BASIC FIBROBLAST GROWTH-FACTOR CAUSES GROWTH ARREST IN MCF-7 HUMAN BREAST-CANCER CELLS WHILE INDUCING BOTH MITOGENIC AND INHIBITORY G(1) EVENTS

Basic fibroblast growth factor (bFGF), a classical mitogen in fibrobla sts and endothelial cells, inhibits the proliferation of MCF-7 and oth er human breast cancer cell lines. To explain this paradoxic effect, w e investigated the effects of hFGF on cyclins and protein members of c yclin complexes that exert positive and negative control on the progre ssion of cells through the G(1) phase of the cell cycle. bFGF induced an increase in cyclin DI, cyclin E, and cyclin-dependent kinase 4 (cdk 4) protein levels in a bFGF dose-dependent manner. However, bFGF also induced a heat-stable, transferable cytoplasmic factor in MCF-7 cells that inhibited the histone H1 kinase activity of reconstituted cyclin E-cdk2 and cyclin A-cdk2 complexes from Mv1Lu mink lung epithelial cel ls. The appearance of this inhibitor correlated with a bFGF dose- and time-dependent increase in the levels of cdk inhibitor p21(WAF1/CIP1) mRNA and protein. The increase in the level of p21(WAF1/CIP1) was asso ciated with the disappearance of the rapidly migrating, activated form of cdk2 from cell lysates, dephosphorylation of the retinoblastoma pr otein (Rb), and a decrease in cyclin A levels. These changes were repr esented in the cyclin D1 and E complexes by an increased association w ith p21(WAF1/CIP1), proliferating cell nuclear antigen (PCNA), and the inactive form of cdk2, without an absolute change in cellular PCNA le vels and by a switch in the association of cyclin D1 complexes with th e hyperphosphorylated form to the dephosphorylated form of Rb. These e xperiments demonstrate that stimulation of MCF-7 cells with bFGF, alth ough resulting in up-regulation of G(1) proteins responsible for mitog enic events, also induces a concomitant decrease in cyclin A levels an d an increase in p21(WAF1/CIP1) mRNA and protein and results in inacti vation of cdk2, dephosphorylation of Rb, and a segregation of PCNA to the G(1) cyclin complexes. The dual, conflicting signaling by bFGF res ults in a net inhibitory phenotype in these cells. These experiments s uggest a pleiotropic role for bFGF in breast cancer.