Subcellular distribution of glycosylphosphatidylinositol-specific phospholipase D in rat liver

Nuclear localization of folate receptor alpha: a new role as a transcription factor

Folic acid (FA) has traditionally been associated with prevention of neural tube defects; more recent work suggests that it may also be involved in in the prevention of adult onset diseases. As the role of FA in human health and disease expands, it also becomes more critical to understand the mechanisms behind FA action. In this work we examined the hypothesis that folate receptor alpha (FRα) acts as a transcription factor. FRα is a GPI-anchored protein and a component of the caveolae fraction. The work described here shows that FRα translocates to the nucleus, where it binds to cis-regulatory elements at promoter regions of Fgfr4 and Hes1, and regulates their expression. The FRα recognition domain mapped to AT rich regions on the promoters. Until this time FRα has only been considered as a folate transporter, these studies describe a novel role for FRα as a transcription factor.

Genetic variation of GPLD1 associates with serum GPI-PLD levels: a preliminary study

HDL is a heterogeneous mixture of lipoprotein particles varying in composition, size, and function. We and others have described a small (7.0nm), minor (0.1% of total apolipoprotein AI) particle containing apolipoprotein AI, AIV and glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) in humans the function of which is not entirely known. Circulating GPI-PLD levels are regulated by multiple factors including genetics. To determine if genetic variation in GPLD1 affects circulating GPI-PLD levels, we examined the relationship between 32 SNPS upstream, within, and downstream of GPLD1 and circulating GPI-PLD levels in Caucasians (n=77) and African-Americans (n=99). The genotype distribution among races differed at 13 SNPs. Nine SNPS were associated with circulating GPI-PLD levels in Caucasians but not African-Americans. These results suggest that genetic variation of GPLD1 appears to associate with circulating GPI-PLD levels. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

Thematic review series: Lipid Posttranslational Modifications. Lysosomal metabolism of lipid-modified proteins

Much is now understood concerning the synthesis of prenylated and palmitoylated proteins, but what is known of their metabolic fate? This review details metabolic pathways for the lysosomal degradation of S-fatty acylated and prenylated proteins. Central to these pathways are two lysosomal enzymes, palmitoyl-protein thioesterase (PPT1) and prenylcysteine lyase (PCL). PPT1 is a soluble lipase that cleaves fatty acids from cysteine residues in proteins during lysosomal protein degradation. Notably, deficiency in the enzyme causes a neurodegenerative lysosomal storage disorder, infantile neuronal ceroid lipofuscinosis. PCL is a membrane-associated flavin-containing lysosomal monooxygenase that metabolizes prenylcysteine to prenyl aldehyde through a completely novel mechanism. The eventual metabolic fates of other lipidated proteins (such as glycosylphosphatidylinositol-anchored and N-myristoylated proteins) are poorly understood, suggesting directions for future research.

Evidence for CEA release from human colon cancer cells by an endogenous GPI-PLD enzyme

Elevated carcinoembryonic antigen (CEA) blood levels are found in a wide variety of epithelial neoplasms. The precise mechanism of the spontaneous CEA release from normal and cancer cells has not been established yet. In this study we investigated 'in vitro' the role of an endogenous glycosylphosphatidyl inositol phospholipase D (GPI-PLD) in spontaneous CEA release from human colon carcinoma cells. We detected GPI-PLD-specific transcript expression in four human colorectal tumor cell lines, LS180, HT29, HT29/219, and SW742 by RT-PCR. Furthermore, CEA release could be activated and inhibited by incubation of LS180 cells with suramin and 1,10-phenanthroline, compounds known to activate and inhibit GPI-PLD activity, respectively. The results suggest a mechanism for the involvement of an endogenous GPI-PLD in spontaneous CEA release from human colon cancer cells.

GPI-specific phospholipase D (GPI-PLD) is expressed during mouse development and is localized to the extracellular matrix of the developing mouse skeleton

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is abundant in serum and has a well-characterized biochemistry; however, its physiological role is completely unknown. Previous investigations into GPI-PLD have focused on the adult animal or on in vitro systems and a putative role in development has been neither proposed nor investigated. We describe the first evidence of GPI-PLD expression during mouse embryonic ossification. GPI-PLD expression was detected predominantly at sites of skeletal development, increasing during the course of gestation. GPI-PLD was observed during both intramembraneous and endochondral ossification and localized predominantly to the extracellular matrix of chondrocytes and to primary trabeculae of the skeleton. In addition, the mouse chondrocyte cell line ATDC5 expressed GPI-PLD after experimental induction of differentiation. These results implicate GPI-PLD in the process of bone formation during mouse embryogenesis.

Lysosomal metabolism of glycoproteins

The lysosomal catabolism of glycoproteins is part of the normal turnover of cellular constituents and the cellular homeostasis of glycosylation. Glycoproteins are delivered to lysosomes for catabolism either by endocytosis from outside the cell or by autophagy within the cell. Once inside the lysosome, glycoproteins are broken down by a combination of proteases and glycosidases, with the characteristic properties of soluble lysosomal hydrolases. The proteases consist of a mixture of endopeptidases and exopeptidases, which act in concert to produce a mixture of amino acids and dipeptides, which are transported across the lysosomal membrane into the cytosol by a combination of diffusion and carrier-mediated transport. Although the glycans of all mature glycoproteins are probably degraded in lysosomes, the breakdown of N-linked glycans has been studied most intensively. The catabolic pathways for high-mannose, hybrid, and complex glycans have been established. They are bidirectional with concurrent sequential removal of monosaccharides from the nonreducing end by exoglycosidases and proteolysis and digestion of the carbohydrate-polypeptide linkage at the reducing end. The process is initiated by the removal of any core and peripheral fucose, which is a prerequisite for the action of the peptide N-glycanase aspartylglucosaminidase, which hydrolyzes the glycan-peptide bond. This enzyme also requires free alpha carboxyl and amino groups on the asparagine residue, implying extensive prior proteolysis. The catabolism of O-linked glycans has not been studied so intensively, but many lysosomal glycosidases appear to act on the same linkages whether they are in N- or O-linked glycans, glycosaminoglycans, or glycolipids. The monosaccharides liberated during the breakdown of N- and O-linked glycans are transported across the lysosomal membrane into the cytosol by a combination of diffusion and carrier-mediated transport. Defects in these pathways lead to lysosomal storage diseases. The structures of some of the oligosaccharides that accumulate in these diseases are not digestion intermediates in the lysosomal catabolic pathways but correspond to intermediates in the biosynthetic pathway for N-linked glycans, suggesting another route of delivery of glycans to the lysosome. Incorrectly folded or glycosylated proteins that are rejected by the quality control mechanism are broken down in the ER and cytoplasm and the end product of the cytosolic degradation of N-glycans is delivered to the lysosomes. This route is enhanced in cells actively secreting glycoproteins or producing increased amounts of aberrant glycoproteins. Thus interaction between the lysosome and proteasome is important for the regulation of the biosynthesis and distribution of N-linked glycoproteins. Another example of the extralysosomal function of lysosomal enzymes is the release of lysosomal proteases into the cytosol to initiate the lysosomal pathway of apoptosis.

GPI-anchored protein cleavage in the regulation of transmembrane signals

The structure of covalently-linked glycosylphosphatidylinositol (GPI) anchors of membrane proteins displayed on the cell surface is described. Evidence of how the GPI-anchors are sorted into membrane rafts in the plasma membrane is reviewed. Proteins are released by hydrolysis of the linkage to the GPI anchor and phospholipases from different sources involved in this process are characterised. The regulation of protein conformation and function resulting from phospholipase cleavage of the GPI anchor is discussed in the context of its role in signal transduction by insulin. In this signalling system, re-distribution of critical membrane components, including GPI-anchored proteins and non-receptor tyrosine kinases, between different raft domains appears to play a central role in the signal transduction pathway.

Anti-Mouse GPI-PLD Antisera Highlight Structural Differences between Murine and Bovine GPI-PLDs

Despite its well characterised biochemistry, the physiological role of glycosylphosphatidylinositolspecific phospholipase D (GPIPLD) is unknown. Most of the previous studies investigating the distribution of GPI-PLD have focused on the human and bovine forms of the enzyme. Studies on mouse GPI-PLD are rare, partly due to the lack of a specific antimouse GPI-PLD antibody, but also due to the apparent low reactivity of existing antibodies to rodent GPI-PLDs. Here we describe the isolation of a mouse liver cDNA, the construction and expression of a recombinant enzyme and the generation of an affinitypurified rabbit antimouse GPI-PLD antiserum. The antibody shows good reactivity to partially purified murine and purified bovine GPI-PLD. In contrast, a rat antibovine GPI-PLD antibody shows no reactivity with the mouse enzyme and the two antibodies recognise different proteolytic fragments of the bovine enzyme. Comparison between the rodent, bovine and human enzymes indicates that small changes in the amino acid sequence of a short peptide in the mouse and bovine GPI-PLDs may contribute to the different reactivities of the two antisera. We discuss the implications of these results and stress the importance of antibody selection while investigating GPI-PLD in the mouse.

Glycosylphosphatidylinositol-anchored proteins: structure, function, and cleavage by phosphatidylinositol-specific phospholipase C

A wide variety of proteins are tethered by a glycosylphosphatidylinositol (GPI) anchor to the extracellular face of eukaryotic plasma membranes, where they are involved in a number of functions ranging from enzymatic catalysis to adhesion. The exact function of the GPI anchor has been the subject of much speculation. It appears to act as an intracellular signal targeting proteins to the apical surface in polarized cells. GPI-anchored proteins are sorted into sphingolipid- and cholesterol-rich microdomains, known as lipid rafts, before transport to the membrane surface. Their localization in raft microdomains may explain the involvement of this class of proteins in signal transduction processes. Substantial evidence suggests that GPI-anchored proteins may interact closely with the bilayer surface, so that their functions may be modulated by the biophysical properties of the membrane. The presence of the anchor appears to impose conformational restraints, and its removal may alter the catalytic properties and structure of a GPI-anchored protein. Release of GPI-anchored proteins from the cell surface by specific phospholipases may play a key role in regulation of their surface expression and functional properties. Reconstitution of GPI-anchored proteins into bilayers of defined phospholipids provides a powerful tool with which to explore the interactions of these proteins with the membrane and investigate how bilayer properties modulate their structure, function, and cleavage by phospholipases.

Intracellular cleavage of glycosylphosphatidylinositol by phospholipase D induces activation of protein kinase Calpha

Many proteins are anchored to the cell membrane by glycosylphosphatidylinositol (GPI). One of the functions proposed for the GPI anchor is as a possible mediator in signal transduction through its hydrolysis. GPI-specific phospholipase D (GPI-PLD) is a secretory protein that is suggested to be involved in the release of GPI-anchored protein from the membrane. In the present study we examined how GPI-PLD is involved in signal transduction. When introduced exogenously and overexpressed in cells, GPI-PLD cleaved the GPI anchors in the early secretory pathway, possibly in the endoplasmic reticulum, resulting in an increased production of diacylglycerol. Experiments in vitro and in vivo showed that the association of protein kinase Calpha (PKCalpha) with membranes was increased markedly by expression of GPI-PLD in cells. Furthermore, sucrose-density-gradient centrifugation and immunofluorescence microscopy demonstrated that PKCalpha was translocated to the endoplasmic reticulum membrane in cells expressing GPI-PLD, in contrast with its association with the plasma membrane in cells treated with PMA. We also confirmed that the phosphorylation of c-Fos as well as PKCalpha itself was greatly enhanced by the expression of GPI-PLD. Taken together, these results suggest that GPI-PLD is involved in intracellular cleavage of the GPI anchor, which is a new potential source of diacylglycerol production to activate PKCalpha.

In vitro phosphorylation of purified glycosylphosphatidylinositol-specific phospholipase D

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) was phosphorylated in vitro by cAMP-dependent protein kinase (PKA) and by tyrosine kinase. Phosphorylation by PKA occurred in the 110 kDa native form of GPI-PLD as well as in multiple proteolytic degradation products and caused a significant decrease in enzyme activity. Dephosphorylation by treatment with alkaline phosphatase completely restored GPI-PLD activity. In addition, incubation of GPI-PLD with trypsin, which results in the generation of distinct peptide fragments, resulted in complete dephosphorylation of radiolabeled GPI-PLD. The site of phosphorylation by PKA was assigned to Thr-286. Tyrosine phosphorylation was only observed in a proteolytically processed fragment of GPI-PLD but not in the 110 kDa native form and had no effect on GPI-PLD activity.

Glycosylphosphatidylinositol-specific phospholipase D in blood serum: is the liver the only source of the enzyme?

In cases of systemic inflammatory response syndrome, sepsis, and septic shock, the activity of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) in serum amounts to 20 to 25% of the activity found in a healthy control group. The activity of serum GPI-PLD is positively correlated with inflammatory markers and counts of monocytes and stab cells (bands) and negatively correlated with polymorphonuclear neutrophils and lymphocytes in severe diseases. This indicates a yet unknown involvement of the inflammatory system in GPI-PLD liberation and suggests that the liver is not the only source of the plasma enzyme. Plasma was shown to contain an effective inhibitor of GPI-PLD which is soluble in organic solvents. Its concentration in capillary plasma is 20-fold higher than in venous plasma. To find possible other sources of plasma GPI-PLD besides the liver, the GPI-degrading activity was measured in different organs of the rat. Product formation was analysed using [125I]TID-labeled GPI-AP.

S-phase DNA damage checkpoint in budding yeast

Eukaryotic cells must be able to coordinate DNA repair, replication and cell cycle progression in response to DNA damage. A failure to activate the checkpoints which delay the cell cycle in response to internal and external cues and to repair the DNA lesions results in an increase in genetic instability and cancer predisposition. The use of the yeast Saccharomyces cerevisiae has been invaluable in isolating many of the genes required for the DNA damage response, although the molecular mechanisms which couple this regulatory pathway to different DNA transactions are still largely unknown. In analogy with prokaryotes, we propose that DNA strand breaks, caused by genotoxic agents or by replication-related lesions, trigger a replication coupled repair mechanism, dependent upon recombination, which is induced by the checkpoint acting during S-phase.

Characteristics of smooth muscle cell lipoprotein binding proteins (p105/p130) as T-cadherin and regulation by positive and negative growth regulators

Smooth muscle cells (SMC) express atypical surface low density lipoprotein (LDL) binding proteins of M(r)105 and M(r)130 (p105 and p130) which have been putatively identified as the cell adhesion glycoprotein T-cadherin. Using cultured human and rat aortic SMC and analysis by ligand (LDL)- and immuno-blotting techniques we now confirm identity of p105 and p130 as T-cadherin, as adjudged by sensitivity to PI-PLC cleavage, insensitivity to trypsin degradation in the presence of calcium, and immunoreactivity to anti-T-cadherin peptide antisera. The function of T-cadherin (p105/p130) in the vasculature is unknown. The proteins were downmodulated by the peptide growth factors PDGF-BB, IGF, EGF, and bFGF, but not by vasoactive peptide hormones (angiotensin II, vasopressin, bradykinin, and endothelin). TGF beta, a recognized inhibitor of SMC proliferation, per se had no effect but inhibited growth factor-induced p105/p130 downmodulation. Expression of p105/p130 in quiescent SMC and growth-stimulated SMC (respectively, in serum-free and serum or PDGF-BB containing culture conditions) was increased by forskolin and 8-Br-cyclic GMP, both anti-mitogenic substances, but was unaffected by phorbol ester, calcium ionophores, or calcium antagonists. The findings are compatible with a function for the lipoprotein binding proteins (T-cadherin) in negative regulation of SMC growth.

The role of glycosyl-phosphatidylinositol in signal transduction

Glycosyl-phosphatidylinositol (GPI) lipids have a structural role as protein anchors to the cell surface. In addition, they are implicated in hormone, growth factor and cytokine signal transduction. Their phosphodiesteric hydrolysis mediated by an activated phospholipase results in the generation of water soluble oligosaccharide species termed the inositol phosphoglycan (IPG). This product has been demonstrated to possess biological properties when added exogenously to cells mimicking the biological effects of a variety of extracellular ligands. This may be accomplished since IPG is generic for a family of closely related species which are released in a tissue-specific manner and additionally have cell-specific targets. Micro-organic synthesis has recently been able to shed new light on this topic by the introduction of defined oligosaccharide analogues of IPG for the assessment of their biological activity. These have complemented the findings observed with purified IPG from biological sources thus strengthening the belief that the GPI/IPG signalling system represents a truly novel aspect of transmembrane signalling.

Glycosyl-phosphatidylinositol-phospholipase type D: a possible candidate for the generation of second messengers

Membrane associated glycosyl-phosphatidylinositols have been shown to be the precursors of inositol phosphoglycan second messengers. Extraction of human liver membranes and purification by serial thin layer chromatography revealed three glycolipids which co-migrated with glycosyl-phosphatidylinositol from rat liver. These lipidic fractions were partially sensitive to treatment with nitrous acid and to hydrolysis by glycosyl-phosphatidylinositol-specific phospholipase D from bovine serum. In parallel, glycosyl-phosphatidylinositol isolated from rat liver was found to be a substrate for the enzyme generating a biologically active inositol phosphoglycan species (determined by measuring inhibition of protein kinase A activity and stimulation of cell proliferation within the chicken embryo cochleovestibular ganglion). This molecule was recognised by an anti-inositol phosphoglycan antibody. Hence, we propose that glycosyl-phosphatidylinositol-specific phospholipase D could be implicated in cellular signalling.

Uptake and intracellular stability of glycosylphosphatidylinositol-specific phospholipase D in neuroblastoma cells

Glycosylphosphatidylinositol-specific phospholipase D from mammalian serum has been described to be relatively stable towards the action of proteases in vitro, and it has been speculated that the enzyme may only be active on glycosylphosphatidylinositol-anchored substrates after its proteolytic processing in an intracellular compartment following uptake from body fluids. To test this hypothesis, we studied the possible uptake and intracellular processing of purified glycosylphosphatidylinositol-specific phospholipase D into the mouse neuroblastoma cell line N2A. We found that after incubation of neuroblastoma cells with glycosylphosphatidylinositol-specific phospholipase D at 37 degrees C the amount of cell-associated glycosylphosphatidylinositol-specific phospholipase D activity increased in a concentration- and time-dependent way. A similar uptake was also observed with 125I-labeled intact and trypsin-treated form of glycosylphosphatidylinositol-specific phospholipase D. We found that the incorporated radiolabeled proteins were processed intracellularly to distinct low molecular mass products, and that this process was in part inhibited by the presence of chloroquine during incubation.