Background: Somatostatin analogues are very useful in the treatment of symptomatic neuroendocrine tumours, but effects on proliferation remain unclear. Overexpression of the proto-oncogene protein kinase Akt has been demonstrated in certain endocrine tumours, and activates downstream proteins including mTOR and p70S6K, which play a significant role in cell growth and proliferation. We have therefore explored the site of action of somatostatin in causing inhibition of proliferation in a neuroendocrine cell line.
Aims: To confirm the anti-proliferative effects of SS analogue treatment in a rat insulinoma cell line (INS-1), and to investigate whether the SS analogues act on the PI3K-Akt-p70S6K pathway.
Methods: RT-PCR was used to demonstrate SS receptors (SSTR) in the INS-1 cell lines. MTS and thymidine incorporation were used to determine the effects of the SS analogues octreotide (SSTR2 agonist) and pasireotide (SOM230, Novartis; activation of SSTR-1, 2, 3 and 5) on cell proliferation. Western blotting was used to characterise phosphorylated-Akt and p70S6K expression in the SS-treated cells.
Results: The INS-1 cells expressed SSTR 1, 2, 3 and 5. Treatment with octreotide and pasireotide caused significant dose-responsive inhibition of proliferation. No difference in phospho-Akt (either Ser473 or Ser308) expression was detected in the octreotide-treated INS-1 cell lysates. However, phospho-p70S6K (Thr389) expression was significantly reduced at 10 minutes-6 hours treatment with octreotide 10−9M (P=0.01), while no effect on phospho-p70S6K (Thr229) expression was observed at 30 and 60 minutes. It is known that Thr229 site of phosphorylation is affected by PDK1 upstream of Akt. Treatment with IGF-1 (10nM) increased both phospho-p70S6K (Thr389) and phospho-Akt expression.
Conclusions: Octreotide and pasireotide treatment inhibited proliferation of INS-1 cells and, at a concentration achieved in clinical human use, octreotide attenuated p70S6K (Thr389) phosphorylation, but not Akt phosphorylation. We conclude that SS analogues acts downstream of Akt to inhibit the mTOR-p70S6K pathway.