Sequence and expression of Tangier apoA-I gene

PPAR-alpha effects on the heart and other vascular tissues

Peroxisome proliferator-activated receptor (PPAR)-alpha is a member of a large nuclear receptor superfamily whose main role is to activate genes involved in fatty acid oxidation in the liver, heart, kidney, and skeletal muscle. While currently used mainly as hypolipidemic agents, the cardiac effects and anti-inflammatory actions of PPAR-alpha agonists in arterial wall cells suggest other potential cardioprotective and antiatherosclerotic effects of these agents. This review summarizes current knowledge regarding the effects of PPAR-alpha agonists on lipid and lipoprotein metabolism, the heart, and the vessel wall and introduces some of the insights gained in these areas from studying PPAR-alpha-deficient mice. The introduction of new and more potent PPAR-alpha agonists will provide important insights into the overall benefits of activating PPAR-alpha clinically for the treatment of dyslipidemia and prevention of vascular disease.

Nuclear receptors and the control of metabolism

The metabolic nuclear receptors act as metabolic and toxicological sensors, enabling the organism to quickly adapt to environmental changes by inducing the appropriate metabolic genes and pathways. Ligands for these metabolic receptors are compounds from dietary origin, intermediates in metabolic pathways, drugs, or other environmental factors that, unlike classical nuclear receptor ligands, are present in high concentrations. Metabolic receptors are master regulators integrating the homeostatic control of (a) energy and glucose metabolism through peroxisome proliferator-activated receptor gamma (PPARgamma); (b) fatty acid, triglyceride, and lipoprotein metabolism via PPARalpha, beta/delta, and gamma; (c) reverse cholesterol transport and cholesterol absorption through the liver X receptors (LXRs) and liver receptor homolog-1 (LRH-1); (d) bile acid metabolism through the farnesol X receptor (FXR), LXRs, LRH-1; and (e) the defense against xeno- and endobiotics by the pregnane X receptor/steroid and xenobiotic receptor (PXR/SXR). The transcriptional control of these metabolic circuits requires coordination between these metabolic receptors and other transcription factors and coregulators. Altered signaling by this subset of receptors, either through chronic ligand excess or genetic factors, may cause an imbalance in these homeostatic circuits and contribute to the pathogenesis of common metabolic diseases such as obesity, insulin resistance and type 2 diabetes, hyperlipidemia and atherosclerosis, and gallbladder disease. Further studies should exploit the fact that many of these nuclear receptors are designed to respond to small molecules and turn them into therapeutic targets for the treatment of these disorders.

Subpopulations of high density lipoproteins in homozygous and heterozygous Tangier disease

Tangier disease (TD) is characterized by severe high-density lipoproteins (HDL) deficiency, hypercatabolism of HDL constituents, impaired cellular cholesterol efflux, and mutations in the gene of ATP-binding cassette 1 (ABC-1). In the present study, we determined plasma lipid and apolipoprotein levels, and HDL subpopulations, in 110 subjects from a large TD kindred in which the proband was homozygous for an A-->C missense mutation at nucleotide 5338 of the ABC-1 transcript. In the proband HDL-C, apoA-I, and apoA-II concentrations were 2, 1, and 2 mg/dl, respectively, apoA-I was present only in prebeta(1), while apoA-II was found free of apoA-I in two distinct alpha mobility subpopulations with different sizes. The smaller size particles contained only apoA-II while the larger one contained apoA-II and apo(a). Relative to unaffected male relatives (n=30), male heterozygotes (n=21) had significant reductions (P<0.001) in plasma HDL-C (-45%), apoA-I (-34%), apoA-II (-59%), apoA-IV (-40%), Lp(a) (-62%), and apoB (-55%) concentrations, and a significant increase (P<0.05, +33%) in plasma apoC-III levels. Female heterozygotes (n=11) similarly had significant reductions (P<0.001) in the concentrations of plasma HDL-C (-42%), apoA-I (-27%), apoA-II (-52%), Lp(a) (-27%), and (P<0.01) apoA-IV (-28%), apoB (-13%), and a significant increase (P<0.05) in plasma apoE levels (+29%) as compared to unaffected female relatives (n=41). Large size HDL subpopulations, especially the two LpA-I particles: alpha(1) and prealpha(1) were dramatically reduced in both male and female heterozygotes relative to their unaffected family members. Since apoA-II decreased more than apoA-I in both male and female heterozygotes, the ratios of apoA-I/apoA-II were significantly (P<0.01) increased. The prevalence of CHD was 60% higher in the 32 heterozygotes than in the 71 unaffected relatives even though the latter group was on average 7 years older. We conclude that TD homozygotes have only prebeta(1) apoA-I-containing HDL subpopulations, while heterozygotes have HDL that is selectively depleted in the large alpha(1), prealpha(1), and alpha(2), prealpha(2) subpopulations, resulting in HDL particles that are small in size, poor in cholesterol, but relatively enriched in apoA-I compared to those of their unaffected relatives. These abnormalities appear to result in a higher risk of CHD in heterozygotes than in unaffected controls.

NFY transcription factor binds to regulatory element AIC and transactivates the human apolipoprotein A-I promoter in HEPG2 cells

Previous studies have shown that the regulatory element AIC of apolipoprotein A-I is recognized by both positive and negative regulators which bind to over-lapping domains. One of these activities has been designated AIC1. Competition experiments showed that AIC1 could be competed out by oligonucleotides containing the binding site of the transcription factor NFY. In the present study, DNA binding gel electrophoresis and competition assays showed that NFY and AIC1 recognized the same binding site on element AIC. This site contains a CCACT motif and differs by one residue from the consensus CCAAT binding motif of NFY. Cotransfection of HepG2 cells with both the -177 to -148 apoA-I CAT constructs and plasmid expressing NFY alpha and NFY beta, transactivated the apoA-I promoter by 1.8 fold, indicating that NFY is a positive activator of the apoA-I gene.

Binding specificity and modulation of the ApoA-I promoter activity by homo- and heterodimers of nuclear receptors

Three proximal regulatory elements, AIB, AIC, and AID, of the apoA-I gene are necessary and sufficient for its hepatic expression in vivo and in vitro. DNA binding and competition assays showed that elements AIB and AID contain hormone response elements composed of imperfect direct repeats that support the binding of the hepatic nuclear factor-4, other nuclear orphan receptors, and the ligand-dependent nuclear receptors retinoic X receptor (RXRalpha), RXRalpha/RARalpha, and RXRalpha/T3Rbeta. Substitution mutations on repeats 1 and 2 in the hormone response sites of elements AIB and AID, respectively, abolished the binding of all nuclear receptors and reduced promoter activity to background levels, indicating the importance of both hormone response elements for the hepatic expression of the apoA-I gene. Cotransfection experiments in HepG2 cells with normal and mutated promoter constructs and plasmids expressing nuclear hormone receptors showed that RXRalpha homodimers transactivated the wild type promoter 150% of control, in the presence of 9-cis-retinoic acid (RA), whereas RXR alpha/T3R beta heterodimers repressed transcription to 60% of control, in the presence of T3. RXR alpha/RAR alpha and hepatic nuclear factor-4 did not affect the transcription, driven by the proximal apoA-I promoter. Potassium permanganate and dimethyl sulfate interference experiments showed that RXRalpha homodimers, RXRalpha/RARalpha, and RXRalpha/T3Rbeta heterodimers participate in protein-DNA interactions with 12, 13, and 11 out of the 14 nucleotides, respectively, that span repeats 1 and 2 and the spacer region separating them on the hormone response element of element AID. The binding of RXRalpha homodimers and RXRalpha/T3Rbeta heterodimers is associated with ligand-dependent activation by 9-cis-RA or repression by T3. Upon deletion or mutation of repeat 1, homodimeric binding of RXRalpha is lost whereas heterodimeric binding is retained. This heterodimeric binding to the mutated element AID is mediated solely by interactions with repeat 2 and one adjacent nucleotide and is confined to a heptameric core recognition motif. The interactions of the RXRalpha heterodimers with repeat 2 are associated with low levels of ligand-independent transcriptional activity. The findings suggest that the specific types of homo- and heterodimers of nuclear hormone receptors occupying the hormone response elements of apoA-I and the availability of the ligand may play an important role in the transcriptional regulation of the human apoA-I gene.

Severe familial HDL deficiency in French-Canadian kindreds. Clinical, biochemical, and molecular characterization

A decreased level of HDL cholesterol (HDL-C) is the most common lipoprotein abnormality seen in people with premature coronary artery disease (CAD). In many cases, HDL-C reduction in patients with CAD may be the result of increased apo B-containing lipoprotein production by the liver with secondary hypoalphalipoproteinemia. Primary hypoalphalipoproteinemia is seen in approximately 4% of people with CAD. We report findings in four subjects with severe familial HDL deficiency (HDL-C << 5th percentile for age and sex; 0.08 to 0.38 mmol/L) in three French-Canadian kindreds with autosomal codominant inheritance. By inclusion criteria, all four subjects had normal fasting triglycerides and none were diabetic. HDL particle size by gradient gel electrophoresis revealed small HDL particles (estimated Stokes' diameter, 8.14 to 8.30 nm). Apo AI analysis by polyacrylamide gel electrophoresis and use of isoelectrofocusing gels in affected subjects revealed normal molecular weight (28.3 kD) and normal isoelectrofocusing point but a relative increase in proapoliprotein AI, with near-normal levels of proapolipoprotein AI in plasma, suggesting normal secretion of apo AI. Quantitative Southern blot analysis of the apo AI-CIII-AIV gene cluster reveals no gene rearrangements or allele deletion. Haplotypes of the apo AI gene, determined by use of the restriction enzymes Pst I, Xmn I, and Sst I and of the apo AII gene by use of the enzyme Msp I, did not reveal segregation of the low HDL-C trait with either the apo AI or the AII gene. Sequence analysis of the promoter region of the apo AI gene reveals heterozygosity for guanine-to-adenine substitution at position 76 in two kindreds with no evidence of segregation with the low HDL trait. None of the patients had mutations of the lipoprotein lipase gene common in subjects of French-Canadian descent. Haplotype analysis of the lipoprotein lipase gene did not show segregation with the low HDL trait. Plasma lecithin: cholesterol acyltransferase (LCAT) activity was found to be within normal levels in affected subjects and in nonaffected first-degree relatives. None of the affected subjects had clinical manifestations of Tangier disease. Two of the four cases examined, both men, had severe CAD and had undergone revascularization procedures. The third is a younger brother of one of these probands and the fourth is a 30-year-old woman, and both were free of clinical CAD. However, in none of the families did the low HDL trait unequivocally cosegregate with CAD.(ABSTRACT TRUNCATED AT 400 WORDS)

Homozygous Tangier disease and cardiovascular disease

Decreased levels of plasma high density lipoprotein (HDL) cholesterol have been associated with premature cardiovascular disease (CVD). Tangier disease is an autosomal co-dominant disorder in which homozygotes have a marked deficiency of HDL cholesterol and apolipoprotein (apo) A-I levels (both < 10 mg/dl), decreased low density lipoprotein (LDL) cholesterol levels (about 40% of normal), and mild hypertriglyceridemia. Homozygotes develop cholesterol ester deposition in tonsils (orange tonsils), liver, spleen, gastrointestinal tract, lymph nodes, bone marrow, and Schwann cells. Our purpose was to assess the prevalence of CVD in Tangier disease. We reviewed published clinical information on 51 cases of homozygous Tangier disease, report 3 new cases and provide autopsy information on 3 cases. Mean (+/- S.D.) lipid values of all cases were as follows: total cholesterol 68 +/- 30 mg/dl (32% of normal), triglycerides 201 +/- 118 mg/dl (162% of normal), HDL cholesterol 3 +/- 3 mg/dl (6% of normal) and LDL cholesterol 50 +/- 38 mg/dl (37% of normal). The most common clinical finding in these subjects (n = 54) was peripheral neuropathy which was observed in 54% of cases versus < 1% of control subjects (n = 3130). CVD was observed in 20% of Tangier patients versus 5% of controls (P < 0.05), and in those that were between 35 and 65 years of age, 44% (11 of 25) had evidence of CVD (either angina, myocardial infarction or stroke) versus 6.5% in 1533 male controls and 3.2% in 1597 female controls in this age group (P < 0.01). In 9 patients who died, 2 died prior to age 20 of probable infectious diseases, 3 of documented coronary heart disease at ages 48, 64, and 72, 2 of stroke at ages 56 and 69, one of valvular heart disease, and 1 of cancer. In three autopsy cases, significant diffuse atherosclerosis was observed in one at age 64, moderate atherosclerosis and cerebral infarction in another at age 56, but no atherosclerosis was noted in the third case who died of lymphoma at age 62. In one patient with established coronary heart disease, none of the lipid lowering agents used (niacin, gemfibrozil, estrogen or lovastatin) raised HDL cholesterol levels above 5 mg/dl. However, these agents did have significant effects on lowering triglyceride and LDL cholesterol levels. Our data indicate that there may be heterogeneity in these patients with regard to CVD risk, that peripheral neuropathy is a major problem in many patients, and that CVD is a significant clinical problem in middle aged and elderly Tangier homozygotes.(ABSTRACT TRUNCATED AT 400 WORDS)

Heterogeneity within the nonstructural protein 5-encoding region of hepatitis C viruses from a single patient

Nucleotide (nt) sequence heterogeneity of the hepatitis C virus (HCV) genome derived from a single carrier was investigated. A polymerase chain reaction (PCR) product of 311 bp in the putative nonstructural protein 5-encoding region was directly sequenced, while part of a PCR product was cloned, and sequence analyses were carried out for 27 independent clones. Although 14 of the 27 clones were conserved, ten other types of nt sequences were found. The difference was at most 3 nt (1.1%). A directly determined sequence showed the major sequence of the cloned products. Since most of the nt changes occurred in the third letter of a codon, these nt changes might not have originated from random misincorporation during the PCR. These results of natural divergence of genome population in a single carrier suggest that HCV is a typical RNA virus with a quasi-species nature due to high mutation rates.

Characterization of the promoter elements required for hepatic and intestinal transcription of the human apoB gene: definition of the DNA-binding site of a tissue-specific transcriptional factor

The promoter elements important for intestinal and hepatic transcription of the human apoB gene have been localized downstream of nucleotide -150. Footprinting analysis using hepatic nuclear extracts identified four protected regions, -124 to -100, -97 to -93, -86 to -33, and +33 to +52. Gel electrophoretic mobility shift assays showed that multiple factors interact with the apoB sequence -86 to -33, while the region -88 to -61 binds a single nuclear factor. Methylation interference analysis and nucleotide substitution mutagenesis identified the binding site of the factor between residues -78 and -68. Binding competition experiments indicate that this factor recognizes the regulatory elements of other liver-specific genes.

Characterization of the promoter elements required for hepatic and intestinal transcription of the human apoB gene: definition of the DNA-binding site of a tissue-specific transcriptional factor

The promoter elements important for intestinal and hepatic transcription of the human apoB gene have been localized downstream of nucleotide -150. Footprinting analysis using hepatic nuclear extracts identified four protected regions, -124 to -100, -97 to -93, -86 to -33, and +33 to +52. Gel electrophoretic mobility shift assays showed that multiple factors interact with the apoB sequence -86 to -33, while the region -88 to -61 binds a single nuclear factor. Methylation interference analysis and nucleotide substitution mutagenesis identified the binding site of the factor between residues -78 and -68. Binding competition experiments indicate that this factor recognizes the regulatory elements of other liver-specific genes.

Synthesis of apolipoprotein A-1 in pig brain microvascular endothelial cells

In an approach toward the identification of hitherto unknown proteins involved in the function of the blood-brain barrier, we constructed a pig brain microvessel-derived cDNA library that is enriched in blood-brain barrier specific sequences by means of subtractive cloning. Sequence analysis of selected clones revealed that one of the cDNAs encoded porcine apolipoprotein (apo) A-1. The identity of apo A-1 mRNA was further confirmed by in vitro translation of RNA from brain microvascular endothelial cells and subsequent immunoprecipitation with an antibody against human apo A-1. We further investigated the expression of apo A-1 mRNA in several tissues and in endothelial cells of the pig. It is shown that cultured brain microvascular endothelial cells provide an in vitro model to study the expression and function of apo A-1 in the microvasculature of the brain.