ALTERNATIVE SPLICING OF THE D-2 DOPAMINE-RECEPTOR MESSENGER-RIBONUCLEIC-ACID IS MODULATED BY ACTIVATED SEX STEROID-RECEPTORS IN THE MMQ PROLACTIN CELL-LINE

The two isoforms of the D-2 dopamine receptor are generated by alterna tive splicing of the exon 6 of the premessenger RNA (pre-mRNA), changi ng the length of the third cytoplasmic loop involved in the coupling t o G proteins. In the MMQ PRL cell line, sex steroid hormones modulated the proportion of the two D-2 receptor isoforms. Under controlled cul ture conditions, 17 beta-estradiol (E-2) strongly favored the producti on of the long isoform of D-2 mRNA over the short one, whereas both is oforms were equally abundant when culture medium was hormone depleted. In the presence of progesterone (P), E-2 action was inhibited, and eq ual amounts of each D-2 receptor isoform were produced in the cells. H ormone treatments never modified either the total amount of D-2 recept or mRNA and D-2 receptor binding sites or D-2 receptor-mediated inhibi tion of adenylyl cyclase. Specific antagonists demonstrated that the a ctivity of each hormone depended on their nuclear receptors. Inhibitor s of gene transcription or translation also showed that their activity required protein synthesis. The expression of the short D-2 receptor isoform was never prominent, even at the single cell level. Analysis o f the intron sequence flanking alternative exon 6 showed that only the upstream intron presented two sequence tracts known to be targets for splicing factors. Taken together, these results provide converging ev idence for a physiologically relevant mechanism by which sex steroid r eceptors could regulate the expression of a splicing factor favoring t he production of the long dopamine D-2 receptor isoform.