Carbachol stimulates transactivation of epidermal growth factor receptor and mitogen-activated protein kinase in T84 cells. Implications for carbachol-stimulated chloride secretion

Long-term functional regeneration of radiation-damaged salivary glands through delivery of a neurogenic hydrogel

Salivary gland acinar cells are severely depleted after radiotherapy for head and neck cancer, leading to loss of saliva and extensive oro-digestive complications. With no regenerative therapies available, organ dysfunction is irreversible. Here, using the adult murine system, we demonstrate that radiation-damaged salivary glands can be functionally regenerated via sustained delivery of the neurogenic muscarinic receptor agonist cevimeline. We show that endogenous gland repair coincides with increased nerve activity and acinar cell division that is limited to the first week after radiation, with extensive acinar cell degeneration, dysfunction, and cholinergic denervation occurring thereafter. However, we found that mimicking cholinergic muscarinic input via sustained local delivery of a cevimeline-alginate hydrogel was sufficient to regenerate innervated acini and retain physiological saliva secretion at nonirradiated levels over the long term (>3 months). Thus, we reveal a previously unknown regenerative approach for restoring epithelial organ structure and function that has extensive implications for human patients.

Genome-wide identification and expression analysis of mitogen-activated protein kinase (MAPK) genes in response to salinity stress in channel catfish (Ictalurus punctatus)

The mitogen-activated protein kinase (MAPK) gene family has been systematically described in several fish species, but less so in channel catfish (Ictalurus punctatus), which is an important global aquaculture species. In this study, 16 MAPK genes were identified in the channel catfish genome and classified into three subfamilies based on phylogenetic analysis, including six extracellular signal regulated kinase (ERK) genes, six p38-MAPK genes and four C-Jun N-terminal kinase (JNK) genes. All MAPK genes were distributed unevenly across 10 chromosomes, of which three (IpMAPK8, IpMAPK12 and IpMAPK14) underwent teleost-specific whole genome duplication during evolution. Gene expression profiles in channel catfish during salinity stress were analysed using transcriptome sequencing and qRT-PCR (quantitative reverse transcription PCR). Results from reads per kilobase million (RPKM) analysis showed IpMAPK13, IpMAPK14a and IpMAPK14b genes were differentially expressed when compared with other genes between treatment and control groups. Furthermore, three of these genes were validated by qRT-PCR, of which IpMAPK14a expression levels were significantly upregulated in treatment groups (high and low salinity) when compared with the control group, with the highest expression levels in the low salinity group (P < 0.05). Therefore, IpMAPK14a may have important response roles to salinity stress in channel catfish.

Intestinal secretory mechanisms and diarrhea

One of the primary functions of the intestinal epithelium is to transport fluid and electrolytes to and from the luminal contents. Under normal circumstances, absorptive and secretory processes are tightly regulated such that absorption predominates, thereby enabling conservation of the large volumes of water that pass through the intestine each day. However, in conditions of secretory diarrhea, this balance becomes dysregulated, so that fluid secretion, driven primarily by Cl- secretion, overwhelms absorptive capacity, leading to increased loss of water in the stool. Secretory diarrheas are common and include those induced by pathogenic bacteria and viruses, allergens, and disruptions to bile acid homeostasis, or as a side effect of many drugs. Here, we review the cellular and molecular mechanisms by which Cl- and fluid secretion in the intestine are regulated, how these mechanisms become dysregulated in conditions of secretory diarrhea, currently available and emerging therapeutic approaches, and how new strategies to exploit intestinal secretory mechanisms are successfully being used in the treatment of constipation.

Potentiation of calcium-activated chloride secretion and barrier dysfunction may underlie EGF receptor tyrosine kinase inhibitor-induced diarrhea

Epidermal growth factor receptor tyrosine kinase inhibitors (EGFr TKIs) are first‐line therapies for various cancers, and cause dose‐limiting severe diarrhea in many patients. We hypothesized that diarrhea caused by EGFr TKIs might reflect actions on epithelial transport, barrier function, or both, which we tested using cell cultures including murine and human enteroid‐derived monolayers (EDMs), analyzed using electrophysiological and other relevant methods. EGFr TKIs (such as afatinib, erlotinib, and osimertinib) reversed the acute inhibitory effect of EGF on chloride secretion induced by carbachol (CCh) across T84 human colonic epithelial cells, which correlated with the diarrhea‐inducing effect of each agent clinically. EGFr TKIs also reduced transepithelial electrical resistance (TEER), whereas co‐treatment with CCh delayed the decrease in TEER compared with that of cells co‐treated with EGF. Furthermore, afatinib and erlotinib prevented EGF‐ or CCh‐induced EGFr phosphorylation. EGFr TKIs also suppressed phosphorylation of extracellular signal‐regulated kinase (Erk)1/2 in response to EGF, whereas they had weaker effects on CCh‐induced Erk1/2 phosphorylation. In human EDMs, EGF potentiated ion transport induced by CCh, whereas afatinib reversed this effect. The ability of EGFr TKIs to reverse the effects of EGF on calcium‐dependent chloride secretion could contribute to the diarrheal side effects of these agents, and their disruption of epithelial barrier dysfunction is likely also pathophysiologically significant. CCh‐activated Erk1/2 phosphorylation was relatively insensitive to EGFr TKIs and delayed the deleterious effects of EGFr TKIs on barrier function. These findings confirm and extend those of other authors, and may be relevant to designing strategies to overcome the diarrheal side effects of EGFr TKIs.

Epidermal growth factor receptor inhibitor-induced diarrhea: clinical incidence, toxicological mechanism, and management

The epidermal growth factor receptor (EGFR) family is a class of receptor tyrosine kinase playing a central role in carcinogenesis and cancer progression. The members of this family, particularly EGFR and human epidermal growth factor receptor 2 (HER2), are the most extensively studied drug targets for malignancy. Today, numerous tyrosine kinase inhibitors targeting EGFR family have been developed to combat non-small-cell lung cancer and breast cancer. However, severe gastrointestinal (GI) toxicity leading to dose reduction and treatment discontinuation hampers the therapeutic outcome of EGFR inhibitors. Diarrhea is one of the most frequent GI side effects, especially when it comes to second-generation EGFR inhibitors. Enterocytes apoptosis and increased inflammation accompany with many oral EGFR inhibitors. Loperamide and budesonide are the first-line treatment to manage such adverse effects. However, current prophylaxis and management are all empirical interventions to relieve the symptom. They do not specifically target the toxicological mechanism of EGFR inhibitors. Hereby, those anti-diarrhea agents do not work well when used in cancer patients experiencing EGFR inhibitor-induced diarrhea. On the other hand, the toxicological mechanism of EGFR inhibitor-induced diarrhea is poorly understood. Thus, determining the mechanism behind such diarrhea is urgently in need for developing genuinely effective anti-diarrhea agents. This review aims to call attention to EGFR inhibitor-induced diarrhea, a highly occurring and devastating cancer drug toxicity.

Intestinal TMEM16A control luminal chloride secretion in a NHERF1 dependent manner

TMEM16A (Transmembrane protein 16A or Anoctamin1) is a calcium-activated chloride channel.

(CaCC),that exerts critical roles in epithelial secretion. However, its localization, function, and regulation in intestinal chloride (Cl⁻) secretion remain obscure. Here, we show that TMEM16A protein abundance correlates with Cl⁻ secretion in different regions of native intestine activated by the Ca²⁺-elevating muscarinic agonist carbachol (CCH). Basal, as well as both cAMP- and CCH-stimulated Isc, was largely reduced in Ano1 ± mouse intestine. We found CCH was not able to increase Isc in the presence of apical to serosal Cl⁻ gradient, strongly supporting TMEM16A as primarily a luminal Cl⁻ channel. Immunostaining demonstrated apical localization of TMEM16A where it colocalized with NHERF1 in mouse colonic tissue. Cellular depletion of NHERF1 in human colonic T84 cells caused a significant reduction of both cAMP- and CCH-stimulated Isc. Immunoprecipitation experiments revealed that NHERF1 forms a complex with TMEM16A through a PDZ-based interaction. We conclude that TMEM16A is a luminal Cl⁻ channel in the intestine that functionally interacts with CFTR via PDZ-based interaction of NHERF1 for efficient and specific cholinergic stimulation of intestinal Cl⁻ secretion.

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TMEM16A express apically and operate Cl⁻ secretion in mouse intestinal tissue.

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TMEM16A potentially interacts with NHERF1 via its C-terminal PDZ binding motif.

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TMEM16A-NHERF1 complex is requisite for cAMP and Ca²⁺ mediated apical Cl⁻ secretion.

Cholinergic-induced anion secretion in murine jejunal enteroids involves synergy between muscarinic and nicotinic pathways

Acetylcholine induces robust electrogenic anion secretion in mammalian intestine and it has long been hypothesized that it mediates the epithelial response through the M3 and, to a lesser extent, the M1 muscarinic receptors in the mouse. However, nicotinic receptors have recently been identified in intestinal enterocytes by quantitative real-time (qRT)-PCR/RNAseq, although any direct influence on intestinal transport has not been identified. We tested the hypothesis that cholinergic-induced anion secretion in the intestine is a result of both muscarinic and nicotinic pathways that are intrinsic to the intestinal epithelia. We developed a method to generate mouse jejunal enteroid monolayers which were used to measure active electrogenic anion secretion by the Ussing chamber/voltage-clamp technique. Here, we show that the cholinergic agonist carbachol (CCh) and the muscarinic agonist bethanechol (BCh) stimulate short-lived, concentration-dependent anion secretion in the epithelial cell-only enteroid monolayers. The muscarinic antagonist atropine completely inhibited CCh- and BCh-induced secretion, while the nicotinic antagonist hexamethonium reduced the CCh response by ~45%. While nicotine alone did not alter anion secretion, it increased the BCh-induced increase in short-circuit current in a concentration-dependent manner; this synergy was prevented by pretreatment with hexamethonium. In addition to being sensitive to hexamethonium, monolayers express both classes of cholinergic receptor by qRT-PCR, including 13 of 16 nicotinic receptor subunits. Our findings indicate that an interaction between muscarinic and nicotinic agonists synergistically stimulates anion secretion in mouse jejunal epithelial cells and identify a role for epithelial nicotinic receptors in anion secretion.

Intestinal epithelial potassium channels and CFTR chloride channels activated in ErbB tyrosine kinase inhibitor diarrhea

Diarrhea is a major side effect of ErbB receptor tyrosine kinase inhibitors (TKIs) in cancer chemotherapy. Here, we show that the primary mechanism of ErbB TKI diarrhea is activation of basolateral membrane potassium (K+) channels and apical membrane chloride (Cl-) channels in intestinal epithelia and demonstrate the efficacy of channel blockers in a rat model of TKI diarrhea. Short-circuit current in colonic epithelial cells showed that the TKIs gefitinib, lapatinib, and afatinib do not affect basal secretion but amplify carbachol-stimulated secretion by 2- to 3-fold. Mechanistic studies with the second-generation TKI afatinib showed that the amplifying effect on Cl- secretion was Ca2+ and cAMP independent, was blocked by CF transmembrane conductance regulator (CFTR) and K+ channel inhibitors, and involved EGFR binding and ERK signaling. Afatinib-amplified activation of basolateral K+ and apical Cl- channels was demonstrated by selective membrane permeabilization, ion substitution, and channel inhibitors. Rats that were administered afatinib orally at 60 mg/kg/day developed diarrhea with increased stool water from approximately 60% to greater than 80%, which was reduced by up to 75% by the K+ channel inhibitors clotrimazole or senicapoc or the CFTR inhibitor (R)-BPO-27. These results indicate a mechanism for TKI diarrhea involving K+ and Cl- channel activation and support the therapeutic efficacy of channel inhibitors.

Autoantibody Signaling in Pemphigus Vulgaris: Development of an Integrated Model

Pemphigus vulgaris (PV) is an autoimmune skin blistering disease effecting both cutaneous and mucosal epithelia. Blister formation in PV is known to result from the binding of autoantibodies (autoAbs) to keratinocyte antigens. The primary antigenic targets of pathogenic autoAbs are known to be desmoglein 3, and to a lesser extent, desmoglein 1, cadherin family proteins that partially comprise the desmosome, a protein structure responsible for maintaining cell adhesion, although additional autoAbs, whose role in blister formation is still unclear, are also known to be present in PV patients. Nevertheless, there remain large gaps in knowledge concerning the precise mechanisms through which autoAb binding induces blister formation. Consequently, the primary therapeutic interventions for PV focus on systemic immunosuppression, whose side effects represent a significant health risk to patients. In an effort to identify novel, disease-specific therapeutic targets, a multitude of studies attempting to elucidate the pathogenic mechanisms downstream of autoAb binding, have led to significant advancements in the understanding of autoAb-mediated blister formation. Despite this enhanced characterization of disease processes, a satisfactory explanation of autoAb-induced acantholysis still does not exist. Here, we carefully review the literature investigating the pathogenic disease mechanisms in PV and, taking into account the full scope of results from these studies, provide a novel, comprehensive theory of blister formation in PV.

Cytokine responses and epithelial function in the intestinal mucosa

Inflammatory diseases of mucosal organs are significantly influenced by the microenvironment in which they reside. Cytokines found within this microenvironment contribute significantly to endpoint functions of the mucosa. Studies dating back to the 1990s have revealed that epithelial cells are both a source as well as a target for numerous cytokines and that such signaling can substantially influence the outcome of mucosal disease, such as inflammatory bowel disease. Here, we will review literature regarding intestinal epithelial cells as sources and responders to cytokines found in the intestinal milieu. These studies highlight the dynamic nature of these pathways and lend insight into the complexity of treating mucosal inflammation.

Existence of muscarinic acetylcholine receptor (mAChR) and fibroblast growth factor receptor (FGFR) heteroreceptor complexes and their enhancement of neurite outgrowth in neural hippocampal cultures

BACKGROUND

Recently, it was demonstrated that G-protein-coupled receptors (GPCRs) can transactivate tyrosine kinase receptors in absence of their ligands. In this work, driven by the observation that mAChRs and fibroblast growth factor receptors (FGFRs) share signalling pathways and regulation of brain functions, it was decided to explore whether mAChRs activation may transactivate FGFRs and, if so, to characterize the related trophic effects in cultured hippocampal neurons.

METHODS

Oxotremorine-M transactivation of FGFRs and related trophic effects were tested in primary hippocampal neurons. Western blotting and in situ proximity ligation assay (PLA) were used to detect FGFR phosphorylation (pFGFR) levels and M₁R-FGFR1 heteroreceptor complexes, respectively.

RESULTS

Oxotremorine-M, a non-selective mAChRs agonist, was able to transactivate FGFR and this transactivation was blocked by Src inhibitors. Oxotremorine-M treatment produced a significant increase in the primary neurite outgrowth that was blocked by pre-treatment with the pFGFR inhibitor SU5402 and Src inhibitors. This trophic effect was almost similar to that induced by fibroblast growth factor-2 (FGF-2). By using atropine as nonselective mAChRs or pirenzepine as selective antagonist for M₁ receptor (M₁R) we could show that mAChRs are involved in modulating the pFGFRs. Using PLA, M₁R-FGFR1 heteroreceptor complexes were identified in the hippocampus and cerebral cortex.

CONCLUSION

The current findings, by showing functional mAChR-FGFR interactions, will contribute to advance the understanding of the mechanisms involved in the actions of cholinergic drugs on neuronal plasticity.

GENERAL SIGNIFICANT

Data may help to develop novel therapeutic strategies not only for neurodegenerative diseases but also for depression-induced atrophy of hippocampal neurons.

Endogenous and exogenous control of gastrointestinal epithelial function: building on the legacy of Bayliss and Starling

Transport of fluid and electrolytes in the intestine allows for appropriate adjustments in luminal fluidity while reclaiming water used in digesting and absorbing a meal, and is closely regulated. This article discusses various endogenous and exogenous mechanisms whereby transport is controlled in the gut, placing these in the context of the ideas about the neurohumoral control of alimentary physiology that were promulgated by William Bayliss and Ernest Starling. The article considers three themes. First, mechanisms that intrinsically regulate chloride secretion, centred on the epidermal growth factor receptor (EGFr), are discussed. These may be important in ensuring that excessive chloride secretion, with the accompanying loss of fluid, is not normally stimulated by intestinal distension as the meal passes through the gastrointestinal tract. Second, mechanisms whereby probiotic microorganisms can impart beneficial effects on the gut are described, with a focus on targets at the level of the epithelium. These findings imply that the commensal microbiota exert important influences on the epithelium in health and disease. Finally, mechanisms that lead to diarrhoea in patients infected with an invasive pathogen, Salmonella, are considered, based on recent studies in a novel mouse model. Diarrhoea is most likely attributable to reduced expression of absorptive transporters and may not require the influx of neutrophils that accompanies infection. Overall, the goal of the article is to highlight the many ways in which critical functions of the intestinal epithelium are regulated under physiological and pathophysiological conditions, and to suggest possible targets for new therapies for digestive disease states.

Acetylcholine inhibits tumor necrosis factor α activated endoplasmic reticulum apoptotic pathway via EGFR-PI3K signaling in cardiomyocytes

Previous findings have shown that acetylcholine (ACh) decreased hypoxia-induced tumor necrosis factor alpha (TNF α) production, thus protected against cardiomyocyte injury. However, whether and how ACh affects TNF α-induced endoplasmic reticulum (ER) stress and cell apoptosis remain poorly defined. This study was aimed at determining the effect of ACh in H9c2 cells after TNF α stimulation. Presence of ER stress was verified using the ER stress protein markers glucose regulatory protein 78 (GRP78) and C/EBP homologous protein (CHOP). Cell apoptosis was shown by caspase-3 activation and terminal deoxynucleotidyl transferase mediated dUTP-biotin nick end labeling. Exogenously administered ACh significantly decreased these TNF α-induced changes. Moreover, when the cells were exposed to nonspecific muscarinic receptor (M AChR) inhibitor atropine, methoctramine (M2 AChR inhibitor) or the epidermal growth factor receptor (EGFR) inhibitor AG1478, the cardioprotection elicited by ACh was diminished. Furthermore, the above effects were also blocked by M2 AChR or EGFR siRNA, indicating that EGFR transactivation by M2 AChR may be the major pathway responsible for the benefits of ACh. In addition, LY294002, a phosphatidylinositol-3-kinase (PI3K) inhibitor, displayed the similar trends as AG1478, suggesting that PI3K/Akt signaling may be the downstream of EGFR in ACh-elicited anti-apoptotic property. Together, these data indicate that EGFR-PI3K/Akt signaling is involved in M2 AChR-mediated ER apoptotic pathway suppression and the subsequent survival of H9c2 cardiomyocytes. We have identified a novel pathway underlying the cardioprotection afforded by ACh.

Cholinergic transactivation of the EGFR in HaCaT keratinocytes stimulates a flotillin-1 dependent MAPK-mediated transcriptional response

Acetylcholine and its receptors regulate numerous cellular processes in keratinocytes and other non-neuronal cells. Muscarinic acetylcholine receptors are capable of transactivating the epidermal growth factor receptor (EGFR) and, downstream thereof, the mitogen-activated protein kinase (MAPK) cascade, which in turn regulates transcription of genes involved in cell proliferation and migration. We here show that cholinergic stimulation of human HaCaT keratinocytes results in increased transcription of matrix metalloproteinase MMP-3 as well as several ligands of the epidermal growth factor family. Since both metalloproteinases and the said ligands are involved in the transactivation of the EGFR, this transcriptional upregulation may provide a positive feed-forward loop for EGFR/MAPK activation. We here also show that the cholinergic EGFR and MAPK activation and the upregulation of MMP-3 and EGF-like ligands are dependent on the expression of flotillin-1 which we have previously shown to be a regulator of MAPK signaling.

Muscarinic cholinoceptor-mediated activation of JNK negatively regulates intestinal secretion in mice

Regulation of intestinal secretion is important for body fluid homeostasis. We investigated the role of three MAP kinases (MAPKs) as negative regulators in muscarinic cholinoceptor (mAChR)-mediated intestinal secretion in mice. Electrophysiological analyses revealed that mAChR stimulation enhanced intestinal chloride secretion, which was further augmented by the inhibition of JNK but not by that of ERK or p38 with specific inhibitors SP600125, U0126 or SB203580, respectively. Immunoblot analyses in colonic mucosa showed that mAChR stimulation increased MAPKs phosphorylation that was suppressed by the specific inhibitor for each MAPK. This suggests that JNK is a major negative regulator in mAChR-induced intestinal secretion.

Effect of orexin A on apoptosis in BGC-823 gastric cancer cells via OX1R through the AKT signaling pathway

Orexins are a class of peptides involved in the regulation of food intake, energy homeostasis, the sleep‑wake cycle and gastrointestinal function. Recent studies have demonstrated that orexin A may influence apoptosis and proliferation in numerous types of cancer cells. However, the effect of orexin A on gastric cancer cells and its mechanisms of action remain elusive. In the present study, BGC‑823 gastric cancer cells were treated with orexin A (10‑10‑10‑6 M) in vitro and the expression levels of orexin receptor 1 (OX1R) protein in cells was then determined. The proliferation, viability and apoptosis of BGC‑823 cells were detected. In addition, BGC‑823 cells were treated with AKT inhibitor PF‑04691502 or OX1R‑specific antagonist SB334867 in combination with orexin A, in order to examine the activation of AKT and caspase‑3. The results showed that orexin A (10‑10‑10‑6 M) stimulated the OX1R protein expression in BGC‑823 cells, which improved the proliferation and viability of the cells as well as protected them from apoptosis. Phosphorylated AKT protein was significantly increased in BGC‑823 cells following treatment with orexin A. Moreover, 10‑8 M orexin A reduced the proapoptotic activity of caspase‑3 (by ≤30%). The OX1R antagonist SB334867 (10‑6 M) and AKT antagonist PF‑04691502 (10‑6 M), when used individually or in combination, abolished the effect of orexin A (10‑8 M) on BGC-823 cells. In conclusion, the results of the present study demonstrated that orexin A inhibited gastric cancer cell apoptosis via OX1R through the AKT signaling pathway.

Epidermal growth factor receptor transactivation is required for mitogen-activated protein kinase activation by muscarinic acetylcholine receptors in HaCaT keratinocytes

Non-neuronal acetylcholine plays a substantial role in the human skin by influencing adhesion, migration, proliferation and differentiation of keratinocytes. These processes are regulated by the Mitogen-Activated Protein (MAP) kinase cascade. Here we show that in HaCaT keratinocytes all five muscarinic receptor subtypes are expressed, but M₁ and M₃ are the subtypes involved in mitogenic signaling. Stimulation with the cholinergic agonist carbachol leads to activation of the MAP kinase extracellular signal regulated kinase, together with the protein kinase Akt. The activation is fully dependent on the transactivation of the epidermal growth factor receptor (EGFR), which even appears to be the sole pathway for the muscarinic receptors to facilitate MAP kinase activation in HaCaT cells. The transactivation pathway involves a triple-membrane-passing process, based on activation of matrix metalloproteases, and extracellular ligand release; whereas phosphatidylinositol 3-kinase, Src family kinases or protein kinase C do not appear to be involved in MAP kinase activation. Furthermore, phosphorylation, ubiquitination and endocytosis of the EGF receptor after cholinergic transactivation are different from that induced by a direct stimulation with EGF, suggesting that ligands other than EGF itself mediate the cholinergic transactivation.

A pathophysiologic role for epidermal growth factor receptor in pemphigus acantholysis

The pemphigus family of autoimmune bullous disorders is characterized by autoantibody binding to desmoglein 1 and/or 3 (dsg1/dsg3). In this study we show that EGF receptor (EGFR) is activated following pemphigus vulgaris (PV) IgG treatment of primary human keratinocytes and that EGFR activation is downstream of p38 mitogen-activated protein kinase (p38). Inhibition of EGFR blocked PV IgG-triggered dsg3 endocytosis, keratin intermediate filament retraction, and loss of cell-cell adhesion in vitro. Significantly, inhibiting EGFR prevented PV IgG-induced blister formation in the passive transfer mouse model of pemphigus. These data demonstrate cross-talk between dsg3 and EGFR, that this cross-talk is regulated by p38, and that EGFR is a potential therapeutic target for pemphigus. Small-molecule inhibitors and monoclonal antibodies directed against EGFR are currently used to treat several types of solid tumors. This study provides the experimental rationale for investigating the use of EGFR inhibitors in pemphigus.

G Protein-Coupled Receptors in cancer: biochemical interactions and drug design

G Protein-Coupled Receptors (GPCRs) share the same topology made of seven-transmembrane segments and represent the largest family of membrane receptors. Initially associated with signal transduction in differentiated cells, GPCRs and heterotrimeric G proteins were shown to behave as proto-oncogenes whose overexpression or activating mutations confer transforming properties. The first part of this review focuses on the link between biochemical interactions of a GPCR with other receptors, such as dimerization or multiprotein complexes, and their oncogenic properties. Alteration of these interactions or deregulation of transduction cascades can promote uncontrolled cell proliferation or cell transformation that leads to tumorigenicity and malignancy. The second part concerns the design of drugs specifically targeting these complex interactions and their promise in cancer therapy.

Muscarinic type 3 receptor induces cytoprotective signaling in salivary gland cells through epidermal growth factor receptor transactivation

Muscarinic type 3 receptor (M3R) plays a pivotal role in the induction of glandular fluid secretions. Although M3R is often the target of autoantibodies in Sjögren's syndrome (SjS), chemical agonists for M3R are clinically used to stimulate saliva secretion in patients with SjS. Aside from its activity in promoting glandular fluid secretion, however, it is unclear whether activation of M3R is related to other biological events in SjS. This study aimed to investigate the cytoprotective effect of chemical agonist-mediated M3R activation on apoptosis induced in human salivary gland (HSG) cells. Carbachol (CCh), a muscarinic receptor-specific agonist, abrogated tumor necrosis factor α/interferon γ-induced apoptosis through pathways involving caspase 3/7, but its cytoprotective effect was decreased by a M3R antagonist, a mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (ERK) inhibitor, a phosphatidylinositol 3-kinase/Akt inhibitor, or an epidermal growth factor receptor (EGFR) inhibitor. Ligation of M3R with CCh transactivated EGFR and phosphorylated ERK and Akt, the downstream targets of EGFR. Inhibition of intracellular calcium release or protein kinase C δ, both of which are involved in the cell signaling of M3R-mediated fluid secretion, did not affect CCh-induced ERK or Akt phosphorylation. CCh stimulated Src phosphorylation and binding to EGFR. A Src inhibitor attenuated the CCh/M3R-induced cytoprotective effect and EGFR transactivation cascades. Overall, these results indicated that CCh/M3R induced transactivation of EGFR through Src activation leading to ERK and Akt phosphorylation, which in turn suppressed caspase 3/7-mediated apoptotic signals in HSG cells. This study, for the first time, proposes that CCh-mediated M3R activation can promote not only fluid secretion but also survival of salivary gland cells in the inflammatory context of SjS.

Interferon-γ alters downstream signaling originating from epidermal growth factor receptor in intestinal epithelial cells: functional consequences for ion transport

The epidermal growth factor receptor (EGFr) regulates many cellular functions, such as proliferation, apoptosis, and ion transport. Our aim was to investigate whether long term treatment with interferon-γ (IFN-γ) modulates EGF activation of downstream signaling pathways in intestinal epithelial cells and if this contributes to dysregulation of epithelial ion transport in inflammation. Polarized monolayers of T(84) and HT29/cl.19A colonocytes were preincubated with IFN-γ prior to stimulation with EGF. Basolateral potassium transport was studied in Ussing chambers. We also studied inflamed colonic mucosae from C57BL/6 mice treated with dextran sulfate sodium or mdr1a knock-out mice and controls. IFN-γ increased intestinal epithelial EGFr expression without increasing its phosphorylation. Conversely, IFN-γ caused a significant decrease in EGF-stimulated phosphorylation of specific EGFr tyrosine residues and activation of ERK but not Akt-1. In IFNγ-pretreated cells, the inhibitory effect of EGF on carbachol-stimulated K(+) channel activity was lost. In inflamed colonic tissues, EGFr expression was significantly increased, whereas ERK phosphorylation was reduced. Thus, although it up-regulates EGFr expression, IFN-γ causes defective EGFr activation in colonic epithelial cells via reduced phosphorylation of specific EGFr tyrosine residues. This probably accounts for altered downstream signaling consequences. These observations were corroborated in the setting of colitis. IFN-γ also abrogates the ability of EGF to inhibit carbachol-stimulated basolateral K(+) currents. Our data suggest that, in the setting of inflammation, the biological effect of EGF, including the inhibitory effect of EGF on Ca(2+)-dependent ion transport, is altered, perhaps contributing to diarrheal and other symptoms in vivo.

Lysophosphatidic acid 5 receptor induces activation of Na(+)/H(+) exchanger 3 via apical epidermal growth factor receptor in intestinal epithelial cells

Na(+) absorption is a vital process present in all living organisms. We have reported previously that lysophosphatidic acid (LPA) acutely stimulates Na(+) and fluid absorption in human intestinal epithelial cells and mouse intestine by stimulation of Na(+)/H(+) exchanger 3 (NHE3) via LPA(5) receptor. In the current study, we investigated the mechanism of NHE3 activation by LPA(5) in Caco-2bbe cells. LPA(5)-dependent activation of NHE3 was blocked by mitogen-activated protein kinase kinase (MEK) inhibitor PD98059 and U0126, but not by phosphatidylinositol 3-kinase inhibitor LY294002 or phospholipase C-β inhibitor U73122. We found that LPA(5) transactivated the epidermal growth factor receptor (EGFR) and that inhibition of EGFR blocked LPA(5)-dependent activation of NHE3, suggesting an obligatory role of EGFR in the NHE3 regulation. Confocal immunofluorescence and surface biotinylation analyses showed that LPA(5) was located mostly in the apical membrane. EGFR, on the other hand, showed higher expression in the basolateral membrane. However, inhibition of apical EGFR, but not basolateral EGFR, abrogated LPA-induced regulation of MEK and NHE3, indicating that LPA(5) selectively activates apical EGFR. Furthermore, transactivation of EGFR independently activated the MEK-ERK pathway and proline-rich tyrosine kinase 2 (Pyk2). Similarly to MEK inhibition, knockdown of Pyk2 blocked activation of NHE3 by LPA. Furthermore, we showed that RhoA and Rho-associated kinase (ROCK) are involved in activation of Pyk2. Interestingly, LPA(5) did not directly activate RhoA but was required for transactivation of EGFR. Together, these results unveil a pivotal role of apical EGFR in NHE3 regulation by LPA and show that the RhoA-ROCK-Pyk2 and MEK-ERK pathways converge onto NHE3.

Estrogen inhibits chloride secretion caused by cholera and Escherichia coli enterotoxins in female rat distal colon

Excessive Cl(-) secretion is the driving force for secretory diarrhea. 17β-Estradiol has been shown to inhibit Cl(-) secretion in rat distal colon through a nongenomic pathway. We examined whether 17β-estradiol inhibits Cl(-) secretion in an animal model of secretory diarrhea and the downstream effectors involved. The effect of 17β-estradiol on cholera toxin and heat-stable enterotoxin induced Cl(-) secretion in rat colonic mucosal sheets was studied by current-voltage clamping. Selective permeabilization of apical or basolateral membranes with amphotericin B or nystatin was used to isolate basolateral K(+) channel and apical Cl(-) channel activity, respectively. 17β-Estradiol dose-dependently inhibited secretory responses to both toxins with IC(50) values of approximately 1nM. This effect was female-gender specific, with no inhibition observed in male tissues. 17β-Estradiol responses were insensitive to the pure anti-estrogen ICI 182,720. 17β-Estradiol exerted its effects downstream of enterotoxin-induced production of second messengers (cAMP and cGMP) but was dependent on PKCδ activation. In nystatin-permeabilized tissues, apical Cl(-) currents were unaffected by 17β-estradiol treatment while basolateral K(+) current was profoundly inhibited by the hormone. This current was sensitive to the specific KCNQ1 channel inhibitors chromanol 293B and HMR-1556. In conclusion, 17β-estradiol inhibits enterotoxin-induced Cl(-) secretion via a PKCδ-dependent mechanism involving inhibition of basolateral KCNQ1 channels. These data elucidate mechanisms of 17β-estradiol inhibition of Cl(-) secretion induced by enterotoxins in intestinal epithelia, which may be relevant for the treatment of diarrheal diseases.

Chronic regulation of colonic epithelial secretory function by activation of G protein-coupled receptors

BACKGROUND

Enteric neurotransmitters that act at G protein-coupled receptors (GPCRs) are well known to acutely promote epithelial Cl(-) and fluid secretion. Here we examined if acute GPCR activation might have more long-term consequences for epithelial secretory function.

METHODS

Cl(-) secretion was measured as changes in short-circuit current across voltage-clamped T(84) colonic epithelial cells. Protein expression was measured by western blotting and intracellular Ca(2+) levels by Fura-2 fluorescence.

KEY RESULTS

While acute (15 min) treatment of T(84) cells with a cholinergic G(q) PCR agonist, carbachol (CCh), rapidly stimulated Cl(-) secretion, subsequent CCh-induced responses were attenuated in a biphasic manner. The first phase was transient and resolved within 6 h but this was followed by a chronic phase of attenuated responsiveness that was sustained up to 48 h. CCh-pretreatment did not chronically alter responses to another G(q)PCR agonist, histamine, or to thapsigargin or forskolin which elevate intracellular Ca(2+) and cAMP, respectively. This chronically acting antisecretory mechanism is not shared by neurotransmitters that activate G(s)PCRs. Conditioned medium from CCh-pretreated cells mimicked its chronic antisecretory actions, suggesting involvement of an epithelial-derived soluble factor but further experimentation ruled out the involvement of epidermal growth factor receptor ligands. Acute CCh exposure did not chronically alter surface expression of muscarinic M(3) receptors but inhibited intracellular Ca(2+) mobilization upon subsequent agonist challenge.

CONCLUSIONS & INFERENCES

These data reveal a novel, chronically acting, antisecretory mechanism that downregulates epithelial secretory capacity upon repeated G(q)PCR agonist exposure. This mechanism involves release of a soluble factor that uncouples receptor activation from downstream prosecretory signals.

Crosstalk between VEGFR2 and muscarinic receptors regulates the mTOR pathway in serum starved SK-N-SH human neuroblastoma cells

Muscarinic acetylcholine receptors (mAchRs) are guanosine nucleotide-binding protein (G protein) coupled receptors that crosstalk with receptor tyrosine kinases (RTKs) to signal mitogenic pathways. In particular, mAchRs are known to couple with RTKs for several growth factors to activate the mammalian target of rapamycin (mTOR)/Akt pathway, a regulator of protein synthesis. The RTK for the vascular endothelial growth factor (VEGF), VEGFR2, can signal protein synthesis but whether it cooperates with mAchRs to mediate mTOR activation has not been demonstrated. Using serum starved SK-N-SH neuroblastoma cells, we show that the muscarinic receptor agonists carbachol and pilocarpine enhance the activation of the mTOR substrate p70 S6 Kinase (S6K) and its target ribosomal protein S6 (S6) in a VEGFR2 dependent manner. Treatments with carbachol increased VEGFR2 phosphorylation, suggesting that mAchRs stimulate VEGFR2 transactivation to enhance mTOR signaling. Inhibitor studies revealed that phosphatidylinositol 3 kinase resides upstream from S6K, S6 and Akt phosphorylation while protein kinase C (PKC) functions in an opposing fashion by positively regulating S6K and S6 phosphorylation and suppressing Akt activation. Treatments with the phosphatase inhibitors sodium orthovanadate and okadaic acid increase S6, Akt and to a lesser extent S6K phosphorylation, indicating that tyrosine and serine/threonine dephosphorylation also regulates their activity. However, okadaic acid elicited a far greater increase in phosphorylation, implicating phosphatase 2A as a critical determinant of their function. Finally, pilocarpine but not carbachol induced a time and dose dependent cell death that was associated with caspase activation and oxidative stress but independent of S6K and S6 activation through VEGFR2. Accordingly, our findings suggest that mAchRs crosstalk with VEGFR2 to enhance mTOR activity but signal divergent effects on survival through alternate mechanisms.

Apical and basolateral pools of proteinase-activated receptor-2 direct distinct signaling events in the intestinal epithelium

Studies suggest that there are two distinct pools of proteinase-activated receptor-2 (PAR₂) present in intestinal epithelial cells: an apical pool accessible from the lumen, and a basolateral pool accessible from the interstitial space and blood. Although introduction of PAR₂ agonists such as 2-furoyl-LIGRL-O-NH₂ (2fAP) to the intestinal lumen can activate PAR₂, the presence of accessible apical PAR₂ has not been definitively shown. Furthermore, some studies have suggested that basolateral PAR₂ responses in the intestinal epithelium are mediated indirectly by neuropeptides released from enteric nerve fibers, rather than by intestinal PAR₂ itself. Here we identified accessible pools of both apical and basolateral PAR₂ in cultured Caco2-BBe monolayers and in mouse ileum. Activation of basolateral PAR₂ transiently increased short-circuit current by activating electrogenic Cl⁻ secretion, promoted dephosphorylation of the actin filament-severing protein, cofilin, and activated the transcription factor, AP-1, whereas apical PAR₂ did not. In contrast, both pools of PAR₂ activated extracellular signal-regulated kinase 1/2 (ERK1/2) via temporally and mechanistically distinct pathways. Apical PAR₂ promoted a rapid, biphasic PLCβ/Ca²(+)/PKC-dependent ERK1/2 activation, resulting in nuclear localization, whereas basolateral PAR₂ promoted delayed ERK1/2 activation which was predominantly restricted to the cytosol, involving both PLCβ/Ca²(+) and β-arrestin-dependent pathways. These results suggest that the outcome of PAR₂ activation is dependent on the specific receptor pool that is activated, allowing for fine-tuning of the physiological responses to different agonists.

Loss of protein tyrosine phosphatase N2 potentiates epidermal growth factor suppression of intestinal epithelial chloride secretion

The Crohn's disease candidate gene, protein tyrosine phosphatase nonreceptor type 2 (PTPN2), has been shown to regulate epidermal growth factor (EGF)-induced phosphatidylinositol 3-kinase (PI3K) activation in fibroblasts. In intestinal epithelial cells (IECs), EGF-induced EGF receptor (EGFR) activation and recruitment of PI3K play a key role in regulating many cellular functions including Ca(2+)-dependent Cl(-) secretion. Moreover, EGFR also serves as a conduit for signaling by other non-growth factor receptor ligands such as the proinflammatory cytokine, IFN-γ. Here we investigated a possible role for PTPN2 in the regulation of EGFR signaling and Ca(2+)-dependent Cl(-) secretion in IECs. PTPN2 knockdown enhanced EGF-induced EGFR tyrosine phosphorylation in T(84) cells. In particular, PTPN2 knockdown promoted EGF-induced phosphorylation of EGFR residues Tyr-992 and Tyr-1068 and led subsequently to increased association of the catalytic PI3K subunit, p110, with EGFR and elevated phosphorylation of the downstream marker, Akt. As a functional consequence, loss of PTPN2 potentiated EGF-induced inhibition of carbachol-stimulated Ca(2+)-dependent Cl(-) secretion. In contrast, PTPN2 knockdown affected neither IFN-γ-induced EGFR transactivation nor EGF- or IFN-γ-induced phosphorylation of ERK1/2. In summary, our data establish a role for PTPN2 in the regulation of EGFR signaling in IECs in response to EGF but not IFN-γ. Knockdown of PTPN2 directs EGFR signaling toward increased PI3K activation and increased suppression of epithelial chloride secretory responses. Moreover, our findings suggest that PTPN2 dysfunction in IECs leads to altered control of intestinal epithelial functions regulated by EGFR.

Calcium mobilization triggered by the chemokine CXCL12 regulates migration in wounded intestinal epithelial monolayers

Restitution of intestinal epithelial barrier damage involves the coordinated remodeling of focal adhesions in actively migrating enterocytes. Defining the extracellular mediators and the intracellular signaling pathways regulating those dynamic processes is a key step in developing restitution-targeted therapies. Previously we have determined that activation of the chemokine receptor CXCR4 by the cognate ligand CXCL12 enhances intestinal epithelial restitution through reorganization of the actin cytoskeleton. The aim of these studies was to investigate the role of calcium effectors in CXCL12-mediated restitution. CXCL12 stimulated release of intracellular calcium in a dose-dependent manner. Inhibition of intracellular calcium flux impaired CXCL12-mediated migration of IEC-6 and CaCo2 cells. Pharmacological blockade and specific shRNA depletion of the phospholipase-C (PLCbeta3) isoform attenuated CXCL12-enhanced migration, linking receptor activation with intracellular calcium flux. Immunoblot analyses demonstrated CXCL12 activated the calcium-regulated focal adhesion protein proline-rich tyrosine kinase-2 (Pyk2) and the effector proteins paxillin and p130(Cas). Interruption of Pyk2 signaling potently blocked CXCL12-induced wound closure. CXCL12-stimulated epithelial cell migration was enhanced on laminin and abrogated by intracellular calcium chelation. These results suggest CXCL12 regulates restitution through calcium-activated Pyk2 localized to active focal adhesions. Calcium signaling pathways may therefore provide a novel avenue for enhancing barrier repair.

P2Y2 nucleotide receptors mediate metalloprotease-dependent phosphorylation of epidermal growth factor receptor and ErbB3 in human salivary gland cells

The G protein-coupled receptor P2Y(2) nucleotide receptor (P2Y(2)R) has been shown to be up-regulated in a variety of tissues in response to stress or injury. Recent studies have suggested that P2Y(2)Rs may play a role in immune responses, wound healing, and tissue regeneration via their ability to activate multiple signaling pathways, including activation of growth factor receptors. Here, we demonstrate that in human salivary gland (HSG) cells, activation of the P2Y(2)R by its agonist induces phosphorylation of ERK1/2 via two distinct mechanisms, a rapid, protein kinase C-dependent pathway and a slower and prolonged, epidermal growth factor receptor (EGFR)-dependent pathway. The EGFR-dependent stimulation of UTP-induced ERK1/2 phosphorylation in HSG cells is inhibited by the adamalysin inhibitor tumor necrosis factor-alpha protease inhibitor or by small interfering RNA that selectively silences ADAM10 and ADAM17 expression, suggesting that ADAM metalloproteases are required for P2Y(2)R-mediated activation of the EGFR. G protein-coupled receptors have been shown to promote proteolytic release of EGFR ligands; however, neutralizing antibodies to known ligands of the EGFR did not inhibit UTP-induced EGFR phosphorylation. Immunoprecipitation experiments indicated that UTP causes association of the EGFR with another member of the EGF receptor family, ErbB3. Furthermore, stimulation of HSG cells with UTP induced phosphorylation of ErbB3, and silencing of ErbB3 expression inhibited UTP-induced phosphorylation of both ErbB3 and EGFR. UTP-induced phosphorylation of ErbB3 and EGFR was also inhibited by silencing the expression of the ErbB3 ligand neuregulin 1 (NRG1). These results suggest that P2Y(2)R activation in salivary gland cells promotes the formation of EGFR/ErbB3 heterodimers and metalloprotease-dependent neuregulin 1 release, resulting in the activation of both EGFR and ErbB3.

JNK mitogen-activated protein kinase limits calcium-dependent chloride secretion across colonic epithelial cells

Neuroimmune agonists induce epithelial Cl(-) secretion through elevations in intracellular Ca2+ or cAMP. Previously, we demonstrated that epidermal growth factor receptor (EGFR) transactivation and subsequent ERK MAPK activation limits secretory responses to Ca2+-dependent, but not cAMP-dependent, agonists. Although JNK MAPKs are also expressed in epithelial cells, their role in regulating transport function is unknown. Here, we investigated the potential role for JNK in regulating Cl(-) secretion in T(84) colonic epithelial cells. Western blot analysis revealed that a prototypical Ca2+-dependent secretagogue, carbachol (CCh; 100 microM), induced phosphorylation of both the 46-kDa and 54-kDa isoforms of JNK. This effect was mimicked by thapsigargin (TG), which specifically elevates intracellular Ca2+, but not by forskolin (FSK; 10 microM), which elevates cAMP. CCh-induced JNK phosphorylation was attenuated by the EGFR inhibitor, tyrphostin-AG1478 (1 microM). Pretreatment of voltage-clamped T(84) cells with SP600125 (2 microM), a specific JNK inhibitor, potentiated secretory responses to both CCh and TG but not to FSK. The effects of SP600125 on CCh-induced secretion were not additive with those of the ERK inhibitor, PD98059. Finally, in apically permeabilized T(84) cell monolayers, SP600125 potentiated CCh-induced K+ conductances but not Na+/K+ATPase activity. These data demonstrate a novel role for JNK MAPK in regulating Ca2+ but not cAMP-dependent epithelial Cl(-) secretion. JNK activation is mediated by EGFR transactivation and exerts its antisecretory effects through inhibition of basolateral K+ channels. These data further our understanding of mechanisms regulating epithelial secretion and underscore the potential for exploitation of MAPK-dependent signaling in treatment of intestinal transport disorders.

Carbachol induces p70S6K1 activation through an ERK-dependent but Akt-independent pathway in human colonic epithelial cells

Stimulation of human colonic epithelial T84 cells with the muscarinic receptor agonist carbachol, a stable analog of acetylcholine, induced Akt, p70S6K1 and ERK activation. Treatment of T84 cells with the selective inhibitor of EGF receptor (EGFR) tyrosine kinase AG1478 abrogated Akt phosphorylation on Ser(473) induced by either carbachol or EGF, indicating that carbachol-induced Akt activation is mediated through EGFR transactivation. Surprisingly, AG1478 did not suppress p70S6K1 phosphorylation on Thr(389) in response to carbachol, indicating the G protein-coupled receptor (GPCR) stimulation induces p70S6K1 activation, at least in part, via an Akt-independent pathway. In contrast, treatment with the selective MEK inhibitor U0126 (but not with the inactive analog U0124) inhibited carbachol-induced p70S6K1 activation, indicating that the MEK/ERK/RSK pathway plays a critical role in p70S6K1 activation in GPCR-stimulated T84 cells. These findings imply that GPCR activation induces p70S6K1 via ERK rather than through the canonical PI 3-kinase/Akt/TSC/mTORC1 pathway in T84 colon carcinoma cells.

Substance P receptor in U373 MG human astrocytoma cells activates mitogen-activated protein kinases ERK1/2 through Src

Substance P (SP) acting through substance P receptor (SPR) increases the proliferation of glioblastoma cells. At the molecular level, stimulation of SPR in human U373 MG glioblastoma cells results in phosphorylation of mitogen-activated protein kinases ERK1/2. Examination of the underlying mechanism reveals that SPR mediates ERK1/2 phosphorylation in a calcium-dependent manner. Surprisingly, blockade of epidermal growth factor receptor (EGFR), which is transactivated by SPR, has a minimal effect on SPR-mediated ERK1/2 phosphorylation. However, SPR-mediated ERK1/2 phosphorylation is significantly reduced by the Src kinase inhibitor PP2. Interestingly, ERK1/2 in U373 MG cells is also activated by several other mitogenic G protein-coupled receptors (GPCRs) including alpha(1B)-adrenergic, M(3)-muscarinic, and H(1)-histaminergic in an Src-dependent manner. We conclude that c-Src is a mediator of SP-stimulated ERK1/2 phosphorylation in human U373 MG glioblastoma cells.

Physiological concentrations of bile acids down-regulate agonist induced secretion in colonic epithelial cells

In patients with bile acid malabsorption, high concentrations of bile acids enter the colon and stimulate Cl(-) and fluid secretion, thereby causing diarrhoea. However, deoxycholic acid (DCA), the predominant colonic bile acid, is normally present at lower concentrations where its role in regulating transport is unclear. Thus, the current study set out to investigate the effects of physiologically relevant DCA concentrations on colonic epithelial secretory function. Cl(-) secretion was measured as changes in short-circuit current across voltage-clamped T(84) cell monolayers. At high concentrations (0.5-1 mM), DCA acutely stimulated Cl(-) secretion but this effect was associated with cell injury, as evidenced by decreased transepithelial resistance (TER) and increased lactate dehydrogenase (LDH) release. In contrast, chronic (24 hrs) exposure to lower DCA concentrations (10-200 microM) inhibited responses to Ca(2+) and cAMP-dependent secretagogues without altering TER, LDH release, or secretagogue-induced increases in intracellular second messengers. Other bile acids - taurodeoxycholic acid, chenodeoxycholic acid and cholic acid - had similar antisecretory effects. DCA (50 microM) rapidly stimulated phosphorylation of the epidermal growth factor receptor (EGFr) and both ERK and p38 MAPKs (mitogen-activated protein kinases). The EGFr inhibitor, AG1478, and the protein synthesis inhibitor, cycloheximide, reversed the antisecretory effects of DCA, while the MAPK inhibitors, PD98059 and SB203580, did not. In summary, our studies suggest that, in contrast to its acute prosecretory effects at pathophysiological concentrations, lower, physiologically relevant, levels of DCA chronically down-regulate colonic epithelial secretory function. On the basis of these data, we propose a novel role for bile acids as physiological regulators of colonic secretory capacity.

Regulation of intestinal electroneutral sodium absorption and the brush border Na+/H+ exchanger by intracellular calcium

The intestinal electroneutral Na(+) absorptive processes account for most small intestinal Na(+) absorption in the period between meals and also for the great majority of the increase in ileal Na(+) absorption that occurs postprandially. In most diarrheal diseases, there is inhibition of neutral NaCl absorption. Elevated levels of intracellular calcium ([Ca(2+)](i)) are known to inhibit NaCl absorption and involve multiple components of the Ca(2+) signaling pathway. The BB Na(+)/H(+) exchanger NHE3 accounts for most of the recognized digestive changes in neutral NaCl absorption, as well as most of the changes in Na(+) absorption that occur in diarrheal diseases. Previous studies have examined several aspects of Ca(2+) regulation of NHE3 activity. These include phosphorylation, protein trafficking, and multiprotein complex formation. In addition, recent studies have demonstrated the role of the NHERF family of PDZ domain-containing proteins in Ca(2+) regulation of NHE3 activity, thereby adding a new level of complexity to understanding Ca(2+)-dependent inhibition of Na(+) absorption. In this article, we will review the current understanding of (1) Ca(2+) signaling events in intestinal epithelial cells; (2) Ca(2+) regulation of intestinal electroneutral sodium absorption, which includes NHE3; and (3) the role of the NHERF family of PDZ domain-containing proteins in Ca(2+) regulation of NHE3 activity. We will also present new data on using advanced imaging showing rapid BB NHE3 endocytosis in response to elevated [Ca(2+)](i).

Decoding epithelial signals: critical role for the epidermal growth factor receptor in controlling intestinal transport function

The intestinal epithelium engages in bidirectional transport of fluid and electrolytes to subserve the physiological processes of nutrient digestion and absorption, as well as the elimination of wastes, without excessive losses of bodily fluids that would lead to dehydration. The overall processes of intestinal ion transport, which in turn drive the secretion or absorption of water, are accordingly carefully regulated. We and others have identified the epidermal growth factor receptor (EGFr) as a critical regulator of mammalian intestinal ion transport. In this article, we focus on our studies that have uncovered the intricate signalling mechanisms downstream of EGFr that regulate both chloride secretion and sodium absorption by colonocytes. Emphasis will be placed on the EGFr-associated regulatory pathways that dictate the precise outcome to receptor activation in response to signals that may seem, on their face, to be quite similar if not identical. The concepts to be discussed underlie the ability of the intestinal epithelium to utilize a limited set of signalling effectors to produce a variety of outcomes suitable for varying physiological and pathophysiological demands. Our findings therefore are relevant not only to basic biological principles, but also may ultimately point to new therapeutic targets in intestinal diseases where ion transport is abnormal.

Induction of Na+/K+/2Cl- cotransporter expression mediates chronic potentiation of intestinal epithelial Cl- secretion by EGF

Alterations in EGF receptor (EGFR) signaling occur in intestinal disorders associated with dysregulated epithelial transport. In the present study, we investigated a role for the EGFR in the chronic regulation of intestinal epithelial secretory function. Epithelial Cl(-) secretion was measured as changes in short-circuit current (Isc) across voltage-clamped monolayers of T84 cells in Ussing chambers. Acute treatment of T84 cells with EGF (100 ng/ml, 15 min) chronically enhanced Isc responses to a broad range of secretagogues. This effect was apparent within 3 h, maximal by 6 h, and sustained for 24 h after treatment with EGF. The Na+/K+/2Cl(-) cotransporter (NKCC1) inhibitor bumetanide (100 microM) abolished the effect of EGF, indicating increased responses are due to potentiated Cl(-) secretion. Neither basal nor agonist-stimulated levels of intracellular Ca2+ or PKA activity were altered by EGF, implying that the effects of the growth factor are not due to chronic alterations in levels of second messengers. EGF increased the expression of NKCC1 with a time course similar to that of its effects on Cl(-) secretion. This effect of EGF was maximal after 6 h, at which time NKCC1 expression in EGF-treated cells was 199.9 +/- 21.9% of that in control cells (n = 21, P < 0.005). EGF-induced NKCC1 expression was abolished by actinomycin D, and RT-PCR analysis demonstrated EGF increased expression of NKCC1 mRNA. These data increase our understanding of mechanisms regulating intestinal fluid and electrolyte transport and reveal a novel role for the EGFR in the chronic regulation of epithelial secretory capacity through upregulation of NKCC1 expression.

Distinct pathways of ERK activation by the muscarinic agonists pilocarpine and carbachol in a human salivary cell line

Cholinergic-muscarinic receptor agonists are used to alleviate mouth dryness, although the cellular signals mediating the actions of these agents on salivary glands have not been identified. We examined the activation of ERK1/2 by two muscarinic agonists, pilocarpine and carbachol, in a human salivary cell line (HSY). Immunoblot analysis revealed that both agonists induced transient activation of ERK1/2. Whereas pilocarpine induced phosphorylation of the epidermal growth factor (EGF) receptor, carbachol did not. Moreover, ERK activation by pilocarpine, but not carbachol, was abolished by the EGF receptor inhibitor AG-1478. Downregulation of PKC by prolonged treatment of cells with the phorbol ester PMA diminished carbachol-induced ERK phosphorylation but had no effect on pilocarpine responsiveness. Depletion of intracellular Ca2+ ([Ca2+]i by EGTA did not affect ERK activation by either agent. In contrast to carbachol, pilocarpine did not elicit [Ca2+]i mobilization in HSY cells. Treatment of cells with the muscarinic receptor subtype 3 (M3) antagonist N-(3-chloropropyl)-4-piperidnyl diphenylacetate decreased ERK responsiveness to both agents, whereas the subtype 1 (M1) antagonist pirenzepine reduced only the carbachol response. Stimulation of ERKs by pilocarpine was also decreased by M3, but not M1, receptor small interfering RNA. The Src inhibitor PP2 blocked pilocarpine-induced ERK activation and EGF receptor phosphorylation, without affecting ERK activation by carbachol. Our results demonstrate that the actions of pilocarpine and carbachol in salivary cells are mediated through two distinct signaling mechanisms-pilocarpine acting via M3 receptors and Src-dependent transactivation of EGF receptors, and carbachol via M1/M3 receptors and PKC-converging on the ERK pathway.

New ways of thinking about (and teaching about) intestinal epithelial function

This article summarizes a presentation made at the Teaching Refresher Course of the American Physiological Society, which was held at the Experimental Biology meeting in 2007. The intestinal epithelium has important ion transport and barrier functions that contribute pivotally to normal physiological functioning of the intestine and other body systems. These functions are also frequently the target of dysfunction that, in turn, results in specific digestive disease states, such as diarrheal illnesses. Three emerging concepts are discussed with respect to ion transport: the complex interplay of intracellular signals that both activate and inhibit chloride secretion; the role of multiprotein complexes in the regulation of ion transport, taking sodium/hydrogen exchange as an example; and acute and chronic regulation of colonic sodium absorption, involving both sodium channel internalization and de novo synthesis of new channels. Similarly, recently obtained information about the molecular components of epithelial tight junctions and the ways in which tight junctions are regulated both in health and disease are discussed to exemplify ways to teach about intestinal barrier properties. Finally, both genetically determined intestinal diseases and those arising as a result of infections and/or inflammation are described, and these can be used as the means to enhance the basic and clinical relevance of teaching about intestinal epithelial physiology as well as the impact that the understanding of such physiology has had on associated therapeutics. The article also indicates, where relevant, how different approaches may be used effectively to teach related concepts to graduate versus medical/professional student audiences.

Anion secretion evoked by Pasteurella multocida toxin across rat colon

Stimulation of muscarinic receptors is known to have a biphasic effect on colonic Cl(-) secretion: a short-lasting activation, which is followed by a long-lasting inhibition. In order to find out, which role Gq proteins play in both processes, Pasteurella multocida toxin was used, a known activator of G alpha q. This toxin (1.5 microg/ml) had a dual action on short-circuit current (Isc) across rat distal colon: it stimulated transiently Isc and subsequently down-regulated the Isc evoked by Ca2+-dependent secretagogues such as acetylcholine or ATP. The inactive mutant (P. multocida toxin C1165S), which does not stimulate G alpha q), was ineffective. Cl(-) dependence and sensitivity against bumetanide, a blocker of the Na+-K+-2Cl(-) cotransporter, confirmed that the increase in Isc evoked by the toxin represented Cl(-) secretion. The effect of P. multocida toxin was suppressed by YM-254890 (10(-7) M), a blocker of G alpha q. Experiments with apically permeabilized tissues revealed that the secretory response to P. multocida toxin was concomitant with an increase in basolateral K+ conductance as it is observed for other agonists inducing Ca2+-dependent anion secretion. Consequently, these results suggest that Gq proteins are not only involved in the activation of secretion, e.g. after stimulation of muscarinic or purinergic receptors, but also play a central role in the long-term down-regulation of intestinal secretion after activation of these types of receptors.

Hydrogen peroxide inhibits Ca2+-dependent chloride secretion across colonic epithelial cells via distinct kinase signaling pathways and ion transport proteins

Reactive oxygen species (ROS) are key mediators in a number of inflammatory conditions, including inflammatory bowel disease (IBD). ROS, including hydrogen peroxide (H(2)O(2)), modulate intestinal epithelial ion transport and are believed to contribute to IBD-associated diarrhea. Intestinal crypt fluid secretion, driven by electrogenic Cl(-) secretion, hydrates and sterilizes the crypt, thus reducing bacterial adherence. Here, we show that pathophysiological concentrations of H(2)O(2) inhibit Ca(2+)-dependent Cl(-) secretion across T(84) colonic epithelial cells by elevating cytosolic Ca(2+), which contributes to activation of two distinct signaling pathways. One involves recruitment of the Ca(2+)-responsive kinases, Src and Pyk-2, as well as extracellular signal-regulated kinase (ERK). A separate pathway recruits p38 MAP kinase and phosphoinositide 3-kinase (PI3-K) signaling. The ion transport response to Ca(2+)-dependent stimuli is mediated in part by K(+) efflux through basolateral K(+) channels and Cl(-) uptake by the Na(+)-K(+)-2Cl(-) cotransporter, NKCC1. We demonstrate that H(2)O(2) inhibits Ca(2+)-dependent basolateral K(+) efflux and also inhibits NKCC1 activity independently of inhibitory effects on apical Cl(-) conductance. Thus, we have demonstrated that H(2)O(2) inhibits Ca(2+)-dependent Cl(-) secretion through multiple negative regulatory signaling pathways and inhibition of specific ion transporters. These findings increase our understanding of mechanisms by which inflammation disturbs intestinal epithelial function and contributes to intestinal pathophysiology.

Dynamic and differential regulation of NKCC1 by calcium and cAMP in the native human colonic epithelium

The capacity of the intestine to secrete fluid is dependent on the basolateral Na(+)-K(+)-2Cl(-) co-transporter (NKCC1). Given that cAMP and Ca(2+) signals promote sustained and transient episodes of fluid secretion, respectively, this study investigated the differential regulation of functional NKCC1 membrane expression in the native human colonic epithelium. Tissue sections and colonic crypts were obtained from sigmoid rectal biopsy tissue samples. Cellular location of NKCC1, Na(+)-K(+)-ATPase, M3 muscarinic acetylcholine receptor (M(3)AChR) and lysosomes was examined by immunolabelling techniques. NKCC1 activity (i.e. bumetanide-sensitive uptake), intracellular Ca(2+) and cell volume were assessed by 2',7'-bis(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF), Fura-2 and differential interference contrast/calcein imaging. Unstimulated NKCC1 was expressed on basolateral membranes and exhibited a topological expression gradient, predominant at the crypt base. Cholinergic Ca(2+) signals initiated at the crypt base and spread along the crypt axis. In response, NKCC1 underwent a Ca(2+)-dependent 4 h cycle of recruitment to basolateral membranes, activation, internalization, degradation and re-expression. Internalization was prevented by the epidermal growth factor receptor kinase inhibitor tyrphostin-AG1478, and re-expression was prohibited by the protein synthesis inhibitor cylcoheximide; the lysosome inhibitor chloroquine promoted accumulation of NKCC1 vesicles. NKCC1 internalization and re-expression were accompanied by secretory volume decrease and bumetanide-sensitive regulatory volume increase, respectively. In contrast, forskolin (i.e. cAMP elevation)-stimulated NKCC1 activity was sustained, and membrane expression and cell volume remained constant. Co-stimulation with forskolin and acetylcholine promoted dramatic recruitment of NKCC1 to basolateral membranes and prolonged the cycle of co-transporter activation, internalization and re-expression. In conclusion, persistent NKCC1 activation by cAMP is constrained by a Ca(2+)-dependent cycle of co-transporter internalization, degradation and re-expression; this is a novel mechanism to limit intestinal fluid loss.

EGF receptor transactivation and PI3-kinase mediate stimulation of ERK by alpha(2A)-adrenoreceptor in intestinal epithelial cells: a role in wound healing

Intestinal cells express alpha(2A)-adrenoreceptors that stimulate sodium and peptide absorption and promote cell proliferation. Involved mechanisms are poorly understood and are not fully related to inhibition of cAMP production. Previous study using a clone of CaCo2 cells expressing the human alpha(2A)-adrenoreceptor (CaCo2-3B) showed that alpha(2)-adrenoreceptor agonists cause extracellular signal-regulated kinase (ERK) phosphorylation. Present work examines the signaling pathway triggering ERK activation and investigates the consequence of alpha(2A)-adrenoreceptor stimulation on cell migration. Treatment of CaCo2-3B with the alpha(2)-adrenoreceptor agonist 5-bromo-6-(2-imidazolin-2-ylamino) quinoxaline (UK14304) induces not only ERK, but also Akt phosphorylation. Both effects are strongly attenuated by inhibition or desensitization of epidermal growth factor (EGF) receptor, matrix metalloproteinase (MMP) blockade, heparin-binding-EGF neutralization or phosphatidylinositol 3-kinase (PI3-kinase) inhibitors. Conditioned medium from UK14304-treated CaCo2-3B stimulates ERK in parental CaCo2 by a mechanism sensitive to EGF receptor and PI3-kinase inhibitors. Exposure of CaCo2-3B to UK14304 accelerates the wound healing. This effect is abolished by heparin-binding-EGF neutralization but not by mitomycin C, indicating that it results probably from increased cell spreading and/or migration. In conclusion, alpha(2A)-adrenoreceptor activates ERK and Akt in intestinal cells by a common pathway which depends on PI3-kinase activation and results from EGF receptor transactivation, via an autocrine/paracrine pathway implying MMP activation and heparin-binding-EGF shedding. Therefore, alpha(2A)-adrenoreceptor could have a positive role in intestinal regeneration in vivo.

Transactivation of the epidermal growth factor receptor by formylpeptide receptor exacerbates the malignant behavior of human glioblastoma cells

The G protein-coupled formylpeptide receptor (FPR), which mediates leukocyte migration in response to bacterial and host-derived chemotactic peptides, promotes the chemotaxis, survival, and tumorigenesis of highly malignant human glioblastoma cells. Because glioblastoma cells may also express other receptors for growth signals, such as the epidermal growth factor (EGF) receptor (EGFR), we investigated the role of EGFR in the signaling cascade of FPR and how two receptors cross-talk to exacerbate tumor growth. We found that N-formyl-methionyl-leucyl-phenylalanine, an FPR agonist peptide, rapidly induced EGFR phosphorylation at tyrosine residue (Tyr) 992, but not residues 846, 1068, or 1173, in glioblastoma cells, whereas all these residues were phosphorylated after only EGF treatment. The FPR agonist-induced EGFR phosphorylation in tumor cells was dependent on the presence of FPR as well as Galphai proteins, and was controlled by Src tyrosine kinase. The transactivation of EGFR contributes to the biological function of FPR in glioblastoma cells because inhibition of EGFR phosphorylation significantly reduced FPR agonist-induced tumor cell chemotaxis and proliferation. Furthermore, depletion of both FPR and EGFR by short interference RNA abolished the tumorigenesis of the glioblastoma cells. Our study indicates that the glioblastoma-promoting activity of FPR is mediated in part by transactivation of EGFR and the cross-talk between two receptors exacerbates the malignant phenotype of tumor cells. Thus, targeting both receptors may yield antiglioblastoma agents superior to those targeting one of them.

Macrophage-inflammatory protein-3alpha mediates epidermal growth factor receptor transactivation and ERK1/2 MAPK signaling in Caco-2 colonic epithelial cells via metalloproteinase-dependent release of amphiregulin

Previously, we reported that normal colonocytes produce the memory CD4(+) T cell-directed chemokine MIP-3alpha, and that epithelial MIP-3alpha levels are elevated in inflammatory bowel disease. Interestingly, the unique receptor for MIP-3alpha, CCR6, is expressed by a variety of cell types including colonocytes, suggesting that MIP-3alpha may regulate additional biological activities in the intestine. The aim of this study was to determine whether MIP-3alpha can induce intestinal epithelial cell proliferation and to examine the signaling mechanisms that mediate this response. We show that nonstimulated Caco-2 and HT-29 colonic epithelial cells express CCR6, and that stimulation of Caco-2 cells by MIP-3alpha can dose dependently increase cell proliferation as well as activate the epidermal growth factor receptor (EGFR) and ERK1/2 MAPK. MIP-3alpha-mediated ERK1/2 activation in Caco-2 cells appeared to require metalloproteinase-dependent release of the endogenous EGFR ligand amphiregulin and transactivation of the EGFR. Moreover, blockade of amphiregulin bioactivity using a neutralizing polyclonal Ab significantly reduced MIP-3alpha-mediated, but not EGF-mediated Caco-2 cell proliferation. Taken together, our findings indicate that MIP-3alpha can regulate mitogenic signaling in colonic epithelial cells and thus may serve an important homeostatic function in the intestine by regulating tissue turnover and maintenance of the epithelium, in addition to its role in regulating leukocyte recruitment.

Consequences of Direct Versus Indirect Activation of Epidermal Growth Factor Receptor in Intestinal Epithelial Cells Are Dictated by Protein-tyrosine Phosphatase 1B

The epidermal growth factor receptor (EGFR) is an integral regulator of many cellular functions. EGFR also acts as a central conduit for extracellular signals involving direct activation of the receptor by EGFR ligands or indirect activation by G protein-coupled receptor (GPCR)-stimulated transactivation of the EGFR. We have previously shown that EGFR negatively regulates epithelial chloride secretion as a result of transforming growth factor-alpha-mediated EGFR transactivation in response to muscarinic GPCR activation. Here we show that direct activation of the EGFR by EGFR ligands produces a different pattern of EGFR tyrosine phosphorylation and downstream phosphatidylinositol 3-kinase recruitment than GPCR-stimulated transactivation of the EGFR occurring via paracrine EGFR ligand release. Moreover, we demonstrate that this differential signaling and its consequences depend on protein-tyrosine phosphatase 1B activity. Thus protein-tyrosine phosphatase 1B governs differential recruitment of signaling pathways involved in EGFR regulation of epithelial ion transport. Our findings furthermore establish how divergent signaling outcomes can arise from the activation of a single receptor.

Preconditioning-mimetics bradykinin and DADLE activate PI3-kinase through divergent pathways

We previously reported that pharmacological preconditioning of rabbit hearts with acetylcholine involves activation of phosphatidylinositol 3-kinase (PI3-K) through transactivation of the epidermal growth factor receptor (EGFR). Transactivation is thought to be initiated by cleavage of membrane-bound pro-heparin-binding EGF-like growth factor (HB-EGF) by a membrane metalloproteinase thus releasing HB-EGF which binds to the EGFR. This pathway leads to redox signaling with the generation of reactive oxygen species (ROS) by mitochondria. We tested whether preconditioning's physiological triggers, bradykinin and opioid, also signal through the EGFR. Both bradykinin and the synthetic delta-opioid agonist DADLE increased ROS production in isolated cardiomyocytes by approximately 50%. DADLE's effect was abrogated by either metalloproteinase inhibitor III (MPI) or the diphtheria toxin mutant CRM-197 which blocks heparin-binding EGF shedding indicating that DADLE signals through EGFR transactivation. MPI also blocked DADLE's infarct-sparing effect in whole hearts. Additionally, blocking Src kinase (a component of the EGFR's signaling complex) with PP2 or PI3-K with wortmannin blocked DADLE's effect on cardiomyocyte ROS production and PP2 blocked DADLE's salvage of ischemic myocardium. Finally, DADLE increased phosphorylation of Akt and extracellular signal-regulated protein kinases (ERK) 1/2 in left ventricular myocardium, and this increase was blocked by the EGFR antagonist AG1478. On the other hand, neither MPI nor CRM-197 prevented bradykinin from increasing ROS production, and MPI did not affect bradykinin's infarct-sparing effect in intact hearts. Conversely, both PP2 and wortmannin blocked bradykinin's effect on ROS generation and also aborted bradykinin's cardioprotective effect in intact hearts. While bradykinin also increased phosphorylation of Akt and ERK in myocardium, that increase was not affected by AG1478. Hence bradykinin, unlike acetylcholine or opioid, does not transactivate EGFR, although all 3 agonists do signal through Src and PI3-K.