Interaction of oestrogen and peroxisome proliferator-activated receptors with apolipoprotein(a) gene enhancers

The tissue-specific metabolic effects of the PPARα agonist ciprofibrate in insulin-resistant male individuals: a double-blind, randomized, placebo-controlled crossover study

OBJECTIVE

Insulin resistance is characterized by ectopic fat accumulation leading to cardiac diastolic dysfunction and nonalcoholic fatty liver disease. The objective of this study was to determine whether treatment with the peroxisome proliferator-activated receptor-α (PPARα) agonist ciprofibrate has direct effects on cardiac and hepatic metabolism and can improve insulin sensitivity and cardiac function in insulin-resistant volunteers.

METHODS

Ten insulin-resistant male volunteers received 100 mg/d of ciprofibrate and placebo for 5 weeks in a randomized double-blind crossover study. Insulin-stimulated metabolic rate of glucose (MRgluc) was measured using dynamic 18 F-fluorodeoxyglucose-positron emission tomography (18 F-FDG-PET). Additionally, cardiac function, whole-body insulin sensitivity, intrahepatic lipid content, skeletal muscle gene expression, 24-hour blood pressure, and substrate metabolism were measured.

RESULTS

Whole-body insulin sensitivity, energy metabolism, and body composition were unchanged after ciprofibrate treatment. Ciprofibrate treatment decreased insulin-stimulated hepatic MRgluc and increased hepatic lipid content. Myocardial net MRgluc tended to decrease after ciprofibrate treatment, but ciprofibrate treatment had no effect on cardiac function and cardiac energy status. In addition, no changes in PPAR-related gene expression in muscle were found.

CONCLUSIONS

Ciprofibrate treatment increased hepatic lipid accumulation and lowered MRgluc, without affecting whole-body insulin sensitivity. Furthermore, parameters of cardiac function or cardiac energy status were not altered upon ciprofibrate treatment.

Lipoprotein(a) beyond the kringle IV repeat polymorphism: The complexity of genetic variation in the LPA gene

High lipoprotein(a) [Lp(a)] concentrations are one of the most important genetically determined risk factors for cardiovascular disease. Lp(a) concentrations are an enigmatic trait largely controlled by one single gene (LPA) that contains a complex interplay of several genetic elements with many surprising effects discussed in this review. A hypervariable coding copy number variation (the kringle IV type-2 repeat, KIV-2) generates >40 apolipoprotein(a) protein isoforms and determines the median Lp(a) concentrations. Carriers of small isoforms with up to 22 kringle IV domains have median Lp(a) concentrations up to 5 times higher than those with large isoforms (>22 kringle IV domains). The effect of the apo(a) isoforms are, however, modified by many functional single nucleotide polymorphisms (SNPs) distributed over the complete range of allele frequencies (<0.1% to >20%) with very pronounced effects on Lp(a) concentrations. A complex interaction is present between the apo (a) isoforms and LPA SNPs, with isoforms partially masking the effect of functional SNPs and, vice versa, SNPs lowering the Lp(a) concentrations of affected isoforms. This picture is further complicated by SNP-SNP interactions, a poorly understood role of other polymorphisms such as short tandem repeats and linkage structures that are poorly captured by common R² values. A further layer of complexity derives from recent findings that several functional SNPs are located in the KIV-2 repeat and are thus not accessible to conventional sequencing and genotyping technologies. A critical impact of the ancestry on correlation structures and baseline Lp(a) values becomes increasingly evident.

This review provides a comprehensive overview on the complex genetic architecture of the Lp(a) concentrations in plasma, a field that has made tremendous progress with the introduction of new technologies. Understanding the genetics of Lp(a) might be a key to many mysteries of Lp(a) and booster new ideas on the metabolism of Lp(a) and possible interventional targets.

Inhibition of phosphodiesterase type 9 reduces obesity and cardiometabolic syndrome in mice

Central obesity with cardiometabolic syndrome (CMS) is a major global contributor to human disease, and effective therapies are needed. Here, we show that cyclic GMP-selective phosphodiesterase 9A inhibition (PDE9-I) in both male and ovariectomized female mice suppresses preestablished severe diet-induced obesity/CMS with or without superimposed mild cardiac pressure load. PDE9-I reduces total body, inguinal, hepatic, and myocardial fat; stimulates mitochondrial activity in brown and white fat; and improves CMS, without significantly altering activity or food intake. PDE9 localized at mitochondria, and its inhibition in vitro stimulated lipolysis in a PPARα-dependent manner and increased mitochondrial respiration in both adipocytes and myocytes. PPARα upregulation was required to achieve the lipolytic, antiobesity, and metabolic effects of PDE9-I. All these PDE9-I-induced changes were not observed in obese/CMS nonovariectomized females, indicating a strong sexual dimorphism. We found that PPARα chromatin binding was reoriented away from fat metabolism-regulating genes when stimulated in the presence of coactivated estrogen receptor-α, and this may underlie the dimorphism. These findings have translational relevance given that PDE9-I is already being studied in humans for indications including heart failure, and efficacy against obesity/CMS would enhance its therapeutic utility.

Estrogen modulates plasminogen promoter activity

Postmenopausal women treated with estrogen hormone replacement therapy and female patients with hypoplasminogenemia receiving oral contraceptives show increasing plasminogen (PLG) concentrations. The elevated PLG levels are in contrast to the estrogen dependent decline of lipoptrotein(a) [Lp(a)], whose main protein component apolipoprotein(a) [APO(a)] is highly homologous to PLG in protein and gene structure and is also located in its immediate vicinity on chromosome 6q26. The intergenic region between both genes comprises several transcription-regulatory regions with enhancer sequences that increase the basal activity of the PLG core promoter. Using luciferase reporter assays we demonstrate that the minimal PLG promoter is insensitive to estrogen. However, an estrogen response element located 11.5 kb upstream of the PLG transcription start site is able to convey a dramatic estrogen-dependent elevation of PLG-minimal promoter driven reporter gene expression. In contrast, the activating effect of two additional enhancer elements, among them an DNase I hypersensitivity region that has been shown to regulate the APO(a) minimal promoter activity, is abrogated by estrogen. Thus, the identified estrogen-responsive elements provide a gene and tissue specific framework by which PLG expression is regulated and whose activity is orchestrated by yet unknown accessory factors.

Association between Low-Molecular Weight Apolipoprotein(a) Isoforms and Obesity in Italian Women

OBJECTIVE

Low-molecular weight (MW) apolipoprotein(a) [apo(a)] isoforms are closely associated with an increased incidence of atherothrombotic disease, prevalence of which is higher in obese individuals, particularly in women. The hypothesis of this study was to assess whether there are differences in the distribution of apo(a) phenotypes between obese patients and healthy controls.

RESEARCH METHODS AND PROCEDURES

One hundred three obese Italian women (BMI > or = 30.0 kg/m2) were enrolled in the study, and apo(a) phenotyping was performed in all subjects. The prevalence of low-MW apo(a) isoforms, detected in plasma samples of our obese women, was compared with that found in a control group of 84 normal-weight, never-obese (BMI < 25.0 kg/m(2)), age-matched women.

RESULTS

The distribution of apo(a) isoforms in the population of obese women was significantly different from that found in normal-weight female subjects. In particular, the percentage of subjects in the obese group with at least one apo(a) isoform of low MW was significantly higher than that in the control group (51.4% vs. 32.1%, p = 0.0079).

DISCUSSION

Our results seem to suggest the possibility that small-sized apo(a) isoforms may be used together with other traditional risk factors to better assess the overall predisposition to atherothrombotic disease in obese women.

Detection of variability in apo(a) gene transcription regulatory sequences using the DGGE method

BACKGROUND

Increased lipoprotein(a), Lp(a), concentration is an independent risk factor for premature atherosclerosis. Apolipoprotein(a), apo(a), determines properties of the lipoprotein and its production rate is the limiting step in Lp(a) particle formation.

METHODS

Subjects covering the whole range of Lp(a) concentration were separated into quintiles. A randomly chosen sample from each quintile was derived, there being a total number of 713 individuals. The DGGE method was used to scan the known transcription regulatory regions of apo(a) gene (promoter; DHII and DHIII enhancers) for variability and its distribution across quintiles.

RESULTS

Besides 5 previously reported nucleotide substitutions (+121 G>A; +93 C>T; -1712 G>T; -1617 C>A; -1230 A>G) 16 unreported rare sequence variants were detected. All polymorphic variants were distributed throughout the quintiles with several significant differences. The novel +62 C variant was found only among individuals with Lp(a) levels over 16 mg/dl.

CONCLUSION

The apo(a) gene transcription regulatory regions were not revealed to be extremely polymorphic. However, we should consider a combined effect of all polymorphic sites from the whole apo(a) gene locus, including the apo(a) gene length polymorphism, when dealing with high population variability of Lp(a) levels.

The V227A polymorphism at the PPARA locus is associated with serum lipid concentrations and modulates the association between dietary polyunsaturated fatty acid intake and serum high density lipoprotein concentrations in Chinese women

Peroxisome proliferators activated receptor alpha (PPARalpha) regulates the transcription of several proteins involved in human lipoprotein metabolism. We screened the PPARA locus for polymorphisms in 20 unrelated subjects from each of three ethnic groups (Chinese, Malays and Asian Indians). Only the V227A polymorphism was observed. We genotyped 4248 subjects (2899 Chinese, 761 Malay and 588 Asian Indians) and found allele frequencies for the A227 allele of 0.04 in Chinese, 0.006 in Malays and 0.003 in Asian Indians. We examined the associations between this polymorphism and serum lipid concentrations in Chinese. In women, but not in men, the presence of the A227 allele was associated with lower serum concentrations of total cholesterol [5.38mmol/l (95%CI: 5.22-5.54) versus 5.21mmol/l (95%CI: 4.99-5.43), p=0.047] and triglycerides [1.19mmol/l (95%CI: 1.10-1.28) versus 1.09mmol/l (95%CI: 0.98-1.21), p=0.048]. We also found that the V227A polymorphism modulates the association between dietary polyunsaturated fatty acid intake and serum high density lipoprotein concentration (p-value for interaction=0.049). Our findings implicate PPARalpha in the lipid lowering associated with diets high in PUFA and suggests that genetic variation at the PPARA locus may determine the lipid response to changes in PUFA intake.

Reduced LPA expression after peroxisome proliferator-activated receptor alpha (PPARα) activation in LPA-YAC transgenic mice

BACKGROUND:: Apolipoprotein(a) (apo(a)), which is part of the atherogenic lipoprotein Lp(a), shares structural homology with plasminogen (plg). Genes coding for plasminogen (PLG) and apo(a) (LPA) are linked and situated 40kb apart in the telomeric region of the long arm of chromosome 6. LPA is naturally expressed only in primates and hedgehogs. Thus, access to knowledge regarding the mechanism by which LPA expression is regulated is limited due to shortage of appropriate animal models. However, mice transgenic for the human LPA gene have been produced. Lp(a) levels in man are genetically determined and not altered significantly by dietary changes. In contrast, mice transgenic for LPA-yeast artificial chromosome (LPA-YAC) have markedly reduced apo(a) levels after maintenance on a high-fat diet. LPA-YAC carries the 40kb LPA-PLG intergenic region, which includes a putative binding site for peroxisome proliferator-activated receptor alpha (PPARalpha). Therefore, we examined if fibrates, which exert their effect via PPARalpha, could alter LPA expression in transgenic mice. METHODS:: Two LPA transgenic mouse lines with or without the LPA-PLG intergenic region we fed either PPARalpha agonist fenofibrate (FF) or 4-chloro-6-(2,3-xylidino)-2-pyrimidinylthioacetic acid (WY 14643) containing diets for 3 weeks. For the study of serum apo(a) levels, blood were sampled prior the experiment and when the animals were sacrificed. For the study of gene expression pattern pieces of livers were collected and submerged in RNAlater buffer and stored at -70 degrees C until analysis by quantitative PCR. RESULTS AND CONCLUSIONS:: The results showed that fibrates reduce LPA expression in LPA-YAC transgenic mice, but have no impact on hepatic apo(a) mRNA or serum apo(a) protein levels in LPA-cDNA transgenic mice, which lack the LPA-PLG intergenic region. This suggests that the effect of fibrates on LPA expression is mediated upstream of the LPA gene. However, on the basis of current data it is not possible to conclude that PPARalpha is the primary factor that represses LPA expression in LPA-YAC transgenic mice. Negative correlation between FXR and apo(a) mRNA levels, in addition to putative FXR DNA binding sequence in LPA-PLG intergenic region, suggest that it is equally likely that reduced expression of LPA could be a secondary consequence of PPARalpha activation on other genes, such as FXR.

Human apoB contributes to increased serum total apo(a) level in LPA transgenic mice

Background

The Lp(a) lipoprotein (Lp(a)) consists of the polymorphic glycoprotein apolipoprotein(a) (apo(a)), which is attached by a disulfide bond to apolipoprotein B (apoB). Apo(a), which has high homology with plasminogen, is present only in primates and hedgehogs. However, transgenic mice and rabbits with high serum apo(a) levels exist. Liver is the main site for apo(a) synthesis, but the site of removal is uncertain. To examine differences between transgenic mice expressing the LPA gene and mice capable of forming Lp(a) particles, LPA-YAC transgenic mice and hAPOB transgenic mice were crossed and their offspring examined.

Results

Comparison of LPA-YAC with LPA-YAC/hAPOB transgenic mice showed that LPA-YAC/hAPOB transgenic mice have higher serum total apo(a) and total cholesterol level than mice lacking the hAPOB gene. However, hepatic apo(a) mRNA level was higher in LPA-YAC transgenic mice than in LPA-YAC/hAPOB transgenic mice. Feeding of a high-cholesterol/high-fat diet to male LPA-YAC transgenic mice with or without the hAPOB gene resulted in reduced serum total apo(a) and hepatic apo(a) mRNA level.

Conclusion

In conclusion, the higher serum total apo(a) level in LPA-YAC/hAPOB transgenic mice than in LPA-YAC transgenic mice is not caused by increased apo(a) synthesis. Lower hepatic apo(a) mRNA level in LPA-YAC/hAPOB than in LPA-YAC transgenic mice may suggest that the increase in total apo(a) level is a result of apo(a) accumulation in serum. Furthermore, observed higher serum total cholesterol level in LPA-YAC/hAPOB transgenic mice than either in wild type or LPA-YAC transgenic mice may further suggest that human APOB transgenicity is a factor that contributes to increased serum total apo(a) and cholesterol levels. Our results on reduced serum total apo(a) and hepatic apo(a) mRNA levels in HCHF fed male LPA-YAC transgenic mice confirm earlier findings in females, and show that there are no sex difference in mechanisms for lowering apo(a) level in response to HCHF feeding.

The effect of rosiglitazone on serum lipoprotein(a) levels in Korean patients with type 2 diabetes mellitus

The aim of the study was to determine if rosiglitazone increases serum levels of lipoprotein(a) [Lp(a)] in Korean patients with type 2 diabetes mellitus. A total of 118 patients were divided into 2 groups: those with rosiglitazone (rosiglitazone group, n = 49) and those without rosiglitazone (control group, n = 69). The rosiglitazone group was given rosiglitazone (4 mg/d) with previous treatment, insulin, or sulfonylurea, for 12 weeks, whereas the control group continued previous treatment with some dose modification for glycemic control. The patients had their blood glucose, lipid levels, as well as Lp(a) levels assessed to obtain a baseline, which were remeasured 12 weeks later. The fasting blood glucose and glycosylated hemoglobin (HbA(1c)) levels decreased significantly in both groups as compared with the baseline. The fasting glucose and HbA(1c) levels in both groups were similar at 12 weeks. The total cholesterol levels increased significantly in the rosiglitazone group (190.6 +/- 32.4 to 212.2 +/- 47.2 mg/dL, P =.002), while they were unchanged in the control group (185.4 +/- 36.8 to 188.0 +/- 35.8 mg/dL, P =.615). The triglyceride levels did not change in either group. Significant increases in high-density lipoprotein (HDL) cholesterol levels were observed in the rosiglitazone group as compared with the baseline (41.7 +/- 10.6 to 45.9 +/- 11.4 mg/dL, P =.004). The low-density lipoprotein (LDL) cholesterol levels increased significantly in the rosiglitazone group (120.5 +/- 29.9 to 136.3 +/- 40.0 mg/dL, P =.012), while they did not change in the control group (113.0 +/- 29.1 to 118.3 +/- 31.7 mg/dL, P =.234). Significant increases in Lp(a) levels were observed in the rosiglitazone group as compared with the baseline (22.4 +/- 17.4 to 25.7 +/- 20.5 mg/dL, P =.015), approximately a 15% increase in average values. In contrast, there was no change in Lp(a) levels in the control group. There was no correlation between the changes in Lp(a) and changes in fasting blood glucose or HbA(1c) levels in all study subjects. In summary, rosiglitazone increased serum total cholesterol, LDL cholesterol, as well as Lp(a) levels in patients with type 2 diabetes mellitus. Considering that patients with type 2 diabetes mellitus have increased risks for cardiovascular disease, caution should be taken when prescribing rosiglitazone to patients who already have other risk factors, such as hypertension and smoking.

Sequence and functional changes in a putative enhancer region upstream of the apolipoprotein(a) gene

Two enhancer regions upstream of the apolipoprotein(a) (apo(a)) gene were amplified and sequenced from subjects who were known to express abnormally high or low amounts of lipoprotein(a) (Lp(a)). No base changes were found in the DHII region situated 28 kb from the apo(a) gene. Three common base substitutions were found in the DHIII region about 20 kb from the gene. One, an A to G change at position -1230, increased the activity of reporter-gene constructs approximately 2.5-fold. The other two, a C to A change at -1617 and a G to T change at -1712, decreased reporter activity by 30 and 40%, respectively. The sites at -1230 and -1617 were in linkage disequilibrium with each other and also with a polymorphic site near the DHII enhancer and sites in the apo(a) promoter and gene. The rarer G variant at -1230 was associated with smaller alleles. After correcting for the effect of allele size, values of Lp(a) concentration for alleles associated with the G variant at -1230 were 70% higher than those associated with the more common A variant. In contrast, the corrected values for alleles associated with the rare T variant at -1712 were 40% lower than those associated with the common G variant. Thus, overall the changes observed in this enhancer could influence apo(a) gene transcription up to 4-fold and could provide a significant contribution to the variation in Lp(a) concentrations in plasma.