Cisplatin increases TNF-alpha mRNA stability in kidney proximal tubule cells

Cisplatin induces acute renal injury in part by increasing the production of TNF-alpha. However, the mechanism by which cisplatin increases renal TNF-alpha expression is not known. The transcription, translation, and stability of TNF-alpha mRNA are sites of regulation of TNF-alpha production. This study investigated the effects of cisplatin on TNF-alpha mRNA stability and the role of MAP kinases in this process in cultured renal proximal tubule cells. Cisplatin increased the expression of TNF-alpha mRNA by proximal tubule cells in a time- and dose-dependent manner, as well as activated p42/44 ERK kinase, p38 MAP kinase, and JNK in a dose-dependent manner. The inhibition of these pathways reduced TNF-alpha expression significantly. Cisplatin also increased the stability of TNF-alpha mRNA, but this effect was not mediated by MAP kinases and did not require the synthesis of a new protein. The treatment of cells with cisplatin induced the formation of complexes of cytosolic proteins and the AU-rich region of the TNF-alpha 3'UTR. These results are consistent with the view that cisplatin increases TNF-alpha mRNA stability in a MAP kinase-independent manner. The stabilization of TNF-alpha mRNA by cisplatin may involve the binding of certain proteins to AU-rich regions in the 3'UTR.

Endotoxin and cisplatin synergistically stimulate TNF-alpha production by renal epithelial cells

Acute renal failure often occurs in the clinical setting of multiple renal insults. Tumor necrosis factor-alpha (TNF-alpha) has been implicated in the pathogenesis of cisplatin nephrotoxicity, ischemia-reperfusion injury, and endotoxin-induced acute renal failure. The current studies examined the interactions between cisplatin and endotoxin with particular emphasis on TNF-alpha production. Treatment of cultured murine proximal tubule cells (TKPTS cells) with cisplatin resulted in a modest production of TNF-alpha, while treatment with endotoxin did not result in any TNF-alpha production. However, the combination of cisplatin and endotoxin resulted in large amounts of TNF-alpha synthesis and secretion. The stimulation of TNF-alpha production was dependent on cisplatin-induced activation of p38 MAPK and was associated with phosphorylation of the translation initiation factor eIF4E and its upstream kinase Mnk1. Inhibition of p38 MAPK and, to a lesser extent, ERK, reduced cisplatin+endotoxin-stimulated TNF-alpha production and phosphorylation of Mnk1 and eIF4E. Synergy between cisplatin and endotoxin was also observed in certain tumor cell lines, but not in macrophages. In macrophages, in contrast to TKPTS cells, endotoxin alone activated p38 MAPK and stimulated TNF-alpha production with no added impact by cisplatin. The combination of cisplatin and endotoxin did not result in synergistic production of other cytokines, e.g., MCP-1 and MIP2, by TKPTS cells. In summary, these studies indicate that cisplatin sensitizes renal epithelial cells to endotoxin and dramatically increases the translation of TNF-alpha mRNA in a p38 MAPK-dependent manner. These interactions between cisplatin and endotoxin may be relevant to the pathogenesis of cisplatin nephrotoxicity in humans.

p38 MAP kinase inhibition ameliorates cisplatin nephrotoxicity in mice

Cisplatin is an important chemotherapeutic agent but can cause acute renal injury. Part of this acute renal injury is mediated through tumor necrosis factor-alpha (TNF-alpha). The pathway through which cisplatin mediates the production of TNF-alpha and injury is not known. Cisplatin activates p38 MAPK and induces apoptosis in cancer cells. p38 MAPK activation leads to increased production of TNF-alpha in ischemic injury and in macrophages. However, little is known concerning the role of p38 MAPK in cisplatin-induced renal injury. Therefore, we examined the effect of cisplatin on p38 MAPK activity and the role of p38 MAPK in mediating cisplatin-induced TNF-alpha production and renal injury. In vitro, cisplatin caused a dose-dependent activation of p38 MAPK in proximal tubule cells. Inhibition of p38 MAPK activation led to inhibition of TNF-alpha production. In vivo, mice treated with a single dose of cisplatin (20 mg/kg body wt) developed severe renal dysfunction at 72 h [blood urea nitrogen (BUN): 154 +/- 34 mg/dl, creatinine: 1.4 +/- 0.4 mg/dl], which was accompanied by an increase in kidney p38 MAPK activity and an increase in infiltrating leukocytes. However, animals treated with the p38 MAPK inhibitor SKF-86002 along with cisplatin showed less renal dysfunction (BUN: 55 +/- 14 mg/dl, creatinine: 0.3 +/- 0.02 mg/dl, P < 0.05), less severe histological damage, and fewer leukocytes compared with cisplatin+vehicle-treated animals. Serum levels of TNF-alpha, sTNFRI, and sTNFRII also increased significantly in cisplatin-treated mice compared with SKF-86002-treated mice (P < 0.05). Kidney mRNA levels of TNF-alpha were significantly increased in cisplatin-treated mice compared with either SKF-86002- or saline-treated animals. The hydroxyl radical scavenger DMTU (100 mg.kg body wt(-1).day(-1)) prevented the activation of p38 MAPK by cisplatin both in vitro and in vivo. DMTU also completely prevented cisplatin-induced renal injury (BUN: 140 +/- 27 vs. 22 +/- 2 mg/dl, P < 0.005) and the increase in serum TNF-alpha (33 +/- 7 vs. 4 +/- 2 pg/ml, P < 0.005) and kidney TNF-alpha mRNA in vivo. We conclude that hydroxyl radicals, either directly or indirectly, activate p38 MAPK and that p38 MAPK plays an important role in mediating cisplatin-induced acute renal injury and inflammation, perhaps through production of TNF-alpha.

Inflammatory cytokines in acute renal failure

A growing body of evidence indicates that inflammatory mechanisms contribute to toxin-induced acute renal failure as well as ischemia/reperfusion injury. A role for tumor necrosis factor-alpha (TNF-alpha) in mediating the inflammatory injury in cisplatin-induced acute renal failure has recently been established. Cisplatin induces the expression of TNF-alpha and TNF receptor subtype 2 (TNFR2) within the kidney. Genetic deletion of either TNF-alpha or TNFR2 substantially reduces cisplatin-induced renal failure and also necrosis and apoptosis within the kidney. Studies will be required to determine if pharmacologic inhibition of TNF-alpha might reduce cisplatin-induced renal failure in humans.

Salicylate reduces cisplatin nephrotoxicity by inhibition of tumor necrosis factor-α

BACKGROUND

Salicylate was recently shown to provide protection against cisplatin nephrotoxicity in rats. We have demonstrated that enhanced tumor necrosis factor-alpha (TNF-alpha) production mediates, in part, cisplatin nephrotoxicity. The purpose of this study was to determine if the protective effects of salicylate were mediated through inhibition of TNF-alpha in vivo and to explore the mechanism of inhibition in vitro.

METHODS

The effects of treatment with cisplatin alone and in combination with sodium salicylate in mice on renal function, histology, and gene expression were determined. The effects of cisplatin and salicylate on TNF-alpha expression, nuclear factor kappa B (NF-kappa B) activity, and apoptosis were determined in vitro using cultured murine proximal tubule cells.

RESULTS

Salicylate significantly reduced both the functional and histologic evidence of cisplatin renal injury. Cisplatin increased the renal expression of TNF-alpha, monocyte chemoattractant protein-1 (MCP-1), intercellular adhesion molecule (ICAM)-1, heme oxygenase-1, TNF receptor 1 (TNFR1), and TNF receptor 2 (TNFR2). Treatment with sodium salicylate blunted the increase in TNF-alpha mRNA and also reduced serum TNF-alpha protein levels. Salicylate had little protective effect when administered with cisplatin to TNF-alpha-deficient mice. Cisplatin increased the degradation of I kappa B (I kappa B) in a time-dependent manner and also increased nuclear NF-kappa B binding activity. Salicylate inhibited I kappa B degradation and NF-kappa B binding activity in the presence of cisplatin. In addition, salicylate inhibited the cisplatin induced TNF-alpha mRNA increase in mouse proximal tubule epithelial (TKPT) cells.

CONCLUSION

These results indicate that salicylate acts via inhibition of TNF-alpha production to reduce cisplatin nephrotoxicity. The inhibition of TNF-alpha production may be mediated via stabilization of I kappa B.

TNFR2-mediated apoptosis and necrosis in cisplatin-induced acute renal failure

Cisplatin produces acute renal failure in humans and mice. Previous studies have shown that cisplatin upregulates the expression of TNF-alpha in mouse kidney and that inhibition of either the release or action of TNF-alpha protects the kidney from cisplatin-induced nephrotoxicity. In this study, we examined the effect of cisplatin on the expression of TNF receptors TNFR1 and TNFR2 in the kidney and the role of each receptor in mediating cisplatin nephrotoxicity. Injection of cisplatin into C57BL/6 mice led to an upregulation of TNFR1 and TNFR2 mRNA levels in the kidney. The upregulation of TNFR2 but not TNFR1 was blunted in TNF-alpha-deficient mice, indicating ligand-dependent upregulation of TNFR2. To study the roles of each receptor, we administered cisplatin to TNFR1- or TNFR2-deficient mice. TNFR2-deficient mice developed less severe renal dysfunction and showed reduced necrosis and apoptosis and leukocyte infiltration into the kidney compared with either TNFR1-deficient or wild-type mice. Moreover, renal TNF-alpha expression, ICAM-1 expression, and serum TNF-alpha levels were lower in TNFR2-deficient mice compared with wild-type or TNFR1-deficient mice treated with cisplatin. These results indicate that TNFR2 participates in cisplatin-induced renal injury in mice and may play an important role in TNF-alpha-mediated inflammation in the kidney in response to cisplatin.

TNF-alpha mediates chemokine and cytokine expression and renal injury in cisplatin nephrotoxicity

The purpose of these studies was to examine the role of cytokines in the pathogenesis of cisplatin nephrotoxicity. Injection of mice with cisplatin (20 mg/kg) led to severe renal failure. The expression of cytokines, chemokines, and ICAM-1 in kidney was measured by ribonuclease protection assays and RT-PCR. We found significant upregulation of TNF-alpha, TGF-beta, RANTES, MIP-2, MCP-1, TCA3, IL-1beta, and ICAM-1 in kidneys from cisplatin-treated animals. In addition, serum, kidney, and urine levels of TNF-alpha measured by ELISA were increased by cisplatin. Inhibitors of TNF-alpha production (GM6001, pentoxifylline) and TNF-alpha Ab's reduced serum and kidney TNF-alpha protein levels and also blunted the cisplatin-induced increases in TNF-alpha, TGF-beta, RANTES, MIP-2, MCP-1, and IL-1beta, but not ICAM-1, mRNA. In addition, the TNF-alpha inhibitors also ameliorated cisplatin-induced renal dysfunction and reduced cisplatin-induced structural damage. Likewise, TNF-alpha-deficient mice were resistant to cisplatin nephrotoxicity. These results indicate cisplatin nephrotoxicity is characterized by activation of proinflammatory cytokines and chemokines. TNF-alpha appears to play a central role in the activation of this cytokine response and also in the pathogenesis of cisplatin renal injury.

Chloride channels in the loop of Henle

Cl- transport in the loop of Henle is responsible for reclamation of 25-40% of the filtered NaCl load and for the formation of dilute urine. Our understanding of the physiologic and molecular mechanisms responsible for Cl- reabsorption in both the thin ascending limb and thick ascending limb of Henle's loop has increased greatly over the last decade. Plasma membrane Cl- channels are known to play an integral role in transcellular Cl- transport in both the thin and thick ascending limbs. This review focuses on the functional characteristics and molecular identities of these Cl- channels, as well as the role of these channels in the pathophysiology of disease.

Cl(-) channels in basolateral TAL membranes XV. Molecular heterogeneity between cortical and medullary channels

We have isolated two new and highly homologous cDNAs, mmClC-Ka from mouse outer medulla and mcClC-Ka from mouse cortex. In both cases, mRNA was obtained from the indicated region and subjected to RT-PCR using primers from the nucleotide sequence of rbClC-Ka, which encodes basolateral Cl(-) channels (termed rbClC-Ka) in rabbit MTAL. The predicted protein products of mmClC-Ka and mcClC-Ka, mmClC-Ka and mcClC-Ka, respectively, were 85% homologous and had predicted molecular weights of 75 kDa. The predicted protein sequences for mmClC-Ka and rbClC-Ka had three cytosolic sites-threonine 185, threonine 187 and serine 270-which were absent in mcClC-Ka. These three moieties represent potential sites for phosphorylation of mmClC-Ka and rbClC-Ka, but not of mcClC-Ka, and may account for the failure of (ATP + PKA) to increase the open time probability P(o) in basolateral CTAL Cl(-) channels. We prepared antisense oligonucleotides specific for nonhomologous regions of these two cDNAs, mmAntisense for mmClC-Ka and mcAntisense for mcClC-Ka. Using anti-rbClC-Ka, a polyclonal antibody to rbClC-Ka, we found that, when transfected into cultured mouse MTAL and CTAL cells, mmAntisense suppressed the appearance of the 75 kDa band by 50% in vesicles from MTAL but not CTAL cells, while transfection of MTAL and CTAL cells with mcAntisense suppressed appearance of the 75 kDa band in vesicles from CTAL but not MTAL cells. mmAntisense transfection also prolonged the half-time (T(1/2), sec) for (36)Cl(-) efflux in cultured MTAL cells from 82.4 +/- 6.8 sec (sem) to 187.8 +/- 9.5 sec (n = 5; P = 0.0001) while mcAntisense transfection had no such effect. Conversely, in cultured CTAL cells, mcAntisense transfection prolonged the T(1/2) for (36)Cl(-) efflux from 80.9 +/- 6.3 sec to 191.8 +/- 6.5 sec (n = 5; P = 0.00005), while mmAntisense had no such effect. We conclude that mmClC-Ka and mcClC-Ka may encode the basolateral Cl(-) channels mediating net Cl(-) absorption in mouse MTAL and CTAL, respectively.

Effects of chloride channel inhibitors on H(2)O(2)-induced renal epithelial cell injury

Oxidative stress contributes to renal epithelial cell injury in certain settings. Chloride influx has also been proposed as an important component of acute renal epithelial cell injury. The present studies examined the role of Cl(-) in H(2)O(2)-induced injury to LLC-PK(1) renal epithelial cells. Exposure of LLC-PK(1) cells to 1 mM H(2)O(2) resulted in the following: depletion of intracellular ATP content; DNA damage; lipid peroxidation; and a loss of membrane integrity to both small molecules, e.g., trypan blue, and macromolecules, e.g., lactate dehydrogenase (LDH), and cell death. Substitution of Cl(-) by isethionate or the inclusion of certain Cl(-) channel blockers, e.g., diphenylamine-2-carboxylate (DPC), 5-nitro-2-(3-phenylpropylamino). benzoate (NPPB), and niflumic acid, prevented the H(2)O(2)-induced loss of membrane integrity to LDH. In addition, the H(2)O(2)-induced loss of membrane integrity was prevented by raising the osmolality of the extracellular solutions, by depletion of cell ATP, and by inhibitors of volume-sensitive Cl(-) channels. However, these maneuvers did not prevent the H(2)O(2)-induced permeability to small molecules or H(2)O(2)-induced ATP depletion, DNA damage, lipid peroxidation, or cell death. These results support the view that volume-sensitive Cl(-) channels play a role in the progressive loss of cell membrane integrity during injury.

Ehrlichia chaffeensis in a renal transplant recipient

Since its first description in human beings in 1986, ehrlichiosis is now increasingly recognized as a cause of tick-borne febrile illnesses. However, the disease has been reported only rarely in immunosuppressed patients. We report a case of human ehrlichiosis in a patient with a cadaveric renal transplant. The diagnosis was confirmed initially by a positive polymerase chain reaction (PCR) for E. chaffeensis. The antibody titer became positive several weeks later. The patient responded promptly to treatment with doxycycline. Ehrlichiosis should be considered in the differential diagnosis of an acute febrile illness in transplant recipients. PCR provides a rapid means to confirm the diagnosis, particularly in settings in which antibody response may be suppressed.

Inhibition of PARP prevents oxidant-induced necrosis but not apoptosis in LLC-PK1 cells

Oxidant-induced cell injury has been implicated in the pathogenesis of several forms of acute renal failure. The present studies examined whether activation of poly(ADP-ribose)polymerase (PARP) by oxidant-induced DNA damage contributes to oxidant injury of renal epithelial cells. H2O2 exposure resulted in an increase in PARP activity and decreases in cell ATP and NAD content. These changes were significantly inhibited by 10 mM 3-aminobenzamide (3-ABA), a PARP inhibitor. In contrast, H2O2-induced DNA damage was not prevented by 3-ABA. Exposure of LLC-PK(1) cells to 1 mM H2O2 for 2 h induced necrotic cell death as measured by increased lactate dehydrogenase (LDH) release. 3-ABA completely prevented the H2O2-induced LDH release. Live/dead fluorescent staining confirmed the protection by 3-ABA. These results are consistent with the view that oxidant-induced DNA damage activates PARP and that the subsequent ATP and NAD depletion contribute to necrotic cell death. Of note, although protected from necrosis, cells treated with H2O2 and 3-ABA underwent apoptosis as evidenced by DNA fragmentation and bis-benzimide staining. In conclusion, activation of PARP contributes to oxidant-induced ATP depletion and necrosis in LLC-PK1 cells. However, PARP inhibition may target cells toward an apoptotic form of cell death.

Cl- channels in basolateral TAL membranes. XIV. Kinetic properties of a basolateral MTAL Cl- channel

BACKGROUND

This article reports studies on the kinetics of chloride (Cl-) conductance in Cl- channels fused into bilayers from basolaterally enriched vesicles from rabbit outer medulla. A considerable body of evidence indicates that these channels represent rbClC-Ka, a 77 kDa kidney-specific protein of the ClC family of Cl- channels. rbClC-Ka, a candidate channel for mediating net Cl- absorption in the medullary thick ascending limb (MTAL), has been cloned from rabbit outer medulla and localized by immunofluorescence to basolateral membranes of the MTAL. Thus, this is the first account, to our knowledge, of the kinetics of ion permeation through a renal Cl- channel mediating net basolateral Cl- absorption in the thick ascending limb of Henle (TALH), and this channel may represent rbClC-Ka.

METHODS

The electrophysiological properties of these channels were studied by fusing basolaterally enriched MTAL vesicles into planar bilayer membranes.

RESULTS

Cl- conductance through these channels was concentration dependent and saturable. The relationship between gCl (pS) and symmetrical aqueous Cl- concentrations could be expressed in terms of the Michaelis equation with a limiting conductance (GClmax, pS) of 114 pS at infinitely high aqueous Cl- concentrations and a K1/2 of 163 mM Cl-. A log-log plot of the conductance-Cl- concentration relations, in the nonsaturating Cl- concentration range, had a slope of 0.91, that is, virtually unity. The relatively impermeant anion I- produced a voltage-dependent conductance blockade that could be overcome at high electric field strengths.

CONCLUSIONS

The experimental data described earlier here fulfill the traditional criteria for a first-order process with a single Cl- ion occupying these channels at a given time. Although the channels may contain multiple ion binding sites, the latter function, in integral kinetic terms, as a single rate-limiting locus.

Developmental expression of sodium entry pathways in rat nephron

During the past several years, sites of expression of ion transport proteins in tubules from adult kidneys have been described and correlated with functional properties. Less information is available concerning sites of expression during tubule morphogenesis, although such expression patterns may be crucial to renal development. In the current studies, patterns of renal axial differentiation were defined by mapping the expression of sodium transport pathways during nephrogenesis in the rat. Combined in situ hybridization and immunohistochemistry were used to localize the Na-Pi cotransporter type 2 (NaPi2), the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2), the thiazide-sensitive Na-Cl cotransporter (NCC), the Na/Ca exchanger (NaCa), the epithelial sodium channel (rENaC), and 11beta-hydroxysteroid dehydrogenase (11HSD). The onset of expression of these proteins began in post-S-shape stages. NKCC2 was initially expressed at the macula densa region and later extended into the nascent ascending limb of the loop of Henle (TAL), whereas differentiation of the proximal tubular part of the loop of Henle showed a comparatively retarded onset when probed for NaPi2. The NCC was initially found at the distal end of the nascent distal convoluted tubule (DCT) and later extended toward the junction with the TAL. After a period of changing proportions, subsegmentation of the DCT into a proximal part expressing NCC alone and a distal part expressing NCC together with NaCa was evident. Strong coexpression of rENaC and 11HSD was observed in early nascent connecting tubule (CNT) and collecting ducts and later also in the distal portion of the DCT. Ontogeny of the expression of NCC, NaCa, 11HSD, and rENaC in the late distal convolutions indicates a heterogenous origin of the CNT. These data present a detailed analysis of the relations between the anatomic differentiation of the developing renal tubule and the expression of tubular transport proteins.

Cl- channels in basolateral TAL membranes: XIII. Heterogeneity between basolateral MTAL and CTAL Cl- channels

BACKGROUND

Antidiuretic hormone (ADH) or adenosine 3', 5'-cyclic phosphate (cAMP) analogues augment net NaCl absorption in microperfused mouse medullary thick ascending limb (MTAL) segments but not in cortical thick ascending limb (CTAL) segments. This ADH-dependent MTAL effect is due to increased apical Na+/K+/2Cl- admittance and apical K+ recycling accompanied by a rise in calculated intracellular Cl- concentrations and by a threefold rise in basolateral Cl- conductance. rbClC-Ka, a 75.2 member of the ClC family of Cl- channels, mediates net Cl- absorption in the MTAL. The gating characteristics of rbClC-Ka channels from their intracellular surfaces are, to our knowledge, unique among Cl- channels. The channels are activated by small increases in intracellular Cl- (K1/2 = 10 mM Cl-). Adenosine triphosphate plus the catalytic subunit of protein kinase A (ATP + PKA) gate rbClC-Ka when cytosolic Cl- concentrations are 25 mM. Thus, in mouse MTAL segments, ADH-dependent rises in cytosolic Cl- are primarily responsible for basolateral Cl- conductance increases.

METHODS

These experiments compared the properties of Cl- channels fused into bilayers from basolaterally enriched vesicles from cultured mouse CTAL cells with rbClC-Ka channels.

RESULTS

The key findings were that anti-rbClC-Ka, antibody that recognizes and blocks rbClC-Ka, recognized and blocked basolateral Cl- channels in CTAL cells, that the extracellular faces of the CTAL channels were, like rbClC-Ka, substrate gated with a K1/2 of approximately 170 mM Cl-, and that, unlike rbClC-Ka channels, cytosolic faces of basolateral CTAL Cl- channels were not gated by either increasing cytosolic Cl- concentrations or cytosolic (ATP + PKA). This failure of activation of basolateral CTAL Cl- channels was confirmed using excised patch clamp studies. Finally, on Western blots, anti-rbClC-Ka recognized a 74 kDa band on basolateral CTAL vesicles.

CONCLUSIONS

Basolateral CTAL Cl- channels probably share a high degree of structural homology and possibly molecular mass with rbClC-Ka channels. However, significant differences between rbClC-Ka channels and CTAL Cl- channels account for the inability of increasing either cytosolic Cl- or (PKA + ATP) to raise Po in CTAL basolateral Cl- channels.

11Beta-hydroxysteroid dehydrogenase, mineralocorticoid receptor, and thiazide-sensitive Na-Cl cotransporter expression by distal tubules

Mineralocorticoid hormones regulate salt transport along the distal nephron by binding to intracellular receptors and activating gene transcription. Previous experiments showed that systemic aldosterone infusions stimulate thiazide-sensitive Na and Cl transport by distal convoluted tubule (DCT) cells; this effect could have been direct or secondary to systemic hormonal effects. Aldosterone target tissues express both mineralocorticoid receptors and the metabolic enzyme 11beta-hydroxysteroid dehydrogenase type 2. Mineralocorticoid receptors have been localized to the DCT in some experiments, but not in others. Expression of 11beta-hydroxysteroid dehydrogenase type 2 by DCT cells has not been investigated. The present experiments were designed to test the hypothesis that rat DCT cells are targets of aldosterone action. Patterns of mineralocorticoid receptor, 11beta-hydroxysteroid dehydrogenase, thiazide-sensitive Na-Cl cotransporter, and Na/Ca exchanger expression along the distal tubule were examined. A polyclonal antibody was generated to localize the thiazide-sensitive Na-Cl cotransporter. Thiazide-sensitive Na-Cl cotransporter and 11beta-hydroxysteroid dehydrogenase expression were examined using both in situ hybridization and immunocytochemistry; Na/Ca exchanger and mineralocorticoid receptor expression were examined by immunocytochemistry. The results indicate that 11beta-hydroxysteroid dehydrogenase is expressed by DCT cells, as well as connecting tubule cells and principal cells of the collecting duct; expression levels are low near the junction with the thick ascending limb and rise near the transition to the connecting tubule. Mineralocorticoid receptors are expressed by DCT cells, as well as along the thick ascending limb, connecting tubule, and collecting duct. The results indicate that components of the mineralocorticoid receptor system are expressed by DCT cells, suggesting that these cells are targets of aldosterone action.

Functional and molecular evidence for P2X receptors in LLC-PK1 cells

Extracellular ATP affects a wide variety of cells via purinergic membrane receptors. One class of purinergic receptors, P2X, consists of ATP-gated, calcium-permeable, cation-selective channels. We performed whole cell patch-clamp studies, intracellular calcium concentration ([Ca2+]i) measurements, and reverse transcription-polymerase chain reaction (RT-PCR) to determine whether P2X receptors are expressed in LLC-PK1 cells. First, in patch-clamp studies, 100 microM ATP depolarized the cell membrane and increased the whole cell conductance of LLC-PK1 cells. This response was dose dependent and inhibited by 100 microM suramin, a P2 receptor antagonist. The ATP-induced conductance was cation selective but did not discriminate between Na+ and K+. ADP, alpha,beta-methylene-ATP, and beta,gamma-methylene-ATP had no effect on the whole cell conductance. Next, 10 microM ATP caused a rapid rise in [Ca2+]i in LLC-PK1 cells. This effect of ATP was inhibited by the absence of extracellular calcium and by suramin but not by pretreatment with pertussis toxin. ADP and beta,gamma-methylene-ATP had little or no effect on [Ca2+]i. Finally, RT-PCR produced a 330-bp fragment from LLC-PK1 cell RNA, whose sequence was 80% identical to the rat P2X1 receptor. We conclude that LLC-PK1 cells express purinergic receptors of the P2X class, which mediate depolarization and calcium entry when activated.

Cl- channels in basolateral renal medullary membranes. XII. Anti-rbClC-Ka antibody blocks MTAL Cl- channels

Cl- channels in the medullary thick ascending limb (MTAL) studied by either patch-clamp technique or reconstitution into lipid bilayers are activated by increases in intracellular Cl- concentrations. rbClC-Ka, a ClC Cl- channel, may represent this channel. We therefore evaluated the role of rbClC-Ka in transcellular MTAL Cl- transport in two separate ways. First, an antibody was raised against a fusion protein containing a 153-amino acid fragment of rbClC-Ka. Immunostaining of rabbit kidney sections with the antibody was localized to basolateral regions of MTAL and cortical thick ascending limb (CTAL) segments and also to the cytoplasm of intercalated cells in the cortical collecting duct. Second, Cl- uptake and efflux were measured in suspensions of mouse MTAL segments. Cl- uptake was bumetanide sensitive and was stimulated by treatment with a combination of vasopressin + forskolin + dibutyryl adenosine 3',5-cyclic monophosphate (DBcAMP). Cl- efflux was also increased significantly by vasopressin + forskolin + DBcAMP from 114 +/- 20 to 196 +/- 36 nmol.mg protein-1.45 s-1 (P = 0.003). Cl- efflux was inhibited by the Cl- channel blocker diphenylamine-2-carboxylate (154 +/- 26 vs. 70 +/- 21 nmol.mg protein-1.45 s-1, P = 0.003). An anti-rbClC-Ka antibody, which inhibits the activity of MTAL Cl- channels in lipid bilayers, reduced Cl- efflux from intact MTAL segments (154 +/- 28 vs. 53 +/- 14 nmol.mg protein-1.45 s-1, P = 0.02). These results support the view that rbClC-Ka is the basolateral membrane Cl- channel that mediates vasopressin-stimulated net Cl- transport in the MTAL segment.

N-glycosylation is not essential for enzyme activity of 11beta-hydroxysteroid dehydrogenase type 2

11Beta-hydroxysteroid dehydrogenase (11beta-HSD) catalyzes the oxidation of cortisol and corticosterone to cortisone and 11-dehydrocorticosterone, respectively. NAD-dependent 11beta-HSD is expressed at high levels in the distal nephron and contributes to mineralocorticoid specificity in that region. The present studies determined whether N-glycosylation is necessary for the activity of NAD-dependent 11beta-HSD (11beta-HSD2). First, cultured human colonic epithelial cells (T84 cells), which express native 11beta-HSD2 activity, were grown in medium with and without tunicamycin, an inhibitor of N-glycosylation. Tunicamycin had no effect on the enzyme activity. Next, the only putative N-glycosylation site (Asn394-Leu395-Ser396) of the cloned human kidney enzyme was eliminated by site-directed mutagenesis. Chinese hamster ovary (CHO) cells transfected with either the wild-type or the mutant cDNA construct showed no difference in the expressed enzyme activity, and Western blot analysis showed that the 11beta-HSD2 protein was the same size in cells expressing either the wild-type or the N394D mutant. Likewise, the molecular mass of the 11beta-HSD2 protein in T84 cells was not altered by treatment with peptide-N-glycosidase F or tunicamycin. We conclude that human 11beta-HSD2 is not a N-glycoprotein and N-glycosylation is not essential for the expression of enzyme activity.