Divergent effects of bryostatin 1 and phorbol myristate acetate on cell cycle arrest and maturation in human myelomonocytic leukemia cells (U937).

Bryostatin 1 and the phorbol ester, phorbol myristate acetate (PMA), both bind to and activate protein kinase C (PKC) but exhibit divergent biological actions. Bryostatin 1 exerts variable effects on leukemic cell differentiation, and has been reported by some investigators to inhibit the proliferation of the monocytic leukemic cell line U937. In this study, we have compared the efficacy of bryostatin 1 and PMA with respect to U937 cell maturation, with a major emphasis on differential actions on the cell cycle arrest machinery. At equimolar concentrations (10 nM), PMA, in contrast to bryostatin 1, induced cellular differentiation of U937 cells, reflected by growth inhibition, increased plastic adhesion, and expression of the monocytic differentiation marker, CD11b. Consistent with these results, bryostatin 1 was less effective in inducing G0/G1 arrest and inhibiting cyclin-dependent kinase 2 (CDK2) activity. Bryostatin 1, unlike PMA, failed to induce expression of the cyclin-dependent kinase inhibitor (CDKI), p21CIP1/WAF1, and blocked the ability of PMA to induce this protein. Bryostatin 1 exposure resulted in increased expression of the CDKI p27KIP1 in these cells, although the kinetics differed from PMA. In addition, bryostatin 1 was less effective than PMA in dephosphorylating pRb, modifying E2F complexes, and downregulating c-Myc. Co-administration of bryostatin 1 with PMA antagonized the latter's differentiation-inducing capacity and anti-proliferative effects, actions that were accompanied by a reduction in PMA-mediated p21CIP1/WAF1 induction, CDK2 inhibition, pRb dephosphorylation, and c-Myc downregulation. Antagonistic effects of bryostatin 1 on PMA-related cell cycle events were mimicked by the specific PKC inhibitor GF109203X. Together, these studies indicate that bryostatin 1 is a considerably weaker stimulus than PMA for U937 cell differentiation, and raise the possibility that this deficiency arises from its failure to induce p21CIP1/WAF1 and trigger cell cycle arrest.