Arginine deiminase: a potential inhibitor of angiogenesis and tumour growth

Rational design to improve the catalytic efficiency and stability of arginine deiminase

Arginine deiminase (ADI) has garnered significant interest because of its ability to objectively eradicate cancer cells and produce L-citrulline. To meet the production demands, this study focused on enhancing the enzyme activity and thermal stability of ADI. In this study, 24 ADI mutants were obtained through computer aid site-specific mutation in the ADI of Enterobacter faecalis. Notably, the specific enzyme activities of F44W, N163P, E220I, E220L, N318E, A336G, T340I, and N382F increased, reaching 1.33-2.53 times that of the original enzyme. This study confirmed that site-specific mutations are critical for optimizing enzyme function. Additionally, the F44W, N163P, E220I, T340I, and A336G mutants demonstrated good thermal stability. The optimal pH for mutant F44W increased to 8, whereas mutants E220I, I244V, A336G, T340I, and N328F maintained an optimal pH of 7.5. Conversely, the M109L, N163P, E220L, I244L, and N318E mutants shad an optimal pH of 7. This study revealed that mutant enzymes with increased activity were more likely to contain mutation sites situated near the four loops associated with catalytic residues, whereas mutations at the dimer junction sites had a higher tendency to enhance enzyme stability. These findings contribute to the development of ADI industrial applications and its modifications.

Serum Metabolomics of Retinoblastoma: Assessing the Differential Serum Metabolic Signatures of Unilateral and Bilateral Patients

Retinoblastoma (Rb) is the most common pediatric eye cancer. To identify the biomarkers for early diagnosis and monitoring the progression of Rb in patients, mapping of the alterations in their metabolic profiles is essential. The present study aims at exploring the metabolic disparity in serum from Rb patients and controls using NMR-based metabolomics. A total of 72 metabolites, including carbohydrates, amino acids, and organic acids, were quantified in serum samples from 24 Rb patients and 26 controls. Distinct clusters of Rb patients and controls were obtained using the partial least-squares discriminant analysis (PLS-DA) model. Further, univariate and multivariate analyses of unilateral and bilateral Rb patients with respect to their age-matched controls depicted their distinct metabolic fingerprints. Metabolites including 2-phosphoglycerate, 4-aminobutyrate, proline, O-phosphocholine, O-phosphoethanolamine, and Sn-glycero-3-phosphocholine (Sn-GPC) showed significant perturbation in both unilateral and bilateral Rb patients. However, metabolic differences among the bilateral Rb cases were more pronounced than those in unilateral Rb cases with respect to controls. In addition to major discriminatory metabolites for Rb, unilateral and bilateral Rb cases showed specific metabolic changes, which might be the result of their differential genetic/somatic mutational backgrounds. This further suggests that the aberrant metabolic perturbation in bilateral patients signifies the severity of the disease in Rb patients. The present study demonstrated that identified serum metabolites have potential to serve as a noninvasive method for detection of Rb, discriminate bilateral from unilateral Rb patients, and aid in better understanding of the RB tumor biology.

Streptococcal arginine deiminase regulates endothelial inflammation, mTOR pathway and autophagy

Endothelial cells (EC) are active participants in the inflammation process. During the infection, the change in endothelium properties provides the leukocyte infiltrate formation and restrains pathogen dissemination due to coagulation control. Pathogenic microbes are able to change the endothelium properties and functions in order to invade the bloodstream and disseminate in the host organism. Arginine deiminase (ADI), a bacterial arginine-hydrolyzing enzyme, which causes the amino acid deficiency, important for endothelium biology. Previous research implicates altered metabolism of arginine in the development of endothelial dysfunction and inflammation. It was shown that arginine deficiency, as well as overabundance affects the balance of mechanical target of rapamycin (mTOR)/S6 kinase (S6K) pathway, arginase and endothelial nitric oxide synthase (eNOS) resulted in reactive oxygen species (ROS) production and EC activation. ADI creating a deficiency of arginine can interfere cellular arginine-dependent processes. Thus, this study was aimed at investigation of the influence of streptococcal ADI on the metabolism and inflammations of human umbilical vein endothelial cells (HUVEC). The action of ADI was studied by comparing the effect Streptococcus pyogenes M49-16 paternal strain expressing ADI and its isogenic mutant M49-16delArcA with the inactivated gene ArcA. Based on comparison of the parental and mutant strain effects, it can be concluded, that ADI suppressed mTOR signaling pathway and enhanced autophagy. The processes failed to return to the basic level with arginine supplement. Our study also demonstrates that ADI suppressed endothelial proliferation, disrupted actin cytoskeleton structure, increased phospho-NF-κB p65, CD62P, CD106, CD54, CD142 inflammatory molecules expression, IL-6 production and lymphocytes-endothelial adhesion. In spite of the ADI-mediated decrease in arginine concentration in the cell-conditioned medium, the enzyme enhanced the production of nitric oxide in endothelial cells. Arginine supplementation rescued proliferation, actin cytoskeleton structure, brought NO production to baseline and prevented EC activation. Additional evidence for the important role of arginine bioavailability in the EC biology was obtained. The results allow us to consider bacterial ADI as a pathogenicity factor that can potentially affect the functions of endothelium.

Predictive markers for efficiency of the amino-acid deprivation therapies in cancer

Amino acid deprivation therapy (AADT) is a promising strategy for developing novel anticancer treatments, based on variations in metabolism of healthy and malignant cells. L-asparaginase was the first amino acid-degrading enzyme that received FDA approval for the treatment of acute lymphoblastic leukemia (ALL). Arginase and arginine deiminase were effective in clinical trials for the treatment of metastatic melanomas and hepatocellular carcinomas. Essential dependence of certain cancer cells on methionine explains the anticancer efficacy of methionine-g-lyase. Along with significant progress in identification of metabolic vulnerabilities of cancer cells, new amino acid-cleaving enzymes appear as promising agents for cancer treatment: lysine oxidase, tyrosine phenol-lyase, cysteinase, and phenylalanine ammonia-lyase. However, sensitivity of specific cancer cell types to these enzymes differs. Hence, search for prognostic and predictive markers for AADT and introduction of the markers into clinical practice are of great importance for translational medicine. As specific metabolic pathways in cancer cells are determined by the enzyme expression, some of these enzymes may define the sensitivity to AADT. This review considers the known predictors for efficiency of AADT, emphasizing the importance of knowledge on cancer-specific amino acid significance for such predictions.

Targeting amino acid metabolism in cancer

Metabolic rewiring is a characteristic hallmark of cancer cells. This phenomenon sustains uncontrolled proliferation and resistance to apoptosis by increasing nutrients and energy supply. However, reprogramming comes together with vulnerabilities that can be used against tumor and can be applied in targeted therapy. In the last years, the genetic background of tumors has been identified thoroughly and new therapies targeting those mutations tested. Nevertheless, we propose that targeting the phenotype of cancer cells could be another way of treatment aiming to avoid drug resistance and non-responsiveness of cancer patients. Amino acid metabolism is part of the altered processes in cancer cells. Amino acids are building blocks and also sensors of signaling pathways regulating main biological processes. In this comprehensive review, we described four amino acids (asparagine, arginine, methionine, and cysteine) which have been actively investigated as potential targets for anti-tumor therapy. Asparagine depletion is successfully used for decades in the treatment of acute lymphoblastic leukemia and there is a strong implication to apply it to other types of tumors. Arginine auxotrophic tumors are great candidates for arginine-starvation therapy. Higher requirement for essential amino acids such as methionine and cysteine point out promising targetable weaknesses of cancer cells.

Synergy effects of copper and L-arginine on osteogenic, angiogenic, and antibacterial activities

Copper (Cu) ions have been found to exert antibacterial and angiogenic effects. However, some studies have indicated that it inhibits osteogenesis at high concentrations. On the other hand, L-arginine (Arg) is a semi-essential amino acid required for various biological processes, including osteogenic and angiogenic activities. As a result, we hypothesized that combining Arg with Cu ions would reduce its inhibitory effects on osteogenesis while increasing its angiogenic and antibacterial capabilities. To assess osteogenic and angiogenic activities, we employed rat bone marrow mesenchymal stem cells (MSCs) and human umbilical vein endothelial cells (HUVECs), respectively. The gram-positive bacteria Staphylococcus epidermidis (S. epidermidis), Staphylococcus aureus (S. aureus), and the gram-negative bacterium Escherichia coli (E. coli) were used to investigate bacterial behaviors. According to ALP activity and calcium deposition outcomes, copper ions inhibited osteogenic development of MSCs at 100 µM; however, Arg supplementation somewhat mitigated the inhibitory effects. Furthermore, Copper and Arg synergistically stimulated migration and tube formation of HUVECs. According to our findings, copper ions and Arg in the range of 1-100 µM had no antibacterial effect on any examined bacteria. However, at a dose of 20 mM, copper demonstrated antibacterial activity, which was boosted by Arg. Overall, these findings suggest that a combination of copper and Arg may be more beneficial for bone regeneration than either copper or Arg alone.

Clinical significance of plasma-free amino acids and tryptophan metabolites in patients with non-small cell lung cancer receiving PD-1 inhibitor: a pilot cohort study for developing a prognostic multivariate model

Background

Amino acid metabolism is essential for tumor cell proliferation and regulation of immune cell function. However, the clinical significance of free amino acids (plasma-free amino acids (PFAAs)) and tryptophan-related metabolites in plasma has not been fully understood in patients with non-small cell lung cancer (NSCLC) who receive immune checkpoint inhibitors.

Methods

We conducted a single cohort observational study. Peripheral blood samples were collected from 53 patients with NSCLC before treatment with PD-1 (Programmed cell death-1) inhibitors. The plasma concentrations of 21 PFAAs, 14 metabolites, and neopterin were measured by liquid chromatography–mass spectrometry. Using Cox hazard analysis with these variables, a multivariate model was established to stratify patient overall survival (OS). Gene expression in peripheral blood mononuclear cells (PBMCs) was compared between the high-risk and low-risk patients by this multivariate model.

Results

On Cox proportional hazard analysis, higher concentrations of seven PFAAs (glycine, histidine, threonine, alanine, citrulline, arginine, and tryptophan) as well as lower concentrations of three metabolites (3h-kynurenine, anthranilic acid, and quinolinic acid) and neopterin in plasma were significantly correlated with better OS (p<0.05). In particular, the multivariate model, composed of a combination of serine, glycine, arginine, and quinolinic acid, could most efficiently stratify patient OS (concordance index=0.775, HR=3.23, 95% CI 2.04 to 5.26). From the transcriptome analysis in PBMCs, this multivariate model was significantly correlated with the gene signatures related to immune responses, such as CD8 T-cell activation/proliferation and proinflammatory immune responses, and 12 amino acid-related genes were differentially expressed between the high-risk and low-risk groups.

Conclusions

The multivariate model with PFAAs and metabolites in plasma might be useful for stratifying patients who will benefit from PD-1 inhibitors.

Investigation of the Lethal Effect of Purified Arginine Deiminase Purified from Lactobacillus plantarum p5 on Murine Mammary Adenocarcinoma and Vero cell Lines

Cancer is one of the most serious diseases facing humanity; accordingly, it is urgent to find a cure that is rarely harmful to the patient as much as possible. It has been approved that arginine deiminase (ADI) can hydrolyze the plasma arginine to citrulline. This hydrolysis activity and reduction in the amount of intercellular arginine suppress lipopolysaccharide-induced nitric oxide synthesis. On the other hand, arginine depletion arrests the cell cycle at the G1 phase; therefore, ADI has been considered a powerful anticancer agent. The current study aimed to investigate the lethal effects of ADI purified from the Lactobacillus plantarum p5 strain on murine mammary adenocarcinoma and Vero cell lines. Anti-proliferative activity of ADI against murine mammary adenocarcinoma) AMN3) cell line was evaluated after different incubation times (3, 6, 24, 48, and 72 h) of exposure to 1 µg/mL of ADI, compared to Vero (non-cancer cell line) transformed cell line with same conditions. The autophagy process in cancer cells was recognized after three hours of incubation with ADI which was clearly observed in the AMN3 cell line under an inverted microscope. The first stages of the programmed cell death (apoptosis) pathway were only observed in AMN3 cells after 24 h of incubation with ADI, and this process continued with the time until they reached the last stages of apoptosis after 72 h of incubation. The results of the current study showed that the AMN3 cell line was auxotrophic for arginine because it could not produce it in the presence of enzyme which had a robust activity to kill these cancer cells; however, Vero non-cancer cell line survived in the presence of ADI because it had the ability to produce arginine.

Metabolomics Analyses of Mouse Retinas in Oxygen-Induced Retinopathy

Purpose

Retinal neovascularization is a severe pathological process leading to irreversible blindness. This study aims to identify the altered metabolites and their related pathways that are involved in retinal neovascularization.

Methods

To reveal the global metabolomic profile change in the retinal neovascularization process, an untargeted metabolomics analysis of oxygen-induced retinopathy (OIR) mice retinas was carried out first, followed by the validation of amino acids and their derivatives through a targeted metabolomics analysis. The involved pathways were predicted by bioinformatic analysis.

Results

By untargeted metabolomics, a total of 58 and 49 metabolites altered significantly in OIR retinas under cationic and anionic modes, respectively. By bioinformatics analysis, “ABC transporters,” “central carbon metabolism in cancer.” and “alanine, aspartate, and glutamate metabolism” were the most enriched Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways associated with the changed metabolites. By targeted metabolomics, no significant change was found in the assessed amino acids and their derivatives at postnatal day (P) 12, whereas significantly altered amino acids and their derivatives were recognized at P13, P17, and P42 in OIR retinas.

Conclusions

The metabolomic profile was significantly altered in the neovascularized retinas. In particular, numerous amino acids and their derivatives were significantly changed in OIR retinas. These altered metabolites, together with their associated pathways, might be involved in the pathogenesis of retinal neovascular diseases.

The prognostic and therapeutic implications of distinct patterns of argininosuccinate synthase 1 (ASS1) and arginase-2 (ARG2) expression by cancer cells and tumor stroma in non-small-cell lung cancer

Background

Arginine (Arg) is essential for cancer cell growth and also for the activation of T cells. Thus, therapies aiming to reduce Arg utilization by cancer may prove detrimental for the immune response.

Methods

We examined the expression of two major enzymes involved in arginine depletion and replenishment, namely arginase ARG2 and argininosuccinate synthase ASS1, respectively, in a series of 98 NSCLCs. Their association with immune infiltrates and the postoperative outcome were also studied.

Results

ARG2 was expressed mainly by cancer-associated fibroblasts (CAFs) (58/98 cases; 59.2%), while ASS1 by cancer cells (75/98 cases; 76.5%). ASS1 and ARG2 expression patterns were not related to hypoxia markers. Auxotrophy, implied by the lack of expression of ASS1 in cancer cells, was associated with high angiogenesis (p < 0.02). ASS1 expression by cancer cells was associated with a high density of iNOS-expressing tumor-infiltrating lymphocytes (^iNOS+TILs). ARG2 expression by CAFs was inversely related to the TIL-density and linked with poorer prognosis (p = 0.02). Patients with ASS1 expression by cancer cells had a better prognosis especially when CAFs did not express ARG2 (p = 0.004).

Conclusions

ARG2 and ASS1 enzymes are extensively expressed in NSCLC stroma and cancer cells, respectively. Auxotrophic tumors have a poor prognosis, potentially by utilizing Arg, thus reducing Arg-dependent TIL anti-tumor activity. ASS1 expression in cancer cells would allow Arg fueling of ^iNOS+TILs and enhance anti-tumor immunity. However, upregulation of ARG2 in CAFs may divert Arg from TILs, allowing immune escape. Identification of these three distinct phenotypes may be useful in the individualization of Arg-targeting therapies and immunotherapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40170-021-00264-7.

Enhancing the Effect of Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand Signaling and Arginine Deprivation in Melanoma

Melanoma as a very aggressive type of cancer is still in urgent need of improved treatment. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and arginine deiminase (ADI-PEG20) are two of many suggested drugs for treating melanoma. Both have shown anti-tumor activities without harming normal cells. However, resistance to both drugs has also been noted. Studies on the mechanism of action of and resistance to these drugs provide multiple targets that can be utilized to increase the efficacy and overcome the resistance. As a result, combination strategies have been proposed for these drug candidates with various other agents, and achieved enhanced or synergistic anti-tumor effect. The combination of TRAIL and ADI-PEG20 as one example can greatly enhance the cytotoxicity to melanoma cells including those resistant to the single component of this combination. It is found that combination treatment generally can alter the expression of the components of cell signaling in melanoma cells to favor cell death. In this paper, the signaling of TRAIL and ADI-PEG20-induced arginine deprivation including the main mechanism of resistance to these drugs and exemplary combination strategies is discussed. Finally, factors hampering the clinical application of both drugs, current and future development to overcome these hurdles are briefly discussed.

The Mechanisms of L-Arginine Metabolism Disorder in Endothelial Cells

L-arginine is a key metabolite for nitric oxide production by endothelial cells, as well as signaling molecule of the mTOR signaling pathway. mTOR supports endothelial cells homeostasis and regulates activity of L-arginine-metabolizing enzymes, endothelial nitric oxide synthase, and arginase II. Disruption of the L-arginine metabolism in endothelial cells leads to the development of endothelial dysfunction. Conflicting results of the use of L-arginine supplement to improve endothelial function reveals a controversial role of the amino acid in the endothelial cell biology. The review is aimed at analysis of the current data on the role of L-arginine metabolism in the development of endothelial dysfunction.

Mono-PEGylation of a Thermostable Arginine-Depleting Enzyme for the Treatment of Lung Cancer

L-arginine (L-Arg) depletion induced by randomly PEGylated arginine deiminase (ADI-PEG20) can treat arginosuccinate synthase (ASS)-negative cancers, and ADI-PEG20 is undergoing phase III clinical trials. Unfortunately, ASS-positive cancers are resistant to ADI-PEG20. Moreover, the yield of ADI production is low because of the formation of inclusion bodies. Here, we report a thermostable arginine-depleting enzyme, Bacillus caldovelox arginase mutant (BCA-M: Ser¹⁶¹->Cys¹⁶¹). An abundant amount of BCA-M was easily obtained via high cell-density fermentation and heat treatment purification. Subsequently, we prepared BCA-M-PEG20, by conjugating a single 20 kDa PEG monomer onto the Cys¹⁶¹ residue via thio-chemistry. Unlike ADI-PEG20, BCA-M-PEG20 significantly inhibited ASS-positive lung cancer cell growth. Pharmacodynamic studies showed that a single intraperitoneal injection (i.p). administration of 250 U/mouse of BCA-M-PEG20 induced low L-Arg level over 168 h. The mono-PEGylation of BCA-M prolonged its elimination half-life from 6.4 to 91.4 h (a 14-fold increase). In an A549 lung cancer xenograft model, a weekly administration of 250 U/mouse of BCA-M-PEG20 suppressed tumor growth significantly. We also observed that BCA-M-PEG20 did not cause any significant safety issue in mouse models. Overall, BCA-M-PEG20 showed excellent results in drug production, potency, and stability. Thereby, it has great potential to become a promising candidate for lung cancer therapy.

Targeting extracellular nutrient dependencies of cancer cells

BACKGROUND

Cancer cells rewire their metabolism to meet the energetic and biosynthetic demands of their high proliferation rates and environment. Metabolic reprogramming of cancer cells may result in strong dependencies on nutrients that could be exploited for therapy. While these dependencies may be in part due to the nutrient environment of tumors, mutations or expression changes in metabolic genes also reprogram metabolic pathways and create addictions to extracellular nutrients.

SCOPE OF REVIEW

This review summarizes the major nutrient dependencies of cancer cells focusing on their discovery and potential mechanisms by which metabolites become limiting for tumor growth. We further detail available therapeutic interventions based on these metabolic features and highlight opportunities for restricting nutrient availability as an anti-cancer strategy.

MAJOR CONCLUSIONS

Strategies to limit nutrients required for tumor growth using dietary interventions or nutrient degrading enzymes have previously been suggested for cancer therapy. The best clinical example of exploiting cancer nutrient dependencies is the treatment of leukemia with l-asparaginase, a first-line chemotherapeutic that depletes serum asparagine. Despite the success of nutrient starvation in blood cancers, it remains unclear whether this approach could be extended to other solid tumors. Systematic studies to identify nutrient dependencies unique to individual tumor types have the potential to discover targets for therapy.

Multilevel algorithms and evolutionary hybrid tools for enhanced production of arginine deiminase from Pseudomonas furukawaii RS3

In the present study a high arginine deiminase (ADI) yielding bacterium was isolated from soil samples of Haryana, India and identified as Pseudomonas furukawaii. The specific enzyme activity was optimized to 1.420 IU/ml by OFAT and further enhanced to 2.708 IU/ml (an increase of 90.7%) with the help of statistical parametric optimization approaches using GA-ANN and GA-ANFIS. The obtained value of the coefficient of correlation (R = 0.88) for ANN and epoch error (0.12) for ANFIS, indicates the prediction accuracy and strength of these data training models. ADI production was improved significantly in simple super broth media supplemented with 1.5% fructose and 1.75% arginine at pH 7 at 37 °C using multilevel algorithms and evolutionary hybrid tools. The native enzyme was partially purified (ten-fold) up to a specific enzyme activity of 29.559 IU/mg.

Exploiting Arginine Auxotrophy with Pegylated Arginine Deiminase (ADI-PEG20) to Sensitize Pancreatic Cancer to Radiotherapy via Metabolic Dysregulation

Distinct metabolic vulnerabilities of cancer cells compared with normal cells can potentially be exploited for therapeutic targeting. Deficiency of argininosuccinate synthetase-1 (ASS1) in pancreatic cancers creates auxotrophy for the semiessential amino acid arginine. We explored the therapeutic potential of depleting exogenous arginine via pegylated arginine deiminase (ADI-PEG20) treatment as an adjunct to radiotherapy. We evaluated the efficacy of treatment of human pancreatic cancer cell lines and xenografts with ADI-PEG20 and radiation via clonogenic assays and tumor growth delay experiments. We also investigated potential mechanisms of action using reverse-phase protein array, Western blotting, and IHC and immunofluorescence staining. ADI-PEG20 potently radiosensitized ASS1-deficient pancreatic cancer cells (MiaPaCa-2, Panc-1, AsPc-1, HPAC, and CaPan-1), but not ASS1-expressing cell lines (Bxpc3, L3.6pl, and SW1990). Reverse phase protein array studies confirmed increased expression of proteins related to endoplasmic reticulum (ER) stress and apoptosis, which were confirmed by Western blot analysis. Inhibition of ER stress signaling with 4-phenylbutyrate abrogated the expression of ER stress proteins and reversed radiosensitization by ADI-PEG20. Independent in vivo studies in two xenograft models confirmed significant tumor growth delays, which were associated with enhanced expression of ER stress proteins and apoptosis markers and reduced expression of proliferation and angiogenesis markers. ADI-PEG20 augmented the effects of radiation by triggering the ER stress pathway, leading to apoptosis in pancreatic tumor cells.

Amino Acid Degrading Enzymes and their Application in Cancer Therapy

BACKGROUND

Amino acids are essential components in various biochemical pathways. The deprivation of certain amino acids is an antimetabolite strategy for the treatment of amino acid-dependent cancers which exploits the compromised metabolism of malignant cells. Several studies have focused on the development and preclinical and clinical evaluation of amino acid degrading enzymes, namely L-asparaginase, L-methionine γ-lyase, L-arginine deiminase, L-lysine α-oxidase. Further research into cancer cell metabolism may therefore define possible targets for controlling tumor growth.

OBJECTIVE

The purpose of this review was to summarize recent progress in the relationship between amino acids metabolism and cancer therapy, with a particular focus on Lasparagine, L-methionine, L-arginine and L-lysine degrading enzymes and their formulations, which have been successfully used in the treatment of several types of cancer.

METHODS

We carried out a structured search among literature regarding to amino acid degrading enzymes. The main aspects of search were in vitro and in vivo studies, clinical trials concerning application of these enzymes in oncology.

RESULTS

Most published research are on the subject of L-asparaginase properties and it's use for cancer treatment. L-arginine deiminase has shown promising results in a phase II trial in advanced melanoma and hepatocellular carcinoma. Other enzymes, in particular Lmethionine γ-lyase and L-lysine α-oxidase, were effective in vitro and in vivo.

CONCLUSION

The findings of this review revealed that therapy based on amino acid depletion may have the potential application for cancer treatment but further clinical investigations are required to provide the efficacy and safety of these agents.

Expression and Characterization of Serotype 2 Streptococcus suis Arginine Deiminase

BACKGROUND

Arginine deiminase (ArcA) has been speculated to facilitate the intracellular survival of Streptococcus suis under acidic conditions. However, the physical and biological properties and function of SS2-ArcA have not yet been elucidated.

METHODS

Recombinant SS2-ArcA (rSS2-ArcA) was expressed and purified using Ni-NTA affinity chromatography. Under various pH and temperature conditions, the enzymatic properties of purified rSS2-ArcA and crude native SS2-ArcA were determined.

RESULTS

The SS2-arcA-deduced amino acid sequence contained a conserved catalytic triad (Cys399-His273-Glu218). The optimum temperature and pH of 47-kDa rSS2-ArcA and crude native SS2-ArcA were 42°C and pH 7.2. The rSS2-ArcA and crude native SS2-ArcA were stable for 3 h at 4 and 25°C, respectively. The pH stability and dependency tests suggested that rSS2-ArcA and crude native SS2-ArcA were functionally active in acidic conditions. The L-arginine substrate binding affinity (Km) values of rSS2-ArcA (specific activity 16.00 U/mg) and crude native SS2-ArcA (specific activity 0.23 U/mg) were 0.058 and 0.157 mM, respectively. rSS2-ArcA exhibited a weak binding affinity with the common ArcA inhibitors L-canavanine and L-NIO. Furthermore, the partial inactivation of SS2-ArcA significantly impaired the viability and growth of SS2 at pH 4.0, 6.0, and 7.5.

CONCLUSIONS

This study profoundly demonstrated the involvement of ArcA enzymatic activity in S. suis survival under acidic conditions.

Mitochondria in endothelial cells: Sensors and integrators of environmental cues

The involvement of angiogenesis in disease and its potential as a therapeutic target have been firmly established over recent decades. Endothelial cells (ECs) are central elements in vessel homeostasis and regulate the passage of material and cells into and out of the bloodstream. EC proliferation and migration are modified by alterations to mitochondrial biogenesis and dynamics resulting from several signals and environmental cues, such as oxygen, hemodynamics, and nutrients. As intermediary signals, mitochondrial ROS are released as important downstream modulators of the expression of angiogenesis-related genes. In this review, we discuss the physiological actions of these signals and aberrant responses during vascular disorders.

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Mitochondria in EC act as integrators of environmental cues.

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Circulating signals modify mitochondrial dynamics, altering EC phenotype.

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ROS release by EC mitochondria regulates expression of vascular genes.

Arginine dependence of tumor cells: targeting a chink in cancer's armor

Arginine, one among the 20 most common natural amino acids, has a pivotal role in cellular physiology as it is being involved in numerous cellular metabolic and signaling pathways. Dependence on arginine is diverse for both tumor and normal cells. Because of decreased expression of argininosuccinate synthetase and/or ornithine transcarbamoylase, several types of tumor are auxotrophic for arginine. Deprivation of arginine exploits a significant vulnerability of these tumor cells and leads to their rapid demise. Hence, enzyme-mediated arginine depletion is a potential strategy for the selective destruction of tumor cells. Arginase, arginine deiminase and arginine decarboxylase are potential enzymes that may be used for arginine deprivation therapy. These arginine catabolizing enzymes not only reduce tumor growth but also make them susceptible to concomitantly administered anti-cancer therapeutics. Most of these enzymes are currently under clinical investigations and if successful will potentially be advanced as anti-cancer modalities.

Biochemical and biological activity of arginine deiminase from Streptococcus pyogenes M22

Streptococcus pyogenes (group A Streptococcus; GAS) is an important gram-positive extracellular bacterial pathogen responsible for a number of suppurative infections. This micro-organism has developed complex virulence mechanisms to avoid the host’s defenses. We have previously reported that SDSC from GAS type M22 causes endothelial-cell dysfunction, and inhibits cell adhesion, migration, metabolism, and proliferation in a dose-dependent manner, without affecting cell viability. This work aimed to isolate and characterize a component from GAS type M22 supernatant that suppresses the proliferation of endothelial cells (EA.hy926). In the process of isolating a protein possessing antiproliferative activity we identified arginine deiminase (AD). Further study showed that this enzyme is most active at pH 6.8. Calculating Km and Vmax gave the values of 0.67 mmol·L–1 and 42 s−1, respectively. A distinctive feature of AD purified from GAS type M22 is that its optimum activity and the maximal rate of the catalytic process is close to neutral pH by comparison with enzymes from other micro-organisms. AD from GAS type M22 suppressed the proliferative activity of endothelial cells in a dose-dependent mode. At the same time, in the presence of AD, the proportion of cells in G0/G1 phase increased. When l-Arg was added at increasing concentrations to the culture medium containing AD (3 μg·mL–1), the enzyme’s capacity to inhibit cell proliferation became partially depressed. The proportion of cells in phases S/G2 increased concomitantly, although the cells did not fully recover their proliferation activity. This suggests that AD from GAS type M22 has potential for the suppression of excessive cell proliferation.

Arginine deiminase augments the chemosensitivity of argininosuccinate synthetase-deficient pancreatic cancer cells to gemcitabine via inhibition of NF-κB signaling

Background

Pancreatic cancer is a leading cause of cancer-related deaths in the world with a 5-year survival rate of less than 6%. Currently, there is no successful therapeutic strategy for advanced pancreatic cancer, and new effective strategies are urgently needed. Recently, an arginine deprivation agent, arginine deiminase, was found to inhibit the growth of some tumor cells (i.e., hepatocellular carcinoma, melanoma, and lung cancer) deficient in argininosuccinate synthetase (ASS), an enzyme used to synthesize arginine. The purpose of this study was to evaluate the therapeutic efficacy of arginine deiminase in combination with gemcitabine, the first line chemotherapeutic drug for patients with pancreatic cancer, and to identify the mechanisms associated with its anticancer effects.

Methods

In this study, we first analyzed the expression levels of ASS in pancreatic cancer cell lines and tumor tissues using immunohistochemistry and RT-PCR. We further tested the effects of the combination regimen of arginine deiminase with gemcitabine on pancreatic cancer cell lines in vitro and in vivo.

Results

Clinical investigation showed that pancreatic cancers with reduced ASS expression were associated with higher survivin expression and more lymph node metastasis and local invasion. Treatment of ASS-deficient PANC-1 cells with arginine deiminase decreased their proliferation in a dose- and time-dependent manner. Furthermore, arginine deiminase potentiated the antitumor effects of gemcitabine on PANC-1 cells via multiple mechanisms including induction of cell cycle arrest in the S phase, upregulation of the expression of caspase-3 and 9, and inhibition of activation of the NF-κB survival pathway by blocking NF-κB p65 signaling via suppressing the nuclear translocation and phosphorylation (serine 536) of NF-κB p65 in vitro. Moreover, arginine deiminase can enhance antitumor activity of gemcitabine-based chemotherapy in the mouse xenograft model.

Conclusions

Our results suggest that arginine deprivation by arginine deiminase, in combination with gemcitabine, may offer a novel effective treatment strategy for patients with pancreatic cancer and potentially improve the outcome of patients with pancreatic cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-686) contains supplementary material, which is available to authorized users.

Targeting arginine-dependent cancers with arginine-degrading enzymes: opportunities and challenges

Arginine deprivation is a novel antimetabolite strategy for the treatment of arginine-dependent cancers that exploits differential expression and regulation of key urea cycle enzymes. Several studies have focused on inactivation of argininosuccinate synthetase 1 (ASS1) in a range of malignancies, including melanoma, hepatocellular carcinoma (HCC), mesothelial and urological cancers, sarcomas, and lymphomas. Epigenetic silencing has been identified as a key mechanism for loss of the tumor suppressor role of ASS1 leading to tumoral dependence on exogenous arginine. More recently, dysregulation of argininosuccinate lyase has been documented in a subset of arginine auxotrophic glioblastoma multiforme, HCC and in fumarate hydratase-mutant renal cancers. Clinical trials of several arginine depletors are ongoing, including pegylated arginine deiminase (ADI-PEG20, Polaris Group) and bioengineered forms of human arginase. ADI-PEG20 is furthest along the path of clinical development from combinatorial phase 1 to phase 3 trials and is described in more detail. The challenge will be to identify tumors sensitive to drugs such as ADI-PEG20 and integrate these agents into multimodality drug regimens using imaging and tissue/fluid-based biomarkers as predictors of response. Lastly, resistance pathways to arginine deprivation require further study to optimize arginine-targeted therapies in the oncology clinic.

Role of endothelial cell metabolism in vessel sprouting

Endothelial cells (ECs) are quiescent for years but can plastically switch to angiogenesis. Vascular sprouting relies on the coordinated activity of migrating tip cells at the forefront and proliferating stalk cells that elongate the sprout. Past studies have identified genetic signals that control vascular branching. Prominent are VEGF, activating tip cells, and Notch, which stimulates stalk cells. After the branch is formed and perfused, ECs become quiescent phalanx cells. Now, emerging evidence has accumulated indicating that ECs not only adapt their metabolism when switching from quiescence to sprouting but also that metabolism regulates vascular sprouting in parallel to the control by genetic signals.

Negative argininosuccinate synthetase expression in melanoma tumours may predict clinical benefit from arginine-depleting therapy with pegylated arginine deiminase

Background:

Arginine-depleting therapy with pegylated arginine deiminase (ADI-PEG20) was reported to have activity in advanced melanoma in early phase I–II trial, and clinical trials are currently underway in other cancers. However, the optimal patient population who benefit from this treatment is unknown.

Methods:

Advanced melanoma patients with accessible tumours had biopsy performed before the start of treatment with ADI-PEG20 and at the time of progression or relapse when amenable to determine whether argininosuccinate synthetase (ASS) expression in tumour was predictive of response to ADI-PEG20.

Results:

Twenty-seven of thirty-eight patients treated had melanoma tumours assessable for ASS staining before treatment. Clinical benefit rate (CBR) and longer time to progression were associated with negative expression of tumour ASS. Only 1 of 10 patients with ASS-positive tumours (ASS+) had stable disease, whereas 4 of 17 (24%) had partial response and 5 had stable disease, when ASS expression was negative (ASS−), giving CBR rates of 52.9 vs 10%, P=0.041. Two responding patients with negative ASS expression before therapy had rebiopsy after tumour progression and the ASS expression became positive. The survival of ASS− patients receiving at least four doses at 320 IU m⁻² was significantly better than the ASS+ group at 26.5 vs 8.5 months, P=0.024.

Conclusion:

ADI-PEG20 is safe and the drug is only efficacious in melanoma patients whose tumour has negative ASS expression. Argininosuccinate synthetase tumour positivity is associated with drug resistance and tumour progression.

Arginine deiminase modulates endothelial tip cells via excessive synthesis of reactive oxygen species

ADI (arginine deiminase), an enzyme that hydrolyses arginine, has been reported as an anti-angiogenesis agent. However, its molecular mechanism is unclear. We have demonstrated for the first time that ADI modulates the angiogenic activity of endothelial tip cells. By arginine depletion, ADI disturbs actin filament in endothelial tip cells, causing disordered migratory direction and decreased migration ability. Furthermore, ADI induces excessive synthesis of ROS (reactive oxygen species), and activates caspase 8-, but not caspase 9-, dependent apoptosis in endothelial cells. These findings provide a novel mechanism by which ADI inhibits tumour angiogenesis through modulating endothelial tip cells.

Arginine deprivation and argininosuccinate synthetase expression in the treatment of cancer

Arginine, a semi-essential amino acid in humans, is critical for the growth of human cancers, particularly those marked by de novo chemoresistance and a poor clinical outcome. In addition to protein synthesis, arginine is involved in diverse aspects of tumour metabolism, including the synthesis of nitric oxide, polyamines, nucleotides, proline and glutamate. Tumoural downregulation of the enzyme argininosuccinate synthetase (ASS1), a recognised rate-limiting step in arginine synthesis, results in an intrinsic dependence on extracellular arginine due to an inability to synthesise arginine for growth. This dependence on extracellular arginine is known as arginine auxotrophy. Several tumours are arginine auxotrophic, due to variable loss of ASS1, including hepatocellular carcinoma, malignant melanoma, malignant pleural mesothelioma, prostate and renal cancer. Importantly, targeting extracellular arginine for degradation in the absence of ASS1 triggers apoptosis in arginine auxotrophs. Several phase I/II clinical trials of the arginine-lowering drug, pegylated arginine deiminase, have shown encouraging evidence of clinical benefit and low toxicity in patients with ASS1-negative tumours. In part, ASS1 loss is due to epigenetic silencing of the ASS1 promoter in various human cancer cell lines and tumours, and it is this silencing that confers arginine auxotrophy. In relapsed ovarian cancer, this is associated with platinum refractoriness. In contrast, several platinum sensitive tumours, including primary ovarian, stomach and colorectal cancer, are characterised by ASS1 overexpression, which is regulated by proinflammatory cytokines. This review examines the prospects for novel approaches in the prevention, diagnosis and treatment of malignant disease based on ASS1 pathophysiology and its rate-limiting product, arginine.

Phase II Study of Pegylated Arginine Deiminase for Nonresectable and Metastatic Hepatocellular Carcinoma

Purpose

It is well known that hepatocellular carcinoma (HCC) is an arginine auxotroph due to argininosuccinate synthetase I deficiency. This study's purpose was to evaluate the effects of pegylated arginine deiminase (ADI) in terms of toxicity, tumor response, α-fetoprotein (AFP) levels, and serum arginine levels.

Patients and Methods

Eighty patients were randomly assigned to receive either 80 IU/m2 or 160 IU/m2 of ADI weekly for up to 6 months. Adverse events, serum arginine, AFP levels, and antibody production against ADI were measured on a regular basis. In addition, disease response and time to progression according to the Response Evaluation Criteria in Solid Tumors (RECIST) and survival rates were evaluated.

Results

Four patients were excluded from the survival analysis because they developed exclusion criteria after randomization, but before first treatment. The number of patients in the two cohorts were similar (n = 37 in the low-dose cohort, n = 39 in the high-dose cohort). Mean (±SE) survival for all subjects was 15.8 months (474 days ± 39 days) from time of diagnosis of unresectable disease. Arginine levels remained below baseline for 50 days while antibodies against ADI reached a plateau at approximately the same time. There were no deaths attributed to ADI treatment. Only two patients were withdrawn for immunogenic-related adverse events. Grade 2, 3, or 4 toxicities were recorded in 92, 19, and 0 patients, respectively.

Conclusion

Pegylated ADI is a promising drug that capitalizes on a significant enzymatic deficiency in HCC. It is safe, well tolerated, and may benefit patients with unresectable HCC.

The Chick Embryo Chorioallantoic Membrane as an In Vivo Assay to Study Antiangiogenesis

Antiangiogenesis, e.g., inhibition of blood vessel growth, is being investigated as a way to prevent the growth of tumors and other angiogenesis-dependent diseases. Pharmacological inhibition interferes with the angiogenic cascade or the immature neovasculature with synthetic or semi-synthetic substances, endogenous inhibitors or biological antagonists.The chick embryo chorioallantoic membrane (CAM) is an extraembryonic membrane, which serves as a gas exchange surface and its function is supported by a dense capillary network. Because its extensive vascularization and easy accessibility, CAM has been used to study morphofunctional aspects of the angiogenesis process in vivo and to study the efficacy and mechanism of action of pro- and anti-angiogenic molecules. The fields of application of CAM in the study of antiangiogenesis, including our personal experience, are illustrated in this review article.

Reduced argininosuccinate synthetase is a predictive biomarker for the development of pulmonary metastasis in patients with osteosarcoma

Pulmonary metastasis is the most significant prognostic determinant for osteosarcoma, but methods for its prediction and treatment have not been established. Using oligonucleotide microarrays, we compared the global gene expression of biopsy samples between seven osteosarcoma patients who developed pulmonary metastasis within 4 years after neoadjuvant chemotherapy and curative resection, and 12 patients who did not relapse. We identified argininosuccinate synthetase (ASS) as a gene differentially expressed with the highest statistical significance (Welch's t test, P = 2.2 x 10(-5)). Immunohistochemical analysis of an independent cohort of 62 osteosarcoma cases confirmed that reduced expression of ASS protein was significantly correlated with the development of pulmonary metastasis after surgery (log-rank test, P < 0.05). Cox regression analysis revealed that ASS was the sole significant predictive factor (P = 0.039; hazard ratio, 0.319; 95% confidence interval, 0.108-0.945). ASS is one of the enzymes required for the production of a nonessential amino acid, arginine. We showed that osteosarcoma cells lacking ASS expression were auxotrophic for arginine and underwent G(0)-G(1) arrest in arginine-free medium, suggesting that an arginine deprivation therapy could be effective in patients with osteosarcoma. Recently, phase I and II clinical trials in patients with melanoma and hepatocellular carcinoma have shown the safety and efficacy of plasma arginine depletion by stabilized arginine deiminase. Our data indicate that in patients with osteosarcoma, reduced expression of ASS is not only a novel predictive biomarker for the development of metastasis, but also a potential target for pharmacologic intervention.

Growth inhibition of human melanoma cells by a recombinant arginine deiminase expressed in Escherichia coli

We have cloned the arginine deiminase (ADI) gene from Mycoplasma hominis PG21 genomic DNA by polymerase chain reaction, and changed four TGA tryptophan codons (stop codon in E. coli) to TGG codons in the coding region by site-directed mutagenesis in order to express in E. coli. The recombinant ADI (rADI) was purified to apparent homogeneity by Ni-affinity chromatography after extraction from inclusion bodies followed by refolding. The rADI expressed in E. coli was estimated to be 50 kDa. Dimeric forms of rADI exerted enzymatic activity. We found that high concentration of potassium dihydrogenphosphate (PDP) and L-arginine addition in refolding reaction increases the enzyme activity. The specific activity of rADl was calculated as 0.618 U/mg. In addition, the enzyme activity of purified rADI remained for at least one month in 100 mM PDP solution (pH 6.5), but diminished within one week in 100 mM PDP solution (pH 7.4). Anti-tumor activity of the purified rADI was estimated to be 0.036 U/ml as 50% growth inhibitory activity against human melanoma cell line G-361.

PEG conjugates in clinical development or use as anticancer agents: an overview

During the almost forty years of PEGylation, several antitumour agents, either proteins, peptides or low molecular weight drugs, have been considered for polymer conjugation but only few entered clinical phase studies. The results from the first clinical trials have shared and improved the knowledge on biodistribution, clearance, mechanism of action and stability of a polymer conjugate in vivo. This has helped to design conjugates with improved features. So far, most of the PEG conjugates comprise of a protein, which in the native form has serious shortcomings that limit the full exploitation of its therapeutic action. The main issues can be short in vivo half-life, instability towards degrading enzymes or immunogenicity. PEGylation proved to be effective in shielding sensitive sites at the protein surface, such as antigenic epitopes and enzymatic degradable sequences, as well as in prolonging the drug half-life by decreasing the kidney clearance. In this review PEG conjugates of proteins or low molecular weight drugs, in clinical development or use as anticancer agents, will be taken into consideration. In the case of PEG-protein derivatives the most represented are depleting enzymes, which act by degrading amino acids essential for cancer cells. Interestingly, PEGylated conjugates have been also considered as adjuvant therapy in many standard anticancer protocols, in this regard the case of PEG-G-CSF and PEG-interferons will be presented.

Arginine deiminase originating from Lactococcus lactis ssp. lactis American Type Culture Collection (ATCC) 7962 induces G1-phase cell-cycle arrest and apoptosis in SNU-1 stomach adenocarcinoma cells

There are multiple lines of evidence that lactic acid bacteria (LAB) exert cancer-preventive effects. However, the underlying mechanisms are poorly understood. In the present study we found that the cytoplasmic fraction of Lactococcus lactis ssp. lactis American Type Culture Collection (ATCC) 7962 exerted the strongest antiproliferative effects (half-maximal inhibitory concentration (IC50) = 17 microg/ml) in SNU-1 human stomach cancer cells and arginine deiminase (ADI; EC 3.5.3.6) activity. We also cloned, expressed and purified ADI from L. lactis ssp. lactis ATCC 7962 (LADI). Both purified ADI from L. lactis (PADI; IC50 = 2 microg/ml) and recombinant ADI originating from LADI (IC50 = 0.6 microg/ml) inhibited the proliferation of SNU-1 cells. LADI induced G0/G1-phase arrest, sub-G1 accumulation, DNA condensation and DNA fragmentation in SNU-1 cells. 4',6-Diamidino-2-phenylindole (DAPI) staining and DNA fragmentation data provide evidence that LADI induces apoptosis in SNU-1 cells. LADI increased the expressions of p53 and p27Kip1, and decreased the expressions of cyclin D1, c-myc and Bcl-xL in SNU-1 cells. However, LADI had no effects on the expressions of p21Cip1 and Bcl-2. Collectively, these data indicate that ADI induces apoptosis and G0/G1-phase arrest of SNU-1 cells, which might contribute to the chemopreventive potential of LAB.

Dependence of leukemic cell proliferation and survival on H2O2 and L-arginine

The proliferation and/or survival of a variety of cells is dependent on cellular hydrogen peroxide (H(2)O(2)) production. We tested whether this was true of leukemic cells, using cell lines from leukemic patients (CEM, 697, Mn-60, and Tanoue). We found that addition of catalase inhibited proliferation of all cell lines and induced death in two. However, this turned out to be due to arginase contamination of the catalase. Pure arginase inhibited cell proliferation and survival, which was reversible by adding L-arginine, demonstrating the L-arginine dependency of these cells. The glutathione peroxidase mimetic ebselen killed the cells by a novel, rapid form of death, preceded by cell blebbing and prevented by N-acetylcysteine, suggesting toxicity is not due to ebselen's antioxidant activity. Addition of N-acetylcysteine to remove endogenous H(2)O(2) stimulated survival and proliferation, suggesting that basal levels of H(2)O(2) promoted cell death. Consistent with this, leukemic cell death was induced by adding as little as 5 microM H(2)O(2). Ascorbic acid, even at 100 microM, induced death through H(2)O(2) production. Thus H(2)O(2) does not promote proliferation and survival, rather the opposite, and previous literature may have misinterpreted the effects of antioxidants. Arginase, H(2)O(2), ascorbic acid, and ebselen might be useful in the treatment of leukemia.

CHR-2797: an antiproliferative aminopeptidase inhibitor that leads to amino acid deprivation in human leukemic cells

CHR-2797 is a novel metalloenzyme inhibitor that is converted into a pharmacologically active acid product (CHR-79888) inside cells. CHR-79888 is a potent inhibitor of a number of intracellular aminopeptidases, including leucine aminopeptidase. CHR-2797 exerts antiproliferative effects against a range of tumor cell lines in vitro and in vivo and shows selectivity for transformed over nontransformed cells. Its antiproliferative effects are at least 300 times more potent than the prototypical aminopeptidase inhibitor, bestatin. However, the mechanism by which inhibition of these enzymes leads to proliferative changes is not understood. Gene expression microarrays were used to profile changes in mRNA expression levels in the human promyelocytic leukemia cell line HL-60 treated with CHR-2797. This analysis showed that CHR-2797 treatment induced a transcriptional response indicative of amino acid depletion, the amino acid deprivation response, which involves up-regulation of amino acid synthetic genes, transporters, and tRNA synthetases. These changes were confirmed in other leukemic cell lines sensitive to the antiproliferative effects of CHR-2797. Furthermore, CHR-2797 treatment inhibited phosphorylation of mTOR substrates and reduced protein synthesis in HL-60 cells, both also indicative of amino acid depletion. Treatment with CHR-2797 led to an increase in the concentration of intracellular small peptides, the substrates of aminopeptidases. It is suggested that aminopeptidase inhibitors, such as CHR-2797 and bestatin, deplete sensitive tumor cells of amino acids by blocking protein recycling, and this generates an antiproliferative effect. CHR-2797 is orally bioavailable and currently undergoing phase II clinical investigation in the treatment of myeloid leukemia.

Arginine deiminase, a potential anti-tumor drug

Arginine deiminase (ADI; EC 3.5.3.6), an arginine-degrading enzyme, has been studied as a potential anti-tumor drug for the treatment of arginine-auxotrophic tumors, such as hepatocellular carcinomas (HCCs) and melanomas. Studies with human lymphatic leukemia cell lines further suggest that ADI is a potential anti-angiogenic agent and is effective in the treatment of leukemia. For instance ADI-PEG-20, patented by Pheonix Pharmacologic Inc., is currently in clinical trials for the treatment of HCC (Phase II/III) and melanoma (Phase I/II). This review summarizes results on recombinant expression, structural analysis, PEG (polyethylene glycerol) modification, in vivo anti-cancer activities, and clinical studies of ADI. Discussions on heterogeneous expression of ADI, directed evolution for improving enzymatic properties, and HSA-fusion for increased in vivo activity conclude this review.

Arginine deprivation as a targeted therapy for cancer

Certain cancers may be auxotrophic for a particular amino acid, and amino acid deprivation is one method to treat these tumors. Arginine deprivation is a novel approach to target tumors which lack argininosuccinate synthetase (ASS) expression. ASS is a key enzyme which converts citrulline to arginine. Tumors which usually do not express ASS include melanoma, hepatocellular carcinoma, some mesotheliomas and some renal cell cancers. Arginine can be degraded by several enzymes including arginine deiminase (ADI). Although ADI is a microbial enzyme from mycoplasma, it has high affinity to arginine and catalyzes arginine to citrulline and ammonia. Citrulline can be recycled back to arginine in normal cells which express ASS, whereas ASS(-) tumor cells cannot. A pegylated form of ADI (ADI-PEG20) has been formulated and has shown in vitro and in vivo activity against melanoma and hepatocellular carcinoma. ADI-PEG20 induces apoptosis in melanoma cell lines. However, arginine deprivation can also induce ASS expression in certain melanoma cell lines which can lead to in vitro drug resistance. Phase I and II clinical trials with ADI-PEG20 have been conducted in patients with melanoma and hepatocellular carcinoma, and antitumor activity has been demonstrated in both cancers. This article reviews our laboratory and clinical experience as well as that from others with ADI-PEG20 as an antineoplastic agent. Future direction in utilizing this agent is also discussed.

In vivo loss of expression of argininosuccinate synthetase in malignant pleural mesothelioma is a biomarker for susceptibility to arginine depletion

PURPOSE

Malignant pleural mesothelioma (MPM) is an increasing health burden on many societies worldwide and, being generally resistant to conventional treatment, has a poor prognosis with a median survival of <1 year. Novel therapies based on the biology of this tumor seek to activate a proapoptotic cellular pathway. In this study, we investigated the expression and biological significance of argininosuccinate synthetase (AS), a rate-limiting enzyme in arginine production.

EXPERIMENTAL DESIGN

Initially, we documented down-regulation of AS mRNA in three of seven MPM cell lines and absence of AS protein in four of seven MPM cell lines. We confirmed that the 9q34 locus, the site of the AS gene, was intact using a 1-Mb comparative genomic hybridization array; however, there was aberrant promoter CpG methylation in cell lines lacking AS expression, consistent with epigenetic regulation of transcription. To investigate the use of AS negativity as a therapeutic target, arginine was removed from the culture medium of the MPM cell lines.

RESULTS

In keeping with the cell line data, 63% (52 of 82) of patients had tumors displaying reduced or absent AS protein, as assessed using a tissue microarray. Cell viability declined markedly in the AS-negative cell lines 2591 and MSTO but not in the AS-positive cell line, 28. This response was apparent by day 4 and maintained by day 9 in vitro. Arginine depletion induced BAX conformation change and mitochondrial inner membrane depolarization selectively in AS-negative MPM cells.

CONCLUSIONS

In summary, we have identified AS negativity as a frequent event in MPM in vivo, leading to susceptibility to cytotoxicity following restriction of arginine. A phase II clinical trial is planned to evaluate the role of arginine depletion in patients with AS-negative MPM.

Identification of a signalling molecule involved in bacterial intergeneric communication

The development of complex multispecies communities such as biofilms is controlled by interbacterial communication systems. We have previously reported an intergeneric communication between two oral bacteria, Streptococcus cristatus and Porphyromonas gingivalis, that results in inhibition of fimA expression. Here, we demonstrate that a surface protein, arginine deiminase (ArcA), of S. cristatus serves as a signal that initiates intergeneric communication. An ArcA-deficient mutant of S. cristatus is unable to communicate with P. gingivalis. Furthermore, arginase activity is not essential for the communication, and ArcA retains the ability to repress expression of fimA in the presence of arginine deiminase inhibitors. These results present a novel mechanism by which intergeneric communication in dental biofilms is accomplished.

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.

Expression, purification, and characterization of arginine deiminase from Lactococcus lactis ssp. lactis ATCC 7962 in Escherichia coli BL21

The arcA gene that encodes arginine deiminase (ADI, EC 3.5.3.6)--a key enzyme of the ADI pathway--was cloned from Lactococcus lactis ssp. lactis ATCC 7962. The deduced amino acid sequence of the arcA gene showed high homology with the arcA gene from Lactobacillus plantarum (99%) and from Lactobacillus sakei (60%), respectively. The arcA gene from Lc. lactis spp. lactis ATCC 7962 was expressed in soluble fraction of recombinant Escherichia coli BL21. ADI produced from Lc. lactis spp. lactis ATCC 7962 (LADI) in E. coli BL21 (DE3) was purified using sequential Q-Sepharose anion exchange and Sephacryl S-200 gel filtration column chromatography. The final yield of LADI in the purification procedure was 63.5%, and the specific activity was 140.27 U/mg. The presence of purified LADI was confirmed by N-terminal sequencing and determination of the molecular mass. The LADI had a molecular mass of about 140 kDa, and comprised a homotrimer of 46 kDa in the native condition. LADI exhibited only 35% amino acid sequence homology with ADI from Mycoplasma arginini. However, LADI shared a similar three dimensional structure. The K(M) and V(max) values for arginine were 8.67+/-0.045 mM (mean+/-SD) and 344.83+/-1.79 micromol/min/mg, respectively, and the optimum temperature and pH for the production of LADI were 60 degrees C and 7.2.

Molecular Dynamics and Density Functional Studies of Substrate Binding and Catalysis of Arginine Deiminase

The active-site dynamics of arginine deiminase (ADI) complexed with the arginine substrate are investigated with ns molecular dynamics for the wildtype ADI and several mutants. It is shown that the substrate is held in the active site by an extensive hydrogen bond network, which may be weakened by substitution of active-site residues. In addition, the initial step of the catalysis is explored in several truncated active-site models with density functional theory. Evidence is presented in support of the hypothesis that the nucleophilic attack of the ADI Cys thiol at the guanidino carbon of the substrate is initiated by substrate-mediated proton transfer to a His residue in the catalytic triad (Cys-His-Glu). In addition, the active-site residues are found to strongly influence the reaction profile, consistent with their important role in catalysis.

Nitric oxide, a double edged sword in cancer biology: Searching for therapeutic opportunities

Nitric oxide (NO) is a pleiotropic molecule critical to a number of physiological and pathological processes. The last decade has witnessed major advances in dissecting NO biology and its role in cancer pathogenesis. However, the complexity of the interactions between different levels of NO and several aspects of tumor development/progression has led to apparently conflicting findings. Furthermore, both anti-NO and NO-based anticancer strategies appear effective in several preclinical models. This paradoxical dichotomy is leaving investigators with a double challenge: to determine the net impact of NO on cancer behavior and to define the therapeutic role of NO-centered anticancer strategies. Only a comprehensive and dynamic view of the cascade of molecular and cellular events underlying tumor biology and affected by NO will allow investigators to exploit the potential antitumor properties of drugs interfering with NO metabolism. Available data suggest that NO should be considered neither a universal target nor a magic bullet, but rather a signal transducer to be modulated according to the molecular makeup of each individual cancer and the interplay with conventional antineoplastic agents.