Prostatic acid phosphatase in carcinoid tumors. Immunohistochemical and immunoblot studies.

The immunohistochemical demonstration of prostatic acid phosphatase (PAcP) and/or prostate-specific antigen (PSA) has been accepted as being reliable in identifying metastatic adenocarcinoma of prostate origin. However, islet cell tumors, especially hindgut-derived carcinoid tumors, have occasionally been reported to be positive for PAcP. We therefore studied a series of carcinoid tumors of the lung and gastrointestinal tract immunohistochemically for PAcP expression by using two polyclonal antibodies and one monoclonal antibody. Thirty-three carcinoid tumors were examined. All five rectal carcinoids in the series showed convincing PAcP positivity with at least two of the three anti-PAcP antibodies. No significant PAcP positivity was observed in the remaining 28 foregut- and midgut-derived carcinoid tumors, except for weak focal positivity in one lung carcinoid. PSA antibody reacted negatively in all cases. Western blots of an aqueous cell lysate from one rectal carcinoid revealed protein bands in the region of 45-55 kd that immunoreacted with anti-PAcP antibodies, confirming the validity of the immunostains. These results suggest that PAcP positivity is common in rectal carcinoid tumors and that it most likely represents true PAcP expression. This seemingly aberrant protein expression may be explained by the shared cloacal derivation of the rectum and prostate, giving rise to cells with both endocrine and partial prostatic epithelial differentiation.