Inhibitory effect of pasireotide and octreotide on lymphocyte activation

Abstract

Somatostatin (SST) regulates the function of the central and peripheral nervous system, the endocrine and exocrine organs, as well as the vascular and immune system. These actions are mediated by five specific membrane somatostatin receptors. This study compares the effects on human lymphocytes of two long-acting somatostatin analogues that have different receptor affinity: octreotide and pasireotide. Both analogues have an antiproliferative effect on human lymphocyte proliferation, but they act at different concentration and, while octreotide enhances IL10 and inhibits gamma IFN pasireotide inhibits IL2 and gamma IFN. In both sets of experiment the different behaviour of the two analogues could be due to their different affinity to the SSTR subtypes. Finally this study suggest that the growth inhibitory action of somatostatin analogues is an apoptotic phenomenon and it can be mediated by SSTR2a, in the case of octreotide, and by SSTR3 when pasireotide is used or it can be mediated by the heterodimerization of the two receptor.

Introduction

Somatostatin (SS) acts as a neurotransmitter and paracrine/autocrine regulator or, more specifically, as a hormone which regulates several physiological processes, such as secretion, neuromodulation, smooth muscle contractility, nutrient absorption and cell growth (Guillemin, 1978, Lamberts et al., 1996, Patel, 1999). The physiological effects of SS are predominantly inhibitory and all the actions are mediated by interaction with five different types of high-affinity plasma membrane receptors (SSTRs), which are widely expressed through human tissues (Patel, 1999). SSTR1 to 5 are encoded by five non-allelic genes and are intronless, apart from SSTR2, which displays two different splice variants (Patel, 1999, Meyerhof, 1998). SSTRs have seven α-helical transmembrane domains and are coupled to guanine–nucleotide-binding proteins (G-proteins) (Patel, 1999, Meyerhof, 1998) which may trigger several pathways on activation. Down-stream signalling includes modulation of adenylyl cyclase (AC) (SSTR1-5), phosphotyrosine phosphatases (PTPs) (SSTR1, 2 and 3) and mytogen activated protein kinase (MAPK) (SSTR4), as well as calcium and potassium channels and the sodium-proton antiporter (Bruns et al., 1994). All SSTRs inhibit AC, resulting in decreased intracellular cyclic AMP levels (Patel, 1999) which, in turn, influence the activity of protein kinase A (PKA) and, subsequently, of the cAMP response-element binding protein, CREB (Florio et al., 1996). Accordingly, all human SSTR subtypes can be coupled to different phospholipase C (PLC) isoforms (Akbar et al., 1994). In most cells, calcium signalling is down-regulated by SSTR activation, resulting in membrane hyperpolarization (Patel, 1999). However, the signalling events prevailing in specific tissues depend on the distribution of SSTRs and on the correlated signalling elements. The somatostatin receptors (SSTR1-5) exert their antiproliferative action by inducing G0–G1 cell cycle arrest (Cheung and Boyages, 1995) or G2-M apoptosis (Srikant, 1995). Somatostatin modulates cell proliferation, differentiation and apoptosis and stops cell cycle progression by inducing the cyclin-dependent kinase inhibitor p27/Kip1 (Pages et al., 1999, Medina et al., 1999). The mechanisms of somatostatin-induced apoptosis are still mostly unexplained. SRIF has a very short plasma half-life of < 3 min (Patel, 1983), but metabolically stable compounds with SRIF-like properties have been synthesised and are currently used in clinical practice. In 1990, octreotide (SMS 201-995) was one of the first SRIF analogues to be synthesised and used in clinical practice. Octreotide triggers cytostatic and cytotoxic effects and has a general inhibitory effect on the secretion mediated through interaction with somatostatin receptors. The long-acting SST analogues, such pasireotide and SMS 201-995, have been reported to inhibit human lymphocyte proliferation (Perego et al., 1998). These analogues have different affinities for SSTR subtypes: pasireotide has a high affinity for SSTRs 1, 2, 3, and 5 (Bruns et al., 2002) while octreotide (SMS 201-995) binds SSTRs 2 and 5 and, to a lesser extent, SSTR3 (Patel, 1999). Compared with octreotide, pasireotide has an affinity which is 30 times greater with SSTR1, 5 times greater with SSTR3 and 40-times greater with SST5 receptors while its affinity to SSTR2 is 2.5 times lower (Hipkin et al., 1997). Moreover, SSTR internalization, desensitization and/or receptor crosstalk play an important role in determining the tissue specific response to SRIF and its analogues (Liu et al., 2005).

The aims of the present study are two-fold and are to compare the ability of these long-acting somatostatin analogues to inhibit lymphocyte proliferation and to shed some light on the mechanism involved.