Purification of a phosphatidylinositol-glycan-specific phospholipase C from liver plasma membranes: a possible target of insulin action

Role of Oxidative Stress in Drug-Induced Kidney Injury

The kidney plays a primary role in maintaining homeostasis and detoxification of numerous hydrophilic xenobiotics as well as endogenous compounds. Because the kidney is exposed to a larger proportion and higher concentration of drugs and toxins than other organs through the secretion of ionic drugs by tubular organic ion transporters across the luminal membranes of renal tubular epithelial cells, and through the reabsorption of filtered toxins into the lumen of the tubule, these cells are at greater risk for injury. In fact, drug-induced kidney injury is a serious problem in clinical practice and accounts for roughly 20% of cases of acute kidney injury (AKI) among hospitalized patients. Therefore, its early detection is becoming more important. Usually, drug-induced AKI consists of two patterns of renal injury: acute tubular necrosis (ATN) and acute interstitial nephritis (AIN). Whereas AIN develops from medications that incite an allergic reaction, ATN develops from direct toxicity on tubular epithelial cells. Among several cellular mechanisms underlying ATN, oxidative stress plays an important role in progression to ATN by activation of inflammatory response via proinflammatory cytokine release and inflammatory cell accumulation in tissues. This review provides an overview of drugs associated with AKI, the role of oxidative stress in drug-induced AKI, and a biomarker for drug-induced AKI focusing on oxidative stress.

Actovegin®: a biological drug for more than 5 decades

Actovegin(®) is a biological drug manufactured from a natural source: it is a calf blood hemodialysate. Its therapeutic benefits stem from a variety of pharmacodynamic actions that can be summarized to a common goal, i.e. the enhancement of cellular metabolism; this results from an insulin-like activity mediated by Inositol-phospho-oligosaccharides. Actovegin(®) results in beneficial effects in several pathophysiological clinical settings including malfunction of the blood circulation and trophic disturbances in the brain, impairment of peripheral blood circulation and associated diseases, dermal transplants and acute and chronic wounds. Here, we give an overview of the pharmacodynamic actions of calf-blood hemidialysate and its beneficial effects in a variety of clinical settings.

Treatment of symptomatic polyneuropathy with actovegin in type 2 diabetic patients

OBJECTIVE To evaluate the efficacy and safety of actovegin in patients with diabetic polyneuropathy. RESEARCH DESIGN AND METHODS In this multicenter, randomized, double-blind trial, 567 patients with type 2 diabetes received 20 intravenous infusions of actovegin (2,000 mg/day) (n = 281) or placebo (n = 286) once daily followed by three tablets of actovegin (1,800 mg/day) or placebo three times daily for 140 days. Total symptom score (TSS) of the lower limbs and vibration perception threshold (VPT) were used as coprimary outcome measures, computed as the area under the curve (AUC) from repeated scores and divided by duration of exposure. Secondary end points included individual TSS symptoms, neuropathy impairment score of the lower limbs (NIS-LL), and quality of life (short form [SF]-36). RESULTS TSS was significantly improved during actovegin treatment compared with placebo, as assessed by AUC (-0.56 points [95% CI -0.85 to -0.27]; P = 0.0003), and from baseline to 160 days (-0.86 points [-1.22 to -0.50]; P < 0.0001). VPT (five sites per foot) decreased by 3% (95% CI 0-6; P = 0.084) with actovegin than placebo, as assessed by AUC, and by 5% (1-9; P = 0.017) after 160 days. NIS-LL sensory function, as assessed by AUC, was significantly improved with actovegin versus placebo (-0.25 [95% CI -0.46 to -0.04]; P = 0.021), as was the SF-36 mental health domain. There were no differences in the incidence of adverse events between the groups. CONCLUSIONS Sequential intravenous and oral actovegin treatment over 160 days improved neuropathic symptoms, VPT, sensory function, and quality of life in type 2 diabetic patients with symptomatic polyneuropathy.

The glycosylphosphatidylinositol-PLC in Trypanosoma brucei forms a linear array on the exterior of the flagellar membrane before and after activation

Bloodstream forms of Trypanosoma brucei contain a glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC) that cleaves the GPI-anchor of the variable surface glycoprotein (VSG). Its location in trypanosomes has been controversial. Here, using confocal microscopy and surface labelling techniques, we show that the GPI-PLC is located exclusively in a linear array on the outside of the flagellar membrane, close to the flagellar attachment zone, but does not co-localize with the flagellar attachment zone protein, FAZ1. Consequently, the GPI-PLC and the VSG occupy the same plasma membrane leaflet, which resolves the topological problem associated with the cleavage reaction if the VSG and the GPI-PLC were on opposite sides of the membrane. The exterior location requires the enzyme to be tightly regulated to prevent VSG release under basal conditions. During stimulated VSG release in intact cells, the GPI-PLC did not change location, suggesting that the release mechanism involves lateral diffusion of the VSG in the plane of the membrane to the fixed position of the GPI-PLC.

African trypanosomes cause sleeping sickness, for which current therapy is inadequate. The parasite protects its surface from the host immune system by regularly switching its surface coat. The glycosylphosphatidylinositol-PLC only occurs in the bloodstream form, where it removes the surface coat after it enters the tsetse fly vector. Activation of the enzyme in the bloodstream would be fatal for the parasite and it is, therefore, a potential drug target. However, therapeutic strategies have been hampered by confusion over the location of the GPI-PLC despite great effort by many labs. We have used a wide variety of techniques, including one completely novel method, that exploits the dependence of detection for partially buried surface proteins on the temperature of fixation, to identify the location of the GPI-PLC in relation to other markers unequivocally. All approaches consistently show that the GPI-PLC is located exclusively in the outer leaflet of the plasma membrane covering the flagellum, where it is confined to a narrow linear array adjacent to the flagellar attachment zone. Our data have resolved the question of how enzyme and substrate meet and also suggest that chemotherapeutic agents would be able to target the GPI-PLC in its exterior location.

Insulin and the stimulation of glycogen synthesis. The road from glycogen structure to glycogen synthase to cyclic AMP-dependent protein kinase to insulin mediators

The enhanced phosphorylations via cAMP, Ca2+ mobilization, and diacyl glycerol formation via the activation of the respective kinases is now classical. The decreased phosphorylation via inhibition of adenylate cyclase via the alpha adrenergic receptor is also becoming understood. What the insulin studies on the control of glycogen synthesis have taught us is that the rate limiting enzyme glycogen synthase is regulated by multiple covalent phosphorylation in an elegant but complex manner. The overall pattern of dephosphorylation is influenced by effecting both phosphatase and kinase activities in a set of interrelated mechanisms. In the presence of glucose, in muscle, fat, and liver under physiological conditions G-6-P acts as a signal to stimulate the phosphatase. An additional stimulation could occur via a novel insulin phosphatase stimulatory mediator. The phosphatase is also stimulated by at least three covalent mechanisms involving altered phosphorylation state. In one there is a decreased phosphorylation of the phosphatase inhibitor 1 potentially related to decreased cAMP-dependent protein kinase activity. In the second, there is decreased phosphorylation of the deinhibitor also potentially related to decreased cAMP-dependent protein kinase phosphorylation. In the third, an increased activity of casein kinase 2 could activate the ATP-Mg dependent phosphatase by an increased phosphorylation of phosphatase inhibitor 2 (modulatory subunit). In the liver, allosteric control of the phosphatase by G-6-P and nucleotides is of great importance. Insulin also stimulates the phosphatase in long-term experiments via increased protein synthesis. It is clear that future work will be required to determine which species of the various classes of phosphatases are regulated in short-term and long-term regulation by insulin. In terms of kinases, the effects of insulin to inactivate and desensitize the cAMP-dependent protein kinase are established. The molecular mechanisms of this effect remain to be worked out. The enhanced activity of MAP and S-6 kinase would appear to be part of a cascade of reactions perhaps originating in the autophosphorylation and activation of the insulin receptor tyrosine kinase. The mechanism of the short-term activation of casein kinase 2 remains to be elucidated. A cAMP-dependent protein kinase inhibitory mediator, which also inhibits adenylate cyclase is an important element in the regulation of kinase and adenylate cyclase activity by insulin. Its physiological significance must be established in the future, in terms of its control of glycogen synthase activation by insulin. Clearly this kinase inhibitor as well as the phosphatase stimulator are potential regulators of glycogen synthase activity by insulin.

Regulation of lipid raft proteins by glimepiride- and insulin-induced glycosylphosphatidylinositol-specific phospholipase C in rat adipocytes

The insulin receptor-independent insulin-mimetic signalling provoked by the antidiabetic sulfonylurea drug, glimepiride, is accompanied by the redistribution and concomitant activation of lipid raft-associated signalling components, such as the acylated tyrosine kinase, pp59(Lyn), and some glycosylphosphatidylinositol-anchored proteins (GPI-proteins). We now found that impairment of glimepiride-induced lipolytic cleavage of GPI-proteins in rat adipocytes by the novel inhibitor of glycosylphosphatidylinositol-specific phospholipase C (GPI-PLC), GPI-2350, caused almost complete blockade of (i) dissociation from caveolin-1 of pp59(Lyn) and GPI-proteins, (ii) their redistribution from high cholesterol- (hcDIGs) to low cholesterol-containing (lcDIGs) lipid rafts, (iii) tyrosine phosphorylation of pp59(Lyn) and insulin receptor substrate-1 protein (IRS-1) and (iv) stimulation of glucose transport as well as (v) inhibition of isoproterenol-induced lipolysis in response to glimepiride. In contrast, blockade of the moderate insulin activation of the GPI-PLC and of lipid raft protein redistribution by GPI-2350 slightly reduced insulin signalling and metabolic action, only. Importantly, in response to both insulin and glimepiride, lipolytically cleaved hydrophilic GPI-proteins remain associated with hcDIGs rather than redistribute to lcDIGs as do their uncleaved amphiphilic versions. In conclusion, GPI-PLC controls the localization within lipid rafts and thereby the activity of certain GPI-anchored and acylated signalling proteins. Its stimulation is required and may even be sufficient for insulin-mimetic cross-talking to IRS-1 in response to glimepiride via redistributed and activated pp59(Lyn).

Cysteine-less glycosylphosphatidylinositol-specific phospholipase C is inhibited competitively by a thiol reagent: evidence for glyco-mimicry by p-chloromercuriphenylsulphonate

Glycosylphosphatidylinositol (GPI)-specific phospholipases are highly valuable for studying the structure and function of GPIs. GPI-specific phospholipase C (GPI-PLC) from Trypanosoma brucei and phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus cereus are the most widely studied of this class of phospholipases C. Inhibition of protein activity by thiol reagents is indicative of the participation of cysteine residues in biochemical events. The thiol reagent p-chloromercuriphenylsulphonate (pCMPS) inhibits T. brucei GPI-PLC, which has eight cysteine residues. Surprisingly, we found that the activity of B. cereus PI-PLC is also blocked by pCMPS, although the protein does not contain cysteine residues. Inhibition of B. cereus PI-PLC was reversed when pCMPS was size-separated from a preformed pCMPS.PI-PLC complex. In contrast, no activity was recovered when T. brucei GPI-PLC was subjected to a similar protocol. Equimolar beta-mercaptoethanol (beta-ME) reversed the inhibition of PI-PLC activity in a pCMPS.PI-PLC complex. For T. brucei GPI-PLC, however, ultrafiltration of the pCMPS.GI-PLC complex and addition of a large excess of beta-ME was necessary for partial recovery of enzyme activity. Thus T. brucei GPI-PLC is susceptible to inactivation by covalent modification with pCMPS, whereas PI-PLC is not. Kinetic analysis indicated that pCMPS was a competitive inhibitor of PI-PLC when a GPI was a substrate. Curiously, with phosphatidylinositol as substrate, inhibition was no longer competitive. These data suggest that pCMPS is a glyco-mimetic that occupies the glycan binding site of PI-PLC, from where, depending on the substrate, it inhibits catalysis allosterically or competitively.

Quantitation of soluble and skeletal alkaline phosphatase, and insoluble alkaline phosphatase anchor-hydrolase activities in human serum

BACKGROUND

The current studies were intended to compare the circulating levels of total and anchorless (soluble) skeletal and hepatic ALP isoenzyme activities, and insoluble ALP anchor-hydrolase activity in serum of postmenopausal women.

METHODS

Preliminary studies of the insoluble ALP anchor-hydrolase activity in serum revealed a pH optimum of pH 5-6.5, a sensitivity to inactivation by heat at temperatures >45 degrees C (t(1/2)=8-9 min at 60 degrees C), and an apparent K(M) (at pH 7.5) of 40-45 mU/ml of insoluble skeletal ALP activity.

RESULTS

Serum analyses showed that 94.5+/-0.5% (mean+/-SEM) of the ALP activity in serum was in the anchorless, soluble form. The data were also consistent with the notion that the amount of insoluble ALP anchor-hydrolase activity in serum, 52.8+/-0.8 U/l (mean+/-SEM), was sufficient for the conversion of anchor-intact (insoluble) ALP into the anchorless, soluble form, assuming activation by serum lipids and/or bile salts. Distributions of results for total, skeletal, hepatic, and insoluble ALP anchor-hydrolase activity were skewed toward the higher range and leptokurtotic (p<0.01 for each). Total ALP activity ranged from 42% to 208% of the group mean value; skeletal, hepatic, and insoluble ALP anchor-hydrolase activities ranged from 5% to 306%, 33% to 277%, and 2% to 325%, respectively. In contrast, the soluble ALP fraction only ranged from 71% to 106% of the group mean value.

CONCLUSIONS

The correlations between the total and both skeletal (r=0.711, p<0.001) and hepatic (r=0.782, p<0.001) ALP isoform activities were predictive. Although correlations were also observed between insoluble ALP anchor-hydrolase activity and total (r=0.197, p<0.001), hepatic (r=0.184, p<0.001) and skeletal ALP activities (r=0.118, p<0.05), those relationships were not predictive (r(2)<0.04).

Endogenous glycosylphosphatidylinositol-specific phospholipase C releases renal dipeptidase from kidney proximal tubules in vitro

Spontaneous enzymic release of renal dipeptidase (RDPase; EC 3.4.13.19), a glycosylphosphatidylinositol (GPI)-linked ectoenzyme, was observed in vitro during incubation of porcine proximal tubules at 37 degrees C. Triton X-114 phase separation of the released RDPase showed that the majority of the enzyme activity partitioned into the aqueous phase, indicating its hydrophilic nature. Immunoblot analyses using an antibody against the cross-reacting determinant (CRD) inositol 1,2-cyclic monophosphate, the epitope formed by phospholipase C (PLC) cleavage of the GPI anchor on a protein, detected the released RDPase. Reprobing the immunoblot with an anti-RDPase serum showed the RDPase band co-migrating with the CRD band. The release of RDPase from the proximal tubules was a Ca(2+)-dependent process and had a pH optimum of 9.0. These results indicate that RDPase is released from the proximal tubules by the action of a distinct endogenous GPI-specific PLC.

Erythropoietin induces glycosylphosphatidylinositol hydrolysis. Possible involvement of phospholipase c-gamma(2)

We showed that erythropoietin induced rapid glycosylphosphatidylinositol (GPI) hydrolysis and tyrosine phosphorylation of phospholipase C (PLC)-gamma(2) in FDC-P1 cells transfected with the wild-type erythropoietin-receptor. Erythropoietin-induced tyrosine phosphorylation of PLC-gamma(2) was time- and dose-dependent. By using FDC-P1 cells transfected with an erythropoietin receptor devoid of tyrosine residues, we showed that both effects required the tyrosine residues of intracellular domain on the erythropoietin receptor. Erythropoietin-activated PLC-gamma(2) hydrolyzed purified [(3)H]GPI indicating that GPI hydrolysis and PLC-gamma(2) activation under erythropoietin stimulation were correlated. Results obtained on FDC-P1 cells transfected with erythropoietin receptor mutated on tyrosine residues suggest that tyrosines 343, 401, 464, and/or 479 are involved in erythropoietin-induced GPI hydrolysis and tyrosine phosphorylation of PLC-gamma(2), whereas tyrosines 429 and/or 431 seem to be involved in an inhibition of both effects. Thus, our results suggest that erythropoietin regulates GPI hydrolysis via tyrosine phosphorylation of its receptor and PLC-gamma(2) activation.

Purification and analysis of the neutral glycan moiety of glycosyl phosphatidylinositol from porcine thyroid cells

Metabolic labelling of inositolphosphate glycan with radioactive precursors is not sufficient to characterize and assess the involvement of the glycosyl phosphatidylinositol/inositolphosphate glycan (GPI/IPG) system in porcine thyroid cell signal transduction machinery. A protocol is described for the isolation and purification of free GPI using differential polarity of lipids and sequential thin layer chromatography. The purification until homogeneity of GPI constitutes a required step for gas chromatographic analysis. Next, successive chemical treatments allowed us to remove the neutral glycan moiety of thyroidal GPI, and its composition was obtained by gas chromatography. The proposed structure is consistent with data available for GPI anchor, but differs from compositional analysis data reported for insulin-sensitive GPI. Our results support the existence in porcine thyroid cells of the GPI/IPG system, which can take part in TSH-dependent signal transduction processes.

Posttranslational modification of glycosylphosphatidylinositol (GPI)-specific phospholipase D and its activity in cleavage of GPI anchors

Human glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) was exogenously expressed in mammalian CHO cells and in insect H5 cells. GPI-PLD was initially synthesized as a 105-kDa form and then secreted as a mature 115-kDa form from the CHO cells, whereas it was secreted as an immature 98-kDa form from the H5 cells. The difference of the molecular forms was caused by its oligosaccharide processing in the two cell lines. These forms showed a different reactivity to anti-C-terminal peptide of GPI-PLD; the 105-kDa and 98-kDa forms were directly recognized by the antibodies, whereas the 115-kDa form was immunoreactive only after being denatured. In an in vitro assay, the 98-kDa form but not the 115-kDa form was able to release a significant amount of GPI-anchored proteins from intact membranes, although the two forms had the same GPI-anchor cleavage activity in the presence of detergents. In addition, a GPI-anchored protein, when coexpressed in CHO cells, was intracellularly cleaved by GPI-PLD in the secretory pathway. Taken together, these results suggest that GPI-PLD undergoes a conformational change by posttranslational modification, which affects its immunoreactive and enzymatic properties.

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.

Insulin stimulates the release of the glycosyl phosphatidylinositol-anchored membrane dipeptidase from 3T3-L1 adipocytes through the action of a phospholipase C

Membrane dipeptidase (MDP; EC 3.4.13.19) enzymic activity that was inhibited by cilastatin has been detected on the surface of 3T3-L1 cells. On differentiation of the cells from fibroblasts to adipocytes the activity of MDP increased 12-fold. Immunoelectrophoretic blot analysis indicated that on adipogenesis the increase in the amount of MDP preceded the appearance of GLUT-4. MDP on 3T3-L1 adipocytes was anchored in the bilayer by a glycosyl phosphatidylinositol (GPI) moiety as evidenced by its release into the medium in a hydrophilic form on treatment of the cells with bacterial phosphatidylinositol-specific phospholipase C and the appearance of the inositol 1,2-cyclic monophosphate cross-reacting determinant. Incubation of 3T3-L1 adipocytes with either insulin or the sulphonylurea glimepiride led to a rapid concentration- and time-dependent release of MDP from the cell surface. The hydrophilic form of MDP released from the cells on stimulation with insulin was recognized by antibodies against the inositol 1,2-cyclic monophosphate cross-reacting determinant, indicating that it had been generated by cleavage of its GPI anchor through the action of a phospholipase C.

Insulin promotes and cyclic adenosine 3',5'-monophosphate impairs functional insertion of insulin receptors in the plasma membrane of rat adipocytes: evidence for opposing effects of tyrosine and serine/threonine phosphorylation

The aim of the present study was to elucidate events in the plasma membrane (PM) associated with the previously described effect of insulin to rapidly enhance the number of cell surface insulin binding sites in rat adipocytes. [125I]insulin was cross-linked to cell surface insulin receptors of intact cells that had been preincubated with or without insulin. Subsequently prepared PM displayed a approximately 3-fold increase in bound [125I]insulin when cells had been pretreated with 6 nM insulin for 20 min compared to membranes from control cells, and SDS-PAGE with autoradiography showed that this occurred at the insulin receptor alpha-subunit. The magnitude of the effect was similar to that found for insulin binding to intact cells that had been preincubated with insulin. In contrast, the insulin binding capacity in the PM was not affected by prior treatment of cells with insulin when assessed with the addition of [125I]insulin directly to solubilized PM; this suggests an unchanged total number of PM receptors. Thus, the enhancement of cell surface insulin binding capacity produced by insulin is not due to the translocation of receptors, but instead appears to be confined to receptors already present in the PM. The addition of phospholipase C (from Clostridium perfringens), which cleaves PM phospholipids, mimicked the effect of insulin to enhance cell surface binding in adipocytes, and this suggests a pool of cryptic PM receptors. Both the nonmetabolizable cAMP analog N6-monobutyryl cAMP (N6-mbcAMP) and the serine/threonine phosphatase inhibitor okadaic acid abolished the effect of concomitant insulin treatment to increase binding capacity. In contrast, the tyrosine phosphatase inhibitor vanadate increased insulin binding even in the presence of okadaic acid or N6-mbcAMP. The effect of N6-mbcAMP to impair cell surface insulin binding was also evident in the presence of a peptide derived from the major histocompatibility complex type I that effectively impairs receptor internalization, but the amount of PM receptors assessed by immunoblot was unaltered. Taken together, the data suggest that insulin exposure leads to the uncovering of cryptic receptors associated with the PM. It is also suggested that tyrosine phosphorylation promotes this process, whereas enhanced serine phosphorylation, e.g. produced by cAMP, impairs the functional insertion of the receptors, rendering them unable to bind insulin.

Biosynthesis of the endogenous cyclic adenosine monophosphate (AMP) antagonist, prostaglandylinositol cyclic phosphate (cyclic PIP), from prostaglandin E and activated inositol polyphosphate in rat liver plasma membranes

The endogenous cyclic adenosine monophosphate (AMP) antagonist, cyclic PIP, has been identified as a prostaglandylinositol cyclic phosphate. It inhibits protein kinase A 100% and activates protein serine phosphatase about sevenfold. It is biosynthesized by an enzyme of the plasma membrane when the assay mixture contains adenosine triphosphate (ATP), Mg2+, prostaglandin E and a novel inositol polyphosphate, which cannot be substituted by commercially available inositol phosphates. This novel inositol polyphosphate is a very labile compound. On anion exchange chromatography it elutes in the range of ATP, which may indicate the presence of three phosphate groups. It adsorbs on charcoal, which suggests the presence of a hydrophobic component, possibly a guanosine. Pyrophosphates obtained from inositol 1,4- and inositol 2,4-bisphosphate are accepted by cyclic PIP synthetase for the synthesis of cyclic PIP. The biosynthesis is characterized by enzyme kinetic parameters like dependence on time, enzyme and substrate concentration. The pH optimum of the enzyme is in the range 7.5-8. The enzyme functions optimally with prostaglandin E and poorly with prostaglandin A as the substrate. The presence of fluoride in the assay causes a three- to fourfold increase in cyclic PIP synthesis, which may be correlated with activation via G proteins. These data support previous reports on the chemical structure and action of cyclic PIP. With respect to the possible isomers of cyclic PIP, these indicate that it is most likely the C4-hydroxyl group of the inositol which binds the C15-hydroxyl group of prostaglandin E. A model of hormone-stimulated synthesis of cyclic PIP is proposed: phospholipase A2 and phospholipase C, activated by G proteins upon alpha-adrenergic stimulation, liberate either unsaturated fatty acids or inositol phosphates, which are transformed to prostaglandins and to novel inositol polyphosphate with an energy-rich bond. The cyclic PIP synthetase combines these two substrates to cyclic PIP.

Phosphatidylinositol hydrolysis by Trypanosoma brucei glycosylphosphatidylinositol phospholipase C

Detergent-solubilized glycosylphosphatidylinositol (GPI)-anchored structures can be cleaved by C-type phospholipases isolated from peanuts and bloodstream cells of the African trypanosome, Trypanosoma brucei. The two enzymes differ in their reported ability to hydrolyze phosphatidylinositol (PI); while the peanut enzyme readily hydrolyzes PI in vitro, the T. brucei enzyme was reported to be virtually inactive against PI and consequently named GPI-specific phospholipase C (GPI-PLC). In this paper, we describe experiments in which we reinvestigated the substrate specificity of T. brucei GPI-PLC by incubating the purified enzyme with Triton X-100/PI-mixed micelles and by studying PI hydrolysis. We found that PI hydrolysis occurred in a detergent-dependent fashion over the range of concentrations tested (5 microM to 1 mM PI). At 5 microM PI, hydrolysis was maximal at 0.005% Triton X-100, whereas at 1 mM PI, maximal hydrolysis required 0.05% Triton X-100. Hydrolysis of both PI and GPI was strongly affected by the presence of phospholipids. Endogenous PI was hydrolyzed during osmotic and detergent lysis of trypanosomes under conditions used to obtain quantitative hydrolysis of the GPI-anchored trypanosome variant surface glycoprotein. PI hydrolysis in the lysates was inhibited by sodium p-chloromercuriphenylsulfonate but unaffected by EGTA, consistent with the proposal that hydrolysis is due to GPI-PLC. These results suggest that the function of T. brucei GPI-PLC may be to regulate PI as well as (or instead of) GPI levels.

Construction and characterization of secreted and chimeric transmembrane forms of Drosophila acetylcholinesterase: a large truncation of the C-terminal signal peptide does not eliminate glycoinositol phospholipid anchoring

Despite advances in understanding the cell biology of glycoinositol phospholipid (GPI)-anchored proteins in cultured cells, the in vivo functions of GPI anchors have remained elusive. We have focused on Drosophila acetylcholinesterase (AChE) as a model GPI-anchored protein that can be manipulated in vivo with sophisticated genetic techniques. In Drosophila, AChE is found only as a GPI-anchored G2 form encoded by the Ace locus on the third chromosome. To pursue our goal of replacing wild-type GPI-anchored AChE with forms that have alternative anchor structures in transgenic files, we report the construction of two secreted forms of Drosophila AChE (SEC1 and SEC2) and a chimeric form (TM-AChE) anchored by the transmembrane and cytoplasmic domains of herpes simplex virus type 1 glycoprotein C. To confirm that the biochemical properties of these AChEs were unchanged from GPI-AChE except as predicted, we made stably transfected Drosophila Schneider Line 2(S2) cells expressing each of the four forms. TM-AChE, SEC1, and SEC2 had the same catalytic activity and quaternary structure as wild type. TM-AChE was expressed as an amphiphilic membrane-bound protein resistant to an enzyme that cleaves GPI-AChE (phosphatidylinositol-specific phospholipase C), and the same percentage of TM-AChE and GPI-AChE was on the cell surface according to immunofluorescence and pharmacological data. SEC1 and SEC2 were constructed by truncating the C-terminal signal peptide initially present in GPI-AChE: in SEC1 the last 25 residues of this 34-residue peptide were deleted while in SEC2 the last 29 were deleted. Both SEC1 and SEC2 were efficiently secreted and are very stable in culture medium; with one cloned SEC1-expressing line, AChE accumulated to as high as 100 mg/liter. Surprisingly, 5-10% of SEC1 was attached to a GPI anchor, but SEC2 showed no GPI anchoring. Since no differences in catalytic activity were observed among the four AChEs, and since the same percentage of GPI-AChE and TM-AChE were on the cell surface, we contend that in vivo experiments in which GPI-AChE is replaced can be interpreted solely on the basis of the altered anchoring domain.

Glucose-induced sequential processing of a glycosyl-phosphatidylinositol-anchored ectoprotein in Saccharomyces cerevisiae

Transfer of spheroplasts from the yeast Saccharomyces cerevisiae to glucose leads to the activation of an endogenous (glycosyl)-phosphatidylinositol-specific phospholipase C ([G]PI-PLC), which cleaves the anchor of at least one glycosyl-phosphatidylinositol (GPI)-anchored protein, the cyclic AMP (cAMP)-binding ectoprotein Gce1p (G. Müller and W. Bandlow, J. Cell Biol. 122:325-336, 1993). Analyses of the turnover of two constituents of the anchor, myo-inositol and ethanolamine, relative to the protein label as well as separation of the two differently processed versions of Gce1p by isoelectric focusing in spheroplasts demonstrate the glucose-induced conversion of amphiphilic Gce1p first into a lipolytically cleaved hydrophilic intermediate, which is then processed into another hydrophilic version lacking both myo-inositol and ethanolamine. When incubated with unlabeled spheroplasts, the lipolytically cleaved intermediate prepared in vitro is converted into the version lacking all anchor constituents, whereby the anchor glycan is apparently removed as a whole. The secondary cleavage ensues independently of the carbon source, attributing the key role in glucose-induced anchor processing to the endogenous (G)PI-PLC. The secondary processing of the lipolytically cleaved intermediate of Gce1p at the plasma membrane is correlated with the emergence of a covalently linked high-molecular-weight form of a cAMP-binding protein at the cell wall. This protein lacks anchor components, and its protein moiety appears to be identical with double-processed Gce1p detectable at the plasma membrane in spheroplasts. The data suggest that glucose-induced double processing of GPI anchors represents part of a mechanism of regulated cell wall expression of proteins in yeast cells.

The limbic system-associated membrane protein (LAMP) selectively mediates interactions with specific central neuron populations

The limbic system-associated membrane protein (LAMP) is a 64–68 × 10(3) M(r) glycoprotein that is expressed by subsets of neurons that are functionally interconnected. LAMP exhibits characteristics that are indicative of a developmentally significant protein, such as an early and restricted pattern of expression and the ability to mediate specific fiber-target interactions. A potential, selective adhesive mechanism by which LAMP may regulate the formation of specific circuits is investigated in the present experiments. LAMP is readily released from intact membranes by phosphatidyl inositol-specific phospholipase C. Purified, native LAMP, isolated by PI-PLC digestion and immunoaffinity chromatography, is capable of mediating fluorescent Covasphere aggregation via homophilic binding. To test the ability of LAMP to selectively facilitate substrate adhesion and growth of neurons from LAMP-positive, in contrast to LAMP-negative regions of the developing brain, purified LAMP was dotted onto nitrocellulose-coated dishes and test cells plated. Limbic neurons from perirhinal cortex bind specifically to substrate-bound LAMP within 4 hours, forming small cell aggregates with short neuritic processes that continue to grow through a 48 hour period of monitoring. Preincubation of cells with anti-LAMP has a modest effect on cell binding but significantly reduces initiation of process growth. Non-limbic neurons from somatosensory cortex and olfactory bulb fail to bind or extend processes on the LAMP substrate to any significant extent. All cell populations bind equally well and form neurites on poly-D-lysine and laminin. The present results provide direct evidence that LAMP can specifically facilitate interactions with select neurons in the CNS during development. The data support the concept that patterned expression of unique cell adhesion molecules in functionally related regions of the mammalian brain can regulate circuit formation.

Activation of the glycosyl-phosphatidylinositol-anchored membrane dipeptidase upon release from pig kidney membranes by phospholipase C

Incubation of pig kidney microvillar membranes with Bacillus thuringiensis or Staphylococcus aureus phosphatidylinositol-specific phospholipase C (PI-PLC) resulted in the release of a number of glycosyl-phosphatidylinositol (GPI)-anchored hydrolases, including alkaline phosphatase (EC 3.1.3.1), amino-peptidase P (EC 3.4.11.9), membrane dipeptidase (EC 3.4.13.19), 5'-nucleotidase (EC 3.1.3.5) and trehalase (EC 3.2.1.28). Of these five ectoenzymes only for membrane dipeptidase was there a significant (approx. 100%) increase in enzymic activity upon release from the membrane. Maximal activation occurred at a PI-PLC concentration 10-fold less than that required for maximal release. In contrast solubilization of the membranes with n-octyl beta-D-glucopyranoside had no effect on the enzymic activity of membrane dipeptidase. A competitive e.l.i.s.a. with a polyclonal antiserum to membrane dipeptidase indicated that the increase in enzymic activity was not due to an increase in the amount of membrane dipeptidase protein. Although PI-PLC cleaved the GPI anchor of the affinity-purified amphipathic form of pig membrane dipeptidase there was no concurrent increase in enzymic activity. In the absence of PI-PLC, membrane dipeptidase in the microvillar membranes hydrolysed Gly-D-Phe with a Km of 0.77 mM and a Vmax. of 602 nmol/min per mg of protein. However, in the presence of a concentration of PI-PLC which caused maximal release from the membrane and maximal activation of membrane dipeptidase the Km was decreased to 0.07 mM while the Vmax. remained essentially unchanged at 624 nmol/min per mg of protein. Overall these results suggest that cleavage by PI-PLC of the GPI anchor on membrane dipeptidase may relax conformational constraints on the active site of the enzyme which exist when it is anchored in the lipid bilayer, thus resulting in an increase in the affinity of the active site for substrate.

Generation by limited proteolysis of a catalytically active 39-kDa protein from the 115-kDa form of phosphatidylinositol-glycan-specific phospholipase D from bovine serum

It has been suggested previously that small amounts of the mature 115-kDa form of phosphatidylinositol (PtdIns)-glycan-specific phospholipase D from bovine serum may exist as a 47-kDa form which can also be generated in vitro by treatment with proteases. In this study, we investigated the possible proteolytic processing by trypsin of partially purified PtdIns-glycan- specific phospholipase D from bovine serum and found that tryptic digestion caused an apparent activation of the enzyme when assayed in the presence of 0.1% (mass/vol.) Triton X-100. Trypsin cleaved the 115-kDa form of PtdIns-glycan-specific phospholipase D into three major polypeptides with molecular masses of 33, 39, and 47 kDa. Under non-denaturing conditions, the polypeptides remained tightly but noncovalently associated with each other. However, in the presence of 6 M urea, the polypeptides could be separated by anion-exchange chromatography. After renaturation, PtdIns-glycan-specific phospholipase D activity was found to be associated with a 39-kDa fragment. Based on its size and its amino acid sequence, the active-site-containing fragment consisted of approximately 275 residues of the N-terminal region of PtdIns-glycan-specific phospholipase D. The active 39-kDa fragment hydrolyzed the PtdIns-glycan-anchors of solubilized acetylcholinesterase from bovine erythrocytes and variant surface glycoprotein from blood stream trypanosomes. However, this fragment was inactive on membrane-associated acetylcholinesterase and PtdIns.

Expression of a glycosylphosphatidylinositol-anchored Trypanosoma brucei transferrin-binding protein complex in insect cells

The expression site-associated gene ESAG 6 was previously implicated in transferrin binding in the protozoan parasite Trypanosoma brucei. ESAG 6 and the closely related ESAG 7 of T. brucei strain AnTat1.3 have now been expressed in insect cells using the baculovirus expression system. Expression of ESAG 6 alone in insect cells gives rise to a glycosylated protein of approximately 52 kDa, which is cell surface-associated through a glycosylphosphatidylinositol anchor at its C terminus. The ESAG 7 product of about 42 kDa is also glycosylated, but lacks the glycosylphosphatidylinositol modification, and is located intracellularly. No transferrin-binding activity is observed when either ESAG is expressed independently. However, their expression results in a cell surface complex of ESAG 6 and 7 products that specifically binds transferrin. This shows that both ESAG 6 and 7 products are necessary and sufficient for binding to transferrin.

Lysophosphoinositide-specific phospholipase C in rat brain synaptic plasma membranes

A membrane preparation from rat brain catalyzed the hydrolysis of [2-3H]glycerol-labeled lysophosphatidylinositol (lysoPI) to yield monoacylglycerol (MG) and inositolphosphates. This phospholipase C activity had an optimal pH of 8.2. The membrane preparation did not require the addition of Ca2+ for its maximum activity, but the activity was inhibited by addition of 0.1 mM EDTA to the assay mixture and was restored by simultaneous addition of 0.2 mM Ca2+. The activity was found to be localized in synaptic plasma membranes prepared by Ficoll and Percoll density gradients. The phospholipase C was highly specific for lysoPI; diacylglycerol formation from phosphatidylinositol, and MG formation from lysophosphatidylcholine, lysophosphatidylethanolamine, and lysophosphatidylserine were below 5% of that observed with lysoPI under the conditions used. We concluded that there is a pathway for phosphatidylinositol metabolism in brain synaptic membranes which is different from the well-characterized phosphoinositide-specific phospholipase C pathway.

Photoaffinity labelling of cardiac membrane GTP-binding proteins in response to insulin

Plasma membranes from rat cardiac ventricular tissue and insulin receptors partially purified by wheat-germ-agglutinin chromatography were subjected to direct photoaffinity labelling with [alpha-32P]GTP in order to elucidate the presence of insulin-receptor-coupled GTP-binding proteins. In plasma membranes three proteins have been identified that exhibit an enhanced photolabelling with the nucleotide in response to insulin. The apparent molecular masses of these proteins were found to be 56, 60 and 74 kDa. Photolabelling of partially purified insulin receptors showed the copurification of the 60-kDa species, whereas the 56-kDa and 74-kDa proteins could not be detected. Furthermore, the 60-kDa G-protein was found to be specifically co-immunoprecipitated with the insulin receptor. Incubation of insulin receptors with insulin increased the labelling of the 60-kDa band to 205 +/- 27% (n = 5) of control. Immuno- and ligand-blotting experiments revealed the additional presence of a 39-kDa G(o)-like protein and two G-proteins with molecular masses of 24 and 26 kDa in the receptor preparation. Under basal conditions the insulin receptor and the 60-kDa G-protein exhibited an apparent inverse distribution between plasma and microsomal membranes with the G-protein being extensively labelled in the microsomal fraction. In conclusion, our data show that, in its native environment, the cardiac insulin receptor couples to at least three GTP-binding proteins. Out of these, a 60-kDa species of microsomal origin, copurifies with the insulin receptor. It is suggested that this G-protein is associated with the insulin receptor and may be involved in insulin receptor signalling in target cells.

Distribution of glycosylphosphatidylinositol-specific phospholipase D mRNA in bovine tissue sections

It has been reported that mammalian serum, and to a lower extent mammalian liver, brain, pancreas, udder, and milk, contain glycosylphosphatidylinositol-specific phospholipase D activity. However, the sites of synthesis have not been determined. In order to study in which cell(s) of the organism synthesis of glycosylphosphatidylinositol-specific phospholipase D takes place, we undertook a systematic screening of 12 different bovine tissues. In situ hybridization experiments with a specific anti-sense RNA probe, derived from a bovine liver cDNA, revealed that glycosylphosphatidylinositol-specific phospholipase D mRNA is present in mast cells of the adrenal gland, lung, and liver. On the other hand, our specific probe detected no mRNA in bovine pancreas, brain, and udder, although enzyme activity has been reported in these tissues. Northern blot analysis of total bovine liver RNA demonstrated two distinct glycosylphosphatidylinositol-specific phospholipase D mRNAs of approximately 3.3 kb and 4 kb length suggesting that two forms of the enzyme may exist.

Glycosylphosphatidylinositol-specific phospholipase D is localized in keratinocytes

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is abundant in mammalian plasma, but little is known of its cellular and tissue distribution. In this study frozen sections of perfused tissues from adult rats were stained with monoclonal antibodies against GPI-PLD. The most intense staining was observed in the stratified squamous epithelium of the forestomach. Staining was also observed in the esophagus, the tongue, the hard palate, and the skin but not in most other tissues including the columnar epithelium of the stomach or the lower gastrointestinal tract. GPI-PLD expression was also detected in several keratinocyte cell lines. Biochemical assays of glycosylphosphatidylinositol-degrading activity using [3H]myristate-labeled variant surface glycoprotein as substrate provided independent evidence for the presence of GPI-PLD. Expression of GPI-PLD by keratinocytes was not affected by culture in serum-free media, indicating that it does not originate by uptake of serum GPI-PLD in the media. These data suggest that keratinocytes are an important site of action of GPI-PLD and possibly a contributor to the plasma GPI-PLD pool.

5'-Nucleotidase: molecular structure and functional aspects

Images

Soluble low-Km 5'-nucleotidase from electric-ray (Torpedo marmorata) electric organ and bovine cerebral cortex is derived from the glycosyl-phosphatidylinositol-anchored ectoenzyme by phospholipase C cleavage

Soluble and membrane-bound low-Km 5'-nucleotidase was isolated from high-speed supernatants and membrane fractions derived from the electric organ of the electric ray (Torpedo marmorata) or from bovine brain cerebral cortex. Purification of both enzymes included chromatography on concanavalin A-Sepharose and AMP-Sepharose. The contribution to the total of soluble enzyme activity was lower in electric organ (1.6%) than in bovine cerebral cortex (27.9%). Membrane-bound and soluble forms have very similar Km values for AMP and are inhibited by micromolar concentrations of ATP. Both forms cross-react with, and are inhibited by, an antibody against the membrane-bound surface-located (ecto-) 5'-nucleotidase from electric organ. The HNK-1 carbohydrate epitope is present on both forms of the Torpedo enzyme, but is entirely absent from bovine cerebral-cortex 5'-nucleotidase. An antibody specific for the inositol 1,2-(cyclic)monophosphate that is formed on phospholipase C cleavage of an intact glycosyl-phosphatidylinositol (GPI) anchor binds to the soluble, but not to the membrane-bound, form of the enzyme from both sources. Our results suggest that soluble low-Km 5'-nucleotidase in both electric organ and bovine brain is derived from the membrane-bound GPI-anchored form of the enzyme by the action of a phospholipase C and is not a soluble cytoplasmic enzyme.

Phosphatidylinositol-glycan-specific phospholipase D is an amphiphilic glycoprotein that in serum is associated with high-density lipoproteins

Phosphatidylinositol (PtdIns)-glycan-specific phospholipase D was purified from bovine and human serum by phase separation in Triton X-114 and by chromatography on DEAE-cellulose, octyl-Sepharose, concanavalin-A-Sepharose, and hydroxyapatite. The purification of the two enzymes was approximately 1200-fold with a recovery of 3-5%. Bovine serum contained about 40 micrograms/ml of PtdIns-glycan-specific phospholipase D, about 10 times more than the amount determined in human serum. PtdIns-glycan-specific phospholipase D is also present in mammalian cerebrospinal fluid and in mammalian milk but to a much lesser extent than in serum. Enzyme from bovine and human serum displayed amphiphilic properties as revealed by sucrose density gradient centrifugation and gel filtration in the absence and presence of detergent. On density gradient centrifugation, both enzymes sedimented with an apparent sedimentation coefficient of about 6.0 S in the presence of 0.1% Triton X-100, and formed aggregates up to 14.5 S in the absence of detergent. Upon gel filtration, the bovine and human enzymes migrated with a Stokes' radius of 6.5 nm and 6.6 nm, respectively, in the presence of Triton X-100. In the absence of Triton X-100, both enzymes gave a Stokes' radius of 8.8 nm. Serial centrifugation of serum at increasing NaBr concentrations revealed that the majority of the enzyme is contained in the high-density lipoprotein fraction. PtdIns-glycan-specific phospholipase D from bovine and human serum contained 27 and 28 N-acetylglucosamine residues, respectively. Treatment with N-glycosidase F decreased the apparent molecular mass of the bovine and human enzyme from 115 and 123 kDa to 91 and 87 kDa, respectively. Sequence analysis of peptides derived from PtdIns-glycan-specific phospholipase D of bovine serum by CNBr cleavage gave 100% identity to the sequence published for the bovine liver enzyme while there was 83% similarity and 74% identity to the sequence of peptides obtained from the human serum enzyme.

Delivery of an accessory signal for cell activation by exogenous phosphatidylinositol-specific phospholipase C

Digestion of phosphatidylinositol (PI) or glycosylphosphatidylinositol (GPI) anchors of membrane proteins on the external cell surface with exogenous PI-specific phospholipase C (PIPLC) from Bacillus thuringiensis was shown to transmit a signal into the thymocyte to modulate the TCR/CD3 complex-induced signal delivery for cell activation. This was demonstrated for very early protein tyrosine phosphorylation, early c-fos transcription and late DNA synthesis. For this effect preincubation of the cells with PIPLC was required, but there was no evidence of involvement of any soluble products released from the cell surface by PIPLC in the signaling, suggesting a crucial role of the membrane-bound counterpart (diacylglycerol or diradylglycerol) of the PI/GPI hydrolysate. A possible role for this accessory signal in the microorganism-linked control of the (diacylglycerol or diradylglycerol) of the PI/GPI hydrolysate. A possible role for this accessory signal in the microorganism-linked control of the T cell receptor function is discussed.

Membrane association and shedding of the GPI-anchored Ca-MOv18 antigen in human ovary carcinoma cells

The antigen recognized by the MOv18 MAb (Ca-MOv18) was recently shown to be a glycosylphosphatidylinositol (GPI)-linked protein. In this report we show that GPI-anchorage is not limited to IGROVI cells nor to other ovary carcinoma cell lines, but Ca-MOv18 was also found to be sensitive to phosphatidylinositol-specific phospholipase C (PI-PLC) treatment on fresh ovarian cancer cells. Furthermore, we found a heterogeneous sensitivity of Ca-MOv18 to PI-PLC cleavage, not only among the different cells studied but also in different experiments performed on the same cell line, during extended periods of time in culture. Sensitivity to PI-PLC cleavage was determined by immunofluorescence on live cells and by double-determinant radioimmunoassay of the antigen released in the supernatant. The specificity of the PI-PLC cleavage was demonstrated as follows: (a) TX114 solubilized Ca-MOv18 shifts from the detergent to the aqueous phase after treatment with PI-PLC; (b) on membrane preparations, PI-PLC specifically released a fraction of the antigen, which is distinct from the weakly associated form released by high-salt treatment; (c) Ca-MOv18 from IGROVI expressed the cross-reacting determinant (CRD), which is characteristic of GPI-linked molecules. The absence of CRD expression on the spontaneously released protein and the possibility of artificially inducing antigen shedding during the resynthesis of Ca-MOv18 which follows bacterial PI-PLC treatment are interesting points which need to be further investigated in order to understand the physiology of the Ca-MOv18 tumor antigen.

Control of alkaline phosphatase activity in C3H10T1/2 cells: role of retinoic acid and cell density

The enzyme alkaline phosphatase (AP) has been shown to be lost or inappropriately expressed during carcinogenesis in some tissues. Because retinoic acid (RA) appears to play a role in the normal regulation of the enzyme (RA up-regulates AP in a variety of cell types) we have suggested that altered AP expression in some cancers may be caused by a defect in the ability of the cells to respond normally to retinoid. We have begun to use the chemically transformable mouse embryo fibroblast cell, C3H10T1/2, to investigate this possibility. In this initial study we characterized AP regulation in normal C3H10T1/2 cells and show that: (1) 10(-7) M RA increases AP activity within 3-4 h in serum-free medium; (2) serum inhibits short-term induction (0-8 h) in a concentration-dependent manner (10% serum causes complete inhibition); (3) during long-term RA exposure (24 h and 48 h), induction can be detected in serum-containing medium; (4) AP induction is dose related at RA concentrations from 10(-10) M to 10(-6) M in serum-free medium; (5) 10(-5) M RA is ineffective at inducing AP in serum-free medium during 8 h but is the most effective concentration in serum-containing medium during 24 h and 48 h exposures; (6) AP inducibility by RA requires near-confluent cell densities; and (7) when cultures become confluent, cells become constitutive for AP and no longer require RA for enzyme expression. The effects of serum and cell density on AP inducibility by RA and implications of the RA up-regulation of AP for teratogenesis are discussed.

Release of cell surface-associated basic fibroblast growth factor by glycosylphosphatidylinositol-specific phospholipase C

Heparan sulfate proteoglycans (HSPG) are ubiquitous constituents of mammalian cell surfaces and most extracellular matrices. A portion of the cell surface HSPG is anchored via a covalently linked glycosyl-phosphatidylinositol (Pl) residue, which can be released by treatment with a glycosyl-Pl specific phospholipase C (Pl-PLC). We report that exposure of bovine aortic endothelial and smooth muscle cells to Pl-PLC resulted in release of cell surface-associated, growth-promoting activity that was neutralized by antibasic fibroblast growth factor (bFGF) antibodies. Active bFGF was also released by treating the cells with bacterial heparitinase. Under the same conditions there was no release of mitogenic activity from cells (BHK-21, NIH/3T3, PF-HR9) that expressed little or no bFGF, as opposed to Pl-PLC-mediated release of active bFGF from the same cells transfected with the bFGF gene. The released bFGF competed with recombinant bFGF in a radioreceptor assay. Addition of Pl-PLC to sparsely seeded vascular endothelial cells resulted in a marked stimulation of cell proliferation, but there was no mitogenic effect of Pl-PLC on 3T3 fibroblasts. Studies with exogenously added 125I-bFGF revealed that about 6.5% and 20% of the cell surface-bound bFGF were released by treatment with Pl-PLC and heparitinase, respectively. Both enzymes also released sulfate-labeled heparan sulfate from metabolically labeled 3T3 fibroblasts. Pl-PLC failed to release 125I-bFGF from the subendothelial extracellular matrix (ECM), as compared to release of 60% of the ECM-bound bFGF by heparitinase. Our results indicate that 3-8% of the total cellular content of bFGF is associated with glycosyl-Pl anchored cell surface HSPG. This FGF may exert both autocrine and paracrine effects, provided that it is released by Pl-PLC and adequately presented to high affinity bFGF cell surface receptor sites.

Insulin receptor signaling through non-tyrosine kinase pathways: evidence from anti-receptor antibodies and insulin receptor mutants

Although there is general agreement that insulin receptor tyrosine kinase activity mediates many of the actions of insulin, two types of studies suggest that non-tyrosine kinase dependent pathways may also exist. First, both monoclonal and polyclonal antibodies to the receptor have been shown to mediate many of insulin's actions with little or no stimulation of receptor kinase. Second, insulin receptor mutants, with reduced or no tyrosine kinase activity, have been shown to mediate several actions of insulin. Non-tyrosine kinase pathways that could signal insulin effects through the insulin receptor include non-covalent activation of G proteins, phospholipase Cs, or docking proteins such as IRS-1. Further studies on the chemical structures of phospholipids and their hydrolysis products involved in insulin action will be required to sort out the underlying mechanisms of insulin action via non-tyrosine kinase dependent pathways.

The insulin receptor: signalling mechanism and contribution to the pathogenesis of insulin resistance

The insulin receptor is a heterotetrameric structure consisting of two alpha-subunits of Mr 135 kilodalton on the outside of the plasma membrane connected by disulphide bonds to beta-subunits of Mr 95 kilodalton which are transmembrane proteins. Insulin binding to the alpha-subunit induces conformational changes which are transduced to the beta-subunit. This leads to the activation of a tyrosine kinase activity which is intrinsic to the cytoplasmatic domains of the beta-subunit. Activation of the tyrosine kinase activity of the insulin receptor represents an essential step in the transduction of an insulin signal across the plasma membrane of target cells. Signal transduction on the post-kinase level is not yet understood in detail, possible mechanisms involve phosphorylation of substrate proteins at tyrosine residues, activation of serine kinases, the interaction with G-proteins, phospholipases and phosphatidylinositol kinases. Studies in multiple insulin-resistant cell models have demonstrated that an impaired response of the tyrosine kinase to insulin stimulation is one potential mechanism causing insulin resistance. An impairment of the insulin effect on tyrosine kinase activation in all major target tissues of insulin, in particular the skeletal muscle was demonstrated in Type 2 (non-insulin-dependent) diabetic patients. There is no evidence that the impaired tyrosine kinase response in the skeletal muscle is a primary defect, however, it is likely that this abnormality of insulin signal transduction contributes significantly to the pathogenesis of the insulin-resistant state in Type 2 diabetes.

Factors affecting the ability of glycosylphosphatidylinositol-specific phospholipase D to degrade the membrane anchors of cell surface proteins

Mammalian serum and plasma contain high levels of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD). Previous studies with crude serum or partially purified GPI-PLD have shown that this enzyme is capable of degrading the GPI anchor of several purified detergent-solubilized cell surface proteins yet is unable to act on GPI-anchored proteins located in intact cells. Treatment of intact ROS17/2.8, WISH or HeLa cells (or membrane fractions prepared from them) with GPI-PLD purified from bovine serum by immunoaffinity chromatography gave no detectable release of alkaline phosphatase into the medium. However, when membranes were treated with GPI-PLD in the presence of 0.1% Nonidet P-40 substantial GPI anchor degradation (as measured by Triton X-114 phase separation) was observed. The mechanism of this stimulatory effect of detergent was further investigated using [3H]myristate-labelled variant surface glycoprotein and human placental alkaline phosphatase reconstituted into phospholipid vesicles. As with the cell membranes the reconstituted substrates exhibited marked resistance to the action of purified GPI-PLD which could be overcome by the inclusion of Nonidet P-40. Similar results were obtained when crude bovine serum was used as the source of GPI-PLD. These data indicate that the resistance of cell membranes to the action of GPI-PLD is not entirely due to the action of serum or membrane-associated inhibitory factors. A more likely explanation is that, in common with many other eukaryotic phospholipases, the action of GPI-PLD is restricted by the physical state of the phospholipid bilayer in which the substrates are embedded. These data may account for the ability of endothelial and blood cells to retain GPI-anchored proteins on their surfaces in spite of the high levels of GPI-PLD present in plasma.

Insulin activates GTP binding to a 40 kDa protein in fat cells

The first steps in insulin action are binding of insulin to its receptor and activation of the insulin receptor kinase. As there is indirect evidence that further signal transduction might involve a guanine-nucleotide-binding protein (G-protein), we studied whether insulin modulates GTP binding to plasma membrane proteins of fat cells and skeletal muscle. We found that insulin rapidly increased (30 s) binding of guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in a dose dependent manner (0.03-2.0 nM). This effect was not altered by pertussis toxin, but it was abolished by cholera toxin treatment of fat cells. Scatchard analysis of the binding data showed that the increased GTP[S] binding is due to a decrease in the Kd for GTP from 100 nM to 50 nM. Furthermore, binding of GTP to these plasma membranes inhibited both the binding of 125I-insulin to the insulin receptor and the stimulation of the insulin receptor kinase, suggesting a feedback interaction between the insulin-stimulated GTP-binding site and the insulin receptor. In order to identify this insulin-stimulated GTP-binding site, plasma membranes were labelled with the photoreactive GTP analogue [alpha-32P]GTP gamma-azidoanilide. We found that insulin selectively stimulated GTP binding to a 40 kDa protein. In conclusion, in plasma membranes of fat cells and skeletal muscle, the insulin receptor interacts with a 40 kDa GTP-binding site. We speculate that this 40 kDa GTP-binding site might be a G-protein which is involved in insulin signal transmission.

Enzymatic properties of phosphatidylinositol-glycan-specific phospholipase C from rat liver and phosphatidylinositol-glycan-specific phospholipase D from rat serum

Using phosphatidylinositol-glycan (PtdIns-glycan) anchored acetylcholinesterase from bovine erythrocytes as substrate, we found PtdIns-glycan-anchor-degrading activity in rat liver and serum [corrected]. The hepatic enzyme was only soluble in detergents, whereas the serum enzyme occurs as soluble, slightly amphiphilic protein. Using 3-trifluoromethyl-3-(m- [125I]iodophenyl)diazirine-labelled acetylcholinesterase as substrate, we showed that the hepatic anchor-degrading enzyme had a cleavage specificity of a phospholipase C, whereas the serum enzyme was a phospholipase D. Both enzyme exhibited maximal activity in slightly acidic conditions and at low ionic strength. They had a high affinity for the PtdIns-glycan anchor of the substrate (Km = 0.1 microM and 0.16 microM, respectively). Both hepatic PtdIns-glycan-specific phospholipase C and serum PtdIns-glycan-specific phospholipase D gave a large increase in activity between 0.1-10 microM Ca2+, indicating that PtdIns-glycan-specific phospholipases are only marginally active at physiological intracellular Ca2+ concentrations. The enzymes were inhibited by heavy metal chelating agents such as 1,10-phenanthroline and 2,2'-bipyridyl but not by the corresponding Fe2+ complexes or non-chelating analogues, indicating that they both require a heavy metal ion for the expression of catalytic activity in addition to Ca2+. Another interesting property of PtdIns-glycan-specific phospholipases is their inactivation by bicarbonate and cyanate. The inactivation was time- and pH-dependent and could be reversed by dialysis. These observations are in agreement with a covalent modification of the enzymes by carbamoylation.

The role of glycosyl-phosphoinositides in hormone action

Despite significant advances in the past few years on the chemistry and biology of insulin and its receptor, the molecular events that couple the insulin-receptor interaction to the regulation of cellular metabolism remain uncertain. Progress in this area has been complicated by the pleiotropic nature of insulin's actions. These most likely involve a complex network of pathways resulting in the coordination of mechanistically distinct cellular effects. Since the well-recognized mechanisms of signal transduction (i.e., cyclic nucleotides, ion channels) appear not to be central to insulin action, investigators have searched for a novel second messenger system. A low-molecular-weight substance has been identified that mimics certain actions of insulin on metabolic enzymes. This substance has an inositol glycan structure, and is produced by the insulin-sensitive hydrolysis of a glycosyl-phosphatidylinositol in the plasma membrane. This hydrolysis reaction, which is catalyzed by a specific phospholipase C, also results in the production of a structurally distinct diacylglycerol that may selectively regulate one or more of the protein kinases C. The glycosyl-phosphatidylinositol precursor for the inositol glycan enzyme modulator is structurally analogous to the recently described glycosyl-phosphatidylinositol membrane protein anchor. Preliminary studies suggest that a subset of proteins anchored in this fashion might be released from cells by a similar insulin-sensitive, phospholipase-catalyzed reaction. Future efforts will focus on the precise role of the metabolism of glycosyl-phosphatidylinositols in insulin action.

The soluble 'low-Km' 5'-nucleotidase of rat kidney represents solubilized ecto-5'-nucleotidase

A soluble 'low-Km' 5'-nucleotidase has been described previously in several organs. It has been presumed to be of cytosolic origin and thus to play a role in the intracellular production of adenosine. Its catalytic properties are similar to those of the ecto-5'-nucleotidase of cell membranes. In the present study we compared molecular properties of the two enzymes in the kidney of the rat. The Mr of the main peak of soluble 'low-Km' 5'-nucleotidase in gel-filtration chromatography was similar to that of the ecto-5'-nucleotidase solubilized by a phosphatidylinositol-specific phospholipase C from renal brush-border membranes. In phase-partition experiments using Triton X-114, the soluble enzyme appeared to be hydrophobic. Its hydrophobicity was decreased on treatment with a phosphatidylinositol-specific phospholipase C, suggesting that the soluble 'low-Km' 5'-nucleotidase contains the phosphatidylinositol anchor which is characteristic for the ecto-enzyme. An anti-ecto-5'-nucleotidase antiserum provoked an almost complete inhibition of the soluble enzyme. Immunoblotting using anti-ecto-5'-nucleotidase antiserum revealed in the high-speed supernatants a polypeptide with a similar Mr to the subunit of the ecto-5'-nucleotidase. The soluble 'low-Km' 5'-nucleotidase, like the ecto-5'-nucleotidase, bound specifically to concanavalin A. We conclude that the soluble 'low-Km' 5'-nucleotidase is not a cytosolic enzyme, but that it most probably originates from the solubilization of the ecto-5'-nucleotidase, and that it therefore cannot participate in the intracellular production of adenosine.

Short-term action of insulin on Aplysia neurons: generation of a possible novel modulator of ion channels

In mollusks as in other animals, peptides can act as hormones, growth factors, and neurotransmitters. The presence of insulin in vertebrate brain as well as its actions on nerve cells led us to examine the electrophysiological effects of the mammalian hormone on Aplysia neurons. Application of insulin extracellularly causes hyperpolarization of L14 and L10, identified neurons of the abdominal ganglion. This hyperpolarization is associated with a decreased membrane conductance that reverses at -35 mV. We also injected inositol phosphate glycan (IPG) into the identified neurons. This complex sugar, which was purified from rat liver and which is a putative second messenger for insulin in nonneural vertebrate cells (Saltiel and Cuatrecasas, 1986; Saltiel, Osterman, and Darnell, 1988), causes hyperpolarization with decreased membrane conductance in L14 and L10 similar to the effects of insulin. Furthermore, exposure of isolated ganglia to insulin results in the generation of IPG with a compensating decrease in its glycosyl-phosphatidylinositol precursor. We suggest that, in addition to its other roles, insulin may function as a neuropeptide transmitter using IPG as a second messenger.

Correction of abnormal small intestinal cytosolic protein kinase C activity in streptozotocin-induced diabetes by insulin therapy

Diabetes was induced in rats by administration of a single intraperitoneal injection of streptozotocin (50 mg/kg body wt). After 7 days, one group of diabetic animals was treated with insulin for an additional 5 days. Control, diabetic and diabetic + insulin rats were then killed, their distal small intestines were removed and the epithelial cells were examined and compared with respect to polyphosphoinositide turnover, total protein kinase C activity and cellular distribution, and 1,2-diacylglycerol mass and production. The results of these experiments demonstrated that, compared with their control counterparts, the intestines from diabetic rats had a decreased turnover of polyphosphoinositides, but an increase in 1,2-diacylglycerol mass which was a result, at least in part, of an increase in the synthesis of this lipid de novo. Total protein kinase C activity was decreased in the diabetic rats due to a decrease in cytosolic activity, with no significant change in particulate activity. Moreover, insulin administration for 5 days to diabetic animals did not affect their lowered intestinal polyphosphoinositide turnover, but did further accentuate their increased 1,2-diacylglycerol mass and synthesis de novo; this treatment also corrected total protein kinase C activity by increasing the cytosolic activity of this enzyme. These results indicate that signalling mechanisms involving polyphosphoinositides, 1,2-diacylglycerol and protein kinase C are abnormal in the intestines of diabetic rats and that some of these biochemical parameters can be modulated by insulin administration in vivo.