Sphingosine enhances apoptosis of radiation-resistant prostate cancer cells

Ceramide has been implicated as an important component of radiation-induced apoptosis of human prostate cancer tells. We examined the role of the sphi ngolipid metabolites-ceramide, sphingosine, and sphingosine-1-phosphate-in susceptibility to radiation-induced apoptosis in prostate cancer cell lines with different sensitivities to gamma-irradiation, Exposure of radiation-s ensitive TSU-Prl cells to 8-Gy irradiation led to a sustained increase in c eramide, beginning after 12 h of treatment and increasing to 2.5- to 3-fold within 18 h. Moreover, irradiation of TSU-Pr1 cells also produced a marked and rapid 50% decrease in the activity of sphingosine kinase, the enzyme t hat phosphorylates sphingosine to form sphingosine-1-phosphate, In contrast , the radiation-insensitive cell line, LNCaP, had sustained sphingosine kin ase activity and did not produce elevated ceramide levels on 8-Gy irradiati on. Although LNCaP cells are highly resistant to gamma-irradiation-induced apoptosis. they are sensitive to the death-inducing effects of tumor necros is factor alpha, which also increases ceramide levels in these cells (K. Ki mura et al., Cancer Res., 59: 1606-1614, 1999), Moreover, we found that alt hough irradiation alone did not increase sphingosine levels in LNCaP cells, tumor necrosis factor alpha plus irradiation induced significantly higher sphingosine levels and markedly reduced intracellular levels of sphingosine -1-phosphate. The elevation of sphingosine levels either by exogenous sphin gosine or by treatment with the sphingosine kinase inhibitor N,N-dimethylsp hingosine induced apoptosis and also sensitized LNCaP cells to gamma-irradi ation-induced apoptosis. Our data suggest that the relative levels of sphin golipid metabolites may play a role in determining the radiosensitivity of prostate cancer cells, and that the enhancement of ceramide and sphingosine generation could be of therapeutic value.