The Modulation of Melatonin in Tumor-Bearing Animals: Underlying Mechanisms and Possible Significance for Prognosis

Earlier clinical studies showed that circulating melatonin is depressed in patients with primary tumors of different histological types including endocrine-dependent (mammary, endometrial, prostate) as well as endocrine-independent (lung, gastric) malignancies. The depression of circulating melatonin is most pronounced in patients with advanced localized primary tumors, where a negative correlation with the size of the tumor is found. In contrast, patients with a high risk to develop breast cancer or with early stages of prostate cancer show a very pronounced secretion of melatonin. Also a considerable number of patients with ovarian cancer exhibit a high melatonin production. The underlying mechanisms involved in the modulation of circulating melatonin in cancer patients is poorly understood and therefore studies on experimental tumor-bearing animals were performed in order to better understand this phenomenon.