Epigenetic Upregulation of Large-Conductance Ca2+Activated K+ Channel Expression in Uterine Vascular Adaptation to Pregnancy

Our previous study demonstrated that pregnancy increased large-conductance Ca2+-activated potassium channel beta 1 subunit (BK beta 1) expression and large-conductance Ca2+-activated potassium channel activity in uterine arteries, which were abrogated by chronic hypoxia. The present study tested the hypothesis that promoter methylation/demethylation is a key mechanism in epigenetic reprogramming of BK beta 1 expression patterns in uterine arteries. Ovine BK beta 1 promoter of 2315 bp spanning from -2211 to +104 of the transcription start site was cloned, and an Sp1(-380) binding site that contains CpG dinucleotide in its core binding sequences was identified. Site-directed deletion of the Sp1 site significantly decreased the BK beta 1 promoter activity. Estrogen receptor-a bound to the Sp1 site through tethering to Sp1 and upregulated the expression of BK beta 1. The Sp1 binding site at BK beta 1 promoter was highly methylated in uterine arteries of nonpregnant sheep, and methylation inhibited transcription factor binding and BK beta 1 promoter activity. Pregnancy caused a significant decrease in CpG methylation at the Sp1 binding site and increased Sp1 binding to the BK beta 1 promoter and BK beta 1 mRNA abundance. Chronic hypoxia during gestation abrogated this pregnancy-induced demethylation and upregulation of BK beta 1 expression. The results provide evidence of a novel mechanism of promoter demethylation in pregnancy-induced reprogramming of large-conductance Ca2+-activated potassium channel expression and function in uterine arteries and suggest new insights of epigenetic mechanisms linking gestational hypoxia to aberrant uteroplacental circulation and increased risk of preeclampsia.