ROLE OF CERAMIDE IN MITOGENESIS INDUCED BY EXOGENOUS SPHINGOID BASES

Dihydrosphingosine, an intermediate in the de novo synthesis of cerami de, induced proliferation of Swiss 3T3 cells. The proliferative effect s of this lipid were much more potent than those of sphingosine, a bre akdown product of ceramide. The maximal proliferative response to dihy drosphingosine occurred at relatively low concentrations (1 mu M), whi le sphingosine produced its maximal effect at much higher concentratio ns (15 mu M), The cell-permeable ceramide, N-hexanoylsphingosine (C-6- ceramide), which was also a mitogen in these cells (at 1 mu M), caused a striking morphological change when added to the cells at concentrat ions of 5-10 mu M. This shape change was reversible with the removal o f ceramide. Exogenous dihydrosphingosines and sphingosines have at lea st two metabolic fates in Swiss 3T3 cells, conversion to ceramide or t o sphingosine 1-phosphate, Surprisingly, both the synthetic threo- iso mer and the naturally occurring erythro- isomer of dihydrosphingosine and sphingosine (D-erythro-sphingosine, L-threo-sphingosine, DL-threo- dihydrosphingosine, and DL-erythro-dihydrosphingosine) were readily ph osphorylated in in tact Swiss 3T3 cells. This substrate specificity ma y be an indication of a sphingosine kinase activity which is distinct from that of platelets or rat brain. Although sphingosine 1-phosphate and ceramide were both produced upon the addition of sphingosine and d ihydrosphingosine, no sphingosine 1-phosphate was produced when Swiss 3T3 cells were treated with mitogenic concentrations of C-6-ceramide. These data are consistent with the formation of ceramide and not sphin gosine 1-phosphate being required for the mitogenesis produced by exog enous sphingoid bases.