Breast cancer is frequently a hormone-dependent tumour, and several studies have suggested that the pineal gland hormone melatonin may influence the growth and development of this malignancy. Subcutaneous injections of melatonin have been shown to inhibit, and pinealectomy to enhance, the development of dimethyl benz(a)anthracene (DMBA)-induced mammmary tumours in rats. Use of the psychotropic drug thorazine, which increases plasma melatonin levels, has been associated with a decreased incidence of breast cancer in psychiatric patients. Calcification of the pineal gland has been correlated with an increased incidence of breast cancer in women. While the mechanism by which melatonin influences these tumours is unknown, both human breast cancer and DMBA-induced tumour contain oestrogen receptors (ER) and respond to changes in the oestrogen milieu. We therefore wondered whether melatonin might be altering ER binding activity of these tumours. We report here that in vitro incubation of MCF-7 human breast cancer cells with melatonin in physiological conditions increased the cytoplasmic and nuclear ER activity of these cells within 40 min, giving no change in the equilibrium dissociation constant (K(d)) of the receptor. This induction was blocked by cycloheximide, and thus requires continuous protein synthesis. The modulation of ER binding activity of breast cancer by another endogenous hormone may be important for understanding the behaviour and treatment of this disease, and may provide insight into the factors regulating the synthesis and metabolism of steroid hormone receptors.
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