Caenorhabditis elegans cyclin D/CDK4 and cyclin E/CDK2 induce distinct cell cycle re-entry programs in differentiated muscle cells

SEM-2/SoxC regulates multiple aspects of C. elegans postembryonic mesoderm development

Development of multicellular organisms requires well-orchestrated interplay between cell-intrinsic transcription factors and cell-cell signaling. One set of highly conserved transcription factors that plays diverse roles in development is the SoxC group. C. elegans contains a sole SoxC protein, SEM-2. SEM-2 is essential for embryonic development, and for specifying the sex myoblast (SM) fate in the postembryonic mesoderm, the M lineage. We have identified a novel partial loss-of-function sem-2 allele that has a proline to serine change in the C-terminal tail of the highly conserved DNA-binding domain. Detailed analyses of mutant animals harboring this point mutation uncovered new functions of SEM-2 in the M lineage. First, SEM-2 functions antagonistically with LET-381, the sole C. elegans FoxF/C forkhead transcription factor, to regulate dorsoventral patterning of the M lineage. Second, in addition to specifying the SM fate, SEM-2 is essential for the proliferation and diversification of the SM lineage. Finally, SEM-2 appears to directly regulate the expression of hlh-8, which encodes a basic helix-loop-helix Twist transcription factor and plays critical roles in proper patterning of the M lineage. Our data, along with previous studies, suggest an evolutionarily conserved relationship between SoxC and Twist proteins. Furthermore, our work identified new interactions in the gene regulatory network (GRN) underlying C. elegans postembryonic development and adds to the general understanding of the structure-function relationship of SoxC proteins.

Cell cycle perturbation uncouples mitotic progression and invasive behavior in a post-mitotic cell

The acquisition of the post-mitotic state is crucial for the execution of many terminally differentiated cell behaviors during organismal development. However, the mechanisms that maintain the post-mitotic state in this context remain poorly understood. To gain insight into these mechanisms, we used the genetically and visually accessible model of C. elegans anchor cell (AC) invasion into the vulval epithelium. The AC is a terminally differentiated uterine cell that normally exits the cell cycle and enters a post-mitotic state before initiating contact between the uterus and vulva through a cell invasion event. Here, we set out to identify the set of negative cell cycle regulators that maintain the AC in this post-mitotic, invasive state. Our findings revealed a critical role for CKI-1 (p21CIP1/p27KIP1) in redundantly maintaining the post-mitotic state of the AC, as loss of CKI-1 in combination with other negative cell cycle regulators-including CKI-2 (p21CIP1/p27KIP1), LIN-35 (pRb/p107/p130), FZR-1 (Cdh1/Hct1), and LIN-23 (β-TrCP)-resulted in proliferating ACs. Remarkably, time-lapse imaging revealed that these ACs retain their ability to invade. Upon examination of a node in the gene regulatory network controlling AC invasion, we determined that proliferating, invasive ACs do so by maintaining aspects of pro-invasive gene expression. We therefore report that the requirement for a post-mitotic state for invasive cell behavior can be bypassed following direct cell cycle perturbation.

Cell cycle perturbation uncouples mitotic progression and invasive behavior in a post-mitotic cell

The acquisition of the post-mitotic state is crucial for the execution of many terminally differentiated cell behaviors during organismal development. However, the mechanisms that maintain the post-mitotic state in this context remain poorly understood. To gain insight into these mechanisms, we used the genetically and visually accessible model of C. elegans anchor cell (AC) invasion into the vulval epithelium. The AC is a terminally differentiated uterine cell that normally exits the cell cycle and enters a post-mitotic state, initiating contact between the uterus and vulva through a cell invasion event. Here, we set out to identify the set of negative cell cycle regulators that maintain the AC in this post-mitotic, invasive state. Our findings revealed a critical role for CKI-1 (p21CIP1/p27KIP1) in redundantly maintaining the post-mitotic state of the AC, as loss of CKI-1 in combination with other negative cell cycle regulators-including CKI-2 (p21CIP1/p27KIP1), LIN-35 (pRb/p107/p130), FZR-1 (Cdh1/Hct1), and LIN-23 (β-TrCP)-resulted in proliferating ACs. Remarkably, time-lapse imaging revealed that these ACs retain their ability to invade. Upon examination of a node in the gene regulatory network controlling AC invasion, we determined that proliferating, invasive ACs do so by maintaining aspects of pro-invasive gene expression. We therefore report that the requirement for a post-mitotic state for invasive cell behavior can be bypassed following direct cell cycle perturbation.

What programs the size of animal cells?

The human body is programmed with definite quantities, magnitudes, and proportions. At the microscopic level, such definite sizes manifest in individual cells - different cell types are characterized by distinct cell sizes whereas cells of the same type are highly uniform in size. How do cells in a population maintain uniformity in cell size, and how are changes in target size programmed? A convergence of recent and historical studies suggest - just as a thermostat maintains room temperature - the size of proliferating animal cells is similarly maintained by homeostatic mechanisms. In this review, we first summarize old and new literature on the existence of cell size checkpoints, then discuss additional advances in the study of size homeostasis that involve feedback regulation of cellular growth rate. We further discuss recent progress on the molecules that underlie cell size checkpoints and mechanisms that specify target size setpoints. Lastly, we discuss a less-well explored teleological question: why does cell size matter and what is the functional importance of cell size control?

Initial characterization of gap phase introduction in every cell cycle of C. elegans embryogenesis

Early embryonic cell cycles usually alternate between S and M phases without any gap phase. When the gap phases are developmentally introduced in various cell types remains poorly defined especially during embryogenesis. To establish the cell-specific introduction of gap phases in embryo, we generate multiple fluorescence ubiquitin cell cycle indicators (FUCCI) in C. elegans. Time-lapse 3D imaging followed by lineal expression profiling reveals sharp and differential accumulation of the FUCCI reporters, allowing the systematic demarcation of cell cycle phases throughout embryogenesis. Accumulation of the reporters reliably identifies both G1 and G2 phases only in two embryonic cells with an extended cell cycle length, suggesting that the remaining cells divide either without a G1 phase, or with a brief G1 phase that is too short to be picked up by our reporters. In summary, we provide an initial picture of gap phase introduction in a metazoan embryo. The newly developed FUCCI reporters pave the way for further characterization of developmental control of cell cycle progression.

From cyclins to CDKIs: Cell cycle regulation of skeletal muscle stem cell quiescence and activation

After extensive proliferation during development, the adult skeletal muscle cells remain outside the cell cycle, either as post-mitotic myofibers or as quiescent muscle stem cells (MuSCs). Despite its terminally differentiated state, adult skeletal muscle has a remarkable regeneration potential, driven by MuSCs. Upon injury, MuSC quiescence is reversed to support tissue growth and repair and it is re-established after the completion of muscle regeneration. The distinct cell cycle states and transitions observed in the different myogenic populations are orchestrated by elements of the cell cycle machinery. This consists of i) complexes of cyclins and Cyclin-Dependent Kinases (CDKs) that ensure cell cycle progression and ii) their negative regulators, the Cyclin-Dependent Kinase Inhibitors (CDKIs). In this review we discuss the roles of these factors in developmental and adult myogenesis, with a focus on CDKIs that have emerging roles in stem cell functions.

MiR-24-3p Conservatively Regulates Muscle Cell Proliferation and Apoptosis by Targeting Common Gene CAMK2B in Rat and Cattle

Skeletal muscle plays an important role in the growth and development of meat animals. MicroRNAs (miRNAs) can participate in the regulation of muscle development-related functions; however, there have been few reports on whether there are related miRNAs that conservatively regulate muscle development among different species. In this study, the miRNA transcriptome sequencing data of the muscle tissue of cattle, rat, goat, and pig showed that miR-24-3p may conservatively regulate muscle development in these species. Furthermore, mmu-miR-24-3p can positively regulate C2C12 cell proliferation and apoptosis by regulating key proliferation and apoptosis genes in muscle development, which was verified by CCK-8 and RT-qPCR. Bta-miR-24-3p can also positively regulate the proliferation and apoptosis of bovine muscle primary cells by regulating key proliferation and apoptosis genes in the process of muscle development, as verified by CCK-8 and RT-qPCR. The target genes of miR-24-3p in cattle, rat, goat, and pig, which include a large proportion of target genes shared among the four species, are enriched in multiple cell functions and signal pathways that are closely related to muscle development, as revealed by GO and KEGG enrichment analysis. A double luciferase test showed that the shared target genes WNT4, CAMK2B, and TCF7 were targeted by mmu-miR-24-3p in rat and bta-miR-24-3p in cattle. These three shared target genes WNT4, CAMK2B, and TCF7 are involved in the Wnt signaling pathway, which showed that miR-24-3p plays an important role in rat and cattle. The shared target gene (CAMK2B) in rat and cattle increased significantly after the inhibition of miR-24-3p by RT-qPCR. The findings of this study contribute to a better understanding of the role of miR-24-3p in the regulation of muscle development.

Cell size homeostasis is maintained by CDK4-dependent activation of p38 MAPK

While molecules that promote the growth of animal cells have been identified, it remains unclear how such signals are orchestrated to determine a characteristic target size for different cell types. It is increasingly clear that cell size is determined by size checkpoints-mechanisms that restrict the cell cycle progression of cells that are smaller than their target size. Previously, we described a p38 MAPK-dependent cell size checkpoint mechanism whereby p38 is selectively activated and prevents cell cycle progression in cells that are smaller than a given target size. In this study, we show that the specific target size required for inactivation of p38 and transition through the cell cycle is determined by CDK4 activity. Our data suggest a model whereby p38 and CDK4 cooperate analogously to the function of a thermostat: while p38 senses irregularities in size, CDK4 corresponds to the thermostat dial that sets the target size.

Identification of the critical replication targets of CDK reveals direct regulation of replication initiation factors by the embryo polarity machinery in C. elegans

During metazoan development, the cell cycle is remodelled to coordinate proliferation with differentiation. Developmental cues cause dramatic changes in the number and timing of replication initiation events, but the mechanisms and physiological importance of such changes are poorly understood. Cyclin-dependent kinases (CDKs) are important for regulating S-phase length in many metazoa, and here we show in the nematode Caenorhabditis elegans that an essential function of CDKs during early embryogenesis is to regulate the interactions between three replication initiation factors SLD-3, SLD-2 and MUS-101 (Dpb11/TopBP1). Mutations that bypass the requirement for CDKs to generate interactions between these factors is partly sufficient for viability in the absence of Cyclin E, demonstrating that this is a critical embryonic function of this Cyclin. Both SLD-2 and SLD-3 are asymmetrically localised in the early embryo and the levels of these proteins inversely correlate with S-phase length. We also show that SLD-2 asymmetry is determined by direct interaction with the polarity protein PKC-3. This study explains an essential function of CDKs for replication initiation in a metazoan and provides the first direct molecular mechanism through which polarization of the embryo is coordinated with DNA replication initiation factors.

How and when a cell divides changes as the cell assumes different fates. How these changes in cell division are brought about are poorly understood, but are critical to ensure that cells do not over-proliferate leading to cancer. The nematode C. elegans is an excellent system to study the role of cell cycle changes during animal development. Here we show that two factors SLD-2 and SLD-3 are critical to control the decision to begin genome duplication. We show that these factors are differently distributed to different cell lineages in the early embryo, which may be a key event in determining the cell cycle rate in these cells. For the first time we show that, PKC-3, a key component of the machinery that determines the front (anterior) from the back (posterior) of the embryo directly controls SLD-2 distribution, which might explain how the polarisation of the embryo causes changes in the proliferation of different cell lineages. As PKC-3 is frequently mutated in human cancers, how this factor controls cell proliferation may be important to understand tumour progression.

Effect of cell cycle synchronization on cadmium-induced apoptosis and necrosis in NRK-52E cells

Heavy metal pollution is a problem that cannot be ignored. Due to the prevalence of cadmium in the environment and its harmful effects on humans, cadmium pollution has become a research hotspot recently. The mechanism of cadmium-induced toxicity has also drawn much attention and most studies have been conducted using whole cells, but the toxicological mechanism of cadmium remains unclear. In this study, we aimed to obtain NRK-52E cells at different growth stages by various methods and analyze the differences in cadmium toxicity. The results show that the cadmium sensitivity of cells in each phase was different and the late apoptotic rate was increased significantly after 5 µM Cd treatment. In addition, cadmium easily induces apoptosis of G0- and S-phase cells, as well as necrosis of S- and M-phase cells, but has no significant effect on G1-phase cells. Overall, we first explored the differences in the effects of cadmium on NRK-52E cells at various growth phases. Besides, the findings of this study might provide a theoretical basis for further exploration of the toxicological mechanism of cadmium.Abbreviations Cd: cadmium; CDK: cyclin-dependent kinases; DAPI 2-(4-amidinophenyl)-1H-indole-6-carboxamidine; TBST: Tris-buffered saline with Tween-20; PI: propidium iodide; DMEM: Dulbecco's Modified Eagle Medium; BCA: bicinchoninic acid.

Changes in chromatin accessibility ensure robust cell cycle exit in terminally differentiated cells

During terminal differentiation, most cells exit the cell cycle and enter into a prolonged or permanent G0 in which they are refractory to mitogenic signals. Entry into G0 is usually initiated through the repression of cell cycle gene expression by formation of a transcriptional repressor complex called dimerization partner (DP), retinoblastoma (RB)-like, E2F and MuvB (DREAM). However, when DREAM repressive function is compromised during terminal differentiation, additional unknown mechanisms act to stably repress cycling and ensure robust cell cycle exit. Here, we provide evidence that developmentally programmed, temporal changes in chromatin accessibility at a small subset of critical cell cycle genes act to enforce cell cycle exit during terminal differentiation in the Drosophila melanogaster wing. We show that during terminal differentiation, chromatin closes at a set of pupal wing enhancers for the key rate-limiting cell cycle regulators Cyclin E (cycE), E2F transcription factor 1 (e2f1), and string (stg). This closing coincides with wing cells entering a robust postmitotic state that is strongly refractory to cell cycle reactivation, and the regions that close contain known binding sites for effectors of mitogenic signaling pathways such as Yorkie and Notch. When cell cycle exit is genetically disrupted, chromatin accessibility at cell cycle genes remains unaffected, and the closing of distal enhancers at cycE, e2f1, and stg proceeds independent of the cell cycling status. Instead, disruption of cell cycle exit leads to changes in accessibility and expression of a subset of hormone-induced transcription factors involved in the progression of terminal differentiation. Our results uncover a mechanism that acts as a cell cycle–independent timer to limit the response to mitogenic signaling and aberrant cycling in terminally differentiating tissues. In addition, we provide a new molecular description of the cross talk between cell cycle exit and terminal differentiation during metamorphosis.

The longer a cell remains in G0, the more refractory it becomes to re-entering the cell cycle. This study shows that in terminally differentiated cells in vivo, regulatory elements at genes encoding just three key cell cycle regulators (cycE, e2f1 and stg) become inaccessible, limiting their aberrant activation and maintaining a prolonged, robust G0.

6,7-Dihydroxy-2-(4'-hydroxyphenyl)naphthalene induces HCT116 cell apoptosis through activation of endoplasmic reticulum stress and the extrinsic apoptotic pathway

Background

Colorectal cancer is the third leading cause of cancer-related deaths worldwide, and therefore, the development of novel drugs for its prevention and therapy are urgently required. This study aimed to determine the molecular mechanism of 6,7-dihydroxy-2-(4′-hydroxyphenyl) naphthalene (PNAP-6)-induced cytotoxicity in human colorectal cancer (HCT116) cells.

Methods

The effects of 2-phenylnaphthalene derivatives on HCT116 cell growth and viability were assessed by MTT assays. The mechanisms involved in the regulation of the extrinsic apoptosis and endoplasmic reticulum (ER) stress pathways by PNAP-6 were analyzed by annexin-V/propidium iodide flow cytometric analysis, Hoechst 33342 fluorescent staining, and Western blotting.

Results

PNAP-6 was shown to have an IC₅₀ value 15.20 μM. It induced G₂/M phase arrest in HCT116 cells, associated with a marked decrease in cyclin B and CDK1 protein expression and increased caspase activation, PARP cleavage, chromatin condensation, and sub-G₁ apoptosis. Moreover, we found that the apoptotic effects of PNAP-6 proceeded through extrinsic apoptosis and ER stress pathways, by increasing the expression of Fas protein and ER stress markers, including PERK, ATF4, CHOP, p-IRE1α, and XBP-1s.

Conclusion

These results suggest that 2-phenylnaphthalene derivatives, such as PNAP-6, have potential as new treatments for colorectal cancer.

Developmental Control of the Cell Cycle: Insights from Caenorhabditis elegans

During animal development, a single fertilized egg forms a complete organism with tens to trillions of cells that encompass a large variety of cell types. Cell cycle regulation is therefore at the center of development and needs to be carried out in close coordination with cell differentiation, migration, and death, as well as tissue formation, morphogenesis, and homeostasis. The timing and frequency of cell divisions are controlled by complex combinations of external and cell-intrinsic signals that vary throughout development. Insight into how such controls determine in vivo cell division patterns has come from studies in various genetic model systems. The nematode Caenorhabditis elegans has only about 1000 somatic cells and approximately twice as many germ cells in the adult hermaphrodite. Despite the relatively small number of cells, C. elegans has diverse tissues, including intestine, nerves, striated and smooth muscle, and skin. C. elegans is unique as a model organism for studies of the cell cycle because the somatic cell lineage is invariant. Somatic cells divide at set times during development to produce daughter cells that adopt reproducible developmental fates. Studies in C. elegans have allowed the identification of conserved cell cycle regulators and provided insights into how cell cycle regulation varies between tissues. In this review, we focus on the regulation of the cell cycle in the context of C. elegans development, with reference to other systems, with the goal of better understanding how cell cycle regulation is linked to animal development in general.

The putative mature peptide of piscidin-1 modulates global transcriptional profile and proliferation of splenic lymphocytes in orange-spotted grouper (Epinephelus coioides)

Piscidins are important components in protecting microbial infections in teleost. The present study purified and identified a truncated peptide, whose sequence was very close to that of putative mature peptide of epinecidin-1 (piscidin-1) in orange-spotted grouper (Epinephelus coioides), Epi-1 (also named as short form of ecPis-1, ecPis-1S). The immunomodulatory effects of ecPis-1S on splenic lymphocytes of orange-spotted grouper were explored in vitro. The transcriptome study was carried out by De novo transcriptome sequencing (RNA-Seq) in splenic lymphocytes of orange-spotted grouper. Regarding the profiles of gene expressions, 2994 genes were up-regulated and 2679 genes were down-regulated in the splenic lymphocytes stimulated by ecPis-1S. In the case of differential expression genes, 330 genes were involved in immune related pathways. Among them, 34 genes were involved in T cell receptor signaling pathway, 31 genes in natural killer cell mediated cytotoxicity and 23 genes in leukocyte transendothelial migration, respectively. Immune-related genes selected for qRT-PCR verification, such as interleukin-1β (il-1b), tumor necrosis factor α (tnfa), T cell antigen receptor (tcr), major histocompatibility complex class I (mhc I), and mhc II were significantly up-regulated by ecPis-1S (p < 0.05). ecPis-1S could significantly enhance the proliferation of splenic lymphocytes of orange-spotted grouper in vitro (p < 0.05). In addition, the result of qRT-PCR revealed that ecPis-1S also significantly up-regulated cell cycle-related genes, including cyclin A (cyca), cyclin-dependent kinase 2 (cdk2), cdk4, cell division cycle protein 6 (cdc6), and transforming growth factor β (tgfb) (p < 0.05), which suggested that ecPis-1S promoted the proliferation of lymphocytes by activating cell division cycle. In conclusion, the results indicated that the mature peptide of piscidin-1 in orange-spotted grouper could act as immune modulator and play an important role in regulation of the immune response in fish.

JNK/STAT signalling pathway is involved in fluoride-induced follicular developmental dysplasia in female mice

Excessive fluoride (F) intake decreases the development of potential oocytes by inducing oxidative stress and apoptosis in female mice in our previous study. This study aims to investigate the underlying mechanisms of F-induced follicular developmental dysplasia. Pathomorphological changes in the ovary tissues were observed under light and transmission electron microscopes. DNA damage and proliferation in granulosa cells were analysed by TUNEL staining and BrdU measurement. The protein expression of cell proliferation related regulatory factors including JNK, STAT3, STAT5, CDK2, CDK4, PCNA and Ki67 in the ovary tissues was measured by immunohistochemistry and Western blot analyses. Results indicated that the structure of granulosa cells in the ovary was seriously damaged by excessive F, evident by the swollen endoplasmic reticulum, mitochondria with vacuoles and nucleus shrinkage. F treatment also considerably enhanced the apoptosis and inhibited the proliferation of granulosa cells. The number of granulosa cells around the oocyte decreased after F treatment. The expression levels of STAT3, CDK2, CDK4 and Ki67 in the ovary tissues were up-regulated, and STAT5 and PCNA did not change significantly after F treatment, whereas JNK expression was down-regulated with increasing F dose. In summary, changes in the expression levels of JNK, STAT3, STAT5, CDK2, CDK4, PCNA and Ki67 in the JNK/STAT signalling pathway are involved in F-induced follicular dysplasia in the ovary.

OrthoList 2: A New Comparative Genomic Analysis of Human and Caenorhabditis elegans Genes

OrthoList, a compendium of Caenorhabditis elegans genes with human orthologs compiled in 2011 by a meta-analysis of four orthology-prediction methods, has been a popular tool for identifying conserved genes for research into biological and disease mechanisms. However, the efficacy of orthology prediction depends on the accuracy of gene-model predictions, an ongoing process, and orthology-prediction algorithms have also been updated over time. Here we present OrthoList 2 (OL2), a new comparative genomic analysis between C. elegans and humans, and the first assessment of how changes over time affect the landscape of predicted orthologs between two species. Although we find that updates to the orthology-prediction methods significantly changed the landscape of C. elegans-human orthologs predicted by individual programs and-unexpectedly-reduced agreement among them, we also show that our meta-analysis approach "buffered" against changes in gene content. We show that adding results from more programs did not lead to many additions to the list and discuss reasons to avoid assigning "scores" based on support by individual orthology-prediction programs; the treatment of "legacy" genes no longer predicted by these programs; and the practical difficulties of updating due to encountering deprecated, changed, or retired gene identifiers. In addition, we consider what other criteria may support claims of orthology and alternative approaches to find potential orthologs that elude identification by these programs. Finally, we created a new web-based tool that allows for rapid searches of OL2 by gene identifiers, protein domains [InterPro and SMART (Simple Modular Architecture Research Tool], or human disease associations ([OMIM (Online Mendelian Inheritence in Man], and also includes available RNA-interference resources to facilitate potential translational cross-species studies.

Subunits of the DNA polymerase alpha-primase complex promote Notch-mediated proliferation with discrete and shared functions in C. elegans germline

Notch receptor signaling is a highly conserved cell communication system in most multicellular organisms and plays a critical role at several junctures in animal development. In Caenorhabditis elegans,GLP-1/Notch signaling is essential for both germline stem cell maintenance and germ cell proliferation during gonad development. Here, we show that subunits (POLA-1, DIV-1, PRI-1, and PRI-2) of the DNA polymerase alpha-primase complex are required for germ cell proliferation in response to GLP-1/Notch signaling in different tissues at different developmental stages. Specifically, genetic and functional analyses demonstrated that (a) maternally contributed DIV-1 (regulatory subunit) is indispensable non-cell autonomously for GLP-1/Notch-mediated germ cell proliferation during early larval development, whereas POLA-1 (catalytic subunit) and two primase subunits, PRI-1 and PRI-2, do not appear to be essential; (b) germline POLA-1, PRI-1, and PRI-2 play a crucial role in GLP-1/Notch-mediated maintenance of proliferative cell fate during adulthood, while DIV-1 is dispensable; and (c) germline POLA-1, DIV-1, PRI-1, and PRI-2 function in tandem with PUF (Pumilio/FBF) RNA-binding proteins to maintain germline stem cells in the adult gonad. These findings suggest that the subunits of the DNA polymerase alpha-primase complex exhibit both discrete and shared functions in GLP-1/Notch or PUF-mediated germ cell dynamics in C. elegans. These findings link the biological functions of DNA replication machineries to signals that maintain a stem cell population, and may have further implications for Notch-dependent tumors.

A population of G2-arrested cells are selected as sensory organ precursors for the interommatidial bristles of the Drosophila eye

Cell cycle progression and differentiation are highly coordinated during the development of multicellular organisms. The mechanisms by which these processes are coordinated and how their coordination contributes to normal development are not fully understood. Here, we determine the developmental fate of a population of precursor cells in the developing Drosophila melanogaster retina that arrest in G2 phase of the cell cycle and investigate whether cell cycle phase-specific arrest influences the fate of these cells. We demonstrate that retinal precursor cells that arrest in G2 during larval development are selected as sensory organ precursors (SOPs) during pupal development and undergo two cell divisions to generate the four-cell interommatidial mechanosensory bristles. While G2 arrest is not required for bristle development, preventing G2 arrest results in incorrect bristle positioning in the adult eye. We conclude that G2-arrested cells provide a positional cue during development to ensure proper spacing of bristles in the eye. Our results suggest that the control of cell cycle progression refines cell fate decisions and that the relationship between these two processes is not necessarily deterministic.

Coordinating cell proliferation and differentiation: Antagonism between cell cycle regulators and cell type-specific gene expression

Cell proliferation and differentiation show a remarkable inverse relationship. Precursor cells continue division before acquiring a fully differentiated state, while terminal differentiation usually coincides with proliferation arrest and permanent exit from the division cycle. Mechanistic insight in the temporal coordination between cell cycle exit and differentiation has come from studies of cells in culture and genetic animal models. As initially described for skeletal muscle differentiation, temporal coordination involves mutual antagonism between cyclin-dependent kinases that promote cell cycle entry and transcription factors that induce tissue-specific gene expression. Recent insights highlight the contribution of chromatin-regulating complexes that act in conjunction with the transcription factors and determine their activity. In particular SWI/SNF chromatin remodelers contribute to dual regulation of cell cycle and tissue-specific gene expression during terminal differentiation. We review the concerted regulation of the cell cycle and cell type-specific transcription, and discuss common mutations in human cancer that emphasize the clinical importance of proliferation versus differentiation control.

Structure-Activity Relationship of Synthetic 2-Phenylnaphthalenes with Hydroxyl Groups that Inhibit Proliferation and Induce Apoptosis of MCF-7 Cancer Cells

In this study, six 2-phenylnaphthalenes with hydroxyl groups were synthesized in high yields by the demethylation of the corresponding methoxy-2-phenylnaphthalenes, and one 2-phenylnaphthalene with an amino group was obtained by hydrogenation. All of the 2-phenylnaphthalene derivatives were evaluated for cytotoxicity, and the structure-activity relationship (SAR) against human breast cancer (MCF-7) cells was also determined. The SAR results revealed that cytotoxicity was markedly promoted by the hydroxyl group at the C-7 position of the naphthalene ring. The introduction of hydroxyl groups at the C-6 position of the naphthalene ring and the C-4' position of the phenyl ring fairly enhanced cytotoxicity, but the introduction of a hydroxyl group at the C-3' position of the phenyl ring slightly decreased cytotoxicity. Overall, 6,7-dihydroxy-2-(4'-hydroxyphenyl)naphthalene (PNAP-6h) exhibited the best cytotoxicity, with an IC50 value of 4.8 μM against the MCF-7 cell line, and showed low toxicity toward normal human mammary epithelial cells (MCF-10A). PNAP-6h led to cell arrest at the S phase, most likely due to increasing levels of p21 and p27 and decreasing levels of cyclin D1, CDK4, cyclin E, and CDK2. In addition, PNAP-6h decreased CDK1 and cyclin B1 expression, most likely leading to G2/M arrest, and induced morphological changes, such as nuclear shrinkage, nuclear fragmentation, and nuclear hypercondensation, as observed by Hoechst 33342 staining. PNAP-6h induced apoptosis, most likely by the promotion of Fas expression, increased PARP activity, caspase-7, caspase-8, and caspase-9 expression, the Bax/Bcl-2 ratio, and the phosphorylation of p38, and decreased the phosphorylation of ERK. This study provides the first demonstration of the cytotoxicity of PNAPs against MCF-7 cells and elucidates the mechanism underlying PNAP-induced cytotoxicity.

Early and Late G1/S Cyclins and Cdks Act Complementarily to Enhance Authentic Human β-Cell Proliferation and Expansion

β-Cell regeneration is a key goal of diabetes research. Progression through the cell cycle is associated with retinoblastoma protein (pRb) inactivation via sequential phosphorylation by the "early" cyclins and cyclin-dependent kinases (cdks) (d-cyclins cdk4/6) and the "late" cyclins and cdks (cyclin A/E and cdk1/2). In β-cells, activation of either early or late G1/S cyclins and/or cdks is an efficient approach to induce cycle entry, but it is unknown whether the combined expression of early and late cyclins and cdks might have synergistic or additive effects. Thus, we explored whether a combination of both early and late cyclins and cdks might more effectively drive human β-cell cell cycle entry than either group alone. We also sought to determine whether authentic replication with the expansion of adult human β-cells could be demonstrated. Late cyclins and cdks do not traffic in response to the induction of replication by early cyclins and cdks in human β-cells but are capable of nuclear translocation when overexpressed. Early plus late cyclins and cdks, acting via pRb phosphorylation on distinct residues, complementarily induce greater proliferation in human β-cells than either group alone. Importantly, the combination of early and late cyclins and cdks clearly increased human β-cell numbers in vitro. These findings provide additional insight into human β-cell expansion. They also provide a novel tool for assessing β-cell expansion in vitro.

Scalloped and Yorkie are required for cell cycle re-entry of quiescent cells after tissue damage

Regeneration of damaged tissues typically requires a population of active stem cells. How damaged tissue is regenerated in quiescent tissues lacking a stem cell population is less well understood. We used a genetic screen in the developing Drosophila melanogaster eye to investigate the mechanisms that trigger quiescent cells to re-enter the cell cycle and proliferate in response to tissue damage. We discovered that Hippo signaling regulates compensatory proliferation after extensive cell death in the developing eye. Scalloped and Yorkie, transcriptional effectors of the Hippo pathway, drive Cyclin E expression to induce cell cycle re-entry in cells that normally remain quiescent in the absence of damage. Ajuba, an upstream regulator of Hippo signaling that functions as a sensor of epithelial integrity, is also required for cell cycle re-entry. Thus, in addition to its well-established role in modulating proliferation during periods of tissue growth, Hippo signaling maintains homeostasis by regulating quiescent cell populations affected by tissue damage.

G1/S Inhibitors and the SWI/SNF Complex Control Cell-Cycle Exit during Muscle Differentiation

The transition from proliferating precursor cells to post-mitotic differentiated cells is crucial for development, tissue homeostasis, and tumor suppression. To study cell-cycle exit during differentiation in vivo, we developed a conditional knockout and lineage-tracing system for Caenorhabditis elegans. Combined lineage-specific gene inactivation and genetic screening revealed extensive redundancies between previously identified cell-cycle inhibitors and the SWI/SNF chromatin-remodeling complex. Muscle precursor cells missing either SWI/SNF or G1/S inhibitor function could still arrest cell division, while simultaneous inactivation of these regulators caused continued proliferation and a C. elegans tumor phenotype. Further genetic analyses support that SWI/SNF acts in concert with hlh-1 MyoD, antagonizes Polycomb-mediated transcriptional repression, and suppresses cye-1 Cyclin E transcription to arrest cell division of muscle precursors. Thus, SWI/SNF and G1/S inhibitors provide alternative mechanisms to arrest cell-cycle progression during terminal differentiation, which offers insight into the frequent mutation of SWI/SNF genes in human cancers.

Rb and FZR1/Cdh1 determine CDK4/6-cyclin D requirement in C. elegans and human cancer cells

Cyclin-dependent kinases 4 and 6 (CDK4/6) in complex with D-type cyclins promote cell cycle entry. Most human cancers contain overactive CDK4/6-cyclin D, and CDK4/6-specific inhibitors are promising anti-cancer therapeutics. Here, we investigate the critical functions of CDK4/6-cyclin D kinases, starting from an unbiased screen in the nematode Caenorhabditis elegans. We found that simultaneous mutation of lin-35, a retinoblastoma (Rb)-related gene, and fzr-1, an orthologue to the APC/C co-activator Cdh1, completely eliminates the essential requirement of CDK4/6-cyclin D (CDK-4/CYD-1) in C. elegans. CDK-4/CYD-1 phosphorylates specific residues in the LIN-35 Rb spacer domain and FZR-1 amino terminus, resembling inactivating phosphorylations of the human proteins. In human breast cancer cells, simultaneous knockdown of Rb and FZR1 synergistically bypasses cell division arrest induced by the CDK4/6-specific inhibitor PD-0332991. Our data identify FZR1 as a candidate CDK4/6-cyclin D substrate and point to an APC/C^FZR1 activity as an important determinant in response to CDK4/6-inhibitors.

In most human tumours, the cell cycle regulators Cdk4/6-cyclinD are overactive. Here the authors use C. elegans as a model system to identify downstream regulators that are critical in the response of tumour cells to Cdk4/6 inhibitors.

Oxygen plasma immersion ion implantation treatment enhances the human bone marrow mesenchymal stem cells responses to titanium surface for dental implant application

OBJECTIVES

The present investigation utilized a novel oxygen plasma immersion ion implantation (O-PIII) treatment to create a dense and thin oxide layer on a titanium (Ti) surface for dental implant application.

MATERIALS AND METHODS

This study evaluated the behavior of human bone marrow mesenchymal stem cells (hMSCs) on O-PIII-treated Ti. The O-PIII treatments were performed using different oxygen ion doses (T(L): 1 × 10(16); T(M): 4 × 10(16); T(H): 1 × 10(17) ions/cm(2)).

RESULTS

Analysis using an X-ray photoelectron spectrometer (XPS) and high resolution X-ray diffractometer (HR-XRD) indicated that the O-PIII-treated specimen T(M) had the highest proportion of rutile phase TiO2 component. The O-PIII-treated specimen T(M) had the greatest protein adsorption capability of the test Ti surfaces using XPS analysis and bicinchoninic acid (BCA) protein assay. Immunofluorescent staining revealed that hMSCs had the best cell adhesion on the O-PIII-treated specimen T(M), whereas green fluorescent protein (GFP)-labeled hMSCs experienced the fastest cell migration based on a wound healing assay. Other assays, including MTT assay, Alizarin red S staining and Western blot analysis, demonstrated that the adhered hMSCs exhibited the greatest cell proliferation, mineralization, and differentiation capabilities on the TM specimen.

CONCLUSIONS

Oxidated Ti (primarily rutile TiO2 ) was produced using a facile and rapid O-PIII treatment procedure, which enhances the biocompatibility of the Ti surface with potential implications for further dental implant application.

Signaling pathways that control cell proliferation

Cells decide to proliferate or remain quiescent using signaling pathways that link information about the cellular environment to the G1 phase of the cell cycle. Progression through G1 phase is controlled by pRB proteins, which function to repress the activity of E2F transcription factors in cells exiting mitosis and in quiescent cells. Phosphorylation of pRB proteins by the G1 cyclin-dependent kinases (CDKs) releases E2F factors, promoting the transition to S phase. CDK activity is primarily regulated by the binding of CDK catalytic subunits to cyclin partners and CDK inhibitors. Consequently, both mitogenic and antiproliferative signals exert their effects on cell proliferation through the transcriptional regulation and ubiquitin-dependent degradation of cyclins and CDK inhibitors.

Re-thinking cell cycle regulators: the cross-talk with metabolism

Analysis of genetically engineered mice deficient in cell cycle regulators, including E2F1, cdk4, and pRB, showed that the major phenotypes are metabolic perturbations. These key cell cycle regulators contribute to lipid synthesis, glucose production, insulin secretion, and glycolytic metabolism. It has been shown that deregulation of these pathways can lead to metabolic perturbations and related metabolic diseases, such as obesity and type II diabetes. The cyclin–cdk–Rb–E2F1 pathway regulates adipogenesis in addition to its well-described roles in cell cycle regulation and cancer. It was also shown that E2F1 directly participates in the regulation of pancreatic growth and function. Similarly, cyclin D3, cdk4, and cdk9 are also adipogenic factors with strong effects on whole organism metabolism. These examples support the emerging notion that cell cycle regulatory proteins also modulate metabolic processes. These cell cycle regulators are activated by insulin and glucose, even in non-proliferating cells. Most importantly, these cell cycle regulators trigger the adaptive metabolic switch that normal and cancer cells require in order to proliferate. These changes include increased lipid synthesis, decreased oxidative metabolism, and increased glycolytic metabolism. In summary, these factors are essential regulators of anabolic biosynthetic processes, blocking at the same time oxidative and catabolic pathways, which is reminiscent of cancer cell metabolism.