Purinergic responses in HT29 colonic epithelial cells are mediated by G protein alpha -subunits

The zinc sensing receptor, ZnR/GPR39, triggers metabotropic calcium signalling in colonocytes and regulates occludin recovery in experimental colitis

Impaired epithelial barrier function is a hallmark of inflammatory bowel diseases, such as colitis, contributing to diarrhoea and perpetuating inflammation. We show that the zinc sensing receptor, ZnR/GPR39, triggers intracellular Ca(2+) signalling in colonocytes thereby inducing occludin expression. Moreover, ZnR/GPR39 is essential for epithelial barrier recovery in the dextran sodium sulfate (DSS) ulcerative colitis model. Loss of ZnR/GPR39 results in increased susceptibility to DSS-induced inflammation, owing to low expression of the tight junction protein occludin and impaired epithelial barrier. Recovery of wild-type (WT) mice from the DSS insult was faster than that of ZnR/GPR39 knockout (KO) mice. Enhanced recovery of the epithelial layer and increased crypt regeneration were observed in WT mice compared with ZnR/GPR39 KO, suggesting that ZnR/GPR39 is promoting epithelial barrier integrity following DSS insult. Indeed, cell proliferation and apical expression of occludin, following the DSS-induced epithelial erosion, were increased in WT tissue but not in ZnR/GPR39 KO tissue. Importantly, survival following DSS treatment was higher in WT mice compared with ZnR/GPR39 KO mice. Our results support a direct role for ZnR/GPR39 in promoting epithelial renewal and barrier function following DSS treatment, thereby affecting the severity of the disease. We suggest ZnR/GPR39 as a novel therapeutic target that can improve epithelial barrier function in colitis.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Purinergic signalling and cancer

Receptors for extracellular nucleotides are widely expressed by mammalian cells. They mediate a large array of responses ranging from growth stimulation to apoptosis, from chemotaxis to cell differentiation and from nociception to cytokine release, as well as neurotransmission. Pharma industry is involved in the development and clinical testing of drugs selectively targeting the different P1 nucleoside and P2 nucleotide receptor subtypes. As described in detail in the present review, P2 receptors are expressed by all tumours, in some cases to a very high level. Activation or inhibition of selected P2 receptor subtypes brings about cancer cell death or growth inhibition. The field has been largely neglected by current research in oncology, yet the evidence presented in this review, most of which is based on in vitro studies, although with a limited amount from in vivo experiments and human studies, warrants further efforts to explore the therapeutic potential of purinoceptor targeting in cancer.

Purinergic signalling in the gastrointestinal tract and related organs in health and disease

Purinergic signalling plays major roles in the physiology and pathophysiology of digestive organs. Adenosine 5'-triphosphate (ATP), together with nitric oxide and vasoactive intestinal peptide, is a cotransmitter in non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. P2X and P2Y receptors are widely expressed in myenteric and submucous enteric plexuses and participate in sympathetic transmission and neuromodulation involved in enteric reflex activities, as well as influencing gastric and intestinal epithelial secretion and vascular activities. Involvement of purinergic signalling has been identified in a variety of diseases, including inflammatory bowel disease, ischaemia, diabetes and cancer. Purinergic mechanosensory transduction forms the basis of enteric nociception, where ATP released from mucosal epithelial cells by distension activates nociceptive subepithelial primary afferent sensory fibres expressing P2X3 receptors to send messages to the pain centres in the central nervous system via interneurons in the spinal cord. Purinergic signalling is also involved in salivary gland and bile duct secretion.

NHERF-1 regulation of EGF and neurotensin signalling in HT-29 epithelial cells

Neurotensin receptors (NT-R) and the epidermal growth factor receptors (EGF-R) are commonly overexpressed in many epithelial origin tumours. In addition to their role as mitogenic mediators through specific cell signalling, recent studies indicate that the activity/expression of scaffold proteins responsible for the assembly and coordination of the signalling complexes may also have central roles in epithelial transformation. In particular, the "epithelial" PSD-95/Dlg/Zo-1 (PDZ) scaffold/adapter protein, Na(+)/H(+) exchanger regulatory factor isoform one (NHERF-1), has been identified as a potential regulator of cellular transformation. NHERF-1 is a known regulator of EGF-R function and plays numerous roles in G-protein-coupled receptor signalling. Because of the synergistic signalling between these two potent mitogens, we investigated a potential role for NHERF-1 in the molecular mechanism linking the aberrant proliferative phenotype initiated by some G-Protein-coupled receptor activators in the colon adenocarcinoma HT-29 cell line. Knockdown (80%) of endogenous NHERF-1 leads to significant reduction in proliferation rate; an effect that could not be recovered by exogenous application of either NT or EGF. Inhibition of the EGF-R with AG1487 also inhibited proliferation and this effect could not be recovered with NT. Knockdown of NHERF-1 significantly altered the expression of the EGF-R, and almost completely abolished the NT-mediated increases in intracellular free Ca(2+). Knockdown of NHERF-1 also attenuated UTP-mediated purinergic Ca(2+) signalling. Taken together, these data suggest that NHERF-1 plays a more central role in cell proliferation by modulating Gq-mediated signalling pathways.

Negative regulation of Ca(2+) influx during P2Y(2) purinergic receptor activation is mediated by Gbetagamma-subunits

We have previously reported that P2Y(2) purinoceptors and muscarinic M(3) receptors trigger Ca(2+) responses in HT-29 cells that differ in their timecourse, the Ca(2+) response to P2Y(2) receptor activation being marked by a more rapid decline of intracellular Ca(2+) concentration ([Ca(2+)](i)) after the peak response and that this rapid decline of [Ca(2+)](i) was slowed in cells expressing heterologous beta-adrenergic receptor kinase (betaARK). In the present study, we demonstrate that, during P2Y(2) receptor activation, betaARK expression increases the rate of Gd(3+)-sensitive Mn(2+) influx, a measure of the rate of store-operated Ca(2+) entry from the extracellular space, during P2Y(2) activation and that this effect of betaARK is mimicked by exogenous alpha-subunits of G(q), G(11) and G(i2). The effect of betaARK on the rate of Mn(2+) influx is thus attributable to its ability to scavenge G protein betagamma-subunits released during activation of P2Y(2) receptor. We further find that the effect of betaARK on the rate of Mn(2+) influx during P2Y(2) receptor activation can be overcome by arachidonic acid. In addition, the UTP-induced Mn(2+) influx rate was significantly increased by inhibitors of phospholipase A(2) (PLA(2)) and an siRNA directed against PLA(2)beta, but not by an siRNA directed against PLA(2)alpha or by inhibitors of arachidonic acid metabolism. These findings provide evidence for the existence of a P2Y(2) receptor-activated signalling system that acts in parallel with depletion of intracellular Ca(2+) stores to inhibit Ca(2+) influx across the cell membrane. This signalling process is mediated via Gbetagamma and involves PLA(2)beta and arachidonic acid.

Purinergic regulation of the epithelial Na+ channel

1. The epithelial Na(+) channel (ENaC) is a major conductive pathway that transports Na(+) across the apical membrane of the distal nephron, the respiratory tract, the distal colon and the ducts of exocrine glands. The ENaC is regulated by hormonal and humoral factors, including extracellular nucleotides that are available from the epithelial cells themselves. 2. Extracellular nucleotides, via the P2Y2 receptors (P2Y2Rs) at the basolateral and apical membrane of the epithelia, trigger signalling systems that inhibit the activity of the ENaC and activate Ca(2+) -dependent Cl(-) secretion. 3. Recent data from our laboratory suggest that stimulation of the P2Y2Rs at the basolateral membrane inhibits ENaC activity by a signalling mechanism that involves G beta gamma subunits freed from a pertussis toxin (PTX)-sensitive G-protein and phospholipase C (PLC) beta 4. A similar signalling mechanism is also partially responsible for inhibition of the ENaC during activation of apical P2Y2Rs. 4. Stimulation of apical P2Y2Rs also activates an additional signalling mechanism that inhibits the ENaC and involves the activated Galpha subunit of a PTX-insensitive G-protein and activation of an unidentified PLC. The effect of this PTX-insensitive system requires the activity of the basolateral Na(+)/K(+)/2Cl(-) cotransporter.

Expression of P2Y2 purinoceptors in MCG 101 murine sarcoma cells, and HT-29 human colon carcinoma cells

We investigated how agonists at purinoceptors may affect tumour cell metabolism. This was investigated in vitro in tumour cell lines by microphysiometry, which method monitors extracellular acidification rate (ECAR), on-line. The cell lines investigated were the murine sarcoma, MCG 101, and the human colon cancer, HT-29. In MCG 101, adenosine-5'-triphosphate (ATP) or uridine-5'-triphosphate (UTP) caused a concentration-dependent increase in ECAR, most likely due to the ligation of P2Y(2) receptors, which response was blocked by suramin. In HT-29, ATP or UTP elicited a concentration-dependent, biphasic change in ECAR (increase/decrease). The pharmacological analysis suggests the involvement of P2Y(2) receptors, although other P2 receptor subtypes cannot be entirely excluded. This biphasic response to UTP or ATP was resistant to suramin. The expression of P2Y(2) receptors was demonstrated in both cell lines by immunocytochemistry and Western blot. The current study, thus, shows the functional and morphological expression of a purinoceptor subtype with partly different effects on metabolism in two different tumour cell lines.

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.

Studying heterotrimeric G-protein-linked signal transduction using replication-deficient adenoviruses

Plasma membrane-spanning G-protein-linked receptors transduce approximately 60% of all extracellular stimuli in higher animals. Many G-protein-linked receptor pathways are yet to be elucidated, with the receptor, G-protein or effector system as yet unidentified. In addition, many fundamental issues pertaining to G-protein signalling remain unresolved, such as the factors governing the specificity of G-protein receptor coupling and the control of signal amplitude in response to G-protein activation. In order to address some of these issues, the use of replication-deficient adenoviruses as gene transfer vectors for investigations of G-protein signalling has been developed, facilitating dissection of G-protein-linked signal transduction pathways in an extensive range of cultured cells, as well as in vivo. The present review focuses on the versatility and utility of adenoviruses for the investigation of signalling by heterotrimeric G-proteins and explores some of the recent advances in adenoviral technology as they relate to the study of signal transduction.