Reassessment of sst5 Somatostatin Receptor Expression in Normal and Neoplastic Human Tissues Using the Novel Rabbit Monoclonal Antibody UMB-4

Abstract

Objective: The frequent overexpression of somatostatin receptors (sst) in neuroendocrine tumors provides the molecular basis for the diagnostic and therapeutic application of stable somatostatin analogs. Whereas octreotide acts mainly via the sst₂ receptor, the novel pan-somatostatin analog pasireotide exhibits particular high affinity for the sst₅ receptor. To determine whether a patient is a candidate for octreotide or pasireotide therapy, it is important to evaluate the somatostatin receptor status. However, so far highly specific rabbit monoclonal antibodies have been developed for the sst₂ receptor only (clone UMB-1). Methods: Here, we have extensively characterized a novel rabbit monoclonal antibody for the human sst₅ receptor (clone UMB-4). In a comparative immunohistochemical study, the expression of sst₅ and sst₂ receptors was assessed using UMB-4 and UMB-1, respectively. Results: Western blot experiments unequivocally demonstrated that UMB-4 selectively detected its cognate sst₅ receptor and did not cross-react with other proteins present in crude tissue homogenates. UMB-4 yielded a highly effective immunostaining of distinct cell populations in formalin-fixed, paraffin-embedded human tissues with a predominance of plasma membrane staining. In the pituitary, sst₅ was present on all growth hormone (GH)- and adrenocorticotropin hormone (ACTH)-producing cells whereas sst₂ was only observed on a subpopulation of GH-positive cells. Consequently, sst₅ was detectable on the majority of GH and ACTH adenomas. In contrast, sst₂ was only seen on GH but not on ACTH adenomas. Conclusions:The rabbit monoclonal antibodies UMB-4 and UMB-1 will facilitate the assessment of the somatostatin receptor status of human tumors during routine histopathological examinations.