Internalized basic fibroblast growth factor translocates to nuclei of venular endothelial cells

To begin to understand the molecular mechanisms by which basic fibroblast growth factor (bFGF) stimulates proliferation of coronary venular endothelial cells (CVEC), we have characterized the kinetic interactions of bFGF with various binding sites on CVEC and determined the kinetics of nuclear translocation of bFGF. We report that bFGF rapidly binds to its receptor and is immediately internalized at 37 degrees C with a half-time for receptor binding of 0.9 min. After internalization bFGF is processed by two kinetically and biochemically distinguishable pathways. Up to 40-50% of total internalized bFGF is translocated to the nuclei of serum-starved, quiescent cells at early time points (0-2 h). This proportion declines to less than or equal to 20% by 24 h. Cytoplasmic accumulation continued to increase for up to 24 h. Nuclear-bound 125I-labeled bFGF consisted primarily of the intact 18-kDa species with small amounts of a 16-kDa degradation fragment. Nuclear-bound 125I-labeled bFGF showed little evidence of degradation even after 24 h, whereas cytoplasmic 125I-labeled bFGF showed increased degradation to smaller fragments with time. Nuclear-binding of bFGF reached equilibrium by 8 h, just before initiation of DNA synthesis, which began 9-12 h after growth factor addition. These results suggest that nuclear-bound bFGF may function in triggering division (proliferation) of CVEC subsequent to binding of the growth factor to cell surface receptors.