Abstract
The transglutaminase (TG) family comprises eight isozymes that form the isopeptide bonds between glutamine and lysine residues and contribute to the fibrotic diseases via crosslinking-mediated stabilization of ECM and the activation of TGF-β in several tissues. However, despite a growing body of evidence implicating TG2 as a key enzyme in fibrosis, the causative role of TG2 and the involvement of the other isozymes have not yet been fully elucidated. Therefore, here we clarified the distributions of TG isozymes and their in situ activities and identified the isozyme-specific possible substrates for both TG1 and TG2 using their substrate peptides in mouse fibrotic liver. We found that TG1 activity was markedly enhanced intracellularly over a widespread area, whereas TG2 activity increased in the extracellular space. In total, 43 and 42 possible substrates were identified for TG1 and TG2, respectively, as involved in chromatin organization and cellular component morphogenesis. These included keratin 18, a biomarker for hepatic injury, which was accumulated in the fibrotic liver and showed the partly similar distribution with TG1 activity. These findings suggest that TG1 activity may be involved in the functional modification of intracellular proteins, whereas TG2 activity contributes to the stabilization of extracellular proteins during liver fibrosis.
Highlights
Mainly involved in skin formation, contributing to the barrier function of the outermost layers via cooperative crosslinking of structural proteins in keratinocytes[3], whereas TG2 is widely distributed and plays multiple roles, including apoptosis, several types of signal transduction, matrix stabilization, wound healing, and angiogenesis[17]
To elucidate the detailed mechanism whereby which TG1 and TG2 contribute to liver fibrotic diseases, here we developed a system able comprehensively to identify the possible substrate proteins incorporated in each isozyme-specific substrate peptide for TG1 and TG2, as well as biotinylated pentylamine (BPA)
This led to several possible substrates being identified, among which we chose to focus on keratin 8 (K8) and keratin 18 (K18), which are coexpressed as complementary partners, and are known biomarkers for hepatic apoptosis and steatohepatitis[18]
Summary
Mainly involved in skin formation, contributing to the barrier function of the outermost layers via cooperative crosslinking of structural proteins in keratinocytes[3], whereas TG2 is widely distributed and plays multiple roles, including apoptosis, several types of signal transduction, matrix stabilization, wound healing, and angiogenesis[17]. To elucidate the detailed mechanism whereby which TG1 and TG2 contribute to liver fibrotic diseases, here we developed a system able comprehensively to identify the possible substrate proteins incorporated in each isozyme-specific substrate peptide for TG1 and TG2, as well as biotinylated pentylamine (BPA). We evaluated the enhanced activity of each TG isozyme, and globally identified the Lys-donor substrates for each TG isozyme and the Gln-donor substrates for the entire TG family during the induction of liver fibrosis This showed that the modification of K18 and K8 via crosslinking by TGs might be involved in liver fibrosis and in the accompanying hepatocytes apoptosis. These results provide a novel insight into the mechanisms of tissue fibrosis, identify a useful target for antifibrotic therapy, and will be helpful in elucidating the physiological and pathological functions of TGs
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
