Improvement of LXR-mediated lipid metabolism in nephrotic model kidney accompanied by suppression of inflammation and fibrosis.

Kidney disease affects millions of people worldwide. Chronic kidney diseases, such as diabetic nephropathy, are often accompanied by nephrotic syndrome, which causes a large amount of protein and lipid to leak out into the urine. Leaked lipids are well known to accumulate in the proximal tubules as lipid droplets. However, the role of lipid metabolism in the kidney has not been thoroughly studied, and the relationship between accumulated lipid and pathological progression is often unknown. In this study, we showed that reducing accumulated lipids by exerting an agonistic effect on Liver X receptor, one of the nuclear receptors known to play an important role in lipid metabolism, suppressed the development of pathological conditions, such as inflammation and fibrosis, in a nephrosis model. Until now, many renal disease treatments have focused on suppressing the inflammatory response. However, it is now clear that even if the direct anti-inflammatory response is weak, the spread of inflammation and fibrosis can be suppressed by reducing accumulated lipids. Our results suggest that reducing abnormal lipid accumulation in the kidney could lead to disease treatment.

Each liver X receptor (LXR) type has a different purpose in different situations.

LXRs, which are nuclear receptors, have 2 isoforms-LXRα and LXRß. Generally, LXRα is expressed in the liver, kidney, and a limited number of other organs, whereas LXRß is thought to be expressed ubiquitously. Nevertheless, no clear consensus has been reached on the role of each in kidney lipid metabolism. Many researchers have reported that lipids accumulate in renal tubular epithelial cells during nephrosis. The nephrosis model we used showed the presence of urinary protein 4 days after the induction of illness. Additionally, the model maintained high levels of urinary protein from day 7-14. Lipid accumulation was clearly verified at day 4 and extreme accumulation was observed at day 7. We observed increased expression of LXRα from an early stage of nephrosis. To explore the role of increased LXRα in diseased kidney in vitro, NRK52E, normal kidney tubular epithelial cells, were forced to overexpress LXRα. These cells showed significantly lower lipid accumulation than mock cells did. In contrast, LXRß knockdown lead to increased lipid accumulation in mock cells, and constancy in overexpressing cells. In normal kidneys, LXRß is expressed stably to control mainly the intracellular lipids. However, with increasing intracellular lipid accumulation, expression of LXRα and its downstream gene, ABCA1, was upregulated, followed by lipid excretion in an LXRα-dependent manner. This phenomenon strongly suggests the importance of LXRα in lipid metabolism in the diseased kidney.

Synthetic construction of sugar-amino acid hybrid polymers involving globotriaose or lactose and evaluation of their biological activities against Shiga toxins produced by Escherichia coli O157:H7.

Synthetic assembly of sugar moieties and amino acids in order to create "sugar-amino acid hybrid polymers" was accomplished by means of simple radical polymerization of carbohydrate monomers having an amino acid-modified polymerizable aglycon. Amines derived from globotriaoside and lactoside as glycoepitopes were condensed with known carbobenzyloxy derivatives, including Z-Gly, Z-l-Ala and Z-ß-Ala, which had appropriate spacer ability and a chiral center to afford fully protected sugar-amino acid hybrid compounds in good yields. After deprotection followed by acryloylation, the water-soluble glycomonomers were polymerized with or without acrylamide in the presence of a radical initiator in water to give corresponding copolymers and homopolymers, which were shown by SEC analysis to have high molecular weights. Evaluation of the biological activities of the glycopolymers against Shiga toxins (Stxs) was carried out, and the results suggested that glycopolymers having highly clustered globotriaosyl residues had high affinity against Stx2 (KD = 2.7∼4.0 µM) even though other glycopolymers did not show any affinity or showed very weak binding affinity. When Stx1 was used for the same assay, all of the glycopolymers having globotriaosyl residues showed high affinity (KD = 0.30∼1.74 µM). Interestingly, couple of glycopolymers having lactosyl moieties had weaker binding affinity against Stx1. In addition, when cytotoxicity assays were carried out for both Stxs, glycopolymers having highly clustered globotriaosyl residues showed higher affinity than that of the copolymers, and only highly clustered-type glycopolymers displayed neutralization potency against Stx2.

Reduction of Membrane Protein CRIM1 Decreases E-Cadherin and Increases Claudin-1 and MMPs, Enhancing the Migration and Invasion of Renal Carcinoma Cells.

CRIM1 is a membrane protein that has been reported to be related to cell proliferation. CRIM1 is expressed in renal carcinoma cells, but its involvement in proliferation and malignant transformation remains unclear. We analyzed whether alterations in the characteristics of cancer cells are observed following knockdown of CRIM1. Decreased expression of CRIM1 did not affect proliferation or anchorage-independent growth. The results of wound healing and invasion assays showed that reduced expression of CRIM1 increased cells' migratory and invasive abilities. Expression analysis of factors involved in migration and invasion in CRIM1-knockdown cells revealed that expression of the cell adhesion factor E-cadherin declined and expression of claudin-1, which is upregulated in metastatic cancer cells, increased. In addition, increased expression of matrix metalloproteinase (MMP) 2 and MMP9, protease essential for cancer cell invasiveness, was observed. Furthermore, an increase in phosphorylated focal adhesion kinase (FAK), which increases cell migration, was observed. Increased expression of the E-cadherin transcription repressors Snail, Slug, and ZEB-1 were observed, and mRNA levels of E-cadherin were decreased. Therefore, expression of E-cadherin is thought to be decreased by both suppression of E-cadherin mRNA expression and promotion of degradation of the E-cadherin protein. In addition, expression of CRIM1 was decreased in renal cancer cells undergoing epithelial-mesenchymal transition (EMT) stimulated by tumor necrosis factor alpha (TNF-α). Thus, CRIM1 regulates the expression of several EMT-related factors and appears to play a role in suppressing migration and invasion through control of EMT.

Impact of anti-glomerular basement membrane antibodies and glomerular neutrophil activation on glomerulonephritis in experimental myeloperoxidase-antineutrophil cytoplasmic antibody vasculitis.

BACKGROUND: Antineutrophil cytoplasmic antibody (ANCA) and neutrophil interactions play important roles in ANCA-associated vasculitis (AAV) pathogenesis. However, mechanisms underlying the pathogenesis of crescent formation in ANCA-associated vasculitis have not been completely elucidated. To ascertain the involvement of these interactions in necrotizing crescentic glomerulonephritis (NCGN), we used an AAV rat model and investigated the effects of the anti-myeloperoxidase (MPO) antibody (Ab) titer, tumor necrosis factor α (TNF-α), granulocyte colony-stimulating factor (G-CSF) and subnephritogenic anti-glomerular basement membrane (GBM) Abs, as proinflammatory stimuli. METHODS: NCGN was induced in Wistar Kyoto rats by human MPO (hMPO) immunization. Renal function, pathology, and glomerular cytokine and chemokine expression were evaluated in hMPO-immunized rats with/without several co-treatments (TNF-α, G-CSF or subnephritogenic anti-GBM Abs). Rat neutrophils activation by IgG purified from rat serum in each group was examined in vitro. RESULTS: The hMPO-immunized rats had significantly higher level of anti-hMPO Ab production. The induced anti-hMPO Abs cross-reacted with TNF-α- or G-CSF-primed rat neutrophils secreting TNF-α and interleukin-1ß in vitro. The reactivity of anti-MPO Abs against rat MPO, crescent formation with neutrophil extracellular traps and glomerular-activated neutrophil infiltration in the rat model were significantly enhanced by subnephritogenic anti-GBM Ab but not by TNF-α or G-CSF administration. The model rats injected with the subnephritogenic anti-GBM Abs showed increased urinary albumin excretion and serum TNF-α, chemokine (C-X-C) ligand 1 (CXCL1) and CXCL2 levels. TNF-α, CXCL1, CXCL2 and CXCL8 increased in the glomeruli with significant amounts of crescent formation. In addition, in vitro activated neutrophils decreased CXC chemokine receptor 1 (CXCR1) and CXCR2 expressions. CONCLUSIONS: The coexistence of subnephritogenic anti-GBM Abs leads to the inflammatory environment in glomeruli that is amplified by the interaction of ANCA and neutrophils. Development of NCGN in MPO-AAV may be necessary for not only the accumulation of neutrophils in glomeruli, but also the aberrant neutrophil activation on glomerulonephritis.

Angiostatin prevents IL-1ß-induced down-regulation of eNOS expression by inhibiting the NF-κB cascade.

This study aimed to elucidate the protective potential of angiostatin in inflamed endothelial cells in culture. We assessed the effect of angiostatin on the expression of ICAM-1 and eNOS. Angiostatin prevented IL-1ß-induced down-regulation of eNOS expression, but produced no significant changes on IL-1ß-induced up-regulation of ICAM-1. We then explored the effect of angiostatin on IL-1ß-mediated inflammatory signaling and found that angiostatin inhibited IL-1ß-mediated nuclear translocation of NF-κB. Thus, our results suggest that angiostatin prevents IL-1ß-induced down-regulation of eNOS expression via inhibition of the NF-κB cascade; this may be the anti-inflammatory mechanism of angiostatin.

Statin attenuates experimental anti-glomerular basement membrane glomerulonephritis together with the augmentation of alternatively activated macrophages.

Macrophages are heterogeneous and include classically activated M1 and alternatively activated M2 macrophages, characterized by pro- and anti-inflammatory functions, respectively. Macrophages that express heme oxygenase-1 also exhibit anti-inflammatory effects. We assessed the anti-inflammatory effects of statin in experimental anti-glomerular basement membrane glomerulonephritis and in vitro, focusing on the macrophage heterogeneity. Rats were induced anti-glomerular basement membrane glomerulonephritis and treated with atorvastatin (20 mg/kg/day) or vehicle (control). Control rats showed infiltration of macrophages in the glomeruli at day 3 and developed crescentic glomerulonephritis by day 7, together with increased mRNA levels of the M1 macrophage-associated cytokines, interferon-gamma, tumor necrosis factor-alpha, and interleukin-12. In contrast, statin reduced the level of proteinuria, reduced infiltration of macrophages in glomeruli with suppression of monocyte chemotactic protein-1 expression, and inhibited the formation of necrotizing and crescentic lesions. The number of glomerular ED3-positive macrophages decreased with down-regulation of M1 macrophage-associated cytokines. Furthermore, statin augmented ED2-positive M2 macrophages with up-regulation of the M2 macrophage-associated chemokines and cytokines, chemokine (C-C motif) Iigand-17 and interleukin-10. Statin also increased the glomerular interleukin-10-expressing heme oxygenase-1-positive macrophages. Statin inhibited macrophage development, and suppressed ED3-positive macrophages, but augmented ED2-positive macrophages in M2-associated cytokine environment in vitro. We conclude that the anti-inflammatory effects of statin in glomerulonephritis are mediated through inhibition of macrophage infiltration as well as augmentation of anti-inflammatory macrophages.

An orally applicable Shiga toxin neutralizer functions in the intestine to inhibit the intracellular transport of the toxin.

Shiga toxin 2 (Stx2) is a major virulence factor in infections with Stx-producing Escherichia coli (STEC), which causes gastrointestinal diseases and sometimes fatal systemic complications. Recently, we developed an oral Stx2 inhibitor known as Ac-PPP-tet that exhibits remarkable therapeutic potency in an STEC infection model. However, the precise mechanism underlying the in vivo therapeutic effects of Ac-PPP-tet is unknown. Here, we found that Ac-PPP-tet completely inhibited fluid accumulation in the rabbit ileum caused by the direct injection of Stx2. Interestingly, Ac-PPP-tet accumulated in the ileal epithelial cells only through its formation of a complex with Stx2. The formation of Ac-PPP-tet-Stx2 complexes in cultured epithelial cells blocked the intracellular transport of Stx2 from the Golgi apparatus to the endoplasmic reticulum, a process that is essential for Stx2 cytotoxicity. Thus, Ac-PPP-tet is the first Stx neutralizer that functions in the intestine by altering the intracellular transport of Stx2 in epithelial cells.

ANG II receptor blockade enhances anti-inflammatory macrophages in anti-glomerular basement membrane glomerulonephritis.

Macrophages are heterogeneous immune cell populations that include classically activated and alternatively activated (M2) macrophages. We examined the anti-inflammatory effect of ANG II type 1 receptor (AT(1)R) blocker (ARB) on glomerular inflammation in a rat model of anti-glomerular basement membrane (GBM) glomerulonephritis (GN). The study focused on infiltrating CD8(+) and CD4(+) cells and macrophages, as well as the heterogeneity of intraglomerular macrophages. Wistar-Kyoto rats were treated with high-dose olmesartan (3 mg.kg(-1).day(-1)), low-dose olmesartan (0.3 mg.kg(-1).day(-1)), or vehicle (control) 7 days before induction of anti-GBM GN. Control rats showed mainly CD8(+) cells and ED1(+) macrophages, with a few CD4(+) cells infiltrating the glomeruli. Necrotizing and crescentic glomerular lesions developed by day 7 with the increase of proteinuria. AT(1)R was expressed on CD8(+) and CD4(+) cells and on ED1(+) macrophages. Low-dose ARB had no anti-inflammatory effects in anti-GBM GN. However, high-dose ARB reduced glomerular infiltration of CD8(+) cells and ED1(+) macrophages and suppressed necrotizing and crescentic lesions by days 5 to 7 (P < 0.05). In addition, high-dose ARB reduced the numbers of ED3(+)-activated macrophages, suppressed glomerular TNF-alpha and IFN-gamma production, and downregulated M1-related chemokine and cytokines (monocyte chemoattractant protein type 1, IL-6, and IL-12). High-dose ARB also enhanced ED2(+) M2 macrophages by day 7 with upregulation of glomerular IL-4 and IL-13 and augmented CCL17, IL-1 receptor antagonist, and IL-10. We concluded that high-dose ARB inhibits glomerular inflammation by increasing the numbers of M2 macrophages and upregulation of anti-inflammatory cytokines and by suppressing M1 macrophage development with downregulation of M1-related proinflammatory cytokines.

The inducible amphisome isolates viral hemagglutinin and defends against influenza A virus infection.

The emergence of drug-resistant influenza type A viruses (IAVs) necessitates the development of novel anti-IAV agents. Here, we target the IAV hemagglutinin (HA) protein using multivalent peptide library screens and identify PVF-tet, a peptide-based HA inhibitor. PVF-tet inhibits IAV cytopathicity and propagation in cells by binding to newly synthesized HA, rather than to the HA of the parental virus, thus inducing the accumulation of HA within a unique structure, the inducible amphisome, whose production from the autophagosome is accelerated by PVF-tet. The amphisome is also produced in response to IAV infection in the absence of PVF-tet by cells overexpressing ABC transporter subfamily A3, which plays an essential role in the maturation of multivesicular endosomes into the lamellar body, a lipid-sorting organelle. Our results show that the inducible amphisomes can function as a type of organelle-based anti-viral machinery by sequestering HA. PVF-tet efficiently rescues mice from the lethality of IAV infection.

Exosome-associated Shiga toxin 2 is released from cells and causes severe toxicity in mice.

Shiga toxin (Stx), a major virulence factor of enterohemorrhagic Escherichia coli (EHEC), is classified into two subgroups, Stx1 and Stx2. Clinical data clearly indicate that Stx2 is associated with more severe toxicity than Stx1, but the molecular mechanism underlying this difference is not fully understood. Here, we found that after being incorporated into target cells, Stx2, can be transported by recycling endosomes, as well as via the regular retrograde transport pathway. However, transport via recycling endosome did not occur with Stx1. We also found that Stx2 is actively released from cells in a receptor-recognizing B-subunit dependent manner. Part of the released Stx2 is associated with microvesicles, including exosome markers (referred to as exo-Stx2), whose origin is in the multivesicular bodies that formed from late/recycling endosomes. Finally, intravenous administration of exo-Stx2 to mice causes more lethality and tissue damage, especially severe renal dysfunction and tubular epithelial cell damage, compared to a free form of Stx2. Thus, the formation of exo-Stx2 might contribute to the severity of Stx2 in vivo, suggesting new therapeutic strategies against EHEC infections.

A multivalent peptide library approach identifies a novel Shiga toxin inhibitor that induces aberrant cellular transport of the toxin.

Infection with Shiga toxin (Stx)-producing Escherichia coli O157:H7 causes bloody diarrhea and hemorrhagic colitis in humans, sometimes resulting in fatal systemic complications. Among the known Stx family members, Stx2 is responsible for the most severe forms of disease. Stx2 binds to target cells via multivalent interactions between its B-subunit pentamer and globotriaosyl ceramide. After binding, it is first retrogradely transported to the Golgi and then to the endoplasmic reticulum (ER). Using a multivalent peptide library approach, we identified a tetravalent peptide that exhibits a high affinity for the Stx2 B-subunit pentamer (KD = 0.13 microM) and markedly inhibits Stx2 cytotoxicity. The tetravalent peptide exerted its inhibitory effects by inducing aberrant cellular transport of Stx2. Although the tetravalent peptide/Stx2 complex was incorporated into cells and translocated to the Golgi, this process was followed by the effective degradation of Stx2 in an acidic compartment rather than by its transfer to the ER. This peptide thoroughly protected mice from a fatal dose of E. coli O157:H7 even when administered after an established infection. Thus, the multivalent peptide library approach enabled the identification of a peptide-based Stx2 inhibitor that has remarkable therapeutic potency and appears to function by inducing aberrant cellular transport and degradation of Stx2.

Regulation of hepatic cholesterol synthesis by a novel protein (SPF) that accelerates cholesterol biosynthesis.

Supernatant protein factor (SPF) is a novel cholesterol biosynthesis-accelerating protein expressed in liver and small intestine. Here, we report on the physiological role of SPF by using Spf-deficient mice. Although plasma cholesterol levels were similar in chow-fed Spf-/- and wild-type (WT) mice, fasting significantly decreased plasma cholesterol levels in Spf-/- mice but not in WT mice. While fasting reduced hepatic cholesterol synthesis rate in WT mice, a more pronounced reduction was observed in Spf-/- mice. The expression of cholesterogenic enzymes was dramatically suppressed by fasting both in WT and Spf-/- mice. In contrast, hepatic SPF expression of WT mice was up-regulated by fasting in peroxisome proliferator-activated receptor alpha (PPAR-alpha)-dependent manner. These results indicate that in WT mice, the decrease of hepatic cholesterol synthesis under fasting conditions is at least in part compensated by SPF up-regulation. Fibrates, which function as a PPAR-alpha agonist and are widely used as hypotriglycemic drugs, reduced hepatic cholesterol synthesis and plasma cholesterol levels by approximately one-half in Spf-/- mice but not in WT mice. These findings suggest that co-administration of fibrates and an SPF inhibitor may reduce not only plasma triglyceride but also cholesterol levels, indicating that SPF is a promising hypocholesterolemic drug target.

NMR metabolic profiling combined with two-step principal component analysis for toxin-induced diabetes model rat using urine.

Nuclear magnetic resonance-based metabolic profiling (NMR-MP) was applied to evaluate disorder model animals using urine. Diabetic nephropathy was established in this experiment by administering streptozotocin to Wistar rats, which immediately developed diabetes after toxin-treatment and then gradually produced albumin-containing urine (albuminuria). Urine samples were collected for the first 4 weeks after toxin treatment. Predominant urinary sugar signals were seen in (1)H-NMR spectra of diabetes rat urine, and spectra were processed and subjected to multivariate analysis. Principal component analysis (PCA) identified 3 outliers among 20 individuals. Outlier rats did not develop urinary sugar and were later found to be rats insufficient to establish diabetes models. A second PCA was performed excluding the additional glucose-signal region (3.2-6.3 ppm), as glucose signals had a predominant effect that may mask details of other metabolic profiles. Consequently another outlier was revealed. This exceptional rat did not develop albuminuria even after producing glucosuria for 14 weeks. NMR metabolic profiling provides good guidance to evaluate biophysical conditions of animals, enabling detection of abnormalities in the early stage of toxicological experiments.

Development of dialyzer with immobilized glycoconjugate polymers for removal of Shiga-toxin.

The dialyzer for Shiga-toxin elimination was developed and its performance was established. The dialyzer was prepared by immobilization of multivalent ligands. Glycoconjugate polymers having oligosaccharides and amino groups were synthesized to function as Shiga-toxin adsorbents. The amino group was utilized to immobilize the polymer inside the cellulose hollow fiber of the dialyzer. Cellulose hollow fibers packed in the dialyzer were carboxymethylated under moderate conditions. The glycoconjugate polymers were bound covalently to the hollow fibers of the dialyzer by condensation reaction between the amino group of the polymer and the carboxyl group of the cellulose hollow fiber. Shiga-toxin eliminabilities of the prepared dialyzers were evaluated at various conditions. Even at high concentration of protein such as FCS, the dialyzer showed an excellent performance for Shiga-toxin adsorption.

Induction of cytokines by toxins that have an identical RNA N-glycosidase activity: Shiga toxin, ricin, and modeccin.

Shiga toxin (Stx) has an A1-B5 subunit structure, and the A subunit is an RNA N-glycosidase that inhibits cellular protein synthesis. We previously reported that in Caco-2 cells Stx induced cytokines and that the RNA N-glycosidase activity was essential for the cytokine induction. It is known that the binding of the Stx-B subunit to its receptor glycolipid, Gb3, mediates an A subunit-independent signal in some types of cells, but the involvement of this signal in the cytokine induction is unclear. In this study, we investigated whether RNA N-glycosidase itself induces cytokines. IL-8 production was enhanced by Stx, ricin, and modeccin, three toxins that inhibit protein synthesis through an identical RNA N-glycosidase activity, but not by two other types of protein synthesis inhibitors, diphtheria toxin and cycloheximide. The RNA N-glycosidase-type toxins showed a similar induction pattern of cytokine mRNAs. Brefeldin A, a Golgi apparatus inhibitor, completely suppressed the cytokine induction by the toxins. Analysis by using inhibitors of toxin binding and also Stx-B subunit showed that the cytokine-inducing activity was independent of Gb3-mediated signaling. These results indicate that RNA N-glycosidase itself induces the cytokine production and that intracellular transport of toxins through the Golgi apparatus is essential for the activity.

[New drugs that prevent cytotoxicity of Shiga toxins].

Shiga toxin(Stx) produced by enterohemorrhagic E. coli is the virulence factor that causes not only enterohemorrhagic colitis but also fatal complications, such as hemolytic uremic syndrome. To prevent the complications, new strategies targeted to Stx have been tested, mostly using mimics of the trisaccharide structure of neutral lipid Gb3, the receptor for Stx. One group of such new drugs are agents that can bind to Stx in gastrointestinal tract and prevent its spread to extraintestinal sites, and the other group are water-soluble neutralizers that suppress Stx cytotoxicity in the circulation. Although most of these are now under the laboratory investigations, one of these drugs may hopefully be utilized clinically to prevent hemolytic uremic syndrome in future.

Type I platelet-activating factor acetylhydrolase catalytic subunits over-expression induces pleiomorphic nuclei and centrosome amplification.

LIS1, a causative gene product for type I lissencephaly, binds to and regulates the dynein motor and the centrosome. LIS1 also forms a complex with the catalytic subunits alpha1 and alpha2 of type I platelet-activating factor acetylhydrolase [PAF-AH (I)]. However, the cellular function of the catalytic subunits remains unknown. In this study, we showed that over-expression of the catalytic subunits, especially alpha2, in cultured cells induced dramatic phenotypical changes including nuclear shape change, centrosomal amplification and microtubule disorganization. We examined if these effects were due to the catalytic activity and/or binding of alpha2 to LIS1. Substitution of a single amino acid Glu39 of murine alpha1 and alpha2 by Asp (alpha2-E39D) did not affect catalytic activity but completely abolished LIS1 binding. Over-expression of either alpha2-E39D or the catalytically inactive alpha2-S48C revealed that alpha2-E39D, but not alpha2-S48C, lost its ability to induce above-mentioned phenotypic changes. Biochemical analyses showed that LIS1 present in the precipitate fraction of murine brain homogenates could be translocated to the soluble fraction by alpha2, but not by alpha2-E39D. These results suggest that over-expression of the PAF-AH (I) catalytic subunits induces centrosomal amplification and microtubule disorganization by disturbing intracellular localization of LIS1.

Structural analysis of the interaction between Shiga toxin B subunits and linear polymers bearing clustered globotriose residues.

We previously developed linear polymers bearing clustered trisaccharides of globotriaosylceramide (Gb3) as orally applicable Shiga toxin (Stx) neutralizers. Here, using a Gb3 polymer with a short spacer tethering the trisaccharide to the core, we found that shortening the spacer length markedly reduced the binding affinity for Stx2 but not Stx1. Moreover, mutational analysis revealed that the essential binding sites of the terminal trisaccharides were completely different between Stx1 and Stx2. These results provide the molecular basis for the interaction between Stx B subunits and Gb3 polymers.

Biochemical and molecular characterization of a novel choline-specific glycerophosphodiester phosphodiesterase belonging to the nucleotide pyrophosphatase/phosphodiesterase family.

Nucleotide pyrophosphatases/phosphodiesterases (NPPs) are ubiquitous membrane-associated or secreted ectoenzymes that release nucleoside 5'-monophosphate from a variety of nucleotides and nucleotide derivatives. The mammalian NPP family comprises seven members, but only three of these (NPP1-3) have been studied in some detail. Previously we showed that lysophospholipase D, which hydrolyzes lysophosphatidylcholine (LPC) to produce lysophosphatidic acid, is identical to NPP2. More recently an uncharacterized novel NPP member (NPP7) was shown to have alkaline sphingomyelinase activity. These findings raised the possibility that other members of the NPP family act on phospholipids. Here we show that the sixth member of the NPP family, NPP6, is a choline-specific glycerophosphodiester phosphodiesterase. The sequence of NPP6 encodes a transmembrane protein containing an NPP domain with significant homology to NPP4, NPP5, and NPP7/alkaline sphingomyelinase. When expressed in HeLa cells, NPP6 was detected in both the cells and the cell culture medium as judged by Western blotting and by enzymatic activity. Recombinant NPP6 efficiently hydrolyzed the classical substrate for phospholipase C, p-nitrophenyl phosphorylcholine, but not the classical nucleotide phosphodiesterase substrate, p-nitrophenyl thymidine 5'-monophosphate. In addition, NPP6 hydrolyzed LPC to form monoacylglycerol and phosphorylcholine but not lysophosphatidic acid, showing it has a lysophospholipase C activity. NPP6 showed a preference for LPC with short (12:0 and 14:0) or polyunsaturated (18:2 and 20:4) fatty acids. It also hydrolyzed glycerophosphorylcholine and sphingosylphosphorylcholine efficiently. In mice, NPP6 mRNA was predominantly detected in kidney with a lesser expression in brain and heart, and in human it was detected in kidney and brain. The present results suggest that NPP6 has a specific role through the hydrolysis of polyunsaturated LPC, glycerophosphorylcholine, or sphingosylphosphorylcholine in these organs.