Brush border phosphatidylinositol 3-kinase mediates epidermal growth factor stimulation of intestinal NaCl absorption and Na+/H+ exchange

Epidermal growth factor regulation of rat NHE2 gene expression

Epidermal growth factor (EGF) is involved in acute regulation of Na(+)/H(+) exchangers (NHEs), but the effect of chronic EGF administration on NHE gene expression is unknown. The present studies showed that EGF treatment increased NHE2-mediated intestinal brush-border membrane vesicle Na(+) absorption and NHE2 mRNA abundance by nearly twofold in 19-day-old rats. However, no changes were observed in renal NHE2 mRNA or intestinal and renal NHE3 mRNA abundance. To understand the mechanism of this regulation, we developed the rat intestinal epithelial (RIE) cell as an in vitro model to study the effect of EGF on NHE2 gene expression. EGF increased functional NHE2 activity and mRNA abundance in cultured RIE cells, and this stimulation could be blocked by actinomycin D (a transcriptional inhibitor). Additionally, NHE2 promoter reporter gene assays in transiently transfected RIE cells showed an almost twofold increase in promoter activity after EGF treatment. We conclude that rat NHE2 activity can be stimulated by chronic EGF treatment and that this response is at least partially mediated by gene transcription.

Regulation of ClC-2 chloride channels in T84 cells by TGF-α

The almost ubiquitously expressed ClC-2 chloride channel is activated by hyperpolarization and osmotic cell swelling. Osmotic swelling also activates a different class of outwardly rectifying chloride channels, and several reports point to a link between protein tyrosine phosphorylation and activation of these channels. This study examines the possibility that transforming growth factor-α (TGF-α) modulates ClC-2 activity in human colonic epithelial (T84) cells. TGF-α (0.17 nM) irreversibly inhibited ClC-2 current in nystatin-perforated whole cell patch-clamp experiments, whereas a superimposed reversible activation of the current was observed at 8.3 nM TGF-α. Both effects required activation of the intrinsic epidermal growth factor receptor (EGFR) tyrosine kinase activity, of phosphoinositide 3-kinase, and of protein kinase C. With microspectrofluorimetry of the pH-sensitive fluorescent dye 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, TGF-α was shown to reversibly alkalinize T84 cells at 8.3 nM but not at 0.17 nM, suggesting that 8.3 nM TGF-α-induced alkalinization activates ClC-2 current. This study indicates that ClC-2 channels are targets for EGFR signaling in epithelial cells.

Constitutively active phosphatidylinositol 3-kinase and AKT are sufficient to stimulate the epithelial Na+/H+ exchanger 3

Phosphatidylinositol 3-kinase (PI 3-kinase) is a cytoplasmic signaling molecule that is recruited to activated growth factor receptors and has been shown to be involved in regulation of stimulated exocytosis and endocytosis. One of the downstream signaling molecules activated by PI 3-kinase is the protein kinase Akt. Previous studies have indicated that PI 3-kinase is necessary for basal Na(+)/H(+) exchanger 3 (NHE3) transport and for fibroblast growth factor-stimulated NHE3 activity in PS120 fibroblasts. However, it is not known whether activation of PI 3-kinase is sufficient to stimulate NHE3 activity or whether Akt is involved in this PI 3-kinase effect. We used an adenoviral infection system to test the possibility that activation of PI 3-kinase or Akt alone is sufficient to stimulate NHE3 activity. This hypothesis was investigated in PS120 fibroblasts stably expressing NHE3 after somatic gene transfer using a replication-deficient recombinant adenovirus containing constitutively active catalytic subunit of PI 3-kinase or constitutively active Akt. The adenovirus construct used was engineered with an upstream ecdysone promoter to allow time-regulated expression. Adenoviral infection was nearly 100% at 48 h after infection. Forty-eight hours after infection (24 h after activation of the ecdysone promoter), PI 3-kinase and Akt amount and activity were increased. Increases in both PI 3-kinase activity and Akt activity stimulated NHE3 transport. In addition, a membrane-permeant synthetic 10-mer peptide that binds polyphosphoinositides and increases PI 3-kinase activity similarly enhanced NHE3 transport activity and also increased the percentage of NHE3 on the plasma membrane. The magnitudes of stimulation of NHE3 by constitutively active PI 3-kinase, PI 3-kinase peptide, and constitutively active Akt were similar to each other. These results demonstrate that activation of PI 3-kinase or Akt is sufficient to stimulate NHE3 transport activity in PS120/NHE3 cells.

Short-term regulation of NHE3 by EGF and protein kinase C but not protein kinase A involves vesicle trafficking in epithelial cells and fibroblasts

NHE3 is an intestinal epithelial isoform Na+/H+ exchanger that is present in the brush border of small intestinal, colonic, and gallbladder Na(+)-absorbing epithelial cells. NHE3 is acutely up- and downregulated in response to some G protein-linked receptors, tyrosine kinase receptors, and protein kinases when studied in intact ileum, when stably expressed in PS120 fibroblasts, and in the few studies reported in the human colon cancer cell line Caco-2. In most cases this is due to changes in Vmax of NHE3, although in response to cAMP and squalamine there are also changes in the K'(H+)i of the exchanger. The mechanism of the Vmax regulation as shown by cell surface biotinylation and confocal microscopy in Caco-2 cells and biotinylation in PS120 cells involves changes in the amount of NHE3 on the plasma membrane. In addition, in some cases there are also changes in turnover number of the exchanger. In some cases, the change in amount of NHE3 in the plasma membrane is associated with a change in the amount of plasma membrane. A combination of biochemical studies and transport/inhibitor studies in intact ileum and Caco-2 cells demonstrated that the increase in brush border Na+/H+ exchange caused by acute exposure to EGF was mediated by PI 3-kinase. PI 3-kinase was also involved in FGF stimulation of NHE3 expressed in fibroblasts. Thus, NHE3 is another example of a transport protein that is acutely regulated in part by changing the amount of the transporter on the plasma membrane by a process that appears to involve vesicle trafficking and also to involve changes in turnover number.

Epidermal growth factor regulates the transition from basal sodium absorption to anion secretion in cultured endometrial epithelial cells

The objective of this study was to investigate acute and long-term effects of epidermal growth factor (EGF) and transforming growth factor alpha (TGFalpha) on basal ion transport activity of glandular endometrial epithelial cells in primary culture. The effects of EGF on insulin-dependent regulation of Na+ transport across this epithelium was also investigated. Addition of 1.6 nM EGF or 2 nM TGFalpha to the basolateral, but not the apical, solution inhibited both basal and insulin-stimulated Na+ transport with a maximum response within 45-60 min. This effect was mimicked by the calcium ionophore ionomycin. Incubation with EGF for 4 days inhibited insulin-stimulated Na absorption in a concentration-dependent fashion with an IC(50) value of 0.3 nM. Experiments using amphotericin B-permeabilized monolayers demonstrated that EGF inhibited Na transport by decreasing apical membrane Na conductance without affecting insulin-dependent stimulation of the Na+-K+ ATPase. Addition of EGF or TGFalpha for 24 h resulted in increased basal Cl- secretion in addition to inhibition of Na absorption. The EGF-induced increase in Cl- secretion was inhibited in part by indomethacin, suggesting that long-term regulation by EGF involves stimulation of arachidonic acid synthesis and prostaglandin release. The EGF-induced increase in indomethacin-insensitive Cl- secretion was prevented by the protein synthesis inhibitor cyclohexamide, and by the DNA transcription inhibitor actinomycin D indicating that EGF-stimulated anion secretion required DNA transcription and protein synthesis. The results of these studies demonstrated that the basal transport properties of endometrial epithelial cells are differentially regulated by EGF, TGFalpha, and insulin.

Hyposmolality stimulates Na(+)/H(+) exchange and HCO(3)(-) absorption in thick ascending limb via PI 3-kinase

The signal transduction mechanisms that mediate osmotic regulation of Na(+)/H(+) exchange are not understood. Recently we demonstrated that hyposmolality increases HCO(3)(-) absorption in the renal medullary thick ascending limb (MTAL) through stimulation of the apical membrane Na(+)/H(+) exchanger NHE3. To investigate the mechanism of this stimulation, MTALs from rats were isolated and perfused in vitro with 25 mM HCO(3)(-)-containing solutions. The phosphatidylinositol 3-kinase (PI 3-K) inhibitors wortmannin (100 nM) and LY-294002 (20 microM) blocked completely the stimulation of HCO(3)(-) absorption by hyposmolality. In tissue strips dissected from the inner stripe of the outer medulla, the region of the kidney highly enriched in MTALs, hyposmolality increased PI 3-K activity 2. 2-fold. Wortmannin blocked the hyposmolality-induced PI 3-K activation. Further studies examined the interaction between hyposmolality and vasopressin, which inhibits HCO(3)(-) absorption in the MTAL via cAMP and often is involved in the development of plasma hyposmolality in clinical disorders. Pretreatment with arginine vasopressin, forskolin, or 8-bromo-cAMP abolished hyposmotic stimulation of HCO(3)(-) absorption, due to an effect of cAMP to inhibit hyposmolality- induced activation of PI 3-K. In contrast to their effects to block stimulation by hyposmolality, PI 3-K inhibitors and vasopressin have no effect on inhibition of apical Na(+)/H(+) exchange (NHE3) and HCO(3)(-) absorption by hyperosmolality. These results indicate that hyposmolality increases NHE3 activity and HCO(3)(-) absorption in the MTAL through activation of a PI 3-K-dependent pathway that is inhibited by vasopressin and cAMP. Hyposmotic stimulation and hyperosmotic inhibition of NHE3 are mediated through different signal transduction mechanisms.

CYP2C40, a unique arachidonic acid 16-hydroxylase, is the major CYP2C in murine intestinal tract

We recently identified five different murine CYP2C cDNAs from a murine cDNA library. When expressed in a bacterial cDNA expression system, all five recombinant proteins metabolized arachidonic acid but produced distinctly different profiles. In addition, some CYP2C mRNAs were found in extrahepatic tissues, as well as in liver. Immunoblots with an antibody raised against recombinant CYP2C38, which recognizes all five murine CYP2Cs, demonstrated that among extrahepatic tissues, colon and cecum contained the highest amount of CYP2Cs. The highest concentration of CYP2Cs occurred in cecum and colon (cecum >/= proximal colon >> distal colon), with lower levels in duodenum, jejunum, and ileum. Immunohistochemical studies revealed that CYP2Cs were localized principally in epithelial cells and autonomic ganglia in gut and colon. Polymerase chain reaction amplification of reverse-transcribed mRNA using murine CYP2C-specific primers followed by cloning and sequencing identified CYP2C40 as the major CYP2C isoform expressed in murine intestinal tract. Recombinant CYP2C40 metabolized arachidonic acid in a regio- and stereospecific manner to 16(R)-HETE (hydroxyeicosatetraenoic acid) as the major product. To our knowledge, CYP2C40 is the first enzyme known to produce primarily 16-HETE. We conclude that CYP2C40 is one of the major cytochrome P450 proteins in the mouse intestinal tract. In the light of vasoactive and anti-neutrophilic effects of 16-HETE, we hypothesize that CYP2C40 may play an important role in endogenous biological functions in intestine.

Basic fibroblast growth factor stimulates surface expression and activity of Na(+)/H(+) exchanger NHE3 via mechanism involving phosphatidylinositol 3-kinase

Na(+)/H(+) exchanger NHE3 is a plasma membrane (PM) protein, which contributes to Na(+) absorption in the intestine. Growth factors stimulate NHE3 via phosphatidylinositol 3-kinase (PI3-K), but mechanism of this process is not clear. To examine the hypothesis that growth factors stimulate NHE3 by modulating NHE3 recycling, and that PI3-K participates in this mechanism, we used PS120 fibroblasts expressing a fusion protein of NHE3 and green fluorescent protein. At steady state, approximately 25% of cellular NHE3 content was expressed at PM. Inhibition of PI3-K decreased PM expression of NHE3, which correlated with retention of the exchanger in recycling endosomal compartment. In contrast, basic fibroblast growth factor (bFGF) increased PM expression of NHE3, which was associated with a 2-fold increase in rate constant for exit of the exchanger from the recycling compartment. Qualitatively similar effects of bFGF were observed in cells pretreated with PI3-K inhibitors, but their magnitude was only approximately 50% of that in intact cells. These data suggest that: (i) bFGF stimulates NHE3 by increasing PM expression of the exchanger; (ii) PI3-K mediates PM expression of NHE3 in both basal and bFGF-stimulated conditions, and (iii) not all of the effects of bFGF on NHE3 expression are mediated by PI3-K, suggesting additional regulatory mechanisms.

Insulin stimulates NHE1 activity by sequential activation of phosphatidylinositol 3-kinase and protein kinase C zeta in human erythrocytes

The signaling cascade linking insulin receptor stimulation to the activation of Na/H exchanger (NHE) was investigated in human erythrocytes, a simple cell model expressing the NHE1 isoform and protein kinase C (PKC) alpha and zeta isoforms only. Our results demonstrate the presence of phosphatidylinositol (PtdIns) 3-kinase in these cells and its activation by insulin. With a similar time-course, insulin also promoted both the translocation and activation of PKC zeta, but had no effect on PKC alpha. Inhibition of PtdIns 3-kinase with wortmannin prevented the activation of PKC zeta by insulin. Stimulation of NHE1 was observed after 10 min of insulin treatment and persisted for at least 60 min. This effect was totally abolished by wortmannin or GF 109203X, an inhibitor of all PKC isoforms, but not by Gö 6976, a specific inhibitor of conventional and novel PKCs (e.g. PKC alpha). These data indicate that PKC zeta activation is mediated by a PtdIns 3-kinase-dependent mechanism and that NHE1 stimulation involves the sequential activation of PtdIns 3-kinase and PKC zeta. In addition, insulin stimulation of NHE1 occurred without altering the phosphorylation state of the exchanger, suggesting that the phosphorylation of an ancillary protein by PKC zeta would be responsible for activation of the transporter.

Epidermal growth factor regulation in adult rat alveolar type II cells of amiloride-sensitive cation channels

Using the patch-clamp technique, we studied the effects of epidermal growth factor (EGF) on whole cell and single channel currents in adult rat alveolar epithelial type II cells in primary culture in the presence or absence of EGF for 48 h. In symmetrical sodium isethionate solutions, EGF exposure caused a significant increase in the type II cell whole cell conductance. Amiloride (10 microM) produced approximately 20-30% inhibition of the whole cell conductance in both the presence and absence of EGF, such that EGF caused the magnitude of the amiloride-sensitive component to more than double. Northern analysis showed that alpha-, beta- and gamma-subunits of rat epithelial Na(+) channel (rENaC) steady-state mRNA levels were all significantly decreased by EGF. At the single channel level, all active inside-out patches demonstrated only 25-pS channels that were amiloride sensitive and relatively nonselective for cations (P(Na(+))/P(K(+)) approximately 1.0:0.48). Although the biophysical characteristics (conductance, open-state probability, and selectivity) of the channels from EGF-treated and untreated cells were essentially identical, channel density was increased by EGF; the modal channel per patch was increased from 1 to 2. These findings indicate that EGF increases expression of nonselective, amiloride-sensitive cation channels in adult alveolar epithelial type II cells. The contribution of rENaC to the total EGF-dependent cation current under these conditions is quantitatively less important than that of the nonselective cation channels in these cells.

Phosphoinositide 3-kinase is required for aldosterone-regulated sodium reabsorption

Aldosterone, a steroid hormone, regulates renal Na+ reabsorption and, therefore, plays an important role in the maintenance of salt and water balance. In a model renal epithelial cell line (A6) we have found that phosphoinositide 3-kinase (PI 3-kinase) activity is required for aldosterone-stimulated Na+ reabsorption. Inhibition of PI 3-kinase by the specific inhibitor LY-294002 markedly reduces both basal and aldosterone-stimulated Na+ transport. Further, one of the products of PI 3-kinase, phosphatidylinositol 3,4,5-trisphosphate, is increased in response to aldosterone in intact A6 monolayers. This increase occurs just before the manifestation of the functional effect of the hormone and is also inhibited by LY-294002. With the use of blocker-induced noise analysis, it has been demonstrated that inhibition of phosphoinositide formation causes an inhibition of Na+ entry in both control and aldosterone-pretreated cultures by reducing the number of open functional epithelial Na+ channels (ENaCs) in the apical membrane of the A6 cells. These novel observations indicate that phosphoinositides are required for ENaC expression and suggest a mechanism for aldosterone regulation of channel function.

Development of an endogenous epithelial Na(+)/H(+) exchanger (NHE3) in three clones of caco-2 cells

Expression of endogenous Na(+)/H(+) exchangers (NHEs) NHE3 and NHE1 at the apical (AP) and basolateral (BL) membrane domains was investigated in three clones (ATCC, PF-11, and TC-7) derived from the human adenocarcinoma cell line Caco-2. In all three clones, NHE1 was the only isoform detected at the BL domain during 3 to 22 postconfluent days (PCD). In clone PF-11, the BL NHE1 activity increased up to 7 PCD and remained stable thereafter. Both NHE1 and NHE3 were found at the AP domain at 3 PCD and contributed 67 and 33% to the total AP Na(+)/H(+) exchange, respectively. The AP NHE3 activity increased significantly from 3 to 22 PCD, from 93 to 450 microM H(+)/s, whereas AP NHE1 activity decreased from 192 to 18 microM H(+)/s during that time. Similar results were obtained with the ATCC clone, whereas very little AP NHE3 activity was observed in clone TC-7. Surface biotinylation and indirect immunofluorescence confirmed these results and also suggested an increase in the number of cells expressing NHE3 being the major mechanism of the observed overall increase in NHE3 activity in PF-11 and ATCC clones. Phorbol 12-myristate 13-acetate (PMA, 1 microM) acutely inhibited NHE3 activity by 28% of control, whereas epidermal growth factor (EGF, 200 ng/ml) stimulated the activity by 18%. The effect of PMA was abolished by the protein kinase C (PKC) inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, suggesting involvement of PKC in the PMA-induced inhibition of NHE3. Similar magnitude of inhibition by PMA and stimulation by EGF was observed at 7 and 17 PCD, suggesting the development of regulatory mechanisms in the early postconfluent period. Taken together, these data suggest a close similarity of membrane targeting and regulation of endogenous NHE3 between Caco-2 cells and native small intestinal epithelial cells and support the usefulness of some Caco-2 cell clones as an in vitro model for studies on physiology of NHE3 in the intestinal epithelium.

TGF-alpha reduces bradykinin-stimulated ion transport and prostaglandin release in human colonic epithelial cells

The effect of chronic exposure to transforming growth factor-alpha (TGF-alpha) on bradykinin-stimulated acute prostanoid production and ion secretion in monolayers of HCA-7 colony 29 colonic epithelial cells has been studied. Monolayers synthesized prostaglandin E2 (PGE2) at a basal rate of 2.10 +/- 0.31 pg. monolayer-1. min-1 over 24 h. Bradykinin (10(-8)-10(-5) M) dose dependently increased acute PGE2 release by three orders of magnitude. This was associated with a rise in cAMP from 1.60 +/- 0.14 to 2.90 +/- 0.1 pmol/monolayer (P < 0.02) and a dose-dependent increase in short-circuit current (SCC). When monolayers were primed by a 24-h exposure to TGF-alpha, basal PGE2 release rose to 6.31 +/- 0.38 pg. monolayer-1. min-1 (TGF-alpha concn 10 ng/ml; P = 0.001). However, the stimulation of acute prostaglandin release, intracellular cAMP, and increased SCC by bradykinin was significantly reduced by preincubation with TGF-alpha. Priming with PGE2 (10(-8)-10(-6) M) over 24 h mimicked the effect of TGF-alpha on bradykinin-induced changes in cAMP and SCC. These data suggest that enhanced chronic release of prostaglandins in response to stimulation with TGF-alpha may downregulate acute responses to bradykinin. In vivo, TGF-alpha could have an important modulatory function in regulating secretion under inflammatory conditions.

Nerve growth factor inhibits HCO3- absorption in renal thick ascending limb through inhibition of basolateral membrane Na+/H+ exchange

Nerve growth factor (NGF) inhibits transepithelial HCO3- absorption in the rat medullary thick ascending limb (MTAL). To investigate the mechanism of this inhibition, MTALs were perfused in vitro in Na+-free solutions, and apical and basolateral membrane Na+/H+ exchange activities were determined from rates of pHi recovery after lumen or bath Na+ addition. NGF (0.7 nM in the bath) had no effect on apical Na+/H+ exchange activity, but inhibited basolateral Na+/H+ exchange activity by 50%. Inhibition of basolateral Na+/H+ exchange activity with ethylisopropyl amiloride (EIPA) secondarily reduces apical Na+/H+ exchange activity and HCO3- absorption in the MTAL (Good, D. W., George, T., and Watts, B. A., III (1995) Proc. Natl. Acad. Sci. U. S. A. 92, 12525-12529). To determine whether a similar mechanism could explain inhibition of HCO3- absorption by NGF, apical Na+/H+ exchange activity was assessed in physiological solutions (146 mM Na+) by measurement of the initial rate of cell acidification after lumen EIPA addition. Under these conditions, in which basolateral Na+/H+ exchange activity is present, NGF inhibited apical Na+/H+ exchange activity. Inhibition of HCO3- absorption by NGF was eliminated in the presence of bath EIPA or in the absence of bath Na+. Also, NGF blocked inhibition of HCO3- absorption by bath EIPA. We conclude that NGF inhibits basolateral Na+/H+ exchange activity in the MTAL, an effect opposite from the stimulation of Na+/H+ exchange by growth factors in other systems. NGF inhibits transepithelial HCO3- absorption through inhibition of basolateral Na+/H+ exchange, most likely as the result of functional coupling in which primary inhibition of basolateral Na+/H+ exchange activity results secondarily in inhibition of apical Na+/H+ exchange activity. These findings establish a role for basolateral Na+/H+ exchange in the regulation of renal tubule HCO3- absorption.

Growth hormone stimulates, through tyrosine kinase, ion transport and proliferation in human intestinal cells

BACKGROUND

Growth hormone (GH) stimulates intestinal growth and differentiation and promotes water and ion absorption in the rat intestine. Epidermal growth factor has similar effects, which involve tyrosine kinase activity. The effects of growth hormone on ion transport and cell growth and the role of tyrosine kinase in these effects were examined in a human-derived intestinal cell line (Caco-2).

METHODS

For transport study, electrical parameters were measured in human intestinal Caco-2 cell monolayers mounted in Ussing chambers. Cell growth was monitored by counting and 3H-thymidine incorporation in the presence and absence of growth hormone. The role of tyrosine kinase was investigated by using its specific inhibitor genistein.

RESULTS

The addition of growth hormone induced a rapid, Cl- -dependent, decrease in short-circuit current without affecting tissue conductance, which is consistent with an anion-absorptive effect. Incubation with growth hormone increased cell count by 85% and 3H-thymidine incorporation by 64% versus the count in control specimens. The absorptive and trophic effects of growth hormone were dose-dependent, and the maximum effective concentration was identical for each effect. Genistein blocked the growth hormone effect on ion transport and cell growth.

CONCLUSIONS

Growth hormone stimulates ion absorption and cell growth in human enterocytes. Both effects result from a direct growth hormone-enterocyte interaction, and both require tyrosine kinase activity. Growth hormone may have therapeutic potential in intestinal diseases characterized by epithelial atrophy and loss of water and electrolytes.

Role of actin in EGF-induced alterations in enterocyte SGLT1 expression

Na+-glucose cotransporter (SGLT1) expression and the role of actin in epidermal growth factor (EGF)-induced alterations in glucose transport and brush-border surface area were examined in New Zealand White rabbit jejunal loops. In separate experiments, EGF or EGF concurrent with cytochalasin D, an inhibitor of actin polymerization, was administered to the experimental loop and compared with its vehicle control. SGLT1 expression was measured by Western blot in brush-border membrane vesicles (BBMV) after 5-min and 1-h exposure. Glucose kinetics were determined by a rapid filtration technique, and brush-border surface area was examined by electron microscopy after 1-h exposure. The effect of cytochalasin D alone on BBMV glucose kinetics and brush-border surface area was also assessed. EGF resulted in a significant increase in BBMV SGLT1 expression (P < 0.05), glucose maximal uptake (Vmax; P < 0.001), and absorptive brush-border surface area (P < 0.001). These effects were abolished with concurrent cytochalasin D treatment. Cytochalasin D alone had no effect on glucose transport or brush-border surface area. The findings suggest that EGF acutely upregulates jejunal brush-border surface area and the Vmax for jejunal glucose uptake via the recruitment and insertion of SGLT1 from an internal pool into the brush border by a mechanism that is dependent on actin polymerization.

Epoxyeicosatrienoic acids and their sulfonimide derivatives stimulate tyrosine phosphorylation and induce mitogenesis in renal epithelial cells

In our present studies utilizing a well characterized proximal tubule cell line, LLCPKcl4, we determined that all four EET regioisomers (5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET) stimulated [3H]thymidine incorporation, with 14,15-EET being the most potent. In contrast, no mitogenic effects were seen with arachidonic acid, other cP450 arachidonate metabolites (12R-hydroxyeicosatetraenoic acid (12R-HETE), 14,15-dihydroxyeicosatrienoic acid (14,15-DHET), or 20-HETE), or lipoxygenase metabolites (5S-HETE, leukotriene B4, or lipoxin A4). We found that their metabolically more stable sulfonimide (SI) analogs (11,12-EET-SI and 14,15-EET-SI) were also potent mitogens. In addition 14,15-EET-SI also increased cell proliferation as well as expression of both c-fos and egr-1 mRNA. The protein kinase C and A inhibitors, H-7 and H-8, or the cyclooxygenase inhibitor, indomethacin, had no effect upon 14, 15-EET-induced [3H]thymidine incorporation, but the selective tyrosine kinase inhibitor, genistein, significantly inhibited it. Immunoprecipitation and immunoblotting demonstrated increased tyrosine phosphorylation of PI3-kinase and epidermal growth factor receptor (EGFR) within 1 min of EET administration. EETs also stimulated association of PI3-kinase with EGFR. PI3-kinase inhibitors, wortmannin and LY 294002, markedly inhibited 14, 15-EET-SI-stimulated [3H]thymidine incorporation. In addition, 14, 15-EET-SI administration stimulated tyrosine phosphorylation of src homologous and collagen-like protein (SHC) and association of SHC with both growth factor receptor-binding protein (GRB2) and EGFR. Mitogen-activated protein kinase was also activated within 5 min. Pretreatment of the cells with the mitogen-activated protein kinase kinase inhibitor, PD98059, inhibited the 14,15-EET-SI-stimulated [3H]thymidine incorporation. Moreover, immunoblotting indicated that 14,15-EET stimulated tyrosine phosphorylation of the specific pp60(c-src) substrate p120 and c-Src association with EGFR. 14, 15-EET increased src kinase activity within 1 min. Our data indicate that EETs are potent mitogens for renal epithelial cells, and the mitogenic effects of the EETs are mediated, at least in part, by the activation of Src kinase and initiation of a tyrosine kinase phosphorylation cascade.

Effects of orally administered epidermal growth factor on enteropathogenic Escherichia coli infection in rabbits

The increased intestinal absorption induced by epidermal growth factor (EGF) is associated with diffuse lengthening of brush border microvilli. The aim of this study was to examine the in vivo effects of oral administration of EGF during infection with enteropathogenic Escherichia coli. New Zealand White rabbits (4 weeks old) received orogastric EGF daily starting 3 days prior to infection with enteropathogenic E. coli RDEC-1 and were compared with sham-treated infected animals and uninfected controls. Weight gain, food intake, fecal E. coli, and stool consistency were assessed daily. On day 10, segments of jejunum, ileum, proximal, and distal colon were assessed for gram-negative bacterial colonization, disaccharidase activities, and epithelial ultrastructure. Effects of EGF on E. coli RDEC-1 proliferation were studied in vitro. E. coli RDEC-1 caused diarrhea and reduced weight gain. Seven days postinfection, the small and large intestines were colonized with numerous bacteria, brush border microvilli were disrupted, and maltase and sucrase activities were significantly reduced in the jejunum. Daily treatment with EGF prevented the occurrence of diarrhea and reduction of weight gain. These effects were associated with significant inhibition of E. coli colonization in the small and large intestine, improved jejunal maltase and sucrase activities and reduced microvillous injury. EGF did not affect the proliferation of E. coli in vitro. The findings suggest that EGF protects the gastrointestinal tract against colonization by enteropathogenic E. coli.

Endosomal recycling of the Na+/H+ exchanger NHE3 isoform is regulated by the phosphatidylinositol 3-kinase pathway

The NHE3 isoform of the Na+/H+ exchanger localizes to both the plasmalemmal and endosomal compartments in polarized epithelial and transfected Chinese hamster ovary (AP-1) cells. It is unclear how the distribution of NHE3 between these compartments is regulated. In this study, we examined the potential involvement of phosphatidylinositol 3'-kinase (PI3-K) in regulating the activity and distribution of NHE3, as this lipid kinase has been implicated in modulating vesicular traffic in the endosomal recycling pathway. Wortmannin and LY294002, both potent inhibitors of PI3-K, markedly inhibited NHE3-mediated H+ extrusion across the plasma membrane in a concentration- and time-dependent manner. The subcellular distribution of the antiporters was monitored by transfecting epitope-tagged NHE3 into AP-1 cells. In parallel with the inhibition of transport, PI3-K antagonists induced a pronounced loss of NHE3 from the cell surface and its accumulation in an intracellular compartment, as assessed by immunofluorescence microscopy and enzyme-linked immunosorbent assays. Further analysis using cells transfected with antiporters bearing an external epitope tag revealed that the redistribution reflected primarily a decrease in the rate of recycling of intracellular NHE3 to the cell surface. The wortmannin-induced inhibition and redistribution of NHE3 were prevented when cells were incubated at 4 degreesC, consistent with the known temperature dependence of the endocytic process. These observations demonstrate that NHE3 activity is controlled by dynamic endocytic and recycling events that are modulated by PI3-K.

Mechanisms of EGF-induced stimulation of sodium reabsorption by alveolar epithelial cells

We investigated the effects of epidermal growth factor (EGF) on active Na+ absorption by alveolar epithelium. Rat alveolar epithelial cells (AEC) were isolated and cultivated in serum-free medium on tissue culture-treated polycarbonate filters. mRNA for rat epithelial Na+ channel (rENaC) alpha-, beta-, and gamma-subunits and Na+ pump alpha1- and beta1-subunits were detected in day 4 monolayers by Northern analysis and were unchanged in abundance in day 5 monolayers in the absence of EGF. Monolayers cultivated in the presence of EGF (20 ng/ml) for 24 h from day 4 to day 5 showed an increase in both alpha1 and beta1 Na+ pump subunit mRNA but no increase in rENaC subunit mRNA. EGF-treated monolayers showed parallel increases in Na+ pump alpha1- and beta1-subunit protein by immunoblot relative to untreated monolayers. Fixed AEC monolayers demonstrated predominantly membrane-associated immunofluorescent labeling with anti-Na+ pump alpha1- and beta1-subunit antibodies, with increased intensity of cell labeling for both subunits seen at 24 h following exposure to EGF. These changes in Na+ pump mRNA and protein preceded a delayed (>12 h) increase in short-current circuit (measure of active transepithelial Na+ transport) across monolayers treated with EGF compared with untreated monolayers. We conclude that EGF increases active Na+ resorption across AEC monolayers primarily via direct effects on Na+ pump subunit mRNA expression and protein synthesis, leading to increased numbers of functional Na+ pumps in the basolateral membranes.

G protein-coupled receptors in gastrointestinal physiology. III. Asymmetry in plasma membrane signal transduction: lessons from brush-border Na+/H+ exchangers

Signal transduction in epithelial cells adds another level of complexity to the signaling that occurs in symmetrical cells, in the form of the need to coordinate and keep separate signals at the apical and basolateral membranes. Regulation by protein kinases of ileal NaCl absorption and its component brush-border Na+/H+ exchanger are used as an example of how signaling in epithelial cells must deal with spatial localization of signals, protein-protein interactions, signaling molecules, and the involvement of the transport protein being regulated in collecting and focusing the signals generated at the receptor and beyond.

Phosphatidylinositol 3-kinase activation is required for insulin-stimulated sodium transport in A6 cells

Insulin stimulates amiloride-sensitive sodium transport in models of the distal nephron. Here we demonstrate that, in the A6 cell line, this action is mediated by the insulin receptor tyrosine kinase and that activation of phosphatidylinositol 3-kinase (PI 3-kinase) lies downstream of the receptor tyrosine kinase. Functionally, a specific inhibitor of PI 3-kinase, LY-294002, blocks basal as well as insulin-stimulated sodium transport in a dose-dependent manner (IC50 approximately 6 microM). Biochemically, PI 3-kinase is present in A6 cells and is inhibited both in vivo and in vitro by LY-294002. Furthermore, a subsequent potential downstream signaling element, pp70 S6 kinase, is activated in response to insulin but does not appear to be part of the pathway involved in insulin-stimulated sodium transport. Together with previous reports, these results suggest that insulin may induce the exocytotic insertion of sodium channels into the apical membrane of A6 cells in a PI 3-kinase-mediated manner.

Effects of growth factors on acid-induced damage to rat gastric epithelial cells

We previously established a model of cell damage induced by an acidified medium in the rat gastric epithelial cell line RGM1. Treatment of the cells with epidermal growth factor (EGF, 1-10 ng/ml) for 4 h prevented acid-induced cell damage in a concentration-dependent manner. In contrast, basic fibroblast growth factor (1-10 ng/ml) and platelet-derived growth factor BB (1-10 ng/ml) had no effect on cell damage. EGF did not affect DNA synthesis by the cells for 4 h. Pretreatment of the cells with cycloheximide (10 micrograms/ml) for 1.5 h before EGF treatment significantly attenuated the cytoprotective effect of EGF by > 50%. Replacement of Na+ with K+ in the acidified medium totally abolished the effect of EGF. Co-incubation with amiloride (1 mM) had no influence on the protective effect of EGF. These results indicate that the cytoprotective effect of EGF appears to involve both the activation of amiloride-resistant Na+/H+ exchangers and the synthesis of a new protein related to Na+/H+ exchangers.

The epithelial sodium-hydrogen antiporter Na+/H+ exchanger 3 accumulates and is functional in recycling endosomes

Na+/H+ exchangers (NHEs) mediate electroneutral exchange of Na+ for H+ and thereby play a central role in pH regulation and Na+ homeostasis. NHE3, the predominant epithelial isoform, is found in apical membranes of renal and intestinal epithelial cells, where it contributes to NaCl (re)absorption. NHE activity has been detected in endomembrane vesicles of epithelial cells, but the precise compartment involved and its functional role have not been defined. Many aspects of the targeting machinery that defines the compartmentation and polarity of epithelia are also functional in nonepithelial cells. We therefore compared the targeting of NHE1, the basolateral isoform, with that of NHE3 in Chinese hamster ovary cells. To circumvent the confounding effects of endogenous exchangers, epitope-tagged constructs of NHE1 and NHE3 were stably expressed in antiport-deficient (AP-1) cells. While NHE1 was found almost exclusively in the surface membrane, NHE3 was also found intracellularly, accumulating in a juxtanuclear compartment. Confocal microscopy showed this compartment to be distinct from the Golgi, trans-Golgi network, and lysosomes. Instead, NHE3 colocalized with transferrin receptors and with cellubrevin, markers of recycling endosomes. The activity of NHE3 in endomembranes was assessed by targeting pH-sensitive probes to the recycling endosomes using a chimeric cellubrevin construct with an accessible extracellular epitope. Fluorescence ratio imaging indicated that cellubrevin resides intracellularly in an acidic compartment. In AP-1 cells, endosomal acidification was unaffected by omission of Na+ but was dissipated entirely by concanamycin, a blocker of H(+)-ATPases. In contrast, the cellubrevin compartment was more acidic in NHE3 transfectants, and the acidification was only partially reduced by concanamycin. Moreover, removal of extracellular Na+ resulted in a significant alkalization of the endocytic compartment. These results indicate that NHE3 is present and active in recycling endosomes. By recruiting NHE3 to the plasma membrane, modulation of vesicular traffic could contribute to the regulation of Na+ reabsorption across epithelia.

Identification of sites required for down-regulation of Na+/H+ exchanger NHE3 activity by cAMP-dependent protein kinase. phosphorylation-dependent and -independent mechanisms

We recently identified a region within the cytoplasmic C-terminal tail of the Na+/H+ exchanger NHE3 isoform (residues 579 to 684) which is essential for inhibition of transport activity by cAMP-dependent protein kinase (PKA) (Cabado, A. G., Yu, F. H., Kapus, A., Gergely, L., Grinstein, S., and Orlowski, J. (1996) J. Biol. Chem. 271, 3590-3599). To further define determinants of PKA regulation, six serine residues located in potential recognition sequences for PKA within, or adjacent to, this region (positions 552, 605, 634, 661, 690, and 691) were altered either independently or in various combinations using site-directed mutagenesis. Wild type and mutant NHE3s tagged with the influenza virus hemagglutinin epitope were stably expressed in exchanger-deficient Chinese hamster ovary cells (AP-1) for functional studies. Of the individual mutations examined, only substitutions at Ser605 or Ser634 affected sensitivity to forskolin, an activator of adenylate cyclase, although partial inhibition of NHE3 activity by forskolin remained. By contrast, simultaneous mutation of both these serines completely abolished cAMP-mediated inhibition of NHE3 without greatly affecting basal transport activity. Two-dimensional analysis of tryptic digests of immunoprecipitated NHE3 labeled in vivo with [32P]orthophosphate revealed several phosphopeptides under basal conditions. Phosphorylation was increased approximately 3-fold in one of these peptides following forskolin treatment, and this change was eliminated by mutation of residue Ser605. Thus, phosphorylation of Ser605 is essential for cAMP-mediated inhibition of NHE3. In addition, Ser634 is also required for the effect of cAMP, even though this residue does not become phosphorylated upon activation of PKA.

Cytoprotective effect of epidermal growth factor on acid- and pepsin-induced damage to rat gastric epithelial cells: roles of Na+/H+ exchangers

We have previously established a cell damage model, with damage induced by either acid or pepsin treatment for 30 min, involving a rat gastric epithelial cell line (RGM1). In the present study, pretreatment of cells with epidermal growth factor (EGF; 0.1-10 ng/mL) or sucralfate (0.1-3 mg/mL) for 4 h prevented such cell damage in a concentration-dependent manner. Protection of cells by these drugs was not affected by pretreatment with indomethacin (10(-5) mol/L) for 4 h. Removal of Na+, but not Ca2+, from the acidified medium totally abolished the inhibitory effect of EGF, but not that of sucralfate. Genistein (a tyrosine kinase inhibitor) apparently reduced the inhibitory effect of EGF. DNA synthesis by RGM1 cells did not increase when cells were incubated with EGF for 4 h. We conclude that both EGF and sucralfate protect RGM1 cells from acid- and pepsin-induced damage and that the mechanism of protection by EGF against acid-induced damage seems to be via activation of Na+/H+ exchangers.