Modulation of murine phenobarbital-inducible CYP2A5, CYP2B10 and CYP1A enzymes by inhibitors of protein kinases and phosphatases

Effects of marine biotoxins on drug-metabolizing cytochrome P450 enzymes and their regulation in mammalian cells

Marine biotoxins are a heterogenous group of natural toxins, which are able to trigger different types of toxicological responses in animals and humans. Health effects arising from exposure to marine biotoxins are ranging, for example, from gastrointestinal symptoms to neurological effects, depending on the individual toxin(s) ingested. Recent research has shown that the marine biotoxin okadaic acid (OA) can strongly diminish the expression of drug-metabolizing cytochrome P450 (CYP) enzymes in human liver cells by a mechanism involving proinflammatory signaling. By doing so, OA may interfere with the metabolic barrier function of liver and intestine, and thus alter the toxico- or pharmacokinetic properties of other compounds. Such effects of marine biotoxins on drug and xenobiotic metabolism have, however, not been much in the focus of research yet. In this review, we present the current knowledge on the effects of marine biotoxins on CYP enzymes in mammalian cells. In addition, the role of CYP-regulating nuclear receptors as well as inflammatory signaling in the regulation of CYPs by marine biotoxins is discussed. Strong evidence is available for effects of OA on CYP enzymes, along with information about possible molecular mechanisms. For other marine biotoxins, knowledge on effects on drug metabolism, however, is scarce.

Protein Kinase R Restricts the Intracellular Survival of Mycobacterium tuberculosis by Promoting Selective Autophagy

Tuberculosis (TB) is a deadly infectious lung disease caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb). The identification of macrophage signaling proteins exploited by Mtb during infection will enable the development of alternative host-directed therapies (HDT) for TB. HDT strategies will boost host immunity to restrict the intracellular replication of Mtb and therefore hold promise to overcome antimicrobial resistance, a growing crisis in TB therapy. Protein Kinase R (PKR) is a key host sensor that functions in the cellular antiviral response. However, its role in defense against intracellular bacterial pathogens is not clearly defined. Herein, we demonstrate that expression and activation of PKR is upregulated in macrophages infected with Mtb. Immunological profiling of human THP-1 macrophages that overexpress PKR (THP-PKR) showed increased production of IP-10 and reduced production of IL-6, two cytokines that are reported to activate and inhibit IFNγ-dependent autophagy, respectively. Indeed, sustained expression and activation of PKR reduced the intracellular survival of Mtb, an effect that could be enhanced by IFNγ treatment. We further demonstrate that the enhanced anti-mycobacterial activity of THP-PKR macrophages is mediated by a mechanism dependent on selective autophagy, as indicated by increased levels of LC3B-II that colocalize with intracellular Mtb. Consistent with this mechanism, inhibition of autophagolysosome maturation with bafilomycin A1 abrogated the ability of THP-PKR macrophages to limit replication of Mtb, whereas pharmacological activation of autophagy enhanced the anti-mycobacterial effect of PKR overexpression. As such, PKR represents a novel and attractive host target for development of HDT for TB, and our data suggest value in the design of more specific and potent activators of PKR.

Investigating the inhibitory potential of 2-Aminopurine metal complexes against serine/threonine protein kinases from Mycobacterium tuberculosis

Tuberculosis - a disease caused by Mycobacterium tuberculosis (Mtb), is one of the most devastating disease. The discovery of Ser/Thr protein kinases (STPKs) in Mtb opened a new avenue for developing anti-tubercular inhibitors. The in-vivo inhibitory effects of many metal ions have been demonstrated in literature. But, one of the limitations of metal ions as inhibitors is their inability to traverse the hydrophobic membrane due to polar nature and their propensity for non-specific interactions. To overcome this, we attached a metal ion to 2-A9P - an analog derived from a cell permeable scaffold, 2-Aminopurine (2-AP) which is a known kinase inhibitor. We investigated the inhibitory potential of 2-AP and its analog 2-A9P against protein kinase B (PknB) and showed that both of these can inhibit Mtb STPKs. Next, we evaluated the latent inhibitory activity of metal ions and for the first time showed that they can inhibit the phosphotransfer reaction in PknB, PknG and PknL. Subsequently, 6 different metal complexes (MC) of 2-A9P were used for inhibitory studies and their estimated IC₅₀ values show that most MCs inhibited PknB with low micromolar potency. Further, MIC values determined for the six MCs against Mtb showed that MC-4 and MC-6 exhibit whole cell inhibitory activity. Cytotoxicity studies show that MC-4 and MC-6 do not affect cell viability of A549 cell lines, suggesting that these inhibitors can be further developed as anti-tubercular agents.

Mechanism of inhibition of lipopolysaccharide-induced interferon-β production by 2-aminopurine

2-Aminopurine (2-AP) is widely used as an inhibitor for double stranded RNA-dependent protein kinase (PKR). Previously, we reported that 2-AP inhibits Toll-like receptor (TLR) ligand-induced nitric oxide production through the prevention of interferon (IFN)-β production. In this study, we investigated the mechanisms for 2-AP inhibition of lipopolysaccharide (LPS)-induced IFN-β production. A reporter gene assay showed that LPS-induced IFN-β promoter, but not nuclear factor (NF)-κB, activation was significantly inhibited by 2-AP. IFN-β promoter activation induced by the overexpression of Toll/interleukin-1 receptor domain-containing adaptor inducing IFN-β (TRIF) was significantly inhibited by 2-AP in a dose-dependent manner, while TRIF- or myeloid differentiation primary response gene 88-dependent NF-κB activation was not inhibited. IFN-β promoter activation induced by expression of the downstream signaling molecules, tumor necrosis factor receptor-associated factor family member-associated NF-κB activator-binding kinase 1, inhibitor of NF-κB kinase i and a constitutively active mutant of interferon regulatory factor (IRF)-3, was also inhibited by 2-AP. Another PKR inhibitor harboring the imidazolo-oxindole structure, however, did not affect TRIF signaling molecules-induced IFN-β promoter activation, suggesting that the inhibition of IFN-β transcription by 2-AP is independent of PKR inhibition. Further, we examined the effect of 2-AP on LPS-induced IRF-3 activation by immunoblotting. While 2-AP did not affect LPS-induced phosphorylation of IRF-3, nuclear translocation of IRF-3 was inhibited. Moreover, we revealed that LPS-induced phosphorylation of Akt, another key molecule involved in IRF-3 activation, was inhibited by 2-AP. These results suggest that 2-AP inhibits nuclear translocation of phosphorylated-IRF-3 by inhibiting Akt activation.

Unsaturated fatty acid regulation of cytochrome P450 expression via a CAR-dependent pathway

The liver is responsible for key metabolic functions, including control of normal homoeostasis in response to diet and xenobiotic metabolism/detoxification. We have shown previously that inactivation of the hepatic cytochrome P450 system through conditional deletion of POR (P450 oxidoreductase) induces hepatic steatosis, liver growth and P450 expression. We have exploited a new conditional model of POR deletion to investigate the mechanism underlying these changes. We demonstrate that P450 induction, liver growth and hepatic triacylglycerol (triglyceride) homoeostasis are intimately linked and provide evidence that the observed phenotypes result from hepatic accumulation of unsaturated fatty acids, which mediate these phenotypes by activation of the nuclear receptor CAR (constitutive androstane receptor) and, to a lesser degree, PXR (pregnane X receptor). To our knowledge this is the first direct evidence that P450s play a major role in controlling unsaturated fatty acid homoeostasis via CAR. The regulation of P450s involved in xenobiotic metabolism by this mechanism has potentially significant implications for individual responses to drugs and environmental chemicals.

Phosphorylation of xenobiotic-metabolizing cytochromes P450

The regulation of cytochromes P450 (CYPs) by induction mediated by xenobiotics is well known. Our team has discovered an additional important regulation of xenobiotic-metabolizing CYPs by phosphorylation. Individual CYPs are phosphorylated by different protein kinases, leading to CYP isoenzyme-selective changes in the metabolism of individual substrates and consequent profound changes in the control of mutagenic and cytotoxic metabolites. Some CYPs are phosphorylated by protein kinase C and some by the cyclic adenosine monophosphate (cAMP) dependent protein kinase A. We found that cAMP not only leads to drastic changes in the activity of individual CYPs, but also drastic changes in the nuclear localization of the CYP-related transcription factor Ah receptor (AHR). The consequences are very different from those of AHR nuclear translocation mediated by its classic ligands (such as dioxin and many polycyclic aromatic hydrocarbons) and may represent the long-sought physiological function of the AHR. The disturbance of this physiological function of AHR by extremely persistent high-affinity xenobiotic ligands such as dioxin may represent the most important contributing factor for their potent toxicity.

The oxindole/imidazole derivative C16 reduces in vivo brain PKR activation

Inhibition of double-stranded RNA-dependent protein kinase (PKR) represents an interesting strategy for neuroprotection. However, inhibiting this kinase which triggers the apoptotic process could favour in counterpart cell proliferation and tumorigenesis. Here, we use an in vivo model of 7-day-old rat displaying a high activation of brain PKR to investigate the effects of a new PKR inhibitor identified as an oxindole/imidazole derivative (C16). We show for the first time that acute systemic injection of C16 specifically inhibits the apoptotic PKR/eIF2alpha signaling pathway without stimulating the proliferative mTOR/p70S6K signaling mechanism.

Requirement for protein kinase R in interleukin-1alpha-stimulated effects in cartilage

Interleukin-1 (IL-1) has pleiotropic effects in cartilage. The interferon-induced, double stranded RNA-activated protein kinase PKR that phosphorylates eukaryotic initiation factor 2 (eIF2) alpha has been implicated in cytokine effects in chondrocytes. A compound was recently identified that potently suppresses PKR autophosphorylation (IC50 approximately 200 etaM) and partially restores PKR-inhibited translation in a cell-free system with significant effect in the nanomolar range. The objectives of this study were to exploit this potent PKR inhibitor to assess whether PKR kinase activity is required for catabolic and proinflammatory effects of IL-1alpha in cartilage and to determine whether IL-1alpha causes an increase in eIF2alpha phosphorylation that is antagonized by the PKR inhibitor. Cartilage explants were incubated with the PKR inhibitor and IL-1alpha. Culture media were assessed for sulfated glycosaminoglycan as an indicator of proteoglycan degradation and for prostaglandin E(2). Cartilage extracts were analyzed by Western blot for cyclooxygenase-2 and phosphorylated signaling molecules. Nanomolar concentrations of the PKR inhibitor suppressed proteoglycan degradation and cyclooxygenase-2 accumulation in IL-1alpha-activated cartilage. The PKR inhibitor stimulated or inhibited PGE(2) production with a biphasic dose response relationship. IL-1alpha increased the phosphorylation of both PKR and eIF2alpha, and nanomolar concentrations of PKR inhibitor suppressed the IL-1alpha-induced changes in phosphorylation. The results strongly support PKR involvement in pathways activated by IL-1alpha in chondrocytes.

Phosphorylated PKR contributes the induction of GRP94 under ER stress

Phosphorylated double-stranded RNA-dependent protein kinase (PKR) is thought to play an important role during ER stress induced cell death, but its molecular mechanism of action has not yet been clarified completely. To resolve this issue, we employed a PKR inhibitor together with ER stress inducers (tunicamycin, thapsigargin, and 2-deoxyglucose) and found that this treatment applied to SK-N-SH and HepG2 cells suppressed the expressional induction of 94kDa glucose regulated protein (GRP94) but not GRP78 proteins at both protein and mRNA levels. Although GRP94 mRNA increased, no significant difference was observed in the mRNA level of spliced X box binding protein 1 (XBP1) and reporter gene assay using GRP78 and GRP94 promoter with an ER stress response element (ERSE) showed that PKR inhibitor did not affect their activity. These results suggest that a novel mechanism other than ERSE-dependent mRNA transcription is required for the induction of GRP94 and phosphorylation of PKR contributes to the induction of GRP94 under ER stress.

Primary hepatocytes: current understanding of the regulation of metabolic enzymes and transporter proteins, and pharmaceutical practice for the use of hepatocytes in metabolism, enzyme induction, transporter, clearance, and hepatotoxicity studies

This review brings you up-to-date with the hepatocyte research on: 1) in vitro-in vivo correlations of metabolism and clearance; 2) CYP enzyme induction, regulation, and cross-talk using human hepatocytes and hepatocyte-like cell lines; 3) the function and regulation of hepatic transporters and models used to elucidate their role in drug clearance; 4) mechanisms and examples of idiosyncratic and intrinsic hepatotoxicity; and 5) alternative cell systems to primary human hepatocytes. We also report pharmaceutical perspectives of these topics and compare methods and interpretations for the drug development process.

Protein kinase R, IkappaB kinase-beta and NF-kappaB are required for human rhinovirus induced pro-inflammatory cytokine production in bronchial epithelial cells

Rhinovirus infections cause the majority of acute exacerbations of airway diseases such as asthma and chronic obstructive pulmonary disease, with increased pro-inflammatory cytokine production by infected bronchial epithelial cells contributing to disease pathogenesis. Theses diseases are a huge cause of morbidity worldwide, and contribute a major economic burden to healthcare costs. Current steroid based treatments are only partially efficient at controlling virus induced inflammation, which remains an unmet therapeutic goal. Although NF-kappaB has been implicated, the precise mechanisms of rhinovirus induction of pro-inflammatory gene expression in bronchial epithelial cells are unclear. We hypothesised that rhinovirus replication and generation of dsRNA was an important process of pro-inflammatory cytokine induction. Using pharmalogical (2-aminopurine and a new small molecule inhibitor) and genetic inhibition of the dsRNA binding kinase protein kinase R, striking inhibition of dsRNA (polyrIC) and rhinovirus induced CCL5, CXCL8 and IL-6 protein was observed. Using confocal microscopy, rhinovirus induced protein kinase R phosphorylation co-located with NF-kappaB p65 nuclear translocation. Focusing on CXCL8, both rhinovirus infection and dsRNA treatment required IkappaB kinase-beta for induction of CXCL8. Analysis of cis-acting sites in the CXCL8 promoter revealed that both rhinovirus infection and dsRNA treatment upregulated CXCL8 promoter activation via NF-kappaB and NF-IL6 binding sites. Together, the results demonstrate the importance of dsRNA in induction of pro-inflammatory cytokines by rhinoviruses, and suggest that protein kinase R is involved in NF-kappaB mediated gene transcription of pro-inflammatory cytokines via IkappaB kinase-beta. These molecules regulating rhinovirus induction of inflammation represent therapeutic targets.

Molecular pathological analysis for determining the possible mechanism of piperonyl butoxide-induced hepatocarcinogenesis in mice

Piperonyl butoxide (PBO), alpha-[2-(2-butoxyethoxy)ethoxy]-4,5-methylene-dioxy-2-propyltoluene, is widely used as a synergist for pyrethrins. In order to clarify the possible mechanism of non-genotoxic hepatocarcinogenesis induced by PBO, molecular pathological analyses consisting of low-density microarray analysis and real-time reverse transcriptase (RT)-PCR were performed in male ICR mice fed a basal powdered diet containing 6000 or 0 ppm PBO for 1, 4, or 8 weeks. The animals were sacrificed at weeks 1, 4, and 8, and the livers were histopathologically examined and analyzed for gene expression using the microarray at weeks 1 and 4 followed by real-time RT-PCR at each time point. Reactive oxygen species (ROS) products were also measured using liver microsomes. At each time point, the hepatocytes of PBO-treated mice showed centrilobular hypertrophy and increased lipofuscin deposition in Schmorl staining. The ROS products were significantly increased in the liver microsomes of PBO-treated mice. In the microarray analysis, the expression of oxidative and metabolic stress-related genes--cytochrome P450 (Cyp) 1A1, Cyp2A5 (week 1 only), Cyp2B9, Cyp2B10, and NADPH-cytochrome P450 oxidoreductase (Por) was over-expressed in mice given PBO at weeks 1 and 4. Fluctuations of these genes were confirmed by real-time RT-PCR in PBO-treated mice at each time point. In additional real-time RT-PCR, the expression of Cyclin D1 gene, key regulator of cell-cycle progression, and Xrcc5 gene, DNA damage repair-related gene, was significantly increased at each time point and at week 8, respectively. These results suggest the possibility that PBO has the potential to generate ROS via the metabolic pathway and to induce oxidative stress, including oxidative DNA damage, resulting in the induction of hepatocellular tumors in mice.

Regulation of Gender-Dependent CYP2A Expression in Pigs: Involvement of Androgens and CAR

The expression of drug metabolizing cytochrome P4502A (CYP2A) is highly gender-dependent in minipigs with the highest activity in females. In other species, orthologs of CYP2A have been shown to be under the regulation of nuclear receptor constitutive androstane receptor, whereas little is known about regulation in pigs. To investigate the effect of sex hormones on porcine cytochrome P450 CYP2A and CYP3A expression was assessed in liver samples taken before and after castration of sexually mature minipig boars. Removal of the primary androgen source resulted in significant increases of CYP2A mRNA, protein and enzyme activity levels. Likewise, expression of CYP3A was increased, although to a lesser extent. To examine the involvement of constitutive androstane receptor in the regulation of CYP2A, primary porcine hepatocytes were exposed to modulators of murine constitutive androstane receptor and human constitutive androstane receptor activity. The CYP2A activity was significantly increased by exposure to phenobarbital, an indirect activator of constitutive androstane receptor, and the human constitutive androstane receptor-ligand CITCO. In contrast, no effect was seen following exposure to the potent murine constitutive androstane receptor-ligand TCPOBOP and the hormonal murine constitutive androstane receptor-ligands androstenol and oestrone. Thus, the results support that 1) porcine CYP2A is reversibly inhibited by androgens on a transcriptional basis in vivo; 2) the induction profile of CYP2A in vitro shares similarity with that of human constitutive androstane receptor-regulated CYPs, indicating an involvement of a porcine constitutive androstane receptor in the regulation of CYP2A.

Small molecule inhibitors of the RNA-dependent protein kinase

The RNA-dependent protein kinase (PKR) is an interferon-induced serine/threonine protein kinase that phosphorylates the alpha subunit of the eukaryotic initiation factor 2 in response to viral infection. Classical genetic approaches for studying the role of PKR in cell signaling have their limitations due to overlapping but non-redundant pathways. Small molecule inhibitors of PKR will be useful in this regard. We report here, the discovery of a small molecule inhibitor of the kinase reaction of PKR. The inhibitor was discovered by screening a library of 26 different ATP-binding site directed inhibitors of varying structure. We also describe the development of a high-throughput assay for screening a large number of compounds for a PKR inhibitor using a rabbit reticulocyte lysate system and luciferase mRNA. The assay takes advantage of the fact that the reticulocyte lysate is rich in components of the translational machinery, of which PKR is an integral part. This assay can be carried out with added exogenous human PKR to study the effect of various compounds in their ability to rescue the translational block imposed by human PKR.

Lack of association between CYP2A5 induction and apoptosis in mouse primary hepatocytes

Upregulation of mouse hepatic cytochrome P450 2A5 (CYP2A5) is a process closely associated with hepatocellular damage and formation of liver tumours. 2-Aminopurine, a protein kinase inhibitor modulating cell cycle control, was recently shown to strongly induce CYP2A5 in mouse hepatocytes. The objective of this study was to determine the association between CYP2A5 induction and apoptosis in mouse primary hepatocytes. Five well-characterised CYP2A5 inducers were tested for their ability to affect apoptosis rate, determined by immunohistochemical in situ 3'-end-labelling technique, in a primary mouse hepatocyte model. Transforming growth factor beta (TGFbeta) was used as a positive (proapoptotic) control. Phenobarbital, pyrazole and the mitogen-activated protein kinase inhibitor PD98059 did not significantly affect apoptosis rate in hepatocytes. Norcocaine induced apoptosis at 6 hr (1.8-fold) and 2-aminopurine 12 hr (1.4-fold) after treatment, which is considerably earlier than peaks in the amount of CYP2A5 mRNA. TGFbeta reduced CYP2A5 marker activity, coumarin 7-hydroxylase by 74%. These results indicate that in a primary hepatocyte model (a) there is no systematic correlation between apoptosis and CYP2A5 induction; (b) phenobarbital does not significantly affect the rate of apoptosis; and (c) the induction of apoptosis caused by the chemicals tested occurs considerable earlier than elevation of CYP2A5 expression. Thus, no causal link appears to exist between induction of CYP2A5 and apoptotic rate.

A conserved nuclear receptor consensus sequence (DR-4) mediates transcriptional activation of the chicken CYP2H1 gene by phenobarbital in a hepatoma cell line

Phenobarbital-responsive DNA elements were identified in the 5'-flanking region of the chicken CYP2H1 gene by in reporter gene assays in a chicken hepatoma cell line (leghorn male hepatoma (LMH)). A 264-base pair (bp) enhancer sequence (phenobarbital-responsive unit (PBRU)) responded to phenobarbital and a variety of phenobarbital-type inducers. Analysis of putative transcription factor binding sites within the 264-bp element revealed a nuclear receptor half-site repeat (DR-4) neighboring a putative nuclear factor-1 site. This motif resembles phenobarbital response elements in the flanking regions of three phenobarbital-inducible genes, rat CYP2B2, mouse Cyp2b10, and human CYP2B6. Activation of the 264-bp element was eliminated after site-directed mutagenesis of the DR-4 hexamer half-sites. Evidence for evolutionary conservation of this recognition site was indicated by activation in LMH cells of a mouse Cyp2b10 phenobarbital-responsive enhancer by the same spectrum of inducers that activate the CYP2H1 264-bp PBRU. Inhibition of this activation by okadaic acid may explain the reported inhibitory effects on induction of CYP2B1/2 and Cyp2b10 by this phosphatase inhibitor. We show that this inhibition occurs directly on the 264-bp PBRU, whereas the proximal promoter of CYP2H1 is induced by okadaic acid in reporter gene assays. These experiments exploit the unique phenobarbital inducibility of the hepatoma-derived cell line LMH.