Prevention of the TPA-mediated down-regulation of protein kinase C.

Protein kinase C activity in lymphocytes was down-regulated upon exposure of the cells to 12-0-tetradecanoylphorbol-13 acetate. This down-regulation was prevented by preincubating the cells with sphingosine, a reported protein kinase C inhibitor. Two other protein kinase C inhibitors, palmitoylcarnitine and phloretin, were ineffective in preventing down-regulation by the phorbol ester.