CELL-SPECIFIC INHIBITION OF RETINOIC ACID RECEPTOR-ALPHA SILENCING BYTHE AF2 TAU-C ACTIVATION DOMAIN CAN BE OVERCOME BY THE COREPRESSOR SMRT, BUT NOT BY N-COR/

The human retinoic acid receptor alpha (hRAR alpha) exhibits cell-spec ific transcriptional activity. Previously, it was shown that in the ab sence of hormone the wild-type receptor is a transcriptional silencer in L cells, whereas it lacks silencing function and is a weak activato r in CV1 cells. Addition of hormone leads to a further increase in tra nsactivation in CV1 cells. Thus, the retinoic acid response mediated b y RAR alpha is weak in these cells. It was shown that the CV1-specific effect is due to the receptor C terminus. We show, that the failure o f silencing by RAR is not due to a general lack of corepressors in CV1 cells, since the silencing domain of RAR is functionally active and e xhibits active repression in these cells. Furthermore, we show that th e conserved AF2/tau c activation function of RAR is responsible for th e cell-specific inhibition of silencing. Thereby, the CV1 cell specifi city was abolished by replacing AF2/tau c of RAR with the correspondin g sequence of the thyroid hormone receptor. Thus, we find a new role o f the C-terminal conserved activation function AF2/tau c in that, spec ifically, the RAR AF2/tau c-sequence is able to prevent silencing of R AR in a cell-specific manner. In addition, we show that the inhibitory effect of AF2/tau c in CV1 cells can be overcome by expression of the corepressor SMRT (silencing mediator of retinoic acid and thyroid hor mone receptor), but not by that of N-CoR (nuclear receptor corepressor ). The expression of these two corepressors, however, had no measurabl e effect on RAR-mediated silencing in L cells. Thus, the expression of a corepressor can lead to a dramatic increase of hormonal response in a cell-specific manner.