Previous studies on human breast cancer patients showed a decline in circulating melatonin levels corresponding to primary tumor growth and an increase when relapse occurred. The aim of the current investigation was to study in an experimental model possible mechanisms involved. Inbred female F344 Fischer rats were used for serial passages derived from a chemically induced mammary adenocarcinoma. Animals with slow-growing carcinosarcomas at passage 2 showed a significant elevation of nocturnal urinary melatonin (23.00–07.00 h; +50%, p < 0.05) and a nominal increase in plasma melatonin (+41%; 02.00–03.00 h). By contrast, these parameters were significantly depressed in animals with fast-growing sarcomas (urinary melatonin: –22%, p < 0.025; plasma melatonin: –56%, p < 0.01). At passage 2 nocturnal pineal N-acetylserotonin (02.00–03.00 h) was significantly enhanced (+62%, p < 0.05) probably due to an increased activity of serotonin-N-acetyltransferase (SNAT, +45%), the rate-limiting step of pineal melatonin biosynthesis converting serotonin to N-acetylserotonin. The activation of SNAT may be due to a stimulation of the sympathetic nervous system (urinary noradrenaline; NA: +243%, p < 0.005) when the cellular immune system responded towards tumor growth (urinary biopterin, +214%, p < 0.005). At passage 12 SNAT and N-acetylserotonin were unaffected but a depletion of plasma tryptophan (–34%, p < 0.0001), the precursor amino acid of melatonin, was found. The marginal decline in pineal serotonin (–18%, p < 0.05) disputes that the drastic depletion in circulating melatonin (–56%, p < 0.01) can be exclusively explained by a reduced availability of tryptophan. Therefore, the involvement of an additional mechanism has to be postulated, such as a degradation of melatonin via indoleamine 2,3-dioxygenase, an extrahepatic enzyme which has been detected in tumor tissue and is related to tryptophan 2,3-dioxygenase (TDO). TDO occurs only in the liver, is highly specific for L-tryptophan and is induced by glucocorticoids which would account for the observed depletion of plasma tryptophan resulting from a tumor-associated activation of the hypothalamo-pituitary-adrenal axis (urinary corticosterone +208%, p < 0.01). These findings present first explanations for the previously observed modulation of melatonin levels in cancer patients but also illustrate the high degree of complexity of mechanisms involved in the interactions between tumor growth and the immunoneuroendocrine system.
Nocturnal (23.00-07.00 h) urinary melatonin and total biopterin (tBI; after acidic oxidation of reduced biopterins) were analyzed during the growth of two passages of a mammary tumor line in female F344 Fischer rats. In addition, nocturnal (02.00–03.00 h) peak concentrations of pineal melatonin in plasma were analyzed when tumors had reached comparable average tumor volumes of 25-30 cm3. Since tetrahydrobiopterin (BH4) is produced by murine macrophages in response to interferon-γ released by activated T lymphocytes, measurements of tBI can serve to estimate the state of cellular immunity. At passage 2, a slow-growing localized carcinosarcoma, tBI showed a progressing increase during tumor growth reaching more than 200% (p < 0.05–0.005) of controls by the end of the experiment. Urinary and plasma melatonin were elevated by 30–50% (p < 0.05) and 42% respectively. At passage 12, a fast-growing metastasizing sarcoma, a depression of about 20–30% was found for tBI (p < 0.05) and urinary melatonin (p < 0.025); plasma melatonin was depleted by 70% (p < 0.005). Parallel changes of both parameters at each tumor passage indicate a close link between the pineal hormone melatonin and cellular immunity. The opposite trends observed at the two passages indicate a clear stimulation of the immune system and the pineal gland at early but inhibition at advanced stages of cancer.
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