Ceramide acts as a mediator of the antiproliferative and pro-death effects of the natural flavonoid luteolin in human colon cancer cells

The sphingolipid ceramide plays an important role as anticancer cellular mediator and is implicated in chemotherapy-induced apoptosis by different agents. Human colon cancers (the second most leading cause of cancer-related deaths in the western countries) have only half the ceramide content of normal tissue, and these low ceramide levels have been correlated with their increased resistance to apoptosis. As luteolin (3',4',5,7-tetrahydroxyflavone), a common dietary flavonoid, exhibits powerful pro-apoptotic and anti-cancer properties, and sensitizes colon cancer cells to therapeutic-induced cytotoxicity, we addressed the question if a relationship exists between the unknown anticancer mechanism of luteolin and ceramide in colon cancer cells. Using the human colon cancer cell line Caco-2 as cell model, we initially found that luteolin administration was associated to the inhibition of cell proliferation and, at higher doses, to cell death. In further experiments, we investigated the ceramide metabolism in Caco-2 cells and the possible effects of luteolin on it, using radiolabeled sphingosine as precursor. The results showed that Caco-2 cells efficiently and rapidly generate both ceramide and sphingomyelin from sphingosine, and that luteolin treatment induces a dose-dependent reduction of the sphingomyelin biosynthesis, associated to a significant increase of the cellular ceramide. This evidence let us hypothesize that ceramide may be involved in the anti-proliferative and toxic effect of luteolin. On these promises, we evaluated the effect of ceramide on cell survival. We found that either the administration of ceramide analogues or the inhibition of sphingomyelin biosynthesis were able to induce Caco-2 cell death, thus demonstrating that ceramide is able to mimic the cytotoxic effect of luteolin. Overall, these results demonstrate that luteolin can act as an antiproliferative and pro-death agent in Caco-2 cells by up-regulating cellular ceramide through inhibition of its metabolism to sphingomyelin, and suggest a potential role of luteolin/ceramide as anti-cancer agent in colon malignancies.