Based upon earlier reports of synergism in cells of lymphoid origin, we have examined interactions between the organotellurium compound AS101 and the protein kinase C (PKC) activator bryostatin 1 with respect to differentiation and Ara-C-induced apoptosis in human myeloid leukemia 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 microM Ara-C for 6 h, this effect was not enhanced by co-administration of AS101 (1.5 microM). However, while exposure of cells to AS101 or bryostatin 1 alone for 72 h was ineffective in inducing cellular maturation, combined treatment resulted in the induction of differentiated features in a subset of cells, manifested by an increase in cell adherence, CD11b expression, cytoplasmic granularity and cell spreading. In addition, cells exposed to the combination of AS101 and bryostatin 1, in contrast to cells incubated with these agents individually, displayed a significant decline in the S-phase and a corresponding increase in the G0/G1 cell populations. These events were accompanied by an increase in protein expression of the cyclin-dependent kinase inhibitor, p21 (WAF1/CIP1/MDA6), and a decline in expression 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 microM AS101 or 10 nM bryostatin 1 for 24 h exerted minimal growth inhibitory effects, combined exposure to these agents reduced colony formation by over 70%. Finally, although addition of AS101 did not potentiate apoptosis induced by the bryostatin 1/Ara-C combination, it did lead 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-60 cell line, possibly by facilitating bryostatin 1-mediated G1 arrest. They also indicate that AS101 potentiates the antiproliferative effects of bryostatin 1 administered alone or in combination with Ara-C through a mechanism other than, or in addition to, induction of apoptosis.