CDK7 and CDK9 inhibition interferes with transcription, translation, and stemness, and induces cytotoxicity in GBM irrespective of temozolomide sensitivity

BACKGROUND

Glioblastoma (GBM) is refractory to current treatment modalities while side effects of treatments result in neurotoxicity and cognitive impairment. Here we test the hypothesis that inhibiting CDK7 or CDK9 would effectively combat GBM with reduced neurotoxicity.

METHODS

We examined the effect of a CDK7 inhibitor, THZ1, and multiple CDK9 inhibitors (SNS032, AZD4573, NVP2, and JSH150) on GBM cell lines, patient-derived temozolomide (TMZ)-resistant and responsive primary tumor cells and glioma stem cells (GSCs). Biochemical changes were assessed by western blotting, immunofluorescence, multispectral imaging, and RT-PCR. In vivo, efficacy was assessed in orthotopic and subcutaneous xenograft models.

RESULTS

CDK7 and CDK9 inhibitors suppressed the viability of TMZ-responsive and resistant GBM cells and GSCs at low nanomolar concentrations, with limited cytotoxic effects in vivo. The inhibitors abrogated RNA Pol II and p70S6K phosphorylation and nascent protein synthesis. Furthermore, the self-renewal of GSCs was significantly reduced with a corresponding reduction in Sox2 and Sox9 levels. Analysis of TCGA data showed increased expression of CDK7, CDK9, SOX2, SOX9, and RPS6KB1 in GBM; supporting this, multispectral imaging of a TMA revealed increased levels of CDK9, Sox2, Sox9, phospho-S6, and phospho-p70S6K in GBM compared to normal brains. RNA-Seq results suggested that inhibitors suppressed tumor-promoting genes while inducing tumor-suppressive genes. Furthermore, the studies conducted on subcutaneous and orthotopic GBM tumor xenograft models showed that administration of CDK9 inhibitors markedly suppressed tumor growth in vivo.

CONCLUSIONS

Our results suggest that CDK7 and CDK9 targeted therapies may be effective against TMZ-sensitive and resistant GBM.

Ethnic and racial-specific differences in levels of centrosome-associated mitotic kinases, proliferative and epithelial-to-mesenchymal markers in breast cancers

Molecular epidemiology evidence indicates racial and ethnic differences in the aggressiveness and survival of breast cancer. Hispanics/Latinas (H/Ls) and non-Hispanic Black women (NHB) are at higher risk of breast cancer (BC)-related death relative to non-Hispanic white (NHW) women in part because they are diagnosed with hormone receptor-negative (HR) subtype and at higher stages. Since the cell cycle is one of the most commonly deregulated cellular processes in cancer, we propose that the mitotic kinases TTK (or Mps1), TBK1, and Nek2 could be novel targets to prevent breast cancer progression among NHBs and H/Ls. In this study, we calculated levels of TTK, p-TBK1, epithelial (E-cadherin), mesenchymal (Vimentin), and proliferation (Ki67) markers through immunohistochemical (IHC) staining of breast cancer tissue microarrays (TMAs) that includes samples from 6 regions in the Southeast of the United States and Puerto Rico -regions enriched with NHB and H/L breast cancer patients. IHC analysis showed that TTK, Ki67, and Vimentin were significantly expressed in triple-negative (TNBC) tumors relative to other subtypes, while E-cadherin showed decreased expression. TTK correlated with all of the clinical variables but p-TBK1 did not correlate with any of them. TCGA analysis revealed that the mRNA levels of multiple mitotic kinases, including TTK, Nek2, Plk1, Bub1, and Aurora kinases A and B, and transcription factors that are known to control the expression of these kinases (e.g. FoxM1 and E2F1-3) were upregulated in NHBs versus NHWs and correlated with higher aneuploidy indexes in NHB, suggesting that these mitotic kinases may be future novel targets for breast cancer treatment in NHB women.

The Nek2 centrosome-mitotic kinase contributes to the mesenchymal state, cell invasion, and migration of triple-negative breast cancer cells

Nek2 (NIMA-related kinase 2) is a serine/threonine-protein kinase that localizes to centrosomes and kinetochores, controlling centrosome separation, chromosome attachments to kinetochores, and the spindle assembly checkpoint. These processes prevent centrosome amplification (CA), mitotic dysfunction, and chromosome instability (CIN). Our group and others have suggested that Nek2 maintains high levels of CA/CIN, tumor growth, and drug resistance. We identified that Nek2 overexpression correlates with poor survival of breast cancer. However, the mechanisms driving these phenotypes are unknown. We now report that overexpression of Nek2 in MCF10A cells drives CA/CIN and aneuploidy. Besides, enhanced levels of Nek2 results in larger 3D acinar structures, but could not initiate tumors in a p53+/+ or a p53-/- xenograft model. Nek2 overexpression induced the epithelial-to-mesenchymal transition (EMT) while its downregulation reduced the expression of the mesenchymal marker vimentin. Furthermore, either siRNA-mediated downregulation or INH6's chemical inhibition of Nek2 in MDA-MB-231 and Hs578t cells showed important EMT changes and decreased invasion and migration. We also showed that Slug and Zeb1 are involved in Nek2 mediated EMT, invasion, and migration. Besides its role in CA/CIN, Nek2 contributes to breast cancer progression through a novel EMT mediated mechanism.

Tank Binding Kinase 1 modulates spindle assembly checkpoint components to regulate mitosis in breast and lung cancer cells

Error-free progression through mitosis is critical for proper cell division and accurate distribution of the genetic material. The anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase regulates the progression from metaphase to anaphase and its activation is controlled by the cofactors Cdc20 and Cdh1. Additionally, genome stability is maintained by the spindle assembly checkpoint (SAC), which monitors proper attachment of chromosomes to spindle microtubules prior to cell division. We had shown a role for Tank Binding Kinase 1 (TBK1) in microtubule dynamics and mitosis and here we describe a novel role of TBK1 in regulating SAC in breast and lung cancer cells. TBK1 interacts with and phosphorylates Cdc20 and Cdh1 and depletion of TBK1 elevates SAC components. TBK1 inhibition increases the association of Cdc20 with APC/C and BubR1 indicating inactivation of APC/C; similarly, interaction of Cdh1 with APC/C is also enhanced. TBK1 and TTK inhibition reduces cell viability and enhances centrosome amplification and micronucleation. These results indicate that alterations in TBK1 will impede mitotic progression and combining TBK1 inhibitors with other regulators of mitosis might be effective in eliminating cancer cells.

Role of E2F3 and shugoshin I in epithelial-to-mesenchymal transition and cell invasion in breast cancer

e13100

Background: Triple-negative breast cancer (TNBC) is the most aggressive and poorly prognostic breast cancer subtype, yet there are currently no biological therapies against this subtype. Our laboratory is finding the sources of novel biological targets in TNBC by studying the E2F transcription factors, which are essential for cellular proliferation and maintenance of genomic stability. While the deregulated Rb/E2F pathway signals the epithelial-to-mesenchymal transition (EMT), the underlining mechanism of how E2Fs drive EMT in TNBC remains unknown. We recently published that the E2F transcriptional activators (E2Fs) are overexpressed in the vast majority of TNBC and that their overexpression upregulates mitotic kinases such as TTK, which we have shown to induce EMT and invasion in TNBC cells. We also demonstrated that the E2Fs maintain genomic integrity in part through Shugoshin I (SGO1), which normally controls chromosome cohesion; however, the role of SGO1 in EMT in breast cancer is unknown. Our hypothesis is that E2F3 and SGO1 are highly expressed in TNBC and that their overexpression modulates EMT genes, thus promoting cell invasion. Methods: To test our hypothesis, we conducted siRNA transfection to knockdown E2F3 and SGO1 in MDA-MB-231 and Hs578t, which are TNBC cells. After 48 hours, we evaluated mRNA levels of EMT-related genes after E2F3 or SGO1 depletion using RT-PCR analysis. We also evaluated the effects of SGO1 depletion in protein localization by immunofluorescence. Furthermore, we evaluated the invasive behavior of MDA-MB-231 and Hs578t cells after SGO1 depletion using a Boyden Chamber Assay. Results: Our results demonstrate that E2F3 and SGO1 depletion decrease MMP3 mRNA levels. Moreover, E2F3 and SGO1 depletion restore E-cadherin expression and localization. Furthermore, E2F3 and SGO1 depletion significantly reduce cell invasion in MDA-MB-231 and Hs578t cells. Conclusions: Our results suggest that SGO1 is a promising drug target for breast cancer metastasis since EMT and invasion are essential early steps in breast cancer metastasis and E2F3 is presently undruggable.

Abstract 1976: β-arrestin-1 function in CAFs is necessary for enhancement of self-renewal of NSCLC stem-like cells

Background: Cigarette smoking is a major risk factor in the genesis of non-small cell lung cancer (NSCLC). Nicotine, the addictive component of tobacco smoke, promotes growth and metastasis of NSCLCs. The scaffolding protein β-arrestin-1 (ARRB1), mediates the proliferative effects of nicotine through nicotinic acetylcholine receptors (nAChR) signaling. Nicotine induces nuclear translocation of ARRB1 and increases E2F-driven proliferative, EMT and survival genes to promote NSCLC growth. Further, nicotine promotes NSCLC stemness by inducing SCF (Stem cell factor) in a β-arrestin-1-dependent manner. While all these studies describe β-arrestin-1 role in cell-autonomous functions, its role in NSCLC microenvironment is largely unknown. Cancer associated fibroblasts (CAFs) and human mesenchymal stem cells (hMSCs) promote the self-renewal and proliferation of cancer stem cells in vitro and in vivo. In the present study, we address if the function of β-arrestin-1 in CAFs/hMSCs is necessary to enhance the self-renewal of stem like cells from NSCLCs.

Methods: GFP stable expressing NSCLC cell line A549 (A549-GFP) was generated. The side population (SP) cells from A549-GFP was used in 3D co-culture with cytotracker red labeled lung CAFs or hMSCs in stem cell selective medium and the sphere growth was monitored for 10 days. Cytokine arrays were used to test the changes that occur in CAFs/hMSCs upon β-arrestin-1 depletion. RNAseq analysis was carried out for differential expression of RNA in β-arrestin-1 depleted lung CAFs.

Results: Our results show that CAFs/hMSCs can promote the self-renewal of SP cells from NSCLCs, while β-arrestin-1 depleted CAFs/hMSCs are incompetent to promote SP cells self-renewal. Cytokine array results showed reduced levels of multiple cytokines including CXCL1/GROα, CXCL5, ICAM-1, VCAM-1 and CCL5 in β-arrestin-1 depleted lung CAFs/hMSCs. Further, RNAseq analysis of β-arrestin-1 depleted lung CAFs revealed differential expression of 577 genes. Interestingly, the data showed significantly altered expression of 23 genes involved in cytokine-cytokine receptor interaction.

Conclusions: β-arrestin-1 in CAFs/hMSCs appears to enhance the stemness of SP cells from NSCLCs. β-arrestin-1 depletion in CAFs/hMSCs significantly altered the expression of multiple cytokines/growth factors. In good correlation, RNAseq results displayed a significant difference in the expression of genes encoding cytokines/cytokine receptors, suggesting the potential role of CAFs/hMSCs secreted cytokines in mediating β-arrestin-1-mediated self-renewal. Experiments are under way to identify cytokines that act downstream of β-arrestin-1 to enhance NSCLC stemness. These studies will shed new light on the mechanisms by which CAFs/hMSCs promote self-renewal and tumor growth, enabling the identification of pathways downstream of β-arrestin-1 that can potentially be targeted for NSCLC therapy.

Citation Format: Mohan Kumar Durai Raj, Srikumar P. Chellappan. β-arrestin-1 function in CAFs is necessary for enhancement of self-renewal of NSCLC stem-like cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1976.

HDAC Inhibition Enhances the In Vivo Efficacy of MEK Inhibitor Therapy in Uveal Melanoma

PURPOSE

The clinical use of MEK inhibitors in uveal melanoma is limited by the rapid acquisition of resistance. This study has used multiomics approaches and drug screens to identify the pan-HDAC inhibitor panobinostat as an effective strategy to limit MEK inhibitor resistance.Experimental Design: Mass spectrometry-based proteomics and RNA-Seq were used to identify the signaling pathways involved in the escape of uveal melanoma cells from MEK inhibitor therapy. Mechanistic studies were performed to evaluate the escape pathways identified, and the efficacy of the MEK-HDAC inhibitor combination was demonstrated in multiple in vivo models of uveal melanoma.

RESULTS

We identified a number of putative escape pathways that were upregulated following MEK inhibition, including the PI3K/AKT pathway, ROR1/2, and IGF-1R signaling. MEK inhibition was also associated with increased GPCR expression, particularly the endothelin B receptor, and this contributed to therapeutic escape through ET-3-mediated YAP signaling. A screen of 289 clinical grade compounds identified HDAC inhibitors as potential candidates that suppressed the adaptive YAP and AKT signaling that followed MEK inhibition. In vivo, the MEK-HDAC inhibitor combination outperformed either agent alone, leading to a long-term decrease of tumor growth in both subcutaneous and liver metastasis models and the suppression of adaptive PI3K/AKT and YAP signaling.

CONCLUSIONS

Together, our studies have identified GPCR-mediated YAP activation and RTK-driven AKT signaling as key pathways involved in the escape of uveal melanoma cells from MEK inhibition. We further demonstrate that HDAC inhibition is a promising combination partner for MEK inhibitors in advanced uveal melanoma.

Cationic lipoplexes for treatment of cancer stem cell-derived murine lung tumors

Side population (SP) cells with stem-like properties, also known as cancer stem cells (CSC) have been recognized as drivers of the resistance phenotype in many cancers. Central to the characteristic stem-like phenotype of CSCs in cancer is the activity of the SOX2 transcription factor whose upregulation has been associated with enrichment of many oncogenes. This study outlines the fabrication of a lipoplex of SOX2 small interfering RNA (CL-siSOX2) for targeted treatment of SOX2-enriched, CSC-derived orthotopic and xenograft lung tumors in CB-17 SCID mice. CL-siSOX2 induced tumor contraction in cisplatin-naïve and cisplatin-treated groups by 85% and 94% respectively. Reduction in tumor weight and volume following treatment with CL-siSOX2 was associated with reduced protein expression of SOX2 and markers of tumor initiation, inflammation, invasion and metastasis in mice tumor xenografts. In addition, histological staining of lung tumor sections showed reduction in SOX2 expression was associated with inhibition markers of epithelial-to-mesenchymal transition.

Regulation of Sox2 and stemness by nicotine and electronic-cigarettes in non-small cell lung cancer

Background

Lung cancer is the leading cause of cancer related deaths and its incidence is highly correlated with cigarette smoking. Nicotine, the addictive component of tobacco smoke, cannot initiate tumors, but can promote proliferation, migration, and invasion of cells in vitro and promote tumor growth and metastasis in vivo. This nicotine-mediated tumor promotion is facilitated through the activation of nicotinic acetylcholine receptors (nAChRs), specifically the α7 subunit. More recently, nicotine has been implicated in promoting self-renewal of stem-like side-population cells from lung cancers. This subpopulation of cancer stem-like cells has been implicated in tumor initiation, generation of the heterogeneous tumor population, metastasis, dormancy, and drug resistance. Here we describe the molecular events driving nicotine and e-cigarette extract mediated stimulation of self-renewal of stem-like cells from non-small cell lung cancer.

Methods

Experiments were conducted using A549 and H1650 non-small cell lung cancer cell lines and human mesenchymal stem cells according to protocols described in this paper. 2 μM nicotine or e-cigarette extracts was used in all relevant experiments. Biochemical analysis using western blotting, transient transfections, RT-PCR and cell biological analysis using double immunofluorescence and confocal microscopy, as well as proximity ligation assays were conducted.

Results

Here we demonstrate that nicotine can induce the expression of embryonic stem cell factor Sox2, which is indispensable for self-renewal and maintenance of stem cell properties in non-small cell lung adenocarcinoma (NSCLC) cells. We further demonstrate that this occurs through a nAChR-Yap1-E2F1 signaling axis downstream of Src and Yes kinases. Our data suggests Oct4 may also play a role in this process. Over the past few years, electronic cigarettes (e-cigarettes) have been promoted as healthier alternatives to traditional cigarette smoking as they do not contain tobacco; however, they do still contain nicotine. Hence we have investigated whether e-cigarette extracts can enhance tumor promoting properties similar to nicotine; we find that they can induce expression of Sox2 as well as mesenchymal markers and enhance migration and stemness of NSCLC cells.

Conclusions

Our findings shed light on novel molecular mechanisms underlying the pathophysiology of smoking-related lung cancer in the context of cancer stem cell populations, and reveal new pathways involved that could potentially be exploited therapeutically.

Electronic supplementary material

The online version of this article (10.1186/s12943-018-0901-2) contains supplementary material, which is available to authorized users.

In vitro replication assay with mammalian cell extracts

Regulatory mechanisms are crucial to control DNA replication during cell cycle in eukaryotic cells. Cell-free in vitro replication assay (IVRA) is one of the widely used assays to understand the complex mammalian replication system. 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.

Nicotinic acetylcholine receptors induce c-Kit ligand/Stem Cell Factor and promote stemness in an ARRB1/ β-arrestin-1 dependent manner in NSCLC

Lung cancer remains the leading cause of cancer-related deaths worldwide. β-arrestin-1 (ARRB1), a scaffolding protein involved in the desensitization of signals arising from activated G-protein-coupled receptors (GPCRs), has been shown to play a role in invasion and proliferation of cancer cells, including nicotine-induced proliferation of human non–small cell lung cancers (NSCLCs). In this study, we identified genes that are differentially regulated by nicotine in an ARRB1/β-arrestin-1 dependent manner in NSCLC cells by microarray analysis. Among the identified genes, SCF (Stem cell factor) strongly differentiated smokers from non-smokers in the Director's Challenge Set expression data and its high expression correlated with poor prognosis. SCF, a major cytokine is the ligand for the c-Kit proto-oncogene and was found to be over expressed in human lung adenocarcinomas, but not squamous cell carcinomas. Data presented here show that transcription factor E2F1 can induce SCF expression at the transcriptional level and depletion of E2F1 or ARRB1/β-arrestin-1 could not promote self-renewal of SP cells. These studies suggest that nicotine might be promoting NSCLC growth and metastasis by inducing the secretion of SCF, and raise the possibility that targeting signalling cascades that activate E2F1 might be an effective way to combat NSCLC.

The Rb-E2F transcriptional regulatory pathway in tumor angiogenesis and metastasis

The retinoblastoma tumor suppressor protein Rb plays a major role in regulating G1/S transition and is a critical regulator of cell proliferation. Rb protein exerts its growth regulatory properties mainly by physically interacting with the transcriptionally active members of the E2F transcription factor family, especially E2Fs 1, 2, and 3. Given its critical role in regulating cell proliferation, it is not surprising that Rb is inactivated in almost all tumors, either through the mutation of Rb gene itself or through the mutations of its upstream regulators including K-Ras and INK4. Recent studies have revealed a significant role for Rb and its downstream effectors, especially E2Fs, in regulating various aspects of tumor progression, angiogenesis, and metastasis. Thus, components of the Rb-E2F pathway have been shown to regulate the expression of genes involved in angiogenesis, including VEGF and VEGFR, genes involved in epithelial-mesenchymal transition including E-cadherin and ZEB proteins, and genes involved in invasion and migration like matrix metalloproteinases. Rb has also been shown to play a major role in the functioning of normal and cancer stem cells; further, Rb and E2F appear to play a regulatory role in the energy metabolism of cancer cells. These findings raise the possibility that mutational events that initiate tumorigenesis by inducing uncontrolled cell proliferation might also contribute to the progression and metastasis of cancers through the mediation of the Rb-E2F transcriptional regulatory pathway. This review highlights these recent studies on tumor promoting functions of the Rb-E2F pathway.

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.

ChIP on chip and ChIP-Seq assays: genome-wide analysis of transcription factor binding and histone modifications

Deregulation of transcriptional activity of many genes has been causatively linked to human diseases including cancer. Altered patterns of gene expression in normal and cancer cells are the result of inappropriate expression of transcription factors and chromatin modifying proteins. Chromatin immunoprecipitation assay is a well-established tool for investigating the interactions between regulatory proteins and DNA at distinct stages of gene activation. ChIP coupled with DNA microarrays, known as ChIP on chip, or sequencing of DNA associated with the factors (ChIP-Seq) allow us to determine the entire spectrum of in vivo DNA binding sites for a given protein. This has been of immense value because ChIP on chip assays and ChIP-Seq experiments can provide a snapshot of the transcriptional regulatory mechanisms on a genome-wide scale. This chapter outlines the general strategies used to carry out ChIP-chip assays to study the differential recruitment of regulatory molecules based on the studies conducted in our lab as well as other published protocols; these can be easily modified to a ChIP-Seq analysis.

Nicotine-mediated cell proliferation and tumor progression in smoking-related cancers

Tobacco smoke contains multiple classes of established carcinogens including benzo(a)pyrenes, polycyclic aromatic hydrocarbons, and tobacco-specific nitrosamines. Most of these compounds exert their genotoxic effects by forming DNA adducts and generation of reactive oxygen species, causing mutations in vital genes such as K-Ras and p53. In addition, tobacco-specific nitrosamines can activate nicotinic acetylcholine receptors (nAChR) and to a certain extent β-adrenergic receptors (β-AR), promoting cell proliferation. Furthermore, it has been demonstrated that nicotine, the major addictive component of tobacco smoke, can induce cell-cycle progression, angiogenesis, and metastasis of lung and pancreatic cancers. These effects occur mainly through the α7-nAChRs, with possible contribution from the β-ARs and/or epidermal growth factor receptors. This review article will discuss the molecular mechanisms by which nicotine and its oncogenic derivatives such as 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone and N-nitrosonornicotine induce cell-cycle progression and promote tumor growth. A variety of signaling cascades are induced by nicotine through nAChRs, including the mitogen-activated protein kinase/extracellular signal-regulated kinase pathway, phosphoinositide 3-kinase/AKT pathway, and janus-activated kinase/STAT signaling. In addition, studies have shown that nAChR activation induces Src kinase in a β-arrestin-1-dependent manner, leading to the inactivation of Rb protein and resulting in the expression of E2F1-regulated proliferative genes. Such nAChR-mediated signaling events enhance the proliferation of cells and render them resistant to apoptosis induced by various agents. These observations highlight the role of nAChRs in promoting the growth and metastasis of tumors and raise the possibility of targeting them for cancer therapy.

Selective disruption of rb-raf-1 kinase interaction inhibits pancreatic adenocarcinoma growth irrespective of gemcitabine sensitivity

Inactivation of the retinoblastoma (Rb) tumor suppressor protein is widespread in human cancers. Inactivation of Rb is thought to be initiated by association with Raf-1 (C-Raf) kinase, and here we determined how RRD-251, a disruptor of the Rb-Raf-1 interaction, affects pancreatic tumor progression. Assessment of phospho-Rb levels in resected human pancreatic tumor specimens by immunohistochemistry (n = 95) showed that increased Rb phosphorylation correlated with increasing grade of resected human pancreatic adenocarcinomas (P = 0.0272), which correlated with reduced overall patient survival (P = 0.0186). To define the antitumor effects of RRD-251 (50 μmol/L), cell-cycle analyses, senescence, cell viability, cell migration, anchorage-independent growth, angiogenic tubule formation and invasion assays were conducted on gemcitabine-sensitive and -resistant pancreatic cancer cells. RRD-251 prevented S-phase entry, induced senescence and apoptosis, and inhibited anchorage-independent growth and invasion (P < 0.01). Drug efficacy on subcutaneous and orthotopic xenograft models was tested by intraperitoneal injections of RRD-251 (50 mg/kg) alone or in combination with gemcitabine (250 mg/kg). RRD-251 significantly reduced tumor growth in vivo accompanied by reduced Rb phosphorylation and lymph node and liver metastasis (P < 0.01). Combination of RRD-251 with gemcitabine showed cooperative effect on tumor growth (P < 0.01). In conclusion, disruption of the Rb-Raf-1 interaction significantly reduces the malignant properties of pancreatic cancer cells irrespective of their gemcitabine sensitivity. Selective targeting of Rb-Raf-1 interaction might be a promising strategy targeting pancreatic cancer.

βArrestin-1 and Mcl-1 modulate self-renewal growth of cancer stem-like side-population cells in non-small cell lung cancer

Side population (SP) cells have been reported to have properties of cancer stem-like cells (CSCs) in non-small cell lung carcinoma (NSCLC), yet their molecular features have not been fully elucidated. Here we show that, NSCLC-SP cells were enriched in G₀/G­₁ phase of cell cycle, had higher aldehyde dehydrogenase activity as well as higher clonogenic and self-renewing ability compared to main population (MP) cells. Interestingly, SP cells were also able to trans-differentiate into angiogenic tubules in vitro and were highly tumorigenic as compared to MP cells. SP-derived tumors demonstrated the intratumoral heterogeneity comprising of both SP and MP cells, suggesting the self-renewal and differentiation ability of SP cells are manifested in vivo as well. βArrestin-1 (βArr1) is involved in the progression of various cancers including NSCLCs and we find that depletion of βArr1 significantly blocked the SP phenotype; whereas depletion of βArr2 had relatively minor effects. Ectopic expression of βArr1 resulted in increased SP frequency and ABCG2 expression while abrogation of βArr1 expression suppressed the self-renewal growth and expansion of A549 cells. Anti-apoptotic protein Mcl-1 is known to be one of the key regulators of self-renewal of tissue stem cells and is thought to contribute to survival of NSCLC cells. Our experiments show that higher levels of Mcl-1 were expressed in SP cells compared to MP cells at both transcriptional and translational levels. In addition, Obatoclax, a pharmacological inhibitor of Mcl-1, could effectively prevent the self-renewal of both EGFR-inhibitor sensitive and resistant NSCLC cells. In conclusion, our findings suggest that βArr1 and Mcl-1 are involved in the self-renewal and expansion of NSCLC-CSCs and are potential targets for anti-cancer therapy.

IKBKE is induced by STAT3 and tobacco carcinogen and determines chemosensitivity in non-small cell lung cancer

Serine/threonine kinase IKBKE is a newly identified oncogene; however, its regulation remains elusive. Here, we provide evidence that IKBKE is a downstream target of signal transducer and activator of transcription 3 (STAT3) and that tobacco components induce IKBKE expression through STAT3. Ectopic expression of constitutively active STAT3 increased IKBKE mRNA and protein levels, whereas inhibition of STAT3 reduced IKBKE expression. Furthermore, expression levels of IKBKE are significantly associated with STAT3 activation and tobacco use history in non-small cell lung cancer (NSCLC) patients examined. In addition, we show induction of IKBKE by two components of cigarette smoke, nicotine and nicotine-derived nitrosamine ketone (NNK). Upon exposure to nicotine or NNK, cells express high levels of IKBKE protein and mRNA, which are largely abrogated by inhibition of STAT3. Characterization of the IKBKE promoter revealed two STAT3-response elements. The IKBKE promoter directly bound to STAT3 and responded to nicotine and NNK stimulation. Notably, enforcing expression of IKBKE induces chemoresistance, whereas knockdown of IKBKE not only sensitizes NSCLC cells to chemotherapy but also abrogates STAT3- and nicotine-induced cell survival. These data indicate for the first time that IKBKE is a direct target of STAT3 and is induced by tobacco carcinogens through STAT3 pathway. In addition, our study also suggests that IKBKE is an important therapeutic target and could have a pivotal role in tobacco-associated lung carcinogenesis.

Regulation of interlocking gene regulatory network subcircuits by a small molecule inhibitor of retinoblastoma protein (RB) phosphorylation: cancer cell expression of HLA-DR

The induction of the major histocompatibility (MHC), antigen-presenting class II molecules by interferon-gamma, in solid tumor cells, requires the retinoblastoma tumor suppressor protein (Rb). In the absence of Rb, a repressosome blocks the access of positive-acting, promoter binding proteins to the MHC class II promoter. However, a complete molecular linkage between Rb expression and the disassembly of the MHC class II repressosome has been lacking. By treating A549 lung carcinoma cells with a novel small molecule that prevents phosphorylation-mediated, Rb inactivation, we demonstrate that Rb represses the synthesis of an MHC class II repressosome component, YY1. The reduction in YY1 synthesis correlates with the advent of MHC class II inducibility; with loss of YY1 binding to the promoter of the HLA-DRA gene, the canonical human MHC class II gene; and with increased Rb binding to the YY1 promoter. These results support the concept that the Rb gene regulatory network (GRN) subcircuit that regulates cell proliferation is linked to a GRN subcircuit regulating a tumor cell immune function.

EGFR/Src/Akt signaling modulates Sox2 expression and self-renewal of stem-like side-population cells in non-small cell lung cancer

Background

Cancer stem cells are thought to be responsible for the initiation and progression of cancers. In non-small cell lung cancers (NSCLCs), Hoechst 33342 dye effluxing side population (SP) cells are shown to have stem cell like properties. The oncogenic capacity of cancer stem-like cells is in part due to their ability to self-renew; however the mechanistic correlation between oncogenic pathways and self-renewal of cancer stem-like cells has remained elusive. Here we characterized the SP cells at the molecular level and evaluated its ability to generate tumors at the orthotopic site in the lung microenvironment. Further, we investigated if the self-renewal of SP cells is dependent on EGFR mediated signaling.

Results

SP cells were detected and isolated from multiple NSCLC cell lines (H1650, H1975, A549), as well as primary human tumor explants grown in nude mice. SP cells demonstrated stem-like properties including ability to self-renew and grow as spheres; they were able to generate primary and metastatic tumors upon orthotopic implantation into the lung of SCID mice. In vitro study revealed elevated expression of stem cell associated markers like Oct4, Sox2 and Nanog as well as demonstrated intrinsic epithelial to mesenchymal transition features in SP cells. Further, we show that abrogation of EGFR, Src and Akt signaling through pharmacological or genetic inhibitors suppresses the self-renewal growth and expansion of SP-cells and resulted in specific downregulation of Sox2 protein expression. siRNA mediated depletion of Sox2 significantly blocked the SP phenotype as well as its self-renewal capacity; whereas other transcription factors like Oct4 and Nanog played a relatively lesser role in regulating self-renewal. Interestingly, Sox2 was elevated in metastatic foci of human NSCLC samples.

Conclusions

Our findings suggest that Sox2 is a novel target of EGFR-Src-Akt signaling in NSCLCs that modulates self-renewal and expansion of stem-like cells from NSCLC. Therefore, the outcome of the EGFR-Src-Akt targeted therapy may rely upon the expression and function of Sox2 within the NSCLC-CSCs.

A novel five gene signature derived from stem-like side population cells predicts overall and recurrence-free survival in NSCLC

Gene expression profiling has been used to characterize prognosis in various cancers. Earlier studies had shown that side population cells isolated from Non-Small Cell Lung Cancer (NSCLC) cell lines exhibit cancer stem cell properties. In this study we apply a systems biology approach to gene expression profiling data from cancer stem like cells isolated from lung cancer cell lines to identify novel gene signatures that could predict prognosis. Microarray data from side population (SP) and main population (MP) cells isolated from 4 NSCLC lines (A549, H1650, H460, H1975) were used to examine gene expression profiles associated with stem like properties. Differentially expressed genes that were over or under-expressed at least two fold commonly in all 4 cell lines were identified. We found 354 were upregulated and 126 were downregulated in SP cells compared to MP cells; of these, 89 up and 62 downregulated genes (average 2 fold changes) were used for Principle Component Analysis (PCA) and MetaCore™ pathway analysis. The pathway analysis demonstrated representation of 4 up regulated genes (TOP2A, AURKB, BRRN1, CDK1) in chromosome condensation pathway and 1 down regulated gene FUS in chromosomal translocation. Microarray data was validated using qRT-PCR on the 5 selected genes and all showed robust correlation between microarray and qRT-PCR. Further, we analyzed two independent gene expression datasets that included 360 lung adenocarcinoma patients from NCI Director's Challenge Set for overall survival and 63 samples from Sungkyunkwan University (SKKU) for recurrence free survival. Kaplan-Meier and log-rank test analysis predicted poor survival of patients in both data sets. Our results suggest that genes involved in chromosome condensation are likely related with stem-like properties and might predict survival in lung adenocarcinoma. Our findings highlight a gene signature for effective identification of lung adenocarcinoma patients with poor prognosis and designing more aggressive therapies for such patients.

Nicotine, IFN-γ and retinoic acid mediated induction of MUC4 in pancreatic cancer requires E2F1 and STAT-1 transcription factors and utilize different signaling cascades

Background

The membrane-bound mucins are thought to play an important biological role in cell–cell and cell–matrix interactions, in cell signaling and in modulating biological properties of cancer cell. MUC4, a transmembrane mucin is overexpressed in pancreatic tumors, while remaining undetectable in the normal pancreas, thus indicating a potential role in pancreatic cancer pathogenesis. The molecular mechanisms involved in the regulation of MUC4 gene are not yet fully understood. Smoking is strongly correlated with pancreatic cancer and in the present study; we elucidate the molecular mechanisms by which nicotine as well as agents like retinoic acid (RA) and interferon-γ (IFN-γ) induce the expression of MUC4 in pancreatic cancer cell lines CD18, CAPAN2, AsPC1 and BxPC3.

Results

Chromatin immunoprecipitation assays and real-time PCR showed that transcription factors E2F1 and STAT1 can positively regulate MUC4 expression at the transcriptional level. IFN-γ and RA could collaborate with nicotine in elevating the expression of MUC4, utilizing E2F1 and STAT1 transcription factors. Depletion of STAT1 or E2F1 abrogated the induction of MUC4; nicotine-mediated induction of MUC4 appeared to require α7-nicotinic acetylcholine receptor subunit. Further, Src and ERK family kinases also mediated the induction of MUC4, since inhibiting these signaling molecules prevented the induction of MUC4. MUC4 was also found to be necessary for the nicotine-mediated invasion of pancreatic cancer cells, suggesting that induction of MUC4 by nicotine and other agents might contribute to the genesis and progression of pancreatic cancer.

Conclusions

Our studies show that agents that can promote the growth and invasion of pancreatic cancer cells induce the MUC4 gene through multiple pathways and this induction requires the transcriptional activity of E2F1 and STAT1. Further, the Src as well as ERK signaling pathways appear to be involved in the induction of this gene. It appears that targeting these signaling pathways might inhibit the expression of MUC4 and prevent the proliferation and invasion of pancreatic cancer cells.

Abstract 798: Id1 (Inhibitor of differentiation-1) expression demonstrates important translational implications for pancreatic adenocarcinoma

Introduction: Inhibitor of differentiation-1 (Id1) demonstrates a role in tumor progression in a variety of tumors but scarce data exists for pancreatic adenocarcinoma. Previously, we demonstrated that nicotine, the addicting component of tobacco, induces Id1 expression through a Src-dependent signaling pathway promoting proliferation, invasion, and chemoresistance in pancreatic cancer cells. Conversely, silencing of Id1 re-establishes gemcitabine sensitivity in acquired and innate gemcitabine-resistant cell lines. Therefore, we determined the role of Id1 in tumor progression and chemoresistance in vivo and determined the expression status of Id1 in a large cohort of human pancreatic cancer tissues. Methods: Cell lysate analyses (protein analyses, mRNA expression) were determined in a bio-diverse sample of pancreatic cancer cells by western and RT-PCR. Metastatic pancreatic cancer cells were stably transfected with a luciferase expression vector and stable short-hairpin RNA silencing of Id1 gene expression with controls. Orthotopic nude mouse model utilized intraperitoneal injections of nicotine (1mg/kg, M/W/F) and gemcitabine (250 mg/kg, T/Th) with monitoring of in vivo growth and metastases based on luminescent signal concentration. Confirmation of primary tumor and metastases was confirmed by H&E staining. Tissue microarrays (TMAs) constructed from 100 resected pancreatic adenocarcinoma were stained for Id1, phospho-Src, and total Src using immunohistochemistry techniques. Scores based on intensity, percent of positive cells, and pathological grade (1-4) were assigned to each malignant core. Results: In vivo, nicotine significantly promoted tumor growth and metastases in vivo. While gemcitabine reduced the tumor burden in control mice, nicotine abrogates this inhibitory effect. Additionally, knockdown of Id1 results in a significant decrease in tumor burden with no identifiable liver metastases, regardless of nicotine exposure. Analyses of resected pancreatic tumor TMAs demonstrated a statistically significant correlation between Id1 and tumor grade/differentiation (p =0.0368). The correlation of protein levels of Id1 and phospho-Src was also statistically significant (p =0.0024); Comparison of overall survival and tumor grade was also significant (p = 0.05); the correlation between overall-survival and Id1 expression trended toward statistical significance (p =0.06). Conclusions: Id1, through a Src-dependent-Id1 signaling axis, promotes tumorigenesis and chemoresistance in an in vivo pancreatic cancer model. Id1 expression correlates with phospho-Src levels and tumor differentiation in human pancreatic cancer tissues. These results support the clinical assessment of Id1 expression as a biomarker of prognosis and support investigations toward targeted therapies against Id1 for pancreatic cancer patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 798. doi:1538-7445.AM2012-798

Abstract 3007: A novel five gene signature predicts overall and recurrence-free survival in NSCLC

Gene expression profiling has been used to characterize prognosis in various cancers. Applying systems biology approach to cancer stem cell data generated from expression profiling of cell lines, a novel 5 gene signature was identified. Earlier studies had shown that side population cells isolated from established Non-Small Cell Lung Cancer (NSCLC) cell lines exhibit cancer stem cell properties. Microarray data from side population (SP) and main population (MP) cells isolated from 4 NSCLC lines (A549, H1650, H460, H1975) were used to examine gene expression profiles associated with stemness. Total RNA extracted from SP and MP samples were used to generate cRNA targets, which were hybridized to Human Genome U133A Plus 2.0 probe arrays. Raw data was processed and the mean center expression level for each gene was determined. We searched for differentially expressed genes that were over or under-expressed at least two fold commonly in all 4 cell lines. It was found that 354 genes were upregulated and 126 genes were downregulated in SP cells from all the 4 cell lines, compared to MP cells; of these, 89 up and 62 downregulated genes (average 2 fold changes) were used for Principle Component Analysis (PCA) and MetaCoreTM pathway analysis. Pathway analysis demonstrated representation of 4 up regulated genes (TOP2A, AURKB, BRRN1, CDK1) in chromosome condensation pathway and 1 down regulated gene FUS in chromosomal translocation. The microarray data was validated using quantitative RT-PCR on the 5 selected genes and all showed robust correlations between microarray and qRT-PCR expression data. Further, we analyzed two independent microarray gene expression datasets that included 361 lung adenocarcinoma patients from NCI Director's Challenge Set (DCS) for overall survival (OS) and 63 adenocarcinoma patients from Sungkyunkwan University (SKKU) for recurrence free survival (RFS). Statistical analyses were done using R package. Expression values were normalized using MAS5.0 and median expression values were used to dichotomize into low and high expression levels for 5 genes. To determine whether the gene signature correlates with poor prognosis, we performed Kaplan-Meier and log-rank test analysis. Kaplan-Meier analysis for survival showed significant trend for all the 5 genes. These genes predicted poor survival of patients in 361 DCS as well as 63 adenocarcinoma samples from SKKU. Our results suggest that genes involved in chromosome condensation are likely related with cancer stem cell like properties and therefore predict survival in lung adenocarcinoma. Further studies are needed to validate the key genes to provide potential new targets to intervene progression of lung adenocarcinoma. Our findings highlight gene signature for effective identification of lung adenocarcinoma patients with poor prognosis and designing more aggressive or tailored therapeutic properties for such patients.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3007. doi:1538-7445.AM2012-3007

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&lt;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&lt;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 (&gt;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.