Effect of exogenous estrogens on catabolism of VLDL in cholesterol-fed rabbits

To determine the metabolic mechanism of the hypolipidemic response to estrogen in cholesterol-fed rabbits, very low-density lipoprotein (VLDL) apolipoprotein B (apoB) turnover studies were conducted in cholesterol-fed animals with or without estrogen treatment. Autologous VLDL apoB had a more rapid fractional catabolic rate (FCR) in estrogen-treated than in untreated animals, but there was no difference in the radioactivity appearing in the intermediate-(IDL) and low- (LDL) density lipoproteins. Similar differences in the FCR were observed when isologous VLDL from donors in the opposite group was injected, suggesting that estrogen treatment in cholesterol-fed rabbits accelerated the catabolism of cholesterol- and apoE-rich lipoproteins by a mechanism that is not dependent on its conversion to LDL. Furthermore, VLDL apoB from normal untreated donor animals was catabolized more rapidly in the estrogen-treated animals, but most of the radioactivity appeared in LDL in both groups. These observations suggest that estrogen treatment of cholesterol-fed rabbits affected only the efficiency but not the completeness of catabolism of normal VLDL. Thus the catabolism of vLDL in cholesterol-fed animals treated with or without estrogen depended on the composition of VLDL injected and the pool size of plasma VLDL, which was reduced by estrogen treatment.