Endocrine Abstracts

Currently, the multi-ligand somatostatin (SS) analogue pasireotide (SOM230) is the only pituitary-targeted drug used to treat patients with Cushing’s disease. SOM230 displays the highest affinity to somatostatin receptor type 5 (SSTR5) and compared to octreotide resulted more effective in reducing ACTH release. Despite its anti-secretory role, SOM230 has been associated with tumor shrinkage in patients subjected to long term treatment, although to date the key factors involved are poor elucidated. The present work aimed to investigate the molecular mechanisms implicated in SOM230-induced cytostatic and cytotoxic effects in ACTH-secreting primary tumour cultures and murine corticotroph tumour cells line, AtT-20. First, by western blot we found SSTR5 expressed at comparable levels in 17 different ACTH-secreting pituitary samples, whereas SSTR2 was detectable in 15 out of 17 tissues. SSTR5 and SSTR2 were expressed in AtT-20 cells. Then, we tested the effect of 96h stimulation with 1 μM SOM230 on cell proliferation in 6 different ACTH-secreting tumors by measuring 5-bromo-20-deoxyuridine incorporation during DNA synthesis. We found a significant in vitro suppression of cell growth in half of the analyzed samples (−12.1±4.3%, P<0.01). Accordingly, SOM230 significantly inhibited cell growth in a dose-dependent manner in AtT-20 cells (−10.5±7.7% at 10 nM, P<0.05; −3.9±10.9% at 100nM, P<0.05; −26.8±8.9% at 1 μM, P<0.01), whilst octreotide was effective only at 1 μM (−13.3±9.1%, P<0.05). To investigate whether direct antiproliferative actions SOM230-mediated might involve MAPK and cyclins pathways, we evaluated the expression level of phospho-ERK1/2 and CD1 in ACTH-secreting primary cultures exposed to 1 μM of SOM230. SOM230 reduced phospho-ERK1/2 levels in 5 of 8 tumours tested (−36.4±20.5%, P<0.01), whereas no significant difference was found in CD1 expression levels in 3 tumours. These data were further confirmed in AtT-20 cells, where octreotide did not have any effect. Furthermore, we found that 48h incubation with 1 μM SOM230 was able to induce a significant increase of caspase 3/7 activity in 2 of 4 ACTH-secreting primary cultures (17±3.6%, P<0.05). Altogether these data suggest a downstream implication of phospho-ERK1/2 inhibition in ACTH-secreting pituitary tumour cells by SOM230 resulting in cell proliferation suppression and indicating that broader-spectrum SS analogues may play a crucial role in the treatment of tumours where the MAPK pathway is overactivated. Moreover, we describe a pro-apoptotic effect of SOM230. Ongoing experiments are aimed to discriminate the specificity effects played by SSTR5 and SSTR2.