Abstract B33: Iqgap2-/- mouse as a model for advanced human hepatocellular carcinoma
Authors: Gnatenko Dmitri V. , Xu Xiao , LaComb Joseph , Ueberheide Beatrix , Zhu Wei , Schmidt Valentina A. 
Published: May 2015

Hepatocellular carcinoma (HCC) is a highly prevalent and deadly cancer. In the United States, HCC incidence rates tripled between 1975 and 2005. HCC is mostly diagnosed in late stage and treatment of advanced HCC remains an area of high unmet medical need. Discovery of novel, more molecularly targeted therapeutic modalities and also diagnostic and prognostic biomarkers for early detection of HCC is urgent. While many molecules have been implicated in hepatic carcinogenesis over the years, a comprehensive mechanistic understanding of the intricate signaling networks responsible for HCC development is still lacking. To establish the physiological role of IQ-motif containing GTPase-activating-like protein 2 (IQGAP2), we generated a conventional Iqgap2 knockout mouse model. It was discovered that 86% of Iqgap2-/- mice developed spontaneous HCC, while mice deficient in both Iqgap2 and its close homolog Iqgap1 genes (Iqgap1-/-/Iqgap2-/-) displayed relative protection against HCC and improved long-term survival (Schmidt et al, MCB 2008). We also showed that IQGAP2 protein expression was reduced and IQGAP1 expression elevated in the majority of human HCC tumors studied [(N = 82), White et al, BMC Gastroenterology 2010]. These suggested that IQGAP2 may be a novel tumor suppressor and IQGAP1, a bona fide oncogene, antagonizes activity of IQGAP2 in liver.

To delineate the mechanism behind the tumor suppressive action of IQGAP2 in the pathogenesis of HCC, an array of assays was conducted, including Affymetrix RNA microarray, cell-based assays such as proliferation, migration and invasion, and also immunoprecipitation and LC-MS/MS proteomics. Further, to establish the relevance of the Iqgap2-/- model to human disease, a cross-species comparison of human and Iqgap2-/- HCC tumors was performed using Significance Analysis of Microarray (SAM) and unsupervised hierarchical clustering analysis.

We found that IQGAP2 silencing results in PI3K/Akt signaling activation in liver. Insulin stimulation experiments showed that both Iqgap2−/− and Iqgap1−/−/Iqgap2−/− mice had elevated (~3-fold) hepatic levels of the phosphorylated (at Ser473) form of Akt kinase, along with increased levels of the phosphorylated (at Ser9) form of GSK3β compared to wild-type controls. Similar data were obtained in HepG2 cells stably expressing IQGAP2 shRNA and stimulated with either insulin or IGF-1. Next, IQGAP2 was identified as an Akt binding partner in mouse liver lysates. In vitro studies in both HCC cell lines and MEFs showed that lack of IQGAP2 was associated with increased cell proliferation and migration, consistent with PI3K/Akt signaling activation. RNA microarray also identified the Wnt/β-catenin signaling pathway as the top canonical pathway dysregulated in Iqgap2-/- mouse livers. This suggests that IQGAP2, being a scaffolding protein, may realize its tumor suppressing function through cross-linking several signaling pathways in liver. Finally, Iqgap2-/- mouse hepatic tumors shared genetic signatures with HCC tumors from patients with advanced disease as evidenced by a 78% mouse-to-human microarray data set concordance rate with 117 out of 151 identified ortholog genes having similar expression profiles across the two species.

Collectively, our results indicate that the Iqgap2 knockout mouse model closely recapitulates human HCC at the molecular level and supports its further application for the study of this disease. Modulation of both IQGAP1 and IQGAP2 expression represents a new potential therapeutic strategy for liver cancer.

Citation Format: Dmitri V. Gnatenko, Xiao Xu, Joseph LaComb, Beatrix Ueberheide, Wei Zhu, Valentina A. Schmidt. Iqgap2-/- mouse as a model for advanced human hepatocellular carcinoma. [abstract]. In: Proceedings of the AACR Special Conference: The Translational Impact of Model Organisms in Cancer; Nov 5-8, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(11 Suppl):Abstract nr B33.