Glycosylphosphatidylinositol-specific Phospholipase D Research Articles

Upregulated phospholipase D activity toward glycosylphosphatidylinositol-anchored proteins in micelle-like serum complexes in metabolically deranged rats and humans.

Glycosylphosphatidylinositol-anchored proteins (GPI-AP) with the complete glycolipid anchor attached have previously been shown to be released from the outer plasma membrane leaflet of rat adipocytes in positive correlation to cell size and blood glucose/insulin levels of the donor rats. Furthermore, they are present in rat and human serum, however, at amounts that are lower in insulin-resistant/obese rats compared with normal ones. These findings prompted further evaluation of the potential of full-length GPI-AP for the prediction and stratification of metabolically deranged states. A comparison of the signatures of horizontal surface acoustic waves that were generated by full-length GPI-AP in the course of their specific capture by and subsequent dissociation from a chip-based sensor between those from rat serum and those reconstituted into lipidic structures strongly argues for expression of full-length GPI-AP in serum in micelle-like complexes in concert with phospholipids, lysophospholipids, and cholesterol. Both the reconstituted and the rat serum complexes were highly sensitive toward mechanical forces, such as vibration. Furthermore, full-length GPI-AP reconstituted into micelle-like complexes represented efficient substrates for cleavage by serum glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD). These findings raised the possibility that the upregulated release of full-length GPI-AP into micelle-like serum complexes from metabolically deranged cells is compensated by elevated GPI-PLD activity. In fact, serum GPI-PLD activity toward full-length GPI-AP in micelle-like complexes, but not in detergent micelles, was positively correlated to early states of insulin resistance and obesity in genetic and diet-induced rat models as well as to the body weight in humans. Moreover, the differences in the degradation of GPI-AP in micelle-like complexes were found to rely in part on the interaction of serum GPI-PLD with an activating serum factor. These data suggest that serum GPI-PLD activity measured with GPI-AP in micelle-like complexes is indicative of enhanced release of full-length GPI-AP from relevant tissues into the circulation as a consequence of early metabolic derangement in rats and humans.

Impact of glycosylphosphatidylinositol-specific phospholipase D on hepatic diacylglycerol accumulation, steatosis, and insulin resistance in diet-induced obesity.

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is an enzyme that specifically cleaves GPI anchors. Previous human studies suggested the relationship of GPI-PLD to insulin resistance, type 1 and type 2 diabetes, and nonalcoholic fatty liver disease (NAFLD). However, the biological roles of GPI-PLD have not been elucidated. Here, we hypothesized that GPI-PLD impacted on lipid and glucose metabolism, especially in the liver. GPI-PLD mRNA was most highly expressed in the liver, and the hepatic mRNA level and circulating concentration of GPI-PLD were significantly augmented in diabetic mice. To investigate in vivo functions of GPI-PLD, we generated GPI-PLD knockout (GP-KO) mice. Mice lacking GPI-PLD exhibited the amelioration of glucose intolerance and hepatic steatosis under high-fat and high-sucrose diet. Furthermore, diacylglycerol (DAG) content was significantly decreased, and PKCε activity was suppressed in the livers of GP-KO mice. In vitro knockdown and overexpression experiments of GPI-PLD using rat primary hepatocytes showed the GPI-PLD-dependent regulation of intracellular DAG content. Finally, serum GPI-PLD levels were strongly and independently associated with serum alanine transaminase (R = 0.37, P = 0.0006) and triglyceride (R = 0.34, P = 0.001) levels in male subjects with metabolic syndrome. In conclusion, upregulation of hepatic GPI-PLD in diabetic conditions leads to DAG accumulation in the liver by shedding GPI anchors intracellularly, which may play a causal role in impaired hepatic insulin signaling and the progression of NAFLD.

GPI-PLD inhibits the growth of hepatoma cells by down-regulation of PI3K-Akt signaling pathway

To clarify the effect of glycosylphosphatidylinositol-specific phospholipase D (GPIPLD) on hepatoma cells HepG2 and the possible molecular mechanism. MTT, fluorescent staining and Western blot were applied to analyze the effect and molecular mechanism of GPI-PLD on hepatoma cells by transfected high expression GPI-PLD model. We inoculated HepG2 in nude mice models to further clarify the effect of GPI-PLD on hepatoma cells in vivo. Compared with the control groups, PI3K-Akt signaling pathway activity and proliferation of hepatoma cells were significantly inhibited in the GPI-PLD group. Nude mice models showed that the tumor growth and tumor weight [(1.87 ± 0.09) g] of the GPI-PLD group were significantly less than those of the blank control group [(2.20 ± 0.17) g] and the negative control group [(2.15 ± 0.09) g]. AST, ALT and AFP serum concentration in the GPI-PLD group were significantly lower than those of the control groups (P<0.05). GPI-PLD can inhibit the proliferation of hepatoma cells and growth in vivo, and promote the apoptosis of hepatoma cells by reducing the activity of PI3K-Akt signaling pathway.

Discovery of a Novel Circulating Biomarker in Patients with Abdominal Aortic Aneurysm: A Pilot Study Using a Proteomic Approach

Abdominal aortic aneurysm (AAA) is a common condition with high mortality when ruptured. Most clinicians agree that small AAAs are best managed by ultrasonographic surveillance. However, it has been stated in recent reviews that a serum/plasma biomarker that predicts AAA rupture risk would be a powerful tool in stratifying patients with small AAA. Identification of such circulating biomarkers has been to date unsuccessful. In this study, we used a proteomic approach to find new, potential plasma AAA biomarker candidates. Prefractionated plasma samples were analyzed by two-dimensional differential in-gel electrophoresis to identify differentially expressed proteins between four patients with small AAA and four controls without aneurysm. Protein spots that differed significantly between patients and controls were selected and identified by mass spectrometry. Three protein spots had significantly different expression between patients and controls. The most interesting finding was that patients with small AAA had increased levels of the enzyme glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) compared with the controls without aneurysm. In conclusion, by using a proteomic approach, this pilot-study provides evidence of GPI-PLD as a novel potential plasma biomarker for AAA.

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).

Regulation of carcinoembryonic antigen release from colorectal cancer cells

Clinical and experimental evidence suggest that circulating carcinoembryonic antigen (CEA) released from tumor cells has an instrumental role in colorectal cancer-liver metastasis. However, the precise mechanism of the regulation of the CEA release from cancer cells is not known. We investigated if the rate of CEA and another GPI-anchored protein, alkaline phosphatase (AP) release is correlated with cellular glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) expression. We also evaluated the effects of phosphatidic acid (PA), a compound known to inhibit GPI-PLD activity, on the CEA and AP release from colon cancer cells. The expression of CEA, GPI-PLD, and AP in five colon carcinoma cells (LS180, Caco2, SW742, SW1116, and HT29/219) was verified by immunoblot and real-time RT-PCR analysis. The amounts of CEA and AP released into cell culture media were determined using ELISA and a colorimetric assay, respectively. We examined the effects of PA (20-100μM) on CEA and AP release from LS180 cells. All five cancer cell lines analyzed expressed GPI-PLD protein. While there was a positive relationship between AP release and the levels of GPI-PLD transcript expression, we found no direct correlation between CEA released from cancer cells and the GPI-PLD mRNA expression level. However, the rate of CEA release was positively associated with the level of CEA transcript expression. In comparison to controls, the release of GPI-anchored CEA and AP, but not CA19-9 was inhibited significantly by both crude and pure phosphatidic acid (by 56 and 54.5%, respectively). Using PA for inhibiting CEA release from cancer cells may have therapeutic application in preventing CRC-liver metastasis.

Glycosylphosphatidylinositol-specific phospholipase D improves glucose tolerance

Insulin regulation of energy metabolism is complex and involves numerous signaling cascades. Insulin has been suggested to stimulate a phospholipase that cleaves glycosylphosphatidylinositols resulting in the generation of an inositol glycan that serves as an insulin mediator. To determine if glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) may play a role in glucose metabolism, we examined the effect of overexpressing GPI-PLD using adenovirus-mediated gene transfer in C57BL/6 mice. Overexpressing GPI-PLD was associated with a decrease in fasting glucose as well as an improvement in glucose tolerance as determined by an intraperitoneal glucose tolerance test. This effect to improve glucose tolerance does not result from an increase in insulin sensitivity, as overexpressing GPI-PLD does not alter the response to insulin. In contrast, the insulin response during the glucose tolerance test in GPI-PLD–overexpressing mice was increased. Overexpressing GPI-PLD in an insulinoma cell line enhanced glucose-stimulated insulin secretion, suggesting that enhanced insulin secretion in vivo may have contributed to the improved glucose tolerance.

Important roles of glycosylphosphatidylinositol (GPI)-specific phospholipase D and some GPI-anchored proteins in the pathogenesis of hepatocellular carcinoma

Objective To investigate the roles of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) in the pathogenesis of hepatocellular carcinoma (HCC). Methods The expression of the GPI-PLD in HCC was determined. The GPI-PLD gene was stably transfected in HepG2 cells and the proliferation of these cells was detected; CD55, CD59 and apoptotic cells were also analyzed. Results The serum GPI-PLD activities, the protein and mRNA levels of GPI-PLD in HCC patients were decreased by 40%, 60% and 56%, respectively. The killing rate of CDC against the positive clone cells was significantly increased, but the proliferative capacity was obviously decreased. The apoptotic rate in positive clones was increased. Conclusion The expression of GPI-PLD decreases in HCC patients. The over-expression of GPI-PLD in HepG2 cells increases their sensitivity to CDC killing, impairs proliferative capacity and promotes apoptosis.

Plasma glycosylphosphatidylinositol-specific phospholipase D predicts the change in insulin sensitivity in response to a low-fat but not a low-carbohydrate diet in obese women

Although circulating glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD), a minor high-density lipoprotein–associated protein, is elevated in patients with insulin resistance or high triglycerides, no information is available on the effect of weight loss or changes in insulin sensitivity on circulating GPI-PLD levels. The objective of the study was to determine the effect of weight loss and changes in insulin sensitivity on plasma GPI-PLD levels. Forty-two nondiabetic obese women were included in the study, which involved a 3-month dietary intervention randomizing patients to a low-fat or a low-carbohydrate diet. The study's main outcome measures were plasma GPI-PLD levels and insulin sensitivity as estimated by the homeostasis model assessment. The very low carbohydrate diet group lost more weight after 3 months (−7.6 ± 3.2 vs −4.2 ± 3.5 kg, P < .01), although the decrease in insulin resistance was similar between groups. Weight loss with either diet did not alter plasma GPI-PLD levels. However, baseline GPI-PLD levels correlated with the change in insulin sensitivity in response to the low-fat diet, whereas baseline insulin sensitivity correlated with the change in insulin sensitivity in response to the low-carbohydrate diet. Plasma GPI-PLD may serve as a clinical tool to determine the effect of a low-fat diet on insulin sensitivity.

Statin therapy reduces serum levels of glycosylphosphatidylinositol-specific phospholipase D

Statin therapy is associated with changes in low-density, very low-density, and high- density lipoprotein metabolism. The effect of statin therapy on a minor high-density lipoprotein particle containing glycosylphosphatidylinositol-specific phospholipase D has not been examined. Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) has been implicated in triglyceride metabolism. A double-blind, crossover design comparing the effect of simvastatin (80 mg) and atorvastatin (80 mg) on serum lipid and glycosylphosphatidylinositol-specific phospholipase D levels was conducted in 13 patients with low high-density lipoproteins. Both statins reduced cholesterol, triglycerides, and apolipoprotein B and significantly lowered serum glycosylphosphatidylinositol-specific phospholipase D levels (16%). This statin effect seems to occur in the plasma compartment as neither statin altered GPI-PLD mRNA levels in HepG2 cells. Serum glycosylphosphatidylinositol-specific phospholipase D levels are regulated by statins and may represent an additional biochemical mechanism for affecting serum triglyceride levels.

Role of glycosylphosphatidylinositol-specific phospholipase D in the homing of umbilical cord blood, mobilized peripheral blood and bone marrow-derived hematopoietic stem/progenitor cells

Recent studies suggested that glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) correlated with tumor malignancy and prognosis of certain tumors. As hematopoietic stem/progenitor cells (HS/PC) homing was similar to tumor invasion and metastasis in some mechanisms, which arose our intrests in whether GPI-PLD contribution to the homing of HS/PC. In this study, CD34 + cells from umbilical cord blood (UCB), mobilized peripheral blood (MPB), and bone marrow (BM) were assayed for their differences in adhesion, migration, respectively. The expression of GPI-anchored proteins (CD48, CD90) on the cells were analyzed by flow cytometry. Semi-quantitive RT-PCR was used to detect GPI-PLD expression in the three different CD34 + cells. The results showed that GPI-PLD had no effect on the adhesion of CD34 + cells. While, spontaneous and SDF-1 induced migration of UCB and MPB, but not BM CD34 + cells were decreased after 1,10-phenanthroline (an inhibitor of GPI-PLD) pretreatment. Furthermore, we found little difference in GPI-anchored adhesion molecules (CD48, CD90) expression between untreated and pretreated CD34 + cells. GPI-PLD mRNA was low expressed in MPB and undetected in UCB and BM CD34 + cells. Our results suggested that GPI-PLD probably had no contribution to HS/PC homing, which may due to its low or no expression in UCB, BM and MPB CD34 + cells.

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.

Glycosylphosphatidylinositol-Specific Phospholipase D in Nonalcoholic Fatty Liver Disease: A Preliminary Study

Recent studies demonstrated that de novo lipogenesis is increased in patients with nonalcoholic fatty liver disease (NAFLD). Patients with NAFLD also have plasma lipid abnormalities. These lipid abnormalities may in part be related to insulin resistance, which is common in patients with NAFLD. Insulin resistance is associated with alterations in proteins involved in lipid metabolism including glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD), which is involved in triglyceride metabolism. The objective of the study was to determine whether alterations in serum and hepatic levels of GPI-PLD occur in patients with NAFLD. We examined the following: 1) levels of serum GPI-PLD in nondiabetics with nonalcoholic steatohepatitis, compared with matched controls; 2) hepatic expression of GPI-PLD mRNA in patients with normal liver or NAFLD; and 3) effect of overexpressing GPI-PLD vs. beta-galactosidase (control) on global gene expression in a human hepatoma cell line. The serum levels of GPI-PLD were significantly higher in patients with nonalcoholic steatohepatitis than in matched controls (119 +/- 24 vs.105 +/- 15 microg/ml, P = 0.047). The hepatic expression of GPI-PLD mRNA was increased nearly 3-fold in NAFLD patients, compared with patients with normal liver (3.1 +/- 2.6 vs. 1.1 +/- 1.0 arbitrary units per microgram total RNA, P = 0.026). Finally, overexpressing GPI-PLD was associated with an increase in de novo lipogenesis genes. Patients with NAFLD have elevated serum levels and hepatic expression of GPI-PLD, and its overexpression in vitro is associated with increased expression of de novo lipogenesis genes. These results suggest that GPI-PLD may play a role in the pathogenesis of NAFLD and/or its metabolic features and warrants further investigation.

Phospholipase Cleavage of D‐ and L‐chiro‐Glycosylphosphoinositides Asymmetrically Incorporated into Liposomal Membranes

The nature of chiro-inositol-containing inositolphosphoglycans (IPGs), reported to be putative insulin mediators, was studied by examination of the substrate specificities of the phosphatidylinositol-specific phospholipase C (PI-PLC) and the glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) by using a series of synthetic D- and L-chiro-glycosylphosphoinositides. 3-O-alpha-D-Glucosaminyl- (3) and -galactosaminyl-2-phosphatidyl-L-chiro-inositol (4), which show the maximum stereochemical similarity to the 6-O-alpha-D-glucosaminylphosphatidylinositol pseudodisaccharide motifs of GPI anchors, were synthesized and asymmetrically incorporated into phospholipid bilayers in the form of large unilamellar vesicles (LUVs). Similarly, 2-O-alpha-D-glucosaminyl- (5) and -galactosaminyl-1-phosphatidyl-D-chiro-inositol (6), which differ from the corresponding pseudodisaccharide motif of the GPI anchors only in the axial orientation of the phosphatidyl moiety, were also synthesized and asymmetrically inserted into LUVs. The cleavage of these synthetic molecules in the liposomal constructs by PI-PLC from Bacillus cereus and by GPI-PLD from bovine serum was studied with the use of 6-O-alpha-D-glucosaminylphosphatidylinositol (7) and the conserved GPI anchor structure (8) as positive controls. Although PI-PLC cleaved 3 and 4 with about the same efficiency as 7 and 8, this enzyme did not accept 5 or 6. GPI-PLD accepted both the L-chiro- (3 and 4) and the D-chiro- (5 and 6) glycosylinositolphosphoinositides. Therefore, IPGs containing L-chiro-inositol only are expected to be released from chiro-inositol-containing GPIs if the cleavage is effected by a PI-PLC, whereas GPI-PLD cleavage could result in both L-chiro- and D-chiro-inositol-containing IPGs.

Glycosylphosphatidylinositol-specific phospholipase D influences triglyceride-rich lipoprotein metabolism

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is a minor HDL-associated protein. Because many minor HDL-associated proteins exchange between different lipoprotein classes during the postprandial state and are also involved in triglyceride (TG) metabolism, we hypothesized that GPI-PLD may play a role in the metabolism of TG-rich lipoproteins. To test this hypothesis, we examined the distribution of GPI-PLD among lipoprotein classes during a fat tolerance test in C57BL/6 and LDL receptor-deficient (LDLR(-/-)) mice fed either a chow or high-fructose diet. In the fasting state in wild-type mice fed a chow diet, GPI-PLD was only present in HDL, whereas in LDLR(-/-) mice GPI-PLD was present in HDL and intermediate-density lipoproteins (IDL)/LDL. During the fat tolerance test, there was no change in total serum GPI-PLD levels in either model; however, a significant amount of GPI-PLD appeared in both VLDL (0.5-1% of total GPI-PLD) and IDL/LDL (5-10% of total GPI-PLD) in both models. The high-fructose diet increased both fasting and postprandial TG and serum GPI-PLD levels in both strains as well as the amount of GPI-PLD in VLDL. To determine whether GPI-PLD plays a direct role in TG metabolism, we increased liver GPI-PLD expression in C57BL/6 mice by adenovirus-mediated gene transfer, which resulted in a sevenfold increase in serum GPI-PLD levels. This change was associated with an increase in fasting (30%) and postprandial TG (50%) and a twofold reduction in TG-rich lipoprotein catabolism compared with saline or control adenovirus-treated mice. These studies demonstrate that GPI-PLD affects serum TG levels by altering catabolism of TG-rich lipoproteins.

Mutating His29, His125, His133 or His158 abolishes glycosylphosphatidylinositol-specific phospholipase D catalytic activity

Glycosylphosphatidylinositol (GPI)-specific phospholipase D (GPI-PLD) specifically cleaves GPIs. This phospholipase D is a secreted protein consisting of two domains: an N-terminal catalytic domain and a predicted C-terminal b-propeller. Although the biochemical properties of GPI-PLD have been extensively studied, its catalytic site has not been identified. We hypothesized that a histidine residue(s) may play a critical role in the catalytic activity of GPI-PLD, based on the observations that (i) Zn2+, which utilizes histidine residues for binding, is required for GPI-PLD catalytic activity, (ii) a phosphohistidine intermediate is involved in phospholipase D hydrolysis of phosphatidylcholine, (iii) computer modelling suggests a catalytic site containing histidine residues, and (iv) our observation that diethyl pyrocarbonate, which modifies histidine residues, inhibits GPI-PLD catalytic activity. Individual mutation of the ten histidine residues to asparagine in the catalytic domain of murine GPI-PLD resulted in three general phenotypes: not secreted or retained (His56 or His88), secreted with catalytic activity (His34, His81, His98 or His219) and secreted without catalytic activity (His29, His125, His133 or His158). Changing His133 but not His29, His125 or His158 to Cys resulted in a mutant that retained catalytic activity, suggesting that at least His133 is involved in Zn2+ binding. His133 and His158 also retained the biochemical properties of wild-type GPI-PLD including trypsin cleavage pattern and phosphorylation by protein kinase A. Hence, His29, His125, His133 and His158 are required for GPI-PLD catalytic activity.

Characterization of primate trypanosome lytic factors

Humans are one of the few species that resist infection by Trypanosoma brucei brucei because the parasites are killed by lytic factors found in human serum. Trypanosome lytic factors (TLFs) are protein/lipid complexes that contain apolipoprotein A-I (apoA-I), and are therefore a class of high density lipoproteins (HDLs). Haptoglobin-related protein (Hpr) is a unique protein component of TLFs, and its expression has only been demonstrated in humans. Trypanolytic activity has only been found in the sera of five primates: humans, gorillas, mandrills, baboons and sooty mangabeys. We describe here previously unidentified components of highly purified human TLF1: apolipoprotein L-I (apoL-I), human cathelicidin antimicrobial peptide 18 (hCAP18) and glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD). However, we found that hCAP18 and GPI-PLD, along with apoA-I, are common components of both lytic and non-lytic primate HDLs. In contrast, Hpr, which has been previously implicated as the main lytic component of TLF1, was a unique component of all trypanolytic primate HDLs. Furthermore, a polyclonal antiserum to Hpr neutralized the lytic activity from humans and baboons. ApoL-I, a candidate lytic component of human serum, was not immunologically or genetically detectable in two primate species with lytic activity. Polyclonal antiserum to apoL-I also did not neutralize TLF activity in a total human HDL preparation. These findings suggest that apoL-I is not essential in all primate TLFs, and apoL-I alone is not sufficient for optimal trypanosome lytic activity in human TLF.

Glycosylphosphatidylinositol-specific phospholipase D immunoreactivity is present in islet amyloid in type 2 diabetes.

Numerous apolipoproteins associate with amyloid plaques. A minor high-density lipoprotein-associated protein, glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD), has recently been described by the authors and others. Since GPI-PLD is synthesized by, and secreted from, pancreatic islet beta cells, the present study examined the hypothesis that GPI-PLD associates with islet amyloid. GPI-PLD immunoreactivity was examined in pancreatic tissues from type 2 diabetic and non-diabetic humans. GPI-PLD binding to heparan sulphate proteoglycan was determined in the absence or presence of heparan sulphate or heparin. Fibril formation from human islet amyloid polypeptide was determined in the absence or presence of GPI-PLD. In non-diabetics, GPI-PLD immunoreactivity was present and co-localized with insulin, as opposed to co-localizing with amyloid in diabetics. No immunoreactivity for apolipoprotein A-I was present in islet cells or islet amyloid. Heparan sulphate proteoglycan, which is commonly present in most amyloid, bound GPI-PLD in vitro. GPI-PLD inhibited the formation of amyloid fibrils from synthetic islet amyloid polypeptide in vitro. GPI-PLD is therefore present in islet amyloid and appears to derive from local production from islets. This localization likely derives from interaction between GPI-PLD and heparan sulphate proteoglycan. Since GPI-PLD also inhibited islet amyloid polypeptide fibril formation in vitro, it is concluded that GPI-PLD may play a role in islet amyloid formation in type 2 diabetes.

Insulin resistance is associated with increased serum levels of glycosylphosphatidylinositol-specific phospholipase D

The dyslipidemia of the metabolic syndrome is associated with alterations in triglyceride and high-density lipoprotein (HDL) metabolism. We examined the serum levels of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD), a minor HDL-associated protein, in a cohort with a wide range of insulin sensitivity. The mean serum GPI-PLD mass from 109 subjects was 58.9 ± 18.4 μg/mL (mean ± SD). GPI-PLD levels directly correlated with cholesterol, apolipoprotein AI, triglycerides, insulin, and homeostasis model assessment (HOMA) but not C-reactive protein. These results suggest that increased serum GPI-PLD is associated with the insulin resistance.

癌・CEAは分泌型で、正常大腸粘膜上皮・CEAは膜結合型であり、この違いはそれぞれの細胞におけるGPI-PLD発現の有無によるという仮説の提唱

癌・CEAは分泌型で、正常大腸粘膜上皮・CEAは膜結合型であり、この違いはそれぞれの細胞におけるGPI-PLD発現の有無によるという仮説の提唱