EFFECT OF AS101 ON BRYOSTATIN 1-MEDIATED DIFFERENTIATION INDUCTION, CELL-CYCLE ARREST, AND MODULATION OF DRUG-INDUCED APOPTOSIS IN HUMAN MYELOID-LEUKEMIA CELLS

Based upon earlier reports of synergism in cells of lymphoid origin, w e have examined interactions between the organotellurium compound AS10 1 and the protein kinase C (PKC) activator bryostatin 1 with respect t o differentiation and Ara-C-induced apoptosis in human myeloid leukemi a cells (HL-60). Although preincubation with bryostatin 1 (10 nM) for 24 h significantly increased DNA fragmentation and apoptosis in cells subsequently treated with 10 mu M Ara-C for 6 h, this effect was not e nhanced by co-administration of AS101 (1.51 mu M). However, while expo sure of cells to AS101 or bryostatin 1 alone for 72 h was ineffective in inducing cellular maturation, combined treatment resulted in the in duction of differentiated features in a subset of cells, manifested by an increase in cell adherence, CD11b expression, cytoplasmic granular ity and cell spreading. In addition, cells exposed to the combination of AS101 and bryostatin 1, in contrast to cells incubated with these a gents individually, displayed a significant decline in the S-phase and a corresponding increase in the G(0)/G(1) cell populations. These eve nts were accompanied by an increase in protein expression of the cycli n-dependent kinase inhibitor, p21(WAF1/CIP1/MDA6), and a decline in ex pression of the c-myc protein. AS101 failed to increase intracellular free Ca2+ ([Ca2+](i)) in HL-60 cells, or reverse the profound PKC down -regulation induced by bryostatin 1. Whereas treatment of cells with 1 .5 mu M AS101 or 10 nM bryostatin 1 for 24 h exerted minimal growth in hibitory effects, combined exposure to these agents reduced colony for mation by over 70%. Finally, although addition of AS101 did not potent iate apoptosis induced by the bryostatin 1/Ara-C combination, it did l ead to a further reduction in clonogenicity. Together, these findings demonstrate that AS101 partially restores the ability of bryostatin 1 to trigger a differentiation program in an otherwise unresponsive HL-6 0 cell line, possibly by facilitating bryostatin 1-mediated G(1) arres t. They also indicate that AS101 potentiates the antiproliferative eff ects of bryostatin 1 administered alone or in combination with Ara-C t hrough a mechanism other than, or in addition to, induction of apoptos is.