Studies on the PI3K/mTOR pathway as cytostatic treatment target in pituitary adenomas

Pituitary adenomas are benign neoplasms accounting for 15% of all intracranial tumors. They are associated with significant clinical syndromes due to the hormonal excess they produce or to visual/cranial disturbances because of their considerable intracranial mass. Surgery is the primary means for the management of pituitary tumor mass, but it comes with considerable side effects to the patients and their quality of life. Tumor shrinkage by pharmacological agents currently used in neuroenocrinology, such as, somatostatin analogs (SSA) is not observed in a large fraction of pituitary adenomas. Therefore, efforts are taken to investigate how to overcome the resistance to existing treatments and to identify new cytostatic therapeutic agents. The PI3K/Akt/mTOR signaling pathway is frequently overactivated in a variety of tumors rendering them resistant to chemo- and radiotherapy. The present study shows that Akt overactivation confers resistance to the antiproliferative action of the SSA octreotide in pituitary tumor cells. Blocking the Akt pathway downstream with rapamycin rendered those cells sensitive to octeotide’s antiproliferative action. However, the efficacy of the combined treatment was not due to the antiproliferative action of rapamycin, since most of the tumors did not respond to this pharmacological agent. Rapamycin and its analogs (rapalogs) have a cytostatic effect in various tumors. However, resistance to rapalog treatment is reported with increasing frequency due to the elimination of the negative feedback loop exerted by the mTOR substrate p70/S6K on IRS-1. Rapamycin, by inhibiting mTOR and p70/S6K, decreases the inhibitory IRS-1 serine phosphorylation, activates IRS-1 and increases Akt-Ser473 phosphorylation. The present study shows for the first time that activating the G protein-coupled receptor Sstr2 with octreotide blocks the rapamycin-induced IRS-1 activation by increasing its inhibitory serine and decreasing its stimulatory tyrosine phosphorylation. This leads to decreased Akt-Ser473 phosphorylation in a mechanism involving the phosphotyrosine phosphatase SHP-1. Both octreotide and rapamycin are cytostatic agents blocking the G1/S cell cycle transition and herein it is seen that their potent antiproliferative action depends on the more potent upregulation of the Cdk2 inhibitor p27/Kip1. A novel drug able to co-target the PI3K pathway up- and downstream is the dual class PI3K/mTOR inhibitor, NVP-BEZ235, which has shown high antiproliferative efficacy in tumors with the overactivated PI3K pathway. In the present study NVP-BEZ235 treatment dramatically decreased cell viability by suppressing the cell cycle activators cyclin E and Cdk2 and upregulating the cell cycle progression inhibitor p27/Kip1. The remarkable sensitivity of pituitary adenomas to NVP-BEZ235 highlights the importance of the Akt dysregulation in their tumor maintenance. Altogether, these data provide new therapeutic cytostatic schemes that could prove beneficial for the management of pituitary macroadenomas. In addition they provide the biochemical basis for combating resistance to rapalog treatment also in other tumor types by concomitant administration of biologicals able to inhibit the PI3K pathway upstream.