The Initiation-Promotion-Progression Model of Rat Hepatocarcinogenesis

Abstract

Carcinogenesis is a multistage process consisting of the three distinct stages: initiation, promotion, and progression. The initiation-promotion-progression (IPP) protocol models these stages and establishes a method whereby agents that possess a carcinogenic risk can be classified as acting primarily at any one or combination of these stages. In one hepatocarcinogenesis IPP protocol, rats were initiated with 10 mg of diethylnitrosamine/kg body wt at 5 days of age, started on the promoting agent phenobarbital at weaning, subjected to a 70% partial hepatectomy at 6 months, and, at the peak of proliferation, given a putative progressor agent, ethylnitrosourea ([ENU] 100 mg/kg, ip) or hydroxy-urea ([HU] 3 x 150 mg/kg, ip). Administration of the promoting agent was discontinued after the progressor agent was given, and the rats were sacrificed 6 months later. The number and volume fraction of promoter-independent (growth in the absence of the promoting agent) altered hepatic foci (AHF) were then determined by quantitative stereology. The number of such AHF increased with either ENU or HU treatment compared with animals not given a progressor agent. In addition, hepatocytes isolated from animals subjected to an IPP regimen with ENU as the progressor agent exhibited a greater degree of chromosomal breakage and aneuploidy than animals not given a second initiator. A variation of this model, in which the promoting agent was maintained after administration of the progressor agent, was examined. In this IPP model, the number of heterogeneous AHF (foci-in-foci) increased after application of the progressor agent (ENU or HU). An increased incidence of hepatocellular carcinoma was also observed in animals subjected to the IPP protocol when promotion was maintained until sacrifice. Thus, the characteristics of progression--increased chromosomal damage, aneuploidy, growth of AHF in the absence of continued tumor promotion, the presence of foci-in-foci, and an increased incidence of malignant neoplasia--have been used as end points for the demonstration of progressor activity by ENU. In addition, the potential progressor activity of HU and benzene has been demonstrated with the IPP model of rat hepatocarcinogenesis.

Quantitation of Multistage Carcinogenesis in Rat Liver

“…Briefly, rats are treated with initiating, promoting, and progressor agents within the initiation-promotion-progression protocol developed in this (13) and other laboratories (47).…”

A toxicogenomics approach for early assessment of potential non-genotoxic hepatocarcinogenicity of chemicals in rats

“…Chemical carcinogenesis is a multistage process, i.e., initiation, promotion and progression (Dragan et al, 1993;Miller and Miller, 1981;Scott et al, 1984). Based on this mechanism of action, chemical carcinogens are classified as genotoxic (mutagenic) and non-genotoxic (non-mutagenic) agents (Hayashi, 1992;Melnick et al, 1996).…”

Detection of initiating potential of non-genotoxic carcinogens in a two-stage hepatocarcinogenesis study in rats

“…Although the mechanism of chemical-induced carcinogenesis remains unclear, multi-step carcinogenesis consisting of initiation, promotion and progression is an accepted theory (Dragan et al, 1993;Klaunig et al, 2011). In general terms (Gregus and Klaassen, 2001), chemicals that react with DNA might induce damage such as adduct formation, oxidative alteration, and strand breakage.…”