Immunohistochemical localization of arginase II and other enzymes of arginine metabolism in rat kidney and liver

Biochemistry, pharmacology, and in vivo function of arginases

The enzyme arginase catalyzes the hydrolysis of l-arginine into l-ornithine and urea. The 2 existing isoforms Arg1 and Arg2 exhibit different cellular localizations and metabolic functions. Arginase activity is crucial for nitrogen detoxification in the urea cycle, synthesis of polyamines, and control of l-arginine bioavailability and nitric oxide (NO) production. Despite significant progress in the understanding of the biochemistry and function of arginases, several open questions remain. Recent studies have revealed that the regulation and function of Arg1 and Arg2 are cell type-specific, species-specific, and profoundly different in mice and humans. The main differences are in the distribution and function of Arg1 and Arg2 in immune and erythroid cells. Contrary to what was previously thought, Arg1 activity appears to be only partially related to vascular NO signaling under homeostatic conditions in the vascular wall, but its expression is increased under disease conditions and may be targeted by treatment with arginase inhibitors. Arg2 appears to be mainly a catabolic enzyme involved in the synthesis of l-ornithine, polyamine, and l-proline but may play a putative role in blood pressure control, at least in mice. The immunosuppressive role of arginase-mediated arginine depletion is a promising target for cancer treatment. This review critically revises and discusses the biochemistry, pharmacology, and in vivo function of arginases, focusing on the insights gained from the analysis of cell-specific Arg1 and Arg2 knockout mice and human studies using arginase inhibitors or pegylated recombinant arginase. SIGNIFICANCE STATEMENT: Further basic and translational research is needed to deepen our understanding of the regulation of Arg1 and Arg2 in different cell types in consideration of their localization, species-specificity, and multiple biochemical and physiological roles. This will lead to better pharmacological strategies to target arginase activity in liver, cardiovascular, hematological, immune/infectious diseases, and cancer.

Hypoxia Induces Renal Epithelial Injury and Activates Fibrotic Signaling Through Up-Regulation of Arginase-II

The ureohydrolase, type-II arginase (Arg-II), is a mitochondrial enzyme metabolizing L-arginine into urea and L-ornithine and is highly expressed in renal proximal tubular cells (PTC) and upregulated by renal ischemia. Recent studies reported contradictory results on the role of Arg-II in renal injury. The aim of our study is to investigate the function of Arg-II in renal epithelial cell damage under hypoxic conditions. Human renal epithelial cell line HK2 was cultured under hypoxic conditions for 12–48 h. Moreover, ex vivo experiments with isolated kidneys from wild-type (WT) and genetic Arg-II deficient mice (Arg-II^–/–) were conducted under normoxic and hypoxic conditions. The results show that hypoxia upregulates Arg-II expression in HK2 cells, which is inhibited by silencing both hypoxia-inducible factors (HIFs) HIF1α and HIF2α. Treatment of the cells with dimethyloxaloylglycine (DMOG) to stabilize HIFα also enhances Arg-II. Interestingly, hypoxia or DMOG upregulates transforming growth factor β1 (TGFβ1) levels and collagens Iα1, which is prevented by Arg-II silencing, while TGFβ1-induced collagen Iα1 expression is not affected by Arg-II silencing. Inhibition of mitochondrial complex-I by rotenone abolishes hypoxia-induced reactive oxygen species (mtROS) and TGFβ1 elevation in the cells. Ex vivo experiments show elevated Arg-II and TGFβ1 expression and the injury marker NGAL in the WT mouse kidneys under hypoxic conditions, which is prevented in the Arg-II^–/– mice. Taking together, the results demonstrate that hypoxia activates renal epithelial HIFs-Arg-II-mtROS-TGFβ1-cascade, participating in hypoxia-associated renal injury and fibrosis.

Endogenous flux of nitric oxide: Citrulline is preferred to Arginine

Both arginine (Arg) and its precursor citrulline (Cit) have received much interest in the past two decades because of their potential effects on whole-body nitric oxide (NO) production and augmentation of NO-dependent signalling pathways. However, the usefulness of Arg supplementation for NO production is questionable because of its high splanchnic first pass metabolism (FPM), which limits its systemic availability. Both hepatic- and extrahepatic arginases critically limit the availability of Arg for the NO synthase enzymes (NOSs) and therefore, a limited amount of oral Arg can reach the systemic circulation for NO synthesis. Arg also has some undesired effects including induction of arginase activity, an increase of urea levels, a decrease of cellular uptake of Cit and decrease of recycling of Arg from Cit. In contrast, Cit has more availability as an NO precursor because of its high intestinal absorption, low FPM and high renal reabsorption. At the cellular level, co-localization of Cit transport systems and the enzymes involved in the Cit-Arg-NO pathway facilitates channelling of Cit into NO. Furthermore, cells preferably use Cit rather than either intra- or extracellular Arg to improve NO output, especially in high-demand situations. In conclusion, available evidence strongly supports the concept that Cit leads to higher NO production and suggests that Cit may have a better therapeutic effect than Arg for NO-disrupted conditions.

Metabolic reprogramming: A driver of cigarette smoke-induced inflammatory lung diseases

Cigarette smoking is a well-known risk factor for pulmonary diseases, including chronic obstructive pulmonary disease (COPD), asthma and pulmonary fibrosis. Despite major progress in dissecting the mechanisms associated with disease development and progression, findings only represent one aspect of multifaceted disease. A crucial consequence of this approach is that many therapeutic treatments often fail to improve or reverse the disease state as other conditions and variables are insufficiently considered. To expand our understanding of pulmonary diseases, omics approaches, particularly metabolomics, has been emerging in the field. This strategy has been applied to identify putative biomarkers and novel mechanistic insights. In this review, we discuss metabolic profiles of patients with COPD, asthma, and idiopathic pulmonary fibrosis (IPF) with a focus on the direct effects of cigarette smoking in altering metabolic regulation. We next present cell- and animal-based experiments and point out the therapeutic potential of targeting metabolic reprogramming in inflammatory lung diseases. In addition, the obstacles in translating these findings into clinical practice, including potential adverse effects and limited pharmacological efficacy, are also addressed.

What we know about plant arginases?

Nitrogen is one of the essential element required for plant growth and development. In plants, most of the nitrogen is stored in arginine. Hence, metabolism of arginine to urea by arginase and its further hydrolysis to ammonia by urease is involved in nitrogen recycling to meet the metabolic demands of growing plants. In this respect, plant arginases differ from that of animals. Animals excrete urea while plants recycle the urea. However, the studies on the biochemical and biophysical characteristics of plant arginase are limited when compared to animal arginase(s). In this review, the structural and biochemical characteristics of various plant arginases are discussed. Moreover, the significance of arginase in nitrogen recycling is explained and recent literature on function and activation of plant arginases in response to various environmental (biotic and abiotic) insults is also presented.

Is the Arginase Pathway a Novel Therapeutic Avenue for Diabetic Retinopathy?

Diabetic retinopathy (DR) is the leading cause of blindness in working age Americans. Clinicians diagnose DR based on its characteristic vascular pathology, which is evident upon clinical exam. However, extensive research has shown that diabetes causes significant neurovascular dysfunction prior to the development of clinically apparent vascular damage. While laser photocoagulation and/or anti-vascular endothelial growth factor (VEGF) therapies are often effective for limiting the late-stage vascular pathology, we still do not have an effective treatment to limit the neurovascular dysfunction or promote repair during the early stages of DR. This review addresses the role of arginase as a mediator of retinal neurovascular injury and therapeutic target for early stage DR. Arginase is the ureohydrolase enzyme that catalyzes the production of L-ornithine and urea from L-arginine. Arginase upregulation has been associated with inflammation, oxidative stress, and peripheral vascular dysfunction in models of both types of diabetes. The arginase enzyme has been identified as a therapeutic target in cardiovascular disease and central nervous system disease including stroke and ischemic retinopathies. Here, we discuss and review the literature on arginase-induced retinal neurovascular dysfunction in models of DR. We also speculate on the therapeutic potential of arginase in DR and its related underlying mechanisms.

Overexpression of carbamoyl-phosphate synthase 1 significantly improves ureagenesis of human liver HepaRG cells only when cultured under shaking conditions

Hyperammonemia is an important contributing factor to hepatic encephalopathy in end-stage liver failure patients. Therefore reducing hyperammonemia is a requisite of bioartificial liver support (BAL). Ammonia elimination by human liver HepaRG cells occurs predominantly through reversible fixation into amino acids, whereas the irreversible conversion into urea is limited. Compared to human liver, the expression and activity of the three urea cycle (UC) enzymes carbamoyl-phosphate synthase1 (CPS1), ornithine transcarbamoylase (OTC) and arginase1, are low. To improve HepaRG cells as BAL biocomponent, its rate limiting factor of the UC was determined under two culture conditions: static and dynamic medium flow (DMF) achieved by shaking. HepaRG cells increasingly converted escalating arginine doses into urea, indicating that arginase activity is not limiting ureagenesis. Neither was OTC activity, as a stable HepaRG line overexpressing OTC exhibited a 90- and 15.7-fold upregulation of OTC transcript and activity levels, without improvement in ureagenesis. However, a stable HepaRG line overexpressing CPS1 showed increased mitochondrial stress and reduced hepatic differentiation without promotion of the CPS1 transcript level or ureagenesis under static-culturing conditions, yet, it exhibited a 4.3-fold increased ureagenesis under DMF. This was associated with increased CPS1 transcript and activity levels amounting to >2-fold, increased mitochondrial abundance and hepatic differentiation. Unexpectedly, the transcript levels of several other UC genes increased up to 6.8-fold. We conclude that ureagenesis can be improved in HepaRG cells by CPS1 overexpression, however, only in combination with DMF-culturing, suggesting that both the low CPS1 level and static-culturing, possibly due to insufficient mitochondria, are limiting UC.

L-Norvaline, a new therapeutic agent against Alzheimer's disease

Growing evidence highlights the role of arginase activity in the manifestation of Alzheimer’s disease (AD). Upregulation of arginase was shown to contribute to neurodegeneration. Regulation of arginase activity appears to be a promising approach for interfering with the pathogenesis of AD. Therefore, the enzyme represents a novel therapeutic target. In this study, we administered an arginase inhibitor, L-norvaline (250 mg/L), for 2.5 months to a triple-transgenic model (3×Tg-AD) harboring PS1M146V, APPSwe, and tauP301L transgenes. Then, the neuroprotective effects of L-norvaline were evaluated using immunohistochemistry, proteomics, and quantitative polymerase chain reaction assays. Finally, we identified the biological pathways activated by the treatment. Remarkably, L-norvaline treatment reverses the cognitive decline in AD mice. The treatment is neuroprotective as indicated by reduced beta-amyloidosis, alleviated microgliosis, and reduced tumor necrosis factor transcription levels. Moreover, elevated levels of neuroplasticity related postsynaptic density protein 95 were detected in the hippocampi of mice treated with L-norvaline. Furthermore, we disclosed several biological pathways, which were involved in cell survival and neuroplasticity and were activated by the treatment. Through these modes of action, L-norvaline has the potential to improve the symptoms of AD and even interferes with its pathogenesis. As such, L-norvaline is a promising neuroprotective molecule that might be tailored for the treatment of a range of neurodegenerative disorders. The study was approved by the Bar-Ilan University Animal Care and Use Committee (approval No. 82-10-2017) on October 1, 2017.

Arginase 1 promotes retinal neurovascular protection from ischemia through suppression of macrophage inflammatory responses

The lack of effective therapies to limit neurovascular injury in ischemic retinopathy is a major clinical problem. This study aimed to examine the role of ureohydrolase enzyme, arginase 1 (A1), in retinal ischemia-reperfusion (IR) injury. A1 competes with nitric oxide synthase (NOS) for their common substrate l-arginine. A1-mediated l-arginine depletion reduces nitric oxide (NO) formation by NOS leading to vascular dysfunction when endothelial NOS is involved but prevents inflammatory injury when inducible NOS is involved. Studies were performed using wild-type (WT) mice, global A1^+/− knockout (KO), endothelial-specific A1 KO, and myeloid-specific A1 KO mice subjected to retinal IR injury. Global as well as myeloid-specific A1 KO mice showed worsened IR-induced neuronal loss and retinal thinning. Deletion of A1 in endothelial cells had no effect, while treatment with PEGylated (PEG) A1 improved neuronal survival in WT mice. In addition, A1^+/− KO mice showed worsened vascular injury manifested by increased acellular capillaries. Western blotting analysis of retinal tissue showed increased inflammatory and necroptotic markers with A1 deletion. In vitro experiments showed that macrophages lacking A1 exhibit increased inflammatory response upon LPS stimulation. PEG-A1 treatment dampened this inflammatory response and decreased the LPS-induced metabolic reprogramming. Moreover, intravitreal injection of A1 KO macrophages or systemic macrophage depletion with clodronate liposomes increased neuronal loss after IR injury. These results demonstrate that A1 reduces IR injury-induced retinal neurovascular degeneration via dampening macrophage inflammatory responses. Increasing A1 offers a novel strategy for limiting neurovascular injury and promoting macrophage-mediated repair.

A practice-changing culture method relying on shaking substantially increases mitochondrial energy metabolism and functionality of human liver cell lines

Practice-changing culturing techniques of hepatocytes are highly required to increase their differentiation. Previously, we found that human liver cell lines HepaRG and C3A acquire higher functionality and increased mitochondrial biogenesis when cultured in the AMC-Bioartificial liver (BAL). Dynamic medium flow (DMF) is one of the major contributors to this stimulatory effect. Recently, we found that DMF-culturing by shaking of HepaRG monolayers resulted in higher mitochondrial biogenesis. Here we further investigated the effect of DMF-culturing on energy metabolism and hepatic functionality of HepaRG and C3A monolayers. HepaRG and C3A DMF-monolayers were incubated with orbital shaking at 60 rpm during the differentiation phase, while control monolayers were maintained statically. Subsequently, energy metabolism and hepatic functionality were compared between static and DMF-cultures. DMF-culturing of HepaRG cells substantially increased hepatic differentiation; transcript levels of hepatic structural genes and hepatic transcription regulators were increased up to 15-fold (Cytochrome P450 3A4) and nuclear translocation of hepatic transcription factor CEBPα was stimulated. Accordingly, hepatic functions were positively affected, including ammonia elimination, urea production, bile acid production, and CYP3A4 activity. DMF-culturing shifted energy metabolism from aerobic glycolysis towards oxidative phosphorylation, as indicated by a decline in lactate production and glucose consumption, and an increase in oxygen consumption. Similarly, DMF-culturing increased mitochondrial energy metabolism and hepatic functionality of C3A cells. In conclusion, simple shaking of monolayer cultures substantially improves mitochondrial energy metabolism and hepatic differentiation of human liver cell lines. This practice-changing culture method may prove to prolong the in-vitro maintenance of primary hepatocytes and increase hepatic differentiation of stem cells.

Pharmacokinetics and Pharmacodynamics of Promising Arginase Inhibitors

Up-regulation of arginase activity in several chronic disease conditions, including cancer and hypertension, may suggest new targets for treatment. Recently, the number of new arginase inhibitors with promising therapeutic effects for asthma, cancer, hypertension, diabetes mellitus, and erectile dysfunction has shown a remarkable increase. Arginase inhibitors may be chemical substances, such as boron-based amino acid derivatives, α-difluoromethylornithine (DMFO), and Nω-hydroxy-nor-L-arginine (nor-NOHA) or, of plant origin such as sauchinone, salvianolic acid B (SAB), piceatannol-3-O-β-D-glucopyranoside (PG) and obacunone. Despite their promising therapeutic potential, little is known about pharmacokinetics and pharmacodynamics of some of these agents. Several studies were conducted in different animal species and in vitro systems and reported significant differences in pharmacokinetics and pharmacodynamics of arginase inhibitors. Therefore, extra caution should be considered before extrapolating these studies to human. Physicochemical and pharmacokinetic profiles of some effective arginase inhibitors make it challenging to formulate stable and effective formulation. In this article, existing literature on the pharmacokinetics and pharmacodynamics of arginase inhibitors were reviewed and compared together with emphasis on possible drug interactions and solutions to overcome pharmacokinetics challenges and shortage of arginase inhibitors in clinical practice.

Arginase-2 mediates renal ischemia-reperfusion injury

Novel therapeutic interventions for preventing or attenuating kidney injury following ischemia-reperfusion injury (IRI) remain a focus of significant interest. Currently, there are no definitive therapeutic or preventive approaches available for ischemic acute kidney injury (AKI). Our objective is to determine 1) whether renal arginase activity or expression is increased in renal IRI, and 2) whether arginase plays a role in development of renal IRI. The impact of arginase activity and expression on renal damage was evaluated in male C57BL/6J (wild type) and arginase-2 (ARG2)-deficient (Arg2^-/- ) mice subjected to bilateral renal ischemia for 28 min, followed by reperfusion for 24 h. ARG2 expression and arginase activity significantly increased following renal IRI, paralleling the increase in kidney injury. Pharmacological blockade or genetic deficiency of Arg2 conferred kidney protection in renal IRI. Arg2^-/- mice had significantly attenuated kidney injury and lower plasma creatinine and blood urea nitrogen levels after renal IRI. Blocking arginases using S-(2-boronoethyl)-l-cysteine (BEC) 18 h before ischemia mimicked arginase deficiency by reducing kidney injury, histopathological changes and kidney injury marker-1 expression, renal apoptosis, kidney inflammatory cell recruitment and inflammatory cytokines, and kidney oxidative stress; increasing kidney nitric oxide (NO) production and endothelial NO synthase (eNOS) phosphorylation, kidney peroxisome proliferator-activated receptor-γ coactivator-1α expression, and mitochondrial ATP; and preserving kidney mitochondrial ultrastructure compared with vehicle-treated IRI mice. Importantly, BEC-treated eNOS-knockout mice failed to reduce blood urea nitrogen and creatinine following renal IRI. These findings indicate that ARG2 plays a major role in renal IRI, via an eNOS-dependent mechanism, and that blocking ARG2 activity or expression could be a novel therapeutic approach for prevention of AKI.

Effects of the Hydroalcoholic Extract of Zingiber officinale on Arginase I Activity and Expression in the Retina of Streptozotocin-Induced Diabetic Rats

BACKGROUND

Emerging evidence suggests that an increased arginase activity is involved in vascular dysfunction in experimental animals. Zingiber officinale Roscoe, commonly known as ginger, has been widely used in the traditional medicine for treatment of diabetes.

OBJECTIVES

This study aimed at investigating the effects of the hydroalcoholic extract of Z. officinale on arginase I activity and expression in the retina of streptozotocin (STZ)-induced diabetic rats.

METHODS

In this experimental study, 16 male Wistar rats weighing 200 - 250 g were assessed. Diabetes was induced via a single intraperitoneal injection of STZ (60 mg/kg body weight). The rats were randomly allocated into four experimental groups. Untreated healthy and diabetic controls received 1.5 mL/kg distilled water. Treated diabetic rats received 200, and 400 mg/kg of the Z. officinale extract dissolved in distilled water (1.5 mL/kg). Body weight, blood glucose and insulin concentration were measured by standard methods. The arginase I activity and expression were determined by spectrophotometric and western blot analysis, respectively.

RESULTS

Our results showed that blood glucose concentration was significantly decreased in diabetic rats treated with the extract compared to untreated diabetic controls (P < 0.01). Treatment with 400 mg/kg of the extract reduced arginase I activity and expression (P < 0.05). A significant elevation in body weight was observed in diabetic rats treated with the extract. Serum insulin was significantly increased in diabetic rats treated with 400 mg/kg of the extract compared to diabetic controls (P < 0.05).

CONCLUSIONS

Our results suggest that the Z. officinale hydroalcoholic extract may potentially be a promising therapeutic option for treating diabetes-induced vascular disorders, possibly through reducing arginase I activity and expression in the retina.

Arginase 2 promotes neurovascular degeneration during ischemia/reperfusion injury

Retinal ischemia is a major cause of visual impairment and blindness and is involved in various disorders including diabetic retinopathy, glaucoma, optic neuropathies and retinopathy of prematurity. Neurovascular degeneration is a common feature of these pathologies. Our lab has previously reported that the ureahydrolase arginase 2 (A2) is involved in ischemic retinopathies. Here, we are introducing A2 as a therapeutic target to prevent neurovascular injury after retinal ischemia/reperfusion (I/R) insult. Studies were performed with mice lacking both copies of A2 (A2^−/−) and wild-type (WT) controls (C57BL6J). I/R insult was conducted on the right eye and the left eye was used as control. Retinas were collected for analysis at different times (3 h–4 week after injury). Neuronal and microvascular degeneration were evaluated using NeuN staining and vascular digests, respectively. Glial activation was evaluated by glial fibrillary acidic protein expression. Necrotic cell death was studied by propidium iodide labeling and western blot for RIP-3. Arginase expression was determined by western blot and quantitative RT-PCR. Retinal function was determined by electroretinography (ERG). A2 mRNA and protein levels were increased in WT I/R. A2 deletion significantly reduced ganglion cell loss and microvascular degeneration and preserved retinal morphology after I/R. Glial activation, reactive oxygen species formation and cell death by necroptosis were significantly reduced by A2 deletion. ERG showed improved positive scotopic threshold response with A2 deletion. This study shows for the first time that neurovascular injury after retinal I/R is mediated through increased expression of A2. Deletion of A2 was found to be beneficial in reducing neurovascular degeneration after I/R.

An Optimized Method for Protein Extraction from OCT-Embedded Human Kidney Tissue for Protein Quantification by LC-MS/MS Proteomics

The existing biobanks of remnant tissue from clinically indicated kidney biopsies are attractive potential reservoirs for quantification of clinically relevant human tissue proteins by quantitative proteomics. However, a significant caveat of this strategy is that the tissues are often preserved in optimal cutting temperature (OCT) medium. Although OCT is an effective method of preserving the morphologic and immunohistological characteristics of tissues for later study, it significantly impacts efforts to quantify protein expression by liquid chromatography-tandem mass spectrometry methods. We report here a simple, reproducible, and cost-effective procedure to extract proteins from OCT-embedded tissue samples. Briefly, the excess frozen OCT medium was scraped before thawing from the tissue specimens stored at -80°C for ∼3 months. The tissue samples were homogenized and diethyl ether/methanol extraction was performed to remove the remaining OCT medium. The recovered protein was denatured, reduced, and alkylated. The second step of protein extraction and desalting was performed by chloroform/methanol/water extraction of denatured proteins. The resultant protein pellet was trypsin-digested and the marker proteins of various kidney cellular compartments were quantified by targeted selective reaction monitoring proteomics. Upon comparison of peptide signals from OCT-embedded tissue and flash-frozen tissue from the same donors, both individual protein quantities, and their interindividual variabilities, were similar. Therefore, the approach reported here can be applied to clinical reservoirs of OCT-preserved kidney tissue to be used for quantitative proteomics studies of clinically relevant proteins expressed in different parts of the kidney (including drug transporters and metabolizing enzymes).

Arginase inhibition augments nitric oxide production and facilitates left ventricular systolic function in doxorubicin-induced cardiomyopathy in mice

A metabolizing enzyme arginase can decrease nitric oxide (NO) production by competing with NO synthase for arginine as a substrate, but its pathophysiological role in heart failure remains unknown. We aimed to investigate the effect of pharmacological inhibition of arginase on left ventricular function in doxorubicin‐induced cardiomyopathy in mice. Doxorubicin administration for 5 weeks significantly increased protein expression levels or activity of arginase in the lungs and liver, and caused moderate increase in arginase 2 expression in the aorta. In the lungs, accumulated interstitial cells strongly expressed both arginase 1 and arginase 2 by doxorubicin administration. Echocardiography revealed that administration of a potent, reversible arginase inhibitor N‐omega‐hydroxy‐nor‐l‐arginine completely reversed doxorubicin‐induced decrease in the ejection fraction, in parallel with expression levels of BNP mRNA, without affecting apoptosis, hypertrophy, fibrosis, or macrophage infiltration in the left ventricle. Arginase inhibition reversibly lowered systolic blood pressure, and importantly, it recovered doxorubicin‐induced decline in NO concentration in the serum, lungs, and aorta. Furthermore, arginase inhibition stimulated NO secretion from aortic endothelial cells and peritoneal macrophages in vitro. In conclusion, pharmacological inhibition of arginase augmented NO concentration in the serum, lungs, and aorta, promoted NO‐mediated decrease in afterload for left ventricle, and facilitated left ventricular systolic function in doxorubicin‐induced cardiomyopathy in mice.

e12130

This figure shows that administration of an arginase inhibitor nor‐ NOHA facilitates LV systolic function in murine Dox‐induced cardiomyopathy.

Arginase in retinopathy

Ischemic retinopathies, such as diabetic retinopathy (DR), retinopathy of prematurity and retinal vein occlusion are a major cause of blindness in developed nations worldwide. Each of these conditions is associated with early neurovascular dysfunction. However, conventional therapies target clinically significant macula edema or neovascularization, which occur much later. Intra-ocular injections of anti-VEGF show promise in reducing retinal edema, but the effects are usually transient and the need for repeated injections increases the risk of intraocular infection. Laser photocoagulation can control pathological neovascularization, but may impair vision and in some patients the retinopathy continues to progress. Moreover, neither treatment targets early stage disease or promotes repair. This review examines the potential role of the ureahydrolase enzyme arginase as a therapeutic target for the treatment of ischemic retinopathy. Arginase metabolizes l-arginine to form proline, polyamines and glutamate. Excessive arginase activity reduces the l-arginine supply for nitric oxide synthase (NOS), causing it to become uncoupled and produce superoxide and less NO. Superoxide and NO react and form the toxic oxidant peroxynitrite. The catabolic products of polyamine oxidation and glutamate can induce more oxidative stress and DNA damage, both of which can cause cellular injury. Studies indicate that neurovascular injury during retinopathy is associated with increased arginase expression/activity, decreased NO, polyamine oxidation, formation of superoxide and peroxynitrite and dysfunction and injury of both vascular and neural cells. Furthermore, data indicate that the cytosolic isoform arginase I (AI) is involved in hyperglycemia-induced dysfunction and injury of vascular endothelial cells whereas the mitochondrial isoform arginase II (AII) is involved in neurovascular dysfunction and death following hyperoxia exposure. Thus, we postulate that activation of the arginase pathway causes neurovascular injury by uncoupling NOS and inducing polyamine oxidation and glutamate formation, thereby reducing NO and increasing oxidative stress, all of which contribute to the retinopathic process.

A performance evaluation of three drug-induced liver injury biomarkers in the rat: alpha-glutathione S-transferase, arginase 1, and 4-hydroxyphenyl-pyruvate dioxygenase

Alanine aminotransferase (ALT) activity is the most frequently relied upon reference standard for monitoring liver injury in humans and nonclinical species. However, limitations of ALT include a lack of specificity for diagnosing liver injury (e.g., present in muscle and the gastrointestinal tract), its inability to monitor certain types of hepatic injury (e.g., biliary injury), and ambiguity with respect to interpretation of modest or transient elevations (< 3× upper limit of normal). As an initial step to both understand and qualify additional biomarkers of hepatotoxicity that may add value to ALT, three novel candidates have been evaluated in 34 acute toxicity rat studies: (1) alpha-glutathione S-transferase (GSTA), (2) arginase 1 (ARG1), and (3) 4-hydroxyphenylpyruvate dioxygenase (HPD). The performance of each biomarker was assessed for its diagnostic ability to accurately detect hepatocellular injury (i.e., microscopic histopathology), singularly or in combination with ALT. All three biomarkers, either alone or in combination with ALT, improved specificity when compared with ALT alone. Hepatocellular necrosis and/or degeneration were detected by all three biomarkers in the majority of animals. ARG1 and HPD were also sensitive in detecting single-cell necrosis in the absence of more extensive hepatocellular necrosis/degeneration. ARG1 showed the best sensitivity for detecting biliary injury with or without ALT. All the biomarkers were able to detect biliary injury with single-cell necrosis. Taken together, these novel liver toxicity biomarkers, GSTA, ARG1, and HPD, add value (both enhanced specificity and sensitivity) to the measurement of ALT alone for monitoring drug-induced liver injury in rat.

Immunonutrition: does it have a role in improving recovery in patients receiving a stem cell transplant?

Numerous clinical trials have demonstrated that immunomodulating diets (IMDs) reduce treatment complications such as the risk of acquired infections, length of hospital stay, and wound complications in patients receiving planned surgery. These complications are possibly exacerbated by malnutrition at the time of surgery, resulting in decreased cell-mediated and humoral immune responses, which can be improved with the utilization of IMDs both prior to and following surgery. Although numerous randomized studies have investigated IMDs in the surgical setting, IMDs have not been well studied to evaluate whether their use improves outcomes for other patient groups with high incidence of malnutrition and acquired infections. Patients receiving stem cell transplantation following preparative myeloablative chemotherapy for treatment of a hematologic malignancy would be a prime example of another patient group who would share these characteristics. Given the proposed mechanism of action by which IMDs have aided recovery after surgery, it is reasonable to expect that IMDs may aid recovery after stem cell transplantation, and current preclinical and clinical data support the need for further clinical studies.

Expression and function of arginine-producing and consuming-enzymes in the kidney

The kidney plays a key role in arginine metabolism. Arginine production is controlled by argininosuccinate synthetase (ASS) and argininosuccinate lyase (ASL) which metabolize citrulline and aspartate to arginine and fumarate whereas arginine consumption is dependent on arginine:glycine amidinotransferase (GAT), which mediates creatine and ornithine synthesis. Histological and biochemical techniques have been used to study the distribution and activity of these enzymes in anatomically dissected segments, in isolated fragments of tubules and in whole tissues. ASS and ASL mRNAs and proteins are expressed in the proximal tubule. Within this nephron segment, the proximal convoluted tubule has a higher arginine synthesis capacity than the proximal straight tubules. Furthermore, this arginine-synthesizing portion of the nephron matches perfectly with the site of citrulline reabsorption from the glomerular filtrate. The kidney itself can produce citrulline from methylated arginine, but this capacity is limited. Therefore, intestinal citrulline synthesis is required for renal arginine production. Although the proximal convoluted tubule also expresses a significant amount of GAT, only 10% of renal arginine synthesis is metabolized to guanidinoacetic acid, possibly because GAT has a mitochondrial localization. Kidney arginase (AII) is expressed in the cortical and outer medullary proximal straight tubules and does not degrade significant amounts of newly synthesized arginine. The data presented in this review identify the proximal convoluted tubule as the main site of endogenous arginine biosynthesis.

Effect of arginase II on L-arginine depletion and cell growth in murine cell lines of renal cell carcinoma

Background

L-arginine is the common substrate for the two isoforms of arginase. Arginase I, highly expressed in the liver and arginase II mainly expressed in the kidney. Arginase I-producing myeloid derived suppressor cells have been shown to inhibit T-cell function by the depletion of L-arginine. On the other hand, arginase II has been detected in patients with cancer and is thought to metabolize L-arginine to L-ornithine needed to sustain rapid tumor growth; however its role in L-arginine depletion is unclear. Thus, in tumor biology, L-arginine metabolism may play a dual role in tumor growth and in the induction of T cell dysfunction. Therefore, we studied in murine renal cell carcinoma (RCC) cell lines, the effect of arginase II on tumor cell proliferation and L-arginine depletion. The effect of arginase inhibitors on cell proliferation was also tested.

Methods

Three murine renal cell carcinoma (mRCC) cell lines were tested for the presence of arginase. nor-NOHA, an arginase inhibitor was used to substantiate the effect of arginase on cell growth and L-arginine depletion. Amino acid levels were tested by HPLC.

Results

Our results show that mRCC cell lines express only arginase II and were able to deplete L-arginine from the medium. Cell growth was independent of the amount of arginase activity expressed by the cells. nor-NOHA significantly (P = 0.01) reduced arginase II activity and suppressed cell growth in cells exhibiting high arginase activity.

The depletion of L-arginine by mRCC induced the decrease expression of CD3ζ a key element for T-cell function.

Conclusion

The results of this study show for the first time that arginase II produced by RCC cell lines depletes L-arginine resulting in decreased expression of CD3ζ. These results indicate that RCC cell lines expressing arginase II can modulate the L-arginine metabolic pathway to regulate both cell growth and T-cell function. Blocking arginase may lead to a decrease in RCC cell growth and aid in restoring immune function by increasing L-arginine availability for T-cell use. Understanding the interplay between arginase II and its interaction with the immune system may provide future therapeutic benefits to treat patients with RCC.

Reduction of arginase I activity and manganese levels in the liver during exposure of rats to methylmercury: a possible mechanism

The toxicity of methylmercury (MeHg) is, in part, thought to be due to its interaction with thiol groups in a variety of enzymes, but the molecular targets of MeHg are poorly understood. Arginase I, an abundant manganese (Mn)-binding protein in the liver, requires Mn as an essential element to exhibit maximal enzyme activity. In the present study, we examined the effect of MeHg on hepatic arginase I in vivo and in vitro. Subcutaneous administration of MeHg (10 mg/kg) for 8 days to rats resulted in marked suppression of arginase I activity. With purified arginase I, we found that interaction of MeHg with arginase I caused the aggregation of arginase I as evaluated by centrifugation and subsequent precipitation, and then the reduction of catalytic activity. Experiments with organomercury column confirmed that arginase I has reactive thiols that are covalently bound to organomercury. While MeHg inhibited arginase I activity, Mn ions were released from this enzyme. These results suggest that MeHg-mediated suppression of hepatic arginase I activity in vivo is, at least in part, attributable to covalent modification of MeHg or substantial leakage of Mn ions from the active site.

Interpretation of plasma amino acids in the follow-up of patients: the impact of compartmentation

Results of plasma or urinary amino acids are used for suspicion, confirmation or exclusion of diagnosis, monitoring of treatment, prevention and prognosis in inborn errors of amino acid metabolism. The concentrations in plasma or whole blood do not necessarily reflect the relevant metabolite concentrations in organs such as the brain or in cell compartments; this is especially the case in disorders that are not solely expressed in liver and/or in those which also affect nonessential amino acids. Basic biochemical knowledge has added much to the understanding of zonation and compartmentation of expressed proteins and metabolites in organs, cells and cell organelles. In this paper, selected old and new biochemical findings in PKU, urea cycle disorders and nonketotic hyperglycinaemia are reviewed; the aim is to show that integrating the knowledge gained in the last decades on enzymes and transporters related to amino acid metabolism allows a more extensive interpretation of biochemical results obtained for diagnosis and follow-up of patients and may help to pose new questions and to avoid pitfalls. The analysis and interpretation of amino acid measurements in physiological fluids should not be restricted to a few amino acids but should encompass the whole quantitative profile and include other pathophysiological markers. This is important if the patient appears not to respond as expected to treatment and is needed when investigating new therapies. We suggest that amino acid imbalance in the relevant compartments caused by over-zealous or protocol-driven treatment that is not adjusted to the individual patient's needs may prolong catabolism and must be corrected.

Cells for bioartificial liver devices: the human hepatoma-derived cell line C3A produces urea but does not detoxify ammonia

Extrahepatic bioartificial liver devices should provide an intact urea cycle to detoxify ammonia. The C3A cell line, a subclone of the hepatoma-derived HepG2 cell line, is currently used in this context as it produces urea, and this has been assumed to be reflective of ammonia detoxification via a functional urea cycle. However, based on our previous findings of perturbed urea-cycle function in the non-urea producing HepG2 cell line, we hypothesized that the urea produced by C3A cells was via a urea cycle-independent mechanism, namely, due to arginase II activity, and therefore would not detoxify ammonia. Urea was quantified using (15)N-ammonium chloride metabolic labelling with gas chromatography-mass spectrometry. Gene expression was determined by real-time reverse transcriptase-PCR, protein expression by western blotting, and functional activities with radiolabelling enzyme assays. Arginase inhibition studies used N(omega)-hydroxy-nor-L-arginine. Urea was detected in C3A conditioned medium; however, (15)N-ammonium chloride-labelling indicated that (15)N-ammonia was not incorporated into (15)N-labelled urea. Further, gene expression of two urea cycle genes, ornithine transcarbamylase and arginase I, were completely absent. In contrast, arginase II mRNA and protein was expressed at high levels in C3A cells and was inhibited by N(omega)-hydroxy-nor-L-arginine, which prevented urea production, thereby indicating a urea cycle-independent pathway. The urea cycle is non-functional in C3A cells, and their urea production is solely due to the presence of arginase II, which therefore cannot provide ammonia detoxification in a bioartificial liver system. This emphasizes the continued requirement for developing a component capable of a full repertoire of liver function.

L-arginine metabolism in dog kidney and isolated nephron segments

The renal basic amino acid metabolism often differs in rodents, strict carnivores, and omnivore species. Given the pivotal role of L-arginine and L-ornithine in several metabolic pathways and the fact that the dog is closely related to humans, being also an omnivore, we tested whether L-arginine metabolism and L-ornithine catabolism take place in the dog kidney. We examined the metabolism of L-arginine in dog cortical tubules to integrate local L-arginine metabolism into a general physiological and metabolic framework. To achieve these goals, we first ascertained the protein expression of relevant enzymes by Western blot. L-Arginine catabolism was studied in suspensions of canine cortical proximal tubules, medullary thick ascending limbs, and papillary collecting ducts either incubated without exogenous L-arginine being added (small endogenous quantities) or incubated with L-arginine being added in supraphysiological amounts (2 mmol/L with or without the presence of alternative metabolic substrates, 2 mmol/L L-glutamine, or lactate). The results revealed that dog kidneys consumed L-citrulline and released L-arginine and L-ornithine. Argininosuccinate synthetase and lyase, arginase II, and ornithine aminotransferase were detected in the renal cortex. Arginase II activity was found in a suspension of proximal tubules by measuring the amounts of urea and L-ornithine produced. A fraction of this L-ornithine was further partially metabolized through the intramitochondrial ornithine aminotransferase pathway, leading to changes in L-glutamate, glucose, L-alanine, and ammonia metabolism without L-proline accumulation. Medullary thick ascending limbs expressed a very low arginase activity, whereas papillary collecting ducts did not. In conclusion, the dog kidney produces L-arginine. Part of this L-arginine is further catabolized by arginase II, suggesting that its physiological role was to produce L-ornithine for the body.

Downregulation of arginase II and renal apoptosis by inorganic mercury: overexpression of arginase II reduces its apoptosis

Inorganic mercury is a toxic metal that accumulates in the proximal tubules of the kidney, causing apoptosis. Arginase II is known to inhibit apoptosis, but its role in the renal apoptosis caused by inorganic mercury is poorly understood. In the present study, we examined the involvement of arginase II in inorganic mercury-dependent apoptosis. A single exposure to mercuric chloride (HgCl(2), 1 mg/kg) in rats resulted in a dramatic time-dependent reduction in the activity of arginase II in the kidney; for example, the activity at 48 h after exposure was 31% of the control level. The decrease in arginase II activity was due to a decrease in the protein level, not to a reduction in gene expression or to direct inhibition of the activity itself. More interestingly, diminished arginase II activity was well correlated with the induction of apoptosis as evaluated by renal DNA fragmentation (r = 0.99). Overexpression of arginase II in LLC-PK(1) cells blocked cell death during exposure to inorganic mercury. These results suggest that inorganic mercury causes a reduction in protein levels of arginase II, and that impaired arginase II activity is, at least in part, associated with the apoptotic cell damage caused by this heavy metal.

Renal cell carcinoma does not express argininosuccinate synthetase and is highly sensitive to arginine deprivation via arginine deiminase

Recently, pegylated arginine deiminase (ADI; EC 3.5.3.6) has been used to treat the patients with hepatocellular carcinoma or melanoma, in which the level of argininosuccinate synthetase (ASS) activity is low or undetectable. The efficacy of its antitumor activity largely depends on the level of intracellular ASS, which enables tumor cells to recycle citrulline to arginine. Thus, we examined the expression levels of ASS in various cancer cells and found that it is low in renal cell carcinoma (RCC) cells, rendering the cells highly sensitive to arginine deprivation by ADI treatment. Immunohistochemical analysis revealed that in biopsy specimens from RCC patients (n = 98), the expression of ASS is highly demonstrated in the epithelium of normal proximal tubule but not seen in tumor cells. Furthermore, RCC cells treated with ADI showed remarkable growth retardation in a dose dependent manner. ADI also exerted in vivo antiproliferative effect on the allografted renal cell carcinoma (RENCA) tumor cells and prolonged the survival of tumor-bearing mice. Histological examination of the tumors revealed that tumor angiogenesis and vascular endothelial growth factor (VEGF) expression were significantly diminished by ADI administration. Therefore, these findings suggest that arginine deprivation by ADI could provide a beneficial strategy for the treatment of RCC in ways of inhibitions of arginine availability and neovascularization.

Serum level of ornithine carbamoyltransferase is influenced by the state of Kupffer cells

BACKGROUND

The ratio of ornithine carbamoyltransferase (OCT) to alanine aminotransferase (ALT) or glutamate dehydrogenase (GDH) in serum has been suggested as an indicator for the diagnosis of hepatocellular carcinoma and alcoholic liver disease, respectively. However, the mechanisms responsible for the increase in these ratios are still unclear.

METHODS

Wistar rats were pretreated with lipopolysaccharide (LPS) or gadolinium chloride (GD) before being administered with thioacetamide (TAA, 200 mg/kg, ip). Serum OCT and ALT levels were compared with control values. Half-lives of the enzymes in circulation were evaluated after the intravenous injection of the purified enzymes into rats with or without the pretreatment.

RESULTS

The serum level of OCT at 24 h after the administration of TAA was significantly lower in the LPS-treated group, and not influenced by pretreatment with GD. The half-life of OCT was prolonged from 1.06+/-0.14 to 2.07+/-0.29 h (p<0.05) by the pretreatment with GD, but not influenced by the administration of LPS. No change was observed in the clearance of GDH or ALT among the pretreatments.

CONCLUSIONS

Leakage into and clearance from the circulation of OCT are influenced by whether Kupffer cells are activated or not. OCT alone or in combination with other markers may be a useful indicator for Kupffer cell activation as well as mitochondrial damage in hepatic cells.

Advantage of serum type-I arginase and ornithine carbamoyltransferase in the evaluation of acute and chronic liver damage induced by thioacetamide in rats

BACKGROUND

We evaluated the usefulness of serum type-I arginase (ARG) and ornithine carbamoyltransferase (OCT) in thioacetamide (TAA)-induced acute and chronic liver injury in rats.

METHODS

In an acute injury model, we measured the time-courses of serum concentrations of ARG and OCT using ELISA, together with AST and ALT using biochemical enzymatic assays after a single administration of TAA (200 mg/kg, i.p.). In the chronic model, TAA was repeatedly administered (20 mg/kg/day, p.o.) for 16 weeks and serum concentrations of the enzymes were evaluated.

RESULTS

In the acute model, the concentrations of the enzymes were increased in a similar manner, peaking 24 h after the administration, and ARG showed the earliest and greatest increase among the enzymes tested. In the chronic model, the serum concentration of OCT was significantly increased only 1 week after oral treatment, while concentrations of the other enzymes were increased at 8 to 12 weeks. In the histological analysis, TAA treatment damaged hepatocytes in both the acute and chronic model.

CONCLUSIONS

These results clearly show the usefulness of ARG and OCT for the evaluation of acute and chronic liver injury, respectively.

Mitochondrial expression of arginase II in male and female rat inner medullary collecting ducts

Microdissected rat proximal straight tubules (PST) and inner medullary collecting ducts (IMCD) highly produce urea from l-arginine, supporting the expression of the mitochondrial arginase II. However, IMCD contain a very low density of mitochondria compared with PST. Recently, arginase II has been localized by immunohistochemistry in rat PST but not IMCD. This study was designed to verify whether rat IMCD express arginase II and to identify its subcellular localization. We developed an antibody raised against arginase II that allowed the detection of a band of 38 kDa corresponding to arginase II on immunoblots. In male and female rat kidneys, Western blot analyses revealed that arginase II was highly expressed in the inner medulla (IM), the outer stripe of the outer medulla (osOM), and the deep cortex. Immunocytochemistry demonstrated that arginase II was homogeneously expressed in IMCD. Proteins of the cytosolic and mitochondrial fractions extracted from osOM and IM and analyzed by Western blot showed that 86% of arginase II was associated with mitochondria. The molecular weight of arginase II was similar in the cytosolic and mitochondrial fractions. Immunoelectron microscopy confirmed the presence of arginase II in the mitochondria of IMCD. In conclusion, arginase II is expressed in mitochondria of male and female rat IMCD.

Localization and differential expression of arginase II in the kidney of male and female mice

Arginase II (AII) has been almost exclusively studied in male mammalian kidneys. Our investigations were conducted to localize AII gene expression in the female mouse kidney, and to analyze the differential expression of AII gene at the transcriptional and translational levels in the kidneys of female and male mice. Total RNAs and soluble proteins extracted from renal zones and whole kidneys were analyzed by Northern and Western blots, respectively. Mitochondrial and cytosolic proteins were analyzed by Western blot. L-[guanidino-14C]arginine hydrolysis by AII was detected in microdissected tubules and the 14CO2 released from [14C]urea hydrolysis was quantified. The results of these experiments showed that: (1) both AII mRNA and protein were highly expressed in the deep cortex and the outer stripe of the outer medulla, (2) urea was produced mainly in the proximal straight tubules (PST), (3) the 38-kDa AII protein was more abundant in the mitochondria than the cytosol, and (4) the renal content of AII mRNA and protein was about three-fold higher in female than in male mice. In conclusion, in both genders, AII gene expression is restricted to the PST and localized into mitochondria. AII gene is differentially expressed in the kidney of female and male mice since higher levels of AII mRNA, protein and activity were observed in the kidneys of the former than those of the latter. Renal AII gene expression was gender-dependent in mice but not in rats. Finally, in the PST of females, L-arginine-derived ornithine may be a precursor for the renal production of L -glutamate and L-glutamine because high levels of AII, ornithine aminotransferase and glutamine synthetase are expressed in this nephron segment.

Production of arginine by the kidney is impaired in a model of sepsis: early events following LPS

Lipopolysaccharide (LPS) is used experimentally to elicit the innate physiological responses observed in human sepsis. We have previously shown that LPS causes depletion of plasma arginine before inducible nitric oxide synthase (iNOS) activity, indicating that changes in arginine uptake and/or production rather than enhanced consumption are responsible. Because the kidney is the primary source of circulating arginine and renal failure is a hallmark of septicemia, we determined the time course of changes in arginine metabolism and kidney function relative to iNOS expression. LPS given intravenously to anesthetized rats caused a decrease in mean arterial blood pressure after 120 min that coincided with increased plasma nitric oxide end products (NOx) and iNOS expression in lung and liver. Interestingly, impairment of renal function preceded iNOS activity by 30–60 min and occurred in tandem with decreased renal arginine production. The baseline rate of renal arginine production was ∼60 μmol·h−1·kg−1, corresponding to an apparent plasma half-life of ∼20 min, and decreased by one-half within 60 min of LPS. Calculations based on the systemic production and clearance show that normally only 5% of kidney arginine output is destined to become nitric oxide and that <25% of LPS-impaired renal production was converted to NOx in the first 4 h. In addition, we provide novel observations indicating that the kidney appears refractory to iNOS induction by LPS because no discernible enhancement of renal NOx production occurred within 4 h, and iNOS expression in the kidney was muted compared with that in liver or lung. These studies demonstrate that the major factor responsible for the rapid decrease in extracellular arginine content following LPS is impaired production by the kidney, a phenomenon that appears linked to reduced renal perfusion.

Dogmas and controversies in the handling of nitrogenous wastes: expression of arginase Type I and II genes in rainbow trout: influence of fasting on liver enzyme activity and mRNA levels in juveniles

Through analysis of a cDNA library and third-party annotation of available database sequences, we characterized the full-length coding regions of rainbow trout (Oncorhynchus mykiss) Type I, Onmy-ARG01, and Type II, Onmy-ARG02, arginase genes. Two partial related arginase sequences, Onmy-ARG01b and Onmy-ARG02b, and a full-length zebrafish arginase coding region (Danio rerio), Dare-ARG02, are also reported. Comparison of vertebrate arginase sequences shows that both Type I and Type II genes in bony fishes contain a mitochondrial targeting N-terminal domain. This suggests that the cytosolic Type I arginase found in ureotelic vertebrates arose in the common ancestor of amphibia and mammals. Onmy-ARG01 and Onmy-ARG02 mRNA was detected in liver, kidney, gill, intestine, red muscle and heart tissues. Onmy-ARG01 was expressed at a significantly higher level relative to Onmy-ARG02 in liver and red muscle tissue. To investigate whether there was differential regulation of Onmy-ARG01 and Onmy-ARG02, juvenile trout were fasted for 6 weeks and hepatic enzyme activities and mRNA levels were compared with those of fed control fish. There was a 3-fold increase in liver arginase activity and a 2-fold increase in Onmy-ARG02 mRNA levels but no change in Onmy-ARG01 mRNA levels in fasted fish relative to fed fish. These findings indicate that both types of arginase genes are present and expressed in rainbow trout and that the pattern of expression varies between tissues. The increase in liver arginase activity after a 6-week fast is due, in part, to an increase in the expression of Onmy-ARG02 mRNA levels.

Regulation of arginase expression by T-helper II cytokines and isoproterenol

BACKGROUND

Trauma causes a release of catecholamines, transforming growth factor-beta (TGF-beta), and T-helper II cytokines (TH2). Individually, these substances also induce arginase in macrophages. The purpose of this study was to determine the synergistic interactions between isoproterenol, TGF-beta, and TH2 cytokines on arginase expression in macrophages.

METHODS

Confluent RAW 264.7 macrophages were incubated with various combinations of interleukins 4, 10, and 13 (IL-4, IL-10, IL-13), and TGF-beta with isoproterenol over 48 hours. Arginase activity, as well as arginase I expression by Western blot and reverse transcriptase-polymerase chain reaction, were measured.

RESULTS

Although isoproterenol, IL-4, IL-10, and IL-13 individually induced arginase, significant synergy between the combination of isoproterenol with either TGF-beta or the TH2 cytokines was observed. All cytokines except IL-10 also induced arginase I protein and mRNA. Arginase II protein was detected in cells exposed to IL-10.

CONCLUSIONS

We conclude that isoproterenol synergizes with IL-4, IL-13, and TGF-beta to increase arginase I mRNA and protein, as well as arginase activity in RAW 264.7 macrophages. Further, IL-10 synergizes with isoproterenol to increase arginase activity and arginase II protein. These synergistic mechanisms may compete with nitric oxide synthase for l-arginine substrate, thus shunting away available arginine from nitric oxide production and contributing to cellular immunosuppression observed after trauma.

Ornithine metabolism in male and female rat kidney: mitochondrial expression of ornithine aminotransferase and arginase II

In the kidney, l-ornithine is reabsorbed along the proximal convoluted tubule (PCT), transported by basolateral carriers, and produced by arginase II (AII). Here, the renal metabolic fate of l-ornithine was analyzed in male and female rats. Kidneys and renal zones were dissected and used for Western blot analysis, immunofluorescence, and electron microscopic studies. Ornithine aminotransferase (OAT) and AII were localized using specific antibodies. Ornithine oxidation was determined by incubating microdissected tubules with l-[1-14C] or l-[U-14C]ornithine in the presence or absence of energy-providing substrates. Ornithine decarboxylase (ODC) mRNAs were localized by in situ hybridization. The 48-kDa OAT protein was detected in male and female kidneys, but its level was fourfold higher in the latter. OAT relative distribution increased from the superficial cortex toward the outer medulla to reach its highest level. Almost all OAT protein was localized in cortical and medullary proximal straight tubules (CPST and OSPST, respectively). In proximal straight tubule (PST), AII protein distribution overlapped that of OAT. No gender difference in AII protein level was found. OAT and AII were colocalized within PST mitochondria. l-[1-14C]ornithine decarboxylation occurred in all tubules, but predominantly in proximal tubules. l-[1-14C]ornithine decarboxylation was enhanced when l-[1-14C]ornithine was given to tubules as the sole substrate. The use of l-[U-14C]ornithine demonstrated the complete oxidation of ornithine. In conclusion, the OAT gene was expressed more in female rat proximal tubules than in male. Because OAT and AII proteins overlapped in PST mitochondria, l-arginine-derived ornithine may be preferentially converted to l-glutamate, as proven by ornithine oxidation. However, the coexpression of ODC, glutamate decarboxylase, and glutamine synthetase in PST suggests that l-ornithine can also be metabolized to putrescine, GABA, and l-glutamine. The fate of l-ornithine may depend on the cellular context.

Differential gene expression in the kidney of sickle cell transgenic mice: upregulated genes

The S+S-Antilles transgenic mouse used in this study has renal defects similar to those seen in sickle cell anemia patients: congested glomeruli, medullary fibrosis, renal enlargement, vasoocclusion, and a urine concentrating defect. We used gene expression microarrays to identify genes highly up-regulated in the kidneys of these mice and validated their expression by real-time PCR. Kidney hypoxia, as demonstrated by the presence of deoxyhemoglobin, was detected by blood oxygen dependent magnetic resonance imaging (BOLD-MRI). Some of the up-regulated genes included cytochrome P450 4a14, glutathione-S-transferase alpha-1, mitochondrial hydroxymethylglutaryl CoA synthase, cytokine inducible SH-2 containing protein, retinol dehydrogenase type III, arginase II, glycolate oxidase, Na/K ATPase, renin-1, and alkaline phosphatase 2. An increase in enzyme activity was also demonstrated for one of the up-regulated genes (arginase II). These genes can be integrated into several different pathophysiological processes: a hypoxia cascade, a replacement cascade, or an ameliorating cascade, one or all of which may explain the phenotype of this disease. We conclude that microarray technology is a powerful tool to identify genes involved in renal disease in sickle cell anemia and that the identification of various metabolic pathways may open new avenues for therapeutic interventions.

Argininosuccinate synthetase from the urea cycle to the citrulline-NO cycle

Argininosuccinate synthetase (ASS, EC 6.3.4.5) catalyses the condensation of citrulline and aspartate to form argininosuccinate, the immediate precursor of arginine. First identified in the liver as the limiting enzyme of the urea cycle, ASS is now recognized as a ubiquitous enzyme in mammalian tissues. Indeed, discovery of the citrulline-NO cycle has increased interest in this enzyme that was found to represent a potential limiting step in NO synthesis. Depending on arginine utilization, location and regulation of ASS are quite different. In the liver, where arginine is hydrolyzed to form urea and ornithine, the ASS gene is highly expressed, and hormones and nutrients constitute the major regulating factors: (a) glucocorticoids, glucagon and insulin, particularly, control the expression of this gene both during development and adult life; (b) dietary protein intake stimulates ASS gene expression, with a particular efficiency of specific amino acids like glutamine. In contrast, in NO-producing cells, where arginine is the direct substrate in the NO synthesis, ASS gene is expressed at a low level and in this way, proinflammatory signals constitute the main factors of regulation of the gene expression. In most cases, regulation of ASS gene expression is exerted at a transcriptional level, but molecular mechanisms are still poorly understood.

Upregulation of arginase expression in wound-derived fibroblasts

BACKGROUND

Wound-derived fibroblasts (WFBs) are phenotypically different from normal dermal fibroblasts (NFBs). We have previously shown that the wound phenotype correlates with expression of the inducible isoform of nitric oxide synthase (iNOS) in fibroblasts. l-Arginine is the sole substrate for iNOS. Arginase is an alternative pathway of l-arginine metabolism in wounds. To clarify the role of l-arginine in wound healing, we investigated arginase expression and activity in WFB.

METHODS

Male Lewis rats underwent dorsal skin incisions and subcutaneous PVA sponge implantation. WFBs were harvested from sponges retrieved at different days postimplantation. Normal fibroblasts were obtained from uninjured skin by an explant technique. Arginase activity was measured by newly formed urea (nmol/min/mg protein) and protein expression was detected by Western blotting using specific antibodies for type I (AI) and type II (AII). The effect of transforming growth factor beta1 (TGF-beta1), interleukin-4 (IL-4), lipopolysaccharide, and wound fluid on arginase activity was also investigated.

RESULTS

WFB arginase activity was significantly elevated compared with NFB activity at all times postwounding. This was paralleled by increased AI protein expression by Western blotting. AII was not detectable. TGF-beta and IL-4 significantly increased arginase activity and protein expression whereas lipopolysaccharide and wound fluid did not affect it.

CONCLUSIONS

The upregulation of the arginase expression in WFB underlines the distinct regulation of l-arginine metabolism in WFBs. Further work is needed to elucidate the functional implications.

Decreased expression of arginase II in the kidneys of Dahl salt-sensitive rats

Arginase catalyzes the hydrolysis of arginine to urea and ornithine. Urea is not only an important solute for concentrating urine but also inhibits Na-K-2Cl cotransport. To elucidate the roles of arginase in the development of salt-sensitive hypertension, we examined arginase activity and expression in the kidney and other organs of Dahl/Rapp salt-sensitive (SS) and salt-resistant (SR) rats before and after 4 weeks' administration of a 4% NaCl or control diet. At 4 weeks of age, arginase activity in the kidney was lower in SS rats than in SR rats. Kidney arginase activity was lower in SS rats than in SR rats at 8 weeks of age, and salt loading did not alter arginase activity. Arginase II (the dominant isoform in the kidney) mRNA and protein in the kidney of salt-loaded SS rats were also lower than those of salt-loaded SR rats. Arginase activities in the liver and cerebellum did not differ between SS and SR rats. To examine the effect of urea, the product of arginase reaction, on the development of hypertension, SS rats were given a 4% NaCl diet containing 5% kaolin or 5% urea. Six-week urea supplementation attenuated the development of hypertension in SS rats. These findings suggest that decreased arginase expression in the kidney may be at least partially responsible for the salt-sensitive hypertension in SS rats.

Induction of arginase II in livers of bile duct-ligated rats

Nitric oxide (NO) has been implicated in playing a role in liver cirrhosis, but the regulatory mechanisms are still unclear. As arginase shares a common substrate with NO synthase (NOS), the aim of this study was to investigate the expression of arginase I and II in cirrhotic liver. Liver cirrhosis was induced in rats by chronic bile duct ligation (BDL). Controls were sham-operated. Competitive polymerase chain reaction was performed to assay the expression of messenger RNA of arginase I and II. Protein expression was detected by immunohistochemistry and western-blotting. The level of arginine in plasma was lower in BDL rats, while the ornithine level in plasma was correspondingly higher (r= -0.96, P<0.0001). Arginase I messenger RNA was reduced significantly in BDL rats (3.34+/-0.32 vs. 1.32+/-0.21 x 10(4) attomole/microg of total RNA, sham vs. BDL, P<0.001), as well as arginase I protein. In contrast, arginase II mRNA was induced in the livers of BDL rats, with negligible expression in controls (0.35+/-0.11 vs. 3.64+/-0.54 attomole/microg of total RNA, sham vs. BDL, P<0.001). Arginase II protein was localized in some hepatocytes and hyperplastic bile ductular epithelial cells of cirrhotic livers but not in control livers. In conclusion, arginase II was induced in BDL livers, while the expression of arginase I was down-regulated. These data suggest that arginase I and II are regulated differently and may have different functions in the livers of BDL rats. Reduction of arginase I in BDL livers may be responsible for the lowering of arginine levels in the plasma, while induction of arginase II could be important in regulating NO synthesis as well as other important mechanisms involved in liver cirrhosis.

Differential regulation of nitric oxide synthase-2 and arginase-1 by type 1/type 2 cytokines in vivo: granulomatous pathology is shaped by the pattern of L-arginine metabolism

Type 2 cytokines regulate fibrotic liver pathology in mice infected with Schistosoma mansoni. Switching the immune response to a type 1-dominant reaction has proven highly effective at reducing the pathologic response. Activation of NOS-2 is critical, because type 1-deviated/NO synthase 2 (NOS-2)-deficient mice completely fail to control their response. Here, we demonstrate the differential regulation of NOS-2 and arginase type 1 (Arg-1) by type 1/type 2 cytokines in vivo and for the first time show a critical role for arginase in the pathogenesis of schistosomiasis. Using cytokine-deficient mice and two granuloma models, we show that induction of Arg-1 is type 2 cytokine dependent. Schistosome eggs induce Arg-1, while Mycobacterium avium-infected mice develop a dominant NOS-2 response. IFN-gamma suppresses Arg-1 activity, because type 1 polarized IL-4/IL-10-deficient, IL-4/IL-13-deficient, and egg/IL-12-sensitized animals fail to up-regulate Arg-1 following egg exposure. Notably, granuloma size decreases in these type-1-deviated/Arg-1-unresponsive mice, suggesting an important regulatory role for Arg-1 in schistosome egg-induced pathology. To test this hypothesis, we administered difluoromethylornithine to block ornithine-aminodecarboxylase, which uses the product of arginine metabolism, L-ornithine, to generate polyamines. Strikingly, granuloma size and hepatic fibrosis increased in the ornithine-aminodecarboxylase-inhibited mice. Furthermore, we show that type 2 cytokine-stimulated macrophages produce proline under strict arginase control. Together, these data reveal an important regulatory role for the arginase biosynthetic pathway in the regulation of inflammation and demonstrate that differential activation of Arg-1/NOS-2 is a critical determinant in the pathogenesis of granuloma formation.

Arginine metabolism and the synthesis of nitric oxide in the nervous system

The biochemistry and physiology of L-arginine have to be reconsidered in the light of the recent discovery that the amino acid is the only substrate of all isoforms of nitric oxide synthase (NOS). Generation of nitric oxide, NO, a versatile molecule in signaling processes and unspecific immune defense, is intertwined with synthesis, catabolism and transport of arginine which thus ultimately participates in the regulation of a fine-tuned balance between normal and pathophysiological consequences of NO production. The complex composition of the brain at the cellular level is reflected in a complex differential distribution of the enzymes of arginine metabolism. Argininosuccinate synthetase (ASS) and argininosuccinate lyase which together can recycle the NOS coproduct L-citrulline to L-arginine are expressed constitutively in neurons, but hardly colocalize with each other or with NOS in the same neuron. Therefore, trafficking of citrulline and arginine between neurons necessitates transport capacities in these cells which are fulfilled by well-described carriers for cationic and neutral amino acids. The mechanism of intercellular exchange of argininosuccinate, a prerequisite also for its proposed function as a neuromodulator, remains to be elucidated. In cultured astrocytes transcription and protein expression of arginine transport system y(+) and of ASS are upregulated concomittantly with immunostimulant-mediated induction of NOS-2. In vivo ASS-immunoreactivity was found in microglial cells in a rat model of brain inflammation and in neurons and glial cells in the brains of Alzheimer patients. Any attempt to estimate the contributions of arginine transport and synthesis to substrate supply for NOS has to consider competition for arginine between NOS and arginase, the latter enzyme being expressed as mitochondrial isoform II in nervous tissue. Generation of NOS inhibitors agmatine and methylarginines is documented for the nervous system. Suboptimal supply of NOS with arginine leads to production of detrimental peroxynitrite which may result in neuronal cell death. Data have been gathered recently which point to a particular role of astrocytes in neural arginine metabolism. Arginine appears to be accumulated in astroglial cells and can be released after stimulation with a variety of signals. It is proposed that an intercellular citrulline-NO cycle is operating in brain with astrocytes storing arginine for the benefit of neighbouring cells in need of the amino acid for a proper synthesis of NO.