Endocrine Abstracts

Somatostatin is a ubiquitous neuropeptidic inhibitor of various cellular functions including endocrine and exocrine secretion. Moreover, this peptide may control cell proliferation in normal and tumoral tissues. Somatostatin biological effects are mediated by five subtype of G protein-coupled receptor, sst1 through sst5. The somatostatin analogs have high affinity for sst2 receptor and the success of in vivo peptide therapy is correlated with high levels of sst2 in the adenomas.

We previously demonstrated the re-sensitivity of human somatotroph adenomas to a somatostatin analog (octreotide) by sst2 expression after adenoviral transfer in adenomas cells in vitro. A ligand-independent activity of sst2 was also demonstrated in human adenomas in this study through an increase in the mortality and a decrease in secretion. Several GPCRs have been shown to exhibit varying degrees in ligand-independent activity including sst2 in a pancreatic adenocarcinoma models and in a corticotroph context.

To investigate more specifically the sst2 ligand-independant and ligand-dependant activity and their impact on cell physiology as proliferation and secretion in somatolactotroph context, an sst2 expressing cell line was generated after lentiviral transfer in the rat somatolactotroph cell line (GH4C1) and compared in vivo and in vitro to a eGFP-GH4C1 cell line. Sst2 mRNA level, sst2 immunostaining and somatostatin binding sites demonstrated a strong sst2 expression in this cell line. The level of somatostatin binding was closer to that found in human somatotroph adenomas.

A ligand independent impact of sst2 was observed on cAMP level, ERK activity, hormonal secretion and, to a lesser extent, on cell viability in early proliferation step, in vitro and in vivo.

Moreover, the octreotide sensitivity was consistently improved in vitro and in vivo, after sst2 transfer.

In conclusion, we have validated a somatotroph cell line, a useful tool to evaluate in vitro and in vivo, new somatostatin analogs.