Melatonin uptake and growth prevention in rat hepatoma 7288CTC in response to dietary melatonin: melatonin receptor-mediated inhibition of tumor linoleic acid metabolism to the growth signaling molecule 13-hydroxyoctadecadienoic acid and the potential role of phytomelatonin.

Both physiological and pharmacological levels of the pineal hormone melatonin exhibit substantial anticancer activity in tissue-isolated rat hepatoma 7288CTC via melatonin receptor-mediated blockade of tumor uptake of linoleic acid (LA) and its metabolism to the mitogenic signaling molecule 13-hydroxyoctadecadienoic acid (13-HODE). Melatonin is also present in significant amounts in edible plants and is supplied in nutritional supplements. We confirmed the presence of significant quantities of melatonin in 20 varieties of edible plants. In pinealectomized tumor-free rats, 3 weeks of ingestion of either 5 or 50 microg/day of melatonin contained in a semi-purified diet resulted in a dose-dependent elevation in steady-state plasma melatonin levels within the nocturnal physiological range. In pineal-intact tumor-bearing rats, the daily intake of 5 microg/day of melatonin for 3 weeks resulted in an enhanced amplitude and duration of the nocturnal melatonin levels within physiological circulating limits. The nocturnal melatonin amplitude in rats ingesting 500 ng of melatonin/day remained within the physiological range. A dose-related increase in tumor concentrations of melatonin occurred in animals ingesting melatonin from the diet. Perfusion of tumors in situ with physiological, nocturnal blood levels of melatonin resulted in a mean 31% uptake and retention of the melatonin. Chronic ingestion of 50 ng, 500 ng or 5 microg of melatonin/day supplied in a semi-purified 5% corn oil diet led to a significant dose-dependent reduction in the rates of tumor total fatty acid uptake, LA uptake, 13-HODE production and tumor growth. The co-ingestion of melatonin receptor antagonist S20928 completely blocked the effects and prevented the intra-tumoral accumulation of melatonin. Melatonin receptor-mediated suppression of tumor growth, LA uptake and metabolism, and stimulation of tumor melatonin uptake and retention in response to the dietary intake of phytomelatonin from edible plants or melatonin from nutritional supplements, could play an important role in cancer growth prevention.