17β-Estradiol Prevents Fatty Streak Formation in Apolipoprotein E–Deficient Mice

Abstract

The reality of the atheroprotective effect of estrogens is still a matter of debate, and its unknown mechanisms could involve favorable changes in blood lipids and lipoproteins and/or direct action at the level of the arterial wall. We used the recently developed animal model of atherosclerosis constituted by apolipoprotein E-deficient mice in an attempt to clarify these issues. Male and female animals, fed a low-fat chow diet, were treated with increasing doses of 17 beta-estradiol (E2) after castration and compared with testosterone treated and uncastrated (intact) animals. Total serum cholesterol, LDL-cholesterol, and HDL-cholesterol concentrations decreased under E2 treatment in each sex and were weakly correlated with lesion area. However, a highly significant correlation between lesion area and serum E2 levels also suggested a direct action of E2 on cells of the vascular wall. A dose-response curve analysis revealed that these activities were sex-dependent, with females being nearly twice as sensitive to E2 as males. It also revealed that the atheroprotective activity was recruited at higher E2 concentrations than those needed by other E2 target tissues such as uterus or functions such as apoA-1 and LDL production and/or clearance rates.

Estradiol increases urethral tone through the local inhibition of neuronal nitric oxide synthase expression

“…This apparent discrepancy between physiological and supraestrus levels of E 2 was previously reported by our group in other extrareproductive effects, such as atherosclerosis prevention (10) and modulation of invariant natural killer T cells (11).…”

“…Two weeks later, the ovariectomized animals were subcutaneously implanted with either a placebo or a 60-day time release E 2 pellet (0.1 mg E 2 releasing 80 g ⅐ kg Ϫ1 ⅐ day Ϫ1 ; Innovative Research of America, Sarasota, FL). This dose was previously reported to induce plasma levels of 0.3 nM (80 pg/ml), a concentration encountered during pregnancy and ϳ10 times higher than that found during the estrus cycle (10). Six weeks later, the mice received either an intraperitoneal injection of 7-nitroindazole (Acros Organics, Morris Plains, NJ), a potent selective competitive inhibitor of nNOS with no significant effect on endothelial NO synthase (eNOS) in mice (21), sonicated in peanut oil at a dose of 50 mg/kg or the vehicle alone.…”

Predominant Role of Nuclear Versus Membrane Estrogen Receptor α in Arterial Protection: Implications for Estrogen Receptor α Modulation in Cardiovascular Prevention/Safety

“…All experimental data demonstrate major atheroprotective actions of estrogens. In particular, 17β‐estradiol strongly prevents lipid deposition in mouse models of atherosclerosis: apolipoprotein E–deficient6, 7 and low‐density lipoprotein receptor–deficient (LDLr −/− )8 mice. In addition, in several experimental models of hypertension, ovariectomy exacerbates, whereas estrogen replacement attenuates, the course of hypertension9, 10 and reduces arterial stiffening associated with hypertension or aging 11.…”

The AF-1 activation-function of ERα may be dispensable to mediate the effect of estradiol on endothelial NO production in mice

“…Although lipoprotein changes occur in premenopausal women and postmenopausal women receiving hormone replacement therapy, in large-scale studies with adjustments for multiple risk factors, only 25-50% of the beneficial effects of estrogen seem to be caused by lipoprotein effects (4). Several large animal species including non-human primate (5), swine (6), and rabbits (7) and, more recently, apolipoprotein E-deficient mice (8,9) have been used to assess these nonlipid effects of estrogen. They showed that effects on endothelium (10,11) may contribute to the cardioprotection observed in humans.…”

“…America) provide controlled continuous release of a constant level of hormone over a period of 60 days (i.e., 80 g͞kg per day). This E 2 dose has previously been defined as adequate for a maximal effect on vascular functions in female mice (9,19). Mice were killed by an overdose of ketamine after 2 weeks of treatment.…”