Long term growth factor exposure and differential tyrosine phosphorylation are required for DNA synthesis in BALB/c...

The importance of growth factor-mediated immediate-early cellular events to the cell cycle has influenced the development and identity of oncogenes and tumor suppressor genes as well as the concept that growth factors commit mammalian cells to enter a biochemical program that ultimately yields DNA synthesis. However, the mid and late events involved in the regulation of growth factor-induced signal transduction remain largely unknown. In this report we demonstrate that BALB/c 3T3 cells require continuous exposure to fibroblast growth factor (FGF)-1 for a minimum of 12 h to achieve near maximal DNA synthesis. This correlates with the continuous internalization of radiolabeled FGF-1 into the cytosol and nucleus of BALB/c 3T3 cells and the maintenance of a low level of FGF receptors on the cell surface during the entire G1 phase of the cell cycle. Further analysis demonstrates the maintenance of a continuous series of differential FGF-1-induced tyrosine phosphorylation events including the phosphorylation of phospholipase C-gamma as well as novel FGF receptor polypeptide substrates, p60, p85, p90, and p130 throughout the G1 phase of the BALB/c 3T3 cell cycle. The tyrosine phosphorylation events are biphasic during the 12-h period after the administration of FGF-1, and the second phase is characterized by hyper-tyrosine phosphorylation of p60, p85, and p130. Interestingly, NIH 3T3 cells which overexpress the FGF receptor-1 polypeptide demonstrate exaggerated tyrosine phosphorylation of p60 and p85 but not p90 and exhibit growth factor-independent cell proliferation. These results suggest that the initiation of DNA synthesis in BALB/c 3T3 cells by FGF-1 is regulated by a complex biochemical program that involves the continuous tyrosine phosphorylation of known and novel polypeptides throughout the G0 to G1 transition period of the cell cycle.