Heme may not be a positive regulator of cytochrome‐P450 gene expression

Abstract

It has been proposed that transcription of cytochrome-P450 genes is positively regulated by heme, the prosthetic group of cytochrome-P450 proteins. We have investigated this proposal in rats treated with succinylacetone, a known specific inhibitor of the heme biosynthetic pathway. While 2-allyl-2-isopropylacetamide, phenobarbitone, dexamethasone, p-naphthoflavone and clofibrate induced specific cytochrome-P450-mRNA species in rat liver, the levels of these induced mRNAs were not affected by succinylacetone administration. Synthesis of the first enzyme of the heme biosynthetic pathway, 5-aminolevulinate synthase, is known to be regulated by the end-product heme, with heme inhibiting 5-aminolevulinate-synthase-gene transcription. Hepatic 5-aminolevulinate-synthase mRNA was induced by drugs and the level increased further by succinylacetone. Furthermore, treatment of rats with succinylacetone alone resulted in elevated levels of 5-aminolevulinate-synthase mRNA but did not affect cytochrome-P450-mRNA levels.The results show that while lowered heme levels lead to an increase in 5-aminolevulinate-synthase-mRNA synthesis there is no effect on cytochrome-P450-mRNA levels, implying that heme is not required for the cytochrome-P450-gene transcription.Cytochrome P45Os (P450s) are heme-containing proteins which constitute a super-family of enzymes involved in the oxidative metabolism of either endogenous compounds or a vast range of foreign chemicals [l, 21. Considerable progress has been made in understanding the mechanism by which polycyclic aromatic hydrocarbons induce hepatic P450I genes and evidence indicates that a drug-receptor complex mediates induction by interaction with regulatory elements in the 5'-

Heme-Reversible Impairment of CYP2B1/2 Induction in Heme-Depleted Rat Hepatocytes in Primary Culture: Translational Control by a Hepatic α-Subunit of the Eukaryotic Initiation Factor Kinase?

“…However, consistent with previous findings (Srivastava et al, 1989;Sinclair et al, 1990), this impairment was not due to the inhibition of PB-induced transcriptional activation of the CYP2B gene but rather due to inhibition of CYP2B translation. A similar impairment of PB-mediated induction of CYP2B1/2 protein has been observed in succinylacetone-treated rat hepatocytes and is proposed to reflect a plausible heme requirement for P450 mRNA translation (Sinclair et al, 1990).…”

“…Yet, even this profound heme depletion had no effect on PB-mediated transcriptional activation of rat CYP2B1 and CYP2B2 genes. Our studies, which used a model of an even more severe hepatic heme pool depletion and highly sensitive quantitative PCR analyses that could distinguish between the highly homologous CYP2B1 and CYP2B2 genes, support previous conclusions that heme is not required for PB-mediated transcriptional activation of either P450 (Srivastava et al, 1989;Sinclair et al, 1990;Jover et al, 2000), as proposed previously (Dwarki et al, 1987;Rangarajan and Padmanaban, 1989).…”

“…Using these models for acute hepatic heme depletion, we have found that heme is definitely required for PB-mediated induction of CYP2B1/2 protein. However, consistent with the previous findings (Srivastava et al, 1989;Sinclair et al, 1990), such regulation is not exerted at the level of CYP2B1/2 mRNA induction, but rather it occurs at a posttranscriptional/translational step. Indeed, our findings in both DDEP-and NMPP-treated hepatocytes of global suppression of de novo hepatic protein synthesis and enhanced phosphorylation of the ␣-subunit of the eukaryotic initiation factor eIF2 (eIF2␣), as well as the reversal of these effects by heme, suggest that this impaired CYP2B1 induction results from blocked translation stemming partly from the activation of a heme-sensitive eIF2␣ kinase.…”

“…Just one group to date has advocated a positive role for heme in CYP2B transcriptional activation (Dwarki et al, 1987;Rangarajan and Padmanaban, 1989). Two other research teams having examined this possibility concluded that heme is not involved in CYP2B transcriptional activation (Hamilton et al, 1988;Srivastava et al, 1989;Sinclair et al, 1990). Similarly, no role for heme in CYP3A11 transcriptional activation was found in mice treated with the heme synthesis inhibitor lead acetate (Jover et al, 1996).…”

RETRACTION: Hepatic Heme-Regulated Inhibitor (HRI) Eukaryotic Initiation Factor 2α Kinase: A Protagonist of Heme-Mediated Translational Control of CYP2B Enzymes and a Modulator of Basal Endoplasmic Reticulum Stress Tone

“…qRT-PCR analyses of total RNA from C57BL/6J WT and HRI(Ϫ/Ϫ) hepatocytes with a CYP2B10-specific Taqman probe revealed that in both C57BL/6J WT and HRI(Ϫ/Ϫ) hepatocytes, CYP2B mRNA was indeed PB-inducible but was unaffected by concomitant hepatic heme depletion or repletion (Fig. 10B), consistent with previous findings (Srivastava et al, 1989;Sinclair et al, 1990;Jover et al, 2000;Han et al, 2005b). However, irrespective of hepatic heme depletion or repletion, CYP2B mRNA levels were slightly albeit significantly higher in the HRI(Ϫ/Ϫ) hepatocytes than in the corresponding WT hepatocytes (Fig.…”