Structural organization of mammalian lipid phosphate phosphatases: implications for signal transduction

doi:10.1016/S1388-1981(99)00102-X

Biochimica et Biophysica Acta (BBA) - Molecular and Cell Biology of Lipids

Volume 1439, Issue 2, 30 July 1999, Pages 299-316

https://doi.org/10.1016/S1388-1981(99)00102-X Get rights and content

Abstract

This article describes the regulation of cell signaling by lipid phosphate phosphatases (LPPs) that control the conversion of bioactive lipid phosphates to their dephosphorylated counterparts. A structural model of the LPPs, that were previously called Type 2 phosphatidate phosphatases, is described. LPPs are characterized by having no Mg²⁺ requirement and their insensitivity to inhibition by N-ethylmaleimide. The LPPs have six putative transmembrane domains and three highly conserved domains that define a phosphatase superfamily. The conserved domains are juxtaposed to the proposed membrane spanning domains such that they probably form the active sites of the phosphatases. It is predicted that the active sites of the LPPs are exposed at the cell surface or on the luminal surface of intracellular organelles, such as Golgi or the endoplasmic reticulum, depending where various LPPs are expressed. LPPs could attenuate cell activation by dephosphorylating bioactive lipid phosphate esters such as phosphatidate, lysophosphatidate, sphingosine 1-phosphate and ceramide 1-phosphate. In so doing, the LPPs could generate alternative signals from diacylglycerol, sphingosine and ceramide. The LPPs might help to modulate cell signaling by the phospholipase D pathway. For example, phosphatidate generated within the cell by phospholipase D could be converted by an LPP to diacylglycerol. This should change the relative balance of signaling by these two lipids. Another possible function of the LPPs relates to the secretion of lysophosphatidate and sphingosine 1-phosphate by activated platelets and other cells. These exogenous lipids activate phospholipid growth factor receptors on the surface of cells. LPP activities could attenuate cell activation by lysophosphatidate and sphingosine 1-phosphate through their respective receptors.

Introduction

The purpose of this article is to review the role of PA phosphatases and lipid phosphate phosphatases (LPPs) in regulating signaling in mammalian cells by bioactive lipid phosphate esters. These include PA, lysoPA, DGPP, ceramide 1-phosphate and sphingosine 1-phosphate. It is proposed that the LPPs help to regulate the balance of signaling by the lipid phosphate esters versus the dephosphorylation products which in the case of diacylglycerol, ceramide and sphingosine are also bioactive. Thus the LPPs could play an important role in cell activation by modulating the signals from both glycerolipids and sphingolipids.

Section snippets

Cell signaling by bioactive glycerolipids versus their dephosphorylation products

The initial interest in the involvement of PA phosphatase in cell activation was as the second enzyme of the PLD pathway. PA phosphatase is believed to be responsible for the second phase of diacylglycerol signaling after PLD stimulation [1], [2], [3]. PA is also converted to lysoPA by phospholipase A₁ or A₂ activities (Fig. 1). LysoPA can be produced through the action of lysoPLD although this reaction is relatively selective for 1-O-alkylglycerophosphorylcholine rather than

Cell signaling by bioactive sphingolipid phosphates versus their dephosphorylation products

The activation of sphingomyelinases produces several sphingolipids that also have multiple biological functions (Table 2) including the regulation of cell transformation, differentiation and proliferation [5], [27], [34], [35], [36], [37], [38]. Sphingomyelin hydrolysis is stimulated by tumor necrosis fator-α (TNFα), interleukin-1, γ-interferon, glucocorticoids and nerve growth factor (Fig. 2 and [34], [35], [38]). Ceramide generated by neutral sphingomyelinase stimulates serine/threonine

Characterization of phosphatidate phosphatase and lipid phosphate phosphatase

Phosphatidate phosphatase (EC 3.1.3.2) was first identified as being involved in the conversion of phosphatidate to diacylglycerol for triacylglycerol, phosphatidylcholine and phosphatidylethanolamine synthesis [61]. This enzyme is often called PAP-1 indicating a Type 1 phosphatase that is characterized by having an absolute Mg²⁺-requirement and being inhibited by N-ethylmaleimide [62]. PAP-1 is located in the cytosol and is activated by translocation to the endoplasmic reticulum where

Structure and substrate specificities of lipid phosphate phosphatase

LPP (PAP-2) has been purified and characterized by various groups [65], [66], [67], [68], [69], [70]. LPP purified from rat liver dephosphorylates lysophosphatidate, ceramide 1-phosphate, sphingosine 1-phosphate [71] and DGPP [28] with efficiencies similar to PA. These substrates are also mutually competitive, indicating that they are dephosphorylated at the same active site. LPP does not hydrolyze phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol or

The lipid phosphate phosphatases belong to a phosphatase superfamily

The existence of a phosphatase superfamily was first proposed by Stukey and Carman [79] and expanded by Hemricka et al. [80], Neuwald [81] and Brindley and Waggoner [64]. This superfamily includes bacterial non-specific acid phosphatases and DGPPase [82], [83], yeast DGPPase [84], dihydrosphingosine/phytosphingosine phosphate phosphatase [85], [86] and lipid phosphate phosphatase [87], fungal haloperoxidases [88], mammalian G6Pase [89], [90], [91], the Drosophila protein wunen [92], rat Dri42

Regulation of LPP activity

Several potential phosphorylation sites on the proposed cytosolic N- and C-termini (Fig. 3) for the LPPs which are consistent with evidence that LPP is a phosphoprotein [67]. rLPP-1 contains a putative phosphorylation consensus sequence for protein kinase C and casein kinase 2 at S255, and a second putative phosphorylation site for casein kinase 2 at T267 (Fig. 3). In LPP-3, the equivalent phosphorylation sites are not present. In hLPP-2, there is a consensus phosphorylation site at T270 and a

Dephosphorylation of internal lipid phosphate esters by the LPPs

It is expected that the LPPs may play a role in metabolizing PA that is produced following activation of the PLDs or diacylglycerol kinases (Fig. 1). We provided indirect evidence that LPP can dephosphorylate PA generated by PLD. These experiments used ras-transformed fibroblasts which have low LPP activity compared to the parental rat2 fibroblasts. Stimulation of PLD increased the labeling of PA relative to DAG in ras-transformed compared to control fibroblasts [98]. This is compatible with

Degradation of exogenous lipid phosphate esters by the LPPs

In addition to degrading internal lipids phosphate esters, LPPs could also act as ‘ecto-enzymes’ that regulate signaling by exogenous lysoPA and sphingosine 1-phosphate (Table 1, Table 2). This function is compatible with the proposed structure of LPP-1 (Fig. 4) which predicts that the active site is located in the external leaflet of the membrane bilayer when LPP-1 is present in the plasma membrane. In agreement with this prediction, overexpression of mLPP-1 in rat fibroblasts increases the

LPP activity in bacteria and yeast

Bacterial and yeast DGPPases also belong to the phosphatase superfamily [59], [74], [75], [76]. As discussed in Section 2, small quantities of DGPP occur in bacteria and yeast and this lipid is thought to exhibit signaling properties. Yeast DGPPases preferentially dephosphorylate DGPP rather than PA since the specificity constant (K_m/V_max) is about 10 times higher for DGPP than for PA [28], [84], [105]. DGPP is a potent competitive inhibitor of PA hydrolysis, whereas PA does not effectively

Summary and conclusions

At present we are at an exciting point where various isoforms of the LPPs have been identified and the recombinant proteins overexpressed. Little is known about the functions of these enzymes or the consequences for signal transduction of modulating LPP activity. The regulation of the LPPs has also not been investigated in detail and we do not know if the putative phosphorylation sites on the C-terminus of LPP-1 (Fig. 2) control enzyme activity. Another area of uncertainty is whether other LPPs

Acknowledgements

We thank Dr. R. Parker (Alberta Peptide Institute) for his contribution to the development of the three-dimensional model of rLPP-1. Our work on LPPs was supported by a grant from the Medical Research Council of Canada (MT 10504).

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ReviewStructural organization of mammalian lipid phosphate phosphatases: implications for signal transduction

Abstract

Introduction

Section snippets

Cell signaling by bioactive glycerolipids versus their dephosphorylation products

Cell signaling by bioactive sphingolipid phosphates versus their dephosphorylation products

Characterization of phosphatidate phosphatase and lipid phosphate phosphatase

Structure and substrate specificities of lipid phosphate phosphatase

The lipid phosphate phosphatases belong to a phosphatase superfamily

Regulation of LPP activity

Dephosphorylation of internal lipid phosphate esters by the LPPs

Degradation of exogenous lipid phosphate esters by the LPPs

LPP activity in bacteria and yeast

Summary and conclusions

Acknowledgements

Chem. Phys. Lipids

Biochim. Biophys. Acta

J. Biol. Chem.

Methods Enzymol.

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Cell

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Cell

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Biochim. Biophys. Acta

FEBS Lett.

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Cell

Biochim. Biophys. Acta

Cell

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FEBS Lett.

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Review
Structural organization of mammalian lipid phosphate phosphatases: implications for signal transduction