Luteolin induces apoptosis in human colon cancer cells by inhibiting ceramide traffic and metabolism

Ceramide, a bioactive lipid, plays an important role as anticancer cellular mediator, and is implicated in cell apoptotic death induced by different chemotherapeutic agents. In colon cancers, ceramide content is lower than in normal mucosa tissue, and these reduced levels correlate with the increased tumor-resistance to apoptosis. The dietary luteolin, known as a pro-apoptotic agent in some cancer cells, is able to sensitize colon cancer cells to drug-induced cytotoxicity through unclear mechanisms. In this study we investigated the possible role of ceramide in luteolin effects in Caco-2, a colon cancer cell line. We demonstrated that luteolin induces apoptosis in Caco-2 cells, and that ceramide treatment mimicked this cytotoxic effect in a dose-dependent manner. Moreover, the administration of luteolin induced an inhibition of ceramide metabolism, by increasing the endoplasmic reticulum ceramide level, and reducing its use for the biosynthesis of sphingomyelin and glycosphingolipids in the Golgi apparatus. Fluorescence microscopy studies with a ceramide analogue revealed that luteolin was able to inhibit the endoplasmic reticulum to Golgi traffic of ceramide. We next mimicked the effect of luteolin on inducing ceramide accumulation by using an inhibitor of ceramide metabolism for the biosynthesis of sphingomyelin. We found that this treatment was effective in inducing Caco-2 cell toxicity. These findings demonstrate that ceramide acts as mediator of the apoptotic effect of luteolin, and suggest that luteolin/ceramide transport as targets for novel dietary strategies against colon cancer.