Inhibition by Sphingosine of Leukemic Cell Killing by Human Monocytes Activated 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 cells of the immune system. Therefore, we evaluated the effect of sphingosine on activation of human monocytes by interleukin‐2 (IL‐2) for killing of leukemic cells. Monocytes, purified by elutriation and adherence, were activated with IL‐2 or interferon‐gamma (IFN‐γ) in the presence or absence of sphingosine or another inhibitor for 18 h. Then the monocytes were washed and the culture medium was replaced with fresh medium to remove the sphingosine. HL‐60 and K562 leukemic cells were added to the monocyte cultures. Over the next 48 h, the cytotoxic activity of the monocytes towards the leukemic cells was assessed by means of an (111)indium‐releasing assay. IL‐2‐activated monocytes lysed 48±3% of HL‐60 cells and 44±3% of K562 cells. Sphingosine, dihydrosphingosine, N, N‐dimethylsphingosine, and the PKC inhibitor H7 inhibited the activation of monocytes by IL‐2, blocking cytotoxic activity against the leukemic cells by approximately 75%. These inhibitors were not toxic to monocytes at the concentrations used. In a PKC assay, sphingosine and H7 inhibited PKC activity in IL‐2‐treated monocytes. Thus, sphingosines, by inhibiting PKC activity, inhibited activation of monocytes by IL‐2, which inhibited the killing of leukemic cells.