Autocrine and paracrine purinergic signaling in the most lethal types of cancer.

Cancer comprises a collection of diseases that occur in almost any tissue and it is characterized by an abnormal and uncontrolled cell growth that results in tumor formation and propagation to other tissues, causing tissue and organ malfunction and death. Despite the undeniable improvement in cancer diagnostics and therapy, there is an urgent need for new therapeutic and preventive strategies with improved efficacy and fewer side effects. In this context, purinergic signaling emerges as an interesting candidate as a cancer biomarker or therapeutic target. There is abundant evidence that tumor cells have significant changes in the expression of purinergic receptors, which comprise the G-protein coupled P2Y and AdoR families of receptors and the ligand-gated ion channel P2X receptors. Tumor cells also exhibit changes in the expression of nucleotidases and other enzymes involved in nucleotide metabolism, and the concentrations of extracellular nucleotides are significantly higher than those observed in normal cells. In this review, we will focus on the potential role of purinergic signaling in the ten most lethal cancers (lung, breast, colorectal, liver, stomach, prostate, cervical, esophagus, pancreas, and ovary), which together are responsible for more than 5 million annual deaths.

Nucleotides and nucleoside signaling in the regulation of the epithelium to mesenchymal transition (EMT).

The epithelium-mesenchymal transition (EMT) is an important process of cell plasticity, consisting in the loss of epithelial identity and the gain of mesenchymal characteristics through the coordinated activity of a highly regulated informational program. Although it was originally described in the embryonic development, an important body of information supports its role in pathology, mainly in cancerous and fibrotic processes. The purinergic system of inter-cellular communication, mainly based in ATP and adenosine acting throughout their specific receptors, has emerged as a potent regulator of the EMT in several pathological entities. In this context, cellular signaling associated to purines is opening the understanding of a new element in the complex regulatory network of this phenotypical differentiation process. In this review, we have summarized recent information about the role of ATP and adenosine in EMT, as a growing field with high therapeutic potential.

Interplay between ryanodine and IP3 receptors in ATP-stimulated mouse luteinized-granulosa cells.

In mouse luteinized-granulosa cells (MGLC), ATP induces an increase in intracellular Ca2+ concentration by stimulating phospholipase C (PLC) associated with purinergic receptors, leading to production of inositol 1,4,5-trisphosphate (IP3) and subsequent release of Ca2+ from intracellular stores. In this study, we examined the cross-talk between the ryanodine receptors (RyR) and IP3 receptors (IP3R) in response to ATP in MGLC. Specifically, the effect of RyR modulators on ATP response was examined. The results showed that ATP-induced intracellular calcium elevation was abolished by inhibitors of the RyR, such as dantrolene (25 microM) and ryanodine (80 microM). When the MGLC were stimulated with activators of RyR, 2 microM ryanodine and 10 mM caffeine, the ATP-elicited response was decreased. These actions were independent of IP3 production stimulated by ATP. Hence, ATP-induced intracellular Ca2+ mobilization involves the coordinated action of both types of calcium release channels (CRCs). Using fluorescent probes, it was shown that IP3R is uniformly distributed throughout the cell; in contrast, RyR is mainly found around the nuclei. It is concluded that the IP3R and the RyR are functionally associated, and both play a role in the pattern of Ca2+ increase observed during purinergic stimulation of MGLC. This coupling may provide a highly efficient amplification mechanism for ATP stimulation of Ca2+ mobilization.

Dissociation between adipose tissue signals, behavior and the food-entrained oscillator.

Digestive and metabolic processes are entrained by restricted feeding (RFS) schedules and are thought to be potential elements of a food-entrained oscillator (FEO). Due to the close relationship of leptin with metabolic regulation and because leptin is a relevant communication signal of the individual's peripheral metabolic condition with the central nervous system, we explored whether leptin is an endogenous entraining signal from the periphery to a central element of an FEO. First we characterized in the rat the diurnal rhythm of serum leptin (in rats fed ad libitum (AL)), its adjustment to an RFS and the influence of fasting after RFS, or RFS followed by AL feeding and then total food deprivation (RF-AF) in the persistence of this fluctuating pattern. We also explored the response of free fatty acids and stomach weight under the same entraining conditions. We compared the metabolic response with the behavioral expression of drinking anticipatory activity (AA) under the same conditions. Finally, we tested the effect of daily i.c.v administration of leptin as a putative entraining signal for the generation of AA. Metabolic parameters responded to food entrainment by adjusting their phase to mealtime. However, leptin and free fatty acid rhythms persisted only for a few cycles in fasting conditions and readjusted to the light-darkness cycle after an RF-AF protocol. In contrast, behavioral food-entrained rhythms persisted after both fasting manipulations. Daily leptin i.c.v. administration did not produce AA, nor produce changes in the behavioral free-running rhythm. Stomach weight indicated an adaptive process allowing an extreme stomach distension followed by a slow emptying process, which suggests that the stomach may be playing a relevant role as a storage organ. In conclusion, metabolic signals here studied respond to feeding schedules by adjusting their phase to mealtime, but do only persist for a few cycles in fasting. Leptin does not produce AA and thus is not an entraining signal for FEO. The response of metabolic signals to feeding schedules depends on different mechanisms than the expression of AA.

Roles of ryanodine and inositol triphosphate receptors in regulation of plateau potentials in turtle spinal motoneurons.

Generation of plateau potentials in spinal motoneurons depends on activation of voltage sensitive L-type Ca(2+) channels. These channels are facilitated by metabotropic receptors known to promote release of Ca(2+) from intracellular stores. The aim of this study is to determine if Ca(2+)-release receptors in the endoplasmic reticulum (ER) that are sensitive to ryanodine (RyRs) and to inositol triphosphate receptors (IP(3)Rs) contribute to the generation of plateau potentials. The effects of antagonists to RyRs, IP(3)Rs and phospholipase C (PLC) were tested on discharge patterns associated with plateau potentials in motoneurons in slices from the spinal cord of the turtle. Plateau-related discharge patterns, un-facilitated or facilitated by agonists for group I glutamate metabotropic receptors, muscarine-sensitive cholinergic receptors or L-type Ca(2+) channels were inhibited by blockade of RyRs. In contrast, antagonists of IP(3)Rs or PLC preferentially inhibited plateau-related discharge patterns when facilitated by activation of metabotropic receptors but in only half of the cells when promoted in the absence of metabotropic facilitators. Our findings show that RyRs and IP(3)Rs regulate the generation of plateau potentials in motoneurons and suggest that RyRs may be directly involved with activation of the plateau potential.

Potentiation of prolactin secretion following lactotrope escape from dopamine action. II. Phosphorylation of the alpha(1) subunit of L-type, voltage-dependent calcium channels.

Modulation of Ca(2+) channels has been shown to alter cellular functions. It can play an important role in the amplification of signals in the endocrine system, including the pleiotropically regulated pituitary lactotropes. Prolactin (PRL) secretion is tonically inhibited by dopamine (DA), the escape from which triggers acute episodes of hormone secretion. The magnitude of these episodes is explained by a potentiation of the PRL-releasing action of secretagogues such as thyrotropin-releasing hormone (TRH). While the mechanisms of this potentiation are not fully understood, it is known to be mimicked by the dihydropyridine, L-type Ca(2+) channel agonist Bay K 8644 and blocked by nifedipine and methoxyverapamil. The potentiation is also blocked by inhibitors of cyclic AMP-dependent protein kinase and protein kinase C. We recently described that the escape from tonic actions of DA results in increased macroscopic Ca(2+) currents in GH(4)C(1) lactotropic clonal cells transfected with a cDNA encoding the long form of the human D(2)-DA receptor. Here we show that the withdrawal from DA potentiates the PRL-releasing action of TRH in GH(4)C(1)/D(2)-DAR cells to the same extent as in enriched lactotropes in primary culture. In both experimental models a low density of dihydropyridine receptors was shown by (+)-[(3)H]PN200-110 binding. Photoaffinity labelling with the dihydropyridine [(3)H]azidopine revealed a protein consistent with the alpha(1) subunit of L-type Ca(2+) channels of 165-170 kDa. In both experimental models, the facilitation of TRH action triggered by the escape from DA was correlated with an enhanced rate of phosphorylation of this putative alpha(1) subunit, the nature of which was further supported by immunoprecipitation with selective antibodies directed against the alpha(1C) and alpha(1D) subunit of voltage-gated calcium channels. We propose that PKA- and PKC-dependent phosphorylation of the alpha(1) subunit of high voltage activated, L-type Ca(2+) channels is responsible for the facilitation of Ca(2+) currents in lactotropes, and hence for the potentiation of secretagogue-mediated PRL secretion. Thus, phosphorylation of Ca(2+) channels in endocrine cells may be a mechanism for the regulation of various functions including amplification of hormone secretion.

Ryanodine receptor binding constants in skeletal muscle, heart, brain and liver of the Mexican volcano mouse (Neotomodon alstoni alstoni; Rodentia:Cricetidae). Comparison with five other rodent species.

Equilibrium [3H]ryanodine binding assay was applied to total membrane fractions of six rodent species, including the Mexican volcano mouse Neotomodon alstoni alstoni, Wistar rat Rattus norvegicus albinus, golden hamster Mesocritus auratus, gerbil Meriones unguiculatus, guinea-pig Cavia porcellus, and ground squirrel Spermophillus mexicanus. The organs selected for this study were: skeletal muscle, heart, brain and liver. The constants derived from Scatchard analysis show slight variations in their Kd, ranging from 3 to 15 nM, except in the gerbil's skeletal muscle (38 nM) and the hamster's brain (27 nM). Remarkably, the Bmax calculated in guinea-pig muscle was as high as that reported for the rabbit fast twitch muscle (4.6 pmol/mg of protein) using the same membrane fraction preparation. For all the other skeletal muscles, Bmax was similar to the corresponding heart Bmax values (from 0.5 to 1 pmol/mg of protein). Gerbil cardiac Bmax was the highest (1.1 pmol/mg of protein). The ground squirrel was the rodent with more cerebral ryanodine binding sites (0.26 pmol/mg of protein), whereas the rat and the volcano mouse showed the lowest values (0.12 pmol/mg of protein). The richest sources of hepatic ryanodine receptor were the guinea-pig and rat livers (approximately equal to 0.35 pmol/mg of protein), whereas the lowest Bmax corresponded to the hamster liver (0.018 pmol/mg of protein). These results allow us to detect the similarities and differences of the ryanodine receptor binding constants from four different tissues of some of the rodents most widely used as biomedical laboratory animals.

Effects of N omega-nitro-L-arginine and L-arginine on quinolinic acid-induced lipid peroxidation.

The effects of a nitric oxide synthase inhibitor, N omega-nitro-L-arginine (L-NARG), and a nitric oxide precursor, L-arginine (L-ARG), on the lipid peroxidation induced by quinolinic acid (QUIN, an NMDA receptor agonist), were both tested in synaptosomal fractions from whole rat brain. Baseline of lipid peroxidation was found at 2.43 +/- 0.24 fluorescence units/mg protein or 14.27 +/- 1.24 nmoles of TBARS/mg protein (100%). QUIN (100 microM)-induced lipid peroxidation in synaptosomes (256% and 166% vs. control, as measured by lipid fluorescent products and thiobarbituric acid-reactive substances, respectively) was inhibited by concentrations of 10, 40, 100, 200 and 400 microM of L-NARG (74%, 58%, 56%, 48% and 48% vs. quinolinate value, respectively). Coincubation of synaptosomes with QUIN plus L-ARG (100 microM), which alone resulted a potent pro-oxidant (277% vs. control), increased the lipoperoxidative effect induced by QUIN alone in 120% (290% vs. control). Synaptosomes simultaneously exposed to QUIN (100 microM) plus L-ARG (100 microM) plus L-NARG (200 microM) showed levels of lipid peroxidation similar to those of quinolinate alone. These findings suggest that nitric oxide may contribute to the oxidative damage induced in vitro by QUIN.

Temporal variations of adenosine metabolism in human blood.

Eight diurnally active (06:00-23:00 h) subjects were adapted for 2 days to the room conditions where the experiments were performed. Blood sampling for adenosine metabolites and metabolizing enzymes was done hourly during the activity span and every 30 min during sleep. The results showed that adenosine and its catabolites (inosine, hypoxanthine, and uric acid), adenosine synthesizing (S-adenosylhomocysteine hydrolase and 5'-nucleotidase), degrading (adenosine deaminase) and nucleotide-forming (adenosine kinase) enzymes as well as adenine nucleotides (AMP, ADP, and ATP) undergo statistically significant fluctuations (ANOVA) during the 24 h. However, energy charge was invariable. Glucose and lactate chronograms were determined as metabolic indicators. The same data analyzed by the chi-square periodogram and Fourier series indicated ultradian oscillatory periods for all the metabolites and enzymatic activities determined, and 24-h oscillatory components for inosine, hypoxanthine, adenine nucleotides, glucose, and the activities of SAH-hydrolase, 5'-nucleotidase, and adenosine kinase. The single cosinor method showed significant oscillatory components exclusively for lactate. As a whole, these results suggest that adenosine metabolism may play a role as a biological oscillator coordinating and/or modulating the energy homeostasis and physiological status of erythrocytes in vivo and could be an important factor in the distribution of purine rings for the rest of the organism.

Possible mechanism of adenosine protection in carbon tetrachloride acute hepatotoxicity. Role of adenosine by-products and glutathione peroxidase.

Adenosine proved to be an effective hepatoprotector increasing the survival rate of rats receiving lethal doses of CCl4. Searching for the mechanism of action, we found that adenosine transiently prevents the necrotic liver damage associated to an acute CCl4 treatment. The antilipoperoxidative action of the nucleoside was evidenced by a decrease of TBA-reactive products and the diene conjugates elicited by the hepatotoxin. Adenosine's protective effect was demonstrated by reverting the decrease of cytochrome P-450 while preserved intact the activity of the microsomal enzyme glucose-6-phosphatase. CCl4 promoted an increase in the oxidant stress through an enhancement in oxidized glutathione levels. This action was also completely counteracted by the nucleoside. Adenosine was unable to prevent CCl4 activation and, even, increased .CCl3 formation in the presence of PBN in vivo. However, in the presence of the nucleoside, irreversible binding of 14CCl4 to the microsomal lipid fraction of the treated animals was decreased. These results suggest that adenosine protective action might be exerted at the level of the propagation reaction following CCl4 activation. Two possible mechanisms were associated to the nucleoside protection: (1) the peroxide-metabolyzed enzymes, GSH-per, showed a marked increase after 30 minutes of adenosine treatment, which was potentiated by the hepatotoxin, suggesting an important role of this enzyme in the nucleoside's action; (2) the adenosine catabolism induced an increase in uric acid level, and allopurinol, a purine metabolism inhibitor, prevented such elevation as well as the antilipoperoxidative action of adenosine and the increase of GSH-per associated with the nucleoside treatment. These facts strongly suggest that the protective effect elicited by adenosine is not a direct one, but rather is related to its catabolic products, such as uric acid, which has been recognized as a free radical scavenger.

Possible role of cell redox state on collagen metabolism in carbon tetrachloride-induced cirrhosis as evidenced by adenosine administration to rats.

Marked changes in the redox state of liver cells in carbon tetrachloride (CCl4)-induced cirrhosis after chronic treatment with the hepatotoxin (4-8 weeks) were observed. A shift of the redox state towards the reduced side is noticed in both compartments, cytosol and mitochondria. At 8 weeks of treatment an imbalance between these two compartments was evident. The alteration produced by the CCl4 treatment in the cell redox state might be related to the mitochondrial damage elicited by the hepatotoxin. Adenosine treatment to CCl4-poisoned rats was able to counteract the effect of the hepatotoxin on the redox equilibrium; hence, it could be linked to the beneficial action of the nucleoside in the maintenance of mitochondrial function. The changes in the hepatocyte redox state, induced by CCl4 and/or adenosine, seem to modify collagen and nitrogen metabolism, indicating a linear correlation between the redox state and the collagen synthesis rate, whereas an inverse relationship was observed with collagenase activity. The possible role of the changes in cell redox state as signals for communication between parenchymal and mesenchymal liver cells is discussed. The results suggest an important correlation among mitochondrial function, cellular redox state, and regulation of collagen metabolism that could be relevant for the physio-pathology of this model of experimental cirrhosis.