EPIDERMAL GROWTH-FACTOR SECRETED FROM THE SALIVARY-GLAND IS NECESSARY FOR LIVER-REGENERATION

Partial hepatectomy (PH) in rats induces a synchronized burst of DNA replication in the remnant liver that peaks at 24 h post-PH. We report here that removal of the major salivary glands before one-third and two-thirds PH prevents the proliferative response in the remaining liver. Twelve days after one-third PH, the remnant liver is 89% of the normal liver weight in nonsalivectomized rats but only 55% in salivectomized animals. This indicates that salivectomy does not merely delay the first round of cell division but that it prevents actual regeneration. Salivectomy alters the early protooncogene response to partial hepatectomy. In salivectomized rats, the characteristic peak of c-myc mRNA synthesis at 2-4 h after PH is significantly decreased compared with nonsalivectomized rats. The peak of DNA synthesis at 24 h after PH in salivectomized rats is also dramatically decreased. DNA synthesis as measured by [H-3]thymidine incorporation into DNA of hepatic cells is decreased similar to 90% in salivectomized rats vs. nonsalivectomized rats 22-26 h after PH. Ligation of the venous drainage of the salivary gland results in the same inhibitory effect on DNA synthesis, indicating 1) that the salivary gland must release circulating factor(s), and 2) that the early increase in c-myc expression and the subsequent DNA synthesis, both of which reflect the stimulation of cellular proliferation in the regenerating liver, are induced by humoral factor(s) released from the salivary glands. Injection of exogenous epidermal growth factor (EGF) in salivectomized rats results in restoration of both the DNA synthetic and c-myc responses at levels characteristic of those of liver regeneration. Thus our results suggest that salivary gland release of EGF may trigger the early events of liver regeneration.