Tissue-selective expression of enzymes of arginine synthesis

Intracellular arginine-dependent translation sensor reveals the dynamics of arginine starvation response and resistance in ASS1-negative cells

BACKGROUND

Many cancers silence the metabolic enzyme argininosuccinate synthetase 1 (ASS1), the rate-limiting enzyme for arginine biosynthesis within the urea cycle. Consequently, ASS1-negative cells are susceptible to depletion of extracellular arginine by PEGylated arginine deiminase (ADI-PEG20), an agent currently being developed in clinical trials. As the primary mechanism of resistance to arginine depletion is re-expression of ASS1, we sought a tool to understand the temporal emergence of the resistance phenotype at the single-cell level.

METHODS

A real-time, single-cell florescence biosensor was developed to monitor arginine-dependent protein translation. The versatile, protein-based sensor provides temporal information about the metabolic adaptation of cells, as it is able to quantify and track individual cells over time.

RESULTS

Every ASS1-deficient cell analyzed was found to respond to arginine deprivation by decreased expression of the sensor, indicating an absence of resistance in the naïve cell population. However, the temporal recovery and emergence of resistance varied widely amongst cells, suggesting a heterogeneous metabolic response. The sensor also enabled determination of a minimal arginine concentration required for its optimal translation.

CONCLUSIONS

The translation-dependent sensor developed here is able to accurately track the development of resistance in ASS1-deficient cells treated with ADI-PEG20. Its ability to track single cells over time allowed the determination that resistance is not present in the naïve population, as well as elucidating the heterogeneity of the timing and extent of resistance. This tool represents a useful advance in the study of arginine deprivation, while its design has potential to be adapted to other amino acids.

In-vivo evaluation of human recombinant Co-arginase against A375 melanoma xenografts

Metastatic melanoma is a deadly form of cancer with few therapeutic options and the cause of more than 9480 deaths annually in the USA alone. Novel treatment options for this disease are urgently needed. Here we test the efficacy of a novel melanoma drug, the human recombinant Co-arginase (CoArgIPEG), against an aggressive A375 melanoma mouse model. CoArgIPEG is a modification of the naturally occurring human enzyme with improved stability, catalytic activity, and potentially lower immunogenicity compared with current amino acid-depleting drugs. Marked tumor growth reductions (mean P=0.0057) with apoptosis induction and proliferation inhibition are noted with CoArgIPEG treatment, both in the presence and in the absence of supplemental citrulline. Further, improved therapeutic efficacy has been noted against A375 xenografts relative to the naturally occurring human recombinant arginase enzyme at lower doses of CoArgIPEG. Unfortunately, after 1 month, half of the relapsing tumors showed argininosuccinate synthase induction, which correlated with Ser62-phosphorylated cMyc. Although argininosuccinate synthase induction could not be induced in vitro, a drug targeting pathway previously demonstrated to be associated with Ser62 cMyc phosphorylation - U0126 - in combination with CoArgIPEG demonstrated an in-vitro synergistic response (combination indices 0.13±0.10 and 0.14±0.10 with or without citrulline, respectively). Overall, favorable efficacy and potential synergy with other antimelanoma drugs support CoArgIPEG as a potent, novel cancer therapeutic.

Correlations of plasma citrulline levels with clinical and endoscopic score and blood markers according to small bowel involvement in pediatric Crohn disease

OBJECTIVE

Several studies have indicated that plasma citrulline levels reflect the extent of mucosal injury of the small intestine. This study was performed to determine whether plasma citrulline levels correlate with the disease activity in pediatric patients with Crohn disease (CD).

METHODS

A total of 63 CD and 23 ulcerative colitis (UC) patients were included in this study. Disease severity was assessed by pediatric CD activity index (PCDAI), pediatric UC activity index, simplified endoscopic activity score for CD, C-reactive protein (CRP), and erythrocyte sedimentation rate (ESR). The correlations among these variables and plasma citrulline levels were evaluated. We performed subgroup analysis whether correlations between plasma citrulline levels and disease activity depend on small bowel involvement in patients with CD.

RESULTS

The plasma citrulline levels correlated negatively with CRP (r = -0.332, P = 0.008), ESR (r = -0.290, P = 0.022), and PCDAI (r = -0.424, P = 0.001) in patients with CD. The plasma citrulline levels were significantly lower in patients with jejunal involvement than in those without (P = 0.027). In subgroup analysis, patients with CD with jejunal involvement showed significantly negative correlations of plasma citrulline levels with CRP (r = -0.628, P = 0.016) and PCDAI (r = -0.632, P = 0.015); however, patients with CD without jejunal involvement revealed no correlations of plasma citrulline levels with CRP and PCDAI. There were no significant correlations between plasma citrulline levels and simplified endoscopic activity score for CD. There were no significant correlations of plasma citrulline levels with CRP, ESR, and pediatric UC activity index in patients with UC.

CONCLUSIONS

Plasma citrulline levels correlated with disease severity as measured by PCDAI, CRP, and ESR in pediatric patients with CD with jejunal involvement.

Arginase-1: a new immunohistochemical marker of hepatocytes and hepatocellular neoplasms

The distinction of hepatocellular carcinoma (HCC) from metastatic tumor in the liver often presents a diagnostic challenge that carries significant impact on prognostication and therapy. The number of diagnostically useful immunohistochemical markers of hepatocytes is limited to hepatocyte paraffin antigen (HepPar-1), polyclonal carcinoembryonic antigen, and CD10, with alpha-fetoprotein and glypican-3 labeling HCCs. Arginase-1 (Arg-1) is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of arginine to ornithine and urea. We used immunohistochemistry to compare the sensitivity of Arg-1 to that of HepPar-1 in 151 HCCs. We found that the overall sensitivities of Arg-1 and HepPar-1 are 96.0% and 84.1%, respectively. The sensitivities of Arg-1 in well, moderately, and poorly differentiated HCCs are 100%, 96.2%, and 85.7%, respectively, whereas, in comparison, HepPar-1 demonstrated sensitivities of 100%, 83.0%, and 46.4% for well, moderately, and poorly differentiated tumors, respectively. There were no HCCs in our study that were reactive for HepPar-1 but nonreactive for Arg-1. We also examined Arg-1 expression in nonhepatocellular tumors, including many that are potential mimics of HCC (renal cell carcinomas, neuroendocrine tumors, melanomas, gastric adenocarcinomas, and adrenocortical carcinomas) and found that only 2 non-HCC tumors were reactive for Arg-1. Arg-1 represents a sensitive and specific marker of benign and malignant hepatocytes that may ultimately prove to be a useful diagnostic tool in routine surgical pathology practice.

The human neonatal small intestine has the potential for arginine synthesis; developmental changes in the expression of arginine-synthesizing and -catabolizing enzymes

Background

Milk contains too little arginine for normal growth, but its precursors proline and glutamine are abundant; the small intestine of rodents and piglets produces arginine from proline during the suckling period; and parenterally fed premature human neonates frequently suffer from hypoargininemia. These findings raise the question whether the neonatal human small intestine also expresses the enzymes that enable the synthesis of arginine from proline and/or glutamine. Carbamoylphosphate synthetase (CPS), ornithine aminotransferase (OAT), argininosuccinate synthetase (ASS), arginase-1 (ARG1), arginase-2 (ARG2), and nitric-oxide synthase (NOS) were visualized by semiquantitative immunohistochemistry in 89 small-intestinal specimens.

Results

Between 23 weeks of gestation and 3 years after birth, CPS- and ASS-protein content in enterocytes was high and then declined to reach adult levels at 5 years. OAT levels declined more gradually, whereas ARG-1 was not expressed. ARG-2 expression increased neonatally to adult levels. Neurons in the enteric plexus strongly expressed ASS, OAT, NOS1 and ARG2, while varicose nerve fibers in the circular layer of the muscularis propria stained for ASS and NOS1 only. The endothelium of small arterioles expressed ASS and NOS3, while their smooth-muscle layer expressed OAT and ARG2.

Conclusion

The human small intestine acquires the potential to produce arginine well before fetuses become viable outside the uterus. The perinatal human intestine therefore resembles that of rodents and pigs. Enteral ASS behaves as a typical suckling enzyme because its expression all but disappears in the putative weaning period of human infants.

Citrulline as a biomarker of intestinal failure due to enterocyte mass reduction

BACKGROUND & AIMS

In human, citrulline (plasma concentration about 40 micromol/L) is an amino acid involved in intermediary metabolism and that is not incorporated in proteins. Circulating citrulline is mainly produced by enterocytes of the small bowel. For this reason plasma or serum citrulline concentration has been proposed as a biomarker of remnant small bowel mass and function. This article reviews this concept and its metabolic basis.

METHODS

Conditions in which there is a significantly reduced small bowel enterocyte mass and function and a plasma or serum citrulline were measured in adults and children. These studies included patients with a short bowel syndrome, villous atrophy states, Crohn's disease, during monitoring of digestive toxicity of chemotherapy and radiotherapy or follow-up of patients after small bowel transplantation.

RESULTS

In all these situations, with more than 500 studied patients a decreased level of plasma citrulline correlated with the reduced enterocyte mass independently of nutritional and inflammatory status. A close correlation between small bowel remnant length and citrullinemia was found. In addition, diagnosis of intestinal failure was assessed through plasma citrulline levels in severe small bowel diseases in which there is a marked enterocyte mass reduction.

DISCUSSION

The threshold for establishing a diagnosis of intestinal failure is lower in villous atrophy disease (10mumol/L) than in short bowel syndrome (20mumol/L). Compromised renal function is an important factor when considering plasma citrulline levels as a marker of intestinal failure as this potentially can increase circulating citrulline values.

CONCLUSIONS

Reduced plasma citrulline levels are an innovative quantitative biomarker of significantly reduced enterocyte mass and function in different disease states in humans.

Liver transplantation for argininosuccinic aciduria: clinical, biochemical, and metabolic outcome

We report successful liver transplantation in a young adult with argininosuccinic aciduria but without cirrhosis. Plasma amino acid profile normalized and brain magnetic resonance spectroscopy indicated improved metabolism after transplantation. The general well-being of the patient and his quality of life improved. We suggest that orthotopic liver transplantation should be considered for patients with argininosuccinic aciduria even in the absence of cirrhosis, with the aim of correcting (at least in part) central nervous system metabolism, thereby preventing further neurological deterioration.

Simultaneous determination of L-arginine and 12 molecules participating in its metabolic cycle by gradient RP-HPLC method: application to human urine samples

We have developed and described a highly sensitive, accurate and precise reversed-phase high-performance liquid chromatography (RP-HPLC) method for the simultaneous determination of L-arginine and 12 molecules participating in its metabolic cycle in human urine samples. After pre-column derivatization with ortho-phthaldialdehyde (OPA) reagent containing 3-mercaptopropionic acid (3MPA), the fluorescent derivatives were separated by a gradient elution and detected by fluorescence measurement at 338 nm (excitation) and 455 nm (emission). L-Arginine (ARG) and its metabolites: L-glutamine (GLN), N(G)-hydroxy-L-arginine (NOHA), L-citrulline (CIT), N(G)-monomethyl-L-arginine (NMMA), L-homoarginine (HARG), asymmetric N(G),N(G)-dimethyl-L-arginine (ADMA), symmetric N(G),N(G')-dimethyl-L-arginine (SDMA), L-ornithine (ORN), putrescine (PUT), agmatine (AGM), spermidine (SPERMD) and spermine (SPERM) were extracted in a cation-exchange solid-phase extraction (SPE) column and after derivatization separated in a Purospher STAR RP-18e analytical column. The calibration curves of analysed compounds are linear within the range of concentration: 45-825, 0.2-15, 16-225, 12-285, 0.1-32, 15-235, 0.1-12, 0.1-12, 10-205, 0.02-12, 0.1-24, 0.01-10 and 0.01-8 nmol mL(-1) for GLN, NOHA, CIT, ARG, NMMA, HARG, ADMA, SDMA, ORN, PUT, AGM, SPERMD and SPERM, respectively. The correlation coefficients are greater than 0.9980. Coefficients of variation are not higher than 6.0% for inter-day precision. The method has been determined or tested for limits of detection and quantification, linearity, precision, accuracy and recovery. All detection parameters of the method demonstrate that it is a reliable and efficient means of the comprehensive determination of ARG and its 12 main metabolites, making this approach suitable for routine clinical applications. The levels of analysed compounds in human urine can be successfully determined using this developed method with no matrix effect.

Hyperammonemia-induced toxicity for the developing central nervous system

In pediatric patients, hyperammonemia can be caused by various acquired or inherited disorders such as urea cycle deficiencies or organic acidemias. The brain is much more susceptible to the deleterious effects of ammonium during development than in adulthood. Hyperammonemia can provoke irreversible damages to the developing central nervous system that lead to cortical atrophy, ventricular enlargement and demyelination, responsible for cognitive impairment, seizures and cerebral palsy. Until recently, the mechanisms leading to these irreversible cerebral damages were poorly understood. Using experimental models allowing the analysis of the neurotoxic effects of ammonium on the developing brain, these last years have seen the emergence of new clues showing that ammonium exposure alters several amino acid pathways and neurotransmitter systems, as well as cerebral energy metabolism, nitric oxide synthesis, oxidative stress, mitochondrial permeability transition and signal transduction pathways. Those alterations may explain neuronal loss and impairment of axonal and dendritic growth observed in the different models of congenital hyperammonemia. Some neuroprotective strategies such as the potential use of NMDA receptor antagonists, nitric oxide inhibitors, creatine and acetyl-l-carnitine have been suggested to counteract these toxic effects. Unraveling the molecular mechanisms involved in the chain of events leading to neuronal dysfunction under hyperammonemia may be useful to develop new potential strategies for neuroprotection.

Functional consequences of mitochondrial proteome heterogeneity

Potential functional consequences of the differences in protein distribution between the mitochondria of the rat liver, heart, brain, and kidney, as determined in the companion paper in this issue (Johnson DT, French S, Blair PV, You JS, Bemis KG, Wang M, Harris RA, and Balaban RS. The tissue heterogeneity of the mammalian mitochondrial proteome. Am J Physiol Cell Physiol292: C689-C697, 2006), were analyzed using a canonical metabolic pathway approach as well as a functional domain homology analysis. These data were inserted into the Kyoto Encyclopedia of Genes and Genomes pathway framework to give global and metabolic pathway-specific information on the impact of the differential protein distribution on mitochondrial function. Custom pathway analysis was also performed using pathways limited to the mitochondrion. With the use of this approach, several well-known functional differences between these mitochondrial populations were confirmed. These included GABA metabolism in the brain, urea synthesis in the liver, and the domination of oxidative phosphorylation in the heart. By comparing relative protein amounts of mitochondria across tissues, a greater understanding of functional emphasis is possible as well as the nuclear "programming" required to enhance a given function within the mitochondria. For proteins determined to be mitochondrial and lacking a defined role functional domain BLAST analyses were performed. Several proteins associated with DNA structural modification and a novel CoA transferase were identified. A protein was also identified capable of catalyzing the first three steps of de novo pyrimidine synthesis. This analysis demonstrates that the distribution of nuclear encoded proteins significantly modifies the overall functional emphasis of the mitochondria to meet tissue-specific needs. These studies demonstrate the existence of mitochondrial biochemical functions that at present are poorly defined.