Hormone dependent and independent mammary tumor development form N-methyl-N-nitrosourea-treated rat mammary epithelial cell xenografts in the nude mouse: multiple pathways and H-ras activation

Of mice and women: A short history of mouse mammary cancer research with an emphasis on the paradigms inspired by the transplantation method

The ability to transplant mammary epithelial cells of any age or developmental stage to the normal anatomical site of an inbred recipient mouse strain has revolutionized the studies of mammary development and tumor biology over the past 50 years. This simple method has made the mammary gland of the rodent one of the most accessible and studied organs and facilitated our understanding of the fundamental cellular and molecular properties of normal and neoplastic development. This short review outlines the early concepts that led to the development of the transplantation technology and the impact of this method on our understanding for a variety of processes important both for the normal development and differentiation of the gland as well as the phenomena of neoplastic progression.

Isoform-specific ras activation and oncogene dependence during MYC- and Wnt-induced mammary tumorigenesis

We have previously shown that c-MYC-induced mammary tumorigenesis in mice proceeds via a preferred secondary pathway involving spontaneous activating mutations in Kras2 (C. M. D'Cruz, E. J. Gunther, R. B. Boxer, J. L. Hartman, L. Sintasath, S. E. Moody, J. D. Cox, S. I. Ha, G. K. Belka, A. Golant, R. D. Cardiff, and L. A. Chodosh, Nat. Med. 7:235-239, 2001). In contrast, we now demonstrate that Wnt1-induced mammary tumorigenesis proceeds via a pathway that preferentially activates Hras1. In addition, we find that expression of oncogenic forms of Kras2 and Hras1 from their endogenous promoters has markedly different consequences for the progression of tumors to oncogene independence. Spontaneous activating Kras2 mutations occurring in either MYC- or Wnt1-induced tumors were strongly associated with oncogene-independent tumor growth following MYC or Wnt1 downregulation. In contrast, Hras1-mutant Wnt1-induced tumors consistently remained oncogene dependent. Additionally, Kras2-mutant tumors exhibited substantially higher levels of ras-GTP, phospho-Erk1/2, and phospho-Mek1/2 compared to Hras1-mutant tumors, suggesting the involvement of the ras/mitogen-activated protein kinase (MAPK) pathway in the acquisition of oncogene independence. Consistent with this, by use of carcinogen-induced ras mutations as well as knock-in mice harboring a latent activated Kras2 allele, we demonstrate that Kras2 activation strongly synergizes with both c-MYC and Wnt1 in mammary tumorigenesis and promotes the progression of tumors to oncogene independence. Together, our findings support a model for tumorigenesis in which c-MYC and Wnt1 select for the outgrowth of cells harboring mutations in specific ras isoforms and that these secondary mutations, in turn, determine the extent of ras/MAPK pathway activation and the potential for oncogene-independent growth.

A novel rodent mammary window of orthotopic breast cancer for intravital microscopy

Orthotopic and ectopic organ environments differentially influence tumor growth, metastasis, and sensitivity to therapy. In this study we present a novel rodent mammary window of orthotopic breast cancer, which is amenable to study of microvascular function and angiogenesis in this orthotopic site. The skin around the nipple of selected mammary glands of female Fischer 344 rats was removed and the nipple was cut at its base. R3230Ac tumor fragments or cells in Gelfoam were aseptically implanted into the nipple sinus. An acrylic disk was placed on top of the implant and was sutured in place. Histology showed that tumors were well established within 5 days. Similar techniques were also applied to BALB/c mice transplanted with 4T1 murine mammary carcinoma cells. With GFP-expressing tumor cells and serial observations, we demonstrated unique patterns of tumor cell proliferation and vascularization in both tumor models. The images obtained were comparable to those from the dorsal skinfold window chambers. This model will allow for study of tumor microcirculatory function, angiogenesis, tumor cell-host interactions, and evaluation of effects of various treatments.

Synergistic effect of MNU and DMBA in mammary carcinogenesis and H-ras activation in female Sprague-Dawley rats

The combined application of N-methyl-N-nitrosourea (MNU) and 7,12-dimethylbenz[alpha]anthracene (DMBA) was compared with the administration of each carcinogen alone as to the effectiveness of the induction of mammary carcinomas and the influence of H-ras oncogene activation in female Sprague-Dawley rats. At 50 days of age, group 1 received 30 mg/kg MNU intraperitoneally (i.p.), group 2 received 30 mg/kg DMBA i.p., group 3 received 60 mg/kg MNU i.p., group 4 received 60 mg/ kg DMBA i.p., group 5 received 30 mg/kg MNU followed by 30 mg/kg DMBA i.p., group 6 received 30 mg/kg MNU i.p. and then 30 mg/kg DMBA intravenously (i.v.) and group 7 remained untreated. Animals were killed when the largest mammary tumor reached 1-2 cm in diameter or were necropsied when they were 30 weeks of age. MNU i.p. produced no deaths (groups 1 and 3), however, the i.p. administration of DMBA induced death due to peritonitis (groups 2, 4 and 5), whereas the i.v. administration of DMBA suppressed the death (group 6). All of the tumors produced by MNU were adenocarcinomas of mammary origin. In contrast, DMBA produced tumors of other than mammary origin. The combined treatment with DMBA and MNU increased the mammary carcinogenic effect; it significantly increased the mean number of mammary cancers per rat. With either carcinogen alone and in combination, the mammary carcinomas produced identical adenocarcinoma histology. Of the mammary carcinomas induced by the combined application of MNU and DMBA (group 6), all 11 tumors from five rats showed the GGA to GAA transitional mutation in H-ras codon 12 (38%) and all 18 tumors from the other 10 rats remained as wild-type. An H-ras point mutation at codon 61 was not detected.