Cell cycle controls in higher eukaryotic cells: Resting state or a prolonged G1 period?

We express the viewpoint that control over cell growth in higher eukaryotes is achieved predominantly by regular transition of cells from proliferation to rest and vice versa as a result of coordinated interrelationship between intracellular growth inhibitors and extracellular growth factors. The resting state is considered as a special physiological state of a cell where the prereplicative reactions necessary for the onset of DNA synthesis are inhibited. Cells pass into a resting state at each successive cell cycle, with regard to the next cycle, once the threshold level of growth inhibitors has been attained. Cellular rest may thus initiate and proceed in parallel with conventional periods of the cell cycle but in a hidden way. Its termination strictly depends on the appropriate concentration of extracellular growth factors. In the absence of growth factors cells, after completing mitosis, pass into an overt state of rest metabolically different from any period of the cell cycle including G1.     

Growth control of human foreskin fibroblasts and inhibition of extracellular sialidase activity by 2-deoxy-2,3-dehydro-N-acetylneuraminic acid

Normal human fibroblasts have two extreme modes of existence in culture, quiescent and proliferative. The growth and division of these cells are usually well regulated by the action of various endogenously generated stimulators and inhibitors. We have speculated that an extracellular sialidase may be involved in the regulation of growth and that inhibition of this activity might decrease or abolish cell growth. To test this hypothesis, we have incubated preconfluent cultures of fibroblasts in the presence and absence of a potent sialidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid. Treatment of cells with this inhibitor resulted in the inhibition of an extracellular sialidase activity for up to 24 h and had a marked growth inhibitory effect in a concentration-dependent manner. The effect of the inhibitor on cell proliferation was specific and reversible. During a chase period of 48 h after pulse labeling cells with [3H]N-acetylmannosamine and [14C]serine, there was a 15% decrease of [3H]sialic acid in the membrane-bound GM3 with 80 microM inhibitor in the medium, as compared with a 32% decrease in the controls. Our results suggest that an extracellular sialidase may participate in cell-surface modifications that accompany (or control) the changes observed when cells traverse the cell cycle, from the quiescent to the proliferative phase.     

The topoisomerase inhibitors camptothecin and etoposide induce a CD95-independent apoptosis of activated peripheral lymphocytes

The effect of etoposide and camptothecin, two topoisomerase inhibitors directed against topoisomerases II and I, respectively, was evaluated on human peripheral blood lymphocytes. Etoposide and camptothecin induced apoptosis of mitogen-activated but not resting CD4+ and CD8+ T lymphocytes. Cell sensitivity to these agents required G1 to S-phase transition of the cell cycle. Conversely, daunorubicin, an intercalating agent and topoisomerase II inhibitor, induced apoptosis of both resting and activated lymphocytes. Although etoposide and camptothecin induced CD95-ligand mRNA expression, drug-induced apoptosis of activated human lymphocytes was not inhibited by CD95 antagonists. Drug-induced cell death was also not inhibited by p55 TNFR-Ig fusion protein. Activation of the caspases cascade was suggested by the partial inhibitory effect of the tripeptide zVAD-fmk and documented by activation of caspase 3. Finally etoposide and camptothecin induced a rapid production of ceramide in activated but not resting peripheral blood lymphocytes, suggesting that ceramide might initiate the signaling apoptotic cascade in sensitive cells.     

Involvement of mitochondria in changes in fluorescein excitation and emission polarization spectra in living cells.

The comparison of fluorescein polarization spectra in living cells and in isolated subcellular structures identified the mitochondria as the cytoplasmic domain in which on excitation at 470 nm the sharp fluorescein emission polarization peak at 510 nm is formed. Changes in the emission polarization peak during the cell cycle or those induced by growth stimulators and inhibitors reflect structural changes in the mitochondria on their transition from the resting, orthodox into the active, ATP-generating, condensed conformation and vice versa. Possible mechanisms for the formation of the sharp emission polarization peak are discussed.     

Intracellular chloride regulates cell proliferation through the activation of stress‐activated protein kinases in MKN28 human gastric cancer cells

Recently, we reported that reduction of intracellular Cl^? concentration ([Cl^?]_i) inhibited proliferation of MKN28 gastric cancer cells by diminishing the transition rate from G₁ to S cell‐cycle phase through upregulation of p21, cyclin‐dependent kinase inhibitor, in a p53‐independent manner. However, it is still unknown how intracellular Cl^? regulates p21 expression level. In this study, we demonstrate that mitogen‐activated protein kinases (MAPKs) are involved in the p21 upregulation and cell‐cycle arrest induced by reduction of [Cl^?]_i. Culture of MKN28 cells in a low Cl^? medium significantly induced phosphorylation (activation) of MAPKs (ERK, p38, and JNK) and G₁/S cell‐cycle arrest. To clarify the involvement of MAPKs in p21 upregulation and cell growth inhibition in the low Cl^? medium, we studied effects of specific MAPKs inhibitors on p21 upregulation and G₁/S cell‐cycle arrest in MKN28 cells. Treatment with an inhibitor of p38 or JNK significantly suppressed p21 upregulation caused by culture in a low Cl^? medium and rescued MKN28 cells from the low Cl^?‐induced G₁ cell‐cycle arrest, whereas treatment with an ERK inhibitor had no significant effect on p21 expression or the growth of MKN28 cells in the low Cl^? medium. These results strongly suggest that the intracellular Cl^? affects the cell proliferation via activation of p38 and/or JNK cascades through upregulation of the cyclin‐dependent kinase inhibitor (p21) in a p53‐independent manner in MKN28 cells. J. Cell. Physiol. 223:764–770, 2010. © 2010 Wiley‐Liss, Inc.     

Involvement of the ERK signaling cascade in protein kinase C-mediated cell cycle arrest in intestinal epithelial cells

We have reported previously that protein kinase C (PKC) signaling can mediate a program of cell cycle withdrawal in IEC-18 nontransformed intestinal crypt cells, involving rapid disappearance of cyclin D1, increased expression of Cip/Kip cyclin-dependent kinase inhibitors, and activation of the growth suppressor function of pocket proteins. In the current study, we present evidence to support a requisite role for PKC alpha in mediating these effects. Furthermore, analysis of the signaling events linking PKC/PKC alpha activation to changes in the cell cycle regulatory machinery implicate the Ras/Raf/MEK/ERK cascade. PKC/PKC alpha activity promoted GTP loading of Ras, activation of Raf-1, and phosphorylation/activation of ERK. ERK activation was found to be required for critical downstream effects of PKC/PKC alpha activation, including cyclin D1 down-regulation, p21(Waf1/Cip1) induction, and cell cycle arrest. PKC-induced ERK activation was strong and sustained relative to that produced by proliferative signals, and the growth inhibitory effects of PKC agonists were dominant over proliferative events when these opposing stimuli were administered simultaneously. PKC signaling promoted cytoplasmic and nuclear accumulation of ERK activity, whereas growth factor-induced phospho-ERK was localized only in the cytoplasm. Comparison of the effects of PKC agonists that differ in their ability to sustain PKC alpha activation and growth arrest in IEC-18 cells, together with the use of selective kinase inhibitors, indicated that the length of PKC-mediated cell cycle exit is dictated by the magnitude/duration of input signal (i.e. PKC alpha activity) and of activation of the ERK cascade. The extent/duration of phospho-ERK nuclear localization may also be important determinants of the duration of PKC agonist-induced growth arrest in this system. Taken together, the data point to PKC alpha and the Ras/Raf/MEK/ERK cascade as key regulators of cell cycle withdrawal in intestinal epithelial cells.     

Rapamycin decelerates cellular senescence

When the cell cycle is arrested but cellular growth is not, then cells senesce, permanently losing proliferative potential. Here we demonstrated that the duration of cell cycle arrest determines a progressive loss of proliferative capacity. In human and rodent cell lines, rapamycin (an inhibitor of mTOR) dramatically decelerated loss of proliferative potential caused by ectopic p21, p16 and sodium butyrate-induced p21. Thus, when the cell cycle was arrested by these factors in the presence of rapamycin, cells retained the capacity to resume proliferation, once p21, p16 or sodium butyrate were removed. While rapamycin prevented the permanent loss of proliferative potential in arrested cells, it did not force the arrested cells into proliferation. During cell cycle arrest, rapamycin transformed the irreversible arrest into a reversible condition. Our data demonstrate that senescence can be pharmacologically suppressed.     

The Ras/Raf/MEK/extracellular signal-regulated kinase pathway induces autocrine-paracrine growth inhibition via the leukemia inhibitory factor/JAK/STAT pathway

Sustained activation of the Ras/Raf/MEK/extracellular signal-regulated kinase (ERK) pathway can lead to cell cycle arrest in many cell types. We have found, with human medullary thyroid cancer (MTC) cells, that activated Ras or c-Raf-1 can induce growth arrest by producing and secreting an autocrine-paracrine factor. This protein was purified from cell culture medium conditioned by Raf-activated MTC cells and was identified by mass spectrometry as leukemia inhibitory factor (LIF). LIF expression upon Raf activation and subsequent activation of JAK-STAT3 was also observed in small cell lung carcinoma cells, suggesting that this autocrine-paracrine signaling may be a common response to Ras/Raf activation. LIF was sufficient to induce growth arrest and differentiation of MTC cells. This effect was mediated through the gp130/JAK/STAT3 pathway, since anti-gp130 blocking antibody or dominant-negative STAT3 blocked the effects of LIF. Thus, LIF expression provides a novel mechanism allowing Ras/Raf signaling to activate the JAK-STAT3 pathway. In addition to this cell-extrinsic growth inhibitory pathway, we find that the Ras/Raf/MEK/ERK pathway induces an intracellular growth inhibitory signal, independent of the LIF/JAK/STAT3 pathway. Therefore, activation of the Ras/Raf/MEK/ERK pathway can lead to growth arrest and differentiation via at least two different signaling pathways. This use of multiple pathways may be important for "fail-safe" induction and maintenance of cell cycle arrest.     

Synergistic action by multi-targeting compounds produces a potent compound combination for human NSCLC both in vitro and in vivo

By screening a collection of one hundred combinations of thiazolidinone compounds, we identified one combination (M4) that synergistically inhibited the growth of H460 and H460/TaxR cells and tumor growth in H460/TaxR xenograft mice. A whole genome microarray assay showed that genes involved in negative regulation of microtubule polymerization or depolymerization, intracellular protein kinase cascade, positive regulation of histone acetylation, cell cycle arrest and apoptosis were upregulated. Further analysis proved that the four compounds act as either microtubule polymerization inhibitors or histone deacetylase inhibitors. They act synergistically targeting multiple proteins and leading to the regulation of cell cycle checkpoint proteins, including p53, p21, cdc25C and cdc2, the activation of caspases, JNK, p38 cascades and the inactivation of Akt. These events resulted in the G2/M cell cycle arrest and cell apoptosis. These data provide a new strategy for discovering anticancer drugs and drug combinations for drug-resistant cancers.     

p21(cip1) mRNA is controlled by endogenous transforming growth factor-beta1 in quiescent human hematopoietic stem/progenitor cells

Transforming growth factor-beta1 (TGF-beta1) has been described as an efficient growth inhibitor that maintains the CD34(+) hematopoietic progenitor cells in quiescence. The concept of high proliferative potential-quiescent cells or HPP-Q cells has been introduced as a working model to study the effect of TGF-beta1 in maintaining the reversible quiescence of the more primitive hematopoietic stem cell compartment. HPP-Q cells are primitive quiescent stem/progenitor cells on which TGF-beta1 has downmodulated the cytokine receptors. These cells can be released from quiescence by neutralization of autocrine or endogenous TGF-beta1 with a TGF-beta1 blocking antibody or a TGF-beta1 antisense oligonucleotide. In nonhematopoietic systems, TGF-beta1 cooperates with the cyclin-dependent kinase inhibitor, p21(cip1), to induce cell cycle arrest. We therefore analyzed whether endogenous TGF-beta1 controls the expression of the p21(cip1) in the CD34(+) undifferentiated cells using a sensitive in situ hybridization method. We observed that addition of anti-TGF-beta1 is followed by a rapid decrease in the level of p21(cip1) mRNA whereas TGF-beta1 enhances p21(cip1) mRNA expression concurrently with an inhibitory effect on progenitor cell proliferation. These results suggest the involvement of p21(cip1) in the cell cycle control of early human hematopoietic quiescent stem/progenitors and not only in the differentiation of more mature myeloid cells as previously described. The modulation of p21(cip1) observed in response to TGF-beta1 allows us to further precise the working model of high proliferative potential-quiescent cells.     

Inhibition of p21-mediated ROS accumulation can rescue p21-induced senescence

The cyclin-dependent kinase (CDK) inhibitor p21(Waf1/Cip1/Sdi1) was identified initially as a gene induced in senescent cells and itself has been shown to cause permanent growth arrest/senescence. Reactive oxygen species (ROS), a byproduct of oxidative processes, can also induce an irreversible growth arrest similar to senescence. Here we show that p21 increased intracellular levels of ROS both in normal fibroblasts and in p53-negative cancer cells. N-acetyl-L-cysteine, an ROS inhibitor, rescued p21-induced senescence, showing that ROS elevation is necessary for induction of the permanent growth arrest phenotype. p16(Ink4a), a CDK4- and CDK6-specific inhibitor, failed to increase ROS levels, and cell cycle arrest induced by p16 was reversible following its down-regulation, demonstrating the specificity of this p21 effect. A p21 mutant that lacked the ability to bind proliferating cell nuclear antigen (PCNA) retained the ability to induce both ROS and permanent growth arrest. All of these findings establish that p21 mediates senescence by a mechanism involving ROS accumulation which does not require either its PCNA binding or the CDK inhibitory functions shared with p16.     

Lack of a correlation between polyamine synthesis and DNA synthesis by cultured rat liver cells and fibroblasts

Growth stimulation of either fetal rat liver cells or rat embryo fibroblasts in culture results in considerable increases in intracellular polyamine levels as cells proceed through the cell cycle. Treatment of such cell cultures with appropriate levels of two inhibitors of polyamine synthesis, namely α-hydrazino ornithine and methylglyoxal bis(guanylhydrazone), can essentially completely block these increases in cellular polyamine content. Under such conditions, where the elevation in intracellular polyamine content is prevented, cell cultures are nevertheless able to initiate DNA synthesis and subsequently synthesize DNA at rates comparable to untreated control cultures that have been growth-stimulated. These two cell types therefore contain sufficient polyamines when in a resting state (G₁) to enable them to enter from G₁ into S phase and traverse S phase at normal rates in the absence of further polyamine synthesis. The recruitment of cells into the first cell cycle, through serum stimulation of growth, therefore appears not to be mediated or regulated by the increases in intracellular levels of polyamines that occurs under these conditions. Conversely, the arrest of growth of these cell types resulting from serum deprivation is not mediated by a limitation of intracellular polyamine content.     

Differential interaction of branch-specific inhibitors of isoprenoid biosynthesis with cell cycle progression in tobacco BY-2 cells

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase (HMGR), which catalyzes the formation of mevalonic acid (MVA), can be specifically blocked by mevinolin. Inhibition of HMGR in vivo leads to an arrest in cell cycle progression in tobacco ( Nicotiana tabacum L. Bright Yellow 2) cells. As MVA in plants is the common precursor of a myriad of isoprenoid products synthesized in the cytosol and mitochondria, it is difficult to identify among such MVA-dependent molecules those whose lack may lead to cell cycle arrest. In an attempt to do so, branch-specific inhibitors of the cytosolic isoprenoid pathway downstream from MVA were used to study their capacity to block cell cycle progression. The effects of squalestatin (sterol biosynthesis inhibitor), chaetomellic acid A and patulin (protein prenyltransferase (PT) inhibitors) and tunicamycin (inhibitor of dolichol-dependent protein glycosyl transferase, thus mimicking the effect of an absence of dolichol) were compared to those induced by mevinolin. In this way, squalestatin and chaetomellic acid were identified as behaving like true cell cycle inhibitors, in that they led to a specific arrest in the cell cycle. However, they did not exactly mimic the mevinolin-induced effects. Patulin proved to be of high general toxicity, which suggests that it may affect other reactions besides blockage of protein isoprenylation. Finally, tunicamycin efficiently blocked growth of cell suspension cultures, but did not arrest the cells in a specific phase of the cell cycle. Results are discussed in the context of a better understanding of the essential implication of isoprenoids in plant cell cycle progression.     

Role of the retinoblastoma protein in cell cycle arrest mediated by a novel cell surface proliferation inhibitor

A novel cell regulatory sialoglycopeptide (CeReS-18), purified from the cell surface of bovine cerebral cortex cells has been shown to be a potent and reversible inhibitor of proliferation of a wide array of fibroblasts as well as epithelial-like cells and nontransformed and transformed cells. To investigate the possible mechanisms by which CeReS-18 exerts its inhibitory action, the effect of the inhibitor on the posttranslational regulation of the retinoblastoma susceptibility gene product (RB), a tumor suppressor gene, has been examined. It is shown that CeReS-18 mediated cell cycle arrest of both human diploid fibroblasts (HSBP) and mouse fibroblasts (Swiss 3T3) results in the maintenance of the RB protein in the hypophosphorylated state, consistent with a late G1 arrest site. Although their normal nontransformed counterparts are sensitive to cell cycle arrest mediated by CeReS-18, cell lines lacking a functional RB protein, through either genetic mutation or DNA tumor virus oncoprotein interaction, are less sensitive. The refractory nature of these cells is shown to be independent of specific surface receptors for the inhibitor, and another tumor suppressor gene (p53) does not appear to be involved in the CeReS-18 inhibition of cell proliferation. The requirement for a functional RB protein product, in order for CeReS-18 to mediate cell cycle arrest, is discussed in light of regulatory events associated with density-dependent growth inhibition. © 1994 Wiley-Liss, Inc.     

Epithelial to mesenchymal transition in Madin-Darby canine kidney cells is accompanied by down-regulation of Smad3 expression,leading to resistance to transforming growth factor-beta-induced growth arrest

In normal epithelial cells, transforming growth factor-beta (TGF-beta) typically causes growth arrest in the G(1) phase of the cell cycle and may eventually lead to apoptosis. However, transformed cells lose these inhibitory responses and often instead show an increase in malignant character following TGF-beta treatment. In the canine kidney-derived epithelial cell line, MDCK, synergism between activation of the Raf/MAPK pathway and the resulting autocrine production of TGF-beta triggers transition from an epithelial to a mesenchymal phenotype. During this process, these cells become refractive to TGF-beta-induced cell cycle arrest and apoptosis. TGF-beta signals are primarily transduced to the nucleus through complexes of receptor-regulated Smads, Smad2 and Smad3 with the common mediator Smad, Smad4. Here we show that the transition from an epithelial to mesenchymal phenotype is accompanied by gradual down-regulation of expression of Smad3. Restoration of Smad3 to previous levels of expression restores the cell cycle arrest induced by TGF-beta without reverting the cells to an epithelial phenotype or impacting on the MAPK pathway. Regulation of apoptosis is not affected by Smad3 levels. These data attribute to Smad3 a critical role in the control of cell proliferation by TGF-beta, which is lost following an epithelial to mesenchymal transition.     

Multiple modes of ligand recognition: Crystal structures of cyclin-dependent protein kinase 2 in complex with ATP and two inhibitors,olomoucine and isopentenyladenine

Cyclin-dependent kinases (CDKs) are conserved regulators of the eukaryotic cell cycle with different isoforms controlling specific phases of the cell cycle. Mitogenic or growth inhibitory signals are mediated, respectively, by activation or inhibition of CDKs which phosphorylate proteins associated with the cell cycle. The central role of CDKs in cell cycle regulation makes them a potential new target for inhibitory molecules with anti-proliferative and/or anti-neoplastic effects. We describe the crystal structures of the complexes of CDK2 with a weakly specific CDK inhibitor, N6-(δ2-isopentenyl)adenine, and a strongly specific inhibitor, olomoucine. Both inhibitors are adenine derivatives and bind in the adenine binding pocket of CDK2, but in an unexpected and different orientation from the adenine of the authentic ligand ATP. The N6-benzyl substituent in olomoucine binds outside the conserved binding pocket and is most likely responsible for its specificity. The structural information from the CDK2-olomoucine complex will be useful in directing the search for the next generation inhibitors with improved properties. © 1995 Wiley-Liss, Inc.     

Selective effects of interferon on distinct sites of the T lymphocyte triggering process

Lectin- and antigen-induced proliferation of murine T cells consists of two major events, namely, a rapid induction of susceptibility to growth factors and a later-occurring, accessory cell-dependent production of T cell growth factors (TCGF). The mechanism by which interferon (IFN) inhibits T cell responses was studied accordingly. A decrease of Con A-induced proliferation was observed in the presence of increasing amounts of IFN. The reduced proliferative response in such cultures was found to be due to an accumulation of cells in the G0/G1 phase of the cell cycle. Furthermore, the results show that IFN did not inhibit the early events in T cell triggering, because the acquisition of responsiveness of resting T cells to TCGF was unaltered in the presence of IFN, nor did it interfere with production of TCGF. In contrast, IFN was found to interfere with the TCGF-dependent T cell blast growth. Cytofluorometric analysis of the proliferative phase revealed that IFN exerts its effect on T cells, which have entered the proliferative cycle, by a postmitotic accumulation in G0/G1, thus reducing the proliferating population. The results demonstrate that IFN primarily affects the later phase of proliferative activity after T cell triggering, leaving the helper cell functions untouched.