Initiation Of Cytokinesis Research Articles

Opposing prognostic roles of nuclear and cytoplasmic RACGAP1 expression in colorectal cancer patients

Rac GTPase activating protein 1 (RACGAP1) plays a regulatory role in initiation of cytokinesis, control of cell growth and differentiation, and tumor malignancy, making it a potential prognostic biomarker. RACGAP1 is present in the nucleus, but a diffuse distribution in the cytoplasm also occurs. The aim of this study was to determine the impact of nuclear and cytoplasmic expression of RACGAP1 on clinical outcome to provide further evidence of a role in colorectal cancer. RACGAP1 expression was analyzed by immunohistochemistry in 166 cancer specimens from primary colorectal cancer patients. The mean follow-up time after surgery was 5.4 years (range, 0.01-13.10 years). The prognostic value of RACGAP1 on overall survival was validated by Kaplan-Meier analysis and Cox regression models. RACGAP1 is expressed in colorectal specimen and is present in both the nucleus and cytoplasm in different amounts. Colorectal cancer patients had opposite prognoses depending on the site of RACGAP1 expression. Patients with high nuclear RACGAP1 expression had poor outcomes, whereas those with high cytoplasmic RACGAP1 expression had favorable prognosis (P = .003 and P = .001, respectively). Patients with low nuclear but high cytoplasmic RACGAP1 expression had better survival compared with those with other combinations (P < .001). We suggest that RACGAP1 expression levels in the nucleus and cytoplasm, determined by immunohistochemical staining, predict opposite clinical outcomes and that both could be independent prognostic markers for colorectal cancer.

Abstract 5460: Intracellular uptake and metabolism of SH7139: Is it a targeted prodrug for B-cell lymphomas

Abstract Selective high affinity ligands (SHALs) are novel targeting agents composed of lysine/mini-polyethylene glycol scaffolds linking two or three ‘recognition’ ligands together. The tridentate SHAL SH7139 is selectively cytotoxic at sub-nanomolar concentrations to lymphoma cells over-expressing HLA-DR10. Electron microscopy examinations of Burkitt's lymphoma xenografts from mice treated with SH7139 suggest one mechanism for cell killing may involve cell signaling-induced apoptosis analogous to that observed with Lym-1 antibody. An alternative mechanism is that intracellular hydrolysis of the SHAL's amide bonds following its uptake and metabolism by lymphoma cells could lead to the release of the three recognition ligands and the subsequent inhibition (or promotion) of cellular activities that trigger apoptosis or induce cell death and necrosis. Each of the three recognition ligands (Dv, Ct, and Cb) is independently, but non-selectively, toxic to both Raji (HLA-DR10 positive) and Jurkat (HLA-DR10 negative) cells. The Ct ligand, 3-(2-([3-chloro-5-trifluoromethyl)-2-pyridinyl]oxy)-anilino)-3-oxopropanionic acid, is a structural analog of aryloxyphenoxypropionate herbicides that inhibit acetyl-CoA carboxylase (ACC), a key enzyme in fatty acid biosynthesis that is upregulated in many cancers to meet their need for rapid growth and proliferation. Our studies show that although Ct could be accommodated into the herbicide binding pocket of ACC, this ligand was not found to inhibit human and rat ACC1 or ACC2. However, metabolic cleavage of the single amide bond present in Ct would lead to the production of two smaller metabolites, an aniline derivative and malonate, both of which are known to be biologically active and potentially toxic compounds. The Dv ligand, dabsyl-L-valine, belongs to a family of biphenylsulfonamides, many of which have been shown to inhibit matrix metalloproteinases such as MMP9 and MMP14 that are upregulated in tumor cells. The Cb ligand, 4-[4-(4-chlorobenzyl)piperazino]-3-nitrobenzene carboxylic acid, is a structural analog of a large number of inhibitors that have been shown to block the assembly of the contractile ring and the initiation of cytokinesis by inhibiting guanosine nucleoside binding to MgcRacGAP, a protein that participates in the activation of the myosin motor and the initiation of cleavage furrow ingression. A combination of SH7139 metabolism studies, computer modeling, and enzyme inhibition assays have been performed to determine if Ct, Dv and Cb and their metabolites are produced and whether they contribute to lymphoma cell killing. Citation Format: Monique C. Balhorn, Gary Mirick, Dong Cheng, Zhengping Ma, Edmond Y. Lau, Saphon Hok, Gerald L. DeNardo, Rod Balhorn. Intracellular uptake and metabolism of SH7139: Is it a targeted prodrug for B-cell lymphomas. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 5460. doi:10.1158/1538-7445.AM2014-5460

SHCBP1 is required for midbody organization and cytokinesis completion

The centralspindlin complex, which is composed of MKLP1 and MgcRacGAP, is one of the crucial factors involved in cytokinesis initiation. Centralspindlin is localized at the middle of the central spindle during anaphase and then concentrates at the midbody to control abscission. A number of proteins that associate with centralspindlin have been identified. These associating factors regulate furrowing and abscission in coordination with centralspindlin. A recent study identified a novel centralspindlin partner, called Nessun Dorma, which is essential for germ cell cytokinesis in Drosophila melanogaster. SHCBP1 is a human ortholog of Nessun Dorma that associates with human centralspindlin. In this report, we analyzed the interaction of SHCBP1 with centralspindlin in detail and determined the regions that are required for the interaction. In addition, we demonstrate that the central region is necessary for the SHCBP1 dimerization. Both MgcRacGAP and MKLP1 are degraded once cells exit mitosis. Similarly, endogenous and exogenous SHCBP1 were degraded with mitosis progression. Interestingly, SHCBP1 expression was significantly reduced in the absence of centralspindlin, whereas centralspindlin expression was not affected by SHCBP1 knockdown. Finally, we demonstrate that SHCBP1 depletion promotes midbody structure disruption and inhibits abscission, a final stage of cytokinesis. Our study gives novel insight into the role of SHCBP in cytokinesis completion.

De novo fatty acid synthesis at the mitotic exit is required to complete cellular division

Although the regulation of the cell cycle has been extensively studied, much less is known about its coordination with the cellular metabolism. Using mass spectrometry we found that lysophospholipid levels decreased drastically from G2/M to G1 phase, while de novo phosphatidylcholine synthesis, the main phospholipid in mammalian cells, increased, suggesting that enhanced membrane production was concomitant to a decrease in its turnover. In addition, fatty acid synthesis and incorporation into membranes was increased upon cell division. The rate-limiting reaction for de novo fatty acid synthesis is catalyzed by acetyl-CoA carboxylase. As expected, its inhibiting phosphorylation decreased prior to cytokinesis initiation. Importantly, the inhibition of fatty acid synthesis arrested the cells at G2/M despite the presence of abundant fatty acids in the media. Our results suggest that de novo lipogenesis is essential for cell cycle completion. This “lipogenic checkpoint” at G2/M may be therapeutically exploited for hyperproliferative diseases such as cancer.

MAP1S controls microtubule stability throughout the cell cycle in human cells.

Summary Understanding the molecular basis for proper cell division requires a detailed functional analysis of microtubule (MT)-associated proteins. MT-associated protein 1S (MAP1S), the most ubiquitously expressed MAP1 family member, is required for accurate cell division. Here, using quantitative analysis of MT plus-end tracking, we show that MAP1S knockdown alters MT dynamics throughout the cell cycle. Surprisingly, MAP1S downregulation results in faster growing, yet short-lived, MTs in all cell cycle stages and in a global loss of MT acetylation. These aberrations correlate with severe defects in the final stages of cell division. In monopolar cytokinesis assays, we demonstrate that MAP1S guides MT-dependent initiation of cytokinesis. Our data underline the key role of MAP1S as a global regulator of MT stability and demonstrate a new primary function of MAP1S to regulate MT dynamics at the onset of cytokinesis.

Distinct Roles of a Mitogen-Activated Protein Kinase in Cytokinesis between Different Life Cycle Forms of Trypanosoma brucei

Mitogen-activated protein kinase (MAPK) modules are evolutionarily conserved signaling cascades that function in response to the environment and play crucial roles in intracellular signal transduction in eukaryotes. The involvement of a MAP kinase in regulating cytokinesis in yeast, animals, and plants has been reported, but the requirement for a MAP kinase for cytokinesis in the early-branching protozoa is not documented. Here, we show that a MAP kinase homolog (TbMAPK6) from Trypanosoma brucei plays distinct roles in cytokinesis in two life cycle forms of T. brucei. TbMAPK6 is distributed throughout the cytosol in the procyclic form but is localized in both the cytosol and the nucleus in the bloodstream form. RNA interference (RNAi) of TbMAPK6 results in moderate growth inhibition in the procyclic form but severe growth defects and rapid cell death in the bloodstream form. Moreover, TbMAPK6 appears to be implicated in furrow ingression and cytokinesis completion in the procyclic form but is essential for cytokinesis initiation in the bloodstream form. Despite the distinct defects in cytokinesis in the two forms, RNAi of TbMAPK6 also caused defective basal body duplication/segregation in a small cell population in both life cycle forms. Altogether, our results demonstrate the involvement of the TbMAPK6-mediated pathway in regulating cytokinesis in trypanosomes and suggest distinct roles of TbMAPK6 in cytokinesis between different life cycle stages of T. brucei.

Phospho-Regulation of the Neurospora crassa Septation Initiation Network

Proper cell division is essential for growth and development of uni- and multicellular organisms. The fungal septation initiation network (SIN) functions as kinase cascade that connects cell cycle progression with the initiation of cytokinesis. Miss-regulation of the homologous Hippo pathway in animals results in excessive cell proliferation and formation of tumors, underscoring the conservation of both pathways. How SIN proteins interact and transmit signals through the cascade is only beginning to be understood. Moreover, our understanding of septum formation and its regulation in filamentous fungi, which represent the vast majority of the fungal kingdom, is highly fragmentary. We determined that a tripartite kinase cascade, consisting of CDC-7, SID-1 and DBF-2, together with their regulatory subunits CDC-14 and MOB-1, is important for septum formation in the model mold Neurospora crassa. DBF-2 activity and septum formation requires auto-phosphorylation at Ser499 within the activation segment and phosphorylation of Thr671 in the hydrophobic motif by SID-1. Moreover, SID-1-stimulated DBF-2 activity is further enhanced by CDC-7, supporting a stepwise activation mechanism of the tripartite SIN kinase cascade in fungi. However, in contrast to the situation described for unicellular yeasts, the localization of the entire SIN cascade to spindle pole bodies is constitutive and cell cycle independent. Moreover, all SIN proteins except CDC-7 form cortical rings prior to septum initiation and localize to constricting septa. Thus, SIN localization and activity regulation significantly differs in unicellular versus syncytial ascomycete fungi.

The BEG (PP2A-B55/ENSA/Greatwall) Pathway Ensures Cytokinesis follows Chromosome Separation

SummaryCytokinesis follows separase activation and chromosome segregation. This order is ensured in budding yeast by the mitotic exit network (MEN), where Cdc14p dephosphorylates key conserved Cdk1-substrates exemplified by the anaphase spindle-elongation protein Ase1p. However, in metazoans, MEN and Cdc14 function is not conserved. Instead, the PP2A-B55α/ENSA/Greatwall (BEG) pathway controls the human Ase1p ortholog PRC1. In this pathway, PP2A-B55 inhibition is coupled to Cdk1-cyclin B activity, whereas separase inhibition is maintained by cyclin B concentration. This creates two cyclin B thresholds during mitotic exit. Simulation and experiments using PRC1 as a model substrate show that the first threshold permits separase activation and chromosome segregation, and the second permits PP2A-B55 activation and initiation of cytokinesis. Removal of the ENSA/Greatwall (EG) timer module eliminates this second threshold, as well as associated delay in PRC1 dephosphorylation and initiation of cytokinesis, by uncoupling PP2A-B55 from Cdk1-cyclin B activity. Therefore, temporal order during mitotic exit is promoted by the metazoan BEG pathway.

Cep57 Protein Is Required for Cytokinesis by Facilitating Central Spindle Microtubule Organization

Cytokinesis is the final stage of cell division in which the cytoplasm of a cell is divided into two daughter cells after the segregation of genetic material, and the central spindle and midbody are considered to be the essential structures required for the initiation and completion of cytokinesis. Here, we determined that the centrosome protein Cep57, which is localized to the central spindle and midbody, acts as a spindle organizer and is required for cytokinesis. Depletion of Cep57 disrupted microtubule assembly of the central spindle and further led to abnormal midbody localization of MKLP1, Plk1, and Aurora B, which resulted in cytokinesis failure and the formation of binuclear cells. Furthermore, we found that Cep57 directly recruited Tektin 1 to the midbody matrix to regulate microtubule organization. Thus, our data reveal that Cep57 is essential for cytokinesis via regulation of central spindle assembly and formation of the midbody.

Polo-like kinases

Polo-like kinases

Regulation of the Cell Division Cycle in Trypanosoma brucei

The cell division cycle is tightly regulated by the activation and inactivation of a series of proteins that control the replication and segregation of organelles to the daughter cells. During the past decade, we have witnessed significant advances in our understanding of the cell cycle in Trypanosoma brucei and how the cycle is regulated by various regulatory proteins. However, many other regulators, especially those unique to trypanosomes, remain to be identified, and we are just beginning to delineate the signaling pathways that drive the transitions through different cell cycle stages, such as the G(1)/S transition, G(2)/M transition, and mitosis-cytokinesis transition. Trypanosomes appear to employ both evolutionarily conserved and trypanosome-specific molecules to regulate the various stages of its cell cycle, including DNA replication initiation, spindle assembly, chromosome segregation, and cytokinesis initiation and completion. Strikingly, trypanosomes lack some crucial regulators that are well conserved across evolution, such as Cdc6 and Cdt1, which are involved in DNA replication licensing, the spindle motor kinesin-5, which is required for spindle assembly, the central spindlin complex, which has been implicated in cytokinesis initiation, and the actomyosin contractile ring, which is located at the cleavage furrow. Conversely, trypanosomes possess certain regulators, such as cyclins, cyclin-dependent kinases, and mitotic centromere-associated kinesins, that are greatly expanded and likely play diverse cellular functions. Overall, trypanosomes apparently have integrated unique regulators into the evolutionarily conserved pathways to compensate for the absence of those conserved molecules and, additionally, have evolved certain cell cycle regulatory pathways that are either different from its human host or distinct between its own life cycle forms.

Targeting of the RhoGEF Ect2 to the Equatorial Membrane Controls Cleavage Furrow Formation during Cytokinesis

In animal cells, formation of the cytokinetic furrow requires activation of the GTPase RhoA by the guanine nucleotide exchange factor Ect2. How Ect2, which is associated with the spindle midzone, controls RhoA activity at the equatorial cortex during anaphase is not understood. Here, we show that Ect2 concentrates at the equatorial membrane during cytokinesis in live cells. Ect2 membrane association requires a pleckstrin homology domain and a polybasic cluster that bind to phosphoinositide lipids. Both guanine nucleotide exchange function and membrane targeting of Ect2 are essential for RhoA activation and cleavage furrow formation in human cells. Membrane localization of Ect2 is spatially confined to the equator by centralspindlin, Ect2's spindle midzone anchor complex, and is temporally coordinated with chromosome segregation through the activation state of CDK1. We propose that targeting of Ect2 to the equatorial membrane represents a key step in the delivery of the cytokinetic signal to the cortex.

Actin regulation during abscission: unexpected roles of Rab35 and endocytic transport

Actin regulation during abscission: unexpected roles of Rab35 and endocytic transport

At the Right Time and at the Right Place: Control of Cytokinesis in Mammals

Ensuring that cytokinesis occurs at the right time and in the right place is essential to maintain euploidy and cell-cycle progression. Cytokinesis is triggered by the RhoA GEF Ect2, which localizes to the spindle midzone during anaphase and activates RhoA at the overlying cell cortex to stimulate contractile ring formation. Using a chemical inhibitor of the Polo kinase Plk1, Petronczki et al. (2007) showed that Plk1 is essential for cytokinesis, because it specifically promotes Ect2 localization and interaction with its midzone receptor HsCyk-4. I liked this paper for three reasons. As a biologist, I was impressed because the paper explains an important part of biology: how a cell-cycle kinase determines when and where cytokinesis occurs. As a geneticist, I appreciate a tight allele. The paper is a beautiful example of the power of chemical genetics to specifically and rapidly inhibit Polo at a specific cell-cycle stage. Finally, as a yeast cell-cycle researcher, it is always nice to see the dramatic conservation between yeast and mammals. In budding yeast, even though the bud neck and not the spindle midzone determines the site of cytokinesis, Polo kinase phosphorylates Rho GEFs to initiate contractile ring formation and cytokinesis. This PaperPick refers to Polo-like Kinase 1 Triggers the Initiation of Cytokinesis in Human Cells by Promoting Recruitment of the RhoGEF Ect2 to the Central Spindle, by M. Petronczki, M. Glotzer, N. Kraut, and J.-M. Peters, published in May 2007. Video Abstract Here, Jan-Michael Peters discusses the work as a collaboration between Mark Petronczki (in his group), Michael Glotzer of the University of Chicago, and Norbert Kraut of Boehringer Ingelheim, who provided the key reagent: BI 2536, a chemical inhibitor of Plk1. The authors would like to note that their work on the role of Plk1 in cytokinesis was inspired by early observations made in the laboratory of David Glover and that the research groups of Prasad Jallepalli (Sloan Kettering) and Aaron Straight (Stanford) have also identified Plk1 as a key regulator of cleavage furrow formation in mammalian cells.

Phosphorylation-dependent regulation of the F-BAR protein Hof1 during cytokinesis

Spatial and timely coordination of cytokinesis is crucial for the maintenance of organelle inheritance and genome integrity. The mitotic exit network (MEN) pathway controls both the timely initiation of mitotic exit and cytokinesis in budding yeast. Here we identified the conserved F-BAR protein Hof1 as a substrate of the MEN kinase complex Dbf2-Mob1 during cytokinesis. We show that polo-like kinase Cdc5 first phosphorylates Hof1 to allow subsequent phosphorylation by Dbf2-Mob1. This releases Hof1 from the septin ring and facilitates Hof1 binding to the medial actomyosin ring (AMR), where Hof1 promotes AMR contraction and membrane ingression. Domain structure analysis established that the central, unstructured, region of Hof1, named the ring localization sequence (RLS), is sufficient to mediate Hof1's binding to the medial ring in a cell cycle-dependent manner. Genetic and functional data support a model in which Dbf2-Mob1 regulates Hof1 by inducing domain rearrangements, leading to the exposure of the Hof1 RLS domain during telophase.

Spores before sporophytes: hypothesizing the origin of sporogenesis at the algal–plant transition

Fossil spores from mid-Ordovician deposits (475 million yr old) are the first indication of plants on land and predate megafossils of plants by 30-50 million yr. Sporopollenin-walled spores distinguish land plants from algae, which typically have heavy-walled zygotes that germinate via meiosis into motile or protonemal cells. All land plants are embryophytes with spores produced by the sporophyte generation. It is generally assumed that retention of the zygote and delay in meiosis led to matrotrophic embryo development and intercalation of the diploid sporophyte before spore production. However, new data on the cell biology of sporogenesis in extant bryophytes suggest that spores were produced directly from zygotes in protoembryophytes. The mechanism of wall transfer from zygote to meiospores was a three-phase heterochrony involving precocious initiation of cytokinesis, acceleration of meiosis, and concomitant delay in wall deposition. In bryophyte sporogenesis, cytokinesis is typically initiated in advance of meiosis, and quadrilobing of the cytoplasm is followed by development of a bizarre quadripolar spindle that assures coordination of nuclear distribution with predetermined spore domains. This concept of the innovation of sporogenesis at the onset of terrestrialization provides a new perspective for interpreting fossil evidence and understanding the evolution of land plants.

Polo Kinase Interacts with RacGAP50C and Is Required to Localize the Cytokinesis Initiation Complex

The assembly and constriction of an actomyosin contractile ring in cytokinesis is dependent on the activation of Rho at the equatorial cortex by a complex, here termed the cytokinesis initiation complex, between a microtubule-associated kinesin-like protein (KLP), a member of the RacGAP family, and the RhoGEF Pebble. Recently, the activity of the mammalian Polo kinase ortholog Plk1 has been implicated in the formation of this complex. We show here that Polo kinase interacts directly with the cytokinesis initiation complex by binding RacGAP50C. We find that a new domain of Polo kinase, termed the intermediate domain, interacts directly with RacGAP50C and that Polo kinase is essential for localization of the KLP-RacGAP centralspindlin complex to the cell equator and spindle midzone. In the absence of Polo kinase, RacGAP50C and Pav-KLP fail to localize normally, instead decorating microtubules along their length. Our results indicate that Polo kinase directly binds the conserved cytokinesis initiation complex and is required to trigger centralspindlin localization as a first step in cytokinesis.

Cytological particularities in nuclear and nuclear-cytoplasmic male sterile pepper lines

The meiosis in the pollen mother cells (PMCs) of nuclear and nuclear-cytoplasmic male sterile (CMS) pepper lines No.1647 ms 8 ms 8 and No.K587 Srfrf was evaluated to establish the causes of male steril- ity. The investigations of sterile (ms 8 ms 8 ) plants revealed that at diakinesis and metaphase one, along with the bivalents, up to 4 quadrivalents, and between 2 and 16 univalents were observed in 21.2% and in 6.2% of the PMCs, respectively,. In the remaining 72.6% of the cells normal pairing and formation of 12 bivalents was found. Lethality in ms 8 ms 8 plants occurred after the second telophase, as cytokinesis and microspore formation were blocked. Similar meiotic disturbances were not visible in the fertile analogues (Msms) from line No.1647 and that was refl ected in the: presence of cytokinesis after the second telophase; tetrad creation in about 99% of the cells; pollen fertility in the range of 60-100%. In the CMS line No.K587, the existence of 12 bivalents (73.8% of the PMCs), one or two quadrivalents, and up to six univalents (21.9% and 4.3% of the PMCs) were described. The microspore degeneration started either after the tetrad stage, as the callose was not dissolved and the microspores were not released from the sporads, or in some cases at various phases of the microgametogenesis. The chromosome rearrangements and chromosome breakage leading to those aberrations could damage the genetic system responsible for the regular initiation, running and termination of late meiotic stages, especially the termination of TII and initiation of cytokinesis in ms 8 ms 8 plants, and loss of callose dissolving in the CMS plants.

Abstract 4435: Discovery, biological characterization and oral antitumor activity of polo-like kinase 1 (Plk1) selective small molecule inhibitors

Abstract We have identified novel inhibitors of Polo-like kinase 1 (Plk1) that exhibit strong single agent activity in solid tumor and leukemia xenograft models after oral dosing. Plk1 is a serine / threonine protein kinase thought to regulate cell division through promotion of mitotic entry, control of spindle assembly, orchestration of mitotic progression and initiation of cytokinesis. Plk1 has been reported to phosphorylate and deactivate the tumor suppressors p53, p63 and p73 thereby inhibiting apoptosis. Furthermore this repression of p53 family members may be responsible for the survival and tumorigenesis of liver cancer stem cells. Cancer cell proliferation is blocked in vitro and in vivo by antisense oligonucleotides and siRNAs to Plk1. Overexpression of Plk1 is associated with tumor development and many human cancers express elevated Plk1 levels compared to surrounding normal tissue. Numerous studies have shown that Plk1 expression levels correlate with disease progression, invasiveness and poor patient prognosis. Collectively, these observations support the selection of Plk1 as an attractive target for cell cycle-directed cancer therapy. We have employed high throughput screening, in silico screening and de novo ligand design approaches to select an inhibitor scaffold for lead optimization. We have selected a set of ATP-competitive Plk1 inhibitors that exhibit high selectivity for Plk1 and inhibit growth of a broad range of tumor cell lines in vitro with IC50s in the low nanomolar range. Structure-activity relationships (SAR) were determined through iterative targeted analogue synthesis and in vitro testing with SAR rationalized against x-ray crystallographic data. We observed selectivity of the inhibitor scaffold for Plk1 against a panel of over 200 kinases. Treatment of tumor cells with our Plk1 inhibitors induced a phenotype consistent with inhibition of Plk1, accumulation of cells in mitosis and induction of apoptosis. The extent of cytotoxicity was dependent on proliferation as determined by comparative viability of cells arrested in the G1 phase versus proliferating cells. Compound evaluation using a cell-based activity assay for inhibition of Plk1 and pharmacokinetic profiling informed lead optimization studies to generate potential drug candidates with high oral bioavailability and which induce tumor stabilization, regression and tumor-free cures in solid tumor and leukemia xenograft models as single agents. For example, 6% T/C (p = 0.00001) was achieved in the HCT116 human colon carcinoma model. In summary, we describe a set of Plk1 inhibitors which act as potent antiproliferative agents suitable for further development as oral Plk1 selective inhibitors for the treatment of human cancers. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4435.

Regulation of Cytokinesis by the Formin cdc12p

For successful cell division, cytokinesis must be properly timed to occur only after the segregation of chromosomes during mitosis. In the fission yeast Schizosaccharomyces pombe, contractile ring assembly initiates at the onset of mitosis, and ring contraction occurs concomitant with septation at the end of anaphase. Although many of the conserved factors necessary for ring assembly and regulation of cytokinesis have been characterized, still little is known about cell-cycle regulation of events that initiate cytokinesis. The formin cdc12p is an essential ring component with a well-characterized function in F-actin assembly. Here we show that overexpression of a cdc12p fragment bypasses normal cell-cycle controls and induces contractile ring assembly and sometimes even ring contraction and septation, all during interphase. Activation of cytokinesis occurs without the apparent activation of cell-cycle regulators such as polo kinase or the septation initiation network. For this effect, cdc12p contributes at least two separable activities: actin assembly and one or more additional functions in cytokinesis initiation. These observations suggest that the formin cdc12p participates downstream of cell-cycle regulators in a network that drives the initiation of cytokinesis.