Search

Genetic variation in the FAM13A (Family with Sequence Similarity 13 Member A) locus has been associated with several glycemic and metabolic traits in genome-wide association studies (GWAS). Here, we demonstrate that in humans, FAM13A alleles are associated with increased FAM13A expression in subcutaneous adipose tissue (SAT) and an insulin resistance-related phenotype (e.g. higher waist-to-hip ratio and fasting insulin levels, but lower body fat). In human adipocyte models, knockdown of FAM13A in preadipocytes accelerates adipocyte differentiation. In mice, Fam13a knockout (KO) have a lower visceral to subcutaneous fat (VAT/SAT) ratio after high-fat diet challenge, in comparison to their wild-type counterparts. Subcutaneous adipocytes in KO mice show a size distribution shift toward an increased number of smaller adipocytes, along with an improved adipogenic potential. Our results indicate that GWAS-associated variants within the FAM13A locus alter adipose FAM13A expression, which in turn, regulates adipocyte differentiation and contribute to changes in body fat distribution., Genetic variants in the FAM13A locus have been associated with anthropometric and glycemic traits. Here, using fine-mapping, in vitro knockdown studies in pre-adipocytes and in vivo knockout in mice, the authors show that FAM13A is involved in regulating fat distribution and metabolic traits.

We recently identified human N-acetyltransferase 2 (NAT2) as an insulin resistance (IR) gene. Here we examine the cellular mechanism linking NAT2 to IR and find that Nat1 (mouse ortholog of NAT2) is co-regulated with key regulators of mitochondria. RNA-interference mediated silencing of Nat1 led to mitochondrial dysfunction characterized by increased intracellular reactive oxygen species and mitochondrial fragmentation as well as decreased mitochondrial membrane potential, biogenesis, mass, cellular respiration and ATP generation. These effects were consistent in 3T3-L1 adipocytes, C2C12 myoblasts and in tissues from Nat1 deficient mice including white adipose tissue, heart and skeletal muscle. Nat1 deficient mice had changes in plasma metabolites and lipids consistent with a decreased ability to utilize fats for energy and a decrease in basal metabolic rate and exercise capacity without altered thermogenesis. Collectively, our results suggest that Nat1 deficiency results in mitochondrial dysfunction, which may constitute a mechanistic link between this gene and IR.

Zusammenfassung: Lipoprotein(a) [Lp(a)] ist eines der atherogensten Lipoproteine. Es besteht aus einem low density lipoprotein (LDL)-Partikel und dem spezifischen Glykoprotein Apo(a). Apo(a) ist homolog zu Plasminogen, weist jedoch im Gegensatz dazu einen spezifischen Größenpolymorphismus auf. Er beruht darauf, dass die Anzahl von Kringle-IV (K-IV) Domänen zwischen zwei und etwa 50 schwankt. Apo(a) wird fast nur in der Leber synthetisiert, wobei bis heute noch nicht eindeutig geklärt ist, ob Apo(a) und LDL intrazellulär oder im zirkulierenden Blut verknüpft werden. Die Lp(a)-Plasmakonzentration ist rechtsschief verteilt und reicht von weniger als 1 mg/dL bis mehr als 200 mg/dL. Sie ist zu 90 Prozent genetisch determiniert; die Anzahl an K-IV repeats korreliert invers mit der Konzentration von Lp(a). Im Apo(a)-Promoter befinden sich zahllose Bindungsstellen für Transkriptionsfaktoren und nukleäre Rezeptoren, wobei jene für HNF4α eine der wichtigsten ist. Die Bindung von HNF4α wird durch die Aktivierung von FXR unterdrückt, was zu einer signifikanten Reduktion der Apo(a)-Biosynthese führt. Neuere große epidemiologische Studien lassen keinen Zweifel daran, dass Lp(a) ein kausaler Risikofaktor für KHK und Herzinfarkt ist, weshalb neue Ansätze erforscht werden, erhöhte Lp(a)-Konzentrationen medikamentös zu korrigieren. Unter den zugelassenen Lipidsenkern senkt nur Nikotinsäure, dessen molekulare Wirkweise wir kürzlich aufklären konnten, in konsistentem und klinisch bedeutsamem Ausmaß das Lp(a). Neue Medikationen die sich derzeit in Klinischen Tests befinden, unter ihnen CETP-Inhibitoren, PCSK9 Antikörper, der MTP Hemmer Lomitapide und Antisense-Oligonukleotide, erscheinen vielversprechend. Das spezifisch gegen die mRNA des Apo(a) gerichtete Antisense-Oligonukleotide APO(a)Rx ® von ISIS weist die stärkste Wirkung auf. Es eröffnet die Möglichkeit, den Effekt einer selektiven Absenkung des Lp(a) auf klinische Ereignisse zu untersuchen. Lp(a) ist ein wichtiger Parameter zur Ermittlung des Risikos für Atherosklerose. Die Bestimmung im klinischen Labor war für lange Zeit nicht standardisiert. Neue Analysenmethoden, vor allem Enzymimmunoassays mit monoklonalen Antikörpern, die nur ein Epitop im Apo(a) erkennen, sowie nephelometrische und turbidimetrische Methoden, könnten jedoch potentiell vergleichbare Resultate in unterschiedlichen Laboren generieren.

Lipoprotein(a) [Lp(a)] is one of the most atherogenic lipoproteins consisting of a core low-density lipoprotein particle and the specific glycoprotein apo(a). Apo(a) is homologous to plasminogen yet in contrast exhibits a specific size polymorphism. This polymorphism is due to the fact that the number of kringle-IV (K-IV) repeats ranges between two and approximately 50. Apo(a) is synthesized almost exclusively in the liver, and there is still some discussion regarding whether the assembly of Lp(a) occurs intracellularly or in the circulating blood. The plasma Lp(a) concentration is markedly skewed to the right and extends from Rx ®, an antisense oligonucleotide, which is specifically directed against the mRNA for apo(a), has the strongest effect on Lp(a). It offers the opportunity to examine the impact of selective Lp(a) lowering on clinical events. Lp(a) emerged as an important screening parameter to assess the risk for atherosclerosis. Its quantitation in the clinical laboratory had not been standardized for a long period of time. New commercial methods, in particular enzyme immunoassays with monoclonal antibodies that recognize single epitopes in apo(a), or nephelometric and turbidimetric assays hold the potential to warrant comparable results in different laboratories.

Lipoprotein(a) [Lp(a)] consists of a low-density lipoprotein (LDL)-like core and apo(a), a large molecular weight glycoprotein. Apo(a) is highly homologous to plas‐ minogen, yet in contrast exhibits a unique size polymorphism that is characterized by an increasing number of kringle-IV (K-IV) repeats. The number of K-IV repeats ranges from n = 2 to n = 40 or even higher. Apo(a) is synthesized almost exclusively in the liver and there is still some debate whether the assembly of Lp(a) from LDL and apo(a) occurs inside the liver cells or in the circulating blood. The plasma Lp(a) con‐ centration is markedly skewed reaching from 200 mg/dl. The plasma concentration is >90% genetically determined and correlates negatively with the num‐ ber of K-IV repeats. In the apo(a) promoter, there are numerous response elements for transcription factors and nuclear receptors that regulate apo(a) expression. The HNF4α binding sequence appears to be the most important one in that respect, yet further work needs to be done to unravel the key features of apo(a) biosynthesis un‐ der different conditions. Importantly, activation of FXR causes the dissociation of HNF4α α from its response element and in turn a significant downregulation of apo(a) transcription. Undoubtedly, Lp(a) is one of the most atherogenic lipoproteins and recent large epi‐ demiological studies document quite impressively that Lp(a) is an independent causal risk factor for coronary heart disease (CHD) and myocardial infarction. This fact led to the development of specific medications to reduce Lp(a) in patients with high plas‐ ma concentrations. Among the registered lipid-lowering drugs, only nicotinic acid has a consistently significant Lp(a)-lowering effect, and we recently succeeded in elucidat‐ ing the mode of action of this drug. There are numerous medications in the pipeline for the treatment of hyper-Lp(a). Among those that are currently in clinical trials, CETP inhibitors, PCSK9 antibodies, MTP inhibitors as well as antisense oligonucleoti‐ des (ASO), such as the specific APO(a)Rx® from ISIS, which is directed against apo(a)mRNA and appears to be the most promising drug as it lowers Lp(a) levels by more than 90%. Lp(a) emerged as an important screening parameter to assess coronary atherosclerosis risk. Its quantitation in the clinical laboratory was, for a long time, quite problematic © 2015 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. since commonly accepted reference materials and standardized analytical methods were lacking. However, newer commercial assays based on nephelometry or tur‐ bidimetry, or ELISA using monoclonal antibodies that recognize single epitopes in apo(a), warrant comparable interlaboratory results.

Liver X receptors (LXRα and LXRβ) are key transcription factors in cholesterol metabolism that regulate cholesterol biosynthesis/efflux and bile acid metabolism/excretion in the liver and numerous organs. In macrophages, LXR signaling modulates cholesterol handling and the inflammatory response, pathways involved in atherosclerosis. Since regulatory pathways of LXR transcription control are well understood, in the present study we aimed at identifying post-transcriptional regulators of LXR activity. MicroRNAs (miRs) are such post-transcriptional regulators of genes that in the canonical pathway mediate mRNA inactivation. In silico analysis identified miR-206 as a putative regulator of LXRα but not LXRβ. Indeed, as recently shown, we found that miR-206 represses LXRα activity and expression of LXRα and its target genes in hepatic cells. Interestingly, miR-206 regulates LXRα differently in macrophages. Stably overexpressing miR-206 in THP-1 human macrophages revealed an up-regulation and miR-206 knockdown led to a down-regulation of LXRα and its target genes. In support of these results, bone marrow-derived macrophages (BMDMs) from miR-206 KO mice also exhibited lower expression of LXRα target genes. The physiological relevance of these findings was proven by gain- and loss-of-function of miR-206; overexpression of miR-206 enhanced cholesterol efflux in human macrophages and knocking out miR-206 decreased cholesterol efflux from MPMs. Moreover, we show that miR-206 expression in macrophages is repressed by LXRα activation, while oxidized LDL and inflammatory stimuli profoundly induced miR-206 expression. We therefore propose a feed-back loop between miR-206 and LXRα that might be part of an LXR auto-regulatory mechanism to fine tune LXR activity., Graphical abstract, Highlights • Functional differences of miR-206 in the liver and macrophages • In the liver, miR-206 suppresses LXRα expression and signaling. • In macrophages, miR-206 increases LXRα abundance and promotes cholesterol efflux. • In macrophages, LXRα activation represses miR-206 expression. • In macrophages, pro-inflammatory stimuli increase miR-206 expression.

Elevated plasma lipoprotein(a) (LPA) levels are recognized as an independent risk factor for cardiovascular diseases. Our knowledge on LPA metabolism is incomplete, which makes it difficult to develop LPA-lowering medications. Nicotinic acid (NA) is the main drug recommended for the treatment of patients with increased plasma LPA concentrations. The mechanism of NA in lowering LPA is virtually unknown. To study this mechanism, we treated transgenic (tg) APOA mice with NA and measured plasma APOA and hepatic mRNA levels. In addition, mouse and human primary hepatocytes were incubated with NA, and the expression of APOA was followed. Feeding 1% NA reduced plasma APOA and hepatic expression of APOA in tg-APOA mice. Experiments with cultured human and mouse primary hepatocytes in addition to reporter assays performed in HepG2 cells revealed that NA suppresses APOA transcription. The region between −1446 and −857 of the human APOA promoter harboring several cAMP response element binding sites conferred the negative effect of NA. In accordance, cAMP stimulated APOA transcription, and NA reduced hepatic cAMP levels. It is suggested that cAMP signaling might be involved in reducing APOA transcription, which leads to the lowering of plasma LPA.

Objective— Lipoprotein(a) is a highly atherogenic lipoprotein, whose metabolism is poorly understood. Currently no safe drugs exists that lower elevated plasma lipoprotein(a) concentrations. We therefore focused on molecular mechanisms that influence apolipoprotein(a) (APOA) biosynthesis. Methods and Results— Transgenic human APOA mice (tg- APO mice) were injected with 1 mg/kg of recombinant human fibroblast growth factor 19 (FGF19). This led to a significant reduction of plasma APOA and hepatic expression of APOA . Incubation of primary hepatocytes of tg- APOA mice with FGF19 induced ERK1/2 phosphorylation and, in turn, downregulated APOA expression. Repression of APOA by FGF19 was abrogated by specific ERK1/2 phosphorylation inhibitors. The FGF19 effect on APOA was attenuated by transfection of primary hepatocytes with siRNA against the FGF19 receptor 4 (FGFR4). Using promoter reporter assays, mutation analysis, gel shift, and chromatin immune-precipitation assays, an Ets-1 binding element was identified at –1630/–1615bp region in the human APOA promoter. This element functions as an Elk-1 binding site that mediates repression of APOA transcription by FGF19. Conclusion— These findings provide mechanistic insights into the transcriptional regulation of human APOA by FGF19. Further studies in the human system are required to substantiate our findings and to design therapeutics for hyper lipoprotein(a).