L-Citrulline increases nitric oxide and improves control in obese asthmatics

Abstract

BACKGROUNDThe airways of obese asthmatics have been shown to be NO deficient, and this contributes to airway dysfunction and reduced response to inhaled corticosteroids. In cultured airway epithelial cells, L-citrulline, a precursor of L-arginine recycling and NO formation, has been shown to prevent asymmetric dimethyl arginine-mediated (ADMA-mediated) NO synthase (NOS2) uncoupling, restoring NO and reducing oxidative stress.METHODSIn a proof-of-concept, open-label pilot study in which participants were analyzed before and after treatment, we hypothesized that 15 g/d L-citrulline for 2 weeks would (a) increase the fractional excretion of NO (FeNO), (b) improve asthma control, and (c) improve lung function. To this end, we recruited obese (BMI >30) asthmatics on controller therapy, with a baseline FeNO of ≤30 ppb from the University of Colorado Medical Center and Duke University Health System.RESULTSA total of 41 subjects with an average FeNO of 17 ppb (95% CI, 15-19) and poorly controlled asthma (average asthma control questionnaire [ACQ] 1.5 [95% CI, 1.2-1.8]) completed the study. Compared with baseline, L-citrulline increased whereas ADMA and arginase concentration did not (values represent the mean Δ and 95% CI): plasma L-citrulline (190 μM, 84-297), plasma L-arginine (67 μM, 38-95), and plasma L-arginine/ADMA (ratio 117, 67-167). FeNO increased by 4.2 ppb (1.7-6.7 ppb); ACQ decreased by -0.46 (-0.67 to 0.27 points); the forced vital capacity and forced exhalation volume in 1 second, respectively, changed by 86 ml (10-161 ml) and 52 ml (-11 to 132 ml). In a secondary analysis, the greatest FEV1 increments occurred in those subjects with late-onset asthma (>12 years) (63 ml [95% CI, 1-137]), in females (80 ml [95% CI, 5-154]), with a greater change seen in late-onset females (100 ml, [95% CI, 2-177]). The changes in lung function or asthma control were not significantly associated with the changes before and after treatment in L-arginine/ADMA or FeNO.CONCLUSIONShort-term L-citrulline treatment improved asthma control and FeNO levels in obese asthmatics with low or normal FeNO. Larger FEV1 increments were observed in those with late-onset asthma and in females.TRIAL REGISTRATIONClinicalTrials.gov NCT01715844.FUNDINGNIH NHLBI R01 HL146542-01.

Keywords: Asthma; Pulmonology.

Conflict of interest statement

Conflict of interest: The authors have declared that no conflict of interest exists.

Figures

Figure 1. ADMA mediated NOS2 uncoupling in the airway epithelium.

As part of normal protein catabolism, arginine residues in many proteins undergo posttranslational dimethylation by protein arginine N-methyl transferases (PRMT), which use methyl groups donated from L-methionine. Subsequent proteolysis releases free asymmetric dimethyl arginine (ADMA), which can be either metabolized to L-citrulline and dimethylamine by dimethylarginine dimethylaminohydrolase (DDAH) or secreted into the circulation, where it is eventually excreted by the kidneys (48). Arginine succinate synthase (ASS) metabolizes L-citrulline to L-arginine, which can be used by NOS to generate NO. In addition, arginase competes with NOS to generate urea and ornithine from L-arginine. Given that increased oxidative stress is associated with reduced DDAH activity (48), and obesity as well as asthma have been associated with increased arginase activity (dark arrows), the combination of obesity and asthma can result in lowering L-arginine levels while increasing the concentration of ADMA. Having a low L-arginine to ADMA balance, favors NOS2 uncoupling. When this occurs, NOS2 preferentially produces anion superoxide instead of NO (gray arrows). Ultimately, this process may explain why some obese asthmatics have reduced NO airway bioavailability, with greater airway oxidative stress. Based on our previous findings showing that L-citrulline recouples airway epithelial NOS2, preventing these downstream events from occurring, we developed this pilot proof-of-concept study (5).

Figure 2. Consort diagram.

Figure 3. FeNO, ACQ, and plasma L-arginine/ADMA changes after a 2-week 15 g/d treatment with L-citrulline supplementation.

n = 41. FeNO mean Δ 4.2 ppb (95% CI, 1.7–6.7); ACQ mean Δ –0.46 (95% CI, –0.67 to –0.27); L-arginine/ADMA (ratio 117, 67–167).

Figure 4. Mean change in FEV1 before and after L-citrulline treatment by age of asthma onset and sex.

Late-onset asthma females = 24; early-onset females = 7. Δ (mean supplementation difference before and after L-citrulline treatment, 95% CI) FEV1 early onset: –11 ml (–155 to 17; P = 0.8); FEV1 late onset: 63 ml (1–137, P = 0.09); FEV1 females: 80 ml (5–154; P = 0.03); FEV1 males: –52 ml (–187 to 8; P = 0.3); FEV1 female early onset: –30 (–21, 200; P = 0.9); FEV1 female late onset 100 ml (16–184; P = 0.02). *P < 0.05.