Purinergic stimulation of cell division and differentiation: mechanisms and pharmacological implications

Polydeoxyribonucleotide and Shock Wave Therapy Sequence Efficacy in Regenerating Immobilized Rabbit Calf Muscles

This study primarily aimed to investigate the combined effects of polydeoxyribonucleotide (PDRN) and extracorporeal shock wave therapy (ESWT) sequences on the regenerative processes in atrophied animal muscles. Thirty male New Zealand rabbits, aged 12 weeks, were divided into five groups: normal saline (Group 1), PDRN (Group 2), ESWT (Group 3), PDRN injection before ESWT (Group 4), and PDRN injection after ESWT (Group 5). After 2 weeks of cast immobilization, the respective treatments were administered to the atrophied calf muscles. Radial ESWT was performed twice weekly. Calf circumference, tibial nerve compound muscle action potential (CMAP), and gastrocnemius (GCM) muscle thickness after 2 weeks of treatment were evaluated. Histological and immunohistochemical staining, as well as Western blot analysis, were conducted 2 weeks post-treatment. Staining intensity and extent were assessed using semi-quantitative scores. Groups 4 and 5 demonstrated significantly greater calf muscle circumference, GCM muscle thickness, tibial nerve CMAP, and GCM muscle fiber cross-sectional area (type I, type II, and total) than the remaining three groups (p < 0.05), while they did not differ significantly in these parameters. Groups 2 and 3 showed higher values for all the mentioned parameters than Group 1 (p < 0.05). Group 4 had the greatest ratio of vascular endothelial growth factor (VEGF) to platelet endothelial cell adhesion molecule-1 (PECAM-1) in the GCM muscle fibers compared to the other four groups (p < 0.05). Western blot analysis revealed significantly higher expression of angiogenesis cytokines in Groups 4 and 5 than in the other groups (p < 0.05). The combination of ESWT and PDRN injection demonstrated superior regenerative efficacy for atrophied calf muscle tissue in rabbit models compared to these techniques alone or saline. In particular, administering ESWT after PDRN injection yielded the most favorable outcomes in specific parameters.

Regeneration of Chronic Rotator Cuff Tear in a Rabbit Model: Synergetic Benefits of Human Umbilical Cord Blood-Derived Mesenchymal Stem Cells, Polydeoxyribonucleotides, and Microcurrent Therapy

Objective

To investigate synergic therapeutic effects of combined injection of intralesional mesenchymal stem cells derived from human umbilical cord blood (UCB-MSCs) and polydeoxyribonucleotide (PDRN) combined with microcurrent therapy (MIC) on full thickness rotator cuff tendon tear (FTRCTT) in rabbit models.

Methods

Thirty-two rabbit models were assigned to 4 different groups. FTRCTT in the supraspinatus tendon was created. After 6 weeks, 4 types of procedures (0.2 mL normal saline injection, group 1 (G1-NS); 0.2 mL SC injection, group 2 (G2-MSC); 0.2 mL SC and weekly four injections of 0.2 mL PDRN with sham MIC, group 3 (G3-MSC+PDRN+sham MIC); and 0.2 mL SC and weekly four injections of 0.2 mL PDRN with MIC for four weeks, group 4 (G4-MSC+PDRN+MIC)) were performed in FTRCTT. Gross morphologic and histological changes of proliferating cell nuclear antigen (PCNA), vascular endothelial growth factor (VEGF) and platelet endothelial cell adhesion molecule (PECAM-1) and motion analysis were performed.

Results

There was a significant difference in gross morphologic changes between baseline and week 4 posttreatment in group 4 compared to the other three groups (p = 0.01). In groups 3 and 4, all parameters of histochemical and motion analysis have been found to be significantly greater than the ones in groups 1 and 2 (p < 0.05). In group 4, PCNA-, VEGF-, and PECAM-1-stained cells, as well as walking distance, were significantly greater than the ones in group 3 (p < 0.05).

Conclusion

The treatment with UCB-MSCs and PDRN combined with MIC might be the most effective in rabbit models' traumatic FRTCTT.

Synergic regenerative effects of polydeoxyribonucleotide and microcurrent on full-thickness rotator cuff healing in a rabbit model

BACKGROUND

Rotator cuff tendon tears (RCTTs) are common adult injuries. We hypothesized that a local injection of polydeoxyribonucleotide (PDRN) and microcurrent therapy (MIC) would be more effective in regenerating a tendon tear than PDRN administration alone.

OBJECTIVES

To evaluate the effect of PDRN combined with MIC on the regeneration of RCTTs in a rabbit subscapularis tendon chronic RCTT model.

METHODS

Rabbits (n=24) were allocated to 3 groups at 6 weeks after full-thickness RCTT (FTRCTT): 0.2mL normal saline (G1-SAL); 0.2mL PDRN with Sham MIC (G2-PDRN+Sham MIC); and 0.2mL PDRN with MIC (G3-PDRN+MIC). All treatments were performed under ultrasound guidance. PDRN was injected weekly for 4 weeks and sham MIC or MIC was applied daily for 4 weeks after the first PDRN injection.

RESULTS

In the G3-PDRN+MIC group, the mean (SD) subscapularis tendon tear size was continuously reduced from 1 week post-treatment to 4 weeks and was significantly decreased as compared with the other 2 groups [6.0 (1.5) vs. G1: 11.5 (1.8) and G2: 9.1 (1.6) mm²; G3 vs. G1, P<0.001; G3 vs. G2, P=0.018]. The gross morphologic mean tendon tear size was significantly smaller in the G3-PDRN+MIC group than G1-SAL and G2-PDRN+ Sham MIC groups [8.8 (3.5) vs. 15.9 (2.3) and 12.4 (1.6) mm²; G3 vs. G1, P<0.001; G3 vs. G2, P=0.03]. Mean values for regenerated collagen type 1 fibers, angiogenesis, and walking parameters were all greater for the G3-PDRN+MIC group than the other 2 groups based on histological examination and motion analysis [collagen type 1, G3: 1.60 (0.80) vs. G1: 0.45 (0.60), G2: 1.10 (0.74), G3 vs. G1, P<0.001; G3 vs. G2, P=0.002] [angiogenesis, G3: 2.44 (0.73) vs. G1: 0.80 (0.82) and G2: 2.06 (0.81), G3 vs. G1, P<0.001; G3 vs. G2, P=0.006] [walking distance, G3: 6391.4 (196.9) vs. G1: 4852.8 (137.3) and G2: 5514.4 (257.3) cm; G3 vs. G1, P<0.001; G3 vs. G2, P<0.001].

CONCLUSIONS

On gross morphologic, histological, and motion analysis, combined PDRN with MIC therapy was more effective than PDRN alone treating a rabbit model of chronic traumatic FTRCTT.

Therapeutic Effects of Umbilical Cord Blood-Derived Mesenchymal Stem Cells Combined with Polydeoxyribonucleotides on Full-Thickness Rotator Cuff Tendon Tear in a Rabbit Model

While therapies using mesenchymal stem cells (MSCs) to treat rotator cuff tendon tear (RCTT) have yielded some promising preliminary results, MSCs therapy has not yet completely regenerated full-thickness RCTT (FTRCTT). It has recently been reported that polydeoxyribonucleotide (PDRN) is effective in the treatment of chronic rotator cuff disease. We hypothesized that local injection of human umbilical cord blood-derived (UCB)–MSCs with PDRN would be more effective in regenerating tendon tear than UCB-MSCs alone. The purpose of this study was to evaluate the effects of UCB–MSCs combined with different doses of PDRN on the regeneration of RCTT in a chronic RCTT model by using a rabbit model. New Zealand white rabbits (n = 24) with FTRCTT were allocated randomly into three groups (8 rabbits per group). Three different injectants (G1-S, 0.2 mL UCB-MSCs; G2-P1, 0.2 mL UCB-MSCs with one injection of 0.2 mL PDRN; G3-P4, 0.2 mL UCB-MSCs, and four injections of 0.2 mL PDRN per week) were injected into FTRCTT under US-guidance. After the rabbits were euthanized, we evaluated ross morphological and histological change. Motion analysis was also performed. There were significant differences in gross morphological changes between before, and at 4 weeks after injection, in all three groups, but no differences were found among the three groups. Masson’s trichrome (MT) or anti-type 1 collagen antibody (COL-1)-positive cell densities in G2-P1 and G3-P4 were improved significantly compared with those in G1-S, but showed no significant difference between G2-P1 and G3-P4. On motion analysis, walking distance and fast walking time in G2-P1 and G3-P4 were significantly longer/higher than those in G1-S, but showed no significant differences between G2-P1 and G3-P4. These results demonstrated that there was no significant difference in the gross morphologic change of tendon tear between UCB-MSCs only and combination with PDRN injection in rabbit model of chronic traumatic FTRCTT. Furthermore, there were no significant differences in the regenerative effects between high and low doses of (0.8 and 0.2) mL of PDRN.

Antiproliferation effect of guanosine on HCT 116 cells involves MAPK and AMPK pathways

This study aims to investigate the mechanisms associated with the antiproliferation effect of guanosine on human colon carcinoma HCT 116 cells. In this study, guanosine induced more drastic cell cycle arrest effect than cell death effect on HCT 116 cells. The cell cycle arrest effect of guanosine on HCT 116 cells appeared to be associated with the increased activation of mitogen-activated protein kinases (MAPK) such as ERK1/2, p38 and JNK. The decrease of AMP-activated protein kinase (AMPK) activation and cyclin D1 expression was also involved. Thus, the antiproliferation of colon cancer cells of guanosine could be mediated by the disruption of MAPK and AMPK pathways.

ATP mediates neuroprotective and neuroproliferative effects in mouse olfactory epithelium following exposure to satratoxin G in vitro and in vivo

Intranasal aspiration of satratoxin G (SG), a mycotoxin produced by the black mold Stachybotrys chartarum, selectively induces apoptosis in olfactory sensory neurons (OSNs) in mouse olfactory epithelium (OE) through unknown mechanisms. Here, we show a dose-dependent induction of apoptosis 24 h post-SG exposure in vitro as measured by increased activated caspases in the OP6 olfactory placodal cell line and increased propidium iodide staining in primary OE cell cultures. Intranasal aspiration of SG increased TUNEL (Terminal dUTP Nick End Labeling) staining in the neuronal layer of the OE and significantly increased the latency to find a buried food pellet, confirming that SG selectively induces neuronal apoptosis and demonstrating that SG impairs the sense of smell. Next, we investigated whether ATP can prevent SG-induced OE toxicity. ATP did not decrease apoptosis under physiological conditions but significantly reduced SG-induced OSN apoptosis in vivo and in vitro. Furthermore, purinergic receptor inhibition significantly increased apoptosis in OE primary cell culture and in vivo. These data indicate that ATP is neuroprotective against SG-induced OE toxicity. The number of cells that incorporated 5'-bromodeoxyuridine, a measure of proliferation, was significantly increased 3 and 6 days post-SG aspiration. Treatment with purinergic receptor antagonists significantly reduced SG-induced cell proliferation, whereas post-treatment with ATP significantly potentiated SG-induced cell proliferation. These data indicate that ATP is released and promotes cell proliferation via activation of purinergic receptors in SG-induced OE injury. Thus, the purinergic system is a therapeutic target to alleviate or restore the loss of OSNs.

ATP differentially upregulates fibroblast growth factor 2 and transforming growth factor α in neonatal and adult mice: effect on neuroproliferation

Multiple neurotrophic factors play a role in proliferation, differentiation and survival in the olfactory epithelium (OE); however, the signaling cascade has not been fully elucidated. We tested the hypotheses that ATP induces the synthesis and secretion of two neurotrophic factors, fibroblast growth factor 2 (FGF2) and transforming growth factor alpha (TGFα), and that these neurotrophic factors have a role in inducing proliferation. Protein levels of FGF2 and TGFα were increased 20 h post-intranasal instillation of ATP compared to vehicle control in adult Swiss Webster mice. Pre-intranasal treatment with purinergic receptor antagonist pyridoxal-phosphate-6-azophenyl-20,40-disulfonic acid (PPADS) significantly blocked this ATP-induced increase, indicating that upregulation of FGF2 and TGFα expression is mediated by purinergic receptor activation. However, in neonatal mouse, intranasal instillation of ATP significantly increased the protein levels of FGF2, but not TGFα. Likewise, ATP evoked the secretion of FGF2, but not TGFα, from neonatal mouse olfactory epithelial slices and PPADS significantly blocked ATP-evoked FGF2 release. To determine the role of FGF2 and TGFα in inducing proliferation, 5-bromo-2-deoxyuridine (BrdU) incorporation was examined in adult olfactory epithelium. Intranasal treatment with FGF receptor inhibitor PD173074 or epidermal growth factor receptor inhibitor AG1478 following ATP instillation significantly blocked ATP-induced BrdU incorporation. Collectively, these data demonstrate that ATP induces proliferation in adult mouse olfactory epithelium by promoting FGF2 and TGFα synthesis and activation of their receptors. These data suggest that different mechanisms regulate neurogenesis in neonatal and adult OE, and FGF2 and TGFα may have different roles throughout development.

Butyl benzyl phthalate suppresses the ATP-induced cell proliferation in human osteosarcoma HOS cells

Butyl benzyl phthalate (BBP), an endocrine disruptor present in the environment, exerts its genomic effects via intracellular steroid receptors and elicits non-genomic effects by interfering with membrane ion-channel receptors. We previously found that BBP blocks the calcium signaling coupled with P2X receptors in PC12 cells (Liu & Chen, 2006). Osteoblast P2X receptors were recently reported to play a role in cell proliferation and bone remodeling. In this present study, the effects of BBP on ATP-induced responses were investigated in human osteosarcoma HOS cells. These receptors mRNA had been detected, named P2X4, P2X7, P2Y2, P2Y4, P2Y5, P2Y9, and P2Y11, in human osteosarcoma HOS cells by RT-PCR. The enhancement of cell proliferation and the decrease of cytoviability had both been shown to be coupled to stimulation via different concentrations of ATP. BBP suppressed the ATP-induced calcium influx (mainly coupled with P2X) and cell proliferation but not the ATP-induced intracellular calcium release (mainly coupled with P2Y) and cytotoxicity in human osteosarcoma HOS cells. Suramin, a common P2 receptor's antagonist, blocked the ATP-induced calcium signaling, cell proliferation, and cytotoxicity. We suggest that P2X is mainly responsible for cell proliferation, and P2Y might be partially responsible for the observed cytotoxicity. BBP suppressed the calcium signaling coupled with P2X, suppressing cell proliferation. Since the importance of P2X receptors during bone metastasis has recently become apparent, the possible toxic risk of environmental BBP during bone remodeling is a public problem of concern.

Activation of purinergic receptors induces proliferation and neuronal differentiation in Swiss Webster mouse olfactory epithelium

In the central nervous system (CNS), adenosine 5'-triphosphate (ATP) induces the synthesis and release of neurotrophic factors, cell proliferation, and differentiation. The olfactory system is one site where multipotent progenitor cells continue to proliferate and differentiate into neurons throughout life. We tested the hypothesis that ATP initiates proliferation in olfactory epithelium by measuring 5-bromo-2-deoxyuridine incorporation. Adult mice were pre-treated intraperitoneally (i.p.) or intranasally with saline or purinergic receptor antagonists (pyridoxal-phosphate-6-azophenyl-2',4'-disulfonate+suramin) 30 min prior to nasal instillation of ATP, uridine 5'-triphosphate (UTP), adenosine 5'-(3-thiotriphosphate) (ATP gamma S) or saline (0 h). Mice received three injections of 5-bromo-2-deoxyuridine between 42 and 46 h, and were sacrificed at 2, 9 or 16 days post-ATP instillation. ATP, UTP or ATP gamma S significantly increased 5-bromo-2-deoxyuridine incorporation compared to intranasal saline controls in groups pre-treated with saline. Saline, ATP, UTP or ATP gamma S instillation did not significantly increase 5-bromo-2-deoxyuridine incorporation in groups pre-treated with purinergic receptor antagonists. Similar results were observed in neonates and in a cultured slice preparation. Intranasal instillation of ATP also increased the protein levels of proliferating cell nuclear antigen in adults. Pre-treatment with purinergic receptor antagonists inhibited the ATP-induced increase in proliferating cell nuclear antigen. In adults, a subset of the cells that incorporated 5-bromo-2-deoxyuridine was immunoreactive to neuronal markers mammalian achaete-schute homolog 1, growth-associated protein 43, and olfactory marker protein at 2, 9, and 16 days, respectively. Collectively, these data indicate that purinergic receptor activation induces proliferation and neuronal differentiation in the mouse olfactory epithelium. We propose that extracellular ATP released upon injury could induce proliferation and promote the neuroregeneration of the olfactory epithelium.

Purinergic receptor activation evokes neurotrophic factor neuropeptide Y release from neonatal mouse olfactory epithelial slices

One premise regarding the mechanism of injury-evoked neuroregeneration is that injured cells induce the release of neurotrophic factors to trigger neurogenesis. Extracellular purine nucleotides exert multiple neurotrophic actions in the central nervous system mediated via activation of purinergic receptors. However, whether purinergics have a neurotrophic role in the olfactory neuroepithelium has not been investigated. Thus, we monitored the ATP-induced release of neuropeptide Y (NPY), a neuropeptide that increases neuroproliferation in the olfactory epithelium. To visualize NPY release, slices of olfactory epithelium from neonatal mice were cultured on nitrocellulose paper. Immunoassays of the nitrocellulose demonstrated NPY immunoreactivity in regions corresponding to the olfactory epithelium of the nasal cavity. One hour of exposure to exogenous ATP (100, 500 microM) significantly increased the number of olfactory epithelium slices that released NPY from 25% +/- 6% to 60% +/- 7% or 71% +/- 10% (P = 0.001). The purinergic receptor antagonists pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS; 25 microM) and suramin (100 microM) significantly reduced the number of olfactory epithelium slices exhibiting ATP-evoked NPY release to 18% +/- 11% (P = 0.004), indicating that NPY release is mediated by activation of purinergic receptors. Released NPY was quantified by enzyme and radioimmunoassays. Exogenous ATP or UTP significantly increased the amount of NPY released. Overall, this study demonstrates that purinergic receptor activation mediates the release of neurotrophic factor NPY in the olfactory epithelium and provides pharmacological targets to promote regeneration of damaged olfactory epithelium.

Extracellular guanosine and GTP promote expression of differentiation markers and induce S-phase cell-cycle arrest in human SH-SY5Y neuroblastoma cells

SH-SY5Y neuroblastoma cells, a model for studying neuronal differentiation, are able to differentiate into either cholinergic or dopaminergic/adrenergic phenotypes depending on media conditions. Using this system, we asked whether guanosine (Guo) or guanosine-5'-triphosphate (GTP) are able to drive differentiation towards one particular phenotype. Differentiation was determined by evaluating the frequency of cells bearing neurites and assessing neurite length after exposure to different concentrations of Guo or GTP for different durations. After 6 days, 0.3 mM Guo or GTP induced a significant increase in the number of cells bearing neurites and increased neurite length. Western blot analyses confirmed that purines induced differentiation; cells exposed to purines showed increases in the levels of GAP43, MAP2, and tyrosine hydroxylase. Proliferation assays and cytofluorimetric analyses indicated a significant anti-proliferative effect of purines, and a concentration-dependent accumulation of cells in S-phase, starting after 24 h of purine exposure and extending for up to 6 days. A transcriptional profile analysis using gene arrays showed that an up-regulation of cyclin E2/cdk2 evident after 24 h was responsible for S-phase entry, and a concurrent down-regulation of cell-cycle progression-promoting cyclin B1/B2 prevented S-phase exit. In addition, patch-clamp recordings revealed that 0.3 mM Guo or GTP, after 6 day incubation, significantly decreased Na(+) currents. In conclusion, we showed Guo- and GTP-induced cell-cycle arrest in neuroblastoma cells and suggest that this makes these cells more responsive to differentiation processes that favor the dopaminergic/adrenergic phenotype.

Neurotrophic effects of extracellular guanosine

Central nervous system (CNS) astrocytes release guanosine extracellularly, that exerts trophic effects. In CNS, extracellular guanosine (GUO) stimulates mitosis, synthesis of trophic factors, and cell differentiation, including neuritogenesis, is neuroprotective, and reduces apoptosis due to several stimuli. Specific receptor-like binding sites for eGUO in the nervous system may mediate its effects through both MAP kinase and PI3-kinase signalling pathways. Extracellular guanine (eGUA) also exerts several effects; the trophic effects of eGUO are likely regulated by conversion of eGUO to eGUA by a membrane located purine nucleoside phosphorylase (ecto-PNP) and by conversion of eGUA to xanthine by guanine deaminase.

The role of ATP and adenosine in the brain under normoxic and ischemic conditions

By taking advantage of some recently synthesized compounds that are able to block ecto-ATPase activity, we demonstrated that adenosine triphosphate (ATP) in the hippocampus exerts an inhibitory action independent of its degradation to adenosine. In addition, tonic activation of P2 receptors contributes to the normally recorded excitatory neurotransmission. The role of P2 receptors becomes critical during ischemia when extracellular ATP concentrations increase. Under such conditions, P2 antagonism is protective. Although ATP exerts a detrimental role under ischemia, it also exerts a trophic role in terms of cell division and differentiation. We recently reported that ATP is spontaneously released from human mesenchymal stem cells (hMSCs) in culture. Moreover, it decreases hMSC proliferation rate at early stages of culture. Increased hMSC differentiation could account for an ATP-induced decrease in cell proliferation. ATP as a homeostatic regulator might exert a different effect on cell trophism according to the rate of its efflux and receptor expression during the cell life cycle. During ischemia, adenosine formed by intracellular ATP escapes from cells through the equilibrative transporter. The protective role of adenosine A(1) receptors during ischemia is well accepted. However, the use of selective A(1) agonists is hampered by unwanted peripheral effects, thus attention has been focused on A(2A) and A(3) receptors. The protective effects of A(2A) antagonists in brain ischemia may be largely due to reduced glutamate outflow from neurones and glial cells. Reduced activation of p38 mitogen-activated protein kinases that are involved in neuronal death through transcriptional mechanisms may also contribute to protection by A(2A) antagonism. Evidence that A(3) receptor antagonism may be protective after ischemia is also reported.

Purinergic substances promote murine keratinocyte proliferation and enhance impaired wound healing in mice

As membrane-bound receptors for adenosine, purines, and pyrimidines, purinoceptors are expressed in nearly all cell types throughout the mammalian organism. Previous studies showed that purinoceptors are involved in the regulation of proliferation and differentiation of most target cells. The present study was performed to elucidate their role in keratinocyte proliferation and wound healing. The expression of the mRNA of several adenosine and P2Y receptors was shown in the immortalized murine keratinocyte cell line MSC-P5 and primary cultured keratinocytes of four different mouse strains. The nonselective adenosine receptor agonist 5'-(N-ethyl)-carboxamidoadenosine enhanced the growth of MSC-P5 cells in vitro via the A2B receptor. The proliferative stimulus of adenosine triphosphate and uridine triphosphate on this cell line was mediated by the P2Y2 receptor. The mitogenic effect of the purinergic substances was inhibited by simultaneous treatment with respective antagonists. Studies in a mouse model of dexamethasone-induced impaired wound healing showed the in vivo efficacy of the purinoceptor agonists. These studies confirm that pharmacological actions via purinoceptors offer an intriguing possibility in the treatment of impaired wound healing. Nevertheless, further investigations are needed to elucidate fully the role of purinergic mechanisms involved in wound healing.

Polydeoxyribonucleotide (PDRN) promotes human osteoblast proliferation: a new proposal for bone tissue repair

Several researchers have recently shed new light upon the importance of extracellular nucleotides and nucleosides to stimulate cells growth. PDRN, a mixture of deoxyribonucleotides polymers of different lengths, has recently demonstrated to stimulate "in vitro" fibroblast proliferation and collagen production, probably stimulating the purinergic receptor system. In this work we evaluated the effects of PDRN on human cultured osteoblasts, focusing our attention on cell proliferation and alkaline phosphatase activity. PDRN at a concentration of 100 microg/ml induce an increase in osteoblasts growth after 6 days as compared to control (+21%). The addition of DMPX 50 microM and suramine (P2 inhibitor) 10 microM give different results: suramine has no significant effect, while DPMX reduce, even if partially, the PDRN induced cell growth. The alkaline phosphatase activity shows a gradual enhancement starting from day 0 to day 10, even if PDRN treated cells, examined at day 6, present a sensibly lower phosphatase activity when compared to controls. Our data demonstrate that PDRN acts as an osteoblast growth stimulator. Its action is partially due to a stimulation of the purinergic system mediated by A2 purinoreceptors, however we can not exclude the involvement of other mechanism like salvage pathway.

Adenosine-regulated cell proliferation in pituitary folliculostellate and endocrine cells: differential roles for the A(1) and A(2B) adenosine receptors

A(1) and A(2) adenosine receptors have been identified in the pituitary gland, but the cell type(s) on which they are located and their effects on pituitary cell growth are not known. Therefore, we analyzed the expression of A(1) and A(2) receptors in primary rat anterior pituitary cells, two pituitary folliculostellate (TtT/GF and Tpit/F1) and two pituitary endocrine (GH(3) and AtT20) cell lines, and compared their effects on cell proliferation. In anterior pituitary and folliculostellate cells, adenosine and adenosine receptor agonists (5'-N-ethylcarboxamidoadenosine, a universal agonist, and CGS 21680, an A(2A) receptor agonist) stimulated cAMP levels with a rank order of potency that indicates the presence of functional A(2B) receptors. This stimulation, however, was not observed in either GH(3) or AtT20 cells, where adenosine and the A(1) receptor agonist 2-chloro-N(6)-cyclopentyladenosine inhibited VIP/forskolin-stimulated cAMP production. Expression of A(2B) and A(1) receptors in the folliculostellate cells and that of the A(1) receptor in the endocrine cells were confirmed by RT-PCR, immunocytochemistry, and ligand binding. Adenosine and 5'-N-ethylcarboxamidoadenosine dose-dependently (10 nM to 10 microM) stimulated growth in the folliculostellate, but not in the endocrine, cells, whereas in the latter, 100 microM adenosine and 2-chloro-N(6)-cyclopentyladenosine inhibited cell proliferation by slowing cell cycle progression. These data highlight the differential expression of A(1) and A(2B) adenosine receptors in pituitary cells and provide evidence for opposing effects of adenosine on pituitary folliculostellate and endocrine cell growth.

Purinergic signaling and vascular cell proliferation and death

Evidence for the role of purinergic signaling (via P1 and P2Y receptors) in the proliferation of vascular smooth muscle and endothelial cells is reviewed. The involvement of the mitogen-activated protein kinase second-messenger cascade in this action is clearly implicated, although details of the precise intracellular pathways involved still remain to be determined. Synergistic actions of purines and pyrimidines with growth factors occur in promoting cell proliferation. Interaction between purinergic signaling for vascular cell proliferation and cell death mediated by P2X7 receptors is discussed. There is evidence of the release of ATP from endothelial cells, platelets, and sympathetic nerves as well as from damaged cells in atherosclerosis, hypertension, restenosis, and ischemia; furthermore, there is evidence that vascular smooth muscle and endothelial cells proliferate in these pathological conditions. Thus, the involvement of ATP and its breakdown product, adenosine, is implicated; it is hoped that with the development of selective P1 (A2) and P2Y receptor agonists and antagonists, new therapeutic strategies will be explored.

Involvement of astrocytes in purine‐mediated reparative processes in the brain

Astrocytes are involved in multiple brain functions in physiological conditions, participating in neuronal development, synaptic activity and homeostatic control of the extracellular environment. They also actively participate in the processes triggered by brain injuries, aimed at limiting and repairing brain damages. Purines may play a significant role in the pathophysiology of numerous acute and chronic disorders of the central nervous system (CNS). Astrocytes are the main source of cerebral purines. They release either adenine-based purines, e.g. adenosine and adenosine triphosphate, or guanine-based purines, e.g. guanosine and guanosine triphosphate, in physiological conditions and release even more of these purines in pathological conditions. Astrocytes express several receptor subtypes of P1 and P2 types for adenine-based purines. Receptors for guanine-based purines are being characterised. Specific ecto-enzymes such as nucleotidases, adenosine deaminase and, likely, purine nucleoside phosphorylase, metabolise both adenine- and guanine-based purines after release from astrocytes. This regulates the effects of nucleotides and nucleosides by reducing their interaction with specific membrane binding sites. Adenine-based nucleotides stimulate astrocyte proliferation by a P2-mediated increase in intracellular [Ca2+] and isoprenylated proteins. Adenosine also, via A2 receptors, may stimulate astrocyte proliferation, but mostly, via A1 and/or A3 receptors, inhibits astrocyte proliferation, thus controlling the excessive reactive astrogliosis triggered by P2 receptors. The activation of A1 receptors also stimulates astrocytes to produce trophic factors, such as nerve growth factor, S100beta protein and transforming growth factor beta, which contribute to protect neurons against injuries. Guanosine stimulates the output of adenine-based purines from astrocytes and in addition it directly triggers these cells to proliferate and to produce large amount of neuroprotective factors. These data indicate that adenine- and guanine-based purines released in large amounts from injured or dying cells of CNS may act as signals to initiate brain repair mechanisms widely involving astrocytes.

Adenosine- and adenine-nucleotide-mediated inhibition of normal and transformed keratinocyte proliferation is dependent upon dipyridamole-sensitive adenosine transport

Extracellular adenosine and its related nucleotides have been referred to as retaliatory metabolites that can be released into the extracellular environment during inflammation, wounding, and other pathologic states. We have previously reported that these compounds reversibly inhibit the proliferation of normal keratinocyte cultures and we now demonstrate that these compounds also arrest the proliferation of transformed keratinocytes. Although our study shows that keratinocytes express mRNA corresponding to the A2B purinoreceptors and that adenosine or AMP treatment elevates intracellular cAMP in these cells, our study also demonstrates that dipyridamole-inhibitable transport of adenosine into the keratinocyte is central to the mechanism by which adenosine and adenine nucleotides arrest proliferation in these cells. In support of this mechanism, our results demonstrate that human keratinocytes express mRNA corresponding to the recently cloned dipyridamole-sensitive human equilibrative nucleoside transporter. Interestingly, coincubation with adenosine deaminase reverses the antiproliferative action of adenosine and exerts no effect on the antiproliferative activity of the adenine nucleotides, thus supporting a model in which adenine nucleotides are enzymatically converted to adenosine and transported into the keratinocyte in a tightly coupled and adenosine-deaminase-resistant manner. Analysis of adenosine- and adenosine-monophosphate-treated keratinocytes demonstrated that quiescence is induced within 12-24 h, and fluorescence-activated cell sorter analysis suggests that treatment with these compounds may result in the inhibition of keratinocyte proliferation at both G1 and S phases of the cell cycle. In addition to their documented antiproliferative action on other cell types, adenosine, adenine nucleotides, and related analogs may also represent a potential new class of pharmacologic regulators of keratinocyte proliferation in vivo.

Agonist-induced internalization and recycling of the human A(3) adenosine receptors: role in receptor desensitization and resensitization

A(3) adenosine receptors have been proposed to play an important role in the pathophysiology of cerebral ischemia with a regimen-dependent nature of the therapeutic effects probably related to receptor desensitization and down-regulation. Here we studied the agonist-induced internalization of human A(3) adenosine receptors in transfected Chinese hamster ovary cells, and then we evaluated the relationship between internalization and signal desensitization and resensitization. Binding of N(6)-(4-amino-3-[(125)I]iodobenzyl)adenosine-5'-N-methyluronamide to membranes from Chinese hamster ovary cells stably transfected with the human A(3) adenosine receptor showed a profile typical of these receptors in other cell lines (K:(D) = 1.3+/-0.08 nM; B(max) = 400+/-28 fmol/mg of proteins). The iodinated agonist, bound at 4 degrees C to whole transfected cells, was internalized by increasing the temperature to 37 degrees C with a rate constant of 0.04+/-0.034 min(-1). Agonist-induced internalization of A(3) adenosine receptors was directly demonstrated by immunogold electron microscopy, which revealed the localization of these receptors in plasma membranes and intracellular vesicles. Moreover, short-term exposure of these cells to the agonist caused rapid desensitization as tested in adenylyl cyclase assays. Subsequent removal of the agonist led to restoration of the receptor function and recycling of the receptors to the cell surface. The rate constant of receptor recycling was 0.02+/-0.0017 min(-1). Blockade of internalization and recycling demonstrated that internalization did not affect signal desensitization, whereas recycling of internalized receptors was implicated in the signal resensitization.

Vasodilator responses to ATP and UTP are not dependent on nitric oxide release, K+ATP channel activation, or the release of vasodilator prostaglandins in the hindlimb vascular bed of the cat

The effects of the purinergic agonists, ATP, ATPγS, UTP, and 2-Met-Thio AP, were investigated in the hindlimb vascular bed of the cat. Under constant-flow conditions, injections of the purinergic agonists into the perfusion circuit elicited dose-related decreases in perfusion pressure. The order of potency was 2-Met-Thio ATP > ATPγS > ATP > UTP. In contrast, injections of GTPγS, cAMP, UDP, and UMP had no effect. Vasodilator responses to ATP, ATPγS, UTP, and 2-Met-Thio ATP were increased in duration by the cAMP phosphodiesterase inhibitor rolipram, whereas the cGMP phosphodiesterase inhibitor zaprinast had no effect. Responses to the purinergic agonists were not altered by nitric oxide synthase inhibitors, K+ATP channel antagonists, cyclooxygenase inhibitors, or agents that interfere with the actions of the adrenergic nervous system. These data suggest that ATP, ATPγS, UTP, and 2-Met-Thio ATP dilate the hindlimb vascular bed by a direct cAMP-dependent mechanism, and that the release of nitric oxide, vasodilator prostaglandins, K+ATP channel opening, or an inhibitory effect on the adrenergic nervous system play little, if any, role in mediating or modulating responses to the purinergic agonists in the hindlimb circulation of the cat.Key words: purinergic agonists, P2 purinergic receptors, cAMP-dependent vasodilator activity, adrenergic system, nitric oxide prostaglandins.

Extracellular guanosine 5' triphosphate enhances nerve growth factor-induced neurite outgrowth via increases in intracellular calcium

Extracellular guanosine 5' triphosphate (GTP) enhances nerve growth factor-dependent neurite outgrowth from rat pheochromocytoma (PC12) cells; cultures of PC12 cells exposed to GTP and nerve growth factor together contain significantly more neurite-bearing cells than do those exposed to either nerve growth factor or GTP alone [Gysbers J. W. and Rathbone M. P. (1996) Int. J. devl Neurosci. 14, 19-34]. PC12 cells contain specific cell surface binding sites for extracellular GTP, which do not bind ATP or uridine 5' triphosphate. Exposure of PC12 cells to extracellular GTP (300microM) produced a robust and sustained increase in intracellular Ca(2+) ([Ca(2+)](i)), different from the transient response to the addition of ATP. The GTP-induced [Ca(2+)](i) increase was blocked by the L-type calcium channel inhibitor, nifedipine. The L-type Ca(2+) channel inhibitors, nifedipine or verapamil, also inhibited the enhancement of neurite outgrowth by GTP, but did not affect neurite outgrowth stimulated by nerve growth factor alone. Pre-treatment of PC12 cells with ryanodine (0.5-50microM) depleted calcium from internal stores and prevented the further release of calcium by GTP. Similarly, pre-treatment of PC12 cells with thapsigargin (an inhibitor of internal store Ca(2+)/ATPase) or dantrolene (which blocks Ca(2+) release from some of these stores) also reduced the enhancement of neurite outgrowth by GTP. Therefore, Ca(2+)-induced Ca(2+) release from specific stores, present in PC12 cells, is involved in the enhancement of nerve growth factor-induced neurite outgrowth by GTP, possibly acting at specific binding sites on the cell surface. GTP is proving to be an important extracellular trophic modulator in the central nervous system. These studies show that the neuritogenic actions of GTP involve moderate but sustained increases in intracellular Ca(2+) which are likely due to activation of L-type Ca(2+) channels and Ca(2+)-induced Ca(2+) release from intracellular stores. These effects of extracellular GTP are likely mediated at the cell surface and may be related to specific GTP binding sites which are distinct from G-proteins and from hitherto described purine nucleotide (P2) receptors. These data indicate a mechanism whereby the neuritogenic effects of GTP are mediated and emphasize the importance of considering GTP as a neurotrophic mediator.

Trophic effects of purines in neurons and glial cells

In addition to their well known roles within cells, purine nucleotides such as adenosine 5' triphosphate (ATP) and guanosine 5' triphosphate (GTP), nucleosides such as adenosine and guanosine and bases, such as adenine and guanine and their metabolic products xanthine and hypoxanthine are released into the extracellular space where they act as intercellular signaling molecules. In the nervous system they mediate both immediate effects, such as neurotransmission, and trophic effects which induce changes in cell metabolism, structure and function and therefore have a longer time course. Some trophic effects of purines are mediated via purinergic cell surface receptors, whereas others require uptake of purines by the target cells. Purine nucleosides and nucleotides, especially guanosine, ATP and GTP stimulate incorporation of [3H]thymidine into DNA of astrocytes and microglia and concomitant mitosis in vitro. High concentrations of adenosine also induce apoptosis, through both activation of cell-surface A3 receptors and through a mechanism requiring uptake into the cells. Extracellular purines also stimulate the synthesis and release of protein trophic factors by astrocytes, including bFGF (basic fibroblast growth factor), nerve growth factor (NGF), neurotrophin-3, ciliary neurotrophic factor and S-100beta protein. In vivo infusion into brain of adenosine analogs stimulates reactive gliosis. Purine nucleosides and nucleotides also stimulate the differentiation and process outgrowth from various neurons including primary cultures of hippocampal neurons and pheochromocytoma cells. A tonic release of ATP from neurons, its hydrolysis by ecto-nucleotidases and subsequent re-uptake by axons appears crucial for normal axonal growth. Guanosine and GTP, through apparently different mechanisms, are also potent stimulators of axonal growth in vitro. In vivo the extracellular concentration of purines depends on a balance between the release of purines from cells and their re-uptake and extracellular metabolism. Purine nucleosides and nucleotides are released from neurons by exocytosis and from both neurons and glia by non-exocytotic mechanisms. Nucleosides are principally released through the equilibratory nucleoside transmembrane transporters whereas nucleotides may be transported through the ATP binding cassette family of proteins, including the multidrug resistance protein. The extracellular purine nucleotides are rapidly metabolized by ectonucleotidases. Adenosine is deaminated by adenosine deaminase (ADA) and guanosine is converted to guanine and deaminated by guanase. Nucleosides are also removed from the extracellular space into neurons and glia by transporter systems. Large quantities of purines, particularly guanosine and, to a lesser extent adenosine, are released extracellularly following ischemia or trauma. Thus purines are likely to exert trophic effects in vivo following trauma. The extracellular purine nucleotide GTP enhances the tonic release of adenine nucleotides, whereas the nucleoside guanosine stimulates tonic release of adenosine and its metabolic products. The trophic effects of guanosine and GTP may depend on this process. Guanosine is likely to be an important trophic effector in vivo because high concentrations remain extracellularly for up to a week after focal brain injury. Purine derivatives are now in clinical trials in humans as memory-enhancing agents in Alzheimer's disease. Two of these, propentofylline and AIT-082, are trophic effectors in animals, increasing production of neurotrophic factors in brain and spinal cord. Likely more clinical uses for purine derivatives will be found; purines interact at the level of signal-transduction pathways with other transmitters, for example, glutamate. They can beneficially modify the actions of these other transmitters.

Antisense oligodeoxynucleotides targeted to MAG mRNA profoundly alter BP and PLP mRNA expression in differentiating oligodendrocytes: a caution

The applicability of antisense technology to suppress the expression of myelin associated glycoprotein (MAG) in cultured oligodendrocytes was evaluated. Differentiating oligodendrocyte precursor cells obtained by the shake-off method were exposed to nine unmodified antisense oligodeoxynucleotides (ODNs) targeted to the first seven exons of MAG mRNA. After four days, steady-state levels of MAG, proteolipid protein (PLP) and basic protein (BP) mRNAs were determined by Northern blot analysis. Only ODN annealing to 599-618 nt of the MAG mRNA (the junction of exon 5 and 6) resulted in a significant, 75% decrease in the MAG mRNA level. Unexpectedly, six other anti-MAG ODNs which had no significant effect on the MAG message, greatly increased the level of BP mRNA. The highest upregulation of approximately 12 fold was observed with ODN annealing to 139-168 nt (junction of exon 3 and 4). On the other hand, the 997-1016 ODN decreased the levels of BP and PLP messages by 70-80%. The 599-618 ODN also decreased the PLP mRNA by 85%. The results demonstrate that antisense ODNs targeted to one gene may profoundly alter the expression of other genes, and hence, complicate functional analysis of the targeted protein.

Effect of polydeoxyribonucleotides on human fibroblasts in primary culture

The effects of a mixture of oligo- and polydeoxyribonucleotides (PDRN) on the growth and protein secretion of cultured human skin fibroblasts were investigated. Both intact and DNAase-digested PDRN stimulated cell proliferation to a similar extent. When cultured fibroblasts were incubated with radioactive amino acids in the presence of intact or digested PDRN the incorporation of the tracer into secreted proteins increased significantly. This stimulation appears to be specific for certain protein components, including fibronectin. These results are interpreted assuming that PDRN and the nucleotides and nucleosides resulting from its degradation, can act as signal transducers or, alternatively, can be internalized and utilized to provide purine and pyrimidine rings for the salvage pathways.

Polydeoxyribonucleotides enhance the proliferation of human skin fibroblasts: involvement of A2 purinergic receptor subtypes

It is well-known that nucleotides, nucleosides and purine/pyrimidine bases enhance cell proliferation in vitro. Nevertheless, the molecular mechanisms involved in this mitogenic activity is still controversial, since these compounds are reported both to synergize with growth factor, and to act directly on purinergic receptor inducing per se a proliferative response. It was suggested that cell growth enhancement could be mediated by the A2 purinergic receptor activation. Here we report that a polydeoxyribonucleotide (PDRN) and adenosine are able to increase, the growth rate of human skin fibroblasts in primary cultures. The proliferative activity exerted by PDRN was significantly counteracted by the A2 antagonist 3, 7-Dimethyl-1-propargylxanthine (DMPX), but not by the A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (PD 116,948, DPCPX). Accordingly, the trophic action of PDRN was mimicked by the A2 agonist N6-[2-(3,5-Dimethoxyphenyl)-2-(methylphenyl)-ethyl]adenosine (DPMA), while the A1 agonist N6-Cyclopenthyladenosine (CPA) did not show any effect. In microfluorimetric studies, we observed that PDRN and adenosine increased the concentration of cytosolic calcium ions. The PDRN-evoked calcium rise was dose-dependent and DMPX sensitive. Taken together, our results suggest that PDRN may operate as a pro-drug providing the cultured cells with an effective amount of mitogenic deoxyribonucleotides, deoxyribonucleosides and bases; moreover, cell proliferation enhancement that has been induced by PDRN seems to be mediated, at least in part, by the activation of purinergic receptors of the A2 subtype.

Antiproliferative, differentiating and apoptotic effects elicited by imidazo[1,2-a]pyrazine derivatives

1. The activity of two series of imidazo[1,2-a]pyrazine derivatives on cell proliferation and differentiation and on apoptosis was examined in relation to their effects on phosphodiesterase (PDE) activity and on purinoceptors. 2. In the first series SC-18 and SC-51 inhibited mitogen-induced 3H-thymidine incorporation in human lymphocytes. 3. The compounds of the new series PAB13, PAB23 and SCA40 inhibited the proliferation of the HEL cell line. 4. Nine imidazo[1,2-a]pyrazine derivatives of the new series have been studied on the Dami cell proliferation. SCA41 and SCA44 inhibited cell growth, SCA40 and PAB40 were moderately effective, whereas PAB12 and PAB30 were devoid of effect. The antiproliferative effects of these six non-cytotoxic compounds could not be related to their action on PDE or on purinoceptors, but rather to their lipophilicity. Conversely, for PAB13, PAB15, and PAB23, the decrease in cell number was related to their cytotoxic and apoptotic effects through their cAMP-increasing and PDE-inhibitory potency, but unrelated to an effect on purinoceptors. 5. Imidazo[1,2-a]pyrazine derivatives decreased the expression of Glycoprotein (GP)Ib in Dami cells while some of them enhanced that of GPIIb/IIIa. These effects appeared to involve inhibition of both cAMP- and cGMP-PDE. 6. These studies demonstrate the potential interest of imidazo[1,2-a]pyrazine derivatives in the query of novel anticancer drugs.

Cell proliferation induced by 8-oxoguanosine and 8-methylguanosine, two adducts produced by free radical attack on ribonucleosides and RNA

The ability of C8-substituted guanine (Gua) ribonucleosides to induce B cell proliferation has been well documented in the literature. These compounds are analogues of adducts formed from free radical attack on ribonucleosides and RNA. Here we examined the proliferative properties of two of these radical adducts, 8-methylguanosine (8-MeG) and 8-oxo-7,8-dihydroguanosine (8-OxoG) and compared them with those of the well studied B cell mitogen, 8-bromoguanosine (8-BrG). 8-MeG and 8-OxoG were synthesized in the considerable yields of 28, and 55%, respectively, and were characterized by UV, NMR and CG-MS. Their effects upon [3H]thymidine uptake into DNA by Swiss mouse splenocytes, mouse embryo 3T3 fibroblasts (A31) and mouse B16F10 melanoma were examined. Both guanosine (G) radical adducts were shown to increase [3H]thymidine uptake by mouse splenocytes but displayed selectivity in respect to continuous cell lines. 8-MeG acted upon 3T3 fibroblasts whereas 8-OxoG acted upon B16F10 melanoma. The non-physiological analogue 8-BrG acted upon all tested cells. Parallel experiments of cell counting, cytotoxicity,and cell sorting indicated that DNA synthesis induced by the C8-substituted G's reflected cell growth. It is proposed that the compounds act intracellularly because their proliferative effects were blocked in the presence of a nucleoside transport inhibitor but were not inhibited by an antagonist of the A2 purine receptor. The present results, taken together with data from the literature, suggest that in the case of 3T3 fibroblasts and mouse splenocytes the proliferative effects of the compounds are not dependent on metabolism through purine salvage pathways. In the case of melanoma, however, the compounds are likely to become part of the purine nucleoside pool. The demonstration that adducts produced by free radical attack on ribonucleosides and RNA are able to induce cell proliferation opens new perspectives for the understanding of free radical mediated carcinogenesis.

Expression of purine metabolism-related enzymes by microglial cells in the developing rat brain

The nucleoside triphosphatase (NTPase), nucleoside diphosphatase (NDPase), 5'-nucleotidase (5'-Nase), and purine nucleoside phosphorylase (PNPase) activity has been examined in the cerebral cortex, subcortical white matter, and hippocampus from embryonic day (E)16 to postnatal day (P)18. Microglia display all four purine-related enzymatic activities, but the expression of these enzymatic activities differed depending on the distinct microglial typologies observed during brain development. We have identified three main morphologic typologies during the process of microglial differentiation: ameboid microglia (parenchymatic precursors), primitive ramified microglia (intermediate forms), and resting microglia (differentiated cells). Ameboid microglia, which were encountered from E16 to P12, displayed the four enzymatic activities. However, some ameboid microglial cells lacked 5'-Nase activity in gray matter, and some were PNPase-negative in both gray and white matter. Primitive ramified microglia were already observed in the embryonic period but mostly distributed during the first 2 postnatal weeks. These cells expressed NTPase, NDPase, 5'-Nase, and PNPase. Similar to ameboid microglia, we found primitive ramified microglia lacking the 5'-Nase and PNPase activities. Resting microglia, which were mostly distinguishable from the third postnatal week, expressed NTPase and NDPase, but they lacked or displayed very low levels of 5'-Nase activity, and only a subpopulation of resting microglia was PNPase-positive. Apart from cells of the microglial lineage, GFAP-positive astrocytes and radial glia cells were also labeled by the PNPase histochemistry. As shown by our results, the differentiation process from cell precursors into mature microglia is accompanied by changes in the expression of purine-related enzymes. We suggest that the enzymatic profile and levels of the different purine-related enzymes may depend not only on the differentiation stage but also on the nature of the cells. The use of purine-related histoenzymatic techniques as a microglial markers and the possible involvement of microglia in the control of extracellular purine levels during development are also discussed.

Effects of P2 purinergic receptor stimulation in brown adipocytes

Sympathetic stimulation of brown adipocytes plays a major role in body energy homeostasis by activating energy-wasting pathways. Sympathetic neuronal input initiates a variety of metabolic, developmental, and membrane responses in brown fat cells. Many of these actions are mediated by adrenergic pathways mobilized by released norepinephrine. However, since sympathetic stimulation may also release vesicular ATP, we tested brown fat cells for ATP responses. Micromolar concentrations of extracellular ATP had a number of effects on brown adipocytes. We have shown previously that ATP elicits substantial (average of approximately 30%) increases in cell membrane capacitance (P. A. Pappone and S. C. Lee, J. Gen. Physiol. 108: 393-404, 1996). Here, we show that cytosolic calcium levels were increased by ATP, both through release from intracellular stores and through influx, as assessed by fura 2 imaging. In addition, ATP indirectly activated a nonselective cation conductance that was independent of cytosolic calcium levels in patch voltage-clamped brown fat cells. Similar calcium, conductance, and capacitance responses could be activated by 2-methylthio-ATP and ADP, consistent with mediation by a P2 type purinergic receptor. Calorimetric measurements from cell suspensions showed that ATP increased basal heat production of isolated brown fat cells by approximately 40% but had no effect on the greater than fivefold increase in heat production seen with maximal adrenergic stimulation. These myriad responses to extracellular ATP suggest that P2 receptor-mediated signaling is important in brown adipocyte physiology and that sympathetic stimulation may normally activate purinergic as well as adrenergic pathways in brown fat.

Antiproliferative effects of imidazo[1,2-a]pyrazine derivatives on the Dami cell line

Since cyclic 3',5'-adenosine monophosphate (cAMP) is involved in cell proliferation and as previous data showed that imidazo[1,2-alpha]pyrazine derivatives (PAB12, PAB30, PAB40, SCA40, SCA41, and SCA44) inhibited cAMP breakdown by a phosphodiesterase (PDE)-inhibitory effect, the aim of the present study was to investigate the effects of these derivatives on proliferation of the Dami cell line in relation with their actions on cAMP content and on PDE isoenzymes isolated from Dami cells. SCA41 and SCA44 inhibited cell growth in a dose-dependent manner, while SCA40 and PAB40 induced a weak inhibition. Growth inhibitions were 40%, 91%, and 60% for SCA41, SCA44 (at 100 microM), and IBMX (at 100 microM), respectively, and could not be related to their effects on cAMP levels. In addition, although all compounds potentiated cAMP formation by prostaglandin E1 (PGE1), no potentiations were observed when the antiproliferative effects of SCA41 and SCA44 were considered. Investigation of derivatives on PDE isoenzymes III, IV, and V indicated non-selective PDE inhibitory effects for SCA41 and SCA44, while SCA40 elicited preferences for type III, and PAB30 and PAB40 preferences for type IV isoenzymes. These effects could not totally explain the antiproliferative activity of the derivatives. The activation of P2 purinoceptors by imidazo[1,2-a]pyrazine did not lead to their antiproliferative effects. Thus, the mechanism of the antiproliferative effects of the compounds remains to be determined. It does, however, depend on the chemical substitutions of the imidazo[1,2-a]pyrazine skeleton and in particular on the 2-carbonitrile presence and the length of the 8-aminoaliphatic group.

Specific binding of [3H]GppNHp to extracellular membrane receptors in chick cerebellum: possible involvement of kainic acid receptors

Guanine nucleotides (GNs), including GMP, displace [3H]kainic acid binding to chick cerebellar lysed and vesiculated membranes. Saturation studies of [3H]GppNHp binding, under conditions that prevent the occupation of the nucleotide binding sites in G-proteins, demonstrate the existence of extracellular membrane receptors specific for guanine nucleotides. Affinity-labeling of a vesicle preparation with [alpha-32P]GTP gives one single labeled band, upon electrophoresis, with an apparent molecular mass of 50 kDa. Additional experiments with partially purified kainate receptors suggest that the GN extracellular sites may overlap, at least partially, the kainic acid binding sites, being then responsible for the displacement of [3H]kainic acid by GNs. The physiological significance of these findings remains unclear.

Characterization and quantitation of the active polynucleotide fraction (PDRN) from human placenta, a tissue repair stimulating agent

The polydeoxyribonucleotide (PDRN) fraction is an extract which forms the active component in a new formulation of the drug Placentex (a tissue repair stimulating agent), obtained from human placenta through an original proprietory extraction method. From a comparison of the UV, NMR and IR spectra of this fraction (before and after nuclease treatment) with that of a similar standard (Sigma D1501), it was shown that the active substances in the PDRN fraction mainly consist of a mixture of DNA fragments. By gel electrophoresis, the molecular weights of the DNA fragments were shown to range from 50 to 2000 base pairs. Finally, an HPLC method is described, based on an anion-exchange material capable of determining the amount of PDRN in different batches of the extract, which varied from 80 to 90%.

Biochemistry, localization and functional roles of ecto-nucleotidases in the nervous system

Nucleotides such as ATP, ADP, UTP or the diadenosine polyphosphates and possibly even NAD+ are extracellular signaling substances in the brain and in other tissues. Enzymes located on the cell surface catalyze the hydrolysis of these compounds and thus limit their spatio-temporal activity. As a final hydrolysis product they generate the nucleoside and phosphate. The paper discusses the biochemical properties, cellular localization and functional properties of surface-located enzymes that hydrolyse nucleotides released from nervous tissue. This is preceded by a brief discussion of nucleotide receptors, cellular storage and mechanisms of nucleotide release. In nervous tissue nucleoside 5'-triphosphates are hydrolysed by ecto-ATP-diphosphohydrolase and possibly in addition also by ecto-nucleoside triphosphatase and ecto-nucleoside diphosphatase. The molecular identity of the ATP-diphosphohydrolase has now been revealed. The hydrolysis of nucleoside 5'-monophosphates is catalysed by 5'-nucleotidase whose biochemical properties and molecular structure have been studied in detail. Little is known about the molecular properties of the diadenosine polyphosphatases. Surface located enzymes for the extracellular hydrolysis of NAD+ and also ecto-protein kinases are discussed briefly. The cellular localization of the ecto-nucleotidases is only partly defined. Whereas in adult mammalian brain activity for hydrolysis of ATP and ADP may be associated with nerve cells or glial cells 5'-nucleotidase appears to have a preferential glial allocation in the adult mammal. The extracellular hydrolysis of the nucleotides is of functional importance not only during synaptic transmission where it functions in signal elimination. It plays a crucial role also for the survival and differentiation of neural cells in vitro and presumably during neuronal development in vivo.

Tyrphostin inhibition of ATP-stimulated DNA synthesis, cell proliferation and fos-protein expression in vascular smooth muscle cells

1. We and others have shown that extracellular ATP (adenosine triphosphate), released from sympathetic nerves and platelets, stimulates growth of vascular smooth muscle cells (SMC). To study the importance of tyrosine kinases for ATP-mediated proliferation in vascular smooth muscle cells we used tyrphostins, a recently developed group of highly specific inhibitors of tyrosine kinases. 2. ATP induced a powerful concentration-dependent increase in DNA synthesis measured by [3H]-thymidine incorporation in rat aorta SMC (RASMC) and an increase in total cell number after 72 h of incubation as measured by an enzymatic cell proliferation assay. Tyrphostin 25 (10(-5) M) had no effect per se on basal DNA synthesis but reduced ATP-stimulated DNA synthesis and increase in cell number in a dose-dependent manner. Higher concentrations of ATP could not reverse the inhibitory effect of tyrphostin 25. The potency of several (six) other tyrphostins was also examined and found to be slightly greater than tyrphostin 25 with equal efficacy. 3. When RASMC were incubated with 10(-5) M ATP for 2 h, nearly all of the cells (87 +/- 5%) were intensely stained with an antibody to the Fos protein while in the controls only 1 +/- 2% of the cells were weakly stained. Tyrphostin 25 greatly reduced the Fos-protein staining (14 +/- 2%). 4. ATP induced a concentration-dependent increase in 45Ca(2+)-influx and formation of inositol phosphates (IPtotal) in RASMC. These effects were not inhibited by tyrphostin 25. 5. Tyrphostin 25 did not alter ATP-induced contraction in ring segments of rat aorta. 6. In conclusion, tyrphostin 25 inhibited ATP-induced DNA synthesis, cell proliferation and Fos-protein expression, but not ATP-induced 45Ca(2+)-influx, inositolphosphate-production or vasoconstriction. This indicates that the mitogenic effect of ATP on vascular smooth muscle cells is dependent on tyrosine kinases in contrast to the contractile effect of ATP in blood vessels.

GTP and guanosine synergistically enhance NGF-induced neurite outgrowth from PC12 cells

Six per cent of rat pheochromocytoma (PC12) cells extended neurites (processes greater than one cell diameter in length) in the presence of 300 microM extracellular GTP or 300 microM guanosine for 48 hr, compared to only 2.5% of cells in control cultures. In the presence of 40 ng/ml of 2.5S NGF, about 20-35% of PC12 cells had neurites after 48 hr, and the addition of 300 microM guanosine or GTP together with NGF synergistically increased the proportion of cells with neurites to 40-65%. GTP and guanosine also increased the average number of branches per neurite, from 0.6 in NGF-treated cultures to 1.2 (guanosine) or 1.5 (GTP). Neurites formed after exposure to NGF alone had axonal characteristics as determined by immunocytochemistry with antibody, SMI-31, against axonal-specific polyphosphorylated neurofilament epitopes. Neurites generated with the addition of both guanosine or GTP had the same characteristics. GTP probably did not exert its effects via the P2X or P2Y purinoceptors because the adenine nucleotides ATP, ATP gamma S, ADP beta S, and ADP, which are all agonists of these receptors, inhibited rather than enhanced, NGF-induced neurite outgrowth. UTP also enhanced the proportion of cells with neurites, although not to the same degree as did GTP. This may indicate activity through a P2U-like nucleotide receptor. However, the response profile obtained, GTP > UTP >> ATP, does not fit the profile of any known P2Y, P2X or P2U receptor. The poorly hydrolyzable GTP analogues, GTP gamma S and GDP beta s were also unable to enhance the proportion of cells with neurites. This implied that GTP may produce its effects through a GTP-specific ectoenzyme or kinase. This idea was supported by results showing that another poorly hydrolyzable analogue, GMP-PCP, competitively inhibited the effects of GTP on neurite outgrowth. GTP did not exert its effects after hydrolysis to guanosine since the metabolic intermediates GDP and GMP were also ineffective in enhancing the proportion of cells with neurites. Moreover, the effects of GTP and guanosine were mutually additive, implying that these two purines utilized different signal transduction mechanisms. The effects of guanosine were not affected by the nucleoside uptake inhibitors nitrobenzylthioinosine (NBTI) and dipyridamole, indicating that a transport mechanism was not involved. Guanosine also did not activate the purinergic P1 receptors, because the A2 receptor antagonists, 1,3-dipropyl-7-methylxanthine (DPMX) or CGS15943, and the A1 receptor antagonist, 1,3-dipropyl-8-(2-amino-4-chloro)xanthine (PACPX) did not inhibit its reaction. Therefore guanosine enhanced neurite outgrowth by a signal transduction mechanism that does not include the activation of the P1 purinoceptors. The enhancement of the neuritogenic effects of NGF by GTP and guanosine may have physiological implications in sprouting and functional recovery after neuronal injury in the CNS, due to the high levels of nucleosides and nucleotides released from dead or injured cells.

Trophic actions of extracellular nucleotides and nucleosides on glial and neuronal cells

In addition to their well-established roles as neurotransmitters and neuromodulators, growing evidence suggests that nucleotides and nucleosides might also act as trophic factors in both the central and peripheral nervous systems. Specific extracellular receptor subtypes for these compounds are expressed on neurons, glial and endothelial cells, where they mediate strikingly different effects. These range from induction of cell differentiation and apoptosis, mitogenesis and morphogenetic changes, to stimulation of synthesis or release, or both, of cytokines and neurotrophic factors, both under physiological and pathological conditions. Nucleotides and nucleosides might be involved in the regulation of development and plasticity of the nervous system, and in the pathophysiology of neurodegenerative disorders. Receptors for nucleotides and nucleosides could represent a novel target for the development of therapeutic strategies to treat incurable diseases of the nervous system, including trauma- and ischemia-associated neurodegeneration, demyelinating and aging-associated cognitive disorders.

Effects of ATP analogues and basic fibroblast growth factor on astroglial cell differentiation in primary cultures of rat striatum

We have used primary cultures of rat striatum to study the effects of ATP analogues on the elongation of astrocytic processes, a parameter of astroglial cell differentiation. Parallel studies were performed with basic fibroblast growth factor, a known regulator of astroglial cell function. After three days in culture, both the growth factor and alpha beta-methylene-ATP induced dramatic increases in the mean length of astrocytic processes/cell. For both agents, effects were dose-dependent. The effect of alpha beta-methylene-ATP was antagonized by the trypanoside suramin and mimicked by 2-methyl-thio-ATP, suggesting the involvement of a suramin-sensitive P2-purinoceptor. Neither an additive nor a synergistic effect between alpha beta-methylene-ATP and basic fibroblast growth factor on the elongation of processes was detected in cultures exposed to both agents. Indeed, an inhibition with respect to the effects induced by either agent alone was recorded, suggesting that the growth factor and the purine analogue can modulate astrocytic differentiation by activation of common intracellular pathways. It is concluded that, like basic fibroblast growth factor, ATP can promote the maturation of astrocytes towards a more differentiated phenotype characterized by the presence of longer astrocytic processes. These findings might have interesting implications for astroglial cell differentiation during brain development and for ischemia- and trauma-associated hypergliosis.

AIT-082, a unique purine derivative, enhances nerve growth factor mediated neurite outgrowth from PC12 cells

AIT-082 is a novel, metabolically stable, derivative of the purine hypoxanthine. Addition of AIT-082 to cultured PC12 cells enhanced significantly nerve growth factor (NGF)-mediated neurite outgrowth from PC12 cells. These results suggest a cellular mechanism, the enhancement of NGF-action, that might account for the ability of AIT-082 to restore age-induced working memory deficits in mice.

5'-nucleotidase activates and an inhibitory antibody prevents neuritic differentiation of PC12 cells

Ecto-5'-nucleotidase catalyses the hydrolysis of AMP at the surface of a variety of cells whereas it is absent from others. In addition to its catalytic activity, a function in neural development and also its interaction with extracellular matrix proteins has been reported. In order to further elucidate the biological function of ecto-5'-nucleotidase we have investigated the effect of 5'-nucleotidase on nerve growth factor-induced differentiation of PC12 cells. Furthermore, we compared the effect of an inhibitory versus a non-inhibitory monospecific antibody against the enzyme on neuritic differentiation and survival of PC12 cells that constitutively express the enzyme. When coverslips are coated with the soluble form of ecto-5'-nucleotidase in addition to collagen, there is a considerable increase in nerve growth factor-induced neurite length during the first 24 h of culture. Addition of an antibody to a culture medium that inhibits 5'-nucleotidase activity to 33% of control values dramatically reduces the number of neurites per cell within 3 days of culture. The cells round up, cluster and eventually die. On the contrary, another antibody that had no significant effect on enzyme activity affected neither nerve growth factor-induced neurite formation nor survival of PC12 cells. Addition of adenosine (200 nM, 10 or 20 microM) to the culture medium did not influence PC12 cell differentiation. The effects induced by the inhibitory antibody could be only partially prevented by simultaneous application of adenosine. Our results suggest that 5'-nucleotidase is essential for nerve growth factor-induced neurite outgrowth and survival of PC12 cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterisation of an ATP receptor mediating mitogenesis in vascular smooth muscle cells

Adenosine triphosphate (ATP), a co-transmitter in sympathetic nerves and released from platelets, has recently been shown to stimulate growth of vascular smooth muscle cells. It might therefore contribute to the development of vascular hypertrophy seen in hypertension and atherosclerosis. We aimed at characterising the receptor mediating this mitogenic effect in rat aorta smooth muscle cells. The potency of agonists indicates a P2 purinoceptor since ATP > or = ADP >> AMP, adenosine. The P2x-receptor subtype, which is responsible for ATP induced vasoconstriction in rat aorta, does not mediate the mitogenic effect since alpha, beta-methyleneATP had no effect and beta, gamma-methyleneATP had lower potency than ATP. The P2Y-receptor subtype was excluded since the selective agonist 2-methylthioATP had weak effect with lower potency than ATP. When we studied the involvement of other nucleotides similar effects were seen of the purines ATP, GTP and ITP; also the pyrimidine UTP had powerful mitogenic effects (Emax = 52% of ATP) with similar potency. Nucleotides with fewer phosphate groups showed a stepwise fall in mitogenic effect. This indicates involvement of a nucleotide-receptor (P2U). Ap4A were of equal potency and effect as ATP. There was strong correlation between the mitogenic effects of the nucleotides and analogues with both 45Ca(2+)-influx and inositol phosphate (IP) production, indicating that they may participate in mediating the mitogenic response. This is the first study describing the potencies for the mitogenic effects of the selective ATP-analogues and other nucleotides in vascular smooth muscle cells. The receptor characterisation indicates a nucleotide-receptor similar to the receptor which stimulates 45Ca(2+)-influx and inositol phosphate-formation in rat aorta smooth muscle cells. Substances related to ATP such as GTP, ITP, UTP and Ap4A which also can be released extracellularly in vivo stimulate mitogenesis of rat aorta smooth muscle cells through the same receptor.