Somatostatin receptor subtypes: basic pharmacology and tissue distribution

The heptahelical receptor superfamily constitutes the largest single family of transmembrane-signalling molecules that regulate a wide range of physiological processes. The five somatostatin receptors represent a distinct subgroup of this seven transmembrane receptor superfamily. They range in size from 356 to 391 amino acids with 39-57% protein identity between the subtypes with 100 residues strictly conserved among the somatostatin receptor sequences. A high grade of mRNA homology exists in somatostatin receptor subtypes cloned from different species. Following somatostatin receptor binding and functional activity studies, two alternative models of ligand-binding interaction have been hypothesised. One relies on the presence of a binding pocket within the receptor structure constituted by specific amino acids residues, the other denies the presence of such binding structures and suggests that it is the interaction of agonists with specific extracellular and/or transmembrane domains that determine stable receptor structure conformation. Somatostatin receptors, as, indeed, all G-protein-coupled receptors are able to regulate their responsiveness to agonist exposure. This agonist-specific regulation includes three main events, namely, desensitisation, receptor internalisation and receptor degradation. The cell expression of somatostatin receptor subtypes, at the mRNA level, has been characterised in rodent and in human organs with multiple subtype expression in brain and peripheral tissues. (C) 2003 Editrice Gastroenterologica Italiana S.r.l. Published by Elsevier Ltd. All rights reserved

Unlabelled somatostatin analogues in treatment of digestive endocrine tumours

Somatostatin analogues are considered first-line therapy in patients with digestive endocrine tumours. Indeed, several studies have investigated their efficacy in the control of specific symptoms and in the decrease of tumour markers. However, randomised controlled trials are needed in order to better define their role in non functioning tumours and their effect on tumour growth, which have seldom been assessed. Several new drugs have been developed over the last few years such as, for example, new long-acting formulations, universal analogues binding to all five somatostatin receptors subtypes, and cytotoxic analogues, all of which offer a promising therapeutic tool in the near future, even if further studies are needed to determine their efficacy and safety in man. (C) 2003 Editrice Gastroenterologica Italiana S.r.l. Published by Elsevier Ltd. All rights reserved

Macrophage long-term cultures as a cellular model of senescence to study proteostasis in aging immune cells

The cellular theory of aging states that human aging is the result of cellular aging, whereby an increasing proportion of cells reach senescence. During aging humans are more susceptible to infections and to develop pathologies. This could be due to immunosenescence, a condition of decline of the immune system which becomes unable to defend our body in a performing way. Mechanisms of cellular senescence in immune cells have been poorly investigated. We have developed an in vitro model which displays features of senescent cells to study macrophage senescence. This model involves long-term culturing of monocyte-derived human macrophages obtained from buffy coats. After extended culture, macrophages expressed multiple senescent markers: i.e., irreversible cell cycle arrest with increased p21 and p16 levels, lipofuscin accumulation and enlarged flattened shape. They also released pro-inflammatory mediators, as demonstrated by activation of proinflammatory signalling cascades such as NF-kB and p38 MAPK in monocytic THP-1 cells exposed to conditioned medium deriving from senescent cells. In this model we studied whether components of the proteostasis network may change during senescence. Indeed, mounting evidence indicates that the reduced ability to maintain proteostasis is intrinsic to human cell senescence. The Ub/proteasome and the autophagy/lysosomal systems are the main regulator of protein degradation. During macrophage senescence, interconversion between immunoproteasome, typically expressed in immune cells, and constitutive proteasome catalytic subunits was observed by both western immunoblotting and native gel analyses. Moreover, activation of the autophagic pathway occurred as determined by increased LC3II/LC3I and decreased p62 levels. The observed changes may contribute to age-related impairments in macrophage function. Further experiments will be directed to characterize the proteasome and autophagosome systems and their role in macrophage senescence

Human circular colonic smooth muscle cells possess active somatostatin receptors

Background & Aims. Somatostatin alters in vivo colonic motility in different species including humans. Few data are available on a cellular basis that could explain the effects of somatostatin on human colon motility. To address these issues we studied the effects of somatostatin on isolated human circular colonic smooth muscle cells to establish whether its actions are directly or neurally mediated Methods, Circular smooth muscle cells were prepared by enzymatic digestion from surgical specimens of human colon (Sigma) and resuspended in HEPES buffer containing protease inhibitors. Results, Cholecystokinin (1 nM), carbachol (30 nM) and KCI (20 mM) each caused a contraction of 17%, 16.5% and 15%, respectively 1 mu M of either somatostarin-14 somatostatin-28 or SMS 201-995 alone were able to produce a contraction of 5.1 %, 5.7%, and 6.8%, respectively. When smooth muscle cells were preincubated with each of the above-mentioned somatostatin analogs, cholecystokinin-mediated contraction was dose-dependently inhibited only in the presence of antiproteases. The half-maximal effective concentration (EC50) for somatostatin-14, somatostatin-28 and SMS 201-995 were similar (3.5, 5.6 3.2 nM, respectively). Conclusions, Somatostatin acts directly on human circular colonic smooth muscle cells through specific somatostatin receptors. SMS 201-995, a somatostatin receptor subtype-2 preferring analogue, shows a high affinity in inhibiting cholecysrokinin-mediated contraction, suggesting the presence of somatostatin receptor subtype-2 on human circular colonic smooth muscle cells