TRANSCRIPTIONAL REGULATION OF RETINOIC ACID RECEPTOR-BETA IN RETINOIC ACID-SENSITIVE AND ACID-RESISTANT P19-EMBRYOCARCINOMA CELLS

As in other embryocarcinoma (EC) cell lines retinoic acid (RA) rapidly induces expression of the nuclear retinoic acid receptor (RAR) beta in murine P19 EC cells, while RAR-alpha is expressed constitutively. In the RA-resistant P19 EC-derived RAC65 cells, however, there is no such induction and an aberrant (smaller) RAR-alpha transcript is expressed. RAR-gamma-1 is expressed at low levels in both cell lines. To study the regulation of the RAR-beta gene and the possible involvement of RAR-alpha protein in transcriptional activation of the RAR-beta gene we transfected these cells with a construct containing a 1.6 kb promoter fragment of the human RAR-beta gene fused to the CAT gene. Upon transient assays in P19 EC cells CAT activity is enhanced rapidly by RA, to more than 100-fold in a concentration-dependent fashion. On the contrary no activity can be observed in the RA-resistant RAC65 cell; however, co-transfection of hRAR-alpha, hRAR-beta or hRAR-gamma-1 restores the RA-dependent induction of CAT activity. These results clearly show that RAR-alpha and RAR-gamma-1 can transactivate the RAR-beta gene; that RAR-beta can stimulate its own expression and that resistance to RA in RAC65 cells is probably due to the altered RAR-alpha transcript present in these cells.