G1/S regulatory mechanisms from yeast to man

Cyclin D2 is a critical mediator of exercise-induced cardiac hypertrophy

A number of signaling pathways underlying pathological cardiac hypertrophy have been identified. However, few studies have probed the functional significance of these signaling pathways in the context of exercise or physiological pathways. Exercise studies were performed on females from six different genetic mouse models that have been shown to exhibit alterations in pathological cardiac adaptation and hypertrophy. These include mice expressing constitutively active glycogen synthase kinase-3β (GSK-3βS9A), an inhibitor of CaMK II (AC3-I), both GSK-3βS9A and AC3-I (GSK-3βS9A/AC3-I), constitutively active Akt (myrAkt), mice deficient in MAPK/ERK kinase kinase-1 (MEKK1-/-), and mice deficient in cyclin D2 (cyclin D2-/-). Voluntary wheel running performance was similar to NTG littermates for five of the mouse lines. Exercise induced significant cardiac growth in all mouse models except the cyclin D2-/- mice. Cardiac function was not impacted in the cyclin D2-/- mice and studies using a phospho-antibody array identified six proteins with increased phosphorylation (greater than 150%) and nine proteins with decreased phosphorylation (greater than 33% decrease) in the hearts of exercised cyclin D2-/- mice compared to exercised NTG littermate controls. Our results demonstrate that unlike the other hypertrophic signaling molecules tested here, cyclin D2 is an important regulator of both pathologic and physiological hypertrophy. Impact statement This research is relevant as the hypertrophic signaling pathways tested here have only been characterized for their role in pathological hypertrophy, and not in the context of exercise or physiological hypertrophy. By using the same transgenic mouse lines utilized in previous studies, our findings provide a novel and important understanding for the role of these signaling pathways in physiological hypertrophy. We found that alterations in the signaling pathways tested here had no impact on exercise performance. Exercise induced cardiac growth in all of the transgenic mice except for the mice deficient in cyclin D2. In the cyclin D2 null mice, cardiac function was not impacted even though the hypertrophic response was blunted and a number of signaling pathways are differentially regulated by exercise. These data provide the field with an understanding that cyclin D2 is a key mediator of physiological hypertrophy.

A Short-Term Advantage for Syngamy in the Origin of Eukaryotic Sex: Effects of Cell Fusion on Cell Cycle Duration and Other Effects Related to the Duration of the Cell Cycle-Relationship between Cell Growth Curve and the Optimal Size of the Species, and Circadian Cell Cycle in Photosynthetic Unicellular Organisms

The origin of sex is becoming a vexatious issue for Evolutionary Biology. Numerous hypotheses have been proposed, based on the genetic effects of sex, on trophic effects or on the formation of cysts and syncytia. Our approach addresses the change in cell cycle duration which would cause cell fusion. Several results are obtained through graphical and mathematical analysis and computer simulations. (1) In poor environments, cell fusion would be an advantageous strategy, as fusion between cells of different size shortens the cycle of the smaller cell (relative to the asexual cycle), and the majority of mergers would occur between cells of different sizes. (2) The easiest-to-evolve regulation of cell proliferation (sexual/asexual) would be by modifying the checkpoints of the cell cycle. (3) A regulation of this kind would have required the existence of the G2 phase, and sex could thus be the cause of the appearance of this phase. Regarding cell cycle, (4) the exponential curve is the only cell growth curve that has no effect on the optimal cell size in unicellular species; (5) the existence of a plateau with no growth at the end of the cell cycle explains the circadian cell cycle observed in unicellular algae.

Mislocalization of CDK11/PITSLRE, a regulator of the G2/M phase of the cell cycle, in Alzheimer disease

Post-mitotic neurons are typically terminally differentiated and in a quiescent status. However, in Alzheimer disease (AD), many neurons display ectopic re-expression of cell cycle-related proteins. Cyclin-dependent kinase 11 (CDK11) mRNA produces a 110-kDa protein (CDK11^p110) throughout the cell cycle, a 58-kDa protein (CDK11^p58) that is specifically translated from an internal ribosome entry site and expressed only in the G₂/M phase of the cell cycle, and a 46-kDa protein (CDK11^p46) that is considered to be apoptosis specific. CDK11 is required for sister chromatid cohesion and the completion of mitosis. In this study, we found that the expression patterns of CDK11 vary such that cytoplasmic CDK11 is increased in AD cellular processes, compared to a pronounced nuclear expression pattern in most controls. We also investigated the effect of amyloid precursor protein (APP) on CDK11 expression in vitro by using M17 cells overexpressing wild-type APP and APP Swedish mutant phenotype and found increased CDK11 expression compared to empty vector. In addition, amyloid-β_25–35 resulted in increased CDK11 in M17 cells. These data suggest that CDK11 may play a vital role in cell cycle re-entry in AD neurons in an APP-dependent manner, thus presenting an intriguing novel function of the APP signaling pathway in AD.

Phosphorylation of pRb by cyclin D kinase is necessary for development of cardiac hypertrophy

OBJECTIVES

A number of stimuli induce cardiac hypertrophy and may lead to cardiomyopathy and heart failure. It is believed that cardiomyocytes withdraw from the cell cycle shortly after birth and become terminally differentiated. However, cell cycle regulatory proteins take part in the development of hypertrophy, and it is important to elucidate the mechanisms of how these proteins are involved in the hypertrophic response in cardiomyocytes.

MATERIALS AND METHODS, AND RESULTS

In the present study, by immunohistochemistry with a phosphorylation-specific antibody, we found that cyclin D-cdk4/6-phosphorylated retinoblastoma protein (pRb) during hypertrophy and expression of an unphosphorylatable pRb mutant impaired hypertrophic growth in cardiomyocytes. Transcription factor E2F was activated by hypertrophic elicitors but activation was impaired by pharmacological inhibition of cyclin D-cdk4/6. Inhibition of cyclin E-cdk2 complex only partly impaired E2F activity and did not prevent hypertrophic growth, but diminished endoreplication during hypertrophy.

CONCLUSIONS

These results indicate that cyclin D-cdk4/6-dependent phosphorylation of pRb and activation of E2F is necessary for hypertrophic growth in cardiomyocytes, whereas cyclin E-cdk2 kinase is not necessary for hypertrophy but regulates endoreplication in these cells. The data support the notion that hypertrophic growth of cardiomyocytes involves a partial progression through the G1 phase of the cell cycle

Linear discriminant analysis-based estimation of the false discovery rate for phosphopeptide identifications

The development of liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) has made it possible to characterize phosphopeptides in an increasingly large-scale and high-throughput fashion. However, extracting confident phosphopeptide identifications from the resulting large data sets in a similar high-throughput fashion remains difficult, as does rigorously estimating the false discovery rate (FDR) of a set of phosphopeptide identifications. This article describes a data analysis pipeline designed to address these issues. The first step is to reanalyze phosphopeptide identifications that contain ambiguous assignments for the incorporated phosphate(s) to determine the most likely arrangement of the phosphate(s). The next step is to employ an expectation maximization algorithm to estimate the joint distribution of the peptide scores. A linear discriminant analysis is then performed to determine how to optimally combine peptide scores (in this case, from SEQUEST) into a discriminant score that possesses the maximum discriminating power. Based on this discriminant score, the p- and q-values for each phosphopeptide identification are calculated, and the phosphopeptide identification FDR is then estimated. This data analysis approach was applied to data from a study of irradiated human skin fibroblasts to provide a robust estimate of FDR for phosphopeptides. The Phosphopeptide FDR Estimator software is freely available for download at http://ncrr.pnl.gov/software/.

Is the time dimension of the cell cycle re-entry in AD regulated by centromere cohesion dynamics?

Chromosomal involvement is a legitimate, yet not well understood, feature of Alzheimer disease (AD). Firstly, AD affects more women than men. Secondly, the amyloid-β protein precursor genetic mutations, responsible for a cohort of familial AD cases, reside on chromosome 21, the same chromosome responsible for the developmental disorder Down's syndrome. Thirdly, lymphocytes from AD patients display a novel chromosomal phenotype, namely premature centromere separation (PCS). Other documented morphological phenomena associated with AD include the occurrence of micronuclei, aneuploidy, binucleation, telomere instability, and cell cycle re-entry protein expression. Based on these events, here we present a novel hypothesis that the time dimension of cell cycle re-entry in AD is highly regulated by centromere cohesion dynamics. In view of the fact that neurons can re-enter the cell division cycle, our hypothesis predicts that alterations in the signaling pathway leading to premature cell death in neurons is a consequence of altered regulation of the separation of centromeres as a function of time. It is well known that centromeres in the metaphase-anaphase transition separate in a non-random, sequential order. This sequence has been shown to be deregulated in aging cells, various tumors, syndromes of chromosome instability, following certain chemical inductions, as well as in AD. Over time, premature chromosome separation is both a result of, and a driving force behind, further cohesion impairment, activation of cyclin dependent kinases, and mitotic catastrophe, a vicious circle resulting in cellular degeneration and death.

Protein phosphatase 1alpha activity prevents oncogenic transformation

Cyclin-dependent kinase 2 (Cdk2) phosphorylates Thr320 of protein phosphatase 1alpha (PP1alpha) in late G(1), thereby inhibiting its activity. Phosphorylation-resistant PP1alphaT320A, acting as a constitutively active (CA) mutant, causes a late G(1) arrest by preventing the phosphorylation and inactivation of the retinoblastoma protein (pRb). Both PP1alpha-mediated G(1) arrest and PP1alpha phosphorylation in late G(1) require the presence of pRb, indicating that PP1alpha is a crucial regulator of the pRb pathway, which is almost invariably mutated in human cancer. These findings prompted us to investigate whether PP1alpha interferes with oncogenic transformation. The ability of NIH 3T3 cells to form foci after transformation with ras/cyclin D1 was significantly inhibited by co-transfection with PP1alphaT320A, but not PP1alpha. Likewise, cells expressing PP1alphaT320A or PP1alphaT320A fused to green fluorescent protein (GFP) were unable to form colonies in soft agar, regardless of whether PP1alpha constructs were co-transfected with ras/cyclin D1 or transfected into stably transformed cells. Overexpressed wild-type (Wt) PP1alpha and GFP-PP1alpha were phosphorylated in Thr320, most likely explaining its lack of effect. Expression of GFP-PP1alphaT320A was associated with caspase-cleaved pRb in Western blots (WB) and morphological signs of cell death. These findings demonstrate that PP1alpha activity can override oncogenic signaling by causing cell-cycle arrest and/or apoptosis rather than restoring contact inhibition or anchorage dependence.

Interaction of p58(PITSLRE), a G2/M-specific protein kinase, with cyclin D3

The p58(PITSLRE) is a p34(cdc2)-related protein kinase that plays an important role in normal cell cycle progression. Elevated expression of p58(PITSLRE) in eukaryotic cells prevents them from undergoing normal cytokinesis and appears to delay them in late telophase. To investigate the molecular mechanism of p58(PITSLRE) action, we used the yeast two-hybrid system, screened a human fetal liver cDNA library, and identified cyclin D3 as an interacting partner of p58(PITSLRE). In vitro binding assay, in vivo coimmunoprecipitation, and immunofluorescence cell staining further confirmed the association of p58(PITSLRE) with cyclin D3. This binding was observed only in the G(2)/M phase but not in the G(1)/S phase of the cell cycle; meanwhile, no interaction between p110(PITSLRE) and cyclin D3 was observed in all the cell cycle. The overexpression of cyclin D3 in 7721 cells leads to an exclusively accumulation of p58(PITSLRE) in the nuclear region, affecting its cellular distribution. Histone H1 kinase activity of p58(PITSLRE) was greatly enhanced upon interaction with cyclin D3. Furthermore, kinase activity of p58(PITSLRE) was found to increase greatly in the presence of cyclin D3 using a specific substrate, beta-1,4-galactosyltransferase 1. These data provide a new clue to our understanding of the cellular function of p58(PITSLRE) and cyclin D3.

SSeCKS, a major protein kinase C substrate with tumor suppressor activity, regulates G(1)-->S progression by controlling the expression and cellular compartmentalization of cyclin D

SSeCKS, first isolated as a G(1)-->S inhibitor that is downregulated in src- and ras-transformed cells, is a major cytoskeleton-associated PKC substrate with tumor suppressor and kinase-scaffolding activities. Previous attempts at constitutive expression resulted in cell variants with truncated ectopic SSeCKS products. Here, we show that tetracycline-regulated SSeCKS expression in NIH 3T3 cells induces G(1) arrest marked by extracellular signal-regulated kinase 2-dependent decreases in cyclin D1 expression and pRb phosphorylation. Unexpectedly, the forced reexpression of cyclin D1 failed to rescue SSeCKS-induced G(1) arrest. Confocal microscopy analysis revealed cytoplasmic colocalization of cyclin D1 with SSeCKS. Because the SSeCKS gene encodes two potential cyclin-binding motifs (CY) flanking major in vivo protein kinase C (PKC) phosphorylation sites (Ser(507/515)), we addressed whether SSeCKS encodes a phosphorylation-dependent cyclin scaffolding function. Bacterially expressed SSeCKS-CY bound cyclins D1 and E, whereas K-->S mutations within either CY motif ablated binding. Activation of PKC in vivo caused a rapid translocation of cyclin D1 to the nucleus. Cell permeable, penetratin-linked peptides encoding wild-type SSeCKS-CY, but not K-->S or phospho-Ser(507/515) variants, released cyclin D1 from its cytoplasmic sequestration and induced higher saturation density in cyclin D1-overexpressor cells or rat embryo fibroblasts. Our data suggest that SSeCKS controls G(1)-->S progression by regulating the expression and localization of cyclin D1. These data suggest that downregulation of SSeCKS in tumor cells removes gating checkpoints for saturation density, an effect that may promote contact independence.

Roscovitine, a specific inhibitor of cellular cyclin-dependent kinases, inhibits herpes simplex virus DNA synthesis in the presence of viral early proteins

We have previously shown that two inhibitors specific for cellular cyclin-dependent kinases (cdks), Roscovitine (Rosco) and Olomoucine (Olo), block the replication of herpes simplex virus (HSV). Based on these results, we demonstrated that HSV replication requires cellular cdks that are sensitive to these drugs (L. M. Schang, J. Phillips, and P. A. Schaffer. J. Virol. 72:5626-5637, 1998). We further established that at least two distinct steps in the viral replication cycle require cdks: transcription of immediate-early (IE) genes and transcription of early (E) genes (L. M. Schang, A. Rosenberg, and P. A. Schaffer, J. Virol. 73:2161-2172, 1999). Since Rosco inhibits HSV replication efficiently even when added to infected cells at 6 h postinfection, we postulated that cdks may also be required for viral functions that occur after E gene expression. In the study presented herein, we tested this hypothesis directly by measuring the efficiency of viral replication, viral DNA synthesis, and expression of several viral genes during infections in which Rosco was added after E proteins had already been synthesized. Rosco inhibited HSV replication, and specifically viral DNA synthesis, when the drug was added at the time of release from a 12-h phosphonoacetic acid (PAA)-induced block in viral DNA synthesis. Inhibition of DNA synthesis was not a consequence of inhibition of expression of IE or E genes in that Rosco had no effect on steady-state levels of two E transcripts under the same conditions in which it inhibited viral DNA synthesis. Moreover, viral DNA synthesis was inhibited by Rosco even in the absence of protein synthesis. In a second series of experiments, the replication of four HSV mutants harboring temperature-sensitive mutations in genes essential for viral DNA replication was inhibited when Rosco was added at the time of shift-down from the nonpermissive to the permissive temperature. Viral DNA synthesis was inhibited by Rosco under these conditions, whereas expression of viral E genes was not affected. We conclude that cellular Rosco-sensitive cdks are required for replication of viral DNA in the presence of viral E proteins. This requirement may indicate that HSV DNA synthesis is functionally linked to transcription, which requires cdks, or that both viral transcription and DNA replication, independently, require viral or cellular factors activated by Rosco-sensitive cdks.

Differentiation-related mechanisms which suppress DNA replication

Differentiation of mammalian cells implies cessation of DNA replication and cell proliferation; the potential controls of this coupling are examined here. It is clear that the known or proposed mechanisms of down-regulation of replicative cellular activities vary in different lineages of cell differentiation, and occur in all phases of the cell cycle. In G1 these regulators include p21/Cip1 or p27/Kip1, pRb, and p53; the novel, recently reported mechanisms of their action are summarized. In S phase the availability of nucleotide precursors, the origin recognition complex (ORC), and other replication proteins may be important in differentiation, and in G2 phase the cdc2/cyclin B complex and replication licensing factors determine normal G2 traverse versus an arrest or polyploidisation. Other replication-related mechanisms include transcription factors, e.g., Sp1, telomerase, and nuclear matrix changes. Thus, differentiation alters the activity not only of the various checkpoint proteins, but also of the components of the replicative machinery itself.

E2F target genes and cell-cycle checkpoint control

In this review, we will focus on the role played by transcription factors of the E2F/DP family in controlling the expression of genes that carry out important cell-cycle control functions, thereby ensuring ordered progression through the mammalian cell division cycle. The emerging picture is that cell-cycle progression depends on the execution of a regulatory cascade of gene expression, driven by E2F/DP transcription factors, which are in turn regulated by the products of some of these genes. That E2F factors are potent regulators of cell-cycle checkpoints in mammalian cells is supported by experiments demonstrating that ectopic expression of individual E2F family members is sufficient to modulate cell proliferation and apoptosis. It is also clear that deregulation of E2F activity will result in the loss of particular checkpoint controls, thereby predisposing cells to malignant conversion.

Analysis of a Drosophila cyclin E hypomorphic mutation suggests a novel role for cyclin E in cell proliferation control during eye imaginal disc development

We have generated and characterized a Drosophila cyclin E hypomorphic mutation, DmcycEJP, that is homozygous viable and fertile, but results in adults with rough eyes. The mutation arose from an internal deletion of an existing P[w+lacZ] element inserted 14 kb upstream of the transcription start site of the DmcycE zygotic mRNA. The presence of this deleted P element, but not the P[w+lacZ] element from which it was derived, leads to a decreased level of DmcycE expression during eye imaginal disc development. Eye imaginal discs from DmcycEJP larvae contain fewer S phase cells, both anterior and posterior to the morphogenetic furrow. This results in adults with small rough eyes, largely due to insufficient numbers of pigment cells. Altering the dosage of the Drosophila cdk2 homolog, cdc2c, retinoblastoma, or p21(CIP1) homolog dacapo, which encode proteins known to physically interact with Cyclin E, modified the DmcycEJP rough eye phenotype as expected. Decreasing the dosage of the S phase transcription factor gene, dE2F, enhanced the DmcycEJP rough eye phenotype. Surprisingly, mutations in G2/M phase regulators cyclin A and string (cdc25), but not cyclin B1, B3, or cdc2, enhanced the DmcycEJP phenotype without affecting the number of cells entering S phase, but by decreasing the number of cells entering mitosis. Our analysis establishes the DmcycEJP allele as an excellent resource for searching for novel cyclin E genetic interactors. In addition, this analysis has identified cyclin A and string as DmcycEJP interactors, suggesting a novel role for cyclin E in the regulation of Cyclin A and String function during eye development.