Luteolin induces an alteration of the Ceramide/Sphingosine-1-phosphate ratio leading to apoptosis in human colon cancer cells

Ceramide (Cer) and sphingosine-1-phosphate (S1P) have emerged as bioactive signaling molecules, with ceramide as activator of cell death pathways, and S1P as mitogenic, survival agent. Alterations of the S1P/Cer ratio, called "sphingolipid rheostat", influence cell fate. Luteolin, a natural flavonoid, has anticancer properties in several cancer cell lines. However, its anti-cancer effects and its molecular mechanisms in colon cancer remain unknown. We investigated the possible role of the sphingolipid rheostat in the effects of Luteolin on Caco-2 cells, a human colon cancer cell line. We found that Luteolin induces apoptosis of Caco-2 cells, and exposure of cells to Cer analogues mimics this toxic effect. Metabolic experiments with radioactive sphingosine demonstrated that Luteolin induces an alteration of sphingosine metabolism. The flavonoid increases cellular Cer by inhibiting its utilization for the biosynthesis of sphingomyelin and glycosphingolipids. Results obtained with fluorescent Cer and brefeldin A, indicate that Luteolin impairs the traffic of Cer from the endoplasmic reticulum to Golgi apparatus. After Luteolin treatment, we found a significant increase of endogenous long-chain Cer levels. Exposure to an inhibitor of sphingomyelin synthase that augments the intracellular Cer level results in a dose-dependent apoptotic death of Caco-2 cells. Luteolin is able to inhibit the metabolic formation of S1P, thus reducing its intracellular levels. S1P administration to Caco-2 cells is able to reduce the cytotoxic effect of Luteolin. All this demonstrates that Luteolin induces apoptosis in colon cancer cells by altering the sphingolipid rheostat; this suggests the potential benefit of Luteolin in the treatment of colon cancer.