Tissue transglutaminase is the most ubiquitous member of the transglutaminase (TG2, EC 184.108.40.206) family of enzymes that catalyze Ca2+-dependent post-translational modification of proteins by inserting highly stable (ε-[γ-glutamyl] lysine) isopeptide bonds or by conjugating polyamines at selected peptide-bound glutamine residues. In addition to transamidation reaction, TG2 can also hydrolyze GTP/ATP and in response to certain agonist hormones it can serve as a signal transducing G protein. Although predominantly a cytosolic protein, TG2 can translocate to the nucleus with the help of importin-α3 protein or to the membranes in association with integrins. TG2 can exert both pro- and anti-apoptotic effects depending on the type of cell, the nature of death stimuli, and its localization within the cell. Moreover, TG2 can be secreted outside the cell (by yet unknown mechanism) where it crosslinks proteins of the extracellular matrix (ECM) and promotes cell adhesion and spreading. Another important property of TG2 is that it has high binding-affinity for the ECM component protein, fibronectin and thus can promote interaction between cell surface integrin with fibronectin. This interaction is crucial in the development of drug resistance and metastatic phenotype seen in cancer. This review will discuss the critical role of TG2 in malignant transformation, metastasis and drug-resistance phenotypes and examine the validity as well as the therapeutic potential of TG2 as an anti-cancer target.
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