Bryostatin, an activator of the calcium phospholipid-dependent protein kinase, blocks phorbol ester-induced differentiation of human promyelocytic leukemia cells HL-60.

Abstract

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) 1334The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Localization of ‘Candidatus Endobugula sertula’ and the bryostatins throughout the life cycle of the bryozoan Bugula neritina

“…sertula' produces the bryostatins (Davidson et al, 2001), a group of complex polyketides that protect B. neritina larvae from predation (Lindquist and Hay, 1996;Lopanik et al, 2004). The bryostatins, first isolated from B. neritina (Pettit et al, 1982), also show potential for treatment of Alzheimer's disease and a variety of cancers (Kraft et al, 1986;Parkinson et al, 1994;Alkon, 2005, 2006;Kuzirian et al, 2006). Research over the past decade has confirmed the symbiotic source of the bryostatins, identified the bryostatin biosynthetic genes from 'E.…”

Modulation of the expression of Bcl-2 and related proteins in human leukemia cells by protein kinase C activators: relationship to effects on 1-[β-D-arabinofuranosyl]cytosine-induced apoptosis

“…Previous studies have demonstrated that bryostatin 1 blocks certain phorboid-mediated actions that it does not itself exert, including induction of leukemic cell differentiation (Kraft et al, 1986). Co-administration of 10 nM bryostatin 1 prevented 10 nM MZN from triggering HL-60 cell maturation, as reflected by antagonism of plastic adherence, while the combination continued to potentiate ara-C-mediated apoptosis ( Figure 6A; Table 1).…”

Towards selective pharmacological modulation of protein kinase C – opportunities for the development of novel antineoplastic agents

“…They also inhibited the growth of A549 cells, albeit only for a period of 24 h, beyond which growth inhibition was slowly reversed (Dale & Gescher, 1989). Bryostatin 1 failed to induce terminal differentiation of human colon cancer (McBain et al, 1988) and of HL-60 cells (Kraft et al, 1986). Nevertheless morphological or functional evidence of incomplete differentiation was noticed in most of these lines on exposure to bryostatin.…”

“…Nevertheless morphological or functional evidence of incomplete differentiation was noticed in most of these lines on exposure to bryostatin. In contrast, bryostatins blocked the differentiation induced by phorbol esters in HL-60 (Kraft et al, 1986), human colon cancer (McBain et al, 1988) and primary mouse epidermal cells (Sako et al, 1987). They also restored the differentiation response in Friend erythroleukaemia cells, in which drug-induced differentiation was suppressed by phorbol esters (Dell'Aquila et al, 1987).…”