Expression of dopamine receptors and transporter in neuroendocrine gastrointestinal tumor cells

Dopamine in the regulation of glucose and lipid metabolism: a narrative review

OBJECTIVE

Owing to the global obesity epidemic, understanding the regulatory mechanisms of glucose and lipid metabolism has become increasingly important. The dopaminergic system, including dopamine, dopamine receptors, dopamine transporters, and other components, is involved in numerous physiological and pathological processes. However, the mechanism of action of the dopaminergic system in glucose and lipid metabolism is poorly understood. In this review, we examine the role of the dopaminergic system in glucose and lipid metabolism.

RESULTS

The dopaminergic system regulates glucose and lipid metabolism through several mechanisms. It regulates various activities at the central level, including appetite control and decision-making, which contribute to regulating body weight and energy metabolism. In the pituitary gland, dopamine inhibits prolactin production and promotes insulin secretion through dopamine receptor 2. Furthermore, it can influence various physiological components in the peripheral system, such as pancreatic β cells, glucagon-like peptide-1, adipocytes, hepatocytes, and muscle, by regulating insulin and glucagon secretion, glucose uptake and use, and fatty acid metabolism.

CONCLUSIONS

The role of dopamine in regulating glucose and lipid metabolism has significant implications for the physiology and pathogenesis of disease. The potential therapeutic value of dopamine lies in its effects on metabolic disorders.

Somatostatin-Dopamine Chimeric Molecules in Neuroendocrine Neoplasms

Neuroendocrine neoplasms (NENs) are a widely heterogeneous family of neoplasms arising from neuroendocrine cells, which are interspersed throughout the body. Despite NENs are relatively rare, their incidence and prevalence are constantly increasing probably due to the improvement in earlier diagnosis and patients’ management. When surgery is not curative, particularly for patients with metastatic disease, several medical options are available. Somatostatin analogues (SSA) are the first-line medical therapy for well-differentiated NENs. Interestingly, the heterodimerization of somatostatin receptors (SSTs) with dopamine receptors (DRs) has been discovered in NENs. This phenomenon results in hybrid receptors with enhanced functional activity. On these bases, chimeric molecules embracing somatostatin and dopamine features have been recently developed. The aim of this review is to provide a comprehensive overview of the available preclinical and clinical data regarding chimeric somatostatin-dopamine agonists as a new class of “magic bullet” in the therapy of NENs.

Effects of the single and combined treatment with dopamine agonist, somatostatin analog and mTOR inhibitors in a human lung carcinoid cell line: an in vitro study

Somatostatin analogues and mTOR inhibitors have been used as medical therapy in lung carcinoids with variable results. No data are available on dopamine agonists as treatment for lung carcinoids. The main aim of the current study was to evaluate the effect of the combined treatment of somatostatin analogue octreotide and the dopamine agonist cabergoline with mTOR inhibitors in an in vitro model of typical lung carcinoids: the NCI-H727 cell line. In NCI-H727 cell line, reverse transcriptase-quantitative polymerase chain reaction and immunofluorescence were assessed to characterize the expression of the somatostatin receptor 2 and 5, dopamine receptor 2 and mTOR pathway components. Fifteen typical lung carcinoids tissue samples have been used for somatostatin receptor 2, dopamine receptor 2, and the main mTOR pathway component p70S6K expression and localization by immunohistochemistry. Cell viability, fluorescence-activated cell sorting analysis and western blot have been assessed to test the pharmacological effects of octreotide, cabergoline and mTOR inhibitors, and to evaluate the activation of specific cell signaling pathways in NCI-H727 cell line. NCI-H727 cell line expressed somatostatin receptor 2, somatostatin receptor 5 and dopamine receptor 2 and all mTOR pathway components at messenger and protein levels. Somatostatin receptor 2, dopamine receptor 2, and p70S6K (non phosphorylated and phosphorylated) proteins were expressed in most typical lung carcinoids tissue samples. Octreotide and cabergoline did not reduce cell viability as single agents but, when combined with mTOR inhibitors, they potentiate mTOR inhibitors effect after long-term exposure, reducing Akt and ERK phosphorylation, mTOR escape mechanisms, and increasing the expression DNA-damage-inducible transcript 4, an mTOR suppressor. In conclusion, the single use of octreotide and cabergoline is not sufficient to block cell viability but the combined approach of these agents with mTOR inhibitors might reduce the mTOR inhibitors-induced escape mechanisms and/or activate the endogenous mTOR suppressor, potentiating the effect of the mTOR inhibitors in an in vitro model of typical lung carcinoids.

Antisecretory Effects of Chimeric Somatostatin/Dopamine Receptor Ligands on Gastroenteropancreatic Neuroendocrine Tumors

OBJECTIVES

The recent finding that gastroenteropancreatic neuroendocrine tumors expressed the dopaminergic D2 receptor in addition to somatostatin (sst) receptors suggested that multiple targeting approaches might decrease hormone hypersecretion more effectively than sst agonists alone.

METHODS

To test this hypothesis, (i) we measured the expression of sst receptor type 2 (sst2 receptor) and D2 receptor in 11 gastroenteropancreatic neuroendocrine tumors and (ii) we compared the ability of lanreotide, cabergoline, their combination, and sst/D2 chimeric ligands to decrease chromogranin A (CgA), gastrin, or serotonin release in primary cultures derived from these tumors.

RESULTS

Moderate to high positivity was observed for sst2 receptor and D2 receptor, the latter being more expressed in pancreatic tumors. Lanreotide decreased CgA secretion in all cultures, but only 3 tumors responded to cabergoline. No additivity was observed in lanreotide. BIM 23A781 decreased CgA release to the same extent as lanreotide, whereas the other chimeric ligands were less efficient. However, BIM 23A781 was 50 times less potent than lanreotide. Similar patterns were found for gastrin or serotonin.

CONCLUSION

No improvement was brought by the sst/D2 combination or chimeric ligands. Factors that underlie these tissue-specific differences remain to be elucidated.

Antiferroptotic activity of non-oxidative dopamine

Dopamine is a neurotransmitter that has many functions in the nervous and immune systems. Ferroptosis is a non-apoptotic form of regulated cell death that is involved in cancer and neurodegenerative diseases. However, the role of dopamine in ferroptosis remains unidentified. Here, we show that the non-oxidative form of dopamine is a strong inhibitor of ferroptotic cell death. Dopamine dose-dependently blocked ferroptosis in cancer (PANC1 and HEY) and non-cancer (MEF and HEK293) cells following treatment with erastin, a small molecule ferroptosis inducer. Notably, dopamine reduced erastin-induced ferrous iron accumulation, glutathione depletion, and malondialdehyde production. Mechanically, dopamine increased the protein stability of glutathione peroxidase 4, a phospholipid hydroperoxidase that protects cells against membrane lipid peroxidation. Moreover, dopamine suppressed dopamine receptor D4 protein degradation and promoted dopamine receptor D5 gene expression. Thus, our findings uncover a novel function of dopamine in cell death and provide new insight into the regulation of iron metabolism and lipid peroxidation by neurotransmitters.

Somatostatin and dopamine receptor expression in neuroendocrine neoplasms: correlation of immunohistochemical findings with somatostatin receptor scintigraphy visual scores

CONTEXT

The expression of somatostatin (sstr1-5) and dopamine (DR) receptors in neuroendocrine neoplasms (NENs) facilitates diagnosis by tumour visualization with somatostatin receptor scintigraphy (SRS) and directs towards specific treatment with peptide receptor radionuclide therapy (PRRT) with radiolabelled somatostatin analogues.

OBJECTIVE

To investigate the co-expression of sstrs, D2R in relation to pre-operative SRSs in NENs.

DESIGN

Prospective two-centre study.

PATIENTS AND MEASUREMENTS

We analysed pre-operative SRS of 60 patients [44 with gastrointestinal (GI) NENs and 16 with lung NENs] and compared SRS results with immunohistochemical (IHC) reactivity for sstr2, sstr3, sstr5 in sample tissues from primary (n = 54) and metastatic (n = 27) lesions and IHC reactivity for D2R in 23 samples from primary GI-NENs lesions.

RESULTS

Sstr2 was the commonest sstr expressed (65·4%) and was co-expressed with sstr3 and sstr5 in 32·1% and 24·7% of the specimens, respectively. In 67 of 81 specimens (82·7%), there was concordance of sstr2 immunohistochemistry with SRS findings (P < 0·001). D2R was expressed in only 8 of 23 (34·8%) GI-NENs while was co-expressed with sstr2 in all cases. SRS grade, as per Krenning scale, was higher in metastatic foci, large-size (>2 cm) tumours and GI-NENs, whereas sstr2 intensity was greater in GI compared to lung NENs. SRS grade showed higher correlation with sstr2 (r = 0·6, P < 0·001) and D2R (r = 0·5, P < 0·001) IHC intensity scores than tumour size (r = 0·4, P < 0·001) and sstr3 (r = 0·4, P < 0·001) intensity score.

CONCLUSIONS

Sstr2 IHC expression and SRS are useful tools for the diagnosis and management of NENs because they display a high concordance. IHC expression of DR2 seems to be of potential clinical significance in GI-NENs tumours.

Synthesis and characterization of novel zinc phthalocyanines as potential photosensitizers for photodynamic therapy of cancers

Two novel water soluble zinc phthalocyanines (Pcs): tetramethyl tetrakis-2,(3)-[(4-methyl-2-pyridyloxy)phthalocyaninato] zinc(ii) (4) and tetrakis-2,(3)-[(3-carboxylicacid-6-sulfanylpyridine)phthalocyaninato] zinc(ii) (5) were synthesized and characterized.

The role of somatostatin and dopamine D2 receptors in endocrine tumors

Somatostatin (SS) and dopamine (DA) receptors have been highlighted as two critical regulators in the negative control of hormonal secretion in a wide group of human endocrine tumors. Both families of receptors belong to the superfamily of G protein-coupled receptors and share a number of structural and functional characteristics. Because of the generally reported high expression of somatostatin receptors (SSTRs) in neuroendocrine tumors (NET), somatostatin analogs (SSA) have a pronounced role in the medical therapy for this class of tumors, especially pituitary adenomas and well-differentiated gastroenteropancreatic NET (GEP NET). Moreover, NET express not only SSTR but also frequently dopamine receptors (DRs), and DA agonists targeting the D(2) receptor (D(2)) have been demonstrated to be effective in controlling hormone secretion and cell proliferation in in vivo and in vitro studies. The treatment with SSAs combined with DA agonists has already been demonstrated efficacious in a subgroup of patients with GH-secreting pituitary adenomas and few reported cases of carcinoids. The recent availability of new selective and universal SSA and DA agonists, as well as the chimeric SS/DA compounds, may shed new light on the potential role of SSTR and D(2) as combined targets for biotherapy in NET. This review provides an overview of the latest studies evaluating the expression of SSTR and DR in NET, focusing on their co-expression and the possible clinical implications of such co-expression. Moreover, the most recent insights in SSTR and D(2) pathophysiology and the future perspectives for treatment with SSA, DA agonists, and SS/DA chimeric compounds are discussed.

Therapeutic management of patients with gastroenteropancreatic neuroendocrine tumours

Patients with neuroendocrine tumours (NETs) are best managed in a specialist centre as part of a multidisciplinary team comprising gastroenterologists, oncologists, endocrinologists, gastrointestinal and hepatopancreaticobiliary surgeons, pathologists, nuclear medicine physicians and technicians, radiologists, specialist nurses, pharmacists, biochemists and dieticians. This should ideally be led by a clinician with experience and interest in NETs. Although the number of medical treatments and clinical trials has increased in the decade, there is still a lack of prospective randomised trials; thus, management is mainly based on limited often single-centre studies, although there are now formal guidelines based on consensus expert opinion. We have outlined the current optimal management of patients with NETs. We have reviewed therapeutic options including surgery, somatostatin analogues and other biotherapies and peptide receptor-targeted therapy. We have discussed the challenge in managing hepatic metastases including hepatic artery embolisation, ablation and orthotopic liver transplant. In addition, we have briefly reviewed the emerging therapies such as the mammalian target of rapamycin and angiogenic inhibitors and the newer somatostatin analogues.

Somatostatin and dopamine receptor profile of gastroenteropancreatic neuroendocrine tumors: an immunohistochemical study

Somatostatin and its synthetic analogs act through five specific somatostatin receptors (sstr1-5), found on the cell membrane of various tumors, including endocrine ones. Dopamine--a known neurotransmitter--acts through five membranous dopamine receptors (D1R-D5R) which have recently been found to be expressed in endocrine tumors. We evaluated the immunohistochemical expression of the sstrs and D2R in a large series of gastroenteropancreatic neuroendocrine tumors (GEP-NETs). A total of 22 (28.94%) well-differentiated NETs (WDNETs), 6 (7.89%) WDNETs of uncertain biology, 26 (34.21%) well-differentiated neuroendocrine carcinomas, and 22 (28.94%) poorly differentiated neuroendocrine carcinomas were studied. Overall, 76.31% of the tumors were positive for different types of sstrs with variable intensity of the membranous staining whereas 36.95% were positive for D2R alone. The sstr2A was the most frequently expressed, followed by sstr2B, sstr1, and sstr5. Co-expression of sstrs and D2R was seen in 88.23% of positive tumors. The high rates of sstr2A and sstr2B and in a lower extent of sstr5 expression are of great importance for more accurate imaging, staging and targeted therapy of the disease. The co-expression of sstrs and D2R in a significant number of the studied cases offers a potential therapeutic alternative for GEP-NETs.

From targets to treatments: a review of molecular targets in pancreatic neuroendocrine tumors

Pancreatic neuroendocrine tumors (pancreatic NET) are relatively rare, slowly growing tumors, although their incidence is increasing, and patients may survive for several years with metastatic disease. Apart from symptomatic relief, there have been few treatment options for these tumors in the past. More recently, investigators have explored the potential of molecularly targeted agents in treating pancreatic NET, with some success. In this review, we consider the data supporting exploitation of different targets in pancreatic NET, including peptide receptors, receptor tyrosine kinases (involved in tumor angiogenesis and more directly supporting tumor growth), and intracellular targets, such as the mammalian target of rapamycin (mTOR), which has a central role in regulating cell growth, metabolism, and apoptosis. Probably due to the paucity of pancreatic NET, many clinical trials to date have included heterogeneous NET populations, and there are few randomized studies of this specific patient population. Very recently, promising results have been achieved in placebo-controlled, phase III trials with the multitargeted tyrosine kinase inhibitor, sunitinib, and the mTOR inhibitor, everolimus. These agents have been approved or are currently being reviewed by authorities for use in patients with pancreatic NET. Here we review potential molecular targets in pancreatic NET and summarize the available data for targeted agents from phase II and III trials open to patients with this tumor.

D1 and D2 dopamine receptor-mediated inhibition of activated normal T cell proliferation is lost in jurkat T leukemic cells

Dopamine is a catecholamine neurotransmitter, which plays an important role in the regulation of T cell functions. In activated T cells from normal volunteers, stimulation of D(1) and D(2) dopamine receptors inhibit cell proliferation and cytokine secretion. However, there is no report yet regarding the regulatory role of D(1) and D(2) dopamine receptors in abnormally proliferating T cells. The present study investigates the expression and effect of activation of these dopamine receptors in Jurkat cells, a leukemic T cell line showing uncontrolled proliferation. Like normal human T cells, in Jurkat cells, D(1) and D(2) dopamine receptors are also expressed; however, unlike activated normal T cells, stimulation of these dopamine receptors in Jurkat cells fails to inhibit their T cell receptor-induced proliferation. This alteration is due to failure of D(1) dopamine receptor-mediated activation of cyclic AMP signaling and a missense mutation at the third cytoplasmic loop of D(2) dopamine receptors affecting inhibition of phosphorylation of ZAP-70, an important downstream protein transducing signal from the T cell receptor. These results help to understand the biology of abnormal proliferation of T cells in pathophysiological conditions where dopamine plays an important role.

PACAP induces signaling and stimulation of 5-hydroxytryptamine release and growth in neuroendocrine tumor cells

Neuroendocrine tumors, although rare, are currently diagnosed with increasing frequency, owing to improved imaging techniques and a greater clinical awareness of this condition. To date, BON is a very well established and characterized human pancreatic neuroendocrine tumor cell line used to study the signal transduction and genetic regulation of neuroendocrine tumors secretion and growth. The secretory activity of BON cells is known to release peptides, such as chromogranin A, neurotensin, and biogenic amines, as 5-HT, permitting an assessment of their biological activity. The neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP), released from the enteric neurons in the gastrointestinal tract by binding to its high affinity receptor PAC1, has been previously shown to regulate the secretory activity and growth of the neuroendocrine-derived enterochromaffin-like cells in the stomach. This led us to speculate that PACAP might also play an important role in regulating the growth of human neuroendocrine tumors. Accordingly, in the current study, we have shown that BON cells express PAC1 receptors, which are rapidly internalized upon PACAP activation. Furthermore, PAC1 receptor activation, in BON cells, couple to intracellular Ca(2+) as well as cAMP responses and induce the release of intracellular 5-HT, activate mitogen activated protein kinases, and stimulate cellular growth. These data indicate that PACAP functionally can stimulate 5-HT release and promote the growth of the BON neuroendocrine tumor cell line. Therefore, PACAP and its receptors regulate neuroendocrine tumor secretory activity and growth in vivo, and this knowledge will permit the development of novel diagnostic and therapeutic targets for managing neuroendocrine tumors in humans.

6-[F-18]Fluoro-L-dihydroxyphenylalanine positron emission tomography is superior to conventional imaging with (123)I-metaiodobenzylguanidine scintigraphy, computer tomography, and magnetic resonance imaging in localizing tumors causing catecholamine excess

CONTEXT

Catecholamine excess is rare, but symptoms may be life threatening.

OBJECTIVE

The objective of the study was to investigate the sensitivity of 6-[F-18]fluoro-l-dihydroxyphenylalanine positron emission tomography ((18)F-DOPA PET), compared with (123)I-metaiodobenzylguanidine ((123)I-MIBG) scintigraphy and computer tomography (CT)/magnetic resonance imaging (MRI) for tumor localization in patients with catecholamine excess.

DESIGN AND SETTING

All consecutive patients with catecholamine excess visiting the University Medical Center Groningen, Groningen, The Netherlands, between March 2003 and January 2008 were eligible.

PATIENTS

Forty-eight patients were included. The final diagnosis was pheochromocytoma in 40, adrenal hyperplasia in two, paraganglioma in two, ganglioneuroma in one, and unknown in three.

MAIN OUTCOME MEASURES

Sensitivities and discordancy between (18)F-DOPA PET, (123)I-MIBG, and CT or MRI were analyzed for individual patients and lesions. Metanephrines and 3-methoxytyramine in plasma and urine and uptake of (18)F-DOPA with PET were measured to determine the whole-body metabolic burden and correlated with biochemical tumor activity. The gold standard was a composite reference standard.

RESULTS

(18)F-DOPA PET showed lesions in 43 patients, (123)I-MIBG in 31, and CT/MRI in 32. Patient-based sensitivity for (18)F-DOPA PET, (123)I-MIBG, and CT/MRI was 90, 65, and 67% (P < 0.01 for (18)F-DOPA PET vs. both (123)I-MIBG and CT/MRI, P = 1.0 (123)I-MIBG vs. CT/MRI). Lesion-based sensitivities were 73, 48, and 44% (P < 0.001 for (18)F-DOPA PET vs. both (123)I-MIBG and CT/MRI, P = 0.51 (123)I-MIBG vs. CT/MRI). The combination of (18)F-DOPA PET with CT/MRI was superior to (123)I-MIBG with CT/MRI (93 vs. 76%, P < 0.001). Whole-body metabolic burden measured with (18)F-DOPA PET correlated with plasma normetanephrine (r = 0.82), urinary normetanephrine (r = 0.84), and metanephrine (r = 0.57).

CONCLUSION

To localize tumors causing catecholamine excess, (18)F-DOPA PET is superior to (123)I-MIBG scintigraphy and CT/MRI.

Review article: future therapies for management of metastatic gastroenteropancreatic neuroendocrine tumours

BACKGROUND

Gastroenteropancreatic neuroendocrine tumours (GEP NETs) are relatively uncommon tumours that occur anywhere within the gastrointestinal tract. The prevalence of GEP NETs is estimated to be 35 per 100 000 population. Patients often present with metastatic disease and consequently, palliative treatments form the mainstay of therapy.

AIM

To review the current and novel therapeutic options for management of GEP NETs.

METHODS

Searches for all studies related to GEP NETs, NETs and carcinoid tumours in Medline and abstracts from international meetings.

RESULTS

Somatostatin analogues remain the first line therapy for management of symptoms of GEP NETs and may have anti-proliferative action. New somatostatin analogues with different somatostatin receptor affinity have been developed. Radionuclide peptide receptor therapy is established in patients with positive somatostatin scintigraphy. A number of new agents and targeted therapies are currently being evaluated in a phase I and II studies and these include angiogenic inhibitors, mammalian target of rapamycin inhibitors and immune therapies.

CONCLUSIONS

A number of nonsurgical therapies are available for management of gastroenteropancreatic neuroendocrine tumours. It is hoped, the development of some of these promising novel therapies will expand the therapeutic armamentarium.

Reduced tumor growth in a mouse model of schizophrenia, lacking the dopamine transporter

The incidence of cancer in patients with schizophrenia has been reported to be lower that in the general population. On the other hand, it is well established that patients with schizophrenia have a hyper-dopaminergic system and dopamine has the ability to inhibit tumor angiogenesis. Therefore, in order to investigate the molecular mechanisms responsible for the lower cancer risk in schizophrenic patients, we used a mouse model of schizophrenia, which shows hyper-dopaminergic transmission in the nerve terminals of dopaminergic neurons. Here, we hypothesized that tumor growth was reduced in a mouse model of schizophrenia, lacking the dopamine transporter (DAT), and investigated tumor growth and angiogenesis in DAT knockout mice. The subcutaneous tumor in mice inoculated with cancer cells was smaller in DAT-/- mice than in the wild type (p < 0.05); however, the level of plasma dopamine in DAT-/- mice was lower than that of control littermates. Using human umbilical vascular endothelial cells (HUVEC), we examined dopamine signaling through dopamine D(1) receptor (D(1)R) and D(2)R. Dopamine stimulation slightly decreased the surface expression of vascular endothelial growth factor receptor-2 (VEGF-R2) but induced the phosphorylation of VEGF-R2 through Src in HUVEC. In addition, DAT-/- mice had less D(1)R. Both pharmacological and genetic interruption of D(1)R showed inhibited tumor growth. These results suggest that modulation of the dopaminergic system may contribute to cancer therapy.

Tumor biology-guided radiotherapy treatment planning: gross tumor volume versus functional tumor volume

This issue of Seminars in Nuclear Medicine deals with a watershed event in cancer treatment -- the combined use of functional and anatomical information to guide therapeutic interventions. The use of positron emission tomography/computed tomography (PET/CT) in radiation treatment planning and tumor response evaluation brings a paradigm change in the development of image-guided therapies into routine clinical practice. The implications, as seen in the following articles, are not only promising but also groundbreaking. And, as in every new scientific breakthrough, each step forward generates a myriad of additional important clinical and research questions. Functional imaging takes advantage of the subtle differences between normal and malignant tissues at the cellular level to reveal in vivo unique functional characteristics of neoplasms. The ultimate goal of the partnership between nuclear medicine physicians and radiation oncologists is to use this information with absolute clarity in target definition for radiation treatment planning and therapy, as well as response evaluation. Functional imaging can provide metabolic information and behavioral correlation along with the anatomical imaging for correlative target delineation. Additionally, as a purely diagnostic instrument, PET/CT provides a tool for oncologists to make critical decisions regarding radiation treatment planning modifications secondary to changes in tumor staging (up or down), treatment field modifications, localized control, sites of residual and/or metastatic disease and post therapy response evaluation. The articles in this issue of the seminars provide insights into the current state-of-the-art of functional imaging techniques, mostly centered on the use of (18)F-fluorodeoxyglucose PET/CT in image guided oncologic therapies. Because it is a novel science, the future of image-guided functional treatment planning is bright with technologic and biologic innovations, translational research and new clinical applications.

[18F]Fluoro-L-DOPA PET/CT in Congenital Hyperinsulinism

Congenital hyperinsulinism can be divided into diffuse or focal form. The treatment and outcome depend on distinguishing between the 2 forms. Pancreatic venous sampling was the only method available to localize the insulin secretion. [F]Fluoro-levodopa, 3,4-dihydroxy-L-phenylalanine positron emission tomography/computed tomography is a noninvasive imaging investigation and increasingly used to determine the type of hyperinsulinism preoperatively. We present a case of diffuse form of congenital hyperinsulinism demonstrated by the [F]levodopa, 3,4-dihydroxy-L-phenylalanine positron emission tomography/computed tomography preoperatively and review the literature.

Role of 18F-dopa PET/CT imaging in the management of patients with 111In-pentetreotide negative GEP tumours

PURPOSE

To assess whether 18F-dopa PET/CT is able to provide information relevant in changing the clinical management of patients with gastro-enteropancreatic (GEP) tumours where there is negative or inconclusive conventional radiological imaging (ultrasound (US), computed tomography (CT) and magnetic resonance imaging (MRI)) and 111In-pentetreotide scintigraphy.

MATERIALS AND METHODS

From January 2005 to October 2006, 84 patients with clinical and biochemical suspicion of GEP tumours were investigated by US and CT scans, MRI and 111In-pentetreotide scintigraphy. In 13/84 (15.4%) both conventional radiological imaging and 111In-pentetreotide scintigraphy provided negative or inconclusive findings, and patients were referred for 18F-dopa PET/CT imaging. Each patient received 5.3 MBq x kg(-1) 18F-dopa intravenously, and imaged 60 min later using a hybrid PET/CT scanner.

RESULTS

18F-dopa PET/CT detected the primary tumour in all 13 patients (size range, 7-26 mm, mean, 18 mm; SUVmax range, 2.3-16.3, mean, 5.7) and further 12 unsuspected lesions (size range, 12-23 mm, mean 17; SUVmax range 2.8-12.7, mean 4.6). Confirmation of the PET/CT findings was obtained in all patients from histopathological analysis of tissue obtained after surgery and/or biopsy. All the 18F-dopa-positive primary lesions were confirmed as being the primary tumour at histology, whereas of the other 12 unsuspected 18F-dopa-positive lesions, 11 were found to be metastatic deposits and one due to unspecific inflammation (one false positive result). Notably, the results of 18F-dopa PET/CT imaging changed the clinical management in 11/13 patients (84%).

CONCLUSIONS

Our preliminary results suggest that 18F-dopa PET/CT has a promising role in GEP patients with negative or inconclusive findings at conventional radiological imaging and 111In-pentetreotide scintigraphy. The findings were helpful in biopsy guidance and played a major role in changing the management of those patients.

Novel insights in dopamine receptor physiology

The dopaminergic system has a pivotal role in the central nervous system but also plays important roles in the periphery, mainly in the endocrine system. Dopamine exerts its functions via five different receptors, named D(1)-D(5), belonging to the category of G protein coupled membrane receptors. Dopamine receptors are heterogeneously expressed in different cells, tissues and organs, where they stimulate or inhibit different functions, including neurotransmission and hormone synthesis and secretion. In particular, the dopamineric system has a pivotal role in the physiological regulation of the hypothalamus-pituitary-adrenal axis. Recent data have demonstrated the expression and function of dopamine receptors not only in endocrine organs but also in endocrine tumors, mainly those belonging to the hypothalamus-pituitary-adrenal axis, and also in the so-called 'neuroendocrine' tumors. These data confirm the important role of the dopaminergic system in this endocrine axis, as well as in the neuroendocrine system. This review summarizes the main structural and functional characteristics of dopamine receptors, emphasizing the most recent novelties, and focused on the physiological and pathological regulation of the hypothalamus-pituitary-adrenal axis by the dopaminergic system. In addition, the recent findings on the relationship between dopamine receptors and neuroendocrine tumors are summarized.

Cutting Edge: Stimulation of dopamine D4 receptors induce T cell quiescence by up-regulating Kruppel-like factor-2 expression through inhibition of ERK1/ERK2 phosphorylation

The neurotransmitter dopamine (DA) is an important regulator of human T cell functions. Although it has been observed that DA, by acting through the D1/D5, D2, and D3 receptors, can activate resting T cells by stimulating the release of cytokines and the expression of surface integrins and also inhibit the proliferation of activated T cells by down-regulating nonreceptor tyrosine kinases, there is not yet a report indicating the functional significance of the D4 DA receptors present in these cells. The present work, for the first time, demonstrates that the stimulation of D4 DA receptors in human T cells induces T cell quiescence by up-regulating lung Krüppel-like factor-2 expression through the inhibition of ERK1/ERK2 phosphorylation. These results reveal a new link between the nervous system and T cell quiescence and indicate that D4 DA receptor agonists may have a therapeutic value in diseases with uncontrolled T cell proliferation.

Dopamine receptor expression and function in corticotroph ectopic tumors

BACKGROUND

Dopamine receptor (DR) expression and dopamine agonist (DA) effectiveness have never been demonstrated in neuroendocrine tumors associated with ectopic ACTH syndrome (EAS).

AIM

The aim of the current study was to evaluate DR and particularly D2 subtype expression in neuroendocrine tumors associated with EAS and to evaluate the in vivo effectiveness of the DA cabergoline in the treatment of EAS.

PATIENTS AND METHODS

Six ACTH-secreting neuroendocrine tumors, including four lung, one pancreatic, and one thymic carcinoid, were used for the evaluation of D2 expression by immunohistochemistry. DR subtypes and D2 isoforms and number were evaluated by RT-PCR in three cases of persistent EAS after surgery. These patients were treated with cabergoline at the dose of 3.5 mg/wk for 6 months. Clinical parameters, hormonal levels, and tumor size were monitored during the treatment period.

RESULTS

At immunohistochemistry, D2 was expressed in five (83.3%) tumors. At RT-PCR, D2 was confirmed in all three cases but at variable numbers, whereas D4 was expressed in two cases. D(2long) was expressed in all three cases, together with D(2short) in one case. A normalization of urinary cortisol levels was found in two patients (66.7%) after 3 months of treatment. However, treatment escape was demonstrated in one of these patients afterward.

CONCLUSION

The results of this study demonstrated that DR are expressed in neuroendocrine tumors associated with EAS and that cabergoline treatment could be effective in controlling cortisol excess in a subgroup of patients with EAS. Further studies on a larger number of patients are mandatory to confirm the usefulness of DA in EAS.

DRD2/DARPP-32 expression correlates with lymph node metastasis and tumor progression in patients with esophageal squamous cell carcinoma

BACKGROUND

Dopamine receptors (DRs) are members of seven transmembrane domain trimeric guanosine 5'-triphosphate (GTP)-binding protein-coupled receptor family. Through dopamine receptor activation, dopamine plays a significant role in regulating gene expression, such as induced tumor cell migration.

MATERIALS AND METHODS

We investigated DRD1 and DRD2 expressions in patients with esophageal squamous cell carcinoma (ESCC) for immunohistochemistry and analyzed differences between DRD1, DRD2, and DARPP-32 expressions of clinicopathological features in 122 patients with ESCC.

RESULTS

DRD1 immunostaining correlated with the pathologic grade (P = 0.0127), and DRD2 immunostaining correlated with the pathologic stage (P = 0.0432) and pN classification (P = 0.0112). A significant correlation was found between DRD1 and DRD2 expression (P = 0.0292). However, no correlation was observed between DRD1/DRD2 expression and DARPP-32 expression (P = 0.4555 and 0.4774, respectively). No correlation was observed between the DRD1/DRD2 expression and patient prognosis. To find the cooperative role between DRD1, DRD2, and DARPP-32 expressions, patients were classified into the different groups. In the DRD2/DARPP-32 combination, the (+/-) group was significantly correlated with pathologic stage (P = 0.0006), lymph node metastasis (P = 0.0001), pT (P = 0.0287), and tumor size (P = 0.0202). Moreover, patients with this combination showed a lower survival rate compared with the other three groups (P = 0.0287).

CONCLUSIONS

We conclude that DRD2/DARPP-32 expression is associated with tumor progression and that DRD2/DARPP-32 expressions may help predict prognosis in patients with ESCC.

The analysis of quantitative expression of somatostatin and dopamine receptors in gastro-entero-pancreatic tumours opens new therapeutic strategies

OBJECTIVE

Somatostatin (sst) are present in the majority of gastro-entero-pancreatic (GEP) tumours. Effects of somatostatin receptor (sst) analogues are partial and of limited duration. Cell lines derived from GEP express dopaminergic receptors D(2). New chimeric analogues simultaneously recognising sst(2) and sst(5) or sst(2) and D(2) have additive effects in inhibition of GH and prolactin secretion in pituitary adenomas. Our aim was to quantify the expression of sst and D(2) mRNA in human GEP tumours.

DESIGN AND METHODS

mRNA expression of sst(1), sst(2), sst(3) and sst(5) as well as D(2), was analysed using real-time PCR (TaqMan probe) in a series of 35 patients with GEP tumours (pancreas (n = 19) and intestinal (n = 16)). Levels of expression were compared with a group of 13 somatotroph adenomas.

RESULTS

All GEP tumours express sst(1), sst(2) and D(2). Expression of sst(3) and sst(5) was observed in 89 and 76% of tumours respectively with highly variable levels. sst(2) mRNA expression was higher in nonfunctional tumours (P < 0.009) and sst5 was higher in pancreatic than in intestinal tumours (P < 0.02). Whereas sst(2) levels were similar between GEP and somatotroph tumours, levels of sst(5) and D(2) were higher in the former (394.9 +/- 156.1 x 10(-2) vs 69.7 +/- 19.5 x 10(-2) copy/copy beta-Gus (P < 0.0036) and 519.6 +/- 121.2 x 10(-2) vs 50.0 +/- 21.6 x 10(-2) copy/copy beta-Gus (P < 0.0001) respectively). In small tumours ( < 30 mm), sst(2) density appeared as a crucial parameter in somatostatin receptor scintigraphy results, whereas in big tumours, a consistent bias in SRS results was introduced by the size. In pancreatic GEP, high-level sst(3) expression was found in tumours with more active angiogenesis (higher microvessel density and vascular endothelial growth factor expression (P < 0.03)).

CONCLUSIONS

GEP tumours co-express sst(2) and D(2) in 100% of cases and sst(5) in 89% thus supporting the testing of bi-specific agonists (sst(2)/sst(5) or sst(2)/D(2)) in these tumours.

Long-term quiescence of ectopic Cushing's syndrome caused by pulmonary neuroendocrine tumor (typical carcinoid) and tumorlets: spontaneous remission or therapeutic effect of bromocriptine?

In 1990, a 55-yr-old woman was admitted to the Medical Department of our hospital for severe hypercortisolism complicated by secondary diabetes mellitus and serious hypokalemia. Although inferior petrosal sinus sampling did not show any significant difference between central and peripheral ACTH concentration, suggesting an ectopic source of ACTH secretion, diagnostic imaging was negative and Cushing's disease due to hyperplasia of the pituitary intermediate lobe was suspected. Medical treatment with bromocriptine and cyproheptadine led to a rapid and stabile normalization of adrenal function, so that after two months cyproheptadine was stopped and bromocriptine was tapered to a smaller dose. An attempt to discontinue medical treatment, carried out 3 yr later, was followed by a quick increase of ACTH and cortisol levels, which were normalized by the resumption of the bromocriptine. Adrenal function remained normal until 1994 when hypercortisolism relapsed despite the treatment. Chest radiography and computed tomography (CT) scan detected a 6 mm nodule in the middle lobe of the lung which proved to be a neuroendocrine tumor, with immunohistochemical positivity for ACTH. Nests of neuroendocrine cells (tumorlets) were also demonstrated in the surrounding lung tissue. After the lobectomy, the patient recovered completely from Cushing's syndrome and no symptoms and/or signs of recurrence have been observed over the subsequent follow-up period. Although cyclical spontaneous Cushing's syndrome could not be excluded, there was strong evidence that medical treatment with bromocriptine might have played a key role in long-lasting remission. To our knowledge, this is the second case described in literature of Cushing's syndrome caused by neuroendocrine lung tumor responsive to bromocriptine.

Dopamine and noradrenaline control distinct functions in rodent microglial cells

Microglial cells are the immune-competent elements of the brain. They not only express receptors for chemokines and cytokines but also for neurotransmitters such as GABA [Charles et al., Mol. Cell Neurosci. 24 (2003) 214], glutamate [Noda et al., J. Neurosci. 20 (2000) 251], and adrenaline [Mori et al., Neuropharmacology 43 (2002) 1026]. Here we report the functional expression of dopamine receptors in mouse and rat microglia, in culture and brain slices. Using the patch clamp technique as the functional assay we identified D1- and D2-like dopamine receptors using subtype-specific ligands. They triggered the inhibition of the constitutive potassium inward rectifier and activated potassium outward currents in a subpopulation of microglia. Chronic dopamine receptor stimulation enhanced migratory activity and attenuated the lipopolysaccharide (LPS)-induced nitric oxide (NO) release similar as by stimulation of adrenergic receptors. While, however, noradrenaline attenuated the LPS-induced release of TNF-alpha and IL-6, dopamine was ineffective in modulating this response. We conclude that microglia express dopamine receptors which are distinct in function from adrenergic receptors.

Dopamine, through the extracellular signal-regulated kinase pathway, downregulates CD4+CD25+ regulatory T-cell activity: implications for neurodegeneration

Fighting off neuronal degeneration requires a well controlled T-cell response against self-antigens residing in sites of the CNS damage. The ability to evoke this response is normally suppressed by naturally occurring CD4+CD25+ regulatory T-cells (Treg). No physiological compound that controls Treg activity has yet been identified. Here, we show that dopamine, acting via type 1 dopamine receptors (found here to be preferentially expressed by Treg), reduces the suppressive activity and the adhesive and migratory abilities of Treg. Treg activity was correlated with activation of the ERK1/2 (extracellular signal-regulated kinase 1/2) signaling pathway. Systemic injection of dopamine or an agonist of its type 1 receptors significantly enhanced, via a T-cell-dependent mechanism, protection against neuronal death after CNS mechanical and biochemical injury. These findings shed light on the physiological mechanisms controlling Treg and might open the way to novel therapeutic strategies for downregulating Treg activity (e.g., in neuronal degeneration) or for strengthening it (in autoimmune diseases).

Real time non-invasive imaging of receptor-ligand interactions in vivo

Non-invasive longitudinal detection and evaluation of gene expression in living animals can provide investigators with an understanding of the ontogeny of a gene's biological function(s). Currently, mouse model systems are used to optimize magnetic resonance imaging (MRI), positron emission tomography (PET), single-photon emission computed tomography (SPECT), and optical imaging modalities to detect gene expression and protein function. These molecular imaging strategies are being developed to assess tumor growth and the tumor microenvironment. In addition, pre-labeling of progenitor cells can provide invaluable information about the developmental lineage of stem cells both in organogenesis and tumorigenesis. The feasibility of this approach has been extensively tested by targeting of endogenous tumor cell receptors with labeled ligand (or ligand analog) reporters and targeting enzymes with labeled substrate (or substrate analog). We will primarily discuss MRI, PET, and SPECT imaging of cell surface receptors and the feasibility of non-invasive imaging of gene expression using the tumor microenvironment (e.g., hypoxia) as a conditional regulator of gene expression.

A novel approach in the treatment of neuroendocrine gastrointestinal tumors: additive antiproliferative effects of interferon-gamma and meta-iodobenzylguanidine

BACKGROUND

Therapeutic options to effectively inhibit growth and spread of neuroendocrine gastrointestinal tumors are still limited. As both meta-iodobenzylguanidine (MIBG) and interferon-gamma (IFNgamma) cause antineoplastic effects in neuroendocrine gastrointestinal tumor cells, we investigated the antiproliferative effects of the combination of IFNgamma and non-radiolabeled MIBG in neuroendocrine gut STC-1 and pancreatic carcinoid BON tumor cells.

METHODS AND RESULTS

IFNgamma receptors were expressed in both models. IFNgamma dose- and time-dependently inhibited the growth of both STC-1 and of BON tumor cells with IC50-values of 95 +/- 15 U/ml and 135 +/- 10 U/ml, respectively. Above 10 U/ml IFNgamma induced apoptosis-specific caspase-3 activity in a time-dependent manner in either cell line and caused a dose-dependent arrest in the S-phase of the cell cycle. Furthermore, IFNgamma induced cytotoxic effects in NE tumor cells. The NE tumor-targeted drug MIBG is selectively taken up via norepinephrine transporters, thereby specifically inhibiting growth in NE tumor cells. Intriguingly, IFNgamma treatment induced an upregulation of norepinephrine transporter expression in neuroendocrine tumors cells, as determined by semi-quantitative RT-PCR. Co-application of sub-IC50 concentrations of IFNgamma and MIBG led to additive growth inhibitory effects, which were mainly due to increased cytotoxicity and S-phase arrest of the cell cycle.

CONCLUSION

Our data show that IFNgamma exerts antiproliferative effects on neuroendocrine gastrointestinal tumor cells by inducing cell cycle arrest, apoptosis and cytotoxicity. The combination of IFNgamma with the NE tumor-targeted agent MIBG leads to effective growth control at reduced doses of either drug. Thus, the administration of IFNgamma alone and more so, in combination with MIBG, is a promising novel approach in the treatment of neuroendocrine gastrointestinal tumors.

Dopaminergic modulation of oxidative stress and apoptosis in human peripheral blood lymphocytes: evidence for a D1-like receptor-dependent protective effect

Dopamine (DA) is a neurotransmitter in the central and peripheral nervous system, which can be either cytotoxic or cytoprotective under selected conditions. Such effects involve oxidative mechanisms and are likely to play a role in neurodegenerative disorders. Because increasing evidence points to peripheral blood lymphocytes (PBL) as a feasible model for studying DA-related mechanisms of cell death and survival, we have explored in these cells the effects of DA on oxidative metabolism and apoptosis. Our results show that, whereas DA 100-500 microM resulted in increased intracellular reactive oxygen species (ROS) levels and apoptotic cell death through oxidative stress, DA 0.1-5 microM decreased ROS levels and apoptosis. DA (both 1 and 500 microM) partially counteracted the decrease in Cu/Zn superoxide dismutase levels observed in untreated PBL. However, whereas the effect of the low dose lasted for the whole incubation period (24 h), the effect of DA 500 microM was transient. DA-dependent reduction of ROS levels and apoptosis was prevented by D1-like (but not D2-like) receptor antagonism. The present findings add knowledge about the sensitivity of PBL to DA and strengthen the rationale for exploiting these cells as an easily accessible peripheral model for the ex vivo investigation of oxidative stress-related dopaminergic mechanisms underlying human neurodegenerative diseases.

A novel approach in the treatment of neuroendocrine gastrointestinal tumours. Targeting the epidermal growth factor receptor by gefitinib (ZD1839)

Therapeutic options to inhibit the growth and spread of neuroendocrine (NE) gastrointestinal tumours are still limited. Since gefitinib (4-(3-chloro-4-fluoroanilino)-7-methoxy-6-(3-morpholinopropoxy)quinazoline), an inhibitor of epidermal growth factor receptor-sensitive tyrosine kinase (EGFR-TK), had been shown to suppress potently the growth of various non-NE tumour entities, we studied the antineoplastic potency of gefitinib in NE gastrointestinal tumour cells. In human insulinoma (CM) cells, in human pancreatic carcinoid (BON) cells and in NE tumour cells of the gut (STC-1), gefitinib induced a time- and dose-dependent growth inhibition by almost 100%. The antiproliferative potency of gefitinib correlated with the proliferation rate of the tumour cells. So the IC₅₀ value of gefitinib was 4.7±0.6 μM in the fast-growing CM cells, still 16.8±0.4 μM in the moderate-growing BON cells, and up to 31.5±2.5 μM in the slow-growing STC-1 cells. Similarly, the induction of apoptosis and cell-cycle arrest by gefitinib differed according to growth characteristics: fast-growing CM cells displayed a strong G0/G1 arrest in response to gefitinib, while no significant cell-cycle alterations were seen in the slow-growing STC-1. Vice versa, the proapoptotic effects of gefitinib, as determined by caspase-3 activation and DNA fragmentation, were most pronounced in the slow-growing STC-1 cells. Using cDNA microarrays, we found extensive changes in the expression of genes involved in the regulation of apoptosis and cell cycle after incubation with gefitinib. Among them, an upregulation of the growth arrest and DNA damage-inducible gene GADD153 was observed. Phosphorylation of ERK1/2, which inhibits GADD153 expression, was reduced in a time-dependent manner. However, no gefitinib-induced activation of the GADD153-inducing p38 mitogen-activated protein kinase was detected. Our data demonstrate that the inhibition of EGFR-TK by gefitinib induces growth inhibition, apoptosis and cell-cycle arrest in NE gastrointestinal tumour cells. Thus, EGFR-TK inhibition appears to be a promising novel approach for the treatment of NE tumour disease.

Insight into the mechanism of dopamine D1-like receptor activation. Evidence for a molecular interplay between the third extracellular loop and the cytoplasmic tail

A chimeric D1A dopaminergic receptor harboring the cytoplasmic tail (CT) of the D1B subtype (D1A-CTB) has been used previously to show that CT imparts high dopamine (DA) affinity and constitutive activity to the D1B receptors. However, the D1A-CTB chimera, unlike the D1B subtype, exhibits a significantly lower DA potency for stimulating adenylyl cyclase and a drastically lower maximal binding capacity (Bmax). Here, using a functional complementation of chimeric D1-like receptors, we have identified the human D1B receptor regions regulating the intramolecular relationships that lead to an increased DA potency and contribute to Bmax. We demonstrate that the addition of variant residues of the third extracellular loop (EL3) of the human D1B receptor into D1A-CTB chimera leads to a constitutively active mutant receptor displaying an increased DA affinity, potency, and Bmax. These results strongly suggest that constitutively active D1-like receptors can adopt multiple active conformations, notably one that confers increased DA affinity with decreased DA potency and Bmax and another that imparts increased DA affinity with a strikingly increased DA potency and Bmax. Overall, we show that a novel molecular interplay between EL3 and CT regulates multiple active conformations of D1-like receptors and may have potential implications for other G protein-coupled receptor classes.