Brian L. Knight

A role for PPARα in the control of SREBP activity and lipid synthesis in the liver

Inclusion of the PPARalpha (peroxisome-proliferator-activated receptor alpha) activator WY 14,643 in the diet of normal mice stimulated the hepatic expression of not only genes of the fatty acid oxidation pathway, but also those of the de novo lipid synthetic pathways. Induction of fatty acid synthase mRNA by WY 14,643 was greater during the light phase of the diurnal cycle, when food intake was low and PPARalpha expression was high. Hepatic fatty acid pathway flux in vivo showed a similar pattern of increases. The abundance of mRNAs for genes involved in hepatic cholesterol synthesis was also increased by WY 14,643, but was associated with a decrease in cholesterogenic carbon flux. None of these changes were apparent in PPARalpha-null mice. Mice of both genotypes showed the expected decreases in 3-hydroxy-3-methylglutaryl-CoA reductase mRNA levels and cholesterol synthesis in response to an increase in dietary cholesterol. The increase in fatty acid synthesis due to WY 14,643 was not mediated by increased expression of SREBP-1c (sterol regulatory element binding protein-1c) mRNA, but by an increase in cleavage of the protein to the active form. An accompanying rise in stearoyl-CoA desaturase mRNA expression suggested that the increase in lipogenesis could have resulted from an alteration in membrane fatty acid composition that influenced SREBP activation.

Peroxisome-proliferator-activated receptor-α (PPARα) deficiency leads to dysregulation of hepatic lipid and carbohydrate metabolism by fatty acids and insulin

The aim of the present study was to determine whether peroxisome-proliferator-activated receptor-alpha (PPARalpha) deficiency disrupts the normal regulation of triacylglycerol (TAG) accumulation, hepatic lipogenesis and glycogenesis by fatty acids and insulin using PPARalpha-null mice. In wild-type mice, hepatic TAG concentrations increased (P<0.01) with fasting (24 h), with substantial reversal after refeeding (6 h). Hepatic TAG levels in fed PPARalpha-null mice were 2.4-fold higher than in the wild-type (P<0.05), increased with fasting, but remained elevated after refeeding. PPARalpha deficiency also impaired hepatic glycogen repletion (P<0.001), despite normal insulin and glucose levels after refeeding. Higher levels of plasma insulin were required to support similar levels of hepatic lipogenesis de novo ((3)H(2)O incorporation) in the PPARalpha-null mice compared with the wild-type. This difference was reflected by corresponding changes in the relationship between plasma insulin and the mRNA expression of the lipogenic transcription factor sterol-regulatory-element-binding protein-1c, and that of one of its known targets, fatty acid synthase. In wild-type mice, hepatic pyruvate dehydrogenase kinase (PDK) 4 protein expression (a downstream marker of altered fatty acid catabolism) increased (P<0.01) in response to fasting, with suppression (P<0.001) by refeeding. Although PDK4 up-regulation after fasting was halved by PPARalpha deficiency, PDK4 suppression after refeeding was attenuated. In summary, PPARalpha deficiency leads to accumulation of hepatic TAG and elicits dysregulation of hepatic lipid and carbohydrate metabolism, emphasizing the importance of precise control of lipid oxidation for hepatic fuel homoeostasis.

The effect of nicotinic acid and acipimox on lipoprotein(a) concentration and turnover.

This study examines the effect of nicotinic acid (1 g t.d.s.) on serum Lp(a) concentration in a group of patients with type II hyperlipidaemia selected on the basis of a plasma Lp(a) concentration greater than 30 mg/dl. Reductions in total cholesterol, triglyceride, LDL-cholesterol and Lp(a) were 16.3%, 25.5%, 23.7% and 36.4%, respectively, with an increase in HDL cholesterol of 37.3%. The reduction in Lp(a) concentration did not correlate with any other lipoprotein changes. In order to establish the mechanism of the fall in Lp(a) concentration, in vivo turnover of autologous Lp(a) was studied in three subjects before and whilst taking nicotinic acid. The fractional catabolic rate in Lp(a) was unaltered in the subjects on therapy, indicating that nicotinic acid did not increase catabolism of Lp(a) but decreased the synthetic rate. Since nicotinic acid was poorly tolerated we examined the effect of acipimox, an analogue of nicotinic acid on lipoproteins using a placebo controlled double-blind crossover design in a group of hyperlipidaemic patients again selected with plasma Lp(a) concentration greater than 30 mg/dl. Acipimox was better tolerated than nicotinic acid but the percentage changes in lipoprotein concentrations were smaller.

Catabolism of lipoprotein(a) in familial hypercholesterolaemic subjects

The in vivo turnover of autologous lipoprotein(a) (Lp(a)) was studied in four heterozygous familial hypercholesterolaemic (FH) subjects and four subjects who were hyperlipidaemic but not FH. Each of the FH subjects exhibited a much lower fractional catabolic rate (FCR) for LDL than each of the non-FH subjects. Lp(a) was purified by sequential density gradient centrifugations and was radio-iodinated. The labelled Lp(a) ran as a single band on electrophoresis in gradient polyacrylamide gels. Less than 5% of the label was in lipid, with about 40% of the remainder on apolipoprotein B (apo B) and 60% on apo(a). Labelled and unlabelled Lp(a) competed equally poorly with LDL for binding to LDL receptors on cultured fibroblasts. The FCR of Lp(a), calculated from the decay of the specific radioactivity of the Lp(a) isolated from the daily blood samples, was the same in FH subjects as in non-FH subjects. There was no consistent relationship between Lp(a) FCR and the plasma Lp(a) concentration or between FCR and the Lp(a) phenotype, at least within this sample of subjects. There was a strong association between Lp(a) concentration and production rate, with values for non-FH and FH subjects falling on the same line. The rate of decline of radioactivity in whole plasma was consistently slower than the fall in specific radioactivity of the isolated Lp(a). This difference was more marked in FH subjects than in non-FH subjects and resulted from the accumulation of radioactivity derived from the injected Lp(a) at a lower density than Lp(a), in the fractions containing LDL. The amount of radioactivity in this fraction increased for the first few days after injection and then fell, the fall being more rapid in non-FH than in FH subjects. These results provide no evidence for the involvement of LDL receptors in the catabolism of Lp(a) itself but suggest that they could be responsible for some of the clearance of the lipid and apo B components after removal of apo(a) in the circulation.

The effect of peroxisome-proliferator-activated receptor-alpha on the activity of the cholesterol 7 alpha-hydroxylase gene.

Cholesterol 7 alpha-hydroxylase (Cyp7a1) plays a central role in the regulation of bile acid and cholesterol metabolism, and transcription of the gene is controlled by bile acids and hormones acting through a complex interaction with a number of potential steroid-hormone-binding sites. Transcriptional activity of the human CYP7A1 gene promoter transfected into HepG2 cells was decreased in a concentration-dependent manner by co-transfection with an expression vector for peroxisome-proliferator-activated receptor-alpha (PPAR alpha). This effect was augmented by 9-cis-retinoic acid receptor-alpha (RXR alpha) and activators of PPAR alpha to give a maximum inhibition of approx. 80%. The region responsible for this inhibition contained a site known to bind hepatocyte nuclear factor 4 (HNF4), and mutation of this site greatly decreased the effect. Co-expression of HNF4 increased promoter activity and decreased the effect of PPAR alpha. Gel-mobility-shift assays failed to detect any binding of PPAR alpha/RXR alpha dimers to any regions of the promoter containing potential binding sites. Also the hepatic abundance of Cyp7a1 mRNA in mice in which the PPAR alpha gene was disrupted was the same as in normal mice, both during the dark phase, when the animals were feeding, and during the light phase, when mRNA abundance was greatly increased. Cholesterol feeding produced the same increase in hepatic Cyp7a1 mRNA abundance in PPAR alpha-null animals as in normals. It is concluded that, whereas PPAR alpha can affect CYP7A1 gene transcription in vitro through an indirect action, probably by competing for co-factors, this is unlikely to be a major influence on Cyp7a1 activity under normal physiological conditions.

Clinical signs of familial hypercholesterolemia in patients with familial defective apolipoprotein B-100 and normal low density lipoprotein receptor function.

In a previous study (Tybjaerg-Hansen et al, Atherosclerosis 1990;80:235-242), we identified nine patients heterozygous for the apolipoprotein B (apo B) arginine-to-glutamine (Arg3,500----Gln) mutation (familial defective apolipoprotein B-100 [FDB]). Six of these had been diagnosed clinically as familial hypercholesterolemic (FH) heterozygotes. We have since examined low density lipoprotein (LDL) receptor function in the FDB index patients and in three of their families. Skin fibroblasts from seven of seven unrelated FDB patients from whom cell lines were established exhibited normal high-affinity binding and degradation of normal LDL in vitro. In the three families, a raised plasma LDL concentration did not segregate with a haplotype of two polymorphic restriction sites at the LDL receptor locus. We conclude that the clinical and biochemical signs of classical FH can occur in the presence of the FDB mutation and a normal LDL receptor gene. In a four-generation family with 11 proven or presumed FDB heterozygotes, expression of the mutation ranged from normal plasma LDL concentrations and no clinical signs in two individuals, to hypercholesterolemia and death from myocardial infarction at age 31. Variable expression of the FDB mutation could not be explained conclusively by variation in diet, body mass index, smoking habit, apo E genotype, or plasma Lp(a) concentration.

Characterization of deletions in the LDL receptor gene in patients with familial hypercholesterolemia in the United Kingdom.

A sample of 200 patients with a clinical diagnosis of heterozygous (189) or homozygous (11) familial hypercholesterolemia (FH) attending lipid clinics in the London area have been screened for the presence of major gene defects in the low density lipoprotein (LDL) receptor gene by Southern blotting of genomic DNA with specific probes. This study is part of a project to determine the frequency of known mutations in the LDL receptor gene in this population. A new polymorphism for the enzyme Bgl II was identified by hybridization with a probe specific for the promoter plus exon 1 of the LDL receptor gene. The observed frequency of the rare allele, characterized by a Bgl II fragment of 13 kb compared with 10 kb for the common allele, was 0.08 in this group of FH patients. Several individuals who were heterozygous for the rare allele were also heterozygous for a mutation elsewhere in the LDL receptor gene that is known to cause FH. Eight different mutations, seven deletions and one duplication, were detected in a total of nine patients, accounting for 4.5% of the mutant alleles in this group. Three of the mutations are apparently identical to deletions that have been described previously in FH patients of British or European origin, while the remaining five have not been described. Two of these were in patients of Polish and Asian Indian origin, while the other three were in patients of apparently British ancestry.

Comparison of the Genetic Defect with LDL-Receptor Activity in Cultured Cells from Patients With a Clinical Diagnosis of Heterozygous Familial Hypercholesterolemia

In this study we have analyzed the genetic defect in 42 patients with a diagnosis of heterozygous familial hypercholesterolemia (FH) by Southern blotting, SSCP, and sequencing of PCR-amplified fragments of genomic DNA or sequencing of RT-PCR products from mRNA in cultured cells. The apoB Arg3500Gln mutation was identified in five patients. A molecular defect in the LDL-receptor gene was confirmed in 23 patients; 16 of these mutations have not been described before. No defect in the coding region, intron:exon junctions or proximal promoter of the LDL-receptor gene or in the region of the apoB gene coding for the LDL-receptor binding domain was found in the remaining 14 patients. LDL-receptor activity and protein content of cultured lymphoblasts from the patients was significantly lower in cells from patients with severe rather than mild LDL-receptor mutations. Cells from four patients with no detectable defect showed reduced LDL receptor activity compared with eight normal cell lines, whereas six others had reduced LDL-receptor activity but LDL-receptor protein content within the normal range. Cells from four patients appeared to have normal LDL-receptor function. Cells from two patients with a defined defect also had LDL-receptor activity within the normal range. The findings demonstrate the problems involved in the genetic diagnosis of FH in patients.

Detection and quantitation of apolipoprotein(a) mRNA in human liver and its relationship with plasma lipoprotein(a) concentration

Northern blotting and hybridisation with specific probes was used to detect and quantitate apolipoprotein(a) (apo(a)) mRNA in total RNA isolated from 25 human liver samples. A total of 14 different transcripts were identified suggesting that there are at least 15 different alleles at the apo(a) locus including a probable null allele. Apo(a) mRNA sizes were linearly correlated with the electrophoretic mobility of plasma apo(a) glycoprotein isoforms, and differed, in many cases, by the equivalent of one Kringle 4 unit. To investigate the relationship between apo(a) mRNA size and its concentration in the liver, and between hepatic apo(a) mRNA concentration and plasma lipoprotein(a) (Lp(a)) levels, apo(a) mRNA was quantified by densitometric scanning of autoradiograms of Northern blots. Overall, there was a significant inverse correlation between apo(a) mRNA size and its concentration in the liver, despite a marked interindividual variability in the relative amounts of similar-sized transcripts. In each heterozygous individual, the difference in concentration between the two mRNA species was determined by the difference in size. However, there was not a significant relationship between hepatic apo(a) mRNA concentration and plasma Lp(a) levels in this group. These findings emphasise the importance of mechanisms other than the rate of transcription of the apo(a) gene in the regulation of Lp(a) synthesis.

Effect of the StuI polymorphism in the LDL receptor gene (Ala 370 to Thr) on lipid levels in healthy individuals

We have examined the effect on plasma lipid levels of a single base change in exon 8 of the LDL receptor gene that causes an amino acid change Ala 370 to Thr in a sample of 318 Icelandic individuals selected at random from the general population. The change destroys a StuI restriction site and was detected by digestion of pooled samples in groups of 5. The frequency of the loss of the cutting site was 0.05 (95%CI=0.014‐0.054). In men, those with the Thr allele (n=18) had 8.3% higher total cholesterol, 11.8% higher LDL cholesterol and 10.3% higher apolipoprotein B than those with the common Ala allele, whereas in women those with the Thr allele (n=12) had levels lower by 7.4%, 13.3% and 10.1% respectively. These differences reached statistical significance only in the men (p<0.05). Functional analysis of CHO cells transfected with constructs of the LDL receptor cDNA carrying the Ala370 and Thr370 alleles showed that within the limits of the assays there was no difference in function of the LDL receptor protein as measured by uptake and degradation of LDL. The data raise the possibility that amino acid substitutions that could affect LDL receptor function below the limits of detection by conventional assays, may have an effect on plasma lipid levels in the general population.