The prevalence of type 2 diabetes (T2D) poses a significant health challenge, yet the contribution of air pollutants to T2D epidemics remains under-studied. Several studies demonstrated a correlation between exposure to volatile organic compounds (VOCs) in indoor/outdoor environments and T2D. Here, we conducted the first meta-analysis, establishing a robust association between exposure to benzene, a prevalent airborne VOC, and insulin resistance in humans across all ages. We used a controlled benzene exposure system, continuous glucose monitoring approach, and indirect calorimetry in mice, to investigate the underlying mechanisms. Following exposure, disruptions in energy homeostasis, accompanied by modifications in the hypothalamic transcriptome and alterations in insulin and immune signaling, were observed exclusively in males, leading to a surge in blood glucose levels. In agreement, RNA sequencing of microglia revealed increased expression of genes associated with immune response and NF-κB signaling. Selective ablation of IKKβ in immune cells (Cx3cr1GFPΔIKK) or exclusively in microglia (Tmem119ERΔIKK) in adult mice alleviated benzene-induced gliosis, restored energy homeostasis and hypothalamic gene expression, and protected against hyperglycemia. We conclude that the microglial NF-κB pathway plays a critical role in chemical-induced metabolic disturbances, revealing a vital pathophysiological mechanism linking exposure to airborne toxicants and the onset of metabolic diseases.

Diabetic nephropathy (DN) is the leading cause of end-stage renal disease, and effective treatment modalities that fully address its molecular etiology are lacking. Prior studies support that the stress response protein REDD1 (regulated in development and DNA damage 1) contributes to the development of diabetes complications. This study investigated a potential role for REDD1 expression in podocytes in diabetes-induced podocyte loss and compromised glomerular filtration. Podocyte-specific REDD1 deletion protected against renal injury, as evidenced by reduced albuminuria, glomerular hypertrophy, and mesangial matrix deposition in streptozotocin (STZ)-induced diabetic mice. Podocyte-specific REDD1 expression was required for diabetes-induced reduction in slit diaphragm (SD) proteins podocin and nephrin. Notably, podocyte-specific REDD1 deletion protected against podocytopenia and preserved glomerular basement membrane and foot process architecture in diabetic mice. In the kidneys of diabetic mice and in human podocyte cultures exposed to hyperglycemic conditions, REDD1 was necessary for increased expression of the transient receptor potential canonical 6 (TRPC6) channel. More specifically, REDD1 promoted nuclear factor-κB-dependent transcription of TRPC6, intracellular calcium entry, and cytoskeletal remodeling under hyperglycemic conditions. Overall, the findings provide new insight into the role of podocyte-specific REDD1 expression in renal pathology and support the possibility that therapeutics targeting REDD1 in podocytes could be beneficial for DN.

Hypothalamic innate immune responses to dietary fats underpin the pathogenesis of obesity, in which microglia play a critical role. Progranulin (PGRN) is an evolutionarily conserved secretory protein containing seven and a half granulin (GRN) motifs. It is cleaved into GRNs by multiple proteases. In the central nervous system, PGRN is highly expressed in microglia. To investigate the role of microglia-derived PGRN in metabolism regulation, we established a mouse model with a microglia-specific deletion of the Grn gene, which encodes PGRN. Mice with microglia-specific Grn depletion displayed diet-dependent metabolic phenotypes. Under normal diet-fed conditions, microglial Grn depletion produced adverse outcomes, such as fasting hyperglycemia and aberrant activation of hypothalamic microglia. However, when fed a high-fat diet (HFD), these mice exhibited beneficial effects, including less obesity, glucose dysregulation, and hypothalamic inflammation. These differing phenotypes appeared to be linked to increased extracellular cleavage of anti-inflammatory PGRN into proinflammatory GRNs in the hypothalamus during overnutrition. In support of this, inhibiting PGRN cleavage attenuated HFD-induced hypothalamic inflammation and obesity progression. Our results suggest that the extracellular cleavage of microglia-derived PGRN plays a significant role in promoting hypothalamic inflammation and obesity during periods of overnutrition. Therefore, therapies that inhibit PGRN cleavage may be beneficial for combating diet-induced obesity.

Type 1 diabetes (T1D) is a consequence of autoimmune destruction of β-cells, and macrophages (MΦs) have a central role in initiating processes that lead to β-cell demise. We reported that Ca2+-independent phospholipase A2β (iPLA2β)-derived lipid (iDL) signaling contributes to β-cell death. Because MΦs express iPLA2β, we assessed its role in T1D development. We find that selective reduction of myeloid-iPLA2β in spontaneously diabetes-prone NOD mice 1) decreases proinflammatory eicosanoid production by MΦs, 2) favors the anti-inflammatory (M2-like) MΦ phenotype, and 3) diminishes activated CD4+ and CD8+ T-cells phenotype in the pancreatic infiltrate, prior to T1D onset. These outcomes are associated with a significant reduction in T1D. Further, inhibition of select proinflammatory lipid signaling pathways reduces M1-like MΦ polarization and adoptive transfer of M2-like MΦs reduces NOD T1D incidence, suggesting a mechanism by which iDLs impact T1D development. These findings identify MΦ-iPLA2β as a critical contributor to T1D development and potential target to counter T1D onset.

Human genetic and transgenic mouse studies have highlighted a potential liver-adipose tissue endocrine axis, involving activin C (Act-C) and/or Act-E and ALK7, influencing fat distribution and systemic metabolism. We investigated the bidirectional effects between circulating INHBC, which homodimerizes into Act-C, and adiposity traits, insulin resistance, inflammation, and cardiometabolic disease risk. Additionally, we examined whether Act-C is an ALK7 ligand in human adipocytes. We used Mendelian randomization and in vitro studies in immortalized human abdominal and gluteal adipocytes. Circulating INHBC was causally linked to reduced lower-body fat, dyslipidemia, and increased risks of coronary artery disease (CAD) and nonalcoholic fatty liver disease (NAFLD). Conversely, upper-body fat distribution, obesity, hypertriglyceridemia, subclinical inflammation, and type 2 diabetes positively impacted plasma INHBC levels. Mechanistically, an atherogenic lipid profile may partly explain the INHBC-CAD link, while inflammation and hypertriglyceridemia may partly explain how adiposity traits affect circulating INHBC. Phenome-wide Mendelian randomization showed weak causal relationships between higher plasma INHBC and impaired kidney function and higher gout risk. In human adipocytes, recombinant Act-C activated SMAD2/3 signaling via ALK7 and suppressed lipolysis. In summary, INHBC influences systemic metabolism by activating ALK7 in adipose tissue and may serve as a drug target for atherogenic dyslipidemia, CAD, and NAFLD.

Our objective was to test a single dose of the phosphatidylinositol 3-kinase (PI3K) inhibitor alpelisib as a tool for acute modeling of insulin resistance in healthy volunteers. This single-center double-blind phase 1 clinical trial randomly assigned healthy adults to a single oral dose of 300 mg alpelisib (n = 5) or placebo (n = 6) at bedtime, followed by measurement of glucose, insulin, and C-peptide levels after an overnight fast and during a 3-h 75-g oral glucose tolerance test (OGTT). Fasting plasma glucose trended higher with alpelisib (mean ± SD 93 ± 11 mg/dL) versus placebo (84 ± 5 mg/dL); mean fasting serum insulin increased nearly fivefold (23 ± 12 vs. 5 ± 3 μU/mL, respectively), and HOMA of insulin resistance (IR) scores were 5.4 ± 3.1 for alpelisib and 1.1 ± 0.6 for placebo. During OGTT, incremental area under the curve (AUC) for insulin was more than fourfold greater with alpelisib (22 ± 15 mU/mL × min) than with placebo (5 ± 2 mU/mL × min); glucose AUC trended higher with alpelisib. Single-dose alpelisib was well tolerated and produced metabolic alterations consistent with acute induction of IR, validating its use for mechanistic study of insulin action in humans.

Muscle sn-1,2-diacylglycerol (DAG) and C18:0 ceramide accumulation in sarcolemmal and mitochondrial compartments have been proposed to regulate muscle insulin sensitivity. Here, we evaluated whether weight loss-induced improvements in insulin sensitivity were associated with changes in muscle sn-1,2-DAG and ceramide content in people with obesity and type 2 diabetes. We measured skeletal muscle insulin sensitivity, assessed by using the hyperinsulinemic-euglycemic clamp procedure in conjunction with stable isotopically labeled glucose tracer infusion, and skeletal muscle sn-1,2-DAG and ceramide contents by using liquid chromatography-tandem mass spectrometry after subcellular fractionation and DAG isomer separation in 14 adults with obesity and type 2 diabetes before and after marked (18.6 ± 2.1%) weight loss. Whole-body insulin sensitivity doubled after weight loss. Sarcolemmal sn-1,2-DAG and C18:0 ceramide contents after weight loss were not different from values before weight loss. In contrast, mitochondrial-endoplasmic reticulum (ER) C18:0 ceramide content decreased by ∼20% after weight loss (from 2.16 ± 0.08 to 1.71 ± 0.13 nmol/g, P < 0.005). These results suggest a decrease in muscle mitochondrial-ER C18:0 ceramide content could contribute to the beneficial effect of weight loss on skeletal muscle insulin sensitivity.

Melanocortin-4 receptor (Mc4r) is a G protein-coupled receptor that controls systemic energy balance by regulating food intake and energy expenditure. Although the detailed molecular mechanism remains unclear, the activation of cAMP signaling in Mc4r-expressing cells reportedly suppresses food intake and increases energy expenditure. CREBP-regulated transcriptional coactivator-1 (CRTC1) is selectively expressed in neuronal cells and participates in transcriptional control, thereby contributing to neuronal plasticity and energy homeostasis. Considering the cAMP-dependent regulation of CRTC1 activity, CRTC1 in Mc4r-expressing cells may contribute to energy balance regulation through the melanocortin pathway. In this context, we examined the physiological contribution of CRTC1 in Mc4r-expressing cells to energy metabolism. In this study, mice with CRTC1 deficiency in Mc4r-expressing cells exhibited 1) modest obesity, glucose intolerance, insulin resistance, hyperinsulinemia, and hyperlipidemia; 2) decreased systemic energy expenditure and thermogenesis; 3) suppression of melanocortin agonist-induced adaptation of energy expenditure and food intake; 4) impaired thermogenic programs and oxidative pathway in brown adipose tissue and skeletal muscle; and 5) enhanced lipogenic programs in the liver and white adipose tissue. These results provide novel insights into the molecular mechanisms underlying the regulation of energy balance by the melanocortin system.

Evaluation of insulin secretory capacity is essential to understand the pathophysiologic condition of individuals with diabetes and assess the efficacy of drugs used in the treatment of this disease. The 1-mg i.v. glucagon stimulation test (GST) is widely used to evaluate residual β-cell function; we previously reported that GST assessment of insulin secretory capacity is useful in assessing the efficacy of glucagon-like peptide 1 (GLP-1) receptor agonists (GLP-1RAs). However, recent reports have indicated that pharmacologic concentrations of glucagon stimulate insulin secretion through GLP-1 receptors, confounding the issue. The current studies were undertaken to reassess the reliability of the GST for evaluation of insulin secretory capacity under GLP-1RAs and dipeptidyl peptidase 4 inhibitors (DPP-4is). Our first study included individuals receiving GLP-1RA treatment, evaluated by the GST before and after treatment. Although the fasting C-peptide response (CPR) levels were elevated after treatment, the induction of insulin secretion by glucagon was significantly reduced. Our second study compared glucagon-induced insulin secretion between DPP-4i users and nonusers, assessed by the GST after propensity score matching. Although the fasting CPR levels were similar in the two investigations, glucagon-induced insulin secretion was significantly lower with DPP-4i use. These results suggest that the GST might underestimate insulin secretory capacity under incretin-based therapy.

Pancreatic β-cell dysfunction caused by obesity can be associated with alterations in the levels of miRNAs. However, the role of miRNAs in such processes remains elusive. Here, we show that pancreatic islet miR-27a-5p, which is markedly increased in obese mice and impairs insulin secretion, is mainly delivered by visceral adipocyte-derived extracellular vesicles (EVs). Depleting miR-27a-5p significantly improved insulin secretion and glucose intolerance in db/db mice. Supporting the function of EV miR-27a-5p as a key pathogenic factor, intravenous injection of miR-27a-5p-containing EVs showed their distribution in mouse pancreatic islets. Tracing the injected adeno-associated virus (AAV)-miR-27a-5p (AAV-miR-27a) or AAV-FABP4-miR-27a-5p (AAV-FABP4-miR-27a) in visceral fat resulted in upregulating miR-27a-5p in EVs and serum and elicited mouse pancreatic β-cell dysfunction. Mechanistically, miR-27a-5p directly targeted L-type Ca2+ channel subtype CaV1.2 (Cacna1c) and reduced insulin secretion in β-cells. Overexpressing mouse CaV1.2 largely abolished the insulin secretion injury induced by miR-27a-5p. These findings reveal a causative role of EV miR-27a-5p in visceral adipocyte-mediated pancreatic β-cell dysfunction in obesity-associated type 2 diabetes mellitus.

Diabetic retinopathy (DR) is one of the most common complications of diabetes worldwide and is associated with visual loss and blindness. However, effective treatments for both early- and late-stage DR remain lacking. A streptozotocin-induced diabetic mouse model and high glucose (HG)-treated Müller cell model were established. M1/M2 microglia polarization was assessed by immunofluorescence staining and flow cytometry. Expression of long noncoding RNA (lncRNA) OGRU, cytokines, and other key molecules was detected by quantitative RT-PCR or Western blot. ELISA was used to monitor cytokine secretion. Müller cell-derived exosomes were isolated and characterized by nanopartical tracking analysis, Western blot, and transmission electron microscopy, and exosome uptake assay was used to monitor the intercellular transport of exosomes. Associations among lncRNA-miRNA-mRNA networks were validated by RNA pulldown and RNA immunoprecipitation and dual luciferase assays. Increased M1 polarization but decreased M2 polarization of retinal microglia was observed in DR mice. HG-treated Müller cell-derived exosomes transported OGRU into microglia and promoted microglia polarization toward the M1 phenotype. Mechanistically, OGRU served as a competing endogenous RNA for miR-320-3p, miR-221-3p, and miR-574-5p to regulate aldose reductase (AR), PFKFB3, and glucose transporter 1 (GLUT1) expression in microglia, respectively. Loss of miR-320-3p/miR-221-3p/miR-574-5p or reinforced AR/PFKFB3/GLUT1 abrogated OGRU silencing-mediated microglia polarization in vitro. In vivo studies further showed that OGRU/miR-320-3p/AR, OGRU/miR-221-3p/PFKFB3, and OGRU/miR-574-5p/GLUT1 axes regulated microglia polarization in DR mice. Collectively, Müller cell-derived exosomal OGRU regulated microglia polarization in DR by modulating OGRU/miR-320-3p/AR, OGRU/miR-221-3p/PFKFB3, and OGRU/miR-574-5p/GLUT1 axes.

Leptin is a homeostatic regulatory element that signals the presence of adipocyte energy stores, reduces food intake, and increases energy expenditure. Similarly, serotonin (5-HT), a signaling molecule found in both the central and peripheral nervous systems, also controls food intake. Using neuronal tract tracing, pharmacologic and optogenetic approaches, and in vivo microdialysis, combined with behavioral end points, we tested the hypothesis that leptin controls food intake not only by activating hypothalamic leptin receptors (LepRs) but also through activation of LepRs expressed by serotonergic raphe neurons that send projections to the arcuate (ARC). We showed that microinjection of leptin directly into the dorsal raphe nucleus (DRN) reduced food intake in rats. This effect was mediated by LepR-expressing neurons in the DRN, because selective optogenetic activation of these neurons at either their DRN cell bodies or their ARC terminals reduced food intake. Anatomically, we identified a unique population of serotonergic raphe neurons expressing LepRs that send projections to the ARC. Finally, by using in vivo microdialysis, we showed that leptin administration to the DRN increased 5-HT efflux into the ARC, and specific antagonism of the 5-HT2C receptors in the ARC diminished the leptin anorectic effect. Overall, this study identified a novel circuit for leptin-mediated control of food intake through a DRN-ARC pathway, identifying a new level of interaction between leptin and serotonin to control food intake. Characterization of this new pathway creates opportunities for understanding how the brain controls eating behavior and opens alternative routes for the treatment of eating disorders.

The approval of teplizumab to delay the onset of type 1 diabetes is an important inflection point in the decades-long pursuit to treat the cause of the disease rather than its symptoms. The National Institute of Diabetes and Digestive and Kidney Diseases convened a workshop of the Diabetes Mellitus Interagency Coordinating Committee titled "Evolving Concepts in Pathophysiology, Screening, and Prevention of Type 1 Diabetes" to review this accomplishment and identify future goals. Speakers representing Type 1 Diabetes TrialNet (TrialNet) and the Immune Tolerance Network emphasized that the ability to robustly identify individuals destined to develop type 1 diabetes was essential for clinical trials. The presenter from the U.S. Food and Drug Administration described how regulatory approval relied on data from the single clinical trial of TrialNet with testing of teplizumab for delay of clinical diagnosis, along with confirmatory evidence from studies in patients after diagnosis. The workshop reviewed the etiology of type 1 diabetes as a disease involving multiple immune pathways, highlighting the current understanding of prognostic markers and proposing potential strategies to improve the therapeutic response of disease-modifying therapies based on the mechanism of action. While celebrating these achievements funded by the congressionally appropriated Special Diabetes Program, panelists from professional organizations, nonprofit advocacy/funding groups, and industry also identified significant hurdles in translating this research into clinical care.

The translocon-associated protein-δ (TRAPδ) plays a role in insulin biosynthesis within pancreatic β-cells. However, its pathophysiological significance in maintaining islet β-cell function and glucose homeostasis remains unclear. In this study, we generated a mouse model featuring pancreatic β-cell-specific deletion of TRAPδ (TRAPδ βKO). Our findings revealed that TRAPδ βKO resulted in decreased circulating insulin levels in mice fed either a normal chow diet or a high-fat diet. Multiple independent experiments established that although TRAPδ deletion reduced insulin content in the islets, it had no discernible effect on insulin gene expression, the insulin to proinsulin ratio, or the expression and glycosylation of the prohormone enzymes involved in proinsulin processing. These data suggest that TRAPδ does not play a pivotal role in the transcription of the insulin gene or proinsulin processing. However, untranslocated preproinsulin levels were significantly increased when islets were treated with a proteasomal inhibitor, suggesting that TRAPδ deficiency may hinder preproinsulin translocation, resulting in a rapid degradation of untranslocated preproinsulin that accounts for the decreased insulin production. Remarkably, despite the moderate decrease in circulating insulin levels in TRAPδ βKO mice, their glucose levels remained unaffected, indicating the presence of compensatory mechanisms that help maintain glucose homeostasis. Insulin tolerance tests further revealed improved insulin sensitivity, accompanied by upregulation of phosphorylated AKT in the peripheral tissues of TRAPδ βKO mice. Collectively, these data highlight the important role of TRAPδ in insulin biosynthesis and β-cell function. The moderate reduction in circulating insulin appears to promote insulin sensitivity in insulin target tissues.

Diabetic peripheral neuropathy (DPN) affects ∼50% of the 500 million people with type 2 diabetes worldwide and is considered disabling and irreversible. The current study was undertaken to assess the effect of metformin on peripheral neuropathy outcomes in type 2 diabetes. Participants with type 2 diabetes (n = 69) receiving metformin were recruited and underwent clinical assessment, peripheral nerve ultrasonography, nerve conduction studies, and axonal excitability studies. Also concurrently screened were 318 participants who were not on metformin, and 69 were selected as disease control subjects and matched to the metformin participants for age, sex, diabetes duration, BMI, HbA1c, and use of other diabetes therapies. Medical record data over the previous 20 years were analyzed for previous metformin use. Mean tibial nerve cross-sectional area was lower in the metformin group (metformin 14.1 ± 0.7 mm2, nonmetformin 16.2 ± 0.9 mm2, P = 0.038), accompanied by reduction in neuropathy symptom severity (P = 0.021). Axonal excitability studies demonstrated superior axonal function in the metformin group, and mathematical modeling demonstrated that these improvements were mediated by changes in nodal Na+and K+conductances. Metformin treatment is associated with superior nerve structure and clinical and neurophysiological measures. Treatment with metformin may be neuroprotective in DPN.

Peripheral neuropathy (PN) is a prevalent and debilitating complication of obesity, prediabetes, and type 2 diabetes, which remains poorly understood and lacks disease-modifying therapies. Fortunately, diet and/or exercise have emerged as effective treatment strategies for PN. Here, we examined the impact of caloric restriction (CR) and high-intensity interval training (HIIT) interventions, alone or combined (HIIT-CR), on metabolic and PN outcomes in high-fat diet (HFD) mice. HFD feeding alone resulted in obesity, impaired glucose tolerance, and PN. Peripheral nerves isolated from these mice also developed insulin resistance (IR). CR and HIIT-CR, but not HIIT alone, improved HFD-induced metabolic dysfunction. However, all interventions improved PN to similar extents. When examining the underlying neuroprotective mechanisms in whole nerves, we found that CR and HIIT-CR activate the fuel-sensing enzyme AMPK. We then performed complimentary in vitro work in Schwann cells, the glia of peripheral nerves. Treating primary Schwann cells with the saturated fatty acid palmitate to mimic prediabetic conditions caused IR, which was reversed by the AMPK activator, AICAR. Together, these results enhance our understanding of PN pathogenesis, the differential mechanisms by which diet and exercise may improve PN, and Schwann cell-specific contributions to nerve insulin signaling and PN progression.

Excessive formation of macrophage extracellular trap (MET) has been implicated in several autoimmune disease pathogeneses; however, its impact on type 1 diabetes (T1D) and related mechanisms remains enigmatic. We demonstrated the pivotal role of peptidyl arginine deiminase 4 (PAD4) in driving profuse MET formation and macrophage M1 polarization in intestinal inflammation in NOD mice. Genetic knockout of PAD4 or adoptive transfer of METs altered the proportion of proinflammatory T cells in the intestine, subsequently influencing their migration to the pancreas. Combining RNA sequencing and CUT&Tag analysis, we found activated PAD4 transcriptionally regulated CXCL10 expression. This study comprehensively investigated how excessive PAD4-mediated MET formation in the colon increases the aggravation of intestinal inflammation and proinflammatory T-cell migration and finally is involved in T1D progression, suggesting that inhibition of MET formation may be a potential therapeutic target in T1D.