FIBROBLAST GROWTH-FACTOR RECEPTORS (FGFRS) LOCALIZE IN DIFFERENT CELLULAR COMPARTMENTS - A SPLICE VARIANT OF FGFR-3 LOCALIZES TO THE NUCLEUS

We have raised specific antibodies to the second immunoglobulin-like d omain of fibroblast growth factor receptors (FGFRs) and used these to investigate the expression and subcellular localization of FGFR-1, -2, -3, and -4 in breast epithelial cells. All four receptors classes cou ld be detected in breast cell lines; however, FGFR-4 and FGFR-2 appear ed to be expressed at a higher level in breast cancer cell lines than in normal epithelial cells. Surprisingly, FGFR-3 localized in the cell nucleus by immunofluorescence. A second antibody to a separate epitop e confirmed this finding and showed that the form of FGFR-3 present mu st contain an intact kinase domain as well as the growth factor bindin g domain. Western analysis of fractionated cells revealed the presence of two forms of FGFR-3 of 135 and 110 kDa. The 110-kDa form was predo minantly found in the nucleus, whereas the 135 kDa form was sometimes found in the nucleus. RT-PCR analysis of FGFR-3 mRNA showed the presen ce of a splice variant in which exons 7 and 8 are deleted. This result s in the translation of FGFR-3 missing the transmembrane domain but wi th an intact kinase domain, which could be a soluble, intracellular re ceptor. Transfection experiments showed that FGFR-3 containing this de letion and no signal peptide gave an identical nuclear staining patter n to that seen in breast epithelial cells. We conclude that two forms of FGFR-3 are present in breast epithelial cells; a full-length 135-kD a receptor, which has a conventional membrane localization, and a nove l soluble form of 110 kDa.