RETINOIC ACID RESISTANCE OF THE VARIANT EMBRYONAL CARCINOMA CELL-LINE RAC65 IS CAUSED BY EXPRESSION OF A TRUNCATED RAR-ALPHA

P19 embryonal carcinoma (EC) cells differentiate when treated with retinoic acid (RA). The P19 EC-derived mutant cell line RAC65 is resistant to the differentiation-inducing activity of RA. We show that these cells express a truncated retinoic acid receptor alpha(mRAR-alpha-RAC65), probably due to the integration of a transposon-like element in the RAR-alpha-gene. This receptor lacks 71 C-terminal amino acids and terminates in the ligand-binding domain. In CAT assays in RAC65 cells, mRAR-alpha-RAC65 fails to trans-activate the RAR-beta promoter, which contains a RA-response element. In wild-type P19 EC cells mRAR-alpha-RAC65 functions as a dominant-negative repressor of RA-induced RAR-beta activation. Gel retardation assays demonstrate that MRAR-alpha-RAC65 is still able to bind to the RA-response element of the RAR-beta promoter, indicating that competition with functional RARs for the same binding site leads to the observed dominant-negative effect. In addition, in two RAC65 clones in which wild-type hRAR-alpha was stably transfected RA-sensitivity was restored and in one RAR-beta expression could be induced by RA. Taken together, these data show that the primary cause of RA-resistance of RAC65 cells is the expression of a defective RAR-alpha, which prevents the trans-activation of RA-responsive genes and results in a loss of the ability to differentiate.