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A comparison of the platelet‐derived growth factor‐dependent tyrosine kinase activity in sparse and confluent fibroblasts

Confluent (density‐inhibited) human foreskin fibroblasts require a higher concentration of platelet‐derived growth factor (PDGF) to elicit a mitogenic response than do sparse (nondensity‐inhibited) fibroblasts. The PDGF receptor number and apparent affinity were similar in the two preparations of cells. The intrinsic kinase activity of the PDGF receptor from sparse and confluent fibroblasts was therefore examined in an attempt to explain the differential mitogenic response to PDGF. When membranes from sparse and confluent cells containing equal PDGF binding capacity were incubated with increasing concentrations of PDGF, the putative PDGF receptor (a 180‐kD component), was phosphorylated on its tyrosyl residues to a similar extent. The time course of tyrosine phosphorylation of the PDGF receptor from sparse and confluent cell membranes was also found to be similar. To determine whether the phosphorylation of the PDGF receptor from isolated membranes differed from the analogous phosphorylation in intact cells, sparse and confluent fibroblasts were metabolically labeled with [32P]H3PO4, stimulated with PDGF, solubilized, and the cell proteins were immunoprecipitated with a phosphotyrosine‐specific antibody. The extent of PDGF‐dependent tyrosine phosphorylation of the PDGF receptor from sparse vs. confluent fibroblasts was quite similar. The time course of the tyrosine dephosphorylation of the PDGF receptor was also similar in the two populations. Because comparable extents of PDGF‐induced tyrosine phosphorylation of the PDGF receptor were observed despite the differential PDGF‐induced mitogenic response of sparse and confluent fibroblasts, we tentatively conclude that (1) PDGF‐dependent tyrosine phosphorylation of the PDGF receptor is not tightly coupled to the propagation of the mitogenic signal and (2) density‐dependent inhibition of growth does not reflect any measurable change in the quantity of kinase activity of the PDGF receptor.