Somatostatin receptors and their subtypes in human tumors and in peritumoral vessels

Somatostatin receptors are expressed by a large variety of human tumors. In vitro receptor autoradiographic studies have shown that these tumors can express more than one somatostatin receptor subtype. Whereas the majority of tumors bind octreotide with high affinity, some, ie, prostate tumors, bind octreotide with low affinity only. The discovery of five somatostatin receptor subtypes, sst_1–5, by gene cloning has increased our understanding of somatostatin receptor structure and function. Using in situ hybridization techniques, we found that various human tumors, identified as somatostatin receptor—positive in binding studies, expressed sst₂ mRNA in the majority of cases, whereas sst₁ and sst₃ were less frequent. Often, all three sst were expressed simultaneously. In another recent in situ hybridization study, primary prostate cancers were shown to preferentially express sst₁ rather than sst₂ or sst₃. Moreover, a high incidence of sst₅ was found in growth hormone (GH)-producing pituitary adenomas and, to a lesser extent, in active pituitary adenomas; gastroenteropancreatic (GEP) tumors showed all possible combinations, but with a predominance of sst₂. Overall, the presence of sst₂ mRNA and/or sst₅ generally correlated with the presence of octreotide-binding sites, but with exceptions. These results indicate the highly variable abundance of sst mRNAs in individual somatostatin receptor—containing tumors. Somatostatin receptors were not only found in tumoural tissue, but also in the peritumoral vascular system. This was particularly well studied in colorectal carcinomas, where the peritumoral veins were shown to express in all cases a high density of somatostatin receptors, probably of the sst₂ type, binding octreotide with high affinity. Therefore, the host peritumoral vascular system may be a possible target of somatostatin action in tumor development. Somatostatin may act locally on tumor growth through two different mechanisms dependent on local somatostatin receptor expression: through direct action on tumor cells or through action on peritumoral vessels, which may alter the hemodynamics of the tumoral blood circulation.