Carlos A. Aguilar-Salinas

Metabolic modes of action of the statins in the hyperlipoproteinemias

Conflicting results have been published during the past few years regarding the physiologic modes of action of the hydroxymethylglutaryl-CoA (HMG-CoA) reductase inhibitors, generally referred to as statins, using standard doses. Three mechanisms have been described: increased LDL catabolic rate, increased removal of LDL precursors resulting in decreased LDL production and decreased VLDL production. The physiologic effects of statins seem to depend on the underlying pathology of the disorders under therapy. More recent data using either the more potent atorvastatin or larger doses of previously available statins (e.g. simvastatin 80-160 mg/day), suggest that both the potency of the statins and the underlying pathopHysiology are important in determining the predominant physiologic responses of patients. To understand physiologic responses more completely, drug-dose-physiologic response curves of apo B kinetics in various groups of patients are needed. Simultaneous studies of apo B, triglycerides and cholesterol metabolism are also needed and are currently feasible.

Apoprotein B-100 Production Is Decreased in Subjects Heterozygous for Truncations of Apoprotein B

Among individuals who are heterozygous for familial hypobetalipoproteinemia (FHBL) and who have various truncations of apoprotein (apo) B (ie, FHBL with apoB truncation/apoB-100 genotypes), the plasma concentrations of apoB-100 are typically approximately 30% rather than the expected approximately 50% of those in unaffected family members. The metabolic basis for the low apoB-100 levels is unknown. Therefore, we compared the metabolism of apoB-100 in 8 subjects with heterozygous FHBL (2 apoB-89/apoB-100, 2 apoB-75/apoB-100, 2 apoB-54.8/apoB-100, 1 apoB-52/apoB-100, and 1 apoB-31/apoB-100) with the metabolism of apoB-100 in 8 apoB-100/apoB-100 control subjects who were paired with the heterozygotes by gender, age, height, weight, and race. Endogenous labeling of apoB-100 with [13C]leucine and a multicompartmental kinetic model were used to obtain kinetic parameters. FHBL heterozygotes had significantly reduced VLDL apoB-100 production rates (7.7 +/- 3.7 versus 21.2 +/- 6.2 mg.kg-1.d-1, P = .002) and LDL apoB-100 production rates (4.5 +/- 3.12 versus 15.3 +/- 1 mg.kg-1.d-1, P = .05) compared with control subjects. Fractional conversion rates of VLDL to LDL were not significantly different (0.67 +/- 0.36 versus 0.77 +/- 0.17 pools/d), and the respective fractional catabolic rates of apoB-100 in VLDL, IDL, and LDL also were similar in both groups. Thus, FHBL heterozygotes produced apoB-100 at about 30% of the rates of control subjects. We believe these reduced production rates largely account for the lower than expected levels of apoB-100 and LDL cholesterol in the plasma of FHBL heterozygotes.

A Familial Combined Hyperlipidemic Kindred With Impaired Apolipoprotein B Catabolism Kinetics of Apolipoprotein B During Placebo and Pravastatin Therapy

Familial combined hyperlipidemia (FCHL) is a heterogeneous disorder characterized by multiple lipoprotein phenotypes, a high risk for coronary heart disease, and predominance among the LDL fraction of smaller and denser particles. We report on an FCHL kindred (the M-kindred) in which decreased VLDL- and LDL-apoB elimination rates rather than enhanced production rates were the main kinetic abnormalities. Lipoprotein levels and metabolic parameters of all apoB-containing lipoproteins (including light and dense LDLs) were determined during placebo and pravastatin treatment periods. ApoB metabolism was studied by endogenous labeling with stable isotopes and a multicompartmental model. Five members of the M-kindred participated. The study was doubly blinded, randomized, and placebo controlled. Treatment periods of 6 weeks were separated by 2-week washout periods. All subjects had high apoB levels, 2 had a mixed lipemia, 1 had hypercholesterolemia, and 2 had hypertriglyceridemia. Familial dysbetalipoproteinemia, hypercholesterolemia, and defective apoB-100 were excluded by genetic, testing. Kinetic parameters were remarkably similar in the five study subjects during the placebo period, despite their diverse plasma lipid profiles. Compared with nine normolipidemic control subjects, low VLDL-apoB fractional catabolic rates (FCRs) (3.6 +/- .1 versus 9.3 +/- 2.9 pools per day) and low LDL-apoB FCRs (0.19 +/- 0.05 versus 0.41 +/- 0.13 pool per day) were observed in every case. The majority of the LDL particles were identified in the denser fraction (d = 1.036 to 1.063 g/mL). A clear precursor-product relationship was observed from VLDL to IDL to light LDL to dense LDL, ie, there was no metabolic channeling. Light LDL had significantly higher FCR than dense LDL (0.82 +/- 0.21 versus 0.22 +/- 0.08 pool per day). VLDL-apoB production rates were normal (19.7 +/- 6.0 versus 21.6 +/- 6.1 mg/kg per day for control subjects). In contrast, in two subjects drawn from two other FCHL kindreds (the C- and K-kindreds), VLDL-apoB production rates were increased (35.6 and 32.1 mg/kg per day, respectively). In these two, more typical FCHL subjects, FCRs of LDL-apoB were near normal (0.351 and 0.311 pool per day, respectively). Pravastatin (20 mg/d) resulted in significantly lower plasma cholesterol (265 +/- 30 to 218 +/- 16 mg/dL, P < .01), LDL cholesterol (186 +/- 31 to 145 +/- 15 mg/dL, P < .03), and apoB levels (168 +/- 14 to 125 +/- 16 mg/dL, P < .01) in the five FCHL subjects of the M-kindred. No changes were observed in plasma HDL cholesterol, apoA-I, or lipoprotein(a). Pravastatin significantly increased the LDL-apoB FCR (from 0.19 +/- 0.05 to 0.34 +/- 0.04 pool per day). The FCRs of both LDL subclasses increased with treatment. No pravastatin-induced changes were seen in apoB production rates.

Molecular Characterization of the Lipid Genome-Wide Association Study Signal on Chromosome 18q11.2 Implicates HNF4A-Mediated Regulation of the TMEM241 Gene

Objective— We recently identified a locus on chromosome 18q11.2 for high serum triglycerides in Mexicans. We hypothesize that the lead genome-wide association study single-nucleotide polymorphism rs9949617, or its linkage disequilibrium proxies, regulates 1 of the 5 genes in the triglyceride-associated region. Approach and Results— We performed a linkage disequilibrium analysis and found 9 additional variants in linkage disequilibrium (r 2>0.7) with the lead single-nucleotide polymorphism. To select the variants for functional analyses, we annotated the 10 variants using DNase I hypersensitive sites, transcription factor and chromatin states and identified rs17259126 as the lead candidate variant for functional in vitro validation. Using luciferase transcriptional reporter assay in liver HepG2 cells, we found that the G allele exhibits a significantly lower effect on transcription (P<0.05). The electrophoretic mobility shift and ChIPqPCR (chromatin immunoprecipitation coupled with quantitative polymerase chain reaction) assays confirmed that the minor G allele of rs17259126 disrupts an hepatocyte nuclear factor 4 &agr;–binding site. To find the regional candidate gene, we performed a local expression quantitative trait locus analysis and found that rs17259126 and its linkage disequilibrium proxies alter expression of the regional transmembrane protein 241 (TMEM241) gene in 795 adipose RNAs from the Metabolic Syndrome In Men (METSIM) cohort (P=6.11×10−07–5.80×10−04). These results were replicated in expression profiles of TMEM241 from the Multiple Tissue Human Expression Resource (MuTHER; n=856). Conclusions— The Mexican genome-wide association study signal for high serum triglycerides on chromosome 18q11.2 harbors a regulatory single-nucleotide polymorphism, rs17259126, which disrupts normal hepatocyte nuclear factor 4 &agr; binding and decreases the expression of the regional TMEM241 gene. Our data suggest that decreased transcript levels of TMEM241 contribute to increased triglyceride levels in Mexicans.