Thrombospondin-dependent activation of latent TGF-beta under normal or high glucose conditions

Transforming growth factor-β (TGF-β) is a potent regulatory protein that affects cell growth, differentiation, and gene expression. TGF-β is secreted by almost all cells in vitro in a biologically latent form. In order to bind to its receptors and elicit cell responses TGF-β has to be in an active state. There are multiple ways of activating TGF-β in vitro and in vivo. In early studies, our laboratory demonstrated that thrombospondin-1 (TSP-1), an extracellular matrix (ECM) and platelet α-granule glycoprotein, activates bovine aortic endothelial cell-derived latent TGF-β via a mechanism that does not require the presence of proteases or interactions with the cell surface. The KRFK sequence in TSP-1 binds to the N-terminal portion of the latency-associated peptide (LAP) and this interaction leads to the activation of latent TGF-β. LAP, TSP-1, and TGF-β form ternary complexes that retain TGF-β activity. The LAP interacts with the KRFK sequence in the type 1 repeats of TSP-1. By using the molecular recognition theory, the LSKL sequence in the LAP was identified as a possible binding site for TSP-1. Activation of recombinant or endothelial cell-derived latent TGF-β by TSP-1 is inhibited by the exogenous LSKL peptide. This process is not unique only to TGF-β1, because TGF-β2 also can be activated by TSP-1 in an LSKL-dependent manner. Diabetic nephropathy is the main cause of end-stage renal failure in patients with types 1 and 2 diabetes. There is evidence from both in vitro and in vivo studies that TGF-β is a link between elevated glucose levels and glomerulosclerotic changes. We demonstrated that under high glucose conditions TSP-1 induces TGF-β bioactivity in rat and human mesangial cells. TSP-mediated activation of TGF-β results in induction of mesangial cell ECM protein expression under elevated glucose conditions. These studies are important in that they describe a new mechanism for TGF-β activation by TSP-1 under normal and high glucose conditions. Determination of the mechanisms involved in the activation of latent TGF-β may help to establish novel therapeutic approaches for treatment of fibrotic diseases.