Characterization of the effects of free fatty acid receptor 4 (FFA4) agonists on human cancer cells
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Dietary omega-3 fatty acids, such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), have many beneficial effects on human health. These include promising inhibitory effects on cancer cell growth and invasion. The goal of this project was to delineate the mechanisms involved in this inhibitory effect. Omega-3 fatty acids act via several potential molecular mechanisms that include their action as agonists at free fatty acid receptors (FFARs), which are G protein-coupled receptors. In this project, we show that two FFARs activated by omega-3 fatty acids, FFA1 and FFA4 (formerly GPR120), are expressed in the human prostate cancer cell lines DU145 and PC-3. EPA completely inhibits lysophosphatidic acid (LPA)- and epidermal growth factor (EGF)-mediated proliferation and migration in prostate cancer cells. TUG-891, a synthetic and highly selective FFA4 agonist, mimics the inhibitory effects of EPA. The inhibitory effects are readily reversed upon removal of TUG-891 or EPA. Knockdown of FFA4, through the utilization of silencing RNA, abolished the effects of both EPA and TUG-891 in these cells. Positive cross-talk has been shown to occur between LPA receptors and EGF receptors. We therefore hypothesized that FFA4 inhibits the LPA receptor LPA1, via beta-arrestin-2, to inhibit EGF-induced proliferation and migration. Consistent with this hypothesis, when LPA1 is knocked down, FFA4 agonists no longer inhibit EGF-induced proliferation and migration. In addition, we found that beta-arrestin-2 is essential for both LPA- and EGF-mediated signaling in prostate cancer cells. The inhibitory effects of FFAR agonists were also explored in two human breast cancer cell lines, MCF-7 and MDA-MB-231. The results suggest that FFA1, not FFA4, is more likely to be responsible for the inhibitory effects of omega-3 fatty acids in these cells.The results of these studies establish that there are specific receptor(s) responsible for mediating the effects of omega-3 fatty acids on human cancer cells. This finding will facilitate the development of targeted pharmaceutical therapies for prevention and treatment of prostate and breast cancer. These results show for the first time the importance of the FFAR family as receptors for omega-3 fatty acids in human cancer cells.