VITAMIN-D METABOLITES ACTIVATE THE SPHINGOMYELIN PATHWAY AND INDUCE DEATH OF GLIOBLASTOMA CELLS

1 alpha,25-dihydroxyvitamin D3 was previously shown to induce cell dea th in brain tumour cell lines when added to the medium at micromolar c oncentration. In this paper we show that Cholecalciferol, a poor ligan d of the vitamin D receptor, also induces cell death of HU197 human gl ioblastoma cell line and early passages cultures derived from a recurr ent human glioblastoma. This finding suggests that the effects of vita min D metabolites on brain tumour cells are at least partially indepen dent from the activation of the classic nuclear receptor pathway. Vita min D metabolites have been shown to activate the sphingomyelin pathwa y inducing an increase in cellular ceramide concentration. We determin ed the levels of sphingomyelin ceramide and ganglioside GD3 in Hu197 c ells after treatment with cholecalciferol. A significant increase in c eramide concentration and a proportional decrease in sphingomyelin was already present after 6 hours of cholecalciferol treatment when no mo rphological changes were visible in the cultures. Treatment with ceram ides (N-acetyl-sphingosine or natural ceramide from bovine brain) of t he same cells also induces cell death. Similarly, treatment of the sam e cells with bacterial Sphingomyelinase also results in cell death. Th e demonstration of an increase in intracellular ceramide after choleca lciferol treatment and the ability of ceramide to induce cell death su ggest that the sphingomyelin pathway may be implicated in the effect o f vitamin D metabolites on human glioblastoma cells. Inhibition of cer amide biosynthesis by fumonisin B1 treatment did not alter the dose re sponse curve of HU197 cells to cholecalciferol. Insensitivity to fumon isin B1 together with a decrease in sphingomyelin content after cholec alciferol treatment indicate that activation of sphingomyelinase shoul d be responsible for the increase in intracellular ceramide concentrat ion.