somatostatin--n-tyr(1)- and Insulinoma

Somatostatin (SRIF) receptors (sst) are present on normal pancreatic endocrine beta-cells. However, the use of SRIF analogs in the scintigraphic imaging of insulinomas and in the medical management of these tumors seems to be restricted to a subgroup of patients. The aim of this study was to determine the prevalence of sst expression in vitro and characterize sst subtype binding in insulinomas and its correlation with in vivo sst receptor scintigraphy (SRS). In vitro studies were performed on 27 insulinomas from 25 patients: 22 with benign and three with malignant tumors. Semiquantitative RT-PCR of sst mRNAs was performed for 20 of these insulinomas. Sst2 and sst5 were expressed in 70%, sst1 in 50%, and sst3 and sst4 subtypes only in 15-20% of the tumors. (125)I-Tyr(0)DTrp(8)SRIF(14) binding was assessed by quantitative autoradiography in 18 insulinomas, and competition experiments were performed with SRIF(14) and L797-591, L779-976, L796-778, L803-087, L817-818, selective agonists of the five sst subtypes, and BIM23244, a selective agonist of sst2 and sst5. Significant specific binding was observed in 72% of the insulinomas. Displacement experiments with ligands of higher affinity for each of the sst receptors revealed significant binding with the sst2 and sst5 ligands in 72%, sst3 in 44%, sst1 in 44%, and sst4 in 28% of cases. All insulinomas displaying sst2 binding were also sst5 sensitive. However, the ratio of sst5/sst2 displacement was variable and only equal to that for SRIF(14) in experiments with the sst2/sst5 agonist BIM23244. SRS was performed 10 times in nine patients; it detected 60% of the tumors, including metastases of a malignant insulinoma. All the tumors detected by SRS displayed high levels of (125)I-Tyr(0)DTrp(8)SRIF(14) binding. The mechanisms underlying the loss of expression of sst2/sst5 in a third of insulinomas remains to be determined, but this loss of expression may be involved in beta-cell dysfunction.

Topics: Adult; Aged; Autoradiography; Female; Gene Expression Regulation, Neoplastic; Humans; In Vitro Techniques; Insulinoma; Iodine Radioisotopes; Male; Membrane Proteins; Middle Aged; Pancreatic Neoplasms; Radionuclide Imaging; Receptors, Somatostatin; Reverse Transcriptase Polymerase Chain Reaction; RNA, Messenger; Somatostatin

Somatostatin (SRIF) is a neuropeptide which inhibits secretion from a variety of target cells including pancreatic beta-cells. In this study we have used the RINm5F rat insulinoma cell line to characterize high affinity receptors for SRIF. The binding of 0.03 nM [125I-Tyr11]SRIF to RINm5F cells reached a plateau level within 4 h at 37 C at which time 80% of the total binding could be displaced by 100 nM unlabeled SRIF. In contrast, 100 nM concentrations of eight structurally unrelated peptides did not inhibit [125I-Tyr11]SRIF binding. Scatchard analysis indicated that RINm5F cells contained a single class of noninteracting binding sites (910 +/- 190 sites per cell) with high affinity for [125I-Tyr11]SRIF [equilibrium dissociation constant (Kd) = 0.04 +/- 0.01 nM]. Competition experiments with SRIF analogs showed that the binding affinity for [I-Tyr11]SRIF (Kd = 0.03 +/- 0.02 nM) was higher than that for either SRIF (0.24 +/- 0.04 nM) or [Tyr11]SRIF (0.27 +/- 0.04 nM) and that reduced SRIF analogs bound poorly (Kd greater than 50 nM). These results demonstrate that RINm5F cells possess specific, high affinity binding sites for SRIF. Insulin release stimulated by 20 mM leucine or 15 mM glyceraldehyde was inhibited as much as 80% by maximal concentrations (100 nM) of SRIF. The IC50 for SRIF inhibition of leucine-stimulated insulin secretion was 0.43 +/- 0.15 nM, in good agreement with the apparent Kd for binding. In fact, this close correlation between binding affinity and potency to inhibit insulin release was observed for six SRIF analogs, indicating that the characterized binding sites are the receptors which mediate the biological actions of SRIF in RINm5F cells.

Topics: Adenoma, Islet Cell; Animals; Binding, Competitive; Cell Line; Glyceraldehyde; Insulin; Insulin Secretion; Insulinoma; Kinetics; Leucine; Pancreatic Neoplasms; Rats; Receptors, Neurotransmitter; Receptors, Somatostatin; Somatostatin