NUCLEAR ACCUMULATION OF EXOGENOUS BASIC FIBROBLAST GROWTH-FACTOR IN ENDOTHELIAL, FIBROBLAST, AND MYOBLAST CELL-LINES RESULTS IN DIVERSE BIOLOGICAL RESPONSES

During studies comparing I-125-bFGF internalization between endothelia l cells and other cell types, we found, unexpectedly, internalization and nuclear translocation of exogenously added I-125-bFGF in two cell lines: Chinese hamster ovary cells (CHO) and rat L6 myoblasts. These c ell lines were previously reported to be devoid of FGF receptors. Furt hermore, CHO cells showed a weak mitogenic response to added bFGF, whi le L6 cells were mitogenically unresponsive. By comparison, coronary v enular endothelial cells (CVEC), BALB/c 3T3 fibroblasts, and BHK-21 ce lls, demonstrated internalization and nuclear translocation of added I -125-bFGF, and mitogenic responsiveness to the growth factor. Insulin alone stimulated DNA synthesis in all cell types, yet augmented bFGF-d ependent DNA synthesis only in CVEC, 3T3, and BHK. All five cell types expressed FGF receptors as assessed by covalent crosslinking with I-1 25-bFGF,and immunoblotting with anti-FGF receptor antibodies. Differin g rates of cytoplasmic and nuclear accumulation of I-125-bFGF and part ial inhibition of internalization by pretreatment of CVEC with chlorat e support a recent model that bFGF can internalize by two mechanisms. Insulin did not significantly affect I-125-bFGF internalization or met abolism in any cell type. bFGF treatment resulted in weak inhibition o f RNA synthesis in L6 cells, bFGF appears firmly bound to the nuclear matrix as little nuclear-bound I-125-bFGF in CVEC is released by DNAse I or RNAse A digestion, while washes with 0.5 M NaCl result in partia l release. Nuclear bFGF may thus be involved in regulation of nuclear events (e.g., gene transcription and/or DNA replication).