AN INTRONIC SEQUENCE ELEMENT MEDIATES BOTH ACTIVATION AND REPRESSION OF RAT FIBROBLAST GROWTH-FACTOR RECEPTOR-2 PRE-MESSENGER-RNA SPLICING

Alternative splicing of fibroblast growth factor receptor 2 (FGF-R2) i s an example of highly regulated alternative splicing in which exons I IIb and IIIc are utilized in a mutually exclusive manner in different cell types. The importance of this splicing choice is highlighted by s tudies which indicate that deregulation of the FGF-R2 splicing is asso ciated with progression of prostate cancer. Loss of expression of a II Ib exon-containing isoform of FGF-R2 [FGF-R2 (IIIb)] accompanies the t ransition of a well-differentiated, androgen dependent rat prostate ca ncer cell line, DT3, to the more aggressive, androgen-independent AT3 cell line. We have used transfection of rat FGF-R2 minigenes into DT3 and AT3 cancer cell lines to study the mechanisms that control alterna tive splicing of rat FGF-R2. Our results support a model in which an i mportant cis-acting element located in the intron between these altern ative exons mediates activation of splicing using the upstream IIIb ex on and repression of the downstream IIIc exon in DT3 cells. This eleme nt consists of 57 nucleotides (nt) beginning 917 nt downstream of the IIIb exon. Analysis of mutants further demonstrates that an 18-nt ''co re sequence'' within this element is most crucial for its function. Ba sed on our observations, we have termed this sequence element ISAR (fo r intronic splicing activator and repressor), and we suggest that fact ors which bind this sequence are required for maintenance of expressio n of the FGF-R2 (IIIb) isoform.