Ceramide mediates tumor-induced dendritic cell apoptosis

Induction of apoptosis in dendritic cells (DC) is one of the escape mechani sms of tumor cells from the immune surveillance system. This study aimed to clarify the underlying mechanisms of tumor-induced DC apoptosis. The super natants (SN) of murine tumor cell lines B16 (melanoma), MCA207, and MCA102 (fibrosarcoma) increased C16 and C24 ceramide as determined by electrospray mass spectrometry and induced apoptosis in bone marrow-derived DC. N-oleoy lethanolamine or D-L-threo 1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP), which inhibits acid ceramidase or glucosylceramide synthase and th en increases endogenous ceramide, enhanced DC apoptosis and ceramide levels in the presence of tumor SN. Pretreatment with L-cycloserine, an inhibitor of de novo ceramide synthesis, or phorbol ester, 12-O-tetradecanoylphorbol -13-acetate reduced endogenous ceramide levels and protected DC from tumor- induced apoptosis. However, other DC survival factors, including LPS and TN F-alpha, failed to do so. The protective activity of 12-O-tetradecanoylphor bol-13-acetate is abrogated by pretreatment with phosphoinositide 3-kinase (PI3K) inhibitor, LY294002. Therefore, down-regulation of PI3K is the major facet of tumor-induced DC apoptosis. Tumor SN, N-oleoylethanolamine, or PD MP suppressed Akt, NF-kappaB, and bcl-x(L) in DC, suggesting that the accum ulation of ceramide impedes PI3K-mediated survival signals. Taken together, ceramide mediates tumor-induced DC apoptosis by down-regulation of the PI3 K pathway.