Omega 3 fatty acids chemosensitize drug resistant colon cancer cells by downregulating cholesterol synthesis and altering lipid rafts environment

Background. High levels of cholesterol in plasmamembrane and in particular in lipid rafts are critical for the activity of P-glycoprotein (Pgp) and multidrug resistance related protein 1 (MRP1), two drug efflux transporters that determine a multidrug resistant phenotype. It has been reported that omega 3 fatty acids prevent colon cancers and modulate cholesterol homeostasis in gut. The mechanisms by which they affect cholesterol homeostasis, and the effects on membrane environment and transporters activity have not been investigated yet. Aim. We aim to clarify how omega 3 fatty acids modulate the cholesterol metabolism, the membrane properties and the activity of drug efflux transporters in colon cancer. Methods. Human chemosensitive colon cancer HT29 cells and their chemoresistant counterpart HT29-dx were treated with omega 3 docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Results. HT29-dx cells had higher plasmamembrane cholesterol and cholesterol synthesis than HT29 cells: the phenotype of HT29-dx cells was due to the lower expression of the oncosuppressor Trc8, a E3 ligase that ubiquitinates 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCoAR). DHA and EPA lowered membrane- and lipid rafts-associated cholesterol and decreased cholesterol synthesis in HT29-dx cells, by down-regulating HMGCoAR protein in a SREBP2- and LXR-independent way. Of note, they activated Trc8 and restored HMGCoAR ubiquitination. DHA and EPA were also incorporated in membrane and lipid rafts, where they decreased the saturated/unsaturated fatty acids ratio. In consequence of the altered membrane environment, DHA and EPA disassociated Pgp and MRP1 from lipid rafts, decreased transporters activity and restored the antitumor effects of several chemotherapeutic drugs in resistant cells. Conclusions. We propose omega 3 fatty acids as potential chemosensitizer agents in colon cancer, thanks to their effects on cholesterol homeostasis and plasmamembrane microenvironment.