Biomedical subjects

A Bybee

Publications and source records attributed to A Bybee.

10 recordsLinked to original sources

PubMed1992

The synthesis of p58cyclin A and the phosphorylation of p34cdc2 are inhibited in human lymphoid cells arrested in G1 by alpha-interferon.

Daudi cells arrest in G1 in the presence of alpha-interferon. Such cells have little p58cyclin A, probably due to inhibition of p58cyclin A synthesis. The phosphorylation-associated migration shift of p34cdc2 is not seen in alpha-interferon-arrested cells. Cells arrested in late G1 by aphidicolin have abundant p58cyclin A and phosphorylated p34cdc2. Cell sorting showed that p58cyclin A increases in proliferating cells in late G1 and coincides with phosphorylation of p34cdc2.

Aphidicolin↗

PubMed1992

The retinoblastoma protein is partially phosphorylated during early G1 in cycling cells but not in G1 cells arrested with alpha-interferon.

The retinoblastoma protein (pRB) is thought to act as a tumour suppressor which is inactivated by phosphorylation. In quiescent (G0) cells pRB exists in a hypophosphorylated form (pRB110), but proliferating cells in G1 contain a significant proportion of phosphorylated pRB (pRB112-114). Studies of synchronized or elutriated cells have suggested that the phosphorylated forms of pRB disappear as cells pass from G2/M to G0/G1 and that pRB is phosphorylated again to pRB114 at the G1/S border. In this study we used two-parameter flow cytometry and cell sorting to isolate cycling cells in early and late G1 (G1A and G1B), and we show that partially phosphorylated pRB is present in cycling human lymphoid cells even in G1A. These G1A cells contain intermediate forms of pRB which become further phosphorylated to pRB112-114 as cells pass into G1B. Therefore pRB is at least partially phosphorylated from early G1 onwards. Cell cycle arrest by alpha-interferon (alpha-IFN) results in an accumulation of cells in both G1A and G1B, and these cells contain mainly pRB110. Since pRB110 is thought to prevent cell proliferation, the cytostatic effect of alpha-IFN may therefore occur by preventing the initial phosphorylation of pRB during or prior to G1A.

Cell Cycle↗

PubMed1991

Interferon-alpha treatment of Daudi cells down-regulates the octamer binding transcription/DNA replication factors Oct-1 and Oct-2.

Treatment of Daudi cells with alpha-interferon (alpha-IFN) results in a considerable decrease in the levels of the octamer-binding DNA replication/transcription factors Oct-1 and Oct-2 and specifically inhibits gene expression by octamer-containing promoters. The inhibitory effect on octamer-binding proteins also occurs after culturing cells with phorbol 12-myristate 13-acetate but it does not occur following alpha-IFN treatment of an alpha-IFN-resistant variant of the Daudi cell line or of HeLa cells. We discuss the potential role of the decreased levels of octamer-binding proteins in the inhibition of cell proliferation.

Base Sequence↗

PubMed1991

Cell cycle regulation.

Over the past few years there has been a resurgance of interest in the cell cycle. The excitement has mainly been due to the fact that researchers all over the world who had been working on seemingly different processes in yeast, fruit flies, frogs and man have found that many of the processes and individual proteins have been highly conserved. In essence, what regulates the cell cycle in yeast also works in man. This review is biased towards cell cycle regulation in mammalian cells but we have also covered what is known about similar mechanisms in other organisms to provide a broader perspective to the field. We outline what is currently known about the cell cycle and the key points at which cell proliferation is controlled. We summarise recent work on cell cycle control genes and antioncogenes and the post-transcriptional regulation of the proteins for which they code. Finally we cover the relationship between the cell cycle and differentiation.

Cell Cycle↗

PubMed1991

Recovery of hyperplastic responsiveness in rat liver after dosing with the peroxisome proliferator methylclofenapate.

Methylclofenapate (MCP) was administered daily by gavage (25 mg/kg) for 7 days to groups of adult male rats. Dosing was interrupted for 28, 35, 56, 70 or 84 days and then resumed (25 mg/kg by gavage at 0 and 24 h). During the second period of dosing animals were killed in groups of three at 6, 12, 18, 24, 30, 36, 42 and 48 h after the resumption of dosing. Hepatocytes in S-phase, labelled with bromodeoxy-uridine, were analysed by flow cytometry, cell sorting and microscopy. It was observed that total S-phase activity was just significantly elevated (approximately 20% of maximum) over corn oil controls after an interval of 28 days between initial and subsequent dosing periods. After an interval of 35 days total S-phase activity was approximately 65% of maximum, and full hyperplastic responsiveness, equal to that observed in naive animals given MCP, was detected after interruptions in dosing of 56, 70 and 84 days. The recovery of S-phase responsiveness during the interruptions in dosing was accompanied by an increase in the proportion of 2 X 2N hepatocytes from approximately 10% in animals dosed continuously with MCP, to approximately 11.4% after 28 days interruption, 17% after 35 days and control levels (approximately 20%) after 56, 70 and 84 days. Irrespective of the magnitude of the hyperplasia elicited by the second period of dosing with MCP, the proportion of 2 X 2N cells was reduced to the same levels as those observed in animals dosed continuously with MCP (approximately 10%). Very low S-phase activity (0.05%) was observed in animals dosed continuously with MCP, this level of activity being similar to that in animals given corn oil continuously.

Animals↗

PubMed1991

The phosphorylation state of the retinoblastoma (RB) protein in G0/G1 is dependent on growth status.

The product of the retinoblastoma gene (RB) is a nuclear phosphoprotein which is thought to regulate the proliferation of cells. Its phosphorylation state changes with passage through the cell cycle and it has been proposed that RB protein in its hypo-phosphorylated form prevents cells proliferating. We have investigated the phosphorylation state of the RB protein in an actively-dividing human B-lymphoblastoid cell line and after cell cycle arrest caused by alpha-Interferon (alpha-IFN). We show that the phosphorylation state of the RB protein in cells with 2N DNA content depends on whether the cells are actively cycling. Our data is compatible with the proposal that dephosphorylation of the RB protein allows cells to enter a quiescent state. This study sheds light on the molecular mechanisms which may mediate the cytostatic effects of alpha-IFN.

Cell Line↗

PubMed1990

Mitosis and histopathology in rat liver during methylclofenapate-induced hyperplasia.

Liver hyperplasia was induced in rats by daily administration of methylclofenapate (25 mg/kg by gavage). An increase in the incidence of colchicine-arrested metaphases was observed with peaks occurring at 40 h (1.3%), 64 h (6.4%) and 84 h (6.8%) after the start of treatment. This response contrasted with the much larger (21.3%) peak in arrested metaphases at 36 h after partial hepatectomy, but was still unexpectedly large in comparison with the S-phase response to methylclofenapate reported in a previous study. Progressive hypertrophic histopathological changes were apparent during the whole course of treatment.

Animals↗

PubMed1990

Studies on the hyperplastic responsiveness of binucleated rat hepatocytes.

Rats were dosed by gavage for 28 days with 25 mg/kg of 3'methyl-4-dimethylaminoazobenzene (3'M) followed by 4 days dosing with 0.5 ml/100 g corn oil. Livers from animals were taken at intervals during the 4 days of corn oil dosing and examined for S-phase activity and nuclearity. The treatment regime caused an increase in the proportion of diploid (2N) cells in the hepatocyte population, a decrease in the proportion of binucleated (2 x 2N) cells of approximately 50%, and induced an increase in cell replication. A second group of rats was dosed for 28 days with 0.5 ml/100 g corn oil followed by 4 days administration of 25 mg/kg methylclofenapate (MCP). Analysis of hepatocytes taken during the MCP treatment revealed that there was acute hyperplasia, involving mainly the 2 x 2N hepatocytes, resulting in a reduction of approximately 50% of the 2 x 2N cells and an accompanying increase in the proportion of 4N cells. When a third group was given 3'M for 28 days followed by 4 days administration of MCP, there was, at the outset of MCP dosing, a hepatocyte population typical of 3'M dosed animals, with an elevated proportion of 2N cells and a 2 x 2N fraction that was reduced to approximately 50% of control levels. During the 4 days of MCP dosing there was a wave of hyperplasia involving S-phase activity in the remaining 2 x 2N hepatocytes. The proportion of 2 x 2N cells decreased further during this period and there was a concomitant increase in 4N cells. The 2N fraction, already elevated, was not further affected. These results indicate that both genotoxic and non-genotoxic hepatocarcinogens induce acute changes in the rat hepatocyte population that involve 2 x 2N cells, and that the effects appear to involve separate sub-populations of 2 x 2N hepatocytes.

Animals↗

PubMed1989

Rapid proliferative response of rat thyroid gland to a single injection of TSH in vivo.

Mitotic and labelling indices were studied in the thyroid follicular cells of the male rat within the first hour after a single injection of TSH or TSH vehicle, using the metaphase arrest agent vincristine sulphate. There was a significant increase in metaphase index over control values 5, 15, 30 and 60 min after injection of TSH. There was no significant change in the tritiated thymidine labelling index in TSH-treated rats in comparison with vehicle-injected controls. None of the metaphase figures was labelled, showing that G2 in this tissue is longer than 2 h.

Animals↗

PubMed1988

The short-term effects of a single injection of isoproterenol on proliferation in the submandibular gland, parotid gland and oesophagus in vivo.

Mitotic and labelling indices were studied in the submandibular, parotid and oesophageal cells of male mice within the first 6 hr (but particularly within the 1st hr) of a single injection of isoproterenol or saline, using the metaphase arrest agent (vincristine) which was previously tested for efficacy in submandibular gland. There was a significant increase in the metaphase index of the salivary glands over control values 5, 15, 30, 45 and 60 min after isoproterenol. In contrast, there were no significant changes in the metaphase index of basal cells of the oesophagus. There was no significant change in the labelling index in isoproterenol-treated mice in comparison with saline-injected control animals. Possible explanations for the rapid mitotic response in murine salivary glands are considered; a rapid efflux from G2 into mitosis is thought to be the most likely.

Animals↗