The therapeutic value of somatostatin and its analogues

The third intracellular loop of the human somatostatin receptor 5 is crucial for arrestin binding and receptor internalization after somatostatin stimulation

Somatostatin (SS) is a widely distributed polypeptide that exerts inhibitory effects on hormone secretion and cell proliferation by interacting with five different receptors (SST1-SST5). Beta-arrestins have been implicated in regulating SST internalization, but the structural domains mediating this effect are largely unknown. The aim of this study was to characterize the intracellular mechanisms responsible for internalization of human SST5 in the rat pituitary cell line GH3 and to identify the SST5 structural domains involved in this process. To this purpose we evaluated, by fluorescence microscopy and biochemical assay, the ability of wild-type, progressive C-terminal truncated and third cytoplasmatic loop mutants SST5-DsRed to associate with beta-arrestin-enhanced green fluorescent protein and to internalize under SS28 stimulation. The truncated mutants were comparable to the wild-type receptor with respect to recruitment of beta-arrestin-2 and internalization, whereas the third loop mutants R240W, S242A, and T247A showed the abolishment or reduction of arrestin association and a significant reduction of receptor internalization (14.4%, 29%, and 30.9% vs. 52.4% of wild type) and serine phosphorylation upon SS28 stimulation. Moreover, we evaluated the ability of simultaneous mutation of these three residues (R240, S242, and T247) and C-terminal truncated receptors to internalize. The progressive truncation of the C-terminal tail resulted in a progressive increased internalization (21.6%, 36.7%, and 41%, respectively) with respect to the full-length total third-loop mutant (15%). In conclusion, our results indicate the SST5 third intracellular loop as an important mediator of beta-arrestin/receptor interaction and receptor internalization, whereas they suggest that residues 328-347 within the C terminus may play an inhibitory role in receptor internalization.

Long-term management and outcome for pituitary tumors

As we enter the twenty-first century, neurosurgeons and endocrinologists are armed with a greater variety of treatment options for pituitary adenomas, both secreting and nonsecreting. These include an ever-increased availability of different drugs that can be used for suppression of hypersecretion of pituitary adenomas in conjunction with their shrinkage (at least for the duration of the treatment), surgical techniques that have greatly improved, and newer techniques, such as endoscopic microsurgery, that have been added to the surgeon's armamentarium. Radiation therapy techniques have also improved in terms of structuring the radiation field as well as in terms of dosimetry and delivery.

N-terminal acylation of somatostatin analog with long chain fatty acids enhances its stability and anti-proliferative activity in human breast adenocarcinoma cells

The anti-proliferative activity of the somatostatin analog RC-160 is limited by its short serum half life. To circumvent this limitation, fatty acids of chain lengths ranging from 4 to 18 were individually conjugated to the N-terminal residue of RC-160. The lipophilized derivatives of RC-160 were synthesized, purified and characterized. The anti-proliferative activity of lipophilized-RC-160 on the human breast carcinoma cell line MCF-7, was evaluated in vitro. The long chain lipopeptides like pamitoyl-RC-160 exhibited significantly higher anti-proliferative activity on MCF-7 cells (p<0.001), relative to RC-160. The affinity of RC-160 towards somatostatin receptors remained unaltered by pamitoylation. However, the observed increase in bioactivity was manifested within an optimum range of chain length of the lipoppetide. Increasing the peptide hydrophobicity beyond this range reduced the bioactivity of lipophilized-RC-160. Accordingly, stearoyl-RC-160, manifested lower anti-neoplastic activity and receptor affinity relative to pamitoyl-RC-160 and RC-160 itself. The signaling pathways underlying the antineoplastic activity of these lipopeptides were found to be similar to RC-160. Pamitoyl-RC-160 displayed enhanced inhibition of protein tyrosine kinase activity and intracellular cAMP levels in MCF-7 cells, relative to butanoyl-RC-160 or RC-160 itself. Pamitoyl-RC-160 also displayed greater resistance towards trypsin and serum degradation than RC-160. Lipophilization of RC-160 with long chain fatty acids like pamitic acid improves its stability and anti-proliferative activity, thereby improving the scope of enhancing its therapeutic index. However, the optimization of peptide hydrophobicity seems to be a crucial factor governing the efficacy of bioactive lipopeptides.

Long-acting octreotide LAR compared with lanreotide SR in the treatment of acromegaly

The use of somatostatin analogues for the treatment of acromegaly is now well established. Recently long-acting preparations of octreotide and lanreotide have been introduced. In this study we have assessed the efficacy and tolerability of the long acting somatostatin analogue octreotide LAR in patients with acromegaly, and compared it with lanreotide SR. Five patients with active acromegaly were recruited; they were treated with lanreotide SR for 6 months and then, following a wash-out period, received octreotide LAR for 6 months. They were assessed at baseline, 3- and 6-months, by clinical score, GH and IGF1. Adverse effects were carefully monitored. Both treatments effectively reduced GH and IGF1 levels. Four of five patients achieved a mean GH level of < 2.5 ng/ml with both drugs; with octreotide LAR only, these patients also had GH < 1 ng/ml after oral glucose loading. The clinical symptoms score improved significantly with octreotide LAR, as did the ring size; the clinical score correlated significantly with GH. Blood glucose was not adversely affected. All patients experienced minor GI symptoms with lanreotide SR, but less frequently with octreotide LAR. Both drugs caused biliary stasis and had a tendency to form biliary sludge. Octreotide LAR proved effective for the treatment of acromegaly and was well tolerated. Octreotide LAR had some advantages over lanreotide SR, although the differences were not great.