Nuclear exclusion of menin drives functional MEN1 deficiency in non-MEN1 prolactinomas: mouse models and human biopsies

Prolactinomas, the most common secretory pituitary tumour subtype, frequently occur in patients with Multiple Endocrine Neoplasia type 1, caused by germline MEN1 mutations encoding menin. While menin loss is well established in MEN1-associated prolactinomas, its role in sporadic tumours remains unclear. We investigated menin expression, subcellular localization, and downstream signalling in two murine models of non-MEN1 prolactinomas, the dopamine D2-receptor knockout and the hCG{beta}-subunit-overexpressing mice, in which only females develop prolactinoma. Pituitary Men1 expression, analysed by qPCR, remained unchanged despite the genotype, in both sexes. However, in prolactinomas, lactotrophs exhibited a marked loss of nuclear MEN1 immunostained, with protein restricted to the cytoplasm. Male mice pituitaries retained nuclear MEN1 localization regardless their genotype. Loss of nuclear menin in prolactinomas was associated with reduced p27 and Pten expression, increased Ccnd1 expression, and enhanced pAKT. Moreover, by using in vivo pharmacological and surgical approaches we demonstrated that dopamine-agonist treatment preserved nuclear menin in lactotrophs, whereas dopamine blockade or estradiol induced its nuclear loss. Importantly, analysis of human pituitary biopsies confirmed nuclear and cytoplasmic menin localization in lactotrophs from normal pituitaries, and in prolactinomas from both genders following dopamine agonist therapy. However, in a prolactinoma from an untreated female, nuclear menin was partially lost. Therefore, our findings identify a state of functional MEN1-deficiency in sporadic prolactinomas (characterized by preserved MEN1 expression), but its exclusion from the nucleus (linked to activation of proliferative pathways, impaired tumour suppressor signalling, and tumour development) highlights the restoration of nuclear MEN1 localization as a potential therapeutic strategy.