• Journal of Life Science
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• v.28 no.10
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• pp.1233-1243
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• 2018

Sterol regulatory-element binding proteins (SREBPs) are a family of transcription factors that regulate lipid homeostasis and metabolism by controlling the expression of enzymes required for endogenous cholesterol, fatty acid (FA), triacylglycerol, and phospholipid synthesis. The three SREBPs are encoded by two different genes. The SREBP1 gene gives rise to SREBP-1a and SREBP-1c, which are derived from utilization of alternate promoters that yield transcripts in which distinct first exons are spliced to a common second exon. SREBP-2 is derived from a separate gene. Additionally, SREBPs are implicated in numerous pathogenic processes, such as endoplasmic reticulum stress, inflammation, autophagy, and apoptosis. They also contribute to obesity, dyslipidemia, diabetes mellitus, and nonalcoholic fatty liver diseases. Genome-wide analyses have revealed that these versatile transcription factors act as important nodes of biological signaling networks. Changes in cell metabolism and growth are reciprocally linked through SREBPs. Anabolic and growth signaling pathways branch off and connect to multiple steps of SREBP activation and form complex regulatory networks. SREBPs are activated through the PI3K-Akt-mTOR pathway in these processes, but the molecular mechanism remains to be understood. This review aims to provide a comprehensive understanding of the role of SREBPs in physiology and pathophysiology at the cell, organ, and organism levels.

• Journal of Life Science
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• v.31 no.9
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• pp.796-805
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• 2021

Hepatic endoplasmic reticulum (ER) stress contributes to the development of steatosis and insulin resistance. The components of unfolded protein response (UPR) regulate lipid metabolism. Recent studies have reported an association between ER stress and aberrant cellular lipid control; moreover, research has confirmed the involvement of sterol regulatory element-binding proteins (SREBPs)-the central regulators of lipid metabolism-in the process. However, the exact role of SREBPs in controlling lipid metabolism during ER stress and its contribution to fatty liver disease remain unknown. Here, we show that SREBP-1c deficiency protects against ER stress-induced hepatic steatosis in mice by regulating UPR, inflammation, and fatty acid oxidation. SREBP-1c directly regulated inositol-requiring kinase 1α (IRE1α) expression and mediated ER stress-induced tumor necrosis factor-α activation, leading to a reduction in expression of peroxisome proliferator-activated receptor γ coactivator 1-α and subsequent impairment of fatty acid oxidation. However, the genetic ablation of SREBP-1c prevented these events, alleviating hepatic inflammation and steatosis. Although the mechanism by which SREBP-1c deficiency prevents ER stress-induced inflammatory signaling remains to be elucidated, alteration of the IRE1α signal in SREBP-1c-depleted Kupffer cells might be involved in the signaling. Overall, the results suggest that SREBP-1c plays a crucial role in the regulation of UPR and inflammation in ER stress-induced hepatic steatosis.

• BMB Reports
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• v.51 no.1
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• pp.33-38
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• 2018

Sterol regulatory element-binding protein-1c (SREBP1c) plays an important role in triglyceride (TG) homeostasis. Although our previous study showed that hepatitis C virus core-binding protein 6 (HCBP6) regulates SREBP1c expression to maintain intracellular TG homeostasis, the mechanism underlying this regulation is unclear. In the present study, we found that HCBP6 increased intracellular TG levels by upregulating SREBP1c expression. HCBP6 increased SREBP1c transcription by directly binding to the SREBP1c promoter (at the -139- to +359-bp region). Moreover, we observed that HCBP6 interacted with $C/EBP{\beta}-binding$ site in the SREBP1c promoter both in vitro and in vivo. These results indicate that HCBP6 upregulates human SREBP1c expression by binding to the $C/EBP{\beta}-binding$ site in the SREBP1c promoter.

• Journal of Ginseng Research
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• v.29 no.4
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• pp.159-166
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• 2005

Ginsenosides, a group of Panax ginseng saponins, exert the lowering effects of plasma cholesterol levels in animals. We had reported earlier that ginsenoside Rb2 upregulate low-density lipoprotein receptor (LDLR) expression via a mechanism that is dependent of the activation of sterol response element binding protein 2 (SREBP-2) expression. This study was conducted to determine the effects of ginsenoside Rb2 on the expression of the hepatic LDLR expression at cellular levels using HepG2 cells, and to evaluate whether the sterol response element binding protein 1 (SREBP-l) was involved in the regulation of LDLR expression. Incubation of HepG2 cells in serum-free medium supplemented with cholesterol $(10{\mu}g/ml)$ for 8 hours decreased the mRNAs of LDLR mRNA by $12\%$ and SREBP-l mRNA by $35\%$. Ginsenoside Rb2 antagonized the repressive effects of cholesterol and increased both LDLR and SREBP-l mRNA expression to 1.5- and 2-fold, respectively. Furthermore, Western blot and confocal microscopic analyses with SREBP-l polyclonal antibody revealed that ginsenoside Rb2 enhanced the maturation of the SREBP-1 from the inactive precursor form in ER membrane to the active transcription factor form in nucleus. These results suggest that ginsenoside Rb2 upregulates LDLR expression via a mechanism that is dependent of the activation of not only SREBP-2 expression, but also SREBP-1 expression and maturation, and also indicate that the pharmacological value of ginsenoside Rb2 may be distinguished from that of lovastatin which is reported that it upregulate LDLR through SREBP-2 only, not through SREBP-1.

• Journal of Life Science
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• v.25 no.3
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• pp.317-322
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• 2015

Alcohol-induced fatty liver (steatosis) results from excessive generation of reducing equivalents by ethanol metabolism. Generally, chronic ethanol treatment causes hepatosteatosis by regulating sterol regulatory element-binding protein 1c (SREBP-1c), which increases the synthesis of hepatic lipids. The effect of ethanol on SREBP-1c is mediated through mammalian sirtuin-1 (SIRT-1), a NAD⁺-dependent protein deacetylase that regulates hepatic lipid metabolism. Ginseng is a widely used herbal medicine that is used in Asia for its anti-diabetes and anti-obesity effects. The pharmacological and therapeutic effects of ginseng are primarily produced by bioactive constituents known as ginsenosides. Here, we examined the regulatory effects of Korean red ginseng (KRG) extracts on SREBP-1c and SIRT-1 on lipid homeostasis in AML-12 mouse hepatocytes. AML-12 cells were treated with ethanol and/or KRG extracts (0 - 1,000 μg/ml). Lipid droplets were assayed using Oil red O staining, and western blotting was used to measure SIRT-1 and SREBP-1 expression. Treatment with KRG extracts restored SIRT-1 expression and reduced SREBP-1c expression in ethanol-treated cells. We also showed that KRG extract and ginsenosides Rb₂ and Rd significantly decreased SREBP-1 acetylation in ethanol-treated cells. These results show that treatment with KRG extract and its active ginsenoside constituents Rb₂ and Rd protected against alcohol-related hepatosteatosis via regulation of SIRT-1 and downstream acetylation of SREBP-1c, which altered hepatic lipid metabolism.

• Animal cells and systems
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• v.8 no.1
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• pp.49-55
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• 2004

In vertebrates, multisubunit cofactors regulate gene expression through interacting with cell-type- and gene-specific DNA-binding proteins in a chromatin-selective manner. ADD1/SREBP1c regulates fatty acid metabolism and insulin-dependent gene expression through binding to SRE and E-box motif with dual DNA binding specificity. Although its transcriptional and post-translational regulation has been extensively studied, its regulation by interacting proteins is not well understood. To identify cellular proteins that associate with nuclear form of ADD1/SEBP1c, we employed the GST pull-down system with Hela cell nuclei extract. In this study, we demonstrated that Ku proteins interact specifically with ADD1/SREP1c protein. GST pull-down combined with peptide sequencing analysis revealed that Ku80 binds to ADD1/SREBP1c in vitro. Additionally, western blot analysis showed that Ku70, a heterodimerizing partner of Ku80, also associates with ADD1/SREBP1c. Furthermore, co-transfection of Ku70/Ku80 with ADD1/SREBP1c enhanced the transcriptional activity of ADD1/SREBP1c. Taken together, these results suggest that the Ku proteins might be involved in the lipogenic and/or adipogenic gene expression through interacting with ADD1/SREBP1c.

• Toxicological Research
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• v.33 no.3
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• pp.219-224
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• 2017

Lipin1 was identified as a phosphatidate phosphatase enzyme, and it plays a key role in lipid metabolism. Since free radicals contribute to metabolic diseases in the liver, this study investigated the effects of free radicals on the regulation of Lipin1 expression in Huh7 and AML12 cells. Hydrogen peroxide induced mRNA and protein expression of Lipin1 in Huh7 cells, which was assayed by quantitative RT-PCR and immunoblotting, respectively. Induction of Lipin1 by hydrogen peroxide was confirmed in AML12 cells. Hydrogen peroxide treatment significantly increased expression of sterol regulatory element-binding protein (SREBP)-2, but not SREBP-1. Moreover, nuclear translocation of SREBP-2 was detected after hydrogen peroxide treatment. Hydrogen peroxide-induced Lipin1 or SREBP-2 expression was significantly reduced by N-acetyl-$\small{L}$-cysteine treatment, indicating that reactive oxygen species (ROS) were implicated in Lipin1 expression. Next, we investigated whether the hypoxic environments that cause endogenous ROS production in mitochondria in metabolic diseases affect the expression of Lipin1. Exposure to hypoxia also increased Lipin1 expression. In contrast, pretreatment with antioxidants attenuated hypoxia-induced Lipin1 expression. Collectively, our results show that ROS activate SREBP-2, which induces Lipin1 expression.

• BMB Reports
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• v.40 no.4
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• pp.525-531
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• 2007

As a $\beta_2$-adrenergic agonist, clenbuterol decreases body fat, but the molecular mechanism underlying this process is unclear. In the present study, we treated 293T and L-02 cells with clenbuterol and found that clenbuterol downregulates SREBP-1c expression and upregulates CREB1 expression. Considering SREBP-1c has the function of regulating the transcription of several lipogenic enzymes, we considered that the downregulation of SREBP-1c is responsible for body fat reduction by clenbuterol. Many previous studies have found that clenbuterol markedly increases intracellular cAMP levels, therefore, we also investigated whether CREB1 is involved in this process. The data from our experiments indicate that CREB1 overexpression inhibits SREBP-1c transcription, and that this action is antagonized by CREB2, a competitive inhibitor of CREB1. Furthermore, since PPARs are able to repress SREBP-1c transcription, we investigated whether clenbuterol and CREB1 function via a pathway involving PPAR activation. However, our results showed that clenbuterol or CREB1 overexpression suppressed PPARs transcription in 293T and L-02 cells, which suggested that they impair SREBP-1c expression in other ways.

• Herbal Formula Science
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• v.30 no.3
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• pp.137-143
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• 2022

Objectives : Brain-Liver axis is an important target of the chronic human diseases. Hepatic steatosis is one of the most famous disorders in the chronic diseases. This study investigated the moderating effect of beneficial herbs on the fat accumulation, which is mediated by the LXR alpha-SREBP-1c signaling pathway. Methods : In order to confirm the SREBP-1c inhibitory effect, we performed immonoblotting ananlysis using HepG2 cells and Huh 7 cells treated by T0901317, the ligand of LXRα. Results : Forsythiae suspensa water extract (FSE) was not cytotoxicity in cell lines. FSE inhibited SREBP-1c protein expression in HepG2 and Huh7 cells induced by T0901317. In addition, FSE increased the phosphorylation of LKB1, which is associated with LXR-related pathway in HepG2 and Huh 7 cells. Conclusions : These results showed that FSE activated LKB1 to suppress SREBP-1c, which protects the cells against oxidative stress.

• Journal of Ginseng Research
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• v.28 no.2
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• pp.87-93
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• 2004

The effect of ginsenoside-Rb2, one of a major pharmacological component of Panax ginseng C.A. Meyer, on low density lipoprotein (LDL) receptor expression was investigated and compared with hypocholesterolemic drug lovastatin. In HepG2 cell, exogenous cholesterol decreased LDL receptor mRNA expression, but ginsenoside-Rb2 recovered this reduction of LDL receptor mRNA up to normal expression level. Lovastatin also increased LDL receptor mRNA expression as similar as ginsenoside-Rb2 did. The reduction of sterol regulatory element binding protein (SREBP) transcription by exogenous cholesterol was also similarly recovered by ginsenoside-Rb2 and lovastatin addition. Compound K, a metabolite of ginsenoside-Rb2 and -Rb1 by human intestinal bacteria also increased the SREBP mRNA expression in cholesterol-enriched condition. Ginsenoside-Rb2 seems to up-regulate LDL receptor mRNA expression through the induction of de novo SREBP transcription. Therefore, increased expression of SREBP mRNA by ginsenoside-Rb2 elevated the LDL receptor mRNA expression in HepG2 cells, and these inductions possibly drop the plasma cholesterol level in hypercholesterolemia patients, in vivo, as likely in case of lovastatin.