- The bryostatin (Bryo) is a macrocyclic lactone that binds specifically to protein kinase C (PKC) thereby affecting cell growth and differentiation and inhibits phorbol ester-induced tumor promotion. We used human peripheral blood lymphocytes (PBL) and epidermal cells in order to analyze the action mechanism of Bryo and compare it with that of the phorbol ester PMA. Bryo and PMA activated PBL- or T cell-derived PKC in a similar dose-response and induced a similar time kinetic of cytosol-to-membrane translocation of enzymatically active and immunoreactive PKC. In addition, the 2 drugs induced similar patterns of protein phosphorylation and activated the c- fos and c- jun genes that their protein products regulate transcription of TRE-containing genes. In contrast, long-term (20 h) treatment of cells with Bryo resulted in a marked loss of both cytosolic- and membrane-bound PKC while PMA induced only a slight reduction in the amount of cellular PKC. Inhibition of PMA-induced human T-cell proliferation by Bryo correlated with a reduction in the amount of cellular PKC. An opposite effect was observed in human epidurmal cells where Bryo augmented growth and proliferation while PMA induced terminal differentiation and cell death. We propse that at least some of the differences in the biological effects induced by Bryo and PMA are due to distinct regulations of PKC. Thus, although both agents can initially bind to and activate PKC at a later time (∼ 16 h), Bryo, but not PMA, induces rapid PKC degradation and inhibition of PKC-regulated biological responses that are dependent on the continuous presence and/or activation of the enzyme.