Changes in mRNAs for enzymes of glutamine metabolism in kidney and liver during ammonium chloride acidosis

pH-responsive, gluconeogenic renal epithelial LLC-PK1-FBPase+cells: a versatile in vitro model to study renal proximal tubule metabolism and function

Ammoniagenesis and gluconeogenesis are prominent metabolic features of the renal proximal convoluted tubule that contribute to maintenance of systemic acid-base homeostasis. Molecular analysis of the mechanisms that mediate the coordinate regulation of the two pathways required development of a cell line that recapitulates these features in vitro. By adapting porcine renal epithelial LLC-PK1 cells to essentially glucose-free medium, a gluconeogenic subline, termed LLC-PK1-FBPase(+) cells, was isolated. LLC-PK1-FBPase(+) cells grow in the absence of hexoses and pentoses and exhibit enhanced oxidative metabolism and increased levels of phosphate-dependent glutaminase. The cells also express significant levels of the key gluconeogenic enzymes, fructose-1,6-bisphosphatase (FBPase) and phosphoenolpyruvate carboxykinase (PEPCK). Thus the altered phenotype of LLC-PK1-FBPase(+) cells is pleiotropic. Most importantly, when transferred to medium that mimics a pronounced metabolic acidosis (9 mM HCO3 (-), pH 6.9), the LLC-PK1-FBPase(+) cells exhibit a gradual increase in NH4 (+) ion production, accompanied by increases in glutaminase and cytosolic PEPCK mRNA levels and proteins. Therefore, the LLC-PK1-FBPase(+) cells retained in culture many of the metabolic pathways and pH-responsive adaptations characteristic of renal proximal tubules. The molecular mechanisms that mediate enhanced expression of the glutaminase and PEPCK in LLC-PK1-FBPase(+) cells have been extensively reviewed. The present review describes novel properties of this unique cell line and summarizes the molecular mechanisms that have been defined more recently using LLC-PK1-FBPase(+) cells to model the renal proximal tubule. It also identifies future studies that could be performed using these cells.

Effect of collecting duct-specific deletion of both Rh B Glycoprotein (Rhbg) and Rh C Glycoprotein (Rhcg) on renal response to metabolic acidosis

The Rhesus (Rh) glycoproteins, Rh B and Rh C Glycoprotein (Rhbg and Rhcg, respectively), are ammonia-specific transporters expressed in renal distal nephron and collecting duct sites that are necessary for normal rates of ammonia excretion. The purpose of the current studies was to determine the effect of their combined deletion from the renal collecting duct (CD-Rhbg/Rhcg-KO) on basal and acidosis-stimulated acid-base homeostasis. Under basal conditions, urine pH and ammonia excretion and serum HCO3(-) were similar in control (C) and CD-Rhbg/Rhcg-KO mice. After acid-loading for 7 days, CD-Rhbg/Rhcg-KO mice developed significantly more severe metabolic acidosis than did C mice. Acid loading increased ammonia excretion, but ammonia excretion increased more slowly in CD-Rhbg/Rhcg-KO and it was significantly less than in C mice on days 1-5. Urine pH was significantly more acidic in CD-Rhbg/Rhcg-KO mice on days 1, 3, and 5 of acid loading. Metabolic acidosis increased phosphenolpyruvate carboxykinase (PEPCK) and Na(+)/H(+) exchanger NHE-3 and decreased glutamine synthetase (GS) expression in both genotypes, and these changes were significantly greater in CD-Rhbg/Rhcg-KO than in C mice. We conclude that 1) Rhbg and Rhcg are critically important in the renal response to metabolic acidosis; 2) the significantly greater changes in PEPCK, NHE-3, and GS expression in acid-loaded CD-Rhbg/Rhcg-KO compared with acid-loaded C mice cause the role of Rhbg and Rhcg to be underestimated quantitatively; and 3) in mice with intact Rhbg and Rhcg expression, metabolic acidosis does not induce maximal changes in PEPCK, NHE-3, and GS expression despite the presence of persistent metabolic acidosis.

Proximal tubule function and response to acidosis

The human kidneys produce approximately 160-170 L of ultrafiltrate per day. The proximal tubule contributes to fluid, electrolyte, and nutrient homeostasis by reabsorbing approximately 60%-70% of the water and NaCl, a greater proportion of the NaHCO3, and nearly all of the nutrients in the ultrafiltrate. The proximal tubule is also the site of active solute secretion, hormone production, and many of the metabolic functions of the kidney. This review discusses the transport of NaCl, NaHCO3, glucose, amino acids, and two clinically important anions, citrate and phosphate. NaCl and the accompanying water are reabsorbed in an isotonic fashion. The energy that drives this process is generated largely by the basolateral Na(+)/K(+)-ATPase, which creates an inward negative membrane potential and Na(+)-gradient. Various Na(+)-dependent countertransporters and cotransporters use the energy of this gradient to promote the uptake of HCO3 (-) and various solutes, respectively. A Na(+)-dependent cotransporter mediates the movement of HCO3 (-) across the basolateral membrane, whereas various Na(+)-independent passive transporters accomplish the export of various other solutes. To illustrate its homeostatic feat, the proximal tubule alters its metabolism and transport properties in response to metabolic acidosis. The uptake and catabolism of glutamine and citrate are increased during acidosis, whereas the recovery of phosphate from the ultrafiltrate is decreased. The increased catabolism of glutamine results in increased ammoniagenesis and gluconeogenesis. Excretion of the resulting ammonium ions facilitates the excretion of acid, whereas the combined pathways accomplish the net production of HCO3 (-) ions that are added to the plasma to partially restore acid-base balance.

Expression of glutamine synthetase in the mouse kidney: localization in multiple epithelial cell types and differential regulation by hypokalemia

Renal glutamine synthetase catalyzes the reaction of NH4+ with glutamate, forming glutamine and decreasing the ammonia available for net acid excretion. The purpose of the present study was to determine glutamine synthetase's specific cellular expression in the mouse kidney and its regulation by hypokalemia, a common cause of altered renal ammonia metabolism. Glutamine synthetase mRNA and protein were present in the renal cortex and in both the outer and inner stripes of the outer medulla. Immunohistochemistry showed glutamine synthetase expression throughout the entire proximal tubule and in nonproximal tubule cells. Double immunolabel with cell-specific markers demonstrated glutamine synthetase expression in type A intercalated cells, non-A, non-B intercalated cells, and distal convoluted tubule cells, but not in principal cells, type B intercalated cells, or connecting segment cells. Hypokalemia induced by feeding a nominally K+ -free diet for 12 days decreased glutamine synthetase expression throughout the entire proximal tubule and in the distal convoluted tubule and simultaneously increased glutamine synthetase expression in type A intercalated cells in both the cortical and outer medullary collecting duct. We conclude that glutamine synthetase is widely and specifically expressed in renal epithelial cells and that the regulation of expression differs in specific cell populations. Glutamine synthetase is likely to mediate an important role in renal ammonia metabolism.

Proteomic profiling of the effect of metabolic acidosis on the apical membrane of the proximal convoluted tubule

The physiological response to the onset of metabolic acidosis requires pronounced changes in renal gene expression. Adaptations within the proximal convoluted tubule support the increased extraction of plasma glutamine and the increased synthesis and transport of glucose and of NH(4)(+) and HCO(3)(-) ions. Many of these adaptations involve proteins associated with the apical membrane. To quantify the temporal changes in these proteins, proteomic profiling was performed using brush-border membrane vesicles isolated from proximal convoluted tubules (BBMV(PCT)) that were purified from normal and acidotic rats. This preparation is essentially free of contaminating apical membranes from other renal cortical cells. The analysis identified 298 proteins, 26% of which contained one or more transmembrane domains. Spectral counts were used to assess changes in protein abundance. The onset of acidosis produced a twofold, but transient, increase in the Na(+)-dependent glucose transporter and a more gradual, but sustained, increase (3-fold) in the Na(+)-dependent lactate transporter. These changes were associated with the loss of glycolytic and gluconeogenic enzymes that are contained in the BBMV(PCT) isolated from normal rats. In addition, the levels of γ-glutamyltranspeptidase increased twofold, while transporters that participate in the uptake of neutral amino acids, including glutamine, were decreased. These changes could facilitate the deamidation of glutamine within the tubular lumen. Finally, pronounced increases were also observed in the levels of DAB2 (3-fold) and myosin 9 (7-fold), proteins that may participate in endocytosis of apical membrane proteins. Western blot analysis and accurate mass and time analyses were used to validate the spectral counting.

Role of AUF1 and HuR in the pH-responsive stabilization of phosphoenolpyruvate carboxykinase mRNA in LLC-PK₁-F⁺ cells

Onset of metabolic acidosis leads to a rapid and pronounced increase in expression of phosphoenolpyruvate carboxykinase (PEPCK) in rat renal proximal convoluted tubules. This adaptive response is modeled by treating a clonal line of porcine LLC-PK(1)-F(+) cells with an acidic medium (pH 6.9, 9 mM HCO(3)(-)). Measurement of the half-lives of PEPCK mRNA in cells treated with normal (pH 7.4, 26 mM HCO(3)(-)) and acidic medium established that the observed increase is due in part to stabilization of the PEPCK mRNA. The pH-responsive stabilization was reproduced in a Tet-responsive chimeric reporter mRNA containing the 3'-UTR of PEPCK mRNA. This response was lost by mutation of a highly conserved AU sequence that binds AUF1 and is the primary element that mediates the rapid turnover of PEPCK mRNA. However, siRNA knockdown of AUF1 had little effect on the basal levels and the pH-responsive increases in PEPCK mRNA and protein. Electrophoretic mobility shift assays established that purified recombinant HuR, another AU element binding protein, also binds with high affinity and specificity to multiple sites within the final 92-nucleotides of the 3'-UTR of the PEPCK mRNA, including the highly conserved AU-rich element. siRNA knockdown of HuR caused pronounced decreases in basal expression and the pH-responsive increases in PEPCK mRNA and protein. Therefore, basal expression and the pH-responsive stabilization of PEPCK mRNA in LLC-PK(1)-F(+) cells, and possibly in the renal proximal tubule, may require the remodeling of HuR and AUF1 binding to the elements that mediate the rapid turnover of PEPCK mRNA.

Genome-wide gene expression profiling reveals renal genes regulated during metabolic acidosis

Production and excretion of acids are balanced to maintain systemic acid-base homeostasis. During metabolic acidosis (MA) excess acid accumulates and is removed from the body, a process achieved, at least in part, by increasing renal acid excretion. This acid-secretory process requires the concerted regulation of metabolic and transport pathways, which are only partially understood. Chronic MA causes also morphological remodeling of the kidney. Therefore, we characterized transcriptional changes in mammalian kidney during MA to gain insights into adaptive pathways. Total kidney RNA from control and 2- and 7-days NH(4)Cl treated mice was subjected to microarray gene profiling. We identified 4,075 transcripts significantly (P < 0.05) regulated after 2 and/or 7 days of treatment. Microarray results were confirmed by qRT-PCR. Analysis of candidate genes revealed that a large group of regulated transcripts was represented by different solute carrier transporters, genes involved in cell growth, proliferation, apoptosis, water homeostasis, and ammoniagenesis. Pathway analysis revealed that oxidative phosphorylation was the most affected pathway. Interestingly, the majority of acutely regulated genes after 2 days, returned to normal values after 7 days suggesting that adaptation had occurred. Besides these temporal changes, we detected also differential regulation of selected genes (SNAT3, PEPCK, PDG) between early and late proximal tubule. In conclusion, the mammalian kidney responds to MA by temporally and spatially altering the expression of a large number of genes. Our analysis suggests that many of these genes may participate in various processes leading to adaptation and restoration of normal systemic acid-base and electrolyte homeostasis.

Glucocorticoids have a role in renal cortical expression of the SNAT3 glutamine transporter during chronic metabolic acidosis

Glucocorticoids are involved in many aspects of regulation of acid-base homeostasis, including the stimulation of renal ammoniagenesis during chronic metabolic acidosis. Plasma glutamine is the principal substrate for ammoniagenesis under these conditions. Expression of the System N glutamine transporter SNAT3 is increased in the renal proximal tubules during acidosis. In vivo studies in rats using 1) sham and adrenalectomized rats, 2) the glucocorticoid receptor antagonist RU486, and 3) dexamethasone treatment demonstrated involvement of glucocorticoids in regulation of SNAT3 expression. Adrenalectomy attenuated the acidosis-induced increase in renal cortical SNAT3 mRNA approximately 40%, and treatment with dexamethasone (1 mg x kg(-1) x day(-1) sc) partially reversed this effect. RU486 also blunted the acidosis-induced increase in SNAT3 expression approximately 50%. Chronic dexamethasone treatment (0.1 mg x kg(-1) x day(-1) sc, 6 days) of normal rats slightly increased SNAT3 expression. In all cases, renal glutamine arteriovenous difference mirrored SNAT3 expression and activity in the proximal tubules, suggesting that SNAT3 regulates glutamine uptake during acidosis. These studies indicate that glucocorticoids regulate acid-base homeostasis during metabolic acidosis in part by regulating expression of the System N transporter SNAT3.

Complexity of glutamine metabolism in kidney tubules from fed and fasted rats

Glutamine is an important renal glucose precursor and energy provider. In order to advance our understanding of the underlying metabolic processes, we studied the metabolism of variously labelled [13C]glutamine and [14C]glutamine molecules and the effects of fasting in isolated rat renal proximal tubules. Absolute fluxes through the enzymes involved, including enzymes of four different cycles operating concomitantly, were assessed by combining mainly the 13C NMR data with an appropriate model of glutamine metabolism. In both nutritional states, unidirectional glutamine removal by glutaminase was partially masked by the concomitant operation of glutamine synthetase; fasting accelerated glutamine removal by increasing flux solely through glutaminase, without changing that through glutamine synthetase. Fasting stimulated net glutamate degradation only by decreasing flux through glutamate dehydrogenase in the reductive amination direction, but surprisingly did not significantly alter complete oxidation of the glutamine carbon skeleton. Finally, gluconeogenesis from glutamine involved not only substantial recycling through the tricarboxylic acid cycle, but also an important anaplerotic flux through pyruvate carboxylase that was accelerated dramatically by fasting. Thus renal glutamine metabolism follows an unexpectedly complex route that is precisely regulated during fasting.

Effect of starvation on glutamine ammoniagenesis and gluconeogenesis in isolated mouse kidney tubules

It has been shown recently that glutamine is taken up by the mouse kidney in vivo. However, knowledge about the fate of this amino acid and the regulation of its metabolism in the mouse kidney remains poor. Given the physiological and pathophysiological importance of renal glutamine metabolism and the increasing use of genetically modified mice in biological research, we have conducted a study to characterize glutamine metabolism in the mouse kidney. Proximal tubules isolated from fed and 48 h-starved mice and then incubated with a physiological concentration of glutamine, removed this amino acid and produced ammonium ions at similar rates. In agreement with this observation, activities of the ammoniagenic enzymes, glutaminase and glutamate dehydrogenase, were not different in the renal cortex of fed and starved mice, but the glutamate dehydrogenase mRNA level was elevated 4.5-fold in the renal cortex from starved mice. In contrast, glucose production from glutamine was greatly stimulated whereas the glutamine carbon removed, that was presumably completely oxidized in tubules from fed mice, was virtually suppressed in tubules from starved animals. In accordance with the starvation-induced stimulation of glutamine gluconeogenesis, the activities and mRNA levels of glucose-6-phosphatase, and especially of phosphoenolpyruvate carboxykinase, but not of fructose-1,6-bisphosphatase, were increased in the renal cortex of starved mice. On the basis of our in vitro results, the elevated urinary excretion of ammonium ions observed in starved mice probably reflected an increased transport of these ions into the urine at the expense of those released into the renal veins rather than a stimulation of renal ammoniagenesis.

Regulation of expression of the SN1 transporter during renal adaptation to chronic metabolic acidosis in rats

During chronic metabolic acidosis, renal glutamine utilization increases markedly. We studied the expression of the system N1 (SN1) amino acid transporter in the kidney during chronic ammonium chloride acidosis in rats. Acidosis caused a 10-fold increase in whole kidney SN1 mRNA level and a 100-fold increase in the cortex. Acidosis increased Na(+)-dependent glutamine uptake into basolateral and brush-border membrane vesicles (BLMV and BBMV, respectively) isolated from rat cortex (BLMV, 219 +/- 66 control vs. 651 +/- 180 pmol. mg(-1). min(-1) acidosis; BBMV, 1,112 +/- 189 control vs. 1,652 +/- 148 pmol. mg(-1). min(-1) acidosis, both P < 0.05). Na(+)-independent uptake was unchanged by acidosis in BLMV and BBMV. The acidosis-induced increase in Na(+)-dependent glutamine uptake was eliminated by histidine, confirming transport by system N. SN1 protein was detected only in BLMV and BBMV from acidotic rats. After recovery from acidosis, SN1 mRNA and protein and Na(+)-dependent glutamine uptake activity rapidly returned to control levels. These data provide evidence that regulation of expression of the SN1 amino acid transporter is part of the renal homeostatic response to acid-base imbalance.

Differential expression and acid-base regulation of glutaminase mRNAs in gluconeogenic LLC-PK(1)-FBPase(+) cells

LLC-PK(1)-FBPase(+) cells, which are a gluconeogenic substrain of porcine renal LLC-PK(1) cells, exhibit enhanced oxidative metabolism and increased levels of phosphate-dependent glutaminase (PDG) activity. On adaptation to acidic medium (pH 6.9, 9 mM HCO(-)(3)), LLC-PK(1)-FBPase(+) cells also exhibit a greater increase in ammonia production and respond with an increase in assayable PDG activity. The changes in PDG mRNA levels were examined by using confluent cells grown on plastic dishes or on permeable membrane inserts. The latter condition increased the state of differentiation of the LLC-PK(1)-FBPase(+) cells. The levels of the primary porcine PDG mRNAs were analyzed by using probes that are specific for the 5.0-kb PDG mRNA (p2400) or that react equally with both the 4.5- and 5.0-kb PDG mRNAs (p930 and r1500). In confluent dish- and filter-grown LLC-PK(1)-FBPase(+) cells, the predominant 4.5-kb PDG mRNA is increased threefold after 18 h in acidic media. However, in filter-grown epithelia, which sustain an imposed pH and HCO(-)(3) gradient, this adaptive increase is observed only when acidic medium is applied to both the apical and the basolateral sides of the epithelia. Half-life experiments established that induction of the 4. 5-kb PDG mRNA was due to its stabilization. An identical pattern of adaptive increases was observed for the cytosolic PEPCK mRNA. In contrast, no adaptive changes were observed in the levels of the 5. 0-kb PDG mRNA in either cell culture system. Furthermore, cultures were incubated in low-potassium (0.7 mM) media for 24-72 h to decrease intracellular pH while maintaining normal extracellular pH. LLC-PK(1)-FBPase(+) cells again responded with increased rates of ammonia production and increased levels of the 4.5-kb PDG and PEPCK mRNAs, suggesting that an intracellular acidosis is the initiator of this adaptive response. Because all of the observed responses closely mimic those characterized in vivo, the LLC-PK(1)-FBPase(+) cells represent a valuable tissue culture model to study the molecular mechanisms that regulate renal gene expression in response to changes in acid-base balance.