[Thyroid hormones and muscle phenotype: involvement of new signaling pathways]. / Hormones thyroïdiennes et phénotype musculaire : proposition d'implication de nouvelles voies de régulation.

Thyroid hormones (TH) are known to control development, body and muscle growth, as well as to determine muscle phenotype in the adult. TH affect muscle properties through nuclear receptors; they act either by a positive or a negative control on target genes that encode proteins accounting for contractile or metabolic phenotypes. Contractile activity and muscle load also affect muscle phenotype; several intracellular signaling pathways are involved in the transduction of signals related to contractile activity, including the calcineurin/NFAT pathway. Calcineurin activity is negatively controlled by MCIP-1 protein (modulatory calcineurin-interacting protein-1). We recently performed an experiment aimed at examining the specific and combined effects of the pharmacological calcineurin inhibition (using cyclosporin-A CsA administration) and thyroid hormone deficiency. The expected effects of CsA administration were only observed if TH were available, while thyroid deficiency totally blunted the muscle responses to calcineurin inhibition. In conditions of thyroid hormone deficiency, there was no response to the pharmacological inhibition of calcineurin, usually known to induce a slow-to-fast IIA transition associated with an enhancement of mitochondrial biogenesis in normothyroid rats. Moreover, thyroid deficiency markedly decreased the expression of MCIP-1 and MCIP-2 mRNA and proteins, two endogenous calcineurin inhibitors; such results clearly suggest that thyroid hormone and calcineurin pathways are interconnected.

Thyroid hormone is required for the phenotype transitions induced by the pharmacological inhibition of calcineurin in adult soleus muscle of rats.

The present experiment was designed to examine the effects of hypothyroidism and calcineurin inhibition induced by cyclosporin A (CsA) administration on both contractile and metabolic soleus muscle phenotypes, with a novel approach to the signaling pathway controlling mitochondrial biogenesis. Twenty-eight rats were randomly assigned to four groups, normothyroid, hypothyroid, and orally treated with either CsA (25 mg/kg, N-CsA and H-CsA) or vehicle (N-Vh and H-Vh), for 3 wk. Muscle phenotype was estimated by the MHC profile and activities of oxidative and glycolytic enzymes. We measured mRNA levels of the peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha), the major regulator of mitochondrial content. We also studied the expression of the catalytic A-subunit of calcineurin (CnA) both at protein and transcript levels and mRNA levels of modulatory calcineurin inhibitor proteins (MCIP)-1 and -2, which are differentially regulated by calcineurin activity and thyroid hormone, respectively. CsA-administration induced a slow-to-fast MHC transition limited to the type IIA isoform, which is associated with increased oxidative capacities. Hypothyroidism strongly decreased both the expression of fast MHC isoforms and oxidative capacities. Effects of CsA administration on muscle phenotype were blocked in conditions of thyroid hormone deficiency. Changes in the oxidative profile were strongly related to PGC-1 alpha changes and associated with phosphorylation of p38 MAPK. Calcineurin and MCIPs mRNA levels were decreased by both hypothyroidism and CsA without additive effects. Taken together, these results suggest that adult muscle phenotype is primarily under the control of thyroid state. Physiological levels of thyroid hormone are required for the effects of calcineurin inhibition on slow oxidative muscle phenotype.

Mitochondrial and energetic cardiac phenotype in hypothyroid rat. Relevance to heart failure.

Changes in thyroid status are associated with profound alterations in biochemical and physiological functioning of cardiac muscle, although its impact on cardiac energy metabolism is still debated. Similarities between the changes in cardiac gene expression in pathological hypertrophy leading to heart failure and hypothyroidism prompted scientists to suggest a role for thyroid hormone status in the development of metabolic and functional alterations in this disease. We thus investigated the effects of hypothyroidism on cardiac energy metabolism. Hypothyroid state (HYPO) was induced by thyroidectomy and propyl-thio-uracyl in male rats for 3 weeks. We examined the effects of hypothyroid state on oxidative capacity and mitochondrial substrate utilization by measuring oxygen consumption of saponin permeabilized cardiac fibers, mitochondrial biogenesis by reverse transcription polymerase chain reaction and energy metabolism, and energy transfer enzymes by spectrophotometry. The results show that maximal oxidative capacity of the myocardium was decreased from 24.9 +/- 0.9 in control (CT) to 19.3 +/- 0.7 micromol O(2) min(-1) g dry weight(-1) in HYPO. However, protein content and messenger RNA (mRNA) of PGC-1alpha and mRNA of its transcription cascade that is thought to control mitochondrial content in normal myocardium and heart failure, were unchanged in HYPO. Mitochondrial utilization of glycerol-3P (-70%), malate (-45%), and octanoate (-24%) but not pyruvate was decreased in HYPO. Moreover, the creatine kinase system and energy transfer were hardly affected in HYPO. Besides, hypothyroidism decreased the activation of other signaling pathways like p38 mitogen-activated protein kinases, AMP-activated protein kinase, and calcineurin. These results show that cellular hypothyroidism can hardly account for the specific energetic alterations of heart failure.

Role of hypoxia-induced anorexia and right ventricular hypertrophy on lactate transport and MCT expression in rat muscle.

To dissect the independent effects of altitude-induced hypoxemia and anorexia on the capacity for cardiac lactate metabolism, we examined the effects of 21 days of chronic hypobaric hypoxia (CHH) and its associated decrease in food intake and right ventricle (RV) hypertrophy on the monocarboxylate transporter 1 and 4 (MCT) expression, the rate of lactate uptake into sarcolemmal vesicles, and the activity of lactate dehydrogenase isoforms in rat muscles. In comparison with control rats (C), 1 mmol/L lactate transport measured on skeletal muscle sarcolemmal vesicles increased by 33% and 58% in hypoxic (CHH, barometric pressure = 495 hPa) and rats pair-fed an equivalent quantity of food to that consumed by hypoxic animals, respectively. The increased lactate transport was higher in PF than in CHH animals ( P < .05). No associated change in the expression of MCT1 protein was observed in skeletal muscles, whereas MCT1 mRNA decreased in CHH rats, in comparison with C animals (42%, P < .05), partly related to caloric restriction (30%, P < .05). MCT4 mRNA and protein increased during acclimatization to hypoxia only in slow-oxidative muscles (68%, 72%, P < .05, respectively). The MCT4 protein content did not change in the plantaris muscle despite a decrease in transcript levels, related to hypoxia and caloric restriction. In both the left and right ventricles, the MCT1 protein content was unaffected by ambient hypoxia or restricted food consumption. These results suggest that MCT1 and MCT4 gene expression in fast-glycolytic muscles is mainly regulated by posttranscriptional mechanisms. Moreover, the results emphasize the role played by caloric restriction on the control of gene expression in response to chronic hypoxia and suggest that hypoxia-induced right ventricle hypertrophy failed to alter MCT proteins.

Does angiotensin-converting enzyme inhibition improve the energetic status of cardiac and skeletal muscles in heart failure induced by aortic stenosis in rats?

Recently, we have demonstrated that heart failure in rats is associated with a myopathy altering energy metabolism in different muscles, but the origin of this myopathy is still unknown. Here, we studied the possible involvement of increased angiotensin II (Ang II) by treatment with perindopril, an inhibitor of angiotensin-converting enzyme (ACE). The beneficial effects of ACE inhibition could result either from vasodilatation-induced cardiac unloading or from inhibition of the direct angiotensin action on the muscle cells. The model of aortic banding with persisting left ventricular (LV) overload where the cardiac unloading does not occur allows to distinguish between the two effects of ACE inhibition. Four months after aortic clipping (just before the treatment), echocardiographic study showed an impairment of the systolic function (decrease of the LV shortening by 30% and ejection fraction by 21%). Ten-week treatment with perindopril dramatically decreased Ang II plasma level but did not reduce LV hypertrophy though a significant decrease in right ventricular (RV) hypertrophy occurred. Perindopril did not improve alterations in activities of energy metabolism enzymes (creatine kinase, citrate synthase, cytochrome c oxidase, lactate dehydrogenase) either in ventricular or in skeletal (gastrocnemius) muscle. Similarly, ACE inhibition did not improve the main parameters of mitochondrial respiration in permeabilized muscle fibers. These data suggest that the generalized metabolic myopathy induced by the hemodynamic abnormalities conditioned by the continuous LV overload (aorta clipping) does not result from the increase in Ang II level per se. Correction of hemodynamic parameters and LV unloading seem to be the prerequisite for the improvement of muscle energy metabolism abnormalities.

Quantitative and qualitative adaptation of skeletal muscle mitochondria to increased physical activity.

Endurance capacity rely on high muscle oxidative capacity but should also involve a tighter coupling between energy production and utilization within the myocyte. The present study examined the responses of muscle oxidative capacity and the regulation of oxidative phosphorylation by phosphate acceptors in locomotor muscles of voluntary running rats (n = 8), using saponin permeabilized fibers of the deep and superficial parts of plantaris muscle (dPLA and sPLA, respectively). Non-ADP stimulated respiration of skinned fibers increased by 33% (P < 0.05) and 100% (P < 0.001) in sPLA and dPLA, respectively. The maximal ADP-stimulated respiration was 57% (P < 0.001) and 32% (P < 0.01) higher in active rats than in sedentary rats (n = 8), in sPLA and dPLA, respectively. This finding was consistent with a 72% increase in the CS activity in plantaris muscle of exercising rats (P < 0.01). Voluntary running induced a 334% increase in the apparent Km for ADP in sPLA (P < 0.001), and a 61% increase in dPLA (P < 0.05), showing a lower affinity for cytosolic ADP of mitochondria present in both, predominantly glycolytic, and oxidative fibers. There was an increase in the creatine kinase efficacy in both sPLA and dPLA (131%, 75%, P < 0.001, respectively), consistent with an increase in the activity of the mitochondrial isoform of creatine kinase (106%, P < 0.01). It is concluded that, in addition to the well-known increased oxidative capacity, voluntary running is associated with changes in the regulation of oxidative phosphorylation by phosphate acceptors, in both glycolytic and oxidative fibers, in the direction of increased coupling between energy production and energy utilization.