AMPK‐independent pathways regulate skeletal muscle fatty acid oxidation

Abstract

The activation of AMP-activated protein kinase (AMPK) and phosphorylation/inhibition of acetyl-CoA carboxylase 2 (ACC2) is believed to be the principal pathway regulating fatty acid oxidation. However, during exercise AMPK activity and ACC Ser-221 phosphorylation does not always correlate with rates of fatty acid oxidation. To address this issue we have investigated the requirement for skeletal muscle AMPK in controlling aminoimidazole-4-carboxymide-1-β-d-ribofuranoside (AICAR) and contraction-stimulated fatty acid oxidation utilizing transgenic mice expressing a muscle-specific kinase dead (KD) AMPK α2. In wild-type (WT) mice, AICAR and contraction increased AMPK α2 and α1 activities, the phosphorylation of ACC2 and rates of fatty acid oxidation while tending to reduce malonyl-CoA levels. Despite no activation of AMPK in KD mice, ACC2 phosphorylation was maintained, malonyl-CoA levels were reduced and rates of fatty acid oxidation were comparable between genotypes. During treadmill exercise both KD and WT mice had similar values of respiratory exchange ratio. These studies suggested the presence of an alternative ACC2 kinase(s). Using a phosphoproteomics-based approach we identified 18 Ser/Thr protein kinases whose phosphorylation was increased by greater than 25% in contracted KD relative to WT muscle. Utilizing bioinformatics we predicted that extracellular regulated protein-serine kinase (ERK1/2), inhibitor of nuclear factor (NF)-κB protein-serine kinase β (IKKβ) and protein kinase D (PKD) may phosphorylate ACC2 at Ser-221 but during in vitro phosphorylation assays only AMPK phosphorylated ACC2. These data demonstrate that AMPK is not essential for the regulation of fatty acid oxidation by AICAR or muscle contraction.

Reduced AMP-activated protein kinase activity in mouse skeletal muscle does not exacerbate the development of insulin resistance with obesity

“…Muscle AMPK activity and ACC phosphorylation In agreement with previous reports [33,40,41], α2AMPK activity was reduced in the gastrocnemius (~50%), soleus (~80%) and EDL (~95%) muscles from KD mice compared with wild-type (Fig. 1a,c).…”

“…These findings are in agreement with previous studies [38,40,41] and suggest that residual α1AMPK activity is unable to compensate for the reduced α2AMPK activity when it comes to ACC phosphorylation. Surprisingly, this dramatic reduction in resting/basal ACC phosphorylation in KD mice did not result in greater accumulation of muscle lipids or increases in body mass, epididymal fat pad weight or serum leptin between high-fat fed KD and wild-type mice.…”

“…AMPK regulates muscles lipid oxidation through phosphorylation and deactivation of ACC2 [22,40,41]. As previously demonstrated [33,40,41], ACC2 serine 228 phosphorylation was reduced by~90% in KD gastrocnemius muscle (Fig. 1c).…”

“…As AMPK is thought to control fatty acid oxidation via the ACC2→malonyl-CoA→CPT-1 axis [20,46] and activation of this pathway prevents lipid-induced insulin resistance [38,[47][48][49], we investigated whether a deficiency in muscle AMPK would exacerbate obesity-related insulin resistance due to increased accumulation of bioactive lipid species. We used a mouse model specifically overexpressing a KD AMPKα2 in skeletal and heart muscle and characterised by a substantial reduction in ACC2 phosphorylation [40]. We assessed the efficacy with which insulin increased Akt phosphorylation and glucose uptake after 12 weeks of a high-fat diet and found, somewhat unexpectedly, that muscles from KD mice develop insulin resistance to a similar degree to their wild-type littermates and thus do not become more insulinresistant with obesity as initially hypothesised.…”

Intramyocellular lipid accumulation is associated with permanent relocation ex vivo and in vitro of fatty acid translocase (FAT)/CD36 in obese patients

“…Moreover, beneficial effects of physical activity in obese and type 2 diabetic patients are associated with an increase in the oxidative capacity of skeletal muscle potentially through activation of AMPactivated protein kinase (AMPK) [13]. However, skeletal muscle fatty acid oxidation can be regulated independently of AMPK [14]. Moreover, the theory of mitochondrial dysfunction in type 2 diabetes has been refuted by other studies, which have shown that mitochondrial function was not altered in type 2 diabetes skeletal muscle after normalisation to mitochondrial content [15] or when control and type 2 diabetic participants were matched for body composition [16].…”