Triiodothyronine (T3) is considered a major regulator of mitochondrial activity. In this review, we show evidence of the existence of a direct T3 mitochondrial pathway, and try to clarify the respective importance of the nuclear and mitochondrial pathways for organelle activity. Numerous studies have reported short-term and delayed T3 stimulation of mitochondrial oxygen consumption. Convincing data indicate that an early influence occurs through an extra-nuclear mechanism insensitive to inhibitors of protein synthesis. Although it has been shown that diiodothyronines could actually be T3 mediators of this short-term influence, the detection of specific T3-binding sites, probably corresponding to a 28 kDa c-Erb A 1 protein of the inner membrane, also supports a direct T3 influence. The more delayed influence of thyroid hormone upon mitochondrial respiration probably results from mechanisms elicited at the nuclear level, including changes in phospholipid turnover and stimulation of uncoupling protein expression, leading to an increased inner membrane proton leak. However, the involvement of a direct mitochondrial T3 pathway leading to a rapid stimulation of mitochondrial protein synthesis has to be considered.Both pathways are obviously involved in the T3 stimulation of mitochondrial genome transcription. First, a 43 kDa c-Erb A 1 protein located in the mitochondrial matrix (p43), acting as a potent T3-dependent transcription factor of the mitochondrial genome, induces early stimulation of organelle transcription. In addition, T3 increases mitochondrial TFA expression, a mitochondrial transcription factor encoded by a nuclear gene. Similarly, the stimulation of mitochondriogenesis by thyroid hormone probably involves both pathways. In particular, the c-erb A gene simultaneously encodes a nuclear and a mitochondrial T3 receptor (p43), thus ensuring coordination of the expression of the mitochondrial genome and of nuclear genes encoding mitochondrial proteins.Recent studies concerning the physiological importance of the direct mitochondrial T3 pathway involving p43 led to the conclusion that it is not only involved in the regulation of fuel metabolism, but also in the regulation of cell differentiation. As the processes leading to or resulting from differentiation are energy-consuming, p43 coordination of metabolism and differentiation could be of significant importance in the regulation of development.

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We have previously shown that mitochondrial activity is an important regulator of myoblast differentiation, partly through processes targeting myogenin expression. Here, we investigated the possible involvement of c-myc in these processes. Inhibition of mitochondrial activity by chloramphenicol abrogated the decrease in c-myc mRNA and protein levels occurring at the onset of terminal differentiation. Conversely, stimulation of mitochondrial activity by overexpression of the T3 mitochondrial receptor (p43) down-regulated c-myc expression. In addition, c-myc overexpression mimicked the influence of mitochondrial activity inhibition on myoblast differentiation. Moreover, like chloramphenicol, c-myc overexpression strongly inhibited the myogenic influence of p43 overexpression. These data suggest that c-Myc is an important target of mitochondrial activity involved in the myogenic influence of the organelle. Lastly, we found that chloramphenicol influence is negatively related to the frequency of post-mitotic myoblasts in the culture at the onset of treatment, and cell cycle analyses demonstrated that the frequency of myoblasts in G0-G1 phase at cell confluence is increased by p43 overexpression and decreased by chloramphenicol or c-myc overexpression. These results suggest that irreversible myoblast withdrawal from the cell cycle is a target of mitochondrial activity by control of c-Myc expression.

Dietary lipids are known to affect the composition of the biological membrane and functions that are involved in cell death and survival. The mitochondrial respiratory chain enzymes are membrane protein complexes whose function depends on the composition and fluidity of the mitochondrial membrane lipid. The present study aimed at investigating the impact of different nutritional patterns of dietary lipids on liver mitochondrial functions. A total of forty-eight Wistar male rats were divided into six groups and fed for 12 weeks with a basal diet, lard diet or fish oil diet, containing either 50 or 300 g lipid/kg. The 30 % lipid intake increased liver NEFA, TAG and cholesterol levels, increased mitochondrial NEFA and TAG, and decreased phospholipid (PL) levels. SFA, PUFA and unsaturation index (UI) increased, whereas MUFA and trans-fatty acids (FA) decreased in the mitochondrial membrane PL in 30 % fat diet-fed rats compared with 5 % lipid diet-fed rats. PL UI increased with fish oil diet v. basal and lard-rich diets, and PL trans-FA increased with lard diet v. basal and fish oil diets. The 30 % lipid diet intake increased mitochondrial membrane potential, membrane fluidity, mitochondrial respiration and complex V activity, and decreased complex III and IV activities. With regard to lipid quality effects, b-oxidation decreased with the intake of basal or fish oil diets compared with that of the lard diet. The intake of a fish oil diet decreased complex III and IV activities compared with both the basal and lard diets. In conclusion, the characteristics and mitochondrial functions of the rat liver mitochondrial membrane are more profoundly altered by the quantity of dietary lipid than by its quality, which may have profound impacts on the pathogenesis and development of non-alcoholic fatty liver disease. Key words: High-fat diet: Lipid metabolism: Mitochondrial functions: Mitochondrial membrane: Respiratory complexes: RatsIncreasing consumption of fat-rich diets and reduced physical activity are the major contributors to the observed increase in body weight, diabetes and fatty liver diseases in many developed and developing countries, and have become a major public health concern (1 -4) . Lipids play varied and critical roles in cellular metabolism, with functions dramatically modulated by the individual fatty acid (FA) moieties in complex lipid entities. Moreover, biological membranes are composed of more than 50 % of lipids, and the quantity and quality of dietary lipids are known to have an impact on the composition, characteristics and functions of the biological membrane (5 -7) . FA as components of biological membranes strongly influence membrane fluidity, which, in turn, may influence many physiological processes involved in cell death and survival such as signal transduction, protein import, membrane receptor function and metabolite transport (8) . Moreover, the quality of administered lipid seems to play a crucial role in the occurrence (or not) of these alterations and their severity (9) . The...

The importance of mitochondrial activity has recently been extended to the regulation of developmental processes. Numerous pathologies associated with organelle's dysfunctions emphasize their physiological importance. However, regulation of mitochondrial genome transcription, a key element for organelle's function, remains poorly understood. After characterization in the organelle of a truncated form of the triiodothyronine nuclear receptor (p43), a T3-dependent transcription factor of the mitochondrial genome, our purpose was to search for other mitochondrial receptors involved in the regulation of organelle transcription. We show that a 44 kDa protein related to RXRalpha (mt-RXR), another nuclear receptor, is located in the mitochondrial matrix. We found that mt-RXR is produced after cytosolic or intramitochondrial enzymatic cleavage of the RXRalpha nuclear receptor. After mitochondrial import and binding to specific sequences of the organelle genome, mt-RXR induces a ligand-dependent increase in mitochondrial RNA levels. mt-RXR physically interacts with p43 and acts alone or through a heterodimerical complex activated by 9-cis-retinoic acid and T3 to increase RNA levels. These data indicate that hormonal regulation of mitochondrial transcription occurs through pathways similar to those that take place in the nucleus and open a new way to better understand hormone and vitamin action at the cellular level.

The btg1 (B-cell translocation gene 1) gene coding sequence was isolated from a translocation break point in a case of B-cell chronic lymphocytic leukaemia. We have already shown that BTG1, considered as an antiproliferative protein, strongly stimulates myoblast differentiation. However, the mechanisms involved in this influence remained unknown. In cultured myoblasts, we found that BTG1 stimulates the transcriptional activity of nuclear receptors (T3 and all-trans retinoic acid receptors but not RXRa and PPARc), c-Jun and myogenic factors (CMD1, Myf5, myogenin). Immunoprecipitation experiments performed in cells or using in vitro-synthesized proteins and GST pull-down assays established that BTG1 directly interacts with T3 and all-trans retinoic acid receptors and with avian MyoD (CMD1). These interactions are mediated by the transactivation domain of each transcription factor and the A box and C-terminal part of BTG1. NCoR presence induces the ligand dependency of the interaction with nuclear receptors. Lastly, deletion of BTG1 interacting domains abrogates its ability to stimulate nuclear receptors and CMD1 activity, and its myogenic influence. In conclusion, BTG1 is a novel important coactivator involved in the regulation of myoblast differentiation. It not only stimulates the activity of myogenic factors, but also of nuclear receptors already known as positive myogenic regulators.

In previous studies, we have characterized a new hormonal pathway involving a mitochondrial T3 receptor (p43) acting as a mitochondrial transcription factor and consequently stimulating mitochondrial activity and mitochondrial biogenesis. We have established the involvement of this T3 pathway in the regulation of in vitro myoblast differentiation.We have generated mice overexpressing p43 under control of the human α-skeletal actin promoter. In agreement with the previous characterization of this promoter, northern-blot and western-blot experiments confirmed that after birth p43 was specifically overexpressed in skeletal muscle. As expected from in vitro studies, in 2-month old mice, p43 overexpression increased mitochondrial genes expression and mitochondrial biogenesis as attested by the increase of mitochondrial mass and mt-DNA copy number. In addition, transgenic mice had a body temperature 0.8°C higher than control ones and displayed lower plasma triiodothyronine levels. Skeletal muscles of transgenic mice were redder than wild-type animals suggesting an increased oxidative metabolism. In line with this observation, in gastrocnemius, we recorded a strong increase in cytochrome oxidase activity and in mitochondrial respiration. Moreover, we observed that p43 drives the formation of oxidative fibers: in soleus muscle, where MyHC IIa fibers were partly replaced by type I fibers; in gastrocnemius muscle, we found an increase in MyHC IIa and IIx expression associated with a reduction in the number of glycolytic fibers type IIb. In addition, we found that PGC-1α and PPARδ, two major regulators of muscle phenotype were up regulated in p43 transgenic mice suggesting that these proteins could be downstream targets of mitochondrial activity. These data indicate that the direct mitochondrial T3 pathway is deeply involved in the acquisition of contractile and metabolic features of muscle fibers in particular by regulating PGC-1α and PPARδ.