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

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.