PUTRESCINE AS A COMITOGEN OF EPIDERMAL GROWTH-FACTOR IN RAT-LIVER GROWTH

Putrescine can stimulate regeneration of the remnant liver after parti al hepatectomy in rats when exogenously administered, but its mitogeni c action has not been shown in cultured hepatocytes. To find its actio n site(s) in the regulation of hepatocyte proliferation, we examined i ts effect on hepatocyte DNA synthesis in relation to mitogenic action of epidermal growth factor in vitro and in vivo. When putrescine was a dded to the medium of adult rat hepatocytes in primary culture 36 hr a fter plating, DNA synthesis at 50 hr induced by addition of epidermal growth factor at 24 hr was significantly enhanced. This enhancement di sappeared by removal of epidermal growth factor at the time of putresc ine addition. Putrescine added to the medium was taken up in a dose-re lated manner by hepatocytes, irrespective of the presence of epidermal growth factor, whereas I-125-epidermal growth factor binding to hepat ocytes was not affected by addition of putrescine. When rats received epidermal growth factor at 2-hr intervals until 10 hr, 5-bromo-2'-deox yuridine labeled and mitotic hepatocytes were increased in number at 4 8 hr with increased hepatic DNA content. These increases were not affe cted by concomitant administration of putrescine until 10 hr, but sign ificantly enhanced by additional administration of putrescine and epid ermal growth factor from 20 to 30 hr. We conclude that putrescine may stimulate proliferation of hepatocytes that have entered the G(1)-phas e of the cell cycle as a comitogen of epidermal growth factor, probabl y through action at the molecular levels to enhance its mitogenic acti vity.