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Cell-surface proteoglycans have been shown to modulate transforming growth factor (TGF)-beta responsiveness in epithelial cells and other cell types. However, the proteoglycan (heparan sulfate or chondroitin sulfate) involved in modulation of TGF-beta responsiveness and the mechanism by which it modulates TGF-beta responsiveness remain unknown. Here we demonstrate that TGF-beta1 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-beta1 affinity labeling analysis of cell-surface TGF-beta receptors reveals that CHO-K1 and CHO-677 cells exhibit low (<1) and high (>1) ratios of 125I-TGF-beta1 binding to TbetaR-II and TbetaR-I, respectively. Receptor-bound 125I-TGF-beta1 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-beta1), 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-beta1 binding to TbetaR-II and TbetaR-I and enhanced TGF-beta1-induced transcriptional activation of PAI-1. Sucrose density gradient analysis indicates that a significant fraction of TbetaR-I and TbetaR-II is localized in caveolae/lipid-raft fractions in CHO-K1 and Mv1Lu cells whereas the majority of the TGF-beta receptors are localized in non-lipid-raft fractions in CHO-677 cells. These results suggest that heparan sulfate negatively modulates TGF-beta1 responsiveness by decreasing the ratio of TGF-beta1 binding to TbetaR-II and TbetaR-I, facilitating caveolae/lipid-raft-mediated endocytosis and rapid degradation of TGF-beta1, thus diminishing non-lipid-raft-mediated endocytosis and signaling of TGF-beta1 in these epithelial cells.
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Chen et al. (2006) studied this question.
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