PARTICIPATION OF ETS TRANSCRIPTION FACTORS IN THE GLUCOCORTICOID RESPONSE OF THE RAT TYROSINE AMINOTRANSFERASE GENE

We have previously shown that two remote glucocorticoid-responsive uni ts (GRUs) of the rat tyrosine aminotransferase (TAT) gene contain mult iple binding sites for several transcription factor families, includin g the glucocorticoid receptor (GR). We report here the identification of two novel binding sites for members of the Ets family of transcript ion factors in one of these GRUs. One of these binding sites overlaps the major GR-binding site (GRBS), whereas the other is located in its vicinity. Inactivation of the latter binding site leads to a twofold r eduction of the glucocorticoid response, whereas inactivation of the s ite overlapping the GRBS has no detectable effect. In vivo footprintin g analysis reveals that the active site is occupied in a glucocorticoi d-independent manner, in a TAT-expressing cell line, even though it is located at a position where there is a glucocorticoid-dependent alter ation of the nucleosomal structure. This same site is not occupied in a cell line that does not express TAT but expresses Ets-related DNA-bi nding activities, suggesting the existence of an inhibitory effect of chromatin structure at a hierarchical level above the nucleosome. The inactive Ets-binding site that overlaps the GRBS is not occupied even in TAT-expressing cells. However, this same overlapping site can confe r Ets dependent stimulation of both basal and glucocorticoid-induced l evels when it is isolated from the GRU and duplicated. Ets-1 expressio n in COS cells mimics the activity of the Ets-related activities prese nt in hepatoma cells. These Ets-binding sites could participate in the integration of the glucocorticoid response of the TAT gene with signa l transduction pathways triggered by other nonsteroidal extracellular stimuli.