Abstract
Epidermal growth factor (EGF) stimulates membrane protein phosphorylation in a human cell line, A-431. The known hepatic mitogenic action of EGF and the reduction in EGF receptor number that occurs during liver regeneration led us to study whether EGF-dependent protein kinase activity was present in rat liver and whether its activity was altered after partial hepatectomy. Liver membranes, preincubated with or without EGF, were phosphorylated (0 degrees C, 15 sec) and subjected to NaDodSO4/polyacrylamide gel electrophoresis and autoradiography. In microsomal fractions, EGF at 5-2000 ng/ml produced a dose-related stimulation of 32P incorporation into a single 170,000-dalton protein (p170). In plasma membranes, a similar EGF-dependent phosphorylation was present and was substantially enriched relative to the microsomal fraction. Acid hydrolysis of labeled microsomal fraction followed by phosphoamino acid determination revealed that EGF stimulated 32P incorporation into phosphotyrosine residues. The EGF-dependent phosphorylation of p170 was compared in microsomal fractions isolated from rats 36 hr after partial hepatectomy or sham operation. In the absence of EGF, in vitro labeling of p170 was similar. EGF stimulated the labeling of p170 in both groups, but the response was clearly diminished after partial hepatectomy. In the presence of EGF, the labeling of p170 in microsomal fraction from regenerating livers was only 47 +/- 6% of that observed in membranes from sham-operated rats (p less than 0.005). Reduction of EGF-dependent phosphorylation during liver regeneration paralleled the loss of binding of 125I-labeled EGF. An increase in the EGF-independent phosphorylation of a 130,000-dalton protein was also observed after partial hepatectomy. The increase in the amount of this phosphoprotein was roughly equal to the loss of EGF-stimulated p170 phosphorylation. Several additional proteins showed increased phosphorylation in membranes from partially hepatectomized rats. These findings indicate that alterations in membrane tyrosine residue phosphorylation occur during regulated growth in vivo.