The insulin receptor substrate (IRS) proteins are adaptor molecules that integrate signals generated by receptors that are implicated in human breast cancer. We investigated the specific contribution of IRS-2 to mammary tumor progression using transgenic mice that express the polyoma virus middle T antigen (PyV-MT) in the mammary gland and IRS-2-null (IRS-2 ؊/؊ ) mice. PyV-MT-induced tumor initiation and growth were similar in wild-type (WT) and IRS-2 ؊/؊ mice. However, the latency and incidence of metastasis were significantly decreased in the absence of IRS-2 expression. The contribution of IRS-2 to metastasis is intrinsic to the tumor cells, because IRS-2 ؊/؊ mammary tumor cells did not metastasize when grown orthotopically in the mammary fat pads of WT mice. WT and IRS-2 ؊/؊ tumors contained similar numbers of mitotic cells, but IRS-2 ؊/؊ tumors had a higher incidence of apoptosis than did WT tumors. In vitro, IRS-2 ؊/؊ mammary tumor cells were less invasive and more apoptotic in response to growth factor deprivation than their WT counterparts. In contrast, IRS-1 ؊/؊ tumor cells, which express only IRS-2, were highly invasive and were resistant to apoptotic stimuli. Collectively, our findings reveal an important contribution of IRS-2 to breast cancer metastasis.

The insulin receptor substrate (IRS) proteins are cytoplasmic adaptors that organize signaling complexes downstream of activated cell surface receptors. Here, we show that IRS-1 and IRS-2, despite significant homology, play critical yet distinct functions in breast cancer, and we identify specific signaling pathways that are influenced by IRS-1 using the polyoma virus middle-T (PyV-MT) transgenic mouse model of mammary carcinoma and Irs-1 null (Irs1 ؊/؊ ) mice. The absence of Irs-1 expression enhanced metastatic spread significantly without a significant effect on primary tumor growth. Orthotopic transplant studies revealed that the increased metastatic potential of Irs1-deficient tumor cells is cell autonomous. Mammary tumors that developed in PyV-MT::Irs1 ؊/؊ mice exhibited elevated Irs-2 function and enhanced phosphatidylinositol 3-kinase/ Akt/mTor activity, suggesting that one mechanism by which Irs-1 impedes metastasis is to suppress Irs-2-dependent signaling. In support of this mechanism, reduction of Irs-2 expression in Irs1 ؊/؊ tumor cells restored mTor signaling to wild-type levels. PyV-MT::Irs1 ؊/؊ tumors also exhibited a significant increase in vascular endothelial growth factor expression and microvessel density, which could facilitate their dissemination. The significance of our findings for human breast cancer is heightened by our observation that Irs-1 is inactivated in wild-type, metastatic mammary tumors by serine phosphorylation. Collectively, our findings reveal that inactivation of IRS-1 enhances breast cancer metastasis and support the novel hypothesis that IRS-1 has metastasis suppressor functions for breast cancer.The insulin receptor substrate (IRS) proteins are cytoplasmic docking proteins that function as essential signaling intermediates downstream of activated cell surface receptors, including the insulin, insulin-like growth factor 1 (IGF-1), prolactin, growth hormone (GH), and vascular endothelial growth factor (VEGF) receptors, members of the integrin receptor family, and select cytokine receptors (38,68,70,78,81,85). The IRS proteins are recruited to receptors through pleckstrin homology and phosphotyrosine-binding domains in their N termini and mediate their functions by organizing signaling complexes at sites of receptor activation (81). Upon binding they are phosphorylated on tyrosine residues in their C termini, creating multiple phosphotyrosine binding motifs that recruit downstream effectors, including phosphatidylinositol 3-kinase (PI3K), Grb-2, Fyn, and Shp-2 to initiate intracellular signaling cascades (81). The IRS proteins were originally identified as substrates of the insulin receptor, and they have been predominantly studied for their role in metabolic signaling (81). Although the IRS proteins are highly homologous, there is evidence for unique functions for each of the four IRS family members. Irs-1 null mice are born small and remain runted throughout life, and these mice develop insulin resistance. However, Irs-1 null mice do not develop diabetes, because they mai...

Mediator of DNA damage checkpoint protein-1 (MDC1) is a recently identified nuclear protein that participates in DNA-damage sensing and signaling. Here we report that knockdown of MDC1 by RNA interference results in cellular hypersensitivity to the DNA cross-linking agent mitomycin C and ionizing radiation and in impaired homology-mediated repair of double-strand breaks in DNA. MDC1 forms a complex with Rad51 through a direct interaction with the forkhead-associated domain of MDC1, not the BRCA1 C-terminal domain. Depletion of MDC1 results in impaired formation of Rad51 ionizing radiation-induced foci, reduced amounts of nuclear and chromatin-bound Rad51, and a corresponding increase in Rad51 protein degradation. Together, our findings suggest that MDC1 functions in Rad51-mediated homologous recombination by retaining Rad51 in chromatin.

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