Polyunsaturated fatty acids are enriched in the plasma membranes of mitogen-stimulated T-lymphocytes

How Tim proteins differentially exploit membrane features to attain robust target sensitivity

Immune surveillance cells such as T cells and phagocytes utilize integral plasma membrane receptors to recognize surface signatures on triggered and activated cells such as those in apoptosis. One such family of plasma membrane sensors, the transmembrane immunoglobulin and mucin domain (Tim) proteins, specifically recognize phosphatidylserine (PS) but elicit distinct immunological responses. The molecular basis for the recognition of lipid signals on target cell surfaces is not well understood. Previous results suggest that basic side chains present at the membrane interface on the Tim proteins might facilitate association with additional anionic lipids including but not necessarily limited to PS. We, therefore, performed a comparative quantitative analysis of the binding of the murine Tim1, Tim3, and Tim4, to synthetic anionic phospholipid membranes under physiologically relevant conditions. X-ray reflectivity and vesicle binding studies were used to compare the water-soluble domain of Tim3 with results previously obtained for Tim1 and Tim4. Although a calcium link was essential for all three proteins, the three homologs differed in how they balance the hydrophobic and electrostatic interactions driving membrane association. The proteins also varied in their sensing of phospholipid chain unsaturation and showed different degrees of cooperativity in their dependence on bilayer PS concentration. Surprisingly, trace amounts of anionic phosphatidic acid greatly strengthened the bilayer association of Tim3 and Tim4, but not Tim1. A novel mathematical model provided values for the binding parameters and illuminated the complex interplay among ligands. In conclusion, our results provide a quantitative description of the contrasting selectivity used by three Tim proteins in the recognition of phospholipids presented on target cell surfaces. This paradigm is generally applicable to the analysis of the binding of peripheral proteins to target membranes through the heterotropic cooperative interactions of multiple ligands.

Vitamin D receptor activation and downregulation of renin-angiotensin system attenuate morphine-induced T cell apoptosis

Opiates have been reported to induce T cell loss. We evaluated the role of vitamin D receptor (VDR) and the activation of the renin-angiotensin system (RAS) in morphine-induced T cell loss. Morphine-treated human T cells displayed downregulation of VDR and the activation of the RAS. On the other hand, a VDR agonist (EB1089) enhanced T cell VDR expression both under basal and morphine-stimulated states. Since T cells with silenced VDR displayed the activation of the RAS, whereas activation of the VDR was associated with downregulation of the RAS, it appears that morphine-induced T cell RAS activation was dependent on the VDR status. Morphine enhanced reactive oxygen species (ROS) generation in a dose-dependent manner. Naltrexone (an opiate receptor antagonist) inhibited morphine-induced ROS generation and thus, suggested the role of opiate receptors in T cell ROS generation. The activation of VDR as well as blockade of ANG II (by losartan, an AT(1) receptor blocker) also inhibited morphine-induced T cell ROS generation. Morphine not only induced double-strand breaks (DSBs) in T cells but also attenuated DNA repair response, whereas activation of VDR not only inhibited morphine-induced DSBs but also enhanced DNA repair. Morphine promoted T cell apoptosis; however, this effect of morphine was inhibited by blockade of opiate receptors, activation of the VDR, and blockade of the RAS. These findings indicate that morphine-induced T cell apoptosis is mediated through ROS generation in response to morphine-induced downregulation of VDR and associated activation of the RAS.

Activation of JNK and c-Jun is involved in glucose oxidase-mediated cell death of human lymphoma cells

Mitogen-activated protein kinases (MAPK) affect the activation of activator protein-1 (AP-1), which plays an important role in regulating a range of cellular processes. However, the roles of these signaling factors on hydrogen peroxide (H(2)O(2))-induced cell death are unclear. This study examined the effects of H(2)O(2) on the activation of MAPK and AP-1 by exposing the cells to H(2)O(2) generated by either glucose oxidase or a bolus addition. Exposing BJAB or Jurkat cells to H(2)O(2) affected the activities of MAPK differently according to the method of H(2)O(2) exposure. H(2)O(2) increased the AP-1-DNA binding activity in these cells, where continuously generated H(2)O(2) led to an increase in mainly the c-Fos, FosB and c-Jun proteins. The c-Jun-NH(2)-terminal kinase (JNK)-mediated activation of c-Jun was shown to be related to the H(2)O(2)-induced cell death. However, the suppression of H(2)O(2)-induced oxidative stress by either JNK inhibitor or c-Jun specific antisense transfection was temporary in the cells exposed to glucose oxidase but not to a bolus H(2)O(2). This was associated with the disruption of death signaling according to the severe and prolonged depletion of reduced glutathione. Overall, these results suggest that H(2)O(2) may decide differently the mode of cell death by affecting the intracellular redox state of thiol-containing antioxidants, and this depends more closely on the duration exposed to H(2)O(2) than the concentration of this agent.

Apoptosis-inducing factor plays a critical role in caspase-independent, pyknotic cell death in hydrogen peroxide-exposed cells

It has been proposed that continuously generated hydrogen peroxide (H(2)O(2)) inhibits typical apoptosis and instead initiates an alternate, apoptosis-inducing factor (AIF)-dependent process. Aside from the role of AIF, however, the detailed morphological characterization of H(2)O(2)-induced cell death is not complete. This study examined the cellular mechanism(s) by which the continuous presence of H(2)O(2) induces cell death. We also further analyzed the precise role of AIF by inhibiting its expression with siRNA. Exposure of cells to H(2)O(2) generated by glucose oxidase caused mitochondrion-mediated, caspase-independent cell death. In addition, H(2)O(2) exposure resulted in cell shrinkage and chromatin condensation without nuclear fragmentation, indicating that H(2)O(2) stimulates a pyknotic cell death. Further analysis of AIF-transfected cells clearly demonstrated that nuclear translocation of AIF is the most important event required for nuclear condensation, phosphatidyl serine translocation, and ultimately cell death in H(2)O(2)-exposed cells. Furthermore, ATP was rapidly and severely depleted in cells exposed to H(2)O(2) generated by glucose oxidase but not by H(2)O(2) added as a bolus. Suppression of the H(2)O(2)-mediated ATP depletion by 3-aminobenzamide led to a significant increase of nuclear fragmentation in glucose oxidase-exposed cells. Collectively, these findings suggest that an acute energy reduction by H(2)O(2) causes caspase-independent and AIF-dependent cell death.

Lysophospholipid acyltransferases: novel potential regulators of the inflammatory response and target for new drug discovery

Molecular and biochemical analyses of membrane phospholipids have revealed that, in addition to their physico-chemical properties, the metabolites of phospholipids play a crucial role in the recognition, signalling and responses of cells to a variety of stimuli. Such responses are mediated in large part by the removal and/or addition of different acyl chains to provide different phospholipid molecular species. The reacylation reactions, catalysed by specific acyltransferases control phospholipid composition and the availability of the important mediators free arachidonic acid and lysophospholipids. Lysophospholipid acyltransferases are therefore key control points for cellular responses to a variety of stimuli including inflammation. Regulation or manipulation of lysophospholipid acyltransferases may thus provide important mechanisms for novel anti-inflammatory therapies. This review will highlight mammalian lysophospholipid acyltransferases with particular reference to the potential role of lysophosphatidylcholine acyltransferase and its substrates in sepsis and other inflammatory conditions and as a potential target for novel anti-inflammatory therapies.

Oxidative stress signalling in the apoptosis of Jurkat T-lymphocytes

The pathways of transduction of oxidative stress signals have been studied using the Jurkat T cell model. The oxidative stress was induced by exposure of the cells to 100 microM H(2)O(2). DNA damage was detected within 15 min after commencement of treatment. DNA damage repair occurred within about 1 h in cells exposed to oxidative stress for 15 min. In continuous exposure to stress, DNA repair was slower and control levels of DNA integrity were not reached. DNA repair did not involve gene transcription. H(2)O(2) at 100 microM caused cell death by necrosis as well as by apoptosis. Both these processes were induced by 15 min exposure to the stress stimulus. However, some important differences were found between necrosis and apoptosis. Necrosis was more rapid, began within an hour of treatment and continued to increase during the full duration of the experiment. But apoptosis was seen after 4 h from treatment and was conspicuous between 6 and 20 h after the start of treatment. The necrotic phase preceded apoptosis, although these did show an overlap. In the necrotic phase, Bcl-2, Caspase 8 genes were down regulated. The 6-20 h phase characterised by a marked increase in apoptosis is accompanied by the up regulation of both Bcl-2 and Caspase genes. Expression of the Fas and p53 genes was not altered in either phase. We also analysed the levels of expression of the scavenging genes whose gene products are involved in detoxification. No modulation of the antioxidant enzymes, catalase, Cu/Zn superoxide dismutase and glutathione peroxidase was detectable.

R3230AC rat mammary tumor and dietary long-chain (n-3) fatty acids change immune cell composition and function during mitogen activation

Because anticancer immunity declines progressively with tumor growth, a major focus of current research in tumor immunology is the development of means to stimulate the host immune system. This study determined the effects of dietary long-chain (n-3) fatty acids and tumor burden on immune cell phospholipid composition and membrane-mediated immune defense in rats implanted with the R3230AC mammary adenocarcinoma. Fischer 344 rats (145 +/- 2 g) were fed one of two semipurified diets (20 g/100 g fat) for 21 d before and 17 d after tumor implantation. Diets provided long-chain (n-3) fatty acids at 0 or 50 g/kg of total fat. Mammary tumor growth was 31% lower (P = 0.1) in rats fed long-chain (n-3) fatty acids. Dietary long-chain (n-3) fatty acids had beneficial effects on several host immune defenses, including activation of CD8(+) T cells and type-1 cytokine (interferon-gamma and tumor necrosis factor-alpha) production (P < 0.05). Upregulated immune function in tumor-bearing rats fed the high (n-3) diet occurred concurrently with specific changes in the major membrane phospholipids phosphatidylcholine and phosphatidylethanolamine in high (n-3)-fed rats. Because membrane composition plays a critical role in immune function, additional work is needed to determine the relationship between alterations in the phospholipid composition of immune cells during cancer and subsequent upregulation of host defense in the tumor-bearing state.

Docosahexaenoic acid-containing phosphatidylcholine affects the binding of monoclonal antibodies to purified Kb reconstituted into liposomes

Class I major histocompatibility complex (MHC I) molecules are transmembrane proteins that bind and present peptides to T-cell antigen receptors. The role of membrane lipids in controlling MHC I structure and function is not understood, although membrane lipid composition influences cell surface expression of MHC I. We reconstituted liposomes with purified MHC I (Kb) and probed the effect of lipid composition on MHC I structure (monoclonal anti-MHC I antibody binding). Four phospholipids were compared; each had a phosphocholine head group, stearic acid in the sn-1 position, and either oleic, alpha-linolenic, arachidonic, or docosahexaenoic acid (DHA) in the sn-2 position. The greatest binding of monoclonal antibody AF6-88.5, which detects a conformationally sensitive epitope in the extracellular region of the MHC I alpha-chain, was achieved with DHA-containing proteoliposomes. Other epitopes (CTKb, 5041.16.1) showed some sensitivity to lipid composition. The addition of beta2-microglobulin, which associates non-covalently with the alpha-chain and prevents alpha-chain aggregation, did not equalize antibody binding to proteoliposomes of different lipid composition, suggesting that free alpha-chain aggregation was not responsible for disparate antibody binding. Thus, DHA-containing membrane lipids may facilitate conformational change in the extracellular domains of the alpha-chain, thereby modulating MHC I function through effects on that protein's structure.

Substrate specificity of acyl-CoA:Lysophospholipid acyltransferase (LAT) from pig spleen

The present investigation was undertaken to gain insights into the nature of both substrate binding sites of acyl-CoA:lysophospholipid acyltransferase (LAT) which could be potentially useful for the identification and purification of this specific acyltransferase. Therefore, we have investigated the specificity of LAT from crude membranes of pig spleen toward various 1-palmitoyl-glycerophospholipids and 1-acyl-glycerophosphocholines (1-acyl-GPC). The enzyme showed the highest specificity toward 1-acyl-GPC and was able to distinguish between the acyl-chain length of the 1-acyl group within the 1-acyl-GPC molecule. We found preferential reactivity in the order C10:0 < C12:0 << C14:0, C18:0, C16:0 < C18:1 of 1-acyl-GPC. Lysophosphatidic acid or 1-O-alkyl-GPC were only poor substrates for the enzyme. In competition studies we could show that palmitic acid, oleic acid, arachidonic acid, and palmitoyl-CoA competitively inhibited LAT activity, whereas the coenzyme A failed to inhibit LAT enzyme activity in a concentration-dependent manner. We concluded that the ligand acyl-CoA is bound via its acyl chain. The finding that palmitoyl-CoA was a poor substrate as well as an inhibitor was the basis for protein purification. When palmitoyl-CoA-agarose was used as matrix for affinity chromatography, LAT enzyme activity was bound and eluted by high salt concentrations yielding an estimated 10-fold purification of the solubilized LAT enzyme.

The mitogen-induced lysophospholipid:acyl-CoA acyltransferase (LAT) expression in human T-lymphocytes is diminished by hydrocortisone

One of the earliest changes observed in activated lymphocytes is the enhanced incorporation of unsaturated fatty acids into membrane phospholipids catalyzed by phospholipases and acyltransferases. This early membrane phospholipid remodeling has been shown to be independent from protein synthesis. We have investigated the oleic acid incorporation into phospholipids of activated T-lymphocytes within hours and present data that the sustained membrane phospholipid remodeling in activated T-lymphocytes was largely decreased by cycloheximide and actinomycin D treatment while neither protein synthesis inhibitor had an effect on the fatty acid incorporation into phospholipids in resting T-lymphocytes. Lisofylline, an inhibitor of lysophosphatidic acid:acyl-CoA acyltransferase, had no inhibitory activity, indicating that the membrane lipid remodeling was not due to fatty acid incorporation into de novo-synthesized phospholipids. The membrane phospholipid alteration induced by mitogens was also diminished by hydrocortisone (HC) in a concentration-dependent manner. The steroid hormone antagonist RU486 failed to reverse but potentiated this inhibitory activity of HC. HC did not affect the fatty acid uptake, and the decrease of fatty acid incorporation into phospholipids induced by HC was accompanied by an increase of fatty acid incorporation into triglycerides, indicating that the inhibitory activity of HC was specific for fatty acid incorporation into phospholipids catalyzed by lysophospholipidacyl-CoA acyltransferase (LAT). HC did not directly inhibit the LAT enzyme activity. From these data we conclude that LAT gene transcription is induced as an early event following T-cell activation. The inhibitory action of hydrocortisone may give new insights into the regulatory mechanisms involved in LAT expression.

1H-NMR visible neutral lipids in activated T lymphocytes: relationship to phosphatidylcholine cycling

Two-dimensional 1H-NMR spectroscopy was used to compare changes in the concentration of isotropically-tumbling neutral lipid during the activation of splenic and thymic T lymphocytes. The concentration of mobile neutral lipid (MNL) was similar in splenic and thymic T cells after 72 h of activation with phorbol myristate acetate and ionomycin. However, after 120 h of activation, MNL concentrations in splenic T cells were more than 3-fold higher than in thymic T cells. An increase in choline (Cho), phosphocholine (PCho) and glycerophosphocholine (GPC) was also observed in both thymic and splenic T cells after 24 h of activation. However, after 72 h of stimulation, Cho and PCho levels had decreased and continued to decline at 96-120 h, while GPC continued to be maintained at elevated levels. The simultaneous increase in MNL and GPC and the decline in Cho and PCho leads us to propose that the synthesis of NMR-visible MNL in activated lymphocytes is linked to the phosphatidylcholine cycle.

Linoleic acid esterified in low density lipoprotein serves as substrate for increased arachidonic acid synthesis in differentiating monocytic cells

The cellular metabolism of albumin- and lipoprotein-bound 18:2(n - 6) during monocytic differentiation was examined in the human premonocytic U937 and Mono Mac 6 cells. Differentiation for 72 h of U937 cells with retinoic acid (RA, 1 microM) or 1,25-(OH)2-vitamin D3 (1,25-D3, 10 nM) and of Mono Mac 6 cells with RA (1 microM) or lipopolysaccharide (LPS, 10 ng/ml) increased the desaturation and elongation of [1-14C]18:2(n - 6) to [1-14C]20:4(n - 6). In undifferentiated U937 and Mono Mac 6 cells, incubations with human LDL (100 micrograms/ml, 18 h) resulted in a 2.5-fold increase in 18:2(n - 6) levels in the cellular phospholipids. Differentiation of U937 cells with RA or or of Mono Mac 6 cells with LPS prior to LDL addition. Significantly reduced 18:2(n - 6) and elevated 20:4(n - 6) levels in cellular phospholipids. This increase in 20:4(n - 6) was likely not due to an increased incorporation of preformed 20:4(n - 6) esterified in LDL, as the receptor-specific degradation of [125I]LDL was reduced in both the RA-treated U937 and LPS-treated Mono Mac 6 cells. In U937 cells incubated with [1-14C]18:2(n - 6), the synthesis of TXB2, PGE2 and HHT could be detected after differentiation with RA. suggesting the availability of [1-14C]20:4(n - 6), derived from [1-14C]18:2(n - 6), for cyclooxygenase metabolism. Our results show that the conversion of 18:2(n - 6) to 20:4(n - 6) increases during monocyte differentiation. The 18:2(n - 6) supplied to the cells via the receptor-mediated uptake of LDL was utilized as substrate for the increased 20:4(n - 6) synthesis.

Identification of two different lysophosphatidylcholine:acyl-CoA acyltransferases (LAT) in pig spleen with putative distinct topological localization

The lysophosphatidylcholine:acyl-CoA acyltransferase (LAT, EC 2.3.1.23) is an integral membrane protein participating in the membrane turnover and the T-cell activation process. Here, we present data that crude membranes of pig spleen contain two different LAT enzyme activities based on topological localization studies and the enzyme specificities towards various acyl-CoAs. When crude membranes are washed with solutions of high ionic strength the supernatant contains a distinct LAT activity that we refer to as peripheral LAT (pLAT). The majority of LAT activity is found in the membrane pellet also after treatment with CHAPS. The CHAPS-insoluble LAT activity is named integral LAT (iLAT) accordingly. While pLAT prefers arachidonoyl-CoA rather than oleoyl-CoA, iLAT shows no specificity towards both unsaturated acyl-CoAs. Further investigations reveal that the CHAPS-insoluble LAT activity in the membranes can be solubilized by n-octyl glucoside and restored to original activity by reconstitution with artificial membranes. The reconstituted iLAT prefers arachidonoyl-CoA rather than oleoyl-CoA. Despite a great deal of effort by several groups little progress has been made so far in LAT purification because of the enzyme instability. We establish experimental conditions that enhance the stability of both enzyme activities and, therefore, allow further protein purification.

Time-course changes in content and fatty acid composition of phosphatidic acid from rat thymocytes during concanavalin A stimulation

Several studies have shown the potential role of phosphatidic acid (PA) as a second messenger in different cell types. Thus, PA has been shown to mimic physiological agonists leading to various cellular responses, such as neurotransmitter and hormone release, cell proliferation by modulating DNA or RNA synthesis, the expression of several proto-oncogenes and growth factors, and the stimulation of enzyme activities such as phospholipase C (PLC), protein kinases and cyclic AMP (cAMP) phosphodiesterase. Stimulation of [3H]arachidonate-labelled rat thymocytes with the mitogen lectin concanavalin A (con A) resulted in enhanced production of radiolabelled PA after only 5 min of activation. The radiolabelled PA increase corresponded to a real increase in PA mass as determined by GLC quantification of its fatty acid content. In the presence of ethanol (0.5%), formation of phosphatidylethanol was not observed after 5 min of con A activation. Pretreatment of cells with R 59022 (10 microM), a diacylglycerol (DAG) kinase inhibitor, showed an inhibition in the formation of radiolabelled PA and in PA mass. These results suggest that the PLC-DAG kinase may be the pathway for PA synthesis in the first minutes of mitogenic thymocyte activation. A detailed analysis of the fatty acid composition showed that the relative amount of unsaturated fatty acids was increased in PA from stimulated cells concomitantly with a decrease in saturated ones; in particular, arachidonic acid was increased approximately 2-fold only 2 min after con A addition whereas palmitic acid was decreased for the whole period investigated (20 min). These changes favour the hydolysis of phosphoinositides rather than phosphatidylcholines by PLC. As PA remains a minor phospholipid, these changes are unlikely to affect cell membrane fluidity; but PA being now well recognized as a potential second messenger, its increased content as well as its increased unsaturation in the fatty acyl moiety might modulate several signalling pathways or the activity of enzymes such as cyclic nucleotide phosphodiesterase, controlling in this way the cellular level of cAMP, a negative regulator of blastic transformation.

Differential signal transduction pathways regulating interleukin-2 synthesis and interleukin-2 receptor expression in stimulated human lymphocytes

In human peripheral blood lymphocytes stimulated via the T-cell antigen receptor/CD3 complex IL-2 synthesis and cellular proliferation were effectively inhibited by a concentration of ouabain as low as 50 nM, whilst the expression of high affinity IL-2 receptors was not influenced. Binding of the monoclonal antibody, BMA 031 to the T-cell antigen receptor/CD3 complex resulted in a bimodal activation of protein kinase C. The activation of protein kinase C-alpha in the early phase of T-lymphocyte activation was not affected by 50 nM ouabain, in contrast sustained activation of protein kinase C-beta, between 90-240 min of stimulation was completely abolished by the cardiac glycoside. When protein kinase C was directly activated by PMA + ionomycin, 50 nM ouabain was ineffective in inhibiting protein kinase C activation, as well as subsequent IL-2 synthesis, suggesting that the glycoside interfered with signal transducing mechanism(s) upstream of the activation of protein kinase C. Ouabain had no influence on the elevation of intracellular calcium concentration in BMA 031 stimulated lymphocytes, ruling out the possibility that it interfered with the T-cell antigen receptor dependent phosphatidylinositol response. In contrast, lysophosphatide acyltransferase catalysed elevated incorporation of polyunsaturated fatty acids was effectively inhibited by low concentrations of ouabain in BMA 031-stimulated T-lymphocytes, whereas stimulation with PMA + ionomycin had no influence on the plasma membrane phospholipid fatty acid metabolism. These results suggest, that differential signal transduction pathways are involved in the activation of protein kinases C-alpha and -beta. They implicate that elevated incorporation of polyunsaturated fatty acids into plasma membrane phospholipids might contribute to sustained activation of protein kinase C-beta, and establish a link between activation of protein kinase C-beta and induction of IL-2 synthesis in human lymphocytes.

Expression of alpha-fetoprotein and interleukin 2 receptors and impairment of membrane fluidity in peripheral blood mononuclear cells from AIDS and related syndromes

We have previously shown that the expression of alpha-fetoprotein (AFP) receptors is impaired in mitogen-activated peripheral blood mononuclear cells (PBMCs) from HIV+ individuals and that this novel abnormality reflects an unusual proliferation response of PBMCs to mitogenic stimuli. Here we comparatively analyze, in PBMCs from patients with AIDS and related syndromes, (1) changes in membrane fluidity, measured as the cholesterol/phospholipid ratio (CH/PL), and (2) changes in the expression of AFP receptors and of the alpha chain of IL-2 receptor (TAC antigen). Relative to normal cells, the expression of AFP and IL-2 receptors appeared considerably reduced in AIDS-related complex (ARC) and AIDS patients. In asymptomatic HIV+ individuals the amount of AFP receptors was within the normal range, whereas that of IL-2 receptors increased twice. CH/PL ratios were significantly lower in PHA-activated than in quiescent PBMCs from healthy donors, which implies a gain in membrane fluidity. For seropositive groups, no statistically significant changes in CH/PL ratios were appreciated on PHA activation. Nevertheless, in HIV+ asymptomatic individuals, the CH/PL ratio of quiescent PBMCs resembled that of PHA-activated PBMCs from healthy donors, suggesting that quiescent PBMCs are in a partially activated or "preactivated" status. With the worsening of the disease, toward ARC and AIDS stages, however, quiescent PBMCs from these groups showed a considerable loss in membrane fluidity, evidenced by elevated values of the CH/PL ratio. This radical change strongly suggest a severe alteration of the lipid metabolism in these cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Endocytosis of three serum proteins of a multigene family and of arachidonic acid in human lectin-stimulated T lymphocytes

Alpha-fetoprotein (AFP), serum albumin (SA), and vitamin D binding protein (DBP) are members of a multigene family of proteins showing high structural homology. AFP and SA exhibit a reciprocal relation during development and carry mostly fatty acids, while DBP carries vitamin D and its metabolites in the plasma. Covalent conjugates of these proteins with horseradish peroxidase (HRP) were used to follow by cytochemistry, at the electron microscope level, the protein uptake and intracellular pathways in peripheral blood human lymphocytes stimulated to blast formation by phytohemagglutinin (PHA). Transferrin (Tf), an iron-binding plasma protein, was used as a control. Combined with the results of competition and saturability experiments reported elsewhere, the ultrastructural observations are in favour of a specific endocytosis of the four proteins through cell surface receptors. Tf and AFP enter the cells via small vesicles and endosomes and move to multivesicular bodies (MVBs) and tubular vesicular elements located in the Golgi-centrosphere region to be finally recycled back into the medium. A noncovalent conjugate of AFP-HRP with 3H arachidonic acid [3H-(20:4)] is strongly internalized at 37 degrees C in PHA-stimulated lymphocytes; the autoradiographic labelling, localized in cellular membranes and mostly in lipid droplets, was only occasionally associated with organelles where the presence of AFP-HRP was cytochemically detected. SA, which competes with AFP for a common binding site on the surface of activated T cells, is endocytosed through small vesicles, endosomes, and MVBs before being released in a degraded form from the cells, in agreement with the localization of SA-HRP in lysosome-like organelles. DBP-HRP is poorly internalized through noncoated vesicles, endosomes, and MVBs and is finally routed to lysosomes. The physiological role of AFP and SA would be to mediate the transfer of fatty acids into cells, while that of DBP would be to facilitate the intracellular delivery of vitamin D.

Incorporation of fatty acids by concanavalin A-stimulated lymphocytes and the effect on fatty acid composition and membrane fluidity

The fatty acid compositions of the neutral lipid and phospholipid fractions of rat lymph node lymphocytes were characterized. Stimulation of rat lymphocytes with the T-cell mitogen concanavalin A resulted in significant changes in the fatty acid composition of both neutral lipids and phospholipids (a decrease in the proportions of stearic, linoleic and arachidonic acids and an increase in the proportion of oleic acid). Membrane fluidity was measured using nitroxide spin-label e.s.r., and increased during culture with concanavalin A. Culturing the lymphocytes in the absence of mitogen did not affect fatty acid composition or membrane fluidity. The uptake and fate of palmitic, oleic, linoleic and arachidonic acids were studied in detail; there was a time-dependent incorporation of each fatty acid into all lipid classes but each fatty acid had a characteristic fate. Palmitic and arachidonic acids were incorporated principally into phospholipids whereas oleic and linoleic acids were incorporated in similar proportions into phospholipids and triacylglycerols. Oleic acid was incorporated mainly into phosphatidylcholine, palmitic and linoleic acids were incorporated equally into phosphatidylcholine and phosphatidylethanolamine, and arachidonic acid was incorporated mainly into phosphatidylethanolamine. Supplementation of the culture medium with particular fatty acids (myristic, palmitic, stearic, oleic, linoleic, alpha-linolenic, arachidonic, eicosapentaenoic or docosahexaenoic acid) led to enrichment of that fatty acid in both neutral lipids and phospholipids. This generated lymphocytes with phospholipids differing in saturated/unsaturated fatty acid ratio, degree of polyunsaturation, index of unsaturation and n - 6/n - 3 ratio. This method allowed the introduction into lymphocyte phospholipids of fatty acids not normally present (e.g. alpha-linolenic) or usually present in low proportions (eicosapentaenoic and docosahexaenoic). These three n - 3 polyunsaturated fatty acids replaced arachidonic acid in lymphocyte phospholipids. Fatty acid incorporation led to an alteration in lymphocyte membrane fluidity: palmitic and stearic acids decreased fluidity whereas the unsaturated fatty acids increased fluidity. It is proposed that the changes in lymphocyte phospholipid fatty acid composition and membrane fluidity brought about by culture in the presence of polyunsaturated fatty acids are responsible for the inhibition of lymphocyte functions caused by these fatty acids.

Cyclosporin A inhibits T cell receptor-induced interleukin-2 synthesis of human T lymphocytes by selectively preventing a transmembrane signal transduction pathway leading to sustained activation of a protein kinase C isoenzyme, protein kinase C-beta

Stimulation of human peripheral blood lymphocytes via T cell receptor/CD3 complex resulted in a bimodal activation of protein kinase(s) C (PKC). Within 10 min of stimulation PKC-alpha was translocated to, and thus activated in, the plasma membranes of human lymphocytes, followed by a fast dissociation of this isotype from the plasma membrane. This short term activation and translocation PKC-alpha proved to be cyclosporin A (CsA) insensitive. After 90 min of stimulation PKC-beta was translocated to and remained bound to the plasma membranes for up to 4 h. Preincubation of human lymphocytes with 200 ng/ml CsA specifically and completely abolished the sustained activation of PKC-beta. Neither the phorbol ester-induced direct activation of PKC nor the specific activity of the plasma membrane-bound enzyme was influenced by CsA, suggesting that a signal transduction pathway leading to sustained activation of PKC-beta was influenced by the immunosuppressive agent. In fact, CsA inhibited, in a concentration-dependent manner, the activation of lysophosphatid acyltransferase-catalyzed elevated incorporation of cis-polyunsaturated fatty acids into plasma membrane phospholipids. While interleukin-2 (IL-2) synthesis and cellular proliferation were completely inhibited by 200 ng/ml CsA in BMA 030- or BMA 031-stimulated cells, expression of high-affinity IL-2 receptors was not influenced by the immunosuppressive drug. These results suggest that synthesis and expression of high-affinity IL-2 receptors might be regulated by a signal-transducing pathway involving activation and translocation of PKC-alpha. Lysophosphatid acyltransferase-catalyzed incorporation of cis-polyunsaturated fatty acids might represent another mechanism of signal transduction implicated in the activation and translocation of PKC-beta, which is specifically inhibited by CsA. Neutralization of PKC-beta by introducing anti-PKC-beta antibodies prevented IL-2 synthesis and proliferation in stimulated human lymphocytes. The results suggest a possible link between activation of PKC-beta and regulation of IL-2 synthesis in activated human lymphocytes. Thus, inhibition of the activation and translocation of PKC-beta by CsA may result in inhibition of IL-2 gene expression in human lymphocytes.

Effect of cocaine and morphine on neutral endopeptidase activity of human peripheral blood mononuclear cells cultured with lectins

We have tested the effect of alkaloids (cocaine, morphine) and enkephalins on neutral endopeptidase of peripheral blood mononuclear cells activated by lectins. When treated with concanavalin A and cocaine, peripheral blood mononuclear cells showed an enhanced activity (+110 per cent) of the membrane neutral endopeptidase, which was not related to the expression of the common acute lymphoblastic leukemia antigen at the cell surface, although both molecules have the identical amino acid sequence. Phytohemagglutinin-P, morphine and synthetic enkephalins did not induce the activity of neutral endopeptidase nor the expression of common acute lymphoblastic leukemia antigen. Our findings suggested that the drugs of abuse, cocaine and morphine, affected specific membrane constituents without altering proliferation, subcellular localization of membrane enzymes or the surface immune phenotype of peripheral blood mononuclear cells.

Properties of a phosphatidylcholine derivative of diphenyl hexatriene (DPH-PC) in lymphocyte membranes. A comparison with DPH and the cationic derivative TMA-DPH using static and dynamic fluorescence

Using static and dynamic fluorescence we studied the fluorescence properties of a phosphatidylcholine analog of 1,6-diphenyl-1,3,5-hexatriene (DPH-PC) incorporated in lymphocyte plasma membranes with respect to DPH and its cationic derivative 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), in order to study if phospholipid derivatives of DPH may be used to investigate structural and physicochemical properties of specific membrane lipid domains. DPH-PC and TMA-DPH showed similar fluorescence polarization values that were significantly higher with respect to DPH, suggesting a localization of the fluorescent portion of DPH-PC in a more ordered region of the membrane which was probably due to the elecrostatic interactions between phospholipid head-groups. The localization of the fluorescent moiety of DPH-PC near the membrane surface was also supported by the study of the fluorescence decay of the three probes using frequency-domain fluorometry. The main lifetime component of DPH-PC was rather similar to that of TMA-DPH (6.74 versus 6.24, ns) but considerably lower with respect to DPH (10.52 ns), in agreement with data obtained from exponential analysis. In lymphocyte membranes obtained from concanavalin A treated cells, a significant decrease of fluorescence polarization has been shown with DPH and its phosphatidylcholine derivative, but not with TMA-DPH. In liposomes obtained from total lipids extracted from lymphocyte membranes, a decrease of fluorescence polarization has been observed only with DPH. Our results suggest that DPH-PC localizes the fluorescent portion of its molecule in membrane microenvironments of different properties with respect to those probed by DPH and TMA-DPH. The use of DPH-phospholipid derivatives and other DPH-probes may represent an useful tool to study plasma membrane heterogeneity in biological membranes.

Alpha-fetoprotein-mediated uptake of fatty acids by human T lymphocytes

The binding to resting and activated T lymphocytes of two radiolabelled fatty acids (oleic and arachidonic) was studied in the presence or in the absence of alpha-fetoprotein (AFP) as carrier protein. Fatty acid binding by resting and activated T lymphocytes was determined at 4 degrees C as a function of the concentration of fatty acid and AFP. Under the conditions employed, the following observations were made: (1) in the presence of AFP, fatty acids (oleic and arachidonic acid) are bound to cells by a two-component pathway; one is a saturable process, evidenced when the fatty acid to AFP (FA/AFP) molar ratio was fixed at 1 and the concentration of the fatty acid and the protein varied from 0.1 to 3.2 microM, and the second is a nonsaturable function of FA/AFP molar ratio and was linearly related to the unbound fatty acid concentration in the medium over the entire range studied; (2) in the absence of AFP, the nonsaturable process appears to be the only component of fatty acid binding; 3) at all tested concentrations of free (unbound) fatty acid in the medium, net fatty acid binding by either resting or activated T cells was considerably greater in the presence than in the absence of AFP; (4) in the presence of AFP, fatty acid binding was much higher in activated T cells than in resting T cells, whereas in the absence of AFP, nonsignificant differences were observed between activated and resting T cells; and (5) the time course of fatty acid and AFP binding at 4 degrees C revealed that, at equilibrium, the number of fatty acid molecules bound to the cell was much greater than that of AFP suggesting an accelerated dissociation of the fatty acid upon interaction of the AFP-fatty acid complex with putative cell receptors. It is concluded to the existence of an AFP/AFP-receptor pathway that facilitates the binding of fatty acids to T lymphocytes, particularly upon their blast transformation. This pathway may fulfill the increased requirement for fatty acids characteristic of proliferating cells and may serve to regulate the endocytosis of fatty acids with modulatory effects on lymphocyte function and to protect cells from their cytotoxic potential when internalized in excess.

Membrane fatty acid changes during the cell cycle of CV-1 cells

Monolayers of CV-1 cells were synchronized at the G1/S boundary of the cell cycle by a 24-h 2 mM thymidine blockade. Uptake of tritiated thymidine indicated that the peak DNA synthesis occurred 6-8 h after release from the block and that cell cycle time was 18-20 h. The fatty acid composition of phospholipids extracted from cells at 0, 7, and 18 h postblockade was measured by gas chromatography. The results indicate cyclic changes in membrane fatty acids with a significant increase in long-chain polyunsaturated fatty acids during the DNA synthesis phase (S phase) of the cell cycle.

Fatty acid metabolism in human lymphocytes. II. Activation of fatty acid desaturase-elongase systems during blastic transformation

The fatty acid desaturation-elongation ability of human T-lymphocytes during blastic transformation was determined both by gas-liquid chromatography and incubation with radiolabeled precursors. Human peripheral blood mononuclear cells (PBMC) were activated with phytohemagglutinin (PHA) and cultured in media supplemented with different fatty acids (18:0, 18:1(n - 9), 18:2(n - 6), 18:3(n - 3) and 20:4(n - 6)) at a final concentration of 30 microM. All the fatty acids added were elongated by activated PBMC and the maximal activity was observed on 20:4(n - 6) (a 25% of conversion to 22:4(n - 6)). Supplementation with stearic acid increased the proportion of oleic (from 21.4% to 23.7%) and eicosaenoic (from 3.1% to 5.7%) acids in cellular lipids, indicating the existence of a delta 9-desaturase activity. Supplementation with linoleic and linoleic acids increased slightly the cell content in their more unsaturated derivatives. Direct measurement of desaturase activities was performed by incubating quiescent and activated PBMC with [1-14C]stearic, [1-14C]linoleic and [1-14C]linolenic acids. Quiescent cells exhibited a very low delta 9-desaturase and no sign of delta 6-desaturase activity. A moderate and progressive activation of delta 9-, delta 6- and delta 5-desaturases was observed during blastic transformation of human PBMC. Up to 8% of 18:0 was converted to monoenes, 4% and 1.5% of 18:2(n - 6) was converted to trienes and tetraenes, respectively, and 14.5% of 18:3(n - 3) was converted to pentaenes. The maximal relative activities were found after 48 h of PHA-stimulation for delta 9-desaturase (around 90 pmol of 18:0 converted per 10(6) cells in the last 24 h) and at 72 h for delta 6- and delta 5-desaturases (around 75 and 140 pmol of 18:2 and 18:3, respectively, converted per 10(7) cells in the last 24 h). Although these activities are not enough to explain all the changes in fatty acid composition of human PBMC during blastic transformation, they may contribute to a more controlled cell phospholipid composition.

Fatty acid metabolism in human lymphocytes. I. Time-course changes in fatty acid composition and membrane fluidity during blastic transformation of peripheral blood lymphocytes

The time-course changes in fatty acid composition of human T-lymphocytes during blastic transformation were analysed, as well as the variations in membrane fluidity determined by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH), using a fluorescence-activated cell sorter. The more important changes observed, in activated relative to quiescent cells, started after 24 h and consisted in an increase in the proportion of oleic (18:1(n - 9)), docosapentaenoic (22:5(n - 3)) and docosahexaenoic (22:6(n - 3)) acids and a decrease in that of linoleic (18:2(n - 6)) and arachidonic (20:4(n - 6)) acids. This represented a relative increase of 26% for 18:1, 56% for 22:5 and 84% for 22:6 in peripheral blood mononuclear cells (PBMC) and 35%, 182% and 94%, respectively, in purified T-lymphocytes, both activated for 72 h. The decrease in n - 6 fatty acids was of 42% for 18:2 and 14% for 20:4 in PBMC and 30% and 19%, respectively, for 72 h. The decrease in n - 6 fatty acids was of 42% for 18:2 and 14% for 20:4 in PBMC and 30% and phosphatidylethanolamine) rather than neutral lipids. The 18:1/18:0 ratio increased greatly in major cell phospholipids. The proportion of 20:4, 22:5 and 22:6 in phosphatidylinositol was not significantly altered after 72 h of activation. The molar ratio cholesterol/phospholipids was reduced in 72-h-activated lymphocytes (0.29) compared to quiescent cells (0.5). On the other hand, the stimulation of human T-lymphocytes caused a significant decrease in the order parameter (S) of DPH, according to the observed changes in lipid composition. After 72 h in culture, the S value for quiescent and stimulated T-lymphocytes was 0.530 and 0.326, respectively. In conclusion, the blastic transformation of human T-lymphocytes is associated with changes in lipid composition which modify the physical properties of their membranes. These modifications could modulate, in turn, the activity of membrane proteins implicated in the process of blastic transformation.

Altered arachidonic acid metabolism during differentiation of the human monoblastoid cell line U937

The human cell line U937 was used as a model for differentiation along the mononuclear phagocyte lineage. Following treatment with the phorbol ester TPA, PGE2 and TxB2 secretion was induced 50-100-fold, and both PGF2 alpha and PGI2 levels became detectable in the supernatant of TPA-differentiated U937 cells. The content of the prostaglandin precursor, arachidonic acid, remained unchanged in the cellular phospholipids of undifferentiated and TPA-differentiated U937 cells. Of the enzymes involved in the availability and metabolism of arachidonic acid, phospholipase A2 activity was increased 2-fold in the membranes of TPA-differentiated U937 cells, whereas lysophosphatide acyltransferase activity remained unaltered. Cyclooxygenase activity, however, was enhanced 5-10-fold, which was due to enhanced expression of the enzyme as demonstrated by dot-blot analysis. The data suggest that the capacity to secrete prostaglandins is acquired during differentiation with TPA and results mainly from an increased cyclooxygenase activity. Despite the capacity of TPA-differentiated U937 cells to synthesize prostaglandins, none of the known monocytic stimuli further stimulated prostaglandin secretion in TPA-differentiated U937 cells. Generation of leukotrienes appears to represent a later state in the differentiation along the monocyte-macrophage lineage, since neither LTB4 nor cysteinyl-leukotrienes were detectable in the supernatants of either undifferentiated or TPA-differentiated U937 cells.

Effects of linoleic acid and mitogenic stimulation on the fatty acid composition of human lymphocytes

Perturbation of the fatty acid composition of human lymphocytes in vitro was investigated by addition of linoleic acid complexed to bovine serum albumin (BSA-LA) and by mitogenic stimulation with phytohaemagglutinin (PHA). BSA-LA resulted in a 45% increase in linoleic acid in phosphatidylethanolamine (PE) and over 100% in phosphatidylcholine (PC) in peripheral blood cells. Supplementation with BSA-LA in PHA-stimulated lymphocytes produced even greater changes: 100% increase in linoleic acid content for PE and over 300% for PC. There was a large decrease in oleic acid: 40% for PE and almost 100% in PC. Significant decreases in arachidonic acid occurred in both phospholipid fractions. PHA alone also altered membrane phospholipid fatty acid composition, with reductions in palmitic, stearic and linoleic acid for PE and increases in oleic acid and arachidonic acid (almost 100%). For PC, there were large decreases in stearic (40%), linoleic (30%) and arachidonic (40%) acids, together with an increase in oleic acid (65%). Cells supplemented with linoleic acid grown in the presence of PHA, compared with those grown in linoleic acid-supplemented medium alone, showed a 40% decrease in palmitic acid and a 55% increase in arachidonic acid in PE. For PC, there were large decreases in stearic acid (40%) and arachidonic acid (57%). Antibody-induced redistribution of surface molecules ('capping') was inhibited by some 14% after incubation with BSA-LA. However, no consistent alterations in PHA-induced cell proliferation were observed. These data suggest that profound alterations of membrane fatty acid composition occur spontaneously during the mitotic cycle, and may be further induced by experimental manipulation, without gross perturbation of cell function.

Structurally distinct plasma membrane regions give rise to extracellular membrane vesicles in normal and transformed lymphocytes

Shedding of extracellular membranes from the cell surface may be one of the means through which cells communicate with one another. In an attempt to elucidate whether cell surface exfoliation is a directed or random process, we investigated the membrane lipid and protein composition and membrane lipid order of shed extracellular membranes and of plasma membranes from which they arose in normal circulating lymphocytes and in the B-lymphoblastoid cell lines Raji, WI HF2 729 and the T-lymphoblastoid cell line Jurkat. Extracellular membranes derived from transformed cell lines were more rigid as assessed by steady state polarization of 1,6-diphenylhexatriene (DPH) and were highly enriched in cholesterol when compared with the corresponding plasma membrane. The extracellular membranes from normal lymphocytes, on the other hand, were more fluid and contained more polyunsaturated acyl chains than did the plasma membranes from these cells. Our results suggest that extracellular membranes are shed from specialized regions of the lymphocyte plasma membrane and that membrane exfoliation is likely to be a directed event.

Plasma membrane lipid composition of vinblastine sensitive and resistant human leukaemic lymphoblasts

Plasma membranes purified 32- to 45-fold were isolated from leukaemic T-lymphoblasts, both sensitive and resistant to the Vinca alkaloid vinblastine. On development of drug resistance there was a very significant elevation of ether lipid content. 1-0-alkyl phospholipid increased by 200% with a smaller 30% increase in 1-0-alkenyl phospholipid. Cholesterol and phospholipid levels were also found to increase by 50% and 30% respectively, while the lipid to protein ratio increased by 60%. More modest changes were observed in the fatty acid composition of the membranes, with an alteration in the double bond index from 35.3 to 41.2. These lipid changes may have important implications in the changes to membrane permeability that develop with drug resistance.

Alterations of cellular lipids in rat thymocytes during cell cycle progression

This study provides information on the lipid content and composition of rat thymocytes during cell cycle progression. Cultured rat thymocytes were stimulated by concanavalin A and interleukin-2 to enter the cell cycle and to proliferate. At indicated intervals during a 96 h culture, cells were analyzed for lipid content and lipid synthesis. A growth-related 3.8-fold increase in the cellular cholesterol and a 2.8-fold increase in the phospholipid content was observed after 48 h of culture. Cholesterol synthesis, measured by [14C]acetate incorporation, increased 34-fold, whereas the synthesis of the other lipids was enhanced only 15-fold. Concerning the percentage distribution of the total fatty acids, arachidonic acid showed a significant decline between 24 and 72 h of culture, being mainly caused by the release of arachidonic acid from phosphatidylcholine. Addition of actinomycin D, an inhibitor of DNA replication, prevented the decline of cellular arachidonic acid content in a dose-dependent manner. These data suggest that both the pronounced rise of the cholesterol content as well as the decline of arachidonic acid content are closely linked to cell proliferation.