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Keratins: Biomarkers and modulators of apoptotic and necrotic cell death in the liver
Author(s) -
Ku NamOn,
Strnad Pavel,
Bantel Heike,
Omary M. Bishr
Publication year - 2016
Publication title -
hepatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.488
H-Index - 361
eISSN - 1527-3350
pISSN - 0270-9139
DOI - 10.1002/hep.28493
Subject(s) - keratin , epitope , keratin 8 , biology , apoptosis , antibody , caspase , programmed cell death , antigen , microbiology and biotechnology , immunology , biochemistry , genetics
Keratins, formerly known as cytokeratins, are the major epithelial‐specific subgroup of intermediate filament proteins. Adult hepatocytes express keratin polypeptides 8 and 18 (K8/K18), whereas cholangiocytes express K8/K18 and keratins 7 and 19 (K7/K19). Keratins function primarily to protect hepatocytes from apoptosis and necrosis, which was revealed using several genetic mouse models. This cytoprotective function was further clarified by the identification of natural human keratin variants that are normally silent, but become pathogenic by predisposing their carriers to apoptosis during acute or chronic liver injury mediated by toxins, virus infection, or metabolic stress. During apoptosis, caspases cleave K18 and K19 at conserved aspartates (human K18/K19: 235 Val‐Glu‐Val‐Asp ↓ ) and K18 at a unique aspartate (human K18: 394 Asp‐Ala‐Leu‐Asp ↓ ), with the latter exposed epitope becoming recognized by the M30 antibody in blood and tissues. Additional K18‐containing protein backbone epitopes are detected using the M6 and M5 (termed M65) antibodies. Intact K18 and its associated fragments, which are released into blood during apoptosis and necrosis in various diseases, have been analyzed by enzyme‐linked immunosorbent assay using the M30/M65 antibodies or their signal ratios. Furthermore, M30/M65 levels have been used as diagnostic and prognostic biomarkers in acute and chronic liver diseases, including nonalcoholic steatohepatitis and acute liver failure. Other keratin biomarkers include K8/K18/K19‐related tissue polypeptide antigen, K18‐related tissue polypeptide‐specific antigen, and K19‐related CYFRA‐21‐1, which have been evaluated mostly in patients with epithelial tumors. Conclusion : Keratins and their fragments are released into blood during liver and other epithelial tissue injury. The epithelial specificity of K18/K19, epitope unmasking upon caspase digestion, keratin abundance, and relative keratin stability render them useful biomarkers for hepatocyte and cholangiocyte apoptosis and necrosis. However, the precise biochemical nature and release mechanism of circulating keratins remain unknown. (H epatology 2016;64:966‐976)