Serum from pregnant donors induces human beta cell proliferation

Pancreatic beta cells are among the slowest replicating cells in the human body and have not been observed to increase in number except during the fetal and neonatal period, in cases of obesity, during puberty, as well as during pregnancy. Pregnancy is associated with increased beta cell mass to meet heightened insulin demands. This phenomenon raises the intriguing possibility that factors present in the serum of pregnant individuals may stimulate beta cell proliferation and offer insights into expansion of the beta cell mass for treatment and prevention of diabetes. The primary objective of this study was to test the hypothesis that serum from pregnant donors contains bioactive factors capable of inducing human beta cell proliferation. An immortalized human beta cell line with protracted replication (EndoC-βH1) was cultured in media supplemented with serum from pregnant and non-pregnant female and male donors and assessed for differences in proliferation. This experiment was followed by assessment of proliferation of primary human beta cells. Sera from five out of six pregnant donors induced a significant increase in the proliferation rate of EndoC-βH1 cells. Pooled serum from the cohort of pregnant donors also increased the rate of proliferation in primary human beta cells. This study demonstrates that serum from pregnant donors stimulates human beta cell proliferation. These findings suggest the existence of pregnancy-associated factors that can offer novel avenues for beta cell regeneration and diabetes prevention strategies. Further research is warranted to elucidate the specific factors responsible for this effect.

Tyrosine nitration of glucagon impairs its function: Extending the role of heme in T2D pathogenesis

New studies raise the possibility that the higher glucagon (GCG) level present in type 2 diabetes (T2D) is a compensatory mechanism to enhance β-cell function, rather than induce dysregulated glucose homeostasis, due to an important role for GCG that acts directly within the pancreas on insulin secretion by intra-islet GCG signaling. However, in states of poorly controlled T2D, pancreatic α cell mass increases (overproduced GCG) in response to insufficient insulin secretion, indicating decreased local GCG activity. The reason for this decrease is not clear. Recent evidence has uncovered a new role of heme in cellular signal transduction, and its mechanism involves reversible binding of heme to proteins. Considering that protein tyrosine nitration in diabetic islets increases and glucose-stimulated insulin secretion (GSIS) decreases, we speculated that heme modulates GSIS by transient interaction with GCG and catalyzing its tyrosine nitration, and the tyrosine nitration may impair GCG activity, leading to loss of intra-islet GCG signaling and markedly impaired insulin secretion. Data presented here elucidate a novel role for heme in disrupting local GCG signaling in diabetes. Heme bound to GCG and induced GCG tyrosine nitration. Two tyrosine residues in GCG were both sensitive to the nitrating species. Further, GCG was also demonstrated to be a preferred target peptide for tyrosine nitration by co-incubation with BSA. Tyrosine nitration impaired GCG stimulated cAMP-dependent signaling in islet β cells and decreased insulin release. Our results provided a new role of heme for impaired GSIS in the pathological process of diabetes.

Excess homocysteine inhibits pancreatic β-cell secretory function by repressing Zbtb20 expression

Homocysteine (Hcy) is a sulfur-containing amino acid. An elevated level of Hcy is a risk factor for diabetes development. However, the mechanism of its effect on pancreatic β-cell function is unclear. In this study, we constructed a hyperhomocysteinemia (HHcy) mouse model by feeding mice a high methionine diet (HMD). The mice suffered impaired glucose tolerance and reduced insulin secretion. Furthermore, at the cellular level, INS1 cells exhibited impaired insulin secretory function after the Hcy intervention. Transcriptomics revealed that Zbtb20 expression was downregulated and the downstream gene Fbp1 was upregulated in HHcy-induced mice compared with mice fed with normal diet. Insulin secretion could be restored by Zbtb20 overexpression or fructose 1,6-bisphosphatase (FBPase) activity inhibition in INS1 cells. In conclusion, our study suggested that Hcy inhibited the insulin secretory function of pancreatic β-cells by suppressing Zbtb20 expression, leading to the development of diabetes. Zbtb20 may be a key target in the development of diabetes associated with elevated Hcy levels.

Alleviative effects of the parthenolide derivative ACT001 on insulin resistance induced by sodium propionate combined with a high-fat diet and its potential mechanisms

The increasing side effects of traditional medications used to treat type II diabetes have made research into the development of safer and more effective natural medications necessary. ACT001, a derivative of parthenolide, has been shown to have good anti-inflammatory and antitumor effects; however, its role in diabetes is unclear. The short-chain fatty acid propionate is a common food preservative that has been found to cause disturbances in glucose metabolism in mice and humans. This study aimed to investigate whether sodium propionate could aggravate insulin resistance in obese mice and cause diabetes and to study the alleviative effects and potential mechanisms of action of ACT001 on insulin resistance in diabetic mice. Type II diabetic mice were adminietered sodium propionate combined with a high-fat diet (HFD + propionate) by gavage daily for four weeks. Biochemical analysis showed that ACT001 significantly affected blood glucose concentration in diabetic mice, mainly by downregulating the expression of phosphoenolpyruvate carboxykinase 2 and glucose-6-phosphatase. Meanwhile, the level of fatty acid-binding protein 4 in the liver was significantly decreased. ACT001 has a protective effect on the liver and adipose tissue of mice. In addition, the results of the running wheel experiment indicated that ACT001 alleviated the circadian rhythm disorder caused by insulin resistance to a certain extent. This study revealed the potential mechanism by which ACT001 alleviates insulin resistance and provides ideas for developing natural antidiabetic drugs.

Unraveling the significance of PPP1R1A gene in pancreatic β-cell function: A study in INS-1 cells and human pancreatic islets

BACKGROUND AND AIMS

The protein phosphatase 1 regulatory inhibitor subunit 1A (PPP1R1A) has been linked with insulin secretion and diabetes mellitus. Yet, its full significance in pancreatic β-cell function remains unclear. This study aims to elucidate the role of the PPP1R1A gene in β-cell biology using human pancreatic islets and rat INS-1 (832/13) cells.

RESULTS

Disruption of Ppp1r1a in INS-1 cells was associated with reduced insulin secretion and impaired glucose uptake; however, cell viability, ROS, apoptosis or proliferation were intact. A significant downregulation of crucial β-cell function genes such as Ins1, Ins2, Pcsk1, Cpe, Pdx1, Mafa, Isl1, Glut2, Snap25, Vamp2, Syt5, Cacna1a, Cacna1d and Cacnb3, was observed upon Ppp1r1a disruption. Furthermore, silencing Pdx1 in INS-1 cells altered PPP1R1A expression, indicating that PPP1R1A is a target gene for PDX1. Treatment with rosiglitazone increased Ppp1r1a expression, while metformin and insulin showed no effect. RNA-seq analysis of human islets revealed high PPP1R1A expression, with α-cells showing the highest levels compared to other endocrine cells. Muscle tissues exhibited greater PPP1R1A expression than pancreatic islets, liver, or adipose tissues. Co-expression analysis revealed significant correlations between PPP1R1A and genes associated with insulin biosynthesis, exocytosis machinery, and intracellular calcium transport. Overexpression of PPP1R1A in human islets augmented insulin secretion and upregulated protein expression of Insulin, MAFA, PDX1, and GLUT1, while silencing of PPP1R1A reduced Insulin, MAFA, and GLUT1 protein levels.

CONCLUSION

This study provides valuable insights into the role of PPP1R1A in regulating β-cell function and glucose homeostasis. PPP1R1A presents a promising opportunity for future therapeutic interventions.

Amphibian host-defense peptides with potential for Type 2 diabetes therapy - an updated review

Investigations conducted since 2018 have identified several host-defense peptides present in frog skin secretions whose properties suggest the possibility of their development into a new class of agent for Type 2 diabetes (T2D) therapy. Studies in vitro have described peptides that (a) stimulate insulin release from BRIN-BD11 clonal β-cells and isolated mouse islets, (b) display β-cell proliferative activity and protect against cytokine-mediated apoptosis and (c) stimulate production of the anti-inflammatory cytokine IL-10 and inhibit production of the pro-inflammatory cytokines TNF-α and IL-1β. Rhinophrynin-27, phylloseptin-3.2TR and temporin F are peptides with therapeutic potential. Studies in vivo carried out in db/db and high fat-fed mice have shown that twice-daily administration of [S4K]CPF-AM1 and [A14K]PGLa-AM1, analogs of peptides first isolated from the octoploid frog Xenopus amieti, over 28 days lowers circulating glucose and HbA1c concentrations, increases insulin sensitivity and improves glucose tolerance and lipid profile. Peptide treatment produced potentially beneficial changes in the expression of skeletal muscle genes involved in insulin signaling and islet genes involved in insulin secretion in these murine models of T2D. Lead compounds uncovered by the study of frog HDPs may provide a basis for the design of new types of agents that can be used, alone or in combination with existing therapies, for the treatment of T2D.

Lipid nanoparticles of quercetin (QU-Lip) alleviated pancreatic microenvironment in diabetic male rats: The interplay between oxidative stress - unfolded protein response (UPR) - autophagy, and their regulatory miRNA

BACKGROUND

Autophagy is a well-preserved mechanism essential in minimizing endoplasmic reticulum stress (ER)-related cell death. Defects in β-cell autophagy have been linked to type 1 diabetes, particularly deficits in the secretion of insulin, boosting ER stress sensitivity and possibly promoting pancreatic β-cell death. Quercetin (QU) is a potent antioxidant and anti-diabetic flavonoid with low bioavailability, and the precise mechanism of its anti-diabetic activity is still unknown. Aim This study aimed to design an improved bioavailable form of QU (liposomes) and examine the impact of its treatment on the alleviation of type 1 diabetes induced by STZ in rats.

METHODS

Seventy SD rats were allocated into seven equal groups 10 rats of each: control, STZ, STZ + 3-MA, STZ + QU-Lip, and STZ + 3-MA + QU-Lip. Fasting blood glucose, insulin, c-peptide, serum IL-6, TNF-α, pancreatic oxidative stress, TRAF-6, autophagy, endoplasmic reticulum stress (ER stress) markers expression and their regulatory microRNA (miRNA) were performed. As well as, docking analysis for the quercetin, ER stress, and autophagy were done. Finally, the histopathological and immunohistochemical analysis were conducted.

SIGNIFICANCE

QU-Lip significantly decreased glucose levels, oxidative, and inflammatory markers in the pancreas. It also significantly downregulated the expression of ER stress and upregulated autophagic-related markers. Furthermore, QU-Lip significantly ameliorated the expression of several MicroRNAs, which both control autophagy and ER stress signaling pathways. However, the improvement of STZ-diabetic rats was abolished upon combination with an autophagy inhibitor (3-MA). The findings suggest that QU-Lip has therapeutic promise in treating type 1 diabetes by modulating ER stress and autophagy via an epigenetic mechanism.

Four weeks SGLT2 inhibition improves beta cell function and glucose tolerance without affecting muscle free fatty acid or glucose uptake in subjects with type 2 diabetes

AIMS

Sodium glucose co-transporter-2 (SGLT2) inhibition lowers glucose levels independently of insulin, leading to reduced insulin secretion and increased lipolysis, resulting in elevated circulating free fatty acids (FFAs). While SGLT2 inhibition improves tissue insulin sensitivity, the increase in circulating FFAs could reduce insulin sensitivity in skeletal muscle and the liver. We aimed to investigate the effects of SGLT2 inhibition on substrate utilization in skeletal muscle and the liver and to measure beta-cell function and glucose tolerance.

METHODS

Thirteen metformin-treated individuals with type 2 diabetes were randomized to once-daily empagliflozin 25 mg or placebo for 4 weeks in a crossover design. Skeletal muscle glucose and FFA uptake together with hepatic tissue FFA uptake were measured using [18F]FDG positron emission tomography/computed tomography (PET/CT) and [11C]palmitate PET/CT. Insulin secretion and action were estimated using the oral minimal model.

RESULTS

Empagliflozin did not affect glucose (0.73 ± 0.30 vs. 1.16 ± 0.64, μmol/g/min p = 0.11) or FFA (0.60 ± 0.30 vs. 0.56 ± 0.3, μmol/g/min p = 0.54) uptake in skeletal muscle. FFA uptake in the liver (21.2 ± 10.1 vs. 19 ± 8.8, μmol/100 ml/min p = 0.32) was unaffected. Empagliflozin increased total beta-cell responsivity (20 ± 8 vs. 14 ± 9, 10-9 min-1, p < 0.01) and glucose effectiveness (2.6 × 10-2 ± 0.3 × 10-2 vs. 2.4 × 10-2 ± 0.3 × 10-2, dL/kg/min, p = 0.02).

CONCLUSIONS

Despite improved beta-cell function and glucose tolerance, empagliflozin does not appear to affect skeletal muscle FFA or glucose uptake.

Silencing of forkhead box protein O-1 (FOXO-1) enhances insulin-producing cell generation from adipose mesenchymal stem cells for diabetes therapy

AIMS

Generation of mature β-cells from MSCs has been a challenge in the field of stem cell therapy of diabetes. Adipose tissue-derived mesenchymal stem cells (Ad-MSCs) have made their mark in regenerative medicine, and provide several advantages compared to other MSCs sources. Forkhead box protein O-1 (FOXO-1) is an important transcription factor for normal development of β-cells, yet its over expression in β-cells may cause glucose intolerance. In this study, we isolated, characterized Ad-MSCs from rat epididymal fat pads, differentiated these MSCs into insulin producing cells (IPCs) and studied the role of FOXO-1 in such differentiation.

MATERIALS AND METHODS

We examined the expression of FOXO-1 and its nuclear cytoplasmic localization in the generated IPCs. Afterwards we knocked down FOXO-1 using siRNA targeting FOXO-1 (siFOXO-1). The differentiated siFOXO-1 IPCs were compared to non-targeting siRNA (siNT) IPCs regarding expression of β-cell markers by qRT-PCR and western blotting, dithizone (DTZ) staining and glucose stimulated insulin secretion (GSIS).

KEY FINDINGS

Isolated Ad-MSCs exhibited all characteristics of MSCs and can generate IPCs. FOXO-1 was initially elevated during differentiation followed by a decline towards end of differentiation. FOXO-1 was dephosphorylated and localized to the nucleus upon differentiation into IPCs. Knock down of FOXO-1 improved the expression of β-cell markers in final differentiated IPCs, improved DTZ uptake and showed increased insulin secretion upon challenging with increased glucose concentration.

SIGNIFICANCE

These results portray FOXO-1 as a hindering factor of generation of IPCs whose down-regulation can generate more mature IPCs for MSCs therapy of diabetes mellitus.

Abrus precatorius Leaf Extract Stimulates Insulin-mediated Muscle Glucose Uptake: In vitro Studies and Phytochemical Analysis

Diabetes mellitus, linked with insulin resistance and hyperglycaemia, is a leading cause of mortality. Glucose uptake through glucose transporter type 4, especially in skeletal muscle, is crucial for maintaining euglycaemia and is a key pathway targeted by antidiabetic medication. Abrus precatorius is a medicinal plant with demonstrated antihyperglycaemic activity in animal models, but its mechanisms are unclear.This study evaluated the effect of a 50% ethanolic (v/v) A. precatorius leaf extract on (1) insulin-stimulated glucose uptake and (2) related gene expression in differentiated C2C12 myotubes using rosiglitazone as a positive control, and (3) generated a comprehensive phytochemical profile of A. precatorius leaf extract using liquid chromatography-high resolution mass spectrometry to elucidate its antidiabetic compounds. A. precatorius leaf extract significantly increased insulin-stimulated glucose uptake, and insulin receptor substrate 1 and Akt substrate of 160 kDa gene expression; however, it had no effect on glucose transporter type 4 gene expression. At 250 µg/mL A. precatorius leaf extract, the increase in glucose uptake was significantly higher than 1 µM rosiglitazone. Fifty-five phytochemicals (primarily polyphenols, triterpenoids, saponins, and alkaloids) were putatively identified, including 24 that have not previously been reported from A. precatorius leaves. Abrusin, precatorin I, glycyrrhizin, hemiphloin, isohemiphloin, hispidulin 4'-O-β-D-glucopyranoside, homoplantaginin, and cirsimaritin were putatively identified as known major compounds previously reported from A. precatorius leaf extract. A. precatorius leaves contain antidiabetic phytochemicals and enhance insulin-stimulated glucose uptake in myotubes via the protein kinase B/phosphoinositide 3-kinase pathway by regulating insulin receptor substrate 1 and Akt substrate of 160 kDa gene expression. Therefore, A. precatorius leaves may improve skeletal muscle insulin sensitivity and hyperglycaemia. Additionally, it is a valuable source of bioactive phytochemicals with potential therapeutic use for diabetes.

The prostaglandin E2 EP3 receptor has disparate effects on islet insulin secretion and content in β-cells in a high-fat diet-induced mouse model of obesity

Signaling through prostaglandin E2 EP3 receptor (EP3) actively contributes to the β-cell dysfunction of type 2 diabetes (T2D). In T2D models, full-body EP3 knockout mice have a significantly worse metabolic phenotype than wild-type controls due to hyperphagia and severe insulin resistance resulting from loss of EP3 in extra-pancreatic tissues, masking any potential beneficial effects of EP3 loss in the β cell. We hypothesized β-cell-specific EP3 knockout (EP3 βKO) mice would be protected from high-fat diet (HFD)-induced glucose intolerance, phenocopying mice lacking the EP3 effector, Gαz, which is much more limited in its tissue distribution. When fed a HFD for 16 wk, though, EP3 βKO mice were partially, but not fully, protected from glucose intolerance. In addition, exendin-4, an analog of the incretin hormone, glucagon-like peptide 1, more strongly potentiated glucose-stimulated insulin secretion in islets from both control diet- and HFD-fed EP3 βKO mice as compared with wild-type controls, with no effect of β-cell-specific EP3 loss on islet insulin content or markers of replication and survival. However, after 26 wk of diet feeding, islets from both control diet- and HFD-fed EP3 βKO mice secreted significantly less insulin as a percent of content in response to stimulatory glucose, with or without exendin-4, with elevated total insulin content unrelated to markers of β-cell replication and survival, revealing severe β-cell dysfunction. Our results suggest that EP3 serves a critical role in temporally regulating β-cell function along the progression to T2D and that there exist Gαz-independent mechanisms behind its effects.NEW & NOTEWORTHY The EP3 receptor is a strong inhibitor of β-cell function and replication, suggesting it as a potential therapeutic target for the disease. Yet, EP3 has protective roles in extrapancreatic tissues. To address this, we designed β-cell-specific EP3 knockout mice and subjected them to high-fat diet feeding to induce glucose intolerance. The negative metabolic phenotype of full-body knockout mice was ablated, and EP3 loss improved glucose tolerance, with converse effects on islet insulin secretion and content.

Metabolomic and genetic architecture of gestational diabetes subtypes

AIMS/HYPOTHESIS

Physiological gestational diabetes mellitus (GDM) subtypes that may confer different risks for adverse pregnancy outcomes have been defined. The aim of this study was to characterise the metabolome and genetic architecture of GDM subtypes to address the hypothesis that they differ between GDM subtypes.

METHODS

This was a cross-sectional study of participants in the Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study who underwent an OGTT at approximately 28 weeks' gestation. GDM was defined retrospectively using International Association of Diabetes and Pregnancy Study Groups/WHO criteria, and classified as insulin-deficient GDM (insulin secretion <25th percentile with preserved insulin sensitivity) or insulin-resistant GDM (insulin sensitivity <25th percentile with preserved insulin secretion). Metabolomic analyses were performed on fasting and 1 h serum samples in 3463 individuals (576 with GDM). Genome-wide genotype data were obtained for 8067 individuals (1323 with GDM).

RESULTS

Regression analyses demonstrated striking differences between the metabolomes for insulin-deficient or insulin-resistant GDM compared to those with normal glucose tolerance. After adjustment for covariates, 33 fasting metabolites, including 22 medium- and long-chain acylcarnitines, were uniquely associated with insulin-deficient GDM; 23 metabolites, including the branched-chain amino acids and their metabolites, were uniquely associated with insulin-resistant GDM; two metabolites (glycerol and 2-hydroxybutyrate) were associated with the same direction of association with both subtypes. Subtype differences were also observed 1 h after a glucose load. In genome-wide association studies, variants within MTNR1B (rs10830963, p=3.43×10-18, OR 1.55) and GCKR (rs1260326, p=5.17×10-13, OR 1.43) were associated with GDM. Variants in GCKR (rs1260326, p=1.36×10-13, OR 1.60) and MTNR1B (rs10830963, p=1.22×10-9, OR 1.49) demonstrated genome-wide significant association with insulin-resistant GDM; there were no significant associations with insulin-deficient GDM. The lead SNP in GCKR, rs1260326, was associated with the levels of eight of the 25 fasting metabolites that were associated with insulin-resistant GDM and ten of 41 1 h metabolites that were associated with insulin-resistant GDM.

CONCLUSIONS/INTERPRETATION

This study demonstrates that physiological GDM subtypes differ in their metabolome and genetic architecture. These findings require replication in additional cohorts, but suggest that these differences may contribute to subtype-related adverse pregnancy outcomes.

C. elegans insulin-like peptides

Insulin-like peptides are a group of hormones crucial for regulating metabolism, growth, and development in animals. Invertebrates, such as C. elegans, have been instrumental in understanding the molecular mechanisms of insulin-like peptides. Here, we review the 40 insulin-like peptide genes encoded in the C. elegans genome. Despite the large number, there is only one C. elegans insulin-like peptide receptor, called DAF-2. The insulin and insulin-like growth factor signaling (IIS) pathway is evolutionarily conserved from worms to humans. Thus C. elegans provides an excellent model to understand how these insulin-like peptides function. C. elegans is unique in that it possesses insulin-like peptides that have antagonistic properties, unlike all human insulin-like peptides, which are agonists. This review provides an overview of the current literature on C. elegans insulin-like peptide structures, processing, tissue localization, and regulation. We will also provide examples of insulin-like peptide signaling in C. elegans during growth, development, germline development, learning/memory, and longevity.

Cellular mechanisms of incretin hormone secretion

Enteroendocrine cells located along the gastrointestinal epithelium sense different nutrients/luminal contents that trigger the secretion of a variety of gut hormones with different roles in glucose homeostasis and appetite regulation. The incretin hormones glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are involved in the regulation of insulin secretion, appetite, food intake and body weight after their nutrient-induced secretion from the gut. GLP-1 mimetics have been developed and used in the treatment of type 2 diabetes mellitus and obesity. Modulating the release of endogenous intestinal hormones may be a promising approach for the treatment of obesity and type 2 diabetes without surgery. For that reason, current understanding of the cellular mechanisms underlying intestinal hormone secretion will be the focus of this review. The mechanisms controlling hormone secretion depend on the nature of the stimulus, involving a variety of signalling pathways including ion channels, nutrient transporters and G-protein-coupled receptors.

Exploring the modulatory effects of sotagliflozin on dyslipidemia in mice: The role of glucagon, fibroblast growth factor 21 and glucagon-like peptide 1

Sotagliflozin is the first dual SGLT1/2 inhibitor antidiabetic drug approved by the US Food and Drug Administration for the management of heart failure. SGLT1/2 inhibition is observed to potentiate the secretion of the incretin hormone, glucagon-like peptide-1 (GLP-1). The current preclinical research sought to investigate the effect of sotagliflozin on the secretion of fat-regulating peptides such as GLP-1, glucagon and fibroblast growth factor 21 (FGF21) and their prospective association with sotagliflozin's potential beneficial effects on dyslipidaemia. During an oral fat tolerance test in mice, sotagliflozin substantially increased GLP-1 and insulin concentrations. Although sotagliflozin alone did not ameliorate postprandial lipemia, its combination with linagliptin (DPP-IV inhibitor) significantly improved lipid tolerance comparable to orlistat (lipase inhibitor). In a triton-induced hypertriglyceridemia model, sotagliflozin, along with other medications (fenofibrate, exenatide and linagliptin) reduced fat excursion; however, co-administration with linagliptin provided no extra advantage. Furthermore, sotagliflozin stimulated glucagon secretion in the alpha TC1.6 cells and healthy mice, which resulted in an increased circulating FGF21 and β-hydroxybutyrate concentration. Finally, chronic treatment of sotagliflozin in high-fat diet (HFD)-fed obese mice resulted in reduced body weight gain, liver triglyceride, cholesterol, interleukin-6 (IL-6) and tumour necrosis factor alpha (TNF-α) levels compared with the placebo group. However, the addition of linagliptin did not provide any additional benefit. In conclusion, sotagliflozin was found to have an effect on GLP-1 and also stimulate the release of glucagon and FGF21, which are important for regulating fat metabolism. Therefore, sotagliflozin might represent a potential therapeutic approach for the treatment of diabetic dyslipidemia and steatohepatitis.

Characterization of palmitic acid toxicity induced insulin resistance in HepG2 cells

BACKGROUND

An etiology of palmitic acid (PA) induced insulin resistance (IR) is complex for which two mechanisms are proposed namely ROS induced JNK activation and lipid induced protein kinase-C (PKCε) activation. However, whether these mechanisms act alone or in consortium is not clear.

METHODS AND RESULTS

In this study, we have characterized PA induced IR in liver cells. These cells were treated with different concentrations of PA for either 8 or 16 h. Insulin responsiveness of cells treated with PA for 8 h was found to be same as that of control. However, cells treated with PA for 16 h, showed increased glucose output both in the presence and in absence of insulin only at higher concentrations, indicating development of IR. In these, both JNK and PKCε were activated in response to increased ROS and lipid accumulation, respectively. Activated JNK and PKCε phosphorylated IRS1 at Ser-307 resulting in inhibition of AKT which in turn inactivated GSK3β, leading to reduced glycogen synthase activity. Inhibition of AKT also reduced insulin suppression of hepatic gluconeogenesis by activating Forkhead box protein O1 (FOXO1) and increased expression of the gluconeogenic enzymes and their transcription factors.

CONCLUSION

Thus, our data clearly demonstrate that both these mechanisms work simultaneously and more importantly, identified a threshold of HepG2 cells, which when crossed led to the pathological state of IR in response to PA.

Zinc deficiency affects insulin secretion and alters insulin-regulated metabolic signaling in rats

BACKGROUND

The essential trace element zinc is important in stabilizing pancreatic insulin secretion. Zinc not only influences the synthesis of insulin but also affects its activity. Insulin not only exerts a hypoglycemic effect but also regulates glucose and lipid metabolisms in insulin target organs. In this study, we aimed to determine changes to pancreatic β cells and insulin secretion induced by different zinc concentrations and to evaluate the effect of zinc deficiency on glucose intolerance, insulin resistance, and insulin target organs via changing insulin levels.

METHODS

We set up two experimental trials. In the first trial, male Sprague Dawley (SD) rats were divided into three groups. Group one (ZnC) received a standard diet, group two (ZnF) was given a zinc-free diet, and group three (ZnFC) was initially fed a zinc-free diet followed by a reversion to the standard diet. After sacrifice, we observed changes in blood parameters, including insulin, and examined alterations in pancreatic tissue using immunostaining, with focus on the localization of pancreatic β-cells. In the second trial, male SD rats were split into two groups, with one receiving a standard diet and the other a zinc-free diet. Oral glucose tolerance and insulin tolerance tests were then performed. After sacrifice, we evaluated changes in lipid and glucose metabolism within insulin target organs using quantitative polymerase chain reaction.

RESULTS

In the first trial, blood insulin levels and the area of insulin-positive staining in pancreatic β-cells decreased in the ZnF compared to the ZnC group. The ZnFC group did not show recovery in either blood insulin levels or the area of insulin-positive staining in pancreatic β-cells. In the second trial, no differences were observed in glucose tolerance or insulin resistance between the ZnC and ZnF groups. However, changes in the expression of insulin target genes were noted in the liver and adipose tissue in the ZnF group.

CONCLUSION

We reveal that dietary zinc concentrations not only affect the concentration of insulin in the blood but also impact the localization of pancreatic β-cells involved in insulin production. Furthermore, our results suggest that changes in blood insulin levels, induced by different zinc concentrations, could cause metabolic alterations in insulin target organs such as the liver and adipose tissue. This study sheds more light on the role of zinc in insulin-regulated metabolic diseases.

Insulin-stimulated translocation of the fatty acid transporter CD36 to the plasma membrane is mediated by the small GTPase Rac1 in adipocytes

Cluster of differentiation 36 (CD36) is a scavenger receptor (SR), recognizing diverse extracellular ligands in various types of mammalian cells. Long-chain fatty acids (FAs), which are important constituents of phospholipids and triglycerides, also utilize CD36 as a predominant membrane transporter, being incorporated from the circulation across the plasma membrane in several cell types, including cardiac and skeletal myocytes and adipocytes. CD36 is localized in intracellular vesicles as well as the plasma membrane, and its distribution is modulated by extracellular stimuli. Herein, we aimed to clarify the molecular basis of insulin-stimulated translocation of CD36, which leads to the enhanced uptake of long-chain FAs, in adipocytes. To this end, we developed a novel exofacial epitope-tagged reporter to specifically detect cell surface-localized CD36. By employing this reporter, we demonstrate that the small GTPase Rac1 plays a pivotal role in insulin-stimulated translocation of CD36 to the plasma membrane in 3T3-L1 adipocytes. Additionally, phosphoinositide 3-kinase and the protein kinase Akt2 are shown to be involved in the regulation of Rac1. Downstream of Rac1, another small GTPase RalA directs CD36 translocation. Collectively, these results suggest that CD36 is translocated to the plasma membrane by insulin through mechanisms similar to those for the glucose transporter GLUT4 in adipocytes.

Glycolytic enzyme Enolase-1 regulates insulin gene expression in pancreatic β-cell

Enolase-1 (Eno1) plays a critical role in regulating glucose metabolism; however, its specific impact on pancreatic islet β-cells remains elusive. This study aimed to provide a preliminary exploration of Eno1 function in pancreatic islet β-cells. The findings revealed that the expression of ENO1 mRNA in type 2 diabetes donors was significantly increased and positively correlated with HbA1C and negatively correlated with insulin gene expression. A high level of Eno1 in human insulin-secreting rat INS-1832/13 cells with co-localization with intracellular insulin proteins was accordingly observed. Silencing of Eno1 using siRNA or inhibiting Eno1 protein activity with an Eno1 antagonist significantly reduced insulin secretion and insulin content in β-cells, while the proinsulin/insulin content ratio remained unchanged. This reduction in β-cells function was accompanied by a notable decrease in intracellular ATP and mitochondrial cytochrome C levels. Overall, our findings confirm that Eno1 regulates the insulin secretion process, particularly glucose metabolism and ATP production in the β-cells. The mechanism primarily involves its influence on insulin production, suggesting that Eno1 represents a potential target for β-cell protection and diabetes treatment.

Flow cytometry protocol for GLUT4-myc detection on cell surfaces

Insulin and muscle contraction trigger GLUT4 translocation to the plasma membrane, which increases glucose uptake by muscle cells. Insulin resistance and Type 2 diabetes are the result of impaired GLUT4 translocation. Quantifying GLUT4 translocation is essential for comprehending the intricacies of both physiological and pathophysiological processes involved in glucose metabolism. The most commonly used methods for measuring GLUT4 translocation are the ELISA-type assay and the immunofluorescence assay. While some reports suggest that flow cytometry could be useful in quantifying GLUT4 translocation, this technique is not frequently used. Much of our current understanding of the regulation of GLUT4 has been based on experiments using the rat myoblast cell line (L6 cell) which expresses GLUT4 with a myc epitope on the exofacial loop. In the present study, we use the L6-GLUT4myc cell line to develop a flow cytometry-based approach to detect GLUT4 translocation. Flow cytometry offers the advantages of both immunofluorescence and ELISA-based assays. It allows easy identification of separate cell populations in the sample, similar to immunofluorescence, while providing results based on a population-level analysis of multiple individual cells, like an ELISA-based assay. Our results demonstrate a 0.6-fold increase with insulin stimulation compared with basal conditions. Finally, flow cytometry consistently yielded results across different experiments and exhibited sensitivity under the tested conditions.

l-norleucine on high glucose-induced insulin sensitivity and mitochondrial function in skeletal muscle cells

l-norleucine, an isomer of leucine, stimulates the anabolic process of insulin. However, it is not known if and how it improves insulin sensitivity and insulin resistance. This experiment describes the generation of an insulin resistance model using high glucose-induced cells and the administration of 1.0 mmol/L l-norleucine for 48 h, to observe the effects on metabolism and gene expression in skeletal muscle cells. The results showed that l-norleucine significantly increased mitochondrial ATP content, decreased the amount of reactive oxygen species (ROS) and promoted the expression of mitochondrial generation-related genes TFAM, AMPK, PGC-1α in cells under high glucose treatment; at the same time, l-norleucine also increased glucose uptake, suggesting that l-norleucine increased insulin sensitivity and improved insulin resistance. This study suggesting that l-norleucine improves insulin resistance by ameliorating oxidative stress damage of mitochondria, improving mitochondrial function, and improving insulin sensitivity in skeletal muscle cell caused by high glucose, rather than by altering mitochondrial efficiency.

The serine phosphorylations in the IRS-1 PIR domain abrogate IRS-1 and IR interaction

Serine phosphorylations on insulin receptor substrate 1 (IRS-1) by diverse kinases aoccur widely during obesity-, stress-, and inflammation-induced conditions in models of insulin resistance and type 2 diabetes. In this study, we define a region within the human IRS-1, which is directly C-terminal to the PTB domain encompassing numerous serine phosphorylation sites including Ser307 (mouse Ser302) and Ser312 (mouse 307) creating a phosphorylation insulin resistance (PIR) domain. We demonstrate that the IRS-1 PTB-PIR with its unphosphorylated serine residues interacts with the insulin receptor (IR) but loses the IR-binding when they are phosphorylated. Surface plasmon resonance studies further confirm that the PTB-PIR binds stronger to IR than just the PTB domain, and that phosphorylations at Ser307, Ser312, Ser315, and Ser323 within the PIR domain result in abrogating the binding. Insulin-responsive cells containing the mutant IRS-1 with all these four serines changed into glutamates to mimic phosphorylations show decreased levels of phosphorylations in IR, IRS-1, and AKT compared to the wild-type IRS-1. Hydrogen-deuterium exchange mass spectrometry experiments indicating the PIR domain interacting with the N-terminal lobe and the hinge regions of the IR kinase domain further suggest the possibility that the IRS-1 PIR domain protects the IR from the PTP1B-mediated dephosphorylation.

Contemporary Clinical Perspectives on Targeting Remission of Type 2 Diabetes

It has long been known that some patients with type 2 diabetes (T2DM) can experience sustained metabolic improvement to near-normal levels of glycemia either spontaneously or after medical intervention. Now recognized as remission of diabetes, this intriguing state is currently more feasible than ever before due to profound advances in metabolic surgery, pharmacologic therapy, and regimens of lifestyle modification. This enhanced capacity to induce remission has revealed new pathophysiologic insights, including the presence of a reversible component of the pancreatic beta-cell dysfunction that otherwise drives the chronic progressive nature of T2DM. In doing so, it has changed the therapeutic landscape by offering new potential management objectives and considerations for patients and providers. However, the excitement around these developments must also be tempered by the sobering realities of our current understanding of remission, including the recognition that this condition may not be permanent (resulting in glycemic relapse over time) and that beta-cell function may not be normalized in the setting of remission. These limitations highlight both the many gaps in our current understanding of remission and the caution with which clinical discussions must be handled for clear patient-directed communication of the pros and cons of targeting this outcome in practice. In this mini-review, we consider this rapidly growing literature, including its implications and its limitations, and thereby seek to provide objective balanced perspectives on targeting remission of T2DM in current clinical care.

Defective Visceral Adipose Tissue Adaptation in Gestational Diabetes Mellitus

CONTEXT

Gestational diabetes mellitus (GDM) is a complex obstetric condition affecting localized glucose metabolism, resulting in systemic metabolic dysfunction.

OBJECTIVE

This cross-sectional study aimed to explore visceral adipose tissue (VAT) as an integral contributor to GDM, focusing on elucidating the specific contribution of obesity and GDM pathology to maternal outcomes.

METHODS

Fifty-six nulliparous pregnant women were recruited, including normal glucose tolerant (NGT) (n = 30) and GDM (n = 26) participants. Participants were subgrouped as nonobese (BMI <30 kg/m2) or obese (BMI ≥30 kg/m2). Metabolic markers in circulation, VAT, and placenta were determined. Morphological analysis of VAT and immunoblotting of the insulin signaling cascade were performed.

RESULTS

GDM participants demonstrated hyperinsulinemia and elevated homeostatic model assessment for insulin resistance (HOMA-IR) scores relative to NGT participants. The GDM-obese subgroup had significant VAT adipocyte hypoplasia relative to NGT-nonobese tissue. GDM-obese VAT had significantly lower insulin receptor substrate (IRS)-2 expression, with elevated ser312 phosphorylation of IRS-1, relative to NGT-nonobese. GDM-obese participants had significantly elevated circulating leptin levels and placental adipsin secretion, while GDM-nonobese participants had elevated circulating adipsin levels with reduced placental adiponectin secretion.

CONCLUSION

These findings suggest that GDM-obese pregnancy is specifically characterized by inadequate VAT remodeling and dysfunctional molecular signaling, which contribute to insulin resistance and hinder metabolic health.

Dopamine D2 receptor antagonist counteracts hyperglycemia and insulin resistance in diet-induced obese male mice

Obesity leads to insulin resistance (IR) and type 2 diabetes. In humans, low levels of the hormone prolactin (PRL) correlate with IR, adipose tissue (AT) dysfunction, and increased prevalence of T2D. In obese rats, PRL treatment promotes insulin sensitivity and reduces visceral AT adipocyte hypertrophy. Here, we tested whether elevating PRL levels with the prokinetic and antipsychotic drug sulpiride, an antagonist of dopamine D2 receptors, improves metabolism in high fat diet (HFD)-induced obese male mice. Sulpiride treatment (30 days) reduced hyperglycemia, IR, and the serum and pancreatic levels of triglycerides in obese mice, reduced visceral and subcutaneous AT adipocyte hypertrophy, normalized markers of visceral AT function (PRL receptor, Glut4, insulin receptor and Hif-1α), and increased glycogen stores in skeletal muscle. However, the effects of sulpiride reducing hyperglycemia were also observed in obese prolactin receptor null mice. We conclude that sulpiride reduces obesity-induced hyperglycemia by mechanisms that are independent of prolactin/prolactin receptor activity. These findings support the therapeutic potential of sulpiride against metabolic dysfunction in obesity.

Illuminating the complete ß-cell mass of the human pancreas- signifying a new view on the islets of Langerhans

Pancreatic islets of Langerhans play a pivotal role in regulating blood glucose homeostasis, but critical information regarding their mass, distribution and composition is lacking within a whole organ context. Here, we apply a 3D imaging pipeline to generate a complete account of the insulin-producing islets throughout the human pancreas at a microscopic resolution and within a maintained spatial 3D context. These data show that human islets are far more heterogenous than previously accounted for with regards to their size distribution and cellular make up. By deep tissue 3D imaging, this in-depth study demonstrates that 50% of the human insulin-expressing islets are virtually devoid of glucagon-producing α-cells, an observation with significant implications for both experimental and clinical research.

Pancreas Β-Cells in Type 1 and Type 2 Diabetes: Cell Death, Oxidative Stress and Immune Regulation. Recently Appearing Changes in Diabetes Consequences

Diabetes mellitus type 1 (T1D) and type 2 (T2D) develop due to dysfunction of the Langerhans islet β-cells in the pancreas, and this dysfunction is mediated by oxidative, endoplasmic reticulum (ER), and mitochondrial stresses. Although the two types of diabetes are significantly different, β-cell failure and death play a key role in the pathogenesis of both diseases, resulting in hyperglycemia due to a reduced ability to produce insulin. In T1D, β-cell apoptosis is the main event leading to hyperglycemia, while in T2D, insulin resistance results in an inability to meet insulin requirements. It has been suggested that autophagy promotes β-cell survival by delaying apoptosis and providing adaptive responses to mitigate the detrimental effects of ER stress and DNA damage, which is directly related to oxidative stress. As people with diabetes are now living longer, they are more susceptible to a different set of complications. There has been a diversification in causes of death, whereby a larger proportion of deaths among individuals with diabetes is attributable to nonvascular conditions; on the other hand, the proportion of cancer-related deaths has remained stable or even increased in some countries. Due to the increasing cases of both T1D and T2D, these diseases become even more socially significant. Hence, we believe that search for any opportunities for control of this disease is an overwhelmingly important target for the modern science. We focus on two differences that are characteristic of the development of diabetes's last periods. One of them shows that all-cause death rates have declined in several diabetes populations, driven in part by large declines in vascular disease mortality but large increases in oncological diseases. Another hypothesis is that some T2D medications could be repurposed to control glycemia in patients with T1D.

Afriplex GRTTM extract attenuates hepatic steatosis in an in vitro model of NAFLD

BACKGROUND

Currently, it is acknowledged that vitamin E, insulin sensitizers and anti-diabetic drugs are used to manage non-alcoholic fatty liver disease (NAFLD), however, these therapeutic interventions harbour adverse side effects. Pioglitazone, an anti-diabetic drug, is currently the most effective therapy to manage NAFLD. The use of natural medicines is widely embraced due to the lack of evidence of their negative side effects. Rooibos has been previously shown to decrease inflammation and oxidative stress in experimental models of diabetes, however, this is yet to be explored in a setting of NAFLD. This study was aimed at investigating the effects of an aspalathin-rich green rooibos extract (Afriplex GRTTM) against markers of hepatic oxidative stress, inflammation and apoptosis in an in vitro model of NAFLD.

METHODS

Oleic acid [1 mM] was used to induce hepatic steatosis in C3A liver cells. Thereafter, the therapeutic effect of Afriplex GRTTM, with or without pioglitazone, was determined by assessing its impact on cell viability, changes in mitochondrial membrane potential, intracellular lipid accumulation and the expression of genes and proteins (ChREBP, SREBF1, FASN, IRS1, SOD2, Caspase-3, GSTZ1, IRS1 and TNF-α) that are associated with the development of NAFLD.

RESULTS

Key findings showed that Afriplex GRTTM added to the medium alone or combined with pioglitazone, could effectively block hepatic lipid accumulation without inducing cytotoxicity in C3A liver cells exposed oleic acid. This positive outcome was consistent with effective regulation of genes involved in insulin signaling, as well as carbohydrate and lipid metabolism (IRS1, SREBF1 and ChREBP). Interestingly, in addition to reducing protein levels of an inflammatory marker (TNF-α), the Afriplex GRTTM could ameliorate oleic acid-induced hepatic steatotic damage by decreasing the protein expression of oxidative stress and apoptosis related markers such as GSTZ1 and caspase-3.

CONCLUSION

Afriplex GRTTM reduced hepatic steatosis in oleic acid induced C3A liver cells by modulating SREBF1, ChREBP and IRS-1 gene expression. The extract may also play a role in alleviating inflammation by reducing TNF-α expression, suggesting that additional experiments are required for its development as a suitable therapeutic option against NAFLD. Importantly, further research is needed to explore its antioxidant role in this model.

Phenotypes of streptozotocin-induced gestational diabetes mellitus in mice

Gestational diabetes mellitus (GDM) in human patients disrupts glucose metabolism post-pregnancy, affecting fetal development. Although obesity and genetic factors increase GDM risk, a lack of suitable models impedes a comprehensive understanding of its pathology. To address this, we administered streptozotocin (STZ, 75 mg/kg) to C57BL/6N mice for two days before pregnancy, establishing a convenient GDM model. Pregnant mice exposed to STZ (STZ-pregnant) were compared with STZ-injected virgin mice (STZ-virgin), citrate buffer-injected virgin mice (CB-virgin), and pregnant mice injected with citrate buffer (CB-pregnant). STZ-pregnant non-obese mice exhibited elevated blood glucose levels on gestational day 15.5 and impaired glucose tolerance. They also showed fewer normal fetuses compared to CB-pregnant mice. Additionally, STZ-pregnant mice had the highest plasma C-peptide levels, with decreased pancreatic islets or increased alpha cells compared to CB-pregnant mice. Kidneys isolated from STZ-pregnant mice did not display histological alterations or changes in gene expression for the principal glucose transporters (GLUT2 and SGLT2) and renal injury-associated markers. Notably, STZ-pregnant mice displayed decreased gene expression of insulin-receiving molecules (ISNR and IGFR1), indicating heightened insulin resistance. Liver histology in STZ-pregnant mice remained unchanged except for a pregnancy-related increase in lipid droplets within hepatocytes. Furthermore, the duodenum of STZ-pregnant mice exhibited increased gene expression of ligand-degradable IGFR2 and decreased expression of GLUT5 and GLUT12 (fructose and glucose transporters, respectively) compared to STZ-virgin mice. Thus, STZ-pregnant mice displayed GDM-like symptoms, including fetal abnormalities, while organs adapted to impaired glucose metabolism by altering glucose transport and insulin reception without histopathological changes. STZ-pregnant mice offer a novel model for studying mild onset non-obese GDM and species-specific differences in GDM features between humans and animals.

Resveratrol relieves HFD-induced insulin resistance in skeletal muscle tissue through antioxidant capacity enhancement and the Nrf2-Keap1 signaling pathway

BACKGROUND

Resveratrol has received much attention due to its beneficial effects including antioxidant activity. The purpose of this study was to investigate the therapeutic effects of resveratrol treatment on oxidative stress and insulin resistance in the skeletal muscle of high-fat diet (HFD)-fed animals.

METHODS AND RESULTS

A total of 30 six-week-old C57BL/6J mice were randomly allocated to three groups (10 animals in each group): The control group in which mice were fed a normal chow diet (NCD); the HFD group in which mice were fed an HFD for 26 weeks; and the HFD-resveratrol group in which HFD was replaced by a resveratrol supplemented-HFD (400 mg/kg diet) after 10 weeks of HFD feeding. At the end of this period, gastrocnemius muscle samples were examined to determine insulin resistance and the oxidative status in the presence of HFD and resveratrol. Resveratrol supplementation in HFD-fed mice reduced body and adipose tissue weight, improved insulin sensitivity, and decreased oxidative stress as indicated by lower malonaldehyde (MDA) levels and higher total antioxidant capacity. The supplement also increased the expression and activity of antioxidative enzymes in gastrocnemius muscle and modulated Nrf2 and Keap1 expression levels.

CONCLUSIONS

These results suggest that resveratrol is effective in improving the antioxidant defense system of the skeletal muscle in HFD-fed mice, indicating its therapeutic potential to combat diseases associated with insulin resistance and oxidative stress.

Elevated levels of alcohol dehydrogenase aggravate ethanol-evoked cardiac remodeling and contractile anomalies through FKBP5-yap-mediated regulation of ferroptosis and ER stress

Alcohol intake provokes severe organ injuries including alcoholic cardiomyopathy with hallmarks of cardiac remodeling and contractile defects. This study examined the toxicity of facilitated ethanol metabolism in alcoholism-evoked changes in myocardial morphology and contractile function, insulin signaling and various cell death domains using cardiac-selective overexpression of alcohol dehydrogenase (ADH). WT and ADH mice were offered an alcohol liquid diet for 12 weeks prior to assessment of cardiac geometry, function, ER stress, apoptosis and ferroptosis. Alcohol intake provoked pronounced glucose intolerance, cardiac remodeling and contractile anomalies with apoptosis, ER stress, and ferroptosis, the effects were accentuated by ADH with the exception of global glucose intolerance. Hearts from alcohol ingesting mice displayed dampened insulin-stimulated phosphorylation of insulin receptor (tyr1146) and IRS-1 (tyrosine) along with elevated IRS-1 serine phosphorylation, the effect was augmented by ADH. Alcohol challenge dampened phosphorylation of Akt and GSK-3β, and increased phosphorylation of c-Jun and JNK, the effects were accentuated by ADH. Alcohol challenge promoted ER stress, FK506 binding protein 5 (FKBP5), YAP, apoptosis and ferroptosis, the effects were exaggerated by ADH. Using a short-term ethanol challenge model (3 g/kg, i.p., twice in three days), we found that inhibition of FKBP5-YAP signaling or facilitated ethanol detoxification by Alda-1 alleviated ethanol cardiotoxicity. In vitro study revealed that the ethanol metabolite acetaldehyde evoked cardiac contractile anomalies, lipid peroxidation, and apoptosis, the effects of which were mitigated by Alda-1, inhibition of ER stress, FKBP5 and YAP. These data suggest that facilitated ethanol metabolism via ADH exacerbates alcohol-evoked myocardial remodeling, functional defects, and insulin insensitivity possibly through a FKBP5-YAP-associated regulation of ER stress and ferroptosis.

Reviewing critical TRPM2 variants through a structure-function lens

The transient receptor potential melastatin 2 (TRPM2) channel plays a central role in connecting redox state with calcium signaling in living cells. This coupling makes TRPM2 essential for physiological functions such as pancreatic insulin secretion or cytokine production, but also allows it to contribute to pathological processes, including neuronal cell death or ischemia-reperfusion injury. Genetic deletion of the channel, albeit not lethal, alters physiological functions in mice. In humans, population genetic studies and whole-exome sequencing have identified several common and rare genetic variants associated with mental disorders and neurodegenerative diseases, including single nucleotide variants (SNVs) in exonic regions. In this review, we summarize available information on the four best-documented SNVs: one common (rs1556314) and three rare genetic variants (rs139554968, rs35288229, and rs145947009), manifested in amino acid substitutions D543E, R707C, R755C, and P1018L respectively. We discuss existing evidence supporting or refuting the associations between SNVs and disease. Furthermore, we aim to interpret the molecular impacts of these amino acid substitutions based on recently published structures of human TRPM2. Finally, we formulate testable hypotheses and suggest means to investigate them. Studying the function of proteins with rare mutations might provide insight into disease etiology and delineate new drug targets.

Characterization of the zebrafish as a model of ATP-sensitive potassium channel hyperinsulinism

INTRODUCTION

Congenital hyperinsulinism (HI) is the leading cause of persistent hypoglycemia in infants. Current models to study the most common and severe form of HI resulting from inactivating mutations in the ATP-sensitive potassium channel (KATP) are limited to primary islets from patients and the Sur1 -/- mouse model. Zebrafish exhibit potential as a novel KATPHI model since they express canonical insulin secretion pathway genes and those with identified causative HI mutations. Moreover, zebrafish larvae transparency provides a unique opportunity for in vivo visualization of pancreatic islets.

RESEARCH DESIGN AND METHODS

We evaluated zebrafish as a model for KATPHI using a genetically encoded Ca2+ sensor (ins:gCaMP6s) expressed under control of the insulin promoter in beta cells of an abcc8 -/- zebrafish line.

RESULTS

We observed significantly higher islet cytosolic Ca2+ in vivo in abcc8 -/- compared with abcc8 +/+ zebrafish larvae. Additionally, abcc8 -/- larval zebrafish had significantly lower whole body glucose and higher whole body insulin levels compared with abcc8 +/+ controls. However, adult abcc8 -/- zebrafish do not show differences in plasma glucose, plasma insulin, or glucose tolerance when compared with abcc8 +/+ zebrafish.

CONCLUSIONS

Our results identify that zebrafish larvae, but not adult fish, are a demonstrable novel model for advancement of HI research.

Loss of Cardiac PFKFB2 Drives Metabolic, Functional, and Electrophysiological Remodeling in the Heart

BACKGROUND

Phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) is a critical glycolytic regulator responsible for upregulation of glycolysis in response to insulin and adrenergic signaling. PFKFB2, the cardiac isoform of PFK-2, is degraded in the heart in the absence of insulin signaling, contributing to diabetes-induced cardiac metabolic inflexibility. However, previous studies have not examined how the loss of PFKFB2 affects global cardiac metabolism and function.

METHODS AND RESULTS

To address this, we have generated a mouse model with a cardiomyocyte-specific knockout of PFKFB2 (cKO). Using 9-month-old cKO and control mice, we characterized the impacts of PFKFB2 on cardiac metabolism, function, and electrophysiology. cKO mice have a shortened life span of 9 months. Metabolically, cKO mice are characterized by increased glycolytic enzyme abundance and pyruvate dehydrogenase activity, as well as decreased mitochondrial abundance and beta oxidation, suggesting a shift toward glucose metabolism. This was supported by a decrease in the ratio of palmitoyl carnitine to pyruvate-dependent mitochondrial respiration in cKO relative to control animals. Metabolomic, proteomic, and Western blot data support the activation of ancillary glucose metabolism, including pentose phosphate and hexosamine biosynthesis pathways. Physiologically, cKO animals exhibited impaired systolic function and left ventricular dilation, represented by reduced fractional shortening and increased left ventricular internal diameter, respectively. This was accompanied by electrophysiological alterations including increased QT interval and other metrics of delayed ventricular conduction.

CONCLUSIONS

Loss of PFKFB2 results in metabolic remodeling marked by cardiac ancillary pathway activation. This could delineate an underpinning of pathologic changes to mechanical and electrical function in the heart.

Ketogenic diet ameliorates high-fat diet-induced insulin resistance in mouse skeletal muscle by alleviating endoplasmic reticulum stress

OBJECTIVE

Ketogenic diets (KD) have been shown to alleviate insulin resistance (IR) by exerting anti-lipogenic and insulin sensitizing effects in the liver through a variety of pathways. The present study sought to investigate whether a ketogenic diet also improves insulin sensitization in skeletal muscle cells through alleviating endoplasmic reticulum stress.

METHODS

High-fat diet-induced IR mice were allowed to a 2-week ketogenic diet. Insulin resistance and glucose tolerance were evaluated through GTT, ITT, and HOMA-IR. The C2C12 myoblasts exposed to palmitic acid were used to evaluate the insulin sensitization effects of β-hydroxybutyric acid (β-OHB). Molecular mechanisms concerning ER stress signaling activation and glucose uptake were assessed.

RESULTS

The AKT/GSK3β pathway was inhibited, ER stress signaling associated with IRE1, PERK, and BIP was activated, and the number of Glut4 proteins translocated to membrane decreased in the muscle of HFD mice. However, all these changes were reversed after 2 weeks of feeding on a ketogenic diet. Consistently in C2C12 myoblasts, the AKT/GSK3β pathway was inhibited by palmitic acid (PA) treatment. The endoplasmic reticulum stress-related proteins, IRE1, and BIP were increased, and the number of Glut4 proteins on the cell membrane decreased. However, β-OHB treatment alleviated ER stress and improved the glucose uptake of C2C12 cells.

CONCLUSION

Our data reveal that KD ameliorated HFD-induced insulin resistance in skeletal muscle, which was partially mediated by inhibiting endoplasmic reticulum stress. The insulin sensitization effect of β-OHB is associated with up regulation of AKT/GSK3β pathway and the increase in the number of Glut4 proteins on the cell membrane.

Role of human Kallistatin in glucose and energy homeostasis in mice

OBJECTIVE

Kallistatin (KST), also known as SERPIN A4, is a circulating, broadly acting human plasma protein with pleiotropic properties. Clinical studies in humans revealed reduced KST levels in obesity. The exact role of KST in glucose and energy homeostasis in the setting of insulin resistance and type 2 diabetes is currently unknown.

METHODS

Kallistatin mRNA expression in human subcutaneous white adipose tissue (sWAT) of 47 people with overweight to obesity of the clinical trial "Comparison of Low Fat and Low Carbohydrate Diets With Respect to Weight Loss and Metabolic Effects (B-SMART)" was measured. Moreover, we studied transgenic mice systemically overexpressing human KST (hKST-TG) and wild type littermate control mice (WT) under normal chow (NCD) and high-fat diet (HFD) conditions.

RESULTS

In sWAT of people with overweight to obesity, KST mRNA increased after diet-induced weight loss. On NCD, we did not observe differences between hKST-TG and WT mice. Under HFD conditions, body weight, body fat and liver fat content did not differ between genotypes. Yet, during intraperitoneal glucose tolerance tests (ipGTT) insulin excursions and HOMA-IR were lower in hKST-TG (4.42 ± 0.87 AU, WT vs. 2.20 ± 0.27 AU, hKST-TG, p < 0.05). Hyperinsulinemic euglycemic clamp studies with tracer-labeled glucose infusion confirmed improved insulin sensitivity by higher glucose infusion rates in hKST-TG mice (31.5 ± 1.78 mg/kg/min, hKST-TG vs. 18.1 ± 1.67 mg/kg/min, WT, p < 0.05). Improved insulin sensitivity was driven by reduced hepatic insulin resistance (clamp hepatic glucose output: 7.7 ± 1.9 mg/kg/min, hKST-TG vs 12.2 ± 0.8 mg/kg/min, WT, p < 0.05), providing evidence for direct insulin sensitizing effects of KST for the first time. Insulin sensitivity was differentially affected in skeletal muscle and adipose tissue. Mechanistically, we observed reduced Wnt signaling in the liver but not in skeletal muscle, which may explain the effect.

CONCLUSIONS

KST expression increases after weight loss in sWAT from people with obesity. Furthermore, human KST ameliorates diet-induced hepatic insulin resistance in mice, while differentially affecting skeletal muscle and adipose tissue insulin sensitivity. Thus, KST may be an interesting, yet challenging, therapeutic target for patients with obesity and insulin resistance.

LINC MIR503HG Controls SC-β Cell Differentiation and Insulin Production by Targeting CDH1 and HES1

Stem cell-derived pancreatic progenitors (SC-PPs), as an unlimited source of SC-derived β (SC-β) cells, offers a robust tool for diabetes treatment in stem cell-based transplantation, disease modeling, and drug screening. Whereas, PDX1+/NKX6.1+ PPs enhances the subsequent endocrine lineage specification and gives rise to glucose-responsive SC-β cells in vivo and in vitro. To identify the regulators that promote induction efficiency and cellular function maturation, single-cell RNA-sequencing is performed to decipher the transcriptional landscape during PPs differentiation. The comprehensive evaluation of functionality demonstrated that manipulating LINC MIR503HG using CRISPR in PP cell fate decision can improve insulin synthesis and secretion in mature SC-β cells, without effects on liver lineage specification. Importantly, transplantation of MIR503HG-/- SC-β cells in recipients significantly restored blood glucose homeostasis, accompanied by serum C-peptide release and an increase in body weight. Mechanistically, by releasing CtBP1 occupying the CDH1 and HES1 promoters, the decrease in MIR503HG expression levels provided an excellent extracellular niche and appropriate Notch signaling activation for PPs following differentiation. Furthermore, this exhibited higher crucial transcription factors and mature epithelial markers in CDH1High expressed clusters. Altogether, these findings highlighted MIR503HG as an essential and exclusive PP cell fate specification regulator with promising therapeutic potential for patients with diabetes.

What Are the Implications of Cr(III) Serving as an Inhibitor of the Beta Subunit of Mitochondrial ATP Synthase?

A recent report has shown the active site of the beta subunit of mitochondrial ATP synthase is probably the site of action of Cr(III) action, independent of the insulin signaling pathway. This works appears to answer an important question about the mode of action of Cr(III) at a molecular level when supplied in supra-nutritional levels to rodents. However, as with any good research, the research also raises several questions. The relationship between this study and the results of rodent studies of chromium supplementation and between this study and the current understanding the chromium(III) transport and detoxification system are put into perspective.

Sex-based differences in insulin resistance

Sexual dimorphism in energy metabolism is now established and suggested to affect many aspects of metabolic diseases and in particular diabetes and obesity. This is strongly related to sex-based differences in whole-body insulin resistance. Women are more insulin sensitive compared to men, but this metabolic advantage gradually disappears after menopause or when insulin resistance progresses to hyperglycemia and diabetes. In this narrative review, first, we describe the pathophysiology related to insulin resistance and then we present the epidemiological evidence as well as the important biological factors that play a crucial role in sexual dimorphism in insulin sensitivity. We focus particularly on the differences in body fat and muscle mass distribution and function, in inflammation and in sex hormones between males and females. Most importantly, we describe the significant mechanistic differences in insulin sensitivity as well as glucose and lipid metabolism in key metabolic organs: liver, white adipose tissue, and skeletal muscle. Finally, we present the sex-based differences in response to different interventions and discuss important open research questions.

Understanding Cr(III) Action on Mitochondrial ATP Synthase and AMPK Efficacy: Insights from Previous Studies-a Review

Chromium supplementation has been notably recognized for its potential health benefits, especially in enhancing insulin sensitivity and managing glucose metabolism. However, recent studies have begun to shed light on additional mechanisms of action for chromium, expanding our understanding beyond its classical effects on the insulin-signaling pathway. The beta subunit of mitochondrial ATP synthase is considered a novel site for Cr(III) action, influencing physiological effects apart from insulin signaling. The physiological effects of chromium supplementation have been extensively studied, particularly in its role in anti-oxidative efficacy and glucose metabolism. However, recent advancements have prompted a re-evaluation of chromium's mechanisms of action beyond the insulin signaling pathway. The discovery of the beta subunit of mitochondrial ATP synthase as a potential target for chromium action is discussed, emphasizing its crucial role in cellular energy production and metabolic regulation. A meticulous analysis of relevant studies that were earlier carried out could shed light on the relationship between chromium supplementation and mitochondrial ATP synthase. This review categorizes studies based on their primary investigations, encompassing areas such as muscle protein synthesis, glucose and lipid metabolism, and antioxidant properties. Findings from these studies are scrutinized to distinguish patterns aligning with the new hypothesis. Central to this exploration is the presentation of studies highlighting the physiological effects of chromium that extend beyond the insulin signaling pathway. Evaluating the various independent mechanisms of action that chromium impacts cellular energy metabolism and overall metabolic balance has become more important. In conclusion, this review is a paradigm shift in understanding chromium supplementation, paving the way for future investigations that leverage the intricate interplay between chromium and mitochondrial ATP synthase.

Human gestational diabetes mellitus-derived exosomes impair glucose homeostasis in pregnant mice and stimulate functional maturation of offspring-islets

AIMS

Pancreatic islets undergo critical development and functional maturation during the perinatal period when they are highly sensitive to microenvironment. We aim to determine the effects and mechanisms of gestational diabetes mellitus (GDM) hypermetabolic stress on glucose homeostasis in pregnant mice and functional maturation of the islets of their offspring.

MAIN METHODS

Exosomes were extracted from the umbilical vein blood of individuals with or without GDM for administration to pregnant mice. The blood glucose, serum insulin, glycosylated hemoglobin, and lipopolysaccharide levels were measured in pregnant mice. The expression and localization of insulin, glucagon, PC1/3, PDX1, and p-S6 in the islets of neonatal rats were continuously monitored using immunofluorescence to evaluate their functional status. Primary islet cells were cultured and treated with GDM exosomes and exendin to determine the expression of GLP-1R, AKT, p-AKT, and p-S6 via western blotting.

KEY FINDINGS

GDM exosomes induced remarkable oral glucose intolerance, hyperinsulinemia, and metabolic inflammation in pregnant mice. The islets of GDM offspring exhibited high insulin, glucagon, PC1/3, PDX1, and p-S6 expression at and after birth, and activation of the local GLP-1/GLP-1R axis. The functional maturation of normal-offspring islets did not commence until after birth, while it was activated prior to birth in GDM offspring, seriously disrupting the whole process. GDM exosomes activated the GLP-1/GLP-1R axis between α and β cells, and stimulated functional maturation of β cells via the Akt-mTORC1-pS6 pathway.

SIGNIFICANCE

These findings provide preliminary insights into the mechanisms underlying the high incidence of diabetes in the offspring of mothers with GDM.

Estimating insulin sensitivity and β-cell function from the oral glucose tolerance test: validation of a new insulin sensitivity and secretion (ISS) model

Efficient and accurate methods to estimate insulin sensitivity (SI) and β-cell function (BCF) are of great importance for studying the pathogenesis and treatment effectiveness of type 2 diabetes (T2D). Existing methods range in sensitivity, input data, and technical requirements. Oral glucose tolerance tests (OGTTs) are preferred because they are simpler and more physiological than intravenous methods. However, current analytical methods for OGTT-derived SI and BCF also range in complexity; the oral minimal models require mathematical expertise for deconvolution and fitting differential equations, and simple algebraic surrogate indices (e.g., Matsuda index, insulinogenic index) may produce unphysiological values. We developed a new insulin secretion and sensitivity (ISS) model for clinical research that provides precise and accurate estimates of SI and BCF from a standard OGTT, focusing on effectiveness, ease of implementation, and pragmatism. This model was developed by fitting a pair of differential equations to glucose and insulin without need of deconvolution or C-peptide data. This model is derived from a published model for longitudinal simulation of T2D progression that represents glucose-insulin homeostasis, including postchallenge suppression of hepatic glucose production and first- and second-phase insulin secretion. The ISS model was evaluated in three diverse cohorts across the lifespan. The new model had a strong correlation with gold-standard estimates from intravenous glucose tolerance tests and insulin clamps. The ISS model has broad applicability among diverse populations because it balances performance, fidelity, and complexity to provide a reliable phenotype of T2D risk.NEW & NOTEWORTHY The pathogenesis of type 2 diabetes (T2D) is determined by a balance between insulin sensitivity (SI) and β-cell function (BCF), which can be determined by gold standard direct measurements or estimated by fitting differential equation models to oral glucose tolerance tests (OGTTs). We propose and validate a new differential equation model that is simpler to use than current models and requires less data while maintaining good correlation and agreement with gold standards. Matlab and Python code is freely available.

A programmable protease-based protein secretion platform for therapeutic applications

Cell-based therapies represent potent enabling technologies in biomedical science. However, current genetic control systems for engineered-cell therapies are predominantly based on the transcription or translation of therapeutic outputs. Here we report a protease-based rapid protein secretion system (PASS) that regulates the secretion of pretranslated proteins retained in the endoplasmic reticulum (ER) owing to an ER-retrieval signal. Upon cleavage by inducible proteases, these proteins are secreted. Three PASS variants (chemPASS, antigenPASS and optoPASS) are developed. With chemPASS, we demonstrate the reversal of hyperglycemia in diabetic mice within minutes via drug-induced insulin secretion. AntigenPASS-equipped cells recognize the tumor antigen and secrete granzyme B and perforin, inducing targeted cell apoptosis. Finally, results from mouse models of diabetes, hypertension and inflammatory pain demonstrate light-induced, optoPASS-mediated therapeutic peptide secretion within minutes, conferring anticipated therapeutic benefits. PASS is a flexible platform for rapid delivery of therapeutic proteins that can facilitate the development and adoption of cell-based precision therapies.

Association between protein undernutrition and diabetes: Molecular implications in the reduction of insulin secretion

Undernutrition is still a recurring nutritional problem in low and middle-income countries. It is directly associated with the social and economic sphere, but it can also negatively impact the health of the population. In this sense, it is believed that undernourished individuals may be more susceptible to the development of non-communicable diseases, such as diabetes mellitus, throughout life. This hypothesis was postulated and confirmed until today by several studies that demonstrate that experimental models submitted to protein undernutrition present alterations in glycemic homeostasis linked, in part, to the reduction of insulin secretion. Therefore, understanding the changes that lead to a reduction in the secretion of this hormone is essential to prevent the development of diabetes in undernourished individuals. This narrative review aims to describe the main molecular changes already characterized in pancreatic β cells that will contribute to the reduction of insulin secretion in protein undernutrition. So, it will provide new perspectives and targets for postulation and action of therapeutic strategies to improve glycemic homeostasis during this nutritional deficiency.

mTORC1 in energy expenditure: consequences for obesity

In eukaryotic cells, the mammalian target of rapamycin complex 1 (sometimes referred to as the mechanistic target of rapamycin complex 1; mTORC1) orchestrates cellular metabolism in response to environmental energy availability. As a result, at the organismal level, mTORC1 signalling regulates the intake, storage and use of energy by acting as a hub for the actions of nutrients and hormones, such as leptin and insulin, in different cell types. It is therefore unsurprising that deregulated mTORC1 signalling is associated with obesity. Strategies that increase energy expenditure offer therapeutic promise for the treatment of obesity. Here we review current evidence illustrating the critical role of mTORC1 signalling in the regulation of energy expenditure and adaptive thermogenesis through its various effects in neuronal circuits, adipose tissue and skeletal muscle. Understanding how mTORC1 signalling in one organ and cell type affects responses in other organs and cell types could be key to developing better, safer treatments targeting this pathway in obesity.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly stimulates hepatic glucose production (HGP), but for reasons that are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course of glucagon-mediated molecular events and their relevance to metabolic flux in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a sixfold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group, glucose remained at basal, whereas in the other, glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) and largely sustained increase in hepatic cAMP over 4 h, a continued elevation in glucose-6-phosphate (G6P), and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis increased rapidly, peaking at 15 min due to activation of the cAMP/PKA pathway, then slowly returned to baseline over the next 3 h in line with allosteric inhibition by glucose and G6P. Glucagon's stimulatory effect on HGP was sustained relative to the hyperglycemic control group due to continued PKA activation. Hepatic gluconeogenic flux did not increase due to the lack of glucagon's effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, as well as downregulation of genes involved in extracellular matrix assembly and development.NEW & NOTEWORTHY Glucagon rapidly stimulates hepatic glucose production, but these effects are transient. This study links the molecular and metabolic flux changes that occur in the liver over time in response to a rise in glucagon, demonstrating the strength of the dog as a translational model to couple findings in small animals and humans. In addition, this study clarifies why the rapid effects of glucagon on liver glycogen metabolism are not sustained.

Downregulation of Sirt3 contributes to β-cell dedifferentiation via FoxO1 in type 2 diabetic mellitus

AIMS

FoxO1 is an important factor in the β-cell differentiation in type 2 diabetes mellitus (T2DM). Sirt3 is found to be involved in FoxO1 function. This study investigated the role of Sirt3 in the β-cell dedifferentiation and its mechanism.

METHODS

Twelve-week-old db/db mice and INS1 cells transfected with Sirt3-specific short hairpin RNA (shSirt3) were used to evaluate the dedifferentiation of β-cell. Insulin levels were measured by enzyme linked immunosorbent assay. The proteins of Sirt3, T-FoxO1, Ac-FoxO1 and differentiation indexes such as NGN3, OCT4, MAFA were determined by western blot or immunofluorescence staining. The combination of Sirt3 and FoxO1 was determined by the co-immunoprecipitation assay. The transcriptional activity of FoxO1 was detected by dual luciferase reporter assay.

RESULTS

Both the in vivo and in vitro results showed that Sirt3 was decreased along with β-cell dedifferentiation and decreased function of insulin secretion under high glucose conditions. When Sirt3 was knocked down in INS1 cells, increased β-cell dedifferentiation and lowered insulin secretion were observed. This effect was closely related to the amount loss and the decreased deacetylation of FoxO1, which resulted in a reduction in transcriptional activity.

CONCLUSION

Downregulation of Sirt3 contributes to β-cell dedifferentiation in high glucose via FoxO1. Intervention of Sirt3 may be an effective approach to prevent β-cell failure in T2DM.

Neddylation of insulin receptor substrate acts as a bona fide regulator of insulin signaling and its implications for cancer cell migration

Irregularities in insulin signaling have significantly increased the risk of various cancers, yet the precise underlying mechanisms remain unclear. Within our study, we observed that inhibiting neddylation enhances cancer cell migration across different cancer types by activating both insulin receptor substrates 1 and 2 (IRS1 and IRS2), along with the PI3K/AKT signaling pathway. Notably, in the context of high-grade serous carcinoma (HGSC) patients, whether they had type 2 diabetes mellitus or not, IRS1 and IRS2 displayed a parallel relationship with each other while exhibiting an inverse relationship with NEDD8. We also identified C-CBL as an E3 ligase responsible for neddylating IRS1 and IRS2, with clinical evidence further confirming a reciprocal relationship between C-CBL and pAKT, thereby reinforcing the tumor suppressive role of C-CBL. Altogether, these findings suggest that neddylation genuinely participates in IRS1 and IRS2-dependent insulin signaling, effectively suppressing cancer cell migration. Thus, caution is advised when considering neddylation inhibitors as a treatment option for cancer patients, particularly those presenting with insulin signaling dysregulations linked to conditions like obesity-related type 2 diabetes or hyperinsulinemia.

Mechanism of muscle atrophy in a normal-weight rat model of type 2 diabetes established by using a soft-pellet diet

Dietary factors such as food texture affect feeding behavior and energy metabolism, potentially causing obesity and type 2 diabetes. We previously found that rats fed soft pellets (SPs) were neither hyperphagic nor overweight but demonstrated glucose intolerance, insulin resistance, and hyperplasia of pancreatic β-cells. In the present study, we investigated the mechanism of muscle atrophy in rats that had been fed SPs on a 3-h time-restricted feeding schedule for 24 weeks. As expected, the SP rats were normal weight; however, they developed insulin resistance, glucose intolerance, and fat accumulation. In addition, skeletal muscles of SP rats were histologically atrophic and demonstrated disrupted insulin signaling. Furthermore, we learned that the muscle atrophy of the SP rats developed via the IL-6-STAT3-SOCS3 and ubiquitin-proteasome pathways. Our data show that the dietary habit of consuming soft foods can lead to not only glucose intolerance or insulin resistance but also muscle atrophy.