Nitric oxide donor NOR 3 inhibits ketogenesis from oleate in isolated rat hepatocytes by a cyclic GMP-independent mechanism

Involvement of nitric oxide/cyclic GMP signaling pathway in the regulation of fatty acid metabolism in rat hepatocytes

The role of nitric oxide (NO)/guanosine 3',5'-cyclic monophosphate (cGMP) signaling pathway in the regulation of fatty acid metabolism was investigated in rat hepatocytes. Treatment with NO donors, which are known to activate soluble guanylyl cyclase, inhibited in parallel fatty acid synthesis de novo and acetyl-CoA carboxylase activity. This effect was mimicked by 8-Br-cGMP and abolished by KT5823, a selective inhibitor of cGMP-dependent protein kinase (PKG). Furthermore, specific and hydrolysis-resistant activators of PKG, and inhibitors of Ca2+ release from endoplasmic reticulum, were also effective in inhibiting both fatty acid-synthesizing activities. These results suggest that this biological action of NO is regulated by a signaling cascade involving soluble guanylyl cyclase, cGMP, and PKG, and may be mediated, at least in part, by inhibition of Ca2+ release from endoplasmic reticulum. In addition, 8-Br-cGMP was able to stimulate fatty acid oxidation by two different mechanisms: the relieving of malonyl-CoA-dependent inhibition by lowering levels of this product of acetyl-CoA carboxylase, and a malonyl-CoA-independent stimulation of carnitine palmitoyltransferase I. Taken together, results of this study suggest that NO/cGMP signaling pathway is endowed with regulatory properties in fatty acid metabolism, and may have a physiological role in the control of this metabolism in liver.

Hepatoprotective role of ganoderma lucidum polysaccharide against BCG-induced immune liver injury in mice

AIM: To examine the effect of ganoderma lucidum polysaccharide (GLP) on the immune liver injury induced by BCG infection, and investigate the relationship between degrees of hepatic damage and NO production in mice.

METHODS: Immune hepatic injury was markedly induced by BCG-pretreatment (125 mg·kg^-1, 2-week, iv) or by BCG-pretreatment plus lipopolysaccharide (LPS, 125 μg·kg^-1, 12-hour, iv) in mice in vivo. Hepatocellular damage induced by BCG-pretreated plus inflammatory cytokines mixture (CM), which was included TNF-α, IL-1β, IFN-γ and LPS in culture medium in vitro. Administration of GLP was performed by oral or incubating with culture medium at immune stimuli simultaneity. Liver damage was determined by activity of alanine aminotransferase (ALT) in serum and in hepatocytes cultured supernatant, by liver weight changes and histopathological examination. NO production in the cultured supernatant was determined by the Griess reaction. Moreover, inducible nitric oxide synthase (iNOS) protein expression was also examinated by immunohistochemical method.

RESULTS: Immune hepatic injury was markedly induced by BCG or BCG plus inflammatory cytokines in BALB/c mice in vivo and in vitro. Under BCG-stimulated condition, augment of the liver weight and increase of the serum/supernatant ALT level were observed, as well as granuloma forming and inflammatory cells soakage were observed by microscopic analysis within liver tissues. Moreover, NO production was also increased by BCG or/and CM stimuli in the culture supernatant, and a lot of iNOS positive staining was observed in BCG-prestimulated hepatic sections. Application of GLP significantly mitigated hepatic tumefaction, decreased ALT enzyme release and NO production in serum/supernatant, improved the pathological changes of chronic and acute inflammation induced by BCG-stimuli in mice. Moreover, the immunohistochemical result showed that GLP inhibited iNOS protein expression in BCG-immune hepatic damage model.

CONCLUSION: The present study indicates that NO participates in immune liver injury induced by Mycobacterium bovis BCG infection. The mechanisms of protective roles by GLP for BCG-induced immune liver injury may be due to influence NO production in mice.

Nitric oxide scavenger carboxy-PTIO potentiates the inhibition of dopamine uptake by nitric oxide donors

2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO) has been increasingly used as nitric oxide (NO) scavenger. Carboxy-PTIO reacts with NO to form nitric dioxide and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (carboxy-PTI). In rat C6 glioma cells expressing human dopamine transporter, carboxy-PTIO paradoxically potentiated the inhibition of [(3)H]dopamine uptake by two NO donors, diethylamine/NO and (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)-amino]/NO. Further examinations revealed that carboxy-PTI concentration-dependently reduced dopamine uptake, indicating that the formation of carboxy-PTI may account for the failure of carboxy-PTIO to abolish NO elicited effects. These results suggest that caution should be taken in interpreting data obtained using carboxy-PTIO and probably other NO scavengers.

Cytokine-mediated inhibition of ketogenesis is unrelated to nitric oxide or protein synthesis

Cytokines play an important role in the lipid disturbances commonly associated with sepsis. Ketogenesis is inhibited during sepsis, and tumor necrosis factor alpha (TNF alpha) and interleukin-6 (IL-6) have been suggested to mediate this impairment, irrespective of the ketogenic substrate (fatty acid or branched chain ketoacid). However, the underlying mechanism of cytokine action is still unknown. First we investigated the possible role of the induction of nitric oxide (NO) synthesis, using rat hepatocyte monolayers. Hepatocytes were incubated for 6 h, with either alpha -ketoisocaproate (KIC) (1 mM) or oleic acid (0.5 mM) in the presence or absence of TNF alpha (25 microg/L) and IL-6 (15 microg/L). In some experiments, cells were incubated with NO synthase (NOS) inhibitors. The ketone body (beta -hydroxybutyrate and acetoacetate) production and nitrite production were measured in the incubation medium. Our results indicated no involvement of nitric oxide in the inhibitory action of cytokines on ketogenesis. Secondly, we showed that cycloheximide (10(-4)M) did not counteract the cytokine-mediated ketogenesis decrease; hence, the effects of cytokines on ketogenesis are not protein synthesis-dependent. The cytokine-mediated inhibition of ketogenesis is therefore unrelated to either NO production or protein synthesis.

Effects of aminoguanidine on nitric oxide production induced by inflammatory cytokines and endotoxin in cultured rat hepatocytes

AIM: To study the effects of aminoguanidine (AG) and two L-arginine analogues N^ω-nitro-L-arginine methyl ester (L-NAME) and N^ω-nitro-L-arginine (L-NNA) on nitric oxide (NO) production induced by cytokines (TNF-α, IL-1β, and IFN-γ) and bacterial lipopolysaccharide (LPS) mixture (CM) in the cultured rat hepatocytes, and examine their mechanisms action.

METHODS: Rat hepatocytes were incubated with AG, L-NAME, L-NNA, Actinomycin D (ActD) and dexamethasone in a medium containing CM (LPS plus TNF-α, IL-1β, and IFN-γ) for 24 h. NO production in the cultured supernatant was measured with the Griess reaction. Intracellular cGMP level was detected with radioimmunoassy.

RESULTS: NO production was markedly blocked by AG and L-NAME in a dose-dependent manner under inflammatory stimuli condition triggered by CM in vitro. The rate of the maximum inhibitory effects of L-NAME (38.9%) was less potent than that obtained with AG (53.7%, P < 0.05). There was no significant difference between the inhibitory effects of AG and two L-arginine analogues on intracellular cGMP accumulation in rat cultured hepatocytes. Non-specific NOS expression inhibitor dexamethasone(DEX) and iNOS mRNA transcriptional inhibitor ActD also significantly inhibited CM-induced NO production. AG (0.1 mmol·L^-1) and ActD (0.2 ng·L^-1) were equipotent in decreasing NO production induced by inflammatory stimuli in vitro, and both effects were more potent than that induced by non-selectivity NOS activity inhibitor L-NAME (0.1 mmol·L^-1) under similar stimuli conditions (P < 0.01).

CONCLUSION: AG is a potent selective inhibitor of inducible isoform of NOS, and the mechanism of action may be not only competitive inhibition in the substrate level, but also the gene expression level in rat hepatocytes.