title: Ubiquitin-dependent proteasomal degradation of human liver cytochrome P450 2E1: identification of sites targeted for phosphorylation and ubiquitination.
abstract: Human liver CYP2E1 is a monotopic, endoplasmic reticulum-anchored cytochrome P450 responsible for the biotransformation of clinically relevant drugs, low molecular weight xenobiotics, carcinogens, and endogenous ketones. CYP2E1 substrate complexation converts it into a stable slow-turnover species degraded largely via autophagic lysosomal degradation. Substrate decomplexation/withdrawal results in a fast turnover CYP2E1 species, putatively generated through its futile oxidative cycling, that incurs endoplasmic reticulum-associated ubiquitin-dependent proteasomal degradation (UPD). CYP2E1 thus exhibits biphasic turnover in the mammalian liver. We now show upon heterologous expression of human CYP2E1 in Saccharomyces cerevisiae that its autophagic lysosomal degradation and UPD pathways are evolutionarily conserved, even though its potential for futile catalytic cycling is low due to its sluggish catalytic activity in yeast. This suggested that other factors (i.e. post-translational modifications or "degrons") contribute to its UPD. Indeed, in cultured human hepatocytes, CYP2E1 is detectably ubiquitinated, and this is enhanced on its mechanism-based inactivation. Studies in Ubc7p and Ubc5p genetically deficient yeast strains versus corresponding isogenic wild types identified these ubiquitin-conjugating E2 enzymes as relevant to CYP2E1 UPD. Consistent with this, in vitro functional reconstitution analyses revealed that mammalian UBC7/gp78 and UbcH5a/CHIP E2-E3 ubiquitin ligases were capable of ubiquitinating CYP2E1, a process enhanced by protein kinase (PK) A and/or PKC inclusion. Inhibition of PKA or PKC blocked intracellular CYP2E1 ubiquitination and turnover. Here, through mass spectrometric analyses, we identify some CYP2E1 phosphorylation/ubiquitination sites in spatially associated clusters. We propose that these CYP2E1 phosphorylation clusters may serve to engage each E2-E3 ubiquitination complex in vitro and intracellularly.
authors.affiliation: Department of Cellular and Molecular Pharmacology, University of California, San Francisco, California 94158-2517, USA.
authors.forename: YongQiang
authors.initials: Y
authors.lastname: Wang
authors.forename: Shenheng
authors.initials: S
authors.lastname: Guan
authors.forename: Poulomi
authors.initials: P
authors.lastname: Acharya
authors.forename: Dennis R
authors.initials: DR
authors.lastname: Koop
authors.forename: Yi
authors.initials: Y
authors.lastname: Liu
authors.forename: Mingxiang
authors.initials: M
authors.lastname: Liao
authors.forename: Alma L
authors.initials: AL
authors.lastname: Burlingame
authors.forename: Maria Almira
authors.initials: MA
authors.lastname: Correia
mesh_terms: D000818:Animals; D001343:Autophagy / Q000502:physiology; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism; D019392:Cytochrome P-450 CYP2E1 / Q000235:genetics / Q000378:metabolism*; D022781:Hepatocytes / Q000166:cytology / Q000201:enzymology*; D006801:Humans; D008099:Liver / Q000166:cytology / Q000201:enzymology*; D008247:Lysosomes / Q000235:genetics / Q000378:metabolism; D010766:Phosphorylation / Q000502:physiology; D046988:Proteasome Endopeptidase Complex / Q000235:genetics / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D060285:Receptors, Autocrine Motility Factor; D018121:Receptors, Cytokine / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000201:enzymology / Q000235:genetics; D044763:Ubiquitin-Conjugating Enzymes / Q000235:genetics / Q000378:metabolism; D044767:Ubiquitin-Protein Ligases / Q000235:genetics / Q000378:metabolism; D054875:Ubiquitination / Q000502:physiology*
chemical_list: D018121:Receptors, Cytokine; D019392:Cytochrome P-450 CYP2E1; C097308:UBE2D1 protein, human; D044763:Ubiquitin-Conjugating Enzymes; C555000:AMFR protein, human; D060285:Receptors, Autocrine Motility Factor; D044767:Ubiquitin-Protein Ligases; D017868:Cyclic AMP-Dependent Protein Kinases; D046988:Proteasome Endopeptidase Complex