Contribution of G protein activation to fluoride stimulation of phosphoinositide hydrolysis in human neuroblastoma cells

Toxicity of fluoride: critical evaluation of evidence for human developmental neurotoxicity in epidemiological studies, animal experiments and in vitro analyses

Recently, epidemiological studies have suggested that fluoride is a human developmental neurotoxicant that reduces measures of intelligence in children, placing it into the same category as toxic metals (lead, methylmercury, arsenic) and polychlorinated biphenyls. If true, this assessment would be highly relevant considering the widespread fluoridation of drinking water and the worldwide use of fluoride in oral hygiene products such as toothpaste. To gain a deeper understanding of these assertions, we reviewed the levels of human exposure, as well as results from animal experiments, particularly focusing on developmental toxicity, and the molecular mechanisms by which fluoride can cause adverse effects. Moreover, in vitro studies investigating fluoride in neuronal cells and precursor/stem cells were analyzed, and 23 epidemiological studies published since 2012 were considered. The results show that the margin of exposure (MoE) between no observed adverse effect levels (NOAELs) in animal studies and the current adequate intake (AI) of fluoride (50 µg/kg b.w./day) in humans ranges between 50 and 210, depending on the specific animal experiment used as reference. Even for unusually high fluoride exposure levels, an MoE of at least ten was obtained. Furthermore, concentrations of fluoride in human plasma are much lower than fluoride concentrations, causing effects in cell cultures. In contrast, 21 of 23 recent epidemiological studies report an association between high fluoride exposure and reduced intelligence. The discrepancy between experimental and epidemiological evidence may be reconciled with deficiencies inherent in most of these epidemiological studies on a putative association between fluoride and intelligence, especially with respect to adequate consideration of potential confounding factors, e.g., socioeconomic status, residence, breast feeding, low birth weight, maternal intelligence, and exposure to other neurotoxic chemicals. In conclusion, based on the totality of currently available scientific evidence, the present review does not support the presumption that fluoride should be assessed as a human developmental neurotoxicant at the current exposure levels in Europe.

Increased expression of Galpha(q/11) and of phospholipase-Cbeta1/4 in differentiated human NT2-N neurons: enhancement of phosphoinositide hydrolysis

The CNS is enriched in phosphoinositide-specific phospholipase C (PLC) and in the G proteins linked to its activation. Although the regional distributions of these signaling components within the brain have been determined, neither their cell type-specific localizations (i.e., neuronal versus glial) nor the functional significance of their high expression has been definitively established. In this study, we have examined the expression of phosphoinositide signaling proteins in human NT2-N cells, a well characterized model system for CNS neurons. Retinoic acid-mediated differentiation of NT2 precursor cells to the neuronal phenotype resulted in five- to 15-fold increases in the expression of PLC-beta1, PLC-beta4, and Galpha(q/11) (the prime G protein activator of these isozymes). In contrast, the expression of PLC-beta3 and PLC-gamma1 was markedly reduced following neuronal differentiation. Similar alterations in cell morphology and in the expression of PLC-beta1, PLC-beta3, and Galpha(q/11) expression were observed when NT2 cells were differentiated with berberine, a compound structurally unrelated to retinoic acid. NT2-N neurons exhibited a significantly higher rate of phosphoinositide hydrolysis than NT2 precursor cells in response to direct activation of either G proteins or PLC. These results indicate that neuronal differentiation of NT2 cells is associated with dramatic changes in the expression of proteins of the phosphoinositide signaling system and that, accordingly, differentiated NT2-N neurons possess an increased ability to hydrolyze inositol lipids.

Fluoride-induced depletion of polyphosphoinositides in rat brain cortical slices: a rationale for the inhibitory effects on phospholipase C

Fluoride, which is used commonly as a pharmacological tool to activate phosphoinositide-phospholipase C coupled to the heterotrymeric Gq/11 proteins, inhibited the phosphorylation of phosphatidylinositol (PtdIns) to polyphosphoinositides (PtdIns4P and PtdIns4,5P2) in membranes from rat brain cortex. Fluoride enhanced basal production of 3H-inositol phosphates in membranes prepared from brain cortical slices that had been prelabeled with [3H]inositol, but inhibited the stimulation elicited by carbachol in the presence of GTPgammaS. However in both cases fluoride depleted [3H]PtdIns4P content by 95%. The inhibitory effects of fluoride on the release of 3H-inositol phosphates in slices were not apparent in a pulse [3H]inositol-labeling strategy, but became dramatic in a continuous labeling protocol, particularly at long incubation times. Prelabeling slices with [3H]inositol in the presence of fluoride precluded polyphosphoinositide labeling, and eliminated phospholipase C responsiveness to carbachol under normal or depolarizing conditions, and to the calcium ionophore ionomycin. The lack of response of 3H-polyphosphoinositide-depleted slices to phospholipase C stimuli was not due to fluoride poisoning, unaccessibility of the [3H]inositol label to phospholipase C or desensitization of Gq/11, as the effect of carbachol and GTPgammaS was restored, in the presence of ATP, in membranes prepared from slices that had been labeled in the presence of fluoride. In conclusion, our data show that fluoride, at a concentration similar to that used to stimulate directly Gq/11-coupled phospholipase C, effectively blocks the synthesis of phospholipase C substrates from PtdIns.

ATP-evoked inositol phosphates formation through activation of P2U purinergic receptors in cultured astrocytes: regulation by PKC subtypes alpha, delta, and theta

ATP-induced phosphoinositide (PI) hydrolysis was studied in cultured astrocytes. To characterize the P2 purinergic receptor-mediated effects of ATP, the subtype-specific agonists 2-methylthio ATP (2-MeSATP), UTP, and alpha, beta-methylene ATP were compared. ATP, UTP, or 2-MeSATP induced a dose-dependent increase of inositol phosphates (IP) accumulation; alpha, beta-methylene ATP and adenosine had no effect. The order of potency was ATP > or = UTP >> 2-MeSATP. Cross-desensitization experiments indicated that ATP interacted with both P2U and P2Y receptors. P2U was the predominant P2 receptor in mediating PI hydrolysis in astrocytes. The effect of ATP, UTP, or 2-MeSATP was markedly inhibited by pretreatment of cells with pertussis toxin (PTX), indicating that both P2U and P2Y receptors coupled to phospholipase C through PTX-sensitive G protein. Short-term (10 min) treatment of cells with 1 microM TPA attenuated ATP, UTP, and 2-MeSATP-induced PI breakdown; however, long-term (24 h) pretreatment resulted in marked potentiation of both ATP and UTP, and restoration of 2-MeSATP responses. In a further analysis of the effect of TPA, 10 min and 1.5 h pretreatment attenuated ATP-and UTP-induced PI breakdown, but this inhibitory action was lost after 3 h of treatment. Both 6 and 24 h pretreatments resulted in a potentiation. Western blot analysis showed translocation of protein kinase C (PKC) alpha, -delta, and -theta from the cytosol to the membrane following 10 min and 1.5 h treatments, and restoration to basal levels in the membrane fraction was seen after 3 h of treatment. On the other hand, partial and complete down-regulation of these three isoforms was seen after 6 and 24 h of treatment, respectively. PKC eta was translocated but not down-regulated by TPA. These results suggested that PKC alpha, -delta, and -theta, not -eta may exert tonic inhibition on P2U receptor-mediated PI turnover in unstimulated astrocytes.

Potentiation of bradykinin-induced inositol phosphates production by cyclic AMP elevating agents and endothelin-1 in cultured astrocytes

Cultured astrocytes express bradykinin (BK) receptors, which are coupled to phospholipase C (PLC) through G-protein to mediate phosphoinositide (PI) hydrolysis. The regulation of this BK receptor-G protein-PLC pathway by cAMP and endothelin-1 (ET-1) was explored by short-term (20 min) and long-term (24 h) treatment with 100 mu M dibutyryl cyclic AMP (dBcAMP) or 10 nM ET-1. Short-term treatment of cells with dBcAMP had no effect on BK-induced PI hydrolysis; however, long-term treatment resulted in potentiation of the BK response. Similar effects were seen after 10 mu M forskolin pretreatment of the cells. We further explored the site of action of 24 h dBcAMP pretreatment and found that AlF(4)-, ionomycin- or A3187-induced PI hydrolysis was not affected but (3H)BK binding was increased. These results indicate that the site of action of dBcAMP is the BK receptor and Scatchard plot analysis showed that the Bmax was increased but the Kd decreased. Cycloheximide (0.5 mu M) blocked the increase in (3H)BK binding, indicating that new synthesis of receptor protein might occur during 24 h pretreatment with dBcAMP. Twenty minutes pretreatment of cells with ET-1 resulted in desensitization of the ET-1 induced P1 response, while the BK response was unaffected. After 24 h pretreatment with ET-1, desensitization to ET-1 still occurred, while BK-induced PI hydrolysis was markedly potentiated. (3H)BK binding and AlF(4)--induced but not A23187- or ionomycin-induced PI hydrolysis were increased, indicating that the site of action of long-term ET-1 treatment was the BK receptor and G protein; Scatchard analysis showed an increase in Bmax but no effect on Kd. These effects were blocked by cycloheximide, indicating that new synthesis of both receptor protein and G protein might occur during 24 h pretreatment with ET-1. (3H)Thymidine uptake was inhibited or potentiated by dBcAMP and ET-1, respectively. Possible dBcAMP-induced differentiation and ET-1-induced proliferation may contribute to the increased expression of receptor proteins.

Evidence obtained using single hepatocytes for inhibition by the phospholipase C inhibitor U73122 of store-operated Ca2+ inflow

The ability of 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17- yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122), an inhibitor of phospholipase C (Smith et al., J Pharmacol Exp Ther 253:688-697, 1992), to inhibit agonist-stimulated and store-operated Ca2+ inflow in single hepatocytes was investigated with the aim of testing whether the activation of phospholipase C is a necessary step in the process of agonist-stimulated Ca2+ inflow in this cell type. U73122 inhibited the release of Ca2+ from intracellular stores and plasma membrane Ca2+ inflow induced by vasopressin. An inactive analogue of U73122, 1-[6-[[17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]- 2,5-pyrrolidone-dione (U73433), did not inhibit vasopressin-induced release of Ca2+ from intracellular stores, but did partially inhibit Ca2+ inflow. Neither U73122 nor 'inactive' analogue U73433 inhibited the release of Ca2+ from intracellular stores when this was initiated by the photolysis of 'caged' guanosine (5'-[gamma-thio]triphosphate (GTP gamma S) introduced to the cytoplasmic space by microinjection. However, both compounds inhibited GTP gamma S-stimulated Ca2+ inflow. U73122 also inhibited the actions of glycerophosphoryl-myo-inositol-4,5-diphosphate (GPIP2), a slowly-hydrolysed analogue of inositol 1,4,5-triphosphate (InsP3) which is released by photolysis of 'caged' 1-(alpha-glycerophosphoryl)-myo-inositol-4,5-diphosphate, P4(5)-1-(2-nitrophenyl)ethyl ester, and thapsigargin in stimulating Ca2+ inflow. U73122 did not inhibit GPIP2-stimulated release of Ca2+ from intracellular stores, but did partially inhibit the ability of thapsigargin to induce Ca2+ release. It is concluded that, while U73122 does inhibit phospholipase C beta in hepatocytes, complete inhibition of this enzyme in situ requires an intracellular concentration of U73122 higher than that achieved in the present experiments. Moreover, both U73122 and 'inactive' analogue U73433 have one or possibly two additional sites of action. These are likely to be the hepatocyte plasma membrane Ca2+ inflow channel protein (or a protein involved in the activation of this channel by the InsP3-sensitive intracellular Ca2+ store), and a protein involved in thapsigargin action.

Interaction of organophosphorus compounds with muscarinic receptors in SH-SY5Y human neuroblastoma cells

Human neuroblastoma cells (line SH-SY5Y) were used to examine the interaction of single exposure to organophosphorus compounds (OPs) with muscarinic receptors. In this study, SH-SY5Y cells were exposed for 30 min to concentrations of paraoxon, diisopropyl phosphorofluoridate (DFP), phenyl saligenin cyclic phosphate (PSP), and mipafox (N,N'-diisopropyl phosphorodiamide fluoridate) that ranged between 10(-9) M and 10(-3) M (10(-2) M for mipafox). Ability to interfere with muscarinic receptor binding was determined by change in the binding of the nonspecific antagonist [3H]-N-methylscopolamine (3H-NMS). Concentrations of paraoxon > 0.5 x 10(-3) M and PSP 1 x 10(-3) M significantly inhibited the binding of a saturating concentration of 3H-NMS. Concentrations of > 10(-5) M paraoxon or PSP could significantly inhibit the binding of a half-saturating concentration of 3H-NMS. Studies using specific antagonists for muscarinic subtypes (pirenzepine for M1, AFDX-116 for M2, and 4-DAMP for M3) indicated that SH-SY5Y cells have muscarinic receptors most sensitive to the specific antagonist for the M3 subtype (IC50 of 10(-8) M for 4-DAMP compared to 2.5 x 10(-6) M and 2.7 x 10(-5) M for pirenzepine and AFDX-116, respectively). As M3 receptor stimulation results in formation of inositol phosphates from membrane phosphoinositides the capability of OPs to alter levels of inositol phosphates and agonist-stimulated increases in inositol phosphate formation was examined. Intact cells were prelabeled with [3H]myo-inositol and then incubated for 15 min with the OPs before addition of 10(-5) M to 10(-3) M carbachol. Levels of inositol phosphates were determined as the amount of aqueous soluble radiolabeled product extracted from the reaction mixture. Paraoxon and PSP, but not mipafox or DFP, decreased basal levels of inositol phosphates in a concentration-related manner. This could be overcome in cells stimulated with carbachol, a muscarinic agonist, and with sodium fluoride, which does not act at muscarinic receptors. These results indicate that certain OPs, upon acute exposure, interact with muscarinic receptors, but that they also have effects on levels of inositol phosphates that may be associated with another site of action in SH-SY5Y cells.

Acute and long-term inhibition of agonist-stimulated phosphoinositide hydrolysis by pulse treatment of cerebellar granule cells with TPA

Acute pretreatment (30 min) of primary cultures of cerebellar granule cells with TPA (10 nM) resulted in a decrease in carbachol-and glutamate-stimulated phosphoinositide hydrolysis, but not in basal levels of PI hydrolysis. To investigate the mechanism of TPA action, phospholipase C was assayed in membranes prepared from cerebellar granule cells acutely treated with TPA. TPA had no effect on basal, GTP gamma S-, NaF-, and calcium-stimulated phospholipase C when compared with membranes prepared from vehicle-treated cells. The effects of pulsing with TPA (30-min pulse, 10 nM) on agonist-stimulated PI hydrolysis were studied 1, 3, and 5 or 6 d after TPA treatment. TPA treatment results in a statistically significant decrease in glutamate-stimulated PI hydrolysis, and a slight reduction of carbachol-stimulated PI hydrolysis when compared to temporally matched controls. Measurements in membranes prepared from TPA-treated vs control cells 1, 3, and 5 d after treatment showed that calcium- and NaF-stimulated phospholipase C activity was significantly decreased at all days tested, whereas GTP gamma S-stimulated phospholipase C activity was significantly decreased only at d 3. These data demonstrate differences in the acute vs long-term effects of TPA treatment on agonist-stimulated PH hydrolysis, and suggest that the acute effects may be mediated at the level of the receptor, whereas long-term effects of TPA on PI hydrolysis may be mediated by deficits in effector function.

G-proteins coupled to phosphoinositide hydrolysis in the cochlear and vestibular sensory epithelia of the rat are insensitive to cholera and pertussis toxins

In the cochlear (CSE) and vestibular sensory epithelia (VSE), phosphoinositides are hydrolyzed in response to stimulation of phospholipase C (PLC) by cholinergic muscarinic and purinergic P2y agonists. Such receptor-mediated activation of PLC is expected to be coupled through guanine nucleotide-binding proteins (G-proteins). Although several classes of G-proteins have been identified in the inner ear, nothing is known about the type of G-proteins associated with the phosphoinositide second messenger system in CSE and VSE. Phosphoinositide hydrolysis was determined by the release of radiolabeled inositol phosphates (InsPs). Ten mM NaF plus 10 microM AlCl3 increased basal InsPs accumulation 2-fold in both CSE and VSE of the rat. Release of InsPs was also enhanced by guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) in saponin-permeabilized tissues. Furthermore, release of InsPs stimulated by both carbamylcholine (CCh) and adenosine 5'-O-[3-thiotriphosphate] (ATP-gamma-S) was significantly inhibited by 100 microM guanosine 5'-O-[2-thiodiphosphate] (GDP-beta-S). These results strongly suggest the involvement of G-proteins in the receptor-PLC coupling in CSE and VSE. ADP-ribosylation in membrane fractions of CSE and VSE in the presence of cholera toxin (CTX) or pertussis toxin (PTX) indicated the existence of Gs- and G(i)-type G-proteins. However, neither CTX nor PTX affected basal or agonist-stimulated release of InsPs. These observations suggest that muscarinic and P2y purinergic receptors are coupled to PLC via CTX- and PTX-insensitive G-proteins in CSE and VSE.