Intestinal Anion Absorption

57.1.2.2.2.1

Electroneutral NaCl Absorption: Collaborative Interplay of SLC26A3, NHE3, and Accessory Proteins

Since, the expression and characteristics of Na ⁺ and Cl ⁻ transporters vary remarkably along the length of human intestine, a functional coupling to mediate NaCl absorption requires that NHE3 be coexpressed with SLC26A3 along the apical surface of the intestinal epithelial cells. Interestingly, similar to SLC26A3, NHE3 expression also exhibits regional variations, where it is predominantly expressed on the apical membrane of surface epithelial cells. Recent studies utilizing quantitative immunofluorescence microscopy in consecutive colon segments of rats and mice proposed a topographical model pertaining to spatial distribution of NHE3 and SLC26A3 along the colonic axis. In cecum, proximal colon, and distal colon, NHE3 and SLC26A3 exhibited reciprocal expression profiles ( Fig. 57.4 ). Unpaired NHE3 expression in proximal colon favored H ⁺ secretion with acidic mucosal surface pH (pHms) of 6.2, whereas unpaired SLC26A3 activity in the cecum and distal colon resulted in alkaline mucosal surface pH (~ 7.5, pHms) consistent with net HCO ₃ ⁻ secretion ( Fig. 57.4 ). Based on this model, NHE3 and SLC26A3 coexist only in the mid colon resulting in neutral pHms, favoring coupled electroneutral NaCl absorption. However, direct studies systematically evaluating the function and expression of both NHE3 and SLC26A3 across the length of the human intestine are not available. Additionally, in vivo studies utilizing transgenic mice also suggested coupling of SLC26A3 with NHE3. Direct coupling of SLC26A3 with NHE3 was also demonstrated in Caco-2.BBe cells infected with tetracycline-off-inducible SLC26A3 transgene. The heterologous expression of SLC26A3 in Caco2.BBe cells increased Cl ⁻ uptake that was dependent on both CA and apical NHE activity, suggesting that apical NHE3 and SLC26A3 when coexpressed mediate coupled electorneutral NaCl absorption in the intestine. Further, pharmacological inhibition of NHE3 with EIPA or of SLC26A3 with niflumic acid did not alter pHi, but coordinately inhibited both the exchangers in mouse jejunal epithelium, indicating functional coupling. Hence, both in vitro and in vivo studies point to coupling of the NHE3 and SLC26A3 exchangers, presumably via changes in intracellular pH. In fact, SLC26A3 has been suggested to possess a pHi sensor in the c-terminal domain. Such a pH _i sensor is expected to sense sufficiency of intracellular HCO ₃ ⁻ concentration and to inhibit SLC26A3-mediated Cl ⁻ /HCO ₃ ⁻ exchange before pH _i is acidified as seen in Xenopus oocytes where intracellular acidification inhibited hSLC26A3-mediated Cl ⁻ transport. In parallel, SLC26A3 is stimulated by alkaline pHi. Thus, each component of the NHE3-DRA dyad is allosterically stimulated by the transport activity of the other. Where intracellular acidification in response to Cl ⁻ /HCO ₃ ⁻ exchanger by DRA favors stimulation of NHE3 and alkalinizing effect of NHE3 contributes to activate DRA, emphasizing their cooperative mode of operation.

Further, a model was proposed where a direct structural link between SLC26A3 and NHE3 was established via the PDZ adapter protein E3KARP (NHERF2) through its dimerization. In this model, SLC26A3 protein was shown to interact specifically with the second PDZ domain of E3KARP, through the PDZ interaction motif located in C-terminal region of DRA. As NHE3 also binds to the second PDZ domain of E3KARP, such an interaction enables DRA to be in close proximity with NHE3 attesting the functional coupling of the two transporters ( Fig. 57.3 ). Also, immunofluorescence studies showed DRA, NHE3, and E3KARP to be colocalized in the apical compartment of the human proximal colon indicative of the physiological relevance of this multimolecular assembly. Recent studies have also shown the interaction of SLC26A3 with other members of NHE3 regulatory factor (NHERF) family of PDZ adaptor proteins such as PDZK1 (NHERF3), E3KARP (NHERF2), and IKEPP (NHERF4) in the specialized compartments of plasma membrane known as lipid rafts. Lipid rafts (LR) are dynamic sphingolipid- and cholesterol-rich membrane micro-domains, which are resistant to detergent solubilization. Both NHE3 and SLC26A3 are lipid-raft-associated proteins ; however, whether their functional coupling depends on integrity of intact lipid rafts is not yet entirely understood.

57.1.2.2.2.2

Coupling of SLC26A3 with CA and Interactions with Accessory Proteins

DRA absorbs Cl ⁻ and exports HCO ₃ ⁻ with equal affinity in a Carbonic Anhydrase II (CAII)-dependent fashion, demonstrating that its activity is regulated by intracellular and extracellular ion composition. CA drives the generation of H ⁺ and HCO ₃ ⁻ ions from hydration of CO ₂ and plays a crucial role in coupling of DRA and NHE3 for mediating electroneutral NaCl absorption. CA enzymes are abundantly expressed in the intestine and are present in both the apical membrane (CAIV) and the cytoplasm (CAI and CAII) of enterocytes. The administration of CA inhibitors reduces colonic chloride absorption indicating a functional relationship of CAs and Cl ⁻ /HCO ₃ ⁻ exchangers in intestinal ion homeostasis. The physical and functional complex of a bicarbonate transporter with CA is referred to as “bicarbonate transport metabolon,” which facilitates coupled CO ₂ /HCO ₃ ⁻ metabolism and HCO ₃ ⁻ transport. Indeed, SLC26A3-mediated bicarbonate transport is functionally coupled to the activity of the cytosolic carbonic anhydrase CAII. Also, inhibition of CAII activity by acetazolamide reduced SLC26A3 activity in HEK-293 cells (transiently transfected with SLC26A3 cDNA). However, in contrast to other bicarbonate transporters, SLC26A3 lacks a consensus CAII-binding motif in its C-terminal and does not physically interact with CAII. It was proposed that in the physiological context, an accessory protein binds CAII and localizes the enzyme in the proximity of DRA for optimal function ( Fig. 57.3 ).

57.1.2.2.2.3

Role of SLC26A3 in HCO ₃ ⁻ Secretion-Coupling With CFTR and Preservation of Epithelial Barrier Integrity

Bicarbonate secretion is essential for the maintenance of pH and formation of alkaline mucus barrier against gastric acid in the luminal membrane of the intestine. As mentioned above, the coexistence of SLC26A3 and NHE3 points to a functional coupling between NHE3 and SLC26A3 to drive electroneutral NaCl absorption both in the ileum and colon. However, SLC26A3 is suggested to exhibit multiple transport modes based on its biological environment. In duodenum and pancreas, SLC26A3 activity has been shown to favor duodenal electroneutral HCO ₃ ⁻ secretion compared to NaCl absorption. Functional studies utilizing KO mice supported the role of SLC26A3 in both basal HCO ₃ ⁻ secretion and cAMP-stimulated HCO ₃ ⁻ secretion in duodenum. Therefore, dudodenal bicarbonate secretion is in part electrogenic mediated by CFTR and partly electroneutral due to secretory activity of SLC26A3. The predominant expression of SLC26A3 in the crypt region in duodenum further indicates its role in duodenal HCO ₃ ⁻ secretion. The secretory activity of SLC26A3 might be of relevance in pathophysiology of duodenal disorders including ulcer disease and cystic fibrosis (CF). Cystic fibrosis (CF) is an autosomal recessive disease characterized by aberrant HCO ₃ ⁻ transport and results from mutational inactivation of CFTR, the cAMP-sensitive Cl ⁻ channel. In this regard, studies utilizing knockout mice demonstrated that a functionally active CFTR is necessary for SLC26 (SLC26A3 or SLC26A6)-mediated HCO ₃ ⁻ secretion but not for SLC26-mediated fluid absorption. SLC26A3 interacts directly and functionally with CFTR. Coupling of SLC26A3 with CFTR at the plasma membrane involves interaction between the STAS domain of SLC26A3 and regulatory (R) domain of CFTR resulting in simultaneous increase or decline of ion transport by both the proteins ( Fig. 57.3 ). The indirect association between the two proteins is facilitated by PDZ domain proteins such as E3KARP and CAP70.

Further, SLC26A3 deficiency was correlated to a significant reduction in the rate of HCO ₃ ⁻ secretion in the chronically inflamed distal ileum and colon of TNF ^+/ΔARE mice both by in vitro (Ussing chamber) and in vivo perfusion studies. Also, genetic ablation of DRA resulted in severely reduced colonic HCO ₃ ⁻ secretory rate and the absence of an adherent inner mucus layer in colon of the knockout mice. Normal mucus release requires the presence of HCO ₃ ⁻ anion for efficient sequestration of mucin bound cations thereby allowing its expansion. The absence of protective colonic mucus layer is considered as a potential mechanism involved in the development of colonic inflammation in both murine colitis models and IBD patients. This was also evident from the increased susceptibility of SLC26A3 ^−/− mice to DSS-induced colitis in this study. Based on the above observations, it can be stated that SLC26A3 deficiency not only results in decreased absorptive efficiency of colon but may also lead to impaired barrier function and aggravated inflammation associated with IBD.

57.1.2.2.2.4

Anion Specificity and Inhibitor Sensitivity of Slc26a3

Although, the role of SLC26A3 in mediating apical anion transport in mammalian intestinal epithelial cells is supported from studies performed in Xenopus oocytes, cell lines, CLD patients, and knockout mice models; function of this transporter still remains controversial with respect to anion specificity and inhibition profile. For example, initial studies using microinjected Xenopus oocytes as an assay system showed that mouse SLC26A3 encoded for a Na ⁺ -independent transporter for both sulfate and oxalate. Human and mouse SLC26A3 proteins were shown to mediate Cl ⁻ /OH ⁻ and Cl ⁻ /HCO ₃ ⁻ exchange processes when expressed in Xenopus oocytes and human embryonic kidney (HEK) 293 cell line, respectively. Another report showed hDRA to mediate Cl ⁻ /Cl ⁻ and Cl ⁻ /HCO ₃ ⁻ exchange but not the transport of sulfate or OH ⁻ , when expressed in Xenopus oocytes. Additionally, hSLC26A3 stably transfected in HEK-293 cells was demonstrated to function as an electroneutral Cl ⁻ /HCO ₃ ⁻ exchanger with low Cl ⁻ /OH ⁻ exchange and minimal SO ₄ ^{2 −} /HCO ₃ ⁻ exchange activity. Contrary to this report, a recent study established that in mouse cecum DRA also contributes to SO ₄ ^{2 −} secretion in exchange for HCO ₃ ⁻ in addition to its role as a primary Cl ⁻ /HCO ₃ ⁻ exchanger. In another study, transepithelial Cl ⁻ and oxalate fluxes measured in short-circuited segments of ileum, cecum and distal colon from wild-type and DRA knockout mice revealed that DRA significantly contributed to net oxalate absorption and that its deficiency led to a significant reduction in urinary oxalate excretion. It appears that, in addition to its primary role as Cl ⁻ /HCO ₃ ⁻ exchanger, DRA may also contribute to oxalate and sulfate transport.

In addition, there are significant conflicting reports in the literature with regard to the inhibition of SLC26A3-mediated transport by stilbene derivatives, such as DIDS. For example, SLC26A3-mediated sulfate transport in Xenopus oocytes and Sf9 cells as well as Cl ⁻ transport was highly sensitive to inhibition by DIDS. On the other hand, some studies suggest very low sensitivity of SLC26A3-mediated Cl ⁻ transport to DIDS, when expressed in Xenopus oocytes or HEK 293 cells. The differences in these studies with regard to DIDS sensitivity are not clearly understood yet, and will require more exhaustive analysis of the inhibition profile of this transporter. Potent inhibition of SLC26A3 by the antiinflammatory drugs, such as tenidap and niflumic acid, is also reported.

57.1.2.3.2.1

Role in Cl ⁻ Absorption and Bicarbonate Secretion

SLC26A3 and A6 are the candidate genes mediating apical Cl ⁻ /HCO ₃ ⁻ exchange. In this regard, it has been shown that both mouse and human SLC26A6 function as Cl ⁻ /HCO ₃ ⁻ exchanger, when expressed in Xenopus oocytes. Also, a decrease in net Cl ⁻ and Na ⁺ fluxes observed in jejunum of SLC26A6 KO mice further attested that SLC26A6 couples with NHE3 in mediating electroneutral absorption in the small intestine. Therefore, NHE3 couples with both SLC26A3 and SLC26A6 to mediate electroneutral NaCl absorption ( Fig. 57.3 ). However, function and expression studies suggest that these two exchangers do not exhibit parallel activities but rather show predominance over one another in different regions of the intestine. For example, measurement of apical Cl ⁻ /HCO ₃ ⁻ exchange showed dominance of SLC26A6 activity over SLC26A3 activity in the duodenal upper villus epithelium, whereas; dominant activity of SLC26A3 was observed in mouse jejunal villus epithelium with little contribution by SLC26A6. Also, in proximal jejunum with high CO ₂ tension and HCO ₃ ⁻ concentration, SLC26A6 is involved in Na ⁺ HCO ₃ ⁻ absorption and acid/base balance, and SLC26A3 mainly mediates the absorption of Cl ⁻ . Therefore, both SLC26A3 and A6 have distinct roles in basal and CO ₂ /HCO ₃ ⁻ -stimulated fluid and electrolyte absorption in mouse jejunum, thereby ensuring the absorption of Cl ⁻ and HCO ₃ ⁻ under varying luminal concentrations of these anions. This further highlights the physiological relevance for the presence of two anion transporters with similar ion exchange activity in the same cellular location. These studies together with the mild expression of SLC26A3 in the mouse small intestine compared to the colon, suggest that SLC26A6 is important for the apical Cl ⁻ base exchange in the upper GI tract and SLC26A3 in the lower GI tract. Intriguingly, unlike diarrheal phenotype of SLC26A3-null mice, mice with deletion of SLC26A6 appeared normal at steady state. This may be due to compensatory upregulation of other anion exchangers in knockout mice. Of note, the mRNA expression of DRA remained unaltered in the small intestine of SLC26A6 knockout mice. In addition, it is suggested that the loss of SLC26A6 may lead to a significant phenotype in pathophysiological states. Thus, it appears that although SLC26A6 is involved in mediating Cl ⁻ absorption, it is not directly coupled to water movement as seen with SLC26A3.

Besides Cl ⁻ absorption, SLC26A6 also mediates bicarbonate secretion. As mentioned before, duodenal bicarbonate secretion (DBS) plays an important role in preventing mucosal injury by the acidic chyme secreted from the stomach. Both SLC26A3 and A6 are involved in DBS. Role of SLC26A6 in DBS was further confirmed with reduced baseline bicarbonate secretion and apical Cl ⁻ /HCO ₃ ⁻ exchange activity in the duodenum of SLC26A6-null mice. However, it should be noted that a significant component of HCO ₃ ⁻ secretion was still intact in knockout mice suggesting the involvement of other anion exchangers such as SLC26A3. Unlike SLC26A3, SLC26A6 has been shown to directly bind CAII via the binding site located within the STAS domain ( Fig. 57.3 ). This coupling facilitates the SLC26A6-mediated transmembrane HCO ₃ ⁻ flux by maximizing the HCO ₃ ⁻ concentration at the SLC26A6 transport site. Functionally active CFTR is a prime requirement for SLC26-mediated HCO ₃ ⁻ secretion. However, this does not hold true for SLC26-mediated Cl ⁻ absorption. Therefore, it can be concluded that both SLC26A3 and A6 work in conjunction with NHE3 in Cl ⁻ absorptive mode and require the exit of Cl ⁻ from the cell via CFTR to function in HCO ₃ ⁻ secretory mode ( Fig. 57.3 ). Structural interaction of SLC26A6 and CFTR has been shown to result in reciprocal activation of both transport proteins in vitro.

57.1.2.3.2.2

Role in Oxalate Transport

SLC26A6 serves as a major oxalate-secreting pathway in the small intestine. Mice deficient in SLC26A6 exhibit enhanced oxalate absorption in the small intestine, with subsequent increase in urine oxalate excretion and kidney oxalate stone formation. Additionally, the role of SLC26A6 in oxalate transport was analyzed in Caco-2.BBe monolayers utilizing siRNA knockdown technology. These studies demonstrated that SLC26A6 contributed to about 50% of the apical oxalate exchange. Further, SLC26A6-mediated vectorial transport of oxalate was shown to be net secretory or absorptive (apical oxalate efflux or influx) depending on the magnitude and direction of transepithelial Cl ⁻ gradient. Renal tubule microperfusion studies and renal brush-border vesicles isolated from SLC26A6-null mice demonstrated that Slc26a6 contributes to all oxalate-dependent NaCl transport and a component of formate-dependent NaCl transport. It is interesting to note that mice and humans show differences in susceptibility to nephrolithiasis with mice being highly resistant to nephrolithiasis, whereas humans show significant susceptibility. A recent study demonstrated that the human SLC26A6 and mouse Slc26a6 have divergent anion transport properties that might underlie these variations in susceptibility to oxalate kidney stones. Studies utilizing oocytes injected with the mouse or human RNA demonstrated that mouse Slc26a6 has more affinity for extracellular chloride over the human SLC26A6 in exhibiting oxalate efflux in exchange for chloride influx. The oxalate influx pathways in the intestine and their relevance to kidney stone formation are described elsewhere ( Section 57.4 ).

57.1.3.1.1.1

Hormones and Neuropeptides

Modulatory effects of hormones and neuropeptides on the electrolyte and water absorbing capacity of intestine in various species including human, rat, rabbit, and dog have been reported. For example, an increase in NaCl absorption has been shown to occur in response to catecholamines, glucocorticoids, thyroxine, and the peptide PYY, whereas serotonin was shown to inhibit electroneutral NaCl absorption. Serotonin (100 nM for 1 h) was demonstrated to inhibit apical Cl ⁻ /HCO ₃ ⁻ exchange as well as Na ⁺ /H ⁺ exchange activity in Caco-2 cells. The results of this study indicated that the inhibition of DIDS-sensitive ³⁶ Cl ⁻ uptake by serotonin was mediated via 5-HT3 and 5-HT4 receptors, activation of c-Src kinase and PKCδ-pathway ( Fig. 57.5 ). Whether serotonin inhibits apical Cl ⁻ /HCO ₃ ⁻ exchange by modulating SLC26A3 or SLC26A6 has not yet been delineated.

In contrast to inhibitory effects of serotonin, both in vitro and in vivo studies have shown that proabsorptive hormone NPY increases intestinal Cl ⁻ /HCO ₃ ⁻ exchange activity via ERK1/2 pathway. This stimulation in response to NPY was attributed to increased association of SLC26A3 with lipid-raft fractions with no alteration in its total plasma membrane levels. With respect to the effect of prostaglandins on SLC26A3 or SLC26A6, a recent study demonstrated that Lubiprostone, a prostaglandin E1 derivative increased the apical expression of SLC26A6 in rat jejunum, ileum, and proximal colon, whereas apical SLC26A3 expression was downregulated in these tissues. A significant reduction in the surface levels of NHE3 in rat ileum and colon was also demonstrated in this study. These data indicated that Lubiprostone, suppressed fluid absorption and, therefore, was beneficial for the treatment of chronic constipation.

A number of other molecules, for example, neurotensin, vasoactive intestinal peptide, acetylcholine, guanylin, histamine, bradykinin, leukotrienes, adenosine, and platelet-activating factor have also been shown to be pro-secretory and antiabsorptive in nature. However, direct studies evaluating the effects of these molecules on Cl ⁻ /HCO ₃ ⁻ exchange activity or SLC26A3 and SLC26A6 expression in the mammalian intestine are warranted.

57.1.3.1.1.2

Nitric Oxide and Reactive Oxygen Metabolites (ROMs)

Nitric oxide (NO) is a free radical that plays an important role in regulating several physiological functions of the GI tract, for example, regulation of motility, mucosal permeability, and intestinal ion transport. However, excessive production of nitric oxide and reactive oxygen metabolites including hydrogen peroxide has been implicated in the pathophysiology of inflammatory bowel disorders. With respect to effects on ion transport, NO is shown to exhibit differential effects under normal physiological conditions vs. pathophysiological conditions. NO has been suggested to be a proabsorptive molecule, whereas under pathophysiological conditions it has been shown to stimulate net secretion and may underlie diarrhea associated with IBD. Inhibition of constitutive nitric oxide (cNO) was shown to stimulate crypt cell Cl ⁻ /HCO ₃ ⁻ exchange activity with no effect on the villus cell Cl ⁻ /HCO ₃ ⁻ exchange activity. In contrast, high concentrations of NO were shown to inhibit the apical membrane Cl ⁻ /HCO ₃ ⁻ (OH ⁻ ) exchange activity in Caco-2 cells via both cGMP/PKG and PKC signaling pathways ( Fig. 57.5 ). In parallel studies, apical NHE3 activity was also shown to be inhibited by NO in Caco-2 cells by cGMP but PKC-independent pathways. Whether SLC26A3 or A6 is inhibited by NO has not been investigated and would be of interest. Similar to NO, H ₂ O ₂ was found to acutely inhibit the Cl ⁻ /HCO ₃ ⁻ exchange activity in Caco-2 cells via the involvement of c-Src family kinases including fyn and p60Src, PI3K, and PKCα-mediated pathways. The decrease in Cl ⁻ /HCO ₃ ⁻ exchange activity was independent of alterations in the surface levels of both SLC26A3 and A6. Whether SLC26A3 or A6 are modulated by phosphorylation or lipid rafts mechanism(s) in response to ROM is not yet known.

57.1.3.1.1.3

Bile Acids

Bile acids have been shown to induce diarrhea in various pathological conditions such as inflammatory bowel disease, ileal resection, radiation enteritis, celiac disease, small intestinal bacterial overgrowth (SIBO), chronic pancreatitis, and idiopathic bile acid malabsorption (BAM). Mechanisms underlying diarrhea induced by bile acids are not fully understood and appear to involve a decrease in NaCl absorption with a concomitant increase in Cl ⁻ secretion in the intestine. For example, in ex vivo studies with short-circuited rabbit colon, addition of taurochenodeoxycholic acid (TCDC) from the mucosal side resulted in 50% decrease in net sodium flux and almost complete loss of net chloride flux. In contrast, addition of TCDC to the serosal compartment abolished net sodium flux and stimulated electrogenic Cl ⁻ secretion. Further, TDC (taurodeoxycholic acid) and GCDC (glycochenodeoxycholic acid) were shown to reduce the apical Cl ⁻ /HCO ₃ ⁻ exchange in Caco-2 cells via multiple signal transduction molecules, including cAMP, Ca ^{2 +} , PI3K, and PKC-β1-dependent pathway ( Fig. 57.5 ). Perfusion studies performed in human colon showed induction of water and electrolyte secretion by dihydroxy bile acids. Also, a decrease in Na ⁺ /H ⁺ exchange (as a result of inhibition of NHE1, 2 and 3) with a parallel decrease in Cl ⁻ /HCO ₃ ⁻ exchange activity has been recently demonstrated in colonic crypts from patients having ileum resection/cholecystectomy-associated diarrhea. In this study, impaired ATP production and elevated intracellular Ca ^{2 +} concentration were shown to inhibit NHE activity and Cl ⁻ /HCO ₃ ⁻ exchange in response to acute treatment of chenodeoxycholic acid to colonic crypts isolated from the control subjects. Therefore, strategies to upregulate NaCl absorption offer new avenues for the treatment of diarrhea associated with BAM.

57.1.3.1.1.4

Protein Kinase C

A number of signaling pathways involving different members of PKC subfamily are implicated in inhibition of Cl ⁻ /HCO ₃ ⁻ exchange by various stimuli. For example, phorbol 12-myristate 13-acetate (PMA), an in vitro PKC agonist was shown to inhibit directly the apical membrane Cl ⁻ /HCO ₃ ⁻ exchange activity via the activation of PI3-kinase and its downstream effector, PKCε. Also, Cl ⁻ /HCO ₃ ⁻ exchange is inhibited by PKCδ in response to serotonin as well as by PKCβ1 and PKCα in response to bile acids and H ₂ O ₂ , respectively ( Fig. 57.5 ). Thus, it is becoming apparent that the apical Cl ⁻ /HCO ₃ ⁻ exchangers are directly modulated by protein kinases by complex mechanisms but their direct effects on SLC26A3 in human intestinal epithelial cells (IECs) have not yet been investigated. In this regard, PMA activation had no effect on SLC26A3 function in HEK293 cells. Direct phosphorylation/dephosphorylation of SLC26A3 by protein kinases in intestinal epithelial cells will be of relevance in future studies. Interestingly, mutation of tyrosine residue Y756F has been shown to increase the basal activity of SLC26A3, suggesting a possible role for tyrosine phosphorylation in inhibition of hDRA activity, although signaling pathways have not yet been identified.

Very few studies are available with respect to SLC26A6 regulation. SLC26A6 has been shown to be negatively regulated by adrenergic stimulation and angiotensin II, via PKC. The mechanism of this inhibition by PKC involved displacement of binding of CAII to A6 and consequent disruption of the HCO ₃ ⁻ transport metabolon. Another study demonstrated that PKC activation by PMA inhibits different modes of SlC26A6-mediated transport including Cl ⁻ /formate exchange, Cl ⁻ /oxalate exchange, and Cl ⁻ /Cl ⁻ exchange in Xenopus oocytes via PKC. These studies also demonstrated redistribution of SLC26A6 from plasma membrane to intracellular compartments by PKC. The observation that PKC activation by PMA inhibits Cl ⁻ /oxalate exchange via PKCδ was reconfirmed by another recent study in T-84 cells. These results also validated the use of T-84 cells as a model to study the regulation of SLC26A6 under various stimuli. This study also demonstrated that cholinergic stimulation with carbachol inhibited SLC26A6-mediated Cl ⁻ /oxalate exchange, via the muscaranic acid receptor-3 (M ₃ ), via the involvement of PLC, PKCδ and c-Src ( Fig. 57.6 ). Furthermore, it was shown that carbachol treatment resulted in decreasing the cell surface levels of SLC26A6, whereas total SLC26A6 levels remained unaltered indicating modulation of trafficking of SLC26A6 to the cell surface. In addition, inhibition of Cl ⁻ /oxalate exchange in response to ATP and UTP treatment in Caco-2.BBe cells mediated via purinergic receptor P2Y ₂ was also demonstrated. Both ATP and UTP decreased oxalate transport by reducing surface expression of SLC26A6 through activation of Phospholipase C (PLC) and PKCδ ( Fig. 57.6 ). More studies in the intestinal epithelial cells are needed to examine the detailed membrane trafficking events modulating SLC26A6 and their relevance in oxalate homeostasis, HCO ₃ ⁻ secretion as well as NaCl absorption.

57.1.3.1.1.5

cAMP and Ca ^{2 +}

Similar to NHE3, cAMP-mediated regulation of SLC26A3 appears to involve interaction with the PDZ adapter proteins such as NHERF2 and NHERF3. SLC26A3 is either activated or inhibited by cAMP based on its mode of transport and cofunctioning proteins such as NHE3 or CFTR. For example, SLC26A3 is activated by cAMP, when it is expressed in a complex with CFTR and NHERF3. In contrast, SLC26A3 is inhibited by cAMP in parallel with NHE3 inhibition during NaCl absorption. Since, both NHE3 and SLC26A3 bind to second PDZ domain of NHERF2, it is possible that SLC26A3 inhibition occurs secondary to NHE3. This inhibition may occur via functional coupling by the intracellular pH or may involve endocytosis pathways. In this regard, recent studies utilizing Caco2.BBe monolayers demonstrated that SLC26A3 undergoes endocytosis in response to both cAMP or Ca ^{2 +} . Moreover, detailed trafficking mechanisms modulating SLC26A3 will require in-depth studies utilizing NHE3 and NHERF knockout mice. With respect to SLC26A6, studies have shown that cAMP stimulates Cl ⁻ /base exchange activity in Xenopus oocytes and HEK-293 cells, coexpressing CFTR with SLC26A6. Ca ^{2 +} -mediated inhibition of SLC26A3, however, was shown to be independent of regulation of CFTR and NHE3 and required interaction of SLC26A3 with NHERF3. Similar studies on regulation of SLC26A6 in intestinal epithelial cells are lacking.

57.1.3.1.1.6