Loose Morphology and High Dynamism of OSER Structures Induced by the Membrane Domain of HMG-CoA Reductase.

The membrane domain of eukaryotic HMG-CoA reductase (HMGR) has the conserved capacity to induce endoplasmic reticulum (ER) proliferation and membrane association into Organized Smooth Endoplasmic Reticulum (OSER) structures. These formations develop in response to overexpression of particular proteins, but also occur naturally in cells of the three eukaryotic kingdoms. Here, we characterize OSER structures induced by the membrane domain of Arabidopsis HMGR (1S domain). Immunochemical confocal and electron microscopy studies demonstrate that the 1S:GFP chimera co-localizes with high levels of endogenous HMGR in several ER compartments, such as the ER network, the nuclear envelope, the outer and internal membranes of HMGR vesicles and the OSER structures, which we name ER-HMGR domains. After high-pressure freezing, ER-HMGR domains show typical crystalloid, whorled and lamellar ultrastructural patterns, but with wide heterogeneous luminal spaces, indicating that the native OSER is looser and more flexible than previously reported. The formation of ER-HMGR domains is reversible. OSER structures grow by incorporation of ER membranes on their periphery and progressive compaction to the inside. The ER-HMGR domains are highly dynamic in their formation versus their disassembly, their variable spherical-ovoid shape, their fluctuating borders and their rapid intracellular movement, indicating that they are not mere ER membrane aggregates, but active components of the eukaryotic cell.

Inhibition mechanism of 3-hydroxy-3-methyl-glutaryl-CoA reductase by tocotrienol-rich rice bran fraction optimally extracted with ultrasonic energy.

Tocotrienols (T3) are vitamin E components that inhibit 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGR), a primary target for cholesterol management. T3 was extracted from rice bran (RBE) using ultrasonic energy keeping solute: solvent ratio, power and time on specific energy and T3 concentration as responses as per Box-Behnken Design. The lowest specific energy (52.38 ± 0.14 J mL-1) uptake by the sample was most effective in enhancing the concentration of T3 in RBE (199.34 ± 0.63 µg mL-1). In vitro HMGR kinetics and in silico binding interactions of the identified α-, δ- and γ-T3 fractions were studied. Enzyme kinetic studies revealed an uncompetitive mode of inhibition by α-T3, γ-T3, and RBE and a mixed mode of inhibition for δ-T3. γ-T3 showed lowest IC50 concentration (11.33 µg mL-1) followed by α-T3 (16.73 µg mL-1), RBE (20.45 µg mL-1) and δ-T3 (23.16 µg mL-1). Molecular docking studies highlighted the hydrogen bonding of δ-T3 with Gln766 and α- and γ-T3 with Met655 and Val805 amino acid residues at the NADPH binding site of HMGR. Results indicate the potential use of T3 enriched RBE optimally extracted using ultrasound as potent HMGR inhibitor.

Computational study of conformational changes in human 3-hydroxy-3-methylglutaryl coenzyme reductase induced by substrate binding.

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) is mainly involved in the regulation of cholesterol biosynthesis. HMGR catalyses the reduction of 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) to mevalonate at the expense of two NADPH molecules in a two-step reversible reaction. In the present study, we constructed a model of human HMGR (hHMGR) to explore the conformational changes of HMGR in complex with HMG-CoA and NADPH. In addition, we analysed the complete sequence of the Flap domain using molecular dynamics (MD) simulations and principal component analysis (PCA). The simulations revealed that the Flap domain plays an important role in catalytic site activation and substrate binding. The apo form of hHMGR remained in an open state, while a substrate-induced closure of the Flap domain was observed for holo hHMGR. Our study also demonstrated that the phosphorylation of Ser872 induces significant conformational changes in the Flap domain that lead to a complete closure of the active site, suggesting three principal conformations for the first stage of hHMGR catalysis. Our results were consistent with previous proposed models for the catalytic mechanism of hHMGR. Communicated by Ramaswamy H. Sarma.

Structural Features and Domain Movements Controlling Substrate Binding and Cofactor Specificity in Class II HMG-CoA Reductase.

The key mevalonate pathway enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase (HMGR) uses the cofactor NAD(P)H to reduce HMG-CoA to mevalonate in the production of countless metabolites and natural products. Although inhibition of HMGR by statin drugs is well-understood, several mechanistic details of HMGR catalysis remain unresolved, and the structural basis for the wide range of cofactor specificity for either NADH or NADPH among HMGRs from different organisms is also unknown. Here, we present crystal structures of HMGR from Streptococcus pneumoniae (SpHMGR) alongside kinetic data of the enzyme's cofactor preferences. Our structure of SpHMGR bound with its kinetically preferred NADPH cofactor suggests how NADPH-specific binding and recognition are achieved. In addition, our structure of HMG-CoA-bound SpHMGR reveals large, previously unknown conformational domain movements that may control HMGR substrate binding and enable cofactor exchange without intermediate release during the catalytic cycle. Taken together, this work provides critical new insights into both the HMGR reaction mechanism and the structural basis of cofactor specificity.

De novo leaf and root transcriptome analysis to identify putative genes involved in triterpenoid saponins biosynthesis in Hedera helix L.

Hedera helix L. is an important traditional medicinal plant in Europe. The main active components are triterpenoid saponins, but none of the potential enzymes involved in triterpenoid saponins biosynthesis have been discovered and annotated. Here is reported the first study of global transcriptome analyses using the Illumina HiSeq™ 2500 platform for H. helix. In total, over 24 million clean reads were produced and 96,333 unigenes were assembled, with an average length of 1385 nt; more than 79,085 unigenes had at least one significant match to an existing gene model. Differentially Expressed Gene analysis identified 6,222 and 7,012 unigenes which were expressed either higher or lower in leaf samples when compared with roots. After functional annotation and classification, two pathways and 410 unigenes related to triterpenoid saponins biosynthesis were discovered. The accuracy of these de novo sequences was validated by RT-qPCR analysis and a RACE clone. These data will enrich our knowledge of triterpenoid saponin biosynthesis and provide a theoretical foundation for molecular research on H. helix.

AKIN10, a representative Arabidopsis SNF1-related protein kinase 1 (SnRK1), phosphorylates and downregulates plant HMG-CoA reductase.

HMG-CoA reductase (HMGR) is a key enzyme in the mevalonate pathway for sterols and cytosolic isoprenoid production. Although HMGR kinases from spinach, barley, and cauliflower tissues have been strongly suggested as members of SNF1-related protein kinases 1 (SnRK1), the phosphorylation and inactivation of HMGR by plant SnRK1s has not been demonstrated. In this study, we elucidated that AKIN10, an Arabidopsis SnRK1, acts as an HMGR kinase. The recombinant AKIN10 phosphorylates and inactivates AtHMGR1S using recombinant GRIK1 as the AKIN10 activator. In contrast, AKIN10-GRIK1 fails to inactivate AtHMGR1S-S577A, suggesting that this is achieved through Ser577 phosphorylation. Moreover, phosphorylation is detected not only in AtHMGR1S but also in AtHMGR1S-S577A, suggesting the presence of a novel regulatory mechanism of plant HMGR.

Proliferation and Morphogenesis of the Endoplasmic Reticulum Driven by the Membrane Domain of 3-Hydroxy-3-Methylglutaryl Coenzyme A Reductase in Plant Cells.

The enzyme 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) has a key regulatory role in the mevalonate pathway for isoprenoid biosynthesis and is composed of an endoplasmic reticulum (ER)-anchoring membrane domain with low sequence similarity among eukaryotic kingdoms and a conserved cytosolic catalytic domain. Organized smooth endoplasmic reticulum (OSER) structures are common formations of hypertrophied tightly packed ER membranes devoted to specific biosynthetic and secretory functions, the biogenesis of which remains largely unexplored. We show that the membrane domain of plant HMGR suffices to trigger ER proliferation and OSER biogenesis. The proliferating membranes become highly enriched in HMGR protein, but they do not accumulate sterols, indicating a morphogenetic rather than a metabolic role for HMGR. The N-terminal MDVRRRPP motif present in most plant HMGR isoforms is not required for retention in the ER, which was previously proposed, but functions as an ER morphogenic signal. Plant OSER structures are morphologically similar to those of animal cells, emerge from tripartite ER junctions, and mainly build up beside the nuclear envelope, indicating conserved OSER biogenesis in high eukaryotes. Factors other than the OSER-inducing HMGR construct mediate the tight apposition of the proliferating membranes, implying separate ER proliferation and membrane association steps. Overexpression of the membrane domain of Arabidopsis (Arabidopsis thaliana) HMGR leads to ER hypertrophy in every tested cell type and plant species, whereas the knockout of the HMG1 gene from Arabidopsis, encoding its major HMGR isoform, causes ER aggregation at the nuclear envelope. Our results show that the membrane domain of HMGR contributes to ER morphogenesis in plant cells.

nor-Mevaldic acid surrogates as selective antifungal agent leads against Botrytis cinerea. Enantioselective preparation of 4-hydroxy-6-(1-phenylethoxy)tetrahydro-2H-pyran-2-one.

Solvent-free desymmetrisation of meso-dialdehyde 1 with chiral 1-phenylethan-1-ol, led to preparation of 4-silyloxy-6-alkyloxytetrahydro-2H-pyran-2-one (+)-3a with a 96:4 dr Deprotected lactone (+)-19a and the related racemic lactones 16a-18a present a lactone moiety resembling the natural substrate of HMG-CoA reductase and their antifungal properties have been evaluated against the phytopathogenic fungi Botrytis cinerea and Colletotrichum gloeosporioides. These compounds were selectively active against B. cinerea, while inactive against C. gloeosporioides.

In vitro screening for ß-hydroxy-ß-methylglutaryl-CoA reductase inhibitory and antioxidant activity of sequentially extracted fractions of Ficus palmata Forsk.

Hypercholesterolemia-induced oxidative stress has been strongly implicated in the pathogenesis of atherosclerosis, which is one of the major causes of mortality worldwide. The current work, for the first time, accounts the antioxidant, genoprotective, antilipoperoxidative, and HMG-CoA reductase (EC 1.1.1.34) inhibitory properties of traditional medicinal plant, Ficus palmata Forsk. Our result showed that among sequentially extracted fractions of Ficus palmata Forsk, FPBA (F. palmata bark aqueous extract) and FPLM (F. palmata leaves methanolic extract) extracts have higher phenolic content and also exhibited significantly more radical scavenging (DPPH and Superoxide) and antioxidant (FRAP) capacity. Moreover, FPBA extract also exhibited significantly higher inhibition of lipid peroxidation assay. Additionally, results showed almost complete and partial protection of oxidatively damaged DNA by these plant extracts when compared to mannitol. Furthermore, our results showed that FPBA extract (IC50 = 9.1 ± 0.61 µg/mL) exhibited noteworthy inhibition of HMG-CoA reductase activity as compared to other extracts, which might suggest its role as cardioprotective agent. In conclusion, results showed that FPBA extract not only possess significant antioxidant and genoprotective property but also is able to attenuate the enzymatic activity of HMG-CoA reductase, which might suggest its role in combating various oxidative stress-related diseases, including atherosclerosis.

Molecular cloning, tissue expression and protein structure prediction of the porcine 3-hydroxy-3-methylglutaryl-Coenzyme A reductase (HMGR) gene.

3-hydroxy-3-methylglutaryl-Coenzyme A reductase (HMGR) is the rate-limiting enzyme for cholesterol synthesis. Its activity is regulated via a negative feedback mechanism through sterols and non-sterol metabolites derived from mevalonate, the product of the reaction catalyzed by reductase. Here, we cloned a full-length transcript of porcine HMGR by RT-PCR and RACE. The porcine HMGR cDNA (2864 bp,GenBank accession no. DQ432054) contains a 2658 bp open reading frame and shares 91% identify with those of human and cattle, and 85%, 85% and 84% identify with the HMGR coding sequences of Norway rat, golden hamster, and house mouse, respectively. The deduced porcine HMGR protein consists of 885 amino acids with a calculated molecular mass of 97.15 kDa(GenBank accession no. ABD96089). The amino acid sequence similarities correspond to 95%, 95%, 92%, 92% and 92% when compared with human, cattle, Norway rat, golden hamster and house mouse sequences, respectively. The structure and function of HMGR deduced protein product were predicted by bioinformatic approaches. HMGR-specific transcripts were found in 15 different tissues from pig by RT-PCR and Real-time PCR. The relative expression level of HMGR was high in liver, heart, kidney, bladder and subcutaneous fat, medium in lung, uterus and large intestine, and low in cerebrum, spleen, spinal cord, stomach, ovary, longissimus muscle, and small intestine. The SNPs analysis of HMGR showed that there were five SNPS and three of them are synonymous mutations and the other two are missense mutations. Taken together, our data may lay a ground for further investigation of HMGR's functions and regulatory mechanisms in swine.

Activation of membrane cholesterol by 63 amphipaths.

A few membrane-intercalating amphipaths have been observed to stimulate the interaction of cholesterol with cholesterol oxidase, saponin and cyclodextrin, presumably by displacing cholesterol laterally from its phospholipid complexes. We now report that this effect, referred to as cholesterol activation, occurs with dozens of other amphipaths, including alkanols, saturated and cis- and trans-unsaturated fatty acids, fatty acid methyl esters, sphingosine derivatives, terpenes, alkyl ethers, ketones, aromatics and cyclic alkyl derivatives. The apparent potency of the agents tested ranged from 3 microM to 7 mM and generally paralleled their octanol/water partition coefficients, except that relative potency declined for compounds with >10 carbons. Some small amphipaths activated cholesterol at a membrane concentration of approximately 3 mol per 100 mol of bilayer lipids, about equimolar with the cholesterol they displaced. Lysophosphatidylserine countered the effects of all these agents, consistent with its ability to reduce the pool of active membrane cholesterol. Various amphipaths stabilized red cells against the hemolysis elicited by cholesterol depletion, presumably by substituting for the extracted sterol. The number and location of cis and trans fatty acid unsaturations and the absolute stereochemistry of enantiomer pairs had only small effects on amphipath potency. Nevertheless, potency varied approximately 7-fold within a group of diverse agents with similar partition coefficients. We infer that a wide variety of amphipaths can displace membrane cholesterol by competing stoichiometrically but with only limited specificity for weak association with phospholipids. Any number of other drugs and experimental agents might do the same.

Cloning of the Rhizomucor miehei 3-hydroxy-3-methylglutaryl-coenzyme A reductase gene and its heterologous expression in Mucor circinelloides.

In this study, the gene hmgR encoding the 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMG-CoA reductase) was cloned and characterized in the zygomycete fungus Rhizomucor miehei. The hmgR gene comprises a total of 3,585 bp including the coding sequence of a 1,058 amino acids length putative protein and five introns (137, 83, 59, 60 and 69 bp in length) dispersed in the whole coding region. Southern hybridization analysis revealed that the gene is present only in one copy in the R. miehei genome. The isolated Rhizomucor gene was expressed in the related fungus, Mucor circinelloides. Transformants harbouring the Rhizomucor hmgR gene in an autoreplicative plasmid proved to be more tolerant to statins (e.g. lovastatin, simvastatin, and fluvastatin), the competitive inhibitors of the HMG-CoA reductase, than the original M. circinelloides strain. At the same time, heterologous expression of the Rhizomucor hmgR did not affect the carotenoid production of M. circinelloides.

Cloning and characterization of a root-specific expressing gene encoding 3-hydroxy-3-methylglutaryl coenzyme A reductase from Ginkgo biloba.

3-Hydroxy-3-methylglutaryl coenzyme A reductase (HMGR, EC: 1.1.1.34) catalyzes the first committed step in mevalonic acid (MVA) pathway for biosynthesis of isoprenoids. The full-length cDNA encoding HMGR was isolated from Ginkgo biloba for the first time (designated as GbHMGR, GenBank accession number AY741133), which contained a 1713 bp ORF encoding 571 amino acids. The GbHMGR genomic DNA sequence was also obtained, revealing GbHMGR had four exons and three introns. The deduced GbHMGR protein showed high identity to other plant HMGRs and contained two trans-membrane domains and a catalytic domain. The three dimensional model of GbHMGR represented a typical spatial structure of HMGRs. The Southern blot and RT-PCR assay results indicated that GbHMGR belonged to a small gene family, and expressed in a tissue-specific manner with a low level expression being only found in root. The potential significance of GbHMGR gene was also discussed.

Insig-dependent ubiquitination and degradation of 3-hydroxy-3-methylglutaryl coenzyme a reductase stimulated by delta- and gamma-tocotrienols.

Sterol-regulated ubiquitination marks 3-hydroxy-3-methylglutaryl coenzyme A reductase, a rate-determining enzyme in cholesterol synthesis, for endoplasmic reticulum (ER)-associated degradation by 26 S proteasomes. This degradation, which results from sterol-induced binding of reductase to ER membrane proteins called Insigs, contributes to the complex, multivalent feedback regulation of the enzyme. Degradation of HMG-CoA reductase is also stimulated by various forms of vitamin E, a generic term for alpha-, beta-, delta-, and gamma-tocopherols and tocotrienols, which are primarily recognized for their potent antioxidant activity. Here, we show that delta-tocotrienol stimulates ubiquitination and degradation of reductase and blocks processing of sterol regulatory element-binding proteins (SREBPs), another sterol-mediated action of Insigs. The gamma-tocotrienol analog is more selective in enhancing reductase ubiquitination and degradation than blocking SREBP processing. Other forms of vitamin E neither accelerate reductase degradation nor block SREBP processing. In vitro assays indicate that gamma- and delta-tocotrienol trigger reductase ubiquitination directly and do not require further metabolism for activity. Taken together, these results provide a biochemical mechanism for the hypocholesterolemic effects of vitamin E that have been observed in animals and humans.

Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase promoter by nuclear receptors liver receptor homologue-1 and small heterodimer partner: a mechanism for differential regulation of cholesterol synthesis and uptake.

Cholesterol homeostasis in mammals involves pathways for biosynthesis, cellular uptake, and hepatic conversion to bile acids. Key genes for all three pathways are regulated by negative feedback control. Uptake and biosynthesis are directly regulated by cholesterol through its inhibition of the proteolytic activation of the sterol regulatory element binding proteins. The conversion of cholesterol into bile acids in the liver is regulated through the bile acid-dependent induction of the negatively acting small heterodimer partner nuclear receptor. In this report, we have shown that the small heterodimer partner also directly regulates cholesterol biosynthesis through inhibition of 3-hydroxy-3-methylglutaryl coenzyme A reductase but has no effect on low density lipoprotein receptor expression. This has significant metabolic significance, as it provides both a mechanism to independently regulate cholesterol synthesis from uptake (an essential regulatory feature known to occur in vivo) and a pathway for direct regulation of cholesterol biosynthesis by bile acids. This latter feature ensures that the early phase of bile acid synthesis (pre-cholesterol) is in metabolic communication with the later stages of the pathway to properly regulate whole pathway flux. This highlights an important regulatory feature that is shared with other key branched, multienzyme pathways, such as glycolysis, where pathway outflow through pyruvate kinase is regulated by the concentration of a key early intermediate, fructose 1,6-bisphosphate.