SIRPα Mediates IGF1 Receptor in Cardiomyopathy Induced by Chronic Kidney Disease.

BACKGROUND: Chronic kidney disease (CKD) is characterized by increased myocardial mass despite near-normal blood pressure, suggesting the presence of a separate trigger. A potential driver is SIRPα (signal regulatory protein alpha)-a mediator impairing insulin signaling. The objective of this study is to assess the role of circulating SIRPα in CKD-induced adverse cardiac remodeling. METHODS: SIRPα expression was evaluated in mouse models and patients with CKD. Specifically, mutant, muscle-specific, or cardiac muscle-specific SIRPα KO (knockout) mice were examined after subtotal nephrectomy. Cardiac function was assessed by echocardiography. Metabolic responses were confirmed in cultured muscle cells or cardiomyocytes. RESULTS: We demonstrate that SIRPα regulates myocardial insulin/IGF1R (insulin growth factor-1 receptor) signaling in CKD. First, in the serum of both mice and patients, SIRPα was robustly secreted in response to CKD. Second, cardiac muscle upregulation of SIRPα was associated with impaired insulin/IGF1R signaling, myocardial dysfunction, and fibrosis. However, both global and cardiac muscle-specific SIRPα KO mice displayed improved cardiac function when compared with control mice with CKD. Third, both muscle-specific or cardiac muscle-specific SIRPα KO mice did not significantly activate fetal genes and maintained insulin/IGF1R signaling with suppressed fibrosis despite the presence of CKD. Importantly, SIRPα directly interacted with IGF1R. Next, rSIRPα (recombinant SIRPα) protein was introduced into muscle-specific SIRPα KO mice reestablishing the insulin/IGF1R signaling activity. Additionally, overexpression of SIRPα in myoblasts and cardiomyocytes impaired pAKT (phosphorylation of AKT) and insulin/IGF1R signaling. Furthermore, myotubes and cardiomyocytes, but not adipocytes treated with high glucose or cardiomyocytes treated with uremic toxins, stimulated secretion of SIRPα in culture media, suggesting these cells are the origin of circulating SIRPα in CKD. Both intracellular and extracellular SIRPα exert biologically synergistic effects impairing intracellular myocardial insulin/IGF1R signaling. CONCLUSIONS: Myokine SIRPα expression impairs insulin/IGF1R functions in cardiac muscle, affecting cardiometabolic signaling pathways. Circulating SIRPα constitutes an important readout of insulin resistance in CKD-induced cardiomyopathy.

Muscle Atrophy in CKD: A Historical Perspective of Advancements in Its Understanding.

OBJECTIVE: This perspective reviews the seminal clinical and experimental observations that led to today's current mechanistic model of muscle protein loss (wasting) in patients with chronic kidney disease (CKD). RESULTS AND CONCLUSION: Early International Society of Renal Nutrition and Metabolism (ISRNM) meetings facilitated discussions and hypotheses about the causes of muscle wasting in CKD. It became widely recognized that wasting is common and correlated with increased risks of mortality and morbidity. Although anorexia and dietary restrictions contribute to muscle loss, several features of CKD-associated wasting cannot be explained by malnutrition alone. The protein catabolism-inducing actions of metabolic acidosis, inflammation, insulin resistance, endocrine disorders and uremic toxins were progressively identified. Continued research to understand the interactions of inflammation, anabolic resistance, mitochondrial dysfunction, exercise, and nutrition on muscle protein turnover in patients with CKD will hopefully accelerate discoveries and treatments to ameliorate muscle wasting as well as the progression of CKD.

Mechanisms Regulating Muscle Protein Synthesis in CKD.

BACKGROUND: CKD induces loss of muscle proteins partly by suppressing muscle protein synthesis. Muscles of mice with CKD have increased expression of nucleolar protein 66 (NO66), as do muscle biopsy specimens from patients with CKD or those undergoing hemodialysis. Inflammation stimulates NO66 expression and changes in NF-κB mediate the response. METHODS: Subtotal nephrectomy created a mouse model of CKD with BUN >80 mg/dl. Crossing NO66flox/flox with MCK-Cre mice bred muscle-specific NO66 (MCK-NO66) knockout mice. Experiments assessed the effect of removing NO66. RESULTS: Muscle-specific NO66 knockout in mice blocks CKD-induced loss of muscle mass and improves protein synthesis. NO66 suppression of ribosomal biogenesis via demethylase activity is the mechanism behind these responses. In muscle cells, expression of NO66, but not of demethylase-dead mutant NO66, decreased H3K4me3 and H3K36me3 and suppressed pre-rRNA expression. Knocking out NO66 increased the enrichment of H3K4me3 and H3K36me3 on ribosomal DNA. In primary muscle cells and in muscles of mice without NO66, ribosomal RNA, pre-rRNA, and protein synthesis all increased. CONCLUSIONS: CKD suppresses muscle protein synthesis via epigenetic mechanisms that NO66 mediates. Blocking NO66 could suggest strategies that counter CKD-induced abnormal muscle protein catabolism.

Pharmacokinetics and pharmacodynamics of TTI-101, a STAT3 inhibitor that blocks muscle proteolysis in rats with chronic kidney disease.

Loss of muscle proteins increases the morbidity and mortality of patients with chronic kidney disease (CKD), and there are no reliable preventive treatments. We uncovered a STAT3/CCAAT-enhancer-binding protein-δ to myostatin signaling pathway that activates muscle protein degradation in mice with CKD or cancer; we also identified a small-molecule inhibitor of STAT3 (TTI-101) that blocks this pathway. To evaluate TTI-101 as a treatment for CKD-induced cachexia, we measured TTI-101 pharmacokinetics and pharmacodynamics in control and CKD rats that were orally administered TTI-101or its diluent. The following two groups of gavage-fed rats were studied: sham-operated control rats and CKD rats. Plasma was collected serially (0, 0.25, 0.5, 1, 2, 4, 8, and 24 h) following TTI-101 administration (at oral doses of 0, 10, 30, or 100 mg/kg). Plasma levels of TTI-101 were measured by LC-MS/MS, and pharmacokinetic results were analyzed with the PKSolver program. Plasma TTI-101 levels increased linearly with doses; the maximum plasma concentrations and time to maximal plasma levels (~1 h) were similar in sham-operated control rats and CKD rats. Notably, gavage treatment of TTI-101 for 3 days produced TTI-101 muscle levels in sham control rats and CKD rats that were not significantly different. CKD rats that received TTI-101 for 7 days had suppression of activated STAT3 and improved muscle grip strength; there also was a trend for increasing body and muscle weights. TTI-101 was tolerated at doses of 100 mg·kg-1·day-1 for 7 days. These results with TTI-101 in rats warrant its development as a treatment for cachexia in humans.

Stat3 activation induces insulin resistance via a muscle-specific E3 ubiquitin ligase Fbxo40.

Cellular mechanisms causing insulin resistance (IR) in chronic kidney disease (CKD) are poorly understood. One potential mechanism is that CKD-induced inflammation activates the signal transducer and activator of transcription 3 (Stat3) in muscle. We uncovered increased p-Stat3 in muscles of mice with CKD or mice fed high-fat diet (HFD). Activated Stat3 stimulates the expression of Fbxo40, a muscle-specific E3 ubiquitin ligase that stimulates ubiquitin conjugation leading to degradation of insulin receptor substrate 1 (IRS1). Evidence that Stat3 activates Fbxo40 includes 1) potential Stat3 binding sites in Fbxo40 promoters; 2) Stat3 binding to the Fbxo40 promoter; and 3) constitutively active Stat3 stimulating both Fbxo40 expression and its promoter activity. We found that IL-6 activates Stat3 in myotubes, increasing Fbxo40 expression with reduced IRS1 and p-Akt. Knockdown Fbxo40 using siRNA from myotubes results in higher levels of IRS1 and p-Akt despite the presence of IL-6. We treated mice with a small-molecule inhibitor of Stat3 (TTI-101) and found improved glucose tolerance and insulin signaling in skeletal muscles of mice with CKD or fed an HFD. Finally, we uncovered improved glucose tolerance in mice with muscle-specific Stat3 KO versus results in Stat3f/f mice in response to the HFD. Thus Stat3 activation in muscle increases IR in mice. Inhibition of Stat3 by TTI-101 could be developed into clinical strategies to improve muscle insulin signaling in inflammation and other catabolic diseases.

Effects of Sodium Bicarbonate in CKD Stages 3 and 4: A Randomized, Placebo-Controlled, Multicenter Clinical Trial.

RATIONALE & OBJECTIVE: Metabolic acidosis associated with chronic kidney disease (CKD) may contribute to muscle dysfunction and bone disease. We aimed to test whether treatment with sodium bicarbonate improves muscle and bone outcomes. STUDY DESIGN: Multicenter, randomized, placebo-controlled, clinical trial. SETTING & PARTICIPANTS: 149 patients with CKD stages 3 and 4 between July 2011 and April 2016 at 3 centers in Cleveland, OH, and the Bronx, NY. INTERVENTION: Sodium bicarbonate (0.4 mEq per kg of ideal body weight per day) (n=74) or identical-appearing placebo (n=75). OUTCOMES: Dual primary outcomes were muscle function assessed using sit-to-stand test and bone mineral density. Muscle biopsies were performed at baseline and 2 months. Participants were seen at baseline and 2, 6, 12, and 24 months. RESULTS: Mean baseline serum bicarbonate level was 24.0±2.2 (SD) mEq/L and mean baseline estimated glomerular filtration rate was 36.3±11.2mL/min/1.73m2. Baseline characteristics did not differ between groups. Mean serum bicarbonate levels in the intervention arm during follow-up were 26.4±2.2, 25.5±2.3, 25.6±2.6, and 24.4±2.8 mEq/L (at 2, 6, 12, and 24 months). These were significantly higher than in the placebo group (P<0.001). Compared to the placebo group, participants randomly assigned to sodium bicarbonate treatment had no significant differences in sit-to-stand time (5 repetitions: P=0.1; and 10 repetitions P=0.07) or bone mineral density (P=0.3). Sodium bicarbonate treatment caused a decrease in serum potassium levels that was of borderline statistical significance (P=0.05). There were no significant differences in estimated glomerular filtration rates, blood pressure, weight, serious adverse events, or levels of muscle gene expression between the randomly assigned groups. LIMITATIONS: Initial mean serum bicarbonate level was in the normal range. CONCLUSIONS: Sodium bicarbonate therapy in patients with CKD stages 3 and 4 significantly increases serum bicarbonate and decreases potassium levels. No differences were found in muscle function or bone mineral density between the randomly assigned groups. Larger trials are required to evaluate effects on kidney function. FUNDING: National Institutes of Health grant. TRIAL REGISTRATION: Registered at ClinicalTrials.gov with study number NCT01452412.

Phosphoinositide 3-kinase γ deficiency attenuates kidney injury and fibrosis in angiotensin II-induced hypertension.

BACKGROUND: We have shown that the CXCL16/CXCR6 axis plays a critical role in recruiting inflammatory cells and bone marrow-derived fibroblasts into the kidney leading to renal injury and fibrosis. However, the underlying signaling mechanisms are not known. METHODS: In the present study, we examined the role of phosphoinositide-3 kinase γ (PI3Kγ) signaling in the recruitment of inflammatory cells and bone marrow-derived fibroblasts into the kidney and development of renal injury and fibrosis in an experimental model of hypertension induced by angiotensin II. RESULTS: Blood pressure was comparable between wild-type (WT) and PI3Kγ knockout (KO) mice at baseline. Angiotensin II treatment led to an increase in blood pressure that was similar between WT and PI3Kγ KO mice. Compared with WT mice, PI3Kγ KO mice were protected from angiotensin II-induced renal dysfunction and injury and developed less proteinuria. PI3Kγ deficiency suppressed bone marrow-derived fibroblast accumulation and myofibroblast formation in the kidney and inhibited total collagen deposition and extracellular matrix protein production in the kidney in response to angiotensin II. PI3Kγ deficiency inhibited the infiltration of F4/80+ macrophages and CD3+ T cells into the kidney and reduced gene expression levels of pro-inflammatory cytokines in the kidney following angiotensin II treatment. Finally, inhibition of PI3Kγ suppressed CXCL16-induced monocyte migration in vitro. CONCLUSION: These results indicate that PI3Kγ mediates the influx of macrophages, T cells and bone marrow-derived fibroblasts into the kidney resulting in kidney injury and fibrosis.

Notch signaling in bone marrow-derived FSP-1 cells initiates neointima formation in arteriovenous fistulas.

Neointima formation is a major contributor to arteriovenous fistula (AVF) failure. We have previously shown that activation of the Notch signaling pathway contributes to neointima formation by promoting the migration of vascular smooth muscle cells (VSMCs) into the venous anastomosis. In the current study we investigated the mechanisms underlying the dedifferentiation and migration of VSMCs, and in particular the role of bone marrow-derived fibroblast specific protein 1 (FSP-1)+ cells, another cell type found in models of vascular injury. Using VSMC-specific reporter mice, we found that most of the VSMCs participating in AVF neointima formation originated from dedifferentiated VSMCs. We also observed infiltration of bone marrow-derived FSP-1+ cells into the arterial anastomosis where they could interact with VSMCs. In vitro, conditioned media from FSP-1+ cells stimulated VSMC proliferation and phenotype switching. Activated Notch signaling transformed FSP-1+ cells into type I macrophages and stimulated secretion of cytokines and growth factors. Pretreatment with a Notch inhibitor or knockout of the canonical downstream factor RBP-Jκ in bone marrow-derived FSP1+ cells decreased FSP1+ cell infiltration into murine AVFs, attenuating VSMC dedifferentiation and neointima formation. Our results suggest that targeting Notch signaling could provide a new therapeutic strategy to improve AVF patency.

Successful Kidney Transplantation Is Associated With Weight Gain From Truncal Obesity and Insulin Resistance.

OBJECTIVE: The objective of this study is to compare changes in body composition, lifestyle factors, and metabolic responses occurring in living kidney transplant recipient patients after transplantation. DESIGN AND METHODS: The study was a single-site, prospective, observational study. To identify metabolic responses during the initial years after transplantation, we obtained state-of-the-art, high-resolution measurements of body composition from a 4-compartment model using dual-energy X-ray absorptiometry, air displacement plethysmography, and total body potassium and nitrogen counters. We also assessed dietary recalls and actigraphy before transplantation and 3- and 12-month after transplantation. The study was conducted at a quaternary care hospital outpatient transplant center and a United States Department of Agriculture Agricultural Research Service center. Thirty-one adults receiving a living donor kidney allograft were studied. The main outcome measures were change in body composition at 3 months and 1 year after transplantation, and this was correlated with the occurrence of insulin resistance. RESULTS: In patients receiving a successful kidney transplant from living donors treated with standard immunosuppression, significant increases in body weight were detected at 3 and 12 months after transplantation (2.2 kg, P = .03 and 6.6 kg, P < .0001, respectively). Weight gain was principally due to adipose tissue accumulation in the truncal region. There was no increase in muscle mass or fluid accumulation. Weight gain was not associated with changes in resting energy expenditure or physical activity. Notably, increases in visceral and subcutaneous adipose tissue were positively correlated with insulin resistance. CONCLUSION: Successful transplantation was associated with increased insulin resistance and weight gain without increases in muscle or fluid. This metabolic pattern suggests potential interventions that could prevent or mitigate the consequences of adipose tissue accumulation in transplant recipients.

AMP-activated protein kinase/myocardin-related transcription factor-A signaling regulates fibroblast activation and renal fibrosis.

Chronic kidney disease is a major cause of death, and renal fibrosis is a common pathway leading to the progression of this disease. Although activated fibroblasts are responsible for the production of the extracellular matrix and the development of renal fibrosis, the molecular mechanisms underlying fibroblast activation are not fully defined. Here we examined the functional role of AMP-activated protein kinase (AMPK) in the activation of fibroblasts and the development of renal fibrosis. AMPKα1 was induced in the kidney during the development of renal fibrosis. Mice with global or fibroblast-specific knockout of AMPKα1 exhibited fewer myofibroblasts, developed less fibrosis, and produced less extracellular matrix protein in the kidneys following unilateral ureteral obstruction or ischemia-reperfusion injury. Mechanistically, AMPKα1 directly phosphorylated cofilin leading to cytoskeleton remodeling and myocardin-related transcription factor-A nuclear translocation resulting in fibroblast activation and extracellular matrix protein production. Thus, AMPK may be a critical regulator of fibroblast activation through regulation of cytoskeleton dynamics and myocardin-related transcription factor-A nuclear translocation. Hence, AMPK signaling may represent a novel therapeutic target for fibrotic kidney disease.

Cystatin C Is a Gender-Neutral Glomerular Filtration Rate Biomarker in Patients with Cirrhosis.

BACKGROUND: Lower serum Cr levels in women as compared to men result in underestimation of renal dysfunction and lower model for end-stage liver disease-sodium scores leading to reduced access to liver transplantation in women compared to men with comparable hepatic dysfunction. AIM: The aim of this study was to determine the gender differences in serum Cr, cystatin C, and other endogenous glomerular filtration rate (GFR) biomarkers, measured and estimated GFR, Cr clearance, and Cr production rates. METHODS: We measured GFR by iothalamate plasma clearance in 103 patients with cirrhosis and assessed gender differences in GFR, Cr clearance and production rate, serum Cr, cystatin C and other endogenous GFR biomarkers including beta-trace protein, beta-2 microglobulin, and dimethylarginines. RESULTS: Comparison of men and women showed significantly lower values for mean serum Cr (0.97 vs. 0.82 mg/dl, P = 0.023), and Cr production rate (13.37 vs. 11.02 mg/kg/day, P = 0.022). In contrast to the serum Cr and Cr production rate, men and women exhibited no significant differences in the means of serum cystatin C and other GFR biomarkers, measured GFR, GFR estimated using Cr-cystatin C GFR equation for cirrhosis, measured and estimated Cr clearances. After controlling for age, race, weight, height, and GFR, female gender remained associated with lower serum Cr levels (P = 0.003). Serum cystatin C levels were not associated with gender, age, race, weight, height, C-reactive protein, and history of hypothyroidism. CONCLUSIONS: Our results suggest that cystatin C and endogenous GFR biomarkers other than Cr, measured GFR, GFR estimated by Cr-cystatin C GFR equation for cirrhosis, measured and estimated Cr clearance minimized between-gender biases in accounting for renal function in patients with cirrhosis. Therefore, serum cystatin C should be measured as a complementary test to serum Cr when renal function is assessed in patients with cirrhosis, particularly in women and those with sarcopenia.

The pathway to muscle fibrosis depends on myostatin stimulating the differentiation of fibro/adipogenic progenitor cells in chronic kidney disease.

Fibrosis in skeletal muscle develops after injury or in response to chronic kidney disease (CKD), but the origin of cells becoming fibrous tissue and the initiating and sustaining mechanisms causing muscle fibrosis are unclear. We identified muscle fibro/adipogenic progenitor cells (FAPs) that potentially differentiate into adipose tissues or fibrosis. We also demonstrated that CKD stimulates myostatin production in muscle. Therefore, we tested whether CKD induces myostatin, which stimulates fibrotic differentiation of FAPs leading to fibrosis in skeletal muscles. We isolated FAPs from mouse muscles and found that myostatin stimulates their proliferation and conversion into fibrocytes. In vivo, FAPs isolated from EGFP-transgenic mice (FAPs-EGFP) were transplanted into muscles of mice with CKD or into mouse muscles that were treated with myostatin. CKD or myostatin stimulated FAPs-EGFP proliferation in muscle and increased α-smooth muscle actin expression in FAP-EGFP cells. When myostatin was inhibited with a neutralizing peptibody (a chimeric peptide-Fc fusion protein), the FAP proliferation and muscle fibrosis induced by CKD were both suppressed. Knocking down Smad3 in cultured FAPs interrupted their conversion into fibrocytes, indicating that myostatin directly converts FAPs into fibrocytes. Thus, counteracting myostatin may be a strategy for preventing the development of fibrosis in skeletal muscles of patients with CKD.

The IL-4 receptor α has a critical role in bone marrow-derived fibroblast activation and renal fibrosis.

Renal fibrosis is a common pathway leading to the progression of chronic kidney disease, and bone marrow-derived fibroblasts contribute significantly to the development of renal fibrosis. However, the signaling mechanisms underlying the activation of these fibroblasts are not completely understood. Here, we examined the role of IL-4 receptor α (IL-4Rα) in the activation of myeloid fibroblasts in two experimental models of renal fibrosis. Compared with wild-type mice, IL-4Rα knockout mice accumulated fewer bone marrow-derived fibroblasts and myofibroblasts in their kidneys. IL-4Rα deficiency suppressed the expression of α-smooth muscle actin, extracellular matrix proteins and the development of renal fibrosis. Furthermore, IL-4Rα deficiency inhibited the activation of signal transducer and activator of transcription 6 (STAT6) in the kidney. Moreover, wild-type mice engrafted with bone marrow cells from IL-4Rα knockout mice exhibited fewer myeloid fibroblasts in the kidney and displayed less severe renal fibrosis following ureteral obstructive injury compared with wild-type mice engrafted with wild-type bone marrow cells. In vitro, IL-4 activated STAT6 and stimulated expression of α-smooth muscle actin and fibronectin in mouse bone marrow monocytes. This was abolished in the absence of IL-4Rα. Thus, IL-4Rα plays an important role in bone marrow-derived fibroblast activation, resulting in extracellular matrix protein production and fibrosis development. Hence, the IL-4Rα/STAT6 signaling pathway may serve as a novel therapeutic target for chronic kidney disease.

Parathyroid hormone stimulates adipose tissue browning: a pathway to muscle wasting.

PURPOSE OF REVIEW: Studying organ-to-organ communications (i.e. crosstalk) uncovers mechanisms regulating metabolism in several tissues. What is missing is identification of mediators of different catabolic conditions contributing to losses of adipose and muscle tissues. Identifying mediators involved in organ-to-organ crosstalk could lead to innovative therapeutic strategies because several disorders such as chronic kidney disease (CKD), cancer cachexia, and other catabolic conditions share signals of worsening metabolism and increased risk of mortality. RECENT FINDINGS: A recent breakthrough published in Cell Metabolism leads to the conclusion that parathyroid hormone (PTH) and parathyroid hormone-related peptide (PTHrP) cause 'browning' of white adipose tissue plus energy production via activation of uncoupling protein-1. Browning was associated with muscle wasting in mouse models of cancer and CKD. The pathway to browning includes PTH/PTHrP activation of protein kinase A (PKA) and lost muscle mass via the ubiquitin proteasome proteolytic system (UPS). SUMMARY: The results suggest that crosstalk between muscle and fat contributes in a major way to tissue catabolism. The pathway initiated by PTH or PTHrP is novel and it suggests potential interrelationships that control metabolism in other catabolic conditions. Identifying how the parathyroid hormone-PKA-UPS axis relates to obesity, type 2 diabetes, and other insulin-resistant conditions remains unclear.

CKD Stimulates Muscle Protein Loss Via Rho-associated Protein Kinase 1 Activation.

In patients with CKD, muscle wasting is common and is associated with morbidity and mortality. Mechanisms leading to loss of muscle proteins include insulin resistance, which suppresses Akt activity and thus stimulates protein degradation via the ubiquitin-proteasome system. However, the specific factors controlling CKD-induced suppression of Akt activity in muscle remain undefined. In mice with CKD, the reduction in Akt activity in muscle exceeded the decrease in upstream insulin receptor substrate-1-associated phosphatidylinositol 3-kinase activity, suggesting that CKD activates other pathways that suppress Akt. Furthermore, a CKD-induced increase uncovered caspase-3 activity in muscle in these mice. In C2C12 muscle cells, activated caspase-3 cleaves and activates Rho-associated protein kinase 1 (ROCK1), which enhances the activity of phosphatase and tensin homolog (PTEN) and reduces Akt activity. Notably, constitutive activation of ROCK1 also led to increased caspase-3 activity in vitro. In mice with either global ROCK1 knockout or muscle-specific PTEN knockout, CKD-associated muscle proteolysis was blunted. These results suggest ROCK1 activation in CKD and perhaps in other catabolic conditions can promote loss of muscle protein via a negative feedback loop.

Inhibition of Stat3 activation suppresses caspase-3 and the ubiquitin-proteasome system, leading to preservation of muscle mass in cancer cachexia.

Cachexia occurs in patients with advanced cancers. Despite the adverse clinical impact of cancer-induced muscle wasting, pathways causing cachexia are controversial, and clinically reliable therapies are not available. A trigger of muscle protein loss is the Jak/Stat pathway, and indeed, we found that conditioned medium from C26 colon carcinoma (C26) or Lewis lung carcinoma cells activates Stat3 (p-Stat3) in C2C12 myotubes. We identified two proteolytic pathways that are activated in muscle by p-Stat3; one is activation of caspase-3, and the other is p-Stat3 to myostatin, MAFbx/Atrogin-1, and MuRF-1 via CAAT/enhancer-binding protein δ (C/EBPδ). Using sequential deletions of the caspase-3 promoter and CHIP assays, we determined that Stat3 activation increases caspase-3 expression in C2C12 cells. Caspase-3 expression and proteolytic activity were stimulated by p-Stat3 in muscles of tumor-bearing mice. In mice with cachexia caused by Lewis lung carcinoma or C26 tumors, knock-out of p-Stat3 in muscle or with a small chemical inhibitor of p-Stat3 suppressed muscle mass losses, improved protein synthesis and degradation in muscle, and increased body weight and grip strength. Activation of p-Stat3 stimulates a pathway from C/EBPδ to myostatin and expression of MAFbx/Atrogin-1 and increases the ubiquitin-proteasome system. Indeed, C/EBPδ KO decreases the expression of MAFbx/Atrogin-1 and myostatin, while increasing muscle mass and grip strength. In conclusion, cancer stimulates p-Stat3 in muscle, activating protein loss by stimulating caspase-3, myostatin, and the ubiquitin-proteasome system. These results could lead to novel strategies for preventing cancer-induced muscle wasting.

In experimental chronic kidney disease or cancer, parathyroid hormone is a novel mediator of cachexia.

Hyperparathyroidism plays a central role in the disordered bone mineral metabolism of chronic kidney disease, and has been associated with increased cardiovascular morbidity and mortality in that setting. A recent study suggests a novel role for parathyroid hormone and its receptor in muscle wasting and cachexia occurring in advanced chronic kidney disease.

Inhibition of myostatin in mice improves insulin sensitivity via irisin-mediated cross talk between muscle and adipose tissues.

BACKGROUND/OBJECTIVE: In mice, a high-fat diet (HFD) induces obesity, insulin resistance and myostatin production. We tested whether inhibition of myostatin in mice can reverse these HFD-induced abnormalities. SUBJECTS/METHODS: C57BL/6 mice were fed a HFD for 16 weeks including the final 4 weeks some mice were treated with an anti-myostatin peptibody. Body composition, the respiratory exchange ratio plus glucose and insulin tolerance tests were examined. Myostatin knock down in C2C12 cells was performed using small hairpin RNA lentivirus. Adipose tissue-derived stem cells were cultured to measure their responses to conditioned media from C2C12 cells lacking myostatin, or to recombinant myostatin or irisin. Isolated peritoneal macrophages were treated with myostatin or irisin to determine whether myostatin or irisin induce inflammatory mechanisms. RESULTS: In HFD-fed mice, peptibody treatment stimulated muscle growth and improved insulin resistance. The improved glucose and insulin tolerances were confirmed when we found increased muscle expression of p-Akt and the glucose transporter, Glut4. In HFD-fed mice, the peptibody suppressed macrophage infiltration and the expression of proinflammatory cytokines in both the muscle and adipocytes. Inhibition of myostatin caused the conversion of white (WAT) to brown adipose tissue, whereas stimulating fatty acid oxidation and increasing energy expenditure. The related mechanism is a muscle-to-fat cross talk mediated by irisin. Myostatin inhibition increased peroxisome proliferator-activated receptor gamma, coactivator 1α expression and irisin production in the muscle. Irisin then stimulated WAT browning. Irisin also suppresses inflammation and stimulates macrophage polarization from M1 to M2 types. CONCLUSIONS: These results uncover a metabolic pathway from an increase in myostatin that suppresses irisin leading to the activation of inflammatory cytokines and insulin resistance. Thus, myostatin is a potential therapeutic target to treat insulin resistance of type II diabetes as well as the shortage of brown/beige fat in obesity.

Protective role of insulin-like growth factor-1 receptor in endothelial cells against unilateral ureteral obstruction-induced renal fibrosis.

Insulin-like growth factor-1 receptor (IGF-1R) can regulate vascular homeostasis and endothelial function. We studied the role of IGF-1R in oxidative stress-induced endothelial dysfunction. Unilateral ureteral obstruction (UUO) was performed in wild-type (WT) mice and mice with endothelial cell (EC)-specific IGF-1R knockout (KO). After UUO in endothelial IGF-1R KO mice, endothelial barrier dysfunction was more severe than in WT mice, as seen by increased inflammatory cell infiltration and vascular endothelial (VE)-cadherin phosphorylation. UUO in endothelial IGF-1R KO mice increased interstitial fibroblast accumulation and enhanced extracellular protein deposition as compared with the WT mice. Endothelial barrier function measured by transendothelial migration in response to hydrogen peroxide (H2O2) was impaired in ECs. Silencing IGF-1R enhanced the influence of H2O2 in disrupting the VE-protein tyrosine phosphatase/VE-cadherin interaction. Overexpression of IGF-1R suppressed H2O2-induced endothelial barrier dysfunction. Furthermore, by using the piggyBac transposon system, we expressed IGF-1R in VE cells in mice. The expression of IGF-1R in ECs also suppressed the inflammatory cell infiltration and renal fibrosis induced by UUO. IGF-1R KO in the VE-cadherin lineage of bone marrow cells had no significant effect on the UUO-induced fibrosis, as compared with control mice. Our results indicate that IGF-1R in the endothelium maintains the endothelial barrier function by stabilization of the VE-protein tyrosine phosphatase/VE-cadherin complex. Decreased expression of IGF-1R impairs endothelial function and increases the fibrosis of kidney disease.