A disease- and phosphorylation-related nonmechanical function for keratin 8

Nam-On Ku, and M. Bishr Omary

Department of Medicine, Palo Alto VA Medical Center and Stanford University School of Medicine, Palo Alto, CA 94304

Correspondence to Nam-On Ku: namonku{at}stanford.edu

Abstract: Keratin 8 (K8) variants predispose to human liver injury via poorly understood mechanisms. We generated transgenic mice that overexpress the human disease-associated K8 Gly61-to-Cys (G61C) variant and showed that G61C predisposes to liver injury and apoptosis and dramatically inhibits K8 phosphorylation at serine 73 (S73) via stress-activated kinases. This led us to generate mice that overexpress K8 S73-to-Ala (S73A), which mimicked the susceptibility of K8 G61C mice to injury, thereby providing a molecular link between K8 phosphorylation and disease-associated mutation. Upon apoptotic stimulation, G61C and S73A hepatocytes have persistent and increased nonkeratin proapoptotic substrate phosphorylation by stress-activated kinases, compared with wild-type hepatocytes, in association with an inability to phosphorylate K8 S73. Our findings provide the first direct link between patient-related human keratin variants and liver disease predisposition. The highly abundant cytoskeletal protein K8, and possibly other keratins with the conserved S73-containing phosphoepitope, can protect tissue from injury by serving as a phosphate "sponge" for stress-activated kinases and thereby provide a novel nonmechanical function for intermediate filament proteins.

Abbreviations used in this paper: Ab, antibody; CREB, cAMP response element binding protein; EBS, epidermolysis bullosa simplex; HSE, high salt extraction; IF, intermediate filament; K, keratin; MLR, microcystin-LR; SAPK, stress-activated protein kinase; WT, wild type.

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