Internalized Somatostatin Receptor Subtype 2 in Neuroendocrine Tumors of Octreotide-Treated Patients

Context: Somatostatin receptor subtype 2 (sst2) is widely expressed in neuroendocrine tumors and can be visualized immunohistochemically at the cell membrane for diagnostic purposes. Recently, it has been demonstrated in animal sst2 tumor models in vivo that somatostatin analog treatment was able to induce a complete internalization of the tumor sst2. Patients and Methods: In the present study, we evaluated whether sst2 expressed in neuroendocrine tumors of patients treated with octreotide are also internalized. Tumor samples were assessed in patients that were treated with various octreotide modalities before and during surgery and compared with tumor samples from untreated patients. Sst2 immunohistochemistry was performed in all samples with three different sst2 antibodies (R2-88, UMB-1, and SS-800). Sst2 receptor expression was confirmed by immunoblotting and in vitro receptor autoradiography. Results: Patients receiving a high dose of octreotide showed predominantly internalized sst2, and patients with a low dose of octreotide had a variable ratio of internalized vs. membranous sst2, whereas untreated patients had exclusively membranous sst2. The internalized sst2 receptor corresponded to a single sst2 band in immunoblots and to sst2 receptors in in vitro receptor autoradiography. Although generally found in endosome-like structures, internalized sst2 receptors were also identified to a small extent in lysosomes, as seen in colocalization experiments. Conclusion: It is the first evidence showing that sst2 receptors can be internalized in sst2-expressing neuroendocrine tumors in patients under octreotide therapy, providing clues about sst2 receptor biology and trafficking dynamics in patients.

Comparison of ANP binding and sensitivity in brains from hypertensive and normotensive rats

We compared the abundance and sensitivity of atrial natriuretic peptides (ANP) receptors in the brains of spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats and examined the effect of blood pressure on the abundance of brain ANP receptors in several other experimental rat models. Brain slices from SHR generated more guanosine 3',5'-cyclic monophosphate in response to ANP than brain slices from WKY rats. No differences were found in brain particulate guanylate cyclase activity in both strains of rats. In rat brain homogenates, we observed that ANP bound in a specific and saturable fashion to samples from WKY rats, but not in samples from SHR. In vitro receptor autoradiography revealed that ANP binding was reduced in the subfornical organ, the choroid plexus, and the paraventricular nucleus of SHR compared with WKY rat brains. Correction of hypertension in SHR or induction of hypertension in other strains did not affect ANP binding in any of these brain regions. Altogether, our data suggest that the increased sensitivity of SHR brains to the action of ANP may be a consequence of factors other than the abundance of receptors and that it is not secondary to the elevation of blood pressure.