Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: UNIPROT:Q8NEX9 (reductase)
 26,410 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The relationship between the inhibition of cholesterol biosynthesis and occurrence of myopathy was studied in L6 myoblasts using two lines of cholesterol biosynthesis inhibitors, 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor (simvastatin) and squalene epoxidase inhibitors (TU-2078 and NB-598). All inhibitors completely inhibited the cholesterol synthesis in L6 myoblasts at doses of 1 and 3 microM. Simvastatin (3 microM) inhibited the fusion reaction of L6 myoblasts followed by the severe cellular damage. The myoblasts also had failed actin fiber formation and creatinine phosphokinase (CPK) production. Additionally, this agent also caused apoptotic cell death in differentiated L6 muscle fiber, indicating that skeletal myopathy by HMG-CoA reductase inhibitors seems to occur not only in differentiating immature myoblasts but also in matured skeletal myotubes. In contrast, TU-2078 and NB-598 had no effect on the fusion reaction of differentiating myoblasts or on the cellular viability of muscle fiber at 3 microM, enough to completely inhibit cholesterol biosynthesis. It is conceivable that the mevalonate depletion and subsequent failure of ras farnesylation induced by simvastatin might cause the defects in differentiation and maintenance of the muscle fiber. Squalene epoxidase inhibitors did not show this adverse effect presumably because of the enzyme inhibition downstream of farnesyl synthesis. The present findings suggest the safe use of squalene epoxidase inhibitors in lipid-lowering therapy.
 ...
PMID:Inhibition of cholesterol biosynthesis by squalene epoxidase inhibitor avoids apoptotic cell death in L6 myoblasts. 930 Jul 86

The cDNA for human squalene monooxygenase, a key enzyme in the committed pathway for cholesterol biosynthesis, was amplified from a human liver cDNA library and cloned, and the protein was expressed in Escherichia coli and purified. Kinetic analysis of the purified enzyme revealed an apparent K(m) for squalene of 7.7 microM and an apparent k(cat) of 1.1 min(-1). For FAD the apparent K(m) is 0.3 microM, consistent with a loosely bound flavin. The apparent K(m) for NADPH-cytochrome P450 reductase, the requisite electron transfer partner, is 14 nM. The amount of reductase needed for maximal activity is about threefold less than the amount of squalene monooxygenase present in the assay; thus, electron transfer to the monooxygenase is not likely to be rate limiting. Previous reports have implicated inhibition of this enzyme as the cause of a peripheral demyelination seen in weanling rats fed a diet containing tellurium. As no data were available for humans, the ability of a number of tellurium and related elemental compounds to inhibit the recombinant human enzyme was examined. Tellurite, tellurium dioxide, selenite, and selenium dioxide were inhibitory; the tellurium compounds were more potent than the selenium compounds, as indicated by their IC(50) values (17 and 37 microM, respectively). Kinetic analysis of the inhibition by tellurite suggests multiple sites of interaction with the enzyme in a noncompetitive manner with respect to squalene.
 ...
PMID:Cloning, heterologous expression, and enzymological characterization of human squalene monooxygenase. 1066 21

NADPH-cytochrome P-450 reductase is the electron transfer partner for the cytochromes P-450, heme oxygenase, and squalene monooxygenase and is a component of the nitric-oxide synthases and methionine-synthase reductase. P-450 reductase shows very high selectivity for NADPH and uses NADH only poorly. Substitution of tryptophan 677 with alanine has been shown to yield a 3-fold increase in turnover with NADH, but profound inhibition by NADP(+) makes the enzyme unsuitable for in vivo applications. In the present study site-directed mutagenesis of amino acids in the 2'-phosphate-binding site of the NADPH domain, coupled with the W677A substitution, was used to generate a reductase that was able to use NADH efficiently without inhibition by NADP(+). Of 11 single, double, and triple mutant proteins, two (R597M/W677A and R597M/K602W/W677A) showed up to a 500-fold increase in catalytic efficiency (k(cat)/K(m)) with NADH. Inhibition by NADP(+) was reduced by up to 4 orders of magnitude relative to the W677A protein and was equal to or less than that of the wild-type reductase. Both proteins were 2-3-fold more active than wild-type reductase with NADH in reconstitution assays with cytochrome P-450 1A2 and with squalene monooxygenase. In a recombinant cytochrome P-450 2E1 Ames bacterial mutagenicity assay, the R597M/W677A protein increased the sensitivity to dimethylnitrosamine by approximately 2-fold, suggesting that the ability to use NADH afforded a significant advantage in this in vivo assay.
 ...
PMID:Modification of the nucleotide cofactor-binding site of cytochrome P-450 reductase to enhance turnover with NADH in Vivo. 1238 19

Squalene monooxygenase is a microsomal enzyme that catalyzes the conversion of squalene to 2,3(s)-oxidosqualene, the immediate precursor to lanosterol in the cholesterol biosynthesis pathway. Unlike other flavoprotein monooxygenases that obtain electrons directly from NAD(P)H, squalene monooxygenase requires a redox partner, and for many years it has been assumed that NADPH-cytochrome P450 reductase is this requisite redox partner. However, our studies with hepatic cytochrome P450-reductase-null mice have revealed a second microsomal reductase for squalene monooxygenase. Inhibition studies with antibody to P450 reductase indicate that this second reductase supports up to 40% of the monooxygenase activity that is obtained with microsomes from normal mice. Studies carried out with hepatocytes from CPR-null mice demonstrate that this second reductase is active in whole cells and leads to the accumulation of 24-dihydrolanosterol; this lanosterol metabolite also accumulates in the livers of CPR-null mice, indicating that cholesterol synthesis is blocked at lanosterol demethylase, a cytochrome P450.
 ...
PMID:Hepatic cytochrome P450 reductase-null mice reveal a second microsomal reductase for squalene monooxygenase. 1737 57

Hypercholesterolemia is a major risk factor for the development of atherosclerotic vascular diseases. The most popular agents for cholesterol reduction are the statin drugs, which are competitive inhibitors of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase, the primary rate-limiting enzyme in the hepatic biosynthesis of cholesterol. Although relatively safe and effective, the available statins can cause elevations in liver enzymes and myopathy. Squalene synthase is another enzyme that is downstream to HMG-CoA reductase in the cholesterol synthesis pathway and modulates the first committed step of hepatic cholesterol biosynthesis at the final branch point of the cholesterol biosynthetic pathway. Squalene epoxidase and oxidosqualene cyclase are other enzymes that act distally to squalene synthase. Pharmacologic inhibitors of these downstream enzymes have been developed, which may reduce low-density lipoprotein cholesterol and reduce the myopathy side effect seen with upstream inhibition of HMG-CoA. At this juncture, one squalene synthase inhibitor, lapaquistat (TAK-475) is in active clinical trials as a monotherapy, but there have been suggestions of increased hepatotoxicity with the drug.
 ...
PMID:Pharmacologic inhibition of squalene synthase and other downstream enzymes of the cholesterol synthesis pathway: a new therapeutic approach to treatment of hypercholesterolemia. 1936 48

NADPH-cytochrome P450 oxidoreductase (POR) is the primary electron donor for cytochromes P450, dehydrocholesterol reductase, heme oxygenase, and squalene monooxygenase. Human patients with specific mutations in POR exhibit severe developmental malformations including disordered steroidogenesis, sexual ambiguities and various bone defects, similar to those seen in patients with Antley-Bixler syndrome (ABS). To probe the role of POR during bone development, we generated a conditional knockout mouse (CKO) by cross breeding Por (lox/lox) and Dermo1 Cre mice. CKO mice were smaller than their littermate controls and exhibited significant craniofacial and long bone abnormalities. Differential staining of the CKO mice skull bases shows premature fusion of the sphenooccipital and basioccipital-exoccipital synchondroses. Class III malocclusion was noted in adult knockout mice with an unusual overgrowth of the lower incisors. Shorter long bones were observed along with a reduction in the bone volume fraction, measured by microCT, in the Por-deleted mice compared to age- and sex-matched littermate controls. Concerted up- or down-regulation of proteins in the FGF signaling pathway observed by immunohistochemistry in the tibia samples of CKO mice compared to wild type controls shows a decrease in the FGF signaling pathway. To our knowledge, this is the first report of a mouse model that recapitulates both skull and long bone defects upon Por deletion, offering an approach to study the sequelae of POR mutations. This unique model demonstrates that P450 metabolism in bone itself is potentially important for proper bone development, and that an apparent link exists between the POR and FGF signaling pathways, begging the question of how an oxidation-reduction flavoprotein affects developmental and cellular signaling processes.
...
PMID:Conditional deletion of cytochrome p450 reductase in osteoprogenitor cells affects long bone and skull development in mice recapitulating antley-bixler syndrome: role of a redox enzyme in development. 2408 98

The mevalonate pathway is used by cells to produce sterol and nonsterol metabolites and is subject to tight metabolic regulation. We recently reported that squalene monooxygenase (SM), an enzyme controlling a rate-limiting step in cholesterol biosynthesis, is subject to cholesterol-dependent proteasomal degradation. However, the E3-ubiquitin (E3) ligase mediating this effect was not established. Using a candidate approach, we identify the E3 ligase membrane-associated RING finger 6 (MARCH6, also known as TEB4) as the ligase controlling degradation of SM. We find that MARCH6 and SM physically interact, and consistent with MARCH6 acting as an E3 ligase, its overexpression reduces SM abundance in a RING-dependent manner. Reciprocally, knockdown of MARCH6 increases the level of SM protein and prevents its cholesterol-regulated degradation. Additionally, this increases cell-associated SM activity but is unexpectedly accompanied by increased flux upstream of SM. Prompted by this observation, we found that knockdown of MARCH6 also controls the level of 3-hydroxy-3-methyl-glutaryl coenzyme A reductase (HMGCR) in hepatocytes and model cell lines. In conclusion, MARCH6 controls abundance of both SM and HMGCR, establishing it as a major regulator of flux through the cholesterol synthesis pathway.
...
PMID:The E3 ubiquitin ligase MARCH6 degrades squalene monooxygenase and affects 3-hydroxy-3-methyl-glutaryl coenzyme A reductase and the cholesterol synthesis pathway. 2444 66

The synthesis of cholesterol requires more than 20 enzymes, many of which are intricately regulated. Post-translational control of these enzymes provides a rapid means for modifying flux through the pathway. So far, several enzymes have been shown to be rapidly degraded through the ubiquitin proteasome pathway in response to cholesterol and other sterol intermediates. Additionally, several enzymes have their activity altered through phosphorylation mechanisms. Most work has focused on the two rate-limiting enzymes: 3-hydroxy-3-methyl-glutaryl coenzyme A reductase and squalene monooxygenase. Here, we review current literature in the area to define some common themes in the regulation of the entire cholesterol synthesis pathway. We highlight the rich variety of inputs controlling each enzyme, discuss the interplay that exists between regulatory mechanisms, and summarize findings that reveal an intricately coordinated network of regulation along the cholesterol synthesis pathway. We provide a roadmap for future research into the post-translational control of cholesterol synthesis, and no doubt the road ahead will reveal further twists and turns for this fascinating pathway crucial for human health and disease.
 ...
PMID:Post-translational control of the long and winding road to cholesterol. 3305 Dec 3