Stop codons affect 5' splice site selection by surveillance of splicing

Pre-mRNA splicing involves recognition of a consensus sequence at the 5' splice site (SS). However, only some of the many potential sites that conform to the consensus are true ones, whereas the majority remain silent and are not normally used for splicing. We noticed that in most cases the utilization of such a latent intronic 5' SS for splicing would introduce an in-frame stop codon into the resultant mRNA. This finding suggested a link between SS selection and maintenance of an ORF within the mRNA. Here we tested this idea by analyzing the splicing of pre-mRNAs in which in-frame stop codons upstream of a latent 5' SS were mutated. We found that splicing with the latent site is indeed activated by such mutations. Our findings predict the existence of a checking mechanism, as a component of the nuclear pre-mRNA splicing machine, to ensure the maintenance of an ORF. This notion is highly important for accurate gene expression, as perturbations that would lead to splicing at these latent sites are expected to introduce in-frame stop codons into the majority of mRNAs.

Lovastatin enhances ecto-5'-nucleotidase activity and cell surface expression in endothelial cells: implication of rho-family GTPases

Extracellular adenosine production by the GPI-anchored Ecto-5'-Nucleotidase (Ecto-5'-Nu) plays an important role in the cardiovascular system, notably in defense against hypoxia. It has been previously suggested that HMG-CoA reductase inhibitors (HRIs) could potentiate the hypoxic stimulation of Ecto-5'Nu in myocardial ischemia. In order to elucidate the mechanism of Ecto-5'-Nu stimulation by HRIs, Ecto-5'-Nu activity and expression were determined in an aortic endothelial cell line (SVAREC) incubated with lovastatin. Lovastatin enhanced Ecto-5'-Nu activity in a dose-dependent manner. This increase was not supported by de novo synthesis of the enzyme because neither the mRNA content nor the total amount of the protein were modified by lovastatin. By contrast, lovastatin enhanced cell surface expression of Ecto-5'-Nu and decreased endocytosis of Ecto-5'-Nu, as evidenced by immunostaining. This effect appeared unrelated to modifications of cholesterol content or Ecto-5'-Nu association with detergent-resistant membranes. The effect of lovastatin was reversed by mevalonate, the substrate of HMG-CoA reductase, by its isoprenoid derivative, geranyl-geranyl pyrophosphate, and by cytotoxic necrotizing factor, an activator of Rho-GTPases. Stimulation of Ecto-5'-Nu by lovastatin enhanced the inhibition of platelet aggregation induced by endothelial cells. In conclusion, lovastatin enhances Ecto-5'-Nu activity and membrane expression in endothelial cells. This effect seems independent of lowering cholesterol content but could be supported by an inhibition of Ecto-5'-Nu endocytosis through a decrease of Rho-GTPases isoprenylation.

Hypoxia reduces alveolar epithelial sodium and fluid transport in rats: reversal by beta-adrenergic agonist treatment

In cultured alveolar epithelial cells, hypoxia induces a downregulation of the two main Na proteins, the epithelial Na channel (ENaC) and the Na,K-ATPase. However, the in vivo effects of hypoxia on alveolar epithelial transport have not been well studied. Therefore, the objectives of this study were to investigate in an in vivo rat model if hypoxia induces a reduction in vectorial Na and fluid transport across the alveolar epithelium in vivo, and if a change in net fluid transport is associated with modification in the expression and/or activity of Na transport proteins. Rats were exposed to 8% O(2) from 3 to 24 h. Hypoxia induced a progressive decrease in alveolar liquid clearance (ALC) reaching 50% at 24 h, an effect that was related primarily to a decrease in amiloride-sensitive transepithelial Na transport. On RNase protection assay of alveolar type II (ATII) cells isolated immediately after hypoxic exposure, steady state levels of mRNA were increased for alpha-rENaC and beta(1)-Na, K-ATPase, whereas the levels of gamma-rENaC and alpha(1)-Na,K-ATPase were unchanged. On Western blots of ATII cell membranes, alpha-ENaC subunit protein slightly increased, whereas the amount of alpha(1)- and beta(1)-Na,K-ATPase protein were unchanged with hypoxia. Thus, the decrease in transepithelial Na transport was not explained by a parallel change in gene expression or the quantity of transport proteins. Interestingly, hypoxia-induced decrease in ALC was completely reversed by intra-alveolar administration of the beta(2) agonist, terbutaline (10(-4) M). These results suggest that hypoxia-induced decrease in Na transport is not simply related to a downregulation of Na transport proteins but rather to a decrease in Na protein activity by either internalization of the proteins and/or direct alteration of the protein in the membrane. The dramatic increase of ALC with beta(2)-agonist therapy indicates that the decrease of transepithelial Na and fluid transport during hypoxia is rapidly reversible, a finding of major clinical significance.

Mechanical strains induced by tubular flow affect the phenotype of proximal tubular cells

The effects of flow-induced mechanical strains on the phenotype of proximal tubular cells were addressed in vivo and in vitro by subjecting LLC-PK(1) and mouse proximal tubular cells to different levels of flow. Laminar flow (1 ml/min) induced a reorganization of the actin cytoskeleton and significantly inhibited the expression of plasminogen activators [tissue-type (tPA) activity: 25% of control cells; tPA mRNA: 70% of control cells; urokinase (uPA) mRNA: 56% of control LLC-PK(1) cells]. In vivo, subtotal nephrectomy (Nx) decreased renal fibrinolytic activity and uPA mRNA content detectable in proximal tubules. Nx also induced a reinforcement of the apical domain of the actin cytoskeleton analyzed by immunofluorescence. These effects of flow on tPA and uPA mRNA were prevented in vitro when reorganization of the actin cytoskeleton was blocked by cytochalasin D and were associated, in vitro and in vivo, with an increase in shear stress-responsive element binding activity detected by an electrophoretic mobility shift assay in proximal cell nuclear extracts. These results demonstrate that tubular flow affects the phenotype of renal epithelial cells and suggest that flow-induced mechanical strains could be one determinant of tubulointerstitial lesions during the progression of renal diseases.

P-glycoprotein inhibitors stimulate renal phosphate reabsorption in rats

BACKGROUND

Dipyridamole (Dip) was previously shown to increase renal phosphate (Pi) reabsorption in humans. However, the mechanism(s) underlying this renal tubular effect is not fully elucidated. It is known that Dip inhibits the activity of the P-glycoprotein (Pgp) multidrug resistance protein 1 (MDR1) expressed on the apical membrane of renal proximal tubular cells where the Na-Pi cotransporter (NPT2) is also expressed. We hypothesized that Dip could increase renal Pi reabsorption by inhibiting Pgp activity.

METHODS

To test this hypothesis, the effects of Dip, verapamil (Ver), and cyclosporine A (CsA), three unrelated Pgp inhibitors, were studied on the renal Pi reabsorption in rats.

RESULTS

All three drugs decreased the fractional excretion of Pi (FE(Pi)) in a dose-dependent manner within one hour after beginning the drug infusion, without altering the glomerular filtration rate or serum parathyroid hormone concentration. Sodium-dependent Pi uptake but not Na-glucose transport was increased in brush-border membrane vesicles (BBMVs) when comparing treated with untreated rats. Western blot analysis showed that NPT2 protein was increased in BBMVs from treated rats. Dip and Ver had no effect when applied directly to BBMVs prepared from untreated rats. Pretreatment of rats with colchicine prevented the effects of Dip on the FE(Pi) and NPT2 expression in brush-border membranes.

CONCLUSIONS

Our results suggest that inhibition of Pgp in the proximal tubule increases Pi uptake and NPT2 translocation to the apical membrane.

Proliferation and remodeling of the peritubular microcirculation after nephron reduction: association with the progression of renal lesions

Little is known about the serial changes that might occur in renal capillaries after reduction of renal mass. In the current study, our aim was to document potential alterations in the morphology and proliferation of the renal cortical peritubular microcirculation at specific time points (7 and 60 days) after experimental 75% surgical nephron reduction using two strains of mice that we here demonstrate react differently to the same initial insult: one strain (C57BL6xDBA2/F1 mice) undergoes compensatory growth alone, whereas the other (FVB/N mice) additionally develops severe tubulo-interstitial lesions. Our data demonstrate that significant remodeling and proliferation occur in renal cortical peritubular capillaries after experimental nephron reduction, as assessed by microangiography using infusion of fluorescein isothiocyanate-labeled dextran, expression of the endothelial markers CD34 and Tie-2, and co-expression of CD34 and proliferating cell nuclear antigen, a surrogate marker of cell proliferation. This was accompanied by an increase of renal vascular endothelial growth factor protein levels and a change in distribution of this protein within the kidney itself. Moreover, most of these responses were accentuated in FVB/N mice in the presence of progressive renal disease and positively correlated with tubular epithelial cell proliferation. Hence, we have made three significant novel observations that illuminate the complex pathophysiology of chronic kidney damage after nephron reduction: 1) cortical peritubular capillaries grow by proliferation and remodeling, 2) vascular endothelial growth factor expression is altered, and 3) the development of tubulo-interstitial disease is genetically determined.

Frequency of renal phosphate leak among patients with calcium nephrolithiasis

BACKGROUND

Nephrolithiasis is a frequent disorder affecting 10 to 15% of the population in Europe and the United States. More than 80% of renal stones are made of calcium oxalate and calcium phosphate. The main identified risks for calcium renal stone formation are hypercalciuria and urinary saturation. A urine phosphate (Pi) loss is often associated with hypercalciuria; furthermore, hyperphosphaturia increases urinary saturation.

METHODS

To determine whether urinary phosphate loss is associated with calcium urolithiasis, we measured renal Pi threshold (TmPi) in 207 stone formers with normal parathyroid hormone (PTH) serum concentration and in 105 control subjects.

RESULTS

The TmPi followed a normal distribution in both groups. The mean TmPi was significantly lower in stone formers versus controls (0.72 +/- 0.13 vs. 0.87 +/- 0.18 mmol/L, P < 0.0001) because of a shift to the left of the TmPi distribution curve in the stone former population, with no evidence for bimodal distribution. Five percent of the controls had a TmPi <0.63 versus 19% of the stone formers. Daily urinary calcium excretion was significantly higher in stone formers than in controls. Calcium excretion was also significantly higher in stone formers with TmPi <0.63 mmol/L compared with those with TmPi > or =0.63. Serum PTH and ionized calcium concentrations were not different in stone formers and in control subjects, whatever the TmPi value.

CONCLUSIONS

: A low TmPi is more frequently encountered in stone formers with a normal PTH concentration than in control subjects and is associated with a high urinary Ca excretion. The hypophosphatemia induced by a renal phosphate leak may predispose the subject to calcium stone formation by increasing the serum calcitriol level, calcium excretion, and urinary saturation.

Two apical multidrug transporters, P-gp and MRP2, are differently altered in chronic renal failure

Tubular function is altered in chronic renal failure (CRF). Whether drug secretion by renal tubules is modified in CRF is questioned because of frequent accumulation of various toxins in CRF. This function mainly involves ATP-dependent drug transporters, particularly P-glycoprotein (P-gp) and multidrug resistance-associated protein (MRP) 2, both present in apical membrane of epithelial cells. The present study was aimed at determining the changes in P-gp and MRP2 expression induced by experimental CRF in kidney and liver. The relationship between MRP2 and glutathione metabolism changes was examined because MRP2 transports GSSG and glutathione conjugates. Rats underwent either 80% subtotal nephrectomy (Nx) or sham operation, and determinations were performed 3 and 6 wk later. CRF induced a 70--200% rise in protein and mRNA expression of MRP2 after 3 and 6 wk post-Nx in remnant kidney and after 6 wk in liver. However, P-gp expression was unchanged by CRF. Relative to whole kidney mass, total MRP2 levels decreased by only 27% in Nx rats whereas total P-gp levels were reduced by 60%. Renal GSSG and total glutathione levels were increased by 30% in Nx rats, but glutathione-S-transferase (GST) activity was normal; liver GSSG levels and GST activity were reduced in Nx rats. In conclusion, CRF resulted in specific overexpression of MRP2 in kidney and liver. This could be an adaptative response to some elevated circulating toxins. The later MRP2 induction and different glutathione changes in liver compared with kidney suggest different mechanisms for MRP2 induction and/or action in these two tissues.

Location and function of the epithelial Na channel in the cochlea

In the cochlea, endolymph is a K-rich and Na-poor fluid. The purpose of the present study was to check the presence and to assess the role of epithelial Na channel (ENaC) in this organ. alpha-, beta-, and gamma-ENaC subunit mRNA, and proteins were detected in rat cochlea by RT-PCR and Western blot. alpha-ENaC subunit mRNA was localized by in situ hybridization in both epithelial (stria vascularis, spiral prominence, spiral limbus) and nonepithelial structures (spiral ligament, spiral ganglion). The alpha-ENaC-positive tissues were also positive for beta-subunit mRNA (except spiral ganglion) or for gamma-subunit mRNA (spiral limbus, spiral ligament, and spiral ganglion), but the signals of beta- and gamma-subunits were weaker than those observed for alpha-subunit. In vivo, the endocochlear potential was recorded in guinea pigs under normoxic and hypoxic conditions after endolymphatic perfusion of ENaC inhibitors (amiloride, benzamil) dissolved either in K-rich or Na-rich solutions. ENaC inhibitors altered the endocochlear potential when Na-rich but not when K-rich solutions were perfused. In conclusion, ENaC subunits are expressed in epithelial and nonepithelial cochlear structures. One of its functions is probably to maintain the low concentration of Na in endolymph.

H+,K+-ATPase is not involved in endolymph pH homeostasis

In endolymph, the calculated electrochemical gradients suggest the presence of active transport of H+ and K+. The presence of H+,K+-ATPase, an active transporter that mediates the exchange of intracellular H+ for extracellular K+, was investigated using reverse transcription-polymerase chain reaction on rat microdissected inner ear tissues. Distal colon and kidney were used as positive controls. The two H+,K+-ATPase colonic or gastric isoforms were not detected in the rat cochlea and endolymphatic sac. This result suggests that H+,K+-ATPase is not involved in H+ and K+ endolymph homeostasis.

Sulfate homeostasis, NaSi-1 cotransporter, and SAT-1 exchanger expression in chronic renal failure in rats

BACKGROUND

It is known that hypersulfatemia, like hyperphosphatemia, occurs in chronic renal failure (CRF). The aim of this study was to assess the effects of CRF on sulfate homeostasis and on sodium sulfate cotransport (NaSi-1) and sulfate/oxalate-bicarbonate exchanger (Sat-1) expression in the kidney. In addition, sulfate homeostasis was compared with phosphate homeostasis.

METHODS

Experimental studies were performed in adult male rats at three and six weeks after 80% subtotal nephrectomy (Nx) or sham-operation (S) (N = 9 per group). Transporter protein and mRNA expressions were measured by Western blot and RNase protection assay (RPA), respectively. Results were quantitated by densitometric scanning (Western) and electronic autoradiography (RPA), and were expressed in densitometric units (DUs; Western) and cpm (RPA).

RESULTS

Creatinine clearance was lower in Nx-3 compared with S-3 rats (0.23 vs. 0.51 mL/min/100 g body weight, P < 0.001) and was further impaired in Nx-6 rats (0.15 vs. 0.48, P < 0.001). Sulfatemia was significantly higher in Nx-3 rats (1.08 vs. 0.84 mmol/L, P < 0.05) and further increased in Nx-6 rats (1.42 vs. 0.90 mmol/L, P < 0.01). Fractional sulfate excretion (FESO4) was increased by twofold in Nx-3 and Nx-6 rats compared with corresponding S rats. Phosphatemia did not differ between Nx-3 rats and controls, but was increased in Nx-6 rats (P < 0.01). Total amounts of both NaSi-1 and Sat-1 proteins were significantly decreased in both Nx-3 and Nx-6 rats when compared with controls. However, NaSi-1 protein and mRNA densities did not significantly change in Nx-3 rats, but were significantly increased in Nx-6 rats when compared with controls (4.8 vs. 3.7 DU/microg protein, P < 0.05, and 7.1 vs. 2.8 cpm/microg RNA, P < 0.01, respectively, for protein and mRNA). In contrast to NaSi-1, Sat-1 protein density was significantly decreased both in Nx-3 (2.9 vs. 3.6 DU/microg protein, P < 0.05) and Nx-6 rats (2.4 vs. 3.4 DU/microg protein, P < 0.05), and Sat-1 mRNA density significantly decreased in Nx-6 rats (10.7 vs. 14.7 cpm/microg RNA, P < 0.05). Na-PO4 cotransporter (NaPi-2) protein total abundance and density were decreased at three and six weeks in Nx rats.

CONCLUSIONS

These results demonstrate that both NaSi-1 and Sat-1 total protein abundances are decreased in CRF, which may contribute to the increase in fractional sulfate excretion. Strikingly, NaSi-1 density was not decreased in CRF three weeks after Nx, and furthermore, increased six weeks after Nx, in contrast to NaPi-2 density, which was decreased at both times. The significance of this difference remains to be determined, but may explain why hypersulfatemia occurs earlier than hyperphosphatemia in CRF.

Using transgenic mice to analyze the mechanisms of progression of chronic renal failure

An understanding of the mechanisms underlying the formation of renal lesions is necessary for the development of strategies aiming to delay the progression of chronic renal failure. The generation of transgenic mice in the past 20 years has contributed significantly to the study of this phenomenon. Overexpression and/or inactivation of single factors in renal tissue demonstrated that molecules such as growth factors, proto-oncogenes, and renin-angiotensin system elements play major roles in renal deterioration. Several mouse models of renal injury have been developed in the past 10 yr. Transgenic mice that exhibit a normal phenotype under physiologic conditions allow analysis of the roles of single factors in the progression of chronic renal failure when renal injury models are used. Using this strategy, it was demonstrated that vascular adaptation, which is a process that involves the endothelin/nitric oxide balance, is essential for the survival of mice after nephron reduction and that the epidermal growth factor/activator protein-1/Bcl-2 pathway is involved in the development of renal lesions after renal injury, possibly via adjustment of the proliferation/apoptosis balance. Moreover, it was demonstrated that selective inhibition of epidermal growth factor signaling in the kidney successfully prevents the progression of chronic renal failure. These results indicate the power of transgenesis for elucidation of the pathogenesis of renal disease.

HNF1alpha controls renal glucose reabsorption in mouse and man

Recently it has been shown that dominant mutations in the human hepatocyte nuclear factor 1alpha (HNF1alpha) gene, encoding for a homeoprotein that is expressed in liver, kidney, pancreas and intestine, result in maturity onset diabetes of the young type 3 (MODY3). HNF1alpha-null mice are diabetic, but at the same time suffer from a renal Fanconi syndrome characterized by urinary glucose loss. Here we show that MODY3 patients are also characterized by a reduced tubular reabsorption of glucose. The renal murine defect is due to reduced expression of the low affinity/high capacity glucose cotransporter (SGLT2). Our results show that HNF1alpha directly controls SGLT2 gene expression. Together these data indicate that HNF1alpha plays a key role in glucose homeostasis in mammals.

Targeted expression of a dominant-negative EGF-R in the kidney reduces tubulo-interstitial lesions after renal injury

The role of EGF in the evolution of renal lesions after injury is still controversial. To determine whether the EGF expression is beneficial or detrimental, we generated transgenic mice expressing a COOH-terminal-truncated EGF-R under the control of the kidney-specific type 1 gamma-glutamyl transpeptidase promoter. As expected, the transgene was expressed exclusively at the basolateral membrane of proximal tubular cells. Under basal conditions, transgenic mice showed normal renal morphology and function. Infusion of EGF to transgenic animals revealed that the mutant receptor behaved in a dominant-negative manner and prevented EGF-signaled EGF-R autophosphorylation. We next evaluated the impact of transgene expression on the development of renal lesions in two models of renal injury. After 75% reduction of renal mass, tubular dilations were less severe in transgenic mice than in wild-type animals. After prolonged renal ischemia, tubular atrophy and interstitial fibrosis were reduced in transgenic mice as compared with wild-type mice. The beneficial effect of the transgene included a reduction of tubular cell proliferation, interstitial collagen accumulation, and mononuclear cell infiltration. In conclusion, functional inactivation of the EGF-R in renal proximal tubular cells reduced tubulo-interstitial lesions after renal injury. These data suggest that blocking the EGF pathway may be a therapeutic strategy to reduce the progression of chronic renal failure.

[Growth factors. Role in the progression of renal lesions]

FROM PATHOPHYSIOLOGY TO THERAPEUTICS: Nephrologists are faced with the continuing problem of helping patients avoid the onset or retard the development of end-stage renal failure. Despite the treatments available, the risk is still high for patients and the cost a heavy burden for the public health budget. These facts underline the importance of a detailed understanding of the mechanisms leading to the destruction of renal parenchyma in order to develop therapeutic strategies capable of slowing the inevitable progression of kidney lesions. GROWTH FACTORS: It is currently recognized that a major reduction in the number of functional nephrons, whatever the initial cause, leads in itself to a progressive deterioration of healthy nephrons and finally to complete destruction of the kidney. The underlying mechanisms remain largely unknown. One possible mechanism would involve an overexpression of several growth factors in the damaged renal parenchyma. We present in this review experimental data obtained with various approaches, including pharmacological and/or dietetic modulations and the establishment of transgenic mouse lines, to demonstrate the key role played by growth factors in the progression of renal lesions. The pathways followed by these growth factors in the process of renal destruction as well as certain elements leading to their overexpression are also discussed.

Sodium restriction decreases AP-1 activation after nephron reduction in the rat: role in the progression of renal lesions

Renal hyperplasia and hypertrophy are early events after nephron reduction which precede progressive destruction of the remnant kidney. Restriction of dietary sodium content was shown to reduce renal lesions following nephron reduction. AP-1 is a transcription factor, resulting from heterodimerization of fos and jun proteins, which mediates the effects of mitogenic growth factors. To elucidate the role of AP-1 in growth processes involved in renal deterioration, we evaluated whether restriction of dietary sodium content (0.25 vs. 0.50% sodium w/w) affected AP-1-DNA binding and hyperplasia in the remnant kidney after nephron reduction (70% nephrectomy). Cell proliferation, evaluated by PCNA immunostaining, increased progressively from day 7 to day 60 in glomeruli, proximal and distal tubules and loops of Henle of nephrectomized (Nx) rats compared to control sham-operated (C) animals. AP-1-DNA binding activity increased 7 and 14 days after surgery, but it was reduced below C values at day 60. c-fos and c-jun expression were also reduced in Nx rats at day 60. Sodium restriction significantly reduced the number of PCNA-stained cells in glomeruli and tubules at days 14 and 60, but not at day 7, whereas it decreased AP-1 activation at all times of the study. This effect was associated to a marked reduction of renal lesions in Nx rats. In conclusion, we showed that, after nephron reduction, the beneficial effect of sodium restriction was associated with a reduction of hyperplasia and AP-1 activation, but that the latter did not parallel delayed cell proliferation rate in remaining nephrons. Thus, we propose that different transduction pathways are involved in cell proliferation after nephron reduction, according to the time of evolution of renal lesions.

[Inhibitors of HMG CoA reductase: new modes of action, new indications?]

3-hydroxy-3-methylglutaryl Coenzyme A reductase inhibitors, or statins, are hypocholesterolaemic agents used in the treatment of hypercholesterolaemia and cardiovascular diseases. Their hypocholesterolaemic action results from the inhibition of the intracellular synthesis of cholesterol via the mevalonate pathway. Recent studies have demonstrated that cholesterol is not the only intracellular target of statins. Indeed, statins have been shown to inhibit protein isoprenylation, a post-translational modification involved in membrane localization and protein activity. By inhibiting isoprenylation of Ras protein, statins induce the inhibition of the Ras-AP1 pathway and the decrease in cell proliferation. Similarly, inhibition of Rho protein insoprenylation leads to the disorganization of the cytoskeleton and the induction of the fibrinolytic system. All these effects need to be confirmed in vivo and in the human, and open new areas for these therapeutics agents.

NaPO(4) cotransport type III (PiT1) expression in human embryonic kidney cells and regulation by PTH

The aim of the present study was to characterize the type(s) of NaPO(4) cotransporter expressed in the human renal cell line HEK-293 and its regulation by parathyroid hormone (PTH) in wild-type cells and in cells transfected by the PTH/PTH-related protein (PTHrP) receptor. The results showed that human embryonic kidney HEK-293 cells expressed NaPO(4) cotransporter type III (PiT1) mRNA and protein. In contrast, type I (NPT1) or II (NPT2) cotransporter mRNA were not expressed. Na(+)-dependent phosphate uptake followed a Michaelis-Menten model (apparent maximal transport rate and affinity constant: 23.32 +/- 0.69 nmol PO(4). mg protein(-1). 10 min(-1) and 0.147 +/- 0.014 mM KH(2)PO(4), respectively), was stimulated by phosphate deprivation (maximal increase 24.5 +/- 0.8%, P < 0.001, after 15 h of phosphate deprivation), and was inhibited by increasing pH (3.6 +/- 0.2-fold decrease at pH 8.5, P < 0.0001). It was inhibited in a time- and concentration-dependent fashion by PTH in HEK-293 cells stably transfected by PTH/PTHrP receptors but not in parental HEK-293 cells. Maximal inhibition of Na(+)-dependent phosphate transport was observed at 30 min after the addition of 72 nM PTH-(1-34) (31.5 +/- 2.4% inhibition, P < 0.01). PTH inhibition of phosphate transport was maintained in phosphate-deprived cells and reversed by both GF109203X (10(-6) M) or staurosporine (5.5 nM), two protein kinase C inhibitors. Na(+)-dependent phosphate uptake was also significantly inhibited by phorbol 12-myristate 13-acetate (20.9 +/- 3.9% inhibition, P < 0.001) but not by dibutyril-cAMP (10(-4) M) or forskolin (50 microM). The physiological role played by type III NaPO(4) cotransport expression in the overall renal regulation of phosphate homeostasis remains to be established.

Renal tubular cells cultured from genetically modified animals

The culture of renal tubular cells from genetically modified animals opens the opportunity of biochemical, cell biology and physiological studies under strictly controlled conditions. Either primary cultures or cell lines can be used. Through two examples of primary cultures of proximal tubular cells obtained from knock-out mice, important information about the function of proteins were obtained. Mice lacking vimentin, an intermediate filament normally reexpressed in tubular cells during regeneration and culture, have a normal tubular function under basal conditions. Proximal cells grown from these animals exhibit a defect in sodium-glucose cotransport activity, most likely related to alterations in the dimer/monomer ratio of the transporter in the apical membranes. These alterations may be important in terms of tubular function during the recovery phase following acute tubular necrosis. The situation is strikingly different with regard to mice lacking HNF-1, a transactivator involved in the transcription of multiple genes. These animals suffer from severe Fanconi syndrome related to decreased expression of proximal transporters including isoforms of sodium-glucose (SGLT2) and sodium-phosphate (NPT1) cotransporters. Whereas transport defects are observed in isolated tubules, they are no longer apparent in cultured proximal cells because the expression of these isoforms is suppressed under culture conditions. These observations illustrate the interest and limits of the in vitro models for studying renal function in transgenic animals.

Halothane stimulates a Na+H+ antiporter involved in the regulation of intracellular pH in alveolar epithelial cells

Changes in intracellular pH (pHi) of alveolar type II (ATII) cells have been involved in the pathophysiology of pulmonary edema. ATII cells have evolved several ions transporters to regulate their pHi, including a Na+H+ antiporter. Because halothane alters the activity of ion transporters in various cells types, it may also affect the activity of this Na+H+ antiporter. This study was performed 1) to characterize a Na+H+ antiporter in a model of ATII cells and 2) to investigate the effect of halothane on the activity of this antiporter. ATII cells were obtained from primary rat ATII cells transfected with a mutant of simian virus SV40 large T antigen (SV40-T2), and their pHi was monitored using the pH-sensitive fluorescent probe 2'-7' (bis carboxyethyl)-5(6')-carboxyfluorescein. We demonstrated in vitro that 1) a Na+H+ antiporter (apparent Km 6.8 +/- 3.4 mM, Vmax 0.0105 +/- 0.0013 delta UpHi/s) regulates the pHi of SV40-T2 cells and 2) at clinically relevant concentrations (10(-3) to 10(-5) M) and for a short exposure duration (60 min), halothane enhances the activity of this antiporter. Because ATII cell acidification has been associated with alterations in the alveolar epithelial barrier, halothane-induced intracellular alkalinization might exhibit some protective effect in clinical situations, such as aspiration pneumonia.

IMPLICATIONS

In vitro, halothane induces an intracellular alkalinization of pneumocytes II via the activation of a Na+H+ antiporter. Because acidification of these cells has been associated with alterations in the alveolar epithelial barrier, halothane might exhibit some protective effect in clinical situations, such as aspiration pneumonia.

Sphingolipid depletion increases formation of the scrapie prion protein in neuroblastoma cells infected with prions

Sphingolipid-rich rafts play an essential role in the posttranslational (Borchelt, D. R., Scott, M., Taraboulos, A., Stahl, N., and Prusiner, S. B. (1990) J. Cell Biol. 110, 743-752)) formation of the scrapie prion protein PrP(Sc) from its normal conformer PrP(C) (Taraboulos, A., Scott, M., Semenov, A., Avrahami, D., Laszlo, L., Prusiner, S. B., and Avraham, D. (1995) J. Cell Biol. 129, 121-132). We investigated the importance of sphingolipids in the metabolism of the PrP isoforms in scrapie-infected ScN2a cells. The ceramide synthase inhibitor fumonisin B(1) (FB(1)) reduced both sphingomyelin (SM) and ganglioside GM1 in cells by up to 50%, whereas PrP(Sc) increased by 3-4-fold. Whereas FB(1) profoundly altered the cell lipid composition, the raft residents PrP(C), PrP(Sc), caveolin 1, and GM1 remained insoluble in Triton X-100. Metabolic radiolabeling demonstrated that PrP(C) production was either unchanged or slightly reduced in FB(1)-treated cells, whereas PrP(Sc) formation was augmented by 3-4-fold. To identify the sphingolipid species the decrease of which correlates with increased PrP(Sc), we used two other reagents. When cells were incubated with sphingomyelinase for 3 days, SM levels decreased, GM1 was unaltered, and PrP(Sc) increased by 3-4-fold. In contrast, the glycosphingolipid inhibitor PDMP reduced PrP(Sc) while increasing SM. Thus, PrP(Sc) seems to correlate inversely with SM levels. The effects of SM depletion contrasted with those previously obtained with the cholesterol inhibitor lovastatin, which reduced PrP(Sc) and removed it from detergent-insoluble complexes.

Effect of lipid-lowering strategies on tubular cell biology

BACKGROUND

Interstitial fibrosis and the development of renal cysts are crucial phenomena in renal disease progression. While 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors has been shown to reduce the progression of several experimental nephropathies, the mechanism of their potential protective effect remaines unclear.

METHODS

The antiproliferative, apoptotic, and fibrinolytic effects of HMG-CoA reductase inhibitors were assessed in primary cultured rat (rPTCs) and mouse proximal tubule cells (mPTCs), in isolated rat proximal tubules, and in vivo in 5/6 nephrectomized rats (Nx).

RESULTS

In vitro, lovastatin inhibited rPTC proliferation in a manner selectively prevented by mevalonate, farnesyl-, or geranylgeranyl-pyrophosphate (FPP or GGPP). Lovastatin reduced membrane-bound p21ras and fetal calf serum-induced c-fos and c-jun protein expression. Gel shift assay showed that lovastatin reduced activated protein-1 (AP-1) binding activity. In vivo, lovastatin inhibited tubular cell proliferation after Nx, as measured by proliferative cell nuclear antigen staining. Lovastatin-treated mPTCs displayed nucleus cleavage and DNA ladder formation, which were prevented by GGPP. Like C3 exoenzyme, lovastatin induced actin filament disruption, which preceded evidence of apoptosis. Lovastatin increased tissue-type plasminogen activator (PA) and decreased PA inhibitor activities and antigens; these effects were prevented by mevalonate and GGPP but not FPP, and were reproduced by C3 exoenzyme in a manner insensitive to GGPP.

CONCLUSIONS

HMG-CoA reductase inhibitors decreased proliferation, increased apoptosis, and enhanced fibrinolytic activity of renal tubular cells via modulation of different isoprenylated proteins. These effects could participate to reduce the progression of renal diseases.