Estrogens are cardioprotective agents with many beneficial cardiovascular effects. One unusual action is the direct in vitro antioxidant protection of low-density lipoprotein (LDL). Oxidized LDL is a known atherogenic agent but LDL protection in vitro requires uM concentrations of estrogens. Since this is over 1,000 times the concentration in blood, this action is usually considered to be biologically irrelevant. Recently, this view has been questioned because it has been shown that the LDL isolated from plasma that has been previously incubated in vitro with physiological concentrations of estradiol (E2) is protected from oxidation. This increased sensitivity is caused by lecithin: cholesterol acyltransferase (LCAT) esterification of E2. The acyltransferase produces a family of fatty acid esters of E2, lipoidal derivatives of estradiol (LE2), known to circulate in female blood in low concentration. We propose to investigate the hypothesis that LDL protection, by E2 and its metabolites, is regulated through LCAT esterification. Very little is known about the LCAT esterification of steroids, especially estrogens. We will investigate the structural requirements for steroid substrates to determine whether other estrogens, including metabolites, especially inactive estrogens, are esterified thus, producing potent antioxidants; and whether other steroids can regulate LDL oxidation by inhibiting the esterification of E2. Model systems will be studied to assess the antioxidant mechanism of LE2. We will design and synthesize non-estrogenic alkylhydroxy substituted phenols as substrates for LCAT and inhibitors of LDL oxidation. We will determine whether exogenous E2-esters can uncover an oxidative resistance of LDL related to gender or menopausal status; whether estrogen-esters can provide sensitive antioxidant protection in vivo using LDL receptor-null and LCAT-null mice as models. The antioxidant action of LCAT esterified estrogens illuminates their physiology and a novel estrogenic effect. These experiments will contribute insights into a previously unknown non-genomic action of estrogens and provide new therapeutic agents for cardiovascular disease.
