Abstract
We have used adenovirus-mediated gene transfer in mice to investigate low density lipoprotein receptor (LDLR) and LDLR-related protein (LRP)-independent mechanisms that control the metabolism of chylomicron and very low density lipoprotein (VLDL) remnants in vivo. Overexpression of receptor-associated protein (RAP) in mice that lack both LRP and LDLR (MX1cre(+)LRP(flox/flox)LDLR(-/-)) in their livers elicited a marked hypertriglyceridemia in addition to the pre-existing hypercholesterolemia in these animals, resulting in a shift in the distribution of plasma lipids from LDL-sized lipoproteins to large VLDL-sized particles. This dramatic increase in plasma lipids was not due to a RAP-mediated inhibition of a unknown hepatic high affinity binding site involved in lipoprotein metabolism, because no RAP binding could be detected in livers of MX1cre(+)LRP(flox/flox)LDLR(-/-) mice using both membrane binding studies and ligand blotting experiments. Remarkably, RAP overexpression also resulted in a 7-fold increase (from 13.6 to 95.6 ng/ml) of circulating, but largely inactive, lipoprotein lipase (LPL). In contrast, plasma hepatic lipase levels and activity were unaffected. In vitro studies showed that RAP binds to LPL with high affinity (K(d) = 5 nM) but does not affect its catalytic activity, in vitro or in vivo. Our findings suggest that an extrahepatic RAP-sensitive process that is independent of the LDLR or LRP is involved in metabolism of triglyceride-rich lipoproteins. There, RAP may affect the functional maturation of LPL, thus causing the accumulation of triglyceride-rich lipoproteins in the circulation.
Highlights
Hypertriglyceridemia, combined with the accumulation of remnant lipoproteins in the circulation, is a major risk factor for atherosclerosis and coronary artery disease
Our findings suggest that an extrahepatic receptor-associated protein (RAP)-sensitive process that is independent of the low density lipoprotein receptor (LDLR) or LDL receptor-related protein (LRP) is involved in metabolism of triglyceride-rich lipoproteins
Plasma Lipid and Lipoprotein Levels after Adenovirus-mediated Gene Transfer of RAP in low density lipoprotein (LDL) Receptor and/or LRPdeficient Mice—We have previously reported the use of the Cre/loxP recombination system to achieve inducible disruption of the LRP gene in adult mice
Summary
VLDL, very low density lipoproteins; LDL, low density lipoproteins; apo, apolipoprotein; LDLR, LDL receptor; LRP, LDL receptor-related protein; RAP, receptor-associated protein; Ad, adenoviral vector; LPL, lipoprotein lipase; HL, hepatic lipase; PFU, plaque-forming units; PBS, phosphate-buffered saline; BSA, bovine serum albumin; ELISA, enzyme-linked immunosorbent assay; HRP, horseradish peroxidase; -Gal, -galactosidase. As in the RAP overexpression experiments [14], LRP gene disruption in LDL receptor-deficient mice did cause the accumulation of cholesterol-rich, apoB48-containing remnants. These remnants were smaller and contained significantly less triglyceride than those that accumulated in the RAP overexpressing animals. These observations suggest that a novel and hitherto unsuspected RAP-sensitive process is involved in the metabolism of triglyceride-rich lipoproteins. The biochemical basis underlying this process may play a role in some of the complex genetic traits that cause hypertriglyceridemia in man
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