Somatostatin receptor gene expression and inhibitory effects of octreotide on primary cultures of orbital fibroblasts from graves' ophthalmopathy

Abstract

To explore the mechanism underlying the effects of the somatostatin (SST) analogue octreotide in Graves' ophthalmopathy (GO), we investigated the expression of SST and of SST receptor (sst1-5) genes in primary cultures of fibroblasts established from retroorbital tissue of GO patients and of control subjects. We determined also SST specific binding sites by competitive binding of [125I-Tyr11]SST-14 and the effect of octreotide on cell growth, cAMP accumulation, Bcl-2 intracellular levels and apoptosis in GO fibroblast primary cultures. All primary cultures expressed the SST gene transcript and one or more ssts that have a high affinity for the two analogues (class 1 sst). The sst2 transcript was found in nine, sst3 in five and sst5 in eight out of ten GO cell cultures. Sst2 was detected in all six, and sst3 in four out of the six control cell cultures. Sst4 was absent from all samples, and sst1 was found only in six out of the ten GO samples. SST-14 and octreotide inhibited the binding of [125I-Tyr11]SST-14 with a half-maximal inhibition of binding (IC50) of 0·80 ± 0·37 and 33·7 ± 33·1 nmol/l respectively in GO cell cultures, and with an IC50 of 0·9 and 1·5 nmol/l in control cultures. Octreotide (10-6 and 10-7 M) significantly decreased (P<0·001) forskolin-induced but not basal cAMP accumulation; at both doses for 72 h it inhibited cell growth (20 and 55% respectively), and induced apoptosis (20 and 40%), and abolished Bcl-2 protein in cell lysates. In conclusion, SST and sst transcripts are expressed and functional in cultured retroorbital fibroblasts. The presence of class 1 sst in GO tissue and the inhibition exerted by octreotide on retroorbital cell growth and activity in vitro may account for the effects of SST analogue administration in vivo in GO.