The phosphatidate-phosphoinositide cycle: an intracellular messenger system in the action of hormones and neurotransmitters

A review of the role of inositols in conditions of insulin dysregulation and in uncomplicated and pathological pregnancy

Inositols, a group of 6-carbon polyols, are highly bioactive molecules derived from diet and endogenous synthesis. Inositols and their derivatives are involved in glucose and lipid metabolism and participate in insulin-signaling, with perturbations in inositol processing being associated with conditions involving insulin resistance, dysglycemia and dyslipidemia such as polycystic ovary syndrome and diabetes. Pregnancy is similarly characterized by substantial and complex changes in glycemic and lipidomic regulation as part of maternal adaptation and is also associated with physiological alterations in inositol processing. Disruptions in maternal adaptation are postulated to have a critical pathophysiological role in pregnancy complications such as gestational diabetes and pre-eclampsia. Inositol supplementation has shown promise as an intervention for the alleviation of symptoms in conditions of insulin resistance and for gestational diabetes prevention. However, the mechanisms behind these affects are not fully understood. In this review, we explore the role of inositols in conditions of insulin dysregulation and in pregnancy, and identify priority areas for research. We particularly examine the role and function of inositols within the maternal-placental-fetal axis in both uncomplicated and pathological pregnancies. We also discuss how inositols may mediate maternal-placental-fetal cross-talk, and regulate fetal growth and development, and suggest that inositols play a vital role in promoting healthy pregnancy.

Metabolic control and future opportunities for growth regulation

The last 10 years have seen a significant expansion in the scope of attempts to manipulate the growth of animals (Buttery, Lindsay and Haynes, 1986). The expansion of interest has been driven by a number of factors, both economic and theoretical. At the economic level the need to develop energetically and economically efficient strategies of animal production has been coupled with a renewed awareness of the implications for human health of excessive intakes of saturated fats. Emphasis then has switched from the maximization of weight gain as an end in itself towards a need to promote protein deposition at any given intake and, at the same time, to reduce the fat content of meat and meat products. These twin objectives might be achieved by one of three strategies: the promotion of protein deposition alone, because at any given rate of weight gain this will tend to minimize the rate of fat deposition (the so-called repartitioning effect); the reduction of fat gain (an approach that has received particularly close attention by those concerned primarily with human obesity); or ideally the simultaneous promotion of protein accretion and depression of that of fat.

Involvement of calcium-independent phospholipase A2 in uterine stromal cell phospholipid remodelling

The role of Ca2+-independent phospholipase A2 (iPLA2) in arachidonic (AA) and docosahexaenoic (DHA) acid incorporation and phospholipid remodelling in rat uterine stromal cells (UIII cells) was studied. Incorporation of AA and DHA into UIII cell phospholipids was Ca2+-independent. Bromoenollactone (BEL), a potent inhibitor of iPLA2, reduced lysophosphatidylcholine level and AA incorporation into phospholipids by approximately 20%. DHA incorporation was not affected by BEL, indicating that the pathways for AA and DHA incorporation are partially different. In control cells, the transfer of AA occurred mainly from diacyl-glycerophosphocholine (GroPCho) to alkenylacyl-glycerophosphoethanolamine (GroPEtn) and to a lesser extent from diacyl-GroPCho to diacyl-GroPEtn. [3H]DHA was redistributed from diacyl-GroPCho and alkylacyl-GroPEtn to alkenylacyl-GroPEtn. BEL treatment inhibited completely the redistributrion of AA within diacyl-GroPCho and diacyl -GroPEtn and reduced the [3H]DHA content of diacyl-GroPEtn, indicating that a BEL-sensitive iPLA2 controls the redistribution of polyunsaturated fatty acids to diacyl-GroPEtn. In contrast the redistribution of radioactive AA and DHA to alkenylacyl-GroPEtn was almost insensitive to BEL. The analysis of substrate specificity and BEL sensitivity of iPLA2 activity indicates that UIII cells exhibit at least two isoforms of iPLA2, one of which is BEL-sensitive and quite selective of diacyl species, and another one that is insensitive to BEL and selective for alkenylacyl-GroPEtn. Taken together, these results suggest that several iPLA2 participate independently in the remodelling of UIII cell phospholipids.

Studies of the role of group VI phospholipase A2 in fatty acid incorporation, phospholipid remodeling, lysophosphatidylcholine generation, and secretagogue-induced arachidonic acid release in pancreatic islets and insulinoma cells

An 84-kDa group VI phospholipase A2 (iPLA2) that does not require Ca2+ for catalysis has been cloned from Chinese hamster ovary cells, murine P388D1 cells, and pancreatic islet beta-cells. A housekeeping role for iPLA2 in generating lysophosphatidylcholine (LPC) acceptors for arachidonic acid incorporation into phosphatidylcholine (PC) has been proposed because iPLA2 inhibition reduces LPC levels and suppresses arachidonate incorporation and phospholipid remodeling in P388D1 cells. Because islet beta-cell phospholipids are enriched in arachidonate, we have examined the role of iPLA2 in arachidonate incorporation into islets and INS-1 insulinoma cells. Inhibition of iPLA2 with a bromoenol lactone (BEL) suicide substrate did not suppress and generally enhanced [3H]arachidonate incorporation into these cells in the presence or absence of extracellular calcium at varied time points and BEL concentrations. Arachidonate incorporation into islet phospholipids involved deacylation-reacylation and not de novo synthesis, as indicated by experiments with varied extracellular glucose concentrations and by examining [14C]glucose incorporation into phospholipids. BEL also inhibited islet cytosolic phosphatidate phosphohydrolase (PAPH), but the PAPH inhibitor propranolol did not affect arachidonate incorporation into islet or INS-1 cell phospholipids. Inhibition of islet iPLA2 did not alter the phospholipid head-group classes into which [3H]arachidonate was initially incorporated or its subsequent transfer from PC to other lipids. Electrospray ionization mass spectrometric measurements indicated that inhibition of INS-1 cell iPLA2 accelerated arachidonate incorporation into PC and that inhibition of islet iPLA2 reduced LPC levels by 25%, suggesting that LPC mass does not limit arachidonate incorporation into islet PC. Gas chromatography/mass spectrometry measurements indicated that BEL but not propranolol suppressed insulin secretagogue-induced hydrolysis of arachidonate from islet phospholipids. In islets and INS-1 cells, iPLA2 is thus not required for arachidonate incorporation or phospholipid remodeling and may play other roles in these cells.

The ontogenesis of lithium-induced effects on suckling: inhibition and facilitation

The ontogenesis of the effect of lithium on suckling behavior was assessed by administering lithium carbonate directly and acutely to 15-, 20-, 30-, and 35-day-old rat pups. Lithium significantly interfered with nipple attachment in 15-day-old rat pups in a dose-dependent pattern, but it facilitated attachment at some doses (40, 60, 80 mg/kg) in weanling-age rat pups. Furthermore, lithium pretreatment reversed quipazine-induced interference of attachment in weanling-age rats. These effects are similar to those previously reported with serotonergic antagonists, suggesting a similar mechanism, perhaps via the inositol phosphate second messenger system.

Possible role of phospholipase C in the regulation of cell division in normal and neoplastic cells

It is proposed that a phosphatidylinositol-specific phospholipase C (PLC) enzyme may be present in abnormally high concentrations in certain cancer cells, and that the elevated activity may explain many, if not all, of the neoplastic characteristics of the cancer cells. There have thus far, been two reports in which PLC activity has been found to be elevated several fold in neoplastic cells. The products of the action of PLC on the phosphoinositides, including diglycerides and inositol phosphates, have been shown to activate the process of cell division by elevating the intracellular concentration of calcium ions and by stimulating the activity of protein kinase C. An elevated content of PLC in at least certain neoplastic cells could thus explain uncontrolled proliferative processes in those cells.

Tumor promoters as probes of protein kinase C in dog thyroid cell: inhibition of the primary effects of carbamylocholine and reproduction of some distal effects

The acute effects of phorbol esters, used as probes of protein kinase C activation, were studied on dog thyroid slices incubated in vitro. The derivatives used were: tetradecanoylphorbol acetate (TPA), phorbol-12,13, didecanoate (PDD), phorbol-12,13-diacetate (PDA), and phorbol dibutyrate (PDBu) and as inactive controls, phorbol itself, phorbol-12, myristate and phorbol-13, acetate, in concentrations ranging from 5.10(-8) to 5.10(-6) mol/L. The active phorbol esters had no effect on basal cyclic AMP concentrations; they inhibited cyclic AMP accumulation induced by prostaglandin E1 but not that induced by thyrotropin (TSH) 1 mU/mL and forskolin 10 mumol/L. Phorbol esters like carbamylcholine acutely stimulated iodide organification and inhibited the stimulation of hormone secretion resulting from TSH, Cholera Toxin, forskolin, and Bu2-cyclic AMP action. These metabolic effects did not require the presence of extracellular Ca++, and could not be antagonized by Ca++ depletion or manganese addition. The active phorbol esters abolished the cyclic AMP independent increased PI turnover induced by TSH 10 mU/mL or carbamylcholine (Cchol) 10(-6) mol/L but did not affect the basal incorporation of 32P into phosphatidylinositol. They reduced the 45Ca efflux from preloaded slices below basal levels and blocked the increased 45Ca release induced by TSH and Cchol. They also inhibited the increase in cyclic GMP concentrations resulting from Cchol action but not the effect of the ionophore A23187 (10(-5) mol/L) nor the basal levels of cyclic GMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissociation of phosphatidylinositol hydrolysis and insulin secretion of cultured mouse pancreatic islets

The correlation between hydrolysis of phosphatidylinositol (PI) and insulin secretion was investigated in cultured mouse pancreatic islets. After prelabeling with [3H] glycerol or [3H] inositol, islets were incubated in the presence of different insulin secretagogues. Carbamylcholine induced a rapid decrease of PI-bound radioactivity concurrent with stimulation of insulin secretion, and both these responses were blocked by atropine. After culture at a low (3.3 mM), glucose concentration, carbamylcholine evoked PI hydrolysis, but no insulin secretory response was observed. Carbamylcholine induced PI hydrolysis also under Ca2+-free conditions, which blocked insulin secretion. Stimulation of insulin secretion with high glucose (16.7 mM), K+(25 mM), or arginine (10 mM), or addition of theophylline (5 mM) to high (16.7 mM) glucose, were associated with unchanged rates of PI hydrolysis. In labelling experiments, both carbamylcholine and glucose (16.7 mM) were found to stimulate the incorporation of [3H] inositol into islet PI. These results demonstrate that stimulation of muscarinic cholinergic receptors increases islet PI turnover. There was, however, no general correlation between islet PI turnover and insulin secretion.

The phosphatidylinositide-Ca2+ hypothesis does not apply to the steroidogenic action of corticotropin

The hypothesis that ACTH (corticotropin) stimulates steroidogenesis by a mechanism that involves breakdown of polyphosphoinositides and increase in intracellular Ca2+ (called here the 'phosphatidylinositide-Ca2+ mechanism') was tested in Y-1 adrenal-tumour cells and in bovine fasciculata cells, by using incorporation of 32P and myo-[3H]inositol to study phospholipid metabolism, and quin-2 and fura 2 to measure intracellular Ca2+. As a positive control, we repeated experiments showing that angiotensin II stimulates glomerulosa cells by way of the phosphatidylinositide-Ca2+ mechanism, by using the same methods. With Y-1 and fasciculata cells, no change was observed in the incorporation of either of the labelled precursors into any phosphatidylinositide or into any of three major phosphoinositols, i.e. inositol phosphate, bisphosphate and trisphosphate. Moreover, no change in mass of any of these compounds was seen. No change was observed in the concentration of intracellular Ca2+ in Y-1 or fasciculata cells on addition of ACTH, by using either quin-2 or fura 2. By contrast, decreased incorporation of 32P into phosphatidylinositol bisphosphate and an increase in intracellular Ca2+ were seen when glomerulosa cells were treated with angiotensin II. It is concluded that the phosphatidylinositide-Ca2+ mechanism is not used by Y-1 adrenal or bovine fasciculata cells in the steroidogenic response to ACTH unless the mechanism is radically different from that seen with all other hormones so far tested in which this mechanism occurs.

Arachidonic acid stimulates 45calcium efflux and hPL release in isolated trophoblast cells

Previous investigations from this laboratory have indicated that arachidonic acid stimulates a rapid, dose-dependent and reversible increase in hPL release which is not dependent on cyclooxygenase or lipoxygenase metabolism. To investigate further the mechanism by which arachidonic acid stimulates the release of hPL, the effect of arachidonic acid on the release of 45Ca from perifused cells prelabelled with 45CaCl was examined in an enriched cell culture population of term human syncytiotrophoblast. Arachidonic acid (10-100 microM) stimulated a dose-dependent, rapid, and reversible increase in the release of both 45Ca and hPL from the perifused placental cells. On the other hand, palmitic acid had little effect on either hPL release or 45Ca release even at concentrations as high as 100 microM. Ionophore A23187 (1-10 microM) also stimulated a dose-dependent and reversible increase in hPL release. Since arachidonic acid increases the mobilization of cellular calcium, as reflected by the increased 45calcium efflux, and since an increase in the intracellular calcium concentration appears to stimulate an increase in hPL release, these results suggest that the stimulation of hPL release by arachidonic acid may be due, at lease in part, to the effects of the fatty acid on cellular calcium mobilization.

Presence of membrane-associated phosphatidate phosphohydrolase activity in cultured islets and its stimulation by glucose

The cellular location at which exogenous phosphatidic acid is hydrolysed in cultured neonatal rat islets was examined. Phosphatidate phosphohydrolase activity could be demonstrated in both whole cell sonicates and isolated plasma membranes. In the whole cell fraction phosphatidic acid hydrolysis to diacylglycerol was stimulated 43% by the presence of Mg2+. The activity present in isolated membranes was totally dependent on the presence of Mg2+ and was increased in plasma membranes from glucose-stimulated islets. Following exposure of islets to low glucose concentrations, raising the Ca2+ concentration from 150 nM to 40 microM in the presence of Mg2+ did not affect the formation of diacylglycerol in whole cell fractions or plasma membranes. These results indicate the presence within the islet of membrane-bound phosphatidate phosphohydrolase activity and demonstrate its activation by glucose.

Exogenous steroids alter steroidogenesis in cultured Y-1 adrenal tumor cells by actions preceding cyclic AMP

Using cultured Y-1 mouse adrenal tumor cells which produce 20 alpha-hydroxy-4-pregnen-3-one (20-DHP), it was found that 0.01 mM corticosterone and deoxycorticosterone increased basal and inhibited ACTH-induced 20-DHP production during consecutive 30 and 120 min incubations. Steroid effects were concentration-dependent and reversible. Six other steroids tested did not stimulate 20-DHP production and varied in ability to inhibit ACTH-stimulated steroidogenesis. Experiments demonstrated that 20-DHP production following treatment with cholera toxin, N,0'-dibutyryl cyclic AMP (dbcAMP), or pregnenolone was not inhibited by exogenous steroids. Corticosterone (0.01 mM) increased basal and inhibited ACTH-induced intracellular cyclic AMP (cAMP) production. Cytochalasin D, a microfilament perturbing agent, inhibited steroid-stimulated 20-DHP production, suggesting that ACTH and steroid stimulation mechanisms were similar. These findings taken together suggest that exogenous steroids can alter steroidogenesis by modifying plasma membrane adenylate cyclase activity.

Phospholipids, sterol carrier protein2 and adrenal steroidogenesis

Rat adrenocortical cells and preparations of plasma membrane and mitochondria have been employed to assess the effects of phospholipids and of sterol carrier protein2 (SCP2) on specific aspects of adrenal steroidogenesis. With intact cells, liposomal dispersions of cardiolipin caused significant stimulation of corticosterone output, while preparations of phosphatidylcholine, phosphatidylinositol, or the 4'-phosphate and the 4',5'-diphosphate derivatives of phosphatidylinositol were without effect. With the adrenal plasma membrane preparation, none of the added phospholipids affected either sodium fluoride or ACTH-responsive adenylate cyclase activity. With intact mitochondria, only cardiolipin, among the various phospholipids, tested, caused a concentration-dependent stimulation of pregnenolone production. However, even at the highest concentration of cardiolipin tested (500 microM), the stimulatory effect was only half that observed with 0.7 microM SCP2, and the two effectors were not synergistic. SCP2 caused a redistribution of cholesterol from mitochondrial outer to inner membranes, while cardiolipin, which is an activator of cytochrome P-450scc, had no effect on distribution of mitochondrial membrane cholesterol.

Mechanism of action of gonadotropin-releasing hormone stimulated Leydig cell steroidogenesis. III. The role of arachidonic acid and calcium/phospholipid dependent protein kinase

Gonadotropin-releasing hormone agonist (GnRHa) markedly increased testosterone formation from 2.35 +/- 0.13 ng/ml of the controls to 14.92 +/- 0.33 ng/ml (mean +/- SE) in isolated and purified rat Leydig cells. GnRHa-induced testosterone formation was completely blocked by phospholipase A2 inhibitor (chloroquin, 10(-4) M), but was potentiated by the addition of either cyclo-oxygenase inhibitor (indomethacin) or lipoxygenase inhibitor (nordihydroguaiaretic acid, NDGA). Arachidonic acid also directly stimulated Leydig cell steroidogenesis and activated Ca/phospholipid dependent protein kinase. Steroidogenic effects of arachidonic acid were also potentiated by the addition of either indomethacin or NDGA. These results suggest that arachidonic acid may be important in mediating direct stimulatory effects of GnRH on Leydig cell steroidogenesis, and the conversion of arachidonic acid to either prostaglandins or leukotrienes is not required for its steroidogenic effect.

alpha-Adrenoceptor stimulated lymphocytes trigger the mechanical response of vas deferens: participation of arachidonic acid metabolites

Normal human lymphocytes (L) (8 X 10(5) ml-1) incubated with methoxamine (Me) (1 X 10(-7) M) (Me-L) triggered the mechanical response of the isolated vas deferens of the rat. L or Me alone did not modify this contractile activity at the concentrations cited above. Me alone (10(-6) to 10(-3) M) increased the tension of the vas. In the presence of L (8 X 10(5) ml-1) the dose-response curve to Me shifted to the left and the efficacy of Me was enhanced. Inhibitors of alpha 1-adrenoceptors completely blocked the reaction between Me and L while drugs that block alpha 1 and alpha 2-adrenoceptors reduced the reaction between Me-L and the vas deferens. Direct contact of Me-L with the assay organ was not necessary. Cell-free supernatants of L exposed to Me (Me-L supernatants) elicited the reaction in the same way as Me-L. This effect required the continuous presence of Me since dialyzed Me-L supernatants were inactive. Inhibitors of lipoxygenase(s) completely blocked the positive inotropic effect of Me-L or of Me-L supernatants. Inhibitors of cyclo-oxygenase potentiated this effect. These results suggest that Me reacts with alpha 1-adrenoceptors of L. From this reaction, soluble factors are released that potentiate the alpha-adrenoceptor stimulatory effect of Me on the vas deferens as a consequence of the release of oxidative products of the lipoxygenase/s pathway of arachidonic acid.

Rapid mobilization of Ca2+ from rat insulinoma microsomes by inositol-1,4,5-trisphosphate

Several hormones and neurotransmitters raise the cytosolic free Ca2+ concentration by stimulating the influx of Ca2+ and/or by mobilizing stored Ca2+. However, the link between the agonist receptor on the cell surface and the organelle(s) from which Ca2+ is mobilized is unknown. One feature of the agonists that increase cytosolic Ca2+ is their rapid induction of phosphatidylinositol turnover and polyphosphoinositide hydrolysis; in some tissues this leads, within seconds, to a marked accumulation of the water-soluble products, inositol 1,4-bisphosphate ( Ins1 , 4P2 ) and inositol-1,4,5- trisphosphate ( Ins1 ,4, 5P3 ), suggesting that these might mediate Ca2+ mobilization from internal pools. Such an action of Ins1 ,4, 5P3 has recently been inferred from studies with permeabilized pancreatic acinar cells and hepatocytes. Here we show directly that Ins1 ,4, 5P3 rapidly releases Ca2+ from a microsomal fraction of rat insulinoma but not from mitochondria or secretory granules. Moreover, this response is transient and desensitizes the microsomes to subsequent Ins1 ,4, 5P3 additions. These results suggest that Ins1 ,4, 5P3 functions as a cellular messenger inducing early mobilization of Ca2+ from the endoplasmic reticulum.

Insulin activates phospholipase C in fat cells: similarity with the activation of pyruvate dehydrogenase

Phospholipase C (PHL-C) activity determined in homogenates of fat cells treated with physiological concentrations of insulin showed a 2-3-fold increase as compared to controls in the absence of insulin. The changes of PHL-C and pyruvate dehydrogenase (PDH) activity which was measured concomitantly exhibited very similar characteristics as to insulin sensitivity, saturability, time dependence and glucose requirement. Exogenous PHL-C as an activator of PDH in fat cells (Honeyman et al., 1983) also showed a striking similarity to insulin. Our findings strongly suggest that, in fat cells, PHL-C is susceptible to short-term activation by insulin. This effect may be relevant to the mechanism of PDH activation and perhaps to other metabolic actions of insulin.

Suppression of phagocytic function and phospholipid metabolism in macrophages by phosphatidylinositol liposomes

Increased phagocytosis of complement-opsonized vesicles was accompanied by increased phosphatidylinositol (PtdIns) turnover in murine macrophages. However when PtdIns was also present as one of the lipids in the opsonized liposomes, it reduced both phagocytosis and stimulation of endogenous PtdIns turnover. These suppressive effects did not occur with liposomes containing PtdIns phosphate (PtdIns-P). When a monoclonal IgM "anti-PtdIns-P" antibody that bound to inositol phosphate was substituted for antigalactosyl ceramide antibodies for activating complement in the opsonizing process, enhanced phagocytosis occurred normally but increased cellular PtdIns turnover did not occur. Therefore the data show that, although PtdIns-P cannot replace PtdIns for suppressing PtdIns turnover, PtdIns-P can be induced to be suppressive after specific binding to an antibody that recognizes inositol phosphate. We conclude that ingestion of complement-opsonized liposomes by macrophages and complement-induced turnover of cellular PtdIns are separate but related phenomena that can be independently modulated by the polar group of liposomal PtdIns.