- Release of large soluble NH2-terminal fragments of the amyloid precursor protein (APP) of Alzheimer's disease was measured in two Swiss 3T3 fibroblast cell lines (designated SF1.4 and SF3.2), overexpressing the alpha subtype of protein kinase C, and in two control cell lines (SC1 and SC2) (Eldar, H., Zisman, Y., Ullrich, A., and Livneh, E. (1990) J. Biol. Chem. 265, 13290-13296). Basal release of APP was significantly increased in SF1.4 cells, but not in SF3.2 cells, relative to controls. Phorbol 12-myristate 13-acetate, an activator of protein kinase C, elicited a concentration-dependent increase in APP release in all four cell lines. However, the estimated EC50 for this effect was lower in the two cell lines overexpressing protein kinase C-alpha (7 and 6 nM, in SF1.4 and SF3.2 cells, respectively) than in control SC1 and SC2 cells (56 and 22 nM, respectively). The absolute amount of APP released by maximal concentrations of phorbol ester was not altered by overexpression of protein kinase C alpha. The protein kinase C inhibitor H-7 (1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride) significantly reduced the response to phorbol esters in control (SC1) cells but not in cells (SF1.4) that overexpress protein kinase C alpha. Levels of cell-associated APP were slightly elevated, and rates of APP turnover were unchanged, in SF1.4 cells relative to controls. However, cell-associated APP levels were lower in SF3.2 cells than in controls. The results demonstrate that protein kinase C alpha regulates APP release in Swiss 3T3 fibroblasts, and perhaps in other tissues, including brain, and may be the isozyme that mediates receptor-evoked release of APP.