Mitosis Entry Research Articles

Key Proteins of Replication Stress Response and Cell Cycle Control as Cancer Therapy Targets.

Replication stress (RS) is a characteristic state of cancer cells as they tend to exchange precision of replication for fast proliferation and increased genomic instability. To overcome the consequences of improper replication control, malignant cells frequently inactivate parts of their DNA damage response (DDR) pathways (the ATM-CHK2-p53 pathway), while relying on other pathways which help to maintain replication fork stability (ATR-CHK1). This creates a dependency on the remaining DDR pathways, vulnerability to further destabilization of replication and synthetic lethality of DDR inhibitors with common oncogenic alterations such as mutations of TP53, RB1, ATM, amplifications of MYC, CCNE1 and others. The response to RS is normally limited by coordination of cell cycle, transcription and replication. Inhibition of WEE1 and PKMYT1 kinases, which prevent unscheduled mitosis entry, leads to fragility of under-replicated sites. Recent evidence also shows that inhibition of Cyclin-dependent kinases (CDKs), such as CDK4/6, CDK2, CDK8/19 and CDK12/13 can contribute to RS through disruption of DNA repair and replication control. Here, we review the main causes of RS in cancers as well as main therapeutic targets-ATR, CHK1, PARP and their inhibitors.

Genetically Encoded Fluorescence Resonance Energy Transfer Biosensor for Live-Cell Visualization of Lamin A Phosphorylation at Serine 22.

Extensive phosphorylation at serine 22 (pSer22) on lamin A is the hallmark of cell mitosis, which contributes to the breakdown of nuclear envelope. In the interphase, pSer22 lamin A exists in low abundance and is involved in mechanotransduction, virus infection, and gene expression. Numerous evidences emerge to support lamin A regulation on cell function and fate by phosphorylation. However, live-cell imaging tools for visualizing the dynamics of pSer22 lamin A are yet to be established. Herein, we developed a novel lamin A phosphorylation sensor (LAPS) based on fluorescence resonance energy transfer (FRET) with high sensitivity and specificity. We observed the dynamic lamin A phosphorylation during the cell cycle progression in single living cells: the increase of pSer22 modification when cells entered the mitosis and recovered upon the mitosis exit. Our biosensor also showed the gradual reduction of pSer22 modification during cell adhesion and in response to hypotonic environment. By applying LAPS, we captured the propagation of pSer22 modification from inside to outside of the inner nuclear membrane, which further led to the breakdown of nuclear envelope. Meanwhile, we found the synchronous phosphorylation of pSer22 lamin A and H3S10ph at mitosis entry. Inhibition of Aurora B, the responsible kinase for H3S10ph, could shorten the mitotic period without obvious effect on the pSer22 modification level of lamin A. Thus, LAPS allows the spatiotemporal visualization of the lamin A pSer22, which will be useful for elucidating the molecular mechanisms underlying cell mitosis and mechanoresponsive processes.

Combination therapy with WEE1 inhibition and trifluridine/tipiracil against esophageal squamous cell carcinoma.

Despite advanced therapeutics, esophageal squamous cell carcinoma (ESCC) remains one of the deadliest cancers. Here, we propose a novel therapeutic strategy based on synthetic lethality combining trifluridine/tipiracil and MK1775 (WEE1 inhibitor) as a treatment for ESCC. This study demonstrates that trifluridine induces single-strand DNA damage in ESCC cells, as evidenced by phosphorylated replication protein 32. The DNA damage response includes cyclin-dependent kinase 1 (CDK1) (Tyr15) phosphorylation as CDK1 inhibition and a decrease of the proportion of phospho-histone H3 (p-hH3)-positive cells, indicating cell cycle arrest at the G2 phase before mitosis entry. The WEE1 inhibitor remarkedly suppressed CDK1 phosphorylation (Try15) and reactivated CDK1, and also increased the proportion of p-hH3-positive cells, which indicates an increase of the number of cells into mitosis. Trifluridine combined with a WEE1 inhibitor increased trifluridine-mediated DNA damage, namely DNA double-strand breaks, as shown by increased γ-H2AX expression. Moreover, the combination treatment with trifluridine/tipiracil and a WEE1 inhibitor significantly suppressed tumor growth of ESCC-derived xenograft models. Hence, our novel combination treatment with trifluridine/tipiracil and a WEE1 inhibitor is considered a candidate treatment strategy for ESCC.

The histone methyltransferase NSD3 contributes to sister chromatid cohesion and to cohesin loading at mitotic exit.

Sister chromatid cohesion is a multi-step process implemented throughout the cell cycle to ensure the correct transmission of chromosomes to daughter cells. Although cohesion establishment and mitotic cohesion dissolution have been extensively explored, the regulation of cohesin loading is still poorly understood. Here, we report that the methyltransferase NSD3 is essential for mitotic sister chromatid cohesion before mitosis entry. NSD3 interacts with the cohesin loader complex kollerin (composed of NIPBL and MAU2) and promotes the chromatin recruitment of MAU2 and cohesin at mitotic exit. We also show that NSD3 associates with chromatin in early anaphase, prior to the recruitment of MAU2 and RAD21, and dissociates from chromatin when prophase begins. Among the two NSD3 isoforms present in somatic cells, the long isoform is responsible for regulating kollerin and cohesin chromatin-loading, and its methyltransferase activity is required for efficient sister chromatid cohesion. Based on these observations, we propose that NSD3-dependent methylation contributes to sister chromatid cohesion by ensuring proper kollerin recruitment and thus cohesin loading.

Mechanistic Study of Wee1 Kinase Inhibition with AZD1775 Exposes Drug Targetable Vulnerabilities in Acute B-Lymphoblastic Leukemia

Targeted therapies exploiting vulnerabilities of cancer cells hold promise for improving patient outcome and reducing side effects of chemotherapy. Despite a high response rate to induction chemotherapy in acute lymphoblastic leukemia (ALL) there is an urgent need to identify effective combination therapy to overcome treatment resistance that associates with disease relapse. ALL cells are acutely highly sensitive to the WEE1 inhibitor AZD1775 that compromises the G2/M-phase cell cycle checkpoint. Here, we show that AZD1775 selectively inhibits recovery of proliferation in KMT2A/MLL-rearranged (MLL-r) ALL, a group of leukemias that respond poorly to current chemotherapy and have dismal prognosis compared to other leukemia subgroups. This vulnerability could be distinguished by elevated WEE1 expression in MLL-r ALL and through biochemical analysis of the resulting mitotic catastrophe upon WEE1 inhibition as assessed by PARP cleavage and flow cytometric analysis of DNA metabolism. Extensive genomics profiling of the drug response by joint single-cell transcriptome and chromatin accessibility profiling revealed that AZD1775 led to a strong p53-driven gene regulatory response resulting in induction of apoptosis and senescence and disrupts a crucial hematopoietic TF-network involving RUNX1, MYC and GATA2. We further demonstrate that RUNX1 protein was degradaded in a CDK-dependent manner. Leukemia relapse may arise from rare cells that escape treatment through genetic mutations, transcriptional reprogramming or signaling rewiring. We here demonstrate that upon the pre-mature mitosis entry resulting from AZD1775 treatment, a rapid transcriptional reprogramming is triggered involving activation of NfKB and transcription factors regulating lipid metabolism, BCL6 and pre-BCR signaling that resulted in a small surviving cell population. Single-cell characterization of the AZD1775 response in a MLL-r patient-derived xenograft (PDX) model supported the in vivo relevance of this drug resistance mechanism. Informed by the genomic profiles, sequential administration of AZD1775 with inhibitors of pre-BCR signaling or fatty acid synthesis was designed to target the resistant cell state and this approach effectively prevented cell survival and recovery of ALL cells. Collectively, our findings provide new insights into the tight connectivity of gene regulatory programs associated with cell survival and cell fate regulation in response to drugs targeting cell cycle regulation and provide a rationale for combination therapies with low toxicity drugs such as dasatinib or fatostatin to counteract drug resistance that occurs through cell state switching.

Bortezomib Is Effective in Treating T-ALL, Inducting G2/M Cell Cycle Arrest and WEE1 Downregulation

T lymphoblastic leukaemia (T-ALL) is an aggressive haematological malignancy with poor outcome, especially for relapse/refractory disease. Early T- cell precursor acute lymphoblastic leukaemia (ETP-ALL) is a recently identified subtype of T-ALL with worse treatment outcome compared with other subtypes of T-ALL and treatment options are limited. T-ALL frequently harbors genetic aberrations leading to cell cycle dysregulation and it is one of the major molecular pathogenesis of T-ALL. WEE1 is a protein kinase that is responsible for inhibiting mitosis with unrepaired damaged DNA via inactivating CDK1. WEE1 is highly express in adult T-ALL and its overexpression is associated with adverse prognosis in various cancers. Inhibiting WEE1 expression is a novel approach of therapy. Bortezomib is a 26S proteosome inhibitor and it is FDA approved for treating plasma cell myeloma and mantle cell lymphoma. Bortezomib had been demonstrated therapeutic efficacy in clinical setting for relapse/refractory paediatric T-ALL and B-ALL when combined with chemotherapy. Despite its therapeutic efficacy in clinical studies, the mechanism of action of Bortezomib in T-ALL remain uncertain. The role of Bortezomib in cell cycle modulation had not been established in T-ALL. Moreover, it had not been demonstrated that the effect of Bortezomib in WEE1 expression in T-ALL. Here, we present our study that demonstrated the therapeutic efficacy of Bortezomib in treating T-ALL via cell cycle modulation and downregulation of WEE1 by Bortezomib.T-ALL cell lines including MOLT16, MOLT4, LOUCY and CEM were used in the study. Cell viability was measured by trypan blue. Apoptosis and cell cycle analysis were measured by flow cytometry. Western blot of WEE1, p53, cyclin B1, p21 and p27 were performed.Our result showed that Bortezomib reduce the cell viability of T-ALL cell lines in dose and time-dependent manner. Bortezomib was also sensitive towards LOUCY, a T-ALL cell line with ETP-ALL phenotype. It implied that Bortezomib could be a promising therapy for ETP-ALL. Bortezomib also triggered apoptosis in various T-ALL and the effect of apoptosis was more pronounced after 72 hours of treatment when compared with 24-hour. Again, Bortezomib was able to induce apoptosis in LOUCY cell line. G2/M cell cycle arrest was observed in various T-ALL upon treatment of Bortezomib. The effect on cell cycle modulation was also observed in LOUCY cell line. The protein expression of p21 and p27 were increased after the treatment of Bortezomib. The level of cyclin B1 was increased also. There was upregulation of p53 after Bortezomib treatment. Strikingly, the protein expression level of WEE1 was reduced. The findings of WEE1 downregulation by Bortezomib is a novel findings. We also showed that Bortezomib downregulate WEE1 mRNA expression by quantitative PCR.Our study showed that Bortezomib is active against T-ALL cell lines, including ETP-ALL cell line, LOUCY and modulates cell cycle with G2/M arrest. Bortezomib had been shown to increase the level of p21, p27 and cyclin B1 and induced G2/M cell cycle arrest in glioblastoma cells. However, studies on cell cycle modulation by Bortezomib in T-ALL are scarce. Here, we demonstrated Bortezomib stabilized p21, p27 and upregulation of cyclin B1 in T-ALL as well, which could account for the G2/M cell cycle arrest. We first showed that downregulation of WEE1 after treatment with Bortezomib, in protein level as well as in mRNA level. Recent study showed that inhibition of WEE1 is a novel target of therapy in T-ALL. WEE1 is upregulated in T-ALL to prevent entry of mitosis with unrepaired damaged DNA. The downregulation of WEE1 by Bortezomib as showed by our study could reverse its effect and leads to apoptosis of leukaemic cells.In summary, our study provides the insight on mechanism of action of Bortezomib in modulating cell cycle in T-ALL. Moreover, it is the first study to demonstrate WEE1 downregulation by Bortezomib in T-ALL. These findings not only enhance our understanding of mechanism of action of Bortezomib in T-ALL, but also rationalized the use of certain synergistics combination therapy with Bortezomib in treating T-ALL, e.g., chemotherapeutic agents, PARP inhibitors which could damage DNA of leukaemic cells. Further research is needed to explore those combination therapy in T-ALL and molecular mechanism of downregulation of WEE1 by Bortezomib in T-ALL. [Display omitted] DisclosuresNo relevant conflicts of interest to declare.

Regulation of Pkc1 Hyper-Phosphorylation by Genotoxic Stress.

The cell wall integrity (CWI) signaling pathway is best known for its roles in cell wall biogenesis. However, it is also thought to participate in the response to genotoxic stress. The stress-activated protein kinase Mpk1 (Slt2, is activated by DNA damaging agents through an intracellular mechanism that does not involve the activation of upstream components of the CWI pathway. Additional observations suggest that protein kinase C (Pkc1), the top kinase in the CWI signaling cascade, also has a role in the response to genotoxic stress that is independent of its recognized function in the activation of Mpk1. Pkc1 undergoes hyper-phosphorylation specifically in response to genotoxic stress; we have found that this requires the DNA damage checkpoint kinases Mec1 (Mitosis Entry Checkpoint) and Tel1 (TELomere maintenance), but not their effector kinases. We demonstrate that the casein kinase 1 (CK1) ortholog, Hrr25 (HO and Radiation Repair), previously implicated in the DNA damage transcriptional response, associates with Pkc1 under conditions of genotoxic stress. We also found that the induced association of Hrr25 with Pkc1 requires Mec1 and Tel1, and that Hrr25 catalytic activity is required for Pkc1-hyperphosphorylation, thereby delineating a pathway from the checkpoint kinases to Pkc1. We used SILAC mass spectrometry to identify three residues within Pkc1 the phosphorylation of which was stimulated by genotoxic stress. We mutated these residues as well as a collection of 13 phosphorylation sites within the regulatory domain of Pkc1 that fit the consensus for CK1 sites. Mutation of the 13 Pkc1 phosphorylation sites blocked hyper-phosphorylation and diminished RNR3 (RiboNucleotide Reductase) basal expression and induction by genotoxic stress, suggesting that Pkc1 plays a role in the DNA damage transcriptional response.

Exploring the ATR-CHK1 pathway in the response of doxorubicin-induced DNA damages in acute lymphoblastic leukemia cells

Doxorubicin (Dox) is one of the most commonly used anthracyclines for the treatment of solid and hematological tumors such as B−/T cell acute lymphoblastic leukemia (ALL). Dox compromises topoisomerase II enzyme functionality, thus inducing structural damages during DNA replication and causes direct damages intercalating into DNA double helix. Eukaryotic cells respond to DNA damages by activating the ATM-CHK2 and/or ATR-CHK1 pathway, whose function is to regulate cell cycle progression, to promote damage repair, and to control apoptosis. We evaluated the efficacy of a new drug schedule combining Dox and specific ATR (VE-821) or CHK1 (prexasertib, PX) inhibitors in the treatment of human B−/T cell precursor ALL cell lines and primary ALL leukemic cells. We found that ALL cell lines respond to Dox activating the G2/M cell cycle checkpoint. Exposure of Dox-pretreated ALL cell lines to VE-821 or PX enhanced Dox cytotoxic effect. This phenomenon was associated with the abrogation of the G2/M cell cycle checkpoint with changes in the expression pCDK1 and cyclin B1, and cell entry in mitosis, followed by the induction of apoptosis. Indeed, the inhibition of the G2/M checkpoint led to a significant increment of normal and aberrant mitotic cells, including those showing tripolar spindles, metaphases with lagging chromosomes, and massive chromosomes fragmentation. In conclusion, we found that the ATR-CHK1 pathway is involved in the response to Dox-induced DNA damages and we demonstrated that our new in vitro drug schedule that combines Dox followed by ATR/CHK1 inhibitors can increase Dox cytotoxicity against ALL cells, while using lower drug doses.Graphical abstract• Doxorubicin activates the G2/M cell cycle checkpoint in acute lymphoblastic leukemia (ALL) cells.• ALL cells respond to doxorubicin-induced DNA damages by activating the ATR-CHK1 pathway.• The inhibition of the ATR-CHK1 pathway synergizes with doxorubicin in the induction of cytotoxicity in ALL cells.• The inhibition of ATR-CHK1 pathway induces aberrant chromosome segregation and mitotic spindle defects in doxorubicin-pretreated ALL cells.

The Apparent Requirement for Protein Synthesis during G2 Phase Is due to Checkpoint Activation

SUMMARYProtein synthesis inhibitors (e.g., cycloheximide) block mitotic entry, suggesting that cell cycle progression requires protein synthesis until right before mitosis. However, cycloheximide is also known to activate p38 mitogen-activated protein kinase (MAPK), which can delay mitotic entry through a G2/M checkpoint. Here, we ask whether checkpoint activation or a requirement for protein synthesis is responsible for the cycloheximide effect. We find that p38 inhibitors prevent cycloheximide-treated cells from arresting in G2 phase and that G2 duration is normal in approximately half of these cells. The Wee1 inhibitor MK-1775 and Wee1/Myt1 inhibitor PD0166285 also prevent cycloheximide from blocking mitotic entry, raising the possibility that Wee1 and/or Myt1 mediate the cycloheximide-induced G2 arrest. Thus, protein synthesis during G2 phase is not required for mitotic entry, at least when the p38 checkpoint pathway is abrogated. However, M phase progression is delayed in cycloheximide-plus-kinase-inhibitor-treated cells, emphasizing the different requirements of protein synthesis for timely entry and completion of mitosis.

Higher AURKA and PLK1 expression are associated with inferior overall survival in patients with myelofibrosis

Aurora-kinase-A (AURKA), BORA and Polo-like-kinase-1 (PLK1) are regulating cell-cycle control and promotion of mitosis entry. AURKA contributes to Janus-kinase-2 (JAK2) activation and increased AURKA protein levels were reported in CD34+ and CD41+ cells of myeloproliferative neoplasm patients, leading to aneuploidy and aberrant megakaryopoiesis. We aimed to investigate AURKA, BORA and PLK1 mRNA expression in unfractionated bone-marrow aspirates of 43 patients with myelofibrosis (28 primary-/PMF, 15 secondary-myelofibrosis/SMF) and 12 controls and to assess their clinical correlations. AURKA expression did not significantly differ between myelofibrosis and controls (P = 0.466). Higher AURKA expression was significantly associated with higher absolute monocyte-count (P = 0.024) and shorter overall survival (HR = 3.77; P = 0.012). Patients with both PMF and SMF had lower BORA expression than controls (P = 0.009). Higher BORA expression was significantly associated with absence of constitutional symptoms (P = 0.049), absence of circulatory blasts (P = 0.047), higher monocyte- (P = 0.040) and higher eosinophil-counts (P = 0.016) and had neutral effect on survival (P > 0.05). PLK1 expression did not significantly differ between myelofibrosis and controls (P = 0.103). Higher PLK1 expression was significantly associated with higher white-blood-cell-count (P = 0.042) and inferior overall survival (HR = 5.87; P = 0.003). In conclusion, AURKA, BORA and PLK1 are involved in pathogenesis of myelofibrosis and may affect survival. Future studies investigating these interesting associations are warranted.

The UFM1 cascade times mitosis entry associated with microcephaly.

Posttranslational modifications enhance the functional diversity of the proteome by modifying the substrates. The UFM1 cascade is a novel ubiquitin-like modification system. The mutations in UFM1, its E1 (UBA5) and E2 (UFC1), have been identified in patients with microcephaly. However, its pathological mechanisms remain unclear. Herein, we observed the disruption of the UFM1 cascade in Drosophila neuroblasts (NBs) decreased the number of NBs, leading to a smaller brain size. The lack of ufmylation in NBs resulted in an increased mitotic index and an extended G2/M phase, indicating a defect in mitotic progression. In addition, live imaging of the embryos revealed an impaired E3 ligase (Ufl1) function resulted in premature entry into mitosis and failed cellularization. Even worse, the embryonic lethality occurred as early as within the first few mitotic cycles following the depletion of Ufm1. Knockdown of ufmylation in the fixed embryos exhibited severe phenotypes, including detached centrosomes, defective microtubules, and DNA bridge. Furthermore, we observed that the UFM1 cascade could alter the level of phosphorylation on tyrosine-15 of CDK1 (pY15-CDK1), which is a negative regulator of the G2 to M transition. These findings yield unambiguous evidence suggesting that the UFM1 cascade is a microcephaly-causing factor that regulates the progression of the cell cycle at mitosis phase entry.

DIS3 Mutation in RUNX1-Mutated AML1 Confers a Highly Dismal Prognosis in AML By Repressing Sister Chromatid Cohesion

Defective in Chromatid Joining (DIS3) is a highly conserved exoribonuclease, involved in the 3'-5' degradation of RNAs, which has a major role as a post-transcriptional checkpoint in eukaryotic cells. The controlled degradation of unnecessary RNA transcripts from the cell by the DIS3 family enzymes has been shown to be essential for cell division and mitosis. Exome- sequencing analyses have demonstrated the presence of DIS3 mutations in several cancers with high incidence in patients with multiple myeloma but its role in acute myeloid leukemia (AML) has not been studied. AML is a malignancy characterized by clonal expansion and differentiation arrest of myeloid progenitors in the bone marrow and peripheral blood. M0 and M1 FAB subtypes are both poorly differentiated AML and RUNX1 mutations are associated with poorer outcome. In this work we asked whether we can confirm in the TCGA AML cohort, that RUNX1 mutations were associated with a prognostic significance and whether other co-existing mutations can influence the overall survival (OS). In this cohort of 193 patients, we have found that the presence of RUNX1 mutation is present in patients with worst OS (log rank p-value=0.036, median OS 227 versus 335 months). We then asked if the association with other mutations has an impact on clinical point of view. With a minimum 3 mutations per gene present in the cohort (Fig.1A), RUNX1 was found to be significantly co-associated with mutations present in 4 other genes: PHF6, TTN, ASXL1 and DIS3. RUNX1 associated mutations affecting ASXL1, TTN and PHF6 did not appear to modify prognosis. However, patients presenting both RUNX1 and DIS3 mutations had a dramatically reduced overall survival (log rank p-value=6E-4, median OS 61 versus 334 months) (Fig.1B) suggesting a dramatic co-association of these two mutations independently of TTN, PHF6, and ASXL1. The two patients presenting a co-association of RUNX1 and DIS3 mutations were found to have a leukemia of M1 subtype. One patient with AML-M3 presented an isolated DIS3 mutation without co-association with a RUNX1 mutation (Fig.1C). DIS3 missense mutations found in the cohort affected VacB RNB protein domain, which is implicated in ribonuclease activity and RNA binding of the molecule (Fig.1D). Removing M3 subtype from the TCGA cohort, RUNX1 fusions were found to be associated with M2 subtype, RUNX1 mutations with M0 & M4 subtypes, and double mutant RUNX1/DIS3 in essentially in M1subgroup (p-value = 0.006976). Without M3, patients with double RUNX1/DIS3 mutations were found to exhibit the worst prognosis respectively as compared to patients presenting mutations or fusion of RUNX1 alone (log rank p-value=0.0042). We then compared the transcriptome of M1 patients with double DIS3-RUNX1 mutations as compared to M1 patients with simple RUNX1 mutations. As can be seen in Fig.1F, the first group harbored an important down-regulation of genes involved in the cohesin complex such as SMC1A, RAD21, STAG2, suggesting a major reduction of sister chromatid cohesion functionality (Normalized enrichment score = -2.017, FDR q-value=0.004). There was also a down-regulation of molecules implicated in DNA double strand repair such as MRE11A, RAD21, as well as that of MAD2L1 (a mitosis checkpoint) and centromere proteins such as CENPF. These results suggest that DIS3 mutations are associated with an important loss of control for the entry in mitosis. Overall, we show here for the first time to our knowledge, that the occurrence of DIS3 mutation in AML-M1 patients presenting a RUNX1 mutation confers a dismal prognosis in terms of overall survival. The transcriptome of these patients harbors a loss of functionality for sister chromatid cohesion. The search for DIS3 mutations is therefore warranted in all AML at diagnosis and could be of decisional value for therapeutic strategies. Disclosures Turhan: Incyte: Consultancy, Honoraria; novartis: Honoraria, Research Funding.

The Dual Cell Cycle Kinase Inhibitor JNJ-7706621 Reverses Resistance to CD37 Targeted Radioimmunotherapy in Activated B Cell like Diffuse Large B Cell Lymphoma Cell Lines

The CD37 targeting radioimmunoconjugate 177Lu-lilotomab satetraxetan (Betalutin®) is currently being evaluated as monotherapy in a clinical phase 2b trial for patients with follicular lymphoma (FL) and in a phase 1 trial for patients with diffuse large B-cell lymphoma (DLBCL), as well as in a phase 1b trial in combination with rituximab for patients with relapsed/refractory FL. Herein we have investigated the effect of 177Lu-lilotomab satetraxetan in seven activated B-cell like (ABC) DLBCL cell lines. Although the radioimmunoconjugate showed anti-tumor activity, primary resistance was observed in a subset of cell lines: U-2932 and RIVA. Both cell lines are representative for TP53 deficient Double Expressor (DE) DLBCL. Importantly, resistance was not a consequence of reduced binding of the radioimmunoconjugate to cell surface expressed CD37. Thus, we set out to identify drugs able to overcome the resistance to 177Lu-lilotomab satetraxetan in both resistant ABC-DLBCL cell lines. We performed a viability-based screen combining 177Lu-lilotomab satetraxetan with the 384-compound Cambridge Cancer Compound Library. Drug combinations were scored using Bliss and Chou-Talalay algorithms. We identified and characterized the dual-specific CDK1/2 and AURA/B kinase inhibitor JNJ-7706621 as compound able to revert the resistance to radioimmunotherapy (RIT), alongside topoisomerase and histone deacetylases (HDAC) inhibitors. Kinetic studies of the effect of mono- and combination therapy of U-2932 and RIVA cells with JNJ-7706621 and 177Lu-lilotomab satetraxetan are suggestive of a model in which radiation damage induced G2-arrested lymphoma cells eventually enter mitosis (repair or escape) and mitotic entry, progression and exit are impaired by JNJ-7706621 mediated inhibition of CDK1/2 and AURKA/B. Extended residence-time of cells in mitosis due to chromosome condensation and congression defects as well as spindle and mid-spindle assembly failure is likely pivotal for the increased sensitivity to persistent 177Lu-lilotomab satetraxetan deposited DNA damage, ultimately promoting cytokinesis failure (multinucleation, aneuploidy, increased cell size) and cell death. In conclusion, CD37-targeting 177Lu-lilotomab satetraxetan RIT showed activity in several ABC-DLBCL lymphoma cell lines. CD37-independent RIT-resistance was identified in two cell lines representative of aggressive DE ABC-DLBCLs with inactive TP53, and reversed by subsequent inhibition of CDK1/2 and AURKA/B by JNJ-7706621. These findings may be of potential relevance for ongoing clinical trials of 177Lu-lilotomab satetraxetan in relapsed, ASCT-non-eligible DLBCL, and may also be more generally applicable to other 177Lu-based RITs and alternative radionuclide utilizing targeted therapies. Future pre-clinical investigations are required to elucidate the potential application of CDK1/2 and AURKA/B inhibitors as a strategy to revert RIT resistance in TP53 deficient cancers. Disclosures Rødland: Nordic Nanovector ASA: Patents & Royalties, Research Funding. Melhus:Nordic Nanovector ASA: Employment, Equity Ownership, Patents & Royalties. Generalov:Nordic Nanovector ASA: Employment, Equity Ownership, Patents & Royalties. Bertoni:Nordic Nanovector ASA: Research Funding; Oncology Therapeutic Development: Research Funding; PIQUR Therapeutics AG: Other: travel grant, Research Funding; HTG: Other: Expert Statements ; Amgen: Other: travel grants; Astra Zeneca: Other: travel grants; Jazz Pharmaceuticals: Other: travel grants; NEOMED Therapeutics 1: Research Funding; Acerta: Research Funding; ADC Therapeutics: Research Funding; Bayer AG: Research Funding; Cellestia: Research Funding; CTI Life Sciences: Research Funding; EMD Serono: Research Funding; Helsinn: Consultancy, Research Funding; ImmunoGen: Research Funding; Menarini Ricerche: Consultancy, Research Funding. Dahle:Nordic Nanovector ASA: Employment, Equity Ownership, Patents & Royalties. Syljuåsen:Nordic Nanovector ASA: Patents & Royalties, Research Funding. Patzke:Nordic Nanovector ASA: Employment, Patents & Royalties.

Electron tomography reveals changes in spatial distribution of UBTF1 and UBTF2 isoforms within nucleolar components during rRNA synthesis inhibition

Upstream binding transcription factor (UBTF) is a co-regulator of RNA polymerase I by constituting an initiation complex on rRNA genes. UBTF plays a role in rDNA bending and its maintenance in “open” state. It exists as two splicing variants, UBTF1 and UBTF2, which cannot be discerned with antibodies raised against UBTF. We investigated the ultrastructural localization of each variant in cells synthesizing GFP-tagged UBTF1 or UBTF2 by using anti-GFP antibodies and pre-embedding nanogold strategy. Detailed 3D distribution of UBTF1 and 2 was also studied by electron tomography. In control cells, the two isoforms are very abundant within fibrillar centers, but their repartition strongly differs. Electron tomography shows that UBTF1 is disposed as fibrils that are folded in coils whereas UBTF2 is localized homogenously, preferentially at their cortical area. As UBTF is a useful marker to trace rDNA genes, we used these data to improve our previous model of 3D organization of active transcribing rDNA gene within fibrillar centers. Finally, when rRNA synthesis is inhibited during actinomycin D treatment or entry in mitosis, UBTF1 and UBTF2 show a similar distribution along extended 3D loop-like structures. Altogether these data suggest new roles for UBTF1 and UBTF2 isoforms in the organization of active and inactive rDNA genes.

Anti-angiogenic effects of crenolanib are mediated by mitotic modulation independently of PDGFR expression

BackgroundCrenolanib is a tyrosine kinase inhibitor targeting PDGFR-α, PDGFR-β and Fms related tyrosine kinase-3 (FLT3) that is currently evaluated in several clinical trials. Although platelet-derived growth factor receptor (PDGFR) signalling pathway is believed to play an important role in angiogenesis and maintenance of functional vasculature, we here demonstrate a direct angiostatic activity of crenolanib independently of PDGFR signalling.MethodsThe activity of crenolanib on cell viability, migration, sprouting, apoptosis and mitosis was assessed in endothelial cells, tumour cells and fibroblasts. Alterations in cell morphology were determined by immunofluorescence experiments. Flow-cytometry analysis and mRNA expression profiles were used to investigate cell differentiation. In vivo efficacy was investigated in human ovarian carcinoma implanted on the chicken chorioallantoic membrane (CAM).ResultsCrenolanib was found to inhibit endothelial cell viability, migration and sprout length, and induced apoptosis independently of PDGFR expression. Treated cells showed altered actin arrangement and nuclear aberrations. Mitosis was affected at several levels including mitosis entry and centrosome clustering. Crenolanib suppressed human ovarian carcinoma tumour growth and angiogenesis in the CAM model.ConclusionsThe PDGFR/FLT3 inhibitor crenolanib targets angiogenesis and inhibits tumour growth in vivo unrelated to PDGFR expression. Based on our findings, we suggest a broad mechanism of action of crenolanib.

Extensive loss of cell-cycle and DNA repair genes in an ancient lineage of bipolar budding yeasts.

Cell-cycle checkpoints and DNA repair processes protect organisms from potentially lethal mutational damage. Compared to other budding yeasts in the subphylum Saccharomycotina, we noticed that a lineage in the genus Hanseniaspora exhibited very high evolutionary rates, low Guanine–Cytosine (GC) content, small genome sizes, and lower gene numbers. To better understand Hanseniaspora evolution, we analyzed 25 genomes, including 11 newly sequenced, representing 18/21 known species in the genus. Our phylogenomic analyses identify two Hanseniaspora lineages, a faster-evolving lineage (FEL), which began diversifying approximately 87 million years ago (mya), and a slower-evolving lineage (SEL), which began diversifying approximately 54 mya. Remarkably, both lineages lost genes associated with the cell cycle and genome integrity, but these losses were greater in the FEL. E.g., all species lost the cell-cycle regulator WHIskey 5 (WHI5), and the FEL lost components of the spindle checkpoint pathway (e.g., Mitotic Arrest-Deficient 1 [MAD1], Mitotic Arrest-Deficient 2 [MAD2]) and DNA-damage–checkpoint pathway (e.g., Mitosis Entry Checkpoint 3 [MEC3], RADiation sensitive 9 [RAD9]). Similarly, both lineages lost genes involved in DNA repair pathways, including the DNA glycosylase gene 3-MethylAdenine DNA Glycosylase 1 (MAG1), which is part of the base-excision repair pathway, and the DNA photolyase gene PHotoreactivation Repair deficient 1 (PHR1), which is involved in pyrimidine dimer repair. Strikingly, the FEL lost 33 additional genes, including polymerases (i.e., POLymerase 4 [POL4] and POL32) and telomere-associated genes (e.g., Repressor/activator site binding protein-Interacting Factor 1 [RIF1], Replication Factor A 3 [RFA3], Cell Division Cycle 13 [CDC13], Pbp1p Binding Protein [PBP2]). Echoing these losses, molecular evolutionary analyses reveal that, compared to the SEL, the FEL stem lineage underwent a burst of accelerated evolution, which resulted in greater mutational loads, homopolymer instabilities, and higher fractions of mutations associated with the common endogenously damaged base, 8-oxoguanine. We conclude that Hanseniaspora is an ancient lineage that has diversified and thrived, despite lacking many otherwise highly conserved cell-cycle and genome integrity genes and pathways, and may represent a novel, to our knowledge, system for studying cellular life without them.

Radotinib inhibits mitosis entry in acute myeloid leukemia cells via suppression of Aurora kinase A expression.

Aurora kinases play critical roles in regulating several processes pivotal for mitosis. Radotinib, which is approved in South Korea as a second-line treatment for chronic myeloid leukemia, inhibits the tyrosine kinase BCR-ABL and platelet-derived growth factor receptor. However, the effects of radotinib on Aurora kinase expression in acute myeloid leukemia are not well studied. Interestingly, the cytotoxicity of acute myeloid leukemia cells was increased by radotinib treatment. Radotinib significantly decreased the expression of cyclin-dependent kinase 1 and cyclin B1, the key regulators of G2/M phase, and inhibited the expression of Aurora kinase A and Aurora kinase B in acute myeloid leukemia cells. In addition, radotinib decreased the expression and binding between p-Aurora kinase A and TPX2, which are required for spindle assembly. Furthermore, it reduced Aurora kinase A and polo-like kinase 1 phosphorylation and suppressed the expression of α-, β-, and γ-tubulin in acute myeloid leukemia cells. Furthermore, radotinib significantly suppressed the key regulators of G2/M phase including cyclin B1 and Aurora kinase A in a xenograft animal model. Therefore, our results suggest that radotinib can abrogate acute myeloid leukemia cell growth both in vitro and in vivo and may serve as a candidate agent or a chemosensitizer for treating acute myeloid leukemia.

Quantitative Proteomics of the Mitotic Chromosome Scaffold Reveals the Association of BAZ1B with Chromosomal Axes*[S

In mitosis, chromosomes achieve their characteristic shape through condensation, an essential process for proper segregation of the genome during cell division. A classical model for mitotic chromosome condensation proposes that non-histone proteins act as a structural framework called the chromosome scaffold. The components of the chromosome scaffold, such as DNA topoisomerase IIα (TOP2A) and structural maintenance of chromosomes protein 2 (SMC2), are necessary to generate stable mitotic chromosomes; however, the existence of this scaffold remains controversial. The aim of this study was to determine the protein composition of the chromosome scaffold. We used the DT40 chicken cell line to isolate mitotic chromosomes and extract the associated protein fraction, which could contain the chromosome scaffold. MS revealed a novel component of the chromosome scaffold, bromodomain adjacent to zinc finger 1B (BAZ1B), which was localized to the mitotic chromosome axis. Knocking out BAZ1B caused prophase delay because of altered chromosome condensation timing and mitosis progression errors, and the effect was aggravated if BAZ1A, a BAZ1B homolog, was simultaneously knocked out; however, protein composition of prometaphase chromosomes was normal. Our results suggest that BAZ1 proteins are essential for timely chromosome condensation at mitosis entry. Further characterization of the functional role of BAZ1 proteins would provide new insights into the timing of chromosome condensation.

MicroRNA-520a suppresses HBV replication in HepG2.2.15 cells by inactivating AKT.

ObjectiveTo investigate whether the mechanism by which a microRNA, miR-520a, suppresses the replication of hepatitis B virus (HBV) involves the regulation of the serine/threonine kinase (AKT) gene.MethodsThe effects of miR-520a on the proliferation, mitotic index and apoptosis of the HBV-replicating human hepatocellular carcinoma cell line HepG2.2.15 were measured using standard laboratory methods including flow cytometry. The effects of miR-520a on HBV transcription and replication were assessed using methods including immunoassays and reverse transcription–polymerase chain reaction. The effect of small interfering RNA (siRNA) to AKT on the levels of AKT mRNA and protein were also evaluated.ResultsIn HepG2.2.15 cells, miRNA-520a reduced HBV transcription and replication by reducing AKT levels. MiRNA-520a decreased cell proliferation and mitosis entry of cells and increased apoptosis in HepG2.2.15 cells. AKT levels were reduced significantly by its siRNA, which resulted in suppression of HBV replication in HepG2.2.15 cells.ConclusionsMiRNA-520a inhibited AKT gene expression and suppressed HBV transcription and replication. These findings suggest that miRNA-520a may be a novel target for the treatment of HBV infection because miRNA-520a reduced HepG2.2.15 cell survival and inhibited HBV replication associated with the AKT signalling pathway.

Abstract 1843: Combination therapy targeting Rb/Wee1 kinase pathways for rhabdomyosarcoma treatment

Abstract Introduction Rhabdomyosarcoma (RMS) is the most common pediatric soft tissue sarcoma. New therapeutic strategies are needed particularly for high-risk patients where the survival rate is 20-40%. The FDA approved CDK4/6 inhibitor palbociclib (PD), and the WEE1 kinase inhibitor MK-1775 have anticancer effects when used as single agents. PD causes cell arrest at G1 phase (requiring intact Rb), while MK-1775 promotes cell death by abrogating DNA damage during the G2/M checkpoint phase by forcing premature mitosis entry. As the two drugs target tumor cells at different phases of the cell cycle, cells that are not in the targeted phase may escape single therapy. We hypothesized that sequential combination therapy will increase therapeutic efficacy. The use of PD first, will induce the cells to go under G1 arrest. Upon recovery, more cells will synchronize in the S/G2 phase, increasing their susceptibility to MK1775, resulting in synergistic cell killing. Methods Rb+ RMS (Rh-30) and Rb- osteosarcoma (OS) cell lines (CCH-D, LM7, and SASO2) were treated with increasing doses of PD for 6 days, then stained with propidium iodide (PI) and analyzed for cell cycle status to determine whether PD induced G1 arrest. To determine the optimal recovery time for Rb+ cells to be synchronized in S/G2 phase, cells were treated with PD (IC50-IC75) for 6 days and then were harvested either immediately or after 3-24 h recovery in drug free medium. Cells were stained with PI for cell cycle analysis. A 12-day long combination therapy assay was performed with or without recovery time between the two treatments. Combination index (CI) was calculated by using the statistical program calcusyn to determine whether the combinatorial drug treatment had acted in an antagonistic, additive or synergistic manner. Results Six days of treatment with PD resulted in reversible G1 arrest only in Rb+ cell lines, confirming that Rb, the downstream target of CDK4, must be intact to induce G1 arrest by palbociclib. Six days of treatment with PD followed by 9-12 h recovery in fresh medium (the optimal recovery time prior to treatment with MK-1775), led to re-entry of the Rb+ Rh-30 cells into the cell cycle and successful synchronization at S/G2 phase. By contrast, Rb- cells failed to arrest in G1 phase following treatment with PD and were not synchronized at S/G2 phase. Since cells synchronized in S/G2 phase are more vulnerable to MK1775, combination therapy resulted in a synergistic effect in the Rb+ cells but an antagonistic effect in the Rb- cells. Conclusion: Our results suggest that the combination therapy strategy of PD preceding MK1775 may be a novel therapeutic approach for patients with Rb+ RMS. This strategy will be investigated in xenograft and PDX RMS model. Citation Format: Simin Kiany, Gangxiong Huang, Melanie Justice, Khandan Keyomarsi, Eugenie Kleinerman. Combination therapy targeting Rb/Wee1 kinase pathways for rhabdomyosarcoma treatment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1843.