Tiam1/Vav2-Rac1 axis: A tug-of-war between islet function and dysfunction

S-Nitrosylation of Septin2 Exacerbates Aortic Aneurysm and Dissection by Coupling the TIAM1-RAC1 Axis in Macrophages

BACKGROUND

S-Nitrosylation (SNO), a prototypic redox-based posttranslational modification, is involved in cardiovascular disease. Aortic aneurysm and dissection are high-risk cardiovascular diseases without an effective cure. The aim of this study was to determine the role of SNO of Septin2 in macrophages in aortic aneurysm and dissection.

METHODS

Biotin-switch assay combined with liquid chromatography-tandem mass spectrometry was performed to identify the S-nitrosylated proteins in aortic tissue from both patients undergoing surgery for aortic dissection and Apoe-/- mice infused with angiotensin II. Angiotensin II-induced aortic aneurysm model and β-aminopropionitrile-induced aortic aneurysm and dissection model were used to determine the role of SNO of Septin2 (SNO-Septin2) in aortic aneurysm and dissection development. RNA-sequencing analysis was performed to recapitulate possible changes in the transcriptome profile of SNO-Septin2 in macrophages in aortic aneurysm and dissection. Liquid chromatography-tandem mass spectrometry and coimmunoprecipitation were used to uncover the TIAM1-RAC1 (Ras-related C3 botulinum toxin substrate 1) axis as the downstream target of SNO-Septin2. Both R-Ketorolac and NSC23766 treatments were used to inhibit the TIAM1-RAC1 axis.

RESULTS

Septin2 was identified S-nitrosylated at cysteine 111 (Cys111) in both aortic tissue from patients undergoing surgery for aortic dissection and Apoe-/- mice infused with Angiotensin II. SNO-Septin2 was demonstrated driving the development of aortic aneurysm and dissection. By RNA-sequencing, SNO-Septin2 in macrophages was demonstrated to exacerbate vascular inflammation and extracellular matrix degradation in aortic aneurysm. Next, TIAM1 (T lymphoma invasion and metastasis-inducing protein 1) was identified as a SNO-Septin2 target protein. Mechanistically, compared with unmodified Septin2, SNO-Septin2 reduced its interaction with TIAM1 and activated the TIAM1-RAC1 axis and consequent nuclear factor-κB signaling pathway, resulting in stronger inflammation and extracellular matrix degradation mediated by macrophages. Consistently, both R-Ketorolac and NSC23766 treatments protected against aortic aneurysm and dissection by inhibiting the TIAM1-RAC1 axis.

CONCLUSIONS

SNO-Septin2 drives aortic aneurysm and dissection through coupling the TIAM1-RAC1 axis in macrophages and activating the nuclear factor-κB signaling pathway-dependent inflammation and extracellular matrix degradation. Pharmacological blockade of RAC1 by R-Ketorolac or NSC23766 may therefore represent a potential treatment against aortic aneurysm and dissection.

Photoactivatable Nanobody Conjugate Dimerizer Temporally Resolves Tiam1-Rac1 Signaling Axis

The precise spatiotemporal dynamics of protein activities play a crucial role in cell signaling pathways. To control cellular functions in a spatiotemporal manner, a powerful method called photoactivatable chemically induced dimerization (pCID) is used. In this study, photoactivatable nanobody conjugate inducers of dimerization (PANCIDs) is introduced, which combine pCID with nanobody technology. A PANCID consists of a nanobody module that directly binds to an antigenic target, a photocaged small molecule ligand, and a cyclic decaarginine (cR10 *) cell-penetrating peptide (CPP) for efficient nonendocytic intracellular delivery. Therefore, PANCID photodimerizers also benefit from nanobodies, such as their high affinities (in the nm or pm range), specificities, and ability to modulate endogenous proteins. Additionally it is demonstrated that the nanobody moiety can be easily replaced with alternative ones, expanding the potential applications. By using PANCIDs, the dynamics of the Tiam1-Rac1 signaling cascade is investigated and made an interesting finding. It is found that Rac1 and Tiam1 exhibit distinct behaviors in this axis, acting as time-resolved "molecular oscillators" that transit between different functions in the signaling cascade when activated either slowly or rapidly.

Tiam1 methylation by NSD2 promotes Rac1 signaling activation and colon cancer metastasis

Metastasis is a major cause of cancer therapy failure and mortality. However, targeting metastatic seeding and colonization remains a significant challenge. In this study, we identified NSD2, a histone methyltransferase responsible for dimethylating histone 3 at lysine 36, as being overexpressed in metastatic tumors. Our findings suggest that NSD2 overexpression enhances tumor metastasis both in vitro and in vivo. Further analysis revealed that NSD2 promotes tumor metastasis by activating Rac1 signaling. Mechanistically, NSD2 combines with and activates Tiam1 (T lymphoma invasion and metastasis 1) and promotes Rac1 signaling by methylating Tiam1 at K724. In vivo and in vitro studies revealed that Tiam1 K724 methylation could be a predictive factor for cancer prognosis and a potential target for metastasis inhibition. Furthermore, we have developed inhibitory peptide which was proved to inhibit tumor metastasis through blocking the interaction between NSD2 and Tiam1. Our results demonstrate that NSD2-methylated Tiam1 promotes Rac1 signaling and cancer metastasis. These results provide insights into the inhibition of tumor metastasis.

Regulatory roles of CARD9-BCL10-Rac1 (CBR) signalome in islet β-cell function in health and metabolic stress: Is there room for MALT1?

It is widely accepted that pancreatic islet β-cell failure and the onset of type 2 diabetes (T2DM) constitute an intricate interplay between the genetic expression of the disease and a host of intracellular events including increased metabolic (oxidative, endoplasmic reticulum) stress under the duress of glucolipotoxicity. Emerging evidence implicates unique roles for Caspase Recruitment Domain containing protein 9 (CARD9) in the onset of metabolic diseases, including obesity and insulin resistance. Mechanistically, CARD9 has been implicated in the regulation of p38MAPK and NFkB signaling pathways culminating in cellular dysfunction. Several regulatory factors, including B-cell lymphoma/leukemia 10 (BCL10) have been identified as modulators of CARD9 function in multiple cell types. Despite this evidence on regulatory roles of CARD9-BCL10 signalome in the onset of various pathological states, putative roles of this signaling module in islet β-cell dysfunction in metabolic stress remain less understood. This brief review is aimed at highlighting roles for CARD9 in islet β-cell function under acute (physiological insulin secretion) and long-term (cell dysfunction) exposure to glucose. Emerging roles of other signaling proteins, such as Rac1, BCL10 and MALT1 as contributors to CARD9 signaling in the islet β-cells are also reviewed. Potential avenues for future research toward the development of novel therapeutics for the prevention CARD9-BCL10-Rac1 (CBR) signalome-induced β-cell defects under metabolic stress are discussed.

Rac1: A Regulator of Cell Migration and A Potential Target for Cancer Therapy

Cell migration is crucial for physiological and pathological processes such as morphogenesis, wound repair, immune response and cancer invasion/metastasis. There are many factors affecting cell migration, and the regulatory mechanisms are complex. Rac1 is a GTP-binding protein with small molecular weight belonging to the Rac subfamily of the Rho GTPase family. As a key molecule in regulating cell migration, Rac1 participates in signal transduction from the external cell to the actin cytoskeleton and promotes the establishment of cell polarity which plays an important role in cancer cell invasion/metastasis. In this review, we firstly introduce the molecular structure and activity regulation of Rac1, and then summarize the role of Rac1 in cancer invasion/metastasis and other physiological processes. We also discuss the regulatory mechanisms of Rac1 in cell migration and highlight it as a potential target in cancer therapy. Finally, the current state as well as the future challenges in this area are considered. Understanding the role and the regulatory mechanism of Rac1 in cell migration can provide fundamental insights into Rac1-related cancer progression and further help us to develop novel intervention strategies for cancer therapy in clinic.

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.

Underappreciated roles for Rho GDP dissociation inhibitors (RhoGDIs) in cell function: Lessons learned from the pancreatic islet β-cell

Rho subfamily of G proteins (e.g., Rac1) have been implicated in glucose-stimulated insulin secretion from the pancreatic β-cell. Interestingly, metabolic stress (e.g., chronic exposure to high glucose) results in sustained activation of Rac1 leading to increased oxidative stress, impaired insulin secretion and β-cell dysfunction. Activation-deactivation of Rho G proteins is mediated by three classes of regulatory proteins, namely the guanine nucleotide exchange factors (GEFs), which facilitate the conversion of inactive G proteins to their active conformations; the GTPase-activating proteins (GAPs), which convert the active G proteins to their inactive forms); and the GDP-dissociation inhibitors (GDIs), which prevent the dissociation of GDP from G proteins. Contrary to a large number of GEFs (82 members) and GAPs (69 members), only three members of RhoGDIs (RhoGDIα, RhoGDIβ and RhoGDIγ) are expressed in mammalian cells.Even though relatively smaller in number, the GDIs appear to play essential roles in G protein function (e.g., subcellular targeting) for effector activation and cell regulation. Emerging evidence also suggests that the GDIs are functionally regulated via post-translational modification (e.g., phosphorylation) and by lipid second messengers, lipid kinases and lipid phosphatases. We highlight the underappreciated regulatory roles of RhoGDI-Rho G protein signalome in islet β-cell function in health and metabolic stress. Potential knowledge gaps in the field, and directions for future research for the identification of novel therapeutic targets to loss of functional β-cell mass under the duress of metabolic stress are highlighted.

Multiple Guanine Nucleotide Exchange Factors Mediate Glucose-Induced Rac1 Activation and Insulin Secretion: Is It Precise Regulatory Control or a Case of Two Peas from the Same Pod?

Glucose-stimulated insulin secretion involves G protein (Rac1)-mediated cytoskeletal remodeling and vesicular transport and fusion with the plasma membrane. Recent evidence implicates at least three guanine nucleotide exchange factors (GEFs), namely, Tiam1, Vav2, and P-Rex1, in glucose-induced activation of Rac1 and insulin secretion. This Viewpoint highlights potential mechanisms underlying Tiam1/Vav2/P-Rex1 sensitive Rac1-mediated insulin secretion in the glucose-stimulated β-cell.

CARD9 mediates glucose-stimulated insulin secretion in pancreatic beta cells

Caspase recruitment domain containing protein 9 (CARD9) plays key regulatory role(s) in innate and adaptive immune responses. Recent evidence implicates CARD9 in the onset of metabolic diseases including insulin resistance. However, potential contributory roles of CARD9 in glucose-stimulated insulin secretion (GSIS) remain unknown. Herein, we report that CARD9 is expressed in human islets, rat islets, mouse islets and clonal INS-1 832/13 cells. Subcellularly, CARD9 is predominantly cytosolic (~75%) in INS-1 832/13 cells. siRNA-mediated depletion of CARD9 expression significantly (~50%) suppressed GSIS in INS-1 832/13 cells. Interestingly, glucose-induced activation of Rac1, a small G-protein, which is a requisite for GSIS to occur, is unaffected in CARD9-si transfected cells, suggesting that CARD9-mediates GSIS in a Rac1-independent fashion. Furthermore, insulin secretion promoted by KCl or mastoparan (a global G protein activator), remained resistant to CARD9 depletion in INS-1 832/13 cells. In addition, pharmacological inhibition (BRD5529) of interaction between CARD9 and TRIM62, its ubiquitin ligase, exerted no significant effects on GSIS. Lastly, depletion of CARD9 prevented glucose-induced p38, not ERK1/2 phosphorylation in beta cells. Based on these observations, we propose that CARD9 might regulate GSIS via a Rac1-independent and p38-dependent signaling module.

Paradoxical regulation of glucose-induced Rac1 activation and insulin secretion by RhoGDIβ in pancreatic β-cells

Small GTPases (e.g., Rac1) play key roles in glucose-stimulated insulin secretion (GSIS) in the β-cell. We investigated regulation by RhoGDIβ of glucose-induced activation of Rac1 and insulin secretion. RhoGDIβ is expressed in INS-1 832/13 cells, rodent and human islets. siRNA-mediated knockdown of RhoGDIβ in INS-1 832/13 cells significantly attenuated glucose-induced Rac1 activation without affecting its translocation and membrane association. Further, suppression of RhoGDIβ expression exerted minimal effects on GSIS at the height of inhibition of Rac1 activation, suggesting divergent effects of RhoGDIβ on Rac1 activation and insulin secretion in the glucose-stimulated β-cell. We provide the first evidence for the expression of RhoGDIβ in rodent and human β-cells, and its differential regulatory roles of this protein in G protein activation and GSIS. Abbreviations: Arf6: ADP ribosylation factor; Cdc42: Cell Division Cycle; GAP: GTPase-activating protein; GDI: GDP dissociation inhibitor; GDIα: GDP dissociation inhibitorα; GDIβ: GDP dissociation inhibitorβ; GEF: Guanine nucleotide exchange factor; GSIS: Glucose-stimulated insulin secretion; Rac1: Ras-Related C3 Botulinum Toxin Substrate 1.

Mechanisms and consequences of dysregulation of the Tiam family of Rac activators in disease

The Tiam family proteins - Tiam1 and Tiam2/STEF - are Rac1-specific Guanine Nucleotide Exchange Factors (GEFs) with important functions in epithelial, neuronal, immune and other cell types. Tiam GEFs regulate cellular migration, proliferation and survival, mainly through activating and directing Rac1 signalling. Dysregulation of the Tiam GEFs is significantly associated with human diseases including cancer, immunological and neurological disorders. Uncovering the mechanisms and consequences of dysregulation is therefore imperative to improving the diagnosis and treatment of diseases. Here we compare and contrast the subcellular localisation and function of Tiam1 and Tiam2/STEF, and review the evidence for their dysregulation in disease.

Tiam1 mediates Rac1 activation and contraction-induced glucose uptake in skeletal muscle cells

Contraction-stimulated glucose uptake in skeletal muscle requires Rac1, but the molecular mechanism of its activation is not fully understood. Treadmill running was applied to induce C57BL/6 mouse hind limb skeletal muscle contraction in vivo and electrical pulse stimulation contracted C2C12 myotube cultures in vitro. The protein levels or activities of AMPK or the Rac1-specific GEF, Tiam1, were manipulated by activators, inhibitors, siRNA-mediated knockdown, and adenovirus-mediated expression. Activated Rac1 was detected by a pull-down assay and immunoblotting. Glucose uptake was measured using the 2-NBD-glucose fluorescent analog. Electrical pulse stimulated contraction or treadmill exercise upregulated the expression of Tiam1 in skeletal muscle in an AMPK-dependent manner. Axin1 siRNA-mediated knockdown diminished AMPK activation and upregulation of Tiam1 protein expression by contraction. Tiam1 siRNA-mediated knockdown diminished contraction-induced Rac1 activation, GLUT4 translocation, and glucose uptake. Contraction increased Tiam1 gene expression and serine phosphorylation of Tiam1 protein via AMPK. These findings suggest Tiam1 is part of an AMPK-Tiam1-Rac1 signaling pathway that mediates contraction-stimulated glucose uptake in skeletal muscle cells and tissue.

Roles of GTP and Rho GTPases in pancreatic islet beta cell function and dysfunction

A growing body of evidence implicates requisite roles for GTP and its binding proteins (Rho GTPases) in the cascade of events leading to physiological insulin secretion from the islet beta cell. Interestingly, chronic exposure of these cells to hyperglycaemic conditions appears to result in sustained activation of specific Rho GTPases (e.g. Rac1) leading to significant alterations in cellular functions including defects in mitochondrial function and nuclear collapse culminating in beta cell demise. One of the objectives of this review is to highlight our current understanding of the regulatory roles of GTP and Rho GTPases in normal islet function (e.g. proliferation and insulin secretion) as well potential defects in these signalling molecules and metabolic pathways that could contribute islet beta cell dysfunction and loss of functional beta cell mass leading to the onset of diabetes. Potential knowledge gaps in this field and possible avenues for future research are also highlighted.

ABBREVIATIONS

ARNO: ADP-ribosylation factor nucleotide binding site opener; CML: carboxyl methylation; Epac: exchange protein directly activated by cAMP; ER stress: endoplasmic reticulum stress; FTase: farnesyltransferase; GAP: GTPase activating protein; GDI: GDP dissociation inhibitor; GEF: guanine nucleotide exchange factor; GGTase: geranylgeranyltransferase; GGpp: geranylgeranylpyrophosphate; GGPPS: geranylgeranyl pyrophosphate synthase; GSIS: glucose-stimulated insulin secretion; HGPRTase: hypoxanthine-guanine phosphoribosyltransferase; IMPDH: inosine monophosphate dehydrogenase; α-KIC: α-ketoisocaproic acid; MPA: mycophenolic acid; MVA: mevalonic acid; NDPK: nucleoside diphosphate kinase; NMPK: nucleoside monophosphate kinase; Nox2: phagocyte-like NADPH oxidase; PAK-I: p21-activated kinase-I; β-PIX: β-Pak-interacting exchange factor; PRMT: protein arginine methyltransferase; Rac1: ras-related C3 botulinum toxin substrate 1; Tiam1: T-cell lymphoma invasion and metastasis-inducing protein 1; Trx-1: thioredoxin-1; Vav2: vav guanine nucleotide exchange factor 2.

A mutation associated with stress resistance in mice is associated with human grip strength and mortality

Hand grip strength (GS) is a valid and reliable predictor of future morbidity and mortality and is considered a useful indicator of aging. In this paper, we use results from the genetic analysis in animal studies to evaluate associations for GS, frailty, and subsequent mortality among humans. Specifically, we use data from the Health and Retirement Survey (HRS) to investigate the association between three polymorphisms in a candidate frailty gene (Tiam1) and GS. Results suggest that the A allele in rs724561 significantly reduces GS among older adults in the US (b = -0.340; p < .006) and is significantly associated with self-reported weakness (b = 0.221; p = .036). This same polymorphism was weakly associated (one-tailed) with an increased risk of mortality (b = 1.091; p < .093) and adjustments for GS rendered this association statistically non-significant (b = 1.048; p < .361). Overall, our results provide tentative evidence that the Tiam1 gene may be associated with frailty development, but we encourage further studies.

Deimination Protein Profiles in Alligator mississippiensis Reveal Plasma and Extracellular Vesicle-Specific Signatures Relating to Immunity, Metabolic Function, and Gene Regulation

Alligators are crocodilians and among few species that endured the Cretaceous-Paleogene extinction event. With long life spans, low metabolic rates, unusual immunological characteristics, including strong antibacterial and antiviral ability, and cancer resistance, crocodilians may hold information for molecular pathways underlying such physiological traits. Peptidylarginine deiminases (PADs) are a group of calcium-activated enzymes that cause posttranslational protein deimination/citrullination in a range of target proteins contributing to protein moonlighting functions in health and disease. PADs are phylogenetically conserved and are also a key regulator of extracellular vesicle (EV) release, a critical part of cellular communication. As little is known about PAD-mediated mechanisms in reptile immunology, this study was aimed at profiling EVs and protein deimination in Alligator mississippiensis. Alligator plasma EVs were found to be polydispersed in a 50-400-nm size range. Key immune, metabolic, and gene regulatory proteins were identified to be posttranslationally deiminated in plasma and plasma EVs, with some overlapping hits, while some were unique to either plasma or plasma EVs. In whole plasma, 112 target proteins were identified to be deiminated, while 77 proteins were found as deiminated protein hits in plasma EVs, whereof 31 were specific for EVs only, including proteins specific for gene regulatory functions (e.g., histones). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis revealed KEGG pathways specific to deiminated proteins in whole plasma related to adipocytokine signaling, while KEGG pathways of deiminated proteins specific to EVs included ribosome, biosynthesis of amino acids, and glycolysis/gluconeogenesis pathways as well as core histones. This highlights roles for EV-mediated export of deiminated protein cargo with roles in metabolism and gene regulation, also related to cancer. The identification of posttranslational deimination and EV-mediated communication in alligator plasma revealed here contributes to current understanding of protein moonlighting functions and EV-mediated communication in these ancient reptiles, providing novel insight into their unusual immune systems and physiological traits. In addition, our findings may shed light on pathways underlying cancer resistance, antibacterial and antiviral resistance, with translatable value to human pathologies.

A genetic screen identifies Crat as a regulator of pancreatic beta-cell insulin secretion

Objectives

Glucose-stimulated insulin secretion is a critical function in the regulation of glucose homeostasis, and its deregulation is associated with the development of type 2 diabetes. Here, we performed a genetic screen using islets isolated from the BXD panel of advanced recombinant inbred (RI) lines of mice to search for novel regulators of insulin production and secretion.

Methods

Pancreatic islets were isolated from 36 RI BXD lines and insulin secretion was measured following exposure to 2.8 or 16.7 mM glucose with or without exendin-4. Islets from the same RI lines were used for RNA extraction and transcript profiling. Quantitative trait loci (QTL) mapping was performed for each secretion condition and combined with transcriptome data to prioritize candidate regulatory genes within the identified QTL regions. Functional studies were performed by mRNA silencing or overexpression in MIN6B1 cells and by studying mice and islets with beta-cell-specific gene inactivation.

Results

Insulin secretion under the 16.7 mM glucose plus exendin-4 condition was mapped significantly to a chromosome 2 QTL. Within this QTL, RNA-Seq data prioritized Crat (carnitine O-acetyl transferase) as a strong candidate regulator of the insulin secretion trait. Silencing Crat expression in MIN6B1 cells reduced insulin content and insulin secretion by ∼30%. Conversely, Crat overexpression enhanced insulin content and secretion by ∼30%. When islets from mice with beta-cell-specific Crat inactivation were exposed to high glucose, they displayed a 30% reduction of insulin content as compared to control islets. We further showed that decreased Crat expression in both MIN6B1 cells and pancreatic islets reduced the oxygen consumption rate in a glucose concentration-dependent manner.

Conclusions

We identified Crat as a regulator of insulin secretion whose action is mediated by an effect on total cellular insulin content; this effect also depends on the genetic background of the RI mouse lines. These data also show that in the presence of the stimulatory conditions used the insulin secretion rate is directly related to the insulin content.

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A QTL analysis in BXD mice identifies Crat as a regulator of insulin secretion.

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Crat regulates insulin content in MIN6B1 cells and pancreatic islets.

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Crat regulates glucose oxidation in MIN6B1 cells and pancreatic islets.

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Crat links glucose metabolism to the control of beta-cell insulin content.

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Insulin content limits insulin secretion in response to high glucose and exendin-4 level.

Liver Transcriptomic Reveals Novel Pathways of Empagliflozin Associated With Type 2 Diabetic Rats

The hypoglycaemic target of empagliflozin (EMP), as a novel inhibitor of sodium-glucose cotransporter (SGLT2), is clear. However, recent studies have shown that EMP also has an important role in lipid metabolism and cardiovascular diseases. The liver plays an important role in the development of type 2 diabetes (T2D), although whether EMP affects liver glucose metabolism is currently not reported. This study was designed to evaluate the effect of EMP on hepatic glucose metabolism in T2D and the underlying mechanism. A model of T2D was established by a high-fat and glucose diet (HFD) combined with streptozotocin (30 mg/kg) in male Wistar rats. Serum samples were collected to measure biochemical indicators, and liver samples were extracted for RNA-seq assay. Quantitative real-time PCR (qPCR) was used to further verify the gene expression levels detected by the RNA-seq assay. The EMP group showed significantly decreased blood glucose, triglyceride, cholesterol, non-esterified fatty acid and low-density lipoprotein cholesterol levels, and increased high-density lipoprotein cholesterol levels in serum compared with the type 2 diabetes model (MOD) group. Furthermore, EMP decreased the levels of inflammatory factors IL-1β, IL-6, and IL-8 in the serum compared to the MOD. Liver transcriptome analysis showed EMP affects a large number of upregulated and downregulated genes. Some of these genes are novel and involve in the metal ion binding pathway and the negative regulation of transcription from the RNA polymerase II promoter pathway, which are also closely related to glucolipid metabolism and insulin signaling. Our study provides new knowledge about the mechanism through which SGLT inhibitor can offer beneficial effects in T2D and especially in the hepatic metabolism. These genes found in this study also laid a solid foundation for further research on the new roles and mechanisms of EMP.

GPCRs, G Proteins, and Their Impact on β-cell Function

Glucose-induced (physiological) insulin secretion from the islet β-cell involves interplay between cationic (i.e., changes in intracellular calcium) and metabolic (i.e., generation of hydrophobic and hydrophilic second messengers) events. A large body of evidence affirms support for novel regulation, by G proteins, of specific intracellular signaling events, including actin cytoskeletal remodeling, transport of insulin-containing granules to the plasma membrane for fusion, and secretion of insulin into the circulation. This article highlights the following aspects of GPCR-G protein biology of the islet. First, it overviews our current understanding of the identity of a wide variety of G protein regulators and their modulatory roles in GPCR-G protein-effector coupling, which is requisite for optimal β-cell function under physiological conditions. Second, it describes evidence in support of novel, noncanonical, GPCR-independent mechanisms of activation of G proteins in the islet. Third, it highlights the evidence indicating that abnormalities in G protein function lead to islet β-cell dysregulation and demise under the duress of metabolic stress and diabetes. Fourth, it summarizes observations of potential beneficial effects of GPCR agonists in preventing/halting metabolic defects in the islet β-cell under various pathological conditions (e.g., metabolic stress and inflammation). Lastly, it identifies knowledge gaps and potential avenues for future research in this evolving field of translational islet biology. Published 2020. Compr Physiol 10:453-490, 2020.

An AMPK/Axin1-Rac1 signaling pathway mediates contraction-regulated glucose uptake in skeletal muscle cells

Contraction stimulates skeletal muscle glucose uptake predominantly through activation of AMP-activated protein kinase (AMPK) and Rac1. However, the molecular details of how contraction activates these signaling proteins are not clear. Recently, Axin1 has been shown to form a complex with AMPK and liver kinase B1 during glucose starvation-dependent activation of AMPK. Here, we demonstrate that electrical pulse-stimulated (EPS) contraction of C2C12 myotubes or treadmill exercise of C57BL/6 mice enhanced reciprocal coimmunoprecipitation of Axin1 and AMPK from myotube lysates or gastrocnemius muscle tissue. Interestingly, EPS or exercise upregulated total cellular Axin1 levels in an AMPK-dependent manner in C2C12 myotubes and gastrocnemius mouse muscle, respectively. Also, direct activation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleotide treatment of C2C12 myotubes or gastrocnemius muscle elevated Axin1 protein levels. On the other hand, siRNA-mediated Axin1 knockdown lessened activation of AMPK in contracted myotubes. Further, AMPK inhibition with compound C or siRNA-mediated knockdown of AMPK or Axin1 blocked contraction-induced GTP loading of Rac1, p21-activated kinase phosphorylation, and contraction-stimulated glucose uptake. In summary, our results suggest that an AMPK/Axin1-Rac1 signaling pathway mediates contraction-stimulated skeletal muscle glucose uptake.

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.

Functional Regulation of an Oxidative Stress Mediator, Rac1, in Diabetic Retinopathy

PURPOSE

Early activation of cytosolic NADPH oxidase-2 (Nox2) in diabetes increases retinal ROS production, damaging their mitochondria. The assembly of Nox2 holoenzyme requires activation of a small molecular weight G protein Rac1. Rac1 activation is regulated by guanine exchange factors and guanine nucleotide-dissociation inhibitors, and post-translational modifications assist in its association with exchange factors and dissociation inhibitors. The goal of this study is to investigate the mechanisms of Rac1 activation in the development of diabetic retinopathy.

METHODS

The levels of the dissociation inhibitor, prenylating enzyme (farnesyltransferase, FNTA), and exchange factor Vav2 were quantified in human retinal endothelial cells, incubated in normal or high glucose for 96 h. The roles of prenylation and Vav2 in Rac1-Nox2-ROS mitochondrial damage were confirmed in FNTA-siRNA-transfected cells and using the Vav2 inhibitor EHop, respectively. Retinal histopathology and functional changes associated with diabetic retinopathy were analyzed in diabetic mice receiving EHop for 6 months. Key parameters of Rac1 activation were confirmed in the retinal microvasculature from human donors with diabetic retinopathy.

RESULTS

In HRECs, glucose increased FNTA and Vav2 and decreased the dissociation inhibitor. FNTA-siRNA and EHop inhibited glucose-induced activation of Rac1-Nox2-ROS signaling. In diabetic mice, EHop ameliorated the development of retinopathy and functional/structural abnormalities and attenuated Rac1-Nox2-mitochondrial damage. Similar alterations in Rac1 regulators were observed in retinal microvasculature from human donors with diabetic retinopathy. In diabetes, Rac1 prenylation and its interactions with Vav2 contribute to Nox2-ROS-mitochondrial damage, and the pharmacological inhibitors to attenuate Rac1 interactions with its regulators could have the potential to halt/inhibit the development of diabetic retinopathy. Graphical Abstract Activation of prenylating enzyme farnesyltransferase (FNTA) in diabetes, prenylates Rac1. The binding of Rac1 with guanine nucleotide-dissociation inhibitor (GDI) is decreased, but its association with the guanine exchange factor, Vav2, is increased, resulting in Rac1 activation. Active Rac1 helps in the assembly of Nox2 holoenzyme, and Nox2 activation increases cytosolic ROS production, damaging the mitochondria. Damaged mitochondria accelerate capillary cell apoptosis, and ultimately, results in the development of diabetic retinopathy.

The Regulatory Role of Rac1, a Small Molecular Weight GTPase, in the Development of Diabetic Retinopathy

Diabetic retinopathy, a microvascular complication of diabetes, remains the leading cause of vision loss in working age adults. Hyperglycemia is considered as the main instigator for its development, around which other molecular pathways orchestrate. Of these multiple pathways, oxidative stress induces many metabolic, functional and structural changes in the retinal cells, leading to the development of pathological features characteristic of this blinding disease. An increase in cytosolic reactive oxygen species (ROS), produced by cytosolic NADPH oxidase 2 (Nox2), is an early event in the pathogenesis of diabetic retinopathy, which leads to mitochondrial damage and retinal capillary cell apoptosis. Activation of Nox2 is mediated through an obligatory small molecular weight GTPase, Ras-related C3 botulinum toxin substrate 1 (Rac1), and subcellular localization of Rac1 and its activation are regulated by several regulators, rendering it a complex biological process. In diabetes, Rac1 is functionally activated in the retina and its vasculature, and, via Nox2-ROS, contributes to mitochondrial damage and the development of retinopathy. In addition, Rac1 is also transcriptionally activated, and epigenetic modifications play a major role in this transcriptional activation. This review focusses on the role of Rac1 and its regulation in the development and progression of diabetic retinopathy, and discusses some possible avenues for therapeutic interventions.

miR-29a Negatively Affects Glucose-Stimulated Insulin Secretion and MIN6 Cell Proliferation via Cdc42/β-Catenin Signaling

BACKGROUND

Diabetes is a progressive metabolic disease characterized by hyperglycemia. Functional impairment of islet β cells can occur to varying degrees. This impairment can initially be compensated for by proliferation and metabolic changes of β cells. Cell division control protein 42 (Cdc42) and the microRNA (miRNA) miR-29 have important roles in β-cell proliferation and glucose-stimulated insulin secretion (GSIS), which we further explored using the mouse insulinoma cell line MIN6.

METHODS

Upregulation and downregulation of miR-29a and Cdc42 were accomplished using transient transfection. miR-29a and Cdc42 expression was detected by real-time PCR and western blotting. MIN6 proliferation was detected using a cell counting kit assay. GSIS under high-glucose (20.0 mM) or basal-glucose (5.0 mM) stimulation was detected by enzyme-linked immunosorbent assay. The miR-29a binding site in the Cdc42 mRNA 3'-untranslated region (UTR) was determined using bioinformatics and luciferase reporter assays.

RESULTS

miR-29a overexpression inhibited proliferation (P < 0.01) and GSIS under high-glucose stimulation (P < 0.01). Cdc42 overexpression promoted proliferation (P < 0.05) and GSIS under high-glucose stimulation (P < 0.05). miR-29a overexpression decreased Cdc42 expression (P < 0.01), whereas miR-29a downregulation increased Cdc42 expression (P < 0.01). The results showed that the Cdc42 mRNA 3'-UTR is a direct target of miR-29a in vitro. Additionally, Cdc42 reversed miR-29a-mediated inhibition of proliferation and GSIS (P < 0.01). Furthermore, miR-29a inhibited β-catenin expression (P < 0.01), whereas Cdc42 promoted β-catenin expression (P < 0.01).

CONCLUSION

By negatively regulating Cdc42 and the downstream molecule β-catenin, miR-29a inhibits MIN6 proliferation and insulin secretion.

Kalirin/Trio Rho GDP/GTP exchange factors regulate proinsulin and insulin secretion

Key features for progression to pancreatic β-cell failure and disease are loss of glucose responsiveness and an increased ratio of secreted proinsulin to insulin. Proinsulin and insulin are stored in secretory granules (SGs) and the fine-tuning of hormone output requires signal mediated recruitment of select SG populations according to intracellular location and age. The GTPase Rac1 coordinates multiple signaling pathways that specify SG release and Rac1 activity is controlled in part by GDP/GTP exchange factors (GEFs). To explore the function of two large multidomain GEFs, Kalirin and Trio in β-cells, we manipulated their Rac1-specific GEF1 domain activity by using small molecule inhibitors and by genetically ablating Kalirin. We examined age related secretory granule behavior employing radiolabeling protocols. Loss of Kalirin/Trio function attenuated radioactive proinsulin release by reducing constitutive-like secretion and exocytosis of 2-hour old granules. At later chase times or at steady state, Kalirin/Trio manipulations decreased glucose stimulated insulin output. Finally, use of a Rac1 FRET biosensor with cultured β-cell lines, demonstrated that Kalirin/Trio GEF1 activity was required for normal rearrangement of Rac1 to the plasma membrane in response to glucose. Rac1 activation can be evoked by both glucose metabolism and signaling through the incretin glucagon-like peptide 1 (GLP-1) receptor. GLP-1 addition restored Rac1 localization/activity and insulin secretion in the absence of Kalirin, thereby assigning Kalirin's participation to stimulatory glucose signaling.

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.

RACking up ceramide-induced islet β-cell dysfunction

The International Diabetes Federation predicts that by 2045 the number of individuals afflicted with diabetes will increase to 629 million. Furthermore, ∼352 million individuals with impaired glucose tolerance are at increased risk for developing diabetes. Several mechanisms have been proposed for the onset of metabolic dysfunction and demise of the islet β-cell leading to the pathogenesis of diabetes. It is widely accepted that the onset of type 2 diabetes is due to an intricate interplay between genetic expression of the disease and a multitude of factors including increased oxidative and endoplasmic reticulum stress consequential to glucolipotoxicity and inflammation. Compelling experimental evidence from in vitro and in vivo studies implicates intracellular generation of ceramide (CER), a biologically-active sphingolipid, as a trigger in the onset of β-cell demise under above pathological conditions. Recent pharmacological and molecular biological evidence affirms regulatory roles for Ras-related C3 botulinum toxin substrate 1 (Rac1), a small G protein, in the islet β-cell function in health and diabetes. In this Commentary, we overviewed the emerging evidence implicating potential cross-talk between Rac1 and ceramide signaling pathways in the onset of metabolic dysregulation of the islet β-cell culminating in impaired physiological insulin secretion, loss of β-cell mass and the onset of diabetes. Further, we propose a model depicting contributory roles of defective protein lipidation (prenylation) pathway in the induction of metabolic defects in the β-cell under metabolic stress conditions. Potential avenues for the identification of novel therapeutic targets for the prevention/treatment of diabetes and its associated complications are highlighted.

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.