Smad7 induces G0/G1 cell cycle arrest in mesenchymal cells by inhibiting the expression of G1 cyclins

The major Smad pathways serve in regulating the expression of genes downstream of TGFbeta signals. In this study, we examined the effects of sustained Smad7 expression in cultured cells. Interestingly, Smad7 caused various mesenchymal cells, including NIH3T3 fibroblast and ST2 bone-marrow stromal cells, to undergo a marked morphological alteration into a flattened cell shape, but kept them alive for as long as 60 days. Furthermore, Smad7 arrested the proliferation of the cells even before they reached confluence. These cells became quiescent in G0/G1 phase and accumulated a hypophosphorylated form of retinoblastoma. The cytostatic effect of Smad7 was closely associated with a preceding decrease in the levels of G1 cyclins, such as cyclin D1 and cyclin E. Accordingly, ectopic cyclin E was able to overcome the Smad7-induced arrest of proliferation. These results indicate that Smad7 functions upstream of G1 cyclins and suggest a novel role for Smad7 as an antiproliferative factor. In contrast to the growth of mesenchymal cells, that of epithelial cells was little susceptible to Smad7. The present findings raise the possibility that a link between Smad7 and the G1 to S phase transition may also contribute to the cell cycle control by certain Smad7-inducing stimuli in a cell-type-dependent fashion.     

A mesoscale model of G1/S phase transition in liver regeneration

The liver regenerates and maintains its function and size after injury by counterbalancing cell death with compensatory cell division. During liver regeneration, injured sites release cytokines, which stimulate normally quiescent hepatocytes to re-enter cell division cycle. Using a mesoscale approach, we have implemented the first mathematical model that describes cytokine-induced dedifferentiation of hepatocytes and the subsequent initiation of DNA synthesis (G0/G1 and G1/S phase transitions of the cell cycle). The model accurately reproduces experimentally measured kinetics of various signaling intermediates and DNA synthesis in hepatocytes for varying degrees of liver damage, in both wild type and knockout backgrounds. Liver regeneration is known to be a robust process, as liver mass reconstitution still occurs in various knockout mice (albeit with different kinetics). We analyze the robustness of the model using methods of control analysis. Moreover, we discuss the system's bandpass filtering properties and delays, which arise from feedbacks and nested feed-forward loops.     

Differential expression of D-type G1 cyclins during mouse development and liver regeneration in vivo

D-type Gl cyclins are the primary cell cycle regulators of G1/S transition in eukaryotic cells, and are differentially expressed in a variety of cell lines in vitro. Little is known, however, about the expression patterns of D-type G1 cyclins in normal mouse in vivo. Thus, in the present study, tissue-specific expressions of cyclin D1 and D3 genes were examined in several tissues derived from adult male mice, and stage-specific expression of cyclin genes was studied in brain, liver, and kidney of developing mice from embryonic day 13 to postnatal day 11. Cell cycle-dependent expression of cyclins was also examined in regenerating livers following partial hepatectomy. Our results indicate that (l) cyclins Dl and D3 are expressed in a tissue-specific manner, with cyclin Dl being highly expressed in kidney and D3 in thymus; (2) cyclin D3 mRNA is abundantly expressed in young proliferating tissues and is gradually reduced during development, whereas cyclin Dl mRNA fluctuates during development; and (3) compensatory regeneration of liver induces cyclin Dl gene expression 12 hr after partial hepatectomy, and cyclin D3 gene expression from 36 to 42 hr (at the time of G1/S transition). In conclusion, this study indicates that cyclin D1 and D3 genes are differentially expressed in vivo in a tissue-specific, developmental stage-dependent, and cell cycle-dependent manner. © 1996 Wiley-Liss, Inc.     

Hydroxyurea inhibits ODC induction, but not the G1 to S phase transition.

Chinese hamster ovary cells, selected in mitosis and plated into medium containing hydroxyurea, can progress through G₁ and enter S phase although bulk DNA synthesis is prevented. As the cells progress through G₁ in the presence of hydroxyurea, ornithine decarboxylase activity remains low while general protein synthesis appears unaffected. After hydroxyurea is removed, ornithine decarboxylase activity increases, but only after approximately 20% of the DNA has been replicated. These results suggest that ornithine decarboxylase induction is not essential for cellular progression into S phase but is required for the completion of DNA synthesis.     

Inhibition of G1 phase cyclin dependent kinases by transforming growth factor β1

Transforming growth factor β1 (TGFβ1) inhibits epithelial cell proliferation late in the G1 phase of the cell cycle. We examined the effect of TGFβ1 on known late G1 cell cycle regulators in an attempt to determine the molecular mechanism of growth inhibition by this physiological inhibitor. The results demonstrate the TGFβ1 inhibits the late G1 and S phase specific histone H1 kinase activity of p33^cdk2. This inhibitiion is not dur to TGFβ1's effect on p33^cdk2 synthesis, but rather due to its negative effect on the late G1 phosphorylation of p33^cdk2. It is also shown that TGFβ1 inhibits both late G1 cyclin A and cyclin E associated histon H1 kinase activities. The inhibitor has no effects on the synthesis of cyclin E but to inhibit the synthesis of cyclin A protein in a cell cycle dependent manner. If TGFβ1 is added to cells which have progressed futher than 8 hours into G1, then it is without inhibitory effect on cyclin A synthesis. These effect on TGFβ1 on late G1 cell cycle regulators correlate well with its inhibitory effects on cellular growth and suggest that these G1 cyclin dependent kinases might serve as targets for TGFβ1-mediated growth arrest.     

Overexpression of DOC-1R Inhibits Cell Cycle G1/S Transition by Repressing CDK2 Expression and Activation

DOC-1R (deleted in oral cancer-1 related) is a novel putative tumor suppressor. This study investigated DOC-1R antitumor activity and the underlying molecular mechanisms. Cell phenotypes were assessed using flow cytometry, BrdU incorporation and CDK2 kinase assays in DOC-1R overexpressing HeLa cells. In addition, RT-PCR and Western blot assays were used to detect underlying molecular changes in these cells. The interaction between DOC-1R and CDK2 proteins was assayed by GST pull-down and immunoprecipitation-Western blot assays. The data showed that DOC-1R overexpression inhibited G1/S phase transition, DNA replication and suppressed CDK2 activity. Molecularly, DOC-1R inhibited CDK2 expression at the mRNA and protein levels, and there were decreased levels of G1-phase cyclins (cyclin D1 and E) and elevated levels of p21, p27, and p53 proteins. Meanwhile, DOC-1R associated with CDK2 and inhibited CDK2 activation by obstructing its association with cyclin E and A. In conclusion, the antitumor effects of DOC-1R may be mediated by negatively regulating G1 phase progression and G1/S transition through inhibiting CDK2 expression and activation.     

The G1 phase optical reporter serves as a sensor of CDK4/6 inhibition in vivo

Visualization of cell-cycle G1 phase for monitoring the early response of cell cycle specific drug remains challenging. In this study, we developed genetically engineered bioluminescent reporters by fusing full-length cyclin E to the C-terminal luciferase (named as CycE-Luc and CycE-Luc2). Next, HeLa cell line or an ER-positive breast cancer cell line MCF-7 was transfected with these reporters. In cellular assays, the bioluminescent signal of CycE-Luc and CycE-Luc2 was accumulated in the G1 phase and decreased after exiting from the G1 phase. The expression of CycE-Luc and CycE-Luc2 fusion protein was regulated in a cell cycle-dependent manner, which was mediated by proteasome ubiquitination and degradation. Next, our in vitro and in vivo experiment confirmed that the cell cycle arrested by anti-cancer agents (palbociclib or 5-FU) was monitored quantitatively and dynamically by bioluminescent imaging of these reporters in a real-time and non-invasive manner. Thus, these optical reporters could reflect the G1 phase alternation of cell cycle, and might become a future clinically translatable approach for predicting and monitoring response to palbociclib in patients with ER-positive breast cancer.     

Alteration of G1/S transition regulators influences recurrences in head and neck squamous carcinomas

Head-Neck Squamous Cell Carcinoma (HN-SCC) is a clinically challenging disease associated with a high mortality rate. The chemo-radiotherapy treatments that aim to preserve the organ represent the current gold standard therapy for advanced laryngeal disease, reserving surgery only for non-responsive or relapsed cases. Despite these aggressive approaches, local persistent or recurrent disease remains the primary cause of treatment failure but we still do not have known factors and/or markers able to predict the outcome of the disease and in particular the risk of local relapse. Here we address this point on a series of 54 cases of HN-SCC for whom the presence of local relapse was known. Using immunohistochemistry (IHC) analysis to evaluate protein expression and localization in the recurrence free and recurrence positive samples, we studied the expression of key cell cycle regulators including p53, p16, p27, pRB, Cyclin D1, Cyclin D3, and Stathmin. Overall by analyzing seven different cell cycle regulators we can hypothesize that the alteration of G1/S regulation represents a fundamental event in the onset/progression of HN-SCC cancers and that the associate use of Cyclin D1/p16 expression should be considered as a possible biomarker toward the identification of those patients that will probably develop a recurrent disease and thus should benefit of a more aggressive treatment.     

ARTD1 regulates cyclin E expression and consequently cell-cycle re-entry and G1/S progression in T24 bladder carcinoma cells

ADP-ribosylation is involved in a variety of biological processes, many of which are chromatin-dependent and linked to important functions during the cell cycle. However, any study on ADP-ribosylation and the cell cycle faces the problem that synchronization with chemical agents or by serum starvation and subsequent growth factor addition already activates ADP-ribosylation by itself. Here, we investigated the functional contribution of ARTD1 in cell cycle re-entry and G₁/S cell cycle progression using T24 urinary bladder carcinoma cells, which synchronously re-enter the cell cycle after splitting without any additional stimuli. In synchronized cells, ARTD1 knockdown, but not inhibition of its enzymatic activity, caused specific down-regulation of cyclin E during cell cycle re-entry and G₁/S progression through alterations of the chromatin composition and histone acetylation, but not of other E2F-1 target genes. Although Cdk2 formed a functional complex with the residual cyclin E, p27^{Kip1 Murray AH, Hunt T. The cell cycle: an introduction. New York: Oxford University Press, 1993. [Google Scholar]} protein levels increased in G₁ upon ARTD1 knockdown most likely due to inappropriate cyclin E-Cdk2-induced phosphorylation-dependent degradation, leading to decelerated G₁/S progression. These results provide evidence that ARTD1 regulates cell cycle re-entry and G₁/S progression via cyclin E expression and p27^{Kip1 Murray AH, Hunt T. The cell cycle: an introduction. New York: Oxford University Press, 1993. [Google Scholar]} stability independently of its enzymatic activity, uncovering a novel cell cycle regulatory mechanism.     

Regulation of the G1 leads to S phase transition in chick embryo fibroblasts with alpha-keto acids and L-alanine.

Temporal inhibition of protein synthesis with cycloheximide prevents subsequent insulin, but not serum-stimulated DNA synthesis in G1-arrested chick embryo fibroblasts (CEF). The inhibition is measured by the incorporation of 3H-thymidine into acid insoluble material and confirmed by chemical estimate of the DNA content of inhibited and uninhibited cells. Cycloheximide treatment is without effect if the cell cultures are maintained at 4 degrees C while exposed to the drug. Several alpha-keto acids (pyruvate, oxaloacetate, alpha-ketobutyrate) at 0.5-1 mM concentrations restore DNA synthesis in previously inhibited cells when combined with insulin. L-alanine (D-alanine is inert) is even more effective than the keto acids in stimulating DNA synthesis after cycloheximide treatment. Glucose transport was unaffected by cycloheximide treatment while lactate levels in medium from inhibited, insulin-stimulated CEF were reduced 70% compared to uninhibited counterparts. We speculate that cycloheximide treatment may lead to the decay of a glycolytic enzyme which compromises the ability of inhibited cells to synthesize pyruvate from glucose, and thus induces an exogenous requirement for alpha-keto acid or L-alanine. A serum component(s) with a molecular weight of about 100 permitted insulin-stimulated DNA synthesis in inhibited cells.     

Developmental changes in cyclin B1 and cyclin-dependent kinase 1 (CDK1) levels in the different populations of spermatogenic cells of the post-natal rat testis

Spermatogenesis is a highly ordered process which requires mitotic and meiotic divisions. In this work, we studied the relative changes in the levels of the two components of the M-phase promoting factor (MPF): the regulatory subunit cyclin B1 (CycB1) and its catalytic subunit cdk1, in spermatogenic cells of rats between 16 and 90 days of life. A multivariate flow cytometry analysis of forward scatter (FSC), side scatter (SSC) and DNA content was used to identify six populations of rat germ cells: spermatogonia with preleptotene spermatocytes, young pachytene spermatocytes, middle to late pachytene spermatocytes, secondary spermatocytes with doublets of round spermatids, round spermatids, and elongated spermatids. For any population studied no significant difference in the relative cellular content of CycB1 or cdk1 proteins between animals of different ages was observed. By contrast, CycB1 and cdk1 levels were different between the different populations of germ cells. CycB1 and cdk1 were rather high in young pachytene spermatocytes and culminated in late spermatocytes, i.e. just before the first meiotic division. The relative levels of the two proteins remained high in secondary spermatocytes then decreased in round spermatids at the exit of meiosis. Similar results were obtained by Western-blot analysis of total proteins obtained from lysates of elutriated fractions of spermatocytes and spermatids. MPF activity was assessed in lysates of germ cells from 32-day-old rats or adult animals using p13suc1 agarose and histone H1 as an exogenous substrate. H1 kinase activity was higher in pachytene spermatocytes than in round spermatid fractions from both adult and young rats. These results indicate that the meiotic G2/M transition is associated to high levels of CycB1 and cdk1 leading to high MPF activity irrespective of the age of the animals.     

Epigenetic regulation of Thy‐1 gene expression by histone modification is involved in lipopolysaccharide‐induced lung fibroblast proliferation

Lipopolysaccharide (LPS)‐induced pulmonary fibrosis is characterized by aberrant proliferation and activation of lung fibroblasts. Epigenetic regulation of thymocyte differentiation antigen 1 (Thy‐1) is associated with lung fibroblast phenotype transformation that results in aberrant cell proliferation. However, it is not clear whether the epigenetic regulation of Thy‐1 expression is required for LPS‐induced lung fibroblast proliferation. To address this issue and better understand the relative underlying mechanisms, we used mouse lung fibroblasts as model to observe the changes of Thy‐1 expression and histone deacetylation after LPS challenge. The results showed that cellular DNA synthesis, measured by BrdU incorporation, was impacted less in the early stage (24 hrs) after the challenge of LPS, but significantly increased at 48 or 72 hrs after the challenge of LPS. Meanwhile, Thy‐1 expression, which was detected by real‐time PCR and Western blot, in lung fibroblasts decreased with increased time after LPS challenge and diminished at 72 hrs. We also found that the acetylation of either histone H3 or H4 decreased in the LPS‐challenged lung fibroblasts. ChIP assay revealed that the acetylation of histone H4 (Ace‐H4) decreased in the Thy‐1 promoter region in response to LPS. In addition, all the above changes could be attenuated by depletion of TLR4 gene. Our studies indicate that epigenetic regulation of Thy‐1 gene expression by histone modification is involved in LPS‐induced lung fibroblast proliferation.     

蛋白激酶C-细胞外信号调节激酶1/2通路介导精氨酸升压素对成年大鼠心肌成纤维细胞的促增殖作用

精氨酸升压素(arginine vasopressin, AVP)是高血压和心力衰竭时激活的神经体液和血流动力学因子,同时,它还具有直接的生长刺激作用.我们以往的研究显示AVP可诱导新生大鼠心肌成纤维细胞(cardiac fibroblasts, CFs)增殖.本研究旨在进一步观察AVP是否对成年大鼠CFs具有促增殖作用,并探计其机制.采用组织块法培养成年大鼠CFs,用[3H]-TdR掺入法和流式细胞仪方法观察AVP作用下CFs的DNA合成和细胞周期分布.根据特异性底物髓磷脂基质蛋白(myelin basic protein, MBP)的磷酸化水平测定细胞外信号调节激酶1/2 (extracellular signal-regulated kinase 1/2, ERK1/2)的活性.用Western blot检测ERK1/2的磷酸化和p27Kip1、细胞周期蛋白D1、 A、 E的表达.结果显示,AVP(0.1μmol/L)可促进成年大鼠CFs的DNA合成,该作用可被V1受体拮抗剂d(CH2)5[Tyr2(Me),Arg8]-vasopressin (0.1μmol/L)阻断,而不受V2受体拮抗剂desglycinamide [d(CH2)5, D-Ile2, Ile4, Arg8]-vasopressin (0.1μmol/L)的影响.AVP可激活ERK1/2,用蛋白激酶C(protein kinase C, PKC)激动剂佛波酯(phorbol 12-myristate 13-acetate, PMA, 30nmol/L, 5min)急性刺激可模拟该作用,而PMA持续慢性作用(2.5μmol/L,24h)耗竭PKC后则抑制AVP对ERK1/2的激活.AVP可抑制p27Kip1的蛋白表达,升高细胞周期蛋白D1、 A和E的表达,同时促进细胞周期由G0/G1期进入S期.ERK1/2抑制剂PD98059 (30μmol/L)阻断AVP对DNA合成、p27Kip1、细胞周期蛋白D1、A和E蛋白表达的作用,并抑制细胞周期进程.以上结果表明,AVP可促进成年大鼠CFs增殖,该作用由V1受体和PKC-ERK1/2通路介导.AVP可通过ERK1/2调控p27Kip1、细胞周期蛋白D1、A和E的表达,从而促进成年大鼠CFs的细胞周期进程.     

Synthesis of protein and mRNA is necessary for transition of suspension-cultured Catharanthus roseus cells from the G1 to the S phase of the cell cycle

The effects of inhibition of the synthesis of protein, mRNA or rRNA on the progression of the cell cycle have been analyzed in cultures of Catharanthus roseus in which cells were induced to divide in synchrony by the double phosphate starvation method. The partial inhibition of protein synthesis at the G₁ phase by anisoniycio or cycloheximide caused the arrest of cells in the G₁ phase or delayed the entry of cells into the S phase. When protein synthesis was partially inhibited at the S phase, cell division occurred to about the same extent as in the control. When asynchronously dividing cells were treated with cycloheximide, cells accumulated in the G₁ phase, as shown by flow-cytometric analysis. The partial inhibition of mRNA synthesis by α-amanitin at the G₁ phase caused the arrest of cells in the G₁ phase, although partial inhibition of mRNA synthesis at the S phase had little effect on cell division. In the case of inhibition of synthesis of rRNA by actinomycin D at the G₁ phase, initiation of DNA synthesis was observed, but no subsequent DNA synthesis or the division of cells occurred. However, the addition of actinomycin D during the S phase had no effect on cell division. These results suggest that specific protein(s), required for the progression of the cell cycle, are synthesized in the G₁ phase, and that the mRNA(s) that encode these proteins are also synthesized at the G₁ phase.     

The miR-17-5p microRNA is a key regulator of the G1/S phase cell cycle transition

Background

MicroRNAs are modifiers of gene expression, acting to reduce translation through either translational repression or mRNA cleavage. Recently, it has been shown that some microRNAs can act to promote or suppress cell transformation, with miR-17-92 described as the first oncogenic microRNA. The association of miR-17-92 encoded microRNAs with a surprisingly broad range of cancers not only underlines the clinical significance of this locus, but also suggests that miR-17-92 may regulate fundamental biological processes, and for these reasons miR-17-92 has been considered as a therapeutic target.

Results

In this study, we show that miR-17-92 is a cell cycle regulated locus, and ectopic expression of a single microRNA (miR-17-5p) is sufficient to drive a proliferative signal in HEK293T cells. For the first time, we reveal the mechanism behind this response - miR-17-5p acts specifically at the G1/S-phase cell cycle boundary, by targeting more than 20 genes involved in the transition between these phases. While both pro- and anti-proliferative genes are targeted by miR-17-5p, pro-proliferative mRNAs are specifically up-regulated by secondary and/or tertiary effects in HEK293T cells.

Conclusion

The miR-17-5p microRNA is able to act as both an oncogene and a tumor suppressor in different cellular contexts; our model of competing positive and negative signals can explain both of these activities. The coordinated suppression of proliferation-inhibitors allows miR-17-5p to efficiently de-couple negative regulators of the MAPK (mitogen activated protein kinase) signaling cascade, promoting growth in HEK293T cells. Additionally, we have demonstrated the utility of a systems biology approach as a unique and rapid approach to uncover microRNA function.