Stimulation of a Mr 80,000 protein phosphorylation by EGF in EGF receptor‐hyperproducing human tumor cells

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Abstract
NA and Ca9‐22 cells derived from squamous cell carcinomas of the tongue possess a large number of epidermal growth factor (EGF) receptors (2.0 × 106 and 1.3 × 106 receptors/cell, respectively). In these cell lines, EGF stimulated receptor autophosphorylation and phosphatidylinositol (PI) turnover. Furthermore, EGF enhanced the phosphorylation of an acidic protein of Mr 80,000. Phosphorylation of this protein was also stimulated by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), a phorbol ester tumor promoter, and was mainly at serine residues. Phosphopeptide mapping using protease V8 or trypsin indicated that Mr 80,000 proteins isolated from the EGF‐ and TPA‐treated cells were identical. The Mr 80,000 protein was present mainly in the cytosol, but it became closely associated with the membrane as a phosphorylated form upon EGF or TPA stimulation. These results suggest that the EGF‐stimulated phosphorylation of the Mr 80,000 acidic phosphoprotein in EGF receptor‐hyperproducing tumor cells is mediated through the activation of PI turnover and protein kinase C.

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