Fibroblastic reticular cells in lymph node potentiate white adipose tissue beiging through neuro-immune crosstalk in male mice.

Lymph nodes (LNs) are always embedded in the metabolically-active white adipose tissue (WAT), whereas their functional relationship remains obscure. Here, we identify fibroblastic reticular cells (FRCs) in inguinal LNs (iLNs) as a major source of IL-33 in mediating cold-induced beiging and thermogenesis of subcutaneous WAT (scWAT). Depletion of iLNs in male mice results in defective cold-induced beiging of scWAT. Mechanistically, cold-enhanced sympathetic outflow to iLNs activates ß1- and ß2-adrenergic receptor (AR) signaling in FRCs to facilitate IL-33 release into iLN-surrounding scWAT, where IL-33 activates type 2 immune response to potentiate biogenesis of beige adipocytes. Cold-induced beiging of scWAT is abrogated by selective ablation of IL-33 or ß1- and ß2-AR in FRCs, or sympathetic denervation of iLNs, whereas replenishment of IL-33 reverses the impaired cold-induced beiging in iLN-deficient mice. Taken together, our study uncovers an unexpected role of FRCs in iLNs in mediating neuro-immune interaction to maintain energy homeostasis.

YY1 Regulates Glucose Homeostasis Through Controlling Insulin Transcription in Pancreatic ß-Cells.

To date, identification of nonislet-specific transcriptional factors in the regulation of insulin gene expression has been little studied. Here, we report that the expression level of the transcription factor YY1 is increased dramatically in both human and mouse pancreatic ß-cells after birth. Nevertheless, the physiological role of YY1 during ß-cell development and its regulatory mechanism in ß-cell function remain largely unknown. After ß-cell ablation of Yy1, we observed rapid onset of hyperglycemia, impaired glucose tolerance, and reduced ß-cell mass in neonatal and adult mice. These mice also had hypoinsulinemia with normal insulin sensitivity compared with their wild-type littermates, manifesting as a type 1 diabetic phenotype. Mechanistically, genome-wide RNA sequencing has defined dysregulated insulin signaling and defective glucose responsiveness in ß-cells devoid of YY1. Integrative analyses coupled with chromatin immunoprecipitation assays targeting YY1, and histone modifications, including H3K4me1, H3K27ac, and H3K27me3, have further identified Ins1 and Ins2 as direct gene targets of YY1. Luciferase reporter assays and loss- and gain-of-function experiments also demonstrated that YY1 binds to the enhancer regions in exon 2 of Ins1 and Ins2, activating insulin transcription and, therefore, proinsulin and insulin production in pancreatic ß-cells. YY1 also directly interacts with RNA polymerase II, potentially stabilizing the enhancer-promoter interaction in the multiprotein-DNA complex during transcription initiation. Taken together, our findings suggest a role for YY1 as a transcriptional activator of insulin gene expression, assisting ß-cell maturation and function after birth. These analyses may advance our understanding of ß-cell biology and provide clinically relevant insights targeting the pathophysiological origins of diabetes.

PM20D1 is a circulating biomarker closely associated with obesity, insulin resistance and metabolic syndrome.

OBJECTIVE: Peptidase M20 domain containing 1 (PM20D1), a secreted enzyme catalysing condensation of fatty acids and amino acids into the bioactive lipids N-acyl amino acids (NAAA), induces uncoupling protein 1 (UCP1)-independent adaptive thermogenesis in brown/beige adipocytes in mice. This study aimed to explore the associations of the circulating levels of PM20D1 and major NAAA with obesity-related metabolic complications in humans. DESIGN AND METHODS: Serum concentrations of PM20D1 and NAAA (C18:1-Leu and C18:1-Phe) in 256 Chinese subjects, including 78 lean and 178 overweight/obese individuals with or without diabetes, were measured with immunoassays and liquid chromatography-mass spectrometry, respectively. The impact of sulfonylurea and rosiglitazone on their circulating levels was examined in 62 patients with type 2 diabetes. RESULTS: Serum PM20D1 level was significantly elevated in overweight/obese individuals and was closely associated with circulating levels of C18:1-Leu and C18:1-Phe. Furthermore, serum PM20D1, C18:1-Leu and C18:1-Phe concentrations correlated positively with several parameters of adiposity as well as fasting and 2 h postprandial glucose, HbA1c, fasting insulin and HOMA-IR independent of BMI and age. Moreover, a significant elevation in PM20D1, C18:1-Leu and C18:1-Phe concentrations corresponding with increases in the number of components of the metabolic syndrome (MetS) was observed. Treatment with sulfonylurea significantly decreased circulating PM20D1, C18:1-Leu and C18:1-Phe in patients with type 2 diabetes. CONCLUSIONS: Increased serum levels of PM20D1 and its catalytic products NAAA are closely associated with obesity-related glucose dysregulation, insulin resistance and MetS and can be potentially used as clinical biomarkers for diagnosing and monitoring these disorders.

The adaptor protein APPL2 controls glucose-stimulated insulin secretion via F-actin remodeling in pancreatic ß-cells.

Filamentous actin (F-actin) cytoskeletal remodeling is critical for glucose-stimulated insulin secretion (GSIS) in pancreatic ß-cells, and its dysregulation causes type 2 diabetes. The adaptor protein APPL1 promotes first-phase GSIS by up-regulating soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein expression. However, whether APPL2 (a close homology of APPL1 with the same domain organization) plays a role in ß-cell functions is unknown. Here, we show that APPL2 enhances GSIS by promoting F-actin remodeling via the small GTPase Rac1 in pancreatic ß-cells. ß-cell specific abrogation of APPL2 impaired GSIS, leading to glucose intolerance in mice. APPL2 deficiency largely abolished glucose-induced first- and second-phase insulin secretion in pancreatic islets. Real-time live-cell imaging and phalloidin staining revealed that APPL2 deficiency abolished glucose-induced F-actin depolymerization in pancreatic islets. Likewise, knockdown of APPL2 expression impaired glucose-stimulated F-actin depolymerization and subsequent insulin secretion in INS-1E cells, which were attributable to the impairment of Ras-related C3 botulinum toxin substrate 1 (Rac1) activation. Treatment with the F-actin depolymerization chemical compounds or overexpression of gelsolin (a F-actin remodeling protein) rescued APPL2 deficiency-induced defective GSIS. In addition, APPL2 interacted with Rac GTPase activating protein 1 (RacGAP1) in a glucose-dependent manner via the bin/amphiphysin/rvs-pleckstrin homology (BAR-PH) domain of APPL2 in INS-1E cells and HEK293 cells. Concomitant knockdown of RacGAP1 expression reverted APPL2 deficiency-induced defective GSIS, F-actin remodeling, and Rac1 activation in INS-1E cells. Our data indicate that APPL2 interacts with RacGAP1 and suppresses its negative action on Rac1 activity and F-actin depolymerization thereby enhancing GSIS in pancreatic ß-cells.

APPL2 Negatively Regulates Olfactory Functions by Switching Fate Commitments of Neural Stem Cells in Adult Olfactory Bulb via Interaction with Notch1 Signaling.

Adult olfactory neurogenesis plays critical roles in maintaining olfactory functions. Newly-generated neurons in the subventricular zone migrate to the olfactory bulb (OB) and determine olfactory discrimination, but the mechanisms underlying the regulation of olfactory neurogenesis remain unclear. Our previous study indicated the potential of APPL2 (adaptor protein, phosphotyrosine interacting with PH domain and leucine zipper 2) as a modulating factor for neurogenesis in the adult olfactory system. In the present study, we report how APPL2 affects neurogenesis in the OB and thereby mediates olfactory discrimination by using both in vitro neural stem cells (NSCs) and an in vivo animal model-APPL2 transgenic (Tg) mice. In the in vitro study, we found that APPL2 overexpression resulted in NSCs switching from neuronal differentiation to gliogenesis while APPL2 knockdown promoted neurogenesis. In the in vivo study, APPL2 Tg mice had a higher population of glial cells and dampened neuronal production in the olfactory system, including the corpus callosum, OB, and rostral migratory stream. Adult APPL2 Tg mice displayed impaired performance in olfactory discrimination tests compared with wild-type mice. Furthermore, we found that an interaction of APPL2 with Notch1 contributed to the roles of APPL2 in modulating the neurogenic lineage-switching and olfactory behaviors. In conclusion, APPL2 controls olfactory discrimination by switching the fate choice of NSCs via interaction with Notch1 signaling.

Obestatin and growth hormone reveal the interaction of central obesity and other cardiometabolic risk factors of metabolic syndrome.

Metabolic syndrome (MetS) is a multi-factorial disorder including central obesity (CO), insulin resistance, hyperglycemia, dyslipidemia and hypertension which increases the risk of diabetes mellitus and cardiovascular diseases. CO is considered as an essential component of MetS according to International Diabetes Federation (IDF), which may further modulate distinct signalling pathways compared with the other four MetS risk factors. Given that ghrelin signalling and the growth hormone/insulin-like growth factor-1 (GH/IGF-1) axis regulates energy balance and metabolic homeostasis, this study examined the changes in various ghrelin products and circulating hormones in response to the interaction between CO and other MetS components including blood pressure, fasting blood glucose, triglycerides, and high-density lipoprotein cholesterol in 133 Hong Kong Chinese adults. Circulating obestatin and GH were increased and reduced, respectively, by either CO or the other 4-risk factor cluster. These changes were further augmented by the presence of all MetS risk factors. However, changes of ghrelin levels were not mediated by CO but the other MetS risk factors. Our findings suggest that CO does not predict all the dysregulation of signalling pathways in individuals with MetS. Although CO and other MetS may share common signalling targets (i.e., obestatin and GH), CO does not contribute to the perturbation of ghrelin signalling.

The role of adipose tissue senescence in obesity- and ageing-related metabolic disorders.

Adipose tissue as the largest energy reservoir and endocrine organ is essential for maintenance of systemic glucose, lipid and energy homeostasis, but these metabolic functions decline with ageing and obesity. Adipose tissue senescence is one of the common features in obesity and ageing. Although cellular senescence is a defensive mechanism preventing tumorigenesis, its occurrence in adipose tissue causatively induces defective adipogenesis, inflammation, aberrant adipocytokines production and insulin resistance, leading to adipose tissue dysfunction. In addition to these paracrine effects, adipose tissue senescence also triggers systemic inflammation and senescence as well as insulin resistance in the distal metabolic organs, resulting in Type 2 diabetes and other premature physiological declines. Multiple cell types including mature adipocytes, immune cells, endothelial cells and progenitor cells gradually senesce at different levels in different fat depots with ageing and obesity, highlighting the heterogeneity and complexity of adipose tissue senescence. In this review, we discuss the causes and consequences of adipose tissue senescence, and the major cell types responsible for adipose tissue senescence in ageing and obesity. In addition, we summarize the pharmacological approaches and lifestyle intervention targeting adipose tissue senescence for the treatment of obesity- and ageing-related metabolic diseases.

Activation of G0/G1 switch gene 2 by endoplasmic reticulum stress enhances hepatic steatosis.

BACKGROUND: Perturbed endoplasmic reticulum (ER) homeostasis and increased levels of G0/G1 Switch Gene 2 (G0S2) have been documented in animal models with fatty liver disease. In this study, we investigated whether G0S2 is regulated by branch of the unfolded protein response (UPR) and contributes to ER stress-induced hepatic steatosis. METHODS: We first analyzed G0S2 expression and the state of the three canonical UPR branches in several hepatic steatosis models, tunicamycin-treated C57BL/6J mice and HepG2 cells, where ER homeostasis was perturbed. We pretreated HepG2 cells with tauroursodeoxycholic acid (TUDCA) to validate whether G0S2 was the downstream target of ER stress. Loss or gain function analysis was conducted to identify which UPR branch specifically linked to G0S2 transcription. The transcription mechanism was estimated by luciferase reporter assay and ChIP assay. RESULTS: Here we showed that the activation of ER stress was accompanied by elevation of G0S2 expression in the occurrence of fatty liver disease. Furthermore, G0S2 was found to be a novel target gene of activating transcription factor 4(ATF4). We also localized one conserved ATF4-binding sequence in the 5' regulatory region of G0S2, which was responsible for transcriptional activating G0S2 by ATF4. CONCLUSION: G0S2 is regulated by the PERK-eIF2α-ATF4 branch of the UPR and mediates ER stress-induced hepatic steatosis.

Adipokines demonstrate the interacting influence of central obesity with other cardiometabolic risk factors of metabolic syndrome in Hong Kong Chinese adults.

OBJECTIVE: Metabolic syndrome (MetS) or prediabetes is a complex disorder that is defined by a clustering of cardiometabolic risk factors, including obesity, hypertriglyceridemia, reduced high-density lipoprotein (HDL) cholesterol, hypertension, and insulin resistance. Among cardiometabolic risk factors, central obesity plays a key role in the development of MetS through alterations in the secretion of adipokines and interacts with other MetS risk factors to unfavorably influence overall cardiometabolic risk. Obesity has grasped epidemic proportions in Asia, which has the highest number of people with diabetes in the world. But, the importance of central obesity in the clustering of all four MetS risk factors or vice versa in predicting severity of MetS has not yet been investigated in Asian population. Therefore, the present study examined the influence of central obesity on circulating levels of adipokines through its interaction with the clustering of cardiometabolic risk factors of MetS including hyperglycemia, hypertriglyceridemia, dyslipidemia and hypertension in Hong Kong Chinese adults. SUBJECTS: Blood samples from 83 Hong Kong Chinese adults, who were previously screened for MetS according to the guideline of the United States National Cholesterol Education Program Expert Panel Adult Treatment Panel III criteria were selected. Insulin and adipokines, including visfatin, chemerin, plasminogen activator inhibitor-1 (PAI-1), resistin, C-C motif chemokine ligand 2 (CCL-2), interleukin-6 (IL-6), interleukin-8 (IL-8), interleukin-10 (IL-10), tumour necrosis factor-α (TNF-α), leptin and adiponectin were assessed. RESULTS: The interacting effect of central obesity with all of the other four MetS risk factors increased the proinflammatory status of adipokines (TNF-α, leptin) and decreased the anti-inflammatory status of adipokine (adiponectin). CONCLUSION: Our results indicate that the inflammatory status of MetS may be more severe in the presence of central obesity. Adipokines, as biomarkers for pathophysiological changes, may help to improve early patient identification and to predict MetS-associated morbidity and mortality.

The Dysfunctional MDM2-p53 Axis in Adipocytes Contributes to Aging-Related Metabolic Complications by Induction of Lipodystrophy.

Profound loss and senescence of adipose tissues are hallmarks of advanced age, but the underlying cause and their metabolic consequences remain obscure. Proper function of the murine double minute 2 (MDM2)-p53 axis is known to prevent tumorigenesis and several metabolic diseases, yet its role in regulation of adipose tissue aging is still poorly understood. In this study, we show that the proximal p53 inhibitor MDM2 is markedly downregulated in subcutaneous white and brown adipose tissues of mice during aging. Genetic disruption of MDM2 in adipocytes triggers canonical p53-mediated apoptotic and senescent programs, leading to age-dependent lipodystrophy and its associated metabolic disorders, including type 2 diabetes, nonalcoholic fatty liver disease, hyperlipidemia, and energy imbalance. Surprisingly, this lipodystrophy mouse model also displays premature loss of physiological integrity, including impaired exercise capacity, multiple organ senescence, and shorter life span. Transplantation of subcutaneous fat rejuvenates the metabolic health of this aging-like lipodystrophy mouse model. Furthermore, senescence-associated secretory factors from MDM2-null adipocytes impede adipocyte progenitor differentiation via a non-cell-autonomous manner. Our findings suggest that tight regulation of the MDM2-p53 axis in adipocytes is required for adipose tissue dynamics and metabolic health during the aging process.

Adipokine Profiling in Adult Women With Central Obesity and Hypertension.

Central obesity and hypertension are common risk factors for the metabolic syndrome, cardiovascular and renal diseases. Studies have shown that it is more difficult to control blood pressure and prevent end-organ damage in obese individuals with hypertension compared to their non-obese counterparts, especially among women. Obese females have a 6 times higher risk of developing hypertension than non-obese females while obese males are at a 1.5 times higher risk of developing hypertension, compared to their non-obese counterparts. Indeed, the inter-relationship between obesity and hypertension is unclear. Adipokines have been proposed to play a mediating role in the relationship between obesity and hypertension and are involved in the pathogenesis of metabolic diseases. Therefore, this study sought to determine the role of adipokines (adiponectin, plasminogen activator inhibitor-1, leptin, and tumor necrosis factor-α) in hypertensive Hong Kong Chinese women with central obesity. A total of 387 women aged 58 ± 11 years who were examined with a 2 × 2 factorial design for central obesity (waist circumference ≥ 80 cm) and hypertension (blood pressure ≥ 140/90 mmHg), were recruited from a pool of 1,492 Hong Kong Chinese adults who were previously screened for metabolic syndrome. Subjects with hyperglycemia, hypertriglyceridemia, and dyslipidemia were excluded to eliminate confounding effects. Our findings revealed that hypertensive women with central obesity had a lower anti-inflammatory status (adiponectin) and a higher pro-inflammatory status (TNF-α) than obese alone or hypertensive alone women. Also, women with central obesity had higher circulatory PAI-1 and leptin concentrations than their non-obese counterparts. We conclude that obesity may shift toward a more pro-inflammatory state and may become more severe in the presence of hypertension or vice versa.

Yoga training modulates adipokines in adults with high-normal blood pressure and metabolic syndrome.

Metabolic syndrome (MetS) is associated with diabetes mellitus and cardiovascular diseases. Our previous study indicated that people with MetS showed a decrease in waist circumference and a decreasing trend in blood pressure after 1-year yoga. This study investigated the effect of yoga on MetS people with high-normal blood pressure by exploring modulations in proinflammatory adipokines (leptin, chemerin, visfatin, and plasminogen activator inhibitor-1 or PAI-1) and an anti-inflammatory adipokine (adiponectin). A total of 97 Hong Kong Chinese individuals aged 57.6 ± 9.1 years with MetS and high-normal blood pressure were randomly assigned to control (n = 45) and yoga groups (n = 52). Participants in the control group were not given any intervention but were contacted monthly to monitor their health status. Participants in the yoga group underwent a yoga training program with three 1-hour yoga sessions weekly for 1 year. The participants' sera were harvested and assessed for adipokines. Generalized estimating equation (GEE) was used to examine the interaction effect between 1-year time (pre vs post), and intervention (control vs yoga). GEE analyses revealed significant interaction effects between 1-year time and yoga intervention for the decreases in leptin and chemerin and the increase in adiponectin concentration in the sera examined. These results demonstrated that 1-year yoga training decreased proinflammatory adipokines and increased anti-inflammatory adipokine in adults with MetS and high-normal blood pressure. These findings support the beneficial role of yoga in managing MetS by favorably modulating adipokines.

Adipocyte Fatty Acid Binding Protein Potentiates Toxic Lipids-Induced Endoplasmic Reticulum Stress in Macrophages via Inhibition of Janus Kinase 2-dependent Autophagy.

Lipotoxicity is implicated in the pathogenesis of obesity-related inflammatory complications by promoting macrophage infiltration and activation. Endoplasmic reticulum (ER) stress and adipocyte fatty acid binding protein (A-FABP) play key roles in obesity and mediate inflammatory activity through similar signaling pathways. However, little is known about their interplay in lipid-induced inflammatory responses. Here, we showed that prolonged treatment of palmitic acid (PA) increased ER stress and expression of A-FABP, which was accompanied by reduced autophagic flux in macrophages. Over-expression of A-FABP impaired PA-induced autophagy associating with enhanced ER stress and pro-inflammatory cytokine production, while genetic ablation or pharmacological inhibition of A-FABP reversed the conditions. PA-induced expression of autophagy-related protein (Atg)7 was attenuated in A-FABP over-expressed macrophages, but was elevated in A-FABP-deficient macrophages. Mechanistically, A-FABP potentiated the effects of PA by inhibition of Janus Kinase (JAK)2 activity, thus diminished PA-induced Atg7 expression contributing to impaired autophagy and further augmentation of ER stress. These findings suggest that A-FABP acts as autophagy inhibitor to instigate toxic lipids-induced ER stress through inhibition of JAK2-dependent autophagy, which in turn triggers inflammatory responses in macrophages. A-FABP-JAK2 axis may represent an important pathological pathway contributing to obesity-related inflammatory diseases.

APPL1 prevents pancreatic beta cell death and inflammation by dampening NFκB activation in a mouse model of type 1 diabetes.

AIMS/HYPOTHESIS: Beta cell inflammation and demise is a feature of type 1 diabetes. The insulin-sensitising molecule 'adaptor protein, phosphotyrosine interacting with PH domain and leucine zipper 1' (APPL1), which contains an NH2-terminal Bin/Amphiphysin/Rvs domain, a central pleckstrin homology domain and a COOH-terminal phosphotyrosine-binding domain, has been shown to modulate inflammatory response in various cell types but its role in regulating beta cell mass and inflammation in type 1 diabetes remains unknown. Thus, we investigated whether APPL1 prevents beta cell apoptosis and inflammation in diabetes. METHODS: Appl1-knockout mice and their wild-type littermates, as well as C57BL/6N mice injected with adeno-associated virus encoding APPL1 or green fluorescent protein, were treated with multiple-low-dose streptozotocin (MLDS) to induce experimental type 1 diabetes. Their glucose metabolism and beta cell function were assessed. The effect of APPL1 deficiency on beta cell function upon exposure to a diabetogenic cytokine cocktail (CKS; consisting of TNF-α, IL-1ß and IFN-γ) was assessed ex vivo. RESULTS: Expression of APPL1 was significantly reduced in pancreatic islets from mouse models of type 1 diabetes or islets treated with CKS. Hyperglycaemia, beta cell loss and insulitis induced by MLDS were exacerbated by genetic deletion of Appl1 but were alleviated by beta cell-specific overexpression of APPL1. APPL1 preserved beta cell mass by reducing beta cell apoptosis upon treatment with MLDS. Mechanistically, APPL1 deficiency potentiate CKS-induced phosphorylation of NFκB inhibitor, α (IκBα) and subsequent phosphorylation and transcriptional activation of p65, leading to a dramatic induction of NFκB-regulated apoptotic and proinflammatory programs in beta cells. Pharmacological inhibition of NFκB or inducible NO synthase (iNOS) largely abrogate the detrimental effects of APPL1 deficiency on beta cell functions. CONCLUSIONS/INTERPRETATION: APPL1 negatively regulates inflammation and apoptosis in pancreatic beta cells by dampening the NFκB-iNOS-NO axis, representing a promising target for treating type 1 diabetes.

Adipose-specific inactivation of JNK alleviates atherosclerosis in apoE-deficient mice.

Both atherosclerosis and obesity, an independent atherosclerotic risk factor, are associated with enhanced systemic inflammation. Obesity is also characterized by increased adipose tissue inflammation. However, the molecular mechanism underlying the accelerated atherosclerosis in obesity remains unclear. In obesity, activation of c-Jun N-terminal kinase (JNK) contributes to adipose tissue inflammation. The present study investigated whether the suppression of fat inflammation through adipose-specific JNK inactivation could protect against atherosclerosis in mice. ApoE-/- mice were cross-bred with transgenic mice with adipose-specific expression of a dominant negative form of JNK (dnJNK) to generate apoE-/-/dnJNK (ADJ) mice. ADJ mice treated with a high-fat-high-cholesterol diet exhibited significant attenuations of visceral fat and systemic inflammation without changes in lipid or glucose metabolism, and were protected against atherosclerosis, when compared with apoE-/- mice. Lean apoE-/- mice that received transplantation of visceral fat from obese wild-type donor mice for 4 weeks showed exacerbated systemic inflammation and atherosclerotic plaque formation. Conversely, apoE-/- recipients carrying a visceral fat graft from obese dnJNK donors were protected against enhanced systemic inflammation and atherogenesis. The beneficial effects of adipose-specific JNK inactivation on atherogenesis in apoE-/- recipients were significantly compromised by continuous infusion of recombinant adipocyte-fatty acid-binding protein (A-FABP), previously shown to interact with JNK via a positive feedback loop to modulate inflammatory responses. Together these data suggested that enhanced atherosclerosis in obesity can be attributed, at least in part, to a distant cross-talk between visceral fat and the vasculature, mediated by the release of proinflammatory cytokines, such as A-FABP, from the inflamed visceral adipose tissue with JNK activation.

The MDM2-p53-pyruvate carboxylase signalling axis couples mitochondrial metabolism to glucose-stimulated insulin secretion in pancreatic ß-cells.

Mitochondrial metabolism is pivotal for glucose-stimulated insulin secretion (GSIS) in pancreatic ß-cells. However, little is known about the molecular machinery that controls the homeostasis of intermediary metabolites in mitochondria. Here we show that the activation of p53 in ß-cells, by genetic deletion or pharmacological inhibition of its negative regulator MDM2, impairs GSIS, leading to glucose intolerance in mice. Mechanistically, p53 activation represses the expression of the mitochondrial enzyme pyruvate carboxylase (PC), resulting in diminished production of the TCA cycle intermediates oxaloacetate and NADPH, and impaired oxygen consumption. The defective GSIS and mitochondrial metabolism in MDM2-null islets can be rescued by restoring PC expression. Under diabetogenic conditions, MDM2 and p53 are upregulated, whereas PC is reduced in mouse ß-cells. Pharmacological inhibition of p53 alleviates defective GSIS in diabetic islets by restoring PC expression. Thus, the MDM2-p53-PC signalling axis links mitochondrial metabolism to insulin secretion and glucose homeostasis, and could represent a therapeutic target in diabetes.

The adaptor protein APPL2 inhibits insulin-stimulated glucose uptake by interacting with TBC1D1 in skeletal muscle.

Insulin stimulates glucose uptake by promoting the trafficking of GLUT4 to the plasma membrane in muscle cells, and impairment of this insulin action contributes to hyperglycemia in type 2 diabetes. The adaptor protein APPL1 potentiates insulin-stimulated Akt activation and downstream actions. However, the physiological functions of APPL2, a close homolog of APPL1, in regulating glucose metabolism remain elusive. We show that insulin-evoked plasma membrane recruitment of GLUT4 and glucose uptake are impaired by APPL2 overexpression but enhanced by APPL2 knockdown. Likewise, conditional deletion of APPL2 in skeletal muscles enhances insulin sensitivity, leading to an improvement in glucose tolerance. We identified the Rab-GTPase-activating protein TBC1D1 as an interacting partner of APPL2. Insulin stimulates TBC1D1 phosphorylation on serine 235, leading to enhanced interaction with the BAR domain of APPL2, which in turn suppresses insulin-evoked TBC1D1 phosphorylation on threonine 596 in cultured myotubes and skeletal muscle. Substitution of serine 235 with alanine diminishes APPL2-mediated inhibition on insulin-dependent TBC1D1 phosphorylation on threonine 596 and the suppressive effects of TBC1D1 on insulin-induced glucose uptake and GLUT4 translocation to the plasma membrane in cultured myotubes. Therefore, the APPL2-TBC1D1 interaction is a key step to fine tune insulin-stimulated glucose uptake by regulating the membrane recruitment of GLUT4 in skeletal muscle.