TRANSFORMING GROWTH-FACTOR-BETA-1 DOWN-REGULATES THE PLATELET-DERIVEDGROWTH-FACTOR ALPHA-RECEPTOR SUBTYPE ON HUMAN LUNG FIBROBLASTS IN-VITRO

Fibroblasts are the central target cell in pulmonary fibrotic diseases , and their proliferation is mediated largely by platelet-derived grow th factor (PDGF) isoforms secreted by activated lung macrophages. Seve ral other macrophage-derived cytokines that are increased during fibro genesis, including interleukin-1 beta and transforming growth factor-b eta 1 (TGF-beta 1), could potentially modulate the mitogenic and chemo tactic activity of PDGF by altering the expression of cell-surface PDG F receptors on fibroblasts, The PDGF receptor system on fibroblasts fr om a variety of tissues shows heterogeneous responses to TGF-beta 1. L ung fibroblasts have not been investigated in this regard. TGF-beta 1 downregulated the gene expression of the 6.5 kb PDGF-alpha receptor (P DGF-R alpha) transcript in normal human lung fibroblasts in a concentr ation dependent fashion that was maximal at 3 ng/ml TGF-beta 1; this c orresponded with a decrease in cell-surface PDGF-R alpha as measured b y radioligand binding assays using [I-125]PDGF-AA. The TGF-beta 1-indu ced downregulation of the PDGF-R alpha gene was rapid (maximal suppres sion by 2 h post-treatment) and preceded the decrease in cell-surface alpha-receptor (maximal reduction by 6 h post-treatment). TGF-beta 1 t reatment did not alter the rate of PDGF-R alpha mRNA degradation follo wing the inhibition of transcription using actinomycin D, indicating t hat TGF-beta 1 increases PDGF-R alpha transcription. Scatchard analysi s of saturation binding data showed that TGF-beta 1 decreased the numb er of [I-125]PDGF-AA binding sites 5-fold without affecting receptor a ffinity. [I-125]PDGF-AB binding sites were downregulated approximately 25%, and the number of [I-125]PDGF-BB binding sites was not changed b y TGF-beta 1 treatment, indicating that the PDGF-beta receptor was not affected, TGF-beta 1 reduced the mitogenic and chemotactic response t o PDGF-AA by > 90%, whereas these biologic response to PDGF-AB and PDG F-BB were inhibited 50% to 80%. The proliferative and chemotactic resp onses of fibroblasts during tissue remodeling or during lung fibrosis are likely controlled by a complex network involving PDGF isoforms and cytokines that modify the PDGF receptor system.