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Frøbert AM, Brohus M, Roesen TS, Kindberg J, Fröbert O, Conover CA, Overgaard MT. Circulating insulin-like growth factor system adaptations in hibernating brown bears indicate increased tissue IGF availability. Am J Physiol Endocrinol Metab 2022; 323:E307-E318. [PMID: 35830688 DOI: 10.1152/ajpendo.00429.2021] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Brown bears conserve muscle and bone mass during 6 mo of inactive hibernation. The molecular mechanisms underlying hibernation physiology may have translational relevance for human therapeutics. We hypothesize that protective mechanisms involve increased tissue availability of insulin-like growth factors (IGFs). In subadult Scandinavian brown bears, we observed that mean plasma IGF-1 and IGF-2 levels during hibernation were reduced to 36 ± 10% and 56 ± 15%, respectively, compared with the active state (n = 12). Western ligand blotting identified IGF-binding protein (IGFBP)-3 as the major IGFBP in the active state, whereas IGFBP-2 was codominant during hibernation. Acid labile subunit (ALS) levels in hibernation were reduced to 41±16% compared with the active state (n = 6). Analysis of available grizzly bear RNA sequencing data revealed unaltered liver mRNA IGF-1, IGFBP-2, and IGFBP-3 levels, whereas ALS levels were significantly reduced during hibernation (n = 6). Reduced ALS synthesis and circulating levels during hibernation should prompt a shift from ternary IGF/IGFBP/ALS to smaller binary IGF/IGFBP complexes, thereby increasing IGF tissue availability. Indeed, size-exclusion chromatography of bear plasma demonstrated a shift to lower molecular weight IGF-containing complexes in the hibernating versus the active state. Furthermore, we note that the major IGF-2 mRNA isoform expressed in livers in both Scandinavian brown bears and grizzly bears was an alternative splice variant in which Ser29 is replaced with a tetrapeptide possessing a positively charged Arg residue. Homology modeling of the bear IGF-2/IGFBP-2 complex showed the tetrapeptide in proximity to the heparin-binding domain involved in bone-specific targeting of this complex. In conclusion, this study provides data which suggest that increased IGF tissue availability combined with tissue-specific targeting contribute to tissue preservation in hibernating bears.NEW & NOTEWORTHY Brown bears shift from circulating ternary IGF/IGFBP/ALS complexes in the active state to binary IGF/IGFBP complexes during hibernation, indicating increased tissue IGF-bioactivity. Furthermore, brown bears use a splice variant of IGF-2, suggesting increased bone-specific targeting of IGF anabolic signaling.
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Affiliation(s)
- Anne Mette Frøbert
- Department of Chemistry and Bioscience, Faculty of Engineering and Science, Aalborg University, Aalborg, Denmark
| | - Malene Brohus
- Department of Chemistry and Bioscience, Faculty of Engineering and Science, Aalborg University, Aalborg, Denmark
| | - Tinna S Roesen
- Department of Chemistry and Bioscience, Faculty of Engineering and Science, Aalborg University, Aalborg, Denmark
| | - Jonas Kindberg
- Department of Wildlife, Fish and Environmental Studies, Swedish University of Agricultural Sciences, Umeå, Sweden
- Norwegian Institute for Nature Research, Trondheim, Norway
| | - Ole Fröbert
- Department of Cardiology, Faculty of Health, Örebro University, Örebro, Sweden
- Department of Clinical Medicine, Aarhus University Health, Aarhus, Denmark
- Department of Clinical Pharmacology, Aarhus University Hospital, Aarhus, Denmark
- Steno Diabetes Center Aarhus, Aarhus University Hospital, Aarhus, Denmark
| | - Cheryl A Conover
- Division of Endocrinology, Metabolism, and Nutrition, Mayo Clinic, Rochester, Minnesota
| | - Michael T Overgaard
- Department of Chemistry and Bioscience, Faculty of Engineering and Science, Aalborg University, Aalborg, Denmark
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