[en] Purpose/Objective: The growth regulatory gene epidermal growth factor-receptor (EGF-R) and transforming growth factor-α (TGF-α) have been identified as radiation late response genes because of their permanent up-regulation after repeated radiation exposures. The steady-state mRNA levels of the same genes are up-regulated after single radiation exposures in the dose range of 2 - 5 Gy. This identifies EGF-R and TGF-α as genes critical in radiation responses and provides a unique opportunity to study the signal transduction cascade between immediate early and late responses. This study examines the role of protein tyrosine kinase receptors and immediate down-stream events in radiation-induced signal transduction. Material and Methods: For the analyses of EGF-R and TGF-α mRNA induction and EGF-R autophosphorylation MCF-7 mammary and A431 squamous carcinoma cells were grown in full medium without refeeding to 90% confluence within 4 days. EGF-R tyrosine phosphorylation (Y^P) after EGF exposures up to 60 min was compared to radiation-induced increases in EGF-R Y^P levels using Y^P-specific monoclonal antibody (Ab-5) for immune precipitation and immunoblotting with secondary antibodies. The phosphorylation pattern of EGF-R after radiation and EGF exposures was examined after metabolic labeling with ³²P-ATP, immune precipitation and HPLC tryptic peptide mapping. The activation of phospholipase-C (PL-C), generating the secondary messengers (IP₃) and diacylglycerol, was monitored by quantifying EGF- or radiation-induced IP₃ production with a radio-immune assay kit. Results: Under the experimental conditions used, EGF induced dose-dependent 5- to 15-fold increases in autophosphorylation of EGF-R within 5 min of EGF exposure. The increased EGF-R Y^P levels were short-lived and reversed to less than base line levels within 60 min, most likely due to EGF-R turnover in the plasma membrane. Similar to EGF, single radiation exposures in the 1 to 4 Gy dose range induced an about 3-fold increase in EGF-R Y phosphorylation. The effect maximized at 2 - 4 Gy and was not seen at radiation doses of ≥ 10 Gy. Time-course experiments revealed maximum EGF-R autophosphorylation within 5 min after exposure with a return to baseline levels within 60 min. Both cell lines, MCF-7 and A431 cells expressing very low and very high numbers of EGF-R molecules, respectively, exhibited EGF- and radiation-induced EGF-F autophosphorylation. However, in both cell lines the radiation-induced effect was less suggesting that only a selected number of Y residues were phosphorylated. This conclusion was supported further by tryptic peptide analyses of ³²P labeled EGF-R. Similar to these labeling experiments, the two cell lines furnished different EGF and radiation responses with respect to PL-C activation and IP₃ generation. Conclusions: The results presented indicate that EGF-R, a radiation late response gene, is involved in immediate early radiation responses. These reactions involve autophosphorylation of EGF-R and immediate down-stream events, such as PL-C activation and IP₃ production. However, there appears to be heterogeneity in the described responses depending on the expression levels of EGF-R. In addition, radiation induces qualitatively and quantitatively different EGF-R Y phosphorylation relative to EGF. The experimental system described provides an opportunity to characterize radiation-induced signal transduction pathways initiated by receptor tyrosine kinase activation