Acid detection by taste receptor cells

Sourness is a primary taste quality that evokes an innate rejection response in humans and many other animals. Acidic stimuli are the unique sources of sour taste so a rejection response may serve to discourage ingestion of foods spoiled by acid producing microorganisms. The investigation of mechanisms by which acids excite taste receptor cells (TRCs) is complicated by wide species variability and within a species, apparently different mechanisms for strong and weak acids. The problem is further complicated by the fact that the receptor cells are polarized epithelial cells with different apical and basolateral membrane properties. The cellular mechanisms proposed for acid sensing in taste cells include, the direct blockage of apical K(+) channels by protons, an H(+)-gated Ca(2+) channel, proton conduction through apical amiloride-blockable Na(+) channels, a Cl(-) conductance blocked by NPPB, the activation of the proton-gated channel, BNC-1, a member of the Na(+) channel/degenerin super family, and by stimulus-evoked changes in intracellular pH. Acid-induced intracellular pH changes appear to be similar to those reported in other mammalian acid-sensing cells, such as type-I cells of the carotid body, and neurons found in the ventrolateral medulla, nucleus of the solitary tract, the medullary raphe, and the locus coceuleus. Like type-I carotid body cells and brainstem neurons, isolated TRCs demonstrate a linear relationship between intracellular pH (pH(i)) and extracellular pH (pH(o)) with slope, DeltapH(i)/DeltapH(o) near unity. Acid-sensing cells also appear to regulate pH(i) when intracellular pH changes occur under iso-extracellular pH conditions, but fail to regulate their pH when pH(i) changes are induced by decreasing extracellular pH. We shall discuss the current status of proposed acid-sensing taste mechanisms, emphasizing pH-tracking in receptor cells.

A novel pharmacological probe links the amiloride-insensitive NaCl, KCl, and NH(4)Cl chorda tympani taste responses

Chorda tympani taste nerve responses to NaCl can be dissected pharmacologically into amiloride-sensitive and -insensitive components. It is now established that the amiloride-sensitive, epithelial sodium channel acts as a sodium-specific ion detector in taste receptor cells (TRCs). Much less is known regarding the cellular origin of the amiloride-insensitive component, but its anion dependence indicates an important role for paracellular shunts in the determination of its magnitude. However, this has not precluded the possibility that undetected apical membrane ion pathways in TRCs may also contribute to its origin. Progress toward making such a determination has suffered from lack of a pharmacological probe for an apical amiloride-insensitive taste pathway. We present data here showing that, depending on the concentration used, cetylpyridinium chloride (CPC) can either enhance or inhibit the amiloride-insensitive response to NaCl. The CPC concentration giving maximal enhancement was 250 microM. At 2 mM, CPC inhibited the entire amiloride-insensitive part of the NaCl response. The NaCl response is, therefore, composed entirely of amiloride- and CPC-sensitive components. The magnitude of the maximally enhanced CPC-sensitive component varied with the NaCl concentration and was half-maximal at [NaCl] = 62 +/- 11 (SE) mM. This was significantly less than the corresponding parameter for the amiloride-sensitive component (268 +/- 71 mM). CPC had similar effects on KCl and NH(4)Cl responses except that in these cases, after inhibition with 2 mM CPC, a significant CPC-insensitive response remained. CPC (2 mM) inhibited intracellular acidification of TRCs due to apically presented NH(4)Cl, suggesting that CPC acts on an apical membrane nonselective cation pathway.

Decrease in rat taste receptor cell intracellular pH is the proximate stimulus in sour taste transduction

Taste receptor cells (TRCs) respond to acid stimulation, initiating perception of sour taste. Paradoxically, the pH of weak acidic stimuli correlates poorly with the perception of their sourness. A fundamental issue surrounding sour taste reception is the identity of the sour stimulus. We tested the hypothesis that acids induce sour taste perception by penetrating plasma membranes as H(+) ions or as undissociated molecules and decreasing the intracellular pH (pH(i)) of TRCs. Our data suggest that taste nerve responses to weak acids (acetic acid and CO(2)) are independent of stimulus pH but strongly correlate with the intracellular acidification of polarized TRCs. Taste nerve responses to CO(2) were voltage sensitive and were blocked with MK-417, a specific blocker of carbonic anhydrase. Strong acids (HCl) decrease pH(i) in a subset of TRCs that contain a pathway for H(+) entry. Both the apical membrane and the paracellular shunt pathway restrict H(+) entry such that a large decrease in apical pH is translated into a relatively small change in TRC pH(i) within the physiological range. We conclude that a decrease in TRC pH(i) is the proximate stimulus in rat sour taste transduction.

Effects of osmolarity on taste receptor cell size and function

Osmotic effects on salt taste were studied by recording from the rat chorda tympani (CT) nerve and by measuring changes in cell volume of isolated rat fungiform taste receptor cells (TRCs). Mannitol, cellobiose, urea, or DMSO did not induce CT responses. However, the steady-state CT responses to 150 mM NaCl were significantly increased when the stimulus solutions also contained 300 mM mannitol or cellobiose, but not 600 mM urea or DMSO. The enhanced CT responses to NaCl were reversed when the saccharides were removed and were completely blocked by addition of 100 microM amiloride to the stimulus solution. Exposure of TRCs to hyperosmotic solutions of mannitol or cellobiose induced a rapid and sustained decrease in cell volume that was completely reversible, whereas exposure to hypertonic urea or DMSO did not induce sustained reductions in cell volume. These data suggest that the osmolyte-induced increase in the CT response to NaCl involves a sustained decrease in TRC volume and the activation of amiloride-sensitive apical Na(+) channels.

Costello syndrome: phenotype, natural history, differential diagnosis, and possible cause

We describe 8 patients affected with Costello syndrome including an affected sib pair and review the literature on 29 previously reported cases. We emphasize an association with advanced parental age, which is consistent with autosomal dominant inheritance with germline mosaicism. The pathogenesis appears to involve metabolic dysfunction, with growth disturbance, storage disorder appearance, acanthosis nigricans, hypertrophic cardiomyopathy, and occasional abnormalities of glucose metabolism. Although the cause is currently unknown, Costello syndrome is interesting because of a potential genetic-metabolic etiology.

Acid-induced responses in hamster chorda tympani and intracellular pH tracking by taste receptor cells

HCl- and NaCl-induced hamster chorda tympani nerve responses were recorded during voltage clamp of the lingual receptive field. Voltage perturbations did not influence responses to HCl. In contrast, responses to NaCl were decreased by submucosal-positive and increased by submucosal-negative voltage clamp. Responses to HCl were insensitive to the Na+ channel blockers, amiloride and benzamil, and to methylisobutylamiloride (MIA), an Na+/H+ exchange blocker. Responses to NaCl were unaffected by MIA but were suppressed by benzamil. Microfluorometric and imaging techniques were used to monitor the relationship between external pH (pHo) and the intracellular pH (pHi) of fungiform papilla taste receptor cells (TRCs) following 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein loading. TRC pHi responded rapidly and monotonically to changes in pHo. This response was unaffected by Na+ removal or the presence of amiloride, benzamil, or MIA. The neural records and the data from isolated TRCs suggest that the principal transduction pathway for acid taste in hamster is similar to that in rat. This may involve the monitoring of changes in TRC pHi mediated through amiloride-insensitive H+ transport across TRC membranes. This is an example of cell monitoring of environmental pH through pH tracking, i.e., a linear change in pHi in response to a change in pHo, as has been proposed for carotid bodies. In taste, the H+ transport sites may be concentrated on the basolateral membranes of TRCs and, therefore, are responsive to an attenuated H+ concentration from diffusion of acids across the tight junctions.

Effects of extracellular pH, PCO2, and HCO3- on intracellular pH in isolated rat taste buds

We studied the effects of changing external pH (pHo), external bicarbonate concentration ([HCO3-]o), and PCO2 on taste receptor cell (TRC) intracellular pH (pHi) in taste bud fragments (TBFs) isolated from rat circumvallate and fungiform papillae with the pH-sensitive fluoroprobe 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) using microfluorometric and imaging techniques. In N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solutions, TRC pHi responded rapidly and monotonically to changes in pHo between 6.5 and 8.0. The relationship between pHi and pHo was steep, with slopes varying between 0.8 and 1.2. Similarly, varying pHo by changing PCO2 at constant [HCO3-]o or changing [HCO3-]o at constant PCO2 led to rapid, monotonic changes in pHi. The relationship between pHi and pHo was once again steep, with slopes varying between 0.8 and 1.2. However, simultaneous changes in PCO2 and [HCO3-]o at constant pHo did not cause any significant changes in steady-state pHi. In imaging studies, single, isolated TRCs responded to changes in pHo, with parallel changes in pHi in the soma and apical process. In addition, changes in pHo induced parallel changes in pHi throughout TBFs. These data suggest that the steady-state TRC pHi is a function of pHo. Changes in TRC pHi may be involved in acid sensing, and salivary [HCO3-] may play a role in the maintainance of steady-state TRC pHi and in the neutralization of acid-induced changes in pHi.

Human monocyte binding to fibronectin enhances IFN-gamma-induced early signaling events

Leukocyte integrins are fundamentally important in modulating adhesion to extracellular matrix components and to other cells. This integrin-mediated adhesion controls leukocyte arrest and extravasation during the onset of inflammatory responses. Moreover, integrin-ligand interactions trigger signaling pathways that may influence leukocyte phenotype and function at sites of inflammation. In the current studies, we evaluated the combinatorial effects of monocyte adhesion and IFN-gamma on intracellular signaling pathways. IFN-gamma triggers a well-defined signal transduction pathway, which although not directly stimulated by monocyte adherence to fibronectin or arginine-glycine-aspartate (RGD)-coated substrata, was enhanced significantly in these matrix-adherent cells. Compared with monocytes in suspension or adherent on plastic surfaces, monocytes adherent to fibronectin or RGD exhibited a greater than threefold increase in steady state levels of IFN-gamma-induced mRNA for the high affinity Fc gammaRI receptor. By electrophoretic mobility shift assays, this increase in mRNA was associated with a 5- to 10-fold increase in the STAT1-containing DNA-binding complex that binds to Fc gammaRI promoter elements. Furthermore, the tyrosine phosphorylation of STAT1 and the tyrosine kinases JAK1 and JAK2 was enhanced significantly in RGD-adherent monocytes compared with control cells. These results suggest a novel mechanism by which integrin-mediated cell adhesion can modulate the magnitude of cytokine-induced signal transduction pathways, thereby amplifying cellular events leading to monocyte activation and inflammation.

STAT5A-deficient mice demonstrate a defect in granulocyte-macrophage colony-stimulating factor-induced proliferation and gene expression

Responses of cells to cytokines typically involve the activation of a family of latent DNA binding proteins, referred to as signal transducers and activators of transcription (STAT) proteins, which are critical for the expression of early response genes. Of the seven known STAT proteins, STAT5 (originally called mammary gland factor) has been shown to be activated by several cytokines, such as granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5, which are known to play important roles in growth and differentiation of hematopoietic precursors. In this report we have used mice that are deficient in STAT5A (one of two homologues of STAT5) to study the role of STAT5A in GM-CSF stimulation of cells. When bone marrow-derived macrophages were generated by differentiation with macrophage-CSF (M-CSF), exposure of cells from wild-type mice to GM-CSF resulted in a typical pattern of assembly of DNA binding proteins specific for the gamma activation sequence (GAS) element within the beta-casein promoter. However, in cells from the STAT5A null mouse one of the shifted bands was absent. Immunoblotting analysis in the null mice showed that lack of STAT5A protein resulted in no alteration in activation of STAT5B by tyrosine phosphorylation. Proliferation experiments revealed that, when exposed to increasing concentrations of GM-CSF, cells derived from the null mice grew considerably more slowly than cells derived from the wild-type mice. Moreover, expression of GM-CSF-dependent genes, CIS and A1, was markedly inhibited in cells derived from null mice as compared with those of wild-type mice. The decreased expression observed with A1, a bcl-2 like gene, may account in part for the suppression of growth in cells from the null mice. These data suggest that the presence of STAT5A during the GM-CSF-induced assembly of STAT5 dimers is critical for the formation of competent transcription factors that are required for both gene expression and cell proliferation.

Human monocyte binding to fibronectin enhances IFN-gamma-induced early signaling events

Leukocyte integrins are fundamentally important in modulating adhesion to extracellular matrix components and to other cells. This integrin-mediated adhesion controls leukocyte arrest and extravasation during the onset of inflammatory responses. Moreover, integrin-ligand interactions trigger signaling pathways that may influence leukocyte phenotype and function at sites of inflammation. In the current studies, we evaluated the combinatorial effects of monocyte adhesion and IFN-gamma on intracellular signaling pathways. IFN-gamma triggers a well-defined signal transduction pathway, which although not directly stimulated by monocyte adherence to fibronectin or arginine-glycine-aspartate (RGD)-coated substrata, was enhanced significantly in these matrix-adherent cells. Compared with monocytes in suspension or adherent on plastic surfaces, monocytes adherent to fibronectin or RGD exhibited a greater than threefold increase in steady state levels of IFN-gamma-induced mRNA for the high affinity Fc gammaRI receptor. By electrophoretic mobility shift assays, this increase in mRNA was associated with a 5- to 10-fold increase in the STAT1-containing DNA-binding complex that binds to Fc gammaRI promoter elements. Furthermore, the tyrosine phosphorylation of STAT1 and the tyrosine kinases JAK1 and JAK2 was enhanced significantly in RGD-adherent monocytes compared with control cells. These results suggest a novel mechanism by which integrin-mediated cell adhesion can modulate the magnitude of cytokine-induced signal transduction pathways, thereby amplifying cellular events leading to monocyte activation and inflammation.

Net H+ and K+ fluxes across the apical surface of rat distal colon

The distal colon absorbs K+ (JK) and secretes H+ (JH) by what is thought to be an H(+)-K(+)-adenosinetriphosphatase (H(+)-K(+)-ATPase). However, the colonic ATPase differs structurally and functionally from the gastric H(+)-K(+)-ATPase. To evaluate the link between JH and JK, JH and JK were simultaneously measured with ion-specific electrodes in segments of rat distal colon. JH and JK averaged 0.40 +/- 0.03 and 0.30 +/- 0.03 mu eq.h-1.cm-2 (n = 191), but JH and JK did not correlate (r = 0.005, not significant). The gastric H(+)-K+ pump inhibitors SCH-28080 (100 microM) and omeprazole (100 microM), as well as a vacuolar H(+)-ATPase inhibitor, bafilomycin A1 (10 microM), did not affect JH or JK. However, the Na(+)-K(+)-ATPase inhibitors ouabain (1 mM) and N-ethylmaleimide (10 microM) inhibited JK but not JH. Although 1 mM orthovanadate inhibited both JH and JK, at lower concentrations orthovanadate only affected JK. Furthermore, removing K+ from the medium did not affect JH. Secondary hyperaldosteronism increased both JH and JK; however, ouabain (1 mM) reduced JK but not JH. Cl(-)-free medium inhibited voltage-insensitive JH and voltage-sensitive JK. Medium pH affected JH, but that effect was contrary to the effect that pH had on Rb+ flux. These data failed to identify a relationship between JH and JK and appear to suggest that JH and JK occur by separate pathways.

Regulation of leukocyte adhesion and signaling in inflammation and disease

Cell adhesion molecules provide the foundation for cell communication, trafficking, and immune surveillance central to host defense. These adhesion molecules which include selectins, integrins and members of the Ig superfamily, provide a recognition system between leukocytes, endothelial cells and matrix molecules. Leukocyte-endothelial interactions initiate recruitment at sites of injury, infection and inflammation. Cell-cell and cell-matrix interactions also influence leukocyte phenotype and function. Dysregulation of these adhesion and signal transduction pathways can contribute to continued recruitment and persistent leukocyte activation with unresolved inflammation. Based on the pivotal role adhesive interactions play, the adhesion molecules provide potential targets for intervention. Selected synthetic fibronectin peptides, which inhibit leukocyte integrin binding and signal transduction in vitro, block recruitment and activation to limit inflammation in vivo.

Regulation of apical Na+ conductive transport in epithelia by pH

Alterations in extracellular (pHo) and/or intracellular pH (pHi) have significant effects on the apical Na+ conductive transport in tight epithelia. They influence apical membrane Na+ conductance via a direct effect on amiloride-sensitive apical Na+ channel activity and indirectly through effects on the basolateral Na+/K(+)-ATPase. Changes in pH also modulate the hormonal regulation of apical Na+ conductive transport. The pH sensitive steps in hormone action include: (i) hormone-receptor binding, (ii) increase in intracellular cyclic 3',5'-adenosine monophosphate (cAMP), (iii) mobilization of intracellular free Ca2+ ([Ca2+]i), and (iv) incorporation of new channels into the apical membrane or recruitment of existing channels. Alternately, changes in pH induce secondary effects via alterations in [Ca2+]i. A reciprocal relationship between pHi and [Ca2+]i has been demonstrated in renal epithelial cells. Natriferic hormones induce a significant increase in pHi. There is a strong temporal relation between hormone-induced increase in pHi and overall increase in transepithelial Na+ transport. This suggests that changes in pHi act as an intermediate in the second messenger cascade initiated by the hormones. Several natriferic hormones activate Na(+)-H+ exchanger, H(+)-ATPase, H+/K(+)-ATPase, H+ conductive pathways in cell membranes or potential-induced changes in pHi. However, changes in pHi do not seem to be essential for the hormone effect on Na+ conductive transport. It is suggested that the role of pHi changes during hormone action is permissive rather than strictly obligatory.

IgG immune complexes inhibit IFN-gamma-induced transcription of the Fc gamma RI gene in human monocytes by preventing the tyrosine phosphorylation of the p91 (Stat1) transcription factor

Immune complexes (IC) modulate Ag-driven immune responses in part by their ability to inhibit IFN-gamma-dependent MHC class II expression. Because many genes, including MHC class II Ags, transcriptionally activated by IFN-gamma require the tyrosine phosphorylation of the transcription factor p91 (Stat1), we examined whether IC could suppress IFN-gamma-induced expression of the Fc gamma receptor I gene (Fc gamma RI) in human monocytes and whether this occurred through inhibition of p91 phosphorylation. Preincubation of monocytes on gamma-globulin-coated dishes resulted in a 80% reduction in steady state levels of RNA for the Fc gamma RI gene. Nuclear run-on analysis confirmed that the inhibition was at the level of transcription. Treatment with IC resulted in no change in the IFN-gamma receptor number. In monocytes pretreated with IC, there was a 79% reduction in the formation of FcRF gamma, a p91-containing DNA binding protein complex that is rapidly activated by IFN-gamma, and which recognizes the gamma response region enhancer within the promoter of the Fc gamma RI gene. Furthermore, there was a marked reduction in the tyrosine phosphorylation of p91. Pretreatment with IC resulted in the inhibition of the tyrosine phosphorylation of the tyrosine kinases, Jak1 and Jak2, both of which are involved in IFN-gamma signal transduction. Therefore, culture of monocytes on IC inhibits IFN-gamma-induced expression of the Fc gamma RI gene by preventing tyrosine phosphorylation of p91, probably by the associated inhibition of the tyrosine kinases Jak1 and Jak2.

IgG immune complexes inhibit IFN-gamma-induced transcription of the Fc gamma RI gene in human monocytes by preventing the tyrosine phosphorylation of the p91 (Stat1) transcription factor

Immune complexes (IC) modulate Ag-driven immune responses in part by their ability to inhibit IFN-gamma-dependent MHC class II expression. Because many genes, including MHC class II Ags, transcriptionally activated by IFN-gamma require the tyrosine phosphorylation of the transcription factor p91 (Stat1), we examined whether IC could suppress IFN-gamma-induced expression of the Fc gamma receptor I gene (Fc gamma RI) in human monocytes and whether this occurred through inhibition of p91 phosphorylation. Preincubation of monocytes on gamma-globulin-coated dishes resulted in a 80% reduction in steady state levels of RNA for the Fc gamma RI gene. Nuclear run-on analysis confirmed that the inhibition was at the level of transcription. Treatment with IC resulted in no change in the IFN-gamma receptor number. In monocytes pretreated with IC, there was a 79% reduction in the formation of FcRF gamma, a p91-containing DNA binding protein complex that is rapidly activated by IFN-gamma, and which recognizes the gamma response region enhancer within the promoter of the Fc gamma RI gene. Furthermore, there was a marked reduction in the tyrosine phosphorylation of p91. Pretreatment with IC resulted in the inhibition of the tyrosine phosphorylation of the tyrosine kinases, Jak1 and Jak2, both of which are involved in IFN-gamma signal transduction. Therefore, culture of monocytes on IC inhibits IFN-gamma-induced expression of the Fc gamma RI gene by preventing tyrosine phosphorylation of p91, probably by the associated inhibition of the tyrosine kinases Jak1 and Jak2.

HCO3- secretion by rat distal colon: effects of inhibitors and extracellular Na+

BACKGROUND/AIMS

The large intestine secretes HCO3- via a Cl-/HCO3- exchange mechanism located in the apical membrane of colonocytes. However, an additional transport system(s) must facilitate HCO3- (OH-) entry or H+ exit across the basolateral cell surface. The aim of this study was to determine that mechanism(s).

METHODS

A modified Ussing apparatus was used to measure net HCO3- secretion in segments of rat distal colon.

RESULTS

When added to the serosal solution, 10 mmol/L 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS), 1 mmol/L SITS and 0.1 mmol/L diisothiocyanostilbene-2,2'-disulfonic acid, inhibited HCO3- secretion by 88%, 51%, and 30%, respectively. However, the Na+/H+ exchange inhibitors, amiloride (1 mmol/L), dimethylamiloride (0.1 mmol/L), ethylisopropylamiloride (0.1 mmol/L), failed to affect HCO3- secretion. Acetazolamide (1 mmol/L) blocked HCO3- secretion by approximately 60% when in the serosal solution but had little effect when in the mucosal solution. Ion substitution studies showed that HCO3- secretion required Na+ in the serosal solution (K0.5 approximately 12 mmol/L). HCO3- secretion was unaffected by depolarizing the basolateral membrane potential with K(+)-rich medium.

CONCLUSIONS

These data are consistent with Na+ linked HCO3- transport across the colonocyte basolateral membrane, which appears to be electroneutral.

Prolactin activates the interferon-regulated p91 transcription factor and the Jak2 kinase by tyrosine phosphorylation

The prolactin (PRL) receptor is a member of the family of cytokine receptors that lack intrinsic tyrosine kinase activity but contain two conserved cysteines in their N-terminal regions and a WSXWS motif adjacent to their transmembrane domains. In a manner similar to the interferons (IFNs), exposure of cells to PRL results in tyrosine phosphorylation of several cellular proteins and the rapid transcriptional induction of the IFN regulatory factor 1 gene. In this communication, we demonstrate that treatment of rat Nb2 lymphoma cells with PRL activates a latent protein factor so that it binds to an enhancer in the IFN regulatory factor 1 gene. This enhancer has been shown to be required for IFN-gamma-activated expression of this gene. PRL-induced assembly of the DNA binding complex, PRL-stimulated factor, required tyrosine phosphorylation. PRL-stimulated factor contained at least one protein that was antigenically similar to the p91 transcription factor, a component of several transcription complexes required for cytokine-activated gene expression. PRL not only induced the tyrosine phosphorylation of p91 but also induced tyrosine phosphorylation of Jak2, a tyrosine kinase required for IFN-gamma-activated gene expression. These results provide evidence for a signaling mechanism, some of whose components are shared by both PRL and IFN-gamma receptors, that results in the expression of early response genes.

Identification of Na+/H+ exchange on the apical side of surface colonocytes using BCECF

Colonocytes must regulate intracellular pH (pHi) while they transport H+ and HCO3-. To investigate the membrane transport processes involved in pHi regulation, colonocyte pHi was measured with 2,'7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) in intact segments of rat distal colon mounted on a holder that fits into a standard fluorometer cuvette and allows independent superfusion of mucosal and serosal surfaces. When NCECF-acetoxymethyl ester was in the mucosal solution only, BCECF loaded surface colonocytes with a high degree of selectivity. In HEPES-buffered solutions, basal pHi was 7.31 +/- 0.01 (n = 68), and pHi was dependent on extracellular Na+. Cells acidified in Na(+)-free solution, and pHi rapidly corrected when Na+ was returned. pHi recovered at 0.22 +/- 0.01 pH/min (n = 6) when Na+ was introduced into the mucosal solution and at 0.02 +/- 0.01 pH/min (n = 7) when Na+ was absent from the mucosal solution. The presence or absence of Na+ in the serosal solution did not affect pHi. This indicated that the Na(+)-dependent pHi recovery process is located in the apical cell membrane, but not in the basolateral membrane. Because amiloride (1 mM) inhibited Na(+)-dependent pHi recovery by 75%, Na+/H+ exchange appears to be present in the apical membrane. Because Na(+)-independent pHi recovery was not affected by K(+)-free media, 50 microM SCH-28080, 100 nM bafilomycin A1, or Cl(-)-free media, this transport mechanism does not involve a gastriclike H(+)-K(+)-ATPase, a vacuolar H(+)-ATPase, or a Cl-/base exchanger. In summary, pHi was selectively measured in surface colonocytes by this technique. In these cells, the Na+/H+ exchange activity involved in pHi regulation was detected in the apical membrane, but not in the basolateral membrane.

Cytokines that associate with the signal transducer gp130 activate the interferon-induced transcription factor p91 by tyrosine phosphorylation

Interleukin-6, leukemia inhibitory factor, and oncostatin M exert a broad range of similar biological activities through association of their receptors with the signal-transducing component gp130. Although it is known that these cytokines trigger rapid tyrosine phosphorylation of a common set of cellular proteins as well as induction of several of the same early response genes, the mechanisms by which these genes are activated is not well understood. In this report, we show that interleukin-6, leukemia inhibitory factor, and oncostatin M stimulate the assembly of protein complexes that recognize conserved sequences within the enhancers of two genes (interferon regulatory factor 1 and Fc gamma receptor type I) that are rapidly activated by these cytokines. These enhancers are known to be required for transcriptional induction of these genes by interferon-gamma. Assembly of the DNA-binding protein complexes occurs within minutes after ligand addition and depends upon tyrosine phosphorylation. These complexes contain the p91 transcription factor, which is tyrosine-phosphorylated in response to these cytokines. An additional tyrosine-phosphorylated protein of 93 kDa can be coimmunoprecipitated with antibodies against p91. These findings further expand the network of cytokines known to activate p91 and, in addition, support the concept that sets of tyrosine-phosphorylated proteins may be responsible for the cytokine-regulated expression of early response genes.

Growth hormone and erythropoietin differentially activate DNA-binding proteins by tyrosine phosphorylation

Binding of growth hormone (GH) and erythropoietin (EPO) to their respective receptors results in receptor clustering and activation of tyrosine kinases that initiate a cascade of events resulting not only in the rapid tyrosine phosphorylation of several proteins but also in the induction of early-response genes. In this report, we show that GH and EPO induce the tyrosine phosphorylation of cellular proteins with molecular masses of 93 kDa and of 91 and 84 kDa, respectively, and that these proteins form DNA-binding complexes which recognize an enhancer that has features in common with several rapidly induced genes such as c-fos. Assembly of the protein complexes required tyrosine phosphorylation, which occurred within minutes after addition of ligand. The activated complexes translocated from the cytoplasm to the nucleus. The protein activated by GH is antigenically similar to p91, a protein common to several transcription complexes that are activated by interferons and other cytokines. In contrast, the proteins activated by EPO are distinct from p91. These findings establish the outlines for a cytokine-induced intracellular signaling pathway, which begins with ligand-induced receptor clustering that activates one or more tyrosine kinases. These data are the first to demonstrate that GH- and EPO-activated tyrosine-phosphorylated proteins can specifically recognize a well-defined enhancer and therefore provide a mechanism for rapidly transducing signals from the membrane to the nucleus.

Interferon-gamma antagonizes interleukin-6-induced expression of interleukin-4 receptors in murine myeloid cells by a transcriptional mechanism

The murine myeloid leukemia cell line M1 induced by interleukin-6 (IL-6) is a model system to study the differentiation of blast cells to mature macrophages. We have recently shown that IL-6 induces the expression of the IL-4 receptor (IL-4R) in these cells. In the present study we investigate the mechanism of action of interferon-gamma (IFN-gamma), an antagonist of IL-4 in numerous cells and a cofactor in both induction and suppression of myelopoiesis, on the expression of IL-4R. Flow cytometry shows that IFN-gamma downregulates the IL-6-induced expression of IL-4R whereas it has no such effect on the high-affinity receptors for monomeric IgG2a (Fc gamma RI). As demonstrated by Scatchard analysis, the number of IL-4R decreases by more than 50% after IFN-gamma treatment whereas the receptor affinity remains unchanged. Northern analysis shows that this decrease is paralleled by a decrease in IL-4R mRNA but not Fc gamma RI or lysozyme mRNA. Nuclear run-on analysis shows that IFN-gamma suppresses the IL-6-induced transcription of the IL-4R gene, whereas actinomycin-D chase experiments showed no change of IL-4R mRNA stability. Furthermore, the production of soluble IL-4R protein is suppressed by IFN-gamma as well. These data explain how IL-4R can be modulated by IFN-gamma in myeloid cells and are consistent with the myelosuppressive capacity of IFN-gamma.

Tyrosine phosphorylation of DNA binding proteins by multiple cytokines

Interferon-alpha (IFN-alpha) and IFN-gamma regulate gene expression by tyrosine phosphorylation of several transcription factors that have the 91-kilodalton (p91) protein of interferon-stimulated gene factor-3 (ISGF-3) as a common component. Interferon-activated protein complexes bind enhancers present in the promoters of early response genes such as the high-affinity Fc gamma receptor gene (Fc gamma RI). Treatment of human peripheral blood monocytes or basophils with interleukin-3 (IL-3), IL-5, IL-10, or granulocyte-macrophage colony-stimulating factor (GM-CSF) activated DNA binding proteins that recognized the IFN-gamma response region (GRR) located in the promoter of the Fc gamma RI gene. Although tyrosine phosphorylation was required for the assembly of each of these GRR binding complexes, only those formed as a result of treatment with IFN-gamma or IL-10 contained p91. Instead, complexes activated by IL-3 or GM-CSF contained a tyrosine-phosphorylated protein of 80 kilodaltons. Induction of Fc gamma RI RNA occurred only with IFN-gamma and IL-10, whereas pretreatment of cells with GM-CSF or IL-3 inhibited IFN-gamma induction of Fc gamma RI RNA. Thus, several cytokines other than interferons can activate putative transcription factors by tyrosine phosphorylation.

Soluble interleukin-4 receptor production by murine myeloid progenitor cells: induction by interleukin-6 and interferon-gamma

Soluble cytokine receptors, molecules that can selectively modulate the effects of a single cytokine, have generated great interest both as indicators of disease and as targeted immunotherapeutic agents. However, cellular sources for soluble cytokine receptors are not well characterized and the regulation of soluble receptor production is not clear. We recently found that interleukin 6 (IL-6) induces the expression of membrane-bound interleukin 4 (IL-4) receptors (mIL-4R) in the murine myeloid leukemia M1 cell line. In the present study we show that IL-6 induces the expression and secretion of the soluble form of the IL-4 receptor (sIL-4R) in these cells as well, with the protein accumulating for up to 72 hours in the cell supernatants. This inducible production of sIL-4R protein is accompanied by the induction of mRNA specific for sIL-4R. In mature bone marrow (BM)-derived macrophages sIL-4R expression can be induced by exposure to either IL-6 or interferon-gamma (IFN gamma). The data suggest that myeloid cells exposed to inflammatory stimuli are one possible source for sIL-4R enabling them to modulate the effects of IL-4 on the immune response by the regulated production of the soluble receptor for this cytokine.

Trisomy 17p associated with Charcot-Marie-Tooth neuropathy type 1A phenotype: evidence for gene dosage as a mechanism in CMT1A

Charcot-Marie-Tooth neuropathy type 1A (CMT1A) is associated with a DNA duplication on chromosome 17, band p11.2, resulting in partial trisomy for this region in CMT1A patients. The 17p11.2 duplication may lead to the CMT1A phenotype either through disruption of a gene at the duplication breakpoint junction or by trisomic dosage and overexpression of a gene within the duplication. To test the latter model, we evaluated a patient with complete translocation trisomy 17p for signs of CMT1A. In addition to the dysmorphic features seen in trisomy 17p, a neurologic examination and electrophysiologic studies detected a demyelinating neuropathy, compatible with CMT1A. A karyotype on the patient's father found a balanced translocation [t(14;17)] with breakpoints on chromosome 17 in either band p11.1 or proximal p11.2. An analysis of the patient's DNA confirmed trisomy 17p and mapped the translocation breakpoint to a region in 17p11.2, proximal to the duplication breakpoint in CMT1A. Our observations in this patient with trisomy 17p are relevant to an understanding of the genetic mechanism in CMT1A and provide strong evidence that gene dosage through segmental trisomy for 17p11.2 results in the CMT1A phenotype.

Comparison between acetate and bicarbonate dialysis for the treatment of lithium intoxication

Lithium is used to treat manic-depressive disorders, but toxic side effects commonly occur. The preferred treatment of severe lithium intoxication is hemodialysis. No data, however, exist comparing the effectiveness of acetate to bicarbonate dialysis for the removal of lithium. We present a case of lithium intoxication treated with both. During acetate dialysis, lithium removal occurred exclusively from the extracellular fluid space, while during bicarbonate dialysis, lithium removal occurred equally from both the extracellular and intracellular fluid spaces. We hypothesize that acetate but not bicarbonate activates the sodium-hydrogen antiporter on cell membranes, and that lithium, substituting for sodium, is driven into cells. This may explain the rebound in lithium levels commonly noted after conventional dialysis. We recommend bicarbonate hemodialysis as the therapy of choice for severe lithium intoxication.

Regulation of interleukin-4 receptors on murine myeloid progenitor cells by interleukin-6

Interleukin-4 (IL-4) is a T-cell-derived cytokine that regulates induction of proliferation of resting B cells and acts on various other immunocompetent cells, such as monocytes/macrophages and mast cells, as well as hematopoietic progenitor cells. On hematopoietic progenitor cells, cooperation with another cytokine (such as granulocyte-macrophage colony-stimulating factor [GM-CSF], G-CSF, IL-3, or IL-6) is required to render the cells responsive to IL-4. The present study was undertaken to determine if such an interaction entails induction of IL-4 receptor (IL-4R) expression. Using the murine myeloid leukemia M1 cell line and mature, bone marrow (BM)-derived macrophages, we investigated whether IL-4R expression can be induced during differentiation. We detected no high-affinity IL-4R on the surface of either cell, but with exposure to IL-6 a significant induction of IL-4R was measured on both cell types by fluorescence-activated cell sorter analysis. This increase in IL-4R was first noted 6 hours after exposure of the cells to IL-6 and continued to increase up to 48 hours. By RNase protection analysis we found that the expression of IL-4R mRNA also appeared within 6 hours, continuing to increase up to 48 hours. Nuclear run-on assays showed that this increase in steady-state level of IL-4R mRNA results from a transcriptional activation of the IL-4R gene. These data suggest that regulation of IL-4R expression by IL-6 is under transcriptional control.

Expression of high-affinity interleukin 4 receptors on murine sarcoma cells and receptor-mediated cytotoxicity of tumor cells to chimeric protein between interleukin 4 and Pseudomonas exotoxin

The presence of interleukin 4 receptor (IL-4R) on methylcholanthrene (MCA-106, MCA-102, and MC-38)- and viral DNA (G-2TS and 14-2TS)-induced murine sarcoma cells was demonstrated. MCA-106 tumor cells express about 500 to 1348 (median, 800) interleukin 4 (IL-4) binding sites/cell with a dissociation constant (Kd) of 115 +/- 26 pM (mean +/- SD, n = 4). By Northern blot analysis, tumor cells exhibited a single mRNA species of 3.9 kilobases. Other murine sarcoma (MCA-102), colon adenocarcinoma (MC-38), G-2TS, and 14-2TS tumor cells express low numbers of IL-4R. By immunoperoxidase staining, 81 to 92% of the cells from fresh MCA-106 tumors were positive for IL-4 receptors, while only 7 to 10% of tumor-infiltrating cells were Thy 1.2 and less than 1% Mac-1 positive. Using a chimeric protein composed of IL-4 and Pseudomonas exotoxin (IL-4-PE40), we observed that IL-4-PE40 was cytotoxic (determined by inhibition of protein synthesis by [3H]leucine uptake) to MCA-106 tumor cells in a dose-dependent manner. A nonchimeric protein (PE40) that cannot bind to the IL-4R did not inhibit protein synthesis in tumor cells. A chimeric mutant protein (IL4-PE40 asp553) that can bind to IL-4 receptors but does not have the capability to inhibit protein synthesis was not cytotoxic to tumor cells. These studies strongly suggest that IL-4R on murine MCA-106 sarcoma cells is internalized when occupied by IL-4 PE40. Furthermore, a neutralizing antibody (11B11) to IL-4 completely abolished the protein synthesis-inhibitory activity of IL-4-PE40. G-2TS tumor cells which expressed low numbers of IL-4 receptors were not vulnerable to cytotoxicity by IL-4-PE40. Taken together, these data suggest that IL-4 receptor may be a target for IL-4-toxin therapy.

Suppression of bacterial cell wall-induced polyarthritis by recombinant gamma interferon

Group A streptococcal cell wall fragments (SCW) induce erosive polyarthritis, characterized by synovial cell hyperplasia and intense mononuclear cell infiltration, in susceptible rats. Because of the known antiproliferative and immunomodulatory effects of interferon (IFN), we evaluated the effect of systemically administered alpha, beta and gamma IFN on the evolution of these destructive lesions. Treatment with gamma IFN not only reduced the acute response, but had an even greater suppressive effect on the chronic mononuclear cell-mediated destructive phase of the disease (articular index 10.2 +/- 1.2 for SCW only versus 3.8 +/- 0.7 for SCW + gamma IFN; p less than 0.01). Treatment with gamma IFN was more effective in the suppression of the arthritis than alpha, beta IFN. Histopathologic evaluation of the joints demonstrated that gamma IFN-treated animals had significantly fewer inflammatory cells, and less synovial hyperplasia and erosions than the SCW controls. gamma IFN suppression of mononuclear cell prostaglandin synthesis and synovial fibroblast proliferation was consistent with its anti-arthritic effects. These data indicate that the pathophysiology of SCW-induced erosive polyarthritis is subject to regulatory control by gamma IFN and that the mechanisms of suppression may be relevant in the treatment of rheumatoid arthritis.

Induction and regulation of IL-4 receptor expression on murine macrophage cell lines and bone marrow-derived macrophages by IFN-gamma

IL-4 is a T cell-derived cytokine that regulates the induction of proliferation of resting B cells, and appears to act on various other cells involved in the immune response. The pluripotential effects of IL-4 are dependent on the interaction of IL-4 with its receptor (IL-4R). Although the regulation and metabolism of these receptors have been examined on B and T cells, little is known about the metabolism or regulation of the IL-4R on macrophages. In studying the dynamics of IL-4R expression on the murine macrophage-like cell line J774.16, we detected the presence of low numbers of high affinity IL-4R (234 +/- 38, Kd = 110 +/- 11 pM) on the surface membrane. However, upon exposure to IFN-gamma, a potent macrophage activating cytokine, there was a rapid upregulation (within 45 min) of IL-4R (2750 +/- 178) on the cell surface, with no change in receptor affinity (Kd = 205 +/- 37 pM). Maximum expression occurred at 2 to 4 h with no further increase in IL-4R expression over the next 48 h. Cells pulsed with IFN-gamma for 45 min displayed maximum IL-4R expression by 4 h. The induction of IL-4R by IFN-gamma was dose dependent: as little as 0.5 ng/ml of IFN-gamma was capable of inducing IL-4R expression, with optimal induction at 10 ng/ml. The addition of the metabolic inhibitors actinomycin D and cycloheximide before the addition of IFN-gamma indicated that both RNA transcription and protein translation were required for this upregulation to occur.

Induction and regulation of IL-4 receptor expression on murine macrophage cell lines and bone marrow-derived macrophages by IFN-gamma

IL-4 is a T cell-derived cytokine that regulates the induction of proliferation of resting B cells, and appears to act on various other cells involved in the immune response. The pluripotential effects of IL-4 are dependent on the interaction of IL-4 with its receptor (IL-4R). Although the regulation and metabolism of these receptors have been examined on B and T cells, little is known about the metabolism or regulation of the IL-4R on macrophages. In studying the dynamics of IL-4R expression on the murine macrophage-like cell line J774.16, we detected the presence of low numbers of high affinity IL-4R (234 +/- 38, Kd = 110 +/- 11 pM) on the surface membrane. However, upon exposure to IFN-gamma, a potent macrophage activating cytokine, there was a rapid upregulation (within 45 min) of IL-4R (2750 +/- 178) on the cell surface, with no change in receptor affinity (Kd = 205 +/- 37 pM). Maximum expression occurred at 2 to 4 h with no further increase in IL-4R expression over the next 48 h. Cells pulsed with IFN-gamma for 45 min displayed maximum IL-4R expression by 4 h. The induction of IL-4R by IFN-gamma was dose dependent: as little as 0.5 ng/ml of IFN-gamma was capable of inducing IL-4R expression, with optimal induction at 10 ng/ml. The addition of the metabolic inhibitors actinomycin D and cycloheximide before the addition of IFN-gamma indicated that both RNA transcription and protein translation were required for this upregulation to occur.

H+ and HCO3- flux across apical surface of rat distal colon

Colonic ion transport is postulated to occur via simultaneous operation of Na(+)-H+ exchange and Cl(-)-HCO3- exchange. Accordingly H+ and HCO3- should be transported simultaneously by the colon. To assess simultaneous H+ and HCO3- transport, net acid-base flux was measured in isolated segments of rat distal colon. When both tissue surfaces were bathed in symmetrical solutions containing Cl-, net base was secreted (-1.0 +/- 0.1 mu eq.cm-2.h-1). Cl- substitution with gluconate in the mucosal medium caused net base flux to switch from secretion to absorption (2.0 +/- 0.2 mu eq.cm-2.h-1). To evaluate whether base absorption was dependent on H+ secretion via Na(+)-H+ exchange, mucosal Na+ was substituted with N-methylglucamine, and amiloride, an inhibitor of Na(+)-H+ exchange, was applied. Na+ substitution and 1 mM amiloride inhibited base absorption by 37 and 38%, respectively, suggesting operation of Na(+)-H+ exchange. Because base absorption persisted, an additional mechanism was considered, HCO3- absorption via Cl(-)-HCO3- exchange. This was evaluated with an inhibitor of Cl(-)-HCO3- exchange 4-acetamido-4'-isothiostilbene-2,2'-disulfonic acid (SITS). SITS (1 mM) inhibited HCO3- absorption by 40%. The effects of amiloride and SITS were additive, suggesting that the Na(+)-H+ and Cl(-)-HCO3- exchangers operate simultaneously. Amiloride also inhibited H+ secretion when net HCO3- was secreted, suggesting that the direction of HCO3- movement does not influence Na(+)-H+ exchange activity. These data suggest that the colon transports both H+ and HCO3- across the apical surface via Na(+)-H+ exchange and Cl(-)-HCO3- exchange; H+ is secreted via Na(+)-H+ exchange, whereas HCO3- can be secreted or absorbed via Cl(-)-HCO3- exchange.

Base secretion in rat distal colon: ionic requirements

To evaluate the ionic requirements of colonic base secretion, segments of rat distal colon were studied under short-circuited conditions. Net base flux was composed of an active secretory component and a diffusive component. Studied in the absence of a transepithelial HCO3- concentration gradient, active base secretion was dependent on the HCO3- concentration of the bathing solution but was not influenced by the CO2 tension or pH. Base secretion appeared to saturate with a Km of 33 +/- 9 mM and was inhibited by ouabain. The diffusive component was characterized by an apparent permeability coefficient to HCO3- of 8.9 +/- 0.9 x 10(-6) cm/s. In addition to requiring HCO3- on the serosal surface, net base secretion was inhibited by reducing the Na+ concentration in the serosal medium and the Cl- concentration in the mucosal medium. These data suggest that colonic base secretion involves HCO3- entry across the basolateral surface, energized by the Na+ gradient, and HCO3- exit across the apical surface in exchange for Cl-.

Physiologic oxygen tensions limit oxidant-mediated killing of schistosome eggs by inflammatory cells and isolated granulomas

Explanted hepatic granulomas, eosinophils obtained from the peritoneal cavity of schistosome-infected mice, schistosome egg granuloma macrophages, alveolar macrophages, and activated peritoneal macrophages obtained from Listeria-infected mice were miracidicidal when cultured at 21% oxygen. This activity was markedly attenuated at physiologic oxygen concentrations (1-15%). Catalase and superoxide dismutase blocked the miracidicidal activity of inflammatory cells but did not prevent granuloma-mediated egg killing. However, the biomimetic superoxide dismutase, copper (II) [diisopropyl salicylate]2, inhibited granuloma-mediated egg killing in a dose-dependent, apparently nontoxic manner. Thioglycollate-elicited macrophages did not kill schistosome egg miracidia even when cultured in 21% oxygen, unless pretreated with lipopolysaccharide. Isolated schistosome eggs initiated an oxidative burst in macrophages, as measured by superoxide anion production. This burst was suppressed at reduced oxygen concentrations. Thus schistosome egg miracidia can be killed nonspecifically by macrophages through the release of cytotoxic reactive oxygen intermediates triggered by the egg. This activity is not supported by the oxygen concentrations found in most tissues, with the possible exception of the lung. Schistosoma mansoni eggs, injected intraveneously and lodged in the pulmonary vasculature of mice, were killed rapidly, with a half life of 3.5 days. Eggs, injected into the mesenteric veins and lodged in the liver, remained fully viable for several weeks. The data suggest that the high oxygen tension of the lung allows for the increased production of reactive oxygen intermediates (ROI) by local inflammatory cells, which in turn increases their miracidicidal efficiency. Conversely, the relatively hypoxic environment of the liver decreases ROI production by local inflammatory cells and decreases their miracidicidal efficiency.

Propionate induces cell swelling and K+ accumulation in shark rectal gland

Small organic anions have been reported to induce cell solute accumulation and swelling. To investigate the mechanism of swelling, we utilized preparations of rectal gland cells from Squalus acanthias incubated in medium containing propionate. Propionate causes cells to swell by diffusing across membranes in its nonionic form, acidifying cell contents, and activating the Na+-H+ antiporter. The Na+-H+ exchange process tends to correct intracellular pH (pHi), and thus it maintains a favorable gradient for propionic acid diffusion and allows propionate to accumulate. Activation of the Na+-H+ antiport also facilitates Na+ entry into the cell and Nai accumulation. At the same time Na+-K+-ATPase activity, unaffected by propionate, replaces Nai with Ki, whereas the K+ leak rate, decreased by propionate, allows Ki to accumulate. As judged by 86Rb+ efflux, the reduction in K+ leak was not due to propionate-induced cell acidification or reduction in Cli concentration. Despite inducing cell swelling, propionate did not disrupt cell structural elements and F actin distribution along cell membranes.