Rac1-NADPH oxidase signaling promotes CD36 activation under glucotoxic conditions in pancreatic beta cells

CD36 inhibition corrects lipid-FetuinA mediated insulin secretory defects by preventing intracellular lipid accumulation and inflammation in the pancreatic beta cells

CD36 is a multifunctional protein involved in long chain fatty acid uptake and immune modulation in different cells. Recently it was reported that increased expression of CD36 is evident in the islets of diabetic obese individuals. In this present study we investigated the role of CD36 in regulating intracellular lipid accumulation and inflammation in beta cells and its implication on secretory dysfunction. Additionally, we have elucidated the potential role of fetuinA, a circulatory glycoprotein and an endogenous ligand of TLR4, for aggravating lipid accumulation and insulin secretory defects in beta cells. MIN6 mouse insulinoma cells when incubated with palmitate and fetuinA together showed activation of TLR4-NFkB inflammatory cascade and increased uptake of palmitate, which was rescued by CD36 functional inhibition or knockdown. Moreover, glucose stimulated insulin secretion was restored with consequent downregulation of IL-1β secretion. TLR4 inhibition also decreased intracellular lipid content with a reduction of CD36, suggesting functional crosstalk between them. At physiological level, excess fetuinA in the islet milieu of HFD fed C57BL/6J mice or exogenous fetuinA administration (i.p.) promoted lipid accumulation in the islets resulting in decreased insulin secretion with increased CD36 expression. Interestingly, CD36 inhibition in HFD mice with a pharmacological inhibitor Salvianolic acid B attenuated inflammation, reduced intracellular lipid accumulation in beta cells and restored insulin secretory function. Therefore, our results suggest that inhibition of CD36 protects beta cells from the derogatory effects of lipid and fetuinA and restores secretory function and can be considered as a therapeutic target for obesity mediated beta cell dysfunction.

Overcoming β-Cell Dysfunction in Type 2 Diabetes Mellitus: CD36 Inhibition and Antioxidant System

Type 2 diabetes mellitus (T2DM) is marked by chronic hyperglycemia, gradually worsening β-cell failure, and insulin resistance. Glucotoxicity and oxidative stress cause β-cell failure by increasing reactive oxygen species (ROS) production, impairing insulin secretion, and disrupting transcription factors such as pancreatic and duodenal homeobox 1 (PDX-1) and musculoaponeurotic fibrosarcoma oncogene family A (MafA). Cluster determinant 36 (CD36), an essential glycoprotein responsible for fatty acid uptake, exacerbates oxidative stress and induces the apoptosis of β-cells under hyperglycemic conditions through pathways involving ceramide, thioredoxin-interacting protein (TXNIP), and Rac1-nicotinamide adenine dinucleotide phosphate oxidase (NOX)-mediated redoxosome formation. Targeting CD36 pathways has emerged as a promising therapeutic strategy. Oral hypoglycemic agents, such as metformin, teneligliptin, and pioglitazone, have shown protective effects on β-cells by enhancing antioxidant defenses. These agents reduce glucotoxicity via mechanisms such as suppressing CD36 expression and stabilizing mitochondrial function. Additionally, novel insights into the glutathione antioxidant system and its role in β-cell survival underscore its therapeutic potential. This review focuses on the key contribution of oxidative stress and CD36 to β-cell impairment, the therapeutic promise of antioxidants, and the need for further research to apply these findings in clinical practice. Promising strategies targeting these mechanisms may help preserve β-cell function and slow T2DM progression.

Glucose metabolism partially regulates β-cell function through epigenomic changes

The β-cell relies predominantly on glucose utilization to generate adenosine triphosphate, which is crucial for both cell viability and insulin secretion. The β-cell has evolved remarkable metabolic flexibility to productively respond to shifts in environmental conditions and changes in glucose availability. Although these adaptive responses are important for maintaining optimal cellular function, there is emerging evidence that the resulting changes in cellular metabolites can impact the epigenome, causing transient and lasting alterations in gene expression. This review explores the intricate interplay between metabolism and the epigenome, providing valuable insights into the molecular mechanisms leading to β-cell dysfunction in diabetes. Understanding these mechanisms will be critical for developing targeted therapeutic strategies to preserve and enhance β-cell function, offering potential avenues for interventions to improve glycemic control in individuals with diabetes.

Implication of Rac1 GTPase in molecular and cellular mitochondrial functions

Rac1 is a member of the Rho GTPase family which plays major roles in cell mobility, polarity and migration, as a fundamental regulator of actin cytoskeleton. Signal transduction by Rac1 occurs through interaction with multiple effector proteins, and its activity is regulated by guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs). The small protein is mainly anchored to the inner side of the plasma membrane but it can be found in endocellular compartments, notably endosomes and cell nuclei. The protein localizes also into mitochondria where it contributes to the regulation of mitochondrial dynamics, including both mitobiogenesis and mitophagy, in addition to signaling processes via different protein partners, such as the proapoptotic protein Bcl-2 and chaperone sigma-1 receptor (σ-1R). The mitochondrial form of Rac1 (mtRac1) has been understudied thus far, but it is as essential as the nuclear or plasma membrane forms, via its implication in regulation of oxidative stress and DNA damages. Rac1 is subject to diverse post-translational modifications, notably to a geranylgeranylation which contributes importantly to its mitochondrial import and its anchorage to mitochondrial membranes. In addition, Rac1 contributes to the mitochondrial translocation of other proteins, such as p53. The mitochondrial localization and functions of Rac1 are discussed here, notably in the context of human diseases such as cancers. Inhibitors of Rac1 have been identified (NSC-23766, EHT-1864) and some are being developed for the treatment of cancer (MBQ-167) or central nervous system diseases (JK-50561). Their effects on mtRac1 warrant further investigations. An overview of mtRac1 is provided here.

Functional Conservation of the Small GTPase Rho5/Rac1-A Tale of Yeast and Men

Small GTPases are molecular switches that participate in many essential cellular processes. Amongst them, human Rac1 was first described for its role in regulating actin cytoskeleton dynamics and cell migration, with a close relation to carcinogenesis. More recently, the role of Rac1 in regulating the production of reactive oxygen species (ROS), both as a subunit of NADPH oxidase complexes and through its association with mitochondrial functions, has drawn attention. Malfunctions in this context affect cellular plasticity and apoptosis, related to neurodegenerative diseases and diabetes. Some of these features of Rac1 are conserved in its yeast homologue Rho5. Here, we review the structural and functional similarities and differences between these two evolutionary distant proteins and propose yeast as a useful model and a device for high-throughput screens for specific drugs.

Oxidative stress signaling in the pathogenesis of diabetic cardiomyopathy and the potential therapeutic role of antioxidant naringenin

Diabetes mellitus (DM) is one of the most prevalent metabolic disorders that poses a global threat to human health. It can lead to complications in multiple organs and tissues, owing to its wide-ranging impact on the human body. Diabetic cardiomyopathy (DCM) is a specific cardiac manifestation of DM, which is characterized by heart failure in the absence of coronary heart disease, hypertension and valvular heart disease. Given that oxidative stress is a key factor in the pathogenesis of DCM, intervening to mitigate oxidative stress may serve as a therapeutic strategy for managing DCM. Naringenin is a natural product with anti-oxidative stress properties that can suppress oxidative damage by regulating various oxidative stress signaling pathways. In this review, we address the relationship between oxidative stress and its primary signaling pathways implicated in DCM, and explores the therapeutic potential of naringenin in DCM.

The role of candidate transport proteins in β-cell long-chain fatty acid uptake: Where are we now?

Type 2 diabetes (T2D) in humans is typically preceded by elevated levels of circulatory long-chain free fatty acids (LC-FFA). These excess LC-FFA are widely thought to be taken up by pancreatic β-cells, contributing to their dysfunction and death during the development of T2D; a process that has been termed lipotoxicity. Depending on their degree of saturation and carbon chain length, LC-FFA can exert different effects on pancreatic β-cells viability and function in vitro. Long-chain saturated fatty acids (LC-SFA) are thought to be toxic, whereas monounsaturated fatty acids are not and may even offer protection against the toxic effects of LC-SFAs. However, the mechanism of LC-FFA uptake into pancreatic β-cells is poorly understood, partly because it has been an understudied area of research. Determining how LC-FFA are taken up into β-cells is crucial for later formulation of therapies to prevent potential cellular overload of LC-FFA, thereby slowing the onset of T2D. In this work, we detail more than 40 years of literature investigating the role of membrane-associated transport proteins in LC-FFA uptake. By focussing on what is known in other cell types, we highlight where we can extrapolate our current understanding of protein-mediated transport to β-cells and uncover where further understanding is required.

Perfluorooctane sulfonate-induced oxidative stress contributes to pancreatic β-cell apoptosis by inhibiting cyclic adenosine monophosphate pathway: Prevention by pentoxifylline

Endocrine-disrupting chemical perfluorooctane sulfonate (PFOS) acute exposure stimulates insulin secretion from pancreatic β-cells. However, chronic exposure to PFOS on pancreatic β-cells, its role in insulin secretion, and the underlying mechanisms have not been studied. We used rat insulinoma INS-1 and human 1.1b4 islet cells to investigate the chronic effects of PFOS on glucose-stimulated insulin secretion and toxicity implicated in the downregulation of β-cell functionality. Chronic exposure of INS-1 cells or human pancreatic 1.1b4 β-cells to PFOS stimulated the small G-protein RAC1-guanosine triphosphate-dependent nicotinamide adenine dinucleotide phosphate oxidase (NOX2/gp91phox) subunit expression and activation. Upregulated NOX2/gp91phox activation led to elevated reactive oxygen species (ROS) production with a decrease in the cyclic adenosine monophosphate/protein kinase A (cAMP/PKA) pathway in both cell types. Inhibition of cAMP/PKA signaling induces β-cell mitochondrial dysfunction and endoplasmic stress via the loss of PDX1-SERCA2B and glucose-stimulated insulin release. Inhibiting RAC1-NOX2/gp91phox activation or elevating cAMP by pentoxifylline, a Food and Drug Administration-approved phosphodiesterase inhibitor, significantly reduced PFOS-induced ROS production and restored insulin secretory function of pancreatic β-cells. Enhanced secretory function in pentoxifylline-treated cells was associated with increased stability of PDX1-SERCA2B protein levels. Intriguingly, inhibition of cAMP/PKA signaling impaired pentoxifylline-induced insulin secretion caused by the activation of ROS production and mitochondrial dysfunction. Overall, our findings show that PFOS has a new and first-ever direct chronic effect on pancreatic β-cell failure through increased RAC1-NOX2/gp91phox activation and pentoxifylline-induced cAMP/PKA signaling, which inhibits PFOS-mediated mitochondrial dysfunction.

Association between RAC1 gene variation, redox homeostasis and type 2 diabetes mellitus

BACKGROUND

Increased production of reactive oxygen species (ROS) and oxidative stress are known to play a key role in the pathogenesis of type 2 diabetes (T2D); however, the relationship between genes encoding a multi-subunit ROS-generated enzyme NADPH oxidase and disease susceptibility remains unexplored.

AIMS

The present pilot study investigated whether single-nucleotide polymorphisms (SNP) at the RAC1 gene (Rac family small GTPase 1), a molecular switcher of NADPH oxidase, are associated with the risk of T2D, glucose metabolism and redox homeostasis.

MATERIALS & METHODS

DNA samples from 3206 unrelated Russian subjects (1579 T2D patients and 1627 controls) were genotyped for six common SNPs rs4724800, rs7784465, rs10951982, rs10238136, rs836478 and rs9374 of RAC1 using the MassArray-4 system.

RESULTS

SNP rs7784465 was associated with an increased risk of T2D (p = .0003), and significant differences in the RAC1 haplotypes occurred between the cases and controls (p = .005). Seventeen combinations of RAC1 genotypes showed significant associations with T2D risk (FDR <0.05). Associations of RAC1 polymorphisms with T2D were modified by environmental factors such as sedentary lifestyle, psychological stresses, a dietary deficit of fresh fruits/vegetables and increased carbohydrate intake. RAC1 polymorphisms were associated with biochemical parameters in diabetics: rs7784465 (p = .015) and rs836478 (p = .028) with increased glycated haemoglobin, rs836478 (p = .005) with increased fasting blood glucose, oxidized glutathione (p = .012) and uric acid (p = .034). Haplotype rs4724800A-rs7784465C-rs10951982G-rs10238136A-rs836478C-rs9374G was strongly associated with increased levels of hydrogen peroxide (p < .0001).

CONCLUSION

Thus, polymorphisms in the RAC1 gene represent novel genetic markers of type 2 diabetes, and their link with glucose metabolism and disease pathogenesis is associated with the changes in redox homeostasis.

Daidzein attenuates urinary bladder dysfunction in streptozotocin-induced diabetes in rats by NOX-4 and RAC-1 inhibition

Reactive oxygen species via NADPH oxidase (NOX) activation are involved in the pathogenesis of many disease conditions such as diabetes and its complications. In the present study, we have examined the effect of daidzein in the management of diabetic cystopathy. Diabetes was induced in male Sprague Dawley rats via intraperitoneal injection of streptozotocin (STZ) at a dose of 55 mg/kg. After 6 weeks of diabetes induction, animals were treated with daidzein orally at a dose of 25, 50, and 100 mg/kg for 4 weeks. Diabetic animals showed increase (p < 0.001) in bladder capacity (4.32 ± 0.43 mL) and residual volume (2.53 ± 0.19 mL) when compared with normal control animals (2.10 ± 0.40 mL and 0.51 ± 0.12 mL res). Treatment with daidzein at dose of 50 and 100 mg/kg significantly reduced the elevated bladder capacity (2.91 ± 0.11 mL, p < 0.01 and 2.65 ± 1.13 mL, p < 0.001) and residual volume (1.40 ± 0.15 mL, p < 0.001 and 1.15 ± 0.05 mL, p < 0.001). Daidzein-treated animals also showed improvement in voiding efficiency. Elevated threshold and baseline pressure were also found to be reduced in diabetic animals after 4 weeks of daidzein treatment. Daidzein treatment also prevented the loss of antioxidant enzymes in the urinary bladder and also reduced the expression of NOX-4 and RAC-1 in the bladder. From the results, it can be concluded that daidzein showed a beneficial effect on urinary bladder dysfunction in diabetic animals.

Lipotoxicity and β-Cell Failure in Type 2 Diabetes: Oxidative Stress Linked to NADPH Oxidase and ER Stress

A high caloric intake, rich in saturated fats, greatly contributes to the development of obesity, which is the leading risk factor for type 2 diabetes (T2D). A persistent caloric surplus increases plasma levels of fatty acids (FAs), especially saturated ones, which were shown to negatively impact pancreatic β-cell function and survival in a process called lipotoxicity. Lipotoxicity in β-cells activates different stress pathways, culminating in β-cells dysfunction and death. Among all stresses, endoplasmic reticulum (ER) stress and oxidative stress have been shown to be strongly correlated. One main source of oxidative stress in pancreatic β-cells appears to be the reactive oxygen species producer NADPH oxidase (NOX) enzyme, which has a role in the glucose-stimulated insulin secretion and in the β-cell demise during both T1 and T2D. In this review, we focus on the acute and chronic effects of FAs and the lipotoxicity-induced β-cell failure during T2D development, with special emphasis on the oxidative stress induced by NOX, the ER stress, and the crosstalk between NOX and ER stress.

Investigating the association of CD36 gene polymorphisms (rs1761667 and rs1527483) with T2DM and dyslipidemia: Statistical analysis, machine learning based prediction, and meta-analysis

CD36 (cluster of differentiation 36) is a membrane protein involved in lipid metabolism and has been linked to pathological conditions associated with metabolic disorders, such as diabetes and dyslipidemia. A case-control study was conducted and included 177 patients with type-2 diabetes mellitus (T2DM) and 173 control subjects to study the involvement of CD36 gene rs1761667 (G>A) and rs1527483 (C>T) polymorphisms in the pathogenesis of T2DM and dyslipidemia among Jordanian population. Lipid profile, blood sugar, gender and age were measured and recorded. Also, genotyping analysis for both polymorphisms was performed. Following statistical analysis, 10 different neural networks and machine learning (ML) tools were used to predict subjects with diabetes or dyslipidemia. Towards further understanding of the role of CD36 protein and gene in T2DM and dyslipidemia, a protein-protein interaction network and meta-analysis were carried out. For both polymorphisms, the genotypic frequencies were not significantly different between the two groups (p > 0.05). On the other hand, some ML tools like multilayer perceptron gave high prediction accuracy (≥ 0.75) and Cohen's kappa (κ) (≥ 0.5). Interestingly, in K-star tool, the accuracy and Cohen's κ values were enhanced by including the genotyping results as inputs (0.73 and 0.46, respectively, compared to 0.67 and 0.34 without including them). This study confirmed, for the first time, that there is no association between CD36 polymorphisms and T2DM or dyslipidemia among Jordanian population. Prediction of T2DM and dyslipidemia, using these extensive ML tools and based on such input data, is a promising approach for developing diagnostic and prognostic prediction models for a wide spectrum of diseases, especially based on large medical databases.

CD36 Signal Transduction in Metabolic Diseases: Novel Insights and Therapeutic Targeting

The cluster of differentiation 36 (CD36) is a scavenger receptor present on various types of cells and has multiple biological functions that may be important in inflammation and in the pathogenesis of metabolic diseases, including diabetes. Here, we consider recent insights into how the CD36 response becomes deregulated under metabolic conditions, as well as the therapeutic benefits of CD36 inhibition, which may provide clues for developing strategies aimed at the treatment or prevention of diabetes associated with metabolic diseases. To facilitate this process further, it is important to pinpoint regulatory mechanisms that are relevant under physiological and pathological conditions. In particular, understanding the mechanisms involved in dictating specific CD36 downstream cellular outcomes will aid in the discovery of potent compounds that target specific CD36 downstream signaling cascades.

Targeting Small GTPases and Their Prenylation in Diabetes Mellitus

A fundamental role of pancreatic β-cells to maintain proper blood glucose level is controlled by the Ras superfamily of small GTPases that undergo post-translational modifications, including prenylation. This covalent attachment with either a farnesyl or a geranylgeranyl group controls their localization, activity, and protein–protein interactions. Small GTPases are critical in maintaining glucose homeostasis acting in the pancreas and metabolically active tissues such as skeletal muscles, liver, or adipocytes. Hyperglycemia-induced upregulation of small GTPases suggests that inhibition of these pathways deserves to be considered as a potential therapeutic approach in treating T2D. This Perspective presents how inhibition of various points in the mevalonate pathway might affect protein prenylation and functioning of diabetes-affected tissues and contribute to chronic inflammation involved in diabetes mellitus (T2D) development. We also demonstrate the currently available molecular tools to decipher the mechanisms linking the mevalonate pathway’s enzymes and GTPases with diabetes.

NADPH Oxidase (NOX) Targeting in Diabetes: A Special Emphasis on Pancreatic β-Cell Dysfunction

In type 2 diabetes, metabolic stress has a negative impact on pancreatic β-cell function and survival (T2D). Although the pathogenesis of metabolic stress is complex, an imbalance in redox homeostasis causes abnormal tissue damage and β-cell death due to low endogenous antioxidant expression levels in β-cells. Under diabetogenic conditions, the susceptibility of β-cells to oxidative damage by NADPH oxidase has been related to contributing to β-cell dysfunction. Here, we consider recent insights into how the redox response becomes deregulated under diabetic conditions by NADPH oxidase, as well as the therapeutic benefits of NOX inhibitors, which may provide clues for understanding the pathomechanisms and developing strategies aimed at the treatment or prevention of metabolic stress associated with β-cell failure.

The Imbalance of MMP-2/TIMP-2 and MMP-9/TIMP-1 Contributes to Collagen Deposition Disorder in Diabetic Non-Injured Skin

The importance of the early diagnosis and treatment of diabetes and its cutaneous complications has become increasingly recognized. When diabetic non-injured skin was stained with Masson's trichrome, its dermal collagen was found to be disordered, its density was variable, and it was dispersed or arranged in vague fascicles. The collagen type I sequencing results of RNA sequencing-based transcriptome analysis of three primary human skin cell types-dermal fibroblasts, dermal microvascular endothelial cells, and epidermal keratinocytes-under high glucose were analyzed. The results showed that both COL1A1 and COL1A2 mRNA expressions were reduced in human dermal fibroblasts (HDFs). The ratio of matrix metalloproteinase (MMP)-2/tissue inhibitors of metalloproteinase (TIMP)-2 and MMP-9/TIMP-1 in HDFs increased when treated with high glucose. By inhibiting MMP-2 and MMP-9 with SB-3CT, collagen deposition disorder of the skin in streptozotocin-induced diabetes mice was alleviated. The imbalance of MMP2/TIMP2 and MMP9/TIMP1 contributes to the non-injured skin disorder of collagen deposition in diabetes, suggesting a possibility for early treatment of diabetes skin complications.

High glucose-induced PRDX3 acetylation contributes to glucotoxicity in pancreatic β-cells: Prevention by Teneligliptin

Chronic hyperglycemia has deleterious effects on pancreatic β-cell function and survival in type 2 diabetes (T2D) due to the low expression level of endogenous antioxidants in the β-cells. Peroxiredoxin-3 (PRDX3) is a mitochondria specific H202 scavenger and protects the cell from mitochondrial damage. However, nothing is known about how glucotoxicity influences PRDX3 function in the pancreatic beta cells. Exposure of rat insulinoma INS-1 cells and human beta cells (1.1B4) to high glucose conditions (30mM) stimulated acetylation of PRDX3 which facilitates its hyper-oxidation causing mitochondrial dysfunction by SIRT1 degradation. SIRT1 deficiency induces beta cell apoptosis via NOX-JNK-p66Shc signalosome activation. Herein we investigated the direct effect of Teneligliptin, a newer DPP-4 inhibitor on beta-cell function and survival in response to high glucose conditions. Teneligliptin treatment enhances SIRT1 protein levels and activity by USP22, an ubiquitin specific peptidase. Activated SIRT1 prevents high glucose-induced PRDX3 acetylation by SIRT3 resulted in inhibition of PRDX3 hyper-oxidation thereby strengthening the mitochondrial antioxidant defense. Notably, we identify PRDX3 as a novel SIRT3 target and show their physical interaction. Intriguingly, inhibition of SIRT1 activity by EX-527 or SIRT1 siRNA knockdown exacerbated the SIRT3 mediated PRDX3 deacetylation which leads to peroxiredoxin-3 hyper-oxidation and beta-cell apoptosis by the activation of NOX-JNK-p66Shc signalosome. Collectively, our results unveil a novel and first direct effect of high glucose on PRDX3 acetylation on beta-cell dysfunction by impaired antioxidant defense and SIRT1 mediated SIRT3-PRDX3 activation by Teneligliptin suppresses high glucose-mediated mitochondrial dysfunction.

The Multifunctionality of CD36 in Diabetes Mellitus and Its Complications-Update in Pathogenesis, Treatment and Monitoring

CD36 is a multiligand receptor contributing to glucose and lipid metabolism, immune response, inflammation, thrombosis, and fibrosis. A wide range of tissue expression includes cells sensitive to metabolic abnormalities associated with metabolic syndrome and diabetes mellitus (DM), such as monocytes and macrophages, epithelial cells, adipocytes, hepatocytes, skeletal and cardiac myocytes, pancreatic β-cells, kidney glomeruli and tubules cells, pericytes and pigment epithelium cells of the retina, and Schwann cells. These features make CD36 an important component of the pathogenesis of DM and its complications, but also a promising target in the treatment of these disorders. The detrimental effects of CD36 signaling are mediated by the uptake of fatty acids and modified lipoproteins, deposition of lipids and their lipotoxicity, alterations in insulin response and the utilization of energy substrates, oxidative stress, inflammation, apoptosis, and fibrosis leading to the progressive, often irreversible organ dysfunction. This review summarizes the extensive knowledge of the contribution of CD36 to DM and its complications, including nephropathy, retinopathy, peripheral neuropathy, and cardiomyopathy.

The Role of CD36 in Type 2 Diabetes Mellitus: β-Cell Dysfunction and Beyond

Impaired β-cell function is the key pathophysiology of type 2 diabetes mellitus, and chronic exposure of nutrient excess could lead to this tragedy. For preserving β-cell function, it is essential to understand the cause and mechanisms about the progression of β-cells failure. Glucotoxicity, lipotoxicity, and glucolipotoxicity have been suggested to be a major cause of β-cell dysfunction for decades, but not yet fully understood. Fatty acid translocase cluster determinant 36 (CD36), which is part of the free fatty acid (FFA) transporter system, has been identified in several tissues such as muscle, liver, and insulin-producing cells. Several studies have reported that induction of CD36 increases uptake of FFA in several cells, suggesting the functional interplay between glucose and FFA in terms of insulin secretion and oxidative metabolism. However, we do not currently know the regulating mechanism and physiological role of CD36 on glucolipotoxicity in pancreatic β-cells. Also, the downstream and upstream targets of CD36 related signaling have not been defined. In the present review, we will focus on the expression and function of CD36 related signaling in the pancreatic β-cells in response to hyperglycemia and hyperlipidemia (ceramide) along with the clinical studies on the association between CD36 and metabolic disorders.

Plasma CD36 and Incident Diabetes: A Case-Cohort Study in Danish Men and Women

BACKGROUND

Membrane CD36 is a fatty acid transporter implicated in the pathogenesis of metabolic disease. We aimed to evaluate the association between plasma CD36 levels and diabetes risk and to examine if the association was independent of adiposity among Danish population.

METHODS

We conducted a case-cohort study nested within the Danish Diet, Cancer and Health study among participants free of cardiovascular disease, diabetes and cancer and with blood samples and anthropometric measurements (height, weight, waist circumference, and body fat percentage) at baseline (1993 to 1997). CD36 levels were measured in 647 incident diabetes cases that occurred before December 2011 and a total of 3,515 case-cohort participants (236 cases overlap).

RESULTS

Higher plasma CD36 levels were associated with higher diabetes risk after adjusting for age, sex and other lifestyle factors. The hazard ratio (HR) comparing high versus low tertile of plasma CD36 levels was 1.36 (95% confidence interval [CI], 1.00 to 1.86). However, the association lost its significance after further adjustment for different adiposity indices such as body mass index (HR, 1.23; 95% CI, 0.87 to 1.73), waist circumference (HR, 1.21; 95% CI, 0.88 to 1.68) or body fat percentage (HR, 1.20; 95% CI, 0.86 to 1.66). Moreover, raised plasma CD36 levels were moderately associated with diabetes risk among lean participants, but the association was not present among overweight/obese individuals.

CONCLUSION

Higher plasma CD36 levels were associated with higher diabetes risk, but the association was not independent of adiposity. In this Danish population, the association of CD36 with diabetes risk could be either mediated or confounded by adiposity.

In Vivo and In Vitro Models of Diabetes: A Focus on Pregnancy

Diabetes in pregnancy is associated with an increased risk of poor outcomes, both for the mother and her offspring. Although clinical and epidemiological studies are invaluable to assess these outcomes and the effectiveness of potential treatments, there are certain ethical and practical limitations to what can be assessed in human studies.Thus, both in vivo and in vitro models can aid us in the understanding of the mechanisms behind these complications and, in the long run, towards their prevention and treatment. This review summarizes the existing animal and cell models used to mimic diabetes, with a specific focus on the intrauterine environment. Summary of this review.

Weighted Gene Coexpression Network Analysis Identified MicroRNA Coexpression Modules and Related Pathways in Type 2 Diabetes Mellitus

Objective

Type 2 diabetes mellitus (T2DM) is a metabolic disease with high incidence, which has seriously affected human life and health. MicroRNA, a short-chain noncoding RNA, plays an important role in T2DM. Identification of meaningful microRNA modules and the role of microRNAs provide a basis for searching potential biomarkers of T2DM.

Materials and Methods

In this study, three newly diagnosed patients with T2DM and three controls were selected for Whole Peripheral Blood RNA Sequencing to establish a microRNA library. Weighted gene coexpression network analysis (WGCNA) was applied to construct coexpression modules and to detect the trait-related microRNA modules; then, KEGG enrichment analysis was performed to predict the biological function of the interest modules, and candidate hub microRNAs were screened out by the value of module membership (MM) and protein-protein interaction (PPI) network.

Result

Four microRNA modules (blue, brown, magenta, and turquoise) were highly associated with the T2DM; the number of miRNAs in these modules ranged from 41 to 469. The Fc gamma R-mediated phagocytosis pathway, Rap1 signaling pathway, MAPK signaling pathway, and Lysosome pathway were common pathways in three of the four modules. RPS27A, UBC, and RAC1 were the top three proteins in our study; their corresponding RNAs were miR-1271-5p, miR-130a-3p, miR-130b-3p, and miR-574-3p.

Conclusion

In summary, this study identified blood miRNAs in human T2DM using RNA sequencing. The findings may be the foundation for understanding the potential role of miRNAs in T2DM.

Contribution of Oxidative Stress and Impaired Biogenesis of Pancreatic β-Cells to Type 2 Diabetes

Significance: Type 2 diabetes development involves multiple changes in β-cells, related to the oxidative stress and impaired redox signaling, beginning frequently by sustained overfeeding due to the resulting lipotoxicity and glucotoxicity. Uncovering relationships among the dysregulated metabolism, impaired β-cell "well-being," biogenesis, or cross talk with peripheral insulin resistance is required for elucidation of type 2 diabetes etiology. Recent Advances: It has been recognized that the oxidative stress, lipotoxicity, and glucotoxicity cannot be separated from numerous other cell pathology events, such as the attempted compensation of β-cell for the increased insulin demand and dynamics of β-cell biogenesis and its "reversal" at dedifferentiation, that is, from the concomitantly decreasing islet β-cell mass (also due to transdifferentiation) and low-grade islet or systemic inflammation. Critical Issues: At prediabetes, the compensation responses of β-cells, attempting to delay the pathology progression-when exaggerated-set a new state, in which a self-checking redox signaling related to the expression of Ins gene expression is impaired. The resulting altered redox signaling, diminished insulin secretion responses to various secretagogues including glucose, may lead to excretion of cytokines or chemokines by β-cells or excretion of endosomes. They could substantiate putative stress signals to the periphery. Subsequent changes and lasting glucolipotoxicity promote islet inflammatory responses and further pathology spiral. Future Directions: Should bring an understanding of the β-cell self-checking and related redox signaling, including the putative stress signal to periphery. Strategies to cure or prevent type 2 diabetes could be based on the substitution of the "wrong" signal by the "correct" self-checking signal.

Myricetin prevents thapsigargin-induced CDK5-P66Shc signalosome mediated pancreatic β-cell dysfunction

Chronic endoplasmic reticulum (ER) stress has deleterious effects on pancreatic β-cell function and survival in type 2 diabetes (T2D). Cyclin-dependent kinase 5 (CDK5) plays a critical role in β-cell failure under diabetic milieu conditions. However, little information is available on CDK5's ability to impair the function of β-cells via a chemical ER stress inducer thapsigargin. Myricetin, a natural flavonoid, has therapeutic potential for the treatment of type 2 diabetes mellitus. Therefore, we examined the effect of CDK5 on thapsigargin-induced β-cell apoptosis, and explored the relationship between myricetin and CDK5. Exposure of beta cells with thapsigargin, induced a Src-mediated redox signaling (VAV2-Rac1-NOX) formation and CDK5 activation. Activated CDK5 induced antiapoptotic protein myeloid cell leukemia sequence 1 (Mcl-1) degradation which was associated with p66Shc serine 36 phosphorylation, causing beta cell apoptosis via mitochondrial dysfunction. Exposure of beta cells to myricetin resulted in an acute inhibition of Src-mediated redox signaling (VAV2-Rac1-NOX) formation and CDK5 activation. Myricetin inhibited CDK5 activation by directly binding to its ATP-binding pocket. Treatment with myricetin also enhanced the stability of Mcl-1 after thapsigargin treatment. Inhibition of CDK5 with myricetin or roscovitine, a CDK5 inhibitor attenuates thapsigargin induced p66Shc serine 36 phosphorylation and also reduced mitochondrial dysfunction by decreasing mitochondrial ROS and caspase-3 activation. In addition, myricetin was observed to enhance PDX-1 and insulin mRNA expression and potentiate glucose stimulated insulin secretion (GSIS). Taken together, these findings indicate that thapsigargin-induced early molecular events lead to CDK5-p66Shc signalosome contributes to thapsigargin-induced pancreatic β-cell dysfunction. Myricetin blocked thapsigargin induced CDK5-p66Shc signalosome formation and prevented pancreatic beta cell dysfunction. In this study, we demonstrated for the first time that thapsigargin initiated CDK5-p66Shc signalosome mediates the pancreatic beta cell dysfunction and myricetin protects the pancreatic beta cells through the inhibition of CDK5-p66Shc signalosome.

Quantitative proteomics reveals novel interaction partners of Rac1 in pancreatic β-cells: Evidence for increased interaction with Rac1 under hyperglycemic conditions

Rac1, a small G protein, regulates physiological insulin secretion from the pancreatic β-cell. Interestingly, Rac1 has also been implicated in the onset of metabolic dysfunction of the β-cell under the duress of hyperglycemia (HG). This study is aimed at the identification of interaction partners of Rac1 in β-cells under basal and HG conditions. Using co-immunoprecipitation and UPLC-ESI-MS/MS, we identified 324 Rac1 interaction partners in INS-1832/13 cells, which represent the largest Rac1 interactome to date. Furthermore, we identified 27 interaction partners that exhibited increased association with Rac1 in β-cells exposed to HG. Western blotting (INS-1832/13 cells, rat islets and human islets) and co-immunoprecipitation (INS-1832/13 cells) further validated the identity of these Rac1 interaction partners including regulators of GPCR-G protein-effector coupling in the islet. These data form the basis for future investigations on contributory roles of these Rac1-specific signaling pathways in islet β-cell function in health and diabetes.

CD36 dependent redoxosomes promotes ceramide-mediated pancreatic β-cell failure via p66Shc activation

Altered metabolism is implicated in the pathogenesis of beta-cell failure in type 2 diabetes (T2D). Plasma and tissue levels of ceramide species play positive roles in inflammatory and oxidative stress responses in T2D. However, oxidative targets and mechanisms underlying ceramide signaling are unclear. We investigated the role of CD36-dependent redoxosome (redox-active endosome), a membrane-based signaling agent, in ceramide-induced beta-cell dysfunction and failure. Exposure of beta cells to C2-ceramide (N-acetyl-sphingosine) induced a CD36-dependent non-receptor tyrosine kinase Src-mediated redoxosome (Vav2-Rac1-NOX) formation. Activated Rac1-GTP-NADPH oxidase complex induced c-Jun-N-terminal kinase (JNK) activation and nuclear factor (NF)-kB transcription, which was associated with thioredoxin-interacting protein (TXNIP) upregulation and thioredoxin activity suppression. Upregulated JNK expression induced p66Shc serine36 phosphorylation and peroxiredoxin-3 hyperoxidation, causing beta-cell apoptosis via mitochondrial dysfunction. CD36 inhibition by sulfo-N-succinimidyl oleate (SSO) or CD36 siRNA blocked C2-ceramide-induced redoxosome activation, thereby decreasing JNK-dependent p66Shc serine36 phosphorylation. CD36 inhibition downregulated TXNIP expression and promoted thioredoxin activity via enhanced thioredoxin reductase activity, which prevented peroxiredoxin-3 oxidation. CD36 inhibition potentiated glucose-stimulated insulin secretion and prevented beta-cell apoptosis. Our results reveal a new role of CD36 during early molecular events that lead to Src-mediated redoxosome activation, which contributes to ceramide-induced pancreatic beta-cell dysfunction and failure.

Reactive Oxygen Species and Oncoprotein Signaling-A Dangerous Liaison

SIGNIFICANCE

There is evidence to implicate reactive oxygen species (ROS) in tumorigenesis and its progression. This has been associated with the interplay between ROS and oncoproteins, resulting in enhanced cellular proliferation and survival. Recent Advances: To date, studies have investigated specific contributions of the crosstalk between ROS and signaling networks in cancer initiation and progression. These investigations have challenged the established dogma of ROS as agents of cell death by demonstrating a secondary function that fuels cell proliferation and survival. Studies have thus identified (onco)proteins (Bcl-2, STAT3/5, RAS, Rac1, and Myc) in manipulating ROS level as well as exploiting an altered redox environment to create a milieu conducive for cancer formation and progression.

CRITICAL ISSUES

Despite these advances, drug resistance and its association with an altered redox metabolism continue to pose a challenge at the mechanistic and clinical levels. Therefore, identifying specific signatures, altered protein expressions, and modifications as well as protein-protein interplay/function could not only enhance our understanding of the redox networks during cancer initiation and progression but will also provide novel targets for designing specific therapeutic strategies.

FUTURE DIRECTIONS

Not only a heightened realization is required to unravel various gene/protein networks associated with cancer formation and progression, particularly from the redox standpoint, but there is also a need for developing more sensitive tools for assessing cancer redox metabolism in clinical settings. This review attempts to summarize our current knowledge of the crosstalk between oncoproteins and ROS in promoting cancer cell survival and proliferation and treatment strategies employed against these oncoproteins. Antioxid. Redox Signal.

Glucotoxicity promotes aberrant activation and mislocalization of Ras-related C3 botulinum toxin substrate 1 [Rac1] and metabolic dysfunction in pancreatic islet β-cells: reversal of such metabolic defects by metformin

Emerging evidence suggests that long-term exposure of insulin-secreting pancreatic β-cells to hyperglycemic (HG; glucotoxic) conditions promotes oxidative stress, which, in turn, leads to stress kinase activation, mitochondrial dysfunction, loss of nuclear structure and integrity and cell apoptosis. Original observations from our laboratory have proposed that Rac1 plays a key regulatory role in the generation of oxidative stress and downstream signaling events culminating in the onset of dysfunction of pancreatic β-cells under the duress of metabolic stress. However, precise molecular and cellular mechanisms underlying the metabolic roles of hyperactive Rac1 remain less understood. Using pharmacological and molecular biological approaches, we now report mistargetting of biologically-active Rac1 [GTP-bound conformation] to the nuclear compartment in clonal INS-1 cells, normal rat islets and human islets under HG conditions. Our findings also suggest that such a signaling step is independent of post-translational prenylation of Rac1. Evidence is also presented to highlight novel roles for sustained activation of Rac1 in HG-induced expression of Cluster of Differentiation 36 [CD36], a fatty acid transporter protein, which is implicated in cell apoptosis. Finally, our findings suggest that metformin, a biguanide anti-diabetic drug, at a clinically relevant concentration, prevents β-cell defects [Rac1 activation, nuclear association, CD36 expression, stress kinase and caspase-3 activation, and loss in metabolic viability] under the duress of glucotoxicity. Potential implications of these findings in the context of novel and direct regulation of islet β-cell function by metformin are discussed.

Cellular models for beta-cell function and diabetes gene therapy

Diabetes is characterized by the destruction and/or relative dysfunction of insulin-secreting beta-cells in the pancreatic islets of Langerhans. Consequently, considerable effort has been made to understand the physiological processes governing insulin production and secretion in these cells and to elucidate the mechanisms involved in their deterioration in the pathogenesis of diabetes. To date, considerable research has exploited clonal beta-cell lines derived from rodent insulinomas. Such cell lines have proven to be a great asset in diabetes research, in vitro drug testing, and studies of beta-cell physiology and provide a sustainable, and in many cases, more practical alternative to the use of animals or primary tissue. However, selection of the most appropriate rodent beta cell line is often challenging and no single cell line entirely recapitulates the properties of human beta-cells. The generation of stable human beta-cell lines would provide a much more suitable model for studies of human beta-cell physiology and pathology and could potentially be used as a readily available source of implantable insulin-releasing tissue for cell-based therapies of diabetes. In this review, we discuss the history, development, functional characteristics and use of available clonal rodent beta-cell lines, as well as reflecting on recent advances in the generation of human-derived beta-cell lines, their use in research studies and their potential for cell therapy of diabetes.

CD36 mediates lipid accumulation in pancreatic beta cells under the duress of glucolipotoxic conditions: Novel roles of lysine deacetylases

The cluster of differentiation 36 (CD36) is implicated in the intake of long-chain fatty acids and fat storage in various cell types including the pancreatic beta cell, thus contributing to the pathogenesis of metabolic stress and diabetes. Recent evidence indicates that CD36 undergoes post-translational modifications such as acetylation-deacetylation. However, putative roles of such modifications in its functional activation and onset of beta cell dysregulation under the duress of glucolipotoxicity (GLT) remain largely unknown. Using pharmacological approaches, we validated, herein, the hypothesis that acetylation-deacetylation signaling steps are involved in CD36-mediated lipid accumulation and downstream apoptotic signaling in pancreatic beta (INS-1832/13) cells under GLT. Exposure of these cells to GLT resulted in significant lipid accumulation without affecting the CD36 expression. Sulfo-n-succinimidyl oleate (SSO), an irreversible inhibitor of CD36, significantly attenuated lipid accumulation under GLT conditions, thus implicating CD36 in this metabolic step. Furthermore, trichostatin A (TSA) or valproic acid (VPA), known inhibitors of lysine deacetylases, markedly suppressed GLT-associated lipid accumulation with no discernible effects on CD36 expression. Lastly, SSO or TSA prevented caspase 3 activation in INS-1832/13 cells exposed to GLT conditions. Based on these findings, we conclude that an acetylation-deacetylation signaling step might regulate CD36 functional activity and subsequent lipid accumulation and caspase 3 activation in pancreatic beta cells exposed to GLT conditions. Identification of specific lysine deacetylases that control CD36 function should provide novel clues for the prevention of beta-cell dysfunction under GLT.

Advanced oxidation protein products induce S-phase arrest of hepatocytes via the ROS-dependent, β-catenin-CDK2-mediated pathway

Liver regeneration has important clinical importance in the setting of partial hepatectomy (PH). Following PH, quiescent hepatocytes can reenter cell cycle to restore liver mass. Hepatocyte cell cycle progression, as the basic motivations of liver regeneration, can be disrupted by multiple pathological factors such as oxidative stress. This study aimed to evaluate the role of advanced oxidation protein products (AOPP) in S-phase arrest in hepatocytes. Serum AOPP level were measured during the perioperative period of PH in 33 patients with hepatocellular carcinoma (HCC). Normal Sprague Dawley rats, human and murine liver cell line (HL-7702 and AML-12) were challenged with AOPP prepared by incubation of rat serum albumin (RSA) with hypochlorous acid, and the effect of AOPP on hepatocytes cell cycle progression and liver regeneration was studied after PH. AOPP levels were increased following partial hepatectomy (PH) in patients with primary liver cancer. AOPP treatment impaired liver regeneration in rats following 70% partial hepatectomy. S-phase arrest was induced by AOPP administration in hepatocytes derived from the remnant liver at controlled times following partial hepatectomy in rats, and in HL-7702 and AML-12 cells. The effect of AOPP on hepatocyte S phase arrest was mainly mediated by a nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-dependent reactive oxygen species (ROS) generation, downregulation of downstream β-catenin signaling and decreased cyclin-dependent kinase 2 (CDK2) expression, which inhibited S-phase progression in hepatocytes. This study provides preliminary evidence that AOPP can induce S-phase arrest in hepatocytes via the ROS-dependent, β-catenin-CDK2-mediated pathway. These findings suggest a novel pathogenic role of AOPP contributing to the impaired liver regeneration and may provide the basis for developing new strategies to improve liver regeneration in patients undergoing PH.

Role of G-proteins in islet function in health and diabetes

Glucose-stimulated insulin secretion (GSIS) involves interplay between metabolic and cationic events. Seminal contributions from multiple laboratories affirm essential roles for small G-proteins (Rac1, Cdc42, Arf6, Rab27A) in GSIS. Activation of these signalling proteins promotes cytoskeletal remodeling, transport and docking of insulin granules on the plasma membrane for exocytotic secretion of insulin. Evidence in rodent and human islets suggests key roles for lipidation (farnesylation and geranylgeranylation) of these G-proteins for their targeting to appropriate cellular compartments for optimal regulation of effectors leading to GSIS. Interestingly, however, inhibition of prenylation appears to cause mislocalization of non-prenylated, but (paradoxically) activated G-proteins, in "inappropriate" compartments leading to activation of stress kinases and onset of mitochondrial defects, loss in GSIS and apoptosis of the islet β-cell. This review highlights our current understanding of roles of G-proteins and their post-translational lipidation (prenylation) signalling networks in islet function in normal health, metabolic stress (glucolipotoxicity and ER stress) and diabetes. Critical knowledge gaps that need to be addressed for the development of therapeutics to halt defects in these signalling steps in β-cells in models of impaired insulin secretion and diabetes are also highlighted and discussed.

Salvianolic acid B inhibits intermittent high glucose-induced INS-1 cell apoptosis through regulation of Bcl-2 proteins and mitochondrial membrane potential

Blood glucose fluctuations, also referred to as intermittent high glucose, have been validated to be more harmful than sustained high glucose in exacerbating pancreatic dysfunction by inducing β cell apoptosis. Salvianolic acid B (Sal B), an aqueous component of Salvia miltiorrhiza, has been proved beneficial to pancreatic islet function in diabetes, but the underlying mechanisms remain to be elucidated. The present study investigated the protective effect of Sal B on INS-1 cells exposed to intermittent high glucose and the possible mechanisms implicated. The results indicated that Sal B was able to restore cell viability and suppress INS-1 cell apoptosis induced by intermittent high glucose. Preincubation with Sal B led to a significant decrease of caspase-9 and caspase-3 activity and poly ADP-ribose polymerase (PARP) cleavage. Exposure to intermittent high glucose induced significant up-regulation of proapoptotic proteins, down-regulation of antiapoptotic protein and depolarization of mitochondrial membrane potential (MMP) in INS-1 cells, while these changes were reversed effectively in Sal B treated groups. In addition, Sal B markedly attenuated intermittent high glucose-induced oxidative stress as manifested by notably decreased levels of intracellular reactive oxygen species and malondialdehyde (MDA). Taken together, these results indicate that Sal B is able to suppress intermittent high glucose-induced INS-1 cell apoptosis, which might be ascribed to regulation of Bcl-2 family protein expression and preservation of mitochondrial membrane potential.