Abstract #4402: Specific contributions of Irs-1 and Irs-2 in regulating mammary tumor metastasis
Authors: Shannon Gibson Pankratz, Zhefu Ma, Yumiko Fine, Jenny Janusis, Leslie Shaw
Journal: Cancer Research (2009)

The Insulin Receptor Substrate (IRS) proteins are docking molecules that function as essential signaling intermediates downstream of receptors that have been associated with cancer progression, including the Insulin-like Growth Factor-1 receptor (IGF-IR) and the \#945;6\#946;4-integrin receptor. We have previously demonstrated that IRS-1 and IRS-2 play distinct roles in mammary tumor progression. Specifically, Irs-2-/- tumors are less metastatic, and Irs-1-/- tumors are more metastatic, than tumors from wild-type mice. The results from our in vivo studies support that IRS-1 negatively impacts mammary tumor metastasis whereas IRS-2 positively regulates metastasis, and underscore the importance of identifying the specific contributions of each protein to tumor metastasis. Irs-1-/- tumors exhibit increased Irs-2 expression and activation, but a corresponding increase in Irs-1 expression is not observed in Irs-2-/- tumors. The upregulation of Irs-2 in Irs-1-/- tumors correlates with an increase in S6 Kinase (S6K) activation. Suppression of Irs-1 expression by RNAi also led to an upregulation of Irs-2 activity as well as an increase in S6K activation. When Irs-2 expression was knocked down to wild-type levels in Irs-1-/- cells, S6K activation was also restored to the level observed in wild-type cells, indicating that Irs-2 compensates for the loss of Irs-1 and mediates the activation of the Akt/mTOR signaling pathway. In light of these results, we hypothesized that IRS-1 inactivation is required for mammary tumor metastasis and that the inactivation of IRS-1 induces an upregulation of IRS-2-dependent signaling pathways that promote metastasis. To this end, we have examined the invasive potential of Irs-1-/- cells in which Irs-2 expression has been stably knocked down via shRNA (Irs-1-/-,Irs-2sh). Our results indicate that loss of Irs-2 in Irs-1-/- cells restores the level of invasion to that of wild-type cells, again indicating that Irs-2 promotes invasion and metastasis in the absence of Irs-1. Likewise, we used shRNA to stably knock down Irs-1 in Irs-2-/- cells (Irs-2-/-,Irs-1sh). Suppression of Irs-1 in Irs-2-/- cells resulted in only a modest increase in invasion, which contrasts with the marked increase in invasion we observed when Irs-2 is expressed. We have also examined both mammary tumor growth rate and metastasis to the lungs after orthotopic injection of cells into the mammary fat pad. Although Irs-2-/-,Irs-1sh cells exhibited a diminished tumor growth rate, the frequency of tumor metastasis was approximately 3-fold higher than that observed for Irs-2-/-,GFPsh cells. Together our data provide evidence that both Irs-1 and Irs-2 play active roles in regulating tumor progression with Irs-1 suppressing and Irs-2 promoting metastasis. (Supported by the American Cancer Society, New England Division-Virginia Cochary Award for Excellence in Breast Cancer Research)   Citation Information:  In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 4402.