Anticancer effects of vitamin E forms and their long-chain metabolites via modulation of sphingolipid metabolism
Cancer is one of the leading causes of death. Studies have shown that vitamin E forms including gamma-tocopherol (γT), delta-tocopherol (δT), and gamma-tocotrienol (γTE) exhibited potent anticancer activities in various types of cancer cells. But molecular mechanisms underlying anticancer actions of γTE are not completely understood. 13’-carboxychromanols (13’-COOHs), major fecal excreted long-chain metabolites of vitamin E, have recently been shown to induce apoptosis in liver cancer cells. However, it is not clear whether 13’-COOHs have anticancer effects on other types of cancer. In the current study, we investigated the anticancer effects and mechanisms of γTE and 13’-COOHs; δT-13’-COOH and γTE-13’-COOH, which are metabolites of δT or delta-tocotrienol (δTE), respectively. Like γTE, 13’-COOHs inhibited the growth and induced apoptosis and autophagy in human colon, breast, and pancreatic cancer cells in a time- and dose-dependent manner. In these activities, 13’-COOHs were similar or more potent than γTE, both of which were much stronger than γT and δT. Since we have previously shown that γTE and δT induce prostate cancer cell death by modulation of sphingolipid metabolism, we investigated whether γTE and 13’-COOHs have effects on the levels of sphingolipids in cancer cells using liquid chromatography tandem mass spectrometry. Treatment of human colon cancer HCT-116 cells with γTE or δT-13’-COOH significantly increased in intracellular dihydroceramides (dhCers) and dihydrosphingosine (dhSph), sphingoid bases in de novo synthesis pathway of sphingolipids, but decreased in C16:0-ceramide (Cer) during shorter treatment. During longer treatment, γTE or δT-13’-COOH increased in C16:0- and C18:0-Cers while decreased in SMs. To investigate potential effects on de novo synthesis of sphingolipids, we used 13C3, 15N-labeled L-serine, which condensed with palmitoyl-CoA to form the first sphingolipid intermediate in the de novo synthesis pathway. We found that compared with controls, γTE or δT-13’-COOH treatment increased labeled dhCers and dhSph, but led to decrease in labeled Cers. These results strongly suggest that γTE or δT-13’-COOH inhibit dihydroceramide desaturase (DEGS)-catalyzed reactions and may activate sphingomyelin hydrolysis to enhance Cer levels. Consistently, we found that γTE or δT-13’-COOH inhibited the DEGS activity, while they did not affect DEGS expression. The importance of sphingolipid modulation was further supported by blocking the increase of these sphingolipids, which resulted in a partial counteraction of γTE or 13’-COOHs-induced cell death. In agreement with these cell-based studies, δTE-13’-COOH showed anticancer activities in a preclinical model in mice. In addition, we found that various phytochemicals including curcumin, resveratrol, and epigallocatechin gallate, etc. also modulated sphingolipid metabolism in cancer cells. Overall, our studies demonstrate that γTE and 13’-COOHs have potent anticancer effects by modulating enzyme activities in sphingolipid metabolism.
Jiang, Purdue University.
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