ONTOGENY OF EPIDERMAL GROWTH-FACTOR RECEPTOR TYROSINE KINASE IN RAT-LIVER

The binding of epidermal growth factor (EGF) to its receptor and the activity of the receptor intrinsic protein-tyrosine kinase were studied during the ontogeny of rat liver. The number of EGF receptors during pre- and postnatal development was first compared in crude liver plasma membranes using 1) specific binding of I-125-labeled EGF and 2) immunoblot analysis using any antireceptor polyclonal rabbit antibody. Both methods detected the expression of the EGF receptor in fetal rat liver on day 17 of gestation, but in an amount markedly less than the adult. Within 24 h, there was a more than twofold increase in EGF binding to plasma membranes as well as a marked increase in receptor immunoreactivity. However, after birth, there was a precipitous drop in receptor number to < 20% of the adult level by the end of the first postnatal day (P < 0.001). Next, the presence of EGF-stimulated tyrosine kinase activity (autophosphorylation) was determined during the same stages of development. Electrophoresis of membranes phosphorylated in the presence or absence of EGF followed by autoradiography demonstrated autokinase activity stimulated by EGF in day 18 and 19 fetal liver plasma membranes, but not in membranes on day 17 of gestation. Similar to the pattern observed with EGF binding, there was a decrease in autokinase activity in early neonatal plasma membranes followed by an increase to near adult levels by 7 days postnatally. Quantitiation of the amount of P-32 radioactivity associated with the EGF receptor bands in each age group, correlated with the degree of autophosphorylation assessed by autoradiography. We conclude from these studies that EGF binding to its receptor and the ligand-stimulated autophosphorylation of the receptor undergo significant and parallel changes during ontogenesis of the rat liver.