CYP2E1 overexpression up-regulates both non-specific δ-aminolevulinate synthase and heme oxygenase-1 in the human hepatoma cell line HLE/2E1

Cytochrome P450 (CYP) is known to turn over rapidly both in vivo in the liver, and in vitro in cultured hepatoma cells expressing CYP. We examined changes in heme metabolism by analyzing gene expression of the non-specific 6-aminolevulinate synthase (ALAS-N), and heme oxygenase-1 (HO-1), the rate limiting enzyme in heme synthesis and catabolism, respectively, in the human hepatoma cell line HLE/2E1, in which CYP2E1 was overexpressed by transfection of its expression vector. Both ALAS-N mRNA and HO-1 mRNA levels were found to be markedly upregulated in HLE/2E1 cells as compared with those in nontransfected cells (HLE), or in mock- transfected cells (HLE/MOCK). Treatment of HLE/2E1 cells with succinylacetone (SA), a potent inhibitor of 8-aminolevulinate dehydratase and thereby heme synthesis, resulted in a further increase in ALAS-N mRNA but a decrease in HO-1 mRNA levels. In contrast, treatment of cells with heme, as heme arginate, to SA-pretreated HLE/2E1 cells restored both mRNA levels to the untreated control level. These findings suggest that the overexpression of CYP2E1 results in the up-regulation of ALAS-N in order to meet with an increased demand for heme synthesis for CYP2E1 formation, while it also results in the up-regulation of HO-1 presumably by enzyme induction by free heme released from CYP2E1, which then results in the elimination of toxic excess free heme and ultimately restores the physiologic milieu.