FXR/Menin-mediated epigenetic regulation of E2F3 expression controls β-cell proliferation and is increased in islets from diabetic GK rats after RYGB

Farnesoid X receptor (FXR) improves the function of islets, especially in the setting of Roux-en-Y gastric bypass (RYGB). Here we investigated how FXR activation regulates β-cell proliferation and explored the potential link between FXR signaling and the menin pathway in controlling E2F3 expression, a key transcription factor for controlling adult β-cell proliferation. Stimulation with the FXR agonist GW4064 or chenodeoxycholic acid (CDCA) increased E2F3 expression and β-cell proliferation. Consistently, E2F3 knockdown abolished GW4064-induced proliferation. Treatment with GW4064 increased E2F3 expression in β-cells via enhancing Steroid receptor coactivator-1 (SRC1) recruitment, increasing the pro-transcriptional acetylation of histone H3 at the E2f3 promoter. GW4064 treatment also decreased the association between FXR and menin, leading to the induction of FXR-mediated SRC1 recruitment. Mimicking the impact of FXR agonists, RYGB also increased E2F3 expression and β-cell proliferation in GK rats and SD rats. These findings unravel the crucial role of the FXR/menin signaling in epigenetically controlling E2F3 expression and β-cell proliferation, a mechanism possibly underlying RYGB-induced β-cell proliferation.

E2F3/5/8 serve as potential prognostic biomarkers and new therapeutic direction for human bladder cancer

OBJECTS

Human bladder cancer (BC) is the most common urogenital system malignancy. E2F transcription factors (E2Fs) have been reported to be involved in the growth of various cancers. However, the expression patterns, prognostic value and immune infiltration in the tumor microenvironment of the 8 E2Fs in BC have yet fully to be explored.

METHODS AND STRATEGY

We investigated the differential expression of E2Fs in BC patients, the prognostic value and correlation with immune infiltration by analyzing a range of databases.

RESULTS

We found that the mRNA expression levels of E2F1/2/3/4/5/7/8 were significantly higher in BC patients than that of control tissues. And the increased mRNA expression levels of all E2Fs were associated with tumor stage of BC. The survival analysis revealed that the elevated mRNA expression levels of E2F3/5/8 were significantly correlated with the overall survival (OS) of BC patients. And the genetic changes of E2Fs in BC patients were associated with shorter overall survival (OS) and progression-free survival (PFS). In addition, we revealed that the E2F3/5/8 expressions were closely correlated with tumor-infiltrating lymphocytes (TILs).

CONCLUSIONS

E2F3/5/8 might serve as promising prognostic biomarkers and new therapeutic direction for BC patients.

E2F3 induces DNA damage repair, stem-like properties and therapy resistance in breast cancer

Therapy resistance is a major hurdle to the treatment of human malignant tumors. Both DNA damage repair and stem-like properties contribute to chemoresistance and radioresistance. E2F transcription factor 3 (E2F3) is overexpressed in breast cancer tissues, and promotes proliferation of breast cancer cells. Higher E2F3 level is associated with shorter survival of breast cancer patients. Functional studies further showed that E2F3 promotes S-phage entry, DNA replication, DNA damage repair and stem-like properties. Accordingly, E2F3 knockdown sensitizes breast cancer cells to DNA-damaging agents Adriamycin, Cisplatin, Olaparib and X-ray. Forkhead box M1 (FOXM1) is a downstream molecule of E2F3 signaling, mediating the effects of E2F3 on breast cancer cells. In an m6A methyltransferase METTL14-dependent manner, YTH RNA binding protein F2 (YTHDF2) increase E2F3 mRNA stability and expression, promotes DNA damage repair and induces therapy resistance. These data demonstrate that YTHDF2-E2F3 pathway is a novel target to overcome chemoresistance and radioresistance in breast cancer.

E2F3/CDCA2 reduces radiosensitivity in gastric adenocarcinoma by activating PI3K/AKT pathway

OBJECTIVES

Gastric adenocarcinoma is primarily responsible for tumor-associated deaths and its incidence is increasing global. CDCA2 is a nuclear protein binding to protein phosphatase one γ (PP1γ) and plays a pro-oncogenic role in tumors. This study aimed to elucidate the biological function of CDCA2 in gastric adenocarcinoma progression and radiosensitivity, as well as its potential mechanisms.

METHODS

Differentially expressed mRNAs in gastric adenocarcinoma were obtained by bioinformatics and upstream regulatory factors were predicted. The correlation between their expressions was analyzed. The expressions of E2F3 and CDCA2 in cells were assayed by qRT-PCR and their regulatory relationship was validated by molecular experiments. Cell viability was tested via CCK-8. Cell proliferation and survival after radiotherapy were determined by colony formation assay. The expressions of PI3K/AKT pathway-related proteins were assessed through western blot.

RESULTS

CDCA2 was significantly upregulated in gastric adenocarcinoma tissues and cells, promoted cell proliferation, and reduced radiosensitivity. The impact of CDCA2 on cell proliferation and radiosensitivity was reversed by the PI3K/AKT inhibitor. Furthermore, the upstream transcription factor of CDCA2 was found to be E2F3, which was highly expressed in gastric adenocarcinoma. The binding relationship between the two was validated by dual luciferase and ChIP experiments. The rescue experiment showed that E2F3 activated CDCA2 to drive cell proliferation and reduce radiosensitivity through PI3K/AKT pathway in gastric adenocarcinoma.

CONCLUSION

In summary, this study found that E2F3 activated CDCA2 to drive cell proliferation and reduce radiosensitivity in gastric adenocarcinoma through the PI3K/AKT pathway, suggesting that E2F3/CDCA2 axis is a new therapeutic target for gastric adenocarcinoma.

ADVANCES IN KNOWLEDGE

1. CDCA2 reduced the radiosensitivity of gastric adenocarcinoma cells;2. CDCA2 reduced the radiosensitivity of gastric adenocarcinoma cells through the PI3K/AKT pathway;3. E2F3 activated CDCA2 to reduce the radiosensitivity of gastric adenocarcinoma cells through the PI3K/AKT pathway.

Retinoblastoma-E2F Transcription Factor Interplay Is Essential for Testicular Development and Male Fertility

The retinoblastoma (RB) protein family members (pRB, p107 and p130) are key regulators of cell cycle progression, but also play crucial roles in apoptosis, and stem cell self-renewal and differentiation. RB proteins exert their effects through binding to E2F transcription factors, which are essential developmental and physiological regulators of tissue and organ homeostasis. According to the canonical view, phosphorylation of RB results in release of E2Fs and induction of genes needed for progress of the cell cycle. However, there are eight members in the E2F transcription factor family with both activator (E2F1-3a) and repressor (E2F3b-E2F8) roles, highlighting the functional diversity of RB-E2F pathway. In this review article we summarize the data showing that RB-E2F interaction is a key cell-autonomous mechanism responsible for establishment and maintenance of lifelong male fertility. We also review the expression pattern of RB proteins and E2F transcription factors in the testis and male germ cells. The available evidence supports that RB and E2F family members are widely and dynamically expressed in the testis, and they are known to have versatile roles during spermatogenesis. Knowledge of the function and significance of RB-E2F interplay for testicular development and spermatogenesis comes primarily from gene knock-out (KO) studies. Several studies conducted in Sertoli cell-specific pRB-KO mice have demonstrated that pRB-mediated inhibition of E2F3 is essential for Sertoli cell functional maturation and cell cycle exit, highlighting that RB-E2F interaction in Sertoli cells is paramount to male fertility. Similarly, ablation of either pRB or E2F1 in the germline results in progressive testicular atrophy due to germline stem cell (GSC) depletion, emphasizing the importance of proper RB-E2F interplay for germline maintenance and lifelong sperm production. In summary, while balanced RB-E2F interplay is essential for cell-autonomous maintenance of GSCs and, the pRB-E2F3 system in Sertoli cells is critical for providing GSC niche thus laying the basis for spermatogenesis.

Long noncoding RNA MAPKAPK5-AS1 promoted lipopolysaccharide-induced inflammatory damage in the myocardium by sponging microRNA-124-3p/E2F3

BACKGROUND

Myocardial dysfunction caused by sepsis (SIMD) leads to high mortality in critically ill patients. We investigated the function and mechanism of long non-coding RNA MAPKAPK5-AS1 (lncRNA MAPKAPK-AS1) on lipopolysaccharide (LPS)-induced inflammation response in vivo and in vitro.

METHOD

Male SD rats were utilized for in vivo experiments. Rat cardiomyocytes (H9C2) were employed for in vitro experiments. Western blotting was employed to measure protein expression, and RT-PCR was performed to measure mRNA expression of inflammation factors. TUNEL and flow cytometry were carried out to evulate cell apoptosis.

RESULT

The results showed that the expression of MAPKAPK5-AS1 was increased, while the expression of miR-124-3p was decreased in the inflammatory damage induced by LPS in vivo and in vitro. Knockdown of MAPKAPK5-AS1 reduced LPS-induced cell apoptosis and inflammation response, while overexpression of miR-124-3p weakened the effects of MAPKAPK5-AS1 knockdown on LPS-induced cell apoptosis and inflammation response. Moreover, miR-124-3p was identified as a downstream miRNA of MAPKAPK5-AS1, and E2F3 was a target of miR-214-3p. MAPKAPK5-AS1 knockdown increased the expression of miR-124-3p, while miR-124-3p overexpression reduced the expression of MAPKAPK5-AS1. In addition, miR-124-3p was found to downregulate E2F3 expression in H9C2 cells.

CONCLUSION

MAPKAPK5-AS1/miR-124-3p/E2F3 axis regulates LPS-related H9C2 cell apoptosis and inflammatory response.

Cloning and Functional Analysis of Rat Tweety-Homolog 1 Gene Promoter

Tweety-homolog 1 protein (Ttyh1) is abundantly expressed in neurons in the healthy brain, and its expression is induced under pathological conditions. In hippocampal neurons in vitro, Ttyh1 was implicated in the regulation of primary neuron morphology. However, the mechanisms that underlie transcriptional regulation of the Ttyh1 gene in neurons remain elusive. The present study sought to identify the promoter of the Ttyh1 gene and functionally characterize cis-regulatory elements that are potentially involved in the transcriptional regulation of Ttyh1 expression in rat dissociated hippocampal neurons in vitro. We cloned a 592 bp rat Ttyh1 promoter sequence and designed deletion constructs of the transcription factors specificity protein 1 (Sp1), E2F transcription factor 3 (E2f3), and achaete-scute homolog 1 (Ascl1) that were fused upstream of a luciferase reporter gene in pGL4.10[luc2]. The luciferase reporter gene assay showed the possible involvement of Ascl1, Sp1, and responsive cis-regulatory elements in Ttyh1 expression. These findings provide novel information about Ttyh1 gene regulation in neurons.

Knockdown of long non-coding RNA LINC00467 inhibits glioma cell progression via modulation of E2F3 targeted by miR-200a

Studies have found that LINC00467 is an important regulator of cancer. However, the function of LINC00467 in glioma cell is unclear. Therefore, this experimental design based on LINC00467 to explore its mechanism of action in glioma cell. RT-qPCR was used to detect the expression of LINC0046 and miR-200a in glioma cell lines. MTT assay, Edru assay and Transwell assay and flow cytometry were used to detect the effects of LINC0046 and miR-200a on PC cell proliferation, migration and apoptosis. Target gene prediction and screening, luciferase reporter assays were used to validate downstream target genes for LINC0046 and miR-200a. Western blotting was used to detect the protein expression of E2F3. The tumor changes in mice were detected by in vivo experiments in nude mice. LINC00467 was up-regulated in glioma cells. Knockdown of LINC00467 inhibited the viability, migration and invasion of glioma cells. In glioma cells, miR-200a was significantly reduced, while E2F3 was significantly rised. LINC00467 negatively regulated the expression of miR-200a in gliomas, while miR-200a negatively regulated the expression of E2F3 in gliomas. INC00467 promoted the development of glioma by inhibiting miR-200a and promoting E2F3 expression. LINC00467 may be a potential therapeutic target for gliomas.

A novel LncRNA transcript, RBAT1, accelerates tumorigenesis through interacting with HNRNPL and cis-activating E2F3

BACKGROUND

Long non-coding RNAs (lncRNAs) have been identified as important epigenetic regulators that play critical roles in human cancers. However, the regulatory functions of lncRNAs in tumorigenesis remains to be elucidated. Here, we aimed to investigate the molecular mechanisms and potential clinical application of a novel lncRNA, retinoblastoma associated transcript-1 (RBAT1), in tumorigenesis.

METHODS

RBAT1 expression was determined by real-time PCR in both retinoblastoma (Rb) and bladder cancer (BCa) cell lines and clinical tissues. Chromatin isolation using RNA purification (ChIRP) assays were performed to identify RBAT1-interacting proteins. Patient-derived xenograft (PDX) retinoblastoma models were established to test the therapeutic potential of RBAT1-targeting GapmeRs.

RESULTS

Here, we found that RBAT1 expression was significantly higher in Rb and BCa tissues than that in adjacent tissues. Functional assays revealed that RBAT1 accelerated tumorigenesis both in vitro and in vivo. Mechanistically, RBAT1 recruited HNRNPL protein to E2F3 promoter, thereby activating E2F3 transcription. Therapeutically, GapmeR-mediated RBAT1 silencing significantly inhibited tumorigenesis in orthotopic xenograft retinoblastoma models derived from Rb cell lines and Rb primary cells.

CONCLUSIONS

RBAT1 overexpression upregulates a known oncogene, E2F3, via directly recruiting HNPNPL to its promoter and cis-activating its expression. Our finding provides a novel mechanism of lncRNA biology and provides potential targets for diagnosis and treatment of Rb and BCa.

Downregulation of LINC00665 confers decreased cell proliferation and invasion via the miR-138-5p/E2F3 signaling pathway in NSCLC

Non-small cell lung cancer (NSCLC) is a type of malignant tumor which threatens human health and life. Recently, some researches on long non-coding RNAs (lncRNAs) in NSCLC has elucidated critical regulatory roles in cell proliferation, migration, and invasion, the relative clinical significance and mechanisms of action are still unclear. This study focuses on the important role of a novel lncRNA LINC00665 in the development of NSCLC. Long intergenic non-protein coding RNA 665 gene (LINC00665) was found through microarray analysis and was measured by real-time quantitative PCR (RT-qPCR). The interactions between LINC00665 and miR-138-5p as well as the interactions between miR-138-5p and E2F3 (E2F transcription factor 3) were explored by bioinformatics analysis and dual-luciferase assays. CCK-8, transwell and mouse xenograft assays were performed to investigate the effects of LINC00665 and miR-138-5p on NSCLC proliferation and invasion. As a result, LINC00665 expression was upregulated in NSCLC lung tissues and cells. Downregulated LINC00665 could arrest A549 and H1299 cell proliferation and invasion in vitro, and this finding was recapitulated in vivo. LINC00665 directly regulated the expression of miR-138-5p. Additionally, E2F3 was one of the targets of miR-138-5p; E2F3 without 3'UTR could reverse the inhibitory effects of downregulated LINC00665 on proliferation and invasion in A549 and H1299 cells. In conclusion, dysregulation of LINC00665 plays a vital role in NSCLC progression, indicating that its downregulation may confer decreased cell proliferation and invasion via the miR-138-5p/E2F3 signaling pathway.

Dying tumor cell-derived exosomal miR-194-5p potentiates survival and repopulation of tumor repopulating cells upon radiotherapy in pancreatic cancer

BACKGROUND

Tumor repopulation is a major cause of radiotherapy failure. Previous investigations highlighted that dying tumor cells played vital roles in tumor repopulation through promoting proliferation of the residual tumor repopulating cells (TRCs). However, TRCs also suffer DNA damage after radiotherapy, and might undergo mitotic catastrophe under the stimulation of proliferative factors released by dying cells. Hence, we intend to find out how these paradoxical biological processes coordinated to potentiate tumor repopulation after radiotherapy.

METHODS

Tumor repopulation models in vitro and in vivo were used for evaluating the therapy response and dissecting underlying mechanisms. RNA-seq was performed to find out the signaling changes and identify the significantly changed miRNAs. qPCR, western blot, IHC, FACS, colony formation assay, etc. were carried out to analyze the molecules and cells.

RESULTS

Exosomes derived from dying tumor cells induced G1/S arrest and promoted DNA damage response to potentiate survival of TRCs through delivering miR-194-5p, which further modulated E2F3 expression. Moreover, exosomal miR-194-5p alleviated the harmful effects of oncogenic HMGA2 under radiotherapy. After a latent time, dying tumor cells further released a large amount of PGE2 to boost proliferation of the recovered TRCs, and orchestrated the repopulation cascades. Of note, low-dose aspirin was found to suppress pancreatic cancer repopulation upon radiation via inhibiting secretion of exosomes and PGE2.

CONCLUSION

Exosomal miR-194-5p enhanced DNA damage response in TRCs to potentiate tumor repopulation. Combined use of aspirin and radiotherapy might benefit pancreatic cancer patients.

Mitochondrial protein E2F3d, a distinctive E2F3 product, mediates hypoxia-induced mitophagy in cancer cells

Mitochondrial damage is caused by changes in the micro-environmental conditions during tumor progression. Cancer cells require mechanisms for mitochondrial quality control during this process; however, how mitochondrial integrity is maintained is unclear. Here we show that E2F3d, a previously unidentified E2F3 isoform, mediates hypoxia-induced mitophagy in cancer cells. Aberrant activity and expression of the E2F3 transcription factor is frequently observed in many cancer cells. Loss of retinoblastoma (Rb) protein family function increases the expression of E2F3d and E2F3a. E2F3d localizes to the outer mitochondrial membrane and its cytosolic domain contains an LC3-interacting region motif. Overexpression of E2F3d induces mitochondrial fragmentation and mitophagy, suggesting that E2F3d plays an important role in mitophagy. Furthermore, depletion of E2F3s attenuates hypoxia-induced mitophagy and increases intracellular levels of reactive oxygen species, which is reversed by the reintroduction of E2F3d. This study presents another key player that regulates mitochondrial quality control in cancer cells.

MiR-194-5p inhibits cell migration and invasion in bladder cancer by targeting E2F3

PURPOSE

MicroRNA (miR)-194-5p is downregulated in bladder cancer (BC), but its role in BC has not been determined mechanistically.

METHODS

The expression levels of miR-194-5p and E2F transcription factor 3 (E2F3) were determined by means of quantitative reverse transcription and polymerase chain reaction in BC specimens. In addition, T24 BC cells were transfected with a miR-194-5p mimic, a miR-194-5p inhibitor, or E2F3 small interfering (si)RNA, and the level of E2F3 protein expressed by these cells was assessed by western blotting. A dual luciferase reporter assay was applied to verify the binding site between miR-194-5p and the 3' untranslated region of E2F3. Transwell assays were performed to examine cell migration and invasion.

RESULTS

Dysregulation of miR-194-5p in BC was closely associated with node metastasis and differentiation. In BC specimens and cell lines, miR-194-5p mRNA was downregulated, while E2F3 mRNA was upregulated. Overexpression of miR-194-5p suppressed the expression of E2F3 mRNA and protein. By regulating E2F3, miR-194-5p inhibited cell migration and invasion in BC. Treatment of BC cells with E2F3 siRNA had the same effect as did overexpression of miR-194-5p.

CONCLUSIONS

MiR-194-5p directly targets E2F3 and inhibits cell migration and invasion in BC.

Transcription factor compensation during mammary gland development in E2F knockout mice

The E2F transcription factors control key elements of development, including mammary gland branching morphogenesis, with several E2Fs playing essential roles. Additional prior data has demonstrated that loss of individual E2Fs can be compensated by other E2F family members, but this has not been tested in a mammary gland developmental context. Here we have explored the role of the E2Fs and their ability to functionally compensate for each other during mammary gland development. Using gene expression from terminal end buds and chromatin immunoprecipitation data for E2F1, E2F2 and E2F3, we noted both overlapping and unique mammary development genes regulated by each of the E2Fs. Based on our computational findings and the fact that E2Fs share a common binding motif, we hypothesized that E2F transcription factors would compensate for each other during mammary development and function. To test this hypothesis, we generated RNA from E2F1^-/-, E2F2^-/- and E2F3^+/- mouse mammary glands. QRT-PCR on mammary glands during pregnancy demonstrated increases in E2F2 and E2F3a in the E2F1^-/- mice and an increase in E2F2 levels in E2F3^+/- mice. During lactation we noted that E2F3b transcript levels were increased in the E2F2^-/- mice. Given that E2Fs have previously been noted to have the most striking effects on development during puberty, we hypothesized that loss of individual E2Fs would be compensated for at that time. Double mutant mice were generated and compared with the single knockouts. Loss of both E2F1 and E2F2 revealed a more striking phenotype than either knockout alone, indicating that E2F2 was compensating for E2F1 loss. Interestingly, while E2F2 was not able to functionally compensate for E2F3^+/- during mammary outgrowth, increased E2F2 expression was observed in E2F3^+/- mammary glands during pregnancy day 14.5 and lactation day 5. Together, these findings illustrate the specificity of E2F family members to compensate during development of the mammary gland.

miR-152 regulates the proliferation and differentiation of C2C12 myoblasts by targeting E2F3

The development of skeletal muscle is a complex process involving the proliferation, differentiation, apoptosis, and changing of muscle fiber types in myoblasts. Many reports have described the involvement of microRNAs in the myogenesis of myoblasts. In this study, we found that the expression of miR-152 was gradually down-regulated during myoblast proliferation, but gradually up-regulated during the differentiation of myoblasts. Transfection with miR-152 mimics restrained cell proliferation and decreased the expression levels of cyclin E, CDK4, and cyclin D1, but promoted myotube formation and significantly increased the mRNA expression levels of MyHC, MyoD, MRF4, and MyoG in C2C12 myoblasts. However, treatment with miR-152 inhibitors promoted cell proliferation and restrained differentiation. Moreover, over-expression of miR-152 significantly decreased E2F3 production in C2C12 myoblasts. A luciferase assay confirmed that miR-152 could bind to the 3' UTR of E2F3. In conclusion, this study showed that miR-152 inhibited proliferation and promoted myoblast differentiation by targeting E2F3.

[miR-503-5p inhibits the proliferation of T24 and EJ bladder cancer cells by interfering with the Rb/E2F signaling pathway]

Objective To observe the effect of microRNA-503-5p (miR-503-5p) on the growth of T24 and EJ bladder cancer cells, and explore the possible molecular mechanism. Methods The miR-504-5p mimics or miR-NC was transfected into T24 and EJ cells. The target gene of miR-503-5p was predicted by bioinformatics. The expressions of E2F transcription factor 3 (E2F3) mRNA and Rb/E2F signaling pathway mRNA were detected by the real-time quantitative PCR (qPCR). The expressions of Rb/E2F signal pathway proteins E2F3, cyclin E, CDK2, Rb and p-Rb were detected by Western blotting. The cell cycle of bladder cancer cell lines was determined by flow cytometry. MTT assay and plate cloning assay were performed to observe the proliferation ability of bladder cancer cells. Results After miR-503-5p mimics transfection, the expression of miR-503-5p in bladder cancer cells significantly increased. The increased expression of miR-503-5p significantly reduced the expressions of E2F3 mRNA and Rb/E2F signaling pathway mRNA in bladder cancer cells. What's more, the expressions of Rb/E2F signal pathway proteins were down-regulated. The bladder cancer cells were arrested in G0/G1 phase, and their growth was significantly inhibited by miR-503-5p. Conclusion The miR-503-5p over-expression can inhibit the growth of bladder cancer cell lines T24 and EJ by down-regulating the expression of the Rb/E2F signaling pathway.

MicroRNA-217 functions as a prognosis predictor and inhibits pancreatic cancer cell proliferation and invasion via targeting E2F3

OBJECTIVE

Pancreatic cancer (PC) is the most malignant tumor among all the tumors in the digestive system. MiR-217 has been reported to take a critical part in various malignant tumors. The aim of this study was to explore the function of MiR-217 in pancreatic cancer and its target genes.

PATIENTS AND METHODS

Twenty pairs of PC tissues and matched normal adjacent pancreatic tissues were collected. The expression of miR-217 in PC tissues and normal pancreatic tissues was detected by Real-time polymerase chain reaction (PCR). PC cells were transfected with miR-217 mimics, inhibitors and negative control, respectively. Cell Counting Kit-8 (CCK-8) assay was used to detect cell viability. Cell apoptosis was checked via Annexin V-FITC/PI apoptosis kit. The protein expression of E2F3 was detected by Western blot. To detect repression by miR-217, HEK293T cells were co-transfected with the indicated E2F3 3'-UTR luciferase reporter.

RESULTS

The expression of miR-217 was reduced in PC tissues comparing to normal pancreatic tissues. Meantime, the in-vitro study revealed that miR-217 suppressed PC cell growth, invasion but promoted apoptosis. Next, we proved that E2F3 was the target of miR-217 on PC cell function.

CONCLUSIONS

miR-217 suppresses PC cell growth, invasion but promotes apoptosis in vitro through targeting E2F3. The miR-217-E2F3 axis may be used for PC therapy.

MicroRNA-17-92 Regulates the Transcription Factor E2F3b during Myogenesis In Vitro and In Vivo

Myogenic differentiation, which occurs during muscle development, is a highly ordered process that can be regulated by E2F transcription factors. Available data show that E2F3b, but not E2F3a, is upregulated and required for myogenic differentiation. However, the regulation of E2F3b expression in myogenic differentiation is not well understood. To investigate whether E2Fb expression is controlled by miRNAs, we used bioinformatics to combine the database of microRNAs downregulated during myogenesis and those predicted to target E2F3. This identified miR-17 and miR-20a as miRNAs potentially involved in E2F3 regulation. We found that miR-17-92 controls the expression of E2F3b in C2C12 cells during myogenic differentiation. Moreover, we confirmed that miR-20a regulates the expression of E2F3b proteins in vivo using a muscle regeneration model.

Dosage-dependent copy number gains in E2f1 and E2f3 drive hepatocellular carcinoma

Disruption of the retinoblastoma (RB) tumor suppressor pathway, either through genetic mutation of upstream regulatory components or mutation of RB1 itself, is believed to be a required event in cancer. However, genetic alterations in the RB-regulated E2F family of transcription factors are infrequent, casting doubt on a direct role for E2Fs in driving cancer. In this work, a mutation analysis of human cancer revealed subtle but impactful copy number gains in E2F1 and E2F3 in hepatocellular carcinoma (HCC). Using a series of loss- and gain-of-function alleles to dial E2F transcriptional output, we have shown that copy number gains in E2f1 or E2f3b resulted in dosage-dependent spontaneous HCC in mice without the involvement of additional organs. Conversely, germ-line loss of E2f1 or E2f3b, but not E2f3a, protected mice against HCC. Combinatorial mapping of chromatin occupancy and transcriptome profiling identified an E2F1- and E2F3B-driven transcriptional program that was associated with development and progression of HCC. These findings demonstrate a direct and cell-autonomous role for E2F activators in human cancer.

Long non-coding RNA NEAT1 promotes non-small cell lung cancer progression through regulation of miR-377-3p-E2F3 pathway

Recently, the long non-coding RNA (lncRNA) NEAT1 has been identified as an oncogenic gene in multiple cancer types and elevated expression of NEAT1 was tightly linked to tumorigenesis and cancer progression. However, the molecular basis for this observation has not been characterized in progression of non-small cell lung cancer (NSCLC). In our studies, we identified NEAT1 was highly expressed in patients with NSCLC and was a novel regulator of NSCLC progression. Patients whose tumors had high NEAT1 expression had a shorter overall survival than patients whose tumors had low NEAT1 expression. Further, NEAT1 significantly accelerates NSCLC cell growth and metastasis in vitro and tumor growth in vivo. Additionally, by using bioinformatics study and RNA pull down combined with luciferase reporter assays, we demonstrated that NEAT1 functioned as a competing endogenous RNA (ceRNA) for hsa-miR-377-3p, antagonized its functions and led to the de-repression of its endogenous targets E2F3, which was a core oncogene in promoting NSCLC progression. Taken together, these observations imply that the NEAT1 modulated the expression of E2F3 gene by acting as a ceRNA, which may build up the missing link between the regulatory miRNA network and NSCLC progression.

miR-503 inhibits cell proliferation and induces apoptosis in colorectal cancer cells by targeting E2F3

OBJECTIVE

Colorectal cancer (CRC) is one of the major healthcare problems worldwide. A lot of miRNAs are aberrantly expressed in CRC and involved in its development and progression. The purpose of this study was to investigate the expression and function of miR-503 in CRC.

METHODS

miR-503 expression was detected in CRC tissues and cell lines by Quantitative real-time PCR. Cell proliferation was assessed by MTT assay. Cell apoptosis and cell cycle distribution were measured by flow cytometry. Moreover, luciferase reporter assay and western blot were performed to determine the potential target of miR-503 in CRC cells.

RESULTS

miR-503 was significantly decreased in CRC tissues and cell lines in comparison with controls. Overexpression of miR-503 in CRC cells remarkably inhibited cell proliferation and induced apoptosis. Furthermore, E2F3 was identified as a direct target of miR-503 in CRC cells and down-regulation of E2F3 had a similar effect as miR-503 overexpression on CRC cells. In addition, the expression of E2F3 was negatively correlated with miR-503 level in CRC tissues.

CONCLUSIONS

miR-503 inhibits cell proliferation and induces apoptosis by directly targeting E2F3 in CRC cells, indicating its potential application in CRC diagnosis and therapy.

The 1,2-Diaminocyclohexane Carrier Ligand in Oxaliplatin Induces p53-Dependent Transcriptional Repression of Factors Involved in Thymidylate Biosynthesis

Platinum-based chemotherapeutic drugs are widely used as components of combination chemotherapy in the treatment of cancer. One such drug, oxaliplatin, exerts a synergistic effect against advanced colorectal cancer in combination with 5-fluorouracil (5-FU) and leucovorin. In the p53-proficient colorectal cancer cell line HCT116, oxaliplatin represses the expression of deoxyuridine triphosphatase (dUTPase), a ubiquitous pyrophosphatase that catalyzes the hydrolysis of dUTP to dUMP and inhibits dUTP-mediated cytotoxicity. However, the underlying mechanism of this activity has not been completely elucidated, and it remains unclear whether factors other than downregulation of dUTPase contribute to the synergistic effect of 5-FU and oxaliplatin. In this study, we found that oxaliplatin and dachplatin, platinum-based drugs containing the 1,2-diaminocyclohexane (DACH) carrier ligand, repressed the expression of nuclear isoform of dUTPase (DUT-N), whereas cisplatin and carboplatin did not. Oxaliplatin induced early p53 accumulation, upregulation of primary miR-34a transcript expression, and subsequent downregulation of E2F3 and E2F1. Nutlin-3a, which activates p53 nongenotoxically, had similar effects. Introduction of miR-34a mimic also repressed E2F1 and DUT-N expression, indicating that this miRNA plays a causative role. In addition to DUT-N, oxaliplatin repressed, in a p53-dependent manner, the expression of genes encoding enzymes involved in thymidylate biosynthesis. Consequently, oxaliplatin significantly decreased the level of dTTP in the dNTP pool in a p53-dependent manner. These data indicate that the DACH carrier ligand in oxaliplatin triggers signaling via the p53-miR-34a-E2F axis, leading to transcriptional regulation that ultimately results in accumulation of dUTP and reduced dTTP biosynthesis, potentially enhancing 5-FU cytotoxicity.

Over-expression of miR-125a-5p inhibits proliferation in C2C12 myoblasts by targeting E2F3

MicroRNAs (miRNAs) are a class of small non-coding RNAs of 20-25 nucleotides in length. It has been shown that miRNAs play important roles in the proliferation of many types of cells, including myoblasts. In this study, we used real-time quantitative polymerase chain reaction, western blotting, EdU, flow cytometry, and CCK-8 assay to explore the role of miR-125a-5p during the proliferation of C2C12 myoblasts. It was found that the expression of miR-125a-5p was decreased during C2C12 myoblast proliferation. Over-expression of miR-125a-5p inhibited C2C12 myoblast proliferation as indicated by EdU staining, flow cytometry, and CCK8 assay. It was also found that miR-125a-5p could negatively regulate E2F3 expression at posttranscriptional level, via a specific target site in the 3' untranslated region. Knockdown of E2F3 showed a similar inhibitory effect on C2C12 myoblast proliferation. Thus, our findings suggest that miR-125a-5p may act as a negative regulator of C2C12 myoblast proliferation by targeting E2F3.

EWS-FLI1 employs an E2F switch to drive target gene expression

Cell cycle progression is orchestrated by E2F factors. We previously reported that in ETS-driven cancers of the bone and prostate, activating E2F3 cooperates with ETS on target promoters. The mechanism of target co-regulation remained unknown. Using RNAi and time-resolved chromatin-immunoprecipitation in Ewing sarcoma we report replacement of E2F3/pRB by constitutively expressed repressive E2F4/p130 complexes on target genes upon EWS-FLI1 modulation. Using mathematical modeling we interrogated four alternative explanatory models for the observed EWS-FLI1/E2F3 cooperation based on longitudinal E2F target and regulating transcription factor expression analysis. Bayesian model selection revealed the formation of a synergistic complex between EWS-FLI1 and E2F3 as the by far most likely mechanism explaining the observed kinetics of E2F target induction. Consequently we propose that aberrant cell cycle activation in Ewing sarcoma is due to the de-repression of E2F targets as a consequence of transcriptional induction and physical recruitment of E2F3 by EWS-FLI1 replacing E2F4 on their target promoters.

E2F inhibition synergizes with paclitaxel in lung cancer cell lines

The CDK/Rb/E2F pathway is commonly disrupted in lung cancer, and thus, it is predicted that blocking the E2F pathway would have therapeutic potential. To test this hypothesis, we have examined the activity of HLM006474 (a small molecule pan-E2F inhibitor) in lung cancer cell lines as a single agent and in combination with other compounds. HLM006474 reduces the viability of both SCLC and NSCLC lines with a biological IC₅₀ that varies between 15 and 75 µM, but with no significant difference between the groups. Combination of HLM006474 with cisplatin and gemcitabine demonstrate little synergy; however, HLM006474 synergizes with paclitaxel. Surprisingly, we discovered that brief treatment of cells with HLM006474 led to an increase of E2F3 protein levels (due to de-repression of these promoter sites). Since paclitaxel sensitivity has been shown to correlate with E2F3 levels, we hypothesized that HLM006474 synergy with paclitaxel may be mediated by transient induction of E2F3. To test this, H1299 cells were depleted of E2F3a and E2F3b with siRNA and treated with paclitaxel. Assays of proliferation showed that both siRNAs significantly reduced paclitaxel sensitivity, as expected. Taken together, these results suggest that HLM006474 may have efficacy in lung cancer and may be useful in combination with taxanes.

A novel negative regulator of adipogenesis: microRNA-363

The differentiation of adipose tissue-derived stromal cells (ADSCs) into adipocytes involves a highly orchestrated series of events that includes cell lineage commitment, mitotic clonal expansion, growth arrest, and terminal differentiation. However, the molecular mechanisms controlling adipogenesis are not yet completely understood. In this study, we investigated whether microRNAs (miRNAs) play a role in adipocyte differentiation. Microarray analysis was performed to determine the miRNA expression profile during ADSC differentiation, and miR-363 was found to be one of the most significantly downregulated miRNAs. We show that the overexpression of miR-363 in ADSCs inhibited mitotic clonal expansion and terminal differentiation. Furthermore, ectopic introduction of miR-363 into ADSCs markedly reduced the levels of E2F3, a key transcription factor that regulates growth and proliferation during mitotic clonal expansion. Finally, using an EGFP/RFP reporter assay, we demonstrate that miR-363 can directly target the 3'UTR of E2F3. Taken together, these results suggest that miR-363 regulates the transition from mitotic clonal expansion to terminal differentiation during adipogenesis in ADSCs, at least in part, by targeting E2F3.

Tumor suppressor microRNAs, miR-100 and -125b, are regulated by 1,25-dihydroxyvitamin D in primary prostate cells and in patient tissue

MiR-100 and miR-125b are lost in many cancers and have potential function as tumor suppressors. Using both primary prostatic epithelial cultures and laser capture-microdissected prostate epithelium from 45 patients enrolled in a vitamin D3 randomized trial, we identified miR-100 and -125b as targets of 1,25-dihydroxyvitamin D3 (1,25D). In patients, miR-100 and -125b levels were significantly lower in tumor tissue than in benign prostate. Similarly, miR-100 and -125b were lower in primary prostate cancer cells than in cells derived from benign prostate. Prostatic concentrations of 1,25D positively correlated with these miRNA levels in both prostate cancer and benign epithelium, showing that patients with prostate cancer may still benefit from vitamin D3. In cell assays, upregulation of these miRNAs by 1,25D was vitamin D receptor dependent. Transfection of pre-miR-100 and pre-miR-125b in the presence or absence of 1,25D decreased invasiveness of cancer cell, RWPE-2. Pre-miR-100 and pre-miR-125b decreased proliferation in primary cells and cancer cells respectively. Pre-miR-125b transfection suppressed migration and clonal growth of prostate cancer cells, whereas knockdown of miR-125b in normal cells increased migration indicates a tumor suppressor function. 1,25D suppressed expression of previously bona fide mRNA targets of these miRNAs, E2F3 and Plk1, in a miRNA-dependent manner. Together, these findings show that vitamin D3 supplementation augments tumor suppressive miRNAs in patient prostate tissue, providing evidence that miRNAs could be key physiologic mediators of vitamin D3 activity in prevention and early treatment of prostate cancer.

The microRNA-29 plays a central role in osteosarcoma pathogenesis and progression

Osteosarcoma is the most common type of bone cancer, with a peak incidence in the early childhood. The relationship between microRNAs (miRNAs) and cancer development attracted more and more attention over the last few years. Members of the miRNA-29 family, including miRNA-29a, miRNA-29b, and miRNA-29c were shown to participate in the development of rhabdomyosarcoma and hepatocarcinogenesis. Here, it has been demonstrated miRNA-29a and miRNA-29b expression levels to be downregulated in most of the osteosarcoma tissues (23 from 30). Besides, miRNA-29a displayed ability to induce apoptosis in both U2OS and SAOS-2 osteoblastic cells. While miRNA-29 members induced apoptosis through p53 gene activation, the effect of miRNA-29a on osteoblastic cells was independent on p53 expression level. Moreover, Bcl-2 and Mcl-1 were earlier demonstrated to be the direct targets of miRNA-29 in many types of cancer tissues and cancers. In both U2OS and SAOS-2 osteoblastic cell types, overexpression of miRNA-29a also downregulated Bcl-2 and Mcl-1, while silencing of miRNA-29a increased their expression. In addition, enhanced expression of miRNA-29a increased the expression of two tumor suppressor genes, E2F1 and E2F3. In summary, data obtained highlight the role of miRNA-29a in the regulation of osteoblastic cell apoptosis by silencing Bcl-2 and Mcl-1 and inducing E2F1 and E2F3 expression.

Induction, modulation and potential targets of miR-210 in pancreatic cancer cells

BACKGROUND

MiR-210 is induced by hypoxia and plays different roles in the development of certain cancers. However, little is known about its role in pancreatic cancer (PC). This study aimed to explore the induction and modulation of PC by miR-210 and its potential molecular targets.

METHODS

PC cells were cultured under normoxic and hypoxic conditions. Expression of miR-210 and hypoxia-inducible factor (HIF)-1alpha was detected using quantitative reverse-transcription polymerase chain reaction. Cancer cells were transiently transfected with HIF-1alpha small interfering RNA (siRNA) and miR-210 mimics, and cell proliferation was measured using the CCK-8 assay. Potential targets for miR-210 were then identified using a dual luciferase reporter assay.

RESULTS

Hypoxic conditions induced miR-210 expression in six PC cell lines (AsPC-1, BxPC-3, MIAPaCa-2, PANC-1, Su86.86 and SW1990), but not in Capan-1 or T3M4 cells. Transfection of HIF-1alpha siRNA into PANC-1 cells markedly inhibited HIF-1alpha expression, and subsequently down-regulated miR-210 expression under hypoxic conditions. MiR-210 had no observable impact on the proliferation of PANC-1 or Su86.86 cells and dual luciferase reporter assays showed significantly reduced luciferase activity in the wild-type E2F3, EFNA3, GIT2, MNT, ZNF462 and EGR3 constructs, compared to the corresponding mutants, but not in HOXA3.

CONCLUSIONS

These results suggest that miR-210 expression in PC cells is induced by hypoxia through a HIF-1alpha-dependent pathway, but does not influence PC cell proliferation. Also, E2F3, EFNA3, GIT2, MNT, ZNF462 and EGR3 may be potential miR-210 targets in PC.

The interleukin 6 receptor is a direct transcriptional target of E2F3 in prostate tumor derived cells

BACKGROUND

The E2F/RB pathway is frequently disrupted in multiple human cancers. E2F3 levels are elevated in prostate tumors and E2F3 overexpression independently predicts clinical outcome. The goals of this study were to identify direct transcriptional targets of E2F3 in prostate tumor derived cells.

METHODS

Expression array studies identified the interleukin 6 receptor (IL-6R) as an E2F3 target. E2F3-dependent expression of IL-6R was analyzed by real time PCR and Western immunoblot analysis in several cell lines. Chromatin immunoprecipitation (ChIP) and IL-6R-luciferase reporter plasmid studies were used to characterize the IL-6R promoter.

RESULTS

Expression array studies identified genes that were regulated by E2F3 in prostate tumor derived cell lines. The network most significantly associated with E2F3-regulated transcripts was cytokine signaling and the IL-6R was a component of several of the most prominent E2F3-regulated pathways. The transcriptional regulation of IL-6R by E2F3 knockdown was validated in several prostate tumor-derived cell lines at the RNA level and protein level. The IL-6R regulatory region containing ChIP-identified E2F3 binding sites was cloned into a reporter and co-transfected with an E2F3a expression plasmid. The luciferase assay showed that E2F3a transactivated the IL-6R promoter in a dose dependent manner. The functional consequence of IL-6R decrease was a reduction in the levels of ERK1/2 phosphorylation, indicating that IL-6R initiated signaling was altered.

CONCLUSION

These studies connect the E2F and IL-6 signaling cascade, thus providing the mechanistic link between two major regulatory networks that are perturbed during prostate tumorigenesis.

Retinoblastoma-independent regulation of cell proliferation and senescence by the p53-p21 axis in lamin A /C-depleted cells

The expression of A-type lamin is downregulated in several cancers, and lamin defects are the cause of several diseases including a form of accelerated aging. We report that depletion of lamin A/C expression in normal human cells leads to a dramatic downregulation of the Rb family of tumor suppressors and a defect in cell proliferation. Lamin A/C-depleted cells exhibited a flat morphology and accumulated markers of cellular senescence. This senescent phenotype was accompanied by engagement of the p53 tumor suppressor and induction of the p53 target gene p21 and was prevented by small hairpin RNAs against p53, p21, or by the oncoprotein Mdm2. The expression of E2F target genes, normally required for cell cycle progression, was downregulated after lamin A/C depletion but restored after the inactivation of p53. A similar senescence response was observed in myoblasts from a patient with a lamin A mutation causing muscular dystrophy. We thus reveal a previously unnoticed mechanism of controlling cell cycle genes expression, which depends on p53 but does not require the retinoblastoma family of tumor suppressors and that can be relevant to understand the pathogenesis of laminopathies and perhaps aging.

The E2F3-Oncomir-1 axis is activated in Wilms' tumor

Oncomir-1 is an oncogenic cluster of microRNAs (miRNA) located on chromosome 13. Previous in vitro studies showed that it is transcriptionally regulated by the transcription factor E2F3. In this report, we combine expression profiling of both mRNA and miRNAs in Wilms' tumor (WT) samples to provide the first evidence that the E2F3-Oncomir-1 axis, previously identified in cell culture, is deregulated in primary human tumors. Analysis of RNA expression signatures showed that an E2F3 gene signature was activated in all WT samples analyzed, in contrast to other kidney tumors. This finding was validated by immunohistochemistry on the protein level. Expression of E2F3 was lowest in early-stage tumors and highest in metastatic tissue. Expression profiling of miRNAs in WT showed that expression of each measured member of the Oncomir-1 family was highest in WT relative to other kidney tumor subtypes. Quantitative PCR confirmed that these miRNAs were overexpressed in WT relative to normal kidney tissue. These results suggest that the E2F3-Oncomir-1 axis is activated in WT. Our study also shows the utility of integrated genomics combining gene signature analysis with miRNA expression profiling to identify protein-miRNA interactions that are perturbed in disease states.

Specific tumor suppressor function for E2F2 in Myc-induced T cell lymphomagenesis

Deregulation of the Myc pathway and deregulation of the Rb pathway are two of the most common abnormalities in human malignancies. Recent in vitro experiments suggest a complex cross-regulatory relationship between Myc and Rb that is mediated through the control of E2F. To evaluate the functional connection between Myc and E2Fs in vivo, we used a bitransgenic mouse model of Myc-induced T cell lymphomagenesis and analyzed tumor progression in mice deficient for E2f1, E2f2, or E2f3. Whereas the targeted inactivation of E2f1 or E2f3 had no significant effect on tumor progression, loss of E2f2 accelerated lymphomagenesis. Interestingly, loss of a single copy of E2f2 also accelerated tumorigenesis, albeit to a lesser extent, suggesting a haploinsufficient function for this locus. The combined ablation of E2f1 or E2f3, along with E2f2, did not further accelerate tumorigenesis. Myc-overexpressing T cells were more resistant to apoptosis in the absence of E2f2, and the reintroduction of E2F2 into these tumor cells resulted in an increase of apoptosis and inhibition of tumorigenesis. These results identify the E2f2 locus as a tumor suppressor through its ability to modulate apoptosis.

Rb-mediated neuronal differentiation through cell-cycle-independent regulation of E2f3a

It has long been known that loss of the retinoblastoma protein (Rb) perturbs neural differentiation, but the underlying mechanism has never been solved. Rb absence impairs cell cycle exit and triggers death of some neurons, so differentiation defects may well be indirect. Indeed, we show that abnormalities in both differentiation and light-evoked electrophysiological responses in Rb-deficient retinal cells are rescued when ectopic division and apoptosis are blocked specifically by deleting E2f transcription factor (E2f) 1. However, comprehensive cell-type analysis of the rescued double-null retina exposed cell-cycle–independent differentiation defects specifically in starburst amacrine cells (SACs), cholinergic interneurons critical in direction selectivity and developmentally important rhythmic bursts. Typically, Rb is thought to block division by repressing E2fs, but to promote differentiation by potentiating tissue-specific factors. Remarkably, however, Rb promotes SAC differentiation by inhibiting E2f3 activity. Two E2f3 isoforms exist, and we find both in the developing retina, although intriguingly they show distinct subcellular distribution. E2f3b is thought to mediate Rb function in quiescent cells. However, in what is to our knowledge the first work to dissect E2f isoform function in vivo we show that Rb promotes SAC differentiation through E2f3a. These data reveal a mechanism through which Rb regulates neural differentiation directly, and, unexpectedly, it involves inhibition of E2f3a, not potentiation of tissue-specific factors.

The retinoblastoma protein (Rb), an important tumor suppressor, blocks division and death by inhibiting the E2f transcription factor family. In contrast, Rb is thought to promote differentiation by potentiating tissue-specific transcription factors, although differentiation defects in Rb null cells could be an indirect consequence of E2f-driven division and death. Here, we resolve different mechanisms by which Rb controls division, death, and differentiation in the retina. Removing E2f1 rescues aberrant division of differentiating Rb-deficient retinal neurons, as well as death in cells prone to apoptosis, and restores both normal differentiation and function of major cell types, such as photoreceptors. However, Rb-deficient starburst amacrine neurons differentiate abnormally even when E2f1 is removed, providing an unequivocal example of a direct role for Rb in neuronal differentiation. Rather than potentiating a cell-specific factor, Rb promotes starburst cell differentiation by inhibiting another E2f, E2f3a. This cell-cycle–independent activity broadens the importance of the Rb–E2f pathway, and suggests we should reassess its role in the differentiation of other cell types.

The retinoblastoma protein (Rb), a tumor suppressor, promotes the differentiation of starburst amacrine cells in the retina by inhibiting the transcription factor E2f3a, whereas it suppresses retinal cell division and death by inhibiting E2f1.

Unique requirement for Rb/E2F3 in neuronal migration: evidence for cell cycle-independent functions

The cell cycle regulatory retinoblastoma (Rb) protein is a key regulator of neural precursor proliferation; however, its role has been expanded to include a novel cell-autonomous role in mediating neuronal migration. We sought to determine the Rb-interacting factors that mediate both the cell cycle and migration defects. E2F1 and E2F3 are likely Rb-interacting candidates that we have shown to be deregulated in the absence of Rb. Using mice with compound null mutations of Rb and E2F1 or E2F3, we asked to what extent either E2F1 or E2F3 interacts with Rb in neurogenesis. Here, we report that E2F1 and E2F3 are both functionally relevant targets in neural precursor proliferation, cell cycle exit, and laminar patterning. Each also partially mediates the Rb requirement for neuronal survival. Neuronal migration, however, is specifically mediated through E2F3, beyond its role in cell cycle regulation. This study not only outlines overlapping and distinct functions for E2Fs in neurogenesis but also is the first to establish a physiologically relevant role for the Rb/E2F pathway beyond cell cycle regulation in vivo.

Proteomic and transcriptomic analyses of retinal pigment epithelial cells exposed to REF-1/TFPI-2

PURPOSE

The authors previously reported a growth-promoting factor, REF-1/TFPI-2, that is specific to retinal pigment epithelial (RPE) cells. The purpose of this study was to determine the genes and proteins of human RPE cells that are altered by exposure to TFPI-2.

METHODS

Human primary RPE cells were cultured with or without TFPI-2. Cell extracts and isolated RNA were subjected to proteomic and transcriptomic analyses, respectively. Proteins were separated by two-dimensional gel electrophoresis followed by gel staining and ion spray tandem mass spectrometry analyses. Transcriptomic analysis was performed using a DNA microarray to detect 27,868 gene expressions.

RESULTS

Proteomic analysis revealed c-Myc binding proteins and ribosomal proteins L11 preferentially induced by TFPI-2 in human RPE cells. Transcriptomic analysis detected 10,773 of 33,096 probes in the TFPI-2 treated samples, whereas only 2186 probes were detected in the nontreated samples. Among the genes up-regulated by TFPI-2 at the protein level were c-myc, Mdm2, transcription factor E2F3, retinoblastoma binding protein, and the p21 gene, which is associated with the c-myc binding protein and ribosomal protein L11.

CONCLUSIONS

The mechanisms by which TFPI-2 promotes the proliferation of RPE cells may be associated with augmented c-myc synthesis and the activation of E2F in the retinoblastoma protein (Rb)/E2F pathway at the G1 phase of the RPE cells. Activation of ribosomal protein L11 and the Mdm2 complex of the p53 pathway may be counterbalanced by the hyperproliferative conditions.

Rb is critical in a mammalian tissue stem cell population

The inactivation of the retinoblastoma (Rb) tumor suppressor gene in mice results in ectopic proliferation, apoptosis, and impaired differentiation in extraembryonic, neural, and erythroid lineages, culminating in fetal death by embryonic day 15.5 (E15.5). Here we show that the specific loss of Rb in trophoblast stem (TS) cells, but not in trophoblast derivatives, leads to an overexpansion of trophoblasts, a disruption of placental architecture, and fetal death by E15.5. Despite profound placental abnormalities, fetal tissues appeared remarkably normal, suggesting that the full manifestation of fetal phenotypes requires the loss of Rb in both extraembryonic and fetal tissues. Loss of Rb resulted in an increase of E2f3 expression, and the combined ablation of Rb and E2f3 significantly suppressed Rb mutant phenotypes. This rescue appears to be cell autonomous since the inactivation of Rb and E2f3 in TS cells restored placental development and extended the life of embryos to E17.5. Taken together, these results demonstrate that loss of Rb in TS cells is the defining event causing lethality of Rb(-/-) embryos and reveal the convergence of extraembryonic and fetal functions of Rb in neural and erythroid development. We conclude that the Rb pathway plays a critical role in the maintenance of a mammalian stem cell population.

Selective requirements for E2f3 in the development and tumorigenicity of Rb-deficient chimeric tissues

The tumor suppressor function of the retinoblastoma protein pRB is largely dependent upon its capacity to inhibit the E2F transcription factors and thereby cell proliferation. Attempts to study the interplay between pRB and the E2Fs have been hampered by the prenatal death of Rb; E2f nullizygous mice. In this study, we isolated Rb; E2f3 mutant embryonic stem cells and generated Rb(-/-); E2f3(-/-) chimeric mice, thus bypassing the lethality of the Rb(-/-); E2f3(-/-) germ line mutant mice. We show that loss of E2F3 has opposing effects on two of the known developmental defects arising in Rb(-/-) chimeras; it suppresses the formation of cataracts while aggravating the retinal dysplasia. This model system also allows us to assess how E2f3 status influences tumor formation in Rb(-/-) tissues. We find that E2f3 is dispensable for the development of pRB-deficient pituitary and thyroid tumors. In contrast, E2f3 inactivation completely suppresses the pulmonary neuroendocrine hyperplasia arising in Rb(-/-) chimeric mice. This hyperproliferative state is thought to represent the preneoplastic lesion of small-cell lung carcinoma. Therefore, our observation highlights a potential role for E2F3 in the early stages of this tumor type.

E2F regulates DDB2: consequences for DNA repair in Rb-deficient cells

DDB2, a gene mutated in XPE patients, is involved in global genomic repair especially the repair of cyclobutane pyrimidine dimers (CPDs), and is regulated by p53 in human cells. We show that DDB2 is expressed in mouse tissues and demonstrate, using primary mouse epithelial cells, that mouse DDB2 is regulated by E2F transcription factors. Retinoblastoma (Rb), a tumor suppressor critical for the control of cell cycle progression, regulates E2F activity. Using Cre-Lox technology to delete Rb in primary mouse hepatocytes, we show that DDB2 gene expression increases, leading to elevated DDB2 protein levels. Furthermore, we show that endogenous E2F1 and E2F3 bind to DDB2 promoter and that treatment with E2F1-antisense or E2F1-small interfering RNA (siRNA) decreases DDB2 transcription, demonstrating that E2F1 is a transcriptional regulator for DDB2. This has consequences for global genomic repair: in Rb-null cells, where E2F activity is elevated, global DNA repair is increased and removal of CPDs is more efficient than in wild-type cells. Treatment with DDB2-siRNA decreases DDB2 expression and abolishes the repair phenotype of Rb-null cells. In summary, these results identify a new regulatory pathway for DDB2 by E2F, which does not require but is potentiated by p53, and demonstrate that DDB2 is involved in global repair in mouse epithelial cells.

Regulation of the Arf/p53 tumor surveillance network by E2F

Deregulation of the cell cycle machinery plays a critical role in tumorigenesis. In particular, functional inactivation of the retinoblastoma protein (pRB) is a key event. pRB's tumor suppressive activity is at least partially dependent on its ability to regulate the activity of the E2F transcription factors. E2F controls the expression of genes that encode the cellular proliferation machinery. E2F can also trigger apoptosis when it is inappropriately expressed. Here we present evidence that E2F acts to directly regulate the Arf/p53 tumor surveillance network. In normal cells, a single member of the E2F family, E2F3, participates in the transcriptional silencing of Arf. In response to oncogenic stress, the activating E2Fs, E2F1, 2, and E2F3A, all associate with Arf and promote its transcription. These findings raise the possibility that E2F acts as a sensor of inappropriate versus normal proliferative signals and determines whether or not the Arf/p53 tumor surveillance network is engaged.

Nuclear overexpression of the E2F3 transcription factor in human lung cancer

BACKGROUND

The E2F3 transcription factor has an established role in controlling cell cycle progression. In previous studies we have provided evidence that nuclear E2F3 overexpression represents a mechanism that drives the development of human bladder cancer and that determines aggressiveness in human prostate cancer. We have proposed a model in which E2F3 overexpression co-operates with removal of the E2F inhibitor pRB to facilitate cancer development. Since small cell lung cancers (SCLC) have one of the highest reported frequencies of functional abnormalities in the pRB protein (90%) of any human cancer, we wish to assess to what extent E2F3 would be overexpressed in this and other classes of human lung cancer.

METHODS

Immunohistochemical techniques were used to assess the E2F3 status in 428 samples of lung cancers, lung carcinoids, normal bronchial epithelium and normal lung tissue.

RESULTS

E2F3 is overexpressed in 55-70% of squamous cell carcinomas and 79% of adenocarcinomas of the lung. In addition very high level expression of nuclear E2F3 is found in almost all small cell lung cancers analysed. When considered together with published data our observations indicate that co-operation between pRB functional knockouts and E2F3 overexpression may represent a mechanism of development of SCLC.

Identification of nuclear localization signal that governs nuclear import of BRD7 and its essential roles in inhibiting cell cycle progression

BRD7, a novel bromodomain gene, is identified to be associated with nasopharyngeal carcinoma (NPC). Decreased or loss of expression of BRD7 was detected in NPC biopsies and cell lines. Overexpression of BRD7 could inhibit NPC cell growth and arrest cells in cell cycle by transcriptionally regulating some important molecules involved in ras/MEK/ERK and Rb/E2F pathway, and downregulate the promoter activity of E2F3. In the present study, the subcellular localization of BRD7 was investigated. It was found that BRD7 was mainly localized in nucleus without distinct cell-specific difference between COS7 and HNE1. Furthermore, a functional nuclear localization signal (NLS) sequence ranging from amino acid 65 to 96 was identified and characterized. The NLS is composed of a cluster of four bipartite nuclear targeting sequences, which are tightly linked and extremely overlapped. We found that whether the entire NLS or the four bipartite nuclear targeting sequences could respectively determine the nuclear import of green fluorescent protein (GFP). The most important is that NLS-deleted BRD7 shifted the nuclear localization to be mostly in cytoplasm, and failed or reduced to negatively regulate the expression of cell cycle related molecules, cyclin D1 and E2F3, and cell cycle progression from G1 to S phase. In conclusion, NLS is an essential motif affecting BRD7 nuclear distribution, and the nuclear localization of BRD7 is critical for the expression of cell cycle related molecules and cell biological function.

Differentiation and injury-repair signals modulate the interaction of E2F and pRB proteins with novel target genes in keratinocytes

E2F transcription factors are central to epidermal morphogenesis and regeneration after injury. The precise nature of E2F target genes involved in epidermal formation and repair has yet to be determined. Identification of these genes is essential to understand how E2F proteins regulate fundamental aspects of epidermal homeostasis and transformation. We have conducted a genome-wide screen using CpG island microarray analysis to identify novel promoters bound by E2F3 and E2F5 in human keratinocytes. We further characterized several of these genes, and determined that multiple E2F and retinoblastoma (pRb) family proteins associate with them in exponentially proliferating cells. We also assessed the effect on E2F and pRb binding to those genes in response to differentiation induced by bone morphogenetic protein-6 (BMP-6), or to activation of repair mechanisms induced by transforming growth factor-beta (TGF-beta). These studies demonstrate promoter- and cytokine-specific changes in binding profiles of E2F and/or pRb family proteins. For example, E2F1, 3, 4 and p107 were recruited to the N-myc promoter in cells treated with BMP-6, whereas E2F1, 3, 4, 5, p107 and p130 were bound to this promoter in the presence of TGF-beta. Functionally, these different interactions resulted in transcriptional repression by BMP-6 and TGF-beta of the N-myc gene, via mechanisms that involved E2F binding to the promoter and association with pRb-family proteins. Thus, multiple combinations of E2F and pRb family proteins may associate with and transcriptionally regulate a given target promoter in response to differentiation and injury-repair stimuli in epidermal keratinocytes.

E2F3a stimulates proliferation, p53-independent apoptosis and carcinogenesis in a transgenic mouse model

Mutation or inactivation of the retinoblastoma (Rb) tumor suppressor occurs in most human tumors and results in the deregulation of several members of the E2F family of transcription factors. Among the E2F family, E2F3 has been implicated as a key regulator of cell proliferation and E2F3 gene amplification and overexpression is detected in some human tumors. To study the role of E2F3 in tumor development, we established a transgenic mouse model expressing E2F3a in a number of epithelial tissues via a keratin 5 (K5) promoter. Transgenic expression of E2F3a leads to hyperproliferation, hyperplasia and increased levels of p53-independent apoptosis in transgenic epidermis. Consistent with data from human cancers, the E2F3a transgene is found to have a weak oncogenic activity on its own and to significantly enhance the response to a skin carcinogenesis protocol. The phenotype of K5 E2F3a transgenic mice is distinct from similar transgenic mice expressing E2F1 or E2F4. In particular, E2F3a has a unique apoptotic activity and lacks the tumor suppressive property of E2F1 in this model system.