Molecular mechanism of insulin-induced degradation of insulin receptor substrate 1

Genetic Underpinnings of Fasting and Oral Glucose-stimulated Based Insulin Sensitivity Indices

CONTEXT

Insulin sensitivity (IS) is an important factor in type 2 diabetes (T2D) and can be estimated by many different indices.

OBJECTIVE

We aimed to compare the genetic components underlying IS indices obtained from fasting and oral glucose-stimulated plasma glucose and serum insulin levels.

METHODS

We computed 21 IS indices, classified as fasting, OGTT0,120, and OGTT0,30,120 indices, using fasting and oral glucose tolerance test (OGTT) data in 2 cohorts. We used data from a family cohort (n = 313) to estimate the heritability and the genetic and phenotypic correlations of IS indices. The population cohort, Inter99 (n = 5343), was used to test for associations between IS indices and 426 genetic variants known to be associated with T2D.

RESULTS

Heritability estimates of IS indices ranged between 19% and 38%. Fasting and OGTT0,30,120 indices had high genetic (ρG) and phenotypic (ρP) pairwise correlations (ρG and ρP: 0.88 to 1) The OGTT0,120 indices displayed a wide range of pairwise correlations (ρG: 0.17-1.00 and ρP: 0.13-0.97). We identified statistically significant associations between IS indices and established T2D-associated variants. The PPARG rs11709077 variant was associated only with fasting indices and PIK3R rs4976033 only with OGTT0,30,120 indices. The variants in FAM63A/MINDY1, GCK, C2CD4A/B, and FTO loci were associated only with OGTT0,120 indices.

CONCLUSION

Even though the IS indices mostly share a common genetic background, notable differences emerged between OGTT0,120 indices. The fasting and OGTT-based indices have distinct associations with T2D risk variants. This work provides a basis for future large-scale genetic investigations into the differences between IS indices.

mTOR in the Development of Hypoxic Pulmonary Hypertension Associated with Cardiometabolic Risk Factors

The pathophysiology of pulmonary hypertension is complex and multifactorial. It is a disease characterized by increased pulmonary vascular resistance at the level due to sustained vasoconstriction and remodeling of the pulmonary arteries, which triggers an increase in the mean pulmonary artery pressure and subsequent right ventricular hypertrophy, which in some cases can cause right heart failure. Hypoxic pulmonary hypertension (HPH) is currently classified into Group 3 of the five different groups of pulmonary hypertensions, which are determined according to the cause of the disease. HPH mainly develops as a product of lung diseases, among the most prevalent causes of obstructive sleep apnea (OSA), chronic obstructive pulmonary disease (COPD), or hypobaric hypoxia due to exposure to high altitudes. Additionally, cardiometabolic risk factors converge on molecular mechanisms involving overactivation of the mammalian target of rapamycin (mTOR), which correspond to a central axis in the development of HPH. The aim of this review is to summarize the role of mTOR in the development of HPH associated with metabolic risk factors and its therapeutic alternatives, which will be discussed in this review.

Mechanism of Obesity-Related Lipotoxicity and Clinical Perspective

The link between cellular exposure to fatty acid species and toxicity phenotypes remains poorly understood. However, structural characterization and functional profiling of human plasma free fatty acids (FFAs) analysis has revealed that FFAs are located either in the toxic cluster or in the cluster that is transcriptionally responsive to lipotoxic stress and creates genetic risk factors. Genome-wide short hairpin RNA screen has identified more than 350 genes modulating lipotoxicity. Hypertrophic adipocytes in obese adipose are both unable to expand further to store excess lipids in the diet and are resistant to the antilipolytic action of insulin. In addition to lipolysis, the inability of packaging the excess lipids into lipid droplets causes circulating fatty acids to reach toxic levels in non-adipose tissues. Deleterious effects of accumulated lipid in non-adipose tissues are known as lipotoxicity. Although triglycerides serve a storage function for long-chain non-esterified fatty acid and their products such as ceramide and diacylglycerols (DAGs), overloading of palmitic acid fraction of saturated fatty acids (SFAs) raises ceramide levels. The excess DAG and ceramide load create harmful effects on multiple organs and systems, inducing chronic inflammation in obesity. Thus, lipotoxic inflammation results in β cells death and pancreatic islets dysfunction. Endoplasmic reticulum stress stimuli induce lipolysis by activating cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) and extracellular signal-regulated kinase (Erk) 1/2 signaling in adipocytes. However, palmitic acid-induced endoplasmic reticulum stress-c-Jun N-terminal kinase (JNK)-autophagy axis in hypertrophic adipocytes is a pro-survival mechanism against endoplasmic reticulum stress and cell death induced by SFAs. Endoplasmic reticulum-localized acyl-coenzyme A (CoA): glycerol-3-phosphate acyltransferase (GPAT) enzymes are mediators of lipotoxicity, and inhibiting these enzymes has therapeutic potential for lipotoxicity. Lipotoxicity increases the number of autophagosomes, which engulf palmitic acid, and thus suppress the autophagic turnover. Fatty acid desaturation promotes palmitate detoxification and storages into triglycerides. As therapeutic targets of glucolipotoxicity, in addition to caloric restriction and exercise, there are four different pharmacological approaches, which consist of metformin, glucagon-like peptide 1 (GLP-1) receptor agonists, peroxisome proliferator-activated receptor-gamma (PPARγ) ligands thiazolidinediones, and chaperones are still used in clinical practice. Furthermore, induction of the brown fat-like phenotype with the mixture of eicosapentanoic acid and docosahexaenoic acid appears as a potential therapeutic application for treatment of lipotoxicity.

Human cytomegalovirus attenuates AKT activity by destabilizing insulin receptor substrate proteins

IMPORTANCE

Human cytomegalovirus (HCMV) requires inactivation of AKT to efficiently replicate, yet how AKT is shut off during HCMV infection has remained unclear. We show that UL38, an HCMV protein that activates mTORC1, is necessary and sufficient to destabilize insulin receptor substrate 1 (IRS1), a model insulin receptor substrate (IRS) protein. Degradation of IRS proteins in settings of excessive mTORC1 activity is an important mechanism for insulin resistance. When IRS proteins are destabilized, PI3K cannot be recruited to growth factor receptor complexes, and hence, AKT membrane recruitment, a rate limiting step in its activation, fails to occur. Despite its penchant for remodeling host cell signaling pathways, our results reveal that HCMV relies upon a cell-intrinsic negative regulatory feedback loop to inactivate AKT. Given that pharmacological inhibition of PI3K/AKT potently induces HCMV reactivation from latency, our findings also imply that the expression of UL38 activity must be tightly regulated within latently infected cells to avoid spontaneous reactivation.

Ascorbic acid enhances chondrocyte differentiation of ATDC5 by accelerating insulin receptor signaling

Chondrogenesis is strictly regulated by several factors, including cytokines, hormones, and extracellular matrix proteins. Mouse teratocarcinoma-derived lineage cells, differentiate into chondrocytes in the presence of insulin. Although ascorbic acid promotes chondrogenic differentiation, the detailed regulative mechanisms underlying its role in chondrogenesis remain unclear. Therefore, in this study, we evaluated the effects of ascorbic acid on insulin-induced chondrogenic differentiation of ATDC5 cells and the underlying intracellular signaling. The results revealed that insulin-stimulated collagen deposition, matrix formation, calcification, and expression of chondrogenic differentiation marker genes in ATDC5 cells. This enhancement by insulin was amplified with the addition of ascorbic acid. Molecular analysis revealed that the activation of insulin-induced phosphoinositide 3-kinase (PI3K)/Akt signaling was enhanced in the presence of ascorbic acid. In contrast, Wnt/β-catenin signaling was suppressed during chondrocyte differentiation via upregulation of the Wnt agonist, secreted Frizzled-related protein 1 (sFRP-1) and 3 (sFRP-3). Notably, ascorbic acid upregulated the expression of insulin receptors and their substrates (IRS-1 and IRS-2). Furthermore, ascorbic acid reversed the suppression of IRS-1 and IRS-2 protein by insulin. These results indicate that ascorbic acid positively regulates the chondrogenic differentiation of ATDC5 cells via enhancement of insulin signaling. Our findings provide a substantial basis for further elucidation of the regulatory mechanisms of chondrocyte differentiation and the pathophysiology of OA, thus aiding in development of effective treatment strategies.

Effects of early life overnutrition and hyperandrogenism on spatial learning and memory in a rat model of polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a complex disorder characterized by endocrine and metabolic abnormalities such as obesity and insulin resistance. PCOS is also associated with psychiatric disorders and cognitive impairment. The animal model of PCOS was induced by treating rats with 5α-dihydrotestosterone (5α-DHT) and additionally modified to induce adiposity by litter size reduction (LSR). Spatial learning and memory were assessed using the Barnes Maze test, and striatal markers of synaptic plasticity were analyzed. Striatal insulin signaling was estimated by the levels of insulin receptor substrate 1 (IRS1), its inhibitory phosphorylation at Ser307, and glycogen synthase kinase-3α/β (GSK3α/β) activity. Both LSR and DHT treatment significantly decreased striatal protein levels of IRS1, followed by increased GSK3α/β activity in small litters. Results of the behavioral study showed that LSR had a negative effect on learning rate and memory retention, whereas DHT treatment did not induce impairment in memory formation. While protein levels of synaptophysin, GAP43, and postsynaptic density protein 95 (PSD-95) were not altered by the treatments, DHT treatment induced an increase in phosphorylation of PSD-95 at Ser295 in both normal and small litters. This study revealed that LSR and DHT treatment suppressed insulin signaling by downregulating IRS1 in the striatum. However, DHT treatment did not have an adverse effect on learning and memory, probably due to compensatory elevation in pPSD-95-Ser295, which had a positive effect on synaptic strength. This implies that hyperandrogenemia in this setting does not represent a threat to spatial learning and memory, opposite to the effect of overnutrition-related adiposity.

Islr regulates insulin sensitivity by interacting with Psma4 to control insulin receptor alpha levels in obese mice

Insulin resistance is the leading cause of type 2 diabetes (T2D), and dysfunctional insulin receptor signaling is a major manifestation of this insulin resistance. In T2D, the corresponding insulin receptor levels are aberrantly down-regulated, which is one of the major factors underlying obesity-induced insulin resistance in adipose tissue. However, the precise mechanism of insulin receptor impairment in obese individuals remains unclear. In the current study, we established that immunoglobulin superfamily containing leucine-rich repeat (Islr) is highly expressed in adipocytes of mice fed a high-fat diet. We further demonstrated that Islr mediates the ubiquitin-independent proteasomal degradation of insulin receptor alpha (Insrα) by specifically interacting with proteasome subunit alpha type 4 (Psma4). Islr knockout increased the corresponding Insrα subunit levels and enhanced insulin sensitivity in adipocytes, ultimately improving systemic metabolism. Further, siRNA-mediated down-regulation of Islr expression in the white adipose tissue of obese mice increased insulin sensitivity. Overall, Islr regulates insulin sensitivity by interacting with Psma4 to control the ubiquitin-independent proteasomal degradation of Insrα in obese mice, indicating that Islr may be a potential therapeutic target for ameliorating insulin resistance.

Human cytomegalovirus attenuates AKT activity by destabilizing insulin receptor substrate proteins

The phosphoinositide 3-kinase (PI3K)/AKT pathway plays crucial roles in cell viability and protein synthesis and is frequently co-opted by viruses to support their replication. Although many viruses maintain high levels of AKT activity during infection, other viruses, such as vesicular stomatitis virus and human cytomegalovirus (HCMV), cause AKT to accumulate in an inactive state. To efficiently replicate, HCMV requires FoxO transcription factors to localize to the infected cell nucleus (Zhang et. al. mBio 2022), a process directly antagonized by AKT. Therefore, we sought to investigate how HCMV inactivates AKT to achieve this. Subcellular fractionation and live cell imaging studies indicated that AKT failed to recruit to membranes upon serum-stimulation of infected cells. However, UV-inactivated virions were unable to render AKT non-responsive to serum, indicating a requirement for de novo viral gene expression. Interestingly, we were able to identify that UL38 (pUL38), a viral activator of mTORC1, is required to diminish AKT responsiveness to serum. mTORC1 contributes to insulin resistance by causing proteasomal degradation of insulin receptor substrate (IRS) proteins, such as IRS1, which are necessary for the recruitment of PI3K to growth factor receptors. In cells infected with a recombinant HCMV disrupted for UL38 , AKT responsiveness to serum is retained and IRS1 is not degraded. Furthermore, ectopic expression of UL38 in uninfected cells induces IRS1 degradation, inactivating AKT. These effects of UL38 were reversed by the mTORC1 inhibitor, rapamycin. Collectively, our results demonstrate that HCMV relies upon a cell-intrinsic negative feedback loop to render AKT inactive during productive infection.

Effects of antidiabetics and exercise therapy on suppressors of cytokine signaling-1, suppressors of cytokine signaling-3, and insulin receptor substrate-1 molecules in diabetes and obesity

OBJECTIVE

Pathological destruction of insulin signaling molecules such as insulin receptor substrate, especially due to the increase in suppressors of cytokine signaling molecules, has been demonstrated in experimental diabetes. The contribution of suppressors of cytokine signaling proteins to the development of insulin resistance and the effects of antidiabetic drugs and exercise on suppressors of cytokine signaling proteins are not clearly known.

METHODS

A total of 48 Wistar albino adult male rats were divided into six groups: control group, obese group with diabetes, obese diabetic rats treated with metformin, obese diabetic rats treated with pioglitazone, obese diabetic rats treated with exenatide, and obese diabetic rats with applied exercise program. Immunohistochemical staining was performed in both the liver and adipose tissue.

RESULTS

There was a statistically significant decrease in suppressors of cytokine signaling-1, a decrease in suppressors of cytokine signaling-3, an increase in insulin receptor substrate-1, and a decrease in immunohistochemical staining in the obese group treated with metformin and exenatide compared to the obese group without treatment in the liver tissue (p<0.05). A statistically significant decrease in immunohistochemical staining of suppressors of cytokine signaling-1 and suppressors of cytokine signaling-3 was found in the obese group receiving exercise therapy compared to the obese group without treatment in visceral adipose tissue (p<0.05). Likewise, no significant immunohistochemistry staining was seen in diabetic obese groups.

CONCLUSION

Metformin or exenatide treatment could prevent the degradation of insulin receptor substrate-1 protein by reducing the effect of suppressors of cytokine signaling-1 and suppressors of cytokine signaling-3 proteins, especially in the liver tissue. In addition, exercise can play a role as a complementary therapy by reducing suppressors of cytokine signaling-1 and suppressors of cytokine signaling-3 proteins in visceral adipose tissue.

Study on the mechanism of lupenone for treating type 2 diabetes by integrating pharmacological evaluation and network pharmacology

CONTEXT

Lupenone (LUP) is the active ingredient of Musa basjoo Sieb. et Zucc. (Musaceae) with antidiabetes effects, but an unclear underlying mechanism of action.

OBJECTIVE

Animal experiments combined with network pharmacology were used to explore the mechanism of LUP for treating diabetes.

MATERIALS AND METHODS

Insulin resistance (IR) in male Sprague-Dawley rats with type 2 diabetic was induced using a high-fat diet and streptozotocin. The selected rats were divided into normal group, model group, positive group and LUP (2.0, 4.0 and 8.0 mg/kg) groups, and orally administrated twice daily with Tween 80, rosiglitazone or LUP. Fasting blood glucose (FBG), oxidative stress index, blood lipids and IR-related targets were detected. A network pharmacology analysis was performed.

RESULTS

Compared to the model group, LUP (8.0 mg/kg) significantly decreased the levels of FBG (22.3%), LEP (9.5%), HbA1c (14.9%) and MDA (12.3%), increased the ADPN (24.2%) levels and GSH-PX activity (12.4%) (p < 0.05), improved oxidative stress, lipid metabolism disorders and pancreas pathological changes, increased the mRNA and protein expression of InsR (3.7-fold and 1.3-fold), IRS-1 (3-fold and 2-fold), IRS-2 (2-fold and 1.6-fold), GLUT-4 (2-fold and 2.4-fold) in skeletal muscle and IRS-1 (6-fold and 1.6-fold), IRS-2 (5.8-fold and 1.5-fold), GLUT-4 (2.5-fold and 1.7-fold) and PPAR-γ (7-fold and 1.4-fold) in adipose tissue (p < 0.05). Network pharmacology analysis revealed that LUP improves IR by multiple targets and signal pathways.

CONCLUSIONS

The mechanism of LUP for treating diabetes is related to improving IR. LUP has the potential to be developed as a new drug for treating type 2 diabetes.

Neddylation-dependent protein degradation is a nexus between synaptic insulin resistance, neuroinflammation and Alzheimer's disease

BACKGROUND

The metabolic syndrome is a consequence of modern lifestyle that causes synaptic insulin resistance and cognitive deficits and that in interaction with a high amyloid load is an important risk factor for Alzheimer's disease. It has been proposed that neuroinflammation might be an intervening variable, but the underlying mechanisms are currently unknown.

METHODS

We utilized primary neurons to induce synaptic insulin resistance as well as a mouse model of high-risk aging that includes a high amyloid load, neuroinflammation, and diet-induced obesity to test hypotheses on underlying mechanisms.

RESULTS

We found that neddylation and subsequent activation of cullin-RING ligase complexes induced synaptic insulin resistance through ubiquitylation and degradation of the insulin-receptor substrate IRS1 that organizes synaptic insulin signaling. Accordingly, inhibition of neddylation preserved synaptic insulin signaling and rescued memory deficits in mice with a high amyloid load, which were fed with a 'western diet'.

CONCLUSIONS

Collectively, the data suggest that neddylation and degradation of the insulin-receptor substrate is a nodal point that links high amyloid load, neuroinflammation, and synaptic insulin resistance to cognitive decline and impaired synaptic plasticity in high-risk aging.

Network Pharmacology and Transcriptomics Reveal the Mechanism of GuaLouQuMaiWan in Treatment of Type 2 Diabetes and Its Active Small Molecular Compound

The main pathophysiological abnormalities in type 2 diabetes (T2D) include pancreatic β-cell dysfunction and insulin resistance. Due to hyperglycemia, patients receive long-term treatment. However, side effects and drug tolerance usually lead to treatment failure. GuaLouQuMaiWan (GLQMW), a common traditional Chinese medicine (TCM) prescription, has positive effects on controlling blood sugar and improving quality of life, but the mechanism is still unclear. To decipher their molecular mechanisms, we used a novel computational systems pharmacology-based approach consisting of bioinformatics analysis, network pharmacology, and drug similarity comparison. We divided the participants into nondisease (ND), impaired glucose tolerance (IGT), and type 2 diabetes groups according to the WHO's recommendations for diabetes. By analyzing the gene expression profile of the ND-IGT-T2D (ND to IGT to T2D) process, we found that the function of downregulated genes in the whole process was mainly related to insulin secretion, while the upregulated genes were related to inflammation. Furthermore, other genes in the ND-IGT (ND to IGT) process are mainly related to inflammation and lipid metabolic disorders. We speculate that 17 genes with a consistent trend may play a key role in the process of ND-IGT-T2D. We further performed target prediction for 50 compounds in GLQMW that met the screening criteria and intersected the differentially expressed genes of the T2D process with the compounds of GLQMW; a total of 18 proteins proved potential targets for GLQMW. Among these, RBP4 is considerably related to insulin resistance. GO/KEGG enrichment analyses of the target genes of GLQMW showed enrichment in inflammation- and T2D therapy-related pathways. Based on the RDKit tool and the DrugBank database, we speculate that (-)-taxifolin, dialoside A_qt, spinasterol, isofucosterol, and 11,14-eicosadienoic acid can be used as potential drugs for T2D via molecular docking and drug similarity comparison.

Role of skeletal muscle lipids in the pathogenesis of insulin resistance of obesity and type 2 diabetes

Obesity predisposes individuals to the development of insulin resistance, which is a risk factor for type 2 diabetes, and muscle plays a central role in this phenomenon. Insulin resistance is associated with: (i) a metabolic inflexibility characterized by a reduced impaired switching from free fatty acid (FA) to carbohydrate substrates; and (ii) an ectopic accumulation of triglyceride in skeletal muscle, generating a cellular "lipotoxicity", but triglyceride per se, does not contribute to insulin resistance ("athlete's paradox"). A large body of evidence supports the idea that a decreased mitochondrial capacity to oxidize FA leads to an accretion of intracellular triglyceride and an accumulation of acyl-CoAs, which are used to synthesize diacylglycerol and ceramide. These lipid derivatives activate serine kinases, leading to increase of insulin receptor substrate 1 serine phosphorylation, which impairs insulin signaling. A second model proposes that insulin resistance arises from an excessive mitochondrial FA oxidation. Studies have shown that the type of FA, unsaturated or saturated, is critical in the development of insulin resistance. It should be also stressed that FA oversupply activates inflammatory signals, induces endoplasmic reticulum stress, increases mitochondrial oxidative stress and influences the regulation of genes that contributes to impaired glucose metabolism. These cellular insults are thought to engage stress-sensitive serine kinases disrupting insulin signaling. In conclusion, reduced dietary lipid intake in association with physical exercise could be a therapeutic option to improve insulin sensitivity.

Relationship between Functional miR-143/145 Cluster Variants and Susceptibility to Type 2 Diabetes Mellitus: A Preliminary Case-Control Study and Bioinformatics Analyses

Purpose: To investigate the link between two variants (rs4705342 and rs4705343) in the promoter of the miR-143/145 cluster with Type 2 diabetes mellitus (T2DM) risk. Methods:A total of 1200 subjects were genotyped using the ARMS-PCR method. Results: The rs4705342 variant enhanced the risk of T2DM under codominant CC (OR = 3.24; 95% CI: 1.89-5.60), recessive TT+TC (OR = 3.02; 95% CI: 1.77-5.17), and dominant TC+CC (OR = 1.35; 95% CI: 1.08-1.71) genetic models. Individuals carrying the C allele of rs4705342 conferred a 1.43 fold increased risk of T2DM. As regards rs4705343, decreased risk of T2DM was observed under codominant TC (OR = 0.53; 95% CI: 0.42-0.67), over-dominant TT+CC (OR = 0.51; 95% CI: 0.40-0.64), and dominant TC+CC (OR = 0.59; 95% CI: 0.48-0.75) models. Haplotype analysis of the variants showed a 1.941-fold increased risk of T2DM regarding the C T combination. Significant associations were noticed between different haplotypes and lipid indices of T2DM patients. There were no notable changes in p-values after adjustment for BMI. Computational analysis revealed that miR143 and/or miR145 target important genes involved in glucose and lipid metabolism. Conclusions: Functional miR-143/145 variants might influence the risk of T2DM. Hence, clarifying the precise regulatory mechanisms of gene expression in the development of T2DM will significantly guide researchers to find a novel target for therapeutic intervention.

The Key Role of Akt Protein Kinase in Metabolic-Inflammatory Pathways Cross-Talk: TNF-α Down-Regulation and Improving of Insulin Resistance in HepG2 Cell Line

BACKGROUND

Elevation of plasma free fatty acids as a principal aspect of type 2 diabetes maintains etiologically insulin insensitivity in target cells. TNF-α inhibitory effects on key insulin signaling pathway elements remain to be verified in insulinresistant hepatic cells. Thus, TNF-α knockdown effects on the key elements of insulin signaling were investigated in the palmitate-induced insulin-resistant hepatocytes. The Akt serine kinase, a key protein of the insulin signaling pathway, phosphorylation was monitored to understand the TNF-α effect on probable enhancing of insulin resistance.

METHODS

Insulin-resistant HepG2 cells were produced using 0.5 mM palmitate treatment and shRNA-mediated TNF-α gene knockdown and its down-regulation confirmed using ELISA technique. Western blotting analysis was used to assess the Akt protein phosphorylation status.

RESULTS

Palmitate-induced insulin resistance caused TNF-α protein overexpression 1.2-, 2.78, and 2.25- fold as compared to the control cells at post-treatment times of 8 h, 16 h, and 24 h, respectively. In the presence of palmitate, TNF-α expression showed around 30% reduction in TNF-α knockdown cells as compared to normal cells. In the TNF-α down-regulated cell, Akt phosphorylation was approximately 62% more than control cells after treatment with 100 nM insulin in conjugation with 0.5 mM palmitate.

CONCLUSIONS

The obtained data demonstrated that TNF-α protein expression reduction improved insulin-stimulated Akt phosphorylation in the HepG2 cells and decreased lipidinduced insulin resistance of the diabetic hepatocytes.

Build-UPS and break-downs: metabolism impacts on proteostasis and aging

Perturbation of metabolism elicits cellular stress which profoundly modulates the cellular proteome and thus protein homeostasis (proteostasis). Consequently, changes in the cellular proteome due to metabolic shift require adaptive mechanisms by molecular protein quality control. The mechanisms vitally controlling proteostasis embrace the entire life cycle of a protein involving translational control at the ribosome, chaperone-assisted native folding, and subcellular sorting as well as proteolysis by the proteasome or autophagy. While metabolic imbalance and proteostasis decline have been recognized as hallmarks of aging and age-associated diseases, both processes are largely considered independently. Here, we delineate how proteome stability is governed by insulin/IGF1 signaling (IIS), mechanistic target of Rapamycin (TOR), 5′ adenosine monophosphate-activated protein kinase (AMPK), and NAD-dependent deacetylases (Sir2-like proteins known as sirtuins). This comprehensive overview is emphasizing the regulatory interconnection between central metabolic pathways and proteostasis, indicating the relevance of shared signaling nodes as targets for future therapeutic interventions.

Mitochondria and T2D: Role of Autophagy, ER Stress, and Inflammasome

Type 2 diabetes (T2D) is one of the main current threats to human health. Both T2D and its numerous clinical complications are related to mitochondrial dysfunction and oxidative stress. Over the past decade, great progress has been made in extending our knowledge about the signaling events regulated by mitochondria. However, the links among mitochondrial impairment, oxidative stress, autophagy, endoplasmic reticulum (ER) stress, and activation of the inflammasome still need to be clarified. In light of this deficit, we aim to provide a review of the existing literature concerning the complicated crosstalk between mitochondrial impairment, autophagy, ER stress, and the inflammasome in the molecular pathogenesis of T2D.

Mechanisms of action of metformin in type 2 diabetes: Effects on mitochondria and leukocyte-endothelium interactions

Type 2 diabetes (T2D) is a very prevalent, multisystemic, chronic metabolic disorder closely related to atherosclerosis and cardiovascular diseases. It is characterised by mitochondrial dysfunction and the presence of oxidative stress. Metformin is one of the safest and most effective anti-hyperglycaemic agents currently employed as first-line oral therapy for T2D. It has demonstrated additional beneficial effects, unrelated to its hypoglycaemic action, on weight loss and several diseases, such as cancer, cardiovascular disorders and metabolic diseases, including thyroid diseases. Despite the vast clinical experience gained over several decades of use, the mechanism of action of metformin is still not fully understood. This review provides an overview of the existing literature concerning the beneficial mitochondrial and vascular effects of metformin, which it exerts by diminishing oxidative stress and reducing leukocyte-endothelium interactions. Specifically, we describe the molecular mechanisms involved in metformin's effect on gluconeogenesis, its capacity to interfere with major metabolic pathways (AMPK and mTORC1), its action on mitochondria and its antioxidant effects. We also discuss potential targets for therapeutic intervention based on these molecular actions.

Role of oxidative stress in the pathogenesis of nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) has emerged as the most common chronic liver disease worldwide and is strongly associated with the presence of oxidative stress. Disturbances in lipid metabolism lead to hepatic lipid accumulation, which affects different reactive oxygen species (ROS) generators, including mitochondria, endoplasmic reticulum, and NADPH oxidase. Mitochondrial function adapts to NAFLD mainly through the downregulation of the electron transport chain (ETC) and the preserved or enhanced capacity of mitochondrial fatty acid oxidation, which stimulates ROS overproduction within different ETC components upstream of cytochrome c oxidase. However, non-ETC sources of ROS, in particular, fatty acid β-oxidation, appear to produce more ROS in hepatic metabolic diseases. Endoplasmic reticulum stress and NADPH oxidase alterations are also associated with NAFLD, but the degree of their contribution to oxidative stress in NAFLD remains unclear. Increased ROS generation induces changes in insulin sensitivity and in the expression and activity of key enzymes involved in lipid metabolism. Moreover, the interaction between redox signaling and innate immune signaling forms a complex network that regulates inflammatory responses. Based on the mechanistic view described above, this review summarizes the mechanisms that may account for the excessive production of ROS, the potential mechanistic roles of ROS that drive NAFLD progression, and therapeutic interventions that are related to oxidative stress.

Advances in cancer cachexia: Intersection between affected organs, mediators, and pharmacological interventions

Advanced cancer patients exhibit cachexia, a condition characterized by a significant reduction in the body weight predominantly from loss of skeletal muscle and adipose tissue. Cachexia is one of the major causes of morbidity and mortality in cancer patients. Decreased food intake and multi-organ energy imbalance in cancer patients worsen the cachexia syndrome. Cachectic cancer patients have a low tolerance for chemo- and radiation therapies and also have a reduced quality of life. The presence of tumors and the current treatment options for cancer further exacerbate the cachexia condition, which remains an unmet medical need. The onset of cachexia involves crosstalk between different organs leading to muscle wasting. Recent advancements in understanding the molecular mechanisms of skeletal muscle atrophy/hypertrophy and adipose tissue wasting/browning provide a platform for the development of new targeted therapies. Therefore, a better understanding of this multifactorial disorder will help to improve the quality of life of cachectic patients. In this review, we summarize the metabolic mediators of cachexia, their molecular functions, affected organs especially with respect to muscle atrophy and adipose browning and then discuss advanced therapeutic approaches to cancer cachexia.

Developing a Robust Assay to Monitor and Quantify Key Players of Metabolic Pathways during Adipogenesis by Targeted Proteomics

Targeted data acquisition using nano liquid chromatrography (nano-LC) coupled mass spectrometry is an emerging approach when there is a need to quantify proteins with high accuracy, sensitivity, and reproducibility. Nevertheless, creating assays meeting all those criteria still remains a laborious task, especially when investigating low abundant proteins and small concentration changes. In this work a targeted data acquisition workflow is developed reducing time and effort to target and investigate key players of metabolic pathways during the process of adipocyte differentiation. This leads to accurate and sensitive quantification of proteins involved in the synthesis of fatty acids, glycerolipids, glycerophospholipids, sphingolipids, the production of energy and reduction equivalents. Additionally low abundant signaling molecules part of the peroxisome proliferator-activated receptor gamma (PPARγ) and insulin signaling pathway with ≈400 for the insulin receptor substrate and 1100 copies per cell for PPARγ are determined.

Cullin-RING E3 Ubiquitin Ligase 7 in Growth Control and Cancer

CRL7^Fbxw8 is an E3 ubiquitin ligase complex, containing cullin7 (CUL7) as a scaffold, the F-box protein Fbxw8 as a substrate receptor, the Skp1 adaptor, and the ROC1/Rbx1 RING finger protein for working with E2 enzyme to facilitate ubiquitin transfer. This chapter provides an update on studies linking CRL7^Fbxw8 to hereditary human growth retardation disease, as at least 64 cul7 germ line mutations were found in patients with autosomal recessive 3-M syndrome. CRL7^Fbxw8 interacts with two additional 3-M associated proteins OBSL1 and CCDC8, leading to subcellular localization of the E3 complex to regions including plasma membrane, centrosome, and Golgi. At least ten mammalian cellular proteins were identified or implicated as CRL7^Fbxw8 substrates. Discussion focuses on the possible impact of CRL7^Fbxw8-mediated proteolytic or non-proteolytic pathways in growth control and cancer.

Dual-specificity tyrosine phosphorylation-regulated kinase 1A ameliorates insulin resistance in neurons by up-regulating IRS-1 expression

Insulin resistance in the brain is a pathological mechanism that is shared between Alzheimer's disease (AD) and type 2 diabetes mellitus (T2DM). Although aberrant expression and phosphorylation of insulin receptor substrate 1 (IRS-1) contribute to insulin resistance, the underlying mechanism remains elusive. In this study, we used several approaches, including adeno-associated virus-based protein overexpression, immunoblotting, immunoprecipitation, immunohistochemistry, and in situ proximal ligation assays, to investigate the function of dual-specificity tyrosine phosphorylation-regulated kinase 1A (DYRK1A) in IRS-1 regulation and the downstream insulin signaling in neurons. We found that DYRK1A overexpression up-regulated IRS-1 expression by slowing turnover of the IRS-1 protein. We further observed that DYRK1A directly interacted with IRS-1 and phosphorylated IRS-1's multiple serine residues. Of note, DYRK1A and IRS-1 were coordinately up-regulated in the prefrontal cortex of db/db mice brain. Furthermore, DYRK1A overexpression ameliorated chronic high insulin-induced insulin resistance in SH-SY5Y cells as well as in primary rat neurons. These findings suggest that DYRK1A protects against insulin resistance in the brain by elevating IRS-1 expression.

Astaxanthin: A Potential Mitochondrial-Targeted Antioxidant Treatment in Diseases and with Aging

Oxidative stress is characterized by an imbalance between prooxidant and antioxidant species, leading to macromolecular damage and disruption of redox signaling and cellular control. It is a hallmark of various diseases including metabolic syndrome, chronic fatigue syndrome, neurodegenerative, cardiovascular, inflammatory, and age-related diseases. Several mitochondrial defects have been considered to contribute to the development of oxidative stress and known as the major mediators of the aging process and subsequent age-associated diseases. Thus, mitochondrial-targeted antioxidants should prevent or slow down these processes and prolong longevity. This is the reason why antioxidant treatments are extensively studied and newer and newer compounds with such an effect appear. Astaxanthin, a xanthophyll carotenoid, is the most abundant carotenoid in marine organisms and is one of the most powerful natural compounds with remarkable antioxidant activity. Here, we summarize its antioxidant targets, effects, and benefits in diseases and with aging.

Deficiency of the autophagy gene ATG16L1 induces insulin resistance through KLHL9/KLHL13/CUL3-mediated IRS1 degradation

Connections between deficient autophagy and insulin resistance have emerged, however, the mechanism through which reduced autophagy impairs insulin-signaling remains unknown. We examined mouse embryonic fibroblasts lacking Atg16l1 (ATG16L1 KO mouse embryonic fibroblasts (MEFs)), an essential autophagy gene, and observed deficient insulin and insulin-like growth factor-1 signaling. ATG16L1 KO MEFs displayed reduced protein content of insulin receptor substrate-1 (IRS1), pivotal to insulin signaling, whereas IRS1myc overexpression recovered downstream insulin signaling. Endogenous IRS1 protein content and insulin signaling were restored in ATG16L1 KO mouse embryonic fibroblasts (MEF) upon proteasome inhibition. Through proximity-dependent biotin identification (BioID) and co-immunoprecipitation, we found that Kelch-like proteins KLHL9 and KLHL13, which together form an E3 ubiquitin (Ub) ligase complex with cullin 3 (CUL3), are novel IRS1 interactors. Expression of Klhl9 and Klhl13 was elevated in ATG16L1 KO MEFs and siRNA-mediated knockdown of Klhl9, Klhl13, or Cul3 recovered IRS1 expression. Moreover, Klhl13 and Cul3 knockdown increased insulin signaling. Notably, adipose tissue of high-fat fed mice displayed lower Atg16l1 mRNA expression and IRS1 protein content, and adipose tissue KLHL13 and CUL3 expression positively correlated to body mass index in humans. We propose that ATG16L1 deficiency evokes insulin resistance through induction of Klhl9 and Klhl13, which, in complex with Cul3, promote proteasomal IRS1 degradation.

Identification of novel mutations of Insulin Receptor Substrate 1 (IRS1) in tumor samples of non-small cell lung cancer (NSCLC): Implications for aberrant insulin signaling in development of cancer

Lung cancer is the leading cause of cancer-related death, and NSCLC constitutes nearly 85%-90% of all cases. The IRS proteins function as adaptors and transmit signals from multiple receptors. Upon binding of insulin to the insulin receptor (IR), IRS1 is phosphorylated at several YXXM motifs creating docking sites for the binding of PI3Kp85, which activates AKT kinase. Therefore, we thought that gain of function mutantions of IRS1 could be related to development of lung cancer. In line with this, we wanted determine whether the IRS1 gene was mutated in the coding regions surrounding YXXM motifs. We sequenced the coding regions surrounding YXXM motifs of IRS1 using tumor samples of 42 NSCLC patients and 40 matching controls and found heterozygote p.S668T mutation in nine of 42 samples and four of nine also had the p.D674H mutation. We generated IRS1 expression vectors harboring p.S668T, p.D674H and double mutants. Expression of the mutants differentially affected insulin-induced phosphorylation of IRS1, AKT, ERK, and STAT3. Also, our mutants induced proliferation, glucose uptake, inhibited the migration of 293T cells and affected the responsiveness of the cells to cisplatin and radiation. Our results suggest that these novel mutations play a role in the phenotype of lung cancer.

Mitochondria, the NLRP3 Inflammasome, and Sirtuins in Type 2 Diabetes: New Therapeutic Targets

SIGNIFICANCE

Type 2 diabetes mellitus and hyperglycemia can lead to the development of comorbidities such as atherosclerosis and microvascular/macrovascular complications. Both type 2 diabetes and its complications are related to mitochondrial dysfunction and oxidative stress. Type 2 diabetes is also a chronic inflammatory condition that leads to inflammasome activation and the release of proinflammatory mediators, including interleukins (ILs) IL-1β and IL-18. Moreover, sirtuins are energetic sensors that respond to metabolic load, which highlights their relevance in metabolic diseases, such as type 2 diabetes. Recent Advances: Over the past decade, great progress has been made in clarifying the signaling events regulated by mitochondria, inflammasomes, and sirtuins. Nod-like receptor family pyrin domain containing 3 (NLRP3) is the best characterized inflammasome, and the generation of oxidant species seems to be critical for its activation. NLRP3 inflammasome activation and altered sirtuin levels have been observed in type 2 diabetes. Critical Issue: Despite increasing evidence of the relationship between the NLRP3 inflammasome, mitochondrial dysfunction, and oxidative stress and of their participation in type 2 diabetes physiopathology, therapeutic strategies to combat type 2 diabetes that target NLRP3 inflammasome and sirtuins are yet to be consolidated.

FUTURE DIRECTIONS

In this review article, we attempt to provide an overview of the existing literature concerning the crosstalk between mitochondrial impairment and the inflammasome, with particular attention to cellular and mitochondrial redox metabolism and the potential role of the NLRP3 inflammasome and sirtuins in the pathogenesis of type 2 diabetes. In addition, we discuss potential targets for therapeutic intervention based on these molecular interactions. Antioxid. Redox Signal. 29, 749-791.

Distinct signalling properties of insulin receptor substrate (IRS)-1 and IRS-2 in mediating insulin/IGF-1 action

Insulin/IGF-1 action is driven by a complex and highly integrated signalling network. Loss-of-function studies indicate that the major insulin/IGF-1 receptor substrate (IRS) proteins, IRS-1 and IRS-2, mediate different biological functions in vitro and in vivo, suggesting specific signalling properties despite their high degree of homology. To identify mechanisms contributing to the differential signalling properties of IRS-1 and IRS-2 in the mediation of insulin/IGF-1 action, we performed comprehensive mass spectrometry (MS)-based phosphoproteomic profiling of brown preadipocytes from wild type, IRS-1^-/- and IRS-2^-/- mice in the basal and IGF-1-stimulated states. We applied stable isotope labeling by amino acids in cell culture (SILAC) for the accurate quantitation of changes in protein phosphorylation. We found ~10% of the 6262 unique phosphorylation sites detected to be regulated by IGF-1. These regulated sites included previously reported substrates of the insulin/IGF-1 signalling pathway, as well as novel substrates including Nuclear Factor I X and Semaphorin-4B. In silico prediction suggests the protein kinase B (PKB), protein kinase C (PKC), and cyclin-dependent kinase (CDK) as the main mediators of these phosphorylation events. Importantly, we found preferential phosphorylation patterns depending on the presence of either IRS-1 or IRS-2, which was associated with specific sets of kinases involved in signal transduction downstream of these substrates such as PDHK1, MAPK3, and PKD1 for IRS-1, and PIN1 and PKC beta for IRS-2. Overall, by generating a comprehensive phosphoproteomic profile from brown preadipocyte cells in response to IGF-1 stimulation, we reveal both common and distinct insulin/IGF-1 signalling events mediated by specific IRS proteins.

Family with sequence similarity 13, member A modulates adipocyte insulin signaling and preserves systemic metabolic homeostasis

Adipose tissue dysfunction is causally implicated in the impaired metabolic homeostasis associated with obesity; however, detailed mechanisms underlying dysregulated adipocyte functions in obesity remain to be elucidated. Here we searched for genes that provide a previously unknown mechanism in adipocyte metabolic functions and identified family with sequence similarity 13, member A (Fam13a) as a factor that modifies insulin signal cascade in adipocytes. Fam13a was highly expressed in adipose tissue, predominantly in mature adipocytes, and its expression was substantially reduced in adipose tissues of obese compared with lean mice. We revealed that Fam13a accentuated insulin signaling by recruiting protein phosphatase 2A with insulin receptor substrate 1 (IRS1), leading to protection of IRS1 from proteasomal degradation. We further demonstrated that genetic loss of Fam13a exacerbated obesity-related metabolic disorders, while targeted activation of Fam13a in adipocytes ameliorated it in association with altered adipose tissue insulin sensitivity in mice. Our data unveiled a previously unknown mechanism in the regulation of adipocyte insulin signaling by Fam13a and identified its significant role in systemic metabolic homeostasis, shedding light on Fam13a as a pharmacotherapeutic target to treat obesity-related metabolic disorders.

What Is Lipotoxicity?

Enlarged fat cells in obese adipose tissue diminish capacity to store fat and are resistant to the anti-lipolytic effect of insulin. Insulin resistance (IR)-associated S-nitrosylation of insulin-signaling proteins increases in obesity. In accordance with the inhibition of insulin-mediated anti-lipolytic action, plasma free fatty acid (FFA) levels increase. Additionally, endoplasmic reticulum stress stimuli induce lipolysis by activating cyclic adenosine monophosphate/Protein kinase A (cAMP/PKA) and extracellular signal-regulated kinase ½ (ERK1/2) signaling in adipocytes. Failure of packaging of excess lipid into lipid droplets causes chronic elevation of circulating fatty acids, which can reach to toxic levels within non-adipose tissues. Deleterious effects of lipid accumulation in non-adipose tissues are known as lipotoxicity. In fact, triglycerides may also serve a storage function for long-chain non-esterified fatty acids and their products such as ceramides and diacylglycerols (DAGs). Thus, excess DAG, ceramide and saturated fatty acids in obesity can induce chronic inflammation and have harmful effect on multiple organs and systems. In this context, chronic adipose tissue inflammation, mitochondrial dysfunction and IR have been discussed within the scope of lipotoxicity.

Skeletal muscle insulin resistance: role of mitochondria and other ROS sources

At present, obesity is one of the most important public health problems in the world because it causes several diseases and reduces life expectancy. Although it is well known that insulin resistance plays a pivotal role in the development of type 2 diabetes mellitus (the more frequent disease in obese people) the link between obesity and insulin resistance is yet a matter of debate. One of the most deleterious effects of obesity is the deposition of lipids in non-adipose tissues when the capacity of adipose tissue is overwhelmed. During the last decade, reduced mitochondrial function has been considered as an important contributor to 'toxic' lipid metabolite accumulation and consequent insulin resistance. More recent reports suggest that mitochondrial dysfunction is not an early event in the development of insulin resistance, but rather a complication of the hyperlipidemia-induced reactive oxygen species (ROS) production in skeletal muscle, which might promote mitochondrial alterations, lipid accumulation and inhibition of insulin action. Here, we review the literature dealing with the mitochondria-centered mechanisms proposed to explain the onset of obesity-linked IR in skeletal muscle. We conclude that the different pathways leading to insulin resistance may act synergistically because ROS production by mitochondria and other sources can result in mitochondrial dysfunction, which in turn can further increase ROS production leading to the establishment of a harmful positive feedback loop.

mTORC1 Plays an Important Role in Skeletal Development by Controlling Preosteoblast Differentiation

The mammalian target of rapamycin complex 1 (mTORC1) is activated by extracellular factors that control bone accrual. However, the direct role of this complex in osteoblast biology remains to be determined. To investigate this question, we disrupted mTORC1 function in preosteoblasts by targeted deletion of Raptor (Rptor) in Osterix-expressing cells. Deletion of Rptor resulted in reduced limb length that was associated with smaller epiphyseal growth plates in the postnatal skeleton. Rptor deletion caused a marked reduction in pre- and postnatal bone accrual, which was evident in skeletal elements derived from both intramembranous and endochondrial ossification. The decrease in bone accrual, as well as the associated increase in skeletal fragility, was due to a reduction in osteoblast function. In vitro, osteoblasts derived from knockout mice display a reduced osteogenic potential, and an assessment of bone-developmental markers in Rptor knockout osteoblasts revealed a transcriptional profile consistent with an immature osteoblast phenotype suggesting that osteoblast differentiation was stalled early in osteogenesis. Metabolic labeling and an assessment of cell size of Rptor knockout osteoblasts revealed a significant decrease in protein synthesis, a major driver of cell growth. These findings demonstrate that mTORC1 plays an important role in skeletal development by regulating mRNA translation during preosteoblast differentiation.

Disruption of the Phosphate Transporter Pit1 in Hepatocytes Improves Glucose Metabolism and Insulin Signaling by Modulating the USP7/IRS1 Interaction

The liver plays a central role in whole-body lipid and glucose homeostasis. Increasing dietary fat intake results in increased hepatic fat deposition, which is associated with a risk for development of insulin resistance and type 2 diabetes. In this study, we demonstrate a role for the phosphate inorganic transporter 1 (PiT1/SLC20A1) in regulating metabolism. Specific knockout of Pit1 in hepatocytes significantly improved glucose tolerance and insulin sensitivity, enhanced insulin signaling, and decreased hepatic lipogenesis. We identified USP7 as a PiT1 binding partner and demonstrated that Pit1 deletion inhibited USP7/IRS1 dissociation upon insulin stimulation. This prevented IRS1 ubiquitination and its subsequent proteasomal degradation. As a consequence, delayed insulin negative feedback loop and sustained insulin signaling were observed. Moreover, PiT1-deficient mice were protected against high-fat-diet-induced obesity and diabetes. Our findings indicate that PiT1 has potential as a therapeutic target in the context of metabolic syndrome, obesity, and diabetes.

Heregulin-1ß and HER3 in hepatocellular carcinoma: status and regulation by insulin

Background

The heregulin-1ß/HER3-driven pathway is implicated in several epithelial malignancies and its blockade is currently undergoing clinical investigation. Paradoxically, the status and the regulation of this pathway is poorly known in hepatocellular carcinoma (HCC).

Methods

Using 85 HCC obtained after tumour resection, heregulin-1ß and HER3 expression was evaluated by real-time RT-PCR, ELISA and/or immunohistochemistry. Statistics were performed to analyze associations between gene expression and clinicopathological parameters. The effects of insulin on the heregulin-1ß/HER3 pathway was investigated in four HCC cell lines.

Results

HER3 mRNA was upregulated in 52 % of tumours, while heregulin-1ß mRNA was downregulated in 82 %. Hepatitis B and C viral infections were respectively associated with high and low HER3 mRNA expression. No association was seen between neither HER3 or heregulin-1ß mRNA and prognostic factors, survival or recurrence. Immunohistochemistry showed predominant cytoplasmic staining of HER3 in tumours but the staining was nonreproducible. HER3 mRNA and protein levels were not correlated in liver tissues. In HCC cells, insulin promoted HER3 proteasomal degradation and inhibited heregulin-1ß stimulation of cell migration. HER3 and insulin receptor co-immunoprecipitated in these cells. The loss of insulin receptor expression by RNA interference sensitized cells to heregulin-1ß-induced AKT phosphorylation.

Conclusions

Autocrine heregulin-1ß loop is uncommon in HCC and HER3 mRNA expression is differentially influenced by hepatitis viruses. Insulin is a negative regulator of HER3 protein expression and function in HCC cells. Altogether these data may explain why HER3 and heregulin-1ß expression have no prognostic value and suggest that HCC patients are unlikely to derive benefit from HER3-targeted monotherapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s13046-016-0402-3) contains supplementary material, which is available to authorized users.

Chapter Nine - Cellular Roles of Beta-Arrestins as Substrates and Adaptors of Ubiquitination and Deubiquitination

β-Arrestin1 and β-arrestin2 are homologous adaptor proteins that are ubiquitously expressed in mammalian cells. They belong to a four-member family of arrestins that regulate the vast family of seven-transmembrane receptors that couple to heterotrimeric G proteins (7TMRs or GPCRs), and that modulate 7TMR signal transduction. β-Arrestins were originally identified in the context of signal inhibition via the 7TMRs because they competed with and thereby blocked G protein coupling to 7TMRs. Currently, in addition to their role as desensitizers of signaling, β-arrestins are appreciated as multifunctional adaptors that mediate trafficking and signal transduction of not only 7TMRs, but a growing list of additional receptors, ion channels, and nonreceptor proteins. β-Arrestins' interactions with their multifarious partners are based on their dynamic conformational states rather than particular domain-domain interactions. β-Arrestins adopt activated conformations upon 7TMR association. In addition, β-arrestins undergo various posttranslational modifications that are choreographed by activated 7TMRs, including phosphorylation, ubiquitination, acetylation, nitrosylation, and SUMOylation. Ubiquitination of β-arrestins is critical for their high-affinity interaction with 7TMRs as well as with endocytic adaptor proteins and signaling kinases. β-Arrestins also function as critical adaptors for ubiquitination and deubiquitination of various cellular proteins, and thereby affect the longevity of signal transducers and the intensity of signal transmission.

Serine 302 Phosphorylation of Mouse Insulin Receptor Substrate 1 (IRS1) Is Dispensable for Normal Insulin Signaling and Feedback Regulation by Hepatic S6 Kinase

Constitutive activation of the mammalian target of rapamycin complex 1 and S6 kinase (mTORC1→ S6K) attenuates insulin-stimulated Akt activity in certain tumors in part through "feedback" phosphorylation of the upstream insulin receptor substrate 1 (IRS1). However, the significance of this mechanism for regulating insulin sensitivity in normal tissue remains unclear. We investigated the function of Ser-302 in mouse IRS1, the major site of its phosphorylation by S6K in vitro, through genetic knock-in of a serine-to-alanine mutation (A302). Although insulin rapidly stimulated feedback phosphorylation of Ser-302 in mouse liver and muscle, homozygous A302 mice (A/A) and their knock-in controls (S/S) exhibited similar glucose homeostasis and muscle insulin signaling. Furthermore, both A302 and control primary hepatocytes from which Irs2 was deleted showed marked inhibition of insulin-stimulated IRS1 tyrosine phosphorylation and PI3K binding after emetine treatment to raise intracellular amino acids and activate mTORC1 → S6K signaling. To specifically activate mTORC1 in mouse tissue, we deleted hepatic Tsc1 using Cre adenovirus. Although it moderately decreased IRS1/PI3K association and Akt phosphorylation in liver, Tsc1 deletion failed to cause glucose intolerance or promote hyperinsulinemia in mixed background A/A or S/S mice. Moreover, Tsc1 deletion failed to stimulate phospho-Ser-302 or other putative S6K sites within IRS1, whereas ribosomal S6 protein was constitutively phosphorylated. Following acute Tsc1 deletion from hepatocytes, Akt phosphorylation, but not IRS1/PI3K association, was rapidly restored by treatment with the mTORC1 inhibitor rapamycin. Thus, within the hepatic compartment, mTORC1 → S6K signaling regulates Akt largely through IRS-independent means with little effect upon physiologic insulin sensitivity.

Protein phosphatase 4 (PP4) functions as a critical regulator in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance

Protein phosphatase 4 (PP4) was shown to participate in multiple cellular processes, including DNA damage response, cell cycle and embryo development. Recent studies demonstrated a looming role of PP4 in glucose metabolism. However, whether PP4 is involved in hepatic insulin resistance remains poorly understood. The objective of this study was to estimate the role of PP4 in tumor necrosis factor (TNF)-α-induced hepatic insulin resistance. db/db mice and TNF-α-treated C57BL/6J mice were used as hepatic insulin resistance animal models. In vitro models were established in both HepG2 cells and primary hepatocytes by TNF-α treatment. We found that increased expression and activity of PP4 occurred in the livers of db/db mice and TNF-α-induced hepatic insulin resistance both in vitro and in vivo. Actually, PP4 silencing and suppression of PP4 activity ameliorated TNF-α-induced hepatic insulin resistance, whereas over-expression of PP4 caused insulin resistance. We then further investigated the prodiabetic mechanism of PP4 in TNF-α-induced insulin resistance. We found that PP4 formed a complex with IRS-1 to promote phosphorylation of IRS-1 on serine 307 via JNK activation and reduce the expression of IRS-1. Thus, PP4 is an important regulator in inflammatory related insulin resistance.

Anti-diabetic efficacy of KICG1338, a novel glycogen synthase kinase-3β inhibitor, and its molecular characterization in animal models of type 2 diabetes and insulin resistance

Selective inhibition of glycogen synthase kinase-3 (GSK3) has been targeted as a novel therapeutic strategy for diabetes mellitus. We investigated the anti-diabetic efficacy and molecular mechanisms of KICG1338 (2-(4-fluoro-phenyl)-3H-imidazo[4,5-b]pyridine-7-carboxylic acid(4-methyl-pyridin-3-yl)-amide), a GSK3β inhibitor, in three animal models: Otsuka Long-Evans Tokushima Fatty (OLETF) rats, leptin receptors-deficient db/db mice, and diet-induced obese (DIO) mice. Biochemical parameters including glucose tolerance tests and gene expressions associated with glucose metabolism were investigated. Glucose excursion decreased significantly by KICG1338-treated OLETF rats, accompanied by increase in insulin receptor substrate-1 and glucose transporter (GLUT)-4 expressions in muscle and decreased GLUT-2 expression in liver. Glucose-lowering effects were similarly observed in KICG1338-treated db/db and DIO mice. KICG1338 treatment increased adiponectin levels and decreased TNF-α levels. KICG1338 therapy also led to greater β-cell preservation and less hepatic fat infiltration with decreased expressions of genes involved in inflammation and endoplasmic reticulum stress. These data demonstrate anti-diabetic efficacy of KICG1338, a novel GSK3β inhibitor.

Effect of Coptidis Rhizoma extracts in a water-based solution on insulin resistance in 3T3-L1 adipocytes

The present study was designed to investigate effects and molecular mechanisms of Coptidis Rhizoma extracts (CRE) on the improvement of insulin resistance induced by tumor necrosis factor-α (TNF-α) in adipocytes. We examined whether CRE administration could directly influence the insulin resistance in 3T3-L1 adipocytes. Potential roles of CRE in glucose consumption, mRNA expression of peroxisome proliferators activated receptor (PPAR-γ), expression of insulin receptor substrate-1 (IRS-1) protein, and phosphorylation of IRS-1 Ser307 were also investigated in the present study. Our data demonstrated that TNF-α significantly reduced levels of glucose consumption and IRS-1 protein expression, while TNF-α increased the phosphorylation of IRS-1 Ser307 in adipocytes 24 h after the challenge, suggesting that TNF-α induced a condition with the occurrence of insulin resistance. Those alterations induced by TNF-α were prevented and the mRNA expression of PPAR-γ was up-regulated by the administration of CRE. Thus, our results indicate that CRE can be used to prevent from the TNF-α-induced insulin resistance through PPAR-γ pathways.

Cell lysis-free quantum dot multicolor cellular imaging-based mechanism study for TNF-α-induced insulin resistance

Background

TNF-α is an inflammatory cytokine that plays an important role in insulin resistance observed in obesity and chronic inflammation. Many cellular components involved in insulin signaling cascade are known to be inhibited by TNF-α. Insulin receptor substrate (IRS)-1 is one of the major targets in TNF-α-induced insulin resistance. The serine phosphorylation of IRS-1 enables the inhibition of insulin signaling. Until now, many studies have been conducted to investigate the mechanism of TNF-α-induced insulin resistance based on Western blot. Intracellular protein kinase crosstalk is commonly encountered in inflammation-associated insulin resistance. The crosstalk among the signaling molecules obscures the precise role of kinases in insulin resistance. We have developed a cell lysis-free quantum dots (QDots) multicolor cellular imaging to identify the biochemical role of multiple kinases (p38, JNK, IKKβ, IRS1^ser, IRS1^tyr, GSK3β, and FOXO1) in inflammation-associated insulin resistance pathway with a single assay in one run. QDot-antibody conjugates were used as nanoprobes to simultaneously monitor the activation/deactivation of the above seven intracellular kinases in HepG2 cells. The effect of the test compounds on the suppression of TNF-α-induced insulin resistance was validated through kinase monitoring. Aspirin, indomethacin, cinnamic acid, and amygdalin were tested.

Results

Through the measurement of the glycogen level in HepG2 cell treated with TNF-α, it was found that aspirin and indomethacin increased glycogen levels by almost two-fold compared to amygdalin and cinnamic acid. The glucose production assay proved that cinnamic acid was much more efficient in suppressing glucose production, compared with MAP kinase inhibitors and non-steroidal anti-inflammatory drugs. QDot multicolor cellular imaging demonstrated that amygdalin and cinnamic acid selectively acted via the JNK1-dependent pathway to suppress the inflammation-induced insulin resistance and improve insulin sensitivity.

Conclusion

The regulatory function of multiple kinases could be monitored concurrently at the cellular level. The developed cellular imaging assay provides a unique platform for the understanding of inflammation and insulin resistance signaling pathways in type II diabetes mellitus and how they regulate each other. The results showed that amygdalin and cinnamic acid inhibit serine phosphorylation of IRS-1 through targeting JNK serine kinase and enhance insulin sensitivity.

Overexpression of insulin receptor partially improves obese and diabetic phenotypes in db/db mice

Type 2 diabetes mellitus (T2DM) is one of the major health concern among the world. Several treatment options for T2DM are in clinical use, including injecting insulin, promoting insulin secretion by insulin secretagogues, and improving insulin sensitivity by insulin sensitizers. However, increasing the amount of insulin receptor in insulin-target tissues has not been explored. In order to test the efficacy of insulin receptor overexpression for improving glucose control, we established a transgenic mouse line expressing human insulin receptor (INSR). We analyzed, growth, energy balance, and glucose control of INSR-overexpressing db/db mice (INSR; db/db), which we produced by mating INSR transgenic mice with db/db mice, a genetic model of obesity due to insufficient leptin signaling. Compared to db/db mice, INSR; db/db mice were rescued from hyperphagia and obesity, leading to improved blood glucose levels. Unexpectedly, however, INSR; db/db mice presented with stunted growth, accompanied by decreased plasma levels of free IGF1 and IGFBP-3, indicating the down-regulation of GH/IGF1 axis. These phenotypes were observed in INSR; db/db mice but not in INSR littermates. Meanwhile, bone defects observed in db/db male mice were not rescued. Moreover, improved blood glucose was not accompanied by improved insulin sensitivity. Therefore, overexpression of insulin receptor improves obese and diabetic phenotypes in db/db mice, with consequences on growth.

The Novel Functions of High-Molecular-Mass Complexes Containing Insulin Receptor Substrates in Mediation and Modulation of Insulin-Like Activities: Emerging Concept of Diverse Functions by IRS-Associated Proteins

Insulin-like peptides, such as insulin-like growth factors (IGFs) and insulin, induce a variety of bioactivities, such as growth, differentiation, survival, increased anabolism, and decreased catabolism in many cell types and in vivo. In general, IGFs or insulin bind to IGF-I receptor (IGF-IR) or insulin receptor (IR), activating the receptor tyrosine kinase. Insulin receptor substrates (IRSs) are known to be major substrates of receptor kinases, mediating IGF/insulin signals to direct bioactivities. Recently, we discovered that IRSs form high-molecular-mass complexes (referred to here as IRSomes) even without IGF/insulin stimulation. These complexes contain proteins (referred to here as IRSAPs; IRS-associated proteins), which modulate tyrosine phosphorylation of IRSs by receptor kinases, control IRS stability, and determine intracellular localization of IRSs. In addition, in these complexes, we found not only proteins that are involved in RNA metabolism but also RNAs themselves. Thus, IRSAPs possibly contribute to modulation of IGF/insulin bioactivities. Since it is established that disorder of modulation of insulin-like activities causes various age-related diseases including cancer, we could propose that the IRSome is an important target for treatment of these diseases.

Beta hydroxy beta methylbutyrate supplementation impairs peripheral insulin sensitivity in healthy sedentary Wistar rats

AIM

Investigate, in healthy sedentary rats, the potential mechanisms involved on the effects of beta hydroxy beta methylbutyrate (HMB) supplementation upon the glycaemic homeostasis, by evaluating the insulin sensitivity in liver, skeletal muscle, and white adipose tissue.

METHODS

Rats were supplemented with either beta hydroxy beta methylbutyrate (320 mg kg(-1)  BW) or saline by gavage for 4 weeks. After the experimental period, the animals were subjected to the glucose tolerance test (GTT) and plasma non-esterified fatty acids (NEFA) concentration measurements. The soleus skeletal muscle, liver and white adipose tissue were removed for molecular (western blotting and RT-PCR) and histological analysis.

RESULTS

The beta hydroxy beta methylbutyrate supplemented rats presented: (i) higher ratio between the area under the curve (AUC) of insulinaemia and glycaemia during glucose tolerance test; (ii) impairment of insulin sensitivity on liver and soleus skeletal muscle after insulin overload; (iii) reduction of glucose transporter 4 (GLUT 4) total and plasma membrane content on soleus; (iv) increased hormone-sensitive lipase (HSL) mRNA and protein expression on white adipose tissue and plasma NEFA levels and (v) reduction of fibre cross-sectional area of soleus muscle.

CONCLUSION

The data altogether indicate that beta hydroxy beta methylbutyrate supplementation impairs insulin sensitivity in healthy sedentary rats, which, in the long-term, could lead to an increased risk of developing type 2 diabetes.

High dietary lipid level is associated with persistent hyperglycaemia and downregulation of muscle Akt-mTOR pathway in Senegalese sole (Solea senegalensis)

High levels of dietary lipids are incorporated in feeds for most teleost fish to promote growth and reduce nitrogen waste. However, in Senegalese sole (Solea senegalensis) previous studies revealed that increasing the level of dietary lipids above 8% negatively affect growth and nutrient utilization regardless of dietary protein content. It has been shown that glucose regulation and metabolism can be impaired by high dietary fat intake in mammals, but information in teleost fish is scarce. The aim of this study was to assess the possible effect of dietary lipids on glucose metabolism in Senegalese sole with special emphasis on the regulation of proteins involved in the muscle insulin-signalling pathway. Senegalese sole juveniles (29 g) were fed two isonitrogenous diets (53% dry matter) for 88 days. These two diets were one with a high lipid level (∼17%, HL) and a moderate starch content (∼14%, LC), and the other being devoid of fish oil (4% lipid, LL) and with high starch content (∼23%, HC). Surprisingly, feeding Senegalese sole the HL/LC diet resulted in prolonged hyperglycaemia, while fish fed on LL/HC diet restored basal glycaemia 2 h after feeding. The hyperglycaemic phenotype was associated with greater glucose-6-phosphatase activity (a key enzyme of hepatic glucose production) and lower citrate synthase activity in the liver, with significantly higher liver glycogen content. Sole fed on HL/LC diet also had significantly lower hexokinase activity in muscle, although hexokinase activity was low with both dietary treatments. The HL/LC diet was associated with significant reductions in muscle AKT, p70 ribosomal S6-K1 Kinase (S6K-1) and ribosomal protein S6 (S6) 2 h after feeding, suggesting down regulation of the AKT-mTOR nutrient signalling pathway in these fish. The results of this study show for the first time that high level of dietary lipids strongly affects glucose metabolism in Senegalese sole.

Influence of miRNA in insulin signaling pathway and insulin resistance: micro-molecules with a major role in type-2 diabetes

The prevalence of type-2 diabetes (T2D) is increasing significantly throughout the globe since the last decade. This heterogeneous and multifactorial disease, also known as insulin resistance, is caused by the disruption of the insulin signaling pathway. In this review, we discuss the existence of various miRNAs involved in regulating the main protein cascades in the insulin signaling pathway that affect insulin resistance. The influence of miRNAs (miR-7, miR-124a, miR-9, miR-96, miR-15a/b, miR-34a, miR-195, miR-376, miR-103, miR-107, and miR-146) in insulin secretion and beta (β) cell development has been well discussed. Here, we highlight the role of miRNAs in different significant protein cascades within the insulin signaling pathway such as miR-320, miR-383, miR-181b with IGF-1, and its receptor (IGF1R); miR-128a, miR-96, miR-126 with insulin receptor substrate (IRS) proteins; miR-29, miR-384-5p, miR-1 with phosphatidylinositol 3-kinase (PI3K); miR-143, miR-145, miR-29, miR-383, miR-33a/b miR-21 with AKT/protein kinase B (PKB) and miR-133a/b, miR-223, miR-143 with glucose transporter 4 (GLUT4). Insulin resistance, obesity, and hyperlipidemia (high lipid levels in the blood) have a strong connection with T2D and several miRNAs influence these clinical outcomes such as miR-143, miR-103, and miR-107, miR-29a, and miR-27b. We also corroborate from previous evidence how these interactions are related to insulin resistance and T2D. The insights highlighted in this review will provide a better understanding on the impact of miRNA in the insulin signaling pathway and insulin resistance-associated diagnostics and therapeutics for T2D.

Host insulin stimulates Echinococcus multilocularis insulin signalling pathways and larval development

BACKGROUND

The metacestode of the tapeworm Echinococcus multilocularis is the causative agent of alveolar echinococcosis, a lethal zoonosis. Infections are initiated through establishment of parasite larvae within the intermediate host's liver, where high concentrations of insulin are present, followed by tumour-like growth of the metacestode in host organs. The molecular mechanisms determining the organ tropism of E. multilocularis or the influences of host hormones on parasite proliferation are poorly understood.

RESULTS

Using in vitro cultivation systems for parasite larvae we show that physiological concentrations (10 nM) of human insulin significantly stimulate the formation of metacestode larvae from parasite stem cells and promote asexual growth of the metacestode. Addition of human insulin to parasite larvae led to increased glucose uptake and enhanced phosphorylation of Echinococcus insulin signalling components, including an insulin receptor-like kinase, EmIR1, for which we demonstrate predominant expression in the parasite's glycogen storage cells. We also characterized a second insulin receptor family member, EmIR2, and demonstrated interaction of its ligand binding domain with human insulin in the yeast two-hybrid system. Addition of an insulin receptor inhibitor resulted in metacestode killing, prevented metacestode development from parasite stem cells, and impaired the activation of insulin signalling pathways through host insulin.

CONCLUSIONS

Our data indicate that host insulin acts as a stimulant for parasite development within the host liver and that E. multilocularis senses the host hormone through an evolutionarily conserved insulin signalling pathway. Hormonal host-parasite cross-communication, facilitated by the relatively close phylogenetic relationship between E. multilocularis and its mammalian hosts, thus appears to be important in the pathology of alveolar echinococcosis. This contributes to a closer understanding of organ tropism and parasite persistence in larval cestode infections. Furthermore, our data show that Echinococcus insulin signalling pathways are promising targets for the development of novel drugs.