MOLECULAR-BASIS FOR EFFECTS OF CARCINOGENIC HEAVY-METALS ON INDUCIBLEGENE-EXPRESSION

Certain forms of the heavy metals arsenic and chromium are considered human carcinogens, although they are believed to act through very diff erent mechanisms. Chromium(VI) is believed to act as a classic genotox ic and mutagenic agent, and DNA/chromatin appears to be the principal target for its effects. In contrast, arsenic(III) is considered nongen otoxic, but is able to target specific cellular proteins, principally through sulfhydryl interactions. We had previously shown that various genotoxic chemical carcinogens, including chromium(VI), preferentially altered expression of several inducible genes but had little or no ef fect on constitutive gene expression. We were therefore interested in whether these carcinogenic heavy metals might target specific but dist inct sites within cells, leading to alterations in gene expression tha t might contribute to the carcinogenic process. Arsenic(III) and chrom ium(VI) each significantly altered both basal and hormone-inducible ex pression of a model inducible gene, phosphoenolpyruvate carboxykinase (PEPCK), at nonovertly toxic doses in the chick embryo in vivo and rat hepatoma H4IIE cells in culture. We have recently developed two paral lel cell culture approaches for examining the molecular basis for thes e effects. First, we are examining the effects of heavy metals on expr ession and activation of specific transcription factors known to be in volved in regulation of susceptible inducible genes, and have recently observed significant but different effects of arsenic(III) and chromi um(Vl) on nuclear transcription factor binding. Second, we have develo ped cell lines with stably integrated PEPCK promoter-luciferase report er gene constructs to examine effects of heavy metals on promoter func tion, and have also recently seen profound effects induced by both chr omium(VI) and arsenic(III) in this system. These model systems should enable us to be able to identify the critical cis (DNA) and trans (pro tein) cellular targets of heavy metal exposure leading to alterations in expression of specific susceptible genes. It is anticipated that su ch information will provide valuable insight into the mechanistic basi s for these effects as well as provide sensitive molecular biomarkers for evaluating human exposure.