17β-Estradiol modulates hMT₁ melatonin receptor function

Estrogen modulates expression and function of G-protein-coupled receptors. The goal of this study was to assess the effect of 17β-estradiol (10 nM) exposure for 1 (E1) or 6 (E6) days on density and function of hMT₁ and hMT₂ melatonin receptors expressed in Chinese hamster ovary (CHO) cells (CHO-MT₁/CHO-MT₂ cells). This strain of CHO cells expressed both estrogen receptor alpha and beta mRNAs, as determined by RT-PCR amplification. 17β-Estradiol treatment did not modify the affinity of either receptor; however, it significantly increased the density of 2-[ ¹²⁵I]iodomelatonin-binding sites in CHO-MT₂ cells. 17β-Estradiol treatment (1-6 days) did not affect the potency of melatonin to inhibit forskolin stimulation of cAMP formation through activation of either MT₁ or MT₂ receptors; however, it significantly attenuated the maximal inhibition of forskolin-stimulated cAMP formation induced by melatonin (0.01-1 μM) in CHO-MT₁ cells. Melatonin stimulation of [³⁵S]GTPγS binding to CHO-MT₁ cell membranes was also attenuated following estradiol treatment. The inverse agonist luzindole reduced basal [³⁵S]GTPγS binding in estradiol-treated cells but not in control CHO-MT₁ cells, suggesting that estradiol promotes constitutive activity of MT₁ melatonin receptors. We suggest that 17β-estradiol differentially affects MT₁ and MT₂ melatonin receptor functions, attenuates melatonin responses through activation of MT₁ receptors, and increases the MT₂ receptors density.