Abstract 4865: Pak1 links the Wnt/β-catenin pathway to ErbB2 signaling in breast cancer cells

Abstract

Abstract Several studies suggest that Pak1 plays an important role in the genesis of breast cancer, but despite the similarity in their sequences, the contribution of the closely related enzyme Pak2 to this disease remains unclear. To establish the function of Paks in ErbB2 driven transformation, we used a three-dimensional tissue culture system of human mammary epithelial cells that stably co-express a chemically-activatable form of ErbB2 and doxycycline (DOX) inducible shRNAs against Pak1 or Pak2. We observed that silencing of Pak1 but not of Pak2 has a dramatic effect in cell proliferation and apoptosis. As expected, activation of ErbB2 induced growth factor-independent proliferation and disruption of 3D acinar-like structures via ERK and Akt signaling. In contrast, silencing of Pak1 compromised activation of ERK, resulting in reversion of the malignant phenotype and restoration of normal acinar architecture. Surprisingly, silencing of Pak2 also compromised ERK activation, but cannot restore normal morphology. Further, we found that ErbB2-amplified breast cancer cells expressing a DOX inducible shRNA against Pak1 formed significantly smaller tumors than cells expressing either GFP or a DOX inducible shRNA against Pak2 in SCID mice as the result of the inhibition of ERK. A phosphoproteomic analysis showed that several signaling pathways, e.g. Wnt and Mdm2-p53, are down regulated in our Pak1 deficient cell line. We found that the shPak1 cell line, but not the shPak2 deficient cell line has a dramatic reduction in total β-catenin levels, and that blocking β-catenin with chemical inhibitors restored normal acinar architecture. Finally, we found that targeting Pak and/or β-catenin in combination with trastuzumab sensitized trastuzumab-resistant breast cancer cells to trastuzumab, suggesting the potential clinical application of this strategy to overcome trastuzumab resistance. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4865. doi:1538-7445.AM2012-4865