Phosphorylation and redistribution of the phosphatidylinositol-transfer protein in phorbol 12-myristate 13-acetate- and bombesin-stimulated Swiss mouse 3T3 fibroblasts

Overexpression of phosphatidylinositol transfer protein β in NIH3T3 cells has a stimulatory effect on sphingomyelin synthesis and apoptosis

Phosphatidylinositol transfer proteins (PI-TPs) consist of two isoforms (PI-TPalpha and PI-TPbeta), which differ in phospholipid transfer properties and intracellular localization. Both PI-TP isoforms are substrates for protein kinase C and contain a minor phosphorylation site (Ser166 in PI-TPalpha; Ser165 in PI-TPbeta). Only PI-TPbeta contains a major phosphorylation site at Ser262, which must be phosphorylated for PI-TPbeta to be associated with the Golgi. The PI-TP isoforms are completely conserved between mammals. Although their function is still not clear, their importance follows from knock-out studies, showing that mice lacking PI-TPalpha die soon after birth and that embryonic stems cells lacking PI-TPbeta cannot be generated [Mol. Biol. Cell 13 (2002) 739]. We determined the levels of the PI-TP isoforms in various mouse tissues by immunoblotting. PI-TPalpha is present in all tissues investigated, with highest levels in brain (167 ng/100 microg total protein). The levels of PI-TPbeta are 50-100 times lower than those of PI-TPalpha, with relatively high levels found in liver and brain (1.2 and 1.8 ng/100 microg of total protein, respectively). In contrast to NIH3T3 cells overexpressing PI-TPalpha, cells overexpressing PI-TPbeta (SPIbeta cells) were able to maintain steady-state levels of sphingomyelin in plasma membrane under conditions where this lipid is degraded by exogenous sphingomyelinase. This process of rapid sphingomyelin replenishment is dependent on PI-TPbeta being associated with the Golgi as cells overexpressing a mutant PI-TPbeta in which the major phosphorylation site is replaced (PI-TPbeta(S262A) behave as wild-type NIH3T3 cells. Since the SPIbeta cells display a decreased growth rate (35 h as compared to 21 h for wtNIH3T3 cells), we have investigated the sensitivity of these cells towards UV-induced apoptosis. We have found that the SPIbeta cells, but not the cells overexpressing PI-TPbeta(S262A), are very sensitive. We are currently investigating whether a relationship exists between PI-TPbeta being involved in maintaining plasma membrane sphingomyelin levels and the enhanced sensitivity towards apoptosis.

The Golgi localization of phosphatidylinositol transfer protein beta requires the protein kinase C-dependent phosphorylation of serine 262 and is essential for maintaining plasma membrane sphingomyelin levels

Recombinant mouse phosphatidylinositol transfer protein (PI-TP)beta is a substrate for protein kinase C (PKC)-dependent phosphorylation in vitro. Based on site-directed mutagenesis and two-dimensional tryptic peptide mapping, Ser(262) was identified as the major site of phosphorylation and Ser(165) as a minor phosphorylation site. The phospholipid transfer activities of wild-type PI-TP beta and PI-TP beta(S262A) were identical, whereas PI-TP beta(S165A) was completely inactive. PKC-dependent phosphorylation of Ser(262) also had no effect on the transfer activity of PI-TP beta. To investigate the role of Ser(262) in the functioning of PI-TP beta, wtPI-TP beta and PI-TP beta(S262A) were overexpressed in NIH3T3 fibroblast cells. Two-dimensional PAGE analysis of cell lysates was used to separate PI-TP beta from its phosphorylated form. After Western blotting, wtPI-TP beta was found to be 85% phosphorylated, whereas PI-TP beta(S262A) was not phosphorylated. In the presence of the PKC inhibitor GF 109203X, the phosphorylated form of wtPI-TP beta was strongly reduced. Immunolocalization showed that wtPI-TP beta was predominantly associated with the Golgi membranes. In the presence of the PKC inhibitor, wtPI-TP beta was distributed throughout the cell similar to what was observed for PI-TP beta(S262A). In contrast to wtPI-TP beta overexpressors, cells overexpressing PI-TP beta(S262A) were unable to rapidly replenish sphingomyelin in the plasma membrane upon degradation by sphingomyelinase. This implies that PKC-dependent association with the Golgi complex is a prerequisite for PI-TP beta to express its effect on sphingomyelin metabolism.

The protein kinase C-dependent phosphorylation of serine 166 is controlled by the phospholipid species bound to the phosphatidylinositol transfer protein alpha

The charge isomers of bovine brain PI-TPalpha (i.e. PI-TPalphaI containing a phosphatidylinositol (PI) molecule and PI-TPalphaII containing a phosphatidylcholine (PC) molecule) were phosphorylated in vitro by rat brain protein kinase C (PKC) at different rates. From the double-reciprocal plot, it was estimated that the V(max) values for PI-TPalphaI and II were 2.0 and 6.0 nmol/min, respectively; the K(m) values for both charge isomers were about equal, i.e. 0.7 micrometer. Phosphorylation of charge isomers of recombinant mouse PI-TPalpha confirmed that the PC-containing isomer was the better substrate. Phosphoamino acid analysis of in vitro and in vivo (32)P-labeled PI-TPalphas showed that serine was the major site of phosphorylation. Degradation of (32)P-labeled PI-TPalpha by cyanogen bromide followed by high pressure liquid chromatography and sequence analysis yielded one (32)P-labeled peptide (amino acids 104-190). This peptide contained Ser-148, Ser-152, and the consensus PKC phosphorylation site Ser-166. Replacement of Ser-166 with an alanine residue confirmed that indeed this residue was the site of phosphorylation. This mutation completely abolished PI and PC transfer activity. This was also observed when Ser-166 was replaced with Asp, implying that this is a key amino acid residue in regulating the function of PI-TPalpha. Stimulation of NIH3T3 fibroblasts by phorbol ester or platelet-derived growth factor induced the rapid relocalization of PI-TPalpha to perinuclear Golgi structures concomitant with a 2-3-fold increase in lysophosphatidylinositol levels. This relocalization was also observed for Myc-tagged wtPI-TPalpha expressed in NIH3T3 cells. In contrast, the distribution of Myc-tagged PI-TPalpha(S166A) and Myc-tagged PI-TPalpha(S166D) were not affected by phorbol ester, suggesting that phosphorylation of Ser-166 was a prerequisite for the relocalization to the Golgi. A model is proposed in which the PKC-dependent phosphorylation of PI-TPalpha is linked to the degradation of PI.

Phospholipid transfer proteins and physiological functions

Issues of how cells generate and maintain unique lipid compositions in distinct intracellular membrane systems remain the subject of much study. A ubiquitous class of soluble proteins capable of transporting phospholipid monomers from membrane to membrane across an aqueous milieu has been thought to define part of the mechanism by which lipids are sorted in cells. Progress in the study of these phospholipid transfer proteins (PLTPs) raises questions regarding their physiological functions in cells and the mechanisms by which these proteins execute them. It is now clear that across the eukaryotic kingdom, members of this protein family exert essential roles in the regulation of phospholipid metabolism and central aspects of phospholipid-mediated signaling. Indeed, it is now known that dysfunction of specific PLTPs defines the basis of inherited diseases in mammals, and this list is expected to grow. Phospholipid transfer proteins, their biochemical properties, and the emerging clues regarding their physiological functions are reviewed.

Elevation of cyclic AMP decreases phosphoinositide turnover and inhibits thrombin-induced secretion in human platelets

Elevation of cyclic AMP (cAMP) in platelets inhibits agonist-induced, G protein-mediated responses and activation of polyphosphoinositide-specific phospholipase C (PLC) by ill-defined mechanism(s). Signal transduction steps downstream of PLC are inhibited by elevated cAMP, suggesting an inhibitory effect of cAMP, via protein kinase A, on PLC. In [32P]i-prelabeled platelets, forskolin increased intracellular cAMP (104 nmol/1011 cells at 10-5 M forskolin) and [32P]phosphatidylinositol 4-phosphate (Delta[32P]PIP) (30% at 10-7-10-6 M forskolin). The thrombin-induced (0.1 U/ml) increase in production of [32P]PA, 'overshoots' in [32P]PIP and [32P]PIP2 ([32P]phosphatidylinositol 4,5-bisphosphate), and the increase in [32P]PI and secretion of ADP+ATP were abolished by forskolin (10-7 M). Forskolin stimulated total [32P]Pi uptake in resting platelets (48%), increased 32P incorporation into PIP (110%), and inhibited 32P incorporation into PI (50%). The latter inhibition was most likely considerably greater due to the forskolin-induced stimulation of [32P]Pi uptake. The changes in radioactive PA, PIP and PIP2 are regarded as being proportional with their masses in the prelabeled platelets, while the increase in PI (phosphatidylinositol) is regarded as a change in specific radioactivity, and hence in its synthesis. The results suggest that cAMP elevation inhibits the flux in the polyphosphoinositide cycle through both inhibition of PIP 5-kinase and PI synthesis. The inverse relation between forskolin-produced DeltaPIP and [32P]PA production suggests that the PLC reaction is inhibited by elevated cAMP through reduction of substrate (PIP2) resynthesis, and not by inhibition of the PLC enzyme.

Phosphatidylinositol transfer proteins: a requirement in signal transduction and vesicle traffic

Phosphatidylinositol transfer protein (PITP) was originally identified and named because of its ability to transport phosphatidylinositol through the aqueous phase from one membrane compartment to another. Recent data, however, indicate unanticipated roles for PITP in the coupling of PIP2 synthesis to signal transduction reactions and to membrane traffic in mammalian cells. PITP was recently purified on the basis of its ability to restore cellular functions in permeabilized cells depleted of cytosolic proteins. These functions include cell-surface receptor-regulated hydrolysis of PIP2 by phospholipases C beta- and gamma-isozymes, regulated release of secretory granules, and the budding of constitutive secretory vesicles and immature secretory granules from the trans-Golgi network. In the yeast Saccharomyces cerevisiae, a PITP was identified from a mutant strain with a defect in the secretory pathway (SEC14) and therefore required for cell viability; in Yarrowia lipolytica, PITP is required for differentiation from a yeast to a mycelial growth form. We are just beginning to unravel the intriguing mechanisms by which PITP/SEC14 may accomplish its function in eukaryotic cells in signal transduction and membrane trafficking.

Phospholipid transfer proteins revisited

Phosphatidylinositol transfer protein (PI-TP) and the non-specific lipid transfer protein (nsL-TP) (identical with sterol carrier protein 2) belong to the large and diverse family of intracellular lipid-binding proteins. Although these two proteins may express a comparable phospholipid transfer activity in vitro, recent studies in yeast and mammalian cells have indicated that they serve completely different functions. PI-TP (identical with yeast SEC14p) plays an important role in vesicle flow both in the budding reaction from the trans-Golgi network and in the fusion reaction with the plasma membrane. In yeast, vesicle budding is linked to PI-TP regulating Golgi phosphatidylcholine (PC) biosynthesis with the apparent purpose of maintaining an optimal PI/PC ratio of the Golgi complex. In mammalian cells, vesicle flow appears to be dependent on PI-TP stimulating phosphatidylinositol 4,5-bisphosphate (PIP2) synthesis. This latter process may also be linked to the ability of PI-TP to reconstitute the receptor-controlled PIP2-specific phospholipase C activity. The nsL-TP is a peroxisomal protein which, by its ability to bind fatty acyl-CoAs, is most likely involved in the beta-oxidation of fatty acids in this organelle. This protein constitutes the N-terminus of the 58 kDa protein which is one of the peroxisomal 3-oxo-acyl-CoA thiolases. Further studies on these and other known phospholipid transfer proteins are bound to reveal new insights in their important role as mediators between lipid metabolism and cell functions.

Growth factor-dependent phosphoinositide signalling

A wide variety of messages, in the form of diffusible growth factors, hormones and cytokines, are carried throughout multicellular organisms to coordinate important physiological properties of target cells, such as proliferation, differentiation, migration, apoptosis and metabolism. Most messengers bind to cognate receptors on target cells, which initiate a characteristic cascade of reactions within the cell, ultimately leading to the desired response. The cellular response is defined by the combination of signalling components whose individual activity depends upon the number and type of surface receptors. Consequently the responses of different cell types to one or more stimuli can be quite disparate. A molecular understanding of the signalling pathways employed by each type of receptor therefore underlies the ability to rationalize many cellular functions and to correct disfunctions. As a well studied example of the primary signalling events that take place on the cytoplasmic leaflet of the plasma membrane following receptor activation, we will discuss how the widely expressed receptor for epidermal growth factor (EGF) causes the phosphorylation and hydrolysis of a signalling precursor, the membrane lipid phosphatidylinositol. This paradigm will be used to illustrate certain general principles of signalling, including formation of multienzyme complexes, compartmentation of second messengers and intermediates, and cross-talk between different signalling pathways.

Contrasting effects of phorbol ester and agonist-mediated activation of protein kinase C on phosphoinositide and Ca2+ signalling in a human neuroblastoma

The effects of protein kinase C (PKC) activation on muscarinic receptor-mediated phosphoinositide and Ca2+ signalling were examined in the human neuroblastoma, SH-SY5Y. Carbachol evoked rapid transient elevations of Ins(1,4,5)P3 and intracellular [Ca2+] followed by lower sustained elevations. Phorbol 12,13-dibutyrate (PDBu) preferentially attenuated transient phases. Removal of the transplasmalemmal Ca2+ gradient coupled with depletion of intracellular Ca2+ stores with thapsigargin also reduced carbachol-mediated Ins(1,4,5)P3 accumulation. Under these conditions, PDBu virtually abolished Ins(1,4,5)P3 responses to carbachol thereby implicating both Ca(2+)- and PKC-sensitive components. PDBu also reduced agonist-mediated accumulation of inositol phosphates and depletion of lipids, thereby eliminating an effect of PKC on Ins(1,4,5)P3 metabolism or phosphoinositide synthesis. In electroporated cells, PDBu inhibited Ins(1,4,5)P3 accumulation mediated by carbachol or guanosine 5'-[gamma-thio]-triphosphate, the latter indicating that some PDBu-sensitive elements were downstream of the receptor. The PKC inhibitor, Ro-318220, protected against PDBu but did not enhance responses to maximal concentrations of carbachol, indicating no feedback inhibition by agonist-activated PKC. Muscarinic antagonist activity of Ro-318220 complicated such assessment at low agonist concentrations. Carbachol or PDBu induced cytosol to membrane translocation of PKC alpha. This was faster and possibly greater with PDBu, which may explain the lack of feedback by agonist-activated PKC. These results indicate that, in SH-SY5Y cells, PDBu activation of PKC preferentially inhibits rapid muscarinic receptor-mediated phosphoinositide and Ca2+ responses via suppression of PtdIns(4,5)P2 hydrolysis. This is at least partially through inhibition of Gq-protein/phosphoinositidase C coupling. However, at least at high agonist concentrations, a major agonist-mediated PKC feedback is not present in these cells.

Mutant rat phosphatidylinositol/phosphatidylcholine transfer proteins specifically defective in phosphatidylinositol transfer: implications for the regulation of phospholipid transfer activity

The mammalian phosphatidylinositol/phosphatidylcholine transfer proteins (PI-TPs) catalyze exchange of phosphatidylinositol (PI) or phosphatidylcholine (PC) between membrane bilayers in vitro. We find that Ser-25, Thr-59, Pro-78, and Glu-248 make up a set of rat (r) PI-TP residues, substitution of which effected a dramatic reduction in the relative specific activity for PI transfer activity without significant effect on PC transfer activity. Thr-59 was of particular interest as it is a conserved residue in a highly conserved consensus protein kinase C phosphorylation motif in metazoan PI-TPs. Replacement of Thr-59 with Ser, Gln, Val, Ile, Asn, Asp, or Glu effectively abolished PI transfer capability but was essentially silent with respect to PC transfer activity. These findings identify rPI-TP residues that likely cooperate to form a PI head-group binding/recognition site or that lie adjacent to such a site. Finally, the selective sensitivity of the PI transfer activity of rPI-TP to alteration of Thr-59 suggests a mechanism for in vivo regulation of rPI-TP activity.

An isoform of the phosphatidylinositol-transfer protein transfers sphingomyelin and is associated with the Golgi system

An isoform of the phosphatidylinositol-transfer protein (PI-TP) was identified in the cytosol fraction of bovine brain. This protein, designated PI-TP beta, has an apparent molecular mass of 36 kDa and an isoelectric point of 5.4. The N-terminal amino acid sequence (21 residues) is 90% similar to that of bovine brain PI-TP, henceforth designated PI-TP alpha (molecular mass 35 kDa and pI 5.5). As observed for PI-TP alpha, PI-TP beta has a distinct preference for phosphatidylinositol over phosphatidylcholine. In addition, it expresses a high transfer activity towards sphingomyelin. PI-TP alpha lacks this activity completely. By indirect immunofluorescence we demonstrated that, in Swiss mouse 3T3 fibroblasts, PI-TP beta is preferentially associated with the Golgi system whereas PI-TP alpha is predominantly present in the cytoplasm and the nucleus. In cytosol-depleted HL60 cells, both PI-TP alpha and PI-TP beta were equally effective at reconstituting guanosine 5'-[gamma-thio]triphosphate-mediated phospholipase C beta activity.

Regulation of phospholipase C-beta isozymes by G-proteins

Phospholipase C (PLC) isozymes are known to be regulated, in part, by heterotrimeric GTP-binding protein (G-protein) subunits, including Galpha subunits of the G(q) family and Gbetagamma subunits. New data show that PLC can also be regulated by a high molecular weight G-protein that doubles as a cellular transglutaminase. Furthermore, a soluble phosphatidylinositol transfer protein (PITP) has been implicated in sustaining the activity of PLC by delivering substrate to the plasma membrane. Such diverse regulatory mechanisms imply that the PLC isozymes are precisely controlled and have specific roles in generating second messengers in response to various external stimuli.

Characterization of mouse phosphatidylinositol transfer protein expressed in Escherichia coli

The cDNA encoding mouse phosphatidylinositol transfer protein (PI-TP) was isolated by means of reverse transcriptase polymerase chain reaction. The nucleotide sequence of this cDNA has a high similarity (98%) with that of rat PI-TP; the predicted amino acid sequence is 99.6% identical to that of rat PI-TP. The cDNA encoding mouse PI-TP was cloned into the expression vector pET3d and the Escherichia coli strain BL21(DE3) was transformed with the resulting plasmid. After induction of the bacteria with isopropyl-beta-D-thiogalactopyranoside, PI-TP was efficiently expressed in the E. coli strain. It was estimated that 5% of the total soluble cell protein consisted of PI-TP. The recombinant mouse PI-TP was purified from the bacterial lysate in four steps: ammonium sulphate precipitation, anion-exchange chromatography, heparin-Sepharose affinity and gel filtration chromatography. Fractionation on the heparin-Sepharose affinity column yielded two forms: PI-TP Hepa1 and Hepa2. These two proteins have the same molecular mass of 35 kDa, both contain a phosphatidylglycerol molecule and both are recognized by anti-PI-TP antibody. Both recombinant proteins have an isoelectric point of 5.4 as compared to 5.5 for bovine brain PI-TP. Sequence analysis of the first 25 N-terminal amino acid residues showed that both forms are identical, except that PI-TP Hepa1 contains the initiator methionine which is lacking from PI-TP Hepa2. The two PI-TP forms have similar phospholipid-binding and transfer activity, comparable to that of bovine brain PI-TP. Both forms and bovine brain PI-TP are phosphorylated equally well in a Ca2+/phospholipid-dependent way by protein kinase C.

Phospholipid-transfer proteins and their mRNAs in developing rat lung and in alveolar type-II cells

Gene expression of non-specific lipid-transfer protein (nsL-TP; identical with sterol carrier protein 2) and phosphatidylinositol-transfer protein (PI-TP) was investigated in developing rat lung. During the late prenatal period (between days 17 and 22) there is a 7-fold increase in the level of nsL-TP and a 2-fold rise in that of PI-TP. The prenatal increases in the levels of nsL-TP and PI-TP are accompanied by parallel increases in the levels of their mRNAs, indicating pretranslational regulation. Compared with whole lung, isolated alveolar type-II cells are enriched in nsL-TP and its mRNA, but not in PI-TP and its mRNA. The observation that the levels of nsL-TP and its mRNA in rat lung show a pronounced increase in the period of accelerated surfactant formation, together with the observation that the surfactant-producing type-II cells are enriched in nsL-TP and its mRNA, suggest that nsL-TP plays a role in the metabolism of pulmonary surfactant.

Transport and sorting of membrane lipids

The lipid composition of cellular membranes may seem unnecessarily complex. However, the lipid composition of each membrane is carefully regulated by local metabolism and specificity in transport, marking the functional significance for the cell. Recent research has revealed unexpected discoveries concerning the topology of lipid synthesis, specificity in lipid transport, and the function of lipid and protein microdomains in sorting.