Estrogen stimulates SREBP2 expression in hepatic cell lines via an estrogen response element in the SREBP2 promoter

Objective Hypoestrogenism in women is strongly associated with menopause and it can lead to lipid disorder, which predisposes people to premature cardiovascular disease. However, the mechanism of lipid disorder remains unclear. Sterol regulatory element-binding protein 2 (SREBP2) is the key transcription factor regulating cholesterol metabolism. We hypothesize that estrogen regulates SREBP2 transcription through an estrogen response element (ERE) in the SREBP2 promoter region. Methods Human hepatoblastoma cells (HepG2) were treated with dose-dependent concentrations of estradiol (E2) for 24 h. Then, SREBP2 expression was determined via real-time PCR and immunofluorescence. The expressions of the SREBP2 downstream target genes HMGCR and LDLR were determined via real-time PCR. Lipid secretion in the culture media of HepG2 cells was measured using ELISA. Through bioinformatics analysis, we identified high-scoring ERE-like sequences in the SREBP2 gene promoter. Chromatin immunoprecipitation analysis was used to confirm the ERE. DNA fragments of the putative or mutated ERE-like sequence were synthesized and ligated into pGL3-basic plasmid to construct the SREBP2 promoter luciferase reporter systems. SREBP2-Luciferase (SREBP2-Luc), SREBP2-Mutation (SREBP2-Mut) and the blank control were transfected into hepatic cell lines. Luciferase activities were measured using the dual-luciferase reporter assay system. Chromatin immunoprecipitation analysis and the luciferase reporter assay were repeated in human hepatoma cells (HuH-7). Results We found that E2 dose-dependently increased the expression of SREBP2 in HepG2 cells and that the increased levels were blocked when treated with an estrogen receptor-alpha antagonist. Additionally, E2 increased both HMGCR and LDLR expression and lipid secretion in HepG2 cells. Notably, we identified a functional ERE in the SREBP2 gene promoter, to which E2 could specifically bind and induce transcription. Conclusions An ERE was identified in the SREBP2 gene promoter. It mediates the regulation of SREBP2 expression by estrogen in hepatocytes. This study provides a mechanism to link cardiovascular disease with estrogen.

Prenylflavonoid Icariin Induces Estrogen Response Element–Independent Estrogenic Responses in a Tissue-Selective Manner

Icariin, a flavonoid phytoestrogen derived from Herba epimedii, has been reported to exert estrogenic effects in bone and activate phosphorylation of estrogen receptor (ER) α in osteoblastic cells. However, it is unclear whether icariin selectively exerts estrogenic activities in bone without inducing undesirable effects in other estrogen-sensitive tissues. The present study aimed to investigate the tissue-selective estrogenic activities of icariin in estrogen-sensitive tissues in vivo and in vitro. Long-term treatment with icariin effectively prevented bone of ovariectomized (OVX) rats from estrogen deficiency–induced osteoporotic changes in bone structure, bone mineral density, and trabecular properties. Moreover, icariin regulated the transcriptional events of estrogen-responsive genes related to bone remodeling and prevented dopaminergic neurons against OVX-induced changes by rescuing expression of estrogen-regulated tyrosine hydroxylase and dopamine transporter in the striatum. Unlike estrogen, icariin did not induce estrogenic effects in the uterus and breast in mature OVX rats or immature CD-1 mice. In vitro studies demonstrated that icariin exerted estrogen-like activities and regulated the expression of estrogen-responsive genes but did not induce estrogen response element–dependent luciferase activities in ER-positive cells. Our results support the hypothesis that icariin, through its distinct mechanism of actions in activating ER, selectively exerts estrogenic activities in different tissues and cell types.

FBXO4 as a novel ubiquitin ligase that targets Cyclin D in the pathogenesis of breast cancer.

e14722 Background: Members of ubiquitin ligase family are reported to be associated with cancer. Genome wide RNAi screen using Dharmacon on target Plus RNAi library identified FBXO4 as one of the potential hit . We therefore aimed to study its detailed role on proliferation of MCF7 breast cancer cells with special focus on cell cycle genes. We want to expand it further and study its involvement in context to E2 signaling in breast cancer. We hypothesized that increase in CCND1 expression, could mimic some of the E2 transcriptional regulation and that CCND1 overexpression could lead to E2 independent ER alpha binding to estrogen response element (ERE). Methods: Cell lines: MCF7 grown in MEM alpha supplemented with 10% FBS . For stimulation studies, cells were maintained in phenol free MEM alpha supplemented with 10% FBS and 10 nM estradiol. RNAi knockdown studies: On-target plus siRNA were used to knockdown FBXO4 in MCF7 cells. Western Blot: Western Blot was performed following standard protocols. IHC/Tissue Array: Paraffin embedded malignant breast cancer (ER/PR/EGFR +ve) tissue samples that are scored by pathologist were used for the study. Gene Expression Studies: Microarray and NGS based approach. Results: FBXO4 has been identified as a potential hit in our RNAi screen showing a hyper-proliferative phenotype in MCF7 cellsupon its knockdown. Western blot and ubiquitination assay identified cyclin D (CCND1) as a dedicated FBXO4 target as revealed by confocal microscopy where FBXO4 knockdown showed an enrichment of CCND1 in cytoplasm. Ingenuity pathway analysis and ChIP assay revealed an exploitation of E2 signaling and enhanced recruitment of ER alpha to estrogen response element (ERE) in FBXO4 depleted cells. CCND1 further seems to co-occupy ESR binding motifs thereby representing ER functions. These effects were observed even in absence of any E2 stimulation. Further we observed a significant overlap of differentially expressed genes in E2 stimulated and FBXO4 knockdown cells that were nullified by CCND1 siRNA. Conclusions: Our study elucidated that CCND1 is spared from degradation following loss of FBXO4 which in turn leads to a hyper-proliferative state. Our findings co-related with TCGA and METABRIC databases indicating a poor patient survival and relapse in FBXO4 low and CCND1 high cohorts of ER +ve breast cancer thereby implying a diagnostic value. Along with traditional hormonal therapy, an effective targeted therapy in breast cancer should be aimed to restore FBXO4 levels.

Pancreatic Beta-Cell Proliferation Induced by Estradiol-17β is Foxo1 Dependent

Estradiol-17β (E2) and the Foxo1 transcription factor have each been implicated in the regulation of β-cell proliferation. Interaction between Foxo1and estrogen receptor alpha (ERα), effecting cell cycle, has been demonstrated in breast cancer cells, but has not been studied thus far in β-cells. Using human islets and the INS1-E β-cell line, this study investigated the contribution of Foxo1 to E2-mediated β-cell replication. Foxo1 expression was knocked down in INS1-E cells using siRNA and Foxo1 activity was inhibited in human islets with a specific Foxo1 inhibitor (AS1842856). Cells were treated with E2 and the ERα agonist PPT and evaluated for proliferation by 3[H]-thymidine incorporation and for transcriptional activity through the estrogen response element by the luciferase assay. As Foxo1 activity is regulated by post-translational modifications, the effect of E2 on phosphorylation was also assessed. In INS1-E cells, knock down of Foxo1 abrogated the proliferative response to E2 and PPT. In human islets, inhibition of Foxo1 abrogated E2-mediated proliferation and attenuated the response to PPT. Foxo1 knock down and inhibition reduced activity through the estrogen response element by 25% (p<0.05) and 50% (p<0.01) respectively, in INS1-E cells. E2 increased Foxo1 phosphorylation in a time dependent manner in INS1-E and human islets (p<0.01, p<0.05, respectively). These findings suggest that Foxo1 is involved in E2-mediated proliferation in INS1-E cells and human islets. This may have implications vis-à-vis variations in circulating endogenous E2 concentrations in diabetes.