Keratins let liver live: Mutations predispose to liver disease and crosslinking generates Mallory-Denk bodies

Abstract

Keratin polypeptides 8 and 18 (K8/K18) are the cytoskeletal intermediate filament proteins of hepatocytes while K8/K18/K19 are the keratins of hepatobiliary ductal cells. Hepatocyte K8/K18 are highly abundant and behave as stress proteins with injury-inducible expression. Human association studies show that K8/K18 germline heterozygous mutations predispose to end-stage liver disease of multiple etiologies (Ϸ3 fold increased risk), and to liver disease progression in patients with chronic hepatitis C infection. These findings are supported by extensive transgenic mouse and ex vivo primary hepatocyte culture studies showing that K8 or K18 mutations predispose the liver to acute or subacute injury and promote apoptosis and fibrosis. Mutation-associated predisposition to liver injury is likely related to mechanical and nonmechanical keratin functions including maintenance of cell integrity, protection from apoptosis and oxidative injury, serving as a phosphate sponge, regulation of mitochondrial organization/function and protein targeting. These functions are altered by mutationinduced changes in keratin phosphorylation, solubility and filament organization/reorganization. Keratins are also the major constituents of Mallory-Denk bodies (MDBs). A toxin-induced K8>K18 ratio, and keratin crosslinking by transglutaminase-2 play essential roles in MDB formation. Furthermore, intracellular or cell-released K18 fragments, generated by caspase-mediated proteolysis during apoptosis serve as markers of liver injury. Therefore, K8 and K18 are cytoprotective stress proteins that play a central role in guarding hepatocytes from apoptosis. Keratin involvement in liver disease is multi-faceted and includes modulating disease progression upon mutation, formation of MDBs in response to unique forms of injury, and serving as markers of epithelial cell death. (HEPATOLOGY 2007; 46:1639-1649.)

Cytokeratins of the Liver and Intestine Epithelial Cells During Development and Disease

“…Under certain stress conditions, increased CK expression may contribute to important cytoprotection provided by CK8 and CK18 in the liver. However, the importance of such upregulation has not been directly demonstrated (Ku, Strnad et al 2007). These findings are supported by the observation of CK8 and CK18 over expression after injury in patients with primary biliary cirrhosis (Fickert, Trauner et al 2003).…”

“Toxic memory” via chaperone modification is a potential mechanism for rapid mallory-denk body reinduction

“…9 MDBs are characteristic of alcoholic steatohepatitis but are also found in other diseases, including nonalcoholic steatohepatitis, primary biliary cirrhosis, and Wilson disease. 9,12,13 MDBs consist mainly of the IFs keratin 8 and 18 (K8/K18) that have undergone several posttranslational modifications, including hyperphosphorylation and transamidation. 9,13 MDB formation requires an alteration in the normal equimolar K8:K18 ratio (under basal conditions) to disproportional K8ϾK18 levels that ultimately lead to K8 cross-linking and insolubility.…”

“…9,12,13 MDBs consist mainly of the IFs keratin 8 and 18 (K8/K18) that have undergone several posttranslational modifications, including hyperphosphorylation and transamidation. 9,13 MDB formation requires an alteration in the normal equimolar K8:K18 ratio (under basal conditions) to disproportional K8ϾK18 levels that ultimately lead to K8 cross-linking and insolubility. 9,13 Our understanding of MDB pathogenesis has been facilitated by mouse models whereby feeding mice the hepatotoxic drugs griseofulvin or 3,5-diethoxycarbonyl-1,4-dihydrocollidin (DDC) for 2 to 5 months leads to MDB formation.…”

“…9,13 MDB formation requires an alteration in the normal equimolar K8:K18 ratio (under basal conditions) to disproportional K8ϾK18 levels that ultimately lead to K8 cross-linking and insolubility. 9,13 Our understanding of MDB pathogenesis has been facilitated by mouse models whereby feeding mice the hepatotoxic drugs griseofulvin or 3,5-diethoxycarbonyl-1,4-dihydrocollidin (DDC) for 2 to 5 months leads to MDB formation. Similar to the human situation, murine MDB formation is reversible, and MDBs largely disappear after switching the animals to a normal diet for 1 month.…”

“…14 Despite the diagnostic importance of MDBs, the precise mechanisms underlying their formation are not completely understood. 9,13 Similar to other nonhepatocyte protein aggregates, MDB formation is thought to be facilitated by altered protein conformation that leads to exposure of hydrophobic amino acids. 7,15 A change in protein conformation might be caused by a genetic defect or by protein misfolding as a consequence of oxidative stress.…”