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.
