miR-34a is upregulated inAIP-mutated somatotropinomas and promotes octreotide resistance

Abstract

Pituitary adenomas (PAs) are intracranial tumors associated with significant morbidity due to hormonal dysregulation, mass effects and have a heavy treatment burden. Growth hormone (GH)-secreting PAs (somatotropinomas) cause acromegaly-gigantism. Genetic forms of somatotropinomas due to germlineAIPmutations (AIPmut+) have an early onset and are aggressive and resistant to treatment with somatostatin analogs (SSAs), including octreotide. The molecular underpinnings of these clinical features remain unclear. We investigated the role of miRNA dysregulation inAIPmut+ vsAIPmut- PA samples by array analysis. miR-34a and miR-145 were highly expressed inAIPmut+ vsAIPmut- somatotropinomas. Ectopic expression ofAIPmut (p.R271W) inAip(-/-)mouse embryonic fibroblasts (MEFs) upregulated miR-34a and miR-145, establishing a causal link betweenAIPmut and miRNA expression. In PA cells (GH3), miR-34a overexpression promoted proliferation, clonogenicity, migration and suppressed apoptosis, whereas miR-145 moderately affected proliferation and apoptosis. Moreover, high miR-34a expression increased intracellular cAMP, a critical mitogenic factor in PAs. Crucially, high miR-34a expression significantly blunted octreotide-mediated GH inhibition and antiproliferative effects. miR-34a directly targetsGnai2encoding G alpha i2, a G protein subunit inhibiting cAMP production. Accordingly, G alpha i2 levels were significantly lower inAIPmut+ vsAIPmut- PA. Taken together, somatotropinomas withAIPmutations overexpress miR-34a, which in turn downregulates G alpha i2 expression, increases cAMP concentration and ultimately promotes cell growth. Upregulation of miR-34a also impairs the hormonal and antiproliferative response of PA cells to octreotide. Thus, miR-34a is a novel downstream target of mutantAIPthat promotes a cellular phenotype mirroring the aggressive clinical features ofAIPmut+ acromegaly.