TRANSFORMING GROWTH-FACTOR-BETA-1 DOWN-REGULATES THE PLATELET-DERIVED GROWTH-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 growth factor (PDGF) isoforms secreted by activated lung macrophages. Several other macrophage-derived cytokines that are increased during fibrogenesis, including interleukin-1 beta and transforming growth factor-beta 1 (TGF-beta 1), could potentially modulate the mitogenic and chemotactic activity of PDGF by altering the expression of cell-surface PDGF receptors on fibroblasts, The PDGF receptor system on fibroblasts from a variety of tissues shows heterogeneous responses to TGF-beta 1. Lung fibroblasts have not been investigated in this regard. TGF-beta 1 downregulated the gene expression of the 6.5 kb PDGF-alpha receptor (PDGF-R alpha) transcript in normal human lung fibroblasts in a concentration dependent fashion that was maximal at 3 ng/ml TGF-beta 1; this corresponded with a decrease in cell-surface PDGF-R alpha as measured by radioligand binding assays using [I-125]PDGF-AA. The TGF-beta 1-induced downregulation of the PDGF-R alpha gene was rapid (maximal suppression by 2 h post-treatment) and preceded the decrease in cell-surface alpha-receptor (maximal reduction by 6 h post-treatment). TGF-beta 1 treatment did not alter the rate of PDGF-R alpha mRNA degradation following the inhibition of transcription using actinomycin D, indicating that TGF-beta 1 increases PDGF-R alpha transcription. Scatchard analysis of saturation binding data showed that TGF-beta 1 decreased the number of [I-125]PDGF-AA binding sites 5-fold without affecting receptor affinity. [I-125]PDGF-AB binding sites were downregulated approximately 25%, and the number of [I-125]PDGF-BB binding sites was not changed by TGF-beta 1 treatment, indicating that the PDGF-beta receptor was not affected, TGF-beta 1 reduced the mitogenic and chemotactic response to PDGF-AA by > 90%, whereas these biologic response to PDGF-AB and PDGF-BB were inhibited 50% to 80%. The proliferative and chemotactic responses 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.