Hepatic cytochromes P450: structural degrons and barcodes, posttranslational modifications and cellular adapters in the ERAD-endgame

Abstract

The endoplasmic reticulum (ER)-anchored hepatic cytochromes P450 (P450s) are enzymes that metabolize endo- and xenobiotics i.e. drugs, carcinogens, toxins, natural and chemical products. These agents modulate liver P450 content through increased synthesis or reduction via inactivation and/or proteolytic degradation, resulting in clinically significant drug-drug interactions. P450 proteolytic degradation occurs via ER-associated degradation (ERAD) involving either of two distinct routes: Ubiquitin (Ub)-dependent 26S proteasomal degradation (ERAD/UPD) or autophagic lysosomal degradation (ERAD/ALD). CYP3A4, the major human liver/intestinal P450, and the fast-turnover CYP2E1 species are degraded via ERAD/UPD entailing multisite protein phosphorylation and subsequent ubiquitination by gp78 and CHIP E3 Ub-ligases. We are gaining insight into the nature of the structural determinants involved in CYP3A4 and CYP2E1 molecular recognition in ERAD/UPD [i.e. K₄₈-linked polyUb chains and linear and/or “conformational” phosphodegrons consisting either of consecutive sequences on surface loops and/or disordered regions, or structurally-assembled surface clusters of negatively charged acidic (Asp/Glu) and phosphorylated (Ser/Thr) residues, within or vicinal to which, Lys-residues are targeted for ubiquitination]. Structural inspection of select human liver P450s reveals that such linear or conformational phosphodegrons may indeed be a common P450-ERAD/UPD feature. By contrast, although many P450s such as the slow-turnover CYP2E1 species and rat liver CYP2B1 and CYP2C11 are degraded via ERAD/ALD, little is known about the mechanism of their ALD-targeting. On the basis of our current knowledge of ALD-substrate targeting, we propose a tripartite conjunction of K₆₃-linked Ub-chains, P450 structural “LIR” motifs and selective cellular “cargo receptors” as plausible P450-ALD determinants.

Keywords:

• Cytochromes P450
• ERAD
• E2/E3 ubiquitin ligases
• 26S proteasome
• autophagic lysosomal degradation
• CHIP
• gp78 E3
• phosphodegrons
• K48 & K63 ubiquitination
• p97
• p62

Acknowledgements

M. A. Correia wishes to gratefully acknowledge the substantial intellectual and/or technical contributions of many of her past coworkers that were crucial in sustaining this at times challenging project. We acknowledge the expert contributions of Drs. Shenheng Guan, Mike Trnka, Kathi Medzihradszky and Arnie Falick in the mass spectrometric analyses and their interpretation provided through the Bio-Organic Biomedical Mass Spectrometry Resource at UCSF.

Disclosure statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article. We thank Mr. Chris Her for hepatocyte isolation and the UCSF Liver Center Core on Cell and Tissue Biology (Dr. J. J. Maher, Director), supported by the National Institute of Digestive Diseases and Kidney Center Grant [P30DK26743]. The UCSF Bio-Organic Biomedical Mass Spectrometry Resource (Prof. A. L. Burlingame, Director) was funded by NIGMS grant 8P41GM103481. These studies were supported by NIH Grants, GM44037 and DK26506 to MAC.