Intraluminal calcium of the liver endoplasmic reticulum stimulates the glucuronidation of p-nitrophenol

CRISPR screen identified that UGT1A9 was required for bisphenols-induced mitochondria dyshomeostasis

Exposure to bisphenols chemicals could cause various adverse health effects, including non-alcoholic fatty liver disease (NAFLD), which have been associated with cellular mitochondria stress. However, the biological mechanism underlying the mitochondria stress-mediated cell death by bisphenols was poorly understood. Here, CRISPR screens were performed to identify the critical genes which were involved in cell death caused by exposure to four bisphenols (BPA, BPB, BPE and BPS). Results of CRISPR screens showed that UGT1A9 was the primary genetic factor facilitating cell death induced by all of the four bisphenols. Systematic toxicological tests demonstrated that UGT1A9 was required for BPA-induced mitochondria dyshomeostasis in vitro and in vivo, and UGT1A9-mediated mitochondria dyshomeostasis was an important cause of facilitating cell death. Liver injury caused by exposure to BPA in wild-type mice was accompanied with suppression of mitophagy and increased expression of C-Caspase 3, but UGT1A9 knockout attenuated these adverse effects induced by BPA. Finally, molecular epidemiology analysis suggested that the five genetic variants of UGT1A9 could be potential genetic risk factors of NAFLD when people were exposed to BPA. The biological mechanism uncovered here provided mechanistic evidence for identification of susceptible populations of liver injury associated with exposure to BPA.

Changes in nucleoporin domain topology in response to chemical effectors

Nucleoporins represent the molecular building blocks of nuclear pore complexes (NPCs), which mediate facilitated macromolecular trafficking between the cytoplasm and nucleus of eukaryotic cells. Phenylalanine-glycine (FG) repeat motifs are found in about one-third of the nucleoporins, and they provide major binding or docking sites for soluble transport receptors. We have shown recently that localization of the FG-repeat domains of vertebrate nucleoporins Nup153 and Nup214 within the NPC is influenced by its transport state. To test whether chemical effectors, such as calcium and ATP, influence the localization of the FG-repeat domains of Nup153 and Nup214 within the NPC, we performed immuno-electron microscopy of Xenopus oocyte nuclei using domain-specific antibodies against Nup153 and Nup214, respectively. Ca2+ and ATP are known to induce conformational changes in the NPC architecture, especially at the cytoplasmic face, but also at the nuclear basket of the NPC. We have found concentrations of calcium in the micromolar range or 1 mM ATP in the surrounding buffer leaves the spatial distribution of the FG-repeat of Nup153 and Nup214 largely unchanged. In contrast, ATP depletion, calcium store depletion by EGTA or thapsigargin, and high concentrations of divalent cation (i.e. 2 mM Ca2+ and 2 mM Mg2+) constrain the distribution of the FG-repeats of Nup153 and Nup214. Our data suggest that the location of the FG-repeat domains of Nup153 and Nup214 is sensitive to chemical changes within the near-field environment of the NPC.

Ca2+-mediated activation of ERK in hepatocytes by norepinephrine and prostaglandin F2 alpha: role of calmodulin and Src kinases

BACKGROUND

Previous studies have shown that several agents that stimulate heptahelical G-protein coupled receptors activate the extracellular signal regulated kinases ERK1 (p44mapk) and ERK2 (p42mapk) in hepatocytes. The molecular pathways that convey their signals to ERK1/2 are only partially clarified. In the present study we have explored the role of Ca2+ and Ca2+-dependent steps leading to ERK1/2 activation induced by norepinephrine and prostaglandin (PG)F2alpha.

RESULTS

Pretreatment of the cells with the Ca2+ chelators BAPTA-AM or EGTA, as well as the Ca2+ influx inhibitor gadolinium, resulted in a partial decrease of the ERK response. Furthermore, the calmodulin antagonists W-7, trifluoperazine, and J-8 markedly decreased ERK activation. Pretreatment with KN-93, an inhibitor of the multifunctional Ca2+/calmodulin-dependent protein kinase, had no effect on ERK activation. The Src kinase inhibitors PP1 and PP2 partially diminished the ERK responses elicited by both norepinephrine and PGF2alpha.

CONCLUSION

The present data indicate that Ca2+ is involved in ERK activation induced by hormones acting on G protein-coupled receptors in hepatocytes, and suggest that calmodulin and Src kinases might play a role in these signaling pathways.

The role of hepatic and extrahepatic UDP-glucuronosyltransferases in human drug metabolism

At present, the methods and enzymology of the UDP-glucuronosyltransferases (UGTs) lag behind that of the cytochromes P450 (CYPs). About 15 human UGTs have been identified, and knowledge about their regulation, substrate selectivity, and tissue distribution has progressed recently. Alamethicin has been characterized as a treatment to remove the latency of microsomal glucuronidations. Most UGT isoforms appear to have a distinct hepatic and/or extrahepatic expression, resulting in significant expression in kidney, intestine, and steroid target tissues. The gastrointestinal tract possesses a complex expression pattern largely containing members of the UGT1A subfamily. Thus, these forms are poised to participate in the first pass metabolism of oral drugs. The authors and others have identified a significant expression of UGT1A1 in human small intestine, an enzyme possessing considerable allelic variability and a polymorphic expression pattern in intestine. Intestinal glucuronidation therefore plays a major role not only in first pass metabolism, but also in the degree of interindividual variation in overall oral bioavailability. Due to issues such as significant genetic variability and tissue localization in first-pass organs, clearance due to UGT1A1 should be minimized for new drugs.

Beta-glucuronidase latency in isolated murine hepatocytes

The physiological function of microsomal beta-glucuronidase is unclear. Substrates may be either glucuronides produced in the lumen of endoplasmic reticulum (ER) or those taken up by hepatocytes. In the latter case, efficient inward transport of glucuronides at the plasma membrane and the ER membrane would be required. Therefore, the potential role of beta-glucuronidase in ER was investigated. Isolated mouse hepatocytes and mouse and rat liver microsomal vesicles were used in the experiments. Selective permeabilization of the plasma membrane of isolated hepatocytes with saponin or digitonin resulted in an almost 4-fold elevation in the rate of beta-nitrophenol glucuronide hydrolysis, while the permeabilization of plasma membrane plus ER membrane by Triton X-100 caused a further 2-fold elevation. In microsomal vesicles, the p-nitrophenol glucuronide or phenolphthalein glucuronide beta-glucuronidase activity showed about 50% latency as revealed by alamethicin or Triton X-100 treatment. A light-scattering study indicated that the microsomes are relatively impermeable to both glucuronides and to glucuronate. On the basis of our results, the role of liver microsomal beta-glucuronidase in the deconjugation of glucuronides taken up by the liver seems unlikely. Hydrolysis of the glucuronides produced in the ER lumen may play a role in substrate supply for ascorbate synthesis or in "proofreading" of glucuronidation.

Maturation and secretion of rat hepatic lipase is inhibited by alpha1B-adrenergic stimulation through changes in Ca2+ homoeostasis: thapsigargin and EGTA both mimic the effect of adrenaline

In rats, the daily changes in hepatic lipase (HL) activity in the liver follow the diurnal rhythm of the catecholamines. To study the underlying mechanism, the effect of adrenaline on maturation and secretion of HL was determined in freshly isolated rat hepatocytes. Adrenaline (10 microM) acutely inhibited the secretion of HL. This effect was abolished by 0.1 microM prazosin, but not by 1 microM propranolol, indicating the involvement of the alpha1-adrenergic pathway. Prazosin was at least 1000-fold more potent than WB4101, a selective alpha1A-antagonist. Adrenaline had no effect on HL secretion in hepatocytes pretreated with chloroethylclonidine, an irreversible alpha1B-selective antagonist. Inhibition of HL was not induced by 10 microM methoxamine, a alpha1A-selective agonist. Thus, adrenaline inhibited HL secretion through activation of the alpha1-adrenoceptors subtype B, which have been shown to signal through Ca2+ as well as cAMP. A similar reduction in HL secretion was induced by the Ca2+-mobilizing hormones angiotensin II (100 nM) and vasopressin (12 nM), the Ca2+ ionophore A23187 (2 microM), and by thapsigargin (1 microM), which inhibits the ER Ca2+-ATPase pump. HL secretion was unaffected by elevating cAMP with 10 microM forskolin or 1 microM 8-Br-cAMP. These results suggest that the alpha1B-adrenergic effects on HL expression are mainly mediated through elevation of intracellular Ca2+. Chelation of extracellular Ca2+ and subsequent lowering of intracellular Ca2+ with EGTA also inhibited HL secretion. In pulse-chase experiments, adrenaline was shown to inhibit the maturation of HL from the 53 kDa, Endo H-sensitive precursor to the Endo H-resistant, catalytically active protein of 58 kDa. In addition, adrenaline induced intracellular degradation of newly synthesized HL. Similar post-translational effects, both qualitatively and quantitatively, were observed with A23187, thapsigargin and EGTA. We conclude that the inhibition of HL maturation and increase in intracellular degradation induced by catecholamines, A23187, thapsigargin and EGTA is evoked by changes in Ca2+ homoeostasis, possibly through lowering ER Ca2+.

Regulation of glucuronidation by glutathione redox state through the alteration of UDP-glucose supply originating from glycogen metabolism

The effect of altered redox state of glutathione was investigated on p-nitrophenol glucuronidation in isolated mouse hepatocytes. Decrease of GSH/GSSG ratio provoked by various agents caused increased glucuronidation which was accompanied by stimulated glycogenolysis and elevated UDP-glucose content. The stimulation of glycogenolysis and glucuronidation by glutathione consumption could be prevented by the reduction of oxidized glutathione with dithiothreitol and by the glycogenolysis inhibitor fructose. In permeabilized hepatocytes glycogen metabolism, bypassed by the addition of UDP-glucose, stimulated glucuronidation which was insensitive to glutathione depletion. In liver microsomes either UDP-glucuronosyltransferase activity or UDP-glucuronic acid transport was not influenced by GSH/GSSG ratio. These results suggest that alteration of the GSH/GSSG ratio regulates glucuronidation by affecting enzymes of the glycogen metabolism via the modification of UDP-glucuronate supply.

Inhibition of glucuronidation by an acyl-CoA-mediated indirect mechanism

The mechanism of the inhibition of glucuronidation by long-chain fatty acyl-CoAs was studied in rat liver microsomal membranes and in isolated hepatocytes. Palmitoyl- and oleoyl-CoA did not affect p-nitrophenol UDP-glucuronosyltransferase activity in native microsomes but were inhibitory in permeabilised vesicles. The extent of inhibition was dependent on the effectiveness of permeabilisation and was constant in time in fully permeabilised microsomes. Fatty acyl-CoAs mobilised calcium from calcium-loaded microsomes. Elevation of the intracellular acyl-CoA level by the addition of palmitate or oleate inhibited the glucuronidation of p-nitrophenol in isolated hepatocytes. This effect could be abolished by emptying the intracellular calcium stores. Therefore, it is concluded that fatty acyl-CoAs inhibit glucuronidation indirectly, presumably via calcium mobilisation.

What is the concentration of calcium ions in the endoplasmic reticulum?

Consideration of the data from a number of sources indicates that the concentration of Ca2+ in the endoplasmic reticulum is very high and perhaps in the mM range. A number of implications flow from this-an important one being that the magnitude of Ca2+ gradients across the endoplasmic and plasma membranes are very similar.

Evidence for an UDP-glucuronic acid/phenol glucuronide antiport in rat liver microsomal vesicles

The transport of glucuronides synthesized in the luminal compartment of the endoplasmic reticulum by UDP-glucuronosyltransferase isoenzymes was studied in rat liver microsomal vesicles. Microsomal vesicles were loaded with p-nitrophenol glucuronide (5 mM), phenolphthalein glucuronide or UDP-glucuronic acid, by a freeze-thawing method. In was shown that: (i) the loading procedure resulted in millimolar intravesicular concentrations of the different loading compounds; (ii) addition of UDP-glucuronic acid (5 mM) to the vesicles released both intravesicular glucuronides within 1 min; (iii) glucuronides stimulated the release of UDP-glucuronic acid from UDP acid-loaded microsomal vesicles; (iv) trans-stimulation of UDP-glucuronic acid entry by loading of microsomal vesicles with p-nitrophenol glucuronide, phenolphthalein glucuronide, UDP-glucuronic acid and UDP-N-acetyl-glucosamine almost completely abolished the latency of UDP-glucuronosyltransferase, although mannose 6-phosphatase latency remained unaltered; (v) the loading compounds by themselves did not stimulate UDP-glucuronosyltransferase activity. This study indicates that glucuronides synthesized in the lumen of endoplasmic reticulum can leave by an antiport, which concurrently transports USP-glucuronic acid into the lumen of the endoplasmic reticulum.

Ascorbate synthesis-dependent glutathione consumption in mouse liver

Ascorbate synthesis causes glutathione consumption in the liver. Addition of gulonolactone resulted in an increase of ascorbate production in isolated murine hepatocytes. At the same time, a decrease in reduced glutathione (GSH) level was observed. In hepatic microsomal membranes, ascorbate synthesis stimulated by gulonolactone caused an almost equimolar consumption of GSH. This effect could be counteracted by the addition of catalase or mercaptosuccinate, indicating the role of hydrogen peroxide formed during ascorbate synthesis in the depletion of GSH. The observed phenomenon may be one of the reasons why the evolutionary loss of ascorbate synthesis could be advantageous.

The lysosomal compartment as intracellular calcium store in MDCK cells: a possible involvement in InsP3-mediated Ca2+ release

To test for a possible role of lysosomes in intracellular Ca2+ homeostasis, the effects of glycyl-L-phenylalanine-beta-naphthylamide (GPN), known to permeabilize these organelles by osmotic swelling, were studied in single MDCK cells. Fluorescence of acridine orange, rhodol green dextran, lysotracker green and FITC-dextran indicated that GPN (0.2 mmol/l) elicited a reversible permeabilization of lysosomes. Cytosolic Ca2+ ([Ca2+]i) as determined by Fura-2 fluorescence increased from 60 +/- 11 to 534 +/- 66 nmol/l (n = 41) in the presence of GPN. Whereas only a single intracellular Ca2+ release could be induced by GPN in a Ca(2+)-free perfusate, repetitive release could be evoked in Ca2+ containing solutions suggesting reuptake of Ca2+ into lysosomal stores. GPN-induced Ca2+ release was blunted after pretreatment with thapsigargin (TG), an inhibitor of Ca(2+)-ATPase, or repeated applications of ATP inducing Ca2+ release from inositol trisphosphate (InsP3) sensitive Ca2+ stores. The effect of ATP on Ca2+ release was, however, not abolished by preceding GPN treatment. GPN-induced Ca2+ release from lysosomes was independent of InsP3 formation or Ca(2+)-induced Ca2+ release, since it was unaffected by the phospholipase C inhibitor U-73, 122 or by caffeine and ruthenium red. These results suggest that Ca2+ largely accumulates in lysosomal vesicles. Moreover, these organelles seem to be part or functionally coupled with InsP3-sensitive Ca2+ stores.

Inhibition of p-nitrophenol glucuronidation by calcium mobilizing hormones

1. Vasopressin and phenylephrine markedly inhibited the glucuronidation of p-nitrophenol in isolated murine hepatocytes. 2. After longer preincubation of hepatocytes in the presence of vasopressin or phenylephrine the rate of conjugation began to return to the control values indicating the reversibility of the inhibition caused by these agents. 3. The inhibitory effect of both agents was dependent on the Ca2+ filled state of the intracellular stores. 4. The inhibition caused by the alpha 1 receptor agonist phenylephrine was receptor mediated because it could be prevented by the addition of alpha 1 antagonist prazosin. 5. The data support the theory that the maintenance of the intralumenal Ca2+ concentration is necessary for the optimal activity of p-nitrophenol UDP-glucuronosyl-transferase.

Increased oxidation and decreased conjugation of drugs in the liver caused by starvation. Altered metabolism of certain aromatic compounds and acetone

Starvation causes several changes in the various processes of biotransformation. The focus of this review is on biotransformation of various aromatic and other compounds whose metabolism is catalyzed in phase I by isozymes belonging to the CYP2E1 gene subfamily, while in phase II phenol-UDPGT or conjugation with GSH play a dominant role. The other ways of conjugation are beyond the scope of this review. The reason why this aspect has been chosen is that the capacity of these reactions is profoundly altered by nutritional conditions. There is a balance between the two phases of biotransformation. Therefore, under standard circumstances in a well-fed state the intermediate formed in the course of phase I is converted to a conjugated compound rapidly, as a result of phase II. However, in starvation the pattern of drug metabolism is altered and the balance between the two phases is changed. This alteration of drug metabolism upon starvation is partly connected to the changes of cofactor supplies due to the metabolic state.

Endotoxin inhibits glucuronidation in the liver. An effect mediated by intercellular communication

Endotoxin [lipopolysaccharide (LPS) 50 micrograms/mL] added to the perfusion medium increased glucose production and inhibited the glucuronidation of p-nitrophenol in perfused mouse liver both in recirculating and non-recirculating systems, while sulfation of p-nitrophenol was unchanged. The effects of endotoxin could be prevented by the addition of cyclooxygenase inhibitors, while PGD2 and PGE2 also caused a decrease in p-nitrophenol glucuronidation in perfused liver. In isolated hepatocytes endotoxin failed to affect p-nitrophenol conjugation, while PGD2 and PGE2 decreased the rate of it. Our results suggest that endotoxin inhibits glucuronidation through an intercellular communication presumably mediated by eicosanoids.

Thapsigargin selectively stimulates synthesis of phosphatidylglycerol in N1E-115 neuroblastoma cells and phosphatidylinositol in C6 glioma cells

Phospholipid metabolism was studied in N1E-115 neuroblastoma and C6 glioma cells exposed to thapsigargin, a selective inhibitor of endoplasmic reticulum Ca(2+)-ATPase that raises the cytosolic free Ca2+ concentration [Ca2+]i. Thapsigargin caused only a transient increase of [Ca2+]i (< 1 min) in N1E-115 cells similar in magnitude and duration to agonist-induced calcium release mediated by inositol trisphosphate. Sustained elevation of [Ca2+]i due to influx of extracellular calcium, as occurs in most other cell lines including C6 cells, did not occur in N1E-115 cells. Increased uptake of inorganic phosphate (Pi) associated calcium influx was observed in C6 but not in N1E-115 cells. Thapsigargin affected phospholipid synthesis in both cell lines, most likely by inhibiting phosphatidic acid phosphohydrolase as indicated by diversion of [3H]oleic acid incorporation from triacylglycerol to phospholipid synthesis and stimulation of [32P]Pi incorporation into anionic phospholipids at the expense of phosphatidylcholine synthesis. The response to increased phosphatidate/phosphatidyl-CMP availability was cell specific. Thapsigargin (> 100 nM) selectively stimulated phosphatidylglycerol synthesis 20-30-fold in N1E-115 neuroblastoma cells while phosphatidylinositol synthesis was increased < 2-fold. In contrast, phosphatidylglycerol was not affected in C6 glioma cells and phosphatidylinositol synthesis was stimulated 8-fold by thapsigargin (> 1 microM). Agonist-stimulated calcium release did not increase phosphatidylglycerol synthesis in N1E-115 cells. Thapsigargin-stimulated phosphatidylglycerol synthesis and agonist-stimulated phosphatidylinositol synthesis could occur at the same time. Similar results were obtained with TMB-8, an inhibitor of intracellular Ca2+ release that decreases diacylglycerol utilization by blocking choline uptake and phosphatidylcholine synthesis without affecting resting [Ca2+]i. Thus [Ca2+]i does not directly mediate the effects of thapsigargin, TMB-8 or agonist stimulation on anionic phospholipid metabolism. These additional effects may limit the use of thapsigargin to assess Ca(2+)-dependence of phospholipid metabolism associated with Ca(2+)-mediated signal transduction.

Dual role of divalent cations in the bile acid:CoA ligase catalyzed reaction

The role of divalent cations in the bile acid:CoA ligase catalyzed reaction of cholic acid, CoA and ATP to yield cholyl-CoA was investigated using guinea pig liver microsomes as the source of enzyme. EDTA treatment completely eliminated activity indicating an absolute requirement for divalent cation for enzyme activity. Analysis of this requirement revealed that it was twofold. First, the data suggested that ATP which was not complexed with a divalent cation did not appreciably bind to the enzyme and thus a divalent cation complex of ATP is the form of ATP that is the substrate for the enzyme. Further, this was shown to be the basis for the absolute requirement for divalent cation in the reaction. In addition, analysis revealed that there is a secondary site which binds divalent cations with relatively low affinity, and results in a rate enhancement. Binding at this secondary site is estimated to increase the rate by greater than 60%.

Ascorbic acid synthesis is stimulated by enhanced glycogenolysis in murine liver

Ascorbic acid synthesis was stimulated by glucagon, dibutyryl cyclic AMP, as well as phenylephrine vasopressin or okadaic acid, in hepatocytes prepared from fed mice. However, no such effect was observed in glycogen-depleted cells from starved animals, either in the presence or absence of glucose. The rate of ascorbate synthesis showed close correlation with the glucose release by hepatocytes. In mice the injection of glucagon increased plasma ascorbate concentration fifteenfold, and caused a sixfold elevation of the ascorbate content of the liver. These results show that hepatic ascorbate synthesis is dependent on glycogenolysis, and indicate a regulatory role of ascorbate released by the liver.