Showing papers on "Mitochondrial biogenesis published in 2004"

Transcriptional regulatory circuits controlling mitochondrial biogenesis and function

Abstract: We are witnessing a period of renewed interest in the biology of the mitochondrion. The mitochondrion serves a critical function in the maintenance of cellular energy stores, thermogenesis, and apoptosis. Moreover, alterations in mitochondrial function contribute to several inherited and acquired human diseases and the aging process. This review summarizes our understanding of the transcriptional regulatory mechanisms involved in the biogenesis and energy metabolic function of mitochondria in higher organisms.

Regulation of muscle fiber type and running endurance by PPARδ

Abstract: Endurance exercise training can promote an adaptive muscle fiber transformation and an increase of mitochondrial biogenesis by triggering scripted changes in gene expression. However, no transcription factor has yet been identified that can direct this process. We describe the engineering of a mouse capable of continuous running of up to twice the distance of a wild-type littermate. This was achieved by targeted expression of an activated form of peroxisome proliferator-activated receptor δ (PPARδ) in skeletal muscle, which induces a switch to form increased numbers of type I muscle fibers. Treatment of wild-type mice with PPARδ agonist elicits a similar type I fiber gene expression profile in muscle. Moreover, these genetically generated fibers confer resistance to obesity with improved metabolic profiles, even in the absence of exercise. These results demonstrate that complex physiologic properties such as fatigue, endurance, and running capacity can be molecularly analyzed and manipulated.

Mitochondrial transcription factor A regulates mtDNA copy number in mammals

Abstract: Mitochondrial DNA (mtDNA) copy number regulation is altered in several human mtDNA-mutation diseases and it is also important in a variety of normal physiological processes. Mitochondrial transcription factor A (TFAM) is essential for human mtDNA transcription and we demonstrate here that it is also a key regulator of mtDNA copy number. We initially performed in vitro transcription studies and determined that the human TFAM protein is a poor activator of mouse mtDNA transcription, despite its high capacity for unspecific DNA binding. Next, we generated P1 artificial chromosome (PAC) transgenic mice ubiquitously expressing human TFAM. The introduced human TFAM gene was regulated in a similar fashion as the endogenous mouse Tfam gene and expression of the human TFAM protein in the mouse did not result in down-regulation of the endogenous expression. The PAC-TFAM mice thus had a net overexpression of TFAM protein and this resulted in a general increase of mtDNA copy number. We used a combination of mice with TFAM overexpression and TFAM knockout and demonstrated that mtDNA copy number is directly proportional to the total TFAM protein levels also in mouse embryos. Interestingly, the expression of human TFAM in the mouse results in up-regulation of mtDNA copy number without increasing respiratory chain capacity or mitochondrial mass. It is thus possible to experimentally dissociate mtDNA copy number regulation from mtDNA expression and mitochondrial biogenesis in mammals in vivo. In conclusion, our results provide genetic evidence for a novel role for TFAM in direct regulation of mtDNA copy number in mammals.

The estrogen-related receptor α (ERRα) functions in PPARγ coactivator 1α (PGC-1α)-induced mitochondrial biogenesis

Abstract: Estrogen-related receptor α (ERRα) is one of the first orphan nuclear receptors to be identified, yet its physiological functions are still unclear. We show here that ERRα is an effector of the transcriptional coactivator PGC-1α [peroxisome proliferator-activated receptor γ (PPARγ) coactivator 1α], and that it regulates the expression of genes involved in oxidative phosphorylation and mitochondrial biogenesis. Inhibition of ERRα compromises the ability of PGC-1α to induce the expression of genes encoding mitochondrial proteins and to increase mitochondrial DNA content. A constitutively active form of ERRα is sufficient to elicit both responses. ERRα binding sites are present in the transcriptional control regions of ERRα/PGC-1α-induced genes and contribute to the transcriptional response to PGC-1α. The ERRα-regulated genes described here have been reported to be expressed at reduced levels in humans that are insulin-resistant. Thus, changes in ERRα activity could be linked to pathological changes in metabolic disease, such as diabetes.

Mitochondrial biogenesis by NO yields functionally active mitochondria in mammals

Abstract: We recently found that long-term exposure to nitric oxide (NO) triggers mitochondrial biogenesis in mammalian cells and tissues by activation of guanylate cyclase and generation of cGMP. Here, we report that the NO/cGMP-dependent mitochondrial biogenesis is associated with enhanced coupled respiration and content of ATP in U937, L6, and PC12 cells. The observed increase in ATP content depended entirely on oxidative phosphorylation, because ATP formation by glycolysis was unchanged. Brain, kidney, liver, heart, and gastrocnemius muscle from endothelial NO synthase null mutant mice displayed markedly reduced mitochondrial content associated with significantly lower oxygen consumption and ATP content. In these tissues, ultrastructural analyses revealed significantly smaller mitochondria. Furthermore, a significant reduction in the number of mitochondria was observed in the subsarcolemmal region of the gastrocnemius muscle. We conclude that NO/cGMP stimulates mitochondrial biogenesis, both in vitro and in vivo, and that this stimulation is associated with increased mitochondrial function, resulting in enhanced formation of ATP.

Cardiac-Specific Induction of the Transcriptional Coactivator Peroxisome Proliferator-Activated Receptor γ Coactivator-1α Promotes Mitochondrial Biogenesis and Reversible Cardiomyopathy in a Developmental Stage-Dependent Manner

Abstract: Recent evidence has identified the peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) as a regulator of cardiac energy metabolism and mitochondrial biogenesis. We describe the dev...

Suppression of mitochondrial respiration through recruitment of p160 myb binding protein to PGC-1α: modulation by p38 MAPK

Abstract: The transcriptional coactivator PPAR gamma coactivator 1 α (PGC-1α) is a key regulator of metabolic processes such as mitochondrial biogenesis and respiration in muscle and gluconeogenesis in liver. Reduced levels of PGC-1α in humans have been associated with type II diabetes. PGC-1α contains a negative regulatory domain that attenuates its transcriptional activity. This negative regulation is removed by phosphorylation of PGC-1α by p38 MAPK, an important kinase downstream of cytokine signaling in muscle and β-adrenergic signaling in brown fat. We describe here the identification of p160 myb binding protein (p160MBP) as a repressor of PGC-1α. The binding and repression of PGC-1α by p160MBP is disrupted by p38 MAPK phosphorylation of PGC-1α. Adenoviral expression of p160MBP in myoblasts strongly reduces PGC-1α's ability to stimulate mitochondrial respiration and the expression of the genes of the electron transport system. This repression does not require removal of PGC-1α from chromatin, suggesting that p160MBP is or recruits a direct transcriptional suppressor. Overall, these data indicate that p160MBP is a powerful negative regulator of PGC-1α function and provide a molecular mechanism for the activation of PGC-1α by p38 MAPK. The discovery of p160MBP as a PGC-1α regulator has important implications for the understanding of energy balance and diabetes.

PGC-1α mRNA expression is influenced by metabolic perturbation in exercising human skeletal muscle

Abstract: Endurance training leads to many adaptational changes in several tissues. In skeletal muscle, fatty acid usage is enhanced and mitochondrial content is increased. The exact molecular mechanisms regulating these functional and structural changes remain to be elucidated. Contractile activity-induced metabolic perturbation has repeatedly been shown to be important for the induction of mitochondrial biogenesis. Recent reports suggest that the peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha)/mitochondrial transcription factor A (Tfam) pathway is involved in exercise-induced mitochondrial biogenesis. In the present study, nine healthy men performed two 45-min bouts of one-legged knee extension exercise: one bout with restricted blood flow, and the other with nonrestricted blood flow to the working muscle. Muscle biopsies were obtained from the vastus lateralis muscle before exercise and at 0, 30, 120, and 360 min after the exercise bout. Biopsies were analyzed for whole muscle, as well as fiber-type specific mRNA expression of myocyte-enriched calcineurin interacting protein (MCIP)-1, PGC-1alpha, and downstream mitochondrial transcription factors. A novel finding was that, in human skeletal muscle, PGC-1alpha mRNA increased more after exercise with restricted blood flow than in the nonrestricted condition. No changes were observed for the mRNA of NRF-1, Tfam, mitochondrial transcription factor B1, and mitochondrial transcription factor B2. Muscle fiber type I and type II did not differ in the basal PGC-1alpha mRNA levels or in the expression increase after ischemic training. Another novel finding was that there was no difference between the restricted and nonrestricted exercise conditions in the increase of MCIP-1 mRNA, a marker for calcineurin activation. This suggests that calcineurin may be activated by exercise in humans and does not exclude that calcineurin could play a role in PGC-1 transcription activation in human skeletal muscle.

Cardiac mitochondrial damage and biogenesis in a chronic model of type 1 diabetes

Abstract: Diabetic cardiomyopathy is a common complication leading to heightened risk of heart failure and death. In the present report, we performed proteomic analysis on total cardiac proteins from the OVE26 mouse model of type 1 diabetes to identify protein changes that may contribute to diabetic cardiomyopathy. This analysis revealed that a surprising high proportion (12 of 20) of the altered proteins that could be identified by mass spectrometry were of mitochondrial origin. All but one of these proteins were upregulated by diabetes. Quantitative RT-PCR, performed for two of these proteins, indicated that part of the upregulation was attributed to increased messenger RNA levels. Morphological study of diabetic hearts showed significantly increased mitochondrial area and number as well as focal regions with severe damage to mitochondria. Diabetic mitochondria also showed reduced respiratory control ratio (9.63 +/- 0.20 vs. 6.13 +/- 0.41, P < 0.0001), apparently due to reduced state 3 rate, and diminished GSH level (5.5 +/- 0.9 vs. 8.2 +/- 2.5 micromol/mg protein, P < 0.05), indicating impaired mitochondrial function and increased oxidative stress. Further examination revealed increased mitochondrial DNA (1.03 +/- 0.18 vs. 0.69 +/- 0.13 relative copy number, P < 0.001) and a tendency to higher protein yield in OVE26 cardiac mitochondria, as well as increased mRNA level for mitochondrial transcription factor A and two mitochondrial encoded proteins. Taken together, these results show that mitochondria are a primary target in the diabetic heart, probably due to oxidative stress, and that this damage coincides with and may stimulate mitochondrial biogenesis.

Somatic mutations in the D-loop and decrease in the copy number of mitochondrial DNA in human hepatocellular carcinoma

Abstract: Somatic mutations in mitochondrial DNA (mtDNA) have been detected in many human cancers, including hepatocellular carcinoma (HCC) The D-loop region was found to be a "hot spot" for mutation in mtDNA of the tumors However, effects of the D-loop mutations on the copy number of mtDNA in tumor tissues are poorly understood Using direct sequencing, we examined mutations in the D-loop region of mtDNA in 61 HCCs and the corresponding non-tumor liver tissues The results revealed that 393% of the HCCs carried somatic mutation(s) in the D-loop of mtDNA, and most of these mutations were homoplasmic Moreover, 370% (10/27) of these mutations were T-to-C and G-to-A transitions and 407% (11/27) of them were located in the polycytidine stretch between nucleotide position (np) 303 and 309 of mtDNA In addition, we found that mtDNA copy number of HCC was significantly decreased in 605% of the patients with hepatoma, especially in those with somatic mutation(s) in the D-loop of mtDNA (17/24) This decrease in mtDNA copy number was highly associated with the occurrence of point mutations near the replication origin of the heavy-strand of mtDNA Interestingly, we found that 429% (6/14) of the HCCs without mutation in the D-loop had a reduced copy number of mtDNA, indicating that other unidentified factors involved in mitochondrial biogenesis might be defective in the tumor The results obtained in this study strongly suggest that somatic mutations in the D-loop together with the decrease in the copy number of mtDNA may be an important event during the early phase of liver carcinogenesis

Role of calcium and AMP kinase in the regulation of mitochondrial biogenesis and GLUT4 levels in muscle

Abstract: Contractile activity induces mitochondrial biogenesis and increases glucose transport capacity in muscle. There has been much research on the mechanisms responsible for these adaptations. The present paper reviews the evidence, which indicates that the decrease in the levels of high-energy phosphates, leading to activation of AMP kinase (AMPK), and the increase in cytosolic Ca(2+), which activates Ca(2+)/calmodulin-dependent protein kinase (CAMK), are signals that initiate these adaptative responses. Although the events downstream of AMPK and CAMK have not been well characterized, these events lead to activation of various transcription factors, including: nuclear respiratory factors (NRF) 1 and 2, which cause increased expression of proteins of the respiratory chain; PPAR-alpha, which up regulates the levels of enzymes of beta oxidation; mitochondrial transcription factor A, which activates expression of the mitochondrial genome; myocyte-enhancing factor 2A, the transcription factor that regulates GLUT4 expression. The well-orchestrated expression of the multitude of proteins involved in these adaptations is mediated by the rapid activation of PPAR gamma co-activator (PGC) 1, a protein that binds to various transcription factors to maximize transcriptional activity. Activating AMPK using 5-aminoimidizole-4-carboxamide-1-beta-D-riboside (AICAR) and increasing cytoplasmic Ca(2+) using caffeine, W7 or ionomycin in L6 myotubes increases the concentration of mitochondrial enzymes and GLUT4 and enhances the binding of NRF-1 and NRF-2 to DNA. AICAR and Ca-releasing agents also increase the levels of PGC-1, mitochondrial transcription factor A and myocyte-enhancing factors 2A and 2D. These results are similar to the responses seen in muscle during the adaptation to endurance exercise and show that L6 myotubes are a suitable model for studying the mechanisms by which exercise causes the adaptive responses in muscle mitochondria and glucose transport.

Lipopolysaccharide induces oxidative cardiac mitochondrial damage and biogenesis

Abstract: Objective : The responses to bacterial lipopolysaccharide (LPS) damage mitochondria by generating oxidative stress within the organelles. We postulated that LPS damages heart mitochondrial DNA and protein by oxidation, and that this is recovered by oxidative mechanisms of mitochondrial biogenesis. Methods and results : Systemic crude E. coli LPS administration decreased mtDNA copy number and mtDNA gene transcription in rat heart caused by oxidant deletion of mtDNA. The fall in copy number was reflected in proteomic expression of several mitochondria-encoded subunits of Complexes I, IV, and V. Recovery of mtDNA copy number involved biogenesis as indicated by mitochondrial transcription factor A (Tfam) and DNA polymerase-γ expression. The transcriptional response also included nuclear accumulation of peroxisome proliferator-activated receptor-γ co-activator 1 (PGC-1) and mRNA expression for redox-regulated nuclear respiratory factors (NRF-1 and -2). Conclusions : These novel findings disclose a duality of reactive oxygen species (ROS) effect in the heart’s response to LPS in which oxidative mitochondrial damage is opposed by oxidant stimulation of biogenesis.

Rapid transformation of white adipocytes into fat-oxidizing machines

Abstract: Adenovirus-induced hyperleptinemia rapidly depletes body fat in normal rats without increasing free fatty acids and ketogenesis, implying that fat-storing adipocytes are oxidizing the fat. To analyze the ultrastructural changes of adipocytes accompanying this functional transformation, we examined the fat tissue by electron microscopy. After 14 days of hyperleptinemia, adipocytes had become shrunken, fatless, and encased in a thick basement-membrane-like matrix. They were crowded with mitochondria that were much smaller than those of brown adipocytes. Their gene expression profile revealed striking up-regulation of peroxisome proliferator-activated receptor γ coactivator 1α (an up-regulator of mitochondrial biogenesis not normally expressed in white fat), increased uncoupling proteins-1 and -2, and down-regulation of lipogenic enzymes. Phosphorylation of both acetyl CoA carboxylase and AMP-activated protein kinase was increased, thus explaining the increase in fatty acid oxidation. The ability to transform adipocytes into unique fat-burning cells may suggest novel therapeutic strategies for obesity.

A switch in metabolism precedes increased mitochondrial biogenesis in respiratory chain-deficient mouse hearts.

Abstract: We performed global gene expression analyses in mouse hearts with progressive respiratory chain deficiency and found a metabolic switch at an early disease stage. The tissue-specific mitochondrial transcription factor A (Tfam) knockout mice of this study displayed a progressive heart phenotype with depletion of mtDNA and an accompanying severe decline of respiratory chain enzyme activities along with a decreased mitochondrial ATP production rate. These characteristics were observed after 2 weeks of age and became gradually more severe until the terminal stage occurred at 10-12 weeks of age. Global gene expression analyses with microarrays showed that a metabolic switch occurred early in the progression of cardiac mitochondrial dysfunction. A large number of genes encoding critical enzymes in fatty acid oxidation showed decreased expression whereas several genes encoding glycolytic enzymes showed increased expression. These alterations are consistent with activation of a fetal gene expression program, a well-documented phenomenon in cardiac disease. An increase in mitochondrial mass was not observed until the disease had reached an advanced stage. In contrast to what we have earlier observed in respiratory chain-deficient skeletal muscle, the increased mitochondrial biogenesis in respiratory chain-deficient heart muscle did not increase the overall mitochondrial ATP production rate. The observed switch in metabolism is unlikely to benefit energy homeostasis in the respiratory chain-deficient hearts and therefore likely aggravates the disease. It can thus be concluded that at least some of the secondary gene expression alterations in mitochondrial cardiomyopathy do not compensate but rather directly contribute to heart failure progression.

Alteration of the copy number and deletion of mitochondrial DNA in human hepatocellular carcinoma

Abstract: Somatic mutations in mitochondrial DNA (mtDNA) have been detected in hepatocellular carcinoma (HCC). However, it remains unclear whether mtDNA copy number and mitochondrial biogenesis are altered in HCC. In this study, we found that mtDNA copy number and the content of mitochondrial respiratory proteins were reduced in HCCs as compared with the corresponding non-tumorous livers. MtDNA copy number was significantly reduced in female HCC but not in male HCC. Expression of the peroxisome proliferator-activated receptor gamma coactivator-1 was significantly repressed in HCCs (P<0.005), while the expression of the mitochondrial single-strand DNA-binding protein was upregulated, indicating that the regulation of mitochondria biogenesis is disturbed in HCC. Moreover, 22% of HCCs carried a somatic mutation in the mtDNA D-loop region. The non-tumorous liver of the HCC patients with a long-term alcohol-drinking history contained reduced mtDNA copy number (P<0.05) and higher level of the 4977 bp-deleted mtDNA (P<0.05) as compared with non-alcohol patients. Our results suggest that reduced mtDNA copy number, impaired mitochondrial biogenesis and somatic mutations in mtDNA are important events during carcinogenesis of HCC, and the differential alterations in mtDNA of male and female HCC may contribute to the differences in the clinical manifestation between female and male HCC patients.

Regulation of mitochondrial proliferation in the heart: power-plant failure contributes to cardiac failure in hypertrophy

Abstract: During hypertrophy, proliferation of mitochondria does not keep pace with the increasing energy demand of the heart. This probably contributes importantly to cardiac failure, together with other phenotypic changes occurring during the growth process. The problem may be even aggravated if defects of mitochondrial function itself and not external factors cause the hypertrophic process. Here we review the basic mechanisms controlling mitochondrial biogenesis, especially the pathways coordinating expression of nuclear encoded mitochondrial genes and the small mitochondrial genome, and how these mechanisms may be connected to the cardiomyocyte differentiation program during development as well as under physiological and pathological circumstances.

Involvement of PPARγ co-activator-1, nuclear respiratory factors 1 and 2, and PPARα in the adaptive response to endurance exercise

Abstract: Endurance exercise training induces an increase in the respiratory capacity of muscle, resulting in an increased capacity to generate ATP as well as improved efficiency of muscle contraction. Such adaptations are largely the result of a coordinated genetic response that increases mitochondrial proteins, fatty acid oxidation enzymes and the exercise- and insulin-stimulated glucose transporter GLUT4, and shifts the contractile and regulatory proteins to their more efficient isoforms. In recent years a number of the transcriptional regulators involved in this genetic response have been identified and these factors can be classified into two different groups. The first group comprises transcription factors such as nuclear respiratory factors (NRF) 1 and 2 and PPAR alpha that bind DNA in a sequence-specific manner. The second group, referred to as transcriptional co-activators, alter transcription without directly binding to DNA. The PPAR gamma co-activator (PGC) family of proteins have been identified as the central family of transcriptional co-activators for induction of mitochondrial biogenesis. PGC-1 alpha is activated by exercise, and is sufficient to produce the endurance phenotype through direct interactions with NRF-1 and PPAR alpha, and potentially NRF-2. Furthering the understanding of the activation of PGC proteins following exercise has implications beyond improving athletic performance, including the possibility of providing targets for the treatment of frailty in the elderly, obesity and diseases such as mitochondrial myopathies and diabetes.

Real-time imaging of peroxisome proliferator-activated receptor-γ coactivator-1α promoter activity in skeletal muscles of living mice

Abstract: In response to sustained increase in contractile activity, mammalian skeletal muscle undergoes adaptation with enhanced mitochondrial biogenesis and fiber type switching. The peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) was recently identified as a key regulator for these adaptive processes. To investigate the sequence elements in the PGC-1alpha gene that are responsible for activity-dependent transcriptional activation, we have established a unique system to analyze promoter activity in skeletal muscle of living mice. Expression of PGC-1alpha-firefly luciferase reporter gene in mouse tibialis anterior muscle transfected by electric pulse-mediated gene transfer was assessed repeatedly in the same muscle by using optical bioluminescence imaging analysis before and after low-frequency (10 Hz) motor nerve stimulation. Nerve stimulation (2 h) resulted in a transient 3-fold increase (P < 0.05) in PGC-1alpha promoter activity along with a 1.6-fold increase (P < 0.05) in endogenous PGC-1alpha mRNA. Mutation of two consensus myocyte enhancer factor 2 (MEF2) binding sites (-2901 and -1539) or a cAMP response element (CRE) (-222) completely abolished nerve stimulation-induced increase in PGC-1alpha promoter activity. These findings provide direct evidence that contractile activity-induced PGC-1alpha promoter activity in skeletal muscle is dependent on the MEF2 and the CRE sequence elements. The experimental methods used in the present study have general applicability to studies of gene regulation in muscle.

Nitric oxide-dependent mitochondrial biogenesis generates Ca2+ signaling profile of lupus T cells.

Abstract: Abnormal T cell activation and cell death underlie the pathology of systemic lupus erythematosus. Although mitochondrial hyperpolarization (MHP) represents an early and reversible checkpoint of T cell activation and apoptosis, lupus T cells exhibit persistent MHP. NO has recently been recognized as a key signal of mitochondrial biogenesis and mediator of MHP in human T lymphocytes. In this study, we show that persistent MHP was associated with increased mitochondrial mass (+47.7 +/- 2.8%; p = 0.00017) and increased mitochondrial (+21.8 +/- 4.1%; p = 0.016) and cytoplasmic Ca2+ content in T cells from 19 systemic lupus erythematosus patients with respect to 11 control donors (+38.0 +/- 6.4%; p = 0.0023). Electron microscopy revealed that lupus lymphocytes contained 8.76 +/- 1.0 mitochondria, while control donors contained 3.18 +/- 0.28 mitochondria per cell (p = 0.0009). Increased mitochondrial mass in T cells was associated with 2.08 +/- 0.09-fold enhanced NO production by lupus monocytes (p = 0.0023). Activation of T cells through the TCR initiates a biphasic elevation in cytosolic free Ca2+ concentration, a rapid initial peak observed within minutes, and a plateau phase lasting up to 48 h. In response to CD3/CD28 costimulation, rapid Ca2+ fluxing was enhanced while the plateau phase was diminished in lupus T cells. NO-induced mitochondrial biogenesis in normal T cells enhanced the rapid phase and reduced the plateau of Ca2+ influx upon CD3/CD28 costimulation, thus mimicking the Ca2+ signaling profile of lupus T cells. Mitochondria constitute major Ca2+ stores and NO-dependent mitochondrial biogenesis may account for altered Ca2+ handling by lupus T cells.

The role of calcium and calcium/calmodulin-dependent kinases in skeletal muscle plasticity and mitochondrial biogenesis.

Abstract: Intracellular Ca(2+) plays an important role in skeletal muscle excitation-contraction coupling and also in excitation-transcription coupling. Activity-dependent alterations in muscle gene expression as a result of increased load (i.e. resistance or endurance training) or decreased activity (i.e. immobilization or injury) are tightly linked to the level of muscle excitation. Differential expression of genes in slow- and fast-twitch fibres is also dependent on fibre activation. Both these biological phenomena are, therefore, tightly linked to the amplitude and duration of the Ca(2+) transient, a signal decoded downstream by Ca(2+)-dependent transcriptional pathways. Evidence is mounting that the calcineurin-nuclear factor of activated T-cells pathway and the Ca(2+)/calmodulin-dependent kinases (CaMK) II and IV play important roles in regulating oxidative enzyme expression, mitochondrial biogenesis and expression of fibre-type specific myofibrillar proteins. CaMKII is known to decode frequency-dependent information and is activated during hypertrophic growth and endurance adaptations. Thus, it was hypothesized that CaMKII, and possibly CaMKIV, are down regulated during muscle atrophy and levels of expression of CaMKII alpha, -II beta, -II gamma and -IV were assessed in skeletal muscles from young, aged and denervated rats. The results indicate that CaMKII gamma, but not CaMKIIalpha or -beta, is up regulated in aged and denervated soleus muscle and that CaMKIV is absent in skeletal but not cardiac muscle. Whether CaMKII gamma up-regulation is part of the pathology of wasting or a result of some adaptational response to atrophy is not known. Future studies will be important in determining whether insights from the adaptational response of muscle to increased loads will provide pharmacological approaches for increasing muscle strength or endurance to counter muscle wasting.

Effect of a C/EBP gene replacement on mitochondrial biogenesis in fat cells

Abstract: At its simplest level, obesity is a disorder of energy balance, where energy intake exceeds energy expenditure; as a consequence, the excess energy is stored in the form of fat in adipocytes. The primary onset mechanisms are dietary and/or genetic (Kopelman 2000). Of those known genetic factors involved in determining adiposity in animals, many are regulatory and exert their effect directly on adipocyte differentiation and development (Rosen et al. 2000; Flier 2004). These factors include CCAAT/enhancer-binding proteins (C/EBPs), SREBP, and PPARγ, whose concerted action during adipogenesis leads to the development of fat-laden mature adipocytes (Rosen et al. 2000). The C/EBP family consists of five members, of which C/EBPα, C/EBPβ, and C/EBPδ have a profound impact on fat cell differentiation (Yeh et al. 1995). During adipogenesis, C/EBP family functions in a transcriptional cascade, in which the early and transiently expressed C/EBPβ and C/EBPδ activate transcription of PPARγ. PPARγ is then responsible for the expression of C/EBPα (Wu et al. 1996; Rosen et al. 1999). Subsequently, PPARγ and C/EBPα, together with SREBP, work synergistically to transactivate expression of most or all of the genes encoding for factors, such as fatty-acid synthase and adipocyte-specific fatty acid-binding protein aP2, which characterize the fat cell phenotype (Speigelman et al. 1993). In this study, we used previously manipulated gene knock-in mice, known as β/β mice, in which the C/EBPα gene has been replaced by the C/EBPβ gene (referred here as β/β allele) to study their function in tissues (Chen et al. 2000). β/β mice are lean, and despite markedly reduced fat storage in their fat cells, they do not develop hyperlipidemia or fatty liver, commonly found in the forced leaness that typically causes the liver to take up and store fatty acids when their circulating levels are elevated (Moitra et al. 1998; Shimomura et al. 1998; Chen et al. 2000). We monitored closely the physiology and lifespan of the lean β/β mice and undertook mechanistic studies to understand the effect of β/β allele in energy metabolism. We found that the β/β allele caused an increase in mitochondrial biogenesis only in fat cells of white adipose tissues (WAT), and this WAT specifically enhanced mitochondrial biogenesis was possibly elicited by the markedly elevated expression of G protein α stimulatory subunit (Gαs).

Increased mitochondrial biogenesis in primary leukemia cells: the role of endogenous nitric oxide and impact on sensitivity to fludarabine

Abstract: B cell chronic lymphocytic leukemia (CLL) is the most prevalent adult leukemia in the Western hemisphere, yet many biological and molecular features of the disease remain undefined. CLL cells generate increased levels of radical species such as superoxide and nitric oxide (NO), which is associated with mitochondrial DNA mutations. Considering that NO levels can affect mitochondrial biogenesis, we hypothesized that the inherent nitrosative stress in CLL cells may lead to hyperactive mitochondrial biogenesis. Here we report that primary CLL cells contained significantly more mitochondria than normal lymphocytes and that their mitochondrial mass was significantly related to endogenous NO levels. Expression of the mitochondrial biogenesis factors nuclear respiratory factor-1 and mitochondrial transcription factor A was elevated in most CLL specimens examined and appeared to be related to cellular NO levels. Treatment of B cells with exogenous NO caused a substantial increase in mitochondrial mass. In vitro sensitivity of CLL cells to fludarabine was highly related to mitochondrial mass in that cells with greater mitochondrial mass were less sensitive to the drug. Taken together, our results suggest that NO is a key mediator of mitochondrial biogenesis in CLL and that modulation of mitochondrial biogenesis by NO may alter cellular sensitivity to fludarabine.

Transcriptional profiling reveals coordinated up-regulation of oxidative metabolism genes in thyroid oncocytic tumors.

Abstract: Oncocytomas are large cell tumors characterized by an abnormal proliferation of mitochondria. To investigate this phenomenon in thyroid oncocytomas, we determined gene expression profiles of 87 samples using microarrays of 6720 PCR products from cDNA clones. Samples included 29 thyroid oncocytomas and six papillary carcinomas, the remainder representing other thyroid pathologies or mitochondria-rich tumor samples, normal thyroid samples, and two thyroid cell lines. Hierarchical clustering and supervised analysis identified two specific oncocytic clusters and 163 distinctly regulated genes between oncocytoma and normal thyroid. Differential expression of five selected genes (APOD, BCL-2, COX, CTSB, and MAP2) was confirmed by immunohistochemistry. The two specific oncocytic clusters were rich in mitochondrial genes and revealed coordinated expression of nuclear and mitochondrial respiratory chain genes. We also observed the up-regulation of genes involved in mitochondrial biogenesis, such as nuclear respiratory factor 1 and the endothelial nitric oxide synthase. Several oxidative metabolism genes were overexpressed in oncocytomas, including those from the tricarboxylic acid cycle (MDH1) and cytosolic glycolysis (GAPD, ENO1, and GPI). On the contrary, the lactate dehydrogenase A gene, involved in anaerobic metabolism, was down-regulated. Our results suggest that, unlike a large number of solid tumors, thyroid oncocytomas produce energy through an aerobic pathway.