Reassessment of sst₃ Somatostatin Receptor Expression in Human Normal and Neoplastic Tissues Using the Novel Rabbit Monoclonal Antibody UMB-5

Abstract

Background: Among the five somatostatin receptors (sst₁-sst₅), the sst₃ receptor displays a distinct pharmacological profile. Like sst₂, the sst₃ receptor efficiently internalizes radiolabeled somatostatin analogs. Unlike sst₂, however, internalized sst₃ receptors are rapidly transferred to lysosomes for degradation. Apart from this, very little is known about the clinical relevance of the sst₃ receptor, which may in part be due to the lack of specific monoclonal sst₃ antibodies. Methods: Here, we have extensively characterized the novel rabbit monoclonal anti-human sst₃ antibody UMB-5 using transfected cells and receptor-expressing tissues. UMB-5 was then subjected to immunohistochemical staining of a series of 190 formalin-fixed, paraffin-embedded normal and neoplastic human tissues. Results: Specificity of UMB-5 was demonstrated by detection of a broad band migrating at a molecular weight of 70,000–85,000 in immunoblots from human pituitary. After enzymatic deglycosylation, the size of this band decreased to a molecular weight of 45,000. Tissue immunostaining was completely abolished by pre-adsorption of UMB-5 with its immunizing peptide. In addition, UMB-5 detected distinct cell populations in human tissues like pancreatic islands, anterior pituitary, adrenal cortex, adrenal medulla, and enteric ganglia, similar to that seen with a rabbit polyclonal antibody generated against a different carboxyl-terminal epitope of the sst₃ receptor. In a comparative immunohistochemical study, UMB-5 yielded predominant plasma membrane staining in the majority of pituitary adenomas, pheochromocytomas, and a subset of neuroendocrine tumors. The sst₃ receptor was also present in many glioblastomas, pancreatic, breast, cervix, and ovarian carcinomas. Conclusion: The rabbit monoclonal antibody UMB-5 may prove of great value in the identification of sst₃-expressing tumors during routine histopathological examinations. Given its unique trafficking properties, these tumors may be potential candidates for sst₃-directed receptor radiotherapy.

Factors predicting pasireotide responsiveness in somatotroph pituitary adenomas resistant to first-generation somatostatin analogues: an immunohistochemical study

“…The introduction of highly specific rabbit monoclonal antibodies has allowed the reassessment of the expression of SSTR1, SSTR2a, SSTR3 and SSTR5 in normal and neoplastic tissues (36,37,38,39), including pituitary adenomas (15,17,40,41). We confirmed a positive …”

A Critical Evaluation of sst3 and sst5 Immunohistochemistry in Human Pituitary Adenomas

“…Immunohistochemistry is the preferable laboratory method because of its wide availability. Promising immunohistochemical monoclonal antibodies have been developed in the last years, including UMB-1 for sst2, UMB-5 for sst3, and UMB-4 for sst5 [22,23]. UMB-1 has been extensively tested and found to be of high clinical utility [20,34,35].…”

“…UMB-1 has been extensively tested and found to be of high clinical utility [20,34,35]. As for the other 2 antibodies, UMB-4 has been evaluated in multiple studies, while UMB-5 is less investigated [21][22][23][24]36]. Several lines of evidence of specificity have been provided, and adequate positivity rates and predominantly membranous staining characteristics, at least for UMB-4, were reported.…”

“…Shortly thereafter, the sst3 antibody UMB-5 and the sst5 antibody UMB-4 were introduced [21][22][23][24]. In comparison with UMB-1, there is less experience with these antibodies regarding their diagnostic value in clinical practice.…”

AIP and the somatostatin system in pituitary tumours

“…Somatotroph and lactotroph adenomas frequently have SSTR1, SSTR5 and SSTR2 expression (Hofland et al 2004, Thodou et al 2006, Taboada et al 2007, Fusco et al 2008, Cuny et al 2012, Ibáñez-Costa et al 2016; and corticotroph adenomas typically express SSTR5 (Arnaldi & Boscaro 2010, Feelders et al 2010, Colao et al 2012, Golor et al 2012, Lu et al 2013, Ibáñez-Costa et al 2016. Classic SSAs aimed to SSTR2 are not effective although corticotroph tumours express high levels of SSTR5 followed by SSTR2 (Hofland et al 2005, Batista et al 2006, de Bruin et al 2009b, Tateno et al 2009, Lupp et al 2012, van der Pas et al 2013, Ibáñez-Costa et al 2016, probably, at least partly, because high levels of glucocorticoids decrease SSTR2 levels (de Bruin et al 2009a). Several studies focused on the role of SSA on gonadotrophderived tumours, particularly on non-functioning pituitary adenomas, tumours with gonadotroph origin but without hormone hypersecretion.…”