The Putative Oncoprotein Bcl-3 Induces Cyclin D1 To Stimulate G₁ Transition

Abstract

Bcl-3 is a distinctive member of the IκB family of NF-κB inhibitors because it can function to coactivate transcription. A potential involvement of Bcl-3 in oncogenesis is highlighted by the fact that it was cloned due to its location at a breakpoint junction in some cases of human B-cell chronic lymphocytic leukemia and that it is highly expressed in human breast tumor tissue. To analyze the effects of Bcl-3 dysregulation in breast epithelial cells, we created stable immortalized human breast epithelial cell lines either expressing Bcl-3 or carrying the corresponding vector control plasmid. Analysis of the Bcl-3-expressing cells suggests that these cells have a shortened G₁ phase of the cell cycle as well as a significant increase in hyperphosphorylation of the retinoblastoma protein. Additionally, the cyclin D1 gene was found to be highly expressed in these cells. Upon further analysis, Bcl-3, acting as a coactivator with NF-κB p52 homodimers, was demonstrated to directly activate the cyclin D1 promoter through an NF-κB binding site. Therefore, our results demonstrate that dysregulated expression of Bcl-3 potentiates the G₁ transition of the cell cycle by stimulating the transcription of the cyclin D1 gene in human breast epithelial cells.

ACKNOWLEDGMENTS

We are grateful to Carolyn Sartor for providing us with the H16N2 cells. We also thank Nancy Rice for kindly providing the p52 antibody and Timothy McKeithan for kindly providing the Bcl-3 antibody used in the EMSA reactions. The cyclin D1 promoter reporter constructs and cyclin D1 antisense expression vector were provided by Richard Pestell. We thank Denis Guttridge for providing reagents and experimental advice. We also thank Jayne Keifer, Denis Guttridge, and Raquel Sitcheran for a critical reading of the manuscript and the members of the Baldwin laboratory for helpful discussions.

This work was supported by grants from the American Cancer Society (PF-00–023-01-MGO) to S.D.W. and from the NIH (CA73756 and CA75080) and the Leukemia and Lymphoma Society to A.S.B.