Involvement of AMPK in regulating slow-twitch muscle atrophy during hindlimb unloading in mice

Abstract

-AMPK is considered to have a role in regulating skeletal muscle mass. However, there are no studies investigating the function of AMPK in modulating skeletal muscle mass during atrophic conditions. In the present study, we investigated the difference in unloading-associated muscle atrophy and molecular functions in response to 2-wk hindlimb suspension between transgenic mice overexpressing the dominant-negative mutant of AMPK (AMPK-DN) and their wild-type (WT) littermates. Male WT (n ϭ 24) and AMPK-DN (n ϭ 24) mice were randomly divided into two groups: an untreated preexperimental control group (n ϭ 12 in each group) and an unloading (n ϭ 12 in each group) group. The relative soleus muscle weight and fiber cross-sectional area to body weight were decreased by ϳ30% in WT mice by hindlimb unloading and by ϳ20% in AMPK-DN mice. There were no changes in puromycin-labeled protein or Akt/70-kDa ribosomal S6 kinase signaling, the indicators of protein synthesis. The expressions of ubiquitinated proteins and muscle RING finger 1 mRNA and protein, markers of the ubiquitin-proteasome system, were increased by hindlimb unloading in WT mice but not in AMPK-DN mice. The expressions of molecules related to the protein degradation system, phosphorylated forkhead box class O3a, inhibitor of B␣, microRNA (miR)-1, and miR-23a, were decreased only in WT mice in response to hindlimb unloading, and 72-kDa heat shock protein expression was higher in AMPK-DN mice than in WT mice. These results imply that AMPK partially regulates unloading-induced atrophy of slow-twitch muscle possibly through modulation of the protein degradation system, especially the ubiquitin-proteasome system. AMP-activated protein kinase; protein degradation; autophagy; ubiquitin-proteasome; microRNA; heat shock protein SKELETAL MUSCLE IS THE LARGEST TISSUE IN THE BODY, accounting for ϳ40% of the total body mass, and has a crucial role in metabolism as well as locomotion. Skeletal muscle has a high ability to adapt to multiple stimuli. Increased loading, such as resistance training and mechanical stretching, leads to skeletal muscle hypertrophy (18,75). In contrast, aging, poor nutrition, several diseases such as diabetes, cancer, sepsis, and chronic renal failure, and decreased loading, such as inactivity, lead to skeletal muscle atrophy (23,34,39). Skeletal muscle atrophy occurs as a result of changes in protein turnover: decreased protein synthesis, increased protein degradation, or a combination of both (17). The coordination of protein turnover in the atrophic process is regulated by complicated molecular responses, and the molecular mechanism involved in this process in skeletal muscle is not yet completely understood and remains to be elucidated.5=-AMP-activated protein kinase (AMPK) is well established as a metabolic sensor that helps maintain cellular energy homeostasis by modulating glucose, lipid, and protein metabolism (16,26,28). AMPK is a heterotrimeric kinase comprising a catalytic ␣-subunit and two regulatory subunits, the ␤-and ␥-subunits. Two ...

Protein arginine methyltransferase expression, localization, and activity during disuse-induced skeletal muscle plasticity

“…PGC-1α, AMPK, and p38 expression and/or activity are affected during skeletal muscle atrophy (37,57,58,59,60). Moreover, these powerful phenotype-bending molecules also participate in mediating disuse-induced remodelling (61,62,63,64,65). We examined these proteins in an effort to enhance their temporal resolution during the early stages of muscle disuse.…”

“…Several studies have reported increased p38 activity in response to immobilization, hind limb suspension and denervation (53,54,55,56,57,62,66,67,69). In support of the notion that elevation of p38 activity promotes protein degradation, numerous papers have showed that induction of MuRF1 can be blocked by p38 inhibitors and that constitutive activation of p38 induces MuRF1 gene expression in vivo and in vitro (57,63,65).…”

“…Furthermore, downstream events are currently unclear with respect to p38. Several studies have found that activation of p38 induces muscle RING finger 1 (MuRF1) and muscle atrophy F-box (MAFbx) (57,60,61,63,64,65,69) expression, while others have shown the opposite outcome (70,71). In response to disuse, the inhibitory effect of PGC-1α and AKT on forkhead box O3a (FOXO3a) is attenuated (43,94), while augmented AMPK activation leads to increased FOXO3a transcriptional activity (76,80).…”

Rapid decline in MyHC I(β) mRNA expression in rat soleus during hindlimb unloading is associated with AMPK dephosphorylation

“…As for the findings of Egawa et al . (), they were done on murine soleus muscle, which is quite different from the rat one (40–50% of slow‐twitch fibres vs . 85% in rat and humans).…”

“…Egawa et al . () did not find any changes in AMPK phosphorylation as well as in acetyl‐CoA carboxylase (ACC) phosphorylation levels in murine soleus muscle after 10 days of hindlimb unloading. Recently, we have found a significant decrease in the content of phosphorylated AMPK in rat soleus after 24‐h HU (Mirzoev et al .…”