Extracellular ATP and UTP increase cytosolic free calcium by activating a common P2u-receptor in single human thyrocytes

Clinico-epidemiological profile and treatment outcomes in patients with squamous cell carcinoma of the esophagus following docetaxel-based neoadjuvant chemotherapy: experience from a cancer care center in Northeast India

BACKGROUND

Squamous cell carcinoma of the esophagus ranks as the most common cause of cancer incidence and mortality in males and the second most common in females. Surgery alone is associated with poor long-term survival. Neoadjuvant chemoradiation and perioperative chemotherapy without radiation have been tried to improve survival rates.

METHODS

We retrospectively evaluated the neoadjuvant chemotherapy in forty-eight patients with non-metastatic, non-cervical squamous cell carcinoma of the esophagus with a docetaxel-based three-drug regimen to improve complete pathological response rates.

RESULTS

The median age of presentation was 52 years, with male preponderance. All the patients received three cycles of docetaxel-cisplatin-fluorouracil-based chemotherapy. A complete pathological response to neoadjuvant chemotherapy was seen in 8 patients (17%). Rates of grade 3 hematological toxicities were seen in 12% of patients, with no observed grade 4 toxicity. The most common non-hematological toxicity was grade 3 alopecia (seen in 40%) and grade 2 nausea/vomiting in 8% of patients. At a median follow-up of 26.5 months, 2-year survival for the patients receiving chemotherapy and surgery is 66%.

CONCLUSIONS

Preoperative chemotherapy with a taxane-based triple-drug regimen is a reasonable approach in squamous cell carcinoma of the esophagus, associated with improvement in complete pathological response rates, increases complete resection rates, with manageable toxicity.

Apical iodide efflux in thyroid

Thyroid follicular epithelial cells produce thyroxine (T4) and its physiologically active derivative, 3,3',5-triiodothyronine (T3), hormones that regulate critical developmental and metabolic functions. In order for the thyroid to form hormone precursor, iodide, the defining element in thyroid hormone, must cross both blood-facing and luminal sides of the follicular epithelium. The pathway for uptake from blood is well understood, but the mechanism(s) that enable iodide to cross the luminally facing apical membrane remain obscure. This chapter considers the physiological properties of several molecularly characterized anion transport proteins, all of which potentially contribute to the overall mechanism of apical iodide efflux.

Purinergic signalling in endocrine organs

There is widespread involvement of purinergic signalling in endocrine biology. Pituitary cells express P1, P2X and P2Y receptor subtypes to mediate hormone release. Adenosine 5'-triphosphate (ATP) regulates insulin release in the pancreas and is involved in the secretion of thyroid hormones. ATP plays a major role in the synthesis, storage and release of catecholamines from the adrenal gland. In the ovary purinoceptors mediate gonadotrophin-induced progesterone secretion, while in the testes, both Sertoli and Leydig cells express purinoceptors that mediate secretion of oestradiol and testosterone, respectively. ATP released as a cotransmitter with noradrenaline is involved in activities of the pineal gland and in the neuroendocrine control of the thymus. In the hypothalamus, ATP and adenosine stimulate or modulate the release of luteinising hormone-releasing hormone, as well as arginine-vasopressin and oxytocin. Functionally active P2X and P2Y receptors have been identified on human placental syncytiotrophoblast cells and on neuroendocrine cells in the lung, skin, prostate and intestine. Adipocytes have been recognised recently to have endocrine function involving purinoceptors.

The birth and postnatal development of purinergic signalling

The purinergic signalling system is one of the most ancient and arguably the most widespread intercellular signalling system in living tissues. In this review we present a detailed account of the early developments and current status of purinergic signalling. We summarize the current knowledge on purinoceptors, their distribution and role in signal transduction in various tissues in physiological and pathophysiological conditions.

Increased P2X7 receptor expression and function in thyroid papillary cancer: a new potential marker of the disease?

Nucleotides are increasingly recognized as nonredundant extracellular signals for chemotaxis, cell growth, and cytokine release. Effects of extracellular nucleotides are mediated by P2 receptors, among which the P2X(7) subtype is attracting increasing attention for its involvement in apoptosis, cell growth, and cytokine release. Recent studies showed that P2X(7) is overexpressed in chronic lymphocytic leukemia and breast and prostate cancer. The aim of the present study was to better understand the clinical significance of P2X(7) receptor expression in normal and cancer human thyroid tissues. P2X(7) receptor message and protein expression and functional activity were tested in two cell lines (FB1 and FB2) established from either anaplastic or papillary primary thyroid cancer and in several histological samples of human papillary cancer. We show here that human thyroid papillary carcinoma, whether of the classical or follicular variant, expresses the P2X(7) receptor (P2X(7)R) to a much higher level than normal thyroid tissue. The P2X(7)R was similarly up-regulated in FB1 and FB2 cell lines. In contrast to normal thyroid cells, both cell lines responded to extracellular nucleotide stimulation with a large increase in intracellular Ca(2+) and secretion of IL-6. Ca(2+) increase was attenuated and release of IL-6 was fully blocked by P2X(7)R inhibitors. Finally, the thyroid carcinoma cell lines had at least a 3-fold higher intracellular ATP concentration and maintained at least a 3-fold higher extracellular ATP level, compared with control cells. These data suggest that an enhanced P2X(7)R function might be a feature of human thyroid cancer.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

P2X7 receptor stimulation of membrane internalization in a thyrocyte cell line

Using fluorescent membrane markers, we have previously shown that extracellular ATP stimulates both exocytosis and membrane internalization in the Fisher rat thyroid cell line FRTL. In this study, we examine the actions of ATP using whole-cell recording conditions that favor stimulation of membrane internalization. ATP stimulation of the P2X(7) receptor activated a reversible, Ca(2+)-permeable, cation conductance that slowly increased in size without changes in ion selectivity. ATP also induced a delayed irreversible decrease in cell capacitance (C(m)) that was equivalent to an 8% decrease in membrane surface area. Addition of guanosine 5'-0-2-thiodiphosphate to the pipette solution inhibited the ATP-induced decrease in C(m) without affecting channel activation. The effects of ATP on membrane conductance were mimicked by 2',3'-O-(4-benzoylbenzoyl)-ATP, but not by UTP, adenosine, or 2-methylthio-ATP, and were inhibited by pyridoxal phosphate-6-azophenyl-2'4'-disulfonic acid, adenosine 5'-triphosphate-2'3'-dialdehyde, and Cu(2+). The capacitance decrease persisted in Na(+)-, Ca(2+)- and Cl(-)-free external saline or with Ca(2+)-free pipette solution. It is concluded that ATP activation of the inotropic P2X(7) receptor stimulates membrane internalization by a mechanism that involves intracellular GTP, but does not require internal Ca(2+) or influx of Na(+) or Ca(2+) through the receptor-gated channel.

A P2X7 receptor stimulates plasma membrane trafficking in the FRTL rat thyrocyte cell line

Thyroid cells express a variety of P2Y and P2X purinergic receptor subtypes. G protein-coupled P2Y receptors influence a wide variety of thyrocyte-specific functions; however, functional P2X receptor-gated channels have not been observed. In this study, we used whole cell patch-clamp recording and fluorescence imaging of the plasma membrane marker FM1-43 to examine the effects of extracellular ATP on membrane permeability and trafficking in the Fisher rat thyroid cell line FRTL. We found a cation-selective current that was gated by ATP and 2',3'-O-(4-benzoylbenzoyl)-ATP but not by UTP. The ATP-evoked currents were inhibited by pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid, adenosine 5'-triphosphate-2',3'-dialdehyde, 100 microM Zn(2+), and 50 microM Cu(2+). Fluorescence imaging revealed pronounced, temperature-sensitive stimulation of exocytosis and membrane internalization by ATP with the same pharmacological profile as observed for activation of current. The EC(50) for ATP stimulation of internalization was 440 microM in saline containing 2 mM Ca(2+) and 2 mM Mg(2+), and 33 microM in low-Mg(2+), nominally Ca(2+)-free saline. Overall, the results are most consistent with activation of a P2X(7) receptor by ATP(4-). However, low permeability to N-methyl-d-glucamine(+) and the propidium cation YO-PRO-1 indicates absence of the cytolytic pore that often accompanies P2X(7) receptor activation. ATP stimulation of internalization occurs in Na(+)-free, Ca(2+)-free, or low-Mg(2+) saline and therefore does not depend on cation influx through the ATP-gated channel. We conclude that ATP activation of a P2X(7) receptor stimulates membrane internalization in FRTL cells via a transduction pathway that does not depend on cation influx.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Increase of [Ca(2+)](i) via activation of ATP receptors in PC-Cl3 rat thyroid cell line

In PC-Cl3 rat thyroid cell line, ATP and UTP provoked a transient increase in [Ca(2+)](i), followed by a lower sustained phase. Removal of extracellular Ca(2+) reduced the initial transient response and completely abolished the plateau phase. Thapsigargin (TG) caused a rapid rise in [Ca(2+)](i) and subsequent addition of ATP was without effect. The transitory activation of [Ca(2+)](i) was dose-dependently attenuated in cells pretreated with the specific inhibitor of phospholipase C (PLC), U73122. These data suggest that the ATP-stimulated increment of [Ca(2+)](i) required InsP(3) formation and binding to its specific receptors in Ca(2+) stores. Desensitisation was demonstrated with respect to the calcium response to ATP and UTP in Fura 2-loaded cells. Further studies were performed to investigate whether the effect of ATP on Ca(2+) entry into PC-Cl3 cells was via L-type voltage-dependent Ca(2+) channels (L-VDCC) and/or by the capacitative pathway. Nifedipine decreased ATP-induced increase on [Ca(2+)](i). Addition of 2 mM Ca(2+) induced a [Ca(2+)](i) rise after pretreatment of the cells with TG or with 100 microM ATP in Ca(2+)-free medium. These data indicate that Ca(2+) entry into PC-Cl3 stimulated with ATP occurs through both an L-VDCC and through a capacitative pathway. Using buffers with differing Na(+) concentrations, we found that the effects of ATP were dependent of extracellular Na(+), suggesting that a Na(+)/Ca(2+) exchange mechanism is also operative. These data suggest the existence, in PC-Cl3 cell line, of a P2Y purinergic receptor able to increase the [Ca(2+)](i) via PLC activation, Ca(2+) store depletion, capacitative Ca(2+) entry and L-VDCC activation.

Inhibition of store-operated Ca2+ entry by extracellular ATP in rat brown adipocytes

1. Modulation of intracellular free Ca2+ concentration ([Ca2+]i) by extracellular ATP was investigated in cultured adult rat brown adipocytes using the fluorescent Ca2+ indicator fura-2. 2. Bath application of ATP in micromolar concentrations caused a large increase in [Ca2+]i in cells previously stimulated with noradrenaline. This ATP-induced [Ca2+]i increase exhibited a monotonic decline to near the resting levels within approximately 2 min, even in the continued presence of the agonist. 3. The magnitude and time course of the [Ca2+]i increase in response to ATP were not significantly affected by removal of extracellular Ca2+, suggesting that a mobilization of intracellular Ca2+ primarily contributes to the increase. 4. The [Ca2+]i increase in response to ATP was sensitive to inhibition by suramin, suggesting the involvement of P2 purinoceptors in the response. 5. Thapsigargin (100 nM) evoked a gradual and irreversible increase in [Ca2+]i which was entirely dependent upon extracellular Ca2+, providing functional evidence for the expression of store-operated Ca2+ entry in these brown adipocytes. 6. Extracellular ATP at a concentration of 10 microM depressed this thapsigargin (100 nM)-induced [Ca2+]i increase by 92 +/- 3 % (n = 8 cells), strongly suggesting that ATP inhibits an influx of Ca2+ across the plasma membrane through the store-operated pathway. Bath application of phorbol 12-myristate 13-acetate (PMA, 5 microM) did not affect the thapsigargin-induced [Ca2+]i increase, indicating that the inhibitory action of ATP is not mediated by activation of protein kinase C (PKC). 7. These results indicate that extracellular ATP not only mobilizes Ca2+ from the intracellular stores but also exerts a potent inhibitory effect on the store-operated Ca2+ entry process in adult rat brown adipocytes.

Regulation of arachidonic acid release and prostaglandin E2 production in thymic epithelial cells by ATPgammaS and transforming growth factor-alpha

The arachidonic acid metabolites produced by thymic epithelial cells play a pivotal role in thymocyte development. We have discovered that ATP and TGF-alpha regulate the arachidonic acid metabolism in TEA3A1 rat thymic epithelial cells by activating phospholipase A2 enzymatic activity. Our present study demonstrates that ATP and its nonhydrolyzable analog ATPgammaS stimulate both prostaglandin E2 production and Ca2+ influx in TEA3A1 cells. The stimulation of prostaglandin E2 production and Ca2+ influx by ATP is inhibited by pertussis toxin treatment, indicating that ATP mediates its effect by binding to a G-protein-coupled purinergic receptor. Treatment of cells with ATPgammaS and transforming growth factor-alpha results in a synergistic activation of phospholipase A2 and stimulation of prostaglandin E2 production. Results from experiments using an inhibitor of receptor-mediated Ca2+ influx indicate that the synergistic stimulation of prostaglandin E2 production by ATPgammaS and transforming growth factor-alpha requires ATPgammaS-mediated Ca2+ influx. The inhibitor of tyrosine kinase genistein also blocked both ATPgammaS- and ATPgammaS plus transforming growth factor-alpha-mediated stimulation of prostaglandin E2 production, indicating that the activation of phospholipase A2 may involve a protein tyrosine phosphorylation step.

Cell interactions with collagen matrices in vivo and in vitro depend on phosphatidylinositol 3-kinase and free cytoplasmic calcium

We have investigated the role of phosphatidylinositol 3-kinase (PI3-kinase) in cellular interactions with collagenous matrices. Platelet-derived growth factor-BB (PDGF-BB) elicited a mobilization of intracellular Ca2+ in pig aortic endothelial (PAE) cells transfected with wild type PDGF beta-receptor. This response was greatly reduced in PAE cells transfected with PDGF beta-receptors mutated at positions Y740 and Y751 to prevent PI3-kinase binding. The experimental drug 1D-myo-inositol 1,2,6-trisphosphate (alpha-trinositol) induced a rapid increase and subsequent oscillations of the cytoplasmic Ca2+ concentration in cultured fibroblasts. This response was not due to an effect of alpha-trinositol on inositol 1,4,5-trisphosphate (IP3) receptors. alpha-Trinositol did not influence PDGF-BB elicited chemotaxis through collagen-coated membranes of PAE cells transfected with the wild-type PDGF beta-receptor, but restored PDGF-BB elicited chemotaxis of PAE cells transfected with the PI3-kinase binding-site mutated PDGF beta-receptor. Collagen gel contraction has been suggested to serve as a model for cellular control of interstitial fluid pressure (PIF) in dermis. The PI3-kinase inhibitors wortmannin (50 nM) and LY294002 (5 microM) inhibited the stimulation of fibroblast-mediated collagen gel contraction by 0.4 nM PDGF-BB. Injection of wortmannin in rat paw skin induced a lowering of PIF, and this effect was abolished in animals pre-treated with alpha-trinositol. Pretreatment of rats with alpha-trinositol abolished the decrease in PIF induced by injecting monoclonal anti-rat alpha 2 beta 1 integrin IgG in rat paw skin. Taken together our data indicate that cell-collagen interactions in vivo and in vitro depend on PI3-kinase, and that this dependence can be bypassed by a drug eliciting intracellular Ca2+ mobilization.

Differential characterization of binding sites for adenine and uridine nucleotides in membranes from rat lung as possible tools for studying P2 receptors in lung

Nucleotide receptors (P2 receptors) are involved in stimulating Cl- secretion in airway epithelia. These receptors may play a key role in development of new therapeutic strategies in the treatment of cystic fibrosis. However, the diversity of nucleotide binding sites in lung tissue has not yet been clarified. Here we studied the characteristics of various nucleotide binding sites in rat lung membranes by equilibrium binding analysis of several P2 receptor specific ligands. Displacement studies revealed a recognition site for adenosine 5'-O-(1-thiotriphosphate) ([35S]ATPalphaS; Kd 243 nM). From this site the ligand is readily displaced by adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), a typical agonist for P2Y1 receptors and also by alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-MeATP), a typical agonist for P2X receptors. [3H]alpha,beta-MeATP labelled specific binding sites (Kd 56 nM) in rat lung membranes. Analysis of binding of [3H]UTP to lung membranes revealed a high-affinity binding site (Kd 44 nM). Membrane-bound [3H]UTP was not displaced even by high concentrations of ATP, indicating no common binding site for UTP and ATP. Furthermore, specific binding of P-1,P-4-di(adenosine 5')tetraphosphate ([3H]Ap4A; Kd 91 nM) was found in lung membranes. Thus, we demonstrate at least four distinct types of nucleotide binding sites in lung membranes: Two have characteristics comparable to P2X and P2Y1 receptors, while two further sites still have to be identified, one binding Ap4A and the other binding UTP very specifically.

Impairment of ATP-induced Ca2+ -signalling in human thyroid cancer cells

Extracellular nucleotides like ATP that activate the Ca2+ -phosphatidylinositol (PI) signalling pathway have been suggested to participate in the regulation of normal human thyroid function. We examined, whether P2y-purinergic receptors are expressed on human thyroid cancer cells and whether post-receptor Ca2+ signalling is altered by malignant transformation. Extracellular ATP caused a biphasic increase in cytosolic free Ca2+ ([Ca2+]i) in normal human thyrocytes and in human follicular (FTC) and papillary (PTC) thyroid carcinoma cells. In FTC and PTC cell lines the dose-response curves for ATP-induced changes in [Ca2+]i were shifted to the right when compared with normal thyrocytes, whereas in undifferentiated thyroid carcinoma (UTC) cells even high concentrations of ATP (500 microM) failed to stimulate a rise in [Ca2+]i. By contrast, ATP stimulated inositol 1,4,5-trisphosphate (IP3) formation and capacitative Ca2+ entry was operational as judged by thapsigargin in normal thyrocytes and all thyroid cancer cells. Thus, P2y-purinergic receptors are expressed on thyroid tumor cells independent of degree of differentiation. In UTC cells, however, impairment in the Ca2+ -phosphatidylinositol (PI) signalling cascade occurs distal to the formation of IP3 and proximal to the activation of capacitative Ca2+ entry. Disturbed ATP-induced Ca2+ -signalling and alterations in the Ca2+ -PI signalling cascade may contribute to decreased expression or loss of specific thyroid functions in thyroid cancer cells.

Uridine nucleotide receptors and their ligands: structural, physiological, and pathophysiological aspects, with special emphasis on the nervous system

This review presents data on metabotropic uridine nucleotide receptor subtypes (P2UR) activated by UTP, sometimes also by UDP and/or ATP. Some chemical details of receptor subtypes and ligand interactions are described. Ligand-activated P2UR subtypes may couple to different second messengers, yet little is known about the nature of the coupling G-proteins. Data evaluating UTP as a physiological ligand include UTP origin, release and metabolism and illuminate especially roles for P2UR in the nervous system. No evidence shows UTP as a synaptic transmitter; sympathetic neurons may, however, carry P2UR allowing UTP-stimulation of norepinephrine release. UTP and derivatives act as therapeutic agents in several diseases involving mutated genes of transepithelial conductance regulators, including cystic fibrosis. This focuses interest to the synthesis of new compounds. Further, therapeutically used pyrimidine and pyrimidine analogues are suspected to have CNS-pathological effects. The presently scarce information in these areas strongly underlines the need for and importance of intense research on the suspected pyrimidine derivative triggered pathology as well as on the role of P2UR receptors in physiology and pathophysiology.

Calcium-mobilizing purine receptors on the surface of mammalian articular chondrocytes

If we are to fully understand mechanisms of cartilage homeostasis, it is essential that we know the full catalogue of receptors present on the surface of a chondrocyte and the pathways regulated by ligands that bind to these receptors. In this study, we describe chondrocyte responses to adenosine 5'-triphosphate and related molecules. Adenosine 5'-triphosphate stimulated a statistically significant, dose-dependent, transient rise in the concentration of calcium ions in Fura 2-loaded, differentiated, primary chondrocytes. The increase occurred in the absence of extracellular calcium, indicating a mobilization from intracellular stores. The increase in concentration of cytoplasmic calcium ions induced by adenosine 5'-triphosphate was mimicked by uridine 5'-triphosphate but not by 2-methylthioadenosine 5'-triphosphate, cytidine 5'-triphosphate, or adenosine. Heterologous desensitization experiments demonstrated that chondrocytes showed no subsequent response to uridine 5'-triphosphate after initial stimulation with adenosine 5'-triphosphate nor did they respond to adenosine 5'-triphosphate in inverse conditions, thereby indicating competition for the same receptor site. Together, these results are consistent with the presence of a P2U receptor on the cell surface of chondrocytes. Purine-induced calcium mobilization in passaged chondrocytes showed the same pharmacological profile with respect to agonist sensitivity, but responses were of greater magnitude than responses in primary differentiated chondrocytes, suggesting upregulation of the receptor with time in culture. Adenosine 5'-triphosphate and uridine 5'-triphosphate (1-100 microM) did not alter cartilage matrix synthesis as measured by rate of incorporation of [35S]sulfate into glycosaminoglycan by cartilage explants or primary chondrocytes. Matrix degradation, measured by release of glycosaminoglycan from cartilage explants, was also unaltered by adenosine 5'-triphosphate or uridine 5'-triphosphate (1-100 microM). Production of prostaglandin E2 was upregulated by incubation with either adenosine 5'-triphosphate or uridine 5'-triphosphate. These data demonstrate the presence of a functional P2U-like purine receptor on the surface of primary articular chondrocytes and support the hypothesis that altered concentrations of extracellular purines may influence chondrocyte metabolism.