Calnuc is a
calcium (Ca2+)
binding protein found in both Golgi and cytoplasm, and it may play a role in
G protein- and Ca2+-regulated signal transduction events. This study was designed to investigate the possibility of whether
Calnuc protein might be a
tumor-associated
antigen (TAA) that induces
autoantibody response in human
cancers, and to evaluate the feasibility of the
Calnuc antigen-antibody system as a marker in
cancer detection. Purified full-length recombinant
Calnuc protein was used as an
antigen in
enzyme-linked immunoassay and Western blotting for the detection of
autoantibodies in
cancers. Sera from 447 patients with 9 different types of
cancer were analyzed. Although the frequency of
autoantibody to
Calnuc was found to be 4.7% in total groups of
cancer, it was not significantly different to that of normal individuals (1.2%). However, the frequency of
autoantibody to
Calnuc in
colon cancer (11.5%) was significantly higher than that in normal individuals (1.2%). The expression analysis of
Calnuc in multiple
colon cancer tissues by immunohistochemistry on tissue array further confirmed the high specificity of
Calnuc in
colon cancer. Of 69
colon cancer tissue specimens examined, 41 tissues (59.4%) overexpressed
Calnuc, while normal colon tissues did not show any expression of
Calnuc. The subcellular distribution analysis of
Calnuc examined by subcellular fractionation and immunofluorescence indicates that
Calnuc is a
membrane associated protein and mostly distributed in Golgi, which is consistent with previous reports. With adding
Calnuc into a TAA array (including p53, c-myc,
cyclin B1,
cyclin D1), the cumulative frequency of antibody to multiple TAAs in
colon cancer was raised to 65.4% which is significantly higher than the cumulative frequency in normal individuals (6.1%). This indicates that a mini-array of multiple TAAs which includes
Calnuc might provide a novel non-invasive approach to enhance antibody detection for
colon cancer diagnosis.