Nicotine-mediated induction of E-selectin in aortic endothelial cells requires Src kinase and E2F1 transcriptional activity

Smoking is highly correlated with enhanced likelihood of atherosclerosis by inducing endothelial dysfunction. In endothelial cells, various cell-adhesion molecules including E-selectin, are shown to be upregulated upon exposure to nicotine, the addictive component of tobacco smoke; however, the molecular mechanisms underlying this induction are poorly understood. Here we demonstrate that nicotine-induced E-selectin transcription in human aortic endothelial cells (HAECs) could be significantly blocked by α7-nAChR subunit inhibitor, α-BT, Src-kinase inhibitor, PP2, or siRNAs against Src or β-Arrestin-1 (β-Arr1). Further, chromatin immunoprecipitations show that E-selectin is an E2F1 responsive gene and nicotine stimulation results in increased recruitment of E2F1 on E-selectin promoter. Inhibiting E2F1 activity using RRD-251, a disruptor of the Rb-Raf-1 kinase interaction, could significantly inhibit the nicotine-induced recruitment of E2F1 to the E-selectin promoter as well as E-selectin expression. Interestingly, stimulation of HAECs with nicotine results in increased adhesion of U937 monocytic cells to HAECs and could be inhibited by pre-treatment with RRD-251. Similarly, depletion of E2F1 or Src using RNAi blocked the increased adhesion of monocytes to nicotine-stimulated HAECs. These results suggest that nicotine-stimulated adhesion of monocytes to endothelial cells is dependent on the activation of α7-nAChRs, β-Arr1 and cSrc regulated increase in E2F1-mediated transcription of E-selectin gene. Therefore, agents such as RRD-251 that can target activity of E2F1 may have potential therapeutic benefit against cigarette smoke induced atherosclerosis.

TNF-α-mediated proliferation of vascular smooth muscle cells involves Raf-1-mediated inactivation of Rb and transcription of E2F1-regulated genes

Atherosclerosis is characterized by hyperplastic neointima and an inflammatory response with cytokines such as TNFα. TNFα is a pleiotropic cytokine that mediates inflammatory, proliferative, cytostatic and cytotoxic effects in a variety of cell types, including endothelial cells and vascular smooth muscle cells (VSMCs). Interestingly, TNFα has been shown to play two very opposing roles in these cell types; it inhibits proliferation and induces apoptosis in endothelial cells, while it enhances the proliferation and migration of VSMCs. Here we show that TNFα is capable of stimulating proliferation of rat VSMCs as well as human VSMCs in a Raf-1/MAP K-dependent manner. TNFα could increase the expression of E2F-regulated proliferative cdc6, Thymidylate synthase (TS) and cdc25A genes in Aortic smooth muscle cells (AoSMC), as seen by real time PCR assays. There is an activation of the stress-induced kinase, JNK1, in VSMCs upon TNFα stimulation. TNFα was capable of inducing binding of the Raf-1 kinase to Rb, and treatment with the Rb-Raf-1 inhibitor, RRD-251, could prevent TNFα-induced S-phase entry in AoSMCs. In addition, inhibition of Raf-1 or Src kinases using pharmacologic inhibitors could also prevent S-phase entry, while inhibition of JNK was not as effective. These results suggest that inhibiting the Rb-Raf-1 interaction is a potential avenue to prevent VSMC proliferation associated with atherosclerosis.

Regulation of matrix metalloproteinase genes by E2F transcription factors: Rb-Raf-1 interaction as a novel target for metastatic disease

The retinoblastoma (Rb)-E2F transcriptional regulatory pathway plays a major role in cell-cycle regulation, but its role in invasion and metastasis is less well understood. We find that many genes involved in the invasion of cancer cells, such as matrix metalloproteinases (MMP), have potential E2F-binding sites in their promoters. E2F-binding sites were predicted on all 23 human MMP gene promoters, many of which harbored multiple E2F-binding sites. Studies presented here show that MMP genes such as MMP9, MMP14, and MMP15 which are overexpressed in non-small cell lung cancer, have multiple E2F-binding sites and are regulated by the Rb-E2F pathway. Chromatin immunoprecipitation assays showed the association of E2F1 with the MMP9, MMP14, and MMP15 promoters, and transient transfection experiments showed that these promoters are E2F responsive. Correspondingly, depletion of E2F family members by RNA interference techniques reduced the expression of these genes with a corresponding reduction in collagen degradation activity. Furthermore, activating Rb by inhibiting the interaction of Raf-1 with Rb by using the Rb-Raf-1 disruptor RRD-251 was sufficient to inhibit MMP transcription. This led to reduced invasion and migration of cancer cells in vitro and metastatic foci development in a tail vein lung metastasis model in mice. These results suggest that E2F transcription factors may play a role in promoting metastasis through regulation of MMP genes and that targeting the Rb-Raf-1 interaction is a promising approach for the treatment of metastatic disease.

Abstract 1972: Nicotine regulates DNA-binding protein inhibitor (Id1) through a Src-dependent pathway promoting tumorigenic properties and chemoresistance in pancreatic adenocarcinoma

Nicotine, an addicting component of an established risk factor (tobacco) for pancreatic cancer is identified as a tumor promoter potentially acting through activation of oncogenes such as Src, a non-receptor protein tyrosine kinase. Recent studies from our laboratory have shown that the Id1 transcription factor (inhibitor of DNA binding/Differentiation) is induced by nicotine in NSCLC cells. Id1 has been shown to play a role in tumor progression by promoting angiogenesis, invasion, and chemoresistance. Further, Src and its family of non-receptor protein tyrosine kinases (SFKs) play a role in angiogenesis, tumor progression, and chemoresistance. Based on our observations on NSCLC cells, we hypothesize nicotine-induces Id1 expression in a Src-dependent fashion and promotes tumorigenesis and chemoresistance. Protein analysis, mRNA expression, and further experiments were determined in a panel of pancreatic cancer cells, PANC-1, Mia-Paca-2, L3.6pl, and L3.6plGemRes by western blot analysis and RT-PCR respectively. Our results demonstrate that stimulation with nicotine (1 µM), and activation of nAChR, resulted in phosphorylation of Src, indicative of activation, and its family of kinases. Nicotine also induced Id1 expression at similar concentrations. Inhibition of SFK activity with Dasatinib (500nM) did not allow serum or nicotine-induced SFK activity and subsequent Id1 expression. Specific inhibition of c-Src expression with short-interfering RNA (siRNA) decreased constitutive and nicotine-induced Id1 expression and depletion of Id1 expression by siRNA or short-hairpin RNA (shRNA) significantly reduced the proliferative (80%) and invasive property (75%) of pancreatic cancer cells, a property often mediated by Src. Additionally, inhibition of Src and Id1 expression in innate or established gemcitabine resistant cancer cells resulted in gemcitabine sensitization at IC50, 250 ng/ml. To determine if nicotine contributes to gemcitabine chemoresistance potentially through a Src-dependent Id1 signaling axis, we exposed gemcitabine-sensitive cells to nicotine with subsequent exposure to gemcitabine. Nicotine promoted a 2-fold increase in cell proliferation and a 4.5-fold reduction in apoptosis. Further, nicotine induced phosphorylation of key Src-dependent signaling enzymes involved in proliferation and apoptosis, Erk1/2 and Akt respectively. We demonstrate that Id1, by a nicotine promoting Src-dependent pathway, contributes to signaling pathways involved in tumor progression and establishment of a chemoresistant phenotype in pancreatic cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1972. doi:10.1158/1538-7445.AM2011-1972

Abstract 2590: Selective disruption of Rb-Raf-1 kinase interaction is a suitable therapeutic option for pancreatic adenocarcinoma

Retinoblastoma tumor suppressor binding protein, Rb, is a major regulator of the mammalian cell cycle progression. Inactivation of Rb by a cascade of phosphorylation events leads to its inactivation, facilitating release of transcriptionally active E2F and S-phase entry. Rb phosphorylaiton is mediated mainly by CDK, but it was observed Raf-1 could bind and phosphorylate Rb early in the cell cycle, facilitating phosphorylation events. Although previous reports have suggested that K-Ras, which is mutated in over 80% of pancreatic cancers, activates MAPK through Raf/Mek signaling, inhibition of this pathway have proven clinically unsuccessful. We demonstrated that increased binding of Raf-1 to Rb, independent of the Mek/MAPK pathway, has contributed to tumor progression and inhibition of this interaction might be a suitable strategy for cancer therapies. Therefore, we examined whether targeting Rb-Raf-1 interaction with a small molecule inhibitor RRD-251 is a viable strategy against pancreatic cancer. To test our hypothesis, we initially probed a panel of pancreatic cancer cell lines (PANC-1, MiaPaCa-2, CD18/HPAF, L3.6pl) and demonstrated the disruption of Rb-Raf-1 interaction by RRD-251, using immunoprecipitation western blot experiments. To fully define the anti-tumor effects of RRD-251, cell cycle analyses, proliferation/cell viability (MTT), cell migration (wound healing assay), and invasion (Boyden chamber assay) experiments were performed. Animal studies were performed in nude mice with subcutaneous and orthotopic pancreas models with intraperitoneal injections of RRD-251 at 50 mpk/daily. RRD-251 significantly reduced proliferation by 3-fold and significantly reduced the ability of metastatic variant, L3.6pl, and other pancreatic cancer cell lines to migrate; there was also a a 5-fold decrease in invasion (p<0.05). Prolonged exposure of RRD-251 (48 hours) demonstrated that disruption of the Rb-Raf-1 interaction would induce apoptosis as defined by cleavage of PARP protein, decreased anti-apoptotic proteins Mcl-1 and Bcl-2, and an increase in pro-apoptotic protein Bax. Additionally, combination of RRD-251 with gemcitabine (125ng/ml) induced a synergistic effect on induction of apoptosis, proliferation/cell viability (7.5 fold decrease), migration, and almost complete loss of the invasive capacity of pancreatic cancer cells (p<0.05). In vivo, RRD-251 significantly abrogated primary tumor growth after subcutaneous and orthotopic injections of metastatic variant, L3.6pl. Additionally, there was a significant reduction (>80%) of liver metastasis in the RRD-251 treated group. In this study, we demonstrate a disruption of the Rb-Raf-1 kinase interaction with RRD-251 significantly affects the malignant properties of pancreatic cancer cells. Treatment with combination RRD-251 and gemcitabine appears to be a viable strategy against pancreatic cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2590. doi:10.1158/1538-7445.AM2011-2590

Abstract 5048: Targeting regulators of Chk1 to enhance cytotoxic efficacy of HDAC inhibitors

Introduction: Predicting and overcoming drug resistance is a major difficulty facing the efficacy of drugs in clinical trials. Histone deacetylase inhibitors (HDACis) are known to induce cell death and have been used in clinic trials as anticancer drugs, however, the cytotoxic effects of HDACis remains to be limited with no clear mechanism of resistance has been elucidated to explain this limited efficacy. In this study we showed that downregulation of the G2 checkpoint kinase 1 (Chk1) plays an essential role in inducing cell death by HDACis. We identified E2F as a main regulator of Chk1 expression in cancer cells, suggesting that inhibition of Chk1 through E2F may enhance the cytotoxic effects of HDACis in clinical studies.

Methods: A549, H1299, PC9, HCT116, HT1080, MNNG, H2172, Saos-2, MG63, U2OS, p21-/- and p21+/+ MEF cell lines were grown in appropriate medium and treated with the HDAC inhibitors, SaHa, LBH589 or MS275. Western blot and qRT-PCR experiments were performed to assess the effect of HDACis on Chk1/G2-M checkpoint pathways and on E2F-mediated gene expression profiles. Additionally, knock out and gain of function experiments were done to better understand the interaction between Chk1, E2F and p21Waf1 in HDACi mediated cell death. The results of the in vitro cell lines studies were corroborated in the ex vivo experiments with patient tumor tissue.

Results: We showed that HDACi treatment leads to inhibition of Chk1 expression and consequently to increased activity of CDC25 and CDC2 (CDK1) enzymes causing premature mitotic entry and cell death. In the knock-down experiments we found that inhibition of E2F1 or E2F3 leads to reduction of Chk1 expression and potentiation of cell death by HDACis. Similar results were observed using patient-derived tumor samples treated with SaHa and an E2F inhibitor (HLM006474). Additionally, over-expression of E2F1 or E2F3 dramatically decreased HDACi-mediated cell death. In time course experiments, Chk1 as well as other E2F-responsive genes were down regulated after HDACi treatment accompanied by an expected p21Waf1 induction. In p21Waf1 +/+ and -/- MEFs we showed that lack of p21Waf1 results in increased apoptosis, suggesting that early induction of p21Waf1 by HDACis negatively regulates HDACi-induced cell death, which likely contributes to HDACi resistance in tumor cells by preventing cells to undergo mitotic cell death.

Conclusions: Our results demonstrate that E2F-mediated downregulation of Chk1 plays a key role in HDACi-induced cell. We provide evidence that lack of Chk1 downregulation and activation of CDC2 is associated with resistance to HDACis indicating that Chk1 may represent a clinically relevant biomarker to assess the efficacy of HDACis in patient tumor samples. Taken together our data lends insight into the molecular mechanism of sensitivity and resistance to HDACi-induced cell death and will hopefully provide a stronger foundation for future clinical translation with HDACis.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5048. doi:10.1158/1538-7445.AM2011-5048

ARRB1-mediated regulation of E2F target genes in nicotine-induced growth of lung tumors

BACKGROUND

Nicotine induces the proliferation of non-small cell lung cancer (NSCLC) cells via nicotinic acetylcholine receptors and the arrestin, β1 (ARRB1) protein. However, whether ARRB1 translocates to the nucleus upon nicotinic acetylcholine receptor activation and how it regulates growth of human NSCLCs are not known.

METHODS

We investigated nuclear localization of ARRB1 in human NSCLC cell lines (A549 and H1650), normal lung cell lines (NHBE and SAEC), and lung cancer tissue microarray. A549 cells were transfected with ARRB1-specific short hairpin RNA (A549-sh) to knockdown ARRB1 expression, or with empty vector (A549-EV), to examine the role of ARRB1 in the mitogenic and antiapoptotic effects of nicotine, binding of ARRB1 to E2F transcription factors, and the role of ARRB1 in nicotine-induced expression of E2F-regulated survival and proliferative genes cell division cycle 6 homolog (CDC6), thymidylate synthetase (TYMS), and baculoviral IAP repeat-containing 5 (BIRC5). Real-time polymerase chain reaction was performed for quantitative analysis of mRNA expression. Chromatin immunoprecipitation assays were performed on A549 cells and fresh-frozen human NSCLC tumors (n = 8) to examine the binding of ARRB1, E1A binding protein (EP300), and acetylated histone 3 (Ac-H3) on the E2F-regulated genes. All statistical tests were two-sided.

RESULTS

Nicotine induced the nuclear translocation of ARRB1 in NSCLC and normal lung cells, and lung tumor tissues from smokers showed an increased nuclear localization. The mitogenic and antiapoptotic effects of nicotine were reduced in A549-sh cells. Nuclear ARRB1 bound to E2F transcription factors in normal lung cells, NSCLC cells, and tumors. Nicotine treatment induced a statistically significant increased expression of E2F-regulated genes in A549-EV but not in A549-sh cells; the maximum difference being observed in BIRC5 (A549-EV vs A549-sh, mean fold-increase in mRNA level upon nicotine treatment = 20.7-fold, 95% confidence interval = 19.2- to 22.2-fold, vs mean = 0.8-fold, 95% confidence interval= 0.78- to 0.82-fold, P < .001). Furthermore, nicotine induced the binding of ARRB1, EP300, and Ac-H3 on E2F-regulated genes.

CONCLUSION

Nicotine induced the nuclear translocation of ARRB1 and showed increased expression of proliferative and survival genes, thereby contributing to the growth and progression of NSCLCs.

Abstract 2948: Nicotine-induced expression of MUC4 in pancreatic cancer cells is mediated by E2F1 and STAT-1 upregulation: Relevance to smoking and pancreatic cancer

The membrane-bound mucins belong to an ever-increasing family of O-glycoproteins. Based on their structure and localization at the cell surface they are thought to play important biological roles in cell-cell and cell-matrix interactions and cell signaling. The human mucin gene MUC4 is overexpressed in pancreatic cancer and cancer cell lines, while remaining undetectable in the normal pancreas, indicating its potential role in pancreatic cancer pathogenesis. Smoking has been correlated with pancreatic cancer, even though the underlying molecular mechanisms are unknown. Attempts were made to elucidate the molecular mechanisms involved in the regulation of MUC4 gene expression by nicotine as well as cytokines. In this report, we used pancreatic cancer cell lines CD18 as well as PANC1 stimulated with nicotine to identify the signaling pathways and transcription factors regulating MUC4 expression. Through chromatin immunoprecipiation assay and real-time PCR, we found that transcription factors E2F1 and STAT1 can positively regulate MUC4 expression. Additionally, the effect is abrogated by short interfering RNA (siRNA)-mediated inhibition of E2F1 and STAT-1 expression, supporting pivotal role played by E2F1 and STAT-1 in MUC4 regulation. The same molecules were responsible for MUC4 induction by nicotine in combination with IFN- and RA. Inhibiting the acetylacholinergic receptors with inhibitors reversed the effects of nicotine stimulation. Similarly, the induction was dependent on the PI3 kinase as well as Src signaling pathways. Additionally, nicotine treatment alone and in combination with IFN- and RA activates and relocalizes signal transducer and activator of transcription-1 (STAT-1) via phosphorylating Jak2 at the tyrosine residue to induce MUC4. These results suggest that nicotine alone and also in combination with IFN- and RA, upregulates the MUC4 expression involving JAK-STAT pathway. In conclusion, our findings identify novel mechanisms for MUC4 regulation in pancreatic cancer cells and will be helpful understanding the transcriptional regulation of MUC4 in response to exposure to nicotine and tobacco smoke.

Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 2948.

CDKN1C negatively regulates RNA polymerase II C-terminal domain phosphorylation in an E2F1-dependent manner

CDKN1C is a cyclin-dependent kinase inhibitor and is a candidate tumor suppressor gene. We previously found that the CDKN1C protein represses E2F1-driven transcription in an apparent negative feedback loop. Herein, we explore the mechanism by which CDKN1C represses transcription. We find that adenoviral-mediated overexpression of CDKN1C leads to a dramatic reduction in phosphorylation of the RNA polymerase II (pol II) C-terminal domain (CTD). RNA interference studies demonstrate that this activity is not an artifact of CDKN1C overexpression, because endogenous CDKN1C mediates an inhibition of RNA pol II CTD phosphorylation in HeLa cells upon treatment with dexamethasone. Surprisingly, we find that CDKN1C-mediated repression of RNA pol II phosphorylation is E2F1-dependent, suggesting that E2F1 may direct CDKN1C to chromatin. Chromatin immunoprecipitation assays demonstrate that CDKN1C is associated with E2F1-regulated promoters in vivo and that this association can dramatically reduce the level of RNA pol II CTD phosphorylation at both Ser-2 and Ser-5 of the C-terminal domain repeat. In addition, we show that CDKN1C interacts with both CDK7 and CDK9 (putative RNA pol II CTD kinases) and that CDKN1C blocks their ability to phosphorylate a glutathione S-transferase-CTD fusion protein in vitro. E2F1 and CDKN1C are found to form stable complexes both in vivo and in vitro. Molecular studies demonstrate that the E2F1-CDKN1C interaction is mediated by two E2F domains. A central E2F1 domain interacts directly with CDKN1C, whereas a C-terminal E2F1 domain interacts with CDKN1C via interaction with Rb. The results presented in this report highlight a novel mechanism of tumor suppression by CDKN1C.

Chromatin immunoprecipitation assays: analyzing transcription factor binding and histone modifications in vivo

Studies in the past decade have shown that differential gene expression depends not only on the binding of specific transcription factors to discrete promoter elements but also on the epigenetic modification of the DNA as well as histones associated with the promoter. While techniques like electrophoretic mobility shift assays could detect and characterize the binding of specific transcription factors present in cell lysates to DNA sequences in in vitro binding conditions, they were not effective in assessing the binding in intact cells. Development of chromatin immunoprecipitation technique in the past decade enabled the analysis of the association of regulatory molecules with specific promoters or changes in histone modifications in vivo, without overexpressing any component. ChIP assays can provide a snapshot of how a regulatory transcription factor affects the expression of a single gene or a variety of genes at the same time. Availability of high-quality antibodies that recognizes histones modified in a specific fashion further expanded the use of ChIP assays to analyze even minute changes in histone modification and nucleosomes structure. This chapter outlines the general strategies and protocols used to carry out ChIP assays to study the differential recruitment of transcription factors as well as histone modifications.

In vitro replication assay with mammalian cell extracts

Regulatory mechanisms for DNA replication are crucial to the control of the cell cycle in eukaryotic cells. One of the widely used assays to understand the complex mammalian replication system is the cell-free in vitro replication assay (IVRA). IVRA can provide a snapshot of the regulatory mechanisms controlling replication in higher eukaryotes by using a single plasmid, pEPI-1. This chapter outlines the general strategies and protocols used to perform IVRA to study the differential recruitment of replication factors either independently or in combination, based on the experience in studying the role of prohibitin in replication as well as other published protocols. This method can be employed to identify not only proteins that assist replication but also proteins that inhibit replication of mammalian genome.

Chromatin immunoprecipitation assays: molecular analysis of chromatin modification and gene regulation

Gene expression pattern in cancer cells differ significantly from their normal counter parts, owing to mutations in oncogenes and tumor suppressor genes, their downstream targets, or owing to increased proliferation, and altered apoptotic potential. Various microarray based techniques have been widely utilized to study the differential expression of genes in cancer in recent years. Along with this, attempts have been made to study the transcriptional regulatory mechanisms and chromatin modifications facilitating such differential gene expression. One of the widely used assays for this purpose is the chromatin immunoprecipitation (ChIP) assay, which enables the analysis of the association of regulatory molecules with specific promoters or changes in histone modifications in vivo, without overexpressing any component. This has been of immense value, because ChIP assays can provide a snapshot of the regulatory mechanisms involved in the expression of a single gene, or a variety of genes at the same time. This review article outlines the general strategies and protocols used to carry out ChIP assays to study the differential recruitment of transcription factors, based on the experience in studying E2F1 and histone modifications as well as other published protocols. In addition, the use of ChIP assays to carry out global analysis of transcription factor recruitment is also addressed.

Nicotine-mediated cell proliferation and angiogenesis: new twists to an old story

Tobacco smoking is one of the major etiologic factors associated with cancer. While there are many carcinogenic compounds present in tobacco smoke, its main addictive component, nicotine, is not carcinogenic by itself. The addictive properties of nicotine are achieved through the nicotinic acetylcholine receptors (nAChRs) that are widely distributed in the brain and neuromuscular junctions; at the same time, they were found to be expressed in a variety of non-neuronal tissues in the body including those of the lung. Recent studies show that these non-neuronal nAChRs can induce cell proliferation and angiogenesis. Analysis of the molecular mechanisms underlying nicotine-mediated cell proliferation showed the involvement of Src kinase and the scaffolding protein beta-arrestin-1. Further, nAChRs were found to activate the basic components of the cell cycle machinery similar to growth factor receptors. This involved increased binding of Raf-1 kinase to the Rb protein, activation of cyclins D and E as well as induction of proliferative promoters. This article describes pathway involved in nicotine-induced cell proliferation and angiogenesis and the potential steps that are amenable for developing novel anti-cancer therapies.

Regulation of neuron survival and death by p130 and associated chromatin modifiers

E2F-mediated gene repression plays a key role in regulation of neuron survival and death. However, the key molecules involved in such regulation and the mechanisms by which they respond to apoptotic stimuli are largely unknown. Here we show that p130 is the predominant Rb family member associated with E2F in neurons, that its major partner for repression of pro-apoptotic genes is E2F4, and that the p130-E2F4 complex recruits the chromatin modifiers HDAC1 and Suv39H1 to promote gene silencing and neuron survival. Apoptotic stimuli induce neuron death by sequentially causing p130 hyperphosphorylation, dissociation of p130-E2F4-Suv39H1-HDAC complexes, altered modification of H3 histone and gene derepression. Experimental suppression of such events blocks neuron death while interference with the synthesis of E2F4 or p130, or with the interaction of E2F4-p130 with chromatin modifiers, induces neuron death. Thus, neuron survival and death are dependent on the integrity of E2F4-p130-HDAC/Suv39H1 complexes.

Disruption of the Rb--Raf-1 interaction inhibits tumor growth and angiogenesis

The retinoblastoma tumor suppressor protein (Rb) plays a vital role in regulating mammalian cell cycle progression and inactivation of Rb is necessary for entry into S phase. Rb is inactivated by phosphorylation upon growth factor stimulation of quiescent cells, facilitating the transition from G(1) phase to S phase. Although the signaling events after growth factor stimulation have been well characterized, it is not yet clear how these signals contact the cell cycle machinery. We had found previously that growth factor stimulation of quiescent cells lead to the direct binding of Raf-1 kinase to Rb, leading to its inactivation. Here we show that the Rb-Raf-1 interaction occurs prior to the activation of cyclin and/or cyclin-dependent kinases and facilitates normal cell cycle progression. Raf-1-mediated inactivation of Rb is independent of the mitogen-activated protein kinase cascade, as well as cyclin-dependent kinases. Binding of Raf-1 seemed to correlate with the dissociation of the chromatin remodeling protein Brg1 from Rb. Disruption of the Rb-Raf-1 interaction by a nine-amino-acid peptide inhibits Rb phosphorylation, cell proliferation, and vascular endothelial growth factor-mediated capillary tubule formation. Delivery of this peptide by a carrier molecule led to a 79% reduction in tumor volume and a 57% reduction in microvessel formation in nude mice. It appears that Raf-1 links mitogenic signaling to Rb and that disruption of this interaction could aid in controlling proliferative disorders.

A putative coiled-coil domain of prohibitin is sufficient to repress E2F1-mediated transcription and induce apoptosis

Prohibitin is a potential tumor suppressor protein that can repress E2F-mediated transcription and arrest cell proliferation. We had shown previously that prohibitin could bind to the Rb protein as well as E2F and this binding was necessary to suppress cell proliferation. Here we show that the E2F1 binding domain of prohibitin has the potential to fold into a coiled-coil structure. This coiled-coil domain by itself could physically interact with E2F1 and block its transcriptional activity. Like full-length prohibitin, the coiled-coil domain also recruited histone deacetylase 1 to repress E2F1. The coiled-coil domain also exhibited growth suppressive properties and we observed a 64% reduction in colony numbers when transfected into T47D cells. Interestingly, a synthetic peptide corresponding to the coiled-coil domain induced apoptosis in four different human cell lines. It is possible that agents that can mimic this peptide would be of value in controlling proliferative disorders.

Prohibitin co-localizes with Rb in the nucleus and recruits N-CoR and HDAC1 for transcriptional repression

The potential tumor suppressor protein prohibitin can prevent cell proliferation and this required its binding to the Rb protein. Prohibitin could repress the transcriptional activity of E2F family members and this required a part of the marked box region of E2F. The sub-cellular localization of prohibitin has been variously attributed to the mitochondria as well as the inner cell membrane. Here we show that a subset of prohibitin molecules are present in the nucleus where it co-localizes with the Rb protein. Deletion of a putative amino-terminal membrane-docking domain of prohibitin had no effect on its ability to suppress cell proliferation or inhibit E2F activity. Our experiments show that a 53 amino-acid stretch of E2F1 is sufficient for being targeted by prohibitin; fusion of this region to GAL4-VP16 construct could make it susceptible to prohibitin-mediated, but not Rb-mediated repression. Prohibitin, like Rb, could repress transcription from SV40 and major late promoters when recruited directly to DNA. Prohibitin mediated transcriptional repression required histone-deacetylase activity, but unlike Rb, additional co-repressors like N-CoR are also involved. Repression by prohibitin correlates with histone deacetylation on promoters and this was reversed by IgM stimulation of cells; IgM did not affect Rb-mediated repression or deacetylation of the promoters. Prohibitin thus appears to repress E2F-mediated transcription utilizing different molecular mediators and facilitate channeling of specific signaling pathways to the cell cycle machinery.

The cyclin-dependent kinase inhibitor flavopiridol disrupts sodium butyrate-induced p21WAF1/CIP1 expression and maturation while reciprocally potentiating apoptosis in human leukemia cells

Interactions between the cyclin-dependent kinase inhibitor flavopiridol (FP) and the histone deacetylase inhibitor sodium butyrate (SB) have been examined in human leukemia cells (U937) in relation to differentiation and apoptosis. Whereas 1 mM of SB or 100 nM of FP minimally induced apoptosis (4% and 10%, respectively) at 24 h, simultaneous exposure of U937 cells to these agents dramatically increased cell death (e.g., approximately 60%), reflected by both morphological and Annexin/propidium iodide-staining features, procaspase 3 activation, and poly(ADP-ribose) polymerase cleavage. Similar interactions were observed in human promyelocytic (HL-60), B-lymphoblastic (Raji), and T-lymphoblastic (Jurkat) leukemia cells. Coadministration of FP opposed SB-mediated accumulation of cells in G0G1 and differentiation, reflected by reduced CD11b expression, but instead dramatically increased procaspase-3, procaspase-8, Bid, and poly(ADP-ribose) polymerase cleavage, as well as mitochondrial damage (e.g., loss of mitochondrial membrane potential and cytochrome c release). FP also blocked SB-related p21WAF1-CIP1 induction through a caspase-independent mechanism and triggered the caspase-mediated cleavage of p27KIP1 and retinoblastoma protein. The latter event was accompanied by a marked reduction in retinoblastoma protein/E2F1 complex formation. However, FP did not modify the extent of SB-associated acetylation of histones H3 and H4. Treatment of cells with FP/SB also resulted in the caspase-mediated cleavage of Bcl-2 and caspase-independent down-regulation of Mcl-1. Levels of cyclins A, D1, and E, and X-linked inhibitor of apoptosis also declined in SB/FP-treated cells. Finally, FP/SB coexposure potently induced apoptosis in two primary acute myelogenous leukemia samples. Together, these findings demonstrate that FP, when combined with SB, induces multiple perturbations in cell cycle and apoptosis regulatory proteins, which oppose leukemic cell differentiation but instead promote mitochondrial damage and apoptosis.

HOG on the promoter: regulation of the osmotic stress response

Members of the mitogen-activated protein (MAP) kinase family regulate transcription through phosphorylation of specific transcription factors. New studies indicate that this process may be more complex than previously anticipated. The yeast Hog1 protein kinase (a homolog of the mammalian p38 MAP kinase) interacts with transcription factors and perhaps with the general transcription machinery at target promoters. Chellappan discusses the recent results and their implications for understanding control of transcription by stress-activated MAP kinases.

Rb dephosphorylation and suppression of E2F activity in human breast tumor cells exposed to a pharmacological concentration of estradiol

This report characterizes the influence of a pharmacological concentration of estradiol on growth arrest and cell death in MCF-7 breast tumor cells, with a focus on elements of the Rb-E2F cell-cycle regulatory pathway. Continuous exposure of MCF-7 breast tumor cells to 100 microM estradiol produces a marked reduction in the G1 and S phase populations and a corresponding increase in the G2/M population within 24 h; after 48 h, accumulation of cells in G1 becomes evident while after 72 h the cells appear to be equally distributed between the G1 and G2/M phases. The accumulation of cells in G1 is temporally associated with dephosphorylation of the Rb protein and suppression of E2F activity. Estradiol also produces an initial burst of cell death with loss of approximately 40% of the tumor cell population within 24 h; however, there is no tangible evidence for the occurrence of apoptosis based on terminal transferase end-labeling of DNA, DNA fragmentation analysis by alkaline unwinding, cell-cycle analysis or cell morphology. In addition to the lack of caspase-3 in MCF-7 cells, the absence of apoptosis could be related, at least in part, to the fact that estradiol promotes a rapid reduction in levels of the E2F-1 and Myc proteins. Overall, these studies are consistent with the concept that alterations in the levels and/or activity of the E2F family of proteins as well as proteins interacting with the E2F family may influence the nature of the antiproliferative and cytotoxic responses of the breast tumor cell.

The cyclin-dependent kinase inhibitor (CDKI) flavopiridol disrupts phorbol 12-myristate 13-acetate-induced differentiation and CDKI expression while enhancing apoptosis in human myeloid leukemia cells

Interactions between the cyclin-dependent kinase inhibitor (CDKI) flavopiridol (FP) and phorbol 12-myristate 13-acetate (PMA) were examined in U937 human leukemia cells in relation to differentiation and apoptosis. Simultaneous, but not sequential, exposure of U937 cells to 100 nM FP and 10 nM PMA significantly increased apoptosis manifested by characteristic morphological features, mitochondrial dysfunction, caspase activation, and poly(ADP-ribose) polymerase cleavage while markedly inhibiting cellular differentiation, as reflected by diminished plastic adherence and CD11b expression. Enhanced apoptosis in U937 cells was associated with an early caspase-independent increase in cytochrome c release and accompanied by a substantial decline in leukemic cell clonogenicity. Moreover, PMA/FP cotreatment significantly increased apoptosis in HL-60 promyelocytic leukemia cells and in U937 cells ectopically expressing the Bcl-2 protein. In U937 cells, coadministration of FP blocked PMA-induced expression and reporter activity of the CDKI p21WAF/CIP1 and triggered caspase-mediated cleavage of the CDKI p27KIP1. Coexposure to FP also resulted in a more pronounced and sustained activation of the mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinase cascade after PMA treatment, although disruption of this pathway by the mitogen-activated protein kinase kinase 1 inhibitor U0126 did not prevent potentiation of apoptosis. FP accelerated PMA-mediated dephosphorylation of the retinoblastoma protein (pRb), an event followed by pRb cleavage culminating in the complete loss of underphosphorylated pRb (approximately Mr 110,000) by 24 h. Finally, gel shift analysis revealed that coadministration of FP with PMA for 8 h led to diminished E2F/pRb binding compared to the effects of PMA alone. Collectively, these findings indicate that FP modulates the expression/activity of multiple signaling and cell cycle regulatory proteins in PMA-treated leukemia cells and that such alterations are associated with mitochondrial damage and apoptosis rather than maturation. These observations also raise the possibility that combining CDKIs and differentiation-inducing agents may represent a novel antileukemic strategy.

HLA class I-mediated induction of cell proliferation involves cyclin E-mediated inactivation of Rb function and induction of E2F activity

Chronic rejection of transplanted organs is manifested as atherosclerosis of the blood vessels of the allograft. HLA class I Ags have been implicated to play a major role in this process, since signaling via HLA class I molecules can induce the proliferation of aortic endothelial as well as smooth muscle cells. In this study, we show that HLA class I-mediated induction of cell proliferation correlates with inactivation of the Rb protein in the T cell line Jurkat as well as human aortic endothelial cells. HLA class I-mediated inactivation of Rb can be inhibited specifically by neutralizing Abs to basic fibroblast growth factor (bFGF), suggesting a role for FGF receptors in the signaling process. Signaling through HLA class I molecules induced cyclin E-associated kinase activity within 4 h in quiescent endothelial cells, and appeared to mediate the inactivation of Rb. A cdk2 inhibitor, Olomoucine, as well as a dominant-negative cdk2 construct prevented HLA class I-mediated inactivation of Rb; in contrast, dominant-negative cdk4 and cdk6 constructs had no effect. Furthermore, there was no increase in cyclin D-associated kinase activity upon HLA class I ligation, suggesting that cyclin E-dependent kinase activity mediates Rb inactivation, leading to E2F activation and cell proliferation.

HLA Class I-Mediated Induction of Cell Proliferation Involves Cyclin E-Mediated Inactivation of Rb Function and Induction of E2F Activity

Chronic rejection of transplanted organs is manifested as atherosclerosis of the blood vessels of the allograft. HLA class I Ags have been implicated to play a major role in this process, since signaling via HLA class I molecules can induce the proliferation of aortic endothelial as well as smooth muscle cells. In this study, we show that HLA class I-mediated induction of cell proliferation correlates with inactivation of the Rb protein in the T cell line Jurkat as well as human aortic endothelial cells. HLA class I-mediated inactivation of Rb can be inhibited specifically by neutralizing Abs to basic fibroblast growth factor (bFGF), suggesting a role for FGF receptors in the signaling process. Signaling through HLA class I molecules induced cyclin E-associated kinase activity within 4 h in quiescent endothelial cells, and appeared to mediate the inactivation of Rb. A cdk2 inhibitor, Olomoucine, as well as a dominant-negative cdk2 construct prevented HLA class I-mediated inactivation of Rb; in contrast, dominant-negative cdk4 and cdk6 constructs had no effect. Furthermore, there was no increase in cyclin D-associated kinase activity upon HLA class I ligation, suggesting that cyclin E-dependent kinase activity mediates Rb inactivation, leading to E2F activation and cell proliferation.

Raf-1 physically interacts with Rb and regulates its function: a link between mitogenic signaling and cell cycle regulation

Cells initiate proliferation in response to growth factor stimulation, but the biochemical mechanisms linking signals received at the cell surface receptors to the cell cycle regulatory molecules are not yet clear. In this study, we show that the signaling molecule Raf-1 can physically interact with Rb and p130 proteins in vitro and in vivo and that this interaction can be detected in mammalian cells without overexpressing any component. The binding of Raf-1 to Rb occurs subsequent to mitogen stimulation, and this interaction can be detected only in proliferating cells. Raf-1 can inactivate Rb function and can reverse Rb-mediated repression of E2F1 transcription and cell proliferation efficiently. The region of Raf-1 involved in Rb binding spanned residues 1 to 28 at the N terminus, and functional inactivation of Rb required a direct interaction. Serum stimulation of quiescent human fibroblast HSF8 cells led to a partial translocation of Raf-1 into the nucleus, where it colocalized with Rb. Further, Raf-1 was able to phosphorylate Rb in vitro quite efficiently. We believe that the physical interaction of Raf-1 with Rb is a vital step in the growth factor-mediated induction of cell proliferation and that Raf-1 acts as a direct link between cell surface signaling cascades and the cell cycle machinery.