Cellular Biochemistry Lab., Graduate School of Pharmaceutical Sciences, Nagoya University, Furo-cho, Chikusa, Nagoya 464-8601, Japan.
Sci Rep. 2017 Mar 22;7:45049. doi: 10.1038/srep45049.
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.
转谷氨酰胺酶（TG）家族包括 8 种同工酶，它们通过交联介导的 ECM 稳定化和 TGF-β在几种组织中的激活，形成谷氨酰胺和赖氨酸残基之间的异肽键，从而导致纤维化疾病。然而，尽管越来越多的证据表明 TG2 是纤维化的关键酶，但 TG2 的致病作用和其他同工酶的参与尚未完全阐明。因此，我们在这里阐明了 TG 同工酶的分布及其原位活性，并使用其在小鼠纤维化肝脏中的底物肽鉴定了 TG1 和 TG2 的同工酶特异性可能的底物。我们发现，TG1 活性在广泛的区域内明显增强，而 TG2 活性在细胞外空间增加。总共鉴定出 43 个和 42 个可能的 TG1 和 TG2 底物，分别涉及染色质组织和细胞成分形态发生。其中包括作为肝损伤标志物的角蛋白 18，它在纤维化肝脏中积累，并与 TG1 活性具有部分相似的分布。这些发现表明，TG1 活性可能参与细胞内蛋白质的功能修饰，而 TG2 活性则有助于肝纤维化过程中外泌体蛋白质的稳定化。
