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A Short-term Rat Mammary Carcinogenesis Model for the Prevention of Hormonally Responsive and Nonresponsive In situ Carcinomas

Authors' Affiliations: ¹ McArdle Laboratory for Cancer Research, ² Research Animal Resource Center, and ³ Paul P. Carbone Comprehensive Cancer Center, University of Wisconsin, Madison, Wisconsin

Requests for reprints: Michael N. Gould, McArdle Laboratory for Cancer Research, University of Wisconsin, 1400 University Avenue, Madison, WI 53706. Phone: 608-263-6615; Fax: 608-262-2824; E-mail: gould{at}oncology.wisc.edu.

Preclinical models that accurately reproduce specific aspects of human disease etiology are invaluable for the initial development and evaluation of chemopreventive agents. We developed a novel, short-term prevention model, which is particularly useful for assessing the efficacy of a compound to prevent hormonally responsive and nonresponsive in situ carcinomas. In this model, carcinogenesis is induced by a high titer of neu-containing, replication-defective retrovirus. The multiplicity and size of the resulting in situ carcinomas are scored in whole-mounted, aluminum carmine–stained mammary glands at 15 days postinfusion. These in situ carcinomas represent a distinct biological time point in the development of neu-induced mammary cancer in the rat. They are characterized by high rates of proliferation (40.0%; P < 0.0001) and apoptosis (2.8%; P < 0.005) compared with mammary carcinomas. The majority of in situ carcinomas regress spontaneously after 20 days postinfusion. The in situ carcinomas at 15 days postinfusion exhibit hormonal responsiveness. The effects of the chemoprevention agents tamoxifen, celecoxib, and targretin on hormonally responsive and nonresponsive in situ carcinomas recapitulate those observed on mammary carcinomas at 12 and 18 weeks postinfusion for intact and ovariectomized rats, respectively. Neu-induced in situ carcinomas in the rat represent etiologically relevant intermediate time points of mammary carcinogenesis. Our prevention model represents a cost-efficient in vivo system to determine whether the preventive effects of a compound extend to hormonally nonresponsive mammary lesions, for which new chemoprevention approaches are needed.

Key Words: neu-induced retroviral in situ carcinoma rat model • mammary carcinogenesis • chemoprevention • hormonal responsiveness • tamoxifen • celecoxib • targretin