Somatostatin receptors in malignant lymphomas: targets for radiotherapy?

Somatostatin (SS) receptor (sst) scintigraphy is widely used in the visualization of neuroendocrine tumors expressing sst, and radiotherapy using radionuclide-labeled SS analogs has been introduced for treatment of patients with neuroendocrine tumors. Previous sst scintigraphy studies revealed that malignant lymphomas can also be visualized using this technique. The question has been addressed whether lymphomas might also be possible targets for radiotherapy using radionuclide-labeled SS analogs. Therefore, we investigated in vitro the characteristics of lymphoma tissues and lymphoid cell lines to evaluate whether lymphomas can be targets for radiotherapy.

METHODS

Six orbital lymphomas, 2 Hodgkin's lymphomas, and 2 non-Hodgkin's lymphomas from the neck region were collected. Reverse transcriptase polymerase chain reaction (RT-PCR) and quantitative RT-PCR were performed to detect and quantify the expression of sst(1-5) mRNA. Receptor autoradiography studies using [(125)I-Tyr(3)]octreotide were performed to evaluate binding to sst on cryostat sections of lymphomas. Immunohistochemistry was used to investigate expression of sst(2) and sst(3). Membrane binding studies and in vitro internalization experiments using [(125)I-Tyr(3)]octreotide were performed to study binding and uptake of [(125)I-Tyr(3)]octreotide by lymphoid cell lines (JY, TMM, APD) and primary cells derived from a B-cell-derived chronic lymphatic leukemia.

RESULTS

A selective expression of sst(2) and sst(3) messenger RNA (mRNA) was demonstrated. By quantitative RT-PCR, expression levels of sst(2) and sst(3) mRNA were relatively low. Autoradiography studies revealed low binding of [(125)I-Tyr(3)]octreotide, whereas immunoreactivity could not be detected for sst(2) and sst(3) by immunohistochemistry. On the lymphoid cell lines only low numbers of high-affinity SS binding sites were found. In vitro, uptake of [(125)I-Tyr(3)]octreotide by these cells was also very low.

CONCLUSION

On the basis of our findings, we conclude that lymphomas do not appear to be candidates for radiotherapy using radionuclide-labeled SS analogs. However, lymphomas are highly radiosensitive tumors and further clinical studies should be performed to evaluate whether the low receptor density is sufficient for targeting treatment in these tumors.

Expression of somatostatin, cortistatin, and somatostatin receptors in human monocytes, macrophages, and dendritic cells

Increasing evidence suggests that neuropeptides play a role in the regulatory mechanisms between the neuroendocrine and immune systems. A differential expression of the five known somatostatin (SS) receptors (sst1-5) has been demonstrated in human immune cells and tissues. However, little is known concerning regulation and expression of sst1-5 and the peptide SS. Therefore, we investigated the expression and the time-dependent regulation of sst1-5, SS, and cortistatin (CST), a novel SS-like peptide, in human monocytes (MO), monocyte-derived macrophages (MP), and dendritic cells (DC) in the basal and lipopolysaccharide (LPS)-activated state. MO, MP, and DC selectively expressed sst2 mRNA. SS mRNA was not detectable, whereas all samples expressed CST mRNA. Expression levels of sst2 and CST mRNA showed marked differences and were in the rank order of MP>>DC>>>MO. LPS stimulation did not induce expression of SS or sst1,3,4,5. However, sst2 mRNA expression was upregulated significantly by stimulation with LPS. CST mRNA was upregulated as well. During differentiation of MO in MP or DC, time-dependent, significantly increasing sst2 and CST mRNA levels were found. By confocal microscopy, the presence of sst2 receptors was demonstrated on MP, but not on DC. This study demonstrates for the first time a selective and inducible expression of the recently discovered CST, as well as sst2, in human monocyte-derived cells, suggesting a role for a CST-sst2 system rather than a SS-sst2 system in these immune cell types.

Crucial role for somatostatin receptor subtype 2 in determining the uptake of [111In-DTPA-D-Phe1]octreotide in somatostatin receptor-positive organs

Human somatostatin (SS) receptor (sst)-positive tumors can be visualized by gamma camera scintigraphy after the injection of [(111)In-diethylenetriaminepentaacetic acid (DTPA)-D-Phe(1)] octreotide. Uptake of [(111)In-DTPA-D-Phe(1)]octreotide is dependent on sst-mediated internalization of the radioligand by the tumor cells. Human sst-positive tumors frequently express multiple sst subtypes. In vitro studies have demonstrated that the 5 sst subtypes (sst(1-5)) differentially internalize sst-bound ligand. The present study was performed to evaluate the role of sst(2) in vivo in determining the uptake of [(111)In-DTPA-D-Phe(1)]octreotide, as well as of the more "universal" ligand [(111)In-DTPA]SS-14, by sst-positive organs expressing multiple sst subtypes.

METHODS

Wild-type and sst(2) knockout mice (n = 4 per treatment group) were injected intravenously with 1 MBq (0.1 micro g) [(111)In-DTPA-D-Phe(1)]octreotide or [(111)In-DTPA]SS-14. After 24 h, the animals were sacrificed and radioactivity in the organs under investigation was determined. In addition, the sst subtype messenger RNA (mRNA) expression pattern in these organs was determined by reverse transcriptase polymerase chain reaction (RT-PCR) analysis.

RESULTS

RT-PCR analysis demonstrated the presence of all 5 sst subtype mRNAs in the adrenals and pituitary of wild-type mice but no sst(2) in the knockout mice. The thymus expressed mRNA for sst(2) and sst(4) mRNA in wild-type mice, whereas no sst(2) was detected in knockout mice. In wild-type mice, the in vivo uptake values (in percentage injected dose per gram of tissue) of [(111)In-DTPA-D-Phe(1)]octreotide for the pituitary, adrenals, pancreas, and thymus amounted to 1.2 +/- 0.2, 0.26 +/- 0.03, 0.18 +/- 0.03, and 0.30 +/- 0.05, respectively, in wild-type mice. Compared with wild-type mice, sst(2) knockout mice had dramatically lower uptake values in these organs-lower by 97%, 83%, 96%, and 94%, respectively (P < 0.01 vs. wild type). Comparable differences in the uptake of radioactivity between wild-type and knockout mice were found using [(111)In-DTPA]SS-14 as the radiotracer. Interestingly, in some organs expressing sst(2) mRNA (liver, muscle, and peripheral blood mononuclear cells), no specific binding of [(111)In-DTPA-D-Phe(1)]octreotide or [(111)In-DTPA]SS-14 to sst in vivo was found, suggesting that the sst(2) protein expression level was very low in these tissues.

CONCLUSION

The uptake of [(111)In-DTPA-D-Phe(1)]octreotide and [(111)In-DTPA]SS-14 in sst-positive organs is determined predominantly by sst(2).

Luteinizing hormone (LH)-responsive Cushing's syndrome: the demonstration of LH receptor messenger ribonucleic acid in hyperplastic adrenal cells, which respond to chorionic gonadotropin and serotonin agonists in vitro

In a substantial part of adrenal adenomas and hyperplasias from patients with Cushing's syndrome, cortisol production is controlled by the expression of aberrant hormone receptors on adrenocortical cells. We present in vivo and in vitro data of two patients with a LH-responsive Cushing's syndrome based on ACTH-independent bilateral adrenal hyperplasia. Patients 1 and 2 are women who presented with Cushing's syndrome and bilateral adrenal hyperplasia. Endocrine testing demonstrated absence of cortisol diurnal rhythm, insufficient cortisol suppression after 1 mg dexamethasone orally, and undetectable ACTH levels in both patients. Both patients were treated by laparoscopic biadrenalectomy. In in vivo testing, in patients 1 and 2, a profound cortisol rise was found after administration of GnRH [change in cortisol (Delta F), 118 and 106%, respectively], human CG (Delta F, 133 and 44%), LH (Delta F, 73 and 43%), ACTH (Delta F, 89 and 181%), and the 5-hydroxy-tryptamine receptor type 4 (5-HT(4)) agonists cisapride (Delta F, 141 and 148%) and metoclopramide (Delta F, 189 and 95%). In in vitro testing, adrenal cells from patient 2 responded, in a dose-dependent fashion, with cortisol production after exposure to human CG (Delta F, 45%), cisapride (Delta F, 68%), and metoclopramide (Delta F, 81%). ACTH induced cortisol production by cells from both patients (Delta F, 135 and 159%). In receptor studies, LH receptor mRNA was demonstrated in adrenal tissue of both patients but also in control adrenal tissue of two patients with persisting pituitary-dependent Cushing's syndrome treated by biadrenalectomy. In neither patient were mutations found in the ACTH receptor gene. LH-responsive Cushing's syndrome associated with bilateral adrenal hyperplasia may result from aberrant (or possibly increased) adrenal LH receptor expression. This variant is further characterized by adrenal responsiveness to 5-HT4 receptor agonists, possibly pointing to an interaction between LH and serotonin in the regulation of cortisol secretion. Despite the regulatory potential of LH and 5-HT4 receptor agonists on cortisol production in our patients, their adrenals seemed to be still sensitive to ACTH, both in vivo and in vitro.

Cortistatin rather than somatostatin as a potential endogenous ligand for somatostatin receptors in the human immune system

Cells of the human immune system have been shown to express somatostatin receptors (sst). The expression of sst suggests a functional role of the peptide somatostatin (SS). However, SS expression has not been demonstrated yet in different human immune tissues. Therefore, we investigated by RT-PCR the expression of both SS and cortistatin (CST), a SS-like peptide, in various human lymphoid tissues and immune cells. We detected SS mRNA expression in the human thymus only, while not in thymocytes. CST mRNA was clearly expressed in the immune cells, lymphoid tissues, and bone marrow. Using quantitative RT-PCR, significant differences in expression levels between tissues were demonstrated. Expression of CST mRNA was up-regulated during differentiation of monocytes into macrophages and dendritic cells and could be up-regulated by lipopolysaccharide stimulation. Two differently sized cDNA fragments of CST were detected in the majority of cells and tissues. However, although both fragments were detected in nearly all T-cell lines (7 of 8), most of the B-cell lines expressed the short fragment only (8 of 10). Using autoradiography, we showed that CST displaced [125I-Tyr3]octreotide binding with relatively high affinity on human thymic tissue and sst2-expressing cells. This is the first extensive study demonstrating that human lymphoid tissues and immune cells express different levels of CST mRNA and that its expression can be regulated. On the basis of these observations, we hypothesize a role for CST as an endogenous ligand of at least the sst2 receptor in the human immune system, rather than SS itself.