The p110 Isoform of the CDP/Cux Transcription Factor Accelerates Entry into S Phase

Abstract

The CDP/Cux transcription factor was previously found to acquire distinct DNA binding and transcriptional properties following a proteolytic processing event that takes place at the G₁/S transition of the cell cycle. In the present study, we have investigated the role of the CDP/Cux processed isoform, p110, in cell cycle progression. Populations of cells stably expressing p110 CDP/Cux displayed a faster division rate and reached higher saturation density than control cells carrying the empty vector. p110 CDP/Cux cells reached the next S phase faster than control cells under various experimental conditions: following cell synchronization in G₀ by growth factor deprivation, synchronization in S phase by double thymidine block treatment, or enrichment in G₂ by centrifugal elutriation. In each case, duration of the G₁ phase was shortened by 2 to 4 h. Gene inactivation confirmed the role of CDP/Cux as an accelerator of cell cycle progression, since mouse embryo fibroblasts obtained from Cutl1^z/z mutant mice displayed a longer G₁ phase and proliferated more slowly than their wild-type counterparts. The delay to enter S phase persisted following immortalization by the 3T3 protocol and transformation with H-Ras^V12. Moreover, CDP/Cux inactivation hindered both the formation of foci on a monolayer and tumor growth in mice. At the molecular level, expression of both cyclin E2 and A2 was increased in the presence of p110 CDP/Cux and decreased in its absence. Overall, these results establish that p110 CDP/Cux functions as a cell cycle regulator that accelerates entry into S phase.

We thank Meinrad Busslinger for the kind gift of the Cutl1^+/z mice and Scott Lowe for the pBabe-H-Ras^V12 plasmid.

A.N. is the recipient of a scholarship from the Fonds de la Recherche en Santé du Québec. L.S. is the recipient of studentships from the Royal Victoria Hospital Research Institute. Exchanges between the Bertoglio and Nepveu laboratories were made possible through a grant from FRSQ and INSERM. This research was supported by grant 014288 from the National Cancer Institute of Canada to A.N.