Down regulation of epidermal growth factor receptors in liver proliferation induced by a mixture of triiodothyronine, amino acids, glucagon, and heparin (TAGH).

This study investigated the mechanisms by which TAGH solution (a mixture of triiodothyronine, amino acids, glucagon, and heparin) induces DNA synthesis in hepatocytes in the liver of intact rats, with particular reference to events at the epidermal growth factor (EGF) receptor. Both partial hepatectomy and infusion of TAGH stimulated DNA synthesis at 24 hours and both procedures resulted in a reduction of EGF receptors assessed in plasma membranes isolated from rat liver at this time. In cell cultures, while EGF strongly stimulated DNA synthesis and started EGF receptor down regulation, TAGH had only a minor effect (1.5 x basal) on DNA synthesis and did not interact with or down regulate the EGF receptor. Membrane phosphorylation studies, however, showed that TAGH induced phosphorylation of tyrosine residues in the EGF receptor. The in vivo action of TAGH seems to entail recruitment of similar changes in the EGF receptor to those that occur after partial hepatectomy.