Synergistic action of GA-Binding protein and glucocorticoid receptor in transcription from the mouse mammary tumor virus promoter

B lymphocytes are among the first cells to be infected by mouse mammary tum or virus (MMTV) and they play a crucial role in its life cycle. To study tr anscriptional regulation of MMTV in B cells, we have analyzed two areas of the long terminal repeat (LTR) next to the glucocorticoid receptor binding site, fp1 (at position -139 to -146 from the cap site) and fp2 (at -157 to -164). Both showed B-cell-specific protection in DNase I in vitro footprint ing assays and contain binding sites for Ets transcription factors, a large family of proteins involved in cell proliferation and differentiation and oncogenic transformation. In gel retardation assays, fp1 and fp2 bound the heterodimeric Ets factor GA-binding protein (GABP) present in B-cell nuclea r extracts, which was identified by various criteria: formation of dimers a nd tetramers, sensitivity to pro-oxidant conditions, inhibition of binding by specific antisera, and comigration of complexes with those formed by rec ombinant GABP, Mutations which prevented complex formation in vitro abolish ed gIucocorticoid-stimulated transcription from an MMTV LTR linked to a rep orter gene in transiently transfected B-cell lines, whereas they did not af fect the basal level, Exogenously expressed GABP resulted in an increased l evel of hormone response of the LTR reporter plasmid and produced a synergi stic effect with the coexpressed glucocorticoid receptor, indicating cooper ation between the two, This is the first example of GABP cooperation with a steroid receptor, providing the opportunity for studying the integration o f their intracellular signaling pathways.