Transglutaminase 2 (TG2) is a multifunctional mammalian protein with transamidase and signaling properties. Using selective TG2 inhibitors and tagged nucleophilic amine substrates, we show that the majority of extracellular TG2 is inactive under normal physiological conditions in cell culture and in vivo. However, abundant TG2 activity was detected around the wound in a standard cultured fibroblast scratch assay. To demonstrate wounding-induced activation of TG2 in vivo, the toll-like receptor 3 ligand, polyinosinic-polycytidylic acid (poly(I:C)), was injected in mice to trigger small intestinal injury. Although no TG2 activity was detected in vehicle-treated mice, acute poly(I:C) injury resulted in rapid TG2 activation in the small intestinal mucosa. Our findings provide a new basis for understanding the role of TG2 in physiology and disease.

Transglutaminase 2 (TG2, a.k.a. tissue transglutaminase) belongs to a family of transglutaminase enzymes that stabilize proteins by affecting covalent crosslinking via formation of amide bonds. Cell surface TG2 is directly involved as an adhesive receptor in cell–extracellular matrix (ECM) interactions. Here, we show that TG2 activity is elevated in glioblastomas compared with non-neoplastic brain. Immunofluorescent studies showed increased staining of fibronectin colocalized with TG2 in the ECM in glioblastomas. In addition, small clusters of invading human glioblastoma cells present in non-neoplastic brain parenchyma secrete high levels of TG2 and fibronectin that distinguish them from normal brain stroma. Downregulation of TG2 in U87MG glioblastoma cells with RNAi demonstrated decreased assembly of fibronectin in the ECM. Treatment with KCC009 blocked the remodeling of fibronectin in the ECM in glioblastomas in both in vitro and in vivo studies. KCC009 treatment in mice harboring orthotopic glioblastomas (DBT-FG) sensitized the tumors to N,N′-bis(2-chloroethyl)-N-nitrosourea chemotherapy, as measured by reduced bioluminescence, increased apoptosis and prolonged survival. The ability of KCC009 to interfere with the permissive remodeling of fibronectin in the ECM in glioblastomas suggests a novel target to enhance sensitivity to chemotherapy directed not only at the tumor mass, but also invading glioblastoma cells.