Suppression of Docetaxel induced apoptosis in Survivin-depleted cells due to failure of sustained mitotic block

The antitumor effect of taxanes have been attributed to their ability to induce mitotic arrest through activation of the spindle assembly checkpoint. Cell death following prolonged mitotic arrest is mediated by the intrinsic apoptosis pathway. Accordingly, factors that influence the robustness of mitotic arrest or disrupt the apoptotic machinery might confer drug resistance. Survivin is an inhibitor of apoptosis protein. Its overexpression has been associated with resistance to multiple anticancer agents including taxanes, and its targeting led to drug sensitization. On the other hand, Survivin is a key regulator of mitosis, which is shown to be required for stable activation of the spindle assembly checkpoint. Since the sensitivity of taxanes depends on a functional spindle checkpoint, inhibition of Survivin may lead to drug resistance, which is the opposite effect of its anti-apoptotic function. Here we show that Survivin-depleted cells escape the mitotic block following Docetaxel treatment, thereby evading drug induced apoptosis. Moreover, Survivin depletion increases the level of mitotic catastrophe and cellular senescence induced by Docetaxel and enhanced its efficacy against clonogenic survival of tumor cells. Our finding suggests that inhibition of Survivin promotes non-apoptotic mechanisms following Docetaxel treatment rather than increases the sensitivity of apoptosis.

Mitotic ER exit site dissociation and reassembly is regulated by TANGO1 phosphorylation status

Golgi fragmentation and ER exit site dissociation are considered as the leading causes of mitotic block of secretion from the ER. Although the mechanisms of Golgi fragmentation have been extensively characterized, ER exit block early in mitosis is not well-understood. We previously found that TANGO1 organizes ER exit sites by directly interacting with Sec16. Here, we showed that TANGO1 is phosphorylated by casein kinase 1 (CK1) during mitosis. Interestingly, the interaction with Sec16 was abrogated by phosphorylation of TANGO1, leading to dissociation of the ER exit sites. Moreover, a TANGO1 mutant deficient in phosphorylation inhibited the mitotic dissociation of ER exit sites. In contrast, a TANGO1 mutant mimicking CK1-mediated phosphorylation dissociated ER exit sites in interphase cells. Although CK1 activity remains constant throughout the cell cycle, PP1, a phosphatase for which activity decreases during mitosis, participates in the regulation of TANGO1 phosphorylation. This is the first report demonstrating the mechanisms of ER exit site dissociation during mitosis.

Incorporation of thymidine into onion root meristematic cell nuclei in presence of hydroxyurea and its role in recovery of mitotic activity

Hydroxyurea treatment of onion roots induced mitotic block which was released by transfer of bulbs to water, and also to some extent by addition of cold or ³H-thymidine to hydroxyurea solutions. In presence of hydroxyurea there was noted very intense incorporation of ³H-thymidine into cell nuclei, giving labelling index of 40-70%. However, all the mitotic figures appearing in presence of hydroxyurea and ³H-thymidine were unlabelled. On the other hand, labelled mitotic figures were obtained when roots incubated with ³H-thymidine in presence of hydroxyurea had been transferred to water. Incorporation of ³H-uridine was unaffected by hydroxyurea. The results show that hydroxyurea arrests onion root meristematic cells, either in the S phase and the G₂ phase. Enhanced incorporation of ³H-thymidine in the presence of hydroxyurea, and release by added thymidine of the mitotic block indicate that hydroxyurea induces in onion root meristematic cells a particular shortage of thymidylate.

A Rationale to Enhance the Response to Antimitotic Therapy in Acute Myeloid Leukemia

Abstract 1332 Acute myeloid leukemia (AML) is known to respond only moderately to antimitotic therapy while acute lymphoblastic leukemias can be efficiently targeted using spindle-disrupting agents. The underlying molecular cause for this clinical phenomenon is unknown. Recent evidence suggests that response to antimitotic therapy substantially depends on the stability of the critical mitotic regulator cyclin B. The ability to keep cyclin B expression levels stable during a mitotic block is associated with a good response leading to cell death in mitosis. At the metaphase to anaphase transition of an unperturbed cell division, cyclin B is targeted for degradation by the anaphase-promoting complex/cyclosome (APC/C) to trigger chromosome separation. The spindle assembly checkpoint (SAC) is a surveillance mechanism to ensure that APC/C-mediated ubiquitylation is restricted to cells that show proper attachment of all chromosomes to a functional mitotic spindle. In case of spindle disruption or unattached chromosomes, the spindle checkpoint stays active which leads to interference with APC/C-dependent proteolysis of cyclin B blocking cells in prometaphase until every chromosome is attached to the mitotic spindle. We recently developed a cell line-based reporter system which allows monitoring of cyclin B degradation under various conditions (Schnerch et al. Cell Cycle 2012). Here, we identified a pattern of slow degradation of cyclin B which continues through a mitotic block in case of chromosomal misalignment in unperturbed cell cycles. Remarkably, we also found prolonged slow degradation to trigger aberrant exit from mitosis in such cells giving rise to tetraploid cells. Therefore, a reduction in slow degradation appears as a promising rationale to foster a mitotic arrest and enhance cell death in mitosis during antimitotic therapy by preventing such mitotic slippage. We exposed our reporter cells to low concentrations of proteasome inhibitor during a spindle poison induced mitotic block to assess whether proteasome inhibition is capable of modulating slow degradation. Importantly, very low doses of proteasome inhibitor were sufficient to reduced the extent of cyclin B slow degradation during the mitotic block. Moreover, we demonstrate that low doses of proteasome inhibitor render the AML cell line Kasumi-1 responsive to low, non-disruptive concentrations of spindle poison (nocodazole and vincristine) leading to remarkable increases in the G2M-fraction. To the best of our knowledge there is no evidence so far that low doses of proteasome inhibitor exert antimitotic effects by interference with protein degradation during mitosis. Importantly, concentration of bortezomib of 1–2ng/ml (such as found in the serum of patients for up to 72h following administration of 1.3mg/m2 bortezomib subcutaneously) were found to exert synergistic effects with antimitotic therapy. Increases in the percentage of G2M cells by 38% were observed in Kasumi-1 cells for the combination of vincristine and bortezomib. Based on these findings, we currently apply our system to probe combinations of proteasome inhibitor with modern tailored therapies that exert their antimitotic effects by activation of the SAC, such as inhibitors of the motor protein Eg5 or of the mitotic kinases Polo-like kinase 1 (Plk1) or Aurora A and B. Using our cell line-based reporter system, we provide evidence in the in vitro setting that modulating slow degradation during antimitotic therapy by proteasome inhibition is a promising rationale to enhance the efficacy of antimitotic drugs. Drug concentrations used are based on published pharmacokinetics in humans and suggest feasibility of the drug combination in vivo. Our approach of targeted drug combinations may provide highly efficient treatment alternatives for patients that are not eligible for induction treatment. Disclosures: No relevant conflicts of interest to declare.

Combined Inhibition of Janus and Aurora Kinases by the Novel Small Molecule Inhibitor AZD1480 Induces Cell Death and Downregulates PD-L1 and PD-L2 Expression In Hodgkin Lymphoma

Abstract 2848 Hodgkin Lymphoma (HL) cell proliferation and survival is sustained by a complex network of cytokine signaling, involving the Hodgkin and Reed-Sternberg cells and tumor microenvironment. Following cytokine stimulation, JAK-STAT activation promotes the transcription of target genes involved in proliferation, survival, and immune escape. Programmed Death-ligands 1 and 2 (PD-L1 and PD-L2) and the Th2 chemokine TARC are immune-modulators involved in immune evasion, respectively through inhibition of effector T cell function (PD-L1, PD-L2) and attraction and homing of Th2 cells (TARC). Aurora kinases are frequently overexpressed in human cancers and play essential functions in chromosome alignment and cytokinesis. The role of Aurora kinases in Hodgkin lymphomagenesis is not defined yet. In this study we report the activity profile of the JAK2 inhibitor AZD1480 in HL cell lines (HD-LM2, L-428, KM-H2, L-540). To assess the effect of AZD1480 on cell proliferation, cells were incubated with increasing concentrations of AZD1480 (from 0.1 to 10 μM) for 24, 48 and 72 hours (hrs). A significant growth inhibition was evident after 72 hrs of incubation, specially using the high doses of AZD1480 (5μM). The L-540 cell line showed the highest sensitivity, with a decrease in cell viability close to 50% following incubation with AZD1480 1μM. Inhibition of STAT3, STAT5 and STAT6 phosphorylation in the L-540, L-428 and HD-LM2 cell lines was observed with concentrations equal to 0.1 μM or higher. Using Annexin V- propidium iodide staining, we found that AZD1480 induced cell death by apoptosis in a dose dependent manner after 72 hrs of incubation when a high concentration (5μM) of the drug was used. Lower concentrations of AZD1480 (1μM) promoted a statistically significant increase in cell death only in the L-540 and to a lesser extent in the L-428 cell line. Consistent with this data, also caspase 9, 3 and PARP cleavage was observed in all the cell lines exposed to AZD1480 5 μM. AZD1480 5μM promoted a marked increase in the G2/M fraction in all the cell lines as soon as 24 hrs after incubation, especially in the HD-LM2 and L-428 cell lines. Treatment with lower doses (1μM) did not affect significantly the cell cycle. Since AZD1480 was also reported to inhibit Aurora A kinase at nanomolar concentrations in enzymatic assays, we assessed if the significant increase in the G2/M fraction was related to the inhibition of the Aurora A kinase. We evaluated the levels of autophosphorylation on Thr-288 by western blotting. Cells were pretreated with Nocodazole 400 ng/ml for 18 hrs in order to achieve a mitotic block, and then exposed to AZD1480 (1-5μM) and/or the proteasome inhibitor MG132 (20μM) (in order to prevent the potential overriding of the Nocodazole induced mitotic block), for 3 hours. A dose-dependent inhibition of Aurora A was detected in all the cell lines, with a complete abrogation when higher doses of AZD1480 were used (5μM). These findings are consistent with the analysis of the cell cycle fractions, showing dose-dependent changes of the cell cycle at 24 hrs following incubation with AZD1480. AZD1480 also decreased the secretion of key cytokines involved autocrine and paracrine survival loops and immune escape. Following incubation with AZD1480 1μM for 72 hrs cell culture supernatants were analyzed by ELISA: decreased levels of IL-6, IL-13, TARC, and IL-21 were observed in HD-LM2, L-428 and L-540 cells. Moreover we assessed the expression of PD-L1 and PD-L2 by flow cytometry and observed significant downregulation in the PD-L1/PD-L2 overexpressing cell lines (L-540 and HD-LM2). These data suggest that AZD1480 has a pleiotropic mechanism of action in HL by targeting the JAK-STAT and the Aurora kinase pathway, and by altering the pattern of cytokine and chemokine secretion and the expression of factors involved in immune escape. Our study provides the rationale for further clinical investigation of AZD1480 in HL. Disclosures: No relevant conflicts of interest to declare.