The import of the transcription factor STAT3 into mitochondria depends on GRIM-19, a component of the electron transport chain

T helper cell dysregulation with hepatitis B and rebalance with glucocorticoids

AIM: To investigate T helper 17/regulatory T cell alterations in early severe hepatitis B and the effect of glucocorticoids.

METHODS: The study included 20 patients in the early stage of severe hepatitis B (SHB) and 11 healthy controls. All patients had elevated T helper 17 (Th17) levels, decreased regulatory T (Treg) cell levels, and significant Th17/Treg ratios.

RESULTS: After glucocorticoid treatment, 16 patients showed improvement with significant decreases in Th17 levels, increases in Treg, and rebalanced Th17/Treg ratios. The four patients who showed no improvement had increases in both Th17 and Treg levels and an even higher Th17/Treg ratio than before.

CONCLUSION: Glucocorticoid treatment can rectify Th17/Treg dysregulation in patients with SHB.

Stat3: friend or foe in colitis and colitis-associated cancer?

Chronic inflammation predisposes tissue to cancer development. Individuals afflicted with inflammatory bowel diseases are at an increased risk of developing colorectal cancer depending on disease severity, duration, and management. The intestinal epithelium exhibits mitochondrial dysfunction during colitis and colitis-associated cancer. Signal Transducer and Activator of Transcription (Stat)-3 is a transcription factor involved in growth-promoting and antiapoptotic signaling pathways. In addition to its activities as a transcription factor, Stat3 resides in the mitochondria of cells where it is required for optimal electron transport chain activity and protects against stress-induced mitochondrial dysfunction. The function of mitochondrial Stat3 is not completely understood; dichotomous roles include protecting against cellular injury but also supporting malignant transformation. This review discusses the roles of Stat3 in the regulation of intestinal epithelial cell fate during colitis and colorectal cancer with an emphasis on mitochondrial dysfunction and the potential involvement of mitochondrial Stat3 during disease progression.

Methionine sulfoxide reductase 2 reversibly regulates Mge1, a cochaperone of mitochondrial Hsp70, during oxidative stress

Methionine sulfoxide reductases are important regulators of oxidative stress, as they reduce oxidized methionine in proteins. Mge1, a cochaperone of mtHsp70, is a physiological substrate of Mxr2 and regulates reversibly to maintain mitochondrial protein homeostasis and oxidative stress.

Peptide methionine sulfoxide reductases are conserved enzymes that reduce oxidized methionines in protein(s). Although these reductases have been implicated in several human diseases, there is a dearth of information on the identity of their physiological substrates. By using Saccharomyces cerevisiae as a model, we show that of the two methionine sulfoxide reductases (MXR1, MXR2), deletion of mitochondrial MXR2 renders yeast cells more sensitive to oxidative stress than the cytosolic MXR1. Our earlier studies showed that Mge1, an evolutionarily conserved nucleotide exchange factor of Hsp70, acts as an oxidative sensor to regulate mitochondrial Hsp70. In the present study, we show that Mxr2 regulates Mge1 by selectively reducing MetO at position 155 and restores the activity of Mge1 both in vitro and in vivo. Mge1 M155L mutant rescues the slow-growth phenotype and aggregation of proteins of mxr2Δ strain during oxidative stress. By identifying the first mitochondrial substrate for Mxrs, we add a new paradigm to the regulation of the oxidative stress response pathway.

Peroxiredoxin-2 and STAT3 form a redox relay for H2O2 signaling

Hydrogen peroxide (H(2)O(2)) acts as a signaling messenger by oxidatively modifying distinct cysteinyl thiols in distinct target proteins. However, it remains unclear how redox-regulated proteins, which often have low intrinsic reactivity towards H(2)O(2) (k(app) ∼1-10 M(-1) s(-1)), can be specifically and efficiently oxidized by H(2)O(2). Moreover, cellular thiol peroxidases, which are highly abundant and efficient H(2)O(2) scavengers, should effectively eliminate virtually all of the H(2)O(2) produced in the cell. Here, we show that the thiol peroxidase peroxiredoxin-2 (Prx2), one of the most H(2)O(2)-reactive proteins in the cell (k(app) ∼10(7)-10(8) M(-1) s(-1)), acts as a H(2)O(2) signal receptor and transmitter in transcription factor redox regulation. Prx2 forms a redox relay with the transcription factor STAT3 in which oxidative equivalents flow from Prx2 to STAT3. The redox relay generates disulfide-linked STAT3 oligomers with attenuated transcriptional activity. Cytokine-induced STAT3 signaling is accompanied by Prx2 and STAT3 oxidation and is modulated by Prx2 expression levels.

STAT3 as a possible therapeutic target in human malignancies: lessons from acute myeloid leukemia

STAT3 is important for transcriptional regulation in human acute myeloid leukemia (AML). STAT3 has thousands of potential DNA binding sites but usually shows cell type specific binding preferences to a limited number of these. Furthermore, AML is a very heterogeneous disease, and studies of the prognostic impact of STAT3 in human AML have also given conflicting results. A more detailed characterization of STAT3 functions and the expression of various isoforms in human AML will therefore be required before it is possible to design clinical studies of STAT3 inhibitors in this disease, and it will be especially important to investigate whether the functions of STAT3 differ between patients. Several other malignancies also show extensive biological heterogeneity, and the present discussion and the suggested scientific approaches may thus be relevant for other cancer patients.

Contribution of chaperones to STAT pathway signaling

Aberrant STAT signaling is associated with the development and progression of many cancers and immune related diseases. Recent findings demonstrate that proteostasis modulators under clinical investigation for cancer therapy have a significant impact on STAT signaling, which may be critical for mediating their anti-cancer effects. Chaperones are critical for protein folding, stability and function and, thus, play an essential role in the maintenance of proteostasis. In this review we discuss the role of chaperones in STAT and tyrosine kinase (TK) protein folding, modulation of STAT and TK activity, and degradation of TKs. We highlight the important role of chaperones in STAT signaling, and how this knowledge has provided a framework for the development of new therapeutic avenues of targeting STAT signaling related pathologies.

STAT3 Activities and Energy Metabolism: Dangerous Liaisons

STAT3 mediates cytokine and growth factor receptor signalling, becoming transcriptionally active upon tyrosine 705 phosphorylation (Y-P). Constitutively Y-P STAT3 is observed in many tumors that become addicted to its activity, and STAT3 transcriptional activation is required for tumor transformation downstream of several oncogenes. We have recently demonstrated that constitutively active STAT3 drives a metabolic switch towards aerobic glycolysis through the transcriptional induction of Hif-1α and the down-regulation of mitochondrial activity, in both MEF cells expressing constitutively active STAT3 (Stat3^C/C) and STAT3-addicted tumor cells. This novel metabolic function is likely involved in mediating pre-oncogenic features in the primary Stat3^C/C MEFs such as resistance to apoptosis and senescence and rapid proliferation. Moreover, it strongly contributes to the ability of primary Stat3^C/C MEFs to undergo malignant transformation upon spontaneous immortalization, a feature that may explain the well known causative link between STAT3 constitutive activity and tumor transformation under chronic inflammatory conditions. Taken together with the recently uncovered role of STAT3 in regulating energy metabolism from within the mitochondrion when phosphorylated on Ser 727, these data place STAT3 at the center of a hub regulating energy metabolism under different conditions, in most cases promoting cell survival, proliferation and malignant transformation even though with distinct mechanisms.

Cellular senescence and protein degradation: breaking down cancer

Autophagy and the ubiquitin-proteasome pathway (UPP) are the major protein degradation systems in eukaryotic cells. Whereas the former mediate a bulk nonspecific degradation, the UPP allows a rapid degradation of specific proteins. Both systems have been shown to play a role in tumorigenesis, and the interest in developing therapeutic agents inhibiting protein degradation is steadily growing. However, emerging data point to a critical role for autophagy in cellular senescence, an established tumor suppressor mechanism. Recently, a selective protein degradation process mediated by the UPP was also shown to contribute to the senescence phenotype. This process is tightly regulated by E3 ubiquitin ligases, deubiquitinases, and several post-translational modifications of target proteins. Illustrating the complexity of UPP, more than 600 human genes have been shown to encode E3 ubiquitin ligases, a number which exceeds that of the protein kinases. Nevertheless, our knowledge of proteasome-dependent protein degradation as a regulated process in cellular contexts such as cancer and senescence remains very limited. Here we discuss the implications of protein degradation in senescence and attempt to relate this function to the protein degradation pattern observed in cancer cells.

Novel distribution of calreticulin to cardiomyocyte mitochondria and its increase in a rat model of dilated cardiomyopathy

BACKGROUND

Calreticulin (CRT), a Ca(2+)-binding chaperone of the endoplasmic reticulum, can also be found in several other locations including the cytosol, nucleus, secretory granules, the outer side of the plasma membrane, and the extracellular matrix. Whether CRT is localized at mitochondria of cardiomyocytes and whether such localization is affected under DCM are still unclear.

METHODS AND RESULTS

The DCM model was generated in rats by the daily oral administration of furazolidone for thirty weeks. Echocardiographic and hemodynamic studies demonstrated enlarged left ventricular dimensions and reduced systolic and diastolic function in DCM rats. Immuno-electron microscopy and Western blot showed that CRT was present in cardiomyocyte mitochondria and the mitochondrial content of CRT was increased in DCM hearts (P<0.05). Morphometric analysis showed notable myocardial apoptosis and mitochondrial swelling with fractured or dissolved cristae in the DCM hearts. Compared with the control group, the mitochondrial membrane potential level of the freshly isolated cardiac mitochondria and the enzyme activities of cytochrome c oxidase and succinate dehydrogenase in the model group were significantly decreased (P<0.05), and the myocardial apoptosis index and the caspase activities of caspase-9 and caspase-3 were significantly increased (P<0.05). Pearson linear correlation analysis showed that the mitochondrial content of CRT had negative correlations with the mitochondrial function, and a positive correlation with myocardial apoptosis index (P<0.001). The protein expression level of cytochrome c and the phosphorylation activity of STAT3 in the mitochondrial fraction were significantly decreased in the model group compared with the control group (P<0.05).

CONCLUSIONS

These data demonstrate that CRT is localized at cardiomyocyte mitochondria and its mitochondrial content is increased in DCM hearts.

Gene associated with retinoid-interferon-induced mortality 19 attenuates murine autoimmune arthritis by regulation of th17 and treg cells

OBJECTIVE

STAT-3 is a key transcriptional factor in the interleukin-6 (IL-6)-mediated differentiation of Th17 cells. Because Th17 is believed to be a central player in rheumatoid arthritis (RA), we sought to evaluate whether an endogenous inhibitor of the STAT3 gene, GRIM-19 (gene associated with retinoid-interferon-induced mortality 19), could attenuate the progression and severity of murine collagen-induced arthritis (CIA) through suppression of Th17 cells and, reciprocally, could increase expression of Treg cells.

METHODS

Overexpression of GRIM-19 was produced either by intravenous/intramuscular administration of a GRIM-19 overexpression vector in DBA1/J mice or by development of GRIM-19-transgenic (Tg) mice on a C57BL/6 background. Clinical signs were scored for arthritis severity, and mouse splenocytes, serum, and joint tissue were obtained for immunostaining and histologic analyses.

RESULTS

The numbers of CD4+IL-17+ cells and CD4+pSTAT3+ cells were decreased, while the numbers of CD4+CD25+Foxp3+ cells and CD4+pSTAT5+ cells were increased, in both GRIM-19 vector-transfected and GRIM-19-Tg mice. Administration of the GRIM-19 overexpression vector into mice with CIA markedly suppressed the clinical and histologic signs of arthritis in the affected joints. Similarly, when CIA was induced in GRIM-19-Tg mice, the arthritis phenotype was markedly attenuated and the expression of inflammatory cytokines (IL-1β, IL-6, tumor necrosis factor α, and IL-17) in the arthritic joints was also significantly reduced. Moreover, bone marrow-derived monocyte/macrophages obtained from GRIM-19-Tg mice showed attenuated RANKL-induced osteoclastogenesis in vitro.

CONCLUSION

GRIM-19 improved the clinical and histologic features of CIA and also inhibited osteoclast formation. These findings suggest that GRIM-19 may be a novel treatment agent for RA.

Inhibition of Janus kinase signaling during controlled mechanical ventilation prevents ventilation-induced diaphragm dysfunction

Controlled mechanical ventilation (CMV) is associated with the development of diaphragm atrophy and contractile dysfunction, and respiratory muscle weakness is thought to contribute significantly to delayed weaning of patients. Therefore, therapeutic strategies for preventing these processes may have clinical benefit. The aim of the current study was to investigate the role of the Janus kinase (JAK)/signal transducer and activator of transcription 3 (STAT3) signaling pathway in CMV-mediated diaphragm wasting and weakness in rats. CMV-induced diaphragm atrophy and contractile dysfunction coincided with marked increases in STAT3 phosphorylation on both tyrosine 705 (Tyr705) and serine 727 (Ser727). STAT3 activation was accompanied by its translocation into mitochondria within diaphragm muscle and mitochondrial dysfunction. Inhibition of JAK signaling during CMV prevented phosphorylation of both target sites on STAT3, eliminated the accumulation of phosphorylated STAT3 within the mitochondria, and reversed the pathologic alterations in mitochondrial function, reduced oxidative stress in the diaphragm, and maintained normal diaphragm contractility. In addition, JAK inhibition during CMV blunted the activation of key proteolytic pathways in the diaphragm, as well as diaphragm atrophy. These findings implicate JAK/STAT3 signaling in the development of diaphragm muscle atrophy and dysfunction during CMV and suggest that the delayed extubation times associated with CMV can be prevented by inhibition of Janus kinase signaling.-Smith, I. J., Godinez, G. L., Singh, B. K., McCaughey, K. M., Alcantara, R. R., Gururaja, T., Ho, M. S., Nguyen, H. N., Friera, A. M., White, K. A., McLaughlin, J. R., Hansen, D., Romero, J. M., Baltgalvis, K. A., Claypool, M. D., Li, W., Lang, W., Yam, G. C., Gelman, M. S., Ding, R., Yung, S. L., Creger, D. P., Chen, Y., Singh, R., Smuder, A. J., Wiggs, M. P., Kwon, O.-S., Sollanek, K. J., Powers, S. K., Masuda, E. S., Taylor, V. C., Payan, D. G., Kinoshita, T., Kinsella, T. M. Inhibition of Janus kinase signaling during controlled mechanical ventilation prevents ventilation-induced diaphragm dysfunction.

Toward a new STATe: the role of STATs in mitochondrial function

Signal Transducers and Activators of Transcription (STATs) have been studied extensively and have been associated with virtually every biochemical pathway. Until recently, however, they were thought to exert these effects solely as a nuclear transcription factor. The finding that STAT3 localizes to the mitochondria and modulates respiration has opened up a new avenue through which STATs may regulate the cell. Recently, other members of the STAT family (STAT1, STAT2, STAT5, and STAT6) have also been shown to be present in the mitochondria. Coordinate regulation at the nucleus and mitochondria by these proteins places them in a unique position to drive cellular processes to achieve a specific response. This review summarizes recent findings that have led to our current understanding of how STATs influence mitochondrial function in health and disease.

Mitoneet mediates TNFα-induced necroptosis promoted by exposure to fructose and ethanol

Fructose and ethanol are metabolized principally in the liver and are both known to contribute to the development of hepatic steatosis that can progress to hepatic steatohepatitis. The present study indentifies a synergistic interaction between fructose and ethanol in promoting hepatocyte sensitivity to TNFα-induced necroptosis. Concurrent exposure to fructose and ethanol induces the overexpression of the CDGSH iron-sulfur domain-containing protein 1 (CISD1 or mitoneet), which is localized to the outer mitochondrial membrane. The increased expression of mitoneet primes the hepatocyte for TNFα-induced cytotoxicity. Treatment with TNFα induces the translocation of a Stat3-Grim-19 complex to the mitochondria, which binds to mitoneet and promotes the rapid release of its 2Fe-2S cluster, causing an accumulation of mitochondrial iron. The dramatic increase of mitochondrial iron provokes a surge in formation of reactive oxygen species, resulting in mitochondrial injury and cell death. Additionally, mitoneet is constitutively expressed at high levels in L929 fibrosarcoma cells and is required for L929 cells to undergo TNFα-induced necroptosis in the presence of caspase inhibition, indicating the importance of mitoneet to the necroptotic form of cell death.

Diabetes impairs an interleukin-1β-dependent pathway that enhances neurite outgrowth through JAK/STAT3 modulation of mitochondrial bioenergetics in adult sensory neurons

Background

A luminex-based screen of cytokine expression in dorsal root ganglia (DRG) and nerve of type 1 diabetic rodents revealed interleukin-1 (IL-1α) and IL-1β to be significantly depressed. We, therefore, tested the hypothesis that impaired IL-1α and IL-1β expression in DRG may contribute to aberrant axon regeneration and plasticity seen in diabetic sensory neuropathy. In addition, we determined if these cytokines could optimize mitochondrial bioenergetics since mitochondrial dysfunction is a key etiological factor in diabetic neuropathy.

Results

Cytokines IL-1α and IL-1β were reduced 2-fold (p<0.05) in DRG and/or nerve of 2 and 5 month streptozotocin (STZ)-diabetic rats. IL-2 and IL-10 were unchanged. IL-1α and IL-1β induced similar 2 to 3-fold increases in neurite outgrowth in cultures derived from control or diabetic rats (p<0.05). STAT3 phosphorylation on Tyr705 or Ser727 was depressed in DRG from STZ-diabetic mice and treatment of cultures derived from STZ-diabetic rats with IL-1β for 30 min raised phosphorylation of STAT3 on Tyr705 and Ser727 by 1.5 to 2-fold (p<0.05). shRNA-based or AG490 inhibition of STAT3 activity or shRNA blockade of endogenous IL-1β expression completely blocked neurite outgrowth. Cultured neurons derived from STZ-diabetic mice were treated for 24 hr with IL-1β and maximal oxygen consumption rate and spare respiratory capacity, both key measures of bioenergetic fidelity that were depressed in diabetic compared with control neurons, were enhanced 2-fold. This effect was blocked by AG490.

Conclusions

Endogenous synthesis of IL-1β is diminished in nerve tissue in type 1 diabetes and we propose this defect triggers reduced STAT3 signaling and mitochondrial function leading to sup-optimal axonal regeneration and plasticity.

Mitochondrial retrograde signaling at the crossroads of tumor bioenergetics, genetics and epigenetics

Mitochondria play a central role not only in energy production but also in the integration of metabolic pathways as well as signals for apoptosis and autophagy. It is becoming increasingly apparent that mitochondria in mammalian cells play critical roles in the initiation and propagation of various signaling cascades. In particular, mitochondrial metabolic and respiratory states and status on mitochondrial genetic instability are communicated to the nucleus as an adaptive response through retrograde signaling. Each mammalian cell contains multiple copies of the mitochondrial genome (mtDNA). A reduction in mtDNA copy number has been reported in various human pathological conditions such as diabetes, obesity, neurodegenerative disorders, aging and cancer. Reduction in mtDNA copy number disrupts mitochondrial membrane potential (Δψm) resulting in dysfunctional mitochondria. Dysfunctional mitochondria trigger retrograde signaling and communicate their changing metabolic and functional state to the nucleus as an adaptive response resulting in an altered nuclear gene expression profile and altered cell physiology and morphology. In this review, we provide an overview of the various modes of mitochondrial retrograde signaling focusing particularly on the Ca(2+)/Calcineurin mediated retrograde signaling. We discuss the contribution of the key factors of the pathway such as Calcineurin, IGF1 receptor, Akt kinase and HnRNPA2 in the propagation of signaling and their role in modulating genetic and epigenetic changes favoring cellular reprogramming towards tumorigenesis.

Mitochondrial p53 mediates a transcription-independent regulation of cell respiration and interacts with the mitochondrial F₁F0-ATP synthase

We and others previously reported that endogenous p53 can be located at mitochondria in the absence of stress, suggesting that p53 has a role in the normal physiology of this organelle. The aim of this study was to characterize in unstressed cells the intramitochondrial localization of p53 and identify new partners and functions of p53 in mitochondria. We find that the intramitochondrial pool of p53 is located in the intermembrane space and the matrix. Of note, unstressed HCT116 p53(+/+) cells simultaneously show increased O₂ consumption and decreased mitochondrial superoxide production compared with their p53-null counterpart. This data was confirmed by stable H1299 cell lines expressing low levels of p53 specifically targeted to the matrix. Using immunoprecipitation and mass spectrometry, we identified the oligomycin sensitivity-conferring protein (OSCP), a subunit of the F₁F₀-ATP synthase complex, as a new partner of endogenous p53, specifically interacting with p53 localized in the matrix. Interestingly, this interaction seems implicated in mitochondrial p53 localization. Moreover, p53 localized in the matrix promotes the assembly of F₁F₀-ATP synthase. Taking into account that deregulations of mitochondrial respiration and reactive oxygen species production are tightly linked to cancer development, we suggest that mitochondrial p53 may be an important regulator of normal mitochondrial and cellular physiology, potentially exerting tumor suppression activity inside mitochondria.

Dancing rhinos in stilettos: The amazing saga of the genomic and nongenomic actions of STAT3 in the heart

A substantial body of evidence has shown that signal transducer and activator of transcription 3 (STAT3) has an important role in the heart in protecting the myocardium from ischemia and oxidative stress. These actions are attributed to STAT3 functioning as a transcription factor in upregulating cardioprotective genes. Loss of STAT3 has been implicated as well in the pathogenesis of heart failure and, in that context and in addition to the loss of a cardioprotective gene program, nuclear STAT3 has been identified as a transcriptional repressor important for the normal functioning of the ubiquitin-proteasome system for protein degradation. The later finding establishes a genomic role for STAT3 in controlling cellular homeostasis in cardiac myocytes independent of stress. Surprisingly, although a well-studied area, very few downstream gene targets of STAT3 in the heart have been definitively identified. In addition, STAT3 is now known to induce gene expression by noncanonical means that are not well characterized in the heart. On the other hand, recent evidence has shown that STAT3 has important nongenomic actions in cardiac myocytes that affect microtubule stability, mitochondrial respiration, and autophagy. These extranuclear actions of STAT3 involve protein–protein interactions that are incompletely understood, as is their regulation in both the healthy and injured heart. Moreover, how the diverse genomic and nongenomic actions of STAT3 crosstalk with each other is unchartered territory. Here we present an overview of what is and is not known about both the genomic and nongenomic actions of STAT3 in the heart from a structure-function perspective that focuses on the impact of posttranslational modifications and oxidative stress in regulating the actions and interactions of STAT3. Even though we have learnt a great deal about the role played by STAT3 in the heart, much more awaits to be discovered.

Mge1, a nucleotide exchange factor of Hsp70, acts as an oxidative sensor to regulate mitochondrial Hsp70 function

Yeast Mge1, the cochaperone of mitochondrial heat shock protein 70 (mHsp70), is essential for exchanging ATP for ADP on mHsp70 and thus for recycling of mHsp70 for mitochondrial protein import and folding. Mge1 acts as an oxidative sensor to regulate mHsp70 function.

Despite the growing evidence of the role of oxidative stress in disease, its molecular mechanism of action remains poorly understood. The yeast Saccharomyces cerevisiae provides a valuable model system in which to elucidate the effects of oxidative stress on mitochondria in higher eukaryotes. Dimeric yeast Mge1, the cochaperone of heat shock protein 70 (Hsp70), is essential for exchanging ATP for ADP on Hsp70 and thus for recycling of Hsp70 for mitochondrial protein import and folding. Here we show an oxidative stress–dependent decrease in Mge1 dimer formation accompanied by a concomitant decrease in Mge1–Hsp70 complex formation in vitro. The Mge1-M155L substitution mutant stabilizes both Mge1 dimer and Mge1–Hsp70 complex formation. Most important, the Mge1-M155L mutant rescues the slow-growth phenomenon associated with the wild-type Mge1 strain in the presence of H₂O₂ in vivo, stimulation of the ATPase activity of Hsp70, and the protein import defect during oxidative stress in vitro. Furthermore, cross-linking studies reveal that Mge1–Hsp70 complex formation in mitochondria isolated from wild-type Mge1 cells is more susceptible to reactive oxygen species compared with mitochondria from Mge1-M155L cells. This novel oxidative sensor capability of yeast Mge1 might represent an evolutionarily conserved function, given that human recombinant dimeric Mge1 is also sensitive to H₂O₂.