Purification of a novel calcium-independent phospholipase A2 from rabbit kidney

Synthesis and Significance of Arachidonic Acid, a Substrate for Cyclooxygenases, Lipoxygenases, and Cytochrome P450 Pathways in the Tumorigenesis of Glioblastoma Multiforme, Including a Pan-Cancer Comparative Analysis

Glioblastoma multiforme (GBM) is one of the most aggressive gliomas. New and more effective therapeutic approaches are being sought based on studies of the various mechanisms of GBM tumorigenesis, including the synthesis and metabolism of arachidonic acid (ARA), an omega-6 polyunsaturated fatty acid (PUFA). PubMed, GEPIA, and the transcriptomics analysis carried out by Seifert et al. were used in writing this paper. In this paper, we discuss in detail the biosynthesis of this acid in GBM tumors, with a special focus on certain enzymes: fatty acid desaturase (FADS)1, FADS2, and elongation of long-chain fatty acids family member 5 (ELOVL5). We also discuss ARA metabolism, particularly its release from cell membrane phospholipids by phospholipase A₂ (cPLA₂, iPLA₂, and sPLA₂) and its processing by cyclooxygenases (COX-1 and COX-2), lipoxygenases (5-LOX, 12-LOX, 15-LOX-1, and 15-LOX-2), and cytochrome P450. Next, we discuss the significance of lipid mediators synthesized from ARA in GBM cancer processes, including prostaglandins (PGE₂, PGD₂, and 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂)), thromboxane A₂ (TxA₂), oxo-eicosatetraenoic acids, leukotrienes (LTB₄, LTC₄, LTD₄, and LTE₄), lipoxins, and many others. These lipid mediators can increase the proliferation of GBM cancer cells, cause angiogenesis, inhibit the anti-tumor response of the immune system, and be responsible for resistance to treatment.

The cyclin D1-CDK4 oncogenic interactome enables identification of potential novel oncogenes and clinical prognosis

Overexpression of cyclin D1 and its catalytic partner, CDK4, is frequently seen in human cancers. We constructed cyclin D1 and CDK4 protein interaction network in a human breast cancer cell line MCF7, and identified novel CDK4 protein partners. Among CDK4 interactors we observed several proteins functioning in protein folding and in complex assembly. One of the novel partners of CDK4 is FKBP5, which we found to be required to maintain CDK4 levels in cancer cells. An integrative analysis of the extended cyclin D1 cancer interactome and somatic copy number alterations in human cancers identified BAIAPL21 as a potential novel human oncogene. We observed that in several human tumor types BAIAPL21 is expressed at higher levels as compared to normal tissue. Forced overexpression of BAIAPL21 augmented anchorage independent growth, increased colony formation by cancer cells and strongly enhanced the ability of cells to form tumors in vivo. Lastly, we derived an Aggregate Expression Score (AES), which quantifies the expression of all cyclin D1 interactors in a given tumor. We observed that AES has a prognostic value among patients with ER-positive breast cancers. These studies illustrate the utility of analyzing the interactomes of proteins involved in cancer to uncover potential oncogenes, or to allow better cancer prognosis.

Calcium-independent phospholipases A2 and their roles in biological processes and diseases

Among the family of phospholipases A2 (PLA2s) are the Ca(2+)-independent PLA2s (iPLA2s) and they are designated group VI iPLA2s. In relation to secretory and cytosolic PLA2s, the iPLA2s are more recently described and details of their expression and roles in biological functions are rapidly emerging. The iPLA2s or patatin-like phospholipases (PNPLAs) are intracellular enzymes that do not require Ca(2+) for activity, and contain lipase (GXSXG) and nucleotide-binding (GXGXXG) consensus sequences. Though nine PNPLAs have been recognized, PNPLA8 (membrane-associated iPLA2γ) and PNPLA9 (cytosol-associated iPLA2β) are the most widely studied and understood. The iPLA2s manifest a variety of activities in addition to phospholipase, are ubiquitously expressed, and participate in a multitude of biological processes, including fat catabolism, cell differentiation, maintenance of mitochondrial integrity, phospholipid remodeling, cell proliferation, signal transduction, and cell death. As might be expected, increased or decreased expression of iPLA2s can have profound effects on the metabolic state, CNS function, cardiovascular performance, and cell survival; therefore, dysregulation of iPLA2s can be a critical factor in the development of many diseases. This review is aimed at providing a general framework of the current understanding of the iPLA2s and discussion of the potential mechanisms of action of the iPLA2s and related involved lipid mediators.

Comparative study of serine-plasmalogens in human retina and optic nerve: identification of atypical species with odd carbon chains

The objective of this work was to detect and identify phosphatidylserine plasmalogen species in human ocular neurons represented by the retina and the optic nerve. Plasmalogens (vinyl-ether bearing phospholipids) are commonly found in the forms of phosphatidylcholine and phosphatidylethanolamine in numerous mammalian cell types, including the retina. Although their biological functions are unclear, the alteration of cellular plasmalogen content has been associated with several human disorders such as rhizomelic chondrodysplasia punctata Type 2 and primary open-angle glaucoma. By using liquid chromatography coupled to high-resolution and tandem mass spectrometry, we have identified for the first time several species of phosphatidylserine plasmalogens, including atypical forms having moieties with odd numbers of carbons and unsaturation in sn-2 position. Structural elucidation of the potential phosphatidylserine ether linked species was pursued by performing MS(3) experiments, and three fragments are proposed as marker ions to deduce which fatty acid is linked as ether or ester on the glycerol backbone. Interpretation of the fragmentation patterns based on this scheme enabled the assignment of structures to the m/z values, thereby identifying the phosphatidylserine plasmalogens.

Hsp90 and client protein maturation

Heat-shock protein 90 (Hsp90) is a molecular chaperone that assists in the maturation of a limited set of substrate proteins that are collectively referred to as clients. The majority of identified Hsp90 clients are involved in signal transduction, including many steroid hormone receptors and kinases. A handful of Hsp90 clients can be classified as nonsignal transduction proteins, including telomerase, cystic fibrosis transmembrane conductance regulator, and antigenic peptides destined for major histocompatibility complex. Because Hsp90 clients are causative agents in cancer and cystic fibrosis, research on Hsp90 has intensified in recent years. We review the historical path of Hsp90 research within each class of client (kinase, hormone receptor, and nonsignal transduction clients) and highlight current areas of active investigation.

Calcium-independent phospholipase A2-catalyzed plasmalogen hydrolysis in hypoxic human coronary artery endothelial cells

Thrombin stimulation of human coronary artery endothelial cells (HCAEC) results in activation of a membrane-associated, calcium-independent phospholipase A2(iPLA2) that selectively hydrolyzes membrane plasmalogen phospholipids. Rupture of an atherosclerotic plaque and occlusion of the coronary vasculature results in a coronary ischemic event in which HCAEC in the ischemic area would be exposed to dramatic decreases in oxygen tension in addition to thrombin exposure. We exposed HCAEC to hypoxia in the presence or absence of thrombin stimulation and measured iPLA2activation, membrane phospholipid hydrolysis, and the accumulation of biologically active phospholipid metabolites. HCAEC exposed to hypoxia, thrombin stimulation, or a combination of the two conditions demonstrated an increase in iPLA2activity and an increase in arachidonic acid release from plasmenylcholine. Thrombin stimulation of normoxic HCAEC did not result in an accumulation of choline lysophospholipids, but hypoxia alone and in combination with thrombin stimulation led to a significant accumulation of lysoplasmenylcholine (LPlsCho). We propose that the presence of hypoxia inhibits LPlsCho catabolism, at least in part, as a result of the accumulation of long-chain acylcarnitines. The combination of increased production and decreased catabolism of LPlsCho is necessary for its accumulation. Pretreatment with bromoenol lactone to inhibit iPLA2blocked membrane phospholipid hydrolysis and production of membrane phospholipid-derived metabolites. The increase in iPLA2activity and the subsequent accumulation of membrane phospholipid-derived metabolites in HCAEC exposed to hypoxia or thrombin stimulation alone, and particularly in combination, have important implications in inflammation and arrhythmogenesis in atherosclerosis/thrombosis and subsequent myocardial ischemia.

Inhibitors of brain phospholipase A2 activity: their neuropharmacological effects and therapeutic importance for the treatment of neurologic disorders

The phospholipase A(2) family includes secretory phospholipase A(2), cytosolic phospholipase A(2), plasmalogen-selective phospholipase A(2), and calcium-independent phospholipase A(2). It is generally thought that the release of arachidonic acid by cytosolic phospholipase A(2) is the rate-limiting step in the generation of eicosanoids and platelet activating factor. These lipid mediators play critical roles in the initiation and modulation of inflammation and oxidative stress. Neurological disorders, such as ischemia, spinal cord injury, Alzheimer's disease, multiple sclerosis, prion diseases, and epilepsy are characterized by inflammatory reactions, oxidative stress, altered phospholipid metabolism, accumulation of lipid peroxides, and increased phospholipase A(2) activity. Increased activities of phospholipases A(2) and generation of lipid mediators may be involved in oxidative stress and neuroinflammation associated with the above neurological disorders. Several phospholipase A(2) inhibitors have been recently discovered and used for the treatment of ischemia and other neurological diseases in cell culture and animal models. At this time very little is known about in vivo neurochemical effects, mechanism of action, or toxicity of phospholipase A(2) inhibitors in human or animal models of neurological disorders. In kainic acid-mediated neurotoxicity, the activities of phospholipase A(2) isoforms and their immunoreactivities are markedly increased and phospholipase A(2) inhibitors, quinacrine and chloroquine, arachidonyl trifluoromethyl ketone, bromoenol lactone, cytidine 5-diphosphoamines, and vitamin E, not only inhibit phospholipase A(2) activity and immunoreactivity but also prevent neurodegeneration, suggesting that phospholipase A(2) is involved in the neurodegenerative process. This also suggests that phospholipase A(2) inhibitors can be used as neuroprotectants and anti-inflammatory agents against neurodegenerative processes in neurodegenerative diseases.

Heat-shock protein 90 inhibitors in cancer therapy: 17AAG and beyond

Heat-shock protein 90 (HSP90) has diverse functions in mammalian cells. It acts as molecular chaperone, together with several co-chaperone molecules (such as Hop, Hip, p23, cdc37, Aha, and immunophilins). HSP90 binds to its client proteins (such as steroid receptors, AKT, Bcr-Abl, Apaf-1, survivin, cyclin dependent kinases which are involved in signal transduction that regulate cell cycle, survival, and death, and promote their proper protein folding, assembly, and transportation across different cellular compartments. Failure of Hsp90 chaperone activity leads to misfolding of client proteins, which leads to ubiquitination and proteasome degradation, and this deregulating cellular homeostasis. Since tumor cells frequently overexpress the active form of HSP90, which is more susceptible to inhibition by small molecules such as geldanamycin and its analogs, HSP90 became an attractive target for cancer therapy. This paper will review the recent advances in HSP90-biology and will discuss the emerging role of the HSP90 inhibitors such as 17-allylamino-17 demethoxy-geldanamycin and other HSP-90-directed small molecules in cancer therapy.

A client-binding site of Cdc37

The molecular chaperone Hsp90 is distinct from Hsp70 and chaperonin in that client proteins are apparently restricted to a subset of proteins categorized as cellular signaling molecules. Among these, many specific protein kinases require the assistance of Hsp90 and its co-chaperone Cdc37/p50 for their biogenesis. A series of Cdc37 deletion mutants revealed that all mutants capable of binding Raf-1 possess amino acid residues between 181 and 200. The 20-residue region is sufficient and, in particular, a five-residue segment (residue 191-195) is essential for binding to Raf-1. These five residues are present in one alpha helix (residues 184-199) in the middle of Cdc37, which is unexpectedly nested within the Hsp90-interacting domain of Cdc37, which was recently determined by crystallography, but does not seem to contribute to direct contact with Hsp90. Furthermore, an N-terminally truncated mutant of Cdc37 composed of residues 181-378 was shown to bind the N-terminal portion of Raf-1 (subdomains I-IV). This mutant can bind not only other Hsp90 client protein kinases, Akt1, Aurora B and Cdk4, but also Cdc2 and Cdk2, which to date have not been shown to physically interact with Cdc37. These results suggest that a region of Cdc37 other than the client-binding site may be responsible for discriminating client protein kinases from others.

The roles of ADP and TXA in botrocetin/VWF-induced aggregation of washed platelets

BACKGROUND

Binding of von Willebrand factor (VWF) to the platelet membrane glycoprotein (GP) Ib-IX-V complex initiates a cascade of events leading to alphaIIbbeta3 activation and platelet aggregation. The roles of ADP and thromboxane A2 (TXA2) in agglutination-induced GPIbalpha-mediated platelet activation have not been fully described.

METHODS

Botrocetin and human VWF were used to stimulate washed mouse platelets. Platelets deficient in TXA2 receptors, Galphaq, or alphaIIbbeta3, and inhibitors and chelating agents were used to investigate the roles of TXA2, ADP, alphaIIbbeta3 and Ca2+ in botrocetin/VWF-induced signaling.

RESULTS

Our data demonstrate that botrocetin/VWF/GPIbalpha-mediated agglutination results in calcium-independent protein kinase C (PKC) and phospholipase A2 (PLA2) activities required for GPIbalpha-elicited TXA2 production that in turn causes dense granule secretion. Aggregation of washed platelets requires TXA2-induced alphaIIbbeta3 activation and ADP signaling. TXA2 or ADP can activate alphaIIbbeta3, but both are required for alpha-granule secretion and aggregation. Botrocetin/VWF-induced dense granule secretion is Galphaq-dependent. alpha-Granule secretion requires initial ADP signaling through P2Y1 and subsequent signaling through P2Y12. Signaling initiated by agglutination is propagated and amplified in an alphaIIbbeta3-dependent manner.

CONCLUSIONS

In contrast to adhesion or shear stress-induced GPIb-elicited signaling, agglutination-elicited GPIb signaling that activates alphaIIbbeta3 requires TXA2. Agglutination-elicited TXA2 production is independent of Ca2+ influx and mobilization of internal Ca2+ stores. Therefore, our results demonstrate that agglutination-elicited GPIb signaling causes alphaIIbbeta3 activation by a mechanism that is distinct from those used by adhesion, or shear stress-induced GPIb signaling.

Studies on the endogenous phospholipids of mammalian kidney and theirin vitro hydrolysis by endogenous phospholipases: a thin layer chromatographic and densitometric study

The phosphoglycerides profile of six species of mammalian kidney (guinea pig, pig, cat, dog, mouse and rat) and their in vitro response to the endogenous phospholipases were determined by TLC technology in conjunction with densitometric measurements. Changes in their phospholipids profile subsequent to in vitro incubation of whole tissue homogenate of these kidneys for 60 min, at pH 7.4, 38 degrees C, and prior to phospholipids extraction have shown that the deacylation of the endogenous cardiolipin (CL) is the most prevalent lipolytic event of all mammalian kidneys studied. Concurrent with the deacylation of CL, there was also formation of monolysocardiolipin (MLCL) and a reduction in CL level. To a much lesser extent, lyso alkenyl phosphatidyl ethanolamine (LPE) was also produced concomitant with a decrease of the endogenous alkenyl phosphatidyl ethanolamine (PE) level. The deacylation of PE plasmalogen to its lyso form confirms the action of endogenous PLA(2) releasing sn-2 fatty acids.

Functions and biosynthesis of plasmalogens in health and disease

Plasmalogens (1-O-alk-1'-enyl-2-acyl glycerophospholipids) constitute a special class of phospholipids characterized by the presence of a vinyl-ether bond at the sn-1 position. Although long considered as biological peculiarities, interest in this group of phospholipids has grown in recent years, thanks to the realization that plasmalogens are involved in different human diseases. In this review, we summarize the current state of knowledge with respect to the enzymatic synthesis of plasmalogens, the characteristic topology of the enzymes involved and the biological roles that have been assigned to plasmalogens.

Age-related changes of the endogenous cardiolipin and plasmalogens of guinea pig kidney and theirin vitro hydrolysis by endogenous phospholipases: a thin layer chromatographic analysis in conjunction with densitometric measurement

The phosphoglycerides profile of guinea pig kidney, fetal, young adult, and aged, and their in vitro response to the endogenous lipolytic enzymes, mainly in the phospholipase group were determined by TLC technology in conjunction with densitometric measurement. Changes in phosphoglycerides profile subsequent to in vitro incubation of these tissues at pH 7.4, and 38 degrees C for 45 min and prior to phospholipid extraction has provided evidence relating to their respective lipolytic enzymes capabilities and age. These changes are mainly related to endogenous cardiolipin (CL), alkenyl phospholipids (phosphatidyl ethanolamine and phosphatidyl choline) and their endogenous deacylation to their respective lyso derivatives monolysocardiolipin (MLCL), lyso alkenyl phosphatidyl ethanolamine (LPE), and lyso alkenyl phosphatidyl choline (LPC) by endogenous phospholipases. The hydrolysis of the plasmalogen confirms the action of endogenous PLA(2) on sn-2 fatty acids of these compounds.

Role of an endoplasmic reticulum Ca2+-independent phospholipase A2 in cisplatin-induced renal cell apoptosis

It has been demonstrated recently that rabbit renal proximal tubule cells (RPTC) express a novel Ca(2+)-independent phospholipase A(2) (iPLA(2)) whose activity localizes to the endoplasmic reticulum (ER-iPLA(2)) and is similar to group VIB PLA(2). In this study, the expression of group VIB PLA(2) was examined and the role of ER-iPLA(2) in cisplatin-induced apoptosis was determined. Cisplatin induced both time- and concentration-dependent RPTC apoptosis as determined by p53 nuclear localization, annexin V staining, caspase 3 activity, and chromatin condensation. Inhibition of ER-iPLA(2) with bromoenol lactone (5 microM) reduced cisplatin-induced annexin V binding 40%, chromatin condensation 55%, and caspase 3 activity 42%, but had no effect on p53 nuclear localization. Treatment of RPTC with the protein kinase C stimulator phorbol 12-myristate 13-acetate increased the activity of ER-iPLA(2) 2-fold and increased cisplatin-induced RPTC apoptosis. These studies demonstrate that group VIB PLA(2) is expressed in RPTC and suggest that RPTC ER-iPLA(2) is the rabbit homolog of group VIB PLA(2). These data also demonstrate that ER-iPLA(2) acts downstream of p53 and upstream of caspase 3 to mediate cisplatin-induced RPTC apoptosis. Finally, ER-iPLA(2) seems to be regulated by protein kinase C.

Role of an endoplasmic reticulum Ca(2+)-independent phospholipase A(2) in oxidant-induced renal cell death

Phospholipase A(2) (PLA(2)) hydrolyzes the sn-2 ester bond in phospholipids, releasing a fatty acid and a lysophospholipid. Recently, a novel 85-kDa membrane-bound-Ca(2+)-independent PLA(2) (iPLA(2)) was identified in insect and bacterial cells transfected with candidate PLA(2) sequences. However, few data exist demonstrating a membrane-bound-iPLA(2) in mammalian cells, its subcellular localization, or its physiological role. Herein, we demonstrate the expression of an 85-kDa endoplasmic reticulum (ER)-Ca(2+)-iPLA(2) (ER-iPLA(2)) in rabbit renal proximal tubule cells (RPTC) that is plasmalogen selective and is inhibited by the specific Ca(2+)-iPLA(2) inhibitor bromoenol lactone (BEL). RPTC exposed to tert-butylhydroperoxide for 24 h exhibited 20% oncosis compared with 2% in controls. Inhibition of ER-iPLA(2) with BEL before tert-butylhydroperoxide exposure resulted in 50% oncosis. To determine whether this effect was common to oxidants, we tested the ability of BEL to potentiate oncosis induced by cumene hydroperoxide, menadione, duraquinone, cisplatin, and the nonoxidant antimycin A. All oxidants tested produced oncosis after 24 h, and prior inhibition of ER-iPLA(2) potentiated oncosis at least twofold. In contrast, inhibition of ER-iPLA(2) did not alter antimycin A-induced oncosis. Lipid peroxidation increased from 1.4- to 5.2-fold in RPTC treated with BEL before oxidant exposure, whereas no change was seen in antimycin A-treated RPTC. These results are the first to demonstrate the expression and subcellular localization of an ER-iPLA(2). These results also suggest that ER-iPLA(2) functions to protect against oxidant-induced lipid peroxidation and oncosis.

Increasing plasmalogen levels protects human endothelial cells during hypoxia

Supplementation of cultured human pulmonary arterial endothelial cells (PAEC) with sn-1-O-hexadecylglycerol (HG) resulted in an approximately twofold increase in cellular levels of plasmalogens, a subclass of phospholipids known to have antioxidant properties; this was due, primarily, to a fourfold increase in the choline plasmalogens. Exposure of unsupplemented human PAEC to hypoxia (PO(2) = 20-25 mmHg) caused an increase in cellular reactive oxygen species (ROS) over a period of 5 days with a coincident decrease in viability. In contrast, HG-supplemented cells survived for at least 2 wk under these conditions with no evidence of increased ROS. Hypoxia resulted in a selective increase in the turnover of the plasmalogen plasmenylethanolamine. Human PAEC with elevated plasmalogen levels were also more resistant to H(2)O(2), hyperoxia, and the superoxide generator plumbagin. This protection was seemingly specific to cellular stresses in which significant ROS were generated because the sensitivity to lethal heat shock or glucose deprivation was not altered in HG-treated human PAEC. HG, by itself, was not sufficient for protection; HG supplementation of bovine PAEC had no effect upon plasmalogen levels and did not rescue these cells from the cytotoxic effects of hypoxia. This is the initial demonstration that plasmalogen content can be substantially enhanced in a normal cell. These data also demonstrate that HG can protect cells during hypoxia and other ROS-mediated stress, likely due to the resulting increase in these antioxidant phospholipids.

On the physiological role of casein kinase II in Saccharomyces cerevisiae

Casein kinase II (CKII) is a highly conserved serine/threonine protein kinase that is ubiquitous in eukaryotic organisms. This review summarizes available data on CKII of the budding yeast Saccharomyces cerevisiae, with a view toward defining the possible physiological role of the enzyme. Saccharomyces cerevisiae CKII is composed of two catalytic and two regulatory subunits encoded by the CKA1, CKA2, CKB1, and CKB2 genes, respectively. Analysis of null and conditional alleles of these genes identifies a requirement for CKII in at least four biological processes: flocculation (which may reflect an effect on gene expression), cell cycle progression, cell polarity, and ion homeostasis. Consistent with this, isolation of multicopy suppressors of conditional cka mutations has identified three genes that have a known or potential role in either the cell cycle or cell polarity: CDC37, which is required for cell cycle progression in both G1 and G2/M; ZDS1 and 2, which appear to have a function in cell polarity; and SUN2, which encodes a protein of the regulatory component of the 26S protease. The identity and properties of known CKII substrates in S. cerevisiae are also reviewed, and advantage is taken of the complete genomic sequence to predict globally the substrates of CKII in this organism. Although the combined data do not yield a definitive picture of the physiological role of CKII, it is proposed that CKII serves a signal transduction function in sensing and/or communicating information about the ionic status of the cell to the cell cycle machinery.

Apical endocytosis of ricin in MDCK cells is regulated by the cyclooxygenase pathway

Addition of arachidonic acid or stimulation of arachidonic acid production by secretory phospholipase A2 selectively upregulated apical endocytosis of ricin in MDCK cells without affecting basolateral endocytosis. Electron microscopic studies revealed that MDCK cells treated with secretory phospholipase A2 and incubated with horseradish peroxidase had an increased number of normal appearing peroxidase-labeled endosomes and no sign of membrane ruffling. Moreover, inhibition of basal arachidonic acid release, either by decreasing the cytosolic phospholipase A(2) activity or the diacylglycerol lipase activity, reduced the rate of apical endocytosis. Furthermore, indomethacin, an inhibitor of the cyclooxygenase pathway, counteracted the stimulation of endocytosis seen with both secretory phospholipase A2 and arachidonic acid, suggesting that formation of eicosanoids such as prostaglandins could be essential for the regulation. This idea was supported by the finding that prostaglandin E2, the predominant prostaglandin formed in kidney, also upregulated ricin uptake. The regulatory effect of the cyclooxygenase pathway on apical endocytosis of ricin was found to be independent of protein kinases A and C, which are known to selectively control apical clathrin-independent endocytosis in polarized cells.

Role of phospholipase A2 in the cytotoxic effects of oxalate in cultured renal epithelial cells

BACKGROUND

Oxalate, a common constituent of kidney stones, is cytotoxic for renal epithelial cells. Although the exact mechanism of oxalate-induced cell death remains unclear, studies in various cell types, including renal epithelial cells, have implicated phospholipase A2 (PLA2) as a prominent mediator of cellular injury. Thus, these studies examined the role of PLA2 in the cytotoxic effects of oxalate.

METHODS

The release of [3H]-arachidonic acid (AA) or [3H]-oleic acid (OA) from prelabeled Madin-Darby canine kidney (MDCK) cells was measured as an index for PLA2 activity. The cell viability was assessed by the exclusion of ethidium homodimer-1.

RESULTS

Oxalate exposure (175 to 550 microM free) increased the release of [3H]-AA in MDCK cells but had no effect on the release of [3H]-OA. Oxalate-induced [3H]-AA release was abolished by arachidonyl trifluoromethyl ketone (AACOCF3), a selective inhibitor of cytosolic PLA2 (cPLA2), but was not affected by selective inhibitors of secretory PLA2 and calcium-independent PLA2. The [3H]-AA release could be demonstrated within 15 minutes after exposure to oxalate, which is considerably earlier than the observed changes in cell viability. Furthermore, AACOCF3 significantly reduced oxalate toxicity in MDCK cells.

CONCLUSIONS

Oxalate increases AA release from MDCK cells by a process involving cPLA2. In addition, based on the evidence obtained using a selective inhibitor of this isoform, it would appear that the activity of this enzyme is responsible, at least in part, for the cytotoxic effects of oxalate. The finding that oxalate can trigger a known lipid-signaling pathway may provide new insight into the initial events in the pathogenesis of nephrolithiasis.

Regional distribution, ontogeny, purification, and characterization of the Ca2+-independent phospholipase A2 from rat brain

We purified an 80-kDa Ca2+-independent phospholipase A2 (iPLA2) from rat brain using octyl-Sepharose, ATP-agarose, and calmodulin-agarose column chromatography steps. This procedure gave a 30,000-fold purification and yielded 4 microg of a near-homogeneous iPLA2 with a specific activity of 4.3 micromol/min/mg. Peptide sequences of the rat brain iPLA2 display considerable homology to sequences of the iPLA2 from P388D1 macrophages, Chinese hamster ovary cells, and human B lymphocytes. Under optimal conditions, the iPLA2 revealed the following substrate preference toward the fatty acid chain in the sn-2 position of phosphatidylcholine: linoleoyl > palmitoyl > oleoyl > arachidonoyl. The rat brain iPLA2 also showed a head group preference for choline > or = ethanolamine >> inositol. The iPLA2 is inactivated when exposed to pure phospholipid vesicles. The only exception is vesicles composed of phosphatidylcholine and phosphatidylinositol 4,5-bisphosphate. Studies on the regional distribution and ontogeny of various phospholipase A2 (PLA2) types in rat brain indicate that the iPLA2 is the dominant PLA2 activity in the cytosolic fraction, whereas the group IIA secreted PLA2 is the dominant activity in the particulate fraction. The activities of these two enzymes change during postnatal development.

Role of fatty acid beta-oxidation and calcium-independent phospholipase A2 in ischemic acute renal failure

Membrane phospholipolysis during ischemic cell injury is accompanied by the activation of a novel calcium-independent phospholipase A2 in the proximal tubule. Long-chain fatty acid metabolic products produced by phospholipase A2 activation accumulate during ischemia as a result of the inhibition of fatty acid beta-oxidation on the mitochondria and peroxisomes. Altogether, lysophospholipids, long-chain acyl carnitines, and long-chain acyl coenzyme A inhibit proximal tubule Na+K(+)-ATPase. Metabolic regulation of the gene expression of fatty acid beta-oxidation enzymes during ischemic acute renal failure may represent a novel therapeutic maneuver to enhance the recovery of kidney function during ischemia.

Inhibitors of intracellular phospholipase A2 activity: their neurochemical effects and therapeutical importance for neurological disorders

Intracellular phospholipases A2 (PLA2) are a diverse group of enzymes with a growing number of members. These enzymes hydrolyze membrane phospholipids into fatty acid and lysophospholipids. These lipid products may serve as intracellular second messengers or can be further metabolized to potent inflammatory mediators, such as eicosanoids and platelet-activating factors. Several inhibitors of nonneural intracellular PLA2 have been recently discovered. However, nothing is known about their neurochemical effects, mechanism of action or toxicity in human or animal models of neurological disorders. Elevated intracellular PLA2 activities, found in neurological disorders strongly associated with inflammation and oxidative stress (ischemia, spinal cord injury, and Alzheimer's disease), can be treated with specific, potent and nontoxic inhibitors of PLA2 that can cross blood-brain barrier without harm. Currently, potent intracellular PLA2 inhibitors are not available for clinical use in human or animal models of neurological disorders, but studies on this interesting topic are beginning to emerge. The use of nonspecific intracellular PLA2 inhibitors (quinacrine, heparin, gangliosides, vitamin E) in animal model studies of neurological disorders in vivo has provided some useful information on tolerance, toxicity, and effectiveness of these compounds.

Sequence features and phylogenetic analysis of the stress protein hsp90alpha in chinook salmon (Oncorhynchus tshawytscha), a poikilothermic vertebrate

We cloned and sequenced a chinook salmon Hsp90 cDNA; sequence analysis shows it to be Hsp90alpha. Phylogenetic analysis supports the hypothesis that alpha and beta paralogs of Hsp90 arose as a result of a gene duplication event and that they diverged early in the evolution of vertebrates, before tetrapods separated from the teleost lineage. Among several differences distinguishing poikilothermic Hsp90alpha sequences from their bird and mammal orthologs, the teleost versions specifically lack a characteristic QTQDQP phosphorylation site near the N-terminus. We used the cDNA to develop an RNA (Northern) blot to quantify cellular Hsp90 mRNA levels. Chinook salmon embryonic (CHSE-214) cells responded to heat shock with a rapid rise in Hsp90 mRNA through 4 h, followed by a gradual decline over the next 20 h. Hsp90 mRNA level may be useful as a stress indicator, especially in a laboratory setting or in response to acute heat stress.

Cellular responses to excess phospholipid

Phosphatidylcholine (PtdCho) is the major membrane phospholipid in mammalian cells, and its synthesis is controlled by the activity of CDP:phosphocholine cytidylyltransferase (CCT). Enforced CCT expression accelerated the rate of PtdCho synthesis. However, the amount of cellular PtdCho did not increase as a result of the turnover of both the choline and glycerol components of PtdCho. Metabolic labeling experiments demonstrated that cells compensated for elevated CCT activity by the degradation of PtdCho to glycerophosphocholine (GPC). Phospholipase D-mediated PtdCho hydrolysis and phosphocholine formation were unaffected. Most of the GPC produced in response to excess phospholipid production was secreted into the medium. Cells also degraded the excess membrane PtdCho to GPC when phospholipid formation was increased by exposure to exogenous lysophosphatidylcholine or lysophosphatidylethanolamine. The replacement of the acyl moiety at the 1-position of PtdCho with a non-hydrolyzable alkyl moiety prevented degradation to GPC. Accumulation of alkylacyl-PtdCho was associated with the inhibition of cell proliferation, demonstrating that alternative pathways of degradation will not substitute. GPC formation was blocked by bromoenol lactone, implicating the calcium-independent phospholipase A2 as a key participant in the response to excess phospholipid. Owing to the fact that PtdCho is biosynthetically converted to PtdEtn, excess PtdCho resulted in overproduction and exit of GPE as well as GPC. Thus, general membrane phospholipid homeostasis is achieved by a balance between the opposing activities of CCT and phospholipase A2.

A specific human lysophospholipase: cDNA cloning, tissue distribution and kinetic characterization

Lysophospholipases are critical enzymes that act on biological membranes to regulate the multifunctional lysophospholipids; increased levels of lysophospholipids are associated with a host of diseases. Herein we report the cDNA cloning of a human brain 25 kDa lysophospholipid-specific lysophospholipase (hLysoPLA). The enzyme (at both mRNA and protein levels) is widely distributed in tissues, but with quite different abundances. The hLysoPLA hydrolyzes lysophosphatidylcholine in both monomeric and micellar forms, and exhibits apparent cooperativity and surface dilution kinetics, but not interfacial activation. Detailed kinetic analysis indicates that the hLysoPLA binds first to the micellar surface and then to the substrate presented on the surface. The kinetic parameters associated with this surface dilution kinetic model are reported, and it is concluded that hLysoPLA has a single substrate binding site and a surface recognition site. The apparent cooperativity observed is likely due to the change of substrate presentation. In contrast to many non-specific lipolytic enzymes that exhibit lysophospholipase activity, hLysoPLA hydrolyzes only lysophospholipids and has no other significant enzymatic activity. Of special interest, hLysoPLA does not act on plasmenylcholine. Of the several inhibitors tested, only methyl arachidonyl fluorophosphonate (MAFP) potently and irreversibly inhibits the enzymatic activity. The inhibition by MAFP is consistent with the catalytic mechanism proposed for the enzyme - a serine hydrolase with a catalytic triad composed of Ser-119, Asp-174 and His-208.

Identification and characterization of alkenyl hydrolase (lysoplasmalogenase) in microsomes and identification of a plasmalogen-active phospholipase A2 in cytosol of small intestinal epithelium

A lysoplasmalogenase (EC 3.3.2.2; EC 3.3.2.5) that liberates free aldehyde from 1-alk-1'-enyl-sn-glycero-3-phospho-ethanolamine or -choline (lysoplasmalogen) was identified and characterized in rat gastrointestinal tract epithelial cells. Glycerophosphoethanolamine was produced in the reaction in equimolar amounts with the free aldehyde. The microsomal membrane associated enzyme was present throughout the length of the small intestines, with the highest activity in the jejunum and proximal ileum. The rate of alkenyl ether bond hydrolysis was dependent on the concentrations of microsomal protein and substrate, and was linear with respect to time. The enzyme hydrolyzed both ethanolamine- and choline-lysoplasmalogens with similar affinities; the Km values were 40 and 66 microM, respectively. The enzyme had no activity with 1-alk-1'-enyl-2-acyl-sn-glycero-3-phospho-ethanolamine or -choline (intact plasmalogen), thus indicating enzyme specificity for a free hydroxyl group at the sn-2 position. The specific activities were 70 nmol/min/mg protein and 57 nmol/min/mg protein, respectively, for ethanolamine- and choline-lysoplasmalogen. The pH optimum was between 6.8 and 7.4. The enzyme required no known cofactors and was not affected by low mM levels of Ca2+, Mg2+, EDTA, or EGTA. The detergents, Triton X-100, deoxycholate, and octyl glucoside inhibited the enzyme. The chemical and physical properties of the lysoplasmalogenase were very similar to those of the enzyme in liver and brain microsomes. In developmental studies the specific activities of the small intestinal and liver enzymes increased markedly, 11.1- and 3.4-fold, respectively, in the first approximately 40 days of postnatal life. A plasmalogen-active phospholipase A2 activity was identified in the cytosol of the small intestines (3.3 nmol/min/mg protein) and liver (0.3 nmol/min/mg protein) using a novel coupled enzyme assay with microsomal lysoplasmalogenase as the coupling enzyme.

On the expression of cytosolic calcium-independent phospholipase A2 (88kDa) in immature and mature myeloid cells and its role in leukotriene synthesis in human granulocytes

The human calcium-independent phospholipase A2 (iPLA2; 88 kDa) has recently been cloned (Larsson, P.K.A., Claesson, H.-E. and Kennedy, B.P. (1998) J. Biol. Chem. 272, 207-214). Here we demonstrate the expression of the human iPLA2 mRNA and its splice variants in blood progenitor cells, immature leukemic cells and mature granulocytes. Chromatographical resolvable iPLA2 activity was found in the cytosolic fraction of granulocytes and the activity was inhibited by the iPLA2 inhibitor bromoenol lactone. This drug also inhibited leukotriene synthesis in human granulocytes, induced by low concentration of calcium ionophore A23187 (0.10-0.15 microM) or opsonized zymosan. These results suggest that iPLA2 is involved in the regulation of the pool of arachidonic acid destined for leukotriene synthesis in human granulocytes.

Characterization of acidic Ca(2+)-independent phospholipase A2 of bovine lung

An acidic Ca(2+)-independent phospholipase A2 (aiPLA2) has been isolated previously from rat lung and a human cDNA has been described. This study applied the method to larger scale isolation of the native protein from the bovine lung. A polyclonal antibody was generated to a 15 amino acid synthetic peptide based on a conserved rat/human sequence. This antibody recognized a single protein band with an estimated molecular mass of approximately 29 kDa in a soluble fraction obtained from bovine lung homogenate. A 29 kDa protein that reacted with the aiPLA2 antipeptide antibody was detected in fractions containing aiPLA2 activity on sequential column chromatographies. The partially purified enzyme showed 176-fold increase over the homogenate in Ca(2+)-independent PLA2 activity at pH 4. Activity was maximal with phosphatidylcholine substrate and was significantly less with phosphatidylethanolamine and anionic phospholipids. The enzyme had no acyl group preference in phosphatidylcholine and showed no preference for oxidized substrate, but activity was less with 1-O-alkyl phosphatidylcholine. aiPLA2 activity was inhibited by a transition state phospholipid analog (MJ33, 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol), serine protease inhibitors, and the anti-peptide antibody but was insensitive to arachidonoyl trifluoromethyl ketone, bromoenol lactone, p-bromophenacyl bromide, and ATP. Analysis of N-terminal amino acid sequence for the 29 kDa protein demonstrated its high homology to human 26 kDa aiPLA2. These was no significant change in molecular mass of the protein following treatment with endoglycosidase F. Western blot of subcellular fractions from rat lung indicated aiPLA2 immunoreactivity with lamellar body, lysosomal, and cytosolic fractions. These results indicate isolation from bovine lung of a 29 kDa acidic Ca(2+)-independent phospholipase A2 homologue of the rat and human enzyme and provide evidence for specificity in the metabolism of lung surfactant phosphatidylcholine.

Purification and characterization of rat kidney sphingosine kinase

Sphingosine kinase catalyzes the formation of the bioactive sphingolipid metabolite sphingosine 1-phosphate, which plays important roles in numerous physiological processes, including growth, survival, and motility. We have purified rat kidney sphingosine kinase 6 x 10(5)-fold to apparent homogeneity. The purification procedure involved ammonium sulfate precipitation followed by chromatography on an anion exchange column. Partially purified sphingosine kinase was found to be stabilized by the presence of high salt, and thus, a scheme was developed to purify sphingosine kinase using sequential dye-ligand chromatography steps (since the enzyme bound to these matrices even in the presence of salt) followed by EAH-Sepharose chromatography. This 385-fold purified sphingosine kinase bound tightly to calmodulin-Sepharose and could be eluted in high yield with EGTA in the presence of 1 M NaCl. After concentration, the calmodulin eluate was further purified by successive high pressure liquid chromatography separations on hydroxylapatite, Mono Q, and Superdex 75 gel filtration columns. Purified sphingosine kinase has an apparent molecular mass of approximately 49 kDa under denaturing conditions on SDS-polyacrylamide gel, which is similar to the molecular mass determined by gel filtration, suggesting that the active form is a monomer. Sphingosine kinase shows substrate specificity for D-erythro-sphingosine and does not catalyze the phosphorylation of phosphatidylinositol, diacylglycerol, ceramide, DL-threo-dihydrosphingosine, or N,N-dimethylsphingosine. However, the latter two sphingolipids were potent competitive inhibitors. With sphingosine as substrate, the enzyme had a broad pH optimum of 6.6-7.5 and showed Michaelis-Menten kinetics, with Km values of 5 and 93 microM for sphingosine and ATP, respectively. This study provides the basis for molecular characterization of a key enzyme in sphingolipid signaling.

Cloning and expression of rat lung acidic Ca(2+)-independent PLA2 and its organ distribution

A clone for a rat acidic Ca(2+)-independent phospholipase A2 (aiPLA2) was isolated from a cDNA library prepared from rat granular pneumocytes with a probe based on the human aiPLA2 sequence (T.S. Kim, C.S. Sundaresh, S. I. Feinstein, C. Dodia, W. R. Skach, M. K. Jain, T. Nagase, N. Seki, K. Ishikawa, N. Nomura, and A. B. Fisher. J. Biol. Chem. 272: 2542-2550, 1997). In addition, a consensus sequence for mouse aiPLA2 was constructed from several mouse cDNA clones in the GenBank and dbEST databases. Each sequence codes for a 224-amino acid protein with 88% identity of the amino acids among the three species and conservation of a putative lipase motif (GDSWG). Translation of mRNA produced from the rat clone in a wheat germ system resulted in expression of PLA2 activity with properties similar to those of the human enzyme, i.e., acidic pH optimum and Ca2+ independence. The localization of aiPLA2 in rat tissues was studied with the human cDNA probe, polyclonal and monoclonal antibodies, and aiPLA2 activity. aiPLA2 is present in the lung as evidenced by high levels of mRNA and protein expression and by enzymatic activity that is inhibited by anti-PLA2 antibody and by the transition state analog 1-hexadecyl-3-trifluoroethylglycero-sn-2-phosphomethanol (MJ33). Immunocytochemistry showed the presence of aiPLA2 in alveolar type II cells, alveolar macrophages, and bronchiolar epithelium. In the brain, heart, liver, kidney, spleen, and intestine, aiPLA2 mRNA content was < 50% of that in the lung, immunoreactive protein was not detectable, and enzymatic activity was not inhibited by MJ33 or aiPLA2 antibody. These results show marked enrichment of aiPLA2 in the lung compared with the other organs and suggest translational control of aiPLA2 expression.

Purification and characterization of 1-O-acylceramide synthase, a novel phospholipase A2 with transacylase activity

A novel pathway for ceramide metabolism, 1-O-acylceramide formation, was previously reported (Abe, A., Shayman, J. A., and Radin, N. S. (1996) J. Biol. Chem. 271, 14383-14389). In this pathway a fatty acid in the sn-2 position of phosphatidylethanolamine or phosphatidylcholine is transferred to the 1-hydroxyl position of ceramide. An enzyme that catalyzes the esterification of N-acetylsphingosine was purified from the postmitochondrial supernatant of calf brain through consecutive steps, including ammonium sulfate fractionation, DEAE-Sephacel, phenyl-Sepharose, S-Sepharose, Sephadex G-75, concanavalin A-agarose, and heparin-Sepharose chromatography. The molecular mass of the enzyme was determined to be 40 kDa by gel filtration on Sephadex G-75. The enzyme bound to concanavalin A-agarose column was eluted with the buffer containing 500 mM alpha-methyl-D-mannopyranoside. Further purification by heparin-Sepharose chromatography resulted in separation of two peaks of enzyme activity. Coincidence between the transacylase activity and a stained protein of a molecular mass of 40 kDa was observed, as determined by SDS-polyacrylamide gel electrophoresis and recovery after separation over an acidic native gel. The second peak of activity from the heparin-Sepharose chromatography represented a purification of 193,000-fold. These results are consistent with the enzyme being a glycoprotein of a molecular mass of about 40 kDa with a single polypeptide chain. The purified enzyme had a pH optimum at pH 4.5. The divalent cations Ca2+ and Mg2+ enhanced but were not essential for the transacylase activity. Neither activation nor inactivation of the enzyme activity was observed in the presence of 2 mM ATP or 2 mM dithiothreitol. Preincubation of the enzyme with 1 mM N-ethylmaleimide, 1 mM phenylmethylsulfonyl fluoride, or 3.1 microM bromoenol lactone, a potent inhibitor of cytosolic Ca2+-independent phospholipase A2, had no significant effect on the enzyme activity. The enzyme activity was completely abolished in the presence of greater than 773 microM Triton X-100. Partial inhibition of the enzyme activity was observed in the presence of 10-100 microg/ml heparin. In the absence of N-acetylsphingosine, the enzyme acted as a phospholipase A2. These results strongly suggest that 1-O-acylceramide synthase is both a transacylase and a novel phospholipase A2.

Isolation and properties of a novel phospholipase A from rat brain that hydrolyses fatty acids at sn-1 and sn-2 positions

A Ca(2+)-independent phospholipase A that releases various fatty acids from sn-1 and sn-2 positions was partially purified from rat brain soluble fraction. The enzyme showed an approximate molecular mass of 300 kDa on gel filtration column chromatography. Its enzymatic properties are distinct from those of well characterized phospholipase A2 enzymes; by using a series of synthetic phosphatidylcholines, the enzyme cleaved oleic, linoleic, and arachidonic acids like phospholipase A2, and released palmitic and stearic acids like phospholipase A1. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidic acid were hydrolysed with almost equal efficiencies by this enzyme. These results indicate that the enzyme isolated is a novel Ca(2+)-independent intracellular phospholipase A that might be responsible for production of various fatty acids from membrane phospholipids.

Cyclin-dependent kinases: engines, clocks, and microprocessors

Cyclin-dependent kinases (Cdks) play a well-established role in the regulation of the eukaryotic cell division cycle and have also been implicated in the control of gene transcription and other processes. Cdk activity is governed by a complex network of regulatory subunits and phosphorylation events whose precise effects on Cdk conformation have been revealed by recent crystallographic studies. In the cell, these regulatory mechanisms generate an interlinked series of Cdk oscillators that trigger the events of cell division.

Multiple splice variants of the human calcium-independent phospholipase A2 and their effect on enzyme activity

Recently, the cloning of a novel Ca2+-independent phospholipase A2 (iPLA2) from Chinese hamster ovary cells as well as from mouse and rat sources containing a C-terminal lipase motif and eight N-terminal ankyrin repeats has been described. In this report we describe the cloning of the human iPLA2 cDNA and its expression in B-cells and show that the iPLA2 gene undergoes extensive alternative splicing generating multiple isoforms that contribute to a novel mechanism to control iPLA2 activity. The full-length cDNA clone encodes a 806-amino acid protein with a calculated molecular mass of 88 kDa. The protein contains a lipase motif, GXSXG, and ankyrin repeats, as described for the hamster and rodent forms of the enzyme but has an additional 54-amino acid proline-rich insertion in the last of the eight ankyrin repeats (residues 395-449). Furthermore, at least three additional isoforms most likely due to alternative splicing were identified. One that is present as a partial cDNA in the expressed sequence tag data base is similar to iPLA2 but terminates just after the lipase active site, and two other isoforms contain only the iPLA2 ankyrin repeat sequence (ankyrin-iPLA2-1 and -2). Ankyrin repeats are involved in protein-protein interactions and because the purified iPLA2 enzyme exists as a multimeric complex of 270-350 kDa, the expression of just the ankyrin-iPLA2 sequence suggested that these may also interact with the iPLA2 oligomeric complexes and perhaps modulate PLA2 activity. Transfection of the human iPLA2 cDNA into COS cells resulted in a substantial increase in calcium-independent PLA2 activity in cell lysate. No activity above background was observed following ankyrin-iPLA2-1 cDNA transfection. However, co-transfection of the ankyrin-iPLA2-1 and the iPLA2 cDNAs resulted in a 2-fold reduction in activity compared with iPLA2 alone. A similar co-transfection of ankyrin-iPLA2-1 cDNA with the cPLA2 cDNA had no effect on PLA2 activity. These results suggest that the ankyrin-iPLA2 sequence can function as a negative regulator of iPLA2 activity and that the alternative splicing of the iPLA2 gene can have a direct effect on the attenuation of enzyme activity.

Bradykinin-stimulated cPLA2 phosphorylation is protein kinase C dependent in rabbit CCD cells

We have used an established cell line of rabbit cortical collecting duct (RCCD) epithelial cells representing a mixed population of principal and intercalated cell types to determine which phospholipase A2 (PLA2) enzyme therein is responsible for bradykinin (BK)-stimulated arachidonic acid (AA) release and how its activation is regulated. BK-stimulated AA release was reduced 92% by arachidonyl trifluoromethyl ketone, an inhibitor of cytosolic PLA2 (cPLA2). Examination of PLA2 activity in vitro demonstrated that BK stimulation resulted in a greater than twofold increase in PLA2 activity and that this activity was dithiothreitol insensitive and was inhibited by an antibody directed against cPLA2. To determine a possible role for protein kinase C (PKC) in the BK-mediated activation of cPLA2, we used the PKC-specific inhibitor Ro31-8220 and examined its effects on AA release, cPLA2 activity, and phosphorylation. Ro31-8220 reduced BK-stimulated AA release and cPLA2 activity by 51 and 58%, respectively. cPLA2 activity stimulated by phorbol ester [phorbol 12-myristate 13-acetate (PMA)] displayed a similar degree of activation and was associated with an increase in serine phosphorylation identical to that caused by BK. The phosphorylation-induced activation of this enzyme was confirmed by the phosphatase-mediated reversal of both BK- and PMA-stimulated cPLA2 activity. In addition, we have also found that PMA stimulation did not cause a synergistic potentiation of BK-stimulated AA release as did calcium ionophore. This occurred despite membrane PKC activity increasing 93% in response to PMA vs. 42% in response to BK. These data, taken together, indicate that cPLA2 is the enzyme responsible for BK-mediated AA release, and, moreover, they indicate that PKC is involved in the onset responses of cPLA2 to BK.

Apical ANG II-stimulated PLA2 activity and Na+ flux: a potential role for Ca2+-independent PLA2

Type 1 angiotensin II (ANG II) receptors (AT1R), which mediate proximal tubule (PT) salt and water reabsorption, undergo endocytosis and recycling. Prior studies in a PT-like model (LLC-PKcl4 cells expressing rabbit AT1R) (LLC-PK-AT1R cells) determined that quinacrine, a nonspecific phospholipase A2 (PLA2) inhibitor, and the haloenol lactone suicide substrate (HELSS), a Ca2+-independent PLA2 inhibitor, attenuated apical (AP) AT1R recycling. Further studies were undertaken to examine the association between AT1R endocytotic movement and PLA2 activity in this model. AP ANG II (100 nM) increased [3H]arachidonic acid ([3H]AA) release 4.4 +/- 0.38-fold in LLC-PK-AT1R cells cultured on permeable supports. Basolateral (BL) ANG II had no significant effect. Reversed-phase high-performance liquid chromatography confirmed that AP ANG II stimulated free [3H]AA release. Quinacrine, HELSS, and palmitoyl trifluoromethyl ketone, another Ca2+-independent PLA2 inhibitor, inhibited AP ANG II-stimulated [3H]AA release, as did inhibiting AP AT1R internalization with phenylarsine oxide. The role of HELSS-inhibitable AA release in ANG II-mediated 22Na flux was examined, given the effects of AT1R-mediated PLA2 activity on salt and water reabsorption. AP ANG II (100 nM) stimulated 22Na flux (AP--> BL), a response inhibited by HELSS. Thus, in this model, AP AT1R activated PLA2 with concomitant 22Na flux (AP --> BL), suggesting a link between AP AT1R endocytotic movement, AT1R-stimulated PLA2 activity, and 22Na flux in this model. The effects of HELSS suggest that Ca2+-independent PLA2 activity may be involved in this AP ANG II response.

A molecular clamp in the crystal structure of the N-terminal domain of the yeast Hsp90 chaperone

Hsp90 is a highly specific chaperone for many signal transduction proteins, including steroid hormone receptors and a broad range of protein kinases. The crystal structure of the N-terminal domain of the yeast Hsp90 reveals a dimeric structure based on a highly twisted sixteen stranded beta-sheet, whose topology suggests a possible 30-domain-swapped structure for the intact Hsp90 dimer. The opposing faces of the beta-sheets in the dimer define a potential peptide-binding cleft, suggesting that the N-domain may serve as a molecular 'clamp' in the binding of ligand proteins to Hsp90.

Cdc37: a protein kinase chaperone?

The activity of most protein kinases is highly regulated, typically via phosphorylation and/or subunit association. However, the folding of protein kinases into an active state or a form capable of activation is now emerging as another important step through which they can be regulated. The 50-kDa protein Cdc37 and the associated heat-shock protein Hsp90 have been found to bind to, and be required for the activity of, diverse protein kinases, including Cdk4, v-Src, Raf and SEVENLESS. Together, Cdc37 and Hsp90 may act as a general chaperone for protein kinases, in particular those involved in signal-transduction pathways and cell-cycle control.

Identity between the Ca2+-independent phospholipase A2 enzymes from P388D1 macrophages and Chinese hamster ovary cells

A novel Ca2+-independent phospholipase A2 (iPLA2) has recently been purified and characterized from P388D1 macrophages (Ackermann, E. J., Kempner, E. S., and Dennis, E. A. (1994) J. Biol. Chem. 269, 9227-9233). This enzyme appears to play a key role in regulating basal phospholipid remodeling reactions. Also an iPLA2 from Chinese hamster ovary (CHO) cells has been purified, molecularly cloned, and expressed (Tang, J., Kriz, R., Wolfman, N., Shaffer, M., Seehra, J., and Jones, S. S. (1997) J. Biol. Chem. 272, 8567-8575). We report herein that the cloned CHO iPLA2 is equivalent to the mouse enzyme purified from P388D1 cells. Polymerase chain reaction amplification of cDNA fragments from P388D1 cells using primers based on the CHO iPLA2 sequence, revealed a high degree of homology between the mouse and hamster enzymes at both the nucleotide and amino acid levels (92 and 95%, respectively). Identity between the two proteins was further demonstrated by using immunochemical, pharmacological, and biochemical approaches. Thus, an antiserum generated against the CHO enzyme recognized the P388D1 cell enzyme and gave similar molecular masses (about 83 kDa) for the two enzymes under the same experimental conditions. Further, the CHO enzyme has exactly the same sensitivity to inhibition by a variety of compounds previously shown to inhibit the P388D1 enzyme, including bromoenol lactone, palmitoyl trifluoromethyl ketone, and methyl arachidonyl fluorophosphonate. Additionally, covalent modification of the CHO enzyme by [3H]bromoenol lactone is dependent on active enzyme as is the P388D1 iPLA2. Finally, both enzymes have the same specific activities under identical experimental conditions.

A novel cytosolic calcium-independent phospholipase A2 contains eight ankyrin motifs

We report the purification, molecular cloning, and expression of a novel cytosolic calcium-independent phospholipase A2 (iPLA2) from Chinese hamster ovary cells, which lacks extended homology to other phospholipases. iPLA2 is an 85-kDa protein that exists as a multimeric complex of 270-350 kDa with a specific activity of 1 micromol/min/mg. The full-length cDNA clone encodes a 752-amino acid cytoplasmic protein with one lipase motif (GXS465XG) and eight ankyrin repeats. Expression of the cDNA in mammalian cells generates an active 85-kDa protein. Mutagenesis studies show that Ser465 and the ankyrin repeats are required for activity. We demonstrate that iPLA2 selectively hydrolyzes the sn-2 over sn-1 fatty acid by 5-fold for 1,2-dipalmitoyl phosphatidylcholine in a mixed micelle. Moreover, we found the fatty acid preference at the sn-2 position to be highly dependent upon substrate presentation. However, iPLA2 does have a marked preference for 1,2-dipalmitoyl phosphatidic acid presented in a vesicle, generating the lipid second messenger lysophosphatidic acid. Finally the enzyme is able to hydrolyze the acetyl moiety at the sn-2 position of platelet-activating factor.

Depletion of intracellular calcium stores activates smooth muscle cell calcium-independent phospholipase A2. A novel mechanism underlying arachidonic acid mobilization

Herein we present multiple lines of evidence which demonstrate that depletion of internal calcium stores is both necessary and sufficient for the activation of calcium-independent phospholipase A2 during arginine vasopressin (AVP)-mediated mobilization of arachidonic acid in A-10 smooth muscle cells. First, AVP-induced [3H]arachidonic acid release was independent of increases in cytosolic calcium yet was decreased by pharmacological inhibition of the release of calcium ion from internal stores. Second, thapsigargin induced the dramatic release of [3H]arachidonic acid from A-10 cells at a similar rate as the AVP-induced release of arachidonic acid, and the release of arachidonic acid by either AVP or thapsigargin was entirely inhibited by (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one (BEL). Third, the magnitude of thapsigargin-induced [3H]arachidonic acid release was entirely independent of alterations in cytosolic calcium concentration. Fourth, A23187 resulted in the BEL-inhibitable release of [3H]arachidonic acid from A-10 cells even when ionophore-induced increases in cytosolic calcium were completely prevented by calcium chelators. Fifth, pretreatment of A-10 cells with a calmodulin antagonist (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, HCl) resulted in the time-dependent decrease of subsequent thapsigargin-induced [3H]arachidonic acid release. Collectively, these results identify a novel paradigm which links alterations in calcium homeostasis to the calmodulin-mediated regulation of calcium-independent phospholipase A2 through the depletion of internal calcium stores.