Calcium-regulated changes in mitochondrial phenotype in skeletal muscle cells

Exercise-Regulated Mitochondrial and Nuclear Signalling Networks in Skeletal Muscle

Exercise perturbs energy homeostasis in skeletal muscle and engages integrated cellular signalling networks to help meet the contraction-induced increases in skeletal muscle energy and oxygen demand. Investigating exercise-associated perturbations in skeletal muscle signalling networks has uncovered novel mechanisms by which exercise stimulates skeletal muscle mitochondrial biogenesis and promotes whole-body health and fitness. While acute exercise regulates a complex network of protein post-translational modifications (e.g. phosphorylation) in skeletal muscle, previous investigations of exercise signalling in human and rodent skeletal muscle have primarily focused on a select group of exercise-regulated protein kinases [i.e. 5' adenosine monophosphate-activated protein kinase (AMPK), protein kinase A (PKA), Ca2+/calmodulin-dependent protein kinase (CaMK) and mitogen-activated protein kinase (MAPK)] and only a small subset of their respective protein substrates. Recently, global mass spectrometry-based phosphoproteomic approaches have helped unravel the extensive complexity and interconnection of exercise signalling pathways and kinases beyond this select group and phosphorylation and/or translocation of exercise-regulated mitochondrial and nuclear protein substrates. This review provides an overview of recent advances in our understanding of the molecular events associated with acute endurance exercise-regulated signalling pathways and kinases in skeletal muscle with a focus on phosphorylation. We critically appraise recent evidence highlighting the involvement of mitochondrial and nuclear protein phosphorylation and/or translocation in skeletal muscle adaptive responses to an acute bout of endurance exercise that ultimately stimulate mitochondrial biogenesis and contribute to exercise's wider health and fitness benefits.

Advancing cancer cachexia diagnosis with -omics technology and exercise as molecular medicine

Muscle atrophy exacerbates disease outcomes and increases mortality, whereas the preservation of skeletal muscle mass and function play pivotal roles in ensuring long-term health and overall quality-of-life. Muscle atrophy represents a significant clinical challenge, involving the continued loss of muscle mass and strength, which frequently accompany the development of numerous types of cancer. Cancer cachexia is a highly prevalent multifactorial syndrome, and although cachexia is one of the main causes of cancer-related deaths, there are still no approved management strategies for the disease. The etiology of this condition is based on the upregulation of systemic inflammation factors and catabolic stimuli, resulting in the inhibition of protein synthesis and enhancement of protein degradation. Numerous necessary cellular processes are disrupted by cachectic pathology, which mediate intracellular signalling pathways resulting in the net loss of muscle and organelles. However, the exact underpinning molecular mechanisms of how these changes are orchestrated are incompletely understood. Much work is still required, but structured exercise has the capacity to counteract numerous detrimental effects linked to cancer cachexia. Primarily through the stimulation of muscle protein synthesis, enhancement of mitochondrial function, and the release of myokines. As a result, muscle mass and strength increase, leading to improved mobility, and quality-of-life. This review summarises existing knowledge of the complex molecular networks that regulate cancer cachexia and exercise, highlighting the molecular interplay between the two for potential therapeutic intervention. Finally, the utility of mass spectrometry-based proteomics is considered as a way of establishing early diagnostic biomarkers of cachectic patients.

The mitochondrial adenine nucleotide transporters in myogenesis

Adenine Nucleotide Translocator isoforms (ANTs) exchange ADP/ATP across the inner mitochondrial membrane, are also voltage-activated proton channels and regulate mitophagy and apoptosis. The ANT1 isoform predominates in heart and muscle while ANT2 is systemic. Here, we report the creation of Ant mutant mouse myoblast cell lines with normal Ant1 and Ant2 genes, deficient in either Ant1 or Ant2, and deficient in both the Ant1 and Ant2 genes. These cell lines are immortal under permissive conditions (IFN-γ + serum at 32 °C) permitting expansion but return to normal myoblasts that can be differentiated into myotubes at 37 °C. With this system we were able to complement our Ant1 mutant studies by demonstrating that ANT2 is important for myoblast to myotube differentiation and myotube mitochondrial respiration. ANT2 is also important in the regulation of mitochondrial biogenesis and antioxidant defenses. ANT2 is also associated with increased oxidative stress response and modulation for Ca⁺⁺ sequestration and activation of the mitochondrial permeability transition (mtPTP) pore during cell differentiation.

Exercise, CaMKII, and type 2 diabetes

Individuals who exercise regularly are protected from type 2 diabetes and other metabolic syndromes, in part by enhanced gene transcription and induction of many signaling pathways crucial in correcting impaired metabolic pathways associated with a sedentary lifestyle. Exercise activates Calmodulin-dependent protein kinase (CaMK)II, resulting in increased mitochondrial oxidative capacity and glucose transport. CaMKII regulates many health beneficial cellular functions in individuals who exercise compared with those who do not exercise. The role of exercise in the regulation of carbohydrate, lipid metabolism, and insulin signaling pathways are explained at the onset. Followed by the role of exercise in the regulation of glucose transporter (GLUT)4 expression and mitochondrial biogenesis are explained. Next, the main functions of Calmodulin-dependent protein kinase and the mechanism to activate it are illustrated, finally, an overview of the role of CaMKII in regulating GLUT4 expression, mitochondrial biogenesis, and histone modification are discussed.

Early Growth Response Gene Upregulation in Epstein-Barr Virus (EBV)-Associated Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) is a chronic multisystem disease exhibiting a variety of symptoms and affecting multiple systems. Psychological stress and virus infection are important. Virus infection may trigger the onset, and psychological stress may reactivate latent viruses, for example, Epstein-Barr virus (EBV). It has recently been reported that EBV induced gene 2 (EBI2) was upregulated in blood in a subset of ME/CFS patients. The purpose of this study was to determine whether the pattern of expression of early growth response (EGR) genes, important in EBV infection and which have also been found to be upregulated in blood of ME/CFS patients, paralleled that of EBI2. EGR gene upregulation was found to be closely associated with that of EBI2 in ME/CFS, providing further evidence in support of ongoing EBV reactivation in a subset of ME/CFS patients. EGR1, EGR2, and EGR3 are part of the cellular immediate early gene response and are important in EBV transcription, reactivation, and B lymphocyte transformation. EGR1 is a regulator of immune function, and is important in vascular homeostasis, psychological stress, connective tissue disease, mitochondrial function, all of which are relevant to ME/CFS. EGR2 and EGR3 are negative regulators of T lymphocytes and are important in systemic autoimmunity.

Acute sprint exercise transcriptome in human skeletal muscle

Aim

To examine global gene expression response to profound metabolic and hormonal stress induced by acute sprint exercise.

Methods

Healthy men and women (n = 14) performed three all-out cycle sprints interspersed by 20 min recovery. Muscle biopsies were obtained before the first, and 2h and 20 min after last sprint. Microarray analysis was performed to analyse acute gene expression response and repeated blood samples were obtained.

Results

In skeletal muscle, a set of immediate early genes, FOS, NR4A3, MAFF, EGR1, JUNB were markedly upregulated after sprint exercise. Gene ontology analysis from 879 differentially expressed genes revealed predicted activation of various upstream regulators and downstream biofunctions. Gene signatures predicted an enhanced turnover of skeletal muscle mass after sprint exercise and some novel induced genes such as WNT9A, FZD7 and KLHL40 were presented. A substantial increase in circulating free fatty acids (FFA) was noted after sprint exercise, in parallel with upregulation of PGC-1A and the downstream gene PERM1 and gene signatures predicting enhanced lipid turnover. Increase in growth hormone and insulin in blood were related to changes in gene expressions and both hormones were predicted as upstream regulators.

Conclusion

This is the first study reporting global gene expression in skeletal muscle in response to acute sprint exercise and several novel findings are presented. First, in line with that muscle hypertrophy is not a typical finding after a period of sprint training, both hypertrophy and atrophy factors were regulated. Second, systemic FFA and hormonal and exposure might be involved in the sprint exercise-induced changes in gene expression.

Inositol trisphosphate receptor-mediated Ca2+ signalling stimulates mitochondrial function and gene expression in core myopathy patients

Core myopathies are a group of childhood muscle disorders caused by mutations of the ryanodine receptor (RyR1), the Ca²⁺ release channel of the sarcoplasmic reticulum. These mutations have previously been associated with elevated inositol trisphosphate receptor (IP3R) levels in skeletal muscle myotubes derived from patients. However, the functional relevance and the relationship of IP3R mediated Ca²⁺ signalling with the pathophysiology of the disease is unclear. It has also been suggested that mitochondrial dysfunction underlies the development of central and diffuse multi-mini-cores, devoid of mitochondrial activity, which is a key pathological consequence of RyR1 mutations. Here we used muscle biopsies of central core and multi-minicore disease patients with RyR1 mutations, as well as cellular and in vivo mouse models of the disease to characterize global cellular and mitochondrial Ca²⁺ signalling, mitochondrial function and gene expression associated with the disease. We show that RyR1 mutations that lead to the depletion of the channel are associated with increased IP₃-mediated nuclear and mitochondrial Ca²⁺ signals and increased mitochondrial activity. Moreover, western blot and microarray analysis indicated enhanced mitochondrial biogenesis at the transcriptional and protein levels and was reflected in increased mitochondrial DNA content. The phenotype was recapitulated by RYR1 silencing in mouse cellular myotube models. Altogether, these data indicate that remodelling of skeletal muscle Ca²⁺ signalling following loss of functional RyR1 mediates bioenergetic adaptation.

The impact of pollen consumption on honey bee (Apis mellifera) digestive physiology and carbohydrate metabolism

Carbohydrate-active enzymes play an important role in the honey bee (Apis mellifera) due to its dietary specialization on plant-based nutrition. Secretory glycoside hydrolases (GHs) produced in worker head glands aid in the processing of floral nectar into honey and are expressed in accordance with age-based division of labor. Pollen utilization by the honey bee has been investigated in considerable detail, but little is known about the metabolic fate of indigestible carbohydrates and glycosides in pollen biomass. Here, we demonstrate that pollen consumption stimulates the hydrolysis of sugars that are toxic to the bee (xylose, arabinose, mannose). GHs produced in the head accumulate in the midgut and persist in the hindgut that harbors a core microbial community composed of approximately 10⁸ bacterial cells. Pollen consumption significantly impacted total and specific bacterial abundance in the digestive tract. Bacterial isolates representing major fermentative gut phylotypes exhibited primarily membrane-bound GH activities that may function in tandem with soluble host enzymes retained in the hindgut. Additionally, we found that plant-originating β-galactosidase activity in pollen may be sufficient, in some cases, for probable physiological activity in the gut. These findings emphasize the potential relative contributions of host, bacteria, and pollen enzyme activities to carbohydrate breakdown, which may be tied to gut microbiome dynamics and associated host nutrition.

Impact of Aging and Exercise on Mitochondrial Quality Control in Skeletal Muscle

Mitochondria are characterized by its pivotal roles in managing energy production, reactive oxygen species, and calcium, whose aging-related structural and functional deteriorations are observed in aging muscle. Although it is still unclear how aging alters mitochondrial quality and quantity in skeletal muscle, dysregulation of mitochondrial biogenesis and dynamic controls has been suggested as key players for that. In this paper, we summarize current understandings on how aging regulates muscle mitochondrial biogenesis, while focusing on transcriptional regulations including PGC-1α, AMPK, p53, mtDNA, and Tfam. Further, we review current findings on the muscle mitochondrial dynamic systems in aging muscle: fusion/fission, autophagy/mitophagy, and protein import. Next, we also discuss how endurance and resistance exercises impact on the mitochondrial quality controls in aging muscle, suggesting possible effective exercise strategies to improve/maintain mitochondrial health.

Unravelling the mechanisms regulating muscle mitochondrial biogenesis

Skeletal muscle is a tissue with a low mitochondrial content under basal conditions, but it is responsive to acute increases in contractile activity patterns (i.e. exercise) which initiate the signalling of a compensatory response, leading to the biogenesis of mitochondria and improved organelle function. Exercise also promotes the degradation of poorly functioning mitochondria (i.e. mitophagy), thereby accelerating mitochondrial turnover, and preserving a pool of healthy organelles. In contrast, muscle disuse, as well as the aging process, are associated with reduced mitochondrial quality and quantity in muscle. This has strong negative implications for whole-body metabolic health and the preservation of muscle mass. A number of traditional, as well as novel regulatory pathways exist in muscle that control both biogenesis and mitophagy. Interestingly, although the ablation of single regulatory transcription factors within these pathways often leads to a reduction in the basal mitochondrial content of muscle, this can invariably be overcome with exercise, signifying that exercise activates a multitude of pathways which can respond to restore mitochondrial health. This knowledge, along with growing realization that pharmacological agents can also promote mitochondrial health independently of exercise, leads to an optimistic outlook in which the maintenance of mitochondrial and whole-body metabolic health can be achieved by taking advantage of the broad benefits of exercise, along with the potential specificity of drug action.

Deceleration of liver regeneration by knockdown of augmenter of liver regeneration gene is associated with impairment of mitochondrial DNA synthesis in mice

Hepatic stimulator substance, also known as augmenter of liver regeneration (ALR), is a novel hepatic mitogen that stimulates liver regeneration after partial hepatectomy (PH). Recent work has indicated that a lack of ALR expression inhibited liver regeneration in rats, and the mechanism seems to be related to increased cell apoptosis. The mitochondria play an important role during liver regeneration. Adequate ATP supply, which is largely dependent on effective mitochondrial biogenesis, is essential for progress of liver regeneration. However, ALR gene expression during liver regeneration, particularly its function with mitochondrial DNA synthesis, remains poorly understood. In this study, ALR expression in hepatocytes of mice was suppressed with ALR short-hairpin RNA interference or ALR deletion (knockout, KO). The ALR-defective mice underwent PH, and the liver was allowed to regenerate for 1 wk. Analysis of liver growth and its correlation with mitochondrial biogenesis showed that both ALR mRNA and protein levels increased robustly in control mice with a maximum at days 3 and 4 post-PH. However, ALR knockdown inhibited hepatic DNA synthesis and decelerated liver regeneration after PH. Furthermore, both in the ALR-knockdown and ALR-KO mice, expression of mitochondrial transcription factor A and peroxisome proliferator-activated receptor-γ coactivator-1α were reduced, resulting in impaired mitochondrial biogenesis. In conclusion, ALR is apparently required to ensure appropriate liver regeneration following PH in mice, and deletion of the ALR gene may delay liver regeneration in part due to impaired mitochondrial biogenesis.

Regulation of mitochondrial biogenesis and GLUT4 expression by exercise

Endurance exercise training can induce large increases mitochondria and the GLUT4 isoform of the glucose transporter in skeletal muscle. For a long time after the discovery in the 1960s that exercise results in an increase in muscle mitochondria, there was no progress in elucidation of the mechanisms involved. The reason for this lack of progress was that nothing was known regarding how expression of the genes-encoding mitochondrial proteins is coordinately regulated. This situation changed rapidly after discovery of transcription factors that control transcription of genes-encoding mitochondrial proteins and, most importantly, the discovery of peroxisome proliferator-gamma coactivator-1α (PGC-1α). This transcription coactivator binds to and activates transcription factors that regulate transcription of genes-encoding mitochondrial proteins. Thus, PGC-1α activates and coordinates mitochondrial biogenesis. It is now known that exercise rapidly activates and induces increased expression of PGC-1α. The exercise-generated signals that lead to PGC-1α activation and increased expression are the increases in cytosolic Ca(2+) and decreases in ATP and creatine phosphate (∼P). Ca(2+) mediates its effect by activating CAMKII, while the decrease in ∼P mediates its effect via activation of AMPK. Expression of the GLUT4 isoform of the glucose transporter is regulated in parallel with mitochondrial biogenesis via the same signaling pathways. This review describes what is known regarding the regulation of mitochondrial biogenesis and GLUT4 expression by exercise. A major component of this review deals with the physiological and metabolic consequences of the exercise-induced increase in mitochondria and GLUT4.

Redox regulation of mitochondrial biogenesis

The cell renews, adapts, or expands its mitochondrial population during episodes of cell damage or periods of intensified energy demand by the induction of mitochondrial biogenesis. This bigenomic program is modulated by redox-sensitive signals that respond to physiological nitric oxide (NO), carbon monoxide (CO), and mitochondrial reactive oxygen species production. This review summarizes our current ideas about the pathways involved in the activation of mitochondrial biogenesis by the physiological gases leading to changes in the redox milieu of the cell, with an emphasis on the responses to oxidative stress and inflammation. The cell's energy supply is protected from conditions that damage mitochondria by an inducible transcriptional program of mitochondrial biogenesis that operates in large part through redox signals involving the nitric oxide synthase and the heme oxygenase-1/CO systems. These redox events stimulate the coordinated activities of several multifunctional transcription factors and coactivators also involved in the elimination of defective mitochondria and the expression of counterinflammatory and antioxidant genes, such as IL10 and SOD2, as part of a unified damage-control network. The redox-regulated mechanisms of mitochondrial biogenesis schematically outlined in the graphical abstract link mitochondrial quality control to an enhanced capacity to support the cell's metabolic needs while improving its resistance to metabolic failure and avoidance of cell death during periods of oxidative stress.

Exercise alters mRNA expression of telomere-repeat binding factor 1 in skeletal muscle via p38 MAPK

Telomeres protect chromosome ends and shorten with age in most tissues. Integral to the maintenance of telomeres is the protein complex shelterin. The gene expression regulation of shelterin proteins to physiological stressors is not understood in vivo. We have recently reported increased telomere-repeat binding factor 1 (TRF1) protein expression and longer telomere length in skeletal muscle of sedentary compared with chronically active mice. These provocative observations led us to examine the effects of acute physiological stress on shelterin expression in vivo in mice and to further define potential mechanisms associated with gene regulation of shelterin. Three groups of female C57Bl/6 mice were studied: one control group and two groups that underwent a 30-min treadmill running bout and were killed either immediately following or 1-h after the exercise. Following the exercise bout, mRNA expression of Trf1 was significantly reduced in the plantaris muscle, and this reduction was paralleled by significant increases in p38 MAPK phosphorylation. To determine if p38 mediated the decreases in Trf1 mRNA expression, C2C12 myotubes were treated with the calcium ionophore, A23187. In response to the A23187, Trf1 gene expression was significantly reduced, coupled with significant increases in p38 phosphorylation, similar to in vivo data. C2C12 myotubes pretreated with a p38 inhibitor (SB-202190) prevented the A23187-induced decrease in Trf1 mRNA expression, indicating a link between Trf1 gene expression and p38 MAPK activation. While it is too early to definitively report the effect of exercise on telomere biology in rodents or humans, these data provide important mechanistic insights into the paradoxical telomere shortening that occurs in skeletal muscle in response to chronic exercise in mice.

Inverse regulation of the cytosolic Ca²⁺ buffer parvalbumin and mitochondrial volume in muscle cells via SIRT1/PGC-1α axis

Skeletal muscles show a high plasticity to cope with various physiological demands. Different muscle types can be distinguished by the force, endurance, contraction/relaxation kinetics (fast-twitch vs. slow-twitch muscles), oxidative/glycolytic capacity, and also with respect to Ca²⁺-signaling components. Changes in Ca²⁺ signaling and associated Ca²⁺-dependent processes are thought to underlie the high adaptive capacity of muscle fibers. Here we investigated the consequences and the involved mechanisms caused by the ectopic expression of the Ca²⁺-binding protein parvalbumin (PV) in C2C12 myotubes in vitro, and conversely, the effects caused by its absence in in fast-twitch muscles of parvalbumin null-mutant (PV⁻/⁻) mice in vivo. The absence of PV in fast-twitch muscle tibialis anterior (TA) resulted in an increase in the peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and of its positive regulator, the deacetylase sirtuin 1 (SIRT1). TA muscles from PV⁻/⁻ mice also have an increased mitochondrial volume. Mild ionophore treatment of control (PV-devoid) C2C12 myotubes causing a moderate elevation in [Ca²⁺](c) resulted in an increase in mitochondrial volume, together with elevated PGC-1α and SIRT1 expression levels, whilst it increased PV expression levels in myotubes stably transfected with PV. In PV-expressing myotubes the mitochondrial volume, PGC-1α and SIRT1 were significantly lower than in control C2C12 myotubes already at basal conditions and application of ionophore had no effect on either one. SIRT1 activation causes a down-regulation of PV in transfected myotubes, whilst SIRT1 inhibition has the opposite effect. We conclude that PV expression and mitochondrial volume in muscle cells are inversely regulated via a SIRT1/PGC-1α signaling axis.

Transcriptional control of mitochondrial biogenesis and its interface with inflammatory processes

BACKGROUND

Cells avoid major mitochondrial damage and energy failure during systemic inflammatory states, such as severe acute infections, by specific targeting of the inflammatory response and by inducing anti-inflammatory and anti-oxidant defenses. Recent evidence indicates that these cell defenses also include mitochondrial biogenesis and the clearance of damaged mitochondria through autophagy.

SCOPE OF REVIEW

This review addresses a group of transcriptional signaling mechanisms that engage mitochondrial biogenesis, including energy-sensing and redox-regulated transcription factors and co-activators, after major inflammatory events.

MAJOR CONCLUSIONS

Stimulation of the innate immune system by activation of toll-like receptors (TLR) generates pro-inflammatory mediators, such as tumor necrosis factor-α (TNF-α)and interleukin-1β (IL-1β), necessary for optimal host defense, but which also contribute to mitochondrial damage through oxidative stress and other mechanisms. To protect its energy supply, host cells sense mitochondrial damage and initiate mitochondrial biogenesis under the control of an inducible transcriptional program that also activates anti-oxidant and anti-inflammatory gene expression. This multifunctional network not only increases cellular resistance to metabolic failure, oxidative stress, and cell death, but promotes immune tolerance as shown in the graphical abstract.

GENERAL SIGNIFICANCE

The post-inflammatory induction of mitochondrial biogenesis supports metabolic function and cell viability while helping to control inflammation. In clinical settings, patients recovering from severe systemic infections may develop transient immune suppression, placing them at risk for recurrent infection, but there may be therapeutic opportunities to enhance mitochondrial quality control that would improve the resolution of life-threatening host responses to such infections.

Reduction in inflammatory gene expression in skeletal muscle from Roux-en-Y gastric bypass patients randomized to omentectomy

Objectives

To examine the effects of Roux-en-Y gastric bypass (RYGB) surgery with and without laparoscopic removal of omental fat (omentectomy) on the temporal gene expression profiles of skeletal muscle.

Design

Previously reported were the whole-body metabolic effects of a randomized, single-blinded study in patients receiving RYGB surgery stratified to receive or not receive omentectomy. In this follow up study we report on changes in skeletal muscle gene expression in a subset of 21 patients, for whom biopsies were collected preoperatively and at either 6 months or 12 months postoperatively.

Methodology/Principal Findings

RNA isolated from skeletal muscle biopsies of 21 subjects (8 without omentectomy and 13 with omentectomy) taken before RYGB or at 6 and 12 months postoperatively were subjected to gene expression profiling via Exon 1.0 S/T Array and Taqman Low Density Array. Robust Multichip Analysis and gene enrichment data analysis revealed 84 genes with at least a 4-fold expression difference after surgery. At 6 and 12 months the RYGB with omentectomy group displayed a greater reduction in the expression of genes associated with skeletal muscle inflammation (ANKRD1, CDR1, CH25H, CXCL2, CX3CR1, IL8, LBP, NFIL3, SELE, SOCS3, TNFAIP3, and ZFP36) relative to the RYGB non-omentectomy group. Expressions of IL6 and CCL2 were decreased at all postoperative time points. There was differential expression of genes driving protein turnover (IGFN1, FBXW10) in both groups over time and increased expression of PAAF1 in the non-omentectomy group at 12 months. Evidence for the activation of skeletal muscle satellite cells was inferred from the up-regulation of HOXC10. The elevated post-operative expression of 22 small nucleolar RNAs and the decreased expression of the transcription factors JUNB, FOS, FOSB, ATF3 MYC, EGR1 as well as the orphan nuclear receptors NR4A1, NR4A2, NR4A3 suggest dramatic reorganizations at both the cellular and genetic levels.

Conclusions/Significance

These data indicate that RYGB reduces skeletal muscle inflammation, and removal of omental fat further amplifies this response.

Trial Registration

ClinicalTrials.gov NCT00212160

Nitric oxide and calcium participate in the fine regulation of mitochondrial biogenesis in follicular thyroid carcinoma cells

Members of the peroxisome proliferator-activated receptor γ coactivator-1 family (i.e. PGC-1α, PGC-1β, and the PGC-1-related coactivator (PRC)) are key regulators of mitochondrial biogenesis and function. These regulators serve as mediators between environmental or endogenous signals and the transcriptional machinery governing mitochondrial biogenesis. The FTC-133 and RO82 W-1 follicular thyroid carcinoma cell lines, which present significantly different numbers of mitochondria, metabolic mechanisms, and expression levels of PRC and PGC-1α, may employ retrograde signaling in response to respiratory dysfunction. Nitric oxide (NO) and calcium have been hypothesized to participate in this activity. We investigated the effects of the S-nitroso-N-acetyl-DL-penicillamine-NO donor, on the expression of genes involved in mitochondrial biogenesis and cellular metabolic functions in FTC-133 and RO82 W-1 cells by measuring lactate dehydrogenase and cytochrome c oxidase (COX) activities. We studied the action of ionomycin and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-acetoxymethyl ester (BAPTA/AM) (i.e. a calcium ionophore and a cytosolic calcium chelator) on whole genome expression and mitochondrial biogenesis in RO82 W-1 cells. COX activity and the dynamics of endoplasmic reticulum and mitochondrial networks were analyzed in regard to calcium-modulating treatments. In the FTC-133 and RO82 W-1 cells, the mitochondrial biogenesis induced by NO was mainly related to PRC expression as a retrograde mitochondrial signaling. Ionomycin diminished COX activity and negatively regulated PRC-mediated mitochondrial biogenesis in RO82 W-1 cells, whereas BAPTA/AM produced the opposite effects with a reorganization of the mitochondrial network. This is the first demonstration that NO and calcium regulate mitochondrial biogenesis through the PRC pathway in thyroid cell lines.

Gene regulation mediating fiber-type transformation in skeletal muscle cells is partly glucose- and ChREBP-dependent

Adaptations in the oxidative capacity of skeletal muscle cells can occur under several physiological or pathological conditions. We investigated the effect of increasing extracellular glucose concentration on the expression of markers of energy metabolism in primary skeletal muscle cells and the C2C12 muscle cell line. Growth of myotubes in 25mM glucose (high glucose, HG) compared with 5.55mM led to increases in the expression and activity of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a marker of glycolytic energy metabolism, while oxidative markers peroxisome proliferator-activated receptor γ coactivator 1α and citrate synthase decreased. HG induced metabolic adaptations as are seen during a slow-to-fast fiber transformation. Furthermore, HG increased fast myosin heavy chain (MHC) IId/x but did not change slow MHCI/β expression. Protein phosphatase 2A (PP2A) was shown to mediate the effects of HG on GAPDH and MHCIId/x. Carbohydrate response element-binding protein (ChREBP), a glucose-dependent transcription factor downstream of PP2A, partially mediated the effects of glucose on metabolic markers. The glucose-induced increase in PP2A activity was associated with an increase in p38 mitogen-activated protein kinase activity, which presumably mediates the increase in MHCIId/x promoter activity. Liver X receptor, another possible mediator of glucose effects, induced only an incomplete metabolic shift, mainly increasing the expression of the glycolytic marker. Taken together, HG induces a partial slow-to-fast transformation comprising metabolic enzymes together with an increased expression of MHCIId/x. This work demonstrates a functional role for ChREBP in determining the metabolic type of muscle fibers and highlights the importance of glucose as a signaling molecule in muscle.

Implications of mitochondrial DNA mutations and mitochondrial dysfunction in tumorigenesis

Alterations in oxidative phosphorylation resulting from mitochondrial dysfunction have long been hypothesized to be involved in tumorigenesis. Mitochondria have recently been shown to play an important role in regulating both programmed cell death and cell proliferation. Furthermore, mitochondrial DNA (mtDNA) mutations have been found in various cancer cells. However, the role of these mtDNA mutations in tumorigenesis remains largely unknown. This review focuses on basic mitochondrial genetics, mtDNA mutations and consequential mitochondrial dysfunction associated with cancer. The potential molecular mechanisms, mediating the pathogenesis from mtDNA mutations and mitochondrial dysfunction to tumorigenesis are also discussed.

Exercise: it's the real thing!

The epidemic emergence of modern chronic diseases largely stems from the adoption of a sedentary lifestyle and excess energy intake. However, it has long been known that regular physical activity induces multiple adaptations within skeletal muscle and other organs and these adaptations have positive outcomes for the prevention and treatment of many metabolic disorders. In recognition of such benefits, a recent goal of industry-funded research is to discover orally active compounds that mimic the effects of exercise training, so-called "exercise pills". This article provides an overview of the role of skeletal muscle in health and disease and discusses whether "exercise mimetics" have any potential to combat metabolic diseases.

Adipose tissue and skeletal muscle plasticity modulates metabolic health

Obesity, accumulation of adipose tissue, develops when energy intake exceeds energy expenditure. Adipose tissue is essential for buffering the differences between energy intake and expenditure by accumulating lipids while skeletal muscle is the energy burning machine. Here we adopted the concept that (i) adipose tissue ability to regulate the storage capacity for lipids as well as (ii) dynamic regulation of muscle and adipose tissue secretory and metabolic activity is important for maintaining the metabolic health. This might be at least in part related to tissue plasticity, a phenomenon enabling dynamic modulation of the tissue phenotype in different physiological and pathophysiological situations. Recent advances in our understanding of the complex endocrine function of adipose tissue in regulating lipid metabolism, adipogenesis, angiogenesis, extracellular matrix remodelling, inflammation and oxidative stress prompted us to review the role of tissue plasticity--dynamic changes in adipose tissue and skeletal muscle metabolic and endocrine phenotype--in determining the difference between metabolic health and disease.

Effect of thyroid hormone on mitochondrial properties and oxidative stress in cells from patients with mtDNA defects

Mitochondrial (mt)DNA mutations contribute to various disease states characterized by low ATP production. In contrast, thyroid hormone [3,3',5-triiodothyronine (T(3))] induces mitochondrial biogenesis and enhances ATP generation within cells. To evaluate the role of T(3)-mediated mitochondrial biogenesis in patients with mtDNA mutations, three fibroblast cell lines with mtDNA mutations were evaluated, including two patients with Leigh's syndrome and one with hypertrophic cardiomyopathy. Compared with control cells, patient fibroblasts displayed similar levels of mitochondrial mass, peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), mitochondrial transcription factor A (Tfam), and uncoupling protein 2 (UCP2) protein expression. However, patient cells exhibited a 1.6-fold elevation in ROS production, a 1.7-fold elevation in cytoplasmic Ca2+ levels, a 1.2-fold elevation in mitochondrial membrane potential, and 30% less complex V activity compared with control cells. Patient cells also displayed 20-25% reductions in both cytochrome c oxidase (COX) activity and MnSOD protein levels compared with control cells. After T(3) treatment of patient cells, ROS production was decreased by 40%, cytoplasmic Ca2+ was reduced by 20%, COX activity was increased by 1.3-fold, and ATP levels were elevated by 1.6-fold, despite the absence of a change in mitochondrial mass. There were no significant alterations in the protein expression of PGC-1alpha, Tfam, or UCP2 in either T(3)-treated patient or control cells. However, T(3) restored the mitochondrial membrane potential, complex V activity, and levels of MnSOD to normal values in patient cells and elevated MnSOD levels by 21% in control cells. These results suggest that T(3) acts to reduce cellular oxidative stress, which may help attenuate ROS-mediated damage, along with improving mitochondrial function and energy status in cells with mtDNA defects.

Kinase-specific responsiveness to incremental contractile activity in skeletal muscle with low and high mitochondrial content

Muscle contractions activate protein kinases, leading to signal transduction. We hypothesized that kinase activation would be influenced by mitochondrial content, as well as by contractile activity-induced increases in muscle O(2) consumption (Vo(2)). Kinase phosphorylation in high-oxidative red and low-oxidative white tibialis anterior (TA) muscle (RTA and WTA, respectively) with 2.5-fold differences in mitochondrial content were compared. Stimulation of the TA muscle elicited large increases in Vo(2) (3- to 6-fold and 4- to 60-fold above resting levels in WTA and RTA, respectively). At rest, AMP-activated protein kinase (AMPK), p38, p42, and p44 activation were nearly twofold greater in WTA than in RTA, suggesting an inverse relationship between mitochondrial content and kinase activation in resting muscle. During contractions, similar degrees of phosphorylation in RTA and WTA were evident as a function of Vo(2) for p38 and p42. During increases in Vo(2) up to sixfold above rest, greater responses were observed in RTA than in WTA for AMPK and p44, whereas Akt activation was greater in WTA. In RTA, elevations in Vo(2) elicited increases in AMPK and p44 activation, whereas Akt, p38, and p42 were less sensitive to increments in Vo(2). Reactive oxygen species (ROS) production was greater in mitochondria from white muscle, but when it was calculated in the context of the whole muscle, ROS production was twofold greater in red than in white myofibers. Thus mitochondrial content influences ROS production and is inversely related to kinase activation in resting muscle. During contractions, kinases are differentially sensitive to contraction-induced increments in Vo(2), suggesting that muscle mitochondrial content is important, but it is not the sole determinant of kinase activation during exercise of different intensities.

Evidence for the involvement of CaMKII and AMPK in Ca2+-dependent signaling pathways regulating FA uptake and oxidation in contracting rodent muscle

Calcium-calmodulin/dependent protein kinase II (CaMKII), AMP-activated protein kinase (AMPK), and extracellular signal-regulated kinase (ERK1/2) have each been implicated in the regulation of substrate metabolism during exercise. The purpose of this study was to determine whether CaMKII is involved in the regulation of FA uptake and oxidation and, if it is involved, whether it does so independently of AMPK and ERK1/2. Rat hindquarters were perfused at rest with (n = 16) or without (n = 10) 3 mM caffeine, or during electrical stimulation (n = 14). For each condition, rats were subdivided and treated with 10 muM of either KN92 or KN93, inactive and active CaMKII inhibitors, respectively. Both caffeine treatment and electrical stimulation significantly increased FA uptake and oxidation. KN93 abolished caffeine-induced FA uptake, decreased contraction-induced FA uptake by 33%, and abolished both caffeine- and contraction-induced FA oxidation (P < 0.05). Caffeine had no effect on ERK1/2 phosphorylation (P > 0.05) and increased alpha(2)-AMPK activity by 68% (P < 0.05). Electrical stimulation increased ERK1/2 phosphorylation and alpha(2)-AMPK activity by 51% and 3.4-fold, respectively (P < 0.05). KN93 had no effect on caffeine-induced alpha(2)-AMPK activity, ERK1/2 phosphorylation, or contraction-induced ERK1/2 phosphorylation (P > 0.05). Alternatively, it decreased contraction-induced alpha(2)-AMPK activity by 51% (P < 0.05), suggesting that CaMKII lies upstream of AMPK. These results demonstrate that regulation of contraction-induced FA uptake and oxidation occurs in part via Ca(2+)-independent activation of ERK1/2 as well as Ca(2+)-dependent activation of CaMKII and AMPK.

Large-scale chemical dissection of mitochondrial function

Mitochondrial oxidative phosphorylation (OXPHOS) is under the control of both mitochondrial (mtDNA) and nuclear genomes and is central to energy homeostasis. To investigate how its function and regulation are integrated within cells, we systematically combined four cell-based assays of OXPHOS physiology with multiplexed measurements of nuclear and mtDNA gene expression across 2,490 small-molecule perturbations in cultured muscle. Mining the resulting compendium revealed, first, that protein synthesis inhibitors can decouple coordination of nuclear and mtDNA transcription; second, that a subset of HMG-CoA reductase inhibitors, combined with propranolol, can cause mitochondrial toxicity, yielding potential clues about the etiology of statin myopathy; and, third, that structurally diverse microtubule inhibitors stimulate OXPHOS transcription while suppressing reactive oxygen species, via a transcriptional mechanism involving PGC-1alpha and ERRalpha, and thus may be useful in treating age-associated degenerative disorders. Our screening compendium can be used as a discovery tool both for understanding mitochondrial biology and toxicity and for identifying novel therapeutics.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Mechanisms of calcium-induced mitochondrial biogenesis and GLUT4 synthesis

Regularly performed aerobic exercise leads to increases in skeletal muscle mitochondria and glucose transporter 4 (GLUT4) protein content, resulting in an enhanced capacity to oxidize substrates and improvements in insulin- and contraction-mediated glucose uptake. Although the specific mechanisms governing these adaptive responses have not been fully elucidated, accumulating evidence suggests that the increase in cytosolic Ca2+ that occurs with each wave of sacrolemmal depolarization is a key component of these processes. Treating L6 muscle cells with agents that increase Ca2+ without causing reductions in ~P or the activation of 5′-AMP-activated protein kinase leads to increases in GLUT4 and mitochondrial protein contents. This effect is likely controlled through calcium/calmodulin-dependent protein kinase (CaMK), since KN93, a specific CaMK inhibitor, blocks these adaptive responses. Recent findings provide evidence that the activation of p38 mitogen-activated protein kinase (MAPK) is involved in the pathway through which Ca2+/CaMK mediates mitochondrial and GLUT4 biogenesis. p38 MAPK initiates GLUT4 and mitochondrial biogenesis through the activation      of transcription factors and transcriptional coactivators such as myocyte enhancer factor 2 (MEF2) and peroxisome proliferator-activated receptor gamma coactivator 1 alpha (PGC-1α). Subsequent increases in the content of these proteins further enhance Ca2+-induced GLUT4 and mitochondrial biogenesis. Since decreases in mitochondrial and GLUT4 contents are associated with skeletal muscle insulin resistance, an understanding of the mechanisms by which these processes can be normalized will aid in the prevention and treatment of type 2 diabetes.

The molecular bases of training adaptation

Skeletal muscle is a malleable tissue capable of altering the type and amount of protein in response to disruptions to cellular homeostasis. The process of exercise-induced adaptation in skeletal muscle involves a multitude of signalling mechanisms initiating replication of specific DNA genetic sequences, enabling subsequent translation of the genetic message and ultimately generating a series of amino acids that form new proteins. The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise. Prolonged endurance training elicits a variety of metabolic and morphological changes, including mitochondrial biogenesis, fast-to-slow fibre-type transformation and substrate metabolism. In contrast, heavy resistance exercise stimulates synthesis of contractile proteins responsible for muscle hypertrophy and increases in maximal contractile force output. Concomitant with the vastly different functional outcomes induced by these diverse exercise modes, the genetic and molecular mechanisms of adaptation are distinct. With recent advances in technology, it is now possible to study the effects of various training interventions on a variety of signalling proteins and early-response genes in skeletal muscle. Although it cannot presently be claimed that such scientific endeavours have influenced the training practices of elite athletes, these new and exciting technologies have provided insight into how current training techniques result in specific muscular adaptations, and may ultimately provide clues for future and novel training methodologies. Greater knowledge of the mechanisms and interaction of exercise-induced adaptive pathways in skeletal muscle is important for our understanding of the aetiology of disease, maintenance of metabolic and functional capacity with aging, and training for athletic performance. This article highlights the effects of exercise on molecular and genetic mechanisms of training adaptation in skeletal muscle.

Effect of endurance exercise training on Ca2+ calmodulin-dependent protein kinase II expression and signalling in skeletal muscle of humans

Here the hypothesis that skeletal muscle Ca(2+)-calmodulin-dependent kinase II (CaMKII) expression and signalling would be modified by endurance training was tested. Eight healthy, young men completed 3 weeks of one-legged endurance exercise training with muscle samples taken from both legs before training and 15 h after the last exercise bout. Along with an approximately 40% increase in mitochondrial F(1)-ATP synthase expression, there was an approximately 1-fold increase in maximal CaMKII activity and CaMKII kinase isoform expression after training in the active leg only. Autonomous CaMKII activity and CaMKII autophosphorylation were increased to a similar extent. However, there was no change in alpha-CaMKII anchoring protein expression with training. Nor was there any change in expression or Thr(17) phosphorylation of the CaMKII substrate phospholamban with training. However, another CaMKII substrate, serum response factor (SRF), had an approximately 60% higher phosphorylation at Ser(103) after training, with no change in SRF expression. There were positive correlations between the increases in CaMKII expression and SRF phosphorylation as well as F(1)ATPase expression with training. After training, there was an increase in cyclic-AMP response element binding protein phosphorylation at Ser(133), but not expression, in muscle of both legs. Taken together, skeletal muscle CaMKII kinase isoform expression and SRF phosphorylation is higher with endurance-type exercise training, adaptations that are restricted to active muscle. This may contribute to greater Ca(2+) mediated regulation during exercise and the altered muscle phenotype with training.

Calcineurin A and CaMKIV transactivate PGC-1α promoter, but differentially regulate cytochrome c promoter in rat skeletal muscle

In skeletal muscle, slow-twitch fibers are highly dependent on mitochondrial oxidative metabolism suggesting the existence of common regulatory pathways in the control of slow muscle-specific protein expression and mitochondrial biogenesis. In this study, we determined whether peroxisome proliferator-activated receptor gamma co-activator-1alpha (PGC-1alpha) could transactivate promoters of nuclear-encoded mitochondrial protein (cytochrome c) and muscle-specific proteins (fast troponin I, MyoD). We also investigated if calcineurin A (CnA) and calcium/calmodulin kinase IV (CaMKIV) were involved in the regulation of PGC-1alpha and cytochrome c promoter. For this purpose, we took advantage of the gene electrotransfer technique, which allows acute expression of a gene of interest. Electrotransfer of a PGC-1alpha expression vector into rat Tibialis anterior muscle induced a strong transactivation of cytochrome c promoter (P < 0.001) independent of nuclear respiratory factor 1. PGC-1alpha gene electrotransfer did not transactivate fast troponin I promoter, whereas it did transactivate MyoD promoter (P < 0.05). Finally, whereas electrotransfers of CnA or CaMKIV expression vectors transactivated PGC-1alpha promoter (P < 0.001), gene electrotransfer of CaMKIV was only able to transactivate cytochrome c promoter. Taken together, these data suggest that CnA triggers PGC-1alpha promoter transactivation to drive the expression of non-mitochondrial proteins.

Energy sensing and regulation of gene expression in skeletal muscle

Major modifications in energy homeostasis occur in skeletal muscle during exercise. Emerging evidence suggests that changes in energy homeostasis take part in the regulation of gene expression and contribute to muscle plasticity. A number of energy-sensing molecules have been shown to sense variations in energy homeostasis and trigger regulation of gene expression. The AMP-activated protein kinase, hypoxia-inducible factor 1, peroxisome proliferator-activated receptors, and Sirt1 proteins all contribute to altering skeletal muscle gene expression by sensing changes in the concentrations of AMP, molecular oxygen, intracellular free fatty acids, and NAD+, respectively. These molecules may therefore sense information relating to the intensity, duration, and frequency of muscle exercise. Mitochondria also contribute to the overall response, both by modulating the response of energy-sensing molecules and by generating their own signals. This review seeks to examine our current understanding of the roles that energy-sensing molecules and mitochondria can play in the regulation of gene expression in skeletal muscle.

Coordination of metabolic plasticity in skeletal muscle

Skeletal muscle is a highly malleable tissue, capable of pronounced metabolic and morphological adaptations in response to contractile activity (i.e. exercise). Each bout of contractile activity results in a coordinated alteration in the expression of a variety of nuclear DNA and mitochondrial DNA (mtDNA) gene products, leading to phenotypic adaptations. This results in an increase in muscle mitochondrial volume and changes in organelle composition, referred to as mitochondrial biogenesis. The functional consequence of this biogenesis is an improved resistance to fatigue. Signals initiated by the exercise bout involve changes in intracellular Ca2+ as well as alterations in energy status (i.e. ATP/ADP ratio) and the consequent activation of downstream kinases such as AMP kinase and Ca2+-calmodulin-activated kinases. These kinases activate transcription factors that bind DNA to affect the transcription of genes, the most evident manifestation of which occurs during the post-exercise recovery period when energy metabolism is directed toward anabolism, rather than contractile activity. An important protein that is affected by exercise is the transcriptional coactivator PGC-1alpha, which cooperates with multiple transcription factors to induce the expression of nuclear genes encoding mitochondrial proteins. Once translated in the cytosol, these mitochondrially destined proteins are imported into the mitochondrial outer membrane, inner membrane or matrix space via specific import machinery transport components. Contractile activity affects the expression of the import machinery, as well as the kinetics of import, thus facilitating the entry of newly synthesized proteins into the expanding organelle. An important set of proteins that are imported are the mtDNA transcription factors, which influence the expression and replication of mtDNA. While mtDNA contributes only 13 proteins to the synthesis of the organelle, these proteins are vital for the proper assembly of multi-subunit complexes of the respiratory chain, when combined with nuclear-encoded protein subunits. The expansion of skeletal muscle mitochondria during organelle biogenesis involves the assembly of an interconnected network system (i.e. a mitochondrial reticulum). This expansion of membrane size is influenced by the balance between mitochondrial fusion and fission. Thus, mitochondrial biogenesis is an adaptive process that requires the coordination of multiple cellular events, including the transcription of two genomes, the synthesis of lipids and proteins and the stoichiometric assembly of multisubunit protein complexes into a functional respiratory chain. Impairments at any step can lead to defective electron transport, a subsequent failure of ATP production and an inability to maintain energy homeostasis.

Ca2+-calmodulin-dependent protein kinase expression and signalling in skeletal muscle during exercise

Ca2+ signalling is proposed to play an important role in skeletal muscle function during exercise. Here, we examined the expression of multifunctional Ca2+-calmodulin-dependent protein kinases (CaMK) in human skeletal muscle and show that CaMKII and CaMKK, but not CaMKI or CaMKIV, are expressed. Furthermore, the effect of exercise duration and intensity on skeletal muscle CaMKII activity and phosphorylation of downstream targets was examined. Eight healthy men exercised at approximately 67% of peak pulmonary O2 uptake(VO2peak) with muscle samples taken at rest and after 1, 10, 30, 60 and 90 min of exercise. Ten other men exercised for three consecutive 10 min bouts at 35%, 60% and 85% VO2peak with muscle samples taken at rest, at the end of each interval and 30 min post-exercise. There was a rapid and transient increase in autonomous CaMKII activity and CaMKII phosphorylation at Thr287 in skeletal muscle during exercise. Furthermore, the phosphorylation of phospholamban (PLN) at Thr17, which was identified as a CaMKII substrate in skeletal muscle, was rapidly (< 1 min) increased by exercise, and remained phosphorylated 5-fold above basal level during 90 min of exercise. The phosphorylation of serum response factor at Ser103, a putative CaMKII substrate, was higher after 30 min of exercise. PLN phosphorylation at Thr17 was higher with increasing exercise intensities. These data indicate that CaMKII is the major multifunctional CaMK in skeletal muscle and its activation occurs rapidly and is sustained during continuous exercise, with the activation being greater during intense exercise.

Dinitrophenol-induced mitochondrial uncoupling in vivo triggers respiratory adaptation in HepG2 cells

Here, we show that 3 days of mitochondrial uncoupling, induced by low concentrations of dinitrophenol (10 and 50 microM) in cultured human HepG2 cells, triggers cellular metabolic adaptation towards oxidative metabolism. Chronic respiratory uncoupling of HepG2 cells induced an increase in cellular oxygen consumption, oxidative capacity and cytochrome c oxidase activity. This was associated with an upregulation of COXIV and ANT3 gene expression, two nuclear genes that encode mitochondrial proteins involved in oxidative phosphorylation. Glucose consumption, lactate and pyruvate production and growth rate were unaffected, indicating that metabolic adaptation of HepG2 cells undergoing chronic respiratory uncoupling allows continuous and efficient mitochondrial ATP production without the need to increase glycolytic activity. In contrast, 3 days of dinitrophenol treatment did not change the oxidative capacity of human 143B.TK(-) cells, but it increased glucose consumption, lactate and pyruvate production. Despite a large increase in glycolytic metabolism, the growth rate of 143B.TK(-) cells was significantly reduced by dinitrophenol-induced mitochondrial uncoupling. We propose that chronic respiratory uncoupling may constitute an internal bioenergetic signal, which would initiate a coordinated increase in nuclear respiratory gene expression, which ultimately drives mitochondrial metabolic adaptation within cells.

Involvement of Egr-1 in lens epithelial cell death induced by selenite

Selenite-overdose cataract in young rats may be caused by an initial insult to the lens epithelial cells. Our previous DNA array analysis revealed a significant increase in the expression of mRNA for early growth response protein-1 (Egr-1) in lens epithelial cells after injection of selenite. This suggested that up-regulation of Egr-1 mRNA may be involved in lens epithelial cell death. The purpose of the present experiment was to further clarify the involvement of Egr-1 in lens epithelial cell death induced by selenite. Rat lens epithelial explants were cultured with sodium selenite. Selenite caused epithelial explants to leak LDH into the medium. During LDH leakage, increased expression of mRNA for Egr-1 was observed by RT-PCR. To further test the involvement of Egr-1 in selenite-induced cell death, mouse lens epithelial cell line (alpha-TN4 cells) was treated with antisense oligonucleotide for Egr-1. Antisense oligonucleotide for Egr-1 significantly diminished expression of Egr-1 protein and leakage of LDH. These results suggested that increased activity of Egr-1 may be a factor in lens epithelial cell death induced by selenite.

How is Mitochondrial Biogenesis Affected in Mitochondrial Disease?

Mitochondrial biogenesis occurs when the tissue energy demand is chronically increased to stress the ATP producing capacity of the preexisting mitochondria. In muscle, endurance training is a metabolic stress that is capable of inducing mitochondrial biogenesis, the consequence of which is improved performance during exercise. Expansion of the mitochondrial volume requires the coordinated response of the nuclear and mitochondrial genomes. During acute exercise, the initial signaling events are the perturbations in ATP turnover and calcium (Ca) concentrations caused by the contractile process. These alterations activate signal transduction pathways which target transcription factors involved in gene expression. Nuclear gene products are then posttranslationally imported into mitochondria. One of these, Tfam, is important for the regulation of mitochondrial DNA (mtDNA) gene expression. In muscle, a broad range of mitochondrial-specific diseases due to mutations in nuclear DNA or mtDNA exist, termed mitochondrial myopathies. These mutations result in dysfunctional mitochondrial assembly which ultimately leads to reduced ATP production. Mitochondrial myopathy patients exhibit a variety of compensatory responses which attempt to reconcile this energy deficiency, but the extent and the type of compensatory adaptations are disease-specific. Understanding the role of exercise in mediating these compensatory responses leading to mitochondrial biogenesis could help us in prescribing exercise designed to improve mitochondrial function in patients with mitochondrial myopathies. In addition, numerous other diseases (e.g., neurological disorders, cancer, diabetes, and cardiomyopathies), as well as the aging process, have etiologies or consequences attributed, in part, to mitochondrial dysfunction. Thus, insight gained by investigating the steps involved in exercise-induced mitochondrial biogenesis may help us to understand the underlying basis of these other disease states.

Application of Animal Models: Chronic Electrical Stimulation-Induced Contractile Activity

Unilateral, chronic low-frequency electrical stimulation (CLFS) is an experimental model that evokes numerous biochemical and physiological adaptations in skeletal muscle. These occur within a short time frame and are restricted to the stimulated muscle. The humoral effects of whole body exercise are eliminated and the nonstimulated contralaterai limb can often be used as a control muscle, if possible effects on the contralateral side are considered. CLFS induces a fast-to-slow transformation of muscle because of alterations in calcium dynamics and myofibrillar proteins, and a white-to-red transformation because of changes in mitochondrial enzymes, myoglobin, and the induction of angiogenesis. These adaptations occur in a coordinated time-dependent manner and result from altered gene expression, including transcriptional and posttranscriptional processes. CLFS techniques have also been applied to myocytes in cell culture, which provide a greater opportunity for the delivery of pharmacological agents or for the application of gene transfer methodologies. Clinical applications of the CLFS technique have been limited, but they have shown potential therapeutic value in patients in whom voluntary muscle contraction is not possible due to debilitating disease and/or injury. Thus the CLFS technique has great value for studying various aspects of muscle adaptation, and its wider scientific application to a variety of neuromuscular-based disorders in humans appears to be warranted. Key words: skeletal muscle, muscle plasticity, endurance training, mitochondrial biogenesis, fiber types

Establishment of a near-standard two-dimensional human urine proteomic map

A proteomic map for human urine on two-dimensional (2-D) gels has been developed. Initial studies demonstrated that the urine proteins prepared by conventional methods showed interference and poor reproducibility in 2-D electrophoresis (2-DE). To address this issue, urine samples were dialyzed to remove any interfering molecules. The dialysis of urine proteins and the concentration by lyophilization without fractionation significantly improved the reproducibility and resolution and likely represents the total urine proteins on a 2-D gel. In addition, removing albumin from urine using Affi-Gel Blue helped to identify the low-abundant proteins. Using the developed method, we prepared proteins from urine collected from healthy females and males. The large inter- and intra-subject variation in protein profiles on 2-D gels made it difficult to establish a normal human urine proteomic 2-D map. To resolve this problem, urinary proteins were prepared from the pooled urine collected from 20 healthy females and males, respectively. The established male and female urine proteomes separated on 2-D gels were almost identical except for some potential sex-dependent protein spots. We have annotated 113 different proteins on the 2-D gel by peptide mass fingerprinting (PMF). We propose that the established total urine proteome can be used for 2-DE analysis, liquid chromatography-tandem mass spectrometry (LC-MS/MS), and identification of novel disease-specific biomarkers.

Regulation of Egr-1, SRF, and Sp1 mRNA expression in contracting skeletal muscle cells

The early cellular signals associated with contractile activity initiate the activation and induction of transcription factors that regulate changes in skeletal muscle phenotype. The transcription factors Egr-1, Sp1, and serum response factor (SRF) are potentially important mediators of mitochondrial biogenesis based on the prevalence of binding sites for them in the promoter regions of genes encoding mitochondrial proteins, including PGC-1α, the important regulator of mitochondrial biogenesis. Thus, to further define a role for transcription factors at the onset of contractile activity, we examined the time-dependent alterations in Egr-1, Sp1, and SRF mRNA and the levels in electrically stimulated mouse C2C12skeletal muscle cells. Early transient increases in Egr-1 mRNA levels within 30 min ( P < 0.05) of contractile activity led to threefold increases ( P < 0.05) in Egr-1 protein by 60 min. The increase in Egr-1 mRNA was not because of increased stability, as Egr-1 mRNA half-life after 30 min of stimulation showed only a 58% decline. Stimulation of muscle cells had no effect on Sp1 mRNA but led to progressive increases ( P < 0.05) in SRF mRNA by 30 and 60 min. This was not matched by increases in SRF protein but occurred coincident with increases ( P < 0.05) in SRF-serum response element DNA binding at 30 and 60 min as a result of SRF phosphorylation on serine-103. To assess the importance of the recovery period, 12 h of continuous contractile activity was compared with four successive 3-h bouts, with an intervening 21-h recovery period after each bout. Continuous contractile activity led to a twofold increase ( P < 0.05) in Egr-1 mRNA, no change in SRF mRNA, and a 43% decrease in Sp1 mRNA expression. The recovery period prevented the decline in Sp1 mRNA, produced a decrease in Egr-1 mRNA, and had no effect on SRF mRNA. Thus continuous and intermittent contractile activity evoked different specific transcription factor expression patterns, which may ultimately contribute to divergent qualitative, or temporal patterns of, phenotypic adaptation in muscle.

Mitochondrial signaling: the retrograde response

Mitochondrial retrograde signaling is a pathway of communication from mitochondria to the nucleus that influences many cellular and organismal activities under both normal and pathophysiological conditions. In yeast it is used as a sensor of mitochondrial dysfunction that initiates readjustments of carbohydrate and nitrogen metabolism. In both yeast and animal cells, retrograde signaling is linked to TOR signaling, but the precise connections are unclear. In mammalian cells, mitochondrial dysfunction sets off signaling cascades through altered Ca(2+) dynamics, which activate factors such as NFkappaB, NFAT, and ATF. Retrograde signaling also induces invasive behavior in otherwise nontumorigenic cells implying a role in tumor progression.