The Galphai and Galphaq proteins mediate the effects of melatonin on steroid/thyroid hormone receptor transcriptional activity and breast cancer cell proliferation.

Abstract	Melatonin, via its MT1 receptor, but not the MT2 receptor, can modulate the transcriptional activity of various nuclear receptors - estrogen receptor alpha (ERalpha) and retinoic acid receptor alpha (RARalpha), but not ERbeta- in MCF-7, T47D, and ZR-75-1 human breast cancer cell lines. The anti-proliferative and nuclear receptor modulatory actions of melatonin are mediated via the MT1 G protein-coupled receptor expressed in human breast cancer cells. However, the specific G proteins and associated pathways involved in the nuclear receptor transcriptional regulation by melatonin are not yet clear. Upon activation, the MT1 receptor specifically couples to the G(alphai2), G(alphai3), G(alphaq), and G(alphall) proteins, and via activation of G(alphai2) proteins, melatonin suppresses forskolin-induced 3',5'-cyclic adenosine monophosphate production, while melatonin activation of G(alphaq), is able to inhibit phospholipid hydrolysis and ATP's induction of inositol triphosphate production in MCF-7 breast cancer cells. Employing dominant-negative and dominant-positive) forms of these G proteins, we demonstrate that G(alphai2) proteins mediate the suppression of estrogen-induced ERalpha transcriptional activity by melatonin, while the G(q) protein mediates the enhancement of retinoid-induced RARalpha transcriptional activity by melatonin. However, the growth-inhibitory actions of melatonin are mediated via both G(alphai2) and G(alphaq) proteins.
Authors	Ling Lai, Lin Yuan, Qi Chen, Chunmin Dong, Lulu Mao, Brian Rowan, Tripp Frasch, Steven M Hill
Journal	Journal of pineal research (J Pineal Res) Vol. 45 Issue 4 Pg. 476-88 (Nov 2008) ISSN: 1600-079X [Electronic] England
PMID	18705646 (Publication Type: Journal Article, Research Support, N.I.H., Extramural)
Chemical References	Estrogens Phosphorus Radioisotopes RARA protein, human Receptor, Melatonin, MT1 Receptors, Estrogen Receptors, Retinoic Acid Receptors, Thyroid Hormone Retinoic Acid Receptor alpha Colforsin Cyclic AMP Luciferases GTP-Binding Protein alpha Subunits, Gi-Go Cyclic GMP Melatonin
Topics	Blotting, Western Breast Neoplasms (metabolism, pathology) Cell Line, Tumor Cell Proliferation Colforsin (pharmacology) Cyclic AMP (analysis) Cyclic GMP (analysis) Estrogens (physiology) Female GTP-Binding Protein alpha Subunits, Gi-Go (genetics, physiology) Gene Expression Regulation Humans Immunoprecipitation Luciferases Melatonin (physiology) Phosphorus Radioisotopes Radioimmunoassay Receptor, Melatonin, MT1 (physiology) Receptors, Estrogen (genetics, metabolism) Receptors, Retinoic Acid (genetics, metabolism) Receptors, Thyroid Hormone (genetics) Retinoic Acid Receptor alpha Transcription, Genetic Transfection

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