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

Abstract

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.

Molecular basis for effects of carcinogenic heavy metals on inducible gene expression.

“…Both the basal and inducible expression of the inducible 5'-aminolevulinate synthase, cytochrome P450 CYP2HI, and phosphoenolpyruvate carboxykinase (PEPCK) genes were markedly affected by the chromium treatment, whereas the albumin, transferrin, and ,-actin genes were refractory to this treatment. The effects on expression of the inducible genes were seen at both the steady-state mRNA and transcriptional levels, and the time courses for these effects closely matched the time course for chromium-induced DNA damage and repair (10,12,14). Interestingly, certain effects most closely correlated with chromium-DNA monoadduct formation, whereas other effects were more closely associated with chromium-DNA cross-link formation (10,12,14).…”

“…Like many organic carcinogens such as benzo [a]pyrene and aflatoxin B1, chromium(VI) appears to act as a classic initiator in these test systems, and this is believed to be the principal mechanism by which chromium(VI) increases carcinogenic risk. Our laboratory has hypothesized that genotoxic carcinogens, including chromium(VI), exert preferential effects on the expression of a specific dass of genes, i.e., inducible genes, as a result of nonrandom DNA damage targeted to members of this gene class (8)(9)(10)(11)(12). Previous studies in our laboratory have shown that a number of different genotoxic chemical carcinogens that induce different types of DNA damage, including the genotoxic metals chromium and nickel, significantly alter both basal and inducible expression of several model inducible genes but have no effect on expression of constitutive genes.…”

“…The highest doses of chromium(VI) and arsenic(III) that caused no overt toxicity or lethality in dose-response experiments were determined to be 50 imol/kg and 100 pmol/kg, respectively (10,12,42).…”

“…At these doses there is little or no effect on overall DNA, RNA, or protein synthesis, and the embryos can be hatched normally with no obvious toxic effects (10,12,42). but had no effect on expression of ,-actin ( Figure 1C).…”

Chromate-induced epimutations in mammalian cells.

“…The complex intracellular reduction chemistry of Cr can produce DNA damage, Cr-mediated DNA adducts, DNA-DNA and DNA-protein crosslinks, and mutations [reviewed in DeFlora et al (4) and Klein (5)]. Cr can also inhibit DNA replication and repair [reviewed in Snow (6)], alter gene expression (7,8), activate stressresponse pathways (2,9), trigger apoptosis (10,11), and damage chromosomes [reviewed in DeFlora et al (4) and Klein (5)]. …”