DNA synthesis of WF-1 fibroblasts derived from a patient with Werner's syndrome was stimulated by fetal calf serum and adult human serum but not by various mitogens including epidermal growth factor, platelet-derived growth factor (PDGF), fibroblast growth factor, insulin and 12-O-tetradecanoylphorbol-13-acetate (TPA). To clarify the cause of nonrespon-siveness to these mitogens, we compared the rate of protein phosphorylation in normal fibroblasts HF-O and Werner's WF-1 cells. PDGF and TPA enhanced the phosphorylation of a Mr 80 K protein, which is known to be a substrate for protein kinase C, both in HF-O and WF-1 cells. This indicates that the pathway involving PDGF receptor, phosphatidylinositol turnover and protein kinase C activation is operational in WF-1 cells. Several species of phosphoproteins of Mr 250 K, 135 K, 110 K, 78 K and 42 K were detected in normal HF-O cells by immunoprecipitation using an anti-phosphotyrosine antibody. The same species of phosphoproteins were detected in Werner's WF-1 cells at passage 6, but only when treated with various mitogens and were not detected in WF-1 cells at passage 10 even after the PDGF- or TPA-treatment. These results suggest that the reduction of phosphorylation of these target proteins may be in part responsible for the diminished mitogenic responsiveness of Werner's fibroblasts.
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