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. Strikingly, extensive lung metastases arising from Neu-induced mammary tumors also rapidly and fully regress following the abrogation of Neu expression. However, despite the near universal dependence of both primary tumors and metastases on Neu transgene expression, most animals bearing fully regressed Neu-induced tumors ultimately develop recurrent tumors that have progressed to a Neu-independent state.

Aberrant activation of Wnt signaling is oncogenic and has been implicated in a variety of human cancers. We have developed a doxycycline-inducible Wnt1 transgenic mouse model to determine the dependence of established mammary adenocarcinomas on continued Wnt signaling. Using this model we show that targeted down-regulation of the Wnt pathway results in the rapid disappearance of essentially all Wnt-initiated invasive primary tumors as well as pulmonary metastases. Tumor regression does not require p53 and occurs even in highly aneuploid tumors. However, despite the dependence of primary mammary tumors and metastases on continued Wnt signaling and the dispensability of p53 for tumor regression, we find that a substantial fraction of tumors progress to a Wnt-independent state and that p53 suppresses this process. Specifically, loss of one p53 allele dramatically facilitates the progression of mammary tumors to a Wnt1-independent state both by impairing the regression of primary tumors following doxycycline withdrawal and by promoting the recurrence of fully regressed tumors in the absence of doxycycline. Thus, although p53 itself is dispensable for tumor regression, it nevertheless plays a critical role in the suppression of tumor recurrence. Our findings demonstrate that although even advanced stages of epithelial malignancy remain dependent upon continued Wnt signaling for maintenance and growth, loss of p53 facilitates tumor escape and the acquisition of oncogene independence.

The insulin receptor substrate-1 (IRS-1) is one of the major substrates of both the insulin and IGF-I receptors and is generally localized in the cytosol/membrane fraction of the cell. We show here that a substantial fraction of IRS-1 is translocated to the nucleus in mouse embryo fibroblasts (MEF) expressing the simian virus 40 T antigen. Nuclear translocation of IRS-1 occurs also in MEF stimulated with IGF-I or in MEF expressing the oncogene v-src. Nuclear translocation of IRS-1 can be demonstrated by confocal microscopy, immunohistochemistry, or subcellular fractionation. An antibody to IRS-1 immunoprecipitates from nuclear fractions (but not from cytosolic fractions) the upstream binding factor, which is a key regulator of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In agreement with this finding, in 32D murine hemopoietic cells, nuclear translocation of IRS-1 correlates with a markedly increased rRNA synthesis. Our experiments suggest that nuclear IRS-1 may play a specialized role in rRNA synthesis and/or processing.The insulin receptor substrate (IRS) 1 proteins are a family of docking proteins, which include IRS-1-4, Gab-1, and p62 dok (1). IRS-1 was the first to be identified as a docking protein for both the insulin and the IGF-I receptors. It transmits a signal from both receptors by interacting with a number of partners, including phosphatidylinositol 3-kinase, SHP2, Grb2, Crk, and others (2, 3). Tyrosine kinase activity of the receptors and phosphorylation of IRS-1 are essential steps in signal transduction. Of the downstream signals generated by IRS-1, the best studied is the phosphatidylinositol 3-kinase signaling pathway, which plays a major role in a number of biological responses to growth factors (1, 4). IRS-1 interacts directly with both the insulin and the IGF-I receptors, and the domains required for their interaction have also been identified (5). Because of its direct interaction with the receptors, its size, and its downstream signaling, it has been generally assumed that IRS-1 is an exclusively cytosolic (or plasma membrane) protein (6, 7). However, IRS-1 is known to interact with the SV40 T antigen (8, 9) and nucleolin (10). T antigen and nucleolin are predominantly nuclear proteins, although minor fractions of either protein can be found in the cytosol (11-13). It has been therefore tacitly assumed that IRS-1, anchored to the receptor, was interacting with the minor cytosolic fractions of T antigen and nucleolin. There is evidence, however, that signal-transduction molecules can translocate to the nucleus. They include mitogen-activated protein kinase (14), p70 S6K /TOR (15, 16), the STAT proteins (17, 18), Akt (19), ␤-catenin (20), the epidermal growth factor receptor (21), phosphatases (22), IRS-3 (23), and a cleaved ErbB-4 receptor (24). Indeed, Jans and Hassan (25) have summarized in a review the evidence that growth factors and their receptors can accumulate in the nuclei of cells. IGF-I, IGF-IR, and IRS-1 are not mentioned in that review, but insulin is (26).T...