Transforming growth factor-β receptor expression on human skin fibroblasts: Dimeric complex formation of type I and type II receptors and identification of glycosyl phosphatidylinositol-anchored transforming growth factor-β binding proteins

Fibroblasts play a critical role in wound repair and in the development of fibrotic diseases, and transforming growth factor-beta (TGF-beta) has been shown to profoundly modulate fibroblast function. However, there is limited information on the TGF-beta receptor types, isoform specificity, and complex formation in skin fibroblasts. Here, we report that normal adult human skin fibroblasts display two isoform-specific, cell surface glycosyl phosphatidylinositol (GPI)-anchored, TGF-beta binding proteins in addition to the type I, II, and III TGF-beta receptors. The identities of these proteins are confirmed on the basis of their affinity for TGF-beta isoforms, immunoprecipitation with specific antireceptor antibodies, and other biochemical analyses. Immunoprecipitation results also indicated oligomeric complex formation between type I and II and between type II and III TGF-beta receptors. Furthermore, by using affinity labeling and two-dimensional electrophoresis, we demonstrate the occurrence of type I and II heterodimers and type I homodimers of TGF-beta receptors on these cells. Because the type I receptor does not bind TGF-beta in the absence of type II receptor, these results indicate that one molecule of TGF-beta induces the formation of a heterooligomeric complex containing more than one molecule each of type I and II TGF-beta receptors on these cells. These cells respond to TGF-beta by markedly down-regulating all five binding proteins and by potently augmenting DNA synthesis. These results allow the expansion of the proposed heteromeric TGF-beta receptor signaling paradigm using mutant cells that are unresponsive to TGF-beta and cell lines that have been transfected to overexpress these receptors, to include normal TGF-beta-responsive cells. In addition, the definition of TGF-beta receptor profiles in human skin fibroblasts provides important information for studying their alterations in these cells in various skin diseases.