A mammalian mutant cell lacking detectable lanosterol 14 alpha-methyl demethylase activity

Role of Akt and mammalian target of rapamycin signalling in insulin-like growth factor 1-mediated cell proliferation in porcine Sertoli cells

The critical role of insulin-like growth factor (IGF) 1 in promoting Sertoli cell proliferation invivo and invitro has been established, but its downstream signalling mechanisms remain unknown. In addition to mitogenic effects, a role for IGF1 in mediating cholesterol biosynthesis within testes has been implied. The aims of this study were to investigate the roles of: (1) phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin (mTOR) signalling in IGF1-mediated Sertoli cell proliferation; and (2) IGF1 in mediating cholesterol biosynthesis in Sertoli cells. Primary cultures of Sertoli cells were prepared from 1-week-old porcine testes. On Day 3 of culture, Sertoli cells were treated with 300ng mL-1 IGF1, alone or in combination with inhibitors of IGF1 receptor (2μM picropodophyllotoxin), Akt (1μM wortmannin) or mTOR (200nM rapamycin). Cells were cultured for 30min and phosphorylation levels of Akt, mTOR and p70 ribosomal protein S6 kinase (p70S6K) were determined by immunoblotting. Cell proliferation and quantitative polymerase chain reaction assays were conducted using cells cultured for 24h. IGF1 increased phosphorylation of Akt, mTOR and p70S6K and cell proliferation, and these effects were inhibited by inhibitors of IGF1R, Akt and mTOR. Furthermore, IGF1 upregulated the expression of cholesterol biosynthetic genes (3-hydroxy-3-methylglutaryl-CoA reductase (HMGCR), 3-hydroxy-3-methylglutaryl-CoA synthase (HMGCS1) and cytochrome P450, family 5, subfamily A, polypeptide 1 (CYP5A1)), but not sterol regulatory element-binding transcription factor 1 (SREBF1). Increased phosphorylation of p70S6K, a major downstream target of mTOR, and upregulated expression of genes involved in cholesterol biosynthesis are indicative of the key role played by IGF1 in regulating the synthesis of cholesterol, the precursor for steroid hormones.

The Role of Pi, Glutamine and the Essential Amino Acids in Modulating the Metabolism in Diabetes and Cancer

Purpose

Re-examine the current metabolic models.

Methods

Review of literature and gene networks.

Results

Insulin activates Pi uptake, glutamine metabolism to stabilise lipid membranes. Tissue turnover maintains the metabolic health. Current model of intermediary metabolism (IM) suggests glucose is the source of energy, and anaplerotic entry of fatty acids and amino acids into mitochondria increases the oxidative capacity of the TCA cycle to produce the energy (ATP). The reduced cofactors, NADH and FADH2, have different roles in regulating the oxidation of nutrients, membrane potentials and biosynthesis. Trans-hydrogenation of NADH to NADPH activates the biosynthesis. FADH2 sustains the membrane potential during the cell transformations. Glycolytic enzymes assume the non-canonical moonlighting functions, enter the nucleus to remodel the genetic programmes to affect the tissue turnover for efficient use of nutrients. Glycosylation of the CD98 (4F2HC) stabilises the nutrient transporters and regulates the entry of cysteine, glutamine and BCAA into the cells. A reciprocal relationship between the leucine and glutamine entry into cells regulates the cholesterol and fatty acid synthesis and homeostasis in cells. Insulin promotes the Pi transport from the blood to tissues, activates the mitochondrial respiratory activity, and glutamine metabolism, which activates the synthesis of cholesterol and the de novo fatty acids for reorganising and stabilising the lipid membranes for nutrient transport and signal transduction in response to fluctuations in the microenvironmental cues. Fatty acids provide the lipid metabolites, activate the second messengers and protein kinases. Insulin resistance suppresses the lipid raft formation and the mitotic slippage activates the fibrosis and slow death pathways.

Disrupting Hepatocyte Cyp51 from Cholesterol Synthesis Leads to Progressive Liver Injury in the Developing Mouse and Decreases RORC Signalling

Development of mice with hepatocyte knockout of lanosterol 14α-demethylase (H^Cyp51−/−) from cholesterol synthesis is characterized by the progressive onset of liver injury with ductular reaction and fibrosis. These changes begin during puberty and are generally more aggravated in the knockout females. However, a subgroup of (pre)pubertal knockout mice (runts) exhibits a pronounced male prevalent liver dysfunction characterized by downregulated amino acid metabolism and elevated Casp12. RORC transcriptional activity is diminished in livers of all runt mice, in correlation with the depletion of potential RORC ligands subsequent to CYP51 disruption. Further evidence for this comes from the global analysis that identified a crucial overlap between hepatic Cyp51^−/− and Rorc^−/− expression profiles. Additionally, the reduction in RORA and RORC transcriptional activity was greater in adult H^Cyp51−/− females than males, which correlates well with their downregulated amino and fatty acid metabolism. Overall, we identify a global and sex-dependent transcriptional de-regulation due to the block in cholesterol synthesis during development of the Cyp51 knockout mice and provide in vivo evidence that sterol intermediates downstream of lanosterol may regulate the hepatic RORC activity.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Feedback regulation of cholesterol synthesis: sterol-accelerated ubiquitination and degradation of HMG CoA reductase

3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase produces mevalonate, an important intermediate in the synthesis of cholesterol and essential nonsterol isoprenoids. The reductase is subject to an exorbitant amount of feedback control through multiple mechanisms that are mediated by sterol and nonsterol end-products of mevalonate metabolism. Here, I will discuss recent advances that shed light on one mechanism for control of reductase, which involves rapid degradation of the enzyme. Accumulation of certain sterols triggers binding of reductase to endoplasmic reticulum (ER) membrane proteins called Insig-1 and Insig-2. Reductase-Insig binding results in recruitment of a membrane-associated ubiquitin ligase called gp78, which initiates ubiquitination of reductase. This ubiquitination is an obligatory reaction for recognition and degradation of reductase from ER membranes by cytosolic 26S proteasomes. Thus, sterol-accelerated degradation of reductase represents an example of how a general cellular process (ER-associated degradation) is used to control an important metabolic pathway (cholesterol synthesis).

Insig-mediated degradation of HMG CoA reductase stimulated by lanosterol, an intermediate in the synthesis of cholesterol

Feedback control of cholesterol synthesis is mediated in part by sterol-induced binding of HMG CoA reductase to Insig proteins in the endoplasmic reticulum (ER). Binding leads to ubiquitination and proteasomal degradation of reductase, a rate-controlling enzyme in cholesterol synthesis. Using in vitro and in vivo assays, we show that lanosterol, the first sterol intermediate in cholesterol synthesis, potently stimulates ubiquitination of reductase, whereas cholesterol has no effect at 10-fold higher concentrations. Lanosterol is not effective in mediating the other action of Insigs, namely to promote ER retention of SCAP-SREBP complexes, a reaction that is mediated directly by cholesterol. A pair of methyl groups located in the C4 position of lanosterol confers this differential response. These data indicate that buildup of cholesterol synthesis intermediates represses the pathway selectively at reductase and reveal a previously unappreciated link between feedback inhibition of reductase and carbon flow through the cholesterol synthetic pathway.

Effects of distal cholesterol biosynthesis inhibitors on cell proliferation and cell cycle progression

Cholesterol is a major lipid component of the plasma membrane in animal cells. In addition to its structural requirement, cholesterol is essential in cell proliferation and other cell processes. The aim of the present study was to elucidate the stringency of the requirement for cholesterol as a regulator of proliferation and cell cycle progression, compared with other sterols of the cholesterol biosynthesis pathway. Human promyelocytic HL-60 cells were cultured in cholesterol-free medium and treated with different distal inhibitors of cholesterol biosynthesis (zaragozic acid, SKF 104976, SR 31747, BM 15766, and AY 9944), which allow the synthesis of isoprenoid derivatives and different sets of sterol intermediates, but not cholesterol. The results showed that only the inhibition of sterol Delta7-reductase was compatible with cell proliferation. Blocking cholesterol biosynthesis upstream of this enzyme resulted in the inhibition of cell proliferation and cell cycle arrest selectively in G2/M phase.

Cholesterol is essential for mitosis progression and its deficiency induces polyploid cell formation

As an essential component of mammalian cell membranes, cells require cholesterol for proliferation, which is either obtained from plasma lipoproteins or synthesized intracellularly from acetyl-CoA. In addition to cholesterol, other non-sterol mevalonate derivatives are necessary for DNA synthesis, such as the phosphorylated forms of isopentane, farnesol, geranylgeraniol, and dolichol. The aim of the present study was to elucidate the role of cholesterol in mitosis. For this, human leukemia cells (HL-60) were incubated in a cholesterol-free medium and treated with SKF 104976, which inhibits cholesterol biosynthesis by blocking sterol 14alpha-demethylase, and the expression of relevant cyclins in the different phases of the cell cycle was analyzed by flow cytometry. Prolonged cholesterol starvation induced the inhibition of cytokinesis and the formation of polyploid cells, which were multinucleated and had mitotic aberrations. Supplementing the medium with cholesterol completely abolished these effects, demonstrating they were specifically due to cholesterol deficiency. This is the first evidence that cholesterol is essential for mitosis completion and that, in the absence of cholesterol, the cells fail to undergo cytokinesis, entered G1 phase at higher DNA ploidy (tetraploidy), and then progressed through S (rereplication) into G2, generating polyploid cells.

Oxysterols: modulators of cholesterol metabolism and other processes

Oxygenated derivatives of cholesterol (oxysterols) present a remarkably diverse profile of biological activities, including effects on sphingolipid metabolism, platelet aggregation, apoptosis, and protein prenylation. The most notable oxysterol activities center around the regulation of cholesterol homeostasis, which appears to be controlled in part by a complex series of interactions of oxysterol ligands with various receptors, such as the oxysterol binding protein, the cellular nucleic acid binding protein, the sterol regulatory element binding protein, the LXR nuclear orphan receptors, and the low-density lipoprotein receptor. Identification of the endogenous oxysterol ligands and elucidation of their enzymatic origins are topics of active investigation. Except for 24, 25-epoxysterols, most oxysterols arise from cholesterol by autoxidation or by specific microsomal or mitochondrial oxidations, usually involving cytochrome P-450 species. Oxysterols are variously metabolized to esters, bile acids, steroid hormones, cholesterol, or other sterols through pathways that may differ according to the type of cell and mode of experimentation (in vitro, in vivo, cell culture). Reliable measurements of oxysterol levels and activities are hampered by low physiological concentrations (approximately 0.01-0.1 microM plasma) relative to cholesterol (approximately 5,000 microM) and by the susceptibility of cholesterol to autoxidation, which produces artifactual oxysterols that may also have potent activities. Reports describing the occurrence and levels of oxysterols in plasma, low-density lipoproteins, various tissues, and food products include many unrealistic data resulting from inattention to autoxidation and to limitations of the analytical methodology. Because of the widespread lack of appreciation for the technical difficulties involved in oxysterol research, a rigorous evaluation of the chromatographic and spectroscopic methods used in the isolation, characterization, and quantitation of oxysterols has been included. This review comprises a detailed and critical assessment of current knowledge regarding the formation, occurrence, metabolism, regulatory properties, and other activities of oxysterols in mammalian systems.

Cholesterol starvation decreases p34(cdc2) kinase activity and arrests the cell cycle at G2

As a major component of mammalian cell plasma membranes, cholesterol is essential for cell growth. Accordingly, the restriction of cholesterol provision has been shown to result in cell proliferation inhibition. We explored the potential regulatory role of cholesterol on cell cycle progression. MOLT-4 and HL-60 cell lines were cultured in a cholesterol-deficient medium and simultaneously exposed to SKF 104976, which is a specific inhibitor of lanosterol 14-alpha demethylase. Through HPLC analyses with on-line radioactivity detection, we found that SKF 104976 efficiently blocked the [(14)C]-acetate incorporation into cholesterol, resulting in an accumulation of lanosterol and dihydrolanosterol, without affecting the synthesis of mevalonic acid. The inhibitor also produced a rapid and intense inhibition of cell proliferation (IC(50) = 0.1 microM), as assessed by both [(3)H]-thymidine incorporation into DNA and cell counting. Flow cytometry and morphological examination showed that treatment with SKF 104976 for 48 h or longer resulted in the accumulation of cells specifically at G2 phase, whereas both the G1 traversal and the transition through S were unaffected. The G2 arrest was accompanied by an increase in the hyperphosphorylated form of p34(cdc2) and a reduction of its activity, as determined by assaying the H1 histone phosphorylating activity of p34(cdc2) immunoprecipitates. The persistent deficiency of cholesterol induced apoptosis. However, supplementing the medium with cholesterol, either in the form of LDL or free cholesterol dissolved in ethanol, completely abolished these effects, whereas mevalonate was ineffective. Caffeine, which abrogates the G2 checkpoint by preventing p34(cdc2) phosphorylation, reduced the accumulation in G2 when added to cultures containing cells on transit to G2, but was ineffective in cells arrested at G2 by sustained cholesterol starvation. Cells arrested in G2, however, were still viable and responded to cholesterol provision by activating p34(cdc2) and resuming the cell cycle. We conclude that in both lymphoblastoid and promyelocytic cells, cholesterol availability governs the G2 traversal, probably by affecting p34(cdc2) activity.

Biochemical basis for a cholesterol-lowering activity of 2-[2"-(1",3"-dioxolane)]-2-methyl-4-(2'-oxo-1'-pyrrolidinyl)-6- nitro-2H-1-benzopyran (SKP-450), a novel antihypertensive agent

Administration (p.o.) of SKP-450, 2-[2"-(1",3"-dioxolane)]-2-methyl-4-(2'-oxo-1'-pyrrolidinyl)-6-nitro-2H- 1-benzopyran, a novel antihypertensive agent, to hypercholesterolemic Syrian hamsters led to a significant reduction in plasma lipids in a dose-dependent manner, i.e., a 10.8% to 29% reduction in low-density lipoprotein cholesterol at doses of 0.3 to 10 mg/kg of SKP-450. SKP-450 was found to specifically inhibit the hepatic microsomal lanosterol 14alpha-methyl demethylase (14alpha-DM) in a competitive manner (Ki:2.65 microM). Furthermore, a dose-dependent decrease in the 14alpha-DM activity by SKP-450 parallelled the cholesterol synthetic rate in vitro in both the rat hepatic S10 fractions (supernatants at 10,000 g; IC50:20 microM) and Chinese hamster ovary cells (IC50:23 microM). However, this phenomenon was not seen in AR45 cells, which are deficient in 14alpha-DM, suggesting that 14alpha-DM is the major target for the inhibitory action of SKP-450 in regard to cholesterol biosynthesis.

Induction of apoptosis in p53-null HL-60 cells by inhibition of lanosterol 14-alpha demethylase

To determine the role of cholesterol deprivation in cell proliferation and, eventually, in apoptosis, HL-60 promyelocytic cells were incubated in a cholesterol-depleted medium in the presence of SKF 104976, a specific inhibitor of lanosterol 14-alpha demethylase. As expected, SKF 104976 efficiently blocked the [14C]-acetate incorporation into cholesterol, whereas it induced the accumulation of both lanosterol and, especially, dihydrolanosterol. As a consequence, cell proliferation was greatly depressed at 24 h of treatment with the drug, and clear signs of apoptosis--annexin V binding, condensed and fragmented nuclei and DNA ladder--were observed thereafter. Provided that the HL-60 cell line does not express p53, it may be concluded that apoptosis induced by cholesterol deprivation is not dependent on this tumor suppressor protein. Supplementing the incubation medium with LDL-cholesterol or pure free cholesterol, fully prevented cell growth inhibition and apoptosis induction, whereas mevalonate was ineffective. These results indicate that cholesterol plays a specific role in cell proliferation, a function that is not shared by its precursors lanosterol and dihydrolanosterol.

Effects of 15-oxa-32-vinyl-lanost-8-ene-3 beta,32 diol on the expression of 3-hydroxy-3-methylglutaryl coenzyme A reductase and low density lipoprotein receptor in rat liver

The mechanisms by which oxylanosterols regulate expression of hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase and lower serum cholesterol levels were examined by using a novel nonmetabolizable oxylanosterol mimic, 15-oxa-32-vinyl-lanost-8-ene-3 beta, 32 diol (DMP 565). This compound, unlike other nonmetabolizable oxylanosterols, is not a substrate for lanosterol 14 alpha-methyl demethylase. Feeding rats a diet supplemented with 0.02% DMP 565 markedly decreased HMG-CoA reductase immunoreactive protein and enzyme activity levels without affecting mRNA levels. The rate of reductase protein degradation was unaffected. However, the rate of translation was reduced to less than 20% of control. Thus, DMP 565 appears to regulate hepatic HMG-CoA reductase gene expression primarily at the level of translation. The pronounced inhibition of HMG-CoA reductase by DMP 565 resulted in a compensatory increase in the functioning of the hepatic low density lipoprotein (LDL) receptor, possibly by increased cycling, as evidenced by a marked increase in the rate of degradation of the LDL receptor. The half-life of the receptor was decreased from over 7 h to only 1 h in animals receiving DMP 565. This increase in the rate of degradation occurred without a change in the steady state level of the receptor. Addition of dietary cholesterol attenuated the increased turnover of the LDL receptor. These effects on the hepatic LDL receptor have also been observed with HMG-CoA reductase inhibitors (G. C. Ness et al., 1996, Arch. Biochem, Biophys. 325, 242-248). However, the effect of DMP 565 on the rate of degradation of the hepatic LDL receptor was of a greater magnitude when equal doses of the drugs were used. These regulatory actions of DMP 565 provide, in part, an explanation for the observed hypocholesterolemic action of this compound.

Review of progress in sterol oxidations: 1987-1995

Material dealing with the chemistry, biochemistry, and biological activities of oxysterols is reviewed for the period 1987-1995. Particular attention is paid to the presence of oxysterols in tissues and foods and to their physiological relevance.

Primary structure of the cytochrome P450 lanosterol 14 alpha-demethylase gene from Candida tropicalis

We report the nucleotide sequence of the gene and flanking DNA for the cytochrome P450 lanosterol 14 alpha-demethylase (14DM) from the yeast Candida tropicalis ATCC750. An open reading frame (ORF) of 528 codons encoding a 60.9-kD protein is identified. This ORF includes a characteristic heme-binding domain, HR2, common to all P450 proteins. This protein and the 14DM from Saccharomyces cerevisiae share 66.5% identical and 23.1% conservatively replaced amino acids in a 516-amino-acid alignment, and thus are orthologous forms of the P450LIA1 gene. Conversely, C. tropicalis 14DM shares relatively little sequence similarity with P450alk, the predominant P450 protein present when this organism is grown on n-alkanes. Sequence information of these three yeast P450s will be useful for structure-function analyses in the future.