Uncarboxylated osteocalcin (ucOC) stimulates muscle glucose uptake in mice EDL and soleus muscles. However, whether ucOC also exerts a similar effect in insulin-stimulated muscles in a muscle type-specific manner is currently unclear. We aimed to test the hypothesis that, with insulin stimulation, ucOC per se has a greater effect on oxidative muscle compared with glycolytic muscle, and to explore the underlying mechanisms. Mouse (C57BL6, male 9–12 weeks) extensor digitorum longus (EDL) and soleus muscles were isolated and longitudinally split into halves. Muscle samples were treated with varying doses of recombinant ucOC (0, 0.3, 1, 3, 30 ng/ml), followed by insulin addition. Muscle glucose uptake, protein phosphorylation and total expression of protein kinase B (Akt), Akt substrate of 160 kDa (AS160), extracellular signal-regulated kinase isoform 2 (ERK2), and adenosine monophosphate-activated protein kinase subunit α (AMPKα) were assessed. ucOC treatment at 30 ng/ml enhanced muscle glucose uptake in insulin-stimulated soleus, a mainly oxidative muscle (17.5%, p < 0.05), but not in EDL—a mostly glycolytic muscle. In insulin-stimulated soleus only, ucOC treatment (3 and 30 ng/ml) increased phosphorylation of AS160 and ERK2, but not Akt or AMPKα. The ucOC-induced increase in ERK2 phosphorylation in soleus was not associated with the increase in glucose uptake or AS160 phosphorylation. ucOC enhances glucose uptake and AS160 phosphorylation in insulin-stimulated oxidative but not glycolytic muscle, via upstream mechanisms which appear to be independent of ERK or AMPK.
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Associate Professor Itamar Levinger is a Future Leader Fellow supported by Australian Heart Foundation (ID: 100040).
XL, XZ, TS, and IL designed the study; XL, LP, and EM performed experiments; XL and LP analyzed the data; XL, LP, AH, GM, TS, and IL interpreted the results; XL wrote the first draft of the paper; XL, LP, XZ, AH, GM, TS, and IL edited and revised the manuscript; All authors approved final version of the manuscript.
Compliance with Ethical Standards
Conflict of interest
Xuzhu Lin, Lewan Parker, Emma Mclennan, Xinmei Zhang, Alan Hayes, Glenn McConell, Tara C. Brennan-Speranza, and Itamar Levinger declare no conflict of interest.
Human and Animal Rights and Informed Consent
All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. All procedures performed in studies involving animals were in accordance with the ethical standards of the institution or practice at which the studies were conducted.
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