Biomedical subjects

Josephine White

Publications and source records attributed to Josephine White.

5 recordsLinked to original sources

PubMed2005

Developmental activation of the Rb-E2F pathway and establishment of cell cycle-regulated cyclin-dependent kinase activity during embryonic stem cell differentiation.

To understand cell cycle control mechanisms in early development and how they change during differentiation, we used embryonic stem cells to model embryonic events. Our results demonstrate that as pluripotent cells differentiate, the length of G(1) phase increases substantially. At the molecular level, this is associated with a significant change in the size of active cyclin-dependent kinase (Cdk) complexes, the establishment of cell cycle-regulated Cdk2 activity and the activation of a functional Rb-E2F pathway. The switch from constitutive to cell cycle-dependent Cdk2 activity coincides with temporal changes in cyclin A2 and E1 protein levels during the cell cycle. Transcriptional mechanisms underpin the down-regulation of cyclin levels and the establishment of their periodicity during differentiation. As pluripotent cells differentiate and pRb/p107 kinase activities become cell cycle dependent, the E2F-pRb pathway is activated and imposes cell cycle-regulated transcriptional control on E2F target genes, such as cyclin E1. These results suggest the existence of a feedback loop where Cdk2 controls its own activity through regulation of cyclin E1 transcription. Changes in rates of cell division, cell cycle structure and the establishment of cell cycle-regulated Cdk2 activity can therefore be explained by activation of the E2F-pRb pathway.

Animals↗

PubMed2005

Cell cycle control of embryonic stem cells.

Embryonic stem cells have the capacity for unlimited proliferation while retaining their potential to differentiate into a wide variety of cell types. Murine, primate and human embryonic stem cells (ESCs) exhibit a very unusual cell cycle structure, characterized by a short G1 phase and a high proportion of cells in S-phase. In the case of mESCs, this is associated with a unique mechanism of cell cycle regulation, underpinned by the precocious activity of cyclin dependent protein kinase (Cdk) activities. As ES cells differentiate, their cell cycle structure changes dramatically so as to incorporate a significantly longer G1 phase and their mechanism of cell cycle regulation changes to that typically seen in other mammalian cells. The unique cell cycle structure and mechanism of cell cycle control indicates that the cell cycle machinery plays a role in establishment or maintenance of the stem cell state. This idea is supported by the frequent involvement of cell cycle regulatory molecules in cell immortalization.

Animals↗

PubMed2004

Follicular lymphoma international prognostic index.

The prognosis of follicular lymphomas (FL) is heterogeneous and numerous treatments may be proposed. A validated prognostic index (PI) would help in evaluating and choosing these treatments. Characteristics at diagnosis were collected from 4167 patients with FL diagnosed between 1985 and 1992. Univariate and multivariate analyses were used to propose a PI. This index was then tested on 919 patients. Five adverse prognostic factors were selected: age (> 60 years vs < or = 60 years), Ann Arbor stage (III-IV vs I-II), hemoglobin level (< 120 g/L vs > or = 120 g/L), number of nodal areas (> 4 vs < or = 4), and serum LDH level (above normal vs normal or below). Three risk groups were defined: low risk (0-1 adverse factor, 36% of patients), intermediate risk (2 factors, 37% of patients, hazard ratio [HR] of 2.3), and poor risk (> or = 3 adverse factors, 27% of patients, HR = 4.3). This Follicular Lymphoma International Prognostic Index (FLIPI) appeared more discriminant than the International Prognostic Index proposed for aggressive non-Hodgkin lymphomas. Results were very similar in the confirmation group. The FLIPI may be used for improving treatment choices, comparing clinical trials, and designing studies to evaluate new treatments.

Adult↗

PubMed2004

Cdk6-cyclin D3 activity in murine ES cells is resistant to inhibition by p16(INK4a).

Through a screen aimed at identifying genes that are specifically upregulated in embryomic stem (ES) cells but not primitive ectoderm, we identified cyclin D3. This was surprising since cyclin D activity is generally believed to be inactive in ES cells even though retinoblastoma tumor suppressor protein (pRb) accumulates in a predominantly hyperphosphorylated state. Cdk6 is the major catalytic partner for cyclin D3 in ES cells and exhibits robust pRb kinase activity that is downregulated during the early stages of ES embryoid body differentiation. To investigate the basis underlying the insensitivity of ES cells to ectopic p16 expression, we show that Cdk6-cyclin D3 complexes are not subject to inhibition by p16, similar to Cdk-viral cyclin complexes. These observations show that specificity exists between Cdk4/6-cyclin D complexes and their ability to be targeted by p16. Our data suggest that Cdk6-cyclin D3 activity in other cell types, including tumors, may also be refractory to p16-mediated growth inhibition and raises the possibility of additional specificity within the INK4 family.

Animals↗

PubMed2002

Pluripotent cell division cycles are driven by ectopic Cdk2, cyclin A/E and E2F activities.

Pluripotent cells of embryonic origin proliferate at unusually rapid rates and have a characteristic cell cycle structure with truncated gap phases. To define the molecular basis for this we have characterized the cell cycle control of murine embryonic stem cells and early primitive ectoderm-like cells. These cells display precocious Cdk2, cyclin A and cyclin E kinase activities that are conspicuously cell cycle independent. Suppression of Cdk2 activity significantly decreased cycling times of pluripotent cells, indicating it to be rate-limiting for rapid cell division, although this had no impact on cell cycle structure and the establishment of extended gap phases. Cdc2-cyclin B was the only Cdk activity that was identified to be cell cycle regulated in pluripotent cells. Cell cycle regulation of cyclin B levels and Y(15) regulation of Cdc2 contribute to the temporal changes in Cdc2-cyclin B activity. E2F target genes are constitutively active throughout the cell cycle, reflecting the low activity of pocket proteins such as p107 and pRb and constitutive activity of pRb-kinases. These results show that rapid cell division cycles in primitive cells of embryonic origin are driven by extreme levels of Cdk activity that lack normal cell cycle periodicity.

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