The biochemical basis for the regulation of fibre-type determination in skeletal muscle is not well understood. In addition to the expression of particular myofibrillar proteins, type I (slow-twitch) fibres are much higher in mitochondrial content and are more dependent on oxidative metabolism than type II (fast-twitch) fibres. We have previously identified a transcriptional co-activator, peroxisome-proliferator-activated receptor-gamma co-activator-1 (PGC-1 alpha), which is expressed in several tissues including brown fat and skeletal muscle, and that activates mitochondrial biogenesis and oxidative metabolism. We show here that PGC-1 alpha is expressed preferentially in muscle enriched in type I fibres. When PGC-1 alpha is expressed at physiological levels in transgenic mice driven by a muscle creatine kinase (MCK) promoter, a fibre type conversion is observed: muscles normally rich in type II fibres are redder and activate genes of mitochondrial oxidative metabolism. Notably, putative type II muscles from PGC-1 alpha transgenic mice also express proteins characteristic of type I fibres, such as troponin I (slow) and myoglobin, and show a much greater resistance to electrically stimulated fatigue. Using fibre-type-specific promoters, we show in cultured muscle cells that PGC-1 alpha activates transcription in cooperation with Mef2 proteins and serves as a target for calcineurin signalling, which has been implicated in slow fibre gene expression. These data indicate that PGC-1 alpha is a principal factor regulating muscle fibre type determination.

PGC-1alpha is a coactivator of nuclear receptors and other transcription factors that regulates several metabolic processes, including mitochondrial biogenesis and respiration, hepatic gluconeogenesis, and muscle fiber-type switching. We show here that, while hepatocytes lacking PGC-1alpha are defective in the program of hormone-stimulated gluconeogenesis, the mice have constitutively activated gluconeogenic gene expression that is completely insensitive to normal feeding controls. C/EBPbeta is elevated in the livers of these mice and activates the gluconeogenic genes in a PGC-1alpha-independent manner. Despite having reduced mitochondrial function, PGC-1alpha null mice are paradoxically lean and resistant to diet-induced obesity. This is largely due to a profound hyperactivity displayed by the null animals and is associated with lesions in the striatal region of the brain that controls movement. These data illustrate a central role for PGC-1alpha in the control of energy metabolism but also reveal novel systemic compensatory mechanisms and pathogenic effects of impaired energy homeostasis.

Here we demonstrate that the ABC transporter ABCG1 plays a critical role in lipid homeostasis by controlling both tissue lipid levels and the efflux of cellular cholesterol to HDL. Targeted disruption of Abcg1 in mice has no effect on plasma lipids but results in massive accumulation of both neutral lipids and phospholipids in hepatocytes and in macrophages within multiple tissues following administration of a high-fat and -cholesterol diet. In contrast, overexpression of human ABCG1 protects murine tissues from dietary fat-induced lipid accumulation. Finally, we show that cholesterol efflux to HDL specifically requires ABCG1, whereas efflux to apoA1 requires ABCA1. These studies identify Abcg1 as a key gene involved in both cholesterol efflux to HDL and in tissue lipid homeostasis.

The multidrug resistance-associated protein 2 (MRP2, ABCC2), mediates the efflux of several conjugated compounds across the apical membrane of the hepatocyte into the bile canaliculi. We identified MRP2 in a screen designed to isolate genes that are regulated by the farnesoid X-activated receptor (FXR, NR1H4). MRP2 mRNA levels were induced following treatment of human or rat hepatocytes with either naturally occurring (chenodeoxycholic acid) or synthetic (GW4064) FXR ligands. In addition, we have shown that MRP2 expression is regulated by the pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3). Thus, treatment of rodent hepatocytes with PXR or CAR agonists results in a robust induction of MRP2 mRNA levels. The dexamethasone-and pregnenolone 16␣-carbonitrile-dependent induction of MRP2 expression was not evident in hepatocytes derived from PXR null mice. In contrast, induction of MRP2 by phenobarbital, an activator of CAR, was comparable in wild-type and PXR null mice. An unusual 26-bp sequence was identified 440 bp upstream of the MRP2 transcription initiation site that contains an everted repeat of the AGTTCA hexad separated by 8 nucleotides (ER-8). PXR, CAR, and FXR bound with high affinity to this element as heterodimers with the retinoid X receptor ␣ (RXR␣, NR2B1). Luciferase reporter gene constructs containing 1 kb of the rat MRP2 promoter were prepared and transiently transfected into HepG2 cells. Luciferase activity was induced in a PXR-, CAR-, or FXR-dependent manner. Furthermore, the isolated ER-8 element was capable of conferring PXR, CAR, and FXR responsiveness on a heterologous thymidine kinase promoter. Mutation of the ER-8 element abolished the nuclear receptor response. These studies demonstrate that MRP2 is regulated by three distinct nuclear receptor signaling pathways that converge on a common response element in the 5-flanking region of this gene.Members of the nuclear receptor superfamily of ligand-activated transcription factors have critical roles in many aspects of development and adult physiology, including cholesterol homeostasis, bile acid biosynthesis and transport, and xenobiotic metabolism. Recently, two orphan nuclear receptors, the farnesoid X-activated receptor (FXR, 1 NRIH4) and the pregnane X receptor (PXR, NR1I2) were shown to be activated by an overlapping spectrum of bile acids (1-5). These results indicate that bile acids function as hormonal ligands, in addition to their well established roles in the solubilization and absorption of lipids and fat-soluble vitamins from the intestinal lumen.

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The PGC-1 family of coactivators stimulates the activity of certain transcription factors and nuclear receptors. Transcription factors in the sterol responsive element binding protein (SREBP) family are key regulators of the lipogenic genes in the liver. We show here that high-fat feeding, which induces hyperlipidemia and atherogenesis, stimulates the expression of both PGC-1beta and SREBP1c and 1a in liver. PGC-1beta coactivates the SREBP transcription factor family and stimulates lipogenic gene expression. Further, PGC-1beta is required for SREBP-mediated lipogenic gene expression. However, unlike SREBP itself, PGC-1beta reduces fat accumulation in the liver while greatly increasing circulating triglycerides and cholesterol in VLDL particles. The stimulation of lipoprotein transport upon PGC-1beta expression is likely due to the simultaneous coactivation of the liver X receptor, LXRalpha, a nuclear hormone receptor with known roles in hepatic lipid transport. These data suggest a mechanism through which dietary saturated fats can stimulate hyperlipidemia and atherogenesis.

Peroxisome proliferator-activated receptor ␥ coactivator-1 (PGC-1) plays a critical role in regulating multiple aspects of energy metabolism, including adaptive thermogenesis, mitochondrial biogenesis, and fatty acid ␤-oxidation. Recently, this coactivator of nuclear receptors/transcription factors has been shown to control hepatic gluconeogenesis, an important component of the pathogenesis of both type-1 and type-2 diabetes. We described here the cloning of a novel bona fide homologue of PGC-1, PGC-1␤ (PGC-1 was renamed as PGC-1␣), first identified through searches of new data base entries. Despite the fact that PGC-1␣ and -1␤ share similar tissue distributions with highest levels of expression in brown fat and heart, their mRNAs are differentially regulated in the brown adipose tissue upon cold exposure and during brown fat cell differentiation. Like PGC-1␣, PGC-1␤ mRNA levels are increased significantly in the liver during fasting, suggesting a possible role for this factor in the regulation of hepatic gluconeogenesis and/or fatty acid oxidation. Consistent with this, PGC-1␤ was shown to physically interact and potently coactivate hepatic nuclear factor 4 and peroxisome proliferator-activated receptor ␣, nuclear receptors that are essential for hepatic adaptation to fasting. Finally, using sequence comparisons between PGC-1␣ and -1␤, we have identified a conserved amino acid motif that serves as a docking site for host cell factor, a cellular protein implicated in cell cycle regulation and viral infection. HCF is shown to bind to both PGC-1␣ and -1␤ and augment their transcriptional activity.

Peroxisome proliferator-activated receptor ␥ coactivator (PGC)-1␣ is a coactivator of nuclear receptors and other transcription factors that regulates several components of energy metabolism, particularly certain aspects of adaptive thermogenesis in brown fat and skeletal muscle, hepatic gluconeogenesis, and fiber type switching in skeletal muscle. PGC-1␣ has been shown to induce mitochondrial biogenesis when expressed in muscle cells, and preliminary analysis has suggested that this molecule may specifically increase the fraction of uncoupled versus coupled respiration. In this paper, we have performed detailed bioenergetic analyses of the function of PGC-1␣ and its homolog PGC-1␤ in muscle cells by monitoring simultaneously oxygen consumption and membrane potential. Cells expressing PGC-1␣ or PGC-1␤ display higher proton leak rates at any given membrane potential than control cells. However, cells expressing PGC-1␣ have a higher proportion of their mitochondrial respiration linked to proton leak than cells expressing PGC-1␤. Although these two proteins cause a similar increase in the expression of many mitochondrial genes, PGC-1␤ preferentially induces certain genes involved in the removal of reactive oxygen species, recently recognized as activators of uncoupling proteins. Together, these data indicate that PGC-1␣ and PGC-1␤ profoundly alter mitochondrial metabolism and suggest that these proteins are likely to play different physiological functions.Mitochondria play a central role in metabolism by coupling cellular respiration to the production of ATP. However, this coupling is not perfectly tight. Indeed, it is estimated that approximately 20% of the standard metabolic rate in mammals is due to a leak of protons across the mitochondrial inner membrane in a manner that uncouples cellular respiration from ATP production, thereby generating heat (1). This cycle is called basal proton leak. In addition to this basal leak, there is an inducible leak of protons catalyzed by uncoupling protein 1 (UCP1) 1 in brown fat. Two close homologs of this protein have been discovered, UCP2 and UCP3 (2-6). Although the function of these homologs is not clear, recent work suggests that they might have an important role in the protection against reactive oxygen species (ROS) (7, 8) and the modulation of cellular ATP levels, especially in insulin-secreting ␤ cells (9, 10). Interestingly, none of the genetic studies using either knockout mice or mice overexpressing moderate levels of UCP2 and UCP3 show a significant effect of these proteins in determining standard metabolic rate by uncoupling cellular respiration (11, 12).Many changes in the cellular environment result in modulation of mitochondrial metabolism. Basal proton leak rate changes in response to hormonal status and metabolic depression (13-19). Also, small mammals with high standard metabolic rates have higher proton leak rates than large mammals with low standard metabolic rates (20,21). Furthermore, proton leak rates differ between phylogenetic groups; it is higher i...