Cell signaling and nuclear receptors: new opportunities for molecular pharmaceuticals in liver disease

[Pharmacokinetic changes related to acute infection. Examples from the SARS-CoV-2 pandemic]

The knowledge of factors of pharmacokinetic variability is important in order to personalize pharmacological treatment, particularly for drugs with a narrow therapeutic range for which pharmacological therapeutic monitoring is recommended. Inflammation is a protective response against acute infections and injuries that contributes to intra- and inter-individual variability in drug exposure by modulating the activity of enzymes involved in drug metabolism, and by altering the binding of drugs to plasma proteins. The understanding of the impact of inflammation on drug metabolism and the related clinical consequences allow to better take into consideration the effect of inflammation on the variability of drug exposure. We first summarized the molecular mechanisms by which inflammation contributes to the inhibition of drug metabolism enzymes. We then presented an updated overview of the consequences of the outcome of acute infectious event on pharmacokinetic exposure of drugs with a narrow therapeutic range and that are substrates of cytochrome P450, and the related clinical consequences. Finally, in the context of the COVID-19 pandemic, we reported examples of drug overexposures in COVID- 19 infected patients.

The Proposal of Molecular Mechanisms of Weak Organic Acids Intake-Induced Improvement of Insulin Resistance in Diabetes Mellitus via Elevation of Interstitial Fluid pH

Blood contains powerful pH-buffering molecules such as hemoglobin (Hb) and albumin, while interstitial fluids have little pH-buffering molecules. Thus, even under metabolic disorder conditions except severe cases, arterial blood pH is kept constant within the normal range (7.35~7.45), but the interstitial fluid pH under metabolic disorder conditions becomes lower than the normal level. Insulin resistance is one of the most important key factors in pathogenesis of diabetes mellitus, nevertheless the molecular mechanism of insulin resistance occurrence is still unclear. Our studies indicate that lowered interstitial fluid pH occurs in diabetes mellitus, causing insulin resistance via reduction of the binding affinity of insulin to its receptor. Therefore, the key point for improvement of insulin resistance occurring in diabetes mellitus is development of methods or techniques elevating the lowered interstitial fluid pH. Intake of weak organic acids is found to improve the insulin resistance by elevating the lowered interstitial fluid pH in diabetes mellitus. One of the molecular mechanisms of the pH elevation is that: (1) the carboxyl group (R-COO-) but not H⁺ composing weak organic acids in foods is absorbed into the body, and (2) the absorbed the carboxyl group (R-COO-) behaves as a pH buffer material, elevating the interstitial fluid pH. On the other hand, high salt intake has been suggested to cause diabetes mellitus; however, the molecular mechanism is unclear. A possible mechanism of high salt intake-caused diabetes mellitus is proposed from a viewpoint of regulation of the interstitial fluid pH: high salt intake lowers the interstitial fluid pH via high production of H⁺ associated with ATP synthesis required for the Na⁺,K⁺-ATPase to extrude the high leveled intracellular Na⁺ caused by high salt intake. This review article introduces the molecular mechanism causing the lowered interstitial fluid pH and insulin resistance in diabetes mellitus, the improvement of insulin resistance via intake of weak organic acid-containing foods, and a proposal mechanism of high salt intake-caused diabetes mellitus.

The critical role of His48 in mouse cytosolic sulfotransferase SULT2A8 for the 7α-hydroxyl sulfation of bile acids

Members of the cytosolic sulfotransferase (SULT) SULT2A subfamily are known to be critically involved in the homeostasis of steroids and bile acids. SULT2A8, a 7α-hydroxyl bile acid-preferring mouse SULT, has been identified as the major enzyme responsible for the mouse-specific 7-O-sulfation of bile acids. Interestingly, SULT2A8 lacks a conservative catalytic His residue at position 99th. The catalytic mechanism underlying the SULT2A8-mediated 7-O-sulfation of bile acids thus remained unclear. In this study, we performed a mutational analysis in order to gain insight into this yet-unresolved issue. Results obtained revealed two amino acid residues, His48 and Leu99, that are unique to the mouse SULT2A8, but not other SULTs, are essential for its 7-O-sulfating activity toward bile acids. These findings suggested that substitutions of two amino acids, which might have occurred during the evolution of the mouse SULT2A8 gene, endowed mouse SULT2A8 the capacity to catalyze the 7-O-sulfation of bile acids.

Role of c-Jun-N-Terminal Kinase in Pregnane X Receptor-Mediated Induction of Human Cytochrome P4503A4 In Vitro

Cytochrome P450 CYP3A4 is the most abundant drug-metabolizing enzyme and is responsible for the metabolism of ∼50% of clinically available drugs. Induction of CYP3A4 impacts the disposition of its substrates and leads to harmful clinical consequences, such as failure of therapy. To prevent such undesirable consequences, the molecular mechanisms of regulation of CYP3A4 need to be fully understood. CYP3A4 induction is regulated primarily by the xenobiotic nuclear receptor pregnane-X receptor (PXR). After ligand binding, PXR is translocated to the nucleus, where it binds to the CYP3A4 promoter and induces its gene expression. PXR function is modulated by phosphorylation(s) by multiple kinases. In this study, we determined the role of the c-Jun N-terminal kinase (JNK) in PXR-mediated induction of CYP3A4 enzyme in vitro. Human liver carcinoma cells (HepG2) were transfected with CYP3A4 luciferase and PXR plasmids, followed by treatment with JNK inhibitor (SP600125; SP) and PXR activators rifampicin (RIF) or hyperforin. Our results indicate that SP treatment significantly attenuated PXR-mediated induction of CYP3A4 reporter activity, as well as gene expression and enzyme activity. JNK knockdown by siRNA (targeting both JNK 1 and 2) also attenuated CYP3A4 induction by RIF. Interestingly, SP treatment attenuated JNK activation by RIF. Furthermore, treatment with RIF increased PXR nuclear levels and binding to the CYP3A4 promoter; SP attenuated these effects. This study shows that JNK is a novel mechanistic regulator of CYP3A4 induction by PXR.

High glucose upregulates BACE1-mediated Aβ production through ROS-dependent HIF-1α and LXRα/ABCA1-regulated lipid raft reorganization in SK-N-MC cells

There is an accumulation of evidence indicating that the risk of Alzheimer’s disease is associated with diabetes mellitus, an indicator of high glucose concentrations in blood plasma. This study investigated the effect of high glucose on BACE1 expression and amyloidogenesis in vivo, and we present details of the mechanism associated with those effects. Our results, using ZLC and ZDF rat models, showed that ZDF rats have high levels of amyloid-beta (Aβ), phosphorylated tau, BACE1, and APP-C99. In vitro result with mouse hippocampal neuron and SK-N-MC, high glucose stimulated Aβ secretion and apoptosis in a dose-dependent manner. In addition, high glucose increased BACE1 and APP-C99 expressions, which were reversed by a reactive oxygen species (ROS) scavenger. Indeed, high glucose increased intracellular ROS levels and HIF-1α expression, associated with regulation of BACE1 and Liver X Receptor α (LXRα). In addition, high glucose induced ATP-binding cassette transporter A1 (ABCA1) down-regulation, was associated with LXR-induced lipid raft reorganization and BACE1 localization on the lipid raft. Furthermore, silencing of BACE1 expression was shown to regulate Aβ secretion and apoptosis of SK-N-MC. In conclusion, high glucose upregulates BACE1 expression and activity through HIF-1α and LXRα/ABCA1-regulated lipid raft reorganization, leading to Aβ production and apoptosis of SK-N-MC.

A SUMO-acetyl switch in PXR biology

Post-translational modification (PTM) of nuclear receptor superfamily members regulates various aspects of their biology to include sub-cellular localization, the repertoire of protein-binding partners, as well as their stability and mode of degradation. The nuclear receptor pregnane X receptor (PXR, NR1I2) is a master-regulator of the drug-inducible gene expression in liver and intestine. The PXR-mediated gene activation program is primarily recognized to increase drug metabolism, drug transport, and drug efflux pathways in these tissues. The activation of PXR also has important implications in significant human diseases including inflammatory bowel disease and cancer. Our recent investigations reveal that PXR is modified by multiple PTMs to include phosphorylation, SUMOylation, and ubiquitination. Using both primary cultures of hepatocytes and cell-based assays, we show here that PXR is modified through acetylation on lysine residues. Further, we show that increased acetylation of PXR stimulates its increased SUMO-modification to support active transcriptional suppression. Pharmacologic inhibition of lysine de-acetylation using trichostatin A (TSA) alters the sub-cellular localization of PXR in cultured hepatocytes, and also has a profound impact upon PXR transactivation capacity. Both the acetylation and SUMOylation status of the PXR protein is affected by its ability to associate with the lysine de-acetylating enzyme histone de-acetylase (HDAC)3 in a complex with silencing mediator of retinoic acid and thyroid hormone receptor (SMRT). Taken together, our data support a model in which a SUMO-acetyl 'switch' occurs such that acetylation of PXR likely stimulates SUMO-modification of PXR to promote the active repression of PXR-target gene expression. This article is part of a Special Issue entitled: Xenobiotic nuclear receptors: New Tricks for An Old Dog, edited by Dr. Wen Xie.

Threonine-408 Regulates the Stability of Human Pregnane X Receptor through Its Phosphorylation and the CHIP/Chaperone-Autophagy Pathway

The human pregnane X receptor (hPXR) is a xenobiotic-sensing nuclear receptor that transcriptionally regulates drug metabolism-related genes. The aim of the present study was to elucidate the mechanism by which hPXR is regulated through threonine-408. A phosphomimetic mutation at threonine-408 (T408D) reduced the transcriptional activity of hPXR and its protein stability in HepG2 and SW480 cells in vitro and mouse livers in vivo. Proteasome inhibitors (calpain inhibitor I and MG132) and Hsp90 inhibitor geldanamycin, but not Hsp70 inhibitor pifithrin-μ, increased wild-type (WT) hPXR in the nucleus. The translocation of the T408D mutant to the nucleus was significantly reduced even in the presence of proteasome inhibitors, whereas the complex of yellow fluorescent protein (YFP)-hPXR T408D mutant with heat shock cognate protein 70/heat shock protein 70 and carboxy terminus Hsp70-interacting protein (CHIP; E3 ligase) was similar to that of the WT in the cytoplasm. Treatment with pifithrin-μ and transfection with anti-CHIP small-interfering RNA reduced the levels of CYP3A4 mRNA induced by rifampicin, suggesting the contribution of Hsp70 and CHIP to the transactivation of hPXR. Autophagy inhibitor 3-methyladenine accumulated YFP-hPXR T408D mutant more efficiently than the WT in the presence of proteasome inhibitor lactacystin, and the T408D mutant colocalized with the autophagy markers, microtubule-associated protein 1 light chain 3 and p62, which were contained in the autophagic cargo. Lysosomal inhibitor chloroquine caused the marked accumulation of the T408D mutant in the cytoplasm. Protein kinase C (PKC) directly phosphorylated the threonine-408 of hPXR. These results suggest that hPXR is regulated through its phosphorylation at threonine-408 by PKC, CHIP/chaperone-dependent stability check, and autophagic degradation pathway.

Serine 350 of human pregnane X receptor is crucial for its heterodimerization with retinoid X receptor alpha and transactivation of target genes in vitro and in vivo

The human pregnane X receptor (hPXR), a member of the nuclear receptor superfamily, senses xenobiotics and controls the transcription of genes encoding drug-metabolizing enzymes and transporters. The regulation of hPXR's transcriptional activation of its target genes is important for xenobiotic detoxification and endobiotic metabolism, and hPXR dysregulation can cause various adverse drug effects. Studies have implicated the putative phosphorylation site serine 350 (Ser(350)) in regulating hPXR transcriptional activity, but the mechanism of regulation remains elusive. Here we investigated the transactivation of hPXR target genes in vitro and in vivo by hPXR with a phosphomimetic mutation at Ser(350) (hPXR(S350D)). The S350D phosphomimetic mutation reduced the endogenous expression of cytochrome P450 3A4 (an hPXR target gene) in HepG2 and LS180 cells. Biochemical assays and structural modeling revealed that Ser(350) of hPXR is crucial for formation of the hPXR-retinoid X receptor alpha (RXRα) heterodimer. The S350D mutation abrogated heterodimerization in a ligand-independent manner, impairing hPXR-mediated transactivation. Further, in a novel humanized transgenic mouse model expressing the hPXR(S350D) transgene, we demonstrated that the S350D mutation alone is sufficient to impair hPXR transcriptional activity in mouse liver. This transgenic mouse model provides a unique tool to investigate the regulation and function of hPXR, including its non-genomic function, in vivo. Our finding that phosphorylation regulates hPXR activity has implications for development of novel hPXR antagonists and for safety evaluation during drug development.

Threonine-290 regulates nuclear translocation of the human pregnane X receptor through its phosphorylation/dephosphorylation by Ca2+/calmodulin-dependent protein kinase II and protein phosphatase 1

The human pregnane X receptor (hPXR) is recognized as a xenobiotic-sensing nuclear receptor that transcriptionally regulates the gene expression of drug-metabolizing enzymes and transporters. Our study elucidates the mechanism by which the localization of hPXR is regulated through threonine-290. A phosphomimetic mutation at threonine-290 (T290D) retained hPXR in the cytoplasm of HepG2, HuH6, and SW480 cells in vitro and the mouse liver in vivo even after treatment with rifampicin, and a phosphodeficient mutation (T290A) translocated from the cytoplasm to the nucleus as the wild-type hPXR. The amount of the unphosphorylated wild-type yellow fluorescent protein-hPXR fusion protein but not the T290A mutant increased on Phos-tag gels in response to stimulations with rifampicin and cyclin-dependent kinase 2 inhibitor roscovitine, and a marked increase was observed in the unphosphorylated levels of the T290A mutant in nontreated cells. The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN93 [2-[N-(2-hydroxyethyl)]-N-(4-methoxybenzenesulfonyl)]amino-N-(4-chlorocinnamyl)-N-methylbenzylamine)] and transfection with anti-CaMKII small-interfering RNA (siRNA) enhanced the unphosphorylated levels of the wild-type protein. CaMKII directly phosphorylated the threonine-290 of hPXR, and the T290A mutant conferred resistance to CaMKII. The protein phosphatase (PP) inhibitor okadaic acid (100 nM) and transfection with anti-PP1 siRNA but not anti-PP2A siRNA led to reduced expression of CYP3A4 mRNA. After the rifampicin and roscovitine stimulations, PP1 was recruited to the wild-type hPXR but not the T290A mutant. These results suggest that phosphorylation at threonine-290 by CaMKII may impair the function of hPXR by repressing its translocation to the nucleus, and dephosphorylation by PP1 is necessary for the xenobiotic-dependent nuclear translocation of hPXR.

Expression profiles uncover the relationship between erythropoietin and cell proliferation in rat hepatocytes after a partial hepatectomy

Erythropoietin (EPO) has a beneficial effect on hepatic cell proliferation during liver regeneration. However, the underlying mechanism has not yet been elucidated. To uncover the proliferation response of EPO in rat liver regeneration after partial hepatectomy (PH) at the cellular level, hepatocytes (HCs) were isolated using Percoll density gradient centrifugation. The genes of the EPO-mediated signaling pathway and the target genes of the transcription factor (TF) in the pathway were identified in a pathway and TF database search. Their expression profiles were then detected using Rat Genome 230 2.0 Microarray. The results indicated that the EPO-mediated signaling pathway is involved in 19 paths and that 124 genes participate, of which 32 showed significant changes and could be identified as liver regeneration-related genes. In addition, 443 targets regulated by the TFs of the pathway and 60 genes associated with cell proliferation were contained in the array. Subsequently, the synergetic effect of these genes in liver regeneration was analyzed using the E(t) mathematical model based on their expression profiles. The results demonstrated that the E(t) values of paths 3, 8, 12 and 14-17 were significantly strengthened in the progressing phase of liver regeneration through the RAS/MEK/ERK or PI3K/AκT pathways. The synergetic effect of the target genes, in parallel with target-related cell proliferation, was also enhanced 12-72 h after PH, suggesting a potential positive effect of EPO on HC proliferation during rat liver regeneration. These data imply that the EPO receptor may allow EPO to promote HC proliferation through paths 3, 8, 12 and 14-17, mediating the RAS/MEK/ERK and PI3K/AκT pathways in rat liver regeneration after PH.

Protection of primary neurons and mouse brain from Alzheimer's pathology by molecular tweezers

Alzheimer's disease is a devastating cureless neurodegenerative disorder affecting >35 million people worldwide. The disease is caused by toxic oligomers and aggregates of amyloid β protein and the microtubule-associated protein tau. Recently, the Lys-specific molecular tweezer CLR01 has been shown to inhibit aggregation and toxicity of multiple amyloidogenic proteins, including amyloid β protein and tau, by disrupting key interactions involved in the assembly process. Following up on these encouraging findings, here, we asked whether CLR01 could protect primary neurons from Alzheimer's disease-associated synaptotoxicity and reduce Alzheimer's disease-like pathology in vivo. Using cell culture and brain slices, we found that CLR01 effectively inhibited synaptotoxicity induced by the 42-residue isoform of amyloid β protein, including ∼80% inhibition of changes in dendritic spines density and long-term potentiation and complete inhibition of changes in basal synaptic activity. Using a radiolabelled version of the compound, we found that CLR01 crossed the mouse blood-brain barrier at ∼2% of blood levels. Treatment of 15-month-old triple-transgenic mice for 1 month with CLR01 resulted in a decrease in brain amyloid β protein aggregates, hyperphosphorylated tau and microglia load as observed by immunohistochemistry. Importantly, no signs of toxicity were observed in the treated mice, and CLR01 treatment did not affect the amyloidogenic processing of amyloid β protein precursor. Examining induction or inhibition of the cytochrome P450 metabolism system by CLR01 revealed minimal interaction. Together, these data suggest that CLR01 is safe for use at concentrations well above those showing efficacy in mice. The efficacy and toxicity results support a process-specific mechanism of action of molecular tweezers and suggest that these are promising compounds for developing disease-modifying therapy for Alzheimer's disease and related disorders.

Regulation of pregnane X receptor (PXR) function and UGT1A1 gene expression by posttranslational modification of PXR protein

Human UDP-glucuronosyltransferase (UGT) 1A1 is a critical enzyme responsible for detoxification and metabolism of endogenous and exogenous lipophilic compounds such as bilirubin. The present study shows how cyclin-dependent kinase (CDK) inhibitor roscovitine stimulated the expression of UGT1A1 in HepG2 cells. Pregnane X receptor (PXR)-mediated transactivation of UGT1A1 reporter gene was more prominently enhanced by roscovitine, compared with the basal-, constitutive androstane receptor (CAR)-, and aryl hydrocarbon receptor-mediated activities. We determined the regulatory mechanism of UGT1A1 expression through PXR's stimulation by roscovitine. Although phosphomimetic mutations at Thr290 and Thr408 retained the PXR protein in cytoplasm and attenuated the induction of UGT1A1 expression by both roscovitine and rifampicin, a mutation at Ser350 specifically reduced the activity of PXR induced by roscovitine. Immunoprecipitation analysis revealed that the T290D but not T408D mutant protein remained in cytoplasm by forming a complex with heat shock protein 90 and cytoplasmic CAR retention protein, whereas treatment with proteasome inhibitor MG-132 accumulated the T408D mutant protein in cytoplasm. Transfection with anti-CDK2 small interfering RNA (siRNA) but not anti-CDK1 or CDK5 siRNA led to enhanced expression of UGT1A1. S350D yellow fluorescent protein-PXR fusion protein could translocate from cytoplasm to nucleus similar to the wild-type protein but was detected as an acetylated protein, whose binding with retinoid X receptor (RXR) and histone deacetylase was impaired. Cotransfection with coactivator steroid receptor coactivator (SRC) 2 but not SRC-1 partly recovered its PXR activity. These results indicate that roscovitine stimulated the expression of UGT1A1 by inhibiting CDK2, which phosphorylated PXR at Ser350 to suppress binding with RXR and coactivator and maintain the acetylation of PXR protein.

Gold nanoparticles functionalized with peptides for specific affinity aggregation assays of estrogen receptors and their agonists

Nuclear receptors regulate the transcription of genes and various functions such as development, differentiation, homeostasis, and behavior by formation of complexes with ligand and co-activator. Recent findings have shown that agonists of a ligand may have a toxic effect on cellular/tissular function through improper activation of nuclear receptors. In this study, a simple assay system of hetero-complexes of three different molecules (estrogen receptor, ligand, and co-activator peptide) has been developed. This assay system employs functionalized gold nanoparticles (GNPs: 15 nm in diameter). The surfaces of the GNPs were modified by a 12- or 20-amino-acid peptide that contains the sequence of co-activator for activating nuclear receptor by an agonist ligand. Owing to the affinity of the peptide, the functionalized GNPs aggregate faster when the nuclear receptor and the agonist ligand are also present. The aggregation of GNPs can be identified by shifts in adsorption spectrum, which give information about the specificity of agonist ligands. Similarly, this spectrum shift can measure concentration of known agonist ligand. This simple agonist screening will be employed as high through-put analysis (HTA) in the discovery of drugs that act through nuclear receptors.

Fatty liver is associated with impaired activity of PPARγ-coactivator 1α (PGC1α) and mitochondrial biogenesis in mice

Accumulating evidence indicates that mitochondria have a key role in non-alcoholic fatty liver disease (NAFLD). C57BL/6J mice were fed a choline-deficient, ethionine-supplemented (CDE) diet. Histological studies demonstrated accumulation of fat vacuoles in up to 90% of hepatocytes in mice fed the CDE diet for 14 days. In addition, a decrease in mitochondrial levels, together with an increase in superoxide radicals' levels were observed, indicating elevation of oxidative stress in hepatocytes. ATP levels were decreased in livers from CDE-fed mice after overnight fasting. This was accompanied by a compensative and significant increase in peroxisome-proliferator-activated receptor-γ coactivator 1α (PGC1α) mRNA levels in comparison to control livers. However, there was a reduction in PGC1α protein levels in CDE-treated mice. Moreover, the expression of mitochondrial biogenesis genes nuclear respiratory factor 1 (NRF-1), mitochondrial transcription factor A (TFAM), mitochondrial transcription factor B1 (TFB1M) and mitochondrial transcription factor B2 (TFB2M), which are all regulated by PGC1α activity, remained unchanged in fasted CDE-treated mice. These results indicate impaired activity of PGC1α. The impaired activity was further confirmed by chromatin immunoprecipitation analysis, which demonstrated decreased interaction of PGC1α with promoters containing NRF-1 and NRF-2 response elements in mice fed the CDE diet. A decrease in PGC1α ability to activate the expression of the gluconeogenic gene phosphoenol-pyruvate carboxykinase was also observed. This study demonstrates, for the first time, that attenuated mitochondrial biogenesis in steatotic livers is associated with impaired biological activity of PGC1α.

Role of high-fat diet in regulation of gene expression of drug metabolizing enzymes and transporters

AIM

Our aim is to investigate the molecular mechanism of regulation of gene expression of drug metabolizing enzymes (DMEs) and transporters in diet-induced obesity.

MAIN METHODS

Adult male CD1 mice were fed diets containing 60% kcal fat (HFD) or 10% kcal fat (LFD) for 14 weeks. RNA levels of hepatic DMEs, transporters and their regulatory nuclear receptors (NRs) were analyzed by real-time PCR. Activation of cell-signaling components (JNK and NF-κΒ) and pro-inflammatory cytokines (IL-1β, IL-6 and TNFα) were measured in the liver. Finally, the pharmacodynamics of drugs metabolized by DMEs was measured to determine the clinical relevance of our findings.

KEY FINDINGS

RNA levels of the hepatic phase I (Cyp3a11, Cyp2b10, Cyp2a4) and phase II (Ugt1a1, Sult1a1, Sultn) enzymes were reduced ~30-60% in HFD compared to LFD mice. RNA levels of Cyp2e1, Cyp1a2 and the drug transporters, multidrug resistance proteins, (Mrp)2, Mrp3 and multidrug resistant gene (Mdr)1b were unaltered in HFD mice. Gene expression of the NRs, PXR and CAR and nuclear protein levels of RXRα was reduced in HFD mice. Cytokines, JNK and NF-κΒ were induced in HFD mice. Thus reduction in hepatic gene expression in obesity may be modulated by cross-talk between NRs and inflammation-induced cell-signaling. Sleep time of Midazolam (Cyp3a substrate) was prolonged in HFD mice, while Zoxazolamine (Cyp1a2 and Cyp2e1 substrate)-induced sleep time was unaltered.

SIGNIFICANCE

This study demonstrates that gene-specific reductions in DMEs can affect specific drugs metabolized by these enzymes, thus providing a rationale to monitor the effectiveness of drug therapy in obese individuals.

The cross-sectional association between insulin resistance and circulating complement C3 is partly explained by plasma alanine aminotransferase, independent of central obesity and general inflammation (the CODAM study)

BACKGROUND

Complement C3, a central component of the innate immune system is increased in subjects with obesity and type 2 diabetes and is a novel risk factor for cardiovascular disease. We hypothesized that the strong association between insulin resistance and circulating amounts of C3 may be related to hepatic fat accumulation -independent of central obesity itself and of a general low-grade inflammatory response.

RESEARCH QUESTION

To what extent is the association between insulin resistance and C3 explained by plasma levels of alanine aminotransferase (ALT) as a surrogate of hepatic fat accumulation.

METHODS

Cross-sectional analyses conducted in the Cohort on Diabetes and Atherosclerosis Maastricht (CODAM) study. Analyses were restricted to subjects with none-to-moderate alcohol consumption (n = 453, 61·4% men). Multiple linear regression analyses were used to investigate the association between HOMA2IR (main determinant) and circulating complement C3 (main outcome), and the mediating role of ALT herein. All analyses were adjusted for age, sex, presence of type 2 diabetes mellitus or heart disease, use of medication, smoking, alcohol consumption, waist circumference and inflammation.

RESULTS

Insulin resistance (estimated as HOMA2IR) was strongly associated with circulating C3 (standardized regression coefficient β 0·40 [95% CI: 0·30; 0·49]) and also with ALT (β 0·44 [0·34; 0·54]), both adjusted for the above-mentioned covariates. The association between HOMA2IR and C3 was attenuated after further adjustment for ALT (β decreased to 0·34 [0·24; 0·44]).

CONCLUDING REMARKS

Plasma ALT can explain 14·2% of the strong association between insulin resistance and circulating concentrations of complement C3, independent of central obesity and general inflammation.

Zinc and alcoholic liver disease

Zinc deficiency is one of the most consistent nutritional/biochemical observations in alcoholic liver disease (ALD). The objectives of our research are to determine how alcohol interferes with cellular zinc homeostasis and if zinc deficiency is a causal factor in the development of ALD. Metallothionein (MT) is a major protein responsible for cellular zinc homeostasis. MT-transgenic (MT-TG) mice with hepatic overexpression of MT and elevation of zinc level were resistant to ethanol-induced liver injury. MT-knockout (MT-KO) mice with a reduction of hepatic zinc were more susceptible to alcohol toxicity. However, zinc treatment also provided beneficial effects on alcohol hepatoxicity in MT-KO mice, suggesting a MT-independent action. Dietary zinc supplementation normalized hepatic zinc level and attenuated the pathological changes in the liver of mice chronically fed alcohol. Several mechanisms were involved in zinc action against alcoholic cytotoxicity. Zinc enhanced cellular antioxidant capacity and corrected alcohol metabolic switch from alcohol dehydrogenase to cytochrome P4502E1. Zinc attenuated cytokine production and TNF-α receptor- and Fas-mediated cell death pathways. Zinc restored activities of hepatocyte nuclear factor-4α (HNF-4α) and peroxisome proliferation activator-α (PPAR-α), and enhanced hepatic fatty acid β-oxidation and lipid secretion. Hepatoma cell cultures showed that zinc deprivation induces lipid accumulation via inactivating HNF-4α and PPAR-α. These results suggest that alcohol exposure interferes with hepatic zinc homeostasis, leading to cellular zinc deprivation. Inactivation of zinc proteins due to zinc release is likely an important molecular mechanism in the pathogenesis of ALD.

Post-translational modification of pregnane x receptor

Pregnane x receptor (PXR, NR1I2) was originally characterized as a broad spectrum entero-hepatic xenobiotic 'sensor' and master-regulator of drug inducible gene expression. A compelling description of ligand-mediated gene activation has been unveiled in the last decade that firmly establishes this receptor's central role in the metabolism and transport of xenobiotics in mammals. Interestingly, pharmacotherapy with potent PXR ligands produces several profound side effects including decreased capacities for gluconeogenesis, lipid metabolism, and inflammation; likely due to PXR-mediated repression of gene expression programs underlying these pivotal physiological functions. An integrated model is emerging that reveals a sophisticated interplay between ligand binding and the ubiquitylation, phosphorylation, SUMOylation, and acetylation status of this important nuclear receptor protein. These discoveries point to a key role for the post-translational modification of PXR in the selective suppression of gene expression, and open the door to the study of completely new modes of regulation of the biological activity of PXR.

PGC-1alpha negatively regulates hepatic FGF21 expression by modulating the heme/Rev-Erb(alpha) axis

FGF21 is a hormone produced in liver and fat that dramatically improves peripheral insulin sensitivity and lipid metabolism. We show here that obese mice with genetically reduced levels of a key hepatic transcriptional coactivator, PGC-1alpha, have improved whole-body insulin sensitivity with increased levels of hepatic and circulating FGF21. Gain- and loss-of-function studies in primary mouse hepatocytes show that hepatic FGF21 levels are regulated by the expression of PGC-1alpha. Importantly, PGC-1alpha-mediated reduction of FGF21 expression is dependent on Rev-Erbalpha and the expression of ALAS-1. ALAS-1 is a PGC-1alpha target gene and the rate-limiting enzyme in the synthesis of heme, a ligand for Rev-Erbalpha. Modulation of intracellular heme levels mimics the effect of PGC-1alpha on FGF21 expression, and inhibition of heme biosynthesis completely abrogates the down-regulation of FGF21 in response to PGC-1alpha. Thus, PGC-1alpha can impact hepatic and systemic metabolism by regulating the levels of a nuclear receptor ligand.

Long-term virus-induced alterations of CYP3A-mediated drug metabolism: a look at the virology, immunology and molecular biology of a multi-faceted problem

Virus infections are on the rise. Although the first description of CYP expression during virus infection was recorded 50 years ago, mechanistic studies of this phenomenon only began to appear in the last decade due to breakthroughs in molecular biology, genomic and transgenic technology. This review describes the relationship(s) among CYP-mediated drug metabolism, virus infection and the immune response and evaluates in vitro and in vivo models for mechanistic studies. The first studies that assessed CYP expression during infection focused on inflammatory mediators and the innate immune response at early time points. Recent studies assessing virus infection and its effect on hepatic CYP expression noted more long-term effects. An obvious approach toward understanding how viruses affect hepatic CYP3A expression and function would be to assess key regulators of CYP during infection. Improvements in techniques to identify post-translational modifications of CYP and systems that focus on virus-receptor interactions which allow subtraction and addition of immunological and regulatory elements that drive CYP will demonstrate that long-term changes in drug metabolism start from the time the virus enters the circulation, are reinforced by virus binding to cellular targets and further solidified by changes in cellular processes long after the virus is cleared.

A systematic analysis of predicted phosphorylation sites within the human pregnane X receptor protein

The pregnane X receptor (PXR, NR1I2) regulates the expression of genes that encode drug-metabolizing enzymes and drug transporter proteins in liver and intestine. Understanding the molecular mechanisms that modulate PXR activity is therefore critical for the development of effective therapeutic strategies. Several recent studies have implicated the activation of kinase signaling pathways in the regulation of PXR biological activity, although direct evidence and molecular mechanisms are currently lacking. We therefore sought to characterize potential phosphorylation sites within the PXR protein by use of a rational, comprehensive, and systematic site-directed mutagenesis approach to generate phosphomimetic mutations (Ser/Thr --> Asp) and phospho-deficient mutations (Ser/Thr --> Ala) at 18 predicted consensus kinase recognition sequences in the human PXR protein. Here, we identify amino acid residues Ser8, Thr57, Ser208, Ser305, Ser350, and Thr408 as being critical for biological activity of the PXR protein. Mutations at positions 57 and 408 abolish ligand-inducible PXR activity. Mutations in the extreme N terminus and in the PXR ligand-binding domain at positions Ser8, Ser305, Ser350, and Thr408 decrease the ability of PXR to form heterodimers with retinoid X receptor alpha. Mutations at positions Ser208, Ser305, Ser350, and Thr408 alter PXR-protein cofactor interactions. Finally, the subcellular localization of the PXR protein is profoundly affected by mutations at position Thr408. These data suggest that PXR activity can potentially be regulated by phosphorylation at specific amino acid residues within several predicted consensus kinase recognition sequences to differentially affect PXR biological activity.

Bile acids as regulatory molecules

In the past, bile acids were considered to be just detergent molecules derived from cholesterol in the liver. They were known to be important for the solubilization of cholesterol in the gallbladder and for stimulating the absorption of cholesterol, fat-soluble vitamins, and lipids from the intestines. However, during the last two decades, it has been discovered that bile acids are regulatory molecules. Bile acids have been discovered to activate specific nuclear receptors (farnesoid X receptor, preganane X receptor, and vitamin D receptor), G protein coupled receptor TGR5 (TGR5), and cell signaling pathways (c-jun N-terminal kinase 1/2, AKT, and ERK 1/2) in cells in the liver and gastrointestinal tract. Activation of nuclear receptors and cell signaling pathways alter the expression of numerous genes encoding enzyme/proteins involved in the regulation of bile acid, glucose, fatty acid, lipoprotein synthesis, metabolism, transport, and energy metabolism. They also play a role in the regulation of serum triglyceride levels in humans and rodents. Bile acids appear to function as nutrient signaling molecules primarily during the feed/fast cycle as there is a flux of these molecules returning from the intestines to the liver following a meal. In this review, we will summarize the current knowledge of how bile acids regulate hepatic lipid and glucose metabolism through the activation of specific nuclear receptors and cell signaling pathways.

Sensitivity of lipid metabolism and insulin signaling to genetic alterations in hepatic peroxisome proliferator-activated receptor-gamma coactivator-1alpha expression

OBJECTIVE

The peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1 family of transcriptional coactivators controls hepatic function by modulating the expression of key metabolic enzymes. Hepatic gain of function and complete genetic ablation of PGC-1alpha show that this coactivator is important for activating the programs of gluconeogenesis, fatty acid oxidation, oxidative phosphorylation, and lipid secretion during times of nutrient deprivation. However, how moderate changes in PGC-1alpha activity affect metabolism and energy homeostasis has yet to be determined.

RESEARCH DESIGN AND METHODS

To identify key metabolic pathways that may be physiologically relevant in the context of reduced hepatic PGC-1alpha levels, we used the Cre/Lox system to create mice heterozygous for PGC-1alpha specifically within the liver (LH mice).

RESULTS

These mice showed fasting hepatic steatosis and diminished ketogenesis associated with decreased expression of genes involved in mitochondrial beta-oxidation. LH mice also exhibited high circulating levels of triglyceride that correlated with increased expression of genes involved in triglyceride-rich lipoprotein assembly. Concomitant with defects in lipid metabolism, hepatic insulin resistance was observed both in LH mice fed a high-fat diet as well as in primary hepatocytes.

CONCLUSIONS

These data highlight both the dose-dependent and long-term effects of reducing hepatic PGC-1alpha levels, underlining the importance of tightly regulated PGC-1alpha expression in the maintenance of lipid homeostasis and glucose metabolism.

Possible regulation of genes associated with intracellular signaling cascade in rat liver regeneration

OBJECTIVE

The importance of signal transduction in cell activities has been generally accepted. The purpose of this study was to analyze the regulatory effect of intracellular signaling cascade-associated genes on rat liver regeneration (LR) at transcriptional level.

MATERIAL AND METHODS

The associated genes were originally obtained through a search of the databases and related scientific publications; their expression profiles were then checked in rat LR using the Rat Genome 230 2.0 array. The LR-associated genes were identified by comparing the discrepancy in gene expression changes between the partial hepatectomy (PH) group and the sham operation (SO) group.

RESULTS

A total of 566 genes associated with the intracellular signaling cascade were LR related. The genes involved in nine signaling pathways including intracellular receptor-, second messenger-, nitric oxide-, hormone-, carbohydrate-mediated, protein kinase, small GTPase, ER-nuclear and target of rapamycin (TOR) signaling pathways were detected to be enriched in a cluster characterized by up-regulated expression in LR. According to their expression similarity and time relevance, they were separately classified into 5 and 5 groups.

CONCLUSIONS

It is presumed that following PH, the second messenger-mediated signaling pathway inhibits the inflammatory response, while the protein kinase cascade and small GTPase-mediated signal transduction stimulate the immune response; the intracellular receptor-, second messenger-, small GTPase-mediated signal transduction and protein kinase cascade coordinately control cell replication; the intracellular receptor-, second messenger-mediated and ER-nuclear signaling pathways facilitate cell differentiation; the MAPK cascade and small GTPase-mediated signal transduction play a role in cytoskeletal reconstruction and cell migration; the second messenger-, small GTPase-mediated and IkappaB kinase/NFkappaB cascades take care of protein transport, etc., in LR.

Cyclic AMP-dependent protein kinase signaling modulates pregnane x receptor activity in a species-specific manner

Pregnane x receptor is a ligand-activated transcription factor that regulates drug-inducible expression of several key cytochrome P450 enzymes and drug transporter proteins in liver and intestine in a species-specific manner. Activation of this receptor modulates several key biochemical pathways, including gluconeogenesis, beta-oxidation of fatty acids, fatty acid uptake, cholesterol homeostasis, and lipogenesis. It is of current interest to determine whether the interaction between pregnane x receptor and these key biochemical pathways is evolutionarily conserved. We show here that activation of the cyclic AMP-dependent protein kinase signaling pathway synergizes with pregnane x receptor-mediated gene activation in mouse hepatocytes. Conversely, cyclic AMP-dependent protein kinase signaling has a repressive effect upon pregnane x receptor-mediated gene activation in rat and human hepatocytes. We show that the human pregnane x receptor protein can serve as an effective substrate for catalytically active cyclic AMP-dependent protein kinase in vitro. Metabolic labeling of the protein in vivo indicates that human pregnane x receptor exists as a phosphoprotein and that activation of cyclic AMP-dependent protein kinase signaling modulates the phosphorylation status of pregnane x receptor. Activation of cyclic AMP-dependent protein kinase signaling also modulates the interactions of pregnane x receptor with protein cofactors. Our results define the species-specific impact of cyclic AMP-dependent protein kinase signaling on pregnane x receptor and provide a molecular explanation of cyclic AMP-dependent protein kinase-mediated repression of human pregnane x receptor activity. Taken together, our results identify a novel mode of regulation of pregnane x receptor activity and highlight prominent functional differences in the process across species.

Bile Acid sulfation: a pathway of bile acid elimination and detoxification

Sulfotransferase-2A1 catalyzes the formation of bile acid-sulfates (BA-sulfates). Sulfation of BAs increases their solubility, decreases their intestinal absorption, and enhances their fecal and urinary excretion. BA-sulfates are also less toxic than their unsulfated counterparts. Therefore, sulfation is an important detoxification pathway of BAs. Major species differences in BA sulfation exist. In humans, only a small proportion of BAs in bile and serum are sulfated, whereas more than 70% of BAs in urine are sulfated, indicating their efficient elimination in urine. The formation of BA-sulfates increases during cholestatic diseases. Therefore, sulfation may play an important role in maintaining BA homeostasis under pathologic conditions. Farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and vitamin D receptor are potential nuclear receptors that may be involved in the regulation of BA sulfation. This review highlights current knowledge about the enzymes and transporters involved in the formation and elimination of BA-sulfates, the effect of sulfation on the pharmacologic and toxicologic properties of BAs, the role of BA sulfation in cholestatic diseases, and the regulation of BA sulfation.