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

The importance of transglutaminase 2 (TG2) in angiogenesis has been highlighted in recent studies, but other roles of this multifunctional 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 beta 1 (TGF beta 1) and Smad signalling. We demonstrate that inhibition of tubule formation by TG2 inhibitors can be restored by add-back of exogenous TGF beta 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 beta 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 beta signalling through entrapment of active TGF beta 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 beta 1 signalling means it can be involved in both endothelial tubule formation and tubule rarefaction.