Abstract
Basic fibroblast growth factor (bFGF), a classical mitogen in fibroblasts and endothelial cells, inhibits the proliferation of MCF-7 and other human breast cancer cell lines. To explain this paradoxic effect, we investigated the effects of bFGF on cyclins and protein members of cyclin complexes that exert positive and negative control on the progression of cells through the G1 phase of the cell cycle. bFGF induced an increase in cyclin D1, cyclin E, and cyclin-dependent kinase 4 (cdk4) 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 cells. The appearance of this inhibitor correlated with a bFGF dose- and time-dependent increase in the levels of cdk inhibitor p21WAF1/CIP1 mRNA and protein. The increase in the level of p21WAF1/CIP1 was associated with the disappearance of the rapidly migrating, activated form of cdk2 from cell lysates, dephosphorylation of the retinoblastoma protein (Rb), and a decrease in cyclin A levels. These changes were represented in the cyclin D1 and E complexes by an increased association with p21WAF1/CIP1, proliferating cell nuclear antigen (PCNA), and the inactive form of cdk2, without an absolute change in cellular PCNA levels and by a switch in the association of cyclin D1 complexes with the hyperphosphorylated form to the dephosphorylated form of Rb. These experiments demonstrate that stimulation of MCF-7 cells with bFGF, although resulting in up-regulation of G1 proteins responsible for mitogenic events, also induces a concomitant decrease in cyclin A levels and an increase in p21WAF1/CIP1 mRNA and protein and results in inactivation of cdk2, dephosphorylation of Rb, and a segregation of PCNA to the G1 cyclin complexes. The dual, conflicting signaling by bFGF results in a net inhibitory phenotype in these cells. These experiments suggest a pleiotropic role for bFGF in breast cancer.
Footnotes
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↵1 Supported in part by Career Development Award DAMD17-94-J-4463 from the United States Army Breast Cancer Research Program (to R. W.) and Grant 16–95 from the Foundation of the University of Medicine and Dentistry of New Jersey (to R. W.).
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↵2 To whom requests for reprints should be addressed, at University of Medicine and Dentistry of New Jersey-New Jersey Medical School, Division of Medical Oncology, MSB I-689, 185 South Orange Avenue, Newark, NJ 07103. Phone: (201) 982-4871; Fax: (201) 982-2384.
- Received October 17, 1996.
- Accepted March 1, 1997.
- ©1997 American Association for Cancer Research.
Volume 57, Issue 9
