Radiosensitive target in the mouse embryo chimera assay: Implications that the target involves autocrine growth factor function

Mouse preimplantation embryos express at least two functional cell surface growth factor receptors that are radiosensitive in other cell types, the epidermal growth factor receptor (EGF receptor) and the insulin-like growth factor I receptor (IGF-I receptor). These embryos also express ligands that bind to and activate these receptors, including transforming growth factor alpha (TGF-α) and insulin-like growth factor II (IGF-II), which bind to the EGF receptor and IGF-I receptor, respectively. Embryo-expressed IGF-II and TGF-α increase embryo cell number-a measure of proliferation rate- and stimulate blastocoele formation-a measure of cell differentiation-allowing the embryo to self-modulate cell proliferation and morphogenesis into a blastocyst (Paria and Dey, Proc. Natl. Acad. Sci. USA 87, 4756-4760, 1990; Dardik and Schultz, Development 113, 919-930, 1991: Rappolee et al., Genes Dev. 6, 939-952, 1992). In this work, we tested the hypothesis that IGF-I receptor and/or EGF receptor function may be impaired to produce the radiation-induced competitive cell proliferation disadvantage that is expressed by irradiated embryos that are aggregated with nonirradiated embryos in chimeras. Cleavage-stage embryos were irradiated with ¹³⁷Cs γ rays (0.5 or 1.0 Gy) and paired with nonirradiated same-stage embryos to form groups of chimeras that were cultured in control medium or medium containing IGF-II, insulin, EGF or TGF-α. The cell proliferation disadvantage expressed by the irradiated embryos within chimeras was completely eliminated by IGF- II or insulin. In contrast to the rescue action of IGF-II or insulin in chimeras, neither EGF nor TGF-α could prevent the cell proliferation disadvantage exhibited by irradiated embryos paired with nonirradiated embryos in chimeras. For irradiated conventionally cultured zona-enclosed embryos, IGF-II and TGF-α did not increase mean embryo cell number significantly, although both IGF-II and TGF-α did increase blastocoele formation significantly. Collectively, these results support the following conclusions: (1) Ligands for the IGF-I receptor can rescue irradiated embryos from competitive cell proliferation disadvantage in chimeras, while ligands for the EGF receptor cannot; (2) IGF-I receptor function and EGF receptor function are affected differently by ionizing radiation with respect to competitive cell proliferation and are affected similarly by ionizing radiation with respect to blastocoele formation; (3) EGF receptor-dependent stimulation of competitive cell proliferation and cell differentiation are affected differently by ionizing radiation in preimplantation embryos.