Presence of sst5TMD4, a truncated splice variant of the somatostatin receptor subtype 5, is associated to features of increased aggressiveness in pancreatic neuroendocrine tumors

Abstract

PurposeGastroenteropancreatic neuroendocrine tumors (GEP-NETs) are rare and heterogeneous tumors, and their biological behavior is not well known. We studied the presence and potential functional roles of somatostatin receptors (sst1-5), focusing particularly on the truncated variants (sst5TMD4, sst5TMD5) and on their relationships with the angiogenic system (Ang/Tie-2 and VEGF) in human GEP-NETs.Experimental DesignWe evaluated 42 tumor tissue samples (26 primary/16 metastatic) from 26 patients with GEP-NETs, and 30 non-tumoral tissues (26 from adjacent non-tumor regions and 4 from normal controls) from a single center. Expression of sst1-5, sst5TMD4, sst5TMD5, Ang1-2, Tie-2 and VEGF was analyzed using real-time qPCR, immunofluorescence and immunohistochemistry. Expression levels were associated with tumor characteristics and clinical outcomes. Functional role of sst5TMD4 was analyzed in GEP-NET cell lines.Resultssst1 exhibited the highest expression in GEP-NET, whilst sst2 was the most frequently observed sst-subtype (90.2%). Expression levels of sst1, sst2, sst3, sst5TMD4, and sst5TMD5 were significantly higher in tumor tissues compared to their adjacent non-tumoral tissue. Lymph-node metastases expressed higher levels of sst5TMD4 than in its corresponding primary tumor tissue. sst5TMD4 was also significantly higher in intestinal tumor tissues from patients with residual disease of intestinal origin compared to those with non-residual disease. Functional assays demonstrated that the presence of sst5TMD4 was associated to enhanced malignant features in GEP-NET cells. Angiogenic markers correlated positively with sst5TMD4, which was confirmed by immunohistochemical/fluorescence studies.Conclusionssst5TMD4 is overexpressed in GEP-NETs and is associated to enhanced aggressiveness, suggesting its potential value as biomarker and target in GEP-NETs.

The oncogenic role of the spliced somatostatin receptor sst5TMD4 variant in prostate cancer

“…sst5TMD4 immunohistochemical analysis sst5TMD4 analysis by immunohistochemistry (IHC) was performed by using a custom-made and previously validated polyclonal rabbit anti-human sst5TMD4 Ab (clone name: 66498; 1:400) (20) using standard procedures (15,17,20). The validity and specificity of this Ab are further supported by the results presented in Supplemental Fig.…”

“…In PCa, ssts are expressed and are capable of mediating such functions (5); however, some initial but limited studies that used SSAs reported no benefit in overall survival in patients with PCa (11,26), and the mechanistic reasons of this clinical failure are still unknown. Given that aberrant alternative splicing is a cancer hallmark (13,27) and our group has demonstrated the presence and relevant pathologic function of the spliced sst5TMD4 variant in other cancer types (15,(17)(18)(19)(20), we hypothesized that sst5TMD4 could be present and play a role in the development and/or progression of PCa or in the response to SSAs in PCa.…”

“…Details of real-time quantitative PCR (qPCR) analysis are reported elsewhere (15,21). Specifically, expression levels-absolute mRNA copy number/50 ng of sample-of sst5TMD4, b-actin, and glyceraldehyde 3-phosphate dehydrogenase (GAPDH) were measured by using validated primers as previously reported (15,17,20,21). For fresh samples and PCa cell lines, a normalization factor was calculated with the expression levels of ACTB and GAPDH using GeNorm 3.3 (22).…”

“…In addition, in contrast to the predominant plasma membrane localization of other ssts, sst5TMD4 demonstrates a preferential intracellular localization wherein it may disrupt the function of other ssts, mainly sst2 and sst5 (14). Of more interest, sst5TMD4 is barely expressed in normal tissues but is highly overexpressed in several tumors and cancers, including pituitary adenomas (15,16), breast cancer (17), thyroid carcinoma (18,19), and NETs (20), where it correlates with aggressive clinical parameters and promotes cell proliferation, migration, invasion, and hormone secretion (15,17,19,20). Indeed, sst5TMD4 expression is negatively correlated with the ability of SSAs to reduce growth hormone secretion in pituitary adenomas (16) and inhibits the ability of sst2-transfected cells to respond to SST and/or SSAs (17).…”

WOMEN IN CANCER THEMATIC REVIEW: Systemic therapies in neuroendocrine tumors and novel approaches toward personalized medicine

“…The expression of a truncated splice variant of sstr-5 was associated with poor biochemical response to octreotide in acromegaly (Marina et al 2015). The finding that tumor expression of the truncated sstr-5 was associated with more aggressive behavior in pancreatic NET warrants further investigation in NET (Sampedro-Núñez et al 2016). The value of SSA for NET of other origin, such as gastric or rectal NET remains unclear, their use, however, seems justified based on the expression of somatostatin receptors, and low grade or slow growth in the absence of any approved drugs.…”