Multiple Roles of Canonical Wnt Signaling in Cell Cycle Progression and Cell Lineage Specification in Neural Development

Signaling by the canonical Wnt pathway has multiple functions in stem cells. It caneither control stem cell expansion or –as we have recently demonstrated withneural crest stem cells– influence cell lineage decisions by promoting specific fatesat the expense of others. Thus, the role of canonical Wnt in stem cells is dependenton cell-intrinsic properties that determine how a cell responds to Wnt. Themolecular basis for the functional diversity of Wnt in different stem cell typesremains to be elucidated.     

Dynamic Organization of Microtubules and Microfilaments during Cell Cycle Progression in Higher Plant Cells

Abstract: The cytoskeleton, which mainly consists of microtubules (MTs) and actin microfilaments (MFs), plays various significant roles that are indispensable for eukaryotic viability, including determination of cell shape, cell movement, nuclear division, and cytokinesis. In animal cells, MFs appear to be of more importance than MTs, except for spindle formation in nuclear division. In contrast, higher plants have a rigid cell wall around their cells, and have thus evolved elegant systems of MTs to control the direction of cellulose microfibrils (CMFs) deposited in the cell wall, and to divide centrifugally in a physically limited space. Dynamic changes in MTs during cell cycle progression in higher plant cells have been observed over several decades, including cortical MTs (CMTs) during interphase, preprophase bands (PPBs) from late G₂ phase to prophase, spindles from prometaphase to anaphase, and phragmoplasts at telophase. The MFs also show some changes not as obvious as MT dynamics. However, questions regarding the process of formation of these arrays, and the precise mechanisms by which they fulfill their roles, remain unsolved. In this article, we present an outline of the changes in the cytoskeleton based on our studies with highly-synchronized tobacco BY-2 cells. Some candidate molecules that could play roles in cytoskeletal dynamics are discussed. We also hope to draw attention to recent attempts at visualization of cytoskeletons with molecular techniques, and to some examples of genetic approaches in this field.     

Okadaic Acid as a Probe to Analyse the Cell Cycle Progression in Plant Cells

Previously we have demonstrated the dynamic change of microtubules (MTs) during cell cycle progression using highly synchronized tobacco BY-2 cells and characterized the specific transition points of MT organization (Hasezawa and Nagata, 1991). In this study the effect of okadaic acid (OA), a specific inhibitor of protein phosphatase 1 and 2A, on such changes of MTs during cell cycle was examined. These experiments revealed that cell cycle was arrested before the formation of the preprophase band (PPB), at anaphase and at the border of M/G₁. Although the block at the anaphase seemed to be analogous to that observed in animal cells (Yamashita et al., 1990), the other two blocks were specific to plant cells. It is interesting that these two blocks coincided with the transition points of MT organization, as revealed in the previous study (Hasezawa and Nagata, 1991). Thus it is proposed that phosphorylation is involved in MT organization, although the effect of OA has been shown mainly to be the activation of cdc-2/histone H1 kinase in animal cells. Another inhibitor of protein phosphatase 1 and 2A, calyculin A (CLA), showed very similar effects on the cell cycle progression. The use of such inhibitors to dissect the cell cycle progression of plant cells is discussed.     

植物细胞周期及其调控

概述了植物细胞周期的特点及其调控.     

Polycomb Group Proteins in Cell Cycle Progression and Cancer

Epigenetic deregulation of gene expression is emerging as key mechanism in tumorigenesis. Deregulated activity of the chromatin remodeling Polycomb Repressive Complex 2 (PRC2) has recently been shown to be a frequent event in human tumors. Here we discuss these findings and speculate on the role of the PRC2 complex in controlling gene expression during normal cellular proliferation and cancer development.     

DDAH1 Deficiency Attenuates Endothelial Cell Cycle Progression and Angiogenesis

Asymmetric dimethylarginine (ADMA) is an endogenous inhibitor of nitric oxide (NO) synthase (NOS). ADMA is eliminated largely by the action of dimethylarginine dimethylaminohydrolase1 (DDAH1). Decreased DDAH activity is found in several pathological conditions and is associated with increased risk of vascular disease. Overexpression of DDAH1 has been shown to augment endothelial proliferation and angiogenesis. To better understand the mechanism by which DDAH1 influences endothelial proliferation, this study examined the effect of DDAH1 deficiency on cell cycle progression and the expression of some cell cycle master regulatory proteins. DDAH1 KO decreased in vivo Matrigel angiogenesis and depressed endothelial repair in a mouse model of carotid artery wire injury. DDAH1 deficiency decreased VEGF expression in HUVEC and increased NF1 expression in both HUVEC and DDAH1 KO mice. The expression of active Ras could overcome the decreased VEGF expression caused by the DDAH1 depletion. The addition of VEGF and knockdown NF1 could both restore proliferation in cells with DDAH1 depletion. Flow cytometry analysis revealed that DDAH1 sRNAi knockdown in HUVEC caused G1 and G2/M arrest that was associated with decreased expression of CDC2, CDC25C, cyclin D1 and cyclin E. MEF cells from DDAH1 KO mice also demonstrated G2/M arrest that was associated with decreased cyclin D1 expression and Akt activity. Our findings indicate that DDAH1 exerts effects on cyclin D1 and cyclin E expression through multiple mechanisms, including VEGF, the NO/cGMP/PKG pathway, the Ras/PI3K/Akt pathway, and NF1 expression. Loss of DDAH1 effects on these pathways results in impaired endothelial cell proliferation and decreased angiogenesis. The findings provide background information that may be useful in the development of therapeutic strategies to manipulate DDAH1 expression in cardiovascular diseases or tumor angiogenesis.     

Cell Cycle Progression and Proliferation Despite 4BP-1 Dephosphorylation

Proliferation and cell cycle progression in response to growth factors require de novo protein synthesis. It has been proposed that binding of the eukaryotic translation initiation factor 4E (eIF-4E) to the inhibitory protein 4BP-1 blocks translation by preventing access of eIF-4G to the 5' cap of the mRNA. The signal for translation initiation is thought to involve phosphorylation of 4BP-1, which causes it to dissociate from eIF-4E and allows eIF-4G to localize to the 5' cap. It has been suggested that the ability of the macrolide antibiotic rapamycin to inhibit 4BP-1 phosphorylation is responsible for the potent antiproliferative property of this drug. We now show that rapamycin-resistant cells exhibited normal proliferation despite dephosphorylation of 4BP-1 that allows it to bind to eIF-4E. Moreover, despite rapamycin-induced dephosphorylation of 4BP-1, eIF-4E-eIF-4G complexes (eIF-4F) were still detected. In contrast, amino acid withdrawal, which caused a similar degree of 4BP-1 dephosphorylation, resulted in dissociation of the eIF-4E-eIF-4G complex. Thus, 4BP-1 dephosphorylation is not equivalent to eIF-4E inactivation and does not explain the antiproliferative property of rapamycin.     

细胞周期因子与植物根系发育

植物根系的发育是一个非常复杂且被精确调控的过程,受到多种信号的调控,其中对细胞分裂水平调控的研究已经成为细胞生物学研究的热点之一.文章介绍了植物细胞周期因子和植物根系发育相关的细胞周期调控机制以及根系细胞周期激素调节的研究进展.     

A Mathematical Model of Cell Cycle Progression Applied to the MCF-7 Breast Cancer Cell Line

In this paper, we present a model of cell cycle progression and apply it to cells of the MCF-7 breast cancer cell line. We consider cells existing in the three typical cell cycle phases determined using flow cytometry: the G1, S, and G2/M phases. We further break each phase up into model phases in order to capture certain features such as cells remaining in phases for a minimum amount of time. The model is also able to capture the environmentally responsive part of the G1 phase, allowing for quantification of the number of environmentally responsive cells at each point in time. The model parameters are carefully chosen using data from various sources in the biological literature. The model is then validated against a variety of experiments, and the excellent fit with experimental results allows for insight into the mechanisms that influence observed biological phenomena. In particular, the model is used to question the common assumption that a ‘slow cycling population’ is necessary to explain some results. Finally, an extension is proposed, where cell death is included in order to accurately model the effects of tamoxifen, a common first line anticancer drug in breast cancer patients. We conclude that the model has strong potential to be used as an aid in future experiments to gain further insight into cell cycle progression and cell death.     

Fate Specification and Tissue-specific Cell Cycle Control of the Caenorhabditis elegans Intestine

Coordination between cell fate specification and cell cycle control in multicellular organisms is essential to regulate cell numbers in tissues and organs during development, and its failure may lead to oncogenesis. In mammalian cells, as part of a general cell cycle checkpoint mechanism, the F-box protein β-transducin repeat-containing protein (β-TrCP) and the Skp1/Cul1/F-box complex control the periodic cell cycle fluctuations in abundance of the CDC25A and B phosphatases. Here, we find that the Caenorhabditis elegans β-TrCP orthologue LIN-23 regulates a progressive decline of CDC-25.1 abundance over several embryonic cell cycles and specifies cell number of one tissue, the embryonic intestine. The negative regulation of CDC-25.1 abundance by LIN-23 may be developmentally controlled because CDC-25.1 accumulates over time within the developing germline, where LIN-23 is also present. Concurrent with the destabilization of CDC-25.1, LIN-23 displays a spatially dynamic behavior in the embryo, periodically entering a nuclear compartment where CDC-25.1 is abundant.     

B-Raf and Insulin Synergistically Prevent Apoptosis and Induce Cell Cycle Progression in Hematopoietic Cell

FDC-P1 hematopoietic cells were conditionally transformed to grow in response to ΔB-Raf:ER, ΔRaf-1:ER or ΔA-Raf:ER in which the hormone binding domain of the estrogen receptor (ER) was linked to the N-terminal truncated (Δ) Raf genes. When these cells were deprived of IL-3 or β-estradiol for 24 hrs, they exited the cell cycle and underwent apoptosis. FD/ΔRaf-1:ER and FD/ΔA-Raf:ER, but not FD/ΔB-Raf:ER cells, were readily induced to re-enter the cell cycle after addition of β-estradiol or IL-3. Deprived FD/ΔRaf-1:ER, but not FD/ΔB-Raf:ER cells, expressed activated forms of MEK1 and ERK after β-estradiol or IL-3 stimulation. Insulin or β-estradiol alone did not induce FD/ΔB-Raf:ER cells to re-enter the cell cycle, whereas cell cycle entry was observed upon their co-addition. Apoptosis was prevented in FD/ΔB-Raf:ER cells when they were cultured in the presence of IL-3 or β-estradiol, whereas they underwent apoptosis in their absence. Insulin by itself did not prevent apoptosis, however, upon ΔB-Raf:ER or ΔRaf-1:ER activation and addition of insulin, more than an additive effect was observed in both lines indicating that these pathways synergized to prevent apoptosis. Raf isoforms differ in their abilities to control apoptosis and cell cycle progression and B-Raf requires insulin-activated pathways for full antiapoptotic and proliferative activity.