Chronic Endurance Exercise Impairs Cardiac Structure and Function in Middle-Aged Mice with Impaired Nrf2 Signaling.

Gobinath Shanmugam,Ronald P Mason,Ashutosh Kumar,Ramalingam Sankaran,John R Hoidal,Namakkal S Rajasekaran,Thiagarajan Sairam,Madhusudhanan Narasimhan,Robbie L Conley

doi:10.3389/fphys.2017.00268

Abstract

Nuclear factor erythroid 2 related factor 2 (Nrf2) signaling maintains the redox homeostasis and its activation is shown to suppress cardiac maladaptation. Earlier we reported that acute endurance exercise (2 days) evoked antioxidant cytoprotection in young WT animals but not in aged WT animals. However, the effect of repeated endurance exercise during biologic aging (WT) characterized by an inherent deterioration in Nrf2 signaling and pathological aging (pronounced oxidative susceptibility—Nrf2 absence) in the myocardium remains elusive. Thus, the purpose of our study was to determine the effect of chronic endurance exercise-induced cardiac adaptation in aged mice with and without Nrf2. Age-matched WT and Nrf2-null mice (Nrf2−/−) (>22 months) were subjected to 6 weeks chronic endurance exercise (25 meter/min, 12% grade). The myocardial redox status was assessed by expression of antioxidant defense genes and proteins along with immunochemical detection of DMPO-radical adduct, GSH-NEM, and total ubiquitination. Cardiac functions were assessed by echocardiography and electrocardiogram. At sedentary state, loss of Nrf2 resulted in significant downregulation of antioxidant gene expression (Nqo1, Ho1, Gclm, Cat, and Gst-α) with decreased GSH-NEM immuno-fluorescence signals. While Nrf2−/− mice subjected to CEE showed an either similar or more pronounced reduction in the transcript levels of Gclc, Nqo1, Gsr, and Gst-α in relation to WT littermates. In addition, the hearts of Nrf2−/− on CEE showed a substantial reduction in specific antioxidant proteins, G6PD and CAT along with decreased GSH, a pronounced increase in DMPO-adduct and the total ubiquitination levels. Further, CEE resulted in a significant upregulation of hypertrophy genes (Anf, Bnf, and β-Mhc) (p < 0.05) in the Nrf2−/− hearts in relation to WT mice. Moreover, the aged Nrf2−/− mice exhibited a higher degree of cardiac remodeling in association with a significant decrease in fractional shortening, pronounced ST segment, and J wave elevation upon CEE compared to age-matched WT littermates. In conclusion, our findings indicate that while the aged WT and Nrf2 knockout animals both exhibit hypertrophy after CEE, the older Nrf2 knockouts showed ventricular remodeling coupled with profound cardiac functional abnormalities and diastolic dysfunction.

Highlights

Heart failure is one of the major health problems in the globe and is the leading cause of death in the United States, accounting for 1 in every 4 deaths
Immunoblotting analyses revealed that the expression of cytoprotective antioxidant proteins, namely GPX1 and SOD1 remained unaltered in the hearts of Nrf2−/− mice subjected to chronic endurance exercise (CEE) compared to its WT counterparts
Our findings indicate that ablation of Nrf2 leads to (i) diminished levels of a subset of antioxidant genes and proteins in the heart, (ii) augmented oxidative stress (OS) associated with a profound increase in the extent of ubiquitination observed in Nrf2−/− mice subjected to CEE, (iii) pronounced cardiac remodeling, (iv) abnormal cardiac electrophysiological characteristics following CEE compared to its WT counterparts, (v) both WT and Nrf2−/− mice exhibit significant decrease in systolic function (EF) post CEE, but Nrf2−/− showed pronounced diastolic dysfunction

Summary

Introduction

Heart failure is one of the major health problems in the globe and is the leading cause of death in the United States, accounting for 1 in every 4 deaths. Age-associated oxidative stress (OS) is considered to be one of the principal risk factors for cardiovascular disease (CVD) due to its ability to induce changes including but not limited to signaling, inflammation, structural, and functional processes in cardiomyocytes (Bayes et al, 2006; Cui et al, 2012) These alterations when occurring for a chronic period under aging blunts both the adaptive as well as the regenerative ability of the heart leading to pathological remodeling and heart failure (Munzel et al, 2015; Rungatscher et al, 2015; Jeong et al, 2016; Kumar et al, 2016). Pathogenic consequences of OS on ventricular dysfunction in failing hearts have received clinical attention (Takimoto and Kass, 2007; Fraccarollo et al, 2015), the mechanisms associated with antioxidant transcriptional signaling in response to a physical activity in an aging heart remains largely unknown

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