TRANSLOCATION OF BFGF TO THE NUCLEUS IS G1 PHASE CELL-CYCLE SPECIFIC IN BOVINE AORTIC ENDOTHELIAL-CELLS

Primary cultures of adult bovine aortic endothelial (ABAE) cells require bFGF to grow. G1-arrested cells, obtained after 48 h without serum and bFGF, were found to enter S phase and grow synchronously for at least two generations on addition of bFGF. In growing cells bFGF was detected both in the cytoplasm (90%) and in the nucleus (10%) where it accumulates in the nucleolus. It was not detected in the nucleus of confluent cells. bFGF uptake was continuous in the cytoplasm throughout the cell cycle with a maximum in G2, while nuclear uptake occurred only in late G1. Cytoplasmic bFGF (18.4 kd) is cleaved into a 16.5 kd peptide in G1 (t( 1/2 ) = 30 min). In the nucleus the 18.4 kd form was the only one detected 2 h following bFGF addition and was then cleaved into the 16.5 kd in early S phase. These results are consistent with the possibility that in addition to the classical pathway of signal transduction, bFGF is directly translocated to the nucleus in late G1, and could play a role in replication and/or in transcription of rDNA.