We investigated transglutaminase-induced crosslinking of cytokeratin polypeptides in liver and hepatoma cells. To overcome the difficulties in the biochemical analysis of highly cross-linked polymers and aggregates of cytokeratins, cross-linked cytokeratin dimers were analyzed by immunoblotting to evaluate the degree of cross-linking of cytokeratins. Covalently cross-linked cytokeratin dimers were not detectable in normal rat liver cells. However, cytokeratin dimers and high-molecular-weight cytokeratin polymers were detected in liver tissue with histological evidence of coagulative necrosis induced by ischemia or carbon tetrachloride. Treatment of cultured hepatoma cells with the Ca2+ ionophore A23187 showed a dosedependent, time-dependent decrease of cell viability.
The appearance of cytokeratin dimers was shown to be correlated with cell death. These results suggest that the transglutaminase-induced cross-linking of cytokeratin polypeptides in liver and hepatoma cells is closely associated with the process of cell degeneration and death. (HEPATOLOGY 1993;17:118-124.)
We have reported the presence of covalently crosslinked cytokeratin (CK) polypeptides in transplantable Morris hepatoma 7777 cells (1). These cross-linked CKs were not detectable in normal rat liver cells or hepatoma cells grown in uitro. However, identical cross-linked CK multimers were demonstrated in rat liver cells when they were homogenized in solutions containing Ca2 + .
The reaction was shown to be mediated by the action of tissue transglutaminases (TGs; E.C. 2,3,2,13), a Ca2 +-dependent protein cross-linking enzyme (2). This finding suggests that the cross-linking of CKs is caused spontaneously by endogenous tissue TG after destruction of cell integrity or organization. We speculate that the reaction is associated with the process of degeneration or death of liver cells.
Hepatocytes are host to considerable TG activity (3,