Cell proliferation, apoptosis and accumulation of lipid droplets in U937-1 cells incubated with eicosapentaenoic acid

The monocytic cell line U937-1 was cultured in the presence of eicosapentaenoic acid (20:5, n-3) (EPA) or oleic acid (18:1, n-9) (OA). EPA caused a dose-dependent inhibition of cell proliferation, whereas OA had no effect. At the highest EPA concentrations, 120 and 240 mu M, inhibition of cell proliferation was accompanied by initiation of apoptosis. A concentration of 60 mu M EPA caused a 35 % reduction in cell proliferation without inducing apoptosis, and was therefore used for further studies. Addition of antioxidants or inhibitors of eicosanoid synthesis had no influence on the reduced cell proliferation after EPA treatment. The inhibition required continuous presence of EPA in the incubation medium as the cells resumed a normal proliferation rate when they were placed in EPA-free medium. The inhibition of proliferation was not accompanied by differentiation into macrophage-like cells, as expression of serglycin and the ability to perform respiratory burst was unaffected by EPA. Expression of CD23 mRNA increased when the cells were incubated with EPA, but to a smaller extent than after retinoic acid (RA) or PMA treatment. Furthermore, expression of the monocytic differentiation markers CD36 and CD68 was lower in cells treated with EPA or OA when compared with untreated cells. The cell cycle distribution of U937-1 cells was similar in cells incubated with EPA or PMA, whereas RA-treated cells accumulated in the G(1) phase. Side scatter increased in cells incubated with EPA and OA, which was ascribed to an accumulation of lipid droplets after examination of the cells by electron microscopy. The number of droplets per cell was higher in cells exposed to EPA than OA, The cellular triacylglycerol (TAG) increased 5,5- and 15.5-fold after incubation with OA and EPA respectively. No difference in the cellular content of cholesterol compared with untreated cells was observed. The TAG fraction in EPA-treated cells contained high amounts of EPA and docosapentaenoic acid and minor amounts of docosahexaenoic acid, whereas OA-treated cells had high levels of OA in the TAG. In cells incubated with a sulphur-substituted EPA, only minor effects on cell proliferation and no accumulation of cellular TAG were observed. These findings may indicate the existence of other mechanisms for regulation of cell behaviour by very-long-chain polyunsaturated n-3 fatty acids than the well established lipid peroxide and eicosanoid pathways.