Mitochondrial fatty acid oxidation alterations in heart failure, ischaemic heart disease and diabetic cardiomyopathy.

Heart disease is a leading cause of death worldwide. In many forms of heart disease, including heart failure, ischaemic heart disease and diabetic cardiomyopathies, changes in cardiac mitochondrial energy metabolism contribute to contractile dysfunction and to a decrease in cardiac efficiency. Specific metabolic changes include a relative increase in cardiac fatty acid oxidation rates and an uncoupling of glycolysis from glucose oxidation. In heart failure, overall mitochondrial oxidative metabolism can be impaired while, in ischaemic heart disease, energy production is impaired due to a limitation of oxygen supply. In both of these conditions, residual mitochondrial fatty acid oxidation dominates over mitochondrial glucose oxidation. In diabetes, the ratio of cardiac fatty acid oxidation to glucose oxidation also increases, although primarily due to an increase in fatty acid oxidation and an inhibition of glucose oxidation. Recent evidence suggests that therapeutically regulating cardiac energy metabolism by reducing fatty acid oxidation and/or increasing glucose oxidation can improve cardiac function of the ischaemic heart, the failing heart and in diabetic cardiomyopathies. In this article, we review the cardiac mitochondrial energy metabolic changes that occur in these forms of heart disease, what role alterations in mitochondrial fatty acid oxidation have in contributing to cardiac dysfunction and the potential for targeting fatty acid oxidation to treat these forms of heart disease.

AMP-activated protein kinase response to contractions and treatment with the AMPK activator AICAR in young adult and old skeletal muscle.

One characteristic of ageing skeletal muscle is a decline in mitochondrial function. Activation of AMP-activated protein kinase (AMPK) occurs in response to an increased AMP/ATP ratio, which is one potential result of mitochondrial dysfunction. We have previously observed higher AMPK activity in old (O; 30 months) vs young adult (YA; 8 months) fast-twitch muscle in response to chronic overload. Here we tested the hypothesis that AMPK would also be hyperactivated in O vs YA fast-twitch extensor digitorum longus muscles from Fischer(344) x Brown Norway (FBN) rats (n = 8 per group) in response to high-frequency electrical stimulation of the sciatic nerve (HFES) or injection of AICAR, an activator of AMPK. Muscles were harvested immediately after HFES (10 sets of six 3-s contractions, 10 s rest between contractions, 1 min rest between sets) or 1 h after AICAR injection (1 mg (g body weight)(-1) subcutaneously). The phosphorylations of AMPKalpha and acetyl-CoA carboxylase (ACC2; a downstream AMPK target) were both greatly increased (P

Thyroid hormone effects on LKB1, MO25, phospho-AMPK, phospho-CREB, and PGC-1alpha in rat muscle.

Expression of all of the isoforms of the subunits of AMP-activated protein kinase (AMPK) and AMPK activity is increased in skeletal muscle of hyperthyroid rats. Activity of AMPK in skeletal muscle is regulated principally by the upstream kinase, LKB1. This experiment was designed to determine whether the increase in AMPK activity is accompanied by increased expression of the LKB1, along with binding partner proteins. LKB1, MO25, and downstream targets were determined in muscle extracts in control rats, in rats given 3 mg of thyroxine and 1 mg of triiodothyronine per kilogram chow for 4 wk, and in rats given 0.01% propylthiouracil (PTU; an inhibitor of thyroid hormone synthesis) in drinking water for 4 wk (hypothyroid group). LKB1 and MO25 increased in the soleus of thyroid hormone-treated rats vs. the controls. In other muscle types, LKB1 responses were variable, but MO25 increased in all. In soleus, MO25 mRNA increased with thyroid hormone treatment, and STRAD mRNA increased with PTU treatment. Phospho-AMPK and phospho-ACC were elevated in soleus and gastrocnemius of hyperthyroid rats. Thyroid hormone treatment also increased the amount of phospho-cAMP response element binding protein (CREB) in the soleus, heart, and red quadriceps. Four proteins having CREB response elements (CRE) in promoter regions of their genes (peroxisome proliferator-activated receptor-gamma coactivator-1alpha, uncoupling protein 3, cytochrome c, and hexokinase II) were all increased in soleus in response to thyroid hormones. These data provide evidence that thyroid hormones increase soleus muscle LKB1 and MO25 content with subsequent activation of AMPK, phosphorylation of CREB, and expression of mitochondrial protein genes having CRE in their promoters.

AMP-activated protein kinase phosphorylates transcription factors of the CREB family.

AMP-activated protein kinase (AMPK) has been identified as a regulator of gene transcription, increasing mitochondrial proteins of oxidative metabolism as well as hexokinase expression in skeletal muscle. In mice, muscle-specific knockout of LKB1, a component of the upstream kinase of AMPK, prevents contraction- and 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR)-induced activation of AMPK in skeletal muscle, and the increase in hexokinase II protein that is normally observed with chronic AICAR activation of AMPK. Since previous reports show a cAMP response element in the promoter region of the hexokinase II gene, we hypothesized that the cAMP-response element (CRE) binding protein (CREB) family of transcription factors could be targets of AMPK. Using radioisotopic kinase assays, we found that recombinant and rat liver and muscle AMPK phosphorylated CREB1 at the same site as cAMP-dependent protein kinase (PKA). AMPK was also found to phosphorylate activating transcription factor 1 (ATF1), CRE modulator (CREM), and CREB-like 2 (CREBL2), but not ATF2. Treatment of HEK-293 cells stably transfected with a CREB-driven luciferase reporter with AICAR increased luciferase activity approximately threefold over a 24-h time course. This increase was blocked with compound C, an AMPK inhibitor. In addition, AICAR-induced activation of AMPK in incubated rat epitrochlearis muscles resulted in an increase in both phospho-acetyl-CoA carboxylase and phospho-CREB. We conclude that CREB and related proteins are direct downstream targets for AMPK and are therefore likely involved in mediating some effects of AMPK on expression of genes having a CRE in their promoters.

LKB1 and the regulation of malonyl-CoA and fatty acid oxidation in muscle.

5'-AMP-activated protein kinase (AMPK), by way of its inhibition of acetyl-CoA carboxylase (ACC), plays an important role in regulating malonyl-CoA levels and the rate of fatty acid oxidation in skeletal and cardiac muscle. In these tissues, LKB1 is the major AMPK kinase and is therefore critical for AMPK activation. The purpose of this study was to determine how the lack of muscle LKB1 would affect malonyl-CoA levels and/or fatty-acid oxidation. Comparing wild-type (WT) and skeletal/cardiac muscle-specific LKB1 knockout (KO) mice, we found that the 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranoside (AICAR)-stimulated decrease in malonyl-CoA levels in WT heart and quadriceps muscles was entirely dependent on the presence of LKB1, as was the AICAR-induced increase in fatty-acid oxidation in EDL muscles in vitro, since these responses were not observed in KO mice. Likewise, the decrease in malonyl-CoA levels after muscle contraction was attenuated in KO gastrocnemius muscles, suggesting that LKB1 plays an important role in promoting the inhibition of ACC, likely by activation of AMPK. However, since ACC phosphorylation still increased and malonyl-CoA levels decreased in KO muscles (albeit not to the levels observed in WT mice), whereas AMPK phosphorylation was entirely unresponsive, LKB1/AMPK signaling cannot be considered the sole mechanism for inhibiting ACC during and after muscle activity. Regardless, our results suggest that LKB1 is an important regulator of malonyl-CoA levels and fatty acid oxidation in skeletal muscle.