Involvement of receptor for advanced glycation end products in microgravity-induced skeletal muscle atrophy in mice

Highlights

• AGEs-RAGE axis in skeletal muscle was upregulated by 1-week hindlimb suspension.

• AGE accumulation in skeletal muscle correlated with loss of muscle mass.

• RAGE inhibition attenuated hindlimb suspension-induced skeletal muscle atrophy.

• Inflammatory response might contribute to RAGE-associated skeletal muscle atrophy.

Abstract

The accumulation of advanced glycation end-products (AGEs) may be involved in the mechanism of skeletal muscle atrophy. However, the involvement of the receptor for AGEs (RAGE) axis in microgravity-induced skeletal muscle atrophy has not been investigated. Therefore, the purpose of the present study was to investigate the effect of RAGE inhibition on microgravity-induced skeletal muscle atrophy and the related molecular responses. Male C57BL/6NCr mice subjected to a 1-week hindlimb suspension lead to muscle atrophy in soleus and plantaris but not extensor digitorum longus muscle, accompanied by increases in RAGE expression. However, treatment with a RAGE antagonist (FPS-ZM1, intraperitoneal, 1 mg/kg/day) during hindlimb suspension ameliorated the atrophic responses in soleus muscle. Further, muscle mass inversely correlated with the accumulation of AGEs (methylglyoxal-modified proteins and Nε-(carboxymethyl) lysine-modified proteins) in soleus muscle. The expression of proinflammatory cytokines, tumor necrosis factor-α, interleukin-1β, and interleukin-6 in soleus muscle was enhanced in response to hindlimb suspension, but these changes were attenuated by FPS-ZM1 treatment. Protein ubiquitination and ubiquitin E3 ligase (muscle RING finger 1) expression in soleus muscle were elevated following hindlimb suspension, and these increments were suppressed by FPS-ZM1 treatment. Our findings indicate that the AGE-RAGE axis is upregulated in unloaded atrophied skeletal muscle, and that RAGE inhibition ameliorates microgravity-induced skeletal muscle atrophy by reducing proinflammatory cytokine expression and ubiquitin-proteasome system activation.

Introduction

Skeletal muscle mass is regulated by the balance between protein synthesis and degradation, which increase and reduce it, respectively. Skeletal muscle atrophy can be caused by several physiological and pathological conditions, such as unloading, aging, malnutrition, burns, diabetes, cancer cachexia, sepsis, chronic renal failure, and chronic obstructive pulmonary disease [1]. In particular, exposure to microgravity environments has been well reported to result in marked skeletal muscle atrophy accompanied by the changes of morphological, metabolic, and contractile properties [2].

Recently, glycation can been shown to be involved in the mechanism of skeletal muscle atrophy [3,4]. Glycation is non-enzymatic reaction between reducing sugars or aldehydes with proteins, DNA, or lipids, resulting in the formation of glycation adducts and advanced glycation end-products (AGEs). Glycation results in cell and tissue damage by inhibiting the biological functions of proteins and activating the AGE receptor (receptor for advanced glycation end-products, RAGE) [5]. Epidemiological studies have shown that AGE accumulation is associated with low skeletal muscle quality [6,7]. In addition, experimental studies have demonstrated that the treatment of cultured muscle cells with AGEs induces muscle atrophy [3,8] and that long-term consumption of an AGE-containing diet results in the accumulation of AGEs in skeletal muscle and muscle dysfunction [9].

AGEs stimulate several signaling pathways via a series of cell surface receptors, the most studied of which is RAGE, a multi-ligand member of the immunoglobulin superfamily. The involvement of the AGE-RAGE axis in several diseases, including diabetic complications, cardiovascular disease, Alzheimer's disease, and osteoporosis is well established [10]. In this context, a variety of RAGE antagonists are now available for preclinical and clinical studies [11,12]. For example, TTP488 (azeliragon), which is an orally-active small-molecule antagonist of RAGE, improves cognitive function in Alzheimer disease patients by inhibiting inflammation and amyloid-β accumulation [13]. FPS-ZM1, which was identified by screening 5000 compounds for their ability to inhibit RAGE and amyloid-β interaction, can block amyloid-β-induced cellular stress in RAGE-expressing brain endothelium, neurons, and microglia [14].

It has been shown that AGEs induce muscle atrophy via RAGE-mediated signaling in cultured muscle cells [3] and that AGE-induced impairment in insulin signaling is mediated by RAGE in cultured muscle cells and rats [15]. Furthermore, a recent study has shown that pharmacological inhibition of RAGE ameliorates the aging-induced loss of muscle mass in middle-aged mice [16]. These evidences suggest that inhibition of the AGE-RAGE axis may be an effective means of treating skeletal muscle atrophy under conditions in which the AGE-RAGE axis is activated. However, it has not been investigated whether the AGE-RAGE axis is activated on microgravity environment, or whether inhibition of the AGE-RAGE axis ameliorates microgravity-induced skeletal muscle atrophy and the related molecular responses. In the present study, therefore, we investigated the involvement of AGE-RAGE axis, by using the RAGE antagonist, FPS-ZM1, in skeletal muscle atrophy following hindlimb suspension, which is a well-established approach to create a ground-based model of microgravity.