The quantitation of altered hepatic foci during multistage hepatocarcinogenesis in the rat: transforming growth factor alpha expression as a marker for the stage of progression. 
The experimental three-stage hepatocarcinogenesis protocol of initiation, promotion, and progression, coupled with the analytical technique of stereology, permits quantitative analysis of the carcinogenic process, including the derivation of biologically based risk assessment models.  The aberrant expression of the placental isozyme of glutathione S-transferase (PGST) is an efficient marker for initiated, preneoplastic, and neoplastic hepatocytes.  Putatively initiated cells and their clonal progeny can be identified, enumerated, and their growth characteristics determined on the basis of their aberrant expression of this protein.  A lack of suitable markers has made the identification and quantitation of hepatocytes in the early stage of progression more difficult.  One characteristic of cells in the stage of progression is the evolution of relatively autonomous growth.  The alteration of growth factor signalling pathways may provide one mechanism for this observation.  The expression of transforming growth factor alpha (TGF alpha) is seen in many malignancies.  The initiation-promotion-progression protocol has been used to induce progression in the rat liver.  The focal expression of TGF alpha was found to correlate with areas of progression in rats subjected to this protocol.  The ability to identify and quantitate cells in the stage of progression should facilitate application of the Moolgavkar-Venzon-Knudson model for assessing human risk from carcinogens active at each of these three stages.  Validation of this model will require determination of the number and growth characteristics of hepatocytes in the stage of progression.