miR-375 regulation of SSTR2 expression in corticotroph pituitary cells: somatostatin receptor ligands effects

: Glucocorticoids (GCs) long-term exposure downregulates SSTR2 expression in corticotroph tumours, limiting the efficacy of octreotide (OCT) in the treatment of Cushing's disease (CD). In AtT20 cells, dexamethasone (DEX) increased miR-375 expression supporting the hypothesis that excessive GCs exposure can lead to epigenetic SSTR2 downregulation. The current study aims to evaluate miR-375 levels by RT-qPCR in CD patients' sera, human corticotroph pituitary tumours and normal pituitaries, AtT20/D16 and GH3 cells, and miR-375 impact on SSTR2 expression in AtT20/D16 and human corticotroph pituitary tumours. SSTR2 protein expression and localization were evaluated by WB and IF in AtT20/D16 and human primary cultures. Proliferation assay and flow cytometry were assessed to investigate the impact of miR-375 regulation on OCT treatment in AtT20/D16. miR-375 levels were higher in CD patients' sera compared to healthy subjects, as well as in human corticotroph pituitary tumours than in normal pituitaries. AtT20/D16 and GH3 exhibited an inverse expression pattern, with SSTR2 mRNA low levels and miR-375 high levels in AtT20/D16 and an opposite expression pattern in GH3. DEX treatment significantly reduced SSTR2 gene expression, while miR-375 inhibition significantly increased SSTR2 membranous protein expression in AtT20/D16 and primary cultures. Receptor internalization appeared stronger when OCT was combined with miR-375 inhibitor. The decreased cell proliferation induced by OCT was potentiated by miR-375 inhibition, increasing cells in early and late apoptosis, by inducing PARP, Caspase3 and ERK1/2 phosphorylation. In conclusion, SSTR2 protein expression can be epigenetically downregulated by GC-induced miR-375 expression, at least partially influencing OCT action in corticotroph pituitary tumours.