The Gfi-1B Proto-Oncoprotein Represses p21^WAF1 and Inhibits Myeloid Cell Differentiation

ABSTRACT

Gfi-1 is a cellular proto-oncogene that was identified as a target of provirus integration in T-cell lymphoma lines selected for interleukin-2 (IL-2) independence in culture and in primary retrovirus-induced lymphomas. Gfi-1 encodes a zinc finger protein that functions as a transcriptional repressor. Here we show that Gfi-1B, a Gfi-1 related gene expressed in bone marrow and spleen, also encodes a transcriptional repressor. IL-6-induced G₁ arrest and differentiation of the myelomonocytic cell line M1 were linked to the downregulation of Gfi-1B and the parallel induction of the cyclin-dependent kinase inhibitor p21^WAF1. Experiments addressing the potential mechanism of the apparent coordinate regulation of these genes revealed that Gfi-1B represses p21^WAF1 directly by binding to a high-affinity site at −1518 to −1530 in the p21^WAF1 promoter. Forced expression ofGfi-1B, but not of Gfi-1B deletion mutants lacking the repressor domain, blocked the IL-6-mediated induction of p21^WAF1 and inhibited G₁ arrest and differentiation. We conclude that Gfi-1B is a direct repressor of the p21^WAF1 promoter, the first such repressor identified to date, and that sustained expression of Gfi-1B blocks IL-6-induced G₁ arrest and differentiation of M1 cells perhaps because it prevents p21^WAF1 induction by IL-6.

ACKNOWLEDGMENTS

We thank H. Mikkers and A. Berns for communicating their data prior to publication. We also thank M. Flubacher (Fox Chase Cancer Center [FCCC]) for providing Northern blots for the initial examination of Gfi-1B expression, K. Stamatakis for help with the generation of the Gfi-1BΔSNAG and Gfi-1BΔNter constructs, J. Sherley and Y. Liu (FCCC) for the 10-1 cell line, B. Calabretta and T. Skorski (Kimmel Cancer Center) for the c-myc and c-myb probes, and P. Bateman for secretarial assistance.

This work was supported by the Public Health Service grant RO1 CA-56110. Additional support was provided by Public Health Service grant CA-06927 and by an appropriation from the Commonwealth of Pennsylvania to the Fox Chase Cancer Center. W.S.E.-D. is an Assistant Investigator of the Howard Hughes Medical Institute. T.-Y.Y. was supported by an L. Greenwald Postdoctoral Fellowship (FCCC). B.T. was a graduate student in the Cell and Molecular Biology program of the University of Pennsylvania School of Medicine.