Transglutaminase 2 is secreted from smooth muscle cells by transamidation-dependent microparticle formation

Abstract

Transglutaminase 2 (TG2) is a pleiotropic enzyme involved in both intra- and extracellular processes. In the extracellular matrix, TG2 stabilizes the matrix by both covalent cross-linking and disulfide isomerase activity. These functions become especially apparent during matrix remodeling as seen in wound healing, tumor development and vascular remodeling. However, TG2 lacks the signal sequence for a classical secretory mechanism, and the cellular mechanism of TG2 secretion is currently unknown. We developed a green fluorescent TG2 fusion protein to study the hypothesis that TG2 is secreted via microparticles. Characterization of TG2/eGFP, using HEK/293T cells with a low endogenous TG2 expression, showed that cross-linking activity and fibronectin binding were unaffected. Transfection of TG2/eGFP into smooth muscle cells resulted in the formation of microparticles (MPs) enriched in TG2, as detected both by immunofluorescent microscopy and flow cytometry. The fraction of TG2-positive MPs was significantly lower for cross-linking-deficient mutants of TG2, implicating a functional role for TG2 in the formation of MPs. In conclusion, the current data suggest that TG2 is secreted from the cell via microparticles through a process regulated by TG2 cross-linking.Electronic supplementary materialThe online version of this article (doi:10.1007/s00726-011-1010-3) contains supplementary material, which is available to authorized users.

Cellular Functions of Tissue Transglutaminase

“…Also, in the case of cancer-derived microvesicles, TG2 and fibronectin were implicated in the transformation of the recipient normal fibroblasts. Evidently, similar TG2-containing microparticles were also described in normal smooth muscle cells (van den Akker et al, 2011). The activities and biological functions of microvesicular TG2 remain to be characterized.…”

“…Although the mechanistic details and regulation of microvesicle-dependent secretion and transfer of TG2 to neighboring cells remain largely unknown, this process might be highly important for cell transformation and cancer progression in vivo (Antonyak et al, 2011). In addition, a novel microparticle-dependent process of TG2 secretion was recently described in normal smooth muscle cells (van den Akker et al, 2011). This process required the transamidating function of the protein.…”

“…In addition, the formation of ternary integrin–TG2–fibronectin complexes is thought to facilitate the anchoring of ovarian cancer cells to the mesothelial lining of the peritoneal cavity and promote a subsequent metastasis during the progression of this type of cancer (Satpathy et al, 2007). Notably, the interaction of integrin-bound TG2 with fibronectin on the surface of activated astrocytes was recently implicated in the recruitment of the cells to multiple sclerosis lesions and, consequently, the progression of multiple sclerosis (van Strien et al, 2011). Thus, targeting the TG2–fibronectin interaction might be a new promising venue for developing novel therapeutics that block the cell–ECM adhesion of tumor cells in ovarian cancer and activated astrocytes in multiple sclerosis.…”

Extracellular Vesicles and Vascular Injury: New Insights for Radiation Exposure

“…Different studies have implicated Rho kinase, an upstream regulator of myosin light-chain kinase and cytoskeletal dynamics, in the formation of MPs from ECs, a process which may involve caspase 2 (21, 29, 30). Finally, transglutaminase 2, an enzyme that catalyzes protein cross-linking and governs cytoskeletal reorganization, has recently been implicated in MP release from vascular smooth muscle cells (SMCs) (48). Thus, although the precise machinery necessary for MP formation is not fully understood and indeed may differ among various cell populations, cytoskeletal reorganization appears to represent a critical step in MP formation.…”

Role of polyamines, their analogs and transglutaminases in biological and clinical perspectives

“…Throughout the TGase field, it has remained an enigma how TGase2 is secreted, and the vascular system is no exception to this. Now, van den Akker et al (2011a) provide evidence that TGase2 is secreted from vascular smooth muscle cells by transamidation-dependent microparticle formation. Together with the recent work on the redox balance in TGase2 activation (van den Akker et al 2011b), novel insight into the vascular actions of TGase2 are gained that open ways for pharmacological interference in cardiovascular diseases.…”