Cell confluence-induced activation of signal transducer and activator of transcription-3 (Stat3) triggers epithelial dome formation via augmentation of sodium hydrogen exchanger-3 (NHE3) expression

Roads to Stat3 Paved with Cadherins

The engagement of cadherins, cell-to-cell adhesion proteins, triggers a dramatic increase in the levels and activity of the Rac/Cdc42 GTPases, through the inhibition of proteasomal degradation. This leads to an increase in transcription and secretion of IL6 family cytokines, activation of their common receptor, gp130, in an autocrine manner and phosphorylation of the signal transducer and activator of transcription-3 (Stat3) on tyrosine-705 by the Jak kinases. Stat3 subsequently dimerizes, migrates to the nucleus and activates the transcription of genes involved in cell division and survival. The Src oncogene also increases Rac levels, leading to secretion of IL6 family cytokines and gp130 activation, which triggers a Stat3-ptyr705 increase. Interestingly, at the same time, Src downregulates cadherins in a quantitative manner, while cadherins are required to preserve gp130 levels for IL6 family signalling. Therefore, a fine balance between Src527F/Rac/IL6 and Src527F/cadherin/gp130 levels is in existence, which is required for Stat3 activation. This further demonstrates the important role of cadherins in the activation of Stat3, through preservation of gp130 function. Conversely, the absence of cadherin engagement correlates with low Stat3 activity: In sparsely growing cells, both gp130 and Stat3-ptyr705 levels are very low, despite the fact that cSrc is active in the FAK (focal adhesion kinase)/cSrc complex, which further indicates that the engagement of cadherins is important for Stat3 activation, not just their presence. Furthermore, the caveolin-1 protein downregulates Stat3 through binding and sequestration of cadherins to the scaffolding domain of caveolin-1. We hypothesize that the cadherins/Rac/gp130 axis may be a conserved pathway to Stat3 activation in a number of systems. This fact could have significant implications in Stat3 biology, as well as in drug testing and development.

Chemical and molecular bases of dome formation in human colorectal cancer cells mediated by sulphur compounds from Cucumis melo var. conomon

Colorectal cancer was the third most commonly diagnosed malignant tumor and the fourth leading cause of cancer deaths worldwide in 2012. A human colorectal cancer cell line, RCM‐1, was established from a colon cancer tissue diagnosed as a well‐differentiated rectum adenocarcinoma. RCM‐1 cells spontaneously form ‘domes’ (formerly designated ‘ducts’) resembling villiform structures. Two sulphur‐containing compounds from Cucumis melo var. conomon (Katsura‐uri, or Japanese pickling melon), referred to as 3‐methylthiopropionic acid ethyl ester (MTPE) and methylthioacetic acid ethyl ester (MTAE), can induce the differentiation of the unorganized cell mass of an RCM‐1 human colorectal cancer cell culture into a dome. However, the underlying molecular mechanisms of such dome formation have not been previously reported. Here, we performed a structure–activity relationship analysis, which indicated that methylthioacetic acid (MTA) was the lowest molecular weight compound with the most potent dome‐inducing activity among 37 MTPE and MTAE analogues, and the methylthio group was essential for this activity. According to our microarray analysis, MTA resulted in down‐regulation of 537 genes and up‐regulation of 117 genes. Furthermore, MTA caused down‐regulation of many genes involved in cell‐cycle control, with the cyclin E2 (CCNE2) and cell division cycle 25A (CDC25A) genes being the most significantly reduced. Pharmacological analysis showed that the administration of two cell‐cycle inhibitors for inactivating CDC25A phosphatase (NSC95397) and the cyclin E2/cyclin‐dependent kinase 2 complex (purvalanol A) increased the dome number independently of MTA. Altogether, our results indicate that MTA is the minimum unit required to induce dome formation, with the down‐regulation of CDC25A and possibly CCNE2 being important steps in this process.

Effect of luminal flow on doming of mpkCCD cells in a 3D perfusable kidney cortical collecting duct model

The cortical collecting duct (CCD) of the mammalian kidney plays a major role in the maintenance of total body electrolyte, acid/base, and fluid homeostasis by tubular reabsorption and excretion. The mammalian CCD is heterogeneous, composed of Na⁺-absorbing principal cells (PCs) and acid-base-transporting intercalated cells (ICs). Perturbations in luminal flow rate alter hydrodynamic forces to which these cells in the cylindrical tubules are exposed. However, most studies of tubular ion transport have been performed in cell monolayers grown on or epithelial sheets affixed to a flat support, since analysis of transepithelial transport in native tubules by in vitro microperfusion requires considerable expertise. Here, we report on the generation and characterization of an in vitro, perfusable three-dimensional kidney CCD model (3D CCD), in which immortalized mouse PC-like mpkCCD cells are seeded within a cylindrical channel embedded within an engineered extracellular matrix and subjected to luminal fluid flow. We find that a tight epithelial barrier composed of differentiated and polarized PCs forms within 1 wk. Immunofluorescence microscopy reveals the apical epithelial Na⁺ channel ENaC and basolateral Na⁺/K⁺-ATPase. On cessation of luminal flow, benzamil-inhibitable cell doming is observed within these 3D CCDs consistent with the presence of ENaC-mediated Na⁺ absorption. Our 3D CCD provides a geometrically and microphysiologically relevant platform for studying the development and physiology of renal tubule segments.

Transformation Foci in IDH1-mutated Gliomas Show STAT3 Phosphorylation and Downregulate the Metabolic Enzyme ETNPPL, a Negative Regulator of Glioma Growth

IDH1-mutated gliomas are slow-growing brain tumours which progress into high-grade gliomas. The early molecular events causing this progression are ill-defined. Previous studies revealed that 20% of these tumours already have transformation foci. These foci offer opportunities to better understand malignant progression. We used immunohistochemistry and high throughput RNA profiling to characterize foci cells. These have higher pSTAT3 staining revealing activation of JAK/STAT signaling. They downregulate RNAs involved in Wnt signaling (DAAM2, SFRP2), EGFR signaling (MLC1), cytoskeleton and cell-cell communication (EZR, GJA1). In addition, foci cells show reduced levels of RNA coding for Ethanolamine-Phosphate Phospho-Lyase (ETNPPL/AGXT2L1), a lipid metabolism enzyme. ETNPPL is involved in the catabolism of phosphoethanolamine implicated in membrane synthesis. We detected ETNPPL protein in glioma cells as well as in astrocytes in the human brain. Its nuclear localization suggests additional roles for this enzyme. ETNPPL expression is inversely correlated to glioma grade and we found no ETNPPL protein in glioblastomas. Overexpression of ETNPPL reduces the growth of glioma stem cells indicating that this enzyme opposes gliomagenesis. Collectively, these results suggest that a combined alteration in membrane lipid metabolism and STAT3 pathway promotes IDH1-mutated glioma malignant progression.

Desmoglein-3 acts as a pro-survival protein by suppressing reactive oxygen species and doming whilst augmenting the tight junctions in MDCK cells

Kidney disease prevalence increases with age, with a common feature of the disease being defects in the epithelial tight junctions. Emerging evidence suggests that the desmosomal adhesion protein Desmoglein-3 (Dsg3) functions beyond the desmosomal adhesion and plays a role in regulating the fundamental pathways that govern cell fate decisions in response to environmental chemical and mechanical stresses. In this study, we explored the role of Dsg3 on dome formation, reactive oxygen species (ROS) production and transepithelial electrical resistance (TER) in MDCK cells, a kidney epithelial cell model widely used to study cell differentiation and tight junction formation and integrity. We show that overexpression of Dsg3 constrained nuclear ROS production and cellular doming in confluent cell cultures and these features coincided with augmented TER and enhanced tight junction integrity. Conversely, cells expressing dominant-negative Dsg3ΔC mutants exhibited heightened ROS production and accelerated doming, accompanied by increased apoptosis, as well as cell proliferation, with massive disruption in F-actin organization and accumulation, and alterations in tight junctions. Inhibition of actin polymerization and protein synthesis was able to sufficiently block dome formation in mutant populations. Taken together, these findings underscore that Dsg3 has a role in controlling cellular viability and differentiation as well as the functional integrity of tight junctions in MDCK cells.

Characterization of Adult Canine Kidney Epithelial Stem Cells That Give Rise to Dome-Forming Tubular Cells

Dome formation can occur in cultured tubular epithelial cells originating from various tissues, including the mammary gland and the kidney. The isolation and characterization of normal kidney epithelial stem cells that give rise to dome-forming tubular cells have never been reported. We attempted to isolate and characterize canine kidney epithelial stem cells using a simple cell culture method that we have previously used to isolate other adult human stem cells. Dome-forming kidney epithelial cells were derived from dissociated adult canine kidney tissues that were cultured in a modified keratinocyte serum-free medium supplemented with N-acetyl-l-cysteine, l-ascorbic acid 2-phosphate, nicotinamide, and fetal bovine serum. These cells exhibited high self-renewal capacity in long-term culture (growth for >13 months and 30 cumulative population doublings) and exhibited characteristics of stem cells, including (1) deficiency in gap junctional intercellular communication, (2) anchorage-independent growth, (3) expression of stem cell markers octamer-binding transcription factor 4 and SRY (sex determining region Y)-box 2, (4) expression of cell surface markers CD24 and CD133, and (5) multipotent differentiation into osteoblasts, adipocytes, chondrocytes, and dome-forming tubular cells. Most of these characteristics are shared by the well-known canine renal tubule-derived immortalized Madin-Darby Canine Kidney cell line. Furthermore, the putative canine kidney stem cells developed in this study formed budding tubule-like organoids on Matrigel and required high cell density (>4,000 cells/cm²) for sustained growth and confluency for dome formation. The signal transducer and activator of transcription-3 (STAT3) phosphorylation inhibitor, AG490, inhibited colony-forming efficiency and dome formation, whereas lipopolysaccharide, an activator of STAT3, increased colony-forming efficiency in a dose-dependent manner. These results are consistent with the hypothesis that high cell density induces STAT3 expression, which promotes both stem cell self-renewal and differentiation into tubular cells. Our novel cell culture method should be useful for the future development of normal human kidney stem cells for clinical applications and for studying mechanisms of nephrotoxicity.

Genome Engineering Renal Epithelial Cells for Enhanced Volume Transport Function

Introduction

Bioengineering an implantable artificial kidney (IAK) will require renal epithelial cells capable of reabsorption of salt and water. We used genome engineering to modify cells for improved Na⁺/H⁺ exchange and H₂O reabsorption. The non-viral piggyBac transposon system enables genome engineering cells to stably overexpress one or more transgenes simultaneously.

Methods

We generated epitope-tagged human sodium hydrogen exchanger 3 (NHE3) and aquaporin-1 (AQP1) cDNA expressing piggyBac transposon vectors. Transgene expression was evaluated via western blot and immunofluorescence. Flow cytometry analysis was used to quantitate transporter expression in a library of genome engineered clones. Cell surface biotinylation was used evaluate surface protein localization. Blister formation assays were used to monitor cellular volumetric transport.

Results

piggyBac enabled stable transposon integration and overexpression of cumate-inducible NHE3 and/or constitutively expressing AQP1 in cultured renal (MDCK) epithelial cells. Cell surface delivery of NHE3 and AQP1 was confirmed using cell surface biotinylation assays. Flow cytometry of a library of MDCK clones revealed varying expression of AQP1 and NHE3. MDCK cells expressing AQP1 and cumate-inducible NHE3 demonstrated increased volumetric transport.

Conclusions

Our results demonstrate that renal epithelial cells an be genome engineered for enhanced volumetric transport that will be needed for an IAK device. Our results lay the foundation for future studies of genome engineering human kidney cells for renal tubule cell therapy.

MicroRNA-34a/IL-6R pathway as a potential therapeutic target for ovarian high-grade serous carcinoma

Accumulating evidence has indicated that microRNAs play a critical role in the pathogenesis of human cancers. microRNA-34a (miR-34a) has been shown to be a key regulator of tumor suppression by targeting several cancer-related signals, including the interleukin-6 receptor (IL-6R)/Signal Transducers and Activator of Transcription 3 (STAT3) signaling pathway. Previously, we determined that miR-34a expression was frequently reduced in high-grade serous carcinoma (HGSC), the major subtype of epithelial ovarian cancer (EOC). Considering that the IL-6R/STAT3 signaling pathway is upregulated and believed to be a potential therapeutic target in EOC, we investigated the biological significance of reduced miR-34a expression in HGSC with regard to IL-6R signaling. Additionally, we evaluated the viability of miR-34a as a therapeutic application for HGSC both in vitro and in vivo. Accordingly, we found that the ectopic expression of miR-34a significantly reduced tumor proliferation and invasion through downregulation of IL-6R expression, suggesting that reduced miR-34a expression might play an important role in the malignant potential of HGSC through upregulation of the IL-6R/STAT3 signaling pathway. Moreover, we demonstrated that replacement of miR-34a reduced tumorigenicity of HGSC in vivo. Therefore, this study may provide the rationale for miR-34a replacement as a promising therapeutic strategy for HGSC.

PI3k and Stat3: Oncogenes that are Required for Gap Junctional, Intercellular Communication

Gap junctional, intercellular communication (GJIC) is interrupted in cells transformed by oncogenes such as activated Src. The Src effector, Ras, is required for this effect, so that Ras inhibition restores GJIC in Src-transformed cells. Interestingly, the inhibition of the Src effector phosphatidyl-inositol-3 kinase (PI3k) or Signal Transducer and Activator of Transcription-3 (Stat3) pathways does not restore GJIC. In the contrary, inhibition of PI3k or Stat3 in non-transformed rodent fibroblasts or epithelial cells or certain human lung carcinoma lines with extensive GJIC inhibits communication, while mutational activation of PI3k or Stat3 increases GJIC. Therefore, it appears that oncogenes such as activated Src have a dual role upon GJIC; acting as inhibitors of communication through the Ras pathway, and as activators through activation of PI3k or Stat3. In the presence of high Src activity the inhibitory functions prevail so that the net effect is gap junction closure. PI3k and Stat3 constitute potent survival signals, so that their inhibition in non-transformed cells triggers apoptosis which, in turn, has been independently demonstrated to suppress GJIC. The interruption of gap junctional communication would confine the apoptotic event to single cells and this might be essential for the maintenance of tissue integrity. We hypothesize that the GJIC activation by PI3k or Stat3 may be linked to their survival function.

NHA2 promotes cyst development in an in vitro model of polycystic kidney disease

KEY POINTS

Significant and selective up-regulation of the Na+ /H+ exchanger NHA2 (SLC9B2) was observed in cysts of patients with autosomal dominant polycystic kidney disease. Using the MDCK cell model of cystogenesis, it was found that NHA2 increases cyst size. Silencing or pharmacological inhibition of NHA2 inhibits cyst formation in vitro. Polycystin-1 represses NHA2 expression via Ca2+ /NFAT signalling whereas the dominant negative membrane-anchored C-terminal fragment (PC1-MAT) increased NHA2 levels. Drugs (caffeine, theophylline) and hormones (vasopressin, aldosterone) known to exacerbate cysts elicit NHA2 expression. Taken together, the findings reveal NHA2 as a potential new player in salt and water homeostasis in the kidney and in the pathogenesis of polycystic kidney disease.

ABSTRACT

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutations in PKD1 and PKD2 encoding polycystin-1 (PC1) and polycystin-2 (PC2), respectively. The molecular pathways linking polycystins to cyst development in ADPKD are still unclear. Intracystic fluid secretion via ion transporters and channels plays a crucial role in cyst expansion in ADPKD. Unexpectedly, we observed significant and selective up-regulation of NHA2, a member of the SLC9B family of Na+ /H+ exchangers, that correlated with cyst size and disease severity in ADPKD patients. Using three-dimensional cultures of MDCK cells to model cystogenesis in vitro, we showed that ectopic expression of NHA2 is causal to increased cyst size. Induction of PC1 in MDCK cells inhibited NHA2 expression with concordant inhibition of Ca2+ influx through store-dependent and -independent pathways, whereas reciprocal activation of Ca2+ influx by the dominant negative membrane-anchored C-terminal tail fragment of PC1 elevated NHA2. We showed that NHA2 is a target of Ca2+ /NFAT signalling and is transcriptionally induced by methylxanthine drugs such as caffeine and theophylline, which are contraindicated in ADPKD patients. Finally, we observed robust induction of NHA2 by vasopressin, which is physiologically consistent with increased levels of circulating vasopressin and up-regulation of vasopressin V2 receptors in ADPKD. Our findings have mechanistic implications on the emerging use of vasopressin V2 receptor antagonists such as tolvaptan as safe and effective therapy for polycystic kidney disease and reveal a potential new regulator of transepithelial salt and water transport in the kidney.

Analysis of opossum kidney NaPi-IIc sodium-dependent phosphate transporter to understand Pi handling in human kidney

BACKGROUND

The role of Na⁺-dependent inorganic phosphate (Pi) transporters in the human kidney is not fully clarified. Hereditary hypophosphatemic rickets with hypercalciuria (HHRH) is caused by loss-of-function mutations in the IIc Na⁺-dependent Pi transporter (NPT2c/Npt2c/NaPi-IIc) gene. Another Na⁺-dependent type II transporter, (NPT2A/Npt2a/NaPi-IIa), is also important for renal Pi reabsorption in humans. In mice, Npt2c deletion does not lead to hypophosphatemia and rickets because Npt2a compensates for the impaired Pi reabsorption. To clarify the differences between mouse and human, we investigated the relation between NaPi-IIa and NaPi-IIc functions in opossum kidney (OK) cells.

METHODS

We cloned NaPi-IIc from OK cells and created opossum NaPi-IIc (oNaPi-IIc) antibodies. We used oNaPi-IIc small interference (si)RNA and investigated the role of NaPi-IIc in Pi transport in OK cells.

RESULTS

We cloned opossum kidney NaPi-IIc cDNAs encoding 622 amino acid proteins (variant1) and examined their pH- and sodium-dependency. The antibodies reacted specifically with 75-kDa and 150-kDa protein bands, and the siRNA of NaPi-IIc markedly suppressed endogenous oNaPi-IIc in OK cells. Treatment with siRNA significantly suppressed the expression of NaPi-4 (NaPi-IIa) protein and mRNA. oNaPi-IIc siRNA also suppressed Na⁺/H⁺ exchanger regulatory factor 1 expression in OK cells.

CONCLUSION

These findings suggest that NaPi-IIc is important for the expression of NaPi-IIa (NaPi-4) protein in OK cells. Suppression of Npt2c may downregulate Npt2a function in HHRH patients.

Networks of Transcription Factors for Oct4 Expression in Mice

The present review aimed to assess the networks of transcription factors regulating the Oct4 expression in mice. Through a comprehensive analysis of the binding sites and the interrelationships of the transcription factors of Oct4, it is found that transcription factors of Oct4 form three regulating complexes centered by Oct4-Sox2, Nanog, and Lrh1. They bind on CR4, CR2, and CR1 regions of Oct4 promoter/enhancer, respectively, to activate Oct4 transcription synergistically. This article also discusses the mechanisms of fine-tuning the Oct4 expression. These findings have important implications in the field of stem cell and developmental biology.

Antisecretory factor peptide AF-16 inhibits the secreted autotransporter toxin-stimulated transcellular and paracellular passages of fluid in cultured human enterocyte-like cells

Both the endogenous antisecretory factor (AF) protein and peptide AF-16, which has a sequence that matches that of the active N-terminal region of AF, inhibit the increase in the epithelial transport of fluid and electrolytes induced by bacterial toxins in animal and ex vivo models. We conducted a study to investigate the inhibitory effect of peptide AF-16 against the increase of transcellular passage and paracellular permeability promoted by the secreted autotransporter toxin (Sat) in a cultured cellular model of the human intestinal epithelial barrier. Peptide AF-16 produced a concentration-dependent inhibition of the Sat-induced increase in the formation of fluid domes, in the mucosal-to-serosal passage of D-[1-(14)C]mannitol, and in the rearrangements in the distribution and protein expression of the tight junction (TJ)-associated proteins ZO-1 and occludin in cultured human enterocyte-like Caco-2/TC7 cell monolayers. In addition, we show that peptide AF-16 also inhibits the cholera toxin-induced increase of transcellular passage and the Clostridium difficile toxin-induced effects on paracellular permeability and TJ protein organization in Caco-2/TC7 cell monolayers. Treatment of cell monolayers by the lipid raft disorganizer methyl-β-cyclodextrin abolished the inhibitory activity of peptide AF-16 at the transcellular passage level and did not modify the effect of the peptide at the paracellular level.

Epithelial cells in culture: injured or differentiated cells?

Isolation of epithelial cells for cell culture is based on destruction of epithelial integrity. The consequences are manifold: cell polarity and specific cell functions are lost; cells acquire non-epithelial characteristics and start to proliferate. This situation may also occur in situ when parts of the epithelium are lost, either by apoptosis or necrosis by organ or tissue injury. During recovery from this injury, surviving epithelial cells proliferate and may restore epithelial integrity and finally re-differentiate into functional epithelial cells. In vitro, this re-differentiation is mostly not complete due to sub-optimal culture conditions. Therefore cultured epithelial cells resemble wounded or injured epithelia rather than healthy and well differentiated epithelia. The value of an in vitro cell model is the extent to which it helps to understand the function of the cells in situ. A variety of parameters influence the state of differentiation of cultured cells in vitro. Although each of these parameters had been studied, the picture how they co-ordinately influence the state of differentiation of epithelial cells in vitro is incomplete. Therefore we discuss the influence of the isolation method and cell culture on epithelial cells, and outline strategies to achieve highly differentiated epithelial cells for the use as an in vitro model.

SH2B1β interacts with STAT3 and enhances fibroblast growth factor 1-induced gene expression during neuronal differentiation

Neurite outgrowth is an essential process during neuronal differentiation as well as neuroregeneration. Thus, understanding the molecular and cellular control of neurite outgrowth will benefit patients with neurological diseases. We have previously shown that overexpression of the signaling adaptor protein SH2B1β promotes fibroblast growth factor 1 (FGF1)-induced neurite outgrowth (W. F. Lin, C. J. Chen, Y. J. Chang, S. L. Chen, I. M. Chiu, and L. Chen, Cell. Signal. 21:1060-1072, 2009). SH2B1β also undergoes nucleocytoplasmic shuttling and regulates a subset of neurotrophin-induced genes. Although these findings suggest that SH2B1β regulates gene expression, the nuclear role of SH2B1β was not known. In this study, we show that SH2B1β interacts with the transcription factor, signal transducer, and activator of transcription 3 (STAT3) in neuronal PC12 cells, cortical neurons, and COS7 fibroblasts. By affecting the subcellular distribution of STAT3, SH2B1β increased serine phosphorylation and the concomitant transcriptional activity of STAT3. As a result, overexpressing SH2B1β enhanced FGF1-induced expression of STAT3 target genes Egr1 and Cdh2. Chromatin immunoprecipitation assays further reveal that, in response to FGF1, overexpression of SH2B1β promotes the in vivo occupancy of STAT3-Sp1 heterodimers at the promoter of Egr1 and Cdh2. These findings establish a central role of SH2B1β in orchestrating signaling events to transcriptional activation through interacting and regulating STAT3-containing complexes during neuronal differentiation.

Engaged for survival: From cadherin ligation to STAT3 activation

In normal tissues or tumors, cells have extensive opportunities for adhesion to their neighbors. This state is mimicked by dense cell cultures. In this review, we integrate some recent findings on a key signal transducer, STAT3 (signal transducer and activator of transcription-3), whose activity is dramatically increased following cadherin-mediated cell to cell adhesion. Cadherin engagement, favored in dense cell cultures, causes a dramatic increase in total Rac/Cdc42 protein levels through inhibition of proteasomal degradation, which is followed by activation of IL-6 and STAT3. The cadherin/Rac/IL-6/STAT3 axis offers a potent survival signal that is a prerequisite for neoplastic transformation, as well as normal tissue function.

The regulation of NHE₁ and NHE₃ activity by angiotensin II is mediated by the activation of the angiotensin II type I receptor/phospholipase C/calcium/calmodulin pathway in distal nephron cells

Angiotensin II (Ang II), acting via the AT1 receptor, induces an increase in intracellular calcium [Ca(2+)]i that then interacts with calmodulin (CaM). The Ca(2+)/CaM complex directly or indirectly activates sodium hydrogen exchanger 1 (NHE1) and phosphorylates calmodulin kinase II (CaMKII), which then regulates sodium hydrogen exchanger 3 (NHE3) activity. In this study, we investigated the cellular signaling pathways responsible for Ang II-mediated regulation of NHE1 and NHE3 in Madin-Darby canine kidney (MDCK) cells. The NHE1- and NHE3-dependent pHi recovery rates were evaluated by fluorescence microscopy using the fluorescent probe BCECF/AM, messenger RNA was evaluated with the reverse transcription polymerase chain reaction (RT-PCR), and protein expression was evaluated by immunoblot. We demonstrated that treatment with Ang II (1pM or 1 nM) for 30 min induced, via the AT1 but not the AT2 receptor, an equal increase in NHE1 and NHE3 activity that was reduced by the specific inhibitors HOE 694 and S3226, respectively. Ang II (1 nM) did not change the total expression of NHE1, NHE3 or calmodulin, but it induced CaMKII, cRaf-1, Erk1/2 and p90(RSK) phosphorylation. The stimulatory effects of Ang II (1 nM) on NHE1 or NHE3 activity or protein abundance was reduced by ophiobolin-A (CaM inhibitor), KN93 (CaMKII inhibitor) or PD98059 (Mek inhibitor). These results indicate that after 30 min, Ang II treatment may activate G protein-dependent pathways, including the AT1/PLC/Ca(2+)/CaM pathway, which induces CaMKII phosphorylation to stimulate NHE3 and induces cRaf-1/Mek/Erk1/2/p90(RSK) activity to stimulate NHE1.

Acidosis-mediated regulation of the NHE1 isoform of the Na⁺/H⁺ exchanger in renal cells

The mammalian Na⁺/H⁺ exchanger isoform 1 (NHE1) is a ubiquitous plasma membrane protein that regulates intracellular pH by removing a proton in exchange for extracellular sodium. Renal tissues are subject to metabolic and respiratory acidosis, and acidosis has been shown to acutely activate NHE1 activity in other cell types. We examined if NHE1 is activated by acute acidosis in HEK293 and Madin-Darby canine kidney (MDCK) cells. Acute sustained intracellular acidosis (SIA) activated NHE1 in both cell types. We expressed wild-type and mutant NHE1 cDNAs in MDCK cells. All the cDNAs had a L163F/G174S mutation, which conferred a 100-fold resistance to EMD87580, an NHE1-specific inhibitor. We assayed exogenous NHE1 activity while inhibiting endogenous activity with EMD87580 and while inhibiting the NHE3 isoform of the Na⁺/H⁺ exchanger using the isoform-specific inhibitor S3226. We examined the activation and phosphorylation of the wild-type and mutant NHE1 proteins in response to SIA. In MDCK cells we demonstrated that the amino acids Ser⁷⁷¹, Ser⁷⁷⁶, Thr⁷⁷⁹, and Ser⁷⁸⁵ are important for NHE1 phosphorylation and activation after acute SIA. SIA activated ERK-dependent pathways in MDCK cells, and this was blocked by treatment with the MEK inhibitor U0126. Treatment with U0126 also blocked activation of NHE1 by SIA. These results suggest that acute acidosis activates NHE1 in mammalian kidney cells and that in MDCK cells this activation occurs through an ERK-dependent pathway affecting phosphorylation of a distinct set of amino acids in the cytosolic regulatory tail of NHE1.

Potential role of sodium-proton exchangers in the low concentration arsenic trioxide-increased intracellular pH and cell proliferation

Arsenic main inorganic compound is arsenic trioxide (ATO) presented in solution mainly as arsenite. ATO increases intracellular pH (pHi), cell proliferation and tumor growth. Sodium-proton exchangers (NHEs) modulate the pHi, with NHE1 playing significant roles. Whether ATO-increased cell proliferation results from altered NHEs expression and activity is unknown. We hypothesize that ATO increases cell proliferation by altering pHi due to increased NHEs-like transport activity. Madin-Darby canine kidney (MDCK) cells grown in 5 mmol/L D-glucose-containing DMEM were exposed to ATO (0.05, 0.5 or 5 µmol/L, 0–48 hours) in the absence or presence of 5-N,N-hexamethylene amiloride (HMA, 5–100 µmol/L, NHEs inhibitor), PD-98059 (30 µmol/L, MAPK1/2 inhibitor), Gö6976 (10 µmol/L, PKCα, βI and μ inhibitor), or Schering 28080 (10 µmol/L, H⁺/K⁺ATPase inhibitor) plus concanamycin (0.1 µmol/L, V type ATPases inhibitor). Incorporation of [³H]thymidine was used to estimate cell proliferation, and counting cells with a hemocytometer to determine the cell number. The pHi was measured by fluorometry in 2,7-bicarboxyethyl-5,6-carboxyfluorescein loaded cells. The Na⁺-dependent HMA-sensitive NHEs-like mediated proton transport kinetics, NHE1 protein abundance in the total, cytoplasm and plasma membrane protein fractions, and phosphorylated and total p42/44 mitogen-activated protein kinases (p42/44^mapk) were also determined. Lowest ATO (0.05 µmol/L, ∼0.01 ppm) used in this study increased cell proliferation, pHi, NHEs-like transport and plasma membrane NHE1 protein abundance, effects blocked by HMA, PD-98059 or Gö6976. Cell-buffering capacity did not change by ATO. The results show that a low ATO concentration increases MDCK cells proliferation by NHEs (probably NHE1)-like transport dependent-increased pHi requiring p42/44^mapk and PKCα, βI and/or μ activity. This finding could be crucial in diseases where uncontrolled cell growth occurs, such as tumor growth, and in circumstances where ATO, likely arsenite, is available at the drinking-water at these levels.

Shear stress reverses dome formation in confluent renal tubular cells

BACKGROUND/AIMS

It has been shown that MDCK cells, a cell line derived from canine renal tubules, develop cell domes due to fluid pumped under cell monolayer and focal detachment from the adhesion surface. In vitro studies have shown that primary cilia of kidney tubular epithelial cells act as mechanosensors, increasing intracellular calcium within seconds upon changes in fluid shear stress (SS) on cell membrane. We then studied the effect of prolonged SS exposure on cell dome formation in confluent MDCK cell monolayers.

METHODS

A parallel plate flow chamber was used to apply laminar SS at 2 dynes/cm(2) to confluent cell monolayers for 6 hours. Control MDCK cell monolayers were maintained in static condition. The effects of Ca(2+) blockade and cell deciliation on SS exposure were also investigated.

RESULTS

Seven days after reaching confluence, static cultures developed liquid filled domes, elevating from culture plate. Exposure to SS induced almost complete disappearance of cell domes (0.4±0.8 vs. 11.4±2.8 domes/mm(2), p < 0.01, n=14). SS induced dome disappearance took place within minutes to hours, as shown by time-lapse videomicroscopy. Exposure to SS importantly affected cell cytoskeleton altering actin stress fibers expression and organization, and the distribution of tight junction protein ZO-1. Dome disappearance induced by flow was completely prevented in the presence of EGTA or after cell deciliation.

CONCLUSIONS

These data indicate that kidney tubular cells are sensitive to apical flow and that these effects are mediated by primary cilia by regulation of Ca(2+) entry in to the cell. SS induced Ca(2+) entry provokes contraction of cortical actin ring that tenses cell-cell borders and decreases basal stress fibers. These processes may increase paracellular permeability and decrease basal adhesion making dome disappear. Elucidation of the effects of apical fluid flow on tubular cell function may open new insights on the pathophysiology of kidney diseases associated with cilia dysfunction.

Transcriptional regulation of the intestinal luminal Na⁺ and Cl⁻ transporters

The epithelial apical membrane Na+/H+ exchangers [NHE (sodium hydrogen exchanger)2 and NHE3] and Cl-/HCO3- exchangers [DRA (down-regulated in adenoma) and PAT-1 (putative anion transporter 1)] are key luminal membrane transporters involved in electroneutral NaCl absorption in the mammalian intestine. During the last decade, there has been a surge of studies focusing on the short-term regulation of these electrolyte transporters, particularly for NHE3 regulation. However, the long-term regulation of the electrolyte transporters, involving transcriptional mechanisms and transcription factors that govern their basal regulation or dysregulation in diseased states, has only now started to unfold with the cloning and characterization of their gene promoters. The present review provides a detailed analysis of the core promoters of NHE2, NHE3, DRA and PAT-1 and outlines the transcription factors involved in their basal regulation as well as in response to both physiological (butyrate, protein kinases and probiotics) and pathophysiological (cytokines and high levels of serotonin) stimuli. The information available on the transcriptional regulation of the recently identified NHE8 isoform is also highlighted. Therefore the present review bridges a gap in our knowledge of the transcriptional mechanisms underlying the alterations in the gene expression of intestinal epithelial luminal membrane Na+ and Cl- transporters involved in electroneutral NaCl absorption. An understanding of the mechanisms of the modulation of gene expression of these transporters is important for a better assessment of the pathophysiology of diarrhoea associated with inflammatory and infectious diseases and may aid in designing better management protocols.

Dual conformation of H2H3 domain of prion protein in mammalian cells

The concept of prion is applied to protein modules that share the ability to switch between at least two conformational states and transmit one of these through intermolecular interaction and change of conformation. Although much progress has been achieved through the understanding of prions from organisms such as Saccharomyces cerevisiae, Podospora anserina, or Aplysia californica, the criteria that qualify a protein module as a prion are still unclear. In addition, the functionality of known prion domains fails to provide clues to understand the first identified prion, the mammalian infectious prion protein, PrP. To address these issues, we generated mammalian cellular models of expression of the C-terminal two helices of PrP, H2 and H3, which have been hypothesized, among other models, to hold the replication and conversion properties of the infectious PrP. We found that the H2H3 domain is an independent folding unit that undergoes glycosylations and glycosylphosphatidylinositol anchoring similar to full-length PrP. Surprisingly, in some conditions the normally folded H2H3 was able to systematically go through a conversion process and generate insoluble proteinase K-resistant aggregates. This structural switch involves the assembly of amyloid structures that bind thioflavin S and oligomers that are reactive to A11 antibody, which specifically detects protein oligomers from neurological disorders. Overall, we show that H2H3 is a conformational switch in a cellular context and is thus suggested to be a candidate for the conversion domain of PrP.

The R(h)oads to Stat3: Stat3 activation by the Rho GTPases

The signal transducer and activator of transcription-3 (Stat3) is a member of the STAT family of cytoplasmic transcription factors. Overactivation of Stat3 is detected with high frequency in human cancer and is considered a molecular abnormality that supports the tumor phenotype. Despite concerted investigative efforts, the molecular mechanisms leading to the aberrant Stat3 activation and Stat3-mediated transformation and tumorigenesis are still not clearly defined. Recent evidence reveals a crosstalk close relationship between Stat3 signaling and members of the Rho family of small GTPases, including Rac1, Cdc42 and RhoA. Specifically, Rac1, acting in a complex with the MgcRacGAP (male germ cell RacGAP), promotes tyrosine phosphorylation of Stat3 by the IL6-receptor family/Jak kinase complex, as well as its translocation to the nucleus. Studies have further revealed that the mutational activation of Rac1 and Cdc42 results in Stat3 activation, which occurs in part through the upregulation of IL6 family cytokines that in turn stimulates Stat3 through the Jak kinases. Interestingly, evidence also shows that the engagement of cadherins, cell to cell adhesion molecules, specifically induces a striking increase in Rac1 and Cdc42 protein levels and activity, which in turn results in Stat3 activation. In this review we integrate recent findings clarifying the role of the Rho family GTPases in Stat3 activation in the context of malignant progression.

Signal transducer and activator of transcription 3 activation up-regulates interleukin-6 autocrine production: a biochemical and genetic study of established cancer cell lines and clinical isolated human cancer cells

Background

Spontaneous interleukin-6 (IL-6) production has been observed in various tumors and implicated in the pathogenesis, progression and drug resistance in cancer. However, the regulation of IL-6 autocrine production in cancer cells is not fully understood. IL-6 is auto-regulated in many types of cell. Two of the three major downstream pathways of IL-6, MEK/extracellular signal-related kinase (Erk) pathway and phosphatidylinositol 3-kinase (PI3-K)/Akt pathway, have been shown to regulate IL-6 expression through the activation of AP-1 and NF-κB. However, it is not clear what the role of Janus kinase (Jak) 2/signal transducer and activator of transcription (Stat) 3 pathway. This study was designed to determine the role of Jak2/Stat3 pathway in the regulation of IL-6 autocrine production in cancer cells.

Results

Inhibitors of Jak2/Stat3, MEK/Erk and PI3-K/Akt pathways down-regulated IL-6 secretion in the lung adenocarcinoma PC14PE6/AS2 (AS2) cells, which spontaneously secreted IL-6 and possessed constitutively activated Stat3. Transfection with dominant-negative Stat3, Stat3 siRNA, or Stat3 shRNA decreased IL-6 expression in AS2 cells. Conversely, transfection with constitutively-activated Stat3 increased the production of IL-6. In AS2 derived cells, resistance to paclitaxel was positively correlated with Stat3 activation status and the expression of IL-6, which is commonly secreted in drug resistant cancer cells. The pharmacological inhibition of NF-κB, PI3-K/Akt and MEK/Erk and the pharmacological inhibition and genetic inhibition (Stat3 siRNA) of Jak2/Stat3 pathway decreased IL-6 autocrine production in various drug resistant cancer cell lines and similarly decreased IL-6 autocrine production in clinically isolated lung cancer cells.

Conclusions

This study is the first to directly address the role Stat3 plays on the autocrine production of IL-6, which occurs through a positive-feedback loop. Our biochemical and genetic studies clearly demonstrated that Jak2/Stat3, in combination with other IL-6 downstream pathways, contributed frequently and substantially to IL-6 autocrine production in a broad spectrum of cancer cell lines as well as in clinical cancer samples. Our findings suggest that Stat3 could potentially be regulated to suppress IL-6 autocrine production in cancer cells to inhibit the progression of cancer and reduce drug resistance.

Transforming growth factor-{beta}1 induces Smad3-dependent {beta}1 integrin gene expression in epithelial-to-mesenchymal transition during chronic tubulointerstitial fibrosis

Transforming growth factor-β1 (TGF-β1)-induced epithelial-to-mesenchymal transition (EMT) contributes to the pathophysiological development of kidney fibrosis. Although it was reported that TGF-β1 enhances β(1) integrin levels in NMuMG cells, the detailed molecular mechanisms underlying TGF-β1-induced β(1) integrin gene expression and the role of β(1) integrin during EMT in the renal system are still unclear. In this study, we examined the role of β(1) integrin in TGF-β1-induced EMT both in vitro and in vivo. TGF-β1-induced augmentation of β(1) integrin expression was required for EMT in several epithelial cell lines, and knockdown of Smad3 inhibited TGF-β1-induced augmentation of β(1) integrin. TGF-β1 triggered β(1) integrin gene promoter activity as assessed by luciferase activity assay. Both knockdown of Smad3 and mutation of the Smad-binding element to block binding to the β(1) integrin promoter markedly reduced TGF-β1-induced β(1) integrin promoter activity. Chromatin immunoprecipitation assay showed that TGF-β1 enhanced Smad3 binding to the β(1) integrin promoter. Furthermore, induction of unilateral ureteral obstruction triggered increases of β(1) integrin in both renal epithelial and interstitial cells. In human kidney with chronic tubulointerstitial fibrosis, we also found a concomitant increase of β(1) integrin and α-smooth muscle actin in tubule epithelia. Blockade of β(1) integrin signaling dampened the progression of fibrosis. Taken together, β(1) integrin mediates EMT and subsequent tubulointerstitutial fibrosis, suggesting that inhibition of β(1) integrin is a possible therapeutic target for prevention of renal fibrosis.

Master stem cell transcription factors and signaling regulation

The present article reviews master stem cell transcription factors, their expression regulation network, and related signaling pathways with the aim of understanding the molecular mechanisms of pluripotent cell fate decisions. Oct4, Sox2, and Nanog are master transcription factors for maintenance of the undifferentiated state and self-renewal of embryonic stem cells (ESCs). In the mouse, they form a regulatory circuitry with coregulators, such as beta-catenin, Stat3, Myc, Klfs, Sall4, and Esrrb to control the expression of pluripotency-related genes including themselves. The threshold expression of Oct4, Sox2, and Nanog for sustaining ESC properties depends on the synergistic effects among Stat3, beta-catenin, and Smad signaling pathway under the specific conditions of the ESC cytoplasmic microenvironment. Some of the salient differences in human ESC signaling pathways affecting their fate commitment are highlighted.

ZIP4 regulates pancreatic cancer cell growth by activating IL-6/STAT3 pathway through zinc finger transcription factor CREB

PURPOSE

Recent studies indicate a strong correlation of zinc transporter ZIP4 and pancreatic cancer progression; however, the underlying mechanisms are unclear. We have recently found that ZIP4 is overexpressed in pancreatic cancer. In this study, we investigated the signaling pathway through which ZIP4 regulates pancreatic cancer growth.

EXPERIMENTAL DESIGN

The expression of cyclin D1, interleukin 6 (IL-6), and signal transducer and activator of transcription 3 (STAT3) in pancreatic cancer xenografts and cells were examined by real-time PCR, Bio-Plex cytokine assay, and Western blot, respectively. The activity of cAMP response element-binding protein (CREB) is examined by a promoter activity assay.

RESULTS

Cyclin D1 was significantly increased in the ZIP4 overexpressing MIA PaCa-2 cells (MIA-ZIP4)-injected orthotopic xenografts and was downregulated in the ZIP4-silenced ASPC-1 (ASPC-shZIP4) group. The phosphorylation of STAT3, an upstream activator of cyclin D1, was increased in MIA-ZIP4 cells and decreased in ASPC-shZIP4 cells. IL-6, a known upstream activator for STAT3, was also found to be significantly increased in the MIA-ZIP4 cells and xenografts and decreased in the ASPC-shZIP4 group. Overexpression of ZIP4 led to a 75% increase of IL-6 promoter activity and caused increased phosphorylation of CREB.

CONCLUSIONS

Our study suggest that ZIP4 overexpression causes increased IL-6 transcription through CREB, which in turn activates STAT3 and leads to increased cyclin D1 expression, resulting in increased cell proliferation and tumor progression in pancreatic cancer. These results elucidated a novel pathway in ZIP4-mediated pancreatic cancer growth and suggest new therapeutic targets, including ZIP4, IL-6, and STAT3, in pancreatic cancer treatment.

Beyond structure, to survival: activation of Stat3 by cadherin engagement

Cells in normal tissues or in tumors have extensive opportunities for adhesion to their neighbors and the importance of cell to cell contact in the study of fundamental cellular processes is beginning to emerge. In this review, we discuss recent evidence of dramatic changes in the activity of an important signal transducer found to be profoundly affected by cell to cell adhesion, the signal transducer and activator of transcription-3 (Stat3). Direct cadherin engagement, growth of cells to postconfluence, or formation of multicellular aggregates were found to induce a striking increase in the levels of Stat3 activity, Rac1/Cdc42, and members of the IL6 receptor family in different settings. This activation was specific to Stat3, in that the levels of the extracellular signal regulated kinase (Erk1/2), a signal transducer often coordinately activated with Stat3 by a number of growth factors or oncogenes, remained unaffected by cell density. Density-dependent Stat3 activation may play a key role in survival, and could contribute to the establishment of cell polarity. It is clear that at any given time the total Stat3 activity levels in a cell are the sum of the effects of cell to cell adhesion plus the conventional Stat3 activating factors present.

Activated Rac1 requires gp130 for Stat3 activation, cell proliferation and migration

Rac1 (Rac) is a member of the Rho family of small GTPases which controls cell migration by regulating the organization of actin filaments. Previous results suggested that mutationally activated forms of the Rho GTPases can activate the Signal Transducer and Activator of Transcription-3 (Stat3), but the exact mechanism is a matter of controversy. We recently demonstrated that Stat3 activity of cultured cells increases dramatically following E-cadherin engagement. To better understand this pathway, we now compared Stat3 activity levels in mouse HC11 cells before and after expression of the mutationally activated Rac1 (Rac(V12)), at different cell densities. The results revealed for the first time a dramatic increase in protein levels and activity of both the endogenous Rac and Rac(V12) with cell density, which was due to inhibition of proteasomal degradation. In addition, Rac(V12)-expressing cells had higher Stat3, tyrosine-705 phosphorylation and activity levels at all densities, indicating that Rac(V12) is able to activate Stat3. Further examination of the mechanism of Stat3 activation showed that Rac(V12) expression caused a surge in mRNA of Interleukin-6 (IL6) family cytokines, known potent Stat3 activators. Knockdown of gp130, the common subunit of this family reduced Stat3 activity, indicating that these cytokines may be responsible for the Stat3 activation by Rac(V12). The upregulation of IL6 family cytokines was required for cell migration and proliferation induced by Rac(V12), as shown by gp130 knockdown experiments, thus demonstrating that the gp130/Stat3 axis represents an essential effector of activated Rac for the regulation of key cellular functions.

Cadherin-cadherin engagement promotes cell survival via Rac1/Cdc42 and signal transducer and activator of transcription-3

Signal transducer and activator of transcription-3 (Stat3) is activated by a number of receptor and nonreceptor tyrosine kinases, whereas a constitutively active form of Stat3 alone is sufficient to induce neoplastic transformation. In the present report, we show that Stat3 can also be activated through homophilic interactions by the epithelial (E)-cadherin. Indeed, by plating cells onto surfaces coated with fragments encompassing the two outermost domains of this cadherin, we clearly show that cadherin engagement can activate Stat3, even in the absence of direct cell-to-cell contact. Most importantly, our results also reveal for the first time an unexpected and dramatic surge in total Rac1 and Cdc42 protein levels triggered by cadherin engagement and an increase in Rac1 and Cdc42 activity, which is responsible for the Stat3 stimulation observed. Inhibition of cadherin interactions using a peptide, a soluble cadherin fragment, or genetic ablation induced apoptosis, points to a significant role of this pathway in cell survival signaling, a finding that could also have important therapeutic implications. (Mol Cancer Res 2009;7(8):1310-27).

Cell confluency-induced Stat3 activation regulates NHE3 expression by recruiting Sp1 and Sp3 to the proximal NHE3 promoter region during epithelial dome formation

Activation of signal transducer and activator of transcription-3 (Stat3) during cell confluency is related to its regulatory roles in cell growth arrest- or survival-related physiological or developmental processes. We previously demonstrated that this signaling event triggers epithelial dome formation by transcriptional augmentation of sodium hydrogen exchanger-3 (NHE3) expression. However, the detailed molecular mechanism remained unclear. By using serial deletions, site-directed mutagenesis, and EMSA analysis, we now demonstrate Stat3 binding to an atypical Stat3-response element in the rat proximal NHE3 promoter, located adjacent to a cluster of Sp cis-elements (SpA/B/C), within -77/-36 nt of the gene. SpB (-58/-55 nt) site was more effective than SpA (-72/-69 nt) site for cooperative binding of Sp1/Sp3. Increasing cell density had no effect on Sp1/Sp3 expression but resulted in their increased binding to the SpA/B/C probe along with Stat3 and concurrently with enhanced nuclear pTyr705-Stat3 level. Immunoprecipitation performed with the nuclear extracts demonstrated physical interaction of Stat3 and Sp1/Sp3 triggered by cell confluency. Stat3 inhibition by overexpression of dominant-negative Stat3-D mutant in MDCK cells or by small interfering RNA-mediated knockdown in Caco-2 cells resulted in inhibition of the cell density-induced NHE3 expression, Sp1/Sp3 binding, and NHE3 promoter activity and in decreased dome formation. Thus, during confluency, ligand-independent Stat3 activation leads to its interaction with Sp1/Sp3, their recruitment to the SpA/B/C cluster in a Stat3 DNA-binding domain-dependent fashion, increased transcription, and expression of NHE3, to coordinate cell density-mediated epithelial dome formation.