MULTIPLE ISOFORMS OF THE MOUSE RETINOIC ACID RECEPTOR ALPHA ARE GENERATED BY ALTERNATIVE SPLICING AND DIFFERENTIAL INDUCTION BY RETINOIC ACID

Together with the previously described mouse retinoic acid receptor alpha-1 (mRAR-alpha-1, formerly mRAR-alpha-0), we have isolated and characterized here a total of seven mRAR-alpha cDNA isoforms (mRAR-alpha-1 to alpha-7). These isoforms are generated from mRAR-alpha primary transcript(s) of a single gene by alternative splicing of at least eight different exons with the exon which encodes the amino acid sequence of their common B region. All of these isoforms differ in their 5'-untranslated regions (5'-UTRs) and, in the case of mRAR-alpha-1 and alpha-2, also in the sequences encoding the N-terminal A region which is known to be important for differential trans-activation by other members of the nuclear receptor superfamily. In addition, the sequences encoding the open reading frames (ORFs) of mRAR-alpha-3 and alpha-4 cDNA isoforms remain open to their very 5' ends, which suggests that these two isoforms may also encode RAR-alpha-s with unique A region amino acid sequences. The two predominant isoforms, mRAR-alpha-1 and alpha-2, were found to be differentially expressed in mouse adult and fetal tissues, as well as in P19 and F9 embryonal carcinoma (EC) cell lines. Interestingly, the expression of mRAR-alpha-2, in contrast to that of the mRAR-alpha-1 isoform, was induced by retinoic acid (RA) in EC cells, thus suggesting the presence of two promoters in the 5' region of the mRAR-alpha gene, which differ in their response to RA. The conservation between mouse and human RAR-alpha-1 and alpha-2 cDNA isoform sequences, as seen by cross-hybridization in Southern blots or by DNA sequence analysis, together with their differential patterns of expression, strongly suggests that they perform specific functions during embryogenesis and in the adult.