Combination of mTOR Inhibitors Augments Potency while Activating PI3K Signaling in Pituitary Tumors

Background: Despite the success in treating some cancers, the efficacy of the mTOR inhibitors rapalogs as anti-cancer therapeutics has been limited. Aims: We undertook to examine the effects of Torin1, a second-generation selective ATP-competitive mTOR inhibitor, in non-functioning pituitary tumor cells. During characterization of the molecular mechanisms that mediate Torin1 actions, there seemed to be a rationale for combining it with rapalogs. Methods: Proliferation assays, flow cytometry and Western blotting were applied to assess the effects of Torin1, RAD001 and their combination on an MtT/E pituitary cell line and human-derived non-functioning pituitary tumor cells. Results: Combined long treatments of Torin1 and RAD001 induced a pronounced reduction in cell growth and viability of both MtT/E pituitary cells and human-derived non-functioning pituitary tumor cells, superior to each drug alone. This was remarkable in the 10 nM combination and was reflected in a triggered decrease of cyclin D3 and p21/CIP expression. Interestingly, Akt-Thr308 and SIN1-Thr86 phosphorylations were robustly elevated in the combined treatment, accompanied by a reduction in PTEN expression. Phosphorylation of p70S6K was abolished in all individual and combined treatments. Akt-Ser473 phosphorylation, induced by RAD001, was reduced by the combined treatment to the same extent as when treated by Torin1 alone. Conclusions: Our results suggest that the differential signaling mechanisms induced by these compounds eventually converge to lead to an efficient blockade of the PI3K/Akt/mTOR pathway in pituitary tumor cells and may facilitate a reduction in treatment dosage.