Specificity and functional analysis of the pH-responsive element within renal glutaminase mRNA

HuR controls glutaminase RNA metabolism

Glutaminase (GLS) is directly related to cell growth and tumor progression, making it a target for cancer treatment. The RNA-binding protein HuR (encoded by the ELAVL1 gene) influences mRNA stability and alternative splicing. Overexpression of ELAVL1 is common in several cancers, including breast cancer. Here we show that HuR regulates GLS mRNA alternative splicing and isoform translation/stability in breast cancer. Elevated ELAVL1 expression correlates with high levels of the glutaminase isoforms C (GAC) and kidney-type (KGA), which are associated with poor patient prognosis. Knocking down ELAVL1 reduces KGA and increases GAC levels, enhances glutamine anaplerosis into the TCA cycle, and drives cells towards glutamine dependence. Furthermore, we show that combining chemical inhibition of GLS with ELAVL1 silencing synergistically decreases breast cancer cell growth and invasion. These findings suggest that dual inhibition of GLS and HuR offers a therapeutic strategy for breast cancer treatment.

Impacts and mechanisms of alternative mRNA splicing in cancer metabolism, immune response, and therapeutics

Alternative pre-mRNA splicing (AS) provides the potential to produce diversity at RNA and protein levels. Disruptions in the regulation of pre-mRNA splicing can lead to diseases. With the development of transcriptome and genome sequencing technology, increasing diseases have been identified to be associated with abnormal splicing of mRNAs. In tumors, abnormal alternative splicing frequently plays critical roles in cancer pathogenesis and may be considered as new biomarkers and therapeutic targets for cancer intervention. Metabolic abnormalities and immune disorders are important hallmarks of cancer. AS produces multiple different isoforms and diversifies protein expression, which is utilized by the immune and metabolic reprogramming systems to expand gene functions. The abnormal splicing events contributed to tumor progression, partially due to effects on immune response and metabolic reprogramming. Herein, we reviewed the vital role of alternative splicing in regulating cancer metabolism and immune response. We discussed how alternative splicing regulates metabolic reprogramming of cancer cells and antitumor immune response, and the possible strategies to targeting alternative splicing pathways or splicing-regulated metabolic pathway in the context of anticancer immunotherapy. Further, we highlighted the challenges and discuss the perspectives for RNA-based strategies for the treatment of cancer with abnormally alternative splicing isoforms.

Glutaminase C overexpression in the brain induces learning deficits, synaptic dysfunctions, and neuroinflammation in mice

Glutaminolysis, a metabolic process that converts glutamine to glutamate, is particularly important for the central nervous system since glutamate is the major transmitter of excitatory synapses. Glutaminase is the mitochondrial enzyme that catalyzes the first step of glutaminolysis. Two genes encode at least four isoforms of glutaminase in humans. Gls1 gene encodes isoforms kidney-type glutaminase (KGA) and glutaminase C (GAC) through alternative splicing, whereas Gls2 gene encodes liver-type glutaminase isoforms. KGA and GAC have been associated with several neurological diseases. However, it remains unclear whether changes in their expressions can directly cause brain abnormalities. Using a transgenic approach, we generated mice that overexpressed GAC in the brain. The resulting transgenic mice had severe impairments in spatial and fear learning compared with littermate controls. The learning deficits were consistent with diminished hippocampal long-term potentiation in the hippocampal slices of the GAC transgenic mice. Furthermore, we found increases in astrocyte and microglia markers, inflammatory factors, and a decrease in synapse marker synaptophysin, suggesting neuroinflammation and synaptic changes in the GAC transgenic mouse brains. In conclusion, these findings provide the first evidence that GAC overexpression in the brain has deleterious effects on learning and synaptic integrity in vivo.

Role of post-transcriptional regulation of mRNA stability in renal pathophysiology: focus on chronic kidney disease

Chronic kidney disease (CKD) represents an important public health problem. Its progression to end-stage renal disease is associated with increased morbidity and mortality. The determinants of renal function decline are not fully understood. Recent progress in the understanding of post-transcriptional regulation of mRNA stability has helped the identification of both the trans- and cis-acting elements of mRNA as potential markers and therapeutic targets for difficult-to-diagnose and -treat diseases, including CKDs such as diabetic nephropathy. Human antigen R (HuR), a trans-acting element of mRNA, is an RNA binding factor (RBF) best known for its ability to stabilize AU-rich-element-containing mRNAs. Deregulated HuR subcellular localization or expression occurs in a wide range of renal diseases, such as metabolic acidosis, ischemia, and fibrosis. Besides RBFs, recent evidence revealed that noncoding RNA, such as microRNA and long noncoding RNA, participates in regulating mRNA stability and that aberrant noncoding RNA expression accounts for many pathologic renal conditions. The goal of this review is to provide an overview of our current understanding of the post-transcriptional regulation of mRNA stability in renal pathophysiology and to offer perspectives for this class of diseases. We use examples of diverse renal diseases to illustrate different mRNA stability pathways in specific cellular compartments and discuss the roles and impacts of both the cis- and trans-activating factors on the regulation of mRNA stability in these diseases.-Feigerlová, E., Battaglia-Hsu, S.-F. Role of post-transcriptional regulation of mRNA stability in renal pathophysiology: focus on chronic kidney disease.

Increased Rate of NAD Metabolism Shortens Plant Longevity by Accelerating Developmental Senescence in Arabidopsis

NAD is a well-known co-enzyme that mediates hundreds of redox reactions and is the basis of various processes regulating cell responses to different environmental and developmental cues. The regulatory mechanism that determines the amount of cellular NAD and the rate of NAD metabolism remains unclear. We created Arabidopsis thaliana plants overexpressing the NAD synthase (NADS) gene that participates in the final step of NAD biosynthesis. NADS overexpression enhanced the activity of NAD biosynthesis but not the amounts of NAD⁺, NADH, NADP⁺ or NADPH. However, the amounts of some intermediates were elevated, suggesting that NAD metabolism increased. The NAD redox state was greatly facilitated by an imbalance between NAD generation and degradation in response to bolting. Metabolite profiling and transcriptional analysis revealed that the drastic modulation of NAD redox homeostasis increased tricarboxylic acid flux, causing the ectopic generation of reactive oxygen species. Vascular bundles suffered from oxidative stress, leading to a malfunction in amino acid and organic acid transportation that caused early wilting of the flower stalk and shortened plant longevity, probably due to malnutrition. We concluded that the mechanism regulating the balance between NAD synthesis and degradation is important in the systemic plant response to developmental cues during the growth-phase transition.

CFIm25 regulates glutaminase alternative terminal exon definition to modulate miR-23 function

Alternative polyadenylation (APA) and alternative splicing (AS) provide mRNAs with the means to avoid microRNA repression through selective shortening or differential usage of 3'UTRs. The two glutaminase (GLS) mRNA isoforms, termed KGA and GAC, contain distinct 3'UTRs with the KGA isoform subject to repression by miR-23. We show that depletion of the APA regulator CFIm25 causes a strong shift to the usage of a proximal poly(A) site within the KGA 3'UTR and also alters splicing to favor exclusion of the GAC 3'UTR. Surprisingly, we observe that while miR-23 is capable of down-regulating the shortened KGA 3'UTR, it has only minor impact on the full-length KGA 3'UTR, demonstrating that additional potent negative regulation of GLS expression exists beyond this single microRNA targeting site. Finally, we show that the apoptosis induced upon down-regulation of the GAC isoform can be alleviated through concurrent reduction in CFIm25 expression, revealing the sensitivity of glutaminase expression to the levels of RNA processing factors. These results exemplify the complex interplay between RNA processing and microRNA repression in controlling glutamine metabolism in cancer cells.

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.

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.

Response of the mitochondrial proteome of rat renal proximal convoluted tubules to chronic metabolic acidosis

Metabolic acidosis is a common clinical condition that is caused by a decrease in blood pH and bicarbonate concentration. Increased extraction and mitochondrial catabolism of plasma glutamine within the renal proximal convoluted tubule generates ammonium and bicarbonate ions that facilitate the excretion of acid and partially restore acid-base balance. Previous studies identified only a few mitochondrial proteins, including two key enzymes of glutamine metabolism, which are increased during chronic acidosis. A workflow was developed to characterize the mitochondrial proteome of the proximal convoluted tubule. Based upon the increase in specific activity of cytochrome c oxidase, the isolated mitochondria were enriched eightfold. Two-dimensional liquid chromatography coupled with mass spectrometry was utilized to compare mitochondrial-enriched samples from control and chronic acidotic rats. Proteomic analysis identified 901 proteins in the control and acidotic samples. Further analysis identified 37 peptides that contain an N-ε-acetyl-lysine; of these, 22 are novel sites. Spectral counting analysis revealed 33 proteins that are significantly altered in abundance in response to chronic metabolic acidosis. Western blot analysis was performed to validate the calculated changes in abundance. Thus the current study represents the first comprehensive analysis of the mitochondrial proteome of the rat renal proximal convoluted tubule and its response to metabolic acidosis.

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.

Renal response to metabolic acidosis: role of mRNA stabilization

The renal response to metabolic acidosis is mediated, in part, by increased expression of the genes encoding key enzymes of glutamine catabolism and various ion transporters that contribute to the increased synthesis and excretion of ammonium ions and the net production and release of bicarbonate ions. The resulting adaptations facilitate the excretion of acid and partially restore systemic acid-base balance. Much of this response may be mediated by selective stabilization of the mRNAs that encode the responsive proteins. For example, the glutaminase mRNA contains a direct repeat of 8-nt AU sequences that function as a pH-response element (pHRE). This element is both necessary and sufficient to impart a pH-responsive stabilization to chimeric mRNAs. The pHRE also binds multiple RNA-binding proteins, including zeta-crystallin (zeta-cryst), AU-factor 1 (AUF1), and HuR. The onset of acidosis initiates an endoplasmic reticulum (ER)-stress response that leads to the formation of cytoplasmic stress granules. zeta-cryst is transiently recruited to the stress granules, and concurrently, HuR is translocated from the nucleus to the cytoplasm. On the basis of the cumulative data, a mechanism for the stabilization of selective mRNAs is proposed. This hypothesis suggests multiple experiments that should define better how cells in the kidney sense very slight changes in intracellular pH and mediate this essential adaptive response.

TGF-beta signaling and its effect on glutaminase expression in LLC-PK1-FBPase+ cells

During systemic acidosis, renal proximal tubular cells exhibit enhanced rates of bicarbonate and ammonium ion synthesis and undergo extensive hypertrophy. The former adaptations are accomplished, in part, by increased expression of glutaminase (GA). LLC-PK(1)-FBPase+ cells, a gluconeogenic line of porcine kidney cells, exhibit a rapid activation of the ERK1/2 and p38 MAPK pathways and a two- to threefold increase in GA mRNA when transferred to acidic medium (pH 6.9). Transforming growth factor-beta (TGF-beta), a potent activator of MAPK and Smad signaling cascades, also causes extensive renal hypertrophy. Thus the potential role of TGF-beta in the renal response to metabolic acidosis was investigated. Western blot analyses established that in LLC-PK(1)-FBPase+ cells, TGF-beta activated the ERK1/2, p38 MAPK, and Smad1/5/8 pathways, but not the JNK and Smad2/3 pathways. Addition of TGF-beta to cells cultured in normal medium (pH 7.4) produced a steady increase in GA mRNA, resulting in a twofold induction after 18 h. Western blot analysis indicated that treatment with either TGF-beta or acidic medium resulted in an increased level of fibronectin. However, the effects of the two treatments on both GA mRNA and fibronectin levels occurred with different time courses and were additive. In addition, the rates of ammonia production were decreased slightly by addition of TGF-beta. Finally, a GA-luciferase reporter construct, which is activated 3.5-fold by treatment with acidic medium, is not affected by TGF-beta. Therefore, TGF-beta and metabolic acidosis activate some of the same signaling pathways in LLC-PK(1)-FBPase+ cells, but produce separate effects on GA expression.

Proteomic analysis of the adaptive response of rat renal proximal tubules to metabolic acidosis

Proximal tubules were isolated from control and acidotic rats by collagenase digestion and Percoll density gradient centrifugation. Western blot analysis indicated that the tubules were ∼95% pure. The samples were analyzed by two-dimensional difference gel electrophoresis (DIGE) and DeCyder software was used to quantify the temporal changes in proteins that exhibit enhanced or reduced expression. The mass-to-charge ratios and the amino acid sequences of the recovered tryptic peptides were determined by MALDI-TOF/TOF mass spectrometry and the proteins were identified using Mascot software. This analysis confirmed the well-characterized adaptive responses in glutaminase (GA), glutamate dehydrogenase (GDH), and phospho enolpyruvate carboxykinase (PEPCK). This approach also identified 17 previously unrecognized proteins that are increased with ratios of 1.5 to 5.6 and 16 proteins that are decreased with ratios of 0.67 to 0.03 when tubules from 7-day acidotic vs. control rats were compared. Some of these changes were confirmed by Western blot analysis. Temporal studies identified proteins that were induced either with rapid kinetics similar to PEPCK or with more gradual profiles similar to GA and GDH. All of the mRNAs that encode the latter proteins contain an AU sequence that is homologous to the pH response element found in GA mRNA. Thus selective mRNA stabilization may be a predominant mechanism by which protein expression is increased in response to acidosis.

Role of deadenylation and AUF1 binding in the pH-responsive stabilization of glutaminase mRNA

During chronic metabolic acidosis, increased expression of renal glutaminase (GA) results from selective stabilization of the GA mRNA. This response is mediated by a direct repeat of an 8-base adenylate-uridylate (AU) sequence that binds zeta-crystallin and functions as a pH response element (pH-RE). A tetracycline-responsive promoter system was developed in LLC-PK(1)-F(+) cells to perform pulse-chase analysis of the turnover of a chimeric beta-globin (betaG) mRNA that contains 960 bp of the 3'-UTR of GA mRNA including the pH-RE. The betaG-GA mRNA exhibits a 14-fold increase in half-life when the LLC-PK(1)-F(+) cells are transferred to acidic medium. RNase H cleavage and Northern blot analysis of the 3'-ends established that rapid deadenylation occurred concomitantly with the rapid decay of the betaG-GA mRNA in cells grown in normal medium. Stabilization of the betaG-GA mRNA in acidic medium is associated with a pronounced decrease in the rate of deadenylation. Mutation of the pH-RE within the betaG-GA mRNA blocked the pH-responsive stabilization, but not the rapid decay, whereas insertion of only a 29-bp segment containing the pH-RE was sufficient to produce both a rapid decay and a pH-responsive stabilization. Various kidney cells express multiple isoforms of AUF1, an AU-binding protein that enhances mRNA turnover. RNA gel-shift assays demonstrated that the recombinant p40 isoform of AUF1 binds to the pH-RE with high affinity and specificity. Thus AUF1 may mediate the rapid turnover of the GA mRNA, whereas increased binding of zeta-crystallin during acidosis may inhibit degradation and result in selective stabilization.

Glutamine metabolism: Role in acid-base balance*

The intent of this review is to provide a broad overview of the interorgan metabolism of glutamine and to discuss in more detail its role in acid-base balance. Muscle, adipose tissue, and the lungs are the primary sites of glutamine synthesis and release. During normal acid-base balance, the small intestine and the liver are the major sites of glutamine utilization. The periportal hepatocytes catabolize glutamine and convert ammonium and bicarbonate ions to urea. In contrast, the perivenous hepatocytes are capable of synthesizing glutamine. During metabolic acidosis, the kidney becomes the major site of glutamine extraction and catabolism. This process generates ammonium ions that are excreted in the urine to facilitate the excretion of acids and bicarbonate ions that are transported to the blood to partially compensate the acidosis. The increased renal extraction of glutamine is balanced by an increased release from muscle and liver and by a decreased utilization in the intestine. During chronic acidosis, this adaptation is sustained, in part, by increased renal expression of genes that encode various transport proteins and key enzymes of glutamine metabolism. The increased levels of phosphoenolpyruvate carboxykinase result from increased transcription, while the increase in glutaminase and glutamate dehydrogenase activities result from stabilization of their respective mRNAs. Where feasible, this review draws upon data obtained from studies in humans. Studies conducted in model animals are discussed where available data from humans is either lacking or not firmly established. Because there are quantitative differences in tissue utilization and synthesis of glutamine in different mammals, the review will focus more on common principles than on quantification.

Differential regulation of branched-chain alpha-ketoacid dehydrogenase kinase expression by glucocorticoids and acidification in LLC-PK1-GR101 cells

Acidosis and glucocorticoids (GC) are two catabolic signals associated with chronic renal disease. Previously, we reported that these signals stimulate branched-chain amino acid (BCAA) oxidation in renal tubule cells by increasing both the amount and activation state of branched-chain alpha-ketoacid dehydrogenase (BCKD). Activation of the BCKD complex could result from decreased expression of BCKD kinase, which inhibits BCKD by phosphorylating its E1 alpha subunit. To investigate this possibility, we examined how dexamethasone and acidification (pH 7.0) influence BCKD kinase expression in LLC-PK(1)-GR101 cells. Dexamethasone, a synthetic GC, decreased BCKD kinase protein by 65% (P < 0.05 vs. control), whereas a low pH (i.e., pH 7.0) decreased the amount of kinase by 71% (P < 0.05 vs. control). Either GC or acidification reduced BCKD kinase mRNA by 46% (P < 0.05 vs. control), but the two signals together did not reduce kinase mRNA more than either signal alone. To examine the mechanism(s) leading to lower kinase mRNA, kinase transcription was evaluated by transiently transfecting LLC-PK(1)-GR101 cells with BCKD kinase promoter-luciferase mini-genes containing approximately 3.5 kb of proximal rat kinase promoter. GC, but not acidification, decreased luciferase activity 42% (P < 0.05 vs. control). Nuclear run-on assays confirmed that GC decrease kinase mRNA by attenuating its transcription. Thus two catabolic signals associated with renal failure, GC and acidification, reduce BCKD kinase expression by different mechanisms. These responses lead to an increase in the activation state of BCKD and a resulting acceleration of BCAA degradation.

pH-responsive stabilization of glutamate dehydrogenase mRNA in LLC-PK1-F+ cells

During chronic metabolic acidosis, the adaptive increase in rat renal ammoniagenesis is sustained, in part, by increased expression of mitochondrial glutaminase (GA) and glutamate dehydrogenase (GDH) enzymes. The increase in GA activity results from the pH-responsive stabilization of GA mRNA. The 3'-untranslated region (3'-UTR) of GA mRNA contains a direct repeat of an eight-base AU-rich element (ARE) that binds zeta-crystallin/NADPH:quinone reductase (zeta-crystallin) with high affinity and functions as a pH-response element. RNA EMSAs established that zeta-crystallin also binds to the full-length 3'-UTR of GDH mRNA. This region contains four eight-base sequences that are 88% identical to one of the two GA AREs. Direct binding assays and competition studies indicate that the two individual eight-base AREs from GA mRNA and the four individual GDH sequences bind zeta-crystallin with different affinities. Insertion of the 3'-UTR of GDH cDNA into a beta-globin expression vector (pbetaG) produced a chimeric mRNA that was stabilized when LLC-PK1-F+ cells were transferred to acidic medium. A pH-responsive stabilization was also observed using a betaG construct that contained only the single GDH4 ARE and a destabilizing element from phosphoenolpyruvate carboxykinase mRNA. Therefore, during acidosis, the pH-responsive stabilization of GDH mRNA may be accomplished by the same mechanism that affects an increase in GA mRNA.

Complexity and species variation of the kidney-type glutaminase gene

Increased expression of rat kidney-type glutaminase (KGA) during metabolic acidosis results from selective mRNA stabilization. This process is mediated by an 8-base AU-sequence that functions as a pH-response element (pHRE). LLC-PK1-FBPase+ cells, a pH-responsive porcine kidney cell line, express four distinct GA mRNAs. RNase H mapping indicated that three of the GA mRNAs are generated by use of alternative polyadenylation sites and are homologs of the rat KGA mRNA, while the fourth contains a different COOH-terminal coding and 3'-untranslated sequence. PCR cloning and sequencing established that the latter GA mRNA is the homolog of the human GAC mRNA. A rat GAC cDNA was also cloned from a rat kidney library. The 3'-untranslated regions of the GAC mRNAs, but not the porcine or human KGA mRNAs, contain identifiable pHREs. The human KGA gene spans 82 kb and is composed of 19 exons. The unique sequence from the hGAC cDNA is contained in a single exon. Thus in humans, alternative splicing of the initial transcript could produce two GA mRNAs, only one of which may be increased during acidosis.

Mechanism of increased renal gene expression during metabolic acidosis

Increased renal catabolism of plasma glutamine during metabolic acidosis generates two ammonium ions that are predominantly excreted in the urine. They function as expendable cations that facilitate the excretion of acids. Further catabolism of alpha-ketoglutarate yields two bicarbonate ions that are transported into the venous blood to partially compensate for the acidosis. In rat kidney, this adaptation is sustained, in part, by the induction of multiple enzymes and various transport systems. The pH-responsive increases in glutaminase (GA) and phosphoenolpyruvate carboxykinase (PEPCK) mRNAs are reproduced in LLC-PK(1)-fructose 1,6-bisphosphatase (FBPase) cells. The increase in GA activity results from stabilization of the GA mRNA. The 3'-untranslated region of the GA mRNA contains a direct repeat of an eight-base AU sequence that functions as a pH-response element. This sequence binds zeta-crystallin/NADPH:quinone reductase with high affinity and specificity. Increased binding of this protein during acidosis may initiate the pH-responsive stabilization of the GA mRNA. In contrast, induction of PEPCK occurs at the transcriptional level. In LLC-PK(1)-FBPase(+) kidney cells, a decrease in intracellular pH leads to activation of the p38 stress-activated protein kinase and subsequent phosphorylation of transcription factor ATF-2. This transcription factor binds to cAMP-response element 1 within the PEPCK promoter and may enhance its transcription during metabolic acidosis.

Isolation and characterization of the promoter region of the rat kidney-type glutaminase gene

A lambdaEMBL3 rat genomic library was screened to clone a phage that contained the promoter region of the kidney-type mitochondrial glutaminase gene. The resulting lambdaGA1 phage contained 13.7 kb of genomic DNA that was mapped by Southern blotting and restriction analysis. The 2.22 kb and 0.83 kb SacI fragments of lambdaGA1 were sequenced and the transcription initiation site was identified by RNase mapping. The reported sequence contains 2287 bp of the promoter, the entire exon 1 (542 bp), and 223 bp of the initial intron of the glutaminase gene. The initial exon contains 141 bp of 5'-nontranslated sequence and 401 bp of coding sequence that encodes the 72-amino acid mitochondrial targeting presequence and 61 amino acids from the N-terminus of the mature 66 kDa glutaminase subunit. Various segments of the GA promoter were cloned into a chloramphenicol acetyltransferase (CAT) expression vector. The resulting GA-CAT constructs were transfected into LLC-PK(1)-F(+) kidney cells to assess the promoter function of the isolated genomic DNA. The GA(-402)CAT construct produced a 10-fold greater CAT activity than the promoter-less pCAT vector. Analysis of various deletion constructs indicated that elements located between -402 and -63 bp must act in synergy with more proximal elements to create a functional promoter. The initial 402 bp segment lacks a TATA sequence but is GC-rich and contains two CCAAT boxes and two Sp1 sites.