Prolactin is a tumor promoter in rat liver.

Since prolactin, like the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, induces ornithine decarboxylase and plasminogen activator activities, biochemical markers of a trophic response, this hormone might likewise promote neoplasia. To test this theory, rats were initiated with a hepatocarcinogen followed by six weeks of ovine prolactin. This regimen caused hepatomegaly and the development of enzyme-altered foci. Promotion with prolactin for 23 weeks further increased the numbers of enzyme-altered foci. We suggest that prolactin is an endogenous tumor promoter for chemically initiated cells.