In vivo and in vitro expression of somatostatin receptors in two human thymomas with similar clinical presentation and different histological features.

Abstract

[(111)In-DTPA0]octreotide scintigraphy allows the in vivo visualization of several types of SS receptor (SSR)-expressing tumors. Among these, thymomas have been recently detected. Here we report on 2 patients admitted for myasthenia gravis and radiological evidence of thymic mass. Although these patients had similar clinical presentation, in vivo SSR scintigraphy displayed a difference in the degree of the [(111)In-DTPA0]octreotide uptake. Considering that both thymic masses had comparable volume, [(111)In-DTPA0]octreotide level was significantly higher in one of the 2 tumors (tumor/background ratio of 5.7 vs 2.6). The SSR subtype expression pattern was studied in vitro on the surgically resected specimens by ligand binding techniques, quantitative reverse transcriptase polymerase chain reaction (RT-PCR) and immunohistochemistry. According to the recent World Health Organization classification, the 2 tumors were classified A and B2 thymomas respectively. In membrane homogenates, we found a higher number of high affinity [125I-Tyr11]-SS-14 binding sites in the B2 thymomas (23.5+/-2.5 vs 12.0+/-0.4 fmol/mg membrane protein; p<0.05). RT-PCR analysis showed sst1, sst2A and sst3 mRNA in the 2 thymoma tissues, whereas SS mRNA was detectable only in the A thymoma. Quantitative evaluation of RT-PCR data showed a comparable expression of the relative amount of sst2A mRNA in both tumors, whereas a significant higher expression of sst3 mRNA in the B2 thymoma. Sst2A immunoreactivity was localized mainly on the endothelium of intratumoral vessels, whereas sst3 was present on either tumoral epithelial cells or normal reactive thymocytes. The expression of sst2A receptors in these tumors is in line with the in vivo visualization by [(111)In-DTPA0]octreotide, which is considered a sst2-preferring ligand. However, since radioligand uptake was significantly higher in the B2 thymoma, which expressed the largest sst3 mRNA levels, it might be possible that this subtype is involved in determining the tumor visualization during SSR scintigraphy. Apart from the affinity of the radioligand for the receptor, also the efficacy of the internalization of the radioligand-receptor complex might play a role in radioactivity accumulation during in vivo SSR scintigraphy. In fact, although octreotide binds with lower affinity to sst3 receptors, this subtype displayed the highest amount of agonist-dependent receptor internalization compared to the other SSR subtypes. Moreover, sst3 was localized on both tumor cells and reactive thymocytes, and these latter cells are characterized by a very active turnover of membrane molecules. Finally, although more cases need to be evaluated, the lack of detection of SS mRNA in the tumor presenting a more aggressive phenotype (B2 thymoma) might have physiopathological or prognostic significance.