AMP-activated protein kinase agonists increase mRNA content of the muscle-specific ubiquitin ligases MAFbx and MuRF1 in C₂C₁₂ cells

Abstract

hypothesis of the present study was that exposure of differentiated muscle cells to agonists of the AMP-activated protein kinase (AMPK) would increase the mRNA content of the muscle-specific ubiquitin ligases muscle atrophy F-box (MAFbx) and muscle RING finger 1 (MuRF1). C 2C12 cells were incubated with incremental doses of 5-aminoimidazol-4-carboximide ribonucleoside (AICAR) or metformin for 24 h. Both MAFbx and MuRF1 mRNA increased dose dependently in response to these AMPK activators. AICAR, metformin, and 2-deoxy-D-glucose produced time-dependent alterations in ubiquitin ligase expression, typified by a biphasic pattern of expression marked by an acute repression followed by a sustained induction. AMPK-activating treatments in conjunction with dexamethasone produced a pronounced synergistic effect on ligase mRNA expression at later time points. This cooperative response occurred in the absence of a dexamethasone-dependent increase in AMPK expression or activity, as determined by immunoblotting for phosphorylation and expression of AMPK␣ and its downstream target acetyl-CoA carboxylase (ACC). These responses elicited by AMPK activation singly or in combination with dexamethasone did not extend to the mRNA expression of the UBR box family E3s UBR1/E3␣I and UBR2/E3␣II. Treatment with the AMPK inhibitor compound C prevented increases in MAFbx and MuRF1 mRNA in response to serum deprivation, as well as AICAR and dexamethasone treatment individually or jointly. Stimulation of AMPK activity in vivo via AICAR injection increased both MAFbx and MuRF1 mRNA in murine skeletal muscle. These data suggest that activation of AMPK in skeletal muscle results in a specific upregulation of MAFbx and MuRF1, responses that are reminiscent of the proposed atrophic transcriptional program executed under various conditions of skeletal muscle wasting. Therefore, AMPK may be a critical component of the intercalated network of signaling pathways governing skeletal muscle atrophy, where its input acts to modify anti-and proatrophic signals to influence gene expression in reaction to catabolic perturbations.atrogin-1; proteolysis; 2-deoxyglucose FOR PROPER GROWTH AND SURVIVAL, all organisms require means of responding to diverse inputs originating from a discontinuous environmental milieu. Failure to both properly integrate these signals and accordingly adjust metabolism within favorable physiological limits may ultimately result in maladaptation, disease, and death. One critical component of this control system in mammalian systems is the AMP-activated protein kinase (AMPK). AMPK functions as a heterotrimer (consisting of a catalytic ␣-subunit and regulatory ␤-and ␥-subunits), which senses alterations in the AMP-to-ATP ratio within cells (15,25,35). In response to energy-depleting stressors (which consequently increase cellular AMP concentrations), AMPK acts to balance energy consumption with production by suppressing ATP-expensive processes and activating ATP-repleting ones. Regulation of its activity is achieved in an elegant fashion...

AICAR-induced activation of AMPK negatively regulates myotube hypertrophy through the HSP72-mediated pathway in C2C12skeletal muscle cells

“…Some studies have reported that these miRNAs promote proliferation and differentiation of muscle cells (3,18,29), whereas miR-1 and miR-133a seem to be downregulated during skeletal muscle hypertrophy (17,25). Furthermore, upregulations of miR-1 in dexamethasone-mediated muscle atrophy in mice and atrophic signals, which are integrated by miR-1, have been reported (19). In addition, it has been reported that miR-1 targets HSP72 during dexamethasone-mediated myotube atrophy (21).…”

“…Negative correlation between AMPK activity and the degree of hypertrophy in rat muscle have also been reported (35,39). These responses appear to be related to the deactivation of the signals in the protein synthesis pathway, mammalian target of rapamycin (mTOR)/p70 S6 kinase (p70 S6K ) (2, 22, 28), and to the activation of the signals in the protein degradation pathway, Forkhead box O transcription factors (Foxo) (31, 36, 41) and muscle-specific E3 ubiquitin ligases, such as muscle RING-finger 1 (MuRF1) and atrogin-1/muscle atrophy F-box (MAFbx) (19,30). Taken together, AMPK would act as a negative regulator for skeletal muscle mass through downregulation of protein synthesis and upregulation of protein degradation pathways.…”

Activation of AMPK inhibits cardiomyocyte hypertrophy by modulating of the FOXO1/MuRF1 signaling pathway in vitro

“…AMPK activation can influence the metabolism of cardiomyocytes by increasing glucose transport and glycolysis, as well as by regulating the oxidative phosphorylation of fatty acids [32] . Recent studies have shown AMPK activation enhances MuRF1 expression in skeletal muscle [21][22][23] . However, the relationship between AMPK and MuRF1 expression in the process of cardiac hypertrophy is still unclear.…”

“…Evidence from skeletal muscle suggests that AMPK activation up-regulates the expression of MuRF1 [21][22][23] . However, the role of AMPK activation on atrophy-related signals in cardiomyocytes has not been established.…”

Novel investigational drugs mimicking exercise for the treatment of cachexia

“…For example, AMPK activation induces the expression of muscle-specific ubiquitin ligases and stimulates autophagy (62) (63) . Genetic AMPK inactivation in the muscle results in reduced mitochondrial mass, loss of type I fibers and reduced lipid metabolism.…”