Epidermal growth factor induces activation of protein kinase F(A) and ATP · Mg-dependent protein phosphatase in A431 cells

The cytosolic fractions from epidermal growth factor (EGF)-treated A431 cells exhibit a marked increase in activities of ATP · Mg-dependent protein phosphatase and its activating factor (protein kinase F(A)) when compared to controls in the absence of EGF. By contrast, the Triton X-100-solubilized membrane fractions from the same EGF-treated cells exhibit a corresponding decrease in protein-kinase F(A) activity. The EGF-dependent activation of protein kinase F(A) and ATP · Mg-dependent protein phosphatase occurred within physiological concentrations of EGF (ED₅₀ = 5 x 10^-10 M). The changes of kinase and phosphatase activities which were measured concomitantly exhibit very similar characteristics as to EGF sensitivity and time dependence. The EGF-induced kinase and phosphatase activation occurred very rapidly, reaching the maximal activity levels within 3 min. Moreover, the EGF effect is transient; both EGF-stimulated phosphatase and kinase activities returned to control levels within 30 min. Taken together, the results suggest that EGF may induce the activation of kinase F(A) in the membrane and thereby promotes the activation of ATP · Mg-dependent phosphatase in the cytosol. Exposure of A431 cells to exogenous phosholipase C also resulted in the activation of endogenous kinase F(A) and ATP · Mg-dependent phosphatase in a similar pattern produced by EGF. This further suggests that phospholipase C can mimic EGF to mediate the activation of kinase F(A) and ATP · Mg-dependent phosphatase in A431 cells. By its dual role as a multisubstrate protein kinase and as an activating factor of multisubstrate protein phosphatase, protein kinase F(A) may represent a transmembrane signal of EGF.