The pineal hormone, melatonin, has repeatedly been shown to inhibit carcinogen-induced mammary tumorigenesis. Several laboratories have shown that melatonin significantly inhibits the proliferation of estrogen receptor (ER)-positive, but not ER-negative human breast cancer cells. The applicant has established that melatonin decreases overall ER levels in MCF-7 breast tumor cells through its suppressive effects on ER gene transcription. Melatonin's suppression of ER gene transcription appears to be at least one of the key events by which melatonin inhibits breast cancer cell growth; however, the pathway(s) through which melatonin regulates the transcription of the ER gene has not yet been determined. The recent cloning of two types of melatonin receptors, the membrane bound -protein-coupled melatonin receptors (Mel1a and Mel1b) and retinoic acid Z or orphan receptors (RZR-beta/ROR-alpha) provides fresh insight into the possible mechanisms by which melatonin inhibits breast tumor cell growth and regulates ER expression. Dr. Hill's most recent studies, described in this proposal, provide evidence that there is cross-talk between the melatonin and other cellular signaling pathways (insulin and epidermal growth factor) to modulate ER function. Considering the central and important mitogenic role of estrogen in breast cancer, the fact that melatonin is able to regulate both ER function and expression attests to its potential usefulness as a therapeutic agent. With the availability of cDNA probes to the ER, Mel1a, Mel1b, ROR alpha and various estrogen regulated genes, antibodies to the Mel1a receptor and ER, and a 3.5-kb fragment of the upstream regulatory region of the ER gene cloned from MCF-7 breast cancer cells, Dr. Hill plans to expand his analysis of the mechanism(s) involved in melatonin's regulation of breast cancer cell growth by examining its effects on ER function and expression. The major issues to be addressed in this application are: (1) to determine which melatonin receptors, Mel1a and/or ROR-alpha, are expressed in breast cancer cells and whether they can be modulated by melatonin or estrogen; (2) to determine the cellular signaling pathway(s) transducing melatonin's modulation of ER function; and (3) to determine the molecular mechanism(s) by which melatonin interacts with the ER gene 5'-flanking region to repress ER gene transcription. The recent increase in melatonin use as a sleep aid by the general public, combined with melatonin's effects on the estrogen-response pathway, particularly in breast cancer, but also in other endocrine-responsive neoplasms, further substantiates the immediate need for a clearer understanding of the effects and mechanism(s) of melatonin's actions.
