Enkephalins modulate differentiation of normal human keratinocytes in vitro

Evaluation of Kynu, Defb2, Camp, and Penk Expression Levels as Psoriasis Marker in the Imiquimod-Induced Psoriasis Model

Background

Psoriasis is a noncontagious auto-inflammatory chronic skin disease. So far, some of the inflammatory genes were upregulated in mouse model of psoriasis. This study examined changes in skin mRNA expression of L-kynureninase (Kynu), cathelicidin antimicrobial peptide (Camp), beta-defensin 2 (Defb2), and proenkephalin (Penk) in a mouse model of imiquimod-induced psoriasis.

Materials and Methods

Tree groups of C57BL/6 female mice were allocated. The imiquimod (IMQ) cream was administered to the mice dorsal skin of the two groups to induce psoriatic inflammation. In the treatment group, IMQ was administered 10 min after hydrogel-containing M7 anti-IL-17A aptamer treatment. Vaseline (Vas) was administered to the negative control group. The psoriatic skin lesions were evaluated based on the psoriasis area severity index (PASI) score, histopathology, and mRNA expression levels of Kynu, Camp, Defb2, and Penk using real-time PCR. In order to assess the systemic response, the spleen and lymph node indexes were also evaluated.

Results

The PASI and epidermal thickness scores were 6.01 and 1.96, respectively, in the IMQ group, and they significantly decreased after aptamer administration to 1.15 and 0.90, respectively (P < 0.05). Spleen and lymph node indexes showed an increase in the IMQ group, followed by a slight decrease after aptamer treatment (P > 0.05). Additionally, the mRNA expression levels of Kynu, Defb2, Camp, and Penk genes in the IMQ-treated region showed a significant 2.70, 4.56, 3.29, and 2.61-fold increase relative to the Vas mice, respectively (P < 0.05). The aptamer-treated region exhibited a significant decrease in these gene expression levels (P < 0.05). A positive correlation was found between Kynu, Penk, and Camp expression levels and erythema, as well as Camp expression with PASI, scaling, and thickness (P < 0.05).

Conclusion

According to our results, it seems that Kynu, Camp, and Penk can be considered appropriate markers for the evaluation of psoriasis in IMQ-induced psoriasis. Also, the anti-IL-17 aptamer downregulated these important genes in this mouse model.

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.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Opioidergic Signaling-A Neglected, Yet Potentially Important Player in Atopic Dermatitis

Atopic dermatitis (AD) is one of the most common skin diseases, the prevalence of which is especially high among children. Although our understanding about its pathogenesis has substantially grown in recent years, and hence, several novel therapeutic targets have been successfully exploited in the management of the disease, we still lack curative treatments for it. Thus, there is an unmet societal demand to identify further details of its pathogenesis to thereby pave the way for novel therapeutic approaches with favorable side effect profiles. It is commonly accepted that dysfunction of the complex cutaneous barrier plays a central role in the development of AD; therefore, the signaling pathways involved in the regulation of this quite complex process are likely to be involved in the pathogenesis of the disease and can provide novel, promising, yet unexplored therapeutic targets. Thus, in the current review, we aim to summarize the available potentially AD-relevant data regarding one such signaling pathway, namely cutaneous opioidergic signaling.

The Local Neuropeptide System of Keratinocytes

Neuropeptides have been known for over 50 years as chemical signals in the brain. However, it is now well established that the synthesis of this class of peptides is not restricted to neurons. For example, human skin not only expresses several functional receptors for neuropeptides but, also, can serve as a local source of neuroactive molecules such as corticotropin-releasing hormone, melanocortins, and β-endorphin. In contrast, an equivalent of the hypothalamic-pituitary axis in the oral mucosa has not been well characterized to date. In view of the differences in the morphology and function of oral mucosal and skin cells, in this review I surveyed the existing evidence for a local synthesis of hypothalamic-pituitary, opiate, neurohypophyseal, and neuroendocrine neuropeptides in both epidermal and oral keratinocytes.

β-Neoendorphin Enhances Wound Healing by Promoting Cell Migration in Keratinocyte

The skin is the largest and a remarkably plastic organ that serves as a protective barrier against environmental stimuli and injuries throughout life. Skin injuries are serious health problems, and wound healing is a critical process to replace devitalized cellular and tissue structures. Although some endogenous opioids are known to be involved in the modulation of wound healing, it remains to be determined whether the β-neoendorphin (β-NEP), an endogenous opioid, has beneficial effects on wound repair in human keratinocyte. In this study, we found that β-NEP accelerated wound repair through activation of mitogen-activated protein kinase (MAPK)/Erk1/2 signaling pathways in human keratinocytes. Moreover, the wound healing effect of β-NEP is mainly through the acceleration of keratinocyte migration without affecting cell proliferation. Therefore, our studies reveal that β-NEP plays an important role in the regulation of wound repair and suggest a therapeutic strategy to promote wound healing using β-NEP.

Recent advances in understanding the molecular mechanisms of cholestatic pruritus: A review

Cholestasis, a condition characterized by an abnormal decrease in bile flow, is accompanied by various symptoms such as pruritus. Although cholestatic pruritus is a prominent condition, its precise mechanisms have largely been elusive. Recently, advancements have been made for understanding the etiology and pathogenesis of cholestatic pruritus. The current review therefore focuses on summarizing the overall progress made in the elucidation of its molecular mechanisms. We have reviewed the available animal models on cholestasis to compare the differences between them, characterized potential pruritogens involved in cholestatic pruritus, and have summarized the receptor and ion channels implicated in the condition. Finally, we have discussed the available treatment options for alleviation of cholestatic pruritus. As our understanding of the mechanisms of cholestatic pruritus deepens, novel strategies to cure this condition are awaited.

High expression of proenkephalin is associated with favorable outcomes in patients with gastrointestinal stromal tumors

Purpose

The aim of this study was to elucidate the prognostic value of proenkephalin (PENK) in gastrointestinal stromal tumors (GISTs).

Patients and methods

We collected data on 268 eligible postoperative patients diagnosed with GIST between January 1, 2002, and December 31, 2011. PENK expression was detected in GIST tissues classified using the United States National Institutes of Health (NIH) risk classification system. The associations between high PENK expression and the clinicopathological characteristics were assessed. Overall survival (OS) and recurrence-free survival (RFS) were estimated by Kaplan–Meier analysis, and the log-rank test was used to compare the differences between groups. Univariate and multivariate Cox regression analyses were conducted to assess the prognostic value of PENK in GIST patients.

Results

High PENK expression was more common in the low- and intermediate-risk GIST groups compared with the high-risk group (P<0.05). Additionally, PENK expression was associated with tumor size, mitosis count per 50 high-power fields, and tumor rupture (P<0.05). Kaplan–Meier analysis revealed that high PENK expression was associated with superior OS and RFS, while low PENK expression was associated with worse OS and RFS. Furthermore, PENK was shown to be an independent predictor of OS and RFS in the overall population (for OS, hazard ratio [HR], 1.596, 95% confidence interval [CI], 1.006–2.914, P<0.001; for RFS, HR, 1.910, 95% CI, 0.977–3.089, P<0.001).

Conclusion

PENK expression in GIST is closely associated with NIH risk grade and prognosis, indicating that PENK may act as a tumor suppressor and may serve as a new biomarker for predicting prognosis in postoperative GIST patients.

Investigating endogenous µ-opioid receptors in human keratinocytes as pharmacological targets using novel fluorescent ligand

Opioids in skin function during stress response, regeneration, ageing and, particularly in regulating sensation. In chronic pruritus, topical treatment with Naltrexone changes μ-opioid receptor (μ-OR) localization to relieve itch. The molecular mechanisms behind the effects of Naltrexone on μ-OR function in reduction of itching behavior has not been studied. There is an immediate need to understand the endogenous complexity of μ-OR dynamics in normal and pathological skin conditions. Here we evaluate real-time behavior of μ-OR-Endomorphine complexes in the presence of agonist and antagonists. The μ-OR ligand Endomorphine-1 (EM) was conjugated to the fluorescent dye Tetramethylrhodamine (TAMRA) to investigate the effects of agonist and antagonists in N/TERT-1 keratinocytes. The cellular localization of the EM-TAMRA was followed through time resolved confocal microscopy and population analysis was performed by flow cytometry. The in vitro analyses demonstrate fast internalization and trafficking of the endogenous EM-TAMRA-μ-OR interactions in a qualitative manner. Competition with Endomorphine-1, Naltrexone and CTOP show both canonical and non-canonical effects in basal and differentiated keratinocytes. Acute and chronic treatment with Naltrexone and Endomorphine-1 increases EM-TAMRA binding to skin cells. Although Naltrexone is clinically effective in relieving itch, the mechanisms behind re-distribution of μ-ORs during clinical treatments are not known. Our study has given insight into cellular mechanisms of μ-OR ligand-receptor interactions after opioid agonist and antagonist treatments in vitro. These findings potentially offer opportunities in using novel treatment strategies for skin and peripheral sensory disorders.

Leucine-enkephalin promotes wound repair through the regulation of hemidesmosome dynamics and matrix metalloprotease

The skin responds to environmental stressors by coordinated actions of neuropeptides and their receptors. An endogenous peptide for δ-opioid receptor (DOPr), Leu-enkephalin (L-ENK), is expressed in the skin and its expression is altered in pathological conditions. Although the importance of DOPr is rapidly gaining recognition, the molecular mechanisms underlying its effects on wound healing are largely undefined. We show here that L-ENK induced activation of Erk, P90(RSK), and Elk-1 and promoted the disruption of hemidesmosomes and the expression of matrix metalloprotease (MMP)-2 and MMP-9, important processes for wound healing. Treatment with Erk inhibitor blocked activation of P90(RSK) and Elk-1 and significantly blunted wound repair. Therefore, our results suggest that activation of Erk and its downstream effectors, P90(RSK) and Elk-1, are critical for DOPr-mediated skin homeostasis.

Featured Article: Selective blockade of the OGF–OGFr pathway by naltrexone accelerates fibroblast proliferation and wound healing

Naltrexone (NTX) is an opioid receptor antagonist that acts at classical and non-classical opioid receptors including the opioid growth factor receptor (OGFr). Animal models of type 1 and type 2 diabetes, as well as normal rodents, have shown that topical NTX enhances the healing rates of corneal epithelium and full-thickness cutaneous wounds. The mechanism of this general opioid antagonist on growth, and in particular the specific receptor pathway involved, is not understood. Tissue culture studies using NIH 3T3 fibroblasts and primary rat auricular fibroblasts were established to evaluate growth following opioid receptor antagonist treatment. Treatment of cells with CTOP, naltrindole, or nalmefene, selective antagonists for mu, delta, and kappa opioid receptors, respectively, did not accelerate cell replication. Addition of the classical opioid receptor peptides DAMGO, DPDPE, or EKC did not alter cell growth, suggesting that the classical opioid receptors were not involved in cutaneous wound healing. However, NTX (10−6 M) increased the growth of NIH 3T3 fibroblasts in culture over a 96-h period, and the specific ligand OGF decreased cell growth, supporting that the OGF-OGFr axis is tonically active and constitutively expressed in fibroblasts, the primary cell type in granulation tissue of the skin. Transfection of NIH 3T3 cells with OGFr siRNA reduced receptor protein; subsequent treatment with NTX did not accelerate cell proliferation. These data indicate that blockade of the OGFr pathway enhances proliferation of fibroblasts in vitro, and in a primary culture of auricular fibroblasts, suggesting that the effect of NTX on growth is mediated through the OGF-OGFr axis. Finally, antagonists for classical opioid receptors as well as NTX were topically applied to cutaneous wounds in type 1 diabetic rats; only NTX accelerated wound closure. These studies indicate that the mechanistic pathway underlying the effects of NTX to enhance cutaneous wound closure in diabetic and nondiabetic subjects is specific blockade of the OGF–OGFr regulatory axis.

Protease-activated receptors as therapeutic targets in visceral pain

The protease-activated receptors (PARs) play a pivotal role in inflammatory and nociceptive processes. PARs have raised considerable interest because of their capacity to regulate numerous aspects of viscera physiology and pathophysiology. The present article summarizes research on PARs and proteases as signalling molecules in visceral pain. In particular, experiments in animal models suggest that PAR2 is important for visceral hypersensitivity. Moreover, endogenous PAR2 agonists seem to be released by colonic tissue of patients suffering from irritable bowel syndrome, suggesting a role for this receptor in visceral pain perception. Thus, PARs, together with proteases that activate them, represent exciting targets for therapeutic intervention on visceral pain.

Role of neuropeptides, neurotrophins, and neurohormones in skin wound healing

Due to the close interactions between the skin and peripheral nervous system, there is increasing evidence that the cutaneous innervation is an important modulator of the normal wound healing process. The communication between sensory neurons and skin cells involves a variety of molecules (neuropeptides, neurohormones, and neurotrophins) and their specific receptors expressed by both neuronal and nonneuronal skin cells. It is well established that neurotransmitters and nerve growth factors released in skin have immunoregulatory roles and can exert mitogenic actions; they could also influence the functions of the different skin cell types during the wound healing process.

Topical treatment with the opioid antagonist naltrexone accelerates the remodeling phase of full-thickness wound healing in type 1 diabetic rats

Wound repair involves a series of overlapping phases that include inflammation, proliferation, and tissue remodeling, with the latter phase requiring months for proper healing. Delays in any of these processes can result in infection, chronic ulceration, and possible amputation. Diabetes is a major risk factor for improper wound repair, and impaired wound healing is a major complication for more than 26 million people in the US diagnosed with diabetes. Previous studies have demonstrated that the opioid antagonist naltrexone (NTX) dissolved in moisturizing cream reverses delays in wound closure in streptozotocin-induced type 1 diabetic (T1D) rats. NTX accelerated DNA synthesis and increased the number of epithelial and mast cells, as well as new blood vessel formation. In this study, remodeling was evaluated in T1D rats up to eight weeks after initial wounding. Twenty days following wounding, diabetic rats treated with vehicle had elevated numbers of MMP-2+ fibroblasts, suggesting delayed healing processes; birefringence of granulation tissue stained with Sirius red revealed diminished collagen formation and maturation. Wound tissue from NTX-treated T1D rats had comparable numbers of MMP-2+ fibroblasts to control specimens, as well as accelerated maturation of granulation tissue. The integrity of wounded skin was evaluated by tensile strength measurements. T1D resulted in delayed wound healing, and wounded skin that displayed reduced tensile strength relative to normal rats. Topical NTX applied to wounds in T1D rats resulted in enhanced collagen formation and maturation over a 60-day period of time. Moreover, the force required to tear skin of NTX-treated T1D rats was elevated relative to the force necessary to tear the skin of vehicle-treated T1D rats, and comparable to that for normal rats. These data reveal that complications in wound healing associated with T1D involve the novel OGF-OGFr pathway, and that topical NTX is an effective treatment to enhance wound healing.

Topical naltrexone accelerates full-thickness wound closure in type 1 diabetic rats by stimulating angiogenesis

Delays in wound healing often result in infection, chronic ulceration, and possible amputation of extremities. Impaired wound healing is a major complication of the 23 million people in the USA with diabetes, and financial and medical burdens are demanding new treatments for wound healing. Previous studies have demonstrated that topical application of the opioid antagonist naltrexone (NTX) dissolved in moisturizing cream reverses delays in wound closure in rats with streptozotocin-induced type 1 diabetes. A target of NTX's action is DNA synthesis and cell proliferation. In this study, granulation tissue was evaluated to ascertain the specific cellular targets that were impaired in diabetic wounds, as well as those that were enhanced following NTX application. Mast cell number as well as the number of new blood vessels immunoreactive to fibroblast growth factor-2 (FGF-2), vascular endothelial growth factor (VEGF), and alpha smooth muscle actin (α-SMA) antibodies were recorded at 3, 5, 8, 10, 15, and 20 days following creation of full-thickness dorsal cutaneous wounds in normal and type 1 diabetic rats. Diabetic rats displayed delays in wound closure as well as a reduction in the number of mast cells responding to the injury, and delays in the spatial and temporal expression of FGF-2, VEGF, and α-SMA in capillaries. Topical NTX accelerated the rate of wound closure and stimulated expression of angiogenic factors within granulation tissue of diabetic rats relative to control animals receiving saline in moisturizing cream. These data support observations that a novel biological pathway is impaired under diabetic conditions and can be modulated by topical NTX to enhance proliferative events in wound healing.

Zinc inhibits apoptosis and maintains NEP downregulation, induced by ropivacaine, in HaCaT cells

Zinc (Zn), a cell-protective metal against various toxic compounds, is the key agent for neutral endopeptidase (NEP) functional structure. NEP is a zinc metalloenzyme which degrades endogenous opioids and is expressed in human keratinocytes (HaCaT). Ropivacaine, a widely used opiate local anaesthetic, exerts cell toxic and apoptotic effects against HaCaT cells. The aim of the present study is to investigate whether zinc modulates the effects of ropivacaine on proliferation, viability, apoptosis and NEP expression in HaCaT cells. To investigate the role of ropivacaine in NEP function, HaCaT cells overexpressing NEP were generated via cell transfection with plasmids carrying NEP cDNA. Ropivacaine's anti-proliferative effect was tested by Neubauer's chamber cell counting, and induction of cell death was demonstrated by trypan blue exclusion assay. Apoptosis due to ropivacaine was tested via DNA fragmentation and poly-ADP-ribose-polymerase (PARP) cleavage. NEP and PARP expression was performed by western blot analysis. Results showed that zinc (15 μΜ) inhibited proliferation and cell death induction by ropivacaine (0.5, 1 and 2 mM) (p < 0.05) as well as apoptosis induced by the drug (0.5 and 1 mM) in HaCaT cells. Ropivacaine (1.0, 2.0 and 5.0 mM) downregulated NEP expression in the presence of zinc (15 μΜ) while NEP overexpression enhanced ropivacaine's apoptotic effect. In conclusion, the abilities of zinc to inhibit the toxic and apoptotic effects of ropivacaine, to maintain NEP downregulation induced by the drug and, consequently, to enhance its anaesthetic result suggest that zinc may have a significant role in pain management and tissue protection.

Opioid system and Alzheimer's disease

The opioid system may be involved in the pathogenesis of AD, including cognitive impairment, hyperphosphorylated tau, Aβ production, and neuroinflammation. Opioid receptors influence the regulation of neurotransmitters such as acetylcholine, norepinephrine, GABA, glutamate, and serotonin which have been implicated in the pathogenesis of AD. Opioid system has a close relation with Aβ generation since dysfunction of opioid receptors retards the endocytosis and degradation of BACE1 and γ-secretase and upregulates BACE1 and γ-secretase, and subsequently, the production of Aβ. Conversely, activation of opioid receptors increases the endocytosis of BACE1 and γ-secretase and downregulates BACE1 and γ-secretase, limiting the production of Aβ. The dysfunction of opioid system (opioid receptors and opioid peptides) may contribute to hyperphosphorylation of tau and neuroinflammation, and accounts for the degeneration of cholinergic neurons and cognitive impairment. Thus, the opioid system is potentially related to AD pathology and may be a very attractive drug target for novel pharmacotherapies of AD.

Update on pruritic mechanisms of hypertrophic scars in postburn patients: the potential role of opioids and their receptors

Although itching (or pruritus) in a scar is a very common and distressing symptom and is increasingly being recognized as a significant obstacle in burn rehabilitation, the exact mechanisms underlying this symptom have not been elucidated; hence, a reliable therapy has not been established. Recent findings have suggested that itching caused by inflammatory dermatosis can be reduced by antihistamines, but histamine antagonists cannot block all types of pruritus (eg, neuropathic itch). This indicates the presence of a histamine-independent pathway for itch. Itch or pruritus may also be evoked by direct activation of opioid receptors, which have recently been identified in the skin. This article aims to assess the current state of knowledge regarding the role of opioids in the generation of itch in hypertrophic scars in postburn patients. To this end, the authors have reviewed the relevant literature and present some clinical data. The authors hope that this review will form the basis for future research to elucidate the mechanism and treatment of itch.

Topical treatment with the opioid antagonist naltrexone facilitates closure of full-thickness wounds in diabetic rats

A major problem associated with diabetes is the complication of chronic non-healing wounds that can lead to the formation of debilitating ulcers, and can progress to more serious problems including amputation. There is no fully effective prevention of these complications, constituting an unmet medical need to understand the pathophysiology and treatment of wound healing in diabetes. This study determined whether blockade of opioid receptors from opioid peptides, known to inhibit cell proliferation and be overexpressed in diabetes, by topical application of the opioid antagonist naltrexone (NTX) reverses delays in wound closure. Rats with streptozotocin-induced type 1 diabetes (T1D) received topical applications of NTX (10(-4)-10(-6) mol/L) or vehicle in a variety of carriers; DNA synthesis was evaluated 12 h later. DNA synthesis in the epithelium of T1D rats was significantly reduced from normal animals. Both systemic and topical application of NTX increased DNA synthesis (up to 2-fold higher) within 12 h of administration. In a second study, diabetic and normal rats received full-thickness cutaneous wounds and were treated three times daily with either 10(-5) mol/L NTX or vehicle in topical carriers. Wound sizes were analyzed, and BrdU (5-bromo-2'-deoxyuridine) labeling in the skin was evaluated to determine DNA synthesis. Application of NTX in a variety of carriers to rats with full-thickness wounds resulted in significantly smaller wound areas relative to T1D animals receiving vehicle, and comparable to that of normal rats. Wound contraction in T1D animals was 50% of that in normal rats, with NTX-treated wounds restoring wound contraction to that of normal cohorts. DNA synthesis was also enhanced in NTX-treated T1D animals compared with T1D vehicle controls. These data suggest that topical application of NTX is a non-toxic and efficacious facilitator for healing full thickness wounds in T1D, with wound contraction serving as a particular target of NTX action.

Regulated proenkephalin expression in human skin and cultured skin cells

Skin responds to environmental stressors via coordinated actions of the local neuroimmunoendocrine system. Although some of these responses involve opioid receptors, little is known about cutaneous proenkephalin expression, its environmental regulation, and alterations in pathology. The objective of this study was to assess regulated expression of proenkephalin in normal and pathological skin and in isolated melanocytes, keratinocytes, fibroblasts, and melanoma cells. The proenkephalin gene and protein were expressed in skin and cultured cells, with significant expression in fibroblasts and keratinocytes. Mass spectroscopy confirmed Leu- and Met-enkephalin in skin. UVR, Toll-like receptor (TLR)4, and TLR2 agonists stimulated proenkephalin gene expression in melanocytes and keratinocytes in a time- and dose-dependent manner. In situ Met/Leu-enkephalin peptides were expressed in differentiating keratinocytes of the epidermis in the outer root sheath of the hair follicle, in myoepithelial cells of the eccrine gland, and in the basement membrane/basal lamina separating epithelial and mesenchymal components. Met/Leu-enkephalin expression was altered in pathological skin, increasing in psoriasis and decreasing in melanocytic tumors. Not only does human skin express proenkephalin, but this expression is upregulated by stressful stimuli and can be altered by pathological conditions.

Thrombin receptor: An endogenous inhibitor of inflammatory pain, activating opioid pathways

Serine proteases such as thrombin, trypsin and mast cell tryptase can act on different cell types through protease-activated receptors (PARs). These receptors have been shown to be implicated in several phenomena such as inflammation, platelet activation, immune response and atherosclerosis. Several studies recently reported PARs expression on neurons and some of them demonstrated that these receptors could interfere with nociception. The contribution of PAR(1) to inflammatory pain and the mechanism involved in this phenomenon were investigated. Intraplantar injection of PAR(1) agonist increased withdrawal latency and reduced response frequency to von Frey filaments, thus inhibiting nociceptive response to both mechanical and thermal stimuli in mice. PAR(1) agonist also reduced carrageenan-induced inflammatory hyperalgesia. The anti-nociceptive effects of PAR(1) agonist were mediated by endogenous opioids, as this effect was inhibited by local injection of naloxone methiodide, and because intraplantar injection of PAR(1) agonist increased mRNA expression of the endogenous opioid precursor proenkephalin. However, PAR(1) agonist was not able to inhibit calcium signals in isolated sensory neurons exposed to pro-nociceptive agents. Finally, despite similar inflammatory parameters, PAR(1)-deficient mice showed a strong potentiation of inflammatory hyperalgesia induced by the intraplantar injection of either formalin or carrageenan, or in the chronic model of collagen-induced arthritis, compared to wild-type mice. This study highlights a previously unknown endogenous mechanism of analgesia, showing a central role for the thrombin receptor PAR(1) in the regulation of inflammatory pain and as an activator of opioid pathways.

Peripheral mechanisms of pain and analgesia

This review summarizes recent findings on peripheral mechanisms underlying the generation and inhibition of pain. The focus is on events occurring in peripheral injured tissues that lead to the sensitization and excitation of primary afferent neurons, and on the modulation of such mechanisms. Primary afferent neurons are of particular interest from a therapeutic perspective because they are the initial generator of noxious impulses traveling towards relay stations in the spinal cord and the brain. Thus, if one finds ways to inhibit the sensitization and/or excitation of peripheral sensory neurons, subsequent central events such as wind-up, sensitization and plasticity may be prevented. Most importantly, if agents are found that selectively modulate primary afferent function and do not cross the blood-brain-barrier, centrally mediated untoward side effects of conventional analgesics (e.g. opioids, anticonvulsants) may be avoided. This article begins with the peripheral actions of opioids, turns to a discussion of the effects of adrenergic co-adjuvants, and then moves on to a discussion of pro-inflammatory mechanisms focusing on TRP channels and nerve growth factor, their signaling pathways and arising therapeutic perspectives.

Deletion of delta-opioid receptor in mice alters skin differentiation and delays wound healing

In addition to their well-known antinociceptive action, opioids can modulate non-neuronal functions, such as immune activity and physiology of different cell types. Several findings suggest that the delta-opioid receptor (DOR) and its endogenous ligands (enkephalins) are important players in cell differentiation and proliferation. Here we show the expression of DOR in mouse skin and human skin cultured fibroblasts and keratinocytes using RT-PCR. In DOR knock-out (KO) mice, a phenotype of thinner epidermis and higher expression of cell differentiation marker cytokeratin 10 (CK 10) were observed compared with wild type (WT). Using a burn wound model, significant wound healing delay (about 2 days) and severe epidermal hypertrophy were shown at the wound margin of DOR KO mice. This wound healing delay was further investigated by immunohistochemistry using markers for proliferation, differentiation, re-epithelialization, and dermal repair (CK 6, CK 10, and collagen IV). The levels of all these markers were increased in wounds of KO mice compared with WT. During the wound healing, the epidermal thickness in KO mice augments faster and exceeds that of the WT by day 3. These results suggest an essential role of DOR in skin differentiation, proliferation, and migration, factors that are important for wound healing.

Opioids and differentiation in human cancer cells

This study was designed to examine the role of opioids on cell differentiation, with an emphasis on the mechanism of opioid growth factor (OGF, [Met5]-enkephalin)-dependent growth inhibition. Three human cancer cell lines (SK-N-SH neuroblastoma and SCC-1 and CAL-27 squamous cell carcinoma of the head and neck), along with OGF and the opioid antagonist naltrexone (NTX) at a dosage (10(-6) M) known to repress or increase, respectively, cell replication, were utilized. The effects on differentiation (neurite formation, process lengths, betaIII-tubulin, involucrin) were investigated in cells exposed to OGF or NTX for up to 6 days. In addition, the influence of a variety of other natural and synthetic opioids on differentiation was examined. OGF, NTX, naloxone, [D-Pen2,5]-enkephalin, dynorphin A1-8, beta-endorphin, endomorphin-1 and -2, [D-Ala2, MePhe4, Glycol5]-enkephalin (DAMGO), morphine, and U69,593 at concentrations of 10(-6) M did not alter cell differentiation of any cancer cell line. In NTX-treated SK-N-SH cells, cellular area was increased 23%, and nuclear area was decreased 17%, from control levels; no changes in cell or nuclear area were recorded in OGF-exposed cells. F-actin concentration was increased 40% from control values in SK-N-SH cells subjected to NTX, whereas alpha-tubulin was decreased 53% in OGF-treated cells. These results indicate that the inhibitory or stimulatory actions of OGF and NTX, respectively, on cell growth in tissue culture are not due to alterations in differentiation pathways. However, exposure to OGF and NTX modified some aspects of cell structure, but this was independent of differentiation. The absence of effects on cancer cell differentiation by a variety of other opioids supports the previously reported lack of growth effects of these compounds.

Expression of CD13/aminopeptidase N and CD10/neutral endopeptidase on cultured human keratinocytes

Keratinocytes actively participate in immune response and inflammation by secreting cytokines and chemokines. Membrane-bound peptidases serve as negative loop in controlling concentration of peptide signalling molecules. Recently, they have also been proposed as additional mechanism of cell-to-cell interaction and as signalling molecules. In this study, we examined expression of two membrane-bound peptidases: aminopeptidase N (APN; EC 3.4.11.2; CD13) and neutral endopeptidase (NEP; EC 3.4.24.11; CD10) on nonstimulated cultured human keratinocytes obtained from healthy skin. Membrane expression of CD13 and CD10 was analysed by FACS and fluorescent microscope. Functional properties of CD13 and CD10 were examined by testing their enzymatic activity towards selective substrates. The data were compared to those obtained on cultured nonstimulated human skin fibroblasts expressing both CD13/APN and CD10/NEP. Approximately one-third (i.e. 31.7+/-2.8%; n=3) of cultured keratinocyte express CD13 as compared to fibroblasts which are 100% CD13(+) (n=3). Density of CD13 on keratinocytes is several times lower than on fibroblasts. Membrane CD13 expression on keratinocytes was associated with significant enzyme activity, which on the basis of substrate (L-Ala-betaNA) and inhibitor (bestatin, actinonin) selectivity could be ascribed to aminopeptidase N. Kinetic parameter V(max) revealed lower APN activity expressed on keratinocytes than on fibroblasts (V(max)=1.49+/-0.08 microM/60 min/5 x 10(4) cells for keratinocytes, n=3 versus V(max)=4.09+/-0.76 microM/60 min/5 x 10(4) cells for fibroblasts, n=3). Likewise, K(m) value of APN on keratinocytes was lower as compared to fibroblasts (K(m)=0.307+/-0.090 mM for keratinocytes, n=3 versus K(m)=0.766+/-0.065 mM for fibroblasts, n=3). CD13 demonstrated on cultured keratinocytes, is at least partly due to its constitutive expression since it was also found on freshly prepared epidermal skin cells. Inhibitors of APN, actinonin, bestatin and substance-P, as well as the APN blocking antibody WM-15, decreased keratinocytes growth. In contrast to membrane CD13 associated with APN enzyme activity, neither membrane CD10, nor its enzyme (NEP) activity could be found on the same keratinocyte samples. In conclusion, functional CD13, associated with APN activity, was found on about one third of cultured, non-stimulated keratinocytes, whereas no CD10/NEP was found on the same keratinocyte samples. Role of APN in regulation of keratinocyte growth is suggested, as its inhibition resulted in decreased keratinocyte growth.

Efficacy of naltrexone on acetylcholine-induced alloknesis in atopic eczema

Atopic eczema (AE) is a chronically pruritic inflammatory skin disease. Although the mediators and exact mechanisms eliciting and sustaining pruritus are not completely known, AE patients in clinical trials have been shown to benefit under treatment with morphine antagonists. Naltrexone (NAL) is a relatively pure morphine antagonist that blocks the effects of opioids twice as much as naloxone. NAL exhibits minimal pharmacological activity and displaces endorphines at mu- and kappa-receptors without its own intrinsic activity. NAL's excellent oral bioavailability and linear increases in the area under plasma concentration-time curve make it ideal for use in experimental studies. We designed our present experiments similar to former experiments evaluating both peripheral cutaneous sensations and central itch procession in order to gain more information about the possible distribution of opioid receptors and their involvement in the pathophysiology of pruritus. Eleven AE patients participated in our double-blind study. Either 25 mg of NAL (Nemexin) or a placebo (PLA) was given to the participants 60 min prior to the acetylcholine (ACH) injection [intracutaneous (i.c.) injection of 0.02 ml of 0.55 M]. A PLA stimulus with buffered saline served as control on the opposite forearm. We used laser Doppler flowmetry to measure the vasomotoric changes after ACH injection and recorded the duration and intensity of itch with a visual analogue scale (VAS). Following the evaluation of wheal and flare sensation, we obtained the area of itchy skin around the injection site (alloknesis) by gently stroking the surrounding skin with a brush in the centripetal direction towards the injection site. The results were planimetrically evaluated. Oral NAL reduced the perifocal itch significantly (P < 0.009). In four of our observations the area of alloknesis completely disappeared. Itch duration was reduced by 20 s and the intensity of itch was diminished, yet not significantly. NAL had no significant effects on cholinergic vasoreactions measured by the laser Doppler (P > 0.50) and especially failed to decrease the initial flux response, which is a typical sign of an altered vascular reaction (P > 0.25). The decrease of wheal (P = 0.008) and flare (P = 0.01) extension indicates an appropriate dosage of our treatment for this experiment. The most significant effects of NAL were observed in parameters of itch processing such as alloknesis (P = 0.009) and flare extension (P = 0.01). Therefore we favour the concept that NAL might have a stronger impact on central nervous mechanisms than on peripheral nociceptive structures.

Cell-density-regulated chemotactic responsiveness of keratinocytes in vitro

Keratinocytes represent the main constituents of the epidermis and have been found to play a regulatory role in a variety of inflammatory skin diseases. The functional activity of keratinocytes is highly heterogeneous, and depends on the cell localization in the epidermal architecture, and the maturation or differentiation state of the cells. Spontaneously proliferating HaCaT cells, showing several similarities to basal epidermal keratinocytes, were found to respond to external chemoattractants, including the chemokines RANTES (regulated on activation normal T cell expressed and secreted) and interleukin-8 and the mu-opioid agonist DAMGO ([d-ala2, N-Me-Phe4, Gly-ol5]enkephalin) in migration assays. The chemotactic responsiveness was highly dependent on the cell density of the monolayer, with greatest chemotactic activity at the highest cell density. Whereas RANTES was found to be the most potent chemoattractant, constitutive RANTES production was also detected in the HaCaT cultures. We found an inverse correlation between constitutive RANTES production and chemotactic responsiveness toward external RANTES, suggesting a possible functional down-modulation of the RANTES receptors, CC chemokine receptor 1 and CC chemokine receptor 5, during culture. Results from confocal laser scanning microscopy showed reduced CC chemokine receptor 1, but not CC chemokine receptor 5, expression by HaCaT cells at low cell densities, which was abolished in the presence of neutralizing antibodies against RANTES. The total CC chemokine receptor 1 pool (surface and intracellular receptors), however, showed no significant change during in vitro culture. Chemotactic responsiveness toward RANTES was directly correlated with the level of CC chemokine receptor 1 surface expression. Taken together these results show that with keratinocyte proliferation and the progressive increase in cell density there are dramatic alterations in keratinocyte function.

Molecular characterization and distribution of the opioid growth factor receptor (OGFr) in mouse

The native opioid growth factor (OGF), [Met(5)]-enkephalin, is a tonic inhibitory peptide that modulates cell proliferation and tissue organization during development, cancer, cellular renewal, wound healing, and angiogenesis. OGF action is mediated by a receptor mechanism. The receptor for OGF, OGFr, has been cloned and sequenced in humans and rats. Using primers based on the rat OGFr cDNA, and a mouse embryo expressed sequence tag, the full-length 2.1 kb mouse OGFr cDNA was sequenced. The open reading frame was found to encode a protein of 634 amino acids, and 14 imperfect repeats of 9 amino acids each were a prominent feature. The molecular weight of OGFr was calculated as 70679, and the isoelectric point was 4.5. Northern blot analysis revealed a 2.1 kb OGFr mRNA transcript in adult mouse brain, heart, lung, liver, kidney, and triceps surae muscle. The amino acids for mouse and rat OGFr were 93% similar and 91% identical, but the mouse and human shared only a 70% similarity and a 58% identity. These results emphasize the molecular validity of OGFr, and explain the interaction of OGF with respect to normal and abnormal growth in mouse cells and tissues.

Neuroendocrinology of the skin

The classical observations of the skin as a target for melanotropins have been complemented by the discovery of their actual production at the local level. In fact, all of the elements controlling the activity of the hypothalamus-pituitary-adrenal axis are expressed in the skin including CRH, urocortin, and POMC, with its products ACTH, alpha-MSH, and beta-endorphin. Demonstration of the corresponding receptors in the same cells suggests para- or autocrine mechanisms of action. These findings, together with the demonstration of cutaneous production of numerous other hormones including vitamin D3, PTH-related protein (PTHrP), catecholamines, and acetylcholine that share regulation by environmental stressors such as UV light, underlie a role for these agents in the skin response to stress. The endocrine mediators with their receptors are organized into dermal and epidermal units that allow precise control of their activity in a field-restricted manner. The skin neuroendocrine system communicates with itself and with the systemic level through humoral and neural pathways to induce vascular, immune, or pigmentary changes, to directly buffer noxious agents or neutralize the elicited local reactions. Therefore, we suggest that the skin neuroendocrine system acts by preserving and maintaining the skin structural and functional integrity and, by inference, systemic homeostasis.

A delta opioid receptor lacking the third cytoplasmic loop is generated by atypical mRNA processing in human malignomas

delta Opioid receptors were identified in human melanomas by RT-PCR and radioligand binding. In all tumors an additional PCR amplificate was detected in which 144 bp within the third exon were deleted. This fragment corresponded to the third cytoplasmic domain of the receptor protein. The short variant resulted from atypical mRNA processing. There were no common splice recognition sequences around the deleted fragment; instead its excision resembled the removal of a transposon. The deletion was not detected in normal human melanocytes nor in human or rat brain. However, it was present in a human neuroblastoma cell line (SH-SY5Y). Thus, it appears that the occurrence of the short delta opioid receptor is correlated to malignancy.

The opioid growth factor, [Met5]-enkephalin, inhibits DNA synthesis during recornification of mouse tail skin

Opioid peptides serve as tonically active negative growth regulators in renewing and regenerating epithelia. To examine the involvement of opioids in renewal of the stratum corneum after tape stripping of tail skin, C57BL/6 J mice were given systemic injections of the potent opioid antagonist, naltrexone (NTX, 20 mg/kg i.p.) following injury. Blockade of opioid-receptor interaction by NTX for 4 h resulted in an elevation of 36-66% in basal cell DNA synthesis measured 24 h after injury. Injection of the endogenous opioid peptide, [Met5]-enkephalin (OGF, 10 mg/kg i.p.) 4 h before termination, suppressed radiolabelled thymidine incorporation in the basal cell layer by 37-46% at 24 h after wounding. The magnitude of the effects on DNA synthesis of OGF, but not NTX, depended on the timing of administration with respect to injury. OGF maximally depressed basal cell labelling (72%) when given 16 h after tape stripping. Concomitant administration of naloxone (10 mg/kg) with OGF blocked the inhibition of DNA synthesis; naloxone alone at the dosage utilized had no effect on cell labelling. Both OGF and its receptor, OGFr, were detected by immunocytochemistry in the basal and suprabasal cell layers, but not the cornified layer of tape stripped and uninjured tail skin. These results indicate: (a) a native opioid peptide and its receptor are expressed in epidermal cells of injured and uninjured mouse tail skin; (b) removal of the stratum corneum by tape stripping does not disrupt the function of the endogenous opioid growth system; (c) the proliferative response to wounding of the tail is tonically inhibited by the receptor-mediated action of an endogenous opioid peptide; and (d) DNA synthesis by basal cells can be elevated by disrupting opioid peptide receptor interactions.

beta-endorphin stimulates cytokeratin 16 expression and downregulates mu-opiate receptor expression in human epidermis

It has been reported that opioid peptides modulate the differentiation of normal human keratinocytes and that mu-opiate receptors are expressed in human epidermis. The regulation of keratinocyte differentiation is particularly important in psoriasis, and one of the markers for hyperproliferative and differentiating skin diseases is cytokeratin 16. The finding that the endogenous mu-opiate receptor ligand beta-endorphin is increased in serum of patients with psoriasis indicates that the mu-opiate system may play an important role in the pathophysiology of the skin. In this study, we addressed the question whether there is a link between mu-opiate receptor regulation and cytokeratin 16 expression in normal and psoriatic skin. Firstly, we demonstrate that beta-endorphin concentrations between 16 and 1000 nM significantly downregulate mu-opiate receptor expression in epidermis of cultured human skin after 48 h. Secondly, we show that beta-endorphin regulates cytokeratin 16 expression in the epidermis of skin organ cultures exposed to 41-125 nM beta-endorphin for 48 h, leading to elevated cytokeratin 16 production. As expected, the expression of cytokeratin 16 was detected primarily in the suprabasal layer. The same pattern was observed in psoriatic lesional skin, i.e., mu-opiate receptor expression was significantly downregulated and cytokeratin 16 expression upregulated. These results suggest that the mu-opiate receptor system and its ligand beta-endorphin are involved in the pathogenesis of psoriasis, especially in terms of differentiation.

Opioid growth factor and organ development in rat and human embryos

In addition to neurotransmission, the native opioid peptide, [Met5]enkephalin, is a tonically active inhibitory growth molecule that is termed opioid growth factor (OGF). OGF interacts with the zeta (zeta) opioid receptor to influence cell proliferation and tissue organization. We now identify OGF and the zeta receptor in embryonic derivatives including ectoderm, mesoderm, and endoderm of the rat on gestation day 20. Messenger RNA for preproenkephalin (PPE), the precursor of OGF, was detected in the developing cells, suggesting an autocrine production of this peptide. Acute exposure of the pregnant female to OGF resulted in a decrease in DNA synthesis in cells of organs representing all three germ layers, and did so in a receptor-mediated fashion. The influence of OGF was direct, as evidenced in organ culture studies. Blockade of endogenous opioid interaction using naltrexone (NTX) produced an increase in DNA synthesis, indicating the constitutive and functional nature of opioid activity on growth during prenatal life. Human fetal cells contained OGF and the zeta receptor. These data support the hypothesis that endogenous opioid modulation of organ development is a fundamental principle of mammalian embryogenesis, and that OGF has a profound influence on ontogeny. Irregularities in the role of opioids as growth regulators in relationship to the more than 500,000 newborns suffering from birth defects each year in the US needs to be examined.

Increased levels of enkephalin following natural sunlight (combined with salt water bathing at the Dead Sea) and ultraviolet A irradiation

The opioid peptides enkephalins have been shown to modulate inflammatory responses and keratinocyte proliferation and differentiation. Furthermore, increased levels of enkephalin are present in psoriatic lesions. The purpose of the present study was to determine the effect of natural sunlight combined with salt water bathing in the Dead Sea on the methionine-enkephalin (e.n.k.) level in psoriatic skin. Ten patients were treated at the Dead Sea for 4 weeks, and keratotome biopsies were obtained before and after treatment. The amount of enkephalin extracted from the biopsies was measured by radioimmunoassay. Treatment at the Dead Sea resulted in a complete clinical clearance of psoriasis, and immunohistochemical stainings of lesional skin showed that the treatment decreased both epidermal thickness/parakeratosis and the dermal infiltration of CD3- and CD68-positive cells, although the number of CD3- and CD68-positive cells became normal in only two of the 10 cases. However, there was only a slight decrease in the mean enk levels (21%). Furthermore, the level of enk was high in non-lesional psoriatic skin after treatment at the Dead Sea, and immunostaining showed that, in some patients, the treatment induced a mild epidermal hyperplasia and a dermal infiltration of CD3- and CD68-positive cells. Enkephalin-like immunoreactivity was detected in the cytoplasm of both epidermal keratinocytes and dermal infiltrating cells. To determine whether the relatively high skin enk levels after treatment at the Dead Sea was caused by ultraviolet (UV) radiation, normal volunteers were exposed to a single dose of UVA and UVB (2 minimal erythema doses). UVA, but not UVB, irradiation stimulated the mean enk level in the irradiated skin by about sixfold. Furthermore, multiple whole-body UVA irradiations not only resulted in increased skin levels of enk, but also in increased plasma levels. In conclusion, natural sunlight combined with salt water bathing cleared psoriasis without causing a significant decrease in lesional enk levels. Furthermore, non-lesional enk levels were increased. These findings may be the result of a direct stimulatory effect of UVA irradiation on enk formation in the skin. It is possible that the increased circulating levels of enk after UV exposure may contribute to the beneficial effects of UVA irradiation.