earticle

영어

Keratins 8 and 18 (K8/K18) are heteropolymeric intermediate filament (IF) phospho-glycoproteins of simple-type epithelia1,2. K8/K18 protect hepatocytes from apoptosis1,3 and their mutations predispose to liver disease4,5. K18 undergoes dynamic single O-linked N-acetylglucosamine (O-GlcNAc)-type glycosylation at Ser30/31/49, the function of which is unknown6. O-GlcNAcylated proteins are modulated on Ser/Thr by O-GlcNAc transferase (OGT) and N-acetyl-D-glucosaminidase (O-GlcNAcase) via GlcNAc addition or removal, respectively7,8. O-GlcNAcylation involves numerous nuclear/cytoplasmic proteins, and regulates several functions including protein turnover, transcription and stress responses7-12. O-GlcNAcylation and phosphorylation frequently occur at adjacent or identical Ser/Thr, and each modification can interfere with the other7-9, 13. Exposure of animal models to streptozotocin (STZ), an O-GlcNAcase inhibitor, results in accumulation of O-GlcNAc-modified proteins, diabetes and neurodegeneration7. Similarly, adipocyte exposure to O-(2-acetamido-2-deoxy-D- glucopyranosylidene) amino-N-phenylcarbamate (PUGNAc), another O-GlcNAcase inhibitor, leads to insulin resistance 14 but no reported studies tested the function of site-specific glycosylation in animal models. We addressed the function of K18 glycosylation by generating mice that overexpress K18 S30/31/49A (K18-Gly-), and compared their susceptibility to STZ- or PUGNAc/Fas-mediated injury with nontransgenic and three other control mouse lines that overexpress wild-type K1815, phospho-mutant S53A K182, or R90C K18 which disrupts keratin filaments and predisposes hepatocytes to apoptosis16. In contrast with the other control mouse lines, K18-Gly- mice are significantly more susceptible to STZ- or PUGNAc/Fas-induced liver and pancreatic injury, including prominent hepatocyte apoptosis which is specific to O-GlcNAcase inhibition since Fas-ligand alone induces apoptosis similarly in K18-Gly- and control mice. The enhanced apoptosis in K18-Gly- mice involves Akt1 and protein kinase Cθ inactivation due to site-specific hypophosphorylation. Furthermore, Akt1-T308 plays an important role in reciprocal Akt1 glycosylation and phosphorylation. Therefore, K18 glycosylation provides a unique protective role in epithelial injury by promoting the phosphorylation and activation of cell survival kinases.