Abstract 3973: Identification of novel transduceome in TGFβ mediated IAP downregulation in colorectal cancer
Authors: Chowdhury Sanjib , Smith Carol A. , Brattain Michael G. , Howell Gillian M. 
Published: February 2011

Survivin is associated with progression, metastases and poor prognosis in colorectal cancer (CRC). Its key role in promoting cell survival in concert with XIAP makes control of its expression and/or activity a potential therapeutic target in this disease. Our laboratory has previously demonstrated that survivin expression is repressed by the TGFβ signaling pathway along with AKT inactivation in CRC cells (Wang et al 2008). Both mutational and epigenetic silencing of TGFβ RII is a well-documented early event in CRC. Rescue of specific tumor suppressor aspects of the TGFβ signaling pathway may provide therapeutic tumor suppressor benefit without promoting the cell survival and metastatic effects of TGFβ signaling. Therefore, we have studied the molecular mechanisms involved in the TGFβ mediated downregulation of survivin and XIAP in order to identify potential new therapeutic targets for CRC disease.

Here we report that the TGFβ targeted downregulation of survivin and the X-linked IAP (XIAP) is mediated through cAMP-independent PKA activation within 4h of TGFβ treatment. The PKA inhibitor H89 inhibits both PKA activation and the proteasomal mediated survivin/XIAP degradation. Stable knowndown of Smad 2 and Smad 3 in FET colon cancer cells revealed that Smad 3 mediated PKA activation and was independent of Smad 2. The protein phosphatase 2A (PP2A) appears to be downstream of the TGFβ mediated activation of PKA as the PP2A selective inhibitor okadaic acid (OA) rescued XIAP loss and AKT dephosphorylation while the PP1 selective inhibitor calyculin A did not. Further, both H89 and OA abrogated the ubiquitination of XIAP which accompanied TGFβ treatment. We report for the first time an association of XIAP and pAKT which is significantly regulated by TGFβ in a PKA dependent mechanism. Dissociation of the XIAP-pAKT complex also results in the dissociation of the XIAP-survivin complex in a PKA dependent manner. Metastatic (CBS and GEO) colon cancer cells showed little PKA mediated TGFβ effect on the IAP's. However, complementary cell lines in which TGFβ signaling was restored by ectopic expression of TGFβ receptors inhibited their metastatic capability in orthotopic xenografts in association with TGFβ mediated reduction of the IAP's as described above. The PKA scaffolding protein A-kinase anchoring protein (AKAP) is also required for the PKA complex mediated TGFβ downregulation of XIAP and survivin and AKT deactivation as the AKAP inhibitor Ht31 inhibits these effects.

Identification of this novel transduceome provides a new platform for studying metastasis in colorectal cancer. If these TGFβ tumor suppressor effects can be mimicked in late stage tumors therapeutic values against metastatic CRC might be obtained.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3973.