Immunohistochemical detection of somatostatin receptor subtypes sst1 and sst2A in human somatostatin receptor positive tumors

Although in situ hybridization has been used to examine the distribution of messenger RNA for somatostatin receptor subtypes (sst) in human tumors, the cellular localization of sst(1) and sst(2A) receptors has not been reported. In this study, we describe the cellular localization of human sst(1) and sst(2A) receptor proteins in both cryostat- and paraffin-embedded sections of 25 human tumor tissues using two recently developed polyclonal antibodies. Six somatostatin (SS) receptor (SSR) positive tumors (two gastrinomas, three carcinoids, one pheockromocytoma) and one SSR negative tumor (renal cell carcinoma), selected by positive and negative SSR autoradiography, respectively, were studied by both immunohistochemistry and Western blot analysis. The six SSR positive tumors expressed sst(2A), while 4 of 5 expressed sst(1) as well. The SSR negative tumor did not express either sst(1) or sst(2A). Western blot analysis of wheat germ agglutinin purified membrane proteins confirmed the presence of the sst(1) and sst(2A) glycosylated receptors. The paraffin-embedded sections gave best information with respect to the subcellular localization Sst(1) immunoreactivity was observed both on the membrane and in the cytoplasm, while sst(2A) showed predominantly membrane-associated immunoreactivity. This subcellular distribution of sst(1) or sst(2A) receptors was confirmed in paraffin-embedded sections of 8 additional intestinal carcinoids, 5 gastrinomas and 5 pheochromocytomas. Sst(1) receptors were detected in 7 out of 8 carcinoids, in all gastrinomas, and in 4 out of 5 pheochromocytomas, white 6 out of 8 carcinoids, all gastrinomas, and 3 out of 5 pheochromocytomas expressed sst(2A) receptors. In conclusion, sst(1) and sst(2A) receptors show a differential subcellular localization in human SSR positive tumors. The use of SSR subtype selective antibodies to detect the subcellular distribution of SSR subtypes in individual tumor cells is an important step forward to understand more about the pathophysiological role of the different SSR subtypes in human tumors.