The nuclear trafficking of extracellular fibroblast growth factor (FGF)-1 correlates with the perinuclear association of the FGF receptor-1α isoforms but not the FGF receptor-1β isoforms

The alternatively spliced fibroblast growth factor receptor (FGFR)-1 isoforms, FGFR-1α and FGFR-1β, are characterized by the presence of either three or two Ig-like loops in the extracellular domain and are differentially expressed during embryonic development and tumor progression. We have previously shown that in cells irreversibly committed to DNA synthesis by FGF-1, approximately 15% of cell surface FGFR-1 traffics to a perinuclear locale as a structurally intact and functional tyrosine kinase (Prudovsky, I., Savion, N, Zhan, X., Friesel, R., Xu, J., Hou, J., McKeehan, W. L., and Maciag, T. (1994) J. Biol. Chem. 269, 31720-31724). In order to define the structural requirement for association of FGFR-1 with the nucleus, the expression and trafficking of FGFR-1 in FGFR-1α and FGFR-1β L6 myoblast transfectants was studied. Although FGFR-1α was expressed as p145 and p125 forms, FGFR-1β was expressed as p120 and p100 forms in the L6 myoblast transfectants. Tunicamycin and N-glyconase experiments suggest that these forms of FGFR-1α and FGFR-1β are the result of differential glycosylation. However, only the p145 form of FGFR-1α and the p120 form of FGFR-1β were able to bind FGF-1 and activate tyrosine phosphorylation. Pulse-chase analysis of FGFR-1 biosynthesis suggests that the p125 and p100 proteins are the precursor forms of p145 FGFR-1α and p120 FGFR-1β, respectively. Because ligand-chase analysis demonstrated that FGFR-1β L6 myoblast transfectants exhibited a reduced efficiency of nuclear translocation of exogenous FGF-1 when compared with FGFR-1α transfectants, the intracellular trafficking of the FGFR-1α and FGFR-1β isoforms was studied using an in vitro kinase assay to amplify immunoprecipitated FGFR-1. Indeed, the appearance of the FGFR-1α but not FGFR-1β isoform in the nuclear fraction of L6 myoblast transfectants suggests that the distal Ig-like loop in FGFR-1α mediates the differential nuclear association of FGFR-1α as a structurally intact and functional tyrosine kinase. Further, the FGFR-1β L6 myoblast transfectants but not the FGFR-1α myoblast transfectants exhibited a pronounced morphologic change in response to exogenous FGF-1. Because this phenotype change involves the induction of a rounded cellular shape, it is possible that the FGFR-1α and FGFR-1β may ultimately exhibit differential trafficking to adhesion sites.