Identification of glycosylphosphatidylinositol-specific phospholipases C in mouse brain membranes

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.

Dual fluorescent protein reporters for studying cell behaviors in vivo

Fluorescent proteins (FPs) are useful tools for visualizing live cells and their behaviors. Protein domains that mediate subcellular localization have been fused to FPs to highlight cellular structures. FPs fused with histone H2B incorporate into chromatin allowing visualization of nuclear events. FPs fused to a glycosylphosphatidylinositol anchor signal sequence label the plasma membrane, highlighting cellular shape. Thus, a reporter gene containing both types of FP fusions would allow for effective monitoring of cell shape, movement, mitotic stage, apoptosis, and other cellular activities. Here, we report a binary color-coding system using four differently colored FP reporters that generates 16 distinct color codes to label the nuclei and plasma membranes of live cells in culture and in transgenic mice. As an initial test of this system in vivo, the promoter of the human Ubiquitin C (UBC) gene was used to widely express one of the color-code reporters. Widespread expression of the reporter was attained in embryos; however, both male and female transgenic mice were infertile. In contrast, the promoter of the mouse Oct4/Pou5f1 gene linked to two different color-code reporters specifically labeled blastocysts, primordial germ cells, and postnatal germ cells, and these mice were fertile. Time-lapse movies of fluorescently-labeled primordial germs cells demonstrate the utility of the color-code system to visualize cell behaviors. This set of new FP reporters should be a useful tool for labeling distinct cell populations and studying their behaviors in complex tissues in vivo.

Release of renal dipeptidase from Glycosylphosphatidylinositol anchor by insulin-triggered phospholipase c/intracellular Ca2+

Glycosylphosphatidylinositol (GPI) anchored proteins appear to be released from the plasma membrane due to various extracellular stimuli. To determine the signaling pathway from insulin to GPI-protein, the release of GPI-renal dipeptidase (RDPase, EC 3.4.13.19) from porcine proximal tubules, stimulated by insulin, was explored. Insulin stimulated the release of RDPase in a concentration-dependent manner (half maximal release at 0.58 nM), which peaked at 10-20 min. Western blot analysis, with antibody against the cross-reacting determinant (CRD), revealed that RDPase was released by a GPI-specific phospholipase C (GPI-PLC), and was shown to be Ca2+-dependent. A PI-PLC inhibitor, U73122, effectively blocked the effect of insulin on the release of RDPase, suggesting insulin is associated with an intracellular PI-PLC. Insulin treatment increased the production of intracellular Ca2+ from porcine proximal tubules. Intracellular Ca2+, coupled with insulin, facilitated the releases of RDPase, an inhibitor of inositol trisphosphate-dependent Ca2+ from the endoplasmic reticulum, and a Ca2+ channel blocker that blocked the effect of insulin. Taken together, these results suggest that insulin, in part, may activate a GPI-PLC, via PI-PLC/intracellular Ca2+, which may consequently stimulate the release of RDPase.

The role of repulsive guidance molecules in the embryonic and adult vertebrate central nervous system

During the development of the nervous system, outgrowing axons often have to travel long distances to reach their target neurons. In this process, outgrowing neurites tipped with motile growth cones rely on guidance cues present in their local environment. These cues are detected by specific receptors expressed on growth cones and neurites and influence the trajectory of the growing fibres. Neurite growth, guidance, target innervation and synapse formation and maturation are the processes that occur predominantly but not exclusively during embryonic or early post-natal development in vertebrates. As a result, a functional neural network is established, which is usually remarkably stable. However, the stability of the neural network in higher vertebrates comes at an expensive price, i.e. the loss of any significant ability to regenerate injured or damaged neuronal connections in their central nervous system (CNS). Most importantly, neurite growth inhibitors prevent any regenerative growth of injured nerve fibres. Some of these inhibitors are associated with CNS myelin, others are found at the lesion site and in the scar tissue. Traumatic injuries in brain and spinal cord of mammals induce upregulation of embryonic inhibitory or repulsive guidance cues and their receptors on the neurites. An example for embryonic repulsive directional cues re-expressed at lesion sites in both the rat and human CNS is provided with repulsive guidance molecules, a new family of directional guidance cues.

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.

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.

Cell signalling by inositol phosphoglycans from different species

The discovery of glycosyl-phosphatidylinositol (GPI) molecules and their products has given new insight into the field of signal transduction. In the last decade a novel mechanism of protein attachment to membranes has emerged, which involves a covalent linkage of the protein to the glycan moiety of a GPI. The discovery that GPI-anchored proteins are ubiquitous throughout the eukaryotes was followed by the observation that uncomplexed GPI molecules are implicated in signal transduction for a diversity of hormones and growth factors. The hydrolysis of free-GPI generates a novel second messenger: the inositol phosphoglycan (IPG). The aim of this article is to review the role of IPG and IPG-like molecules in signal transduction and to discuss future research directions.

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.

Expression of a glycosyl phosphatidylinositol-anchored adhesion molecule, the glycoprotein F3, in the adult rat hypothalamo-neurohypophysial system

The F3 cell surface glycoprotein consists of six immunoglobulin-like domains, four fibronectin type III repeats and a glycosylphosphatidylinositol anchor and is found in membrane-bound and soluble form. Until now, it has been localized mainly on axons of subsets of developing and postnatal neurons and has been implicated in axonal growth and synaptogenesis. We here examined its expression in the adult rat hypothalamo-neurohypophysial system composed of magnocellular neurons whose axons can undergo remodelling in adulthood in response to lesion or physiological stimulation. Immunoblot analyses demonstrated high levels of F3 immunoreactivity in the hypothalamic nuclei containing the somata of the neurons, in the median eminence, through which pass their axons and in the neurohypophysis, where they terminate. The amount of F3 detected in the latter was 2-fold that in the hypothalamus. In addition, soluble forms predominated in the neurohypophysis and GPI-linked forms in the hypothalamus. Immunocytochemistry revealed a strong F3 immunoreactivity throughout the neurohypophysis and internal layer of the median eminence, characterized by a punctate labeling of fibers and dense filling of dilatations. In the hypothalamic nuclei, staining of variable intensity was visible in the cytoplasm of some magnocellular somata. In contrast, in colchicine-treated rats, all magnocellular somata throughout the hypothalamus displayed intense labeling while staining in the neurohypophysis was greatly reduced. Our observations reveal that neurons of the adult hypothalamo-neurohypophysial system express high level of F3, even under normal conditions. In view of its distribution and the differing proportions of membrane-bound and soluble forms, we propose that, after synthesis in the hypothalamus, F3 is targeted to the neurohypophysis where it accumulates in neurosecretory terminals or is released into the extracellular space. It remains to be seen whether its expression is linked to the secretion of the neurohypophysial peptides and in particular, to the ability of these neurons to undergo structural remodelling in adulthood.

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.

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.

Axonal glycoproteins with immunoglobulin- and fibronectin type III-related domains in vertebrates: structural features, binding activities, and signal transduction

The L1- and F11-like axonal glycoproteins, implicated in neurite outgrowth and fasciculation, are members of the Ig superfamily comprising multiple fibronectin type III-like domains. Their Ig-like and fibronectin type III-related domains are likely to be composed of seven beta-strands arranged in two opposing beta-sheets of highly similar topology. Whereas the F11-like molecules lack a transmembrane sequence and are anchored in the plasma membrane by a glycosylphosphatidylinositol, the L1-like molecules comprise cytoplasmic domains with highly conserved sequence motifs. Most of the latter proteins occur in different isoforms generated by alternative pre-mRNA splicing, which has not been documented for molecules of the F11 subgroup. L1-like proteins undergo heterophilic as well as homophilic interactions, whereas only the former mode of binding was observed for F11-like proteins. Evidence is accumulating that these Ig superfamily molecules with fibronectin type III-like domains are interacting in a complex manner with each other and molecules of the extracellular matrix. Investigations assigning structure to function reveal that their individual extracellular domains serve distinct binding activities. Recent studies also suggest that L1 and NCAM are implicated in the transduction of transmembrane signals.

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.

Detection and partial characterization of glycosylphosphatidylinositol-specific phospholipase activities from Fasciola hepatica and Schistosoma mansoni

The tegument of trematodes serves as a dynamic host-parasite interface where surface antigens are shed in a process of immune evasion. Phospholipases, which could provide an enzymatic mechanism for release of glycosylphosphatidylinositol (GPI)-anchored proteins, were detected in detergent extracts of adult worms of Fasciola hepatica and cercaria and adult worms of Schistosoma mansoni. The enzymatic activities were partially characterized from both adult worm species and demonstrated a preference for [3H]GPI substrate over [3H]PI. Lipase activities from both species were sensitive to sulfhydryl-modifying reagents and the detergents CHAPS and n-octylglucoside. The presence of 1 M ammonium sulfate increased the enzyme activity in adult worms of both species by 8-11-fold and in cercaria by 146-fold, whereas other conditions of high ionic strength were inhibitory. Such stimulation suggested dissociation of a negative inhibitor which is prominent in the cercarial stage. The schistosome extract, which was partially sensitive to cation chelators and o-phenanthroline, contained a GPI-phospholipase D activity. In contrast, the F. hepatica extract contained a cation-independent phospholipase C activity which was partially purified and shown by gel filtration to have a molecular mass of 30,000-80,000.

Membrane-bound choline-O-acetyltransferase in rat hippocampal tissue is anchored by glycosyl-phosphatidylinositol

In an earlier study, we presented evidence to suggest that some of the particulate choline-O-acetyltransferase (ChAT) in rat hippocampal tissue might be linked to membranes by a glycosyl-phosphatidylinositol (GPI) anchor. In the present report, we attempted to determine if any of this GPI-anchored ChAT might be intracellular. Internalization of phosphatidylinositol-specific phospholipase C (PI-PLC) from Bacillus thuringiensis into rat hippocampal synaptosomes by the DMSO (dimethyl sulfoxide) freeze/thawing procedure caused an increase in cytosolic and a decrease in membrane-bound ChAT. Incubation of a plasma membrane enriched subcellular fraction at 16 degrees C relative to 4 degrees C led to a conversion of the membrane-bound, amphiphilic ChAT into hydrophilic ChAT. This conversion was blocked by zinc, an inhibitor of GPI-PLC. The cytosolic fraction of ChAT immunoreacted on western blots with an antibody directed against the cross-reacting determinant (CRD) of the GPI anchor. We suggest that some of the membrane-bound ChAT in rat hippocampal tissue is GPI-anchored intracellularly; also, that an endogenous GPI-PLC-like enzyme acts to release it into the cytosol.

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.

Removal of C6 astrocytoma cell surface molecules with phosphatidylinositol phospholipase C: effect on regulation of neural cell adhesion molecule isoforms

Different membrane-associated isoforms of the neural cell adhesion molecules have been described. One of them, N-CAM120, has been shown to be anchored to the membranes by a complex glycan-phosphatidylinositol group and to be releasable, under soluble form, by the bacterial enzyme phosphatidylinositol-phospholipase C. We used the C6 rat astrocytoma cell line expressing both N-CAM120 and the transmembrane isoform N-CAM140 as a model system. We investigated whether artificial depletion of cell membrane N-CAM120 influences the synthesis and the messenger RNA transcript levels of the isoforms of the neural cell adhesion molecules. Our results showed an increase in the rate of N-CAM120 protein synthesis, whereas the expression of N-CAM140 decreased. Additionally, perturbations in the levels of the 6.7-kb messenger RNA encoding for N-CAM140 were observed, whereas the 2.7-kb transcript encoding for N-CAM120 remained stable. Examination of the time course for the reexpression of N-CAM120 showed that control levels were recovered after 24 h. We provide evidence that N-CAM120 spontaneously released in the culture medium is not incorporated into the extracellular matrix; however, its concentration is important because, if the medium was changed, cells rapidly released a new pool.

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.