Circadian genes are expressed in virtually all cells and tissues, and circadian rhythms influence many bodily processes, including reproductive physiology. The expression of hMTNR1B is suppressed during pregnancy until late in term (much like the oxytocin receptor), at which time it is up-regulated to allow for the nocturnal melatonin/oxytocin synergy, which promotes strong nocturnal contractions. Little is currently known about the regulation of hMNTR1b, nor about its functional significance in the myometrium. We, therefore, aimed to elucidate some of the transcription factors that regulate hMNTR1b gene expression in the human myometrium and to determine if hMNTR1b is under circadian control. In this study, we used immortalized and primary myometrial cells that were assessed for circadian gene expression rhythms using real-time bioluminometry and quantitative PCR. Chromatin immunoprecipitation examined the binding of the clock gene product brain and muscle aryl hydrocarbon receptor nuclear translocator (ARNT)-like protein 1 (BMAL1) to the promoter of the hMTNR1B gene. Overexpression studies tested the role of circadian locomotor output cycles kaput (CLOCK) and its partner BMAL1 in regulating hMTNR1B expression. We confirmed circadian clock gene expression in both immortalized human myometrial cells and primary myometrial cell cultures. We further showed that the hBMAL1 protein binds to an E-box motif in the proximal promoter of the hMTNR1B gene. Overexpression studies demonstrated that the BMAL1/CLOCK complex activates expression of hMTNR1B leading to a circadian rhythm in phase with the E-box driven clock gene hPER2 (Period 2). These results indicate, for the first time, the presence of a functional circadian clock in the human myometrium with the hMTNR1B gene as a clock controlled target. Further investigations could open new vistas for understanding the regulation of uterine contractions and the timing of human labor.
