sources. We also looked for signs of transformation in cells treated with Pb2+. We found that the expressions ERCC3, XRCC14, and RAD 51 remain unchanged in control and Pb2+ treated samples (Figures 4 and 5).

fig4
Figure 4: ((a) and (b)) Gene expression analysis of bone marrow stem cells (BM-MSCs), deciduous stem cells (SCDs), periodontal ligament stem cells (PDLs), and permanent stem cells (DPSCs) for control and after treatment of Pb2+ at 160 μmol. BM-MSCs, SCDs, DPSCs, and PDLs were tested against gene repair enzyme (ERCC3, XRCC14, RAD 51), stemness and self-renewal (Nanog, Rex 1, Oct 4), endoderm lineage (HNF-4α, SOX 17), and ectoderm lineage (SOX 1, KRT-15, NURR1). The lower a cycle threshold value is, the more copies are present in the specific sample. Values are presented after being normalized to 18s mRNA levels. The average of 3 replicates is displayed.
235941.fig.005
Figure 5: Soft agar assay of bone marrow stem cells (BM-MSCs), deciduous stem cells (SCDs), periodontal ligament stem cells (PDLs), and permanent stem cells (DPSCs) with HeLa cells as control after treatment of Pb2+ at 160 μM. Phase contrast microscope; 10x magnification of BM-MSCs, SCDs, PDLs, and DPSCs in presence of various concentrations of Pb2+.

4. Discussion

For centuries, Pb2+ poisoning had posed a serious environment threat for human health in all societies [29]. Children have been more vulnerable to Pb2+ exposure than adults for many potential reasons, including their exposure to Pb2+, favoured by the habit of eating unhealthy food coupled with the evidence that a child’s intestine absorbs Pb2+ much faster than that of an adult [30]. Among the detrimental effects of Pb2+ are the disruption of the peripheral and central nervous system [31, 32], blood, and skeletal systems [33, 34]. Pb2+ is stored mainly in the skeletal systems and reenters the blood circulation depending on bone turnover rates, which in turn depend on the type of bone (compact or trabecular) as well as on physiological and pathological conditions that affect the bone turnover rates [35, 36]. Given the multifaceted effects of Pb2+, it is important that we have a strong and continuous dataset for a better understanding of Pb2+ toxicity. Conventionally, laboratory animal-based systems have been used for toxicology studies, but conclusions based on animal testing raise questions due to numerous species-specific differences [37]. Alternatives such as whole embryo cultures and cellular models using primary or immortal cell lines have been developed. In this regard, stem cells from a dental origin could be an ideal cell source for Pb2+ screening and testing. Apart from noninvasive procedures in obtaining the cells, dental enamel is known to accumulate high amounts of Pb2+ on its surface [13, 3843]. This artificial localization of Pb2+ turns dental enamel into a potentially interesting marker of exposure to Pb2+. Indeed, some studies have revealed that there is a relationship between the Pb2+ in surface enamel (Pb-enamel) and environmental lead exposure in permanent teeth [3842] and in primary teeth [13, 43].

In this investigation, we have shown that Pb2+exposure inhibited adhesion with the highest being in BM-MSCs followed by SCDs, DPSCs, and PDLs as well as it increased the biological aging of the cells in a dose-dependent manner. Surprisingly, we noticed that dental derived stem cells are resistant to Pb2+ compared to BM-MSCs. One of the reasons is due to dissimilarities in age and gender of the donors. It should be noted that proliferation of stem cells gradually decreases as the age increases [44, 45]. But, at the mechanistic level, Pb2+ and ionic calcium (Ca2+) have a similar mechanism in entering and leaving the bone. Further, Pb2+ also follows the movement of Ca2+ in the body as it utilises the same ion transporter as calcium, acting like a competitive inhibitor [46]. The ion transporter and binding site of Ca2+ will recognize Pb2+ instead of Ca2+ and Pb2+ is released from the bone cells, along with Ca2+, when the bone is demineralised (as reviewed by [47]). But we postulate that the demineralisation process is slow in teeth allowing the cells of dental origin to adapt for more resistance to Pb2+ toxicity. In this study we identified that the concentration of Pb2+ (40 to 160 μM) suppressed the proliferation of stem cells rather than inducing cell death. One possible explanation is that platelets are present in significant amounts in stem cells or bone marrow mononuclear cell cultures [48, 49] and Pb2+ is thought to influence platelets or lysate-based platelets by regulating the levels of growth chemotactic factors [50]. Thus, we speculate that Pb2+ inhibits growth factors related to the proliferation of stem cells resulting in slow growth of cells.

Next, we demonstrated that all four sources of MSCs were capable of differentiating into osteoblast, chrondocytes, and adipocytes in control samples. All four levels of Pb2+ exposed to MSCs cultured in an osteogenesis medium showed a reduction in osteogenesis differentiation capacity but a higher prevalence was observed in BM-MSCs.

Pb2+ has been shown in various tissues to block calcium signalling and inhibit Ca2+/phosphorylation and activation. There are also reports suggesting that Pb2+ suppressed the expression of osteogenic genes such as osteocalcin, alkaline phosphatase, and type I collagen [51].

In conclusion, we suggest that Pb2+ disturbed the osteogenic pathways, inhibiting the osteoblast differentiation. On the other hand, the results we obtained showed that the chondrogenesis was not greatly affected. [52, 53].

Pb2+ was found to induce chondogenesis in the presence of the transforming growth factor-β (TGF-β) and bone morphogenetic proteins (BMP) in MSCs [5457].

The expression of CD166 was affected in Pb2+ exposed SCDs. This cell surface antigen is a known putative cancer stem cell marker derived in colorectal cancer [58]. It was speculated that the loss of expression of these markers contributed to metastasis. We suggest that Pb2+ did not significantly affect the rest of the cell surface antigen markers. This is because the target area for Pb2+ is in the chromatin and DNA of the cells which are located in the nucleus. Studies have been reported that Pb2+ interacts with histone protein in DNA and affects its integrity by forming cross links and eventually forms soluble complexes [59]. These complexes in turn may decrease the fidelity of DNA [60] and inhibits DNA and RNA synthesis, the process that introduces Pb2+ toxicity at the chromatin level [59]. Nevertheless, Pb2+ on cell surfaces is reported to increase B cell surface expression of murine MHC class II molecules [61].

Our results on gene expressions show that the gene repair enzyme did not significantly change the control and Pb2+ treated samples for all four MSC sources. This finding supports an earlier study stating that stem cells display an enhanced capacity to repair multiple forms of DNA damage including H2O2, UV-C, and ionizing radiation [62].

Recent studies have shown that a chronic low level of Pb2+ exposure may inhibit neurogenesis especially in the hippocampal formation and affect the differentiation/maturation of the newly generated neurons [6365].

It has been reported that the heavy metals including Pb2+ interferes or inhibits gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter binding to GABA receptors, a member of the ligand-gated ion channel super family [66]. Likewise, we hypothesize that activation of a similar pathway induced by GABA-mediated inhibition leads to selective silencing of neuronal markers such as SOX 1 and NURR1 in the present study. We observed a sharp decreased expression of HNF-4α in the cells treated with Pb2+ from BM-MSCs. In contrast, the expression of HNF-4α, a hepatic marker, had somewhat similar effects on SCDs, DPSCs, and PDLs. We observed that BM-MSCs have lower expression of HNF-4α marker compared to the other dental-derived MSCs sources. This is possibly due to the effect of Pb2+ displacing metal ions from proteins by altering the homeostasis of metals which could explain the effect of Pb2+ on gene expression. Our results are in agreement with the report demonstrating the regulation of Pb2+ in hepatocytes [67], indicating a hepatotoxic potential of Pb2+.

5. Conclusions

We propose that the dental derived stem cells especially PDLs are an ideal source for in vitro heavy metal screening since it can withstand the toxicity of Pb2+ better than the other cell lines making it as one of the final frontiers to evaluate the extremisms of Pb2+ toxicity. Nevertheless, few factors need to be taken into consideration to avoid misinterpretation of data: (a) establishment of a good quality and quantity of cell lines is crucial to get a legitimate endpoint of heavy metal toxicity studies; (b) exposure of heavy metals should be prolonged to cover subchronic or chronic effects; (c) a combined battery of experiments covering physiology and biochemistry should be run concurrently to understand the synergic and agonistic effects of heavy metals.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of the paper.

Acknowledgments

This work is part of a research collaboration between the Faculty of Dentistry University of Malaya and Hygieia Innovation Sdn Bhd. This work is supported by the University of Malaya High Impact Research Grant (UM.C/625/1/HIR/065).

References

  1. H. Needleman, “Lead poisoning,” Annual Review of Medicine, vol. 55, pp. 209–222, 2004. View at Publisher · View at Google Scholar · View at Scopus
  2. T. W. Clarkson, “Effects—general principles underlying the toxic action of metals,” in Handbook on the Toxicology of Metals, L. Friberg, G. F. Nordberg, and V. B. Vouk, Eds., pp. 128–148, Elsevier, Amsterdam, The Netherlands, 1990. View at Google Scholar
  3. L. R. Goldman, “Children—unique and vulnerable. Environmental risks facing children and recommendations for response,” Environmental Health Perspectives, vol. 103, no. 6, pp. 13–18, 1995. View at Google Scholar · View at Scopus
  4. I. M. Shapiro, B. Dobkin, O. C. Tuncay, and H. L. Needleman, “Lead levels in dentine and circumpulpal dentine of deciduous teeth of normal and lead poisoned children,” Clinica Chimica Acta, vol. 46, no. 2, pp. 119–123, 1973. View at Google Scholar · View at Scopus
  5. H. T. Delves and M. J. Campbell, “Identification and apportionment of sources of lead in human tissue,” Environmental Geochemistry and Health, vol. 15, no. 2-3, pp. 75–84, 1993. View at Publisher · View at Google Scholar · View at Scopus
  6. H. L. Needleman and I. M. Shapiro, “Dentine lead levels in asymptomatic Phildelphia school children: subclinical exposure in high and low risk groups,” Environmental Health Perspectives, vol. 7, pp. 27–31, 1974. View at Google Scholar · View at Scopus
  7. A. Al-Haddad, F. Al-Saleh, and F. Al-Mahroos, “Levels of cadmium, copper and iron in deciduous teeth of children living in Bahrain,” International Journal of Environmental Health Research, vol. 9, no. 4, pp. 261–268, 1999. View at Google Scholar · View at Scopus
  8. J. Appleton, K. M. Lee, K. Sawicka Kapusta, M. Damek, and M. Cooke, “The heavy metal content of the teeth of the bank vole (Clethrionomys glareolus) as an exposure marker of environmental pollution in Poland,” Environmental Pollution, vol. 110, no. 3, pp. 441–449, 2000. View at Publisher · View at Google Scholar · View at Scopus
  9. I. Baranowska, L. Barchański, M. B̧k, B. Smolec, and Z. Mzyk, “X-ray fluorescence spectrometry in multielemental analysis of hair and teeth,” Polish Journal of Environmental Studies, vol. 13, no. 6, pp. 639–646, 2004. View at Google Scholar · View at Scopus
  10. M. L. Carvalho, C. Casaca, T. Pinheiro, J. P. Marques, P. Chevallier, and A. S. Cunha, “Analysis of human teeth and bones from the chalcolithic period by X-ray spectrometry,” Nuclear Instruments and Methods in Physics Research B, vol. 168, no. 4, pp. 559–565, 2000. View at Publisher · View at Google Scholar · View at Scopus
  11. J. Gdula-Argasińska, J. Appleton, K. Sawicka-Kapusta, and B. Spence, “Further investigation of the heavy metal content of the teeth of the bank vole as an exposure indicator of environmental pollution in Poland,” Environmental Pollution, vol. 131, no. 1, pp. 71–79, 2004. View at Publisher · View at Google Scholar · View at Scopus
  12. R. F. Gerlach, D. B. Toledo, P. D. Novaes, J. Merzel, and S. R. P. Line, “The effect of lead on the eruption rates of incisor teeth in rats,” Archives of Oral Biology, vol. 45, no. 11, pp. 951–955, 2000. View at Publisher · View at Google Scholar · View at Scopus
  13. V. E. Gomes, M. D. L. R. de Sousa, F. Barbosa Jr. et al., “In vivo studies on lead content of deciduous teeth superficial enamel of preschool children,” Science of the Total Environment, vol. 320, no. 1, pp. 25–35, 2004. View at Publisher · View at Google Scholar · View at Scopus
  14. B. Nowak and J. Chmielnicka, “Relationship of lead and cadmium to essential elements in hair, teeth, and nails of environmentally exposed people,” Ecotoxicology and Environmental Safety, vol. 46, no. 3, pp. 265–274, 2000. View at Publisher · View at Google Scholar · View at Scopus
  15. M. L. Sargentini-Maier, R. M. Frank, M. J. Leroy, and J. C. Turlot, “A method of lead determination in human teeth by energy dispersive X-ray fluorescence (EDXRF),” Journal of Trace Elements and Electrolytes in Health and Disease, vol. 2, no. 4, pp. 221–226, 1988. View at Google Scholar · View at Scopus
  16. I. M. Sharon, “The significance of teeth in pollution detection,” Perspectives in Biology and Medicine, vol. 32, no. 1, pp. 124–131, 1988. View at Google Scholar · View at Scopus
  17. R. Lansdown, “Lead, intelligence, attainment and behavior,” in The Lead Debate: The Environment, Toxicology and Child Health, R. Lansdown and W. Yule, Eds., pp. 235–270, Croom Helm, London, UK, 1986. View at Google Scholar
  18. J. E. Fergusson and N. G. Purchase, “The analysis and levels of lead in human teeth: a review,” Environmental Pollution, vol. 46, no. 1, pp. 11–44, 1987. View at Google Scholar · View at Scopus
  19. S. J. Pocock, M. Smith, and P. Baghurst, “Environmental lead and children's intelligence: a systematic review of the epidemiological evidence,” The British Medical Journal, vol. 309, no. 6963, pp. 1189–1197, 1994. View at Google Scholar · View at Scopus
  20. K. R. Mahaffey, J. McKinney, and J. R. Reigart, “Lead and compounds,” in Environmental Toxicants Human Exposure and Their Health Effects, M. Lippmann, Ed., pp. 360–391, Van Nostrand Reinhold, New York, NY, USA, 1992. View at Google Scholar
  21. Y. Hayama, “Effect of heavy metal ions on the cells derived from human periodontal ligament. Effects of Pb and Cd,” The Journal of the Kanagawa Odontological Society, vol. 24, no. 4, pp. 671–691, 1990. View at Google Scholar · View at Scopus
  22. H. M. Tvinnereim, R. Eide, and T. Riise, “Heavy metals in human primary teeth: some factors influencing the metal concentrations,” Science of the Total Environment, vol. 255, no. 1–3, pp. 21–27, 2000. View at Publisher · View at Google Scholar · View at Scopus
  23. S. Thaweboon, P. Chunhabundit, R. Surarit, S. Swasdison, and P. Suppukpatna, “Effects of lead on the proliferation, protein production, and osteocalcin secretion of human dental pulp cells in vitro,” The Southeast Asian Journal of Tropical Medicine and Public Health, vol. 33, no. 3, pp. 654–661, 2002. View at Google Scholar · View at Scopus
  24. R. Pal, M. Hanwate, M. Jan, and S. Totey, “Phenotypic and functional comparison of optimum culture conditions for upscaling of bone marrow-derived mesenchymal stem cells,” Journal of Tissue Engineering and Regenerative Medicine, vol. 3, no. 3, pp. 163–174, 2009. View at Publisher · View at Google Scholar · View at Scopus
  25. V. Govindasamy, V. S. Ronald, S. Totey et al., “Micromanipulation of culture niche permits long-term expansion of dental pulp stem cells-an economic and commercial angle,” In Vitro Cellular and Developmental Biology, vol. 46, no. 9, pp. 764–773, 2010. View at Publisher · View at Google Scholar · View at Scopus
  26. X. Shen, K. Lee, and R. König, “Effects of heavy metal ions on resting and antigen-activated CD4+ T cells,” Toxicology, vol. 169, no. 1, pp. 67–80, 2001. View at Publisher · View at Google Scholar · View at Scopus
  27. L. Braydich-Stolle, S. Hussain, J. J. Schlager, and M.-C. Hofmann, “In vitro cytotoxicity of nanoparticles in mammalian germline stem cells,” Toxicological Sciences, vol. 88, no. 2, pp. 412–419, 2005. View at Publisher · View at Google Scholar · View at Scopus
  28. W. W. Hwang-Verslues, W.-H. Kuo, P.-H. Chang et al., “Multiple lineages of human breast cancer stem/progenitor cells identified by profiling with stem cell markers,” PloS ONE, vol. 4, no. 12, article e8377, 2009. View at Publisher · View at Google Scholar · View at Scopus
  29. R. Fuller, “Lead exposures from car batteries—a global problem,” Environmental Health Perspectives, vol. 117, no. 12, article A535, 2009. View at Publisher · View at Google Scholar · View at Scopus
  30. J. Huang, K. Huang, L. Shang et al., “Chronic lead exposure reduces doublecortin-expressing immature neurons in young adult guinea pig cerebral cortex,” BMC Neuroscience, vol. 13, article 82, 2012. View at Publisher · View at Google Scholar
  31. R. A. Goyer and H. G. Seiler, Handbook on Toxicity of Inorganic Compounds, vol. 359, Marcel Dekker, New York, NY, USA, 1988.
  32. D. C. Bellinger and A. M. Bellinger, “Childhood lead poisoning: the torturous path from science to policy,” Journal of Clinical Investigation, vol. 116, no. 4, pp. 853–857, 2006. View at Publisher · View at Google Scholar · View at Scopus
  33. K. Nemoto, S. Miyata, F. Nemoto et al., “Gene expression of neurotrophins and their receptors in lead nitrate-induced rat liver hyperplasia,” Biochemical and Biophysical Research Communications, vol. 275, no. 2, pp. 472–476, 2000. View at Publisher · View at Google Scholar · View at Scopus
  34. D. G. Hicks, R. J. O'Keefe, K. J. Reynolds et al., “Effects of lead on growth plate chondrocyte phenotype,” Toxicology and Applied Pharmacology, vol. 140, no. 1, pp. 164–172, 1996. View at Publisher · View at Google Scholar · View at Scopus
  35. H. Hu, M. Rabinowitz, and D. Smith, “Bone lead as a biological marker in epidemiologic studies of chronic toxicity: conceptual paradigms,” Environmental Health Perspectives, vol. 106, no. 1, pp. 1–8, 1998. View at Google Scholar · View at Scopus
  36. A. C. Todd, J. G. Wetmur, J. M. Moline, J. H. Godbold, S. M. Levin, and P. J. Landrigan, “Unraveling the chronic toxicity of lead: an essential priority for environmental health,” Environmental Health Perspectives, vol. 104, no. 1, pp. 141–146, 1996. View at Google Scholar · View at Scopus
  37. G. Scholz, I. Pohl, E. Genschow, M. Klemm, and H. Spielmann, “Embryotoxicity screening using embryonic stem cells in vitro: correlation to in vivo teratogenicity,” Cells Tissues Organs, vol. 165, no. 3-4, pp. 203–211, 1999. View at Google Scholar · View at Scopus
  38. R. Cleymaet, P. Bottenberg, D. H. Retief, D. Slop, Y. Michotte, and D. Coomans, “In vivo use of a dual acid etch biopsy for the evaluation of lead profiles in human surface enamel,” Caries Research, vol. 25, no. 4, pp. 256–263, 1991. View at Google Scholar · View at Scopus
  39. R. Cleymaet, P. Bottenberg, D. Slop, R. Clara, and D. Coomans, “Study of lead and cadmium content of surface enamel of schoolchildren from an industrial area in Belgium,” Community Dentistry and Oral Epidemiology, vol. 19, no. 2, pp. 107–111, 1991. View at Google Scholar · View at Scopus
  40. R. Cleymaet, K. Collys, D. H. Retief et al., “Relation between lead in surface tooth enamel, blood, and saliva from children residing in the vicinity of a non-ferrous metal plant in Belgium,” British Journal of Industrial Medicine, vol. 48, no. 10, pp. 702–709, 1991. View at Google Scholar · View at Scopus
  41. R. Cleymaet, E. Quartier, D. Slop, D. H. Retief, J. Smeyers-Verbeke, and D. Coomans, “Model for assessment of lead content in human surface enamel,” Journal of Toxicology and Environmental Health, vol. 32, no. 2, pp. 111–127, 1991. View at Google Scholar · View at Scopus
  42. R. Cleymaet, D. H. Retief, E. Quartier, D. Slop, D. Coomans, and Y. Michotte, “A comparative study of the lead and cadmium content of surface enamel of Belgian and Kenyan children,” Science of the Total Environment, vol. 104, no. 3, pp. 175–189, 1991. View at Google Scholar · View at Scopus
  43. G. R. C. D. Almeida, M. D. C. Pereira Saraiva, F. Barbosa Jr. et al., “Lead contents in the surface enamel of deciduous teeth sampled in vivo from children in uncontaminated and in lead-contaminated areas,” Environmental Research, vol. 104, no. 3, pp. 337–345, 2007. View at Publisher · View at Google Scholar · View at Scopus
  44. D. L. Jones and T. A. Rando, “Emerging models and paradigms for stem cell ageing,” Nature Cell Biology, vol. 13, no. 5, pp. 506–512, 2011. View at Publisher · View at Google Scholar · View at Scopus
  45. D. R. Bell and G. van Zant, “Stem cells, aging, and cancer: inevitabilities and outcomes,” Oncogene, vol. 23, no. 43, pp. 7290–7296, 2004. View at Publisher · View at Google Scholar · View at Scopus
  46. E. M. Alissa and G. A. Ferns, “Heavy metal poisoning and cardiovascular disease,” Journal of Toxicology, vol. 2011, Article ID 870125, 21 pages, 2011. View at Publisher · View at Google Scholar · View at Scopus
  47. N. C. Papanikolaou, E. G. Hatzidaki, S. Belivanis, G. N. Tzanakakis, and A. M. Tsatsakis, “Lead toxicity update. A brief review,” Medical Science Monitor, vol. 11, no. 10, pp. RA329–RA336, 2005. View at Google Scholar · View at Scopus
  48. B. Assmus, T. Tonn, F. H. Seeger et al., “Red blood cell contamination of the final cell product impairs the efficacy of autologous bone marrow mononuclear cell therapy,” Journal of the American College of Cardiology, vol. 55, no. 13, pp. 1385–1394, 2010. View at Publisher · View at Google Scholar · View at Scopus
  49. M. Prokopi, G. Pula, U. Mayr et al., “Proteomic analysis reveals presence of platelet microparticles in endothelial progenitor cell cultures,” Blood, vol. 114, no. 3, pp. 723–732, 2009. View at Publisher · View at Google Scholar · View at Scopus
  50. M. Müller, O. Raabe, K. Addicks, S. Wenisch, and S. Arnhold, “Effects of non-steroidal anti-inflammatory drugs on proliferation, differentiation and migration in equine mesenchymal stem cells,” Cell Biology International, vol. 35, no. 3, pp. 235–248, 2011. View at Publisher · View at Google Scholar · View at Scopus
  51. G. J. Long, J. F. Rosen, and J. G. Pounds, “Cellular lead toxicity and metabolism in primary and clonal osteoblastic bone cells,” Toxicology and Applied Pharmacology, vol. 102, no. 2, pp. 346–361, 1990. View at Publisher · View at Google Scholar · View at Scopus
  52. M. J. Zuscik, L. Ma, T. Buckley et al., “Lead induces chondrogenesis and alters transforming growth factor-β and bone morphogenetic protein signaling in mesenchymal cell populations,” Environmental Health Perspectives, vol. 115, no. 9, pp. 1276–1282, 2007. View at Publisher · View at Google Scholar · View at Scopus
  53. J. J. Carmouche, J. E. Puzas, X. Zhang et al., “Lead exposure inhibits fracture healing and is associated with increased chondrogenesis, delay in cartilage mineralization, and a decrease in osteoprogenitor frequency,” Environmental Health Perspectives, vol. 113, no. 6, pp. 749–755, 2005. View at Publisher · View at Google Scholar · View at Scopus
  54. P. Chen, J. L. Carrington, R. G. Hammonds, and A. H. Reddi, “Stimulation of chondrogenesis in limb bud mesoderm cells by recombinant human bone morphogenetic protein 2B (BMP-2B) and modulation by transforming growth factor β1 and β2,” Experimental Cell Research, vol. 195, no. 2, pp. 509–515, 1991. View at Publisher · View at Google Scholar · View at Scopus
  55. D. A. Frenz, W. Liu, J. D. Williams et al., “Induction of chondrogenesis: requirement for synergistic interaction of basic fibroblast growth factor and transforming growth factor-beta,” Development, vol. 120, no. 2, pp. 415–424, 1994. View at Google Scholar · View at Scopus
  56. M. Iwasaki, K. Nakata, H. Nakahara et al., “Transforming growth factor-β1 stimulates chondrogenesis and inhibits osteogenesis in high density culture of periosteum-derived cells,” Endocrinology, vol. 132, no. 4, pp. 1603–1608, 1993. View at Publisher · View at Google Scholar · View at Scopus
  57. M. E. Joyce, A. B. Roberts, M. B. Sporn, and M. E. Bolander, “Transforming growth factor-β and the initiation of chondrogenesis and osteogenesis in the rat femur,” The Journal of Cell Biology, vol. 110, no. 6, pp. 2195–2207, 1990. View at Publisher · View at Google Scholar · View at Scopus
  58. P. Dalerba, S. J. Dylla, I.-K. Park et al., “Phenotypic characterization of human colorectal cancer stem cells,” Proceedings of the National Academy of Sciences of the United States of America, vol. 104, no. 24, pp. 10158–10163, 2007. View at Publisher · View at Google Scholar · View at Scopus
  59. A. Rabbani-Chadegani, S. Abdosamadi, N. Fani, and S. Mohammadian, “A comparison of the effect of lead nitrate on rat liver chromatin, DNA and histone proteins in solution,” Archives of Toxicology, vol. 83, no. 6, pp. 565–570, 2009. View at Publisher · View at Google Scholar · View at Scopus
  60. L. A. Loeb and R. A. Zakour, “Metals and genetic miscoding,” in Nucleic Acid-Metal Ion InteractIon, T. G. Spiro, Ed., p. 115, John Wiley & Sons, New York, NY, USA, 1980. View at Google Scholar
  61. M. J. McCabe Jr. and D. A. Lawrence, “The heavy metal lead exhibits B cell-stimulatory factor activity by enhancing B cell Ia expression and differentiation,” The Journal of Immunology, vol. 145, no. 2, pp. 671–677, 1990. View at Google Scholar · View at Scopus
  62. S. Maynard, A. M. Swistowska, J. W. Lee et al., “Human embryonic stem cells have enhanced repair of multiple forms of DNA damage,” Stem Cells, vol. 26, no. 9, pp. 2266–2274, 2008. View at Publisher · View at Google Scholar · View at Scopus
  63. M. E. Gilbert, M. E. Kelly, T. E. Samsam, and J. H. Goodman, “Chronic developmental lead exposure reduces neurogenesis in adult rat hippocampus but does not impair spatial learning,” Toxicological Sciences, vol. 86, no. 2, pp. 365–374, 2005. View at Publisher · View at Google Scholar · View at Scopus
  64. K. Jaako-Movits, T. Zharkovsky, O. Romantchik et al., “Developmental lead exposure impairs contextual fear conditioning and reduces adult hippocampal neurogenesis in the rat brain,” International Journal of Developmental Neuroscience, vol. 23, no. 7, pp. 627–635, 2005. View at Publisher · View at Google Scholar · View at Scopus
  65. T. Verina, C. A. Rohde, and T. R. Guilarte, “Environmental lead exposure during early life alters granule cell neurogenesis and morphology in the hippocampus of young adult rats,” Neuroscience, vol. 145, no. 3, pp. 1037–1047, 2007. View at Publisher · View at Google Scholar · View at Scopus
  66. T. M. DeLorey and R. W. Olsen, “γ-Aminobutyric acid(A) receptor structure and function,” The Journal of Biological Chemistry, vol. 267, no. 24, pp. 16747–16750, 1992. View at Google Scholar · View at Scopus
  67. N. Milosevic and P. Maier, “Lead stimulates intercellular signalling between hepatocytes and Kupffer cells,” European Journal of Pharmacology, vol. 401, no. 3, pp. 317–328, 2000. View at Publisher · View at Google Scholar · View at Scopus