Abstract B19: IQGAP1 in the tumor microenvironment suppresses TGF-beta mediated myofibroblastic activation and ensuing tumor growth

Introduction: Transforming growth factor beta (TGF-β) within the tumor microenvironment induces transdifferentiation of quiescent pericytes and related stromal cells into myofibroblasts that promote tumor growth. However mechanisms governing TGF-β mediated myofibroblastic activation remain poorly understood. IQ motif containing GTPase activating protein 1 (IQGAP1) exerts diverse cellular functions, but its role in the tumor microenvironment is unexplored. In this study, we tested a hypothesis that IQGAP1 may interact with TGF-β receptor II (TβRII) and regulate its signaling in mesenchymal-type cells that activate into tumor associated myofibroblasts such as hepatic stellate cells (HSCs), which are resident liver pericytes, and that IQGAP1 of HSCs regulates liver metastatic growth. Experimental procedures: Retroviral and lentiviral based vectors were used to deliver TβRII, IQGAP1 or IQGAP1 shRNA into human primary HSCs. IQGAP1/TβRII interactions were determined by immunofluorescence staining, immunoprecipitation, in vitro GST pull down and in vitro protein binding assays. TGF-β1 activation of HSCs into myofibroblasts was determined by α-SMA immunofluorescence and Western Blot analyses for α-SMA, fibronectin and P-Smad2. The role of stellate cell IQGAP1 in tumorigenesis was assessed by in vitro assays of conditioned media, a HSC/tumor cell co-implantation mouse model and experimental liver metastasis model performed on IQGAP1 knockout mice. Additionally, IQGAP1 protein levels in the myofibroblasts of 29 human colorectal liver metastases were investigated by immunofluorescence staining. Results: IQGAP1 is recruited to TβRII by TGF-β1 with the C-terminal a.a. 1503-1657 region of IQGAP1 mediating IQGAP1/TβRII binding. Through scaffolding the E3 ubiquitin ligase Smurf1 and TβRII, IQGAP1 promotes ubiquitination, lysosomal and proteasomal degradation of TβRII, thus suppressing TβRII and TGF-β1 activation of HSCs into myofibroblasts in vitro. In mice, co-implantation of IQGAP1 knockdown HSCs promotes myofibroblastic activation of HSCs, tumor implantation and growth. In an experimental liver metastasis model, IQGAP1 deficiency in the tumor microenvironment promotes myofibroblastic activation and liver metastatic growth. Indeed, IQGAP1 knockdown in HSCs upregulates paracrine signaling molecules such as stromal derived factor 1 (SDF-1/CXCL12) and hepatic growth factor (HGF) that confer a stimulatory effect on proliferation, migration and survival of tumor cells. Additionally, we found that 24 out of 29 colorectal cancer patients display varying degrees of reduction of IQGAP1 protein in the myofibroblasts of their liver metastases as compared to IQGAP1 expression levels observed in activated HSCs and portal myofibroblasts of the adjacent non-tumorous control liver. Conclusion: Our studies demonstrate that IQGAP1 in the tumor microenvironment suppresses TβRII and the TGFβ1 dependent myofibroblastic activation switch with these events constraining tumor growth and metastasis. Citation Format: Chunsheng Liu, Haitham Abdelhakim, Vijay H. Shah, Ningling Kang. IQGAP1 in the tumor microenvironment suppresses TGF-beta mediated myofibroblastic activation and ensuing tumor growth. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr B19.