Various Phases Of Cell Cycle Research Articles

Quantitative Structure-Cytotoxicity Relationship of 2-Arylazolylchromones and 2-Triazolylchromones.

4H-1-Benzopyran-4-one (chromone), present in various flavonoids as a backbone structure, has been used for the synthesis of anticancer drugs. The study aimed at investigating the cytotoxicity of eight 2-arylazolylchromones and twelve 2-triazolylchromones against four human oral squamous cell carcinoma (OSCC) cell lines and three human normal mesenchymal oral cells, and then performed a quantitative structure-activity relationship (QSAR) analysis. Cell viability was determined by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide method. The distribution of cells to various phases of cell cycle was determined by cell cycle analysis. A total of 3,218 physicochemical, structural and quantum chemical features were calculated for QSAR analysis from the most stabilized structure optimized using CORINA. 2-[4-(4-fluorophenyl)-1H-imidazol-1-yl]-4H-1-benzopyran-4-one [6] had the highest tumor-specificity (TS), comparable with that of 5-flurouracil (5-FU) and doxorubicin, inducing cytostatic growth inhibition, accumulation of G2+M phase cells with no cells in the G1 phase. All eight 2-triazolylchromones showed much lower tumor-specificity, confirming our previous finding. Tumor-specificity was also correlated with 3D shape, topological shape, size, ionization potential, and the presence of more than two aromatic rings in the molecule and imidazole ring in the nitrogen-containing heterocyclic ring. [6] can be a lead compound for designing anticancer drugs.

Abstract 940: Identification of Palbociclib response signature across indications

Abstract Cellular proliferation is dependent on an orderly movement through the various phases of cell cycle. Progression through the G1 phase in particular requires phosphorylation of the retinoblastoma (Rb) protein which in turn releases E2F transcription factors resulting in transcriptional activation of response gene necessary for progression into S-phase. The cyclin D-CDK4/6 signaling pathway represent a critical regulatory pathway controlling transition from G1 into S-phase and greater than 90% of human tumors have mutations in this pathway. Palbociclib is a potent, selective, and orally bioavailable inhibitor of Cdk4/6. In human tumor xenograft models, Palbociclib has significant antitumor activities. The clinical activities of Palbociclib have also be demonstrated in the phase II PALOMA-1 trial for dramatic efficacy of postmenopausal patients with locally advanced or newly diagnosed estrogen receptor (ER)-positive, HER2-negative metastatic breast cancer in combination with letrozole. To better understand the molecular mechanisms of Palbociclib response, we have identified a common set of gene signatures for ER+ BC as well as melanoma models. The physiological role of these genes regulated by Palbociclib is associated with DNA replication and repair, cell cycle, signal transduction, and Mitosis. Many of these genes have been previously identified as E2F signatures. In this study, we are aiming to expand this analysis to include two additional indications including Head and Neck squamous cell carcinoma (HNSCC) as well as squamous cell lung carcinoma (Sq lung) using Illumina RNASeq technology platform. Results from these studies will be presented and core Palbo-response signature will be discussed. In addition, we will also present the pharmacological effects of Letrozole on the gene expression changes for these core signatures in ER+ BC models. Citation Format: Xianxian Zheng, Mark Ozeck, Zhou Zhu, Keith Ching, David Shields, James Hardwick, Paul Rejto, Todd VanArsdale. Identification of Palbociclib response signature across indications. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 940. doi:10.1158/1538-7445.AM2015-940

Arsenic trioxide (As2O3) induces apoptosis and necrosis mediated cell death through mitochondrial membrane potential damage and elevated production of reactive oxygen species in PLHC-1 fish cell line

Several environmental pollutants, including metals can induce toxicological effect on aquatic animal species. Most studies to understand the toxicity of arsenic compounds were performed in mammalian cells; however, the study of the arsenic toxicity to the aquatic animals’ species, including fish, is limited. So the objective of this study was first to investigate the effects of As2O3 induced toxicity particularly on apoptosis and necrosis mediated cell death in fish cell PLHC-1 as compared to the mechanism of toxicity from known mammalian cell lines, secondly to relate in vitro effects in fish to those demonstrated by in vivo systems. To conduct this study, PLHC-1 cells were exposed to various concentrations of As2O3 (0–100μM) for 10, 20 and 40h. The results indicate that As2O3 exposure promoted apoptotic and necrotic mediated cell death in a concentration and time dependent manner. Cell death (apoptotic and necrotic) induced by As2O3 was further confirmed by changes in various phases of cell cycle, DNA fragmentation (necro- comet and apo-comet) in the comet assay, alteration in mitochondrial membrane potential and formation of increased reactive oxygen species (ROS). Apoptotic mediated cell death was confirmed further by observing the increased caspase-3 activity and elevated expression of p53, cytochrome c and Bax proteins levels in the same experimental conditions. PLHC-1 cells were shown to be a good model for evaluating biochemical/cytotoxic effects following exposure to various reference chemicals and environmental contaminants. In vitro data obtained from this study provides a comprehensive approach for the elucidating the actual molecular mechanism for As2O3 induced toxicity particularly apoptosis and necrosis mediated cell death in PLHC-1 cell line.

Cyclin Dependent Kinase as Significant Target for Cancer Treatment

Cyclin Dependent Kinase (CDKs) regulates cell cycle commitment and DNA synthesis. Cell division in mammalian cells is driven by protein kinase that regulates progression through the various phases of cell cycle. The activity of cyclins and their associated CDKs are frequently deranged in human cancers. For this reason, Cyclin-CDK complexes have been considered as very promising therapeutic targets in human malignancies. An obvious concern whether, blocking cyclin-CDK function would preferentially affects cancer cells but not normal and non-transformed cells. The cell cycle represents a series of tightly integrated events that allow the cell to grow and proliferate. Critical part of the cell cycle machinery is the CDK, which, when activated, provide a means for the cell to move from one phase of the cell cycle to next. The cell cycle also serves to protect the cell from DNA damage. Thus, cell cycle arrest represents a survival mechanism that provides tumour cell, to repair its own damaged DNA Thus, abrogation of cell cycle checkpoints, before DNA repair is complete can activate the apoptotic cascade leading to cell death. Misregulation of CDK is one of the most frequent alterations in human cancer. CDK are critical regulators of cell cycle progression and RNA transcription. A series of targeted agents that directly inhibit the CDKs, inhibit unrestricted cell growth, and induce growth arrest. Recent attention has also focused on these drugs as inhibitors of transcription. In this review we are summarizing that why CDK is important target for cancer chemotherapy and why finding out the best and potent kinase inhibitor is essential. Keywords: Cancer, CAK, Cell cycle, CDK, Cyclin, Kinases, Phosporylation, Threonine

Purified alkaloid extract of Scutellaria barbata inhibits proliferation of hepatoma HepG-2 cells by inducing apoptosis and cell cycle arrest at G 2 /M phase

Scutellaria barbata was widely used as an antitumor agent in traditional Chinese medicine. However, its antitumor components and mechanism remained unclear. In the present study, the effects of purified alkaloid extract of Scutellaria barbata (PAESB) on the cultured hepatoma HepG-2 cells were investigated. 3-(4,5-Dimethylthiazol-2-Yl)-2,5-Diphenyltetrazolium Bromide (MTT) assay showed that PAESB could inhibit the proliferation of HepG-2 cells in a dose- and time-dependent manner. Annexin V-FITC and propidium iodide (PI) staining showed that PAESB had a positive effect on apoptosis of HepG-2 cells. After treatment by PAESB (0.75 mg/ml) for 48 h, the HepG-2 cells apoptosis rate (25.12±0.91%) had statistical significance as compared with the negative group (2.97±0.49%) (P<0.01). Among the various phases of cell cycle, significant increase in the percentage of G2/M phase occurred after treated with PAESB (0.75 mg/ml) for 48 h by comparison with the corresponding values for HepG-2 cells without PAESB (P<0.01). These results indicated that PAESB exhibited potential anticancer activity against hepatoma HepG-2 cells through induction of apoptosis and G2/M phase cell cycle arrest. Key words: Scutellaria barbata, purified alkaloid extract, apoptosis, cell cycle arrest, hepatoma HepG-2 cells.

Purified alkaloid extract of Scutellaria barbata inhibits proliferation of nasopharyngeal carcinoma CNE-1 cells by inducing apoptosis and cell cycle arrest at S phase

Scutellaria barbata was widely used as an antitumor agent in traditional Chinese Medicine. However, its antitumor components and mechanism remained unclear. In the present study, the effects of purified alkaloid extract of S. barbata (PAESB) on the cultured nasopharyngeal carcinoma CNE-1 cells were investigated. MTT assay showed that PAESB could inhibit the proliferation of CNE-1 cells in a dose- and time-dependent manner. Annexin V-FITC and propidium iodide (PI) staining showed that PAESB had a positive effect on apoptosis of CNE-1 cells. After treated by PAESB (0.75 mg/ml) for 48 h, the CNE-1 cells apoptosis rate (5.13 ± 0.70%) had statistical significance as compared with the negative group (1.42 ± 0.26%) (P < 0.05). Among the various phases of cell cycle, significant increase in the percentage of S phase occurred after treated with PAESB (0.75 mg/ml) for 48 h while the percentage of cells at G0/G1 and G2/M phases were decreased by comparison with the corresponding values for CNE-1 cells without PAESB (P < 0.05). These results indicated that PAESB exhibited potential anticancer activity against nasopharyngeal carcinoma CNE-1 cells through induction of apoptosis and S phase cell cycle arrest. Key words: Scutellaria barbata, purified alkaloid extract, apoptosis, cell cycle arrest, nasopharyngeal carcinoma CNE-1 cells.

HSP90 Controls SIR2 Mediated Gene Silencing

In recent years, Hsp90 is found to interact with several telomeric proteins at various phases of cell cycle. The Hsp90 chaperone system controls assembly and disassembly of telomere structures and thus maintains the dynamic state of telomere. Here, for the first time we report that the activity of another telomeric protein Sir2p is modulated by Hsp82, the ortholog of Hsp90 from budding yeast (Saccharomyces cerevisiae). In a temperature sensitive Hsp90 deficient yeast strain (iG170Dhsp82), less abundant Sir2p is observed, resulting in de-repression of telomere silencing and a complete loss of mating type silencing. Intriguingly, over expression of Hsp90, either by exposing cells to heat shock or by introducing HSP82 overexpression plasmid also yields reduced level of Sir2p, with a consequential loss of telomere silencing. Thus, Hsp90 homeostasis maintains the cellular pool of Sir2p and thereby controls the reversible nature of telomere silencing. Interestingly, such regulation is independent of one of its major co-chaperones Sba1 (human ortholog of p23).

Abstract A38: Chemopreventive effect of different ratios of fish oil and corn oil in experimental colon carcinogenesis

Abstract Aim: The present study was designed to study the chemopreventive effect of different ratios of n-3 PUFA rich fish oil and n-6 PUFA rich corn oil on prognostic markers, DNA damage and cell cycle phases in experimentally induced colon carcinoma. Methods: Male Wistar rats were divided into control, dimethylhydrazine dihydrochloride (DMH) treated, FO+CO(1:1)+DMH and FO+CO(2.5:1)+DMH. All the groups, except control received a weekly injection of DMH for 4 weeks. The animals were sacrificed either 48 hrs later (initiation phase) or kept for 16 weeks (post initiation phase). The chemopreventive efficacy was evaluated by analyzing the regression of preneoplastic lesions, total sialic acid (TSA), lipid associated sialic acid (LASA). To study the underlying mechanisms DNA damage, distribution of cells in various phases of cell cycle, cyclin D1 and p53 levels were assessed in the isolated colonocytes. Results: The animals treated with DMH in the initiation phase and post initiation phase studies showed increase in Multiple plaque lesions (MPL), total sialic acid (TSA), lipid associated sialic acid (LASA), DNA damage and cell proliferation with increased percentage of cells in G2M and S phase. However, levels of p53 and cyclin D 1 were not significantly altered in the initiation phase while levels were significantly elevated in the post initiation phase study on treatment with DMH. The FO+CO(1:1)+DMH treatment in the initiation phase and post initiation phase led to a decrease in the MPL, DNA damage, TSA, LASA as compared to the control and DMH treated group. The cells were arrested in the G1 phase with a decrease in G2M and S phase. The levels of p53 and cyclin D1 were not significantly altered while a decrease in the levels was observed in the post initiation phase as compared to the DMH treated animals. Treatment with FO+CO(2.5:1)+DMH in the initiation phase study and post initiation phase study led to a decrease in the MPL, TSA, LASA and DNA damage as compared to the DMH treated animals. The levels of p53 and cyclin D1 remain unaltered on treatment with FO+CO(2.5:1)+DMH in the initiation phase study. However a decrease was observed in the post initiation phase as compared to the DMH treated animals. The cells were arrested in the G1 phase in the initiation phase and revert to normal in the post initiation phase showing the protective effect. There was more protective effect in the FO+CO(2.5:1)+DMH treated animals as compared to the FO+CO(1:1)+DMH treated animals in the initiation phase and the post initiation phase showing the dose and time dependent effect of n-3 PUFAs. Conclusion: The results of the present study suggest that fish oil and corn oil in the ration of 2.5;1 is more effective in chemoprevention of experimental colon carcinogenesis. Citation Information: Cancer Prev Res 2010;3(12 Suppl):A38.

Scutellaria barbate extract induces apoptosis of hepatoma H22 cells via the mitochondrial pathway involving caspase-3

To study the growth inhibitory and apoptotic effects of Scutellaria barbata D.Don (S. barbata) and to determine the underlying mechanism of its antitumor activity in mouse liver cancer cell line H22. Proliferation of H22 cells was examined by MTT assay. Cellular morphology of PC-2 cells was observed under fluorescence microscope and transmission electron microscope (EM). Mitochondrial transmembrane potential was determined under laser scanning confocal microscope (LSCM) with rhodamine 123 staining. Flow cytometry was performed to analyze the cell cycle of H22 cells with propidium iodide staining. Protein level of cytochrome C and caspase-3 was measured by semi-quantitive RT-PCR and Western blot analysis. Activity of caspase-3 enzyme was measured by spectrofluorometry. MTT assay showed that extracts from S. barbata (ESB) could inhibit the proliferation of H22 cells in a time-dependent manner. Among the various phases of cell cycle, the percentage of cells in S phase was significantly decreased, while the percentage of cells in G(1) phase was increased. Flow cytometry assay also showed that ESB had a positive effect on apoptosis. Typical apoptotic morphologies such as condensation and fragmentation of nuclei and blebbing membrane of apoptotic cells could be observed under transmission electron microscope and fluorescence microscope. To further investige the molecular mechanism behind ESB-induced apoptosis, ESB-treated cells rapidly lost their mitochondrial transmembrane potential, released mitochondrial cytochrome C into cytosol, and induced caspase-3 activity in a dose-dependent manner. ESB can effectively inhibit the proliferation and induce apoptosis of H22 cells involving loss of mitochondrial transmembrane potential, release of cytochrome C, and activation of caspase-3.

Comparative sensitivity of barley ( Hordeum vulgare L.) to insecticide and fungicide on different stages of cell cycle

In this study, the comparison of cytogenetic effects of insecticide and fungicide in different phases of cell cycle was investigated in the root tip cells of barley ( Hordeum vulgare L.). The seeds of H. vulgare L. Var. Karan 16 were treated with different concentrations (0.05%, 0.1% and 0.5%) of insecticide Profenophos (PF) and fungicide Mancozeb (MZ) for 6 h after presoaking durations of 7, 17 and 27 h.The different presoaking durations were used to bring the cells in various phases of cell cycle. Negative control was run parallel in distilled water. Cytogenetic examinations of root meristems exposed to the PF and MZ showed significant inhibition of mitotic index (MI) as well as significant increase in chromosomal aberrations (CAs). These parameters were dependent on the concentrations of insecticide and fungicide. The present study shows that PF and MZ both caused more damage in S phase of cell cycle which indicates that S phase is more sensitive in comparison to other phases.

Double minute chromosomes in mouse methotrexate-resistant cells studied by atomic force microscopy

Double minute chromosomes (DMs) are acentric, autonomously replicating extra-chromosomes and frequently mediate gene amplification in tumor and drug resistant cells. Atomic force microscopy (AFM) is a powerful tool in microbiology. We used AFM to explore the ultrastructure of DMs in mouse fibroblasts 3T3R500. DMs in various phases of cell cycle were also studied in order to elucidate the mechanisms of their duplication and separation. Metaphase spread and induced premature condensed chromosomes (PCCs) were observed under the AFM. DMs were detected to be composed of two compact spheres linked by fibers. The fibers of DMs directly connected with metaphase chromosomes were observed. Many single-minutes and few DMs were detected in G1 PCCs, while more DMs were detected in S PCCs than in G1 PCCs. Besides, all of the DMs in G2 PCCs were coupled. Our present results suggested that DMs might divide into single-minutes during or before G1-phase, followed by duplication of the single-minutes in S-phase. Moreover, we introduced a new powerful tool to study DMs and got some ideal results.

Cytosolic chaperonin-containing t-complex polypeptide 1 changes the content of a particular subunit species concomitant with substrate binding and folding activities during the cell cycle.

The chaperonin-containing t-complex polypeptide 1 (CCT) is a cytosolic molecular chaperone composed of eight subunits that assists in the folding of actin, tubulin and other cytosolic proteins. We show here that the content of particular subunits of CCT within mammalian cells decreases concomitantly with the reduction of chaperone activity during cell cycle arrest at M phase. CCT recovers chaperone activity upon resumption of these subunits after release from M phase arrest or during arrest at S phase. The levels of alpha, delta and zeta-1 subunits decreased more rapidly than the other subunits during M phase arrest by colcemid treatment and recovered after release from the arrest. Gel filtration chromatography or native (nondenaturing) PAGE analysis followed by immunoblotting indicated that the alpha and delta subunit content in the 700- to 900-kDa CCT complex was appreciably lower in the M phase cells than in asynchronous cells. In vivo, the CCT complex of M-phase-arrested cells was found to bind lower amounts of tubulin than that of asynchronous cells. In vitro, the CCT complex of M phase-arrested cells was less active in binding and folding denatured actin than that of asynchronous cells. On the other hand, the CCT complex of asynchronous cells (a mixture of various phases of cell cycle) exhibited lower alpha and delta subunit content and lower chaperone activity than that of S-phase-arrested cells obtained by excess thymidine treatment. In addition, turnover (synthesis and degradation) rates of the alpha and delta subunits in vivo were more rapid than those of most other subunits. These results suggest that the content of alpha and delta subunits of CCT reduces from the complete active complex in S phase cells to incomplete inactive complex in M phase cells.

Xeroderma pigmentosum variant: from a human genetic disorder to a novel DNA polymerase.

DNA replication in eukaryotic cells is a complex process requiring a variety of proteins to synthesize the leading and lagging strands of DNA in an asymmetric and coordinated manner (Bambara et al. 1997; Waga andStillman 1998). A myriad of lesions in DNA are causedby ubiquitous environmental and endogenous genotoxicagents. These lesions can interfere with normal DNAmetabolism including DNA replication, eventually resulting in mutations that lead to carcinogenesis and/orcell death. To maintain the integrity of the genetic material, cells possess multiple pathways including checkpoint mechanisms in various phases of cell cycle and repair mechanisms such as nucleotide excision and baseexcision repair pathways (Friedberg et al. 1995). However, not all lesions on the genome can be repaired efficiently by these processes in time for DNA replication,and some types of lesions are repaired very inefficiently.To prevent acute cell death through arrested DNA replication at unrepaired lesions, cells acquired a variety ofmechanisms to avoid interference of replication at the lesions so-called postreplication repair during evolution(Friedberg et al. 1995)...

Cell cycle arrest, apoptosis and p53 expression in nickel(II) acetate-treated Chinese hamster ovary cells.

Nickel(II) compounds are known human and animal carcinogens. In this study, the effects of nickel(II) acetate on cell cycle, apoptosis and p53 expression were investigated in order to unveil the elements of early cellular responses to the metal. Chinese hamster ovary (CHO) cells were grown for 72 h in Ham's F-12 medium containing 0, 40, 80, 160, 240, 320, 480 or 640 microM nickel(II) acetate. DNA fragmentation, representative of apoptosis, was examined by agarose gel electrophoresis. The distribution of cells among various phases of cell cycle was determined by DNA flow cytometry. Expression of p53 protein was measured by the Western blotting technique. DNA fragmentation was detectable in cells treated with > or = 160 microM nickel(II) and its intensity increased with increasing nickel(II) concentration. The proportion of cells at S phase declined in a nickel(II) concentration-dependent manner. The decline was accompanied by an increase of cell proportion in G2/M phase and the increase became statistically significant in cells exposed to at least 480 microM nickel(II). Expression of p53 protein was not different from that in the control among samples treated with < or = 480 microM nickel(II). However, an extra fraction that migrated close to the p53 protein fraction was detected in cells treated with 640 microM nickel(II). Our findings suggest that nickel(II) modulates cellular response through effectors involved in both G2/M arrest and apoptosis regulatory pathways. The proportion of cells arrested at G2/M phase or undergoing apoptosis depends directly on nickel(II) concentration. High concentration of nickel(II) appears to up-regulate protein(s) other than the common form of p53 protein.

Activin A-induced differentiation in K562 cells is associated with a transient hypophosphorylation of RB protein and the concomitant block of cell cycle at G1 phase.

The human erythroleukemic cell line, K562, can be induced to differentiate by the addition of activin A, a newly purified protein belonging to the TGF-beta 1 family. The present studies used flow cytometric cell cycle analysis, indirect immunofluorescence staining of the proliferating cell nuclear antigen (PCNA), and thymidine incorporation assay of cell proliferation to study the effects of activin A on the cell cycle during differentiation in K562 cells. Activin A-treated K562 cells were found to undergo a transient block in cell cycle, temporarily halting progression from G1 to S phase. The latter can be observed after approximately 24 hr of incubation with activin A and then disappears after this early stage of induction of differentiation. Cell cycle kinetics analysis using synchronized K562 cells also confirms that in the presence of activin A, K562 cells progress normally through various phases of cell cycle, except that there is prolongation of the G1 phase between 10 to 24 hr of culture. Furthermore, this transient arrest in G1 is correlated with dephosphorylation of a nucleoprotein, the RB gene product, which occurs within 9-24 hr of incubation with activin A; and phosphorylation of RB protein then develops afterward. In addition, these cell cycle-related events are observed to occur earlier than the accumulation of hemoglobins in K562 cells. It is concluded that transient dephosphorylation of RB protein and prolongation of G1 phase of cell cycle precede and accompany erythroid differentiation caused by activin A and chemical inducers, thus constituting part of the mechanism for induction of differentiation in the erythroleukemia cells.

Increase of UV-induced DNA repair synthesis during blast transformation of human lymphocytes

UV-induced DNA repair synthesis, measured as unscheduled DNA synthesis, was studied in human peripheral lymphocytes in various phases of the cell cycle. Mitogen transformation of the lymphocytes was effected with phytohemagglutinin (PHA), and the stage in the cell cycle was determined by measuring the Feulgen DNA content and the dry mass in individual cells by cytophotometry. The initial rate of repair was determined by autoradiography after UV-light irradiation (19.2 J/m 2) and incubation of the cells for 30 min with [ 3H]thymidine. When the cells progressed from the G0 to the G1 phase there was a 3-fold increase in the grain count. The correlation between the grain count and the dry mass indicated an increase in the initial rate of repair during the progression of cells from G0 to G2 phase. G2 cells were more heavily labelled than those in G1, but there did not seem to be any difference between these two phases as regards the relationship between grain count and DNA content. The results indicate that the initial rate of UV-induced DNA repair may differ in various phases of the lymphocyte cell cycle.

Radiosensitivity and recovery of mouse L cells during the cell cycle.

Mouse fibroblasts, subline L-929 F were synchronized by mitotic detachment. The synchronized cell cultures were irradiated with 200 kVp X-rays at different time after mitosis, and age response functions and dose effect curves were determined using the colony test. The cell age in the mitotic cycle was obtained from a computer analysis of flow cytometric DNA histograms. Both intrinsic radiosensitivity 1/D0 and extrapolation number n were found to vary during the cell cycle. The D0 has a maximum value of 176 +/- 1 rad in the middle of G1 phase and minimum of 71 +/- 1 rad at the S/G2 transition, while the extrapolation number is rather constant from the beginning of G1 phase (1.9 +/- 0.1) to the middle of S phase (2.3 +/- 0.1) and reaches a steep maximum of 9.3 +/- 1.1 at S/G2 transition. The values of n in the various phases of cell cycle are compared with the respective values of the recovery factor gamma determined after fractionated irradiation. --Cell survival after a single dose of 616 rad has minima for irradiation at G1/S transition and in early G2 phase; the survival in early G2 being about 40 times smaller than in early G1 phase. Implications for a cell cycle specific therapy are discussed.

Cytological evaluation of dose-rate effects of radiation on mutation frequency of silkworm gonia: II. Evidence of G 2 accumulation of primary spermatogonia after acute irradiation

The effects of acute irradiation on the kinetic indices of primary spermatogonia of the silkworm have been studied in an attempt to gain cytological insight into the mechanisms of dose-fractionation effects on the induction of mutation. 1. ( 1) Kinetic studies, with the use of colchicine treatment and the [ 3H]thymidine-labeling technique, on primary spermatogonial populations in unirradiated larvae showed that primary spermatogonia of this insect had a mean generation time of 34 h, of which the relative time occupied by the various phases of cell cycle was roughly as follows: G 1, ∽ 40%; S, ∽ 40%; G 2, ∽ 18%; and M, ∽ 2%. 2. ( 2) Cytokinetic studies on the effect of acute irradiation (1000 R) on the population of primary spermatogonia showed that during the first 24 h after irradiation the relative frequency of cells in different phases of the cell cycle changed noticeably. Thus, at 24 h as many as 58% of the total population was shown to be in the G 2 population in contrast to 18% of the unirradiated primary spermatogonial population. Furthermore, only 3%, instead of 40%, in the control, was in the S phase at this time. 3. ( 3) It is tentatively concluded that enhanced mutation frequency in dose-fractionation experiments was primarily due to an increase in the relative frequency of cells in the G 2 population. However, the possibility remains that accumulation of cells in late G 1 might also be responsible for the enhancement of mutation frequency by dose fractionations.