The 5'¯anking region of the C3(1) component of the rat prostate steroid binding protein (PSBP) has been used to successfully target the expression of the SV40 large Tantigen (Tag) to the epithelium of both the mammary and prostate glands resulting in models of mammary and prostate cancers which histologically resemble the human diseases. Atypia of the mammary ductal epithelium develops at about 8 weeks of age, progressing to mammary intraepithelial neoplasia (resembling human ductal carcinoma in situ [DCIS]) at about 12 weeks of age with the development of invasive carcinomas at about 16 weeks of age in 100% of female mice. The carcinomas share features to what has been classi®ed in human breast cancer as in®ltrating ductal carcinomas. All FVB/N female mice carrying the transgene develop mammary cancer with about a 15% incidence of lung metastases. Approximately 10% of older male mice develop anaplastic mammary carcinomas. Unlike many other transgenic models in which hormones and pregnancy are used to induce a mammary phenotype, C3(1)/Tag mice develop mammary tumors in the mammary epithelium of virgin animals without hormone supplementation or pregnancy. Although mammary tumor development appears hormone-responsive at early stages, invasive carcinomas are hormone-independent, which corresponds to the loss of estrogen receptor-a expression during tumor progression. Molecular and biologic factors related to mammary tumor progression can be studied in this model since lesions evolve over a predictable time course. Genomic alterations have been identi®ed during tumor progression, including an ampli®cation of the distal portion of chromosome 6 containing ki-ras and loss of heterozygosity (LOH) in other chromosomal regions. We have demonstrated that stage speci®c alterations in the expression of genes which are critical regulators of the cell cycle and apoptosis are functionally important in vivo. C3(1)/Tag mice appear useful for testing particular therapies since growth of the mammary tumors can be reduced using chemopreventive agents, cytokines, and an anti-angiogenesis agent.

The dramatic increase in apoptosis observed during the development of preneoplastic mammary lesions is associated with a significant elevation in Bax expression in C3(1)/SV40 large T antigen (TAg) transgenic mice. The significance of Bax expression during tumor progression in vivo was studied by generating double-transgenic mice carrying the C3(1)/TAg transgene and mutant alleles for bax. C3(1)/TAg transgenic mice carrying mutant bax alleles exhibited accelerated rates of tumor growth, increased tumor numbers, larger tumor mass and decreased survival rates compared with mice carrying wild-type bax. Accelerated tumorigenesis associated with the bax+/- genotype did not require the loss of function of the second bax allele. Thus, haploid insufficiency of bax is enough to accelerate tumor progression, suggesting that the protective effect of Bax is dose-dependent. While levels of apoptosis in the preneoplastic lesions, but not carcinomas, were reduced in bax+/- or bax-/- mice compared with bax+/+ mice, rates of cellular proliferation in mammary lesions were similar among all bax genotypes. These data demonstrate that bax is a critical suppressor of mammary tumor progression at the stage of preneoplastic mammary lesion development through the upregulation of apoptosis, but that this protective effect is lost during the transition from preneoplasia to invasive carcinoma.

The effects of lovastatin, a potent inhibitor of HMG CoA reductase, on experimental mammary and prostate oncogenesis, were studied in vitro and in vivo. Lovastatin inhibited cell growth in vitro in a dose-dependent manner for both mammary and prostate cancer cell lines, which was associated with p53-independent apoptosis. Flow cytometric analyses of lovastatin-treated mammary and prostate cancer cells demonstrated cell-cycle G(1) arrest, as well as decreases in S and G(2)/M fractions. p21(Waf1) and p27(Kip1) were induced by lovastatin in both types of cancer cells. Gene expression profiling of cells treated with lovastatin, however, was remarkable for a paucity of transcriptional changes induced by lovastatin. Treatment with lovastatin for 4 weeks did inhibit the formation of pre-neoplastic mammary intraepithelial neoplasias (MIN) in vivo, but not invasive carcinomas in the C3(1)/SV40 TAg transgenic model of mammary cancer. The decreased multiplicity of MIN lesions was associated with increased levels of apoptosis in these lesions. However, cell proliferation in the mammary lesions was not significantly different between lovastatin-treated and control mice 1 day after lovastatin treatment. In female mice treated with lovastatin for 12 weeks, there was a tendency for reduced tumor volume, which did not reach statistical significance. However, lovastatin did not suppress any lesion formation in the prostate of C3(1)/SV40 TAg transgenic male mice. Our results suggest that as lovastatin exerts an inhibitory effect on the development of early mammary lesions of mammary carcinogenesis, this compound may be useful for the chemoprevention of mammary cancer and might have utility as an adjuvant in breast cancer therapy. The chemopreventive effects of lovastatin in vivo, however, may be tissue-specific.

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Mammary carcinomas that develop in C3 (1)/SV40 T- antigen (TAg) transgenic mice have lost the p53-mediated induction of p21, leading to increased cellular proliferation and significant elevations of cyclins and Cdks. To test whether p21 could serve as a target for anticancer therapy for this mammary cancer model, a retroviral delivery system for the inducible expression of p21 was developed. We demonstrate that overexpression of p21 in C3(1)/TAg mammary tumor cells using the retroviral inducible p21 expression system results in increased apoptosis, reduced cell proliferation in vitro and reduced tumor growth in vivo associated with reduced expression of cyclins D1 and E, and Cdks 2, 4, and 6. Reciprocal changes in the expression of p21 and p27(Kip1), another cell-cycle regulator, were also observed. Because reduced p21 expression occurs frequently in human breast cancer, restoration of the Cdk inhibitor p21 by gene therapy approaches may provide a method for inhibiting mammary tumor progression.