ABSTRACT
TGF-ß is a multifunctional cytokine affecting many cell types and implicated in tissue remodeling processes. Due to its many functions and cell-specific effects, the consequences of TGF-ß signaling are process-and stage-dependent, and it is not uncommon that TGF-ß exerts distinct and sometimes opposing effects on a disease progression depending on the stage and on the pathological changes associated with the stage. The mechanisms underlying cell- and process-specific effects of TGF-ß are poorly understood. We are describing a novel pathway that mediates induction of angiogenesis in response to TGF-ß1. We found that in endothelial cells (EC) thrombospondin-4 (TSP-4), a secreted extracellular matrix (ECM) protein, is upregulated in response to TGF-ß1 and mediates the effects of TGF-ß1 on angiogenesis. Upregulation of TSP-4 does not require the synthesis of new protein, is not caused by decreased secretion of TSP-4, and is mediated by activation of SMAD3. Using Thbs4-/- mice and TSP-4 shRNA, we found that TSP-4 mediated pro-angiogenic functions in cultured EC and angiogenesis in vivo in response to TGF-ß1. We observed~3-fold increases in tumor mass and levels of angiogenesis markers in animals injected with TGF-ß1, and these effects did not occur in Thbs4-/- animals. Injections of an inhibitor of TGF-ß1 signaling SB-431542 also decreased the weights of tumors and cancer angiogenesis. Our results from in vivo angiogenesis models and cultured EC document that TSP-4 mediates upregulation of angiogenesis by TGF-ß1. Upregulation of pro-angiogenic TSP-4 and selective effects of TSP-4 on EC may contribute to stimulation of tumor growth by TGF-ß despite the inhibition of cancer cell proliferation.
