Somatostatin analog SMS 201995 inhibits proliferation in human leukemia T-cell line: relevance of the adenylyl cyclase stimulation

The Role of Neuropeptide-Stimulated cAMP-EPACs Signalling in Cancer Cells

Neuropeptides are autocrine and paracrine signalling factors and mainly bind to G protein-coupled receptors (GPCRs) to trigger intracellular secondary messenger release including adenosine 3′, 5′-cyclic monophosphate (cAMP), thus modulating cancer progress in different kind of tumours. As one of the downstream effectors of cAMP, exchange proteins directly activated by cAMP (EPACs) play dual roles in cancer proliferation and metastasis. More evidence about the relationship between neuropeptides and EPAC pathways have been proposed for their potential role in cancer development; hence, this review focuses on the role of neuropeptide/GPCR system modulation of cAMP/EPACs pathways in cancers. The correlated downstream pathways between neuropeptides and EPACs in cancer cell proliferation, migration, and metastasis is discussed to glimmer the direction of future research.

Synthesis and evaluation of biodegradable PCL/PEG nanoparticles for neuroendocrine tumor targeted delivery of somatostatin analog

PURPOSE

Neuroendocrine tumors often present a diagnostic and therapeutic challenge. We have aimed to synthesize and develop biodegradable nanoparticles of somatostatin analogue, octreotide for targeted therapy of human neuroendocrine pancreatic tumor.

METHODS

Direct solid phase peptide synthesis of octreotide was done. Octreotide loaded PCL/PEG nanoparticles were prepared by solvent evaporation method and characterized for transmission electron microscopy, differential scanning calorimetery (DSC), Zeta potential measurement studies. The nanoparticles were evaluated in vitro for release studies and peptide content. For biological evaluations, receptor binding & cytotoxicity studies were done on BON-1 neuroendocrine tumor cell line. Biodistribution of radiolabeled peptide and nanoparticles, tumor regression studies were performed on tumor-bearing mouse models.

RESULTS

We have synthesized and purified octreotide with the purity of 99.96% in our laboratory. PEG/PCL nanoparticles with an average diameter of 130-195 nm having peptide loading efficiency of 66-84% with a negative surface charge were obtained with the formulation procedure. Octreotide nanoparticles have a negative action on the proliferation of BON-1 cells. In vivo biodistribution studies exhibited major accumulation of octreotide nanoparticles in tumor as compared to plain octreotide. Octreotide nanoparticles inhibited tumor growth more efficiently than free octreotide.

CONCLUSIONS

Thus, it was concluded that the PCL/PEG nanoformulation of octreotide showed high tumor uptake due to the enhanced permeation and retention (EPR) effect and then peptide ligand imparts targetability to the sst2 receptor and there by showing increase tumor growth inhibition. Selective entry of nanoparticles to the tumor also give the reduce side effects both in vivo and in vitro.

Treatment of transplanted tumor of lung adenocarcinoma A549 transfected by human somatostatin receptor subtype 2 (hsstr2) gene with 188Re-RC-160

BACKGROUND AND AIM

Radionuclide-labeled somatostatin analogues selectively target somatostatin receptor (SSTR)-expressing tumors as a basis for diagnosis and treatment of these tumors. To those tumors without somatostatin receptor expressed, the hSSTR2 gene was transfected. Express of the hSSTR2 receptor was imaging and the radiotherapeutic effect was evaluated with (188)Re-RC-160.

METHODS

The stable hSSTR2-expressing A549 cells (pcDNA3-hSSTR2 A549) and non-somatostatin receptor expressing A549 cells (pcDNA3 A549) were selected by western blot. Later, a corresponding animal tumor model was established. Expression of the hSSTR2 reporter was imaged using (188)Re-RC-160 recognition. Tumors were evaluated for somatostatin receptor expression using immunohistochemistry. The distribution of (188)Re-RC-160 in the animal tumor model was measured and the inhibitory effects of (188)Re-RC-160 were evaluated by measurement of tumor growth and hematoxylin and eosin and TdT mediated dUTP nick end labeling (TUNEL) staining.

RESULTS

In vivo radioimaging revealed specific targeting of (188)Re-RC-160 to tumors derived from pcDNA3- hSSTR2 A549 cells, compared to those from pcDNA3 A549 cells. pcDNA3- hSSTR2 A549 tumor growth inhibition was significantly higher in the single 7.4 MBq (188)Re-RC-160 treatment group than in the 2×7.4 MBq rhenium-188, RC-160 group, control group, and pcDNA3 A549 tumors (P<.05). Furthermore, treatment fractionation group (2 × 7.4 MBq (188)Re-RC-160), induced significantly increased tumor-growth inhibition compare with single 7.4 MBq (188)Re-RC-160 treatment (P<.05).

CONCLUSION

These studies showed that (188)Re-RC-160 could be effectively used for targeting therapy the A549-derived tumors exogenously expressing hSSTR2, which will offers a potential therapeutic strategy for the treatment of somatostatin receptor-negative cancers.

Modulation of the neuronal response to ischaemia by somatostatin analogues in wild-type and knock-out mouse retinas

Somatostatin acts at five G protein-coupled receptors, sst(1)-sst(5). In mouse ischaemic retinas, the over-expression of sst(2) (as in sst(1) knock-out mice) results in the reduction of cell death and glutamate release. In this study, we reported that, in wild-type retinas, somatostatin, the multireceptor ligand pasireotide and the sst(2) agonist octreotide decreased ischaemia-induced cell death and that octreotide also decreased glutamate release. In contrast, cell death was increased by blocking sst(2) with cyanamide. In sst(2) over-expressing ischaemic retinas, somatostatin analogues increased cell death, and octreotide also increased glutamate release. To explain this reversal of the anti-ischaemic effect of somatostatin agonists in the presence of sst(2) over-expression, we tested sst(2) desensitisation because of internalisation or altered receptor function. We observed that (i) sst(2) was not internalised, (ii) among G protein-coupled receptor kinases (GRKs) and regulators of G protein signalling (RGSs), GRK1 and RGS1 expression increased following ischaemia, (iii) both GRK1 and RGS1 were down-regulated by octreotide in wild-type ischaemic retinas, (iv) octreotide down-regulated GRK1 but not RGS1 in sst(2) over-expressing ischaemic retinas. These results demonstrate that sst(2) activation protects against retinal ischaemia. However, in the presence of sst(2) over-expression sst(2) is functionally desensitised by agonists, possibly because of sustained RGS1 levels.

An update on somatostatin receptor signaling in native systems and new insights on their pathophysiology

The peptide somatostatin (SRIF) has important physiological effects, mostly inhibitory, which have formed the basis for the clinical use of SRIF compounds. SRIF binding to its 5 guanine nucleotide-binding proteins-coupled receptors leads to the modulation of multiple transduction pathways. However, our current understanding of signaling exerted by receptors endogenously expressed in different cells/tissues reflects a rather complicated picture. On the other hand, the complexity of SRIF receptor signaling in pathologies, including pituitary and nervous system diseases, may be studied not only as alternative intervention points for the modulation of SRIF function but also to exploit new chemical space for drug-like molecules.

SMS 201-995 enhances S-phase block induced by 5-fluorouracil in a human colorectal cancer cell line

The action of the somatostatin analog SMS-201.995 (SMS) was tested in monotherapy and in combined therapy with the cytotoxic agent 5-fluorouracil (5-FU) on cell cycle kinetics of the human colon cancer cell line WiDr, expressing a mutant p53 (mp53). The data, obtained by flow cytometric DNA analysis, showed that SMS at 0.2 microg/ml increased apoptosis, augmenting the proportion of cells with subdiploid DNA content by 65 and 48% after 3 and 6 h, respectively. In cultures lasting 24 and 36 h, it also decreased the percentages of cells in G0/G1 phase by 22.9 and 14.3%; whereas at a dose of 0.1 microg/ml, SMS decreased the percentage of cells in G2/M by 14.3%. In contrast to SMS, 5-FU (0.1 microg/ml) augmented the apoptosis at 12 h, and markedly increased the fraction of cells in S phase, increasing its value from 24 and 72 h by 108 and 234%, respectively, in comparison to the control. The most evident finding after the combination of SMS (0.2 microg/ml) and 5-FU (0.1 microg/ml) was a potentiation of 5-FU-induced S-phase block by a further 7.9, 12.9 and 42.1% at 24, 36 and 72 h, respectively. Treatment with 5-FU also upregulated HLA class I expression of the cancer cells. In this sense, SMS was less effective and when given in combination with 5-FU did not change the effects induced by 5-FU. The data emphasize that SMS exhibits pro-apoptotic and anti-proliferative effects, which in proper dose combinations might enhance the effects of 5-FU on human colorectal cancer cells expressing mp53.

A cyclic peptide, L1AD3, induces early signs of apoptosis in human leukemic T-cell lines

L1AD3 is a small cyclic synthetic peptide designed to resemble the first loop of a cobra venom cytotoxin. Instead of inducing membrane disruption similar to that caused by the parent toxin, L1AD3 promotes extensive and unusually rapid apoptosis in leukemic T-cells without making the plasma membrane permeable to small fluorescent dyes. Within 4 h, micromolar concentrations of L1AD3 almost totally inhibit thymidine incorporation, and ATP levels decrease significantly. By contrast, normal human white blood cells are not affected by L1AD3, nor is heart cell function affected by it. If L1AD3 kills by interacting with targets that are different from those of currently applied agents, this peptide, or a derivative of it, could become a useful adjunct for cancer chemotherapy.

Antiproliferative effect of octreotide on gastric cancer cells mediated by inhibition of Akt/PKB and telomerase

AIM: To investigate the antiproliferative effect of octreotide, a long-acting analogue of somatostatin, on gastric cancer cell line SGC7901 and its possible molecular mechanisms.

METHODS: Gastric cancer cell line SGC7901 employed in the study was treated with 0.008, 0.04, 0.2, 1, 5 and 25 μg·mL^-1 of octreotide respectively for 24 h to evaluate the antiproliferative effect of somatostatin analog on the tumor cells by MTT assay method. To elucidate the underlying mechanism, the cells were exposed to 1 μg·mL^-1 of octreotide for 0, 12, 24 and 48 h, when their Akt/PKB and telomerase activities were respectively determined using PCR-ELSIA and nonradioactive protein kinase assay protocols. The same experimental procedures were also performed in the control cells that were treated with corresponding vehicles instead of somatostatin analog.

RESULTS: After exposed to octreotide for 24 h at the concentrations of more than 1 μg·mL^-1, SGC7901 cells exhibited a dose-dependent inhibition of growth with the inhibiting rate to be as high as 34.66% when 25 μg·mL^-1 of octreotide was applied. The Akt/PKB and telomerase activity of SGC7901 cells was significantly inhibited when the cells were exposed to 1 μg·mL^-1 of octreotide for 12, 24 and 48 h compared with that of their control counterparts (P < 0.01), both of which exhibited in a time-dependent manner.

CONCLUSION: The antiproliferative effect of octreotide on SGC7901 cells might be mediated by the inhibition of Akt/PKB and telomerase.

T cell receptor-independent CD4 signalling: CD4-MHC class II interactions regulate intracellular calcium and cyclic AMP

CD4 is a coreceptor on T helper (Th) cells that interacts with MHC class II molecules (MHCII). The mechanisms mediating the effects of CD4 on responses by T helper cells to stimulation of the antigen-specific T cell receptor (TCR) are still poorly understood. Here, we demonstrate T cell costimulation via CD4 signalling independent of T cell receptor-mediated signals. Incubation of T helper cells with peptide mimetics of the CD4-binding region on the MHC class II beta2 domain caused intracellular calcium mobilization in the absence of antigen or other T cell receptor stimuli. Engagement of CD4 by peptide mimetics or wild-type MHC class II, but not by mutant MHC class II molecules incapable of engaging CD4, inhibited the T cell receptor-mediated increase in cyclic AMP (cAMP) concentrations in T helper cells. CD4-mediated signals activated cyclic AMP phosphodiesterases (PDEs) and inhibited adenylyl cyclase. Full activation and clonal expansion of antigen-stimulated T helper cells required the CD4-mediated regulation of cyclic AMP. Our results suggest a costimulatory mechanism of CD4 function that acts on the second messengers, calcium and cyclic AMP.

Biological activity of somatostatin receptors in GC rat tumour somatotrophs: evidence with sst1-sst5 receptor-selective nonpeptidyl agonists

The physiological actions of somatostatin-14 (SRIF: somatotrophin release inhibitory factor) receptor subtypes (sst(1)-sst(5)), which are endogenously expressed in growth cells (GC cells), have not yet been elucidated, although there is evidence that sst(2) receptors are negatively coupled to cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and adenosine 3,5'-cyclic monophosphate (cAMP) accumulation. In addition, both sst(1) and sst(2) receptors are negatively coupled to growth hormone (GH) secretion in GC cells. Here we report on studies concerning the expression, the pharmacology and the functional role of native SRIF receptors in GC cells with the use of five nonpeptidyl agonists, highly selective for each of the SRIF receptors. Radioligand binding studies show that sst(2) and sst(5) receptors are present at different relative densities, while the presence of sst(3) and sst(4) receptors appears to be negligible. The absence of sst(1) receptor binding was unexpected in view of sst(1) receptor functional effects on GH secretion. This suggests very efficient receptor-effector coupling of a low-density population of sst(1) receptors. Functionally, only sst(2) receptors are coupled to the inhibition of [Ca(2+)](i) and cAMP accumulation and the selective activation of sst(5) receptors facilitates the stimulation of adenylyl cyclase activity through G(i/o) proteins. This effect was not observed when sst(2) and sst(5) receptors were simultaneously activated, suggesting that there is a functional interaction between sst(2) and sst(5) receptors. In addition, sst(1), sst(2) and sst(5) receptor activation inhibits GH release, further indicating that SRIF can modulate GH secretion in GC cells through mechanisms both dependent and independent on [Ca(2+)](i) and cAMP-dependent pathways. The present data suggest SRIF-mediated functional effects in GC cells to be very diverse and provides compelling arguments to propose that multiple native SRIF receptors expressed in the same cells are not simply redundant, but contribute to marked signalling diversity.

Human immune cells express ppMCH mRNA and functional MCHR1 receptor

Melanin-concentrating hormone (MCH) is highly expressed in the brain and modulates feeding behavior. It is also expressed in some peripheral tissues where its role remains unknown. We have investigated MCH function in human and mouse immune cells. RT-PCR analysis revealed a low expression of prepro-MCH and MCH receptor 1 (MCHR1) but not of MCHR2 transcript in tissular and peripheral blood immune cells. FACS and in vitro assay studies demonstrated that MCHR1 receptor expression on most cell types can trigger, in the presence of MCH, cAMP synthesis and calcium mobilization in peripheral blood mononuclear cells (PBMCs). Moreover, MCH treatment decreases the CD3-stimulated PBMC proliferation in vitro. Accordingly, our data indicate for the first time that MCH and MCHR1 may exert immunomodulatory functions.

Somatostatin receptor (SSTR) expression and function in normal and leukaemic T-cells. Evidence for selective effects on adhesion to extracellular matrix components via SSTR2 and/or 3

We have examined normal T-cells and T-cell lines with respect to expression of various somatostatin receptor subtypes (SSTR1--5) using RT-PCR and PCR. To evaluate the function of these receptors we have further studied the effects of subtype specific signalling on T-cell adhesion using somatostatin analogs specific for various receptors as probes. Human T-lymphocytes showed SSTR expression related to activation and stage of differentiation. Normal T-cells (peripheral blood, T-cell clone) and T-leukaemia cell lines expressed SSTR2, SSTR3 and SSTR4. Normal T-cells expressed SSTR1 and SSTR5 while T-leukaemia lines did not. SSTR5 was selectively expressed in activated normal T-cells. T-lymphocytes produced no somatostatin themselves. Somatostatin and somatostatin analogs specific for SSTR2 and/or SSTR3 enhanced adhesion of T-cells to fibronectin (FN), and to a certain extent, also to collagen type IV (CIV) and laminin (LAM). T-lymphocytes express multiple SSTR and somatostatin may therefore regulate lymphocyte functions via distinct receptor subtypes as shown here for adhesion to extracellular matrix components (ECM) via SSTR2 and SSTR3. SSTR expression also distinguishes normal and leukaemic T-cells. Our findings suggest that SSTR subtypes may be useful targets for therapy during inflammatory diseases and malignancies affecting lymphocytes.