Glucose induces de novo lipogenesis in rat muscle satellite cells through a sterol-regulatory-element-binding-protein-1c-dependent pathway

Unraveling the intricate relationship between lipid metabolism and oncogenic signaling pathways

Lipids, the primary constituents of the cell membrane, play essential roles in nearly all cellular functions, such as cell-cell recognition, signaling transduction, and energy provision. Lipid metabolism is necessary for the maintenance of life since it regulates the balance between the processes of synthesis and breakdown. Increasing evidence suggests that cancer cells exhibit abnormal lipid metabolism, significantly affecting their malignant characteristics, including self-renewal, differentiation, invasion, metastasis, and drug sensitivity and resistance. Prominent oncogenic signaling pathways that modulate metabolic gene expression and elevate metabolic enzyme activity include phosphoinositide 3-kinase (PI3K)/AKT, MAPK, NF-kB, Wnt, Notch, and Hippo pathway. Conversely, when metabolic processes are not regulated, they can lead to malfunctions in cellular signal transduction pathways. This, in turn, enables uncontrolled cancer cell growth by providing the necessary energy, building blocks, and redox potentials. Therefore, targeting lipid metabolism-associated oncogenic signaling pathways could be an effective therapeutic approach to decrease cancer incidence and promote survival. This review sheds light on the interactions between lipid reprogramming and signaling pathways in cancer. Exploring lipid metabolism as a target could provide a promising approach for creating anticancer treatments by identifying metabolic inhibitors. Additionally, we have also provided an overview of the drugs targeting lipid metabolism in cancer in this review.

Targeting fatty acid uptake and metabolism in cancer cells: A promising strategy for cancer treatment

Despite scientific development, cancer is still a fatal disease. The development of cancer is thought to be significantly influenced by fatty acids. Several mechanisms that control fatty acid absorption and metabolism are reported to be altered in cancer cells to support their survival. Cancer cells can use de novo synthesis or uptake of extracellular fatty acid if one method is restricted. This factor makes it more difficult to target one pathway while failing to treat the disease properly. Side effects may also arise if several inhibitors simultaneously target many targets. If a viable inhibitor could work on several routes, the number of negative effects might be reduced. Comparative investigations against cell viability have found several potent natural and manmade substances. In this review, we discuss the complex roles that fatty acids play in the development of tumors and the progression of cancer, newly discovered and potentially effective natural and synthetic compounds that block the uptake and metabolism of fatty acids, the adverse side effects that can occur when multiple inhibitors are used to treat cancer, and emerging therapeutic approaches.

The roles of DGAT1 and DGAT2 in human myotubes are dependent on donor patho-physiological background

The roles of DGAT1 and DGAT2 in lipid metabolism and insulin responsiveness of human skeletal muscle were studied using cryosections and myotubes prepared from muscle biopsies from control, athlete, and impaired glucose regulation (IGR) cohorts of men. The previously observed increases in intramuscular triacylglycerol (IMTG) in athletes and IGR were shown to be related to an increase in lipid droplet (LD) area in type I fibers in athletes but, conversely, in type II fibers in IGR subjects. Specific inhibition of both diacylglycerol acyltransferase (DGAT) 1 and 2 decreased fatty acid (FA) uptake by myotubes, whereas only DGAT2 inhibition also decreased fatty acid oxidation. Fatty acid uptake in myotubes was negatively correlated with the lactate thresholds of the respective donors. DGAT2 inhibition lowered acetate uptake and oxidation in myotubes from all cohorts whereas DGAT1 inhibition had no effect. A positive correlation between acetate oxidation in myotubes and resting metabolic rate (RMR) from fatty acid oxidation in vivo was observed. Myotubes from athletes and IGR had higher rates of de novo lipogenesis from acetate that were normalized by DGAT2 inhibition. Moreover, DGAT2 inhibition in myotubes also resulted in increased insulin-induced Akt phosphorylation. The differential effects of DGAT1 and DGAT2 inhibition suggest that the specialized role of DGAT2 in esterifying nascent diacylglycerols and de novo synthesized FA is associated with synthesis of a pool of triacylglycerol, which upon hydrolysis results in effectors that promote mitochondrial fatty acid oxidation but decrease insulin signaling in skeletal muscle cells.

Kidney Proximal Tubule GLUT2-More than Meets the Eye

Tubulopathy plays a central role in the pathophysiology of diabetic kidney disease (DKD). Under diabetic conditions, the kidney proximal tubule cells (KPTCs) are exposed to an extensive amount of nutrients, most notably glucose; these nutrients deteriorate KPTCs function and promote the development and progression of DKD. Recently, the facilitative glucose transporter 2 (GLUT2) in KPTCs has emerged as a central regulator in the pathogenesis of DKD. This has been demonstrated by identifying its specific role in enhancing glucose reabsorption and glucotoxicity, and by deciphering its effect in regulating the expression of the sodium-glucose transporter 2 (SGLT2) in KPTCs. Moreover, reduction/deletion of KPTC-GLUT2 has been recently found to ameliorate DKD, raising the plausible idea of considering it as a therapeutic target against DKD. However, the underlying molecular mechanisms by which GLUT2 exerts its deleterious effects in KPTCs remain vague. Herein, we review the current findings on the proximal tubule GLUT2 biology and function under physiologic conditions, and its involvement in the pathophysiology of DKD. Furthermore, we shed new light on its cellular regulation during diabetic conditions.

Effect of Different Basal Culture Media and Sera Type Combinations on Primary Broiler Chicken Muscle Satellite Cell Heterogeneity during Proliferation and Differentiation

Simple Summary

Little consistency in the literature exists for optimal culture conditions for proliferating and differentiating primary broiler chicken muscle satellite cells regarding basal culture media, proliferation sera, and differentiation sera. This experiment assessed primary satellite cell proliferation and differentiation when cultured in different combinations of basal media and sera. Cells were cultured in different basal media: low glucose Dulbecco’s Modified Eagle’s medium, McCoy’s 5A, and high glucose Dulbecco’s Modified Eagle’s medium. Each media was supplemented with 15% chicken serum, or a combination of 5% horse serum + 10% chicken serum during proliferation while 3% horse serum or 3% chicken serum were supplemented during differentiation. Cultures were immunofluorescence stained for myogenic regulatory factors at different time points during proliferation and differentiation. During proliferation and differentiation, cells cultured in Dulbecco’s Modified Eagle’s medium tended to have higher proportions of myogenic cells expressing myogenic regulatory factors and promoted satellite cell fusion into myotubes compared with McCoy’s 5A. Low glucose media, glucose concentration similar to circulating glucose concentrations in broilers, combined with sera published in the literature may be the optimal culture media to promote satellite cell proliferation and differentiation.

Abstract

The objective of this experiment was to access primary satellite cell (SC) proliferation and differentiation when cultured in different combinations of basal media and sera due to little consistency being published on the optimal culture media for primary broiler chicken satellite cells. Cells were cultured in one of three different basal media: McCoy’s 5A, high glucose Dulbecco’s Modified Eagle’s medium (DMEM), and low glucose DMEM. Media were supplemented with 15% chicken serum (CS) or a combination of 5% horse serum (HS) + 10% CS during proliferation while 3% HS or 3% CS were added to the media during differentiation. Cultures were immunofluorescence stained for myogenic regulatory factors (MRF) at 48, 72, and 96 h post-plating for proliferation (Pax7, MyoD, and Myf-5) and 96 h post-proliferation during differentiation (Pax7 and MyoD), including MF20 to assess fusion. Cells cultured in Dulbecco’s Modified Eagle’s medium tended to have higher proportions of myogenic cells expressing MRF during proliferation and promoted fusion into myotubes compared with McCoy’s 5A during differentiation. Culturing primary SC in low glucose media, glucose concentrations similar to circulating glucose concentrations in broilers, HSCS during proliferation and CS during differentiation, appears to be optimal for promoting broiler chicken satellite cell proliferation and differentiation.

STAT3 as a mediator of oncogenic cellular metabolism: Pathogenic and therapeutic implications

The oncogenic transcription factor signal transducer and activator of transcription 3 (STAT3) is activated constitutively in a wide array of human cancers. It is an appealing molecular target for novel therapy as it directly regulates expression of genes involved in cell proliferation, survival, angiogenesis, chemoresistance and immune responsiveness. In addition to these well-established oncogenic roles, STAT3 has also been found to mediate a wide array of functions in modulating cellular behavior. The transcriptional function of STAT3 is canonically regulated through tyrosine phosphorylation. However, STAT3 phosphorylated at a single serine residue can allow incorporation of this protein into the inner mitochondrial membrane to support oxidative phosphorylation (OXPHOS) and maximize the utility of glucose sources. Conflictingly, its canonical transcriptional activity suppresses OXPHOS and favors aerobic glycolysis to promote oncogenic behavior. Apart from mediating the energy metabolism and controversial effects on ATP production, STAT3 signaling modulates lipid metabolism of cancer cells. By mediating fatty acid synthesis and beta oxidation, STAT3 promotes employment of available resources and supports survival in the conditions of metabolic stress. Thus, the functions of STAT3 extend beyond regulation of oncogenic genes expression to pleiotropic effects on a spectrum of essential cellular processes. In this review, we dissect the current knowledge on activity and mechanisms of STAT3 involvement in transcriptional regulation, mitochondrial function, energy production and lipid metabolism of malignant cells, and its implications to cancer pathogenesis and therapy.

Effects of energy supplements on the differentiation of skeletal muscle satellite cells

To investigate the effects of the activator of AMPK and high glucose on the differentiation of mouse SMSCs, primary SMSCs were isolated from mouse extensor digitorum longus muscle and grown to near confluence (80%). Postconfluent cells were cultured in a growth medium with different inductors: AICAR, glucose, and AICAR mixed with glucose. The specific protein expressions of SMSCs, myoblasts, adipocytes, and brown adipocytes were analyzed on days 0, 3, 5, 7, and 10. The results showed treatment with AICAR in SMSCs markedly activated AMPK phosphorylation (p < .05) and increased protein expression of Pax7 and MyoD (p < .05), high concentrations of intracellular glucose upregulated UCP-1 protein expression and enhanced lipid accumulation (p < .05), the cowork of AICAR and glucose affected a decrease on MyoD, PPARg, and UCP-1 expression (p < .05) and an increase on Pax7. The present study indicated that the certain energy supplements influence the direction of SMSC differentiation which may contribution on the structure of muscle and meat quality, sequentially.

Role of the Sterol Regulatory Element Binding Protein Pathway in Tumorigenesis

Metabolic changes are a major feature of tumors, including various metabolic forms, such as energy, lipid, and amino acid metabolism. Sterol regulatory element binding proteins (SREBPs) are important modules in regulating lipid metabolism and play an essential role in metabolic diseases. In the previous decades, the regulatory range of SREBPs has been markedly expanded. It was found that SREBPs also played a critical role in tumor development. SREBPs are involved in energy supply, lipid supply, immune environment and inflammatory environment shaping in tumor cells, and as a protective umbrella to support the malignant proliferation of tumor cells. Natural medicine and traditional Chinese medicine, as an important part of drug therapy, demonstrates the multifaceted effects of SREBPs regulation. This review summarizes the core processes in the involvement of SREBPs in tumors and provides a comprehensive understanding of the pathways through which natural drugs target the SREBP pathway and regulate tumor progression.

SREBP1 silencing inhibits the proliferation and motility of human esophageal squamous carcinoma cells via the Wnt/β-catenin signaling pathway

Sterol regulatory element-binding protein 1 (SREBP1) is dysregulated in a variety of types of human cancer. However, the functional roles of SREBP1 in esophageal squamous cell carcinoma (ESCC) remain poorly understood. The present study investigated the function of SREBP1 in cell proliferation and motility. Microarray datasets in Oncomine, reverse transcription-quantitative PCR and western blot analysis revealed that SREBP1 was overexpressed in ESCC tumors when compared with normal tissues. In addition, SREBP1 overexpression was significantly associated with tumor differentiation, lymphatic metastasis and Ki67 expression. Results suggested that silencing SREBP1 inhibited the proliferation, migration and invasion of ESCC cells, whereas overexpression of SREBP1 had opposite effects on proliferation and metastasis. In addition, loss of SREBP1 significantly increased E-cadherin and decreased N-cadherin, Vimentin, Snail, matrix metalloproteinase 9 and vascular endothelial growth factor C expression levels, which were restored via SREBP1-overexpression. Mechanistically, loss of SREBP1 suppressed T-cell factor 1/lymphoid enhancer factor 1 (TCF1/LEF1) activity and downregulated TCF1/LEF1 target proteins, including CD44 and cyclin D1. Moreover, knockdown of SREBP1 downregulated the expression levels of stearoyl-CoA desaturase 1 (SCD1), phosphorylated glycogen synthase kinase-3β and nuclear β-catenin. Furthermore, the inhibitors of SREBP1 and/or SCD1 and small interfering RNA-SCD1 efficiently inhibited the activation of the Wnt/β-catenin pathway driven by constitutively active SREBP1. Finally, in vivo results indicated that SREBP1-knockdown suppressed the proliferation and metastasis of ESCC. Taken together, these findings demonstrated that SREBP1 exerts oncogenic effects in ESCC by promoting proliferation and inducing epithelial-mesenchymal transition via the SCD1-induced activation of the Wnt/β-catenin signaling pathway.

Dietary high calories from sunflower oil, sucrose and fructose sources alters lipogenic genes expression levels in liver and skeletal muscle in rats

INTRODUCTION AND OBJECTIVES

The objectives of this study were to investigate the underlying mechanism of PPARα, LXRα, ChREBP, and SREBP-1c at the level of gene and protein expression with high-energy diets in liver and skeletal muscle.

MATERIALS AND METHODS

Metabolic changes with consumption of high fat (Hfat), high sucrose (Hsuc) and high fructose (Hfru) diets were assessed. Levels of mRNA and protein of PPARα, LXRα, ChREBP, and SREBP-1c were investigated. Body weight changes, histological structure of liver and plasma levels of some parameters were also examined.

RESULTS

In Hfru group, body weights were higher than other groups (P<0.05). In liver, LXRα levels of Hsuc and Hfru groups were upregulated as 1.87±0.30 (P<0.05) and 2.01±0.29 (P<0.01). SREBP-1c levels were upregulated as 4.52±1.25 (P<0.05); 4.05±1.11 (P<0.05) and 3.85±1.04 (P<0.05) in Hfat, Hsuc, and Hfru groups, respectively. In skeletal muscle, LXRα and SREBP-1c were upregulated as 1.77±0.30 (P<0.05) and 2.71±0.56 (P<0.05), in the Hfru group. Protein levels of ChREBP (33.92±8.84ng/mg protein (P<0.05)) and SREBP-1c (135.16±15.57ng/mg protein (P<0.001)) in liver were higher in Hfru group. In skeletal muscle, LXRα, ChREBP and SREBP-1c in Hfru group were 6.67±0.60, 7.11±1.29 and 43.17±6.37ng/mg, respectively (P<0.05; P<0.01; P<0.05). The rats in Hfru group had the most damaged livers.

CONCLUSION

Besides liver, fructose consumption significantly effects skeletal muscle and leads to weight gain, triggers lipogenesis and metabolic disorders.

Recent Insights Into SREBP as a Direct Mediator of Kidney Fibrosis via Lipid-Independent Pathways

Sterol regulatory-element binding proteins (SREBPs) are classical regulators of cellular lipid metabolism in the kidney and other tissues. SREBPs are currently recognized as versatile transcription factors involved in a myriad of cellular processes. Meanwhile, SREBPs have been recognized to mediate lipotoxicity, contributing to the progression of kidney diseases. SREBP1 has been shown to bind to the promoter region of TGFβ, a major pro-fibrotic signaling mechanism in the kidney. Conversely, TGFβ activates SREBP1 transcriptional activity suggesting a positive feedback loop of SREBP1 in TGFβ signaling. Public ChIP-seq data revealed numerous non-lipid transcriptional targets of SREBPs that plausibly play roles in progressive kidney disease and fibrosis. This review provides new insights into SREBP as a mediator of kidney fibrosis via lipid-independent pathways.

Excess Accumulation of Lipid Impairs Insulin Sensitivity in Skeletal Muscle

Both glucose and free fatty acids (FFAs) are used as fuel sources for energy production in a living organism. Compelling evidence supports a role for excess fatty acids synthesized in intramuscular space or dietary intermediates in the regulation of skeletal muscle function. Excess FFA and lipid droplets leads to intramuscular accumulation of lipid intermediates. The resulting downregulation of the insulin signaling cascade prevents the translocation of glucose transporter to the plasma membrane and glucose uptake into skeletal muscle, leading to metabolic disorders such as type 2 diabetes. The mechanisms underlining metabolic dysfunction in skeletal muscle include accumulation of intracellular lipid derivatives from elevated plasma FFAs. This paper provides a review of the molecular mechanisms underlying insulin-related signaling pathways after excess accumulation of lipids.

Reprogramming of fatty acid metabolism in cancer

A common feature of cancer cells is their ability to rewire their metabolism to sustain the production of ATP and macromolecules needed for cell growth, division and survival. In particular, the importance of altered fatty acid metabolism in cancer has received renewed interest as, aside their principal role as structural components of the membrane matrix, they are important secondary messengers, and can also serve as fuel sources for energy production. In this review, we will examine the mechanisms through which cancer cells rewire their fatty acid metabolism with a focus on four main areas of research. (1) The role of de novo synthesis and exogenous uptake in the cellular pool of fatty acids. (2) The mechanisms through which molecular heterogeneity and oncogenic signal transduction pathways, such as PI3K-AKT-mTOR signalling, regulate fatty acid metabolism. (3) The role of fatty acids as essential mediators of cancer progression and metastasis, through remodelling of the tumour microenvironment. (4) Therapeutic strategies and considerations for successfully targeting fatty acid metabolism in cancer. Further research focusing on the complex interplay between oncogenic signalling and dysregulated fatty acid metabolism holds great promise to uncover novel metabolic vulnerabilities and improve the efficacy of targeted therapies.

Disruption of Cancer Metabolic SREBP1/miR-142-5p Suppresses Epithelial-Mesenchymal Transition and Stemness in Esophageal Carcinoma

: Elevated activity of sterol regulatory element-binding protein 1 (SREBP1) has been implicated in the tumorigenesis of different cancer types. However, the functional roles of SREBP1 in esophageal cancer are not well appreciated. Here, we aimed to investigate the therapeutic potential of SREBP1 and associated signaling in esophageal cancer. Our initial bioinformatics analyses showed that SREBP1 expression was overexpressed in esophageal tumors and correlated with a significantly lower overall survival rate in patients. Additionally, tumor suppressor miR-142-5p was predicted to target SREBP1/ZEB1 and a lower miR-142-5p was correlated with poor prognosis. We then performed in vitro experiments and showed that overexpressing SREBP1 in OE33 cell line led to increased abilities of colony formation, migration, and invasion; the opposite was observed in SREBP1-silenced OE21cells and SREBP1-silencing was accompanied by the reduced mesenchymal markers, including vimentin (Vim) and ZEB1, while E-cadherin and tumor suppressor miR-142-5p were increased. Subsequently, we first demonstrated that both SREBP1 and ZEB1 were potential targets of miR-142-5p, followed by the examination of the regulatory circuit of miR-142-5p and SREBP1/ZEB1. We observed that increased miR-142-5p level led to the reduced tumorigenic properties, such as migration and tumor sphere formation, and both observations were accompanied by the reduction of ZEB1 and SREBP1, and increase of E-cadherin. We then explored the potential therapeutic agent targeting SREBP1-associated signaling by testing fatostatin (4-hydroxytamoxifen, an active metabolite of tamoxifen). We found that fatostatin suppressed the cell viability of OE21 and OE33 cells and tumor spheres. Interestingly, fatostatin treatment reduced CD133+ population in both OE21 and OE33 cells in concert of increased miR-142-5p level. Finally, we evaluated the efficacy of fatostatin using a xenograft mouse model. Mice treated with fatostatin showed a significantly lower tumor burden and better survival rate as compared to their control counterparts. The treatment of fatostatin resulted in the reduced staining of SREBP1, ZEB1, and Vim, while E-cadherin and miR-142-5p were increased. In summary, we showed that increased SREBP1 and reduced miR-142-5p were associated with increased tumorigenic properties of esophageal cancer cells and poor prognosis. Preclinical tests showed that suppression of SREBP1 using fatostatin led to the reduced malignant phenotype of esophageal cancer via the reduction of EMT markers and increased tumor suppressor, miR-142-5p. Further investigation is warranted for the clinical use of fatostatin for the treatment of esophageal malignancy.

ROS and Lipid Droplet accumulation induced by high glucose exposure in healthy colon and Colorectal Cancer Stem Cells

Lipid Droplets (LDs) are emerging as crucial players in colon cancer development and maintenance. Their expression has been associated with high tumorigenicity in Cancer Stem Cells (CSCs), so that they have been proposed as a new functional marker in Colorectal Cancer Stem Cells (CR-CSCs). They are also indirectly involved in the modulation of the tumor microenvironment through the production of pro-inflammatory molecules. There is growing evidence that a possible connection between metabolic alterations and malignant transformation exists, although the effects of nutrients, primarily glucose, on the CSC behavior are still mostly unexplored. Glucose is an essential fuel for cancer cells, and the connections with LDs in the healthy and CSC populations merit to be more deeply investigated. Here, we showed that a high glucose concentration activated the PI3K/AKT pathway and increased the expression of CD133 and CD44v6 CSC markers. Additionally, glucose was responsible for the increased amount of Reactive Oxygen Species (ROS) and LDs in both healthy and CR-CSC samples. We also investigated the gene modulations following the HG treatment and found out that the healthy cell gene profile was the most affected. Lastly, Atorvastatin, a lipid-lowering drug, induced the highest mortality on CR-CSCs without affecting the healthy counterpart.

PGC1β Regulates Breast Tumor Growth and Metastasis by SREBP1-Mediated HKDC1 Expression

Background: Breast cancer is a very common cancer with significant premature mortality in women. In this study, we show that HKDC1 expression in breast cancer cells is increased significantly. We aim to investigate the detailed mechanism for the regulation of HKDC1 expression and its potential contribution to tumorigenesis.

Methods: Gene expression was evaluated by real time PCR, western blotting, and immunohistochemistry. The mechanism for PGC1β/SREBP1-mediated HKDC1 expression was investigated using luciferase reporter assay, chromatin immunoprecipitation, and siRNA techniques. In addition, HKDC1 was overexpressed or knocked down by lentivirus to evaluate the potential effect on in vitro cell proliferation, glucose uptake, mitochondrial function, apoptosis, and reactive oxygen species (ROS) formation. Furthermore, an in vivo xenograft tumor development study was employed to investigate the effect of HKDC1 on tumor growth and mouse survival.

Results: HKDC1 is highly expressed in both breast cancer cells and clinical tumor tissues. HKDC1 expression is upregulated and co-activated by PGC1β through SREBP1 binding motif on the HKDC1 promoter. HKDC1 is located on the mitochondrial membrane and regulates the permeability transition pore opening by binding with VDAC1, subsequently modulating glucose uptake and cell proliferation. Overexpression of HKDC1 increases while knockdown of HKDC1 decreases in vitro breast cancer cell proliferation and in vivo tumor growth, metastasis, and mouse survival.

Conclusions: PGC1β regulates breast cancer tumor growth and metastasis by SREBP1-mediated HKDC1 expression. This provides a novel therapeutic strategy through targeting the PGC1β/HKDC1 signaling pathway for breast cancer treatment.

Salvianolic acids improve liver lipid metabolism in ovariectomized rats via blocking STAT-3/SREBP1 signaling

Postmenopausal women, who have reduced circulating estrogen levels, are more prone to develop obesity and related metabolic diseases than premenopausal women. The absence of safe and effective treatments for postmenopausal obesity has changed the focus to natural products as alternative remedies. Total salvianolic acids (TSA) are the major water-soluble ingredients of Danshen. Salvianolic acid (SA) is the major constituent of the TSA. Salvianolic acids, including TSA and SA, are widely used in traditional Chinese medicine. In the present study, ovariectomized rats and LO2 cells were used to study the effects of salvianolic acids on body weight gain and hepatic steatosis. Salvianolic acids reduced ovariectomy (OVX)-induced body weight gain, attenuated the expressions of hepatic lipogenic genes, such as sterol regulatory element binding protein (SREBP)1, fatty acid synthase (FAS), and stearoyl-CoA desaturase (SCD)1, and decreased the liver triglyceride (TG) and total cholesterol (TC). For the molecular mechanisms, OVX and high glucose-induced phosphorylation of signal transducer and activator of transcription (STAT)-3 was inhibited by salvianolic acids treatment. In LO2 cells, inhibition of STAT-3 by siRNA attenuated the increased expression of SREBP1 and TG induced by high glucose. Salvianolic acids reduced the upregulation of SREBP1 and TG induced by high glucose in LO2 cells. In conclusion, these findings illustrated that salvianolic acids markedly alleviated the lipid metabolism disorders and protected against the postmenopausal obesity. The underlying mechanism was probably associated with the regulation of STAT-3 signaling.

Adipogenesis and lipid production in adipocytes subjected to sustained tensile deformations and elevated glucose concentration: a living cell-scale model system of diabesity

Adipocyte fate commitment is characterized by morphological changes of fibroblastic pre-adipocyte cells, and specifically by accumulation of lipid droplets (LDs) as part of the adipogenesis metabolism. Formation of LDs indicates the production of triglycerides from glucose through an insulin-regulated glucose internalization process. In obesity, adipocytes typically become insulin resistant, and glucose transport into the cells is impaired, resulting in type 2 diabetes. In the present study, we monitored the adipogenesis in 3T3-L1 cultured cells exposed to high (450 mg/dL hyperglycemia) and low (100 mg/dL physiological) glucose concentrations, in a novel cell culture model system of diabesity. In addition to glucose conditions, cells were concurrently exposed to different substrate tensile strains (12% and control) based on our prior work which revealed that adipogenesis is accelerated in cultures subjected to static, chronic substrate tensile deformations. Phase-contrast images were taken throughout the adipogenesis process (3 weeks) and were analyzed by an image processing algorithm which quantitatively monitors cell differentiation and lipid accumulation (number of LDs per cell and their radius as well as cell size and shape). The results indicated that high glucose concentrations and substrate tensile strains delivered to adipocytes accelerated lipid production by 1.7- and 1.4-fold, respectively. In addition, significant changes in average cell projected area and in other morphological attributes were observed during the differentiation process. The importance of this study is in characterizing the adipogenesis parameters as potential read-outs that can predict the occurrence of insulin resistance in the development of diabesity.

Insulin restores UCP3 activity and decreases energy surfeit to alleviate lipotoxicity in skeletal muscle

An early insulin regimen ameliorates glucotoxicity but also lipotoxicity in type 2 diabetes; however, the underlying mechanism remains elusive. In the present study, we investigated the role of mitochondria in lipid regulation following early insulin administration in insulin-resistant skeletal muscle cells. Male C57BL/6 mice, fed a high-fat diet (HFD) for 8 weeks, were treated with insulin for 3 weeks, and L6 myotubes cultured with palmitate (PA) for 24 h were incubated with insulin for another 12 h. The results showed that insulin facilitated systemic glucose disposal and attenuated muscular triglyceride accumulation in vivo. Recovery of AMP-activated protein kinase (AMPK) phosphorylation, inhibition of sterol-regulated element binding protein-1c (SREBP-1c) and increased carnitine palmitoyltransferase‑1B (CPT1B) expression were observed after insulin administration. Moreover, increased ATP concentration and cellular energy charge elicited by over-nutrition were suppressed by insulin. Despite maintaining respiratory complex activities, insulin restored muscular uncoupling protein 3 (UCP3) protein expression in vitro and in vivo. By contrast, knockdown of UCP3 abrogated insulin-induced restoration of AMPK phosphorylation in vitro. Importantly, the PA-induced decrease in UCP3 was blocked by the proteasome inhibitor MG132, and insulin reduced UCP3 ubiquitination, thereby prohibiting its degradation. Our findings, focusing on energy balance, provide a mechanistic understanding of the promising effect of early insulin initiation on lipotoxicity. Insulin, by recovering UCP3 activity, alleviated energy surfeit and potentiated AMPK-mediated lipid homeostasis in skeletal muscle cells following exposure to PA and in gastrocnemius of mice fed HFD.

Kukoamine A attenuates insulin resistance and fatty liver through downregulation of Srebp-1c

Nonalcoholic fatty liver disease (NAFLD) refers to a pathological condition of hepatic steatosis. Insulin resistance is believed to be the key mechanism mediating initial accumulation of fat in the liver, resulting in hepatic steatosis. Kukoamine A (KuA), a spermine alkaloid, is a major bioactive component extracted from the root barks of Lycium chinense (L. chinense) Miller. In the current study, we aimed to explore the possible effect of KuA on insulin resistance and fatty liver. We showed that KuA significantly inhibited the increase of fasting blood glucose level and insulin level, and the glucose levels in response to glucose and insulin load in HFD-fed mice, which was in a dose-dependent manner. KuA dose-dependently decreased the histological injury of liver, levels of hepatic triglyceride (TG), and serum AST and ALT activities in HFD-fed mice. The increase of serum levels of TNFɑ, IL-1β, IL-6 and C reactive protein in HFD-fed mice was inhibited by KuA. HFD feeding-induced increase of hepatic expression of Srebp-1c and its target genes, including fatty acid synthase (FAS) and acetyl CoA carboxylase 1 (ACC1), was significantly inhibited by KuA. Moreover, upregulation of Srebp-1c notably inhibited KuA-induced improvement of insulin-stimulated glucose uptake, decrease of lipid accumulation and H₂O₂ level in palmitic acid-treated AML-12 cells. In conclusion, we reported that KuA inhibited Srebp-1c and downstream genes expression and resulted in inhibition of lipid accumulation, inflammation, insulin resistance and oxidative stress. Overall, our results provide a better understanding of the pharmacological activities of KuA against insulin resistance and hepatic steatosis.

Skeletal muscle insulin resistance in hamsters with diabetes developed from obesity is involved in abnormal skeletal muscle LXR, PPAR and SREBP expression

Diabetic ‘lipotoxicity’ theory suggests that fat-induced skeletal muscle insulin resistance (FISMIR) in obesity induced by a high-fat diet (HFD), which leads to ectopic lipid accumulation in insulin-sensitive tissues, may play a pivotal role in the pathogenesis of type 2 diabetes. However, the changes in gene expression and the molecular mechanisms associated with the pathogenesis of FISMIR have not yet been fully elucidated. In the present study the changes in skeletal muscle gene expression were examined in FISMIR in obese insulin-resistant and diabetic hamster models induced by HFD with or without low-dose streptozotocin-treatment. Microarray technology and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) were used to explore the potential underlying molecular mechanisms. The pathophysiological and metabolic features of obesity and type 2 diabetes in humans are closely resembled by these hamster models. The results of microarray analysis showed that the differentially expressed genes associated with metabolism were mostly related to the abnormal regulation and changes in the gene expression of liver X receptor (LXR), peroxisome proliferator-activated receptor (PPAR) and sterol regulatory element-binding protein (SREBP) transcriptional programs in the skeletal muscle from insulin-resistant and diabetic hamsters. The microarray findings confirmed by RT-qPCR indicated that the increased expression of SREBPs and LXRβ and the decreased expression of LXRα and PPARs were involved in the molecular mechanisms of FISMIR pathogenesis in insulin-resistant and diabetic hamsters. A significant difference in the abnormal expression of skeletal muscle LXRs, PPARs and SREBPs was found between insulin-resistant and diabetic hamsters. It may be concluded that the combined abnormal expression of LXR, PPAR and SREBP transcriptional programs may contribute to the development of FISMIR mediated by skeletal muscle lipid accumulation resulting from abnormal skeletal muscle glucose and lipid metabolism in these HFD- and streptozotocin injection-induced insulin-resistant and diabetic hamsters.

Advanced Glycation End Products (AGE) Potently Induce Autophagy through Activation of RAF Protein Kinase and Nuclear Factor κB (NF-κB)

Advanced glycation end products (AGE) accumulate in diabetic patients and aging people because of high amounts of three- or four-carbon sugars derived from glucose, thereby causing multiple consequences, including inflammation, apoptosis, obesity, and age-related disorders. It is important to understand the mechanism of AGE-mediated signaling leading to the activation of autophagy (self-eating) that might result in obesity. We detected AGE as one of the potent inducers of autophagy compared with doxorubicin and TNF. AGE-mediated autophagy is inhibited by suppression of PI3K and potentiated by the autophagosome maturation blocker bafilomycin. It increases autophagy in different cell types, and that correlates with the expression of its receptor, receptor for AGE. LC3B, the marker for autophagosomes, is shown to increase upon AGE stimulation. AGE-mediated autophagy is partially suppressed by inhibitor of NF-κB, PKC, or ERK alone and significantly in combination. AGE increases sterol regulatory element binding protein activity, which leads to an increase in lipogenesis. Although AGE-mediated lipogenesis is affected by autophagy inhibitors, AGE-mediated autophagy is not influenced by lipogenesis inhibitors, suggesting that the turnover of lipid droplets overcomes the autophagic clearance. For the first time, we provide data showing that AGE induces several cell signaling cascades, like NF-κB, PKC, ERK, and MAPK, that are involved in autophagy and simultaneously help with the accumulation of lipid droplets that are not cleared effectively by autophagy, therefore causing obesity.

Functional characterization of retromer in GLUT4 storage vesicle formation and adipocyte differentiation

Insulin-stimulated translocation of glucose transporter 4 (GLUT4) storage vesicles (GSVs), the specialized intracellular compartments within mature adipocytes, to the plasma membrane (PM) is a fundamental cellular process for maintaining glucose homeostasis. Using 2 independent adipocyte cell line models, human primary Simpson-Golabi-Behmel syndrome and mouse 3T3-L1 fibroblast cell lines, we demonstrate that the endosome-associated protein-sorting complex retromer colocalizes with GLUT4 on the GSVs by confocal microscopy in mature adipocytes. By use of both confocal microscopy and differential ultracentrifugation techniques, retromer is redistributed to the PM of mature adipocytes upon insulin stimulation. Furthermore, stable knockdown of the retromer subunit-vacuolar protein-sorting 35, or the retromer-associated protein sorting nexin 27, by lentivirus-delivered small hairpin RNA impaired the adipogenesis process when compared to nonsilence control. The knockdown of retromer decreased peroxisome proliferator activated receptor γ expression during differentiation, generating adipocytes with decreased levels of GSVs, lipid droplet accumulation, and insulin-stimulated glucose uptake. In conclusion, our study demonstrates a role for retromer in the GSV formation and adipogenesis.

Glucose-Mediated N-glycosylation of SCAP Is Essential for SREBP-1 Activation and Tumor Growth

Tumorigenesis is associated with increased glucose consumption and lipogenesis, but how these pathways are interlinked is unclear. Here, we delineate a pathway in which EGFR signaling, by increasing glucose uptake, promotes N-glycosylation of sterol regulatory element-binding protein (SREBP) cleavage-activating protein (SCAP) and consequent activation of SREBP-1, an ER-bound transcription factor with central roles in lipid metabolism. Glycosylation stabilizes SCAP and reduces its association with Insig-1, allowing movement of SCAP/SREBP to the Golgi and consequent proteolytic activation of SREBP. Xenograft studies reveal that blocking SCAP N-glycosylation ameliorates EGFRvIII-driven glioblastoma growth. Thus, SCAP acts as key glucose-responsive protein linking oncogenic signaling and fuel availability to SREBP-dependent lipogenesis. Targeting SCAP N-glycosylation may provide a promising means of treating malignancies and metabolic diseases.

LipiD-QuanT: a novel method to quantify lipid accumulation in live cells

Lipid droplets (LDs) are the main storage organelles for triglycerides. Elucidation of lipid accumulation mechanisms and metabolism are essential to understand obesity and associated diseases. Adipogenesis has been well studied in murine 3T3-L1 and human Simpson-Golabi-Behmel syndrome (SGBS) preadipocyte cell lines. However, most techniques for measuring LD accumulation are either not quantitative or can be destructive to samples. Here, we describe a novel, label-free LD quantification technique (LipiD-QuanT) to monitor lipid dynamics based on automated image analysis of phase contrast microscopy images acquired during in vitro human adipogenesis. We have applied LipiD-QuanT to measure LD accumulation during differentiation of SGBS cells. We demonstrate that LipiD-QuanT is a robust, nondestructive, time- and cost-effective method compared with other triglyceride accumulation assays based on enzymatic digest or lipophilic staining. Further, we applied LipiD-QuanT to measure the effect of four potential pro- or antiobesogenic substances: DHA, rosiglitazone, elevated levels of D-glucose, and zinc oxide nanoparticles. Our results revealed that 2 µmol/l rosiglitazone treatment during adipogenesis reduced lipid production and caused a negative shift in LD diameter size distribution, but the other treatments showed no effect under the conditions used here.

Age-related obesity is a heritage of the evolutionary past

In the process of human aging, an increase in the total amount of fat is observed mainly due to accumulation of lipids in non-adipose tissues. Insulin resistance, provoked by the intracellular accumulation of triglycerides, is often associated with development of such age-related diseases as atherosclerosis, type 2 diabetes, cancer, osteoporosis, and also with systemic inflammation and lipo- and glucose toxicity. Accumulation of lipids and lipophilic compounds is a biological phenomenon common for both prokaryotes and eukaryotes. Initially, it arose as an adaptation to starvation and shortage of nitrogen-containing nutrients, but later it converted into a depot of membrane material, needed on recommencement of cell division. In rodents and humans, the accumulation of non-metabolized fat in non-adipose tissues can be regarded as an adaptation to changes in the internal medium on a certain stage of ontogenesis as a result of age-related dysfunction of adipose tissue.

Egg yolks inhibit activation of NF-κB and expression of its target genes in adipocytes after partial delipidation

How composition of egg yolk (EY) influences NF-κB, a key transcription pathway in inflammation, remains unclear. We performed partial delipidation of EY that removed 20-30% of cholesterol and triglycerides. The resulting polar and nonpolar fractions were termed EY-P and EY-NP. NF-κB activation in response to EY from different suppliers and their fractions was examined in 3T3-L1 adipocytes using a NF-κB response element reporter assay and by analyzing expression of 248 inflammatory genes. Although EY-P and EY contained similar level of vitamins, carotenoids, and fatty acids, only delipidated EY-P fraction suppressed NF-κB via down-regulation of toll like receptor-2 and up-regulation of inhibitory toll interacting protein (Tollip) and lymphocyte antigen 96 (Ly96). Our data suggest that anti-inflammatory activity of lutein and retinol were blunted by nonpolar lipids in EY, likely via crosstalk between SREBP and NF-κB pathways in adipocytes. Thus, moderate delipidation may improve the beneficial properties of regular eggs.

Pathophysiological role of enhanced bone marrow adipogenesis in diabetic complications

Diabetes leads to complications in select organ systems primarily by disrupting the vasculature of the target organs. These complications include both micro- (cardiomyopathy, retinopathy, nephropathy, and neuropathy) and macro-(atherosclerosis) angiopathies. Bone marrow angiopathy is also evident in both experimental models of the disease as well as in human diabetes. In addition to vascular disruption, bone loss and increased marrow adiposity have become hallmarks of the diabetic bone phenotype. Emerging evidence now implicates enhanced marrow adipogenesis and changes to cellular makeup of the marrow in a novel mechanistic link between various secondary complications of diabetes. In this review, we explore the mechanisms of enhanced marrow adipogenesis in diabetes and the link between changes to marrow cellular composition, and disruption and depletion of reparative stem cells.

The effect of nutritional status and muscle fiber type on myogenic satellite cell fate and apoptosis

Satellite cells (SC) are multipotential stem cells that can be induced by nutrition to alter their cellular developmental fate, which may vary depending on their fiber type origin. The objective of the current study was to determine the effect of restricting protein synthesis on inducing adipogenic transdifferentiation and apoptosis of SC originating from fibers of the fast glycolytic pectoralis major (p. major) and fast oxidative and glycolytic biceps femoris (b. femoris) muscles of the chicken. The availability of the essential sulfur amino acids Met and Cys was restricted to regulate protein synthesis during SC proliferation and differentiation. The SC were cultured and treated with 1 of 6 Met/Cys concentrations: 60/192, 30/96 (control), 7.5/24, 3/9.6, 1/3.2, or 0/0 mg/L. Reductions in Met/Cys concentrations from the control level resulted in increased lipid staining and expression of the adipogenic marker genes peroxisome proliferator-activated receptor gamma and stearoyl-CoA desaturase during differentiation in the p. major SC. Although b. femoris SC had increased lipid staining at lower Met/Cys concentrations, there was no increase in expression of either adipogenic gene. For both muscle types, SC Met/Cys, concentration above the control increased the expression of peroxisome proliferator-activated receptor gamma and stearoyl-CoA desaturase during differentiation. As Met/Cys concentration was decreased during proliferation, a dose-dependent decline in all apoptotic cells occurred except for early apoptotic cells in the p. major, which had no treatment effect (P < 0.05). During differentiation, decreasing Met/Cys concentration caused an increase in early apoptotic cells in both fiber types and no effect on late apoptotic cells except for an increase in the p. major 7.5/24 mg/L of Met/Cys treatment. In general, the viability of the SC was unaffected by the Met/Cys concentration except during proliferation in the p. major 0/0 mg/L of Met/Cys treatment, which increased SC viability. These data demonstrate the effect of nutrition on SC transdifferentiation to an adipogenic lineage and apoptosis, and the effect of fiber type on this response in an in vitro context.

Role for sterol regulatory element binding protein-1c activation in mediating skeletal muscle insulin resistance via repression of rat insulin receptor substrate-1 transcription

AIMS/HYPOTHESIS

Sterol regulatory element binding protein-1c (SREBP-1c) is a master regulator of fatty acid synthase and controls lipogenesis. IRS-1 is the key insulin signalling mediator in skeletal muscle. In the present study, we investigated the role of SREBP-1c in the regulation of IRS-1 in skeletal muscle cells.

METHODS

L6 muscle cells were treated with palmitic acid (PA) or metformin. Adenovirus vectors expressing Srebp-1c (also known as Srebf1) and small interfering RNA (siRNA) against Srebp-1c were transfected into the L6 cells. Protein-DNA interactions were assessed by luciferase reporter analysis, electrophoretic mobility shift assay and chromatin immunoprecipitation assay.

RESULTS

We found that both gene and protein expression of SREBP-1c was increased in contrast to IRS-1 expression in PA-treated L6 cells. SREBP-1c overproduction decreased Irs-1 mRNA and IRS-1 protein expression in a dose-dependent manner, and suppressed the resultant insulin signalling, whereas SERBP-1c knockdown by Serbp-1c siRNA blocked the downregulation of IRS-1 induced by PA. Protein-DNA interaction studies demonstrated that SREBP-1c was able to bind to the rat Irs-1 promoter region, thereby repressing its gene transcription. Of particular importance, we found that metformin treatment downregulated Srebp-1c promoter activity, decreased the specific binding of SREBP-1c to Irs-1 promoter and upregulated Irs-1 promoter activity in PA-cultured L6 cells.

CONCLUSIONS/INTERPRETATION

Our data indicate for the first time that SREBP-1c activation participates in skeletal muscle insulin resistance through a direct effect of suppressing Irs-1 transcription. These findings imply that SREBP-1c could serve as an attractive therapeutic target for insulin resistance.

Similar changes in muscle lipid metabolism are induced by chronic high-fructose feeding and high-fat feeding in C57BL/J6 mice

The aim of the present study was to investigate the effects of high fructose and high fat feeding on muscle lipid metabolism and to illustrate the mechanisms by which the two different dietary factors induce muscle lipid accumulation. C57BL/J6 mice were fed either a standard, high-fructose (HFru) or high-fat diet. After 16 weeks feeding, mice were killed and plasma triglyceride (TG) and free fatty acid (FFA) levels were detected. In addition, muscle TG and long chain acyl CoA (LCACoA) content was determined, glucose tolerance was evaluated and the protein content of fatty acid translocase CD36 (FATCD36) in muscle was measured. Mitochondrial oxidative function in the muscle was evaluated by estimating the activity of oxidative enzymes, namely cytochrome oxidase (COx), citrate synthase (CS) and β-hydroxyacyl CoA dehydrogenase (β-HAD), and the muscle protein content of carnitine palmitoyltransferase-1 (CPT-1), cyclo-oxygenase (COX)-1 and proliferator-activated receptor coactivator (PGC)-1α was determined. Finally, sterol regulatory element-binding protein-1c (SREBP-1c) gene expression and fatty acid synthase (FAS) protein content were determined in muscle tissues. After 16 weeks, plasma TG and FFA levels were significantly increased in both the HFru and HF groups. In addition, mice in both groups exhibited significant increases in muscle TG and LCACoA content. Compared with mice fed the standard diet (control group), those in the HFru and HF groups developed glucose intolerance and exhibited increased FATCD36 protein levels, enzyme activity related to fatty acid utilization in the mitochondria and protein expressions of CPT-1, COX-1 and PGC-1α in muscle tissue. Finally, mice in both the HFru and HF groups exhibited increase SREBP-1c expression and FAS protein content. In conclusion, high fructose and high fat feeding lead to similar changes in muscle lipid metabolism in C57BL/J6 mice. Lipid accumulation in the muscle may be associated with increased expression of proteins related to lipid transportation and synthesis.

Role of stearoyl-CoA desaturase-1 in skeletal muscle function and metabolism

Stearoyl-CoA desaturase-1 (SCD1) converts saturated fatty acids (SFA) into monounsaturated fatty acids and is necessary for proper liver, adipose tissue, and skeletal muscle lipid metabolism. While there is a wealth of information regarding SCD1 expression in the liver, research on its effect in skeletal muscle is scarce. Furthermore, the majority of information about its role is derived from global knockout mice, which are known to be hypermetabolic and fail to accumulate SCD1's substrate, SFA. We now know that SCD1 expression is important in regulating lipid bilayer fluidity, increasing triglyceride formation, and enabling lipogenesis and may protect against SFA-induced lipotoxicity. Exercise has been shown to increase SCD1 expression, which may contribute to an increase in intramyocellular triglyceride at the expense of free fatty acids and diacylglycerol. This review is intended to define the role of SCD1 in skeletal muscle and discuss the potential benefits of its activity in the context of lipid metabolism, insulin sensitivity, exercise training, and obesity.

High-content screening of human primary muscle satellite cells for new therapies for muscular atrophy/dystrophy

Myoblast proliferation and differentiation are essential for normal skeletal muscle growth and repair. Muscle recovery is dependent on the quiescent population of muscle stem cells - satellite cells. During muscle injury, satellite cells become mitotically active and begin the repair process by fusing with each other and/or with myofibers. Aging, prolonged inactivity, obesity, cachexia and other muscle wasting diseases are associated with a decreased number of quiescent and proliferating satellite cells, which impedes the repair process.

A high-content/high-throughput platform was developed and utilized for robust phenotypic evaluation of human primary satellite cells in vitro for the discovery of chemical probes that may improve muscle recovery. A 1600 compound pilot screen was developed using two highly annotated small molecule libraries. This screen yielded 15 dose responsive compounds that increased proliferation rate in satellite cells derived from a single obese human donor. Two of these compounds remained dose responsive when counter-screened in 3-donor obese superlot. The Alk-5 inhibitor LY364947, was used as a positive control for assessing satellite cell proliferation/delayed differentiation. A multivariate approach was utilized for exploratory data analysis to discover proliferation vs. differentiation-dependent changes in cellular phenotype. Initial screening efforts successfully identified a number of phenotypic outcomes that are associated with desired effect of stimulation of proliferation and delayed differentiation.

Independent and combined effects of acute physiological hyperglycaemia and hyperinsulinaemia on metabolic gene expression in human skeletal muscle

Physiological hyperglycaemia and hyperinsulinaemia are strong modulators of gene expression, which underpins some of their well-known effects on insulin action and energy metabolism. The aim of the present study was to examine whether acute in vivo exposure of healthy humans to hyperinsulinaemia and hyperglycaemia have independent or additive effects on expression of key metabolic genes in skeletal muscle. On three randomized occasions, seven young subjects underwent a 4 h (i) hyperinsulinaemic (50 m-units·m−2·min−1) hyperglycaemic (10 mmol/l) clamp (HIHG), (ii) hyperglycaemic (10 mmol/l) euinsulinaemic (5 m-units·m−2·min−1) clamp (LIHG) and (iii) hyperinsulinaemic (50 m-units·m−2·min−1) euglycaemic (4.5 mmol/l) clamp (HING). Muscle biopsies were obtained before and after each clamp for the determination of expression of genes involved in energy metabolism, and phosphorylation of key insulin signalling proteins. Hyperinsulinaemia and hyperglycaemia exerted independent effects with similar direction of modulation on PI3KR1 (phosphatidylinositol 3-kinase, regulatory 1), LXRα (liver X receptor α), PDK4 (pyruvate dehydrogenase kinase 4) and FOXO1 (forkhead box O1A) and produced an additive effect on PI3KR1, the gene that encodes the p85α subunit of PI3K in human skeletal muscle. Acute hyperglycaemia itself altered the expression of genes involved in fatty acid transport and oxidation [fatty acid transporter (CD36), LCAD (long-chain acyl-CoA dehydrogenase) and FOXO1], and lipogenesis [LXRα, ChREBP (carbohydrate-responseelement-binding protein), ABCA1 (ATP-binding cassette transporter A1) and G6PD (glucose-6-phosphate dehydrogenase). Surperimposing hyperinsulinaemia on hyperglycaemia modulated a number of genes involved in insulin signalling, glucose metabolism and intracellular lipid accumulation and exerted an additive effect on PI3KR1. These may be early molecular events that precede the development of glucolipotoxicity and insulin resistance normally associated with more prolonged periods of hyperglycaemia and hyperinsulinaemia.

Elevated lipogenesis in epithelial stem-like cell confers survival advantage in ductal carcinoma in situ of breast cancer

Upregulation of lipogenesis is a hallmark of cancer and blocking the lipogenic pathway is known to cause tumor cell death by apoptosis. However, the exact role of lipogenesis in tumor initiation is as yet poorly understood. We examined the expression profile of key lipogenic genes in clinical samples of ductal carcinoma in situ (DCIS) of breast cancer and found that these genes were significantly upregulated in DCIS. We also isolated cancer stem-like cells (CSCs) from DCIS.com cell line using cell surface markers (CS24(-)CD44(+)ESA(+)) and found that this cell population has significantly higher tumor-initiating ability to generate DCIS compared with the non-stem-like population. Furthermore, the CSCs showed significantly higher level of expression of all lipogenic genes than the counterpart population from non-tumorigenic breast cancer cell line, MCF10A. Importantly, ectopic expression of SREBP1, the master regulator of lipogenic genes, in MCF10A significantly enhanced lipogenesis in stem-like cells and promoted cell growth as well as mammosphere formation. Moreover, SREBP1 expression significantly increased the ability of cell survival of CSCs from MCF10AT, another cell line that is capable of generating DCIS, in mouse and in cell culture. These results indicate that upregulation of lipogenesis is a pre-requisite for DCIS formation by endowing the ability of cell survival. We have also shown that resveratrol was capable of blocking the lipogenic gene expression in CSCs and significantly suppressed their ability to generate DCIS in animals, which provides us with a strong rationale to use this agent for chemoprevention against DCIS.

Actions and interactions of AMPK with insulin, the peroxisomal-proliferator activated receptors and sirtuins

AMP-activated protein kinase (AMPK) activity responds to a requirement to increase cellular ATP production and/or to conserve available ATP. AMPK is therefore central to the mechanisms of adjustment to fluctuating energy demand or metabolic substrate supply. AMPK has important actions in several insulin-responsive tissues, as well as in the pancreatic β cell, through which it can modulate glycemic control, insulin action and metabolic substrate selection and disposal. We review recent novel findings elucidating the mechanisms by which AMPK activation can correct impaired insulin action. However, we also emphasize not only the similarities, but also the differences in the actions of insulin and AMPK. We focus on metabolic interfaces between AMPK, peroxisomal proliferator-activated receptors, sirtuins and mTORC.

SREBP-1 activation by glucose mediates TGF-β upregulation in mesangial cells

Glomerular matrix accumulation is a hallmark of diabetic nephropathy. Recent studies showed that overexpression of the transcription factor sterol-responsive element-binding protein (SREBP)-1 induces pathology reminiscent of diabetic nephropathy, and SREBP-1 upregulation was observed in diabetic kidneys. We thus studied whether SREBP-1 is activated by high glucose (HG) and mediates its profibrogenic responses. In primary rat mesangial cells, HG activated SREBP-1 by 30 min, seen by the appearance of its cleaved nuclear form (nSREBP-1), EMSA, and by activation of an SREBP-1 response element (SRE)-driven green fluorescent protein construct. Activation was dose dependent and not induced by an osmotic control. Site 1 protease was required, since its inhibition by AEBSF prevented SREBP-1 activation. SCAP, the ER-associated chaperone for SREBP-1, was also necessary since its inhibitor fatostatin also blocked SREBP-1 activation. Signaling through the EGFR/phosphatidylinositol 3-kinase (PI3K) pathway, which we previously showed mediates HG-induced TGF-β1 upregulation, and through RhoA, were upstream of SREBP-1 activation (Wu D, Peng F, Zhang B, Ingram AJ, Gao B, Krepinsky JC. Diabetologia 50: 2008–2018, 2007; Wu D, Peng F, Zhang B, Ingram AJ, Kelly DJ, Gilbert RE, Gao B, Krepinsky JC. J Am Soc Nephrol 20: 554–566, 2009). Fatostatin and AEBSF prevented HG-induced TGF-β1 upregulation by Northern blot analysis, and HG-induced TGF-β1 promoter activation was inhibited by both fatostatin and dominant negative SREBP-1a. Chromatin immunoprecipitation analysis confirmed that HG led to SREBP-1 binding to the TGF-β1 promoter in a region containing a putative SREBP-1 binding site (SRE). Thus HG-induced SREBP-1 activation requires EGFR/PI3K/RhoA signaling and SCAP-mediated transport to the Golgi for its proteolytic cleavage. Activated SREBP-1 binds to the TGF-β promoter, resulting in TGF-β1 upregulation in response to HG. SREBP-1 thus provides a potential novel therapeutic target for the treatment of diabetic nephropathy.

Transcriptional analysis reveals a high impact of conjugated linoleic acid on stearoyl-Coenzyme A desaturase 1 mRNA expression in mice gastrocnemius muscle

We examined the potential implication of skeletal muscle in the fat-lowering effect observed in mice treated with moderate doses of CLA. In experiment 1, mice fed with a standard-fat diet were orally treated with sunflower oil (control) and 3 or 10 mg CLA mixture/day for 37 days. In experiment 2, mice were fed with a high-fat diet for 65 days. For the first 30 days, they received the same doses as in experiment 1 and, from that time onwards, animals received double doses. Gene expression of key proteins involved in fatty acid transport, oxidation, regulation of lipid and carbohydrate utilization, composition of muscle fiber, and thermogenesis were determined and, in most of them, no major impact of CLA was seen. Therefore, enhancement of fatty acid oxidation in muscle did not seem to contribute to the antiobesity effect of CLA as seen in other studies with higher CLA doses. However, a strong induction of classically associated lipogenic genes such as Fasn (up to twofold) and, particularly, Scd1 (up to ninefold) was found. This activation could contribute to a protective role in muscle cells, since expression of ER stress markers was decreased and inversely correlated with the induction of Scd1.

Lipid metabolism in mammalian tissues and its control by retinoic acid

Evidence has accumulated that specific retinoids impact on developmental and biochemical processes influencing mammalian adiposity including adipogenesis, lipogenesis, adaptive thermogenesis, lipolysis and fatty acid oxidation in tissues. Treatment with retinoic acid, in particular, has been shown to reduce body fat and improve insulin sensitivity in lean and obese rodents by enhancing fat mobilization and energy utilization systemically, in tissues including brown and white adipose tissues, skeletal muscle and the liver. Nevertheless, controversial data have been reported, particularly regarding retinoids' effects on hepatic lipid and lipoprotein metabolism and blood lipid profile. Moreover, the molecular mechanisms underlying retinoid effects on lipid metabolism are complex and remain incompletely understood. Here, we present a brief overview of mammalian lipid metabolism and its control, introduce mechanisms through which retinoids can impact on lipid metabolism, and review reported activities of retinoids on different aspects of lipid metabolism in key tissues, focusing on retinoic acid. Possible implications of this knowledge in the context of the management of obesity and the metabolic syndrome are also addressed. This article is part of a Special Issue entitled Retinoid and Lipid Metabolism.

Skeletal muscle satellite cells are committed to myogenesis and do not spontaneously adopt nonmyogenic fates

The developmental potential of skeletal muscle stem cells (satellite cells) remains controversial. The authors investigated satellite cell developmental potential in single fiber and clonal cultures derived from MyoD(iCre/+);R26R(EYFP/+) muscle, in which essentially all satellite cells are permanently labeled. Approximately 60% of the clones derived from cells that co-purified with muscle fibers spontaneously underwent adipogenic differentiation. These adipocytes stained with Oil-Red-O and expressed the terminal differentiation markers, adipsin and fatty acid binding protein 4, but did not express EYFP and were therefore not of satellite cell origin. Satellite cells mutant for either MyoD or Myf-5 also maintained myogenic programming in culture and did not adopt an adipogenic fate. Incorporation of additional wash steps prior to muscle fiber plating virtually eliminated the non-myogenic cells but did not reduce the number of adherent Pax7+ satellite cells. More than half of the adipocytes observed in cultures from Tie2-Cre mice were recombined, further demonstrating a non-satellite cell origin. Under adipogenesis-inducing conditions, satellite cells accumulated cytoplasmic lipid but maintained myogenic protein expression and did not fully execute the adipogenic differentiation program, distinguishing them from adipocytes observed in muscle fiber cultures. The authors conclude that skeletal muscle satellite cells are committed to myogenesis and do not spontaneously adopt an adipogenic fate.

Metabolic imaging allows early prediction of response to vandetanib

The RET (rearranged-during-transfection protein) protooncogene triggers multiple intracellular signaling cascades regulating cell cycle progression and cellular metabolism. We therefore hypothesized that metabolic imaging could allow noninvasive detection of response to the RET inhibitor vandetanib in vivo.

METHODS

The effects of vandetanib treatment on the full-genome expression and the metabolic profile were analyzed in the human medullary thyroid cancer cell line TT. In vitro, transcriptional changes of pathways regulating cell cycle progression and glucose, dopa, and thymidine metabolism were correlated to the results of cell cycle analysis and the uptake of (3)H-deoxyglucose, (3)H-3,4-dihydroxy-L-phenylalanine, and (3)H-thymidine under vandetanib treatment. In vivo, the tumor metabolism under vandetanib was monitored by small-animal PET of tumor-bearing mice.

RESULTS

Vandetanib treatment resulted in the transcriptional downregulation of various effector pathways with consecutive downregulation of cyclin expression and a G(0)/G(1) arrest. In vitro, vandetanib treatment resulted in the decreased expression of genes regulating glucose, 3,4-dihydroxy-L-phenylalanine, and thymidine metabolism, with a subsequent reduction in the functional activity of the corresponding pathways. In vivo, metabolic imaging with PET was able to assess changes in the tumoral glucose metabolism profile as early as 3 d after initiation of vandetanib treatment.

CONCLUSION

We describe a metabolic imaging approach for the noninvasive detection of successful vandetanib treatment. Our results suggest that PET may be useful for identifying patients who respond to vandetanib early in the course of treatment.

Laminopathies: the molecular background of the disease and the prospects for its treatment

Laminopathies are rare human degenerative disorders with a wide spectrum of clinical phenotypes, associated with defects in the main protein components of the nuclear envelope, mostly in the lamins. They include systemic disorders and tissue-restricted diseases. Scientists have been trying to explain the pathogenesis of laminopathies and find an efficient method for treatment for many years. In this review, we discuss the current state of knowledge about laminopathies, the molecular mechanisms behind the development of particular phenotypes, and the prospects for stem cell and/or gene therapy treatments.

Regulation of fasting fuel metabolism by toll-like receptor 4

OBJECTIVE

Toll-like receptor 4 (TLR4) has been reported to induce insulin resistance through inflammation in high-fat-fed mice. However, the physiological role of TLR4 in metabolism is unknown. Here, we investigated the involvement of TLR4 in fasting metabolism.

RESEARCH DESIGN AND METHODS

Wild-type and TLR4 deficient (TLR4(-/-)) mice were either fed or fasted for 24 h. Glucose and lipid levels in circulation and tissues were measured. Glucose and lipid metabolism in tissues, as well as the expression of related enzymes, was examined.

RESULTS

Mice lacking TLR4 displayed aggravated fasting hypoglycemia, along with normal hepatic gluconeogenesis, but reversed activity of pyruvate dehydrogenase complex (PDC) in skeletal muscle, which might account for the fasting hypoglycemia. TLR4(-/-) mice also exhibited higher lipid levels in circulation and skeletal muscle after fasting and reversed expression of lipogenic enzymes in skeletal muscle but not liver and adipose tissue. Adipose tissue lipolysis is normal and muscle fatty acid oxidation is increased in TLR4(-/-) mice after fasting. Inhibition of fatty acid synthesis in TLR4(-/-) mice abolished hyperlipidemia, hypoglycemia, and PDC activity increase, suggesting that TLR4-dependent inhibition of muscle lipogenesis may contribute to glucose and lipid homeostasis during fasting. Further studies showed that TLR4 deficiency had no effect on insulin signaling and muscle proinflammatory cytokine production in response to fasting.

CONCLUSIONS

These data suggest that TLR4 plays a critical role in glucose and lipid metabolism independent of insulin during fasting and identify a novel physiological role for TLR4 in fuel homeostasis.

Gene regulatory network reveals oxidative stress as the underlying molecular mechanism of type 2 diabetes and hypertension

Background

The prevalence of diabetes is increasing worldwide. It has been long known that increased rates of inflammatory diseases, such as obesity (OBS), hypertension (HT) and cardiovascular diseases (CVD) are highly associated with type 2 diabetes (T2D). T2D and/or OBS can develop independently, due to genetic, behavioral or lifestyle-related variables but both lead to oxidative stress generation. The underlying mechanisms by which theses complications arise and manifest together remain poorly understood. Protein-protein interactions regulate nearly every living process. Availability of high-throughput genomic data has enabled unprecedented views of gene and protein co-expression, co-regulations and interactions in cellular systems.

Methods

The present work, applied a systems biology approach to develop gene interaction network models, comprised of high throughput genomic and PPI data for T2D. The genes differentially regulated through T2D were 'mined' and their 'wirings' were studied to get a more complete understanding of the overall gene network topology and their role in disease progression.

Results

By analyzing the genes related to T2D, HT and OBS, a highly regulated gene-disease integrated network model has been developed that provides useful functional linkages among groups of genes and thus addressing how different inflammatory diseases are connected and propagated at genetic level. Based on the investigations around the 'hubs' that provided more meaningful insights about the cross-talk within gene-disease networks in terms of disease phenotype association with oxidative stress and inflammation, a hypothetical co-regulation disease mechanism model been proposed. The results from this study revealed that the oxidative stress mediated regulation cascade is the common mechanistic link among the pathogenesis of T2D, HT and other inflammatory diseases such as OBS.

Conclusion

The findings provide a novel comprehensive approach for understanding the pathogenesis of various co-associated chronic inflammatory diseases by combining the power of pathway analysis with gene regulatory network evaluation.

Increased mitochondrial fatty acid oxidation is sufficient to protect skeletal muscle cells from palmitate-induced apoptosis

The mechanisms underlying the protective effect of monounsaturated fatty acids (e.g. oleate) against the lipotoxic action of saturated fatty acids (e.g. palmitate) in skeletal muscle cells remain poorly understood. This study aimed to examine the role of mitochondrial long-chain fatty acid (LCFA) oxidation in mediating oleate's protective effect against palmitate-induced lipotoxicity. CPT1 (carnitine palmitoyltransferase 1), which is the key regulatory enzyme of mitochondrial LCFA oxidation, is inhibited by malonyl-CoA, an intermediate of lipogenesis. We showed that expression of a mutant form of CPT1 (CPT1mt), which is active but insensitive to malonyl-CoA inhibition, in C2C12 myotubes led to increased LCFA oxidation flux even in the presence of high concentrations of glucose and insulin. Furthermore, similar to preincubation with oleate, CPT1mt expression protected muscle cells from palmitate-induced apoptosis and insulin resistance by decreasing the content of deleterious palmitate derivates (i.e. diacylglycerols and ceramides). Oleate preincubation exerted its protective effect by two mechanisms: (i) in contrast to CPT1mt expression, oleate preincubation increased the channeling of palmitate toward triglycerides, as a result of enhanced diacylglycerol acyltransferase 2 expression, and (ii) oleate preincubation promoted palmitate oxidation through increasing CPT1 expression and modulating the activities of acetyl-CoA carboxylase and AMP-activated protein kinase. In conclusion, we demonstrated that targeting mitochondrial LCFA oxidation via CPT1mt expression leads to the same protective effect as oleate preincubation, providing strong evidence that redirecting palmitate metabolism toward oxidation is sufficient to protect against palmitate-induced lipotoxicity.

Human adenovirus 36 decreases fatty acid oxidation and increases de novo lipogenesis in primary cultured human skeletal muscle cells by promoting Cidec/FSP27 expression

BACKGROUND

It has been well documented that human adenovirus type 36 (Ad-36) is associated with obesity. However, the underlying molecular mechanism of Ad-36 inducing obesity remains unknown. We sought to investigate the effect of Ad-36 infection on Cidec, AMPK pathway and lipid metabolism in primary cultured human skeletal muscle cells.

METHODS

Cidec/fat-specific protein 27 (FSP27), fatty acid oxidation, AMPK signaling and the abundance of proteins involved in lipid synthesis were determined in muscle cells infected with various doses (1.9-7.6 MOI) of Ad-36 and non-lipogenic adenovirus type 2 (Ad-2) as a negative control as well as an uninfected control. Cidec/FSP27 siRNA transfection was performed in Ad-36-infected muscle cells.

RESULTS

Our data show that Ad-36 significantly reduced fatty acid oxidation in a dose-dependent manner (all P values are <0.01), but Ad-2 did not affect fatty acid oxidation. Ad-36 substantially increased Cidec/FSP27, ACC, sterol regulatory element-binding protein 1c (SREBP-1c), SREBP-2 and 3-hydroxy-3-methylglutaryl-CoA reductase protein abundance, but significantly reduced AMPK activity, mitochondrial mass and uncoupling protein 3 (UCP3) abundance in comparison with control cells (all P values are <0.01). Oil Red O staining revealed that there was substantial fat accumulation in the Ad-36-infected muscle cells. Furthermore, Cidec/FSP27 siRNA transfection significantly reduced FSP27 expression and partially restored AMPK signaling, increased UCP3 and decreased SERBP 1c and perilipin proteins in Ad-36-infected muscle cells. Interestingly, neither Ad-36 nor Ad-2 affected peroxisome proliferator-activated receptor γ protein expression in muscle cells.

CONCLUSION

This study suggests that Ad-36 induced lipid droplets in the cultured skeletal muscle cells and this process may be mediated by promoting Cidec/FSP27 expression.