Objective: The protective effect of estrogen against early atherosclerosis in animal models is well documented, but the mechanisms responsible for this effect are not well understood. The earliest recognizable event in the pathogenesis of atherosclerosis is an increased recruitment of macrophages into the arterial subendothelium. Macrophages first play a protective role by removing low-density lipoproteins, but when the cholesterol is in excess, macrophages are converted into foam cells and form atheromas. Recent human and animal data indicate that the recruitment of macrophages to the arterial wall is mediated by monocyte chemotactic protein-1 (MCP-1). We hypothesized that one of the mechanisms of estrogen's protective effect against atherosclerosis may be the down-regulation of MCP-1 expression in the arterial wall.
Design: Human coronary artery smooth muscle cells were replicated to confluence in smooth muscle cell basal medium supplemented with growth factors and 5% fetal bovine serum. Before each experiment, cells were incubated for 24 h with phenol red-free medium containing 5% charcoal-stripped calf serum, and then they were treated with various concentrations of 17beta-estradiol as well as selective estrogen receptor (ER) modulators, raloxifene and tamoxifen. MCP-1 messenger ribonucleic acid (mRNA) levels were quantified by Northern blots. MCP-1 protein was quantified using an enzyme-linked immunosorbent assay. ER expression was evaluated by reverse transcriptase-polymerase chain reaction.
Results: Human coronary artery smooth muscle cells expressed MCP-1 mRNA and produced MCP-1 protein. Estradiol induced up to 40% inhibition in mRNA expression at concentrations 10-9 M and higher. Raloxifene and tamoxifen also resulted in an inhibition, but the inhibition was less than when induced by estradiol. Estradiol also inhibited the MCP-1 protein production in a concentration-dependent manner (p < 0.05). Coronary smooth muscle cells expressed both ERalpha and ERbeta.
Conclusion: Our findings suggest that one of the mechanisms by which estrogen prevents atherosclerosis is by down-regulating MCP-1 expression, thus decreasing macrophage recruitment to the arterial wall.