Identification and characterization of infiltrating macrophages in acetaminophen-induced liver injury

The dual role of osteopontin in acetaminophen hepatotoxicity

AIM

Osteopontin (OPN), a multifunctional protein, has been reported to be protoxicant in acetaminophen hepatotoxicity. In this study, the mechanisms underlying the detrimental role of OPN in acetaminophen toxicity were explored.

METHODS

Male C57BL/6 (wild-type, WT) and OPN(-/-) mice were administered with acetaminophen (500 mg/kg, ip). After the treatment, serum transaminase (ALT), as well as OPN expression, histology changes, oxidative stress and inflammation response in liver tissue were studied. Freshly isolated hepatocytes of WT and OPN(-/-) mice were prepared.

RESULTS

Acetaminophen administration significantly increased OPN protein level in livers of WT mice. OPN expression was mainly localized in hepatic macrophages 6 h after the administration. In OPN(-/-) mice, acetaminophen-induced serum ALT release was reduced, but the centrilobular hepatic necrosis was increased. In OPN(-/-) mice, the expression of CYP2E1 and CYP1A2 in livers was significantly increased; GSH depletion and lipid peroxidation in livers were enhanced. On the other hand, OPN(-/-) mice exhibited less macrophage and neutrophil infiltration and reduced expression of proinflammatory cytokines TNF-α and IL-1α in livers. An anti-OPN neutralizing antibody significantly reduced acetaminophen-induced serum ALT level and inflammatory infiltration in livers of WT mice.

CONCLUSION

OPN plays a dual role in acetaminophen toxicity: OPN in hepatocytes inhibits acetaminophen metabolism, while OPN in macrophages enhances acetaminophen toxicity via recruitment of inflammatory cells and production of proinflammatory cytokines.

Source and characterization of hepatic macrophages in acetaminophen-induced acute liver failure in humans

Acetaminophen-induced acute liver failure (AALF) is associated with innate immunity activation, which contributes to the severity of hepatic injury and clinical outcome. A marked increase in hepatic macrophages (h-mφ) is observed in experimental models of AALF, but controversy exists regarding their role, implicating h-mφ in both aggravation and resolution of liver injury. The role of h-mφ in human AALF is virtually unexplored. We sought to investigate the role of chemokine (C-C motif) ligand 2 (CCL2) in the recruitment of circulating monocytes to the inflamed liver and to determine how the h-mφ infiltrate and liver microenvironment may contribute to tissue repair versus inflammation in AALF. We evaluated circulating monocytes, their chemokine (C-C motif) receptor 2 (CCR2) expression, and serum CCL2 levels in patients with AALF. Cell subsets and numbers of circulation-derived (MAC387+) or resident proliferating (CD68/Ki67+) h-mφ in hepatic immune infiltrates were determined by immunohistochemistry. Inflammatory cytokine levels were determined in whole and laser microdissected liver tissue by proteome array. In AALF, circulating monocytes were depleted, with the lowest levels observed in patients with adverse outcomes. CCL2 levels were high in AALF serum and hepatic tissue, and circulating monocyte subsets expressed CCR2, suggesting CCL2-dependent hepatic monocyte recruitment. Significant numbers of both MAC387+ and CD68+ h-mφ were found in AALF compared with control liver tissue with a high proportion expressing the proliferation marker Ki67. Levels of CCL2, CCL3, interleukin (IL)-6, IL-10, and transforming growth factor-β1 were significantly elevated in AALF liver tissue relative to chronic liver disease controls.

CONCLUSION

In AALF, the h-mφ population is expanded in areas of necrosis, both through proliferation of resident cells and CCL2-dependent recruitment of circulating monocytes. The presence of h-mφ within an anti-inflammatory/regenerative microenvironment indicates that they are implicated in resolution of inflammation/tissue repair processes during AALF.

The evolving story of macrophages in acute liver failure

Acute liver failure (ALF) remains a worldwide problem. The innate immune system acts as an important regulator of ALF. Kupffer cells (KCs), the resident macrophages in liver, play a key role in liver innate immune response. Recent researches have shown that macrophages display a remarkable plasticity and can differentiate into functionally diverse subsets. However, the dynamic polarized phenotypes and functional status of macrophages at different stage of ALF are not clear. In this paper, we present a review of evidence that KCs play a significant role in the pathogenesis of ALF, including the phenotype and functions of macrophages, signaling pathways involved in macrophage functional status and cell-crosstalks of KCs with other immune cells. More information on macrophages will promote a better understanding of the cellular molecular mechanisms of ALF and provide new insights for the development of therapeutic targets for ALF.

Spred-2 deficiency exacerbates acetaminophen-induced hepatotoxicity in mice

MAPKs are involved in acetaminophen (APAP)-hepatotoxicity, but the regulatory mechanism remains unknown. Here, we explored the role of Spred-2 that negatively regulates Ras/ERK pathway in APAP-hepatotoxicity. Spred-2 knockout (KO) mice demonstrated exacerbated liver injury, an event that was associated with increased numbers of CD4(+) T, CD8(+) T and NK cells in the liver compared to the control. Levels of CXCL9/CXCL10 that attract and activate these cells were increased in Spred-2 KO-liver. Kupffer cells isolated from Spred-2 KO mice after APAP challenge expressed higher levels of CXCL9/CXCL10 than those from the control. Upon stimulation with APAP or IFNγ, naïve Kupffer cells from Spred-2 KO mice expressed higher levels of CXCL9/CXCL10. NK cell-depletion attenuated APAP-hepatotoxicity with lowered hepatic IFNγ and decreased numbers of not only NK cells but also CD4(+) T and CD8(+) T cells in the liver. These results suggest that Spred-2 negatively regulates APAP-hepatotoxicity under the control of Kupffer cells and NK cells.

The role of chemokines in acute liver injury

Chemokines are small molecular weight proteins primarily known to drive migration of immune cell populations. In both acute and chronic liver injury, hepatic chemokine expression is induced resulting in inflammatory cell infiltration, angiogenesis, and cell activation and survival. During acute injury, massive parenchymal cell death due to apoptosis and/or necrosis leads to chemokine production by hepatocytes, cholangiocytes, Kupffer cells, hepatic stellate cells, and sinusoidal endothelial cells. The specific chemokine profile expressed during injury is dependent on both the type and course of injury. Hepatotoxicity by acetaminophen for example leads to cellular necrosis and activation of Toll-like receptors while the inciting insult in ischemia reperfusion injury produces reactive oxygen species and subsequent production of pro-inflammatory chemokines. Chemokine expression by these cells generates a chemoattractant gradient promoting infiltration by monocytes/macrophages, NK cells, NKT cells, neutrophils, B cells, and T cells whose activity are highly regulated by the specific chemokine profiles within the liver. Additionally, resident hepatic cells express chemokine receptors both in the normal and injured liver. While the role of these receptors in normal liver has not been well described, during injury, receptor up-regulation, and chemokine engagement leads to cellular survival, proliferation, apoptosis, fibrogenesis, and expression of additional chemokines and growth factors. Hepatic-derived chemokines can therefore function in both paracrine and autocrine fashions further expanding their role in liver disease. More recently it has been appreciated that chemokines can have diverging effects depending on their temporal expression pattern and the type of injury. A better understanding of chemokine/chemokine receptor axes will therefore pave the way for development of novel targeted therapies for the treatment of liver disease.

Kupffer cells express a unique combination of phenotypic and functional characteristics compared with splenic and peritoneal macrophages

The immunostimulatory role of Kupffer cells in various inflammatory liver diseases is still not fully understood. In this study, phenotypic and functional aspects of Kupffer cells from healthy C57BL/6 mice were analyzed and compared with those of splenic and peritoneal macrophages to generate a blueprint of the cells under steady-state conditions. In the mouse liver, only one population of Kupffer cells was identified as F4/80(high)CD11b(low) cells. We observed that freshy isolated Kupffer cells are endocytic and show a relatively high basal ROS content. Interestingly, despite expression of TLR mRNA on Kupffer cells, ligation of TLR4, TLR7/8, and TLR9 resulted in a weak induction of IL-10, low or undetectable levels of IL-12p40 and TNF, and up-regulation of CD40 on the surface. Kupffer cells and splenic macrophages show functional similarities, in comparison with peritoneal macrophages, as reflected by comparable levels of TLR4, TLR7/8, and TLR9 mRNA and low or undetectable levels of TNF and IL-12p40 produced upon TLR ligation. The unique, functional characteristics of Kupffer cells, demonstrated in this study, suggest that Kupffer cells under steady-state conditions are specialized as phagocytes to clear and degrade particulates and only play a limited immunoregulatory role via the release of soluble mediators.

Regulation of alternative macrophage activation in the liver following acetaminophen intoxication by stem cell-derived tyrosine kinase

Stem cell-derived tyrosine kinase (STK) is a transmembrane receptor reported to play a role in macrophage switching from a classically activated/proinflammatory phenotype to an alternatively activated/wound repair phenotype. In the present studies, STK⁻/⁻ mice were used to assess the role of STK in acetaminophen-induced hepatotoxicity as evidence suggests that the pathogenic process involves both of these macrophage subpopulations. In wild type mice, centrilobular hepatic necrosis and increases in serum transaminase levels were observed within 6h of acetaminophen administration (300 mg/kg, i.p.). Loss of STK resulted in a significant increase in sensitivity of mice to the hepatotoxic effects of acetaminophen and increased mortality, effects independent of its metabolism. This was associated with reduced levels of hepatic glutathione, rapid upregulation of inducible nitric oxide synthase, and prolonged induction of heme oxygenase-1, suggesting excessive oxidative stress in STK⁻/⁻ mice. F4/80, a marker of mature macrophages, was highly expressed on subpopulations of Kupffer cells in livers of wild type, but not STK⁻/⁻ mice. Whereas F4/80⁺ macrophages rapidly declined in the livers of wild type mice following acetaminophen intoxication, they increased in STK⁻/⁻ mice. In wild type mice hepatic expression of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and IL-12, products of classically activated macrophages, increased after acetaminophen administration. Monocyte chemotactic protein-1 (MCP-1) and its receptor, CCR2, as well as IL-10, mediators involved in recruiting and activating anti-inflammatory/wound repair macrophages, also increased in wild type mice after acetaminophen. Loss of STK blunted the effects of acetaminophen on expression of TNFα, IL-1β, IL-12, MCP-1 and CCR2, while expression of IL-10 increased. Hepatic expression of CX3CL1, and its receptor, CX3CR1 also increased in STK⁻/⁻ mice treated with acetaminophen. These data demonstrate that STK plays a role in regulating macrophage recruitment and activation in the liver following acetaminophen administration, and in hepatotoxicity.

Acetaminophen hepatotoxicity and repair: the role of sterile inflammation and innate immunity

Acetaminophen (APAP) hepatotoxicity because of overdose is the most frequent cause of acute liver failure in the western world. Metabolic activation of APAP and protein adduct formation, mitochondrial dysfunction, oxidant stress, peroxynitrite formation and nuclear DNA fragmentation are critical intracellular events in hepatocytes. However, the early cell necrosis causes the release of a number of mediators such as high-mobility group box 1 protein, DNA fragments, heat shock proteins (HSPs) and others (collectively named damage-associated molecular patterns), which can be recognized by toll-like receptors on macrophages, and leads to their activation with cytokine and chemokine formation. Although pro-inflammatory mediators recruit inflammatory cells (neutrophils, monocytes) into the liver, neither the infiltrating cells nor the activated resident macrophages cause any direct cytotoxicity. In contrast, pro- and anti-inflammatory cytokines and chemokines can directly promote intracellular injury mechanisms by inducing nitric oxide synthase or inhibit cell death mechanisms by the expression of acute-phase proteins (HSPs, heme oxygenase-1) and promote hepatocyte proliferation. In addition, the newly recruited macrophages (M2) and potentially neutrophils are involved in the removal of necrotic cell debris in preparation for tissue repair and resolution of the inflammatory response. Thus, as discussed in detail in this review, the preponderance of experimental evidence suggests that the extensive sterile inflammatory response during APAP hepatotoxicity is predominantly beneficial by limiting the formation and the impact of pro-inflammatory mediators and by promoting tissue repair.

Macrophages and tissue injury: agents of defense or destruction?

The past several years have seen the accumulation of evidence demonstrating that tissue injury induced by diverse toxicants is due not only to their direct effects on target tissues but also indirectly to the actions of resident and infiltrating macrophages. These cells release an array of mediators with cytotoxic, pro- and anti-inflammatory, angiogenic, fibrogenic, and mitogenic activity, which function to fight infections, limit tissue injury, and promote wound healing. However, following exposure to toxicants, macrophages can become hyperresponsive, resulting in uncontrolled or dysregulated release of mediators that exacerbate acute tissue injury and/or promote the development of chronic diseases such as fibrosis and cancer. Evidence suggests that the diverse activity of macrophages is mediated by distinct subpopulations that develop in response to signals within their microenvironment. Understanding the precise roles of these different macrophage populations in the pathogenic response to toxicants is key to designing effective treatments for minimizing tissue damage and chronic disease and for facilitating wound repair.

Role of the Nalp3 inflammasome in acetaminophen-induced sterile inflammation and liver injury

Acetaminophen (APAP) overdose is the leading cause of acute liver failure in the US and UK. Recent studies implied that APAP-induced injury is partially mediated by interleukin-1β (IL-1β), which can activate and recruit neutrophils, exacerbating injury. Mature IL-1β is formed by caspase-1, dependent on inflammasome activation. The objective of this invetstigation was to evaluate the role of the Nalp3 inflammasome on release of damage associated molecular patterns (DAMPs), hepatic neutrophil accumulation and liver injury (ALT, necrosis) after APAP overdose. Mice deficient for each component of the Nalp3 inflammasome (caspase-1, ASC and Nalp3) were treated with 300mg/kg APAP for 24h; these mice had similar neutrophil recruitment and liver injury as APAP-treated C57Bl/6 wildtype animals. In addition, plasma levels of DAMPs (DNA fragments, keratin-18, hypo- and hyper-acetylated forms of high mobility group box-1 protein) were similarly elevated with no significant difference between wildtype and gene knockout mice. In addition, aspirin treatment, which has been postulated to attenuate cytokine formation and the activation of the Nalp3 inflammasome after APAP, had no effect on release of DAMPs, hepatic neutrophil accumulation or liver injury. Together, these data confirm the release of DAMPs and a sterile inflammatory response after APAP overdose. However, as previously reported minor endogenous formation of IL-1β and the activation of the Nalp3 inflammasome have little impact on APAP hepatotoxicity. It appears that the Nalp3 inflammasome is not a promising therapeutic target to treat APAP overdose.

Current issues with acetaminophen hepatotoxicity--a clinically relevant model to test the efficacy of natural products

There is a significant need to evaluate the therapeutic potential of natural products and other compounds purported to be hepatoprotective. Acetaminophen-induced liver injury, especially in mice, is an attractive and widely used model for this purpose because it is both clinically relevant and experimentally convenient. However, the pathophysiology of liver injury after acetaminophen overdose is complex. This review describes the multiple steps and signaling pathways involved in acetaminophen-mediated cell death. The toxicity is initiated by the formation of a reactive metabolite, which depletes glutathione and binds to cellular proteins, especially in mitochondria. The resulting mitochondrial oxidant stress and peroxynitrite formation, in part through amplification by c-jun-N-terminal kinase activation, leads to mitochondrial DNA damage and opening of the mitochondrial permeability transition pore. Endonucleases from the mitochondrial intermembrane space and lysosomes are responsible for nuclear DNA fragmentation. Despite the oxidant stress, lipid peroxidation is not a relevant mechanism of injury. The mitochondrial dysfunction and nuclear DNA damage ultimately cause oncotic necrotic cell death with release of damage-associated molecular patterns that trigger a sterile inflammatory response. Current evidence supports the hypothesis that innate immune cells do not contribute to injury but are involved in cell debris removal and regeneration. This review discusses the latest mechanistic aspects of acetaminophen hepatotoxicity and demonstrates ways to assess the mechanisms of drug action and design experiments needed to avoid pitfalls and incorrect conclusions. This review should assist investigators in the optimal use of this model to test the efficacy of natural compounds and obtain reliable mechanistic information.

Critical care "normality": individualized versus protocolized care

Patients with critical illness are heterogeneous, with differing physiologic requirements over time. Goal-directed therapy in the emergency room demonstrates that protocolized care could result in improved outcomes. Subsequent studies have confirmed benefit with such a "bundle-based approach" in the emergency room and in preoperative and postoperative scenarios. However, this cannot be necessarily extrapolated to the medium-term and long-term care pathway of the critically ill patient. It is likely that the development of mitochondrial dysfunction could result in goal-directed types of approaches being detrimental. Equally, arterial pressure aims are likely to be considerably different as the patient's physiology moves toward "hibernation." The agents we utilize as sedative and pressor agents have considerable effects on immune function and the inflammatory profile, and should be considered as part of the total clinical picture. The role of gut failure in driving inflammation is considerable, and the drive to feed enterally, regardless of aspirate volume, may be detrimental in those with degrees of ileus, which is often a difficult diagnosis in the critically ill. The pathogenesis of liver dysfunction may be, at least in part, related to venous engorgement that will contribute toward portal hypertension and gut edema. This, in association with loss of the hepatosplanchnic buffer response, it is likely to contribute to venous pooling in the abdominal cavity, impaired venous return, and decreased central blood volumes. Therapies such as those used in "small-for-size syndrome" may have a role in the chronic stages of septic vascular failure.

Reactive oxygen and mechanisms of inflammatory liver injury: Present concepts

Liver cell death induced by stresses such as ischemia-reperfusion, cholestasis and drug toxicity can trigger a sterile inflammatory response with activation of innate immune cells through release of damage-associated molecular patterns (DAMPs). A similar inflammatory response can be induced by pathogen-associated molecular patterns (PAMPs) such as endotoxin. Both DAMPs and PAMPs activate through toll-like receptors the resident macrophages (Kupffer cells) and recruit activated neutrophils and monocytes into the liver. Central to this inflammatory response is promotion of reactive oxygen species (ROS) formation by these phagocytes. ROS are the principal toxic mediators by which inflammatory cells kill their targets, e.g. bacteria during host defense but also hepatocytes and other liver cells. The mechanism of ROS-induced cell killing during inflammation involves the promotion of mitochondrial dysfunction through an intracellular oxidant stress in hepatocytes leading mainly to oncotic necrosis and less apoptosis. The additional release of cell contents amplifies the inflammatory injury. However, an inflammatory oxidant stress insufficient to directly cause cell damage can induce transcription of stress defence genes including antioxidant genes. This preconditioning effect of ROS enhances the resistance against future inflammatory oxidant stress and promotes the initiation of tissue repair processes. Despite the substantial progress in our understanding of mechanisms of inflammatory liver injury during the last decade, more research is necessary to better understand the role of ROS in acute liver inflammation and to develop clinically applicable therapeutic strategies that selectively target the detrimental effects of oxidant stress without compromising the vital function of ROS in host defense.

Vascular endothelial growth factor receptor-1 signaling promotes liver repair through restoration of liver microvasculature after acetaminophen hepatotoxicity

Vascular endothelial growth factor (VEGF) and its receptors promote liver regeneration. The objective of the present study was to examine the role of VEGF receptor 1 (VEGFR1) signaling in hepatic tissue repair after acetaminophen (N-acetyl-para-aminophenol) (APAP)-induced liver injury. To do this, we treated VEGFR1 tyrosine kinase knockout (VEGFR1 TK(-/-)) and wild-type (WT) mice with APAP (300 mg/kg, ip). In WT mice, serum levels of alanine aminotransferase (ALT) and the necrotic area peaked between 8 and 24 h and then declined. In VEGFR1 TK(-/-) mice, ALT levels remained high at 48 h and extensive hepatic necrosis and hemorrhage were observed, as well as high mortality. Downregulation of hepatic messenger RNA expression of VEGFR1 and VEGFR2 was also noted in VEGFR1 TK(-/-) mice. VEGFR1 TK(-/-) mice displayed lower expression of proliferating cell nuclear antigen and of growth factors including hepatocyte growth factor, CD31, and basic fibroblast growth factor than WT. The hepatic microvasculature in VEGFR1 TK(-/-) was compromised as evidenced by impaired sinusoidal perfusion, suppressed endocytosis in liver sinusoidal endothelial cells (LSECs), and the formation of large gaps in LSECs. In WT mice, immunofluorescence revealed that recruited VEGFR1(+) cells in the necrotic area were positive for CD11b. VEGFR1 TK(-/-) exhibited fewer VEGFR1(+) and VEGFR2(+) cells. These results suggest that VEGFR1 signaling facilitates liver recovery from APAP hepatotoxicity by preventing excessive hemorrhage and reconstituting the sinusoids through recruitment of VEGFR1-expressing macrophages to the injured area and also through affecting expression of genes including hepatotrophic and pro-angiogenic growth factors.

Innate immunity of the liver microcirculation

The liver is a complex organ with a unique microcirculation and both synthetic and immune functions. Innate immune responses have been studied in response to single inflammatory mediators and several clinically relevant models of infection and injury. While standard histological techniques have been used in many models, the liver microcirculation is also amenable to in vivo examination using epifluorescent, confocal and transillumination intravital microscopy. These techniques have begun to clarify not only the molecular mechanisms but also the specific cell populations involved in the liver inflammation. In this review, we discuss the cells and mediators involved in hepatic innate immunity in simple and complex models of injury and infection, and present the view that the liver microcirculation utilizes non-classical pathways for leukocyte recruitment.

Inflammatory responses are not sufficient to cause delayed neuronal death in ATP-induced acute brain injury

BACKGROUND

Brain inflammation is accompanied by brain injury. However, it is controversial whether inflammatory responses are harmful or beneficial to neurons. Because many studies have been performed using cultured microglia and neurons, it has not been possible to assess the influence of multiple cell types and diverse factors that dynamically and continuously change in vivo. Furthermore, behavior of microglia and other inflammatory cells could have been overlooked since most studies have focused on neuronal death. Therefore, it is essential to analyze the precise roles of microglia and brain inflammation in the injured brain, and determine their contribution to neuronal damage in vivo from the onset of injury.

METHODS AND FINDINGS

Acute neuronal damage was induced by stereotaxic injection of ATP into the substantia nigra pars compacta (SNpc) and the cortex of the rat brain. Inflammatory responses and their effects on neuronal damage were investigated by immunohistochemistry, electron microscopy, quantitative RT-PCR, and stereological counting, etc. ATP acutely caused death of microglia as well as neurons in a similar area within 3 h. We defined as the core region the area where both TH(+) and Iba-1(+) cells acutely died, and as the penumbra the area surrounding the core where Iba-1(+) cells showed activated morphology. In the penumbra region, morphologically activated microglia arranged around the injury sites. Monocytes filled the damaged core after neurons and microglia died. Interestingly, neither activated microglia nor monocytes expressed iNOS, a major neurotoxic inflammatory mediator. Monocytes rather expressed CD68, a marker of phagocytic activity. Importantly, the total number of dopaminergic neurons in the SNpc at 3 h (∼80% of that in the contralateral side) did not decrease further at 7 d. Similarly, in the cortex, ATP-induced neuron-damage area detected at 3 h did not increase for up to 7 d.

CONCLUSIONS

Different cellular components (microglia, astrocytes, monocytes, and neutrophils) and different factors (proinflammatory and neurotrophic) could be produced in inflammatory processes depending on the nature of the injury. The results in this study suggest that the inflammatory responses of microglia and monocytes in response to ATP-induced acute injury could not be neurotoxic.

Role of caspase-1 and interleukin-1beta in acetaminophen-induced hepatic inflammation and liver injury

Acetaminophen (APAP) overdose can result in serious liver injury and potentially death. Toxicity is dependent on metabolism of APAP to a reactive metabolite initiating a cascade of intracellular events resulting in hepatocellular necrosis. This early injury triggers a sterile inflammatory response with formation of cytokines and innate immune cell infiltration in the liver. Recently, IL-1beta signaling has been implicated in the potentiation of APAP-induced liver injury. To test if IL-1beta formation through caspase-1 is critical for the pathophysiology, C57Bl/6 mice were treated with the pan-caspase inhibitor Z-VD-fmk to block the inflammasome-mediated maturation of IL-1beta during APAP overdose (300 mg/kg APAP). This intervention did not affect IL-1beta gene transcription but prevented the increase in IL-1beta plasma levels. However, APAP-induced liver injury and neutrophil infiltration were not affected. Similarly, liver injury and the hepatic neutrophilic inflammation were not attenuated in IL-1-receptor-1 deficient mice compared to wild-type animals. To evaluate the potential of IL-1beta to increase injury, mice were given pharmacological doses of IL-1beta after APAP overdose. Despite increased systemic activation of neutrophils and recruitment into the liver, there was no alteration in injury. We conclude that endogenous IL-1beta formation after APAP overdose is insufficient to activate and recruit neutrophils into the liver or cause liver injury. Even high pharmacological doses of IL-1beta, which induce hepatic neutrophil accumulation and activation, do not enhance APAP-induced liver injury. Thus, IL-1 signaling is irrelevant for APAP hepatotoxicity. The inflammatory cascade is a less important therapeutic target than intracellular signaling pathways to attenuate APAP-induced liver injury.

Forkhead box M1 transcription factor is required for macrophage recruitment during liver repair

Acute liver injury results from exposure to toxins, pharmacological agents, or viral infections, contributing to significant morbidity and mortality worldwide. While hepatic inflammation is critical for liver repair, the transcriptional mechanisms required for the recruitment of inflammatory cells to the liver are not understood. Forkhead box M1 (Foxm1) transcription factor is a master regulator of hepatocyte proliferation, but its role in inflammatory cells remains unknown. In this study, we generated transgenic mice in which Foxm1 was deleted from myeloid-derived cells, including macrophages, monocytes, and neutrophils. Carbon tetrachloride liver injury was used to demonstrate that myeloid-specific Foxm1 deletion caused a delay in liver repair. Although Foxm1 deficiency did not influence neutrophil infiltration into injured livers, the total numbers of mature macrophages were dramatically reduced. Surprisingly, Foxm1 deficiency did not influence the proliferation of macrophages or their monocytic precursors but impaired monocyte recruitment during liver repair. Expression of L-selectin and the CCR2 chemokine receptor, both critical for monocyte recruitment to injured tissues, was decreased. Foxm1 induced transcriptional activity of the mouse CCR2 promoter in cotransfection experiments. Adoptive transfer of monocytes to Foxm1-deficient mice restored liver repair and rescued liver function. Foxm1 is critical for liver repair and is required for the recruitment of monocytes to the injured liver.

Peroxiredoxin: a central player in immune modulation

Peroxiredoxins (Prx) are a family of anti-oxidants that protect cells from metabolically produced reactive oxygen species (ROS). The presence of these enzymes in the secretomes of many parasitic helminths suggests they provide protection against ROS released by host immune effector cells. However, we recently reported that helminth-secreted Prx also contribute to the development of Th2-responses via a mechanism involving the induction of alternatively activated macrophages. In this review, we discuss the role helminth Prx may play in modulating the immune responses of their hosts.

Tip-DC development during parasitic infection is regulated by IL-10 and requires CCL2/CCR2, IFN-gamma and MyD88 signaling

The development of classically activated monocytic cells (M1) is a prerequisite for effective elimination of parasites, including African trypanosomes. However, persistent activation of M1 that produce pathogenic molecules such as TNF and NO contributes to the development of trypanosome infection-associated tissue injury including liver cell necrosis in experimental mouse models. Aiming to identify mechanisms involved in regulation of M1 activity, we have recently documented that during Trypanosoma brucei infection, CD11b⁺Ly6C⁺CD11c⁺ TNF and iNOS producing DCs (Tip-DCs) represent the major pathogenic M1 liver subpopulation. By using gene expression analyses, KO mice and cytokine neutralizing antibodies, we show here that the conversion of CD11b⁺Ly6C⁺ monocytic cells to pathogenic Tip-DCs in the liver of T. brucei infected mice consists of a three-step process including (i) a CCR2-dependent but CCR5- and Mif-independent step crucial for emigration of CD11b⁺Ly6C⁺ monocytic cells from the bone marrow but dispensable for their blood to liver migration; (ii) a differentiation step of liver CD11b⁺Ly6C⁺ monocytic cells to immature inflammatory DCs (CD11c⁺ but CD80/CD86/MHC-II^low) which is IFN-γ and MyD88 signaling independent; and (iii) a maturation step of inflammatory DCs to functional (CD80/CD86/MHC-II^high) TNF and NO producing Tip-DCs which is IFN-γ and MyD88 signaling dependent. Moreover, IL-10 could limit CCR2-mediated egression of CD11b⁺Ly6C⁺ monocytic cells from the bone marrow by limiting Ccl2 expression by liver monocytic cells, as well as their differentiation and maturation to Tip-DCs in the liver, showing that IL-10 works at multiple levels to dampen Tip-DC mediated pathogenicity during T. brucei infection. A wide spectrum of liver diseases associates with alteration of monocyte recruitment, phenotype or function, which could be modulated by IL-10. Therefore, investigating the contribution of recruited monocytes to African trypanosome induced liver injury could potentially identify new targets to treat hepatic inflammation in general, and during parasite infection in particular.

Most infections are associated with host inflammatory responses that can result in multiple organ failure and death. It is therefore essential to understand the mechanisms balancing host immune response and tissue damage. Mouse models of African trypanosome infection represent valuable tools to study the mechanisms contributing to the inflammatory (pathogenic) or anti-inflammatory (anti-pathogenic) immune response. We recently identified TNF and NO producing DCs (Tip-DCs) as major contributors to liver pathogenicity in Trypanosoma brucei infected mice. Herein, the role of different chemokine and cytokines in the generation of Tip-DCs was investigated. Tip-DCs originated from bone marrow derived monocytes that egressed to the blood in a CCR2 chemokine receptor dependent manner. Then, monocytes extravasated to inflamed liver where IFN-γ and MyD88 signaling promoted their maturation to Tip-DCs. Both the egression of monocytes from bone marrow and their IFN-γ/MyD88 dependent maturation to Tip-DCs was counteracted by IL-10, hereby reducing liver pathogenicity. Liver injury, affecting millions of persons worldwide with often lethal consequences, frequently results from uncontrolled activation of recruited monocyte-derived cells that can be modulated by IL-10. Thus, the mechanisms regulating liver immunopathogenicity during parasitic infection identified herein could lead to new therapeutic policies in the field of hepatic inflammation.

Increased mitochondrial stress and modulation of mitochondrial respiratory enzyme activities in acetaminophen-induced toxicity in mouse macrophage cells

Overdose of acetaminophen (APAP) causes tissue injury particularly in the liver. However, the precise mechanism of APAP toxicity is not clear. Glutathione (GSH) depletion and oxidative stress are believed to be the main cause of APAP toxicity. The role of macrophages in APAP-induced tissue injury is controversial. Using mouse macrophage J774.2 cells, we recently demonstrated that like in animal models, APAP reduces GSH pool and alters GSH metabolism by increasing the production of reactive oxygen species (ROS). In the present study, we show that APAP-induced cytotoxicity and apoptosis in macrophages are associated with increased mitochondrial metabolic and oxidative stress, alterations in the mitochondrial membrane potential and activities of the respiratory enzyme complexes. APAP treatment also altered ROS/NO production and inhibited the expression of COX-2 and iNOS in LPS-stimulated macrophages. Electron microscopic studies also confirmed morphological changes associated with apoptosis at the lower dose of APAP, while at the higher dose late apoptosis/necrotic changes were visible. These results suggest that mitochondrial metabolic and oxidative stress are the main causes of cytotoxicity and cell death in APAP treated macrophages. The study may have long term implications to better understand the role of macrophages in the toxicology and pharmacology of APAP.

Mechanisms of immune-mediated liver injury

Hepatic inflammation is a common finding during a variety of liver diseases including drug-induced liver toxicity. The inflammatory phenotype can be attributed to the innate immune response generated by Kupffer cells, monocytes, neutrophils, and lymphocytes. The adaptive immune system is also influenced by the innate immune response leading to liver damage. This review summarizes recent advances in specific mechanisms of immune-mediated hepatotoxicity and its application to drug-induced liver injury. Basic mechanisms of activation of lymphocytes, macrophages, and neutrophils and their unique mechanisms of recruitment into the liver vasculature are discussed. In particular, the role of adhesion molecules and various inflammatory mediators in this process are explored. In addition, the authors describe mechanisms of liver cell damage by these inflammatory cells and critically evaluate the functional significance of each cell type for predictive and idiosyncratic drug-induced liver injury. It is expected that continued advances in our understanding of immune mechanisms of liver injury will lead to an earlier detection of the hepatotoxic potential of drugs under development and to an earlier identification of susceptible individuals at risk for predictive and idiosyncratic drug toxicities.

Role of monocytes and macrophages in experimental and human acute liver failure

Acute liver failure (ALF) is a devastating clinical syndrome characterised by progressive encephalopathy, coagulopathy, and circulatory dysfunction, which commonly leads to multiorgan failure and death. Central to the pathogenesis of ALF is activation of the immune system with mobilisation of cellular effectors and massive production of cytokines. As key components of the innate immune system, monocytes and macrophages are postulated to play a central role in the initiation, progression and resolution of ALF. ALF in humans follows a rapidly progressive clinical course that poses inherent difficulties in delineating the role of these pivotal immune cells. Therefore, a number of experimental models have been used to study the pathogenesis of ALF. Here we consider the evidence from experimental and human studies of ALF on the role of monocytes and macrophages in acute hepatic injury and the ensuing extrahepatic manifestations, including functional monocyte deactivation and multiple organ failure.

Identification of the cellular sensor that stimulates the inflammatory response to sterile cell death

Cell death provokes a robust inflammatory response. We have previously shown that this response is dependent on IL-1alpha. In this study, we investigate the cellular mechanism used by a host to sense cell death, produce IL-1alpha and also the role of IL-1beta in this response. In almost all cases examined, the IL-1 that stimulated the death-induced inflammatory response came from the host rather than the cell that was dying. In these situations, host bone marrow-derived cells were the key source of the IL-1alpha that was required for the inflammatory response. Conditional cellular depletion and reconstitution in CD11b promoter-driven diphtheria toxin receptor transgenic mice revealed that host macrophages played an essential role in the generation of the inflammatory response and were the source of the required IL-1alpha. In addition, we found a role for IL-1beta in the death-induced inflammatory response and that this cytokine was generated by both bone marrow-derived and radioresistant host cells. The one exception to these findings was that when dendritic cells were injected into mice, they provided a portion of the IL-1 that stimulated inflammation, and this was observed whether the dendritic cells were live or necrotic. Together, these findings demonstrate that macrophages play a key role as the primary sentinels that are required to sense and report cell death in ways that initiate the inflammatory response. One key way they accomplish this important task is by producing IL-1alpha that is needed to initiate the inflammatory response.

CCR2 mediates hematopoietic stem and progenitor cell trafficking to sites of inflammation in mice

HSCs are BM-derived, self-renewing multipotent cells that develop into circulating blood cells. They have been implicated in the repair of inflamed parenchymal tissue, but the signals that regulate their trafficking to sites of inflammation are unknown. As monocytes are recruited to sites of inflammation via chemoattractants that activate CCR2 on their surface, we investigated whether HSCs are also recruited to sites of inflammation through CCR2. Initial analysis indicated that in mice, CCR2 was expressed on subsets of HSCs and hematopoietic progenitor cells (HPCs) and that freshly isolated primitive hematopoietic cells (Lin-c-Kit+ cells) responded to CCR2 ligands in vitro. In vivo analysis indicated that after instillation of thioglycollate to cause aseptic inflammation and after administration of acetaminophen to induce liver damage, endogenous HSCs/HPCs were actively recruited to the peritoneum and liver, respectively, in WT but not Ccr2-/- mice. HSCs/HPCs recovered from the peritoneum successfully engrafted into the BM of irradiated primary and secondary recipients, confirming their self renewal and multipotency. Importantly, administration of exogenous WT, but not Ccr2-/-, HSCs/HPCs accelerated resolution of acetaminophen-induced liver damage and triggered the expression of genes characteristic of the macrophage M2 or repair phenotype. These findings reveal what we believe to be a novel role for CCR2 in the homing of HSCs/HPCs to sites of inflammation and suggest new functions for chemokines in promoting tissue repair and regeneration.

Discovery of (Z)-5-(4-methoxybenzylidene)thiazolidine-2,4-dione, a readily available and orally active glitazone for the treatment of concanavalin A-induced acute liver injury of BALB/c mice

A large amount of evidence suggests that monocytes/macrophages infiltration is implicated in a variety of inflammatory diseases including acute liver injury. Monocyte chemoattractant protein 1 (MCP-1) plays a crucial role in the process of macrophages recruitment. We herein presented a small-molecule library and a feasible quick screening method of evaluating potency of inhibition of chemotaxis of RAW264.7 cells stimulated by MCP-1. Fifty-three small molecules were synthesized and screened, and four compounds (2g, 2h, 4f, and 6h) showed inhibitory effects with IC(50) values range from 0.72 to 20.47 microM, with compound 4f being the most efficient. Further in vivo studies demonstrated that oral administration of 2g, 2h, 4f, or 6h decreases, most significantly for 4f, the serum levels of alanine aminotransaminase (ALT) and asparate aminotransaminase (AST) in ConA-induced acute livery injury BALB/c mice. Histopathological evaluation liver sections confirmed 4f as a potent, orally active compound for hepatoprotective effects against ConA-induced acute liver injury in BALB/c mice.

Potential role of caveolin-1 in acetaminophen-induced hepatotoxicity

Caveolin-1 (Cav-1) is a membrane scaffolding protein, which functions to regulate intracellular compartmentalization of various signaling molecules. In the present studies, transgenic mice with a targeted disruption of the Cav-1 gene (Cav-1(-/-)) were used to assess the role of Cav-1 in acetaminophen-induced hepatotoxicity. Treatment of wild-type mice with acetaminophen (300 mg/kg) resulted in centrilobular hepatic necrosis and increases in serum transaminases. This was correlated with decreased expression of Cav-1 in the liver. Acetaminophen-induced hepatotoxicity was significantly attenuated in Cav-1(-/-) mice, an effect that was independent of acetaminophen metabolism. Acetaminophen administration resulted in increased hepatic expression of the oxidative stress marker, lipocalin 24p3, as well as hemeoxygenase-1, but decreased glutathione and superoxide dismutase-1; no differences were noted between the genotypes suggesting that reduced toxicity in Cav-1(-/-) mice is not due to alterations in antioxidant defense. In wild-type mice, acetaminophen increased mRNA expression of the pro-inflammatory cytokines, interleukin-1beta, and monocyte chemoattractant protein-1 (MCP-1), as well as cyclooxygenase-2, while 15-lipoxygenase (15-LOX), which generates anti-inflammatory lipoxins, decreased. Acetaminophen-induced changes in MCP-1 and 15-LOX expression were greater in Cav-1(-/-) mice. Although expression of tumor necrosis factor-alpha, a potent hepatocyte mitogen, was up-regulated in the liver of Cav-1(-/-) mice after acetaminophen, expression of proliferating cell nuclear antigen and survivin, markers of cellular proliferation, were delayed, which may reflect the reduced need for tissue repair. Taken together, these data demonstrate that Cav-1 plays a role in promoting inflammation and toxicity during the pathogenesis of acetaminophen-induced injury.

The quest for liver progenitor cells: a practical point of view

Many chronic liver diseases can lead to hepatic dysfunction with organ failure. At present, orthotopic liver transplantation represents the benchmark therapy of terminal liver disease. However this practice is limited by shortage of donor grafts, the need for lifelong immunosuppression and very demanding state-of-the-art surgery. For this reason, new therapies have been developed to restore liver function, primarily in the form of hepatocyte transplantation and artificial liver support devices. While already offered in very specialized centers, both of these modalities still remain experimental. Recently, liver progenitor cells have shown great promise for cell therapy, and consequently they have attracted a lot of attention as an alternative or supportive tool for liver transplantation. These liver progenitor cells are quiescent in the healthy liver and become activated in certain liver diseases in which the regenerative capacity of mature hepatocytes and/or cholangiocytes is impaired. Although reports describing liver progenitor cells are numerous, they have not led to a consensus on the identity of the liver progenitor cell. In this review, we will discuss some of the characteristics of these cells and the different ways that have been used to obtain these from rodents. We will also highlight the challenges that researchers are facing in their quest to identify and use liver progenitor cells.

Drug-induced liver injury: what was new in 2008?

BACKGROUND

Given the number of publications appearing annually regarding drug-induced liver injury (DILI), there remains a need to concisely summarize each year's new crop of case series and reports as well as the advances in mechanisms of liver injury and in the field of pharmacogenomics relating to DILI.

OBJECTIVE

To present an up-to-date review of the past year's most important clinical studies and reports of DILI, placing them into context of previous publications.

METHODS

A Medline search was conducted of all manuscripts appearing in the fields "hepatotoxicity" and "drug-induced liver injury" during the calendar year 2008. The most clinically relevant English language case reports and studies exploring mechanisms and risk factors for DILI were then chosen for review, and supplemented with older literature where appropriate.

CONCLUSIONS

As in past years, 2008 was replete with publications dealing with virtually all facets of DILI, including updated incidence and prevalence data, as well as the latest information regarding mechanisms of liver injury. Data from the first 300 patients in the National Institute of Health-sponsored DILI Network registry of > 100 non-acetaminophen causes were presented. Antimicrobials and CNS drugs were responsible for > 60% of cases, with herbals and dietary supplements being increasingly reported. Identification of genetic predispositions to DILI is coming of age with the FDA calling for the testing of human leukocyte antigen B(*)5701 before the use of abacavir to reduce the risk of hypersensitivity reactions. Several groups emphasized the pitfalls in utilizing Roussel Uclaf Causality Assessment Method and other causality assessment methodologies, and an updated review appeared on the use of potentially hepatotoxic medications in patients with underlying liver disease.

Macrophages and inflammatory mediators in chemical toxicity: a battle of forces

Macrophages function as control switches of the immune system, providing a balance between pro- and anti-inflammatory responses. To accomplish this, they develop into different subsets: classically (M1) or alternatively (M2) activated macrophages. Whereas M1 macrophages display a cytotoxic, proinflammatory phenotype, much like the soldiers of The Dark Side of The Force in the Star Wars movies, M2 macrophages, like Jedi fighters, suppress immune and inflammatory responses and participate in wound repair and angiogenesis. Critical to the actions of these divergent or polarized macrophage subpopulations is the regulated release of inflammatory mediators. When properly controlled, M1 macrophages effectively destroy invading pathogens, tumor cells, and foreign materials. However, when M1 activation becomes excessive or uncontrolled, these cells can succumb to The Dark Side, releasing copious amounts of cytotoxic mediators that contribute to disease pathogenesis. The activity of M1 macrophages is countered by The Force of alternatively activated M2 macrophages, which release anti-inflammatory cytokines, growth factors, and mediators involved in extracellular matrix turnover and tissue repair. It is the balance in the production of mediators by these two macrophage subpopulations that ultimately determines the outcome of the tissue response to chemical toxicants.

Hepatic sinusoidal cells in health and disease: update from the 14th International Symposium

This review aims to give an update of the field of the hepatic sinusoid, supported by references to presentations given at the 14th International Symposium on Cells of the Hepatic Sinusoid (ISCHS2008), which was held in Tromsø, Norway, August 31-September 4, 2008. The subtitle of the symposium, 'Integrating basic and clinical hepatology', signified the inclusion of both basal and applied clinical results of importance in the field of liver sinusoidal physiology and pathophysiology. Of nearly 50 oral presentations, nine were invited tutorial lectures. The authors of the review have avoided writing a 'flat summary' of the presentations given at ISCHS2008, and instead focused on important novel information. The tutorial presentations have served as a particularly important basis in the preparation of this update. In this review, we have also included references to recent literature that may not have been covered by the ISCHS2008 programme. The sections of this review reflect the scientific programme of the symposium (http://www.ub.uit.no/munin/bitstream/10037/1654/1/book.pdf): 1. Liver sinusoidal endothelial cells. 2. Kupffer cells. 3. Hepatic stellate cells. 4. Immunology. 5. Tumor/metastasis. Symposium abstracts are referred to by a number preceded by the letter A.