Age-dependent induction of hepatic tumor regression by the tissue microenvironment after transplantation of neoplastically transformed rat liver epithelial cells into the liver.

Age is the biggest risk factor associated with the development of cancer. Whereas the incidence of neoplastic disease increases dramatically in aging humans and experimental animals, the effects of aging on tumorigenesis are poorly understood. Using a rodent model, we have previously shown that the microenvironment of the hepatic parenchyma regulates hepatic tumor formation from transplanted neoplastic cells in an age-dependent manner. In the current study, we have investigated the mechanistic basis for the age-dependent suppression of tumor formation by transplanted BAG2-GN6TF rat liver epithelial tumor cells. Examination of liver tissue at 7 and 14 days after transplantation of liver tumor cells revealed the presence of injection-site tumors in both young and old animals. With time, these tumors spontaneously regressed from young adult livers, leaving no tumor remnant and without evidence of injury to the parenchyma. In contrast, tumors detected in old animals at early time points after transplantation persisted for the remainder of the life of the host. Reduced cell proliferation and increased apoptotic cell death were detected in hepatic tumors in young rats relative to hepatic tumors in old rats. These observations suggest that the regression of hepatic tumors from young rats was the direct result of an increased ratio of cell death to cell birth, whereas the persistence and expansion of hepatic tumors in old rats was related to increased cell proliferation relative to cell death. Because young adult rats developed persistent (nonregressing) tumors after transplantation of BAG2-GN6TF cells to extrahepatic sites, the consistent regression of BAG2-GN6TF tumors from livers of young rats seemed to be largely a result of interactions between tumor cells and factors specific to the liver microenvironment. These data indicate that the hepatic microenvironment of young rats can negatively regulate the growth of transformed liver epithelial cells, but with increasing age, the ability of the hepatic microenvironment to suppress the growth of neoplastic tissue deteriorates. Age-associated alterations in tissue microenvironments may thus permit the development of tumors late in life.