Rates of apoptosis and proliferation vary with caloric intake and may influence incidence of spontaneous hepatoma in C57BL/6 x C3H F1 mice

Abstract

Although the dysregulation of physiological signals and mechanisms controlling cell proliferation has been a major focus in cancer research, recent evidence suggests that explicit evaluation of apoptosis or physiological cell death may be equally important in understanding multistage carcinogenesis. Dietary restriction of rodents is well known to reproducibly retard development of spontaneous and chemically induced tumors. We reasoned that the decrease in metabolic and hormonal trophic factors induced with this intervention could promote selective cell deletion via apoptosis. To pursue this possibility, we quantified the spontaneous apoptotic rate in liver sections from diet-restricted (DR) and ad libitum-fed (AL) 12-month-old male C57BL/6 x C3H F1 mice, a murine strain known to develop a high incidence of spontaneous liver tumors by 18 months of age. The identification of hepatocyte apoptotic bodies was facilitated by in situ end-labeling immunohistochemistry. The basal rate of proliferation of hepatocytes was quantified utilizing proliferating cell nuclear antigen immunohistochemistry. The incidence of apoptotic bodies and total proliferating cell nuclear antigen-positive cells was enumerated in 14 mice/group by scoring 50,000 random hepatocytes/liver and expressed as the mean incidence/100 cells. When the comparison was made between diet groups, the apoptotic rate was significantly higher in the DR mice relative to the AL mice, while the proliferation rate was significantly lower (P < 0.01 and P < 0.05, respectively). The increase in spontaneous level of apoptosis and the decrease in proliferation rate in livers of DR mice were associated with a significantly lower rate of spontaneous hepatoma over a 36-month period. In summary, the results suggest that caloric intake may modulate the basal turnover rates of cell death and proliferation in a direction consistent with a cancer-protective effect in the DR mice and a cancer-promoting effect in AL mice.