Transglutaminase 2 mediates lung inflammation and remodeling by transforming growth factor beta 1 via alveolar macrophage modulation

Tissue transglutaminase: a multifunctional and multisite regulator in health and disease

Tissue transglutaminase (TG2) is a widely distributed multifunctional protein involved in a broad range of cellular and metabolic functions carried out in a variety of cellular compartments. In addition to transamidation, TG2 also functions as a Gα signaling protein, a protein disulfide isomerase (PDI), a protein kinase, and a scaffolding protein. In the nucleus, TG2 modifies histones and transcription factors. The PDI function catalyzes the trimerization and activation of heat shock factor-1 in the nucleus and regulates the oxidation state of several mitochondrial complexes. Cytosolic TG2 modifies proteins by the addition of serotonin or other primary amines and in this way affects cell signaling. Modification of protein-bound glutamines reduces ubiquitin-dependent proteasomal degradation. At the cell membrane, TG2 is associated with G protein-coupled receptors (GPCRs), where it functions in transmembrane signaling. TG2 is also found in the extracellular space, where it functions in protein cross-linking and extracellular matrix stabilization. Of particular importance in transglutaminase research are recent findings concerning the role of TG2 in gene expression, protein homeostasis, cell signaling, autoimmunity, inflammation, and hypoxia. Thus, TG2 performs a multitude of functions in multiple cellular compartments, making it one of the most versatile cellular proteins. Additional evidence links TG2 with multiple human diseases including preeclampsia, hypertension, cardiovascular disease, organ fibrosis, cancer, neurodegenerative diseases, and celiac disease. In conclusion, TG2 provides a multifunctional and multisite response to physiological stress.

Transglutaminases in fibrosis-overview and recent advances

Transglutaminases (TGs) are a family of protein cross-linking enzymes that are capable of stiffening and insolubilizing proteins and creating protein networks, and thereby altering biological functions of proteins. Their role in fibrosis progression has been widely investigated with a focus on kidney, lung, liver, and heart where activity is triggered by various stimuli including hypoxia, inflammation, and hyperglycemia. TG2 has been considered one of the key enzymes in the pathogenesis of fibrosis mainly through transforming growth factor beta (TGF-beta) signaling and matrix cross-linking mechanisms. Although TG2 has been most widely studied in this context, the involvement of other TGs, TG1 and Factor XIII-A (FXIII-A), is beginning to emerge. This mini-review highlights the major steps taken in the TG and fibrosis research and summarizes the most recent advances and contributions of TG2, TG1, and FXIII-A to the progression of fibrosis in various animal models. Also, their mechanisms of action as well as therapeutic prospects are discussed.