LncSIK1 enhanced the sensitivity of AML cells to retinoic acid by the E2F1/autophagy pathway.

OBJECTIVES: This study aimed to investigate the biological impacts and possible mechanisms of a novel lncRNA, LncSIK1, in AML progression and retinoic acid-regulated AML cell development. MATERIALS AND METHODS: The expression pattern of LncSIK1 was evaluated by qPCR and fluorescence in situ hybridization. CCK-8 assay, immunofluorescence, Wright-Giemsa staining, flow cytometry and Western blotting were performed to assess cell proliferation and differentiation. Bioluminescence imaging and H&E staining were used to detect AML progression in vivo. RNA or chromatin immunoprecipitation assays were conducted to measure the interaction of E2F1 and LncSIK1 or the LC3 and DRAM promoters. Autophagy was measured by transmission electron microscopy and Western blotting. RESULTS: LncSIK1 was silenced in bone marrow mononuclear cells from AML patients compared with those from healthy donors. LncSIK1 strengthened the effect of retinoic acid in inducing cell differentiation and inhibiting cell proliferation in AML cells. Moreover, the silencing of LncSIK1 was critical to maintaining AML leukaemogenesis, as LncSIK1 enhancement retarded AML progression in vivo. Mechanistically, in NB4 cells, LncSIK1 recruited the E2F1 protein to the promoters of LC3 and DRAM and induced autophagy-dependent degradation of the oncoprotein PML-RARa. However, LncSIK1 blocked E2F1 expression and the E2F1-mediated transcription of LC3 and DRAM, thereby relieving aggressive autophagy in Molm13 cells. CONCLUSIONS: Taken together, these data indicated that LncSIK1 was an important regulator of AML development through regulating the E2F1/autophagy signalling pathway.

BET Bromodomain Inhibition Blocks an AR-Repressed, E2F1-Activated Treatment-Emergent Neuroendocrine Prostate Cancer Lineage Plasticity Program.

PURPOSE: Lineage plasticity in prostate cancer-most commonly exemplified by loss of androgen receptor (AR) signaling and a switch from a luminal to alternate differentiation program-is now recognized as a treatment resistance mechanism. Lineage plasticity is a spectrum, but neuroendocrine prostate cancer (NEPC) is the most virulent example. Currently, there are limited treatments for NEPC. Moreover, the incidence of treatment-emergent NEPC (t-NEPC) is increasing in the era of novel AR inhibitors. In contradistinction to de novo NEPC, t-NEPC tumors often express the AR, but AR's functional role in t-NEPC is unknown. Furthermore, targetable factors that promote t-NEPC lineage plasticity are also unclear. EXPERIMENTAL DESIGN: Using an integrative systems biology approach, we investigated enzalutamide-resistant t-NEPC cell lines and their parental, enzalutamide-sensitive adenocarcinoma cell lines. The AR is still expressed in these t-NEPC cells, enabling us to determine the role of the AR and other key factors in regulating t-NEPC lineage plasticity. RESULTS: AR inhibition accentuates lineage plasticity in t-NEPC cells-an effect not observed in parental, enzalutamide-sensitive adenocarcinoma cells. Induction of an AR-repressed, lineage plasticity program is dependent on activation of the transcription factor E2F1 in concert with the BET bromodomain chromatin reader BRD4. BET inhibition (BETi) blocks this E2F1/BRD4-regulated program and decreases growth of t-NEPC tumor models and a subset of t-NEPC patient tumors with high activity of this program in a BETi clinical trial. CONCLUSIONS: E2F1 and BRD4 are critical for activating an AR-repressed, t-NEPC lineage plasticity program. BETi is a promising approach to block this program.

The carcinogen cadmium elevates CpG-demethylation and enrichment of NFYA and E2F1 in the promoter of oncogenic PRMT5 and EZH2 methyltransferases resulting in their elevated expression in vitro.

Cadmium (Cd) is considered as a carcinogenic chemical with potential to endanger normal cellular functioning. The present study was aimed to investigate the impact of Cd on the expression of two oncogenic epigenetic regulators, viz., protein arginine methyltransferase 5 (PRMT5) and the polycomb repressive complex 2 (PRC2) member enhancer of Zeste homolog 2 (EZH2). Our results indicate that Cd at 1 µM concentration increases the viability of HepG2 and MCF7 cells and significantly upregulates the expression of PRMT5 and EZH2, leading to an increased global level of symmetric dimethylarginine (SDMA), H4R3me2s, and H3K27me3. The luciferase reporter assay showed that the promoter activity of PRMT5 and EZH2 is significantly enhanced in both cell lines. Furthermore, Cd exposure induces global DNA hypomethylation due to a decrease in DNA methyltransferases (DNMTs) expression. Methylation-specific and bisulfite sequencing PCR reveal that the proximal promoters of PRMT5 and EZH2, which harbour CpG islands, are almost demethylated when exposed to Cd. The Cd exposure also increases the protein level of transcription factors NFYA and E2F1; consistently, the two transcription factors are found to be enriched at the PRMT5 and EZH2 promoter in chromatin immunoprecipitation experiments. The alterations induced by Cd in the two cancer cell lines were also observed in a non-cancerous cell line (HEK-293). In conclusion, we propose that Cd increases the expression of two oncogenic methyltransferases, possibly with a DNA methylation-dependent mechanism. Further studies focused on the epigenetic alterations induced by Cd would provide mechanistic insights on the carcinogenicity of this metal toxicant at the molecular level.

Inhibition of Rac1 activity induces G1/S phase arrest through the GSK3/cyclin D1 pathway in human cancer cells.

Rac1 has been shown to regulate the cell cycle in cancer cells. Yet, the related mechanism remains unclear. Thus, the present study aimed to investigate the mechanism involved in the regulation of G1/S phase transition by Rac1 in cancer cells. Inhibition of Rac1 by inhibitor NSC23766 induced G1/S phase arrest and inhibited the proliferation of A431, SW480 and U2-OS cells. Suppression of GSK3 by shRNA partially rescued G1/S phase arrest and inhibition of proliferation. Incubation of cells with NSC23766 reduced p-AKT and inactivated p-GSK3α and p-GSK3ß, increased p-cyclin D1 expression and decreased the level of cyclin D1 protein. Consequently, cyclin D1 targeting transcriptional factor E2F1 expression, which promotes G1 to S phase transition, was also reduced. In contrast, constitutive active Rac1 resulted in increased p-AKT and inactivated p-GSK3α and p-GSK3ß, decreased p-cyclin D1 expression and enhanced levels of cyclin D1 and E2F1 expression. Moreover, suppression of GSK3 did not alter p-AKT or Rac1 activity, but decreased p-cyclin D1 and increased total cyclin D1 protein. However, neither Rac1 nor GSK3 inhibition altered cyclin D1 at the RNA level. Moreover, after inhibition of Rac1 or GSK3 following proteasome inhibitor MG132 treatment, cyclin D1 expression at the protein level remained constant, indicating that Rac1 and GSK3 may regulate cyclin D1 turnover through phosphorylation and degradation. Therefore, our findings suggest that inhibition of Rac1 induces cell cycle G1/S arrest in cancer cells by regulation of the GSK3/cyclin D1 pathway.

Antiproliferative effects of artemisinin on human breast cancer cells requires the downregulated expression of the E2F1 transcription factor and loss of E2F1-target cell cycle genes.

Artemisinin, a sesquiterpene phytolactone derived from Artemisia annua, is a potent antimalarial compound with promising anticancer properties, although the mechanism of its anticancer signaling is not well understood. Artemisinin inhibited proliferation and induced a strong G1 cell cycle arrest of cultured MCF7 cells, an estrogen-responsive human breast cancer cell line that represents an early-stage cancer phenotype, and effectively inhibited the in-vivo growth of MCF7 cell-derived tumors from xenografts in athymic nude mice. Artemisinin also induced a growth arrest of tumorigenic human breast cancer cell lines with preneoplastic and late stage cancer phenotypes, but failed to arrest the growth of a nontumorigenic human mammary cell line. Concurrent with the cell cycle arrest of MCF7 cells, artemisinin selectively downregulated the transcript and protein levels of the CDK2 and CDK4 cyclin-dependent kinases, cyclin E, cyclin D1, and the E2F1 transcription factor. Analysis of CDK2 promoter-luciferase reporter constructs showed that the artemisinin ablation of CDK2 gene expression was accounted for by the loss of CDK2 promoter activity. Chromatin immunoprecipitation revealed that artemisinin inhibited E2F1 interactions with the endogenous MCF7 cell CDK2 and cyclin E promoters. Moreover, constitutive expression of exogenous E2F1 prevented the artemisinin-induced cell cycle arrest and downregulation of CDK2 and cyclin E gene expression. Taken together, our results demonstrate that the artemisinin disruption of E2F1 transcription factor expression mediates the cell cycle arrest of human breast cancer cells and represents a critical transcriptional pathway by which artemisinin controls human reproductive cancer cell growth.

Methylated chrysin reduced cell proliferation, but antagonized cytotoxicity of other anticancer drugs in acute lymphoblastic leukemia.

The efficacy of 5,7-dimethoxyflavone (DMF), a methylated analog of chrysin, as a therapeutic agent to treat acute lymphoblastic leukemia (ALL) was investigated. Using a panel of ALL cell lines, the IC50 (half-maximal inhibitory concentration) of DMF varied between 2.8 and 7.0 µg/ml. DMF induced G0/G1 cell cycle arrest, concomitant with a decreased expression of phosphorylated retinoblastoma-associated protein 1. DMF increased the rate of apoptosis, although it was apparent only after a long period of exposure (96 h). The accumulation of oxidative stress was not involved in the growth-inhibitory effects of DMF. As DMF reduced the intracellular levels of glutathione, the combination effects of DMF with other anticancer drugs were evaluated using the improved Isobologram and the combination index method. In the simultaneous drug combination assay, DMF antagonized the cytotoxicity of 4-hydroperoxy-cyclophosphamide, cytarabine, vincristine, and L-asparaginase in all tested ALL cells. This study demonstrated that DMF, a methylated flavone, was an effective chemotherapy agent that could inhibit cell cycle arrest and induce apoptosis in ALL cell lines. However, combination therapy with DMF and other anticancer drugs is not recommended.

Estrogen receptor regulates E2F1 expression to mediate tamoxifen resistance.

Antiestrogen resistance often develops with prolonged exposure to hormone therapies, including tamoxifen, and is a major problem in the treatment of breast cancer. Understanding the mechanisms involved in the development of antiestrogen resistance is an important step in the development of new targeted therapies. Two forms of antiestrogen resistance exist: de novo resistance and acquired resistance. To mimic acquired resistance, we have established a tamoxifen-resistant breast cancer cell line (MCF-7TamR) by treating parental MCF-7 cells with tamoxifen over a period of 6 months to select for cells with the resistant phenotype. Characterization of the MCF-7TamR cells under normal, hormone-deprived, and tamoxifen-treated conditions suggests that these cells continue to grow in the presence of tamoxifen. Additionally, a greater percentage of resistant cells enter the S phase under tamoxifen conditions, compared with parental MCF-7 cells. Consistent with these growth results, molecular analysis indicates that tamoxifen-resistant cells express higher levels of cyclin E1, cdk2, ACTR, and E2F1. Faslodex or ICI 182, 780 (ICI)-mediated degradation of estrogen receptor (ER) reduced the proliferation of these cells, as well as the level of E2F1 expression in tamoxifen-resistant cells, suggesting that tamoxifen resistance and E2F1 expression are in part dependent on ER. We further showed that tamoxifen enhances the ERalpha/Sp-1 interaction and promotes the recruitment of ERalpha and Sp-1 to the proximal promoter of E2F1 in resistant cells. Collectively, our findings suggest that tamoxifen resistance is a result of increased ERalpha/Sp-1 interaction, which enhances the expression of E2F1 to promote tamoxifen resistance.

E2F3 is a mediator of DNA damage-induced apoptosis.

The E2F transcription factors have emerged as critical apoptotic effectors. Herein we report that the E2F family member E2F3a can be induced by DNA damage through transcriptional and posttranslational mechanisms. We demonstrate that the posttranslational induction of human E2F3a is dependent on the checkpoint kinases. Moreover, we show that human E2F3a is a substrate for the checkpoint kinases (chk kinases) and that mutation of the chk phosphorylation site eliminates the DNA damage inducibility of the protein. Furthermore, we demonstrate that E2F1 and E2F2 are transcriptionally induced by DNA damage in an E2f3-dependent manner. Finally, using both in vitro and in vivo approaches, we establish that E2f3 is required for DNA damage-induced apoptosis. Thus, our data reveal the novel ability of E2f3 to function as a master regulator of the DNA damage response.

Delta 9-tetrahydrocannabinol inhibits cell cycle progression by downregulation of E2F1 in human glioblastoma multiforme cells.

BACKGROUND: The active components of Cannabis sativa L., Cannabinoids, traditionally used in the field of cancer for alleviation of pain, nausea, wasting and improvement of well-being have received renewed interest in recent years due to their diverse pharmacologic activities such as cell growth inhibition, anti-inflammatory activity and induction of tumor regression. Here we used several experimental approaches, which identified delta-9-tetrahydrocannabinol (Delta(9)-THC) as an essential mediator of cannabinoid antitumoral action. METHODS AND RESULTS: Administration of Delta(9)-THC to glioblastoma multiforme (GBM) cell lines results in a significant decrease in cell viability. Cell cycle analysis showed G(0/1) arrest and did not reveal occurrence of apoptosis in the absence of any sub-G(1) populations. Western blot analyses revealed a THC altered cellular content of proteins that regulate cell progression through the cell cycle. The cell content of E2F1 and Cyclin A, two proteins that promote cell cycle progression, were suppressed in both U251-MG and U87-MG human glioblastoma cell lines, whereas the level of p16(INK4A), a cell cycle inhibitor was upregulated. Transcription of thymidylate synthase (TS) mRNA, which is promoted by E2F1, also declined as evident by QRT-PCR. The decrease in E2F1 levels resulted from proteasome mediated degradation and was prevented by proteasome inhibitors. CONCLUSIONS: Delta(9)-THC is shown to significantly affect viability of GBM cells via a mechanism that appears to elicit G(1) arrest due to downregulation of E2F1 and Cyclin A. Hence, it is suggested that Delta(9)-THC and other cannabinoids be implemented in future clinical evaluation as a therapeutic modality for brain tumors.

Inhibition of ataxia telangiectasia-p53-E2F-1 pathway in neurons as a target for the prevention of neuronal apoptosis.

Over the last few decades, understanding of the mechanisms involved in the process of neuronal cell death has grown. Recent findings have established that DNA damage, and specifically ataxia telangiectasia mutated protein (ATM), is key to the cascade of regulation of neuronal apoptosis. Another characteristic common to all neurodegenerative diseases is oxidative stress. Likewise, a common feature in the brain of patients with neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and other neurological disorders is the expression of proteins involved in cell-cycle control. In the process of re-entry in the cell cycle, an additional component, transcription factor E2F-1, also involved in the regulation of apoptosis, is expressed. Finally, in this complex puzzle, mitochondrial activation with the release of proteins and the activation of cystein proteases, specifically caspase-3, is prominent in the last step of neuronal apoptosis. This review focuses on the role of ATM activation and its re-entry into the cell cycle in neurons as a potential target for the prevention of neuronal apoptosis. We suggest the mechanisms by which ATM and E2F-1 orchestrate the apoptotic process. Among them, p53 could be a common point on this apoptotic route. Finally, we put forward drugs that are now being studied experimentally, such as p53 inhibitors, ATM inhibitors and cyclin-dependent kinase (CDKs) inhibitors, for the treatment of neurodegenerative diseases.

The MDM-2 antagonist nutlin-3 promotes the maturation of acute myeloid leukemic blasts.

The small-molecule inhibitor of murine double minute (MDM-2), Nutlin-3, induced variable apoptosis in primary acute myeloid leukemia (AML) blasts and promoted myeloid maturation of surviving cells, as demonstrated by analysis of CD11b and CD14 surface antigens and by morphologic examination. Although the best-characterized activity of Nutlin-3 is activation of the p53 pathway, Nutlin-3 induced maturation also in one AML sample characterized by p53 deletion, as well as in the p53(-/-) human myeloblastic HL-60 cell line. At the molecular level, the maturational activity of Nutlin-3 in HL-60 cells was accompanied by the induction of E2F1 transcription factor, and it was significantly counteracted by specific gene knockdown with small interfering RNA for E2F1. Moreover, Nutlin-3, as well as tumor necrosis factor (TNF) alpha, potentiated the maturational activity of recombinant TNF-related apoptosis-inducing ligand (TRAIL) in HL-60 cells. However, although TNF-alpha significantly counteracted the proapoptotic activity of TRAIL, Nutlin-3 did not interfere with the proapoptotic activity of TRAIL. Taken together, these data disclose a novel, potentially relevant therapeutic role for Nutlin-3 in the treatment of both p53 wild-type and p53(-/-) AML, possibly in association with recombinant TRAIL.

Transcriptional upregulation of p57 (Kip2) by the cyclin-dependent kinase inhibitor BMS-387032 is E2F dependent and serves as a negative feedback loop limiting cytotoxicity.

In spite of the fact that cyclin-dependent kinase (cdk) inhibiting drugs are potent transcriptional repressors, we discover that p57 (Kip2, CDKN1C) transcription is significantly upregulated by three small molecule cdk inhibitors, including BMS-387032. Treatment of MDA-MB-231 breast cancer cells with BMS-387032 led to a stabilization of the E2F1 protein that was accompanied by significant increases in the p57 mRNA and protein. This increase did not occur in an E2F1-deficient cell line. An E2F1-estrogen receptor fusion protein activated the endogenous p57 promoter in response to hydroxytamoxifen treatment in the presence of cycloheximide. Luciferase constructs driven by the p57 promoter verified that upregulation of p57 mRNA by BMS-387032 is transcriptional and dependent on E2F-binding sites in the promoter. Expression of exogenous p57 significantly decreased the fraction of cells in S phase. Furthermore, p57-deficient MDA-MB-231 cell lines were significantly more sensitive to BMS-387032-induced apoptosis than controls. The results presented in this manuscript demonstrate that small molecule cdk inhibitors transcriptionally activate p57 dependent upon E2F1 and that this activation in turn serves to limit E2F1's death-inducing activity.

Nucleophosmin/B23 regulates transcriptional activation of E2F1 via modulating the promoter binding of NF-kappaB, E2F1 and pRB.

Expression of nucleophosmin/B23 and E2F1 and E2F1-dependent transcription increased in U1 bladder cancer cells upon serum stimulation from quiescence. Nucleophosmin/B23-siRNA treatment abrogated such increase of E2F1-dependent transcriptional activity. In identifying physiologically important factors that may occupy E2F1 promoter and regulate its activity in vivo, we found that the pattern of NF-kappaB, E2F1 and pRB recruitment to E2F1 promoter changed in a strikingly dynamic fashion as cells progressed from quiescence into serum-stimulated growth. E2F1 promoter activity in quiescent cells was associated with recruitment of NF-kappaB. NF-kappaB was replaced largely by E2F1 in concert with gene activation during the early stage (12 h) of serum stimulation. At late stage (24 h) of serum stimulation, pRB was then recruited to the E2F1-promoter complex to counterbalance its activity. Upon siRNA-mediated reduction of intracellular nucleophosmin/B23, E2F1 and pRB were recruited to the promoter with the dissociation of NF-kappaB concomitant with gene inactivation. Based on immunoprecipitation experiments, nucleophosmin/B23 was found to be associated with NF-kappaB in cells grown in serum-supplemented but not in serum-deprived medium. Furthermore, nucleophosmin/B23 could also be co-immunoprecipitated with ppRB at the early stage (12 h) but not at the late stage (24 h) of serum stimulation. The results demonstrate a novel mechanism for transcriptional regulation of E2F1 and identify the functional role of nucleophosmin/B23 in modulating the binding of NF-kappaB, E2F1 and pRB to activate E2F1 promoter.