EGF activates autocrine TGFα to induce prolonged egf receptor signaling and hepatocyte proliferation

EGF receptor is a main participant in the regulation of liver regeneration. In primary hepatocyte cultures, EGF or TGFα binding to EGF receptor activates Erk1/2 and PI3K pathways, induces cyclin D1 and thus initiates DNA synthesis. We have explored mechanisms by which prolonged EGF receptor activation induces hepatocyte proliferation.

EGF receptor activation, as well as Erk1/2 and PI3K signaling were explored in EGF-stimulated primary hepatocyte cultures by Western blotting and immunocytochemistry. TGFα release to the medium was quantified by ELISA. Effects of a neutralizing antibody to TGFα on EGF receptor signaling and proliferation were explored.

Inhibitors of PI3K or Erk1/2 inhibited cyclin D1 expression and G1 progression when added 12 hours after EGF stimulation, whereas depletion of EGF from the medium at this time point did not. ELISA demonstrated that EGF induced TGFα release to the medium. Cyclin D1 induction and cellular proliferation were efficiently inhibited when a neutralizing antibody to TGFα was added to the medium. This also occurred when the antibody was added 12 hours after EGF stimulation.

Sustained EGF receptor activity and signaling through both Erk1/2 and PI3K pathways were necessary for proliferation. This was achieved by EGF activation of autocrine TGFα.