Growth factor- and dexamethasone-induced proteins in Swiss 3T3 cells. Relationship to DNA synthesis.
Dexamethasone synergistically enhances the stimulation of DNA synthesis in quiescent Swiss 3T3 cells by cartilage-derived growth factor (CDGF) while having no consistent effect when added with platelet-derived growth factor (PDGF) or serum. We examined the hypothesis that this difference might be attributed to selective synthesis of individual proteins early in the G1 phase of the cell cycle. Swiss 3T3 cells were treated with CDGF, PDGF, and fetal bovine serum for 3 h, with or without dexamethasone, and [35S]methionine-labeled proteins were separated by two-dimensional electrophoresis on giant gels. Over 3300 proteins could be distinguished; 34 of these were consistently induced more than 3-fold by all three factors, while an additional 30 inductions were variably present. Dexamethasone by itself induced 8 other proteins, and at least 9 growth factor inductions were synergistically enhanced by addition of the hormone. To identify proteins intimately associated with growth control, we looked for inductions that reflected the dexamethasone synergy with CDGF on DNA synthesis and lack of such an effect with PDGF. The induction of only one group of proteins, the Band 1 isoforms (44-46 kDa, pI 6.1-5.9) displayed such selective synergy. The majority of the other growth factor inductions were inhibited by dexamethasone, even in the context of maximal DNA synthesis, implying that their increased synthesis is not required for growth. When 3T3 cells were treated with increasing doses of CDGF with and without dexamethasone, autoradiographic densities of induced proteins varied in a dose-responsive fashion. However, only levels of the Band 1 proteins bore a constant linear relationship to DNA synthesis, suggesting that they play an important role in early control of the cell cycle.