In vivo metabolism of apolipoproteins A-I and E in patients with abetalipoproteinemia: implications for the roles of apolipoproteins B and E in HDL metabolism

Abstract

The metabolism of high density lipoproteins (HDL) is tightly linked to the metabolism of apoB-containing lipoproteins through the exchange and transfer of lipids and apolipoproteins within the plasma compartment. Abetalipoproteinemia (ABL), a genetic disease in which apoB is absent from the plasma and HDL are the sole plasma lipoproteins, is a model for the investigation of HDL metabolism without modification by apoB-containing lipoproteins. Apolipoproteins A-I and E are two of the major apolipoproteins in HDL. Plasma apoA-I levels, but not apoE levels, have been reported to be decreased in patients with ABL. Furthermore, HDL from ABL patients is enriched in apoE compared with normal subjects. The purpose of the present study was: 1) to elucidate the metabolic basis of the low apoA-I levels in ABL; 2) to determine whether in vivo apoE production rates are normal in the absence of apoB-lipoprotein secretion; and 3) to test the hypothesis that apoE influences apoA-I and HDL catabolism in ABL. 131I-labeled apoA-I and 125I-labeled apoE were reassociated with autologous lipoproteins and injected into two unrelated ABL patients and control subjects. The mean residence time of apoA-I in ABL (2.4 days) was significantly decreased by nearly 50% compared with control subjects (4.7 +/- 0.6 days). ApoA-I production rates were also significantly decreased by 40% in ABL (7.1 mg/kg-d) compared with control subjects (11.8 +/- 1.7 mg/kg-d). The mean residence time of apoE in ABL (0.50 days) was somewhat shorter than that of control subjects (0.66 +/- 0.15 days), whereas the mean apoE production rate in ABL (2.14 mg/kg-d) was not substantially different from that of control subjects (1.55 +/- 0.62 mg/kg-d). HDL subfractions LpA-I and LpA-I:A-II were isolated using immunoaffinity chromatography. In contrast to the normal metabolism, apoA-I in LpA-I:A-II particles was catabolized at a faster rate than apoA-I in LpA-I, accounting for the greater decrease of plasma LpA-I:A-II relative to LpA-I in the ABL patients. HDL subfractions with and without apoE were also isolated using anti-apoE immunoaffinity chromatography. Labeled apoA-I in apoE-containing HDL was catabolized faster than that in HDL without apoE. Among the three different forms of apoE, the apoE monomer was catabolized at the fastest rate, the apoE homodimer at an intermediate rate, and the apoE-A-II heterodimer had the slowest rate of catabolism.(ABSTRACT TRUNCATED AT 400 WORDS)