Early Signals in the Mitogenic Response of Swiss 3T3 Cells: A Comparative Study of Purified PDGF Homodimers

Abstract

Platelet-derived growth factor (PDGF) occurs as three dimeric isoforms, AA, BB, and AB. Two distinct receptor subunits, α and β, have been identified which bind either all three isoforms of PDGF (α) or PDGF-BB only (β). Here, we have compared the effect of purified PDGF homodimers on the early intracellular signaling events and mitogenesis in Swiss 3T3 cells, which possess equivalent numbers of the α and β subunits. Both PDGF-AA and PDGF-BB stimulated receptor phosphorylation, inositol phosphate formation, activation of protein kinase C, calcium mobilization, EGF receptor transmodulation, sodium uptake, arachidonic acid release, cyclic AMP accumulation, and c-fos induction in a comparable, dose-dependent manner (half-maximal values for all these responses were in the 2–10 ng/ml range for both homodimers). At high concentrations of PDGF (> 10 ng/ml), the BB homo-dimer effect on early membrane and cytosolic signals was 20–30% greater than PDGF-AA, reflecting the greater number of available binding sites for PDGF-BB. DNA synthesis studies indicated that PDGF-AA and PDGF-BB were potent mitogens for Swiss 3T3 cells, displaying identical dose-response effects. Moreover, the mitogenic activities of both homodimers were equally potentiated in the presence of insulin. These results indicate that both PDGF-AA and PDGF-BB stimulate the full complement of molecular responses required for the synergistic interactions mediating long-term mitogenesis. We conclude that α and β receptor subunits do not differ in their ability to transduce PDGF-mediated signals leading to DNA synthesis in Swiss 3T3 cells.