Expression of gastrin-releasing peptide receptors in endometrial cancer

Targeting GRPR in urological cancers--from basic research to clinical application

Gastrin releasing peptide (GRP) is a regulatory peptide that acts through its receptor (GRPR) to regulate physiological functions in various organs. GRPR is overexpressed in neoplastic cells of most prostate cancers and some renal cell cancers and in the tumoral vessels of urinary tract cancers. Thus, targeting these tumours with specifically designed GRP analogues has potential clinical application. Potent and specific radioactive, cytotoxic or nonradioactive GRP analogues have been designed and tested in various animal tumour models with the aim of receptor targeting for tumour diagnosis or therapy. All three categories of compound were found suitable for tumour targeting in animal models. The cytotoxic and nonradioactive GRP analogues have not yet shown convincing tumour-reducing effects in human trials; however, the first clinical studies of radioactive GRP analogues--both agonists and antagonists--suggest promising opportunities for both diagnostic tumour imaging and radiotherapy of prostate and other GRPR-expressing cancers.

Old and new peptide receptor targets in cancer: future directions

A precise definition of the tumor tissue targets to be selected for in vivo peptide receptor targeting, namely to know which peptide receptor is expressed in which type of cancer, is an important prerequisite for successful clinical application of this technology. In this short review, I give three selected examples of new and promising peptide receptor targets. In the somatostatin receptor field, based on in vitro receptor autoradiography experiments showing that much more sst(2) binding sites are detected in tumors using a (177)Lu-labeled sst(2) antagonist than a (177)Lu-labeled agonist, it can be proposed that, in addition to neuroendocrine tumors, nonneuroendocrine tumors with lower sst(2) levels such as breast carcinomas, renal cell carcinomas, and non-Hodgkin lymphomas may become potential candidates for sst(2) antagonist targeting. In the gastrin-releasing peptide receptor field, recent in vitro data show that not only tumor cells may overexpress gastrin-releasing peptide receptors but also neoangiogenic tumoral vessels, making tumors expressing high levels of gastrin-releasing peptide receptors in tumor vessels, such as ovarian or urinary tract cancers, attractive new candidates for gastrin-releasing peptide receptor targeting. In the incretin receptor field, it was found in vitro that, apart from glucagon-like peptide 1 receptors overexpressed in benign insulinomas, incretin receptors, especially the glucose-dependent insulinotropic polypeptide receptors, can be overexpressed in medullary thyroid cancers, an unexpected finding making also these tumors potential novel candidates for incretin receptor targeting. Due to the abundance of peptide receptors in various cancers, it may be possible in the future to define for each tumor type a corresponding overexpressed peptide receptor suitable for targeting.

Concomitant vascular GRP-receptor and VEGF-receptor expression in human tumors: molecular basis for dual targeting of tumoral vasculature

Gastrin-releasing peptide (GRP) and GRP receptors (GRPR) play a role in tumor angiogenesis. Recently, GRPR were found to be frequently expressed in the vasculature of a large variety of human cancers. Here, we characterize these GRPR by comparing the vascular GRPR expression and localization in a selection of human cancers with that of an established biological marker of neoangiogenesis, the vascular endothelial growth factor (VEGF) receptor. In vitro quantitative receptor autoradiography was performed in parallel for GRPR and VEGF receptors (VEGFR) in 32 human tumors of various origins, using ¹²⁵I-Tyr-bombesin and ¹²⁵I-VEGF₁₆₅ as radioligands, respectively. Moreover, VEGFR-2 was evaluated immunohistochemically. All tumors expressed GRPR and VEGFR in their vascular system. VEGFR were expressed in the endothelium in the majority of the vessels. GRPR were expressed in a subpopulation of vessels, preferably in their muscular coat. The vessels expressing GRPR were all VEGFR-positive whereas the VEGFR-expressing vessels were not all GRPR-positive. GRPR expressing vessels were found immunohistochemically to co-express VEGFR-2. Remarkably, the density of vascular GRPR was much higher than that of VEGFR. The concomitant expression of GRPR with VEGFR appears to be a frequent phenomenon in many human cancers. The GRPR, localized and expressed in extremely high density in a subgroup of vessels, may function as target for antiangiogenic tumor therapy or angiodestructive targeted radiotherapy with radiolabeled bombesin analogs alone, or preferably together with VEGFR targeted therapy.

Targeted Therapy of Breast and Gynecological Cancers with Cytotoxic Analogues of Peptide Hormones

Gynecological cancers such as breast, ovarian, and endometrial carcinoma express receptors for luteinizing hormone-releasing hormone (LHRH), bombesin/gastrin-releasing peptide (BN/GRP), and somatostatin (SST). These tumors are therefore suitable candidates for targeted therapy with cytotoxic hybrid molecules consisting of a cytotoxic radical and a peptide hormone analogue as a carrier. These compounds have been shown to be more active and less toxic in vivo than nontargeted chemotherapy in models of various human cancers which express the respective receptors. The current review summarizes experimental and clinical findings with cytotoxic peptide hormone analogues of LHRH (AN-152 [AEZS 108], AN-207), BN/GRP (AN-215), and SST (AN-238) in breast, ovarian, and endometrial cancers.

Gastrin-releasing peptide and cancer

Over the past 20 years, abundant evidence has been collected to suggest that gastrin-releasing peptide (GRP) and its receptors play an important role in the development of a variety of cancers. In fact, the detection of GRP and the GRP receptor in small cell lung carcinoma (SCLC), and the demonstration that anti-GRP antibodies inhibited proliferation in SCLC cell lines, established GRP as the prototypical autocrine growth factor. All forms of GRP are generated by processing of a 125-amino acid prohormone; recent studies indicate that C-terminal amidation of GRP18-27 is not essential for bioactivity, and that peptides derived from residues 31 to 125 of the prohormone are present in normal tissue and in tumors. GRP receptors can be divided into four classes, all of which belong to the 7 transmembrane domain family and bind GRP and/or GRP analogues with affinities in the nM range. Over-expression of GRP and its receptors has been demonstrated at both the mRNA and protein level in many types of tumors including lung, prostate, breast, stomach, pancreas and colon. GRP has also been shown to act as a potent mitogen for cancer cells of diverse origin both in vitro and in animal models of carcinogenesis. Other actions of GRP relevant to carcinogenesis include effects on morphogenesis, angiogenesis, cell migration and cell adhesion. Future prospects for the use of radiolabelled and cytotoxic GRP analogues and antagonists for cancer diagnosis and therapy appear promising.

Gastrin-releasing peptide receptors in normal and neoplastic human uterus: involvement of multiple tissue compartments

CONTEXT

Bombesin-like neuropeptides including gastrin-releasing peptide (GRP) and their corresponding receptors, mediate multiple physiological actions and have biological significance in cancer. However, information about the function of these neuropeptides and the incidence, distribution, density, and subtype of their receptors in human uterine tissues is scarce.

OBJECTIVE

The objective of the study was to investigate normal and neoplastic human uterine tissues for their bombesin receptor status.

DESIGN

In vitro subtype-specific bombesin receptor autoradiography was used in this study.

PATIENTS

The following tissue samples were taken immediately after surgery: myometrium (n = 41), endometrium (n = 29), leiomyomas (n = 26), leiomyosarcomas (n = 6), endometrial adenocarcinomas (n = 28), and carcinosarcoma (n = 1).

RESULTS

Normal uterine tissues expressed GRP receptors (GRP-Rs) in the myometrium, in subsets of secretory endometrial glands, and in subsets of endometrial blood vessels of the late proliferative and the secretory phase. Most leiomyomas (20 of 26) expressed GRP-R but not the leiomyosarcomas. GRP-Rs were also detected in 10 of 28 adenocarcinomas, one of one carcinosarcoma, and in blood vessels surrounding the adenocarcinomas. No other bombesin receptor subtypes (neuromedin B receptors and bb3) were detected.

CONCLUSIONS

These findings may be of physiological and pathophysiological significance. The expression of GRP-R in glands and vessels during specific phases of the cycle suggests a timely precise physiological action of GRP in these targets; in certain uterine neoplasms, the GRP-R overexpression may contribute to tumor development because GRP is a potent growth factor. Furthermore, these findings may be diagnostically and therapeutically relevant. The expression of GRP-R in leiomyomas may allow distinguishing them from receptor-negative leiomyosarcomas; GRP-R in leiomyomas, in a subset of endometrial adenocarcinomas, carcinosarcomas, and in peritumoral vessels may be candidates for receptor targeting in vivo.

Experimental therapy of human endometrial cancers with a targeted cytotoxic bombesin analog AN-215: Low induction of multidrug resistance proteins

In this study we have investigated the efficacy and toxicity of targeted cytotoxic bombesin (BN) analog AN-215 and its effects on the expression of three multidrug resistance proteins in experimental human endometrial cancers. Nude mice bearing HEC-1A, RL-95-2 and AN3CA tumours were treated with AN-215 and its cytotoxic radical (AN-201). The BN receptor expression in tumours was followed by RT-PCR analysis and radioligand binding assays. Expression of drug resistance proteins MDR-1, MRP-1 and BCRP were measured by realtime PCR. AN-215 significantly (P<0.05) inhibited the growth of HEC-1A, RL-95-2 and AN3CA tumours while AN-201 was ineffective. The expression of BN receptors was demonstrated in all three tumour models. AN-215 caused a lower induction of MDR-1 in HEC-1A and RL-95-2 cancers than AN-201. MRP-1 and BCRP were not induced by AN-215 or AN-201. Thus, targeted chemotherapy with AN-215 powerfully inhibits the growth of human BN receptor-positive endometrial cancers.

Targeting gastrin-releasing peptide receptors for cancer treatment

Growth factor receptors play critical roles in cancer cell proliferation and progression. A number of such receptors have been targeted for cancer treatment by either a monoclonal antibody or a specifically designed small molecule to inhibit the receptor function. Bombesin/gastrin-releasing peptide receptors (BN/GRP-Rs) are expressed in a variety of cancer cells and have limited distribution in normal human tissue. Inhibition of BN/GRP-Rs has been shown to block small cell lung cancer growth in vitro. Early phase clinical trials targeting human GRP-R showed anti-cancer activity. This review will focus on the study of the distribution of BN/GRP-Rs in normal and malignant tissues, and various approaches to targeting BN-GRP-Rs for cancer diagnosis and treatment.