Shifting the LDL-receptor paradigm in familial hypercholesterolemia: novel insights from recent kinetic studies of apolipoprotein B-100 metabolism

Clinical and pathophysiological evidence supporting the safety of extremely low LDL levels-The zero-LDL hypothesis

While the impact of very low concentrations of low-density lipoprotein cholesterol (LDL-C) on cardiovascular prevention is very reassuring, it is intriguing to know what effect these extremely low LDL-C concentrations have on lipid homoeostasis. The evidence supporting the safety of extremely low LDL levels comes from genetic studies and clinical drug trials. Individuals with lifelong low LDL levels due to mutations in genes associated with increased LDL-LDL receptor (LDLR) activity reveal no safety issues. Patients achieving extremely low LDL levels in the IMPROVE-IT and FOURIER, and the PROFICIO and ODYSSEY programs seem not to have an increased prevalence of adverse effects. The main concern regarding extremely low LDL-C plasma concentrations is the adequacy of the supply of cholesterol, and other molecules, to peripheral tissues. However, LDL proteomic and kinetic studies reaffirm that LDL is the final product of endogenous lipoprotein metabolism. Four of 5 LDL particles are cleared through the LDL-LDLR pathway in the liver. Given that mammalian cells have no enzymatic systems to degrade cholesterol, the LDL-LDLR pathway is the main mechanism for removal of cholesterol from the body. Our focus, therefore, is to review, from a physiological perspective, why such extremely low LDL-C concentrations do not appear to be detrimental. We suggest that extremely low LDL-C levels due to increased LDLR activity may be a surrogate of adequate LDL-LDLR pathway function.

Genetics and kinetics of familial hypercholesterolemia, with the special focus on FH-(Marburg) p.W556R

OBJECTIVE

Familial hypercholesterolemia (FH) is an autosomal dominant inherited disorder, caused by mutations in the low density lipoprotein receptor (LDLR) gene. FH is characterized by elevated plasma LDL cholesterol, premature atherosclerosis and high risk of premature myocardial infarction. Extended work has been done to understand both, the primary genetic defect as well as the in vivo kinetic consequences of this disease. Both approaches, genetics and kinetics, are challenging but also fruitful approaches for a better understanding of this devastating disease. For this we reviewed the recent literature and used our in vitro and in vivo data on one of the most frequently occurring types of FH, the FH(Marburg) p.W556R.

METHODS

To identify the primary genetic defect of the FH(Marburg) we used denaturing gradient gel electrophoresis (DGGE) mutation analysis. In vivo kinetic studies were performed in a heterozygote FH(Marburg) subject and in 5 healthy control subjects utilizing a stable isotope tracer kinetic approach with 3D-leucine.

RESULTS

DGGE screening of the LDLR gene identified a tryptophan (W) to arginine (R) substitution at residue 556 (p.W556R) in the fifth conserved YWTD repeat of the LDLR-beta-propeller in FH(Marburg). In vivo kinetic studies in a heterozygote FH subject for FH(Marburg) and in 5 healthy control subjects demonstrated a severe decrease in LDL FCR and a mild increase of LDL PR in FH compared to healthy controls.

CONCLUSIONS

The LDLR mutation p.W556R is a frequent and severe defect for FH. This defect has a major influence on the in vivo lipoprotein kinetics and lipid levels. In a heterozygote FH patient we found a dual defect for the increase in LDL cholesterol, namely a decrease in the fractional catabolic rate (FCR) of LDL but also an increase in LDL production rate (PR). By this a well defined, single genetic defect may have a series of different in vivo metabolic consequences which could be used for potential therapeutic approaches to this disease.

Apolipoprotein A1 Is a Stronger Prognostic Marker Than Are HDL and LDL Cholesterol for Cardiovascular Disease and Mortality in Elderly Men

The aim of this study was to compare apolipoprotein A1 (ApoA1) and B (ApoB) with high-density lipoprotein cholesterol (HDL-C) and low-density lipoprotein cholesterol (LDL-C) as markers for cardiovascular mortality and morbidity in elderly men. We analyzed serum ApoA1, ApoB, total cholesterol, HDL-C, and LDL-C in a group of 77-year-old men (n = 785). The results were correlated with data from the Swedish cause of death registry. Receiver-operating characteristic curves showed that, of the studied serum markers, ApoA1 was the best predictor for ischemic heart disease mortality (area under the curve = 0.724, 95% confidence interval, 0.691-0.755). There were also significant correlations between the apolipoproteins and other known risk markers for cardiovascular disease such as triglycerides, high-sensitivity C-reactive protein (hsCRP), and cystatin C. Serum ApoA1 is a better risk marker than are ApoB, ApoB/ApoA1 ratio, HDL-C, and LDL-C for cardiovascular disease and mortality in elderly men.

Plasma kinetics of free and esterified cholesterol in familial hypercholesterolemia: Effects of simvastatin

The objective of this study was to evaluate the kinetics of both free and esterified forms of cholesterol contained in a emulsion that binds to LDL receptors (LDE) in subjects with heterozygous familial hypercholesterolemia (FH), and the same subjects under the effects of high-dose simvastatin treatment, as compared with a control normolipidemic group (NL). Twenty-one FH patients (44.0 +/- 13.0 yr, 12 females, LDL cholesterol levels 6.93 +/- 1.60 mmol/L) and 22 normolipidemic patients (44.0 +/- 15.0, 10 females, LDL cholesterol levels 3.15 +/- 0.62 mmol/L) were injected intravenously with 14C-cholesteryl ester and 3H-cholesterol. FH patients were also evaluated after 2 mon of 40 or 80 mg/d simvastatin treatment, and plasma samples were collected over 24 h to determine the residence time (RT, in h) of both LDE labels, expressed as the median (25%; 75%). The RT of both 14C-cholesteryl ester and 3H-cholesterol were greater in FH than in NL [FH: 36.0 (20.5; 1191.0), NL: 17.0 (12.0-62.5), P = 0.015; and FH: 52.0 (30.0; 1515.0); NL 20.5 (14.0-30.0) P < 0.0001]. Treatment reduced LDL cholesterol by 36% (P < 0.0001), RT of 14C-cholesteryl ester by 49% (P = 0.0029 vs. baseline), and 3H-cholesterol RT by 44% (P = 0.019 vs. baseline). After treatment, the RT values of 14C-cholesteryl ester in the FH group approached the NL values (P = 0.58), but the RT of 3H-cholesterol was still greater than those for the NL group (P = 0.01). The removal of LDE cholesteryl esters and free cholesterol was delayed in FH patients. Treatment with a high dose of simvastatin normalized the removal of cholesterol esters but not the removal of free cholesterol.

Effects of ezetimibe on plasma lipoproteins in severely hypercholesterolemic patients treated with regular LDL-apheresis and statins

Ezetimibe, a cholesterol absorption inhibitor, can be combined with statins to lower LDL-cholesterol. We evaluated additional ezetimibe (10 mg/day) in a placebo-controlled, double blind, randomized cross-over study in 20 patients (age 56+/-9 years, m:f 10:10, BMI 27.5+/-4.0 kg/m(2)) suffering from severe hypercholesterolemia and CHD who were treated by statins and regular LDL-apheresis. Lipoproteins (cholesterol, triglycerides, LDL-cholesterol, HDL-cholesterol, VLDL-cholesterol, VLDL-triglycerides, lipoprotein(a)) were determined twice (before and during ezetimibe/placebo, each given for 5 weeks), dietary behaviour was analyzed once (3-days-protocol) during each treatment period. During ezetimibe the mean (+/-S.D.) preapheresis LDL-cholesterol concentration decreased from 159+/-26 mg/dl (4.11+/-0.67 mmol/l) to 133+/-28 mg/dl (3.44+/-0.72 mmol/l) (-16+/-11%, P<0.0001, Wilcoxon test) and the postapheresis LDL-cholesterol from 51+/-9 mg/dl (1.32+/-0.23 mmol/l) to 43+/-8 mg/dl (1.11+/-0.21 mmol/l) (-14+/-25%, P<0.05), while there was no significant change during placebo. Mean VLDL-cholesterol fell by 18+/-71% (P<0.05) during ezetimibe and was not significantly changed by placebo (+19+/-70%). Furthermore, during ezetimibe less plasma volume was treated (3725+/-1560 versus 3870+/-1549 ml, P<0.05). Ezetimibe had no effect on pre- and postapheresis triglyceride, HDL-cholesterol and lipoprotein(a) levels. The effect of ezetimibe was independent of the statin dose. Dietary behaviour did not change and no side effects were observed. Thus, in patients with severe LDL-hypercholesterolemia and CHD the addition of ezetimibe to intensive lipid lowering therapy (statins and LDL-apheresis) resulted in a further, clinically significant decrease of LDL-cholesterol.

Hepatic secretion of small lipoprotein particles in apobec-1-/- mice is regulated by the LDL receptor

Recent studies have examined the role of the LDL receptor (LDLR) in regulating murine hepatic lipoprotein production and apolipoprotein B (apoB) secretion, with divergent conclusions from in vivo versus in vitro approaches. We have re-examined this question, both in vivo and in vitro, using apobec-1-/- mice to model the pattern of human hepatic apoB-100 secretion. Hepatic triglyceride production in vivo (using Triton WR-1339) was unchanged in wild-type (WT) C57BL/6, apobec-1-/-, ldlr-/-, and [apobec-1-/-, ldlr-/-] mice, while apoB-100 production (using [35S]methionine incorporation) was increased > 2-fold in [apobec-1-/-, ldlr-/-] mice. Although > 90% of newly synthesized apoB floated within the d < 1.006 fraction of serum from all genotypes, fast-performance liquid chromatography separation revealed that nascent triglyceride-rich particles from [apobec-1-/-, ldlr-/-] mice, but not WT, apobec-1-/-, or ldlr-/- mice, distributed into smaller (intermediate and LDL-sized) particles. Studies in isolated hepatocytes from these different genotypes confirmed secretion of smaller particles exclusively from [apobec-1-/-, ldlr-/-] mice, and pulse-chase analysis demonstrated increased secretion of apoB-100 with virtual elimination of posttranslational degradation. These results directly support the suggestion that the LDLR regulates hepatic apoB-100 production and modulates secretion of small, triglyceride-rich particles, both in vivo and in vitro.