Insulin and IGF-1 receptors regulate complex I-dependent mitochondrial bioenergetics and supercomplexes via FoxOs in muscle.

Decreased skeletal muscle strength and mitochondrial dysfunction are characteristic of diabetes. The actions of insulin and IGF-1 through the insulin receptor (IR) and IGF-1 receptor (IGF1R) maintain muscle mass via suppression of forkhead box O (FoxO) transcription factors, but whether FoxO activation coordinates atrophy in concert with mitochondrial dysfunction is unknown. We show that mitochondrial respiration and complex I activity were decreased in streptozotocin (STZ) diabetic muscle, but these defects were reversed in muscle-specific FoxO1, -3, and -4 triple-KO (M-FoxO TKO) mice rendered diabetic with STZ. In the absence of systemic glucose or lipid abnormalities, muscle-specific IR KO (M-IR-/-) or combined IR/IGF1R KO (MIGIRKO) impaired mitochondrial respiration, decreased ATP production, and increased ROS. These mitochondrial abnormalities were not present in muscle-specific IR, IGF1R, and FoxO1, -3, and -4 quintuple-KO mice (M-QKO). Acute tamoxifen-inducible deletion of IR and IGF1R also decreased muscle pyruvate respiration, complex I activity, and supercomplex assembly. Although autophagy was increased when IR and IGF1R were deleted in muscle, mitophagy was not increased. Mechanistically, RNA-Seq revealed that complex I core subunits were decreased in STZ-diabetic and MIGIRKO muscle, and these changes were not present with FoxO KO in STZ-FoxO TKO and M-QKO mice. Thus, insulin-deficient diabetes or loss of insulin/IGF-1 action in muscle decreases complex I-driven mitochondrial respiration and supercomplex assembly in part by FoxO-mediated repression of complex I subunit expression.

Loss of FoxOs in muscle reveals sex-based differences in insulin sensitivity but mitigates diet-induced obesity.

OBJECTIVE: Gender influences obesity-related complications, including diabetes. Females are more protected from insulin resistance after diet-induced obesity, which may be related to fat accumulation and muscle insulin sensitivity. FoxOs regulate muscle atrophy and are targets of insulin action, but their role in muscle insulin sensitivity and mitochondrial metabolism is unknown. METHODS: We measured muscle insulin signaling, mitochondrial energetics, and metabolic responses to a high-fat diet (HFD) in male and female muscle-specific FoxO1/3/4 triple knock-out (TKO) mice. RESULTS: In male TKO muscle, insulin-stimulated AKT activation was decreased. AKT2 protein and mRNA levels were reduced and insulin receptor protein and IRS-2 mRNA decreased. These changes contributed to decreased insulin-stimulated glucose uptake in glycolytic muscle in males. In contrast, female TKOs maintain normal insulin-mediated AKT phosphorylation, normal AKT2 levels, and normal glucose uptake in glycolytic muscle. When challenged with a HFD, fat gain was attenuated in both male and female TKO mice, and associated with decreased glucose levels, improved glucose homeostasis, and reduced muscle triglyceride accumulation. Furthermore, female TKO mice showed increased energy expenditure, relative to controls, due to increased lean mass and maintenance of mitochondrial function in muscle. CONCLUSIONS: FoxO deletion in muscle uncovers sexually dimorphic regulation of AKT2, which impairs insulin signaling in male mice, but not females. However, loss of FoxOs in muscle from both males and females also leads to muscle hypertrophy and increases in metabolic rate. These factors mitigate fat gain and attenuate metabolic abnormalities in response to a HFD.

FoxO Transcription Factors Are Critical Regulators of Diabetes-Related Muscle Atrophy.

Insulin deficiency and uncontrolled diabetes lead to a catabolic state with decreased muscle strength, contributing to disease-related morbidity. FoxO transcription factors are suppressed by insulin and thus are key mediators of insulin action. To study their role in diabetic muscle wasting, we created mice with muscle-specific triple knockout of FoxO1/3/4 and induced diabetes in these M-FoxO-TKO mice with streptozotocin (STZ). Muscle mass and myofiber area were decreased 20-30% in STZ-Diabetes mice due to increased ubiquitin-proteasome degradation and autophagy alterations, characterized by increased LC3-containing vesicles, and elevated levels of phosphorylated ULK1 and LC3-II. Both the muscle loss and markers of increased degradation/autophagy were completely prevented in STZ FoxO-TKO mice. Transcriptomic analyses revealed FoxO-dependent increases in ubiquitin-mediated proteolysis pathways in STZ-Diabetes, including regulation of Fbxo32 (Atrogin1), Trim63 (MuRF1), Bnip3L, and Gabarapl. These same genes were increased 1.4- to 3.3-fold in muscle from humans with type 1 diabetes after short-term insulin deprivation. Thus, FoxO-regulated genes play a rate-limiting role in increased protein degradation and muscle atrophy in insulin-deficient diabetes.