In vitro testing of new somatostatin analogs on pituitary tumor cells

Molecular Characterization of Growth Hormone-producing Tumors in the GC Rat Model of Acromegaly

Acromegaly is a disorder resulting from excessive production of growth hormone (GH) and consequent increase of insulin-like growth factor 1 (IGF-I), most frequently caused by pituitary adenomas. Elevated GH and IGF-I levels results in wide range of somatic, cardiovascular, endocrine, metabolic, and gastrointestinal morbidities. Subcutaneous implantation of the GH-secreting GC cell line in rats leads to the formation of tumors. GC tumor-bearing rats develop characteristics that resemble human acromegaly including gigantism and visceromegaly. However, GC tumors remain poorly characterized at a molecular level. In the present work, we report a detailed histological and molecular characterization of GC tumors using immunohistochemistry, molecular biology and imaging techniques. GC tumors display histopathological and molecular features of human GH-producing tumors, including hormone production, cell architecture, senescence activation and alterations in cell cycle gene expression. Furthermore, GC tumors cells displayed sensitivity to somatostatin analogues, drugs that are currently used in the treatment of human GH-producing adenomas, thus supporting the GC tumor model as a translational tool to evaluate therapeutic agents. The information obtained would help to maximize the usefulness of the GC rat model for research and preclinical studies in GH-secreting tumors.

GEP-NETs update: Biotherapy for neuroendocrine tumours

Neuroendocrine tumours (NETs) represent a less frequent and heterogeneous group of tumours, which has experienced, in recent years, a significant increase in effective therapeutic possibilities overcoming the disappointing results from chemotherapy. Initial improvements in treatment strategies came from somatostatin analogues (SSAs) that have widely demonstrated a significant improvement in symptomatic relief and tumour control growth by a complex mechanism of action over cell survival, angiogenesis and immunomodulation. Recent investigations have pointed out novel SSAs with a wider binding profile (pasireotide), chimeric molecules against somatostatin receptors and dopamine receptors and the combination with targeted agents, such as mTOR inhibitors or antiangiogenic agents. Immunotherapy is the second cornerstone in NET treatment and has been represented with interferon alpha for a long time, with a demonstrated activity on tumour and clinical response. Its less manageable adverse events have limited its usage. However, different checkpoints in immune system regulation have been effectively targeted in different solid tumours, and novel approaches are currently arising in NETs. In conclusion, biotherapy remains an active treatment strategy for initial approach in patients with NETs. Further investigation on patients' selection, molecular profiles, treatment sequence or combination and optimisation of current and novel biotherapy agents is required.

Somatostatin dramatically stimulates growth hormone release from primate somatotrophs acting at low doses via somatostatin receptor 5 and cyclic AMP

Somatostatin and cortistatin have been shown to act directly on pituitary somatotrophs to inhibit growth hormone (GH) release. However, previous results from nonprimate species indicate that these peptides can also directly stimulate GH secretion, at low concentrations. The relevance of this phenomenon in a nonhuman primate model was investigated in the present study by testing the impact of somatostatin/cortistatin on GH release in primary pituitary cell cultures from baboons. High doses (> 10(-10) m) of somatostatin/cortistatin did not alter basal GH secretion but blocked GH-releasing hormone (GHRH)- and ghrelin-induced GH release. However, at low concentrations (10(-17)-10(-13) m), somatostatin/cortistatin dramatically stimulated GH release to levels comparable to those evoked by GHRH or ghrelin. Use of somatostatin receptor (sst) specific agonists/antagonists, and signal transduction blockers indicated that sst2 and sst1 activation via intact adenylate cylcase and mitogen-activated protein kinase systems mediated the inhibitory actions of high-concentration somatostatin. By contrast, the stimulatory actions of low-dose somatostatin on GH release were mediated by sst5 signalling through adenylate cylcase/cAMP/protein kinase A and intracellular Ca(2+) pathways, and were additive with ghrelin (not GHRH). Notably, low-concentrations of somatostatin, similar to sst5-agonists, inhibited prolactin release. These results clearly demonstrate that the ultimate impact of somatostatin/cortistatin on hormone release is dose-dependent, cell type-selective and receptor-specific, where the stimulatory effects of low-concentration somatostatin/cortistatin on GH release extend to primates, thereby supporting the notion that this action is relevant in regulating GH secretion in humans.

Antiproliferative effects of somatostatin analogs in endocrine tumours

Somatostatin has been discovered as a somatotroph release inhibitory factor (SRIF), and it has been demonstrated that SRIF and its analogs can inhibit hormone secretion and control the neoplastic bulk of several endocrine tumours. In vitro studies have contributed to the current knowledge of the mechanisms by which SRIF and its analogs may influence endocrine tumour proliferation, opening the way to new possible therapeutic strategies. Here, we focus on the studies concerning the antiproliferative effects of SRIF and its analogs that provide the basis for future investigations, both at basic and clinical levels, into the application of SRIF analogs in the endocrine field.