Regulation of adipogenesis and fat storage by IRX3 and IRX5
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Obesity is a highly prevalent disease underlying several chronic diseases including Type 2 diabetes (T2D) and cardiovascular diseases (CVDs). Thus, increasing levels of obesity is associated with a series of co-morbidities and elevated risk of premature death. Obesity results from a chronic positive energy balance, causing white adipose tissue dysfunction, which in turn promotes dyslipidemia, systemic lipotoxicity and insulin resistance, eventually leading to ectopic fat accumulation and chronic diseases in multiple organs, including the heart, liver, arteries and kidneys. Unlike white adipocytes, beige fat cells are capable of disposing of excess energy by heat dissipation, thus protecting against obesity-related disease. Recently, two developmental transcription factors, IRX3 and IRX5, were shown to inhibit beige adipogenesis via an obesity associated risk genotype-dependent activation in preadipocytes. The aim of this study was therefore to investigate whether reducing IRX3 or IRX5 expression in adipose tissue offers protection from obesity, and if so, by which mechanisms. In paper I, we randomized wild type (WT) and Irx5 knock-out (KO) mice to a control or high-fat diet, and measured body weight, fat mass and global gene expression in adipose tissue. We found Irx5-KO mice to be lean and completely protected from diet-induced obesity. This was found to be partially attributable to increased mitochondrial respiration and thermogenesis due to reduction of Irx5 and App specifically in adipocytes. In papers II and III, we investigated the role of Irx3 in transcriptional regulation of adipogenesis, using WT and CRISPR-Cas9 mediated KO of Irx3 in preadipocytes, followed by RNA-, ATAC- and ChIP-sequencing. We found Irx3 to be critical for adipogenic identity and the ability of precursor cells to differentiate into mature adipocytes. Moreover, this lineage control was found to be mediated by direct transcriptional regulation of chromatin remodeling factors by Irx3. In conclusion, genetic repression of Irx3 or Irx5 offers strong protection against obesity, and reduces adipose tissue mass partially by increasing thermogenesis and improving mitochondrial respiration in existing adipocytes, and partially by preventing the formation of new adipocytes. This work has implications for identifying patients with genetic predisposition to obesity, who could benefit from potential therapeutic intervention targeting IRX3 or IRX5.
Has partsPaper I: Bjune JI, Haugen C, Gudbrandsen O, Nordbø OP, Nielsen HJ, Våge V, Njølstad PR, Sagen JV, Dankel SN and Mellgren G. IRX5 regulates adipocyte amyloid precursor protein and mitochondrial respiration in obesity. Int J Obes 2019;43: 2151-62. The article is not available in BORA due to publisher restrictions. The published version is available at: https://doi.org/10.1038/s41366-018-0275-y
Paper II: Bjune JI, Dyer L, Røsland GV, Tronstad KJ, Njølstad PR, Sagen JV, Dankel SN and Mellgren G. The homeobox factor Irx3 maintains adipogenic identity. Metabolism 2020;103: 154014. The article is available in the main thesis. The article is also available at: https://doi.org/10.1016/j.metabol.2019.154014
Paper III: Bjune JI, Laber S, Dyer L, Njølstad PR, Sagen JV, Claussnitzer M, Mellgren G, Cox R and Dankel SN. Epigenetic control of adipogenesis by Irx3. The article is not available in BORA.