CHRYSOTILE ASBESTOS STIMULATES PLATELET-DERIVED GROWTH FACTOR-AA PRODUCTION BY RAT LUNG FIBROBLASTS IN-VITRO - EVIDENCE FOR AN AUTOCRINE LOOP

We have investigated the mitogenic and chemotactic role of platelet-de rived growth factor (PDGF) in pulmonary fibrogenesis induced by chryso tile asbestos. Since fibroblasts phagocytize asbestos in the lung inte rstitium, we have sought to learn whether the fibers alter the product ion of PDGF-like molecules by rat lung fibroblasts or induce mitogenes is of these fibroblasts in vitro. Conditioned medium as well as cell l ysates from fibroblasts exposed to asbestos contained approximately 4- fold more PDGF than unexposed cells as detected by Western blot. Two d istinct molecular weight forms of PDGF (36 and 18 kD) were detected by Western blotting. We postulate that these PDGF-like molecules are hom ologues of human PDGF-AA since we could not detect any PDGF in a sensi tive enzyme immunoassay that recognized only PDGF-BB and PDGF-AB. Furt hermore, PDGF-A chain mRNA was readily detected by Northern analysis, whereas PDGF-B chain mRNA was not detected by conventional Northern an alysis. However, message amplification using a reverse transcriptase p olymerase chain reaction allowed detection of the B-chain message. A s ignificant dose-dependent mitogenic effect of asbestos was found by us ing both a cell proliferation assay and nuclear labeling with bromodeo xyuridine when fibroblasts were exposed under serum-free conditions. T his mitogenesis induced directly by asbestos was blocked almost entire ly with an anti-PDGF antibody that neutralized all three PDGF isoforms . Thus, these data support our hypothesis that an autocrine loop for P DGF-AA is operative in vitro following exposure to asbestos in lung fi broblasts, and we suggest that this signaling pathway could be signifi cant in the pathogenesis of pulmonary fibrosis.