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Department of Oncology, University of Turin, Orbassano, Turin, Italy


and of manipulating SSTR signaling as a therapeutic strategy, that will be discussed extensively in the following sections of this book.

methods to identify sstr in tissues

Before discussing the current knowledge on SSTR expression in tumors and other pathologies, a short reappraisal of the different methods available and reported to determine SSTR at the tissue level is mandatory to critically consider the expression data currently available.

Several methods have been used to determine the expression of SSTR in tissue samples. All of them have intrinsically limitations and advantages, and therefore, the most powerful data have been generated by the combination of two or more of them. A comparison of the three most relevant methods is repre-sented in Table 3.1.

SSTR tissue localization had originally been demonstrated by means of binding assays of radiolabeled SS analogues [2–4]. however, this method that has the unique property of tracing the presence of “functionally active” receptors is affected by a scarce reproducibility, is applicable to high‐quality frozen material only, and does not recognize the SSTR subtype expressed, unless using highly subtype‐specific analogues [5].

Since the cloning of the five genes, between 1992 and 1994, several studies detected the specific mRnA expression of SSTR in normal tissue and tumors by means of alternative techniques such as the northern blot [6], in situ hybridization [7, 8], or

tAble 3.1 comparison between different methods of tissue identification of sstr

Method Pros Cons Applicability


reverse transcriptase‐polymerase chain reaction (RT‐PCR), either qualitative or quantitative [9–11]. however, in general terms, mRnA expression from tissue extracts is variably affected by signals determined by SSTR‐expressing cells different from those that represent the target, and the quality of the tissue sample is again a relevant clue for obtaining adequate results.

In parallel with mRnA determination, several groups aimed at the development of SSTR‐specific antibodies. The vast majority of them are polyclonal antisera and have been determined either against the n‐ or the C‐terminus of the protein [12–17]. Most of these antibodies are working nicely on paraffin‐embedded tissue, are raised against all or most of SSTR receptor subtypes (although with variable reliability), are com-mercially available, and, therefore, allowed extensive investigations on large archival case series. Moreover, some monoclonal antibodies have also been developed, with special reference to SSTR subtype 2A [18–21], although not commercially available yet at the time of this manuscript preparation.

Besides research purposes, the advantages of immunohistochemical SSTR detec-tion in the clinical practice include a high cost/benefit ratio, high reproducibility in pathology laboratories worldwide, possibility of tissue localization of tumor cells, recognition of the different SSTR subtypes, and, last but not least, applicability on retrospective archival material. In addition, immunohistochemical methods may be applied to preoperative fine‐needle aspiration of cytological or biopsy material [22], allowing SSTR demonstration in inoperable tumors or offering specific information to diagnostic or therapeutic decisions before surgery (Fig. 3.1). Several studies have also documented that immunohistochemistry correlates with other methods in a

(a) (b)

figure 3.1 SSTR type 2A determination in a cellblock preparation from a liver metastasis of a well‐differentiated neuroendocrine carcinoma of unknown primary. (a) hematoxylin and eosin; (b) immunoperoxidase; (a and b) original magnification 400×. (See insert for color rep-resentation of the figure.)

significant proportion of cases, with only minor discrepancies [10, 13, 18, 21–23].

however, a major caveat of immunohistochemistry for SSTR, with special reference to its use as a diagnostic tool, is represented by the lack of standardization, although some scoring systems have been proposed [24, 25]. In general terms, a fine membranous staining is considered the most specific [26], although a cytoplasmic staining may be a result of receptor internalization, especially in the case of tumors coexpressing both SSTR and their natural ligand or in the cases of patients treated with SS analogues [27].

sstr exPression in humAn mAlignAncies

A range of different tumors overexpress SSTR, as compared to nontransformed cells.

The underlying stimuli that induce this overexpression as well as the functional meaning for the biology of tumor cells have not been conclusively explained. It is possible that the upregulation of SSTR serves as homeostatic growth inhibitory auto-crine/paracrine response to the deregulated tumor cell proliferation [28]. nevertheless, with all the limitations related to the complexity of the role of SSTR in cancer biology, it can be asserted that SSTR and their intracellular signaling pathways should generally be considered as tumor suppressive.

sstr exPression in neuroendocrine neoPlAsms

neuroendocrine neoplasms originate from a normal cell population that is physio-logically a target of SS, thus expressing SSTR. Therefore, the generally high level of SSTR expression in these groups of tumors is not surprising.

A tentative list of all data on SSTR‐positive neuroendocrine neoplasms would greatly fail to be comprehensive. Wide literature data (see for review [29–32]) show that SSTR are highly expressed in pituitary adenomas; neuroendocrine tumors of the  pancreas, gastrointestinal tract, and lung; paragangliomas/pheochromocytomas;

Merkel cell carcinomas; neuroblastomas; and medullary thyroid carcinomas. hundreds of such tumors have been analyzed by means of various techniques, including binding assays, immunohistochemistry, and mRnA analysis. A wide heterogeneity in SSTR subtype expression in the different tumor types, in different cases of the same tumor type, and even in different cell populations within individual lesions has been reported.

Such high heterogeneity of SSTR distribution partially explains some discrepancies in the clinical features and response to SS analogue therapy observed in neuroendocrine tumors from various sites. According to such literature data, in most cases, individual tumors coexpress different SSTR subtypes, SSTR type 2 being the most frequently represented in all locations, followed by types 3, 5, and 1. Subtype 4 is generally poorly expressed. higher SSTR expression is generally observed in “well‐differentiated”

tumors although a considerable proportion of positive cases might be observed also in poorly differentiated—highly aggressive—lesions, such as small cell lung cancer [33].

Although poorly elucidated from a biological point of view, hormonal secretion by the

SSTR eXPReSSIon In nonneURoendoCRIne MALIgnAnCIeS 25

tumor is also associated with different patterns of expression of SSTR subtypes, such as in the case of pancreatic insulin‐producing tumors that show a low SSTR type 2 expression [34].

Apart from detailing a prevalence of expression, SSTR determination at the tissue level is potentially a complementary approach to better define the diagnostic and therapeutic strategies in patients affected by neuroendocrine tumors. A recent literature focused on the comparison between SSTR tissue determination (mainly by means of immunohistochem-istry) and in vivo imaging using different SS analogue‐based methodologies [24, 25, 35–37], with in general a good correlation. In a previous paper by our group, a relatively high correlation was observed with the response to SS analogue treatment [24]. Moreover, some studies claimed a prognostic value of SSTR type 2 determination in neuroendocrine tumors of the gastrointestinal tract and pancreas [38–40].

Future studies are therefore needed to validate SSTR testing as a relevant marker of clinical usefulness in neuroendocrine neoplasms.

sstr exPression in nonneuroendocrine mAlignAncies A wide spectrum of solid or hematological malignancies has been demonstrated to variably express SSTR [41, 42]. A variety of carcinomas showed in vivo and tissue localization‐based evidence of SSTR expression: such tumors include, among others, cancers from the breast [43–45], lung [18, 22], kidney [45], pancreatobili-ary tract [46], stomach [47], liver [48], colorectum [49], ovpancreatobili-ary [50], thyroid follic-ular cells [51], and prostate [52–55] (Fig. 3.2). Unpublished data from our group onto cell lines (Table 3.2) are consistent with what was reported on tissues earlier.

In some cases, an antiproliferative activity of SS analogues could be demonstrated

figure 3.2 SSTR type 2A clonal expression in prostatic adenocarcinoma (immunoperoxi-dase; original magnification 400×). (See insert for color representation of the figure.)

in vitro in hormone independent prostate cancer models [56]. SSTR subtypes have also been detected in meningiomas, medulloblastomas, and gliomas, in soft tissue sarcomas, and in malignant melanomas; this distribution correlated with either scintigraphic imaging or in vitro tests on SS analogue response [57–59]. Promising clinical applications of SS analogues have also been reported in lymphohemato-logical malignancies [60] and in thymomas [61].

sstr in nonneoPlAstic diseAses

There is strong evidence that selected nontumoral lesion may also express SSTR. For instance, active granulomas in sarcoidosis express SSTR on epithelioid cells [62], whereas inactive or successfully treated fibrosing granulomas devoid of epithelioid cells lack SSTR. Inflamed joints in active rheumatoid arthritis express SSTR, prefer-entially located in the proliferating synovial vessel [63].

Furthermore, inflammatory bowel disease is characterized by an overexpression of SSTR in the vascular system [64] of the altered parts of the gastrointestinal tract.

Concerning SSTR presence and possible applications of SS analogues in nonneoplas-tic diseases, SS actions in modulating the immunological response and angiogenesis together with the high density of expression of SSTR (viz., type 1 and 2) in the retina represent the baseline of very promising applications for therapy in various retinal diseases, from macular edema or macular degeneration to thyrotoxic orbitopathy, ret-inal ischemic damage, and proliferative diabetic retinopathy [65, 66].

unmet clues

despite the wide body of evidence on the presence and function of SSTR in several neoplastic and nonneoplastic human diseases, several issues still deserve further investigation and elucidation.

tAble 3.2 sstr expression at mrnA level in nonneuroendocrine cell linesa

Cell line derivation SSTR mRnA expressed

MonoMAC Monoblastic leukemia 4

MCF7 Breast cancer 2, 5

T47d Breast cancer, apocrine 2, 5

MdAMB231 Breast cancer 2, 4

CALU‐1 Lung cancer, squamous 3

KATo III gastric cancer 1, 2, 5

hT29 Colon cancer 1, 2, 5

h716 Colon cancer (with neuroendocrine features)

1, 2, 3, 5

Mog UVW glioblastoma 2

a Volante, M., unpublished.

ReFeRenCeS 27

The correct methodology for SSTR determination that could be applied in the clinical practice is not well established, so far. Immunohistochemistry seems the most promising but is still limited by the scarce availability of reliable and clinically validated reagents (mostly related to SSTR subtype 2), as well as by the lack of stan-dardization in the interpretation. Moreover, novel molecules with a wider spectrum of affinity to different SSTR subtypes than those currently available [67] claim a reinterpretation of the data available with a better understanding of coexpression modalities of the different SSTR subtypes, also taking into consideration the capa-bility of different subtype to form functionally active homo‐ or heterodimers that modify significantly the activation of intracellular signaling pathways also due to altered agonist‐induced desensitization [68]. Moreover, the SS/SSTR axis is more complex, due to both other peptides with selective affinity to SSTR such as cor-tistatin [31] and due to the capability of SSTR to heterodimerize with other receptors such as dopamine receptors [69]. In this respect, recent studies claim that the coex-pression of SSTR and dopamine receptors (type 2) might open to novel therapeutic strategies with chimeric molecules [70–72].


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Somatostatin Analogues: From Research to Clinical Practice, First Edition. Edited by Alicja Hubalewska-Dydejczyk, Alberto Signore, Marion de Jong, Rudi A. Dierckx, John Buscombe, and Christophe Van de Wiele.

© 2015 John Wiley & Sons, Inc. Published 2015 by John Wiley & Sons, Inc.


NET neuroendocrine tumor

SRS somatostatin receptor scintigraphy SST somatostatin

SSTR somatostatin receptor

Nuclear medicine has always been a powerful tool for the study of biological functions and, more recently, for the molecular and histological characterization of tissues under physiological and pathological conditions. The growing knowledge of the biology of normal and pathological tissues leads to the discovery of several biologically active peptides that mediate their function by binding to external membrane‐bound receptors with high affinity. This property made peptide/receptor complex a potential target to be used for molecular characterization of tissues in vivo.

Peptides have good characteristics as radiopharmaceuticals: they are small in size and have a fast renal clearance and easily penetrate into tissues with consequent low

the Use of rAdiolAbeled somAtostAtin AnAlogUe in medicAl diAgnosis: