Characterization of SF3B1 role in prolactin-secreting pituitary tumors

Somatic mutations in the gene encoding splicing factor 3B subunit 1 (SF3B1), a key component of the splicing machinery, have been described in patients with PRL-secreting pituitary neuroendocrine tumors (PRL-PitNET) and associated with aggressiveness and resistance to pharmacological therapy with dopamine agonists (DAs). Dopamine receptor type 2 (DRD2) represents the main target for PRL-PitNETs treatment with DAs, even if about 10% of patients is resistant. Aims of the study were to: 1) test the effects of SF3B1 inhibitor pladienolide B in tumoral lactotroph cells expressing wild-type or mutated SF3B1R625H; 2) investigate SF3B1 impact on tumoral cells responsiveness to DRD2 agonist cabergoline. Pladienolide B treatment reduced cell proliferation (-45.1(15.3)%,p<0.001), PRL secretion (-19.25(35.1)%,p<0.05) and increased apoptosis (+2.4(2.5)-fold,p<0.05) in rat tumoral MMQ cells. Pladienolide B antimitotic, proapoptotic and antisecretory effects were maintained in primary cultured cells from both resistant and responsive PRL-PitNETs. SF3B1R625H overexpression increased tumoral lactotrophs proliferation and migration. Moreover, the antimitotic efficacy of pladienolide B, but not of cabergoline, was maintained in MMQ cells transfected with SF3B1R625H. Cabergoline effects on cell proliferation, AKT activation, cyclin D3 and p27 were abolished in MMQ cells silenced for SF3B1. Accordingly, SF3B1R625H overexpression and SF3B1 silencing reduced DRD2 expression at both protein and transcript level, an effect reverted by nonsense-mediated decay inhibitor NMDI14. In conclusion, we demonstrated a relevant role of SF3B1 in PRL-PitNET. Indeed, SF3B1 inhibitor pladienolide B exerted antitumoral actions in PRL-PitNET cells bearing wild-type or mutated SF3B1. Moreover, both SF3B1R625H overexpression and SF3B1 genetic silencing reduced DRD2 expression and signaling.