Antifibrogenic role of the cannabinoid receptor CB2 in the liver

The endocannabinoid system as a key mediator during liver diseases: new insights and therapeutic openings

Chronic liver diseases represent a major health problem due to cirrhosis and its complications. During the last decade, endocannabinoids and their receptors have emerged as major regulators of several pathophysiological aspects associated with chronic liver disease progression. Hence, hepatic cannabinoid receptor 2 (CB(2)) receptors display beneficial effects on alcoholic fatty liver, hepatic inflammation, liver injury, regeneration and fibrosis. Cannabinoid receptor 1 (CB(1)) receptors have been implicated in the pathogenesis of several lesions such as alcoholic and metabolic steatosis, liver fibrogenesis, or circulatory failure associated with cirrhosis. Although the development of CB(1) antagonists has recently been suspended due to the high incidence of central side effects, preliminary preclinical data obtained with peripherally restricted CB(1) antagonists give real hopes in the development of active CB(1) molecules devoid of central adverse effects. CB(2) -selective molecules may also offer novel perspectives for the treatment of liver diseases, and their clinical development is clearly awaited. Whether combined treatment with a peripherally restricted CB(1) antagonist and a CB(2) agonist might result in an increased therapeutic potential will warrant further investigation.

Mechanisms and biomarkers of apoptosis in liver disease and fibrosis

Liver fibrosis and cirrhosis are a major cause of morbidity and mortality worldwide. Development of the fibrotic scar is an outcome of chronic liver diseases of varying aetiologies including alcoholic liver disease (ALD) nonalcoholic liver disease (NAFLD) including non-alcoholic steatohepatitis (NASH) viral hepatitis B and C (HBV, HCV). The critical step in the development of scar is activation of hepatic stellate cells (HSCs), which become the primary source of extracellular matrix. Aberrant apoptosis is a feature of chronic liver diseases and is associated with worsening stages of fibrosis. However, apoptosis is also the main mechanism promoting the resolution of fibrosis, and spontaneous or targeted apoptosis of HSC is associated with regression of fibrosis in animal models and patients with chronic liver disease. Given the importance of apoptosis in disease progression and resolution, there is much interest in precisely delineating the mechanisms involved and also developing biomarkers that accurately reflect the underlying pathogenesis. Here, we review the mechanisms driving apoptosis in development of liver disease and use of apoptosis -related biomarkers to aid in clinical diagnosis. Finally, we will also examine the recent literature regarding new insights into mechanisms involved in apoptosis of activated HSCs as possible method of fibrosis regression.

Metabolic disorders and steatosis in patients with chronic hepatitis C: metabolic strategies for antiviral treatments

It has been reported that hepatitis C virus (HCV) infection is closely associated with hepatic metabolic disorders. Hepatic steatosis and insulin resistance are both relatively common in patients with chronic hepatitis C. Recent investigations suggest that HCV infection changes the expression profile of lipid-metabolism-associated factors in the liver, conferring advantages to the life cycle of HCV. Moreover, insulin resistance and steatosis are independent predictors of impaired response to antiviral treatment in chronic hepatitis C. In this paper, we summarize our current knowledge of hepatic metabolic disorders and describe how HCV leads to and exploits these hepatic disorders. We also discuss the clinical significance of insulin sensitizers used to improve insulin resistance and lipid modulators used to manage lipid metabolism as potential treatment options for chronic hepatitis C.

Liver precursor cells increase hepatic fibrosis induced by chronic carbon tetrachloride intoxication in rats

Hepatic fibrosis, the major complication of virtually all types of chronic liver damage, usually begins in portal areas, and its severity has been correlated to liver progenitor cells (LPC) expansion from periportal areas, even if the primary targets of injury are intralobular hepatocytes. The aim of this study was to determine the potential fibrogenic role of LPC, using a new experimental model in which rat liver fibrosis was induced by chronic carbon tetrachloride (CCl(4)) administration for 6 weeks, in combination with chronic acetylaminofluorene treatment (AAF), which promotes activation of LPC compartment. Treatment with CCl(4) alone caused a significant increase in serum transaminase activity as well as liver fibrosis initiating around central veins and leading to formation of incomplete centro-central septa with sparse fibrogenic cells expressing α-smooth muscle actin (αSMA). In AAF/CCl(4)-treated animals, the fibrogenic response was profoundly worsened, with formation of multiple porto-central bridging septa leading to cirrhosis, whereas hepatocellular necrosis and inflammation were similar to those observed in CCl(4)-treated animals. Enhanced fibrosis in AAF/CCl(4) group was accompanied by ductule forming LPC expanding from portal areas, αSMA-positive cells accumulation in the fibrotic areas and increased expression of hepatic collagen type 1, 3 and 4 mRNA. Moreover, CK19-positive LPC expressed the most potent fibrogenic cytokine transforming growth factor-β (TGFβ) without any expression of αSMA, desmin or fibroblast-specific protein-1, demonstrating that LPC did not undergo an epithelial-mesenchymal transition. In this new experimental model, LPC, by expressing TGFβ, contributed to the accumulation of αSMA-positive myofibroblasts in the ductular reaction leading to enhanced fibrosis but also to disease progression and to a fibrotic pattern similar to that observed in humans.

Current status of novel antifibrotic therapies in patients with chronic liver disease

Fibrosis accumulation is a dynamic process resulting from a wound-healing response to acute or chronic liver injury of all causes. The cascade starts with hepatocyte necrosis and apoptosis, which instigate inflammatory signaling by chemokines and cytokines, recruitment of immune cell populations, and activation of fibrogenic cells, culminating in the deposition of extracellular matrix. These key elements, along with pathways of transcriptional and epigenetic regulation, represent fertile therapeutic targets. New therapies include drugs specifically designed as antifibrotics, as well as drugs already available with well-established safety profiles, whose mechanism of action may also be antifibrotic. At the same time, the development of noninvasive fibrogenic markers, and techniques (e.g. fibroscan), as well as combined scoring systems incorporating serum and clinical features will allow improved assessment of therapy response. In aggregate, the advances in the elucidation of the biology of fibrosis, combined with improved technologies for assessment will provide a comprehensive framework for design of antifibrotics and their analysis in well-designed clinical trials. These efforts may ultimately yield success in halting the progression of, or reversing, liver fibrosis.

Cannabinoid CB2 receptors protect against alcoholic liver disease by regulating Kupffer cell polarization in mice

Activation of Kupffer cells plays a central role in the pathogenesis of alcoholic liver disease. Because cannabinoid CB2 receptors (CB2) display potent anti-inflammatory properties, we investigated their role in the pathogenesis of alcoholic liver disease, focusing on the impact of CB2 on Kupffer cell polarization and the consequences on liver steatosis. Wild-type (WT) mice fed an alcohol diet showed an induction of hepatic classical (M1) and alternative (M2) markers. Cotreatment of alcohol-fed mice with the CB2 agonist, JWH-133, decreased hepatic M1 gene expression without affecting the M2 profile. In keeping with this, genetic ablation of CB2 enhanced hepatic induction of M1 gene signature and blunted the induction of M2 markers. CB2 also modulated alcohol-induced fatty liver, as shown by the reduction of hepatocyte steatosis in JWH-133-treated mice and its enhancement in CB2-/- animals. Studies in isolated Kupffer cells and cultured macrophages further demonstrated that CB2 inhibits M1 polarization and favors the transition to an M2 phenotype. In addition, conditioned-medium experiments showed that preventing M1 polarization in CB2-activated macrophages protects from lipid accumulation in hepatocytes. Heme oxygenase-1 (HO-1) mediated the anti-inflammatory effects of CB2 receptors. Indeed, alcohol-fed mice treated with JWH-133 showed increased hepatic expression of macrophage HO-1, as compared to vehicle-treated counterparts. In keeping with this, JWH-133 induced HO-1 expression in cultured macrophages, and the HO-1 inhibitor, zinc protoporphyrin, blunted the inhibitory effect of JWH-133 on lipopolysaccharide-induced nuclear factor-kappa B activation and M1 polarization. Altogether, these findings demonstrate that CB2 receptors display beneficial effects on alcohol-induced inflammation by regulating M1/M2 balance in Kupffer cells, thereby reducing hepatocyte steatosis via paracrine interactions between Kupffer cells and hepatocytes. These data identify CB2 agonists as potential therapeutic agents for the management of alcoholic liver disease.

The endogenous cannabinoid system in the gut of patients with inflammatory bowel disease

Activation of cannabinoid receptors (CBs) by endocannabinoids impacts on a number of gastrointestinal functions. Recent data indicate that CB1 agonists improve 2,4-dinitrobenzene sulfonic acid-induced colitis in mice, thus suggesting a role for the endocannabinoid agonist anandamide (AEA) in protecting the gut against inflammation. We here examined the gut endocannabinoid system in inflammatory bowel disease (IBD) patients, and investigated the ex vivo and in vitro effects of the non-hydrolysable AEA analog methanandamide (MAEA) on the mucosal proinflammatory response. The content of AEA, but not of 2-arachidonoyl-glycerol and N-palmitoylethanolamine, was significantly lower in inflamed than uninflamed IBD mucosa, and this was paralleled by lower activity of the AEA-synthesizing enzyme N-acyl-phosphatidylethanolamine-specific phospholipase D and higher activity of the AEA-degrading enzyme fatty acid amide hydrolase. MAEA significantly downregulated interferon-γ and tumor necrosis factor-α secretion by both organ culture biopsies and lamina propria mononuclear cells. Although these results are promising, further studies are needed to determine the role of cannabinoid pathways in gut inflammation.

Protective role of cannabinoid receptor type 2 in a mouse model of diabetic nephropathy

OBJECTIVE

The cannabinoid receptor type 2 (CB2) has protective effects in chronic degenerative diseases. Our aim was to assess the potential relevance of the CB2 receptor in both human and experimental diabetic nephropathy (DN).

RESEARCH DESIGN AND METHODS

CB2 expression was studied in kidney biopsies from patients with advanced DN, in early experimental diabetes, and in cultured podocytes. Levels of endocannabinoids and related enzymes were measured in the renal cortex from diabetic mice. To assess the functional role of CB2, streptozotocin-induced diabetic mice were treated for 14 weeks with AM1241, a selective CB2 agonist. In these animals, we studied albuminuria, renal function, expression of podocyte proteins (nephrin and zonula occludens-1), and markers of both fibrosis (fibronectin and transforming growth factor-β1) and inflammation (monocyte chemoattractant protein-1 [MCP-1], CC chemokine receptor 2 [CCR2], and monocyte markers). CB2 signaling was assessed in cultured podocytes.

RESULTS

Podocytes express the CB2 receptor both in vitro and in vivo. CB2 was downregulated in kidney biopsies from patients with advanced DN, and renal levels of the CB2 ligand 2-arachidonoylglycerol were reduced in diabetic mice, suggesting impaired CB2 regulation. In experimental diabetes, AM1241 ameliorated albuminuria, podocyte protein downregulation, and glomerular monocyte infiltration, without affecting early markers of fibrosis. In addition, AM1241 reduced CCR2 expression in both renal cortex and cultured podocytes, suggesting that CB2 activation may interfere with the deleterious effects of MCP-1 signaling.

CONCLUSIONS

The CB2 receptor is expressed by podocytes, and in experimental diabetes, CB2 activation ameliorates both albuminuria and podocyte protein loss, suggesting a protective effect of signaling through CB2 in DN.

Cannabinoid receptor type I modulates alcohol-induced liver fibrosis

The cannabinoid system (CS) is implicated in the regulation of hepatic fibrosis, steatosis and inflammation, with cannabinoid receptors 1 and 2 (CB1 and CB2) being involved in regulation of pro- and antifibrogenic effects. Daily cannabis smoking is an independent risk factor for the progression of fibrosis in chronic hepatitis C and a mediator of experimental alcoholic steatosis. However, the role and function of CS in alcoholic liver fibrosis (ALF) is unknown so far. Thus, human liver samples from patients with alcoholic liver disease (ALD) were collected for analysis of CB1 expression. In vitro, hepatic stellate cells (HSC) underwent treatment with acetaldehyde, Δ9-tetrahydrocannabinol H₂O₂, endo- and exocannabinoids (2-arachidonoylglycerol (2-AG) and [THC]), and CB1 antagonist SR141716 (rimonabant). In vivo, CB1 knockout (KO) mice received thioacetamide (TAA)/ethanol (EtOH) to induce fibrosis. As a result, in human ALD, CB1 expression was restricted to areas with advanced fibrosis only. In vitro, acetaldehyde, H₂O₂, as well as 2-AG and THC, alone or in combination with acetaldehyde, induced CB1 mRNA expression, whereas CB1 blockage with SR141716 dose-dependently inhibited HSC proliferation and downregulated mRNA expression of fibrosis-mediated genes PCα1(I), TIMP-1 and MMP-13. This was paralleled by marked cytotoxicity of SR141716 at high doses (5-10 μmol/L). In vivo, CB1 knockout mice showed marked resistance to alcoholic liver fibrosis. In conclusion, CB1 expression is upregulated in human ALF, which is at least in part triggered by acetaldehyde (AA) and oxidative stress. Inhibition of CB1 by SR141716, or via genetic knock-out protects against alcoholic-induced fibrosis in vitro and in vivo.

Essential roles of sphingosine 1-phosphate receptor types 1 and 3 in human hepatic stellate cells motility and activation

The biological roles of sphingosine 1-phosphate (S1P) and S1P receptors (S1PRs) have been broadly investigated. However, at present pathophysiological roles of S1P/S1PRs axis in liver fibrosis are not well defined. Here, we investigated the functions of S1P/S1PRs axis in human hepatic stellate cells (HSC) line, LX-2 cells. We found that S1PR types 1, 2 and 3 (S1PR1-3) are clearly detected in LX-2 cells, as determined by RT-PCR, Western blot and immunocytochemistry analysis. S1P exerted a powerful migratory action on LX-2 cells, as determined in Boyden chambers, and stimulated fibrogenic activity of LX-2 cells, as demonstrated by increase of expression of smooth muscle α-actin, procollagen α1(I) and α1(III) and total hydroxyproline content. Moreover, the effects of S1P were mimicked by S1PR1 agonist SEW2871, and abrogated by W146 (S1PR1 antagonist) and/or silencing S1PR1, three expression with small interfering RNA, suggesting the main roles of S1PR1 and 3. However, studies with S1PR2 antagonist JTE-013 and silencing S1PR2 expression indicated that S1PR2 negatively regulated S1P-induced cell migration. Interestingly, exogenously added S1P induced significant up-regulation of sphingosine kinase-1 and the synthesis of additional S1P, and expression of S1PR1,3, but not S1PR2. In conclusion, our data have identified an additional function regulated by S1P/S1PR1,3 axis involving migration and fibrogenic activation of HSCs. These results suggest that selective modulation of S1PR activity may represent a new antifibrotic strategy.

The highs and lows of cannabinoid receptor expression in disease: mechanisms and their therapeutic implications

Alterations in the endogenous cannabinoid system have been described in almost every category of disease. These changes can alternatively be protective or maladaptive, such as producing antinociception in neuropathic pain or fibrogenesis in liver disease, making the system an attractive therapeutic target. However, the challenge remains to selectively target the site of disease while sparing other areas, particularly mood and cognitive centers of the brain. Identifying regional changes in cannabinoid receptor-1 and -2 (CB(1)R and CB(2)R) expression is particularly important when considering endocannabinoid system-based therapies, because regional increases in cannabinoid receptor expression have been shown to increase potency and efficacy of exogenous agonists at sites of disease. Although there have been extensive descriptive studies of cannabinoid receptor expression changes in disease, the underlying mechanisms are only just beginning to unfold. Understanding these mechanisms is important and potentially relevant to therapeutics. In diseases for which cannabinoid receptors are protective, knowledge of the mechanisms of receptor up-regulation could be used to design therapies to regionally increase receptor expression and thus increase efficacy of an agonist. Alternatively, inhibition of harmful cannabinoid up-regulation could be an attractive alternative to global antagonism of the system. Here we review current findings on the mechanisms of cannabinoid receptor regulation in disease and discuss their therapeutic implications.

Cannabinoids as novel anti-inflammatory drugs

Cannabinoids are a group of compounds that mediate their effects through cannabinoid receptors. The discovery of Δ9-tetrahydrocannabinol (THC) as the major psychoactive principle in marijuana, as well as the identification of cannabinoid receptors and their endogenous ligands, has led to a significant growth in research aimed at understanding the physiological functions of cannabinoids. Cannabinoid receptors include CB1, which is predominantly expressed in the brain, and CB2, which is primarily found on the cells of the immune system. The fact that both CB1 and CB2 receptors have been found on immune cells suggests that cannabinoids play an important role in the regulation of the immune system. Recent studies demonstrated that administration of THC into mice triggered marked apoptosis in T cells and dendritic cells, resulting in immunosuppression. In addition, several studies showed that cannabinoids downregulate cytokine and chemokine production and, in some models, upregulate T-regulatory cells (Tregs) as a mechanism to suppress inflammatory responses. The endocannabinoid system is also involved in immunoregulation. For example, administration of endocannabinoids or use of inhibitors of enzymes that break down the endocannabinoids, led to immunosuppression and recovery from immune-mediated injury to organs such as the liver. Manipulation of endocannabinoids and/or use of exogenous cannabinoids in vivo can constitute a potent treatment modality against inflammatory disorders. This review will focus on the potential use of cannabinoids as a new class of anti-inflammatory agents against a number of inflammatory and autoimmune diseases that are primarily triggered by activated T cells or other cellular immune components.

Role of cannabinoid receptors in alcoholic hepatic injury: steatosis and fibrogenesis are increased in CB2 receptor-deficient mice and decreased in CB1 receptor knockouts

BACKGROUND

Alcohol is a common cause of hepatic liver injury with steatosis and fibrosis. Cannabinoid receptors (CB) modulate steatosis, inflammation and fibrogenesis. To investigate the differences between CB(1) and CB(2) in the hepatic response to chronic alcohol intake, we examined CB knockout mice (CB(1)(-/-), CB(2)(-/-)).

METHODS

Eight- to 10-week-old CB(1)(-/-), CB(2)(-/-) and wild-type mice received 16% ethanol for 35 weeks. Animals receiving water served as controls. We analysed triglyceride and hydroxyproline contents in liver homogenates. mRNA levels of CBs, pro-inflammatory cytokines [tumour necrosis factor (TNF)-α, monocyte chemotactic protein (MCP)-1, interleukin (IL)-1β] and profibrotic factors [α-smooth muscle actin (α-SMA), procollagen-Ia, platelet-derived growth factor β receptor (PDGFβ-R)] were analysed by reverse transcription-polymerase chain reaction (RT-PCR). Histology (hemalaun and eosin, oil-red O, CD3, CD45R, CD45, F4/80, Sirius red) characterized hepatic steatosis, inflammation and fibrosis. Activation of lipogenic pathways, activation and proliferation of hepatic stellate cell (HSC) were assessed by western blot [fatty acid synthase (FAS), sterol regulatory element binding protein 1c (SREBP-1c), α-SMA, proliferating cell nuclear antigen (PCNA), cathepsin D].

RESULTS

Hepatic mRNA levels of the respective CBs were increased in wild-type animals and in CB(1)(-/-) mice after ethanol intake. Ethanol intake in CB(2)(-/-) mice induced much higher steatosis (SREBP-1c mediated) and inflammation (B-cell predominant infiltrates) compared with wild-type animals and CB(1)(-/-) mice. HSC activation and collagen production were increased in all groups after forced ethanol intake, being most pronounced in CB(2)(-/-) mice and least pronounced in CB(1)(-/-) mice.

DISCUSSION

The fact that CB(2) receptor knockout mice exhibited the most pronounced liver damage after ethanol challenge indicates a protective role of CB(2) receptor expression in chronic ethanol intake. By contrast, in CB(1) knockouts, the effect of ethanol was attenuated, suggesting aggravation of fibrogenesis and SREBP-1c-mediated steatosis via CB(1) receptor expression after ethanol intake.

Sphingosine 1-phosphate (S1P)/S1P receptors are involved in human liver fibrosis by action on hepatic myofibroblasts motility

BACKGROUND & AIMS

Directed migration of hepatic myofibroblasts (hMFs) contributes to the development of liver fibrosis. However, the signals regulating the motility of these cells are incompletely understood. We have recently shown that sphingosine 1-phosphate (S1P) and S1P receptors (S1PRs) are involved in mouse liver fibrogenesis. Here, we investigated the role of S1P/S1PRs signals in human liver fibrosis involving motility of human hMFs.

METHODS

S1P level in the liver was examined by high-performance liquid chromatography. Expression of S1PRs was characterized, in biopsy specimens of human liver and cultured hMFs, by immunofluorescence and real-time RT-PCR or Western blot analysis. Cell migration was determined in Boyden chambers, by using the selective S1P receptor agonist or antagonist and silencing of S1PRs expression with small interfering RNA.

RESULTS

S1P level in the human fibrotic liver was increased through up-regulation of sphingosine kinase (SphK), irrespective of the etiology of fibrosis. S1P receptors type 1, 2, and 3 (S1P(1,2,3)) were expressed in human hMFs in vivo and in vitro. Interestingly, S1P(1,3) were strongly induced in human fibrotic samples, whereas expression of S1P(2) was massively decreased. S1P exerted a powerful migratory action on human hMFs. Furthermore, the effect of S1P was mimicked by SEW2871 (an S1P(1) agonist), and blocked by suramin (an S1P(3) antagonist) and by silencing S1P(1,3) expression. In contrast, JTE-013 (an S1P(2) antagonist) and silencing of S1P(2) expression enhanced S1P-induced migration.

CONCLUSIONS

SphK/S1P/S1PRs signaling axis plays an important role in human liver fibrosis and is involved in the directed migration of human hMFs into the damaged areas.

Hepatic stellate cells and astrocytes: Stars of scar formation and tissue repair

Scar formation inhibits tissue repair and regeneration in the liver and central nervous system. Activation of hepatic stellate cells (HSCs) after liver injury or of astrocytes after nervous system damage is considered to drive scar formation. HSCs are the fibrotic cells of the liver, as they undergo activation and acquire fibrogenic properties after liver injury. HSC activation has been compared to reactive gliosis of astrocytes, which acquire a reactive phenotype and contribute to scar formation after nervous system injury, much like HSCs after liver injury. It is intriguing that a wide range of neuroglia-related molecules are expressed by HSCs. We identified an unexpected role for the p75 neurotrophin receptor in regulating HSC activation and liver repair. Here we discuss the molecular mechanisms that regulate HSC activation and reactive gliosis and their contributions to scar formation and tissue repair. Juxtaposing key mechanistic and functional similarities in HSC and astrocyte activation might provide novel insight into liver regeneration and nervous system repair.

Liver fibrogenesis and metabolic factors

Mechanisms of liver fibrosis are complex and varied. Among them, metabolic factors are particularly important in the development of fibrosis associated with nonalcoholic steatohepatitis (NASH). These factors are some of the "multiple parallel hits" responsible for liver damage during NASH. Nonalcoholic fatty liver disease (NAFLD) is the hepatic manifestation of the metabolic syndrome. Major profibrogenic protagonists, such as hepatic stellate cells and Kupffer cells, are activated by insulin resistance, apoptosis and local inflammation. Relations between steatosis, insulin resistance and fibrosis are complex. Initially, simple steatosis may be a way to store deleterious free fatty acid in neutral triglycerides. If the lipid storage threshold is exceeded, steatosis may become associated with lipotoxicity. Similarly, interindividual variations of adipose tissue expandability might explain various phenotypes, ranging from "metabolically obese patients with normal weight" to "metabolically normal morbidly obese patients". The metabolic abnormalities in subcutaneous and visceral adipose tissue are insulin resistance and low-grade inflammation, which are associated with increased release of free fatty acid flux and changes in adipocytokines production such as leptin, adiponectin and interleukin 6. The nuclear transcription factor peroxisome proliferator-activated receptor gamma (PPARγ) and the endocannabinoid system might have important roles in liver fibrogenesis and are potential therapeutic targets. Finally, with the development of new molecular tools, gut microbiota has been recently identified for its pleiotropic functions, including metabolism regulation. Better knowledge of these mechanisms should lead to new strategies for the treatment of metabolic factors that play a key role in liver injuries.

Animal models for the study of liver fibrosis: new insights from knockout mouse models

Fibrosis arises as part of a would-healing response that maintains organ structure and integrity following tissue damage but also contributes to a variety of human pathologies such as liver fibrosis. Liver fibrosis is an abnormal response of the liver to persistent injury with the excessive accumulation of collagenous extracellular matrices. Currently there is no effective treatment, and many patients end up with a progressive form of the disease, eventually requiring a liver transplant. The clarification of mechanisms underlying pathogenesis of liver fibrosis and the development of effective therapy are of clinical importance. Experimental animal models, in particular targeted gene knockouts (loss of function) in mice, have become a powerful resource to address the molecular mechanisms or significance of the targeted gene in hepatic functions and diseases. This review will focus on the recent advances in knowledge obtained from genetically engineered mice that provide novel insights into the pathophysiology of liver fibrosis.

Role of the endocannabinoid system in alcoholic liver disease

Alcohol abuse is a major cause of liver fibrosis and cirrhosis in developed countries. Alcoholic liver disease (ALD) is distinctively characterized by a pronounced inflammatory response due to elevated gut-derived endotoxin plasma levels, an augmented generation of oxidative stress with pericentral hepatic hypoxia and the formation of noxious ethanol metabolites (e.g. acetaldehyde or lipid oxidation products). These factors, based on a complex network of cytokine actions, together result in increased hepatocellular damage and activation of hepatic stellate cells, the key cell type of liver fibrogenesis. Recent studies suggest that the endocannabinoid system is a signaling system that also plays an important role in the pathogenesis of ALD. A study comparing chronic alcohol administration in cannabinoid receptor (CB) 1 or CB2 knockout versus wild-type mice revealed that CB1 signaling aggravated hepatic steatosis and fibrogenesis whereas CB2 protected the liver from ALD. These data suggested a protective role of CB2 (in contrast to CB1) in ALD. Similar results were found in global or hepatocyte-specific CB1 knockout mice that were resistant to ethanol-induced steatosis. Moreover, ethanol feeding upregulated the endocannabinoid 2-arachidonoyl glycerol and its biosynthetic enzyme diacylglycerol lipase-β selectively in hepatic stellate cells and subsequently increased expression of CB1 receptors in hepatocytes of wild-type mice leading to CB1-dependent hepatic steatosis by activation of lipogenic pathways. This ethanol-induced upregulation of CB1 receptors was partly dependent on the ethanol metabolite acetaldehyde. Thus, the hepatic endocannabinoid system offers emerging options for therapeutic exploitation not only for liver disease in general, but also for ALD.

Pathogenesis of liver fibrosis

Liver fibrosis is a major cause of morbidity and mortality worldwide due to chronic viral hepatitis and, more recently, from fatty liver disease associated with obesity. Hepatic stellate cell activation represents a critical event in fibrosis because these cells become the primary source of extracellular matrix in liver upon injury. Use of cell-culture and animal models has expanded our understanding of the mechanisms underlying stellate cell activation and has shed new light on genetic regulation, the contribution of immune signaling, and the potential reversibility of the disease. As pathways of fibrogenesis are increasingly clarified, the key challenge will be translating new advances into the development of antifibrotic therapies for patients with chronic liver disease.

Recent advances in the understanding of the role of the endocannabinoid system in liver diseases

Endocannabinoids are ubiquitous signalling molecules that exert their effects through a number of specific cannabinoid receptors. Recent studies have indicated that this endocannabinoid system is involved in the pathophysiological processes associated with both acute and chronic liver diseases as well as in the complications that arise from these diseases such as hepatic encephalopathy and cardiac problems. Targeting this signalling system has been useful in ameliorating some of the symptoms and consequences in experimental models of these liver diseases. This review summarises the recent advances into our knowledge and understanding of endocannabinoids in liver diseases and highlights potential novel therapeutic strategies that may prove useful to treat these diseases.

Is lipid signaling through cannabinoid 2 receptors part of a protective system?

The mammalian body has a highly developed immune system which guards against continuous invading protein attacks and aims at preventing, attenuating or repairing the inflicted damage. It is conceivable that through evolution analogous biological protective systems have been evolved against non-protein attacks. There is emerging evidence that lipid endocannabinoid signaling through cannabinoid 2 (CB₂) receptors may represent an example/part of such a protective system/armamentarium. Inflammation/tissue injury triggers rapid elevations in local endocannabinoid levels, which in turn regulate signaling responses in immune and other cells modulating their critical functions. Changes in endocannabinoid levels and/or CB₂ receptor expressions have been reported in almost all diseases affecting humans, ranging from cardiovascular, gastrointestinal, liver, kidney, neurodegenerative, psychiatric, bone, skin, autoimmune, lung disorders to pain and cancer, and modulating CB₂ receptor activity holds tremendous therapeutic potential in these pathologies. While CB₂ receptor activation in general mediates immunosuppressive effects, which limit inflammation and associated tissue injury in large number of pathological conditions, in some disease states activation of the CB₂ receptor may enhance or even trigger tissue damage, which will also be discussed alongside the protective actions of the CB₂ receptor stimulation with endocannabinoids or synthetic agonists, and the possible biological mechanisms involved in these effects.

Endocannabinoids in liver disease

Endocannabinoids are lipid mediators of the same cannabinoid (CB) receptors that mediate the effects of marijuana. The endocannabinoid system (ECS) consists of CB receptors, endocannabinoids, and the enzymes involved in their biosynthesis and degradation, and it is present in both brain and peripheral tissues, including the liver. The hepatic ECS is activated in various liver diseases and contributes to the underlying pathologies. In patients with cirrhosis of various etiologies, the activation of vascular and cardiac CB(1) receptors by macrophage-derived and platelet-derived endocannabinoids contributes to the vasodilated state and cardiomyopathy, which can be reversed by CB(1) blockade. In mouse models of liver fibrosis, the activation of CB(1) receptors on hepatic stellate cells is fibrogenic, and CB(1) blockade slows the progression of fibrosis. Fatty liver induced by a high-fat diet or chronic alcohol feeding depends on the activation of peripheral receptors, including hepatic CB(1) receptors, which also contribute to insulin resistance and dyslipidemias. Although the documented therapeutic potential of CB(1) blockade is limited by neuropsychiatric side effects, these may be mitigated by using novel, peripherally restricted CB(1) antagonists.

Antiproliferative effects of cannabinoid agonists on deep infiltrating endometriosis

Deep infiltrating endometriosis (DIE) is characterized by chronic pain, hyperproliferation of endometriotic cells and fibrosis. Since cannabinoids are endowed with antiproliferative and antifibrotic properties, in addition to their psychogenic and analgesic effects, cannabinoid agonists have been evaluated in DIE both in vitro and in vivo. The in vitro effects of the cannabinoid agonist WIN 55212-2 were evaluated on primary endometriotic and endometrial stromal and epithelial cell lines extracted from patients with or without DIE. Cell proliferation was determined by thymidine incorporation and production of reactive oxygen species by spectrofluorometry. ERK and Akt pathways were studied by immunoblotting. Immunoblotting of α-smooth muscle actin was studied as evidence of myofibroblastic transformation. The in vivo effects of WIN 55212-2 were evaluated on Nude mice implanted with human deep infiltrating endometriotic nodules. The in vitro treatment of stromal endometriotic cells by WIN 55212-2 decreased cell proliferation, reactive oxygen species production, and α-smooth muscle actin expression. The decrease in cell proliferation induced by WIN 55212-2 was not associated with a decrease in ERK activation, but was associated with the inhibition of Akt activation. WIN 55212-2 abrogated the growth of endometriotic tissue implanted in Nude mice. Cannabinoid agonists exert anti-proliferative effects on stromal endometriotic cells linked to the inhibition of the Akt pathway. These beneficial effects of cannabinoid agonists on DIE have been confirmed in vivo.

CB2: a cannabinoid receptor with an identity crisis

CB(2) was first considered to be the 'peripheral cannabinoid receptor'. This title was bestowed based on its abundant expression in the immune system and presumed absence from the central nervous system. However, multiple recent reports question the absence of CB(2) from the central nervous system. For example, it is now well accepted that CB(2) is expressed in brain microglia during neuroinflammation. However, the extent of CB(2) expression in neurons has remained controversial. There have been studies claiming either extreme-its complete absence to its widespread expression-as well as everything in between. This review will discuss the reported tissue distribution of CB(2) with a focus on CB(2) in neurons, particularly those in the central nervous system as well as the implications of that presence. As CB(2) is an attractive therapeutic target for pain management and immune system modulation without overt psychoactivity, defining the extent of its presence in neurons will have a significant impact on drug discovery. Our recommendation is to encourage cautious interpretation of data that have been presented for and against CB(2)'s presence in neurons and to encourage continued rigorous study.

Cannabinoid-1 receptor activation induces reactive oxygen species-dependent and -independent mitogen-activated protein kinase activation and cell death in human coronary artery endothelial cells

BACKGROUND AND PURPOSE

Impaired endothelial activity and/or cell death play a critical role in the development of vascular dysfunction associated with congestive heart failure, diabetic complications, hypertension, coronary artery disease and atherosclerosis. Increasing evidence suggests that cannabinoid 1 (CB(1)) receptor inhibition is beneficial in atherosclerosis and cardiovascular inflammation both in experimental models, as well as in humans. Here, we investigated the effects of CB(1) receptor activation with the endocannabinoid anandamide (AEA) or synthetic agonist HU210 on cell death and interrelated signal transduction pathways in human primary coronary artery endothelial cells (HCAECs).

EXPERIMENTAL APPROACH

Cell death, CB(1) receptor expression, reactive oxygen species (ROS) generation and activation of signal transduction pathways in HCAECs were determined by flow cytometry and molecular biology tools.

KEY RESULTS

In HCAECs expressing CB(1) receptors (demonstrated by Western immunoblot and flow cytometry) AEA (5-15 microM) or HU210 (30-1000 nM) triggered concentration- and time-dependent activation of p38 and c-Jun NH(2)-terminal protein kinase (JNK)-mitogen-activated protein kinases (MAPKs), cell death and ROS generation. The AEA- or HU210-induced cell death and MAPK activation were attenuated by CB(1) antagonists [SR141716 (rimonabant) and AM281], inhibitors of p38 and JNK-MAPKs or the antioxidant N-acetylcysteine. N-acetylcysteine alone prevented AEA- or HU210-induced ROS generation, but only partially attenuated MAPK activation and cell death. In contrast, in combination with CB(1) antagonists, N-acetylcysteine completely prevented these effects.

CONCLUSIONS AND IMPLICATIONS

CB(1) receptor activation in endothelial cells may amplify the ROS-MAPK activation-cell death pathway in pathological conditions when the endocannabinoid synthetic or metabolic pathways are dysregulated by excessive inflammation and/or oxidative/nitrosative stress, thereby contributing to the development of endothelial dysfunction and pathophysiology of multiple cardiovascular diseases.

CB2: a cannabinoid receptor with an identity crisis

CB(2) was first considered to be the 'peripheral cannabinoid receptor'. This title was bestowed based on its abundant expression in the immune system and presumed absence from the central nervous system. However, multiple recent reports question the absence of CB(2) from the central nervous system. For example, it is now well accepted that CB(2) is expressed in brain microglia during neuroinflammation. However, the extent of CB(2) expression in neurons has remained controversial. There have been studies claiming either extreme-its complete absence to its widespread expression-as well as everything in between. This review will discuss the reported tissue distribution of CB(2) with a focus on CB(2) in neurons, particularly those in the central nervous system as well as the implications of that presence. As CB(2) is an attractive therapeutic target for pain management and immune system modulation without overt psychoactivity, defining the extent of its presence in neurons will have a significant impact on drug discovery. Our recommendation is to encourage cautious interpretation of data that have been presented for and against CB(2)'s presence in neurons and to encourage continued rigorous study.

CB1 cannabinoid receptors promote oxidative/nitrosative stress, inflammation and cell death in a murine nephropathy model

BACKGROUND AND PURPOSE

Accumulating recent evidence suggests that cannabinoid-1 (CB(1)) receptor activation may promote inflammation and cell death and its pharmacological inhibition is associated with anti-inflammatory and tissue-protective effects in various preclinical disease models, as well as in humans.

EXPERIMENTAL APPROACH

In this study, using molecular biology and biochemistry methods, we have investigated the effects of genetic deletion or pharmacological inhibition of CB(1) receptors on inflammation, oxidative/nitrosative stress and cell death pathways associated with a clinically relevant model of nephropathy, induced by an important chemotherapeutic drug cisplatin.

RESULTS

Cisplatin significantly increased endocannabinoid anandamide content, activation of p38 and JNK mitogen-activated protein kinases (MAPKs), apoptotic and poly (ADP-ribose)polymerase-dependent cell death, enhanced inflammation (leucocyte infiltration, tumour necrosis factor-alpha and interleukin-1beta) and promoted oxidative/nitrosative stress [increased expressions of superoxide-generating enzymes (NOX2(gp91phox), NOX4), inducible nitric oxide synthase and tissue 4-hydroxynonenal and nitrotyrosine levels] in the kidneys of mice, accompanied by marked histopathological damage and impaired renal function (elevated creatinine and serum blood urea nitrogen) 3 days following its administration. Both genetic deletion and pharmacological inhibition of CB(1) receptors with AM281 or SR141716 markedly attenuated the cisplatin-induced renal dysfunction and interrelated oxidative/nitrosative stress, p38 and JNK MAPK activation, cell death and inflammatory response in the kidney.

CONCLUSIONS AND IMPLICATIONS

The endocannabinoid system through CB(1) receptors promotes cisplatin-induced tissue injury by amplifying MAPK activation, cell death and interrelated inflammation and oxidative/nitrosative stress. These results also suggest that inhibition of CB(1) receptors may exert beneficial effects in renal (and most likely other) diseases associated with enhanced inflammation, oxidative/nitrosative stress and cell death.

Practical management of the increasing burden of non-alcoholic fatty liver disease

Obesity-induced liver disease (non-alcoholic fatty liver disease (NAFLD)) describes a spectrum from steatosis through steatohepatitis to cirrhosis. Its prevalence is rising in tandem with societal rates of obesity which through consequent insulin resistance and fat deposition in hepatocytes lead to hepatocyte death and attempts at repair, which if persistent, lead to activation of liver fibrogenic cells. NAFLD, which may also progress to primary liver cancer, is now the most common cause of chronic liver disease in affluent countries. There is currently no single accurate diagnostic test besides a liver biopsy. The decision to consider a liver biopsy will be informed by the presence of insulin resistance determined by comparatively easy-to-measure factors together with other putative markers of progression such as hypertension. If a liver biopsy is performed, patients with steatosis with no evidence of inflammation may be less aggressively managed while those with steatohepatitis, since they have a faster trajectory to cirrhosis, should be managed more robustly. Besides lifestyle changes and increased aerobic exercise other strategies include considering referral to centres with ongoing clinical trials. Emerging treatments include α1 adrenoceptors antagonists, angiotensin receptor blockers, glitazones and vitamin E.

Hepatitis C virus induces the cannabinoid receptor 1

BACKGROUND

Activation of hepatic CB(1) receptors (CB(1)) is associated with steatosis and fibrosis in experimental forms of liver disease. However, CB(1) expression has not been assessed in patients with chronic hepatitis C (CHC), a disease associated with insulin resistance, steatosis and metabolic disturbance. We aimed to determine the importance and explore the associations of CB(1) expression in CHC.

METHODS

CB(1) receptor mRNA was measured by real time quantitative PCR on extracted liver tissue from 88 patients with CHC (genotypes 1 and 3), 12 controls and 10 patients with chronic hepatitis B (CHB). The Huh7/JFH1 Hepatitis C virus (HCV) cell culture model was used to validate results.

PRINCIPAL FINDINGS

CB(1) was expressed in all patients with CHC and levels were 6-fold higher than in controls (P<0.001). CB(1) expression increased with fibrosis stage, with cirrhotics having up to a 2 fold up-regulation compared to those with low fibrosis stage (p<0.05). Even in mild CHC with no steatosis (F0-1), CB(1) levels remained substantially greater than in controls (p<0.001) and in those with mild CHB (F0-1; p<0.001). Huh7 cells infected with JFH-1 HCV showed an 8-fold upregulation of CB(1), and CB(1) expression directly correlated with the percentage of cells infected over time, suggesting that CB(1) is an HCV inducible gene. While HCV structural proteins appear essential for CB(1) induction, there was no core genotype specific difference in CB(1) expression. CB(1) significantly increased with steatosis grade, primarily driven by patients with genotype 3 CHC. In genotype 3 patients, CB(1) correlated with SREBP-1c and its downstream target FASN (SREBP-1c; R=0.37, FASN; R=0.39, p<0.05 for both).

CONCLUSIONS/SIGNIFICANCE

CB(1) is up-regulated in CHC and is associated with increased steatosis in genotype 3. It is induced by the hepatitis C virus.

Beneficial paracrine effects of cannabinoid receptor 2 on liver injury and regeneration

The cannabinoid receptor 2 (CB2) plays a pleiotropic role in innate immunity and is a crucial mediator of liver disease. In this study, we investigated the impact of CB2 receptors on the regenerative process associated with liver injury. Following acute hepatitis induced by carbon tetrachloride (CCl(4)), CB2 was induced in the nonparenchymal cell fraction and remained undetectable in hepatocytes. Administration of CCl(4) to CB2(-/-) mice accelerated liver injury, as shown by increased alanine/aspartate aminotransferase levels and hepatocyte apoptosis, and delayed liver regeneration, as reflected by a retarded induction of hepatocyte proliferating cell nuclear antigen expression; proliferating cell nuclear antigen induction was also delayed in CB2(-/-) mice undergoing partial hepatectomy. Conversely, following treatment with the CB2 agonist JWH-133, CCl(4)-treated WT mice displayed reduced liver injury and accelerated liver regeneration. The CCl(4)-treated CB2(-/-) mice showed a decrease in inducible nitric oxide synthase and tumor necrosis factor-alpha expression, and administration of the nitric oxide donor moldomine (SIN-1) to these animals reduced hepatocyte apoptosis, without affecting liver regeneration. Impaired liver regeneration was consecutive to an interleukin-6 (IL-6)-mediated decrease in matrix metalloproteinase 2 (MMP-2) activity. Indeed, CCl(4)-treated CB2(-/-) mice displayed lower levels of hepatic IL-6 messenger RNA and increased MMP-2 activity. Administration of IL-6 to these mice decreased MMP-2 activity and improved liver regeneration, without affecting hepatocyte apoptosis. Accordingly, administration of the MMP inhibitor CTTHWGFTLC to CCl(4)-treated CB2(-/-) mice improved liver regeneration. Finally, in vitro studies demonstrated that incubation of hepatic myofibroblasts with JWH-133 increased tumor necrosis factor-alpha and IL-6 and decreased MMP-2 expressions.

CONCLUSION

CB2 receptors reduce liver injury and promote liver regeneration following acute insult, via distinct paracrine mechanisms involving hepatic myofibroblasts. These results suggest that CB2 agonists display potent hepatoprotective properties, in addition to their antifibrogenic effects.

Update on the pathophysiology of liver fibrosis

Fibrogenesis represents the main pathophysiological consequence of chronic liver disease and leads to life-threatening clinical consequences. The knowledge in this field has grown exponentially in the past 20 years and, currently, evaluation and treatment of liver fibrosis are central issues in hepatology. Classic mechanisms of liver fibrogenesis have been expanded and consolidated over the past few years. Concomitantly, novel mechanisms have been suggested and demonstrated. The aim of this article is to provide an update on these mechanisms with an attempt to integrate classic and novel pathways responsible for the evolution of the fibrogenic process and, potentially, for its regression.

Peroxisomal and microsomal lipid pathways associated with resistance to hepatic steatosis and reduced pro-inflammatory state

Accumulation of fat in the liver increases the risk to develop fibrosis and cirrhosis and is associated with development of the metabolic syndrome. Here, to identify genes or gene pathways that may underlie the genetic susceptibility to fat accumulation in liver, we studied A/J and C57Bl/6 mice that are resistant and sensitive to diet-induced hepatosteatosis and obesity, respectively. We performed comparative transcriptomic and lipidomic analysis of the livers of both strains of mice fed a high fat diet for 2, 10, and 30 days. We found that resistance to steatosis in A/J mice was associated with the following: (i) a coordinated up-regulation of 10 genes controlling peroxisome biogenesis and β-oxidation; (ii) an increased expression of the elongase Elovl5 and desaturases Fads1 and Fads2. In agreement with these observations, peroxisomal β-oxidation was increased in livers of A/J mice, and lipidomic analysis showed increased concentrations of long chain fatty acid-containing triglycerides, arachidonic acid-containing lysophosphatidylcholine, and 2-arachidonylglycerol, a cannabinoid receptor agonist. We found that the anti-inflammatory CB2 receptor was the main hepatic cannabinoid receptor, which was highly expressed in Kupffer cells. We further found that A/J mice had a lower pro-inflammatory state as determined by lower plasma levels and IL-1β and granulocyte-CSF and reduced hepatic expression of their mRNAs, which were found only in Kupffer cells. This suggests that increased 2-arachidonylglycerol production may limit Kupffer cell activity. Collectively, our data suggest that genetic variations in the expression of peroxisomal β-oxidation genes and of genes controlling the production of an anti-inflammatory lipid may underlie the differential susceptibility to diet-induced hepatic steatosis and pro-inflammatory state.

Thiol redox systems and protein kinases in hepatic stellate cell regulatory processes

Hepatic stellate cells (HSC) are the major producers of collagen in the liver and their conversion from resting cells to a proliferating, contractile and fibrogenic phenotype ('activation') is a critical step, leading to liver fibrosis characterized by deposition of excessive extracellular matrix. Cytokines, growth factors, reactive oxygen and nitrogen species (ROS/RNS), lipid peroxides and their products deriving from hepatocytes, Kupffer cells and other cells converge on HSC and influence their activation. This review focuses on glutathione and thioredoxin pathways, with particular emphasis on their role in HSC. These two systems have been shown to act in the metabolism of hydrogen peroxide, control of thiol redox balance and regulation of signalling pathways. Particular attention is paid to mitochondria and NADPH oxidase. Detailed knowledge of specific signalling, redox conditions and apoptotic processes will be of help in devising proper pharmacological treatments for liver fibrosis.

Pharmacological and non-pharmacological treatment of non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) comprises a disease spectrum ranging from simple steatosis and steatohepatitis to cirrhosis. Based on its strongest risk factors namely visceral obesity and insulin resistance, NAFLD is thought to be the hepatic manifestation of the metabolic syndrome and is considered to be the most common liver disorder in Western countries. Pathophysiological mechanisms include an enlarged pool of fatty acids, subclinical inflammation, oxidative stress and imbalances of various adipocytokines such as adiponectin. Accordingly, targets for therapeutic interventions are miscellaneous: amelioration of obesity by pharmacological, surgical or lifestyle intervention has been evaluated with success in numerous, but not all studies. Some efficacy was reported for metformin and short-term glitazone treatment. In a large recently reported trial, vitamin E supplementation improved biochemical and histological markers in subjects with non-alcoholic steatohepatitis. Blockade of the endocannabinoid system has been proposed to be a promising target in NAFLD; however, very recently the cannabinoid receptor blocker rimonabant has been withdrawn because of central nervous system toxicity. Cytoprotective therapies and statins have been mainly ineffective in NAFLD. New but so far insufficiently studied therapeutic approaches include inhibitors of the renin-angiotensin system as well as incretin mimetics respectively.

Targeting the cannabinoid pathway limits the development of fibrosis and autoimmunity in a mouse model of systemic sclerosis

Our aim was to evaluate the roles of the cannabinoid pathway in the induction and propagation of systemic sclerosis (SSc) in a mouse model of diffuse SSc induced by hypochlorite injections. BALB/c mice injected subcutaneously every day for 6 weeks with PBS or hypochlorite were treated intraperitoneally with either WIN-55,212, an agonist of the cannabinoid receptors 1 (CB1) and receptors 2 (CB2), with JWH-133, a selective agonist of CB2, or with PBS. Skin and lung fibrosis were then assessed by histological and biochemical methods, and the proliferation of fibroblasts purified from diseased skin was assessed by thymidine incorporation. Autoantibodies were detected by ELISA, and spleen cell populations were analyzed by flow cytometry. Experiments were also performed in mice deficient for CB2 receptors (Cnr2(-/-)). Injections of hypochlorite induced cutaneous and lung fibrosis as well as increased the proliferation rate of fibroblasts isolated from fibrotic skin, splenic B cell counts, and levels of anti-DNA topoisomerase-1 autoantibodies. Treatment with WIN-55,212 or with the selective CB2 agonist JWH-133 prevented the development of skin and lung fibrosis as well as reduced fibroblast proliferation and the development of autoantibodies. Experiments performed in CB2-deficient mice confirmed the influence of CB2 in the development of systemic fibrosis and autoimmunity. Therefore, we demonstrate that the CB2 receptor is a potential target for the treatment of SSc because it controls both skin fibroblast proliferation and the autoimmune reaction.

Effect of newer oral antiviral agents on future therapy of chronic hepatitis B

Long-term therapy with oral nucleoside/nucleotide analogues (NAs) is a favoured approach to the treatment of patients with chronic hepatitis B (CHB); however, all oral agents currently approved for the treatment of such patients are associated with some risk for drug resistance. This can lead to a rebound in HBV levels and, eventually, progressive liver disease. Combination therapy is one strategy that has the potential for enhanced antiviral effects and diminished or delayed resistance. The disadvantages of combination therapy include increased cost, the potential for drug interactions and increased toxicity. Additional therapeutic efficacy from combination therapy has not been demonstrated in clinical trials of HBV, and this approach might be less relevant now that potent NAs with excellent drug resistance profiles are available. However, it might be possible to identify subsets of patients (for example, those with extremely high viraemia or low baseline alanine aminotransferase levels) who derive added benefit from combination therapy. This review examines efficacy and resistance data for new low resistance oral NAs and clinical experience to date with de novo combination therapy in patients with CHB. The application of combination therapy in select populations of patients with CHB is also discussed.

Examining the roles of cannabinoids in pain and other therapeutic indications: a review

IMPORTANCE OF THE FIELD

In recent times, our knowledge of cannabinoids and the endocannabinoid system has greatly advanced. With expanding knowledge, synthetic cannabinoids - including nabilone, dronabinol and a combination of synthetic Delta9-THC and cannabidiol - have been developed and tested for benefit in a variety of therapeutic indications.

AREAS COVERED IN THIS REVIEW

The aim of this article is to provide a summative review of the vast amount of clinical trial data now available on these agents.

WHAT THE READER WILL GAIN

To locate clinical trials for review, a literature search was performed using PubMed between the dates of 25 May and 30 June 2009. Search parameters were set to isolate only human randomized controlled trials (RCTs) published between 1990 and 2009. Keywords consistently used for each search include: cannabinoids, marijuana, THC, nabilone and dronabinol. Preferential selection was given to the best-designed trials, focusing on placebo-controlled, double-blind RCTs with the largest patient populations, if available.

TAKE HOME MESSAGE

As efficacy and tolerability of these agents remain questionable, it is important that cannabinoids not be considered 'first-line' therapies for conditions for which there are more supported and better-tolerated agents. Instead, these agents could be considered in a situation of treatment failure with standard therapies or as adjunctive agents where appropriate.

Hepatic expression of endocannabinoid receptors and their novel polymorphisms in primary biliary cirrhosis

BACKGROUND

The endocannabinoid system (EC) has emerged as a crucial mediator in a variety of pathophysiological conditions.

AIMS

To evaluate: (1) whether the EC system is activated in the livers of patients with primary biliary cirrhosis (PBC); (2) if genetic variations in human EC receptor genes (CB1 and CB2) may be associated with a different phenotypic expression of the disease and response to therapy.

METHODS

The expression of CB1 and CB2 receptors was studied by immunohistochemistry in liver biopsy specimens from 13 patients with PBC, and CB1 and CB2 mRNA expression was studied by real-time polymerase chain reaction testing (RT-PCR) in liver samples. In addition, genetic polymorphisms in the EC receptor gene were sought in 68 patients with PBC from Italy, 84 patients who were residents of the United States (US), and 70 controls matched for sex, age, and for geographical area with the Italian PBC patients. Genomic DNA was extracted from peripheral venous blood leucocytes with standard methods. PCR was used to amplify the coding regions of the CB1 and CB2 genes with specific primers.

RESULTS

CB1 was markedly expressed in hepatocytes and biliary epithelial cells in the livers of patients with PBC; conversely in control liver samples, it was virtually absent. CB2 was expressed in hepatocytes and in cholangiocytes, whereas it was absent from mesenchymal cells. The mRNA of both CB1 and CB2 was detected in the PBC liver samples, as demonstrated by RT-PCR. The CB1 polymorphism (1359 G/A) was present in 26.5% of Italian patients, in 22.9% of healthy controls, and in 27.4% of patients from the US (p = n.s.). The CB2 polymorphism (188-189 AA/GG) was present in 24.4 versus 30.4% of Italian and US patients with PBC, respectively, and in 28.0% of Italian controls samples (p = n.s.). Logistic regression analysis showed that advanced histological stage and the lack of response to ursodeoxycholic acid treatment were significantly correlated with the CB1 polymorphism.

CONCLUSIONS

The EC system is markedly up-regulated in the livers of patients with PBC and it may exert a role regulating adaptive mechanisms in cholestasis.

Effect of (-)-Delta(9)-tetrahydrocannabinoid on the hepatic redox state of mice

(-)-Delta(9)-Tetrahydrocannabinol (Delta(9)-THC), a psychoactive component of marijuana, has been reported to induce oxidative damage in vivo and in vitro. In this study, we administered Delta(9)-THC to healthy C57BL/6J mice aged 15 weeks in order to determine its effect on hepatic redox state. Mice were divided into 3 groups: Delta(9)-THC (N = 10), treated with 10 mg/kg body weight Delta(9)-THC daily; VCtrl (N = 10), treated with vehicle [1:1:18, cremophor EL (polyoxyl 35 castor oil)/ethanol/saline]; Ctrl (N = 10), treated with saline. Animals were injected ip twice a day with 5 mg/kg body weight for 10 days. Lipid peroxidation, protein carbonylation and DNA oxidation were used as biomarkers of oxidative stress. The endogenous antioxidant defenses analyzed were glutathione (GSH) levels as well as enzyme activities of superoxide dismutase, catalase, glutathione S-transferase, glutathione reductase, and glutathione peroxidase (GPx) in liver homogenates. The levels of mRNA of the cannabinoid receptors CB1 and CB2 were also monitored. Treatment with Delta(9)-THC did not produce significant changes in oxidative stress markers or in mRNA levels of CB1 and CB2 receptors in the liver of mice, but attenuated the increase in the selenium-dependent GPx activity (Delta(9)-THC: 8%; VCtrl: 23% increase) and the GSH/oxidized GSH ratio (Delta(9)-THC: 61%; VCtrl: 96% increase), caused by treatment with the vehicle. Delta(9)-THC administration did not show any harmful effects on lipid peroxidation, protein carboxylation or DNA oxidation in the healthy liver of mice but attenuated unexpected effects produced by the vehicle containing ethanol/cremophor EL.

Role of cannabinoids in the development of fatty liver (steatosis)

Emerging evidence suggests that cannabinoids play an important role in the modulation of fatty liver, which appears to be mediated via activation of cannabinoid receptors. Steatogenic agents such as ethanol and high-fat diet can upregulate the activity of cannabinoid 1 (CB1) receptors via increasing synthesis of endocannabinoids, 2-arachidonoylglycerol, and anandamide. CB1 receptors can also be upregulated by obesity. CB1 receptor activation results in upregulation of lipogenic transcription factor, sterol regulatory element-binding protein 1c and its target enzymes, acetyl-CoA carboxylase-1, and fatty acid synthase and concomitantly, downregulation of carnitine palmitoyltransferase-1. This leads to increased de novo fatty acid synthesis as well as decreased fatty acid oxidation, culminating into the development of fatty liver. High-fat diet, in addition to CB1 receptor activation, appears to activate CB2 receptors that may also contribute to fatty liver. In non-alcoholic fatty liver disease, CB2 receptor activation is associated with the development of fatty liver. Cannabis smoking can increase the severity of fatty liver in hepatitis C patients although the precise mechanism is unknown. As the mechanisms involved in endocannabinoid receptor signaling are being increasingly well understood and the biosynthetic regulatory elements elucidated, these present good opportunity for the pharmaceutical scientists to design drugs to treat liver diseases, including steatosis, based on the cannabinoids, endocannabinoids, and related templates.

Role of cannabinoids in the development of fatty liver (steatosis)

Emerging evidence suggests that cannabinoids play an important role in the modulation of fatty liver, which appears to be mediated via activation of cannabinoid receptors. Steatogenic agents such as ethanol and high-fat diet can upregulate the activity of cannabinoid 1 (CB1) receptors via increasing synthesis of endocannabinoids, 2-arachidonoylglycerol, and anandamide. CB1 receptors can also be upregulated by obesity. CB1 receptor activation results in upregulation of lipogenic transcription factor, sterol regulatory element-binding protein 1c and its target enzymes, acetyl-CoA carboxylase-1, and fatty acid synthase and concomitantly, downregulation of carnitine palmitoyltransferase-1. This leads to increased de novo fatty acid synthesis as well as decreased fatty acid oxidation, culminating into the development of fatty liver. High-fat diet, in addition to CB1 receptor activation, appears to activate CB2 receptors that may also contribute to fatty liver. In non-alcoholic fatty liver disease, CB2 receptor activation is associated with the development of fatty liver. Cannabis smoking can increase the severity of fatty liver in hepatitis C patients although the precise mechanism is unknown. As the mechanisms involved in endocannabinoid receptor signaling are being increasingly well understood and the biosynthetic regulatory elements elucidated, these present good opportunity for the pharmaceutical scientists to design drugs to treat liver diseases, including steatosis, based on the cannabinoids, endocannabinoids, and related templates.

Oxidation of the endogenous cannabinoid arachidonoyl ethanolamide by the cytochrome P450 monooxygenases: physiological and pharmacological implications

Arachidonoyl ethanolamide (anandamide) is an endogenous amide of arachidonic acid and an important signaling mediator of the endocannabinoid system. Given its numerous roles in maintaining normal physiological function and modulating pathophysiological responses throughout the body, the endocannabinoid system is an important pharmacological target amenable to manipulation directly by cannabinoid receptor ligands or indirectly by drugs that alter endocannabinoid synthesis and inactivation. The latter approach has the possible advantage of more selectivity, thus there is the potential for fewer untoward effects like those that are traditionally associated with cannabinoid receptor ligands. In that regard, inhibitors of the principal inactivating enzyme for anandamide, fatty acid amide hydrolase (FAAH), are currently in development for the treatment of pain and inflammation. However, several pathways involved in anandamide synthesis, metabolism, and inactivation all need to be taken into account when evaluating the effects of FAAH inhibitors and similar agents in preclinical models and assessing their clinical potential. Anandamide undergoes oxidation by several human cytochrome P450 (P450) enzymes, including CYP3A4, CYP4F2, CYP4X1, and the highly polymorphic CYP2D6, forming numerous structurally diverse lipids, which are likely to have important physiological roles, as evidenced by the demonstration that a P450-derived epoxide of anandamide is a potent agonist for the cannabinoid receptor 2. The focus of this review is to emphasize the need for a better understanding of the P450-mediated pathways of the metabolism of anandamide, because these are likely to be important in mediating endocannabinoid signaling as well as the pharmacological responses to endocannabinoid-targeting drugs.

Signal transduction via cannabinoid receptors

The endocannabinoids anandamide and 2-arachidonoylglycerol are lipid mediators that signal via CB(1) and CB(2) cannabinoid receptors and Gi/o-proteins to inhibit adenylyl cyclase and stimulate mitogen-activated protein kinase. In the brain, CB(1) receptors interact with opioid receptors in close proximity, and these receptors may share G-proteins and effector systems. In the striatum, CB(1) receptors function in coordination with D(1) and D(2) dopamine receptors, and combined stimulation of CB(1)-D(2) receptor heteromeric complexes promotes a unique interaction to stimulate cAMP production. CB(1) receptors also trigger growth factor receptor signaling cascades in cells by engaging in cross-talk or interreceptor signal transmission with the receptor tyrosine kinase (RTK) family. Mechanisms for CB(1) receptor-RTK transactivation can include stimulation of signal transduction pathways regulated by second messengers such as phospholipase C, metalloprotease cleavage of membrane-bound precursor proteins such as epidermal growth factor which activate RTKs, RTK autophosphorylation, and recruitment of non-receptor tyrosine kinases. CB(1) and CB(2) receptors are expressed in peripheral tissues including liver and adipose tissue, and are induced in pathological conditions. Novel signal transduction resulting from endocannabinoid regulation of AMP-regulated kinase and peroxisome proliferator-activated receptors have been discovered from studies of hepatocytes and adipocytes. It can be predicted that drug discovery of the future will be based upon these novel signal transduction mechanisms for endocannabinoid mediators.