Effects of the genotoxic carcinogen chromium(VI) on basal and hormone-inducible phosphoenolpyruvate carboxykinase gene expression in vivo: correlation with glucocorticoid- and developmentally regulated expression.

Previous studies have shown that a number of different genotoxic carcinogens that induce different types of DNA damage preferentially alter the expression of inducible genes in vivo. To investigate further the mechanistic basis for these effects, we examined the effects of the human lung carcinogen chromium(VI) on expression of the hormone-inducible cytosolic phosphoenolpyruvate carboxykinase (PEPCK) gene in chick embryo liver. Chromium(VI) pretreatment had significant effects on both basal and glucocorticoid-inducible PEPCK expression in 14-d-old embryo liver. These effects were principally a result of changes in PEPCK transcription. In contrast, treatment with chromium(VI) 1 h after treatment with glucocorticoid had no effect on PEPCK induction, suggesting that an early event in the induction process is the target for carcinogen effects. In 16-d-old liver, in which PEPCK expression is no longer responsive to glucocorticoid induction, both basal and inducible PEPCK expression were also refractory to chromium(VI) effects, indicating that carcinogen responsiveness is a phenotypic rather than an inherent property of inducible genes and is related to their competence for induction. Chromium(VI) had no effect on cAMP induction of PEPCK expression, demonstrating that carcinogens target their effects to specific regulatory pathways. Comparison of the effects of chromium(VI) with those of cycloheximide suggests that chromium(VI) targets its effects to a labile, constitutively expressed repressor involved in PEPCK gene regulation.