Removal of lipid-rich lipoproteins by the liver

Studies were performed to determine the mechanism of hepatic removal of a cholesterol-rich @-migrating lipoprotein. This fraction, designated IDL,, was isolated from the serum of cholesterol-fed diabetic rats by ultracentrifugation at d 1.006- 1.03 g/ml and contained apoproteins B, E,.C, and A-I. When 1251-IDL, ('251-labeled IDL,) was injected into normal chow-fed rats, 40% of the radioactivity was cleared from the plasma within 5 minutes with slight additional removal during the next 25 minutes. The rapid removal phase was due to the clearance of apoB-containing lipoproteins. The slow removal phase was due to transfer of apoA-I and C-apoproteins to HDL which has a considerably slower rate of turnover. The in vivo clearance of total '251-IDL, radioactivity was enhanced by pretreatment of normal rats with l7a-ethinyl estradiol. This appeared to be associated with lack of transfer of apoA-I and C-apoproteins to HDL, and the removal of these apoproteins along with the apoB- containing lipoproteins. Treatment of rats with 17a-ethinyl es- tradiol did not result in an increased rate of removal of 1251- IDL, when their livers were perfused and this suggests that the removal of IDL, is not mediated by the LDL (B, E) receptor whose activity is stimulated by estradiol administration-Ar- beeny, C. M., and H. A. Mer. Removal of lipid-rich lipoproteins by the liver.). Lipid Res. 1983. 24: 1457-1467.