Translocation of bFGF to the nucleus is G1 phase cell cycle specific in bovine aortic endothelial cells

Abstract

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 (t1/2 = 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.