The discovery and development of the radioimmunoassay (RIA) for insulin by Berson and Yalow fundamentally changed biomedical science. The story of this accomplishment began with the pairing of brilliant scientists with complementary expertise who identified a key gap in knowledge they were able to bridge through a series of insightful experiments. Through a succession of important publications over 5 years of work, Berson and Yalow refined the approach to a novel method to measure insulin and demonstrated the power of this method in convincing clinical studies. This culminated in 1960, with three independent papers introducing the insulin RIA and demonstrating the utility in measuring circulating insulin in healthy and diseased states. Two of these papers were published in Diabetes-classics that are revisited here.

The unfolded protein response (UPR) drives events that promote diabetic retinopathy, including vascular endothelial growth factor (VEGF) upregulation in Müller cells. How UPR is activated in vivo in the diabetic retina is not well understood. CD40 is required for development of diabetic retinopathy, but whether CD40 mediates activation of UPR sensors is unknown. CD40 ligation in Müller cells caused phospholipase Cγ1 (PLCγ1)-dependent activation of UPR sensors (PERK, IRE1α, and ATF6α) and VEGF production dependent on PLCγ1 and UPR sensors. Diabetic Cd40-/- mice did not exhibit UPR activation or VEGF upregulation in the retina. These responses were restored in diabetic Cd40-/- mice rescued to express wild-type CD40 in Müller cells but not in mice rescued to express a CD40 mutation unable to recruit TRAF2/3. Intravitreal administration of a cell-permeable CD40-TRAF2/3-disrupting peptide reduced UPR activation, VEGF upregulation, and vascular leakage in diabetic mice. CD40 and TRAF2 in Müller cells from patients with diabetic retinopathy colocalized with activated UPR sensors and VEGF. Our study indicates that CD40 (via TRAF2/3 signaling) is an inducer of UPR activation that triggers VEGF production in Müller cells. This work uncovered inhibition of CD40-TRAF2/3 signaling as a potential approach to impair UPR activation, VEGF upregulation, and vascular leakage in diabetic retinopathy.

Diabetic neuropathic pain is associated with elevated plasma levels of methylglyoxal (MGO). MGO is a metabolite of glycolysis that causes pain hypersensitivity in mice by stimulating the phosphorylation of eukaryotic initiation factor 2α (p-eIF2α) and subsequently activating the integrated stress response (ISR). We first established that Zucker diabetic fatty rats have enhanced MGO signaling, engage ISR, and develop pain hypersensitivity. Since nuclear factor erythroid 2-related factor 2 (Nrf2) regulates the expression of antioxidant proteins that neutralize MGO, we hypothesized that fumarates, like diroximel fumarate (DRF), will stimulate Nrf2 signaling, and prevent MGO-induced ISR and pain hypersensitivity. DRF (100 mg/kg) treated animals were protected from developing MGO (20 ng) induced mechanical and cold hypersensitivity. Mechanistically, DRF treatment protected against MGO-induced increase in p-eIF2α levels in the sciatic nerve and reduced loss of intraepidermal nerve fiber density. Using Nrf2 knockout mice, we demonstrate that Nrf2 is necessary for the antinociceptive effects of DRF. Cotreatment of MGO (1 µmol/L) with monomethyl fumarate (10, 20, and 50 µmol/L), the active metabolite of DRF, prevented ISR in both mouse and human dorsal root ganglia neurons. Our data show that targeting Nrf2 with DRF is a strategy to potentially alleviate pain associated with elevated MGO levels.

Microbial signals trigger the release of neutrophil extracellular traps (NETs) through peptidyl arginine deiminase 4 (PADI4). In turn, NETosis can propagate inflammation to distant tissues. We hypothesize that PADI4 mediates the interactions between diet-modified microbiota and host metabolism. We report that in the adipose tissue of individuals with obesity, NETosis was associated with dysglycemia. In mice, high-fat diet (HFD) induced not only dysmetabolism and metainflammation but also local and systemic signs of NETosis. Deleting Padi4 in hematopoietic cells (Padi4KO) blunted liver and adipose inflammation and improved metabolism under HFD. While NETs were able to disrupt gut epithelial integrity, abrogating NETosis preserved intestinal barrier function and mitigated metabolic endotoxemia due to HFD. Padi4 deletion did not prevent diet-induced dysbiosis, but Padi4KO mice were protected from intestinal hyperpermeability and metabolic impairment due to the transfer of HFD-modified microbiota. As Padi4KO did not blunt the dysmetabolic effects of lipopolysaccharide, we concluded that NETosis operates at the microbiota-intestinal interface, inducing hyperpermeability and the systemic spillover of bacterial-derived products, paving the way to the metabolic consequences of HFD. Finally, pharmacologic PADI4 inhibition recapitulated findings obtained in Padi4KO mice on metabolism and liver steatosis, thereby uncovering a druggable role for PADI4 in mediating the metabolic effects of unhealthy microbiota.

Anti-vascular endothelial growth factor (anti-VEGF) therapies are effective treatment for severe diabetic retinopathy (DR) and macular edema, but a significant subset of people had inadequate response to anti-VEGF intervention. Because elevation or overexpression of retinol binding protein 3 (RBP3) decreases risks for retinal pathologies and progression to severe DR, we compared the therapeutic profiles of RBP3 and anti-VEGF antibody to normalize retinal dysfunctions induced by diabetes. Intravitreous injection of recombinant human RBP3 (rhRBP3) and anti-VEGF antibody (namely, bevacizumab) inhibited retinal vascular permeability in Lewis rats induced by VEGF-A or after 2 months of diabetes induced by streptozotocin, in parallel with reductions of retinal VEGF and VEGF receptor 2 expressions and tyrosine phosphorylation of VEGF receptor. Only rhRBP3 ameliorated diabetes-induced reduction of neural retinal function, measured by electroretinogram. Furthermore, rhRBP3 reduced retinal expressions of inflammatory cytokines (TNF-α and IL-6) in retinal pigmented epithelial and Müller cells exposed to hyperglycemia. Metabolic studies, using a Seahorse flux analyzer, showed only rhRBP3 normalized retinal glycolytic rates in diabetic rats. Thus, both intravitreous anti-VEGF antibody and RBP3 injections normalized retinal vascular dysfunctions caused by diabetes. Only RBP3 targeted both neural and vascular retina to reduce glycolytic rates, reverse neural-retinal dysfunctions, and reduce inflammatory cytokines induced by diabetes, to delay early changes of DR.

Finding no head-to-head research evaluating the cardiovascular and renal benefits of sodium-glucose cotransporter 2 inhibitors (SGLT-2i) and glucagon-like peptide 1 receptor agonists (GLP-1RA) in patients with type 2 diabetes (T2D) at different baseline renal function, we performed a network meta-analysis to compare the two drugs indirectly. Systematic literature searches were conducted of the PubMed, Cochrane Library, Web of Science, and Embase databases, covering their inception until 7 January 2025. Randomized controlled trials (RCTs) comparing the effects of SGLT-2i and GLP-1RA in T2D with different glomerular filtration rates (eGFRs) were selected. Results were reported as risk ratios (RRs) with corresponding 95% CIs. Finally, 10 RCTs involving 87,334 patients with T2D were included. In patients with an eGFR >90 mL/min/1.73 m2, GLP-1RA exhibited a superior ability to reduce the risk of all-cause death compared with SGLT-2i (RR 0.75; 95% CI 0.58, 0.97), but it was less effective in reducing the risk of renal outcome (RR 1.80; 95% CI 1.15, 2.84) in patients with an eGFR 60-90 mL/min/1.73 m2. Conversely, in patients with eGFR 30-60 and 60-90 mL/min/1.73 m2, GLP-1RA did not show an advantage in reducing the risk of hospitalization for heart failure (RR 1.87 [95% CI 1.15, 3.04] and 1.37 [95% CI 1.05, 1.78], respectively).

Sodium-glucose transporter-2 inhibitor (SGLT2i) drugs are widely used for lowering blood glucose levels independent of insulin. Beyond this, these drugs induce various metabolic changes, including weight loss and impaired bone integrity. A significant gap exists in understanding SGLT2i-induced skeletal changes, as SGLT2 is not expressed in osteoblasts or osteocytes, which use glucose to remodel the bone matrix. We studied the impact of 1, 3, or 6 months of canagliflozin (CANA), an SGLT2i treatment, on the skeleton of 6-month-old genetically heterogeneous UM-HET3 mice. Significant metabolic adaptations to CANA were evident as early as 1.5 months after treatment, specifically in male mice. CANA-treated male mice exhibited notable reductions in body weight and decreased proinflammatory and bone remodeling markers associated with reduced cortical bone remodeling indices. Bone tissue metabolome indicated enrichment in metabolites related to amino acid transport and tryptophan catabolism in CANA-treated male mice. In contrast, CANA-treated female mice showed increases in nucleic acid metabolism. An integrOmics approach of source-matched bone tissue metabolome and bone marrow RNA sequencing indicated a positive correlation between the two omics data sets in male mice. Three clusters of transcripts and metabolites involved in energy metabolism, oxidative stress response, and cellular proliferation and differentiation were reduced in CANA-treated male mice. In conclusion, CANA affects bone metabolism mainly via the "glucose restriction state" it induces and impacts bone cell proliferation and differentiation. These findings underline the effects of SGLT2i on bone health and highlight the need to consider sex-specific responses when developing clinical treatments that alter substrate availability.

Insulin regulates glucose uptake and metabolism in muscle via the insulin receptor. Here, we show that Lrtm1 (leucine-rich repeat and transmembrane domain 1), a protein of unknown function enriched in insulin-responsive metabolic tissues, senses changes in insulin signaling in muscle and serves as a regulator of metabolic response. Thus, whole-body Lrtm1-deficient mice exhibit a reduced percentage of fat mass, an increased percentage of lean mass, and an enhanced glucose tolerance and insulin sensitivity compared with control mice under both chow and high-fat diet conditions. Lrtm1 whole-body deficiency also affects dopamine signaling in the brain, leading to hyperactivity. The improvements in glucose and insulin tolerance, but not behavioral or body composition changes, are also observed in skeletal muscle-specific Lrtm1 knockout mice. These effects occur with no change in classical insulin receptor-Akt signaling. Thus, Lrtm1 senses changes in insulin receptor signaling and serves as a novel postreceptor regulator of metabolic and behavioral activity.

Glucolipotoxicity, caused by combined hyperglycemia and hyperlipidemia, results in β-cell failure and type 2 diabetes via cellular stress-related mechanisms. Activating transcription factor 4 (Atf4) is an essential effector of stress response. We show here that Atf4 expression in β-cells is minimally required for glucose homeostasis in juvenile and adolescent mice but it is needed for β-cell function during aging and under obesity-related metabolic stress. Henceforth, Atf4-deficient β-cells older than 2 months after birth display compromised secretory function under acute hyperglycemia. In contrast, they are resistant to acute free fatty acid-induced dysfunction and reduced production of several factors essential for β-cell identity. Atf4-deficient β-cells downregulate genes involved in protein translation. They also upregulate several lipid metabolism or signaling genes, likely contributing to their resistance to free fatty acid-induced dysfunction. These results suggest that Atf4 activation is required for β-cell identity and function under high glucose. But Atf4 activation paradoxically induces β-cell failure in high levels of free fatty acids. Different transcriptional targets of Atf4 could be manipulated to protect β-cells from metabolic stress-induced failure.

Insulin resistance, a hallmark of type 2 diabetes, accelerates muscle breakdown and impairs energy metabolism. However, the role of ubiquitin specific peptidase 2 (USP2), a key regulator of insulin resistance, in sarcopenia remains unclear. Peroxisome proliferator-activated receptor γ (PPAR-γ) plays a critical role in regulating muscle atrophy. The role of deubiquitinase USP2 in mitigating muscle atrophy was investigated. Our findings revealed reduced USP2 expression in skeletal muscles of patients with type 2 diabetes. In mouse models of diabetes- and dexamethasone (DEX)-induced muscle atrophy, USP2 expression was downregulated in skeletal muscles. Usp2 knockout exacerbated muscle loss and functional impairment induced by diabetes or DEX. Moreover, skeletal muscle-specific Usp2 knockout further aggravated muscle loss and functional impairment induced by diabetes. Local injection of adeno-associated virus-Usp2 into the gastrocnemius muscles of diabetic mice increased muscle mass and improved skeletal muscle performance and endurance. It enhanced insulin sensitivity in diabetic mice, shown by lower fasting serum glucose and insulin levels and better glucose tolerance. Mechanistic analysis showed USP2 directly interacted with PPAR-γ by deubiquitinating it, stabilizing its protein levels, enhancing insulin signaling and sensitivity, and maintaining muscle mass. Loss of PPAR-γ abolishes the regulatory effects of USP2 on insulin sensitivity and muscle atrophy. MYOD1 activates USP2 transcription by binding to its promoter region. This study demonstrates the protective role of USP2 in mitigating muscle atrophy by stabilizing PPAR-γ through deubiquitination, particularly in models of diabetic and DEX-induced muscle atrophy. Targeting the USP2-PPAR-γ axis may offer promising therapeutic strategies for metabolic disorders and sarcopenia.

Increasing evidence links coronavirus disease 2019 (COVID-19) infection with heightened type 2 diabetes (T2D) risk; however, the mechanisms underlying this relationship remain poorly understood. We aimed to identify mediating proteins linking COVID-19 infection with T2D, elucidating how COVID-19 might heighten T2D risk. Protein CD209 and central obesity potentially play a crucial role between COVID-19 susceptibility and T2D. Our results highlight CD209 as a potential intervention target for T2D prevention following COVID-19 infection.

An understanding of cell interactions is needed to identify therapeutic targets for diabetic cutaneous ulcers. We explored extracellular vesicles after treatment with advanced glycation end products (AGEs-EVs) derived from macrophages that can suppress diabetic cutaneous wound healing. We found that a novel miRNA enriched in AGEs-EVs (miR-ERIA) suppresses the migration and tube formation of vascular endothelial cells by targeting helicase with zinc finger 2. miR-ERIA offers a potential therapeutic target for diabetic cutaneous ulcers.

Homeobox C4 (HOXC4) links metabolic pathways and correlates inversely with mouse body weight and positively with Ucp1 expression in mouse adipose tissue. Gain- and loss-of-function experiments in mice demonstrated HOXC4's essential role in promoting adipose thermogenesis and providing metabolic benefits. HOXC4 interacts with the nuclear receptor coactivator 1 cofactor via its hexapeptide motif to activate Ucp1 transcription, revealing a novel mechanism of thermogenic gene regulation.