Mitochondrial c-Src regulates cell survival through phosphorylation of respiratory chain components

c-Jun N-terminal kinase mediates mouse liver injury through a novel Sab (SH3BP5)-dependent pathway leading to inactivation of intramitochondrial Src

Sustained c-Jun N-terminal kinase (JNK) activation has been implicated in many models of cell death and tissue injury. Phosphorylated JNK (p-JNK) interacts with the mitochondrial outer membrane SH3 homology associated BTK binding protein (Sab, or SH3BP5). Using knockdown or liver-specific deletion of Sab, we aimed to elucidate the consequences of this interaction on mitochondrial function in isolated mitochondria and liver injury models in vivo. Respiration in isolated mitochondria was directly inhibited by p-JNK + adenosine triphosphate. Knockdown or liver-specific knockout of Sab abrogated this effect and markedly inhibited sustained JNK activation and liver injury from acetaminophen or tumor necrosis factor/galactosamine. We then elucidated an intramitochondrial pathway in which interaction of JNK and Sab on the outside of the mitochondria released protein tyrosine phosphatase, nonreceptor type 6 (SHP1, or PTPN6) from Sab in the inside of the mitochondrial outer membrane, leading to its activation and transfer to the inner membrane, where it dephosphorylates P-Y419Src (active), which required a platform protein, docking protein 4 (DOK4), on the inner membrane. Knockdown of mitochondrial DOK4 or SHP1 inhibited the inactivation of mitochondrial p-Src and the effect of p-JNK on mitochondria.

CONCLUSIONS

The binding to and phosphorylation of Sab by p-JNK on the outer mitochondrial membrane leads to SHP1-dependent and DOK4-dependent inactivation of p-Src on the inner membrane; inactivation of mitochondrial Src inhibits electron transport and increases reactive oxygen species release, which sustains JNK activation and promotes cell death and organ injury. (Hepatology 2016;63:1987-2003).

The Effect of Aging on Mitochondrial Complex I and the Extent of Oxidative Stress in the Rat Brain Cortex

One of the characteristic features of the aging is dysfunction of mitochondria. Its role in the regulation of metabolism and apoptosis suggests a possible link between these cellular processes. This study investigates the relationship of respiratory complex I with aging-related oxidative stress in the cerebral mitochondria. Deterioration of complex I seen in senescent (26-months old) mitochondria was accompanied by decline in total thiol group content, increase of HNE and HNE-protein adducts as well as decreased content of complex I subunits, GRIM-19 and NDUFV2. On the other hand, decline of complex I might be related with the mitochondrial apoptosis through increased Bax/Bcl-2 cascade in 15-month old animal brains. Higher amount of Bcl-2, Bcl-xL with the lower content of GRIM-19 could maintain to some extent elevated oxidative stress in mitochondria as seen in the senescent group. In the cortical M1 region increased presence of TUNEL+ cells and more than 20-times higher density of Fluoro-Jade C+ cells in 26-months old was observed, suggesting significant neurodegenerative effect of aging in the neuronal cells. Our study supports a scenario in which the age-related decline of complex I might sensitize neurons to the action of death agonists, such as Bax through lipid and protein oxidative stimuli in mitochondria. Although aging is associated with oxidative stress, these changes did not increase progressively with age, as similar extent of lesions was observed in oxidative stress markers of the both aged groups.

Regulation of mitochondrial functions by protein phosphorylation and dephosphorylation

The mitochondria are double membrane-bound organelles found in most eukaryotic cells. They generate most of the cell’s energy supply of adenosine triphosphate (ATP). Protein phosphorylation and dephosphorylation are critical mechanisms in the regulation of cell signaling networks and are essential for almost all the cellular functions. For many decades, mitochondria were considered autonomous organelles merely functioning to generate energy for cells to survive and proliferate, and were thought to be independent of the cellular signaling networks. Consequently, phosphorylation and dephosphorylation processes of mitochondrial kinases and phosphatases were largely neglected. However, evidence accumulated in recent years on mitochondria-localized kinases/phosphatases has changed this longstanding view. Mitochondria are increasingly recognized as a hub for cell signaling, and many kinases and phosphatases have been reported to localize in mitochondria and play important functions. However, the strength of the evidence on mitochondrial localization and the activities of the reported kinases and phosphatases vary greatly, and the detailed mechanisms on how these kinases/phosphatases translocate to mitochondria, their subsequent function, and the physiological and pathological implications of their localization are still poorly understood. Here, we provide an updated perspective on the recent advancement in this area, with an emphasis on the implications of mitochondrial kinases/phosphatases in cancer and several other diseases.

Metabolic changes associated with tumor metastasis, part 2: Mitochondria, lipid and amino acid metabolism

Metabolic alterations are a hallmark of cancer controlling tumor progression and metastasis. Among the various metabolic phenotypes encountered in tumors, this review focuses on the contributions of mitochondria, lipid and amino acid metabolism to the metastatic process. Tumor cells require functional mitochondria to grow, proliferate and metastasize, but shifts in mitochondrial activities confer pro-metastatic traits encompassing increased production of mitochondrial reactive oxygen species (mtROS), enhanced resistance to apoptosis and the increased or de novo production of metabolic intermediates of the TCA cycle behaving as oncometabolites, including succinate, fumarate, and D-2-hydroxyglutarate that control energy production, biosynthesis and the redox state. Lipid metabolism and the metabolism of amino acids, such as glutamine, glutamate and proline are also currently emerging as focal control points of cancer metastasis.

Fatty Acid Oxidation-Driven Src Links Mitochondrial Energy Reprogramming and Oncogenic Properties in Triple-Negative Breast Cancer

Transmitochondrial cybrids and multiple OMICs approaches were used to understand mitochondrial reprogramming and mitochondria-regulated cancer pathways in triple-negative breast cancer (TNBC). Analysis of cybrids and established breast cancer (BC) cell lines showed that metastatic TNBC maintains high levels of ATP through fatty acid β oxidation (FAO) and activates Src oncoprotein through autophosphorylation at Y419. Manipulation of FAO including the knocking down of carnitine palmitoyltransferase-1A (CPT1) and 2 (CPT2), the rate-limiting proteins of FAO, and analysis of patient-derived xenograft models confirmed the role of mitochondrial FAO in Src activation and metastasis. Analysis of TCGA and other independent BC clinical data further reaffirmed the role of mitochondrial FAO and CPT genes in Src regulation and their significance in BC metastasis.

NDUFV2 regulates neuronal migration in the developing cerebral cortex through modulation of the multipolar-bipolar transition

Abnormalities during brain development are tightly linked several psychiatric disorders. Mutations in NADH dehydrogenase ubiquinone flavoprotein 2 (NDUFV2) are responsible for schizophrenia, bipolar disorder and Parkinson׳s disease. However, the function of NDUFV2 during brain development remains unclear. Here we reported that ndufv2 is expressed in the developing cerebral cortex. In utero suppression of ndufv2 arrested neuronal migration, leading to accumulation of ectopic neurons in the intermediate zone. ndufv2 inhibition did not affect radial glia scaffold, progenitor cells or neurons survival. However, the loss of ndufv2 impairs neuronal multipolar-bipolar transition in vivo and polarization in vitro. Moreover, ndufv2 affected actin cytoskeleton and tubulin stabilization in cortical neurons. Overall, our findings establish a new NDUFV2 dependent mechanism underlying neuronal migration and psychiatric disorders.

Lymphocyte-specific protein tyrosine kinase (Lck) interacts with CR6-interacting factor 1 (CRIF1) in mitochondria to repress oxidative phosphorylation

BACKGROUND

Many cancer cells exhibit reduced mitochondrial respiration as part of metabolic reprogramming to support tumor growth. Mitochondrial localization of several protein tyrosine kinases is linked to this characteristic metabolic shift in solid tumors, but remains largely unknown in blood cancer. Lymphocyte-specific protein tyrosine kinase (Lck) is a key T-cell kinase and widely implicated in blood malignancies. The purpose of our study is to determine whether and how Lck contributes to metabolic shift in T-cell leukemia through mitochondrial localization.

METHODS

We compared the human leukemic T-cell line Jurkat with its Lck-deficient derivative Jcam cell line. Differences in mitochondrial respiration were measured by the levels of mitochondrial membrane potential, oxygen consumption, and mitochondrial superoxide. Detailed mitochondrial structure was visualized by transmission electron microscopy. Lck localization was evaluated by subcellular fractionation and confocal microscopy. Proteomic analysis was performed to identify proteins co-precipitated with Lck in leukemic T-cells. Protein interaction was validated by biochemical co-precipitation and confocal microscopy, followed by in situ proximity ligation assay microscopy to confirm close-range (<16 nm) interaction.

RESULTS

Jurkat cells have abnormal mitochondrial structure and reduced levels of mitochondrial respiration, which is associated with the presence of mitochondrial Lck and lower levels of mitochondrion-encoded electron transport chain proteins. Proteomics identified CR6-interacting factor 1 (CRIF1) as the novel Lck-interacting protein. Lck association with CRIF1 in Jurkat mitochondria was confirmed biochemically and by microscopy, but did not lead to CRIF1 tyrosine phosphorylation. Consistent with the role of CRIF1 in functional mitoribosome, shRNA-mediated silencing of CRIF1 in Jcam resulted in mitochondrial dysfunction similar to that observed in Jurkat. Reduced interaction between CRIF1 and Tid1, another key component of intramitochondrial translational machinery, in Jurkat further supports the role of mitochondrial Lck as a negative regulator of CRIF1 through competitive binding.

CONCLUSIONS

This is the first report demonstrating the role of mitochondrial Lck in metabolic reprogramming of leukemic cells. Mechanistically, it is distinct from other reported mitochondrial protein tyrosine kinases. In a kinase-independent manner, mitochondrial Lck interferes with mitochondrial translational machinery through competitive binding to CRIF1. These findings may reveal novel approaches in cancer therapy by targeting cancer cell metabolism.

Sepsis-induced brain mitochondrial dysfunction is associated with altered mitochondrial Src and PTP1B levels

Sepsis-induced brain dysfunction (SIBD) is often the first manifestation of sepsis, and its pathogenesis is associated with mitochondrial dysfunction. In this study, we investigated the roles of the tyrosine kinase Src and protein tyrosine phosphatase 1B (PTP1B) in brain mitochondrial dysfunction using a rat model of lipopolysaccharide (LPS)-induced sepsis. We found that there was a gradual and significant increase of PTP1B levels in the rat brain after sepsis induction. In contrast, brain Src levels were reduced in parallel with the PTP1B increase. Sepsis led to significantly reduced tyrosine phosphorylation of mitochondrial oxidative phosphorylation (OXPHOS) complexes I, II and III. Pretreatment of mitochondrial proteins with active PTP1B significantly inhibited complexes I and III activities in vitro, whereas Src enhanced complexes I, II, and III activities. PTP1B and Src were each co-immunoprecipitated with OXPHOS complexes I and III, suggesting direct interactions between both proteins and complexes I and III. Src also directly interacted with complex II. Furthermore, pretreatment of mitochondrial proteins with active PTP1B resulted in overproduction of reactive oxygen species and decreased mitochondrial membrane potential. Pretreatment with active Src produced the opposite effect. These results suggest that brain mitochondrial dysfunction following LPS-induced sepsis in rats is partly attributed to PTP1B and Src mediated decrease in mitochondrial protein tyrosine phosphorylation.

Pathophysiological implications of mitochondrial oxidative stress mediated by mitochondriotropic agents and polyamines: the role of tyrosine phosphorylation

Mitochondria, once merely considered as the "powerhouse" of cells, as they generate more than 90 % of cellular ATP, are now known to play a central role in many metabolic processes, including oxidative stress and apoptosis. More than 40 known human diseases are the result of excessive production of reactive oxygen species (ROS), bioenergetic collapse and dysregulated apoptosis. Mitochondria are the main source of ROS in cells, due to the activity of the respiratory chain. In normal physiological conditions, ROS generation is limited by the anti-oxidant enzymatic systems in mitochondria. However, disregulation of the activity of these enzymes or interaction of respiratory complexes with mitochondriotropic agents may lead to a rise in ROS concentrations, resulting in oxidative stress, mitochondrial permeability transition (MPT) induction and triggering of the apoptotic pathway. ROS concentration is also increased by the activity of amine oxidases located inside and outside mitochondria, with oxidation of biogenic amines and polyamines. However, it should also be recalled that, depending on its concentration, the polyamine spermine can also protect against stress caused by ROS scavenging. In higher organisms, cell signaling pathways are the main regulators in energy production, since they act at the level of mitochondrial oxidative phosphorylation and participate in the induction of the MPT. Thus, respiratory complexes, ATP synthase and transition pore components are the targets of tyrosine kinases and phosphatases. Increased ROS may also regulate the tyrosine phosphorylation of target proteins by activating Src kinases or phosphatases, preventing or inducing a number of pathological states.

Weight loss by Ppc-1, a novel small molecule mitochondrial uncoupler derived from slime mold

Mitochondria play a key role in diverse processes including ATP synthesis and apoptosis. Mitochondrial function can be studied using inhibitors of respiration, and new agents are valuable for discovering novel mechanisms involved in mitochondrial regulation. Here, we screened small molecules derived from slime molds and other microorganisms for their effects on mitochondrial oxygen consumption. We identified Ppc-1 as a novel molecule which stimulates oxygen consumption without adverse effects on ATP production. The kinetic behavior of Ppc-1 suggests its function as a mitochondrial uncoupler. Serial administration of Ppc-1 into mice suppressed weight gain with no abnormal effects on liver or kidney tissues, and no evidence of tumor formation. Serum fatty acid levels were significantly elevated in mice treated with Ppc-1, while body fat content remained low. After a single administration, Ppc-1 distributes into various tissues of individual animals at low levels. Ppc-1 stimulates adipocytes in culture to release fatty acids, which might explain the elevated serum fatty acids in Ppc-1-treated mice. The results suggest that Ppc-1 is a unique mitochondrial regulator which will be a valuable tool for mitochondrial research as well as the development of new drugs to treat obesity.

Dual role of Src kinase in governing neuronal survival

BACKGROUND

Src-family kinases (SFKs) are involved in neuronal survival and their aberrant regulation contributes to neuronal death. However, how they control neuronal survival and death remains unclear.

OBJECTIVE

To define the effect of inhibition of Src activity and expression on neuronal survival.

RESULTS

In agreement with our previous findings, we demonstrated that Src was cleaved by calpain to form a 52-kDa truncated fragment in neurons undergoing excitotoxic cell death, and expression of the recombinant truncated Src fragment induced neuronal death. The data confirm that the neurotoxic signaling pathways are intact in the neurons we used for our study. To define the functional role of neuronal SFKs, we treated these neurons with SFK inhibitors and discovered that the treatment induced cell death, suggesting that the catalytic activity of one or more of the neuronal SFKs is critical to neuronal survival. Using small hairpin RNAs that suppress Src expression, we demonstrated that Src is indispensable to neuronal survival. Additionally, we found that neuronal death induced by expression of the neurotoxic truncated Src mutant, treatment of SFK inhibitors or knock-down of Src expression caused inhibition of the neuroprotective protein kinases Erk1/2, or Akt.

CONCLUSIONS

Src is critical to both neuronal survival and death. Intact Src sustains neuronal survival. However, in the excitotoxic condition, calpain cleavage of Src generates a neurotoxic truncated Src fragment. Both intact Src and the neurotoxic truncated Src fragment exert their biological actions by controlling the activities of neuroprotective protein kinases.

Phosphorylation of flotillin-1 by mitochondrial c-Src is required to prevent the production of reactive oxygen species

We have shown that mitochondrial c-Src regulates reactive oxygen species (ROS) production by phosphorylating the succinate dehydrogenase A of respiratory complex II (CxII). To elucidate the molecular mechanisms underlying ROS production regulated by c-Src in the CxII, we investigated the CxII protein complex derived from cells treated with Src family kinase inhibitor PP2. We identified flotillin-1 as a c-Src target that prevents ROS production from CxII. Phosphorylation-site analysis suggests Tyr56 and Tyr149 on flotillin-1 as sites for phosphorylation by c-Src. A comparison of cells expressing flotillin-1 and its phosphorylation defective mutants confirms the requirement for flotillin-1 phosphorylation for its interaction with CxII and subsequent reduction in ROS production. Our findings suggest a critical role of flotillin-1 in ROS production mediated by c-Src.

Redox circuitries driving Src regulation

SIGNIFICANCE

Here, we review recent advances with regard to the role of Src kinase in the regulation of cytoskeleton organization, cell adhesion, and motility, focusing on redox circuitries engaging this kinase for anchorage and motility, control of cell survival to anoikis, as well as metabolic deregulation, all features belonging to the new hallmarks of cancer.

RECENT ADVANCES

Several recent insights have reported that, alongside the well-known phosphorylation/dephosphorylation control, cysteine oxidation is a further mechanism of enzyme activation for both c-Src kinase and its oncogenic counterparts. Indeed, mounting evidence portrays redox regulation of Src kinase as a compulsory outcome in growth factors/cytokines signaling, integrin engagement, motility and invasiveness of tissues, receptor cross-talking at plasmamembrane, as well as during carcinogenesis and progression toward tumor malignancy or fibrotic disease. In addition, the kinase is an upstream regulator of NADPH oxidase-driven oxidants, a critical step for invadopodia formation and metastatic spread.

CRITICAL ISSUES

Not satisfactorily unraveled yet, the exact role of Src kinase in redox cancer biology needs to be implemented with studies that are aimed at clarifying (i) the exact hierarchy between oxidants sources, Src redox-dependent activation and the regulation of cell motility, and (ii) the actual susceptibility of invading cells to redox-based treatments, owing to the well-recognized ability of cancer cells to find new strategies to adapt to new environments.

FUTURE DIRECTIONS

Once these critical issues are addressed, redox circuitries involving Src kinase should potentially be used as both biomarkers and targets for personalized therapies in the fight against cancer or fibrotic diseases.

Metarhizin A suppresses cell proliferation by inhibiting cytochrome c oxidase activity

AIMS

Metarhizin A was originally isolated from Metarhizium flavoviride as a potent inhibitor of the growth of insect and mammalian cells. In this study, we aimed to understand the molecular targets of metarhizin A involved in its anti-proliferative activity against human cells.

MAIN METHODS

Cell cycle regulators and signaling molecules were examined by immunoblotting using specific antibodies. A mitochondria-enriched fraction was prepared from mouse liver, and mitochondrial activity was monitored using an oxygen electrode. Enzyme activity was measured using purified cytochrome c oxidase and permeabilized cells.

KEY FINDINGS

Metarhizin A inhibits the growth of MCF-7 cells with an IC50 value of ~0.2 μM and other cells in a similar manner; a cell cycle-dependent kinase inhibitor, p21, is selectively induced. Significant amounts of reactive oxygen species (ROS) are generated and ERK1/2 is activated in cells treated with metarhizin A. Metarhizin A completely suppresses oxygen consumption by mitochondria, and potently inhibits the activity of cytochrome c oxidase. It induces cell death when MCF-7 cells are cultured under limiting conditions.

SIGNIFICANCE

Metarhizin A is a potent inhibitor of cytochrome c oxidase and activates the MAPK pathway through the generation of ROS, which induces growth arrest of cells, and, under some conditions, enhances cell death. The cytochrome c oxidase system is a possible molecular target of metarhizin A.

The life of plant mitochondrial complex I

The mitochondrial NADH dehydrogenase complex (complex I) of the respiratory chain has several remarkable features in plants: (i) particularly many of its subunits are encoded by the mitochondrial genome, (ii) its mitochondrial transcripts undergo extensive maturation processes (e.g. RNA editing, trans-splicing), (iii) its assembly follows unique routes, (iv) it includes an additional functional domain which contains carbonic anhydrases and (v) it is, indirectly, involved in photosynthesis. Comprising about 50 distinct protein subunits, complex I of plants is very large. However, an even larger number of proteins are required to synthesize these subunits and assemble the enzyme complex. This review aims to follow the complete "life cycle" of plant complex I from various molecular perspectives. We provide arguments that complex I represents an ideal model system for studying the interplay of respiration and photosynthesis, the cooperation of mitochondria and the nucleus during organelle biogenesis and the evolution of the mitochondrial oxidative phosphorylation system.

The role of mitochondria in the oncogenic signal transduction

Mitochondria are intracellular organelles thought to have evolved from an alphaproteobacterium engulfed by the ancestor of the eukaryotic cell, an archeon, two billion years ago. Although mitochondria are frequently recognised as the "power plant" of the cell, the function of these organelles go beyond the simple generation of ATP. In fact, mounting evidence suggests that mitochondria are involved in several cellular processes, from regulation of cell death to signal transduction. Given this important role in cell physiology, mitochondrial dysfunction has been frequently associated with human diseases including cancer. Importantly, recent evidence suggests that mitochondrial function is directly regulated by oncogenes and tumour suppressors. However, the consequences of deregulation of mitochondrial function in tumour formation are still unclear. In this review, I propose that mitochondria play a pivotal role in shaping the oncogenic signalling cascade and that mitochondrial dysfunction, in some circumstances, is a required step for cancer transformation.

Unraveling the phosphoproteome dynamics in mammal mitochondria from a network perspective

With mitochondrion garnering more attention for its inextricable involvement in pathophysiological conditions, it seems imperative to understand the means by which the molecular pathways harbored in this organelle are regulated. Protein phosphorylation has been considered a central event in cellular signaling and, more recently, in the modulation of mitochondrial activity. Efforts have been made to understand the molecular mechanisms by which protein phosphorylation regulates mitochondrial signaling. With the advances in mass-spectrometry-based proteomics, there is a substantial hope and expectation in the increased knowledge of protein phosphorylation profile and its mode of regulation. On the basis of phosphorylation profiles, attempts have been made to disclose the kinases involved and how they control the molecular processes in mitochondria and, consequently, the cellular outcomes. Still, few studies have focused on mitochondrial phosphoproteome profiling, particularly in diseases. The present study reviews current data on protein phosphorylation profiling in mitochondria, the potential kinases involved and how pathophysiological conditions modulate the mitochondrial phosphoproteome. To integrate data from distinct research papers, we performed network analysis, with bioinformatic tools like Cytoscape, String, and PANTHER taking into consideration variables such as tissue specificity, biological processes, molecular functions, and pathophysiological conditions. For instance, data retrieved from these analyses evidence some homology in the mitochondrial phosphoproteome among liver and heart, with proteins from transport and oxidative phosphorylation clusters particularly susceptible to phosphorylation. A distinct profile was noticed for adipocytes, with proteins form metabolic processes, namely, triglycerides metabolism, as the main targets of phosphorylation. Regarding disease conditions, more phosphorylated proteins were observed in diabetics with some distinct phosphoproteins identified in type 2 prediabetic states and early type 2 diabetes mellitus. Heart-failure-related phosphorylated proteins are in much lower amount and are mainly involved in transport and metabolism. Nevertheless, technical considerations related to mitochondria isolation and protein separation should be considered in data comparison among different proteomic studies. Data from the present review will certainly open new perspectives of protein phosphorylation in mitochondria and will help to envisage future studies targeting the underlying regulatory mechanisms.

Cellular functions regulated by phosphorylation of EGFR on Tyr845

The Src gene product (Src) and the epidermal growth factor receptor (EGFR) are prototypes of oncogene products and function primarily as a cytoplasmic non-receptor tyrosine kinase and a transmembrane receptor tyrosine kinase, respectively. The identification of Src and EGFR, and the subsequent extensive investigations of these proteins have long provided cutting edge research in cancer and other molecular and cellular biological studies. In 1995, we reported that the human epidermoid carcinoma cells, A431, contain a small fraction of Src and EGFR in which these two kinase were in physical association with each other, and that Src phosphorylates EGFR on tyrosine 845 (Y845) in the Src-EGFR complex. Y845 of EGFR is located in the activation segment of the kinase domain, where many protein kinases contain kinase-activating autophosphorylation sites (e.g., cAMP-dependent protein kinase, Src family kinases, transmembrane receptor type tyrosine kinases) or trans-phosphorylation sites (e.g., cyclin-dependent protein kinase, mitogen-activated protein kinase, Akt protein kinase). A number of studies have demonstrated that Y845 phosphorylation serves an important role in cancer as well as normal cells. Here we compile the experimental facts involving Src phosphorylation of EGFR on Y845, by which cell proliferation, cell cycle control, mitochondrial regulation of cell metabolism, gamete activation and other cellular functions are regulated. We also discuss the physiological relevance, as well as structural insights of the Y845 phosphorylation.

Molecular chaperone TRAP1 regulates a metabolic switch between mitochondrial respiration and aerobic glycolysis

TRAP1 (TNF receptor-associated protein), a member of the HSP90 chaperone family, is found predominantly in mitochondria. TRAP1 is broadly considered to be an anticancer molecular target. However, current inhibitors cannot distinguish between HSP90 and TRAP1, making their utility as probes of TRAP1-specific function questionable. Some cancers express less TRAP1 than do their normal tissue counterparts, suggesting that TRAP1 function in mitochondria of normal and transformed cells is more complex than previously appreciated. We have used TRAP1-null cells and transient TRAP1 silencing/overexpression to show that TRAP1 regulates a metabolic switch between oxidative phosphorylation and aerobic glycolysis in immortalized mouse fibroblasts and in human tumor cells. TRAP1-deficiency promotes an increase in mitochondrial respiration and fatty acid oxidation, and in cellular accumulation of tricarboxylic acid cycle intermediates, ATP and reactive oxygen species. At the same time, glucose metabolism is suppressed. TRAP1-deficient cells also display strikingly enhanced invasiveness. TRAP1 interaction with and regulation of mitochondrial c-Src provide a mechanistic basis for these phenotypes. Taken together with the observation that TRAP1 expression is inversely correlated with tumor grade in several cancers, these data suggest that, in some settings, this mitochondrial molecular chaperone may act as a tumor suppressor.

Mitochondrial complex II and genomic imprinting in inheritance of paraganglioma tumors

Germ line heterozygous mutations in the structural subunit genes of mitochondrial complex II (succinate dehydrogenase; SDH) and the regulatory gene SDHAF2 predispose to paraganglioma tumors which show constitutive activation of hypoxia inducible pathways. Mutations in SDHD and SDHAF2 cause highly penetrant multifocal tumor development after a paternal transmission, whereas maternal transmission rarely, if ever, leads to tumor development. This transmission pattern is consistent with genomic imprinting. Recent molecular evidence supports a model for tissue-specific imprinted regulation of the SDHD gene by a long range epigenetic mechanism. In addition, there is evidence of SDHB mRNA editing in peripheral blood mononuclear cells and long-term balancing selection operating on the SDHA gene. Regulation of SDH subunit expression by diverse epigenetic mechanisms implicates a crucial dosage-dependent role for SDH in oxygen homeostasis. This article is part of a Special Issue entitled: Respiratory complex II: Role in cellular physiology and disease.