Arginase Inhibitors: A Rational Approach Over One Century

Arginase inhibitory properties of flavonoid compounds from the leaves of Mulberry (Morus alba, Moraceae)

OBJECTIVES

We aimed to isolate and identify bioactive molecules from Morus alba (Moraceae) leaves having arginase inhibitory activity towards the combat of clinical outcomes related to endothelial dysfunction.

METHOD

Extraction and isolation were carried out by successive macerations, prepurification by using a Solid Phase Extraction (SPE) and separation using preparative PLC. The structures of the isolated components were established and confirmed by spectroscopic analyses, including the ESI-HRMS and NMR spectroscopic investigations. Biological evaluation was performed by using an in vitro assay with liver bovine purified arginase and by an ex vivo aortic ring study.

KEY FINDINGS

We demonstrated that a phenolic extract from the leaves of M. alba possesses mammalian arginase inhibitory capacities. Investigation of the chemical constituents of its leaves results in the isolation and identification of ten compounds investigated in vitro for their arginase inhibitory capacities. Four compounds showed significant inhibition of arginase, with percentage inhibition ranging from 54% to 83% at 100 µm. In isolated rat aortic rings incubated with NO synthase inhibitor, Luteolin-7-diglucoside compound (2) was able to increase acetylcholine-induced relaxation.

CONCLUSIONS

These results demonstrated the attractive ability of M. alba to be a potential source for the discovery of new active products on vascular system.

Insights on the participation of Glu256 and Asp204 in the oligomeric structure and cooperative effects of human arginase type I

Arginase (EC 3.5.3.1) catalyzes the hydrolysis of L-arginine to L-ornithine and urea, and requires a bivalent cation, especially Mn²⁺ for its catalytic activity. It is a component of the urea cycle and regulates the intracellular levels of l-arginine, which makes the arginase a target for treatment of vascular diseases and asthma. Mammalian arginases contain an unusual S-shaped motif located at the intermonomeric interface. Until now, the studies were limited to structural role of the motif. Then, our interest was focused on functional aspects and our hypothesis has been that the motif is essential for maintain the oligomeric state, having Arg308 as a central axis. Previously, we have shown that the R308A mutant is monomeric and re-associates to the trimeric-cooperative state in the presence of low concentrations of guanidine chloride. We have now mutated Asp204 that interacts with Arg308 in the neighbor subunit, and also we mutated Glu256, proposed as important for oligomerization. Concretely, the human arginase I mutants D204A, D204E, E256A, E256Q and E256D were generated and examined. No differences were observed in the kinetic parameters at pH 9.5 or in tryptophan fluorescence. However, the D204A and E256Q variants were monomeric. On the other hand, D204E and E256D proved to be trimeric and kinetically cooperative at pH 7.5, whereas hyperbolic kinetics was exhibited by E256A, also trimeric. The results obtained strongly support the importance of the interaction between Arg255 and Glu256 in the cooperative properties of arginase, and Asp204 would be relevant to maintain the oligomeric state through salt bridges with Arg255 and Arg308.

Naringin regulates erectile dysfunction by abolition of apoptosis and inflammation through NOS/cGMP/PKG signalling pathway on exposure to Bisphenol-A in hypertensive rat model

This study investigated the effect of naringin (NRG) on extracellular metabolism of ATP through the NOS/cGMP/PKG signaling pathway induced by Nω-nitro-L-arginine methyl ester hydrochloride (_L-NAME) on exposure to Bisphenol-A (BPA) in penis. Fifty-six adult male albino rats were randomly distributed into eight (n = 7) groups. Group I: control animals, Group II was treated with 40 mg/kg _L-NAME, Group III was treated with 50 mg/kg BPA, Group IV was treated with 40 mg/kg _L-NAME +50 mg/kg BPA. Group V was administered with 40 mg/kg _L-NAME +80 mg/kg NRG. Group VI was administered with 50 mg/kg BPA + 80 mg/kg NRG. Group VII was administered with 40 mg/kg _L-NAME+50 mg/kg BPA + 80 mg/kg NRG. Lastly, group VIII was treated with 80 mg/kg NRG for 14 days. NRG prevented hypertension and erectile dysfunction by inhibiting the activities of angiotensin-converting enzymes, arginase, and phosphodiesterase-5¹ (PDE-5¹) with corresponding down-regulation of inflammatory markers including TNF-α and IL-B. Additionally, hypertensive erectile dysfunction was remarkably prevented by NRG as manifested by the declined activities of AChE, MAO-A and enzymes of ATP hydrolysis (ATPase, ADPase, AMPase and ADA) with resultant increase in NO level. Also, penile expression of antigen presenting cells, CD43 transcript, caspace-9 and tumor suppressor P53 proteins were repressed on treatment with NRG. This study validates the hypothesis that NRG may be a valuable remedy in abrogating penile inflammatory markers, apoptosis and enzymes of ATP-hydrolysis via NOS/cGMP/PKG signaling pathways in hypertensive rat model on exposure to environmental toxicant.

Myeloid Cell-Derived Arginase in Cancer Immune Response

Amino acid metabolism is a critical regulator of the immune response, and its modulating becomes a promising approach in various forms of immunotherapy. Insufficient concentrations of essential amino acids restrict T-cells activation and proliferation. However, only arginases, that degrade L-arginine, as well as enzymes that hydrolyze L-tryptophan are substantially increased in cancer. Two arginase isoforms, ARG1 and ARG2, have been found to be present in tumors and their increased activity usually correlates with more advanced disease and worse clinical prognosis. Nearly all types of myeloid cells were reported to produce arginases and the increased numbers of various populations of myeloid-derived suppressor cells and macrophages correlate with inferior clinical outcomes of cancer patients. Here, we describe the role of arginases produced by myeloid cells in regulating various populations of immune cells, discuss molecular mechanisms of immunoregulatory processes involving L-arginine metabolism and outline therapeutic approaches to mitigate the negative effects of arginases on antitumor immune response. Development of potent arginase inhibitors, with improved pharmacokinetic properties, may lead to the elaboration of novel therapeutic strategies based on targeting immunoregulatory pathways controlled by L-arginine degradation.

Synthesis, evaluation and molecular modelling of piceatannol analogues as arginase inhibitors

Arginase is involved in a wide range of pathologies including cardiovascular diseases and infectious diseases whilst it is also a promising target to improve cancer immunotherapy. To date, only a limited number of inhibitors of arginase have been reported. Natural polyphenols, among them piceatannol, are moderate inhibitors of arginase. Herein, we report our efforts to investigate catechol binding by quantum chemistry and generate analogues of piceatannol. In this work, we synthesized a novel series of amino-polyphenols which were then evaluated as arginase inhibitors. Their structure-activity relationships were elucidated by deep quantum chemistry modelling. 4-((3,4-Dihydroxybenzyl)amino)benzene-1,2-diol 3t displays a mixed inhibition activity on bovine and human arginase I with IC50 (K i) values of 76 (82) μM and 89 μM, respectively.

Discovery and Optimization of Rationally Designed Bicyclic Inhibitors of Human Arginase to Enhance Cancer Immunotherapy

The action of arginase, a metalloenzyme responsible for the hydrolysis of arginine to urea and ornithine, is hypothesized to suppress immune-cell activity within the tumor microenvironment, and thus its inhibition may constitute a means by which to potentiate the efficacy of immunotherapeutics such as anti-PD-1 checkpoint inhibitors. Taking inspiration from reported enzyme-inhibitor cocrystal structures, we designed and synthesized novel inhibitors of human arginase possessing a fused 5,5-bicyclic ring system. The prototypical member of this class, 3, when dosed orally, successfully demonstrated serum arginase inhibition and concomitant arginine elevation in a syngeneic mouse carcinoma model, despite modest oral bioavailability. Structure-based design strategies to improve the bioavailability of this class, including scaffold modification, fluorination, and installation of active-transport recognition motifs were explored.

Discovery and Pharmacokinetics of Sulfamides and Guanidines as Potent Human Arginase 1 Inhibitors

We designed and synthesized a series of arginase inhibitors as derivatives of the well-known 2-(S)-amino-6-boronohexanoic acid (ABH) with basic and neutral side chains in the α-position relative to the amino acid group. In an effort to improve the pharmacokinetic profile of literature examples and retain potent enzymatic activity, sulfamido moieties were introduced to generate hydrogen bond interaction with the aspartic acid residue in the arginase active site. The compounds with basic guanidine-containing side chains were even more potent arginase inhibitors. Both groups of compounds, as designed, demonstrated low clearance in their pharmacokinetic profile. The most active inhibitor 15aa showed high nanomolar potency with IC50 = 32 nM toward human arginase 1 and demonstrated low clearance (4.2 mL/min/kg), long t 1/2, and moderate volume of distribution in rat pharmacokinetic studies.

Pharmacological Screening Identifies SHK242 and SHK277 as Novel Arginase Inhibitors with Efficacy against Allergen-Induced Airway Narrowing In Vitro and In Vivo

Arginase is a potential target for asthma treatment. However, there are currently no arginase inhibitors available for clinical use. Here, a novel class of arginase inhibitors was synthesized, and their efficacy was pharmacologically evaluated. The reference compound 2(S)-amino-6-boronohexanoic acid (ABH) and >200 novel arginase inhibitors were tested for their ability to inhibit recombinant human arginase 1 and 2 in vitro. The most promising compounds were separated as enantiomers. Enantiomer pairs SHK242 and SHK243, and SHK277 and SHK278 were tested for functional efficacy by measuring their effect on allergen-induced airway narrowing in lung slices of ovalbumin-sensitized guinea pigs ex vivo. A guinea pig model of acute allergic asthma was used to examine the effect of the most efficacious enantiopure arginase inhibitors on allergen-induced airway hyper-responsiveness (AHR), early and late asthmatic reactions (EAR and LAR), and airway inflammation in vivo. The novel compounds were efficacious in inhibiting arginase 1 and 2 in vitro. The enantiopure SHK242 and SHK277 fully inhibited arginase activity, with IC₅₀ values of 3.4 and 10.5 μM for arginase 1 and 2.9 and 4.0 µM for arginase 2, respectively. Treatment of slices with ABH or novel compounds resulted in decreased ovalbumin-induced airway narrowing compared with control, explained by increased local nitric oxide production in the airway. In vivo, ABH, SHK242, and SHK277 protected against allergen-induced EAR and LAR but not against AHR or lung inflammation. We have identified promising novel arginase inhibitors for the potential treatment of allergic asthma that were able to protect against allergen-induced early and late asthmatic reactions. SIGNIFICANCE STATEMENT: Arginase is a potential drug target for asthma treatment, but currently there are no arginase inhibitors available for clinical use. We have identified promising novel arginase inhibitors for the potential treatment of allergic asthma that were able to protect against allergen-induced early and late asthmatic reactions. Our new inhibitors show protective effects in reducing airway narrowing in response to allergens and reductions in the early and late asthmatic response.

Orally administrated D-arginine exhibits higher enrichment in the brain and milk than L-arginine in ICR mice

D-Amino acids exert various physiological functions and are widely present in animals. However, they are absorbed to a lesser extent than L-amino acids. Little is known about D-arginine (D-Arg); however, its isomer L-Arg serves as a substrate for several metabolites and exhibits various functions including promotion of growth hormone secretion. Milk is the only nutrient source for infants; it plays an important role during their initial growth and brain development. No studies have evaluated the availability of D-Arg in the brain and milk in mammals. Here, we have studied the differential availability of orally administered D- and L-Arg in the brain and milk using ICR mice. Our results revealed that without D-Arg administration, D-Arg was undetectable in both plasma and brain samples. However, the plasma D-Arg was about twice the concentration of L-Arg post administration of the same. In the cerebral cortex and hypothalamus, L-Arg concentration remained almost constant for over period of 90 min after L-Arg treatment. Nevertheless, the L-Arg concentration decreased after D-Arg administration with time compared to the case post L-Arg administration. Contrastingly, D-Arg level sharply increased at both the brain regions with time after D-Arg treatment. Furthermore, L-Arg concentration in the milk hardly increased after L-Arg administration. Interestingly, oral administration of D-Arg showed efficient enrichment of D-Arg in milk, compared with L-Arg. Thus, our results imply that D-Arg may be available for brain development and infant nourishment through milk as an oral drug and/or nutrient supplement.

Semiconducting Polymer Nanoreporters for Near-Infrared Chemiluminescence Imaging of Immunoactivation

Real-time in vivo imaging of immunoactivation is critical for longitudinal evaluation of cancer immunotherapy, which, however, is rarely demonstrated. This study reports semiconducting polymer nanoreporters (SPNRs) with superoxide anion (O₂ ^•- )-activatable chemiluminescence signals for in vivo imaging of immunoactivation during cancer immunotherapy. SPNRs are designed to comprise an SP and a caged chemiluminescence phenoxy-dioxetane substrate, which respectively serve as the chemiluminescence acceptor and donor to enable intraparticle chemiluminescence resonance energy transfer. SPNRs are intrinsically fluorescent but only become chemiluminescent upon activation by O₂ ^•- . Representing the first O₂ ^•- -activatable near-infrared chemiluminescent reporter, SPNR3 sensitively differentiates higher O₂ ^•- levels in immune cells from other cells including cancer and normal cells. Following systemic administration, SPNR3 passively accumulates into tumors in living mice and activates the chemiluminescence signals responding to the concentration of O₂ ^•- in the tumor microenvironment. Moreover, the enhancement of in vivo chemiluminescence signal after cancer immunotherapy is correlated with increased population of T cells in the tumor, proving its feasibility in tracking of T cell activation. Thus, SPNRs represent the first kind of chemiluminescent reporters competent for in vivo imaging of immunoactivation.

Structural and functional characterization of C0021158, a high-affinity monoclonal antibody that inhibits Arginase 2 function via a novel non-competitive mechanism of action

Arginase 2 (ARG2) is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of L-arginine. The dysregulated expression of ARG2 within specific tumor microenvironments generates an immunosuppressive niche that effectively renders the tumor 'invisible' to the host's immune system. Increased ARG2 expression leads to a concomitant depletion of local L-arginine levels, which in turn leads to suppression of anti-tumor T-cell-mediated immune responses. Here we describe the isolation and characterization of a high affinity antibody (C0021158) that inhibits ARG2 enzymatic function completely, effectively restoring T-cell proliferation in vitro. Enzyme kinetic studies confirmed that C0021158 exhibits a noncompetitive mechanism of action, inhibiting ARG2 independently of L-arginine concentrations. To elucidate C0021158's inhibitory mechanism at a structural level, the co-crystal structure of the Fab in complex with trimeric ARG2 was solved. C0021158's epitope was consequently mapped to an area some distance from the enzyme's substrate binding cleft, indicating an allosteric mechanism was being employed. Following C0021158 binding, distinct regions of ARG2 undergo major conformational changes. Notably, the backbone structure of a surface-exposed loop is completely rearranged, leading to the formation of a new short helix structure at the Fab-ARG2 interface. Moreover, this large-scale structural remodeling at ARG2's epitope translates into more subtle changes within the enzyme's active site. An arginine residue at position 39 is reoriented inwards, sterically impeding the binding of L-arginine. Arg39 is also predicted to alter the pKA of a key catalytic histidine residue at position 160, further attenuating ARG2's enzymatic function. In silico molecular docking simulations predict that L-arginine is unable to bind effectively when antibody is bound, a prediction supported by isothermal calorimetry experiments using an L-arginine mimetic. Specifically, targeting ARG2 in the tumor microenvironment through the application of C0021158, potentially in combination with standard chemotherapy regimens or alternate immunotherapies, represents a potential new strategy to target immune cold tumors.

Targeting the arginine metabolic brake enhances immunotherapy for leukaemia

Therapeutic approaches which aim to target Acute Myeloid Leukaemia through enhancement of patients' immune responses have demonstrated limited efficacy to date, despite encouraging preclinical data. Examination of AML patients treated with azacitidine (AZA) and vorinostat (VOR) in a Phase II trial, demonstrated an increase in the expression of Cancer-Testis Antigens (MAGE, RAGE, LAGE, SSX2 and TRAG3) on blasts and that these can be recognised by circulating antigen-specific T cells. Although the T cells have the potential to be activated by these unmasked antigens, the low arginine microenvironment created by AML blast Arginase II activity acts a metabolic brake leading to T cell exhaustion. T cells exhibit impaired proliferation, reduced IFN-γ release and PD-1 up-regulation in response to antigen stimulation under low arginine conditions. Inhibition of arginine metabolism enhanced the proliferation and cytotoxicity of anti-NY-ESO T cells against AZA/VOR treated AML blasts, and can boost anti-CD33 Chimeric Antigen Receptor-T cell cytotoxicity. Therefore, measurement of plasma arginine concentrations in combination with therapeutic targeting of arginase activity in AML blasts could be a key adjunct to immunotherapy.

Sources of Vascular Nitric Oxide and Reactive Oxygen Species and Their Regulation

Nitric oxide (NO) is a small free radical with critical signaling roles in physiology and pathophysiology. The generation of sufficient NO levels to regulate the resistance of the blood vessels and hence the maintenance of adequate blood flow is critical to the healthy performance of the vasculature. A novel paradigm indicates that classical NO synthesis by dedicated NO synthases is supplemented by nitrite reduction pathways under hypoxia. At the same time, reactive oxygen species (ROS), which include superoxide and hydrogen peroxide, are produced in the vascular system for signaling purposes, as effectors of the immune response, or as byproducts of cellular metabolism. NO and ROS can be generated by distinct enzymes or by the same enzyme through alternate reduction and oxidation processes. The latter oxidoreductase systems include NO synthases, molybdopterin enzymes, and hemoglobins, which can form superoxide by reduction of molecular oxygen or NO by reduction of inorganic nitrite. Enzymatic uncoupling, changes in oxygen tension, and the concentration of coenzymes and reductants can modulate the NO/ROS production from these oxidoreductases and determine the redox balance in health and disease. The dysregulation of the mechanisms involved in the generation of NO and ROS is an important cause of cardiovascular disease and target for therapy. In this review we will present the biology of NO and ROS in the cardiovascular system, with special emphasis on their routes of formation and regulation, as well as the therapeutic challenges and opportunities for the management of NO and ROS in cardiovascular disease.

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

Gallic Acid Potentiates the Antimicrobial Activity of Tulathromycin Against Two Key Bovine Respiratory Disease (BRD) Causing-Pathogens

Bovine respiratory disease (BRD) is the most common infectious disease in dairy and beef cattle. It is associated with significant morbidity and mortality and causes a huge economic loss each year. In western Canada, a one-time injection of tulathromycin is commonly used as a metaphylactic procedure to reduce BRD incidence and eliminate potential BRD outbreak. With increased global concern on antimicrobial usage in dairy and beef products and bacterial resistance to antimicrobials, it is important to develop a novel strategy to eliminate the usage or decrease the dosage of antimicrobials. In this study, we showed that gallic acid was active against both Mannheimia haemolytica and Pasteurella multocida, two key BRD associated-pathogens, with the minimum inhibitory concentration (MIC) measured at 250 and 500 μg/mL, respectively. Co-administration of tulathromycin and gallic acid exhibited a strong additive or weak synergistic effect toward both M. haemolytic and P. multocida. Tulathromycin, gallic acid and their combination were also effective against the mixed culture of M. haemolytic and P. multocida. Furthermore, we showed that pre-exposure to tulathromycin generated bacterial resistance to the antimicrobial in M. haemolytica but not in P. multocida.

Insights into the Structural Requirements of 2(S)-Amino-6-Boronohexanoic Acid Derivatives as Arginase I Inhibitors: 3D-QSAR, Docking, and Interaction Fingerprint Studies

Human arginase I (hARGI) is an important enzyme involved in the urea cycle; its overexpression has been associated to cardiovascular and cerebrovascular diseases. In the last years, several congeneric sets of hARGI inhibitors have been reported with possible beneficial roles for the cardiovascular system. At the same time, crystallographic data have been reported including hARGI–inhibitor complexes, which can be considered for the design of novel inhibitors. In this work, the structure–activity relationship (SAR) of Cα substituted 2(S)-amino-6-boronohexanoic acid (ABH) derivatives as hARGI inhibitors was studied by using a three-dimensional quantitative structure–activity relationships (3D-QSAR) method. The predictivity of the obtained 3D-QSAR model was demonstrated by using internal and external validation experiments. The best model revealed that the differential hARGI inhibitory activities of the ABH derivatives can be described by using steric and electrostatic fields; the local effects of these fields in the activity are presented. In addition, binding modes of the above-mentioned compounds inside the hARGI binding site were obtained by using molecular docking. It was found that ABH derivatives adopted the same orientation reported for ABH within the hARGI active site, with the substituents at Cα exposed to the solvent with interactions with residues at the entrance of the binding site. The hARGI residues involved in chemical interactions with inhibitors were identified by using an interaction fingerprints (IFPs) analysis.

Chronic Insulin Infusion Down-Regulates Circulating and Urinary Nitric Oxide (NO) Levels Despite Molecular Changes in the Kidney Predicting Greater Endothelial NO Synthase Activity in Mice

Insulin therapy is often needed to overcome insulin receptor resistance in type 2 diabetes; however, the impact of providing additional insulin to already hyperinsulinemic subjects is not clear. We infused male TALLYHO/Jng (TH) mice (insulin resistant) with insulin (50 U/kg·bw/d) or vehicle (control) by osmotic minipump for 14 days. One group of insulin-infused mice was switched to 4% NaCl diet (high-sodium diet, HSD) in the second week. Blood chemistry revealed a significantly higher anion gap and blood sodium concentrations with insulin infusion, i.e., relative metabolic acidosis. Systolic BP and heart rate were slightly (~5 mm Hg) higher in insulin-infused versus control mice. HSD resulted in a modest and transient rise in mean arterial blood pressure (BP), relative to control or insulin-infused, normal-NaCl-fed mice. In kidney, insulin infusion: (1) increased total and phosphorylated (serine-1177) endothelial nitric oxide synthase (eNOS) band densities; (2) reduced band density of the uncoupled form of eNOS; and (3) increased renal homogenate nitric oxide synthase (NOS) activity. Despite this, plasma and urine levels of nitrates plus nitrites (NOx) fell with insulin infusion, by day 14 (40–50%) suggesting worsening of resistance. Overall, insulin infusion ramps up the cellular means in kidney to increase vasodilatory and natriuretic NO, but in the long term may be associated with worsening of insulin receptor resistance.

Deficient arginase II expression without alteration in arginase I expression attenuated experimental autoimmune encephalomyelitis in mice

In the past there have been a multitude of studies that ardently support the role of arginase II (Arg II) in vascular and endothelial disorders; however, the regulation and function of Arg II in autoimmune diseases has thus far remained unclear. Here we report that a global Arg II null mutation in mice suppressed experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. During EAE, both Arg I and Arg II were induced in spinal cords, but only Arg II was induced in spleens and splenic dendritic cells (DCs). DC activation by lipopolysaccharide (LPS), CD40L or TLR8 agonist significantly enhanced Arg II expression without affecting Arg I expression. Conversely, DC differentiating cytokines [IL-4 and granulocyte macrophage-colony-stimulating factor (GM-CSF)] yielded opposite effects. In addition, Arg I and Arg II were regulated differentially during Th1 and Th17 cell polarization. Arg II deficiency in mice delayed EAE onset, ameliorated clinical symptoms and reduced myelin loss, accompanied by a remarkable reduction in the EAE-induced spinal cord expression of Th17 cell markers (IL-17 and RORγt). The abundance of Th17 cells and IL-23+ cells in relevant draining lymph nodes was significantly reduced in Arg II knockout mice. In activated DCs, Arg II deficiency significantly suppressed the expression of Th17-differentiating cytokines IL-23 and IL-6. Interestingly, Arg II deficiency did not lead to any compensatory increase in Arg I expression in vivo and in vitro. In conclusion, Arg II was identified as a factor promoting EAE likely via an Arg I-independent mechanism. Arg II may promote EAE by enhancing DC production of Th17-differentiating cytokines. Specific inhibition of Arg II could be a potential therapy for multiple sclerosis.

Hypoargininemia exacerbates airway hyperresponsiveness in a mouse model of asthma

BACKGROUND

Asthma is a chronic respiratory condition, with airway hyperresponsiveness (AHR) and inflammation as hallmarks. The hypothesis that the substantially increased expression of arginase 1 in activated macrophages limits the availability of L-arginine for nitric oxide synthesis, and thus increases AHR in lungs of mice with experimentally induced allergic asthma was recently refuted by several studies. In the present study, we tested the hypothesis that, instead, a low circulating concentration of arginine aggravates AHR in the same murine asthma model. Female FVB F/A2 tg/tg transgenic mice, which overexpress rat arginase 1 in their enterocytes, exhibit a ~ 50% decrease of their plasma L-arginine concentration.

METHODS

Adult female F/A2 tg/tg mice and their wild-type littermates (F/A2 wt/wt ) were sensitized and challenged with ovalbumin (OVA/OVA). Lung function was assessed with the flexiVent™ system. Adaptive changes in the expression of arginine-metabolizing or -transporting enzymes, chemokines and cytokines, and lung histology were quantified with qPCR, ELISA, and immunohistochemistry, respectively.

RESULTS

Reduction of circulating L-arginine concentration significantly increased AHR in OVA/OVA-treated mice and, to a lesser extent, even in PBS/OVA-treated mice. The pulmonary inflammatory response in OVA/OVA-treated F/A2 tg/tg and F/A2 wt/wt mice was comparable. OVA/OVA-treated F/A2 tg/tg mice differed from similarly treated female mice, in which arginase 1 expression in lung macrophages was eliminated, by a complete absence of an adaptive increase in the expression of arginine-metabolizing or -transporting enzymes.

CONCLUSION

A reduction of the circulating L-arginine concentration rather than the macrophage-mediated increase of arginine catabolism worsens AHR.

Arginase inhibition prevents the development of hypertension and improves insulin resistance in obese rats

This study investigated the temporal activation of arginase in obese Zucker rats (ZR) and determined if arginase inhibition prevents the development of hypertension and improves insulin resistance in these animals. Arginase activity, plasma arginine and nitric oxide (NO) concentration, blood pressure, and insulin resistance were measured in lean and obese animals. There was a chronological increase in vascular and plasma arginase activity in obese ZR beginning at 8 weeks of age. The increase in arginase activity in obese animals was associated with a decrease in insulin sensitivity and circulating levels of arginine and NO. The rise in arginase activity also preceded the increase in blood pressure in obese ZR detected at 12 weeks of age. Chronic treatment of 8-week-old obese animals with an arginase inhibitor or L-arginine for 4 weeks prevented the development of hypertension and improved plasma concentrations of arginine and NO. Arginase inhibition also improved insulin sensitivity in obese ZR while L-arginine supplementation had no effect. In conclusion, arginase inhibition prevents the development of hypertension and improves insulin sensitivity while L-arginine administration only mitigates hypertension in obese animals. Arginase represents a promising therapeutic target in ameliorating obesity-associated vascular and metabolic dysfunction.

Small molecule immuno-oncology therapeutic agents

Treatment of cancer by activation of an antitumor immune response is now a widely practiced and well-accepted approach to therapy. However, despite dramatic responses in some patients, the high proportion of unresponsive patients points to a considerable unmet medical need. Although antibody therapies have led the way, small molecule immuno-oncology agents are close behind. This perspective provides an overview of some of the many small molecule approaches being explored. It encompasses small molecule modulators of validated targets such as programed cell death 1 (PD-1) as well as novel approaches still to be proven clinically.

Arginase Structure and Inhibition: Catalytic Site Plasticity Reveals New Modulation Possibilities

Metalloenzyme arginase is a therapeutically relevant target associated with tumor growth. To fight cancer immunosuppression, arginase activity can be modulated by small chemical inhibitors binding to its catalytic center. To better understand molecular mechanisms of arginase inhibition, a careful computer-aided mechanistic structural investigation of this enzyme was conducted. Using molecular dynamics (MD) simulations in the microsecond range, key regions of the protein active site were identified and their flexibility was evaluated and compared. A cavity opening phenomenon was observed, involving three loops directly interacting with all known ligands, while metal coordinating regions remained motionless. A novel dynamic 3D pharmacophore analysis method termed dynophores has been developed that allows for the construction of a single 3D-model comprising all ligand-enzyme interactions occurring throughout a complete MD trajectory. This new technique for the in silico study of intermolecular interactions allows for loop flexibility analysis coupled with movements and conformational changes of bound ligands. Presented MD studies highlight the plasticity of the size of the arginase active site, leading to the hypothesis that larger ligands can enter the cavity of arginase. Experimental testing of a targeted fragment library substituted by different aliphatic groups validates this hypothesis, paving the way for the design of arginase inhibitors with novel binding patterns.

NOX2-Induced Activation of Arginase and Diabetes-Induced Retinal Endothelial Cell Senescence

Increases in reactive oxygen species (ROS) and decreases in nitric oxide (NO) have been linked to vascular dysfunction during diabetic retinopathy (DR). Diabetes can reduce NO by increasing ROS and by increasing activity of arginase, which competes with nitric oxide synthase (NOS) for their commons substrate l-arginine. Increased ROS and decreased NO can cause premature endothelial cell (EC) senescence leading to defective vascular repair. We have previously demonstrated the involvement of NADPH oxidase 2 (NOX2)-derived ROS, decreased NO and overactive arginase in DR. Here, we investigated their impact on diabetes-induced EC senescence. Studies using diabetic mice and retinal ECs treated with high glucose or H₂O₂ showed that increases in ROS formation, elevated arginase expression and activity, and decreased NO formation led to premature EC senescence. NOX2 blockade or arginase inhibition prevented these effects. EC senescence was also increased by inhibition of NOS activity and this was prevented by treatment with a NO donor. These results indicate that diabetes/high glucose-induced activation of arginase and decreases in NO bioavailability accelerate EC senescence. NOX2-generated ROS contribute importantly to this process. Blockade of NOX2 or arginase represents a strategy to prevent diabetes-induced premature EC senescence by preserving NO bioavailability.

Interactions between integrase inhibitors and human arginase 1

The neuro-pathogenic mechanism(s) underlying HIV-associated neurocognitive disorders are mostly unknown. HIV-infected macrophages and microglial cells play a crucial role and the metabolic fate of l-arginine may be highly relevant to microglia activation. In this context, arginase (ARG), which uses l-arginine as substrate, can be on the same time a target and source of oxidative stress and inflammation. In this study, we investigated whether integrase strand transfer inhibitors share with the other antiretroviral drugs the ability to inhibit ARG activity. We used the previously validated cell model, namely the human microglia cell line, as well as the computational chemistry approach. Furthermore, here we characterized the activity of purified human ARG in a cell-free in vitro system, and investigated the effects of integrase strand transfer inhibitors in this newly validated model. Overall evidence shows that Dolutegravir, Raltegravir and Elvitegravir inhibit ARG activity.