Calorie Restriction-Induced Increase in Skeletal Muscle Insulin Sensitivity Is Not Prevented by Overexpression of the p55α Subunit of Phosphoinositide 3-Kinase.

Vitor F Martins,Simon Schenk,Carrie E Mccurdy,Ji H Kang,Kristoffer Svensson,Wallace S Chick,Byron Hetrick,Shahriar Tahvilian

doi:10.3389/fphys.2018.00789

Abstract

Introduction: The Phosphoinositide 3-kinase (PI3K) signaling pathway plays an important role in skeletal muscle insulin-stimulated glucose uptake. While whole-body and tissue specific knockout (KO) of individual or combinations of the regulatory subunits of PI3K (p85α, p55α, and p50α or p85β); increase insulin sensitivity, no study has examined whether increasing the expression of the individual regulatory subunits would inhibit insulin action in vivo. Therefore, the objective of this study was to determine whether skeletal muscle-specific overexpression of the p55α regulatory subunit of PI3K impairs skeletal muscle insulin sensitivity, or prevents its enhancement by caloric restriction.Methods: We developed a novel “floxed” mouse that, through the Cre-LoxP approach, allows for tamoxifen (TMX)-inducible and skeletal muscle-specific overexpression of the p55α subunit of PI3K (referred to as, ‘p55α-mOX’). Beginning at 10 weeks of age, p55α-mOX mice and their floxed littermates (referred to as wildtype [WT]) either continued with free access to food (ad libitum; AL), or were switched to a calorie restricted diet (CR; 60% of AL intake) for 20 days. We measured body composition, whole-body energy expenditure, oral glucose tolerance and ex vivo skeletal muscle insulin sensitivity in isolated soleus and extensor digitorum longus muscles using the 2-deoxy-glucose (2DOG) uptake method.Results: p55α mRNA and protein expression was increased ∼2 fold in muscle from p55α-mOX versus WT mice. There were no differences in energy expenditure, total activity, or food intake of AL-fed mice between genotypes. Body weight, fat and lean mass, tissue weights, and fasting glucose and insulin were comparable between p55α-mOX and WT mice on AL, and were decreased equally by CR. Interestingly, overexpression of p55α did not impair oral glucose tolerance or skeletal muscle insulin signaling or sensitivity, nor did it impact the ability of CR to enhance these parameters.Conclusion: Skeletal muscle-specific overexpression of p55α does not impact skeletal muscle insulin action, suggesting that p85α and/or p50α may be more important regulators of skeletal muscle insulin signaling and sensitivity.

Highlights

The Phosphoinositide 3-kinase (PI3K) signaling pathway plays an important role in skeletal muscle insulin-stimulated glucose uptake
Development, Generation, and Validation of the p55α-mOX Mouse Model p55α mRNA expression was ∼2-fold higher in skeletal muscle of p55α-mOX versus WT mice, but was comparable in epididymal white adipose tissue (eWAT) and liver. p50α and p85α mRNA expression were comparable between p55α-mOX and WT mice across all tissues (Figure 1B)
Whole-body oxygen consumption (VO2), respiratory exchange ratio (RER), total activity, and food intake were significantly increased in the dark vs. light phase, but were not different between genotypes (Figure 2)

Summary

Introduction

The Phosphoinositide 3-kinase (PI3K) signaling pathway plays an important role in skeletal muscle insulin-stimulated glucose uptake. The objective of this study was to determine whether skeletal muscle-specific overexpression of the p55α regulatory subunit of PI3K impairs skeletal muscle insulin sensitivity, or prevents its enhancement by caloric restriction. A phosphorylation-based signaling cascade is required for insulin action, with phosphoinositide 3-kinase (PI3K) being central for the propagation of insulin signaling to glucose uptake in skeletal muscle (Lee et al, 1995; Yeh et al, 1995). Certain studies propose that the ratio of catalytic-to-regulatory subunits might modulate the regulation of insulin sensitivity (Ueki et al, 2000, 2002a; Brachmann et al, 2005)

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