A Soluble Protein Negatively Regulates Phospholipase D Activity. Partial Purification and Characterization

Purification of phospholipase D from Dacus carota by three-phase partitioning and its characterization

Phospholipase D from Dacus carota (carrot) was purified by subjecting it to three-phase partitioning. The single step of three phase partitioning led to 13-fold purification with an activity recovery of 72%. SDS-PAGE analysis showed a single band with minimum molecular weight corresponding to nearly 60 kDa. The purified enzyme had a pH optimum in the range of 6.0--6.5 and was unstable above 30 degrees C. Kinetic studies showed a K(m) value of 9.5 mM and a V(max) of 0.35 mL min(-1). The enzyme purified by three-phase partitioning was found to resolve into two isoenzymes on a DEAE-cellulose column.

A role for rho-kinase in rho-controlled phospholipase D stimulation by the m3 muscarinic acetylcholine receptor

Stimulation of phospholipase D (PLD) by membrane receptors is now recognized as a major signal transduction pathway involved in diverse cellular functions. Rho proteins control receptor signaling to PLD, and these GTPases have been shown to directly stimulate purified recombinant PLD1 enzymes in vitro. Here we report that stimulation of PLD activity, measured in the presence of phosphatidylinositol 4,5-bisphosphate, by RhoA in membranes of HEK-293 cells expressing the m3 muscarinic acetylcholine receptor (mAChR) is phosphorylation-dependent. Therefore, the possible involvement of the RhoA-stimulated serine/threonine kinase, Rho-kinase, was investigated. Overexpression of Rho-kinase and constitutively active Rho-kinase (Rho-kinase-CAT) but not of kinase-deficient Rho-kinase-CAT markedly increased m3 mAChR-mediated but not protein kinase C-mediated PLD stimulation, similar to overexpression of RhoA. Expression of the Rho-inactivating C3 transferase abrogated the stimulatory effect of wild-type Rho-kinase, but not of Rho-kinase-CAT. Recombinant Rho-kinase-CAT mimicked the phosphorylation-dependent PLD stimulation by RhoA in HEK-293 cell membranes. Finally, the Rho-kinase inhibitor HA-1077 largely inhibited RhoA-induced PLD stimulation in membranes as well as PLD stimulation by the m3 mAChR but not by protein kinase C in intact HEK-293 cells. We conclude that Rho-kinase is involved in Rho-dependent PLD stimulation by the G protein-coupled m3 mAChR in HEK-293 cells. Thus, our findings identify Rho-kinase as a novel player in the receptor-controlled PLD signaling pathway.

Enhancement of phospholipase D activity following baculovirus and adenovirus infection in Sf9 and COS-7 cells

In order to purify the human phospholipase D1 (hPLD1) for analysis of its functional properties, we applied a baculovirus-based high-expression system. As expected, Sf9 cells infected with a baculovirus encoding for the hPLD1 displayed a 7.5-fold increase in PLD activity compared to uninfected cells. Sf9 cells infected with the wild-type (WT) and other recombinant baculoviruses were used as an expression control. Surprisingly, all baculoviruses tested led to a 3-5 fold increase in basal PLD activity when compared to uninfected cells. To further characterize the nature of the increased PLD activity, the influence of ADP-ribosylation factor (ARF) and phorbol 12-myristate 13-acetate (PMA) was studied. In contrast to membranes containing the hPLD1, the PLD activity in membranes from uninfected and WT-infected Sf9 cells was not stimulated by ARF. PMA did not affect the increase in PLD activity in any case. To further study whether the virus-mediated increase in PLD activity is a more general phenomenon, we infected COS-7 cells with recombinant and WT adenoviruses. Only the infection with the WT adenovirus resulted in an approx. 2-fold increase in PLD activity. Our results demonstrate for the first time that a viral infection elevates the PLD activity in insect and mammalian cells.

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.

The elevation of cellular phosphatidic acid levels caused by polyomavirus transformation can be disassociated from the activation of phospholipase D

Middle T (mT), the oncogene of murine polyomavirus, causes transformation of rat fibroblasts by activating a number of signal transducing pathways usually used by polypeptide growth factors and their receptors. Here, we report data regarding the activation of signal transducing pathways involving phospholipase D (PL-D). The hydrolysis of phospholipids by PL-D produces phosphatidic acid (PA), a compound with multiple biological effects. The PA content of cells expressing wild-type mT, introduced via a number of different methods, is approximately 50% higher than their untransformed counterparts. This increase in cellular PA content is associated with an approximately 65% increase in PL-D activity in cells expressing wild-type mT. We have also examined the effects of a number of site-directed mutants of mT, on both cellular PA levels and on PL-D activity. Mutants that do not produce mT (Py808A) or that produce a truncated, nonmembrane bound mT (Py1387T) have PA levels similar to that of control cells. Cells expressing the 322YF mutant of mT (which abolishes interaction of mT with phospholipase C gamma1) show increases in both PA levels and PL-D activity that are similar to those seen with wild-type mT. Expression of mutants that abolish the interaction of mT with either shc or with phosphatidylinositol 3-kinase (250YS and 315YF, respectively) cause an increase in PL-D activity comparable to that seen with wild-type mT. However, the PA content of cells expressing these mutants is not elevated. These results suggest that mT causes activation of cellular PL-D, but this activation alone is not sufficient to cause an increase in cellular PA content. Therefore, wild-type mT must affect another, as yet unknown, step in PA metabolism.

Characterization of two alternately spliced forms of phospholipase D1. Activation of the purified enzymes by phosphatidylinositol 4,5-bisphosphate, ADP-ribosylation factor, and Rho family monomeric GTP-binding proteins and protein kinase C-alpha

We previously reported the cloning of a cDNA encoding human phosphatidylcholine-specific phospholipase D1 (PLD1), an ADP-ribosylation factor (ARF)-activated phosphatidylcholine-specific phospholipase D (Hammond, S. M., Tsung, S., Autschuller, Y., Rudge, S. A., Rose, K., Engebrecht, J., Morris, A. J., and Frohman, M. A. (1995) J. Biol. Chem. 270, 29640-29643). We have now identified an evolutionarily conserved shorter splice variant of PLD1 lacking 38 amino acids (residues 585-624) that arises from regulated splicing of an alternate exon. Both forms of PLD1 (PLD1a and 1b) have been expressed in Sf9 cells using baculovirus vectors and purified to homogeneity by detergent extraction and immunoaffinity chromatography. PLD1a and 1b have very similar properties. PLD1a and 1b activity is Mg2+dependent but insensitive to changes in free Ca2+ concentration. Phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate activate PLD1a and 1b but a range of other acidic phospholipids are ineffective. PLD1a and 1b are highly responsive to activation by GTP-gammaS-liganded ADP-ribosylation factor-1 (ARF-1) and can also be activated to a lesser extent by three purified RHO family monomeric GTP-binding proteins, RHO A, RAC-1, and CDC42. Activation of PLD1a and 1b by the RHO family monomeric GTP-binding proteins is GTP-dependent and synergistic with ARF-1. Purified protein kinase C-alpha activates PLD1a and 1b in a manner that is stimulated by phorbol esters and does not require ATP. Activation of PLD1a and 1b by protein kinase C-alpha is synergistic with ARF and with the RHO family monomeric GTP-binding proteins, suggesting that these three classes of regulators interact with different sites on the enzyme.

Regulation of eukaryotic phosphatidylinositol-specific phospholipase C and phospholipase D

This review focuses on two phospholipase activities involved in eukaryotic signal transduction. The action of the phosphatidylinositol-specific phospholipase C enzymes produces two well-characterized second messengers, inositol 1,4,5-trisphosphate and diacylglycerol. This discussion emphasizes recent advances in elucidation of the mechanisms of regulation and catalysis of the various isoforms of these enzymes. These are especially related to structural information now available for a phospholipase C delta isozyme. Phospholipase D hydrolyzes phospholipids to produce phosphatidic acid and the respective head group. A perspective of selected past studies is related to emerging molecular characterization of purified and cloned phospholipases D. Evidence for various stimulatory agents (two small G protein families, protein kinase C, and phosphoinositides) suggests complex regulatory mechanisms, and some studies suggest a role for this enzyme activity in intracellular membrane traffic.

Inhibition of phospholipase D by a protein factor from bovine brain cytosol. Partial purification and characterization of the inhibition mechanism

A specific protein inhibitor of partially purified bovine brain phospholipase D (PLD) was identified from bovine brain cytosol. The PLD inhibitor has been enriched through several chromatographic steps and characterized with respect to size and mechanism of inhibition. The inhibitor showed an apparent molecular mass of 30 kDa by Superose 12 gel exclusion chromatography and inhibited PLD activity with an IC50 of 7 nM. The inhibitor had neither proteolytic activity nor phospholipid-hydrolyzing activity. Because phosphatidylinositol 4,5-bisphosphate (PIP2), which is included in substrate vesicles, is an essential cofactor for PLD, we examined whether the inhibition might be mediated by sequestration of PIP2. PIP2 hydrolysis by phospholipase C (PLC)-beta1 was not affected by the inhibitor and the inhibitor did not bind to substrate vesicles containing PIP2. In contrast, a PH domain derived from PLC-delta1, which could bind to PIP2, showed a nearly identical inhibition of both PLC-beta1 and PLD activities. Thus, the PLD inhibition by the inhibitor is due to the specific interaction with not PIP2 but PLD. The suppression of PLD activity by the inhibitor was largely eliminated by the addition of ADP-ribosylation factor (ARF) and GTPgammaS. We propose that the inhibitor plays a negative role in regulation of PLD activity by PIP2 and ARF.

Inhibition of phospholipase D activity by fodrin. An active role for the cytoskeleton

Phospholipase D (PLD) is a major enzyme implicated in important cellular processes such as secretion and proliferation. The knowledge of its regulation is essential to understand the control of these phenomena. Several proteins activating PLD have been described in the last years. In this report, we chromatographed bovine brain cytosolic proteins to identify fodrin, the non-erythroid spectrin, as the first described inhibitor of PLD. A cytosolic fraction with an inhibitory effect on PLD activity loses its capacity after immunoprecipitation of fodrin. Moreover, at 1 nM, purified fodrin blocks fully and quickly PLD activity, whatever the stimuli used. In contrast, fodrin has no effect on adenylate cyclase activity. Fodrin-analogous proteins like dimeric or tetrameric erythroid spectrin have the same inhibitory effect on PLD, at higher concentrations. Other cytoskeletal proteins, actin and vimentin, are inefficient on PLD inhibition. The mechanisms implicated in PLD modulation such as post-translational modifications of fodrin and the role of small G-proteins on the cytoskeleton regulation are discussed. In conclusion, this study reveals that fodrin is involved in the control of PLD activity, suggesting that the cytoskeleton could have an active role in control of secretion and proliferation.

Multiple forms of phospholipase D inhibitor from rat brain cytosol. Purification and characterization of heat-labile form

Rat brain cytosol contains proteins that markedly inhibit the activity of partially purified brain membrane phospholipase D (PLD) stimulated by ADP-ribosylation factor (Arf) and phosphatidylinositol 4,5-bisphosphate (PIP2). Sequential chromatography of the brain cytosol yielded four inhibitor fractions, which exhibited different kinetics to heat treatment at 70 degrees C. Purification of the most heat-labile inhibitor to homogeneity yielded two preparations, which displayed apparent molecular masses of 150 kDa and 135 kDa, respectively, on SDS-polyacrylamide gels. Tryptic digests of the 150- and 135-kDa proteins yielded similar elution profiles on a C18 reverse-phase column, suggesting that the 135-kDa form is a truncated form of the 150-kDa form. Sequences of two tryptic peptides were determined. A data base search revealed no proteins with these sequences. The purified 150-kDa inhibitor negated the PLD activity stimulated by Arf, RhoA, or Cdc42. The concentration required for half-maximal inhibition was 0.4 nM. Concentration dependence on the 150-kDa inhibitor was not affected by changes in the concentrations of Arf, PIP2, or phosphatidylcholine used in the assays, suggesting that the inhibition is not due to competition with the activators or substrate for PLD. The purified inhibitor did not affect the PIP2-hydrolyzing activity of a phospholipase C isozyme that was measured with substrate vesicles of lipid composition identical with that used for the PLD assay. Thus, the mechanism of inhibition appears to be a specific allosteric modification of PLD rather than disruption of substrate vesicles.

Phospholipase D regulation by a physical interaction with the actin-binding protein gelsolin

Increases in intracellular phosphatidic acid levels caused by receptor- mediated activation of phospholipase D (PLD) have been implicated in many signal transduction pathways leading to cellular activation. PLD is known to be regulated by several means, including tyrosine kinase activity, increases in Ca2+, receptor-coupled G proteins, small GTP binding proteins, ceramide metabolisms, and protein kinase C. We have investigated a additional regulatory effect on PLD activity involving nucleoside triphosphates (NTPs). A NTP binding protein copurifies with LPD activity from rabbit brains using a GTP-agarose affinity column, and this protein stimulates PLD activity only in the absence of NPTs. The NTP effect is reversible and labile, and the binding protein is separable from the PLD activity by heparin-agarose chromatography. We identified this protein as the actin- binding protein gelsolin by amino acid sequencing following peptide mapping. This finding was verified by the co-immunoprecipitation of gelsolin and PLD activity as well as by the reconstitution of gelsolin- dependent nucleotide sensitive PLD activity by the addition of purified gelsolin-free PLD. Our data indicate that actin rearrangements and PLD signaling are coordinately regulated through the physical association between PLD and gelsolin and that this interaction may also serve to amplify both PLD signaling and actin reorganization.

Regulation of membrane-bound phospholipase D by protein kinase C in HL60 cells. Synergistic action of small GTP-binding protein RhoA

In HL60 cells, the membrane-bound phospholipase D (PLD) was stimulated by 4beta-phorbol 12-myristate 13-acetate (PMA) in the presence of the cytosolic fraction from HL60 cells or rat brain. The cytosolic factor for this PMA-induced PLD activation was subjected to purification from rat brain by sequential chromatographies. The PLD stimulating activity was found in protein kinase C (PKC) fraction containing alpha, betaI, betaII, and gamma isozymes. PKC isozymes were further separated by hydroxylapatite chromatography. PKCalpha and - beta, but not gamma, isozymes were found to activate membrane-bound PLD. PKCalpha was much more effective than PKCbeta for PLD activation. Millimolar concentrations of MgATP were required for the PKC-mediated PLD activation in HL60 membranes. MgATP is utilized to maintain the levels of phosphatidylinositol 4,5-bisphosphate (PIP2) under these assay conditions. The PKC-mediated PLD activation was completely inhibited by neomycin, a high affinity ligand for PIP2, and this suppression was recovered by the addition of exogenous PIP2. Thus, these results suggest that PIP2 is supposed to play a key role in PKC-mediated PLD activity in HL60 membranes. Furthermore, PKCalpha-mediated PLD activation was potentiated by the addition of recombinant RhoA protein in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). The results obtained here indicate that PKCalpha and RhoA (GTP form) exert a synergistic action in the membrane-bound PLD activation in HL60 cells.