Bisphenol A impairs mitochondrial function in the liver at doses below the no observed adverse effect level

Bisphenol A (BPA) has been reported to possess hepatic toxicity. We investigated the hypothesis that BPA, below the no observed adverse effect level (NOAEL), can induce hepatic damage and mitochondrial dysfunction by increasing oxidative stress in the liver. Two doses of BPA, 0.05 and 1.2 mg/kg body weight/day, were administered intraperitoneally for 5 days to mice. Both treatments impaired the structure of the hepatic mitochondria, although oxygen consumption rate and expression of the respiratory complex decreased only at the higher dose. The hepatic levels of malondialdehyde (MDA), a naturally occurring product of lipid peroxidation, increased, while the expression of glutathione peroxidase 3 (GPx3) decreased, after BPA treatment. The expression levels of proinflammatory cytokines such as interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α) also increased. In HepG2 cells, 10 or 100 nM of BPA also decreased the oxygen consumption rate, ATP production, and the mitochondrial membrane potential. In conclusion, doses of BPA below the NOAEL induce mitochondrial dysfunction in the liver, and this is associated with an increase in oxidative stress and inflammation.

SUMO-Specific Protease 2 (SENP2) Is an Important Regulator of Fatty Acid Metabolism in Skeletal Muscle

Small ubiquitin-like modifier (SUMO)-specific proteases (SENPs) that reverse protein modification by SUMO are involved in the control of numerous cellular processes, including transcription, cell division, and cancer development. However, the physiological function of SENPs in energy metabolism remains unclear. Here, we investigated the role of SENP2 in fatty acid metabolism in C2C12 myotubes and in vivo. In C2C12 myotubes, treatment with saturated fatty acids, like palmitate, led to nuclear factor-κB-mediated increase in the expression of SENP2. This increase promoted the recruitment of peroxisome proliferator-activated receptor (PPAR)δ and PPARγ, through desumoylation of PPARs, to the promoters of the genes involved in fatty acid oxidation (FAO), such as carnitine-palmitoyl transferase-1 (CPT1b) and long-chain acyl-CoA synthetase 1 (ACSL1). In addition, SENP2 overexpression substantially increased FAO in C2C12 myotubes. Consistent with the cell culture system, muscle-specific SENP2 overexpression led to a marked increase in the mRNA levels of CPT1b and ACSL1 and thereby in FAO in the skeletal muscle, which ultimately alleviated high-fat diet-induced obesity and insulin resistance. Collectively, these data identify SENP2 as an important regulator of fatty acid metabolism in skeletal muscle and further implicate that muscle SENP2 could be a novel therapeutic target for the treatment of obesity-linked metabolic disorders.

Functional expression of an insect cathepsin B-like counter-defence protein

Insects are capable of readjusting their digestive regimes in response to dietary challenge. Cowpea bruchids (Callosobruchus maculatus) strongly induce C. maculatus cathepsin B-like cysteine protease 1 (CmCatB1) transcripts when fed diet containing a soybean cysteine protease inhibitor soyacystatin N (scN). CmCatB1 shares significant sequence similarity with cathepsin B-like cysteine proteases. In this study, we isolated another cDNA, namely CmCatB2 that encodes a protein sequence otherwise identical to CmCatB1, but lacking a 70-amino-acid internal section. CmCatB1 and CmCatB2 probably resulted from alternate splicing events. Only the CmCatB1 transcript, however, exhibited differential expression in response to dietary scN. Further, this expression was only detectable in larvae, which is the developmental stage associated with food ingestion. The scN-activated and developmentally regulated CmCatB1 expression pattern suggests it may have a unique function in insect counter-defence against antinutritional factors. Heterologously expressed recombinant CmCatB1 protein exhibited enzymatic activity in a pH-dependent manner. Activity of the protein was inhibited by both the cysteine protease inhibitor E-64 and the cathepsin B-specific inhibitor CA-074, verifying its cathepsin B-like cysteine protease nature. Interestingly, the enzymatic activity was unaffected by the presence of scN. Together, we have provided functional evidence suggesting that CmCatB1 confers inhibitor-insensitive enzymatic activity to cowpea bruchids, which is crucial for insect survival when challenged by dietary protease inhibitors.

A Comprehensive Approach to Sample Preparation for Electron Microscopy and the Assessment of Mitochondrial Morphology in Tissue and Cultured Cells

Mitochondria respond to metabolic demands of the cell and to incremental damage, in part, through dynamic structural changes that include fission (fragmentation), fusion (merging of distinct mitochondria), autophagic degradation (mitophagy), and biogenic interactions with the endoplasmic reticulum (ER). High resolution study of mitochondrial structural and functional relationships requires rapid preservation of specimens to reduce technical artifacts coupled with quantitative assessment of mitochondrial architecture. A practical approach for assessing mitochondrial fine structure using two dimensional and three dimensional high-resolution electron microscopy is presented, and a systematic approach to measure mitochondrial architecture, including volume, length, hyperbranching, cristae morphology, and the number and extent of interaction with the ER is described. These methods are used to assess mitochondrial architecture in cells and tissue with high energy demand, including skeletal muscle cells, mouse brain tissue, and Drosophila muscles. The accuracy of assessment is validated in cells and tissue with deletion of genes involved in mitochondrial dynamics.

Thyroid-stimulating hormone improves insulin sensitivity in skeletal muscle cells via cAMP/PKA/CREB pathway-dependent upregulation of insulin receptor substrate-1 expression

Thyroid-stimulating hormone (TSH) receptor is expressed in extrathyroidal tissues such as hepatocytes, adipocytes, and skeletal muscle, which suggests a possible novel role of TSH in various metabolic processes in extrathyroidal tissues independent of thyroid hormones. We investigated whether TSH has any effects on glucose tolerance and insulin sensitivity in the skeletal muscle using diet-induced obesity (DIO) mouse models and rodent skeletal muscle cells. TSH improved glucose tolerance in DIO mice and this was associated with an improvement of skeletal muscle insulin sensitivity resulting from the increased expression of insulin receptor substrate (IRS)-1 protein and mRNA therein. TSH significantly increased both basal and insulin-stimulated glucose transport in rat L6 myotubes and increased the expression of IRS-1 protein and mRNA in these cells as well. TSH also stimulated Irs1 promoter activation; this stimulation was abolished by protein kinase A (PKA) inhibition using H89 or by mutation of the cAMP-response element site located at -1155 to -875 bp of the Irs1 promoter region, supporting a novel role of TSH activated-cAMP/PKA/CREB signaling in the regulation of Irs1 expression. In conclusion, TSH improves insulin sensitivity in skeletal muscle by increasing Irs1 gene expression. This regulatory effect is mediated by a PKA-CREB-dependent pathway.

F-box only protein 9 is an E3 ubiquitin ligase of PPARγ

Peroxisome proliferator-activated receptor gamma (PPARγ) is a critical regulator of carbohydrate and lipid metabolism, adipocyte differentiation and inflammatory response. Post-translational modification of PPARγ and its degradation involve several pathways, including the ubiquitin–proteasome system. Here, we identified F-box only protein 9 (FBXO9) as an E3 ubiquitin ligase of PPARγ. We screened interacting partners of PPARγ using immunoprecipitation and mass spectrometric analysis and identified FBXO9 as an E3 ubiquitin ligase of PPARγ. FBXO9 directly interacted with PPARγ through the activation function-1 domain and ligand-binding domain. FBXO9 decreased the protein stability of PPARγ through induction of ubiquitination. We found that the F-box motif of FBXO9 was required for its ubiquitination function. The activity of PPARγ was significantly decreased by FBXO9 overexpression. Furthermore, FBXO9 overexpression in 3T3-L1 adipocytes resulted in decreased levels of endogenous PPARγ and suppression of adipogenesis. These results suggest that FBXO9 is an important enzyme that regulates the stability and activity of PPARγ through ubiquitination.

Rg3 Improves Mitochondrial Function and the Expression of Key Genes Involved in Mitochondrial Biogenesis in C2C12 Myotubes

BACKGROUND

Panax ginseng has glucose-lowering effects, some of which are associated with the improvement in insulin resistance in skeletal muscle. Because mitochondria play a pivotal role in the insulin resistance of skeletal muscle, we investigated the effects of the ginsenoside Rg3, one of the active components of P. ginseng, on mitochondrial function and biogenesis in C2C12 myotubes.

METHODS

C2C12 myotubes were treated with Rg3 for 24 hours. Insulin signaling pathway proteins were examined by Western blot. Cellular adenosine triphosphate (ATP) levels and the oxygen consumption rate were measured. The protein or mRNA levels of mitochondrial complexes were evaluated by Western blot and quantitative reverse transcription polymerase chain reaction analysis.

RESULTS

Rg3 treatment to C2C12 cells activated the insulin signaling pathway proteins, insulin receptor substrate-1 and Akt. Rg3 increased ATP production and the oxygen consumption rate, suggesting improved mitochondrial function. Rg3 increased the expression of peroxisome proliferator-activated receptor γ coactivator 1α, nuclear respiratory factor 1, and mitochondrial transcription factor, which are transcription factors related to mitochondrial biogenesis. Subsequent increased expression of mitochondrial complex IV and V was also observed.

CONCLUSION

Our results suggest that Rg3 improves mitochondrial function and the expression of key genes involved in mitochondrial biogenesis, leading to an improvement in insulin resistance in skeletal muscle. Rg3 may have the potential to be developed as an anti-hyperglycemic agent.

SUMO-specific protease 2 mediates leptin-induced fatty acid oxidation in skeletal muscle

BACKGROUND AND PURPOSE

In addition to the central nervous system-mediated action, leptin also directly induces fatty acid oxidation in skeletal muscle. Rapid induction of FAO by leptin is mediated by the AMP-activated protein kinase (AMPK) pathway, but the mechanism of prolonged FAO by leptin was previously unknown. In an earlier study, we showed that free fatty acids increase transcription of small ubiquitin-like modifier (SUMO) specific protease 2 (SENP2) in skeletal muscle, and that SENP2 stimulates expression of FAO-associated enzymes by deSUMOylating peroxisome proliferator-activated receptors, PPARδ and PPARγ. In this study, we examine whether SENP2 is involved in prolonged stimulation of FAO by leptin.

METHODS

The Effect of leptin on expression of SENP2 and on SENP2-mediated FAO was investigated by using western blotting and real time qPCR of C2C12 myotubes, and of C2C12 myotubes in which expression of specific genes was knocked down using siRNAs. Additionally, muscle-specific SENP2 knockout mice were generated to test the involvement of SENP2 in leptin-induced FAO in vivo.

RESULTS

We show that leptin treatment of C2C12 myotubes causes signal transducer and activator of transcription 3 (STAT3) to bind to the Senp2 promoter, inducing SENP2 expression. We also show that leptin increases the binding of PPARδ and PPARγ to PPRE sites in the promoters of two FAO-associated genes: long-chain acyl-CoA synthetase 1 (Acsl1) or carnitine palmitoyl transferase 1b (Cpt1b). When SENP2 is knocked down in myotubes, leptin-induced expression of FAO-associated enzymes and prolonged increase of FAO are suppressed, but rapid increase of FAO is unaffected. In addition, leptin-induced expression of FAO-associated enzymes was not observed in muscle tissue of SENP2 knockout mice.

CONCLUSIONS

We demonstrate that the peripheral actions of leptin on FAO are mediated by two different pathways: AMPK causes a rapid increase in FAO, and SENP2 of the STAT3 pathway causes a slow, prolonged increase in FAO.

Retinoid X receptor α overexpression alleviates mitochondrial dysfunction-induced insulin resistance through transcriptional regulation of insulin receptor substrate 1

Mitochondrial dysfunction is associated with insulin resistance and diabetes. We previously showed that retinoid X receptor α (RXRα) played an important role in transcriptional regulation of oxidative phosphorylation (OXPHOS) genes in cells with mitochondrial dysfunction caused by mitochondrial DNA mutation. In this study, we investigated whether mitochondrial dysfunction induced by incubation with OXPHOS inhibitors affects insulin receptor substrate 1 (IRS1) mRNA and protein levels and whether RXRα activation or overexpression can restore IRS1 expression. Both IRS1 and RXRα protein levels were significantly reduced when C2C12 myotubes were treated with the OXPHOS complex inhibitors, rotenone and antimycin A. The addition of RXRα agonists, 9-cis retinoic acid (9cRA) and LG1506, increased IRS1 transcription and protein levels and restored mitochondrial function, which ultimately improved insulin signaling. RXRα overexpression also increased IRS1 transcription and mitochondrial function. Because RXRα overexpression, knock-down, or activation by LG1506 regulated IRS1 transcription mostly independently of mitochondrial function, it is likely that RXRα directly regulates IRS1 transcription. Consistent with the hypothesis, we showed that RXRα bound to the IRS1 promoter as a heterodimer with peroxisome proliferator-activated receptor δ (PPARδ). These results suggest that RXRα overexpression or activation alleviates insulin resistance by increasing IRS1 expression.

Determination of selected reaction monitoring peptide transitions via multiplexed product-ion scan modes

RATIONALE

Although in silico prediction of selected reaction monitoring (SRM) peptide transitions is the most commonly used approach in quantitative proteomics, systematically detectable peptide transitions selected from actual experimental data are desirable. Here, we demonstrated the use of two triple quadrupole mass spectrometry (QqQ-MS) operation modes to identify reliable SRM peptide transitions of target peptides selected from a shotgun proteomic linear ion-trap mass spectrometry (LIT-MS) profiling dataset.

METHODS

Transition ions (Q1 and Q3 ions) of target peptides were selected from the LIT MS/MS spectra. We performed multiplexed SRM blindly for the selected transition ions of target peptides using QqQ-MS and selected peptide transitions for which the chromatographically aligned and correlated ion intensities to the corresponding fragment ions appeared in the LIT MS/MS spectra. The identities of the peptides were further confirmed by MS/MS spectra acquired via SRM-triggered MS/MS on QqQ-MS.

RESULTS

Despite the different MS platforms, we observed similar MS/MS patterns and relative ion abundance using both LIT-MS and QqQ-MS. Therefore, we were able to determine peptide transitions based on matching the chromatographic peak areas of all the selected Q3 ions of target peptides by the order of the corresponding ion intensities in the LIT MS/MS spectra. This approach demonstrated an efficient method to determine SRM peptide transitions, particularly when the target proteins are in low abundance and are therefore not easily detected by the QqQ full MS/MS scan mode. We employed this approach to determine the SRM peptide transitions of mitochondrial oxidative phosphorylation (OXPHOS) proteins involved in mitochondrial ATP synthesis.

CONCLUSIONS

The multiplexed product-ion scan mode using QqQ-MS generates systematically detectable peptide transitions in a single liquid chromatography/MS run, in which we were able to identify SRM peptides that represent known target proteins in complex biological samples. The method presented here is easy to implement and has high-throughput capabilities as a result of the short analysis time. It is therefore well suited for the design of optimal SRM experiments.

ULK1 Regulates Hepatic Lipid Metabolism via Autophagy Independent Mechanisms

Non-alcoholic steatohepatitis (NASH), a major complication of obesity, diabetes, and metabolic syndrome has emerged as a leading cause of chronic liver disease and a risk factor for hepatocellular carcinoma. Autophagy is a critical pathway for the degradation of intracellular components by lysosomes. Established functions for autophagy in hepatic lipid metabolism and insulin sensitivity suggest a mechanistic link between altered autophagy and NASH. However, the interactions between insulin sensitivity, NASH, and autophagy are incompletely understood. The Unc-51 Like Autophagy Activating Kinase 1 (ULK1) is the only serine/threonine kinase in the core autophagy pathway and thus represents an excellent drug target. In this study, we observed that ULK1 may directly regulate insulin signaling and lipid metabolism via mechanisms that might involve modulation of AKT dephosphorylation. Surprisingly, silencing ULK1 did not significantly alter autophagy in hepatocytes despite impairing insulin-stimulated activation of AKT. To further elucidate the autophagy-independent role of ULK1 in hepatic lipid metabolism and insulin action, ULK1 liver-specific knock-out mice were generated. L-ULK1 KO mice exhibited impaired glucose tolerance and insulin resistance on a normal chow diet or 60% high-fat diet (HFD). In young mice (4 weeks after birth), the expression of genes that regulate de novo lipogenesis, such as FAS, SCD1, and SREBP1-c were induced in livers of L-ULK1KO mice even prior to the development of insulin resistance and obesity. Hepatomegaly and lipid accumulation developed in L-ULK1KO on normal chow and was exacerbated relative to wild type mice on a HFD. Serum concentrations of insulin, triglyceride, cholesterol, AST and ALT were significantly increased. In contrast, L-ULK2 KO mice were phenotypically normal. To identify putative novel ULK1 targets, we conducted a phospho-proteomics screen in a ULK1 deficient hepatocyte cell line. We identified a relatively small number of novel proteins whose phosphorylation levels were reduced by ULK1 deficiency. The identification of these targets supports autophagy-independent mechanisms of action of ULK1. Recently, we confirmed that NCOA3, one of the targets regulates hepatic lipid metabolism by interacting directly with ULK1. These data suggest that ULK-1 may regulate cellular targets that regulate hepatic lipid metabolism and insulin sensitivity.

Cold accumulation of SCOF-1 transcripts is associated with transcriptional activation and mRNA stability

Cold acclimation enhances the transcription of several cold regulated (COR) genes. However, little is known about whether the elevation of the transcriptional level of the COR genes is due to transcriptional activation, or mRNA stability by a low temperature. Recently, we cloned a novel cold-inducible zinc finger protein gene from soybean, SCOF-1, which may function as a positive regulator of the COR gene expression . Here we report that the elevation of the SCOF-1 transcript level by cold stress is associated with both transcriptional activation and post-transcriptional mRNA stability under a low temperature. A nuclear run-on assay reveals that cold acclimation elevates the SCOF-1 transcript about three-fold compared to that of non-acclimated soybean nuclei. Furthermore, SCOF-1 transcripts increased substantially by a low temperature in transgenic tobacco plants that constitutively expressed SCOF-1 under the control of a constitutive cauliflower mosaic virus (CaMV) 35S promoter. When a transcription inhibitor, cordycepin, was treated with the deacclimating soybean cell, the decay level of the SCOF-1 transcripts was delayed significantly. This suggests that it may affect de novo protein synthesis, which degrades the SCOF-1 mRNA at room temperature. In addition, a secondary structure may be involved in the mRNA stability of SCOF-1 under a low temperature.

The MICOS Complex Regulates Mitochondrial Structure and Oxidative Stress During Age-Dependent Structural Deficits in the Kidney

The kidney filters nutrient waste and bodily fluids from the bloodstream, in addition to secondary functions of metabolism and hormone secretion, requiring an astonishing amount of energy to maintain its functions. In kidney cells, mitochondria produce adenosine triphosphate (ATP) and help maintain kidney function. Due to aging, the efficiency of kidney functions begins to decrease. Dysfunction in mitochondria and cristae, the inner folds of mitochondria, is a hallmark of aging. Therefore, age-related kidney function decline could be due to changes in mitochondrial ultrastructure, increased reactive oxygen species (ROS), and subsequent alterations in metabolism and lipid composition. We sought to understand if there is altered mitochondrial ultrastructure, as marked by 3D morphological changes, across time in tubular kidney cells. Serial block facing-scanning electron microscope (SBF-SEM) and manual segmentation using the Amira software were used to visualize murine kidney samples during the aging process at 3 months (young) and 2 years (old). We found that 2-year mitochondria are more fragmented, compared to the 3-month, with many uniquely shaped mitochondria observed across aging, concomitant with shifts in ROS, metabolomics, and lipid homeostasis. Furthermore, we show that the mitochondrial contact site and cristae organizing system (MICOS) complex is impaired in the kidney due to aging. Disruption of the MICOS complex shows altered mitochondrial calcium uptake and calcium retention capacity, as well as generation of oxidative stress. We found significant, detrimental structural changes to aged kidney tubule mitochondria suggesting a potential mechanism underlying why kidney diseases occur more readily with age. We hypothesize that disruption in the MICOS complex further exacerbates mitochondrial dysfunction, creating a vicious cycle of mitochondrial degradation and oxidative stress, thus impacting kidney health.

MICOS Complex Loss Governs Age-Associated Murine Mitochondrial Architecture and Metabolism in the Liver, While Sam50 Dictates Diet Changes

The liver, the largest internal organ and a metabolic hub, undergoes significant declines due to aging, affecting mitochondrial function and increasing the risk of systemic liver diseases. How the mitochondrial three-dimensional (3D) structure changes in the liver across aging, and the biological mechanisms regulating such changes confers remain unclear. In this study, we employed Serial Block Face-Scanning Electron Microscopy (SBF-SEM) to achieve high-resolution 3D reconstructions of murine liver mitochondria to observe diverse phenotypes and structural alterations that occur with age, marked by a reduction in size and complexity. We also show concomitant metabolomic and lipidomic changes in aged samples. Aged human samples reflected altered disease risk. To find potential regulators of this change, we examined the Mitochondrial Contact Site and Cristae Organizing System (MICOS) complex, which plays a crucial role in maintaining mitochondrial architecture. We observe that the MICOS complex is lost during aging, but not Sam50. Sam50 is a component of the sorting and assembly machinery (SAM) complex that acts in tandem with the MICOS complex to modulate cristae morphology. In murine models subjected to a high-fat diet, there is a marked depletion of the mitochondrial protein SAM50. This reduction in Sam50 expression may heighten the susceptibility to liver disease, as our human biobank studies corroborate that Sam50 plays a genetically regulated role in the predisposition to multiple liver diseases. We further show that changes in mitochondrial calcium dysregulation and oxidative stress accompany the disruption of the MICOS complex. Together, we establish that a decrease in mitochondrial complexity and dysregulated metabolism occur with murine liver aging. While these changes are partially be regulated by age-related loss of the MICOS complex, the confluence of a murine high-fat diet can also cause loss of Sam50, which contributes to liver diseases. In summary, our study reveals potential regulators that affect age-related changes in mitochondrial structure and metabolism, which can be targeted in future therapeutic techniques.

ATF4-dependent increase in mitochondrial-endoplasmic reticulum tethering following OPA1 deletion in skeletal muscle

Mitochondria and endoplasmic reticulum (ER) contact sites (MERCs) are protein- and lipid-enriched hubs that mediate interorganellar communication by contributing to the dynamic transfer of Ca2+, lipid, and other metabolites between these organelles. Defective MERCs are associated with cellular oxidative stress, neurodegenerative disease, and cardiac and skeletal muscle pathology via mechanisms that are poorly understood. We previously demonstrated that skeletal muscle-specific knockdown (KD) of the mitochondrial fusion mediator optic atrophy 1 (OPA1) induced ER stress and correlated with an induction of Mitofusin-2, a known MERC protein. In the present study, we tested the hypothesis that Opa1 downregulation in skeletal muscle cells alters MERC formation by evaluating multiple myocyte systems, including from mice and Drosophila, and in primary myotubes. Our results revealed that OPA1 deficiency induced tighter and more frequent MERCs in concert with a greater abundance of MERC proteins involved in calcium exchange. Additionally, loss of OPA1 increased the expression of activating transcription factor 4 (ATF4), an integrated stress response (ISR) pathway effector. Reducing Atf4 expression prevented the OPA1-loss-induced tightening of MERC structures. OPA1 reduction was associated with decreased mitochondrial and sarcoplasmic reticulum, a specialized form of ER, calcium, which was reversed following ATF4 repression. These data suggest that mitochondrial stress, induced by OPA1 deficiency, regulates skeletal muscle MERC formation in an ATF4-dependent manner.