Journal of lipid mediators and cell signalling最新文献

Erythropoietin (EPO) is a hormone, as well as a hematopoietic growth factor, that specifically regulates the proliferation and differentiation of erythroid progenitor cells. Although the membrane-bound receptor for EPO has no intrinsic kinase activity, it triggers the activation of protein kinases via phospholipases A₂, C, and D. A cascade of serine and threonine kinases, including Raf-1, MAP kinase and protein kinase C (PKC) is activated following tyrosine phosphorylation. In this study, we have examined whether changes in nuclear PKC and 1,2-diacylglycerol (DAG) are induced following EPO treatment of the murine target cell line, B6SUt.EP. Western blot analysis using isoform-specific antibodies demonstrated the presence of PKCβII, but not PKC α, βI, γ, ε, δ, η, or ζ in the nuclei of cells stimulated with EPO. The increase in nuclear βII levels was accompanied by an immediate rise in DAG mass levels with both of the increases peaking by 1 min. These rapid increases in nuclear DAG and PKCβII expression suggest a mechanism for EPO-induced changes in gene expression necessary for cell proliferation.

Our previous studies have shown that both increase in Ca²⁺ levels and activation of protein kinase C (PKC) are required for monocyte-mediated O₂⁻ production and low density lipoprotein (LDL) peroxidation. Phosphoinositide-specific phospholipase C (phosphoinositidase C or PIC) is believed to mediate release of intracellular Ca²⁺ through InsP₃ formation and activation of PKC through diacylglycerol (DAG). In these studies, we investigated the PIC pathway for its participation in monocytic cell-mediated lipid peroxidation of LDL. We found substantial InsP₃ formation in opsonized zymosan (ZOP)-activated U937-b cells, indicating the activation of PIC. Both inhibition of PIC by the PIC inhibitor U-73122 and reduction of the supply of the precursor lipid by lithium chloride suppressed InsP₃ formation but did not alter LDL lipid peroxidation nor O₂⁻ production by activated cells. Furthermore, we also found that suppression of PIC activity had no substantial inhibitory effect on PKC activity in ZOP-activated human monocytes. Our data suggest that PIC activity is induced upon cell activation resulting in increased levels of InsP₃. The activity of this pathway, however, is not required for cell-mediated O₂⁻ production, PKC activation or LDL oxidation

We studied the structure of N-linked carbohydrates bound to apolipoprotein H by a combination of two methods which make use of lectins. Digoxigenin-labelled lectins are used for the structural characterization of carbohydrate chains of glycoproteins. Concanavalin A lectin affinity chromatography was used to analyse apolipoprotein H according to the characteristics of its carbohydrate chain inner to sialic acid residues. Our results from digoxigenin-labelled lectins analysis showed that apolipoprotein H gave positive bands to SNA, DSA, GNA, PNA and AAA lectins. Apolipoprotein H gave a negative band when reacted with MAA lectin. When we applied apolipoprotein H onto the Concanavalin A lectin column no detectable amounts of protein were eluted with Concanavalin A buffer. After adding a buffer with low sugar concentration (10 mM glucoside) a large amount of apolipoprotein H was recovered. These molecules of apolipoprotein H weakly bound to the lectin. When a higher sugar concentration (500 mM mannoside) was added most of the sample applied was eluted. These molecules of apolipoprotein H firmly bound to the column having high affinity for the lectin. These results combined with those coming from the digoxigen-labeled lectins method enable us to understand the inner structure of carbohydrate chains with their outer branches. Molecules of apolipoprotein H which weakly bind to Concanavalin A could bear complex N-glycans organized in biantennary or truncated hybrid structures. Firmly bound apolipoprotein H referred to molecules rich in N-glycan hybrid structures. They have an outer branch belonging to the high mannose carbohydrate chains which explain the ability to bind to the column and an other main branch bearing the sequence galactose β-(1-4)-N-acetylglucosamine β-(1-2) mannose. Galactose could be the terminal sugar or, alternatively, be masked with sialic acid α-(2-6) terminally linked.

As compared with high dietary linoleate safflower oil, high dietary α-linolenate perilla oil decreased platelet-activating factor (PAF) production by nearly half in calcium ionophore (CaI)-stimulated rat polymorphonuclear leukocytes (PMN). In the CaI-stimulated PMN from the perilla oil group, the accumulated amount of arachidonate (AA) plus eicosapentaenoate (EPA) was 30% less and that of lyso-PAF was 50% less, indicating that the decreased availability of lyso-PAF is a factor contributing to the relatively low PAF production. Consistently, eicosatetraynoic acid (ETYA), a dual inhibitor of cyclooxygenase and lipoxygenase, increased free fatty acids (FFA) and decreased PAF production possibly by decreasing the availability of lyso-PAF. Although, leukotrienes (LTs) have been proposed to stimulate PAF production synergistically, a potent LTB₄ receptor antagonist, ONO-4057, decreased the formation of free fatty acids and LTB₄, but stimulated PAF production somewhat, indicating that LTB₄ may not stimulate PAF production in PMN. Lysophospholipid-induced transacylase (CoA-independent transacylase) activity in PMN homogenates was 25∼30% lower in the perilla oil group but no significant differences were observed in the lyso-PAF acetyltransferase and PAF acetylhydrolase activities between the two dietary groups. Thus, decreased transacylase activity is another factor associated with the relatively low PAF production in the perilla oil group.

Arachidonic acid treatment in isolated liver nuclei resulted in a rapid and transient increase of Ca²⁺concentration in the nucleoplasm which was monitored with the Ca²⁺-sensitive dye fura-2 dextran. This effect was associated with a decrease of Ca²⁺concentration in the nuclear envelope as measured with fura-2 AM. Our results indicate that arachidonic acid causes a Ca²⁺ release from the nuclear envelope to the nucleoplasm similar to that evoked by inositol trisphosphate (IP₃). The arachidonic acid-induced Ca²⁺ mobilization in the nucleus was not due to the metabolites of arachidonic acid. Experiments performed in the presence of ATP and Ca²⁺ indicate that arachidonic acid-induced Ca²⁺ mobilization in the nucleus takes place in a non ATP-dependent way. Taken together, these results suggest that arachidonic acid may contribute to the regulation of nuclear Ca²⁺ mobilization.

The purpose of the present study was to investigate the involvement of nitric oxide (NO) in the modulatory role of platelet-activating factor (PAF, 1-O-hexadecyl-2-acetyl-sn-glyceryl-3-phosphorylcholine), a vasoactive phospholipid mediator synthesized by endothelial cells, on the vascular tone and arterial blood pressure. In pentobarbitone-anaesthetized rabbits, unloading of the carotid sinus baroreceptors by a bilateral carotid artery occlusion elicited a reflex rise in systemic vascular resistance, which was markedly potentiated by pretreating the animals with the PAF receptor antagonist WEB 2086 ([3-4-(2-chlorphenyl-)-9-methyl-6H-thieno-3,2-f-1,2,4-triazolo-4,3-α-1,4-diazepin-2-yl-(4-morpholinyl)-l-propanone]; 5 mg/kg, i.v.). In contrast, the inhibition of the biosynthesis of NO via NO synthase using N^ω-nitro-l-arginine methyl ester (l-NAME) neither affected the systemic vasoconstriction induced by carotid artery occlusion nor modified the potentiating effect of WEB 2086. The haemodynamic alterations induced by l-NAME administration were corrected by continuous infusions of the directly-acting vasodilators sodium nitroprusside or diazoxide. The results of the present study confirm previous studies from our group suggesting the involvement of PAF in a negative feedback mechanism effective in the local regulation of vasomotor tone in anaesthetized rabbits, but exclude the participation of NO in this process.

Insulin resistance is characterized principally by impaired insulin-mediated glucose uptake which provokes a compensatory increase in pancreatic β-cell secretory activity. For a time this may produce well-controlled plasma glucose levels but as the insulin resistance worsens the augmented insulin production becomes inadequate to keep plasma glucose at euglycemia leading to the development of non-insulin dependent diabetes mellitus (NIDDM), accompanied by hyperinsulinemia and hyperglycemia. A number of metabolic defects are associated with NIDDM including obesity, hypercoagulability, cardiovascular disease risk factors such as hypertension and dyslipidemia and these constitute the insulin resistance syndrome. The identity of the biochemical factor that might link all these defects is not yet known. We have hypothsisized that platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF) may be such a link. In this study, we measured plasma acetylhydrolase (EC.1.1.48), which degrades PAF to the inactive metabolise lyso-PAF, as a surrogate for PAF activity in three groups of hypercholesterolemic subjects: lean controls (n=9), non-diabetic obese (n=6) and NIDDM subjects (n=6). The ages and body mass indices of the subjects were 46±3.1 and 24.2±2.2 for the lean controls, 52±2.5 and 28.7±0.9 for the NIDDM subjects and 60±2 and 27.6±2.1 for the obese, non-diabetic subjects (mean±S.E.M.). The measurements were made before and after therapy with the cholesterol-lowering drug lovastatin, a 3-hydroxy 3 methylglutaryl (HMG) coenzyme A reductase inhibitor (40 mg/day) for 3 months. Fasting plasma glucose (FPG) levels were 91±11, 96±3 and 146±11 mg/dl, for the lean, obese and NIDDM subjects, respectively, before therapy began. Lovastatin did not affect FPG in any of the three subject groups. Before treatment, the fasting plasma insulin (FPI) levels were 6.1±0.92, 10.83±2.03 and 14.68±3.64 mU/l for the lean, non-diabetic obese and NIDDM subjects, respectively. After lovastatin therapy only the obese group exhibited a significant change in FPI (15.35±2.47 mU/l) (P<0.05). Total cholesterol levels were similar in all three groups both before and after lovastatin therapy but within each group lovastatin therapy significantly reduced the total cholesterol by 32, 29 and 34% in the lean, obese and NIDDM subject groups respectively (P<0.0001). Lovastatin therapy reduced LDL-cholesterol levels by 40, 32and 46% in the lean, obese and NIDDM subjects, respectively, but produced no significant effect on HDL or triglyceride levels. Before therapy, the plasma acetylyhydrolase activities were 104±7, 164±7 and 179±7 nmol/ml per min in the lean, obese and NIDDM subjects, respectively. Lovastatin therapy reduced plasma acetylhydrolase levels to 70±7, 87±6 and 86±7