Adaptive thermogenesis: orchestrating mitochondrial biogenesis

Effect of acute cold exposure on cardiac mitochondrial function: role of sirtuins

Cardiac function depends mainly on mitochondrial metabolism. Cold conditions increase the risk of cardiovascular diseases by increasing blood pressure. Adaptive thermogenesis leads to increased mitochondrial biogenesis and function in skeletal muscles and adipocytes. Here, we studied the effect of acute cold exposure on cardiac mitochondrial function and its regulation by sirtuins. Significant increase in mitochondrial DNA copy number as measured by the ratio between mitochondrial-coded COX-II and nuclear-coded cyclophilin A gene expression by qRT-PCR and increase in the expression of PGC-1α, a mitochondriogenic factor and its downstream target NRF-1 were observed on cold exposure. This was associated with an increase in the activity of SIRT-1, which is known to activate PGC-1α. Mitochondrial SIRT-3 was also upregulated. Increase in sirtuin activity was reflected in total protein acetylome, which decreased in cold-exposed cardiac tissue. An increase in mitochondrial MnSOD further indicated enhanced mitochondrial function. Further evidence for this was obtained from ex vivo studies of cardiac tissue treated with norepinephrine, which caused a significant increase in mitochondrial MnSOD and SIRT-3. SIRT-3 appears to mediate the regulation of MnSOD, as treatment with AGK-7, a SIRT-3 inhibitor reversed the norepinephrine-induced upregulation of MnSOD. It, therefore, appears that SIRT-3 activation in response to SIRT-1-PGC-1α activation contributes to the regulation of cardiac mitochondrial activity during acute cold exposure.

Enoxacin ameliorates polycystic ovary syndrome by promoting the browning of white adipose tissue and restoring gut dysbiosis

Polycystic ovary syndrome (PCOS) is a complex endocrine disorder syndrome characterized by polycystic ovary, ovulation disorder and hyperandrogenemia, and is often accompanied by metabolic disorders. Enoxacin has been reported to protect against diet-induced obesity and insulin resistance by promoting fat thermogenesis. However, the function of enoxacin in PCOS remains unknown. This study aimed to investigate the impact of the enoxacin on the regulation of PCOS mouse model induced by dehydroepiandrosterone (DHEA). Here, we found that reproductive endocrine disorder, glucose intolerance, and ovarian dysfunction in PCOS mice induced by DHEA were attenuated by enoxacin treatment. Mechanistically, we identified that enoxacin can promote white fat browning and improve metabolic disorders, thus ameliorating DHEA-induced reproductive dysfunction. Moreover, these beneficial effects might be associated with the restoration of gut dysbiosis. These findings provide a novel therapeutic target for enoxacin in the treatment of PCOS.

Expression Profile of Mouse Gm20594, Nuclear-Encoded Humanin-Like Gene

Background

Mitochondrial-derived peptides (MDPs) such as MOTS-c and humanin have been studied for their cytoprotective functions. In mice, humanin-encoding Mtrnr2 is a mitochondrial pseudogene, and the humanin-like peptide is encoded by the nuclear Gm20594 gene. However, endogenous tissue-specific expression profiles of Gm20594 have not yet been identified.

Methods

Mtrnr1 and Gm20594 expression was profiled via reverse transcription using only oligo(dT) primers from tissues of C57BL6/J mice. To analyze altered expression upon mitochondrial biogenesis, C2C12 myocytes and brown adipocytes were differentiated. Mitochondrial DNA copy numbers were quantified for normalization.

Results

Both Mtrnr1 and Gm20594 were highly expressed in brown adipose tissue. When normalized against mitochondrial content, Mtrnr1 was identified as being highly expressed in the duodenum, followed by the jejunum. In models of mitochondrial biogenesis, both Mtrnr1 and Gm20594 were upregulated during myocyte and brown adipocyte differentiation. Increased Mtrnr1 expression during brown adipocyte differentiation remained significant after normalization against mitochondrial DNA copy number, whereas myocyte differentiation exhibited biphasic upregulation and downregulation in early and late phases, respectively.

Conclusion

Nuclear-encoded Gm20594 showed similar expression patterns of mitochondrial-encoded Mtrnr1. Brown adipose tissue presented the highest basal expression levels of Gm20594 and Mtrnr1. When normalized against mitochondrial DNA copy number, gut tissues exhibited the highest expression of Mtrnr1. Upregulation of Mtrnr1 during mitochondrial biogenesis is independent of mitochondrial content.

The diabetes medication canagliflozin promotes mitochondrial remodelling of adipocyte via the AMPK-Sirt1-Pgc-1α signalling pathway

The diabetes medication canagliflozin (Cana) is a sodium glucose cotransporter 2 (SGLT2) inhibitor acting by increasing urinary glucose excretion and thus reducing hyperglycaemia. Cana treatment also reduces body weight. However, it remains unclear whether Cana could directly work on adipose tissue. In the present study, the pharmacological effects of Cana and the associated mechanism were investigated in adipocytes and mice. Stromal-vascular fractions (SVFs) were isolated from subcutaneous adipose tissue and differentiated into mature adipocytes. Our results show that Cana treatment directly increased cellular energy expenditure of adipocytes by inducing mitochondrial biogenesis independently of SGLT2 inhibition. Along with mitochondrial biogenesis, Cana also increased mitochondrial oxidative phosphorylation, fatty acid oxidation and thermogenesis. Mechanistically, Cana promoted mitochondrial biogenesis and function via an Adenosine monophosphate-activated protein kinase (AMPK) - silent information regulator 1 (Sirt1) - peroxisome proliferator-activated receptor γ coactivator-1α (Pgc-1α) signalling pathway. Consistently, in vivo study demonstrated that Cana increased AMPK phosphorylation and the expression of Sirt1 and Pgc-1α. The present study reveals a new therapeutic function for Cana in regulating energy homoeostasis.

ABBREVIATIONS

Ucp-1, uncoupling protein 1; cAMP, cyclic adenosine monophosphate; PKA, cAMP-dependent protein kinase A; SGLT, sodium glucose cotransporter; Cana, canagliflozin; T2DM: type 2 diabetes; Veh, vehicle; Pgc-1α, peroxisome proliferator-activated receptor γ coactivator-1α; SVFs, stromal-vascular fractions; FBS, bovine serum; Ad, adenovirus; mtDNA, mitochondrial DNA; COX2, cytochrome oxidase subunit 2; RT-PCR, real-time PCR; SDS-PAGE, sodium dodecyl sulphate-polyacrylamide gel electrophoresis; Prdm16, PR domain zinc finger protein 16; Cidea, cell death inducing DFFA-like effector A; Pgc-1β, peroxisome proliferator-activated receptor γ coactivator-1β; NRF1, nuclear respiratory factor 1; Tfam, mitochondrial transcription factor A; OXPHOS, oxidative phosphorylation; FAO, fatty acid oxidation; AMPK, Adenosine monophosphate-activated protein kinase; p-AMPK, phosphorylated AMPK; Sirt1, silent information regulator 1; mTOR, mammalian target of rapamycin; WAT, white adipose tissue; Fabp4, fatty acid binding protein 4; Lpl, lipoprotein lipase; Slc5a2, solute carrier family 5 member 2; ERRα, oestrogen related receptor α; Uqcrc2, ubiquinol-cytochrome c reductase core protein 2; Uqcrfs1, ubiquinol-cytochrome c reductase, Rieske iron-sulphur polypeptide 1; Cox4, cytochrome c oxidase subunit 4; Pparα, peroxisome proliferator activated receptor α; NAD⁺, nicotinamide adenine dinucleotide; Dio2, iodothyronine deiodinase 2; Tmem26, transmembrane protein 26; Hoxa9, homeobox A9; FCCP, carbonyl cyanide 4-(trifluoromethoxy) phenylhydrazone; Rot/AA, rotenone/antimycin A; OCR, oxygen consumption rate; Pparγ, peroxisome proliferator activated receptor γ; C/ebp, CCAAT/enhancer binding protein; LKB1, liver kinase B1; AUC, area under the cure; Vd, apparent volume of distribution.

A pivotal role of AMPK signaling in medicarpin-mediated formation of brown and beige

Obesity poses a substantial threat of a worldwide epidemic and requires better understanding of adipose-tissue biology as well as necessitates research into the etiology and therapeutic interventions. In this study, Medicarpin (Med), a natural pterocarpan, was selected (by screening) as a small-molecule inducer of adipocyte differentiation among 854 candidates by using C3H10T1/2 mesenchymal stem cell; a cellular model of adipogenesis. Med induced the expression of brown-adipocyte commitment marker Bmp7 as well as the early regulators of brown fat fate Pparγ, Prdm16, and Pgc-1α during differentiation of C3H10T1/2 mesenchymal stem cells. Med also induced the expression of a key thermogenic marker-uncoupling protein 1 (UCP1)-along with expression of other brown-fat-specific markers and beige-fat-specific markers. Of note, Med significantly reduced the expression of white fat markers too. Furthermore, Med treatment promoted formation of multilocular lipid droplets (LDs), expression of mitochondrial-biogenesis-related genes, and increased oxygen consumption. Gene silencing study revealed that Med promotes the development of brown- and beige-adipocyte characteristics in C3H10T1/2 mesenchymal stem cells through activation of the AMPK pathway, and our data allow us to propose Med as a candidate for therapeutics against obesity or related metabolic disorders. © 2017 BioFactors, 44(2):168-179, 2018.

A novel brown adipocyte-enriched long non-coding RNA that is required for brown adipocyte differentiation and sufficient to drive thermogenic gene program in white adipocytes

The thermogenic activities of brown and beige adipocytes can be exploited to reduce energy surplus and counteract obesity. Recent RNA sequencing studies have uncovered a number of long noncoding RNAs (lncRNAs) uniquely expressed in white and brown adipose tissues (WAT and BAT), but whether and how these lncRNAs function in adipogenesis remain largely unknown. Here, we report the identification of a novel brown adipocyte-enriched LncRNA (AK079912), and its nuclear localization, function and regulation. The expression of AK079912 increases during brown preadipocyte differentiation and in response to cold-stimulated browning of white adipocytes. Knockdown of AK079912 inhibits brown preadipocyte differentiation, manifested by reductions in lipid accumulation and down-regulation of adipogenic and BAT-specific genes. Conversely, ectopic expression of AK079912 in white preadipocytes up-regulates the expression of genes involved in thermogenesis. Mechanistically, inhibition of AK079912 reduces mitochondrial copy number and protein levels of mitochondria electron transport chain (ETC) complexes, whereas AK079912 overexpression increases the levels of ETC proteins. Lastly, reporter and pharmacological assays identify Pparγ as an upstream regulator of AK079912. These results provide new insights into the function of non-coding RNAs in brown adipogenesis and regulating browning of white adipocytes.

Increasing cAMP levels of preadipocytes by cyanidin-3-glucoside treatment induces the formation of beige phenotypes in 3T3-L1 adipocytes

Obesity is a serious health problem and a major risk factor for the onset of several diseases such as heart disease, diabetes, stroke and cancer. The conversion of white adipocytes to brown-like adipocytes, also called beige or brite adipocytes, by pharmacological and dietary compounds has gained attention as an effective treatment for obesity. Cyanidin-3-glucoside (Cy3G), a polyphenolic compound contained in black soybean, blueberry and grape, has several antiobesity effects. However, there are no reports on the role of Cy3G in the induction of differentiation of preadipocytes to beige adipocytes and corresponding phenotypes. Here, the formation of beige adipocyte phenotypes following treatment with Cy3G was evaluated using 3T3-L1 adipocytes. Cy3G induced phenotypic changes to white adipocytes, such as increased multilocular lipid droplets and mitochondrial content. Additionally, the expression of mitochondrial genes (TFAM, SOD2, UCP-1 and UCP-2), UCP-1 protein and beige adipocyte markers (CITED1 and TBX1) in 3T3-L1 adipocytes was increased by Cy3G. Furthermore, Cy3G promoted preadipocyte differentiation by up-regulating of C/EBPβ through the elevation of the intracellular cAMP levels. These results indicated that Cy3G elevates the intracellular cAMP levels, which induces beige adipocyte phenotypes. This is the first report on the effect of Cy3G on induction of differentiation of preadipocytes into beige adipocyte phenotypes.

FSH and bFGF regulate the expression of genes involved in Sertoli cell energetic metabolism

The purpose of this study was to investigate if FSH and bFGF regulate fatty acid (FA) metabolism and mitochondrial biogenesis in Sertoli cells (SC). SC cultures obtained from 20-day-old rats were incubated with 100ng/ml FSH or 30ng/ml bFGF for 6, 12, 24 and 48h. The expression of genes involved in transport and metabolism of FA such as: fatty acid transporter CD36 (FAT/CD36), carnitine-palmitoyltransferase 1 (CPT1), long- and medium-chain 3-hydroxyacyl-CoA dehydrogenases (LCAD, MCAD), and of genes involved in mitochondrial biogenesis such as: nuclear respiratory factors 1 and 2 (NRF1, NRF2) and transcription factor A (Tfam), was analyzed. FSH stimulated FAT/CD36, CPT1, MCAD, NRF1, NRF2 and Tfam mRNA levels while bFGF only stimulated CPT1 expression. A possible participation of PPARβ/δ activation in the regulation of gene expression and lactate production was then evaluated. SC cultures were incubated with FSH or bFGF in the presence of the PPARβ/δ antagonist GSK3787 (GSK; 20μM). bFGF stimulation of CPT1 expression and lactate production were inhibited by GSK. On the other hand, FSH effects were not inhibited by GSK indicating that FSH regulates the expression of genes involved in FA transport and metabolism and in mitochondrial biogenesis, independently of PPARβ/δ activation. FA oxidation and mitochondrial biogenesis as well as lactate production are essential for the energetic metabolism of the seminiferous tubule. The fact that these processes are regulated by hormones in a different way reflects the multifarious regulation of molecular mechanisms involved in Sertoli cell function.

Human brown fat inducible thioesterase variant 2 cellular localization and catalytic function

The mammalian brown fat inducible thioesterase variant 2 (BFIT2), also known as ACOT11, is a multimodular protein containing two consecutive hotdog-fold domains and a C-terminal steroidogenic acute regulatory protein-related lipid transfer domain (StarD14). In this study, we demonstrate that the N-terminal region of human BFIT2 (hBFIT2) constitutes a mitochondrial location signal sequence, which undergoes mitochondrion-dependent posttranslational cleavage. The mature hBFIT2 is shown to be located in the mitochondrial matrix, whereas the paralog "cytoplasmic acetyl-CoA hydrolase" (CACH, also known as ACOT12) was found in the cytoplasm. In vitro activity analysis of full-length hBFIT2 isolated from stably transfected HEK293 cells demonstrates selective thioesterase activity directed toward long chain fatty acyl-CoA thioesters, thus distinguishing the catalytic function of BFIT2 from that of CACH. The results from a protein-lipid overlay test indicate that the hBFIT2 StarD14 domain binds phosphatidylinositol 4-phosphate.

Triiodothyronine induces UCP-1 expression and mitochondrial biogenesis in human adipocytes

Uncoupling protein (UCP)-1 expressed in brown adipose tissue plays an important role in thermogenesis. Recent data suggest that brown-like adipocytes in white adipose tissue (WAT) and skeletal muscle play a crucial role in the regulation of body weight. Understanding of the mechanism underlying the increase in UCP-1 expression level in these organs should, therefore, provide an approach to managing obesity. The thyroid hormone (TH) has profound effects on mitochondrial biogenesis and promotes the mRNA expression of UCP in skeletal muscle and brown adipose tissue. However, the action of TH on the induction of brown-like adipocytes in WAT has not been elucidated. Thus we investigate whether TH could regulate UCP-1 expression in WAT using multipotent cells isolated from human adipose tissue. In this study, triiodothyronine (T3) treatment induced UCP-1 expression and mitochondrial biogenesis, accompanied by the induction of the CCAAT/enhancer binding protein, peroxisome proliferator-activated receptor-γ coactivator-1α, and nuclear respiratory factor-1 in differentiated human multipotent adipose-derived stem cells. The effects of T3 on UCP-1 induction were dependent on TH receptor-β. Moreover, T3 treatment increased oxygen consumption rate. These findings indicate that T3 is an active modulator, which induces energy utilization in white adipocytes through the regulation of UCP-1 expression and mitochondrial biogenesis. Our findings provide evidence that T3 serves as a bipotential mediator of mitochondrial biogenesis.

PGC-1 beta-regulated mitochondrial biogenesis and function in myotubes is mediated by NRF-1 and ERR alpha

The peroxisome proliferator-activated receptor-gamma (PPAR-gamma) coactivator-1 beta (PGC-1 beta) is a well-established regulator of the beta-oxidation of fatty acids and the oxidative phosphorylation in mitochondria. However, the underlying mechanism of PGC-1 beta action remains elusive. This study reveals that PGC-1 beta is highly induced during myogenic differentiation and knockdown of endogenous PGC-1 beta by siRNA leads to a decrease in the expression of several mitochondria-related genes. In consistence, the over-expression of PGC-1 beta stimulates its target genes such as cytochrome c, ATP synthase beta and ALAS-1 by its interaction with two transcriptional factors, NRF-1 and ERR alpha. The deletion or mutation of NRF-1 and/or ERR alpha binding sites in target gene promoters attenuates their activation by PGC-1 beta. Moreover, inhibition of NRF-1 or ERR alpha by siRNA ablated the aforesaid function of PGC-1 beta and compromised the oxidative phosphorylation and mitochondrial biogenesis. Taken together, these results confirm the direct interaction of NRF-1 and ERR alpha with PGC-1 beta, and their participation in mitochondrial biogenesis and respiration.

Regulation of mitochondrial biogenesis during myogenesis

Pathways involved in mitochondrial biogenesis associated with myogenic differentiation are poorly defined. Therefore, C(2)C(12) myoblasts were differentiated into multi-nucleated myotubes and parameters/regulators of mitochondrial biogenesis were investigated. Mitochondrial respiration, citrate synthase- and beta-hydroxyacyl-CoA dehydrogenase activity as well as protein content of complexes I, II, III and V of the mitochondrial respiratory chain increased 4-8-fold during differentiation. Additionally, an increase in the ratio of myosin heavy chain (MyHC) slow vs MyHC fast protein content was observed. PPAR transcriptional activity and transcript levels of PPAR-alpha, the PPAR co-activator PGC-1alpha, mitochondrial transcription factor A and nuclear respiratory factor 1 increased during differentiation while expression levels of PPAR-gamma decreased. In conclusion, expression and activity levels of genes known for their regulatory role in skeletal muscle oxidative capabilities parallel the increase in oxidative parameters during the myogenic program. In particular, PGC-1alpha and PPAR-alpha may be involved in the regulation of mitochondrial biogenesis during myogenesis.

Mitochondrial abnormalities in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a motor neuron disease caused by polyglutamine expansion mutation in the androgen receptor (AR). We investigated whether the mutant protein alters mitochondrial function. We found that constitutive and doxycycline-induced expression of the mutant AR in MN-1 and PC12 cells, respectively, are associated with depolarization of the mitochondrial membrane. This was mitigated by cyclosporine A, which inhibits opening of the mitochondrial permeability transition pore. We also found that the expression of the mutant protein in the presence of ligand results in an elevated level of reactive oxygen species, which is blocked by the treatment with the antioxidants co-enzyme Q10 and idebenone. The mutant protein in MN-1 cells also resulted in increased Bax, caspase 9 and caspase 3. We assessed the effects of mutant AR on the transcription of mitochondrial proteins and found altered expression of the peroxisome proliferator-activated receptor γ coactivator 1 and the mitochondrial specific antioxidant superoxide dismutase-2 in affected tissues of SBMA knock-in mice. In addition, we found that the AR associates with mitochondria in cultured cells. This study thus provides evidence for mitochondrial dysfunction in SBMA cell and animal models, either through indirect effects on the transcription of nuclear-encoded mitochondrial genes or through direct effects of the mutant protein on mitochondria or both. These findings indicate possible benefit from mitochondrial therapy for SBMA.

Why do we still have a maternally inherited mitochondrial DNA? Insights from evolutionary medicine

The human cell is a symbiosis of two life forms, the nucleus-cytosol and the mitochondrion. The nucleus-cytosol emphasizes structure and its genes are Mendelian, whereas the mitochondrion specializes in energy and its mitochondrial DNA (mtDNA) genes are maternal. Mitochondria oxidize calories via oxidative phosphorylation (OXPHOS) to generate a mitochondrial inner membrane proton gradient (DeltaP). DeltaP then acts as a source of potential energy to produce ATP, generate heat, regulate reactive oxygen species (ROS), and control apoptosis, etc. Interspecific comparisons of mtDNAs have revealed that the mtDNA retains a core set of electron and proton carrier genes for the proton-translocating OXPHOS complexes I, III, IV, and V. Human mtDNA analysis has revealed these genes frequently contain region-specific adaptive polymorphisms. Therefore, the mtDNA with its energy controlling genes may have been retained to permit rapid adaptation to new environments.

Expression of mitochondrial biogenesis-signaling factors in brown adipocytes is influenced specifically by 17beta-estradiol, testosterone, and progesterone

Control of mitochondrial biogenesis in brown adipose tissue (BAT), as part of the thermogenesis program, is a complex process that requires the integration of multiple transcription factors to orchestrate mitochondrial and nuclear gene expression. Despite the knowledge of the role of sex hormones on BAT physiology, little is known about the effect of these hormones on the mitochondrial biogenic program. The aim of this study was to determine the effect of testosterone, 17beta-estradiol, and progesterone on the expression of nuclear factors involved in the control of mitochondrial biogenesis and thermogenic function such as ppargamma, pgc1alpha, nrf1, gabpa, and tfam, and also an inhibitor of PI3K-Akt pathway, recently found to be involved in the control of mitochondrial recruitment (pten). For this purpose, an in vitro assay using cell-cultured brown adipocytes was used to address the role of steroid hormones, progesterone, testosterone, and 17beta-estradiol on the mRNA expression of these factors by real-time PCR. Thus 17beta-estradiol seemed to exert a dual effect, activating the PI3K-Akt pathway by inhibiting pten mRNA expression and also inhibiting nrf1 and tfam mRNA expression. Progesterone seemed to positively stimulate mitochondriogenesis and BAT differentiation by increasing the mRNA expression of the gabpa-tfam axis and ppargamma, respectively, but also exerted a negative output by increasing pten mRNA levels. Finally, testosterone inhibited the transcription of pgc1alpha, the master factor involved in UCP1 expression and mitochondrial biogenesis. In conclusion, our results support the idea that sex hormones have direct effects on different mediators of the mitochondriogenesis program.

Complementary action of the PGC-1 coactivators in mitochondrial biogenesis and brown fat differentiation

Mitochondria play an essential role in the ability of brown fat to generate heat, and the PGC-1 coactivators control several aspects of mitochondrial biogenesis. To investigate their specific roles in brown fat cells, we generated immortal preadipocyte lines from the brown adipose tissue of mice lacking PGC-1alpha. We could then efficiently knockdown PGC-1beta expression by shRNA expression. Loss of PGC-1alpha did not alter brown fat differentiation but severely reduced the induction of thermogenic genes. Cells deficient in either PGC-1alpha or PGC-1beta coactivators showed a small decrease in the differentiation-dependant program of mitochondrial biogenesis and respiration; however, this increase in mitochondrial number and function was totally abolished during brown fat differentiation when both PGC-1alpha and PGC-1beta were deficient. These data show that PGC-1alpha is essential for brown fat thermogenesis but not brown fat differentiation, and the PGC-1 coactivators play an absolutely essential but complementary function in differentiation-induced mitochondrial biogenesis.

Expression of the Drosophila melanogaster ATP synthase alpha subunit gene is regulated by a transcriptional element containing GAF and Adf-1 binding sites

Mitochondrial biogenesis is a complex and highly regulated process that requires the controlled expression of hundreds of genes encoded in two separated genomes, namely the nuclear and mitochondrial genomes. To identify regulatory proteins involved in the transcriptional control of key nuclear-encoded mitochondrial genes, we have performed a detailed analysis of the promoter region of the alpha subunit of the Drosophila melanogaster F1F0 ATP synthase complex. Using transient transfection assays, we have identified a 56 bp cis-acting proximal regulatory region that contains binding sites for the GAGA factor and the alcohol dehydrogenase distal factor 1. In vitro mutagenesis revealed that both sites are functional, and phylogenetic footprinting showed that they are conserved in other Drosophila species and in Anopheles gambiae. The 56 bp region has regulatory enhancer properties and strongly activates heterologous promoters in an orientation-independent manner. In addition, Northern blot and RT-PCR analysis identified two alpha-F1-ATPase mRNAs that differ in the length of the 3' untranslated region due to the selection of alternative polyadenylation sites.

The yeast cAMP protein kinase Tpk3p is involved in the regulation of mitochondrial enzymatic content during growth

During aerobic cell growth, mitochondria must meet energy demand either by adjusting cellular mitochondrial content or by adjusting ATP production flux, allowing a constant growth yield. On respiratory substrate, the Ras/cAMP pathway has been shown to be involved in this process in the yeast Saccharomyces cerevisiae. We show that of the three cAMP protein kinase catalytic subunits, Tpk3p is the one specifically involved in the regulation of cellular mitochondrial content when energy demand decreases. In decreased energy demand, the Deltatpk3 mitochondrial enzymatic content decreases leading to a subsequent decrease in the cellular growth rate. Moreover, enzymatic content decreases in the Deltatpk3 isolated mitochondria, suggesting that the amount of cellular mitochondria is not affected, but rather that the mitochondria are modified. Our study points to an important decrease in the cytochrome c content in the Deltatpk3 mitochondria, which leads to a decrease in the slipping process at the level of cytochrome-c-oxidase.

Contribution of the phosphorylable complex I in the growth phase-dependent respiration of C6 glioma cells in vitro

The energy metabolism of rat C6 glioma cells was investigated as a function of the growth phases. Three-dimensional cultures of C6 cells exhibited diminished respiration and respiratory capacity during the early growth phase, before reaching confluence. This decrease in respiration was neither due to changes in the respiratory complex content nor in the mitochondrial mass per se. Nevertheless, a quantitative correlation was found between cellular respiration and the rotenone-sensitive NADH ubiquinone oxidoreductase (i.e. complex I) activity. Immunoblot analysis showed that phosphorylation of the 18 kDa-subunit of this complex was associated with the growth-phase dependent modulation of complex I and respiratory activity in C6 cells. In addition, by using forskolin or dibutyryl cAMP, short-term activation of protein kinases A of C6 cells correlated with increased phosphorylation of the 18-kDa subunit of complex I, activated NADH ubiquinone oxidoreductase activity and stimulated cellular respiration. These findings suggest that complex I of C6 glioma cells is a key regulating step that modulates the oxidative phosphorylation capacity during growth phase transitions.

Saccharomyces cerevisiae Sin3p facilitates DNA double-strand break repair

There are two main pathways in eukaryotic cells for the repair of DNA double-strand breaks: homologous recombination and nonhomologous end joining. Because eukaryotic genomes are packaged in chromatin, these pathways are likely to require the modulation of chromatin structure. One way to achieve this is by the acetylation of lysine residues on the N-terminal tails of histones. Here we demonstrate that Sin3p and Rpd3p, components of one of the predominant histone deacetylase complexes of Saccharomyces cerevisiae, are required for efficient nonhomologous end joining. We also show that lysine 16 of histone H4 becomes deacetylated in the proximity of a chromosomal DNA double-strand break in a Sin3p-dependent manner. Taken together, these results define a role for the Sin3p/Rpd3p complex in the modulation of DNA repair.

Activation of Ras cascade increases the mitochondrial enzyme content of respiratory competent yeast

We investigated the effects of genetic and physiological modulations of the cAMP-protein kinase A pathway on mitochondrial biogenesis of yeast cells grown on lactate. Yeast mutants with over-activated Ras/adenylate cyclase pathway (i.e., Ras2(val19), ira1Delta(ira2)Delta) or with a constitutive downstream activation of protein kinases A (i.e., bcyDelta) showed an increase in the mitochondrial enzyme content. In contrast, loss of Ras activity (i.e., Ras2 mutant) resulted in a slight decrease. The treatment by cAMP of a responsive mutant increased the oxidative phosphorylation capacity of cells and increased the transcript level of nuclear genes encoding for mitochondrial proteins. In contrast, the transcript level of mitochondrial DNA genes was unchanged. It is concluded that the Ras/cAMP/protein kinase A pathway is part of the regulatory circuit controlling biogenesis of the oxidative phosphorylation complexes in yeast cells.

PGC-1, a versatile coactivator

PGC-1 was originally identified as a transcriptional coactivator of the nuclear receptor PPARgamma. The expression pattern and induction by exposure to cold have implicated PGC-1 in the regulation of energy metabolism and adaptive thermogenesis. Remarkably, PGC-1 overexpression can induce mitochondrial biogenesis and functions. Recent studies show that PGC-1 regulates the activity of several nuclear receptors and other transcription factors, and thus acts in a broader context than previously anticipated. Furthermore, PGC-1 displays the striking ability to interact with components of the splicing machinery. PGC-1 could therefore allow coordinated regulation of transcription and splicing in response to signals relaying metabolic needs. These novel findings are discussed in the context of the proposed physiological functions of PGC-1.

Mitochondrial contributions to cancer cell physiology: potential for drug development

Mitochondria make an integral contribution to the regulation of several aspects of cell biology such as energy production, molecular metabolism, redox status, calcium signalling and programmed cell death. In accordance with an endosymbiotic origin, mitochondria rely upon the nucleus for synthesis and function. In addition, these organelles can respond to intra- and extracellular cues independently, and there exists a highly coordinated "cross talk" between mitochondrial and nuclear signals that can greatly influence cell behaviour. This review focuses upon the putative roles of altered mitochondrial physiology in the process of cellular transformation. Discussed are: mitochondria as targets of drug-induced cytotoxicity or cancer promotion, as regulators of apoptosis, as sources of cell signalling through reactive oxygen species, and mitochondrial control of specific nuclear responses.