Transglutaminase 2 in the balance of cell death and survival

Transglutaminase 2 (TG2), a multifunctional enzyme with Ca(2+)-dependent protein crosslinking activity and GTP-dependent G protein functions, is often upregulated in cells undergoing apoptosis. In cultured cells TG2 may exert both pro- and anti-apoptotic effects depending upon the type of cell, the kind of death stimuli, the intracellular localization of the enzyme and the type of its activities switched on. The majority of data support the notion that transamidation by TG2 can both facilitate and inhibit apoptosis, while the GTP-bound form of the enzyme generally protects cells against death. In vivo studies confirm the Janus face of TG2 in the initiation of the apoptotic program. In addition, they reveal a further role: the prevention of inflammation, tissue injury and autoimmunity once the apoptosis has already been initiated. This function of TG2 is partially achieved by being expressed and activated also in macrophages digesting apoptotic cells and mediating a crosstalk between dying and phagocytic cells.