Mitochondria-to-nucleus stress signaling induces phenotypic changes, tumor progression and cell invasion

Can stable introns and noncoding RNAs be harnessed to improve health through activation of mitohormesis?

Ever since their introduction a decade ago, stable introns, a type of noncoding (nc)RNAs, are found to be key players in different important cellular processes acting through regulation of gene expression and feedback loops to maintain cellular homeostasis. Despite being commonly regarded as useless byproducts, recent studies in yeast suggested that stable introns are essential for cell survivability under starvation. In Drosophila, we found that a stable intron, sisR-1, has a direct effect in regulating mitochondrial dynamics during short-term fasting and subsequently improved overall oocyte quality. We speculated that the beneficial effects implicated by sisR-1 is through the activation of mitohormesis, an interesting phenomenon in mitochondrial biology. Mitohormesis is suggested to improve health span and lifespan of cells and organisms, but the involvement of ncRNAs is not well-documented. Here, we discuss the potential role of sisR-1 and other ncRNAs in activating mitohormesis and the possible applications in improving cellular and organismal health.

Mitochondrial signaling pathways and their role in cancer drug resistance

Mitochondria, traditionally known as cellular powerhouses, now emerge as critical signaling centers influencing cancer progression and drug resistance. The review highlights the role that apoptotic signaling, DNA mutations, mitochondrial dynamics and metabolism play in the development of resistance mechanisms and the advancement of cancer. Targeted approaches are discussed, with an emphasis on managing mitophagy, fusion, and fission of the mitochondria to make resistant cancer cells more susceptible to traditional treatments. Additionally, metabolic reprogramming can be used to effectively target metabolic enzymes such GLUT1, HKII, PDK, and PKM2 in order to avoid resistance mechanisms. Although there are potential possibilities for therapy, the complex structure of mitochondria and their subtle role in tumor development hamper clinical translation. Novel targeted medicines are put forth, providing fresh insights on combating drug resistance in cancer. The study also emphasizes the significance of glutamine metabolism, mitochondrial respiratory complexes, and apoptotic pathways as potential targets to improve treatment effectiveness against drug-resistant cancers. Combining complementary and nanoparticle-based techniques to target mitochondria has demonstrated encouraging results in the treatment of cancer, opening doors to reduce resistance and enable individualized treatment plans catered to the unique characteristics of each patient. Suggesting innovative approaches such as drug repositioning and mitochondrial drug delivery to enhance the efficacy of mitochondria-targeting therapies, presenting a pathway for advancements in cancer treatment. This thorough investigation is a major step forward in the treatment of cancer and has the potential to influence clinical practice and enhance patient outcomes.

Mitochondria as intracellular signalling organelles. An update

Traditionally, mitochondria are known as "the powerhouse of the cell," responsible for energy (ATP) generation (by the electron transport chain, oxidative phosphorylation, the tricarboxylic acid cycle, and fatty acid ß-oxidation), and for the regulation of several metabolic processes, including redox homeostasis, calcium signalling, and cellular apoptosis. The extensive studies conducted in the last decades portray mitochondria as multifaceted signalling organelles that ultimately command cells' survival or death. Based on current knowledge, we'll outline the mitochondrial signalling to other intracellular compartments in homeostasis and pathology-related mitochondrial stress conditions here. The following topics are discussed: (i) oxidative stress and mtROS signalling in mitohormesis, (ii) mitochondrial Ca²⁺ signalling; (iii) the anterograde (nucleus-to-mitochondria) and retrograde (mitochondria-to-nucleus) signal transduction, (iv) the mtDNA role in immunity and inflammation, (v) the induction of mitophagy- and apoptosis - signalling cascades, (vi) the mitochondrial dysfunctions (mitochondriopathies) in cardiovascular, neurodegenerative, and malignant diseases. The novel insights into molecular mechanisms of mitochondria-mediated signalling can explain mitochondria adaptation to metabolic and environmental stresses to achieve cell survival.

The Interplay between Dysregulated Metabolism and Epigenetics in Cancer

Cellular metabolism (or energetics) and epigenetics are tightly coupled cellular processes. It is arguable that of all the described cancer hallmarks, dysregulated cellular energetics and epigenetics are the most tightly coregulated. Cellular metabolic states regulate and drive epigenetic changes while also being capable of influencing, if not driving, epigenetic reprogramming. Conversely, epigenetic changes can drive altered and compensatory metabolic states. Cancer cells meticulously modify and control each of these two linked cellular processes in order to maintain their tumorigenic potential and capacity. This review aims to explore the interplay between these two processes and discuss how each affects the other, driving and enhancing tumorigenic states in certain contexts.

Mitochondrial Metabolism in Pancreatic Ductal Adenocarcinoma: From Mechanism-Based Perspectives to Therapy

Pancreatic ductal adenocarcinoma (PDAC), the fourteenth most common malignancy, is a major contributor to cancer-related death with the utmost case fatality rate among all malignancies. Functional mitochondria, regardless of their complex ecosystem relative to normal cells, are essential in PDAC progression. Tumor cells' potential to produce ATP as energy, despite retaining the redox potential optimum, and allocating materials for biosynthetic activities that are crucial for cell growth, survival, and proliferation, are assisted by mitochondria. The polyclonal tumor cells with different metabolic profiles may add to carcinogenesis through inter-metabolic coupling. Cancer cells frequently possess alterations in the mitochondrial genome, although they do not hinder metabolism; alternatively, they change bioenergetics. This can further impart retrograde signaling, educate cell signaling, epigenetic modifications, chromatin structures, and transcription machinery, and ultimately satisfy cancer cellular and nuclear demands. To maximize the tumor microenvironment (TME), tumor cells remodel nearby stromal cells and extracellular matrix. These changes initiate polyclonality, which is crucial for growth, stress response, and metastasis. Here, we evaluate all the intrinsic and extrinsic pathways drawn by mitochondria in carcinogenesis, emphasizing the perspectives of mitochondrial metabolism in PDAC progression and treatment.

LUBAC assembles a ubiquitin signaling platform at mitochondria for signal amplification and transport of NF-κB to the nucleus

Mitochondria are increasingly recognized as cellular hubs to orchestrate signaling pathways that regulate metabolism, redox homeostasis, and cell fate decisions. Recent research revealed a role of mitochondria also in innate immune signaling; however, the mechanisms of how mitochondria affect signal transduction are poorly understood. Here, we show that the NF-κB pathway activated by TNF employs mitochondria as a platform for signal amplification and shuttling of activated NF-κB to the nucleus. TNF treatment induces the recruitment of HOIP, the catalytic component of the linear ubiquitin chain assembly complex (LUBAC), and its substrate NEMO to the outer mitochondrial membrane, where M1- and K63-linked ubiquitin chains are generated. NF-κB is locally activated and transported to the nucleus by mitochondria, leading to an increase in mitochondria-nucleus contact sites in a HOIP-dependent manner. Notably, TNF-induced stabilization of the mitochondrial kinase PINK1 furthermore contributes to signal amplification by antagonizing the M1-ubiquitin-specific deubiquitinase OTULIN. Overall, our study reveals a role for mitochondria in amplifying TNF-mediated NF-κB activation, both serving as a signaling platform, as well as a transport mode for activated NF-κB to the nuclear.

Nuclear-Mitochondrial Interactions

Mitochondria, the cell’s major energy producers, also act as signaling hubs, interacting with other organelles both directly and indirectly. Despite having its own circular genome, the majority of mitochondrial proteins are encoded by nuclear DNA. To respond to changes in cell physiology, the mitochondria must send signals to the nucleus, which can, in turn, upregulate gene expression to alter metabolism or initiate a stress response. This is known as retrograde signaling. A variety of stimuli and pathways fall under the retrograde signaling umbrella. Mitochondrial dysfunction has already been shown to have severe implications for human health. Disruption of retrograde signaling, whether directly associated with mitochondrial dysfunction or cellular environmental changes, may also contribute to pathological deficits. In this review, we discuss known signaling pathways between the mitochondria and the nucleus, examine the possibility of direct contacts, and identify pathological consequences of an altered relationship.

Active mitochondrial respiration in cancer: a target for the drug

The relative contribution of mitochondrial respiration and subsequent energy production in malignant cells has remained controversial to date. Enhanced aerobic glycolysis and impaired mitochondrial respiration have gained more attention in the metabolic study of cancer. In contrast to the popular concept, mitochondria of cancer cells oxidize a diverse array of metabolic fuels to generate a majority of the cellular energy by respiration. Several mitochondrial respiratory chain (MRC) subunits' expressions are critical for the growth, metastasis, and cancer cell invasion. Also, the assembly factors, which regulate the integration of individual MRC complexes into native super-complexes, are upregulated in cancer. Moreover, a series of anti-cancer drugs function by inhibiting respiration and ATP production. In this review, we have specified the roles of mitochondrial fuels, MRC subunits, and super-complex assembly factors that promote active respiration across different cancer types and discussed the potential roles of MRC inhibitor drugs in controlling cancer.

Riddle of the Sphinx: Emerging Role of Transfer RNAs in Human Cancer

The dysregulation of transfer RNA (tRNA) expression contributes to the diversity of proteomics, heterogeneity of cell populations, and instability of the genome, which may be related to human cancer susceptibility. However, the relationship between tRNA dysregulation and cancer susceptibility remains elusive because the landscape of cancer-associated tRNAs has not been portrayed yet. Furthermore, the molecular mechanisms of tRNAs involved in tumorigenesis and cancer progression have not been systematically understood. In this review, we detail current knowledge of cancer-related tRNAs and comprehensively summarize the basic characteristics and functions of these tRNAs, with a special focus on their role and involvement in human cancer. This review bridges the gap between tRNAs and cancer and broadens our understanding of their relationship, thus providing new insights and strategies to improve the potential clinical applications of tRNAs for cancer diagnosis and therapy.

An efficient and concise synthesis of a selective small molecule non-peptide inhibitor of cathepsin L: KGP94

KGP94 is a potent, selective, and competitive inhibitor of the lysosomal endopeptidase enzyme (Cathepsin L) currently in preclinical trials for the treatment of metastatic cancer, which is a leading cause of cancer-associated death. Herein, we report two new synthetic routes for synthesizing the target compound through four consecutive steps, using a Weinreb amide approach starting from a common 3-bromobenzoyl chloride. A key step in the approach is a coupling reaction of a readily available Grignard reagent with amide 4 to produce 6, a previously unreported coupling pattern. These new strategies offer an efficient and alternative approach to synthesis of target compound with an excellent overall yield.

Advances in Intracellular Calcium Signaling Reveal Untapped Targets for Cancer Therapy

Intracellular Ca2+ distribution is a tightly regulated process. Numerous Ca2+ chelating, storage, and transport mechanisms are required to maintain normal cellular physiology. Ca2+-binding proteins, mainly calmodulin and calbindins, sequester free intracellular Ca2+ ions and apportion or transport them to signaling hubs needing the cations. Ca2+ channels, ATP-driven pumps, and exchangers assist the binding proteins in transferring the ions to and from appropriate cellular compartments. Some, such as the endoplasmic reticulum, mitochondria, and lysosomes, act as Ca2+ repositories. Cellular Ca2+ homeostasis is inefficient without the active contribution of these organelles. Moreover, certain key cellular processes also rely on inter-organellar Ca2+ signaling. This review attempts to encapsulate the structure, function, and regulation of major intracellular Ca2+ buffers, sensors, channels, and signaling molecules before highlighting how cancer cells manipulate them to survive and thrive. The spotlight is then shifted to the slow pace of translating such research findings into anticancer therapeutics. We use the PubMed database to highlight current clinical studies that target intracellular Ca2+ signaling. Drug repurposing and improving the delivery of small molecule therapeutics are further discussed as promising strategies for speeding therapeutic development in this area.

Mitochondrial DNA variation and cancer

Variation in the mitochondrial DNA (mtDNA) sequence is common in certain tumours. Two classes of cancer mtDNA variants can be identified: de novo mutations that act as 'inducers' of carcinogenesis and functional variants that act as 'adaptors', permitting cancer cells to thrive in different environments. These mtDNA variants have three origins: inherited variants, which run in families, somatic mutations arising within each cell or individual, and variants that are also associated with ancient mtDNA lineages (haplogroups) and are thought to permit adaptation to changing tissue or geographic environments. In addition to mtDNA sequence variation, mtDNA copy number and perhaps transfer of mtDNA sequences into the nucleus can contribute to certain cancers. Strong functional relevance of mtDNA variation has been demonstrated in oncocytoma and prostate cancer, while mtDNA variation has been reported in multiple other cancer types. Alterations in nuclear DNA-encoded mitochondrial genes have confirmed the importance of mitochondrial metabolism in cancer, affecting mitochondrial reactive oxygen species production, redox state and mitochondrial intermediates that act as substrates for chromatin-modifying enzymes. Hence, subtle changes in the mitochondrial genotype can have profound effects on the nucleus, as well as carcinogenesis and cancer progression.

Mitochondrial Metabolism in Carcinogenesis and Cancer Therapy

Simple Summary

Reprogramming metabolism is a hallmark of cancer. Warburg’s effect, defined as increased aerobic glycolysis at the expense of mitochondrial respiration in cancer cells, opened new avenues of research in the field of cancer. Later findings, however, have revealed that mitochondria remain functional and that they actively contribute to metabolic plasticity of cancer cells. Understanding the mechanisms by which mitochondrial metabolism controls tumor initiation and progression is necessary to better characterize the onset of carcinogenesis. These studies may ultimately lead to the design of novel anti-cancer strategies targeting mitochondrial functions.

Abstract

Carcinogenesis is a multi-step process that refers to transformation of a normal cell into a tumoral neoplastic cell. The mechanisms that promote tumor initiation, promotion and progression are varied, complex and remain to be understood. Studies have highlighted the involvement of oncogenic mutations, genomic instability and epigenetic alterations as well as metabolic reprogramming, in different processes of oncogenesis. However, the underlying mechanisms still have to be clarified. Mitochondria are central organelles at the crossroad of various energetic metabolisms. In addition to their pivotal roles in bioenergetic metabolism, they control redox homeostasis, biosynthesis of macromolecules and apoptotic signals, all of which are linked to carcinogenesis. In the present review, we discuss how mitochondria contribute to the initiation of carcinogenesis through gene mutations and production of oncometabolites, and how they promote tumor progression through the control of metabolic reprogramming and mitochondrial dynamics. Finally, we present mitochondrial metabolism as a promising target for the development of novel therapeutic strategies.

New hypotheses for cancer generation and progression

Since Nixon famously declared war on cancer in 1971, trillions of dollars have been spent on cancer research but the life expectancy for most forms of cancer is still poor. There are many reasons for the partial success of cancer translational research. One of these can be the predominance of certain paradigms that potentially narrowed the vision in interpreting cancer. The main paradigm to explain carcinogenesis is based on DNA mutations, which is well interpreted by the somatic mutation theory (SMT). However, a different theory claims that cancer is instead a tissue disease as proposed by the Tissue Organization Field Theory (TOFT). Here, we propose new hypotheses to explain the origin and pathogenesis of cancer. In this perspective, the systemic-evolutionary theory of cancer (SETOC) is discussed as well as how the microenvironment affects the adaptation of transformed cells and the reversion to a unicellular-like or embryo-like phenotype.

Mitochondrial DNA Depletion in Granulosa Cell Derived Nuclear Transfer Tissues

Somatic cell nuclear transfer (SCNT) is a key technology with broad applications that range from production of cloned farm animals to derivation of patient-matched stem cells or production of humanized animal organs for xenotransplantation. However, effects of aberrant epigenetic reprogramming on gene expression compromise cell and organ phenotype, resulting in low success rate of SCNT. Standard SCNT procedures include enucleation of recipient oocytes before the nuclear donor cell is introduced. Enucleation removes not only the spindle apparatus and chromosomes of the oocyte but also the perinuclear, mitochondria rich, ooplasm. Here, we use a Bos taurus SCNT model with in vitro fertilized (IVF) and in vivo conceived controls to demonstrate a ∼50% reduction in mitochondrial DNA (mtDNA) in the liver and skeletal muscle, but not the brain, of SCNT fetuses at day 80 of gestation. In the muscle, we also observed significantly reduced transcript abundances of mtDNA-encoded subunits of the respiratory chain. Importantly, mtDNA content and mtDNA transcript abundances correlate with hepatomegaly and muscle hypertrophy of SCNT fetuses. Expression of selected nuclear-encoded genes pivotal for mtDNA replication was similar to controls, arguing against an indirect epigenetic nuclear reprogramming effect on mtDNA amount. We conclude that mtDNA depletion is a major signature of perturbations after SCNT. We further propose that mitochondrial perturbation in interaction with incomplete nuclear reprogramming drives abnormal epigenetic features and correlated phenotypes, a concept supported by previously reported effects of mtDNA depletion on the epigenome and the pleiotropic phenotypic effects of mtDNA depletion in humans. This provides a novel perspective on the reprogramming process and opens new avenues to improve SCNT protocols for healthy embryo and tissue development.

Links between mitochondrial retrograde response and mitophagy in pathogenic cell signalling

Preservation of mitochondrial quality is paramount for cellular homeostasis. The integrity of mitochondria is guarded by the balanced interplay between anabolic and catabolic mechanisms. The removal of bio-energetically flawed mitochondria is mediated by the process of mitophagy; the impairment of which leads to the accumulation of defective mitochondria which signal the activation of compensatory mechanisms to the nucleus. This process is known as the mitochondrial retrograde response (MRR) and is enacted by Reactive Oxygen Species (ROS), Calcium (Ca2+), ATP, as well as imbalanced lipid and proteostasis. Central to this mitochondria-to-nucleus signalling are the transcription factors (e.g. the nuclear factor kappa-light-chain-enhancer of activated B cells, NF-κB) which drive the expression of genes to adapt the cell to the compromised homeostasis. An increased degree of cellular proliferation is among the consequences of the MRR and as such, engagement of mitochondrial-nuclear communication is frequently observed in cancer. Mitophagy and the MRR are therefore interlinked processes framed to, respectively, prevent or compensate for mitochondrial defects.In this review, we discuss the available knowledge on the interdependency of these processes and their contribution to cell signalling in cancer.

Alcohol-induced CYP2E1, mitochondrial dynamics and retrograde signaling in human hepatic 3D organoids

Alcohol toxicity is a significant health problem with ~3 million estimated deaths per year globally. Alcohol is metabolized to the toxic metabolite, acetaldehyde by alcohol dehydrogenase or CYP2E1 in the hepatic tissue, and also induces reactive oxygen species (ROS), which together play a pivotal role in cell and tissue damage. Our previous studies with COS-7 cells transduced with unique human CYP2E1 variants that mostly localize to either microsomes or mitochondria revealed that mitochondrially-localized CYP2E1 drives alcohol toxicity through the generation of higher levels of ROS, which has a consequent effect on cytochrome c oxidase (CcO) and mitochondrial oxidative function. Alcohol treatment of human hepatocyte cell line, HepaRG, in monolayer cultures increased ROS, affected CcO activity/stability, and induced mitophagy. Alcohol treatment of 3D organoids of HepaRG cells induced higher levels of CYP2E1 mRNA and activated mitochondrial stress-induced retrograde signaling, and also induced markers of hepatic steatosis. Knock down of CYP2E1 mRNA using specific shRNA, FK506, a Calcineurin inhibitor, and Mdivi-1, a DRP1 inhibitor, ameliorated alcohol-induced mitochondrial retrograde signaling, and hepatic steatosis. These results for the first time present a mechanistic link between CYP2E1 function and alcohol mediated mitochondrial dysfunction, retrograde signaling, and activation of hepatic steatosis in a 3D organoid system that closely recapitulates the in vivo liver response.

The role of mitochondrial ATP synthase in cancer

The mitochondrial ATP synthase is a multi-subunit enzyme complex located in the inner mitochondrial membrane which is essential for oxidative phosphorylation under physiological conditions. In this review, we analyse the enzyme functions involved in cancer progression by dissecting specific conditions in which ATP synthase contributes to cancer development or metastasis. Moreover, we propose the role of ATP synthase in the formation of the permeability transition pore (PTP) as an additional mechanism which controls tumour cell death. We further describe transcriptional and translational modifications of the enzyme subunits and of the inhibitor protein IF1 that may promote adaptations leading to cancer metabolism. Finally, we outline ATP synthase gene mutations and epigenetic modifications associated with cancer development or drug resistance, with the aim of highlighting this enzyme complex as a potential novel target for future anti-cancer therapy.

The Warburg Effect 97 Years after Its Discovery

The deregulation of the oxidative metabolism in cancer, as shown by the increased aerobic glycolysis and impaired oxidative phosphorylation (Warburg effect), is coordinated by genetic changes leading to the activation of oncogenes and the loss of oncosuppressor genes. The understanding of the metabolic deregulation of cancer cells is necessary to prevent and cure cancer. In this review, we illustrate and comment the principal metabolic and molecular variations of cancer cells, involved in their anomalous behavior, that include modifications of oxidative metabolism, the activation of oncogenes that promote glycolysis and a decrease of oxygen consumption in cancer cells, the genetic susceptibility to cancer, the molecular correlations involved in the metabolic deregulation in cancer, the defective cancer mitochondria, the relationships between the Warburg effect and tumor therapy, and recent studies that reevaluate the Warburg effect. Taken together, these observations indicate that the Warburg effect is an epiphenomenon of the transformation process essential for the development of malignancy.

Dysregulation of RyR Calcium Channel Causes the Onset of Mitochondrial Retrograde Signaling

This study shows that multiple modes of mitochondrial stress generated by partial mtDNA depletion or cytochrome c oxidase disruption cause ryanodine receptor channel (RyR) dysregulation, which instigates the release of Ca2+ in the cytoplasm of C2C12 myoblasts and HCT116 carcinoma cells. We also observed a reciprocal downregulation of IP3R channel activity and reduced mitochondrial uptake of Ca2+. Ryanodine, an RyR antagonist, abrogated the mitochondrial stress-mediated increase in [Ca2+]c and the entire downstream signaling cascades of mitochondrial retrograde signaling. Interestingly, ryanodine also inhibited mitochondrial stress-induced invasive behavior in mtDNA-depleted C2C12 cells and HCT116 carcinoma cells. In addition, co-immunoprecipitation shows reduced FKBP12 protein binding to RyR channel proteins, suggesting the altered function of the Ca2+ channel. These results document how the endoplasmic reticulum-associated RyR channels, in combination with inhibition of the mitochondrial uniporter system, modulate cellular Ca2+ homeostasis and signaling under mitochondrial stress conditions.

Mitochondria-targeted paraquat and metformin mediate ROS production to induce multiple pathways of retrograde signaling: A dose-dependent phenomenon

The mitochondrial electron transport chain is a major source of reactive oxygen species (ROS) and is also a target of ROS, with an implied role in the stabilization of hypoxia-inducible factor (HIF) and induction of the AMPK pathway. Here we used varying doses of two agents, Mito-Paraquat and Mito-Metformin, that have been conjugated to cationic triphenylphosphonium (TPP⁺) moiety to selectively target them to the mitochondrial matrix compartment, thereby resulting in the site-specific generation of ROS within mitochondria. These agents primarily induce superoxide (O₂^•-) production by acting on complex I. In Raw264.7 macrophages, C2C12 skeletal myocytes, and HCT116 adenocarcinoma cells, we show that mitochondria-targeted oxidants can induce ROS (O₂^•- and H₂O₂). In all three cell lines tested, the mitochondria-targeted agents disrupted membrane potential and activated calcineurin and the Cn-dependent retrograde signaling pathway. Hypoxic culture conditions also induced Cn activation and HIF1α activation in a temporally regulated manner, with the former appearing at shorter exposure times. Together, our results indicate that mitochondrial oxidant-induced retrograde signaling is driven by disruption of membrane potential and activation of Ca²⁺/Cn pathway and is independent of ROS-induced HIF1α or AMPK pathways.

Clinical Characteristics of Mitochondrial DNA Copy Number in Osteonecrosis of the Femoral Head

Background and objectives: Alterations in mitochondrial DNA (mtDNA) have been observed and studied in various diseases. However, the clinical value of the mtDNA copy number (mtDNA-CN) alterations in osteonecrosis of the femoral head (ONFH) is poorly understood. In the present study, we investigated whether alterations in mtDNA-CNs are associated with clinicopathological parameters in ONFH. Materials and methods: MtDNA-CNs in the synovial tissue of 34 patients with ONFH and 123 control tissues (femoral neck fracture) were measured using quantitative real-time PCR. The present study then analyzed the correlation between the mtDNA-CN and the clinicopathological characteristics of ONFH and fracture patients. Results: The average mtDNA-CN (mean ± standard deviation) was 23.82 ± 22.37 and 25.04 ± 24.27 in ONFH and control tissues, respectively, and was not significantly different between the groups (p = 0.792). The mtDNA-CN was positively associated with age (27.7% vs. 45.9%, p = 0.018) and negatively associated with the erythrocyte sedimentation rate (ESR) (11.8% vs. 39.7%, p = 0.024) in all of the samples. The study also found further associations with age (22.2% vs. 68.8%, p = 0.014), gender (30.0% vs. 64.3%, p = 0.048), and ESR (0% vs. 57.7%, p = 0.043) in ONFH. Conclusions: in this study, we demonstrated that mtDNA-CN might be a significant marker for predicting clinical characteristics in ONFH.

Mitochondrial retrograde signalling in neurological disease

Neuronal mitochondrial dysfunction causes primary mitochondrial diseases and likely contributes to neurodegenerative diseases including Parkinson's and Alzheimer's disease. Mitochondrial dysfunction has also been documented in neurodevelopmental disorders such as tuberous sclerosis complex and autism spectrum disorder. Only symptomatic treatments exist for neurodevelopmental disorders, while neurodegenerative diseases are largely untreatable. Altered mitochondrial function activates mitochondrial retrograde signalling pathways, which enable signalling to the nucleus to reprogramme nuclear gene expression. In this review, we discuss the role of mitochondrial retrograde signalling in neurological diseases. We summarize how mitochondrial dysfunction contributes to neurodegenerative disease and neurodevelopmental disorders. Mitochondrial signalling mechanisms that have relevance to neurological disease are discussed. We then describe studies documenting retrograde signalling pathways in neurons and glia, and in animal models of neuronal mitochondrial dysfunction and neurological disease. Finally, we suggest how specific retrograde signalling pathways can be targeted to develop novel treatments for neurological diseases. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Invasive phenotype induced by low extracellular pH requires mitochondria dependent metabolic flexibility

Metabolic reprogramming is required for tumors to meet the bioenergetic and biosynthetic demands of malignant progression. Numerous studies have established a causal relationship between oncogenic drivers and altered metabolism, most prominently aerobic glycolysis, which supports rapid growth and affects the tumor microenvironment. Less is known about how the microenvironment modulates cancer metabolism. In the present study, we found that low extracellular pH, a common feature of solid tumors, provoked PDAC cells to decrease glycolysis and become resistant to glucose starvation. This was accompanied by increased dependency on mitochondrial metabolism, in which long-chain fatty acids became a primary fuel source. Consistent with previous reports, low pH enhanced tumor cell invasiveness. A novel finding was that limiting PDAC metabolic flexibility by either suppression of oxidative phosphorylation capacity or the pharmacological inhibition of fatty-acid oxidation prevented invasion induced by low extracellular pH. Altogether, our results suggest for the first time that targeting fatty-acid oxidation may be a viable adjunct strategy for preventing metastatic progression of pancreatic cancer mediated by the acidic tumor compartment.

Mitochondrial Dysfunction in Aging and Cancer

Aging is a major risk factor for developing cancer, suggesting that these two events may represent two sides of the same coin. It is becoming clear that some mechanisms involved in the aging process are shared with tumorigenesis, through convergent or divergent pathways. Increasing evidence supports a role for mitochondrial dysfunction in promoting aging and in supporting tumorigenesis and cancer progression to a metastatic phenotype. Here, a summary of the current knowledge of three aspects of mitochondrial biology that link mitochondria to aging and cancer is presented. In particular, the focus is on mutations and changes in content of the mitochondrial genome, activation of mitochondria-to-nucleus signaling and the newly discovered mitochondria-telomere communication.

Cathepsin L induced PC-12 cell apoptosis via activation of B-Myb and regulation of cell cycle proteins

Cathepsin L (CTSL), a cysteine protease, is responsible for the degradation of a variety of proteins. It is known to participate in neuronal apoptosis associated with abnormal cell cycle. However, the mechanisms underlying CTSL-induced cell apoptosis remain largely unclear. We reported here that rotenone caused an activation of CTSL expression in PC-12 cells, while knockdown of CTSL by small interfering RNAs or its inhibitor reduced the rotenone-induced cell cycle arrest and apoptosis. Moreover, elevation of CTSL and increased-apoptosis were accompanied by induction of B-Myb, a crucial cell cycle regulator. We found that B-Myb was increased in rotenone-treated PC-12 cells and knockdown of B-Myb ameliorated rotenone-stimulated cell apoptosis. Further analysis demonstrated that CTSL influenced the expression of B-Myb as suppression of CTSL activity led to a decreased B-Myb expression, whereas overexpression of CTSL resulted in B-Myb induction. Reduction of B-Myb in CTSL-overexpressing cells revealed that regulation of cell cycle-related proteins, including cyclin A and cyclin B1, through CTSL was mediated by the transcription factor B-Myb. In addition, we demonstrated that the B-Myb target, Bim, and its regulator, Egr-1, which was also associated with CTSL closely, were both involved in rotenone-induced apoptosis in PC-12 cells. Our data not only revealed the role of CTSL in rotenone-induced neuronal apoptosis, but also indicated the involvement of B-Myb in CTSL-related cell cycle regulation.

Alteration of mitochondrial DNA content modulates antioxidant enzyme expressions and oxidative stress in myoblasts

Mitochondrial dysfunction is closely associated with reactive oxygen species (ROS) generation and oxidative stress in cells. On the other hand, modulation of the cellular antioxidant defense system by changes in the mitochondrial DNA (mtDNA) content is largely unknown. To determine the relationship between the cellular mtDNA content and defense system against oxidative stress, this study examined a set of myoblasts containing a depleted or reverted mtDNA content. A change in the cellular mtDNA content modulated the expression of antioxidant enzymes in myoblasts. In particular, the expression and activity of glutathione peroxidase (GPx) and catalase were inversely correlated with the mtDNA content in myoblasts. The depletion of mtDNA decreased both the reduced glutathione (GSH) and oxidized glutathione (GSSG) slightly, whereas the cellular redox status, as assessed by the GSH/GSSG ratio, was similar to that of the control. Interestingly, the steady-state level of the intracellular ROS, which depends on the reciprocal actions between ROS generation and detoxification, was reduced significantly and the lethality induced by H₂O₂ was alleviated by mtDNA depletion in myoblasts. Therefore, these results suggest that the ROS homeostasis and antioxidant enzymes are modulated by the cellular mtDNA content and that the increased expression and activity of GPx and catalase through the depletion of mtDNA are closely associated with an alleviation of the oxidative stress in myoblasts.

Esophageal 3D organoids of MPV17-/- mouse model of mitochondrial DNA depletion show epithelial cell plasticity and telomere attrition

Esophageal squamous cell carcinoma (ESCC) is an aggressive cancer with late-stage detection and poor prognosis. This emphasizes the need to identify new markers for early diagnosis and treatment. Altered mitochondrial genome (mtDNA) content in primary tumors correlates with poor patient prognosis. Here we used three-dimensional (3D) organoids of esophageal epithelial cells (EECs) from the MPV17-/- mouse model of mtDNA depletion to investigate the contribution of reduced mtDNA content in ESCC oncogenicity. To test if mtDNA defects are a contributing factor in ESCC, we used oncogenic stimuli such as ESCC carcinogen 4-nitroquinoline oxide (4-NQO) treatment, or expressing p53R175H oncogenic driver mutation. We observed that EECs and 3D-organoids with mtDNA depletion had cellular, morphological and genetic alterations typical of an oncogenic transition. Furthermore, mitochondrial dysfunction induced cellular transformation is accompanied by elevated mitochondrial fission protein, DRP1 and pharmacologic inhibition of mitochondrial fission by mDivi-1 in the MPV17-/- organoids reversed the phenotype to that of normal EEC organoids. Our studies show that mtDNA copy number depletion, activates a mitochondrial retrograde response, potentiates telomere defects, and increases the oncogenic susceptibility towards ESCC. Furthermore, mtDNA depletion driven cellular plasticity is mediated via altered mitochondrial fission-fusion dynamics.

Connecting Histopathology Imaging and Proteomics in Kidney Cancer through Machine Learning

Proteomics data encode molecular features of diagnostic value and accurately reflect key underlying biological mechanisms in cancers. Histopathology imaging is a well-established clinical approach to cancer diagnosis. The predictive relationship between large-scale proteomics and H&E-stained histopathology images remains largely uncharacterized. Here we investigate such associations through the application of machine learning, including deep neural networks, to proteomics and histology imaging datasets generated by the Clinical Proteomic Tumor Analysis Consortium (CPTAC) from clear cell renal cell carcinoma patients. We report robust correlations between a set of diagnostic proteins and predictions generated by an imaging-based classification model. Proteins significantly correlated with the histology-based predictions are significantly implicated in immune responses, extracellular matrix reorganization, and metabolism. Moreover, we showed that the genes encoding these proteins also reliably recapitulate the biological associations with imaging-derived predictions based on strong gene–protein expression correlations. Our findings offer novel insights into the integrative modeling of histology and omics data through machine learning, as well as the methodological basis for new research opportunities in this and other cancer types.

Cytochrome c oxidase dysfunction enhances phagocytic function and osteoclast formation in macrophages

The mitochondria-to-nucleus retrograde signaling (MtRS) pathway aids in cellular adaptation to stress. We earlier reported that the Ca2+- and calcineurin-dependent MtRS induces macrophage differentiation to bone-resorbing osteoclasts. However, mechanisms through which macrophages sense and respond to cellular stress remain unclear. Here, we induced mitochondrial stress in macrophages by knockdown (KD) of subunits IVi1 or Vb of cytochrome c oxidase (CcO). Whereas both IVi1 and Vb KD impair CcO activity, IVi1 KD cells produced higher levels of cellular and mitochondrial reactive oxygen species with increased glycolysis. Additionally, IVi1 KD induced the activation of MtRS factors NF-κB, NFAT2, and C/EBPδ as well as inflammatory cytokines, NOS 2, increased phagocytic activity, and a greater osteoclast differentiation potential at suboptimal RANK-L concentrations. The osteoclastogenesis in IVi1 KD cells was reversed fully with an IL-6 inhibitor LMT-28, whereas there was minimal rescue of the enhanced phagocytosis in these cells. In agreement with our findings in cultured macrophages, primary bone marrow-derived macrophages from MPV17-/- mice, a model for mitochondrial dysfunction, also showed higher propensity for osteoclast formation. This is the first report showing that CcO dysfunction affects inflammatory pathways, phagocytic function, and osteoclastogenesis.-Angireddy, R., Kazmi, H. R., Srinivasan, S., Sun, L., Iqbal, J., Fuchs, S. Y., Guha, M., Kijima, T., Yuen, T., Zaidi, M., Avadhani, N. G. Cytochrome c oxidase dysfunction enhances phagocytic function and osteoclast formation in macrophages.

The secret messages between mitochondria and nucleus in muscle cell biology

Over two thousand proteins are found in the mitochondrial compartment but the mitochondrial genome codes for only 13 proteins. The majority of mitochondrial proteins are products of nuclear genes and are synthesized in the cytosol, then translocated into the mitochondria. Most of the subunits of the five respiratory chain complexes in the inner mitochondrial membrane, which generate a proton gradient across the membrane and produce ATP, are encoded by nuclear genes. Therefore, it is quite clear that import of nuclear-encoded proteins into the mitochondria is essential for mitochondrial function. Nuclear to mitochondrial communication is well studied. However, there is another arm to this communication, mitochondria to nucleus retrograde signaling. This plays an important role in the maintenance of cellular homeostasis, and is less well studied. Several transcription factors, including Sp1, SIRT3 and GSP2, are activated by altered mitochondrial function. These activated transcription factors then translocate to the nucleus. Based on the mitochondrially generated molecular signal, nuclear genes are targeted, which alters transcription of nuclear genes that code for mitochondrial proteins. This review article will mainly focus on this interactive and bi-directional communication between mitochondria and nucleus, and how this communication plays a significant role in muscle cell biology.

The SIAH2-NRF1 axis spatially regulates tumor microenvironment remodeling for tumor progression

The interactions between tumor cells with their microenvironments, including hypoxia, acidosis and immune cells, lead to the tumor heterogeneity which promotes tumor progression. Here, we show that SIAH2-NRF1 axis remodels tumor microenvironment through regulating tumor mitochondrial function, tumor-associated macrophages (TAMs) polarization and cell death for tumor maintenance and progression. Mechanistically, low mitochondrial gene expression in breast cancers is associated with a poor clinical outcome. The hypoxia-activated E3 ligase SIAH2 spatially downregulates nuclear-encoded mitochondrial gene expression including pyruvate dehydrogenase beta via degrading NRF1 (Nuclear Respiratory Factor 1) through ubiquitination on lysine 230, resulting in enhanced Warburg effect, metabolic reprogramming and pro-tumor immune response. Dampening NRF1 degradation under hypoxia not only impairs the polarization of TAMs, but also promotes tumor cells to become more susceptible to apoptosis in a FADD-dependent fashion, resulting in secondary necrosis due to the impairment of efferocytosis. These data represent that inhibition of NRF1 degradation is a potential therapeutic strategy against cancer.

Cathepsin L activated by mutant p53 and Egr-1 promotes ionizing radiation-induced EMT in human NSCLC

BACKGROUND

Ionizing radiation (IR) is one of the major clinical therapies of cancer, although it increases the epithelial-mesenchymal transition (EMT) of non-small cell lung cancer (NSCLC), unexpectedly. The cellular and molecular mechanisms underlying this role are not completely understood.

METHODS

We used NSCLC cell lines as well as tumor specimens from 78 patients with NSCLC to evaluate p53, Cathepsin L (CTSL) and EMT phenotypic changes. Xenograft models was also utilized to examine the roles of mutant p53 (mut-p53) and CTSL in regulating IR-induced EMT of NSCLC.

RESULTS

Expression of CTSL was markedly increased in human NSCLC tissues with mutant p53 (mut-p53), and p53 mutation positively correlated with metastasis of NSCLC patients. In human non-small cell lung cancer cell line, H1299 cells transfected with various p53 lentivirus vectors, mut-p53 could promote the invasion and motility of cells under IR, mainly through the EMT. This EMT process was induced by elevating intranuclear CTSL which was regulated by mut-p53 depending on Early growth response protein-1 (Egr-1) activation. In the subcutaneous tumor xenograft model, IR promoted the EMT of the cancer cells in the presence of mut-p53, owing to increase expression and nuclear transition of its downstream protein CTSL.

CONCLUSION

Taken together, these data reveal the role of the mut-p53/Egr-1/CTSL axis in regulating the signaling pathway responsible for IR-induced EMT.

hnRNPA2 mediated acetylation reduces telomere length in response to mitochondrial dysfunction

Telomeres protect against chromosomal damage. Accelerated telomere loss has been associated with premature aging syndromes such as Werner's syndrome and Dyskeratosis Congenita, while, progressive telomere loss activates a DNA damage response leading to chromosomal instability, typically observed in cancer cells and senescent cells. Therefore, identifying mechanisms of telomere length maintenance is critical for understanding human pathologies. In this paper we demonstrate that mitochondrial dysfunction plays a causal role in telomere shortening. Furthermore, hnRNPA2, a mitochondrial stress responsive lysine acetyltransferase (KAT) acetylates telomere histone H4at lysine 8 of (H4K8) and this acetylation is associated with telomere attrition. Cells containing dysfunctional mitochondria have higher telomere H4K8 acetylation and shorter telomeres independent of cell proliferation rates. Ectopic expression of KAT mutant hnRNPA2 rescued telomere length possibly due to impaired H4K8 acetylation coupled with inability to activate telomerase expression. The phenotypic outcome of telomere shortening in immortalized cells included chromosomal instability (end-fusions) and telomerase activation, typical of an oncogenic transformation; while in non-telomerase expressing fibroblasts, mitochondrial dysfunction induced-telomere attrition resulted in senescence. Our findings provide a mechanistic association between dysfunctional mitochondria and telomere loss and therefore describe a novel epigenetic signal for telomere length maintenance.

The dysregulation of tRNAs and tRNA derivatives in cancer

Transfer RNAs (tRNAs), traditionally considered to participate in protein translation, were interspersed in the entire genome. Recent studies suggested that dysregulation was observed in not only tRNAs, but also tRNA derivatives generated by the specific cleavage of pre- and mature tRNAs in the progression of cancer. Accumulating evidence had identified that certain tRNAs and tRNA derivatives were involved in proliferation, metastasis and invasiveness of cancer cell, as well as tumor growth and angiogenesis in several malignant human tumors. This paper reviews the importance of the dysregulation of tRNAs and tRNA derivatives during the development of cancer, such as breast cancer, lung cancer, and melanoma, aiming at a better understanding of the tumorigenesis and providing new ideas for the treatment of these cancers.

MtDNA depleted PC3 cells exhibit Warburg effect and cancer stem cell features

Reducing mtDNA content was considered as a critical step in the metabolism restructuring for cell stemness restoration and further neoplastic development. However, the connections between mtDNA depletion and metabolism reprograming-based cancer cell stemness in prostate cancers are still lack of studies. Here, we demonstrated that human CRPC cell line PC3 tolerated high concentration of the mtDNA replication inhibitor ethidium bromide (EtBr) and the mtDNA depletion triggered a universal metabolic remodeling process. Failure in completing that process caused lethal consequences. The mtDNA depleted (MtDP) PC3 cells could be steadily maintained in the special medium in slow cycling status. The MtDP PC3 cells contained immature mitochondria and exhibited Warburg effect. Furthermore, the MtDP PC3 cells were resistant to therapeutic treatments and contained greater cancer stem cell-like subpopulations: CD44+, ABCG2+, side-population and ALDH^bright. In conclusion, these results highlight the association of mtDNA content, mitochondrial function and cancer cell stemness features.

Divergent branches of mitochondrial signaling regulate specific genes and the viability of specialized cell types of differentiated yeast colonies

Mitochondrial retrograde signaling mediates communication from altered mitochondria to the nucleus and is involved in many normal and pathophysiological changes, including cell metabolic reprogramming linked to cancer development and progression in mammals. The major mitochondrial retrograde pathway described in yeast includes three activators, Rtg1p, Rtg2p and Rtg3p, and repressors, Mks1p and Bmh1p/Bmh2p. Using differentiated yeast colonies, we show that Mks1p-Rtg pathway regulation is complex and includes three branches that divergently regulate the properties and fate of three specifically localized cell subpopulations via signals from differently altered mitochondria. The newly identified RTG pathway-regulated genes ATO1/ATO2 are expressed in colonial upper (U) cells, the cells with active TORC1 that metabolically resemble tumor cells, while CIT2 is a typical target induced in one subpopulation of starving lower (L) cells. The viability of the second L cell subpopulation is strictly dependent on RTG signaling. Additional co-activators of Rtg1p-Rtg3p specific to particular gene targets of each branch are required to regulate cell differentiation.

Defining the momiome: Promiscuous information transfer by mobile mitochondria and the mitochondrial genome

Mitochondria are complex intracellular organelles that have long been identified as the powerhouses of eukaryotic cells because of the central role they play in oxidative metabolism. A resurgence of interest in the study of mitochondria during the past decade has revealed that mitochondria also play key roles in cell signaling, proliferation, cell metabolism and cell death, and that genetic and/or metabolic alterations in mitochondria contribute to a number of diseases, including cancer. Mitochondria have been identified as signaling organelles, capable of mediating bidirectional intracellular information transfer: anterograde (from nucleus to mitochondria) and retrograde (from mitochondria to nucleus). More recently, evidence is now building that the role of mitochondria extends to intercellular communication as well, and that the mitochondrial genome (mtDNA) and even whole mitochondria are indeed mobile and can mediate information transfer between cells. We define this promiscuous information transfer function of mitochondria and mtDNA as "momiome" to include all mobile functions of mitochondria and the mitochondrial genome. Herein, we review the "momiome" and explore its role in cancer development, progression, and treatment.

Mitochondrial retrograde signaling in the nervous system

Mitochondria generate the majority of cellular ATP and are essential for neuronal function. Loss of mitochondrial activity leads to primary mitochondrial diseases and may contribute to neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Mitochondria communicate with the cell through mitochondrial retrograde signaling pathways. These signaling pathways are triggered by mitochondrial dysfunction and allow the organelle to control nuclear gene transcription. Neuronal mitochondrial retrograde signaling pathways have been identified in disease model systems and targeted to restore neuronal function and prevent neurodegeneration. In this review, we describe yeast and mammalian cellular models that have paved the way in the investigation of mitochondrial retrograde mechanisms. We then discuss the evidence for retrograde signaling in neurons and our current knowledge of retrograde signaling mechanisms in neuronal model systems. We argue that targeting mitochondrial retrograde pathways has the potential to lead to novel treatments for neurological diseases.

Potential Diagnostic and Prognostic Value of Lymphocytic Mitochondrial DNA Deletion in Relation to Folic Acid Status in HCV-Related Hepatocellular Carcinoma

Objective:

We assessed the possibility of using mitochondrial (mt) DNA deletion as a molecular biomarker for disease progression in HCV-related hepatocellular carcinoma (HCC) and to identify its association with folic acid status.

Methods:

Serum folic acid and lymphocytic mtDNA deletions were assessed in 90 patients; 50 with HCC, 20 with liver cirrhosis (LC), and 20 with chronic hepatitis C (CHC) compared to 10 healthy control subjects. The diagnostic accuracy of mtDNA deletions frequency was evaluated using receiver-operating characteristic (ROC) curve analysis Survival analysis was performed using the Kaplan-Meier method. Differences in the survival rates were compared using log-rank test.

Result:

Our data revealed a significant elevation of mtDNA deletions frequency in the HCC group compared to the other groups (P-value <0.01). Also, our data showed a significant correlation between folate deficiency and high frequency of mtDNA deletions in patients with HCV-related HCC when compared to the other groups (r= -0.094 and P-value <0.05). Moreover, the size of the hepatic focal lesion in the HCC patients was positively correlated with mtDNA deletions (r= 0.09 and P-value <0.01). The median survival time for the HCC patients with high frequency of mtDNA deletions (∆Ct ≥3.9; 5.7+ 0.6 months) was significantly shorter than those with low mtDNA deletions frequency (∆Ct < 3.9; 11.9+ 0.04 months, P-value <0.01).

Conclusion:

Our data provided an evidence that lymphocytic mtDNA deletion could be used as non-invasive biomarker for disease progression and patients’ survival in HCV-related HCC. Also, our findings implied a causal relationship between the folate deficiency and the high mtDNA deletions frequency among Egyptian patients with HCV related HCC.

Mitonuclear Interactions Mediate Transcriptional Responses to Hypoxia in Drosophila

Among the major challenges in quantitative genetics and personalized medicine is to understand how gene × gene interactions (G × G: epistasis) and gene × environment interactions (G × E) underlie phenotypic variation. Here, we use the intimate relationship between mitochondria and oxygen availability to dissect the roles of nuclear DNA (nDNA) variation, mitochondrial DNA (mtDNA) variation, hypoxia, and their interactions on gene expression in Drosophila melanogaster. Mitochondria provide an important evolutionary and medical context for understanding G × G and G × E given their central role in integrating cellular signals. We hypothesized that hypoxia would alter mitonuclear communication and gene expression patterns. We show that first order nDNA, mtDNA, and hypoxia effects vary between the sexes, along with mitonuclear epistasis and G × G × E effects. Females were generally more sensitive to genetic and environmental perturbation. While dozens to hundreds of genes are altered by hypoxia in individual genotypes, we found very little overlap among mitonuclear genotypes for genes that were significantly differentially expressed as a consequence of hypoxia; excluding the gene hairy. Oxidative phosphorylation genes were among the most influenced by hypoxia and mtDNA, and exposure to hypoxia increased the signature of mtDNA effects, suggesting retrograde signaling between mtDNA and nDNA. We identified nDNA-encoded genes in the electron transport chain (succinate dehydrogenase) that exhibit female-specific mtDNA effects. Our findings have important implications for personalized medicine, the sex-specific nature of mitonuclear communication, and gene × gene coevolution under variable or changing environments.

DNA damage related crosstalk between the nucleus and mitochondria

The electron transport chain is the primary pathway by which a cell generates energy in the form of ATP. Byproducts of this process produce reactive oxygen species that can cause damage to mitochondrial DNA. If not properly repaired, the accumulation of DNA damage can lead to mitochondrial dysfunction linked to several human disorders including neurodegenerative diseases and cancer. Mitochondria are able to combat oxidative DNA damage via repair mechanisms that are analogous to those found in the nucleus. Of the repair pathways currently reported in the mitochondria, the base excision repair pathway is the most comprehensively described. Proteins that are involved with the maintenance of mtDNA are encoded by nuclear genes and translocate to the mitochondria making signaling between the nucleus and mitochondria imperative. In this review, we discuss the current understanding of mitochondrial DNA repair mechanisms and also highlight the sensors and signaling pathways that mediate crosstalk between the nucleus and mitochondria in the event of mitochondrial stress.

Mitochondrial retrograde signaling connects respiratory capacity to thermogenic gene expression

Mitochondrial respiration plays a crucial role in determining the metabolic state of brown adipose tissue (BAT), due to its direct roles in thermogenesis, as well as through additional mechanisms. Here, we show that respiration-dependent retrograde signaling from mitochondria to nucleus contributes to genetic and metabolic reprogramming of BAT. In mouse BAT, ablation of LRPPRC (LRP130), a potent regulator of mitochondrial transcription and respiratory capacity, triggers down-regulation of thermogenic genes, promoting a storage phenotype in BAT. This retrograde regulation functions by inhibiting the recruitment of PPARγ to the regulatory elements of thermogenic genes. Reducing cytosolic Ca²⁺ reverses the attenuation of thermogenic genes in brown adipocytes with impaired respiratory capacity, while induction of cytosolic Ca²⁺ is sufficient to attenuate thermogenic gene expression, indicating that cytosolic Ca²⁺ mediates mitochondria-nucleus crosstalk. Our findings suggest respiratory capacity governs thermogenic gene expression and BAT function via mitochondria-nucleus communication, which in turn leads to either a thermogenic or storage mode.

Migration of mitochondrial DNA in the nuclear genome of colorectal adenocarcinoma

Background

Colorectal adenocarcinomas are characterized by abnormal mitochondrial DNA (mtDNA) copy number and genomic instability, but a molecular interaction between mitochondrial and nuclear genome remains unknown. Here we report the discovery of increased copies of nuclear mtDNA (NUMT) in colorectal adenocarcinomas, which supports link between mtDNA and genomic instability in the nucleus. We name this phenomenon of nuclear occurrence of mitochondrial component as numtogenesis. We provide a description of NUMT abundance and distribution in tumor versus matched blood-derived normal genomes.

Methods

Whole-genome sequence data were obtained for colon adenocarcinoma and rectum adenocarcinoma patients participating in The Cancer Genome Atlas, via the Cancer Genomics Hub, using the GeneTorrent file acquisition tool. Data were analyzed to determine NUMT proportion and distribution on a genome-wide scale. A NUMT suppressor gene was identified by comparing numtogenesis in other organisms.

Results

Our study reveals that colorectal adenocarcinoma genomes, on average, contains up to 4.2-fold more somatic NUMTs than matched normal genomes. Women colorectal tumors contained more NUMT than men. NUMT abundance in tumor predicted parallel abundance in blood. NUMT abundance positively correlated with GC content and gene density. Increased numtogenesis was observed with higher mortality. We identified YME1L1, a human homolog of yeast YME1 (yeast mitochondrial DNA escape 1) to be frequently mutated in colorectal tumors. YME1L1 was also mutated in tumors derived from other tissues. We show that inactivation of YME1L1 results in increased transfer of mtDNA in the nuclear genome.

Conclusions

Our study demonstrates increased somatic transfer of mtDNA in colorectal tumors. Our study also reveals sex-based differences in frequency of NUMT occurrence and that NUMT in blood reflects NUMT in tumors, suggesting NUMT may be used as a biomarker for tumorigenesis. We identify YME1L1 as the first NUMT suppressor gene in human and demonstrate that inactivation of YME1L1 induces migration of mtDNA to the nuclear genome. Our study reveals that numtogenesis plays an important role in the development of cancer.

Electronic supplementary material

The online version of this article (doi:10.1186/s13073-017-0420-6) contains supplementary material, which is available to authorized users.

Integrating the UPRmt into the mitochondrial maintenance network

Mitochondrial function is central to many different processes in the cell, from oxidative phosphorylation to the synthesis of iron-sulfur clusters. Therefore, mitochondrial dysfunction underlies a diverse array of diseases, from neurodegenerative diseases to cancer. Stress can be communicated to the cytosol and nucleus from the mitochondria through many different signals, and in response the cell can effect everything from transcriptional to post-transcriptional responses to protect the mitochondrial network. How these responses are coordinated have only recently begun to be understood. In this review, we explore how the cell maintains mitochondrial function, focusing on the mitochondrial unfolded protein response (UPR^mt), a transcriptional response that can activate a wide array of programs to repair and restore mitochondrial function.

Prepregnancy maternal diabetes combined with obesity impairs placental mitochondrial function involving Nrf2/ARE pathway and detrimentally alters metabolism of offspring

Metabolic disorders usually increase the level of reactive oxygen species (ROS) and damage mitochondrial function. The placenta supplies nutrients and hormonal signals to the fetus for regulating fetal metabolism, and is also prone to injury by oxidants. The aim of this study was to determine the effect of pre-existing maternal type 2 diabetes mellitus (DM) combined with obesity on placental mitochondrial function and metabolism disorders of offspring. The study included 96 pregnant women. The women were put into the following groups: healthy women (control, n=24), women with DM (DM, n=24), women with obesity (OB, n=24) and women with both DM and obesity (DM+OB, n=24). The ROS level, mitochondrial content, and the mitochondrial respiratory complex activities of the placenta were measured in the four groups. The expression of nuclear factor erythroid 2-related factor 2 (Nrf2) and heme oxygenase-1 (HO-1) was detected by immunofluorescence staining and western blotting. In addition, serum levels of insulin, glucose, leptin, nonesterified fatty acid (NEFA), adiponectin and triglycerides of their offspring were also measured. Maternal DM combined with obesity markedly increased ROS level, reduced mitochondrial DNA (mtDNA) content and mitochondrial respiratory complex I, II-III activities in placenta compared to the placenta from the control group and the DM group. Maternal DM combined with obesity significantly decreased Nrf2 and HO-1 expression. Furthermore, maternal DM combined with obesity influenced the glucose and lipid metabolism in their offspring. In conclusion, women with both DM and obesity detrimentally alter placenta function in oxidative stress regulation, and the Nrf2/ARE (antioxidant responsive element) pathway is involved. This may increase metabolic disturbance susceptibility in their offspring.

Mitochondrial dysfunction and mitochondrial dynamics-The cancer connection

Mitochondrial dysfunction is a hallmark of many diseases. The retrograde signaling initiated by dysfunctional mitochondria can bring about global changes in gene expression that alters cell morphology and function. Typically, this is attributed to disruption of important mitochondrial functions, such as ATP production, integration of metabolism, calcium homeostasis and regulation of apoptosis. Recent studies showed that in addition to these factors, mitochondrial dynamics might play an important role in stress signaling. Normal mitochondria are highly dynamic organelles whose size, shape and network are controlled by cell physiology. Defective mitochondrial dynamics play important roles in human diseases. Mitochondrial DNA defects and defective mitochondrial function have been reported in many cancers. Recent studies show that increased mitochondrial fission is a pro-tumorigenic phenotype. In this paper, we have explored the current understanding of the role of mitochondrial dynamics in pathologies. We present new data on mitochondrial dynamics and dysfunction to illustrate a causal link between mitochondrial DNA defects, excessive fission, mitochondrial retrograde signaling and cancer progression. This article is part of a Special Issue entitled Mitochondria in Cancer, edited by Giuseppe Gasparre, Rodrigue Rossignol and Pierre Sonveaux.