Differential Regulation of Proteasome-Dependent Estrogen Receptor α and β Turnover in Cultured Human Uterine Artery Endothelial Cells

Estrogen-induced loss of estrogen receptor (ER) α expression limits estrogen responsiveness in many target cells. However, whether such a mechanism contributes to changes in vascular endothelial ERα and/or ERβ levels is unclear. Using RT-PCR assays, we did not find any regulation of ERα or ERβ mRNA expression in human uterine artery endothelial cell (HUAEC) nuclear extracts on stimulation with 17β-estradiol for 1 or 2 h. By contrast, Western analysis on HUAEC extracts revealed that 17β-estradiol was capable of down-regulating both ERα and ERβ protein starting 1 h after treatment, an effect that can be blocked by pretreatment with tamoxifen as well as with the proteasome inhibitor lactacystin. The proteolysis inhibitors insulin, cycloheximide, and puromycin impede ERα, but not ERβ, turnover. Ubiquitin, but not its competitive inhibitor methyl-ubiquitin, induces rapid turnover of both ERs in a cell-free system of MCF-7 and HUAEC extracts. We, thus, propose the existence of estrogen-induced ER degradation th...