Triggering Nox2 / ROS Signaling During Disuse-Induced Skeletal Muscle Atrophy

Prolonged periods of reduced mechanical loading lead to physiological changes such as a reduction in muscle mass and strength. These morphological and functional changes are allowed to take place to regulate muscle mass, metabolic demand, and availability of the amino acid pool. Dysregulation of a number of cell signaling pathways have been proposed as triggers of unloading-induced atrophy including Ca^2+, growth factors (e.g., IGF-1), heat shock proteins, and reactive oxygen species (ROS). ROS are causal in key disuse atrophy signaling events such as sarcolemma dislocation of nNOSμ and activation of the atrophy-inducing transcription factor FoxO3a. Upregulation of the Nox2 isoform of NADPH oxidase has been implicated in the disuse-associated increase in ROS production. Evidence indicates that activation of angiotensin II type 1 receptor (AT1R) is an upstream event leading to Nox2 complex formation and activity. In the current study, we used an AT1R blocker (losartan) to test the hypothesis that AT1R activation is an upstream signaling event driving Nox2 complex activity, nNOSμ dislocation, FoxO3a activation, and thus the slow-to-fast fiber type shift and atrophy in the unloaded soleus. Rats were divided into 4 groups: ambulatory control (CON), ambulatory + Losartan (40 mg/kg/day) (CONL), 7-days hindlimb unloaded (HU), and HU + Losartan (HUL). We report that administration of losartan attenuated unloading-induced phenotypic alterations in the soleus muscle, including the shift in fiber type and reductions in muscle fiber cross-sectional area (CSA). Losartan administration ameliorated increased ROS levels and the upregulation and activity of Nox2 following unloading. Furthermore, AT1R blockade during unloading mitigated nNOSμ dislocation from the sarcolemma. In addition, losartan treatment during disuse offered significant protection against the increase in nuclear localized FoxO3a. We conclude that AT1R attenuates disuse atrophy by inhibiting Nox2 signaling, thereby lessening the extent of nNOSμ dislocation, and activation of the pro-atrophic transcription factor FoxO3a. Our findings identify important downstream effects of AT1R activation during mechanical unloading-induced atrophy. Furthermore, these data provide support for the usage of the FDA-approved AT1R blocker, losartan, as a treatment to mitigate the effects of disuse atrophy that occur during instances such as prolonged bed rest, limb casting, and spaceflight