Somatostatin analogues increase AIP expression in somatotropinomas, irrespective of Gsp mutations

Germline aryl hydrocarbon receptor interacting protein (AIP) gene mutations confer a predisposition to pituitary adenoma (PA), predominantly GH-secreting (GH-PA). As recent data suggest a role for AIP in the pathogenesis of sporadic GH-PA and their response to somatostatin analogues (SSA), the expression of AIP and its partner, aryl hydrocarbon receptor (AHR), was determined by semiquantitative immunohistochemistry scoring in 62 sporadic GH-PA (37 treated with SSA preoperatively). The influence of Gsp status was studied in a subset of tumours (n=39, 14 Gsp(+)) and six GH-PA were available for primary cultures. AIP and AHR were detected in most cases, with a positive correlation between AIP and cytoplasmic AHR (P=0.012). Low AIP expression was significantly more frequent in untreated vs SSA-treated tumours (44.0 vs 20.5%, P=0.016). AHR expression or localisation did not differ between the two groups. Similarly, in vitro octreotide induced a median twofold increase in AIP expression (range 1.2-13.9, P=0.027) in GH-PA. In SSA-treated tumours, the AIP score was significantly higher in the presence of preoperative IGF1 decrease or tumour shrinkage (P=0.008 and P=0.014 respectively). In untreated tumours, low AIP expression was significantly associated with invasiveness (P=0.028) and suprasellar extension (P=0.019). The only effect of Gsp status was a significantly lower nuclear AHR score in Gsp(+) vs Gsp(-) tumours (P=0.025), irrespective of SSA. In conclusion, AIP is involved in the aggressiveness of sporadic GH-PA, regardless of Gsp status, and AIP up-regulation in SSA-treated tumours is associated with a better preoperative response, with no clear role for AHR.

Somatostatin receptors: from signaling to clinical practice

Somatostatin is a peptide with a potent and broad antisecretory action, which makes it an invaluable drug target for the pharmacological management of pituitary adenomas and neuroendocrine tumors. Somatostatin receptors (SSTR1, 2A and B, 3, 4 and 5) belong to the G protein coupled receptor family and have a wide expression pattern in both normal tissues and solid tumors. Investigating the function of each SSTR in several tumor types has provided a wealth of information about the common but also distinct signaling cascades that suppress tumor cell proliferation, survival and angiogenesis. This provided the rationale for developing multireceptor-targeted somatostatin analogs and combination therapies with signaling-targeted agents such as inhibitors of the mammalian (or mechanistic) target of rapamycin (mTOR). The ability of SSTR to internalize and the development of rabiolabeled somatostatin analogs have improved the diagnosis and treatment of neuroendocrine tumors.

Knockout of the aryl hydrocarbon receptor results in distinct hepatic and renal phenotypes in rats and mice

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor which plays a role in the development of multiple tissues and is activated by a large number of ligands, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In order to examine the roles of the AHR in both normal biological development and response to environmental chemicals, an AHR knockout (AHR-KO) rat model was created and compared with an existing AHR-KO mouse. AHR-KO rats harboring either 2-bp or 29-bp deletion mutation in exon 2 of the AHR were created on the Sprague-Dawley genetic background using zinc-finger nuclease (ZFN) technology. Rats harboring either mutation type lacked expression of AHR protein in the liver. AHR-KO rats were also insensitive to thymic involution, increased hepatic weight and the induction of AHR-responsive genes (Cyp1a1, Cyp1a2, Cyp1b1, Ahrr) following acute exposure to 25 μg/kg TCDD. AHR-KO rats had lower basal expression of transcripts for these genes and also accumulated ~30-45-fold less TCDD in the liver at 7 days post-exposure. In untreated animals, AHR-KO mice, but not AHR-KO rats, had alterations in serum analytes indicative of compromised hepatic function, patent ductus venosus of the liver and persistent hyaloid arteries in the eye. AHR-KO rats, but not AHR-KO mice, displayed pathological alterations to the urinary tract: bilateral renal dilation (hydronephrosis), secondary medullary tubular and uroepithelial degenerative changes and bilateral ureter dilation (hydroureter). The present data indicate that the AHR may play significantly different roles in tissue development and homeostasis and toxicity across rodent species.

Characteristics of familial isolated pituitary adenomas

The familial occurrence of pituitary adenomas has been recognized for many years and currently accounts for approximately 5% of all cases. Molecular, genetic and clinical features of familial pituitary adenomas have been well characterized in multiple endocrine neoplasia type 1 (MEN-1) and Carney's complex (CNC), which account for the majority of familial pituitary tumor cases. These conditions are caused by MEN1 and PRKAR1A gene mutations, respectively, and the clinical and pathological features of pituitary pathology in these diseases differ from those of sporadic pituitary tumors. Familial acromegaly has been recognized for many years and, more recently, the clinical features of this clinical phenotype, referred to as isolated familial somatotropinoma, have been clarified. Over the past decade, the concept of non-MEN-1/CNC familial pituitary tumors has been expanded significantly to include all phenotypes, a condition known as familial isolated pituitary adenomas (FIPA). In FIPA, tumors can present homogeneously (same phenotype) or heterogeneously (different tumor phenotypes) within the same family. Compared with sporadic pituitary adenomas, patients with FIPA have a younger age at diagnosis and have larger tumors. The clinical features of FIPA differ from those of MEN-1 in terms of a higher frequency of somatotropinomas and a lower frequency of prolactinomas. The recent discovery of the involvement of mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene in association with pituitary tumors has provided new information regarding potential mechanisms of tumorigenesis in FIPA patients. While very infrequent in sporadic pituitary tumors, approximately 15% of FIPA patients have AIP mutations, rising to half of patients with familial acromegaly. In this review, we detail the clinical features of FIPA and discuss tumor pathology and genetic findings in this increasingly recognized clinical condition.

AIP gene in pituitary adenoma predisposition

Germline mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene were recently shown to cause susceptibility to pituitary adenoma predisposition. The purpose of this review is to briefly recapitulate the current knowledge on hereditary susceptibility to pituitary adenomas and what led to the identification of AIP as a novel predisposition gene. We will then concentrate on the data on AIP mutations and pituitary adenoma predisposition phenotype that have accumulated since the gene was identified. Major future challenges, as well as the possibilities for clinical practice based on this recent finding, will also be discussed.