Ascorbic acid enhances 17 β-estradiol-mediated inhibition of oxidized low density lipoprotein formation

Postmenopausal women who use estrogen appear to be protected from coronary heart disease (CHD). Studies have demonstrated that estrogen can lower low-density lipoprotein (LDL) levels and the antioxidant activity of 17 beta-estradiol can prevent the oxidation of this LDL. Ascorbic acid is regarded as a major hydrophylic antioxidant, however, its impact on the prevention of CHD has yet to be clearly demonstrated. Modified low density lipoprotein (LDL-) is an important marker of LDL oxidation in vivo, since it contributes to the oxidative susceptibility of low density lipoprotein, and at physiological levels displays pro-inflammatory and cytotoxic properties. Previously we showed that women taking estrogen replacement therapy have lower LDL- levels along with lower predisposition of the LDL, to oxidize. In this study, we evaluated the potential action of 17 beta-estradiol (E-2) in combination with ascorbic acid (AA) measured on the basis of LDL oxidative susceptibility in vitro and in the presence of cultured cells. High concentrations of E-2 were able to inhibit LDL oxidation, whereas in the presence of ascorbic acid nano- to picomolar levels of E-2 were sufficient to suppress LDL oxidation (P < 0.05). Preconditioning male aortic endothelial cells (RAEC) with 5 ng/ml of E-2 (E(2)RAEC) reduced the formation of LDL- (P < 0.005), and a more extensive inhibition was found in the presence of AA (P < 0.0001). Interestingly, E-2 enhanced the uptake of LDL in the absence or presence of AA, however, this was not seen for the uptake of LDL-. These results provide the first evidence that ascorbic acid can enhance the antioxidant effect of E-2 by preventing LDL oxidation by copper ions or cells. The cytoprotective and antiatherogenic effect of E-2 appears to involve a reduction in the extent of oxidized LDL formation and uptake. The enhanced activity of E-2 in the presence of ascorbate indicates that the antioxidant and antiatherosclerosis activity of E-2 may occur at concentrations within the physiological range. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.