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Reassessment of sst₂ Somatostatin Receptor Expression in Human Normal and Neoplastic Tissues Using the Novel Rabbit Monoclonal Antibody UMB-1

Department of Pharmacology (T.F., C.D., S.J., S.S.), Julius-Maximilians-University, H 97078 Würzburg, Germany; Department of Pharmacology and Toxicology (C.D., S.S.), Friedrich-Schiller-University, 07743 Jena, Germany; and Department of Pharmacology and Toxicology (A.K., R.S.), Otto-von-Guericke-University, 39120 Magdeburg, Germany

Address all correspondence and requests for reprints to: Stefan Schulz, Department of Pharmacology and Toxicology, Friedrich-Schiller-University, Nonnenplan 4, 07743 Jena, Germany. E-mail: Stefan.Schulz{at}mti.uni-jena.de.

Objective: The overexpression of somatostatin receptor 2 (sst₂) in neuroendocrine tumors is the molecular basis for diagnostic and therapeutic application of the stable somatostatin analog octreotide. Recent evidence has shown that the immunocytochemical evaluation of sst_2A status is of value for predicting response to octreotide therapy and disease prognosis. However, due to the lack of monoclonal and limited availability of specific polyclonal anti-sst_2A antibodies, only very few patients can currently benefit from in vitro sst₂ evaluation.

Methods: In the present study, we extensively characterized the novel rabbit monoclonal anti-sst_2A antibody (clone UMB-1) using tissues from sst₂-deficient mice and their wild-type littermates. UMB-1 was then subjected to a comparative study of immunohistochemistry on a series of histological specimens from formalin-fixed, paraffin-embedded human tumors and adjacent normal tissues.

Results: Immunoprecipitation experiments unequivocally demonstrated that UMB-1 selectively detected its cognate sst_2A and did not cross-react with other proteins present in crude tissue homogenates. The UMB-1 monoclonal antibody, when compared with currently available polyclonal antisera, yielded several times more effective immunohistochemical staining of fixed-embedded tissues with a predominance of plasma membrane staining and very low cytoplasmic signal even without heat-based antigen retrieval. In addition, dual immunofluorescence revealed for the first time that the sst_2A is present on not only gastrin-containing but also ghrelin-containing cells in human gastric mucosa.

Conclusion: Thus, the rabbit monoclonal antibody UMB-1 may prove of great value in the assessment of sst_2A status in human neuroendocrine tumors during routine histopathological examination.

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