Hyo Kyun Chung

Crif1 Deficiency Reduces Adipose OXPHOS Capacity and Triggers Inflammation and Insulin Resistance in Mice

Impaired mitochondrial oxidative phosphorylation (OXPHOS) has been proposed as an etiological mechanism underlying insulin resistance. However, the initiating organ of OXPHOS dysfunction during the development of systemic insulin resistance has yet to be identified. To determine whether adipose OXPHOS deficiency plays an etiological role in systemic insulin resistance, the metabolic phenotype of mice with OXPHOS–deficient adipose tissue was examined. Crif1 is a protein required for the intramitochondrial production of mtDNA–encoded OXPHOS subunits; therefore, Crif1 haploinsufficient deficiency in mice results in a mild, but specific, failure of OXPHOS capacity in vivo. Although adipose-specific Crif1-haploinsufficient mice showed normal growth and development, they became insulin-resistant. Crif1-silenced adipocytes showed higher expression of chemokines, the expression of which is dependent upon stress kinases and antioxidant. Accordingly, examination of adipose tissue from Crif1-haploinsufficient mice revealed increased secretion of MCP1 and TNFα, as well as marked infiltration by macrophages. These findings indicate that the OXPHOS status of adipose tissue determines its metabolic and inflammatory responses, and may cause systemic inflammation and insulin resistance.

Growth differentiation factor 15 is a myomitokine governing systemic energy homeostasis

Reduced mitochondrial electron transport chain activity promotes longevity and improves energy homeostasis via cell-autonomous and –non-autonomous factors in multiple model systems. This mitohormetic effect is thought to involve the mitochondrial unfolded protein response (UPRmt), an adaptive stress-response pathway activated by mitochondrial proteotoxic stress. Using mice with skeletal muscle–specific deficiency of Crif1 (muscle-specific knockout [MKO]), an integral protein of the large mitoribosomal subunit (39S), we identified growth differentiation factor 15 (GDF15) as a UPRmt-associated cell–non-autonomous myomitokine that regulates systemic energy homeostasis. MKO mice were protected against obesity and sensitized to insulin, an effect associated with elevated GDF15 secretion after UPRmt activation. In ob/ob mice, administration of recombinant GDF15 decreased body weight and improved insulin sensitivity, which was attributed to elevated oxidative metabolism and lipid mobilization in the liver, muscle, and adipose tissue. Thus, GDF15 is a potent mitohormetic signal that safeguards against the onset of obesity and insulin resistance.

GDF15 Is a Novel Biomarker for Impaired Fasting Glucose

Background Growth differentiation factor-15 (GDF15) is a protein that belongs to the transforming growth factor β superfamily. An elevated serum level of GDF15 was found to be associated with type 2 diabetes mellitus (T2DM). T2DM is an inflammatory disease that progresses from normal glucose tolerance (NGT) to impaired fasting glucose (IFG). Hence, we aimed to validate the relationship between GDF15 and IFG. Methods The participants were divided into the following three groups: NGT (n=137), IFG (n=29), and T2DM (n=75). The controls and T2DM outpatients visited the hospital for routine health check-ups. We used fasting blood glucose to detect IFG in nondiabetic patients. We checked the body mass index (BMI), C-reactive protein level, metabolic parameters, and fasting serum GDF15 level. Results Age, BMI, triglyceride, insulin, glucose, homeostatic model assessment-insulin resistance (HOMA-IR), and GDF15 levels were elevated in the IFG and T2DM groups compared to the NGT group. In the correlation analysis between metabolic parameters and GDF15, age and HOMA-IR had a significant positive correlation with GDF15 levels. GDF15 significantly discriminated between IFG and NGT, independent of age, BMI, and HOMA-IR. The serum levels of GDF15 were more elevated in men than in women. As a biomarker for IFG based on the receiver operating characteristic curve analysis, the cutoff value of GDF15 was 510 pg/mL in males and 400 pg/mL in females. Conclusion GDF15 had a positive correlation with IR independent of age and BMI, and the serum level of GDF15 was increased in the IFG and T2DM groups. GDF15 may be a novel biomarker for detecting IFG in nondiabetic patients.

Thyrotropin-Mediated Repression of Class II Trans-Activator Expression in Thyroid Cells: Involvement of STAT3 and Suppressor of Cytokine Signaling

It has been suggested that class I and class II MHC are contributing factors for numerous diseases including autoimmune thyroid diseases, type 1 diabetes, rheumatoid arthritis, Alzheimer’s disease, and multiple sclerosis. The class II trans-activator (CIITA), which is a non-DNA-binding regulator of class II MHC transcription, regulates the constitutive and inducible expression of the class I and class II genes. FRTL-5 thyroid cells incubated in the presence of IFN-γ have a significantly higher level of cell surface rat MHC class II RTI.B. However, the IFN-γ-induced RT1.B expression was suppressed significantly in cells incubated in the presence of thyrotropin. Thyrotropin (TSH) represses IFN-γ-induced CIITA expression by inhibiting type IV CIITA promoter activity through the suppression of STAT1 activation and IFN regulatory factor 1 induction. This study found that TSH induces transcriptional activation of the STAT3 gene through the phosphorylation of STAT3 and CREB activation. TSH induces SOCS-1 and SOCS-3, and TSH-mediated SOCS-3 induction was dependent on STAT3. The cell line stably expressing the wild-type STAT3 showed a higher CIITA induction in response to IFN-γ and also exhibited TSH repression of the IFN-γ-mediated induction of CIITA. However, TSH repression of the IFN-γ-induced CIITA expression was not observed in FRTL-5 thyroid cells, which stably expresses the dominant negative forms of STAT3, STAT3-Y705F, and STAT3-S727A. This report suggests that TSH is also engaged in immunomodulation through signal cross-talk with the cytokines in thyroid cells.

The protective role of NAD(P)H:quinone oxidoreductase 1 on acetaminophen-induced liver injury is associated with prevention of adenosine triphosphate depletion and improvement of mitochondrial dysfunction

Abstract An overdose of acetaminophen (APAP) causes hepatotoxicity due to its metabolite, N-acetyl-p-benzoquinone imine. NAD(P)H:quinone oxidoreductase 1 (NQO1) is an important enzyme for detoxification, because it catabolizes endogenous/exogenous quinone to hydroquinone. Although various studies have suggested the possible involvement of NQO1 in APAP-induced hepatotoxicity, its precise role in this remains unclear. We investigated the role of NQO1 against APAP-induced hepatotoxicity using a genetically modified rodent model. NQO1 wild-type (WT) and knockout (KO) mice were treated with different doses of APAP, and we evaluated the mortality and toxicity markers for cell death caused by APAP. NQO1 KO mice showed high sensitivity to APAP-mediated hepatotoxicity (as indicated by a large necrotic region) as well as increased levels of nitrotyrosine adducts and reactive oxygen species. APAP-induced cell death in the livers and primary hepatocytes of NQO1 KO mice, which was accompanied by an extensive reduction in adenosine triphosphate (ATP) levels. In accordance with this ATP depletion, cytosolic increases in mitochondrial proteins such as apoptosis-inducing factor, second mitochondria-derived activator of caspases/DIABLO, endonuclease G, and cytochrome c, which indicate severe mitochondrial dysfunction, were observed in NQO1 KO mice but not in WT mice after APAP exposure. Severe mitochondrial depolarization was also greater in hepatocytes isolated from NQO1 KO mice. Collectively, our data suggest that NQO1 plays a critical role in protection against energy depletion caused by APAP, and NQO1 may be useful in the development of therapeutic approaches to effectively diminish the hepatotoxicity caused by an APAP overdose.

The indole derivative NecroX‐7 improves nonalcoholic steatohepatitis in ob/ob mice through suppression of mitochondrial ROS/RNS and inflammation

Nonalcoholic steatohepatitis (NASH) is associated with cirrhosis and hepatocellular carcinoma. Reactive oxygen species (ROS) and reactive nitrogen species (RNS) play key roles in the development of the disease. However, the therapeutic target of NASH has not been fully defined and new treatments are needed. We investigated the protective effects of the antioxidant indole‐derived NecroX‐7 in a NASH mouse model using leptin‐deficient ob/ob and methionine‐ and choline‐deficient (MCD) diet‐fed ob/ob mice.

Mitochondrial oxidative phosphorylation reserve is required for hormone- and PPARγ agonist-induced adipogenesis

Adipocyte differentiation requires the coordinated activities of several nuclear transcription factors. Recently, mitochondria biogenesis was reported to occur during adipocyte differentiation and following treatment with thiazolidinediones in vitro and in vivo. Crif1 is a translational factor for mitochondrial DNA (mtDNA) and is important for transcription of the mitochondrial oxidative phosphorylation (OXPHOS) complex. To investigate the role of OXPHOS in adipogenesis, we analyzed adipocyte differentiation following disruption of Crif1 in vitro and in vivo. The adipose-specific Crif1 knockout mouse had a lower body weight and less fat mass than wild-type mice. Furthermore, adipocytes were smaller and had a dysplastic morphology in the adipose-specific Crif1 knockout mouse. 3T3-L1 adipocytes or adipose-derived stem cells (ADSCs) that lacked Crif1 expressed lower levels of mtDNA-encoded OXPHOS subunits, and adipocyte differentiation was disrupted. Rosiglitazone treatment did not induce adipogenesis or mitochondria biogenesis in Crif1 knockout ADSCs. These results show that mitochondrial OXPHOS and Crif1 are required for rosiglitazone- and hormone-induced adipogenesis.

Thyroid Dysfunction Associated With Follicular Cell Steatosis in Obese Male Mice and Humans

Adult thyroid dysfunction is a common endocrine disorder associated with an increased risk of cardiovascular disease and mortality. A recent epidemiologic study revealed a link between obesity and increased prevalence of hypothyroidism. It is conceivable that excessive adiposity in obesity might lead to expansion of the interfollicular adipose (IFA) depot or steatosis in thyroid follicular cells (thyroid steatosis, TS). In this study, we investigated the morphological and functional changes in thyroid glands of obese humans and animal models, diet-induced obese (DIO), ob/ob, and db/db mice. Expanded IFA depot and TS were observed in obese patients. Furthermore, DIO mice showed increased expression of lipogenesis-regulation genes, such as sterol regulatory element binding protein 1 (SREBP-1), peroxisome proliferator-activated receptor γ (PPARγ), acetyl coenzyme A carboxylase (ACC), and fatty acid synthetase (FASN) in the thyroid gland. Steatosis and ultrastructural changes, including distension of the endoplasmic reticulum (ER) and mitochondrial distortion in thyroid follicular cells, were uniformly observed in DIO mice and genetically obese mouse models, ob/ob and db/db mice. Obese mice displayed a variable degree of primary thyroid hypofunction, which was not corrected by PPARγ agonist administration. We propose that systemically increased adiposity is associated with characteristic IFA depots and TS and may cause or influence the development of primary thyroid failure.

Role of NADH: quinone oxidoreductase-1 in the tight junctions of colonic epithelial cells.

NADH:quinone oxidoreductase 1 (NQO1) is known to be involved in the regulation of energy synthesis and metabolism, and the functional studies of NQO1 have largely focused on metabolic disorders. Here, we show for the first time that compared to NQO1-WT mice, NQO1-KO mice exhibited a marked increase of permeability and spontaneous inflammation in the gut. In the DSS-induced colitis model, NQO1-KO mice showed more severe inflammatory responses than NQO1-WT mice. Interestingly, the transcript levels of claudin and occludin, the major tight junction molecules of gut epithelial cells, were significantly decreased in NQO1-KO mice. The colons of NQO1-KO mice also showed high levels of reactive oxygen species (ROS) and histone deacetylase (HDAC) activity, which are known to affect transcriptional regulation. Taken together, these novel findings indicate that NQO1 contributes to the barrier function of gut epithelial cells by regulating the transcription of tight junction molecules. [BMB Reports 2014;47(9): 494-499]

Thyroid-stimulating hormone transcriptionally regulates the thiol-specific antioxidant gene.

Thiol-specific antioxidant (TSA) plays an important role in regulating cell differentiation and proliferation by modulating the hydrogen peroxide (H2O2) mediated responses in a variety of mammalian cells. Thyroid cells are constantly exposed to the actions of reactive oxygen species (ROS), because they produce high levels of H2O2 in response to the physiological action of TSH (thyroid-stimulating hormone). Thyrocytes have several defense mechanisms against ROS, including TSA and SOD (superoxide anion dismutase). Using Northern blot hybridization, we tested the effects of TSH on TSA gene expression in FRTL-5 cells derived from rat thyroids. TSA mRNA expression increased following treatment of cells with TSH at concentrations greater than 10-9 M. This effect was observed within 6 hours following treatment, and peaked at 8 hours. The effect was blocked by actinomycin D, but not by cycloheximide. The half-life of TSA mRNA was approximately 5.5 hours in the presence or absence of TSH, and that was not affected by TSA mRNA stability. The effects on TSA gene expression were specific to TSH. Other growth factors (e.g., insulin, transferrin and hydrocortisone) did not alter TSA expression. Our results are the first indication that TSH regulates the expression of TSA transcriptionally in thyrocytes.