Adenomatous polyposis coli (APC) is a tumour suppressor protein that is a critical component of the Wnt signalling pathway. APC has been described as 'the gene for colon cancer' , reflecting its importance in this disease. Mutations in APC have been reported in both hereditary and sporadic forms of colorectal cancer, and are seen at the earliest stages of colorectal cancer which can be observed.
There have been many reports of the subcellular localisation of both full length and truncated APC, many of these conflicting. Work presented here characterises the localisation and interactions of APC using a variety of antibodies directed to various epitopes within APC. Characterisation of a panel of APC antibodies shows that many of the conflicting reports of the localisation of APC resulted from use of
antibodies which detect proteins other than full length APC.
Localisation of APC and P-catenin has been shown to be linked to cell density, cell
type and mutation status of APC and P-catenin. The distribution of truncated APC
and P-catenin is closely linked in sub-confluent SW480 cells, with both being
localised to the nucleus. At high cell density nuclear localisation of P-catenin and truncated APC and co-localisation is lost. We postulated that in SW480 cells the decrease in nuclear P-catenin as cell density increases could be due to an increase in P-catenin bound to E-cadherin with formation of adherens junctions. In coimmunoprecipitation assays an increase in binding between P-catenin and Ecadherin, and a corresponding decrease in binding between P-catenin and APC, was observed at high cell density. Although an increase in P-catenin/E-cadherin interaction was seen with cell density, co-localisation of P-catenin and E-cadherin at the cell membrane was not seen in all cells, and membrane E-cadherin did not appear to be necessary for nuclear exclusion of P-catenin.
An apical protein has been identified, the localisation of which had previously been reported as being that of full length APC. Work presented here shows that this apical staining is not due to full length APC, but instead appears to be a 150 kDa protein identified as a potential novel isoform of APC. Unlike full length APC, this protein does not interact with P-catenin. This potential APC isoform shows variable distribution in epithelial and colorectal cancer cell lines, with similarities to the localisation of Drosophila E-APC, suggesting a similar role in spindle orientation.