Abstract-Estrogen therapy increases plasma HDL levels, which may reduce cardiovascular risk in postmenopausal women. The mechanism of action of estrogen in influencing various steps in hepatic HDL and apolipoprotein (apo) A-I synthesis and secretion are not fully understood. In this study, we have used the human hepatoblastoma cell line (Hep G2) as an in vitro model system to delineate the effect of estradiol on multiple regulatory steps involved in hepatic HDL metabolism. Incubation of Hep G2 cells with estradiol resulted in the following statistically significant findings: (1) increased accumulation of apoA-I in the medium without affecting uptake/removal of radiolabeled HDL-protein; (2) accelerated incorporation of [ 3 H]leucine into apoA-I; (3) selective increase in [ 3 H]leucine incorporation into lipoprotein (LP) A-I but not LP A-IϩA-II HDL particles (HDL particles without and with apoA-II, respectively); (4) increased ability of apoA-I-containing particles to efflux cholesterol from fibroblasts; (5) stimulated steady state apoA-I but not apoA-II mRNA expression; and (6) increased newly transcribed apoA-I mRNA message without effect on apoA-I mRNA half-life. The data indicate that estradiol stimulates newly transcribed hepatic apoA-I mRNA, resulting in a selective increase in LP A-I, a subfraction of HDL that is associated with decreased atherosclerotic cardiovascular disease, especially in premenopausal women. (Arterioscler Thromb Vasc Biol. 1998;18:999-1006.) Key Words: estrogen Ⅲ high density lipoproteins Ⅲ apolipoprotein A-I Ⅲ cardiovascular disease A therosclerotic cardiovascular disease is the leading cause of mortality among postmenopausal women. Abnormalities in lipid and lipoprotein metabolism (eg, increased LDL and decreased HDL levels) commonly seen in postmenopausal women have been attributed to the increased coronary heart disease-related mortality in these individuals.1-3 Because the decline in estrogen levels is the primary metabolic alteration observed in postmenopausal women, it has been thought that endogenous concentrations of estrogen may have fundamental roles in lipoprotein-mediated development of atherosclerotic coronary heart disease. Clinical studies have indicated that estrogen therapy significantly elevated plasma HDL levels and decreased LDL concentrations, suggesting a favorable effect on the plasma lipoprotein profile. 4 -6 Recent comparative studies by meta-analysis showed that postmenopausal women on estrogen therapy have a lower relative risk of coronary events than postmenopausal women who are not on estrogen therapy. 7,8 Immunoaffinity techniques have revealed that HDL particles exist in 2 major classes: LP A-I and LP A-IϩA-II (ie, without and with apoA-II, respectively).9 ApoA-I and apoA-II are major proteins of HDL. There is considerable evidence to suggest that LP A-I is more importantly linked to decreased atherosclerosis risk. Premenopausal women have higher levels of LP A-I than age-matched men. 10 Oral estrogen replacement therapy in postmenopausal women was sho...