Phorbol esters bind to and activate a calcium phospholipid-dependent protein kinase (C kinase). Some researchers believe that activation of C kinase is necessary for the induction of phorbol ester biologic effects. Our research indicates that bryostatin, a macrocyclic lactone that binds to the phorbol ester receptor in human polymorphonuclear leukocytes, also binds to this receptor in the human promyelocytic leukemia cell line, HL-60. Bryostatin activates partially purified C kinase from HL-60 cells in vitro, and when applied to HL-60 cells in vivo, it decreases measurable cytoplasmic C kinase activity. Unlike the phorbol esters, bryostatin is unable to induce a macrophage-like differentiation of HL-60 cells; however, bryostatin, in a dose-dependent fashion, blocks phorbol ester-induced differentiation of HL-60 cells and, if applied within 48 hr of phorbol esters, halts further differentiation. These results suggest that activation of the C kinase by some agents is not sufficient for induction of HL-60 cell differentiation and imply that some of the biologic effects of phorbol esters may occur through a more complex mechanism than previously thought.Considerable evidence suggests that the biochemical mechanism of action of tumor-promoting phorbol esters involves the activation of a calcium phospholipid-dependent protein kinase (C kinase) (1). When cell-free preparations were studied, this enzyme was found to copurify with the cellular phorbol ester receptor (2). Further studies have shown that[3H]phorbol 12,13-dibutyrate ([3H]PBt2) binds to homogeneous preparations of C kinase isolated from mouse brain (3-5) and that phorbol esters compete directly with 1,2-diacylglycerols, the natural activators of the C kinase, for binding to this enzyme (6). The addition of phorbol esters to intact cells induces a marked increase in membrane-associated C kinase (7) and in the phosphorylation of specific membrane proteins-e.g., the transferrin and insulin receptors (8, 9). Activation of C kinase by phorbol esters has been implicated in several biologic responses, including platelet and polymorphonuclear leukocyte (PMN) degranulation, Swiss 3T3 cell mitogenesis, and human lymphocyte activation (1,(10)(11)(12)(13)(14) Recently, bryostatin, a macrocyclic lactone with a significantly different structure from phorbol esters (Fig. 1), has been isolated from the marine bryozoan Bugula neritina (25)(26)(27)
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