Phosphorylation of CCAAT/enhancer-binding protein alpha regulates GLUT4 expression and glucose transport in adipocytes

The conserved Mediator subunit cyclin C (CCNC) is required for brown adipocyte development and lipid accumulation

OBJECTIVE

Cyclin C (CCNC) is the most conserved subunit of the Mediator complex, which is an important transcription cofactor. Recently, we have found that CCNC facilitates brown adipogenesis in vitro by activating C/EBPα-dependent transcription. However, the role of CCNC in brown adipose tissue (BAT) in vivo remains unclear.

METHODS

We generated conditional knock-out mice by crossing Ccncflox/flox mice with Myf5Cre, Ucp1Cre or AdipoqCre transgenic mice to investigate the role of CCNC in BAT development and function. We applied glucose and insulin tolerance test, cold exposure and indirect calorimetry to capture the physiological phenotypes and used immunostaining, immunoblotting, qRT-PCR, RNA-seq and cell culture to elucidate the underlying mechanisms.

RESULTS

Here, we show that deletion of CCNC in Myf5+ progenitor cells caused BAT paucity, despite the fact that there was significant neonatal lethality. Mechanistically different from in vitro, CCNC deficiency impaired the proliferation of embryonic brown fat progenitor cells without affecting brown adipogenesis or cell death. Interestingly, CCNC deficiency robustly reduced age-dependent lipid accumulation in differentiated brown adipocytes in all three mouse models. Mechanistically, CCNC in brown adipocytes is required for lipogenic gene expression through the activation of the C/EBPα/GLUT4/ChREBP axis. Consistent with the importance of de novo lipogenesis under carbohydrate-rich diets, high-fat diet (HFD) feeding abolished CCNC deficiency -caused defects of lipid accumulation in BAT. Although insulin sensitivity and response to acute cold exposure were not affected, CCNC deficiency in Ucp1+ cells enhanced the browning of white adipose tissue (beiging) upon prolonged cold exposure.

CONCLUSIONS

Together, these data indicate an important role of CCNC-Mediator in the regulation of BAT development and lipid accumulation in brown adipocytes.

Activating transcription factor 3 coordinates differentiation of cardiac and hematopoietic progenitors by regulating glucose metabolism

The cardiac and hematopoietic progenitors (CPs and HPs, respectively) in the mesoderm ultimately form a well-organized circulation system, but mechanisms that reconcile their development remain elusive. We found that activating transcription factor 3 (atf3) was highly expressed in the CPs, HPs, and mesoderm, in zebrafish. The atf3 ^-/- mutants exhibited atrial dilated cardiomyopathy and a high ratio of immature myeloid cells. These manifestations were primarily caused by the blockade of differentiation of both CPs and HPs within the anterior lateral plate mesoderm. Mechanistically, Atf3 targets cebpγ to repress slc2a1a-mediated glucose utilization. The high rate of glucose metabolism in atf3 ^-/- mutants inhibited the differentiation of progenitors by changing the redox state. Therefore, atf3 could provide CPs and HPs with metabolic adaptive capacity to changes in glucose levels. Our study provides new insights into the role of atf3 in the coordination of differentiation of CPs and HPs by regulating glucose metabolism.

Estradiol stimulates adipogenesis and Slc2a4/GLUT4 expression via ESR1-mediated activation of CEBPA

The ability of adipose tissue to expand is dependent on adipocyte differentiation and adipose tissue glucose disposal. The CCAAT/enhancer-binding protein alpha (CEBPA) enhances the expression of the Slc2a4 gene and GLUT4 protein, which are markers of adipocyte differentiation/glucose disposal. We hypothesized estradiol (E2) facilitates adipocyte differentiation/glucose disposal by an estrogen receptor 1 (ESR1)-dependent and CEBPA-mediated mechanism. Our results suggest that E2 (10 nM) has a positive effect on 3T3-L1 adipocyte differentiation (days 2-8), lipid accumulation, Slc2a4 and Cebpa mRNA expression, total GLUT4 and nuclear CEBPA contents, and CEBP/Slc2a4-binding activity. Esr1 silencing (∼50%) in mature adipocytes abrogates the 24-h E2 effects on nuclear CEBPA content, Slc2a4/GLUT4 expression and GLUT4 translocation to the cell membrane. Thus, E2 stimulates adipocyte differentiation and Slc2a4/GLUT4 expression in an ESR1/CEBPA-mediated pathway. Our data provide mechanistic insight demonstrating E2 participates in adipose-tissue differentiation and glucose transporter expression which ultimately can improve adipose tissue expandability and glycemic control.

Dlk1 expression relates to visceral fat expansion and insulin resistance in male and female rats with postnatal catch-up growth

BACKGROUND

Although prenatal and postnatal programming of metabolic diseases in adulthood is well established, the mechanisms underpinning metabolic programming are not. Dlk1, a key regulator of fetal development, inhibits adipocyte differentiation and restricts fetal growth.

METHODS

Assess DLk1 expression in a Wistar rat model of catch-up growth following intrauterine restriction. Dams fed ad libitum delivered control pups (C) and dams on a 50% calorie-restricted diet delivered pups with low birth weight (R). Restricted offspring fed a standard rat chow showed catch-up growth (R/C) but those kept on a calorie-restricted diet did not (R/R).

RESULTS

Decreased Dlk1 expression was observed in adipose tissue and skeletal muscle of R/C pups along with excessive visceral fat accumulation, decreased circulating adiponectin, increased triglycerides and HOMA-IR (from p < 0.05 to p < 0.0001). Moreover, in R/C pups the reduced Dlk1 expression in adipose tissue and skeletal muscle correlated with visceral fat (r = -0.820, p < 00001) and HOMA-IR (r = -0.745, p = 0.002).

CONCLUSIONS

Decreased Dlk1 expression relates to visceral fat expansion and insulin resistance in a rat model of catch-up growth following prenatal growth restriction. Modulation of Dlk1 expression could be among the targets for the early prevention of fetal programming of adult metabolic disorders.

Conjugated Linoleic Acid Supplementation Improves Maternal High Fat Diet-Induced Programming of Metabolic Dysfunction in Adult Male Rat Offspring

The developmental origins of health and disease hypothesis proposes that an adverse early life environment, including in utero exposure to a maternal obesogenic environment, can lead to an increased long-term risk of obesity and related metabolic complications in offspring. We assessed whether maternal supplementation with conjugated linoleic acid (CLA) could prevent some of these adverse effects in offspring exposed to a maternal high fat diet. Sprague-Dawley dams consumed either a: control (CD), control with CLA (CLA), high fat (HF) or high fat with CLA (HFCLA) diet 10 days prior to mating and throughout pregnancy/lactation. Male offspring were weaned onto a standard chow diet. Body composition was quantified by DXA and oral glucose tolerance tests conducted on adult offspring. Gene/protein expression and histological analysis were conducted in adipose tissue. Offspring from HF dams had increased body weight, body fat deposition, impaired insulin sensitivity and adipocyte hypertrophy; all of which were rescued in HFCLA offspring. Molecular and histological analyses of the adipose tissue suggest that disturbances in adipogenesis may mediate the metabolic dysfunction observed in HF offspring. Therefore, CLA supplementation to a maternal obesogenic diet may be a promising strategy to prevent adverse programming outcomes.

PI3K-resistant GSK3 controls adiponectin formation and protects from metabolic syndrome

Metabolic syndrome is characterized by insulin resistance, obesity, and dyslipidemia. It is the consequence of an imbalance between caloric intake and energy consumption. Adiponectin protects against metabolic syndrome. Insulin-induced signaling includes activation of PI3 kinase and protein kinase B (PKB)/Akt. PKB/Akt in turn inactivates glycogen synthase kinase (GSK) 3, a major regulator of metabolism. Here, we studied the significance of PI3K-dependent GSK3 inactivation for adiponectin formation in diet-induced metabolic syndrome. Mice expressing PI3K-insensitive GSK3 (gsk3(KI)) and wild-type mice (gsk3(WT)) were fed a high-fat diet. Compared with gsk3(WT) mice, gsk3(KI) mice were protected against the development of metabolic syndrome as evident from a markedly lower weight gain, lower total body and liver fat accumulation, better glucose tolerance, stronger hepatic insulin-dependent PKB/Akt phosphorylation, lower serum insulin, cholesterol, and triglyceride levels, as well as higher energy expenditure. Serum adiponectin concentration and the activity of transcription factor C/EBPα controlling the expression of adiponectin in adipose tissue was significantly higher in gsk3(KI) mice than in gsk3(WT) mice. Treatment with GSK3 inhibitor lithium significantly decreased the serum adiponectin concentration of gsk3(KI) mice and abrogated the difference in C/EBPα activity between the genotypes. Taken together, our data demonstrate that the expression of PI3K-insensitive GSK3 stimulates the production of adiponectin and protects from diet-induced metabolic syndrome.

Lipopolysaccharide binding protein is an adipokine involved in the resilience of the mouse adipocyte to inflammation

AIMS/HYPOTHESIS

Lipopolysaccharide (LPS) binding protein (LBP) is a novel 65 kDa adipokine, linked to adipose tissue (AT) inflammation, obesity and insulin resistance, that inhibits adipocyte differentiation. Here, we investigated the molecular mechanisms behind these detrimental effects on adipogenesis through whole-genome transcriptomics and in vitro experiments.

METHODS

Permanent and transient knockdown (KD) and co-culture experiments were performed in 3T3-L1 and 3T3-F442A cell lines during adipocyte differentiation. Microarray gene expression was performed using Genechip Affymetrix technology and validated by real-time PCR.

RESULTS

LBP KD of 3T3-L1 cells led to a potentiated adipocyte differentiation with a dose-response relationship; genes involved in mitochondrial biogenesis, fatty acid metabolism and peroxisome proliferator-activated receptor γ (PPAR-γ) action were dramatically upregulated in parallel to increased insulin signalling. Cells with LBP KD became refractory to proinflammatory cytokines and other inflammatory stimuli (LPS and palmitate). This phenotype, mediated through disrupted nuclear factor κB (NFκB) signalling, was reversed by a soluble factor present in a co-culture with native cells and by exogenous LBP. Double-silencing of LBP and toll-like receptor 4 (TLR4) again rendered these cells insensitive to co-culture, LBP and inflammatory factors.

CONCLUSIONS/INTERPRETATION

In summary, LBP is a proinflammatory soluble adipokine that acts as a brake for adipogenesis, strengthening the negative effects of palmitate and LPS on adipocyte differentiation.

A novel crosstalk between TLR4- and NOD2-mediated signaling in the regulation of intestinal inflammation

Although Toll-like receptor 4 (TLR4)- and nucleotide-binding oligomerization domain 2 (NOD2)-mediated signaling mechanisms have been extensively studied individually, the crosstalk between them in the regulation of intestinal mucosal defense and tissue homeostasis has been underappreciated. Here, we uncover some novel activities of NOD2 by gene expression profiling revealing the global nature of the cross-regulation between TLR4- and NOD2-mediated signaling. Specifically, NOD2 is able to sense the intensity of TLR4-mediated signaling, resulting in either synergistic stimulation of Interluekin-12 (IL-12) production when the TLR signaling intensity is low; or in the inhibition of IL-12 synthesis and maintenance of intestinal mucosal homeostasis when the TLR signaling intensifies. This balancing act is mediated through receptor-interacting serine/threonine kinase 2, and the transcriptional regulator CCAAT/enhancer-binding protein α (C/EBPα) via its serine 248 phosphorylation by Protein Kinase C. Mice deficient in C/EBPα in the hematopoietic compartment are highly susceptible to chemically induced experimental colitis in an IL-12-dependent manner. Additionally, in contrast to the dogma, we find that the major Crohn’s disease-associated NOD2 mutations could cause a primarily immunodeficient phenotype by selectively impairing TLR4-mediated IL-12 production and host defense. To restore the impaired homeostasis would be a way forward to developing novel therapeutic strategies for inflammatory bowel diseases.

Evaluation of protein phosphorylation during adipogenesis

Adipocyte differentiation is a complex process that involves the sequential expression of various adipocyte-specific genes controlled by signaling pathways and transcription factors for which phosphorylation plays a crucial regulatory role. CCAAT/enhancer-binding proteins and peroxisome proliferator-activated receptors are the most important transcriptional regulators in adipogenesis, and the functions of these proteins are regulated by various phosphorylation events. Because cultured 3T3-L1 preadipocytes are commonly used as a model for adipocyte differentiation, we used these cells for a proteomic analysis to identify kinases, phosphatases, and phosphosites that participate in adipogenesis. In addition to the phosphoproteomic analysis, we provide a detailed description of Western blotting, an in vitro phosphorylation assay, enzyme-linked immunosorbent assay, and phosphorylation site mutagenesis to fully characterize the phosphorylation of proteins and verify their roles in adipogenesis.

SREBP-1a activation by HBx and the effect on hepatitis B virus enhancer II/core promoter

Hepatitis B virus (HBV) X protein (HBx) plays an important role in HBV pathogenesis by regulating gene expression. Sterol regulatory element binding protein-1a (SREBP-1a) is a key transcriptional factor for modulating fatty acid and cholesterol synthesis. Here we demonstrated that HBx increased mature SREBP-1a protein level in the nucleus and its activity as a transcription factor. We further showed that the up-regulation of SREBP-1a by HBx occurred at the transcriptional level after ectopic expression and in the context of HBV replication. Deletional analysis using SREBP-1a promoter revealed that the sequence from -436 to -398 in the promoter was required for its activation by HBx. This promoter region possesses the binding sequences for two basic leucine zipper (b-ZIP) transcription factors, namely C/EBP and E4BP4. Mutagenesis of the binding sequences on the SREBP-1a promoter and ectopic expression experiments demonstrated that C/EBPα enhanced SREBP-1a activation by HBx, while E4BP4 had an inhibitory effect. C/EBPα was able to significantly reverse the inhibitory activity of E4BP4 on SREBP-1a promoter. These results demonstrated that HBx activates SREBP-1a activity at the transcription level through a complex mechanism involving two bZIP transcription factors C/EBP and E4BP4 with C/EBP being the dominant positive factor. Finally, we showed that knocking down SREBP-1 abolishes HBV enhancer II/core promoter activation by HBx.

Targeting CDK1 promotes FLT3-activated acute myeloid leukemia differentiation through C/EBPα

Mutations that activate the fms-like tyrosine kinase 3 (FLT3) receptor are among the most prevalent mutations in acute myeloid leukemias. The oncogenic role of FLT3 mutants has been attributed to the abnormal activation of several downstream signaling pathways, such as STAT3, STAT5, ERK1/2, and AKT. Here, we discovered that the cyclin-dependent kinase 1 (CDK1) pathway is also affected by internal tandem duplication mutations in FLT3. Moreover, we also identified C/EBPα, a granulopoiesis-promoting transcription factor, as a substrate for CDK1. We further demonstrated that CDK1 phosphorylates C/EBPα on serine 21, which inhibits its differentiation-inducing function. Importantly, we found that inhibition of CDK1 activity relieves the differentiation block in cell lines with mutated FLT3 as well as in primary patient-derived peripheral blood samples. Clinical trials with CDK1 inhibitors are currently under way for various malignancies. Our data strongly suggest that targeting the CDK1 pathway might be applied in the treatment of FLT3ITD mutant leukemias, especially those resistant to FLT3 inhibitor therapies.

12/15-Lipoxygenase signaling in the endoplasmic reticulum stress response

Central obesity is associated with chronic inflammation, insulin resistance, β-cell dysfunction, and endoplasmic reticulum (ER) stress. The 12/15-lipoxygenase enzyme (12/15-LO) promotes inflammation and insulin resistance in adipose and peripheral tissues. Given that obesity is associated with ER stress and 12/15-LO is expressed in adipose tissue, we determined whether 12/15-LO could mediate ER stress signals. Addition of 12/15-LO lipid products 12(S)-HETE and 12(S)-HPETE to differentiated 3T3-L1 adipocytes induced expression and activation of ER stress markers, including BiP, XBP-1, p-PERK, and p-IRE1α. The ER stress inducer, tunicamycin, upregulated ER stress markers in adipocytes with concomitant 12/15-LO activation. Addition of a 12/15-LO inhibitor, CDC, to tunicamycin-treated adipocytes attenuated the ER stress response. Furthermore, 12/15-LO-deficient adipocytes exhibited significantly decreased tunicamycin-induced ER stress. 12/15-LO action involves upregulation of interleukin-12 (IL-12) expression. Tunicamycin significantly upregulated IL-12p40 expression in adipocytes, and IL-12 addition increased ER stress gene expression; conversely, LSF, an IL-12 signaling inhibitor, and an IL-12p40-neutralizing antibody attenuated tunicamycin-induced ER stress. Isolated adipocytes and liver from 12/15-LO-deficient mice fed a high-fat diet revealed a decrease in spliced XBP-1 expression compared with wild-type C57BL/6 mice on a high-fat diet. Furthermore, pancreatic islets from 12/15-LO-deficient mice showed reduced high-fat diet-induced ER stress genes compared with wild-type mice. These data suggest that 12/15-LO activity participates in ER stress in adipocytes, pancreatic islets, and liver. Therefore, reduction of 12/15-LO activity or expression could provide a new therapeutic target to reduce ER stress and downstream inflammation linked to obesity.

Triptolide inhibits IL-12/IL-23 expression in APCs via CCAAT/enhancer-binding protein alpha

Triptolide is a biologically active component purified from Chinese herbal plant Tripterygium wilfordii Hook F. It is widely used in East Asia for treatment of systemic lupus erythematosus, rheumatoid arthritis, nephritis, Bechect's disease, psoriasis, atopic dermatitis, and asthma. However, its immunological mechanisms are poorly understood. IL-12 and IL-23 are closely related heterodimeric cytokines that share the common subunit p40. They are produced by APCs and are key factors in the generation and effector functions of Th1 and Th17 cells, respectively. They have been strongly implicated in the pathogenesis of several autoimmune disorders. In this study, we investigated the molecular mechanism whereby triptolide inhibits the expression of the p40 gene in APCs. We demonstrate that triptolide does so at the transcriptional level in part through targeting CCAAT/enhancer-binding protein-alpha (C/EBPalpha), which directly interacts with the p40 promoter and inhibits its transcription in inflammatory macrophages. Triptolide can activate the transcription of C/EBPalpha, and phosphorylation of Ser21 and Thr222/226 critical for C/EBPalpha inhibition of p40. Further, activation of C/EBPalpha by triptolide is dependent on upstream kinases ERK1/2 and Akt-GSK3beta. This study provides mechanistic insights into the immunomodulatory capacity of triptolide and has strong implications for its therapeutic applications in autoimmune diseases.

C/EBPalpha and the corepressors CtBP1 and CtBP2 regulate repression of select visceral white adipose genes during induction of the brown phenotype in white adipocytes by peroxisome proliferator-activated receptor gamma agonists

White adipose tissue (WAT) stores energy in the form of triglycerides, whereas brown tissue (BAT) expends energy, primarily by oxidizing lipids. WAT also secretes many cytokines and acute-phase proteins that contribute to insulin resistance in obese subjects. In this study, we have investigated the mechanisms by which activation of peroxisome proliferator-activated receptor gamma (PPARgamma) with synthetic agonists induces a brown phenotype in white adipocytes in vivo and in vitro. We demonstrate that this phenotypic conversion is characterized by repression of a set of white fat genes ("visceral white"), including the resistin, angiotensinogen, and chemerin genes, in addition to induction of brown-specific genes, such as Ucp-1. Importantly, the level of expression of the "visceral white" genes is high in mesenteric and gonadal WAT depots but low in the subcutaneous WAT depot and in BAT. Mutation of critical amino acids within helix 7 of the ligand-binding domain of PPARgamma prevents inhibition of visceral white gene expression by the synthetic agonists and therefore shows a direct role for PPARgamma in the repression process. Inhibition of the white adipocyte genes also depends on the expression of C/EBPalpha and the corepressors, carboxy-terminal binding proteins 1 and 2 (CtBP1/2). The data further show that repression of resistin and angiotensinogen expression involves recruitment of CtBP1/2, directed by C/EBPalpha, to the minimal promoter of the corresponding genes in response to the PPARgamma ligand. Developing strategies to enhance the brown phenotype in white adipocytes while reducing secretion of stress-related cytokines from visceral WAT is a means to combat obesity-associated disorders.

Regulation of cyclin D1 and Wnt10b gene expression by cAMP-responsive element-binding protein during early adipogenesis involves differential promoter methylation

Cyclin D1 expression is elevated and Wnt10b is repressed by cAMP during the first few hours of adipogenesis. cAMP-responsive element-binding protein (CREB) is a primary target for cAMP signaling, and we have shown that activation of CREB promotes adipogenesis and adipocyte survival. Here we tested the impact of CREB on expression of cyclin D1 and wingless-related mouse mammary tumor virus integration site 10b (Wnt10b) in 3T3-L1 cells. Forced depletion of CREB blocked Bt(2)cAMP-stimulated cyclin D1 expression and basal Wnt10b gene expression. Two CREB-binding sites were identified in the Wnt10b promoter region. Ablation of either site partially blocked promoter activity, while mutation of both sites completely suppressed promoter activity. These results suggest that CREB activates transcription from both the cyclin D1 and Wnt10b gene promoters. What accounts for the differential regulation of cyclin D1 and Wnt10b genes by cAMP? Chromatin immunoprecipitation revealed CREB bound to the Wnt10b promoter in untreated preadipocytes but not following treatment with Bt(2)cAMP. CREB binding to the cyclin D1 promoter was detected in untreated cells and post-Bt(2)cAMP. Differences between CREB binding to the two genes correlated with increasing methylation of the Wnt10b promoter following Bt(2)cAMP treatment, whereas no methylation of the cyclin D1 promoter was observed. Treatment of cells with the methylase inhibitor 5-azacytidine restored CREB binding to the Wnt10b gene promoter and prevented the inhibition of Wnt10b RNA expression by Bt(2)cAMP. We conclude that cAMP stimulates phosphorylation and binding of CREB to the cyclin D1 gene promoter. Simultaneously, hypermethylation of the Wnt10b gene promoter suppresses binding of CREB, allowing adipogenesis to proceed.