Quantitative and functional expression of somatostatin receptor subtypes in human thymocytes

Abstract

We recently demonstrated the expression of somatostatin (SS) and SS receptor (SSR) subtype 1 (sst1), sst2A, and sst3 in normal human thymic tissue and of sst1 and sst2A on isolated thymic epithelial cells (TEC). We also found an inhibitory effect of SS and octreotide on TEC proliferation. In the present study, we further investigated the presence and function of SSR in freshly purified human thymocytes at various stages of development. Thymocytes represent a heterogeneous population of lymphoid cells displaying different levels of maturation and characterized by specific cell surface markers. In this study, we first demonstrated specific high-affinity 125I-Tyr(11)-labeled SS-14 binding on thymocyte membrane homogenates. Subsequently, by RT-PCR, sst2A and sst3 mRNA expression was detected in the whole thymocyte population. After separation of thymocytes into subpopulations, we found by quantitative RT-PCR that sst2A and sst3 are differentially expressed in intermediate/mature and immature thymocytes. The expression of sst3 mRNA was higher in the intermediate/mature CD3+ fraction compared with the immature CD2+CD3- one, whereas sst2A mRNA was less abundant in the intermediate/mature CD3+ thymocytes. In 7-day-cultured thymocytes, SSR subtype mRNA expression was lost. SS-14 significantly inhibited [3H]thymidine incorporation in all thymocyte cultures, indicating the presence of functional receptors. Conversely, octreotide significantly inhibited [3H]thymidine incorporation only in the cultures of immature CD2+CD3- thymocytes. Subtype sst3 is expressed mainly on the intermediate/mature thymocyte fraction, and most of these cells generally die by apoptosis. Because SS-14, but not octreotide, induced a significant increase in the percentage of apoptotic thymocytes, it might be that sst3 is involved in this process. Moreover, sst3 has recently been demonstrated on peripheral human T lymphocytes, which derive directly from mature thymocytes, and SS analogs may induce apoptosis in these cells. Interestingly, CD14+ thymic cells, which are cells belonging to the monocyte-macrophage lineage, selectively expressed sst2A mRNA. Finally, SSR expression in human thymocytes seems to follow a developmental pathway. The heterogeneous expression of SSR within the human thymus on specific cell subsets and the endogenous production of SS as well as SS-like peptides emphasize their role in the bidirectional interactions between the main cell components of the thymus involved in intrathymic T cell maturation.

The role of cortistatin in the human immune system

“…Most of the studies have been carried out using native SRIF, octreotide or lanreotide. Murine lymphocyte proliferation is inhibited in the same extent by lanreotide as native SRIF, indicating that the effect is mediated by sst 2 or possibly sst 3 (Agro et al, 1991 A c c e p t e d M a n u s c r i p t 7 thymocytes expressing sst 2 can be inhibited by octreotide whereas more differentiated thymocytes expressing sst 3 can be inhibited by SRIF and not by octreotide which has only a low affinity for sst 3 (Ferone et al, 2002).…”

“…The expression of these different subtypes appears heterogeneous within the thymic tissue and on different cell subsets. A selective expression of sst 1 and sst 2 has been found on cultured thymic epithelial cells (TEC) (Ferone et al, 2002). TEC are thought to be a potential source of SRIF and CST production within the thymus, since their mRNA has been detected in these cells Ferone et al, 2002).…”

“…The sst 2 /sst 3 switch suggests a role during thymocyte differentiation (Ferone et al, 2002). The spleen has not been studied in depth but, PCR studies show that the spleen expresses both sst 2 and sst 3 , whereas the human bone marrow expresses only sst 2 (Oomen et al, 2000).…”

“…Expression of SRIF and cortistatin (CST) and their receptors in the human immune system and malignancies of the immune system (Dalm et al, 2003b;Dalm et al, 2004a;Dalm et al, 2003c;Dalm et al, 2004b;Ferone et al, 2004;Ferone et al, 2002;Lichtenauer-Kaligis et al, 2004;Oomen et al, 2000;ten Bokum et al, 2000;ten Bokum et al, 1999a;ten Bokum et al, 1999c;van Hagen et al, 1999 A c c e p t e d M a n u s c r i p t M a n u s c r i p t M a n u s c r i p t M a n u s c r i p t…”

Function and expression of somatostatin receptors of the endocrine pancreas

“…NOD diabetes is characterized by the invasion of lymphocytes, which destroy pancreatic B-cells (Hayakawa et al, 1991). It was demonstrated that lymphocytes express SSTR2, and SSTR3, and that the expression pattern of these SSTRs depends upon the degree of lymphocyte maturation (Ferone et al, 2002). Thus, the positive correlation of SSTR2 overexpression with the severity of insulitis could correlate with the degree of the lymphocyte invasion.…”