Inhibitory Effects of Specific Apolipoprotein C-III Isoforms on the Binding of Triglyceride-rich Lipoproteins to the Lipolysis-stimulated Receptor

Abstract

ApoC-III overexpression in mice results in severe hypertriglyceridemia due primarily to a delay in the clearance of triglyceride-rich lipoproteins. We have, in primary cultures of rat hepatocytes, characterized a lipolysis-stimulated receptor (LSR). The apparent number of LSR that are available on rat liver plasma membranes is negatively correlated with plasma triglyceride concentrations measured in the fed state. We therefore proposed that the primary physiological role of the LSR is to contribute to the cellular uptake of triglyceriderich lipoproteins. We have now tested the effect of apoC-III on the binding of triglyceride-rich lipoproteins to LSR. Supplementation of 125 I-very low density lipoprotein (VLDL) with apoC-III inhibited the LSR-mediated binding, internalization, and degradation of 125 I-VLDL in primary cultures of rat hepatocytes. Studies using isolated rat liver plasma membranes showed that enrichment of human VLDL and chylomicrons with synthetic or purified human apoC-III decreased their binding to the LSR by about 40%. Supplementation of triglyceride-rich lipoproteins under the same conditions with human apoC-II had no such inhibitory effect, despite the fact that this apoprotein bound as efficiently as apoC-III to these particles.Preincubation of LDL with apoC-III did not modify its binding to LSR. Partitioning studies using 125 I-apoC-III showed that this lack of effect was due to apoC-III's inability to efficiently associate with LDL. Purified human apoC-III 1 was as efficient as the synthetic nonsialylated form of apoC-III in inhibiting binding of VLDL to LSR. However, despite a 2-fold greater binding of apoC-III 2 to VLDL, this isoform was a less efficient inhibitor of the binding of VLDL to LSR than apoC-III 1 or nonsialylated apoC-III. Desialylation of apoC-III 2 by treatment with neuraminidase increased the inhibition of VLDL binding to LSR to a level similar to that observed with apoC-III 1 and nonsialylated apoC-III. We propose that apoC-III regulates in part the rate of removal of triglyceride-rich particles by inhibiting their binding to the LSR, and that the level of inhibition is determined by the degree of apoC-III sialylation.Apolipoprotein (apo)C-III 1 has been shown to modulate the plasma triglyceride (TG) concentrations. ApoC-III is a 79-amino acid polypeptide in the plasma that readily exchanges between the different lipoprotein fractions (1). In subjects with normal plasma lipid levels, 30% of apoC-III is distributed among triglyceride-rich lipoproteins (TGRL), i.e. chylomicrons and very low density lipoprotein (VLDL), and about 60% is bound to high density lipoprotein (1).Biochemical studies showed that apoC-III is an inhibitor of lipase activity (2-4), and it was established in the early 1980's that supplementation of small chylomicrons or apoE-containing triglyceride emulsions with apoC-III inhibited their uptake in perfused rat livers (5-7). An inhibitory effect was also observed with apoC-I and C-II, but was less pronounced and not consistently observed (5, 6). Genetic...

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