Estrogen has been postulated to exert direct anti-atherogenic effects via binding to
estrogen receptors (ERs) in vascular smooth muscle cells (VSMCs). Therefore, we believe it is important to examine the status of ER expression in the human cardiovascular system and its disorders. In this study, we first evaluated the relative abundance of
messenger RNA (
mRNA) of both ER subtypes (
ERalpha and
ERbeta) in the human aorta using reverse transcription followed by quantitative polymerase chain reaction (RT-qPCR). We then examined the immunolocalization of both ERs in VSMCs of human atherosclerotic lesions. In order to examine which ER subtype was associated with the anti-atherogenic effects of
estrogen, we examined the effects of
estrogen in two VSMC cell lines, one positive only for
ERalpha and the other positive only for
ERbeta. The relative abundance of mRNAs for both ERs was higher in female aorta with a mild degree of
atherosclerosis than in female aorta with a severe degree of
atherosclerosis (P < 0.05). In addition, the number of
ERalpha and/or
ERbeta double positive cells in the
neointima was higher in female aorta with a mild degree of
atherosclerosis than in female aorta with severe
atherosclerosis (P < 0.05). Our in vitro study found that
estradiol was able to significantly inhibit the proliferation of
ERalpha positive VSMCs but not
ERbeta positive VSMCs (P < 0.05). Moreover,
estradiol was found to significantly suppress
proliferating cell nuclear antigen (
PCNA)
mRNA levels in
ERalpha positive VSMCs compared to that of
ERbeta positive VSMCs, consistent with the findings of cell proliferation. Results from this study suggest that
estrogens can inhibit the proliferation of VSMCs through
ERalpha, especially in pre-menopausal women. Our study also indicates that decreased levels of ER, especially
ERalpha, in the female atherosclerotic
neointima may be associated with progression of atherosclerotic changes.