Effect of hormone replacement therapy on nitric oxide bioactivity and monocyte chemoattractant protein-1 levels

Abstract

Background: Vascular inflammation plays an important role in the pathogenesis of atherosclerosis. We investigated the effect of hormone replacement therapy (HRT) on vasomotor function and monocyte chemoattractant protein (MCP)-1 levels, an important serological marker of inflammation. Methods: We administered micronized progesterone (MP) 200 mg for 10 days with conjugated equine estrogen (CEE) 0.625 mg for 25 days and remaining 5 days off cyclically during 2 months to 20 healthy postmenopausal women (PMW). We measured NO bioactivity and plasma levels of MCP-1 before and after HRT in 20 PMW. And we measured plasma levels of MCP-1 in each 20 subjects of premenopausal women, men<50, and men>50 years, respectively. Results: MP combined with CEE significantly improved the percent flow-mediated dilator response to hyperemia relative to baseline measurements (P<0.001). PMW receiving HRT had lower levels of MCP-1 than those not receiving HRT (121±38 versus 146±44 pg/ml, P<0.001). In all comparisons, subjects with high estrogen status had significantly lower MCP-1 levels than subjects with low estrogen status (P<0.001 by ANOVA). Premenopausal women had lower levels of MCP-1 than men of a similar age (106±14 versus 164±40 pg/ml, P<0.001). PMW not receiving HRT had similar levels of MCP-1 compared with men of a similar age (146±44 versus 143±29 pg/ml, P=0.816). Premenopausal women had markedly lower levels of MCP-1 than PMW not receiving HRT (106±14 versus 146±44 pg/ml, P=0.001). PMW receiving HRT had similar levels of MCP-1 compared with premenopausal women (121±38 versus 106±14 pg/ml, P=0.323). Conclusion: These findings might provide at least a partial explanation for the protection against cardiovascular disease experienced by premenopausal women, and the loss of that protection following menopause.

Introduction

Although extensive evidence indicates that estrogen is responsible for the markedly decreased cardiovascular risk of premenopausal women [1], the mechanism through which estrogen might exert its protective effect has not been adequately explained: e.g. improving fibrinolysis [2] or reducing soluble cell adhesion molecules [3], [4], [5].

Vascular inflammation plays an important role in the pathogenesis of atherosclerosis and may contribute to increase the risk of myocardial infarction [6]. Monocyte chemoattractant protein (MCP)-1 — having powerful monocyte chemotactic activity for monocytes and thus, correlated with vascular macrophage accumulation — had been demonstrated in primates and human atherosclerotic arteries by immunohistochemistry [7], [8]. MCP-1 expression increased the progression of atherosclerosis by increasing both macrophage numbers and oxidized lipid accumulation in apolipoprotein E-deficient mice [9]. The absence of MCP-1 provided dramatic protection from macrophage recruitment and atherosclerotic lesion formation in apo B transgenic mice [10]. Further, one recent study demonstrated that plasma MCP-1 levels were elevated in patients with acute coronary syndrome and enalapril therapy significantly reduced plasma MCP-1 levels, compared with placebo [11].

A nuclear transcription factor, NF-kB activates transcription of genes encoding chemoattractant factors such as MCP-1 and macrophage stimulatory factor that attract monocytes into the vessel wall. Inflammatory cells such as monocytes, once activated and attracted into the vessel wall, have a variety of proatherogenic effects, including the release of reactive oxygen species, growth factors, prothrombotic factors (such as tissue factor), and transformation into foam cells upon unregulated uptake of oxidized low-density lipoprotein (LDL) via the scavenger receptor [12].

In endothelial cell culture preparations, nitric oxide (NO) donors and antioxidant compounds reduced synthesis of proinflammatory proteins by inhibiting transcriptional activation of target genes [13], [14]. Estrogen increased NO production in endothelial cell culture [15], [16]. We have previously shown that unopposed estrogen improved NO bioactivity in hypercholesterolemic or healthy postmenopausal women (PMW) [3], [4]. Further, estradiol suppressed MCP-1 protein and mRNA expression in ovariectomized rabbits or murine macrophages in culture [17], [18]. Therapies that increase NO bioactivity may reduce synthesis of proinflammatory proteins and prevent or attenuate the transcription and expression of MCP-1 on the endothelial cell surface. In other words, therapies that reduce vascular inflammation may reduce cardiovascular risk. Although inflammation of the arterial wall in the vicinity of atherosclerotic plaques is commonly found at necropsy, this manner of demonstration is hardly useful for identifying patients at risk for myocardial infarction. Thus, we investigated the effect of hormone replacement therapy (HRT) on vasomotor function and plasma MCP-1 levels, an important serological marker of inflammation potentially affected by NO-potentiating properties in PMW.