Somatostatin receptor genes are expressed in lymphocytes from retroorbital tissues in Graves' disease.

Abstract

The radiolabeled somatostatin (SST) analog octreotide accumulates within the orbits of active Graves' ophthalmopathy (GO), and octreotide and lanreotide have been proposed to treat this disorder. It is still unclear which retroorbital cells the SST analogs target. Lymphocytic infiltration of retroorbital tissues is a peculiarity of GO, and labeled octreotide could accumulate at specific sites on retroorbital-activated lymphocytes. The accumulation of radiolabeled analogs is due to the interaction with specific cell surface SST receptors. Five subtypes of somatostatin receptors (SST1-5), member of the G protein-coupled, seven-transmembrane superfamily, are described. It still unknown which SST subtype is expressed in retroorbital activated lymphocytes. The aim of this study was to evaluate the expression of SST1-5 genes in lymphocytes recovered from retroorbital tissues obtained from patients with GO undergoing orbital decompression. Cultured phytohemagglutinin-stimulated lymphocytes from retroorbital blood samples, drawn during orbital surgery in five patients with GO and in two control patients without autoimmune or thyroid diseases and without orbital inflammatory conditions, were also studied. RT-PCR of total RNA extracted from lymphocytes was performed using primers for SST1-5 and, as internal control, for glyceraldehyde-3-phosphate dehydrogenase. All SSTs transcripts were found in lymphocytes both from GO retroorbital tissues and blood samples. The levels of expression of SST1, -2, and -4 mRNA were higher than those of the SST3 and -5 transcripts. In the lymphocytes from control subjects, the SST subtypes with high affinity for octreotide were barely found. The presence, even if at different concentrations, of all SST1-5 receptors in retroorbital lymphocytes from GO shows that they are targeted by SST analogs and could explain the effects described in GO patients treated with SST analogs.