Differential regulation of the expression of Na(+)/H(+) exchanger isoform NHE3 by PKC-alpha in Caco-2 cells

Expression, Localization and Functional Activity of the Major Na⁺/H⁺ Exchange Isoforms Expressed in the Intestinal Cell Line Caco-2BBe

Background/Aims

Enterocytes express a number of NHE isoforms with presumed localization in the apical (NHE2, 3 and 8) or basolateral (NHE1) membrane. Functional activity and localization of enterocyte NHE isoforms were assessed using fully differentiated Caco-2BBe cells, whose genetic expression profile closely resembles mature enterocytes.

Methods

The activity of the different NHEs was analyzed by fluorometric pH_i-metry in a perfusion chamber with separate apical and basolateral perfusion, using specific inhibitors and shRNA knockdown of NHE2. The expression of the NHEs and of other relevant acid extrusion transporters was quantified by qPCR.

Results

Quantitative comparison of the mRNA expression levels of the different NHE isoforms in 14 day-differentiated Caco-2BBe cells showed the following order: NHE2>NHE8>NHE3>NHE1. Acid-activated NHE exchange rates in the basolateral membrane were >6-fold higher than in the apical membrane. 79 ± 3 % of the acid-activated basolateral Na⁺/H⁺ exchange rate displayed a NHE1-typical inhibitor profile, and no NHE2/3/8 typical activity could be observed. Analysis of the apical Na⁺/H⁺ exchange rates revealed that approximately 51 ± 3 % of the total apical activity displayed a NHE2/8-typical inhibitor profile and 31 ± 6 % a NHE3-typical inhibitor profile. Because no selective NHE2 inhibitor is available, a stable NHE2 knockdown cell line (C2NHE2KD) was generated. C2NHE2KD displayed a reduced NHE2-typical apical Na⁺/H⁺ exchange rate and maintained a lower steady-state pH_i, despite high expression levels of other acid extruders, in particular NBCn1 (Slc4a7).

Conclusion

Differentiated Caco-2BBe cells display particularly high mRNA expression levels of NHE2, which can be functionally identified in the apical membrane. Although at low intracellular pH, NHE2 transport rate was far lower than that of NHE1. NHE2 activity was nevertheless essential for the maintenance of the steady-state pH_i of these cells.

Differential regulation of the Na+-Ca2+ exchanger 3 (NCX3) by protein kinase PKC and PKA

Isoform 3 of the Na⁺-Ca²⁺ exchanger (NCX3) participates in the Ca²⁺ fluxes across the plasma membrane. Among the NCX family, NCX3 carries out a peculiar role due to its specific functions in skeletal muscle and the immune system and to its neuroprotective effect under stress exposure. In this context, proper understanding of the regulation of NCX3 is primordial to consider its potential use as a drug target. In this study, we demonstrated the regulation of NCX3 by protein kinase A (PKA) and C (PKC). Disparity in regulation has been previously reported among the splice variants of NCX3 therefore the activity of Ca²⁺ uptake and extrusion of the two murine variants was measured using fura-2-based Ca²⁺ imaging and revealed that both variants are similarly regulated. PKC stimulation diminished the Ca²⁺ uptake performed by NCX3 in the reverse mode, triggered by a rise in [Ca²⁺]_i or [Na⁺]_i, whereas an opposite response was observed upon PKA stimulation, with a significant increase of the Ca²⁺ uptake after a rise in [Ca²⁺]_i. The latter stimulation affected similarly the efflux capacity of NCX3 whereas Ca²⁺ extrusion capacity remained unaffected under activation of PKC. Next, using site-directed mutagenesis, the sensitivity of NCX3 to PKC was abolished by singly mutating its predicted phosphorylation sites T529 or S695. The sensitivity to PKC might be due to the influence of T529 phosphorylation on the Ca²⁺-binding domain 1. Additionally, we showed that stimulation of NCX3 by PKA occurred through residue S524. This effect may well participate in the fight-or-flight response in skeletal muscle and the long-term potentiation in hippocampus.

The role of intestinal oxalate transport in hyperoxaluria and the formation of kidney stones in animals and man

The intestine exerts a considerable influence over urinary oxalate in two ways, through the absorption of dietary oxalate and by serving as an adaptive extra-renal pathway for elimination of this waste metabolite. Knowledge of the mechanisms responsible for oxalate absorption and secretion by the intestine therefore have significant implications for understanding the etiology of hyperoxaluria, as well as offering potential targets for future treatment strategies for calcium oxalate kidney stone disease. In this review, we present the recent developments and advances in this area over the past 10 years, and put to the test some of the new ideas that have emerged during this time, using human and mouse models. A key focus for our discussion are the membrane-bound anion exchangers, belonging to the SLC26 gene family, some of which have been shown to participate in transcellular oxalate absorption and secretion. This has offered the opportunity to not only examine the roles of these specific transporters, revealing their importance to oxalate homeostasis, but to also probe the relative contributions made by the active transcellular and passive paracellular components of oxalate transport across the intestine. We also discuss some of the various physiological stimuli and signaling pathways which have been suggested to participate in the adaptation and regulation of intestinal oxalate transport. Finally, we offer an update on research into Oxalobacter formigenes, alongside recent investigations of other oxalate-degrading gut bacteria, in both laboratory animals and humans.

Hyperosmolality regulates UT-A6 urea transporter expression in the Caco-2 cell line

Gastrointestinal facilitative urea transporters play a significant role in the urea nitrogen salvaging process, which supports the symbiotic relationship between mammals and their gut microbial populations. UT‐A6 urea transporters have been previously reported in the human gastrointestinal tract, specifically in the colon. As renal UT‐A transporters can be regulated by external osmolality, this study investigated whether UT‐A6 expression could also be regulated in this manner. Initial end‐point RT‐PCR experiments confirmed UT‐A6 expression along the human gastrointestinal tract (colon > small intestine ≫ stomach) and also in the Caco‐2 intestinal cell line. Using Caco‐2 cells exposed for 24 hours to changed external osmotic conditions (from 350 to 250, 500, or 600 mOsm), end‐point PCR suggested UT‐A6 expression increased in hyperosmotic conditions. Using quantitative PCR, it was confirmed that 24 h exposure to 600 mOsm stimulated a significant ~15‐fold increase in UT‐A6 expression (P < 0.001, N = 5, ANOVA). Finally, inhibitory experiments suggested that protein kinase C and calcium were involved in this hyperosmotic‐stimulated regulatory pathway. In conclusion, these data demonstrated UT‐A6 expression was indeed regulated by external osmolality. The physiological significance of this regulatory process upon gastrointestinal urea transport has yet to be determined.

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.

Lactobacillus acidophilus upregulates intestinal NHE3 expression and function

A major mechanism of electroneutral NaCl absorption in the human ileum and colon involves coupling of Na(+)/H(+) and Cl(-)/HCO(3)(-) exchangers. Disturbances in these mechanisms have been implicated in diarrheal conditions. Probiotics such as Lactobacillus have been indicated to be beneficial in the management of gastrointestinal disorders, including diarrhea. However, the molecular mechanisms underlying antidiarrheal effects of probiotics have not been fully understood. We have previously demonstrated Lactobacillus acidophilus (LA) to stimulate Cl(-)/HCO3- exchange activity via an increase in the surface levels and expression of the Cl(-)/HCO3- exchanger DRA in vitro and in vivo. However, the effects of LA on NHE3, the Na(+)/H(+) exchanger involved in the coupled electroneutral NaCl absorption, are not known. Current studies were, therefore, undertaken to investigate the effects of LA on the function and expression of NHE3 and to determine the mechanisms involved. Treatment of Caco2 cells with LA or its conditioned culture supernatant (CS) for 8-24 h resulted in a significant increase in Na(+)/H(+) exchange activity, mRNA, and protein levels of NHE3. LA-CS upregulation of NHE3 function and expression was also observed in SK-CO15 cells, a human colonic adenocarcinoma cell line. Additionally, LA treatment increased NHE3 promoter activity, suggesting involvement of transcriptional mechanisms. In vivo, mice gavaged with live LA showed significant increase in NHE3 mRNA and protein expression in the ileum and colonic regions. In conclusion, LA-induced increase in NHE3 expression may contribute to the upregulation of intestinal electrolyte absorption and might underlie the potential antidiarrheal effects of probiotics.

BAX inhibitor-1 enhances cancer metastasis by altering glucose metabolism and activating the sodium-hydrogen exchanger: the alteration of mitochondrial function

The anti-apoptotic protein, BAX inhibitor-1 (BI-1), has a role in cancer/tumor progression. BI-1-overexpressing HT1080 and B16F10 cells produced higher lung weights and tumor volumes after injection into the tail veins of mice. Transfection of BI-1 siRNA into cells before injection blocked lung metastasis. in vitro, the overexpression of BI-1 increased cell mobility and invasiveness, with highly increased glucose consumption and cytosolic accumulation of lactate and pyruvate, but decreased mitochondrial O(2) consumption and ATP production. Glucose metabolism-associated extracellular pH also decreased as cells excreted more H(+), and sodium hydrogen exchanger (NHE) activity increased, probably as a homeostatic mechanism for intracellular pH. These alterations activated MMP 2/9 and cell mobility and invasiveness, which were reversed by the NHE inhibitor, 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting a role for NHE in cancer metastasis. In both in vitro and in vivo experiments, C-terminal deleted (CDeltaBI-1) cells showed similar results to control cells, suggesting that the C-terminal motif is required for BI-1-associated alterations of glucose metabolism, NHE activation and cancer metastasis. These findings strongly suggest that BI-1 reduces extracellular pH and regulates metastasis by altering glucose metabolism and activating NHE, with the C-terminal tail having a pivotal role in these processes.

Vitamin D and differentiation in cancer

This paper reviews the current understanding of the vitamin D-induced differentiation of neoplastic cells, which results in the generation of cells that acquire near-normal, mature phenotype. Examples of the criteria by which differentiation is recognized in each cell type are provided, and only those effects of 1alpha,25-dihydroxyvitamin D(3) (1,25D) on cell proliferation and survival that are associated with the differentiation process are emphasized. The existing knowledge, often fragmentary, of the signaling pathways that lead to vitamin D-induced differentiation of colon, breast, prostate, squamous cell carcinoma, osteosarcoma, and myeloid leukemia cancer cells is outlined. The important distinctions between the different mechanisms of 1,25D-induced differentiation that are cell-type and cell-context specific are pointed out where known. There is a considerable body of evidence that the principal human cancer cells can be suitable candidates for chemoprevention or differentiation therapy with vitamin D. However, further studies are needed to fully understand the underlying mechanisms in order to improve the therapeutic approaches.

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.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Cardiac glycoside downregulates NHE3 activity and expression in LLC-PK1 cells

Ouabain, a cardiotonic steroid and a specific inhibitor of the Na(+)-K(+)-ATPase, has been shown to significantly inhibit transcellular Na(+) transport without altering the intracellular Na(+) concentration ([Na(+)](i)) in the epithelial cells derived from the renal proximal tubules. We therefore studied whether ouabain affects the activity and expression of Na(+)/H(+) exchanger isoform 3 (NHE3) representing the major route of apical Na(+) reabsorption in LLC-PK(1) cells. Chronic basolateral, but not apical, exposure to low-concentration ouabain (50 and 100 nM) did not change [Na(+)](i) but significantly reduced NHE3 activity, NHE3 protein, and mRNA expression. Inhibition of c-Src or phosphoinositide 3-kinase (PI3K) with PP2 or wortmannin, respectively, abolished ouabain-induced downregulation of NHE3 activity and mRNA expression. In caveolin-1 knockdown LLC-PK(1) cells, ouabain failed to downregulate NHE3 mRNA expression and NHE3 promoter activity. Ouabain response elements were mapped to a region between -450 and -1,194 nt, where decreased binding of thyroid hormone receptor (TR) and Sp1 to their cognate cis-elements was documented in vitro and in vivo by protein/DNA array analysis, EMSA, supershift, and chromatin immunoprecipitation. These data suggest that, in LLC-PK(1) cells, ouabain-induced signaling through the Na(+)-K(+)-ATPase-Src pathway results in decreased Sp1 and TR DNA binding activity and consequently in decreased expression and activity of NHE3. These novel findings may represent the underlying mechanism of cardiotonic steroid-mediated renal compensatory response to volume expansion and/or hypertension.

Protein kinase C-alpha regulation of gallbladder Na+ transport becomes progressively more dysfunctional during gallstone formation

Gallbladder Na+ absorption and biliary Ca2+ are both increased during gallstone formation and may promote cholesterol nucleation. Na+/H+ exchange (NHE) is a major pathway for gallbladder Na+ transport. Ca2+-dependent second messengers, including protein kinase C (PKC), inhibit basal gallbladder Na+ transport. Multiple PKC isoforms with species- and tissue-specific expression have been reported. In this study we sought to characterize Ca2+-dependent PKC isoforms in gallbladder and to examine their roles in Na+ transport during gallstone formation. Gallbladders were harvested from prairie dogs fed either nonlithogenic chow or 1.2% cholesterol-enriched diet for varying periods to induce various stages of gallstone formation. PKC was activated with the use of phorboldibutyrate, and we assessed gallbladder NHE regulation by measuring unidirectional Na+ flux and dimethylamiloride-inhibitable 22Na+ uptake. We measured gallbladder PKC activity with the use of histone III-S phosphorylation and used Gö 6976 to determine PKC-alpha contributions. Gallbladder PKC isoform messenger RNA and protein expression were examined with the use of Northern- and Western-blot analysis, respectively. Prairie dog and human gallbladder expresses PKC-alpha, betaII, and delta isoforms. The PKC activation significantly decreased gallbladder J(Na)(ms) and reduced baseline 22Na+ uptake by inhibiting NHE. PKC-alpha mediated roughly 42% of total PKC activity under basal conditions. PKC-alpha regulates basal gallbladder Na+ transport by way of stimulation of NHE isoform NHE-2 and inhibition of isoform NHE-3. PKC-alpha blockade reversed PKC-induced inhibition of J(Na)(ms) and 22Na+ uptake by about 45% in controls but was progressively less effective during gallstone formation. PKC-alpha contribution to total PKC activity is progressively reduced, whereas expression of PKC-alpha mRNA, and protein increases significantly during gallstone formation. We conclude that PKC-alpha regulation of gallbladder NHE becomes progressively more dysfunctional and may in part account for the increased Na+ absorption observed during gallstone formation.

Gallbladder Na+/H+ exchange activity is up-regulated prior to cholesterol crystal formation

BACKGROUND

Gallbladder Na+ and H2O absorption are increased prior to gallstone formation and may promote cholesterol nucleation. Na+/H+ exchange (NHE) isoforms NHE2 and NHE3 are involved in gallbladder Na+ transport in prairie dogs. We examined whether increased gallbladder Na+ absorption observed during early gallstone formation is the result of NHE up-regulation.

MATERIALS AND METHODS

Native gallbladder and primary cultures of gallbladder epithelial cells (GBECs) harvested from prairie dogs fed nonlithogenic (CON) or 1.2% cholesterol diet for varying lengths of time to induce cholesterol-saturated bile (PreCRYS), cholesterol crystals (CRYS), or gallstones (GS) were used. NHE activity was assessed by measuring dimethylamiloride-inhibitable 22Na+ uptake under H+ gradient in primary GBECs. HOE-694 was used to determine NHE2 and NHE3 contributions. NHE protein and mRNA expression were examined by Western and Northern blots, respectively.

RESULTS

Gallbladder total NHE activity was 25.1 +/- 1.3 nmol mg protein(-1) min(-1) in the control and increased during gallstone formation peaking at the PreCRYS stage (98.4 +/- 3.9 nmol mg protein(-1) min(-1)). There was a shift in NHE activity from NHE2 to NHE3 as the animals progressed from no stones through the PreCRYS and CRYS stages to gallstones. The increase in NHE activity was partly caused by an increased Vmax without any change in K(Na)m. Both NHE2 and NHE3 protein increased moderately during the PreCRYS stage without increases in mRNA expression.

CONCLUSIONS

Increased gallbladder Na+ absorption observed prior to crystal formation is in part caused by an increase NHE activity which is not fully accounted for by an increase in NHE proteins and mRNA levels but may be explained by enhanced localization in the membranes and/or altered regulation of NHE.

Serotonin inhibits Na+/H+ exchange activity via 5-HT4 receptors and activation of PKC alpha in human intestinal epithelial cells

BACKGROUND & AIMS

Increased serotonin levels have been implicated in the pathophysiology of diarrhea associated with celiac and inflammatory diseases. However, the effects of serotonin on Na+ /H+ exchange (NHE) activity in the human intestine have not been investigated fully. The present studies examined the acute effects of 5-hydroxytryptamine (5-HT) on NHE activity using Caco-2 cells as an in vitro model.

METHODS

Caco-2 cells were treated with 5-HT (.1 micromol/L, 1 h) and NHE activity was measured as ethyl-isopropyl-amiloride (EIPA)-sensitive 22Na uptake. The effect of 5-HT receptor-specific agonists and antagonists was examined. The role of signaling intermediates in 5-HT-mediated effects on NHE activity was elucidated using pharmacologic inhibitors and immunoblotting.

RESULTS

NHE activity was inhibited significantly (approximately 50%-75%, P < .05) by .1 micromol/L 5-HT via inhibition of maximal velocity (Vmax) without any changes in apparent affinity (Km) for the substrate Na+ . NHE inhibition involved a decrease of both NHE2 and NHE3 activities. Studies using specific inhibitors and agonists showed that the effects of 5-HT were mediated by 5-HT4 receptors. 5-HT-mediated inhibition of NHE activity was dependent on phosphorylation of phospholipase C gamma 1 (PLC gamma 1) via activation of src-kinases. Signaling pathways downstream of PLC gamma 1 involved increase of intracellular Ca 2+ levels and subsequent activation of protein kinase C alpha (PKC alpha). The effects of 5-HT on NHE activity were not cell-line specific because T84 cells also showed NHE inhibition.

CONCLUSIONS

A better understanding of the regulation of Na+ absorption by 5-HT offers the potential for providing insights into molecular and cellular mechanisms involved in various diarrheal and inflammatory disorders.

Differential regulation of Na+/H+ exchange isoform activities by enteropathogenic E. coli in human intestinal epithelial cells

Enteropathogenic Escherichia coli (EPEC) is an important human intestinal foodborne pathogen associated with diarrhea, especially in infants and young children. Although EPEC produces characteristic attaching and effacing lesions and loss of microvilli, the pathophysiology of EPEC-associated diarrhea, particularly during early infection, remains elusive. The present studies were designed to examine the direct effects of EPEC infection on intestinal absorption via Na(+)/H(+) exchanger (NHE) isoforms. Caco-2 cells were infected with EPEC strain E2348/69 or nonpathogenic E. coli HB101 for a period of 60 to 120 min. Total NHE activity was significantly increased at 60 min, reaching approximately threefold increase after 90 min of EPEC infection. Similar findings were seen in HT-29 cells and T84 cells indicating that the response was not cell-line specific. Most surprising was the differential regulation of NHE2 and NHE3 by EPEC. Marked activation of NHE2 (300%) occurred, whereas significant inhibition ( approximately 50%) of NHE3 activity was induced. The activity of basolateral isoform NHE1 was also significantly increased in response to EPEC infection. Mutations that disrupted the type III secretion system (TTSS) ablated the effect of EPEC on the activity of both NHE2 and NHE3. These results suggest that EPEC, through a TTSS-dependent mechanism, exerts differential effects on NHE isoform activity in intestinal epithelial cells. Additionally, NHEs do not appear to play any role in EPEC-mediated inflammation, because the NHE inhibitors amiloride and 5-(N-ethyl-N-isopropyl)amiloride did not prevent EPEC-mediated IkappaBalpha degradation.

Gialpha3 protein-coupled dopamine D3 receptor-mediated inhibition of renal NHE3 activity in SHR proximal tubular cells is a PLC-PKC-mediated event

This study evaluated the transduction pathway associated with type 3 Na(+)/H(+) exchanger (NHE3) activity-induced inhibition during dopamine D(3) receptor activation in immortalized renal proximal tubular epithelial cells from the spontaneously hypertensive rat. The dopamine D(3) receptor agonist 7-OH-DPAT decreased NHE3 activity, which was prevented by the D(2)-like receptor antagonist S-sulpiride, pertussis toxin (PTX; overnight treatment), and the PKC inhibitor chelerythrine, but not by cholera toxin (overnight treatment), the MAPK inhibitor PD-098059, or the p38 inhibitor SB-203580. The PKA inhibitor H-89 abolished the inhibitory effects of forskolin on NHE3 activity, but not that of 7-OH-DPAT. The phospholipase C (PLC) inhibitor U-73122 prevented the inhibitory effects of 7-OH-DPAT, whereas PDBu and 7-OH-DPAT increased PLC activity and reduced NHE3 activity; downregulation of PKC abolished the inhibitory effects of both PDBu and 7-OH-DPAT on NHE activity. The inhibition of NHE3 activity by GTPgammaS and the prevention of the effect of 7-OH-DPAT by PTX suggest an involvement of a G(i/o) protein coupled to the dopamine D(3) receptor. Indeed, the 7-OH-DPAT-induced decrease in NHE3 activity was abolished in cells treated overnight with the anti-G(i)alpha3 antibody, but not in cells treated with antibodies against G(q/11), G(s)alpha, G(beta), and G(i)alpha1,2 proteins. The calcium ionophore A-23187 and the Ca(2+)-ATPase inhibitor thapsigargin increased intracellular Ca(2+) but did not affect NHE3 activity. However, the inhibitory effects of PDBu and 7-OH-DPAT on NHE3 activity were completely abolished by A-23287 and thapsigargin. It is concluded that inhibition of NHE3 activity by dopamine D(3) receptors coupled to G(i)alpha3 proteins is a PLC-PKC-mediated event, modulated by intracellular Ca(2+).

Regulation of butyrate uptake in Caco-2 cells by phorbol 12-myristate 13-acetate

Butyrate and the other short-chain fatty acids (SCFAs) are the most abundant anions in the colonic lumen. Also, butyrate is the preferred energy source for colonocytes and has been shown to regulate colonic electrolyte and fluid absorption. Previous studies from our group have demonstrated that the HCO(3)(-)/SCFA(-) anion exchange process is one of the major mechanisms of butyrate transport across the purified human colonic apical membrane vesicles and the apical membrane of human colonic adenocarcinoma cell line Caco-2 and have suggested that it is mainly mediated via monocarboxylate transporter-1 (MCT-1) isoform. However, little is known regarding the regulation of SCFA transport by various hormones and signal transduction pathways. Therefore, the present studies were undertaken to examine whether hydrocortisone and phorbol 12-myristate 13-acetate (PMA) are involved in a possible regulation of the butyrate/anion exchange process in Caco-2 cells. The butyrate/anion exchange process was assessed by measuring a pH-driven [(14)C]butyrate uptake in Caco-2 cells. Our results demonstrated that 24-h incubation with PMA (1 microM) significantly increased [(14)C]butyrate uptake compared with incubation with 4alphaPMA (inactive form). In contrast, incubation with hydrocortisone had no significant effect on butyrate uptake in Caco-2 cells compared with vehicle (ethanol) alone. Induction of butyrate uptake by PMA appeared to be via an increase in the maximum velocity (V(max)) of the transport process with no significant changes in the K(m) of the transporter for butyrate. Parallel to the increase in the V(max) of [(14)C]butyrate uptake, the MCT-1 protein level was also increased in response to PMA incubation. Our studies demonstrated that the butyrate/anion exchange was increased in response to PMA treatment along with the induction in the level of MCT-1 expression in Caco-2 cells.

Transcriptional regulation of the rat NHE3 gene. Functional interactions between GATA-5 and Sp family transcription factors

Expression of sodium-hydrogen exchanger isoform 3 (NHE3) in the intestinal and renal epithelium plays a critical role in sodium absorption and acid/base homeostasis. To decipher rat NHE3 gene regulation, its cis-acting regulatory elements and associated transcription factors were characterized by transient transfection of Caco-2, IEC-6, Qt6, and Drosophila SL2 cells. Deletion and mutational analyses demonstrated that the atypical TATA box located at bp -26/-31 was not necessary for promoter activity, and that a -20/+8-bp fragment represents a functional initiator. Within the 81-bp upstream region, three Sp transcription factor binding sites were critical because their mutation drastically reduced promoter activity. The roles of Sp1 and Sp3 were further demonstrated by electromobility shift assay and by transactivation of the NHE3 promoter in SL2 cells by forced expression of Sp1 and Sp3. Both of these transcription factors were found to act synergistically with GATA-5 bound to a GATA box in exon 1 (+20/+23 bp). These studies demonstrate that rat NHE3 promoter is initiator-driven and controlled mainly by Sp1 and Sp3, which functionally interact with GATA-5. This interaction represents a novel regulatory mechanism, which is likely to participate in a gradient of intestinal gene expression along the crypt-villus axis.

Inhibition of apical Cl-/OH- exchange activity in Caco-2 cells by phorbol esters is mediated by PKCepsilon

The present studies were undertaken to examine the possible regulation of apical membrane Cl-/OH- exchanger in Caco-2 cells by protein kinase C (PKC). The effect of the phorbol ester phorbol 12-myristate 13-acetate (PMA), an in vitro PKC agonist, on OH- gradient-driven 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS)-sensitive 36Cl uptake in Caco-2 cells was assessed. The results demonstrated that PMA decreased apical Cl-/OH- exchanger activity via phosphatidylinositol 3-kinase (PI3-kinase)-mediated activation of PKCepsilon. The data consistent with these conclusions are as follows: 1) short-term treatment of cells for 1-2 h with PMA (100 nM) significantly decreased Cl-/OH- exchange activity compared with control (4alpha-PMA); 2) pretreatment of cells with specific PKC inhibitors chelerythrine chloride, calphostin C, and GF-109203X completely blocked the inhibition of Cl-/OH- exchange activity by PMA; 3) specific inhibitors for PKCepsilon (Ro-318220) but not PKCalpha (Go-6976) significantly blocked the PMA-mediated inhibition; 4) specific PI3-kinase inhibitors wortmannin and LY-294002 significantly attenuated the inhibitory effect of PMA; and 5) PI3-kinase activators IRS-1 peptide and phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P(3)] mimicked the effects of PMA. These findings provide the first evidence for PKCepsilon-mediated inhibition of Cl-/OH- exchange activity in Caco-2 cells and indicate the involvement of the PI3-kinase-mediated pathways in the regulation of Cl- absorption in intestinal epithelial cells.

Regulation of NHE3 by nitric oxide in Caco-2 cells

The effect of nitric oxide (NO) on Na+/H+ exchange (NHE) activity was investigated utilizing Caco-2 cells as an experimental model. Incubation of Caco-2 cells with 10(-3) M S-nitroso-N-acetylpenicillamine (SNAP), a conventional donor of NO, for 20 min resulted in a approximately 45% dose-dependent decrease in NHE activity, as determined by assay of ethylisopropylamiloride-sensitive 22Na uptake. A similar decrease in NHE activity was observed utilizing another NO-specific donor, sodium nitroprusside. SNAP-mediated inhibition of NHE activity was not secondary to a loss of cell viability. NHE3 activity was significantly reduced by SNAP (P < 0.05), whereas NHE2 activity was essentially unaltered. The effects of SNAP were mediated by the cGMP-dependent signal transduction pathway as follows: 1) LY-83583 and 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ), specific inhibitors of soluble guanylate cyclase, blocked the inhibitory effect of SNAP on NHE; 2) 8-bromo-cGMP mimicked the effects of SNAP on NHE activity; 3) the SNAP-induced decrease in NHE activity was counteracted by a specific protein kinase G inhibitor, KT-5823 (1 microM); 4) chelerythrine chloride (2 microM) or calphostin C (200 nM), specific protein kinase C inhibitors, did not affect inhibition of NHE activity by SNAP; 5) there was no cross activation by the protein kinase A-dependent pathway, as the inhibitory effects of SNAP were not blocked by Rp-cAMPS (25 microM), a specific protein kinase A inhibitor. These data provide novel evidence that NO inhibits NHE3 activity via activation of soluble guanylate cyclase, resulting in an increase in intracellular cGMP levels and activation of protein kinase G.