Jean Claude Reubi

Immunohistochemical Localization of Somatostatin Receptors sst2A in Human Tumors

Human tumors 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 sst2A receptor was investigated using immunohistochemistry with the specific 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-preferring ligand 125I-labeled Tyr3-octreotide were specifically immunostained with R2-88. Conversely, all tumors without somatostatin binding or expressing predominantly other somatostatin receptor subtype mRNAs (sst1 or sst3) were not specifically immunostained by R2-88. Specificity was shown in immunoblots, demonstrating receptor migration as a 70-kd broad band. In immunohistochemical and immunoblotting experiments, the abolition of staining after antibody blockade with antigen peptide was demonstrated. Immunostaining was identified in cryostat and in formalin-fixed, paraffin-embedded sections. Heat-induced epitope retrieval was necessary to visualize sst2A receptors in formalin-fixed sections. Moreover, because of occasional high nonspecific staining, the demonstration of complete abolition of immunostaining by treatment with 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 localization of a somatostatin receptor subtype in human tissues at the cellular level. The sst2A receptor identification and visualization in tumors with simple immunohistochemical methods in formalin-fixed, paraffin-embedded material will open new diagnostic opportunities for pathologists.

Expression of Somatostatin Receptors in Human Tissues in Health and Disease

The knowledge of SSTR expression in tumors and normal tissues is a prerequisite for clinical investigations with somatostatin analogs. Indeed, the high expression of SSTRs in tumors represents the molecular basis for novel diagnostical and radiotherapeutical applications of somatostatin analogs (67, 68). Moreover, the presence of SSTRs may, in some instances, such as neuroblastomas, represent a good prognostic factor for patient survival (69, 70), while the absence of receptors is related to early death. Furthermore, the absence of SSTRs, such as sst2 in pancreatic cancers, may represent a significant growth advantage to these tumors (17). One of the main challenges for the future will be to unravel the various physiological and pathophysiological roles of somatostatin mediated by SSTR subtypes, and identify potential therapeutical indications for the newly developed subtypeselective somatostatin analogs (27–29).

N-Methyl Scan of a sst1-Selective Somatostatin (SRIF) Analog

Tyrosine residue substitutions at position 2, 7 and 11 in des-AA1,2,5-[D-Trp8,IAmp9]-SRIF (CH-275) resulted in peptides not only with improved affinity and selectivity for sstl, but suitable for radioiodination as well [1]. Here, we present the effect of methylation of the backbone nitrogen of all residues in des-AA1,2,5-[D2Nal8,IAmp9]-SRIF (2) (Table 1) on receptor binding affinity. Nα-Methyl amino acid substitutions may limit conformational freedom of a peptide, block its proteolytic enzyme cleavage sites, and as a result yield analogs with increased potency and selectivity [2,3].