U73122 and U73343 inhibit receptor-mediated phospholipase D activation downstream of phospholipase C in CHO cells

The aminosteroid U73122 promotes oligodendrocytes generation and myelin formation

Oligodendrocytes (OLs) are glial cells that ensheath neuronal axons and form myelin in the central nervous system (CNS). OLs are differentiated from oligodendrocyte precursor cells (OPCs) during development and myelin repair, which is often insufficient in the latter case in demyelinating diseases such as multiple sclerosis (MS). Many factors have been reported to regulate OPC-to-OL differentiation, including a number of G protein-coupled receptors (GPCRs). In an effort to search pathways downstream of GPCRs that might be involved in OPC differentiation, we discover that U73122, a phosphoinositide specific phospholipase C (PI-PLC) inhibitor, dramatically promotes OPC-to-OL differentiation and myelin regeneration in experimental autoimmune encephalomyelitis model. Unexpectedly, U73343, a close analog of U73122 which lacks PI-PLC inhibitory activity also promotes OL differentiation, while another reported PI-PLC inhibitor edelfosine does not have such effect, suggesting that U73122 and U73343 enhance OPC differentiation independent of PLC. Although the structures of U73122 and U73343 closely resemble 17β-estradiol, and both compounds do activate estrogen receptors Erα and Erβ with low efficacy and potency, further study indicates that these compounds do not act through Erα and/or Erβ to promote OPC differentiation. RNA-Seq and bioinformatic analysis indicate that U73122 and U73343 may regulate cholesterol biosynthesis. Further study shows both compounds increase 14-dehydrozymostenol, a steroid reported to promote OPC differentiation, in OPC culture. In conclusion, the aminosteroids U73122 and U73343 promote OPC-to-OL generation and myelin formation by regulating cholesterol biosynthesis pathway.

The effects of phospholipase C on oestradiol and progesterone secretion in porcine granulosa cells cultured in vitro

Granulosa cells play important roles in the regulation of ovarian functions. Phospholipase C is crucial in several signalling pathways and could participate in the molecular mechanisms of cell proliferation, differentiation and ageing. The objective of this study was to identify the effects of phospholipase C on the steroidogenesis of oestradiol and progesterone in porcine granulosa cells cultured in vitro. Inhibitor U73122 or activator m-3M3FBS of phospholipase C was added to the in vitro medium of porcine granulosa cells, respectively. The secretion of oestradiol decreased after 2 hr, 8 hr, 12 hr, 24 hr and 48 hr of treatment with 500 nM U73122 (p < .05) and decreased after 2 hr of treatment in the 500 nM m-3M3FBS addition group (p < .05). The secretion of progesterone increased after 4 hr of treatment with 500 nM U73122 (p < .05) and increased after 2 hr and 8 hr of treatment in the 500 nM m-3M3FBS addition group (p < .05). The ratio of oestradiol to progesterone decreased at each time point, except 8 hr after the addition of 500 nM U73122 (p < .05). The ratio of oestradiol to progesterone decreased after 2 hr (p < .05) of treatment with 500 nM m-3M3FBS. In genes that regulate the synthesis of oestradiol or progesterone, the mRNA expression of CYP11A1 was markedly increased (p < .05), and the mRNA expression of other genes did not change significantly in the U73122 treatment group, while the addition of m-3M3FBS did not change those genes significantly despite the contrary trend. Our results demonstrated that phospholipase C can be a potential target to stimulate the secretion of oestradiol and suppress progesterone secretion in porcine granulosa cells cultured in vitro, which shed light on a novel biological function of phospholipase C in porcine granulosa cells.

Structure, Function, Pharmacology, and Therapeutic Potential of the G Protein, Gα/q,11

G protein coupled receptors (GPCRs) are one of the major classes of cell surface receptors and are associated with a group of G proteins consisting of three subunits termed alpha, beta, and gamma. G proteins are classified into four families according to their α subunit; Gα_i, Gα_s, Gα_12/13, and Gα_q. There are several downstream pathways of Gα_q of which the best known is upon activation via guanosine triphosphate (GTP), Gα_q activates phospholipase Cβ, hydrolyzing phosphatidylinositol 4,5-biphosphate into diacylglycerol and inositol triphosphate and activating protein kinase C and increasing calcium efflux from the endoplasmic reticulum. Although G proteins, in particular, the Gα_q/11 are central elements in GPCR signaling, their actual roles have not yet been thoroughly investigated. The lack of research of the role on Gα_q/11 in cell biology is partially due to the obscure nature of the available pharmacological agents. YM-254890 is the most useful Gα_q-selective inhibitor with antiplatelet, antithrombotic, and thrombolytic effects. YM-254890 inhibits Gα_q signaling pathways by preventing the exchange of guanosine diphosphate for GTP. UBO-QIC is a structurally similar compound to YM-254890, which can inhibit platelet aggregation and cause vasorelaxation in rats. Many agents are available for the study of signaling downstream of Gα_q/11. The role of G proteins could potentially represent a novel therapeutic target. This review will explore the range of pharmacological and molecular tools available for the study of the role of Gα_q/11 in GPCR signaling.

G-Protein-Dependent and -Independent Pathways in Denatonium Signal Transduction

To clarify the involvement of G protein in denatonium signal transduction, we carried out a whole-cell patch-clamp analysis with isolated taste cells in mice. Two different responses were observed by applying GDP-beta-S, a G-protein inhibitor. One response to denatonium was reduced by GDP-beta-S (G-protein-dependent), whereas the other was not affected (G-protein-independent). These different patterns were also observed by concurrently inhibiting the phospholipase C beta2 and phosphodiesterase pathways via G protein. These data suggest dual, G-protein-dependent and -independent mechanisms for denatonium. Moreover, the denatonium responses were not attenuated by singly inhibiting the phospholipase C beta2 or phosphodiesterase pathway, implying that both pathways were involved in G-protein-dependent transduction. In the G-protein-independent cells, the response was abolished by the depletion of calcium ions within the intracellular store. These results suggest that Ca2+ release from the intracellular store is an important factor. Our data demonstrate multiple transduction pathways for denatonium in mammalian taste cells.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Involvement of protein kinase D in uridine diphosphate-induced microglial macropinocytosis and phagocytosis

The clearance of tissue debris by microglia is a crucial component of maintaining brain homeostasis. Microglia continuously survey the brain parenchyma and utilize extracellular nucleotides to trigger the initiation of their dynamic responses. Extracellular uridine diphosphate (UDP), which leaks or is released from damaged neurons, has been reported to stimulate the phagocytotic activity of microglia through P2Y(6) receptor activation. However, the intracellular mechanisms underlying microglial P2Y(6) receptor signals have not been identified. In this study, we demonstrated that UDP stimulation induced immediate and long-lasting dynamic movements in the cell membrane. After 60 min of UDP stimulation, there was an upregulation in the number of large vacuoles formed in the cell that incorporate extracellular fluorescent-labeled dextran, which indicates microglial macropinocytosis. In addition, UDP-induced vacuole formation and continuous membrane motility were suppressed by the protein kinase D (PKD) inhibitors, Gö6976 and CID755673, unlike Gö6983, which is far less sensitive to PKD. The inhibition of PKD also reduced UDP-induced incorporation of fluorescent-labeled dextran and soluble β-amyloid and phagocytosis of microspheres. UDP induced rapid phosphorylation and membrane translocation of PKD, which was abrogated by the inhibition of protein kinase C (PKC) with Gö6983. However, Gö6983 failed to suppress UDP-induced incorporation of microspheres. Finally, we found that inhibition of PKD by CID755673 significantly suppressed UDP-induced engulfment of IgG-opsonized microspheres. These data suggest that a PKC-independent function of PKD regulates UDP-induced membrane movement and contributes to the increased uptake of extracellular fluid and microspheres in microglia.

Roles of phospholipase D in phorbol myristate acetate-stimulated neutrophil respiratory burst

The phorbol myristate acetate (PMA) stimulated nutrophil respiratory burst has been considered to simply involve the activation of protein kinase C (PKC). However, the PLD activity was also increased by 10-fold in human neutrophils stimulated with 100 nM PMA. Unexpectedly, U73122, an inhibitor of phospholipase C, was found to significantly inhibit PMA-stimulated respiratory burst in human neutrophils. U73122 at the concentrations, which were sufficient to inhibit the respiratory burst completely, caused partial inhibition of the PLD activity but no inhibition on PKC translocation and activation, suggesting that PLD activity is also required in PMA-stimulated respiratory burst. Using 1-butanol, a PLD substrate, to block phosphatidic acid (PA) generation, the PMA-stimulated neutrophil respiratory burst was also partially inhibited, further indicating that PLD activation, possibly its hydrolytic product PA and diacylglycerol (DAG), is involved in PMA-stimulated respiratory burst. Since GF109203X, an inhibitor of PKC that could completely inhibit the respiratory burst in PMA-stimulated neutrophils, also caused certain suppression of PLD activation, it may suggest that PLD activation in PMA-stimulated neutrophils might be, to some extent, PKC dependent. To further study whether PLD contributes to the PMA stimulated respiratory burst through itself or its hydrolytic product, 1,2-dioctanoyl-sn-glycerol, an analogue of DAG , was used to prime cells at low concentration, and it reversed the inhibition of PMA-stimulated respiratory burst by U73122. The results indicate that U73122 may act as an inhibitor of PLD, and PLD activation is required in PMA-stimulated respiratory burst.

Expression pattern and sub-cellular distribution of phosphoinositide specific phospholipase C enzymes after treatment with U-73122 in rat astrocytoma cells

Phosphoinositide specific phospholipase C (PI-PLC) enzymes interfere with the metabolism of inositol phospholipids (PI), molecules involved in signal transduction, a complex process depending on various components. Many evidences support the hypothesis that, in the glia, isoforms of PI-PLC family display different expression and/or sub cellular distribution under non-physiological conditions such as the rat astrocytes activation during neurodegeneration, the tumoural progression of some neoplasms and the inflammatory cascade activation after lipopolysaccharide administration, even if their role remains not completely elucidated. Treatment of a cultured established glioma cell line (C6 rat astrocytoma cell line) induces a modification in the pattern of expression and of sub cellular distribution of PI-PLCs compared to untreated cells. Special attention require PI-PLC beta3 and PI-PLC gamma2 isoforms, whose expression and sub cellular localization significantly differ after U-73122 treatment. The meaning of these modifications is unclear, also because the use of this N-aminosteroid compound remains controversial, inasmuch it has further actions which might contribute to the global effect recorded on the treated cells.

Coincident regulation of PKCdelta in human platelets by phosphorylation of Tyr311 and Tyr565 and phospholipase C signalling

PKC (protein kinase C)d plays a complex role in platelets, having effects on both positive and negative signalling functions. It is phosphorylated on tyrosine residues in response to thrombin and collagen, and it has recently been shown that Tyr311 is phosphorylated in response to PAR (protease-activated receptor) 1 and PAR4 receptor activation. In the present study, we show that Tyr311 and Tyr565 are phosphorylated in response to thrombin, and have examined the interplay between phosphorylation and the classical lipid-mediated activation of PKCd. Phosphorylation of both Tyr311 and Tyr565 is dependent on Src kinase and PLC (phospholipase C) activity in response to thrombin. Importantly, direct allosteric activation of PKCd with PMA also induced phosphorylation of Tyr311 and Tyr565, and this was dependent on the activity of Src kinases, but not PLC. Membrane recruitment of PKCd is essential for phosphorylation of this tyrosine residue, but tyrosine phosphorylation is not required for membrane recruitment of PKCd. Both thrombin and PMA induce recruitment of PKCd to the membrane, and for thrombin, this recruitment is a PLC-dependent process. In order to address the functional role of tyrosine residue phosphorylation of PKCd, we demonstrate that phosphorylation can potentiate the activity of the kinase, although phosphorylation does not play a role in membrane recruitment of the kinase. PKCd is therefore regulated in a coincident fashion, PLC-dependent signals recruiting it to the plasma membrane and by phosphorylation on tyrosine residues, potentiating its activity.

Involvement of phospholipase C in endothelin 1-induced stimulation of Ca++ channels and basilar artery contraction in rabbits

OBJECT

Endothelin 1 (ET-1) is a major cause of cerebral vasospasm after subarachnoid hemorrhage (SAH), and extracellular Cal++ influx plays an essential role in ET-1-induced vasospasm. The authors recently demonstrated that ET-1 activates two types of Ca"-permeable nonselective cation channels (designated NSCC-1 and NSCC-2) and a store-operated Cal++ channel (SOCC) in vascular smooth-muscle cells located in the basilar arteries (BAs) of rabbits. In the present study, they investigate the effects of phospholipase C (PLC) on ET-1-induced activation of these Ca++ channels and BA contraction by using the PLC inhibitor U73122. Methods. To determine which Cal++ channels are activated via a PLC-dependent pathway, these investigators monitored the intracellular free Cal++ concentration ([Ca++]i). The role of PLC in ET-1-induced vascular contraction was examined by performing a tension study of rabbit BA rings. The U73122 inhibited the ET-1-induced transient increase in [Ca++]i, which resulted from mobilization of Ca++ from the intracellular store. Phospholipase C also inhibited ET-1-induced extracellular Ca++ influx through the SOCC and NSCC-2, but not through the NSCC-1. The U73122 inhibited the ET-1-induced contraction of the rabbit BA rings, which depended on extracellular Cal++ influx through the SOCC and NSCC-2. Conclusions. These results indicate the following. (1) The SOCC and NSCC-2 are stimulated by ET-1 via a PLC-dependent cascade whereas NSCC-1 is stimulated via a PLC-independent cascade. (2) The PLC is involved in the ET-1-induced contraction of rabbit BA rings, which depends on extracellular Ca++ influx through the SOCC and NSCC-2.

Cytoskeletal restraints regulate homotypic ALCAM-mediated adhesion through PKCα independently of Rho-like GTPases

The activated leukocyte cell adhesion molecule (ALCAM) is dynamically regulated by the actin cytoskeleton. In this study we explored the molecular mechanisms and signaling pathways underlying the cytoskeletal restraints of this homotypic adhesion molecule. We observed that ALCAM-mediated adhesion induced by cytoskeleton-disrupting agents is accompanied by activation of the small GTPases RhoA, Rac1 and Cdc42. Interestingly, unlike adhesion mediated by integrins or cadherins, ALCAM-mediated adhesion appears to be independent of Rho-like GTPase activity. By contrast, we demonstrated that protein kinase C (PKC) plays a major role in ALCAM-mediated adhesion. PKC inhibition by chelerythrine chloride and myristoylated PKC pseudosubstrate, as well as PKC downregulation by PMA strongly reduce cytoskeleton-dependent ALCAM-mediated adhesion. Since serine and threonine residues are dispensable for ALCAM-mediated adhesion and ALCAM is not phosphorylated, we can rule out that ALCAM itself is a direct PKC substrate. We conclude that PKCα plays a dominant role in cytoskeleton-dependent avidity modulation of ALCAM.

Phosphatidylinositol promotes cholesterol transport and excretion

Administration of phosphatidylinositol (PI) to New Zealand White rabbits increases HDL negative charge and stimulates reverse cholesterol transport. Intravenously administered PI (10 mg/kg) associated almost exclusively with the HDL fraction in rabbits. PI promoted an increase in the hepatic uptake of plasma free cholesterol (FC) and a 21-fold increase in the biliary secretion of plasma-derived cholesterol. PI also increased cholesterol excretion into the feces by 2.5-fold. PI directly affects cellular cholesterol metabolism. In cholesterol-loaded macrophages, PI stimulated cholesterol mass efflux to lipid-poor reconstituted HDL. PI was about half as effective as cAMP at stimulating efflux, and the effects of cAMP and PI were additive. In cultured HepG2 cells, PI-enriched HDL also enhanced FC uptake from HDL by 3-fold and decreased cellular cholesterol synthesis and esterification. PI enrichment had no effect on the selective uptake of cholesterol esters or on the internalization of HDL particles. PI-dependent metabolic events were efficiently blocked by inhibitors of protein kinase C and the inositol signaling cascade. The data suggest that HDL-PI acts via cell surface ATP binding cassette transporters and signaling pathways to regulate both cellular and intravascular cholesterol homeostasis.

Regulation of phospholipase D in human placental trophoblasts by the P(2) purinergic receptor

Phospholipase D (PLD) is present in human placental tissue. Since purinergic receptor agonists activate PLD in many different cell types, we evaluated the purinergic activation of the enzyme in cultured trophoblasts from the placenta. We found that P(2) receptor agonists stimulate PLD. The preferred ligand for P(2X7) (P(2Z)) receptor subtype, BzBz-ATP (10(-3)M ), induced the enzyme more than ten times over basal (unstimulated) activity, while ATP caused a much smaller increase. ATPgammaS, ADP and UTP were even less effective, compared to BzBz-ATP or ATP. AMP and alpha,beta-methyl-ATP, a P(2X) agonist that is uniquely inactive on the P(2X7) subtype, had no effect. This represents the first suggestion of the presence of the P(2X7) type of receptor in human trophoblasts that was directly confirmed by immunoblot detection. The action of BzBz-ATP was dependent upon the presence of calcium in the culture medium and was inhibited by high (5m M ) Mg(++) concentration. P(2X7) receptor subtype specific antagonists, ATP-2',3'-dialdehyde (o-ATP), CBB and the broad specificity P(2) inhibitor PPADS inhibited the effect of BzBz-ATP. Pertussis toxin treatment did not inhibit the effect. Down-regulation of cPKC/nPKC isoforms by prolonged PMA treatment (36 h, 10(-7)M ) prevented the stimulation of PLD by P(2) agonists or the calcium ionophore A-23187. PLA(2) inhibitors did not block the effect of BzBz-ATP. The possibility for a calcium influx related interdependence of PLC and PLD was evaluated. For PLC activation, UTP and ATP surpassed BzBz-ATP, while ionophore did not elevate PLC (assessed by IP(3) measurements). This suggested the predominance of a P(2Y2) receptor in the whole cell in gross activation of PLC. PLD was affected with a reversed order of potency. These results and the dependence of PLD on PKC activity implies that a restricted, membrane localized calcium flux activates PKC and in turn, mediates the P(2X7) dependent stimulation of PLD. This may have implications for physiologic regulation of trophoblast function.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

Blockade of maitotoxin-induced oncotic cell death reveals zeiosis

BACKGROUND

Maitotoxin (MTX) initiates cell death by sequentially activating 1) Ca2+ influx via non-selective cation channels, 2) uptake of vital dyes via formation of large pores, and 3) release of lactate dehydrogenase, an indication of cell lysis. MTX also causes formation of membrane blebs, which dramatically dilate during the cytolysis phase. To determine the role of phospholipase C (PLC) in the cell death cascade, U73122, a specific inhibitor of PLC, and U73343, an inactive analog, were examined on MTX-induced responses in bovine aortic endothelial cells.

RESULTS

Addition of either U73122 or U73343, prior to MTX, produced a concentration-dependent inhibition of the cell death cascade (IC50 asymptotically equal to 1.9 and 0.66 microM, respectively) suggesting that the effect of these agents was independent of PLC. Addition of U73343 shortly after MTX, prevented or attenuated the effects of the toxin, but addition at later times had little or no effect. Time-lapse videomicroscopy showed that U73343 dramatically altered the blebbing profile of MTX-treated cells. Specifically, U73343 blocked bleb dilation and converted the initial blebbing event into "zeiosis", a type of membrane blebbing commonly associated with apoptosis. Cells challenged with MTX and rescued by subsequent addition of U73343, showed enhanced caspase-3 activity 48 hr after the initial insult, consistent with activation of the apoptotic program.

CONCLUSIONS

Within minutes of MTX addition, endothelial cells die by oncosis. Rescue by addition of U73343 shortly after MTX showed that a small percentage of cells are destined to die by oncosis, but that a larger percentage survive; cells that survive the initial insult exhibit zeiosis and may ultimately die by apoptotic mechanisms.

Inhibition of acetylcholine-activated K(+) currents by U73122 is mediated by the inhibition of PIP(2)-channel interaction

1. We have investigated the effect of U73122, a specific inhibitor of phospholipase C (PLC), on acetylcholine-activated K(+) currents (I(KACh)) in mouse atrial myocytes. 2. In perforated patch clamp mode, I(KACh) was activated by 10 microM acetylcholine. When atrial myocytes were pretreated with U73122 or U73343, I(KACh) was inhibited dose-dependently (half-maximal inhibition at 0.12+/-0.0085 and 0.16+/-0.0176 microM, respectively). The current-voltage relationships for I(KACh) in the absence and in the presence of U73122 showed that the inhibition occurred uniformly from -120 to +40 mV, indicating a voltage-independent inhibition. 3. When U73122 was applied after I(KACh) reached steady-state, a gradual decrease in I(KACh) was observed. The time course of the current decrease was well fitted to a single exponential, and the rate constant was proportional to the concentration of U73122. 4. When K(ACh) channels were directly activated by adding 1 mM GTP gamma S to the bath solution in inside-out patches, U73122 (1 microM) decreased the open probability significantly without change in mean open time. When K(ACh) channels were activated independently of G-protein activation by 20 mM Na(+), open probability was also inhibited by U73122. 5. Voltage-activated K(+) currents and inward rectifying K(+) currents were not affected by U73122. 6. These findings show that inhibition by U73122 and U73343 of K(ACh) channels occurs at a level downstream of the action of G beta gamma or Na(+) on channel activation. The interference with phosphatidylinositol 4,5-bisphosphate (PIP(2))-channel interaction can be suggested as a most plausible mechanism.

Enhanced interaction between renovascular alpha(2)-adrenoceptors and angiotensin II receptors in genetic hypertension

In spontaneously hypertensive rats (SHR), hypertension is mediated in part by an enhanced renovascular response to angiotensin (Ang) II. Pertussis toxin normalizes renovascular responses to Ang II and lowers blood pressure in SHR, suggesting a role for altered G(i) signaling in the enhanced renovascular response to Ang II in SHR. To further investigate this hypothesis, we measured reductions in renal blood flow and increases in renovascular resistance in response to intrarenal infusions of Ang II in the presence and absence of coactivation of alpha(2)-adrenoceptors (ie, receptors selectively coupled to G(i)) with UK 14,304 in adrenalectomized, renal-denervated, captopril-pretreated SHR and normotensive Wistar-Kyoto rats. In SHR, but not Wistar-Kyoto rats, UK 14,304 markedly enhanced renovascular responses to Ang II and vasopressin. However, UK 14,304 did not enhance renovascular responses to methoxamine (alpha(1)-adrenoceptor agonist) in either strain. In uninephrectomized, normotensive Sprague-Dawley animals and in Sprague-Dawley rats with nongenetic hypertension induced by uninephrectomy, chronic administration of deoxycorticosterone acetate, and 1% saline as drinking water, UK 14,304 had little or no effect on renovascular responses to Ang II. In SHR, intrarenal infusions of U73122, a phospholipase C/D inhibitor, blocked the enhancement of renovascular responses to Ang II by UK 14,304. We conclude that activation of alpha(2)-adrenoceptors selectively enhances renovascular responses to Ang II and vasopressin in vivo in animals with genetic hypertensive but not in normotensive animals or animals with acquired hypertension. These results suggest that in SHR, there is a genetically mediated enhanced cross talk between the G(i) signal transduction pathway and signal transduction pathways activated by Ang II and vasopressin, but not methoxamine, and involving phospholipase C and/or D.

Hormonal regulation of phospholipase D activity in Ca(2+) transporting cells of rabbit connecting tubule and cortical collecting duct

Phospholipase D (PLD) is distributed widely in mammalian tissues where it is believed to play an important role in the regulation of cell functions and cell fate by a variety of extracellular signals. In this study, we used primary cultures of rabbit connecting tubule (CNT) and cortical collecting duct (CCD) cells, grown to confluence on a permeable support, to investigate the possible involvement of PLD in the mechanism of action of hormones that regulate Ca(2+) reabsorption. RT-PCR revealed the presence of transcripts of PLD1b and PLD2, but not PLD1a, in these cultures. Moreover, the expression of substantial amounts of PLD1 protein was demonstrated by Western blotting. To measure PLD activity, cells were labelled with [(3)H]myristic acid after which the PLD-catalysed formation of radiolabelled phosphatidylethanol ([(3)H]PtdEth) was measured in the presence of 1% (v/v) ethanol. Deamino-Cys,D-Arg(8)-vasopressin (dDAVP) and N(6)-cyclopentyladenosine (CPA), two potent stimulators of Ca(2+) transport across these monolayers, stimulated PLD activity as was indicated by a marked increase in [(3)H]PtdEth. Similarly, ATP, a potent inhibitor of dDAVP- and CPA-stimulated Ca(2+) transport, increased the formation of [(3)H]PtdEth. PLD activity was furthermore increased by 8Br-cAMP and following acute (30 min) stimulation of protein kinase C (PKC) with a phorbol ester (PMA). Chronic PMA treatment (120 h) to downregulate phorbol ester-sensitive PKC isoforms did not affect PLD activation by dDAVP, CPA and 8Br-cAMP, while markedly decreasing the effect of ATP and abolishing the effect of PMA. The PKC inhibitor chelerythrine significantly reduced PLD activation by dDAVP, CPA and 8Br-cAMP, without changing the effect of ATP. The inhibitor only partially reduced the effect of PMA. This study shows that Ca(2+) transporting cells of CNT and CCD contain a regulated PLD activity. The physiological relevance of this activity, which is not involved in Ca(2+) reabsorption, remains to be established.

U73122 inhibits the dephosphorylation and translocation of cofilin in activated macrophage-like U937 cells

Cofilin, an actin-binding protein, plays an important role in the migration, phagocytosis, and superoxide production of activated phagocytes through cytoskeletal reorganization. In unstimulated phagocytes, cofilin is a major phosphoprotein. However, upon activation, the phosphoprotein is dephosphorylated and translocated from cytosol to plasma membranes. Only the unphosphorylated form of cofilin is an active form that binds actin, whereas the regulatory mechanisms of cofilin have not been elucidated. We found that 1-[6-[[17beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione (U73122), an inhibitor of phospholipase C (PLC), suppressed both opsonized zymosan (OZ)-induced dephosphorylation and translocation of cofilin in macrophage-like U937 cells at 4 microM concentration. OZ triggered an increase in inositol 1,4,5-trisphosphate (IP3), and U73122 inhibited it. 1-[6-[[17beta-3-Methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-pyrrodione-dione (U73343), which was employed as an inactive analogue, had no such inhibitory activities as did U73122. Furthermore, herbimycin A, an inhibitor of src-type tyrosine kinase, also inhibited OZ-triggered IP3 formation. These results suggest that the activity and localization of cofilin are regulated by PLC at the downstream of src-family tyrosine kinase.

Effect of the aminosteroid U73122 on prostaglandin E(2) production in a murine clonal osteoblast-like cell line, MC3T3-E1

Prostaglandin E(2) production stimulated by various agents (arachidonic acid, prostaglandin F(2alpha), ionomycin, the calcium ionophore A23187, and melittin) was investigated after pretreatment of murine osteoblast-like MC3T3-E1 cells with the putative phospholipase C blocker, U73122. The aminosteroid dose dependently inhibited prostaglandin E(2) production induced by all agonists, except arachidonic acid. The results suggest an inhibitory role of U73122 on phospholipase A(2) activity or activation.

Phospholipase C and D in the commitment to survival or death in the early lag phase of tetrahymena cultures

We made three kinds of experiments in order to elucidate aspects of physiological mechanisms involved in a series of specific events leading to either cell death or survival in the lag phase of culture growth. We studied the fate of newly inoculated Tetrahymena cells in small droplets at 'high' (more than 1000 cells ml(-1)) and 'low' cell densities (less than 600 cells ml(-1)) in a nutrionally complete, synthetic nutrient medium. Confirming previous results we found that the cells in high-density cultures multiplied to final densities around 500,000 cells ml(-1) and that cells in low-density cultures died before multiplying. The appearance of the cells was recorded with a video camera at 20 frames per second for 6 h or until they died. The results indicated that the death process took place within milliseconds. We also studied the effects of U 73122, an inhibitor of the phosphatidylinositol-specific phospholipase C, on cell survival at low densities. At low inhibitor concentrations low-density cells were rescued from dying. At high inhibitor concentrations all cells died, and phosphatidylinositol - but not phosphatidylserine and phosphatidylcholine - saved them. The results indicate that the paths leading to either cell death or to cell proliferation separate within the first few minutes after subcultivation into a new medium, since the first cells in each culture died within 4-30 min after inoculation. Our results also indicate that some PLC activity was required for stimulation of phospholipase D, and that cell death during the early lag phase is caused by a shortage in phosphatidylinositol before the phospholipase D activity is upregulated. These experiments are shedding light on the lethal consequences of a cellular depletion of the important signalling compound phosphatidylinositol in an in vivo system, and may help to elucidate mechanisms behind the century-old fact that eukaryote cells die when inoculated at too low a cell density to survive.

Platelet-derived growth factor-induced activation of sphingosine kinase requires phosphorylation of the PDGF receptor tyrosine residue responsible for binding of PLCgamma

Sphingosine-1-phosphate, a sphingolipid metabolite, is involved in the mitogenic response of platelet-derived growth factor (PDGF) and is formed by activation of sphingosine kinase. We examined the effect of PDGF on sphingosine kinase activation in TRMP cells expressing wild-type or various mutant betaPDGF receptors. Sphingosine kinase was stimulated by PDGF in cells expressing wild-type receptors but not in cells expressing kinase-inactive receptors (R634). Cells expressing mutated PDGF receptors with phenylalanine substitutions at five major tyrosine phosphorylation sites 740/751/771/1009/1021 (F5 mutants), which are unable to associate with PLCgamma, phosphatidylinositol 3-kinase, Ras GTPase-activating protein, or protein tyrosine phosphatase SHP-2, not only failed to increase DNA synthesis in response to PDGF but also did not activate sphingosine kinase. Moreover, mutation of tyrosine-1021 of the PDGF receptor to phenylalanine, which impairs its association with PLCgamma, abrogated PDGF-induced activation of sphingosine kinase. In contrast, PDGF was still able to stimulate sphingosine kinase in cells expressing the PDGF receptor mutated at tyrosines 740/751 and 1009, responsible for binding of phosphatidylinositol 3-kinase and SHP-2, respectively. In agreement, PDGF did not stimulate sphingosine kinase activity in F5 receptor 'add-back' mutants in which association with the Ras GTPase-activating protein, phosphatidylinositol 3-kinase, or SHP-2 was individually restored. However, a mutant PDGF receptor that was able to bind PLCgamma (tyrosine-1021), but not other signaling proteins, restored sphingosine kinase sensitivity to PDGF. These data indicate that the tyrosine residue responsible for binding of PLCgamma is required for PDGF-induced activation of sphingosine kinase. Moreover, calcium mobilization downstream of PLCgamma, but not protein kinase C activation, appears to be required for stimulation of sphingosine kinase by PDGF.-Olivera, A., Edsall, J., Poulton, S., Kazlauskas, A., Spiegel, S. Platelet-derived growth factor-induced activation of sphingosine kinase requires phosphorylation of the PDGF receptor tyrosine residue responsible for binding of PLCgamma.

Two compounds commonly used for phospholipase C inhibition activate the nuclear estrogen receptors

The aminosteroid U73122 is generally used as a specific inhibitor of phosphoinositide specific phospholipase C (PLC) and typically, the structurally related compound U73343 is used as control, since it lacks PLC inhibitory activity. We have found that both compounds possess strong estrogenic activity and that this activity is mediated by the estrogen receptors (ER) alpha and beta. Although no direct evidence for binding of U73122 and U73343 to the ER could be provided, the estrogenic activity of the aminosteroids requires an intact ER hormone binding pocket. Given the chemical structure of the two aminosteroids, they may be converted to an estrogenic derivative by chemical degradation or an enzymatic metabolization reaction. Our data indicate that additional care should be taken in the interpretation of the effects of U73122 in cells expressing ER.