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

Previous results suggested that changes in the activity of nitric oxide (NO) can influence metabolism of apo B-containing lipoproteins. Therefore, we studied effects of exogenous NO donors and physiological NO precursors on metabolism of these lipoproteins. In rabbits, addition of 0.03% sodium nitroprusside (NaNP) to a semipurified, cholesterol-free, casein diet counteracted the elevation of LDL cholesterol induced by this diet but did not alter liver lipids after 4 weeks of feeding. In HepG2 cells, addition of nontoxic concentrations of another NO donor, S-nitroso-N-acetylpenicillamine (SNAP) to culture medium caused a dose-dependent reduction of medium apo B after 24 h. At the concentration 0.5 mM, SNAP significantly decreased medium apo B by 50% without altering total synthesis and secretion of proteins and without altering rates of cellular sterol synthesis. In cells incubated with l-arginine, reduction of medium apo B was not associated with increased NO production whereas in those exposed to N–OH–Arg medium apo B levels were not altered. We concluded that synthetic NO donors can reduce hypercholesterolemia by affecting apo B metabolism directly in the liver, via the sterol-independent mechanism.

Dietary 18 and 20-carbon fatty acids of the n-6 and the n-3 families are metabolized to 22:5,n-6 and 22:6,n-3 by a sequence of specific desaturases and chain elongation via 24-carbon intermediates. This pathway is regulated so that more 22:6,n-3 than 22:5,n-6 is found in the tissues. Rat testis is an exception since 22:5,n-6 is present in large proportions in this organ. Therefore rat testis appears to be interesting for studies of the detailed synthesis of 22:5,n-6 compared with that of 22:6,n-3. By using fresh preparations of rat testicular cells from 19-day-old rats enriched in Sertoli cells, we compared the metabolism of 1-¹⁴C-labelled n-3, n-6 and n-9 fatty acids. The testicular cells actively synthesized 22:6,n-3 and 22:5,n-6, but not 22:4,n-9 from the 18 and 20-carbon precursors. Of 200 mol ¹⁴C-labelled C₁₈ and C₂₀ fatty acids added initially, approximately 20-40 mol were found as 24-carbon intermediates after 24 h of incubation. This indicates that the balanced capacity of elongation, desaturation and chain shortening favours the accumulation of 24-carbon intermediates in these cells. One exception was [1-¹⁴C]20:3,n-9 which was efficiently elongated to 22:3,n-9 but not to C₂₄ fatty acids. Our data suggests that the poor elongation of n-9 fatty acids from C_22⁻ to C₂₄ may be an important hindrance in the synthesis of 22:4,n-9. The efficient synthesis of 22:5,n-6 may also partly explain why this is the major 22-carbon fatty acid in rat testis.

We have investigated the role and mechanism of protein kinase C (PKC) isoforms in endothelin-1 (ET-1)-induced arachidonic acid (AA) release in cat iris sphincter smooth muscle (CISM) cells. ET-1 increased AA release in a concentration (EC₅₀=8 nM) and time-dependent (t_1/2=1.2 min) manner. Cytosolic phospholipase A₂ (cPLA₂), but not phospholipase C (PLC), is involved in the liberation of AA in the stimulated cells. This conclusion is supported by the findings that ET-1-induced AA release is inhibited by AACOCF₃, quinacrine and manoalide, PLA₂ inhibitors, but not by U-73122, a PLC inhibitor, or by RHC-80267, a diacylglycerol lipase inhibitor. A role for PKC in ET-1-induced AA release is supported by the findings that the phorbol ester, PDBu, increased AA release by 96%, that prolonged treatment of the cells with PDBu resulted in the selective down regulation of PKCα and the complete inhibition of ET-1-induced AA release, and that pretreatment of the cells with staurosporine or RO 31-8220, PKC inhibitors, blocked the ET-1-induced AA release. Gö-6976, a compound that inhibits PKCα and β specifically, blocked ET-1-induced AA release in a concentration-dependent manner with an IC₅₀ value of 8 nM. Thymeatoxin (0.1 μM), a specific activator of PKCα, β, and γ induced a 150% increase in AA release. Treatment of the cells with ET-1 caused significant translocation of PKCα, but not PKCβ, from cytosol to the particulate fraction. These results suggest that PKCα plays a critical role in ET-1-induced AA release in these cells. Immunochemical analysis revealed the presence of cPLA₂, p42^mapk and p44^mapk in the CISM cells. The data presented are consistent with a role for PKCα, but not for p42/p44 mitogen-activated protein kinase (MAPK), in cPLA₂ activation and AA release in ET-1-stimulated CISM cells since: (i) the PKC inhibitor, RO 31-8220, inhibited ET-1-induced AA release, cPLA₂ phosphorylation and cPLA₂ activity, but had no inhibitory effect on p42/p44 MAPK activation, (ii) genistein, a tyrosine kinase inhibitor, inhibited ET-1-stimulated MAPK activity but had no inhibitory effect on AA release in the ET-1-stimulated cells. We conclude that in CISM cells, ET-1 activates PKCα, which activates cPLA₂, which liberates AA for prostaglandin synthesis.

Chronic inflammatory diseases are often accompanied by intense angiogenesis. A model of inflammatory angiogenesis is the murine air pouch granuloma which has a hyperangiogenic component. Proinflammatory lipid mediator generation is also a hallmark of chronic inflammation and the role of endogenous production of these mediators in angiogenesis is not known. The 14 kDa phospholipase A₂ (PLA₂) deacylates phospholipid, liberating arachidonic acid, which is used for leukotriene production, and lysophospholipid, which can drive the production of platelet-activating factor (PAF). Therefore, SB 203347, an inhibitor of the 14 kDa PLA₂, zileuton, an inhibitor of 5-lipoxygenase, and Ro 24-4736 a PAF receptor antagonist were evaluated for their effects in the murine air pouch granuloma. SB 203347 reduced both LTB₄ and PAF, but not PGD₂ levels measured in the day 6 granuloma. This correlated with a significant reduction in angiogenesis. Zileuton reduced LTB₄ levels as expected, but did not significantly inhibit angiogenesis, whereas Ro 24-4736 potently reduced angiogenesis. These data support the hypothesis that PAF, and to a lesser extent leukotrienes contribute to the angiogenic phenotype in chronic inflammation.

1,2-Dimyristoyloxypropane-3-thiophospho(1d-1-myo-inositol) (d-thio-DMPI) was used as a substrate for the continuous assay of phosphoinositide-specific phospholipase C (PI-PLC). Its activity with a Δ(1–132) deletion mutant of mammalian PI-PLCδ₁ is about one-fourth that with PI under similar conditions. Optimal conditions for the assay include 0.2 mM substrate, 0.2 mM Ca²⁺, and a mole ratio of hexadecylphosphocholine detergent to substrate of 2.0. A minimum of about 60 ng of pure enzyme can be detected. The apparent bulk K_m for PI-PLC with d-thio-DMPI under these conditions is about 6 μM. Enzyme activity as a function of surface concentration of substrate shows no sign of saturation up to the maximum mole fraction.

The crtB gene encoding phytoene synthase from the carotenogenic enterobacterium Erwinia uredovora was overexpressed to about 20% of the total cellular protein in Escherichia coli. Formation of the active phytoene synthase had the effect of suppressing the growth of the expressing strain. Presumably inhibition of growth arose from the depletion of the substrate geranylgeranyl pyrophosphate (GGPP) which, in E. coli, is necessary for the synthesis of essential prenylpyrophosphate derivatives. In order to overcome the poor growth characteristics of the phytoene synthase expressing strain, GGPP levels were increased by co-expressing the isoprenoid biosynthetic genes crtE and idi, encoding the Erwinia GGPP synthase and Rhodobacter isopentenyl pyrophosphate isomerase, respectively. The crude enzyme preparation was partially purified 15-fold by chromatography on a DEAE column. A non-radioactive assay was developed that enabled the conversion of GGPP to phytoene. The reaction product was identified by co-chromatography with authentic standards on HPLC systems and comparison of spectral characteristics. The phytoene formed in vitro was present in both a 15-cis and all-trans isomeric configuration. The essential cofactors required were ATP in combinations with either Mn²⁺ or Mg²⁺. The K_m value for GGPP was determined as 41 μM. Phytoene synthesis was inhibited by phosphate ions and squalestatin. The I₅₀ value for the latter inhibitor was 15 μM. Lineweaver–Burk plots showed constant K_m values in the presence or absence of squalestatin.

The actions of neuromedin B (NMB), a recently discovered mammalian bombesin-related peptide, are mediated by interacting with a distinct receptor; however, little is known about its cellular basis of action. Recent studies show activation of phospholipase D (PLD) is an important transduction cascade for a number of GI hormones, especially for stimulation of growth and protein sorting. The purpose of the present study was to determine whether activation of the NMB receptor causes activation of PLD and to explore whether this activation was coupled to PLC activation. Rat C6 glioblastoma cells (C6 cells), which contain a low density of native NMB receptors and BALB 3T3 cells stably transfected with rat NMB receptors, were used. NMB caused a 3-fold increase in C6 cells and an 11-fold increase in rNMB-R transfected cells in PLD activity. Increases in PLD activity were rapid and NMB was 100-fold more potent than gastrin-releasing peptide (GRP). NMB caused a half-maximal increase in [Ca²⁺]_i at 0.2 nM, in [³H]IP and PLD at 1 nM, and half-maximal receptor occupation at 1.2 nM. TPA increased PLD dose-dependently with a half-maximal effect at 60 nM. The calcium ionophore A23187 (1 μM) alone did not increase PLD activity but potentiated the effect of TPA. The Ca²⁺-ATPase inhibitor, thapsigargin, did not affect NMB- or TPA-stimulated PLD activities, although it blocked completely the NMB-induced increase in [Ca²⁺]_i. The PKC inhibitor GF109203X completely abolished TPA-induced PLD activity, however, it only inhibited NMB-induced PLD activity by 20%. The combination of thapsigargin and GF109203X had the same effect as GF109203X alone. These data indicate that NMB receptor activation is coupled to both PLC and PLD. In contrast to a number of other phospholipase C-coupled receptors, NMB receptor stimulated changes in [Ca²⁺]_i do not contribute to PLD activation. Both PKC-dependent and PKC-independent mechanisms are involved in the NMB-stimulated PLD activation with the PKC-independent pathway predominating.

Stimulation of pancreatic islets with glucose induces phospholipid hydrolysis and accumulation of nonesterified arachidonic acid, which may play signaling or effector roles in insulin secretion. Of enzymes that catalyze phospholipid hydrolysis, islet β-cells express low molecular weight secretory phospholipases A₂ (PLA₂) and a Group VI, Ca²⁺-independent PLA₂ (iPLA₂). Previous studies indicate that islets also express a protein recognized by antibodies against a Group IV, cytosolic, Ca²⁺-dependent PLA₂ (cPLA₂). To further examine the possible expression of cPLA₂ by islets, we screened a rat islet cDNA library with a probe that recognizes cPLA₂ sequence, and isolated a full-length cPLA₂ cDNA. The rat islet cPLA₂-deduced amino acid sequence is 96% identical to those of human and mouse cPLA₂. Transfection of COS-7 cells with cPLA₂ cDNA in an expression vector induced expression of Ca²⁺-dependent PLA₂ activity and of a protein recognized by anti-cPLA₂ antibody. Comparison of recombinant islet cPLA₂ and iPLA₂ activities expressed in transfected COS-7 cells indicated that iPLA₂ but not cPLA₂ is stimulated by ATP. Both activities are similarly sensitive to inhibition by arachidonyltrifluoromethyl ketone, but iPLA₂ is more effectively inhibited by a haloenol lactone suicide substrate than cPLA₂. RT-PCR experiments with RNA from purified islet β-cells and from an α-cell-enriched population prepared by fluorescence-activated cell-sorting indicated that cPLA₂ mRNA is more abundant in the β-cell population. Immunoblotting analyses indicate that islets express cPLA₂-immunoreactive protein, and that interleukin-1 does not affect its expression. The cPLA₂ is thus one of at least three classes of PLA₂ enzymes with distinct properties expressed in β-cells.