Transcriptional Regulation by Nuclear Corepressors and PGC-1α: Implications for Mitochondrial Quality Control and Insulin Sensitivity

Mitoregulin controls mitochondrial function and stress-adaptation response during early phase of endoplasmic reticulum stress in breast cancer cells

The proper regulation of mitochondrial function is important for cellular homeostasis. Especially, in cancer cells, dysregulation of mitochondria is associated with diverse cellular events such as metabolism, redox status, and stress responses. Mitoregulin (MTLN), a micro protein encoded by LINC00116, recently has been reported to control mitochondrial functions in skeletal muscle cells and adipocytes. However, the role of MTLN in cancer cells remains unclear. In the present study, we found that MTLN regulates membrane potential and reactive oxygen species (ROS) generation of mitochondria in breast cancer cells. Moreover, MTLN deficiency resulted in abnormal mitochondria-associated ER membranes (MAMs) formation, which is crucial for stress adaptation. Indeed, the MTLN-deficient breast cancer cells failed to successfully resolve ER (endoplasmic reticulum) stress, and cell vulnerability to ER-stress inducers was significantly enhanced by the downregulation of MTLN. In conclusion, MTLN controls stress-adaptation responses in breast cancer cells as a key regulator of mitochondria-ER harmonization, and thereby its expression level may serve as an indicator of the responsiveness of cancer cells to proteasome inhibitors.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α’s roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington’s Disease, Parkinson’s Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Dysregulation of PGC-1α-Dependent Transcriptional Programs in Neurological and Developmental Disorders: Therapeutic Challenges and Opportunities

Substantial evidence indicates that mitochondrial impairment contributes to neuronal dysfunction and vulnerability in disease states, leading investigators to propose that the enhancement of mitochondrial function should be considered a strategy for neuroprotection. However, multiple attempts to improve mitochondrial function have failed to impact disease progression, suggesting that the biology underlying the normal regulation of mitochondrial pathways in neurons, and its dysfunction in disease, is more complex than initially thought. Here, we present the proteins and associated pathways involved in the transcriptional regulation of nuclear-encoded genes for mitochondrial function, with a focus on the transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator-1alpha (PGC-1α). We highlight PGC-1α's roles in neuronal and non-neuronal cell types and discuss evidence for the dysregulation of PGC-1α-dependent pathways in Huntington's Disease, Parkinson's Disease, and developmental disorders, emphasizing the relationship between disease-specific cellular vulnerability and cell-type-specific patterns of PGC-1α expression. Finally, we discuss the challenges inherent to therapeutic targeting of PGC-1α-related transcriptional programs, considering the roles for neuron-enriched transcriptional coactivators in co-regulating mitochondrial and synaptic genes. This information will provide novel insights into the unique aspects of transcriptional regulation of mitochondrial function in neurons and the opportunities for therapeutic targeting of transcriptional pathways for neuroprotection.

Azelaic Acid Induces Mitochondrial Biogenesis in Skeletal Muscle by Activation of Olfactory Receptor 544

Mouse olfactory receptor 544 (Olfr544) is ectopically expressed in varied extra-nasal organs with tissue specific functions. Here, we investigated the functionality of Olfr544 in skeletal muscle cells and tissue. The expression of Olfr544 is confirmed by RT-PCR and qPCR in skeletal muscle cells and mouse skeletal muscle assessed by RT-PCR and qPCR. Olfr544 activation by its ligand, azelaic acid (AzA, 50 μM), induced mitochondrial biogenesis and autophagy in cultured skeletal myotubes by induction of cyclic adenosine monophosphate-response element binding protein (CREB)-peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α)-extracellular signal-regulated kinase-1/2 (ERK1/2) signaling axis. The silencing Olfr544 gene expression abrogated these effects of AzA in cultured myotubes. Similarly, in mice, the acute subcutaneous injection of AzA induced the CREB-PGC-1α-ERK1/2 pathways in mouse skeletal muscle, but these activations were negated in those of Olfr544 knockout mice. These demonstrate that the induction of mitochondrial biogenesis in skeletal muscle by AzA is Olfr544-dependent. Oral administration of AzA to high-fat-diet fed obese mice for 6 weeks increased mitochondrial DNA content in the skeletal muscle as well. Collectively, these findings demonstrate that Olfr544 activation by AzA regulates mitochondrial biogenesis in skeletal muscle. Intake of AzA or food containing AzA may help to improve skeletal muscle function.

The Essential Oil from Acori Tatarinowii Rhizome (the Dried Rhizome of Acorus tatarinowii Schott) Prevents Hydrogen Peroxide-Induced Cell Injury in PC12 Cells: A Signaling Triggered by CREB/PGC-1α Activation

Acori Tatarinowii Rhizome (ATR, the dried rhizome of Acorus tatarinowii Schott), a well-recognized traditional Chinese herbal medicine, is prescribed to treat neurological disorders. The essential oil is considered as the active fraction of ATR, and the neuroprotection of ATR essential oil (ATEO) is proven, including the protection against oxidative stress. However, the cellular mechanism of ATEO against oxidative stress has not been fully illustrated. In this study, to investigate the cellular mechanism of ATEO, the cytoprotective effect of ATEO against H₂O₂-induced injury was revealed in PC12 cells. ATEO treatment increased the viability of cells affected by H₂O₂-mediated injury, inhibited reactive oxygen species (ROS) accumulation, and induced the expression of several antioxidant proteins (SODs, GPx, and UCPs). The cytoprotective effect of ATEO was related to upregulation of peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α) expression, which was counteracted by PGC-1α specific knockdown. Using inhibitor of protein kinase A (PKA), we found that cAMP-response element binding protein (CREB) activation was involved in ATEO-induced PGC-1α expression. Taken together, we suggest that ATEO effectively prevents H₂O₂-induced cell injury possibly through the activation of CREB/PGC-1α signaling in PC12 cells. The results provide a molecular insight into the effect of ATEO on cytoprotection against oxidative stress.

Autophagy-activating strategies to promote innate defense against mycobacteria

Mycobacterium tuberculosis (Mtb) is a major causal pathogen of human tuberculosis (TB), which is a serious health burden worldwide. The demand for the development of an innovative therapeutic strategy to treat TB is high due to drug-resistant forms of TB. Autophagy is a cell-autonomous host defense mechanism by which intracytoplasmic cargos can be delivered and then destroyed in lysosomes. Previous studies have reported that autophagy-activating agents and small molecules may be beneficial in restricting intracellular Mtb infection, even with multidrug-resistant Mtb strains. Recent studies have revealed the essential roles of host nuclear receptors (NRs) in the activation of the host defense through antibacterial autophagy against Mtb infection. In particular, we discuss the function of estrogen-related receptor (ERR) α and peroxisome proliferator-activated receptor (PPAR) α in autophagy regulation to improve host defenses against Mtb infection. Despite promising findings relating to the antitubercular effects of various agents, our understanding of the molecular mechanism by which autophagy-activating agents suppress intracellular Mtb in vitro and in vivo is lacking. An improved understanding of the antibacterial autophagic mechanisms in the innate host defense will eventually lead to the development of new therapeutic strategies for human TB.

Therapies that promote intracellular digestion of microbes could prove a valuable addition to antibiotic weapons against tuberculosis. Mycobacterium tuberculosis (Mtb) establishes itself within immune cells, and employs a variety of tricks to protect itself as it sickens its host. Researchers led by Eun-Kyeong Jo at Chungnam National University, Daejeon, South Korea, have reviewed efforts to defeat this pathogen by jump-starting a cellular ‘recycling’ pathway called autophagy. Autophagy helps cells break down both biomolecules aggregates and potential invaders, but Mtb can elude such digestion. Jo and colleagues highlight antimycobacterial agents that can potentially render Mtb vulnerable to autophagy, as well as promising cellular targets that may allow researchers to access this process. For example, evidence suggests that agents that activate a regulatory protein such as ERRα or PPARα could stimulate cellular degradation of Mtb.

Nuclear Respiratory Factor 1 Negatively Regulates the P1 Promoter of the Peroxisome Proliferator-Activated Receptor-γ Gene and Inhibits Chicken Adipogenesis

Peroxisome proliferator-activated receptor-γ (PPARγ) is a master regulator of adipogenesis, and alterations in its function are associated with various pathological processes related to metabolic syndrome. Recently, we found that the chicken PPARγ gene is regulated by three alternative promoters (P1, P2 and P3), producing five different transcript isoforms and two protein isoforms. In this study, the P1 promoter structure was characterized. Bioinformatics identified six putative nuclear respiratory factor 1 (NRF1) binding sites in the P1 promoter, and a reporter assay showed that NRF1 inhibited the activity of the P1 promoter. Of the six putative NRF1 binding sites, individual mutations of three of them abolished the inhibitory effect of NRF1 on P1 promoter activity. Furthermore, a ChIP assay indicated that NRF1 directly bound to the P1 promoter, and real-time quantitative RT-PCR analysis showed that NRF1 mRNA expression was negatively correlated with PPARγ1 expression (Pearson’s r = -0.148, p = 0.033). Further study showed that NRF1 overexpression inhibited the differentiation of the immortalized chicken preadipocyte cell line (ICP1), which was accompanied by reduced PPARγ1 mRNA expression. Taken together, our findings indicated that NRF1 directly negatively regulates the P1 promoter of the chicken PPARγ gene and inhibits adipogenesis.

Coregulator-mediated control of skeletal muscle plasticity - A mini-review

Skeletal muscle plasticity is a complex process entailing massive transcriptional programs. These changes are mediated by the action of nuclear receptors and other transcription factors. In addition, coregulator proteins have emerged as important players in this process by linking transcription factors to the RNA polymerase II complex and inducing changes in the chromatic structure. An accumulating body of work highlights the pleiotropic functions of coregulator proteins in the control of tissue-specific and whole body metabolism. In skeletal muscle, several coregulators have been identified as potent modulators of metabolic and myofibrillar plasticity. In this mini-review, we will discuss the control, function and physiological significance of these coregulators in skeletal muscle biology.

Long-term treatment with nicotinamide induces glucose intolerance and skeletal muscle lipotoxicity in normal chow-fed mice: compared to diet-induced obesity

Nicotinamide (NAM), or vitamin B3, is an essential coenzyme for ATP synthesis and an inhibitor of sirtuin 1. Recently, conflicting results were reported regarding the treatment of NAM in type 2 diabetes and obesity. The aim of this study was to determine whether and how long-term treatment with NAM at lower dose would affect insulin sensitivity in mice fed chow diet. We treated mice with NAM (100 mg/kg/day) and normal chow for 8 weeks. Strikingly, NAM induced glucose intolerance and skeletal muscle lipid accumulation in nonobese mice. NAM impaired mitochondrial respiration capacity and energy production in skeletal muscle, in combination with increased expression of the mediators for mitophagy (p62, PINK1, PARK2 and NIX) and autophagy (FOXO3, Bnip3, CTSL, Beclin1 and LC-3b). Next, we treated mice with high-fat diet (HFD) and resveratrol (RSV; 100 mg/kg/day) for 8 weeks. RSV protected against HFD-induced insulin resistance and obesity. HFD increased skeletal muscle lipid content as well as NAM, but this increase was attenuated by RSV. In contrast to NAM, HFD enhanced fatty acid oxidative capacity. Muscle transcript levels of genes for mitophagy and autophagy were largely suppressed by HFD, whereas RSV did not rescue these effects. These differences suggest that skeletal muscle autophagy may represent adaptive response to NAM-induced lipotoxicity, whereas reduced autophagy in skeletal muscle may promote HFD-induced lipotoxicity. Our results demonstrate that chronic NAM supplementation in healthy individuals, although at lower dose than previously reported, is still detrimental to glucose homeostasis and skeletal muscle lipid metabolism.

Chinese medicine Jinlida (JLD) ameliorates high-fat-diet induced insulin resistance in rats by reducing lipid accumulation in skeletal muscle

The present paper reports the effects of Jinlida (JLD), a traditional Chinese medicine which has been given as a treatment for high-fat-diet (HFD)-induced insulin resistance. A randomized controlled experiment was conducted to provide evidence in support of the affects of JLD on insulin resistance induced by HFD. The affect of JLD on blood glucose, lipid, insulin, adiponectin, alanine aminotransferase (ALT), aspartate aminotransferase (AST) and total bilirubin (TBIL) in serum and lipid content in skeletal muscle was measured. Genes and proteins of the AMPK signaling pathway were analyzed by real time RT-PCR and Western blot. Adiponectin receptor 1 and 2 (ADIPOR1, ADIPOR2) and other genes involved in mitochondrial function and fat oxidation were analyzed by real time RT-PCR. Histological staining was also performed. JLD or pioglitazone administration ameliorated fasting plasma levels of glucose, insulin, triglyceride (TG), total cholesterol (TC), ALT, AST and non-esterified fatty acid (NEFA) (P < 0.05). Treatment with JLD or pioglitazone significantly reverted muscle lipid content (P < 0.05). JLD (1.5 g/kg) significantly increased plasma adiponectin concentration by 60.17% and increased AMPK and acetyl-CoA carboxylase (ACC) phosphorylation in skeletal muscle (P < 0.05). JLD administration increased levels of ADIPOR1 and ADIPOR2 by 1.48 and 1.29 respectively. Levels of genes involved in mitochondrial function and fat oxidation were increased. This study provides the molecular mechanism by which JLD ameliorates HFD-induced insulin resistance in rats.

PGC-1α buffers ROS-mediated removal of mitochondria during myogenesis

Mitochondrial biogenesis and mitophagy are recognized as critical processes underlying mitochondrial homeostasis. However, the molecular pathway(s) coordinating the balance between these cellular programs is still poorly investigated. Here, we show an induction of the nuclear and mitochondrial peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PGC-1α) during myogenesis, which in turn co-activates the transcription of nuclear and mtDNA-encoded mitochondrial genes. We demonstrate that PGC-1α also buffers oxidative stress occurring during differentiation by promoting the expression of antioxidant enzymes. Indeed, by downregulating PGC-1α, we observed an impairment of antioxidants expression, which was accompanied by a significant reactive oxygen species (ROS) burst and increase of oxidative damage to proteins. In parallel, we detected a decrease of mitochondrial mass and function as well as increased mitophagy through the ROS/FOXO1 pathway. Upon PGC-1α downregulation, we found ROS-dependent nuclear translocation of FOXO1 and transcription of its downstream targets including mitophagic genes such as LC3 and PINK1. Such events were significantly reverted after treatment with the antioxidant Trolox, suggesting that PGC-1α assures mitochondrial integrity by indirectly buffering ROS. Finally, the lack of PGC-1α gave rise to a decrease in MYOG and a strong induction of atrophy-related ubiquitin ligases FBXO32 (FBXO32), indicative of a degenerative process. Overall, our results reveal that in myotubes, PGC-1α takes center place in mitochondrial homeostasis during differentiation because of its ability to avoid ROS-mediated removal of mitochondria.

Monitoring of neuronal loss in the hippocampus of Aβ-injected rat: autophagy, mitophagy, and mitochondrial biogenesis stand against apoptosis

In the present study, we tried to answer the following questions: which kind of defense pathways are activated after Aβ insult? How defense systems react against noxious effects of Aβ and whether they are able to deal against apoptosis or not? So, we traced some molecular pathways including autophagy, mitophagy, and mitochondrial biogenesis before reaching to the endpoint of apoptosis. Besides, we measured the function of mitochondria after injection of Aβ (1-42) in CA1 area of hippocampus as a model of Alzheimer's disease (AD). Based on our data, autophagy markers reached to their maximum level and returned to the control level as apoptotic markers started to increase. As a specialized form of autophagy, mitophagy markers followed the trend of autophagy markers. Whereas mitochondrial dynamic processes shifted toward fission, mitochondrial biogenesis was severely affected by Aβ and significantly decreased. Alongside suppression of mitochondrial biogenesis, activity of specific enzymes involved in antioxidant defense system, electron transport chain, and tricarboxylic acid cycle (TCA) decreased in response to the Aβ. Activity of antioxidant enzymes increased at first and then decreased significantly compared to the control. TCA enzymes aconitase and malate dehydrogenase activities reduced immediately while citrate synthase and fumarase activities did not change. Based on our finding, monitoring of the master molecules of intracellular cascades and determining their trends before the destructive function of Aβ could be the target of therapeutic issues for AD.