Prolonged G1 Arrest Research Articles

Abstract LB035: The role of nucleophosmin1 mediated caspase2 activation in acute myeloid leukemia cell death and cell survival

Abstract Nucleophosmin 1 (NPM1) mutation is the primary genetic lesion in adult acute myeloid leukemia (AML). Typically, a nucleolar protein, NPM1 mutations cause cytoplasmic delocalization of NPM1(NPM1c+), triggering leukemogenesis. Interestingly, NPM1c+ mutation confers enhanced chemosensitivity. However, the mechanism underlying NPM1c+ mutation-induced oncogenesis or chemosensitivity remains unclear. We previously reported that, in response to genotoxic stimuli, NPM1 forms a platform for caspase-2 activation in the nucleolus. Caspase-2 is a pro-apoptotic protein and a known tumor suppressor. Our results indicate that nucleolar caspase-2 activation is specific to NPM1wt cells. In the NPM1c+ cells, caspase-2 activation is exclusively cytosolic, suggesting that caspase-2 is activated in the same sub-cellular compartment as NPM1 localization. Loss of caspase-2 rescued the increased sensitivity of NPM1c+ cells to apoptotic cell death. This is accompanied by reduced cleavage of the apoptotic substrate, caspase-3. Notably, caspase-2 deficient NPM1wt cells showed no significant difference in the percentage of apoptotic cell death or endogenous substrate cleavage compared to its caspase-2 wild-type counterpart. This suggests that caspase-2 may contribute to the chemosensitivity of NPM1c+ cells. Strikingly, in unstimulated cells, loss of caspase-2 leads to prolonged G1 arrest and progressive loss of cell viability. Immunophenotyping of NPM1c+ cells revealed a remarkable increase in the CD14+population representing monocyte/macrophage-like features, suggesting that loss of caspase-2 results in terminal differentiation. Exogenous expression of NPM1c+ in caspase-2 deficient NPM1wt cells impaired viability, confirming that NPM1c+ cells require caspase-2 for its growth and proliferation. Transcriptomic analysis revealed that loss of caspase-2 significantly downregulates pathways involved in stem cell pluripotency. In particular, FGF2, IGFR, WNT, LIF, and one of the core members of the self-renewal transcription factor, OCT 4, were downregulated in the absence of caspase-2 in NPM1c+ cells. This reveals a novel role for caspase-2 in maintaining NPM1c+ AML stemness. Taken together, our study shows that NPM1c+ mediated caspase-2 activation regulates AML cell death and survival cascades, a key determinant of chemosensitivity and leukemogenesis. Citation Format: Dharaniya Sakthivel, Alexandra Brown-Suedel, Francesca Keane, F. Kenneth McSherry, Chloe I. Charendoff, Kevin Dunne, Dexter Robichaux, Jonathan M. Flanagan, Lisa Bouchier-Hayes. The role of nucleophosmin1 mediated caspase2 activation in acute myeloid leukemia cell death and cell survival [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 2 (Clinical Trials and Late-Breaking Research); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(8_Suppl):Abstract nr LB035.

Abstract 6057: Mechanisms mediating the radioresistance of human glioma cells growing in the murine olfactory bulb

Abstract We have previously reported (Int J Radiat Oncol Biol Phys. 107(1):194-201, 2020) that human GBM-stem like cells (GSCs) implanted into the right striatum of nude mice migrate/invade throughout the right hemisphere (RH) as well as into the olfactory bulb (OB). The highly invasive nature of the GSCs suggested a model system for evaluating the effects of specific brain microenvironments on glioma radioresponse and the potential for topographic heterogeneity in radioresistance. Subsequent analyses revealed that GSCs in the OB were significantly more radioresistant than those in the RH. Because the OB is highly enriched in neurons and given recent data suggesting that neuron/glioma interactions drive GBM growth, in this study we first defined contact between GSCs and neurons in the OB and RH using immunohistochemistry (IHC) and confocal microscopy. In the OB 56% of tumor cells were in direct contact with neurons as compared to 24% of tumor cells in the RH. Moreover, among the tumor cells in contact with neurons 25% had 2 or more neuronal contacts in the OB versus 2% in the RH. These data suggest a significantly greater level of glioma-neurons interaction in the OB. To further investigate the OB as a radioresistant niche, RNAseq was used to define the transcriptome of NSC11 GSCs located in the OB versus the RH. Of the greater than 15,000 genes evaluated, 1887 genes were differentially expressed between the 2 sites: 872 upregulated and 1015 down regulated in the OB versus the RH. Functional analyses (IPA and GSEA) indicated that among the genes overexpressed in the OB GSCs were a group associated with the cell cycle, specifically mitosis and the spindle assembly checkpoint (SAC). Comparison of the SAC genes overexpressed in the OB with GBM clinical outcomes (TCGA) showed a significant correlation with decreased patient survival. These analyses suggested that cell cycle regulation may play a role in the radioresistance of OB GSCs. Accordingly, radiation-induced cell cycle redistribution of GSCs in the OB and the RH was determined by IHC analysis of cell cycle phase markers and flow cytometry (propidium iodide) performed on dissociated tumor cells. Results showed a significantly higher percentage of cells in G2 in the OB at 24h and 48h after 10Gy, suggestive of prolonged G2 arrest. In addition, analysis of radiation-induced DNA DSBs using γH2AX and 53BP1 nuclear foci showed faster and more complete foci dispersal after 10Gy in the OB GSCs versus those in the RH, indicative of increased DSBs repair in the OB. These data suggest that GSC radioresistance in the OB is the result of a more effective DNA damage response. Moreover, results suggest that the murine OB may provide a model system for investigating GBM radioresistance as well as the potential role of glioma/neuron interactions. Citation Format: Charlotte M. Degorre, Ian C. Sutton, Stacey L. Lehman, Uma T. Shankavaram, Kevin A. Camphausen, Philip J. Tofilon. Mechanisms mediating the radioresistance of human glioma cells growing in the murine olfactory bulb [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6057.

Mutations in the acetylation hotspots of Rbl2 are associated with increased risk of breast cancer.

Retinoblastoma like protein-2 (Rbl2) is functionally regulated by phosphorylation and acetylation. Previously, we demonstrated that lysine K1083 (K1079 in human Rbl2) is a potential target for acetylation but its functional role remains elusive. We investigated alterations in human Rbl2 gene specifically targeting exons 19-22 harbouring acetylatable residues i.e. K1072, K1083 and K1115 through single stranded conformation polymorphism (SSCP) in breast cancer patients. The K1083 was found altered into arginine (R) in 51% of the cases but K1072 and K1115 remained conserved. The 'K1083R' mutation impairs the acetylation potential of this motif that may result in functional inactivation of Rbl2. These patients also showed poor survival outcome that highlights prognostic relevance of this residue. NIH3T3 cells expressing glutamine (K1083Q) mutated Rbl2 could not be arrested in G1 by serum starvation, whereas cells expressing Rbl2 with K1083R showed prolonged G1 arrest in fluorescence activated cell sorting (FACS) analysis. This suggests that K1083 acetylation is important for G1/S transition. Further, we performed molecular dynamic simulations (MDS) to analyse kinetics of residue K1083 with Cyc-D1/CDK4. Mutations at K1083 impaired this binding exposing neighbouring residues S1080, P1081, S1082 and R1084, hence enhancing the possibility of accelerated phosphorylation. S1080 has previously been reported as a promising candidate of cell cycle dependent phosphorylation in Rbl2. This highlights significance of mutations in the pocket domain of Rbl2 gene in breast cancer, and also strengthen the supposition that K1083 acetylation is pre-requisite for its phosphorylation.

CDK4/6 inhibitors induce replication stress to cause long‐term cell cycle withdrawal

CDK4/6 inhibitors arrest the cell cycle in G1‐phase. They are approved to treat breast cancer and are also undergoing clinical trials against a range of other tumour types. To facilitate these efforts, it is important to understand why a cytostatic arrest in G1 causes long‐lasting effects on tumour growth. Here, we demonstrate that a prolonged G1 arrest following CDK4/6 inhibition downregulates replisome components and impairs origin licencing. Upon release from that arrest, many cells fail to complete DNA replication and exit the cell cycle in a p53‐dependent manner. If cells fail to withdraw from the cell cycle following DNA replication problems, they enter mitosis and missegregate chromosomes causing excessive DNA damage, which further limits their proliferative potential. These effects are observed in a range of tumour types, including breast cancer, implying that genotoxic stress is a common outcome of CDK4/6 inhibition. This unanticipated ability of CDK4/6 inhibitors to induce DNA damage now provides a rationale to better predict responsive tumour types and effective combination therapies, as demonstrated by the fact that CDK4/6 inhibition induces sensitivity to chemotherapeutics that also cause replication stress.

Active RB causes visible changes in nuclear organization.

RB restricts G1/S progression by inhibiting E2F. Here, we show that sustained expression of active RB, and prolonged G1 arrest, causes visible changes in chromosome architecture that are not directly associated with E2F inhibition. Using FISH probes against two euchromatin RB-associated regions, two heterochromatin domains that lack RB-bound loci, and two whole-chromosome probes, we found that constitutively active RB (ΔCDK-RB) promoted a more diffuse, dispersed, and scattered chromatin organization. These changes were RB dependent, were driven by specific isoforms of monophosphorylated RB, and required known RB-associated activities. ΔCDK-RB altered physical interactions between RB-bound genomic loci, but the RB-induced changes in chromosome architecture were unaffected by dominant-negative DP1. The RB-induced changes appeared to be widespread and influenced chromosome localization within nuclei. Gene expression profiles revealed that the dispersion phenotype was associated with an increased autophagy response. We infer that, after cell cycle arrest, RB acts through noncanonical mechanisms to significantly change nuclear organization, and this reorganization correlates with transitions in cellular state.

The Novel Multitarget Small-Molecule Inhibitor SRX3177 Overcomes Ibrutinib Resistance in Mantle Cell Lymphoma

Mantle cell lymphoma (MCL) is an aggressive, rare, and difficult to treat subtype of non-Hodgkin's Lymphoma (NHL) that accounts for about 6% of all cases. Although there is no defined standard of care for MCL treatment, some combination of chemo-immunotherapy and rituximab maintenance with or without autologous stem cell transplantation is generally employed depending on the age and fitness of the patient. Despite recent development of novel therapeutics, there is inevitable disease relapse with progressively declining efficacy and increasing frequency of resistance with single agent targeted therapy. Here, we describe the novel multitarget inhibitor SRX3177 which simultaneously hits three oncogenic targets: phosphatidylinositol-3 kinase (PI3K), cyclin-dependent kinases 4 and 6 (CDK4/6), and the epigenetic reader protein BRD4. This in silico designed, thieno-pyranone (TP) scaffold-based small molecule inhibitor orthogonally disrupt three targets within the cancer cell with one agent.Targeting the cell cycle with small molecule inhibitors represents a reasonable attempt to treat MCL, as cell cycle-associated genes like ATM, TP53, CDKN2A, CCND1 and CDK4/6 are most frequently mutated in patients. Palbociclib is a well-known single agent CDK4/6 inhibitor that has been employed in both solid and hematological malignancies. Due to its cytostatic nature, treatment with single agent palbociclib often results in the emergence of treatment-resistant clones. Therefore, a combination strategy would theoretically be more effective and can overcome the development of resistance. Moreover, prolonged G1 arrest by CDK4/6 inhibition sensitizes lymphoma cells to PI3K inhibition, suggesting a synthetic lethality relationship between these two agents. Inhibiting the chromatin reader protein BRD4 causes downregulation of target genes c-MYC and BCL2, further increasing cytotoxic capabilities. Hence, we developed SRX3177 as a potent CDK4/6/PI3K/BRD4 triple inhibitor to synergistically inhibit cell cycle progression and induce cancer cell apoptosis.SRX3177 is an ATP competitive CDK4/6 inhibitor (IC 50: CDK4 = 2.54 nM, CDK6 = 3.26 nM), PI3K inhibitor (IC 50: PI3Kα = 79.3 nM, PI3Kδ = 83.4 nM), and BRD4 inhibitor (IC 50: BD1 = 32.9 nM, BD2 = 88.8 nM). We have tested the efficacy of SRX3177 against a panel of MCL cell lines and report that SRX3177 induces a strong antiproliferative activity with maximal IC 50 0f 340 nM in JeKo-1, 29 nM in Mino cells, and 630 nM for Rec-1 cells while IC 50 values for cell lines Granta and JVM-2 were 1.3 µM and 1.5 µM, respectively. Further, we show that SRX3177 is more potent to tumor cells than the individual PI3K (BKM120), BTK (Ibrutinib), BRD4 (JQ1), and CDK4/6 (palbociclib) inhibitors, and dual PI3K/BRD4 inhibitor SF2523 (backbone for SRX3177) in JeKo-1 cells. Next, we examine the cytotoxic effect of SRX3177 in ibrutinib/palbociclib resistant primary MCL cells. Our results show that SRX3177 triggers cytotoxic response at 500 nM and 1000 nM as compared to the lack of cytotoxicity of combination Ibrutinib and palbociclib at 150 nM and 1000 nM (Fig 1). SRX3177 induces a strong apoptotic response and cell cycle arrest in JeKo-1 and Mino cells at 24hrs. Annexin V/7AAD apoptosis staining confirmed the induction of PCD by SRX3177with increase in c-PARP. Western blot analysis shows SRX3177 treatment blocks both PI3K/AKT signaling and Rb phosphorylation. Moreover, analysis by chromatin immunoprecipitation revealed that SRX3177 effectively blocked BRD4 binding to both the promoter and enhancer of c-MYC (p≤0.01 and p≤0.001) and BCL2 (p≤0.05). SRX3177 also suppresses the c-MYC and BCL2 transcriptional program in both a time- and dose-dependent manner. Our findings also demonstrate a SRX3177-dependent reduction in c-MYC half-life via induction of proteasomal-mediated degradation. This degradation is associated with decreased phosphorylation of c-MYC at Ser62 and increased phosphorylation of c-MYC at Thr58 - indicative of differential regulation of c-MYC stability. Finally, we show that SRX3177 overcomes chronic ibrutinib resistance in Jeko-1 cells with a maximal IC 50 of 150 nM as compared to 64 µM with ibrutinib. Hence, the triple inhibitor SRX3177 has superior potency to ibrutinib in MCL cell lines and succeeds in overcoming ibrutinib-resistance at nanomolar doses. Taken together, our data supports the development of SRX3177 as a novel therapeutic agent for treatment of MCL. [Display omitted] DisclosuresMartin: ADCT: Consultancy. Park: Takeda: Research Funding; G1 Therapeutics: Consultancy; Teva: Consultancy, Membership on an entity's Board of Directors or advisory committees; Seattle Genetics: Research Funding, Speakers Bureau; Gilead: Speakers Bureau; Rafael Pharma: Membership on an entity's Board of Directors or advisory committees, Other: Advisory Board; Morphosys: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees, Research Funding. Durden: SignalRx Pharmaceuticals: Current holder of individual stocks in a privately-held company.

CDK1 inhibitor controls G2/M phase transition and reverses DNA damage sensitivity

CDK1 plays key roles in cell cycle progression through the G2/M phase transition and activation of homologous recombination (HR) DNA repair pathway. Accordingly, various CDK1 inhibitors have been developed for cancer therapy that induce prolonged G2 arrest and/or sensitize cells to DNA damaging agents in tumor cells, resulting in cell death. However, CDK1 inhibition can induce resistance to DNA damage in certain conditions. The mechanism of different DNA damage sensitivity is not completely understood. We performed immunofluorescence and flow cytometry analysis to investigate DNA damage responses in human tumor cells during low and high dose treatments with RO-3306, a selective CDK1 inhibitor. This comparative investigation demonstrated that RO-3306-induced G2 arrest prevented cells with DNA double-strand breaks from transitioning into the M-phase and that the cells maintained their DNA repair capacity in G2-phase, even under RO-3306 dose-dependent DNA repair inhibition. These findings reveal that CDK1 inhibitor-induced DNA repair inhibition and cell cycle control, which regulate each other during the G2/M phase transition determine the cellular sensitivity to DNA damage, providing insight useful for developing clinical strategies targeting CDK1 inhibition in tumor cells.

Dual inhibition of DNA-PK and DNA polymerase theta overcomes radiation resistance induced by p53 deficiency.

TP53 deficiency in cancer is associated with poor patient outcomes and resistance to DNA damaging therapies. However, the mechanisms underlying treatment resistance in p53-deficient cells remain poorly characterized. Using live cell imaging of DNA double-strand breaks (DSBs) and cell cycle state transitions, we show that p53-deficient cells exhibit accelerated repair of radiomimetic-induced DSBs arising in S phase. Low-dose DNA-dependent protein kinase (DNA-PK) inhibition increases the S-phase DSB burden in p53-deficient cells, resulting in elevated rates of mitotic catastrophe. However, a subset of p53-deficient cells exhibits intrinsic resistance to radiomimetic-induced DSBs despite DNA-PK inhibition. We show that p53-deficient cells under DNA-PK inhibition utilize DNA polymerase theta (Pol θ)-mediated end joining repair to promote their viability in response to therapy-induced DSBs. Pol θ inhibition selectively increases S-phase DSB burden after radiomimetic therapy and promotes prolonged G2 arrest. Dual inhibition of DNA-PK and Pol θ restores radiation sensitivity in p53-deficient cells as well as in p53-mutant breast cancer cell lines. Thus, combination targeting of DNA-PK- and Pol θ-dependent end joining repair represents a promising strategy for overcoming resistance to DNA damaging therapies in p53-deficient cancers.

Pulsatile MAPK Signaling Modulates p53 Activity to Control Cell Fate Decisions at the G2 Checkpoint for DNA Damage

SummaryCell-autonomous changes in p53 expression govern the duration and outcome of cell-cycle arrest at the G2 checkpoint for DNA damage. Here, we report that mitogen-activated protein kinase (MAPK) signaling integrates extracellular cues with p53 dynamics to determine cell fate at the G2 checkpoint. Optogenetic tools and quantitative cell biochemistry reveal transient oscillations in MAPK activity dependent on ataxia-telangiectasia-mutated kinase after DNA damage. MAPK inhibition alters p53 dynamics and p53-dependent gene expression after checkpoint enforcement, prolonging G2 arrest. In contrast, sustained MAPK signaling induces the phosphorylation of CDC25C, and consequently, the accumulation of pro-mitotic kinases, thereby relaxing checkpoint stringency and permitting cells to evade prolonged G2 arrest and senescence induction. We propose a model in which this MAPK-mediated mechanism integrates extracellular cues with cell-autonomous p53-mediated signals, to safeguard genomic integrity during tissue proliferation. Early steps in oncogene-driven carcinogenesis may imbalance this tumor-suppressive mechanism to trigger genome instability.

Human mesenchymal stem cells are resistant to UV-B irradiation

Albeit being an effective therapy for various cutaneous conditions, UV-B irradiation can cause severe skin damage. While multipotent mesenchymal stem cells (MSCs) may aid the regeneration of UV-B-induced skin injuries, the influence of UV-B irradiation on MSCs remains widely unknown. Here, we show that human MSCs are relatively resistant to UV-B irradiation compared to dermal fibroblasts. MSCs exhibited higher clonogenic survival, proliferative activity and viability than dermal fibroblasts after exposure to UV-B irradiation. Cellular adhesion, morphology and expression of characteristic surface marker patterns remained largely unaffected in UV-irradiated MSCs. The differentiation ability along the adipogenic, osteogenic and chondrogenic lineages was preserved after UV-B treatment. However, UV-B radiation resulted in a reduced ability of MSCs and dermal fibroblasts to migrate. MSCs exhibited low apoptosis rates after UV-B irradiation and repaired UV-B-induced cyclobutane pyrimidine dimers more efficiently than dermal fibroblasts. UV-B irradiation led to prolonged p53 protein stability and increased p21 protein expression resulting in a prolonged G2 arrest and senescence induction in MSCs. The observed resistance may contribute to the ability of these multipotent cells to aid the regeneration of UV-B-induced skin injuries.

Impact of aging on transition of acute kidney injury to chronic kidney disease

Acute kidney injury (AKI) increases the risk of end stage renal disease among the elderly, but the precise underlying mechanism is unknown. We investigated the effects of aging on AKI-to-chronic kidney disease (CKD) transition, focusing on renal inflammation. Aged and young C57BL/6 mice were subjected to bilateral ischemia-reperfusion injury (IRI). Baseline proinflammatory cytokine levels of kidneys were elevated in aged mice. After IRI, aged mice also showed persistent M1 dominant inflammation, with increased proinflammatory cytokines during the recovery phase. Persistent M1 inflammation was associated with blunted activation of CSF-1/IRF4 signal for M1/M2 polarization, but in vitro macrophage polarization with cytokine stimulation was not different between young and aged mononuclear cells. The tubular expressions of cell cycle arrest markers increased in aged mice during recovery phase, and in vitro transwell experiments showed that mononuclear cells or M1 macrophages co-cultured with arrested proximal tubular cells at G1 phase significantly impaired M2 polarization, suggesting that prolonged G1 arrest might be involved in persistent M1 inflammation in aged mice. Finally, M1 dominant inflammation in aged mice resulted in fibrosis progression. Our data show that impaired M2 polarization partially driven by senescent tubule cells with cell-cycle arrest may lead to an accelerated progression to CKD in the elderly.

Cooperative Repression of IRF4 and Induction of IRF7 By CDK4/6 Inhibition Sensitizes Mantle Cell Lymphoma Cells to IMiD-Mediated Apoptosis Independent of p16 Deletion or c-Myc Overexpression

Lenalidomide (Len) and pomalidomide (Pom) are standard of care therapies for multiple myeloma (MM) and also exhibit clinical efficacy in some lymphomas such as diffuse large B cell lymphoma (DLBCL) and mantle cell lymphoma (MCL). As single agents, each rarely achieves complete remission despite a high overall response rate and durability; however, they potentiate the clinical response to diverse partners including Rituximab in MCL. As immunomodulatory drugs (IMiDs), Len and Pom target multiple cell types in vivo and have been shown to promote the expansion of NK cells in MCL. Len in combination with BTK inhibitors apparently leads to synthetic lethality in the activated B cell-like DLBCL cell lines, but the mechanism that mediates IMiD sensitivity in MCL cells remains largely unknown.We found that Len markedly enhanced ACP196 (second generation BTK inhibitor) killing of the CCMCL1 MCL cell line, despite c-Myc overexpression and deletion of p16INK4a, but not the JEKO-1 cells, which harbor TP53 deletion. Therefore Len appears to enhance killing by BTK inhibition but does not confer sensitivity to resistant MCL cells. Next, we investigated how inhibition of CDK4 might sensitize MCL cells to Len or Pom killing, given that dysregulation of cyclin D1 and CDK4 (CDK6 is silenced in MCL) underlies uncontrolled proliferation and disease progression, and inhibition of CDK4 is clinically efficacious in MCL. Indeed, priming of CCMCL1 cells by prior induction of G1 arrest with the selective CDK4/6 inhibitor palbociclib markedly sensitized them to Len-induced apoptosis, but only marginally so in JEKO-1 cells. Kinetic studies revealed that Len reduced the IRF4 protein markedly in CCMCL1 cells arrested in G1 by CDK4 inhibition, but marginally so in cycling CCMCL1 cells and not at all in Len-resistant JEKO-1 or Maver-1 cells. Reduction of IRF4 protein by Len in prolonged G1 arrest was concomitant with induction of IRF7 protein and tightly associated with apoptosis, as evidenced by PARP and caspase-3 cleavage. Moreover, inhibition of CDK4 conferred sensitivity to Pom in the Len-resistant JEKO-1 cells, and it was concurrent with reduction of IRF4 and induction of IRF7.The triple combination of Len, Ibrutinib, and Rituximab in MCL has been shown to be clinically active in relapsed MCL. To explore the underpinning, we found by RNA-seq that CD20 mRNA expression was downregulated in normal primary human peripheral B cells when they were activated into the cell cycle and in primary MCL cells, suggesting an inverse correlation between cell cycling and CD20 mRNA expression. However, surface CD20 expression was not collinear with CD20 mRNA expression and there was no evidence for upregulation of CD20 by BTK inhibition. On this basis, upregulation of the rituximab target CD20 is unlikely to be the primary mechanism for improved clinical response to this triple combination in MCL.In summary, our data suggest cooperative repression of IRF4 and induction of IRF7 as a central mechanism that underlies enhancement of Len and Pom killing by inhibition of BTK or CDK4, independent of deletion of the physiologic CDK4/6 inhibitor p16INK4a or overexpression of c-Myc. Furthermore, it appears that induction of prolonged early G1 arrest by Palbociclib confers Pom sensitivity independent of p53, implicating a potential to combine Palbociclilb with Pom for treatment of MCL with p53 loss or mutation. DisclosuresNo relevant conflicts of interest to declare.

PO-143 Frequent loss of annexin A1 expression in head and neck squamous cell carcinomas by transcriptional regulatory mechanisms

IntroductionColorectal cancer (CRC) is amongst most frequent tumour types worldwide. Survivin, the smallest member of the inhibitors of apoptosis (IAP) family, is a well-established resistance marker, highly expressed in all tumour types including CRC. The aim of our study was to ascertain whether cellular localization of Survivin might affect the response of CRC cells to the first-line treatment with the topoisomerase I inhibitor irinotecan (IT).Material and methodsThe methods used were DNA cloning, site-directed mutagenesis, transfection and generation of stable clones, immune-fluorescence, colony-forming assays, flow cytometry, ELISA, PCR-based homologous recombination (HR) repair activity assays, western blot analysis, γH2AX foci formation and chromosomal aberration assays.Results and discussionsIn order to examine the effects of Survivin expression and localization on the response to IT, we transfected the p53-proficient CRC cell line (SW48) with the recombinant vector expressing Survivin-GFP fusion protein mainly localised to the cytoplasm. We also generated stable SW48 clones mutated in the nuclear export sequence (NES) of Survivin leading to its retention and accumulation in the nucleus. The proper localization of the fusion protein variants (∼42 kDa) i.e. the functional Survivin-GFP nuclear shuttling by passive diffusion and the active nuclear export were proven by leptomycin B (nuclear export inhibitor), leading to accumulation of the fusion protein in the nucleus. In colony-forming assays the striking difference between the clones was observed: the Survivin-NES-GFP clone was hypersensitive towards IT as compared to the Survivin-GFP clone or parental cells. Flow-cytometric cell-cycle distribution analyses showed that IT induced a prolonged G2-arrest (without induction of senescence) and a higher apoptotic frequency in the Survivin-NES clone, as compared to the parental cell line and the Survivin-GFP clone. Most interesting, the NES clone showed a low repair efficiency of DNA double-strand breaks (DSBs) by HR and lots of aberrant mitoses with a high clastogenic frequency, underlying the onset of mitotic catastrophe.ConclusionCytoplasm-localised Survivin with a normal shuttling function enhanced reproductive survival upon IT exposure and showed a reduced amount of IT-induced DSBs, probably supporting the repair by HR, while the export-impaired NES-Survivin led to an extremely toxic response towards IT. Therefore, Survivin localization could be a prognostic marker for topoisomerase I inhibitor-based CRC therapy.

Identification of a Gene Signature Associated to Treatment Response to Palbociclib in Multiple Myeloma

Background: Inpatients with relaspsed/refractoryMultiple Myeloma (MM), outcomes are far from optimal, especially in patients refractory to current treatments Recent studies and clinical trials have highlighted the therapeutic potential of Palbociclib, a CDK4/6 inhibitor, in various cancers including MM. Deregulation of CDK4/6 is involved in the loss of cell cycle control in MM. Response to Palbociclib combined with bortezomib and dexamethasone was acquired in 20% of the relapsed/refractory MM patients, suggesting that biomarkers to identify patients that could benefit from this treatment are needed. Additional studies are required to understand the biological pathways associated with sensitivity or resistance of MM cells to Palbociclib.Methods: 14 human MM cell lines and 12 primary MM samples were tested for response to Palbociclib treatment. The concentration required to inhibit growth by 50% (IC50) was calculated. Gene expression signature associated with multiple myeloma response to Palbociclib, as well as, genes deregulated by the treatment have been analyzed using microarray and RNA-sequencing methods.Results: Palbociclib had an heterogeneous in vitro activity among the 14 human myeloma cell lines tested, which aggregated into three groups based on the distribution of the IC50 values: sensitive (n = 5, IC50: 0.2 - 0.3µM), intermediate (n = 3, IC50: 0.5 - 0.7µM) or more resistant group (n = 6, IC50: 0.9 - 2.4µM). The same holds true when testing the Palbociclib on primary multiple myeloma samples. The evaluation of the Palbociclib effect on cell cycle progression and the induction of the apoptosis, reveals that Palbociclib is essentially cytostatic, inducing prolonged G1 arrest in sensitive cell lines with a strong reduction of the percentage of cells in S phase.To better understand the molecular mechanisms associated with Palbociclib response, we identified a gene expression signature correlated with the response in both MM cell lines and primary myeloma cells from patients. Additionally, we have analyzed differentially expressed genes after Palbociclib treatment in human MM cell lines using RNA sequencing (n = 4). The physiological role of the downregulated genes after Palbociclib treatment is associated with cell cycle, mitosis and E2F mediated regulation of DNA replication. Significantly upregulated genes, after Palbociclib treatment, were enriched in genes encoding proteins involved in glutathione synthesis and recycling, and biological oxidations.Conclusion: Altogether, our data demonstrated a high heterogeneity in the response of MM cells to Palbociclib. We identified a gene expression signature associated with Palbociclib response in MM. These genes could help to identify MM patients that could benefit from Palbociclib treatment and provide novel targets for efficient combination therapy. DisclosuresNo relevant conflicts of interest to declare.

4P IGF-IR, but not EGFR, regulates DNA damage response in HeLa cells following irradiation

Legal entity responsible for the study The roles of receptor tyrosine kinases in DNA damage response (DDR) are still largely unknown. In this study, we examined the possible involvement of insulin-like growth factor I receptor (IGF-IR) and epidermal growth factor receptor (EGFR) in the DNA damage response (DDR) following irradiation. Methods HeLa cells expressing the fluorescence ubiquitination-based cell cycle indicator (Fucci) probes (HeLa-Fucci cells) were used in this study. Kinetics of the Fucci fluorescence were detected by FACS and time-lapse imaging. NVP-AEW541, Tyrphostin AG1478, LY294002, PD98059, NU7026 were used as specific inhibitors for IGF-IR, EGFR, PI3-K, MEK, and DNA-PKcs, respectively. Phosphorylation of ERK1/2 and Akt was detected by western blotting. Double strand breaks (DSBs) were detected by immunofluorescence staining for 53BP1. Cells were irradiated using an RX-650 Cabinet X-radiator system (Faxitron). Results To investigate the possible involvement of EGFR and IGF-IR in G2 arrest, FACS analysis and time-lapse imaging of Fucci fluorescence were performed using specific inhibitors. Results showed that inhibition of IGF-IR, but not that of EGFR, prolonged G2 arrest, which was irrespective of cell cycle phases at irradiation, i.e., red (G1 phase) or green (S/G2 phases) phase in the Fucci system. Similarly, only inhibition of IGF-IR decreased the DSB repair activity. Hereafter, further analysis was focused on IGF-IR-associated events. We next attempted to identify the responsible IGF-IR- downstream signaling pathways. Irradiation phosphorylated both Akt and ERK; however, inhibition of IGF-IR abrogated activation of only Akt. Moreover, inhibition of PI3-K/Akt, but not that of MEK/ERK, prolonged G2 arrest, which mimicked that of IGF-IR. Inhibition of DNA-PKcs, a major factor of non-homologous end joining (NHEJ), also prolonged G2 arrest. Conclusions We conclude that irradiation is likely to preferentially activate the IGF- IR/PI3-K/Akt signaling pathway in HeLa-Fucci cells, which may enhance DSB repair, eventually contributing to reduction of G2 arrest in the DDR following irradiation. Legal entity responsible for the study Tokyo Medical and Dental University Funding Tokyo Medical and Dental University, MEXT Japan Disclosure All authors have declared no conflicts of interest.

Modulation of Pathways Underlying Distinct Cell Death Mechanisms in Two Human Lung Cancer Cell Lines in Response to SN1 Methylating Agents Treatment.

Despite advances in the understanding of tumor biology in recent years, lung cancer remains the leading cause of cancer death worlwide. Non small cell lung cancer (NSCLC; adenocarcinoma, squamous carcinoma, and large cell carcinoma) is the most common type, representing more than 80% of lung cancer diagnoses. Clinical improvements following treatment with traditional chemotherapy have reached a plateau in patients with NSCLC and 5 year survival still remains at 15%. Moreover, the molecularly targeted therapies having already progressed to clinical trials have not proved successful [1,2]. Although NSCLC is very often extremely resistant to chemotherapeutic agents, chemotherapy remains an established treatment for advanced NSCLC. Platinum-based chemotherapy frequently combined with other agents and/or approaches remain the standard in the first-line treatment of patients with advanced NSCLC [3,4]. Methylating agents such as temozolomide, an oral SN1 methylating agent which crosses the blood brain barrier achieving effective concentrations in the Central Nervous System (CNS), have already been used combined with platinum-based regimen and whole brain radiotherapy as prophylaxis against brain metastasis in NSCLC and/or as second-line treatment in NSCLC patients with brain metastases [5–8]. Methylating agents of SN1 type, including temozolomide (TMZ), procarbazine, dacarbazine and streptozotocine, constitute a widely used class of anticancer drugs. These DNA damaging agents are highly cytotoxic, mutagenic, recombinogenic and clastogenic inducing about a dozen DNA methylation adducts [9]. Among them, O6-methylguanine (O6-meG), induced in small amounts (maximally 8% of total methylation products), is the most biologically significant lesion; it mispairs with thymine during DNA replication finally generating G to A transitions in the second round of replication. The DNA mismatches are recognized by the Methyl-directed Mismatch Repair (MMR) system that removes the mis-incorporated base opposite the O6-meG lesion. Repair resynthesis of DNA leads to reinsertion of thymine opposite O6-meG and re-initiation of futile mismatch repair. The DNA double-strand breaks (DSBs) resulting from this process are thought to initiate a series of events including prolonged G2 arrest leading to cell death [9]. A critical factor influencing the cellular response to methylating agents is O6-methylguanine- DNA methyltransferase (MGMT), the DNA repair protein that stoichiometrically and selectively removes methyl lesions from the O6 position of guanine and returns the DNA to its pre-lesioned state [10]. Pre-replicative repair by MGMT as well as post-replicative MMR determine the level of methylating agent-induced genotoxicity and cell death [11–13]. Chemotherapeutic agents inducing DNA damage, such as SN1 methylating agents and cisplatin, may activate cell death by apoptosis or necrosis. They could also induce autophagy, senescence or mitotic catastrophe, which may then be followed by apoptosis or necrosis [13–15]. The molecular basis underlying the decision- making process is currently the subject of intense investigation because a deeper understanding of how a given chemotherapy affects all of the signalling pathways involved in cell death is highly relevant in order to develop more effective therapeutics. Despite their use in combination therapies, the effect of SN1 methylating agents on human NSCLC has not been studied thoroughly. We thus investigated the mechanism of the cell death induced by a model SN1 methylating agent, N-methyl-N-nitrosourea (MNU) in two human NSCLC cell lines, A549 (p53wt) and H157 (p53null) [13] through a time course gene expression profiling study 24, 48 and 72 hours after treatment. The list of differentiated genes, biological processes and cellular pathways were identified using appropriate bioinformatics tools and the results were further validated through RT-PCR of selected genes. MNU induced cell death through distinct responses at the gene expression level in the above cell lines. Our results overall support the use of SN1 methylating agents in platinum-based combination regimen against advanced NSCLC.

Downregulation of vimentin expression increased drug resistance in ovarian cancer cells.

Cisplatin and other platinum-based drugs have been widely used in the treatment of ovarian cancer, but most patients acquire the drug resistance that greatly compromises the efficacy of drugs. Understanding the mechanism of drug resistance is important for finding new therapeutic approaches. In the present study, we found that the expression of vimentin was downregulated in drug-resistant ovarian cancer cell lines A2780-DR and HO-8910 as compared to their respective control cells. Overexpression of vimentin in A2780-DR cells markedly increased their sensitivity to cisplatin, whereas knockdown of vimentin in A2780, HO-8910-PM and HO-8910 cells increased the resistance to cisplatin, demonstrating that vimentin silencing enhanced cisplatin resistance in ovarian cancer cells. Quantitative proteomic analysis identified 95 differentially expressed proteins between the vimentin silenced A2780 cells (A2780-VIM-KN) and the control cells, in which downregulation of endocytic proteins and the upregulation of exocytotic proteins CHMP2B and PDZK1 were proposed to contribute the decreased cisplatin accumulation in vimentin knockdown cells. Silencing of vimentin induced upregulation of cancer stem cell markers and both A2780-DR and A2780-VIM-KN cells were more facile to form spheroids than control cells under serum-free culture condition. Our results also revealed that vimentin knockdown increased the 14-3-3 mediated retention of Cdc25C in the cytoplasm, leading to inactivation of Cdk1 and the prolonged G2 phase arrest that allowed the longer period of time for cells to repair cisplatin-damaged DNA. Taken together, we demonstrated that vimentin silencing enhanced cells' resistance to cisplatin via prolonged G2 arrest and increased exocytosis, suggesting that vimentin is a potential target for treatment of drug resistant ovarian cancer.

Unusual prolongation of radiation-induced G2 arrest in tumor xenografts derived from HeLa cells.

The effect of ionizing radiation on cell cycle kinetics in solid tumors remains largely unknown because of technical limitations and these tumors’ complicated structures. In this study, we analyzed intratumoral cell cycle kinetics after X-irradiation of tumor xenografts derived from HeLa cells expressing the fluorescent ubiquitination-based cell cycle indicator (Fucci), a novel system to visualize cell cycle kinetics in vivo. Cell cycle kinetics after X-irradiation was examined by using tumor sections and in vivo real-time imaging system in tumor xenografts derived from HeLa cells expressing Fucci. We found that G2 arrest was remarkably prolonged, up to 5 days after 10-Gy irradiation, in contrast to monolayer cultures where G2 arrest returned within 24 h. Cells isolated from tumors 5 days after irradiation exhibited a higher surviving fraction than those isolated immediately or one day after irradiation. In this study, we clearly demonstrated unusual post-irradiation cell cycle kinetics in tumor xenografts derived from HeLa-Fucci cells. Our findings imply that prolonged G2 arrest occurring in tumor microenvironments following irradiation may function as a radioresistance mechanism.

Trametinib radiosensitises RAS- and BRAF-mutated melanoma by perturbing cell cycle and inducing senescence

PurposeRadiotherapy (RT) is used frequently in patients with melanoma, but results are suboptimal because the disease is often radioresistant. This may be due to constitutive activation of MAPK pathway signalling through mutations involving RAS/RAF. Thus, we studied whether trametinib, a potent and selective allosteric inhibitor of MEK1/2 could improve the efficacy of RT. Methods and materialsClonogenic survival assays were performed in human BRAF-mutant (A375), NRAS-mutant (D04, WM1631), KRAS-mutant (WM1791c) and wild-type (PMWK) melanoma cell lines. The effects of trametinib with and without radiation on protein levels of MEK effectors were measured by immunoblot analyses. Cell cycle effects, DNA damage repair, mitotic catastrophe and senescence were measured using flow cytometry, γH2Ax staining, nuclear fragmentation and β-galactosidase staining, respectively. Additionally, athymic mice with D04 flank tumours were treated with fractionated RT after gavage with trametinib and monitored for tumour growth. ResultsAll cell lines, except PMWK, exhibited enhanced cytotoxicity when RT was combined with trametinib compared to either agent alone. Sensitiser enhancement ratios were 1.70, 1.32, 1.10, and 1.70 for A375, D04, WM1361 and WM1791c, respectively. Trametinib efficiently blocked RT-induced phosphorylation of ERK at nanomolar concentrations. Increased radiosensitivity correlated with prolonged G1 arrest and reduction in the radioresistant S phase up to 48h following RT. A larger population of senescence-activated β-galactosidase-positive cells was seen in the trametinib pretreated group, and this correlated with activation of two of the major mediators of induced senescence, p53 and pRb. Mice receiving the combination treatment (trametinib 1mg/kg and RT over 3days) showed a reduced mean tumour volume compared with mice receiving trametinib alone (p=0.016), or RT alone (p=0.047). No overt signs of drug toxicity were observed. ConclusionTrametinib radiosensitised RAS-/RAF-mutated melanoma cells by inducing prolonged G1 arrest and premature senescence. In this pre-clinical study we demonstrate that combining trametinib and RT is well tolerated, and reduces tumour growth in vivo.

Oncogenic HRAS Activates Epithelial-to-Mesenchymal Transition and Confers Stemness to p53-Deficient Urothelial Cells to Drive Muscle Invasion of Basal Subtype Carcinomas.

Muscle-invasive urothelial carcinomas of the bladder (MIUCB) exhibit frequent receptor tyrosine kinase alterations, but the precise nature of their contributions to tumor pathophysiology is unclear. Using mutant HRAS (HRAS*) as an oncogenic prototype, we obtained evidence in transgenic mice that RTK/RAS pathway activation in urothelial cells causes hyperplasia that neither progresses to frank carcinoma nor regresses to normal urothelium through a period of one year. This persistent hyperplastic state appeared to result from an equilibrium between promitogenic factors and compensatory tumor barriers in the p19-MDM2-p53-p21 axis and a prolonged G2 arrest. Conditional inactivation of p53 in urothelial cells of transgenic mice expressing HRAS* resulted in carcinoma in situ and basal-subtype MIUCB with focal squamous differentiation resembling the human counterpart. The transcriptome of microdissected MIUCB was enriched in genes that drive epithelial-to-mesenchymal transition, the upregulation of which is associated with urothelial cells expressing multiple progenitor/stem cell markers. Taken together, our results provide evidence for RTK/RAS pathway activation and p53 deficiency as a combinatorial theranostic biomarker that may inform the progression and treatment of urothelial carcinoma.