The cellular forms and functions of the inositol phospholipids and their metabolic derivatives

Inositol, neural tube closure and the prevention of neural tube defects

Susceptibility to neural tube defects (NTDs), such as anencephaly and spina bifida is influenced by genetic and environmental factors including maternal nutrition. Maternal periconceptional supplementation with folic acid significantly reduces the risk of an NTD-affected pregnancy, but does not prevent all NTDs, and "folic acid non-responsive" NTDs continue to occur. Similarly, among mouse models of NTDs, some are responsive to folic acid but others are not. Among nutritional factors, inositol deficiency causes cranial NTDs in mice while supplemental inositol prevents spinal and cranial NTDs in the curly tail (Grhl3 hypomorph) mouse, rodent models of hyperglycemia or induced diabetes, and in a folate-deficiency induced NTD model. NTDs also occur in mice lacking expression of certain inositol kinases. Inositol-containing phospholipids (phosphoinositides) and soluble inositol phosphates mediate a range of functions, including intracellular signaling, interaction with cytoskeletal proteins, and regulation of membrane identity in trafficking and cell division. Myo-inositol has been trialed in humans for a range of conditions and appears safe for use in human pregnancy. In pilot studies in Italy and the United Kingdom, women took inositol together with folic acid preconceptionally, after one or more previous NTD-affected pregnancies. In nonrandomized cohorts and a randomized double-blind study in the United Kingdom, no recurrent NTDs were observed among 52 pregnancies reported to date. Larger-scale fully powered trials are needed to determine whether supplementation with inositol and folic acid would more effectively prevent NTDs than folic acid alone. Birth Defects Research 109:68-80, 2017. © 2016 The Authors Birth Defects Research Published by Wiley Periodicals, Inc.

In vitro interceptive and reparative effects of myo-inositol against copper-induced oxidative damage and antioxidant system disturbance in primary cultured fish enterocytes

Copper (Cu) is essential for normal cellular processes in most eukaryotic organisms but is toxic in excess. Our previous study reported that a nutrient antioxidant, myo-inositol (MI), can protect fish from Cu-induced oxidative injury; however, the mechanisms involved are not fully understood. Therefore, the present study aimed to analyze potential pathways. First, to investigate the hypothesis that MI protects enterocytes against Cu toxicity via the intercept pathway, enterocytes were treated with different concentrations of MI (0-75mg/L medium) in the presence of 6mg/L of Cu for 24h (Experiment 1). Next, we investigated the potential reparative role of MI after a Cu challenge (Experiment 2). The results of Experiment 1 indicated that cells exposed to Cu alone for 24h exhibited increases in lactate dehydrogenase release (LDH), malondialdehyde (MDA) formation and protein oxidation (P<0.05). Notably, a dose-dependent inhibitory effect on LDH release was observed with all doses of MI. Moreover, co-treatment with MI completely inhibited Cu-induced protein carbonyl (PC) formation. However, Cu-induced lipid peroxidation was not altered by MI co-treatment. Additionally, Cu exposure suppressed total-superoxide dismutase (T-SOD), CuZnSOD and catalase (CAT) activities, and these changes were completely blocked by co-treatment with sufficient MI concentrations. In contrast, cells exposed to Cu exhibited adaptive increases in reduced glutathione (GSH) content and the activities of anti-hydroxyl radical (AHR), glutathione peroxidase (GPx), glutathione-S-transferase (GST) and glutathione reductase (GR). Interestingly, the Cu-stimulated increases in these antioxidants were blocked by co-treatment with sufficient MI concentrations. The results of Experiment 2 indicated that cell injury (LDH release), lipid peroxidation (MDA formation) and protein oxidation induced by Cu were reversed by subsequent MI treatment. Meanwhile, Cu-induced decreases in alkaline phosphatase (AKP), anti-superoxide anion (ASA), T-SOD and CuZnSOD activities were completely restored by subsequent MI treatment, while the reduced CAT activity was partially restored. However, MI rescues partially restored the adaptive increase in GPx activity induced by Cu, whereas the adaptive increase in reduced GSH content was completely reversed by 75mg/L of MI. However, subsequent MI treatments did not alter the induction of GST activity by Cu. In conclusion, we demonstrated for the first time that MI not only protected enterocytes from Cu-induced oxidative damage but also increased the repair activity in primary enterocytes after challenge with Cu. Moreover, MI-mediated increases in antioxidant enzyme activities contributed to lipid and protein oxidant repair.

N-3 polyunsaturated fatty acid supplementation alters inositol phosphate metabolism and protein kinase C activity in adult porcine cardiac myocytes

Several mechanisms have been proposed to explain the anti-arrhythmic effects of n-3 polyunsaturated fatty acids. One mechanism is the effect of modifying cell membrane phospholipid and their subsequent effect on intracellular cell signaling via the second messengers, Ins(1,4,5)P(3) and diacylglycerol. Isolated cardiac myocytes from adult pig hearts were used to investigate the effect of n-3 polyunsaturated fatty acids, eicosapentaenoic acid and docosahexaenoic acid, on the inositol phosphate metabolism and protein kinase C activity. Adult porcine cardiac myocytes were grown in media supplemented with 400 µM arachidonic acid, eicosapentaenoic acid and docosahexaenoic acid. After 24 hr, fatty acid analyses of total lipids by TLC in supplemented cells showed that eicosapentaenoic acid and docosahexaenoic acid were selectively incorporated into the phosphatidylinositol fraction. In the diacylglycerol fraction, there was a small incorporation of both eicosapentaenoic acid and docosahexaenoic acid but it was not significantly different from that of controls. To study the effect of membrane phospholipid modification on the phospholipase C mediated inositol lipid cycle, cardiac myocytes were labeled with 4µCi/ml myo-[2-(3)H]Ins for 48 hr. After stimulation with epinephrine and phenylephrine (alpha-receptor agonist) the water soluble [(3)H]Ins products were separated by chromatography on Dowex AG 1-X8 and measured by scintillation counting. After stimulation, the levels of [(3)H]Ins(1,4,5)P(3) and [(3)H]Ins(1,3,4,5)P(4) in eicosapentaenoic acid and docosahexaenoic acid supplemented myocytes were significantly reduced (P < 0.05) compared to arachidonic acid supplemented myocytes. Similarly, eicosapentaenoic acid and docosahexaenoic acid supplemented cells had reduced levels of protein kinase C activity after stimulation compared to arachidonic acid supplemented cells. From these experiments, it is evident that n-3 PUFA supplementation modulates intracellular cell signaling suggesting a possible anti-arrhythmic mechanism.

Manipulation of lipid composition of rat heart myocytes aged in culture and its effect on alpha1-adrenoceptor stimulation

The fatty acid composition of the phosphoinositides was evaluated in cultured neonatal rat cardiomyocytes during the aging-like process in vitro, comparing data obtained from control and gamma-linolenic acid supplemented cardiomyocytes. The response to alpha1 stimulation was evaluated in both control and supplemented cells to verify the relationship between the alterations of the phosphoinositide fatty acid composition concomitant to culture aging and the cell response to exogenous stimuli. Arachidonate level decreased as a function of age in all the phosphoinositides, which appeared to be more saturated as cells aged in culture. Inositol phosphate production in response to alpha1 stimulation decreased as cells aged in culture. Supplementation of culture medium with gamma-linolenic acid caused significant modifications in the fatty acid pattern of the phosphoinositides, which appeared less saturated than the corresponding fractions isolated from unsupplemented cells during the aging-like process. The modifications induced by the supplementation in the phosphoinositide fatty acid composition prevented the age-related reduction of inositol phosphate production upon stimulation. These results clearly indicate a major role for the lipid composition in determining the response to alpha1 stimulation, suggesting a nutritional approach to overcome some of the impairments of molecular events related to the process of aging.

[Influence of an activator of protein kinase C (TPA) and a calcium-mobilizing agonist (A 23187) on zinc metabolism in the rat]

The influence of 12-O-Tetradecanoylphorbol-13-acetate, an activator of proteinkinase C and A 23187, a calcium ionophore increasing cytosolic free calcium concentration on zinc metabolism was investigated in a study with 24 eight-week old rats. Twenty-four hours before killing, the rats (235 g body weight, 8 per group) were either injected intraperitoneally with TPA (1.6 x 10(-7) mol/kg body weight) or A 23187 (1.6 x 10(-6) mol/kg body weight). Control rats received the solvent dimethylsulfoxide. The application of TPA and A 23187 provoked a marked decline in feed intake accompanied by a reduction in body weight and liver mass. Serum concentrations of zinc were reduced significantly after A 23187 injections. TPA and A 23187 increased liver zinc levels by 20 and 30% respectively, if based on fresh and dry weight. The injections, however, did not alter total liver zinc. Liver metallothionein (MT) concentration was elevated 2.4-fold after TPA administration. The increase in response to A 23187 was only 1.5-fold and not significant. Mucosa MT levels were not altered. Serum activity of alkaline phosphatase was significantly reduced (TPA: -23%, A 23187: -31%). There was no change in serum glucose after injections. However, serum creatinine and urea were increased in response to A 23187. In conclusion, TPA and A 23187 had an effect on zinc metabolism of the rat, most marked in the case of MT induction in the liver. There is evidence that the reduced feed intake caused by TPA and A 23187 resulted in effects indistinguishable from those caused by fasting. Further experiments are needed to clarify whether proteinkinase C and cytosolic free calcium are directly involved in the regulation of zinc metabolism.

Occurrence and functions of the phosphatidylinositol cycle in the myocardium

In the last decade a great deal of attention was awarded to a signal transduction pathway which is utilized primarily by 'Ca2+ mobilizing' signal molecules and which involves the hydrolysis of a quantitatively minor phospholipid, phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) by a PtdIns-specific phospholipase C (PLC). The evidence for the existence of receptor-mediated GTP binding protein-coupled PLC in myocardium and its possible functions are briefly summarized. The minireview is concentrated on the following aspects: 1) cellular localization and synthesis of polyphospho-PtdIns from PtdIns, 2) desensitization of the alpha 1-adrenergic agonist and endothelin-1 mediated PtdIns responses, 3) oscillatory Ca2+ transients initiated by PtdIns(4,5)P2 hydrolysis, 4) polyunsaturated fatty acids as constituents of polyphospho-PtdIns and of the protein kinase C activator 1,2-diacylglycerol (DAG), 5) source other than PtdIns(4,5)P2 contributing to the stimulated DAG, 6) role of the PtdIns pathway in cardiomyocyte growth and gene expression during the hypertrophic response.

Protein kinase C activity in neonatal cultured rat cardiomyocytes supplemented with docosahexaenoic acid

In vitro studies have indicated that the 1-stearoyl, 2-arachidonyl diacylglycerol (DAG) is the most effective one for the activation of protein kinase C, although many other DAGs having a different fatty acid composition are active, but to a different extent. Using cultures of neonatal rat ventricular cells, grown in a medium enriched in docosahexaenoic acid (DHA), we previously obtained a cell population that, after alpha 1-adrenoceptor stimulation, produced a DHA enriched DAG. In this study, we have tested the "in vivo" ability of this modified DAG as protein kinase C activator, demonstrating a lower but more persistent translocation of the enzyme from cytosol to particulate fraction in the DHA treated cells. The differences in the PKC activation pattern could be explained by a different metabolism of the DHA enriched DAG by DAG kinase.

Alpha-1-stimulated phosphoinositide breakdown in cultured cardiomyocytes: diacylglycerol production and composition in docosahexaenoic acid supplemented cells

The fatty acid pattern of phosphatidylinositol and other inositol phospholipids is reported to be predominantly 1-stearoyl, 2-arachidonyl. However, literature does not report data about the effect of a modification of this fatty acid composition on the production and acidic pattern of the diacylglycerol (DAG) formed during phosphoinositide hydrolysis. Culturing cardiomyocytes in a docosahexaenoic acid supplemented medium, we obtained an homogeneous cell population whose phospholipid fatty acid pattern was strongly different from control cells, and which produced, after alpha 1-adrenergic stimulation with phenylephrine, an higher amount of DAG. This DAG was different from control DAG in fatty acid composition, too. This structurally different DAG could be responsible for a different activation pattern of protein kinase C.

Regulation of MI transport in retinal pigment epithelium by sugars, amiloride, and pH gradients: potential impairment of pump-leak balance in diabetic maculopathy

Impairment of transport and metabolism of retinal pigment epithelium (RPE) has been recognized to play a role in the development of diabetic macular edema. To understand the mechanism(s) of action of high glucose levels in alteration of RPE metabolism, primary cultures of RPE cells were used as an in vitro model of diabetic retinopathy/maculopathy. RPE cells were grown with 5 mM (control) or 40 mM glucose (a monosaccharide that enters the cells), or 40 mM sucrose (a disaccharide that does not enter the cells), and the extent of Na(+)-dependent active transport of an osmolyte ([3H]-myo-inositol, MI, 10 microM) into cells was determined. While 40 mM glucose down-regulated 3H-MI transport, 40 mM sucrose stimulated it, compared to 5 mM glucose feeding. Addition of 1 mM amiloride, an inhibitor of Na+/H+ exchanger, in the incubation media, significantly inhibited MI transport. Cells treated with high sucrose or high glucose were more sensitive toward amiloride inhibition, compared to controls. Inhibition of either pump or leak pathway alone was not sufficient to completely inhibit MI transport, but simultaneous inhibition of both pathways, by amiloride and ouabain (1 mM each), strongly inhibited osmolyte accumulation. The strongest inhibition of uptake occurred when 150 mM NaCl in the incubation media was replaced by 150 mM choline-Cl, and the percent inhibition of uptake, with choline-Cl, was highest with sucrose-fed cells, compared to normal or high glucose-fed cells. Imposition of a pH gradient [pHi (6.1) less than pH0 (8.0)] across the cell membrane, a condition that stimulates Na+/H+ exchange activity, also reduced MI accumulation. Cellular water content, measured by the extent of [3H]-3-O-methyl glucose uptake, in the presence of balanced salt solution (BSS), BSS containing half the ionic strength (hypotonic solution), or BSS containing 20 mM K+, for induction of cell swelling, varied when cells were fed with various sugars. Cells fed with high glucose were less sensitive toward media tonicity compared to normal. These results suggested that in cultured RPE cells, changes in Na+/H+ exchanger activity (intracellularly or extracellularly), through its inhibition by amiloride, its activation via intracellular acidification, or perhaps by chronic feeding with high sucrose or high glucose, affected the Na(+)-dependent active accumulation of MI. A metabolic factor involved in the development of diabetic macular edema is perhaps associated with glucose-induced alterations in Na+ fluxes (e.g., changes in Na+/H+ exchanger activity), which can secondarily influence osmolyte accumulation, impairment of pump-leak balance, and/or intracellular pH.(ABSTRACT TRUNCATED AT 400 WORDS)

Kinetics of myo-inositol transport in corneal endothelial cells: diverse effects of sugars and implications in corneal deutergensence [corrected]

Kinetics of myo-inositol (MI) uptake into primary cultures of bovine corneal endothelial cells (CEC) were studied. Confluent corneal endothelial cells accumulated 3H-MI in a time dependent and saturable process. At a narrow range of external concentrations of 3H-MI (4-50 microM), the Na(+)-dependent MI uptake followed saturation kinetics. The apparent Km value was 20 microM with a maximum velocity (Vmax) of 16 pmol/20 min/micrograms DNA. At low external 3H-MI concentrations the uptake was dependent on Na ions, but at higher levels the Na(+)-independent fraction of MI uptake significantly increased. The uptake was sensitive to removal of Ca ions and to the presence of inhibitors such as n-ethyl maleimide, phlorizin, ouabain, and amiloride (an inhibitor of Na+/H+ exchanger). The sensitivity of MI uptake toward inhibitors and ionic changes in the bathing media was reduced as external concentrations of 3H-MI increased. Citrate at 0.5 mM increased the uptake, suggesting involvement of mitochondrial oxidative metabolism in the MI uptake. Percent release of radioactivity by 2 min, after an initial 40-min incubation with 20 microM 3H-MI, was 6.6% +/- 0.8 or 35% +/- 4 when release media contained BSS alone or BSS containing 5 mM nonradioactive MI, respectively. Efflux of radioactivity from the cells also was enhanced when release media contained 40 mM glucose. Glucose and galactose as well as nonmetabolizable glucose analogues, such as 3O-methyl glucose or alpha-methyl glucose, at high concentrations (40 mM), acutely (in the incubation media) or chronically (in the growth media) inhibited MI uptake into CEC, and the extent of inhibition was inversely proportional to the external levels of 3H-MI. However, glucose at lower levels (less than or equal to 10 mM) slightly increased MI uptake. These studies indicated that the uptake of MI into corneal endothelial cells was an Na(+)-dependent active process at a narrow range of external radioactive MI concentrations. Higher levels of MI were taken up by the cells via a passive diffusion mechanism, independent of carrier protein(s). Glucose influenced the uptake of MI in a complex manner. The increased MI efflux by glucose or by MI was perhaps due to the limited capacity of CEC for accumulation or compartmentalization of this or other solutes/osmolytes, a phenomenon that may be related to the role of CEC in maintenance of corneal deutergence. High glucose-induced inhibition of Na(+)-dependent MI uptake may be in part due to glucose regulation of Na+ fluxes and cell volume.(ABSTRACT TRUNCATED AT 400 WORDS)

Hydrolysis of phosphatidylinositol by human pancreatic phospholipase A2

Pure human pancreatic phospholipase A2 efficiently hydrolyzed the 2-ester bond of 14C-2-linoleoyl and 14C-2-arachidonyl phosphatidylinositol (PI). The rate of hydrolysis varied markedly with the bile salt (sodium taurocholate to sodium taurodeoxycholate, 3:4 mol/mol) concentration, the hydrolysis being decreased with increasing bile salt to PI ratio. The influence of bile salts was thus similar to that which has earlier been described for the hydrolysis of phosphatidylcholine (PC) with pig pancreatic phospholipase A2. When 2-3H-arachidonyl PC and 2-14C-arachidonyl PI were incorporated into a mixed substrate, PI was hydrolyzed even faster than PC, the hydrolysis of both phospholipids varying in the same manner with bile salt concentration. 2-14C-arachidonyl PI was also efficiently hydrolyzed by human duodenal content, although at a somewhat slower rate than 2-3H-arachidonyl PC. It is concluded that PI is a good substrate for human phospholipase A2. This minor but arachidonate-rich dietary phospholipid may thus be digested and absorbed by pathways similar to those of the major dietary and bile phospholipid, phosphatidylcholine.

Arginine vasopressin stimulates phosphoinositide turnover in an enriched rat Leydig cell preparation

An enriched rat Leydig cell preparation was preincubated with [14C]arachidonic acid. Stimulation of the cells with arginine vasopressin (AVP) (1 microM) for 2 min caused a significant increase in labelled phosphatidic acid and a significant fall in radioactivity in phosphatidylinositol and phosphatidylinositol 4-monophosphate + phosphatidylinositol 4,5-bisphosphate. Preincubation with dibutyryl cyclic AMP had no effect on the AVP-induced phospholipid turnover. Leydig cells were preincubated with myo-[2-3]inositol for 22 h and then with 10 mM LiCl for 10 min. Exposure to AVP (1 microM) induced a rise in labelled inositol phosphates. The response was inhibited when the cells were preincubated with the phorbol ester, 4 beta-phorbol 12-myristate 13-acetate (0.16 microM) for 10 min. These results provide evidence for an AVP-induced phospholipase C stimulation in rat Leydig cells and suggest a protein kinase C-dependent feedback inhibition of the stimulation. Other agonists that might have a regulatory function in the testis were tested for possible effects on phosphoinositide metabolism. Of prostaglandin E2 (10 microns,) angiotensin II (0.1 microM), and bradykinin (0.9 microM), only the latter induced a significant increase in the labelled inositol phosphates. This suggests that Leydig cells possess a bradykinin receptor which can activate phospholipase C.

Acute effects of prostaglandin F2 alpha on inositol phospholipid hydrolysis in the large and small cells of the bovine corpus luteum

The present studies were conducted to determine whether the large or small bovine luteal cell was the site for the stimulatory effect of prostaglandin F2 alpha (PGF) on phospholipase C-catalyzed inositol phospholipid hydrolysis. Corpora lutea were removed from heifers during the luteal phase of the normal estrous cycle. Small luteal cells were isolated by unit-gravity sedimentation and large luteal cells were isolated by flow cytometry using a Becton Dickson FACS 440 cell sorter. PGF provoked rapid (5-30 s) and sustained (up to 30 min) increases in the levels of inositol mono-, bis-, and trisphosphates (IP, IP2, IP3, respectively) in small luteal cells. IP3 was formed more rapidly than IP2 or IP following PGF treatment. The PGF-stimulated increase in IP3 was accompanied by a transient reduction in the levels of 3H-labeled phosphatidylinositol 4,5-bisphosphate. LiCl (10 mM) enhanced inositol phosphate accumulation in response to PGF. Maximal increases in inositol phosphate accumulation were observed with 1-10 microM PGF and half-maximal increases were observed with 60 nM PGF. PGF (1-10 microM) had no effect on cAMP levels but stimulated small increases in progesterone accumulation in 30 min incubations of small luteal cells. PGF also increased the accumulation of inositol phosphates in large luteal cells. The increases were apparent within 5 min of incubation (the earliest time examined) and further increases were observed in incubations lasting 30 min. PGF had no significant effect on cAMP or progesterone in 30 min incubations of large cells.(ABSTRACT TRUNCATED AT 250 WORDS)