DRD2 agonist cabergoline abolished the escape mechanism induced by mTOR inhibitor everolimus in pituitary neuroendocrine tumoral cells

Abstract PI3K-AKT-mTOR inhibitors have been proposed as alternative therapeutic strategy for tumors resistant to conventional drugs. 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) cell growth and vitality in a subset of patients. Sensitivity to everolimus has been demonstrated to be reduced by an escape mechanism that increases AKT phosphorylation (p-AKT), leading to pro-survival pathway activation. This limited efficacy of everolimus due to AKT re-activation could be overtaken by upstream AKT phosphorylation inhibition. Given it was recently demonstrated that DRD2 mediates a reduction of p-AKT in a subset of NF-PitNETs and in rat prolactin-secreting tumor cells MMQ through a β-arrestin 2-dependent mechanism, the aim of the present work was to test the efficacy of adjuvant treatment with cabergoline in increasing sensitivity of human primary cultured NF-PitNET cells to antiproliferative effects of everolimus. In addition, to test a possible role of β-arrestin2. We found that 9 out of 14 NF-PitNETs were resistant to everolimus 1 nM, but the combined treatment with cabergoline inhibited cell proliferation in 7 out of 9 tumors (-31.4 ± 9.9%, p < 0.001 vs basal), increased p27 and reduced cyclin D3 expression. In everolimus unresponsive NF-PitNETs group, 3 h everolimus treatment determined a significant increase of p-AKT/total-AKT ratio (+2.1-fold, p < 0.01, vs basal), and this effect was reverted by cabergoline cotreatment in all tested samples. To investigate the molecular mechanism involved, we used MMQ cells as a model of everolimus escape mechanism. Indeed, 1nM everolimus did not affect MMQ cells proliferation due to an increase of p- AKT/total-AKT ratio (+1.53 ± 24%, p<0.001 vs basal), whereas cabergoline reduced cell proliferation (-22.8 ± 6.8%, p<0.001 vs basal) and AKT phosphorylation. The combined treatment of everolimus and cabergoline induced a significant reduction of both cell proliferation (-34.8 ± 18%, p<0.001 vs basal) and p-AKT/total-AKT ratio (-34.5 ± 14%, p<0.001 vs basal). To test a possible involvement of β-arrestin 2, 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, these data 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 antitumoural activity, paving the way for a potential combined therapy in β-arrestin 2 expressing NF-PitNETs or other PitNETs resistant to conventional treatments. Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.