Evidence for ADP-ribosylation-factor-mediated activation of phospholipase D by m3 muscarinic acetylcholine receptor

The mouse ADP-ribosylation factor-like 4 gene: two separate promoters direct specific transcription in tissues and testicular germ cell

ADP-ribosylation factor-like protein 4 (ARL4) is a Ras-related GTPase that has been cloned from the 3T3-L1 preadipocyte cell line as an adipocyte-specific cDNA [Schürmann, Breiner, Becker, Huppertz, Kainulainen, Kentrup and Joost (1994) J. Biol. Chem. 269, 15683-15688]. The Arl4 gene maps to the proximal region of mouse chromosome 12 linked to Lamb1-1, Hfhbf1 and Sos2. Compared with all other known genes of Ras-related GTPases, the genomic organization of Arl4 is unusual in that its entire coding region, the 3' untranslated region (UTR) and most of the 5' UTR are located on a single exon. This structure suggests that Arl4 has evolved by retroposition of an Arf (ADP-ribosylation factor) or Arf-like gene. Isolation of the 5' UTR by rapid amplification of cDNA ends (RACE)-PCR revealed heterogeneous transcription initiation sites in alternative exons 1. Both 5'-flanking regions exhibited promoter activity when expressed in COS-7 cells, indicating that the expression of Arl4 is directed by two separate promoters. mRNA transcribed under the control of the downstream promoter was isolated by RACE-PCR from all investigated tissues. In contrast, the upstream promoter seems to drive specifically the expression of Arl4 in adult testis. Hybridization of rat testis in situ indicated that Arl4 is expressed in germ cells of puberal and adult testis, but not in prepuberal testis, suggesting that Arl4 is involved in sperm production.

Tyrosine-phosphorylation-dependent and rho-protein-mediated control of cellular phosphatidylinositol 4,5-bisphosphate levels

The polyphosphoinositide PtdIns(4,5)P2, best known as a substrate for phospholipase C isozymes, has recently been recognized to be involved in a variety of other cellular processes. The aim of this study was to examine whether the cellular levels of this versatile phospholipid are controlled by tyrosine phosphorylation. The studies were performed in human embryonic kidney (HEK)-293 cells stably expressing the M3 muscarinic acetylcholine receptor. Inhibition of tyrosine phosphatases by pervanadate induced an up-to-approx.-2. 5-fold increase in the total cellular level of PtdIns(4,5)P2, which was both time- and concentration-dependent. In contrast, the tyrosine kinase inhibitors, genistein and tyrphostin 23, caused a rapid and specific fall in the cellular PtdIns(4,5)P2 level and prevented the stimulatory effect of pervanadate on PtdIns(4,5)P2 formation. Inactivation of Rho proteins by Clostridium difficile toxin B caused a similar fall in the HEK-293 cell PtdIns(4,5)P2 level, which was not altered by additional genistein treatment. Furthermore, toxin B treatment abolished the pervanadate-induced increase in PtdIns(4,5)P2 levels. As PtdIns(4,5)P2 is an essential stimulatory cofactor for phospholipase D (PLD) enzymes, we finally examined the effects of the agents regulating PtdIns(4,5)P2 levels on PLD activity in HEK-293 cells. Inhibition of tyrosine phosphatases by pervanadate caused an increase in PLD activity, which was susceptible to genistein and tyrphostin 23, and which was abolished by prior treatment with toxin B. In conclusion, the data presented indicate that the cellular level of the multifunctional phospholipid, PtdIns(4,5)P2, in HEK-293 cells is controlled by a tyrosine-kinase-dependent mechanism and that this process apparently involves Rho proteins, as found similarly for tyrosine-phosphorylation-induced PLD activation.

The distribution and translocation of the G protein ADP-ribosylation factor 1 in live cells is determined by its GTPase activity

ADP-ribosylation factors (ARF) are small G proteins that play key roles in vesicular transport processes. We have studied the distribution of ARF1 in live cells using chimeras of ARF1 mutants (wild type (wt) ARF1; Q71L-ARF1 (reduced GTPase); T31N (low affinity for GTP); and (Delta)Nwt (deletion of amino acids 2–18)) with green fluorescent protein (GFP). Confocal microscopy studies showed that the wt and Q71L proteins were localized in the Golgi and cytoplasm. The (Delta)Nwt and the T31N mutants were exclusively cytoplasmic. The behavior of the wt and Q71L proteins was studied in detail. About 15% of wt-ARF1-GFP was bound to the Golgi. Bound wt-ARF1-GFP dissociated rapidly after addition of Brefeldin A (BFA). This process did not appear to be a consequence of BFA-induced disappearance of the Golgi. Photobleaching recovery showed that essentially all the ARF-GFP was mobile, although it diffused very slowly. In contrast, about 40–50% of the Q71L mutant was found in the Golgi, and its rate of dissociation in the presence of BFA was slow and biphasic. Q71L-ARF1-GFP diffused more slowly than the wt. We conclude that ARF1 proteins exist in a dynamic equilibrium between Golgi-bound and cytosolic pools, and that the translocation of ARF in live cells requires the hydrolysis of GTP by the Golgi-bound protein.

Functional association between Arf and RalA in active phospholipase D complex

Activation of phospholipase D1 (PLD1) by Arf has been implicated in vesicle transport and membrane trafficking. PLD1 has also been shown to be associated with the small GTPase RalA, which functions downstream from Ras in a Ras-RalA GTPase cascade that facilitates intracellular signal transduction. Although PLD1 associates directly with RalA, RalA has no effect upon the activity of PLD1. However, PLD1 precipitated from cell lysates with immobilized glutathione S-transferase-RalA fusion protein is active. This suggests the presence of an additional activating factor in the active RalA-PLD1 complexes. Because Arf stimulates PLD1, we looked for the presence of Arf in the active RalA-PLD1 complexes isolated from v-Src- and v-Ras-transformed cell lysates. Low levels of Arf protein were detected in RalA-PLD1 complexes; however, if guanosine 5'-[gamma-thio]triphosphate was added to activate Arf and stimulate translocation to the membrane, high levels of Arf were precipitated by RalA from cell lysates. Interestingly, deletion of 11 amino-terminal amino acids unique to Ral GTPases, which abolished the ability of RalA to precipitate PLD activity, prevented the association between RalA and Arf. Brefeldin A, which inhibits Arf GDP-GTP exchange, inhibited PLD activity in v-Src- and v-Ras-transformed cells but not in the nontransformed cells, suggesting that the association of Arf with RalA is required for the increased PLD activity induced by v-Src and v-Ras. These data implicate Arf in the transduction of intracellular signals activated by v-Src and mediated by the Ras-RalA GTPase cascade. Because both Arf and PLD1 stimulate vesicle formation in the Golgi, these data raise the possibility that vesicle formation and trafficking may play a role in the transduction of intracellular signals.

Specific inhibition of phorbol ester-stimulated phospholipase D by Clostridium sordellii lethal toxin and Clostridium difficile toxin B-1470 in HEK-293 cells. Restoration by Ral GTPases

Activation of m3 muscarinic acetylcholine receptor (mAChR), stably expressed in human embryonic kidney (HEK)-293 cells, leads to phospholipase D (PLD) stimulation, a process apparently involving Rho GTPases, as shown by studies with Clostridium botulinum C3 exoenzyme and Clostridium difficile toxin B (TcdB). Direct activation of protein kinase C (PKC) by phorbol esters, such as phorbol 12-myristate 13-acetate (PMA), also induces PLD stimulation, which is additive to the mAChR action and which is only poorly sensitive to inactivation of Rho proteins by TcdB. To study whether Ras-like GTPases are involved in PLD regulation, we studied the effects of the TcdB variant TcdB-1470 and Clostridium sordellii lethal toxin (TcsL), known to inactivate Rac and some members of the Ras protein family, on PLD activities. TcdB-1470 and TcsL did not affect basal PLD activity and PLD stimulation by mAChR or direct G protein activation. In contrast, PMA-induced PLD stimulation was inhibited by TcdB-1470 and TcsL in a time- and concentration-dependent manner, without alteration in immunologically detectable PKC isozyme levels. In membranes of HEK-293 cells pretreated with TcdB-1470 or TcsL, basal and stable GTP analog-stimulated PLD activities measured with exogenous phosphatidylcholine, in the presence or absence of phosphatidylinositol 4,5-bisphosphate, were not altered. In contrast, pretreatment with TcdB-1470 and TcsL, but not TcdB, strongly reduced PMA-stimulated PLD activity. The addition of recombinant Rac1, serving as glucosylation substrate for TcdB, TcsL, and TcdB-1470, did not restore PLD stimulation by PMA. Furthermore, PMA-stimulated PLD activity, suppressed by prior treatment with TcdB-1470 or TcsL, was not rescued by the addition of recombinant Ras (RasG12V) or Rap proteins, acting as glucosylation substrates for TcsL only (Ras) or TcdB-1470 and TcsL (Rap). In contrast, the addition of recombinant Ral proteins (RalA and RalB), glucosylation substrates for TscL and TcdB-1470, but not for TcdB, to membranes of TcdB-1470- or TcsL-treated cells fully restored PLD stimulation by PMA without altering the strict MgATP dependence of PMA-induced PLD stimulation. RalA-mediated restoration of PMA-stimulated PLD activity in membranes of TcsL-treated cells was not enhanced by coaddition of RasG12V. In conclusion, the data presented indicate that TcdB-1470 and TcsL selectively interfere with phorbol ester stimulation of PLD and suggest an essential role of Ral proteins in PKC signaling to PLD in HEK-293 cells.

Rhodopsin-family receptors associate with small G proteins to activate phospholipase D

G-protein-coupled receptors of the rhodopsin family transduce many important neural and endocrine signals. These receptors activate heterotrimeric G proteins and in many cases also cause activation of phospholipase D, an enzyme that can be controlled by the small G proteins ARF and RhoA. Here we show that the activation of phospholipase D that is induced by many, but not all, Ca2+-mobilizing G-protein-coupled receptors is sensitive to inhibitors of ARF and of RhoA. Receptors of this type were co-immunoprecipitated with ARF or RhoA on exposure to agonists, and the effects of GTP analogues on ligand binding to the receptor changed to a profile that is characteristic of small G proteins. These receptors contain the amino-acid sequence AsnProXXTyr in their seventh transmembrane domain, whereas receptors capable of activating phospholipase D without involving ARF contain the sequence AspProXXTyr. Mutation of this latter sequence to AsnProXXTyr in the gonadotropin-releasing hormone receptor conferred sensitivity to an inhibitor of ARF, and the reciprocal mutation in the 5-HT2A receptor for 5-hydroxy-tryptamine reduced its sensitivity to the inhibitor. Receptors carrying the AsnProXXTyr motif thus seem to form functional complexes with ARF and RhoA.

Characterization of a rat brain phospholipase D isozyme

We have recently cloned a cDNA encoding a phospholipase D (PLD) from rat brain and named it rPLD1. It shows 90% amino acid identity with the human PLD isoform hPLD1b. We have expressed rPLD1 as a histidine-tagged fusion protein in insect (Sf9) cells using the expression vector pBlueBacHis and purified the recombinant protein to homogeneity by Ni2+-agarose affinity chromatography. Phosphatidylinositol 4,5-P2 and phosphatidylinositol 3,4,5-P3 activated the PLD equipotently, but other acidic phospholipids were ineffective. The activity of rPLD1 was dependent on both Mg2+ and Ca2+. It was specific for phosphatidylcholine and showed a broad dependence on pH with optimum activity at pH 6.5-7.5. The enzyme was inhibited by oleate and activated by the small G proteins ARF3 and RhoA in the presence of guanosine 5'-3-O-(thio)triphosphate. Protein kinase C (PKC)-alpha and -betaII, but not PKC-gamma, -delta, -epsilon, or -zeta, activated rPLD1 in a manner that was stimulated by phorbol ester but did not require ATP. Neither synergistic interactions between ARF3 and RhoA nor between these G proteins and PKC-alpha or -betaII were observed. Recombinant PKC-alpha and -betaII phosphorylated purified rPLD1 to high stoichiometry in vitro, and the phosphorylated PLD exhibited a mobility shift upon electrophoresis. Phosphorylation of the PLD by PKC was correlated with inhibition of its catalytic activity. rPLD1 bound to concanavalin A-Sepharose beads, and its electrophoretic mobility was altered by treatment with endoglycosidase F. The amount of PLD bound to the beads was decreased in a concentration-dependent manner when tunicamycin was added to the Sf9 expression system. Tunicamycin also decreased membrane localization of rPLD1. These results suggest that rPLD1 is a glycosylated protein and that it is negatively regulated by phosphorylation by PKC in vitro.

Activation of phospholipase D by ADP-ribosylation factor in rat submandibular acinar cells

The hydrolysis of membrane phosphatidylcholine by the enzyme phospholipase D is a key initial step in the intracellular release of the signalling molecules phosphatidic acid, diacylglycerol and arachidonic acid. Guanine nucleotide-dependent pathway leading to PLD activation were investigated in enzymatically dispersed rat submandibular acinar cells. Guanosine 5'-O-[gamma-thio]triphosphate (GTP gamma S) caused the time- and concentration-dependent stimulation of PLD in permeabilized cells. This effect was lost in prepermeabilized cells, from which cytosolic components had been allowed to leak, but was restored when endogenous cytosol, or cytosol from platelets, was added back to such cells. PLD was also activated in cytosol-depleted cells by GTP gamma S in combination with purified ARF (ADP-ribosylation factor), a low M(r) guanine nucleotide-binding protein of the ras superfamily. Additional evidence for the involvement of ARF in PLD activation was the inhibition of carbachol- or GTP gamma S-induced stimulation of the enzyme by brefeldin A, a blocker of ARF activation; and the observed translocation of ARF from cytosol to membrane on GTP gamma S treatment in permeabilized cells. The heterotrimeric G-protein stimulator, AlFn, also activated PLD, and this response, too, was inhibited by brefeldin A, suggesting the downstream involvement of ARF in coupling AlFn action to phospholipase D elevation. PLD activation caused by both GTP gamma S and AlFn was only partially reduced after treatment of cells with U73122, a demonstrated inhibitor of phospholipase C in the Gq-coupled phosphoinositide signal-transduction pathway. It is therefore proposed that in rat submandibular mucous acinar cells, a guanine nucleotide-regulated PLD activation pathway exists that involves the sequential actions of a G heterotrimeric protein and ARF. It is further suggested that this pathway is independent of the Gq/PLC/phosphatidylinositol signal transduction system.

Receptor-mediated endocytosis in kidney proximal tubules: recent advances and hypothesis

Preparation of kidney proximal tubules in suspension allows the study of receptor-mediated endocytosis, protein reabsorption, and traffic of endosomal vesicles. The study of tubular protein transport in vitro coupled with that of the function of endosomal preparation offers a unique opportunity to investigate a receptor-mediated endocytosis pathway under physiological and pathological conditions. We assume that receptor-mediated endocytosis of albumin in kidney proximal tubules in situ and in vitro can be regulated, on the one hand, by the components of the acidification machinery (V-type H+-ATPase, Cl(-)-channel and Na+/H+-exchanger), giving rise to formation and dissipation of a proton gradient in endosomal vesicles, and, on the other hand, by small GTPases of the ADP-ribosylation factor (Arf)-family. In this paper we thus analyze the recent advances of the studies of cellular and molecular mechanisms underlying the identification, localization, and function of the acidification machinery (V-type H+-ATPase, Cl(-)-channel) as well as Arf-family small GTPases and phospholipase D in the endocytotic pathway of kidney proximal tubules. Also, we explore the possible functional interaction between the acidification machinery and Arf-family small GTPases. Finally, we propose the hypothesis of the regulation of translocation of Arf-family small GTPases by an endosomal acidification process and its role during receptor-mediated endocytosis in kidney proximal tubules. The results of this study will not only enhance our understanding of the receptor-mediated endocytosis pathway in kidney proximal tubules under physiological conditions but will also have important implications with respect to the functional consequences under some pathological circumstances. Furthermore, it may suggest novel targets and approaches in the prevention and treatment of various diseases (cystic fibrosis, Dent's disease, diabetes and autosomal dominant polycystic kidney disease).

Cloning and characterization of phospholipase D from rat brain

The regulation of phospholipase D cloned from rat brain (rPLD) was examined in vivo and in vitro. The enzyme was a shorter splice variant of human phospholipase D 1 (Hammond, S. M., Altshuller, Y. M. , Sung, T.-C., Rudge, S. M., Rose, K., Engebrecht, J. A., Morris, A. J., and Frohman, M. A. (1995) J. Biol. Chem. 270, 29640-29643). Its expression in COS-7 cells led to increased phospholipase D (PLD) activity that was further stimulated by constitutively active V14RhoA. V14RhoA had no effect on the endogenous PLD of the COS-7 cells, but constitutively active L71ARF3 increased its activity. In contrast, L71ARF3 did not activate rPLD expressed in the cells. Addition of phorbol ester markedly increased the endogenous PLD activity of COS-7 cells, and there was a further increase in the cells expressing rPLD. In membranes from COS-7 cells expressing rPLD, addition of myristoylated ADP-ribosylation factor (ARF) and RhoA in vitro stimulated PLD activity. The effect of ARF was greater than that of RhoA, although the concentrations for half-maximal stimulation (0.08-0.2 microM) were similar. Membranes isolated from cells expressing rPLD plus L71ARF3 and/or V14RhoA also showed higher PLD activity but no synergism between the two G proteins. Addition of phorbol ester and protein kinase C alpha (PKCalpha) also stimulated PLD activity in membranes from COS-7 cells expressing rPLD, but it had no effect on the activity in control (vector) membranes and did not enhance the effects of constitutively active ARF or Rho. The stimulation by PKCalpha did not require ATP and was not increased by addition of this nucleotide. No synergism between ARF and Rho and between these and PKCalpha on PLD activity was observed when these were added to membranes from cells expressing rPLD. Oleate inhibited the PLD activity of membranes from both control and rPLD-expressing cells. In summary, these results indicate that in vitro, rPLD is stimulated by ARF, RhoA, and PKCalpha and inhibited by oleate. However, in intact COS-7 cells, ARF activates endogenous PLD but not rPLD, whereas the reverse is true for RhoA. In addition, the effects of phorbol ester are much greater in the intact cells. It is concluded that the regulation of rPLD in intact COS-7 cells differs significantly from that seen in vitro; possible reasons for this are discussed.

Effects of brefeldin A on phosphatidylcholine phospholipase D and inositolphospholipid metabolism in HL-60 cells

The involvement of the small GTP-binding protein ADP-ribosylation factor (ARF) in guanosine 5'-[gamma-thio]triphosphate (GTP[S])-dependent activation of phospholipase D (PLD) in HL-60 cells has been well established in vitro. In this study, we tested the effect of brefeldin A, which prevents ARF activation by inhibiting guanine-nucleotide-exchange activity, on PLD stimulation by receptor agonists (formyl-Met-Leu-Phe and ATP) and by the phorbol ester phorbol 12-myristate 13-acetate (PMA) in differentiated HL-60 cells. However, brefeldin A did not affect the activation of PLD at a time (1 h) when it eliminated the activity of the trans-Golgi enzyme galactosyltransferase. It also did not inhibit PLD activity in Golgi-enriched membranes treated with GTP[S] with or without ARF in vitro. However, longer times of brefeldin A treatment (>6 h), progressively and completely inhibited the activation of PLD by formyl-Met-Leu-Phe and partly inhibited (approximately 50%) the activation by PMA. In contrast, long-term brefeldin A treatment did not inhibit the effect of GTP[S] on PLD in permeabilized HL-60 cells. Long-term brefeldin A treatment completely inhibited inositol phosphate production in response to formyl-Met-Leu-Phe and ATP, indicating that it affected inositolphospholipid-specific phospholipase C activity. These data indicate that the rapid inhibitory effect of brefeldin A on Golgi function is not associated with inhibition of receptor-mediated or PMA-mediated PLD activation in HL-60 cells. However, longer-term effects, presumably arising from the disruption of the Golgi, lead to a total inhibition of agonist activation of PLD and inositolphospholipid-specific phospholipase C. In summary, these results do not support a role for brefeldin-A-sensitive ARF in agonist regulation of PLD in HL-60 cells.

Characteristics of protein-kinase-C- and ADP-ribosylation-factor-stimulated phospholipase D activities in human embryonic kidney cells

Phospholipase D (PLD) activity in human embryonic kidney (HEK) cells is stimulated by phorbol-ester-activated protein kinase C (PKC) and by membrane receptors, the latter apparently acting via the GTP-binding proteins, ADP-ribosylation factor (ARF) and Rho. In the present study, performed in cell-free preparations, we have characterized and compared the regulation of HEK cell PLD activity by the stable GTP analogue, guanosine 5'-O-[gamma-thio]triphosphate (GTP[S]), and the phorbol ester, phorbol 12-myristate 13-acetate (PMA). In digitonin-permeabilized HEK cells, prelabeled with [3H]oleic acid, GTP[S] and PMA caused an approximately threefold concentration-dependent increase in the formation of [3H]phosphatidylethanol, measured in the presence of ethanol. Neomycin, which is known to complex with the PLD cofactor, phosphatidylinositol 4,5-bisphosphate, decreased basal and GTP[S]- or PMA-stimulated PLD activities with similar sensitivity. GDP and its analogue, guanosine 5'-O-[beta-thio]diphosphate, inhibited the stimulatory effect of GTP[S], whereas the PMA response was prevented by the nonselective PKC inhibitor, staurosporine, but not vice versa. PLD stimulation by GTP[S], but not by PMA, was markedly reduced upon cytosol depletion and reconstituted by purified recombinant ARF1. In HEK cell membranes, addition of purified recombinant ARNO, a guanine-nucleotide-exchange factor for ARF1. potentiated the GTP[S]-stimulated PLD activity. PLD stimulation by PMA in HEK cell membranes required MgATP and was largely prevented by the selective PKC inhibitors Goe 6976 and bisindolylmaleimide I. Immunoblot analysis demonstrated that both conventional PKC (alpha, beta, gamma) and atypical PKC isozymes (zeta, tau) were present in HEK cell membranes. The results indicate that phorbol ester stimulation of PLD activity in HEK cells apparently occurs by a phosphorylation-dependent mechanism involving membrane-associated PKC isozymes but not ARF proteins, the major targets of GTP[S]' action.

ARF-independent inhibition of the carbachol-induced contractions by brefeldin A in intestinal smooth muscle

The aim of this study was to determine whether an ADP ribosylation factor (ARF)-regulated pathway is involved in the carbachol-induced contraction in rat intestinal smooth muscle. Brefeldin A, a known inhibitor of the guanine nucleotide exchange activity on ARF, reversibly inhibited the carbachol-induced contraction in intact ileal muscle strips, whereas the carbachol- and guanosine 5'-O-(3-thiotriphosphate)-induced increases in the Ca2+ sensitivity of myofilaments in beta-escin-permeabilized strips were not affected. The high-K(+)-induced contraction in intact strips was also inhibited by brefeldin A. In isolated ileal myocytes, brefeldin A inhibited the Ca2+ channel current, indicating that the inhibitory effect of brefeldin A in intact cells is related to an inhibition of voltage-dependent Ca2+ channels. Furthermore, the loading of permeabilized strips with the combination of the recombinant fully myristoylated ARF1, the guanine nucleotide exchange factor ARNO, and guanosine 5'-triphosphate did not change the tone at constant pCa (6.45) and did not modify the carbachol- and guanosine 5'-O-(3-thiotriphosphate)-induced Ca2+ sensitization. Taken together, these findings suggest that an ARF-dependent pathway is not involved in the carbachol-induced contraction.

ARF proteins mediate insulin-dependent activation of phospholipase D

BACKGROUND

ADP-ribosylation factors (ARFs) have been shown to activate phospholipase D (PLD), an enzyme modulated by extracellular signals, including several growth factors and, in particular, insulin. We have tested the hypothesis that ARF proteins are involved specifically in insulin-induced activation of PLD.

RESULTS

We found that in membranes obtained from HIRcB cells, a cell line derived from Rat-1 fibroblasts that overexpresses normal human insulin receptors, binding of the GTP analogue GTPgammaS to purified bovine or recombinant ARF was enhanced in the presence of insulin. Membranes obtained from cells that overexpressed a mutated, nonfunctional insulin receptor failed to stimulate ARF activation. Insulin promoted the association of ARF proteins with membranes in the presence of GTPgammaS in permeabilized cells. Insulin activated PLD in permeabilized HIRcB cells by a process that required GTPgammaS and ARF. Azido-gamma[32P]-GTP labelling of immunoprecipitated receptors revealed the presence of a unique 19 kD band; ARF proteins are approximately this size, and analysis using specific monoclonal antibodies demonstrated that ARF proteins coimmunoprecipitated with the insulin receptor. Coimmunoprecipitation of ARF with the receptor was inhibited by guanine nucleotides and stimulated by insulin. No evidence of the coprecipitation of ARF with mutant receptors could be obtained using azido-gamma[32P]-GTP or anti-ARF antibodies.

CONCLUSIONS

The activation of ARF proteins is stimulated by insulin and this process plays an important role in insulin-mediated regulation of PLD.

Phospholipase D2, a distinct phospholipase D isoform with novel regulatory properties that provokes cytoskeletal reorganization

BACKGROUND

Activation of phospholipase D (PLD) is an important but poorly understood component of receptor-mediated signal transduction responses and regulated secretion. We recently reported the cloning of the human gene encoding PLD1; this enzyme has low basal activity and is activated by protein kinase C and the small GTP-binding proteins, ADP-ribosylation factor (ARF), Rho, Rac and Cdc42. Biochemical and cell biological studies suggest, however, that additional and distinct PLD activities exist in cells, so a search was carried out for novel mammalian genes related to PLD1.

RESULTS

We have cloned the gene for a second PLD family member and characterized the protein product, which appears to be regulated differently from PLD1: PLD2 is constitutively active and may be modulated in vivo by inhibition. Unexpectedly, PLD2 localizes primarily to the plasma membrane, in contrast to PLD1 which localizes solely to peri-nuclear regions (the endoplasmic reticulum, Golgi apparatus and late endosomes), where PLD activity has been shown to promote ARF-mediated coated-vesicle formation. PLD2 provokes cortical reorganization and undergoes redistribution in serum-stimulated cells, suggesting that it may have a role in signal-induced cytoskeletal regulation and/or endocytosis.

CONCLUSIONS

PLD2 is a newly identified mammalian PLD isoform with novel regulatory properties. Our findings suggest that regulated secretion and morphological reorganization, the two most frequently proposed biological roles for PLD, are likely to be effected separately by PLD1 and PLD2.

Cell signalling through guanine-nucleotide-binding regulatory proteins (G proteins) and phospholipases

Phospholipases are important enzymes in cell signal transduction since they hydrolyze membrane phospholipids to generate signalling molecules. Heterotrimeric guanine-nucleotide-binding regulatory proteins (G proteins) play a major role in their regulation by a variety of agonists that activate receptors with seven membrane-spanning domains. Phospholipases of the C type, which hydrolyze inositol phospholipids to yield inositol trisphosphate and diacylglycerol, are regulated by the alpha and betagamma subunits of certain heterotrimeric G proteins as well as by receptor-associated and non-receptor-associated tyrosine kinases. Phospholipases of the D type, which hydrolyze phosphatidylcholine to phosphatidic acid, are regulated by members of the ADP-ribosylation factor and Rho subfamilies of small G proteins, and by protein kinase C and other factors. This review presents recent information concerning the molecular details of G protein regulation of these phospholipases.

Differential effects of the M1-M5 muscarinic acetylcholine receptor subtypes on intracellular calcium and on the incorporation of choline into membrane lipids in genetically modified Chinese hamster ovary cell lines

We compared responses of Chinese hamster ovary (CHO) cell lines stably transfected with human genes for the M1-M5 muscarinic receptor subtypes to several stimuli. While ATP brought about similar increases in the concentration of intracellular Ca2+ ions ([Ca2+]i) in the cell lines expressing all individual receptor subtypes, carbachol acted with much higher potency and efficacy on the cells expressing the M1, M3, and M5 receptor subtypes than on those expressing the M2 and M4 subtypes. The maximum [Ca2+]i responses to ATP corresponded to 41-75% of the maximum responses to carbachol in the cells expressing the M1, M3, and M5 receptor subtypes. The responses to ATP were strongly suppressed (> 75% decrease) by a preliminary administration of a maximally active concentration of carbachol in these three cell lines, whereas the responses to carbachol were less sensitive to the preliminary administration of a maximally active concentration of ATP (< 25% decrease). It appears likely that carbachol and ATP release Ca2+ ions from identical intracellular stores. Tetradecanoylphorbol acetate (TPA) strongly inhibited the responses of [Ca2+]i to both carbachol and ATP and enhanced the incorporation of [14C] choline into lipids in all five CHO cell lines investigated. On the other hand, the incorporation of [14C] choline into lipids was diminished by carbachol in the cell line expressing the M3 receptor subtype and unchanged in the other cell lines. This effect of carbachol was not dependent on the presence of extracellular Ca2+ ions and was not affected by TPA, which diminished the response of [Ca2+]i to muscarinic stimulation. It is suggested that it was due to muscarinic receptor-mediated activation of phospholipase D.

Regulation of phospholipase C and D activities by small molecular weight G proteins and muscarinic receptors

The role of small molecular weight guanine nucleotide-binding proteins (G proteins) of the Rho family in muscarinic acetylcholine receptor (mAChR) signaling to phospholipase C (PLC) and phospholipase D (PLD) was studied in human embryonic kidney (HEK) cells, stably expressing the human m3 receptor subtype. Evidence for the involvement of Rho proteins in m3 mAChR signaling to both phospholipases is based on findings obtained with Clostridium (C.) difficile toxin B and C. botulinum C3 exoenzyme, both of which specifically, although by different mechanisms, inactivate Rho family G proteins. Toxin B potently inhibited both the mAChR-stimulated PLC and PLD activities in intact cells as well as the stimulation of both phospholipases by the stable GTP analog GTPgammaS in permeabilized cells, the latter effect being mimicked by C3 exoenzyme. In contrast, PLC and PLD activities, measured in the presence of exogenous phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], a substrate and cofactor for PLC and PLD, respectively, were not altered. These data suggested that the Rho-inactivating toxins inhibit stimulation of PLC and PLD by reducing the cellular level of PtdIns(4,5)P2, which was indeed found with both toxin B and C3 exoenzyme. In agreement with a crucial role of cellular PtdIns(4,5)P2 supply for PLC signaling, we observed that short-term agonist (carbachol) treatment of HEK cells caused a long-lasting increase in PtdIns(4,5)P2 level, accompanied by a potentiation of receptor- and G protein-stimulated inositol phosphate formation. Finally, studies with tyrosine kinase and tyrosine phosphatase inhibitors strongly suggest that PtdIns(4,5)P2 synthesis and mAChR-stimulated PLD activity in HEK cells apparently also involve a tyrosine phosphorylation-dependent mechanism(s). Thus, m3 mAChR signaling to PLC and PLD in HEK cells requires the concerted action of various intracellular components, most notably the complex regulation of PtdIns(4,5)P2 synthesis.

Restoration of Clostridium difficile toxin-B-inhibited phospholipase D by phosphatidylinositol 4,5-bisphosphate

Receptor signalling to phospholipase D (PLD) in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor apparently involves Rho proteins. Since phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] has been recognized as an essential cofactor for PLD activity and since activated Rho proteins have been reported to stimulate the synthesis of PtdIns(4,5)P2, we studied whether in HEK cells PLD activity is regulated by PtdIns(4,5)P2 and, in particular, whether PtdIns(4,5)P2 can restore PLD activity inhibited by Clostridium difficile toxin B, which inactivates Rho proteins. Addition of MgATP to permeabilized HEK cells increased basal PLD activity and potentiated PLD stimulation by the stable GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP[S]), concomitant with a large increase in PtdIns(4,5)P2. On the other hand, neomycin, which binds to PtdIns(4,5)P2, inhibited basal and GTP[S]-stimulated PLD activities. Addition of PtdIns(4,5)P2 increased PLD activity in HEK cell membranes by 2-3-fold, whereas various other phospholipids were ineffective. Prior treatment of HEK cells with toxin B reduced the level of PtdIns(4,5)P2, measured either in intact cells or in membrane preparations, by about 40%. In membranes of toxin-B-treated cells, basal and GTP[S]-stimulated PLD activities were reduced, when measured with exogenous phosphatidylcholine as enzyme substrate. Inclusion of PtdIns(4,5)P2 with phosphatidylcholine in the substrate vesicles or addition of PtdIns(4,5)P2 fully restored basal and GTP[S]-stimulated PLD activities in membranes of toxin-B-treated cells. In conclusion, the data indicate that PtdIns(4,5)P2 is an essential cofactor for PLD activity in HEK cells and that inhibition of PLD activity by the Rho-inactivating toxin B is apparently caused by depletion of the PLD cofactor, PtdIns(4,5)P2.

Inhibition of receptor signaling to phospholipase D by Clostridium difficile toxin B. Role of Rho proteins

Rho proteins have been reported to activate phospholipase D (PLD) in in vitro preparations. To examine the role of Rho proteins in receptor signaling to PLD, we studied the effect of Clostridium difficile toxin B, which glucosylates Rho proteins, on the regulation of PLD activity in human embryonic kidney (HEK) cells stably expressing the m3 muscarinic acetylcholine receptor (mAChR). Toxin B treatment of HEK cells potently and efficiently blocked mAChR-stimulated PLD. In contrast, basal and phorbol ester-stimulated PLD activities were not or only slightly reduced. Cytochalasin B and Clostridium botulinum C2 toxin, mimicking the effect of toxin B on the actin cytoskeleton but without involving Rho proteins, had no effect on mAChR-stimulated PLD. Toxin B did not alter cell surface mAChR number and mAChR-stimulated binding of (guanosine 5'-O-(thio)triphosphate (GTP gamma S) to G proteins. In addition to mAChR-stimulated PLD, toxin B treatment also inhibited PLD activation by the direct G protein activators, AlF4- and GTP gamma S, studied in intact and permeabilized cells, respectively. Finally, C. botulinum C3 exoenzyme, which ADP-ribosylates Rho proteins, mimicked the inhibitory effect of toxin B on GTP gamma S-stimulated PLD activity. In conclusion, the data presented indicate that toxin B potently and selectively interferes with receptor coupling mechanisms to PLD, and furthermore suggest an essential role for Rho proteins in receptor signaling to PLD.