A model for farnesoid feedback control in the mevalonate pathway

Predictive Modeling of MAFLD Based on Hsp90α and the Therapeutic Application of Teprenone in a Diet-Induced Mouse Model

BACKGROUND AND AIMS

The heat shock protein (Hsp) 90α is induced by stress and regulates inflammation through multiple pathways. Elevated serum Hsp90α had been found in nonalcoholic steatohepatitis (NASH). Geranylgeranylacetone (GGA, also called teprenone) is a terpenoid derivative. It was reported to induce Hsp and alleviate insulin resistance. We aimed to evaluate the Hsp90α as a biomarker in predicting metabolic-associated fatty liver disease (MAFLD) and define the therapeutic effects of geranylgeranylacetone for the disease.

METHODS

A clinical study was conducted to analyze the elements associated with Hsp90α, and a predictive model of MAFLD was developed based on Hsp90α. The histopathological correlation between Hsp90α and MAFLD was investigated through a diet-induced mouse model. Furthermore, GGA was applied to the mouse model.

RESULTS

Serum Hsp90α was increased in patients with MAFLD. A positive linear relationship was found between age, glycosylated hemoglobin (HbA1c), MAFLD, and serum Hsp90α. Meanwhile, a negative linear relationship with body mass index (BMI) was found. A model using Hsp90α, BMI, HbA1c, and ALT was established for predicting MAFLD. The area under the receiver operating characteristic (ROC) curves was 0.94 (95% CI 0.909-0.971, p = 0.000). The sensitivity was 84.1%, and the specificity was 93.1%. In vitro experiments, GGA induced Hsp90α in steatosis cells. In the mice model, Hsp90α decreased in the GGA treatment group. Hepatic steatosis, inflammation, insulin resistance, and glucose intolerance were improved in the GGA-treated group. Serum Hsp90α was positively correlated with steatohepatitis activity according to hepatic histopathology.

CONCLUSIONS

Serum Hsp90α was elevated in MAFLD, and a positive correlation between serum Hsp90α and the grade of activity of steatohepatitis was observed. The model using BMI, HbA1c, and alanine aminotransferase (ALT) had a good value to predict MAFLD. The findings also revealed the effectiveness of GGA in the treatment of MAFLD.

The balance of protein farnesylation and geranylgeranylation during the progression of nonalcoholic fatty liver disease

Protein prenylation is an essential posttranslational modification and includes protein farnesylation and geranylgeranylation using farnesyl diphosphate or geranylgeranyl diphosphate as substrates, respectively. Geranylgeranyl diphosphate synthase is a branch point enzyme in the mevalonate pathway that affects the ratio of farnesyl diphosphate to geranylgeranyl diphosphate. Abnormal geranylgeranyl diphosphate synthase expression and activity can therefore disrupt the balance of farnesylation and geranylgeranylation and alter the ratio between farnesylated and geranylgeranylated proteins. This change is associated with the progression of nonalcoholic fatty liver disease (NAFLD), a condition characterized by hepatic fat overload. Of note, differential accumulation of farnesylated and geranylgeranylated proteins has been associated with differential stages of NAFLD and NAFLD-associated liver fibrosis. In this review, we summarize key aspects of protein prenylation as well as advances that have uncovered the regulation of associated metabolic patterns and signaling pathways, such as Ras GTPase signaling, involved in NAFLD progression. Additionally, we discuss unique opportunities for targeting prenylation in NAFLD/hepatocellular carcinoma with agents such as statins and bisphosphonates to improve clinical outcomes.

Conditional loss of geranylgeranyl diphosphate synthase alleviates acute obstructive cholestatic liver injury by regulating hepatic bile acid metabolism

Previous studies have suggested that metabolites in the mevalonate pathway are involved in hepatic bile acid metabolism, yet the details of this relationship remain unknown. In this study, we found that the hepatic farnesyl pyrophosphate (FPP) level and the ratio of FPP to geranylgeranyl pyrophosphate (GGPP) were increased in mice with acute obstructive cholestasis compared with mice that underwent a sham operation. In addition, the livers of the mice with acute obstructive cholestasis showed lower expression of geranylgeranyl diphosphate synthase (GGPPS), which synthesizes GGPP from FPP. When Ggps1 was conditionally deleted in the liver, amelioration of liver injury, as shown by downregulation of the hepatic inflammatory response and decreased hepatocellular apoptosis, was found after ligation of the common bile duct and cholecystectomy (BDLC). Subsequently, liquid chromatography/mass spectrometry analysis showed that knocking out Ggps1 decreased the levels of hepatic bile acids, including hydrophobic bile acids. Mechanistically, the disruption of Ggps1 increased the levels of hepatic FPP and its metabolite farnesol, thereby resulting in farnesoid X receptor (FXR) activation, which modulated hepatic bile acid metabolism and reduced hepatic bile acids. It was consistently indicated that digeranyl bisphosphonate, a specific inhibitor of GGPPS, and GW4064, an agonist of FXR, could also alleviate acute obstructive cholestatic liver injury in vivo. In general, GGPPS is critical for modulating acute obstructive cholestatic liver injury, and the inhibition of GGPPS ameliorates acute obstructive cholestatic liver injury by decreasing hepatic bile acids, which is possibly achieved through the activation of FXR-induced bile acid metabolism.

SREBP2 is upregulated in esophageal squamous cell carcinoma and co‑operates with c‑Myc to regulate HMGCR expression

Dysregulations of the mevalonate pathway (MVA) have been previously identified. Our previous study demonstrated that 3‑hydroxy‑3‑methylglutaryl‑coenzyme A reductase (HMGCR), the rate‑limiting enzyme of the MVA pathway, was upregulated in esophageal squamous cell carcinoma (ESCC) and statin‑inhibited ESCC tumorigenesis. However, the underlying mechanism of HMGCR regulation in ESCC remains unknown. In the present study, western blotting and immunohistochemistry analysis demonstrated that sterol regulatory element‑binding protein 2 (SREBP2), the master regulator for HMGCR, was upregulated in ESCC clinical samples. Overexpression of SREBP2 expression in ESCC cell lines promoted the growth, migration and colony formation of cancer cells in the MTT, Boyden chamber and soft agar assays, respectively, which was inhibited by lovastatin. Downregulation of SREBP2 expression in ESCC cell lines inhibited the viability, and migration and colony formation abilities of cancer cells. Assessment of the molecular mechanism demonstrated that SREBP2 interacted with c‑Myc and cooperated with c‑Myc to activate HMGCR expression. Collectively, the present study identified SREBP2 as an oncogene associated with the tumorigenesis of ESCC and further demonstrated the therapeutic effects of statins in ESCC.

Farnesol, a sesquiterpene alcohol in essential oils, ameliorates serum allergic antibody titres and lipid profiles in ovalbumin-challenged mice

BACKGROUND

Farnesol, a natural sesquiterpene alcohol in essential oils, was found to have potential for alleviating massive inflammation, oxidative stress and lung injury. However, effects of farnesol supplementation on allergic asthma remain unclear.

OBJECTIVES

To clarify the puzzle, this work investigates the effects of farnesol on allergic asthma using an ovalbumin (OVA)-sensitised and challenged mouse model.

METHODS

Farnesol was administered to OVA-sensitised and challenged mice for 5 weeks. Three farnesol doses, namely 5, 25 and 100mg farnesol/kg BW/day, non-sensitised control, dietary control, and positive control (dexamethasone 3mg/kg BW by gavage) were included. Sera and bronchoalveolar lavage fluids from the experimental mice were collected to measure farnesol concentrations, serum lipid profiles, antibody titres, differential cell counts or Th1/Th2 cytokines levels.

RESULTS

The results showed that farnesol supplementation increased serum farnesol concentration dose-dependently, significantly increased (P<0.05) OVA-specific IgG2a/IgE antibody titre ratios, but decreased total IgE levels. Farnesol supplementation markedly reversed the aberrated LDL-c/HDL-c and HDL-c/TC ratios in the sera of asthmatic mice, suggesting that farnesol supplementation might ameliorate serum lipid profiles in the OVA-sensitised and challenged mice.

CONCLUSION

Our results evidenced that farnesol supplementation might improve serum allergic antibody titres and lipid profiles in asthmatic mice.

Immunohistochemical detection of the retinoid acid receptors (RXR-alpha, -beta, -gamma) and Farnesoid X-activated receptor (FXR) in the marbled newt along the annual cycle

Retinoid acid receptors (RXR-alpha, -beta, -gamma) and Farnesoid X-activated receptor (FXR) expression in the testis of the marbled newt were investigated with special attention to the changes during the annual testicular cycle, using light microscopy immunohistochemistry and Western blot analysis. The annual testicular cycle of the marbled newt (Triturus marmoratus marmoratus) comprises three periods: (a) proliferative period (germ cell proliferation from primordial germ cells to round spermatids, April-June); (b) spermiogenesis period (July-September); and (c) quiescence period (interstitial and follicular cells form the glandular tissue, October-April). In the proliferative period, primordial germ cells and primary spermatogonia immunostained intensely to the three types of RXRs and also to FXR. In the other periods, immunostaining to these antibodies was weak or absent. Secondary spermatogonia stained weakly to the four antibodies in the proliferative period, and only to FXR, also weakly, in the spermiogenesis period. Immunoreactive primary spermatocytes were weakly labeled with the RXR antibodies in the proliferative period. Spermatids and spermatozoa did not stain to any antibody in any period. Follicular cells only immunostained to RXR-gamma and only in the quiescence period when they are forming the glandular tissue, together with the interstitial cells. As follicular cells, interstitial cells only immunostained in the quiescence period; however, they immunoreacted to the three types of RXRs. These findings suggest that in the newt, RXRs and FXR are involved in spermatogenesis control by regulating the proliferation of primordial germ cells and spermatogonia. In addition, RXR-gamma seems to be also involved in the development of the glandular (steroidogenic) tissue.

Cell biology of peroxisomes and their characteristics in aquatic organisms

The general characteristics of peroxisomes in different organisms, including aquatic organisms such as fish, crustaceans, and mollusks, are reviewed, with special emphasis on different aspects of the organelle biogenesis and mechanistic aspects of peroxisome proliferation. Peroxisome proliferation and peroxisomal enzyme inductions elicited by xenobiotics or physiological conditions have become useful tools to study the mechanisms of peroxisome biogenesis. During peroxisome proliferation, the induction of peroxisomal proteins is heterogeneous, enzymes that show increased activity being involved in different aspects of lipid homeostasis. The process of peroxisome biogenesis is coordinately triggered by a whole array of structurally dissimilar compounds known as peroxisome proliferators, and investigating the effect of some of these compounds that commonly appear as pollutants in the environment on the peroxisomes of aquatic animals inhabiting marine and estuarine habitats seems interesting. It is also important to determine whether peroxisome proliferation in these animals is a phenomenon that might occur under normal physiological or season-related conditions and plays a metabolic or functional role. This would help set the basis for understanding the process of peroxisome biogenesis in aquatic animals.

Farnesoid X receptor responds to bile acids and represses cholesterol 7alpha-hydroxylase gene (CYP7A1) transcription

Cholesterol 7alpha-hydroxylase gene (CYP7A1) transcription is repressed by bile acids. The goal of this study is to elucidate the mechanism of CYP7A1 transcription by bile acid-activated farnesoid X receptor (FXR) in its native promoter and cellular context and to identify FXR response elements in the gene. In Chinese hamster ovary cells transfected with retinoid X receptor alpha (RXRalpha)/FXR, only chenodeoxycholic acid (CDCA) and deoxycholic acid (DCA) were able to stimulate a heterologous promoter/reporter containing an ecdysone response element. In HepG2 cells, all bile acids (25 microM) were able to repress CYP7A1/luciferase reporter activity, and only CDCA and DCA further repressed reporter activity when cotransfected with RXRalpha/FXR. The concentration of CDCA required to inhibit 50% of reporter activity (IC(50)) was determined to be approximately 25 microM without FXR and 10 microM with FXR. Deletion analysis revealed that the bile acid response element located between nucleotides -148 and -128 was the FXR response element, but RXRalpha/FXR did not bind to this sequence. These results suggest that bile acid-activated FXR exerts its inhibitory effect on CYP7A1 transcription by an indirect mechanism, in contrast to the stimulation and binding of FXR to intestinal bile acid-binding protein gene promoter. Results also reveal that bile acid receptors other than FXR are present in HepG2 cells.

Induction of endogenous coenzyme Q biosynthesis by administration of peroxisomal inducers

A large number of chemical compounds have been identified which cause peroxisomal proliferation and induce a number of enzymes, mainly those participating in lipid metabolism. Administration of these drugs/chemicals to rats increased coenzyme Q levels in the blood and most of the organs. Levels were raised in all cellular membranes of such organs. The extent of induction of this lipid was 8-fold in young animals but decreased during aging and was absent at 1.5 year of age. One of the regulating factors of the mevalonate pathway is farnesol, which is produced by dephosphorylation of farnesyl-PP and eliminated by phosphorylation including two kinases. Future research will involve a search for modified intermediary metabolites, which increase coenzyme Q synthesis and thereby efficiently elevate the level of this lipid in conditions of deficiency.

Identification of an active site alanine in mevalonate kinase through characterization of a novel mutation in mevalonate kinase deficiency

Sequencing of polymerase chain reaction-amplified cDNAs from cultured cells of three patients with mevalonate kinase deficiency revealed a G --> A transversion at nucleotide 1000 of the coding region, converting alanine to threonine at position 334 (A334T). To characterize this defect, we expressed wild-type and mutant cDNAs in Escherichia coli as the glutathione S-transferase fusion proteins, with purification by affinity chromatography. SDS-polyacrylamide gel electrophoresis analysis for wild-type and mutant fusion proteins indicated an expected molecular mass of 42-43 kDa. Kinetic characterization of the wild-type fusion protein yielded Km values of 150 +/- 23 and 440 +/- 190 microM (mean +/- S.E.) for substrates (RS)-mevalonate and ATP, respectively. Expressed wild-type mevalonate kinase (MKase) had a maximum velocity of 13.6 +/- 1.4 units/mg of protein (n = 22, +/-S.E.), whereas the A334T mutation yielded an enzyme with average Vmax of 0.26 +/- 0.02 unit/mg of protein (n = 6, +/-S.E.), representing a decrease to 1.4% of control Vmax. Restriction digestion with HhaI, in conjunction with direct sequencing of cDNAs, revealed that two patients were homozygous and one heterozygous for the A334T allele, establishing autosomal recessive inheritance within families. Although the A334T enzyme had a normal Km for ATP of 680 +/- 226 microM (n = 3, +/-S.E.), the Michaelis constant for (RS)-mevalonate was increased >30-fold to 4623 +/- 1167 microM (n = 4, +/-S.E.) under standard assay conditions. Comparable kinetic results were obtained using extracts of lymphoblasts, which were homozygous for the A334T allele. Alanine 334 is invariant in MKase from bacteria to man and located in a glycine-rich region postulated to have homology with ATP-binding sequences. Our results indicate that the bacterial expression system for human MKase will provide a useful model system in which to analyze inherited mutations and identify the first active site residue in MKase associated with stabilization of mevalonate binding.

Molecular cloning of human phosphomevalonate kinase and identification of a consensus peroxisomal targeting sequence

Two overlapping cDNAs which encode human liver phosphomevalonate kinase (PMKase) were isolated. The human PMKase cDNAs predict a 191-amino acid protein with a molecular weight of 21,862, consistent with previous reports for mammalian PMKase (Mr = 21,000-22,500). Further verification of the clones was obtained by expression of PMKase activity in bacteria using a composite 1024-base pair cDNA clone. Northern blot analysis of several human tissues revealed a doublet of transcripts at approximately 1 kilobase (kb) in heart, liver, skeletal muscle, kidney, and pancreas and lower but detectable transcript levels in brain, placenta, and lung. Analysis of transcripts from human lymphoblasts subcultured in lipid-depleted sera (LDS) and LDS supplemented with lovastatin indicated that PMKase gene expression is subject to regulation by sterol at the level of transcription. Southern blotting indicated that PMKase is a single copy gene covering less than 15 kb in the human genome. The human PMKase amino acid sequence contains a consensus peroxisomal targeting sequence (PTS-1), Ser-Arg-Leu, at the C terminus of the protein. This is the first report of a cholesterol biosynthetic protein which contains a consensus PTS-1, providing further evidence for the concept that early cholesterol and nonsterol isoprenoid biosynthesis may occur in the peroxisome.