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), b oth bind to and activate protein kinase C (PKC) but exhibit divergent biological actions. Bryostatin 1 exerts variable effects on leukemic c ell differentiation, and has been reported by some investigators to in hibit the proliferation of the monocytic leukemic cell line U937. In t his 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 concentratio ns (10 nM), PMA, in contrast to bryostatin 1, induced cellular differe ntiation of U937 cells, reflected by growth inhibition, increased plas tic adhesion, and expression of the monocytic differentiation marker, CD11b. Consistent with these results, bryostatin 1 was less effective in inducing G(0)/G(1) arrest and inhibiting cyclin-dependent kinase 2 (CDK2) activity. Bryostatin 1, unlike PMA, failed to induce expression of the cyclin-dependent kinase inhibitor (CDKI), p21(CIP1/WAF1), and blocked the ability of PMA to induce this protein. Bryostatin 1 exposu re resulted in increased expression of the CDKI p27(KIP1) these cells, although the kinetics differed from PMA. In addition, bryostatin 1 wa s less effective than PMA in dephosphorylating pRb, modifying E2F comp lexes, and downregulating c-Myc. Go-administration of bryostatin 1 wit h PMA antagonized the latter's differentiation-inducing capacity and a nti-proliferative effects, actions that were accompanied by a reductio n in PMA-mediated p21(CIP1/WAF1) induction, CDK2 inhibition, pRb depho sphorylation, and c-Myc downregulation. Antagonistic effects of bryost atin 1 on PMA-related cell cycle events were mimicked by the specific PKC inhibitor GF109203X. Together, these studies indicate that bryosta tin 1 is a considerably weaker stimulus than PMA for U937 cell differe ntiation, and raise the possibility that this deficiency arises from i ts failure to induce p21(CIP1/WAF1) and trigger cell cycle arrest.