Cellular heparan sulfate negatively modulates transforming growth factor-β1 (TGF-β1) responsiveness in epithelial cells

Abstract

Cell-surface proteoglycans have been shown to modulate transforming growth factor (TGF)-β responsiveness in epithelial cells and other cell types. However, the proteoglycan (heparan sulfate or chondroitin sulfate) involved in modulation of TGF-β responsiveness and the mechanism by which it modulates TGF-β responsiveness remain unknown. Here we demonstrate that TGF-β1 induces transcriptional activation of plasminogen activator inhibitor-1 (PAI-1) and growth inhibition more potently in CHO cell mutants deficient in heparan sulfate (CHO-677 cells) than in wild-type CHO-K1 cells. 125I-TGF-β1 affinity labeling analysis of cell-surface TGF-β receptors reveals that CHO-K1 and CHO-677 cells exhibit low (<1) and high (>1) ratios of 125I-TGF-β1 binding to TβR-II and TβR-I, respectively. Receptor-bound 125I-TGF-β1 undergoes nystatin-inhibitable rapid degradation in CHO-K1 cells but not in CHO-677 cells. In Mv1Lu cells (which, like CHO-K1 cells, exhibit nystatin-inhibitable rapid degradation of receptor-bound 125I-TGF-β1), treatment with heparitinase or a heparan sulfate biosynthesis inhibitor results in a change from a low (<1) to a high (>1) ratio of 125I-TGF- β1 binding to TβR-II and TβR-I and enhanced TGF-β1-induced transcriptional activation of PAI-1. Sucrose density gradient analysis indicates that a significant fraction of TβR-I and TβR-II is localized in caveolae/lipid-raft fractions in CHO-K1 and Mv1Lu cells whereas the majority of the TGF-β receptors are localized in non-lipid-raft fractions in CHO-677 cells. These results suggest that heparan sulfate negatively modulates TGF-β1 responsiveness by decreasing the ratio of TGF-β1 binding to TβR-II and TβR-I, facilitating caveolae/lipid-raft-mediated endocytosis and rapid degradation of TGF-β1, thus diminishing non-lipid-raft-mediated endocytosis and signaling of TGF-β1 in these epithelial cells. © 2006 by The American Society for Biochemistry and Molecular Biology, Inc.