Mitochondria-related transcriptional signature is downregulated in adipocytes in obesity ‒ a study of young healthy MZ twins

Abstract

<p><b>Supplementary data tables<br></b></p><p><br> </p><p><b>Mitochondria-related transcriptional signature is downregulated in adipocytes in obesity ‒ a study of young healthy MZ twins</b></p> <p>Sini Heinonen ¹, Maheswary Muniandy¹, Jana Buzkova², Adil Mardinoglu^3,4, Amaia Rodríguez⁵, Gema Frühbeck⁵, Antti Hakkarainen⁶, Jesper Lundbom^6,7, Nina Lundbom⁶, Jaakko Kaprio^8,9,10,Aila Rissanen^1,11, Kirsi H. Pietiläinen^1,8,12</p> <p><b> </b></p> <p><b>¹</b>Obesity Research Unit, Research Programs Unit, Diabetes and Obesity, University of Helsinki, Helsinki, Finland 00014,²Research Programs Unit, Molecular Neurology, Biomedicum-Helsinki, University of Helsinki, Helsinki, Finland 00290, ³Department of Biology and Biological Engineering, Chalmers University of Technology, SE‑412 96, Gothenburg, Sweden, ⁴Science for Life Laboratory, KTH - Royal Institute of Technology, SE‑171 21, Stockholm, Sweden ⁵Metabolic Research Laboratory, Clínica Universidad de Navarra, &</p> <p> CIBERobn, Instituto de Salud Carlos III, Pamplona, Spain 31008, ⁶HUS Medical Imaging Center, Radiology, University of Helsinki and Helsinki University Hospital, Helsinki, Finland 00014, ⁷Institute for Clinical Diabetology, German Diabetes Center, Leibniz Center for Diabetes Research, Heinrich Heine University, Düsseldorf, Germany, ⁸FIMM, Institute for Molecular Medicine, University of Helsinki, Helsinki, Finland 00014, ⁹Finnish Twin Cohort Study, Department of Public Health, University of Helsinki, Helsinki Finland 00014, ¹⁰National Institute for Health and Welfare, Department of Health, Helsinki, Finland 00271, ¹¹Department of Psychiatry, Helsinki University Central Hospital, Helsinki, Finland,¹²Endocrinology, Abdominal Center, Helsinki University Hospital and University of Helsinki, Helsinki, Finland 00290.</p> <p><b>Abstract </b></p> <p> </p> <p><b>Introduction</b>:</p> <p>Low mitochondrial activity in adipose tissue is suggested to be an underlying factor in obesity and its metabolic complications. It is still unknown however whether this is due to malfunction of adipocytes or of other cells in adipose tissue. </p> <p><b>Methods:</b></p> <p>We studied young adult monozygotic (MZ) twin pairs discordant (<i>n </i>= 14, intrapair difference ΔBMI > 3 kg/m²) and concordant (<i>n </i>= 5, ΔBMI < 3 kg/m²) for BMI, identified from ten birth cohorts of 22-36-year-old Finnish twins. Abdominal body fat distribution (magnetic resonance imaging, MR), liver fat content (MR spectroscopy), insulin sensitivity (oral glucose tolerance test OGTT), hs-CRP, serum lipids and adipokines were measured. Subcutaneous abdominal adipose tissue biopsies were obtained to analyze the transcriptomics patterns (Affymetrix U133 Plus 2.0 chips) of the isolated adipocytes, as well as of the whole adipose tissue. Mitochondrial DNA (mtDNA) transcript levels in adipocytes were measured by qRT-PCR. Western blots of oxidative phosphorylation (OXPHOS) protein levels in adipocytes were performed in heavy and lean unrelated individuals.</p> <p><b>Results:</b></p> <p>The heavier (BMI 29.9 ± 1.0 kg/m²) co-twins of the discordant twin pairs had more subcutaneous, intra-abdominal and liver fat and were more insulin-resistant (<i>p</i><0.01 for all measures) than the leaner (24.1 ± 0.9 kg/m²) co-twins. Altogether 2538 genes in adipocytes and 2135 in adipose tissue were significantly differentially expressed (nominal <i>p</i><0.05) between the co-twins. Pathway-analysis of these transcripts in both isolated adipocytes and adipose tissue revealed that the heavier co-twins had reduced expression of mitochondrial pathways, a result that was replicated when analyzing the pathways behind most consistently downregulated genes in the heavy co-twins (in at least 12 out of 14 pairs). Consistently upregulated genes in adipocytes were related to inflammation.</p> <p>As previously found in adipose tissue, we now confirmed that mitochondrial DNA transcript levels (<i>12S</i> RNA, <i>16S</i> RNA, <i>COX1</i>, <i>ND5</i>, <i>CYTB</i>; qRT-PCR), expression of mitochondrial ribosomal protein transcripts (Affymetrix) and a major mitochondrial regulator PGC-1α were reduced in the heavier co-twins’ adipocytes (<i>p</i><0.05). OXPHOS protein levels of complex I and III in adipocytes were lower in heavier than in leaner individuals.</p> <p><b>Conclusion: </b></p> <p>Subcutaneous abdominal adipocytes in obesity show global expressional downregulation of oxidative pathways, mitochondrial transcripts and OXPHOS protein levels, and upregulation of inflammatory pathways. </p> <p><b>1. Conflict of Interest:</b> None  </p> <p><b>2. Funding:</b>  Orion Foundation, Biomedicum Helsinki Foundation, Paulo Foundation, Finnish Medical Foundation, Maud Kuistila Foundation, Emil Aaltonen Foundation, Academy of Finland (Center of Excellence grant# 265240, 272376, 263278 and 266286), Novo Nordisk Foundation, Jalmari and Rauha Ahokas Foundation, Finnish Diabetes Research Foundation, Finnish Foundation for Cardiovascular Research, Knut and Alice Wallenberg Foundation, Fondo de Investigación Sanitaria-FEDER and CIBEROBN from the Instituto de  Salud Carlos III, Spain, University of Helsinki and Helsinki University Central Hospital grants.</p> <p><b>Keywords:</b></p> <p>Adipocytes, gene expression, mitochondria, obesity, twins </p