INTACT AND FUNCTIONAL FIBROBLAST GROWTH-FACTOR (FGF) RECEPTOR-1 TRAFFICKS NEAR THE NUCLEUS IN RESPONSE TO FGF-1

Exogenous fibroblast growth factor-1 (FGF-1) associates with the nucle us in a receptor-dependent manner during the entire G(1) period of the BALB/c 3T3 cell cycle (Zhan, X., Hu, X., Friesel, R., and Maciag, T. (1993) J. Biol, Chem, 268, 9611-9620). To further study the role of th e FGF receptor (FGFR) during this translocation, the intracellular fat e of FGFR-1 protein and enzymatic activity was examined, Immunoprecipi tation using multiple FGFR-1 antibodies followed by an in vitro tyrosi ne kinase activity assay enabled us to identify FGFR-1 as a 130-kDa ph osphotyrosine-containing protein associated with the nuclear fraction of NIH 3T3 cells exposed to FGF-1. While FGFR-1 tyrosine kinase activi ty could be detected as a nuclear-associated protein after a 2-h expos ure of the NIH 3T3 cells to FGF-1, this activity appeared to be maxima l in the nuclear fraction between 4 and 12 h after FGF-1 treatment, In addition, analysis by confocal immunofluorescence microscopy of quies cent and FGF-1-stimulated NIH 3T3 cells reveal a prominent perinuclear FGFR-1 staining pattern in the cells exposed to FGF-1 but not in the quiescent population, We also observed FGFR-1 associated with the nucl ear fraction in FGFR-1-transfected L6 rat myoblasts, which are known t o be refractive to exogenous FGF-1 and express relatively low levels o f endogenous FGFR-1, In addition, these cells also exhibited the prese nce of a 145-kDa phosphoprotein in the nuclear fraction that was recog nized by FGFR-1 antibodies, These results suggest that the FGFR-1 may be translocated near the nucleus upon interaction with its ligand duri ng the entire G(1) period of the NIH 3T3 cell cycle as a structurally intact and functional tyrosine kinase that may be accessible to perinu clear polypeptides as a regulatory enzyme.