The fatty acid composition of 1,2-diacylglycerol and polyphosphoinositides from human erythrocyte membranes

Effects of ischemia on free fatty acids and diacylglycerols in developing rat brain

Post-decapitative ischemic treatment imposed on the developing rat brain elicited a marked increase in its susceptibility to free fatty acid (FFA) release between 14 and 17 days, an observation similar to that reported by Bazan (Acta Physiol. Lat. Am.21, 15, 1971). Although the level of diacylglycerols (DG) also increased during this period, the extent of the increase was not as obvious as the FFA. Ischemic treatment to rats after 17 days of age elicited increases in DG and FFA enriched in stearic and arachidonic acids. The delayed response in susceptibility of brain tissue to ischemia-induced changes seems to suggest that the biochemical mechanism(s) responsible for the FFA and DG release is better correlated to events commensurating with synaptogenesis than with myelination.

Structural requirements of membrane phospholipids for M-type potassium channel activation and binding

M-channels are voltage-gated potassium channels that regulate cell excitability. They are heterotetrameric assemblies of Kv7.2 and Kv7.3 subunits. Their opening requires the presence of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂). However, the specificity of PI(4,5)P₂ as a binding and activating ligand is unknown. Here, we tested the ability of different phosphoinositides and lipid phosphates to activate or bind to M-channel proteins. Activation of functional channels was measured in membrane patches isolated from cells coexpressing Kv7.2 and Kv7.3 subunits. Channels were activated to similar extents (maximum open probability of ∼0.8 at 0 mV) by 0.1–300 μm dioctanoyl homologs of the three endogenous phosphoinositides, PI(4)P, PI(4,5)P₂, and PI(3,4,5)P₃, with sensitivity increasing with increasing numbers of phosphates. Non-acylated inositol phosphates had no effect up to 100 μm. Channels were also activated with increasing efficacy by 1–300 μm concentrations of the monoacyl monophosphates fingolimod phosphate, sphingosine 1-phosphate, and lysophosphatidic acid but not by phosphate-free fingolimod or sphingosine or by phosphate-masked phosphatidylcholine or phosphatidylglycerol. An overlay assay confirmed that a fusion protein containing the full-length C terminus of Kv7.2 could bind to a broad range of phosphoinositides and phospholipids. A mutated Kv7.2 C-terminal construct with reduced sensitivity to PI(4,5)P showed significantly less binding to most polyphosphoinositides. We concluded that M-channels bind to, and are activated by, a wide range of lipid phosphates, with a minimum requirement for an acyl chain and a phosphate headgroup. In this, they more closely resemble inwardly rectifying Kir6.2 potassium channels than the more PI(4,5)P₂-specific Kir2 channels. Notwithstanding, the data also support the view that the main endogenous activator of M-channels is PI(4,5)P₂.

Triacylglycerol in biomembranes

A better knowledge of the biochemical and biophysical properties of cell membranes has revealed fundamental concepts concerning the regulation of cell functions by intrinsic components of the lipid matrix. Membrane lipids exhibit high chemical heterogeneity, with hundreds of distinct chemical species; studies of structure-function relationships have unraveled new roles for an increasing number of these lipids as determinants of membrane structure, anchors for membrane-associated proteins or signalling agents. Recent observations have confirmed triacylglycerol (TG) as a quantitatively minor intrinsic membrane component which seems to play a specific role in important metabolic events such as cell stimulation or transformation and metastatic processes. The rapid turnover of the acyl chains into TG of cell membranes suggests an active metabolism. In the plasma membrane, TG appears to be implicated in the generation of transient non-bilayer domains suspected to be associated with specific cellular events. This paper summarizes the current information on TG metabolism and focuses on the potential role of this neutral lipid species on the structure and function of cell membranes.

Characterization of structural and functional phosphoinositide domains in human erythrocyte membranes

In the erythrocyte membrane, only a fraction (50-60%) of phosphatidylinositol 4,5-bisphosphate (PIP2) and of phosphatidylinositol 4-phosphate (PIP) is rapidly turned over by specific kinases and phosphatases and accessible to hydrolysis by the polyphosphoinositide (PPI)-specific phospholipase C (PLC). To investigate whether the metabolic segregation of PPI resulted from preferential interactions with proteins, we have measured the accessibility of PPI to bee venom phospholipase A2 (PLA2) in native erythrocyte membranes, or after treatments designed to remove peripheral proteins and cytoplasmic domains of integral proteins. In native membranes, PPI, as well as the other major phospholipids, behaved as two distinct fractions (R1 and R2) differing by their sensitivity to PLA2. Such a behavior was not observed in PIP and PIP2 containing artificial vesicles. Evidence was provided that the highly sensitive fraction of PIP and PIP2 (R1) may be identical to the PLC-sensitive and rapidly metabolized pool. Removal of peripheral proteins, followed by proteolysis of the cytoplasmic domain of integral proteins, mainly glycophorins and band 3, led to a reduction of the R1 fraction of PIP and of PIP2. It is proposed that the rapidly metabolized pool of PIP2 and PIP, involved in the regulation of major cellular functions, would be maintained in its functional state through interactions with integral proteins.

Generation of phosphatidic acid during calcium-loading of human erythrocytes. Evidence for a phosphatidylcholine-hydrolyzing phospholipase D

We have studied the mechanism by which calcium-loading of human erythrocytes stimulates phospholipid turnover and generates diacylglycerol and phosphatidic acid. Using quantitative measurement of individual phospholipid classes, we have demonstrated that the amount of phosphatidic acid generated during calcium-loading of intact red cells exceeds the amount of diacylglycerol formed by phospholipase-C-mediated hydrolysis of the polyphosphoinositol lipids and that addition of the diacylglycerol kinase inhibitor, R59022, only partly inhibited this increase. Thus, in contrast to current explanations, the phosphatidic acid generated following calcium-loading of erythrocytes cannot be solely explained by the action of a polyphosphoinositol-lipid-specific phospholipase C with subsequent phosphorylation of diacylglycerol to phosphatidic acid. Our data demonstrate that calcium-loading of intact erythrocytes, but not of red cell ghost membranes, causes a small but significant decrease in the relative amount of phosphatidylcholine (PtdCho). In order to identify the mechanisms responsible for calcium-mediated hydrolysis of PtdCho, we encapsulated Ptd[Me-14C]Cho-containing rat liver microsomes into erythrocytes and studied the generation of [Me-14C]choline and phospho[Me-14C]choline. We found that choline was the only detectable 14C-labeled product. Furthermore, incubation of erythrocytes with calcium under hypotonic conditions and in the presence of [14C]PtdCho vesicles and ethanol resulted in the formation of [14C]phosphatidylethanol. Together, these results suggest that the loss of PtdCho during calcium-loading of human erythrocytes is caused by a previously unrecognized PtdCho-hydrolyzing phospholipase D, resulting in direct generation of phosphatidic acid. Analysis of the molecular species composition of PtdCho, phosphatidic acid, and diradylglycerol, confirm the simultaneous actions of PtdCho-hydrolyzing and polyphosphoinositol-lipid-hydrolyzing phospholipases in calcium-loaded human erythrocytes.

The role of inositol phospholipids in the association of band 4.1 with the human erythrocyte membrane

Band 4.1 is a major protein of the erythrocyte membrane skeleton. It promotes the binding of spectrin to F-actin and may anchor the skeletal network to the plasma membrane via its association with integral membrane proteins. Here, we have investigated the involvement of inositol phospholipids in the binding of band 4.1 to erythrocyte membranes using membrane vesicles stripped of all peripheral proteins at alkaline pH. Trypsinization of these vesicles allows the discrimination of two classes of band 4.1 binding sites: trypsin-sensitive sites (60-65% of the total), largely or exclusively on band 3, and trypsin-resistant sites (35-40% of the total), composed, at least in part, of the glycophorins. ATP depletion or activation of erythrocyte phosphoinositol phospholipase C led to a reduction in membrane phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] content by 20-70% in different experiments. The resulting decrease of band 4.1 binding to vesicles by was variable, but averaged about 15-20%. The same treatments led to an average decrease in the band 4.1 binding capacity of trypsinized vesicles of 55%. Since this is equivalent to a 20% decrease in the binding capacity of non-trypsinized vesicles (consistent with the above result), it indicates that PtdIns(4,5)P2 regulates the binding of band 4.1 only to trypsin-resistant binding sites (and to only a subset of these) accounting for about 15-20% of total band 4.1 binding sites on membranes. We found that hydrolysis of > 95% of PtdIns(4,5)P2 with exogenous phospholipase C-delta (PLC delta) resulted in no further decrease in band 4.1 binding to vesicles than did hydrolysis of 65-70% of PtdIns(4,5)P2 which is accessible to erythrocyte phosphoinositol phospholipase C. This suggests that only 65-70% of total membrane PtdIns(4,5)P2 is involved in regulating band 4.1 binding. Significantly, the pool of PtdIns(4,5)P2 involved is the same pool which can be hydrolysed by erythrocyte phosphoinositol phospholipase C, and which has been shown to be metabolically labile in erythrocytes. The membrane binding capacity for band 4.1 found in this study (averaging 1000 micrograms/mg vesicle protein) is considerably higher than that found in previous studies. The results are consistent with the existence of a binding site for band 4.1 on each copy of the major transmembrane proteins (band 3 and the glycophorins). These results provide new insights into the involvement of membrane inositol phospholipids in cytoskeletal-membrane interactions.

Ectocytosis caused by sublytic autologous complement attack on human neutrophils. The sorting of endogenous plasma-membrane proteins and lipids into shed vesicles

During sublytic complement attack on human neutrophils, plasma-membrane vesicles are shed from the cell surface as a cell-protection mechanism. By using surface-iodinated neutrophils it was found that less than 2% of surface label was recovered in shed vesicles under conditions where 40% of complement component C9 was shed. SDS/PAGE of 125I-labelled shed vesicles and plasma membranes showed differences in iodination pattern, demonstrating the sorting of membrane proteins into the shed vesicles. Analysis of 32P-labelled phospholipids after labeling of neutrophils with [32P]Pi before sublytic complement attack showed the presence of phosphatidic acid, phosphatidylcholine, phosphatidyl-ethanolamine, phosphatidylinositol and polyphosphoinositides in shed vesicles. Quantitative analysis using [3H]acetic anhydride-labelling method showed that the molar proportions of phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine and sphingomyelin were the same in shed vesicles as in plasma membranes. In contrast, the molar proportions of cholesterol and diacylglycerol relative to sphingomyelin were almost twice those found in plasma membranes. The data demonstrate the existence of protein and lipid sorting mechanisms during the formation of shed vesicles when neutrophils are subject to sublytic complement attack. The term 'ectocytosis' is proposed to describe triggered shedding of right-side-out membrane vesicles from the surface of eukaryotic cells.

The differentiating agent, retinoic acid, causes an early inhibition of inositol lipid-specific phospholipase C activity in HL-60 cells

Retinoic acid, a derivative of vitamin A, is shown to inhibit the levels of inositol phosphates and diacylglycerol by 25-30% when added to intact HL-60 cells at concentrations which induce differentiation. The onset of inhibition occurs after 10 min and reaches a maximum at 45 min. To study the mechanism and the site of action of retinoic acid, the activity of the phosphatidylinositol bisphosphate-specific phospholipase C was studied in cells permeabilized with streptolysin O and in membrane preparations. Phospholipase C activity was stimulated either via the guanine nucleotide regulatory protein (G-protein) or directly by Ca2+. Retinoic acid treatment, in a time- and concentration-dependent manner, led to a decrease in phospholipase C activity when stimulated with either GTP gamma S or NaF, both of which activate the enzyme via the G-protein. By contrast, it had no effect on the enzyme activity when stimulated with Ca2+ alone. This indicates that retinoic acid interferes with the coupling of the G-protein and phospholipase C. A relationship between the inhibition of phospholipase C activity and the induction of differentiation by retinoic acid was investigated. Only a small inhibition of GTP gamma S-stimulated phospholipase C activity was observed when an analogue of retinoic acid, etretine or Ro10-1670, with low differentiating activity, was used. Moreover, no inhibition of the GTP gamma S-stimulated phospholipase C activity was observed in an HL-60 sub-line resistant to retinoic acid. These results suggest that phospholipase C inhibition is an important step in the induction of differentiation.

A method for the quantitative analysis of molecular species of alkylacylglycerol and diacylglycerol

We describe a method for the quantitative analysis of molecular species of diacylglycerol and alkylacylglycerol as their diradylglycerobenzoate derivatives. Synthetic internal standards were used to provide quantitative determinations of the low levels of diacylglycerol and alkylacylglycerol and their individual molecular species in cultured cells. Diradylglycerols were isolated by thin-layer chromatography (TLC), converted to their benzoate derivatives and separated into subclasses by TLC. The molecular species of each subclass were analyzed by reversed-phase high performance liquid chromatography. Thirty-six species of diglyceride-type molecules were identified in Madin-Darby canine kidney cells. These cells were shown to contain 7.88 nmoles of diacylglycerol and 3.97 nmoles of alkylacylglycerol per mumole of phospholipid. Both subclasses contain predominantly monoenoic and saturated species. This technique should be valuable for studies examining the origin and metabolism of these important intracellular mediators.

Insulin increases the synthesis of phospholipid and diacylglycerol and protein kinase C activity in rat hepatocytes

The effects of insulin on phospholipid metabolism and generation of diacylglycerol (DAG) and on activation of protein kinase C in rat hepatocytes were compared to those of vasopressin and angiotension II. Insulin provoked increases in [3H]glycerol labeling of phosphatidic acid (PA), diacylglycerol (DAG), and other glycerolipids within 30 s of stimulation. Similar increases were also noted for vasopressin and angiotensin II. Corresponding rapid increases in DAG mass also occurred with all three hormones. As increases in [3H]DAG (and DAG mass) occurred within 30-60 s of the simultaneous addition of [3H]glycerol and hormone, it appeared that DAG was increased, at least partly, through the de novo synthesis of PA. That de novo synthesis of PA was increased is supported by the fact that [3H]glycerol labeling of total glycerolipids was increased by all three agents. Increases in [3H]glycerol labeling of lipids by insulin were not due to increased labeling of glycerol 3-phosphate, and were therefore probably due to activation of glycerol-3-phosphate acyltransferase. Unlike vasopressin, insulin did not increase the hydrolysis of inositol phospholipids. Insulin- and vasopressin-induced increases in DAG were accompanied by increases in cytosolic and membrane-associated protein kinase C activity. These findings suggest that insulin-induced increases in DAG may lead to increases in protein kinase C activity, and may explain some of the insulin-like effects of phorbol esters and vasopressin on hepatocyte metabolism.

Functional heterogeneity of polyphosphoinositides in human erythrocytes

After labelling of erythrocytes with [32P]P1 for 23 h, the specific radioactivities of the phosphomonoester groups of PtdIns4P and of PtdIns(4,5)P2 approached equilibrium values which were close to that of the gamma-phosphate of ATP (78-85%), showing that almost all of these phosphate groups were metabolically active. Phosphoinositidase C (PIC) activation, using Ca2+ and the ionophore A23187, of 32P-prelabelled erythrocytes was used to investigate a possible functional heterogeneity of the phosphoinositides. Hydrolysis of PtdIns(4,5)P2, measured from its radioactivity, decreased as function of the time of prelabelling up to a constant value equal to that measured from its content. In contrast, hydrolysis of PtdIns4P, determined both from radioactivity and from content, was always the same. These data suggest that newly labelled molecules of PtdIns(4,5)P2, initially accessible to PIC, then moved towards a PIC-resistant pool. This was further confirmed by measuring the fraction of labelled PtdIns(4,5)P2 molecules accessible to PIC after a prelabelling period of 5 min and different times of reincubation. Hydrolysis by PIC was also measured in erythrocytes in which the phosphoinositide content had been modified by activation (Mg2+-enriched cells) or inhibition (ATP-depleted cells) of the phosphoinositide kinases. The sizes of the PIC-resistant pools of polyphosphoinositides were not affected by these treatments, indicating that the kinases (and the phosphatases) act on the PIC-sensitive pools. This was also shown by the decrease in the production of Ins(1,4,5)P3 upon PIC activation in ATP-depleted erythrocytes. A model is presented in which the PIC-sensitive pools of polyphosphoinositides are those which are accessible to the kinases and the phosphatases and are rapidly turned over.

Erythrocytes attached to a wheat germ agglutinin coated surface display an altered phospholipid metabolism

Erythrocytes were bound to a lectin-coated surface; the multivalent attachment to this surface resulted in a severe deformation of the cells and an alteration in the cellular phospholipid metabolism. Human erythrocytes were allowed to bind for 20 min at 20 degrees C to polystyrene beads coated with wheat germ agglutinin (WGA beads). The bound erythrocytes were then lysed to produce stroma bound to WGA beads. Control stroma and stroma-WGA beads were incubated at 37 degrees C with gamma-32P-ATP to examine the phospholipid labeling patterns. The control stroma incorporated 32P-label into phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-bisphosphate, in agreement with earlier studies. However, the stroma-WGA beads showed incorporation of 32P-label into phosphatidic acid in addition to that in the phosphoinositides. The quantity of 32P-phosphatidic acid produced during the 20-min assay was 3.23 +/- 0.84 (n = 7) picomoles/micrograms stromal cholesterol; the amount synthesized, however, was dependent on the procedure used to prepare the stroma-WGA beads. If the erythrocytes were bound to the WGA beads at 0 degrees C instead of 20 degrees C, the quantity of 32P-phosphatidic acid produced during the subsequent 37 degrees C assay with gamma-32P-ATP was decreased 4.2 fold; the phosphoinositide labeling pattern was unchanged. In addition, when the time for binding of intact erythrocytes to the WGA beads was varied from 1 to 20 minutes, there was a time-dependent increase in the amount of 32P-phosphatidic acid produced. This induction of phosphatidic acid synthesis could not be duplicated with fluid phase WGA. Therefore, the multivalent binding of intact erythrocytes to WGA beads causes an alteration in phospholipid metabolism.

Phorbol esters inhibit inositol phosphate and diacylglycerol formation in proliferating HL60 cells. Relationship to differentiation

Phorbol 12-myristate 13-acetate (PMA) induces the differentiation of the human promyelocytic cell line, HL60, towards adherent macrophage-like cells within 2 days. We have examined the early effects of PMA on inositol phosphates and on diacylglycerol production, two second messengers derived from inositol lipids. In proliferating HL60 cells, PMA induced a time- and concentration-dependent decrease in inositol phosphate levels. Maximal effects were seen after 1 h at 10 nM PMA. PMA also induced the translocation of protein kinase C from the cytosol to the membrane. Comparison between the differentiating effects of several phorbol esters and of 1-oleoyl-2-acetylglycerol with their ability to inhibit inositol phosphate formation suggests that the two effects are correlated.

Depletion of arachidonic acid from GH3 cells. Effects on inositol phospholipid turnover and cellular activation

We have adapted rat pituitary GH3 cells to grow in delipidated culture medium. In response, esterfied linoleic acid and arachidonic acid become essentially undetectable, whereas eicosa-5,8,11-trienoic acid accumulates and oleic acid increases markedly. These changes occur in all phospholipid classes, but are particularly pronounced in inositol phospholipids, where the usual stearate/arachidonate profile is replaced with oleate/eicosatrienoate (n - 9) and stearate/eicosatrienoate (n - 9). Incubation of arachidonate-depleted cells with 10 microM-arachidonic acid for only 24 h results in extensive remodelling of phospholipid fatty acids, such that close-to-normal compositions and arachidonic acid content are achieved for the inositol phospholipids. In comparison studies with arachidonic acid-depleted or -repleted cells, it was found that the arachidonate content does not affect thyrotropin-releasing-hormone (TRH)-stimulated responses measured at long time points, including [32P]Pi labelling of phosphatidylinositol and phosphatidic acid, stimulation of protein phosphorylation, and basal or TRH-stimulated prolactin release. However, transient events such as stimulated breakdown of inositol phospholipids and an initial rise in diacylglycerol are enhanced by the presence of arachidonate. These results show that arachidonic acid itself is not required for operation of the phosphatidylinositol cycle and is not an obligatory intermediate in TRH-mediated GH3 cell activation. It is possible that any structural or functional role of arachidonic acid in these processes is largely met by replacement with eicosatrienoate (n - 9). However, since arachidonate in inositol phospholipids facilitates their hydrolysis upon stimulation by TRH, arachidonic acid apparently may have a specific role in the recognition of these lipids by phospholipase C.

Evidence from studies employing radioactively labelled fatty acids that the stimulation of flux through the diacylglycerol pool is an early action of vasopressin on hepatocytes

1. In isolated hepatocytes prelabelled with [14C]-arachidonic, -stearic, -linoleic, -oleic or -palmitic acids, vasopressin increased the amount of radioactivity present in diacylglycerols. The largest increase was observed in cells labelled with arachidonic or stearic acids. 2. In cells prelabelled with [14C]- or [3H]-arachidonic acid, the onset of the increase in radioactivity in diacylglycerols induced by vasopressin was slow, the increase was partly dependent on the presence of extracellular Ca2+, and was associated with an increase in radioactivity present in phosphatidic acid which was more rapid in onset. Vasopressin decreased the amount of [3H]arachidonyl-phosphatidylinositol 4,5-bisphosphate, but the magnitude of this decrease was less than 10% of the observed increase in radioactivity in [3H]arachidonyl-diacylglycerol. 3. The concentration of vasopressin which gave half-maximal increase in [14C]arachidonyl-diacylglycerol at low extracellular Ca2+ was 10-fold higher than that which gave half-maximal stimulation of 45Ca2+ efflux. Phenylephrine, but not glucagon, also increased the amount of [14C]arachidonyl-diacylglycerol. 4. It is concluded that an early action of vasopressin on the liver cell is to increase the flux of carbon from phospholipids, including the phosphoinositides, to diacylglycerols.

Effects of the fatty acid composition of phosphatidylserine and diacylglycerol on the in vitro activity of protein kinase C from rat spleen: influences of (n-3) and (n-6) polyunsaturated fatty acids

Protein kinase C from rat spleen was assayed with different phosphatidylserines (PtdSer) and diacylglycerols (DAG): PtdSer from bovine brain containing 0.8% 20:4 (n-6), 1.0% 20:5 (n-3) and 5.7% 22:6 (n-3); PtdSer from trout liver lacking 20:4 (n-6) and containing 0.6% 20:5 (n-3) and 43% 22:6 (n-3); 1-stearoyl-2-arachidonylglycerol prepared from synthetic 1-stearoyl-2-arachidonyl-sn-glycero-3-phosphocholine; DAG, prepared from cod roe phospholipids, containing 2.1% 20:4 (n-6), 11.7% 20:5 (n-3) and 29.0% 22:6 (n-3); 1,2-dioleoylglycerol. When assayed with Ca2+ in the absence of DAG there was no difference in the activity of protein kinase C between the two PtdSer. When assayed with Ca2+ in the absence of PtdSer the (n-6)-rich DAG was 2 fold more active, and the (n-3)-rich DAG 1.3 fold more active than 1,2-dioleoylglycerol. When assayed in the presence of both PtdSer and DAG, the enzyme was equally active with all of the DAG, but was 1.3 fold more active with the PtdSer from bovine brain than with the PtdSer from trout liver.

Separation of phosphoinositides and other phospholipids by two-dimensional thin-layer chromatography

A simple, rapid, two-dimensional TLC system is presented which resolves the four phosphoinositide cycle phospholipids as well as all commonly encountered major and minor phospholipids. Ca2+-free lipid samples are loaded onto silica gel HL plates and developed first in 48:40:7:5 chloroform:methanol:water:concentrated ammonia, and then in 55:25:5 chloroform:methanol:formic acid. The method was applied successfully to human erythrocytes, human platelets, and BL/VL3 murine lymphoma cells.

Stimulation of phosphoinositide metabolism in hamster brown adipocytes exposed to alpha 1-adrenergic agents and its inhibition with phorbol esters

The present experiments were undertaken to investigate the role of the phosphoinositides phosphatidylinositol 4-phosphate (PtdIns-4-P) and phosphatidylinositol 4,5-biphosphate (PtdIns-4,5-P2) in the alpha 1-adrenergic stimulation of respiration in isolated hamster brown adipocytes. Exposure of isolated brown adipocytes to the alpha-adrenergic-receptor agonist phenylephrine provoked a breakdown of 30-50% of the PtdIns-4-P and PtdIns-4,5-P2 after prelabelling of the cells with [32P]Pi. Coincident with the breakdown of phosphoinositides was an accumulation of labelled phosphatidic acid, which continued for the duration of the cell incubation. The time course of phosphoinositide breakdown was defined more precisely by pulse-chase experiments. Under these conditions, phenylephrine caused radioactivity in phosphatidylinositol, PtdIns-4-P and PtdIns-4,5-P2 to fall by more than 50% within 30 s and to remain at the depressed value for the duration of the incubation (10 min). This phospholipid response to alpha-adrenergic stimulation was blocked by exposure of the cells to phorbol 12-myristate 13-acetate (PMA); likewise phenylephrine stimulation of respiration was prevented by PMA. beta-Adrenergic stimulation of respiration and inhibition of respiration by 2-chloroadenosine and insulin were, however, unaffected by treatment with PMA. On the assumption that PMA is acting in these cells as an activator of protein kinase C, these results suggest the selective interruption of alpha-adrenergic actions in brown adipocytes by activated protein kinase C. These findings suggest that breakdown of phosphoinositides is an early event in alpha-adrenergic stimulation of brown adipocytes which may be important for the subsequent stimulation of respiration. The results from the pulse-chase studies also suggest, however, that phenylephrine-stimulated breakdown of inositol phospholipids is a short-lived event which does not appear to persist for the entire period of exposure to the alpha 1-adrenergic ligand.

Inositol phospholipid arachidonic acid metabolism in GH3 pituitary cells

Inositol phospholipids in cultured GH3 cells, a prolactin secreting, thyrotropin-releasing hormone (TRH) sensitive rat pituitary cell line, exhibit a preferential selectivity for incorporating arachidonic acid. Fatty acid composition data show that all inositol phospholipids are enriched in stearic and arachidonic acids to a much greater degree than other cellular phospholipids. Incubation of GH3 cells with radioactive stearate, oleate, arachidonate, eicosapentaenoate or docosahexaenoate also showed that much more stearate and arachidonate were incorporated into inositol phospholipids. In short term incubations with tracer amounts of radioactive arachidonate, incorporation was initially into phosphatidylinositol (PtdIns), with phosphatidylinositol 4-phosphate (PtdIns4P), and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] being labelled at later times. During longer incubations, all of the inositol phospholipids reach equilibrium at about 10 h, and the resulting specific activities of the three fractions were similar. These findings suggest that arachidonate is incorporated initially into PtdIns and that PtdIns is then phosphorylated. There was no release of either arachidonate or eicosanoid products when GH3 cells were incubated with TRH. However, TRH stimulation of 32P-labelled GH3 cells resulted in rapid breakdown of PtdIns(4,5)P2 and PtdIns4P, with concomitant increases in [32P]phosphatidic acid and [32P]PtdIns. When the [32P]PtdIns was further analysed by argentation chromatography to separate PtdIns molecular species, it was found that tetraenoic (stearate/arachidonate) species accounted for 80% of the stimulated labelling. The selectivity for arachidonate incorporation into inositol phospholipids coupled with turnover of the arachidonate-containing molecular species suggests that inositol phospholipids containing arachidonic acid or the diacylglycerol resulting therefrom may play a vital cellular role in GH3 cells. This role may involve the operation of the PtdIns cycle itself rather than a stimulated release of arachidonate for eicosanoid formation.

Calcium-dependent polyphosphoinositide hydrolysis is associated with exocytosis in vitro

Micromolar calcium ions stimulate both exocytosis and polyphosphoinositide hydrolysis in sea urchin egg plasma membrane in vitro. Strontium and barium ions also stimulate both processes equally. Magnesium ions reduce the calcium sensitivity of both. Neomycin, a drug which prevents phosphoinositide hydrolysis, inhibits exocytosis in vitro. We suggest that hydrolysis of plasma membrane phosphoinositides may be an essential step in the fusion of the secretory granule and plasma membranes.

Ethanol and membrane lipids

Although ethanol is known to exert its primary mode of action on the central nervous system, the exact molecular interaction underlying the behavioral and physiological manifestations of alcohol intoxication has not been elucidated. Chronic ethanol administration results in changes in organ functions. These changes are reflective of the adaptive mechanisms in response to the acute effects of ethanol. Biophysical studies have shown that ethanol in vitro disorders the membrane and perturbs the fine structural arrangement of the membrane lipids. In the chronic state, these membranes develop resistance to the disordering effects. Tolerance development is also accompanied by biochemical changes. Although ethanol-induced changes in membrane lipids have been implicated in both biophysical and biochemical studies, measurements of membrane lipids, such as cholesterol content, fatty acid unsaturation, phospholipid distribution, and ganglioside profiles, have not produced conclusive evidence that any of these parameters are directly involved in the action of ethanol. On the other hand, there is increasing evidence indicating that although ethanol in vitro produces a membrane-fluidizing effect, the chronic response to this effect is not to change the membrane bulk lipid composition. Instead, changes in membrane lipids may pertain to small metabolically active pools located in certain subcellular fractions. Most likely, these lipids are involved in important membrane functions. For example, the increase in PS in brain plasma membranes may provide an explanation for the adaptive increase in synaptic membrane ion transport activity, especially (Na,K)-ATPase. There is also evidence that the lipid pool involved in the deacylation-reacylation mechanism (i.e., PI and PC with 20:4 groups) is altered after ethanol administration. An increase in metabolic turnover of these phospholipid pools may have important implications for the membrane functional changes. Obviously, there are other lipid-metabolizing enzyme systems that may exert similar effects but have not yet been investigated in detail. From the results of these studies, it is concluded that the multiple actions of ethanol are associated with changes in enzymic systems important in the functional expression of the membranes.

The fatty acid composition of phosphatidylinositol, phosphatidate and 1,2-diacylglycerol in stimulated human neutrophils

The fatty acid composition of phosphatidylinositol (PtdIns), phosphatidate (PtdOH) and 1,2-diacylglycerol (DG) was determined in cytochalasin B-treated human neutrophils in the presence and in the absence of formylmethionyl-leucyl-phenylalanine. The compositions of PtdOH and DG in stimulated cells resemble closely that of PtdOH in control cells and are quite different from the composition of PtdIns. DG appears to be produced as a subsequent metabolite of PtdOH and not as an intermediate in the conversion of PtdIns into PtdOH. We conclude that PtdOH is produced directly from a small pool of newly synthesized PtdIns in stimulated neutrophils.

A transient increase in diacylglycerols is associated with the action of vasopressin on hepatocytes

Vasopressin induced a transient increase of 50% in the total concentration of diacylglycerols (determined by g.l.c.) in isolated hepatocytes. The increase was maximal at 0.25 min, and the concentration of diacylglycerols in cells treated with vasopressin had returned to the basal value by 4 min. No change in the concentration of diacylglycerols was observed after the treatment of cells with glucagon. The dependency of this effect on the concentration of vasopressin was similar to that of the effect of the hormone on 45Ca2+ efflux measured at 0.1 mM extracellular Ca2+. Vasopressin increased the proportion of arachidonic acid and stearic acid and decreased the proportion of oleic acid present in the diacylglycerols. In hepatocytes prelabelled with [14C]arachidonic acid, vasopressin increased the amount of [14C]diacylglycerol. The effects of vasopressin on the total concentration of diacylglycerols and [14C]diacylglycerol were mimicked by an exogenous phospholipid phosphodiesterase (phospholipase C) from Clostridium perfringens. The results are consistent with the conclusion that the transient increase in diacylglycerols induced by vasopressin is caused by the rapid hydrolysis of both the phosphoinositides and one or more other phospholipids.

The Ca2+-activated polyphosphoinositide phosphodiesterase of human and rabbit neutrophil membranes

Addition of Ca2+ to a plasma-membrane fraction derived from human or rabbit neutrophils led to the specific breakdown of polyphosphoinositides. The degradation products were identified as diacylglycerol and inositol bis- and tris-phosphate, thus demonstrating the presence of a Ca2+-activated phospholipase C. The newly generated diacylglycerol resembled the polyphosphoinositides in its fatty acid composition, and in the presence of MgATP2- it was converted into phosphatidate. These results therefore demonstrate the presence in neutrophil plasma membranes not only of polyphosphoinositide phosphodiesterase but also of diacylglycerol kinase.