Dihydropyrazine induces endoplasmic reticulum stress and inhibits autophagy in HepG2 human hepatoma cells.

Dihydropyrazines (DHPs) are formed by non-enzymatic glycation reactions in vivo and in food. We recently reported that 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3), which is a methyl-substituted DHP, caused severe oxidative stress and cytotoxicity. However, the molecular mechanisms underlying the cytotoxic pathways of the DHP response remain elusive. Because oxidative stress induces endoplasmic reticulum (ER) stress and autophagy, we investigated the ability of DHP-3 to modulate the ER stress and autophagy pathways. DHP-3 activated the ER stress pathway by increasing inositol-requiring enzyme 1 (IRE1) and PKR-like ER kinase (PERK) phosphorylation and transcription factor 6 (ATF6) expression. Moreover, DHP-3 increased the expression of activating transcription factor 4 (ATF4) and C/EBP homologous protein (CHOP), which are downstream targets of PERK. In addition, DHP-3 inhibited the autophagy pathway by increasing the accumulation of microtubule-associated protein 1 light chain 3 alpha-phosphatidylethanolamine conjugate (LC3-II) and p62/sequestosome 1 (p62), while decreasing autophagic flux. Taken together, these results indicate that DHP-3 activates the ER stress pathway and inhibits the autophagy pathway, suggesting that the resulting removal of damaged organelles is inadequate.

Oxidative stress and cellular toxicity induced by dihydropyrazine: a comparative study with other Maillard reaction products.

Glycation products are generated during the Maillard reaction, a non-enzymatic reaction between reducing sugars and the amino groups of proteins, which accumulate in the body with aging and cause many diseases. Herein, we have focused on dihydropyrazines (DHPs), which are glycation products formed by the dimerization of D-glucosamine or 5-aminolevulinic acid, and have reported that DHPs can produce several kinds of radicals and induce cytotoxicity via oxidative stress. To advance our understanding of DHP-mediated cytotoxicity, we selected a DHP, 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3), and two major Maillard reaction products, Nε-(carboxymethyl)-L-lysine (CML) and acrylamide, and performed comparative experiments focusing on their cytotoxicity and their ability to induce oxidative stress. The order of increasing cytotoxicity was DHP-3, acrylamide, and CML, and the LC50 value could be calculated only for DHP-3 (0.53 mM), indicating that DHP-3 is more toxic than the other Maillard reaction products. However, their toxicities were significantly lower than those of common toxic chemicals. Further, the results of their cytotoxicity assay were consistent with the results of intracellular reactive oxygen species production and activation of oxidative stress response signaling. These results indicate that the acute toxicity of Maillard reaction products is closely related to their ability to induce oxidative stress, and that DHP-3 is a particularly strong inducer of oxidative stress and thus exhibits high cytotoxicity among Maillard reaction products. In addition, we have shown that a comprehensive analysis comparing multiple Maillard reaction products is effective for elucidating their complex and diverse toxicities.

[Protein-Protein Interactions as Underlying Regulatory Mechanisms of Drug-metabolizing Enzyme Function].

Cytochrome P450 (P450, CYP) and uridine 5'-diphospho-glucuronosyltransferase (UGT) are major drug-metabolizing enzymes known to catalyze substrate oxidation and glucuronidation, respectively. Both enzymes are located within the endoplasmic reticulum (ER) membrane; however, their membrane topologies differ, with P450 facing cytosol and a major part of UGTs located on the luminal side. Because of the large differences in the reactions that they catalyze and their membrane topologies, it had been believed that P450 and UGT function separately. However, some chemicals are oxidized by P450 and undergo further conjugation by UGT. Therefore, it is important to consider that P450 and UGT may form a complex within the ER membrane and regulate each other's function through this interaction. To prove this hypothesis, we constructed a co-expression system for CYP3A4, P450 reductase, and UGT2B7/1A9 with a baculovirus-insect cell expression system. This system allowed us to compare CYP3A4 activity in the presence and absence of UGT2B7/1A9 co-expression and revealed that the UGTs suppress CYP3A4 activity. The suppressive effect of UGT2B7 was not limited to the enzymatic activity of CYP3A4 but also extended to the entire catalytic cycle, which may be resulted from the inhibition of substrate-binding to the P450. Analysis using UGT mutants indicated that one of the hydrophobic regions within the luminal portion of the UGT interacts with CYP3A4. Furthermore, we suggested that UGT1A suppresses CYP3A activity in vivo by treating rats with dexamethasone. Thus, the functional interactions between P450 and UGT would advance our understanding of the large inter-individual differences in drug-metabolizing capacity.

Dihydropyrazine suppresses TLR4-dependent inflammatory responses by blocking MAPK signaling in human hepatoma HepG2 cells.

Dihydropyrazines (DHPs), including 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3), are glycation products generated through non-enzymatic reactions in vivo and in food. They are recognized as compounds that are toxic to organisms as they produce radicals. However, our previous study indicated that DHP-3 suppressed Toll-like receptor 4 (TLR4) expression and decreased the phosphorylation of nuclear factor-κB (NF-κB) in lipopolysaccharide (LPS)-treated HepG2 cells. TLR4 signaling is involved in the onset of various inflammatory diseases, and NF-κB and mitogen-activated protein kinase (MAPK) play important roles in TLR4 signaling. Thus, we aimed to elucidate the effects of DHP-3 on MAPK signaling and in turn on the activated TLR4 signaling pathway. In LPS-stimulated HepG2 cells, DHP-3 reduced the phosphorylation of MAPK, extracellular signal-regulated kinase, c-Jun NH2-terminal kinase, and p38. The expression of c-jun, a subunit of activator protein-1, was decreased by DHP-3 treatment. Furthermore, DHP-3-induced suppression of MAPK signaling resulted in a decrease in various inflammatory regulators, such as interleukin-6, CC-chemokine ligand 2, and cyclooxygenase-2. These results suggest that DHP-3 exerts an inhibitory effect on TLR4-dependent inflammatory response by suppressing MAPK signaling.

Use of a Baculovirus-Mammalian Cell Expression-System for Expression of Drug-Metabolizing Enzymes: Optimization of Infection With a Focus on Cytochrome P450 3A4.

Heterologous expression systems are important for analyzing the effects of genetic factors including single nucleotide polymorphisms on the functions of drug-metabolizing enzymes. In this study, we focused on a baculovirus-mammalian cell (Bac-Mam) expression system as a safer and more efficient approach for this purpose. The baculovirus-insect cell expression system is widely utilized in large-scale protein expression. Baculovirus has been shown to also infect certain mammalian cells, although the virus only replicates in insect cells. With this knowledge, baculovirus is now being applied in a mammalian expression system called the Bac-Mam system wherein a gene-modified baculovirus is used whose promotor is replaced with one that can function in mammalian cells. We subcloned open-reading frames of cytochrome P450 3A4 (CYP3A4), UDP-glucuronosyltransferase (UGT) 1A1, and UGT2B7 into a transfer plasmid for the Bac-Mam system, and prepared recombinant Bac-Mam virus. The obtained virus was amplified in insect Sf9 cells and used to infect mammalian COS-1 cells. Expression of CYP3A4, UGT1A1, and UGT2B7 in COS-1 cell homogenates were confirmed by immunoblotting. Optimum infection conditions including the amount of Bac-Mam virus, culture days before collection, and concentration of sodium butyrate, an enhancer of viral-transduction were determined by monitoring CYP3A4 expression. Expressed CYP3A4 showed appropriate activity without supplying hemin/5-aminolevulinic acid or co-expressing with NADPH-cytochrome P450 reductase. Further, we compared gene transfer efficiency between the Bac-Mam system and an established method using recombinant plasmid and transfection reagent. Our results indicate that the Bac-Mam system can be applied to introduce drug-metabolizing enzyme genes into mammalian cells that are widely used in drug metabolism research. The expressed enzymes are expected to undergo appropriate post-translational modification as they are in mammalian bodies. The Bac-Mam system may thus accelerate pharmacogenetics and pharmacogenomics research.

Adenine-related compounds modulate UDP-glucuronosyltransferase (UGT) activity in mouse liver microsomes.

Adenine-related compounds are allosteric inhibitors of UDP-glucuronosyltransferase (UGT) in rat liver microsomes (RLM) and human UGT isoforms treated with detergent or pore-forming peptide, alamethicin.To clarify whether the same is true beyond species, the effects of adenine-related compounds on 4-methylumbelliferone (4-MU) glucuronidation were examined using detergent-treated mouse liver microsomes (MLM).Brij-58 treatment of MLM increased the Vmax and the Michaelis constant, Km, of 4-MU. This study was performed using Brij-58-treated MLM as an enzyme source. ATP- and ADP-inhibited 4-MU glucuronidation. In contrast, AMP caused a 1.5-fold increase in glucuronidation. Oxidised forms, NAD+ and NADP+, potently inhibited 4-MU glucuronidation, whereas the reduced forms, NADH and NADPH, did not. Furthermore, the IC50 values of ATP, ADP, NAD+, and NADP+ were approximately 15 µM.In our previous study, ATP was the strongest inhibitor of UGT activity in RLM. However, in this study, the above-mentioned compounds inhibited 4-MU UGT in a comparable and non-competitive manner. Furthermore, AMP antagonised the inhibitory effects of ATP and ADP.These results suggest that ATP, ADP, NAD+, and NADP+ are common endogenous inhibitors of UGT beyond species.

Molecular mechanism of dihydropyrazine-induced cytotoxicity: the possibility of an independent pathway from the receptor for advanced glycation end products.

Dihydropyrazines (DHPs) are one of glycation products that are non-enzymatically generated in vivo and in food. We had previously revealed that 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3), a methyl-substituted DHP, elicited redox imbalance and cytotoxicity in cultured cells. However, the molecular mechanisms underlying DHP-3-induced cytotoxicity remain unclear. To address this issue, we examined the involvement of the receptor for advanced glycation end products (RAGE) in DHP-3-induced cytotoxicity. To evaluate the role of RAGE, we prepared HeLa cells that constitutively expressed RAGE and its deletion mutant, which lacks the cytoplasmic domain (RAGEΔcyto), using an episomal vector. After transfection with the vector, cells were selected following incubation with multiple concentrations of hygromycin to remove non-transfected cells. The expression of RAGE and RAGEΔcyto in the cells was confirmed by immunoblotting. RAGE and RAGEΔcyto were apparently expressed in transfected cells; however, there were no significant differences in DHP-3-induced cytotoxicity between these cells and mock vector-transfected cells. These results suggested that DHP-3 elicits cytotoxicity in a RAGE-independent manner.

Functional Interaction between Cytochrome P450 and UDP-Glucuronosyltransferase on the Endoplasmic Reticulum Membrane: One of Post-translational Factors Which Possibly Contributes to Their Inter-Individual Differences.

Cytochrome P450 (P450) and uridine 5'-diphosphate (UDP)-glucuronosyltransferase (UGT) catalyze oxidation and glucuronidation in drug metabolism, respectively. It is believed that P450 and UGT work separately because they perform distinct reactions and exhibit opposite membrane topologies on the endoplasmic reticulum (ER). However, given that some chemicals are sequentially metabolized by P450 and UGT, it is reasonable to consider that the enzymes may interact and work cooperatively. Previous research by our team detected protein-protein interactions between P450 and UGT by analyzing solubilized rat liver microsomes with P450-immobilized affinity column chromatography. Although P450 and UGT have been known to form homo- and hetero-oligomers, this is the first report indicating a P450-UGT association. Based on our previous study, we focused on the P450-UGT interaction and reported lines of evidence that the P450-UGT association is a functional protein-protein interaction that can alter the enzymatic capabilities, including enhancement or suppression of the activities of P450 and UGT, helping UGT to acquire novel regioselectivity, and inhibiting substrate binding to P450. Biochemical and molecular bioscientific approaches suggested that P450 and UGT interact with each other at their internal hydrophobic domains in the ER membrane. Furthermore, several in vivo studies have reported the presence of a functional P450-UGT association under physiological conditions. The P450-UGT interaction is expected to function as a novel post-translational factor for inter-individual differences in the drug-metabolizing enzymes.

DAPL1 is a novel regulator of testosterone production in Leydig cells of mouse testis.

Leydig cells in the testes produce testosterone in the presence of gonadotropins. Therefore, male testosterone levels must oscillate within a healthy spectrum, given that elevated testosterone levels augment the risk of cardiovascular disorders. We observed that the expression of death-associated protein-like 1 (DAPL1), which is involved in the early stages of epithelial differentiation and apoptosis, is considerably higher in the testes of sexually mature mice than in other tissues. Accordingly, Dapl1-null mice were constructed to evaluate this variation. Notably, in these mice, the testicular levels of steroidogenic acute regulatory protein (StAR) and serum testosterone levels were significantly elevated on postnatal day 49. The findings were confirmed in vitro using I-10 mouse testis-derived tumor cells. The in vivo and in vitro data revealed the DAPL1-regulated the expression of StAR involving altered transcription of critical proteins in the protein kinase A and CREB/CREM pathways in Leydig cells. The collective findings implicate DAPL1 as an important factor for steroidogenesis regulation, and DAPL1 deregulation may be related to high endogenous levels of testosterone.

LAPTM4α is targeted from the Golgi to late endosomes/lysosomes in a manner dependent on the E3 ubiquitin ligase Nedd4-1 and ESCRT proteins.

Lysosome-associated protein transmembrane 4α (LAPTM4α) is a four transmembrane-spanning protein primarily localized in endosomes and lysosomes and has several putative lysosomal targeting signals at its C-terminal cytoplasmic domain, including tyrosine-based motifs (YxxΦ) and PY motifs (L/PxxY). LAPTM4α has been previously shown to be ubiquitinated by the E3 ubiquitin ligase Nedd4-1 through binding to its PY motifs and sorted to lysosomes, however, the molecular mechanisms underlying the localization of LAPTM4α to endosomes/lysosomes have not yet been fully elucidated. In the present study, we show that LAPTM4α binds Nedd4-1 in a manner dependent on PY motifs, while the PY motifs and Nedd4-1 are not necessarily required for LAPTM4α ubiquitination. The binding of LAPTM4α with Nedd4-1, however, is necessary for an effective sorting of LAPTM4α from the Golgi to late endosomes/lysosomes. An unexpected finding is that LAPTM4α is localized in the lumen, but not in the limiting membrane, of late endosomes, and degraded in lysosomes over time. Interestingly, we further found that siRNA knockdown of endosomal sorting complexes required for transport (ESCRT) components that mediate sorting of ubiquitinated membrane proteins into intralumenal vesicles (ILVs) of endosomes selectively blocks the transport of LAPTM4α to endosomes. Collectively, these results suggest that trafficking of LAPTM4α from the Golgi to endosomes is promoted by the interaction with Nedd4-1, which further requires ESCRT components. Furthermore, our findings highlight a novel function for ESCRT proteins in mediating protein and/or vesicle trafficking from the Golgi to endosomes/lysosomes.

The carboxyl-terminal di-lysine motif is essential for catalytic activity of UDP-glucuronosyltransferase 1A9.

UDP-Glucuronosyltransferase (UGT) is a type I membrane protein localized to the endoplasmic reticulum (ER). UGT has a di-lysine motif (KKXX/KXKXX) in its cytoplasmic domain, which is defined as an ER retention signal. However, our previous study has revealed that UGT2B7, one of the major UGT isoform in human, localizes to the ER in a manner that is independent of this motif. In this study, we focused on another UGT isoform, UGT1A9, and investigated the role of the di-lysine motif in its ER localization, glucuronidation activity, and homo-oligomer formation. Immunofluorescence microscopy indicated that the cytoplasmic domain of UGT1A9 functioned as an ER retention signal in a chimeric protein with CD4, but UGT1A9 itself could localize to the ER in a di-lysine motif-independent manner. In addition, UGT1A9 formed homo-oligomers in the absence of the motif. However, deletion of the di-lysine motif or substitution of lysines in the motif for alanines, severely impaired glucuronidation activity of UGT1A9. This is the first study that re-defines the cytoplasmic di-lysine motif of UGT as an essential peptide for retaining glucuronidation capacity.

The effect of dihydropyrazines on lipopolysaccharide-stimulated human hepatoma HepG2 cells via regulating the TLR4-MyD88-mediated NF-κB signaling pathway.

Dihydropyrazines (DHPs), including 3-hydro-2,2,5,6-tetramethylpyrazine (DHP-3), are glycation products that are spontaneously generated in vivo and ingested via food. DHPs generate various radicals and reactive oxygen species (ROS), which can induce the expression of several antioxidant genes in HepG2 cells. However, detailed information on DHP-response pathways remains elusive. To address this issue, we investigated the effects of DHP-3 on the nuclear factor-κB (NF-κB) pathway, a ROS-sensitive signaling pathway. In lipopolysaccharide-stimulated (LPS-stimulated) HepG2 cells, DHP-3 decreased phosphorylation levels of inhibitor of NF-κB (IκB) and NF-κB p65, and nuclear translocation of NF-κB p65. In addition, DHP-3 reduced the expression of Toll-like receptor 4 (TLR4) and the adaptor protein myeloid differentiation primary response gene 88 (MyD88). Moreover, DHP-3 suppressed the mRNA expression of tumor necrosis factor-alpha (TNFα), and interleukin-1 beta (IL-1ß). Taken together, these results suggest that DHP-3 acts as a negative regulator of the TLR4-MyD88-mediated NF-κB signaling pathway.

Phosphorylation of vaccinia-related kinase 1 at threonine 386 transduces glucose stress signal in human liver cells.

Vaccinia-related kinase 1 (VRK1) is a chromatin-associated Ser-Thr kinase that regulates numerous downstream factors including DNA repair as well as stress factors c-Jun and p53. Both c-Jun and p53 are phosphorylated at Ser63 and Thr18, respectively, in response to low glucose (40 mg/dl of medium) but not high glucose (140 mg/dl of medium) in human hepatoma-derived Huh-7 cells. Here, we have determined the molecular mechanism by which VRK1 phosphorylates these residues in response to glucose in Huh-7 cells. Human VRK1 auto-phosphorylates Ser376 and Thr386 in in vitro kinase assays. In Huh-7 cells, this auto-phosphorylation activity is regulated by glucose signaling; Thr386 is auto-phosphorylated only in low glucose medium, while Ser376 is not phosphorylated in either medium. A correlation of this low glucose response phosphorylation of Thr386 with the phosphorylation of c-Jun and p53 suggests that VRK1 phosphorylated at Thr386 catalyzes this phosphorylation. In fact, VRK1 knockdown by siRNA decreases and over-expression of VRK1 T386D increases phosphorylated c-Jun and p53 in Huh-7 cells. Phosphorylation by VRK1 of c-Jun but not p53 is regulated by cadherin Plakophilin-2 (PKP2). The PKP2 is purified from whole extracts of Huh-7 cells cultured in low glucose medium and is characterized to bind a C-terminal peptide of the VRK1 molecules to regulate its substrate specificity toward c-Jun. siRNA knockdowns show that PKP2 transduces low glucose signaling to VRK1 only to phosphorylate c-Jun, establishing the low glucose-PKP2-VRK1-c-Jun pathway as a glucose stress signaling pathway.

Hetero-oligomer formation of mouse UDP-glucuronosyltransferase (UGT) 2b1 and 1a1 results in the gain of glucuronidation activity towards morphine, an activity which is absent in homo-oligomers of either UGT.

UDP-Glucuronosyltransferase (UGT, Ugt) is a major drug metabolizing enzyme family involved in the glucuronidation and subsequent elimination of drugs and small lipophilic molecules. UGT forms homo- and hetero-oligomers that enhance or suppress UGT activity. In our previous study, we characterized mouse Ugt1a1 and all the Ugt isoform belonging to the Ugt2b subfamily and revealed that mouse Ugt2b1 and Ugt1a1 cannot metabolize morphine. Mouse Ugt2b1 had been believed to function similarly to rat UGT2B1, which plays a major role in morphine glucuronidation in rat liver. Thus, in this study, we hypothesized that hetero-oligomerization with another Ugt isoform may affect Ugt2b1 catalytic ability. We co-expressed Ugt1a1 and Ugt2b1 in a baculovirus-insect cell system, and confirmed hetero-oligomer formation by co-immunoprecipitation. As reported previously, microsomes singly expressing Ugt1a1 or Ugt2b1 were inactive towards the glucuronidation of morphine. Interestingly, in contrast, morphine-3-glucuronide, a major metabolite of morphine was formed, when Ugt2b1 and Ugt1a1 were co-expressed. This effect of hetero-oligomerization of Ugt1a1 and Ugt2b1 was also observed for 17ß-estradiol glucuronidation. This is the first report demonstrating that UGT acquires a novel catalytic ability by forming oligomers. Protein-protein interaction of Ugts may contribute to robust detoxification of xenobiotics by altering the substrate diversity of the enzymes.

UDP-Glucuronosyltransferase (UGT)-mediated attenuations of cytochrome P450 3A4 activity: UGT isoform-dependent mechanism of suppression.

BACKGROUND AND PURPOSE: Cytochrome P450 (CYP, P450) 3A4 is involved in the metabolism of 50% of drugs and its catalytic activity in vivo is not explained only by hepatic expression levels. We previously demonstrated that UDP-glucuronosyltransferase (UGT) 2B7 suppressed CYP3A4 activity through an interaction. In the present study, we target UGT1A9 as another candidate modulator of CYP3A4. EXPERIMENTAL APPROACH: We prepared co-expressed enzymes using the baculovirus-insect cell expression system and compared CYP3A4 activity in the presence and absence of UGT1A9. Wistar rats were treated with dexamethasone and liver microsomes were used to elucidate the role of CYP3A-UGT1A interactions. KEY RESULTS: UGT1A9 and UGT2B7 interacted with and suppressed CYP3A4. Kinetic analyses showed that both of the UGTs significantly reduced Vmax of CYP3A4 activity. In addition, C-terminal truncated mutants of UGT1A9 and UGT2B7 still retained the suppressive capacity. Dexamethasone treatment induced hepatic CYP3As and UGT1As at different magnitudes. Turnover of CYP3A was enhanced about twofold by this treatment. CONCLUSION AND IMPLICATIONS: The changes of kinetic parameters suggested that UGT1A9 suppressed CYP3A4 activity with almost the same mechanism as UGT2B7. The luminal domain of UGTs contains the suppressive interaction site(s), whereas the C-terminal domain may contribute to modulating suppression in a UGT isoform-specific manner. CYP3A-UGT1A interaction seemed to be disturbed by dexamethasone treatment and the suppression was partially cancelled. CYP3A4-UGT interactions would help to better understand the causes of inter/intra-individual differences in CYP3A4 activity.

Investigation of the Endoplasmic Reticulum Localization of UDP-Glucuronosyltransferase 2B7 with Systematic Deletion Mutants.

UDP-Glucuronosyltransferase (UGT) plays an important role in the metabolism of endogenous and exogenous compounds. UGT is a type I membrane protein, and has a dilysine motif (KKXX/KXKXX) in its C-terminal cytoplasmic domain. Although a dilysine motif is defined as an endoplasmic reticulum (ER) retrieval signal, it remains a matter of debate whether this motif functions in the ER localization of UGT. To address this issue, we generated systematic deletion mutants of UGT2B7, a major human isoform, and compared their subcellular localizations with that of an ER marker protein calnexin (CNX), using subcellular fractionation and immunofluorescent microscopy. We found that although the dilysine motif functioned as the ER retention signal in a chimera that replaced the cytoplasmic domain of CD4 with that of UGT2B7, UGT2B7 truncated mutants lacking this motif extensively colocalized with CNX, indicating dilysine motif-independent ER retention of UGT2B7. Moreover, deletion of the C-terminal transmembrane and cytoplasmic domains did not affect ER localization of UGT2B7, suggesting that the signal necessary for ER retention of UGT2B7 is present in its luminal domain. Serial deletions of the luminal domain, however, did not affect the ER retention of the mutants. Further, a cytoplasmic and transmembrane domain-deleted mutant of UGT2B7 was localized to the ER without being secreted. These results suggest that UGT2B7 could localize to the ER without any retention signal, and lead to the conclusion that the static localization of UGT results from lack of a signal for export from the ER.

Glucose elicits serine/threonine kinase VRK1 to phosphorylate nuclear pregnane X receptor as a novel hepatic gluconeogenic signal.

Low glucose stimulated phosphorylation of pregnane X receptor (PXR) at Ser350 in correlation with an increased gluconeogenesis in human hepatoma-derived HepG2 cells. Only glucose, but neither insulin nor glucagon, stimulated this phosphorylation. Here, serine/threonine kinase, vaccinia related kinase 1 (VRK1)-mediated phosphorylation of PXR is now defined as this glucose-elicited novel signal. In low glucose conditions, VRK1 directly phosphorylates PXR at Ser350, enabling PO3-PXR to scaffold protein phosphatase PP2Cα. This PP2Cα dephosphorylates serine/threonine kinase 2 (SGK2) at Thr193. This dephosphorylation dissociates SGK2 from and actives the phosphoenolpyruvate carboxykinase 1 (PCK1) gene as phosphorylated SGK2 binds and represses the gene. Conversely, VRK1 self-represses its activity to phosphorylate PXR through cyclin-dependent kinase 2 (CDK2) in high glucose conditions, resulting in the repression of the PCK1 gene. This PXR phosphorylation was also observed in fasting mouse livers. Thus, the VRK1-CDK2-PXR-PP2Cα-SGK2 pathway can be a novel physiological cell signaling that regulates gluconeogenesis in response to glucose.

Comprehensive Characterization of Mouse UDP-Glucuronosyltransferase (Ugt) Belonging to the Ugt2b Subfamily: Identification of Ugt2b36 as the Predominant Isoform Involved in Morphine Glucuronidation.

UDP-Glucuronosyltransferases (UGTs) are classified into three subfamilies in mice: Ugt1a, 2b, and 2a. In the Ugt1a subfamily, Ugt1a1 and 1a6 appear to correspond to human UGT1A1 and 1A6 The mouse is an important animal for its use in investigations, but the substrate specificities of Ugt isoforms belonging to the 2b subfamily in mice remain largely unknown. To address this issue, we characterized the substrate specificity of all isoforms of the Ugt2b subfamily expressed in the mouse liver. The cDNAs of Ugt1a1, Ugt2a3, and all the Ugt2b isoforms expressed in the liver were reverse-transcribed from the total RNA of male FVB-mouse livers and then amplified. A baculovirus-Sf9 cell system for expressing each Ugt was established. Of all the Ugts examined, Ugt2b34, 2b36, and 2b37 exhibited the ability to glucuronidate morphine with Ugt2b36, the most active in this regard. Ugt1a1, but also Ugt2b34, 2b36, and 2b37 to a lesser extent, preferentially catalyzed the glucuronidation of 17ß-estradiol on the 3-hydroxyl group (E3G). With these isoforms, E3G formation by Ugt1a1 was efficient; however, Ugt2b5 exhibited a preference for the 17ß-hydroxyl group (E17G). Ugt2b1 and Ugt2a3 formed comparable levels of E3G and E17G. Ugt2b1 and 2b5 were the only isoforms involved in chloramphenicol glucuronidation. As Ugt2b36 is highly expressed in the liver, it is most likely that Ugt2b36 is a major morphine Ugt in mouse liver. Regarding E3G formation, Ugt1a1, like the human homolog, seems to play an important role in the liver.

Introduction of an N-Glycosylation Site into UDP-Glucuronosyltransferase 2B3 Alters Its Sensitivity to Cytochrome P450 3A1-Dependent Modulation.

Our previous studies have demonstrated functional protein-protein interactions between cytochrome P450 (CYP) 3A and UDP-glucuronosyltransferase (UGT). However, the role of carbohydrate chains of UGTs in the interaction with CYP is not well understood. To address this issue, we examined whether CYP3A1 modulates the function of UGT2B3 which lacks potential glycosylation sites. We also examined whether the introduction of N-glycosylation to UGT2B3 affects CYP3A-dependent modulation of UGT function. To introduce a potential glycosylation site into UGT2B3, Ser 316 of UGT2B3 was substituted with Asn by site-directed mutagenesis. A baculovirus-Sf-9 cell system for expressing CYP3A1 and UGT2B3/UGT2B3(S316N) was established using a Bac-to-Bac system. Glycosylation of UGT2B3(S316N) was demonstrated in this expression system. The microsomal activity of recombinant UGT was determined using 4-methylumbelliferone as a substrate. The effect of CYP3A1 co-expression on UGT function was examined by comparing the kinetic profiles between single (UGT alone) and double expression (UGT plus CYP) systems. The kinetics of the two expression systems fitted a Michaelis-Menten equation. When the 4-MU concentration was varied, co-expression of CYP3A1 lowered the Vmax of UGT2B3-mediated conjugation. Conversely, for UGT2B3(S316N), the Vmax in the dual expression system was higher than that in the single expression system. The data obtained demonstrate that the introduction of N-glycosylation to UGT2B3 alters its sensitivity to CYP3A1-dependent modulation while CYP3A1 enhanced UGT2B3(S316N) activity, and wild-type UGT2B3 was suppressed by CYP3A1. These data suggest that N-glycosylation of UGT is one of the determinants regulating the interaction between CYP3A and UGT.

Suppression of Cytochrome P450 3A4 Function by UDP-Glucuronosyltransferase 2B7 through a Protein-Protein Interaction: Cooperative Roles of the Cytosolic Carboxyl-Terminal Domain and the Luminal Anchoring Region.

There is a large discrepancy between the interindividual difference in the hepatic expression level of cytochrome P450 3A4 (CYP3A4) and that of drug clearance mediated by this enzyme. However, the reason for this discrepancy remains largely unknown. Because CYP3A4 interacts with UDP-glucuronosyltransferase 2B7 (UGT2B7) to alter its function, the reverse regulation is expected to modulate CYP3A4-catalyzed activity. To address this issue, we investigated whether protein-protein interaction between CYP3A4 and UGT2B7 modulates CYP3A4 function. For this purpose, we coexpressed CYP3A4, NADPH-cytochrome P450 reductase, and UGT2B7 using a baculovirus-insect cell system. The activity of CYP3A4 was significantly suppressed by coexpressing UGT2B7, and this suppressive effect was lost when UGT2B7 was replaced with calnexin (CNX). These results strongly suggest that UGT2B7 negatively regulates CYP3A4 activity through a protein-protein interaction. To identify the UGT2B7 domain associated with CYP3A4 suppression we generated 12 mutants including chimeras with CNX. Mutations introduced into the UGT2B7 carboxyl-terminal transmembrane helix caused a loss of the suppressive effect on CYP3A4. Thus, this hydrophobic region is necessary for the suppression of CYP3A4 activity. Replacement of the hydrophilic end of UGT2B7 with that of CNX produced a similar suppressive effect as the native enzyme. The data using chimeric protein demonstrated that the internal membrane-anchoring region of UGT2B7 is also needed for the association with CYP3A4. These data suggest that 1) UGT2B7 suppresses CYP3A4 function, and 2) both hydrophobic domains located near the C terminus and within UGT2B7 are needed for interaction with CYP3A4.