PDGF‐stimulated fibroblast proliferation is enhanced synergistically by receptor‐recognized α2‐Macroglobulin

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Abstract
α-Macroglobulins derived from plasma or secreted by macrophages are plateletderived growth factor (PDGF) binding proteins that compete with cell-surface receptors on fibroblasts for PDGF binding. α2-Macroglobulin (α2M) 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 α2M, alone or in combination. Receptor-recognized α2M was prepared by treatment with methylamine. Both native α2M and the α2M-methylamine (α2M-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. α2M and α2M-MA alone had no effect on cell proliferation. However, α2M-MA concentrations above 32 μg/ml synergistically enhanced PDGF-stimulated proliferation >100% in the presence of 15 ng/ml PDGF. Native α2M enhanced PDGF-stimulated growth 80–100% above PDGF controls only at low concentrations (32–64 μg/ml α2M). High concentrations of native α2M (128–256 μg/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 α2M. We further demonstrated that early passage RLFs possess specific cell-surface receptors for [125I]-PDGF and [125I]-α2M-MA, and preincubation of RLFs with α2M-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 α2M synergistically enhances the proliferative capacity of PDGF. We postulate that receptor-recognized αMs 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.

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