INHIBITION BY SPHINGOSINE OF LEUKEMIC-CELL KILLING BY HUMAN MONOCYTESACTIVATED WITH INTERLEUKIN-2 - A POSSIBLE ROLE OF PROTEIN-KINASE-C

Sphingosine and its analogs, which inhibit protein kinase C (PKC), are known to be potent inducers of apoptosis in tumor cells. However, we were concerned that sphingosine might also interfere with anti-tumor c ells of the immune system. Therefore, we evaluated the effect of sphin gosine on activation of human monocytes by interleukin-2 (IL-2) for ki lling of leukemic cells, Monocytes, purified by elutriation and adhere nce, were activated with IL-2 or interferon-gamma (IFN-gamma) in the p resence or absence of sphingosine or another inhibitor for 18 h, Then the monocytes were washed and the culture medium was replaced with fre sh medium to remove the sphingosine, HL-60 and K562 leukemic cells wer e added to the monocyte cultures. over the next 48 h, the cytotoxic ac tivity of the monocytes towards the leukemic cells was assessed by mea ns of an (111)indium-releasing assay IL-2-activated monocytes lysed 48 +/-3% of HL-60 cells and 44+/-3% of K562 cells, Sphingosine, dihydrosp hingosine, N,N-dimethylsphingosine, and the PKC inhibitor H7 inhibited the activation of monocytes by IL-2, blocking cytotoxic activity agai nst the leukemic cells by approximately 75%. These inhibitors were not toxic to monocytes at the concentrations used, In a PKC assay, sphing osine and H7 inhibited PKC activity in IL-2-treated monocytes. Thus, s phingosines, by inhibiting PKC activity, inhibited activation of monoc ytes by IL-2, which inhibited the killing of leukemic cells.