The cooperative cell kinetic actions of ET-1 with TGF-α or EGF in normal rat kidney fibroblasts (NRK-49F) and KNRK cells (Kirsten MSV transformed) were analyzed by [3H]-thymidine incorporation assay and flow cytometry. A marked synergistic effect of TGF-α and ET-1 (or EGF and ET-1) on DNA synthesis and G1 to S transition was observed in NRK cells; 15–20% S for TGF-α and 12% S for ET-1 alone but 45–50% S in combination. There was no detectable effect on cell cycle kinetics by TGF-α (1 ng/ml) or EGF (1 ng/ml) plus ET-1 (1 ng/ml) in KNRK cells treated for 22 hours. Insulin, insulin-like growth factor I (IGF-I), fibroblast growth factor (FGF), platelet derived growth factor (PDGF), and transforming growth factor β (TGF-β) were also tested and found to have no significant synergistic effects on ET-1 actions. Our findings suggest that the combination of TGF-α (EGF) and ET-1 is an important part of an intricate network which coordinates progression of G1 to S phase in normal cells.

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