Short-term fasting and the reversal of the stage of promotion in rat hepatocarcinogenesis: Role of cell replication, apoptosis, and gene expression

Studies of the multistage nature of hepatocarcinogenesis in the rat have le d to the development of models having significant potential application to carcinogenesis in other tissues as well as other species. Whereas the initi al and final stages of carcinogenesis-initiation and progression-involve ge netic changes and are operationally irreversible, the intermediate stage of promotion is operationally reversible acid can be modulated by a variety o f environmental factors. Numerous investigations have demonstrated that chr onic caloric restriction modifies neoplastic development, primarily during the stage of promotion, so that fewer lesions develop. Short-term fasting o f rats, initiated with a non-necrogenic dose of diethylnitrosamine (DEN) ac id promoted with 0.05% phenobarbital (PB) for 4 weeks, results in loss of v irtually all of the measurable altered hepatic foci (AHF) after two 5-day p eriods of fasting with an intermediate 2-day period of feeding. This change was accompanied by a marked decrease in bromodeoxyuridine (BrdU) labeling of hepatocytes within AHF together with a significant increase in apoptosis of such cells measured by nick end-labeling. Similar but lesser effects we re noted in surrounding, nonfocal hepatocytes, On refeeding, both the numbe rs and volume percentage of AHF returned within 2 weeks to values seen in n onfasted controls. Administration of PB during the fasting period did not a lter these results, although AHF reappeared more rapidly in such animals on refeeding. Nuclear DNA fragmentation was evident in samples of whole liver from fasted animals. During this same period the expression of c-myc mRNA increased 3- to 9-fold, while levels of albumin and insulin-like growth fac tor I mRNAs decreased significantly. This study demonstrates a model system in which the reversibility of the effects of promoting agents may be rapid ly determined and the effects of chemopreventive inhibitors of promotion ma y be rapidly evaluated.