AUTHOR=Mangili Federica , Esposito Emanuela , Treppiedi Donatella , Catalano Rosa , Marra Giusy , Di Muro Genesio , Barbieri Anna Maria , Locatelli Marco , Lania Andrea G. , Mangone Alessandra , Spada Anna , Arosio Maura , Peverelli Erika , Mantovani Giovanna 

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

JOURNAL=Frontiers in Endocrinology

VOLUME=Volume 13 - 2022

YEAR=2022

URL=https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2022.867822

DOI=10.3389/fendo.2022.867822

ISSN=1664-2392

ABSTRACT=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 tumors (PitNETs) cells 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 β-arrestin 2-dependent mechanism.
The aim of this study was to investigate the efficacy of everolimus combined with DRD2 ago-nist cabergoline in reducing NF-PitNETs primary cells and MMQ cells proliferation, to evaluate AKT phosphorylation and a possible role of β-arrestin 2.
We found that 9 out of 14 NF-PitNETs were resistant to everolimus, but the combined treat-ment with cabergoline inhibited cell proliferation in 7 out of 9 tumors (-31.4±9.9%, p<0.001 vs basal) reduced cyclin D3 expression. In everolimus unresponsive NF-PitNETs group, everolimus deter-mined 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 everoli-mus escape mechanism. Indeed, everolimus did not affect MMQ cells proliferation and increased 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 β-arrestin 2 involvement, silencing experiments were performed in MMQ cells. Our data showed that the lack of β-arrestin 2 prevented everolimus and cabergoline co-treatment inhibitory effects on both p-AKT and cells 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 inhibitors antitumoral activity, paving the way for a potential combined therapy in β-arrestin 2 expressing NF-PitNETs or other PitNETs resistant to conventional treatments.