Mechanisms by which arsenite decreases CYP3A

Arsenic is implicated in numerous human pathologies, including cancer and several forms of liver disease. One contributing mechanism may involve alterations of CYP levels by arsenic. CYP3As are the most abundant CYP proteins in human liver and metabolize roughly half of all currently used drugs. The following studies investigated the effects and underlying mechanisms of acute arsenite exposure on CYP3A in primary rat and human hepatocyte cultures. The concentrations of arsenite used in these studies were non-toxic to the hepatocytes and failed to elicit an oxidative stress response. Arsenite-mediated decreases in CYP3A23 were not due to depletion of heme or glutathione. In addition, arsenite did not enhance degradation of CYP3A23 protein by calpain- or proteasome-mediated proteolysis. In rat hepatocyte cultures, 5 muM arsenite abolished DEX-mediated induction of CYP3A23 protein and activity, yet only reduced DEX-induced CYP3A23 mRNA by 30%. Arsenite caused a 30% decrease in luciferase expression of a CYP3A23-promoter construct and a 50% decrease in CYP3A23 hnRNA. Additionally, arsenite caused up to 85% decreases in association of CYP3A23 mRNA with polyribosomes, and caused 20--30% decreases in luciferase expression of reporter constructs containing either the 5' or 3' UTR of CYP3A23 mRNA. Together, these results suggest that low-level arsenite decreases both transcription and translation of CYP3A23 in rat hepatocyte cultures. Treatment with 2.5 or 5 muM arsenite caused dramatic decreases in Rif- or PB-mediated induction of CYP3A4 mRNA, protein and activity in human hepatocyte cultures. CYP3A4 expression in untreated cells was also decreased following arsenite treatment. We found that arsenite decreased human PXR responsiveness to Rif, with sustained decreases in RXRalpha protein, and transient decreases in PXR protein. These results suggest that arsenite inhibits CYP3A4 expression in human hepatocyte cultures by decreasing PXR- and RXRalpha-mediated transcription. Importantly, a concentration of arsenite (50 nM) below the allowable level set by the EPA for drinking water (10 ppb), significantly decreased induction of CYP3A4 mRNA in human hepatocyte cultures. In summary, these studies show that environmentally and clinically relevant concentrations of arsenite inhibit CYP3A expression, which has the potential to contribute to adverse drug reactions or arsenite-induced disease.