AIP inactivation leads to pituitary tumorigenesis through defective Gαi-cAMP signaling

Abstract

The aryl hydrocarbon receptor interacting protein (AIP) is a tumor-suppressor gene underlying the pituitary adenoma predisposition. Thus far, the exact molecular mechanisms by which inactivated AIP exerts its tumor-promoting action have been unclear. To better understand the role of AIP in pituitary tumorigenesis, we performed gene expression microarray analysis to examine changes between Aip wild-type and knockout mouse embryonic fibroblast (MEF) cell lines. Transcriptional analyses implied that Aip deficiency causes a dysfunction in cyclic adenosine monophosphate (cAMP) signaling, as well as impairments in signaling cascades associated with developmental and immune-inflammatory responses. In vitro experiments showed that AIP deficiency increases intracellular cAMP concentrations in both MEF and murine pituitary adenoma cell lines. Based on knockdown of various G protein α subunits, we concluded that AIP deficiency leads to elevated cAMP concentrations through defective Gαi-2 and Gαi-3 proteins that normally inhibit cAMP synthesis. Furthermore, immunostaining of Gαi-2 revealed that AIP deficiency is associated with a clear reduction in Gαi-2 protein expression levels in human and mouse growth hormone (GH)-secreting pituitary adenomas, thus indicating defective Gαi signaling in these tumors. By contrast, all prolactin-secreting tumors showed prominent Gαi-2 protein levels, irrespective of Aip mutation status. We additionally observed reduced expression of phosphorylated extracellular signal-regulated kinases 1/2 and cAMP response element-binding protein levels in mouse and human AIP-deficient somatotropinomas. This study implies for the first time that a failure to inhibit cAMP synthesis through dysfunctional Gαi signaling underlies the development of GH-secreting pituitary adenomas in AIP mutation carriers.

Prevalence of AIP mutations in a series of Turkish acromegalic patients: are synonymous AIP mutations relevant?

“…A second somatic mutation ('second hit') is necessary to loose the expression of the normal allele and to cause disease. When AIP activity is lost, it has been shown recently that the mechanism of tumorigenesis is related to the loss of inhibition of cAMP synthesis due to alteration in the function of inhibitory G-protein Gai2 [18,19].…”

AIP and the somatostatin system in pituitary tumours

“…AIP expression was associated to higher Gαi-2 and lower Ki67. In vitro Gαi-3 silencing revealed that AIP deficiency might lead to cAMP accumulation via Gαi-3, although its expression at protein level was not affected by the presence or absence of AIP (Tuominen et al 2015).…”

“…AIP was found to be associated with Gα13, Gαq and possibly regulate the abundance of Gαi-2 (Nakata et al 2009, Tuominen et al 2015, Ritvonen et al 2017. These interactions could influence multiple signalling pathways, with the Gαi-cAMP pathway being especially relevant regarding somatotroph cells and somatostatin action (see section below).…”

“…Patients with GNAS mutations respond better to SSAs (Efstathiadou et al 2015), while the presence of AIP and GNAS mutations seems to be mutually exclusive in somatotrophinomas (Hernández-Ramírez et al 2015). Recent studies performed on human and mouse somatotrophinomas demonstrated that AIP is essential to maintain relatively low levels of cAMP in normal somatotrophes, and the lack of AIP causes an accumulation of cAMP through defective Gαi-2 signalling, which leads to a downregulation of phosphorylated extracellular signal-regulated kinases 1/2 (p-ERK1/2) and p-CREB (Tuominen et al 2015). This point was supported by the fact that low Gαi-2 levels were observed in sporadic somatotrophinomas with decreased AIP immunoreactivity, and a positive association between AIP and Gαi-2 was found at protein level, which suggests a combined regulation (Ritvonen et al 2017).…”

AIP mutations impair AhR signaling in pituitary adenoma patients fibroblasts and in GH3 cells

“…The molecular mechanisms of AIP-dependent pituitary tumorigenesis are elusive. Recent in vitro studies implicate the cAMP pathway, which seems to be activated by AIP deficiency (Formosa et al 2013, Tuominen et al 2014, but several lines of evidence also suggest the involvement of AhR. Most pathogenic AIP mutations alter the C-terminal region of the protein, particularly the last few amino acids required for AhR interaction (Bell et al 2000), resulting in loss of protein-protein interactions (Leontiou et al 2008, Morgan et al 2012.…”