Insulin vasodilatation and the "arginine paradox"

Polydopamine-cloaked Fe-based metal organic frameworks enable synergistic multidimensional treatment of osteosarcoma

One of the major challenges in effective cancer therapy arises because of the hypoxic microenvironment in the tumor. This compromises the efficacy of both chemo- and radiotherapy, and thus hinders patient outcomes. To solve this problem, we constructed polydopamine (PDA)-cloaked Fe-based metal organic frameworks (MOFs) loaded with d-arginine (d-Arg), glucose oxidase (GOX), and the chemotherapeutic drug tirapazamine (TPZ). These offer simultaneous multifaceted therapy combining chemodynamic therapy (CDT)/radiotherapy (RT)/starvation therapy (ST)/gas therapy (GT) and chemotherapy. The particles further can act as contrast agents in magnetic resonance imaging. GOX catalyses the conversion of endogenous glucose and O2 to hydrogen peroxide and gluconic acid, blocking the cells' energy supply and providing ST. With the resultant acidification of the local environment, the breakdown of the MOF releases TPZ (for chemotherapy) and Fe3+, which reacts with H2O2 to produce reactive oxygen species and thus stimulates the conversion of d-Arg to NO for GT and RT sensitization. The PDA coating not only seals the pores and chelates Fe3+ to enhance the T1-weighted magnetic resonance imaging (MRI) properties, but also is used to graft folate bovine serum albumin (FA-BSA) and thereby target the tumor site. The combined administration of low doses of X-ray irradiation and nanoparticles reduces the side effects on healthy tissue and can prevent lung metastases in mice. This work highlights the synergistic treatment of osteosarcoma via ST/GT/CDT/RT/MRI/ chemotherapy using a PDA-cloaked MOF system.

D-arginine-loaded metal-organic frameworks nanoparticles sensitize osteosarcoma to radiotherapy

Osteosarcoma is a common type of bone cancers with a high rate of pulmonary recurrence. High-dose radiation therapy is useful for the ablation of unresectable osteosarcoma. However, it may cause severe adverse effects. To address this problem, we developed D-arginine-loaded metal-organic frameworks (MOF) nanoparticles for improving the radiosensitivity of osteosarcoma. D-arginine, a metabolically inert enantiomer of L-arginine, could produce nitric oxide and down-regulate hypoxia-inducible factor-1alpha (HIF-1α) to alleviate tumor hypoxia. In addition, MOF could also generate free radicals to kill the tumor cells. Results demonstrate that D-arginine-loaded nanoparticles enhanced tumor ablation and prevented the lung metastasis in mice upon radiation therapy. Furthermore, the nanoparticles or radiation alone had relatively low toxicity in cells and mice. Therefore, D-arginine-loaded MOF nanoparticles are relatively safe and highly effective in sensitizing osteosarcoma to radiotherapy.

Arginine and Endothelial Function

Arginine (L-arginine), is an amino acid involved in a number of biological processes, including the biosynthesis of proteins, host immune response, urea cycle, and nitric oxide production. In this systematic review, we focus on the functional role of arginine in the regulation of endothelial function and vascular tone. Both clinical and preclinical studies are examined, analyzing the effects of arginine supplementation in hypertension, ischemic heart disease, aging, peripheral artery disease, and diabetes mellitus.

Review: Endothelial dysfunction in type 2 diabetes

Dysfunction of the endothelial cell monolayer leads to increased vascular tone and permeability and a prothrombotic environment. Type 2 diabetes is a state of insulin resistance, hyperglycaemia and dyslipidaemia characterised by high cardiovascular risk and accelerated atherosclerosis. Many mechanisms by which hyperglycaemia can result in endothelial dysfunction have now been identified. However, the presence of endothelial dysfunction in normoglycaemic first-degree relatives and insulin resistant subjects is less well understood and less readily explained by `confounding' variables. We suggest that insulin's effects on glucose transport in classical target tissues and nitric oxide production in the endothelium are fundamentally linked at a molecular level. It is expected that greater understanding of these underlying mechanisms will lead to novel approaches to prevention of cardiovascular disease in both diabetic and non-diabetic subjects.

Insulin rapidly stimulates L-arginine transport in human aortic endothelial cells via Akt

Insulin stimulates endothelial NO synthesis, at least in part mediated by phosphorylation and activation of endothelial NO synthase at Ser1177 and Ser615 by Akt. We have previously demonstrated that insulin-stimulated NO synthesis is inhibited under high culture glucose conditions, without altering Ca(2+)-stimulated NO synthesis or insulin-stimulated phosphorylation of eNOS. This indicates that stimulation of endothelial NO synthase phosphorylation may be required, yet not sufficient, for insulin-stimulated nitric oxide synthesis. In the current study we investigated the role of supply of the eNOS substrate, L-arginine as a candidate parallel mechanism underlying insulin-stimulated NO synthesis in cultured human aortic endothelial cells. Insulin rapidly stimulated L-arginine transport, an effect abrogated by incubation with inhibitors of phosphatidylinositol-3'-kinase or infection with adenoviruses expressing a dominant negative mutant Akt. Furthermore, supplementation of endothelial cells with extracellular L-arginine enhanced insulin-stimulated NO synthesis, an effect reversed by co-incubation with the L-arginine transport inhibitor, L-lysine. Basal L-arginine transport was significantly increased under high glucose culture conditions, yet insulin-stimulated L-arginine transport remained unaltered. The increase in L-arginine transport elicited by high glucose was independent of the expression of the cationic amino acid transporters, hCAT1 and hCAT2 and not associated with any changes in the activity of ERK1/2, Akt or protein kinase C (PKC). We propose that rapid stimulation of L-arginine transport contributes to insulin-stimulated NO synthesis in human endothelial cells, yet attenuation of this is unlikely to underlie the inhibition of insulin-stimulated NO synthesis under high glucose conditions.

Vascular responses to IGF-I and insulin are impaired in aortae of hypertensive rats

OBJECTIVE

Insulin-like growth factor-I (IGF-I) and insulin are important vasoactive peptides but little is known about their effects in hypertension.

DESIGN

We compared the responses of stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto (WKY) rat aortae to IGF-I and insulin.

METHODS

Aortae were removed from WKY and SHRSP, cut into 2-3 mm rings, and contractile responses to phenylephrine and endothelin-1 studied in organ chambers in the presence of vehicle, IGF-I (0.1 micromol/l) or insulin (0.1 micromol/l). In addition, the effects of nitric oxide synthase (NOS) inhibition, phosphatidylinositol 3-kinase (PI3-kinase) inhibition and superoxide scavenging on these responses were investigated.

RESULTS

Incubation with IGF-I and insulin caused attenuation of phenylephrine-induced and endothelin-1-induced vasoconstriction in arteries from normotensive but not hypertensive animals. In the arteries from WKY rats, co-incubation with either wortmannin or LY294002, inhibitors of PI3-kinase, attenuated the effect of IGF-I. The vasorelaxant effect of IGF-I was also abolished by removal of the endothelium or addition of the NOS inhibitor, N-nitro-L-arginine methyl ester (L-NAME). Co-incubation with tiron, a superoxide scavenger, suggested that the attenuation of IGF-I vasodilation in SHRSP arteries was not due to excess superoxide production.

CONCLUSION

In WKY, IGF-I/insulin attenuate phenylephrine-mediated constrictions via PI3-kinase/nitric oxide pathways. In contrast, in SHRSP these pathways are dysfunctional and IGF-I has little effect on vascular responses.

Vasodilator effects of L-arginine are stereospecific and augmented by insulin in humans

The amino acid l-arginine, the precursor of nitric oxide (NO) synthesis, induces vasodilation in vivo, but the mechanism behind this effect is unclear. There is, however, some evidence to assume that the l-arginine membrane transport capacity is dependent on insulin plasma levels. We hypothesized that vasodilator effects of l-arginine may be dependent on insulin plasma levels. Accordingly, we performed two randomized, double-blind crossover studies in healthy male subjects. In protocol 1 (n = 15), subjects received an infusion of insulin (6 mU x kg(-1) x min(-1) for 120 min) or placebo and, during the last 30 min, l-arginine or d-arginine (1 g/min for 30 min) x In protocol 2 (n = 8), subjects received l-arginine in stepwise increasing doses in the presence (1.5 mU x kg(-1) x min(-1)) or absence of insulin. Renal plasma flow and glomerular filtration rate were assessed by the para-aminohippurate and inulin plasma clearance methods, respectively. Pulsatile choroidal blood flow was assessed with laser interferometric measurement of fundus pulsation, and mean flow velocity in the ophthalmic artery was measured with Doppler sonography. l-arginine, but not d-arginine, significantly increased renal and ocular hemodynamic parameters. Coinfusion of l-arginine with insulin caused a dose-dependent leftward shift of the vasodilator effect of l-arginine. This stereospecific renal and ocular vasodilator potency of l-arginine is enhanced by insulin, which may result from facilitated l-arginine membrane transport, enhanced intracellular NO formation, or increased NO bioavailability.

[The arginine paradox]

L-Arginine has attracted major interest because it has been identified as the natural substrate of nitric oxide synthase and is now recognized as a major player in the regulation of biological function. The arginine paradox refers to the phenomenon that exogenous L-arginine causes NO-mediated biological effects despite the fact that nitric oxide synthases (NOS) are theoretically saturated with the substrate L-arginine. There have been several explanations for this phenomenon, although none of them can explain the arginine paradox fully: (1) L-arginine-induced insulin, which has vasodilatory actions. (2) Neither extracellular nor intracellular concentration determines the NOS activity but rather the L-arginine amount transported across the plasma membrane may do so. (3) Endogenous NOS inhibitors reduce the enzyme sensitivity to L-arginine. These inhibitors include, NG, NG-dimethyl-L-arginine, L-citrulline, argininosuccinic acid and agmatine. (4) Intracellular L-citrulline, an NOS product, is a potent inhibitor of NOS so that the cells may need extra L-arginine to compete with L-citrulline inhibition.

Effect of nebulised l- and d-arginine on exhaled nitric oxide in steroid naive asthma

BACKGROUND

Nitric oxide (NO) is a product of the enzyme nitric oxide synthase (NOS) and is found in normal and asthmatic human airways. The administration ofl-arginine results in an increase in airway NO production in asthmatic subjects. This is thought to occur becausel-arginine is the substrate for NOS. However, studies in the systemic vasculature suggest that other mechanisms may be responsible.

METHODS

Eight patients with steroid naive asthma each received 2.5 g l-arginine, 2.5 g d-arginine, and 2.0% saline by ultrasonic nebuliser on separate days in a randomised, single blind manner. Exhaled NO was measured by chemiluminescence and spirometric tests were performed before and for 3 hours after each administration. The mean concentration of NO after exposure was calculated from the area under the curve.

RESULTS

l-arginine,d-arginine, and 2.0% saline induced a mean (95% CI) maximal bronchoconstriction of 11.9% (–1.7 to 25.4), 10.0% (2.8 to 17.2), and 8.5% (–2.5 to 19.5) of the starting forced expiratory volume in one second (FEV1), respectively. Exhaled NO declined in proportion to the degree of bronchoconstriction (r=0.60, p<0.01). Bronchoconstriction and the acute reduction in exhaled NO resolved within 15 minutes. The mean post-exposure concentration of NO was 15.75 parts per billion (ppb) after l-arginine, 15.16 ppb after d-arginine, and 12.74 ppb after 2.0% saline. The mean (95% CI) difference between l-arginine and placebo was 3.01 (0.32 to 5.7) ppb, between d-arginine and placebo 2.42 (0.10 to 4.74) ppb, and between l- and d-arginine 0.59 (–1.56 to 2.74) ppb.

CONCLUSIONS

Exhaled NO decreased with acute bronchoconstriction and returned to baseline with the resolution of bronchoconstriction. Exhaled NO increased following the administration of both l-arginine andd-arginine. Since NOS is stereospecific, this finding suggests that the increase in exhaled NO is not entirely mediated through an increase in NOS enzyme activity. We suggest that arginine may react in a non-stereospecific fashion with reactive oxygen species present in asthmatic airways.

Effect of nebulised L- and D-arginine on exhaled nitric oxide in steroid naive asthma

BACKGROUND

Nitric oxide (NO) is a product of the enzyme nitric oxide synthase (NOS) and is found in normal and asthmatic human airways. The administration of L-arginine results in an increase in airway NO production in asthmatic subjects. This is thought to occur because L-arginine is the substrate for NOS. However, studies in the systemic vasculature suggest that other mechanisms may be responsible.

METHODS

Eight patients with steroid naive asthma each received 2.5 g L-arginine, 2.5 g D-arginine, and 2.0% saline by ultrasonic nebuliser on separate days in a randomised, single blind manner. Exhaled NO was measured by chemiluminescence and spirometric tests were performed before and for 3 hours after each administration. The mean concentration of NO after exposure was calculated from the area under the curve.

RESULTS

L-arginine, D-arginine, and 2.0% saline induced a mean (95% CI) maximal bronchoconstriction of 11.9% (-1.7 to 25.4), 10.0% (2.8 to 17.2), and 8.5% (-2.5 to 19.5) of the starting forced expiratory volume in one second (FEV(1)), respectively. Exhaled NO declined in proportion to the degree of bronchoconstriction (r=0.60, p<0.01). Bronchoconstriction and the acute reduction in exhaled NO resolved within 15 minutes. The mean post-exposure concentration of NO was 15.75 parts per billion (ppb) after L-arginine, 15.16 ppb after D-arginine, and 12.74 ppb after 2.0% saline. The mean (95% CI) difference between L-arginine and placebo was 3.01 (0.32 to 5.7) ppb, between D-arginine and placebo 2.42 (0.10 to 4.74) ppb, and between L- and D-arginine 0.59 (-1.56 to 2.74) ppb.

CONCLUSIONS

Exhaled NO decreased with acute bronchoconstriction and returned to baseline with the resolution of bronchoconstriction. Exhaled NO increased following the administration of both L-arginine and D-arginine. Since NOS is stereospecific, this finding suggests that the increase in exhaled NO is not entirely mediated through an increase in NOS enzyme activity. We suggest that arginine may react in a non-stereospecific fashion with reactive oxygen species present in asthmatic airways.