Downregulation of HMGA-targeting microRNAs has a critical role in human pituitary tumorigenesis

Abstract

Previous studies have demonstrated that high mobility group A proteins have a critical role on the onset of human pituitary adenomas. Indeed, both high mobility group A (HMGA) genes are overexpressed in pituitary adenomas, and consistently transgenic mice overexpressing either the Hmga1 or the Hmga2 gene develop mixed growth hormone/prolactin (GH-PRL)-secreting pituitary adenomas. Trisomy of chromosome 12, where HMGA2 is located, and/or amplification of the HMGA2 gene locus account for the HMGA2 overexpression in most human prolactinomas. Conversely, HMGA1 overexpression is not associated to any rearrangement or amplification of the HMGA1 locus. We have first identified micro RNAs (miRNAs) able to target both HMGA1 and HMGA2 messenger RNAs. Then, all of these miRNAs have been found downregulated in pituitary adenomas of different histotypes, compared with normal pituitary. Interestingly, their downregulation was also observed in nonfunctioning pituitary adenomas where HMGA2 overexpression is not associated to any alteration of the HMGA2 locus. Functional studies show that all these HMGA-targeting miRNAs inhibit the proliferation of the rat pituitary adenoma cell line GH3. Therefore, these results indicate that the downregulation of the miRNAs able to target the HMGA genes could contribute to increase HMGA protein levels in human pituitary adenomas, and then to pituitary tumorigenesis.

Epidrug mediated re-expression of miRNA targeting the HMGA transcripts in pituitary cells

“…Moreover, heterologous expression of these miRNA in pituitary cell lines inhibited cell proliferation, colony forming activity (CFE) and leads to decrease in cell viability. [11] In a follow on study, in this case interrogating GH secreting adenoma, multiple miRNA that target the HMGA and the E2F1 transcripts were also identified and their heterologous expression were also associated with similar end-points. [14] More recent studies, of inappropriate miRNA expression profile in this case in GH and ACTH secreting adenomas also show that particular miRNA can elicit growth suppressive actions in vitro, and also in, in vivo model systems.…”

“…For analysis of HMGA1 and BMP-4 expression, transcript expression was normalized to GAPDH. [24,27,11] Relative quantification was carried out using 2 -ΔΔCT threshold method.…”

“…[11] For each of the adenomas we also determined the histone tail modification commonly associated with active, H3K9Ac, and repressed genes, H3K27Me3. The findings for, miR-26b, miR-196a-2 and let-7a are presented in Figure 1 and for miR-16 and miR34b as supplemental data (supplemental data, Figure, S1).…”

“…[9,10] However, more recent investigations now describe association between microRNA (miRNA) and the inappropriate expression of HMGA2 and also that of the HMGA1 gene. [11] In these cases, increase in HMGA1 and HMGA2 expression is associated with loss or significantly reduced expression of miRNA that target these gene transcripts.…”

“…The mechanistic potential of miRNAs to elicit tumour suppressor activity in a pituitary context was first reported by Palmieri and co-workers. [11] They identified loss or significantly reduced expression of multiple miRNAs that target the HMGA1 and HMGA2 transcripts across each of the major pituitary adenoma subtypes. Moreover, heterologous expression of these miRNA in pituitary cell lines inhibited cell proliferation, colony forming activity (CFE) and leads to decrease in cell viability.…”