Role of TLR-4 in liver macrophage and endothelial cell responsiveness during acute endotoxemia

Protective Effect of the Total Triterpenes of Euscaphis konishii Hayata Pericarp on Bacillus Calmette-Guérin Plus Lipopolysaccharide-Induced Liver Injury

BACKGROUND

Liver injury has been recognized as a primary cause of hepatic morbidity and mortality. Euscaphis konishii Hayata, also called Euscaphis fukienensis Hsu, is usually used as a detumescent and analgesic agent to improve liver function in South China, but its mechanism of action and chemical composition are unclear.

OBJECTIVE

The main aim of the study was to investigate the constituent and potential hepatoprotective mechanism of the total triterpenes of E. konishii pericarp (TTEP).

METHODS

The constituent of TTEP was analyzed by a series of silica gel column to get single compounds and then identified by NMR and MS. In vitro assays were conducted to test the free radical scavenging activity of TTEP. The BCG/LPS-induced immunological livery injury mice model was established to clarify the hepatoprotective effect of TTEP in vivo.

RESULTS

8 pentacyclic triterpene acids were separated and identified by NMR and MS. TTEP treatment (50, 100, and 200 mg/Kg) improved the immune function of the BCG/LPS-infected mice, dose-dependently alleviated the BCG/LPS-induced inflammation and oxidative stress, and ameliorated the hepatocyte apoptosis in the liver tissue.

CONCLUSION

The pericarp of E. konishii may be further considered as a potent natural food for liver disease treatment.

The hepatic innate immune response is lobe-specific in a murine model endotoxemia

The liver plays a central role in the innate immune response to endotoxemia. While previous studies have demonstrated lobe-specific transcriptional responses to various insults, whether this is true in response to endotoxemia is unknown. We sought to assess whether there were significant intra- and inter-lobe differences in the murine hepatic innate immune transcriptional response to endotoxemia. Adult male ICR mice were exposed to i.p. LPS (5 mg/kg, 30 min, 60 min, 5 h) and primary (Tnf, Cxcl1, Nfkbia, Tnfiap3) and secondary (Il6, Nos2) innate immune response gene expression was assessed in the left medial, right medial, left lateral, and right lateral lobes, and the papillary and caudate processes. The expression of all innate immune response genes increased following i.p. LPS challenge. When tested at the early time points (30 and 60 min), the left medial lobe and caudate process consistently demonstrated the highest induction of gene expression. Most inter-lobe differences were attenuated at later time points (5 h). To improve reproducibility of the study of endotoxemia induced by i.p. LPS challenge, inclusion of appropriate methodological details regarding collection of hepatic tissue should be included when reporting scientific results in published manuscripts.

Alveolar-Capillary Membrane-Related Pulmonary Cells as a Target in Endotoxin-Induced Acute Lung Injury

The main function of the lungs is oxygen transport from the atmosphere into the blood circulation, while it is necessary to keep the pulmonary tissue relatively free of pathogens. This is a difficult task because the respiratory system is constantly exposed to harmful substances entering the lungs by inhalation or via the blood stream. Individual types of lung cells are equipped with the mechanisms that maintain pulmonary homeostasis. Because of the clinical significance of acute respiratory distress syndrome (ARDS) the article refers to the physiological role of alveolar epithelial cells type I and II, endothelial cells, alveolar macrophages, and fibroblasts. However, all these cells can be damaged by lipopolysaccharide (LPS) which can reach the airspaces as the major component of the outer membrane of Gram-negative bacteria, and lead to local and systemic inflammation and toxicity. We also highlight a negative effect of LPS on lung cells related to alveolar-capillary barrier and their response to LPS exposure. Additionally, we describe the molecular mechanism of LPS signal transduction pathway in lung cells.

Delayed liver regeneration in C3H/HeJ mice: possible involvement of haemodynamic and structural changes in the hepatic microcirculation

NEW FINDINGS

What is the central question of this study? The liver regenerative process is complex and involves a sequence of signalling events, but the possible involvement of structural and haemodynamic changes in vivo during this process has never been explored. What is the main finding and its importance? Normal sinusoidal blood flow and velocity are crucial for a normal regenerative response, and delays in these haemodynamic events resulted in impaired liver regeneration in lipopolysaccharide-insensitive, C3H/HeJ mice. Toll-like receptor 4 signalling is required for restoration of normal liver architecture during the liver regenerative process. Liver regeneration is delayed in mice with a defective Toll-like receptor 4 (TLR4; C3H/HeJ mice) but is normal in TLR4 knockouts (TLR4^-/- ). Here, we investigated the possible involvement of structural and haemodynamic changes in vivo in the underlying mechanism. In lipopolysaccharide-sensitive (C3H/HeN and C57BL/6) and lipopolysaccharide-insensitive (C3H/HeJ and TLR4^-/- ) mice, a 70% partial hepatectomy (PH) was performed under inhalational anaesthesia. At days 3 and 7 after PH, the hepatic microcirculation was interrogated using intravital microscopy. Delayed liver regeneration was confirmed in C3H/HeJ, but not in C3H/HeN, C57BL/6 (WT) or TLR4^-/- mice by liver weight-to-body-weight ratio, the percentage of proliferating cell nuclear antigen (PCNA)-positive cells and mitotic index data. At day 3 after PH, sinusoidal red blood cell velocity increased by 100% in C3H/HeN mice, but by only 40% in C3H/HeJ mice. Estimated sinusoidal blood flow was significantly higher at day 7 after PH in C3H/HeN than in C3H/HeJ mice. The hepatic cord width was significantly larger in C3H/HeN than in C3H/HeJ mice at day 3 and it was significantly larger in TLR4^-/- than in C57BL/6 WT mice at day 7 after PH. Hepatocyte nucleus density and functional sinusoidal density was significantly reduced at days 3 and 7 after PH in all mouse strains compared with their zero-time controls. Functional sinusoidal density was significantly lower in C3H/HeJ compared with C3H/HeN mice at day 7 after PH. The present study indicates that altered sinusoidal blood flow and velocity in C3H/HeJ mice may contribute to the observed delay in the regenerative response in these mice. In addition, restoration of normal liver architecture may be delayed in TLR4^-/- mice.

Attenuation of LPS-induced cyclooxygenase-2 and inducible NO synthase expression by lysophosphatidic acid in macrophages

LPS can activate the inflammatory cascades by inducing various inflammatory mediators, such as prostaglandin E2 (PGE2) resulting from cyclooxygenase-2 (COX-2), and NO produced by inducible NO synthase (iNOS). Lysophosphatidic acid (LPA) has been demonstrated to participate in inflammation. This study aimed to clarify the impact and the involving mechanisms of LPA on LPS-incurred inflammation in macrophages. First, LPA appeared to attenuate LPS-induced protein and mRNA expression of COX-2 and iNOS genes, as well as production of PGE2 and NO. By using selective inhibitors targeting various signaling players, the inhibitory G protein alpha subunit (Gαi) seemed to be involved in the effect of LPA; p38, ERK and NF-κB were involved in the LPS-mediated COX-2/PGE2 pathway; and p38, JNK, phosphoinositide-3-kinase and NF-κB were involved in the LPS-mediated iNOS/NO pathway. LPA was able to diminish LPS-induced phosphorylation of p38 and Akt, as well as NF-κB p65 nuclear translocation. By utilization of inhibitors of COX-2 and iNOS, there appeared to be no modulation between the COX-2/PGE2 and the iNOS/NO signaling pathways. Our findings demonstrate a clear anti-inflammatory role of LPA acting via Gαi in LPS-mediated inflammatory response in macrophages, owing, at least in part, to its suppressive effect on LPS-induced activation of p38, Akt and NF-κB.

Inflammatory mediators in mastitis and lactation insufficiency

Mastitis is a common inflammatory disease during lactation that causes reduced milk supply. A growing body of evidence challenges the central role of pathogenic bacteria in mastitis, with disease severity associated with markers of inflammation rather than infection. Inflammation in the mammary gland may be triggered by microbe-associated molecular patterns (MAMPs) as well as danger-associated molecular patterns (DAMPs) binding to pattern recognition receptors such as the toll-like receptors (TLRs) on the surface of mammary epithelial cells and local immune cell populations. Activation of the TLR4 signalling pathway and downstream nuclear factor kappa B (NFkB) is critical to mediating local mammary gland inflammation and systemic immune responses in mouse models of mastitis. However, activation of NFkB also induces epithelial cell apoptosis and reduced milk protein synthesis, suggesting that inflammatory mediators activated during mastitis promote partial involution. Perturbed milk flow, maternal stress and genetic predisposition are significant risk factors for mastitis, and could lead to a heightened TLR4-mediated inflammatory response, resulting in increased susceptibility and severity of mastitis disease in the context of low MAMP abundance. Therefore, heightened host inflammatory signalling may act in concert with pathogenic or commensal bacterial species to cause both the inflammation associated with mastitis and lactation insufficiency. Here, we present an alternate paradigm to the widely held notion that breast inflammation is driven principally by infectious bacterial pathogens, and suggest there may be other therapeutic strategies, apart from the currently utilised antimicrobial agents, that could be employed to prevent and treat mastitis in women.

Silencing of triggering receptor expressed on myeloid cells-2 enhances the inflammatory responses of alveolar macrophages to lipopolysaccharide

Triggering receptor expressed on myeloid cells-2 (TREM-2) has been shown to attenuate inflammatory responses in various cell lines including bone marrow-derived macrophages, hepatic macrophages, osteoclasts and dendritic cells. However, its effects on alveolar macrophages remain unknown. Lentivirus-mediated RNA interference (RNAi) is a post-transcriptional gene silencing method, which is capable of degrading target genes specifically and efficiently. In this study, we silenced TREM-2 in murine alveolar macrophages by using lentivirus-mediated short hairpin RNA (shRNA) and evaluated the effects of TREM-2 silencing on expression of toll-like receptor-4 (TLR-4), tumor necrosis factor-α (TNF-α) and interleukin-10 (IL-10) in response to lipopolysaccharide (LPS). Alveolar macrophages were transfected with shRNA targeting TREM-2 by use of lentivirus vector, non-sense shRNA as a negative control or empty lentivirus vector as a blank control. Silencing of TREM-2 was assessed by real‑time fluorescence quantitative PCR and flow cytometry. Following LPS stimulation, the levels of TLR-4, TNF-α and IL-10 expressed in alveolar macrophages were measured by real-time PCR, flow cytometry or ELISA. TREM-2 expression on alveolar macrophages was downregulated significantly by lentivirus-mediated shRNA treatment at the transcriptional and translational levels. However, alveolar macrophages that received non-sense shRNA or empty lentivirus vectors showed no effects on TREM-2 expression. Silencing of TREM-2 enhanced expression of TLR-4, as well as TNF-α and IL-10, by alveolar macrophages following LPS stimulation. These results indicate a significant effect of TREM-2 on attenuating the LPS-induced inflammatory response of murine alveolar macrophages, which may be dependent on TLR-4.

Role of galectin-3 in classical and alternative macrophage activation in the liver following acetaminophen intoxication

Inflammatory macrophages have been implicated in hepatotoxicity induced by the analgesic acetaminophen (APAP). In these studies, we characterized the phenotype of macrophages accumulating in the liver following APAP intoxication and evaluated the role of galectin-3 (Gal-3) in macrophage activation. Administration of APAP (300 mg/kg, i.p.) to wild-type mice resulted in the appearance of two distinct subpopulations of CD11b(+) cells in the liver, which expressed high or low levels of the monocyte/macrophage activation marker Ly6C. Whereas CD11b(+)/Ly6C(hi) macrophages exhibited a classically activated proinflammatory phenotype characterized by increased expression of TNF-α, inducible NO synthase, and CCR2, CD11b(+)/Ly6C(lo) macrophages were alternatively activated, expressing high levels of the anti-inflammatory cytokine IL-10. APAP intoxication was also associated with an accumulation of Gal-3(+) macrophages in the liver; the majority of these cells were Ly6C(hi). APAP-induced increases in CD11b(+)/Ly6C(hi) macrophages were significantly reduced in Gal-3(-/-) mice. This reduction was evident 72 h post APAP and was correlated with decreased expression of the classical macrophage activation markers, inducible NO synthase, IL-12, and TNF-α, as well as the proinflammatory chemokines CCL2 and CCL3, and chemokine receptors CCR1 and CCR2. Conversely, numbers of CD11b(+)/Ly6C(lo) macrophages increased in livers of APAP-treated Gal-3(-/-) mice; this was associated with increased expression of the alternative macrophage activation markers Ym1 and Fizz1, increased liver repair, and reduced hepatotoxicity. These data demonstrate that both classically and alternatively activated macrophages accumulate in the liver following APAP intoxication; moreover, Gal-3 plays a role in promoting a persistent proinflammatory macrophage phenotype.

Classical and alternative activation of rat hepatic sinusoidal endothelial cells by inflammatory stimuli

The ability of rat hepatic sinusoidal endothelial cells (HSEC) to become activated in response to diverse inflammatory stimuli was analyzed. Whereas the classical macrophage activators, IFNγ and/or LPS upregulated expression of iNOS in HSEC, the alternative macrophage activators, IL-10 or IL-4+IL-13 upregulated arginase-1 and mannose receptor. Similar upregulation of iNOS and arginase-1 was observed in classically and alternatively activated Kupffer cells, respectively. Removal of inducing stimuli from the cells had no effect on expression of these markers, demonstrating that activation is persistent. Washing and incubation of IFNγ treated cells with IL-4+IL-13 resulted in decreased iNOS and increased arginase-1 expression, while washing and incubation of IL-4+IL-13 treated cells with IFNγ resulted in decreased arginase-1 and increased iNOS, indicating that classical and alternative activation of the cells is reversible. HSEC were more sensitive to phenotypic switching than Kupffer cells, suggesting greater functional plasticity. Hepatocyte viability and expression of PCNA, β-catenin and MMP-9 increased in the presence of alternatively activated HSEC. In contrast, the viability of hepatocytes pretreated for 2 h with 5 mM acetaminophen decreased in the presence of classically activated HSEC. These data demonstrate that activated HSEC can modulate hepatocyte responses following injury. The ability of hepatocytes to activate HSEC was also investigated. Co-culture of HSEC with acetaminophen-injured hepatocytes, but not control hepatocytes, increased the sensitivity of HSEC to classical and alternative activating stimuli. The capacity of HSEC to respond to phenotypic activators may represent an important mechanism by which they participate in inflammatory responses associated with hepatotoxicity.

Distinct responses of lung and liver macrophages to acute endotoxemia: role of toll-like receptor 4

Exposure to excessive quantities of bacterial-derived lipopolysaccharide (LPS) is associated with injury to the lung and the liver. Macrophages are thought to play a key role in the pathogenic response to LPS by releasing proinflammatory/cytotoxic mediators. Macrophage responses to LPS are mediated in large part by toll-like receptor 4 (TLR4). In the present studies we used C3H/HeJ mice, which possess a mutated nonfunctional TLR4, to examine its role in lung and liver macrophage responses to acute endotoxemia induced by LPS administration. Treatment of control C3H/HeOuJ mice with LPS (3 mg/ml, i.p.) was associated with a significant increase in the number of macrophages in both the lung and the liver. This was most prominent after 48 h, and was preceded by expression of proliferating cell nuclear antigen (PCNA), suggesting that macrophage proliferation contributes to the response. In liver, but not lung macrophages, LPS administration resulted in a rapid (within 3 h) increase in mRNA expression of Mn superoxide dismutase (SOD) and heme oxygenase-1 (HO-1), key enzymes in antioxidant defense. In contrast, HO-1 protein expression decreased 3 h after LPS administration in liver macrophages, while in lung macrophages it increased. mRNA expression of enzymes mediating the biosynthesis of eicosanoids, including cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase-1 (mPGES-1), but not 12/15-lipoxygenase (LOX), was upregulated in liver macrophages 3-24 h after LPS, with no effect on lung macrophages. However, COX-2 protein expression increased in both cell types. Loss of functional TLR4 significantly blunted the effects of LPS. Thus, no major changes were observed after LPS administration in the number of lung and liver macrophages recovered from TLR4 mutant mice, or on expression of PCNA. Increases in HO-1, MnSOD, COX-2 and PGES-1 mRNA expression in liver macrophages were also reduced in these mice. Conversely, in lung macrophages, loss of functional TLR4 resulted in increased expression of COX-2 protein and 12/15-LOX mRNA. These results demonstrate distinct lung and liver macrophage responses to acute endotoxemia are mediated, in part, by functional TLR4.

Genipin attenuates sepsis by inhibiting Toll-like receptor signaling

The pathogenesis of sepsis is characterized by overwhelming inflammatory responses that lead to tissue damage and organ failure. Toll-like receptor (TLR) signaling is crucial for induction of hyperinflammatory responses and tissue injury during sepsis. Genipin, an aglycon of geniposide, has antiinflammatory and antimicrobial activities. The purpose of this study was to test the hypothesis that genipin reduces multiple organ dysfunction and mortality during sepsis through inhibition of TLR signaling. Male ICR were subjected to sepsis by cecal ligation and puncture (CLP) or endotoxemia by lipopolysaccharide (LPS). Various doses of genipin (1, 2.5 and 5 mg/kg) or a vehicle were administered intravenously immediately after CLP or intraperitoneally after LPS treatment. In another set of survival tests, mice were treated with 2.5 mg/kg of genipin 0 and 24 h after CLP. Genipin was found to improve survival and to attenuate multiple organ dysfunction. Genipin attenuated production of proinflammatory cytokines and release of high-mobility group box 1 (HMGB1). Genipin prevented TLR2 and TLR4, myeloid differentiation factor 88 and the Toll/interleukin-1 receptor domain-containing adaptor protein, inducing interferon-β overexpression. Phosphorylation of mitogen-activated protein kinases and interferon regulatory factor 3 and translocation of nuclear factor (NF)-κB were prevented by genipin. Moreover, genipin attenuated increases in serum tumor necrosis factor-α and HMGB1 in LPS-induced endotoxemia. Pam3CSK4- and LPS-mediated production of nitrites and proinflammatory cytokines was suppressed by genipin in RAW264.7 cells. Genipin attenuated mortality and organ injuries during sepsis through interference with TLR signaling. Therefore, genipin might be useful as a potential therapeutic agent for treatment of sepsis.

Toll-like receptor 4 knockout mice are protected from endothelial overactivation in the absence of Kupffer cells after total hepatic ischemia/reperfusion

Kupffer cells (KCs) have been shown to be critical mediators of ischemia/reperfusion (I/R) injury in the murine liver. Using liposomal clodronate (LC), we found that KCs were protective in models of total hepatic ischemia with bowel congestion. We investigated the role of toll-like receptor 4 (TLR4) in the damage that occurs after I/R in KC-depleted livers. We injected 8-week-old C57BL/10J mice and C57BL/10ScN [toll-like receptor 4 knockout (TLR4KO)] mice with LC 48 hours before 35 minutes of warm hepatic ischemia with bowel congestion, which was followed by either 6 or 24 hours of reperfusion. The KC-depleted animals had increased mortality as well as a 10-fold increase in their aminotransferase levels that correlated with increases in centrilobular necrosis. These changes were absent in the TLR4KO animals. Lipopolysaccharide was bound extensively to endothelial cells after I/R, and this binding was diminished in the TLR4KO animals. In conjunction with this, there was an up-regulation of endothelial cell adhesion molecules in the LC-treated animals that was absent in the TLR4KO animals. Finally, there was a dramatic increase in the proinflammatory cytokine levels of the LC-treated animals, and the TLR4KO animals were protected against this increase. In conclusion, TLR4 promotes endothelial overactivation after I/R in the absence of KCs.

Inorganic mercury exposure in prairie voles (Microtus ochrogaster) alters the expression of toll-like receptor 4 and activates inflammatory pathways in the liver in a sex-specific manner

Environmental exposure to mercury can cause a number of adverse effects in humans including the disruption of endocrine function that may result in sex-specific effects. The present study was designed to characterize sex-specific effects of chronic inorganic mercury exposure on toll-like receptor (TLR) 2 and TLR4 and inflammatory signaling in the liver of prairie voles (Microtus ochrogaster). Following 10 weeks of exposure to mercury via drinking water, effects on protein expression levels of TLR2 and TLR4 and the downstream p38 mitogen-activated protein kinase (p38 MAPK) and nuclear factor-kappa (NF-κB) signaling pathways were assessed. Using immunoblot analysis, we found that mercury exposure significantly enhanced the expression of TLR4 and activated p38 MAPK and NF-κB pathways in vole livers. This is the first report indicating that TLR4 may serve as a sensor for chronic mercury exposure in the liver. Further, compared to females, mercury-treated male voles exhibited significant increases in activated p38 MAPK and a greater extent of liver damage. Together, these findings establish sex-specific liver immunomodulation and cellular signaling following chronic inorganic mercury exposure. Furthermore, this study also supports the use of voles as biomarkers of environmental mercury contamination and offers a promising in vivo tool to test various therapeutic strategies for mercury detoxification.

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.

Toll-like receptor 4 signaling in liver injury and hepatic fibrogenesis

Toll-like receptors (TLRs) are a family of transmembrane pattern recognition receptors (PRR) that play a key role in innate and adaptive immunity by recognizing structural components unique to bacteria, fungi and viruses. TLR4 is the most studied of the TLRs, and its primary exogenous ligand is lipopolysaccharide, a component of Gram-negative bacterial walls. In the absence of exogenous microbes, endogenous ligands including damage-associated molecular pattern molecules from damaged matrix and injured cells can also activate TLR4 signaling. In humans, single nucleotide polymorphisms of the TLR4 gene have an effect on its signal transduction and on associated risks of specific diseases, including cirrhosis. In liver, TLR4 is expressed by all parenchymal and non-parenchymal cell types, and contributes to tissue damage caused by a variety of etiologies. Intact TLR4 signaling was identified in hepatic stellate cells (HSCs), the major fibrogenic cell type in injured liver, and mediates key responses including an inflammatory phenotype, fibrogenesis and anti-apoptotic properties. Further clarification of the function and endogenous ligands of TLR4 signaling in HSCs and other liver cells could uncover novel mechanisms of fibrogenesis and facilitate the development of therapeutic strategies.

Role of STK in mouse liver macrophage and endothelial cell responsiveness during acute endotoxemia

Acute endotoxemia is associated with excessive production of proinflammatory mediators by hepatic macrophages and endothelial cells, which have been implicated in liver injury and sepsis. In these studies, we analyzed the role of MSP and its receptor STK in regulating the activity of these cells. Acute endotoxemia, induced by administration of LPS (3 mg/kg) to mice, resulted in increased expression of STK mRNA and protein in liver macrophages and endothelial cells, an effect that was dependent on TLR-4. This was correlated with decreased MSP and increased pro-MSP in serum. In Kupffer cells, but not endothelial cells, MSP suppressed LPS-induced NOS-2 expression, with no effect on COX-2. LPS treatment of mice caused a rapid (within 3 h) increase in the proinflammatory proteins NOS-2, IL-1beta, and TNF-alpha, as well as TREM-1 and TREM-3 and the anti-inflammatory cytokine IL-10 in liver macrophages and endothelial cells. Whereas LPS-induced expression of proinflammatory proteins was unchanged in STK-/- mice, IL-10 expression was reduced significantly. Enzymes mediating eicosanoid biosynthesis including COX-2 and mPGES-1 also increased in macrophages and endothelial cells after LPS administration. In STK-/- mice treated with LPS, mPGES-1 expression increased, although COX-2 expression was reduced. LPS-induced up-regulation of SOD was also reduced in STK-/- mice in liver macrophages and endothelial cells. These data suggest that MSP/STK signaling plays a role in up-regulating macrophage and endothelial cell anti-inflammatory activity during hepatic inflammatory responses. This may be important in protecting the liver from tissue injury.

Murine Kupffer cells are protective in total hepatic ischemia/reperfusion injury with bowel congestion through IL-10

Kupffer cells (KCs) are thought to mediate hepatocyte injury via their production of proinflammatory cytokines and reactive oxygen species in response to stress. In this study, we depleted KCs from the liver to examine their role in total warm hepatic ischemia/reperfusion (I/R) injury with bowel congestion. We injected 8-wk-old C57BL/10J mice with liposome-encapsulated clodronate 48 h before 35 min of hepatic ischemia with bowel congestion, followed by 6 or 24 h of reperfusion. KC-depleted animals had a higher mortality rate than diluent-treated animals and a 10-fold elevation in transaminase levels that correlated with increases in centrilobular necrosis. There was extensive LPS binding to the endothelial cells, which correlated with an upregulation of endothelial adhesion molecules in the KC-depleted animals versus diluent-treated animals. There was an increase in the levels of proinflammatory cytokines in KC-depleted animals, and a concomitant decrease in IL-10 levels. When KC-depleted mice were treated with recombinant IL-10, their liver damage profile in response to I/R was similar to diluent-treated animals, and endothelial cell adhesion molecules and proinflammatory cytokine levels decreased. KCs are protective in the liver subjected to total I/R with associated bowel congestion and are not deleterious as previously thought. This protection appears to be due to KC secretion of the potent anti-inflammatory cytokine IL-10.

The hepatic microcirculation: mechanistic contributions and therapeutic targets in liver injury and repair

The complex functions of the liver in biosynthesis, metabolism, clearance, and host defense are tightly dependent on an adequate microcirculation. To guarantee hepatic homeostasis, this requires not only a sufficient nutritive perfusion and oxygen supply, but also a balanced vasomotor control and an appropriate cell-cell communication. Deteriorations of the hepatic homeostasis, as observed in ischemia/reperfusion, cold preservation and transplantation, septic organ failure, and hepatic resection-induced hyperperfusion, are associated with a high morbidity and mortality. During the last two decades, experimental studies have demonstrated that microcirculatory disorders are determinants for organ failure in these disease states. Disorders include 1) a dysregulation of the vasomotor control with a deterioration of the endothelin-nitric oxide balance, an arterial and sinusoidal constriction, and a shutdown of the microcirculation as well as 2) an overwhelming inflammatory response with microvascular leukocyte accumulation, platelet adherence, and Kupffer cell activation. Within the sequelae of events, proinflammatory mediators, such as reactive oxygen species and tumor necrosis factor-alpha, are the key players, causing the microvascular dysfunction and perfusion failure. This review covers the morphological and functional characterization of the hepatic microcirculation, the mechanistic contributions in surgical disease states, and the therapeutic targets to attenuate tissue injury and organ dysfunction. It also indicates future directions to translate the knowledge achieved from experimental studies into clinical practice. By this, the use of the recently introduced techniques to monitor the hepatic microcirculation in humans, such as near-infrared spectroscopy or orthogonal polarized spectral imaging, may allow an early initiation of treatment, which should benefit the final outcome of these critically ill patients.

Regulation of TREM expression in hepatic macrophages and endothelial cells during acute endotoxemia

Triggering receptor expressed on myeloid cells (TREM) regulates inflammatory responses to lipopolysaccharide (LPS). In these studies, we analyzed the expression of TREM in hepatic macrophages and endothelial cells which play a central role in LPS clearance. LPS administration to C3H/HeOuJ mice resulted in a rapid induction of TREM-1 and TREM-3, but a decrease in TREM-2 in liver macrophages and endothelial cells. The observation that TREM family members are detectable in endothelial cells is novel and demonstrates that their expression is not limited to myeloid cells. LPS-induced alterations in TREM expression were not evident in cells from C3H/HeJ TLR-4 mutant mice, indicating that the response is dependent on TLR-4. IL-1beta and TNFalpha upregulated TREM-1 and TREM-3 expression and suppressed TREM-2 expression in macrophages and endothelial cells. This activity involved PI3-kinase and p38 MAP kinase signaling. Interestingly, no significant differences were noted in TREM expression between wild-type and TNFR1-/- mice treated with LPS. Treatment of macrophages and endothelial cells with LPS upregulated expression of nitric oxide synthase-2 (NOS-2). This was blocked by TREM-1 Fc/fusion protein, indicating that TREM-1 mediates LPS-induced NOS-2 expression. These results suggest that TREM proteins are important in the inflammatory response of hepatic macrophages and endothelial cells to acute endotoxemia.