Effect of pharmacologic manipulation of protein kinase C by phorbol dibutyrate and bryostatin 1 on the clonogenic response of human granulocyte‐macrophage progenitors to recombinant GM‐CSF

Summary. The effects of pharmacologic manipulation of protein kinase C (PK‐C) activity on the response of committed human myeloid progenitor cells (CFU‐GM) to recombinant human granulocyte‐macrophage colony stimulating factor (rGM‐CSF) was assessed. Coadministration of the PK‐C activating agents, phorbol dibutyrate (PDBu) or bryostatin 1, with rGM‐CSF resulted in a dose‐dependent and, under some conditions, highly synergistic increase in the number of CFU‐GM. With optimal combinations, colony formation far exceeded that which could be obtained with high concentrations of rGM‐CSF alone. High concentrations of PDBu (e.g. ≥ 50 nM), but not bryostatin 1, completely inhibited the CFU‐GM response. These inhibitory effects could be reversed by bryostatin 1, but not by high concentrations of rGM‐CSF. Bryostatin 1 also potentiated colony formation in response to rGM‐CSF, and blocked the inhibitory effects of high concentrations of PDBu in bone marrow cells highly enriched for progenitors bearing the MY‐10 antigen. The increase in CFU‐GM induced by PDBu or bryostatin 1 was associated with little change in the morphologic type of colony observed. Continuous exposure of cells to the calcium ionophore, ionomycin (500 nM), reduced the number of granulocyte‐macrophage colonies, but produed little change in the concentration‐response of rGM‐CSF and PK‐C activating agents. Finally, the PK‐C inhibitors H‐7 and tamoxifen, when administered at concentrations exhibiting minimal inhibitory effects in the presence of rGM‐CSF alone, led to no change or small increases in the numbers of colonies formed in response to rGM‐CSF and bryostatin‐1, and a substantial increase in the number of colonies formed in the presence of rGM‐CSF and PDBu. These results suggest that PK‐C activation may play a complex role in regulating the response of normal myeloid progenitors to growth factors such as rGM‐CSF. They also raise the possibility that under some circumstances the phorbol ester PDBu may trigger events that inhibit the growth of myeloid progenitors, and that this process may be blocked by bryostatin 1.