PROMOTER ARCHITECTURE, COFACTORS, AND ORPHAN RECEPTORS CONTRIBUTE TO CELL-SPECIFIC ACTIVATION OF THE RETINOIC ACID RECEPTOR BETA-2 PROMOTER

Expression of retinoic acid receptor beta (RAR beta) is spatially and temporally restricted during embryonal development. Also during retino ic acid (RA)-dependent embryonal carcinoma (EC) cell differentiation, RAR(beta) expression is initially up-regulated, while in later phases of differentiation expression is down-regulated, by an unknown mechani sm. To gain insight into the regulation of RAR beta, we studied the ac tivity of the RAR beta 2 promoter and mutants thereof in various cell lines. While the RAR beta 2 promoter is activated by RA in a limited n umber of cell lines, synthetic RA-responsive reporters are activated i n most cell types. We show that the expression levels of proteins that bind to the beta-retinoic acid response element (RAR/retinoid X recep tors and orphan receptors) and also the differential expression of a n umber of coactivators modulate the RA response on both natural and syn thetic reporters, We further show that cell type-specific activation o f the RAR beta 2 promoter is dependent on the promoter architecture in cluding the spacing between retinoic acid response element and TATA-bo x and initiator sequence (beta INR). Mutation within these regions cau sed a decrease in the activity of this promoter in responsive EC cells , while an increase in activity in non-EC cell lines was observed, Cel l-specific complexes were formed on the beta INR, suggesting that the beta INR contributes to cell-specific activation of the promoter. On t his basis we propose that promoter context-dependent and more general RA response-determining mechanisms contribute to cell-specific RA-depe ndent activation of transcription.