Estradiol may be cardioprotective; however, the mechanisms involved remain
unclear. Recent findings that estradiol attenuates neointima formation in e
strogen receptor knockout mice suggest that the cardioprotective effects of
estradiol may be mediated through estrogen receptor-independent mechanisms
. Because 2-methoxyestradiol, an endogenous metabolite of estradiol with no
affinity for estrogen receptors, is more potent than estradiol in inhibiti
ng vascular smooth muscle cell growth, it is feasible that 2-methoxyestradi
ol mediates in part the cardioprotective effects of estradiol. To address t
his hypothesis, we examined the kinetics of 2-methoxyestradiol synthesis in
vascular smooth muscle cells and endothelial cells. In human aortic smooth
muscle cells, the V-max, K-m, and V-max/K-m ratio values for conversion of
2-hydroxyestradiol to 2-methoxyestradiol were 19 +/-0.69 pmol . min(-1) pe
r 10(6) cells, 0.52 +/-0.085 mu mol/L, and 44 +/-4.9 pmol . min(-1) . mu mo
l/L per 10(6) cells, respectively. In human coronary artery vascular smooth
muscle cells, the V-max, K-m, and V-max/K-m ratio values for conversion of
2-hyelroxyeshadiol to 2-methoxyestradiol were 16 +/-0.59 pmol . min(-1) pe
r 10(6) cells, 0.23 +/-0.011 mu mol/L, and 69 +/-3.6 pmol . min(-1) . mu mo
l/L per 10(6) cells, respectively (all values significantly different compa
red with human aortic smooth muscle cells). Also, in human aortic versus co
ronary artery endothelial cells, the V-max (33 +/-0.24 versus 22 +/-0.33 pm
ol . min(-1) per 10(6) cells, respectively), K-m (0.20 +/-0.010 versus 0.09
9 +/-0.014 mu mol/L, respectively), and V-max/K-m (163 +/-7.7 versus 243 +/
- 41 pmol . min(-1) . mu mol/L per 10(6) cells, respectively) values were s
ignificantly different, Our results indicate that vascular smooth muscle an
d endothelial cells effectively metabolize 2-hydroxyestradiol to 2-methoxye
stradiol. The lower K-m and higher V-max/K-m ratio of human coronary versus
aortic cells indicate a faster rate of local metabolism of 2-hydroxyestrad
iol to 2-methoxyestradiol in the coronary circulation at low levels of 2-hy
droxyestradiol.