Insulin-stimulated endoproteolytic TUG cleavage links energy expenditure with glucose uptake

Sustained caloric restriction potentiates insulin action by activating prostacyclin synthase

OBJECTIVE

Caloric restriction (CR) is known to enhance insulin sensitivity and reduce the risk of metabolic disorders; however, its molecular mechanisms are not fully understood. This study aims to elucidate specific proteins and pathways responsible for these benefits.

METHODS

We examined adipose tissue from participants in the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy Phase 2 (CALERIE 2) study, comparing proteomic profiles from individuals after 12 and 24 months of CR with baseline and an ad libitum group. Biochemical and cell-specific physiological approaches complemented these analyses.

RESULTS

Our data revealed that CR upregulates prostacyclin synthase (PTGIS) in adipose tissue, an enzyme crucial for producing prostacyclin (PGI2). PGI2 improves the ability of insulin to stimulate the tether-containing UBX domain for GLUT4 (TUG) cleavage pathway, which is essential for glucose uptake regulation. Additionally, iloprost, a PGI2 analog, was shown to increase insulin receptor density on cell membranes, increasing glucose uptake in human adipocytes. CR also reduces carbonylation of GLUT4, a modification that is detrimental to GLUT4 function.

CONCLUSIONS

CR enhances insulin sensitivity by promoting PTGIS expression and stimulating the TUG cleavage pathway, leading to increased GLUT4 translocation to the cell surface and decreased GLUT4 carbonylation. These findings shed light on the complex molecular mechanisms through which CR favorably impacts insulin sensitivity and metabolic health.

UBXN9 governs GLUT4-mediated spatial confinement of RIG-I-like receptors and signaling

The cytoplasmic RIG-I-like receptors (RLRs) recognize viral RNA and initiate innate antiviral immunity. RLR signaling also triggers glycolytic reprogramming through glucose transporters (GLUTs), whose role in antiviral immunity is elusive. Here, we unveil that insulin-responsive GLUT4 inhibits RLR signaling independently of glucose uptake in adipose and muscle tissues. At steady state, GLUT4 is trapped at the Golgi matrix by ubiquitin regulatory X domain 9 (UBXN9, TUG). Following RNA virus infection, GLUT4 is released and translocated to the cell surface where it spatially segregates a significant pool of cytosolic RLRs, preventing them from activating IFN-β responses. UBXN9 deletion prompts constitutive GLUT4 translocation, sequestration of RLRs and attenuation of antiviral immunity, whereas GLUT4 deletion heightens RLR signaling. Notably, reduced GLUT4 expression is uniquely associated with human inflammatory myopathies characterized by hyperactive interferon responses. Overall, our results demonstrate a noncanonical UBXN9-GLUT4 axis that controls antiviral immunity via plasma membrane tethering of cytosolic RLRs.

Induction of insulin resistance in female mice due to prolonged phenanthrene exposure: Unveiling the low-dose effect and potential mechanisms

Phenanthrene (Phe) is a commonly occurring polycyclic aromatic hydrocarbon (PAH) found in various food sources and drinking water. Previous studies have shown that long-term exposure to Phe in male mice leads to insulin resistance in a dose-dependent manner. However, the effect of Phe on glucose homeostasis in female mice remains unknown. To address this knowledge gap, female Kunming mice were exposed to Phe through their drinking water at concentrations of 0.05, 0.5, and 5 ng/mL. After 270 d of exposure, we surprisingly discovered a low-dose effect of Phe on insulin resistance in female mice, which differed from the effect observed in male mice and showed sexual dimorphism. Specifically, insulin resistance was only observed in the 0.05 ng/mL treatment, and this low-dose effect was also reflected in the concentration of Phe in white adipose tissue (WAT). Differences in metabolic enzyme activities in the liver may potentially explain this effect. The observed sexual dimorphism in Phe exposure could be attributed to variations in estrogen (E2) level and estrogen receptor beta (ERβ) expression in WAT. These findings highlight the association between environmental factors and the development of insulin resistance, emphasizing the pathogenic effect of even low doses of Phe. Moreover, sex dependent-effect should be given more attention when studying the toxic effects of environmental pollutants.

Er-Dong-Xiao-Ke decoction regulates lipid metabolism via PPARG-mediated UCP2/AMPK signaling to alleviate diabetic meibomian gland dysfunction

ETHNOPHARMACOLOGICAL RELEVANCE

Meibomian gland dysfunction (MGD), complicated by type 2 diabetes, is associated with a high incidence of ocular surface disease, and no effective drug treatment exists. Diabetes mellitus (DM) MGD shows a notable disturbance in lipid metabolism. Er-Dong-Xiao-Ke decoction (EDXKD) has important functions in nourishing yin, clearing heat, and removing blood stasis, which are effective in the treatment of DM MGD.

AIM OF THE STUDY

To observe the therapeutic effect of EDXKD on DM MGD and its underlying molecular mechanism.

MATERIALS AND METHODS

After establishing a type 2 DM (T2DM)-induced MGD rat model, different doses of EDXKD and T0070907 were administered. The chemical constituents of EDXKD were identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS), and the molecular mechanism of EDXKD in treating DM MGD was predicted using network pharmacology. Lipid metabolism in DM meibomian glands (MGs) was analyzed using LC-MS/MS, and lipid biomarkers were screened and identified. Histological changes and lipid accumulation in MGs were detected by staining, and Peroxisome proliferator-activated receptor gamma (PPARG) expression in MG acinar cells was detected by immunofluorescence. The expression of lipid metabolism-related factors was detected by reverse transcription-quantitative polymerase chain reaction (RT-qPCR) or western blotting.

RESULTS

EDXKD reduced lipid accumulation in the MGs and improved the ocular surface index in DM MGD rats. The main active components of EDXKD had advantages in lipid regulation. Additionally, the PPARG signaling pathway was the key pathway of EDXKD in the treatment of DM MGD. Twelve lipid metabolites were biomarkers of EDXKD in the treatment of DM MGD, and glycerophospholipid metabolism was the main pathway of lipid regulation. Moreover, EDXKD improved lipid deposition in the acini and upregulated the expression of PPARG. Further, EDXKD regulated the PPARG-mediated UCP2/AMPK signaling network, inhibited lipid production, and promoted lipid transport.

CONCLUSION

EDXKD is an effective treatment for MGD in patients with T2DM. EDXKD can regulate lipids by regulating the PPARG-mediated UCP2/AMPK signaling network, as it reduced lipid accumulation in the MGs of DM MGD rats, promoted lipid metabolism, and improved MG function and ocular surface indices.

UBXN9 governs GLUT4-mediated spatial confinement of RIG-I-like receptors and signaling

The cytoplasmic RIG-I-like receptors (RLRs) recognize viral RNA and initiate innate antiviral immunity. RLR signaling also triggers glycolytic reprogramming through glucose transporters (GLUTs), whose role in antiviral immunity is elusive. Here, we unveil that insulin-responsive GLUT4 inhibits RLR signaling independently of glucose uptake in adipose and muscle tissues. At steady state, GLUT4 is docked at the Golgi matrix by ubiquitin regulatory X domain 9 (UBXN9, TUG). Following RNA virus infection, GLUT4 is released and translocated to the cell surface where it spatially segregates a significant pool of cytosolic RLRs, preventing them from activating IFN-β responses. UBXN9 deletion prompts constitutive GLUT4 trafficking, sequestration of RLRs, and attenuation of antiviral immunity, whereas GLUT4 deletion heightens RLR signaling. Notably, reduced GLUT4 expression is uniquely associated with human inflammatory myopathies characterized by hyperactive interferon responses. Overall, our results demonstrate a noncanonical UBXN9-GLUT4 axis that controls antiviral immunity via plasma membrane tethering of cytosolic RLRs.

The global perspective on peroxisome proliferator-activated receptor γ (PPARγ) in ectopic fat deposition: A review

Excessive expansion of adipocytes can have unhealthy consequences as excess free fatty acids enter other tissues and cause ectopic fat deposition by resynthesizing triglycerides. This lipid accumulation in various tissues is harmful and can increase the risk of related metabolic diseases such as type II diabetes, cardiovascular disease, and insulin resistance. Peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily that play a key role in energy metabolism as fatty acid metabolism sensors, and peroxisome proliferator-activated receptor γ (PPARγ) is the main subtype responsible for fat cell differentiation and adipogenesis. In this paper, we introduce the main structure and function of PPARγ and its regulatory role in the process of lipogenesis in the liver, kidney, skeletal muscle, and pancreas. This information can serve as a reference for further understanding the regulatory mechanisms and measures of the PPAR family in the process of ectopic fat deposition.

Fibroblast Growth Factor 21: A Fascinating Perspective on the Regulation of Muscle Metabolism

Fibroblast growth factor 21 (FGF21) plays a vital role in normal eukaryotic organism development and homeostatic metabolism under the influence of internal and external factors such as endogenous hormone changes and exogenous stimuli. Over the last few decades, comprehensive studies have revealed the key role of FGF21 in regulating many fundamental metabolic pathways, including the muscle stress response, insulin signaling transmission, and muscle development. By coordinating these metabolic pathways, FGF21 is thought to contribute to acclimating to a stressful environment and the subsequent recovery of cell and tissue homeostasis. With the emphasis on FGF21, we extensively reviewed the research findings on the production and regulation of FGF21 and its role in muscle metabolism. We also emphasize how the FGF21 metabolic networks mediate mitochondrial dysfunction, glycogen consumption, and myogenic development and investigate prospective directions for the functional exploitation of FGF21 and its downstream effectors, such as the mammalian target of rapamycin (mTOR).

The intracellular helical bundle of human glucose transporter GLUT4 is important for complex formation with ASPL

Glucose transporters (GLUTs) are responsible for transporting hexose molecules across cellular membranes. In adipocytes, insulin stimulates glucose uptake by redistributing GLUT4 to the plasma membrane. In unstimulated adipose-like mouse cell lines, GLUT4 is known to be retained intracellularly by binding to TUG protein, while upon insulin stimulation, GLUT4 dissociates from TUG. Here, we report that the TUG homolog in human, ASPL, exerts similar properties, i.e., forms a complex with GLUT4. We describe the structural details of complex formation by combining biochemical assays with cross-linking mass spectrometry and computational modeling. Combined, the data suggest that the intracellular domain of GLUT4 binds to the helical lariat of ASPL and contributes to the regulation of GLUT4 trafficking by cooperative binding.

Hypermetabolism in mice carrying a near-complete human chromosome 21

The consequences of aneuploidy have traditionally been studied in cell and animal models in which the extrachromosomal DNA is from the same species. Here, we explore a fundamental question concerning the impact of aneuploidy on systemic metabolism using a non-mosaic transchromosomic mouse model (TcMAC21) carrying a near-complete human chromosome 21. Independent of diets and housing temperatures, TcMAC21 mice consume more calories, are hyperactive and hypermetabolic, remain consistently lean and profoundly insulin sensitive, and have a higher body temperature. The hypermetabolism and elevated thermogenesis are likely due to a combination of increased activity level and sarcolipin overexpression in the skeletal muscle, resulting in futile sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA) activity and energy dissipation. Mitochondrial respiration is also markedly increased in skeletal muscle to meet the high ATP demand created by the futile cycle and hyperactivity. This serendipitous discovery provides proof-of-concept that sarcolipin-mediated thermogenesis via uncoupling of the SERCA pump can be harnessed to promote energy expenditure and metabolic health.

The effects of sucrose and arsenic on muscular insulin signaling pathways differ between the gastrocnemius and quadriceps muscles

Introduction

Insulin resistance in muscle can originate from a sedentary lifestyle, hypercaloric diets, or exposure to endocrine-disrupting pollutants such as arsenic. In skeletal muscle, insulin stimulates glucose uptake by translocating GLUT4 to the sarcolemma. This study aimed to evaluate the alterations induced by sucrose and arsenic exposure in vivo on the pathways involved in insulinstimulated GLUT4 translocation in the quadriceps and gastrocnemius muscles.

Methods

Male Wistar rats were treated with 20% sucrose (S), 50 ppm sodium arsenite (A), or both (A+S) in drinking water for 8 weeks. We conducted an intraperitoneal insulin tolerance (ITT) test on the seventh week of treatment. The quadriceps and gastrocnemius muscles were obtained after overnight fasting or 30 min after intraperitoneal insulin injection. We assessed changes in GLUT4 translocation to the sarcolemma by cell fractionation and abundance of the proteins involved in GLUT4 translocation by Western blot.

Results

Male rats consuming S and A+S gained more weight than control and Atreated animals. Rats consuming S, A, and A+S developed insulin resistance assessed through ITT. Neither treatments nor insulin stimulation in the quadriceps produced changes in GLUT4 levels in the sarcolemma and Akt phosphorylation. Conversely, A and A+S decreased protein expression of Tether containing UBX domain for GLUT4 (TUG), and A alone increased calpain-10 expression. All treatments reduced this muscle's protein levels of VAMP2. Conversely, S and A treatment increased basal GLUT4 levels in the sarcolemma of the gastrocnemius, while all treatments inhibited insulin-induced GLUT4 translocation. These effects correlated with lower basal levels of TUG and impaired insulin-stimulated TUG proteolysis. Moreover, animals treated with S had reduced calpain-10 protein levels in this muscle, while A and A+S inhibited insulin-induced Akt phosphorylation.

Conclusion

Arsenic and sucrose induce systemic insulin resistance due to defects in GLUT4 translocation induced by insulin. These defects depend on which muscle is being analyzed, in the quadriceps there were defects in GLUT4 retention and docking while in the gastrocnemius the Akt pathway was impacted by arsenic and the proteolytic pathway was impaired by arsenic and sucrose.

Hypermetabolism in mice carrying a near complete human chromosome 21

The consequences of aneuploidy have traditionally been studied in cell and animal models in which the extrachromosomal DNA is from the same species. Here, we explore a fundamental question concerning the impact of aneuploidy on systemic metabolism using a non-mosaic transchromosomic mouse model (TcMAC21) carrying a near complete human chromosome 21. Independent of diets and housing temperatures, TcMAC21 mice consume more calories, are hyperactive and hypermetabolic, remain consistently lean and profoundly insulin sensitive, and have a higher body temperature. The hypermetabolism and elevated thermogenesis are due to sarcolipin overexpression in the skeletal muscle, resulting in futile sarco(endo)plasmic reticulum Ca ²⁺ ATPase (SERCA) activity and energy dissipation. Mitochondrial respiration is also markedly increased in skeletal muscle to meet the high ATP demand created by the futile cycle. This serendipitous discovery provides proof-of-concept that sarcolipin-mediated thermogenesis via uncoupling of the SERCA pump can be harnessed to promote energy expenditure and metabolic health.

Endosomal trafficking in metabolic homeostasis and diseases

The global prevalences of obesity and type 2 diabetes mellitus have reached epidemic status, presenting a heavy burden on society. It is therefore essential to find novel mechanisms and targets that could be utilized in potential treatment strategies and, as such, intracellular membrane trafficking has re-emerged as a regulatory tool for controlling metabolic homeostasis. Membrane trafficking is an essential physiological process that is responsible for the sorting and distribution of signalling receptors, membrane transporters and hormones or other ligands between different intracellular compartments and the plasma membrane. Dysregulation of intracellular transport is associated with many human diseases, including cancer, neurodegeneration, immune deficiencies and metabolic diseases, such as type 2 diabetes mellitus and its associated complications. This Review focuses on the latest advances on the role of endosomal membrane trafficking in metabolic physiology and pathology in vivo, highlighting the importance of this research field in targeting metabolic diseases.

Inhibitors of RNA and protein synthesis cause Glut4 translocation and increase glucose uptake in adipocytes

Insulin stimulates glucose uptake in adipocytes by triggering translocation of glucose transporter 4-containg vesicles to the plasma membrane. Under basal conditions, these vesicles (IRVs for insulin-responsive vesicles) are retained inside the cell via a “static” or “dynamic” mechanism. We have found that inhibitors of RNA and protein synthesis, actinomycin D and emetine, stimulate Glut4 translocation and glucose uptake in adipocytes without engaging conventional signaling proteins, such as Akt, TBC1D4, or TUG. Actinomycin D does not significantly affect endocytosis of Glut4 or recycling of transferrin, suggesting that it specifically increases exocytosis of the IRVs. Thus, the intracellular retention of the IRVs in adipocytes requires continuous RNA and protein biosynthesis de novo. These results point out to the existence of a short-lived inhibitor of IRV translocation thus supporting the “static” model.

Insulin action and resistance are dependent on a GSK3β-FBXW7-ERRα transcriptional axis

Insulin resistance, a harbinger of the metabolic syndrome, is a state of compromised hormonal response resulting from the dysregulation of a wide range of insulin-controlled cellular processes. However, how insulin affects cellular energy metabolism via long-term transcriptional regulation and whether boosting mitochondrial function alleviates insulin resistance remains to be elucidated. Herein we reveal that insulin directly enhances the activity of the nuclear receptor ERRα via a GSK3β/FBXW7 signaling axis. Liver-specific deletion of GSK3β or FBXW7 and mice harboring mutations of ERRα phosphosites (ERRα3SA) co-targeted by GSK3β/FBXW7 result in accumulated ERRα proteins that no longer respond to fluctuating insulin levels. ERRα3SA mice display reprogrammed liver and muscle transcriptomes, resulting in compromised energy homeostasis and reduced insulin sensitivity despite improved mitochondrial function. This crossroad of insulin signaling and transcriptional control by a nuclear receptor offers a framework to better understand the complex cellular processes contributing to the development of insulin resistance.

The role of SERCA and sarcolipin in adaptive muscle remodeling

Sarcolipin (SLN) is a small integral membrane protein that regulates the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) pump. When bound to SERCA, SLN reduces the apparent Ca²⁺ affinity of SERCA and uncouples SERCA Ca²⁺ transport from its ATP consumption. As such, SLN plays a direct role in altering skeletal muscle relaxation and energy expenditure. Interestingly, the expression of SLN is dynamic during times of muscle adaptation, where large increases in SLN content are found in response to development, atrophy, overload and disease. Several groups have suggested that increases in SLN, especially in dystrophic muscle, are deleterious to muscle function and exacerbate already abhorrent intracellular Ca²⁺ levels. However, there is also significant evidence to show that increased SLN content is a beneficial adaptive mechanism which protects the SERCA pump and activates Ca²⁺ signaling and adaptive remodeling during times of cell stress. In this review, we first discuss the role for SLN in healthy muscle during both development and overload, where SLN has been shown to activate Ca²⁺ signaling to promote mitochondrial biogenesis, fibre type shifts and muscle hypertrophy. Then, with respect to muscle disease, we summarize the discrepancies in the literature as to whether SLN upregulation is adaptive or maladaptive in nature. This review is the first to offer the concept of SLN hormesis in muscle disease, wherein both too much and too little SLN are detrimental to muscle health. Finally, the underlying mechanisms which activate SLN upregulation are discussed, specifically acknowledging a potential positive feedback loop between SLN and Ca²⁺ signaling molecules.

Ubiquitin-like processing of TUG proteins as a mechanism to regulate glucose uptake and energy metabolism in fat and muscle

In response to insulin stimulation, fat and muscle cells mobilize GLUT4 glucose transporters to the cell surface to enhance glucose uptake. Ubiquitin-like processing of TUG (Aspscr1, UBXD9) proteins is a central mechanism to regulate this process. Here, recent advances in this area are reviewed. The data support a model in which intact TUG traps insulin-responsive "GLUT4 storage vesicles" at the Golgi matrix by binding vesicle cargoes with its N-terminus and matrix proteins with its C-terminus. Insulin stimulation liberates these vesicles by triggering endoproteolytic cleavage of TUG, mediated by the Usp25m protease. Cleavage occurs in fat and muscle cells, but not in fibroblasts or other cell types. Proteolytic processing of intact TUG generates TUGUL, a ubiquitin-like protein modifier, as the N-terminal cleavage product. In adipocytes, TUGUL modifies a single protein, the KIF5B kinesin motor, which carries GLUT4 and other vesicle cargoes to the cell surface. In muscle, this or another motor may be modified. After cleavage of intact TUG, the TUG C-terminal product is extracted from the Golgi matrix by the p97 (VCP) ATPase. In both muscle and fat, this cleavage product enters the nucleus, binds PPARγ and PGC-1α, and regulates gene expression to promote fatty acid oxidation and thermogenesis. The stability of the TUG C-terminal product is regulated by an Ate1 arginyltransferase-dependent N-degron pathway, which may create a feedback mechanism to control oxidative metabolism. Although it is now clear that TUG processing coordinates glucose uptake with other aspects of physiology and metabolism, many questions remain about how this pathway is regulated and how it is altered in metabolic disease in humans.

The many actions of insulin in skeletal muscle, the paramount tissue determining glycemia

As the principal tissue for insulin-stimulated glucose disposal, skeletal muscle is a primary driver of whole-body glycemic control. Skeletal muscle also uniquely responds to muscle contraction or exercise with increased sensitivity to subsequent insulin stimulation. Insulin's dominating control of glucose metabolism is orchestrated by complex and highly regulated signaling cascades that elicit diverse and unique effects on skeletal muscle. We discuss the discoveries that have led to our current understanding of how insulin promotes glucose uptake in muscle. We also touch upon insulin access to muscle, and insulin signaling toward glycogen, lipid, and protein metabolism. We draw from human and rodent studies in vivo, isolated muscle preparations, and muscle cell cultures to home in on the molecular, biophysical, and structural elements mediating these responses. Finally, we offer some perspective on molecular defects that potentially underlie the failure of muscle to take up glucose efficiently during obesity and type 2 diabetes.