The dynamic localization of the glucocorticoid receptor in rat C6 glioma cell mitochondria

An updated compendium and reevaluation of the evidence for nuclear transcription factor occupancy over the mitochondrial genome

In most eukaryotes, mitochondrial organelles contain their own genome, usually circular, which is the remnant of the genome of the ancestral bacterial endosymbiont that gave rise to modern mitochondria. Mitochondrial genomes are dramatically reduced in their gene content due to the process of endosymbiotic gene transfer to the nucleus; as a result most mitochondrial proteins are encoded in the nucleus and imported into mitochondria. This includes the components of the dedicated mitochondrial transcription and replication systems and regulatory factors, which are entirely distinct from the information processing systems in the nucleus. However, since the 1990s several nuclear transcription factors have been reported to act in mitochondria, and previously we identified 8 human and 3 mouse transcription factors (TFs) with strong localized enrichment over the mitochondrial genome using ChIP-seq (Chromatin Immunoprecipitation) datasets from the second phase of the ENCODE (Encyclopedia of DNA Elements) Project Consortium. Here, we analyze the greatly expanded in the intervening decade ENCODE compendium of TF ChIP-seq datasets (a total of 6,153 ChIP experiments for 942 proteins, of which 763 are sequence-specific TFs) combined with interpretative deep learning models of TF occupancy to create a comprehensive compendium of nuclear TFs that show evidence of association with the mitochondrial genome. We find some evidence for chrM occupancy for 50 nuclear TFs and two other proteins, with bZIP TFs emerging as most likely to be playing a role in mitochondria. However, we also observe that in cases where the same TF has been assayed with multiple antibodies and ChIP protocols, evidence for its chrM occupancy is not always reproducible. In the light of these findings, we discuss the evidential criteria for establishing chrM occupancy and reevaluate the overall compendium of putative mitochondrial-acting nuclear TFs.

The mitochondrial Ahi1/GR participates the regulation on mtDNA copy numbers and brain ATP levels and modulates depressive behaviors in mice

BACKGROUND

Previous studies have shown that depression is often accompanied by an increase in mtDNA copy number and a decrease in ATP levels; however, the exact regulatory mechanisms remain unclear.

METHODS

In the present study, Western blot, cell knockdown, immunofluorescence, immunoprecipitation and ChIP-qPCR assays were used to detect changes in the Ahi1/GR-TFAM-mtDNA pathway in the brains of neuronal Abelson helper integration site-1 (Ahi1) KO mice and dexamethasone (Dex)-induced mice to elucidate the pathogenesis of depression. In addition, a rescue experiment was performed to determine the effects of regular exercise on the Ahi1/GR-TFAM-mtDNA-ATP pathway and depression-like behavior in Dex-induced mice and Ahi1 KO mice under stress.

RESULTS

In this study, we found that ATP levels decreased and mitochondrial DNA (mtDNA) copy numbers increased in depression-related brain regions in Dex-induced depressive mice and Ahi1 knockout (KO) mice. In addition, Ahi1 and glucocorticoid receptor (GR), two important proteins related to stress and depressive behaviors, were significantly decreased in the mitochondria under stress. Intriguingly, GR can bind to the D-loop control region of mitochondria and regulate mitochondrial replication and transcription. Importantly, regular exercise significantly increased mitochondrial Ahi1/GR levels and ATP levels and thus improved depression-like behaviors in Dex-induced depressive mice but not in Ahi1 KO mice under stress.

CONCLUSIONS

In summary, our findings demonstrated that the mitochondrial Ahi1/GR complex and TFAM coordinately regulate mtDNA copy numbers and brain ATP levels by binding to the D-loop region of mtDNA Regular exercise increases the levels of the mitochondrial Ahi1/GR complex and improves depressive behaviors. Video Abstract.

Corticosterone dynamically regulates retrotransposable element expression in the rat hippocampus and C6 cells

The hippocampus is a highly plastic brain region sensitive to environmental stress. It shows dynamic changes in epigenetic marks associated with stress related learning. Previous work has shown that acute stress induces substantial transient changes in histone H3 lysine 9 trimethylation (H3K9me3). Moreover, increased H3K9me3 is enriched in hippocampal gene deserts accumulating within endogenous retroviruses and transposable elements. We have found that in response to acute glucocorticoid treatment, a similar change in global H3K9me3 is observed. However, when localized we found that H3K9me3 is markedly decreased at B2 short interspersed nuclear elements but not within intracisternal-A particle endogenous retroviruses. Further, decreased H3K9me3 valence within B2 elements was associated with increased transcript abundance. These data demonstrate the capacity for acute glucocorticoids to mobilize transposable elements via epigenetic unmasking. Reconciled with previous findings following acute stress, this suggests the capacity for mobile elements to potentially function as novel regulators given their dynamic regulation by stress and glucocorticoids.

Mitochondrial Glucocorticoid Receptors and Their Actions

Mitochondria are membrane organelles present in almost all eukaryotic cells. In addition to their well-known role in energy production, mitochondria regulate central cellular processes, including calcium homeostasis, Reactive Oxygen Species (ROS) generation, cell death, thermogenesis, and biosynthesis of lipids, nucleic acids, and steroid hormones. Glucocorticoids (GCs) regulate the mitochondrially encoded oxidative phosphorylation gene expression and mitochondrial energy metabolism. The identification of Glucocorticoid Response Elements (GREs) in mitochondrial sequences and the detection of Glucocorticoid Receptor (GR) in mitochondria of different cell types gave support to hypothesis that mitochondrial GR directly regulates mitochondrial gene expression. Numerous studies have revealed changes in mitochondrial gene expression alongside with GR import/export in mitochondria, confirming the direct effects of GCs on mitochondrial genome. Further evidence has made clear that mitochondrial GR is involved in mitochondrial function and apoptosis-mediated processes, through interacting or altering the distribution of Bcl2 family members. Even though its exact translocation mechanisms remain unknown, data have shown that GR chaperones (Hsp70/90, Bag-1, FKBP51), the anti-apoptotic protein Bcl-2, the HDAC6- mediated deacetylation and the outer mitochondrial translocation complexes (Tom complexes) co-ordinate GR mitochondrial trafficking. A role of mitochondrial GR in stress and depression as well as in lung and hepatic inflammation has also been demonstrated.

Protein profiling identified mitochondrial dysfunction and synaptic abnormalities after dexamethasone intervention in rats with traumatic brain injury

Dexamethasone has been widely used after various neurosurgical procedures due to its anti-inflammatory property and the abilities to restore vascular permeability, inhibit free radicals, and reduce cerebrospinal fluid production. According to the latest guidelines for the treatment of traumatic brain injury in the United States, high-dose glucocorticoids cause neurological damage. To investigate the reason why high-dose glucocorticoids after traumatic brain injury exhibit harmful effect, rat controlled cortical impact models of traumatic brain injury were established. At 1 hour and 2 days after surgery, rat models were intraperitoneally administered dexamethasone 10 mg/kg. The results revealed that 31 proteins were significantly upregulated and 12 proteins were significantly downregulated in rat models of traumatic brain injury after dexamethasone treatment. The Ingenuity Pathway Analysis results showed that differentially expressed proteins were enriched in the mitochondrial dysfunction pathway and synaptogenesis signaling pathway. Western blot analysis and immunohistochemistry results showed that Ndufv2, Maob and Gria3 expression and positive cell count in the dexamethasone-treated group were significantly greater than those in the model group. These findings suggest that dexamethasone may promote a compensatory increase in complex I subunits (Ndufs2 and Ndufv2), increase the expression of mitochondrial enzyme Maob, and upregulate synaptic-transmission-related protein Gria3. These changes may be caused by nerve injury after traumatic brain injury treatment by dexamethasone. The study was approved by Institutional Ethics Committee of Beijing Neurosurgical Institute (approval No. 201802001) on June 6, 2018.

Bag-1 mediates glucocorticoid receptor trafficking to mitochondria after corticosterone stimulation: Potential role in regulating affective resilience

Molecular abnormalities within the Glucocorticoid Receptor (GR) stress signaling pathway involved in dysfunction of mitochondria and confer vulnerability to stress-related psychiatric disorders. Bcl-2 associated athanogene (Bag-1) is a target for the actions of mood stabilizers. Bag-1 interacts with GR, thereby regulating glucocorticoid function. In this study, we investigate the potential role of Bag-1 in regulating GR translocation into mitochondria. Corticosterone (CORT) treatment significantly enhanced Bag-1/GR complex formation and GR mitochondrial translocation in cultured rat cortical neurons after treatment for 30 min and 24 hr. By contrast, after stimulation with CORT for 3 days, localization of the Bag-1/GR complex and mitochondrial GR were reduced. Similar results were obtained in mice, in which administrated CORT in drinking water for 21 days significantly impaired the GR levels in the mitochondria, while Bag-1 over-expression rescued this reduction. Furthermore, chronic CORT exposure led to anhedonia-like and depression-like behaviors in the sucrose-consumption test and forced swimming test, and these behaviors were rescued by Bag-1 over-expression. These results suggest that Bag-1 mediates GR trafficking to mitochondria and regulates affective resilience in response to a CORT increase and provide potential insight into the mechanisms by which Bag-1 and GR could contribute to the physiology and pathogenesis of psychiatric disorders in response to the change of stress hormone.

Stress and glucocorticoid receptor regulation of mitochondrial gene expression

Glucocorticoids have long been recognized for their role in regulating the availability of energetic resources, particularly during stress. Furthermore, bidirectional connections between glucocorticoids and the physiology and function of mitochondria have been discovered over the years. However, the precise mechanisms by which glucocorticoids act on mitochondria have only recently been explored. Glucocorticoids appear to regulate mitochondrial transcription via activation of glucocorticoid receptors (GRs) with elevated circulating glucocorticoid levels following stress. While several mechanistic questions remain, GR and other nuclear transcription factors appear to have the capacity to substantially alter mitochondrial transcript abundance. The regulation of mitochondrial transcripts by stress and glucocorticoids will likely prove functionally relevant in many stress-sensitive tissues including the brain.

Glucocorticoid signaling and osteoarthritis

Glucocorticoids are steroid hormones synthesized and released by the adrenal cortex. Their main function is to maintain cell homeostasis through a variety of signaling pathways, responding to changes in an organism's environment or developmental status. Mimicking the actions of natural glucocorticoids, synthetic glucocorticoids have been recruited to treat many diseases that implicate glucocorticoid receptor signaling such as osteoarthritis. In osteoarthritis, synthetic glucocorticoids aim to alleviate inflammation and pain. The variation of patients' response and the possibility of complications associated with their long-term use have led to a need for a better understanding of glucocorticoid receptor signaling in osteoarthritis. In this review, we performed a literature search in the molecular pathways that link the osteoarthritic joint to the glucocorticoid receptor signaling. We hope that this information will advance research in the field and propose new molecular targets for the development of more optimized therapies for osteoarthritis.

The Multiple Levels of Mitonuclear Coregulation

Together, the nuclear and mitochondrial genomes encode the oxidative phosphorylation (OXPHOS) complexes that reside in the mitochondrial inner membrane and enable aerobic life. Mitochondria maintain their own genome that is expressed and regulated by factors distinct from their nuclear counterparts. For optimal function, the cell must ensure proper stoichiometric production of OXPHOS subunits by coordinating two physically separated and evolutionarily distinct gene expression systems. Here, we review our current understanding of mitonuclear coregulation primarily at the levels of transcription and translation. Additionally, we discuss other levels of coregulation that may exist but remain largely unexplored, including mRNA modification and stability and posttranslational protein degradation.

Mitochondrial-epigenetic crosstalk in environmental toxicology

Crosstalk between the nuclear epigenome and mitochondria, both in normal physiological function and in responses to environmental toxicant exposures, is a developing sub-field of interest in environmental and molecular toxicology. The majority (∼99%) of mitochondrial proteins are encoded in the nuclear genome, so programmed communication among nuclear, cytoplasmic, and mitochondrial compartments is essential for maintaining cellular health. In this review, we will focus on correlative and mechanistic evidence for direct impacts of each system on the other, discuss demonstrated or potential crosstalk in the context of chemical insult, and highlight biological research questions for future study. We will first review the two main signaling systems: nuclear signaling to the mitochondria [anterograde signaling], best described in regulation of oxidative phosphorylation (OXPHOS) and mitochondrial biogenesis in response to environmental signals received by the nucleus, and mitochondrial signals to the nucleus [retrograde signaling]. Both signaling systems can communicate intracellular energy needs or a need to compensate for dysfunction to maintain homeostasis, but both can also relay inappropriate signals in the presence of dysfunction in either system and contribute to adverse health outcomes. We will first review these two signaling systems and highlight known or biologically feasible epigenetic contributions to both, then briefly discuss the emerging field of epigenetic regulation of the mitochondrial genome, and finally discuss putative "crosstalk phenotypes", including biological phenomena, such as caloric restriction, maintenance of stemness, and circadian rhythm, and states of disease or loss of function, such as cancer and aging, in which both the nuclear epigenome and mitochondria are strongly implicated.

11β-HSD1 Modulates the Set Point of Brown Adipose Tissue Response to Glucocorticoids in Male Mice

Glucocorticoids (GCs) are potent regulators of energy metabolism. Chronic GC exposure suppresses brown adipose tissue (BAT) thermogenic capacity in mice, with evidence for a similar effect in humans. Intracellular GC levels are regulated by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) activity, which can amplify circulating GC concentrations. Therefore, 11β-HSD1 could modulate the impact of GCs on BAT function. This study investigated how 11β-HSD1 regulates the molecular architecture of BAT in the context of GC excess and aging. Circulating GC excess was induced in 11β-HSD1 knockout (KO) and wild-type mice by supplementing drinking water with 100 μg/mL corticosterone, and the effects on molecular markers of BAT function and mitochondrial activity were assessed. Brown adipocyte primary cultures were used to examine cell autonomous consequences of 11β-HSD1 deficiency. Molecular markers of BAT function were also examined in aged 11β-HSD1 KO mice to model lifetime GC exposure. BAT 11β-HSD1 expression and activity were elevated in response to GC excess and with aging. 11β-HSD1 KO BAT resisted the suppression of uncoupling protein 1 (UCP1) and mitochondrial respiratory chain subunit proteins normally imposed by GC excess. Furthermore, brown adipocytes from 11β-HSD1 KO mice had elevated basal mitochondrial function and were able to resist GC-mediated repression of activity. BAT from aged 11β-HSD1 KO mice showed elevated UCP1 protein and mitochondrial content, and a favorable profile of BAT function. These data reveal a novel mechanism in which increased 11β-HSD1 expression, in the context of GC excess and aging, impairs the molecular and metabolic function of BAT.

Zearalenone and 17 β-estradiol induced damages in male rats reproduction potential; evidence for ERα and ERβ receptors expression and steroidogenesis

The estrogen receptors (ERs)-dependent effects of Zearalenone (ZEA) on structure and function of the testis as well as sperm parameters were compared with 17-β estradiol as endogenous substance. For this purpose, 30 mature male rats were assigned into five groups as; control (appropriate volume of normal saline, i. p.), ZEA-received (1, 2 and 4 mg/kg, b. w., i. p.) and 17 β-estradiol (E2)-received (appropriate dose of 0.1 mg/kg, i. p.). Following 28 days, the mRNA levels of estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ) in the testis and sperms and the expression of them at protein levels in testicles were estimated. Mitochondrial content of germinal epithelium, Leydig cells steroid foci, sperm quality parameters and serum level of testosterone were assessed. Fluorescent techniques were used for analyzing apoptosis and mRNA damage in necrotic cells. ZEA reduced the mRNA and protein levels of ERα in testicles while up-regulated the ERβ expression. The mRNA level of ERα decreased in sperms of ZEA and E2-received animals. No remarkable changes were found for ERβ expression in sperms from ZEA and E2-received animals. ZEA reduced the Leydig cells steroidogenesis, mitochondrial content of germinal cells and elevated cellular apoptosis and necrosis dose-dependently. E2 reduced the testosterone concentration, enhanced the apoptosis and reduced sperm quality. Our data suggest that ZEA-induced detrimental effects in the structure and function of testis, may attribute to changing the ERs expression at mRNA and translational level.

The role of mitochondrial DNA copy number, variants, and haplotypes in farm animal developmental outcome

The vast majority of cellular energy is generated through the process of oxidative phosphorylation, which takes place in the electron transport chain in the mitochondria. The electron transport chain is encoded by 2 genomes, the chromosomal and the mitochondrial genomes. Mitochondrial DNA is associated with a number of traits, which include tolerance to heat, growth and physical performance, meat and milk quality, and fertility. Mitochondrial genomes can be clustered into groups known as mtDNA haplotypes. Mitochondrial DNA haplotypes are a potential genetic source for manipulating phenotypes in farm animals. The use of assisted reproductive technologies, such as nuclear transfer, allows favorable chromosomal genetic traits to be mixed and matched with sought after mtDNA haplotype traits. As a result super breeds can be generated.

Cellular respiration: the nexus of stress, condition, and ornamentation

A fundamental hypothesis for the evolution and maintenance of ornamental traits is that ornaments convey information to choosing females about the quality of prospective mates. A diverse array of ornaments (e.g., colors, morphological features, and behaviors) has been associated with a wide range of measures of individual quality, but decades of study of such indicator traits have failed to produce general mechanisms of honest signaling. Here, I propose that efficiency of cellular respiration, as a product of mitochondrial function, underlies the associations between ornamentation and performance for a broad range of traits across taxa. A large biomedical literature documents the fundamental biochemical links between oxidative phosphorylation (OXPHOS) and the production of reactive oxygen species (ROS), the process of metabolism, the function of the immune system, the synthesis of proteins, and the development and function of the nervous system. The production of virtually all ornaments whose expressions have been demonstrated to be condition-dependent is directly affected by the efficiency of cellular respiration, suggesting that the signaling of respiratory efficiency may be the primary function of such traits. Furthermore, the production of ornaments links to stress-response systems, including particularly the neuroendocrine system, through mitochondrial function, thereby makes ornamental traits effective signals of the capacity to withstand environmental perturbations. The identification of a unifying mechanism of honest signaling holds the potential to connect many heretofore-disparate fields of study related to stress and ornamentation, including neuroendocrinology, respiratory physiology, metabolic physiology, and immunology.

Changes in mitochondrial function are pivotal in neurodegenerative and psychiatric disorders: how important is BDNF?

The brain is at the very limit of its energy supply and has evolved specific means of adapting function to energy supply, of which mitochondria form a crucial link. Neurotrophic and inflammatory processes may not only have opposite effects on neuroplasticity, but also involve opposite effects on mitochondrial oxidative phosphorylation and glycolytic processes, respectively, modulated by stress and glucocorticoids, which also have marked effects on mood. Neurodegenerative processes show marked disorders in oxidative metabolism in key brain areas, sometimes decades before symptoms appear (Parkinson's and Alzheimer's diseases). We argue that brain-derived neurotrophic factor couples activity to changes in respiratory efficiency and these effects may be opposed by inflammatory cytokines, a key factor in neurodegenerative processes.

Evidence for site-specific occupancy of the mitochondrial genome by nuclear transcription factors

Mitochondria contain their own circular genome, with mitochondria-specific transcription and replication systems and corresponding regulatory proteins. All of these proteins are encoded in the nuclear genome and are post-translationally imported into mitochondria. In addition, several nuclear transcription factors have been reported to act in mitochondria, but there has been no comprehensive mapping of their occupancy patterns and it is not clear how many other factors may also be found in mitochondria. Here we address these questions by using ChIP-seq data from the ENCODE, mouseENCODE and modENCODE consortia for 151 human, 31 mouse and 35 C. elegans factors. We identified 8 human and 3 mouse transcription factors with strong localized enrichment over the mitochondrial genome that was usually associated with the corresponding recognition sequence motif. Notably, these sites of occupancy are often the sites with highest ChIP-seq signal intensity within both the nuclear and mitochondrial genomes and are thus best explained as true binding events to mitochondrial DNA, which exist in high copy number in each cell. We corroborated these findings by immunocytochemical staining evidence for mitochondrial localization. However, we were unable to find clear evidence for mitochondrial binding in ENCODE and other publicly available ChIP-seq data for most factors previously reported to localize there. As the first global analysis of nuclear transcription factors binding in mitochondria, this work opens the door to future studies that probe the functional significance of the phenomenon.

Glucocorticoid receptors modulate mitochondrial function: A novel mechanism for neuroprotection

It has become increasingly clear that glucocorticoid (GC) signaling not only comprises classic nuclear receptor binding-that is, glucocorticoid receptors (GRs) to their response element in the nucleus-but also involves rapid, non-genomic efforts to regulate signaling cascades and other cell functions in the cytoplasm as well as other cell organelles. In a recent study, we found that GRs form a complex with B-cell lymphoma 2 (Bcl-2), trans- locate to mitochondria in response to corticosterone (CORT), and modulate mitochondrial calcium and oxidation in an inverted U-shaped manner. It is also well established that steroid and thyroid hormone receptors regulate mitochondrial function to protect cells against various challenges and modulate synaptic plasticity. Here, we explore how such work reveals a fundamental mechanism whereby GCs regulate mitochondrial functions, and provides a mechanistic basis for therapeutic methods to rescue mitochondrial dysfunction during chronic stress or related psychiatric and neurodegenerative disorders.

Glucocorticoid receptor signaling in bone cells

Glucocorticoids are used for treating a wide range of diseases including inflammation and autoimmune disorders. However, there are drawbacks, primarily due to adverse effects on bone cells resulting in osteoporosis. Evidence indicates that the ratio of benefits to adverse effects depends greatly on glucocorticoid receptor (GR)-mediated mechanisms. Delineating GR-mediated signaling in bone cells will allow development of selective GR ligands/agonists (SEGRAs), which would dissociate the positive therapeutic (anti-inflammatory) effects from the negative effects on the skeleton. The present review provides an in-depth account of the current knowledge of GR-mediated transcriptional regulation of specific genes and proteins engaged in the proliferation, differentiation, and apoptosis of bone cells (osteoblasts, osteocytes, osteoclasts). We hope this knowledge will advance research in the development of SEGRAs with improved benefit/risk ratios.

Macromolecular synthesis inhibitors perturb glucocorticoid receptor trafficking

The ability of inhibitors of transcription and translation to prevent glucocorticoid-induced apoptosis has been interpreted to indicate that the cell death machinery requires de novo protein synthesis. The transcriptional inhibitors actinomycin D (Act D) and DRB as well as the translational inhibitors CHX and puromycin inhibited early loss of mitochondrial membrane integrity in a dose-dependent manner. This effect was not observed with the transcriptional inhibitor α-amanitin suggesting they may have additional effects. Their role in the glucocorticoid receptor (GR) intracellular trafficking was therefore investigated. Here, we show that Act D and CHX reduced glucocorticoid binding, GR turnover and impaired GR nuclear translocation. We performed the same experiments in different thymocyte subpopulations of Balb/c mice. At the highest dose tested, actinomycin D and cycloheximide abolished glucocorticoid-induced cell death of CD4+CD8+ and CD4+CD8-. In all subsets, Act D, DRB, as well as CHX and puromycin prevented receptor nuclear translocation, indicating a general alteration of GR trafficking. Overall, our data support a direct effect of macromolecular inhibitors on GR activation and trafficking. Finally, direct alterations of the functional properties of the glucocorticoid receptor might be responsible for cell death prevention by actinomycin D, DRB, cycloheximide and puromycin.

Nuclear transcription factors in mammalian mitochondria

Nuclear transcription factors have been detected in mammalian mitochondria and may directly regulate mitochondrial gene expression. Emerging genomics techniques may overcome outstanding challenges in this field.

Manganese superoxide dismutase vs. p53: regulation of mitochondrial ROS

Coordination of mitochondrial and nuclear activities is vital for cellular homeostasis, and many signaling molecules and transcription factors are regulated by mitochondria-derived reactive oxygen species (ROS) to carry out this interorganellar communication. The tumor suppressor p53 regulates myriad cellular functions through transcription-dependent and -independent mechanisms at both the nucleus and mitochondria. p53 affect mitochondrial ROS production, in part, by regulating the expression of the mitochondrial antioxidant enzyme manganese superoxide dismutase (MnSOD). Recent evidence suggests mitochondrial regulation of p53 activity through mechanisms that affect ROS production, and a breakdown of communication amongst mitochondria, p53, and the nucleus can have broad implications in disease development.

Ecdysteroid receptor (EcR) is associated with microtubules and with mitochondria in the cytoplasm of prothoracic gland cells of Rhodnius prolixus (Hemiptera)

We have shown previously that EcR in larval Rhodnius is present in the cytoplasm of various cell types and undergoes daily cycling in abundance in the cytoplasm (Vafopoulou and Steel, 2006. Cell Tissue Res 323:443-455). It is unknown which organelles are associated with EcR. Here, we report that cytoplasmic EcR in prothoracic gland cells is associated with both microtubules and mitochondria, and discuss the implications for both nuclear and non-genomic actions of EcR. EcR was localized immunohistochemically using several antibodies to EcR of Manduca and Drosophila and a confocal laser scanning microscope. Double labels were made to visualize EcR and (1) microtubules (using an antibody to tyrosylated alpha-tubulin) and (2) mitochondria (using a fluorescent MitoTracker probe), both after stabilization of microtubules with taxol. EcR co-localized with both tubulin and mitochondria. All the different EcR antibodies produced similar co-localization patterns. EcR was seen in the perinuclear aggregation of mitochondria, indicating that mitochondria are targets of ecdysone, which could influence mitochondrial gene transcription. EcR was also distributed throughout the microtubule network. Co-localization of EcR with tubulin or mitochondria was maintained after depolymerization of microtubules with colchicine. Treatment with taxol resulted in accumulation of EcR in the cytoplasm and simultaneous depletion of EcR from the nucleus, suggesting that microtubules may be involved in targeted intracellular transport of EcR to the nucleus (genomic action) or may play a role in rapid ecdysone signal transduction in the extranuclear compartment, i.e., in non-genomic actions of ecdysone. These findings align EcR more closely with steroid hormone receptors in vertebrates.

The role of phosphorylated glucocorticoid receptor in mitochondrial functions and apoptotic signalling in brain tissue of stressed Wistar rats

Mitochondrial dysfunction is increasingly recognized as a key component in compromised neuroendocrine stress response and, among other etiological causes, it may also involve action of glucocorticoid hormones. In the current study we followed glucocorticoid receptor and identified its mitochondrial phosphoisophorms in hippocampus and prefrontal brain cortex of Wistar male rats subjected to acute, chronic and combined neuroendocrine stresses. In both brain structures chronic social isolation caused marked increase in mitochondrial glucocorticoid receptor that was preferentially phosphorylated at serine 232 compared to serine 246 or serine 171. This increase corresponded with the decreased expression of mitochondrially encoded cytochrome oxidase subunits 1 and 3 in hippocampus, and with their increased expression in prefrontal brain cortex. Prefrontal brain cortex appeared to be more sensitive to chronic stress, since it exibited higher levels of mitochondrial Bax and cytoplasmic Bcl2 compared to hippocampus. Chronic stress also altered the response of both brain structures to subsequent acute stress according to the studied parameters. Therefore, prolonged social isolation may cause susceptibility to mitochondria triggered proapototic signalling, which at least in part may be mediated by the glucocorticoid receptor dependent mechanism.

Dynamic regulation of mitochondrial function by glucocorticoids

Glucocorticoids play an important biphasic role in modulating neural plasticity; low doses enhance neural plasticity and spatial memory behavior, whereas chronic, higher doses produce inhibition. We found that 3 independent measures of mitochondrial function-mitochondrial oxidation, membrane potential, and mitochondrial calcium holding capacity-were regulated by long-term corticosterone (CORT) treatment in an inverted "U"-shape. This regulation of mitochondrial function by CORT correlated with neuroprotection; that is, treatment with low doses of CORT had a neuroprotective effect, whereas treatment with high doses of CORT enhanced kainic acid (KA)-induced toxicity of cortical neurons. We then undertook experiments to elucidate the mechanisms underlying these biphasic effects and found that glucocorticoid receptors (GRs) formed a complex with the anti-apoptotic protein Bcl-2 in response to CORT treatment and translocated with Bcl-2 into mitochondria after acute treatment with low or high doses of CORT in primary cortical neurons. However, after 3 days of treatment, high, but not low, doses of CORT resulted in decreased GR and Bcl-2 levels in mitochondria. As with the in vitro studies, Bcl-2 levels in the mitochondria of the prefrontal cortex were significantly decreased, along with GR levels, after long-term treatment with high-dose CORT in vivo. These findings have the potential to contribute to a more complete understanding of the mechanisms by which glucocorticoids and chronic stress regulate cellular plasticity and resilience and to inform the future development of improved therapeutics.

Mechanisms regulating the susceptibility of hematopoietic malignancies to glucocorticoid-induced apoptosis

Glucocorticoids (GCs) are commonly used in the treatment of hematopoietic malignancies owing to their ability to induce apoptosis of these cancerous cells. Whereas some types of lymphoma and leukemia respond well to this drug, others are resistant. Also, GC-resistance gradually develops upon repeated treatments ultimately leading to refractory relapsed disease. Understanding the mechanisms regulating GC-induced apoptosis is therefore uttermost important for designing novel treatment strategies that overcome GC-resistance. This review discusses updated data describing the complex regulation of the cell's susceptibility to apoptosis triggered by GCs. We address both the genomic and nongenomic effects involved in promoting the apoptotic signals as well as the resistance mechanisms opposing these signals. Eventually we address potential strategies of clinical relevance that sensitize GC-resistant lymphoma and leukemia cells to this drug. The major target is the nongenomic signal transduction machinery where the interplay between protein kinases determines the cell fate. Shifting the balance of the kinome towards a state where Glycogen synthase kinase 3alpha (GSK3alpha) is kept active, favors an apoptotic response. Accumulating data show that it is possible to therapeutically modulate GC-resistance in patients, thereby improving the response to GC therapy.

Bimodal protein targeting through activation of cryptic mitochondrial targeting signals by an inducible cytosolic endoprotease

Bimodal targeting of the endoplasmic reticular protein, cytochrome P4501A1 (CYP1A1), to mitochondria involves activation of a cryptic mitochondrial targeting signal through endoprotease processing of the protein. Here, we characterized the endoprotease that regulates mitochondrial targeting of CYP1A1. The endoprotease, which was induced by beta-naphthoflavone, was a dimer of 90 kDa and 40 kDa subunits, each containing Ser protease domains. The purified protease processed CYP1A1 in a sequence-specific manner, leading to its mitochondrial import. The glucocorticoid receptor, retinoid X receptor, and p53 underwent similar processing-coupled mitochondrial transport. The inducible 90 kDa subunit was a limiting factor in many cells and some tissues and, thus, regulates the mitochondrial levels of these proteins. A number of other mitochondria-associated proteins with noncanonical targeting signals may also be substrates of this endoprotease. Our results describe a new mechanism of mitochondrial protein import that requires an inducible cytoplasmic endoprotease for activation of cryptic mitochondrial targeting signals.

Steroid and thyroid hormone receptors in mitochondria

Receptors for glucocorticoids, estrogens, androgens, and thyroid hormones have been detected in mitochondria of various cell types by Western blotting, immunofluorescence labeling, confocal microscopy, and immunogold electron microscopy. A role of these receptors in mitochondrial transcription, OXPHOS biosynthesis, and apoptosis is now being revealed. Steroid and thyroid hormones regulate energy production, inducing nuclear and mitochondrial OXPHOS genes by way of cognate receptors. In addition to the action of the nuclearly localized receptors on nuclear OXPHOS gene transcription, a parallel direct action of the mitochondrially localized receptors on mitochondrial transcription has been demonstrated. The coordination of transcription activation in nuclei and mitochondria by the respective receptors is in part realized by their binding to common trans acting elements in the two genomes. Recent evidence points to a role of the mitochondrial receptors in cell survival and apoptosis, exerted by genomic and nongenomic mechanisms. The identification of additional receptors of the superfamily of nuclear receptors and of other nuclear transcription factors in mitochondria increases their arsenal of regulatory molecules and further underlines the central role of these organelles in the integration of growth, metabolic, and cell survival signals.

Approaches to mitochondrially mediated cancer therapy

For some malignant cancers even combined surgical, radiotherapeutic and chemotherapeutic approaches are not curative, indeed, in certain tumour types even a modest survival benefit is difficult to achieve. There are various biological reasons which underlie this profound resistance but the propensity of cancer cells to repair breaks caused by DNA-damaging radiation and cytotoxic drugs is of major significance in this context. Such highly resistant tumours include the malignant gliomas which are intrinsic to and directly affect the brain and spinal cord. In evaluating approaches which do not elicit tumour cell death directly by DNA damage, it is intriguing to consider mitochondrially mediated apoptosis as a potentially effective alternative. Since the mitochondrial membrane potentials in cancer cells are frequently reduced in comparison with those of non-neoplastic cells this allows a window of opportunity for small molecule agents to enter the tumour cell mitochondria and reduce oxygen consumption with subsequent release of cytochrome c and activation of a caspase pathway to apoptosis which is cancer cell specific. In the quest for agents which can selectively destroy neoplastic cells in this manner, whilst leaving normal adjacent cells intact, various tricyclic drugs have come under scrutiny. In a range of laboratory assays we, and others, have established that certain cancers and, in particular, malignant glioma, are intrinsically sensitive to this approach. We have also established the cellular, molecular and biochemical mechanisms underlying this process. While such archival tricyclics as the antidepressants, clomipramine and amitriptyline, have been used in these experiments their commercial development in cancer therapy has not been forthcoming and their clinical use in glioma has been confined to anecdotal cases. In addition, the dose-dependant role of agents such as anticonvulsants and steroids commonly used in glioma patients in modulating efficacy of the tricyclics is a matter for continued investigation. Other ways of targeting the mitochondrion for cancer therapy include exploitation of the 18kDa translocator protein (peripheral-type benzodiazepine receptor) within the mitochondrial permeability transition pore and enzymatic or molecular modification of a species of ganglioside (GD3/GD3(A)) expressed on the surface of neoplastic cells which are determinants of mitochondrially mediated apoptosis. It is hoped that such approaches may lead to clinical programmes which will improve the prognosis for patients suffering from highly resistant neoplasms.

The glucocorticoid receptor signalling in breast cancer

Glucocorticoids (GCs) are provided as co-medication with chemotherapy in breast cancer, albeit several lines of evidence indicate that their use may have diverse effects and in fact may inhibit chemosensitivity. The molecular basis of GC-induced resistance to chemotherapy in breast cancer remains poorly defined. Recent researchers, in an attempt to clarify some aspects of the underlying pathways, provide convincing evidence that GCs induce effects that are dependent upon the glucocorticoid-receptor (GR)-mediated transcriptional regulation of specific genes known to play key roles in cellular/tissue functions, including growth, apoptosis, differentiation, metastasis and cell survival. In this review, we focus on how GC-induced chemoresistance in breast cancer is mediated by the GR, unravelling the molecular interplay of GR signalling with other signalling cascades prevalent in breast cancer. We also include a detailed description of GR structure and function, summarizing data gained during recent years into the mechanism(s) of the cross-talk between the GR and other signalling cascades and secondary messengers, via which GCs exert their pleiotropic effects.

Nuclear receptors and other nuclear transcription factors in mitochondria: regulatory molecules in a new environment

The mitochondrion is the major energy generating organelle of the cell and the site of other basic processes, including apoptosis. The mitochondrial functions are performed in concert with other cell compartments and are regulated by various extracellular and intracellular signals. Several nuclear receptors and other nuclear transcription factors, such as NF-kappa B, AP-1, CREB and p53, involved in growth, metabolic and developmental processes, have been detected in mitochondria. This finding raises the question as to the role of these regulatory molecules in their "new" environment. Experimental evidence supports the action of the mitochondrially localized transcription factors on mitochondrial transcription, energy yield and apoptosis, extending the known nuclear role of these molecules outside the nucleus. A principle of coordination of nuclear and mitochondrial gene transcription has been ascertained as regards the regulatory action of steroid and thyroid hormones on energy yield. Accordingly, the same nuclear receptors, localized in the two compartments-nuclei and mitochondria-regulate transcription of genes serving a common function by way of interaction with common binding sites in the two genomes. This principle is now expanding to encompass other nuclearly and mitochondrially localized transcription factors.

The Mitochondrion as a Primary Site of Action of Regulatory Agents Involved in Neuroimmunomodulation

A major system of neuroimmunomodulation is the hypothalamic-pituitary-adrenocortical (HPA) axis, acting through glucocorticoids and their intracellular signaling components, exerting both stimulatory and inhibitory effects on the immune reaction. Glucocorticoids inhibit the production of proinflammatory cytokines by interacting with nuclear transcription factors (nuclear factor [NF]-kappaB, activated protein [AP]-1) and induce the production of several anti-inflammatory cytokines by gene activation. In some cells and/or in extreme stress conditions, apoptosis is evoked. In most processes related to neuroimmunomodulation a prominent role is emerging for mitochondria. These organelles generate more than 90% of the cell's energy requirements through oxidative phosphorylation (OXPHOS), which is regulated by several agents, including steroid and thyroid hormones. These hormones are inducers of nuclear and mitochondrial OXPHOS gene transcription and they exert a primary action not only on nuclear but also on mitochondrial genes by way of cognate receptors. Recently, additional nuclear transcription factors involved in neuroimmunomodulation have been detected in mitochondria (NF-kappaB, AP-1, p53, calcium/cAMP response element binding protein [CREB]), and binding sites of these and putative binding sites of other nuclear transcription factors have been identified in the mitochondrial genome. The interaction of these factors with mitochondrial regulatory proteins, with receptors and with the genome has been shown and, in some cases, modulation of mitochondrial transcription was observed with possible effects on energy yield. The mitochondria store a host of critical apoptotic activators and inhibitors in their intermembrane space and the release of these factors could be another possible mode of action of the mitochondrially translocated regulatory agents and receptors.

Differential subcellular distribution of estrogen receptor isoforms: localization of ERalpha in the nucleoli and ERbeta in the mitochondria of human osteosarcoma SaOS-2 and hepatocarcinoma HepG2 cell lines

The localization of estrogen receptors alpha (ERalpha) and beta (ERbeta) in osteosarcoma SaOS-2 and hepatocarcinoma HepG2 cells was studied by immunofluorescence labelling and confocal laser scanning microscopy, as well as by subcellular fractionation and immunoblotting of the proteins of the fractions with respective antibodies. In both cell types, ERalpha was localized mainly in the nucleus, particularly concentrated on nuclear structures, which on the basis of their staining with pyronin and with antibodies against the nucleoli-specific Ki67 antigen and C23-nucleolin, were characterized as nucleoli. A faint, diffuse ERalpha staining was also observed in the cytoplasm. ERbeta was specifically enriched at the site of the mitochondria, visualized by labelling with the vital dye CMX and antibody against the mitochondrial-specific cytochrome oxidase subunit I. Immunoblotting experiments corroborated the immunofluorescence labelling distribution of ERalpha and ERbeta. These findings support the concept of a direct action of steroid/thyroid hormones on mitochondrial functions by way of their cognate receptors and also suggest a direct involvement of ERalpha in nucleolar-related processes.

Intracellular localization and nucleocytoplasmic trafficking of steroid receptors: an overview

Subcellular compartmentalization and dynamic movements of steroid receptors are major steps in executing their transcription regulatory function. Though significant progress has been made in understanding the mechanisms underlying nuclear import of NLS-bearing proteins, our general and mechanistic understanding about the nuclear export processes has begun to emerge only recently. The discovery of most commonly utilized CRM1/exportin1 dependent nuclear export pathway is attributed to a potent nuclear export inhibitor leptomycin B that helped dissecting this and other nuclear export pathways. Simultaneously, utilization of green fluorescent protein (GFP)-tagged intracellular steroid receptors has contributed to not only resolving controversial issue of subcellular localization of unliganded hormone receptors but also provided further insight into finer details of receptor dynamics in living cells. With judicious use of leptomycin B and expression of GFP-tagged receptors in living cells, existence of exportin1/CRM1 independent pathway(s), nuclear export signals and receptors for bi-directional translocation that are unique to steroid receptor trafficking have been specified. Currently, we appear to be arriving at a consensus that steroid/nuclear receptors follow dynamic nucleocytoplasmic processes that deviate from the ones commonly utilized by majority of other proteins.

The mitochondrion as a primary site of action of steroid and thyroid hormones: presence and action of steroid and thyroid hormone receptors in mitochondria of animal cells

Mitochondria are key cellular organelles that regulate events related to energy production and apoptosis. These processes are modulated, in turn, by steroid and thyroid hormones in the course of their actions on metabolism, growth and development. In this context, a direct effect of these hormones on the mitochondrial-linked processes, possibly by way of cognate mitochondrial receptors, has been proposed. In this paper we review data from the literature and present new findings supporting this concept. Receptors for steroid hormones, glucocorticoids and estrogens, and for T(3), have been detected in mitochondria by immunofluorescence labeling and confocal laser microscopy, by Western blotting of mitochondrial proteins and by immunogold electron microscopy. Furthermore, the mitochondrial genome contains nucleotide sequences with high similarity to known hormone-responsive elements, which interact with the appropriate receptors to confer hormone-dependent activation of reporter genes in transfection experiments. Thus, thyroid hormone stimulates mitochondrial transcription mediated by the cognate receptor when added to an in organello mitochondrial system, capable of faithful transcription.

Role of mitochondrial glucocorticoid receptor in glucocorticoid-induced apoptosis

The mechanisms by which glucocorticoid receptor (GR) mediates glucocorticoid (GC)-induced apoptosis are unknown. We studied the role of mitochondrial GR in this process. Dexamethasone induces GR translocation to the mitochondria in GC-sensitive, but not in GC-resistant, T cell lines. In contrast, nuclear GR translocation occurs in all cell types. Thymic epithelial cells, which cause apoptosis of the PD1.6 T cell line in a GR-dependent manner, induce GR translocation to the mitochondria, but not to the nucleus, suggesting a role for mitochondrial GR in eliciting apoptosis. This hypothesis is corroborated by the finding that a GR variant exclusively expressed in the mitochondria elicits apoptosis of several cancer cell lines. A putative mitochondrial localization signal was defined to amino acids 558-580 of human GR, which lies within the NH2-terminal part of the ligand-binding domain. Altogether, our data show that mitochondrial and nuclear translocations of GR are differentially regulated, and that mitochondrial GR translocation correlates with susceptibility to GC-induced apoptosis.

Estrogen receptors alpha and beta (ERalpha and ERbeta) and androgen receptor (AR) in human sperm: localization of ERbeta and AR in mitochondria of the midpiece

BACKGROUND

The central role of estrogens and androgens in the male reproductive system has focused attention on the presence and distribution of their cognate receptors [estrogen receptor (ER) alpha, ERbeta and androgen receptor (AR)] in male reproductive tissues and cells. Since the presence of steroid hormone receptors in mitochondria of mammalian cells has been well documented, we investigated the possibility of mitochondrial localization of sex steroid hormone receptors in sperm.

METHODS AND RESULTS

Applying immunofluorescence labelling and confocal laser scanning microscopy we show that the estrogen receptor beta and the AR of human sperm are specifically enriched in the midpiece, at the site of the mitochondria, which were visualized by labelling with the vital dye CMX. Nuclear and mitochondrial localization of AR was also detected in LnCap human prostate cancer cells. Differentially, most of the ERalpha immunostaining is in the form of a compact zone at a region corresponding to the equatorial segment of the upper post-acrosomal region of the sperm head. Immunoblotting experiments using sperm extracts revealed the presence of a 66 and a 45 kDa protein reacting with the ERalpha antibody, one 64 kDa protein reacting with the ERbeta antibody and a 110 and a 90 kDa protein reacting with the antibody against AR.

CONCLUSIONS

Our findings suggest that the differential localization of AR and ER isoforms in human sperm reveals distinct roles of these receptors in the physiology of sperm cells and, perhaps, also in the process of fertilization.

Glucocorticoid receptor isoforms in human hepatocarcinoma HepG2 and SaOS-2 osteosarcoma cells: presence of glucocorticoid receptor alpha in mitochondria and of glucocorticoid receptor beta in nucleoli

In the context of a possible direct action of glucocorticosteroids on mitochondrial transcription and/or apoptosis by way of cognate mitochondrial receptors, the possible localization of glucocorticoid receptors alpha and beta (GRalpha and GRbeta) in mitochondria was explored in human hepatocarcinoma HepG2 and osteosarcoma SaOS-2 cells, in which glucocorticoids exert an anabolic and apoptotic effect, respectively. In both cell types, GRalpha was detected in mitochondria, in nuclei and in cytosol by immunofluorescence labeling and confocal scanning microscopy, by immunogold electron microscopy and by Western blotting. GRbeta was shown to be almost exclusively restricted to the nucleus of the two cell types, being particularly concentrated in nucleoli, pointing to a solely nuclear role of this receptor isoform and to a possible function in nucleoli related processes. Computer analysis identified a putative internal mitochondrial targeting sequence within the glucocorticoid receptor. The demonstration of mitochondrially localized GRalpha in HepG2 and SaOS-2 cells corroborates previous findings in other cell types and further supports a direct role of this receptor in mitochondrial functions.

Regulation of corticosteroid receptors in the rat brain: the role of serotonin and stress

It has been suggested that physiological resistance to repeated stress is associated with increased 5-hydroxytryptamine (5-HT) release in the dorsal hippocampus and that dysregulation of this neuroadaptation may be implicated in the psychopathology of depression. This study used 5,7-dihydroxytryptamine lesions to investigate the role of 5-HT projections to the hippocampus in physiological responses to repeated stress and putative changes in corticosteroid receptor immunoreactivity in the brain. Repeated exposure to elevated open platform stress (1 h/day) caused regionally selective changes in glucocorticoid and mineralocorticoid receptor immunoreactivity in the dorsal hippocampus that were not observed in ventral hippocampus, frontal cortex, hypothalamus or parietal cortex. Glucocorticoid receptor immunoreactivity in the dorsal hippocampus was decreased after 5 days but increased after 20 days of stress. Mineralocorticoid receptor immunoreactivity was increased after 5 or 10 days of stress. The increases in glucocorticoid and mineralocorticoid receptor immunoreactivity, evoked by repeated stress, were abolished by lesions of the principal 5-HT projections to the hippocampus. The lesions abolished the increased defecation observed in stressed animals, but had no effects on the plasma corticosterone response to the stressor or the habituation of this response observed following repeated stress. The experiments have revealed a dissociation in the regulation of corticosteroid receptor expression in the dorsal and ventral hippocampus by repeated stress and 5-HT. The data suggest that adaptation to inescapable stress is associated with regionally selective changes in corticosteroid receptor expression in dorsal hippocampus that are largely 5-HT-dependent, although these changes do not mediate habituation of the pituitary adrenocortical response to the stressor.