Adenosine A2B Receptors - Mediated Induction of Interleukin-6 in Skeletal Muscle Cells

A2B adenosine receptor signaling and regulation

The A2B adenosine receptor (A2BR) is one of the four adenosine-activated G protein-coupled receptors. In addition to adenosine, protein kinase C (PKC) was recently found to activate the A2BR. The A2BR is coupled to both Gs and Gi, as well as Gq proteins in some cell types. Many primary cells and cell lines, such as bladder and breast cancer, bronchial smooth muscle, skeletal muscle, and fat cells, express the A2BR endogenously at high levels, suggesting its potentially important role in asthma, cancer, diabetes, and other conditions. The A2BR has been characterized as both pro- and anti-inflammatory, inducing cell type-dependent secretion of IL-6, IL-8, and IL-10. Theophylline and enprofylline have long been used for asthma treatment, although it is still not entirely clear if their A2BR antagonism contributes to their therapeutic effects or side effects. The A2BR is required in ischemic cardiac preconditioning by adenosine. Both A2BR and protein kinase C (PKC) contribute to cardioprotection, and both modes of A2BR signaling can be blocked by A2BR antagonists. Inhibitors of PKC and A2BR are in clinical cancer trials. Sulforaphane and other isothiocyanates from cruciferous vegetables such as broccoli and cauliflower have been reported to inhibit A2BR signaling via reaction with an intracellular A2BR cysteine residue (C210). A full, A2BR-selective agonist, critical to elucidate many controversial roles of the A2BR, is still not available, although agonist-bound A2BR structures have recently been reported.

Molecular networks underlying cannabinoid signaling in skeletal muscle plasticity

The cannabinoid system is ubiquitously present and is classically considered to engage in neural and immunity processes. Yet, the role of the cannabinoid system in the whole body and tissue metabolism via central and peripheral mechanisms is increasingly recognized. The present review provides insights in (i) how cannabinoid signaling is regulated via receptor-independent and -dependent mechanisms and (ii) how these signaling cascades (might) affect skeletal muscle plasticity and physiology. Receptor-independent mechanisms include endocannabinoid metabolism to eicosanoids and the regulation of ion channels. Alternatively, endocannabinoids can act as ligands for different classic (cannabinoid receptor 1 [CB₁ ], CB₂ ) and/or alternative (e.g., TRPV1, GPR55) cannabinoid receptors with a unique affinity, specificity, and intracellular signaling cascade (often tissue-specific). Antagonism of CB₁ might hold clues to improve oxidative (mitochondrial) metabolism, insulin sensitivity, satellite cell growth, and muscle anabolism, whereas CB₂ agonism might be a promising way to stimulate muscle metabolism and muscle cell growth. Besides, CB₂ ameliorates muscle regeneration via macrophage polarization toward an anti-inflammatory phenotype, induction of MyoD and myogenin expression and antifibrotic mechanisms. Also TRPV1 and GPR55 contribute to the regulation of muscle growth and metabolism. Future studies should reveal how the cannabinoid system can be targeted to improve muscle quantity and/or quality in conditions such as ageing, disease, disuse, and metabolic dysregulation, taking into account challenges that are inherent to modulation of the cannabinoid system, such as central and peripheral side effects.

Therapeutic Potential of Highly Selective A3 Adenosine Receptor Ligands in the Central and Peripheral Nervous System

Ligands of the G_i protein-coupled adenosine A₃ receptor (A₃R) are receiving increasing interest as attractive therapeutic tools for the treatment of a number of pathological conditions of the central and peripheral nervous systems (CNS and PNS, respectively). Their safe pharmacological profiles emerging from clinical trials on different pathologies (e.g., rheumatoid arthritis, psoriasis and fatty liver diseases) confer a realistic translational potential to these compounds, thus encouraging the investigation of highly selective agonists and antagonists of A₃R. The present review summarizes information on the effect of latest-generation A₃R ligands, not yet available in commerce, obtained by using different in vitro and in vivo models of various PNS- or CNS-related disorders. This review places particular focus on brain ischemia insults and colitis, where the prototypical A₃R agonist, Cl-IB-MECA, and antagonist, MRS1523, have been used in research studies as reference compounds to explore the effects of latest-generation ligands on this receptor. The advantages and weaknesses of these compounds in terms of therapeutic potential are discussed.

The Impact of CB1 Receptor on Nuclear Receptors in Skeletal Muscle Cells

Cannabinoids are abundant signaling compounds; their influence predominantly arises via engagement with the principal two G-protein-coupled cannabinoid receptors, CB1 and CB2. One suggested theory is that cannabinoids regulate a variety of physiological processes within the cells of skeletal muscle. Earlier publications have indicated that expression of CB1 receptor mRNA and protein has been recognized within myotubes and tissues of skeletal muscle from both murines and humans, thus representing a potentially significant pathway which plays a role in the control of skeletal muscular activities. The part played by CB1 receptor activation or inhibition with respect to these functions and relevant to targets in the periphery, especially skeletal muscle, is not fully delineated. Thus, the aim of the current research was to explore the influence of CB1 receptor stimulation and inhibition on downstream signaling of the nuclear receptor, NR4A, which regulates the immediate impacts of arachidonyl-2′-chloroethylamide (ACEA) and/or rimonabant in the cells of skeletal muscle. Murine L6 skeletal muscle cells were used in order to clarify additional possible molecular signaling pathways which contribute to alterations in the CB1 receptor. Skeletal muscle cells have often been used; it is well-documented that they express cannabinoid receptors. Quantitative real-time probe-based polymerase chain reaction (qRT-PCR) assays are deployed in order to assess the gene expression characteristics of CB1 receptor signaling. In the current work, it is demonstrated that skeletal muscle cells exhibit functional expression of CB1 receptors. This can be deduced from the qRT-PCR assays; triggering CB1 receptors amplifies both NR4A1 and NR4A3 mRNA gene expression. The impact of ACEA is inhibited by the selective CB1 receptor antagonist, rimonabant. The present research demonstrated that 10 nM of ACEA notably amplified mRNA gene expression of NR4A1 and NR4A3; this effect was suppressed by the addition of 100 nM rimonabant. Furthermore, the CB1 receptor antagonist led to the downregulation of mRNA gene expression of NR4A1, NR4A2 and NR4A3. In conclusion, in skeletal muscle, CB1 receptors were recognized to be important moderators of NR4A1 and NR4A3 mRNA gene expression; these actions may have possible clinical benefits. Thus, in skeletal muscle cells, a possible physiological expression of CB1 receptors was identified. It is as yet unknown whether these CB1 receptors contribute to pathways underlying skeletal muscle biological function and disease processes. Further research is required to fully delineate their role(s).

Impact of Adenosine Analogue, Adenosine-5'-N-Ethyluronamide (NECA), on Insulin Signaling in Skeletal Muscle Cells

Materials and Methods

Rat L6 skeletal muscle cells were cultured in 25 cm² flasks. These differentiated cells were treated, and then, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) (probe-based) was used to measure the relative mRNA expression level for metabolic, inflammatory, and nuclear receptor genes including peroxisome proliferator-activated receptor gamma (PGC-1α), carnitine palmitoyl transferase 1 beta (CPT1B), long-chain acyl-CoA de hydrogenase (LCAD), acetyl-CoA carboxylase beta (ACCβ), pyruvate dehydrogenase kinase 4 (PDK4), hexokinase II (HKII), phosphofructokinase (PFK), interleukin-6 (IL-6), and nuclear receptor subfamily 4, group A (NR4A) at different treatment conditions.

Results

Adenosine-5′-N-ethyluronamide (NECA), a stable adenosine analogue, significantly stimulate inflammatory mediator (IL-6) (p < 0.001) and nuclear receptors (NR4A) (p < 0.05) and significantly modulate metabolic (PFK, LCAD, PGC-1α, and CPT1B) gene expressions in skeletal muscle cells (p < 0.05, p < 0.05, p < 0.001, and p < 0.01, respectively). This present study shows that there is a noteworthy crosstalk between NECA and insulin at various metabolic levels including glycolysis (HKII), fatty acid oxidation (ACCβ), and insulin sensitivity (PDK4).

Conclusions

A novel crosstalk between adenosine analogue and insulin has been demonstrated for the first time; evidence has been gathered in vitro for the effects of NECA and insulin treatment on intracellular signaling pathways, in particular glycolysis and insulin sensitivity in skeletal muscle cells.

Impact of Adenosine A2 Receptor Ligands on BCL2 Expression in Skeletal Muscle Cells

Background: Adenosine plays the role of regulating cell differentiation, proliferation, and apoptosis in various kinds of cells through the B-cell lymphoma 2 (BCL2) pathway. Objectives: Since anti-apoptotic (BCL2) expression plays a role in controlling apoptosis in some cell lines, this study was designed to investigate whether adenosine analogue, NECA (non-selective adenosine receptors agonist), selective adenosine A2B receptor antagonist, PSB 603, and a selective adenosine A2A receptor agonist, CG21680, affect BCL2-gene expression in the skeletal muscle cells of rats. The purpose of this investigation was to test the hypothesis that CG21680 treatment would significantly intensify BCL2 gene expression in rat skeletal muscle. Methods: Flasks measuring 25 cm2 were employed in culturing the rat L6 skeletal muscle cells. After treating these differential cells, the relative mRNA expression level for the BCL2 gene, at varying conditions of treatment, was measured using quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Results: From the qRT-PCR analysis results, it was concluded that BCL2 expression was markedly amplified after selective adenosine A2A receptor agonist, CGS21680 (p < 0.01) treatment. More prospective validation for the adenosine receptors’ contribution in modulating apoptosis by NECA was delivered by the outcomes from the combined pre-treatment of the cells with NECA and PSB 603. These outcomes show that when starved skeletal muscle cells are treated with a combination of NECA and 100 nM PSB 603, there was a substantial decrease in comparison to either treatment used on its own. Conclusions: This study’s results showed, for the first time, an increase in BCL2 gene expression within skeletal muscle after CGS21680 treatment. Hence, the prospective escalation in BCL2 protein expression might have a protective role to play against apoptosis and avert damage to the skeletal muscle.