The protein kinases ERK1/2 and their roles in pancreatic beta cells

Stimulation of insulin secretion induced by low 4-cresol dose involves the RPS6KA3 signalling pathway

4-cresol (4-methylphenol, p-cresol) is a xenobiotic substance negatively correlated with type 2 diabetes and associated with health improvement in preclinical models of diabetes. We aimed at refining our understanding of the physiological role of this metabolite and identifying potential signalling mechanisms. Functional studies revealed that 4-cresol does not deteriorate insulin sensitivity in human primary adipocytes and exhibits an additive effect to that of insulin on insulin sensitivity in mouse C2C12 myoblasts. Experiments in mouse isolated islets showed that 4-cresol potentiates glucose induced insulin secretion. We demonstrated the absence of off target effects of 4-cresol on a panel of 44 pharmacological compounds. Screening large panels of 241 G protein-coupled receptors (GPCRs) and 468 kinases identified binding of 4-cresol only to TNK1, EIF2AK4 (GCN2) and RPS6KA3 (RSK2), a kinase strongly expressed in human and rat pancreatic islets. Islet expression of RPS6KA3 is reduced in spontaneously diabetic rats chronically treated with 4-cresol and Rps6ka3 deficient mice exhibit reduction in both body weight and fasting glycemia, modest improvement in glycemic control and enhanced insulin release in vivo. Similar to low doses of 4-cresol, incubation of isolated rat islets with low concentrations of the RPS6KA3 inhibitor BIX 02565 stimulates both glucose induced insulin secretion and β-cell proliferation. These results provide further information on the role of low 4-cresol doses in the regulation of insulin secretion.

GLP-1 and GIP receptors signal through distinct β-arrestin 2-dependent pathways to regulate pancreatic β cell function

Glucagon-like peptide 1 (GLP-1R) and glucose-dependent insulinotropic polypeptide (GIPR) receptors are G-protein-coupled receptors involved in glucose homeostasis. Diabetogenic conditions decrease β-arrestin 2 (ARRB2) levels in human islets. In mouse β cells, ARRB2 dampens insulin secretion by partially uncoupling cyclic AMP (cAMP)/protein kinase A (PKA) signaling at physiological doses of GLP-1, whereas at pharmacological doses, the activation of extracellular signal-related kinase (ERK)/cAMP-responsive element-binding protein (CREB) requires ARRB2. In contrast, GIP-potentiated insulin secretion needs ARRB2 in mouse and human islets. The GIPR-ARRB2 axis is not involved in cAMP/PKA or ERK signaling but does mediate GIP-induced F-actin depolymerization. Finally, the dual GLP-1/GIP agonist tirzepatide does not require ARRB2 for the potentiation of insulin secretion. Thus, ARRB2 plays distinct roles in regulating GLP-1R and GIPR signaling, and we highlight (1) its role in the physiological context and the possible functional consequences of its decreased expression in pathological situations such as diabetes and (2) the importance of assessing the signaling pathways engaged by the agonists (biased/dual) for therapeutic purposes.

Navigating the ERK1/2 MAPK Cascade

The RAS-ERK pathway is a fundamental signaling cascade crucial for many biological processes including proliferation, cell cycle control, growth, and survival; common across all cell types. Notably, ERK1/2 are implicated in specific processes in a context-dependent manner as in stem cells and pancreatic β-cells. Alterations in the different components of this cascade result in dysregulation of the effector kinases ERK1/2 which communicate with hundreds of substrates. Aberrant activation of the pathway contributes to a range of disorders, including cancer. This review provides an overview of the structure, activation, regulation, and mutational frequency of the different tiers of the cascade; with a particular focus on ERK1/2. We highlight the importance of scaffold proteins that contribute to kinase localization and coordinate interaction dynamics of the kinases with substrates, activators, and inhibitors. Additionally, we explore innovative therapeutic approaches emphasizing promising avenues in this field.

Recent Developments in Islet Biology: A Review With Patient Perspectives

Navigating the coronavirus disease-2019 (COVID-19, now COVID) pandemic has required resilience and creativity worldwide. Despite early challenges to productivity, more than 2,000 peer-reviewed articles on islet biology were published in 2021. Herein, we highlight noteworthy advances in islet research between January 2021 and April 2022, focussing on 5 areas. First, we discuss new insights into the role of glucokinase, mitogen-activated protein kinase-kinase/extracellular signal-regulated kinase and mitochondrial function on insulin secretion from the pancreatic β cell, provided by new genetically modified mouse models and live imaging. We then discuss a new connection between lipid handling and improved insulin secretion in the context of glucotoxicity, focussing on fatty acid-binding protein 4 and fetuin-A. Advances in high-throughput "omic" analysis evolved to where one can generate more finely tuned genetic and molecular profiles within broad classifications of type 1 diabetes and type 2 diabetes. Next, we highlight breakthroughs in diabetes treatment using stem cell-derived β cells and innovative strategies to improve islet survival posttransplantation. Last, we update our understanding of the impact of severe acute respiratory syndrome-coronavirus-2 infection on pancreatic islet function and discuss current evidence regarding proposed links between COVID and new-onset diabetes. We address these breakthroughs in 2 settings: one for a scientific audience and the other for the public, particularly those living with or affected by diabetes. Bridging biomedical research in diabetes to the community living with or affected by diabetes, our partners living with type 1 diabetes or type 2 diabetes also provide their perspectives on these latest advances in islet biology.

Commiphora myrrha stimulates insulin secretion from β-cells through activation of atypical protein kinase C and mitogen-activated protein kinase

ETHNOPHARMACOLOGICAL RELEVANCE

Ayurvedic medicine has been used in the treatment of diabetes mellitus for centuries. In Arabia and some areas of Africa, Commiphora myrrha (CM) has been extensively used as a plant-based remedy. We have previously shown that an aqueous CM resin solution directly stimulates insulin secretion from MIN6 cells, a mouse β-cell line, and isolated mouse and human islets. However, the signaling pathways involved in CM-induced insulin secretion are completely unknown. Insulin secretion is normally triggered by elevations in intracellular Ca²⁺ ([Ca²⁺]_i) through voltage gated Ca²⁺ channels (VGCC) and activation of protein kinases. Protein and lipid kinases such as protein kinase A (PKA), Ca²⁺-calmodulin dependent protein kinase II (CaMKII), phosphoinositide 3-kinases (PI3Ks), protein kinase C (PKC) and mitogen-activated protein kinase (MAPK), specifically extracellular signal-regulated kinases (ERK1/2), may be involved in receptor-operated insulin secretion. Therefore, we hypothesized that CM may induce insulin secretion by modulating the activity of VGCC and/or one or more of the above kinases.

AIM OF THE STUDY

To investigate the possible molecular mechanism of action of CM-induced insulin secretion. The effects of aqueous CM resin extract on [Ca²⁺]_i and protein kinase activation from β-cells were examined.

METHODS

The effect of aqueous CM resin solution on [Ca²⁺]_i was assessed using Ca²⁺ microfluorimetry. The involvement of VGCC in CM-induced insulin secretion was investigated using static and perifusion insulin secretion experiments in the presence of either EGTA, a Ca²⁺ chelator, or nifedipine, a blocker of VGCC. The involvement of kinase activation in the stimulatory effect of CM on insulin secretion was examined by using static and perifusion insulin secretion experiments in the presence of known pharmacological inhibitors and/or downregulation of specific kinases. The effects of CM on phosphorylation of PKCζ and ERK1/2 were also assessed using the Wes™ capillary-based protein electrophoresis.

RESULTS

Ca²⁺ microfluorimetry measurements showed that exposing MIN6 cells to CM (0.5-2 mg/mL) was not associated with changes in [Ca²⁺]_i. Similarly, incubating MIN6 cells and mouse islets with EGTA and nifedipine, respectively, did not attenuate the insulin secretion induced by CM. However, incubating mouse and human islets with CM in the presence of staurosporine, a non-selective protein kinase inhibitor, completely blocked the effect of CM on insulin secretion. Exposing mouse islets to CM in the presence of H89, KN62 and LY294002, inhibitors of PKA, CaMKII and PI3K, respectively, did not reduce CM-induced insulin secretion. However, incubating mouse and human islets with CM in the presence of Ro 31-8220, a pan-PKC inhibitor, diminished insulin secretion stimulated by CM, whereas inhibiting the action of typical PKC (with Go6976) and PLCβ (with U73122) did not affect CM-stimulated insulin secretion. Similarly, downregulating typical and novel PKC by chronic exposure of mouse islets to phorbol 12-myristate 13-acetate (PMA) was also not associated with a decrease in the stimulatory effect of CM on insulin secretion. Interestingly, CM-induced insulin secretion from mouse islets was inhibited in the presence of the PKCζ inhibitor ZIP and a MAPK inhibitor PD 98059. In addition, Wes™ capillary-based protein electrophoresis indicated that expression of the phosphorylated forms of PKCζ and ERK1/2, a MAPK, was significantly increased following exposure of INS-1832/13 cells, a rat insulinoma cell line, to CM.

CONCLUSIONS

Our data indicate that CM directly stimulates insulin secretion through activating known downstream effectors of insulin-stimulus secretion coupling. Indeed, the increase in insulin secretion seen with CM is independent of changes in [Ca²⁺]_i and does not involve activation of VGCC. Instead, the CM stimulatory effect on insulin secretion is completely dependent on protein kinase activation. Our findings indicate that CM could induce insulin exocytosis by stimulating the phosphorylation and activation of PKCζ, which in turn phosphorylates and activates ERK1/2.

RyR2/IRBIT regulates insulin gene transcript, insulin content, and secretion in the insulinoma cell line INS-1

The role of ER Ca2+ release via ryanodine receptors (RyR) in pancreatic β-cell function is not well defined. Deletion of RyR2 from the rat insulinoma INS-1 (RyR2KO) enhanced IP3 receptor activity stimulated by 7.5 mM glucose, coincident with reduced levels of the protein IP3 Receptor Binding protein released with Inositol 1,4,5 Trisphosphate (IRBIT). Insulin content, basal (2.5 mM glucose) and 7.5 mM glucose-stimulated insulin secretion were reduced in RyR2KO and IRBITKO cells compared to controls. INS2 mRNA levels were reduced in both RyR2KO and IRBITKO cells, but INS1 mRNA levels were specifically decreased in RyR2KO cells. Nuclear localization of S-adenosylhomocysteinase (AHCY) was increased in RyR2KO and IRBITKO cells. DNA methylation of the INS1 and INS2 gene promotor regions was very low, and not different among RyR2KO, IRBITKO, and controls, but exon 2 of the INS1 and INS2 genes was more extensively methylated in RyR2KO and IRBITKO cells. Exploratory proteomic analysis revealed that deletion of RyR2 or IRBIT resulted in differential regulation of 314 and 137 proteins, respectively, with 41 in common. These results suggest that RyR2 regulates IRBIT levels and activity in INS-1 cells, and together maintain insulin content and secretion, and regulate the proteome, perhaps via DNA methylation.

Calcineurin/NFATc2 and PI3K/AKT signaling maintains β-cell identity and function during metabolic and inflammatory stress

Pancreatic islets respond to metabolic and inflammatory stress by producing hormones and other factors that induce adaptive cellular and systemic responses. Here we show that intracellular Ca2+ ([Ca2+]i) and ROS signals generated by high glucose and cytokine-induced ER stress activate calcineurin (CN)/NFATc2 and PI3K/AKT to maintain β-cell identity and function. This was attributed in part by direct induction of the endocrine differentiation gene RFX6 and suppression of several β-cell "disallowed" genes, including MCT1. CN/NFATc2 targeted p300 and HDAC1 to RFX6 and MCT1 promoters to induce and suppress gene transcription, respectively. In contrast, prolonged exposure to stress, hyperstimulated [Ca2+]i, or perturbation of CN/NFATc2 resulted in downregulation of RFX6 and induction of MCT1. These findings reveal that CN/NFATc2 and PI3K/AKT maintain β-cell function during acute stress, but β-cells dedifferentiate to a dysfunctional state upon loss or exhaustion of Ca2+/CN/NFATc2 signaling. They further demonstrate the utility of targeting CN/NFATc2 to restore β-cell function.

Beta-Cell Dysfunction Induced by Tacrolimus: A Way to Explain Type 2 Diabetes?

The combination of insulin resistance and β-cells dysfunction leads to the onset of type-2 diabetes mellitus (T2DM). This process can last for decades, as β-cells are able to compensate the demand for insulin and maintain normoglycemia. Understanding the adaptive capacity of β-cells during this process and the causes of its failure is essential to the limit onset of diabetes. Post-transplant diabetes mellitus (PTDM) is a common and serious disease that affects 30% of renal transplant recipients. With the exception of immunosuppressive therapy, the risk factors for T2D are the same as for PTDM: obesity, dyslipidaemia, insulin resistance and metabolic syndrome. Tacrolimus (TAC) is the immunosuppressant of choice after renal transplantation but it has the highest rates of PTDM. Our group has shown that insulin resistance and glucolipotoxicity, without favouring the appearance of apoptosis, modify key nuclear factors for the maintenance of identity and functionality of β-cells. In this context, TAC accelerates or enhances these changes. Our hypothesis is that the pathways that are affected in the progression from pre-diabetes to diabetes in the general population are the same pathways that are affected by TAC. So, TAC can be considered a tool to study the pathogenesis of T2DM. Here, we review the common pathways of β-cells dysfunction on T2DM and TAC-induced diabetes.

Role of mitochondria-associated endoplasmic reticulum membrane (MAMs) interactions and calcium exchange in the development of type 2 diabetes

Glucotoxicity-induced β-cell dysfunction in type 2 diabetes is associated with alterations of mitochondria and the endoplasmic reticulum (ER). Mitochondria and ER form a network in cells that controls cell function and fate. Mitochondria of the pancreatic β cell play a central role in the secretion of insulin in response to glucose through their ability to produce ATP. Both organelles interact at contact sites, defined as mitochondria-associated membranes (MAMs), which were recently implicated in the regulation of glucose homeostasis. Here, we review MAM functions in the cell and we focus on the crosstalk between the ER and Mitochondria in the context of T2D, highlighting the pivotal role played by MAMs especially in β cells through inter-organelle calcium exchange and glucotoxicity-associated β cell dysfunction.

MEK/ERK Signaling in β-Cells Bifunctionally Regulates β-Cell Mass and Glucose-Stimulated Insulin Secretion Response to Maintain Glucose Homeostasis

In diabetic pathology, insufficiency in β-cell mass, unable to meet peripheral insulin demand, and functional defects of individual β-cells in production of insulin are often concurrently observed, collectively causing hyperglycemia. Here we show that the phosphorylation of ERK1/2 is significantly decreased in the islets of db/db mice as well as in those of a cohort of subjects with type 2 diabetes. In mice with abrogation of ERK signaling in pancreatic β-cells through deletion of Mek1 and Mek2, glucose intolerance aggravates under high-fat diet-feeding conditions due to insufficient insulin production with lower β-cell proliferation and reduced β-cell mass, while in individual β-cells dampening of the number of insulin exocytosis events is observed, with the molecules involved in insulin exocytosis being less phosphorylated. These data reveal bifunctional roles for MEK/ERK signaling in β-cells for glucose homeostasis, i.e., in regulating β-cell mass as well as in controlling insulin exocytosis in individual β-cells, thus providing not only a novel perspective for the understanding of diabetes pathophysiology but also a potential clue for new drug development for diabetes treatment.

Intermedin facilitates hepatocellular carcinoma cell survival and invasion via ERK1/2-EGR1/DDIT3 signaling cascade

As one of the most malignant cancer types, hepatocellular carcinoma (HCC) is highly invasive and capable of metastasizing to distant organs. Intermedin (IMD), an endogenous peptide belonging to the calcitonin family, has been suggested playing important roles in cancer cell survival and invasion, including in HCC. However, how IMD affects the behavior of HCC cells and the underlying mechanisms have not been fully elucidated. Here, we show that IMD maintains an important homeostatic state by activating the ERK1/2-EGR1 (early growth response 1) signaling cascade, through which HCC cells acquire a highly invasive ability via significantly enhanced filopodia formation. The inhibition of IMD blocks the phosphorylation of ERK1/2, resulting in EGR1 downregulation and endoplasmic reticulum stress (ER) stress, which is evidenced by the upregulation of ER stress marker DDIT3 (DNA damage-inducible transcript 3). The high level of DDIT3 induces HCC cells into an ER-stress related apoptotic pathway. Along with our previous finding that IMD plays critical roles in the vascular remodeling process that improves tumor blood perfusion, IMD may facilitate the acquisition of increased invasive abilities and a survival benefit by HCC cells, and it is easier for HCC cells to obtain blood supply via the vascular remodeling activities of IMD. According to these results, blockade of IMD activity may have therapeutic potential in the treatment of HCC.

α2-Adrenergic Disruption of β Cell BDNF-TrkB Receptor Tyrosine Kinase Signaling

Adrenergic signaling is a well-known input into pancreatic islet function. Specifically, the insulin-secreting islet β cell expresses the Gi/o-linked α2-adrenergic receptor, which upon activation suppresses insulin secretion. The use of the adrenergic agonist epinephrine at micromolar doses may have supraphysiological effects. We found that pretreating β cells with micromolar concentrations of epinephrine differentially inhibited activation of receptor tyrosine kinases. We chose TrkB as an example because of its relative sensitivity to the effects of epinephrine and due to its potential regulatory role in the β cell. Our characterization of brain-derived neurotrophic factor (BDNF)-TrkB signaling in MIN6 β cells showed that TrkB is activated by BDNF as expected, leading to canonical TrkB autophosphorylation and subsequent downstream signaling, as well as chronic effects on β cell growth. Micromolar, but not nanomolar, concentrations of epinephrine blocked BDNF-induced TrkB autophosphorylation and downstream mitogen-activated protein kinase pathway activation, suggesting an inhibitory phenomenon at the receptor level. We determined epinephrine-mediated inhibition of TrkB activation to be Gi/o-dependent using pertussis toxin, arguing against an off-target effect of high-dose epinephrine. Published data suggested that inhibition of potassium channels or phosphoinositide-3-kinase signaling may abrogate the negative effects of epinephrine; however, these did not rescue TrkB signaling in our experiments. Taken together, these results show that (1) TrkB kinase signaling occurs in β cells and (2) use of epinephrine in studies of insulin secretion requires careful consideration of concentration-dependent effects. BDNF-TrkB signaling in β cells may underlie pro-survival or growth signaling and warrants further study.

Transient receptor potential vanilloid 4 channels as therapeutic targets in diabetes and diabetes-related complications

With an estimated 425 million diabetes patients worldwide in 2019, type 2 diabetes has reached a pandemic proportion and represents a major unmet medical need. A key determinant of the development and progression of type 2 diabetes is pancreatic -cell dysfunction, including the loss of cell mass, the impairment of insulin biosynthesis and inadequate exocytosis. Recent studies have shown that transient receptor potential vanilloid 4 (TRPV4), a Ca2+ -permeable non-selective cation channel, is involved in -cell replication, insulin production and secretion. TRPV4 agonists have insulinotropic activity in pancreatic -cell lines, but the prolonged activation of TRPV4 leads to -cell dysfunction and death. In addition, TRPV4 is involved in a wide variety of pathophysiological activities, and has been reported to play an important role in diabetes-related complications, such as obesity, cardiovascular diseases, diabetic retinopathy, nephropathy and neuropathy. In a rodent type 2 diabetes model, Trpv4 agonists promote vasodilation and improve cardiovascular function, whereas Trpv4 antagonists reduce high-fat diet-induced obesity, insulin resistance, diabetic nephropathy, retinopathy and neuropathy. These findings raise interest in using TRPV4 as a therapeutic target for type 2 diabetes. In this review, we intend to summarize the latest findings regarding the role of TRPV4 in diabetes as well as diabetes-related conditions, and to evaluate its potential as a therapeutic target for diabetes and diabetes-related diseases.

Urolithin C increases glucose-induced ERK activation which contributes to insulin secretion

Polyphenols exert pharmacological actions through protein-mediated mechanisms and by modulating intracellular signalling pathways. We recently showed that a gut-microbial metabolite of ellagic acid named urolithin C is a glucose-dependent activator of insulin secretion acting by facilitating L-type Ca²⁺ channel opening and Ca²⁺ influx into pancreatic β-cells. However, it is still unknown whether urolithin C regulates key intracellular signalling proteins in β-cells. Here, we report that urolithin C enhanced glucose-induced extracellular signal-regulated kinases 1/2 (ERK1/2) activation as shown by higher phosphorylation levels in INS-1 β-cells. Interestingly, inhibition of ERK1/2 with two structurally distinct inhibitors led to a reduction in urolithin C effect on insulin secretion. Finally, we provide data to suggest that urolithin C-mediated ERK1/2 phosphorylation involved insulin signalling in INS-1 cells. Together, these data indicate that the pharmacological action of urolithin C on insulin secretion relies, in part, on its capacity to enhance glucose-induced ERK1/2 activation. Therefore, our study extends our understanding of the pharmacological action of urolithin C in β-cells. More generally, our findings revealed that urolithin C modulated the activation of key multifunctional intracellular signalling kinases which participate in the regulation of numerous biological processes.

Centaurium erythraea extract improves survival and functionality of pancreatic beta-cells in diabetes through multiple routes of action

ETHNOPHARMACOLOGICAL RELEVANCE

Centaurium erythraea Rafn (CE) is used as a traditional medicinal plant in Serbia to treat different ailments due to its antidiabetic, antipyretic, antiflatulent and detoxification effects.

AIM OF THE STUDY

Elucidation of the mechanisms that underlie the antioxidant and pro-survival effects of the CE extract (CEE) in beta-cells and pancreatic islets from streptozotocin (STZ)-treated diabetic rats.

MATERIAL AND METHODS

Diabetes was induced in rats by multiple applications of low doses of STZ (40 mg/kg intraperitoneally (i.p.), for five consecutive days). CEE (100 mg/kg) was administered orally, in the pre-treated group for two weeks before diabetes induction, during the treatments with STZ and for four weeks after diabetes onset, and in the post-treatment group for four weeks after diabetes induction. The impact of CEE on diabetic islets was estimated by histological and immunohistochemical examination of the pancreas. Molecular mechanisms of the effects of CEE were also analyzed in insulinoma Rin-5F cells treated with STZ (12 mM) and CEE (0.25 mg/mL). Oxidative stress was evaluated by assessing the levels of DNA damage, lipid peroxidation, protein S-glutathionylation and enzymatic activities and expression of CAT, MnSOD, CuZnSOD, GPx and GR in beta-cells. The presence and activities of the redox-sensitive and islet-enriched regulatory proteins were also analyzed.

RESULTS

Treatment with CEE ameliorated the insulin level and glycemic control in STZ-induced diabetic rats by improving the structural and functional properties of pancreatic islets through multiple routes of action. The disturbance of islet morphology and islet cell contents in diabetes was reduced by the CEE treatment and was associated with a protective effect of CEE on the levels of insulin, GLUT-2 and p-Akt in diabetic islets. The antioxidant effect of CEE on STZ-treated beta-cells was displayed as reduced DNA damage, lipid peroxidation, protein S-glutathionylation and alleviation of STZ-induced disruption in MnSOD, CuZnSOD and CAT enzyme activities. The oxidative stress-induced disturbance of the transcriptional regulation of CAT, MnSOD, CuZnSOD, GPx and GR enzymes in beta-cells was improved after the CEE treatment, and was observed as readjustment of the presence and activities of redox-sensitive NFκB-p65, FOXO3A, Sp1 and Nrf-2 transcription factors. The observed CEE-mediated induction of proliferative and pro-survival pathways and insulin expression/secretion after STZ-induced oxidative stress in beta-cells could be partially attributed to a fine-tuned modulation of the activities of pro-survival Akt, ERK and p38 kinases and islet-enriched Pdx-1 and MafA regulatory factors.

CONCLUSIONS

The results of this study provide evidence that CEE improves the structural and functional properties of pancreatic beta-cells by correcting the endogenous antioxidant regulatory mechanisms and by promoting proliferative and pro-survival pathways in beta-cells.

The role of microRNAs in the regulation of insulin signaling pathway with respect to metabolic and mitogenic cascades: A review

Insulin resistance (IR) is a shared pathological condition among type 2 diabetes, obesity, cardiovascular disease, and other metabolic disorders. It is growing significantly all over the world and consequently, a substantial effort is needed for developing the potential novel diagnostics and therapeutics. An insulin signaling pathway is tightly modulated by different mechanisms including the epigenetic modifications. Today, a deal of great attention has been shifted towards the regulatory role of noncoding RNAs on target proteins of the insulin signaling pathway. Noncoding RNAs are a major area of the epigenetics which control gene expression at the posttranscriptional levels and include a large class of microRNAs (miRNAs). With this in view, many studies have implicated the mediatory effects of miRNAs on the downstream metabolic and mitogenic proteins of the insulin signaling pathway. Since providing new biomarkers for the early diagnosis of IR and related metabolic traits are very significant, we intended to review the possible role of miRNAs in the regulation of the insulin signaling pathway, with a primary focus on the downstream target proteins of the metabolic and mitogenic cascades.

Asprosin impairs insulin secretion in response to glucose and viability through TLR4/JNK-mediated inflammation

Severe inflammation in the islets is observed in obese patients with type 2 diabetes. Inflammation in the islets is caused by obesity-induced serum free fatty acids. Asprosin is a fasting-induced adipokine, which contributes to hepatic glucose production. However, the effects of asprosin on inflammation and cellular dysfunction in pancreatic β-cells remain to be elucidated. Here, we demonstrated that treatment of mouse insulinoma MIN6 cells and human primary islets containing β-cells with palmitate increased asprosin expression and secretion. Treatment of MIN6 cells and human primary islets with palmitate increased phosphorylation of the inflammatory marker nuclear factor-kappa B (NFκB) and the release of pro-inflammatory cytokines including TNF and MCP-1 and decreased glucose-stimulated insulin secretion and cell viability. However, siRNA-mediated suppression of asprosin reversed these changes. Recombinant asprosin treatment of MIN6 cells and human primary islets augmented the inflammation response, cellular dysfunction, and apoptosis in a dose-dependent manner. Asprosin induced toll-like receptor (TLR) 4 expression and JNK phosphorylation. siRNA for TLR4 or JNK mitigated the effects of asprosin on inflammation and cellular dysfunction. These results suggest that palmitate-derived asprosin secretion from β-cells results in their inflammation and dysfunction through a TLR4/JNK-mediated pathway. This report suggests asprosin as a novel therapeutic target for the treatment of type 2 diabetes through preservation of β-cell function.

Trace amine-associated receptor 1 (TAAR1) promotes anti-diabetic signaling in insulin-secreting cells

Pancreatic β-cell failure in type 2 diabetes mellitus is a serious challenge that results in an inability of the pancreas to produce sufficient insulin to properly regulate blood glucose levels. Trace amine-associated receptor 1 (TAAR1) is a G protein-coupled receptor expressed by β-cells that has recently been proposed as a potential target for improving glycemic control and suppressing binge eating behaviors. We discovered that TAAR1 is coupled to Gα_s-signaling pathways in insulin-secreting β-cells to cause protein kinase A (PKA)/exchange protein activated by cAMP (Epac)-dependent release of insulin, activation of RAF proto-oncogene, Ser/Thr kinase (Raf)-mitogen-activated protein kinase (MAPK) signaling, induction of cAMP response element-binding protein (CREB)-insulin receptor substrate 2 (Irs-2), and increased β-cell proliferation. Interestingly, TAAR1 triggered cAMP-mediated calcium influx and release from internal stores, both of which were required for activation of a MAPK cascade utilizing calmodulin-dependent protein kinase II (CaMKII), Raf, and MAPK/ERK kinase 1/2 (MEK1/2). Together, these data identify TAAR1/Gα_s-mediated signaling pathways that promote insulin secretion, improved β-cell function and proliferation, and highlight TAAR1 as a promising new target for improving β-cell health in type 2 diabetes mellitus.

Antagonistic interaction between Nodal and insulin modulates pancreatic β-cell proliferation and survival

Background

Insulin signaling pathway in β-cell is essential to promote β-cells proliferation and survival, while Nodal–ALK7–Smad3 signaling involves β-cells apoptosis. We attempted to address inter-relationship between Nodal and insulin in modulating β-cell proliferation and apoptosis.

Methods

Using INS-1 β-cells and isolated rat islets, we examined the effects of Nodal, insulin, or the two combined on β-cell proliferation and/or apoptosis.

Results

The β-cells under high-glucose or palmitate conditions showed significant up-regulation of Nodal expression and activation of its downstream signaling pathway resulted in increased cleaved caspase-3. Insulin treatment led to significantly attenuated Nodal-induced cell apoptotic pathway. Similar results were found in directly Nodal-treated β-cell that insulin could partially block Nodal-induced up-regulation of ALK7–Smad3–caspase-3 signaling pathways with significantly attenuated β-cell apoptosis. Interestingly, we found that insulin-induced Akt activation and downstream molecules including GSK-3β, β-catenin and ERK1/2 was significantly attenuated by the co-treatment with Nodal, resulted in decreased cell proliferation. Furthermore, Nodal decreased glucose-evoked calcium influx and played a negative role during glucose-stimulated insulin secretion in the β-cells. Immunocytochemistry studies showed that Nodal treatment translocated Smad3 from cytosol mostly to the nucleus; however, co-treatment with insulin significantly decreased Smad3 nuclear localization. Co-immunoprecipitation experiments showed a directly interaction between Smad3 and Akt, and this interaction was enhanced by co-treatment with insulin.

Conclusions

Our data suggest that the antagonistic interaction between Nodal and insulin has a role in the regulation of β-cell mass and secretion.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0288-0) contains supplementary material, which is available to authorized users.

Prolonged Exposure to Insulin Inactivates Akt and Erk1/2 and Increases Pancreatic Islet and INS1E β-Cell Apoptosis

Chronic hyperinsulinemia, in vivo, increases the resistance of peripheral tissues to insulin by desensitizing insulin signaling. Insulin, in a heterologous manner, can also cause IGF-1 resistance. The aim of the current study was to investigate whether insulin-mediated insulin and IGF-1 resistance develops in pancreatic β-cells and whether this resistance results in β-cell decompensation. Chronic exposure of rat islets or INS1E β-cells to increasing concentrations of insulin decreased Akt^S473 phosphorylation in response to subsequent acute stimulation with 10 nM insulin or IGF-1. Prolonged exposure to high insulin levels not only inhibited Akt^S473 phosphorylation, but it also resulted in a significant inhibition of the phosphorylation of P70S6 kinase and Erk_1/2 phosphorylation in response to the acute stimulation by glucose, insulin, or IGF-1. Decreased activation of Akt, P70S6K, and Erk_1/2 was associated with decreased insulin receptor substrate 2 tyrosine phosphorylation and insulin receptor β-subunit abundance; neither IGF receptor β-subunit content nor its phosphorylation were affected. These signaling impairments were associated with decreased SERCA2 expression, perturbed plasma membrane calcium current and intracellular calcium handling, increased endoplasmic reticulum stress markers such as eIF2α^S51 phosphorylation and Bip (GRP78) expression, and increased islet and β-cell apoptosis. We demonstrate that prolonged exposure to high insulin levels induces not only insulin resistance, but in a heterologous manner causes resistance to IGF-1 in rat islets and insulinoma cells resulting in decreased cell survival. These findings suggest the possibility that chronic exposure to hyperinsulinemia may negatively affect β-cell mass by increasing β-cell apoptosis.

The Regulatory Roles of Mitogen-Activated Protein Kinase (MAPK) Pathways in Health and Diabetes: Lessons Learned from the Pancreatic β-Cell

BACKGROUND

Glucose-stimulated insulin secretion (GSIS) from the pancreatic β-cell involves several intracellular metabolic events which lead to the translocation of insulin granules towards the membrane for fusion and release. It is well established that loss of β-cell function and decreased GSIS underlie the pathogenesis of diabetes. Evidence from several laboratories, including our own, demonstrated requisite roles of Rac1 and phagocyte-like NADPH oxidase (Nox2)-derived reactive oxygen species (ROS) in optimal function of the pancreatic β-cell, including GSIS. However, it is becoming increasingly clear that prolonged exposure of β-cells to hyperglycemic conditions, leads to sustained activation of Rac1-Nox2 signaling axis culminating in excessive generation of intracellular ROS (oxidative stress) and β-cell dysregulation and demise. Such "cytotoxic" effects of ROS appear to be mediated via the stress-activated protein kinases/mitogen-activated protein kinases (SAPK/MAPK) signaling pathways.

OBJECTIVE

This review discusses our current understanding of regulation and functions of the conventional MAPKs, namely, ERK1/2, JNK1/2 and p38MAPK.

CONCLUSION

The MAPK pathways are activated in the presence of various stress stimuli including intracellular ROS, via distinct signaling cascades. Once activated, MAPKs participate in specific intracellular signaling processes via interaction with several downstream kinases including the MAPKactivated protein kinases (MAPKAPKs) and transcription factors including c-jun and p53. We have provided an overview of existing evidence in the islet β-cell on the regulatory roles of these MAPKs in mediating cellular responses to alterations in intracellularly generated ROS, which is mediated by the Rac1-Nox2 signaling module. Additionally, we enlisted recent patents developed to improve β-cell function in diabetes and novel pharmacological agents that target oxidative stress and MAPK pathways.

ERK1 is dispensable for mouse pancreatic beta cell function but is necessary for glucose-induced full activation of MSK1 and CREB

AIMS/HYPOTHESIS

Insufficient insulin secretion from pancreatic beta cells, which is associated with a decrease in beta cell mass, is a characteristic of type 2 diabetes. Extracellular signal-related kinase 1 and 2 (ERK1/2) inhibition in beta cells has been reported to affect insulin secretion, gene transcription and survival, although whether ERK1 and ERK2 play distinct roles is unknown. The aim of this study was to assess the individual roles of ERK1 and ERK2 in beta cells using ERK1 (also known as Mapk3)-knockout mice (Erk1 ^-/- mice) and pharmacological approaches.

METHODS

NAD(P)H, free cytosolic Ca²⁺ concentration and insulin secretion were determined in islets. ERK1 and ERK2 subplasmalemmal translocation and activity was monitored using total internal reflection fluorescence microscopy. ERK1/2, mitogen and stress-activated kinase1 (MSK1) and cAMP-responsive element-binding protein (CREB) activation were evaluated by western blot and/or immunocytochemistry. The islet mass was determined from pancreatic sections.

RESULTS

Glucose induced rapid subplasmalemmal recruitment of ERK1 and ERK2. When both ERK1 and ERK2 were inhibited simultaneously, the rapid transient peak of the first phase of glucose-induced insulin secretion was reduced by 40% (p < 0.01), although ERK1 did not appear to be involved in this process. By contrast, ERK1 was required for glucose-induced full activation of several targets involved in beta cell survival; MSK1 and CREB were less active in Erk1 ^-/- mouse beta cells (p < 0.01) compared with Erk1 ^+/+ mouse beta cells, and their phosphorylation could only be restored when ERK1 was re-expressed and not when ERK2 was overexpressed. Finally, the islet mass of Erk1 ^-/- mice was slightly increased in young animals (4-month-old mice) vs Erk1 ^+/+ mice (section occupied by islets [mean ± SEM]: 0.74% ± 0.03% vs 0.62% ± 0.04%; p < 0.05), while older mice (10 months old) were less prone to age-associated pancreatic peri-insulitis (infiltrated islets [mean ± SEM]: 7.51% ± 1.34% vs 2.03% ± 0.51%; p < 0.001).

CONCLUSIONS/INTERPRETATION

ERK1 and ERK2 play specific roles in beta cells. ERK2 cannot always compensate for the lack of ERK1 but the absence of a clear-cut phenotype in Erk1 ^-/- mice shows that ERK1 is dispensable in normal conditions.

Glucagon-like peptide-1 receptor agonist exendin-4 protects against interleukin-1β-mediated inhibition of glucose-stimulated insulin secretion by mouse insulinoma β cells

The aim of the present study was to investigate the protective effect of the glucagon-like peptide-1 receptor agonist exendin-4 on the interleukin (IL)-1β-induced impairment of glucose-stimulated insulin secretion (GSIS) in β-TC-6 cells. β-TC-6 cells were pretreated with various concentrations of IL-1β (0.15, 1.5 or 15 ng/ml) and exendin-4 (0.1 or 1 mM). Exendin-4 was administered to β-TC-6 cells prior to, during and following pretreatment. Cells were stimulated with various concentrations of glucose (0, 1.38, 5.5 and 11.1 mM), and insulin was measured via radioimmunoassay of the supernatant; furthermore, western blot analysis was used to detect phosphorylated extracellular receptor kinase (ERK)1/2. The insulin levels (151.08±14.34 µIU/ml) and ERK1/2 phosphorylation in β-TC-6 cells peaked in response to 1.38 mM glucose stimulation compared with 0, 5.5 and 11.1 mM glucose stimulation. IL-1β inhibited GSIS in a dose-dependent manner: Insulin levels were 83.76±1.16 µIU/ml when 0.15 ng/ml IL-1β was added under GSIS, 59.46±3.20 µIU/ml when 1.5 ng/ml IL-1β was added under GSIS, and 56.98±1.19 µIU/ml when 15 ng/ml IL-1β was added under GSIS. Exendin-4 exerted a protective effect against IL-1β-induced GSIS inhibition in a dose-dependent manner. The greatest protective effect was observed when exendin-4 was added prior to IL-1β pretreatment, which was statistically significant (P<0.05). These findings suggested that exendin-4 was able to reverse the IL-1β-induced inhibition of ERK1/2 phosphorylation and serves a protective role by impairing GSIS induced by IL-1β in β-TC-6 cells. This mechanism may be associated with the recovery of ERK1/2 activation.

Guanine nucleotide exchange factor Epac2-dependent activation of the GTP-binding protein Rap2A mediates cAMP-dependent growth arrest in neuroendocrine cells

First messenger-dependent activation of MAP kinases in neuronal and endocrine cells is critical for cell differentiation and function and requires guanine nucleotide exchange factor (GEF)-mediated activation of downstream Ras family small GTPases, which ultimately lead to ERK, JNK, and p38 phosphorylation. Because there are numerous GEFs and also a host of Ras family small GTPases, it is important to know which specific GEF-small GTPase dyad functions in a given cellular process. Here we investigated the upstream activators and downstream effectors of signaling via the GEF Epac2 in the neuroendocrine NS-1 cell line. Three cAMP sensors, Epac2, PKA, and neuritogenic cAMP sensor-Rapgef2, mediate distinct cellular outputs: p38-dependent growth arrest, cAMP response element-binding protein-dependent cell survival, and ERK-dependent neuritogenesis, respectively, in these cells. Previously, we found that cAMP-induced growth arrest of PC12 and NS-1 cells requires Epac2-dependent activation of p38 MAP kinase, which posed the important question of how Epac2 engages p38 without simultaneously activating other MAP kinases in neuronal and endocrine cells. We now show that the small GTP-binding protein Rap2A is the obligate effector for, and GEF substrate of, Epac2 in mediating growth arrest through p38 activation in NS-1 cells. This new pathway is distinctly parcellated from the G protein-coupled receptor → G_s → adenylate cyclase → cAMP → PKA → cAMP response element-binding protein pathway mediating cell survival and the G protein-coupled receptor → G_s → adenylate cyclase → cAMP → neuritogenic cAMP sensor-Rapgef2 → B-Raf → MEK → ERK pathway mediating neuritogenesis in NS-1 cells.

Role of TRPV channels in regulating various pancreatic β-cell functions: Lessons from in vitro studies

Pancreatic β-cell functions are regulated by a variety of endogenous and exogenous factors. Calcium is one of the most potent triggers of β-cell growth, insulin production and exocytosis. Recently, others and we showed that TRPV channels are expressed in insulin producing cell lines and/or primary β-cells. These channels modulate calcium ions, insulin secretion and cell proliferation. Besides the classical roles of TRPV channels in the sensory system, there are also novel functions described in non-excitable cells such as in insulin-producing β-cells. This review summarises the current knowledge about the expression and the role of TRPV channels in controlling β-cell functions based upon studies performed in isolated primary β-cells as well as permanent β-cell models.

Role of extracellular signal‑regulated kinase 1/2 signal transduction pathway in insulin secretion by β‑TC6 cells

The present study aimed to investigate the role of the extracellular signal-regulated kinase (ERK)1/2 signal transduction pathway in glucose‑stimulated insulin secretion in β‑TC6 mouse pancreatic cells. Insulin production by β‑TC6 cells was stimulated with various concentrations of glucose, which was dose-dependently inhibited by mitogen‑activated protein kinase inhibitor PD98059, as indicated by a radioimmunoassay. Furthermore, glucose stimulation enhanced the phosphorylation of ERK1/2, which was dose-dependently inhibited by PD98059, as indicated by western blot analysis. These results indicated that the activation of the ERK1/2 signal transduction pathway may have an important role in glucose‑stimulated insulin secretion in β‑TC6 cells.

Ca2+ influx through L-type Ca2+ channels and Ca2+-induced Ca2+ release regulate cAMP accumulation and Epac1-dependent ERK 1/2 activation in INS-1 cells

We previously reported that INS-1 cells expressing the intracellular II-III loop of the L-type Ca(2+) channel Cav1.2 (Cav1.2/II-III cells) are deficient in Ca(2+)-induced Ca(2+) release (CICR). Here we show that glucose-stimulated ERK 1/2 phosphorylation (GSEP) is slowed and reduced in Cav1.2/II-III cells compared to INS-1 cells. This parallels a decrease in glucose-stimulated cAMP accumulation (GS-cAMP) in Cav1.2/II-III cells. Influx of Ca(2+) via L-type Ca(2+) channels and CICR play roles in both GSEP and GS-cAMP in INS-1 cells since both are inhibited by nicardipine or ryanodine. Further, the Epac1-selective inhibitor CE3F4 abolishes glucose-stimulated ERK activation in INS-1 cells, as measured using the FRET-based sensor EKAR. The non-selective Epac antagonist ESI-09 but not the Epac2-selective antagonist ESI-05 nor the PKA antagonist Rp-cAMPs inhibits GSEP in both INS-1 and Cav1.2/II-III cells. We conclude that L-type Ca(2+) channel-dependent cAMP accumulation, that's amplified by CICR, activates Epac1 and drives GSEP in INS-1 cells.

Different downstream signalling of CCK1 receptors regulates distinct functions of CCK in pancreatic beta cells

BACKGROUND AND PURPOSE

Cholecystokinin (CCK) is secreted by intestinal I cells and regulates important metabolic functions. In pancreatic islets, CCK controls beta cell functions primarily through CCK1 receptors, but the signalling pathways downstream of these receptors in pancreatic beta cells are not well defined.

EXPERIMENTAL APPROACH

Apoptosis in pancreatic beta cell apoptosis was evaluated using Hoechst-33342 staining, TUNEL assays and Annexin-V-FITC/PI staining. Insulin secretion and second messenger production were monitored using ELISAs. Protein and phospho-protein levels were determined by Western blotting. A glucose tolerance test was carried out to examine the functions of CCK-8s in streptozotocin-induced diabetic mice.

KEY RESULTS

The sulfated carboxy-terminal octapeptide CCK26-33 amide (CCK-8s) activated CCK1 receptors and induced accumulation of both IP3 and cAMP. Whereas Gq -PLC-IP3 signalling was required for the CCK-8s-induced insulin secretion under low-glucose conditions, Gs -PKA/Epac signalling contributed more strongly to the CCK-8s-mediated insulin secretion in high-glucose conditions. CCK-8s also promoted formation of the CCK1 receptor/β-arrestin-1 complex in pancreatic beta cells. Using β-arrestin-1 knockout mice, we demonstrated that β-arrestin-1 is a key mediator of both CCK-8s-mediated insulin secretion and of its the protective effect against apoptosis in pancreatic beta cells. The anti-apoptotic effects of β-arrestin-1 occurred through cytoplasmic late-phase ERK activation, which activates the 90-kDa ribosomal S6 kinase-phospho-Bcl-2-family protein pathway.

CONCLUSIONS AND IMPLICATIONS

Knowledge of different CCK1 receptor-activated downstream signalling pathways in the regulation of distinct functions of pancreatic beta cells could be used to identify biased CCK1 receptor ligands for the development of new anti-diabetic drugs.

Short-term low-protein diet during pregnancy alters islet area and protein content of phosphatidylinositol 3-kinase pathway in rats

The phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways mediate β cell growth, proliferation, survival and death. We investigated whether protein restriction during pregnancy alters islet morphometry or the expression and phosphorylation of several proteins involved in the phosphatidylinositol 3-kinase and mitogen-activated protein kinase pathways. As controls, adult pregnant and non-pregnant rats were fed a normal-protein diet (17%). Pregnant and non-pregnant rats in the experimental groups were fed a low-protein diet (6%) for 15 days. Low protein diet during pregnancy increased serum prolactin level, reduced serum corticosterone concentration and the expression of both protein kinase B/AKT1 (AKT1) and p70 ribosomal protein S6 kinase (p70S6K), as well as the islets area, but did not alter the insulin content of pancreatic islets. Pregnancy increased the expression of the Src homology/collagen (SHC) protein and the extracellular signal-regulated kinases 1/2 (ERK1/2) independent of diet. ERK1/2 phosphorylation (pERK1/2) was similar in islets from pregnant and non-pregnant rats fed a low-protein diet, and was higher in islets from pregnant rats than in islets from non-pregnant rats fed a normal-protein diet. Thus, a short-term, low-protein diet during pregnancy was sufficient to reduce the levels of proteins in the phosphatidylinositol 3-kinase pathway and affect islet morphometry.

Transcriptional profiling reveals functional links between RasGrf1 and Pttg1 in pancreatic beta cells

BACKGROUND

Our prior characterization of RasGrf1 deficient mice uncovered significant defects in pancreatic islet count and size as well as beta cell development and signaling function, raising question about the mechanisms linking RasGrf1 to the generation of those "pancreatic" phenotypes.

RESULTS

Here, we compared the transcriptional profile of highly purified pancreatic islets from RasGrf1 KO mice to that of WT control animals using commercial oligonucleotide microarrays. RasGrf1 elimination resulted in differential gene expression of numerous components of MAPK- and Calcium-signaling pathways, suggesting a relevant contribution of this GEF to modulation of cellular signaling in the cell lineages integrating the pancreatic islets. Whereas the overall transcriptional profile of pancreatic islets was highly specific in comparison to other organs of the same KO mice, a significant specific repression of Pttg1 was a common transcriptional alteration shared with other tissues of neuroectodermal origin. This observation, together with the remarkable pancreatic phenotypic similarities between RasGrf1 KO and Pttg1 KO mice suggested the possibility of proximal functional regulatory links between RasGrf1 and Pttg1 in pancreatic cell lineages expressing these proteins.Analysis of the mPttg1 promoter region identified specific recognition sites for numerous transcription factors which were also found to be differentially expressed in RasGrf1 KO pancreatic islets and are known to be relevant for Ras-ERK signaling as well as beta cell function. Reporter luciferase assays in BT3 insulinoma cells demonstrated the ability of RasGrf1 to modulate mPttg1 promoter activity through ERK-mediated signals. Analysis of the phenotypic interplay between RasGrf1 and Pttg1 in double knockout RasGrf1/Pttg1 mice showed that combined elimination of the two loci resulted in dramatically reduced values of islet and beta cell count and glucose homeostasis function which neared those measured in single Pttg1 KO mice and were significantly lower than those observed in individual RasGrf1 KO mice.

CONCLUSIONS

The specific transcriptional profile and signaling behavior of RasgGrf1 KO pancreatic islets, together with the dominance of Pttg1 over RasGrf1 with regards to the generation of these phenotypes in mouse pancreas, suggest that RasGrf1 is an important upstream component of signal transduction pathways regulating Pttg1 expression and controlling beta cell development and physiological responses.

New insights into the roles of CHOP-induced apoptosis in ER stress

Endoplasmic reticulum stress (ER stress) is triggered due to a loss of homeostasis in the ER, resulting in accumulation of misfolded proteins in the ER lumen. ER stress activates a series of adaptive mechanisms known as the unfolded protein response. Perturbation of the ER is a powerful inducer of the transcription factor C/EBP homologous protein (CHOP). Although it has been proved that excessive or adverse stress to the ER triggers apoptosis, the specific mechanisms underlying these processes induced by CHOP remain unclear. By now, CHOP-induced apoptosis in ER stress has been implicated in numerous human diseases, such as neurodegenerative diseases, diabetes, ischemic diseases, tumor, and so on. In this review, we summarized the current understanding of the roles of CHOP in the development of several diseases from the laboratory to the clinic.

Transcription factor Ets1 regulates expression of thioredoxin-interacting protein and inhibits insulin secretion in pancreatic β-cells

Long-term activation of extracellular-regulated kinase (ERK1/2) pathway has been shown to cause glucotoxicity and inhibit insulin gene expression in β-cells. Transcription factor Ets1 is activated by ERK1/2-mediated phosphorylation at the Thr38 residue. We hypothesize that Ets1 plays an important role in mediating ERK1/2 induced glucotoxicity in β-cells. We determined the role of Ets1 in Min6 cells and isolated mouse islets using overexpression and siRNA mediated knockdown of Ets1. The results show that Ets1 was localized in insulin-staining positive cells but not in glucagon-staining positive cells. Overexpression of Ets1 reduced glucose-stimulated insulin secretion in primary mouse islets. Overexpression of Ets1 in Min6 β-cells and mouse islets increased expression of thioredoxin-interacting protein (TXNIP). Conversely, knockdown of Ets1 by siRNA reduced expression of TXNIP in Min6 cells. Ets1 was associated with the txnip promoter in min6 cells and transfection of 293 cells with Ets1 and p300 synergistically increased txnip promoter reporter activity. Moreover, overexpression of Ets1 inhibited Min6 cell proliferation. Our results suggest that Ets1, by promoting TXNIP expression, negatively regulates β-cell function. Thus, over-activation of Ets1 may contribute to diet-induced β-cell dysfunction.

Fibrin improves beta (INS-1) cell function, proliferation and survival through integrin αvβ3

Extracellular matrix (ECM)-integrin stimulation can promote beta cell differentiation, proliferation and function. However, beta cells lose their insulin secretion function in response to glucose stimulation, and senesce when cultured with ECM proteins for a long time. Fibrin is a provisional ECM protein that is capable of maintaining beta cell function, yet the mechanisms by which this occurs is unknown. The present study examined how fibrin interacts with integrin receptors to promote beta cell cluster formation, proliferation and function. The rat insulinoma cell line, INS-1, was cultured on tissue-culture polystyrene, or with 2-D or 3-D fibrin gels for up to 4 weeks. Cells cultured with fibrin formed islet-like clusters and showed direct contacts with fibrin determined by scanning electron microscopy. Fibrin-cultured INS-1 cells also had significantly increased glucose-stimulated insulin secretion. A significant increase in integrin αvβ3 protein and phosphorylated FAK, Erk1/2 and Akt levels was observed in fibrin-cultured INS-1 cells, which was associated with significantly increased cell proliferation and decreased cell apoptosis. Integrin αvβ3 blockade affected INS-1 cell spreading on fibrin gels, and resulted in significantly decreased FAK phosphorylation and increased cleaved caspase-3 levels. These results show that fibrin promotes beta cell function, proliferation and survival via integrin αvβ3 interactions.

Off-target effects of MEK inhibitors

The mitogen-activated protein kinases (MAPKs) ERK1/2 regulate numerous cellular processes, including gene transcription, proliferation, and differentiation. The only known substrates of the MAP2Ks MEK1/2 are ERK1/2; thus, MEK inhibitors PD98059, U0126, and PD0325901 have been important tools in determining the functions of ERK1/2. By using these inhibitors and genetically manipulating MEK, we found that ERK1/2 activation is neither sufficient nor necessary for regulated secretion of insulin from pancreatic β cells or secretion of epinephrine from chromaffin cells. We show that both PD98059 and U0126 reduce agonist-induced entry of calcium into cells in a manner independent of their ability to inhibit ERK1/2. Caution should be used when interpreting results from experiments using these compounds.

High glucose induces mitochondrial p53 phosphorylation by p38 MAPK in pancreatic RINm5F cells

Pancreatic β-cell death in type 2 diabetes has been related to p53 subcellular localisation and phosphorylation. However, the mechanisms by which p53 is phosphorylated and its activation in response to oxidative stress remain poorly understood. Therefore, the aim of this study was to investigate mitochondrial p53 phosphorylation, its subcellular localisation and its relationship with apoptotic induction in RINm5F cells cultured under high glucose conditions. Our results show that p53 phosphorylation in the mitochondrial fraction was greater at ser392 than at ser15. This increased phosphorylation correlated with an increase in reactive oxygen species, a decrease in the Bcl-2/Bax ratio, a release of cytochrome c and an increase in the rate of apoptosis. We also observed a decline in ERK 1/2 phosphorylation over time, which is an indicator of cell proliferation. To identify the kinase responsible for phosphorylating p53, p38 mitogen-activated protein kinase (MAPK) activation was analysed. We found that high glucose induced an increase in p38 MAPK phosphorylation in the mitochondria after 24-72 h. Moreover, the phosphorylation of p53 (ser392) by p38 MAPK in mitochondria was confirmed by colocalisation studies with confocal microscopy. The addition of a specific p38 MAPK inhibitor (SB203580) to the culture medium during high glucose treatment blocked p53 mobilisation to the mitochondria and phosphorylation; thus, the release of cytochrome c and the apoptosis rate in RINm5F cells decreased. These results suggest that mitochondrial p53 phosphorylation by p38 MAPK plays an important role in RINm5F cell death under high glucose conditions.

Chronic activation of a designer G(q)-coupled receptor improves β cell function

Type 2 diabetes (T2D) has emerged as a major threat to human health in most parts of the world. Therapeutic strategies aimed at improving pancreatic β cell function are predicted to prove beneficial for the treatment of T2D. In the present study, we demonstrate that drug-mediated, chronic, and selective activation of β cell G(q) signaling greatly improve β cell function and glucose homeostasis in mice. These beneficial metabolic effects were accompanied by the enhanced expression of many genes critical for β cell function, maintenance, and differentiation. By employing a combination of in vivo and in vitro approaches, we identified a novel β cell pathway through which receptor-activated G(q) leads to the sequential activation of ERK1/2 and IRS2 signaling, thus triggering a series of events that greatly improve β cell function. Importantly, we found that chronic stimulation of a designer G(q)-coupled receptor selectively expressed in β cells prevented both streptozotocin-induced diabetes and the metabolic deficits associated with the consumption of a high-fat diet in mice. Since β cells are endowed with numerous receptors that mediate their cellular effects via activation of G(q)-type G proteins, our findings provide a rational basis for the development of novel antidiabetic drugs targeting this class of receptors.

Trefoil factor 2 promotes cell proliferation in pancreatic β-cells through CXCR-4-mediated ERK1/2 phosphorylation

Decreased β-cell mass is a hallmark of type 2 diabetes, and therapeutic approaches to increase the pancreatic β-cell mass have been expected. In recent years, gastrointestinal incretin peptides have been shown to exert a cell-proliferative effect in pancreatic β-cells. Trefoil factor 2 (TFF2), which is predominantly expressed in the surface epithelium of the stomach, plays a role in antiapoptosis, migration, and proliferation. The TFF family is expressed in pancreatic β-cells, whereas the role of TFF2 in pancreatic β-cells has been obscure. In this study, we investigated the mechanism by which TFF2 enhances pancreatic β-cell proliferation. The effects of TFF2 on cell proliferation were evaluated in INS-1 cells, MIN6 cells, and mouse islets using an adenovirus vector containing TFF2 or a recombinant TFF2 peptide. The forced expression of TFF2 led to an increase in bromodeoxyuridine (BrdU) incorporation in both INS-1 cells and islets, without any alteration in insulin secretion. TFF2 significantly increased the mRNA expression of cyclin A2, D1, D2, D3, and E1 in islets. TFF2 peptide increased ERK1/2 phosphorylation and BrdU incorporation in MIN6 cells. A MAPK kinase inhibitor (U0126) abrogated the TFF2 peptide-mediated proliferation of MIN6 cells. A CX-chemokine receptor-4 antagonist also prevented the TFF2 peptide-mediated increase in ERK1/2 phosphorylation and BrdU incorporation in MIN6 cells. These results indicated that TFF2 is involved in β-cell proliferation at least partially via CX-chemokine receptor-4-mediated ERK1/2 phosphorylation, suggesting TFF2 may be a novel target for inducing β-cell proliferation.

Molecular mechanisms of muscarinic acetylcholine receptor-stimulated increase in cytosolic free Ca(2+) concentration and ERK1/2 activation in the MIN6 pancreatic β-cell line

Muscarinic acetylcholine receptor (mAChR) activation of pancreatic β-cells elevates intracellular Ca(2+) and potentiates glucose-stimulated insulin secretion. In addition, it activates a number of signaling molecules, including ERK1/2, whose activation has been shown to play an important role in regulating pancreatic β-cell function and mass. The aim of this work was to determine how mAChR activation elevates intracellular Ca(2+) concentration ([Ca(2+)]( i )) and activates ERK1/2 in the pancreatic β-cell line MIN6. We demonstrate that agonist-stimulated ERK1/2 activation is dependent on the activation of phospholipase C and an elevation in [Ca(2+)]( i ), but is independent of the activation of diacylglycerol-dependent protein kinase C isoenzymes. Using a pharmacological approach, we provide evidence that agonist-induced increases in [Ca(2+)]( i ) and ERK activity require (1) IP(3) receptor-mediated mobilization of Ca(2+) from the endoplasmic reticulum, (2) influx of extracellular Ca(2+) through store-operated channels, (3) closure of K(ATP) channels, and (4) Ca(2+) entry via L-type voltage-operated Ca(2+) channels. Moreover, this Ca(2+)-dependent activation of ERK is mediated via both Ras-dependent and Ras-independent mechanisms. In summary, this study provides important insights into the multifactorial signaling mechanisms linking mAChR activation to increases in [Ca(2+)]( i ) and ERK activity.

Neuronal Cbl controls biosynthesis of insulin-like peptides in Drosophila melanogaster

The Cbl family proteins function as both E3 ubiquitin ligases and adaptor proteins to regulate various cellular signaling events, including the insulin/insulin-like growth factor 1 (IGF1) and epidermal growth factor (EGF) pathways. These pathways play essential roles in growth, development, metabolism, and survival. Here we show that in Drosophila melanogaster, Drosophila Cbl (dCbl) regulates longevity and carbohydrate metabolism through downregulating the production of Drosophila insulin-like peptides (dILPs) in the brain. We found that dCbl was highly expressed in the brain and knockdown of the expression of dCbl specifically in neurons by RNA interference increased sensitivity to oxidative stress or starvation, decreased carbohydrate levels, and shortened life span. Insulin-producing neuron-specific knockdown of dCbl resulted in similar phenotypes. dCbl deficiency in either the brain or insulin-producing cells upregulated the expression of dilp genes, resulting in elevated activation of the dILP pathway, including phosphorylation of Drosophila Akt and Drosophila extracellular signal-regulated kinase (dERK). Genetic interaction analyses revealed that blocking Drosophila epidermal growth factor receptor (dEGFR)-dERK signaling in pan-neurons or insulin-producing cells by overexpressing a dominant-negative form of dEGFR abolished the effect of dCbl deficiency on the upregulation of dilp genes. Furthermore, knockdown of c-Cbl in INS-1 cells, a rat β-cell line, also increased insulin biosynthesis and glucose-stimulated secretion in an ERK-dependent manner. Collectively, these results suggest that neuronal dCbl regulates life span, stress responses, and metabolism by suppressing dILP production and the EGFR-ERK pathway mediates the dCbl action. Cbl suppression of insulin biosynthesis is evolutionarily conserved, raising the possibility that Cbl may similarly exert its physiological actions through regulating insulin production in β cells.

Reduced expression of the LRP16 gene in mouse insulinoma (MIN6) cells exerts multiple effects on insulin content, proliferation and apoptosis

This study assessed the effects of leukemia-related protein 16 (LRP16) on the regulation of pancreatic functions in mouse insulinoma (MIN6) cells. Cells with down-regulated expression of LRP16 were obtained by a shRNA interference strategy. Insulin content and glucose-stimulated insulin secretion (GSIS) were examined by radioimmunoassay. Western blotting was applied to detect protein expression. Glucose-stimulated sub-cellular localization of PDX-1 was immunocytochemically determined. Cell proliferation and apoptosis were detected by flow cytometry. Our results showed that LRP16 regulated insulin content in MIN6 cells by controlling expression of insulin and insulin transcription factors. LRP16 gene silence in MIN6 cells led to reduced cell proliferation and increased apoptosis. The observation of phosphorylation of serine-473 Akt and the localization of PDX-1 to the nucleus under glucose-stimulation exhibited that LRP16 was a component mediating Akt signaling in MIN6 cells. These results suggest that LRP16 plays a key role in maintaining pancreatic β-cell functions and may help us to understand the protective effects of estrogen on the functions of pancreatic β-cells.

Protein phosphorylation involved in the gene expression of the hydrogen sulphide producing enzyme cystathionine γ-lyase in the pancreatic β-cell

Cystathionine γ-lyase (CSE) is one of the major enzymes for the production of hydrogen sulphide (H(2)S), a multifunctional gasotransmitter in the pancreatic β-cell. We examined the mechanisms by which glucose induces CSE expression in mouse pancreatic islets and the insulin-secreting cell line MIN6. CSE expression was increased by anti-diabetic sulphonylureas, and decreased by the ATP-sensitive K(+)-channel opener diazoxide and the voltage-dependent Ca(2+) channel blocker nitrendipine. Application of the synthetic inhibitors of protein kinases revealed the involvement of Ca(2+)/calmodulin-dependent protein kinase (CaMK) II and extracellular signal-regulated protein kinase (ERK) in glucose- and thapsigargin-induced CSE expression. The CaMK IIδ knockdown also suppressed CSE expression. Knockdown of the transcription factors Sp1 and Elk1, both of which can be phosphorylated by ERK, blunted CSE expression. By a reporter assay, we found Sp1 may directly and Elk1 may indirectly regulate CSE expression. These findings suggest Ca(2+)-dependent CSE expression may be mediated via protein phosphorylation of Sp1 and Elk1 in pancreatic β-cells.

Endurance training stimulates growth and survival pathways and the redox balance in rat pancreatic islets

Endurance training has been shown to increase pancreatic β-cell function and mass. However, whether exercise modulates β-cell growth and survival pathways signaling is not completely understood. This study investigated the effects of exercise on growth and apoptotic markers levels in rat pancreatic islets. Male Wistar rats were randomly assigned to 8-wk endurance training or to a sedentary control group. After that, pancreatic islets were isolated; gene expression and the total content and phosphorylation of several proteins related to growth and apoptotic pathways as well as the main antioxidant enzymes were determined by real-time polymerase chain reaction and Western blot analysis, respectively. Reactive oxygen species (ROS) production was measured by fluorescence. Endurance training increased the time to reach fatigue by 50%. Endurance training resulted in increased protein phosphorylation content of AKT (75%), AKT substrate (AS160; 100%), mTOR (60%), p70s6k (90%), and ERK1/2 (50%), compared with islets from control group. Catalase protein content was 50% higher, whereas ROS production was 49 and 77% lower in islets from trained rats under basal and stimulating glucose conditions, respectively. Bcl-2 mRNA and protein levels increased by 46 and 100%, respectively. Bax and cleaved caspase-3 protein contents were reduced by 25 and 50% in islets from trained rats, respectively. In conclusion, these results demonstrate that endurance training favors the β-cell growth and survival by activating AKT and ERK1/2 pathways, enhancing antioxidant capacity, and reducing ROS production and apoptotic proteins content.

DNA methylation profiling identifies epigenetic dysregulation in pancreatic islets from type 2 diabetic patients

In addition to genetic predisposition, environmental and lifestyle factors contribute to the pathogenesis of type 2 diabetes (T2D). Epigenetic changes may provide the link for translating environmental exposures into pathological mechanisms. In this study, we performed the first comprehensive DNA methylation profiling in pancreatic islets from T2D and non-diabetic donors. We uncovered 276 CpG loci affiliated to promoters of 254 genes displaying significant differential DNA methylation in diabetic islets. These methylation changes were not present in blood cells from T2D individuals nor were they experimentally induced in non-diabetic islets by exposure to high glucose. For a subgroup of the differentially methylated genes, concordant transcriptional changes were present. Functional annotation of the aberrantly methylated genes and RNAi experiments highlighted pathways implicated in β-cell survival and function; some are implicated in cellular dysfunction while others facilitate adaptation to stressors. Together, our findings offer new insights into the intricate mechanisms of T2D pathogenesis, underscore the important involvement of epigenetic dysregulation in diabetic islets and may advance our understanding of T2D aetiology.

Multiple signaling pathways involved in stimulation of osteoblast differentiation by N-methyl-D-aspartate receptors activation in vitro

AIM

Glutamate receptors are expressed in osteoblastic cells. The present study was undertaken to investigate the mechanisms underlying the stimulation of osteoblast differentiation by N-methyl-D-aspartate (NMDA) receptor activation in vitro.

METHODS

Primary culture of osteoblasts was prepared from SD rats. Microarray was used to detect the changes of gene expression. The effect of NMDA receptor agonist or antagonist on individual gene was examined using RT-PCR. The activity of alkaloid phosphotase (ALP) was assessed using a commercial ALP staining kit.

RESULTS

Microarray analyses revealed that 10 genes were up-regulated by NMDA (0.5 mmol/L) and down-regulated by MK801 (100 μmol/L), while 13 genes down-regulated by NMDA (0.5 mmol/L) and up-regulated by MK801 (100 μmol/L). Pretreatment of osteoblasts with the specific PKC inhibitor Calphostin C (0.05 μmol/L), the PKA inhibitor H-89 (20 nmol/L), or the PI3K inhibitor wortmannin (100 nmol/L) blocked the ALP activity increase caused by NMDA (0.5 mmol/L). Furthermore, NMDA (0.5 mmol/L) rapidly increased PI3K phosphorylation, which could be blocked by pretreatment of wortmannin (100 nmol/L).

CONCLUSION

The results suggest that activation of NMDA receptors stimulates osteoblasts differentiation through PKA, PKC, and PI3K signaling pathways, which is a new role for glutamate in regulating bone remodeling.

Bimodal effect on pancreatic β-cells of secretory products from normal or insulin-resistant human skeletal muscle

OBJECTIVE

Type 2 diabetes is characterized by insulin resistance with a relative deficiency in insulin secretion. This study explored the potential communication between insulin-resistant human skeletal muscle and primary (human and rat) β-cells.

RESEARCH DESIGN AND METHODS

Human skeletal muscle cells were cultured for up to 24 h with tumor necrosis factor (TNF)-α to induce insulin resistance, and mRNA expression for cytokines was analyzed and compared with controls (without TNF-α). Conditioned media were collected and candidate cytokines were measured by antibody array. Human and rat primary β-cells were used to explore the impact of exposure to conditioned media for 24 h on apoptosis, proliferation, short-term insulin secretion, and key signaling protein phosphorylation and expression.

RESULTS

Human myotubes express and release a different panel of myokines depending on their insulin sensitivity, with each panel exerting differential effects on β-cells. Conditioned medium from control myotubes increased proliferation and glucose-stimulated insulin secretion (GSIS) from primary β-cells, whereas conditioned medium from TNF-α-treated insulin-resistant myotubes (TMs) exerted detrimental effects that were either independent (increased apoptosis and decreased proliferation) or dependent on the presence of TNF-α in TM (blunted GSIS). Knockdown of β-cell mitogen-activated protein 4 kinase 4 prevented these effects. Glucagon-like peptide 1 protected β-cells against decreased proliferation and apoptosis evoked by TMs, while interleukin-1 receptor antagonist only prevented the latter.

CONCLUSIONS

Taken together, these data suggest a possible new route of communication between skeletal muscle and β-cells that is modulated by insulin resistance and could contribute to normal β-cell functional mass in healthy subjects, as well as the decrease seen in type 2 diabetes.