Betaglycan Inhibits TGF-β Signaling by Preventing Type I-Type II Receptor Complex Formation

Abstract

Transforming growth factor (TGF)-β is a multifunctional growth factor with important roles in development, cell proliferation, and matrix deposition. It signals through the sequential activation of two serine/ threonine kinase receptors, the type I and type II receptors. A third cell surface receptor, betaglycan, serves as a co-receptor for TGF-β in some cell types, enhancing TGF-β-mediated signaling. We have examined the function of betaglycan in renal epithelial LLC-PK1 cells that lack endogenous betaglycan. We demonstrate that the expression of betaglycan in LLC-PK1 cells results in inhibition of TGF-β signaling as measured by reporter gene expression, thymidine incorporation, collagen production, and phosphorylation of the down-stream signaling effectors Smad2 and Smad3. In comparison, the expression of betaglycan in L6 myoblasts enhances TGF-β signaling, which is consistent with the published literature. The effects of betaglycan in LL-CPK1 cells are not mediated by ligand sequestration or increased production of a soluble form of the receptor, which has been reported to serve as a ligand antagonist. We demonstrate instead that in LLC-PK1 cells, unlike L6 cells, expression of betaglycan prevents association between the type I and type II TGF-β receptors, which is required for signaling. This is a function of the glycosaminoglycan modifications of betaglycan. Betaglycan in LLC-PK1 cells exhibits higher molecular weight glycosaminoglycan (GAG) chains than in L6 cells, and a GAG- betaglycan mutant does not inhibit TGF-β signaling or type I/type II receptor association in LLC-PK1 cells. Our data indicate that betaglycan can function as a potent inhibitor of TGF-β signaling by a novel mechanism and provide support for an essential but complex role for proteoglycan co-recept6rs in growth factor signaling.