Abstract 3763: Estrogen Mediates the Extracellular Release of the Atheroprotective Heat Shock Protein 27 via Estrogen Receptor Beta
Our laboratory recently demonstrated that Heat Shock Protein 27 (HSP27) is protective against the development of atherosclerosis with a preferential advantage for female mice during the early stages of lesion formation - thereby suggesting an estrogen-mediated phenomenon (Circ Res, in Press). Female HSP27 over-expressing/apoE−/− mice had 10-fold higher serum HSP27 levels compared to males (1805 ± 583 pg/ml vs. 155 ± 88 pg/ml; p≤0.001) and serum HSP27 levels inversely correlated to aortic lesion area (r2=0.78; p≤ 0.001). Hence, we postulated that the release of HSP27 into the extracellular space is a key atheroprotective step. The purpose of this study was to elucidate the role of estrogens in the release of HSP27 using in vitro and in vivo studies. Human macrophages treated in culture with increasing concentrations of 17β-estradiol (0.1–100nM) demonstrated a dose-dependent increase in the extracellular release of HSP27 by Western blot and ELISA (p≤ 0.05 vs. control) - an effect that was blocked with the non-selective estrogen receptor antagonist ICI 182,780. The estrogen receptor beta (ERβ) specific agonist DPN (10nM) caused a 7-fold increase in extracellular HSP27 compared to estrogen alone (p≤ 0.05); an effect that was not observed with the ERα-specific agonist PPT. This estrogen-mediated release of HSP27 was cell-type specific, being present in both human macrophages and aortic endothelial cells but not smooth muscle cells treated with DPN. Membrane co-localization of HSP27, ERβ and E2-BSA (membrane impermeable estrogen bound to bovine serum albumin) was observed in macrophages, and HSP27 and ERβ co-localized with caveolin-1 at the membrane upon treatment with estrogen. Finally, this effect was studied in vivo using female HSP27 overexpressing/apoE−/− and non-overexpressing/apoE−/− mice subjected to ovariectomy before being placed on a high-fat diet for 4 weeks. Lesion analysis demonstrated that the protective effect of HSP27 on atherosclerotic lesion development was abolished with ovariectomy (i.e., in the absence of estrogen). These data indicate that estrogens acting specifically via ERβ at the cell membrane release atheroprotective HSP27 and highlight a novel therapeutic opportunity for attenuating atherogenesis.