BackgroundA high consumption of fructose leads to hepatic steatosis. About 20-30% of triglycerides are synthesized via de novo lipogenesis. Some studies showed that endoplasmic reticulum stress (ERS) is involved in this process, while others showed that a lipotoxic environment directly influences ER homeostasis. Here, our aim was to investigate the causal relationship between ERS and fatty acid synthesis and the effect of X-box binding protein-1 (XBP-1), one marker of ERS, on hepatic lipid accumulation stimulated by high fructose.

MethodsHepG2 cells were incubated with different concentrations of fructose. Upstream regulators of de novo lipogenesis (i.e., carbohydrate response element-binding protein [ChREBP] and sterol regulatory element-binding protein 1c [SREBP-1c]) were measured by polymerase chain reaction and key lipogenic enzymes (acetyl-CoA carboxylase [ACC], fatty acid synthase [FAS], and stearoyl-CoA desaturase-1 [SCD-1]) by Western blotting. The same lipogenesis-associated factors were then evaluated after exposure of HepG2 cells to high fructose followed by the ERS inhibitor tauroursodeoxycholic acid (TUDCA) or the ERS inducer thapsigargin. Finally, the same lipogenesis-associated factors were evaluated in HepG2 cells after XBP-1 upregulation or downregulation through cell transfection.

ResultsExposure to high fructose increased triglyceride levels in a dose- and time-dependent manner and significantly increased mRNA levels of SREBP-1c and ChREBP and protein levels of FAS, ACC, and SCD-1, concomitant with XBP-1 conversion to an active spliced form. Lipogenesis-associated factors induced by high fructose were inhibited by TUDCA and induced by thapsigargin. Triglyceride level in XBP-1-deficient group decreased significantly compared with high-fructose group (4.41 ± 0.54 μmol/g vs. 6.52 ± 0.38 μmol/g, P < 0.001), as mRNA expressions of SREBP-1c (2.92 ± 0.46 vs. 5.08 ± 0.41, P < 0.01) and protein levels of FAS (0.53 ± 0.06 vs. 0.85 ± 0.05, P = 0.01), SCD-1 (0.65 ± 0.06 vs. 0.90 ± 0.04, P = 0.04), and ACC (0.38 ± 0.03 vs. 0.95 ± 0.06, P < 0.01) decreased. Conversely, levels of triglyceride (4.22 ± 0.54 μmol/g vs. 2.41 ± 0.35 μmol/g, P < 0.001), mRNA expression of SREBP-1c (2.70 ± 0.33 vs. 1.00 ± 0.00, P < 0.01), and protein expression of SCD-1 (0.93 ± 0.06 vs. 0.26 ± 0.05, P < 0.01), ACC (0.98 ± 0.09 vs. 0.43 ± 0.03, P < 0.01), and FAS (0.90 ± 0.33 vs. 0.71 ± 0.02, P = 0.04) in XBP-1s-upregulated group increased compared with the untransfected group.

ConclusionsERS is associated with de novo lipogenesis, and XBP-1 partially mediates high-fructose-induced lipid accumulation in HepG2 cells through augmentation of de novo lipogenesis.

Endoplasmic Reticulum Stress ; physiology ; Fatty Liver ; Fructose ; metabolism ; Hep G2 Cells ; Humans ; Lipogenesis ; physiology ; Liver ; Sterol Regulatory Element Binding Protein 1 ; X-Box Binding Protein 1 ; physiology

Endoplasmic reticulum (ER) stress is involved in the process of kidney fibrosis. Spliced X-box binding protein 1 (XBP1S) is the key mediator of ER stress while its role in fibrosis is still poorly understood. This study was aimed to investigate the role of XBP1S in renal fibrosis and evaluate whether valsartan could alleviate fibrosis through XBP1S. Renal interstitial fibrosis was induced by unilateral ureteral obstruction (UUO) in C57BL/6 mice, and UUO mice were daily administered with valsartan (20 mg/kg) through oral gavage. After 7 days of UUO, at euthanasia, left kidney was collected to examine the histological alteration by using haematoxylin-eosin staining, Masson's trichrome staining, Sirius red staining and immunohistochemistry. Western blot was used to assess XBP1S, targets of XBP1S, fibronectin, α-SMA, BAX and BCL2 protein levels. Real-time polymerase chain reaction was performed to assess NADPH oxidase subunits p47-phox and p67-phox mRNA levels. The results showed that XBP1S expression was decreased by about 70% in the UUO mice compared with that in sham mice (P < 0.01), which was reversed by valsartan administration (P < 0.05). Meanwhile, UUO-induced renal interstitial fibrosis was attenuated by valsartan treatment. In addition, the protein levels of fibronectin and α-SMA were upregulated by UUO induction (P < 0.01), and valsartan administration inhibited the protein levels of fibronectin and α-SMA in UUO mice (P < 0.05). Western blot analysis showed that the ratio of BAX to BCL2 protein level was increased in UUO model compared with that in sham mice, and the increment also was diminished by valsartan treatment (P < 0.05). Finally, UUO-induced mRNA levels of p47-phox and p67-phox were significantly attenuated by valsartan administration (P < 0.05). These results showed that valsartan at least partly restores renal interstitial fibrosis by enhancing XBP1S activation through inhibiting oxidative stress and apoptosis in the UUO mice. These results suggest that XBP1S could be a potential therapeutic target for kidney fibrosis.
Animals ; Apoptosis ; Fibronectins ; Fibrosis ; Kidney ; Kidney Diseases ; Mice ; Mice, Inbred C57BL ; NADPH Oxidases ; Oxidative Stress ; Phosphoproteins ; Real-Time Polymerase Chain Reaction ; Ureteral Obstruction ; X-Box Binding Protein 1

The aim of this study was to explore the effect of 2 different spliceosomes of X-box binding protein 1 (XBP-1), the spliced form XBP-1s and unspliced form XBP-1u, on myeloma cell differentiation and its mechanism. The overexpression plasmids pcDNA3.1-C-XBP1u and pcDNA3.1-C-XBP1s were constructed and transfected into myeloma cell line U266, RPMI8226. The morphology of U266 and RPMI 8226 cells was observed by means of light microscope, the expression rate of CD49e on cell surface was detected by flow cytometry, the ELISA was used to determine the changes of light chain protein level in supernatants of cell culture, the Western blot was used to assay the expression changes of XBP1u and XBP1s. The results showed that the overexpression of XBP1u could promote the myeloma cell differentiation morphologically displaying the maturation of plasmocytes, the CD49e positive expression rates on surface of U266 and RPMI8226 cells were obviously up-regulated from 9.02±0.3% and 5.17±0.92% in control group to 27.7±1.14% and 13.97±1.79% respectively (p<0.01), the levels of light chain protein in supernatants of U266 and RPMI 8226 cell cultures increased from 474.75±19.52 ng/ml and 289.44±6.19 ng/ml in control group to 692.34±21.17 ng/ml and 401.55±13.7 ng/ml respectively (p<0.01, p<0.05), while the above-mentioned parameters in the overexpressed XBP-1s showed no significant changes, which indicated no promotive effect of overexpressed XBP1s on myeloma cell differentiation. It is concluded that the up-regulation of XBP-1u expression plays an important role in the differentiation of myeloma cells.
Cell Differentiation ; genetics ; Cell Line, Tumor ; DNA-Binding Proteins ; genetics ; Gene Expression Regulation, Neoplastic ; Humans ; Multiple Myeloma ; genetics ; pathology ; Regulatory Factor X Transcription Factors ; Spliceosomes ; genetics ; Transcription Factors ; genetics ; Transfection ; X-Box Binding Protein 1

This study was purposed to investigate the effect of xbp-1 gene silencing on bortezomib-induced apoptosis in multiple myeloma cell line NCI-H929 (H929). After xbp-1 gene expression was interfered by small hairpin RNA, the cell apoptosis was assayed by flow cytometry with Annexin V-FITC/PI staining, and the expression level of XBP-1 protein was detected by Western blot. The results showed that XBP-1 protein level of H929 cells was inhibited effectively by the PLL3.7 lentiviral vector mediated expression xbp-1 shRNA. The apoptosis rate was significantly higher in xbp-1 shRNA-expressing cells than in untreated control group [(10.13±0.61)% vs (2.5±0.2)%, p<0.05]. After treatment with bortezomib, the apoptosis rate of XBP-1 protein functionally deficient H929 cells was significantly higher than those in vector control group [(45.07±1)% vs (19.53±0.8)%, p<0.05]. It is concluded that xbp-1 gene silencing can significantly enhance the pro-apoptotic activity of bortezomib in multiple myeloma cells.
Apoptosis ; drug effects ; Boronic Acids ; pharmacology ; Bortezomib ; Cell Line, Tumor ; DNA-Binding Proteins ; genetics ; Gene Silencing ; Humans ; Multiple Myeloma ; genetics ; Pyrazines ; pharmacology ; RNA, Small Interfering ; genetics ; Regulatory Factor X Transcription Factors ; Transcription Factors ; genetics ; X-Box Binding Protein 1

OBJECTIVETo study the molecular mechanism of differentiation induction by 2-methoxyestradiol (2ME2) of myeloma cell lines.

METHODSDifferentiation induction effect on myeloma cell lines LP-1, CZ-1 and NCI-H929 which were incubated with 2ME2 and XBP-1, Blimp-1, pax-5 phosphorothioate antisense oligodeoxynucleotide (ASODN) was evaluated by cell morphology, CD49e expression, quantitation of light chain secretion, and the level of pax-5 and XBP-1 mRNA expression.

RESULTS2ME2 caused morphological, immunophenotypic and the supernatant light chain secretion changes typical of differentiation in all the three myeloma cell lines. 2ME2 up-regulated the XBP-1 mRNA expression. XBP-1 and Blimp-1 ASODNs partially inhibited the differentiation of LP-1, CZ-1, NCI-H929 cells induced by 2ME2; whereas pax-5 ASODN did the contrary. After incubated with pax-5 ASODN for 72 hours, LP-1, CZ-1, NCI-H929 cells exhibited characteristic morphologic feature of differentiation. The expression of CD49e was increased statistically (P < 0.05). Light chain secretion in the supernatant was also increased statistically (P < 0.05). After incubation with Blimp-1 ASODN, the level of XBP-1 mRNA was declined, while the level of pax-5 mRNA increased.

CONCLUSION2ME2 could induce cell differentiation and up-regulate XBP-1 mRNA expression in myeloma cell lines. Blimp-1 could help 2ME2 with inducing differentiation of myeloma cells through downregulating pax-5 mRNA and upregulating XBP-1 mRNA.

Cell Differentiation ; drug effects ; Cell Line, Tumor ; DNA-Binding Proteins ; genetics ; metabolism ; Estradiol ; pharmacology ; Humans ; Multiple Myeloma ; genetics ; metabolism ; pathology ; RNA, Messenger ; genetics ; Regulatory Factor X Transcription Factors ; Transcription Factors ; genetics ; metabolism ; X-Box Binding Protein 1

The aim of this study was to investigate the role of S100 calcium binding protein A16 (S100A16) in lipid metabolism in hepatocytes and its possible biological mechanism. HepG2 cells (human hepatoma cell line) were cultured with fatty acid to establish fatty acid culture model. The control model was cultured without fatty acid. Each model was divided into three groups and transfected with S100a16 over-expression, shRNA and vector plasmids, respectively. The concentration of triglyceride (TG) in the cells was measured by kit, and the lipid droplets was observed by oil red O staining. Immunoprecipitation and mass spectrometry were used to find the interesting proteins interacting with S100A16, and the interaction was verified by immunoprecipitation. The further mechanism was studied by Western blot and qRT-PCR. The results showed that the intracellular lipid droplet and TG concentrations in the fatty acid culture model were significantly higher than those in the control model. The accumulation of intracellular fat in the S100a16 over-expression group was significantly higher than that in the vector plasmid transfection group. There was an interaction between heat shock protein A5 (HSPA5) and S100A16. Over-expression of S100A16 up-regulated protein expression levels of HSPA5, inositol-requiring enzyme 1α (IRE1α) and pIREα1, which belong to endoplasmic reticulum stress HSPA5/IRE1α-XBP1 pathway. Meanwhile, over-expression of S100A16 up-regulated the mRNA expression levels of adipose synthesis-related gene Srebp1c, Acc and Fas. In the S100a16 shRNA plasmid transfection group, the above-mentioned protein and mRNA levels were lower than those of vector plasmid transfection group. These results suggest that S100A16 may promote lipid synthesis in HepG2 cells through endoplasmic reticulum stress HSPA5/IRE1α-XBP1 pathway.
Endoplasmic Reticulum Stress ; Endoribonucleases ; physiology ; Heat-Shock Proteins ; physiology ; Hep G2 Cells ; Humans ; Lipid Metabolism ; Protein-Serine-Threonine Kinases ; physiology ; S100 Proteins ; physiology ; Triglycerides ; biosynthesis ; X-Box Binding Protein 1 ; physiology

The purpose of the present study is to explore the effect of oxidized low density lipoprotein (ox-LDL) on the induction of endoplasmic reticulum stress (ERS) and the underlying mechanisms in ox-LDL-induced macrophage foam-forming process. RAW264.7 macrophages were cultured in DMEM medium containing 10% fetal bovine serum, and then treated with ox-LDL (25, 50 and 100 mg/L), anti-CD36 monoclonal antibody+ox-LDL and tunicamycin (TM), respectively. After incubation for 24 h, the cells were collected. The cellular lipid accumulation was showed by oil red O staining and the content of cellular total cholesterol was quantified by enzymatic colorimetry. The expression of glucose-regulated protein 94 (GRP94), a molecular marker of ERS, was determined by immunocytochemistry assay. The levels of GRP94 protein, phosphorylated inositol-requiring enzyme 1 (p-IRE1) and X box binding protein 1 (XBP1) in RAW264.7 cells were detected by Western blotting. The results indicated that after incubation with ox-LDL (25, 50 and 100 mg/L) for 24 h, a large amount of lipid droplets were found in the cytoplasm, and the contents of cellular total cholesterol were increased by 2.1, 2.8 and 3.1 folds compared with the control, respectively. Anti-CD36 antibody decreased markedly the cellular lipid accumulation induced by ox-LDL at 100 mg/L. Both ox-LDL and TM, a specific ERS inducer, could up-regulate the protein expression of GRP94 in a dose-dependent manner. Furthermore, p-IRE1 and XBP1, two key components of the unfolded protein response, were also significantly induced by the treatment with ox-LDL. The up-regulations of the three proteins induced by ox-LDL were inhibited significantly when the macrophages were pre-incubated with anti-CD36 antibody. These results suggest that ox-LDL may induce ERS in a dose-dependent way and subsequently activate the unfolded protein response signaling pathway in RAW264.7 macrophages, which is potentially mediated by scavenger receptor CD36.
Animals ; CD36 Antigens ; physiology ; Cell Line ; Cells, Cultured ; DNA-Binding Proteins ; metabolism ; Endoplasmic Reticulum ; drug effects ; Foam Cells ; cytology ; Lipoproteins, LDL ; pharmacology ; Macrophages ; cytology ; Membrane Glycoproteins ; metabolism ; Membrane Proteins ; metabolism ; Mice ; Protein-Serine-Threonine Kinases ; metabolism ; Regulatory Factor X Transcription Factors ; Stress, Physiological ; drug effects ; Transcription Factors ; metabolism ; X-Box Binding Protein 1

Estrogen receptor alpha (ERalpha) has been a primary target of treatment as well as a prognostic indicator for breast cancer. The level of human X-box binding protein 1 (XBP-1) mRNA was related with that of ERalpha in breast tumors and was over-expressed in some breast tumors. These previous studies suggested that XBP-1 may interact with ERalpha. XBP-1 has two isoforms, XBP-1S and XBP-1U, as the result of unique splicing. GST pull-down assay showed that both XBP-1S and XBP-1U bound to ERalpha in vitro. The binding of XBP-1S to ERalpha was stronger than that of XBP-1U to ERalpha. Co-immunoprecipitation revealed that the binding was in a ligand-independent manner. XBP-1S and XBP-1U interacted with the region of ERalpha that contains a DNA-binding domain. The ERalpha-interacting regions on XBP-1S and XBP-1U have been mapped to two regions, the N-terminal basic region leucine zipper domain (bzip) and the C-terminal activation domain. These findings suggest that XBP-1S and XBP-1U may participate in ERalpha signaling pathway through the mediation of ERalpha.
Breast Neoplasms ; genetics ; metabolism ; Cell Line, Tumor ; DNA-Binding Proteins ; genetics ; metabolism ; Estrogen Receptor alpha ; genetics ; metabolism ; Female ; Humans ; Protein Interaction Domains and Motifs ; physiology ; RNA, Messenger ; biosynthesis ; genetics ; Regulatory Factor X Transcription Factors ; Signal Transduction ; Transcription Factors ; genetics ; metabolism ; X-Box Binding Protein 1

BACKGROUNDPrediabetes is an early stage of β-cell dysfunction presenting as insulin resistance. Evidences suggest that endoplasmic reticulum (ER) stress is involved in the pathogenesis of type 2 diabetes mellitus and prediabetes. In a Chinese population with prediabetes, we investigated single nucleotide polymorphisms (SNPs) in the genes of PERK, JNK, XBP1, BIP and CHOP which encode molecular proteins involved in ER stress pathways.

METHODSNine SNPs at the PERK, JNK, XBP1, BIP and CHOP loci were genotyped by mass spectrometry in 1 448 unrelated individuals. By using a 75 g oral glucose tolerance test (OGTT), 828 subjects were diagnosed as prediabetes and 620 subjects aged 55 years and over as normal controls based on WHO diagnostic criteria (1999) for diabetes mellitus.

RESULTSThe allele C of SNP rs867529 at PERK locus was a risk factor for prediabetes, with the carriers of C allele genotype at a higher risk of prediabetes compared to non-carriers (OR = 1.279, 95% CI: 1.013-1.614, P = 0.039, after adjustment for age, sex and body mass index (BMI). The SNPs rs6750998 at PERK locus was associated with homeostasis model assessments of insulin resistance (HOMA-IR) (P = 0.019), and rs17037621 with BMI (P = 0.044). The allele G of SNP rs10986663 in BIP gene was associated with a decreased risk of prediabetes (OR = 0.699, 95% CI: 0.539-0.907, P = 0.007). The SNP rs2076431 in JNK gene was associated with fasting plasma glucose levels (P = 0.006) and waist-hip ratios (P = 0.019). The SNP rs2239815 in XBP1 gene was associated with 2-hour plasma glucose levels after 75 g oral glucose load (P = 0.048) in the observed population.

CONCLUSIONCommon variants at PERK and BIP loci contributed to the risk of prediabetes, and the genetic variations in JNK and XBP1 genes are associated with diabetes-related clinical parameters in this Chinese population.

Aged ; DNA-Binding Proteins ; genetics ; Diabetes Mellitus, Type 2 ; genetics ; Female ; Genotype ; Humans ; MAP Kinase Kinase 4 ; genetics ; Male ; Middle Aged ; Polymorphism, Single Nucleotide ; genetics ; Prediabetic State ; genetics ; Regulatory Factor X Transcription Factors ; Transcription Factor CHOP ; genetics ; Transcription Factors ; genetics ; X-Box Binding Protein 1 ; eIF-2 Kinase ; genetics

OBJECTIVETo analyze the transcription activation and possible regulation mechanism of human X-box binding protein 1(XBP1)gene 5'upstream DNA sequence in different cell lines.

METHODSSix kinds of XBP1 promoter deletion mutants were cloned into pGEM-Teasy vector, which included XBP1 gene 5' upstream -1039 to 66 bp,-859 to 66 bp,-623 to 66 bp,-351 to 66 bp,-227 to 66 bp,-227 to -45 bp respectively. Every deletion mutant sequence was cut from Teasy-XBP1p by KpnI and Xho I, and subcloned into pCAT3-Basic to produce a set of constructs termed as p1-XBP1p, p2-XBP1p, p3-XBP1p, p4-XBP1p, p5-XBP1p, p6-XBP1p, respectively. The transcription activity of each construct was detected after transiently transfecting K562, HepG2,NIH-3T3 and L0(2)cell with FuGENE 6 transfection reagent. Cells transfected by pCAT3-Basic or pCAT3-Promoter were used as negative and positive controls. The activity of chloramphenicol acetyltransferase(CAT), which reflects the transcription activation of the XBP1 gene promoter, was detected by ELISA after 48 hours of transfection.

RESULTSThe reporter vectors of six kinds of XBP1 promoter deletion mutants were successfully constructed, as confirmed by restriction enzyme digestion and sequencing. The activities of p4-XBP1p and p5-XBP1p were higher than the other deletion mutants in K562 and HepG2. And the activity of p5-XBP1p was the highest in HepG2. There was no activity detected from any transfected NIH-3T3.

CONCLUSIONThe XBP1 gene promoter can transactivate its downstream gene to transcription. The core sequence of XBP1 promoter was implied between -227 bp and 66 bp. This sequence was connected with the transcriptional activity of XBP1 promoter closely. Its transcription activity varies with different cell lines. XBP1 promoter might drive gene expression with cell-type specificity.

3T3 Cells ; 5' Flanking Region ; genetics ; Animals ; Base Sequence ; Cell Line ; Chloramphenicol O-Acetyltransferase ; metabolism ; DNA ; analysis ; DNA-Binding Proteins ; genetics ; Gene Deletion ; Gene Expression Regulation ; physiology ; Genes, Reporter ; Humans ; K562 Cells ; Mice ; Molecular Sequence Data ; Nuclear Proteins ; genetics ; Promoter Regions, Genetic ; genetics ; Regulatory Factor X Transcription Factors ; Transcription Factors ; Transcription, Genetic ; physiology ; Transcriptional Activation ; Transfection ; Tumor Cells, Cultured ; X-Box Binding Protein 1