DRD2 Agonist Cabergoline Abolished the Escape Mechanism Induced by mTOR Inhibitor Everolimus in Tumoral Pituitary Cells

The mammalian target of rapamycin (mTOR) inhibitor everolimus has been shown to display antiproliferative effects on a wide spectrum of tumors. In vitro studies demonstrated that everolimus inhibited pituitary neuroendocrine tumor (PitNET) cell growth in a subset of patients. Sensitivity to everolimus is reduced by an escape mechanism that increases AKT phosphorylation (p-AKT), leading to pro-survival pathway activation. Dopamine receptor type 2 (DRD2) mediates a reduction of p-AKT in a subgroup of non-functioning PitNETs (NF-PitNETs) and in prolactin-secreting tumor cells (MMQ cells) through a beta-arrestin 2-dependent mechanism. The aim of this study was to investigate the efficacy of everolimus combined with DRD2 agonist cabergoline in reducing NF-PitNET primary cells and MMQ cell proliferation and to evaluate AKT phosphorylation and a possible role of beta-arrestin 2. We found that 9 out of 14 NF-PitNETs were resistant to everolimus, but the combined treatment with cabergoline inhibited cell proliferation in 7 out of 9 tumors (-31.4 +/- 9.9%, p < 0.001 vs. basal) and reduced cyclin D3 expression. In the everolimus-unresponsive NF-PitNET group, everolimus determined a significant increase of p-AKT/total-AKT ratio (2.1-fold, p < 0.01, vs. basal) that was reverted by cabergoline cotreatment. To investigate the molecular mechanism involved, we used MMQ cells as a model of everolimus escape mechanism. Indeed everolimus did not affect MMQ cell proliferation and increased the p-AKT/total-AKT ratio (+1.53 +/- 0.24-fold, p < 0.001 vs. basal), whereas cabergoline significantly reduced cell proliferation (-22.8 +/- 6.8%, p < 0.001 vs. basal) and p-AKT. The combined treatment of everolimus and cabergoline induced a reduction of both cell proliferation (-34.8 +/- 18%, p < 0.001 vs. basal and p < 0.05 vs. cabergoline alone) and p-AKT/total-AKT ratio (-34.5 +/- 14%, p < 0.001 vs. basal and p < 0.05 vs. cabergoline alone). To test beta-arrestin 2 involvement, silencing experiments were performed in MMQ cells. Our data showed that the lack of beta-arrestin 2 prevented the everolimus and cabergoline cotreatment inhibitory effects on both p-AKT and cell proliferation. In conclusion, this study revealed that cabergoline might overcome the everolimus escape mechanism in NF-PitNETs and tumoral lactotrophs by inhibiting upstream AKT activation. The co-administration of cabergoline might improve mTOR inhibitor antitumoral activity, paving the way for a potential combined therapy in beta-arrestin 2-expressing NF-PitNETs or other PitNETs resistant to conventional treatments.