Methionine-enkephalin stimulates interleukin-6 mRNA expression: human plasma levels in coronary artery bypass grafting

The Opioid Growth Factor in Growth Regulation and Immune Responses in Cancer

It has become apparent that endogenous opioids act not only as neurotransmitters and neuromodulators, but have multiple functions in the body. Activation of the opioid system by opiate drugs is associated with a risk of cancer development through direct stimulation of tumor cell proliferation and through immunosuppression. In contrast, the endogenous peptide opioid [Met5]-enkephalin, now commonly referred to as Opioid Growth Factor (OGF), negatively regulates cell proliferation in a wide number of cells during development, homeostasis, and neoplasia. This action is mediated through the opioid growth factor receptor, originally designated the zeta (ζ) opioid receptor. Further, contrary to the traditional notion of opiates as immunosuppressive, endogenous OGF has been shown to possess a number of positive immunomodulatory properties and may provide a beneficial effect in cancer by augmenting the activity of cells involved in both innate and acquired immunity. Taken together, the evidence supports consideration of opioid peptides such as OGF as new strategies for cancer therapy.

The paradoxical role of methionine enkephalin in tumor responses

Methionine enkephalin (MENK) is an opioid peptide composed of five amino acids with multiple biological activities. Since its discovery, MENK has become prominent in neuroregulation and immunoregulation. Tumors have increasingly been a spotlight because of their terrible trends and refractory characteristic. The therapeutic potential of MENK was investigated on a large scale, and there are numerous evidences that MENK exerts anti-tumor effects via two mechanisms. The first mechanism explains the enhanced anti-tumor immune effects of MENK. The second mechanism shows that MENK directly inhibits tumor cell proliferation. However, numerous reports have clarified the pro-tumor role of MENK by inhibiting T and B cell proliferation, promoting tumor cell growth by binding to opioid receptors, leading to desensitization of lymphocytes, and inducing tolerance. It is particularly intriguing that dual reactions are triggered when MENK combines with its opioid receptors; thus, anti-tumor response of the whole body is influenced. This review will expound the dual roles of MENK in tumor responses based on immune cells, cytokines, and tumor cells to provide better suggestions for its application in tumor treatment.

Endogenous opioid peptides in regulation of innate immunity cell functions

Endogenous opioid peptides comprise a group of bioregulatory factors involved in regulation of functional activity of various physiological systems of an organism. One of most important functions of endogenous opioids is their involvement in the interaction between cells of the nervous and immune systems. Summary data on the effects of opioid peptides on regulation of functions of innate immunity cells are presented.

Effects of leucine-enkephalin on catalase activity and hydrogen peroxide levels in the haemolymph of the Pacific Oyster (Crassostrea gigas)

The nervous and immune systems of invertebrates can exchange information through neuropeptides. Furthermore, some opioid peptides can function as endogenous immune system messengers and participate in the regulation of the immune responses. The present study was designed to investigate the effects of leucine-enkephalin (L-ENK) on the activity of catalase (CAT) and hydrogen peroxide (H₂O₂) content in the haemolymph of the Pacific Oyster (Crassostrea gigas). The CAT activity and H₂O₂ content were investigated after the haemolymph of the species was exposed to 1, 5, and 50 μg/mL of L-ENK. The results indicate that the intracellular and extracellular CAT activity was increased with increasing concentration of L-ENK, while the intracellular and extracellular H₂O₂ content was decreased with increasing concentration of L-ENK. L-ENK may regulate the intracellular and extracellular CAT activity and H₂O₂ content via binding with opioid neuropeptide receptors on immunocytes of the oysters. The data strongly suggests an involvement of opioid peptides in the regulation of the antioxidant defence systems of Crassostrea gigas.

Effects of endogenous and synthetic opioid peptides on neutrophil function in vitro

BACKGROUND

Opioid peptides released from immunocytes during inflammation and stress in critically ill patients are associated with an altered immune response. Moreover, concentrations of opioid peptides are increased in peripheral blood and at the sites of inflammatory reactions.

METHODS

Using flow cytometric assay of whole human blood, we investigated direct effects of endogenous and synthetic opioid peptides on surface expression of complement receptors CD35 and CD11b/CD18 and Fcã receptor III CD16, and superoxide anion generation of neutrophils.

RESULTS

The endogenous opioid peptides beta-endorphin(1-31) and met-enkephalin, representing the N-terminal fragment of beta-endorphin(1-31), and the synthetic delta opioid receptor agonists D-Ala(2)-D-Leu(5)-enkephalin and D-Pen(2)-enkephalin produced concentration-dependent stimulation of neutrophil activity. Incubation with met-enkephalin 10(-7) M or beta-endorphin(1-31) 10(-7) M led to an increase in receptor expression of up to 10% (met-enkephalin) and 15% (beta-endorphin(1-31)). After incubation with D-Ala(2)-D-Leu(5)-enkephalin or D-Pen(2/5)-enkephalin, receptor expression was increased by up to 30%. This correlated with concentration-dependent stimulation of the production of reactive oxygen intermediates, as shown by an increase of up to 40% in oxidative burst activity. All effects were abolished after preincubation with naloxone or with the selective delta opioid antagonist naltrindole, whereas the selective micro receptor antagonist d-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH(2) showed only partial inhibitory effects.

CONCLUSIONS

Our data suggest a delta opioid receptor-mediated stimulatory effect on neutrophil function. beta-Endorphin(27-31), the C-terminal fragment of beta-endorphin(1-31), did not alter neutrophil function, indicating that beta-endorphin(1-31) mediates its effect on neutrophils via the N-terminal fragment. This study may contribute to a better understanding of neuroimmune interaction.

Synthesis and binding characteristics of a novel enkephalin analogue, [3H]Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe

The endogenous opioid heptapeptide (Tyr-Gly-Gly-Phe-Met-Arg-Phe; MERF) has been shown to interact with multiple opioid as well as non-opioid sites in mammalian brain membranes. To increase the stability and bioavailability of MERF, new synthetic derivatives with D-amino acid substitutions were prepared and studied. One of the new compounds in this series, Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (DADN), had only moderate affinity in competing with [3H]MERF, whereas it displayed the highest potency in producing antinociception following intrathecal administration. DADN was radiolabeled with 41Ci/mmol specific activity. Specific binding of [3H]DADN was saturable, stereoselective and of high affinity. Chemical stability, increased micro-receptor selectivity, and hydrophobicity of the peptide all contribute to the effectiveness observed in biochemical and pharmacological studies.

Absence of preproenkephalin increases the threshold for T cell activation

Certain forms of the neuroendocrine hormone preproenkephalin (PPNK) are produced by T cells, B cells and macrophages. This hormone has been shown to be important in regulating a variety of immune responses; however, the basic mechanisms of this regulation are unknown. Here we examine the ability of CD8 and CD4 PPNK-deficient T lymphocytes to proliferate to antigenic and mitogenic stimuli. We found that lymphocyte activation and proliferation to suboptimal concentrations of both anti-CD3 and antigen was reduced in the absence of PPNK. Proliferation could be rescued by increasing antigen or by co-incubation of PPNK-deficient cells with wild-type cells. These data confirm the importance of neuroendocrine hormones such as PPNK in T cell activation and proliferation and provides a potential mechanism for the regulation of T cell responses by PPNK or its peptide derivatives.

Presence of chromogranin-derived antimicrobial peptides in plasma during coronary artery bypass surgery and evidence of an immune origin of these peptides

Chromogranin A (CGA) and chromogranin B (CGB) are acidic proteins stored in secretory organelles of endocrine cells and neurons. In addition to their roles as helper proteins in the packaging of peptides, they may serve as prohormones to generate biologically active peptides such as vasostatin-1 and secretolytin. These molecules derived from CGA and CGB, respectively, possess antimicrobial properties. The present study demonstrates that plasmatic levels of both vasostatin-1 and secretolytin increase during surgery in patients undergoing cardiopulmonary bypass (CPB). Vasostatin-1 and secretolytin, initially present in plasma at low levels, are released just after skin incision. Consequently, they can be added to enkelytin, an antibacterial peptide derived from proenkephalin A, for the panoply of components acting as a first protective barrier against hypothetical invasion of pathogens, which may occur during surgery. CGA and CGB, more commonly viewed as markers for endocrine and neuronal cells, were also found to have an immune origin. RNA messengers coding for CGB were amplified by reverse transcription-polymerase chain reaction in human monocytes, and immunocytochemical analysis by confocal microscopy revealed the presence of CGA or CGB or both in monocytes and neutrophils. A combination of techniques including confocal microscopic analysis, mass spectrometry measurement, and antibacterial tests allowed for the identification of the positive role of interleukin 6 (IL-6) in the secretolytin release from monocytes in vitro. Because IL-6 release is known to be strongly enhanced during CPB, we suggest a possible relationship between IL-6 and the increased level of secretolytin in patients undergoing CPB.

Involvement of pro-enkephalin-derived peptides in immunity

It is widely accepted that all organisms have processes that maintain their state of health. Failure of these processes, such as those involving the naturally occurring antibacterial peptides, may lead to pathological events. Recent results demonstrate that these peptides, such as peptide B, appear in invertebrates and vertebrates (including humans) immediately after tissue trauma, and maintain themselves for long durations (over 4h). Their degradation products lead to other inflammatory peptides, such as Met-enkephalin-Arg-Phe. These newly described antibacterial peptides, which are released and not induced, are present on neuropeptide precursors such as proenkephalin. This is a new field of research, in that the same protein contains proposed neuropeptides, antibacterial peptides, and immune stimulatory peptides. The focus of this review is to describe how the pro-enkephalin derived peptides participate in immune processes.

Crosstalk between nervous and immune systems through the animal kingdom: focus on opioids

During the course of evolution invertebrates and vertebrates have maintained common signaling molecules, such as neuropeptides. For example, complete hormonal-enzymatic systems for the biosynthesis of opioid peptides have been found in both the CNS and immune systems of these animals. These signaling molecules have been found in the blood circulation and act as immunomodulators. In vertebrates, release of the signaling molecules occurs during stress (cognitive or pathogens), which triggers the hypothalamo-hypophysial-adrenal axis. Similarly, these neuropeptides are used as messengers to initiate and stimulate the innate immune response in invertebrates. Thus, the crosstalk between nervous and immune systems has an ancient evolutionary origin and the messengers used have been conserved during the course of evolution reflecting their vital importance.

Proenkephalin A-derived peptides in invertebrate innate immune processes

Lipopolysaccharides (LPS) injection into the coelomic fluid of the leech Theromyzon tessulatum stimulates release of proenkephalin A (PEA)-derived peptides as determined by immunoprecipitation and Western blot analyses. This release occurs in the first 15 min after LPS exposure and yields a 5.3-kDa peptide fragment corresponding to the C-terminal part of the precursor. This fragment is then cleaved to free an antibacterial peptide related to mammals arginine phenylalanine extended enkelytin: the peptide B. These PEA processing peptides were characterized using a combination of techniques including reversed-phase HPLC, microsequencing and mass spectrometry. The isolated invertebrate peptide B presents a high sequence homology with the bovine's and the same activity against Gram+bacteria. Titrations revealed the simultaneous appearance of Methionine-enkephalin (ME) and peptide B in invertebrates after stimulation by LPS (in a dose-dependent manner), surgical trauma or electrical stimulations to neural tissues of the mussel. Furthermore, peptide B processing in vitro yields Methionine-enkephalin arginine phenylalanine (MERF), which exhibits via the delta receptors, immunocyte excitatory properties, i.e., movement and conformational changes, but no antibacterial activity. We surmise that this unified response to the various stimuli is a survival strategy for organism by providing immediate antibacterial activity and immunocyte stimulation, thereby reducing any immune latency period needed for an adequate immune response.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Invertebrate opioid precursors: evolutionary conservation and the significance of enzymatic processing

Invertebrate tissues contain mammalian-like proenkephalin, prodynorphin, and proopiomelanocortin. Amino acid sequence determination of these opioid gene products reveals the presence of various opioid peptides exhibiting high sequence identity with their mammalian counterparts. These associated peptides are flanked by dibasic amino acid residues, indicating cleavage sites. Together with the presence of various processing enzymes, i.e., neutral endopeptidase 24.11 and angiotensin-converting enzymes, this suggests that opioid precursor processing is also similar to that described in mammals. It is noted that the levels and/or activity of invertebrate neutral endopeptidase 24.11 can be upregulated by signaling molecules shown to perform the same function in mammals, i.e., morphine. Critical to opioid precursor processing are immunocytes that contain the precursors and transport processing enzymes to sites of inflammation, in part, to cleave these peptide precursors, thus liberating immune-stimulating molecules. Furthermore, in response to lipopolysaccharides, Met-enkephalin levels peak immediately and hours after the exposure, revealing a release and induction process. It appears that the opioid precursors and their processing enzymes first evolved in "simple" animals and the have been maintained and embellished during the course of evolution guided by conformational matching.

Opiate, cannabinoid, and eicosanoid signaling converges on common intracellular pathways nitric oxide coupling

Scientific fields as they emerge initially appear to be unrelated to other projects even if they are in a similar area of interest. This is especially true in the case of opiate, cannabinoid, and eicosanoid signaling processes. In this limited speculative review, we attempt to examine aspects of their intracellular cascading signaling systems for their commonalities. We find intracellular calcium mobilization, nuclear factor kappa B involvement, adenylate cyclase activity, and, finally, constitutive nitric oxide release to be converging points for these signaling processes, occurring by separate and distinct receptor-mediated effector systems. Phosphokinase C, mitogen activated protein kinase, and cytosolic phospholipase A2 also represent points of common impact. In this regard, aspirin also appears to be involved in an aspect of this signaling convergence. We conclude that many of the physiological observations regarding the actions of these signaling molecules, for example, immunosuppression, neurotransmission, vasodilation, cellular adherence, and cytotoxicity, can now be understood by considering their converging biochemical cascades.

Delta2 opioid receptor subtype on human vascular endothelium uncouples morphine stimulated nitric oxide release

We demonstrate the presence of both delta and mu opioid receptors on the endothelium of human saphenous vein and internal thoracic artery. Displacement analysis revealed that a variety of opioid peptides were found to be ineffective in displacing specifically bound 3H dihydromorphine and only delta2 ligands were effective in regard to 3H Ala2-met5 enkephalinamide (DAMA), indicating the presence of mu3 and delta2 opioid receptor sites, respectively. Confirming the presence of both mu and delta sites we demonstrated positive immunostaining with anti-delta and anti-mu receptor antibodies. Exposure of these vessels to DAMA significantly enhances granulocyte adherence (P<0.01) even in vessels 5 min later exposed to 10(-6) M morphine. Unlike morphine, DAMA did not stimulate nitric oxide from either blood vessel and human granulocytes. Additionally, DAMA preadministered before morphine exposure to the endothelium or granulocytes, inhibited the morphine-stimulated release of NO in a dose-dependent manner. The data indicate that opioid peptides and opiate alkaloids regulate endothelial function in an antagonistic manner thereby influencing the microvascular environment.