Estradiol 17β inhibition of LDL oxidation and endothelial cell cytotoxicity is opposed by progestins to different degrees

Progestins oppose the effects of estrogens in many biological systems, but it is not known if progestins oppose the antioxidant effects of estrogen and to differing degrees. To test these questions, the effects of various sex steroids on LDL oxidation and cytotoxicity were studied in the absence or presence of endothelial cells. Freshly isolated LDL was incubated in the presence of Cu++ in the absence or presence of cultured bovine aortic endothelial cells in phenol red-free medium and without or with hormones in 0.5% ethanol. The hormones included 17β-estradiol (E2), progesterone (Pg), norgestimate (NGM), levonorgestrel (LNG), and medroxyprogesterone acetate (MPA). LDL oxidation was measured as formation of conjugated dienes, lipid peroxides, and TBARS, and cyotoxicity by tetrazolium salt reduction (MTT reduction). Progestins diminished conjugated diene lag phase, accelerated lipid peroxide and TBARS production in the absence and presence of cells and accelerated cytotoxicity. When E2 and progestin were incubated with cells at a molar ratio of 1:5, lipid peroxides were reduced from baseline by E2 alone 31%, E2/Pg 29%, E2/NGM 16%, E2/LNG 9% (all P<0.05 or more) and E2/MPA 8% (ns) (E2 or E2/Pg>E2/NGM, E2/LNG and E2/MPA [P<0.001]; E2/NGM>E2/LNG or E2/MPA [P<0.05]). MTT reduction followed a similar gradient, greatest with E2 alone, least with E2/MPA. Conclusions: Progestins promote LDL oxidation and, conjointly, endothelial cell cytotoxicity. Progestins oppose the antioxidant and cytoprotective effects of estrogen when given in combination. MPA and LNG have the strongest prooxidant and cytotoxic effects, which may limit the cardiovascular benefit of estrogen during combined administration in vivo.