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)

Enhanced proliferation of fibroblasts is a primary characteristic of lung fibrosis. Macrophage-secreted platelet-derived growth factor (PDGF) is a potent mitogen and chemoattractant for lung fibroblasts. The magnitude of the fibroblast PDGF response is dependent on the number of PDGF receptor alpha (PDGF-R alpha) relative to PDGF-R beta at the cell surface. We recently reported that upregulation of the PDGF-R alpha subtype by interleukin (IL)-1beta results in enhanced lung fibroblast proliferation in response to PDGF-AA, PDGF-AB, and PDGF-BB whereas transforming growth factor (TGF)-beta1 has the opposite effect. Both IL-1beta and TGF-beta1 are produced by particle-activated macrophages in vivo and in vitro. We studied the net effect of macrophage conditioned medium (MOCM), which contains both IL-1beta and TGF-beta1, on the expression of the lung fibroblast PDGF receptor system. MOCM obtained from unstimulated, titanium dioxide (TiO2)-, chrysotile asbestos-, or residual oil fly ash (ROFA)-exposed macrophages in vitro increased [125I]PDGF-AA binding 3-, 6-, 6-, and 20-fold, respectively. These increases correlated with increased PDGF-R alpha mRNA and protein expression as shown by northern and western assays. PDGF-AB and -BB-stimulated [3H]thymidine incorporation by fibroblasts was enhanced 5-, 5-, 10-, and 20-fold by pretreatment with MOCM from unstimulated, TiO2-, asbestos-, and ROFA-exposed macrophages, respectively. [125I]PDGF-AA binding experiments using the IL-1 receptor antagonist blocked the upregulatory effect of all MOCM samples. Latent TGF-beta1 present in MOCM was activated by acid treatment, inhibiting upregulation by approximately 60%, a result similar to experiments with IL-1beta and TGF-beta1 mixtures. Treatment with a TGF-beta neutralizing antibody restored full upregulatory activity to acidified MOCM. Thus activated macrophages increase lung fibroblast PDGF-R alpha primarily due to the secretion of IL-1beta. Intratracheal instillation of ROFA particles in rats induced a 2-fold increase in total lung PDGF-R alpha mRNA in vivo. These findings support the idea that macrophage-derived IL-1beta plays a key role in the initiation of a fibrotic response by increasing fibroblast PDGF-R alpha expression, thereby dramatically potentiating the mitogenic response to PDGF.

alpha-Macroglobulins derived from plasma or secreted by macrophages are platelet-derived growth factor (PDGF) binding proteins that compete with cell-surface receptors on fibroblasts for PDGF binding. alpha 2-Macroglobulin (alpha 2M) derived from bovine plasma was tested for its ability to modulate the PDGF-induced proliferation of primary passage rat lung fibroblasts (RLFs) and a human skin fibroblast cell line (CRL 1508). Fibroblasts were grown in 10% fetal bovine serum (FBS) for 24 hr, then washed with serum-free medium before adding serum-free defined medium (SFDM) containing insulin and transferrin. To this medium were added varying concentrations of human plasma-derived AB-PDGF and alpha 2 M, alone or in combination. Receptor-recognized alpha 2M was prepared by treatment with methylamine. Both native alpha 2M and the alpha 2M-methylamine (alpha 2M-MA) were tested for growth promoting activity in the absence or presence of PDGF. After 3 days, a concentration-dependent growth curve of fibroblast proliferation was demonstrated for PDGF alone, with near maximal stimulation reached at 15-20 ng/ml PDGF. alpha 2M and alpha 2M-MA alone had no effect on cell proliferation. However, alpha 2M-MA concentrations above 32 micrograms/ml synergistically enhanced PDGF-stimulated proliferation greater than 100% in the presence of 15 ng/ml PDGF. Native alpha 2M enhanced PDGF-stimulated growth 80-100% above PDGF controls only at low concentrations (32-64 micrograms/ml alpha 2M). High concentrations of native alpha 2M (128-256 micrograms/ml) either had no effect on growth or were inhibitory to PDGF-stimulated growth, depending on the cell type tested. Rat lung fibroblasts were shown to secrete a factor(s) that inhibited the trypsin-binding capacity of native alpha 2M. We further demonstrated that early passage RLFs possess specific cell-surface receptors for [125I]-PDGF and [125I]-alpha 2M-MA, and preincubation of RLFs with alpha 2M-MA increased the specific binding of [125I]-PDGF to the cell surface of these fibroblasts. Considered together, these data support the view that receptor-recognized alpha 2M synergistically enhances the proliferative capacity of PDGF. We postulate that receptor-recognized alpha Ms enhance PDGF-stimulated growth by increasing the local concentration of PDGF at the cell surface, where the PDGF could be released in close proximity to its own receptors.

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 [125I]PDGF-AA. The TGF-beta 1-induced down-regulation 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 [125I]PDGF-AA binding sites 5-fold without affecting receptor affinity. [125I]PDGF-AB binding sites were downregulated approximately 25%, and the number of [125I]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.

Bronchial smooth muscle cell (SMC) hyperplasia is a key feature in the pathology of asthma. Platelet-derived growth factor (PDGF) isoforms are SMC mitogens. We investigated the effect of basic fibroblast growth factor (bFGF), transforming growth factor-beta 1 (TGF-beta 1), interleukin-1 beta (IL-1 beta), and tumor necrosis factor-alpha (TNF-alpha) on the PDGF receptor system on human bronchial SMC from three different donors. bFGF induced gene expression of the PDGF alpha-receptor (PDGF-R alpha) approximately threefold without altering the PDGF beta-receptor (PDGF-R beta). IL-1 beta and TNF-alpha did not affect the PDGF receptor system. TGF-beta 1 downregulated PDGF-R alpha mRNA approximately 60% without changing PDGF-R beta mRNA levels. Receptor assays showed that bFGF increased the [125I]PDGF-AA binding site approximately twofold, whereas TGF-beta 1 reduced [125I]PDGF-AA binding approximately 60%. TGF-beta 1, but not latent TGF-beta 1, counteracted the bFGF-induced increase in [125I]PDGF-AA binding. PDGF-AA-stimulated tyrosine phosphorylation on the PDGF-R alpha was enhanced after treatment with bFGF, bFGF pretreatment enhanced the mitogenic response of SMC to PDGF-AA and PDGF-AB. These findings suggest that upregulation of the PDGF-R alpha by bFGF could contribute to SMC hyperplasia during chronic airway inflammation in asthma.