Sphingolipid breakdown products, including
ceramide and
sphingosine, regulate cell growth, cell differentiation, and apoptosis. We examined the effect of various agents, including
sphingolipids, on apoptosis induction in human
epidermoid carcinoma KB-3-1 and its multidrug-resistant (MDR) subclone KB-C2 cells which express
P-glycoprotein.
Adriamycin (ADM) induced apoptosis in KB-3-1 cells but not in KB-C2 MDR cells at the concentration of 50 microg/ml. On the other hand, 15 microM
sphingosine or its methylated derivative
N, N-dimethylsphingosine (DMS) induced apoptosis in both cell types in vitro. These results suggested that KB-C2 MDR cells were resistant to apoptosis induction by ADM but sensitive to that by
sphingosine and DMS.
Ceramide and
sphingosine-1-phosphate, the initial metabolites of
sphingosine, failed to induce apoptosis under the same experimental condition as
sphingosine/DMS. The
protein kinase C (PKC) inhibitors H7 and
staurosporine did not induce apoptosis in either cell line, suggesting that PKC-independent signaling is involved in apoptosis induced by
sphingosine and DMS, although both
sphingosine and DMS have been shown to down-regulate PKC. Furthermore, DMS significantly inhibited the growth of KB-3-1 as well as KB-C2 MDR
tumors in vivo, with evidence of increased apoptosis. The intracellular level of exogenously added [3H]
sphingosine or [14C]DMS did not differ between the KB-3-1 parent cell line and its MDR subclone KB-C2, whereas that of [14C]ADM was reduced in KB-C2 MDR cells compared to KB-3-1 cells. These results suggest that
P-glycoprotein acts as a transporter for ADM but not for
sphingosine or DMS. Furthermore, DMS at the concentrations which induce apoptosis in KB-C2 cells did not affect the level of [14C]ADM. Because
sphingosine and DMS induce apoptosis regardless of
P-glycoprotein expression, they may provide a new strategy and a promising approach to the treatment of anticancer
drug-resistant
cancer.