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Dopamine 2 and somatostatin 1 – 5 receptors coexpression in clinically non-functioning pituitary adenomas

Filip Gabalec, Monika Drastikova, David Netuka, Vaclav Masopust, Tomas Cesak, Josef Machac, Josef Marek, Jan Cap, Martin Beranek. 4 Department of Internal Medicine – Hematology, Institute of Clinical Biochemistry and Diagnostics, and Department of Neurosurgery, University Hospital Hradec Kralove and Charles University in Prague, Faculty of Medicine in Hradec Kralove; Department of Neurosurgery, 1 Faculty of Medicine, Charles University in Prague and Central Military Hospital Prague; Dept. of Neurosurgery, University Hospital, Olomouc; 3 Department of Medicine, 1st Faculty of Medicine, Charles University in Prague; Czech Republic.


Introduction
Clinically non-functioning pituitary adenomas (CNFAs) represent one third of human pituitary tumors. An estimated prevalence of all pituitary tumors is 16. 7% (Ezzat et al. 2004).
Diagnosis of CNFAs is often determined when the tumor had grown to the stage of macroadenoma and presents with signs of mass effect. The majority of CNFAs produce either low, non-significant amounts of hormones or defective hormones. No more than 10% of CNFAs are immunohistochemically negative; these are called null-cell adenomas (Jaffe 2006).
Transsphenoidal surgery is the primary option for CNFAs requiring treatment. However, because of frequent supra-or parasellar extension, surgery is infrequently curative, leaving tumor remnants that regrow in 12-58% of patients within 5 years of follow-up (Brochier et al. 2010, Greenman et al. 2003. The only independent predictors for tumor regrowth are: a) preoperative presence of cavernous sinus invasion, and b) postoperative suprasellar extent of residual tumor (Greenman et al. 2003). In postoperative management of patients, there is no clear strategy. We can choose between reoperation, Leksell-gamma knife surgery or pharmacological treatment. Each option has its drawbacks. The presence of somatostatin (SSTR) and dopamine 2 subtype (D2R) receptors in CNFAs has been proved by several methods including radioligands, in-situ hybridization, RT-PCR, and quantitative real-time RT-PCR. However, empirical use of dopamine agonists (DA) or somatostatin analogues (SA) alone or in combination has been effective in only a few cases (Colao et al. 2008). Recently, new chimeric compounds "dopastatins" showed promising efficacy in vitro (Florio et al. 2008). In contrast, pharmacological treatment of CNFAs has not been successful. Highly variable expression or homo/heterodimerization of receptors in the pituitary gland partly could explain this discrepancy (Neto et al. 2009, Renner et al. 1998, Taboada et al. 2007).
Thus, knowledge of receptor mRNA expression patterns would help in the targeted therapy (Babu et al. 2014). The aim of this study was to quantify coexpression of SSTR1-5 and D2R in a large series of CNFAs and to correlate the results with the pathological classification of adenomas.

Patients and methods
Clinically non-functioning adenomas were diagnosed based on the absence of hypersecretory laboratory markers. Specifically, the IGF-I level was normal or low for the particular age, and the serum prolactin level was normal or only slightly elevated to a level considered appropriate to stalk compression seen on MRI. Urinary free cortisol was not elevated. Serum free thyroid hormones were normal or lower. Luteinizing hormone, Follicle-stimulating hormone and testosterone, free testosterone (or estradiol in women) were evaluated.
Diagnosis of adenoma was confirmed by pathological investigation after surgery. We enrolled 198 patients who fulfilled study criteria. Several patients from our previous analysis of D2R expression (Gabalec et al. 2012) from whom we had cDNA stored for further analysis were also included. The study was approved by the Ethics Committee; patients had all signed informed consent forms. Subjects were 113 men and 85 women, aged 15-87 years (mean ± standard deviation 60.4 ± 13.1). All tumors except of one were macroadenomas; sized (at their largest diameters) 7-53 mm (mean ± standard deviation 25.9 ± 8.8 mm; median: 25 mm). The most common indications for operation were bitemporal hemianopsia or visual defect in 89 cases (45%), diplopia due to nerve palsy in 26 cases (13 %), progression of tumor growth on MRI in 18 (9%) and headache in 21 (11%), or in 44 (22%) in combination. Most patients underwent surgery by transsphenoidal endoscopic approach. Three patients needed subfrontal surgery due to the adenoma extent. At initial evaluation, hypopituitarism was present in 50 patients, diabetes insipidus in 4 cases, 26 patients had hypogonadism and only 2 subjects with hypogonadism had substitution prior to diagnosis.

Pathology
Tissue specimens were investigated as described in our previous work. ( Gabalec et al. 2012, Cap et al. 2003 Briefly, adenoma tissue specimens obtained at surgery were immediately fixed in 10% formalin, routinely processed, and subsequently embedded in paraffin. Sections 5 μm thick were stained with hematoxylin-eosin and Goldberg's trichrome. The Gömöri silver impregnation of reticuline was used to enhance staining of fibrous tissue. In total, 133 specimens from 75 men and 58 women, aged 23-87 years (61.4 ± 12.2 years) underwent immunohistochemical analysis. For immunohistochemistry (IHC), serial sections from the same paraffin blocks were usually exposed to a heat-mediated antigen retrieval procedure in citrate buffer, pH 6.0, using a microwave oven. We investigated the following pituitary hormones: ACTH (monoclonal mouse antibodies, Dako, Denmark); GH; prolactin; and specific β-subunits of the dimeric glycoproteins TSH, FSH, LH, and their common α-subunit (monoclonal antibodies, Immunotech, Prague, Czech Republic). We used the avidin-biotin technique and diaminobenzidine staining. Expression of each hormone was described semiquantitatively in 4 stages: 0: absolutely negative; 1: slight positivity in < 15% of cells; 2: positivity in 15-70% of cells; and 3: strong positivity in > 70% of cells.

Quantitative real-time RT-PCR
Details of RNA extraction, reverse transcription and quantification were described previously. Statistics mRNA quantities were expressed both absolutely, in copies per 5 µl of cDNA, and relatively as the ratio of the receptor subtype to housekeeping gene copies. The distribution of values in the whole sample and by groups according to the immunocytochemical classification proved to be non-Gaussian using Kolmogorov-Smirnov test (with Lilliefors' correction). For this reason, the groups were characterized by median and percentiles and the Mann-Whitney rank sum test was used to evaluate differences between individual groups. Comparisons were also done with the use of Kruskal-Wallis analysis of variance on ranks (Dunn's method).
Correlation between various receptors expression was evaluated using Spearman Rank Order Correlation. P < 0.05 was considered significant. SigmaStat version 3.5 software (Systat Software, San Jose, USA) was used.
Expression of D2R correlated positively (Correlation Coefficient 0.199, P 0.01) with adenoma size. No correlation with size was found for other receptors. Receptor expression was not influenced by tumor extension.
Expression of D2R correlated positively with expression of SSTR2 and SSTR3 and inversely with expression of SSTR1 and SSTR5 (Fig. 2, Table 1). D2R expression was high (relative expression > 1) in most CNFAs, whereas expression of individual somatostatin receptor types was much lower, high expression being an exception. (Fig.3) High expression of somatostatin receptors was rare (15/198 for SSTR1,14/198 for SSTR2,8/198 for SSTR3 and 1/198 for SSTR5, respectively; Table 2). Coexpression of both SSTR and D2R in high quantities was even less frequent.
The median of relative mRNA expression showed the same pattern as the complete group. In contrast, SSTR3 and D2R expressions were significantly lower in silent ACTH adenomas.
(Kruskal-Wallis analysis on ranks, P <0.001 and P<0.01, respectively). SSTR1 was expressed significantly more in null cell adenomas than in gonadotrophs (Kruskal-Wallis analysis on ranks, P <0.021). Expression of SSTR1 was also higher in null cell adenomas than other subtypes, but this was not statistically significant. (Fig. 4)

Discussion
Transsphenoidal surgery is a standard approach to treating CNFAs. However  (Neto et al. 2009, Taboada et al. 2007). The effect of somatostatin and its analogues is determined by the binding receptor. Each receptor starts different signal pathways and intracellular response (Hofland et al. 2003, Ferone et al. 2009, Zatelli et al. 2004. Treatment with somatostatin analogues in CNFAs has poor results. Data with short-acting octreotide showed tumor volume reduction only in 12%, whereas most tumors remained stable (Colao et al. 2008). A study with octreotide LAR in CNFAs in vivo showed stable tumor remnants in the majority of cases and tumor volume enlargement in 19% of cases (Fusco et al. 2012). Conversely, in vitro studies with somatostatin and lanreotide showed significantly inhibited cell proliferation in CNFAs (Florio et al. 1999). In another study SA binding SSTR1, SSTR2 and/or SSTR5 inhibited cell viability and alpha-subunit secretion in vitro (Zatelli et al. 2004). In CNFAs, our work supports the findings of Taboada et al. that SSTR3 has the highest expression levels followed by SSTR2. Other authors report predominant and highly variable expression of SSTR2 and SSTR3 (Babu et al. 2014, Florio et al. 2008, Saveanu et al. 2001, Saveanu et al. 2009, Tateno et al. 2009), but SSTR5 was not reported in these papers or had low expression. We observed SSTR5 expression in 61% of cases and SSTR1 expression in all cases, which is more than was reported by Taboada (Taboada et al. 2007). However, Babu reported SSTR5 expression in 82% of patients (Babu et al. 2014). High variability of expression together with preferential binding of current analogues to SSTR2 could explain the low efficacy of SA. To date, no studies that use qRT-PCR correlate the number of receptor copies in CNFAs with response to SA.
Coexpression of dopamine and somatostatin receptors was evaluated only in few studies.
Coexpression of SSTR2, SSTR3 and D2R in most evaluated CNFAs had highly variable expression levels (Neto et al. 2009, Florio et al. 2008, Saveanu et al. 2009, Nishioka et al. 2011). In all series somatostatin receptors are expressed much less than D2 receptors. (Neto et al. 2009, Florio et al. 2008, Saveanu et al. 2009). In our study D2R expression was also higher by one order than SSTR3 or SSTR2 expression. Similar data were reported by Saveanu for 58 CNFAs (Saveanu et al. 2009). The same patterns of D2R and SSTR expression were also seen in CNFAs cell cultures (Florio et al. 2008). High numbers of copies in both somatostatin and dopamine receptors were seen in 3 of 17 patients in the study of Babu (Babu et al. 2014).
Evidence indicates that D2R and SSTR can form heterodimers that influence their binding capacity, signalization and internalization. After heterodimerization of SSTR5 with D2R or SSTR2 with D2R, new receptors presented with new conformation, higher activities and binding capacities for both SA andDA (Baragli et al. 2007, Rocheville et al. 2000).
Adenomas with high expression of somatostatin receptors are rare and those with high coexpression of both somatostatin and dopamine receptors are an exception. Therefore chimeric compound BIM-23A760 that bind both somatostatin and dopamine receptors did not inhibit cell cultures more than DA alone (Florio et al. 2008, Gruszka et al. 2006).
We found negative correlation between D2R expression and expression of SSTR1 and SSTR5, but D2R expression correlated positively with expression of SSTR2 and SSTR3. This is an original finding, different from that of Neto et al. who found no correlation in 14 CNFAs (Neto et al. 2009). The number of samples probably explains the discrepancy, as the relationship is not very close. Some adenomas had low D2R expression and high somatostatin receptor expression, especially of the 1 and 5 subtypes.
With regard to pathological subtypes of CNFAs, we found significantly lower expression of SSTR3 in silent corticotrophs. The median relative SSTR5 expression was higher than in other subtypes, but this difference was not significant. Higher SSTR5 expression was previously reported in corticotroph adenomas (de Bruin et al. 2009). Tateno also reported high SSTR5 expression in corticotroph adenoma, but did not observe significant differences between silent corticotrophs and other CNFAs. SSTR5 expression was even lower than in CNFAs (Tateno et al. 2009). Again, this discrepancy might be due to the number of examined adenomas.
To our knowledge no previous study has correlated D2R and SSTR coexpression to pathological subtype. Compared with gonadotroph and null-cell adenomas, where D2R expression was predominant and significantly higher than SSTR, SSTR3 and SSTR2 were the most expressed receptors (although not significantly) in plurihormonal adenomas and silent corticotrophs, respectively. SSTR1 was statistically more expressed in null-cell adenomas than in other subtypes. Evaluation of other subtypes is limited by small number of cases other than gonadotrophs.
To conclude, in CNFAs, high expression of somatostatin receptors is much less common than that of D2R, and coexpression of both these receptors is exceptional.      types. Null-cell adenomas and gonadotrophs differed significantly (*P = 0.021).