Expression of delta opioid receptors by splenocytes from SEB-treated mice and effects on phosphorylation of MAP kinase

Opiate antagonist prevents μ- and δ-opiate receptor dimerization to facilitate ability of agonist to control ethanol-altered natural killer cell functions and mammary tumor growth

In the natural killer (NK) cells, δ-opiate receptor (DOR) and μ-opioid receptor (MOR) interact in a feedback manner to regulate cytolytic function with an unknown mechanism. Using RNK16 cells, a rat NK cell line, we show that MOR and DOR monomer and dimer proteins existed in these cells and that chronic treatment with a receptor antagonist reduced protein levels of the targeted receptor but increased levels of opposing receptor monomer and homodimer. The opposing receptor-enhancing effects of MOR and DOR antagonists were abolished following receptor gene knockdown by siRNA. Ethanol treatment increased MOR and DOR heterodimers while it decreased the cellular levels of MOR and DOR monomers and homodimers. The opioid receptor homodimerization was associated with an increased receptor binding, and heterodimerization was associated with a decreased receptor binding and the production of cytotoxic factors. Similarly, in vivo, opioid receptor dimerization, ligand binding of receptors, and cell function in immune cells were promoted by chronic treatment with an opiate antagonist but suppressed by chronic ethanol feeding. Additionally, a combined treatment of an MOR antagonist and a DOR agonist was able to reverse the immune suppressive effect of ethanol and reduce the growth and progression of mammary tumors in rats. These data identify a role of receptor dimerization in the mechanism of DOR and MOR feedback interaction in NK cells, and they further elucidate the potential for the use of a combined opioid antagonist and agonist therapy for the treatment of immune incompetence and cancer and alcohol-related diseases.

Immune conditions associated with CD4+ T effector-induced opioid release and analgesia

Effector CD4(+) T lymphocytes generated in response to antigens produce endogenous opioids. Thus, in addition to their critical role in host defenses against pathogens, effector CD4(+) T lymphocytes contribute to relieving inflammatory pain. In this study, we investigated mechanisms of opioid release by antigen-experienced effector CD4(+) T cells that leave draining lymph nodes and come back into the inflammatory site. Effector antigen-primed CD4(+) T lymphocytes generated in vitro were intravenously injected into nude mice previously immunized with either cognate or irrelevant antigens in complete Freund adjuvant (CFA). CFA-induced mechanical hyperalgesia was only reduced in mice immunized with cognate antigen. Thus, antinociceptive activity of effector CD4(+) T cells requires the presence of the antigen for which they are specific within the inflammatory site. Accordingly, analgesia was inhibited by neutralizing cognate T cell receptor-mediated interaction between effector CD4(+) T lymphocytes and antigen-presenting cells at the site of inflammation. Analgesia was observed by transferring effector CD4(+) T lymphocytes with Th1 or Th2 phenotype, suggesting that antinociceptive activity is a fundamental property of effector CD4(+) T lymphocytes irrespective of their effector functions. Based on the use of agonists and antagonists selective for each of the opioid receptor subclasses, we showed that analgesia induced by T cell-derived opioids is elicited via activation of δ-type opioid receptors in the periphery. Thus, the antinociceptive activity is a fundamental property associated with the effector phase of adaptive immunity, which is driven by recognition of the cognate antigen by effector CD4(+) T lymphocytes at the inflammatory site.

Reduction of cell proliferation and potentiation of Fas-induced apoptosis by the selective kappa-opioid receptor agonist U50 488 in the multiple myeloma LP-1 cells

As opioid receptors modulate proliferation and apoptosis of immune cells, we hypothesized that they could reduce malignant haematopoietic cells. After screening, we selected the human multiple myeloma LP-1 cells which express mu- (MOP-) and kappa-opioid receptors (KOP-R). U50 488 produces a modest but significant decrease in viability associated with an arrest in the G0/G1 phase, but not antagonized by NorBNI and not associated with modulation of p21(Cip1), p27(Kip1) or p53 expression. In contrast, no effect was observed with dynorphin, U69 593 and morphine. In conclusion, the anti-proliferative effects of U50 488 are not mediated by KOP-R in the LP-1 cells.

Autoantibodies to the delta-opioid receptor function as opioid agonists and display immunomodulatory activity

In this report, we show that affinity purified human anti-delta opioid receptor (DOR) autoantibodies from IVIG are specific to DOR and possess agonistic properties displayed by their ability to dramatically decrease forskolin stimulated cAMP accumulation. Anti-DOR autoantibody also caused phosphorylation of the opioid receptor. Anti-DOR autoantibody treatment showed a significant reduction in CXCR4 gene expression as well as surface protein expression. In contrast, anti-DOR autoantibody treatment significantly upregulated CCR5 gene and protein expression. The presence of anti-DOR autoantibodies in IVIG and their potent immunomodulatory activity is further evidence to support the cross-talk between the neuroendocrine and immune systems.

Methionine-enkephalin secreted by human colorectal cancer cells suppresses T lymphocytes

The role of methionine-enkephalin (MENK) as an immunomodulator in colorectal carcinomas (CRC) was examined. MENK was produced in CT26, IEC6A, Colo320, and HT29 CRC cell lines but not in IEC6 intestinal cells. MENK secretion was associated with tumorigenicity and metastasis of CRC cells in syngeneic rodent models. The MENK concentration in subcutaneous tumors of CT26 and IEC6A CRC cells exhibited an inverse correlation with the number of tumor-infiltrating T lymphocytes. MENK inhibited the growth of MOLT-4 T-lymphoblastic cells in a dose-dependent manner. Furthermore, it increased the phosphorylation level of c-Jun N-terminal kinase and induced apoptosis in MOLT-4 cells. MENK-induced apoptosis was abrogated by a c-Jun N-terminal kinase inhibitor. Immunohistochemical analysis revealed moderate to strong expression of MENK in 33 (54%) of 61 CRC. MENK expression was associated with Dukes' staging, nodal metastasis, and liver metastasis. The MENK concentration in tumor tissues was higher in Dukes' C cases than in Dukes' B cases. MENK expression was associated with tumor-infiltrating T lymphocytes, especially those belonging to the CD4(+) subset. These findings suggest that MENK secreted by CRC cells caused escape of the host from the effects of immunity.

Opioid receptor blockade increases the number of lymphocytes without altering T cell response in draining lymph nodes in vivo

A number of studies have been dedicated to estimate the consequences on immunity of the clinical use of opioids by focusing on mitogen-induced polyclonal proliferation of T cells from blood or spleen. Here we examined, under physiological conditions, the contribution of endogenous opioids in the development of a CD4(+) T cell response within draining lymph nodes. We show in OVA-primed DO11.10 mice that delta-opioid receptors were up-regulated upon T cell activation in vivo and that opioid receptor neutralization increased the number of specific anti-OVA T lymphocytes without promoting their capacity to proliferate. The sensitivity to Fas-mediated apoptosis of T lymphocytes and the synthesis of homeostatic lymphoid chemokines were not either affected suggesting that opioids operate mainly before the entry of T lymphocytes into lymph nodes.

Role of beta-endorphin, corticotropin-releasing hormone, and autonomic nervous system in mediation of the effect of chronic ethanol on natural killer cell cytolytic activity

BACKGROUND

We have recently shown that alcohol feeding suppresses natural killer (NK) cell cytolytic activity partly by decreasing the function of hypothalamic beta-endorphin (beta-EP) neurons. The neuronal mechanism by which hypothalamic beta-EP communicates with the spleen to regulate the action of ethanol on NK cells is not known. In the present study, we evaluated the roles of beta-EP neurons, corticotropin releasing hormone (CRH) neurons, and the autonomic nervous system (ANS) in regulation of the ethanol effect on splenic NK cell cytolytic function.

METHODS

Male rats were fed an ethanol-containing liquid diet or control diets. These rats were used to determine the hormone release from the paraventricular nuclei (PVN) of the hypothalamus or used to determine the splenic NK cell cytolytic function after PVN administration of CRH or intraperitoneal (i.p.) administration of a ganglionic blocker chlorisondamine. The release of hormones from the PVN was measured using the push-pull perfusion method. Splenic cytolytic activity was determined using the 4-hour (51)Cr release assay against YAC-1 lymphoma target cells.

RESULTS

Alcohol feeding decreased the amount of beta-EP but increased the amount of CRH in the push-pull perfusate (PPP) samples collected from the PVN. When exogenous beta-EP was perfused into the PVN, it suppressed the release of endogenous CRH found in PPP samples of the PVN. Conversely, perfusion of an opiate antagonist naltrexone into the PVN increased the levels of endogenous CRH in PPP samples of the PVN. In addition, administration of exogenous beta-EP in the PVN stimulated the cytolytic function of NK cells, an action that was antagonized by CRH as well as by ethanol. Corticotropin-releasing hormone and ethanol alone also had an inhibitory action on NK cells. Finally, the ganglionic blocker used prevented the effect that ethanol, beta-EP, and CRH had on NK cells. These data suggest that ethanol inhibits the function of NK cells partly by suppressing the influence of the beta-EP-CRH-ANS signal to the spleen.

Nuclear factor kappaB signaling in opioid functions and receptor gene expression

Opiates are the most powerful of all known analgesics. The prototype opiate morphine has been used as a painkiller for several thousand years. Chronic usage of opiates not only causes drug tolerance, dependence, and addiction, but also suppresses immune functions and affects cell proliferation and cell survival. The diverse functions of opiates underscore the complexity of opioid receptor signaling. Several downstream signaling effector systems, including adenylyl cyclase, mitogen-activated protein kinase, Ca2+ channels, K+ channels, and phosphatidylinositol 3-kinase/Akt, have been identified to be critical in opioid functions. Nuclear factor-kappaB (NF-kappaB), one of the most diverse and critical transcription factors, is one of the downstream molecules that may either directly or indirectly transmit the receptor-mediated upstream signals to the nucleus, resulting in the regulation of the NF-kappaB-dependent genes, which are critical for the opioid-induced biological responses of neuronal and immune cells. In this minireview, we focus on current understanding of the involvement of NF-kappaB signaling in opioid functions and receptor gene expression in cells.

delta opioid receptors stimulate Akt-dependent phosphorylation of c-jun in T cells

Activation of naive T cells markedly up-regulates the expression of delta opioid receptors (DORs). These receptors are bound by DOR peptides released by T cells, modulating T cell functions such as interleukin-2 production, cellular proliferation, and chemotaxis. Previous studies have shown that DOR agonists [e.g., [D-Ala(2)-D-Leu(5)]-enkephalin (DADLE)] modulate T cell antigen receptor signaling through mitogen-activated protein kinases (MAPKs; i.e., extracellular signal-regulated kinases 1 and 2) and that DORs directly induce phosphorylation of activating transcription factor-2 (implicated in cytokine gene transcription) and its association with the MAPK c-jun1 NH(2)-terminal kinase (JNK). Such observations suggest that DORs may induce the phosphorylation of c-jun. These experiments were performed to test this hypothesis and determine the potential roles of phosphoinositide 3-kinase (PI3K) and Akt (protein kinase B). DADLE (10(-10) to 10(-6) M) dose-dependently induced c-jun phosphorylation. This was blocked by pertussis toxin and the DOR-specific antagonist naltindole. Fluorescence flow cytometry showed that DADLE significantly stimulated c-jun phosphorylation by T cells. DADLE stimulated phosphorylation of membrane-associated Akt; wortmannin and LY294002 ([2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one]), specific inhibitors of PI3K, abolished the DADLE-induced phosphorylation of c-jun. Finally, inhibitors of Akt and JNK blocked DADLE-induced phosphorylation of c-jun. Thus, activated DORs directly stimulate c-jun phosphorylation through a PI3K-dependent pathway in T cells, apparently involving Akt. This implies that DORs activate JNK through a novel pathway dependent on PI3K and Akt, thereby regulating the function of activator protein-1 transcription complexes containing c-jun and other transcription partners.

Multiple opioid receptors on immune cells modulate intracellular signaling

In 1979, Joseph Wybran reported his insights into the existence of different opioid receptor subtypes on T-cells. He observed that morphine and methionine enkephalin had different effects on human T-cell rosetting to sheep red blood cells. Since that time, a wide array of laboratories have shown that opiate alkyloids and opioid peptides exert pleiotropic effects on immune cell function. These compounds are immunomodulators, modifying immune responses to extracellular stimuli such as mitogens, antigens, and antibodies that cross-link the T-cell receptor. It has been demonstrated that cells involved in host defense and immunity express mRNA transcripts encoding the various opioid receptors originally described in neuronal tissues. Molecular imaging approaches have demonstrated the regulated expression of both delta and kappa opioid receptors, predominantly on T-cells. Moreover, atypical opiate and opioid binding sites are present on these cells. This review will consider the evidence for both classical and atypical opioid receptors and their effects on signaling within immune cells; our emphasis is the T-cell and its delta opioid receptor.

Beta-endorphin modulation of interferon-gamma, perforin and granzyme B levels in splenic NK cells: effects of ethanol

The effects of ethanol and beta-endorphin (beta-EP) on productions of cytolytic factors granzyme B, perforin and IFN-gamma in splenic rat NK cells were determined. Intracranial administration of beta-EP increased protein and mRNA levels of cytolytic factors in NK cells. Chronic ethanol feeding reduced the basal and beta-EP-induced levels of cytolytic factors in NK cells. In vitro treatment of beta-EP on NK cells increased the levels of perforin, granzyme B and IFN-gamma and their mRNA transcripts, whereas ethanol pre-treatment prevented beta-EP effects on cytolytic factors in these cells. These results suggest that beta-EP and ethanol interact to regulate NK cell functions.

The delta-opioid receptor participates in T-cell development by promoting negative selection

The delta-opioid receptor-1 (DOR-1) as well as delta-opioid enkephalin peptides are expressed during maturation of T cells, although the functional significance of their expression remains unclear. Based on results which show that the administration of the highly selective delta-opioid agonist D-Pen2, D-Pen5]enkephalin (DPDPE) induces an altered pattern of T-cell differentiation in fetal thymic organ culture (FTOC), we hypothesized that DOR-1 is involved in the negative selection process. Our results show that superantigen-induced clonal deletion is promoted by DPDPE and significantly impaired in DOR-1-deficient mice. These results suggest that delta-opioids may play a homeostatic role in the negative selection process during T-cell development.

Opioids as modulators of cell death and survival--unraveling mechanisms and revealing new indications

Opioids are powerful analgesics but also drugs of abuse. Because opioid addicts are susceptible to certain infections, opioids have been suspected to suppress the immune response. This was supported by the finding that various immune-competent cells express opioid receptors and undergo apoptosis when treated with opioid alkaloids. Recent evidence suggests that opioids may also effect neuronal survival and proliferation or migrating properties of tumor cells. A multitude of signaling pathways has been suggested to be involved in these extra-analgesic effects of opioids. Growth-promoting effects were found to be mediated through Akt and Erk signaling cascades. Death-promoting effects have been ascribed to inhibition of nuclear factor-kappaB, increase of Fas expression, p53 stabilization, cytokine and chemokine release, and activation of nitric oxide synthase, p38, and c-Jun-N-terminal kinase. Some of the observed effects were inhibited with opioid receptor antagonists or pertussis toxin; others were unaffected. It is still unclear whether these properties are mediated through typical opioid receptor activation and inhibitory G-protein-signaling. The present review tries to unravel controversial findings and provides a hypothesis that may help to integrate diverse results.

Opioid antagonist naltrexone disrupts feedback interaction between mu and delta opioid receptors in splenocytes to prevent alcohol inhibition of NK cell function

Naltrexone, an opioid antagonist, has been used in clinical trials to treat alcoholism. As the opioid peptides beta-endorphin and enkephalin increase splenic NK cell function in laboratory animals, it is anticipated that naltrexone treatment will cause immunosuppression. However, we report in this study that chronic naltrexone administration in laboratory rats increases the cytolytic activity of NK cells. It also prevents alcohol's suppressive effect on these cells. We identified that, in the splenocytes, delta opioid receptor expression is tightly controlled by negative feedback regulation of micro opioid receptors. Naltrexone disrupts this feedback control by reducing micro opioid receptor function, thereby up-regulating delta opioid receptor binding, which results in an enhanced NK cell cytolytic response to delta opioid receptor ligands. We conclude that naltrexone, which has been shown to be a promising agent for the clinical management of alcoholism, may have potential use in the treatment of immune deficiency in alcoholic and nonalcoholic patients.

Phosphorylation of activating transcription factor in murine splenocytes through delta opioid receptors

Delta opioid receptors (DORs) modulate TCR signaling through the mitogen-activated protein kinases (MAPKs), ERKs 1 and 2. These studies determined whether a DOR agonist alone ([D-Ala(2)-D-Leu(5)]enkephalin; DADLE) affects phosphorylation of the activating transcription factor (ATF-2) and its interaction with the MAPK, c-Jun NH(2)-terminal kinase (JNK). DOR expression was induced on murine splenocytes by anti-CD3 and then quiescent cells were treated with DADLE. DADLE, itself, dose-dependently induced maximal phosphorylation of ATF-2 within 5-10min; naltrindole, a specific antagonist, abolished this. Anti-ATF-2 immunoprecipitates from control and DADLE-treated splenocytes showed a dominant 59kDa phosphorylated band and a 71kDa band. DADLE stimulated phosphorylation of both bands, although the 71kDa band was selectively immunoprecipitated by anti-JNK. Thus, DADLE stimulated phosphorylation of 71kDa ATF-2 and its association with JNK, suggesting that JNK is activated through DORs. Along with previous observations, these studies suggest that lymphocyte DORs can affect the activation of MAPKs by TCR-independent stimulation (e.g., JNK) or indirectly by modulating TCR-dependent stimulation (e.g., ERK).

Transcriptional regulation of mouse delta-opioid receptor gene. Ikaros-2 and upstream stimulatory factor synergize in trans-activating mouse delta-opioid receptor gene in T cells

Considerable evidence indicates that transcription of the delta-opioid receptor (dor) gene is correlated with both the expression of DOR on T cells and the capacity of DOR agonists to modulate the immunological functions of the T cell. We previously reported that increased Ikaros (Ik) binding activity over an Ik-binding site at -378 to -374 (with the translation start site designated as +1) in the mouse dor promoter was required for the enhanced transcription of dor gene in phytohemagglutinin-activated EL-4 cells, a mouse T cell line that constitutively expresses DOR. In the present study, we have analyzed further the mouse dor promoter in EL-4 cells and have demonstrated that Ik-2 homodimers bind to the -378/-374 Ik-binding site and exerts a position-dependent trans-activation effect on the dor promoter. Moreover, an E box (-185 to -180) that binds upstream stimulatory factor is essential for the dor promoter activity in both resting and phytohemagglutinin-activated T cells. Furthermore, we have demonstrated that Ik-2 and upstream stimulatory factor synergize in trans-activating the dor promoter via the putative Ik-binding site and the E box, respectively.

CD28 costimulation induces delta opioid receptor expression during anti-CD3 activation of T cells

Previous studies have demonstrated that naive splenic mouse T cells express no or only very low levels of the delta-type opioid receptor (delta OR), but stimulation of mouse splenocytes with Con A results in induction of delta OR mRNA and protein. In this report we have shown that stimulation of highly purified populations of naive mouse T cells with anti-CD3 mAb alone results in T cell activation, as evidenced by sustained IL-2 secretion and cell proliferation, but fails to elicit delta OR expression. However, delta OR expression is induced by costimulation of these very pure T cells with anti-CD3 and anti-CD28 mAbs. The delta OR induction by anti-CD3 and anti-CD28 costimulation was completely blocked by inhibition of phosphatidylinositol 3-kinase with wortmannin. Because phosphatidylinositol 3-kinase activation in T cells is linked to costimulation, these results suggest that induction of delta OR expression during T cell activation is strictly dependent on costimulation. It also appears that costimulatory receptors other than CD28 can provide the signaling required for delta OR expression because delta OR mRNA was induced by Con A stimulation of splenocytes from CD28-deficient mice.

Transcriptional regulation of mouse delta-opioid receptor gene. Role of Ikaros in the stimulated transcription of mouse delta-opioid receptor gene in activated T cells

Delta-opioid receptors (DOR) present on T cells have been shown to mediate the immunomodulatory effects of endogenous and synthetic DOR agonists on T cells. Considerable evidence indicates that there is stimulated transcription of DOR gene in activated T cells, which is correlated with augmented expression of DOR and enhanced capacity of DOR agonists to affect the T-cell's functions. However, the molecular mechanism underlying the stimulated transcription of the DOR gene in activated T cells is still unclear. In the present study, we analyzed a 1.3-kb DNA fragment immediately upstream of the translation start site (-1300 to +1 bp, with the translation start site designated as +1) of the mouse DOR gene in EL-4 cells, a mouse lymphoma T cell line that exhibits enhanced expression of DOR transcripts when activated by phytohemagglutinin. Through both in vivo and in vitro experiments, we have demonstrated that increased binding activity of Ikaros at the Ikaros-binding site (-378 to -374) in the DOR promoter is required for the stimulated transcription of DOR gene in phytohemagglutinin-activated T cells.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 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; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Immunofluorescence detection of delta opioid receptors (DOR) on human peripheral blood CD4+ T cells and DOR-dependent suppression of HIV-1 expression

The delta opioid receptors (DORs) modulate T cell proliferation, IL-2 production, chemotaxis, and intracellular signaling. Moreover, in DOR-transfected Jurkat cells, delta opioids have been shown to suppress HIV-1 p24 Ag expression. These observations led us to characterize the expression of DORs by human peripheral blood T cells and to determine whether a specific DOR agonist, benzamide,4-([2,5-dimethyl-4-(2-propenyl)-1-piperazinyl](3-methoxyphenyl)methyl]-N,-,(2S[1(S*),2alpha,5beta])-(9Cl) (SNC-80), can suppress p24 Ag expression by HIV-1-infected CD4+ T cells obtained from normal donors. By immunofluorescence flow cytometry, PHA stimulated the expression of DOR from 1.94 +/- 0.70 (mean +/- SEM) to 20.70 +/- 1.88% of the PBMC population by 48 h (p < 0.0001). DOR expression was approximately 40% of both the PHA-stimulated CD4+ and CD8+ T cell subsets, and virtually all DORs were found on these subsets. To determine whether activated DORs suppress HIV-1 expression, PBMC were prestimulated with PHA, and then CD4+ T cells were purified, pretreated with SNC-80, and infected with HIV-1. In a concentration-dependent manner, SNC-80 inhibited production of p24 Ag. SNC-80 10(-10) M maximally suppressed (approximately 50%) both lymphocytotropic (HIV-1 MN) and monocytotropic (SF162) strains; higher concentrations were less effective. Naltrindole, a selective DOR antagonist, abolished the inhibitory effects of SNC-80. Kinetic studies indicated that 24-h pre- or postincubation with SNC-80, relative to infection with HIV-1, eliminated its suppressive effects. Thus, stimulating the DORs expressed by activated CD4+ T cells significantly suppressed the expression of HIV-1. These findings suggest that opioid immunomodulation directed at host T cells may be adjunctive to standard antiviral approaches to HIV-1 infection.