Double-stranded RNA-activated protein kinase is a key modulator of insulin sensitivity in physiological conditions and in obesity in mice

A critical role for PKR complexes with TRBP in Immunometabolic regulation and eIF2α phosphorylation in obesity

Aberrant stress and inflammatory responses are key factors in the pathogenesis of obesity and metabolic dysfunction, and the double-stranded RNA-dependent kinase (PKR) has been proposed to play an important role in integrating these pathways. Here, we report the formation of a complex between PKR and TAR RNA-binding protein (TRBP) during metabolic and obesity-induced stress, which is critical for the regulation of eukaryotic translation initiation factor 2 alpha (eIF2α) phosphorylation and c-Jun N-terminal kinase (JNK) activation. We show that TRBP phosphorylation is induced in the setting of metabolic stress, leading to PKR activation. Suppression of hepatic TRBP reduced inflammation, JNK activity, and eIF2α phosphorylation and improved systemic insulin resistance and glucose metabolism, while TRBP overexpression exacerbated the impairment in glucose homeostasis in obese mice. These data indicate that the association between PKR and TRBP integrates metabolism with translational control and inflammatory signaling and plays important roles in metabolic homeostasis and disease.

Protein kinase R and the inflammasome

Protein kinase R (PKR) was first identified as a mediator of the double-stranded RNA (dsRNA)-mediated inhibition of protein synthesis in extracts from interferon-treated cells. In a physiological context, viral replication results in production of dsRNA, activation of PKR by autophosphorylation, and phosphorylation of the protein synthesis initiation factor eIF2α. Subsequent biochemical, structural, and genetic analyses have identified the dsRNA and kinase domain structure of PKR, and shown that its deletion from the germline of mice results in impaired resistance to infection by many different viruses. These studies have also opened up roles for PKR in different signaling pathways, the most recent being regulation of the inflammasome. Here we review evidence for this newly ascribed function for PKR and discuss roles in inflammasome regulation and associated diseases.

Protein-Binding Function of RNA-Dependent Protein Kinase Promotes Proliferation through TRAF2/RIP1/NF-κB/c-Myc Pathway in Pancreatic β cells

Double-stranded RNA-dependent protein kinase (PKR), an intracellular pathogen recognition receptor, is involved both in insulin resistance in peripheral tissues and in downregulation of pancreatic β-cell function in a kinase-dependent manner, indicating PKR as a core component in the progression of type 2 diabetes. PKR also acts as an adaptor protein via its protein-binding domain. Here, the PKR protein-binding function promoted β-cell proliferation without its kinase activity, which is associated with enhanced physical interaction with tumor necrosis factor receptor-associated factor 2 (TRAF2) and TRAF6. In addition, the transcription of the nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB)-dependent survival gene c-Myc was upregulated significantly and is necessary for proliferation. Upregulation of the PKR protein-binding function induced the NF-κB pathway, as observed by dose-dependent degradation of IκBα, induced nuclear translocation of p65 and elevated NF-κB-dependent reporter gene expression. NF-κB-dependent reporter activity and β-cell proliferation both were suppressed by TRAF2-siRNA, but not by TRAF6-siRNA. TRAF2-siRNA blocked the ubiquitination of receptor-interacting serine/threonine-protein kinase 1 (RIP1) induced by PKR protein binding. Furthermore, RIP1-siRNA inhibited β-cell proliferation. Proinflammatory cytokines (TNFα) and glucolipitoxicity also promoted the physical interaction of PKR with TRAF2. Collectively, these data indicate a pivotal role for PKR's protein-binding function on the proliferation of pancreatic β cells through TRAF2/RIP1/NF-κB/c-Myc pathways. Therapeutic opportunities for type 2 diabetes may arise when its kinase catalytic function, but not its protein-binding function, is downregulated.

Defects in TLR3 expression and RNase L activation lead to decreased MnSOD expression and insulin resistance in muscle cells of obese people

Obesity is associated with chronic low-grade inflammation and oxidative stress that blunt insulin response in its target tissues, leading to insulin resistance (IR). IR is a characteristic feature of type 2 diabetes. Skeletal muscle is responsible for 75% of total insulin-dependent glucose uptake; consequently, skeletal muscle IR is considered to be the primary defect of systemic IR development. Interestingly, some obese people stay insulin-sensitive and metabolically healthy. With the aim of understanding this difference and identifying the mechanisms responsible for insulin sensitivity maintenance/IR development during obesity, we explored the role of the latent endoribonuclease (RNase L) in skeletal muscle cells. RNase L is a regulator of innate immunity, of double-stranded RNA sensors and of toll-like receptor (TLR) 4 signaling. It is regulated during inflammation by interferons and its activity is dependent on its binding to 2-5A, an oligoadenylate synthesized by oligoadenylate synthetases (OAS). Increased expression of RNase L or downregulation of its inhibitor (RLI) improved insulin response in mouse myogenic C2C12 cells and in primary human myotubes from normal-weight subjects treated with palmitate, a saturated free fatty acid (FFA) known to induce inflammation and oxidative stress via TLR4 activation. While RNase L and RLI levels remained unchanged, OAS level was decreased in primary myotubes from insulin-resistant obese subjects (OB-IR) compared with myotubes from insulin-sensitive obese subjects (OB-IS). TLR3 and mitochondrial manganese superoxide dismutase (MnSOD) were also underexpressed in OB-IR myotubes. Activation of RNase L by 2-5A transfection allowed to restore insulin response, OAS, MnSOD and TLR3 expression in OB-IR myotubes. Due to low expression of OAS, OB-IR myotubes present a defect in RNase L activation and TLR3 regulation. Consequently, MnSOD level is low and insulin sensitivity is reduced. These results support that RNase L activity limits FFA/obesity-induced impairment of insulin response in muscle cells via TLR3 and MnSOD expression.

The impact of PKR activation: from neurodegeneration to cancer

An inverse association between cancer and neurodegeneration is plausible because these biological processes share several genes and signaling pathways. Whereas uncontrolled cell proliferation and decreased apoptotic cell death governs cancer, excessive apoptosis contributes to neurodegeneration. Protein kinase R (PKR), an interferon-inducible double-stranded RNA protein kinase, is involved in both diseases. PKR activation blocks global protein synthesis through eIF2α phosphorylation, leading to cell death in response to a variety of cellular stresses. However, PKR also has the dual role of activating the nuclear factor κ-B pathway, promoting cell proliferation. Whereas PKR is recognized for its negative effects on neurodegenerative diseases, in part, inducing high level of apoptosis, the role of PKR activation in cancer remains controversial. In general, PKR is considered to have a tumor suppressor function, and some clinical data show a correlation between suppressed or inactivated PKR and a poor prognosis for several cancers. However, other studies show high PKR expression and activation levels in various cancers, suggesting that PKR might contribute to neoplastic progression. Understanding the cellular factors and signals involved in the regulation of PKR in these age-related diseases is relevant and may have important clinical implications. The present review highlights the current knowledge on the role of PKR in neurodegeneration and cancer, with special emphasis on its regulation and clinical implications.

Small-molecule inhibitors of PKR improve glucose homeostasis in obese diabetic mice

Obesity and metabolic diseases appear as clusters, often featuring high risk for insulin resistance and type 2 diabetes, and constitute a major global health problem with limited treatment options. Previous studies have shown that double-stranded RNA-dependent kinase, PKR, plays an important role in the nutrient/pathogen-sensing interface, and acts as a key modulator of chronic metabolic inflammation, insulin sensitivity, and glucose homeostasis in obesity. Recently, pathological PKR activation was also demonstrated in obese humans, strengthening its prospects as a potential drug target. Here, we investigate the use of two structurally distinct small-molecule inhibitors of PKR in the treatment of insulin resistance and type 2 diabetes in cells and in a mouse model of severe obesity and insulin resistance. Inhibition of PKR reduced stress-induced Jun NH2-terminal kinase activation and insulin receptor substrate 1 serine phosphorylation in vitro and in vivo. In addition, treatment with both PKR inhibitors reduced adipose tissue inflammation, improved insulin sensitivity, and improved glucose intolerance in mice after the establishment of obesity and insulin resistance. Our findings suggest that pharmacologically targeting PKR may be an effective therapeutic strategy for the treatment of insulin resistance and type 2 diabetes.

Tyrosine kinase inhibitors as novel drugs for the treatment of diabetes

INTRODUCTION

Some inhibitors of tyrosine kinase, as imatinib, erlotinib and sunitinib have antihyperglycemic effects but the mechanisms are not totally clear.

AREAS COVERED

It is well established that insulin resistance and beta-cell failure are hallmarks of type 2 diabetes mellitus (DM2). The present review will discuss the molecular mechanisms that account for insulin resistance and beta-cell failure in DM2, and also the effect of tyrosine kinase inhibitors in these processes.

EXPERT OPINION

A better understanding of how these drugs improve the two most important mechanisms of DM2 associated with suggestions of clinical studies will lead to improve the treatment of this disease.

Activation of double-stranded RNA-dependent protein kinase inhibits proliferation of pancreatic β-cells

Double-stranded RNA-dependent protein kinase (PKR) is revealed to participate in the development of insulin resistance in peripheral tissues in type 2 diabetes (T2DM). Meanwhile, PKR is also characterized as a critical regulator of cell proliferation. To date, no study has focused on the impact of PKR on the proliferation of pancreatic β-cells. Here, we adopted insulinoma cell lines and mice islet β-cells to investigate: (1) the effects of glucolipotoxicity and pro-inflammatory cytokines on PKR activation; (2) the effects of PKR on proliferation of pancreatic β-cells and its underlying mechanisms; (3) the actions of PKR on pro-proliferative effects of IGF-I and its underlying pathway. Our results provided the first evidence that PKR can be activated by glucolipitoxicity and pro-inflammatory cytokines in pancreatic β-cells, and activated PKR significantly inhibited cell proliferation by arresting cell cycle at G1 phase. Reductions in cyclin D1 and D2 as well as increases in p27 and p53 were associated with the anti-proliferative effects of PKR, and proteasome-dependent degradation took part in the reduction of cyclin D1 and D2. Besides, PKR activation abrogated the pro-proliferative effects of IGF-I by activating JNK and disrupting IRS1/PI3K/Akt signaling pathway. These findings indicate that the anti-proliferative actions of PKR on pancreatic β-cells may contribute to the pathogenesis of T2DM.

Modulation of double-stranded RNA-activated protein kinase in insulin sensitive tissues of obese humans

OBJECTIVE

The double-stranded RNA-dependent protein kinase (PKR) was recently implicated in regulating molecular integration of nutrient- and pathogen-sensing pathways in obese mice. However, its modulation in human tissues in situations of insulin resistance has not been investigated. The present study was performed to first determine the tissue expression and phosphorylation levels of PKR in the liver, muscle, and adipose tissue in obese humans, and also the modulation of this protein in the adipose tissue of obese patients after bariatric surgery.

DESIGN AND METHODS

Eleven obese subjects who were scheduled to undergo Roux-en-Y Gastric Bypass Procedure participated in this study. Nine apparently healthy lean subjects as a control group were also included.

RESULTS

Our data show that PKR is activated in liver, muscle, and adipose tissue of obese humans and, after bariatric surgery, there is a clear reduction in PKR activation accompanied by a decrease in protein kinase-like endoplasmic reticulum kinase, c-Jun N-terminal kinase, inhibitor of kappa β kinase, and insulin receptor substrate-1 serine 312 phosphorylation in subcutaneous adipose tissue from these patients.

CONCLUSION

Thus, it is proposed that PKR is an important mediator of obesity-induced insulin resistance and a potential target for the therapy.

Influence of gut microbiota on subclinical inflammation and insulin resistance

Obesity is the main condition that is correlated with the appearance of insulin resistance, which is the major link among its comorbidities, such as type 2 diabetes, nonalcoholic fatty liver disease, cardiovascular and neurodegenerative diseases, and several types of cancer. Obesity affects a large number of individuals worldwide; it degrades human health and quality of life. Here, we review the role of the gut microbiota in the pathophysiology of obesity and type 2 diabetes, which is promoted by a bacterial diversity shift mediated by overnutrition. Whole bacteria, their products, and metabolites undergo increased translocation through the gut epithelium to the circulation due to degraded tight junctions and the consequent increase in intestinal permeability that culminates in inflammation and insulin resistance. Several strategies focusing on modulation of the gut microbiota (antibiotics, probiotics, and prebiotics) are being experimentally employed in metabolic derangement in order to reduce intestinal permeability, increase the production of short chain fatty acids and anorectic gut hormones, and promote insulin sensitivity to counteract the inflammatory status and insulin resistance found in obese individuals.