The Antiobesity Effects of Rosehip (Rosa canina) Flesh by Antagonizing the PPAR Gamma Activity in High-Fat Diet-Fed Mice

SCOPE

The rosehip (Rosa canina) is a perennial shrub with a reddish pseudofruit that has demonstrated antidiabetic, antiatherosclerotic, and antiobesogenic effects in rodent models but there is low information about the molecular mechanisms underlying these effects on the onset and progression of diet-induced obesity.

METHODS AND RESULTS

Four-week-old C57BL/6J male mice are subjected to a high-fat diet (HFD)-supplemented or not with R. canina flesh for 18 weeks. The results indicated that the R. canina flesh exerts a preventive effect on HFD-induced obesity with a significant reduction in body-weight gain and an improvement of hyperglycemia and insulin resistance caused by a HFD. At the tissue level, subcutaneous white adipose tissue exhibits a higher number of smaller adipocytes, with decreased lipogenesis. On its side, the liver shows a significant decrease in lipid droplet content and in the expression of genes related to lipogenesis, fatty acid oxidation, and glucose metabolism. Finally, the data suggest that most of these effects agree with the presence of a putative Perosxisome proliferator-activated receptor gamma (PPARγ) antagonist in the R. canina flesh.

CONCLUSIONS

R. canina flesh dietary supplementation slows down the steatotic effect of a HFD at least in part through the regulation of the transcriptional activity of PPARγ.

FOXO1 represses PPARα-Mediated induction of FGF21 gene expression

Fibroblast growth factor 21 (FGF21) has emerged as a metabolic regulator that exerts potent anti-diabetic and lipid-lowering effects in animal models of obesity and type 2 diabetes, showing a protective role in fatty liver disease and hepatocellular carcinoma progression. Hepatic expression of FGF21 is regulated by PPARα and is induced by fasting. Ablation of FoxO1 in liver has been shown to increase FGF21 expression in hyperglycemia. To better understand the role of FOXO1 in the regulation of FGF21 expression we have modified HepG2 human hepatoma cells to overexpress FoxO1 and PPARα. Here we show that FoxO1 represses PPARα-mediated FGF21 induction, and that the repression acts on the FGF21 gene promoter without affecting other PPARα target genes. Additionally, we demonstrate that FoxO1 physically interacts with PPARα and that FoxO1/3/4 depletion in skeletal muscle contributes to increased Fgf21 tissue levels. Taken together, these data indicate that FOXO1 is a PPARα-interacting protein that antagonizes PPARα activity on the FGF21 promoter. Because other PPARα target genes remained unaffected, these results suggest a highly specific mechanism implicated in FGF21 regulation. We conclude that FGF21 can be specifically modulated by FOXO1 in a PPARα-dependent manner.

A Mixture of Pure, Isolated Polyphenols Worsens the Insulin Resistance and Induces Kidney and Liver Fibrosis Markers in Diet-Induced Obese Mice

Obesity is a worldwide epidemic with severe metabolic consequences. Polyphenols are secondary metabolites in plants and the most abundant dietary antioxidants, which possess a wide range of health effects. The most relevant food sources are fruit and vegetables, red wine, black and green tea, coffee, virgin olive oil, and chocolate, as well as nuts, seeds, herbs, and spices. The aim of this work was to evaluate the ability of a pure, isolated polyphenol supplementation to counteract the pernicious metabolic effects of a high-fat diet (HFD). Our results indicated that the administration of pure, isolated polyphenols under HFD conditions for 26 weeks worsened the glucose metabolism in diet-induced obese mice. The data showed that the main target organ for these undesirable effects were the kidneys, where we observed fibrotic, oxidative, and kidney-disease markers. This work led us to conclude that the administration of pure polyphenols as a food supplement would not be advisable. Instead, the ingestion of complete "whole" foods would be the best way to get the health effects of bioactive compounds such as polyphenols.

Lyophilized Maqui (Aristotelia chilensis) Berry Administration Suppresses High-Fat Diet-Induced Liver Lipogenesis through the Induction of the Nuclear Corepressor SMILE

The liver is one of the first organs affected by accumulated ectopic lipids. Increased de novo lipogenesis and excessive triglyceride accumulation in the liver are hallmarks of nonalcoholic fatty liver disease (NAFLD) and are strongly associated with obesity, insulin resistance, and type 2 diabetes. Maqui dietary supplemented diet-induced obese mice showed better insulin response and decreased weight gain. We previously described that these positive effects of maqui are partially due to an induction of a brown-like phenotype in subcutaneous white adipose tissue that correlated with a differential expression of Chrebp target genes. In this work, we aimed to deepen the molecular mechanisms underlying the impact of maqui on the onset and development of the obese phenotype and insulin resistance focusing on liver metabolism. Our results showed that maqui supplementation decreased hepatic steatosis caused by a high-fat diet. Changes in the metabolic profile include a downregulation of the lipogenic liver X receptor (LXR) target genes and of fatty acid oxidation gene expression together with an increase in the expression of small heterodimer partner interacting leucine zipper protein (Smile), a corepressor of the nuclear receptor family. Our data suggest that maqui supplementation regulates lipid handling in liver to counteract the metabolic impact of a high-fat diet.

Metabolic Impact of Flavonoids Consumption in Obesity: From Central to Peripheral

The prevention and treatment of obesity is primary based on the follow-up of a healthy lifestyle, which includes a healthy diet with an important presence of bioactive compounds such as polyphenols. For many years, the health benefits of polyphenols have been attributed to their anti-oxidant capacity as free radical scavengers. More recently it has been described that polyphenols activate other cell-signaling pathways that are not related to ROS production but rather involved in metabolic regulation. In this review, we have summarized the current knowledge in this field by focusing on the metabolic effects of flavonoids. Flavonoids are widely distributed in the plant kingdom where they are used for growing and defensing. They are structurally characterized by two benzene rings and a heterocyclic pyrone ring and based on the oxidation and saturation status of the heterocyclic ring flavonoids are grouped in seven different subclasses. The present work is focused on describing the molecular mechanisms underlying the metabolic impact of flavonoids in obesity and obesity-related diseases. We described the effects of each group of flavonoids in liver, white and brown adipose tissue and central nervous system and the metabolic and signaling pathways involved on them.

Mediterranean Tomato-Based Sofrito Sauce Improves Fibroblast Growth Factor 21 (FGF21) Signaling in White Adipose Tissue of Obese ZUCKER Rats

SCOPE

Obesity is a fibroblast growth factor 21 (FGF21)-resistant state. Since FGF21 production and signaling are regulated by some bioactive dietary compounds, we analyze the impact of Mediterranean tomato-based sofrito sauce on: (i) the FGF21 expression and signaling in visceral white adipose tissue (vWAT), and (ii) the insulin sensitivity of obese Zucker rats (OZR).

METHODS AND RESULTS

OZR are fed with a sofrito-supplemented diet or control diet. Insulin sensitivity and FGF21 signaling are determined. We observed that sofrito is able to improve the responsiveness to both hormones in obese rats. Sofrito-supplemented diet increases FGF21 signaling in vWAT by inducing the expression of the FGF receptors (FGFR1 and FGFR4) that promotes the expression of canonical target genes, like Egr-1, c-Fos and uncoupling protein 1 (Ucp1).

CONCLUSIONS

A sofrito-supplemented diet improves insulin and FGF21 sensitivity in OZR, explaining part of sofrito's healthy effects on glucose metabolism. In addition, induction of UCP1 and the unchanged body weight despite the hyperphagic behavior of the sofrito-fed rats suggests that the increase in FGF21 signaling correlates with an increase in energy expenditure (EE). Further studies in humans may help to understand whether sofrito consumption increases the EE in obese individuals.

Nutritional regulation of fibroblast growth factor 21: from macronutrients to bioactive dietary compounds

Obesity is a worldwide health problem mainly due to its associated comorbidities. Fibroblast growth factor 21 (FGF21) is a peptide hormone involved in metabolic homeostasis in healthy individuals and considered a promising therapeutic candidate for the treatment of obesity. FGF21 is predominantly produced by the liver but also by other tissues, such as white adipose tissue (WAT), brown adipose tissue (BAT), skeletal muscle, and pancreas in response to different stimuli such as cold and different nutritional challenges that include fasting, high-fat diets (HFDs), ketogenic diets, some amino acid-deficient diets, low protein diets, high carbohydrate diets or specific dietary bioactive compounds. Its target tissues are essentially WAT, BAT, skeletal muscle, heart and brain. The effects of FGF21 in extra hepatic tissues occur through the fibroblast growth factor receptor (FGFR)-1c together with the co-receptor β-klotho (KLB). Mechanistically, FGF21 interacts directly with the extracellular domain of the membrane bound cofactor KLB in the FGF21- KLB-FGFR complex to activate FGFR substrate 2α and ERK1/2 phosphorylation. Mice lacking KLB are resistant to both acute and chronic effects of FGF21. Moreover, the acute insulin sensitizing effects of FGF21 are also absent in mice with specific deletion of adipose KLB or FGFR1. Most of the data show that pharmacological administration of FGF21 has metabolic beneficial effects. The objective of this review is to compile existing information about the mechanisms that could allow the control of endogenous FGF21 levels in order to obtain the beneficial metabolic effects of FGF21 by inducing its production instead of doing it by pharmacological administration.

Regulation of human class I alcohol dehydrogenases by bile acids

Class I alcohol dehydrogenases (ADH1s) are the rate-limiting enzymes for ethanol and vitamin A (retinol) metabolism in the liver. Because previous studies have shown that human ADH1 enzymes may participate in bile acid metabolism, we investigated whether the bile acid-activated nuclear receptor farnesoid X receptor (FXR) regulates ADH1 genes. In human hepatocytes, both the endogenous FXR ligand chenodeoxycholic acid and synthetic FXR-specific agonist GW4064 increased ADH1 mRNA, protein, and activity. Moreover, overexpression of a constitutively active form of FXR induced ADH1A and ADH1B expression, whereas silencing of FXR abolished the effects of FXR agonists on ADH1 expression and activity. Transient transfection studies and electrophoretic mobility shift assays revealed functional FXR response elements in the ADH1A and ADH1B proximal promoters, thus indicating that both genes are direct targets of FXR. These findings provide the first evidence for direct connection of bile acid signaling and alcohol metabolism.

FGF21 mediates the lipid metabolism response to amino acid starvation

Lipogenic gene expression in liver is repressed in mice upon leucine deprivation. The hormone fibroblast growth factor 21 (FGF21), which is critical to the adaptive metabolic response to starvation, is also induced under amino acid deprivation. Upon leucine deprivation, we found that FGF21 is needed to repress expression of lipogenic genes in liver and white adipose tissue, and stimulate phosphorylation of hormone-sensitive lipase in white adipose tissue. The increased expression of Ucp1 in brown adipose tissue under these circumstances is also impaired in FGF21-deficient mice. Our results demonstrate the important role of FGF21 in the regulation of lipid metabolism during amino acid starvation.

Fsp27/CIDEC is a CREB target gene induced during early fasting in liver and regulated by FA oxidation rate

FSP27 [cell death-inducing DFFA-like effector c (CIDEC) in humans] is a protein associated with lipid droplets that downregulates the fatty acid oxidation (FAO) rate when it is overexpressed. However, little is known about its physiological role in liver. Here, we show that fasting regulates liver expression of Fsp27 in a time-dependent manner. Thus, during the initial stages of fasting, a maximal induction of 800-fold was achieved, whereas during the later phase of fasting, Fsp27 expression decreased. The early response to fasting can be explained by a canonical PKA-CREB-CRTC2 signaling pathway because: i) CIDEC expression was induced by forskolin, ii) Fsp27 promoter activity was increased by CREB, and iii) Fsp27 expression was upregulated in the liver of Sirt1 knockout animals. Interestingly, pharmacological (etomoxir) or genetic (Hmgcs2 interference) inhibition of the FAO rate increases the in vivo expression of Fsp27 during fasting. Similarly, CIDEC expression was upregulated in HepG2 cells by either etomoxir or HMGCS2 interference. Our data indicate that there is a kinetic mechanism of autoregulation between short- and long-term fasting, by which free FAs delivered to the liver during early fasting are accumulated/exported by FSP27/CIDEC, whereas over longer periods of fasting, they are degraded in the mitochondria through the carnitine palmitoyl transferase system.

PGC-1β regulates mouse carnitine-acylcarnitine translocase through estrogen-related receptor α

Carnitine/acylcarnitine translocase (CACT) is a mitochondrial-membrane carrier proteins that mediates the transport of acylcarnitines into the mitochondrial matrix for their oxidation by the mitochondrial fatty acid-oxidation pathway. CACT deficiency causes a variety of pathological conditions, such as hypoketotic hypoglycemia, cardiac arrest, hepatomegaly, hepatic dysfunction and muscle weakness, and it can be fatal in newborns and infants. Here we report that expression of the Cact gene is induced in mouse skeletal muscle after 24h of fasting. To gain insight into the control of Cact gene expression, we examine the transcriptional regulation of the mouse Cact gene. We show that the 5'-flanking region of this gene is transcriptionally active and contains a consensus sequence for the estrogen-related receptor (ERR), a member of the nuclear receptor family of transcription factors. This sequence binds ERRαin vivo and in vitro and is required for the activation of Cact expression by the peroxisome proliferator-activated receptor gamma coactivator (PGC)-1/ERR axis. We also demonstrate that XTC790, the inverse agonist of ERRα, specifically blocks Cact activation by PGC-1β in C2C12 cells.

Different fatty acid metabolism effects of (-)-epigallocatechin-3-gallate and C75 in adenocarcinoma lung cancer

Background

Fatty acid synthase (FASN) is overexpressed and hyperactivated in several human carcinomas, including lung cancer. We characterize and compare the anti-cancer effects of the FASN inhibitors C75 and (−)-epigallocatechin-3-gallate (EGCG) in a lung cancer model.

Methods

We evaluated in vitro the effects of C75 and EGCG on fatty acid metabolism (FASN and CPT enzymes), cellular proliferation, apoptosis and cell signaling (EGFR, ERK1/2, AKT and mTOR) in human A549 lung carcinoma cells. In vivo, we evaluated their anti-tumour activity and their effect on body weight in a mice model of human adenocarcinoma xenograft.

Results

C75 and EGCG had comparable effects in blocking FASN activity (96,9% and 89,3% of inhibition, respectively). In contrast, EGCG had either no significant effect in CPT activity, the rate-limiting enzyme of fatty acid β-oxidation, while C75 stimulated CPT up to 130%. Treating lung cancer cells with EGCG or C75 induced apoptosis and affected EGFR-signaling. While EGCG abolished p-EGFR, p-AKT, p-ERK1/2 and p-mTOR, C75 was less active in decreasing the levels of EGFR and p-AKT. In vivo, EGCG and C75 blocked the growth of lung cancer xenografts but C75 treatment, not EGCG, caused a marked animal weight loss.

Conclusions

In lung cancer, inhibition of FASN using EGCG can be achieved without parallel stimulation of fatty acid oxidation and this effect is related mainly to EGFR signaling pathway. EGCG reduce the growth of adenocarcinoma human lung cancer xenografts without inducing body weight loss. Taken together, EGCG may be a candidate for future pre-clinical development.

New synthetic inhibitors of fatty acid synthase with anticancer activity

Fatty acid synthase (FASN) is a lipogenic enzyme that is highly expressed in different human cancers. Here we report the development of a new series of polyphenolic compounds 5-30 that have been evaluated for their cytotoxic capacity in SK-Br3 cells, a human breast cancer cell line with high FASN expression. The compounds with an IC(50) < 50 μM have been tested for their ability to inhibit FASN activity. Among them, derivative 30 blocks the 90% of FASN activity at low concentration (4 μM), is highly cytotoxic in a broad panel of tumor cells, induces apoptosis, and blocks the activation of HER2, AKT, and ERK pathways. Remarkably, 30 does not activate carnitine palmitoyltransferase-1 (CPT-1) nor induces in mice weight loss, which are the main drawbacks of other previously described FASN inhibitors. Thus, FASN inhibitor 30 may aid the validation of this enzyme as a therapeutic target for the treatment of cancer.

Human HMGCS2 regulates mitochondrial fatty acid oxidation and FGF21 expression in HepG2 cell line

HMGCS2 (hydroxymethylglutaryl CoA synthase 2), the gene that regulates ketone body production, is barely expressed in cultured cell lines. In this study, we restored HMGCS2 expression and activity in HepG2 cells, thus showing that the wild type enzyme can induce fatty acid β-oxidation (FAO) and ketogenesis, whereas a catalytically inactive mutant C166A did not generate either process. Peroxisome proliferator-activated receptor (PPAR) α expression also induces fatty acid β-oxidation and endogenous HMGCS2 expression. Interestingly, PPARα-mediated induction was abolished when HMGCS2 expression was down-regulated by RNAi. These results indicate that HMGCS2 expression is both sufficient and necessary to the control of fatty acid oxidation in these cells. Next, we examined the expression pattern of several PPARα target genes in this now "ketogenic" HepG2 cell line. FGF21 (fibroblast growth factor 21) expression was specifically induced by HMGCS2 activity or by the inclusion of the oxidized form of ketone bodies (acetoacetate) in the culture medium. This effect was blunted by SirT1 (sirtuin 1) RNAi, so we propose a SirT1-dependent mechanism for FGF21 induction by acetoacetate. These data suggest a novel feed-forward mechanism by which HMGCS2 could regulate adaptive metabolic responses during fasting. This mechanism could be physiologically relevant, because fasting-mediated induction of liver FGF21 was dependent on SirT1 activity in vivo.

Novel Inhibitors of Fatty Acid Synthase with Anticancer Activity

PURPOSE: Fatty acid synthase (FASN) is overexpressed in human breast carcinoma. The natural polyphenol (-)-epigallocatechin-3-gallate blocks in vitro FASN activity and leads to apoptosis in breast cancer cells without any effects on carnitine palmitoyltransferase-1 (CPT-1) activity, and in vivo, does not decrease body weight. We synthesized a panel of new polyphenolic compounds and tested their effects on breast cancer models. EXPERIMENTAL DESIGN: We evaluated the in vitro effects of the compounds on breast cancer cell growth (SK-Br3, MCF-7, and MDA-MB-231), apoptosis [as assessed by cleavage of poly(ADP-ribose) polymerase], cell signaling (HER2, ERK1/2, and AKT), and fatty acid metabolism enzymes (FASN and CPT-1). In vivo, we have evaluated their antitumor activity and their effect on body weight in a mice model of BT474 breast cancer cells. RESULTS: Two compounds potently inhibited FASN activity and showed high cytotoxicity. Moreover, the compounds induced apoptosis and caused a marked decrease in the active forms of HER2, AKT, and ERK1/2 proteins. Interestingly, the compounds did not stimulate CPT-1 activity in vitro. We show evidence that one of the FASN inhibitors blocked the growth of BT474 breast cancer xenografts and did not induce weight loss in vivo. CONCLUSIONS: The synthesized polyphenolic compounds represent a novel class of FASN inhibitors, with in vitro and in vivo anticancer activity, that do not exhibit cross-activation of beta-oxidation and do not induce weight loss in animals. One of the compounds blocked the growth of breast cancer xenografts. These FASN inhibitors may represent new agents for breast cancer treatment. (Clin Cancer Res 2009;15(24):7608-15).

A characteristic Glu17 residue of pig carnitine palmitoyltransferase 1 is responsible for the low Km for carnitine and the low sensitivity to malonyl-CoA inhibition of the enzyme

Human carnitine palmitoyltransferase 1B (CPT1B) is a highly malonyl-CoA-sensitive enzyme (IC50=0.097 microm) and has a positive determinant (residues 18-28) of malonyl-CoA inhibition. By contrast, rat carnitine palmitoyltransferase 1A is less sensitive to malonyl-CoA inhibition (IC(50)=1.9 microm), and has both a positive (residues 1-18) and a negative (residues 18-28) determinant of its inhibition. Interestingly, pig CPT1B shows a low degree of malonyl-CoA sensitivity (IC(50)=0.804 microm). Here, we examined whether any additional molecular determinants affect malonyl-CoA inhibition of CPT1B. We show that the malonyl-CoA sensitivity of CPT1B is determined by the length (either 50 or 128 residues) of the N-terminal region constructed by recombining pig and human enzymes. We also show that the N-terminal region of pig CPT1B carries a single positive determinant of malonyl-CoA sensitivity, but that this is located between residues 1 and 18 of the N-terminal segment. Importantly, we found a single amino acid variation (D17E) relevant to malonyl-CoA sensitivity. Thus, Asp17 is specifically involved, under certain assay conditions, in the high malonyl-CoA sensitivity of the human enzyme, whereas the naturally occurring variation, Glu17, is responsible for both the low malonyl-CoA sensitivity and high carnitine affinity characteristics of the pig enzyme. This is the first demonstration that a single naturally occurring amino acid variation can alter CPT1B enzymatic properties.

Green tea catechin inhibits fatty acid synthase without stimulating carnitine palmitoyltransferase-1 or inducing weight loss in experimental animals

BACKGROUND

The enzyme fatty acid synthase (FASN) is highly expressed in many human carcinomas and its inhibition is cytotoxic to human cancer cells. The use of FASN inhibitors has been limited until now by anorexia and weight loss, which is associated with the stimulation of fatty acid oxidation.

MATERIALS AND METHODS

The in vitro effect of (-)-epigallocatechin-3-gallate (EGCG) on fatty acid metabolism enzymes, on apoptosis and on cell signalling was evaluated. In vivo, the effect of EGCG on animal body weight was addressed.

RESULTS

EGCG inhibited FASN activity, induced apoptosis and caused a marked decrease of human epidermal growth factor receptor 2 (HER2), phosphatidylinositol 3-kinase (PI3K)/AKT and extracellular (signal)-regulated kinase (ERK) 1/2 proteins, in breast cancer cells. EGCG did not induce a stimulatory effect on CPT-1 activity in vitro (84% of control), or on animal body weight in vivo (99% of control).

CONCLUSION

EGCG is a FASN inhibitor with anticancer activity which does not exhibit cross-activation of fatty acid oxidation and does not induce weight loss, suggesting its potential use as an anticancer drug.

Fatty acid metabolism in breast cancer cells: differential inhibitory effects of epigallocatechin gallate (EGCG) and C75

Endogenous fatty acid metabolism is crucial to maintain the cancer cell malignant phenotype. Lipogenesis is regulated by the enzyme fatty acid synthase (FASN); and breakdown of fatty acids is regulated by carnitine palmitoyltransferase-1 (CPT-I). FASN is highly expressed in breast cancer and most common human carcinomas. Several compounds can inhibit FASN, although the degree of specificity of this inhibition has not been addressed. We have tested the effects of C75 and (-)-epigallocatechin-3-gallate (EGCG) on fatty acid metabolism pathways, cellular proliferation, induction of apoptosis and cell signalling in human breast cancer cells. Our results show that C75 and EGCG had comparable effects in blocking FASN activity. Treating cancer cells with EGCG or C75 induced apoptosis and caused a decrease in the active forms of oncoprotein HER2, AKT and ERK1/2 to a similar degree. We observed, in contrast, marked differential effects between C75 and EGCG on the fatty acid oxidation pathway. While EGCG had either no effect or a moderate reduction in CPT-I activity, C75 stimulated CPT-I activity (up to 129%), even in presence of inhibitory levels of malonyl-CoA, a potent inhibitor of the CPT-I enzyme. Taken together, these findings indicate that pharmacological inhibition of FASN occurs uncoupled from the stimulation of CPT-I with EGCG but not with C75, suggesting that EGCG might be free of the CPT-I related in vivo weight-loss that has been associated with C75. Our results establish EGCG as a potent and specific inhibitor of fatty acid synthesis (FASN), which may hold promise as a target-directed anti-cancer drug.

KetogenicHMGCS2Is a c-Myc Target Gene Expressed in Differentiated Cells of Human Colonic Epithelium and Down-Regulated in Colon Cancer

HMGCS2, the gene that regulates ketone body production, is expressed in liver and several extrahepatic tissues, such as the colon. In CaCo-2 colonic epithelial cells, the expression of this gene increases with cell differentiation. Accordingly, immunohistochemistry with specific antibodies shows that HMGCS2 is expressed mainly in differentiated cells of human colonic epithelium. Here, we used a chromatin immunoprecipitation assay to study the molecular mechanism responsible for this expression pattern. The assay revealed that HMGCS2 is a direct target of c-Myc, which represses HMGCS2 transcriptional activity. c-Myc transrepression is mediated by blockade of the transactivating activity of Miz-1, which occurs mainly through a Sp1-binding site in the proximal promoter of the gene. Accordingly, the expression of human HMGCS2 is down-regulated in 90% of Myc-dependent colon and rectum tumors. HMGCS2 protein expression is down-regulated preferentially in moderately and poorly differentiated carcinomas. In addition, it is also down-regulated in 80% of small intestine Myc-independent tumors. Based on these findings, we propose that ketogenesis is an undesirable metabolic characteristic of the proliferating cell, which is down-regulated through c-Myc-mediated repression of the key metabolic gene HMGCS2.

An Intronic Peroxisome Proliferator-activated Receptor-binding sequence Mediates Fatty Acid Induction of the Human Carnitine Palmitoyltransferase 1A

The liver plays a central role in the response to fasting. The hormonal profile in this condition, low insulin, and high concentrations of glucagon in plasma, induce the release of large amounts of fatty acids from adipose tissue. Prolonged starvation can therefore induce a dramatic change in the fatty acid oxidative capacity of liver metabolism. Modulation of gene expression by PPARalpha plays a crucial role in this response. While a major role for PPARalpha in the liver is to produce ketone bodies as fuel through beta-oxidation for peripheral tissues during fast, its participation in the control of CPT1A, the rate-limiting step of the pathway, remains controversial. Using Web-based software (VISTA) combining transcription factor binding site database searches with comparative sequence analyses, we have localized a conserved functional PPAR responsive element downstream of the transcriptional start site of the human CPT1A gene. We have shown that this sequence is fundamental for fatty acids or PGC1-induced transcriptional activation of the CPT1A gene. These results corroborate the hypothesis that PPARalpha regulates the limiting step in the oxidation of fatty acids in liver mitochondria.

C75 activates malonyl-CoA sensitive and insensitive components of the CPT system

Carnitine palmitoyltransferase I (CPT-I) and II (CPT-II) enzymes are components of the carnitine palmitoyltransferase shuttle system which allows entry of long-chain fatty acids into the mitochondrial matrix for subsequent oxidation. This system is tightly regulated by malonyl-CoA levels since this metabolite is a strong reversible inhibitor of the CPT-I enzyme. There are two distinct CPT-I isotypes (CPT-Ialpha and CPT-Ibeta), that exhibit different sensitivity to malonyl-CoA inhibition. Because of its ability to inhibit fatty acid synthase, C75 is able to increase malonyl-CoA intracellular levels. Paradoxically it also activates long-chain fatty acid oxidation. To identify the exact target of C75 within the CPT system, we expressed individually the different components of the system in the yeast Pichia pastoris. We show here that C75 acts on recombinant CPT-Ialpha, but also on the other CPT-I isotype (CPT-Ibeta) and the malonyl-CoA insensitive component of the CPT system, CPT-II.

Pig muscle carnitine palmitoyltransferase I (CPTI beta), with low Km for carnitine and low sensitivity to malonyl-CoA inhibition, has kinetic characteristics similar to those of the rat liver (CPTI alpha) enzyme

The outer mitochondrial membrane enzyme carnitine palmitoyltransferase I (CPTI) catalyzes the initial and regulatory step in the beta-oxidation of long-chain fatty acids. There are two well-characterized isotypes of CPTI: CPTIalpha (also known as L-CPTI) and CPTIbeta (also known as M-CPTI) that in human and rat encode for enzymes with very different kinetic properties and sensitivity to malonyl-CoA inhibition. Kinetic hallmarks of the CPTIalpha are high affinity for carnitine and low sensitivity to malonyl-CoA inhibition, while the opposite characteristics, low affinity for carnitine and high sensitivity to malonyl-CoA, are intrinsic to the CPTIbeta isotype. We have isolated the pig CPTIbeta cDNA which encodes for a protein of 772 amino acids that shares extensive sequence identity with human (88%), rat (85%), and mouse (86%) CPTIbeta, while the degree of homology with the CPTIalpha from human (61%), rat (62%), and mouse (60%) is much lower. However, when expressed in the yeast Pichia pastoris, pig CPTIbeta shows kinetic characteristics similar to those of the CPTIalpha isotype. Thus, the pig CPTIbeta, unlike the corresponding human or rat enzyme, has a high affinity for carnitine (K(m) = 197 microM) and low sensitive to malonyl-CoA inhibition (IC(50) = 906 nM). Therefore, the recombinant pig CPTIbeta has unique kinetic characteristics, which makes it a useful model to study the structure-function relationship of the CPTI enzymes.

Functional interaction between peroxisome proliferator-activated receptors-alpha and Mef-2C on human carnitine palmitoyltransferase 1beta (CPT1beta) gene activation

Muscle-type carnitine palmitoyltransferase 1 (CPT1beta) is considered to be the gene that controls fatty acid mitochondrial beta-oxidation. A functional peroxisome proliferator-activated receptor (PPAR) responsive element (PPRE) and a myocite-specific (MEF2) site that binds MEF2A and MEF2C in the promoter of this gene had been previously identified. We investigated the roles of the PPRE and the MEF2 binding sites and the potential interaction between PPARalpha and MEF2C regulating the CPT1beta gene promoter. Mutation analysis indicated that the MEF2 site contributed to the activation of the CPT1beta promoter by PPAR in C2C12 cells. The reporter construct containing the PPRE and the MEF2C site was synergistically activated by co-expression of PPAR, retinoid X receptor (RXR) and MEF2C in non-muscle cells. Moreover, protein-binding assays demonstrated that MEF2C and PPAR specifically bound to one another in vitro. Also for the synergistic activation of the CPT1beta gene promoter by MEF2C and PPARalpha-RXRalpha, a precise arrangement of its binding sites was essential.

A single amino acid change (substitution of the conserved Glu-590 with alanine) in the C-terminal domain of rat liver carnitine palmitoyltransferase I increases its malonyl-CoA sensitivity close to that observed with the muscle isoform of the enzyme

Carnitine palmitoyltransferase I (CPTI) catalyzes the conversion of long-chain fatty acyl-CoAs to acylcarnitines in the presence of l-carnitine. To determine the role of the highly conserved C-terminal glutamate residue, Glu-590, on catalysis and malonyl-CoA sensitivity, we separately changed the residue to alanine, lysine, glutamine, and aspartate. Substitution of Glu-590 with aspartate, a negatively charged amino acid with only one methyl group less than the glutamate residue in the wild-type enzyme, resulted in complete loss in the activity of the liver isoform of CPTI (L-CPTI). A change of Glu-590 to alanine, glutamine, and lysine caused a significant 9- to 16-fold increase in malonyl-CoA sensitivity but only a partial decrease in catalytic activity. Substitution of Glu-590 with neutral uncharged residues (alanine and glutamine) and/or a basic positively charged residue (lysine) significantly increased L-CPTI malonyl-CoA sensitivity to the level observed with the muscle isoform of the enzyme, suggesting the importance of neutral and/or positive charges in the switch of the kinetic properties of L-CPTI to the muscle isoform of CPTI. Since a conservative substitution of Glu-590 to aspartate but not glutamine resulted in complete loss in activity, we suggest that the longer side chain of glutamate is essential for catalysis and malonyl-CoA sensitivity. This is the first demonstration whereby a single residue mutation in the C-terminal region of the liver isoform of CPTI resulted in a change of its kinetic properties close to that observed with the muscle isoform of the enzyme and provides the rationale for the high malonyl-CoA sensitivity of muscle CPTI compared with the liver isoform of the enzyme.

Histone deacetylase inhibitors stimulate mitochondrial HMG-CoA synthase gene expression via a promoter proximal Sp1 site

The expression of mitochondrial HMG-CoA synthase in the colon has been correlated with the levels of butyrate present in this tissue. We report here that the effect of butyrate on mitochondrial HMG-CoA synthase gene expression is exerted in vivo at the transcriptional level, and that trichostatin A (TSA), a specific histone deacetylase inhibitor, also induces transcriptional activity and mRNA expression of the gene in human cell lines derived from colon carcinoma. Using chromatin immunoprecipitation assays, we show that histone deacetylase 1 (HDAC1) is associated with the endogenous mitochondrial HMG-CoA synthase promoter and that TSA induction correlates with hyperacetylation of H4 histone associated with the 5' flanking region of the gene. Overexpression of HDAC1 activity leads consistently to mitochondrial HMG-CoA synthase promoter hypoacetylation and reduces its transcriptional activity. The effect of butyrate and TSA maps to a single Sp1 site present in the proximal promoter of the gene, which is able to bind Sp1 and Sp3 proteins. Interestingly, the binding affinity of Sp1 and Sp3 proteins to the Sp1 site correlates with the TSA responsiveness of the promoter. Using a one-hybrid system (GAL4-Sp1 and GAL4-Sp3), we show that both proteins can mediate responsiveness to TSA in CaCo-2 cells employing distinct mechanisms.

Control of human carnitine palmitoyltransferase II gene transcription by peroxisome proliferator-activated receptor through a partially conserved peroxisome proliferator-responsive element

The expression of several genes involved in fatty acid metabolism is regulated by peroxisome proliferator-activated receptors (PPARs). To gain more insight into the control of carnitine palmitoyltransferase (CPT) gene expression, we examined the transcriptional regulation of the human CPT II gene. We show that the 5'-flanking region of this gene is transcriptionally active and binds PPARalpha in vivo in a chromatin immunoprecipitation assay. In addition, we characterized the peroxisome proliferator-responsive element (PPRE) in the proximal promoter of the CPT II gene, which appears to be a novel PPRE. The sequence of this PPRE contains one half-site which is a perfect consensus sequence (TGACCT) but no clearly recognizable second half-site (CAGCAC); this part of the sequence contains only one match to the consensus, which seems to be irrelevant for the binding of PPARalpha. As expected, other members of the nuclear receptor superfamily also bind to this element and repress the activation mediated by PPARalpha, thus showing that the interplay between several nuclear receptors may regulate the entry of fatty acids into the mitochondria, a crucial step in their metabolism.

Down-regulation of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene by insulin: the role of the forkhead transcription factor FKHRL1

Normal physiological responses to carbohydrate shortages cause the liver to increase the production of ketone bodies from the acetyl-CoA generated from fatty acid oxidation. This allows the use of ketone bodies for energy, thereby preserving the limited glucose for use by the brain. This adaptative response is switched off by insulin rapidly inhibiting the expression of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase (HMGCS2) gene, which is a key control site of ketogenesis. We decided to investigate the molecular mechanism of this inhibition. In the present study, we show that FKHRL1, a member of the forkhead in rhabdosarcoma (FKHR) subclass of the Fox family of transcription factors, stimulates transcription from transfected 3-hydroxy-3-methylglutaryl-CoA synthase promoter-luciferase reporter constructs, and that this stimulation is repressed by insulin. An FKHRL1-responsive sequence AAAAATA, located 211 bp upstream of the HMGCS2 gene transcription start site, was identified by deletion analysis. It binds FKHRL1 in vivo and in vitro and confers FKHRL1 responsiveness on homologous and heterologous promoters. If it is mutated, it partially blocks the effect of insulin in HepG2 cells, both in the absence and presence of overexpressed FKHRL1. These results suggest that FKHRL1 contributes to the regulation of HMGCS2 gene expression by insulin.

Pig liver carnitine palmitoyltransferase. Chimera studies show that both the N- and C-terminal regions of the enzyme are important for the unusual high malonyl-CoA sensitivity

Pig and rat liver carnitine palmitoyltransferase I (L-CPTI) share common K(m) values for palmitoyl-CoA and carnitine. However, they differ widely in their sensitivity to malonyl-CoA inhibition. Thus, pig l-CPTI has an IC(50) for malonyl-CoA of 141 nm, while that of rat L-CPTI is 2 microm. Using chimeras between rat L-CPTI and pig L-CPTI, we show that the entire C-terminal region behaves as a single domain, which dictates the overall malonyl-CoA sensitivity of this enzyme. The degree of malonyl-CoA sensitivity is determined by the structure adopted by this domain. Using deletion mutation analysis, we show that malonyl-CoA sensitivity also depends on the interaction of this single domain with the first 18 N-terminal amino acid residues. We conclude that pig and rat L-CPTI have different malonyl-CoA sensitivity, because the first 18 N-terminal amino acid residues interact differently with the C-terminal domain. This is the first study that describes how interactions between the C- and N-terminal regions can determine the malonyl-CoA sensitivity of L-CPTI enzymes using active C-terminal chimeras.

3-Hydroxy-3-methylglutaryl coenzyme A synthase-1 of Blattella germanica has structural and functional features of an active retrogene

Blattella germanica has two cytosolic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase genes, HMG-CoA synthase-1 and -2. HMG-CoA synthase-1 gene shows several features of processed genes (retroposons): it contains no introns but has a short direct-repeat sequence (ATTATTATT) at both ends. An atypical feature is the presence at both ends of the gene of short inverse repeats flanked by direct repeats. There is neither a TATA box nor a CAAT box in the 5' region. Comparative analysis with other species suggests that the HMG-CoA synthase-1 gene derives from HMG-CoA synthase-2. Cultured embryonic B. germanica UM-BGE-1 cells express HMG-CoA synthase-1 but not HMG-CoA synthase-2, suggesting that the intron-less gene is functional. In addition, it can complement MEV-1 cell line, which is auxotrophic for mevalonate. We show that compactin and mevalonate do not significantly affect the mRNA levels of HMG-CoA synthase-1 in UM-BGE-1 cells. Compactin induces a 6.7-fold increase in HMG-CoA reductase activity, which is restored to normal levels by mevalonate. HMG-CoA synthase activity is not modified by either of these effectors, suggesting that the mevalonate pathway in this insect cell line is regulated by post-transcriptional mechanisms affecting HMG-CoA reductase but not HMG-CoA synthase.

Pig liver carnitine palmitoyltransferase I, with low Km for carnitine and high sensitivity to malonyl-CoA inhibition, is a natural chimera of rat liver and muscle enzymes

The outer mitochondrial membrane enzyme carnitine palmitoyltransferase I (CPTI) catalyzes the initial and regulatory step in the beta-oxidation of fatty acids. The genes for the two isoforms of CPTI-liver (L-CPTI) and muscle (M-CPTI) have been cloned and expressed, and the genes encode for enzymes with very different kinetic properties and sensitivity to malonyl-CoA inhibition. Pig L-CPTI encodes for a 772 amino acid protein that shares 86 and 62% identity, respectively, with rat L- and M-CPTI. When expressed in Pichia pastoris, the pig L-CPTI enzyme shows kinetic characteristics (carnitine, K(m) = 126 microM; palmitoyl-CoA, K(m) = 35 microM) similar to human or rat L-CPTI. However, the pig enzyme, unlike the rat liver enzyme, shows a much higher sensitivity to malonyl-CoA inhibition (IC(50) = 141 nM) that is characteristic of human or rat M-CPTI enzymes. Therefore, pig L-CPTI behaves like a natural chimera of the L- and M-CPTI isotypes, which makes it a useful model to study the structure--function relationships of the CPTI enzymes.

Low Activity of Mitochondrial HMG-CoA Synthase in Liver of Starved Piglets Is Due to Low Levels of Protein Despite High mRNA Levels

The unusually low hepatic ketogenic capacity of piglets has been correlated with lack of expression of the mitochondrial HMG-CoA synthase gene. However, we have shown that starvation of 2-week-old piglets increased the mRNA levels of mitochondrial HMG-CoA synthase to a level similar to that observed in starved rats (S. H. Adams, C. S. Alho, G. Asins, F. G. Hegardt, and P. F. Marrero, 1997, Biochem. J. 324, 65-73). We now report that antibodies against pig mitochondrial HMG-CoA synthase detected the pig enzyme in mitochondria of 2-week-old starved piglets and that the pig mitochondrial HMG-CoA synthase cDNA encodes an active enzyme in the eukaryotic cell line Mev-1, with catalytic behavior similar to that of the rat enzyme when expressed in the same system. We also show that low activity of pig mitochondrial HMG-CoA synthase correlates with low expression of the pig enzyme. The discrepancy in mitochondrial HMG-CoA synthase gene expression between the high levels of mRNA and low levels of enzyme was not associated with differences in transcript maturation, which suggests that an attenuated translation of the pig mRNA is responsible for the diminished ketogenic capacity of pig mitochondria.

Regulation of the rat liver carnitine palmitoyltransferase I gene transcription by thyroid hormone

L-CPT I isotype is the main locus of control for liver LCFA oxidation. T3 levels have been described as controlling L-CPT I gene expression, and in this paper we demonstrate that rat liver CPT I promoter responds to T3. Using deleted reporter constructs we located the thyroid hormone-responsive element between -2935 and -2918, consisting of a DR4. This response is mediated by the binding of the thyroid to this sequence as a monomer, homodimer, or heterodimer with RXR.

Contribution of steroidogenic factor 1 to the regulation of cholesterol synthesis

Steroidogenic factor 1 (SF-1) is an orphan member of the nuclear receptor family expressed in steroidogenic tissues, where it has an essential role in the regulation of the steroid hormone biosynthesis, adrenal and gonadal development and endocrine responses fundamental for reproduction. Here we show that SF-1 regulates the transcription of cytosolic 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene, which is essential for the endogenous synthesis of cholesterol. We have identified an element located 365 bp upstream of the gene for cytosolic HMG-CoA synthase; SF-1 binds as a monomer to this element and confers SF-1 responsiveness to homologous and heterologous promoters. It has been shown that in tissues with a high demand for cholesterol to be used in steroid synthesis, there is a lack of correlation between the cholesterol levels and the activity of the limiting enzymes of the mevalonate pathway. In accord with those results, we observed that cholesterol synthesis from acetate and either cytosolic HMG-CoA mRNA expression or transcriptional activity were not changed in response to 25-hydroxycholesterol in the SF-1-expressing steroidogenic Leydig tumour MA-10 cells. Moreover, the overexpression of SF-1 in non-steroidogenic CV-1 cells renders them less sensitive to the regulatory effects of cholesterol. This observation led to the hypothesis that in steroidogenic tissues the expression of SF-1 permits high levels of endogenous synthesis of cholesterol irrespective of the intracellular levels of this metabolite.

Isolation of pig mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase gene promoter: characterization of a peroxisome proliferator-responsive element

Low expression of the mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase gene during development correlates with an unusually low hepatic ketogenic capacity and lack of hyperketonaemia in piglets. Here we report the isolation and characterization of the 5' end of the pig mitochondrial HMG-CoA synthase gene. The 581 bp region proximal to the transcription start site permits transcription of a reporter gene, confirming the function of the promoter. The pig mitochondrial HMG-CoA synthase promoter is trans-activated by the peroxisomal proliferator-activated receptor (PPAR), and a functional response element for PPAR (PPRE) has been localized in the promoter region. Pig PPRE is constituted by an imperfect direct repeat (DR-1) and a downstream sequence, both of which are needed to confer PPAR-sensitivity to a thymidine kinase promoter and to form complexes with PPAR.retinoid X receptor heterodimers. A role of PPAR trans-activation in starvation-associated induction of gene expression is suggested.

Control of human muscle-type carnitine palmitoyltransferase I gene transcription by peroxisome proliferator-activated receptor

The expression of several genes involved in intra- and extracellular lipid metabolism, notably those involved in peroxisomal and mitochondrial beta-oxidation, is mediated by ligand-activated receptors, collectively referred to as peroxisome proliferator-activated receptors (PPARs). To gain more insight into the control of expression of carnitine palmitoyltransferase (CPT) genes, which are regulated by fatty acids, we have examined the transcriptional regulation of the human MCPT I gene. We have cloned by polymerase chain reaction the 5'-flanking region of this gene and demonstrated its transcriptional activity by transfection experiments with the CAT gene as a reporter. We have also shown that this is a target gene for the action of PPARs, and we have localized a PPAR responsive element upstream of the first exon. These results show that PPAR regulates the entry of fatty acids into the mitochondria, which is a crucial step in their metabolism, especially in tissues like heart, skeletal muscle and brown adipose tissue in which fatty acids are a major source of energy.

Catalytic properties of recombinant 3-hydroxy-3-methylglutaryl coenzyme A synthase-1 from Blattella germanica

Blattella germanica is the first organism in which two cytosolic HMG-CoA synthase genes have been described: HMGS-1 (Martínez-González et al., 1993b) and HMGS-2 (Buesa et al., 1994). The HMGS-1 gene showed special features, which led us to characterize the kinetic properties of the enzyme it encodes. Here we report the expression of recombinant HMGS-1, the protocol of enzyme purification, and the measurement of kinetic parameters. The K(m) for acetyl-CoA is 15.2 microM and the Ki for the other substrate, acetoacetyl-CoA, is 1.26 microM, both similar to that of yeast, ox, and chicken liver enzymes; the Vmax of HMGS-1 measured in this paper is 66 mU, which is the lowest Vmax of the HMG-CoA synthases reported to date.

Gene expression of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase in a poorly ketogenic mammal: effect of starvation during the neonatal period of the piglet

The low ketogenic capacity of pigs correlates with a low activity of mitochondrial 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase. To identify the molecular mechanism controlling such activity, we isolated the pig cDNA encoding this enzyme and analysed changes in mRNA levels and mitochondrial specific activity induced during development and starvation. Pig mitochondrial synthase showed a tissue-specific expression pattern. As with rat and human, the gene is expressed in liver and large intestine; however, the pig differs in that mRNA was not detected in testis, kidney or small intestine. During development, pig mitochondrial HMG-CoA synthase gene expression showed interesting differences from that in the rat: (1) there was a 2-3 week lag in the postnatal induction; (2) the mRNA levels remained relatively abundant through the suckling-weaning transition and at maturity, in contrast with the fall observed in rats at similar stages of development; and (3) the gene expression was highly induced by fasting during the suckling, whereas no such change in mitochondrial HMG-CoA synthase mRNA levels has been observed in rat. The enzyme activity of mitochondrial HMG-CoA synthase increased 27-fold during starvation in piglets, but remained one order of magnitude lower than rats. These results indicate that post-transcriptional mechanism(s) and/or intrinsic differences in the encoded enzyme are responsible for the low activity of pig HMG-CoA synthase observed throughout development or after fasting.

Coordinated expression and activity of 3-hydroxy-3-methylglutaryl coenzyme A synthase and reductase in the fat body of Blattella germanica (L.) during vitellogenesis

Levels of mRNA for the two 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthases, (HMG-S1 and HMG-S2), and for HMG-CoA reductase (HMG-R) of Blattella germanica were analyzed in the fat body during the first gonadotrophic cycle. HMG-S2 and HMG-R showed the highest mRNA levels on day 0 and decreased thereafter, whereas HMG-S1, showed faint expression. Western blot using specific antibodies for HMG-S1 and HMG-S2 showed no detectable levels for HMG-S1 but a clear pattern for HMG-S2. Both results point to a very limited role for HMG-CoA synthase-1 in B. germanica fat body that the functional enzyme in this organ is HMG-CoA synthase-2. HMG-CoA reductase and synthase proteins shared a cyclic pattern (maximum levels at day 4 and minimum levels on days 0 and 8), which was coincident with the pattern of activity. The delay between gene transcription and protein synthesis suggests a finely regulated translation mechanism. Moreover, the pattern of mevalonate synthesis parallels that of vitellogenin production, suggesting a coordinate mechanism between the mevalonate pathway and the production of vitellogenin.

Effects of site-directed mutagenesis of the highly conserved aspartate residues in domain II of farnesyl diphosphate synthase activity

Comparison of the farnesyl diphosphate (FPP) synthase amino acid sequences from four species with amino acid sequences from the related enzymes hexaprenyl diphosphate synthase and geranylgeranyl diphosphate synthase show the presence of two aspartate rich highly conserved domains. The aspartate motif ((I, L, or V)XDDXXD) of the second of those domains has homology with at least 9 prenyl transfer enzymes that utilize an allylic prenyl diphosphate as one substrate. In order to investigate the role of this second aspartate-rich domain in rat FPP synthase, we mutated the first or third aspartate to glutamate, expressed the wild-type and mutant enzymes in Escherichia coli, and purified them to apparent homogeneity using a single chromatographic step. Approximately 12 mg of homogeneous protein was isolated from 120 mg of crude bacterial extract. The kinetic parameters of the purified wild-type recombinant FPP synthase containing the DDYLD motif were as follows: Vmax = 0.84 mumol/min/mg; GPP Km = 1.0 microM; isopentenyl diphosphate (IPP) Km = 2.7 microM. Substitution of glutamate for the first aspartate (EDYLD) decreased the Vmax by over 90-fold. The Km for IPP increased, whereas the Km for GPP remained the same in this D243E mutant. Substitution of glutamate for the third aspartate (DDYLE) did not result in altered enzyme kinetics in the D247E mutant. These results suggest that the first aspartate in the second domain is involved in the catalysis by FPP synthase.

Rat mitochondrial and cytosolic 3-hydroxy-3-methylglutaryl-CoA synthases are encoded by two different genes

We report the isolation and characterization of a 1994-base-pair cDNA that encompasses the entire transcription unit of the mitochondrial 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) synthase (EC 4.1.3.5.) gene from rat. Analysis of the nucleotide sequence reveals that the cDNA encodes a polypeptide of 508 residues and 56,918-Da molecular mass. Identify of the cDNA clone isolated as mitochondrial HMG-CoA synthase was confirmed by the following criteria: (i) Amino acid residues are 65% homologous with hamster cytosolic HMG-CoA synthase. (ii) A 19-amino acid sequence probably corresponding to the catalytic site is highly homologous (90%) to that reported for chicken liver mitochondrial HMG-CoA synthase. (iii) The expression product of the cDNA in Escherichia coli has HMG-CoA synthase activity. (iv) The protein includes a sequence of 37 amino acid residues at the N terminus that is not present in the cytosolic enzyme. The predominantly basic, hydrophobic, and hydroxylated nature of the residues of this sequence suggests that it is a leader peptide to target HMG-CoA synthase inside mitochondria. These data plus the hybridization pattern in genomic Southern blot analysis, the different transcript size (2.0 kilobases versus 3.4 kilobases for the cytosolic enzyme), and the different expression pattern shown in RNA blot experiments suggest the presence of two HMG-CoA synthase genes, one for the cytosolic and another for the mitochondrial enzyme.

Identification of a cholesterol-regulated 180-kDa microsomal protein in rat hepatocytes

The immunoprecipitation by antibodies to 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase of extracts of [35S]methionine-pulse-labelled isolated hepatocytes, followed by electrophoresis and fluorography, showed the presence not only of 97-kDa HMG-CoA reductase, but also of another protein of 180 kDa. Boiling the immunoprecipitates both in the presence and in the absence of 2-mercaptoethanol, followed by SDS/polyacrylamide gel electrophoresis both in the presence and in the absence of 8 M urea, was not found to change the ratio of 180-kDa/97-kDa proteins. These facts suggest that the 180-kDa protein is not an aggregated form of HMG-CoA reductase. A different batch of antibodies obtained from a newly purified HMG-CoA reductase fully titrated the reductase activity, but did not immunoprecipitate the 180-kDa protein, showing that there is no cross-reactivity between these proteins. The 180-kDa polypeptide is a glycoprotein of N-linked high-mannose oligosaccharide chains, which is not processed on the Golgi system. The apparent molecular mass of the carbohydrate is 16 kDa. The incubation of rat hepatocytes with sterols produces, on the one hand, a decrease in the rate of synthesis, and on the other hand, an acceleration in the turnover rate of the 180-kDa protein. In addition, mevalonate is known to decrease its rate of synthesis. The carbohydrate-free 164-kDa protein was found to degrade only a tenth as fast as the glycoprotein and, furthermore, the degradation was no longer accelerated by sterols. These results support the notion that the 180-kDa protein is not a modified form of 97-kDa reductase, but probably a different protein related to cholesterol metabolism, and also that the N-linked, high-mannose chains, which are bound to the glycoprotein, are required for rapid and controlled degradation of the protein.

Phosphorylation of HMG-CoA reductase induced by mevalonate accelerates its rate of degradation in isolated rat hepatocytes

Incubation of rat hepatocytes with 10 mM mevalonate produces a decrease in HMG-CoA reductase activity and in the rate of synthesis of both monomeric and dimeric HMG-CoA reductase, and an increase in the rate of degradation of the monomeric form without significant change in that of the dimeric form. Since mevalonate promotes a short-term phosphorylation of the monomeric form without affecting the dimeric form, it is suggested that the mechanism of degradation of reductase is controlled by its phosphorylation state.

Rat liver HMG CoA reductase, a glycoprotein of the endoplasmic reticulum, is in equilibrium between monomeric and dimeric forms

Incubation of rat hepatocytes with [35S]methionine in pulse and pulse-chase experiments followed by immunoprecipitation of the HMG CoA reductase and SDS-PAGE results in two labelled polypeptides of 104 and 180 Kdaltons. These two polypeptides have half lives of 80 and 46 minutes respectively. When hepatocytes are incubated with mevalonolactone, and a pulse of [35S]methionine is given, the rate of synthesis of both the 180 and 104 Kd peptides is strongly diminished. After treatment of the [35S] labelled immunoprecipitates with endoglycosidase H, the 180 Kd reductase splits into two labelled peptides of 110 and 97 Kd. We suggest that in addition to the 104 Kd reductase, the endoplasmic reticulum contains the dimer of two reductases linked by a carbohydrate chain. The equilibrium monomer-dimer probably regulates the rate of degradation of reductase.