Effect of phosphatidylcholine structure on the adenylate cyclase activity of a murine fibroblast cell line

Pharmacological Inhibition of Monoacylglycerol O-Acyltransferase 2 Improves Hyperlipidemia, Obesity, and Diabetes by Change in Intestinal Fat Utilization

Monoacylglycerol O-acyltransferase 2 (MGAT2) catalyzes the synthesis of diacylglycerol (DG), a triacylglycerol precursor and potential peripheral target for novel anti-obesity therapeutics. High-throughput screening identified lead compounds with MGAT2 inhibitory activity. Through structural modification, a potent, selective, and orally bioavailable MGAT2 inhibitor, compound A (compA), was discovered. CompA dose-dependently inhibited postprandial increases in plasma triglyceride (TG) levels. Metabolic flux analysis revealed that compA inhibited triglyceride/diacylglycerol resynthesis in the small intestine and increased free fatty acid and acyl-carnitine with shorter acyl chains than originally labelled fatty acid. CompA decreased high-fat diet (HFD) intake in C57BL/6J mice. MGAT2-null mice showed a similar phenotype as compA-treated mice and compA did not suppress a food intake in MGAT2 KO mice, indicating that the anorectic effects were dependent on MGAT2 inhibition. Chronic administration of compA significantly prevented body weight gain and fat accumulation in mice fed HFD. MGAT2 inhibition by CompA under severe diabetes ameliorated hyperglycemia and fatty liver in HFD-streptozotocin (STZ)-treated mice. Homeostatic model assessments (HOMA-IR) revealed that compA treatment significantly improved insulin sensitivity. The proximal half of the small intestine displayed weight gain following compA treatment. A similar phenomenon has been observed in Roux-en-Y gastric bypass-treated animals and some studies have reported that this intestinal remodeling is essential to the anti-diabetic effects of bariatric surgery. These results clearly demonstrated that MGAT2 inhibition improved dyslipidemia, obesity, and diabetes, suggesting that compA is an effective therapeutic for obesity-related metabolic disorders.

Vitamin B12 deficiency results in the abnormal regulation of serine dehydratase and tyrosine aminotransferase activities correlated with impairment of the adenylyl cyclase system in rat liver

The aim of the present study was to elucidate the mechanism of the vitamin B(12) deficiency-induced changes of the serine dehydratase (SDH) and tyrosine aminotransferase (TAT) activities in the rat liver. When rats were maintained on a vitamin B(12)-deficient diet, the activities of these two enzymes in the liver were significantly reduced compared with those in the B12-sufficient control rats (SDH 2.8 (sd 0.56) v. 17.5 (sd 6.22) nmol/mg protein per min (n 5); P < 0.05) (TAT 25.2 (sd 5.22) v. 41.3 (sd 8.11) nmol/mg protein per min (n 5); P < 0.05). In the B(12)-deficient rats, the level of SDH induction in response to the administration of glucagon and dexamethasone was significantly lower than in the B(12)-sufficient controls. Dexamethasone induced a significant increase in TAT activity in the primary culture of the hepatocytes prepared from the deficient rats, as well as in the cells from the control rats. However, a further increase in TAT activity was not observed in the hepatocytes from the deficient rats, in contrast to the cells from the controls, when glucagon was added simultaneously with dexamethasone. The glucagon-stimulated production of cAMP was significantly reduced in the hepatocytes from the deficient rats relative to the cells from the control rats. Furthermore, the glucagon-stimulated adenylyl cyclase activity in the liver was significantly lower in the deficient rats than in the controls. These results suggest that vitamin B(12) deficiency results in decreases in SDH and TAT activities correlated with the impairment of the glucagon signal transduction through the activation of the adenylyl cyclase system in the liver.

Influence of the Membrane Lipid Structure on Signal Processing via G Protein-Coupled Receptors

We have recently reported that lipid structure regulates the interaction with membranes, recruitment to membranes, and distribution to membrane domains of heterotrimeric Galphabetagamma proteins, Galpha subunits, and Gbetagamma dimers (J Biol Chem 279:36540-36545, 2004). Here, we demonstrate that modulation of the membrane structure not only determines G protein localization but also regulates the function of G proteins and related signaling proteins. In this context, the antitumor drug daunorubicin (daunomycin) and oleic acid changed the membrane structure and inhibited G protein activity in biological membranes. They also induced marked changes in the activity of the alpha(2A/D)-adrenergic receptor and adenylyl cyclase. In contrast, elaidic and stearic acid did not change the activity of the above-mentioned proteins. These fatty acids are chemical but not structural analogs of oleic acid, supporting the structural basis of the modulation of membrane lipid organization and subsequent regulation of G protein-coupled receptor signaling. In addition, oleic acid (and also daunorubicin) did not alter G protein activity in a membrane-free system, further demonstrating the involvement of membrane structure in this signal modulation. The present work also unravels in part the molecular bases involved in the antihypertensive (Hypertension 43:249-254, 2004) and anticancer (Mol Pharmacol 67:531-540, 2005) activities of synthetic oleic acid derivatives (e.g., 2-hydroxyoleic acid) as well as the molecular bases of the effects of diet fats on human health.

Synergistic effects of highly unsaturated fatty acid-containing phosphatidyl-ethanolamine on differentiation of human leukemia HL-60 cells by dibutyryl cyclic adenosine monophosphate

Highly unsaturated fatty acid-containing phospholipid (HUFA-PL) has many nutritional and medical applications. We investigated the effect of HUFA-PL on differentiation of human leukemia HL-60 cells induced by dibutyryl cyclic adenosine monophosphate (dbcAMP). HUFA-containing phosphatidylethanolamine (HUFA-PE), such as salmon testis PE, significantly enhanced dbcAMP-induced cell differentiation. A combined treatment of 200 mM dbcAMP with 50 mM HUFA-PE increased the nitroblue tetrazolium (NBT)-reducing activity, which is an indicator of differentiation, to a level comparable to that in the case of 500 mM dbcAMP treatment. In contrast, HUFA-lyso PE (a monoacyl form) did not exert an enhancing effect on dbcAMP-induced differentiation. The enhancing effect of HUFA-PE was suppressed by a protein kinase C inhibitor, staurosporine, while a protein kinase A inhibitor, H-8, did not suppress the enhancing effect. These findings suggest that HUFA-PE might enhance dbcAMP-induced differentiation through modulation of the protein kinase C signaling pathway in HL-60 cells.

Does membrane fluidity contribute to thermal compensation of beta-adrenergic signal transduction in isolated trout hepatocytes?

The potential role of compensatory adjustments to membrane components in determining the function of the (β)-adrenergic receptor/adenylyl cyclase ((β)-AR/AC) signal-transduction system was studied in isolated hepatocytes of 5 degrees C- and 20 degrees C-acclimated rainbow trout Oncorhynchus mykiss. Rates of epinephrine-stimulated cyclic AMP (cAMP) production, although slowed (by a factor of 1.6- to 2.4-fold) by an acute drop in assay temperature from 20 to 5 degrees C, were significantly temperature-compensated, being approximately twofold higher in hepatocytes of 5 degrees C- than of 20 degrees C-acclimated trout. Membrane order in the bilayer interior of hepatocyte plasma membranes (as assessed by fluorescence polarization of 1, 6-diphenyl-1,3,5-hexatriene) was consistently lower in cold- than in warm-acclimated trout, reflecting an efficacy of homeoviscous adaptation of approximately 50 %. Temperature-induced changes in plasma membrane fatty acid composition (i.e. an increase in the proportions of polyunsaturated fatty acids with acclimation to 5 degrees C) were consistent with both the observed changes in the order of the bilayer interior and the extent of homeoviscous adaptation. However, isothermal fluidization of the bilayer interior by the addition of benzyl alcohol (30 mmol l(−)(1)) decreased rather than increased the rate of cAMP production. Significantly more (1. 81-fold) beta-adrenergic receptors were present in plasma membranes of hepatocytes from 5 degrees C-acclimated (6. 23×10(4)+/−4206 receptors per cell; mean +/− s.e.m., N=3) than 20 degrees C-acclimated fish (3.44×10(4)+/−4360 receptors per cell; N=3) when assayed at the acclimation temperature, whereas the equilibrium dissociation constants (K(d)) (13.73+/−4.33 nmol l(−)(1) at 5 degrees C; 9.75+/−3.29 nmol l(−)(1) at 20 degrees C; N=3) were similar. On the basis of a strong correlation between beta-adrenoceptor number and the rate of cAMP production (r(2)=0. 956), regardless of assay or acclimation temperature, we conclude that modulation of receptor number is the primary acclimatory response of this signal-transduction pathway to temperature change in trout liver.

Impairment of adenylyl cyclase signal transduction in mecobalamin-deficient rats

This study examined alterations in the beta-adrenoceptor-G5-adenylyl cyclase system in cerebral cortex membranes from vitamin B12-deficient rats fed a diet lacking vitamin B12 (mecobalamin) for 15 weeks. Basal, 5(7)-guanylylimidodiphosphate (GppNHp)-, isoproterenol-, and forskolin-stimulated adenylyl cyclase activities were significantly reduced in mecobalamin-deficient rats compared with those in control rats. However, no significant differences were observed in the amount and function of G5- estimated by immunoblotting and guanine nucleotide photoaffinity labeling, respectively, or in the densities and the dissociation constants of beta-adrenoceptors, estimated by [125I] pindolol binding, between control and the deficient rats. These results indicate that vitamin B12 deficiency results in the impairment of the coupling among the beta-adrenoceptor, G5- and the catalytic subunit of adenylyl cyclase, and in dysfunction of the catalytic subunit of the enzyme, suggesting that vitamin B12 participates in the regulation of neuronal adenylyl cyclase signal transduction.

Lipid changes in HL-60 cells on differentiation into macrophages by treatment with a phorbol ester

We studied changes in lipid composition of human promyelocytic leukemia cells (HL-60) on differentiation to the macrophage/monocytic lineage by treatment with the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate. Differentiation was accompanied by: (i) a decrease in the level of phospholipids; (ii) a greater amount of triacylglycerols; (iii) an increase in 1-alk-1'-enyl-2-acyl- and 1-alkyl-2-acyl-sn-glycero-3-phosphoethanolamine and a decrease in 1-alkyl-2-acyl-sn-glycero-3-phosphocholine; and (iv) an increase in the level of arachidonic acid in ethanolamine phospholipids. The increased levels of ether-linked lipids and of arachidonic acid in ethanolamine phospholipids are consistent with an enhanced biosynthesis of platelet-activating factor and eicosanoids, which are particularly important in the macrophage function.

Polyunsaturated fatty acids recruit brown adipose tissue: increased UCP content and NST capacity

To examine the effect of polyunsaturated fatty acids (PUFA) on brown adipose tissue recruitment and on the capacity for nonshivering thermogenesis (NST), mice were fed a diet with a standard fat content (9%) and a normal PUFA content (1.6%) (controls) or a high-PUFA content (4.3%) for 2-3 wk. The resting metabolic rate was somewhat lower in the high-PUFA group, probably due to a lower physical activity whereas the capacity for NST (estimated as the metabolic response to a norepinephrine injection) was higher in the high-PUFA group. There was no significant effect on brown adipose tissue wet weight, protein content, or cytochrome-c oxidase content, but the amount of the uncoupling protein thermogenin (UCP), measured immunologically, was significantly increased in the high-PUFA mice (totally by 41%). It was concluded that short-term feeding of a high-PUFA diet may recruit brown adipose tissue thermogenic capacity; it was speculated that earlier-reported recruiting effects of high-fat diets may not be due solely to the high total fat content of the diet but perhaps also to a specific recruiting property of the high-PUFA diets generally used in this type of investigation.

Effects of ageing and adrenergic stimulation on alpha 1- and beta-adrenoceptors and phospholipid fatty acids in rat heart

The purpose of this study was to examine the influence of ageing on the alterations in binding characteristics of adrenoceptors and membrane phospholipid fatty acids in rat heart following repeated administration of epinephrine. The maximal number of binding sites (Bmax) and dissociation constant (Kd) of [3H]prazosin and [3H]dihydroalprenolol binding to alpha 1- and beta-adrenoceptors, respectively, changed significantly during ageing. The downregulation of alpha 1- and beta-adrenoceptors after repeated epinephrine administration for one week, did not differ with age, but the response of the affinity (1/Kd) of both alpha 1- and beta-adrenoceptors to epinephrine treatment was age dependent. In 3-month-old rats the affinity of alpha 1-adrenoceptors was decreased after epinephrine treatment but the affinity of beta-adrenoceptors was unchanged. In 10- and 23-month-old rats the affinity of beta-adrenoceptors decreased after epinephrine treatment but the affinity of alpha 1-adrenoceptors did not change. During ageing the linoleic acid (18:2(n-6)) level decreased in phosphatidylcholine and the arachidonic acid (20:4(n-6)) level increased in phosphatidylcholine and phosphatidylethanolamine. After epinephrine administration the 18:2(n-6) level decreased and the docosahexaenoic acid (22:6(n-3)) level increased in phosphatidylcholine and phosphatidylethanolamine and those changes were not age dependent. The 20:4(n-6) level increased in phosphatidylcholine after epinephrine administration, but that increase was smaller in old than in young rats. The results show that both ageing and epinephrine administration simultaneously modify the fatty acid composition of membrane phospholipids and the binding properties of alpha 1- and beta-adrenoceptors in rat heart.