Role of the IQGAP1/IQGAP2 imbalance in Liver Cancer

Background and Aims: The scaffold proteins IQGAP1 and IQGAP2 are involved in the regulation of several cellular processes affecting liver homeostasis. Several studies associated specific cellular functions to the localization and/or the protein levels of IQGAP1 and IQGAP2, which seem to modulate pathways related to liver cancer (e.g. Hippo-YAP, ß-catenin, MAPK). While IQGAP2 is exclusively expressed in the liver, the IQGAP1 protein is ubiquitously expressed, albeit at low levels in healthy adult liver. Recently, a switch in the IQGAP1 and IQGAP2 expression during the progression of liver cancer has been shown, which results in increased IQGAP1 levels and strongly reduced or even abolished IQGAP2 expression in human hepatocellular carcinoma (HCC).

Methods: We took advantage of different HCC-derived cell lines ubiquitously presenting high IQGAP1 expression but varying levels of IQGAP2 (Huh6 and Huh7 high levels, HLE and SK-Hep1 low levels). Expression levels of IQGAP1, IQGAP2 and candidate downstream targets (YAP, ß-catenin, AKT) together with potential common effectors (CDC42, angiomotin (AMOT)) were measured by Western immunoblot and quantitative PCR, respectively. Co-immunprecipitation (Co-IP) experiments were carried out to identify interaction partners of IQGAP1 and IQGAP2. Affinity pull-down assay was used to detect the CDC42 activity in HCC cell lines. As bile acids may affect the IQGAP1 expression level [Cell Rep. 2013 Nov 27;5(4):1060 – 9], HCC cells were treated with chenodeoxycholic acid (CDCA), and the effect on IQGAP1/2 complex composition and CDC42 activity was determined.

Results: Western immunoblot analysis revealed that HCC cells with high IQGAP2 expression (Huh6, Huh7) show high ß-catenin levels, while HCC cells characterized by a high IQGAP1 expression reveal an upregulation of YAP expression. Co-IP analyses revealed that IQGAP1 interacts with YAP and AMOT in a possible functional complex in HLE and Huh6 cells, while IQGAP2 exclusively binds to AMOT in Huh6 cells. Furthermore a binding between IQGAP1 and CDC42 was shown for all HCC cell lines analyzed, whereas the interaction between CDC42 and IQGAP2 could be only demonstrated in the IQGAP2-high expressing cell lines Huh6 and Huh7, respectively. Low dose CDCA (50µM) treatment increased the binding of IQGAP2 to CDC42 in Huh6 and Huh7 with no interaction detectable in HLE cells.

Conclusion and Outlook: This study demonstrated the relevance of the IQGAP1/IQGAP2 ratio for the modulation of downstream signaling pathways involved in liver cancer progression (e.g., YAP, ß-catenin, CDC42). HCC cell lines with a high IQGAP2/IQGAP1 ratio (Huh6, Huh7) showed a higher affinity binding between CDC42 and IQGAP2. Further in vitro and in vivo experiments are needed to understand the dynamic interplay between CDC42 and IQGAP1 respectively IQGAP2, which seem to modulate specific cellular functions and mechanisms critical for the selection of cancer cells during liver carcinogenesis.