Pregnancy Reprograms Large-Conductance Ca 2 +-Activated K + Channel in Uterine Arteries Roles of Ten-Eleven Translocation Methylcytosine Dioxygenase 1 – Mediated Active Demethylation

The large-conductance Ca-activated K (BKCa) channel is essential in the control of vascular tone. The BK Ca channel in vascular smooth muscle cells (VSMCs) exists as an assembly of pore-forming α, and auxiliary β1 subunits, encoded by KCNMA1 and KCNMB1, respectively. Activation of the BK Ca channel in VSMCs is primarily stimulated by an increase in intracellular Ca concentrations. The β1 subunit enhances BK Ca channel activity by increasing the intrinsic Ca sensitivity of the α subunit. Because of its large conductance and high density in VSMCs, activation of the BK Ca channel exhibits an immense effect on membrane potential. Therefore, the BK Ca channel functions as a key determinant in the regulation of vascular tone. Lines of evidence have suggested a critical importance of the β1 subunit in the regulation of BK Ca channel function. Mice deficient in KCNMB1 displayed impaired vasorelaxation and elevated blood pressure. In addition, a gain-of-function mutation of the β1 subunit is associated with a low prevalence of diastolic hypertension in humans. The uterine circulation undergoes significant hemodynamic changes during the course of pregnancy, and uterine blood flow increases dramatically to meet the demand of fetal growth and development. Maladaptation and inadequate perfusion of the uterus and placenta during pregnancy are associated with high incidence of clinical complications, including preeclampsia and fetal intrauterine growth restriction. The upregulation of BK Ca channel β1 subunit expression and enhanced BK Ca channel function plays a key role in the reduction of uterine vascular tone and the hemodynamic adaptation of uterine circulation during pregnancy. Of importance, steroid hormone estrogen was found to be the initiator of pregnancy-induced BK Ca channel β1 subunit upregulation. In addition to pregnancy, a variety of physiological and pathophysiological conditions such as hypertension, aging, diabetes mellitus, chronic hypoxia, shock, Abstract—The large-conductance Ca-activated K (BK Ca ) channel is of critical importance in pregnancy-mediated increase in uterine artery vasodilation and blood flow. The present study tested the hypothesis that active DNA demethylation plays a key role in pregnancy-induced reprogramming and upregulation of BK Ca channel β1 subunit (BKβ1) in uterine arteries. Uterine arteries were isolated from nonpregnant and near-term pregnant sheep. Pregnancy significantly increased the expression of ten-eleven translocation methylcytosine dioxygenase 1 (TET1) in uterine arteries. A half-palindromic estrogen response element was identified at the TET1 promoter, and estrogen treatment increased TET1 promoter activity and TET1 expression in uterine arteries. In accordance, pregnancy and steroid hormone treatment resulted in demethylation of BKβ1 promoter by increasing 5-hydroxymethylcytosine and decreasing 5-methylcytosine at the CpG in the Sp1 -380 binding site that is of critical importance in the regulation of the promoter activity and BKβ1 expression. Inhibition of TET1 with fumarate significantly decreased BKβ1 expression in uterine arteries of pregnant animals and blocked steroid hormone–induced upregulation of BKβ1. Functionally, fumarate treatment inhibited pregnancy and steroid hormone– induced increases in BK Ca channel current density and BK Ca channel–mediated relaxations. In addition, fumarate blocked pregnancy and steroid hormone–induced decrease in pressure-dependent myogenic tone of the uterine artery. The results demonstrate a novel mechanism of estrogen-mediated active DNA demethylation in reprogramming of BK Ca channel expression and function in the adaption of uterine circulation during pregnancy. (Hypertension. 2017;69:1181-1191. DOI: 10.1161/HYPERTENSIONAHA.117.09059.) • Online Data Supplement