Role of c-Myc in nitric oxide-mediated suppression of cytochrome P450 3A4

Regulation of cytochrome P450 enzyme activity and expression by nitric oxide in the context of inflammatory disease

Many hepatic cytochrome P450 enzymes and their associated drug metabolizing activities are down-regulated in disease states, and much of this has been associated with inflammatory cytokines and their signaling pathways. One such pathway is the induction of inducible nitric oxide synthase (NOS2) and generation of nitric oxide (NO) in many tissues and cells including the liver and hepatocytes. Experiments in the 1990s demonstrated that NO could bind to and inhibit P450 enzymes, and suggested that inhibition of NOS could attenuate, and NO generation could mimic, the down-regulation by inflammatory stimuli of not only P450 catalytic activities but also of mRNA expression and protein levels of certain P450 enzymes. This review will summarize and examine the evidence that NO functionally inhibits and down-regulates P450 enzymes in vivo and in vitro, with a particular focus on the mechanisms by which these effects are achieved.

Nitric oxide-regulated proteolysis of human CYP2B6 via the ubiquitin-proteasome system

We showed previously that rat cytochrome P450 CYP2B1 undergoes NO-dependent proteasomal degradation in response to inflammatory stimuli, and that the related human enzyme CYP2B6 is also down-regulated by NO in primary human hepatocytes. To investigate the mechanism of CYP2B6 down-regulation, we made several cell lines (HeLa and HuH7 cells) in which native CYP2B6 or CYP2B6 with a C-terminal V5 tag (CYP2B6V5) are expressed from a lentiviral vector with a cytomegalovirus promoter. Native CYP2B6 protein was rapidly down-regulated in HeLa cells within 3h of treatment with the NO donor (Z)-1-[2-(2-Aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate, while its mRNA level was not down-regulated. Treatment of the cells with the NO donor (Z)-1-[N-(3-aminopropyl)-N-(3-ammoniopropyl)amino]diazen-1-ium-1,2-diolate also resulted in rapid down-regulation of CYP2B6 activity, measured as the formation of 7-hydroxy-4-trifluoromethylcoumarin, as well as 2B6 protein in the CYP2B6 HeLa cell line. CYP2B6V5 was also down-regulated by NO donors in HuH7 cells. Down-regulation was observed in the presence of cycloheximide, demonstrating that this occurs via a post-translational mechanism. We generated a HeLa cell line expressing both CYP2B6V5 and human nitric oxide synthase 2 (NOS2), the latter under positive control by tetracycline. The cellular NO produced by doxycycline treatment also effectively down-regulated CYP2B6 protein, which was blocked by the co-treatment with the NOS2 competitive inhibitor L-N^G-nitroarginine methyl ester (L-NAME). We next investigated the proteolytic enzymes responsible for NO-dependent CYP2B6 degradation. Neither calpain inhibitors (N-Acetyl-L-leucyl-L-leucyl-L-norleucinal, carbobenzoxy-valinyl-phenylalaninal), nor lysosomal protease inhibitors (3-methyladenine and chloroquine) inhibited the NO dependent CYP2B6V5 down-regulation. The proteasome inhibitors MG132 and bortezomib attenuated, but did not completely block the NO-induced down-regulation in the HuH7 cell line. However, when cells were co-treated with NO donor and proteasome inhibitors, high molecular mass species could be detected on native CYP2B6 as well as CYP2B6V5 Western blots. Further investigation demonstrated that CYP2B6 protein was polyubiquitinated and this was dramatically enhanced by co-treatment with NO donor and bortezomib. Taken together, our data demonstrate that CYP2B6 is down-regulated in an NO-dependent manner via ubiquitination and proteasomal degradation.

SjCa8, a calcium-binding protein from Schistosoma japonicum, inhibits cell migration and suppresses nitric oxide release of RAW264.7 macrophages

Background

Schistosomiasis is considered second only to malaria as the most devastating parasitic disease in tropical countries. Schistosome cercariae invade the host by penetrating the skin and migrate though the lungs and portal circulation to their final destination in the hepatic portal system and eventually the mesenteric veins. Previous studies have shown that the cytotoxic pathways that target schistosomulum in the lung-stage involve nitric oxide (NO) produced by macrophages. By contrast, skin-stage schistosomulas can evade clearance, indicating that they might be freed from macrophage NO-mediated cytotoxicity to achieve immune evasion; however, the critical molecules and mechanisms involved remain unknown.

Methods

Recombinant SjCa8 (rSjCa8), an 8-kDa calcium-binding protein that is stage-specifically expressed in cercaria and early skin-stage schistosomulas of Schistosoma japonicum, was incubated with mouse RAW264.7 macrophages. Effects on macrophage proliferation were determined using Cell Counting Kit-8. Next, transwell assay was carried out to further investigate the role of rSjCa8 in macrophage migration. The effects of rSjCa8 on macrophage apoptosis were evaluated using confocal microscopy and flow cytometry. Additional impacts of rSjCa8 on NO release by lipopolysaccharide (LPS)-stimulated macrophages as well as the underlying mechanisms were explored using fluorescent probe, nitric oxide signaling pathway microarray, quantitative real-time PCR, mutagenesis, and neutralizing antibody approaches.

Results

rSjCa8 exhibited a striking inhibitory effect on macrophage migration, but did not markedly increase cell proliferation or apoptosis. Additionally, rSjCa8 potently inhibited NO release by LPS-stimulated macrophages in a dose- and time-dependent manner, and the inhibitory mechanism was closely associated with intracellular Ca²⁺ levels, the up-regulation of catalase expression, and the down-regulation of the expression of 47 genes, including Myc, Gadd45a, Txnip, Fas, Sod2, Nos2, and Hmgb1. Vaccination with rSjCa8 increased NO concentration in the challenging skin area of infected mice and reduced the number of migrated schistosomula after skin penetration by cercariae.

Conclusions

Our findings indicate that SjCa8 might be a novel molecule that plays a critical role in immune evasion by S. japonicum cercaria during the process of skin penetration. The inhibitory impacts of rSjCa8 on macrophage migration and [Ca²⁺]_i-dependent NO release suggest it might represent a novel vaccine candidate and chemotherapeutic target for the prevention and treatment of schistosomiasis.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1119-4) contains supplementary material, which is available to authorized users.

Transcriptomic profiles of Drosophila melanogaster third instar larval midgut and responses to oxidative stress

Oligoarray analysis was used to determine the number and nature of genes expressed in third instar Drosophila melanogaster larval midguts. The majority of transcripts were associated with protein synthesis and metabolism. Serine proteases were the main proteolytic enzymes detected. Some 40% of the cytochrome P450 genes and 74% of the glutathione S transferases (GSTs) in the genome of D. melanogaster were observed to be expressed in the midgut by oligoarray analysis. We also identified potential transcription factor binding motifs (TFBMs) of P450s, GSTs and carboxylesterases. Many of the midgut-expressed GST genes contained candidate TFBMs homologous to TFBMs in mammals that have been associated with responses to oxidative stress. We also investigated the response of GSTs in the midgut to dietary H2O2, which showed a dosage-based differential response.

Roles of nitric oxide in inflammatory downregulation of human cytochromes P450

The purpose of this study was to determine the role of nitric oxide (NO) in the downregulation of human cytochrome P450 (CYP) enzymes and mRNAs by an inflammatory stimulus in cultured human hepatocytes. We focused on CYP2B6 because previous studies showed that rat CYP2B proteins undergo an NO-dependent degradation in response to inflammatory stimuli. To ensure high-level expression of CYP2B6, the inducer phenytoin was present at all times. Stimulation of cells with a mixture of tumor necrosis factor-alpha, interleukin-1, and interferon-gamma (ILmix) downregulated CYP2B6 mRNA and protein to 9 and 19% of control levels. The NO donor NOC-18 downregulated CYP2B6 protein to 30% of control, with only a small effect on CYP2B6 mRNA. Nitric oxide synthase inhibitors attenuated the downregulation of CYP2B6 protein but not mRNA by ILmix. These findings demonstrate that the posttranscriptional NO-dependent downregulation of CYP2B enzymes, observed previously in rat hepatocytes, is conserved in human CYP2B6. This mechanism is specific for CYP2B6 among the enzymes tested. No evidence was found for regulation of CYP2E1 mRNA or protein by NO. NOC-18 treatment downregulated CYP3A4 mRNA to 50% of control. However, NOS inhibitors failed to block the effects of ILmix on CYP3A4 expression.

20-HETE-mediated cytotoxicity and apoptosis in ischemic kidney epithelial cells

20-HETE, a metabolite of arachidonic acid, has been implicated as a mediator of free radical formation and tissue death following ischemia-reperfusion (IR) injury in the brain and heart. The present study examined the role of this pathway in a simulated IR renal injury model in vitro. Modified self-inactivating lentiviral vectors were generated to stably overexpress murine Cyp4a12 following transduction into LLC-PK(1) cells (LLC-Cyp4a12). We compared the survival of control and transduced LLC-PK(1) cells following 4 h of ATP depletion and 2 h of recovery in serum-free medium. ATP depletion-recovery of LLC-Cyp4a12 cells resulted in a significantly higher LDH release (P < 0.05) compared with LLC-enhanced green fluorescent protein (EGFP) cells. Treatment with the SOD mimetic MnTMPyP (100 microM) resulted in decreased cytotoxicity in LLC-Cyp4a12 cells. The selective 20-HETE inhibitor HET-0016 (10 microM) also inhibited cytotoxicity significantly (P < 0.05) in LLC-Cyp4a12 cells. Dihydroethidium fluorescence showed that superoxide levels were increased to the same degree in LLC-EGFP and LLC-Cyp4a12 cells after ATP depletion-recovery compared with control cells and that this increase was inhibited by MnTMPyP. There was a significant increase (P < 0.05) of caspase-3 cleavage, an effector protease of the apoptotic pathway, in the LLC-Cyp4a12 vs. LLC-EGFP cells (P < 0.05). This was abolished in the presence of HET-0016 (P < 0.05) or MnTMPyP (P < 0.01). These results demonstrate that 20-HETE overexpression can significantly exacerbate the cellular damage that is associated with renal IR injury and that the programmed cell death is mediated by activation of caspase-3 and is partially dependent on enhanced CYP4A generation of free radicals.

The effects of culture conditions on CYP3A4 and MDR1 mRNA induction by 1alpha,25-dihydroxyvitamin D(3) in human intestinal cell lines, Caco-2 and LS180

To evaluate an in vitro model suitable for investigating intestinal first-pass drug metabolism, CYP3A4 and MDR1 mRNA induction by 1alpha,25-dihydroxyvitamin D(3) (VD3) was examined in two human intestinal cell lines, Caco-2 and LS180, under various culture conditions. CYP3A4 mRNA expression was induced by 100 nM VD3 at levels between 234-549 times above normal in Caco-2 cells for 2 weeks and by 74-200 times above normal in LS180 cells for 2 days. The CYP3A4 induction effect of 250 nM VD3 was similar to or slightly higher than that of 100 nM VD3 in both Caco-2 and LS180 cells. Also, CYP3A4 was induced in Caco-2 and LS180 cells when they were cultured on a polystyrene plate slightly less than when they were cultured on a porous membrane. The increase in fetal bovine serum (FBS) content in the culture medium resulted in little or only slight increase of CYP3A4 induction in both Caco-2 and LS180 cells. MDR1 mRNA expression was marginally increased by VD3 in LS180 cells, but not in Caco-2 cells, and neither increased FBS content nor use of a porous membrane significantly affected MDR1 induction in LS180 cells. The transepithelial electrical resistance of LS180 cells was almost zero, whereas that of Caco-2 cells was high and was marginally decreased by VD3. These findings indicate that Caco-2 cells cultured on a porous membrane with 100 nM VD3 for 2 weeks may be used as a model to investigate the intestinal absorption and first-pass metabolism of drugs, while LS180 cells may be utilized to elucidate the mechanisms which regulate intestinal CYP3A4 mRNA expression.