Enhancement of calcium uptake and phosphatidylinositol turnover by epidermal growth factor in A-431 cells

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Abstract
Epidermal growth factor (EGF) stimulates the incorporation of 32Pi and [3H]inositol into phosphatidylinositol (5 to 10-fold) in human epidermoid carcinoma A-431 cells. EGF also stimulates the incorporation of 32Pi into phosphatidic acid (up to 10-fold). These effects are attributed to an acceleration of the turnover of phosphatidylinositol as a consequence of the binding of EGF to its membrane receptor. The extent of the phosphatidylinositol response to EGF parallels the extent of hormone binding. The phosphatidylinositol response to EGF appears to be dependent on an influx of Ca since external Ca is required for the enhancement of phosphatidylinositol turnover, the accumulation of 45Ca by A-431 cells is stimulated by EGF, blockage of Ca influx with LaCl3 inhibits stimulation of phosphatidylinositol turnover and Ca influx via ionophore A23187 [calcimycin] is sufficient to stimulate phosphatidylinositol turnover. Since the binding, internalization and degradation of 125I-labeled EGF in A-431, cells are unaffected by the omission of Ca from the medium, external Ca and phosphatidylinositol turnover are not necessary for the internalization and degradation of the EGF-receptor complex.