Randy Y C Poon

The Hong Kong University of Science and Technology, Chiu-lung, Kowloon City, Hong Kong

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Publications (110)563.76 Total impact

  • Shan Huang · Rui Tang · Randy Y C Poon
    [Show abstract] [Hide abstract] ABSTRACT: Microtubule inhibitors including taxanes and vinca alkaloids are among the most widely used anticancer agents. Disrupting the microtubules activates the spindle-assembly checkpoint and traps cells in mitosis. Whether cells subsequently undergo mitotic cell death is an important factor for the effectiveness of the anticancer agents. Given that apoptosis accounts for the majority of mitotic cell death induced by microtubule inhibitors, we performed a systematic study to determine which members of the anti-apoptotic BCL-2 family are involved in determining the duration of mitotic block before cell death or slippage. Depletion of several anti-apoptotic BCL-2-like proteins significantly shortened the time before apoptosis. Among these proteins, BCL-W has not been previously characterized to play a role in mitotic cell death. Although the expression of BCL-W remained constant during mitotic block, it varied significantly between different cell lines. Knockdown of BCL-W with siRNA or disruption of the BCL-W gene with CRISPR-Cas9 speeded up mitotic cell death. Conversely, overexpression of BCL-W delayed mitotic cell death, extending the mitotic block to allow mitotic slippage. Taken together, these results showed that BCL-W contributes to the threshold of anti-apoptotic activity during mitosis.
    No preview · Article · May 2016 · Oncotarget
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    Po Yee Wong · Hoi Tang Ma · Hyun-jung Lee · Randy Y. C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: The G2 DNA damage checkpoint is one of the most important mechanisms controlling G2-mitosis transition. The kinase Greatwall (MASTL in human) promotes normal G2-mitosis transition by inhibiting PP2A via ARPP19 and ENSA. In this study, we demonstrate that MASTL is critical for maintaining genome integrity after DNA damage. Although MASTL did not affect the activation of DNA damage responses and subsequent repair, it determined the timing of entry into mitosis and the subsequent fate of the recovering cells. Constitutively active MASTL promoted dephosphorylation of CDK1(Tyr15) and accelerated mitotic entry after DNA damage. Conversely, downregulation of MASTL or ARPP19/ENSA delayed mitotic entry. Remarkably, APC/C was activated precociously, resulting in the damaged cells progressing from G2 directly to G1 and skipping mitosis all together. Collectively, these results established that precise control of MASTL is essential to couple DNA damage to mitosis through the rate of mitotic entry and APC/C activation.
    Preview · Article · Feb 2016 · Scientific Reports
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    Hoi Tang Ma · Randy Yat Choi Poon
    [Show abstract] [Hide abstract] ABSTRACT: Biochemical studies have indicated that p31comet and TRIP13 are critical for inactivating MAD2. To address unequivocally whether p31comet and TRIP13 are required for mitotic exit at the cellular level, their genes were ablated either individually or together in human cells. Neither p31comet nor TRIP13 were absolutely required for unperturbed mitosis. MAD2 inactivation was only partially impaired in p31comet-deficient cells. In contrast, TRIP13-deficient cells contained MAD2 exclusively in the C-MAD2 conformation. Our results indicate that although p31comet enhanced TRIP13-mediated MAD2 conversion, it was not absolutely necessary for the process. Paradoxically, TRIP13-deficient cells were unable to activate the spindle-assembly checkpoint, revealing that cells lacking the ability to inactivate MAD2 were incapable in mounting a checkpoint response. These results establish a paradigm of the roles of p31comet and TRIP13 in both checkpoint activation and inactivation.
    Preview · Article · Jan 2016 · Cell Reports
  • Randy Y C Poon
    [Show abstract] [Hide abstract] ABSTRACT: The cell cycle is the sequence of events through which a cell duplicates its genome, grows, and divides. Key cell cycle transitions are driven by oscillators comprising cyclin-dependent kinases and other kinases. Different cell cycle oscillators are inextricably linked to ensure orderly activation of oscillators. A recurring theme in their regulation is the abundance of auto-amplifying loops that ensure switch-like and unidirectional cell cycle transitions. The periodicity of many cell cycle oscillators is choreographed by inherent mechanisms that promote automatic inactivation, often involving dephosphorylation and ubiquitin-mediated protein degradation. These inhibitory signals are subsequently suppressed to enable the next cell cycle to occur. Although the activation and inactivation of cell cycle oscillators are in essence autonomous during the unperturbed cell cycle, a number of checkpoint mechanisms are able to halt the cell cycle until defects are addressed. Together, these mechanisms orchestrate orderly progression of the cell cycle to produce more cells and to safeguard genome integrity.
    No preview · Article · Jan 2016 · Methods in molecular biology (Clifton, N.J.)
  • Randy Y.C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: The cell cycle is the sequence of events through which a cell duplicates its genome, grows, and divides. Major cell cycle transitions are driven by cyclin-dependent kinases and other kinases. Ubiquitin-mediated protein degradation is also a major driving force for cell cycle transitions. Checkpoint mechanisms delay the cell cycle to ensure that each stage of the cell cycle is completed before the next stage is initiated. A recurring theme in cell cycle control is the abundance of feedback controls. Together, these mechanisms orchestrate the orderly progression of the cell cycle to maintain genome stability and to produce more cells.
    No preview · Chapter · Dec 2015
  • Wing Ki Wong · Terrenz Kelly · Jingjing Li · Hoi Tang Ma · Randy Y.C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: Shugoshin (SGO1) plays a pivotal role in sister chromatid cohesion during mitosis by protecting the centromeric cohesin from mitotic kinases and WAPL. Mammalian cells contain at least six alternatively spliced isoforms of SGO1. The relationship between the canonical SGO1A with shorter isoforms including SGO1C remains obscure. Here we show that SGO1C was unable to replace the loss of SGO1A. Instead, expression of SGO1C alone induced aberrant mitosis similar to depletion of SGO1A, promoting premature sister chromatid separation, activation of the spindle-assembly checkpoint, and mitotic arrest. In disagreement with previously published data, we found that SGO1C was localized to kinetochores. However, the ability to induce aberrant mitosis did not correlate with its kinetochore localization. SGO1C mutants that abolished binding to kinetochores still triggered premature sister chromatid separation. We provide evidence that SGO1C-mediated mitotic arrest involved the sequestering of PP2A-B56 pool. Accordingly, SGO1C mutants that abolished binding to PP2A localized to kinetochores but did not induce aberrant mitosis. These studies imply that the expression of SGO1C should be tightly regulated to prevent dominant-negative effects on SGO1A and induce genome instability.
    No preview · Article · Oct 2015 · Cell cycle (Georgetown, Tex.)
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    [Show abstract] [Hide abstract] ABSTRACT: Nasopharyngeal carcinoma (NPC) is a rare but highly invasive cancer. As radiotherapy is the primary treatment for NPC, this offers a rationale to investigate if uncoupling the DNA damage responses can sensitize this cancer type. The G2 DNA damage checkpoint is controlled by a cascade of protein kinases: ATM/ATR, which phosphorylates CHK1/CHK2, which in turn phosphorylates WEE1. A number of small molecule inhibitors have been developed against these kinases as potential therapeutic agents. Here we demonstrated that compare to that in immortalized nasopharyngeal epithelial cells, ATR, CHK1, and WEE1 were overexpressed in NPC cell lines. Inhibitors of these kinases were unable to promote extensive mitotic catastrophe in ionizing radiation-treated NPC cells, indicating that they are not very effective radiosensitizer for this cancer. In the absence of prior irradiation, however, mitotic catastrophe could be induced with inhibitors against CHK1 (AZD7762) or WEE1 (MK-1775). NPC cells were more sensitive to WEE1 inactivation than nasopharyngeal epithelial cells. Targeting CHK1 and WEE1 together induced more extensive mitotic catastrophe than the individual components alone. Taken together, our results show that NPC cells depend on CHK1 and WEE1 activity for growth and that inhibitors of these kinases may serve as potential therapeutics for NPC.
    Full-text · Article · May 2015 · Oncotarget
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    [Show abstract] [Hide abstract] ABSTRACT: Mitosis is choreographed by a number of protein kinases including polo-like kinases and Aurora kinases. As these kinases are frequently dysregulated in cancers, small-molecule inhibitors have been developed for targeted anticancer therapies. Given that PLK1 and Aurora kinases possess both unique functions as well as co-regulate multiple mitotic events, whether pharmacological inhibition of these kinases together can enhance mitotic catastrophe remains an outstanding issue to be determined. Using concentrations of inhibitors that did not induce severe mitotic defects on their own, we found that both the metaphase arrest and mitotic slippage induced by inhibitors targeting Aurora A and Aurora B (MK-5108 and Barasertib respectively) were enhanced by a PLK1 inhibitor (BI 2536). We found that PLK1 is overexpressed in cells from nasopharyngeal carcinoma, a highly invasive cancer with poor prognosis, in comparison to normal nasopharyngeal epithelial cells. Nasopharyngeal carcinoma cells were more sensitive to BI 2536 as a single agent and co-inhibition with Aurora kinases than normal cells. These observations underscore the mechanism and potential benefits of targeting PLK1 and Aurora kinases to induce mitotic catastrophe in cancer cells.
    Full-text · Article · Mar 2015 · Oncotarget
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    Joyce P.Y. Mak · Wing Yu Man · Hoi Tang Ma · Randy Y.C. Poon
    Preview · Dataset · Sep 2014
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    Joyce P.Y. Mak · Wing Yu Man · Hoi Tang Ma · Randy Y.C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: The ATR-CHK1-WEE1 kinase cascade's functions in the DNA damage checkpoints are well established. Moreover, its roles in the unperturbed cell cycle are also increasingly being recognized. In this connection, a number of small-molecule inhibitors of ATR, CHK1, and WEE1 are being evaluated in clinical trials. Understanding precisely how cells respond to different concentrations of inhibitors is therefore of paramount importance and has broad clinical implications. Here we present evidence that in the absence of DNA damage, pharmacological inactivation of ATR was less effective in inducing mitotic catastrophe than inhibition of WEE1 and CHK1. Small-molecule inhibitors of CHK1 (AZD7762) or WEE1 (MK-1775) induced mitotic catastrophe, as characterized by dephosphorylation of CDK1Tyr15, phosphorylation of histone H3Ser10, and apoptosis. Unexpectedly, partial inhibition of WEE1 and CHK1 had the opposite effect of accelerating the cell cycle without inducing apoptosis, thereby increasing the overall cell proliferation. This was also corroborated by the finding that cell proliferation was enhanced by kinase-inactive versions of WEE1. We demonstrated that these potential limitations of the inhibitors could be overcome by targeting more than one components of the ATR-CHK1-WEE1 simultaneously. These observations reveal insights into the complex responses to pharmacological inactivation of the ATR-CHK1-WEE1 axis.
    Full-text · Article · Sep 2014 · Oncotarget
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    Xiaoxing Xing · Randy Y.C. Poon · Cesar S C Wong · Levent Yobas
    [Show abstract] [Hide abstract] ABSTRACT: We report the label-free enumeration of human colorectal-carcinoma cells from blood lymphocytes by using interdigitated ring-array microelectrodes; this enumeration was based on the dielectrophoretic selection of cells. Because of the novel design of the device, a continuous flow of cells is uniformly distributed into parallel streams through 300 rings (~40μm in diameter each) that are integrated into the electrode digits. Using this array, 82% of cancer cells were recovered and 99% of blood lymphocytes were removed. Most of the cancer cells recovered were viable (94%) and could be cultivated for >8 days, during which period they retained their normal cell morphology and proliferation rates. The recovery rate correlated closely with cancer-cell loadings in spiked samples and this relationship was linear over a range of at least 2 orders of magnitude. Importantly, because of the 3D structure of the rings, these results were obtained at a high cell-loading concentration (10(7)cells/mL). The rings could be further optimized for use in accurate label-free identification and measurement of circulating tumor cells in cancer research and disease management.
    Full-text · Article · Jun 2014 · Biosensors & Bioelectronics
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    Hoi Tang Ma · Sergio Erdal · Shan Huang · Randy Y.C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: The mitotic kinesin KIF11 (also called Eg5) plays critical roles in spindle functions. Although a number of small-molecule inhibitors of KIF11 are currently in clinical development, drug-resistance could be developed through compensation by another kinesin called KIF15. Using a newly developed infrared-based cell system, we discovered that the effectiveness of one of the latest generations of KIF11 inhibitor (SB743921) could be enhanced with several inhibitors of Aurora A kinase. Evidence including live-cell imaging and isobologram analysis indicated that targeting KIF11 and Aurora A together promoted monoastral spindle formation and mitotic catastrophe synergistically, supporting a model of parallel pathways of centrosome regulation by Aurora A and KIF11. We also developed a KIF15-dependent SB743921-resistance cell model. Significantly, the drug-resistance could also be overcome with Aurora A inhibitors. These results provide a molecular basis for increasing the effectiveness of Aurora A and KIF11 inhibitors and tackling problems of drug resistance.
    Full-text · Article · Jun 2014 · Molecular Oncology
  • Randy Y.C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: Nasopharyngeal carcinoma (NPC) is a highly invasive cancer with poor prognosis. One of the recurring themes of NPC biology and treatments is DNA damage. Epstein–Barr virus infection, which is generally accepted as a key etiological factor for NPC, triggers DNA damage responses. In normal cells, DNA damage checkpoints are able to prevent cell cycle progression following DNA damage and are critical for maintaining genome stability. Main features of the checkpoints include activation of ATM and ATR by sensors of DNA damage, which activates effector kinases CHK1 and CHK2; they in turn targets the CDC25/WEE1–cyclin B1–CDK1 axis to cause G2 arrest, or the p53–p21CIP1/WAF1 and pRb pathways to cause G1 arrest. Significantly, these checkpoints are typically disrupted in NPC cells. While mutations are relatively rare, mechanisms including promoter modifications, miRNAs, and actions of Epstein–Barr virus-encoded proteins such as EBNA3C and LMP1 have been described. Paradoxically, radiation-mediated DNA damage remains the primary treatment of NPC. How dysregulation of the DNA damage checkpoints contribute to NPC tumorigenesis and responses to treatment remain poorly understood. In this review, the current understanding of the molecular mechanisms of the various DNA damage checkpoints and what is known about them in NPC are discussed.
    No preview · Article · May 2014 · Oral Oncology
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    H Chen · S Huang · X Han · J Zhang · C Shan · Y H Tsang · H T Ma · R Y C Poon
    [Show abstract] [Hide abstract] ABSTRACT: Many mitotic kinases are both critical for maintaining genome stability and are important targets for anticancer therapies. We provide evidence that SIK3 (salt-inducible kinase 3), an AMP-activated protein kinase-related kinase, is important for mitosis to occur properly in mammalian cells. Downregulation of SIK3 resulted in an extension of mitosis in both mouse and human cells but did not affect the DNA damage checkpoint. Time-lapse microscopy and other approaches indicated that mitotic exit but not mitotic entry was delayed. Although repression of SIK3 alone simply delayed mitotic exit, it was able to sensitize cells to various antimitotic chemicals. Both mitotic arrest and cell death caused by spindle poisons were enhanced after SIK3 depletion. Likewise, the antimitotic effects due to pharmacological inhibition of mitotic kinases including Aurora A, Aurora B, and polo-like kinase 1 were enhanced in the absence of SIK3. Finally, in addition to promoting the sensitivity of a small-molecule inhibitor of the mitotic kinesin Eg5, SIK3 depletion was able to overcome cells that developed drug resistance. These results establish the importance of SIK3 as a mitotic regulator and underscore the potential of SIK3 as a druggable antimitotic target.
    Full-text · Article · Apr 2014 · Cell Death & Disease
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    Wing Yu Man · Joyce P.Y. Mak · Randy Y.C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: Dovitinib (TKI258; formerly CHIR-258) is an orally bioavailable inhibitor of multiple receptor tyrosine kinases. Interestingly, Dovitinib triggered a G2 /M arrest in cancer cell lines from diverse origins including HeLa, nasopharyngeal carcinoma, and hepatocellular carcinoma. Single-cell analysis revealed that Dovitinib promoted a delay in mitotic exit in a subset of cells, causing the cells to undergo mitotic slippage. Higher concentrations of Dovitinib induced a G2 arrest similar to the G2 DNA damage checkpoint. In support of this, DNA damage was triggered by Dovitinib as revealed by γ-H2AX and comet assays. The mitotic kinase CDK1 was found to be inactivated by phosphorylation in the presence of Dovitinib. Furthermore, the G2 arrest could be overcome by abrogation of the G2 DNA damage checkpoint using small molecule inhibitors of CHK1 and WEE1. Finally, Dovitinib-mediated G2 cell cycle arrest and subsequent cell death could be promoted after DNA damage repair was disrupted by inhibitors of poly(ADP-ribose) polymerases. These results are consistent with the recent finding that Dovitinib can also target topoisomerases. Collectively, these results suggest additional directions for use of Dovitinib, in particular with agents that target the DNA damage checkpoint.
    Full-text · Article · Nov 2013 · Journal of Cellular and Molecular Medicine
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    [Show abstract] [Hide abstract] ABSTRACT: Nasopharyngeal carcinoma (NPC) is a rare but highly invasive cancer. As options of agents for effective combination chemoradiotherapy of advanced NPC are limited, novel therapeutic approaches are desperately needed. The ubiquitin ligase CHFR is known to target poly(ADP-ribose) polymerase 1 (PARP1) for degradation and is epigenetically inactivated in NPC. We present evidence that PARP1 protein is indeed overexpressed in NPC cells in comparison to immortalized normal nasopharyngeal epithelial cells. Tissue microarray analysis also indicated that PARP1 protein is significantly elevated in primary NPC tissues, with strong correlation with all stages of NPC development. We found that the PARP inhibitor AZD2281 (Olaparib) increased DNA damage, cell cycle arrest, and apoptosis in NPC cells challenged with ionizing radiation or temozolomide. Isobologram analysis confirmed that the cytotoxicity triggered by AZD2281 and DNA damaging agents was synergistic. Finally, AZD2281 also enhanced the tumor-inhibitory effects of ionizing radiation in animal xenograft models. These observations implicate that PARP1 overexpression is an early event in NPC development and provide a molecular basis of using PARP inhibitors to potentiate treatment of NPC with radiotherapy and chemotherapy.
    Preview · Article · Aug 2013 · Molecular Cancer Therapeutics
  • M Marxer · H T Ma · WY Man · R Y C Poon
    [Show abstract] [Hide abstract] ABSTRACT: A number of small-molecule inhibitors of Aurora kinases have been developed and are undergoing clinical trials for anti-cancer therapies. Different Aurora kinases, however, behave as very different targets: while inhibition of Aurora A (AURKA) induces a delay in mitotic exit, inhibition of Aurora B (AURKB) triggers mitotic slippage. Furthermore, while it is evident that p53 is regulated by Aurora kinase-dependent phosphorylation, how p53 may in turn regulate Aurora kinases remains mysterious. To address these issues, isogenic p53-containing and -negative cells were exposed to classic inhibitors that target both AURKA and AURKB (Alisertib and ZM447439), as well as to new generation of inhibitors that target AURKA (MK-5108), AURKB (Barasertib) individually. The fate of individual cells was then tracked with time-lapse microscopy. Remarkably, loss of p53, either by gene disruption or small interfering RNA-mediated depletion, sensitized cells to inhibition of both AURKA and AURKB, promoting mitotic arrest and slippage respectively. As the p53-dependent post-mitotic checkpoint is also important for preventing genome reduplication after mitotic slippage, these studies indicate that the loss of p53 in cancer cells represents a major opportunity for anti-cancer drugs targeting the Aurora kinases.Oncogene advance online publication, 19 August 2013; doi:10.1038/onc.2013.325.
    No preview · Article · Aug 2013 · Oncogene
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    Xianxian Han · Randy Y C Poon
    [Show abstract] [Hide abstract] ABSTRACT: Sister chromatid separation depends on the activity of separase, which in turn requires the proteolysis of its inhibitor, securin. It has been speculated that securin also supports the activation of separase. In this study, we found that PTTG1 was the major securin isoform expressed in most normal and cancer cell lines. Remarkably, a highly homologous isoform called PTTG2 was unable to interact with separase. Using chimeras between PTTG1 and PTTG2 and other approaches, we pinpointed a single amino acid that accounted for the loss of securin function in PTTG2. Mutation of the homologous position in PTTG1 (H134) switched PTTG1 from an inhibitor into an activator of separase. In agreement with this, PTTG1 lacking H134 was able to trigger premature sister chromatid separation. Conversely, introduction of H134 into PTTG2 is sufficient to allow it to bind separase. These data demonstrate that while the motif containing H134 has a strong affinity for separase and is involved in inhibiting it, another domain(s) is involved in activating separase and has a weaker affinity for it. Although PTTG2 lacks securin function, its differences from PTTG1 provide evidence of independent inhibitory and activating functions of PTTG1 on separase.
    Preview · Article · Jun 2013 · Molecular and Cellular Biology
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    Randy Y.C. Poon
    Preview · Article · Mar 2013 · Cell cycle (Georgetown, Tex.)
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    Dataset: Figure S2
    Yiu Huen Tsang · Xianxian Han · Wing Yu Man · Nelson Lee · Randy Y. C. Poon
    [Show abstract] [Hide abstract] ABSTRACT: Downregulation of SHP2 enhances CIS-mediated cell death. (A) Downregulation of SHP2 sensitizes cells to CIS. HeLa cells transfected with control, SHP2, or CHK1 siRNAs were treated with either buffer or CIS. After 24 h, cell death was analyzed with fluorescence microscopy (n = 400). Representative images are shown. (B) Time-lapse imaging reveals that downregulation of SHP2 enhances CIS-mediated cell death. HeLa cells stably expressing histone H2B–GFP were transfected with either control or siSHP2. After treatment with CIS, the fate of individual cells were tracked with time-lapse microscopy at 5 min/frame for 24 h (n = 100). Each horizontal line represents one cell. Key: light grey = interphase; black = mitosis (from DNA condensation to anaphase or cell death); truncated bars = cell death. (PDF)
    Full-text · Dataset · Nov 2012

Publication Stats

4k Citations
563.76 Total Impact Points

Institutions

  • 1997-2012
    • The Hong Kong University of Science and Technology
      • Division of Life Science
      Chiu-lung, Kowloon City, Hong Kong
    • Torrey Pines Institute for Molecular Studies
      Port St. Lucie, Florida, United States
  • 2003
    • Kanazawa University
      Kanazawa, Ishikawa, Japan
  • 2001
    • University of California, San Diego
      • Department of Surgery
      San Diego, California, United States