Abstract 1438: Opposing influence of activin and TGFβ on PI3K and MEK/ERK signaling in colon cancer

Introduction: Colorectal cancer (CRC) is the second deadliest cancer in the US. The overall number of new cases and deaths has decreased, most likely due to increased screening. However, once CRC has metastasized, survival rate drops dramatically. In early stage CRC, the Transforming Growth Factor (TGF) β super family is growth suppressive; in advanced disease, high TGFβ serum levels are associated with poor prognosis. Activin and TGFβ regulate cell growth, apoptosis, and cell migration involving a poorly understood switch from growth suppressive to proinvasive actions. Understanding the switch to metastatic behavior and developing therapeutic strategies and biomarkers to target this process are clinical challenges. We recently observed opposing effects of TGFβ and activin on a downstream target, the cell cycle inhibitor p21. TGFβ induces SMAD4-dependent upregulation of p21, while activin downregulation of p21 is SMAD4-independent. Here, we dissect the unique contributions of mitogenic pathways to a TGFβ or activin-induced metastatic phenotype in colon cancer. Methods: Colon cancer cell lines (+/- SMAD4) were assessed for ligand-induced PI3K and MEK/ERK pathway activation. Protein/phospho-isoform expression and association were determined following knockdown and pharmacologic inhibition of pathway members. TGFβ and activin influence on cell migration via mitogenic pathways was assessed through use of small molecule inhibitors of signaling members. Epithelial mesenchymal transition (EMT) was assessed by altered expression of epithelial and mesenchymal markers. Pathway activation was compared in intestinal tumors from AOM/DSS treated ACVR2 knockout and ACVR2 wild type mice. Mitogenic signaling/growth factor status and p21 localization were correlated in primary colon cancers. Results: Activin, but not TGFβ, led to PI3K activation via interaction of ACVR1 and p85 independent of SMAD4, resulting in p21 downregulation. In contrast, TGFβ increased significantly p21 via MEK/ERK/SMAD4 synergy through a SMAD4-dependent mechanism. While activin induced EMT via PI3K, TGFβ induced EMT via MEK/ERK activation. In vivo, loss of ACVR2 resulted in loss of pAkt, consistent with activin-dependent PI3Ksignaling. In primary colon cancers, loss of nuclear p21 correlated with upstream activation of activin/PI3K while nuclear p21 was associated with TGFβ/MEK/ERK pathway activation. Conclusions: Activin and TGFβ diverge in their pro-migratory, SMAD4-independent signaling in colon cancer and utilize distinct mitogenic signaling to affect p21 localization. p21 localization in colon cancer may warrant further validation as a therapeutic biomarker for targeting TGFβ family receptors. Citation Format: Jessica I. Bauer, Naomi Akagi, Ozkan Ozden, Daniel R. Principe, Timothy Carroll, Seung Hyun Baik, Martina E. Spehlmann, Lars Eckmann, Paul J. Grippo, Barbara Jung. Opposing influence of activin and TGFβ on PI3K and MEK/ERK signaling in colon cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1438. doi:10.1158/1538-7445.AM2015-1438