Attenuation of apoptosis in vitro and ischemia/reperfusion injury in vivo in mouse skeletal muscle by P2Y6 receptor activation

d-Pinitol Improves Diabetic Sarcopenia by Regulation of the Gut Microbiome, Metabolome, and Proteome in STZ-Induced SAMP8 Mice

d-Pinitol (DP) is primarily found in Vigna sinensis, which has been shown to have hypoglycemic and protective effects on target organs. However, the mechanism of DP in treating diabetic sarcopenia (DS) is still unclear. To explore the underlying mechanism of DS and the protective targets of DP by high-throughput analysis of 16S rRNA gene, metabolome, and the proteome. Streptozotocin-induced SAMP8 mice were intragastrically administrated DP (150 mg/kg) for 8 weeks. Fecal 16S rRNA gene sequencing and gastrocnemius muscle metabolomic and proteomic analyses were completed to investigate the gut-muscle axis interactions. DP significantly alleviated the muscle atrophy in diabetic mice. Dysfunction of the gut microbiota was observed in the DS mice. DP significantly reduced the Parabacteroides, Akkermansia, and Enterobacteriaceae, while it increased Lachnospiraceae_NK4A136. Metabolome and proteome revealed that 261 metabolites and 626 proteins were significantly changed in the gastrocnemius muscle of diabetic mice. Among these, DP treatment restored 44 metabolites and 17 proteins to normal levels. Functional signaling pathways of DP-treated diabetic mice included nucleotide metabolism, β-alanine, histidine metabolism, ABC transporters, and the calcium signaling pathway. We systematically explored the molecular mechanism of DS and the protective effect of DP, providing new insights that may advance the treatment of sarcopenia.

Mechanical Disturbance of Osteoclasts Induces ATP Release That Leads to Protein Synthesis in Skeletal Muscle through an Akt-mTOR Signaling Pathway

Muscle and bone are tightly integrated through mechanical and biochemical signals. Osteoclasts are cells mostly related to pathological bone loss; however, they also start physiological bone remodeling. Therefore, osteoclast signals released during bone remodeling could improve both bone and skeletal muscle mass. Extracellular ATP is an autocrine/paracrine signaling molecule released by bone and muscle cells. Then, in the present work, it was hypothesized that ATP is a paracrine mediator released by osteoclasts and leads to skeletal muscle protein synthesis. RAW264.7-derived osteoclasts were co-cultured in Transwell^® chambers with flexor digitorum brevis (FDB) muscle isolated from adult BalbC mice. The osteoclasts at the upper chamber were mechanically stimulated by controlled culture medium perturbation, resulting in a two-fold increase in protein synthesis in FDB muscle at the lower chamber. Osteoclasts released ATP to the extracellular medium in response to mechanical stimulation, proportional to the magnitude of the stimulus and partly dependent on the P2X₇ receptor. On the other hand, exogenous ATP promoted Akt phosphorylation (S473) in isolated FDB muscle in a time- and concentration-dependent manner. ATP also induced phosphorylation of proteins downstream Akt: mTOR (S2448), p70S6K (T389) and 4E-BP1 (T37/46). Exogenous ATP increased the protein synthesis rate in FDB muscle 2.2-fold; this effect was blocked by Suramin (general P2X/P2Y antagonist), LY294002 (phosphatidylinositol 3 kinase inhibitor) and Rapamycin (mTOR inhibitor). These blockers, as well as apyrase (ATP metabolizing enzyme), also abolished the induction of FDB protein synthesis evoked by mechanical stimulation of osteoclasts in the co-culture model. Therefore, the present findings suggest that mechanically stimulated osteoclasts release ATP, leading to protein synthesis in isolated FDB muscle, by activating the P2-PI3K-Akt-mTOR pathway. These results open a new area for research and clinical interest in bone-to-muscle crosstalk in adaptive processes related to muscle use/disuse or in musculoskeletal pathologies.

The expression of P2Y6 receptor promotes the quality of mucus in colitic mice

In the intestine, mucins are expressed and secreted by goblet cells and enterocytes in a constitutive manner and in response to secretagogues to form a protective mucus layer. This protective barrier is often lost in inflammatory bowel disease (IBD). Interestingly, extracellular nucleotides, through P2Y receptors, were identified as mucin secretagogues in mucinous epithelia. These nucleotides are found in the intestine's extracellular milieu under basal conditions and in higher concentrations in pathologies such as IBD. It was observed that the mucus layer was affected in P2ry6 knockout mice suffering from dextran sodium sulfate (DSS)-induced colitis. P2ry6^-/- mice were more sensitive to DSS-induced colitis, resulting in larger ulcers and increased disease activity index. Interestingly, the absence of P2Y₆ receptor expression negatively affected the mucus quality, as shown by a reduction in sulfomucin staining and the absence of a dense internal fucosylated mucin layer in P2ry6^-/- mice. Hence, we cannot rule out that the absence of P2Y₆ receptors in knockout animals could negatively impact mucin secretion. However, we did not measure a reduction in the number of goblet cells, as previously reported. Instead, the results suggest that goblet cells rapidly discharged mucins to compensate for the mucus layer's increased lability, which resulted in empty goblet cells that are less visible to mucin staining. This study's results, along with previous reports, point toward a protective role for the P2Y₆ receptor in IBD.

Comparison of Ischemic Preconditioning and Systemic Piracetam for Prevention of Ischemia-Reperfusion Injury in Musculocutaneous Flaps

BACKGROUND

 Ischemia-reperfusion injury plays an important role in flap failure. Ischemic preconditioning technique is the only proven method for preventing ischemia-reperfusion injury, but it is not used widely in daily practice because of difficulties such as prolonging the operation time, need for surgical experience, and increasing the risk of complications. This study has been performed with the assumption that piracetam may be a simple and inexpensive alternative to the preconditioning technique due to its antioxidant, antiaggregant, rheological, anti-inflammatory, antiapoptotic, cytoprotective, and immune modulating effects.

METHODS

 Thirty-two rats were divided into four groups and latissimus dorsi musculocutaneous flaps were raised. No extra procedure was applied, and no treatment was given to the control group. Four hours of ischemia was created by clamping the thoracodorsal pedicle in the second group. The animals in the third group were treated with 10 minutes of ischemia and reperfusion periods as a preconditioning procedure before the 4 hours of ischemia. Animals in the fourth group received systemic piracetam 30 minutes before and 6 days after reperfusion. Nitric oxide and myeloperoxidase levels in serum and tissue, acute inflammatory cell response, and vascular proliferation in tissue were examined at the postoperative 24th hour and 10th day.

RESULTS

 Myeloperoxidase activity in both preconditioning and piracetam groups, was significantly lower than the ischemia-reperfusion group. Acute inflammatory cell response was similarly decreased in both preconditioning and piracetam groups compared with ischemia-reperfusion group. Tissue measurements of nitric oxide were also significantly higher in both preconditioning and piracetam groups than in the ischemia-reperfusion group. However, vascular proliferation increased in the preconditioning group, while it did not show any significant change in the piracetam group.

CONCLUSION

 This study shows that systemic piracetam treatment provides protection against ischemia-reperfusion injury in musculocutaneous flaps and can offer a simple and inexpensive alternative to the preconditioning technique.

Chronic intermittent hypobaric hypoxia attenuates skeletal muscle ischemia-reperfusion injury in mice

AIM

The aim of this study was to investigate the protective effect of chronic intermittent hypobaric hypoxia (CIHH) against skeletal muscle ischemia-reperfusion (IR) injury and to determine the underlying mechanism.

MAIN METHODS

C57BL/6 mice were randomly divided into 3 groups: skeletal muscle IR injury group (IR), CIHH pretreatment following IR group (IR + CIHH), and sham operation group (Sham). The skeletal muscle IR injury model was induced by the unilateral application of a tourniquet on a hind limb for 3 h and then releasing it for 24 h. CIHH pretreatment simulating a 5000-m altitude was applied 6 h per day for 28 days. The functional and morphological performance of IR-injured gastrocnemius muscle was evaluated using contraction force, H&E staining, and transmission electron microscopy. IR injury-induced CD68⁺ macrophage infiltration was assessed by immunofluorescence. TNFα levels in serum and muscle were measured by ELISA and western blotting, respectively. Apoptosis was examined by TUNEL staining and Cleaved Caspase-3 protein expression.

KEY FINDINGS

Acute IR injury resulted in reduced contraction tension, morphological destruction, macrophage infiltration, increased TNFα levels, and apoptosis in gastrocnemius muscle. CIHH pretreatment significantly ameliorated contraction function and morphological performance in IR-injured skeletal muscle. In addition, CIHH pretreatment resulted in marked decreases in CD68⁺ macrophage infiltration, TNFα levels, and apoptosis.

SIGNIFICANCE

These data demonstrated that CIHH has a protective effect against acute IR injury in skeletal muscle via inhibition of inflammation and apoptosis.

Tools and drugs for uracil nucleotide-activated P2Y receptors

P2Y receptors (P2YRs) are a family of G protein-coupled receptors activated by extracellular nucleotides. Physiological P2YR agonists include purine and pyrimidine nucleoside di- and triphosphates, such as ATP, ADP, UTP, UDP, nucleotide sugars, and dinucleotides. Eight subtypes exist, P2Y₁, P2Y₂, P2Y₄, P2Y₆, P2Y₁₁, P2Y₁₂, P2Y₁₃, and P2Y₁₄, which represent current or potential future drug targets. Here we provide a comprehensive overview of ligands for the subgroup of the P2YR family that is activated by uracil nucleotides: P2Y₂ (UTP, also ATP and dinucleotides), P2Y₄ (UTP), P2Y₆ (UDP), and P2Y₁₄ (UDP, UDP-glucose, UDP-galactose). The physiological agonists are metabolically unstable due to their fast hydrolysis by ectonucleotidases. A number of agonists with increased potency, subtype-selectivity and/or enzymatic stability have been developed in recent years. Useful P2Y₂R agonists include MRS2698 (6-01, highly selective) and PSB-1114 (6-05, increased metabolic stability). A potent and selective P2Y₂R antagonist is AR-C118925 (10-01). For studies of the P2Y₄R, MRS4062 (3-15) may be used as a selective agonist, while PSB-16133 (10-06) is a selective antagonist. Several potent P2Y₆R agonists have been developed including 5-methoxyuridine 5'-O-((R_p)α-boranodiphosphate) (6-12), PSB-0474 (3-11), and MRS2693 (3-26). The isocyanate MRS2578 (10-08) is used as a selective P2Y₆R antagonist, although its reactivity and low water-solubility are limiting. With MRS2905 (6-08), a potent and metabolically stable P2Y₁₄R agonist is available, while PPTN (10-14) represents a potent and selective P2Y₁₄R antagonist. The radioligand [³H]UDP can be used to label P2Y₁₄Rs. In addition, several fluorescent probes have been developed. Uracil nucleotide-activated P2YRs show great potential as drug targets, especially in inflammation, cancer, cardiovascular and neurodegenerative diseases.

The G protein-coupled P2Y₆ receptor promotes colorectal cancer tumorigenesis by inhibiting apoptosis

Colorectal tumors are immersed in an array of tumor-promoting factors including extracellular nucleotides such as uridine 5'‑diphosphate (UDP). UDP is the endogenous agonist of the G protein-coupled P2Y₆ receptor (P2Y₆R), which may contribute to the formation of a tumor-promoting microenvironment by coordinating resistance to apoptosis. Colorectal cancer (CRC) was chemically induced in P2ry6 knockout (P2ry6^-/-) mice using azoxymethane and dextran sulfate sodium challenges. Mice were euthanatized and their tumor load determined. Fixed tissues were stained for histological and immunohistochemistry analysis. Tumoroids were also prepared from CRC tumors resected from P2ry6^+/+ mice to determine the role of P2Y₆R in resistance to apoptosis, whereas HT29 carcinoma cells were used to elucidate the signaling mechanism involved in P2Y₆R anti-apoptotic effect. P2ry6^-/- mice developed a reduced number of colorectal tumors with apparent tumors having smaller volumes. Overall dysplastic score was significantly lower in P2ry6^-/- animals. Stimulation of P2Y₆R with the selective agonist MRS2693 protected HT-29 cells from TNFα-induced apoptosis. This protective effect was mediated by the stabilizing phosphorylation of the X-linked inhibitor of apoptosis protein (XIAP) by AKT. Using CRC-derived tumoroids, P2Y₆R activation was found to contribute to chemoresistance since addition of the P2Y₆R agonist MRS2693 significantly prevented the cytotoxic effect of 5-fluorouracil. The present study shows that sustained activation of P2Y₆R may contribute to intestinal tumorigenesis by blocking the apoptotic process and by contributing to chemoresistance, a substantial concern in the treatment of patients with CRC. These results suggest that P2Y₆R may represent a prime target for reducing colorectal carcinogenesis.

P2Y6 regulates cytoskeleton reorganization and cell migration of C2C12 myoblasts via ROCK pathway

Migration of skeletal muscle precursor cells is required for limb muscle development and skeletal muscle repair. This study aimed to examine the role of P2Y6 receptor in C2C12 myoblasts migration. C2C12 myoblasts were treated with P2Y6 agonist UDP, P2Y6 antagonist MRS2578, Ca²⁺ channel blocker BTP2, or ROCK inhibitor GSK269962 or Y27632, and the migration ability of C2C12 cells was assessed by wound healing assay. The cellular Ca²⁺ content was analyzed with fluo-4 probe and the activation of ROCK (phosphorlyation of LIMK and cofilin) was assayed by western blot. The cytoskeleton was labeled with Actin-Tracker Green and Tubulin-Tracker-Red. Silencing P2Y6 expression in C2C12 myoblasts reduced intracellular Ca²⁺ content and cell motility. Whereas UDP increased cellular Ca²⁺ content, actin filaments, and cell migration, MRS2578 had the opposite effects. The effects of UDP were abrogated by BTP2 and GSK269962 (and Y27632). Disruption of P2Y6 signaling pathway caused C2C12 myoblasts to have an elongated morphology. These results demonstrated that P2Y6 signaled through Ca²⁺ influx and RhoA/ROCK to reorganize cytoskeleton and promote migration in myoblasts.

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.

The Protective Effects of Fasciotomy on Reperfusion Injury of Skeletal Muscle of Rabbits

The authors aim to investigate protective effects of fasciotomy against ischemia reperfusion injury of skeletal muscle in rabbit and to compare the treatment effects of prereperfusion + fasciotomy and fasciotomy + postreperfusion against ischemia reperfusion injury of skeletal muscle. 24 healthy male Japanese white rabbits were randomly divided into 3 groups, and 4 hours' ischemia was established in these rabbits through surgery. Six hours' reperfusion was performed in group A; reperfusion + postfasciotomy was performed in group B; and prefasciotomy + reperfusion was performed in group C. Result showed that prefasciotomy and postfasciotomy could protect skeletal muscle against ischemia reperfusion injury, reduced MDA (malondialdehyde) expression, MPO (myeloperoxidase) expression, and apoptosis of muscle in the reperfused areas, increased Bcl-2 expression, and decreased Bax expression. The MDA and MPO levels in group B and group C were significantly lower than those in group A, and MDA and MPO levels in group C were significantly lower than those in group B. Prefasciotomy and postfasciotomy could protect against ischemia reperfusion injury in skeletal muscle. The protective effects of prefasciotomy against ischemia reperfusion injury are better than postfasciotomy.

Current knowledge on the role of P2Y receptors in cardioprotection against ischemia-reperfusion

During ischemia, numerous effective endogenous extracellular mediators have been identified, particularly, nucleosides such as adenosine as well as purinergic and pyrimidinergic nucleotides. They may play important regulatory roles within the cardiovascular system and notably as cardio-protectants. Indeed, the distribution of the P2Y receptors in mammalian heart includes several cellular constituents relevant for the pathophysiology of myocardial ischemia. Beside the well-known cardioprotective effect of adenosine, the additional protective role of P2Y receptors has emerged. However, interpretation of experimental results may be sometimes perplexing. This is due to the variability of: the experimental models, the endpoints criteria, the chemical structure of agonist and antagonist ligands and their concentrations, the sequences of drug administration with respect to the model used (before and/or during and/or after ischemia). The net effect may be in the opposite direction after a transient or a prolonged stimulation. Nevertheless, the overall reading of published data highlights the beneficial role of the P2Y_2/4 receptor stimulation, the useful and synergistic role of P2Y_6/11 receptor activation and even of the P2Y₁₁ receptor alone in cardioprotection. More, the P2Y₁₁ receptor could be involved in counter-regulation of profibrotic processes. Paradoxically, transient P2X₇ receptor stimulation could contribute to the net cardioprotective effect of ATP. Recently, experimental data have shown that blocking the P2Y₁₂ receptor after ischemia confers cardioprotection independently of platelet antiaggregatory effect. This suggests for P2Y receptors an important role in primary prevention and as a therapeutic target in myocardial protection during ischemia and reperfusion.

H89 enhances the sensitivity of cancer cells to glyceryl trinitrate through a purinergic receptor-dependent pathway

High doses of the organic nitrate glyceryl trinitrate (GTN), a nitric oxide (NO) donor, are known to trigger apoptosis in human cancer cells. Here, we show that such a cytotoxic effect can be obtained with subtoxic concentrations of GTN when combined with H89, N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulphonamide.2HCl. This synergistic effect requires the generation of reactive oxygen species (ROS) from H89 and NO from GTN treatment that causes cGMP production and PKG activation. Furthermore, the GTN/H89 synergy was attenuated by inhibition of P2-purinergic receptors with suramin and competition with ATP/UDP. By down-regulating genes with antisense oligonucleotides, P2-purinergic receptors P2X3, P2Y1, and P2Y6 were found to have a role in creating this cytotoxic effect. Thus, H89 likely acts as an ATP mimetic synergizing with GTN to trigger apoptosis in aggressive cancer cells.

Thymidine 5'-O-monophosphorothioate induces HeLa cell migration by activation of the P2Y6 receptor

ATP, ADP, UTP, and UDP acting as ligands of specific P2Y receptors activate intracellular signaling cascades to regulate a variety of cellular processes, including proliferation, migration, differentiation, and cell death. Contrary to a widely held opinion, we show here that nucleoside 5'-O-monophosphorothioate analogs, containing a sulfur atom in a place of one nonbridging oxygen atom in a phosphate group, act as ligands for selected P2Y subtypes. We pay particular attention to the unique activity of thymidine 5'-O-monophosphorothioate (TMPS) which acts as a specific partial agonist of the P2Y6 receptor (P2Y6R). We also collected evidence for the involvement of the P2Y6 receptor in human epithelial adenocarcinoma cell line (HeLa) cell migration induced by thymidine 5'-O-monophosphorothioate analog. The stimulatory effect of TMPS was abolished by siRNA-mediated P2Y6 knockdown and diisothiocyanate derivative MRS 2578, a selective antagonist of the P2Y6R. Our results indicate for the first time that increased stability of thymidine 5'-O-monophosphorothioate as well as its affinity toward the P2Y6R may be responsible for some long-term effects mediated by this receptor.

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

Glucose uptake by peripheral tissues such as skeletal muscles and adipocytes is important in the maintenance of glucose homeostasis. We previously demonstrated that P2Y₆ receptor (P2Y₆R) agonists protect pancreatic islet cells from apoptosis and stimulate glucose-dependent insulin release. Here, we investigated the effects of P2Y₆R activation on glucose uptake in insulin target tissues. An agonist of the P2Y₆R, P¹-(5′-uridine)-P³-(5′-N⁴-methoxycytidine)-triphosphate (MRS2957), significantly increased the uptake of [³H]2-deoxyglucose in mouse C2C12 myotubes and 3T3-L1 adipocytes, and this stimulation was significantly decreased by a selective P2Y₆R antagonist N,N″-1,4-butanediyl-bis[N′-(3-isothiocyanatophenyl)thiourea] (MRS2578). Pre-incubation with Compound C (an inhibitor of 5′-AMP-activated protein kinase, AMPK), or AMPK siRNA abolished the stimulatory effect of MRS2957 on glucose uptake. Also, MRS2957 (60 min incubation) increased recruitment of the facilitated glucose transporter-4 (GLUT4) to the cell membrane, which was blocked by MRS2578. Treatment of C2C12 myotubes with MRS2957 induced significant phosphorylation of AMPK, which increase GLUT4 expression through histone deacetylase (HDAC)5 signaling. Glucose uptake in primary mouse adipocytes from wild-type mice was stimulated upon P2Y₆R activation by either MRS2957 or native agonist UDP, and the P2Y₆R effect was antagonized by MRS2578. However, in adipocytes from P2Y₆R-knockout mice P2Y₆R agonists had no effect on glucose uptake, and there was no change in the glucose uptake by insulin. Our results indicate that the P2Y₆R promotes glucose metabolism in peripheral tissues, which may be mediated through AMPK signaling.

Hydrogen-rich saline attenuates ischemia-reperfusion injury in skeletal muscle

BACKGROUND

To investigate the potential beneficial effect of hydrogen-rich saline (HRS) in ischemia-reperfusion (IR) injury of skeletal muscle.

METHODS

Three experimental groups were established in male Sprague-Dawley rats: (1) sham group, (2) IR with normal saline group, (3) and IR with HRS group. A rat model of skeletal muscle IR injury was induced by 3-h tourniquet occlusion on its left hind limb and 4-h reperfusion. Normal saline and HRS (1.0 mL/100 g) were administered intraperitoneally at 10 min before reperfusion, respectively. Muscle and serum samples were analyzed for detecting the levels of myeloperoxidase (MPO), superoxide dismutase (SOD), malondialdehyde (MDA), and hydroxyl radical (•OH). Muscle samples were assessed by wet/dry rate, hematoxylin and eosin histologic assessment, Bcl2, Bax, cytochrome C, LC3B, terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling, and electron microscopy.

RESULTS

The wet/dry ratio increased significantly in the IR group (P < 0.01 compared with that in the sham group) and decreased significantly in IR with HRS groups (4.12 ± 0.14 versus 4.12 ± 0.14, P < 0.01 compared with that in the IR group). Muscle tissues and serum of the IR group had significantly increased levels of MPO, MDA, •OH content, and decreased SOD activities compared with the sham group (P < 0.01). The activity of SOD in the IR with HRS group was greatly elevated compared with that in the IR group (295.028 ± 9.288 versus 249.190 ± 5.450 in muscle tissues; 91.627 ± 2.604 versus 73.4045 ± 6.487 in serum; P < 0.01), whereas the levels of MPO, MDA, and •OH content were clearly reduced (MPO: 0.5649 ± 0.0724 versus 1.0984 ± 0.0824 in muscle tissues; 0.7257 ± 0.1232 versus 1.3147 ± 0.0531 in serum. MDA: 4.457 ± 0.650 versus 7.107 ± 0.597 in muscle tissues; 2.531 ± 0.434 versus 4.626 ± 0.237 in serum. •OH: 16.451 ± 0.806 versus 19.871 ± 0.594 in muscle tissues; 500.212 ± 7.387 versus 621.352 ± 7.591 in serum, P < 0.01). The integrated optical density of positive amethyst staining increased significantly in the IR group (P < 0.01 compared with that in the sham group) and decreased significantly in IR with HRS group (928.79 ± 234.537 versus 3005.972 ± 83.567, P < 0.01 compared with that in the IR group). Muscle tissues of the IR group had significantly increased levels of Bax, cytochrome C, LC3B content, and decreased Bcl2 activities compared with those in the sham group (P < 0.01). The activity of Bcl2 in the IR with HRS group was greatly elevated compared with that in the IR group (0.2635 ± 0.0704 versus 0.1242 ± 0.0662; P < 0.01), whereas the levels of Bax, cytochrome C, and LC3B content were clearly reduced (Bax: 0.3103 ± 0.0506 versus 0.5122 ± 0.0148; cytochrome C: 0.4194 ± 0.1116 versus 0.8127 ± 0.0166; LC3B: 0.5884 ± 0.0604 versus 1.3758 ± 0.0319; respectively, P < 0.01).

CONCLUSIONS

HRS seems to be effective in attenuating IR injury in skeletal muscle via its antioxidant, anti-apoptosis, and anti-autophagy effect.

5-OMe-uridine-5'-O-(α-boranodiphosphate), a novel nucleotide derivative highly active at the human P2Y(6) receptor protects against death-receptor mediated glial apoptosis

P2Y receptors are activated by nucleotides and involved in numerous physiological/pathophysiological processes. However, investigations of specific P2Y receptor functions have been hampered by lack of suitable receptor agonists-antagonists. Recently, we identified the nucleotide 5-OMe-UDP as potent and selective agonist for human P2Y6 receptors. We studied a series of derivatives of this analog with a Pα-borano group substituting a non-bridging oxygen and found increased potency and receptor specificity. Rp-5-OMe-UDPαB (Rp-5-OMe-uridine 5'-O-α-boranodiphosphate) was most potent and selective in inducing intracellular calcium signaling in 1321N1 astrocytoma cells expressing the human P2Y6 receptor. Here, we investigated whether Rp-5-OMe-UDPαB evokes cell protection through human P2Y6 receptors. We tested a well-established model, tumor necrosis factor α (TNFα)-induced cell death in 1321N1 astrocytoma cells. Rp-5-OMe-UDPαB inhibited TNFα-induced cell death even stronger than UDP. These first data of a neuro-protective activity of the human P2Y6 receptor emphasize the potential of the stable, selective, and potent Rp-5-OMe-UDPαB analog for exploiting P2Y6 receptor-mediated cellular functions, like cytoprotection in human tissues, with suitability for future neuro-protective drug development.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

P2Y receptors in the mammalian nervous system: pharmacology, ligands and therapeutic potential

P2Y receptors for extracellular nucleotides are coupled to activation of a variety of G proteins and stimulate diverse intracellular signaling pathways that regulate functions of cell types that comprise the central nervous system (CNS). There are 8 different subtypes of P2Y receptor expressed in cells of the CNS that are activated by a select group of nucleotide agonists. Here, the agonist selectivity of these 8 P2Y receptor subtypes is reviewed with an emphasis on synthetic agonists with high potency and resistance to degradation by extracellular nucleotidases that have potential applications as therapeutic agents. In addition, the recent identification of a wide variety of subtype-selective antagonists is discussed, since these compounds are critical for discerning cellular responses mediated by activation of individual P2Y receptor subtypes. The functional expression of P2Y receptor subtypes in cells that comprise the CNS is also reviewed and the role of each subtype in the regulation of physiological and pathophysiological responses is considered. Other topics include the role of P2Y receptors in the regulation of blood-brain barrier integrity and potential interactions between different P2Y receptor subtypes that likely impact tissue responses to extracellular nucleotides in the CNS. Overall, current research suggests that P2Y receptors in the CNS regulate repair mechanisms that are triggered by tissue damage, inflammation and disease and thus P2Y receptors represent promising targets for the treatment of neurodegenerative diseases.

4-Alkyloxyimino-cytosine nucleotides: tethering approaches to molecular probes for the P2Y6 receptor

4-Alkyloxyimino derivatives of pyrimidine nucleotides display high potency as agonists of certain G protein-coupled P2Y receptors (P2YRs). In an effort to functionalize a P2Y₆R agonist for fluorescent labeling, we probed two positions (N⁴ and γ-phosphate of cytidine derivatives) with various functional groups, including alkynes for click chemistry. Functionalization of extended imino substituents at the 4 position of the pyrimidine nucleobase of CDP preserved P2Y₆R potency generally better than γ-phosphoester formation in CTP derivatives. Fluorescent Alexa Fluor 488 conjugate 16 activated the human P2Y₆R expressed in 1321N1 human astrocytoma cells with an EC₅₀ of 9 nM, and exhibited high selectivity for this receptor over other uridine nucleotide-activated P2Y receptors. Flow cytometry detected specific labeling with 16 to P2Y₆R-expressing but not to wild-type 1321N1 cells. Additionally, confocal microscopy indicated both internalized 16 (t_1/2 of 18 min) and surface-bound fluorescence. Known P2Y₆R ligands inhibited labeling. Theoretical docking of 16 to a homology model of the P2Y₆R predicted electrostatic interactions between the fluorophore and extracellular portion of TM3. Thus, we have identified the N⁴-benzyloxy group as a structurally permissive site for synthesis of functionalized congeners leading to high affinity molecular probes for studying the P2Y₆R.

UDP made a highly promising stable, potent, and selective P2Y6-receptor agonist upon introduction of a boranophosphate moiety

P2Y(6) nucleotide receptor (P2Y(6)-R) plays important physiological roles, such as insulin secretion and reduction of intraocular pressure. However, this receptor is still lacking potent and selective agonists to be used as potential drugs. Here, we synthesized uracil nucleotides and dinucleotides, substituted at the C5 and/or P(α) position with methoxy and/or borano groups, 18-22. Compound 18A, R(p) isomer of 5-OMe-UDP(α-B), is the most potent and P2Y(6)-R selective agonist currently known (EC(50) 0.008 μM) being 19-fold more potent than UDP and showing no activity at uridine nucleotide receptors, P2Y(2)- and P2Y(4)-R. Analogue 18A was highly chemically stable under conditions mimicking gastric juice acidity (t(1/2) = 16.9 h). It was more stable to hydrolysis by nucleotide pyrophosphatases (NPP1,3) than UDP (15% and 28% hydrolysis by NPP1 and NPP3, respectively, vs 50% and 51% hydrolysis of UDP) and metabolically stable in blood serum (t(1/2) = 17 vs 2.4, 11.9, and 21 h for UDP, 5-OMe-UDP, and UDP(α-B), respectively). This newly discovered highly potent and physiologically stable P2Y(6)-R agonist may be of future therapeutic potential.

P2Y6 receptor contributes to neutrophil recruitment to inflamed intestinal mucosa by increasing CXC chemokine ligand 8 expression in an AP-1-dependent manner in epithelial cells

BACKGROUND

Inflammatory bowel diseases are characterized by the presence of CXCL8 at the site of lesions resulting in neutrophil recruitment and loss of tissue functions. We report that P2Y(6) receptor activation stimulates CXCL8 expression and release by intestinal epithelial cells (IECs). In this context, we investigated if uridine 5'-diphosphate (UDP) enemas stimulate neutrophil recruitment to the mucosa of mice suffering from colitis-like disease and we characterized the signaling events linking P2Y(6) to CXCL8 expression in IEC.

METHODS

Neutrophil recruitment was monitored by immunofluorescence and FACS analysis. Expression of Cxcl1, a mouse functional homolog of CXCL8, was determined by quantitative real-time polymerase chain reaction (qPCR). Pharmacological inhibitors and interfering RNAs were used to characterize the signaling pathway. The outcomes of these treatments on protein phosphorylation and on CXCL8 expression were characterized by western blots, qPCR, luciferase, and chromatin immunoprecipitation (ChIP) assays.

RESULTS

Mutation of the AP-1 site in the CXCL8 core promoter abolished the UDP-stimulating effect. The c-fos/c-jun dimer was identified as the AP-1 complex regulating CXCL8 in response to UDP stimulation. Regulation of CXCL8 expression by P2Y(6) required PKCδ activation upstream of the signaling pathway composed of MEK1/2-ERK1/2 and c-fos. UDP administration to mice suffering from colitis-like disease increased the number of neutrophil infiltrating the mucosa, correlating with Cxcl1 increased expression in IEC and the severity of inflammation.

CONCLUSIONS

This study not only describes the P2Y(6) signaling mechanism regulating CXCL8 expression in IEC, but it also illustrates the potential of targeting P2Y(6) to reduce intestinal inflammation.

Mechanisms underlying altered extracellular nucleotide-induced contractions in mesenteric arteries from rats in later-stage type 2 diabetes: effect of ANG II type 1 receptor antagonism

Little is known about the vascular contractile responsiveness to, and signaling pathways for, extracellular nucleotides in the chronic stage of type 2 diabetes or whether the ANG II type 1 receptor blocker losartan might alter such responses. We hypothesized that nucleotide-induced arterial contractions are augmented in diabetic Goto-Kakizaki (GK) rats and that treatment with losartan would normalize the contractions. Here, we investigated the vasoconstrictor effects of ATP/UTP in superior mesenteric arteries isolated from GK rats (37-42 wk old) that had or had not received 2 wk of losartan (25 mg·kg(-1)·day(-1)). In arteries from GK rats (vs. those from Wistar rats), 1) ATP- and UTP-induced contractions, which were blocked by the nonselective P2 antagonist suramin, were enhanced, and these enhancements were suppressed by endothelial denudation, by cyclooxygenase (COX) inhibitors, or by a cytosolic phospholipase A(2) (cPLA(2)) inhibitor; 2) both nucleotides induced increased release of PGE(2) and PGF(2α); 3) nucleotide-stimulated cPLA(2) phosphorylations were increased; 4) COX-1 and COX-2 expressions were increased; and 5) neither P2Y2 nor P2Y6 receptor expression differed, but P2Y4 receptor expression was decreased. Mesenteric arteries from GK rats treated with losartan exhibited (vs. untreated GK) 1) reduced nucleotide-induced contractions, 2) suppressed UTP-induced release of PGE(2) and PGF(2α), 3) suppressed UTP-stimulated cPLA(2) phosphorylation, 4) normalized expressions of COX-2 and P2Y4 receptors, and 5) reduced superoxide generation. Our data suggest that the diabetes-related enhancement of ATP-mediated vasoconstriction was due to P2Y receptor-mediated activation of the cPLA(2)/COX pathway and, moreover, that losartan normalizes such contractions by a suppressing action within this pathway.

Uridine adenosine tetraphosphate-induced contraction is increased in renal but not pulmonary arteries from DOCA-salt hypertensive rats

Uridine adenosine tetraphosphate (Up(4)A) was reported as a novel endothelium-derived contracting factor. Up(4)A contains both purine and pyrimidine moieties, which activate purinergic (P2)X and P2Y receptors. However, alterations in the vasoconstrictor responses to Up(4)A in hypertensive states remain unclear. The present study examined the effects of Up(4)A on contraction of isolated renal arteries (RA) and pulmonary arteries (PA) from DOCA-salt rats using isometric tension recording. RA from DOCA-salt rats exhibited increased contraction to Up(4)A versus arteries from control uninephrectomized rats in the absence and presence of N(G)-nitro-l-arginine (nitric oxide synthase inhibitor). On the other hand, the Up(4)A-induced contraction in PA was similar between the two groups. Up(4)A-induced contraction was inhibited by suramin (nonselective P2 antagonist) but not by diinosine pentaphosphate pentasodium salt hydrate (Ip(5)I; P2X(1) antagonist) in RA from both groups. Furthermore, 2-thiouridine 5'-triphosphate tetrasodium salt (2-ThioUTP; P2Y(2) agonist)-, uridine-5'-(γ-thio)-triphosphate trisodium salt (UTPγS; P2Y(2)/P2Y(4) agonist)-, and 5-iodouridine-5'-O-diphosphate trisodium salt (MRS 2693; P2Y(6) agonist)-induced contractions were all increased in RA from DOCA-salt rats. Protein expression of P2Y(2)-, P2Y(4)-, and P2Y(6) receptors in RA was similar between the two groups. In DOCA-salt RA, the enhanced Up(4)A-induced contraction was reduced by PD98059, an ERK pathway inhibitor, and Up(4)A-stimulated ERK activation was increased. These data are the first to indicate that Up(4)A-induced contraction is enhanced in RA from DOCA-salt rats. Enhanced P2Y receptor signaling and activation of the ERK pathway together represent a likely mechanism mediating the enhanced Up(4)A-induced contraction. Up(4)A might be of relevance in the pathophysiology of vascular tone regulation and renal dysfunction in arterial hypertension.

Gene profiling analysis reveals the contribution of CD24 and P2Y6R to the susceptibility of young rats to Plasmodium berghei infection

Our previous studies have shown that Plasmodium berghei infection induces distinct clinical, parasitological and immunological states in young susceptible rats versus adult resistant rats. This susceptibility was mainly found to be related to inadequate cellular responses. In this study we first identified the altered genes in young susceptible rats. Unexpectedly, transcriptome analysis did not reveal any alteration of effector cytokines or their receptors. At day 13 p.i., six transcripts corresponding to faim3, mesothelin, gas3 (PMP22), gas7, CD24 and P2Y6R were significantly decreased in young infected rats when compared with adult infected rats. Because CD24 and P2Y6R participate in cellular immune responses, we next evaluated their role in the course of infection. Adoptive transfer experiments showed a transient but robust participation of CD24+ cells in the control of parasitaemia. The role of P2Y6R was investigated via its specific ability to be activated by Uridine di-Phosphate (UDP). Young rats treated with UDP partially restored the expression of P2Y6R, controlled parasitaemia and survived thereafter. In conclusion, this study contributes to the discovery of novel biomarkers in young susceptible rats and suggests that the decrease in their expression could be among the reasons for the development of severe pathology in malaria.

Extracellular ATP signaling during differentiation of C2C12 skeletal muscle cells: role in proliferation

Evidence shows that extracellular ATP signals influence myogenesis, regeneration and physiology of skeletal muscle. Present work was aimed at characterizing the extracellular ATP signaling system of skeletal muscle C2C12 cells during differentiation. We show that mechanical and electrical stimulation produces substantial release of ATP from differentiated myotubes, but not from proliferating myoblasts. Extracellular ATP-hydrolyzing activity is low in myoblasts and high in myotubes, consistent with the increased expression of extracellular enzymes during differentiation. Stimulation of cells with extracellular nucleotides produces substantial Ca(2+) transients, whose amplitude and shape changed during differentiation. Consistently, C2C12 cells express several P2X and P2Y receptors, whose level changes along with maturation stages. Supplementation with either ATP or UTP stimulates proliferation of C2C12 myoblasts, whereas excessive doses were cytotoxic. The data indicate that skeletal muscle development is accompanied by major functional changes in extracellular ATP signaling.

Pyrimidine ribonucleotides with enhanced selectivity as P2Y(6) receptor agonists: novel 4-alkyloxyimino, (S)-methanocarba, and 5'-triphosphate gamma-ester modifications

The P2Y(6) receptor is a cytoprotective G-protein-coupled receptor (GPCR) activated by UDP (EC(50) = 0.30 microM). We compared and combined modifications to enhance P2Y(6) receptor agonist selectivity, including ribose ring constraint, 5-iodo and 4-alkyloxyimino modifications, and phosphate modifications such as alpha,beta-methylene and extension of the terminal phosphate group into gamma-esters of UTP analogues. The conformationally constrained (S)-methanocarba-UDP is a full agonist (EC(50) = 0.042 microM). 4-Methoxyimino modification of pyrimidine enhanced P2Y(6), preserved P2Y(2) and P2Y(4), and abolished P2Y(14) receptor potency, in the appropriate nucleotide. N(4)-Benzyloxy-CDP (15, MRS2964) and N(4)-methoxy-Cp(3)U (23, MRS2957) were potent, selective P2Y(6) receptor agonists (EC(50) of 0.026 and 0.012 microM, respectively). A hydrophobic binding region near the nucleobase was explored with receptor modeling and docking. UTP-gamma-aryl and cycloalkyl phosphoesters displayed only intermediate P2Y(6) receptor potency but had enhanced stability in acid and cell membranes. UTP-glucose was inactive, but its (S)-methanocarba analogue and N(4)-methoxycytidine 5'-triphospho-gamma-[1]glucose were active (EC(50) of 2.47 and 0.18 microM, respectively). Thus, the potency, selectivity, and stability of pyrimidine nucleotides as P2Y(6) receptor agonists may be enhanced by modest structural changes.

Activation of distinct P2Y receptor subtypes stimulates insulin secretion in MIN6 mouse pancreatic beta cells

Extracellular nucleotides and their receptor antagonists have therapeutic potential in disorders such as inflammation, brain disorders, and cardiovascular diseases. Pancreatic beta cells express several purinergic receptors, and reported nucleotide effects on insulin secretion are contradictory. We studied the effect of P2Y receptors on insulin secretion and cell death in MIN6, mouse pancreatic beta cells. Expression of P2Y(1) and P2Y(6) receptors was revealed by total mRNA analysis using RT-PCR. MIN6 cells were stimulated in the presence of 16.7 mM glucose with or without P2Y(1) and P2Y(6) agonists, 2-MeSADP and Up(3)U, respectively. Both the agonists increased insulin secretion with EC(50) values of 44.6+/-7.0 nM and 30.7+/-12.7 nM respectively. The insulin secretion by P2Y(1) and P2Y(6) agonists was blocked by their selective antagonists MRS2179 and MRS2578, respectively. Binding of the selective P2Y(1) receptor antagonist radioligand [125I]MRS2500 in MIN6 cell membranes was saturable (K(D) 4.74+/-0.47 nM), and known P2Y(1) ligands competed with high affinities. Inflammation and glucose toxicity lead to pancreatic beta cell death in diabetes. Flow cytometric analysis revealed that Up(3)U but not 2-MeSADP protected MIN6 cells against TNF-alpha induced apoptosis. Overall, the results demonstrate that selective stimulation of P2Y(1) and P2Y(6) receptors increases insulin secretion that accompanies intracellular calcium release, suggesting potential application of P2Y receptor ligands in the treatment of diabetes.