Nuclear Respiratory Factor-1 Transcription Research Articles

Abstract 4391: Aberrant transcription factor activity of NRF1, NFE2L2, E2F1, RFX1, and MEF2 associated with severity of astrocytoma

Abstract Astrocytoma is the most frequently occurring nervous system cancer in humans. Glioblastoma (GBM) is the most severe form of astrocytoma ([WHO] grade IV), and GBM patients rarely live beyond 5 years. Understanding how transcription factor regulatory networks contribute to the severity of astrocytoma will help identify novel therapeutic strategies for GBM. Nuclear Respiratory Factor 1 (NRF1) is a transcription factor (TF) that we have previously demonstrated as being active in cancer and having the ability to participate in crosstalk with other TFs such as the E2Fs and MYC. We have previously demonstrated that NRF1 gene networks are associated with increased NRF1 gene expression and GBM patient survival outcomes. Here, we examined the TF activity landscape that is differentially expressed in GBM compared to non-tumor tissue and in lower grade (I, II, and III) astrocytoma and their role in disease severity. Activities of 247 TFs including NRF1 were examined using patient mRNA expression data from four independent public datasets including the TCGA. The R package ‘limma' with ‘voom' was used to calculate the differential expression of each gene in the microarray and RNA-Seq datasets included in the present study, and then LRPath was employed to determine TF activity. NRF1 transcription factor (TF) activity is upregulated in glioma, correlates with disease aggressiveness, and is associated with cancer hallmarks and key functional pathways known to be dysregulated in cancer. Further analysis of data from all four patient cohorts including TCGA showed that NRF1 TF activity positively correlates with NFE2L2, E2F1, and RFX1, and negatively correlates with MEF2. Differential activity of these TFs is associated with astrocytoma disease severity. Protein-protein interaction networks (PPIN) were then generated for the TF combinations separately for male and female patients. Downstream gene ontology pathway analysis of the TF target genes revealed that DNA repair (GO:0006281), histone modification (GO:0016570), mitotic cell cycle (GO:0000278), and regulation of neurotransmitter level (GO: 0001505) genes regulated by NFE2L2, E2F1, and RFX1 are upregulated corresponding to increased NRF1 activity, while tissue development (GO: 0009888) genes regulated by MEF2 are down-regulated corresponding to increased NRF1 activity. These findings suggest that aberrant transcription factor activity of NRF1, NFE2L2, E2F1, MEF2, and RFX1 may be involved in the pathogenesis and severity of astrocytoma. Further analysis of these TF-regulated gene signatures combined with patient clinical outcomes will help pave the way for next generation targeted therapies and drug combination strategies for GBM patients. Citation Format: Kaumudi Bhawe, Quentin Felty, Changwon Yoo, Deodutta Roy. Aberrant transcription factor activity of NRF1, NFE2L2, E2F1, RFX1, and MEF2 associated with severity of astrocytoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4391.

The Ability of Carnitine to Act as a Type 1Histone Deacetylase Inhibitor May Explain the Favorable Impact of Carnitine Supplementation on Mitochondrial Biogenesis in the Elderly

A number of studies have shown that carnitine supplementation – alone or in conjunction with supplemental lipoic acid - promotes mitochondrial biogenesis (MB) in skeletal, muscle, and brain of aging rodents; no such effect is seen in younger animals. These findings parallel clinical studies in which supplemental carnitine improves physical and mental energy in elderly humans, while decreasing body fat and increasing lean mass – effects that have not been achieved with carnitine in younger people. The age dependence of these phenomena appears to reflect the fact that tissue carnitine levels, especially those in muscle, decline during aging; carnitine supplementation restores higher, more youthful tissues carnitine levels in aging animals, but has relatively little impact in this regard on younger ones. The effect of supplemental carnitine on MB in aging animals appears to be mediated, in whole or in part, by increased expression of PPARγ-coactivator-1α (PGC-1α), a key driver of MB. There is recent evidence that, in low millimolar intracellular concentrations such as those seen in skeletal muscle, carnitine functions as an inhibitor of type 1 histone deacetylases (HDACs). Moreover, it has been reported that drug inhibitors of these deacetylases boost mRNA and protein expression of PGC-1α, presumably by promoting transcription of the PGC-1a gene; these drugs also amplify MB. It is therefore proposed that intracellular carnitine provides a moderate tonic inhibition of type 1 HDACs that supports PGC1α transcription and that diminishes with age as tissue carnitine levels decline; hence, carnitine supplementation in the elderly restores youthful expression of PGC-1α and promotes MB. The complementary impact of lipoic acid on MB may reflect the fact that the promoter of the gene coding for nuclear respiratory factor-1 (NRF-1) contains antioxidant response elements; hence, NRF-1 transcription is promoted by phase 2 inducers such as lipoic acid. PGC-1α and NRF-1 collaborate in driving the expression of mitochondrial proteins. Additional nutraceutical measures which may likewise support MB – citrulline, taurine, N-acetylcysteine, high-dose biotin, and astaxanthin – are discussed. The adverse impact of metabolic syndrome on MB in skeletal muscle may be mediated by toll-like receptor 4 (TLR4) signaling stimulated by saturated fatty acids; antagonists of TLR4 signaling, possibly including ferulic acid and phycocyanobilin, may therefore promote MB in the context of metabolic syndrome. Restoration of youthful MB in the elderly may have favorable impacts on physical capacity and cognitive function, body composition, insulin sensitivity, and oxidative stress.

CDK2 regulates the NRF1/Ehmt1 axis during meiotic prophase I

Meiosis generates four genetically distinct haploid gametes over the course of two reductional cell divisions. Meiotic divisions are characterized by the coordinated deposition and removal of various epigenetic marks. Here we propose that nuclear respiratory factor 1 (NRF1) regulates transcription of euchromatic histone methyltransferase 1 (EHMT1) to ensure normal patterns of H3K9 methylation during meiotic prophase I. We demonstrate that cyclin-dependent kinase (CDK2) can bind to the promoters of a number of genes in male germ cells including that of Ehmt1 through interaction with the NRF1 transcription factor. Our data indicate that CDK2-mediated phosphorylation of NRF1 can occur at two distinct serine residues and negatively regulates NRF1 DNA binding activity in vitro. Furthermore, induced deletion of Cdk2 in spermatocytes results in increased expression of many NRF1 target genes including Ehmt1 We hypothesize that the regulation of NRF1 transcriptional activity by CDK2 may allow the modulation of Ehmt1 expression, therefore controlling the dynamic methylation of H3K9 during meiotic prophase.

Abstract 2441: Association between NRF1 regulatory gene networks and cancer and aging

Abstract There is a growing convergence between molecular aspects of aging and cancer. Cellular senescence acts as a barrier through preventing the proliferation of cells under oncogenic stress. Therefore, we decided to examine the role of nuclear respiratory factor 1 (NRF1) transcription factor in human brain cancer and brain aging. In contrast to higher expression of NRF1 in human glioma tumors, lower NRF1 levels are observed in Alzheimer’s disease (AD) brain tissue. Using the glioma and AD transcriptomic data of NRF1 target genes, we conducted Transcription Factor Target Enrichment Analysis that detects transcription factor (TF) activity based on quantification of the collective transcriptional activation of their target genes. NRF1 TF activity was significantly lower in AD brain tissues from four independent cohorts compared to non-AD brain tissues. In contrast, we observed higher NRF1 transcription factor activity in glioma. NRF1 activity as well as expression of several cell cycle pathway genes including CDK1,2,6,9 and NRF1 target genes such as NR2E1, APOE and MAPT are significantly altered in these two population cohorts. We also discovered that expression of NRF1, RB1 and E2F influenced survival in glioma. Specifically, the mean overall survival for patients with a high NRF1, E2F1,2,6,7,8 and RB1 gene signature was 20.91 months (95% CI = 9.37-32.46), compared to 101.99 months (95% CI = 74.28-129.70), in high NRF1 expressing patients that lacked the specific gene signature. NRF1 based gene signatures combined with IDH1 mutation status are necessary and sufficient for explaining the observed differences in survival curves. We examined the association of NRF1, the E2Fs and RB1 through Bayesian graphical modeling and observed gender differences in gene networks between NRF1 and E2F family members in the glioma patient sub-populations as compared to normal aging human brain samples. Furthermore, the differentially expressed NRF1 regulated genes are part of the KEGG pathways associated with spliceosome, ribosome and lysosome function as well as a diverse array of other functions are enriched, indicating ways in which NRF1 may be involved in glioma biology. In summary these findings support our postulate that NRF1 is an important transcription factor in the human brain, where it plays an active role in both aging and cancer. Citation Format: Kaumudi Bhawe, Quentin Felty, Changwon Yoo, Deodutta Roy. Association between NRF1 regulatory gene networks and cancer and aging [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2441.

Interplay between NRF1, E2F4 and MYC transcription factors regulating common target genes contributes to cancer development and progression

Nuclear respiratory factor 1 (NRF1), historically perceived as a protein regulating genes controlling mitochondrial biogenesis, is now widely recognized as a multifunctional protein and as a key player in the transcriptional modulation of genes implicated in various cellular functions. Here, we present emerging data supporting novel roles of NRF1 in cancer development and progression through its interplay with the transcription factors E2F4 and MYC. To identify common human NRF1, E2F4 and MYC target genes, we analyzed the Encyclopedia of DNA Elements (ENCODE) NRF1 ChIP-Seq data. By doing so, we identified 9253 common target genes with NRF1, E2F4 and MYC binding motifs. NRF1 binding motifs were found to be present in genes operating in signaling pathways governing all hallmarks of malignant transformation and progression, including proliferation, invasion, self-renewal and apoptosis. In addition to controlling mitochondrial biogenesis NRF1, in conjunction with E2F4 and MYC, may play a critical role in the acquisition of human cancer characteristics. Additionally, NRF1 may orchestrate both MYC and E2F4 to regulate common target genes linked to multiple networks in the development and progression of cancer. A comprehensive understanding of this dynamic interplay will set the stage, not only for the design of novel treatment strategies, but also for the discovery of pan-cellular transcription factor regulatory strategies to predict cancer risk, therapy response and patient prognosis.

Abstract 803: Overexpression of NRF1 leads to the generation of cancer stem-like cells and resistance to anoikis _ pathways to anchorage-independent growth during estrogen-induced malignant transformation

Abstract The actual role nuclear respiratory factor-1 (NRF-1) plays in breast cancer remains the least studied of all transcription factors. A growth role for the NRF1 through regulation of cell cycle genes during estrogen-induced malignant transformation has recently been reported by us; however, the underlying mechanism of its contribution to estrogen-induced breast carcinogenesis is undefined. In this study we provide new mechanistic insight into the mode by which NRF1 may contribute in estrogen-induced neoplastic conversion of cells by suppressing anoikis; identified BNIP3 as a key transcriptional target of NRF1; and survival signals conferred by NRF1 seems to impinge on the hypoxia-inducible death factor BNIP3. ChIP, qPCR, mass spectrometry, redox Western blot, colony formation, cell proliferation, ROS assay, immunofluorescence microscopy were used to study the role of NRF-1. Our data showed that higher expresion of NRF-1 in MCF10A produced large sizes of spheroids with increased level of stem cell markers -CD44+CD24+CD49f+ compared to cells transfected with vector alone. E2-induced increase of spheroid formation, suppression of cell death and cell survival were amplified by expression of NRF1 and these effects were diminished by loss of NRF1 function through expression of dominant negative NRF1. Breast cancer cell lines - MCF7 and MDA-MB231 overexpressed both NRF1 and BNIP3, and formed larger tumor spheroids compared to the MCF10A or vector-transformed breast cancer cells. E2 treatment as well as overexpression of NRF1 increased mRNA and protein levels of BNIP3. In addition, we found the increased transcriptional interaction of NRF-1 transcription factor in the BNIP3 promoter region by ChIP-qPCR assay in cells treated with E2. We also found the increased functional activity of the BNIP3 promoter in cells treated with E2 and this was decreased with co-treatment of ROS scavenger-Ebselen. NRF1 spheroids have the ability to micro-invasion. In summary, overexpression of NRF1 altered the cell morphology towards mesenchymal stem-like shape and promoted cancer stem cell-like phenotype. Findings of this study unveiled the underlying mechanism of NRF1 action through the activation of BNIP3 which is regulated by NRF1 and is presumably responsible for the estrogen-induced malignant transformation of MCF10A cells, including anoikis resistance, anchorage-independent cell growth, and increased cell migration and invasion. This work was in part supported by a VA MERIT Review (VA BX001463) grant to DR. Citation Format: Jayanta Das, Deodutta Roy. Overexpression of NRF1 leads to the generation of cancer stem-like cells and resistance to anoikis _ pathways to anchorage-independent growth during estrogen-induced malignant transformation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 803. doi:10.1158/1538-7445.AM2015-803

Estradiol and tamoxifen regulate NRF-1 and mitochondrial function in mouse mammary gland and uterus

Nuclear respiratory factor-1 (NRF-1) stimulates the transcription of nuclear-encoded genes that regulate mitochondrial (mt) genome transcription and biogenesis. We reported that estradiol (E2) and 4-hydroxytamoxifen (4-OHT) stimulate NRF-1 transcription in an estrogen receptor α (ERα)- and ERβ-dependent manner in human breast cancer cells. The aim of this study was to determine whether E2 and 4-OHT increase NRF-1 in vivo. Here, we report that E2 and 4-OHT increase NRF-1 expression in mammary gland (MG) and uterus of ovariectomized C57BL/6 mice in a time-dependent manner. E2 increased NRF-1 protein in the uterus and MG; however, in MG, 4-OHT increased Nrf1 mRNA but not protein. Chromatin immunoprecipitation assays revealed increased in vivo recruitment of ERα to the Nrf1 promoter and intron 3 in MG and uterus 6 h after E2 and 4-OHT treatment, commensurate with increased NRF-1 expression. E2- and 4-OHT-induced increases in NRF-1 and its target genes Tfam, Tfb1m, and Tfb2m were coordinated in MG but not in uterus due to uterine-selective inhibition of the expression of the NRF-1 coactivators Ppargc1a and Ppargc1b by E2 and 4-OHT. E2 transiently increased NRF-1 and PGC-1α nuclear staining while reducing PGC-1α in uterus. E2, not 4-OHT, activates mt biogenesis in MG and uterus in a time-dependent manner. E2 increased mt outer membrane Tomm40 protein levels in MG and uterus whereas 4-OHT increased Tomm40 only in uterus. These data support the hypothesis of tissue-selective regulation of NRF-1 and its downstream targets by E2 and 4-OHT in vivo.

Diesel exhaust particulate extracts inhibit transcription of nuclear respiratory factor-1 and cell viability in human umbilical vein endothelial cells

Endothelial dysfunction precedes cardiovascular disease and is accompanied by mitochondrial dysfunction. Here we tested the hypothesis that diesel exhaust particulate extracts (DEPEs), prepared from a truck run at different speeds and engine loads, would inhibit genomic estrogen receptor activation of nuclear respiratory factor-1 (NRF-1) transcription in human umbilical vein endothelial cells (HUVECs). Additionally, we examined how DEPEs affect NRF-1-regulated TFAM expression and, in turn, Tfam-regulated mtDNA-encoded cytochrome c oxidase subunit I (COI, MTCO1) and NADH dehydrogenase subunit I (NDI) expression as well as cell proliferation and viability. We report that 17β-estradiol (E(2)), 4-hydroxytamoxifen (4-OHT), and raloxifene increased NRF-1 transcription in HUVECs in an ER-dependent manner. DEPEs inhibited NRF-1 transcription, and this suppression was not ablated by concomitant treatment with E(2), 4-OHT, or raloxifene, indicating that the effect was not due to inhibition of ER activity. While E(2) increased HUVEC proliferation and viability, DEPEs inhibited viability but not proliferation. Resveratrol increased NRF-1 transcription in an ER-dependent manner in HUVECs, and ablated DEPE inhibition of basal NRF-1 expression. Given that NRF-1 is a key nuclear transcription factor regulating genes involved in mitochondrial activity and biogenesis, these data suggest that DEPEs may adversely affect mitochondrial function leading to endothelial dysfunction and resveratrol may block these effects.

Nuclear Respiratory Factor-1 is Involved in Mitochondrial Dysfunction Induced by Benzo(a)pyrene in Human Bronchial Epithelial Cells

In this study, we investigated the role of nuclear respiratory factor-1(NRF-1) in benzo(a)pyrene (BaP)-induced mitochondrial events in human bronchial epithelial cells (16HBE). Cytotoxicity was determined with MTT assay, and apoptosis was measured by flow cytometry. The results showed that BaP inhibited cell proliferation in a dose-dependent manner and induced apoptosis in 16HBE cells. Time-dependent reactive oxygen species (ROS) generation induced by BaP was observed in 16HBE cells. The loss of mitochondrial membrane permeability transition (MPT) was obtained by a laser scanning confocal microscope, and the decreasing ATP level was detected by a Cell-Titer-Glo(®) Luminescent Cell Viability Assay. Results of western blotting assay revealed that both NRF-1 and mitochondrial transcription factor A (mtTFA) decreased in 12-μM BaP-treated cells at both 12 and 24 hr. The results of RT-PCR indicate that NRF-1 and mtTFA mRNA in 16HBE cells were not changed after BaP treatment 12 or 24 hr. Down-regulation of NRF-1 by shRNA further reduced the loss of MPT and increased ROS generation in response to BaP treatment. Therefore, our results demonstrate that NRF-1 is responsible for BaP-induced mitochondrial dysfunction in 16HBE cells and associated with the level of mtTFA protein, loss of MPT and ROS overproduction.

Tamoxifen increases nuclear respiratory factor 1 transcription by activating estrogen receptor β and AP‐1 recruitment to adjacent promoter binding sites

Little is known about endogenous estrogen receptor β (ERβ) gene targets in human breast cancer. We reported that estradiol (E(2)) induces nuclear respiratory factor-1 (NRF-1) transcription through ERα in MCF-7 breast cancer cells. Here we report that 4-hydroxytamoxifen (4-OHT), with an EC(50) of ~1.7 nM, increases NRF-1 expression by recruiting ERβ, cJun, cFos, CBP, and RNA polymerase II to and dismissing NCoR from the NRF1 promoter. Promoter deletion and transient transfection studies showed that the estrogen response element (ERE) is essential and that an adjacent AP-1 site contributes to maximal 4-OHT-induced NRF-1 transcription. siRNA knockdown of ERβ revealed that ERβ inhibits basal NRF-1 expression and is required for 4-OHT-induced NRF-1 transcription. An AP-1 inhibitor blocked 4-OHT-induced NRF-1 expression. The 4-OHT-induced increase in NRF-1 resulted in increased transcription of NRF-1 target CAPNS1 but not CYC1, CYC2, or TFAM despite increased NRF-1 coactivator PGC-1α protein. The absence of TFAM induction corresponds to a lack of Akt-dependent phosphorylation of NRF-1 with 4-OHT treatment. Overexpression of NRF-1 inhibited 4-OHT-induced apoptosis and siRNA knockdown of NRF-1 increased apoptosis, indicating an antiapoptotic role for NRF-1. Overall, NRF-1 expression and activity is regulated by 4-OHT via endogenous ERβ in MCF-7 cells.

Estradiol Stimulates Transcription of Nuclear Respiratory Factor-1 and Increases Mitochondrial Biogenesis

Estrogen has direct and indirect effects on mitochondrial activity, but the mechanisms mediating these effects remain unclear. Others reported that long-term estradiol (E(2)) treatment increased nuclear respiratory factor-1 (NRF-1) protein in cerebral blood vessels of ovariectomized rats. NRF-1 is a transcription factor that regulates the expression of nuclear-encoded mitochondrial genes, e.g. mitochondrial transcription factor A (TFAM), that control transcription of the mitochondrial genome. Here we tested the hypothesis that E(2) increases NRF-1 transcription resulting in a coordinate increase in the expression of nuclear- and mitochondrial- encoded genes and mitochondrial respiratory activity. We show that E(2) increased NRF-1 mRNA and protein in MCF-7 breast and H1793 lung adenocarcinoma cells in a time-dependent manner. E(2)-induced NRF-1 expression was inhibited by the estrogen receptor (ER) antagonist ICI 182,780 and actinomycin D but not by phosphoinositide-3 kinase and MAPK inhibitors, indicating a genomic mechanism of E(2) regulation of NRF-1 transcription. An estrogen response element (ERE) in the NRF-1 promoter bound ER alpha and ER beta in vitro, and E(2) induced ER alpha and ER beta recruitment to this ERE in chromatin immunoprecipitation assays in MCF-7 cells. The NRF-1 ERE activated reporter gene expression in transfected cells. Small interfering RNA to ER alpha and ER beta revealed that ER alpha mediates E(2)-induced NRF-1 transcription. The E(2)-induced increase in NRF-1 was followed by increased TFAM and the transcription of Tfam-regulated mitochondrial DNA-encoded COI and NDI genes and increased mitochondrial biogenesis. Knockdown of NRF-1 blocked E(2) stimulation of mitochondrial biogenesis and activity, indicating a mechanism by which estrogens regulate mitochondrial function by increasing NRF-1 expression.

Increase of mitochondrial DNA content and transcripts in early bovine embryogenesis associated with upregulation of mtTFA and NRF1 transcription factors

BackgroundRecent work has shown that mitochondrial biogenesis and mitochondrial functions are critical determinants of embryonic development. However, the expression of the factors controlling mitochondrial biogenesis in early embryogenesis has received little attention so far.MethodsWe used real-time quantitative PCR to quantify mitochondrial DNA (mtDNA) in bovine oocytes and in various stages of in vitro produced embryos. To investigate the molecular mechanisms responsible for the replication and the transcriptional activation of mtDNA, we quantified the mRNA corresponding to the mtDNA-encoded cytochrome oxidase 1 (COX1), and two nuclear-encoded factors, i.e. the Nuclear Respiratory Factor 1 (NRF1), and the nuclear-encoded Mitochondrial Transcription Factor A (mtTFA).ResultsUnlike findings reported in mouse embryos, the mtDNA content was not constant during early bovine embryogenesis. We found a sharp, 60% decrease in mtDNA content between the 2-cell and the 4/8-cell stages. COX1 mRNA was constant until the morula stage after which it increased dramatically. mtTFA mRNA was undetectable in oocytes and remained so until the 8/16-cell stage; it began to appear only at the morula stage, suggesting de novo synthesis. In contrast, NRF1 mRNA was detectable in oocytes and the quantity remained constant until the morula stage.ConclusionOur results revealed a reduction of mtDNA content in early bovine embryos suggesting an active process of mitochondrial DNA degradation. In addition, de novo mtTFA expression associated with mitochondrial biogenesis activation and high levels of NRF1 mRNA from the oocyte stage onwards argue for the essential function of these factors during the first steps of bovine embryogenesis.

Coordinated reduction of genes of oxidative metabolism in humans with insulin resistance and diabetes: Potential role of PGC1 and NRF1.

Type 2 diabetes mellitus (DM) is characterized by insulin resistance and pancreatic beta cell dysfunction. In high-risk subjects, the earliest detectable abnormality is insulin resistance in skeletal muscle. Impaired insulin-mediated signaling, gene expression, glycogen synthesis, and accumulation of intramyocellular triglycerides have all been linked with insulin resistance, but no specific defect responsible for insulin resistance and DM has been identified in humans. To identify genes potentially important in the pathogenesis of DM, we analyzed gene expression in skeletal muscle from healthy metabolically characterized nondiabetic (family history negative and positive for DM) and diabetic Mexican-American subjects. We demonstrate that insulin resistance and DM associate with reduced expression of multiple nuclear respiratory factor-1 (NRF-1)-dependent genes encoding key enzymes in oxidative metabolism and mitochondrial function. Although NRF-1 expression is decreased only in diabetic subjects, expression of both PPAR gamma coactivator 1-alpha and-beta (PGC1-alpha/PPARGC1 and PGC1-beta/PERC), coactivators of NRF-1 and PPAR gamma-dependent transcription, is decreased in both diabetic subjects and family history-positive nondiabetic subjects. Decreased PGC1 expression may be responsible for decreased expression of NRF-dependent genes, leading to the metabolic disturbances characteristic of insulin resistance and DM.

Invited Review: contractile activity-induced mitochondrial biogenesis in skeletal muscle.

Chronic contractile activity produces mitochondrial biogenesis in muscle. This adaptation results in a significant shift in adenine nucleotide metabolism, with attendant improvements in fatigue resistance. The vast majority of mitochondrial proteins are derived from the nuclear genome, necessitating the transcription of genes, the translation of mRNA into protein, the targeting of the protein to a mitochondrial compartment via the import machinery, and the assembly of multisubunit enzyme complexes in the respiratory chain or matrix. Putative signals involved in initiating this pathway of gene expression in response to contractile activity likely arise from combinations of accelerations in ATP turnover or imbalances between mitochondrial ATP synthesis and cellular ATP demand, and Ca(2+) fluxes. These rapid events are followed by the activation of exercise-responsive kinases, which phosphorylate proteins such as transcription factors, which subsequently bind to upstream regulatory regions in DNA, to alter transcription rates. Contractile activity increases the mRNA levels of nuclear-encoded proteins such as cytochrome c and mitochondrial transcription factor A (Tfam) and mRNA levels of upstream transcription factors like c-jun and nuclear respiratory factor-1 (NRF-1). mRNA level changes are often most evident during the postexercise recovery period, and they can occur as a result of contractile activity-induced increases in transcription or mRNA stability. Tfam is imported into mitochondria and controls the expression of mitochondrial DNA (mtDNA). mtDNA contributes only 13 protein products to the respiratory chain, but they are vital for electron transport and ATP synthesis. Contractile activity increases Tfam expression and accelerates its import into mitochondria, resulting in increased mtDNA transcription and replication. The result of this coordinated expression of the nuclear and the mitochondrial genomes, along with poorly understood changes in phospholipid synthesis, is an expansion of the muscle mitochondrial reticulum. Further understanding of 1) regulation of mtDNA expression, 2) upstream activators of NRF-1 and other transcription factors, 3) the identity of mRNA stabilizing proteins, and 4) potential of contractile activity-induced changes in apoptotic signals are warranted.