Characterization of a high-molecular-weight protein immunoprecipitated by platelet-derived growth factor antisera

Interleukin-1 modulates phosphorylation of proteins in human osteoblastic cells

Interleukin-1 (IL-1) is a potent bone resorbing cytokine with diverse biological effects. We previously reported that IL-1 inhibits PDGF-AA-induced biological activities including PDGF-AA-induced tyrosyl phosphorylation. In the present studies, we first investigated and compared the tyrosyl phosphorylation pattern induced by EGF, IGF-1, PDGF-AA, and bFGF in human osteoblastic cells. We then examined the effect of IL-1 on the tyrosyl phosphoproteins induced by each ligand. Immunoblot analyses show that EGF, IGF-1, and PDGF-AA each elicit a different pattern of tyrosyl phosphorylated proteins in normal human osteoblastic cells. IL-1 beta inhibits PDGF-AA induced autophosphorylation by down-regulation of the PDGF-alpha receptor, as demonstrated by immunoprecipitation experiments. For other ligand-induced tyrosyl phosphoproteins, IL-1 beta reduced the intensity of EGF-induced pp55,000, and IGF-1 induced pp185,000 and pp175,000. These experiments indicate that IL-1 inhibits phosphorylation of specific proteins induced by growth factors. By using inhibitors of secondary message pathways, we determined that the inhibitory effect of IL-1 beta on PDGF-AA receptor binding and receptor tyrosyl autophosphorylation was not dependent on protein kinase A, protein kinase C, or the formation of prostaglandins. These data suggest the existence of an alternative pathway that may participate in IL-1 beta signaling.

Differential migratory response of U-2 OS osteosarcoma cell to the various forms of platelet-derived growth factor

U-2 OS osteosarcoma cells are mesenchymal-derived transformed cells spontaneously expressing both platelet-derived growth factor (PDGF) A- and B-chain genes, and releasing PDGF AA dimers in culture. Using modified Boyden chemotactic chambers, platelet-purified PDGF was shown to be a chemoattractant for U-2 OS cells. More specifically, U-2 OS cells migrated in the presence of PDGF AB and BB dimers but not in the presence of PDGF AA dimers. This pattern of response was similar to that observed with human fibroblasts and this similarity is consistent with the fact that U-2 OS cells express PDGF receptor alpha- and beta-subunits in a similar fashion to human fibroblasts.

Alpha-macroglobulin secreted by alveolar macrophages serves as a binding protein for a macrophage-derived homologue of platelet-derived growth factor

Evidence is presented to support our hypothesis that an alpha-macroglobulin (alpha M) produced by lung macrophages serves as a specific binding protein for platelet-derived growth factor (PDGF) from these same macrophages. Culture medium "conditioned" by alveolar macrophages was fractionated by gel filtration according to molecular weight. Proteins larger than 200 kD were bound to greater than 50% of the macrophage-derived PDGF (MD-PDGF) that was extractable by 1 M acetic acid. Another approximately 25% was bound to fractions at approximately 150 kD, and approximately 20% remained unbound. The two high molecular weight fractions inhibited approximately 40% of specific [125I]PDGF binding to rat lung fibroblasts, whereas other fractions did not block PDGF binding to its receptor. Only the greater than 200 kD fractions inhibited the binding of PDGF antisera to purified human PDGF by 20% of control and exhibited specific complex formation and coelution on a gel filtration column with [125I]PDGF. The macrophage-derived alpha M (MD-alpha M) was separated from other macrophage-derived proteins by nickel-affinity chromatography and exhibited clear characteristics of alpha Ms, i.e., cross-reactivity with antibodies to human alpha 2-macroglobulin (alpha 2M) on immunoblots as well as gel migration corresponding to the electrophoretic mobility of the protease-bound "fast" and protease-unbound "slow" forms of human alpha 2M. Nickel-bound protein identified as an alpha M was bound to greater than 50% of the acid-extractable MD-PDGF in macrophage-conditioned medium, supporting the view that the greater than 200 kD protein separated by gel filtration is an alpha M.(ABSTRACT TRUNCATED AT 250 WORDS)

The potential role of platelet-derived growth factor as an autocrine or paracrine factor for human bone cells

Platelet-derived growth factor, PDGF, is a potent mitogen for cells of mesenchymal origin such as fibroblasts, smooth muscle cells and glial cells. PDGF is thought to have the potential to act as both a paracrine and an autocrine factor. Studies described here extend these observations to human bone-derived cells. Exogenous PDGF induces biologic activity in two human osteogenic sarcoma cell lines and in one of these, the two PDGF genes, PDGF-1 and PDGF-2/c-sis are expressed. In addition, PDGF stimulates proliferation of normal osteoblastic cells derived from adult human cancellous bone. The expression of the PDGF-1 gene but not the PDGF-2/c-sis gene is demonstrated in normal human adult bone-derived cells by Northern blot analysis and synthesis of PDGF is shown by immunoprecipitation with PDGF antisera. These studies indicate that PDGF has the potential to act as a paracrine or autocrine regulator of bone cells.