Cytotoxicity and transcriptional activation of stress genes in human liver carcinoma cells (HepG2) exposed to cadmium chloride

Epidemiological studies have shown that there exists some correlation between cadmium exposure and human cancers. The evidence that cadmium and cadmium compounds are probable human carcinogens is also supported by experimental studies reporting induction of malignant tumors formation in multiple species of laboratory animals exposed to these compounds. In vitro studies with mammalian cells have also shown that cadmium is clastogenic, but its mutagenic potential is rather weak. In this research, we performed the MTT assay for cell viability to assess the cytotoxicity of cadmium chloride (CdCl2), and the CAT-Tox (L) assay to measure the induction of stress genes in thirteen different recombinant cell lines generated from human liver carcinoma cells (HepG(2)), by creating stable transfectants of different mammalian promoter - chloramphenicol acetyltransferase (CAT) gene fusions. Cytotoxicity experiments with the parental cell line yielded a LC50 of 6.1 +/- 0.8 mug/mL, upon 48 h of exposure. Four (metallothionein - HMTIIA, 70-kDa heat shock protein - HSP70, xenobitic response element - XRE, and cyclic adenosine monophosphate response element - CRE) out of the 13 constructs evaluated showed statistically significant inductions (p < 0.05). The induction of these genes was concentration-dependent. Marginal inductions were also recorded for the c-fos, and 153-kDa growth arrest DNA damage (GADD153) promoters, indicating a potential for CdC1(2) to damage DNA. However, no significant inductions (p > 0.05) of gene expression were recorded for cytochrome P450 1A1 - CYP1A1, glutathion-S-transferase Ya subunit - GST Ya, nuclear factor kappa (B site) response element - NFkBRE, tumor suppressor protein response element - p53RE, 45-kDa growth arrest DNA damage - GADD45, 78-kDa glucose regulated protein - GRP78, and retinoic acid response element - RARE. As expected, these results indicate that metallothioneins and heat shock proteins appear to be excellent candidates for biomarkers for detecting cadmium-induced proteotoxic effects at the molecular and cellular levels. Induction of XRE indicates the potential involvement of CdC1(2) in the biotransformation process in the liver, while activation of CRE indicates stimulation of cellular signaling through the protein kinases pathway.