OVEREXPRESSION OF INSULIN-LIKE-GROWTH-FACTOR (IGF)-I RECEPTOR ENHANCES INHIBITION OF DNA-REPLICATION IN MOUSE CELLS EXPOSED TO X-RAYS

Previous studies from our laboratory provided evidence for the operati on of signal transduction pathways involving ms, myc, and staurosporin e-sensitive protein kinases in the regulation of DNA replication in ir radiated cells. Because ras and myc are also involved in the signal tr ansduction elicited in response to ligand activation of growth factor receptors, we wondered whether growth factor receptors are upstream el ements in the regulation of DNA replication in irradiated cells. Here, we report on the role of insulin-like growth factor I receptor (IGF-I R) in the regulation of DNA replication in irradiated cells. We compar e radiation-induced inhibition of DNA replication in BALB/c 3T3 cells with that in P6 cells. P6 cells are derived from BALB/c 3T3 cells by t ransfection with a vector expressing IGF-IR, leading to 30-fold overex pression. We observe a significantly stronger inhibition of DNA replic ation after irradiation in P6 as compared with BALB/c 3T3 cells at all doses examined. Sedimentation in alkaline sucrose gradients shows tha t the increased inhibition in P6 cells is due to an increased inhibiti on of replicon initiation, the main controlling event in DNA replicati on. Staurosporine at 20 nM reduces radiation-induced inhibition of DNA replication in BALB/c 3T3 cells, but has only a small effect in P6 ce lls. Caffeine at a concentration of 1 mM, on the other hand, removes o ver 60% of the inhibition in both cell lines. The results implicate IG F-IR in the regulation of DNA replication in irradiated cells, but als o suggest differences between cells of different origins in the protei ns involved in the regulating signal transduction pathway.