Sterol Regulatory Element Binding Protein-1a Research Articles

Chitosan-Based Sustained Expression of Sterol Regulatory Element-Binding Protein 1a Stimulates Hepatic Glucose Oxidation and Growth in Sparus aurata.

The administration of a single dose of chitosan nanoparticles driving the expression of sterol regulatory element-binding protein 1a (SREBP1a) was recently associated with the enhanced conversion of carbohydrates into lipids. To address the effects of the long-lasting expression of SREBP1a on the growth and liver intermediary metabolism of carnivorous fish, chitosan-tripolyphosphate (TPP) nanoparticles complexed with a plasmid expressing the N terminal active domain of hamster SREBP1a (pSG5-SREBP1a) were injected intraperitoneally every 4 weeks (three doses in total) to gilthead sea bream (Sparus aurata) fed high-protein-low-carbohydrate and low-protein-high-carbohydrate diets. Following 70 days of treatment, chitosan-TPP-pSG5-SREBP1a nanoparticles led to the sustained upregulation of SREBP1a in the liver of S. aurata. Independently of the diet, SREBP1a overexpression significantly increased their weight gain, specific growth rate, and protein efficiency ratio but decreased their feed conversion ratio. In agreement with an improved conversion of dietary carbohydrates into lipids, SREBP1a expression increased serum triglycerides and cholesterol as well as hepatic glucose oxidation via glycolysis and the pentose phosphate pathway, while not affecting gluconeogenesis and transamination. Our findings support that the periodical administration of chitosan-TPP-DNA nanoparticles to overexpress SREBP1a in the liver enhanced the growth performance of S. aurata through a mechanism that enabled protein sparing by enhancing dietary carbohydrate metabolisation.

Non-metabolic enzyme function of PKM2 in hepatocellular carcinoma: A review.

Hepatocellular carcinoma (HCC) is one of the most malignant tumors with the highest incidence and mortality in the world, causing a serious burden on society. Pyruvate kinase M2 (PKM2) is one of the principal metabolic enzymes involved in glycolysis. Studies have shown that PKM2 is highly expressed in HCC and can be translocated to the nucleus, where it interacts with various transcription factors and proteins such as hypoxia-inducible factor-1α, sterol regulatory element-binding protein 1a, signal transducer and activator of transcription 3, nuclear factor erythroid 2-like 2 and histone H3, exerting non-metabolic enzyme functions to regulate the cell cycle, proliferation, apoptosis, immune escape, migration, and invasion, as well as HCC angiogenesis and tumor microenvironment. This review is focused on the recent progress of PKM2 interacting with various transcription factors and proteins affecting the onset and development of HCC, as well as natural drugs and noncoding RNA impacting diverse biological functions of liver cancer cells by regulating PKM2 non-metabolic enzyme functions, thereby providing valuable directions for the prognosis improvement and molecular targeted therapy of HCC in the future.

Overexpression of F-box and WD repeat domain containing 7 prevents tumor growth of bladder cancer cells through regulating SREBP1a.

BackgroundThis study aimed to explore the effect of F-box and WD repeat domain containing 7 (FBXW7) overexpression on bladder cancer, and to determine the regulatory effect of FBXW7 on sterol regulatory element-binding protein 1a (SREBP1a) in bladder cancer.MethodsThe function of F-box and FBXW7 in tumor growth of bladder cancer cells was investigated using in vivo and in vitro models. We constructed and transfected FBXW7 overexpression vectors into T24 and J82 bladder cancer cells. After transfections, the expression of FBXW7 at messenger RNA (mRNA) and protein levels in human bladder cancer cells was confirmed. To verify the effect of FBXW7 on tumor growth, cell proliferation and migration were detected in the tumor cells after overexpression of FBXW7. Additionally, an in vivo tumor model was produced by inoculating the tumor cells and the effect of FBXW7 was evaluated. Finally, the regulation mechanisms were determined.ResultsOur data showed that overexpression of FBXW7 in J28 and T24 cells significantly inhibited the migration and proliferation of J82 and T24 cells, arrested the cells at G0/G1 phase, and up-regulated phosphorylation of glycogen synthase kinase-3β, while suppressing SREBP1a expression. In vivo data also showed that FBXW7 overexpression triggered apoptosis of tumor cells, prevented the pathological changes of tumor tissues, up-regulated p-GSK3β expression, and suppressed SREBP1a expression. In addition, an interaction between FBXW7 and SREBP1a was confirmed by co-immunoprecipitation.ConclusionsTogether, our data indicate that overexpression of FBXW7 prevents tumor growth of bladder cancer cells, likely through suppressing SREBP1a expression.

Apatinib induced ferroptosis by lipid peroxidation in gastric cancer.

Apatinib, a competitive inhibitor of VEGFR2, has anti-angiogenesis and anticancer activities through different mechanisms, but it still cannot fully explain the drug efficacy of apatinib. Ferroptosis, associated with lethal lipid peroxidation, has emerged to play an important role in cancer biology, however, the exact role of ferroptosis in apatinib-mediating anticancer treatment are still not clear. The effects of (1S, 3R)-RSL3 and apatinib were evaluated in different GC cell lines and in normal human gastric epithelial cells. Further, the effects of apatinib and inhibition of antioxidant defense enzyme glutathione peroxidase (GPX4) on cell viability, cell death, glutathione (GSH) levels, lipid ROS production, cellular malondialdehyde (MDA) levels and protein expression were evaluated in vitro as well as in a mouse tumor xenograft model. The expression level of GPX4 in GC tissues and paracancerous tissues was measured by immunohistochemistry. (1S, 3R)-RSL3 selectively killed GC cells, but not normal cells. Apatinib induced ferroptosis in GC cells by decreasing cellular GSH levels and increasing lipid peroxidation levels. This effect was blocked by co-incubation with ferrostatin-1, liproxstatin-1, GSH, or vitamin E. Further investigation revealed that apatinib down-regulated GPX4 expression via inhibition of the transcription factors Sterol regulatory element-binding protein-1a (SREBP-1a). Besides, we found that multi-drug resistant GC cells were vulnerable to apatinib-induced GPX4 inhibition. In summary, we show that apatinib could induce the lipid peroxidation through GPX4 mediated by SREBP-1a, then negatively regulate the GC cell, even the multi-drug-resistantGC cell, ferroptosis.

Pyruvate kinase M2 interacts with nuclear sterol regulatory element–binding protein 1a and thereby activates lipogenesis and cell proliferation in hepatocellular carcinoma

Dysregulation of lipid metabolism is common in cancer cells, but the underlying mechanisms are poorly understood. Sterol regulatory element-binding proteins (SREBPs) stimulate lipid biosynthesis through transcriptional activation of lipogenic enzymes. However, SREBPs' roles and potential interacting partners in cancer cells are not fully defined. Using a biochemical approach, we found here that pyruvate kinase M2 (PKM2) physically interacts with the nuclear form of SREBP-1a (nBP1a), by binding to amino acids 43-56 in nBP1a. We also found that PKM2 activates SREBP target gene expression and lipid biosynthesis by stabilizing nBP1a proteins. Using a competitive peptide inhibitor to block the formation of the SREBP-1a/PKM2 complex, we observed that this blockade inhibited lipogenic gene expression. Of note, nBP1a phosphorylation at Thr-59 enhanced the binding to PKM2 and promoted cancer cell growth. Moreover, we show that PKM2 phosphorylates Thr-59 in vitro Lastly, in human patients with hepatocellular carcinoma, nBP1a phosphorylation at Thr-59 was negatively correlated with clinical outcomes. Together, our results reveal that nBP1a/PKM2 interaction activates lipid metabolism genes in cancer cells and that Thr-59 phosphorylation of SREBP-1a plays an important role in cancer cell proliferation.

Regulation of INSIG2 by microRNA-96

ABSTRACTMature forms of the microRNAs miR-96, -182, and -183 originate from a single genomic locus and have been shown to be elevated approximately 50-fold in the livers of sterol regulatory element-binding protein-1a and -2 (SREBP-1a and -2) transgenic mice. Our study attempted to identify the possible targets of these microRNAs using miRNA target prediction software. This revealed putative sites in insulin-induced genes (INSIGs). The 3′ untranslated region (UTR) of insulin-induced gene 1 (INSIG1) contained sites corresponding to miR-182, and -183, while the 3′ UTR of INSIG2 featured an miR-96 site. Among these putative sites, only miR-96 demonstrated an inhibitory effect that was specific to the 3′ UTR of INSIG2. As INSIG proteins are the main components of SREBP cleavage complexes that act to release active SREBPs, we assessed the effects of miR-96 on INSIG and SREBP levels and activities. We found that miR-96 reduced the levels of INSIG2 in INSIG1 knockout human fibroblasts, resulting in an increase in SREBP-1 and -2 nuclear forms and a subsequent increase in the abundance of the mRNA of their target genes. These results suggest that miR-96, an miRNA induced by SREBP-2 activation, regulates downstream targets of SREBPs and may increase the abundance of active SREBP.

AMFR gene silencing inhibits the differentiation of porcine preadipocytes.

Our study clarifies the role of the autocrine motility factor receptor (AMFR) gene in porcine preadipocyte differentiation. AMFR-siRNA was transfected into porcine preadipocytes and the preadipocytes were induced to differentiation. Subsequently, qRT-PCR was conducted to examine changes in mRNA expression of a series of genes in porcine preadipocytes, including AMFR, sterol-regulatory element-binding protein-1a (SREBP1a), SREBP2, insulin-induced gene 1 (Insig1), and Insig2. Expression changes in the mRNA of genes regulating adipocyte differentiation were also analyzed using qRT-PCR, including peroxisome proliferator-activated receptor gamma (PPARγ), CCAAT/enhancer-binding protein alpha (C/EBPα), and Kruppel-like factor 2 (KLF2). Western blot analysis was conducted to examine the changes in AMFR protein expression in porcine preadipocytes. Additionally, morphological changes in differentiated porcine preadipocytes were examined by oil red O staining, and changes in optical density (OD) values were measured using an ultraviolet spectrophotometer. At 24 h after transfection with AMFR-siRNA, AMFR mRNA expression significantly reduced (P < 0.01), and AMFR protein expression markedly decreased (P < 0.05). The mRNA expression of SREBP1a, SREBP2, Insig1, and C/EBPα was significantly reduced (P < 0.01), whereas the expression of KLF2 mRNA was significantly elevated (P < 0.01). After induction of preadipocyte differentiation, the number of lipid droplets decreased in the AMFR-silenced group, and the OD value markedly reduced (P < 0.05). In addition, the expression of C/EBPα mRNA significantly decreased (P < 0.05), whereas the expression of KLF2 mRNA considerably increased (P < 0.05). Taken together, silencing of the AMFR gene inhibits the differentiation of porcine preadipocytes.

Stronger activation of SREBP-1a by nucleus-localized HBx

We previously showed that hepatitis B virus (HBV) X protein activates the sterol regulatory element-binding protein-1a (SREBP-1a). Here we examined the role of nuclear localization of HBx in this process. In comparison to the wild-type and cytoplasmic HBx, nuclear HBx had stronger effects on SREBP-1a and fatty acid synthase transcription activation, intracellular lipid accumulation and cell proliferation. Furthermore, nuclear HBx could activate HBV enhancer I/X promoter and was more effective on up-regulating HBV mRNA level in the context of HBV replication than the wild-type HBx, while the cytoplasmic HBx had no effect. Our results demonstrate the functional significance of the nucleus-localized HBx in regulating host lipogenic pathway and HBV replication.

Human Sterol Regulatory Element-Binding Protein 1a Contributes Significantly to Hepatic Lipogenic Gene Expression

Background/Aims: Sterol regulatory element-binding protein (SREBP) 1, the master regulator of lipogenesis, was shown to be associated with non-alcoholic fatty liver disease, which is attributed to its major isoform SREBP1c. Based on studies in mice, the minor isoform SREBP1a is regarded as negligible for hepatic lipogenesis. This study aims to elucidate the expression and functional role of SREBP1a in human liver. Methods: mRNA expression of both isoforms was quantified in cohorts of human livers and primary human hepatocytes. Hepatocytes were treated with PF-429242 to inhibit the proteolytic activation of SREBP precursor protein. SREBP1a-specifc and pan-SREBP1 knock-down were performed by transfection of respective siRNAs. Lipogenic SREBP-target gene expression was analyzed by real-time RT-PCR. Results: In human liver, SREBP1a accounts for up to half of the total SREBP1 pool. Treatment with PF-429242 indicated SREBP-dependent auto-regulation of SREBP1a, which however was much weaker than of SREBP1c. SREBP1a-specifc knock-down also reduced significantly the expression of SREBP1c and of SREBP-target genes. Regarding most SREBP-target genes, simultaneous knock-down of both isoforms resulted in effects of only similar extent as SREBP1a-specific knock-down. Conclusion: We here showed that SREBP1a is significantly contributing to the human hepatic SREBP1 pool and has a share in human hepatic lipogenic gene expression.

Renoprotective effect of myricetin restrains dyslipidemia and renal mesangial cell proliferation by the suppression of sterol regulatory element binding proteins in an experimental model of diabetic nephropathy

Myricetin is a natural flavonoid used in various health management systems. In this present study myricetin tested to evaluate the effect on lipids and lipid metabolism enzymes in normal and streptozotocin (STZ) with cadmium (Cd) induced diabetic nephrotoxic rats. Diabetic nephrotoxic rats were significantly (P<0.05) increased the levels of urinary albumin and lipid profiles: total cholesterol (TC), triglycerides (TGs), free fatty acids (FFAs), phospholipids (PLs), low density lipoprotein (LDL), very low-density lipoproteins (VLDL), and decreased in the levels of high-density lipoproteins (HDL). In addition, the activity of lipoprotein lipase (LPL) and lecithin cholesterol acyl transferase (LCAT) were decreased significantly, whereas the 3-hydroxy 3-methylglutaryl coenzyme A (HmgCoA) reductase activity was increased. The upregulation of sterol regulatory element binding protein-1a (SREBP-1a), SREBP-1c, SREBP-2, transforming growth factor-β1 (TGF-β1), vascular endothelial growth factor (VEGF) and downregulation peroxisome proliferator-activated receptor alpha (PPAR-α) proteins expression levels were noticed. An administration of myricetin (1.0 mg/kg body weight (b/w)) for 12 weeks was brought the above parameters towards normal level. Histopathological study of kidney samples showed that extracellular mesangial matrix expansion, glomerulosclerosis and interstitial fibrosis in diabetic nephrotoxic rats was suppressed by myricetin treatment. Further our results indicate that administration of myricetin afforded remarkable protection against STZ-Cd induced alterations in lipid metabolism and thereby reduced the diabetic nephropathy in experimental rats.

Phosphorylation of sterol regulatory element binding protein-1a by protein kinase A (PKA) regulates transcriptional activity

The counter-regulatory hormone glucagon inhibits lipogenesis via downregulation of sterol regulatory element binding protein 1 (SREBP-1). The effect of glucagon is mediated via protein kinase A (PKA). To determine if SREBP-1 is a direct phosphorylation target of PKA, we conducted mass spectrometry analysis of recombinant n-terminal SREBP-1a following PKA treatment in vitro. This analysis identified serines 331/332 as bona-fide phosphorylation targets of PKA. To determine the functional consequences of phosphorylation at these sites, we constructed mammalian expression vector for both nSREBP-1a and 1c isoforms in which the candidate PKA phosphorylation sites were mutated to active phosphomimetic or non-phosphorylatable amino acids. The transcriptional activity of SREBP was reduced by the phosphomimetic mutation of S332 of nSREBP-1a and the corresponding serine (S308) of nSREBP-1c. This site is a strong candidate for mediating the negative regulatory effect of glucagon on SREBP-1 and lipogenesis.

MicroRNA-122: A New Player in the Negative Regulation of LH Receptor Expression by the LH Receptor mRNA Binding Protein (LRBP)

The LH receptor (LHR) plays an essential role in mediating LH actions in the steroidogenic cells of the ovary (theca, granulosa, and luteal cells) and testis (Leydig cells). It belongs to the G protein-coupled superfamily of receptors with seven transmembrane domains (1) and it is one of the most studied peptide hormone receptors. After extensive characterization of its hormone binding properties in the early to mid 1970s (2–10), in the ensuing 40 years or so, tremendous progress has been made in understanding various functions of the LHR, including the regulation of cellular signaling (11–14), expression of steroidogenic acute regulatory protein (StAR), the protein that transports cholesterol to the inner mitochondrial membrane for side-chain cleavage (P450scc, CYP11A) (14–17), endocytic and selective delivery of exogenous cholesterol to the steroidogenic cells mediated by low-density lipoprotein receptor, and scavenger receptor class B, type, respectively (18–21), and the events connected with the mobilization of cholesterol from the intracellular stores (22, 23) and the cellular cholesterol trafficking (17, 24). However, relatively less is known about the molecular and cellular regulation of LHR expression (11, 12). Until recently, based on the available data, it was generally believed that LHR expression is primarily regulated at the level of gene transcription (1, 11). In recent years, however, the seminal studies carried out by Dr Jerry Menon's laboratory at the University of Michigan have led to the realization that LHR is also subject to posttranscription (25, 26). In fact, the overwhelming evidence now suggests that these mechanisms are major contributors to the LH-induced down-regulation of LHR. Initial studies reported by Menon and colleagues demonstrated that the expression of mevalonate kinase (MVK), a key enzyme involved in cholesterol biosynthesis, that catalyzes the conversion of mevalonic acid to 5-phosphomevalonic acid, is up-regulated under the conditions that promote the repression of the LHR expression (27–29). The follow-up studies demonstrated that MVK can directly bind to the coding region of LHR mRNA and inhibits its translational capacity (27–31); because of this specialized function, MVK was renamed by the Menon laboratory as “LHR mRNA binding protein (LRBP).” Both rat and human granulosa cells also express high levels of LRBP and the functional studies demonstrated that it interferes with the LHR expression in a manner similar to that seen in the luteinized ovary (32–34). The presence to MVK (LRBP) activity has been demonstrated in the interstitial cells of rat testis (35) but so far no functional studies have been carried out in LH responsive Leydig cells. Recent evidence suggests that in addition to gonadotropin, MVK/LRBP, a member of the oxysterol responsive gene family, is also regulated transcriptionally by the cholesterol responsive transcription factors, sterol regulatory element-binding protein-1a (SREBP-1a) and SREBP-2 (36). These various findings can be interpreted to suggest that ovarian steroid hormone biosynthesis, which is under LH regulation via LHR and is dependent on the cholesterol metabolism for the availability of cholesterol substrate, are integrated through a common regulatory mechanism acting at the level of LHR mRNA expression.

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.

Eicosapentaenoic acid suppresses palmitate-induced cytokine production by modulating long-chain acyl-CoA synthetase 1 expression in human THP-1 macrophages

BackgroundChronic inflammation caused by macrophages may be associated with progression of arteriosclerosis or obesity, both risk factors for cardiovascular events. In the Japan EPA Lipid Intervention Study (JELIS), eicosapentaenoic acid (EPA), an n-3 polyunsaturated fatty acid, was found to reduce the incidence of cardiovascular events. MethodsThe effect of EPA on the expression of inflammatory factors induced by palmitate, a saturated fatty acid, was investigated using human THP-1 macrophages. ResultsPalmitate induced expression of inflammatory cytokines and activated NF-κB, similar to lipopolysaccharide (LPS). EPA strongly suppressed palmitate-induced up-regulation of inflammatory factors while slightly suppressing LPS-induced factors. Both palmitate and LPS up-regulated expression of long-chain acyl-CoA synthetase (ACSL) 1, while EPA preferentially suppressed palmitate-induced ACSL1 expression. Although an acyl-CoA synthetase inhibitor and ACSL1 siRNA both suppressed palmitate-induced tumor necrosis factor (TNF)-α expression, the former had no effect on LPS-induced TNF-α expression. Palmitate may therefore stimulate cytokine production through a different mechanism than LPS mediated through Toll-like receptor 4, at least partly, and ACSL1 may play an important role in this mechanism. Finally, palmitate induced expression of sterol regulatory element-binding protein-1a and ACSL1, while EPA suppressed the expression of these genes. ConclusionThe suppressive effects of EPA on palmitate-induced cytokine production may be mediated by the suppression of ACSL1 expression, at least partly. This anti-inflammatory effect of EPA may contribute to suppression of chronic inflammation caused by macrophages in atherosclerotic plaques.

Expression of R132H Mutational IDH1 in Human U87 Glioblastoma Cells Affects the SREBP1a Pathway and Induces Cellular Proliferation

Sterol regulatory element-binding protein-1a (SREBP1a) is a member of the SREBP family of transcription factors, which mainly controls homeostasis of lipids. SREBP1a can also activate the transcription of isocitrate dehydrogenase 1 (IDH1) by binding to its promoter region. IDH1 mutations, especially R132H mutation of IDH1, are a common feature of a major subset of human gliomas. There are few data available on the relationship between mutational IDH1 expression and SREBP1a pathway. In this study, we investigated cellular effects and SREBP1a pathway alterations caused by R132H mutational IDH1 expression in U87 cells. Two glioma cell lines, stably expressing mutational (U87/R132H) or wild type (U87/wt) IDH1, were established. A cell line, stably transfected with pcDNA3.1(+) (U87/vector), was generated as a control. Click-iT EdU assay, sulforhodamine B assay, and wound healing assay respectively showed that the expression of R132H induced cellular proliferation, cell growth, and cell migration. Western blot revealed that SREBP1 was increased in U87/R132H compared with that in U87/wt. Elevated SREBP1a and several its target genes, but not SREBP1c, were detected by real-time polymerase chain reaction in U87/R132H. All these findings indicated that R132H mutational IDH1 is involved in the regulation of proliferation, growth, and migration of glioma cells. These effects may partially be mediated by SREBP1a pathway.

Regulation of the bovine SCD5 promoter by EGR2 and SREBP1

In rodents, the transcription factors early growth response 2 (EGR2) and sterol regulatory element binding protein 1a (SREBP1a) regulate transcription of the stearoyl-CoA desaturase 2 (SCD2) gene during peripheral nerve myelination, which may be important for synthesis of the lipid component of myelin. Most non-rodent genomes do not contain the SCD2 gene, but rather express SCD5 in brain and nervous tissues. In this paper, we asked whether bovine SCD5 is regulated in a similar manner to rodent SCD2. Expression of EGR2 did not result in an increase in endogenous SCD5 mRNA expression in JEG3 cells, but did result in activation of truncated bovine SCD5 promoter luciferase reporter constructs. Similar results were obtained with expression of the active form of SREBP1a; however, unlike rodent SCD2, there was no synergistic activation of the bovine SCD5 promoter reporters when EGR2 and SREBP1a were co-expressed. Mutation of the putative EGR2 binding site in the SCD5 promoter abolished activation by SREBP1a, suggesting that EGR2 and SREBP1a bind to the same site in the SCD5 promoter. Finally, we have identified a region of the bovine SCD5 promoter between 505 and 305 base pairs upstream of the transcriptional start site that appears to be important for maintaining basal levels of transcription of this gene. While it appears that there are some differences between the regulation of rodent SCD2 and bovine SCD5, the promoters of both genes can be activated by EGR2 and SREBP1a. This is the first report of potential regulators of SCD5 transcription.

Preventing Phosphorylation of Sterol Regulatory Element-Binding Protein 1a by MAP-Kinases Protects Mice from Fatty Liver and Visceral Obesity

The transcription factor sterol regulatory element binding protein (SREBP)-1a plays a pivotal role in lipid metabolism. Using the SREBP-1a expressing human hepatoma cell line HepG2 we have shown previously that human SREBP-1a is phosphorylated at serine 117 by ERK-mitogen-activated protein kinases (MAPK). Using a combination of cell biology and protein chemistry approach we show that SREBP-1a is also target of other MAPK-families, i.e. c-JUN N-terminal protein kinases (JNK) or p38 stress activated MAP kinases. Serine 117 is also the major phosphorylation site in SREBP-1a for JNK. In contrast to that the major phosphorylation sites of p38 MAPK family are serine 63 and threonine 426. Functional analyses reveal that phosphorylation of SREBP-1a does not alter protein/DNA interaction. The identified phosphorylation sites are specific for both kinase families also in cellular context. To provide direct evidence that phosphorylation of SREBP-1a is a regulatory principle of biological and clinical relevance, we generated transgenic mice expressing mature transcriptionally active N-terminal domain of human SREBP–1a variant lacking all identified phosphorylaton sites designed as alb-SREBP-1aΔP and wild type SREBP-1a designed as alb-SREBP-1a liver specific under control of the albumin promoter and a liver specific enhancer. In contrast to alb-SREBP–1a mice the phosphorylation–deficient mice develop no enlarged fatty livers under normocaloric conditions. Phenotypical examination reveales a massive accumulation of adipose tissue in alb-SREBP-1a but not in the phosphorylation deficient alb-SREBP-1aΔP mice. Moreover, preventing phosphorylation of SREBP-1a protects mice also from dyslipidemia. In conclusion, phosphorylation of SREBP-1a by ERK, JNK and p38 MAPK-families resembles a biological principle and plays a significant role, in vivo.

Linking Lipid Metabolism to the Innate Immune Response in Macrophages through Sterol Regulatory Element Binding Protein-1a

We show that mice with a targeted deficiency in the gene encoding the lipogenic transcription factor SREBP-1a are resistant to endotoxic shock and systemic inflammatory response syndrome induced by cecal ligation and puncture (CLP). When macrophages from the mutant mice were challenged with bacterial lipopolysaccharide, they failed to activate lipogenesis as well as two hallmark inflammasome functions, activation of caspase-1 and secretion of IL-1β. We show that SREBP-1a activates not only genes required for lipogenesis in macrophages but also the gene encoding Nlrp1a, which is a core inflammasome component. Thus, SREBP-1a links lipid metabolism to the innate immune response, which supports our hypothesis that SREBPs evolved to regulate cellular reactions to external challenges that range from nutrient limitation and hypoxia to toxins and pathogens.

Ginseng Leaf Extract Prevents High Fat Diet-Induced Hyperglycemia and Hyperlipidemia through AMPK Activation

This study evaluated the protective effects of ginseng leaf extract (GLE) against high fat-diet-induced hyperglycemia and hyperlipidemia, and explored the potential mechanism underlying these effects in C57BL/6J mice. The mice were randomly divided into four groups: normal control, high fat diet control (HFD), GLE-treated at 250 ㎎/㎏, and GLE-treated at 500 ㎎/㎏. To induce hyperglycemic and hyperlipidemic states, mice were fed a high fat diet for 6 weeks and then administered GLE once daily for 8 weeks. At the end of the treatment, we examined the effects of GLE on plasma glucose, lipid levels, and the expression of genes related to lipogenesis, lipolysis, and gluconeogenesis. Both GLE groups lowered levels of plasma glucose, insulin, triglycerides, total cholesterol, and non-esterified fatty acids when compared to those in HFD group. Histological analysis revealed significantly fewer lipid droplets in the livers of GLE-treated mice compared with HFD mice. To elucidate the mechanism, Western blots and RT-PCR were performed using liver tissue. Compared with HFD mice, GLE-treated mice showed higher levels of phosphorylation of AMP-activated protein kinase (AMPK) and its substrate, acetyl-CoA carboxylase, but no differences in the expression of lipogenic genes such as sterol regulatory element-binding protein 1a, fatty acid synthase, sterol-CoA desaturase 1 and glycerol-3-phosphate acyltransferase. However, the expression levels of lipolysis and fatty acid uptake genes such as peroxisome proliferator-activated receptor-α and CD36 were increased. In addition, phospho-α and CD36 were increased. In addition, phosphoenolpyruvate carboxykinase gene expression was decreased. These results suggest that GLE ameliorates hyperglycemia and hyperlipidemia by inhibiting gluconeogenesis and stimulating lipolysis, respectively, via AMPK activation.

Short communication: Identification of the bovine sterol regulatory element binding protein-1c promoter and its activation by liver X receptor

Sterol regulatory element binding proteins (SREBP) are a family of transcription factors that regulate cholesterogenesis and lipogenesis. Sterol regulatory element binding proteins-1a and -1c are transcribed from the same gene by the use of alternate promoters, and only differ at their first exon. Sterol regulatory element binding protein-1c is hypothesized to be an important regulator of genes involved in milk fat synthesis in the lactating dairy cow. However, the bovine SREBP-1c promoter has not been previously characterized, and studies to date that have investigated the role of SREBP-1 in the bovine mammary gland have not distinguished between isoforms 1a and 1c. The purpose of this study was to characterize the bovine SREBP-1c promoter and to investigate the DNA elements involved in the regulation of SREBP-1c expression by the liver X receptor agonist T0901317 in 2 different bovine mammary epithelial cell lines. Luciferase reporter constructs containing the wild-type SREBP-1c promoter or constructs with mutated liver X receptor response elements or sterol response element were transfected into MacT cells and bovine mammary epithelial (BME-UV) cells. We have demonstrated that the liver X receptor response elements sites in the SREBP-1c promoter are necessary for mediating the T0901317 response, and that stimulation through the sterol response element site plays only a minor role in this pathway. This report describes the bovine SREBP-1c promoter and its regulation by liver X receptor in bovine mammary epithelial cells.