EPIDERMAL GROWTH FACTOR-SPECIFIC PROTEIN TYROSINE PHOSPHORYLATION IN PREIMPLANTATION EMBRYO DEVELOPMENT

We examined whether epidermal growth factor (EGF)-induced preimplantation mouse embryo development and function are mediated by EGF-specific protein tyrosine phosphorylation (PTP). In situ cross-linking and autophosphorylation studies showed that EGF receptor (EGF-R) in Day 4 mouse blastocysts is a protein of approximately 170 kDa that is phosphorylated when exposed to EGF and ATP. Furthermore, EGF induced about a twofold increase in protein tyrosine kinase (PTK) activity in Day 4 blastocysts when incubated in the presence of a peptide substrate with a tyrosine moiety and ATP. RG 50864, a specific inhibitor of EGF-dependent PTK, diminished autophosphorylation of the 170-kDa protein and completely blocked PTK activity in the blastocyst induced by EGF. However, this inhibitor did not affect EGF binding to the embryonic cell surface. In contrast, an inactive tyrphostin compound, RG 50862, did not alter EGF-induced PTK activity in the blastocyst. These findings led us to examine the effects of these tyrphostin compounds on preimplantation mouse embryo development and blastocyst hatching in vitro. RG 50864, in a dose-dependent manner, inhibited EGF-dependent development of 2-cell embryos to blastocysts and the number of cells per blastocyst. This inhibitor also antagonized EGF-induced zona-hatching of blastocysts formed from 8-cell embryos in culture. However, the inhibitor was not effective in deterring transforming growth factor-beta-1-induced blastocyst formation. The inactive compound, RG 50862, had no effects on EGF-dependent blastocyst formation or zona-hatching. The data show that the effects of RG 50864 are specific and mediated by inhibition of EGF-specific PTK activity. Taken together, the results suggest that signal transduction by EGF ligand-receptor-mediated PTP is important for mouse preimplantation embryo development and function.