The neuronal marker protein gene product 9.5 (PGP 9.5) is phenotypically expressed in human breast epithelium, in milk, and in benign and malignant breast tumors.

The aim of this study was to examine the expression of the well-established neuronal marker protein gene product 9.5 (PGP 9.5) in nonneuroendocrine tissues of the human breast. Using antibodies directed against PGP 9.5 in an immunohistological technique, a positive staining of nerves and, surprisingly, a positive cytoplasmic reaction were obtained in normal breast epithelium, in all cases of fibroadenomata (n = 7) and in carcinoma cells in 5 out of 16 cases of breast cancers. In screening several human breast cancer cell lines, a positive immunoreaction was observed in 4 out of 6 of the cell lines. To exclude the possibility that this immunoreactivity was false, positive lysates from the same cell lines and preparations from human breast milk were subjected to NaDodSO4-polyacrylamide gel electrophoresis and Western blotting. This study confirmed the presence of PGP 9.5 in both milk and breast cancer cell lines. Because any contamination with nerve fibers can be excluded having used cell culture material or milk, it can be concluded that the presence of PGP 9.5 in the normal epithelium and in breast cancer cells is genuine. PGP 9.5 expression is, therefore, a feature of normal breast epithelia and breast cancer cells and cannot be regarded as 'neuron' specific.