Declining melatonin levels and MT1 receptor expression in aging rats is associated with enhanced mammary tumor growth and decreased sensitivity to melatonin.

Serum melatonin (MLT) levels have been reported to diminish significantly by the 5th and 6th decades of life as the incidence of breast cancer increases. Given MLT's anti-cancer activity, we hypothesize that age-related decline in pineal MLT production leads to enhanced breast cancer development and growth as women age. In this study, we sought to determine whether the growth of tissue-isolated mammary tumors in young, adult, and old female Buffalo rats relates to the age-related changes in MLT and its MT1 receptor. Significant decreases in the peak nighttime serum MLT levels were observed in old as compared to adult and young rats. Significantly diminished nighttime and early morning levels of MT1-melatonin receptors were observed in uteri from old rats compared to adult and young rats. Growth rates in transplanted, tissue-isolated, carcinogen-induced mammary tumors are significantly increased in old rats as compared to adult or young rats. The growth-suppressive actions of exogenous MLT are diminished in old rats compared to adult and young rats. This decrease in tumor response correlates with reduced expression of the MT1 receptor in old as compared to young and adult rats. Thus, enhanced mammary tumor growth is associated with old age and diminished levels of MLT and MT1 receptor during old age, resulting in reduced sensitivity to exogenous MLT. Finally, our studies demonstrate that the tissue-isolated tumor model is viable model system in which to study the role of aging on breast cancer growth.

Age-related decline in melatonin and its MT1 receptor are associated with decreased sensitivity to melatonin and enhanced mammary tumor growth.

The pineal hormone melatonin (MLT) has potent anti-breast cancer activity, its actions are heavily mediated via the MT1 receptor and subsequent modulation of downstream signaling pathways including cAMP/PKA, Erk/MAPK, p38, and Ca2+/calmodulin. Also, via the MT1 pathway, MLT can repress the transcriptional activity of some mitogenic nuclear receptors including ERα, GR, and RORα, while potentiating the activity of other receptors (RARα and RXRα) involved in differentiation, anti-proliferation, and apoptosis. A review of the literature supports the view that MLT, via its MT1 receptor, can suppress all phases of breast cancer including initiation, promotion, and progression. During the fifth and sixth decades of life, the production of MLT diminishes, concurrently with an increase in the incidence of breast cancer. Inasmuch as MLT has been demonstrated to have anti-cancer activity, we hypothesized that there may be a causal link between the reduction in MLT production in the pineal gland and the incidence of breast cancer which increases with age. We designed this study to establish whether a truly inverse relationship exists between tissue-isolated mammary tumor growth in young (2 months), adult (12 months), and old (20 months) female Buffalo rats and the decrease in both MLT and the MT1 receptor with age, such that a causal link could be found. Serum MLT levels were measured in both the light and dark phases. A significant 29% decrease in serum MLT levels, measured at the nocturnal peak, was found in the adult and senescent rats (75% decrease) in comparison to that in young rats. In young rats, the nocturnal pineal gland MLT content exceeded daytime levels by 19-fold compared to a sevenfold increase in old mice. Also, the MT1 receptor was found to be significantly lower in the nighttime and early morning in the senescent rat uterus as compared to uteri from young and adult rats. Analysis of the rate of growth in transplanted, tissue-isolated, mammary tumors induced by N-nitroso-n-methyl-urea (NMU) showed a significant increase in the senescent rats, but not in the young or adult rats Additionally, diminished response to the inhibitory action on tumor growth of exogenous MLT was noted in senescent rats such that tumor growth was suppressed by only 33% compared to 48% and 66% in adult and young rats, respectively. The diminution of the response of tumors to exogenous MLT was found to correlate with reduced MT1 receptor expression in senescent compared to young and adult rats. These data suggest that the observed age-associated enhanced growth of tumors is related to the much reduced levels of MLT and its receptor in aged animals which reduce the sensitivity of tumors to inhibition by exogenous MLT.