Abstract
Melatonin, the circadian nighttime neurohormone, and eicosapentaenoic acid (EPA) and docosahexaenoic acids (DHA), which are omega-3 fatty acids (FA) found in high concentrations in fish oil (FO) and plants, abrogate the oncogenic effects of linoleic acid (LA), an omega-6 FA, on the growth of rodent tumors and human breast, prostate, and head and neck squamous cell carcinoma (HNSCC) xenografts in vivo. Here we determined and compared the long-term effects of these inhibitory agents on tumor regression and LA uptake and metabolism to the mitogenic agent 13-[S]-hydroxyoctadecadienoic acid (13-[S]-HODE) in human prostate cancer 3 (PC3) and FaDu HNSCC xenografts in tumor-bearing male nude rats. Rats in this study were split into 3 groups and fed one of 2 diets: one diet containing 5% corn oil (CO, high LA), 5% CO oil and melatonin (2 μg/mL) or an alternative diet 5% FO (low LA). Rats whose diet contained melatonin had a faster rate of regression of PC3 prostate cancer xenografts than those receiving the FO diet, while both in the melatonin and FO groups induced the same rate of regression of HNSCC xenografts. The results also demonstrated that dietary intake of melatonin or FO significantly inhibited tumor LA uptake, cAMP content, 13-[S]-HODE formation, [³H]-thymidine incorporation into tumor DNA, and tumor DNA content. Therefore, long-term ingestion of either melatonin or FO can induce regression of PC3 prostate and HNSCC xenografts via a mechanism involving the suppression of LA uptake and metabolism by the tumor cells.