(-)-UB006: A new fatty acid synthase inhibitor and cytotoxic agent without anorexic side effects

C75 is a synthetic anticancer drug that inhibits fatty acid synthase (FAS) and shows a potent anorexigenic side effect. In order to find new cytotoxic compounds that do not impact food intake, we synthesized a new family of C75 derivatives. The most promising anticancer compound among them was UB006 ((4SR,5SR)-4-(hydroxymethyl)-3-methylene-5-octyldihydrofuran-2(3H)-one). The effects of this compound on cytotoxicity, food intake and body weight were studied in UB006 racemic mixture and in both its enantiomers separately. The results showed that both enantiomers inhibit FAS activity and have potent cytotoxic effects in several tumour cell lines, such as the ovarian cell cancer line OVCAR-3. The (-)-UB006 enantiomer's cytotoxic effect on OVCAR-3 was 40-fold higher than that of racemic C75, and 2- and 38-fold higher than that of the racemic mixture and its opposite enantiomer, respectively. This cytotoxic effect on the OVCAR-3 cell line involves mechanisms that reduce mitochondrial respiratory capacity and ATP production, DDIT4/REDD1 upregulation, mTOR activity inhibition, and caspase-3 activation, resulting in apoptosis. In addition, central and peripheral administration of (+)-UB006 or (-)-UB006 into rats and mice did not affect food intake or body weight. Altogether, our data support the discovery of a new potential anticancer compound (-)-UB006 that has no anorexigenic side effects.

Regulation of substrate oxidation preferences in muscle by the peptide hormone adropin

Rigorous control of substrate oxidation by humoral factors is essential for maintaining metabolic homeostasis. During feeding and fasting cycles, carbohydrates and fatty acids are the two primary substrates in oxidative metabolism. Here, we report a novel role for the peptide hormone adropin in regulating substrate oxidation preferences. Plasma levels of adropin increase with feeding and decrease upon fasting. A comparison of whole-body substrate preference and skeletal muscle substrate oxidation in adropin knockout and transgenic mice suggests adropin promotes carbohydrate oxidation over fat oxidation. In muscle, adropin activates pyruvate dehydrogenase (PDH), which is rate limiting for glucose oxidation and suppresses carnitine palmitoyltransferase-1B (CPT-1B), a key enzyme in fatty acid oxidation. Adropin downregulates PDH kinase-4 (PDK4) that inhibits PDH, thereby increasing PDH activity. The molecular mechanisms of adropin's effects involve acetylation (suggesting inhibition) of the transcriptional coactivator PGC-1α, downregulating expression of Cpt1b and Pdk4. Increased PGC-1α acetylation by adropin may be mediated by inhibiting Sirtuin-1 (SIRT1), a PGC-1α deacetylase. Altered SIRT1 and PGC-1α activity appear to mediate aspects of adropin's metabolic actions in muscle. Similar outcomes were observed in fasted mice treated with synthetic adropin. Together, these results suggest a role for adropin in regulating muscle substrate preference under various nutritional states.

Differential pharmacologic properties of the two C75 enantiomers: (+)-C75 is a strong anorectic drug; (-)-C75 has antitumor activity

C75 is a synthetic compound described as having antitumoral properties. It produces hypophagia and weight loss in rodents, limiting its use in cancer therapy but identifying it as a potential anti-obesity drug. C75 is a fatty acid synthase (FAS) inhibitor and, through its coenzyme A (CoA) derivative, it acts as a carnitine palmitoyltransferase (CPT) 1 inhibitor. Racemic mixtures of C75 have been used in all the previous studies; however, the potential different biological activities of C75 enantiomers have not been examined yet. To address this question we synthesized the two C75 enantiomers separately. Our results showed that (-)-C75 inhibits FAS activity in vitro and has a cytotoxic effect on tumor cell lines, without affecting food consumption. (+)-C75 inhibits CPT1 and its administration produces anorexia, suggesting that central inhibition of CPT1 is essential for the anorectic effect of C75. The differential activity of C75 enantiomers may lead to the development of potential new specific drugs for cancer and obesity.

Important role of ventromedial hypothalamic carnitine palmitoyltransferase-1a in the control of food intake

Carnitine palmitoyltransferase-1 (CPT-1) liver isoform, or CPT-1a, is implicated in CNS control of food intake. However, the exact brain nucleus site(s) in mediating this action of CPT-1a has not been identified. In this report, we assess the role of CPT-1a in hypothalamic ventromedial nucleus (VMN). We stereotaxically injected an adenoviral vector containing CPT-1a coding sequence into the VMN of rats to induce overexpression and activation of CPT-1a. The VMN-selective activation of CPT-1a induced an orexigenic effect, suggesting CPT-1a in the VMN is involved in the central control of feeding. Intracerebroventricular administration of etomoxir, a CPT-1 inhibitor, decreases food intake. Importantly, in the animals with VMN overexpression of a CPT-1a mutant that antagonizes the CPT-1 inhibition by etomoxir, the anorectic response to etomoxir was attenuated. This suggests that VMN is involved in mediating the anorectic effect of central inhibition of CPT-1a. In contrast, arcuate nucleus (Arc) overexpression of the mutant did not alter etomoxir-induced inhibition of food intake, suggesting that Arc CPT-1a does not play significant roles in this anorectic action. Furthermore, in the VMN, CPT-1a appears to act downstream of hypothalamic malonyl-CoA action of feeding. Finally, we show that in the VMN CPT-1 activity was altered in concert with fasting and refeeding states, supporting a physiological role of CPT-1a in mediating the control of feeding. All together, CPT-1a in the hypothalamic VMN appears to play an important role in central control of food intake. VMN-selective modulation of CPT-1a activity may therefore be a promising strategy in controlling food intake and maintaining normal body weight.

Hypothalamic ceramide levels regulated by CPT1C mediate the orexigenic effect of ghrelin

Recent data suggest that ghrelin exerts its orexigenic action through regulation of hypothalamic AMP-activated protein kinase pathway, leading to a decline in malonyl-CoA levels and desinhibition of carnitine palmitoyltransferase 1A (CPT1A), which increases mitochondrial fatty acid oxidation and ultimately enhances the expression of the orexigenic neuropeptides agouti-related protein (AgRP) and neuropeptide Y (NPY). However, it is unclear whether the brain-specific isoform CPT1C, which is located in the endoplasmic reticulum of neurons, may play a role in this action. Here, we demonstrate that the orexigenic action of ghrelin is totally blunted in CPT1C knockout (KO) mice, despite having the canonical ghrelin signaling pathway activated. We also demonstrate that ghrelin elicits a marked upregulation of hypothalamic C18:0 ceramide levels mediated by CPT1C. Notably, central inhibition of ceramide synthesis with myriocin negated the orexigenic action of ghrelin and normalized the levels of AgRP and NPY, as well as their key transcription factors phosphorylated cAMP-response element-binding protein and forkhead box O1. Finally, central treatment with ceramide induced food intake and orexigenic neuropeptides expression in CPT1C KO mice. Overall, these data indicate that, in addition to formerly reported mechanisms, ghrelin also induces food intake through regulation of hypothalamic CPT1C and ceramide metabolism, a finding of potential importance for the understanding and treatment of obesity.

New case of mitochondrial HMG-CoA synthase deficiency. Functional analysis of eight mutations

Mitochondrial HMG-CoA synthase deficiency is a rare inherited metabolic disorder that affects ketone-body synthesis. Acute episodes include vomiting, lethargy, hepatomegaly, hypoglycaemia, dicarboxylic aciduria, and in severe cases, coma. This deficiency may have been under-diagnosed owing to the absence of specific clinical and biochemical markers, limitations in liver biopsy and the lack of an effective method of expression and enzyme assay for verifying the mutations found. To date, eight patients have been reported with nine allelic variants of the HMGCS2 gene. We present a new method of enzyme expression and a modification of the activity assay that allows, for first time, the functional study of missense mutations found in patients with this deficiency. Four of the missense mutations (p.V54M, p.R188H, p.G212R and p.G388R) did not produce proteins that could have been detected in soluble form by western blot; three produced a total loss of activity (p.Y167C, p.M307T and p.R500H) and one, variant p.F174L, gave an enzyme with a catalytic efficiency of 11.5%. This indicates that the deficiency may occur with partial loss of activity of enzyme. In addition, we describe a new patient with this deficiency, in which we detected the missense allelic variant, c.1162G>A (p.G388R) and the nonsense variant c.1270C>T (p.R424X).

Characterization of a novel HMG-CoA lyase enzyme with a dual location in endoplasmic reticulum and cytosol

A novel lyase activity enzyme is characterized for the first time: HMG-CoA lyase-like1 (er-cHL), which is a close homolog of mitochondrial HMG-CoA lyase (mHL). Initial data show that there are nine mature transcripts for the novel gene HMGCLL1, although none of them has all its exons. The most abundant transcript is called “variant b,” and it lacks exons 2 and 3. Moreover, a three-dimensional model of the novel enzyme is proposed. Colocalization studies show a dual location of the er-cHL in the endoplasmic reticulum (ER) and cytosol, but not in mitochondria or peroxisomes. Furthermore, the dissociation experiment suggests that it is a nonendoplasmic reticulum integral membrane protein. The kinetic parameters of er-cHL indicate that it has a lower V_max and a higher substrate affinity than mHL. Protein expression and lyase activity were found in several tissues, and were particularly strong in lung and kidney. The occurrence of er-cHL in brain is surprising, as mHL has not been found there. Although mHL activity is clearly associated with energy metabolism, the results suggest that er-cHL is more closely related to another metabolic function, mostly at the pulmonary and brain level.

Cornelia de Lange syndrome with NIPBL mutation and mosaic Turner syndrome in the same individual

Background

Cornelia de Lange syndrome (CdLS) is a dominantly inherited disorder characterized by facial dysmorphism, growth and cognitive impairment, limb malformations and multiple organ involvement. Mutations in NIPBL gene account for about 60% of patients with CdLS. This gene encodes a key regulator of the Cohesin complex, which controls sister chromatid segregation during both mitosis and meiosis. Turner syndrome (TS) results from the partial or complete absence of one of the X chromosomes, usually associated with congenital lymphedema, short stature, and gonadal dysgenesis.

Case presentation

Here we report a four-year-old female with CdLS due to a frameshift mutation in the NIPBL gene (c.1445_1448delGAGA), who also had a tissue-specific mosaic 45,X/46,XX karyotype. The patient showed a severe form of CdLS with craniofacial dysmorphism, pre- and post-natal growth delay, cardiovascular abnormalities, hirsutism and severe psychomotor retardation with behavioural problems. She also presented with minor clinical features consistent with TS, including peripheral lymphedema and webbed neck. The NIPBL mutation was present in the two tissues analysed from different embryonic origins (peripheral blood lymphocytes and oral mucosa epithelial cells). However, the percentage of cells with monosomy X was low and variable in tissues. These findings indicate that, ontogenically, the NIPBL mutation may have appeared before the mosaic monosomy X.

Conclusions

The coexistence in several patients of these two rare disorders raises the issue of whether there is indeed a cause-effect association. The detailed clinical descriptions indicate predominant CdLS phenotype, although additional TS manifestations may appear in adolescence.

Ceramide levels regulated by carnitine palmitoyltransferase 1C control dendritic spine maturation and cognition

The brain-specific isoform carnitine palmitoyltransferase 1C (CPT1C) has been implicated in the hypothalamic regulation of food intake and energy homeostasis. Nevertheless, its molecular function is not completely understood, and its role in other brain areas is unknown. We demonstrate that CPT1C is expressed in pyramidal neurons of the hippocampus and is located in the endoplasmic reticulum throughout the neuron, even inside dendritic spines. We used molecular, cellular, and behavioral approaches to determine CPT1C function. First, we analyzed the implication of CPT1C in ceramide metabolism. CPT1C overexpression in primary hippocampal cultured neurons increased ceramide levels, whereas in CPT1C-deficient neurons, ceramide levels were diminished. Correspondingly, CPT1C knock-out (KO) mice showed reduced ceramide levels in the hippocampus. At the cellular level, CPT1C deficiency altered dendritic spine morphology by increasing immature filopodia and reducing mature mushroom and stubby spines. Total protrusion density and spine head area in mature spines were unaffected. Treatment of cultured neurons with exogenous ceramide reverted the KO phenotype, as did ectopic overexpression of CPT1C, indicating that CPT1C regulation of spine maturation is mediated by ceramide. To study the repercussions of the KO phenotype on cognition, we performed the hippocampus-dependent Morris water maze test on mice. Results show that CPT1C deficiency strongly impairs spatial learning. All of these results demonstrate that CPT1C regulates the levels of ceramide in the endoplasmic reticulum of hippocampal neurons, and this is a relevant mechanism for the correct maturation of dendritic spines and for proper spatial learning.

Malonyl-CoA mediates leptin hypothalamic control of feeding independent of inhibition of CPT-1a

Hypothalamic fatty acid metabolism is involved in central nervous system controls of feeding and energy balance. Malonyl-CoA, an intermediate of fatty acid biosynthesis, is emerging as a significant player in these processes. Notably, hypothalamic malonyl-CoA has been implicated in leptin's feeding effect. Leptin treatment increases malonyl-CoA level in the hypothalamic arcuate nucleus (Arc), and this increase is required for leptin-induced decrease in food intake. However, the intracellular downstream mediators of malonyl-CoA's feeding effect have not been identified. A primary biochemical action of malonyl-CoA is the inhibition of the acyltransferase activity of carnitine palmitoyltransferase-1 (CPT-1). In the hypothalamus, the predominant isoform of CPT-1 that possesses the acyltransferase activity is CPT-1 liver type (CPT-1a). To address the role of CPT-1a in malonyl-CoA's anorectic action, we used a recombinant adenovirus expressing a mutant CPT-1a that is insensitive to malonyl-CoA inhibition. We show that Arc overexpression of the mutant CPT-1a blocked the malonyl-CoA-mediated inhibition of CPT-1 activity. However, the overexpression of this mutant did not affect the anorectic actions of leptin or central cerulenin for which an increase in Arc malonyl-CoA level is also required. Thus, CPT-1a does not appear to be involved in the malonyl-CoA's anorectic actions induced by leptin. Furthermore, long-chain fatty acyl-CoAs, substrates of CPT-1a, dissociate from malonyl-CoA's actions in the Arc under different feeding states. Together, our results suggest that Arc intracellular mechanisms of malonyl-CoA's anorectic actions induced by leptin are independent of CPT-1a. The data suggest that target(s), rather than CPT-1a, mediates malonyl-CoA action on feeding.

Molecular therapy for obesity and diabetes based on a long-term increase in hepatic fatty-acid oxidation

Obesity-induced insulin resistance is associated with both ectopic lipid deposition and chronic, low-grade adipose tissue inflammation. Despite their excess fat, obese individuals show lower fatty-acid oxidation (FAO) rates. This has raised the question of whether burning off the excess fat could improve the obese metabolic phenotype. Here we used human-safe nonimmunoreactive adeno-associated viruses (AAV) to mediate long-term hepatic gene transfer of carnitine palmitoyltransferase 1A (CPT1A), the key enzyme in fatty-acid β-oxidation, or its permanently active mutant form CPT1AM, to high-fat diet-treated and genetically obese mice. High-fat diet CPT1A- and, to a greater extent, CPT1AM-expressing mice showed an enhanced hepatic FAO which resulted in increased production of CO(2) , adenosine triphosphate, and ketone bodies. Notably, the increase in hepatic FAO not only reduced liver triacylglyceride content, inflammation, and reactive oxygen species levels but also systemically affected a decrease in epididymal adipose tissue weight and inflammation and improved insulin signaling in liver, adipose tissue, and muscle. Obesity-induced weight gain, increase in fasting blood glucose and insulin levels, and augmented expression of gluconeogenic genes were restored to normal only 3 months after AAV treatment. Thus, CPT1A- and, to a greater extent, CPT1AM-expressing mice were protected against obesity-induced weight gain, hepatic steatosis, diabetes, and obesity-induced insulin resistance. In addition, genetically obese db/db mice that expressed CPT1AM showed reduced glucose and insulin levels and liver steatosis.

CONCLUSION

A chronic increase in liver FAO improves the obese metabolic phenotype, which indicates that AAV-mediated CPT1A expression could be a potential molecular therapy for obesity and diabetes.

Differential HMG-CoA lyase expression in human tissues provides clues about 3-hydroxy-3-methylglutaric aciduria

3-Hydroxy-3-methylglutaric aciduria is a rare human autosomal recessive disorder caused by deficiency of 3-hydroxy-3-methylglutaryl CoA lyase (HL). This mitochondrial enzyme catalyzes the common final step of leucine degradation and ketogenesis. Acute symptoms include vomiting, seizures and lethargy, accompanied by metabolic acidosis and hypoketotic hypoglycaemia. Such organs as the liver, brain, pancreas, and heart can also be involved. However, the pathophysiology of this disease is only partially understood. We measured mRNA levels, protein expression and enzyme activity of human HMG-CoA lyase from liver, kidney, pancreas, testis, heart, skeletal muscle, and brain. Surprisingly, the pancreas is, after the liver, the tissue with most HL activity. However, in heart and adult brain, HL activity was not detected in the mitochondrial fraction. These findings contribute to our understanding of the enzyme function and the consequences of its deficiency and suggest the need for assessment of pancreatic damage in these patients.

Mutations and variants in the cohesion factor genes NIPBL, SMC1A, and SMC3 in a cohort of 30 unrelated patients with Cornelia de Lange syndrome

Cornelia de Lange syndrome (CdLS) manifests facial dysmorphic features, growth and cognitive impairment, and limb malformations. Mutations in three genes (NIPBL, SMC1A, and SMC3) of the cohesin complex and its regulators have been found in affected patients. Here, we present clinical and molecular characterization of 30 unrelated patients with CdLS. Eleven patients had mutations in NIPBL (37%) and three patients had mutations in SMC1A (10%), giving an overall rate of mutations of 47%. Several patients shared the same mutation in NIPBL (p.R827GfsX2) but had variable phenotypes, indicating the influence of modifiers in CdLS. Patients with NIPBL mutations had a more severe phenotype than those with mutations in SMC1A or those without identified mutations. However, a high incidence of palate defects was noted in patients with SMC1A mutations. In addition, we observed a similar phenotype in both male and female patients with SMC1A mutations. Finally, we report the first patient with an SMC1A mutation and the Sandifer complex.

Carnitine insufficiency caused by aging and overnutrition compromises mitochondrial performance and metabolic control

In addition to its essential role in permitting mitochondrial import and oxidation of long chain fatty acids, carnitine also functions as an acyl group acceptor that facilitates mitochondrial export of excess carbons in the form of acylcarnitines. Recent evidence suggests carnitine requirements increase under conditions of sustained metabolic stress. Accordingly, we hypothesized that carnitine insufficiency might contribute to mitochondrial dysfunction and obesity-related impairments in glucose tolerance. Consistent with this prediction whole body carnitine diminution was identified as a common feature of insulin-resistant states such as advanced age, genetic diabetes, and diet-induced obesity. In rodents fed a lifelong (12 month) high fat diet, compromised carnitine status corresponded with increased skeletal muscle accumulation of acylcarnitine esters and diminished hepatic expression of carnitine biosynthetic genes. Diminished carnitine reserves in muscle of obese rats was accompanied by marked perturbations in mitochondrial fuel metabolism, including low rates of complete fatty acid oxidation, elevated incomplete beta-oxidation, and impaired substrate switching from fatty acid to pyruvate. These mitochondrial abnormalities were reversed by 8 weeks of oral carnitine supplementation, in concert with increased tissue efflux and urinary excretion of acetylcarnitine and improvement of whole body glucose tolerance. Acetylcarnitine is produced by the mitochondrial matrix enzyme, carnitine acetyltransferase (CrAT). A role for this enzyme in combating glucose intolerance was further supported by the finding that CrAT overexpression in primary human skeletal myocytes increased glucose uptake and attenuated lipid-induced suppression of glucose oxidation. These results implicate carnitine insufficiency and reduced CrAT activity as reversible components of the metabolic syndrome.

Ten novel HMGCL mutations in 24 patients of different origin with 3-hydroxy-3-methyl-glutaric aciduria

3-Hydroxy-3-methylglutaric aciduria is a rare autosomal recessive genetic disorder that affects ketogenesis and L-leucine catabolism. The clinical acute symptoms include vomiting, convulsions, metabolic acidosis, hypoketotic hypoglycaemia and lethargy. To date, 33 mutations in 100 patients have been reported in the HMGCL gene. In this study 10 new mutations in 24 patients are described. They include: 5 missense mutations: c.109G>A, c.425C>T, c.521G>A, c.575T>C and c.598A>T, 2 nonsense mutations: c.242G>A and c.559G>T, one small deletion: c.853delC, and 2 mutations in intron regions: c.497+4A>G and c.750+1G>A. Two prevalent mutations were detected, 109G>T (E37X) in 38% of disease alleles analyzed and c.504_505delCT in 10% of them. Although patients are mainly of European origin (71%) and mostly Spanish (54%), the group is ethnically diverse and includes, for the first time, patients from Pakistan, Palestine and Ecuador. We also present a simple, efficient method to express the enzyme and we analyze the possible functional effects of missense mutations. The finding that all identified missense mutations cause a >95% decrease in the enzyme activity, indicates that the disease appears only in very severe genotypes."

Novel role of FATP1 in mitochondrial fatty acid oxidation in skeletal muscle cells

Carnitine palmitoyltransferase 1 (CPT1) catalyzes the first step in long-chain fatty acid import into mitochondria, and it is believed to be rate limiting for β-oxidation of fatty acids. However, in muscle, other proteins may collaborate with CPT1. Fatty acid translocase/CD36 (FAT/CD36) may interact with CPT1 and contribute to fatty acid import into mitochondria in muscle. Here, we demonstrate that another membrane-bound fatty acid binding protein, fatty acid transport protein 1 (FATP1), collaborates with CPT1 for fatty acid import into mitochondria. Overexpression of FATP1 using adenovirus in L6E9 myotubes increased both fatty acid oxidation and palmitate esterification into triacylglycerides. Moreover, immunocytochemistry assays in transfected L6E9 myotubes showed that FATP1 was present in mitochondria and coimmunoprecipitated with CPT1 in L6E9 myotubes and rat skeletal muscle in vivo. The cooverexpression of FATP1 and CPT1 also enhanced mitochondrial fatty acid oxidation, similar to the cooverexpression of FAT/CD36 and CPT1. However, etomoxir, an irreversible inhibitor of CPT1, blocked all these effects. These data reveal that FATP1, like FAT/CD36, is associated with mitochondria and has a role in mitochondrial oxidation of fatty acids.

C75 is converted to C75-CoA in the hypothalamus, where it inhibits carnitine palmitoyltransferase 1 and decreases food intake and body weight

Central nervous system administration of C75 produces hypophagia and weight loss in rodents identifying C75 as a potential drug against obesity and type 2 diabetes. However, the mechanism underlying this effect is unknown. Here we show that C75-CoA is generated chemically, in vitro and in vivo from C75 and that it is a potent inhibitor of carnitine palmitoyltranferase 1 (CPT1), the rate-limiting step of fatty-acid oxidation. Three-D docking and kinetic analysis support the inhibitory effect of C75-CoA on CPT1. Central nervous system administration of C75 in rats led to C75-CoA production, inhibition of CPT1 and lower body weight and food intake. Our results suggest that inhibition of CPT1, and thus increased availability of fatty acids in the hypothalamus, contribute to the pharmacological mechanism of C75 to decrease food intake.

Molecular genetics of HMG-CoA lyase deficiency

3-Hydroxy-3-methylglutaryl-CoA lyase (HL) deficiency is a rare autosomal recessive genetic disorder that affects ketogenesis and l-leucine catabolism, which generally appears during the first year of life. Patients with HL deficiency have a reduced capacity to synthesize ketone bodies. The disease is caused by lethal mutations in the HL gene (HMGCL). To date, up to 30 variant alleles (28 mutations and 2 SNPs) in 93 patients have been reported, with a recognizable population-specific mutational spectrum. This disorder is frequent in Saudi Arabia and the Iberian Peninsula (Portugal and Spain), where two mutations (122G>A and 109G>A) have been identified in 87% and 94% of the cases, respectively. In most countries a few patients have a high level of allelic heterogeneity. The mutations are distributed along the gene sequences, although some clustering was observed in exon 2, conforming a possible hot spot. Recently, the crystal structures of the human and two bacterial HL have been published. These experimentally obtained structures confirmed the overall architecture, previously predicted by our group and others using bioinformatic approaches, which shows the (betaalpha)8-barrel structure of the enzyme. In addition, the crystals confirmed the presence of an additional COOH domain containing important structures and residues for enzyme functionality and oligomerization processes. Here, we review all HMGCL mis-sense mutations identified to date, and their implication in enzyme structure and function is discussed. We found that genotype-phenotype correlations are difficult to establish because the evolution of the disease seems more related to the causes of hypoglycaemia (fasting or acute illness) than to a particular genotype.

A new LC-ESI-MS/MS method to measure long-chain acylcarnitine levels in cultured cells

The quantitative evaluation of long-chain acylcarnitines in lipid extracts from cultured cells or tissues is a prerequisite to study carnitine palmitoyltransferase (CPT) activity. There is thus a need for the accurate measurement of the concentration of long-chain acylcarnitines at the lowest concentration present in lipid extracts. Here we report a fast and reliable quantitative method based on the use of weak acid extraction and liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) to quantify acylcarnitines through hydrophilic interaction chromatography. The method was validated using isotopic dilution and the results allow the analysis of a large number of samples at low concentration levels (down to 0.35 nmol L(-1) for palmitoylcarnitine) with good inter- and intra-day precision. The method was used for the quantitative study of changes in concentration of palmitoylcarnitine and other acylcarnitines in PC-12 cells over-expressing CPT1a gene. It was also used to measure CPT1 activity in mitochondria isolated from transfected cells, giving similar results to the more common radiometric method, but with higher sensitivity.

C-terminal end and aminoacid Lys48 in HMG-CoA lyase are involved in substrate binding and enzyme activity

3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase adopts a (betaalpha)(8) TIM barrel structure with an additional beta9, alpha11 and alpha12 helices. Location of HMG part of the substrate has been suggested but the binding mode for the CoA moiety remains to be resolved. As mutation F305 fs(-2), which involves the last 21 residues of the protein, and mutation K48N caused 3-hydroxy-3-methylglutaric aciduria in two patients, we examined the role of the C-terminal end and Lys(48) in enzyme activity. Expression studies of various C-terminal-end-deleted and K48N-mutated proteins revealed that residues 311-313 (localized in the loop between alpha11 and alpha12 helices) and Lys(48) are essential for enzyme activity. An in silico docking model locating HMG-CoA on the surface of the enzyme implicates Asn(311) and Lys(313) in substrate binding by establishing multiple polar contacts with phosphate and ribose groups of adenosine, and Lys(48) by contacting the carboxyl group of the panthotenic acid moiety.

Definition by functional and structural analysis of two malonyl-CoA sites in carnitine palmitoyltransferase 1A

Carnitine palmitoyltransferase 1 (CPT1) catalyzes the conversion of palmitoyl-CoA to palmitoylcarnitine in the presence of l-carnitine, thus facilitating the entry of fatty acids to mitochondria, in a process that is physiologically inhibited by malonyl-CoA. To examine the mechanism of CPT1 liver isoform (CPT1A) inhibition by malonyl-CoA, we constructed an in silico model of both its NH2- and COOH-terminal domains. Two malonyl-CoA binding sites were found. One of these, the "CoA site" or "A site," is involved in the interactions between NH2- and COOH-terminal domains and shares the acyl-CoA hemitunnel. The other, the "opposite-to-CoA site" or "O site," is on the opposite side of the enzyme, in the catalytic channel. The two sites share the carnitine-binding locus. To prevent the interaction between NH2- and COOH-terminal regions, we produced CPT1A E26K and K561E mutants. A double mutant E26K/K561E (swap), which was expected to conserve the interaction, was also produced. Inhibition assays showed a 12-fold decrease in the sensitivity (IC50) toward malonyl-CoA for CPT1A E26K and K561E single mutants, whereas swap mutant reverts to wild-type IC50 value. We conclude that structural interaction between both domains is critical for enzyme sensitivity to malonyl-CoA inhibition at the "A site." The location of the "O site" for malonyl-CoA binding was supported by inhibition assays of expressed R243T mutant. The model is also sustained by kinetic experiments that indicated linear mixed type malonyl-CoA inhibition for carnitine. Malonyl-CoA alters the affinity of carnitine, and there appears to be an exponential inverse relation between carnitine Km and malonyl-CoA IC50.

Aging results in paradoxical susceptibility of fat cell progenitors to lipotoxicity

Aging is associated with metabolic syndrome, tissue damage by cytotoxic lipids, and altered fatty acid handling. Fat tissue dysfunction may contribute to these processes. This could result, in part, from age-related changes in preadipocytes, since they give rise to new fat cells throughout life. To test this hypothesis, preadipocytes cultured from rats of different ages were exposed to oleic acid, the most abundant fatty acyl moiety in fat tissue and the diet. At fatty acid concentrations at which preadipocytes from young animals remained viable, cells from old animals accumulated lipid in multiple small lipid droplets and died, with increased apoptotic index, caspase activity, BAX, and p53. Rather than inducing apoptosis, oleic acid promoted adipogenesis in preadipocytes from young animals, with appearance of large lipid droplets. CCAAT/enhancer-binding protein-alpha (C/EBPalpha) and peroxisome proliferator-activated receptor-gamma (PPARgamma) increased to a greater extent in cells from young than old animals after oleate exposure. Oleic acid, but not glucose, oxidation was impaired in preadipocytes and fat cells from old animals. Expression of carnitine palmitoyltransferase (CPT)-1, which catalyzes the rate-limiting step in fatty acid beta-oxidation, was not reduced in preadipocytes from old animals. At lower fatty acid levels, constitutively active CPT I expression enhanced beta-oxidation. At higher levels, CPT I was not as effective in enhancing beta-oxidation in preadipocytes from old as young animals, suggesting that mitochondrial dysfunction may contribute. Consistent with this, medium-chain acyl-CoA dehydrogenase expression was reduced in preadipocytes from old animals. Thus preadipocyte fatty acid handling changes with aging, with increased susceptibly to lipotoxicity and impaired fatty acid-induced adipogenesis and beta-oxidation.

CPT I overexpression protects L6E9 muscle cells from fatty acid-induced insulin resistance

Oversupply of lipids to skeletal muscle causes insulin resistance by promoting the accumulation of lipid-derived metabolites that inhibit insulin signaling. In this study, we tested the hypothesis that overexpression of carnitine palmitoyltransferase I (CPT I) could protect myotubes from fatty acid-induced insulin resistance by reducing lipid accumulation in the muscle cell. Incubation of L6E9 myotubes with palmitate caused accumulation of triglycerides, diacylgycerol, and ceramide, produced an activation of PKCtheta and PKCzeta, and blocked insulin-stimulated glucose metabolism, reducing insulin-stimulated PKB activity by 60%. Transduction of L6E9 myotubes with adenoviruses encoding for liver CPT I (LCPT I) wild-type (WT), or a mutant form of LCPT I (LCPT I M593S), which is insensitive to malonyl-CoA, produced a twofold increase in palmitate oxidation when LCPT I activity was increased threefold. LCPT I WT and LCPT I M593S-overexpressing L6E9 myotubes showed normal insulin-stimulated glucose metabolism and an improvement in PKB activity when pretreated with palmitate. Moreover, LCPT I WT- and LCPT I M593S-transduced L6E9 myotubes were protected against the palmitate-induced accumulation of diacylglycerol and ceramide and PKCtheta and -zeta activation. These results suggest that LCPT I overexpression protects L6E9 myotubes from fatty acid-induced insulin resistance by inhibiting both the accumulation of lipid metabolites and the activation of PKCtheta and PKCzeta.

Modulating aroma compounds during wine fermentation by manipulating carnitine acetyltransferases in Saccharomyces cerevisiae

The wine yeast Saccharomyces cerevisiae is central in the production of aroma compounds during fermentation. Some of the most important yeast-derived aroma compounds produced are esters. The esters ethyl acetate and isoamyl acetate are formed from alcohols and acetyl-CoA in a reaction catalysed by alcohol acetyltransferases. The pool of acetyl-CoA available in yeast cells could play a key role in the development of ester aromas. Carnitine acetyltransferases catalyse the reversible reaction between carnitine and acetyl-CoA to form acetylcarnitine and free CoA. This reaction is important in transferring activated acetyl groups to the mitochondria and in regulating the acetyl-CoA/CoA pools within the cell. We investigated the effect of overexpressing CAT2, which encodes the major mitochondrial and peroxisomal carnitine acetyltransferase, on the formation of esters and other flavour compounds during fermentation. We also overexpressed a modified CAT2 that results in a protein that localizes to the cytosol. In general, the overexpression of both forms of CAT2 resulted in a reduction in ester concentrations, especially in ethyl acetate and isoamyl acetate. We hypothesize that overproduction of Cat2p favours the formation of acetylcarnitine and CoA and therefore limits the precursor for ester production. Carnitine acetyltransferase expression could potentially to be used successfully in order to modulate wine flavour.

Novel effect of C75 on carnitine palmitoyltransferase I activity and palmitate oxidation

C75 is a potential drug for the treatment of obesity. It was first identified as a competitive, irreversible inhibitor of fatty acid synthase (FAS). It has also been described as a malonyl-CoA analogue that antagonizes the allosteric inhibitory effect of malonyl-CoA on carnitine palmitoyltransferase I (CPT I), the main regulatory enzyme involved in fatty acid oxidation. On the basis of MALDI-TOF analysis, we now provide evidence that C75 can be transformed to its C75-CoA derivative. Unlike the activation produced by C75, the CoA derivative is a potent competitive inhibitor that binds tightly but reversibly to CPT I. IC50 values for yeast-overexpressed L- or M-CPT I isoforms, as well as for purified mitochondria from rat liver and muscle, were within the same range as those observed for etomoxiryl-CoA, a potent inhibitor of CPT I. When a pancreatic INS(823/13), muscle L6E9, or kidney HEK293 cell line was incubated directly with C75, fatty acid oxidation was inhibited. This suggests that C75 could be transformed in the cell to its C75-CoA derivative, inhibiting CPT I activity and consequently fatty acid oxidation. In vivo, a single intraperitoneal injection of C75 in mice produced short-term inhibition of CPT I activity in mitochondria from the liver, soleus, and pancreas, indicating that C75 could be transformed to its C75-CoA derivative in these tissues. Finally, in silico molecular docking studies showed that C75-CoA occupies the same pocket in CPT I as palmitoyl-CoA, suggesting an inhibiting mechanism based on mutual exclusion. Overall, our results describe a novel role for C75 in CPT I activity, highlighting the inhibitory effect of its C75-CoA derivative.

Mutagenesis of Specific Amino Acids Converts Carnitine Acetyltransferase into Carnitine Palmitoyltransferase

Carnitine acyltransferases catalyze the exchange of acyl groups between carnitine and CoA. The members of the family can be classified on the basis of their acyl-CoA selectivity. Carnitine acetyltransferases (CrATs) are very active toward short-chain acyl-CoAs but not toward medium- or long-chain acyl-CoAs. Previously, we identified an amino acid residue (Met(564) in rat CrAT) that was critical to fatty acyl-chain-length specificity. M564G-mutated CrAT behaved as if its natural substrates were medium-chain acyl-CoAs, similar to that of carnitine octanoyltransferase (COT). To extend the specificity of rat CrAT to other substrates, we have performed new mutations. Using in silico molecular modeling procedures, we have now identified a second putative amino acid involved in acyl-CoA specificity (Asp(356) in rat CrAT). The double CrAT mutant D356A/M564G showed 6-fold higher activity toward palmitoyl-CoA than that of the single CrAT mutant M564G and a new activity toward stearoyl-CoA. We show that by performing two amino acid replacements a CrAT can be converted into a pseudo carnitine palmitoyltransferase (CPT) in terms of substrate specificity. To change CrAT specificity from carnitine to choline, we also prepared a mutant CrAT that incorporates four amino acid substitutions (A106M/T465V/T467N/R518N). The quadruple mutant shifted the catalytic discrimination between l-carnitine and choline in favor of the latter substrate and showed a 9-fold increase in catalytic efficiency toward choline compared with that of the wild-type. Molecular in silico docking supports kinetic data for the positioning of substrates in the catalytic site of CrAT mutants.

A single-residue mutation, G203E, causes 3-hydroxy-3-methylglutaric aciduria by occluding the substrate channel in the 3D structural model of HMG-CoA lyase

3-Hydroxy-3-methylglutaric aciduria is a rare autosomal recessive genetic disorder that affects ketogenesis and leucine metabolism. The disease is caused by mutations in the gene coding for 3-hydroxy-3-methylglutaryl-coenzyme A lyase (HL). To date 26 different mutations have been described. A (betaalpha)(8) TIM barrel structure has been proposed for the protein, and almost all missense mutations identified so far localize in the beta sheets that define the inside cavity. We report an Italian patient who bears homozygously a novel HL mutation, c.608G > A (p. G203E) in beta sheet six. A structural model of the mutated protein suggests that glutamic acid 203 impedes catalysis by blocking the entrance to the inner cavity of the enzyme. Loss of functionality has been confirmed in expression studies in E. coli, which demonstrate that the G203E mutation completely abolishes enzyme activity. Beta sheet six and beta sheet two are the two protein regions that accumulate most missense mutations, indicating their importance in enzyme functionality. A model for the mechanism of enzyme function is proposed.

Refining the diagnosis of mitochondrial HMG-CoA synthase deficiency

Mitochondrial HMG-CoA synthase deficiency is an inherited metabolic disorder caused by a defect in the enzyme that regulates the formation of ketone bodies. Patients present with hypoketotic hypoglycaemia, encephalopathy and hepatomegaly, usually precipitated by an intercurrent infection or prolonged fasting. The diagnosis may easily be missed as previously reported results of routine metabolic investigations, urinary organic acids and plasma acylcarnitines may be nonspecific or normal, and a high index of suspicion is required to proceed to further confirmatory tests. We describe a further acute case in which the combination of urinary organic acids, low free carnitine and changes in the plasma acylcarnitine profile on carnitine supplementation were very suggestive of a defect in ketone synthesis. The diagnosis of mitochondrial HMG-CoA synthase deficiency was confirmed on genotyping, revealing two novel mutations: c.614G > A (R188H) and c.971T > C (M307T). A further sibling, in whom the diagnosis had not been made acutely, was also found to be affected. The possible effects of these mutations on enzyme activity are discussed.

Alteration of the malonyl-CoA/carnitine palmitoyltransferase I interaction in the beta-cell impairs glucose-induced insulin secretion

Carnitine palmitoyltransferase I, which is expressed in the pancreas as the liver isoform (LCPTI), catalyzes the rate-limiting step in the transport of fatty acids into the mitochondria for their oxidation. Malonyl-CoA derived from glucose metabolism regulates fatty acid oxidation by inhibiting LCPTI. To examine directly whether the availability of long-chain fatty acyl-CoA (LC-CoA) affects the regulation of insulin secretion in the beta-cell and whether malonyl-CoA may act as a metabolic coupling factor in the beta-cell, we infected INS(832/13) cells and rat islets with an adenovirus encoding a mutant form of LCPTI (Ad-LCPTI M593S) that is insensitive to malonyl-CoA. In Ad-LCPTI M593S-infected INS(832/13) cells, LCPTI activity increased sixfold. This was associated with enhanced fatty acid oxidation, at any glucose concentration, and a 60% suppression of glucose-stimulated insulin secretion (GSIS). In isolated rat islets in which LCPTI M593S was overexpressed, GSIS decreased 40%. The impairment of GSIS in Ad-LCPTI M593S-infected INS(832/13) cells was not recovered when cells were incubated with 0.25 mmol/l palmitate, indicating the deep metabolic influence of a nonregulated fatty acid oxidation system. At high glucose concentration, overexpression of a malonyl-CoA-insensitive form of LCPTI reduced partitioning of exogenous palmitate into lipid esterification products and decreased protein kinase C activation. Moreover, LCPTI M593S expression impaired K(ATP) channel-independent GSIS in INS(832/13) cells. The LCPTI M593S mutant caused more pronounced alterations in GSIS and lipid partitioning (fat oxidation, esterification, and the level of nonesterified palmitate) than LCPTI wt in INS(832/13) cells that were transduced with these constructs. The results provide direct support for the hypothesis that the malonyl-CoA/CPTI interaction is a component of a metabolic signaling network that controls insulin secretion.

Structural model of carnitine palmitoyltransferase I based on the carnitine acetyltransferase crystal

CPT I (carnitine palmitoyltransferase I) catalyses the conversion of palmitoyl-CoA into palmitoylcarnitine in the presence of L-carnitine, facilitating the entry of fatty acids into mitochondria. We propose a 3-D (three-dimensional) structural model for L-CPT I (liver CPT I), based on the similarity of this enzyme to the recently crystallized mouse carnitine acetyltransferase. The model includes 607 of the 773 amino acids of L-CPT I, and the positions of carnitine, CoA and the palmitoyl group were assigned by superposition and docking analysis. Functional analysis of this 3-D model included the mutagenesis of several amino acids in order to identify putative catalytic residues. Mutants D477A, D567A and E590D showed reduced L-CPT I activity. In addition, individual mutation of amino acids forming the conserved Ser685-Thr686-Ser687 motif abolished enzyme activity in mutants T686A and S687A and altered K(m) and the catalytic efficiency for carnitine in mutant S685A. We conclude that the catalytic residues are His473 and Asp477, while Ser687 probably stabilizes the transition state. Several conserved lysines, i.e. Lys455, Lys505, Lys560 and Lys561, were also mutated. Only mutants K455A and K560A showed decreases in activity of 50%. The model rationalizes the finding of nine natural mutations in patients with hereditary L-CPT I deficiencies.

Molecular basis of 3-hydroxy-3-methylglutaric aciduria

3-Hydroxy-3-methylglutaric aciduria is a human autosomal recessive metabolic disorder that usually appears within the first year of life. The causes of this aciduria are lethal mutations in the gene encoding for 3-hydroxy-3-methylglutaryl coenzyme A lyase (HL). HL is a mitochondrial matrix enzyme that catalyzes the last step of ketogenesis and leucine catabolism. This gene has been mapped to chromosome 1 at locus 1pter-p33 and its genomic organisation comprises 9 exons whose sizes vary between 64-678 bp. The human cDNA sequence was reported in 1993 with the first genetic study of two Acadian-French Canadian siblings. To date, 24 mutations in 36 patients have been described; most of them are single-base substitutions causing amino acid replacements and a variety of splicing defects. In the population studied two mutations appear predominant: g.122GA (8 patients and 15 alleles) frequent in Saudi Arabia, and g.109GT (6 patients and 12 alleles), prevalent in Spain. At least seven mutations are clustered in the second half of exon 2 affecting aminoacids E37, R41 and D42 and conforming a possible hot spot. The genotype-phenotype correlation is difficult to establish since the probands received different treatments, and the onset of an acute episode frequently depends on external factors such as fasting or acute illness.

Redesign of carnitine acetyltransferase specificity by protein engineering

In eukaryotes, L-carnitine is involved in energy metabolism by facilitating beta-oxidation of fatty acids. Carnitine acetyltransferases (CrAT) catalyze the reversible conversion of acetyl-CoA and carnitine to acetylcarnitine and free CoA. To redesign the specificity of rat CrAT toward its substrates, we mutated Met564. The M564G mutated CrAT showed higher activity toward longer chain acyl-CoAs: activity toward myristoyl-CoA was 1250-fold higher than that of the wild-type CrAT, and lower activity toward its natural substrate, acetyl-CoA. Kinetic constants of the mutant CrAT showed modification in favor of longer acyl-CoAs as substrates. In the reverse case, mutation of the orthologous glycine (Gly553) to methionine in carnitine octanoyltransferase (COT) decreased activity toward its natural substrates, medium- and long-chain acyl-CoAs, and increased activity toward short-chain acyl-CoAs. Another CrAT mutant, M564A, was prepared and tested in the same way, with similar results. We conclude that Met564 blocks the entry of medium- and long-chain acyl-CoAs to the catalytic site of CrAT. Three-dimensional models of wild-type and mutated CrAT and COT support this hypothesis. We show for the first time that a single amino acid is able to determine the substrate specificity of CrAT and COT.

Structural (betaalpha)8 TIM barrel model of 3-hydroxy-3-methylglutaryl-coenzyme A lyase

This study describes three novel homozygous missense mutations (S75R, S201Y, and D204N) in the 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase gene, which caused 3-hydroxy-3-methylglutaric aciduria in patients from Germany, England, and Argentina. Expression studies in Escherichia coli show that S75R and S201Y substitutions completely abolished the HMG-CoA lyase activity, whereas D204N reduced catalytic efficiency to 6.6% of the wild type. We also propose a three-dimensional model for human HMG-CoA lyase containing a (betaalpha)8 (TIM) barrel structure. The model is supported by the similarity with analogous TIM barrel structures of functionally related proteins, by the localization of catalytic amino acids at the active site, and by the coincidence between the shape of the substrate (HMG-CoA) and the predicted inner cavity. The three novel mutations explain the lack of HMG-CoA lyase activity on the basis of the proposed structure: in S75R and S201Y because the new amino acid residues occlude the substrate cavity, and in D204N because the mutation alters the electrochemical environment of the active site. We also report the localization of all missense mutations reported to date and show that these mutations are located in the beta-sheets around the substrate cavity.

Identification of conserved amino acid residues in rat liver carnitine palmitoyltransferase I critical for malonyl-CoA inhibition. Mutation of methionine 593 abolishes malonyl-CoA inhibition

Carnitine palmitoyltransferase (CPT) I, which catalyzes the conversion of palmitoyl-CoA to palmitoylcarnitine facilitating its transport through the mitochondrial membranes, is inhibited by malonyl-CoA. By using the SequenceSpace algorithm program to identify amino acids that participate in malonyl-CoA inhibition in all carnitine acyltransferases, we found 5 conserved amino acids (Thr(314), Asn(464), Ala(478), Met(593), and Cys(608), rat liver CPT I coordinates) common to inhibitable malonyl-CoA acyltransferases (carnitine octanoyltransferase and CPT I), and absent in noninhibitable malonyl-CoA acyltransferases (CPT II, carnitine acetyltransferase (CAT) and choline acetyltransferase (ChAT)). To determine the role of these amino acid residues in malonyl-CoA inhibition, we prepared the quintuple mutant CPT I T314S/N464D/A478G/M593S/C608A as well as five single mutants CPT I T314S, N464D, A478G, M593S, and C608A. In each case the CPT I amino acid selected was mutated to that present in the same homologous position in CPT II, CAT, and ChAT. Because mutant M593S nearly abolished the sensitivity to malonyl-CoA, two other Met(593) mutants were prepared: M593A and M593E. The catalytic efficiency (V(max)/K(m)) of CPT I in mutants A478G and C608A and all Met(593) mutants toward carnitine as substrate was clearly increased. In those CPT I proteins in which Met(593) had been mutated, the malonyl-CoA sensitivity was nearly abolished. Mutations in Ala(478), Cys(608), and Thr(314) to their homologous amino acid residues in CPT II, CAT, and ChAT caused various decreases in malonyl-CoA sensitivity. Ala(478) is located in the structural model of CPT I near the catalytic site and participates in the binding of malonyl-CoA in the low affinity site (Morillas, M., Gómez-Puertas, P., Rubi, B., Clotet, J., Ariño, J., Valencia, A., Hegardt, F. G., Serra, D., and Asins, G. (2002) J. Biol. Chem. 277, 11473-11480). Met(593) may participate in the interaction of malonyl-CoA in the second affinity site, whose location has not been reported.

Impaired ketogenesis is a major mechanism for disturbed hepatic fatty acid metabolism in rats with long-term cholestasis and after relief of biliary obstruction

BACKGROUND/AIMS

Rats with long-term cholestasis have reduced ketosis of unknown origin.

METHODS

Fatty acid metabolism was studied in starved rats with biliary obstruction for 4 weeks (bile duct ligated rats = BDL rats), and 3, 7, 14, 28 and 84 days after reversal of biliary obstruction by Roux-en-Y anastomosis (RY rats), and in sham-operated control rats.

RESULTS

BDL rats had reduced beta-hydroxybutyrate concentrations in plasma (0.25 +/- 0.10 vs. 0.75 +/- 0.20 mmol/l) and liver (2.57 +/- 0.20 vs. 4.63 +/- 0.61 micromol/g) which increased after restoring bile flow. Hepatic expression and activity of carnitine palmitoyltransferase I (CPT I) or CPT II were unaffected or decreased in BDL rats, respectively, and increased after restoring bile flow. Oxidative metabolism of different substrates by isolated liver mitochondria and activation of palmitate were reduced in BDL rats and recovered 7-14 days after restoring bile flow. Ketogenesis was decreased in mitochondria from BDL rats and recovered 3 months after restoring bile flow. Both mRNA and protein expression of hydroxymethylglutaryl-coenzyme A synthase (HMG-CoA synthase), the rate-limiting enzyme of ketogenesis, was reduced in livers of BDL rats and increased after reversing biliary obstruction.

CONCLUSIONS

In BDL rats, impairment of hepatic fatty acid metabolism is multifactorial. After reversing biliary obstruction, reduced activity of HMG-CoA synthase is the major factor.

The diagnosis of mitochondrial HMG-CoA synthase deficiency

Deficiency of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase, the only disorder exclusively affecting hepatic ketogenesis, is a cause of hypoglycemic coma. We report that the diagnosis can be made by typical laboratory findings (hypoketosis, elevated free fatty acids, normal acylcarnitines, specific urinary organic acids) during acute episodes.

Adenovirus-mediated overexpression of liver carnitine palmitoyltransferase I in INS1E cells: effects on cell metabolism and insulin secretion

Lipid metabolism in the beta-cell is critical for the regulation of insulin secretion. Pancreatic beta-cells chronically exposed to fatty acids show higher carnitine palmitoyltransferase I (CPT I) protein levels, higher palmitate oxidation rates and an altered insulin response to glucose. We examined the effect of increasing CPT I levels on insulin secretion in cultured beta-cells. We prepared a recombinant adenovirus containing the cDNA for the rat liver isoform of CPT I. The overexpression of CPT I in INS1E cells caused a more than a 5-fold increase in the levels of CPT I protein (detected by Western blotting), a 6-fold increase in the CPT activity, and an increase in fatty acid oxidation at 2.5 mM glucose (1.7-fold) and 15 mM glucose (3.1-fold). Insulin secretion was stimulated in control cells by 15 mM glucose or 30 mM KCl. INS1E cells overexpressing CPT I showed lower insulin secretion on stimulation with 15 mM glucose (-40%; P<0.05). This decrease depended on CPT I activity, since the presence of etomoxir, a specific inhibitor of CPT I, in the preincubation medium normalized the CPT I activity, the fatty-acid oxidation rate and the insulin secretion in response to glucose. Exogenous palmitate (0.25 mM) rescued glucose-stimulated insulin secretion (GSIS) in CPT I-overexpressing cells, indicating that the mechanism of impaired GSIS was through the depletion of a critical lipid. Depolarizing the cells with KCl or intermediary glucose concentrations (7.5 mM) elicited similar insulin secretion in control cells and cells overexpressing CPT I. Glucose-induced ATP increase, glucose metabolism and the triacylglycerol content remained unchanged. These results provide further evidence that CPT I activity regulates insulin secretion in the beta-cell. They also indicate that up-regulation of CPT I contributes to the loss of response to high glucose in beta-cells exposed to fatty acids.

Structural model of a malonyl-CoA-binding site of carnitine octanoyltransferase and carnitine palmitoyltransferase I: mutational analysis of a malonyl-CoA affinity domain

Carnitine octanoyltransferase (COT) and carnitine palmitoyltransferase (CPT) I, which facilitate the transport of medium- and long-chain fatty acids through the peroxisomal and mitochondrial membranes, are physiologically inhibited by malonyl-CoA. Using an "in silico" macromolecular docking approach, we built a model in which malonyl-CoA could be attached near the catalytic core. This disrupts the positioning of the acyl-CoA substrate in the channel in the model reported for both proteins (Morillas, M., Gómez-Puertas, P., Roca, R., Serra, D., Asins, G., Valencia, A., and Hegardt, F. G. (2001) J. Biol. Chem. 276, 45001-45008). The putative malonyl-CoA domain contained His(340), implicated together with His(131) in COT malonyl-CoA sensitivity (Morillas, M., Clotet, J., Rubi, B., Serra, D., Asins, G., Ariño, J., and Hegardt F. G. (2000) FEBS Lett. 466, 183-186). When we mutated COT His(131) the IC(50) increased, and malonyl-CoA competed with the substrate decanoyl-CoA. Mutation of COT Ala(332), present in the domain 8 amino acids away from His(340), decreased the malonyl-CoA sensitivity of COT. The homologous histidine and alanine residues of L-CPT I, His(277), His(483), and Ala(478) were also mutated, which decreased malonyl-CoA sensitivity. Natural mutation of Pro(479), which is also located in the malonyl-CoA predicted site, to Leu in a patient with human L-CPT I hereditary deficiency, modified malonyl-CoA sensitivity. We conclude that this malonyl-CoA domain is present in both COT and L-CPT I proteins and might be the site at which malonyl-CoA interacts with the substrate acyl-CoA. Other malonyl-CoA non-inhibitable members of the family, CPT II and carnitine acetyltransferase, do not contain this domain.

Structural model of the catalytic core of carnitine palmitoyltransferase I and carnitine octanoyltransferase (COT): mutation of CPT I histidine 473 and alanine 381 and COT alanine 238 impairs the catalytic activity

Carnitine palmitoyltransferase I (CPT I) and carnitine octanoyltransferase (COT) catalyze the conversion of long- and medium-chain acyl-CoA to acylcarnitines in the presence of carnitine. We propose a common three-dimensional structural model for the catalytic domain of both, based on fold identification for 200 amino acids surrounding the active site through a threading approach. The model is based on the three-dimensional structure of the rat enoyl-CoA hydratase, established by x-ray diffraction analysis. The study shows that the structural model of 200 amino acids of the catalytic site is practically identical in CPT I and COT with identical distribution of 4 beta-sheets and 6 alpha-helices. Functional analysis of the model was done by site-directed mutagenesis. When the critical histidine residue 473 in CPT I (327 in COT), localized in the acyl-CoA pocket in the model, was mutated to alanine, the catalytic activity was abolished. Mutation of the conserved alanine residue to aspartic acid, A381D (in CPT I) and A238D (in COT), which are 92/89 amino acids far from the catalytic histidine, respectively (but very close to the acyl-CoA pocket in the structural model), decreased the activity by 86 and 80%, respectively. The K(m) for acyl-CoA increased 6-8-fold, whereas the K(m) for carnitine hardly changed. The inhibition of the mutant CPT I by malonyl-CoA was not altered. The structural model explains the loss of activity reported for the CPT I mutations R451A, W452A, D454G, W391A, del R395, P479L, and L484P, all of which occur in or near the modeled catalytic domain.

GAD65-mediated glutamate decarboxylation reduces glucose-stimulated insulin secretion in pancreatic beta cells

Mitochondrial metabolism plays a pivotal role in the pancreatic beta cell by generating signals that couple glucose sensing to insulin secretion. We have demonstrated previously that mitochondrially derived glutamate participates directly in the stimulation of insulin exocytosis. The aim of the present study was to impose altered cellular glutamate levels by overexpression of glutamate decarboxylase (GAD) to repress elevation of cytosolic glutamate. INS-1E cells infected with a recombinant adenovirus vector encoding GAD65 showed efficient overexpression of the GAD protein with a parallel increase in enzyme activity. In control cells glutamate levels were slightly increased by 7.5 mm glucose (1.4-fold) compared with the effect at 15 mm (2.3-fold) versus basal 2.5 mm glucose. Upon GAD overexpression, glutamate concentrations were no longer elevated by 15 mm glucose as compared with controls (-40%). Insulin secretion was stimulated in control cells by glucose at 7.5 mm (2.5-fold) and more efficiently at 15 mm (5.2-fold). INS-1E cells overexpressing GAD exhibited impaired insulin secretion on stimulation with 15 mm glucose (-37%). The secretory response to 30 mm KCl, used to raise cytosolic Ca(2+) levels, was unaffected. Similar results were obtained in perifused rat pancreatic islets following adenovirus transduction. This GAD65-mediated glutamate decarboxylation correlating with impaired glucose-induced insulin secretion is compatible with a role for glutamate as a glucose-derived factor participating in insulin exocytosis.

Post-transcriptional regulation of rat carnitine octanoyltransferase

Carnitine octanoyltransferase (COT) produces three different transcripts in rat through cis- and trans-splicing reactions, which can lead to the synthesis of two proteins. The occurrence of the three COT transcripts in rat has been found in all tissues examined and does not depend on sex, fat feeding, peroxisome proliferators or hyperinsulinaemia. Rat COT exon 2 contains a putative exonic splicing enhancer (ESE) sequence. Mutation of this ESE (GAAGAAG) to AAAAAAA decreased trans-splicing in vitro, from which it is deduced that this ESE sequence is partly responsible for the formation of the three transcripts. The protein encoded by cis-spliced mRNA of rat COT is inhibited by malonyl-CoA and etomoxir. cDNA species encoding full-length wild-type COT and one double mutant COT were expressed in Saccharomyces cerevisiae. The recombinant enzymes showed full activity towards both substrates, carnitine and decanoyl-CoA. The activity of the doubly mutated H131A/H340A enzyme was similar to that of the rat peroxisomal enzyme but was completely insensitive to malonyl-CoA and etomoxir. These results indicate that the histidine residues His-131 and His-340 are the sites responsible for the interaction of these two inhibitors, which inhibit COT by interacting with the same sites.

Localization of an exonic splicing enhancer responsible for mammalian natural trans-splicing

Carnitine octanoyltransferase (COT) produces three different transcripts in rat through cis- and trans-splicing reactions, which may lead to the synthesis of two proteins. Generation of the three COT transcripts in rat does not depend on sex, development, fat feeding, the inclusion of the peroxisome proliferator diethylhexyl phthalate in the diet or hyperinsulinemia. In addition, trans-splicing was not detected in COT of other mammals, such as human, pig, cow and mouse, or in Cos7 cells from monkey. Rat COT exon 2 contains two purine-rich sequences. Mutation of the rat COT exon 2 upstream box does not affect the trans-splicing in vitro between two truncated constructs containing exon 2 and its adjacent intron boundaries. In contrast, mutation of the downstream box from the rat sequence (GAAGAAG) to a random sequence or the sequence observed in the other mammals (AAAAAAA) decreased trans-splicing in vitro. In contrast, mutation of the AAAAAAA box of human COT exon 2 to GAAGAAG increases trans-splicing. Heterologous reactions between COT exon 2 from rat and human do not produce trans-splicing. HeLa cells transfected with minigenes of rat COT sequences produced cis- and trans-spliced bands. Mutation of the GAAGAAG box to AAAAAAA abolished trans-splicing and decreased cis-splicing in vivo. We conclude that GAAGAAG is an exonic splicing enhancer that could induce natural trans-splicing in rat COT.

Genetic basis of mitochondrial HMG-CoA synthase deficiency

Deficiency of mitochondrial 3-hydroxy-3-methylglutaryl-CoA synthase (mHMGS) is a recessive disorder of ketogenesis that has been previously diagnosed in two children with hypoglycaemic hypoketotic coma during fasting periods. Here, we report the results of molecular investigations in a third patient affected by this disease. Sequencing of the entire coding region of the HMGCS2 gene revealed two missense mutations, G212R and R500H. Mendelian inheritance was confirmed by the analysis of parental samples and neither of the mutations was found on 200 control chromosomes. Functional relevance was confirmed by in vitro expression studies in cytosolic HMGS-deficient cells. Whereas wild-type cDNA of the HMGCS2 gene reverted the auxotrophy for mevalonate, the cDNAs of the mutants did not. The disease may be recognised by specific clinical and biochemical features but it is difficult to confirm enzymatically since the gene is expressed only in liver and testis. Molecular studies may facilitate or confirm future diagnoses in affected patients.

Modulation in vitro of H-ras oncogene expression by trans-splicing

In man, activated N-, K- and H-ras oncogenes have been found in around 30% of the solid tumours tested. An exon known as IDX, which has been described previously and is located between exon 3 and exon 4A of the c-H-ras pre-mRNA, allows an alternative splicing process that results in the synthesis of the mRNA of a putative protein named p19. It has been suggested that this alternative pathway is less tumorigenic than that which results in the activation of p21. We have used the mammalian trans-splicing mechanism as a tool with which to modulate this particular pre-mRNA processing to produce mRNA similar to that of mature p19 RNA. The E4A exon of the activated H-ras gene was found to be a good target for external trans-splicing. We reprogrammed the rat carnitine octanoyltransferase exon 2 to specifically invade the terminal region of H-ras. Assays performed with this reprogrammed trans-exon showed that the trans-splicing product was obtained in competition with cis-splicing of the D intron of the H-ras gene, and was associated with concomitant down-modulation of D intron cis-splicing. We also found that the exon 4A of the human c-H-ras gene underwent successive trans-splicing rounds with an external exon.

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.