Metabolic Syndrome Is Associated with Increased Oxo-Nitrative Stress and Asthma-Like Changes in Lungs

Abstract

Epidemiological studies have shown an increased obesity-related risk of asthma. In support, obese mice develop airway hyperresponsiveness (AHR). However, it remains unclear whether the increased risk is a consequence of obesity, adipogenic diet, or the metabolic syndrome (MetS). Altered L-arginine and nitric oxide (NO) metabolism is a common feature between asthma and metabolic syndrome that appears independent of body mass. Increased asthma risk resulting from such metabolic changes would have important consequences in global health. Since high-sugar diets can induce MetS, without necessarily causing obesity, studies of their effect on arginine/NO metabolism and airway function could clarify this aspect. We investigated whether normal-weight mice with MetS, due to high-fructose diet, had dysfunctional arginine/NO metabolism and features of asthma. Mice were fed chow-diet, high-fat-diet, or high-fructose-diet for 18 weeks. Only the high-fat-diet group developed obesity or adiposity. Hyperinsulinemia, hyperglycaemia, and hyperlipidaemia were common to both high-fat-diet and high-fructose-diet groups and the high-fructose-diet group additionally developed hypertension. At 18 weeks, airway hyperresponsiveness (AHR) could be seen in obese high-fat-diet mice as well as non-obese high-fructose-diet mice, when compared to standard chow-diet mice. No inflammatory cell infiltrate or goblet cell metaplasia was seen in either high-fat-diet or high-fructose-diet mice. Exhaled NO was reduced in both these groups. This reduction in exhaled NO correlated with reduced arginine bioavailability in lungs. In summary, mice with normal weight but metabolic obesity show reduced arginine bioavailability, reduced NO production, and asthma-like features. Reduced NO related bronchodilation and increased oxo-nitrosative stress may contribute to the pathogenesis.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Fig 1. The obese and non-obese model of metabolic syndrome in mice.

(A) Total mass, Lean mass, Fat mass and % Fat in control, high fat and high fructose diet groups. (B) Cholesterol levels in blood serum from Control, HFA and HFR diet mice groups. (C) Triglycerides levels in blood serum from Control, HFA and HFR diet mice groups. Data shown here are Mean ± SE of 6 mice in each group.*Denotes statistically significant differences (p

Fig 2. Increased baseline resistance and decreased exhaled NO in mice with metabolic syndrome.

(A-B) Baseline lung resistance (R) and elastance (E) in anesthetized and ventilated control, high fat (HFA) and high fructose (HFR) diet fed mice (Mean ± SE, n = 12 per group, two independent sets of experiments). (C) Non-invasively measured exhaled NO of control, HFA and HFR mice (n = 6 per group). All data are Mean ± SE. n = 6 mice in each group *Denotes statistically significant differences (p

Fig 3. High-fat or high-sugar diets lead to increased ADMA associated oxo-nitrative stress and organelle dysfunction, independent of obesity.

(A) ADMA levels were estimated from CN, HFA and HFR mice in total lung protein (TLP). (B) L-Arigine/ADMA ratio in lung. (C) ADMA levels in serum. (D) Nitrotyrosine levels in lung cytosolic fraction. All data are Mean ± SE. n = 6 mice in each group.*Denotes statistically significant differences (P

Fig 4. Induction of Inducible Nitric oxide synthase (iNOS) in lungs of mice with metabolic syndrome.

(A) Western blot analysis of eNOS. (B) Western blot analysis of iNOS (C) Densitometry of eNOS (D) Densitometry of iNOS (E) Immunohistochemistry of iNOS. Brown colour indicates the positive expressions. HFA and HFR diet fed mice showed high expression of iNOS as compared to Control mice. Representative images are shown from each group. All photographs are at 10X magnification. Scale bar = 100μm. (F) Quantitative analysis for iNOS done using ImageJ software showed significant increase in its expression in HFA and HFR mice as compared to CN.*Denotes statistically significant differences (P

Fig 5. Increase in arginase levels in lungs of mice with metabolic syndrome.

(A) Western blot analysis of arginase 1 (B) Densitometry of arginase 1 (C) Quantitative morphometry for arginase 1 IHC (D) Immunohistochemistry of arginase, Brown colour indicates positive expression. Representative images are shown from each group. All photographs are at 10X magnification. Scale bar = 100μm. *Denotes statistically significant differences (P

Fig 6. Lack of cellular inflammation in lungs of mice with metabolic syndrome.

(A) Lung sections were stained with haematoxylin and eosin to estimate airway inflammation. (B) Inflammation score of the lungs was evaluated by experimentally blind experts and shown as perivascular (PV), peribronchial (PB) and Total (sum of both PV and PB). (Representative images are shown from each group. All photographs are at 10X magnification. Scale bar = 100μm.

Fig 7. Arginine-NO pathway.