Influence of Heme on Hepatic Cytochrome P450 Turnover

University Of California, San Francisco, San Francisco CA

Investigators

Linked publications, trials & patents

Abstract

DESCRIPTION (provided by applicant): Acute hepatic heme deficiency is a biochemical hallmark of acute clinical attacks of hepatic porphyrias, genetic disorders stemming from inherited defects in heme synthesis. In genetically predisposed individuals, such acute attacks are commonly triggered by some prescription drugs, starvation, infections and/or inflammation. Inhibition of heme synthesis in rat or mouse hepatocytes also results in acute heme depletion and consequent autoactivation of hepatic heme-regulated inhibitor HRI eIF2? kinase, an exquisite heme-sensor. The ensuing increased eIF2? phosphorylation causes global translational arrest of de novo synthesis of a myriad of hepatic proteins including cytochromes P450 (P450s), hemoprotein enzymes engaged in the breakdown and clearance of endo- and xenobiotics such as drugs, carcinogens, toxins, natural and chemical products. Such hepatic heme depletion also results in the translationally arrested synthesis of hepatic I?B? (a NF?B-inhibitor) and consequent nuclear activation of NF?B, a proinflammatory factor involved in inflammation, cancer, autophagy apoptosis and immunity. Concurrently, autophagic-lysosomal degradation (ALD) of existing and otherwise longer-lived hepatic P450s is also rapidly accelerated, in a heme-reversible manner. Activation of the other three eIF2? kinases is associated with NF?B-mediated autophagy, thereby indicating that hepatic HRI activation could similarly induce P450 ALD. The specific role of hepatic HRI in NF?B activation and consequent P450 ALD remains to be established and will be examined in hepatocytes from mice with genetic HRI knockout (Hri-/-) relative to wild type (Hri+/+) controls. Relatively little is mechanistically known about P450 ALD other than its recognition as a slow bulk process. Studies are therefore proposed to characterize the molecular determinants and participants operating in normal P450 ALD pathway, and to elucidate those responsible for accelerating P450 degradation upon hepatic heme depletion. Such elucidation will rely on approaches such as site-directed mutagenesis, lentiviral shRNA interference, immunoaffinity capture and proteomic analyses. The liver is a major supplier of proteins, lipids and nutrients to the rest of the body, and thus translational arrest of de novo synthesis coupled with enhanced ALD of hepatic proteins is bound to have far-reaching physiological/ pathophysiological consequences. Our preliminary proteomic analyses reveal that within a few hours of hepatic heme depletion, several hepatic proteins including P450s are decreased >2-fold, while a few are actually increased >2-fold. Studies are proposed in Hri-/- and Hri+/+ hepatocytes using SILAC (stable isotope labeling of amino acids in culture) coupled with proteomics to identify and quantify the hepatic proteins whose synthesis and/or degradation are so dramatically altered. The specific hepatic proteins whose turnover is markedly altered upon acute heme depletion and HRI activation may provide insight into the symptomatology and manifestations of acute hepatic porphyria. Moreover, such global alterations of hepatic P450 turnover would also rationalize the impaired drug clearance clinically observed in porphyric patients.