Question:
Disturbance of cell polarity (CP) has been shown in many types of epithelial cancers and is frequently associated with delocalization of structural proteins, which may alter cellular signaling and cell fate. The scaffold proteins IQGAP1 and IQGAP2 are involved in the regulation of several cellular processes maintaining liver homeostasis. Recently, a switch in the expression levels of IQGAP1 and IQGAP2 during liver cancer progression has been shown, which results in increased IQGAP1 levels and strongly reduced or even abolished IQGAP2 expression. We hypothesize that the decrease of IQGAP2 levels occurs due to a cell selection process, in which the change in compartmentalization of IQGAP2 alters CP and results from liver cell damage in the early stage of hepatocarcinogenesis.
Methods:
Tissue microarrays (TMAs) from human hepatocellular carcinoma (HCC) and surrounding non-neoplastic tissue were stained for cell polarity markers and IQGAP1 and IQGAP2. The relationship between localization of the two scaffold proteins and CP was analyzed in different HCC cell lines, ubiquitously expressing high IQGAP1 and a variable level of IQGAP2 (Huh6 and Huh7 high, HLE low) by immunofluorescence. In order to induce polarization in vitro, cells were grown in collagen double layers. Co-immunoprecipitation studies were performed to identify functionally relevant interactors of IQGAP2. The dynamics of NFκB and JNK signalling was analyzed in cell lines with different cellular localization of IQGAP2.
Results:
TMAs analysis revealed IQGAP2 mislocalization and partial or total loss of CP in HCC samples. Collagen double layer revealed the development of liver-like structures in IQGAP2 high cells (Huh6 and HepG2) in vitro. Time course experiments after TNF-α stimulation revealed an increased NFκB amplitude for cell lines with no/low IQGAP2 expression. Physical interactions between IQGAP2, ASK1 and c-FLIP proteins were demonstrated by IP and suggested an interesting IQGAP2 network downstream of the signalosome.
Conclusion:
The ability to form bile canaliculi correlates with both presence and proper localization of IQGAP2, thus the physiological localization of IQGAP2 may be a prerequisite for the maintenance of CP. Specific interactions between IQGAP2 and proteins involved in the apoptotic/survival process (ASK1/c-FLIP) may support our selection theory during liver cancer development. In the context of inflammatory microenvironment cells with mislocalized IQGAP2 might be prone to cell death, which may explain the elimination of IQGAP2 expressing liver cells during the progression of chronic liver disease. In vivo experiments are needed to validate this hypothesis.
