Na؉ /H ؉ exchanger isoform 3 (NHE3), an epithelial brush border isoform of the Na ؉ /H ؉ exchanger gene family, plays an important role in reabsorption of Na ؉ in the small intestine, the colon, and the kidney. In several cell types, phorbol 12-myristate 13-acetate (PMA) acutely inhibits NHE3 activity by changes in V max , but the mechanism of this inhibition is unknown. We investigated the role of subcellular redistribution of NHE3 in the PMA-induced inhibition of endogenous brush border NHE3 in a model human colon adenocarcinoma cell line, Caco-2. Subcellular localization of NHE3 was examined by confocal morphometric analysis complemented with cell surface biotinylation and compared with NHE3 activity evaluated by fluorometric measurement of intracellular pH. PMA inhibited NHE3 activity by 28% (p < 0.01), which was associated with a decrease of the ratio of the brush border/subapical cytoplasmic compartment of NHE3 from ϳ4.3 to ϳ2.4. This translocation resulted in 10 -15% of the total cell NHE3 being shifted from the brush border pool to the cytoplasmic pool. These effects were mediated by protein kinase C, since they were blocked by the protein kinase C inhibitor H7. We conclude that inhibition of NHE3 by protein kinase C in Caco-2 cells involves redistribution of the exchanger from brush border into a subapical cytoplasmic compartment, and that this mechanism contributes ϳ50% to the overall protein kinase C-induced inhibition of the exchanger.

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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, ϳ25% 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 ϳ50% 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.In the mammalian intestine, sodium and water are reabsorbed by multiple mechanisms which include the activity of Na ϩ /H ϩ exchanger NHE3. 1 This transmembrane protein is expressed in the epithelium of renal tubules, intestine, gall bladder, and salivary gland, where it was localized to the apical microvillar domain and, at least in the kidney and in the intestine, to an yet undefined cytoplasmic compartment (1-4).In the small intestine, NHE3 participates in neutral NaCl absorption, and in the increase in Na ϩ absorption that occurs via neurohormonal stimulation after meals (5). The activity of NHE3 is acutely regulated by multiple mechanisms involving growth factors and protein kinases (6). We and others have shown that stimulation of NHE3 activity by growth factors, okadaic acid, and serum occurs via an increase in the maximal velocity (V max ) of the exchange, whereas phorbol ester and carbachol inhibits NHE3 via a decrease in V max (6). These effects were observed in non-polarized mesenchymal cells as well as in epithelial cells, and they suggested that at least part of the acute regulation might be accomplished by rapid changes in the number of active exchanger molecules at the plasma membrane.Over the last few years, a growing body of evidence has indicated that NHE3 might, indeed, be regulated by redistribution of the exchanger molecules between the cytoplasm and the plasma membrane. Thus, recycling of NHE3 has been suggested in kidney epithelial cells based on the results of subcellular fractionation experiments (7,8), and on the presence of an intracellular compartment accumulating NHE3 (1). Moreover, the protein kinase C-mediated inhibition of endogenous NHE3 in the human colonic adenocarcino...

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 μM H+/s, whereas AP NHE1 activity decreased from 192 to 18 μM 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 μM) 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.

S U M M A R YWe developed a confocal morphometric analysis to quantitate the relative plasma membrane (PM) expression of the Na/H exchanger NHE3 in living PS120 fibroblasts. NHE3 is a membrane transport protein that is acutely regulated by changes in the number of molecules expressed at the PM. To quantitate the PM expression of NHE3 under various experimental conditions, we stably expressed a chimera of rabbit NHE3 and green fluorescent protein (NHE3-GFP) in PS120 fibroblasts. A three-dimensional (3D) map of the intracellular distribution of NHE3-GFP was obtained by confocal laser scanning microscopy (CLSM) of cells superfused with a styryl dye, FM 4-64. This fluorophore rapidly and reversibly labeled the outer lipid layer of the PM, which allowed generation of a digital mask of the PM and calculation of the fraction of a total cellular NHE3-GFP expressed at the PM. This analysis was successfully used to quantitate the relative PM expression of NHE3-GFP in control cells (25%) and a decrease in the expression caused by subsequent exposure of cells to wortmannin (5.1%). Reliability of the method was confirmed by cell surface biotinylation, which yielded very similar results. Confocal morphometric analysis is fast and reproducible and could potentially be used for investigations on regulation of expression of other membrane proteins. The sodium / hydrogen exchanger NHE3 is a transmembrane protein expressed at the apical membrane domain of Na ϩ -absorbing epithelia, predominantly in the kidney and in the small and large intestine. The protein performs Na/H exchange with stoichiometry 1:1 and is driven by the Na ϩ concentration gradient across the plasma membrane (reviewed in Donowitz et al. 1996). In the kidney, NHE3 plays a role in net NaCl, HCO 3 , and NH 4 reabsorption in renal tubules (Paillard 1997). In the intestine, the exchanger participates in both basal and neurohormonally induced postprandial NaCl absorption in the intestine (reviewed in Donowitz et al. 1999). Acute regulation of NHE3 activity by protein kinases and growth factors occurs within minutes and is mediated predominantly by changes in maximal velocity (V max ) of the exchange (Donowitz et al. 1999). Such a mechanism suggests rapid changes either in the number of NHE3 molecules at the plasma membrane (PM) or in the turnover number (number of exchange cycles per molecule per second), or both. Recent evidence obtained from studies on non-epithelial and on polarized epithelial cells suggest that rapid changes in NHE3 activity might indeed be mediated by removal and/or insertion of the exchanger molecules from the plasma membrane (D'Souza et

Sertoli cells in vivo are highly polarized and interact with the inner (tubular) and outer (interstitial) fluids. To simulate this condition in vitro we developed a two-compartment culture system in which confluent Sertoli cell monolayers were grown on permeable supports (Millipore filters) separating the inner and outer fluid compartments. Monolayer permeability to (3H)-inulin decreased by 90% after 5 to 7 days of culture, presumably due to formation of tight junctions. This process was influenced by cell plating density. The cells were highly polarized morphologically, resembling their appearance in vivo, and secreted transferrin bidirectionally into both fluid compartments. The amount of transferrin secreted was 166% to 250% of that secreted by the same number of Sertoli cells cultured in plastic dishes. Testosterone (5 X 10(-8) M) doubled and testosterone + FSH (0.1 microgram/ml) increased transferrin secretion 3.6-fold. These results demonstrate that under suitable culture conditions the Sertoli cells remain both morphologically and functionally polarized, reflecting a more physiologic state.

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Lipid peroxidation products have signaling functions and at higher concentrations are toxic and may trigger cell death. The compounds are metabolized predominantly by glutathione S-transferases exemplified by mGSTA4-4, an enzyme highly efficient in glutathione conjugation of 4-hydroxyalkenals, and possessing glutathione peroxidase activity toward phospholipid hydroperoxides. mGSTA4-4 belongs to the predominant group of "canonical" glutathione S-transferases that are soluble and generally localized in the cytoplasm. The intracellular localization of mGSTA4-4 was examined in hepatocytes of normal mouse liver and in transfected HepG2 cells by fluorescence microscopy and digital deconvolution. mGSTA4-4 was found to be predominantly localized at or near the plasma membrane in transfected HepG2 cells, as well as in hepatocytes endogenously expressing the protein. In vitro, mGSTA4-4 associated with liposomes, and this interaction was potentiated when the liposomes contained negatively charged phospholipids. Mutating lysine 115 to glutamic acid resulted in a loss of the plasma membrane targeting of mGSTA4-4 as well as in a significant reduction of its binding to liposomes in vitro. These data suggest preferential targeting of mGSTA4-4 to the plasma membrane that may contain the major substrate(s) for this enzyme. Lysine 115 is critically important for the membrane association of mGSTA4-4, most likely by entering into an electrostatic interaction with negatively charged phospholipid headgroups.