Delta-opioid peptide receptors in muscles from obese diabetic and normal mice

δ-Opioid receptors stimulate GLUT1-mediated glucose uptake through Src- and IGF-1 receptor-dependent activation of PI3-kinase signalling in CHO cells

BACKGROUND AND PURPOSE

Although opioids have been reported to affect glucose homeostasis, relatively little is known on the role of δ-opioid receptors. We have investigated the regulation of glucose transport by human δ-opioid receptors expressed in Chinese hamster ovary cells.

EXPERIMENTAL APPROACH

The uptake of [(3)H]-2-deoxy-D-glucose and 3-O-[methyl-[(3)H]]-D-glucose in response to δ-opioid receptor ligands and the expression of GLUT1, GLUT3 and GLUT4 glucose transporters were examined. Moreover, the effects of intracellular signal transduction inhibitors on δ-opioid receptor-regulated [(3)H]-2-deoxy-D-glucose uptake and protein phosphorylation were investigated.

KEY RESULTS

Activation of δ-opioid receptors rapidly stimulated [(3)H]-2-deoxy-D-glucose and 3-O-[methyl-[(3)H]]-D-glucose uptakes, which were blocked by the GLUT inhibitors cytochalasin B and phloretin. The stimulation of [(3)H]-2-deoxy-D-glucose uptake that occurred without a change in plasma membrane GLUT1 - required the coupling to G(i) /G(o) proteins - was independent of cAMP and extracellular signal-regulated protein kinases, and was suppressed by blockade of Src and insulin-like growth factor-1 receptor (IGF-1R) tyrosine kinases. Inhibition of phosphatidylinositol 3-kinase (PI3K) by wortmannin or LY294002 and by PI3Kα, but not γ, isoform-selective inhibitors greatly reduced the δ-opioid receptor stimulation of glucose uptake. Moreover, the response was attenuated by overexpressing a dominant-negative kinase-deficient Akt form and by chemical inhibition of Akt. Stimulation of δ-opioid receptors increased protein kinase Cζ/λ (PKCζ/λ) phosphorylation and a selective PKCζ/λ inhibitor slightly reduced opioid stimulation of glucose uptake.

CONCLUSIONS AND IMPLICATIONS

δ-Opioid receptors stimulated glucose transport probably by enhancing GLUT1 intrinsic activity through a signalling cascade involving G(i)/G(o), Src, IGF-1R, PI3Kα, Akt and, to a minor extent, PKCζ/λ. This effect may contribute to the opioid regulation of glucose homeostasis in physio-pathological conditions.

Remote ischemic preconditioning: a novel protective method from ischemia reperfusion injury--a review

BACKGROUND

Restoration of blood supply to an organ after a critical period of ischemia results in parenchymal injury and dysfunction of the organ referred to as reperfusion injury. Ischemia reperfusion injury is often seen in organ transplants, major organ resections and in shock. Ischemic preconditioning (IPC) is an adaptational response of briefly ischemic tissues which serves to protect against subsequent prolonged ischemic insults and reperfusion injury. Ischemic preconditioning can be mechanical or pharmacological. Direct mechanical preconditioning in which the target organ is exposed to brief ischemia prior to prolonged ischemia has the benefit of reducing ischemia-reperfusion injury (IRI) but its main disadvantage is trauma to major vessels and stress to the target organ. Remote (inter organ) preconditioning is a recent observation in which brief ischemia of one organ has been shown to confer protection on distant organs without direct stress to the organ.

AIM

To discuss the evidence for remote IPC (RIPC), underlying mechanisms and possible clinical applications of RIPC. METHODS OF SEARCH: A Pubmed search with the keywords "ischemic preconditioning," "remote preconditioning," "remote ischemic preconditioning," and "ischemia reperfusion" was done. All articles on remote preconditioning up to September 2006 have been reviewed. Relevant reference articles from within these have been selected for further discussion.

RESULTS

Experimental studies have demonstrated that the heart, liver, lung, intestine, brain, kidney and limbs are capable of producing remote preconditioning when subjected to brief IR. Remote intra-organ preconditioning was first described in the heart where brief ischemia in one territory led to protection in other areas. Translation of RIPC to clinical application has been demonstrated by the use of brief forearm ischemia in preconditioning the heart prior to coronary bypass and in reducing endothelial dysfunction of the contra lateral limb. Recently protection of the heart has been demonstrated by remote hind limb preconditioning in children who underwent surgery on cardiopulmonary bypass for congenital heart disease. The RIPC stimulus presumably induces release of biochemical messengers which act either by the bloodstream or by the neurogenic pathway resulting in reduced oxidative stress and preservation of mitochondrial function. Studies have demonstrated endothelial NO, Free radicals, Kinases, Opioids, Catecholamines and K(ATP) channels as the candidate mechanism in remote preconditioning. Experiments have shown suppression of proinflammatory genes, expression of antioxidant genes and modulation of gene expression by RIPC as a novel method of IRI injury prevention.

CONCLUSION

There is strong evidence to support RIPC. The underlying mechanisms and pathways need further clarification. The effective use of RIPC needs to be investigated in clinical settings.

Opioid receptors in fast and slow skeletal muscles of normal and dystrophic mice

The density of beta-endorphin receptors and the proportions of fibres that expressed the receptors was assessed in fast extensor digitorum longus muscle and slow soleus muscles of normal and dystrophic mice using [125I]beta-endorphin and autoradiography. In the EDL the density was approximately 3.5 times higher and the proportion of labelled fibres approximately 2.6 times higher in dystrophic mice than normal mice. In the soleus the density was approximately 6.4 times higher and the proportion of labelled fibres approximately 1.5 times higher in the dystrophic mice than the normal mice. The receptors were of the delta-opioid subtype.

Noninvasive remote ischemic preconditioning for global protection of skeletal muscle against infarction

The aim of this study was to investigate the efficacy and mechanism of action of a noninvasive remote ischemic preconditioning (IPC) technique for the protection of multiple distant skeletal muscles against ischemic necrosis (infarction). It was observed in the pig that three cycles of 10-min occlusion and reperfusion in a hindlimb by tourniquet application reduced the infarction of latissimus dorsi (LD), gracilis (GC), and rectus abdominis (RA) muscle flaps by 55%, 60%, and 55%, respectively, compared with their corresponding control (n = 6, P < 0.01) when they were subsequently subjected to 4 h of ischemia and 48 h of reperfusion. This infarct-protective effect of remote IPC in LD muscle flaps was abolished by an intravenous bolus injection of the nonselective opioid receptor antagonist naloxone (3 mg/kg) 10 min before remote IPC and a continuous intravenous infusion (3 mg/kg) during remote IPC and by an intravenous bolus injection of the selective delta 1-opioid receptor antagonist 7-benzylidenealtrexone maleate (3 mg/kg). However, this infarct-protective effect of remote IPC was not affected by an intravenous bolus injection of the ganglionic blocker hexamethonium chloride (20 mg/kg) or the nonspecific adenosine receptor antagonist 8-(p-sulfophenyl)theophylline (10 mg/kg) or by a local intra-arterial injection of the adenosine1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (3 mg/muscle flap) given 10 min before remote IPC. It was also observed that this remote IPC of skeletal muscle against infarction was associated with a slower rate of muscle ATP depletion during the 4 h of sustained ischemia and a reduced muscle neutrophilic myeloperoxidase activity after 1.5 h of reperfusion. These observations led us to speculate that noninvasive remote IPC by brief cycles of occlusion and reperfusion in a pig hindlimb is effective in global protection of skeletal muscle against infarction. This infarct-protective effect is most likely triggered by the activation of opioid receptors in the skeletal muscle, and remote IPC is associated with an energy-sparing effect during sustained ischemia and attenuation of neutrophil accumulation during reperfusion.

Radiosynthesis and in vivo evaluation of the pseudopeptide delta-opioid antagonist [(125)I]ITIPP(psi)

The radioiodinated tetrapeptide delta-opioid antagonist [(125)I]ITIPP(psi) [H-Tyr(3'I)-Ticpsi[CH2NH]Phe-Phe-OH] (Ki(delta) = 2.08 nM; Ki(micro)/Ki(delta) = 1280) has been synthesized and evaluated as a potential lung tumour imaging agent. [(125)I]ITIPP(psi) was obtained, via electrophilic iodination, in 46% yield (>44,000 MBq/micromol) from the parent TIPP(psi). The biodistribution of [(125)I]ITIPP(psi) in nu/nu mice bearing SCLC-SW210.5 xenographs revealed good uptake and prolonged retention of radioactivity in organs known to possess delta-opioid receptors. Metabolite analysis showed that [(125)I]ITIPP(psi) was largely unmetabolized at 25 min PI and blocking studies showed significant reduction of uptake of the tracer in the brain, liver, intestine and tumor indicating that the iodinated tetrapeptide binds to delta opioid receptors in vivo.

Effect of beta-endorphin C-terminal peptides on glucose uptake in isolated skeletal muscles of the mouse

The uptake of a nonmetabolizable derivative of glucose, [3H]2-deoxy-D-glucose was examined in isolated slow (soleus) and fast (extensor digitorum longus, EDL) muscles of adult mice. An analogue of beta-endorphin (28-31), Ac-Lys-D-Lys-Sar-Glu, which is stable to proteolytic digestion, enhanced the uptake of glucose into the slow and fast muscles. The muscles of male mice were more sensitive to the peptide than those of female mice. The maximum uptake seen in the presence of the peptide was similar to that seen with insulin in the soleus muscle and greater than that seen with insulin in the EDL muscle.

Actions of beta-endorphin peptides on the contractions of mouse diaphragm muscle

The effect of derivatives of beta-endorphin on the contractile response to indirect stimulation in mouse diaphragm muscle was studied to determine whether the action of the peptide to increase muscle tension is an opioid effect. beta-Endorphin (1-27), beta-endorphin (30-31), and a beta-endorphin (28-31) analogue all increased the amplitude of the contractions. The C-terminal peptides were more potent than beta-endorphin or beta-endorphin (1-27). The beta-endorphin (28-31) analogue, like beta-endorphin, decreased the time to peak but beta-endorphin (1-27) did not. The effect of beta-endorphin (1-27), but not that of the beta-endorphin (28-31) analogue, was blocked by naloxone. Thus, beta-endorphin acts on muscle via both opioid and nonopioid receptors.

The role of tumor necrosis factor-alpha in the pathogenesis of anorexia and bulimia nervosa, cancer cachexia and obesity

In this paper a new immunological model of anorexia and bulimia nervosa will be presented in which the inflammatory cytokines are conceived as the fundamental regulators of body metabolism. This conception differs from the conventional view in which the inflammatory cytokines are perceived primarily as peptide molecules utilized by the immune system to control infection, inflammation and tissue or neuronal damage. Given that the inflammatory cytokines are also fundamental regulators of body metabolism, when they become dysregulated they create physiological chaos which results in the development of a number of autoimmune, metabolic and psychiatric disorders. In this proposed immunological model of anorexia and bulimia nervosa, elevated tumor necrosis factor-alpha features as the primary cause of these conditions. Pathophysiological parallels are drawn between anorexia nervosa and cancer cachexia in terms of the causal role the cytokines, neuropeptides and neurotransmitters play in the manifestation of shared symptoms. These shared symptoms include elevated tumour necrosis factor-alpha, down-regulated interleukin-2 and interleukin-4 and depletion of lean body mass. Furthermore, the following neuropeptides are dysregulated in both anorexia nervosa and cancer cachexia: vasoactive intestinal peptide, cholecystokinin, corticotropin-releasing factor, neuropeptide Y, peptide YY and beta-endorphin. In addition, in anorexia and bulimia nervosa, secretion of the neurotransmitter serotonin is inhibited while norepinephrine is enhanced. It will be argued that the causal interplay between the cytokines, neuropeptides and neurotransmitters initiates a cascade of biochemical events which may result in either anorexia or bulimia nervosa, or cancer cachexia. The extent to which these inflammatory cytokines, neuropeptides and neurotransmitters are causally efficacious in the pathogenesis of other autoimmune disorders, such as diabetes mellitus and rheumatoid arthritis, will also be addressed.

Distribution of opioid peptide receptors in muscles of lean and obese-diabetic mice

Autoradiography was used to demonstrate beta-endorphin and delta-opioid receptors in muscles of normal and obese-diabetic mice. The density of the receptors was significantly higher in the obese-diabetic mice. In both normal and diabetic mice, glycolytic and oxidative fibers exhibited the beta-endorphin receptors. However, a significantly greater density of beta-endorphin receptors was observed in the extensor digitorum longus muscles than in the soleus muscles in the diabetic mice. In normal muscles the beta-endorphin receptors were largely restricted to regions where endplates were present, but in the obese-diabetic mice they were densely distributed along the length of the muscle fibers.

Effect of beta-endorphin on the contractile responses in mouse skeletal muscle

Tension development in response to direct and indirect electrical stimulation was studied in an isolated phrenic nerve hemidiaphragm preparation of the mouse. beta-Endorphin (beta-EP) caused an increase in the amplitude and a decrease in the time to peak of muscle contractions in response to low frequency stimulation of the nerve. Upon direct stimulation of the muscle the peptide had no effect. The actions of beta-EP were abolished in the presence of the opioid antagonist naloxone and mimicked by delta opioid agonists. Upon high frequency stimulation of the nerve, beta-EP caused an increase in the initial, maximum, and mean tension. It also prevented the fall in the final tension seen in the control preparations with repeated periods of stimulation. The findings are consistent with beta-EP having a role to improve neuromuscular function and delay fatigue, and indicate the possible therapeutic potential of opioid substances in conditions where muscle weakness is present.

Effect of beta-endorphin and alpha-melanotropin on muscle wasting in mice

The pro-opiomelanocortin-derived peptides beta-endorphin (beta-EP) and alpha-melanocortin (alpha-MSH) were administered to normal and dystrophic C57BL6J mice. All groups of normal and dystrophic mice which had been treated with the two peptides gained significant body weight, as did the normal and dystrophic saline-treated male controls, but the normal and dystrophic female controls did not. The plasma activity of creatine phosphokinase (CPK) was lower in normal mice and dystrophic males which had been treated with the two peptides compared to the corresponding controls. There was no significant difference between the plasma LDH activity in any of the peptide-treated and the corresponding control groups. The activity of CPK was significantly higher in the extensor digitorum longus (EDL) muscles, but not the soleus muscles, of the peptide-treated dystrophic mice compared to the corresponding controls. Administration of alpha-MSH alone or beta-EP alone had no significant effect on the body weight or plasma CPK activity of dystrophic mice compared to the controls. However the activity of CPK was significantly higher in the EDL muscles of the alpha-MSH-treated mice than in the corresponding controls. It is possible that beta-EP and alpha-MSH act synergistically on the neuromuscular system to protect the muscles from damage.