The functional relationship between transglutaminase 2 and transforming growth factor β1 in the regulation of angiogenesis and endothelial–mesenchymal transition

Abstract The importance of transglutaminase 2 (TG2) in angiogenesis has been highlighted in recent studies, but other roles of this multi-functional enzyme in endothelial cell (EC) function still remains to be fully elucidated. We previously showed that the extracellular TG2 is involved in maintaining tubule formation in ECs by a mechanism involving matrix-bound vascular endothelial growth factor (VEGF) signalling. Here, by using the ECs and fibroblast co-culture and ECs 3D culture models, we demonstrate a further role for TG2 in both endothelial tubule formation and in tubule loss, which involves its role in the regulation of transforming growth factor β 1 (TGF β 1) and Smad signalling. We demonstrate that inhibition of tubule formation by TG2 inhibitors can be restored by add-back of exogenous TGF β 1 at pg/ml levels and show that TG2 −/− mouse ECs are unable to form tubules in 3D culture and display negligible Smad signalling compared to wild-type cells. Loss of tubule formation in the TG2 −/− ECs can be reconstituted by transduction with TG2. We demonstrate that extracellular TG2 also has an important role in TGF β 1-induced transition of ECs into myofibroblast-like cells (endothelial–mesenchymal transition), resulting in loss of EC tubules and tubule formation. Our data also indicate that TG2 may have a role in regulating TGF β signalling through entrapment of active TGF β 1 into the extracellular matrix. In conclusion, our work demonstrates that TG2 has multi-functional roles in ECs where its ability to fine-tune of TGF β 1 signalling means it can be involved in both endothelial tubule formation and tubule rarefaction.