Inflammation and atrophy precede prostatic neoplasia in a PhIP-induced rat model

2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine (PhIP) has been implicated as a major mutagenic heterocyclic amine in the human diet and is carcinogenic in the rat prostate. To validate PhIP-induced rat prostatic neoplasia as a model of human prostate cancer progression, we sought to study the earliest histologic and morphologic changes in the prostate and to follow progressive changes over time. We fed sixty-seven 5-week-old male Fischer F344 rats with PhIP (400 ppm) or control diets for 20 weeks, and then sacrificed animals for histomorphologic examination at the ages of 25, 45, and 65 weeks. Animals treated with PhIP showed significantly more inflammation (P = .002, > .001, and .016 for 25, 45, and 65 weeks, respectively) and atrophy (P = .003, > .001, and .006 for 25, 45, and 65 weeks, respectively) in their prostate glands relative to controls. Prostatic intraepithelial neoplasia (PIN) occurred only in PhIP-treated rats. PIN lesions arose in areas of glandular atrophy, most often in the ventral prostate. Atypical cells in areas of atrophy show loss of glutathione S-transferase pi immunostaining preceding the development of PIN. None of the animals in this study developed invasive carcinomas, differing from those in previous reports. Overall, these findings suggest that the pathogenesis of prostatic neoplasia in the PhIP-treated rat prostate proceeds from inflammation to postinflammatory proliferative atrophy to PIN.