Enhancement of glucose uptake in mouse skeletal muscle cells and adipocytes by P2Y6 receptor agonists

Pharmacological interaction and immune response of purinergic receptors in therapeutic modulation

Nucleosides and purine nucleotides serve as transmitter and modulator agents that extend their functions beyond the cell. In this context, purinergic signaling plays a crucial role in regulating energy homeostasis and modulating metabolic alterations in tumor cells. Therefore, it is essential to consider the pharmacological targeting of purinergic receptors (PUR), which encompass the expression and inhibition of P1 receptors (metabotropic adenosine receptors) as well as P2 receptors (extracellular ATP/ADP) comprising P2X and P2Y receptors. Thus, the pharmacological interaction between inhibitors (such as RNA, monoclonal antibodies, and small molecules) and PUR represents a key aspect in facilitating the development of therapeutic interventions. Moreover, this review explores recent advancements in pharmacological inhibitors and the regulation of innate and adaptive immunity of PUR, specifically in relation to immunological and inflammatory responses. These responses encompass the release of pro-inflammatory cytokines (PIC), the production of reactive oxygen and nitrogen species (ROS and RNS), the regulation of T cells, and the activation of inflammasomes in all human leukocytes.

Hypoglycemic effects of dracorhodin and dragon blood crude extract from Daemonorops draco

BACKGROUND

Dragon blood is a red fruit resin from the palm tree Daemonorops draco and is a herbal ingredient used in the traditional Chinese medicine, "Jinchuang Ointment," which is used to treat non-healing diabetic wounds. According to the Taiwan Herbal Pharmacopeia, the dracorhodin content in dragon blood should exceed 1.0%.

RESULTS

Our findings indicate that dracorhodin and dragon blood crude extracts can stimulate glucose uptake in mouse muscle cells (C2C12) and primary rat aortic smooth muscle cells (RSMC). Dracorhodin is not the only active compound in dragon blood crude extracts from D. draco. Next, we orally administered crude dragon blood extracts to male B6 mice. The experimental group displayed a decreasing trend in fasting blood glucose levels from the second to tenth week. In summary, crude extracts of dragon blood from D. draco demonstrated in vivo hypoglycemic effects in B6 male mice.

CONCLUSIONS

We provide a scientific basis "Jinchuang ointment" in treating non-healing wounds in patients with diabetes.

Exploring the Anti-Diabetic Potential of Quercetagitrin through Dual Inhibition of PTPN6 and PTPN9

Protein tyrosine phosphatases (PTPs) are pivotal contributors to the development of type 2 diabetes (T2DM). Hence, directing interventions towards PTPs emerges as a valuable therapeutic approach for managing type 2 diabetes. In particular, PTPN6 and PTPN9 are targets for anti-diabetic effects. Through high-throughput drug screening, quercetagitrin (QG) was recognized as a dual-target inhibitor of PTPN6 and PTPN9. We observed that QG suppressed the catalytic activity of PTPN6 (IC50 = 1 μM) and PTPN9 (IC50 = 1.7 μM) in vitro and enhanced glucose uptake by mature C2C12 myoblasts. Additionally, QG increased the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) and insulin-dependent phosphorylation of Akt in mature C2C12 myoblasts. It further promoted the phosphorylation of Akt in the presence of palmitic acid, suggesting the attenuation of insulin resistance. In summary, our results indicate QG's role as a potent inhibitor targeting both PTPN6 and PTPN9, showcasing its potential as a promising treatment avenue for T2DM.

Dipyridamole activates adenosine A2B receptor and AMPK/cAMP signaling and promotes myogenic differentiation of myoblastic C2C12 cells

Introduction: Sarcopenia is defined as a loss of muscle mass and strength. ATP homeostasis is crucial during myogenesis. We determined how the purinergic system modulates myogenesis using dipyridamole (blocks adenosine taken up by the cells) and tenofovir (inhibits ATP release) in a myoblast cell line. Methods: C2C12 cells were differentiated in the presence/absence of tenofovir/dipyridamole, with/without the A2B selective inhibitor PSB-603. Extra-/intracellular nucleotides were examined via HPLC. The expression of muscle differentiation proteins (Pax7, Mif5, MyoD, MyoG, and MHC), PKA/CREB, adenosine receptors (A1, A2A, A2B, and A3), ATP-channel pannexin-1 and the P2X7 receptor was analyzed via WB and RT-PCR. cAMP and AMPK activation was measured. Results: Tenofovir increased intracellular ATP and reduced extracellular adenosine, decreasing Pax7 expression and increasing MHC expression prematurely. Dipyridamole increased intracellular AMP and extracellular adenosine, counteracting the premature myogenesis promoted by tenofovir. All adenosine receptors were expressed during differentiation with dipyridamole, increasing A2B expression. Tenofovir maintained inactive AMPK and decreased cAMP levels, as well as PKAα and pCREB expression, which were recovered with dipyridamole. Discussion: Adenosine and ATP act as mediators in muscle myogenesis. The blockade of ATP release by tenofovir promotes premature myogenesis, with dipyridamole counteracting the premature differentiation promoted by tenofovir via the adenosine A2B receptor and cAMP/AMPK pathways. Therefore, dipyridamole might be of interest as a therapeutic approach in sarcopenia.

Methyl Syringate Stimulates Glucose Uptake by Inhibiting Protein Tyrosine Phosphatases Relevant to Insulin Resistance

Several protein tyrosine phosphatases (PTPs), particularly PTPN1, PTPN2, PTPN6, PTPN9, PTPN11, PTPRS, and DUSP9, are involved in insulin resistance. Therefore, these PTPs could be promising targets for the treatment of type 2 diabetes. Our previous studies revealed that PTPN2 and PTPN6 are potential antidiabetic targets. Therefore, the identification of dual-targeting inhibitors of PTPN2 and PTPN6 could be a potential therapeutic strategy for the treatment or prevention of type 2 diabetes. In this study, we demonstrate that methyl syringate inhibits the catalytic activity of PTPN2 and PTPN6 in vitro, indicating that methyl syringate acts as a dual-targeting inhibitor of PTPN2 and PTPN6. Furthermore, methyl syringate treatment significantly increased glucose uptake in mature 3T3-L1 adipocytes. Additionally, methyl syringate markedly enhanced phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) in 3T3L1 adipocytes. Taken together, our results suggest that methyl syringate, a dual-targeting inhibitor of PTPN2 and PTPN6, is a promising therapeutic candidate for the treatment or prevention of type 2 diabetes.

Adipocyte purinergic receptors activated by uracil nucleotides as obesity and type 2 diabetes targets

Extracellular uridine nucleotides regulate physiological and pathophysiological metabolic processes through the activation of P2Y₂, P2Y₄, P2Y₆ and P2Y₁₄ purinergic receptors, which play a key role in adipogenesis, glucose uptake, lipolysis and adipokine secretion. Using adipocyte-specific knockout mouse models, it has been demonstrated that lack of the P2Y₆R or P2Y₁₄R can protect against diet-induced obesity and improve whole-body glucose metabolism. The P2Y₂R facilitated adipogenesis and inflammation, and the loss of P2Y₄R or P2Y₁₄R raised the levels of the protective endocrine factor adiponectin. Hence, potent antagonists for these receptors may be tested to identify drug candidates for the treatment of obesity and type 2 diabetes. However, future studies are required to provide insight into purinergic regulation of brown adipocytes and their role in thermogenesis. This review summarizes the current studies on uridine nucleotide-activated P2YRs and their role in adipocyte function, diet-induced obesity and associated metabolic deficits.

Ghrelin is a signal to facilitate the utilization of fatty acids and save glucose by the liver, skeletal muscle, and adipose tissues in chicks

Ghrelin, classically known as a central appetite-stimulating hormone, has recently been recognized to play an important role in peripheral tissue energy metabolism. In chicken, contrary to mammal, ghrelin acts as an anorexia signal, increased by fasting and further elevated after refed. In the present study, the effect of ghrelin on glucose/lipid utilization by peripheral tissues was investigated. Injection of exogenous acyl ghrelin reduced plasma triglyceride and glucose levels of chickens at both fasting and fed status. In the in vitro cultured chicken primary hepatocytes, adipocytes, and myoblasts, ghrelin suppressed glucose uptake, stimulated fatty acids uptake and oxidation, and decreased TG content. In hepatocyte, ghrelin increased the activities of LPL and HL, and upregulated the expression levels of gene ACC, CPT1, and PPARα. Ghrelin treatment markedly increased the protein level of p-ACC, PPARγ, PGC1α, and CPT1 in hepatocytes, adipocytes and myoblasts. Inhibition of AMPK activity by Compound C had no influence on glucose uptake by hepatocyte, adipocyte, and myoblast, but further amplified the stimulated fatty acid uptake of adipocyte by ghrelin. The present result demonstrates that ghrelin facilitates the uptake and oxidation of fatty acid and cut down the utilization of glucose by the liver, muscle, and adipose tissues. The result suggests that ghrelin functions as a signal of fatty acid oxidation. The study provides a vital framework for understanding the intrinsic role of ghrelin as a crucial factor in the concerted regulation of metabolic substrate of hepatocytes, adipocytes, and myoblasts.

The relevance of adhesion G protein-coupled receptors in metabolic functions

G protein-coupled receptors (GPCRs) modulate a variety of physiological functions and have been proven to be outstanding drug targets. However, approximately one-third of all non-olfactory GPCRs are still orphans in respect to their signal transduction and physiological functions. Receptors of the class of Adhesion GPCRs (aGPCRs) are among these orphan receptors. They are characterized by unique features in their structure and tissue-specific expression, which yields them interesting candidates for deorphanization and testing as potential therapeutic targets. Capable of G-protein coupling and non-G protein-mediated function, aGPCRs may extend our repertoire of influencing physiological function. Besides their described significance in the immune and central nervous systems, growing evidence indicates a high importance of these receptors in metabolic tissue. RNAseq analyses revealed high expression of several aGPCRs in pancreatic islets, adipose tissue, liver, and intestine but also in neurons governing food intake. In this review, we focus on aGPCRs and their function in regulating metabolic pathways. Based on current knowledge, this receptor class represents high potential for future pharmacological approaches addressing obesity and other metabolic diseases.

Diabetes and hypertension: Pivotal involvement of purinergic signaling

Diabetes mellitus (DM) and hypertension are highly prevalent worldwide health problems and frequently associated with severe clinical complications, such as diabetic cardiomyopathy, nephropathy, retinopathy, neuropathy, stroke, and cardiac arrhythmia, among others. Despite all existing research results and reasonable speculations, knowledge about the role of purinergic system in individuals with DM and hypertension remains restricted. Purinergic signaling accounts for a complex network of receptors and extracellular enzymes responsible for the recognition and degradation of extracellular nucleotides and adenosine. The main components of this system that will be presented in this review are: P1 and P2 receptors and the enzymatic cascade composed by CD39 (NTPDase; with ATP and ADP as a substrate), CD73 (5'-nucleotidase; with AMP as a substrate), and adenosine deaminase (ADA; with adenosine as a substrate). The purinergic system has recently emerged as a central player in several physiopathological conditions, particularly those linked to inflammatory responses such as diabetes and hypertension. Therefore, the present review focuses on changes in both purinergic P1 and P2 receptor expression as well as the activities of CD39, CD73, and ADA in diabetes and hypertension conditions. It can be postulated that the manipulation of the purinergic axis at different levels can prevent or exacerbate the insurgency and evolution of diabetes and hypertension working as a compensatory mechanism.

Metabolic Changes Induced by Purinergic Signaling: Role in Food Intake

The purinergic signalling has a well-established role in the regulation of energy homeostasis, but there is growing evidence of its implication in the control of food intake. In this review, we provide an integrative view of the molecular mechanisms leading to changes in feeding behaviour within hypothalamic neurons following purinergic receptor activation. We also highlight the importance of purinergic signalling in metabolic homeostasis and the possibility of targeting its receptors for therapeutic purposes.

Metabolic roles of G protein-coupled receptor signaling in obesity and type 2 diabetes

The incidence of obesity and type 2 diabetes (T2D) has been increasing steadily worldwide. It is estimated that by 2045 more than 800 million people will be suffering from diabetes. Despite the advancements in modern medicine, more effective therapies for treating obesity and T2D are needed. G protein-coupled receptors (GPCRs) have emerged as important drug targets for various chronic diseases, including obesity, T2D, and liver diseases. During the past two decades, many laboratories worldwide focused on understanding the role of GPCR signaling in regulating glucose metabolism and energy homeostasis. The information gained from these studies can guide the development of novel therapeutic agents. In this review, we summarize recent studies providing insights into the role of GPCR signaling in peripheral, metabolically important tissues such as pancreas, liver, skeletal muscle, and adipose tissue, focusing primarily on the use of mutant animal models and human data.

Phloridzin Acts as an Inhibitor of Protein-Tyrosine Phosphatase MEG2 Relevant to Insulin Resistance

Inhibition of the megakaryocyte protein tyrosine phosphatase 2 (PTP-MEG2, also named PTPN9) activity has been shown to be a potential therapeutic strategy for the treatment of type 2 diabetes. Previously, we reported that PTP-MEG2 knockdown enhances adenosine monophosphate activated protein kinase (AMPK) phosphorylation, suggesting that PTP-MEG2 may be a potential antidiabetic target. In this study, we found that phloridzin, isolated from Ulmus davidiana var. japonica, inhibits the catalytic activity of PTP-MEG2 (half-inhibitory concentration, IC₅₀ = 32 ± 1.06 μM) in vitro, indicating that it could be a potential antidiabetic drug candidate. Importantly, phloridzin stimulated glucose uptake by differentiated 3T3-L1 adipocytes and C2C12 muscle cells compared to that by the control cells. Moreover, phloridzin led to the enhanced phosphorylation of AMPK and Akt relevant to increased insulin sensitivity. Importantly, phloridzin attenuated palmitate-induced insulin resistance in C2C12 muscle cells. We also found that phloridzin did not accelerate adipocyte differentiation, suggesting that phloridzin improves insulin sensitivity without significant lipid accumulation. Taken together, our results demonstrate that phloridzin, an inhibitor of PTP-MEG2, stimulates glucose uptake through the activation of both AMPK and Akt signaling pathways. These results strongly suggest that phloridzin could be used as a potential therapeutic candidate for the treatment of type 2 diabetes.

Pentadecanoic acid promotes basal and insulin-stimulated glucose uptake in C2C12 myotubes

Background

Saturated fatty acids (SFAs) generally have been thought to worsen insulin-resistance and increase the risk of developing type 2 diabetes mellitus (T2DM). Recently, accumulating evidence has revealed that SFAs are not a single homogeneous group, instead different SFAs are associated with T2DM in opposing directions. Pentadecanoic acid (C15:0, PA) is directly correlated with dairy products, and a negative association between circulating PA and metabolic disease risk was observed in epidemiological studies. Therefore, the role of PA in human health needs to be reinforced. Whether PA has a direct benefit on glucose metabolism and insulin sensitivity needs further investigation.

Objective

The present study aimed to investigate the effect and potential mechanism of action of PA on basal and insulin stimulated glucose uptake in C2C12 myotubes.

Methods

Glucose uptake was determined using a 2-(N-[7-nitrobenz-2-oxa-1,3-diazol-4-yl] amino)-2-deoxyglucose (2-NBDG) uptake assay. Cell membrane proteins were isolated and glucose transporter 4 (GLUT4) protein was detected by western blotting to examine the translocation of GLUT4 to the plasma membrane. The phosphorylation levels of proteins involved in the insulin and 5’-adenosine monophosphate-activated protein kinase (AMPK) pathways were examined by western blotting.

Results

We found that PA significantly promoted glucose uptake and GLUT4 translocation to the plasma membrane. PA had no effect on the insulin-dependent pathway involving insulin receptor substrate 1 (Tyr632) and protein kinase B (PKB/Akt), but increased phosphorylation of AMPK and Akt substrate of 160 kDa (AS160). Compound C (an AMPK inhibitor) blocked PA-induced AMPK activation and reversed PA-induced GLUT4 translocation, indicating that PA promotes glucose uptake via the AMPK pathway in vitro. Moreover, PA significantly promoted insulin-stimulated glucose uptake in myotubes. Under insulin stimulation, PA did not affect the insulin-dependent pathway, but still activated AMPK.

Conclusion

PA, an odd-chain SFA, significantly stimulates glucose uptake via the AMPK-AS160 pathway and exhibits an insulin-sensitizing effect in myotubes.

Ca2+-Dependent Glucose Transport in Skeletal Muscle by Diphlorethohydroxycarmalol, an Alga Phlorotannin: In Vitro and In Vivo Study

Diphlorethohydroxycarmalol (DPHC), a type of phlorotannin isolated from the marine alga Ishige okamurae, reportedly alleviates impaired glucose tolerance. However, the molecular mechanisms of DPHC regulatory activity and by which it exerts potential beneficial effects on glucose transport into skeletal myotubes to control glucose homeostasis remain largely unexplored. The aim of this study was to evaluate the effect of DPHC on cytosolic Ca2+ levels and its correlation with blood glucose transport in skeletal myotubes in vitro and in vivo. Cytosolic Ca2+ levels upon DPHC treatment were evaluated in skeletal myotubes and zebrafish larvae by Ca2+ imaging using Fluo-4. We investigated the effect of DPHC on the blood glucose level and glucose transport pathway in a hyperglycemic zebrafish. DPHC was shown to control blood glucose levels by accelerating glucose transport; this effect was associated with elevated cytosolic Ca2+ levels in skeletal myotubes. Moreover, the increased cytosolic Ca2+ level caused by DPHC can facilitate the Glut4/AMPK pathways of the skeletal muscle in activating glucose metabolism, thereby regulating muscle contraction through the regulation of expression of troponin I/C, CaMKII, and ATP. Our findings provide insights into the mechanism of DPHC activity in skeletal myotubes, suggesting that increased cytosolic Ca2+ levels caused by DPHC can promote glucose transport into skeletal myotubes to modulate blood glucose levels, thus indicating the potential use of DPHC in the prevention of diabetes.

Purinergic signaling in diabetes and metabolism

Purinergic signaling, a concept originally formulated by the late Geoffrey Burnstock (1929-2020), was found to modulate pathways in every physiological system. In metabolic disorders there is a role for both adenosine receptors and P2 (nucleotide) receptors, of which there are two classes, i.e. P2Y metabotropic and P2X ionotropic receptors. The individual roles of the 19 receptors encompassed by this family have been dissected - and in many cases the effects associated with specific cell types, including adipocytes, skeletal muscle, liver cells and immune cells. It is suggested that ligands selective for each of the four adenosine receptors (A₁, A_2A, A_2B and A₃), and several of the P2 subtypes (e.g. P2Y₆ or P2X7 antagonists) might have therapeutic potential for treating diabetes and obesity. This is a developing story with some conflicting conclusions relevant to drug discovery, which we summarize here.

Influence of purinergic signaling on glucose transporters: A possible mechanism against insulin resistance?

Metabolic disorders, such as insulin resistance, affect many people worldwide due to the prevalence of obesity and type 2 diabetes, which are pathologies that impair glycemic metabolism. Glucose is the primary energetic substrate of the body and is essential for cellular function. As the cell membrane is not permeable to glucose molecules, there are two distinct groups of glucose transporters: sodium-glucose-linked transporters (SGLTs) and the glucose transporter (GLUT) family. These transporters facilitate the entry of glucose into the bloodstream or cytoplasm where it functions in the production of adenosine 5 ́-triphosphate (ATP). This nucleotide acts in several cellular mechanisms, such as protein phosphorylation and cellular immune processes. ATP directly and indirectly acts as an agonist for purinergic receptors in high concentrations in the extracellular environment. Composed by P1 and P2 groups, the purinoreceptors cover several cellular mechanisms involving cytokines, tumors, and metabolic signaling pathways. Previous publications have indicated that the purinergic signaling activity in insulin resistance and glucose transporters modulates relevant actions on the deregulations that can affect glycemic homeostasis. Thus, this review focuses on the pharmacological influence of purinergic signaling on the modulation of glucose transporters, aiming for a new way to combat insulin resistance and other metabolic disorders.

Lack of adipocyte purinergic P2Y6 receptor greatly improves whole body glucose homeostasis

Uridine diphosphate (UDP)-activated purinergic receptor P2Y₆ (P2Y₆R) plays a crucial role in controlling energy balance through central mechanisms. However, P2Y₆R's roles in peripheral tissues regulating energy and glucose homeostasis remain unexplored. Here, we report the surprising finding that adipocyte-specific deletion of P2Y₆R protects mice from diet-induced obesity, improving glucose tolerance and insulin sensitivity with reduced systemic inflammation. These changes were associated with reduced JNK signaling and enhanced expression and activity of PPARα affecting downstream PGC1α levels leading to beiging of white fat. In contrast, P2Y₆R deletion in skeletal muscle reduced glucose uptake, resulting in impaired glucose homeostasis. Interestingly, whole body P2Y₆R knockout mice showed metabolic improvements similar to those observed with mice lacking P2Y₆R only in adipocytes. Our findings provide compelling evidence that P2Y₆R antagonists may prove useful for the treatment of obesity and type 2 diabetes.

Pitfalls and challenges of the purinergic signaling cascade in obesity

Obesity is a worldwide health problem which have reached pandemic proportions, now also including low and middle-income countries. Excessive or abnormal fat deposition in the abdomen especially in the visceral compartment is tightly associated with a high metabolic risk for arterial hypertension, type II diabetes, cardiovascular diseases, musculoskeletal disorders (especially articular degeneration) and some cancers. Contrariwise, accumulation of fat in the subcutaneous compartment has been associated with a neutral metabolic impact, favoring a lower risk of insulin resistance. Obesity results more often from an avoidable imbalance between food consumption and energy expenditure. There are several recommended strategies for dealing with obesity, including pharmacological therapies, but their success remains incomplete and may not compensate the associated adverse effects. Purinergic signaling operated by ATP and its metabolite, adenosine, has attracted increasing attention in obesity. The extracellular levels of purines often reflect the energy status of a given cell population. Adenine nucleotides and nucleosides fine tuning control adipogenesis and mature adipocytes function via the activation of P2 and P1 purinoceptors, respectively. These features make the purinergic signaling cascade a putative target for therapeutic intervention in obesity and related metabolic syndromes. There are, however, gaps in our knowledge regarding the role of purines in adipocyte precursors differentiation and mature adipocytes functions, as well as their impact among distinct adipose tissue deposits (e.g. white vs. brown, visceral vs. subcutaneous), which warrants further investigations before translation to clinical trials can be made.

Identification and characterization of adipose surface epitopes

Adipose tissue is a central regulator of metabolism and an important pharmacological target to treat the metabolic consequences of obesity, such as insulin resistance and dyslipidemia. Among the various cellular compartments, the adipocyte cell surface is especially appealing as a drug target as it contains various proteins that when activated or inhibited promote adipocyte health, change its endocrine function and eventually maintain or restore whole-body insulin sensitivity. In addition, cell surface proteins are readily accessible by various drug classes. However, targeting individual cell surface proteins in adipocytes has been difficult due to important functions of these proteins outside adipose tissue, raising various safety concerns. Thus, one of the biggest challenges is the lack of adipose selective surface proteins and/or targeting reagents. Here, we discuss several receptor families with an important function in adipogenesis and mature adipocytes to highlight the complexity at the cell surface and illustrate the problems with identifying adipose selective proteins. We then discuss that, while no unique adipocyte surface protein might exist, how splicing, posttranslational modifications as well as protein/protein interactions can create enormous diversity at the cell surface that vastly expands the space of potentially unique epitopes and how these selective epitopes can be identified and targeted.

Alisol A-24-acetate promotes glucose uptake via activation of AMPK in C2C12 myotubes

BACKGROUND

Alisol A-24-acetate (AA-24-a) is one of the main active triterpenes isolated from the well-known medicinal plant Alisma orientale (Sam.) Juz., which possesses multiple biological activities, including a hypoglycemic effect. Whether AA-24-a is a hypoglycemic-active compound of A. orientale (Sam.) Juz. is unclear. The present study aimed to clarify the effect and potential mechanism of action of AA-24-a on glucose uptake in C2C12 myotubes.

METHOD

Effects of AA-24-a on glucose uptake and GLUT4 translocation to the plasma membrane were evaluated. Glucose uptake was determined using a 2-(N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl) amino)-2-deoxyglucose (2-NBDG) uptake assay. Cell membrane proteins were isolated and glucose transporter 4 (GLUT4) protein was detected by western blotting to examine the translocation of GLUT4 to the plasma membrane. To determine the underlying mechanism, the phosphorylation levels of proteins involved in the insulin and 5'-adenosine monophosphate-activated protein kinase (AMPK) pathways were examined using western blotting. Furthermore, specific inhibitors of key enzymes in AMPK signaling pathway were used to examine the role of these kinases in the AA-24-a-induced glucose uptake and GLUT4 translocation.

RESULTS

We found that AA-24-a significantly promoted glucose uptake and GLUT4 translocation in C2C12 myotubes. AA-24-a increased the phosphorylation of AMPK, but had no effect on the insulin-dependent pathway involving insulin receptor substrate 1 (IRS1) and protein kinase B (PKB/AKT). In addition, the phosphorylation of p38 mitogen-activated protein kinase (MAPK) and the AKT substrate of 160 kDa (AS160), two proteins that act downstream of AMPK, was upregulated. Compound C, an AMPK inhibitor, blocked AA-24-a-induced AMPK pathway activation and reversed AA-24-a-induced glucose uptake and GLUT4 translocation to the plasma membrane, indicating that AA-24-a promotes glucose metabolism via the AMPK pathway in vitro. STO-609, a calcium/calmodulin-dependent protein kinase kinase β (CaMKKβ) inhibitor, also attenuated AA-24-a-induced glucose uptake and GLUT4 translocation. Moreover, STO-609 weakened AA-24-a-induced phosphorylation of AMPK, p38 MAPK and AS160.

CONCLUSIONS

These results indicate that AA-24-a isolated from A. orientale (Sam.) Juz. significantly enhances glucose uptake via the CaMKKβ-AMPK-p38 MAPK/AS160 pathway.

Role of purines in regulation of metabolic reprogramming

Purines, among most influential molecules, are reported to have essential biological function by regulating various cell types. A large number of studies have led to the discovery of many biological functions of the purine nucleotides such as ATP, ADP, and adenosine, as signaling molecules that engage G protein-coupled or ligand-gated ion channel receptors. The role of purines in the regulation of cellular functions at the gene or protein level has been well documented. With the advances in multiomics, including those from metabolomic and bioinformatic analyses, metabolic reprogramming was identified as a key mechanism involved in the regulation of cellular function under physiological or pathological conditions. Recent studies suggest that purines or purine-derived products contribute to important regulatory functions in many fundamental biological and pathological processes related to metabolic reprogramming. Therefore, this review summarizes the role and potential mechanism of purines in the regulation of metabolic reprogramming. In particular, the molecular mechanisms of extracellular purine- and intracellular purine-mediated metabolic regulation in various cells during disease development are discussed. In summary, our review provides an extensive resource for studying the regulatory role of purines in metabolic reprogramming and sheds light on the utilization of the corresponding peptides or proteins for disease diagnosis and therapy.

Identification of chebulinic acid as a dual targeting inhibitor of protein tyrosine phosphatases relevant to insulin resistance

Natural products as antidiabetic agents have been shown to stimulate insulin signaling via the inhibition of the protein tyrosine phosphatases relevant to insulin resistance. Previously, we have identified PTPN9 and DUSP9 as potential antidiabetic targets and a multi-targeting natural product thereof. In this study, knockdown of PTPN11 increased AMPK phosphorylation in differentiated C2C12 muscle cells by 3.8 fold, indicating that PTPN11 could be an antidiabetic target. Screening of a library of 658 natural products against PTPN9, DUSP9, or PTPN11 identified chebulinic acid (CA) as a strong allosteric inhibitor with a slow cooperative binding to PTPN9 (IC₅₀ = 34 nM) and PTPN11 (IC₅₀ = 37 nM), suggesting that it would be a potential antidiabetic candidate. Furthermore, CA stimulated glucose uptake and resulted in increased AMP-activated protein kinase (AMPK) phosphorylation. Taken together, we demonstrated that CA increased glucose uptake as a dual inhibitor of PTPN9 and PTPN11 through activation of the AMPK signaling pathway. These results strongly suggest that CA could be used as a potential therapeutic candidate for the treatment of type 2 diabetes.

Fridamycin A, a Microbial Natural Product, Stimulates Glucose Uptake without Inducing Adipogenesis

Type 2 diabetes is a complex, heterogeneous, and polygenic disease. Currently, available drugs for treating type 2 diabetes predominantly include sulfonylureas, α-glucosidase inhibitors, and biguanides. However, long-term treatment with these therapeutic drugs is often accompanied by undesirable side effects, which have driven interest in the development of more effective and safer antidiabetic agents. To address the urgent need for new chemical solutions, we focused on the analysis of structurally novel and/or biologically new metabolites produced by insect-associated microbes as they have recently been recognized as a rich source of natural products. Comparative LC/MS-based analysis of Actinomadura sp. RB99, isolated from a fungus-growing termite, led to the identification of the type II polyketide synthase-derived fridamycin A. The structure of fridamycin A was confirmed by ¹H NMR data and LC/MS analysis. The natural microbial product, fridamycin A, was examined for its antidiabetic properties in 3T3-L1 adipocytes, which demonstrated that fridamycin A induced glucose uptake in 3T3-L1 cells by activating the AMP-activated protein kinase (AMPK) signaling pathway but did not affect adipocyte differentiation, suggesting that the glucose uptake took place through activation of the AMPK signaling pathway without inducing adipogenesis. Our results suggest that fridamycin A has potential to induce fewer side effects such as weight gain compared to rosiglitazone, a commonly used antidiabetic drug, and that fridamycin A could be a novel potential therapeutic candidate for the management of type 2 diabetes.

Pharmacology of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes divided into two subgroups (P2Y₁, P2Y₂, P2Y₄, P2Y₆, and P2Y₁₁) and (P2Y₁₂, P2Y₁₃, and P2Y₁₄). The P2Y receptors are expressed in various cell types and play important roles in physiology and pathophysiology including inflammatory responses and neuropathic pain. The antagonism of P2Y₁₂ receptors is used in pharmacotherapy for the prevention and therapy of cardiovascular events. The nucleoside analogue ticagrelor and active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel inhibit platelet P2Y₁₂ receptors and reduce thereby platelet aggregation. The P2Y₂ receptor agonist diquafosol is used for the treatment of the dry eye syndrome. The P2Y receptor subtypes differ in their amino acid sequences, their pharmacological profiles and their signaling transduction pathways. Recently, selective receptor ligands have been developed for all subtypes. The published crystal structures of the human P2Y₁ and P2Y₁₂ receptors as well as receptor models will facilitate the development of novel drugs for pharmacotherapy.

The involvement of purinergic signalling in obesity

Obesity is a growing worldwide health problem, with an alarming increasing prevalence in developed countries, caused by a dysregulation of energy balance. Currently, no wholly successful pharmacological treatments are available for obesity and related adverse consequences. In recent years, hints obtained from several experimental animal models support the notion that purinergic signalling, acting through ATP-gated ion channels (P2X), G protein-coupled receptors (P2Y) and adenosine receptors (P1), is involved in obesity, both at peripheral and central levels. This review has drawn together, for the first time, the evidence for a promising, much needed novel therapeutic purinergic signalling approach for the treatment of obesity with a 'proof of concept' that hopefully could lead to further investigations and clinical trials for the management of obesity.

Inhibition of P2Y6 Signaling in AgRP Neurons Reduces Food Intake and Improves Systemic Insulin Sensitivity in Obesity

Uridine-diphosphate (UDP) and its receptor P2Y6 have recently been identified as regulators of AgRP neurons. UDP promotes feeding via activation of P2Y6 receptors on AgRP neurons, and hypothalamic UDP concentrations are increased in obesity. However, it remained unresolved whether inhibition of P2Y6 signaling pharmacologically, globally, or restricted to AgRP neurons can improve obesity-associated metabolic dysfunctions. Here, we demonstrate that central injection of UDP acutely promotes feeding in diet-induced obese mice and that acute pharmacological blocking of CNS P2Y6 receptors reduces food intake. Importantly, mice with AgRP-neuron-restricted inactivation of P2Y6 exhibit reduced food intake and fat mass as well as improved systemic insulin sensitivity with improved insulin action in liver. Our results reveal that P2Y6 signaling in AgRP neurons is involved in the onset of obesity-associated hyperphagia and systemic insulin resistance. Collectively, these experiments define P2Y6 as a potential target to pharmacologically restrict both feeding and systemic insulin resistance in obesity.

Purinergic Receptors in Adipose Tissue As Potential Targets in Metabolic Disorders

Extracellular nucleosides and nucleotides, such as adenosine and adenosine triphosphate (ATP), are involved in many physiological and pathological processes in adipose tissue (AT). It is becoming accepted that, in addition to the well-established sympathetic and hormonal system, purinergic receptors contribute significantly to regulation of adipocyte functions. Several receptor subtypes for both adenosine (P1) and ATP (P2X and P2Y) have been characterized in white adipocytes (WA) and brown adipocytes (BA). The effects mediated by adenosine and ATP on adipocytes are multiple and often differing, depending on specific receptors activated. Using a variety of agonists, antagonists and transgenic animals it has been demonstrated that adenosine and P2 receptors are involved in lipolysis, lipogenesis, adipokines secretion, glucose uptake, adipogenesis, cell proliferation, inflammation, and other processes. Given their central role in regulating many AT functions, purinergic receptors are considered potential therapeutic targets in different pathological conditions, such as obesity and type-2 diabetes. To achieve this goal, specific and potent P1 and P2 receptors activators and inhibitors are being developed and show promising results. However, more insight is needed into the function of P2 receptors in brown and beige adipocytes and their potential role in thermogenesis. This review aims at summarizing current knowledge on the patho-/physiological role of P1, P2X, and P2Y receptors in WA and BA and their potential exploitation for pharmacological intervention. Furthermore, we analyze impact of purinergic signaling in AT - in health and metabolic diseases.

Pyrimidine Nucleotides Containing a (S)-Methanocarba Ring as P2Y6 Receptor Agonists

Both agonists and antagonists of the UDP-activated P2Y₆ receptor (P2Y₆R) have been proposed for therapeutic use, in conditions such as cancer, inflammation, neurodegeneration and diabetes. Uracil nucleotides containing a South-bicyclo[3.1.0]hexane ((S)-methanocarba) ring system in place of the ribose ring were synthesized and shown to be potent P2Y₆R agonists in a calcium mobilization assay. The (S)-methanocarba modification was compatible with either a 5-iodo or 4-methoxyimino group on the pyrimidine, but not with a α,β-methylene 5´-diphosphate. (S)-Methanocarba dinucleotide potency was compatible with a N⁴-methoxy modification on the proximal nucleoside that is assumed to bind at the P2Y₆R similarly to UDP; (N)-methanocarba was preferred on the distal nucleoside moiety. This suggests that the distal dinucleotide P2Y₆R binding site prefers a ribose-like group that can attain a (N) conformation, rather than (S). Dinucleotide binding was modeled by homology modeling, docking and molecular dynamics simulations, which suggested the same ribose conformational preferences found empirically.

Glycolysis and oxidative phosphorylation are essential for purinergic receptor-mediated angiogenic responses in vasa vasorum endothelial cells

Angiogenesis is an energy-demanding process; however, the role of cellular energy pathways and their regulation by extracellular stimuli, especially extracellular nucleotides, remain largely unexplored. Using metabolic inhibitors of glycolysis (2-deoxyglucose) and oxidative phosphorylation (OXPHOS) (oligomycin, rotenone, and FCCP), we demonstrate that glycolysis and OXPHOS are both essential for angiogenic responses of vasa vasorum endothelial cell (VVEC). Treatment with P2R agonists, ATP, and 2-methylthioadenosine diphosphate trisodium salt (MeSADP), but not P1 receptor agonist, adenosine, increased glycolytic activity in VVEC (measured by extracellular acidification rate and lactate production). Stimulation of glycolysis was accompanied by increased levels of phospho-phosphofructokinase B3, hexokinase (HK), and GLUT-1, but not lactate dehydrogenase. Moreover, extracellular ATP and MeSADP, and to a lesser extent adenosine, increased basal and maximal oxygen consumption rates in VVEC. These effects were potentiated when the cells were cultured in 20 mM galactose and 5 mM glucose compared with 25 mM glucose. Treatment with P2R agonists decreased phosphorylation of pyruvate dehydrogenase (PDH)-E1α and increased succinate dehydrogenase (SDH), cytochrome oxidase IV, and β-subunit of F₁F₀ ATP synthase expression. In addition, P2R stimulation transiently elevated mitochondrial Ca²⁺ concentration, implying involvement of mitochondria in VVEC angiogenic activation. We also demonstrated a critical role of phosphatidylinositol 3-kinase and Akt pathways in lactate production, PDH-E1α phosphorylation, and the expression of HK, SDH, and GLUT-1 in ATP-stimulated VVEC. Together, our findings suggest that purinergic and metabolic regulation of VVEC energy pathways is essential for VV angiogenesis and may contribute to pathologic vascular remodeling in pulmonary hypertension.

P2X1-regulated IL-22 secretion by innate lymphoid cells is required for efficient liver regeneration

Paracrine signalling mediated by cytokine secretion is essential for liver regeneration after hepatic resection, yet the mechanisms of cellular crosstalk between immune and parenchymal cells are still elusive. Interleukin-22 (IL-22) is released by immune cells and mediates strong hepatoprotective functions. However, it remains unclear whether IL-22 is critical for the crosstalk between liver lymphocytes and parenchymal cells during liver regeneration after partial hepatectomy (PH). Here, we found that plasma levels of IL-22 and its upstream cytokine, IL-23, are highly elevated in patients after major liver resection. In a mouse model of PH, deletion of IL-22 was associated with significantly delayed hepatocellular proliferation and an increase of hepatocellular injury and endoplasmic reticulum stress. Using Rag1(-/-) and Rag2(-/-) γc(-/) (-) mice, we show that the main producers of IL-22 post-PH are conventional natural killer cells and innate lymphoid cells type 1. Extracellular adenosine triphosphate (ATP), a potent danger molecule, is elevated in patients immediately after major liver resection. Antagonism of the P2-type nucleotide receptors, P2X1 and P2Y6, significantly decreased IL-22 secretion ex vivo. In vivo, specific inhibition of P2X1 was associated with decreased IL-22 secretion, elevated liver injury, and impaired liver regeneration.

CONCLUSION

This study shows that innate immune cell-derived IL-22 is required for efficient liver regeneration and that secretion of IL-22 in the regenerating liver is modulated by the ATP receptor, P2X1. (Hepatology 2016;63:2004-2017).

Hypothalamic UDP Increases in Obesity and Promotes Feeding via P2Y6-Dependent Activation of AgRP Neurons

Activation of orexigenic AgRP-expressing neurons in the arcuate nucleus of the hypothalamus potently promotes feeding, thus defining new regulators of AgRP neuron activity could uncover potential novel targets for obesity treatment. Here, we demonstrate that AgRP neurons express the purinergic receptor 6 (P2Y6), which is activated by uridine-diphosphate (UDP). In vivo, UDP induces ERK phosphorylation and cFos expression in AgRP neurons and promotes action potential firing of these neurons in brain slice recordings. Consequently, central application of UDP promotes feeding, and this response is abrogated upon pharmacologic or genetic inhibition of P2Y6 as well as upon pharmacogenetic inhibition of AgRP neuron activity. In obese animals, hypothalamic UDP content is elevated as a consequence of increased circulating uridine concentrations. Collectively, these experiments reveal a potential regulatory pathway in obesity, where peripheral uridine increases hypothalamic UDP concentrations, which in turn can promote feeding via PY6-dependent activation of AgRP neurons.

Pharmacology and structure of P2Y receptors

P2Y receptors are G-protein-coupled receptors (GPCRs) for extracellular nucleotides. There are eight mammalian P2Y receptor subtypes (P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, P2Y12, P2Y13, and P2Y14). P2Y receptors are widely expressed and play important roles in physiology and pathophysiology. One important example is the ADP-induced platelet aggregation mediated by P2Y1 and P2Y12 receptors. Active metabolites of the thienopyridine compounds ticlopidine, clopidogrel and prasugrel as well as the nucleoside analogue ticagrelor block P2Y12 receptors and thereby platelet aggregation. These drugs are used for the prevention and therapy of cardiovascular events. Moreover, P2Y receptors play important roles in the nervous system. Adenine nucleotides modulate neuronal activity and neuronal fibre outgrowth by activation of P2Y1 receptors and control migration of microglia by P2Y12 receptors. UDP stimulates microglial phagocytosis through activation of P2Y6 receptors. There is evidence for a role for P2Y2 receptors in Alzheimer's disease pathology. The P2Y receptor subtypes are highly diverse in both their amino acid sequences and their pharmacological profiles. Selective receptor ligands have been developed for the pharmacological characterization of the receptor subtypes. The recently published three-dimensional crystal structures of the human P2Y1 and P2Y12 receptors will facilitate the development of therapeutic agents that selectively target P2Y receptors. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.