The role of transglutaminase 2 in regulation of the balance between autophagy and apoptosis in tumor cells

In normal tissue, cellular homeostasis is largely driven by two catabolic pathways: apoptosis and autophagy. Apoptosis, or programmed cell death, is regulated by pro-apoptotic factors, and promotes the removal of problematic cells. Autophagy, which in turn includes three forms: macro-, micro-, and chaperone-mediated autophagy, can promote both cell survival by selectively removing potentially apoptosis-inducing factors and raising the threshold of stress required for the induction of cell death. Recently, evidence has been accumulating suggesting the existence of common molecular pathways between autophagy and apoptosis, as well as the influence of the extracellular matrix on these processes. One of the important enzymes involved in the coordination and regulation of these processes is transglutaminase 2 (TG2). Different types of TG2 activities are involved in maintaining the dynamic balance between extracellular matrix and intracellular autophagy/apoptosis processes, while dysregulation of these processes may contribute to the pathogenesis of various human diseases, including oncogenesis. For example, TG2 can promote the degradation of pro-apoptotic proteins and the survival of renal cell carcinoma cells under nutrient-deficient conditions by modulating the autophagy process. In cells of various tissues deprived of TG2, aggregates of ubiquitinated proteins and damaged mitochondria are observed, which in turn induces proteotoxic stress and cell death. conversely, the transamidase activity of TG2 was observed to inhibit anti-apoptotic  signaling in a human leukemic monocytic lymphoma model. In the present review, a number of important functions of TG2 in oncogenesis are described, along with the dual role of TG2 in modulating such opposite processes as cell survival and cell death.