Endocrine Abstracts

Germline mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene predispose to early onset pituitary adenoma, with a preponderance of somatotrophinomas. Patients harbouring an AIP mutation respond poorly to somatostatin analogue (SSA) treatment. On the other hand, a subset of sporadic somatotrophinomas with no AIP mutations show low AIP protein expression and exhibit a decreased response to SSA treatment as well. microRNAs are small interfering RNAs which can regulate protein expression. miR-34a is predicted to interact with specific target sequences located within the 3′ untranslated region (3′UTR) of AIP.

RNA expression levels of AIP and miR-34a were evaluated in 31 sporadic somatotrophinoma tissues (17 invasive, 14 non-invasive) by RT-qPCR and immunohistochemistry was used to assess AIP protein expression. A luciferase reporter assay was used to examine the in silico predicted target sites of miR-34a in the 3′UTR of AIP. Deletion constructs of AIP 3′UTR were utilised to confirm the binding and regulatory function of miR-34a.

Low AIP protein expression was seen in invasive sporadic somatotrophinomas, whereas the corresponding mRNA levels were not statistically different compared to non-invasive tumours. This suggests that the expression of AIP might be regulated post-transcriptionally by miRNAs, which repress gene expression mainly by inhibiting protein translation. We observed that the expression levels of miR-34a is higher in somototrophinomas with low AIP expression. (P=0.02) Co-transfection of reporter cells with a miR-34a precursor and the luciferase-WT-AIP-3′UTR plasmid construct shows a negative effect on the luciferase expression, suggesting that miR-34a binds to the AIP-3″UTR. By using deletion mutants we have validated miR-34a predicted target sites at AIP-3″UTR.

Conclusions: We have identified and proved that miR-34a is a negative regulator of AIP protein expression and increased miR-34a expression could explain the low AIP expression and resulting tumour invasiveness and decreased SSA-responsiveness of sporadic somatotrophinomas.