Diabetes and insulin resistance (IR) remain major global health challenges, underscoring the need for novel therapeutic strategies. Here, we identify an autophagy-independent role of circulating autophagy-related gene 7 (ATG7) in metabolic regulation. Circulating ATG7 enhances insulin sensitivity and glucose homeostasis by directly interacting with IRS1 and modulating insulin signaling (IS) through liver-muscle crosstalk. Mechanistically, ATG7 binds to IRS1, promoting its activation and the propagation of downstream IS. Notably, we identify an ATG7-derived peptide (Aap2) that recapitulates ATG7's insulin-sensitizing effects and improves glycemic control in both Type 1 and Type 2 diabetic mouse models. These findings establish ATG7 as a key regulator of IS and suggest that targeting ATG7 may represent a promising therapeutic approach for IR and diabetes.

CBL is rapidly phosphorylated upon insulin receptor activation. Mice whole body CBL depletion improved insulin sensitivity and glucose clearance; however, the precise mechanisms remain unknown. We depleted either CBL or its associated protein SORBS1/CAP independently in myocytes and assessed mitochondrial function and metabolism compared to control cells. CBL- and CAP-depleted cells showed increased mitochondrial mass with greater proton leak. Mitochondrial respiratory complex I activity and assembly into respirasomes were reduced. Proteome profiling revealed alterations in proteins involved in glycolysis and fatty acid degradation. Our findings demonstrate CBL/CAP pathway couples insulin signaling to efficient mitochondrial respiratory function and metabolism in muscle.

X Sravan K. Thondam, Christina Daousi, John P. H. Wilding, Jens J. Holst, Gulizar I. Ameen, Chenjing Yang, Catherine Whitmore, Silvia Mora, and Daniel J. Cuthbertson Obesity and Endocrinology Research Group, University Hospital Aintree, Liverpool, United Kingdom; Institute of Ageing and Chronic Disease, University of Liverpool, Liverpool, United Kingdom; Department of Cellular and Molecular Physiology University of Liverpool, Liverpool, United Kingdom; and NovoNordisk Foundation Center for Basic Metabolic Research and Department of Biomedical Sciences, University of Copenhagen, Copenhagen, Denmark

Glucose-dependent insulinotropic polypeptide (GIP) beyond its insulinotropic effects may regulate postprandial lipid metabolism. Whereas the insulinotropic action of GIP is known to be impaired in type 2 diabetes mellitus (T2DM), its adipogenic effect is unknown. We hypothesized that GIP is anabolic in human subcutaneous adipose tissue (SAT) promoting triacylglycerol (TAG) deposition through reesterification of nonesterified fatty acids (NEFA), and this effect may differ according to obesity status or glucose tolerance. Twenty-three subjects categorized into four groups, normoglycemic lean (n = 6), normoglycemic obese (n = 6), obese with impaired glucose regulation (IGR; n = 6), and obese T2DM (n = 5), participated in a double-blind, randomized, crossover study involving a hyperglycemic clamp with a 240-min GIP infusion (2 pmol·kg-1·min-1) or normal saline. Insulin, NEFA, SAT-TAG content, and gene expression of key lipogenic enzymes were determined before and immediately after GIP/saline infusions. GIP lowered NEFA concentrations in the obese T2DM group despite diminished insulinotropic activity (mean NEFA AUC0-4 h ± SE, 41,992 ± 9,843 µmol·l-1·min-1 vs. 71,468 ± 13,605 with placebo, P = 0.039, 95% CI: 0.31-0.95). Additionally, GIP increased SAT-TAG in obese T2DM (1.78 ± 0.4 vs 0.86 ± 0.1-fold with placebo, P = 0.043, 95% CI: 0.1-1.8). Such effect with GIP was not observed in other three groups despite greater insulinotropic activity. Reduction in NEFA concentration with GIP correlated with adipose tissue insulin resistance for all subjects (Pearson, r = 0.56, P = 0.005). There were no significant gene expression changes in key SAT lipid metabolism enzymes. In conclusion, GIP appears to promote fat accretion and thus may exacerbate obesity and insulin resistance in T2DM.

Mitochondrial dysfunction has been associated with insulin resistance, obesity and diabetes. Hyperinsulinaemia and hyperlipidaemia are hallmarks of the insulin-resistant state. We sought to determine the contributions of high insulin and saturated fatty acid exposure to mitochondrial function and biogenesis in cultured myocytes. Differentiated C2C12 myotubes were left untreated or exposed to chronic high insulin or high palmitate. Mitochondrial function was determined assessing: oxygen consumption, mitochondrial membrane potential, ATP content and ROS (reactive oxygen species) production. We also determined the expression of several mitochondrial genes. Chronic insulin treatment of myotubes caused insulin resistance with reduced PI3K (phosphoinositide 3-kinase) and ERK (extracellular-signal-regulated kinase) signalling. Insulin treatment increased oxygen consumption but reduced mitochondrial membrane potential and ROS production. ATP cellular levels were maintained through an increased glycolytic rate. The expression of mitochondrial OXPHOS (oxidative phosphorylation) subunits or Mfn-2 (mitofusin 2) were not significantly altered in comparison with untreated cells, whereas expression of PGC-1α (peroxisome-proliferator-activated receptor γ co-activator-1α) and UCPs (uncoupling proteins) were reduced. In contrast, saturated fatty acid exposure caused insulin resistance, reducing PI3K (phosphoinositide 3-kinase) and ERK (extracellular-signal-regulated kinase) activation while increasing activation of stress kinases JNK (c-Jun N-terminal kinase) and p38. Fatty acids reduced oxygen consumption and mitochondrial membrane potential while up-regulating the expression of mitochondrial ETC (electron chain complex) protein subunits and UCP proteins. Mfn-2 expression was not modified by palmitate. Palmitate-treated cells also showed a reduced glycolytic rate. Taken together, our findings indicate that chronic insulin and fatty acid-induced insulin resistance differentially affect mitochondrial function. In both conditions, cells were able to maintain ATP levels despite the loss of membrane potential; however, different protein expression suggests different adaptation mechanisms.

Type 2 Diabetes (T2D) is a highly prevalent chronic metabolic disease with strong co-morbidity with obesity and cardiovascular diseases. There is growing evidence supporting the notion that a crosstalk between mitochondria and the insulin signaling cascade could be involved in the etiology of T2D and insulin resistance. In this study we investigated the molecular basis of this crosstalk by using systems biology approaches. We combined, filtered, and interrogated different types of functional interaction data, such as direct protein-protein interactions, co-expression analyses, and metabolic and signaling dependencies. As a result, we constructed the mitochondria-insulin (MITIN) network, which highlights 286 genes as candidate functional linkers between these two systems. The results of internal gene expression analysis of three independent experimental models of mitochondria and insulin signaling perturbations further support the connecting roles of these genes. In addition, we further assessed whether these genes are involved in the etiology of T2D using the genome-wide association study meta-analysis from the DIAGRAM consortium, involving 8,130 T2D cases and 38,987 controls. We found modest enrichment of genes associated with T2D amongst our linker genes (p = 0.0549), including three already validated T2D SNPs and 15 additional SNPs, which, when combined, were collectively associated to increased fasting glucose levels according to MAGIC genome wide meta-analysis (p = 8.12 x 10(-5)). This study highlights the potential of combining systems biology, experimental, and genome-wide association data mining for identifying novel genes and related variants that increase vulnerability to complex diseases.

Mitochondrial dysfunction has been associated with insulin resistance, obesity and diabetes. Both hyperinsulinaemia and dyslipidemia are hallmarks of the insulin resistant state. The purpose of this study was to examine the contributions of high insulin and saturated fatty acid exposure on mitochondrial function and gene expression in cultured myocytes. Chronic insulin treatment of myotubes reduced signalling through PI 3‐Kinase/AKT. Insulin‐resistant myocytes exhibited lower mitochondrial membrane potential and ROS production but exhibited ATP levels similar to control cells. Citrate synthase activity was increased in the insulin treated cells. The expression of a number of mitochondrial genes including COX2, COX4, ATP synthase were reduced in the chronic insulin treated cells albeit they were not significant. However, the transcriptional co‐activator PGC1alpha, and uncoupling proteins, UCP2 and UCP3 were significantly reduced in these cells. No changes were detected in Tfam or NRF‐1 transcript levels. Mitofusin2 and Porin protein levels were also not altered by chronic insulin exposure. In contrast, treatment of cells with saturated fatty acid reduced mitochondrial membrane potential, significantly increased ROS production, and increased the mRNA expression of UCP2, UCP3 and ATP synthase. These cells also displayed lower levels PGC1α and PGC1β gene expression compared to controls. Taken together these results suggest that chronic hyperinsulinemia and exposure to saturated fatty acids impair mitochondrial function by different mechanisms.