Transglutaminase 2 Regulates Mallory Body Inclusion Formation and Injury-Associated Liver Enlargement

Background & Aims: Mallory body (MB) inclusions are a characteristic feature of several liver disorders and share similarities with cytoplasmic inclusions observed in neural diseases and myopathies. MBs consist primarily of keratins 8 and 18 (K8/K18), require a K8-greater-than-K18 ratio for their formation, and contain glutamine-lysine cross-links generated by transglutaminase (TG). We hypothesized that protein transamidation is essential for MB formation. Methods: Because TG2 is the most abundant hepatocyte TG, we tested our hypothesis using TG2−/− and their wild-type counterpart mice fed 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC), an established MB inducer. Keratin cross-linking was further examined using recombinant proteins or transgenic mice that overexpress K8 or K18. Results: TG2−/− livers have markedly reduced TG2 activity as compared with TG2+/+ livers. The DDC-fed TG2−/− mice have dramatic decreases in MB formation and liver hypertrophy response as contrasted with DDC-fed TG2+/+ mice. Despite similar hepatocellular damage, TG2−/− mice had more gallstones, jaundice, and ductal proliferation than wild-type mice. Inhibition of MB formation in TG2−/− mice was associated with marked attenuation of ubiquitination and K8-containing protein cross-linking. MB formation and resolution paralleled the generation then disappearance of cross-linked K8, respectively. K8 is a preferential TG2 substrate when compared to K18, as examined in vitro or in DDC-fed transgenic mice that overexpress K8 or K18. Conclusions: We demonstrate an essential role for TG2 in determining injury-mediated liver enlargement and the necessity of K8 and TG2 for generating cross-linked keratins and MBs. The role of TG in inclusion formation might extend to nonkeratin intermediate filament protein-related diseases.