Phorbol ester-mediated protein phosphorylations in S49 mouse lymphoma cells

Using high-resolution 2-dimensional gel electrophoresis to separate proteins from cells labeled in vivo with either [32P]phosphate or [35S]methionine, the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) was shown to stimulate phosphorylation of at least 18 proteins in a subline of S49 mouse lymphoma cells deficient in cyclic AMP-dependent protein kinase. Phosphorylation of these proteins was not altered by phorbol acetate, a phorbol ester inactive in tumor promotion, and stimulation by TPA was half-maximal at less than 16 nM; therefore, these responses appeared to reflect specific interactions of TPA with high-affinity receptors. Treatment of cells with phospholipase C mimicked TPA in stimulating phosphorylation of some of these substrate proteins, thereby suggesting possible involvement of protein kinase C, the calcium-activated phospholipid-dependent protein kinase. Substrates differed in their relative responses to phospholipase C, the kinetics and concentration dependence of their phosphorylation in response to TPA, their extents of TPA-stimulated changes in phosphorylation, and their responses to tetracaine and retinal, two inhibitors of protein kinase C. Using these responses as criteria for classification, the TPA-mediated phosphorylations could be shown to fall into at least three distinct groups. The significance of these results to regulation of intracellular protein phosphorylation, to the relationship of protein kinase C and phorbol ester receptors, and to possible heterogeneity in kinases stimulable by phorbol ester tumor promoters is discussed.