Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism最新文献

In patients with diabetes, non-enzymatic glycation of low-density lipoprotein (LDL) has been suggested to be involved in the development of atherosclerosis. α-Dicarbonyl compounds were identified as intermediates in the non-enzymatic glycation and increased levels were reported in patients with diabetes. We studied the effect of the α-dicarbonyl compound methylglyoxal (MG) on the physicochemical and biological properties of LDL. MG dose-dependently modifies LDL, as indicated by the formation of fluorescent products and the increase of a net negative charge. MG (10 mmol/l) induced major modifications of arginine residues (up to 85%) and minor lysine modifications (less than 6%). MG-LDL preparations generated small amounts of superoxide anion radicals as measured by the reduction of cytochrome c, but this was not accompanied by peroxidation of the polyunsaturated fatty acids of MG-LDL. MG-LDL showed diminished recognition and uptake by the human LDL receptor in cultured cells and a markedly increased plasma clearance rate in vivo in rats. The reduced association and degradation of ¹²⁵I-oxidised LDL by murine macrophages indicates recognition of MG-LDL by a scavenger receptor. Surprisingly, MG-LDL caused significantly less cholesteryl ester synthesis in murine macrophages, as compared to native LDL and oxidised or acetylated LDL. Highly modified MG-LDL did not induce activation of human endothelial cells, as measured by the expression of monocyte chemoattractant protein-1 and vascular cell adhesion molecule-1.

Elevation of cyclic AMP (cAMP) in platelets inhibits agonist-induced, G protein-mediated responses and activation of polyphosphoinositide-specific phospholipase C (PLC) by ill-defined mechanism(s). Signal transduction steps downstream of PLC are inhibited by elevated cAMP, suggesting an inhibitory effect of cAMP, via protein kinase A, on PLC. In [³²P]_i-prelabeled platelets, forskolin increased intracellular cAMP (104 nmol/10¹¹ cells at 10⁻⁵ M forskolin) and [³²P]phosphatidylinositol 4-phosphate (Δ[³²P]PIP) (30% at 10⁻⁷–10⁻⁶ M forskolin). The thrombin-induced (0.1 U/ml) increase in production of [³²P]PA, ‘overshoots’ in [³²P]PIP and [³²P]PIP₂ ([³²P]phosphatidylinositol 4,5-bisphosphate), and the increase in [³²P]PI and secretion of ADP+ATP were abolished by forskolin (10⁻⁷ M). Forskolin stimulated total [³²P]P_i uptake in resting platelets (48%), increased ³²P incorporation into PIP (110%), and inhibited ³²P incorporation into PI (50%). The latter inhibition was most likely considerably greater due to the forskolin-induced stimulation of [³²P]P_i uptake. The changes in radioactive PA, PIP and PIP₂ are regarded as being proportional with their masses in the prelabeled platelets, while the increase in PI (phosphatidylinositol) is regarded as a change in specific radioactivity, and hence in its synthesis. The results suggest that cAMP elevation inhibits the flux in the polyphosphoinositide cycle through both inhibition of PIP 5-kinase and PI synthesis. The inverse relation between forskolin-produced ΔPIP and [³²P]PA production suggests that the PLC reaction is inhibited by elevated cAMP through reduction of substrate (PIP₂) resynthesis, and not by inhibition of the PLC enzyme.

The hypothesis that the capability of agents to mobilize arachidonic acid (AA) could predict increased anandamide (ANA) synthesis in a macrophage cell line has been examined. Lipopolysaccharide (LPS), platelet-activating factor (PAF) and cannabinoids such as Δ⁹-tetrahydrocannabinol (THC) and anandamide were all found to be agonists for the release of AA and led to increased ANA synthesis in RAW264.7 mouse macrophage cells. Nitric oxide, in contrast, stimulated AA release without raising ANA levels. ANA stimulation of its own synthesis indicates the existence of a positive feedback mechanism. The possible involvement of the CB2 receptor in THC-mediated AA release and ANA synthesis is addressed using the antagonist SR144528. ANA synthesis is also increased by the combination of calcium ionophore and indomethacin, suggesting that ANA is metabolized by a cyclooxygenase in this system. The data imply that ANA could play a role in the response of the immune system to cannabinoids and bacterial endotoxins and that AA mobilization is a predictor for increased ANA synthesis.

While UDP-glucuronosyltransferases (UGTs) are known to be expressed at high levels in human liver, relatively little is known about extrahepatic expression. In the present study, UGT2B family isoforms involved in the glucuronidation of steroid hormones and bile acids have been characterized in microsomes prepared from jejunum, ileum and colon from six human subjects. Glucuronidation of androsterone and testosterone was highly significant and increased from proximal to distal intestine. In contrast, hyodeoxycholic acid was glucuronidated at a low level in jejunum and ileum and activity was barely detectable in colon. No significant glucuronidation of lithocholic acid was found. Small phenols were glucuronidated with much lower activity than found in liver. High levels of UGT protein were detected with polyclonal anti-rat androsterone- and testosterone-UGT antibodies, whereas UGT2B4, a major hepatic hyodeoxycholic acid-specific UGT, was undetectable using a highly specific anti-human UGT2B4 antibody. Screening for RNA expression by RT-PCR confirmed the absence of UGT2B4 and UGT1A6 and showed expression of UGT2B7, a hepatic isoform shown to glucuronidate androsterone, in all intestinal segments. To our knowledge, the presence of functional androsterone and testosterone directed isoforms in human intestine is a novel finding which supports the idea that the intestinal tract functions as a steroid-metabolizing organ and plays a significant role in steroid hormone biotransformation.

We showed previously that hypertriglyceridaemia, but not hypercholesterolaemia, is correlated with increases in cholesterol synthesis and apolipoprotein B secretion in patients with secondary hypertriglyceridaemia. The aim of the present study was to compare the rate of cholesterol synthesis, using fasting plasma mevalonic acid (MVA) as an index, in patients with primary mixed hyperlipidaemia (type IIb phenotype, n=45) and primary hypercholesterolaemia (type IIa phenotype, n=92). LDL cholesterol was significantly higher in types IIa (6.38±0.18 mmol/l) and IIb (5.89±0.25 mmol/l) compared to 40 normolipidaemic controls (2.99±0.1 mmol/l, P<0.0001), whereas serum triglyceride was higher in type IIb (2.62 (range 2.2–3.0) mmol/l) than type IIa (1.22 (range 0.85–1.60) mmol/l, P<0.001) and controls (0.90 (range 0.68–1.24) mmol/l, P<0.001). Similarly, MVA was higher in type IIb (7.0±0.46 ng/ml) than IIa (5.6±0.23 ng/ml, P<0.0) and controls (5.6±0.36 ng/ml, P<0.05). Plasma MVA correlated positively with serum triglyceride (r=0.22, P=0.004) and negatively with LDL cholesterol (r=−0.21, P=0.014). These results are in accordance with previous observations that VLDL-apolipoprotein B secretion and cholesterol synthesis are linked and demonstrate that the latter is increased in mixed hyperlipidaemia.

Plasma levels of the cholesterol precursor 7-dehydrocholesterol (7-DHC) were compared with activities of the rate-limiting enzyme in cholesterol biosynthesis, 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase assayed in liver biopsies from patients undergoing cholecystectomy. Some patients were treated with cholestyramine, deoxycholic acid or chenodeoxycholic acid prior to surgery in order to alter the activity of the enzyme. The median level of 7-DHC and the activity of HMG-CoA reductase in the untreated group were 55 ng/ml and 98 pmol/min/mg protein, respectively. The sterol levels and enzyme activities were increased in patients treated with cholestyramine (85 ng/ml and 439 pmol/min/mg protein) and deoxycholic acid (86 ng/ml and 173 pmol/min/mg protein) and decreased in patients treated with chenodeoxycholic acid (38 ng/ml and 51 pmol/min/mg protein). There was a strong positive correlation (r_s=0.75, P<0.0005) between the plasma levels of 7-DHC and the activities of hepatic HMG-CoA reductase in these patients. This correlation was further improved when the plasma levels of 7-DHC were expressed relative to those of cholesterol (r_s=0.90, P<0.0001). The results show that the level of 7-DHC in plasma reflects the activity of HMG-CoA reductase in the liver.

We have prepared a panel of lipidic ammonium tetrafluoroborate salts that contain trifluoromethyl, trichloromethyl, and methyl groups attached to the headgroup. ¹⁹F-NMR analyses of the cationic lipid panel revealed that the differences in electron-withdrawal from the ammonium ion headgroup accounted for differences in ion-pairing. Exchange of the tetrafluoroborate counterion by complexation to DNA-phosphate of a reporter gene enabled us to probe the influence of inductive electron-withdrawal in cationic lipid-mediated DNA transfection. We tested the lipid panel for transfection activity in two cell lines. The results indicate that the inductive effects of electron-withdrawing functionality diminish transfection activity in modest (2–4-fold) increments. The present study suggests that the mechanism whereby poly(alcohol)- or poly(ether)-substituted headgroups improve DNA transfection is not based on electronic activation of the ammonium ion.

We have shown previously that both 1,2-diacylglycerol (AAG) and 1-O-alkyl-2-acylglycerol (EAG) prime neutrophil release of arachidonic acid via uncharacterized phospholipases A₂. Therefore, we investigated the actions of EAG and AAG specifically on neutrophil cytosolic (cPLA₂) and secretory (sPLA₂) phospholipase A₂s. We hypothesized that AAG as a protein kinase activator would activate cPLA₂ via phosphorylation events. EAG is antagonistic to the AAG activation of PKC, thus it was not expected to act via phosphorylation of cPLA₂. Neutrophils were primed with either AAG or EAG and then stimulated with fMLP. When neutrophils were primed with 5–20 μM 1,2-diacylglycerol, a shift was observed in cPLA₂ migration on SDS-PAGE gels, consistent with phosphorylation of the protein. This gel shift was not seen after exposure to EAG. AAG also caused a parallel increase in enzymatic activity of cPLA₂ that was not seen with EAG. We also investigated whether either diglyceride would cause similar priming or direct secretion of sPLA₂. Both AAG and EAG directly caused significant secretion of neutrophil sPLA₂. EAG also increased the release of sPLA₂ in cells subsequently stimulated with fMLP. Thus, AAG activated cPLA₂ and stimulated secretion of sPLA₂. In contrast, EAG did not activate cPLA₂, but directly activated secretion of sPLA₂. We also demonstrated that human synovial fluid sPLA₂ increased AA release from resting and fMLP-stimulated neutrophils. Given that diglycerides prime for release of AA, PAF, and LTB₄, these current data support the hypothesis that such priming may be mediated by phosphorylation dependent (cPLA₂) or phosphorylation independent (e.g. secretion of sPLA₂) events.

The biosynthesis of cholesterol, dolichol and dolichyl-P were investigated in a murine model of Niemann–Pick type C disease using both in vitro and in vivo systems. In vivo incorporation of [³H]mevalonate into squalene, dolichol and dolichyl-P decreased. The amount of dolichyl-P was elevated due to a decrease in the rate of degradation. Labeling of squalene and cholesterol of liver homogenates in vitro was decreased in the diseased mice and a lowering of microsomal activities of both HMG-CoA reductase and squalene synthase were also observed. In experiments with brain homogenate, decreased [³H]mevalonate labeling of squalene, cholesterol and dolichol was found in vitro. The decreases in cis-prenyltransferase and squalene synthase activities were observed at a very early phase of the disease. In contrast to the decreased biosynthesis of cholesterol observed in vitro, the labeling of total liver cholesterol was found to be increased in Niemann–Pick type C liver upon in vivo investigation, possibly due to the accumulation of this lipid as a result of a deficient transport process. In the brain, where in vivo labeling reflects only biosynthesis, a decreased rate of cholesterol synthesis was demonstrated.