Antitumor activity of fish oils against human lung cancer is associated with altered formation of PGE₂ and PGE₃ and regulation of Akt phosphorylation.

A104

Essential fatty acids (EFA, n-3) have been implicated in lowering the incidence of atherosclerosis, inflammation and tumor cell proliferation. The beneficial effects of EFA are believed to be due in part to selective alteration of arachidonic acid metabolism that involves cyclooxygenase enzymes We previously reported that the anti-proliferative effect of EPA in human lung cancer A549 cells is associated with formation of PGE₃, a COX-2 metabolite of EPA (J. Lipid Res. 45: 1030-1039, 2004). We now report the antitumor efficacy of selectively modified fish oil diets in mice bearing human lung cancer A549 (COX-2 over-expressed) or H1299 (without COX-2) xenograft tumors and have explored probable anti-proliferative mechanisms. Mice were fed either menhaden oil (33% n-3 fatty acids; EPA:DHA 3:2) or 'enriched' fish oil (60% n-3 fatty acids; EPA:DHA 4:1) for 28 days. Both menhaden and 'enriched' fish oil diets significantly inhibited the growth of A549 tumors demonstrated with reduction of tumor weight by 44.8 ± 9.0 % and 58.8 ± 7.4 %, respectively. In contrast, only 'enriched' fish oil diet reduced tumor weight by 58.88 ± 16.8% in the H1299 xenograft tumor model (p < 0.05). Interestingly, both menhaden and 'enriched' fish oil inhibited significantly the formation of PGE₂ in A549 xenograft tumor tissues by 53.2 ± 17.2% and 60.2 ± 7.6%, respectively; and by 45.6 ± 16.9 and 63.6 ± 15.2% in H1299 tumor tissues. Concomitantly, the formation of PGE₃ was detected in tumor tissues from mice fed either menhaden or enriched fish oil with levels of 0.05 ± 0.01 and 0.16 ± 0.02 ng PGE₃/mg protein in A549 xenograft tumor tissues, respectively. In contrast, the formation of PGE₃ in H1299 bearing mice was much less than that from the A549 xenograft tumor tissues. However, the PGE₃ to PGE₂ ratio in A549 and H1299 xenograft tumor tissues from mice fed 'enriched' fish oil was much higher (0.88 in A549, 0.56 in H1299) than that in the menhaden oil group (0.16 in A549, 0.06 in H1299). When A549 cells in culture were exposed to PGE₂ or PGE₃ (0.1 to 1 μM), PGE₂ up-regulated expression of pAkt in a concentration-dependent manner, whereas PGE₃ had no effect on pAkt expression. In order to confirm the role of COX-2 in EPA induced anti-proliferative activity in A549 cells, expression of COX-2 in these cells was reduced by transfecting them with COX-2 siRNA. Intriguingly, when the COX-2 expression was reduced by siRNA and then treated with EPA (3.2 μm to 100 μM), the antiproliferative activity of this fish oil was reduced 10-fold with the IC₅₀ increasing to 62.5 μM in comparison to that of non-transfected A549 cells (IC₅₀, 6.25 μM). Taken together, our preliminary data suggest that the ability of EPA to generate PGE₃ through COX-2 enzyme is critical for EPA mediated tumor growth inhibition; the growth inhibitory effect of EPA may also involve regulation of Akt phosphorylation by PGE3. This study is supported in part by NCI Grant CA102411.