Immunofluorometric assay for the metastasis-related protein S100A4: Release of S100A4 from normal blood cells prohibits the use of S100A4 as a tumor marker in plasma and serum

The metastasis-related protein S100A4 is released from tumor cells, and since it is highly expressed in colorectal cancer (CRC), it could be a potential tumor marker in plasma or serum. Monoclonal antibodies (MAbs) were raised against human recombinant S100A4 and shown to detect native and recombinant antigen with high sensitivity and specificity. Using two MAbs, an immunofluorometric assay (IFMA) was established to detect S100A4 in clinical samples with high sensitivity and precision. S100A4 in plasma and serum from patients with CRC was highly influenced by sample hemolysis. Both red blood cells and mononuclear cells were found to contain S100A4, possibly contributing to the measured levels in serum and plasma. Since even very low-level hemolysis influenced the results, a potential contribution from an S100A4-expressing tumor could not be discerned, indicating that S100A4 is not suitable as a plasma or serum tumor marker for CRC. The antibodies and the IFMA may still be useful for research purposes.