Article | Published:

IRX5 regulates adipocyte amyloid precursor protein and mitochondrial respiration in obesity

International Journal of Obesity (2018) | Download Citation

Abstract

Objective

A causal obesity risk variant in the FTO locus was recently shown to inhibit adipocyte thermogenesis via increased adipose expression of the homeobox transcription factors IRX3 and IRX5. However, causal effects of IRX5 on fat storage remain to be shown in vivo, and discovery of downstream mediators may open new therapeutic avenues.

Methods

17 WT and 13 Irx5 knockout (KO) mice were fed low-fat control (Ctr) or high-fat (HF) diet for 10 weeks. Body weight, energy intake and fat mass were measured. Irx5-dependent gene expression was explored by transcriptome analysis of epididymal white adipose tissue (eWAT), confirmatory obesity-dependent expression in human adipocytes in vivo, and in vitro knock-down, overexpression and transcriptional activation assays.

Results

Irx5 knock-out mice weighed less, had diminished fat mass, and were protected from diet-induced fat accumulation. Key adipose mitochondrial genes Pparγ coactivator 1-alpha (Pgc-1α) and uncoupling protein 1 (Ucp1) were upregulated, and a gene network centered on amyloid precursor protein (App) was downregulated in adipose tissue of knock-out mice and in isolated mouse adipocytes with stable Irx5 knock-down. An APP-centered network was also enriched in isolated adipocytes from obese compared to lean humans. IRX5 overexpression increased APP promoter activity and both IRX5 and APP inhibited transactivation of PGC-1α and UCP1. Knock-down of Irx5 or App increased mitochondrial respiration in adipocytes.

Conclusion

Irx5-KO mice were protected from obesity and this can partially be attributed to reduced adipose App and improved mitochondrial respiration. This novel Irx5-App pathway in adipose tissue is a possible therapeutic entry point against obesity.

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Acknowledgements

We thank Carol Cook, Margit Solsvik, Rita Holdhus, Alba Kaci, Regine Åsen Jersin, André Madsen, Mona Nilsen and Tone Flølo for excellent technical assistance. We thank the patients and hospital staff at Voss and Haraldsplass Hospitals for the adipose tissue biopsies. The Irx5-KO mice were a gift from Drs. Kyoung-Han Kim and Chi-chung Hui. The pGL4-phAPP-luc reporter construct was provided by the RIKEN BRC through the National Bio-Resource Project of the MEXT, Japan. The mPgc-1α-luc construct was a gift from Dr. Bruce Spiegelman and the pGL3-UCP1-luc construct was a gift from Dr. Leslie P. Kozak. The ME3 cells were a gift from Dr. Karsten Kristiansen.

Author contributions

SD and GM conceived the study and designed the experiments. JIB, CH, and OG designed and conducted the experiments. HJN and VV obtained the human adipose tissue biopsies and performed the bariatric surgery. OPN made the CRISPR-cells. SD and JIB performed the data analyses. JIB, CH and SD drafted the manuscript. JIB, CH, OG, OPN, HJN, VV, PRN, JVS, SD and GM contributed to data interpretation, discussion and reviewed and edited the manuscript.

Author information

Author notes

  1. These authors contributed equally: Jan-Inge Bjune, Christine Haugen.

  2. These authors contributed equally: Simon N. Dankel, Gunnar Mellgren.

Affiliations

  1. KG Jebsen Center for Diabetes Research, Department of Clinical Science, University of Bergen, 5020, Bergen, Norway

    • Jan-Inge Bjune
    • , Christine Haugen
    • , Ole P. Nordbø
    • , Pål R. Njølstad
    • , Jørn V. Sagen
    • , Simon N. Dankel
    •  & Gunnar Mellgren
  2. Hormone Laboratory, Haukeland University Hospital, 5021, Bergen, Norway

    • Jan-Inge Bjune
    • , Christine Haugen
    • , Ole P. Nordbø
    • , Jørn V. Sagen
    • , Simon N. Dankel
    •  & Gunnar Mellgren
  3. Department of Clinical Medicine, University of Bergen, 5020, Bergen, Norway

    • Oddrun Gudbrandsen
  4. Department of Surgery, Voss Hospital, 5704, Voss, Norway

    • Hans J. Nielsen
    •  & Villy Våge
  5. Department of Pediatrics and Adolescents, Haukeland University Hospital, 5021, Bergen, Norway

    • Pål R. Njølstad

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Funding for this project was provided by Persontilpasset medisin for barn og voksne med diabetes mellitus (PERSON-MED-DIA), the Western Norway Regional Health Authority and Meltzerfondet. PRN was supported by the European Research Council (AdG #293574).

Conflict of interest

The authors declare that they have no conflict of interest.

Corresponding authors

Correspondence to Simon N. Dankel or Gunnar Mellgren.

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DOI

https://doi.org/10.1038/s41366-018-0275-y