TY - JOUR
T1 - Reduction of Cdc25A contributes to cyclin E1-Cdk2 inhibition at senescence in human mammary epithelial cells
AU - Sandhu, Charanjit
AU - Donovan, Jeffrey
AU - Bhattacharya, Nandita
AU - Stampfer, Martha
AU - Worland, Peter
AU - Slingerland, Joyce
N1 - Funding Information:
We thank M Viscardi and S Ku for excellent secretarial support and M Loda for communicating data prior to its publication. We thank Vivi Ann Florenes for technical assistance with Northern blots. We acknowledge the expert assistance of registered cytogenetic technologist, Christine Ruely, in the preparation and scoring of ploidy in metaphase cells. We thank David Epstein at Mitotix for assistance with protocols for work with recombinant Cdc25A. JM Slingerland is supported by Cancer Care Ontario. This work was funded by grants from the Canadian Breast Cancer Research Initiative to JM Sling-erland and by NIH grant CA-24844, and by the oce of Energy Research, Oce of Health and Environmental Research, US Department of Energy, under contract DE-AC03-76SF00098 to MR Stampfer.
PY - 2000/11/9
Y1 - 2000/11/9
N2 - Replicative senescence may be an important tumor suppressive mechanism for human cells. We investigated the mechanism of cell cycle arrest at senescence in human mammary epithelial cells (HMECs) that have undergone a period of 'self-selection', and as a consequence exhibit diminished p16(INK4A) levels. As HMECs approached senescence, the proportion of cells with a 2N DNA content increased and that in S phase decreased progressively. Cyclin D1-cdk4, cyclin E-cdk2 and cyclin A-cdk2 activities were not abruptly inhibited, but rather diminished steadily with increasing population age. In contrast to observations in fibroblast, p21(Cip1) was not increased at senescence in HMECs. There was no increase in p27(Kip1) levels nor in KIP association with targets cdks. While p15(INK4B) and its binding to both cdk4 and cdk6 increased with increasing passage, some cyclin D1-bound cdk4 and cdk6 persisted in senescent cells, whose inhibition could not be attributed to p15(INK4B). The inhibition of cyclin E-cdk2 in senescent HMECs was accompanied by increased inhibitory phosphorylation of cdk2, in association with a progressive loss of Cdc25A. Recombinant Cdc25A strongly reactivated cyclin E-cdk2 from senescent HMECs suggesting that reduction of Cdc25A contributes to cyclin E-cdk2 inhibition and G1 arrest at senescence. Although ectopic expression of Cdc25A failed to extend the lifespan of HMECs, the exogenous Cdc25A appeared to lack activity in these cells, since it neither shortened the G1-to-S phase interval nor activated cyclin E-cdk2. In contrast, in the breast cancer-derived MCF-7 line, Cdc25A overexpression increased both cyclin E-cdk2 activity and the S phase fraction. Thus, mechanisms leading to HMEC immortalization may involve not only the re-induction of Cdc25A expression, but also activation of this phosphatase.
AB - Replicative senescence may be an important tumor suppressive mechanism for human cells. We investigated the mechanism of cell cycle arrest at senescence in human mammary epithelial cells (HMECs) that have undergone a period of 'self-selection', and as a consequence exhibit diminished p16(INK4A) levels. As HMECs approached senescence, the proportion of cells with a 2N DNA content increased and that in S phase decreased progressively. Cyclin D1-cdk4, cyclin E-cdk2 and cyclin A-cdk2 activities were not abruptly inhibited, but rather diminished steadily with increasing population age. In contrast to observations in fibroblast, p21(Cip1) was not increased at senescence in HMECs. There was no increase in p27(Kip1) levels nor in KIP association with targets cdks. While p15(INK4B) and its binding to both cdk4 and cdk6 increased with increasing passage, some cyclin D1-bound cdk4 and cdk6 persisted in senescent cells, whose inhibition could not be attributed to p15(INK4B). The inhibition of cyclin E-cdk2 in senescent HMECs was accompanied by increased inhibitory phosphorylation of cdk2, in association with a progressive loss of Cdc25A. Recombinant Cdc25A strongly reactivated cyclin E-cdk2 from senescent HMECs suggesting that reduction of Cdc25A contributes to cyclin E-cdk2 inhibition and G1 arrest at senescence. Although ectopic expression of Cdc25A failed to extend the lifespan of HMECs, the exogenous Cdc25A appeared to lack activity in these cells, since it neither shortened the G1-to-S phase interval nor activated cyclin E-cdk2. In contrast, in the breast cancer-derived MCF-7 line, Cdc25A overexpression increased both cyclin E-cdk2 activity and the S phase fraction. Thus, mechanisms leading to HMEC immortalization may involve not only the re-induction of Cdc25A expression, but also activation of this phosphatase.
KW - Cdc25A
KW - Cell cycle
KW - Cyclin E-cdk2
KW - HMEC
KW - P15(INK4B)
KW - Senescence
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U2 - 10.1038/sj.onc.1203908
DO - 10.1038/sj.onc.1203908
M3 - Article
C2 - 11103932
AN - SCOPUS:0034626721
VL - 19
SP - 5314
EP - 5323
JO - Oncogene
JF - Oncogene
SN - 0950-9232
IS - 47
ER -