Cdh1: a master G0/G1 regulator

APC/C(Cdh1) controls the G0 and G1 phases of the cell cycle. Using a conditional knockout of the Cdh1 coding gene Fizzy-related (Fzr), a new study demonstrates that Cdh1 is essential for viability and that it functions as a tumour suppressor by preventing genomic instability.     

The G1 period

In previous papers the existence of two cycles of chromosome condensation-decondensation per cell cycle was suggested based on experiments involving nuclear morphometry measurements of Feulgen-stained nuclei. This conclusion can be criticized since its assumption of a relationship between nuclear morphology and chromatin structure is derived from indirect evidence. In this paper, we report simultaneous measurements of nuclear area and nuclear fluorescence intensity on individual cells stained with the intercalating dye, acridine orange (AO). Using cells in various stages of G₁ and synchronized by two different methods, our results demonstrate a linear correlation between nuclear area and fluorescence intensity. They also indicate two cycles of chromatin condensation-decondensation during the G₁ period, as assayed by the number of chromatin primary, intercalating AO binding sites. Finally, they show that the first of these cycles involves a transition in early G₁ from a very small condensed nucleus (immediately after telephase) to a relatively large, dispersed nucleus that occurs abruptly.     

Conjugation Between G1 and G2 Phase Cells in Paramecium tetraurelia1

Cells of Paramecium tetraurelia, stock hr^d, cultured in a micro-capillary containing 1 μl fresh culture medium, expressed mating activity through the whole cell cycle. Mating-reactive G₂ phase cells can conjugate with cells of other phases. The G₂ phase cells, which have double (4C) the normal micronuclear DNA content, undergo pre-meiotic DNA synthesis when conjugated with G₁ phase cells. The micronucleus of the progeny from the cross between a G₁ and a G₂ cell becomes triploid.     

D1 in G2

No Abstract Available

Key Words

D-type Cyclins, Cyclin D1, Ras     

G1 and G2 chalones of the gastric mucosa

A study was made of the action of human gastric mucosa G1 and G2 chalones on cellular regeneration of mouse gastric mucosa and of the duration of their maximal effect. Chalone fractions were obtained from the mucous membranes of 21 stomachs resected for peptic ulcer by the method of fractional ethanol precipitation. The data indicate that the maximal inhibitory action of G1 chalone occurs in 3, whereas that of chalone G2 in 6 hours. Some specificity of the action of chalones was discovered depending on the part of the gastric mucosa from which they were obtained.     

A new method to discriminate G1, S, G2, M, and G1 postmitotic cells

A new flow cytometric method combining light scattering measurements, detection of bromodeoxyuridine (BrdU) incorporation via fluorescent antibody, and quantitation of cellular DNA content by propidium iodide (PI) allows identification of additional compartments in the cell cycle. Thus, while cell staining with BrdU-antibodies and PI reveals the G1, S, and G2 + M phases of the cell cycle, differences in light scattering allow separation of G2 phase cells from M phase cells and subdivision of G1 phase into two compartments, i.e., G1A representing postmitotic cells which mature to G1B cells ready to initiate DNA synthesis. The method involves fixation of cells in 70% ethanol, extraction of histones with HC1, and thermal denaturation of DNA. This treatment appears to enhance the differences in chromatin structure of cells in the various phases of the cell cycle to the extent that cells could be separated on the basis of the 90 degrees scatter. Mitotic cells show much lower scatter than G2 phase cells, and G1 postmitotic cells (G1A) show lower scatter than G1 cells about to enter the S phase (G1B). Light scattering is correlated with chromatin condensation, as judged by microscopic evaluation of cells sorted on the basis of light scatter. The method has the advantage over the parental BrdU/DNA bivariate analysis in allowing the G2 and M phases of the cell cycle to be separated and the G1 phase to be analyzed in more detail. The method may also allow separation of unlabeled S phase cells from mitotic cells and distinguish between labeled and unlabeled mitotic cells.     

Distribution of G1m(1), G1m(2) and G3m(5) allotypes in Aragon.

The distribution of G1m(1), G1m(2) and G3m(5) allotypes was studied in 700 unrelated individuals from Aragon (North-East Spain). The Gm haplotype frequencies were similar to those reported in French areas next to Aragon.     

Sulfhydryl groups during the S phase: comparison of cells from G 1 , plateau-phase G 1 , and G 0

The non-protein sulfhydryl (NPSH) content of cells moving into S from G₁, plateau phase G₁, and G₀ was measured. Chinese hamster ovary (CHO) cells accumulated in G₁ by growth into plateau phase contain only one-fourth the NPSH concentration of cycling C₁ cells or G₁ cells accumulated by brief growth in isoleucine-deficient medium. Upon dilution of plateau cultures with fresh medium, cellular NPSH content increases rapidly, reaching the same level as that in cycling cells within four hours. This increase is prevented by cycloheximide but not by actinomycin D or hydroxyurea. Neither CHO cells cycling in vitro nor salivary gland G₀ cells stimulated with isoproterenol in vivo show significant changes in intracellular NPSH concentrations during S phase. This suggests that the concentration of intracellular NPSH (glutathione) remains constant during the cell cycle except when cells are grown to plateau phase in exhausted or deficient medium, in which case normal degradation exceeds synthesis and the gross level falls until fresh medium is provided and synthesis, apparently on preexisting RNA templates, accelerates.     

G0/G1 Switch Gene 2 Regulates Cardiac Lipolysis

The anabolism and catabolism of myocardial triacylglycerol (TAG) stores are important processes for normal cardiac function. TAG synthesis detoxifies and stockpiles fatty acids to prevent lipotoxicity, whereas TAG hydrolysis (lipolysis) remobilizes fatty acids from endogenous storage pools as energy substrates, signaling molecules, or precursors for complex lipids. This study focused on the role of G₀/G₁ switch 2 (G0S2) protein, which was previously shown to inhibit the principal TAG hydrolase adipose triglyceride lipase (ATGL), in the regulation of cardiac lipolysis. Using wild-type and mutant mice, we show the following: (i) G0S2 is expressed in the heart and regulated by the nutritional status with highest expression levels after re-feeding. (ii) Cardiac-specific overexpression of G0S2 inhibits cardiac lipolysis by direct protein-protein interaction with ATGL. This leads to severe cardiac steatosis. The steatotic hearts caused by G0S2 overexpression are less prone to fibrotic remodeling or cardiac dysfunction than hearts with a lipolytic defect due to ATGL deficiency. (iii) Conversely to the phenotype of transgenic mice, G0S2 deficiency results in a de-repression of cardiac lipolysis and decreased cardiac TAG content. We conclude that G0S2 acts as a potent ATGL inhibitor in the heart modulating cardiac substrate utilization by regulating cardiac lipolysis.     

Posttranslational processing of Uukuniemi virus glycoproteins G1 and G2

A total of 26 polypeptides have been resolved by gel electrophoresis of purified phage PBSX, 3 of which belong to the head and the remainder to the tail. After mitomycin C treatment, synthesis of 11 additional proteins which are not found in the assembled phage particle was demonstrated, all but 4 being under the control of the phage repressor. Existence of a prehead and of a precursor of the main capsid protein (molecular weight, 35,000) suggested phage head maturation which is accompanied by cleavage of the precursor (molecular weight, 36,500). The role of induced proteins related and unrelated to PBSX is discussed. Finally, the estimated phage genome mass of 4 X 10(7) daltons exceeded by more than four times its head capacity, which could explain the defectiveness of the phage.     

G1m(1) and G1m(2) allotypes in Albanian towns of Calabria

The G1m(1) and G1m(2) allotype distribution was analyzed in a population sample from 11 Albanian towns of Calabria. The unusually high frequency of the G1m(1) marker already observed in Calabria as well as the presence of the Gm(2) phenotype were shown. The Calabrian and Albanian populations were similar, but significantly different from other Italian populations.     

A Cdc28 mutant uncouples G1 cyclin phosphorylation and ubiquitination from G1 cyclin proteolysis

Proteolysis of the yeast G(1) cyclins is triggered by their Cdc28-dependent phosphorylation. Phosphorylated Cln1 and Cln2 are ubiquitinated by the SCF-Grr1 complex and then degraded by the 26 S proteasome. In this study, we identified a cak1 allele in a genetic screen for mutants that stabilize the yeast G(1) cyclins. Further characterization showed that Cln2HA was hypophosphorylated, unable to bind Cdc28, and stabilized in cak1 mutants at the restrictive temperature. Hypophosphorylation of Cln2HA could thus explain its stabilization. To test this possibility, we expressed a Cak1-independent mutant of Cdc28 (Cdc28-43244) in cak1 mutants and found that Cln2HA phosphorylation was restored, but surprisingly, the phospho-Cln2HA was stabilized. When bound to Cdc28-43244, Cln2HA was recognized and polyubiquitinated by SCF-Grr1. The Cdc28-43244 mutant thus reveals an unexpected complexity in the degradation of polyubiquitinated Cln2HA by the proteasome.     

Micronuclei and 3AB index in X-irradiated human lymphocytes in G1 and G1 phases

We applied the cytokinesis-block micronucleus assay to observe the radiobiological response of human lymphocytes after X-ray treatment in the G0 and G1 phases. In addition, we used 3-aminobenzamide (3AB) to measure the 3AB index in the two phases. The experimental results show that the at 2 Gy the MN yield and the 3AB index are dependent on the cell phase and show considerable inter-individual variability. The radiation-induced MN frequency obtained for 33 subjects is 0.470 ± 0.063 for the G0 phase and 0.689 ± 0.139 for the G1 phase; the 3AB index values are 0.326 ± 0.144 and 0.067 ± 0.058 for G0 and G1 phases, respectively. At the individual level, the 3AB index for the G1 phase correlations inversely with the cytogenetic effects observed in that phase. We discuss the possibility of applying the MN test combined with the 3AB index to lymphocytes at different phases to study the individual response to radiation (individual radiosensitivity).     

Biosynthetic relationship among aflatoxins B1, B2, G1, and G2

Aspergillus parasiticus NIAH-26, a UV-irradiated mutant of A. parasiticus SYS-4 (NRRL 2999), produces neither aflatoxins nor precursors. When sterigmatocystin (ST) or O-methylsterigmatocystin was fed to this mutant in YES medium, aflatoxins B1 (AFB1) and G1 (AFG1) were produced. When dihydrosterigmatocystin (DHST) or dihydro-O-methylsterigmatocystin was fed to this mold, aflatoxins B2 (AFB2) and G2 (AFG2) were produced. The reactions from ST to AFB1 and DHST to AFB2 were also observed in the cell-free system and were catalyzed stepwise by the methyltransferase and oxidoreductase enzymes. In the feeding experiments of strain NIAH-26, the convertibility from ST to AFB1-AFG1 was found to be remarkably suppressed by the coexistence of DHST in the medium, and the convertibility from DHST to AFB2-AFG2 was also suppressed by the presence of ST. When some other mutants which endogenously produce a small amount of aflatoxins (mainly AFB1 and AFG1) were cultured with DHST, the amounts of AFB1 and AFG1 produced were significantly decreased, whereas AFB2 and AFG2 were newly produced. In similar feeding experiments in which 27 kinds of mutants including these mutants were used, most of the mutants which were able to convert exogenous ST to AFB1-AFG1 were also found to convert exogenous DHST to AFB2-AFG2. These results suggest that the same enzymes may be involved in the both biosynthetic pathways from ST to AFB1-AFG1 and DHST to AFB2-AFG2. The reactions described herein were not observed when the molds had been cultured in the YEP medium.     

Is G1 normally distributed?

The normal distribution parameters were calculated for seven sets of cell cycle data of animal cells in culture. These include two sets of intermitotic times (rat S6/1 cells and mouse fibroblast L 929) and five sets of DNA synthesis (two of mouse fibroblast line L 929, two Chinese hamster CHO lines and Syrian hamster line BHK 21/613).It is demonstrated that within the errors involved the experimental data fit the normal distribution adequately. The Smith-Martin model and the normal distribution are briefly discussed in relationship to the initial curvature observed in a semilogarithmic presentation of such data.     

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.     

Polyclonal hyper-immunoglobulin G1(A1) syndrome

Summary The human homologue of the recently isolated myeloproliferative leukemia virus, a retrovirus that induces myeloproliferative disorder in mouse, has been mapped in man to chromosome band 1p34 by in situ hybridization.