Interleukin-1 is not essential for expression of inducible NOS in hepatocytes induced by lipopolysaccharide in vivo

Hepatic cytochrome P450 metabolism suppressed by mast cells in type 1 allergic mice

Mast cells and Kupffer cells secrete interleukin (IL)-1β, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α, which stimulate excess nitric oxide (NO) producing-inducible NO synthase (iNOS). Unlike Kupffer cells, immunoglobulin E-sensitized mast cells elicit sustained NO production. We investigated the participation of mast cell-released NO and cytokine-derived iNOS activation in type 1 allergy-suppressed hepatic cytochrome P450 (CYP) metabolism. Aminoguanidine, a selective iNOS inhibitor, completely suppressed serum nitrate plus nitrite (NOx) concentrations after primary and secondary sensitization of ICR mice and markedly attenuated allergy-suppressed hepatic CYP1A2, CYP2C, CYP2E1, and CYP3A activities. In the liver, primary and secondary sensitization enhanced iNOS-stimulating IFN-γ (5-15-fold) and TNF-α (3-5-fold) mRNA levels more than IL-1β (2-fold) and F4/80-positive Kupffer cell (2-fold) mRNA levels. When mast cell-deficient (-/-) mice were sensitized, hepatic CYP activities were not suppressed. Serum NOx levels in the sensitized -/- mice were similar with those in saline-treated ICR and -/- mice. In the liver of -/- mice, secondary sensitization markedly enhanced mRNA expression of iNOS (20-fold), IFN-γ (15-fold), and TNF-α (3-fold). However, hepatic total NOS activities in -/- mice were not significantly different between saline treatment and sensitization. Similarly, primary and secondary ICR mice did not significantly enhance total NOS activities in the liver and hepatocytes. The total NOS activities observed did not relate to the high levels of iNOS, IFN-γ, and TNF-α mRNA in the liver. Hepatic c-kit-positive mast cells in sensitized ICR mice were maintained at control levels. Therefore, our data suggest that mast cell-released NO participates in type 1 allergy-suppressed CYP1A2, CYP2C, CYP2E1, and CYP3A metabolism.

Hepatocyte toll-like receptor 4 mediates lipopolysaccharide-induced hepcidin expression

Hepcidin expression is induced by inflammatory molecules such as lipopolysaccharide (LPS) via a macrophage-mediated pathway. Although hepatocytes directly respond to LPS, the molecular mechanism underlying toll-like receptor (TLR)-dependent hepcidin expression by hepatocytes is mostly unknown. Here we show that LPS can directly induce the mRNA expression and secretion of hepcidin by hepatocytes via TLR4 activation. Using hepatocytes deficient in TLR4, myeloid differentiation factor 88 (MyD88) and TIR domain-containing adaptor inducing interferon-β (TRIF), we demonstrated that LPS-induced hepcidin expression by hepatocytes is regulated by its specific receptor, TLR4, via a MyD88-dependent signaling pathway. Hepcidin promoter activity was significantly increased by MyD88-dependent downstream signaling molecules (interleukin-1 receptor-associated kinase (IRAK) and tumor necrosis factor receptor-associated factor 6 (TRAF6), which activate c-Jun N-terminal kinase (JNK) and activator protein-1 (AP-1). We then confirmed that LPS stimulation induced the phosphorylation of JNK and c-Jun, and observed strong occupancy of the hepcidin promoter by c-Jun. Promoter mutation analysis also identified the AP-1-binding site on the hepcidin promoter. Finally, bone marrow transplantation between wild-type and TLR4 knockout mice revealed that hepatic TLR4-dependent hepcidin expression was comparable to macrophage TLR4-dependent hepcidin expression induced by LPS. Taken together, these results suggest that TLR4 expressed by hepatocytes regulates hepcidin expression via the IRAK–TRAF6–JNK–AP-1 axis.

Hepatic Flavin-Containing Monooxygenase 3 Enzyme Suppressed by Type 1 Allergy-Produced Nitric Oxide

Flavin-containing monooxygenases (FMOs) are major mammalian non-cytochrome P450 oxidative enzymes. T helper 2 cell-activated allergic diseases produce excess levels of nitric oxide (NO) that modify the functions of proteins. However, it remains unclear whether allergy-induced NO affects the pharmacokinetics of drugs metabolized by FMOs. This study investigated alterations of hepatic microsomal FMO1 and FMO3 activities in type 1 allergic mice and further examined the interaction of FMO1 and FMO3 with allergy-induced NO. Imipramine (IMP; FMO1 substrate) N-oxidation activity was not altered in allergic mice with high serum NO and immunoglobulin E levels. At 7 days after primary sensitization (PS7) or secondary sensitization (SS7), benzydamine (BDZ; FMO1 and FMO3 substrate) N-oxygenation was significantly decreased to 70% of individual controls. The expression levels of FMO1 and FMO3 proteins were not significantly changed in the sensitized mice. Hepatic inducible NO synthase (iNOS) mRNA level increased 5-fold and 15-fold in PS7 and SS7 mice, respectively, and hepatic tumor necrosis factor-α levels were greatly enhanced. When a selective iNOS inhibitor was injected into allergic mice, serum NO levels and BDZ N-oxygenation activity returned to control levels. NO directly suppressed BDZ N-oxygenation, which was probably related to FMO3-dependent metabolism in comparison with IMP N-oxidation. In hepatic microsomes from PS7 and SS7 mice, the suppression of BDZ N-oxygenation was restored by ascorbate. Therefore, type 1 allergic mice had differentially suppressed FMO3-dependent BDZ N-oxygenation. The suppression of FMO3 metabolism related to reversible S-nitrosyl modifications of iNOS-derived NO. NO is expected to alter FMO3-metabolic capacity-limited drug pharmacokinetics in humans.

Role of Kupffer cells in the pathogenesis of endotoxin-induced liver injury

Endotoxin (lipopolysaccharide, LPS)-induced liver injury was involved in the initiation and development of various liver diseases, and liver macrophages (Kupffer cells) were demonstrated to play important roles in these processes. On one hand, Kupffer cells were activated to release pro-inflammatory factors via the system of LPS-induced signal transduction, and this responsibility was also cooperated by the interaction of other liver cells to mediate liver injury. On the other hand, the activation of Kupffer cells were also inhibited by LPS tolerance and other physiological regulatory mechanisms, which prevented the liver injury mediated by Kupffer cells. Thus, Kupffer cells were simultaneously co-stimulated by factors of activation and inhibition to keep a relative balance. LPS-induced liver injury may occur once the balance is pathologically broken, whereas inhibitory intervention of Kupffer cells' activation is a key strategy to protect the LPS-induced injury.

Endotoxemia-mediated induction of cardiac inducible nitric-oxide synthase expression accounts for the hypotensive effect of ethanol in female rats

We have recently shown that intragastric (i.g.) ethanol lowers blood pressure (BP) in conscious female rats via a reduction in cardiac output (CO). However, the mechanisms implicated in these hemodynamic effects of ethanol are not known. Therefore, we tested the hypothesis that ethanol-evoked endotoxemia mediates the reduction in CO via enhanced myocardial inducible nitric-oxide synthase (iNOS) expression. Immunoblot (myocardial iNOS), biochemical (plasma endotoxin and nitrite/nitrate), and integrative [BP, heart rate, CO, stroke volume (SV), and total peripheral resistance (TPR)] studies were conducted in conscious female rats that received i.g. ethanol (1 g/kg) in the absence or presence of 1400W (N-(3-[aminomethyl]benzyl) acetamidine) or ampicillin to selectively inhibit iNOS and to eliminate endogenous endotoxin, respectively. Ethanol-evoked hypotension coincided with reductions in CO and SV and increases in: 1) TPR, 2) plasma endotoxin and nitrite/nitrate, and 3) myocardial iNOS expression. These effects of ethanol were virtually abolished in rats pretreated with ampicillin (200 mg/kg/day for 2 days by gavage) or with 1400W (5 mg/kg i.p.) except for the increase in plasma endotoxin, which persisted in 1400W-pretreated rats. These findings yield insight into the mechanistic role of endotoxin-myocardial iNOS signaling in the cardiodepressant action of ethanol, which accounts for its hypotensive effect in conscious female rats.