Early-passage Rat Lung Fibroblasts Do Not Migrate In Vitro to Transforming Growth Factor- β

Lung fibrosis has been postulated to be mediated by the production of macrophage-derived growth factors that are both mitogenic and chemotactic for fibroblasts. In vitro studies from our laboratory demonstrated that alveolar and interstitial macrophages treated with iron and asbestos release platelet-derived growth factor (PDGF) and transforming growth factor-β (TGF-β) into the media. This conditioned media was capable of inducing proliferation and Chemotaxis of primary rat lung fibroblasts (RLF). TGF-β is known to be present in the media, and RLF have high-affinity receptors for TGF-β. However, we found that > 95% of the Chemotaxis was blocked by a polyclonal anti-PDGF antibody, whereas anti-TGF-β did not change cell migration. TGF-β has been described previously as a potent chemoattractant for fibroblasts. Thus, we tested the potential of purified TGF-β to induce RLF Chemotaxis in an attempt to address this apparent contradiction in results. Four separate preparations of RLFs from four different rats, Swiss 3T3 cells, human and rat fetal skin fibroblasts, and human foreskin fibroblasts were tested for Chemotaxis using purified porcine TGF-β1 as well as human TGF-β. None of these cells responded chemotactically to TGF-β over a broad range of concentrations used (0.004 pg/ml to 50 ng/ml). RLF plated at different densities also did not respond to TGF-β. On the other hand, all the fibroblast types migrated vigorously to PDGF (4 ng/ml). In addition, all of the TGF-β samples induced rat mononuclear leukocyte Chemotaxis (0.5 pg/ml optimal) and clearly inhibited 3H-thymidine incorporation by the RLF as expected (0.005 ng/ml caused 50% inhibition and 0.1 ng/ml maximal inhibition). Although no clear explanation can be offered for the apparent contradiction with current dogma, variables such as cell type, source of TGF-β, and specific experimental conditions could be contributors to the TGF-β-induced chemotaxis observed by others. Thus, RLF have high-affinity TGF-β receptors, yet they lack a chemotactic response to this factor, suggesting that specific intracellular signaling mechanisms regulating biologic activities must be understood before the role of growth factors as chemotactic agents can be clarified.