Inhibition of the MAP3 kinase Tpl2 protects rodent and human β-cells from apoptosis and dysfunction induced by cytokines and enhances anti-inflammatory actions of exendin-4

Targeting Protein Kinases to Protect Beta-Cell Function and Survival in Diabetes

The prevalence of diabetes is increasing worldwide. Massive death of pancreatic beta-cells causes type 1 diabetes. Progressive loss of beta-cell function and mass characterizes type 2 diabetes. To date, none of the available antidiabetic drugs promotes the maintenance of a functional mass of endogenous beta-cells, revealing an unmet medical need. Dysfunction and apoptotic death of beta-cells occur, in particular, through the activation of intracellular protein kinases. In recent years, protein kinases have become highly studied targets of the pharmaceutical industry for drug development. A number of drugs that inhibit protein kinases have been approved for the treatment of cancers. The question of whether safe drugs that inhibit protein kinase activity can be developed and used to protect the function and survival of beta-cells in diabetes is still unresolved. This review presents arguments suggesting that several protein kinases in beta-cells may represent targets of interest for the development of drugs to treat diabetes.

Jiao-tai-wan and its effective component-berberine improve diabetes and depressive disorder through the cAMP/PKA/CREB signaling pathway

ETHNOPHARMACOLOGICAL RELEVANCE

Jiao-tai-wan (JTW), a classic herbal formula of traditional Chinese medicine recorded in Han Shi Yi Tong, has been used to alleviate sleep disorders since ancient times. In modern pharmacological research, JTW has been adopted for treating diabetes mellitus and even exerts antidepressant effects. However, the potential mechanisms deserve further elucidation.

AIM OF THE STUDY

The prevalence of diabetes mellitus combined with depressive disorder (DD) is continuing to increase, yet it is currently under-recognized and its treatment remains inadequate. The present study aims to explore the underlying therapeutics and mechanisms of JTW on DD.

MATERIALS AND METHODS

Chronic restraint stress was used on db/db mice to construct a mouse model of DD. The therapeutic effects of JTW were assessed by glucolipid metabolic indexes, behavioral tests, and depression-related neurotransmitter levels. The inflammatory status and cell apoptosis of different mice were investigated and the changes in the cAMP/PKA/CREB pathway were detected. Combining the results of fingerprinting with molecular docking, the active components of JTW were screened. A cellular model was constructed by intervention of glucose combined with corticosterone (CORT). The levels of apoptosis and depression-related neurotransmitters in HT-22 cells were examined, and the changes in the cAMP/PKA/CREB pathway were tested. Finally, the activator and inhibitor of the PKA protein were used for reverse validation experiments.

RESULTS

JTW could improve the impaired glucose tolerance, lipid metabolism disorders, and depression-like symptoms in DD mice. Meanwhile, JTW could alleviate the inflammatory status, suppress the microglia activation, and improve hippocampal neuron apoptosis in DD mice. The dual effects of JTW might be associated with the activation of the cAMP/PKA/CREB pathway. Berberine (Ber) was identified for the in vitro experiment, it could reverse the apoptosis of HT-22 cells and up-regulate the depression-related neurotransmitter levels, and the effects of Ber were related to the activation of the cAMP/PKA/CREB pathway as well.

CONCLUSION

JTW could exert both hypoglycemic and antidepressant effects through activating the cAMP/PKA/CREB signaling pathway, its active component, Ber, could improve the damage to HT-22 cells induced by glucose combined with CORT via the activation of the cAMP/PKA/CREB pathway. Ber may be one of the effective components of the dual effects of JTW.

Pharmacological inhibitors of β-cell dysfunction and death as therapeutics for diabetes

More than 500 million adults suffer from diabetes worldwide, and this number is constantly increasing. Diabetes causes 5 million deaths per year and huge healthcare costs per year. β-cell death is the major cause of type 1 diabetes. β-cell secretory dysfunction plays a key role in the development of type 2 diabetes. A loss of β-cell mass due to apoptotic death has also been proposed as critical for the pathogenesis of type 2 diabetes. Death of β-cells is caused by multiple factors including pro-inflammatory cytokines, chronic hyperglycemia (glucotoxicity), certain fatty acids at high concentrations (lipotoxicity), reactive oxygen species, endoplasmic reticulum stress, and islet amyloid deposits. Unfortunately, none of the currently available antidiabetic drugs favor the maintenance of endogenous β-cell functional mass, indicating an unmet medical need. Here, we comprehensively review over the last ten years the investigation and identification of molecules of pharmacological interest for protecting β-cells against dysfunction and apoptotic death which could pave the way for the development of innovative therapies for diabetes.

FGF-23 protects cell function and viability in murine pancreatic islets challenged by glucolipotoxicity

The fibroblast growth factor FGF-23 is a member of the FGF-15/19 subfamily with hormonal functions. Besides its well-known role for bone mineralization, FGF-23 is discussed as a marker for cardiovascular disease. We investigated whether FGF-23 has any effects on the endocrine pancreas of mice by determining insulin secretion, electrical activity, intracellular Ca2+, and apoptosis. Acute application of FGF-23 (10 to 500 ng/ml, i.e., 0.4 to 20 nM) does not affect insulin release of murine islets, while prolonged exposure leads to a 21% decrease in glucose-stimulated secretion. The present study shows for the first time that FGF-23 (100 or 500 ng/ml) partially protects against impairment of insulin secretion and apoptotic cell death induced by glucolipotoxicity. The reduction of apoptosis by FGF-23 is approximately twofold higher compared to FGF-21 or FGF-15/19. In contrast to FGF-23 and FGF-21, FGF-15/19 is clearly pro-apoptotic under control conditions. The beneficial effect of FGF-23 against glucolipotoxicity involves interactions with the stimulus-secretion cascade of beta-cells. Electrical activity and the rise in the cytosolic Ca2+ concentration of islets in response to acute glucose stimulation increase after glucolipotoxic culture (48 h). Co-culture with FGF-23 further elevates the glucose-mediated effects on both parameters. Protection against apoptosis and glucolipotoxic impairment of insulin release by FGF-23 is prevented, when calcineurin is inhibited by tacrolimus or when c-Jun N-terminal kinase (JNK) is blocked by SP600125. In conclusion, our data suggest that FGF-23 can activate compensatory mechanisms to maintain beta-cell function and integrity of islets of Langerhans during excessive glucose and lipid supply.

Tpl2 contributes to IL-1β-induced IL-8 expression via ERK1/2 activation in canine dermal fibroblasts

In autoimmune diseases, fibroblasts produce and secrete various cytokines and act as sentinel immune cells during inflammatory states. However, the contribution of sentinel immune cells (i.e. dermal fibroblasts) in autoimmune diseases of the skin, such as atopic dermatitis, has been obscure. The pro-inflammatory cytokine interleukin 1β (IL-1β) induces the expression of chemokines, such as interleukin 8 (IL-8), in autoimmune diseases of the skin. IL-8 induces the activation and recruitment of innate immune cells such as neutrophils to the site of inflammation. IL-1β-mediated induction of IL-8 expression is important for the pathogenesis of autoimmune diseases; however, the intracellular singling remains to be understood. To elucidate the mechanism of the onset of autoimmune diseases, we established a model for IL-1β-induced dermatitis and investigated MAPK signaling pathways in IL-1β-induced IL-8 expression. We also identified that a MAP3K Tpl2 acts as an upstream modulator of IL-1β-induced ERK1/2 activation in dermal fibroblasts. We observed an increase in the expression of IL-8 mRNA and protein in cells treated with IL-1β. ERK1/2 inhibitors significantly reduced IL-1β-induced IL-8 expression, whereas the inhibitor for p38 MAPK or JNK had no effect. IL-1β induced ERK1/2 phosphorylation, which was attenuated in the presence of an ERK1/2 inhibitor. IL-1β failed to induce IL-8 expression in cells transfected with siRNA for ERK1, or ERK2. Notably, a Tpl2 inhibitor reduced IL-1β-induced IL-8 expression and ERK1/2 phosphorylation. We confirmed that the silencing of Tpl2 in siRNA-transfected fibroblasts prevented both in IL-1β-induced IL-8 expression and ERK1/2 phosphorylation. Taken together, our data indicate the importance of Tpl2 in the modulation of ERK1/2 signaling involved in the IL-1β-induced development of autoimmune diseases affecting the dermal tissue, such as atopic dermatitis.

Mechanisms of Beta-Cell Apoptosis in Type 2 Diabetes-Prone Situations and Potential Protection by GLP-1-Based Therapies

Type 2 diabetes (T2D) is characterized by chronic hyperglycemia secondary to the decline of functional beta-cells and is usually accompanied by a reduced sensitivity to insulin. Whereas altered beta-cell function plays a key role in T2D onset, a decreased beta-cell mass was also reported to contribute to the pathophysiology of this metabolic disease. The decreased beta-cell mass in T2D is, at least in part, attributed to beta-cell apoptosis that is triggered by diabetogenic situations such as amyloid deposits, lipotoxicity and glucotoxicity. In this review, we discussed the molecular mechanisms involved in pancreatic beta-cell apoptosis under such diabetes-prone situations. Finally, we considered the molecular signaling pathways recruited by glucagon-like peptide-1-based therapies to potentially protect beta-cells from death under diabetogenic situations.

IL-33-Induced Transcriptional Activation of LPIN1 Accelerates Breast Tumorigenesis

Phospholipids are crucial materials that are not only required for cell membrane construction but also play significant roles as signaling molecules. LPIN1 is an enzyme that displays phosphatidate phosphatase activity in the triglyceride and phospholipid synthesis pathway. Recent studies have shown that overexpression of LPIN1 is involved in breast tumorigenesis, but the underlying mechanism regulating LPIN1 expression has not been elucidated yet. In the present study, we showed that the IL-33-induced COT-JNK1/2 signaling pathway regulates LPIN1 mRNA and protein expression by recruiting c-Jun to the LPIN1 promoter in breast cancer cells. IL-33 dose-dependently and time-dependently increased LPIN1 mRNA and protein expression. Moreover, IL-33 promoted colony formation and mammary tumorigenesis via induction of LPIN1 expression, while inhibition of LPIN1 disturbed IL-33-induced cell proliferation and mammary tumorigenesis. IL-33-driven LPIN1 expression was mediated by the COT-JNK1/2 signaling pathway, and inhibition of COT or JNK1/2 reduced LPIN1 expression. COT-JNK1/2-mediated IL-33 signaling activated c-Jun and promoted its binding to the promoter region of LPIN1 to induce LPIN1 expression. These findings demonstrated the regulatory mechanism of LPIN1 transcription by the IL-33-induced COT/JNK1/2 pathway for the first time, providing a potential mechanism underlying the upregulation of LPIN1 in cancer.

Activation of c-Jun N-Terminal Kinase, a Potential Therapeutic Target in Autoimmune Arthritis

The c-Jun-N-terminal kinase (JNK) is a critical mediator involved in various physiological processes, such as immune responses, and the pathogenesis of various diseases, including autoimmune disorders. JNK is one of the crucial downstream signaling molecules of various immune triggers, mainly proinflammatory cytokines, in autoimmune arthritic conditions, mainly including rheumatoid arthritis, ankylosing spondylitis, and psoriatic arthritis. The activation of JNK is regulated in a complex manner by upstream kinases and phosphatases. Noticeably, different subtypes of JNKs behave differentially in immune responses. Furthermore, aside from biologics targeting proinflammatory cytokines, small-molecule inhibitors targeting signaling molecules such as Janus kinases can act as very powerful therapeutics in autoimmune arthritis patients unresponsiveness to conventional synthetic antirheumatic drugs. Nevertheless, despite these encouraging therapies, a population of patients with an inadequate therapeutic response to all currently available medications still remains. These findings identify the critical signaling molecule JNK as an attractive target for investigation of the immunopathogenesis of autoimmune disorders and for consideration as a potential therapeutic target for patients with autoimmune arthritis to achieve better disease control. This review provides a useful overview of the roles of JNK, how JNK is regulated in immunopathogenic responses, and the potential of therapeutically targeting JNK in patients with autoimmune arthritis.

Tumor progression locus 2 (TPL2) in tumor-promoting Inflammation, Tumorigenesis and Tumor Immunity

Over the years, tumor progression locus 2 (TPL2) has been identified as an essential modulator of immune responses that conveys inflammatory signals to downstream effectors, subsequently modulating the generation and function of inflammatory cells. TPL2 is also differentially expressed and activated in several cancers, where it is associated with increased inflammation, malignant transformation, angiogenesis, metastasis, poor prognosis and therapy resistance. However, the relationship between TPL2-driven inflammation, tumorigenesis and tumor immunity has not been addressed. Here, we reconcile the function of TPL2-driven inflammation to oncogenic functions such as inflammation, proliferation, apoptosis resistance, angiogenesis, metastasis, immunosuppression and immune evasion. We also address the controversies reported on TPL2 function in tumor-promoting inflammation and tumorigenesis, and highlight the potential role of the TPL2 adaptor function in regulating the mechanisms leading to pro-tumorigenic inflammation and tumor progression. We discuss the therapeutic implications and limitations of targeting TPL2 for cancer treatment. The ideas presented here provide some new insight into cancer pathophysiology that might contribute to the development of more integrative and specific anti-inflammatory and anti-cancer therapeutics.

Exendin‑4 inhibits lipotoxicity‑induced oxidative stress in β‑cells by inhibiting the activation of TLR4/NF‑κB signaling pathway

The present study aimed to investigate the relationship between the protective effects of exendin‑4 (EX‑4) on lipotoxicity‑induced oxidative stress and meta‑inflammation in β‑cells and the toll‑like receptor 4 (TLR4)/NF‑κB signaling pathway. Lipotoxicity, hydrogen peroxide (H2O2)‑induced oxidative stress in β cells, obese Sprague Dawley rats and TLR4 truncation rats were utilized in the present study. The expression levels were detected by western blotting; cell apoptosis was detected by TUNEL assay; and the intracellular reactive oxygen species (ROS) levels were analyzed using a ROS assay kit. The findings of the present study showed that EX‑4 inhibited the expression of TLR4, NF‑κB p65 subunit and p47phox in a concentration‑dependent manner, and decreased the intracellular level of ROS. Additionally, silencing of TLR4 expression enhanced the protective effects of EX‑4, while overexpression of TLR4 attenuated these protective influences. Simultaneously, it was demonstrated that TLR4 was involved in the process of EX‑4 intervention to inhibit H2O2‑induced oxidative stress in islet β‑cells. Moreover, it was found that EX‑4 also inhibited TLR4‑ or NF‑κB agonist‑induced oxidative stress. These results were also confirmed in an animal model of obese rats, in which EX‑4 was able to improve the function of β‑cells, attenuate oxidative stress, and inhibit the expression levels of TLR4 and NF‑κB p65 subunit in the pancreas of the diet‑induced obese rats. Furthermore, truncation of the TLR4 gene in SD rats delayed the aforementioned damage. In summary, EX‑4 may inhibit lipotoxicity‑induced oxidative stress in β‑cells by inhibiting the activation of the TLR4/NF‑κB signaling pathway.

[Diabetes: What are the key targets and the objectives? Preserving and protecting a functional pancreatic beta cell mass]

Diabetes is characterized by chronic hyperglycemia. Type 2 diabetes, which represents 90% of diabetes cases, is the consequence of an insulin resistance and pancreatic beta cell dysfunction combination. Since the beta cells are the only cells of the organism to synthesize and to secrete insulin, it is essential to maintain and to protect their function and survival. It is currently proposed that an ideal and innovative treatment of type 2 diabetes should be based on an approach targeting pancreatic beta-cell dysfunction and death. It is now well described that chronic hyperglycemia is critically involved in the development of beta-cell dysfunction and apoptotic death (Glucotoxicity). Reducing the chronic hyperglycemia is a key objective in the treatment of type 2 diabetes, to attenuate not only the development of micro and macrovascular complications, but also the deleterious effects exerted on the pancreatic beta cells.

TPL2 kinase action and control of inflammation

Tumor progression locus 2 (TPL2, also known as COT or MAP3K8) is a mitogen-activated protein kinase kinase (MAP3K) activated downstream of TNFαR, IL1R, TLR, CD40, IL17R, and some GPCRs. TPL2 regulates the MEK1/2 and ERK1/2 pathways to regulate a cascade of inflammatory responses. In parallel to this, TPL2 also activates p38α and p38δ to drive the production of various inflammatory mediators in neutrophils. We discuss the implications of this finding in the context of various inflammatory diseases.

Autocrine Interleukin-10 Mediates Glucagon-Like Peptide-1 Receptor-Induced Spinal Microglial β-Endorphin Expression

The glucagon-like peptide-1 (GLP-1) receptor agonist exenatide stimulates microglial β-endorphin expression and subsequently produces neuroprotection and antinociception. This study illustrated an unrecognized autocrine role of IL-10 in mediation of exenatide-induced β-endorphin expression. Treatment with exenatide in cultured primary spinal microglia concentration dependently stimulated the expression of the M2 microglial markers IL-10, IL-4, Arg 1, and CD206, but not the M1 microglial markers TNF-α, IL-1β, IL-6, or CD68. Intrathecal exenatide injection also significantly upregulated spinal microglial expression of IL-10, IL-4, Arg 1, and CD206, but not TNF-α, IL-1β, IL-6, or CD68. Intrathecal injection of exenatide stimulated spinal microglial expression of IL-10 and β-endorphin in neuropathic rats. Furthermore, treatment with IL-10 (but not IL-4) stimulated β-endorphin expression in cultured primary microglia, whereas treatment with β-endorphin failed to increase IL-10 expression. The IL-10-neutralizing antibody entirely blocked exenatide-induced spinal microglial expression of β-endorphin in vitro and in vivo and fully blocked exenatide mechanical antiallodynia in neuropathic rats. Moreover, specific cAMP/PKA/p38 signal inhibitors and siRNA/p38β, but not siRNA/p38α, completely blocked exenatide-induced IL-10 expression in cultured primary microglia. Knock-down of IL-10 receptor-α mRNA using siRNA fully inhibited exenatide-induced spinal microglial β-endorphin expression and mechanical antiallodynia in neuropathy. Exenatide also markedly stimulated phosphorylation of the transcription factor STAT3 in cultured primary microglia and β-endorphin stimulation was completely inhibited by the specific STAT3 activation inhibitor. These results revealed that IL-10 in microglia mediated β-endorphin expression after GLP-1 receptor activation through the autocrine cAMP/PKA/p38β/CREB and subsequent IL-10 receptor/STAT3 signal pathways.SIGNIFICANCE STATEMENT Activation of GLP-1 receptors specifically and simultaneously stimulates the expression of anti-inflammatory cytokines IL-10 and IL-4, as well as the neuroprotective factor β-endorphin from microglia. GLP-1 receptor agonism induces β-endorphin expression and antinociception through autocrine release of IL-10. Activation of GLP-1 receptors stimulates IL-10 and β-endorphin expression subsequently through the Gs-cAMP/PKA/p38β/CREB and IL-10/IL-10 receptor-α/STAT3 signal transduction pathways.

TPL2 (Therapeutic Targeting Tumor Progression Locus-2)/ATF4 (Activating Transcription Factor-4)/SDF1α (Chemokine Stromal Cell-Derived Factor-α) Axis Suppresses Diabetic Retinopathy

RATIONALE

Diabetic retinopathy is characterized by vasopermeability, vascular leakage, inflammation, blood-retinal barrier breakdown, capillary degeneration, and neovascularization. However, the mechanisms underlying the association between diabetes mellitus and progression retinopathy remain unclear.

OBJECTIVE

TPL2 (tumor progression locus 2), a serine-threonine protein kinase, exerts a pathological effect on vascular angiogenesis. This study investigated the role of N^ε-(carboxymethyl)lysine, a major advanced glycation end products, and the involved TPL2-related molecular signals in diabetic retinopathy using models of in vitro and in vivo and human samples.

METHODS AND RESULTS

Serum N^ε-(carboxymethyl)lysine levels and TPL2 kinase activity were significantly increased in clinical patients and experimental animals with diabetic retinopathy. Intravitreal administration of pharmacological blocker or neutralizing antibody inhibited TPL2 and effectively suppressed the pathological characteristics of retinopathy in streptozotocin-induced diabetic animal models. Intravitreal VEGF (vascular endothelial growth factor) neutralization also suppressed the diabetic retinopathy in diabetic animal models. Mechanistic studies in primary human umbilical vein endothelial cells and primary retinal microvascular endothelial cells from streptozotocin-diabetic rats, db/db mice, and samples from patients with diabetic retinopathy revealed a positive parallel correlation between N^ε-(carboxymethyl)lysine and the TPL2/chemokine SDF1α (stromal cell-derived factor-α) axis that is dependent on endoplasmic reticulum stress-related molecules, especially ATF4 (activating transcription factor-4).

CONCLUSIONS

This study demonstrates that inhibiting the N^ε-(carboxymethyl)lysine-induced TPL2/ATF4/SDF1α axis can effectively prevent diabetes mellitus-mediated retinal microvascular dysfunction. This signaling axis may include the therapeutic potential for other diseases involving pathological neovascularization or macular edema.

Glucagon and glucagon-like peptide-1 as novel anti-inflammatory and immunomodulatory compounds

Glucagon and glucagon-like peptide-1 (GLP-1) are polypeptide hormones that are produced by pancreatic α-cells and the intestine, respectively, whose main function is to control glucose homeostasis. The glucagon and GLP-1 levels are imbalanced in diabetes. Furthermore, type 1 diabetic patients and animals present with a diminished inflammatory response, which is related to some morbidities of diabetes, such as a higher incidence of infectious diseases, including sepsis. The focus of this review is to briefly summarize the state of the art concerning the effects of glucagon and GLP-1 on the inflammatory response. Here, we propose that glucagon and GLP-1 have anti-inflammatory properties, making them possible prototypes for the design and synthesis of new compounds to treat inflammatory diseases. In addition, glucagon, GLP-1 or their analogues or new derivatives may not only be important for managing inflammatory diseases but may also have the therapeutic potential to prevent, cure or ameliorate diabetes in patients by counteracting the deleterious effects of pro-inflammatory cytokines on the function and viability of pancreatic β-cells. In addition, GLP-1, its analogues or drugs that inhibit GLP-1 metabolism may have a doubly beneficial effect in diabetic patients by inhibiting the inflammatory response and reducing glycaemia.

A Specific Gut Microbiota Dysbiosis of Type 2 Diabetic Mice Induces GLP-1 Resistance through an Enteric NO-Dependent and Gut-Brain Axis Mechanism

Glucagon-like peptide-1 (GLP-1)-based therapies control glycemia in type 2 diabetic (T2D) patients. However, in some patients the treatment must be discontinued, defining a state of GLP-1 resistance. In animal models we identified a specific set of ileum bacteria impairing the GLP-1-activated gut-brain axis for the control of insulin secretion and gastric emptying. Using prediction algorithms, we identified bacterial pathways related to amino acid metabolism and transport system modules associated to GLP-1 resistance. The conventionalization of germ-free mice demonstrated their role in enteric neuron biology and the gut-brain-periphery axis. Altogether, insulin secretion and gastric emptying require functional GLP-1 receptor and neuronal nitric oxide synthase in the enteric nervous system within a eubiotic gut microbiota environment. Our data open a novel route to improve GLP-1-based therapies.