Differential Proliferation of Rat Lung Fibroblasts Induced by the Platelet-derived Growth Factor-AA, -AB, and -BB Isoforms Secreted by Rat Alveolar Macrophages

Abstract

Platelet-derived growth factor (PDGF)-like molecules secreted by alveolar macrophages have been postulated to be mediators of lung fibrogenesis since these cytokines stimulate the proliferation and chemotaxis of lung fibroblasts. We are studying the biology and biochemistry of rat macrophage-derived PDGF and have identified for the first time the specific isoforms of PDGF (-AA, -AB, and -BB) that these macrophages secreted in vitro following activation with either chrysotile asbestos or carbonyl iron spheres. Subsequently, the proliferative response of rat lung fibroblasts (RLF) to the different PDGF isoforms was established. Using several antibodies raised against the distinct isoforms, we established that two different PDGF-like factors with molecular masses of 30 to 34 kD and 16 to 18 kD were contained in alveolar macrophage-conditioned medium. Within each of these molecular mass regions was a mixture of all three PDGF isoforms. We estimated that the 30- to 34-kD PDGF was mainly PDGF-BB (approximately 50%), while the remaining consisted of PDGF-AA (approximately 13%) and PDGF-AB (approximately 37%). Purified recombinant PDGF isoforms were tested for their ability to stimulate the growth of early-passage RLF and Swiss 3T3 cells in a 3-day cell proliferation assay. PDGF-BB and PDGF-AB were the most potent inducers of RLF proliferation and stimulated growth half-maximally at approximately 1 ng/ml and approximately 7 ng/ml, respectively. While these two B-chain-containing dimers stimulated lung fibroblast growth by as much as 150% above control medium, the PDGF-AA homodimer stimulated lung fibroblast proliferation less than 25% above control medium at the highest concentrations tested (20 ng/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Specific Inhibitors of Platelet-Derived Growth Factor or Epidermal Growth Factor Receptor Tyrosine Kinase Reduce Pulmonary Fibrosis in Rats

“…Asbestos fibers also stimulate the secretion of PDGF isoforms and TGF-␣ in vitro and in vivo in rodent models. 10,12,30 Due to the potent mitogenic effect of these growth factors, these studies suggest a role for PDGF and EGF receptor ligands in causing a fibrotic response. However, they do not clearly establish a link between PDGF-or EGF-mediated cell proliferation and the subsequent deposition of collagen, which defines the fibrotic lesion.…”

Interstitial fibrosis and growth factors.

“…PDGFR-a, but not PDGFR-P, mRNA, and peptide are upregulated at the site of asbestos-induced fibrotic lesions (23 (134). Furthermore, asbestos fibers induce proliferation of rat lung fibroblasts in serumfree media through a PDGF-dependent pathway that involves the PDGF-AA isoform and upregulation of the PDGF-a receptor (21,22).…”

“…PDGFR-a expression is increased in rat lung through IL-1 (130). Rat AM secrete more PDGF (all three isoforms( following exposure to asbestos in vitro ( 134). Exposure of mineral dust to tracheal explants results in increased expression of PDGF-A and -B chain gene expression (131).…”

Asbestos-induced lung disease.

“…Human lung macrophages were shown in 1985 to produce PDGF (37), and in 1988 we demonstrated that rat lung macrophages secrete a similar molecule (38). This macrophage product was found to account for the majority of fibroblast proliferation induced by culture medium "conditioned" by macrophages in vitro (39). Thus, PDGF, including the macrophage-derived (MD) molecule, is a potent mesenchymal cell mitogen commonly found in cell secretions.…”

“…We have studied the specific high-affinity receptors for TGF-fl and PDGF on the putative target cells, i.e., the rat lung fibroblasts (39,41 In 1981 (46), we showed that the majority of inhaled chrysotile asbestos fibers which are small enough to pass through the terminal bronchioles is deposited at the first bifurcation of the alveolar ducts in rats. This was accomplished by exposing the animals for only 0.5 or 1 hr, sacrificing them within 4 min after cessation ofexposure, and then counting the fibers at their sites of initial deposition by use of scanning electron microscopy FIGURE 2.…”