Cell-Type-Dependent Activity of the Ubiquitous Transcription Factor USF in Cellular Proliferation and Transcriptional Activation

Abstract

USF1 and USF2 are basic helix-loop-helix transcription factors implicated in the control of cellular proliferation. In HeLa cells, the USF proteins are transcriptionally active and their overexpression causes marked growth inhibition. In contrast, USF overexpression had essentially no effect on the proliferation of the Saos-2 osteosarcoma cell line. USF1 and USF2 also lacked transcriptional activity in Saos-2 cells when assayed by transient cotransfection with USF-dependent reporter genes. Yet, there was no difference in the expression, subcellular localization, or DNA-binding activity of the USF proteins in HeLa and Saos-2 cells. Furthermore, Gal4-USF1 and Gal4-USF2 fusion proteins activated transcription similarly in both cell lines. Mutational analysis and domain swapping experiments revealed that the small, highly conserved USF-specific region (USR) was responsible for the inactivity of USF in Saos-2 cells. In HeLa, the USR serves a dual function. It acts as an autonomous transcriptional activation domain at promoters containing an initiator element and also induces a conformational change that is required for USF activity at promoters lacking an initiator. Taken together, these results suggest a model in which the transcriptional activity of the USF proteins, and consequently their antiproliferative activity, is tightly controlled by interaction with a specialized coactivator that recognizes the conserved USR domain and, in contrast to USF, is not ubiquitous. The activity of USF is therefore context dependent, and evidence for USF DNA-binding activity in particular cells is insufficient to indicate USF function in transcriptional activation and growth control.

ACKNOWLEDGMENTS

We are grateful to Howard C. Towle for the USF2-VP16 construct, to Kuo Ooi for assisting in plasmid construction, and to Marilyn N. Szentirmay and Michael W. Dyke for critical reading of the manuscript.

This work was supported by grants G-1195 from the Robert A. Welch foundation, DMAD17-96-1-6221 from the Department of the Army, and CA79578 from the National Institutes of Health. Additional support was provided by institutional funds and by a postdoctoral fellowship from National Cancer Institute Training Grant CA09299 (T.L.). The confocal microscopy facility at the M.D. Anderson Cancer Center is supported by grant CA16672.