In the study of apoptosis initiated by various signals including
ligands binding to cell membrane receptors such as Fas and TNFRI, the
sphingomyelin pathway and its resulting metabolites, the
sphingolipids, have been suggested to be involved in the signaling pathway. In earlier studies we presented data which indicated that
sphingosine (Sph) itself was increased during apoptosis induced by
phorbol myristate acetate (PMA) in HL60 cells and
tumor necrosis factor (TNF) in neutrophils, and when added exogenously was able to induce apoptosis. We report here that Sph and its methylated derivative
N,N,-dimethylsphingosine (DMS) are able to induce apoptosis in
cancer cells of both hematopoietic and
carcinoma origin. In human leukemic cell lines CMK-7, HL60 and U937, treatment with 20 microM Sph for 6 hr caused apoptosis in up to 90% of cells. Human colonic
carcinoma cells HT29, HRT18, MKN74 and COLO205 were shown to be more susceptible to apoptosis upon addition of DMS (>50%) than of Sph (<50%), yet were weakly or not sensitive to
N,N,N-trimethylsphingosine (TMS). Under the same conditions, in the presence of serum, neither Sph-1-phosphate nor
ceramide analogues C2-, C6- or
C8-ceramide were able to induce apoptosis in any cell lines. However, in the absence of serum,
ceramide analogues induced apoptosis in
leukemia cell lines after 18 hr, yet much less so than Sph or DMS. Furthermore, apoptosis induced by Sph or DMS could not be inhibited by the
ceramide synthase inhibitor
fumonisin B1. Apoptosis was not induced by
sphingolipids in primary culture cells, such as HUVEC or rat mesangial cells, but was apparent in transformed rat mesangial cells. Additionally, apoptosis induced by Sph, DMS or C2Cer was inhibited by
protease inhibitors. Our data further support the evidence that the catabolic pathway of
sphingomyelin involving Sph and other metabolites is an integral part of the apoptosis pathway.