Itch-scratch responses induced by lysophosphatidic acid in mice

Sodium/bile acid co-transporter inhibitors currently in preclinical or early clinical development for the treatment of primary biliary cholangitis

INTRODUCTION

Pruritus is common and often undertreated in patients with primary biliary cholangitis (PBC). Existing treatments largely have an aging and low-quality evidence base, and studies included only small numbers of patients. More recent data that has added to our understanding of pruritus treatments has often come from clinical trials where itching was a secondary outcome measure in a trial designed primarily to assess disease-modifying agents. This area represents an unmet clinical need in the management of PBC.

AREAS COVERED

In this manuscript, we first summarize the proposed mechanisms for PBC-related pruritus and the current treatment paradigm. We then present an appraisal of the existing pre-clinical and clinical evidence for the use of ileal bile acid transporter inhibitors (IBATis) for this indication in PBC patients.

EXPERT OPINION

Evidence for the efficacy of IBATis is promising but limited by the currently available volume of data. Furthermore, larger clinical trials with long-term data on efficacy, safety and tolerability are needed to confirm the role of using IBATis in clinical practice and their place on the itch treatment ladder. Additional focus should also be given to exploring the disease-modifying potential of IBATis in PBC.

Emerging roles of lysophosphatidic acid receptor subtype 5 (LPAR5) in inflammatory diseases and cancer

Lysophosphatidic acid (LPA) is a bioactive lipid mediator that regulates a variety of cellular functions such as cell proliferation, migration, survival, calcium mobilization, cytoskeletal rearrangements, and neurite retraction. The biological actions of LPA are mediated by at least six G protein-coupled receptors known as LPAR1-6. Given that LPAR1-3 were among the first LPARs identified, the majority of research efforts have focused on understanding their biology. This review provides an in-depth discussion of LPAR5, which has recently emerged as a key player in regulating normal intestinal homeostasis and modulating pathological conditions such as pain, itch, inflammatory diseases, and cancer. We also present a chronological overview of the efforts made to develop compounds that target LPAR5 for use as tool compounds to probe or validate LPAR5 biology and therapeutic agents for the treatment of inflammatory diseases and cancer.

Schwann cell stimulation induces functional and structural changes in peripheral nerves

Signal propagation is the essential function of nerves. Lysophosphatidic acid 18:1 (LPA) allows the selective stimulation of calcium signaling in Schwann cells but not neurons. Here, the time course of slowing and amplitude reduction on compound action potentials due to LPA exposure was observed in myelinated and unmyelinated fibers of the mouse, indicating a clear change of axonal function. Teased nerve fiber imaging showed that Schwann cell activation is also present in axon-attached Schwann cells in freshly isolated peripheral rat nerves. The LPA receptor 1 was primarily localized at the cell extensions in isolated rat Schwann cells, suggesting a role in cell migration. Structural investigation of rat C-fibers demonstrated that LPA leads to an evagination of the axons from their Schwann cells. In A-fibers, the nodes of Ranvier appeared unchanged, but the Schmidt-Lanterman incisures were shortened and myelination reduced. The latter might increase leak current, reducing the potential spread to the next node of Ranvier and explain the changes in conduction velocity. The observed structural changes provide a plausible explanation for the functional changes in myelinated and unmyelinated axons of peripheral nerves and the reported sensory sensations such as itch and pain.

Mechanisms of pruritus in cholestasis: understanding and treating the itch

Pruritus in cholestatic liver diseases can be a major burden and dramatically impair the quality of life of those affected. Here, we provide an update on the latest insights into the molecular pathogenesis of and novel therapeutic approaches for cholestasis-associated itch. Endogenous and exogenous small-molecule pruritogen candidates bind to their receptors on unmyelinated itch C-fibres in the skin. Candidate pruritogens in cholestasis include certain lysophospholipids and sulfated progesterone metabolites, among others, whereas total bile acid or bilirubin conjugates seem unlikely to have a dominant role in the pathogenesis of cholestasis-associated pruritus. Transmission of itch signals via primary, secondary and tertiary itch neurons to the postcentral gyrus and activation of scratch responses offer various targets for therapeutic intervention. At present, evidence-based treatment options for pruritus in fibrosing cholangiopathies, such as primary biliary cholangitis and primary sclerosing cholangitis, are the peroxisome proliferator-associated receptor (PPAR) agonist bezafibrate and the pregnane X receptor (PXR) agonist rifampicin. In pruritus of intrahepatic cholestasis of pregnancy, ursodeoxycholic acid is recommended and might be supported in the third trimester by rifampicin if needed. Alternatively, non-absorbable anion exchange resins, such as cholestyramine, can be administered, albeit with poor trial evidence. Liver transplantation for intolerable refractory pruritus has become an extremely rare therapeutic strategy.

Generalized resistance to pruritogen-induced scratching in the C3H/HeJ strain

Previously the effect of the pruritogens, such as histamine and chloroquine, was tested in 11 inbred mouse strains, and this study aimed to identify resistant and sensitive strains, consistent with the observation that underlies the large variability in human populations. In the present study, we used the low responder C3H/HeJ (C3H) and the more sensitive C57BL/6J (C57) strain to find out if resistance and sensitivity to develop pruritus is restricted to only histamine and chloroquine or extends to other known pruritogens as well. We tested five additional commonly known pruritogens. We established dose-response relationships by injecting four concentrations of the pruritogens in the range of 0.3, 1, 3, and ten-fold in the nuchal fold. Then we assessed the scratching behavior for 30 min after injection with an automated custom-designed device based on the bilateral implantation of mini-magnets in the hind paws and on single cages placed within a magnetic coil. We found that the resistance to pruritogens is a general phenotype of the C3H strain and extends to all pruritogens tested, including not only histamine and chloroquine, but also endothelin, trypsin, 5-HT (serotonin), the short peptide SLIGRL, and Lysophosphatidic acid (LPA). C57 was more sensitive to all pruritogens and, in contrast to C3H, dose-response relationships were evident for some of the pruritogens. In general, comparable peak scratch responses were observed for the 0.3-fold concentrations of the pruritogens in C57 whereas C3H required at least the ten-fold concentration and still displayed only between 5 and 33% of the scratch responses observed in C57 for the respective pruritogen. The general resistance to pruritogens and the low level of scratching behavior found in the C3H strain is an interesting trait and represents a model for the study of the heritability of itch. It is accompanied in C3H with a higher sensitivity in assays of nociception.

Serum autotaxin levels correlate with the severity of pruritus in intrahepatic cholestasis of pregnancy

PURPOSE

This study aimed to examine autotaxin (ATX) concentrations in the serum of pregnant women complicated with intrahepatic cholestasis of pregnancy (ICP) and compare them with individuals with uncomplicated healthy pregnancies.

METHODS

This prospective case-control study took place with 83 pregnant women. The study group included 43 pregnant women presenting with a singleton pregnancy diagnosed with ICP in their third trimester of pregnancy. The diagnostic power of the ATX variable was examined by receiver operating characteristic analysis, and the cut-off value calculated according to the Youden index was summarized with the related sensitivity and specificity points.

RESULTS

The mean serum concentration of maternal ATX was significantly higher in the ICP cases (8.91 ± 2.69 pg/mL) compared to the pregnant women in the control group (3.59 ± 1.39 ng/mL, p < 0.001). According to the Youden index, a 5.80 ng/mL cut-off value of serum ATX concentrations can be used to diagnose ICP with 97.7% sensitivity and 97.5% specificity. A significant highly positive correlation was found between maternal serum ATX levels and maternal serum total bile acid levels (r = 0.633 and p < 0.001) and itch intensity, which was objectified by the visual analog scale score (r = 0.951 and p < 0.001).

CONCLUSION

Maternal serum ATX levels were significantly increased in ICP patients as compared with healthy pregnant women. Also, serum ATX activity was highly correlated with the itch intensity. We consider that ATX might represent a robust, accurate, and reliable circulating biomarker to diagnose ICP.

An LPAR5-antagonist that reduces nociception and increases pruriception

Introduction

The G-protein coupled receptor LPAR₅ plays a prominent role in LPA-mediated pain and itch signaling. In this study we focus on the LPAR₅-antagonist compound 3 (cpd3) and its ability to affect pain and itch signaling, both in vitro and in vivo.

Methods

Nociceptive behavior in wild type mice was induced by formalin, carrageenan or prostaglandin E2 (PGE₂) injection in the hind paw, and the effect of oral cpd3 administration was measured. Scratch activity was measured after oral administration of cpd3, in mice overexpressing phospholipase A2 (sPLA2tg), in wild type mice (WT) and in TRPA1-deficient mice (Trpa1 KO). In vitro effects of cpd3 were assessed by measuring intracellular calcium release in HMC-1 and HEK-TRPA1 cells.

Results

As expected, nociceptive behavior (induced by formalin, carrageenan or PGE₂) was reduced after treatment with cpd3. Unexpectedly, cpd3 induced scratch activity in mice. In vitro addition of cpd3 to HEK-TRPA1 cells induced an intracellular calcium wave that could be inhibited by the TRPA1-antagonist A-967079. In Trpa1 KO mice, however, the increase in scratch activity after cpd3 administration was not reduced.

Conclusions

Cpd3 has in vivo antinociceptive effects but induces scratch activity in mice, probably by activation of multiple pruriceptors, including TRPA1. These results urge screening of antinociceptive candidate drugs for activity with pruriceptors.

Lysophosphatidic acid activates nociceptors and causes pain or itch depending on the application mode in human skin

ABSTRACT

Lysophosphatidic acid (LPA) is involved in the pathophysiology of cholestatic pruritus and neuropathic pain. Slowly conducting peripheral afferent C-nerve fibers are crucial in the sensations of itch and pain. In animal studies, specialized neurons ("pruriceptors") have been described, expressing specific receptors e.g. from the Mrgpr family. Human nerve fibers involved in pain signaling ("nociceptors") can elicit itch if activated by focalized stimuli such as cowhage spicules.In this study, we scrutinized the effects of LPA in humans by two different application modes on the level of psychophysics and single nerve fiber recordings (microneurography). In healthy human subjects, intracutaneous LPA microinjections elicited burning pain, whereas LPA application via inactivated cowhage spicules evoked a moderate itch sensation. LPA microinjections induced heat hyperalgesia and hypersensitivity to higher electrical stimulus frequencies. Pharmacological blockade of TRPA1 or TRPV1 reduced heat hyperalgesia but not acute chemical pain. Microneurography revealed an application mode-dependent differential activation of mechano-sensitive (CM) and mechano-insensitive (CMi) C-fibers. LPA microinjections activated a greater proportion of CMi and more strongly than CM fibers; spicule-application of LPA activated CM and CMi fibers to a similar extent but excited CM more and CMi fibers less intensely than microinjections.In conclusion, we show for the first time in humans that LPA can cause pain as well as itch dependent on the mode of application and activates afferent human C-fibers. Itch may arise from focal activation of few nerve fibers with distinct spatial contrast to unexcited surrounding afferents, and a specific combination of activated fiber subclasses might contribute.

Cholestasis-Associated Pruritus and Its Pruritogens

Pruritus is a debilitating symptom of various cholestatic disorders, including primary biliary cholangitis (PBC), primary sclerosing cholangitis (PSC) and inherited progressive familial intrahepatic cholestasis (PFIC). The molecular mechanisms leading to cholestasis-associated pruritus are still unresolved and the involved pruritogens are indecisive. As a consequence of pruritus, patients suffer from sleep deprivation, loss of daytime concentration, auto-mutilation and sometimes even suicidal ideations. Current guideline-approved therapy of cholestasis-associated pruritus includes stepwise administration of several medications, which may alleviate complaints in some, but not all affected patients. Therefore, also experimental therapeutic approaches are required to improve patients' quality of life. This article reviews the current state of research on pruritogens and their receptors, and shortly discusses the most recent experimental therapies.

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.

Neural Mechanisms of Itch

Itch is a unique sensation that helps organisms scratch away external threats; scratching itself induces an immune response that can contribute to more itchiness. Itch is induced chemically in the peripheral nervous system via a wide array of receptors. Given the superficial localization of itch neuron terminals, cells that dwell close to the skin contribute significantly to itch. Certain mechanical stimuli mediated by recently discovered circuits also contribute to the itch sensation. Ultimately, in the spinal cord, and likely in the brain, circuits that mediate touch, pain, and itch engage in cross modulation. Much of itch perception is still a mystery, but we present in this review the known ligands and receptors associated with itch. We also describe experiments and findings from investigations into the spinal and supraspinal circuitry responsible for the sensation of itch.

Pruritus secondary to primary biliary cholangitis: a review of the pathophysiology and management with phototherapy

BACKGROUND

Primary biliary cholangitis (PBC) is an autoimmune hepatobiliary disorder characterized by destruction of liver bile ducts leading to intrahepatic cholestasis. It causes intractable pruritus for which ultraviolet (UV)B phototherapy is an experimental treatment when alternative therapies fail. The pathophysiology of cholestatic itch and the mechanism of action of narrowband UVB in this condition remains poorly understood.

OBJECTIVES

To summarize the current literature and propose testable hypotheses for the mechanism of action of phototherapy in attenuating itch.

METHODS

A focused PubMed search for articles relating to the pathogenesis of itch in cholestatic disease was performed. A total of 3855 articles were screened and 50 were found suitable for literature review. Evidence from this literature review was combined with author expertise in the area.

RESULTS

Formulated hypotheses focus on the role of bile salts, autotaxin and specific receptors including G-protein-coupled bile acid receptor, Gpbar1 (also known as TGR5) and the nuclear transcription factor farnesoid X receptor.

CONCLUSIONS

Several testable mechanisms through which phototherapy may exert its effects are discussed in this review. The next steps are to carry out an objective assessment of the efficacy of phototherapy in cholestatic pruritus, gain further knowledge on the underlying pathways, and subsequently trial its use against current licensed therapies. Such studies could lead to increased mechanistic understanding, identification of novel therapeutic targets and the potential to refine phototherapy protocols, leading to improved control of itch and quality of life in patients with PBC. What's already known about this topic? Primary biliary cholangitis (PBC) is frequently associated with intractable pruritus for which current treatment options are often unsuccessful. Phototherapy is used as an experimental treatment for PBC-associated pruritus when alternative better-studied treatments fail. What does this study add? This study reviews the current literature on the pathophysiology and management of cholestatic pruritus, an area which remains poorly understood. We propose testable hypotheses of the mechanisms behind the attenuation of cholestatic pruritus with phototherapy.

Cholestatic pruritus: Emerging mechanisms and therapeutics

Patients suffering from cholestasis often report experiencing a debilitating, unrelenting itch. In contrast to conditions, such as urticaria, in which histamine primarily drives itch (pruritus), cholestatic pruritus is multifactorial and more difficult to treat. Existing therapies are not always effective and have undesirable adverse effect profiles. Here, we conducted a systematic literature review to evaluate conventional treatment strategy, current pathophysiologic understanding, and the role of new therapies in the context of cholestatic pruritus. We discuss novel findings implicating bile acids, lysophosphatidic acid, and bilirubin as potential important mediators of cholestatic itch. New therapies that aim to remove or modulate pruritogens have been supported in observational cohort studies and randomized controlled trials. Although these new therapies show promise, further research is needed to confirm the pathophysiology of cholestatic pruritus so that targeted therapy can be developed.

Lysophosphatidic acid activates satellite glia cells and Schwann cells

Pruritus is a common and disabling symptom in patients with hepatobiliary disorders, particularly in those with cholestatic features. Serum levels of lysophosphatidic acid (LPA) and its forming enzyme autotaxin were increased in patients suffering from hepatic pruritus, correlated with itch severity and response to treatment. Here we show that in a culture of dorsal root ganglia LPA 18:1 surprisingly activated a large fraction of satellite glia cells, and responses to LPA 18:1 correlated inversely with responses to neuronal expressed transient receptor potential channels. LPA 18:1 caused only a marginal activation of heterologously expressed TRPV1, and responses in dorsal root ganglion cultures from TRPV1-deficient mice were similar to controls. LPA 18:1 desensitized subsequent responsiveness to chloroquine and TGR5 agonist INT-777. The LPA 18:1-induced increase in cytoplasmatic calcium stems from the endoplasmatic reticulum. LPA receptor expression in dorsal root ganglia and Schwann cells, LPAR1 immunohistochemistry, and pharmacological results indicate a signaling pathway through LPA receptor 1. Peripheral rat Schwann cells, which are of glial lineage as the satellite glia cells, were also responsive to LPA 18:1. Summarizing, LPA 18:1 primarily activates rather glial cells than neurons, which may subsequently modulate neuronal responsiveness and sensory sensations such as itch and pain.

The role of lysophosphatidic acid in the physiology and pathology of the skin

Lysophosphatidic acid (LPA) is the simplest phospholipid found in nature. LPA is mainly biosynthesized in tissues and cells by autotoxin and PA-PLA1α/PA-PLA1β and is degraded by lipid phosphate phosphatases (LPPs). It is an important component of biofilm, an extracellular signal transmitter and intracellular second messenger. After targeting to endothelial differentiation gene (Edg) family LPA receptors (LPA1, LPA2, LPA3) and non-Edg family LPA receptors (LPA4, LPA5, LPA6), LPA mediates physiological and pathological processes such as embryonic development, angiogenesis, tumor progression, fibrogenesis, wound healing, ischemia/reperfusion injury, and inflammatory reactions. These processes are induced through signaling pathways including mitogen-activated protein kinase (MAPK), phosphatidylinositol-3-kinase (PI3K)/Akt, protein kinase C (PKC)-GSK3β-β-catenin, Rho, Stat, and hypoxia-inducible factor 1-alpha (HIF-1α). LPA is involved in multiple physiological and pathological processes in the skin. It not only regulates skin function but also plays an important role in hair follicle development, skin wound healing, pruritus, skin tumors, and scleroderma. Pharmacological inhibition of LPA synthesis or antagonization of LPA receptors is a new strategy for the treatment of various skin disorders. This review focuses on the current understanding of the pathophysiologic role of LPA in the skin.

TRP Channels as Drug Targets to Relieve Itch

Although acute itch has a protective role by removing irritants to avoid further damage, chronic itch is debilitating, significantly impacting quality of life. Over the past two decades, a considerable amount of stimulating research has been carried out to delineate mechanisms of itch at the molecular, cellular, and circuit levels. There is growing evidence that transient receptor potential (TRP) channels play important roles in itch signaling. The purpose of this review is to summarize our current knowledge about the role of TRP channels in the generation of itch under both physiological and pathological conditions, thereby identifying them as potential drug targets for effective anti-itch therapies.

Itch and psyche: psychiatric aspects of pruritus

Itch, also referred to as pruritus, is an unpleasant cutaneous sensation provoking the desire to scratch. It is often an uncomfortable, subjective sensation responsible for decreased quality of life in a variety of psychodermatological conditions. Comorbid psychiatric conditions, including depression and anxiety, are frequently associated with itch and scratch cycle. The reciprocal and intricate relationship between the psyche and itch has been widely studied. The neurobiology of itch involves the complexity of specific mediators, itch-related neuronal pathways, and central processing of itch. The connection between itch and the psyche can be grouped under three headings: pruritic diseases with psychosocial sequelae, pruritic diseases aggravated by psychosocial factors, and psychiatric disorders causing pruritus. Itch and pain modulation go together in most circumstances and involve various substances including histamine, interleukins, protease-activated receptors, transient receptor potential receptors, opioids, and cannabinoids. The close interaction between keratinocytes and nerve endings modulating pain and itch also play a major role. Management of itch associated with its psychosomatic components is directed at an underlying cause and adopting a holistic approach to address not only dermatologic and somatosensory aspects, but also the cognitive, emotional, and psychosocial components. An integrated multidisciplinary team consisting of a dermatologist, psychiatrist, psychologist, and social worker is vital in addressing the multifaceted aspects of pruritus.

An update on the physiopathology and therapeutic management of cholestatic pruritus in children

Pruritus is a disabling symptom accompanying chronic cholestasis. In extreme cases, the refractory nature of pruritus can result in a need for invasive therapies including liver transplantation. The pathogenesis of pruritus in cholestatic disease is poorly understood. It may involve a specific neural pathway (similar to that associated with pain) regulated by several pruritogenic substances such as bile acids, opioids, serotonin, and the more recently identified lysophosphatidic acid. While the therapeutic management of cholestatic pruritus is well established in adults, there is no consensus in children, in light of the difficulty of conducting controlled clinical studies. The currently recommended strategy to manage cholestatic pruritus in children is based on several lines of specific therapies that should be associated with skin hydration and with non-specific treatment of cholestasis including ursodeoxycholic acid. Pruritus should be assessed as objectively as possible between each line of therapy. Rifampicin, a potent CYP3A4 inducer, is the first-line treatment of cholestatic pruritus. Second-line therapies require evaluation of the child in an expert center and are discussed on a case-by-case basis depending on the underlying disease and the experience of the center. These include inhibitors of serotonin reuptake (sertraline), opioid antagonists (naloxone), or ASBT inhibitors. Invasive therapies such as biliary diversion or liver transplantation can also be proposed in the most severe cases. The aim of the current update is to review the physiopathologic mechanisms implicated in cholestatic pruritus and to propose potential therapeutic strategies in children.

Lysophosphatidic acid-induced itch is mediated by signalling of LPA5 receptor, phospholipase D and TRPA1/TRPV1

KEY POINTS

Lysophosphatidic acid (LPA) is an itch mediator, but not a pain mediator by a cheek injection model. Dorsal root ganglion neurons directly respond to LPA depending on transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1). LPA-induced itch-related behaviours are decreased in TRPA1-knockout (KO), TRPV1KO or TRPA1TRPV1 double KO mice. TRPA1 and TRPV1 channels are activated by intracellular LPA, but not by extracellular LPA following LPA₅ receptor activation with an activity of Ca²⁺ -independent phospholipase A₂ and phospholipase D. Intracellular LPA interaction sites of TRPA1 are KK672-673 and KR977-978 (K: lysine, R: arginine).

ABSTRACT

Intractable and continuous itch sensations often accompany diseases such as atopic dermatitis, neurogenic lesions, uremia and cholestasis. Lysophosphatidic acid (LPA) is an itch mediator found in cholestatic itch patients and it induces acute itch and pain in experimental rodent models. However, the molecular mechanism by which LPA activates peripheral sensory neurons remains unknown. In this study, we used a cheek injection method in mice to reveal that LPA induced itch-related behaviours but not pain-related behaviours. The LPA-induced itch behaviour and cellular effects were dependent on transient receptor potential ankyrin 1 (TRPA1) and vanilloid 1 (TRPV1), which are important for itch signal transduction. We also found that, among the six LPA receptors, the LPA₅ receptor had the greatest involvement in itching. Furthermore, we demonstrated that phospholipase D (PLD) plays a critical role downstream of LPA₅ and that LPA directly and intracellularly activates TRPA1 and TRPV1. These results suggest a unique mechanism by which cytoplasmic LPA produced de novo could activate TRPA1 and TRPV1. We conclude that LPA-induced itch is mediated by LPA₅ , PLD, TRPA1 and TRPV1 signalling, and thus targeting TRPA1, TRPV1 or PLD could be effective for cholestatic itch interventions.

Hepatitis C virus-associated pruritus: Etiopathogenesis and therapeutic strategies

In addition to its contributing role in the development of chronic liver diseases, chronic hepatitis C virus (HCV) infection is associated with extrahepatic manifestations, particularly, cutaneous-based disorders including those with pruritus as a symptom. Pruritus is frequently associated with the development of chronic liver diseases such as cholestasis and chronic viral infection, and the accumulation of bile acids in patients’ sera and tissues as a consequence of liver damage is considered the main cause of pruritus. In addition to their role in dietary lipid absorption, bile acids can trigger the activation of specific receptors, such as the G protein-coupled bile acid receptor (GPBA/ TGR5). These types of receptors are known to play a crucial role in the modulation of the systemic actions of bile acids. TGR5 expression in primary sensory neurons triggers the activation of the transient receptor potential vanilloid 1 (TRPV1) leading to the induction of pruritus by an unknown mechanism. Although the pathologic phenomenon of pruritus is common, there is no uniformly effective therapy available. Understanding the mechanisms regulating the occurrence of pruritus together with the conduction of large-scale clinical and evidence-based studies, may help to create a standard treatment protocol. This review focuses on the etiopathogenesis and treatment strategies of pruritus associated with chronic HCV infection.

A systematic approach to the management of cholestatic pruritus in primary biliary cirrhosis

Pruritus (itch) is an important symptom of primary biliary cirrhosis (PBC), an archetypal cholestatic liver disease. Cholestatic pruritus can be a debilitating symptom causing significant deterioration in patients' quality of life. Effective management of pruritus in PBC involves awareness among clinicians to adequately assess its severity, and treatment with specific drug therapies in line with current practice guidelines. In PBC, antipruritic drugs are not universally effective and/or have significant side effects, and despite best efforts with various combinations of drugs, some patients remain significantly symptomatic, eventually opting for invasive or experimental treatments. Therefore, there is a clear unmet need for better alternative treatments for patients with refractory or intractable cholestatic pruritus. Recent advances in the understanding of pathogenesis of cholestatic pruritus and bile acid physiology have raised hopes for novel therapies, some of which are currently under trial. In this review, we aim to provide a practical guide to the management of this important and complex problem, discussing current knowledge and recent advances in the pathogenesis, summarise the evidence base for available therapeutic approaches and update potential novel future therapies for the management of pruritus in PBC.

Trp channels and itch

Itch is a unique sensation associated with the scratch reflex. Although the scratch reflex plays a protective role in daily life by removing irritants, chronic itch remains a clinical challenge. Despite urgent clinical need, itch has received relatively little research attention and its mechanisms have remained poorly understood until recently. The goal of the present review is to summarize our current understanding of the mechanisms of acute as well as chronic itch and classifications of the primary itch populations in relationship to transient receptor potential (Trp) channels, which play pivotal roles in multiple somatosensations. The convergent involvement of Trp channels in diverse itch signaling pathways suggests that Trp channels may serve as promising targets for chronic itch treatments.

Serum Autotaxin Activity Correlates With Pruritus in Pediatric Cholestatic Disorders

OBJECTIVE

Pruritus is a common symptom of cholestatic liver disorders. The present study aimed at evaluating autotaxin (ATX), a lysophospholipase recently identified as potential cause for cholestatic pruritus, in pediatric cholestatic diseases presenting with or without itching.

METHODS

A cohort of 45 children consisting of 14 patients experiencing itching (Alagille syndrome [n = 10], complete extrahepatic biliary atresia [n = 2], neonatal sclerosing cholangitis (n = 1), progressive familial intrahepatic cholestasis type 2 [n = 1]), 9 patients with bile acid synthesis defects (3β-hydroxy-C27-steroid-oxidoreductase [n = 7] and Δ-3-oxosteroid-5β-reductase deficiency [n = 2]), and 22 healthy children were studied. Serum ATX activity and total serum bile salt were determined enzymatically, ATX protein content was semiquantified by Western blotting. Using real-time polymerase chain reaction, ATX mRNA expression was studied in HepG2 cells treated with farnesoid-X-receptor agonists or vehicle.

RESULTS

Serum ATX activity was increased in pruritic children with Alagille and other cholestatic syndromes (mean ± standard deviation: 16.1 ± 4.3 nmol · mL · min) compared with children with nonpruritic cholestatic diseases with bile acid synthesis defects (10.4 ± 4.7 nmol · mL · min; P < 0.01) and healthy controls (7.6 ± 2.3 nmol · mL · min; P < 0.001). ATX protein levels closely correlated with serum ATX activity. Serum ATX activity and total serum bile salt showed a linear correlation with itch intensity (r = 0.66, P < 0.001 and r = 0.80, P < 0.001, respectively). No correlation was observed between ATX activity and bilirubin. ATX mRNA expression in HepG2 cells was not induced by farnesoid-X-receptor ligands.

CONCLUSIONS

Serum ATX activity correlated with itch intensity in children with cholestatic diseases. Bile salts did not increase ATX expression in vitro. ATX inhibitors may be useful antipruritic agents in pediatric cholestatic disorders.

Jaundice associated pruritis: A review of pathophysiology and treatment

To review the underlying pathophysiology and currently available treatments for pruritis associated with jaundice. English language literature was reviewed using MEDLINE, PubMed, EMBASE and clinicaltrials.gov for papers and trails addressing the pathophysiology and potential treatments for pruritis associated with jaundice. Recent advances in the understanding of the peripheral anatomy of itch transmission have defined a histamine stimulated pathway and a cowhage stimulated pathway with sensation conveyed centrally via the contralateral spinothalamic tract. Centrally, cowhage and histamine stimulated neurons terminate widely within the thalamus and sensorimotor cortex. The causative factors for itch in jaundice have not been clarified although endogenous opioids, serotonin, steroid and lysophosphatidic acid all play a role. Current guidelines for the treatment of itching in jaundice recommend initial management with biliary drainage where possible and medical management with ursodeoxycholic acid, followed by cholestyramine, rifampicin, naltrexone and sertraline. Other than biliary drainage no single treatment has proved universally effective. Pruritis associated with jaundice is a common but poorly understood condition for which biliary drainage is the most effective therapy. Pharmacological therapy has advanced but remains variably effective.

Lysophosphatidic acid and signaling in sensory neurons

Lysophosphatidic acid is a potent signaling lipid molecule that has initially been characterized as a growth factor. However, later studies have revealed many more functions such as modulation of cell shape, cell migration, prevention of apoptosis, platelet aggregation, wound healing, osteoclast differentiation, vasopressor activity, embryo implantation, angiogenesis, lung fibrosis, hair growth and more. The molecule mainly acts through the activation of a set of at least 6 G-protein-coupled receptors (LPA1-6), but intracellular LPA was also shown to signal through the activation of the nuclear receptor PPARγ. In this short review we discuss the recent observations which suggest that in pathological conditions LPA also modulates signaling in sensory neurons. Thus, LPA has been shown to play a role in the initiation of neuropathic pain and, more recently, a relation was observed between increased LPA levels in the circulation and cholestatic itch. The mechanism by which this occurs remains to be elucidated. This article is part of a Special Issue entitled Linking transcription to physiology in lipodomics.

Advances in pathogenesis and management of pruritus in cholestasis

Chronic pruritus is a burdensome feature of numerous hepatobiliary disorders such as primary biliary cirrhosis, primary sclerosing cholangitis, cholangiocarcinoma, inherited forms of cholestasis and intrahepatic cholestasis of pregnancy. Bile salts, μ-opioids, serotonin, histamine and steroids have been controversially discussed in the pathogenesis of cholestatic pruritus. However, for these substances neither a correlation with itch severity nor a causative link has ever been established. Recent findings indicate that the potent neuronal activator lysophosphatidic acid and autotaxin, the enzyme forming lysophosphatidic acid, may play a key element in the pathogenesis of cholestatic pruritus. Serum activity of autotaxin correlated with itch intensity and response to antipruritic treatment in patients with cholestatic pruritus, but not other forms of pruritus. Autotaxin activity thereby represents the first biomarker for pruritus and had a positive predictive value of 70% in differentiating cholestatic pruritus from other forms of pruritus. Treatment options for patients with cholestatic pruritus include the anion exchange resin colestyramine, the PXR agonist rifampicin, the μ-opioid antagonist naltrexone, and the serotonin reuptake inhibitor sertraline. These drugs are recommended by evidence-based guidelines as a stepwise therapeutic approach. Patients unresponsive to these drugs should be referred to specialized centers to receive experimental approaches such as UVB phototherapy, albumin dialysis, plasmapheresis or nasobiliary drainage. This review discusses pruritogen candidates in cholestasis, gives novel insights into the neuronal signaling pathway of pruritus and summarizes evidence-based treatment options for patients suffering from pruritus in cholestasis.

Pruritus in cholestasis: facts and fiction

Pruritus is a common symptom in patients with cholestatic liver diseases such as primary biliary cirrhosis, primary sclerosing cholangitis, intrahepatic cholestasis of pregnancy, or hereditary pediatric cholestatic disorders and may accompany, although less frequently, many other liver diseases. Recent findings indicate that lysophosphatidic acid (LPA), a potent neuronal activator, and autotaxin (ATX; ectonucleotide pyrophosphatase/phosphodiesterase 2), the enzyme which forms LPA, may form a key element of the long-sought pruritogenic signaling cascade in cholestatic patients suffering from itch. Serum ATX, but no other pruritogen candidate studied so far, correlates with pruritus intensity and responds to therapeutic interventions. In this comprehensive review, we provide a short update on actual insights in signal transmission related to pruritus and discuss pruritogen candidates in cholestasis. We also summarize evidence-based and guideline-approved as well as experimental therapeutic approaches for patients suffering from pruritus in cholestasis.

Neural processing of itch

While considerable effort has been made to investigate the neural mechanisms of pain, much less effort has been devoted to itch, at least until recently. However, itch is now gaining increasing recognition as a widespread and costly medical and socioeconomic issue. This is accompanied by increasing interest in the underlying neural mechanisms of itch, which has become a vibrant and rapidly-advancing field of research. The goal of the present forefront review is to describe the recent progress that has been made in our understanding of itch mechanisms.

Lysophosphatidic acid impairs glucose homeostasis and inhibits insulin secretion in high-fat diet obese mice

AIMS/HYPOTHESIS

Lysophosphatidic acid (LPA) is a lipid mediator produced by adipocytes that acts via specific G-protein-coupled receptors; its synthesis is modulated in obesity. We previously reported that reducing adipocyte LPA production in high-fat diet (HFD)-fed obese mice is associated with improved glucose tolerance, suggesting a negative impact of LPA on glucose homeostasis. Here, our aim was to test this hypothesis.

METHODS

First, glucose tolerance and plasma insulin were assessed after acute (30 min) injection of LPA (50 mg/kg) or of the LPA1/LPA3 receptor antagonist Ki16425 (5 mg kg(-1) day(-1), i.p.) in non-obese mice fed a normal diet (ND) and in obese/prediabetic (defined as glucose-intolerant) HFD mice. Glucose and insulin tolerance, pancreas morphology, glycogen storage, glucose oxidation and glucose transport were then studied after chronic treatment (3 weeks) of HFD mice with Ki16425.

RESULTS

In ND and HFD mice, LPA acutely impaired glucose tolerance by inhibiting glucose-induced insulin secretion. These effects were blocked by pre-injection of Ki16425 (5 mg/kg, i.p.). Inhibition of glucose-induced insulin secretion by LPA also occurred in isolated mouse islets. Plasma LPA was higher in HFD mice than in ND mice and Ki16425 transiently improved glucose tolerance. The beneficial effect of Ki16425 became permanent after chronic treatment and was associated with increased pancreatic islet mass and higher fasting insulinaemia. Chronic treatment with Ki16425 also improved insulin tolerance and increased liver glycogen storage and basal glucose use in skeletal muscle.

CONCLUSIONS/INTERPRETATION

Exogenous and endogenous LPA exerts a deleterious effect on glucose disposal through a reduction of plasma insulin; pharmacological blockade of LPA receptors improves glucose homeostasis in obese/prediabetic mice.

New insights into the mechanisms of itch: are pain and itch controlled by distinct mechanisms?

Itch and pain are closely related but distinct sensations. They share largely overlapping mediators and receptors, and itch-responding neurons are also sensitive to pain stimuli. Itch-mediating primary sensory neurons are equipped with distinct receptors and ion channels for itch transduction, including Mas-related G protein-coupled receptors (Mrgprs), protease-activated receptors, histamine receptors, bile acid receptor, toll-like receptors, and transient receptor potential subfamily V1/A1 (TRPV1/A1). Recent progress has indicated the existence of an itch-specific neuronal circuitry. The MrgprA3-expressing primary sensory neurons exclusively innervate the epidermis of skin, and their central axons connect with gastrin-releasing peptide receptor (GRPR)-expressing neurons in the superficial spinal cord. Notably, ablation of MrgprA3-expressing primary sensory neurons or GRPR-expressing spinal cord neurons results in selective reduction in itch but not pain. Chronic itch results from dysfunction of the immune and nervous system and can manifest as neural plasticity despite the fact that chronic itch is often treated by dermatologists. While differences between acute pain and acute itch are striking, chronic itch and chronic pain share many similar mechanisms, including peripheral sensitization (increased responses of primary sensory neurons to itch and pain mediators), central sensitization (hyperactivity of spinal projection neurons and excitatory interneurons), loss of inhibitory control in the spinal cord, and neuro-immune and neuro-glial interactions. Notably, painful stimuli can elicit itch in some chronic conditions (e.g., atopic dermatitis), and some drugs for treating chronic pain are also effective in chronic itch. Thus, itch and pain have more similarities in pathological and chronic conditions.

Advances in pathogenesis and treatment of pruritus

The pathogenesis of itch during cholestasis is largely unknown and treatment options are limited. Lysophosphatidate, female steroid hormones, and endogenous opioids are among the agents discussed as potential pruritogens in cholestasis. The itch-alleviating action of guideline-based therapeutic interventions with anion exchanger resins, rifampicin, opioid antagonists, and serotonin reuptake inhibitors are studied to unravel the molecular pathogenesis of itch. Still, a considerable part of the patients is in need of alternative experimental therapeutic approaches (eg, UV-B phototherapy, extracorporeal albumin dialysis, nasobiliary drainage), providing additional information about the enigmatic pathophysiology of cholestatic pruritus.

The TGR5 receptor mediates bile acid-induced itch and analgesia

Patients with cholestatic disease exhibit pruritus and analgesia, but the mechanisms underlying these symptoms are unknown. We report that bile acids, which are elevated in the circulation and tissues during cholestasis, cause itch and analgesia by activating the GPCR TGR5. TGR5 was detected in peptidergic neurons of mouse dorsal root ganglia and spinal cord that transmit itch and pain, and in dermal macrophages that contain opioids. Bile acids and a TGR5-selective agonist induced hyperexcitability of dorsal root ganglia neurons and stimulated the release of the itch and analgesia transmitters gastrin-releasing peptide and leucine-enkephalin. Intradermal injection of bile acids and a TGR5-selective agonist stimulated scratching behavior by gastrin-releasing peptide- and opioid-dependent mechanisms in mice. Scratching was attenuated in Tgr5-KO mice but exacerbated in Tgr5-Tg mice (overexpressing mouse TGR5), which exhibited spontaneous pruritus. Intraplantar and intrathecal injection of bile acids caused analgesia to mechanical stimulation of the paw by an opioid-dependent mechanism. Both peripheral and central mechanisms of analgesia were absent from Tgr5-KO mice. Thus, bile acids activate TGR5 on sensory nerves, stimulating the release of neuropeptides in the spinal cord that transmit itch and analgesia. These mechanisms could contribute to pruritus and painless jaundice that occur during cholestatic liver diseases.

Cholestasis-induced pruritus treated with ultraviolet B phototherapy: an observational case series study

BACKGROUND & AIMS

Pruritus is a disabling complication of cholestatic liver disorders. Its management remains challenging. Ultraviolet B (UVB) phototherapy has been successfully used to treat pruritus in other indications.

METHODS

This is an observational case series. The study population consists of 13 patients (10 females, mean age 52 years) with pruritus due to different cholestatic liver disorders: PBC (n=4), PSC (n=2), drug-induced (n=3) and persistent cholestasis after liver transplantation (LT) (n=4). Serum alkaline phosphatase levels were: 686 ± 363 μ/L and serum bile acids levels: 147 ± 15 μmol/L. In all patients, conventional medical treatment had failed to control pruritus. Perception of pruritus was recorded by the visual analogue scale (VAS).

RESULTS

The mean follow-up was 3 years. Ten patients (77%) had more than 60% reduction in perceived pruritus of which 4 had more than an 80% reduction. Median [25-75% percentiles] VAS score before and after treatment decreased from 8.0 [8.0-10] to 2.0 [1.5-2.1] (p<0.001). The mean number of irradiations required to obtain this effect was 26 ± 17 (average duration of phototherapy: 8 weeks). No significant changes in cholestatic serum markers were observed. Four patients (30%) needed an additional phototherapy course because of recurrent pruritus and in all of them again a marked improvement of pruritus was observed. The therapy was well tolerated, except in two patients who developed, during retreatment, pronounced erythema in one case and paresthesia in the other case.

CONCLUSIONS

UVB phototherapy appears to be a promising and well tolerated treatment also for cholestasis-associated pruritus.

Current progress in non-Edg family LPA receptor research

Lysophosphatidic acid (LPA) is the simplest phospholipid yet possesses myriad biological functions. Until 2003, the functions of LPA were thought to be elicited exclusively by three subtypes of the endothelial differentiation gene (Edg) family of G protein-coupled receptors - LPA(1), LPA(2), and LPA(3). However, several biological functions of LPA could not be assigned to any of these receptors indicating the existence of one or more additional LPA receptor(s). More recently, the discovery of a second cluster of LPA receptors which includes LPA(4), LPA(5), and LPA(6) has paved the way for new avenues of LPA research. Analyses of these non-Edg family LPA receptors have begun to fill in gaps to understand biological functions of LPA such as platelet aggregation and vascular development that could not be ascribed to classical Edg family LPA receptors and are also unveiling new biological functions. Here we review recent progress in the non-Edg family LPA receptor research, with special emphasis on the pharmacology, signaling, and physiological roles of this family of receptors. This article is part of a Special Issue entitled Advances in Lysophospholipid Research.

Pathogenesis and management of pruritus in cholestatic liver disease

Patients with cholestatic liver diseases such as primary biliary cirrhosis, primary sclerosing cholangitis and intrahepatic cholestasis of pregnancy commonly complain of pruritus. The underlying pathogenesis remains obscure with several mediators possibly playing an important role; these include lysophosphatidic acid, bile salts, opioids, histamine and progesterone metabolites. We describe in this review novel insights into the pathogenesis and management of pruritus in patients with cholestasis.

Pruritus in chronic cholestatic liver disease

Pruritus is a troublesome complication in patients with cholestatic liver disease. Several links to its pathogenesis have been proposed, including the role of bile acids, endogenous opioid and serotonins, and lysophosphatidic acid. The management of pruritus in cholestasis is challenging. Medical treatment of the underlying cholestatic condition may provide benefit. Extracorporeal albumin dialysis can be pursued for those who have a poor quality of life and failed the various therapeutic interventions, while awaiting liver transplantation. Experimental interventions, and the management of pruritus in certain conditions such as intrahepatic cholestasis of pregnancy and benign recurrent intrahepatic cholestasis, are also briefly reviewed.

Vascular hyperpermeability in response to inflammatory mustard oil is mediated by Rho kinase in mice systemically exposed to arsenic

The mechanisms underlying vascular dysfunction and cardiovascular disease induced by chronic arsenic exposure are not completely understood. We have previously shown that mice chronically fed sodium arsenite are hypersensitive to the permeability-increasing effects of inflammatory mustard oil. The aim of this study was to investigate whether RhoA/Rho kinase (ROCK)-mediated vascular leakage (hyperpermeability) is induced by mustard oil in mice systemically exposed to arsenic. Animals were orally fed water (control group) or sodium arsenite for 8weeks. We compared the blood pressure and microvessel density of the ears between these two groups. Both control and arsenic groups exhibited a similar mean arterial pressure and microvessel density. Microvessel permeability changes that occurred following mustard oil treatment in the presence of Y-27632, a ROCK inhibitor, were quantified using the Evans blue (EB) technique and vascular labeling with carbon particles. Both the excessive leakiness of EB and the high density of carbon-labeled microvessels upon stimulation with mustard oil in the arsenic-fed mice were reduced by Y-27632 treatment. However, RhoA and ROCK2 expression levels were similar between control and arsenic-fed mice. We further investigated ROCK2 levels and ROCK activity in the ears following mustard oil challenge. ROCK2 levels in mouse ears treated with mustard oil were higher in the arsenic group as compared with the control group. Following mustard oil application, ROCK activity was significantly higher in the arsenic-fed mice compared with the control mice. These findings indicate that increased ROCK2 levels and enhanced ROCK activity are responsible for mustard oil-induced vascular hyperpermeability in arsenic-fed mice.

Basic mechanisms of itch

Chronic itch represents a burdensome clinical problem that can originate from a variety of aetiologies. Pruriceptive itch originates following the activation of peripheral sensory nerve endings following damage or exposure to inflammatory mediators and ascends to the brain through the spinal thalamic tract. Much insight has been gained into the understanding of the mechanisms underlying pruriceptive itch through studies using humans and experimental animals. More than one sensory nerve subtype is thought to subserve pruriceptive itch which includes both unmyelinated C-fibres and thinly myelinated Aδ nerve fibres. There are a myriad of mediators capable of stimulating these afferent nerves leading to itch, including biogenic amines, proteases, cytokines, and peptides. Some of these mediators can also evoke sensations of pain and the sensory processing underlying both sensations overlaps in complex ways. Studies have demonstrated that both peripheral and central sensitization to pruritogenic stimuli occur during chronic itch.

Mediators of pruritus during cholestasis

PURPOSE OF REVIEW

Pruritus is a frequent symptom in patients with cholestatic liver diseases. Itching may be excruciating, may seriously impair quality of life and even induce suicidal ideation in the most severe cases.

RECENT FINDINGS

The molecular mechanism of itch signal transduction in cholestasis is largely unclear. It may be caused or potentiated by compounds that accumulate in the circulation during cholestasis, which either directly or indirectly affect signalling in itch fibres. In the past, bile salts and endogenous opioids have been proposed but never been proven to be key factors in itch perception during cholestasis. We have performed a screen for compounds in plasma from patients with various cholestatic conditions for their capacity to activate neuronal cell lines. In these sera, we could identify a potent neuronal activator as lysophosphatidic acid (LPA). LPA is a very potent signalling phospholipid that can activate cells through various LPA receptors. Quite strikingly, samples from itchy cholestatic patients contained higher amounts of LPA. These increased levels of LPA turned out to be caused by elevated levels of serum autotaxin, the enzyme that converts lysophosphatidylcholine into LPA. This is a striking finding, as autotaxin has never been connected to itch perception thus far. We have also shown that LPA, when injected intradermally, caused scratching behaviour in mice.

SUMMARY

On the basis of our results, we hypothesize that during cholestasis expression of autotaxin is induced, which gives rise to increased local formation of LPA near unmyelinated nerve endings of itch fibres. LPA activates these neurons through one of the LPA receptors, which in turn potentiates action potentials along itch fibres leading to the perception of pruritus.