Dose-dependent oncogene-induced senescence in vivo and its evasion during mammary tumorigenesis.

Activating Ras mutations can induce either proliferation or senescence depending on the cellular context. To determine whether Ras activation has context-dependent effects in the mammary gland, we generated doxycycline-inducible transgenic mice that permit Ras activation to be titrated. Low levels of Ras activation - similar to those found in non-transformed mouse tissues expressing endogenous oncogenic Kras2 - stimulate cellular proliferation and mammary epithelial hyperplasias.

The transcriptional repressor Snail promotes mammary tumor recurrence.

Breast cancer recurrence is a fundamental clinical manifestation of tumor progression and represents the principal cause of death from this disease. Using a conditional transgenic mouse model for the recurrence of HER2/neu-induced mammary tumors, we demonstrate that the transcriptional repressor Snail is spontaneously upregulated in recurrent tumors in vivo and that recurrence is accompanied by epithelial-to-mesenchymal transition (EMT). Consistent with a causal role for Snail in these processes, we show that Snail is sufficient to induce EMT in primary tumor cells, that Snail is sufficient to promote mammary tumor recurrence in vivo, and that high levels of Snail predict decreased relapse-free survival in women with breast cancer.

Conditional activation of Neu in the mammary epithelium of transgenic mice results in reversible pulmonary metastasis.

To determine the impact of tumor progression on the reversibility of Neu-induced tumorigenesis, we have used the tetracycline regulatory system to conditionally express activated Neu in the mammary epithelium of transgenic mice. When induced with doxycycline, bitransgenic MMTV-rtTA/TetO-NeuNT mice develop multiple invasive mammary carcinomas, essentially all of which regress to a clinically undetectable state following transgene deinduction. This demonstrates that Neu-initiated tumorigenesis is reversible.