IMMUNOHISTOCHEMICAL LOCALIZATION OF SOMATOSTATIN RECEPTORS SST2A IN HUMAN TUMORS

Human tamers frequently express somatostatin receptors, However, none of the receptor subtype proteins have been individually visualized in normal or neoplastic human tissues. Here, the distribution of the sst2 A receptor was investigated using immunohistochemistry with the specif ic anti-peptide antibody R2-88 in 47 human tumors. All tumors selected for their abundance of sst2 mRNA and/or strong binding of the sst2-pr eferring ligand I-125-labeled Tyr(3)-octreotide were specifically immu nostained with R2-88, Conversely, all tumors without somatostatin bind ing or expressing predominantly other somatostatin receptor subtype mR NAs (sst1 or sst3) were not specifically immunostained by R2-88, Speci ficity was shown in immunoblots, demonstrating receptor migration as a 70-kd broad band. In immunohistochemical and immunoblotting experimen ts, the abolition of staining after antibody blockade with antigen pep tide was demonstrated, Immunostaining was identified in cryostat and i n formalin-fixed, paraffin-embedded sections. Heat-induced epitope ret rieval aias necessary to visualize sst2A receptors in formalin-fixed s ections. Moreover, because of occasional high nonspecific staining, th e demonstration of complete abolition of immunostaining by treatment w ith antigen peptide was a prerequisite for the correct identification of sst2A-positive tumors. The sst2A receptors were clearly located at the membrane of the tumor cells. These results provide the first local ization of a somatostatin receptor subtype in human tissues at the cel lular level. The sst2A receptor identification and visualization in tu mors with simple immunohistochemical methods in formalin-fixed, paraff in-embedded material will open new diagnostic opportunities for pathol ogists.