Density-dependent inhibitory effect of transforming growth factor-β on human fibroblasts involves the down-regulation of platelet-derived growth factor α-receptors

We have previously found that transforming growth factor-β1 (TGF-β1) inhibits the mitogenic activity of platelet-derived growth factor (PDGF) in cultures of human neonatal fibroblasts in a density-dependent fashion. In the present investigation we determined the effect of TGF-β1 on the PDGF α-receptor, which binds all PDGF isoforms, as well as on the β-receptor, which binds only PDGF-BB with high affinity. We found that the inhibitory effect of TGF-β1 on PDGF-AA-induced mitogenesis was density-dependent; when dense cell cultures were preincubated with TGF-β1, there was an complete inhibition of ³H-thymidine incorporation, whereas the effect was less in sparse cultures. A similar density-dependent effect of TGF-β1 was seen in PDGF-BB treated cells, although less pronounced. The binding of ¹²⁵I-labeled PDGF-AA and PDGF-BB to the α-receptor was significantly reduced after treatment with TGF-β1 in dense cultures, whereas the sparse cultures were less affected. A decrease of α-receptor mRNA was also seen. The levels of β-receptor protein and mRNA were unaffected. We conclude that the growth inhibitory effect of TGF-β1 is cell density-dependent and involves down-regulation of PDGF α-receptors. © 1993 Wiley-Liss, Inc.