Superfund Research Program
Effects of Arsenic on Cytochrome P450
Project Leader: Jacqueline F. Sinclair (Dartmouth Medical School)
Grant Number: P42ES007373
Funding Period: 1995 - 2005
Arsenic was one of the earliest hepatotoxins to be identified in humans, and is associated with biliary occlusion, fibrosis and cirrhosis in the liver, as well as hepatocellular carcinoma. These effects may arise, in part, due to arsenic-mediated decreases in cytochromes P450 (CYP), which are involved in the detoxification of numerous exogenous and endogenous chemicals. Treatment of rats with sodium arsenite resulted in a decrease in hepatic levels of CYP (Sardana et al. 1981; Albores et al., 1992). Sodium arsenite also increased heme oxygenase, the rate-limiting step in heme degradation, leading to the hypothesis that the arsenite-mediated decreases in CYP resulted from decreased availability of heme (Sardana et al. 1981). However, a causative relationship between the induction of heme oxygenase and decreases in CYP has not been established. Work in Project 3 showed that arsenite decreased the induction of several forms of CYP in primary cultures of chick and rat hepatocytes. Arsenite increased heme oxygenase concomitant with its effects to decrease these CYPs. However, equivalent increases in heme oxygenase are obtained on treatment with heme, with no decrease in induction of these CYPs. Therefore, the arsenite-mediated decreases in CYPs cannot be solely attributed to increases in heme oxygenase. Provision of exogenous heme did not prevent the arsenite-mediated decreases in CYPs, indicating that such decreases were not due to limiting amounts of heme. Arsenite has also been reported to increase the accumulation of porphyrin intermediates in the heme biosynthetic pathway. However such effects were observed at high concentrations of arsenic and may have been due to effects on the kidney rather than the liver. The purpose of the present study was to examine the consequences of arsenite treatment on heme synthesis, and on the degradation of heme in chick embryo hepatocytes. Arsenite, at concentrations ranging from 1 to 25 µM, did not increase the accumulation of porphyrins, indicating that the enzymes in the pathway between 5-aminolevulinic acid synthase and ferrochelatase were unaffected by arsenite. The rate of heme disappearance from exogenous heme was no different in chick embryo hepatocyte cultures treated simultaneously with arsenite and heme or compared to heme alone. Heme negatively regulates 5-aminolevulinate synthase (ALAS), the first enzyme in heme biosynthetic pathway, mainly by increasing the degradation of its mRNA. A 4 to 6-fold elevation of heme oxygenase by arsenite did not elevate ALAS mRNA or alter the induction of ALAS by phenobarbital, indicating that the regulatory heme pool was not affected by these increases in heme oxygenase. These results suggest, that while low concentrations of arsenite induce heme oxygenase several-fold, this has little effect on heme metabolism in chick embryo hepatocyte cultures.