We have previously shown that disruption of mitochondrial membrane potential by depletion of mitochondrial DNA (mtDNA) or treatment with a mitochondrial ionophore, carbonyl cyanide m-chlorophenylhydrazone, initiates a stress signaling, which causes resistance to apoptosis, and induces invasive behavior in C2C12 myocytes and A549 cells. In the present study we show that calcineurin (Cn), activated as part of this stress signaling, plays an important role in increased glucose uptake and glycolysis. Here we report that, although both insulin and insulin-like growth factor-1 receptor levels (IR and IGF1R, respectively) are increased in response to mitochondrial stress, autophosphorylation of IGF1R was selectively increased suggesting a shift in receptor pathways. Using an approach with FK506, an inhibitor of Cn, and mRNA silencing by small interference RNA we show that mitochondrial stress-activated Cn is critical for increased GLUT 4 and IGF1R expression and activation. The importance of the IGF1R pathway in cell survival under mitochondrial stress is demonstrated by increased apoptosis either by IGF1R mRNA silencing or by treatment with IGF1R inhibitors (AG1024 and picropodophyllin). This study describes a novel mechanism of mitochondrial stress-induced metabolic shift involving Cn with implications in resistance to apoptosis and tumor proliferation.
Acid-sensing ion channel (ASIC) 1a and ASIC2a are acid-sensing ion channels in central and peripheral neurons. ASIC1a has been implicated in long-term potentiation of synaptic transmission and ischemic brain injury, whereas ASIC2a is involved in mechanosensation. Although the biological role and distribution of ASIC1a and ASIC2a subunits in brain have been well characterized, little is known about the intracellular regulation of these ion channels that modulates their function. Using pulldown assays and mass spectrometry, we have identified A kinase-anchoring protein (AKAP)150 and the protein phosphatase calcineurin as binding proteins to ASIC2a. Extended pulldown and co-immunoprecipitation assays showed that these regulatory proteins also interact with ASIC1a. Transfection of rat cortical neurons with constructs encoding green fluorescent protein- or hemagglutinin-tagged channels showed expression of ASIC1a and ASIC2a in punctate and clustering patterns in dendrites that co-localized with AKAP150. Inhibition of protein kinase A binding to AKAPs by Ht-31 peptide reduces ASIC currents in cortical neurons and Chinese hamster ovary cells, suggesting a role of AKAP150 in association with protein kinase A in ASIC function. We also demonstrated a regulatory function of calcineurin in ASIC1a and ASIC2a activity. Cyclosporin A, an inhibitor of calcineurin, increased ASIC currents in Chinese hamster ovary cells and in cortical neurons, suggesting that activity of ASICs is inhibited by calcineurin-dependent dephosphorylation. These data imply that ASIC down-regulation by calcineurin could play an important role under pathological conditions accompanying intracellular Ca2+ overload and tissue acidosis to circumvent harmful activities mediated by these channels.
Cytochrome P450 2E1 (CYP2E1) plays an important role in alcohol-induced toxicity and oxidative stress. Recently, we showed that this predominantly microsomal protein is also localized in rat hepatic mitochondria. In this report, we show that the N-terminal 30 amino acids of CYP2E1 contain a chimeric signal for bimodal targeting of the apoprotein to endoplasmic reticulum (ER) and mitochondria. We demonstrate that the cryptic mitochondrial targeting signal at sequence 21–31 of the protein is activated by cAMP-dependent phosphorylation at Ser-129. S129A mutation resulted in lower affinity for binding to cytoplasmic Hsp70, mitochondrial translocases (TOM40 and TIM44) and reduced mitochondrial import. S129A mutation, however, did not affect the extent of binding to the signal recognition particle and association with ER membrane translocator protein Sec61. Addition of saturating levels of signal recognition particle caused only a partial inhibition of CYP2E1 translation under in vitro conditions, and saturating levels of ER resulted only in partial membrane integration. cAMP enhanced the mitochondrial CYP2E1 (referred to as P450MT5) level but did not affect its level in the ER. Our results provide new insights on the mechanism of cAMP-mediated activation of a cryptic mitochondrial targeting signal and regulation of P450MT5 targeting to mitochondria.
In mammalian cells, phospholipase D activity is tightly regulated by diverse cellular signals, including hormones, neurotransmitters, and growth factors. Multiple signaling pathways converge upon phospholipase D to modulate cellular actions, such as cell growth, shape, and secretion. We examined the kinetics of protein kinase C and G-protein regulation of mammalian phospholipase D1 (PLD1) in order to better understand interactions between PLD1 and its regulators. Activation by Arf-1, RhoA, Rac1, Cdc42, protein kinase Cα, and phosphatidylinositol 4,5-bisphosphate displayed surface dilution kinetics, but these effectors modulated different kinetic parameters. PKCα activation of PLD1 involves N- and C-terminal PLD domains. Rho GTPases were binding activators, enhancing the catalytic efficiency of a purified PLD1 catalytic domain via effects on Km. Arf-1, a catalytic activator, stimulated PLD1 by enhancing the catalytic constant, kcat. A kinetic description of PLD1 activation by multiple modulators reveals a mechanism for apparent synergy between activators. Synergy was observed only when PLD1 was simultaneously stimulated by a binding activator and a catalytic activator. Surprisingly, synergistic activation was steeply dependent on phosphatidylinositol 4,5-bisphosphate and phosphatidylcholine. Together, these findings suggest a role for PLD1 as a signaling node, in which integration of convergent signals occurs within discrete locales of the cellular membrane.
The tumor necrosis family member BAFF is limiting for the survival of follicular B lymphocytes, but excessive BAFF signaling can lead to autoimmunity, suggesting that its activity must be tightly regulated. We have identified a conserved alternate splice isoform of BAFF, called ΔBAFF, which lacks 57 nt encoding the A–A1 loop and is co-expressed with BAFF in many mouse and human myeloid cells. Mouse ΔBAFF appears on the plasma membrane, but unlike BAFF it is inefficiently released by proteolysis. ΔBAFF can associate with BAFF in heteromultimers and diminish BAFF bioactivity and release. Thus, alternative splicing of the BAFF gene suppresses BAFF B cell stimulatory function in several ways, and ΔBAFF may promote other functions as well.
In Plasmodium falciparum, dihydrofolate reductase and thymidylate synthase activities are conferred by a single 70-kDa bifunctional polypeptide (DHFR-TS, dihydrofolate reductase-thymidylate synthase) which assembles into a functional 140-kDa homodimer. In mammals, the two enzymes are smaller distinct molecules encoded on different genes. A 27-kDa amino domain of malarial DHFR-TS is sufficient to provide DHFR activity, but the structural requirements for TS function have not been established. Although the 3′-end of DHFR-TS has high homology to TS sequences from other species, expression of this protein fragment failed to yield active TS enzyme, and it failed to complement TS−
Escherichia coli. Unexpectedly, even partial 5′-deletion of full-length DHFR-TS gene abolished TS function on the 3′-end. Thus, it was hypothesized that the amino end of the bifunctional parasite protein plays an important role in TS function. When the 27-kDa amino domain (DHFR) was provided in trans, a previously inactive 40-kDa carboxyl-domain from malarial DHFR-TS regained its TS function. Physical characterization of the “split enzymes” revealed that the 27- and the 40-kDa fragments of DHFR-TS had reassembled into a 140-kDa hybrid complex. Thus, in malarial DHFR-TS, there are physical interactions between the DHFR domain and the TS domain, and these interactions are necessary to obtain a catalytically active TS. Interference with these essential protein-protein interactions could lead to new selective strategies to treat malaria resistant to traditional DHFR-TS inhibitors.
The developing mammalian embryo is entirely dependent on the maternal circulation for its supply of retinoids (vitamin A and its metabolites). The mechanisms through which mammalian developing tissues maintain adequate retinoid levels in the face of suboptimal or excessive maternal dietary vitamin A intake have not been established. We investigated the role of retinyl ester formation catalyzed by lecithin:retinol acyltransferase (LRAT) in regulating retinoid homeostasis during embryogenesis. Dams lacking both LRAT and retinol-binding protein (RBP), the sole specific carrier for retinol in serum, were maintained on diets containing different amounts of vitamin A during pregnancy. We hypothesized that the lack of both proteins would make the embryo more vulnerable to changes in maternal dietary vitamin A intake. Our data demonstrate that maternal dietary vitamin A deprivation during pregnancy generates a severe retinoid-deficient phenotype of the embryo due to the severe retinoid-deficient status of the double mutant dams rather than to the lack of LRAT in the developing tissues. Moreover, in the case of excessive maternal dietary vitamin A intake, LRAT acts together with Cyp26A1, one of the enzymes that catalyze the degradation of retinoic acid, and possibly with STRA6, the recently identified cell surface receptor for retinol-RBP, in maintaining adequate levels of retinoids in embryonic and extraembryonic tissues. In contrast, the pathway of retinoic acid synthesis does not contribute significantly to regulating retinoid homeostasis during mammalian development except under conditions of severe maternal retinoid deficiency.
Rapamycin is an immunosuppressive drug that binds simultaneously to the 12-kDa FK506- and rapamycin-binding protein (FKBP12, or FKBP) and the FKBP-rapamycin binding domain (FRB) of the mammalian target of rapamycin (mTOR) kinase. The resulting ternary complex has been used to conditionally perturb protein function, and one such method involves perturbation of a protein of interest through its mislocalization. We synthesized two rapamycin derivatives that possess large substituents at the C16 position within the FRB-binding interface, and these derivatives were screened against a library of FRB mutants using a three-hybrid assay in Saccharomyces cerevisiae. Several FRB mutants responded to one of the rapamycin derivatives, and twenty of these mutants were further characterized in mammalian cells. The mutants most responsive to the ligand were fused to yellow fluorescent protein, and fluorescence levels in the presence and absence of the ligand were measured to determine stability of the fusion proteins. Wild-type and mutant FRB domains were expressed at low levels in the absence of the rapamycin derivative, and expression levels rose up to ten-fold upon treatment with ligand. The synthetic rapamycin derivatives were further analyzed using quantitative mass spectrometry, and one of the compounds was found to contain contaminating rapamycin. Furthermore, uncontaminated analogs retain the ability to inhibit mTOR, albeit with diminished potency relative to rapamycin. The ligand-dependent stability displayed by wildtype FRB and FRB mutants as well as the inhibitory potential and purity of the rapamycin derivatives should be considered as potentially confounding experimental variables when using these systems.
Transcription by RNA polymerase II is antagonized by the presence of a nearby tRNA gene in Saccharomyces cerevisiae. To test hypotheses concerning the mechanism of this tRNA gene-mediated (tgm) silencing, the effects of specific gene deletions were determined. The results show that the mechanism of silencing near tRNA genes is fundamentally different from other forms of transcriptional silencing in yeast. Rather, tgm silencing is dependent on the ability to cluster the dispersed tRNA genes in or near the nucleolus, constituting a form of three-dimensional gene control.
d-Tyr-tRNATyr deacylase is an editing enzyme that removes d-tyrosine and other d-amino acids from charged tRNAs, thereby preventing incorrect incorporation of d-amino acids into proteins. A model for the catalytic mechanism of this enzyme is proposed based on the crystal structure of the enzyme from Haemophilus influenzae determined at a 1.64-Å resolution. Structural comparison of this dimeric enzyme with the very similar structure of the enzyme from Escherichia coli together with sequence analyses indicate that the active site is located in the dimer interface within a depression that includes an invariant threonine residue, Thr-80. The active site contains an oxyanion hole formed by the main chain nitrogen atoms of Thr-80 and Phe-79 and the side chain amide group of the invariant Gln-78. The Michaelis complex between the enzyme and d-Tyr-tRNA was modeled assuming a nucleophilic attack on the carbonyl carbon of d-Tyr by the Thr-80 Oγ atom and a role for the oxyanion hole in stabilizing the negatively charged tetrahedral transition states. The model is consistent with all of the available data on substrate specificity. Based on this model, we propose a substrate-assisted acylation/deacylation-catalytic mechanism in which the amino group of the d-Tyr is deprotonated and serves as the general base.
Oxygen homeostasis represents an essential organizing principle of metazoan evolution and biology. Hypoxia-inducible factor 1 (HIF-1) is a master regulator of transcriptional responses to changes in O2 concentration. HIF-1 is a heterodimer of HIF-1α and HIF-1β subunits. O2-dependent degradation of the HIF-1α subunit is mediated by prolyl hydroxylase, von Hippel-Lindau protein (VHL)/Elongin-C E3 ubiquitin ligase, and the proteasome. O2-independent degradation of HIF-1α is regulated by the competition of RACK1 and HSP90 for binding to HIF-1α. RACK1 binding results in the recruitment of the Elongin-C E3 ubiquitin ligase, leading to VHL-independent ubiquitination and degradation of HIF-1α. In this report, we show that calcineurin inhibits the ubiquitination and proteasomal degradation of HIF-1α. Calcineurin is a serine/threonine phosphatase that is activated by calcium and calmodulin. The phosphatase activity of calcineurin is required for its regulation of HIF-1α. RACK1 binds to the catalytic domain of calcineurin and is required for HIF-1α degradation induced by the calcineurin inhibitor cyclosporine A. Elongin-C and HIF-1α each bind to RACK1 and dimerization of RACK1 is required to recruit Elongin-C to HIF-1α. Phosphorylation of RACK1 promotes its dimerization and dephosphorylation by calcineurin inhibits dimerization. Serine 146 within the dimerization domain is phosphorylated and mutation of serine 146 impairs RACK1 dimerization and HIF-1α degradation. These results indicate that intracellular calcium levels can regulate HIF-1α expression by modulating calcineurin activity and RACK1 dimerization.
The MNT1 gene of the human fungal pathogen Candida albicans is involved in O-glycosylation of cell wall and secreted proteins and is important for adherence of C. albicans to host surfaces and for virulence. Here we describe the molecular analysis of CaMNT2, a second member of the MNT1-like gene family in C. albicans. Mnt2p also functions in O-glycosylation. Mnt1p and Mnt2p encode partially redundant α-1,2-mannosyltransferases that catalyze the addition of the second and third mannose residues in an O-linked mannose pentamer. Deletion of both copies of MNT1 and MNT2 resulted in reduction in the level of in vitro mannosyltransferase activity and truncation of O-mannan. Both the mnt2Δ and mnt1Δ single mutants were significantly reduced in adherence to human buccal epithelial cells and Matrigel-coated surfaces, indicating a role for O-glycosylated cell wall proteins or O-mannan itself in adhesion to host surfaces. The double mnt1Δmnt2Δ mutant formed aggregates of cells that appeared to be the result of abnormal cell separation. The double mutant was attenuated in virulence, underlining the importance of O-glycosylation in pathogenesis of C. albicans infections.
Three short hydrophobic loops and a conserved undecapeptide at the tip of domain 4 (D4) of the cholesterol-dependent cytolysins (CDCs) mediate the binding of the CDC monomers to cholesterol-rich cell membranes. But intermedilysin (ILY), from Streptococcus intermedius, does not bind to cholesterol-rich membranes unless they contain the human protein CD59. This observation suggested that the D4 loops, which include loops L1–L3 and the undecapeptide, of ILY were no longer required for its cell binding. However, we show here that membrane insertion of the D4 loops is required for the cytolysis by ILY. Receptor binding triggers changes in the structure of ILY that are necessary for oligomerization, but membrane insertion of the D4 loops is critical for oligomer assembly and pore formation. Defects that prevent membrane insertion of the undecapeptide also block assembly of the prepore oligomer, while defects in the membrane insertion of the L1–L3 loops prevent the conversion of the prepore oligomer to the pore complex. These studies reveal that pore formation by ILY, and probably other CDCs, is affected by an intricate and coupled sequence of interactions between domain 4 and the membrane.
Obesity is associated with a significantly increased risk for cancer suggesting that adipose tissue dysfunctions might play a crucial role therein. Macrophages play important roles in adipose tissue as well as in cancers. Here, we studied whether human adipose tissue macrophages (ATM) modulate cancer cell function.
Therefore, ATM were isolated and compared to monocyte-derived macrophages (MDM) from the same obese patients. ATM, but not MDM, were found to secrete factors inducing inflammation and lipid accumulation in human T47D and HT-29 cancer cells. Gene expression profile comparison of ATM and MDM revealed over-expression of functional clusters, such as cytokine-cytokine receptor interaction (especially CXC-chemokine) signalling as well as cancer-related pathways, in ATM. Comparison with gene expression profiles of human tumour-associated macrophages (TAM) showed that ATM, but not MDM resemble TAM. Indirect co-culture experiments demonstrated that factors secreted by pre-adipocytes, but not mature adipocytes, confer an ATM-like phenotype to MDM. Finally, the concentrations of ATM secreted factors related to cancer are elevated in serum of obese subjects. In conclusion, ATM may thus modulate the cancer cell phenotype.
Adipocytes; cytology; Adipose Tissue; metabolism; Azo Compounds; pharmacology; Cell Line, Tumor; Chemokines; metabolism; Disease Progression; Gene Expression Regulation, Neoplastic; Humans; Immunohistochemistry; methods; Inflammation; Macrophages; cytology; metabolism; Neoplasms; metabolism; Obesity; metabolism; Oligonucleotide Array Sequence Analysis; Phenotype; macrophages; obesity; cancer; adipose tissue; chemokines
The small GTPase Rab2 is required for membrane transport between the endoplasmic reticulum (ER) and the Golgi complex. Rab2 associates with pre-Golgi intermediates (also termed vesicular tubular clusters; VTCs) that sort cargo to the anterograde pathway from recycling proteins retrieved to the ER. Our previous studies have shown that Rab2 stimulates atypical protein kinase Cι/λ (aPKCι/λ) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) recruitment to VTCs. Both aPKCι/λ and GAPDH bind directly to Rab2 and aPKCι/λ and GAPDH interact. Based on the reports demonstrating aPKCι-Src interaction and Src activity in the retrograde pathway (Golgi - ER), studies were initiated to learn whether Rab2 also promoted Src recruitment to VTCs. Using a quantitative membrane binding assay, we found that Rab2 stimulated Src membrane association in a dose dependent manner. The recruited Src binds to aPKCι/λ and GAPDH on the membrane, however Src does not interact with Rab2. The membrane-associated Src tyrosine phosphorylates aPKCι/λ on the VTC. To determine the consequence of aPKCι/λ tyrosine phosphorylation, the membrane binding assay was supplemented with the Src-specific tyrosine kinase inhibitor PP2. Although Rab2, Src, and GAPDH recruitment was not affected, the Rab2-PP2 treated membranes contained a negligible amount of aPKCι/λ. Since Rab2 requires aPKCι/λ for the downstream recruitment of β–coat protein (β-COP) to VTCs, the Rab2-PP2 treated membranes were evaluated for the presence of β-COP. Like aPKCι/λ, the membranes contained a negligible amount of β-COP that was reflected by the drastic reduction in Rab2-dependent COPI-vesicle formation. These data suggest that Src-mediated tyrosine phosphorylation of aPKCι/λ facilitates aPKCι/λ association with Rab2-Src-GAPDH on VTCs, which is ultimately necessary for the downstream recruitment of β-COP and release of Rab2-mediated retrograde-directed vesicles.
As a central regulator for cell cycle arrest, apoptosis, and cellular senescence, p53 requires multiple layers of regulatory control to ensure correct temporal and spatial functions. It is well accepted that Mdm2-mediated ubiquitination plays a crucial role in p53 regulation. In addition to proteasome-mediated degradation, ubiquitination of p53 by Mdm2 acts a key signal for its nuclear export. Nuclear export has previously been thought to require the disassociation of the p53 tetramer and exposure of the intrinsic nuclear export signal. To elucidate the molecular mechanism of degradation-independent repression on p53 by Mdm2, we have developed a two-step approach to purify ubiquitinated forms of p53 induced by Mdm2 from human cells. Surprisingly, however, we found that ubiquitination has no effect on the tetramerization/oligomerization of p53, arguing against this seemingly well accepted model. Moreover, nuclear export of p53 alone is not sufficient to completely abolish p53 activity. Ubiquitination-mediated repression of p53 by Mdm2 acts at least, in part, through inhibiting the sequence-specific DNA binding activity. Thus, our results have important implications regarding the mechanisms by which Mdm2 acts on p53.
Ultraviolet (UV) irradiation rapidly increases tyrosine phosphorylation (i.e. activates) of epidermal growth factor receptors (EGFR) in human skin. EGFR-dependent signaling pathways drive increased expression of matrix-metalloproteinases, whose actions fragment collagen and elastin fibers, the primary structural protein components in skin connective tissue. Connective tissue fragmentation, which results from chronic exposure to solar UV irradiation, is a major determinant of premature skin aging (photoaging). UV irradiation generates reactive oxygen species, which readily react with conserved cysteine residues in the active site of protein tyrosine phosphatases (PTP). We report here that EGFR activation by UV irradiation results from oxidative inhibition of receptor type PTP-kappa (RPTP-κ). RPTP-κ directly counters intrinsic EGFR tyrosine kinase activity, thereby maintaining EGFR in an inactive state. Reversible, oxidative inactivation of RPTP-κ activity by UV irradiation shifts the kinase-phosphatase balance in favor of EGFR activation. These data delineate a novel mechanism of EGFR regulation and identify RPTP-κ as a key molecular target for anti-oxidant protection against skin aging.
Smad7 functions as an endogenous negative regulator of TGF-β/Smad signaling. TGF-β/Smad pathway is a major regulator of collagen production in connective tissue. Reduced expression of Smad7 has been reported in TGF-β-mediated fibrotic diseases, characterized by over production of collagen. Solar ultraviolet (UV) irradiation reduces collagen production by fibroblasts in human skin in vivo. We have investigated regulation of Smad7 gene expression by UV irradiation in human skin fibroblasts. UV irradiation transiently increased Smad7 mRNA and protein levels. Induction of Smad7 mRNA and protein was maximal within five hours, and returned to initial basal levels 24 hours post UV. UV irradiation induced Smad7 promoter reporter activity 3-fold. Smad7 promoter contains functional enhancer sequences that bind transcription factors Smad3 and activator protein-1 (AP-1). UV irradiation reduced protein binding to the Smad3 enhancer, and increased binding to the AP-1 enhancer. Deletion of AP-1 binding site in the Smad7 promoter completely abolished UV stimulation of Smad7 transcription. Deletion of Smad3 element had no effect on UV-induced promoter activity. UV irradiation increased mRNA and protein expression of AP-1 family members, c-Jun and c-Fos, which bound to the AP-1 element in the Smad7 promoter. Furthermore, over-expression of dominant negative c-Jun substantially reduced UV induction of Smad7 transcription. These data demonstrate that induction of Smad7 gene expression by UV irradiation is mediated via induction of transcription factor AP-1 in human skin fibroblasts.
Smad7; Ultraviolet radiation; AP-1
5-Aminolevulinate synthase (ALAS), a pyridoxal 5′-phosphate-dependent enzyme, catalyzes the first, and regulatory, step of the heme biosynthetic pathway in nonplant eukaryotes and some bacteria. 5-Aminolevulinate synthase is a dimeric protein having an ordered kinetic mechanism with glycine binding before succinyl-CoA and with aminolevulinate release after CoA and carbon dioxide. Rapid scanning stopped-flow absorption spectrophotometry in conjunction with multiple turnover chemical quenched-flow kinetic analyses and a newly developed CoA detection method were used to examine the ALAS catalytic reaction and identify the rate-determining step. The reaction of glycine with ALAS follows a three-step kinetic process, ascribed to the formation of the Michaelis complex and the pyridoxal 5′-phosphate-glycine aldimine, followed by the abstraction of the glycine pro-R proton from the external aldimine. Significantly, the rate associated with this third step (k3 = 0.002 s−1) is consistent with the rate determined for the ALAS-catalyzed removal of tritium from [2-3H2]glycine. Succinyl-CoA and acetoacetyl-CoA increased the rate of glycine proton removal ~250,000-and 10-fold, respectively, supporting our previous proposal that the physiological substrate, succinyl-CoA, promotes a protein conformational change, which accelerates the conversion of the external aldimine into the initial quinonoid intermediate (Hunter, G. A., and Ferreira, G. C. (1999) J. Biol. Chem. 274, 12222–12228). Rapid scanning stopped-flow and quenched-flow kinetic analyses of the ALAS reaction under single turnover conditions lend evidence for two quinonoid reaction intermediates and a model of the ALAS kinetic mechanism in which product release is at least the partially rate-limiting step. Finally, the carbonyl and carboxylate groups of 5-aminolevulinate play a major protein-interacting role by inducing a conformational change in ALAS and, thus, possibly modulating product release.
The Group VIA Phospholipase A2 (iPLA2β) is the first recognized cytosolic Ca2+-independent PLA2 and has been proposed to participate in arachidonic acid (20:4) incorporation into glycerophosphocholine lipids, cell proliferation, exocytosis, apoptosis, and other processes. To study iPLA2β functions, we disrupted its gene by homologous recombination to generate mice that do not express iPLA2β. Heterozygous iPLA2β+/− breeding pairs yield a Mendelian 1:2:1 ratio of iPLA2β+/+, iPLA2β+/−, and iPLA2β−/− pups and a 1:1 male:female gender distribution of iPLA2β−/− pups. Several tissues of wild-type mice express iPLA2β mRNA, immunoreactive protein, and activity, and testes express the highest levels. Testes or other tissues of iPLA2β−/− mice express no iPLA2β mRNA or protein, but iPLA2β−/− testes are not deficient in 20:4-containing glycerophosphocholine lipids, indicating that iPLA2β does not play an obligatory role in formation of such lipids in that tissue. Spermatozoa from iPLA2β−/− mice have reduced motility and impaired ability to fertilize mouse oocytes in vitro and in vivo, and inhibiting iPLA2β with a bromoenol lactone suicide substrate reduces motility of wild-type spermatozoa in a time- and concentration-dependent manner.Mating iPLA2β−/− male mice with iPLA2β+/+, iPLA2β+/−, or iPLA2β−/− female mice yields only about 7% of the number of pups produced by mating pairs with an iPLA2β+/+ or iPLA2β+/− male, but iPLA2β−/− female mice have nearly normal fertility. These findings indicate that iPLA2β plays an important functional role in spermatozoa, suggest a target for developing male contraceptive drugs, and complement reports that disruption of the Group IVA PLA2 (cPLA2α) gene impairs female reproductive ability.
An 85-kDa Group VI phospholipase A2 enzyme (iPLA2) that does not require Ca2+ for catalysis has recently been cloned from three rodent species. A homologous 88-kDa enzyme has been cloned from human B-lymphocyte lines that contains a 54-amino acid insert not present in the rodent enzymes, but human cells have not previously been observed to express catalytically active iPLA2 isoforms other than the 88-kDa protein. We have cloned cDNA species that encode two distinct iPLA2 isoforms from human pancreatic islet RNA and a human insulinoma cDNA library. One isoform is an 85-kDa protein (short isoform of human iPLA2 (SH-iPLA2)) and the other an 88-kDa protein (long isoform of human iPLA2 (LH-iPLA2)). Transcripts encoding both isoforms are also observed in human promonocytic U937 cells. Recombinant SH-iPLA2 and LH-iPLA2 are both catalytically active in the absence of Ca2+ and inhibited by a bromoenol lactone suicide substrate, but LH-iPLA2 is activated by ATP, whereas SH-iPLA2 is not. The human iPLA2 gene has been found to reside on chromosome 22 in region q13.1 and to contain 16 exons represented in the LH-iPLA2 transcript. Exon 8 is not represented in the SH-iPLA2 transcript, indicating that it arises by an exon-skipping mechanism of alternative splicing. The amino acid sequence encoded by exon 8 of the human iPLA2 gene is proline-rich and shares a consensus motif of PX5PX8HHPX12NX4Q with the proline-rich middle linker domains of the Smad proteins DAF-3 and Smad4. Expression of mRNA species encoding two active iPLA2 isoforms with distinguishable catalytic properties in two different types of human cells demonstrated here may have regulatory or functional implications about the roles of products of the iPLA2 gene in cell biologic processes.
Insulin-secreting pancreatic islet β-cells express a Group VIA Ca2+-independent phospholipase A2 (iPLA2β) that contains a calmodulin binding site and protein interaction domains. We identified Ca2+/calmodulin-dependent protein kinase IIβ (CaMKIIβ) as a potential iPLA2β-interacting protein by yeast two-hybrid screening of a cDNA library using iPLA2β cDNA as bait. Cloning CaMKIIβ cDNA from a rat islet library revealed that one dominant CaMKIIβ isoform mRNA is expressed by adult islets and is not observed in brain or neonatal islets and that there is high conservation of the isoform expressed by rat and human β-cells. Binary two-hybrid assays using DNA encoding this isoform as bait and iPLA2β DNA as prey confirmed interaction of the enzymes, as did assays with CaMKIIβ as prey and iPLA2β bait. His-tagged CaMKIIβ immobilized on metal affinity matrices bound iPLA2β, and this did not require exogenous calmodulin and was not prevented by a calmodulin antagonist or the Ca2+ chelator EGTA. Activities of both enzymes increased upon their association, and iPLA2β reaction products reduced CaMKIIβ activity. Both the iPLA2β inhibitor bromoenol lactone and the CaMKIIβ inhibitor KN93 reduced arachidonate release from INS-1 insulinoma cells, and both inhibit insulin secretion. CaMKIIβ and iPLA2β can be coimmunoprecipitated from INS-1 cells, and forskolin, which amplifies glucose-induced insulin secretion, increases the abundance of the immunoprecipitatable complex. These findings suggest that iPLA2β and CaMKIIβ form a signaling complex in β-cells, consistent with reports that both enzymes participate in insulin secretion and that their expression is coinduced upon differentiation of pancreatic progenitor to endocrine progenitor cells.
The transcription factor NF-κB plays an important role in both physiological and pathological events in the central nervous system. Nevertheless, the mechanisms of NF-κB-mediated regulation of gene expression, and the signaling molecules participating in the NF-κB pathway in the central nervous system are, to date, poorly understood. To identify such molecules, we conducted a yeast two-hybrid screen of a human brain cDNA library using NIK as bait. As a result, we identified a novel NIK and IKKβ binding protein designated NIBP that is mainly expressed in brain, muscle, heart, and kidney. Interestingly, low levels of expression were detected in immune tissues such as spleen, thymus, and peripheral blood leukocytes, where NF-κB is known to modulate immune function. We demonstrated by immunohistochemistry that NIBP expression in the brain is localized to neurons. NIBP physically interacts with NIK IKKβ, but not IKKα or IKKγ. NIBP overexpression potentiates tumor necrosis factor-α-induced NF-κB activation through increased phosphorylation of the IKK complex and its downstream IκBα and p65 substrates. Finally, knockdown of NIBP expression by small interfering RNA reduces tumor necrosis factor-α-induced NF-κB activation, prevents nerve growth factor-induced neuronal differentiation, and decreases Bcl-xL gene expression in PC12 cells. Our data demonstrate that NIBP, by interacting with NIK and IKKβ, is a new enhancer of the cytokine-induced NF-κB signaling pathway. Because of its neuronal expression, we propose that NIBP may be a potential target for modulating the NF-κB signaling cascade in neuronal pathologies dependent upon abnormal activation of this pathway.
Hookworms, parasitic nematodes that infect nearly one billion people worldwide, are a major cause of anemia and malnutrition. We hypothesize that hookworms actively manipulate the host immune response through the production of specific molecules designed to facilitate infection by larval stages and adult worm survival within the intestine. A full-length cDNA encoding a secreted orthologue of the human cytokine, Macro-phage Migration Inhibitory Factor (MIF) has been cloned from the hookworm Ancylostoma ceylanicum. Elucidation of the three-dimensional crystal structure of recombinant AceMIF (rAceMIF) revealed an overall structural homology with significant differences in the tautomerase sites of the human and hookworm proteins. The relative bioactivities of human and hookworm MIF proteins were compared using in vitro assays of tautomerase activity, macrophage migration, and binding to MIF receptor CD74. The activity of rAceMIF was not inhibited by the ligand ISO-1, which was previously determined to be an inhibitor of the catalytic site of human MIF. These data define unique immunological, structural, and functional characteristics of AceMIF, thereby establishing the potential for selectively inhibiting the hookworm cytokine as a means of reducing parasite survival and disease pathogenesis.
Erythroid-specific, high level expression of the β-globin genes is regulated by the locus control region (LCR), composed of multiple DNase I-hypersensitive sites and located far upstream of the genes. Recent studies have shown that LCR core elements recruit RNA polymerase II (pol II). In the present study we demonstrate the following: 1) pol II and other basal transcription factors are recruited to LCR core hypersensitive elements; 2) pol II dissociates from and re-associates with the globin gene locus during replication; 3) pol II interacts with the LCR but not with the β-globin gene prior to erythroid differentiation in embryonic stem cells; and 4) the erythroid transcription factor NF-E2 facilitates the transfer of pol II from immobilized LCR constructs to a β-globin gene in vitro. The data are consistent with the hypothesis that the LCR serves as the primary attachment site for the recruitment of macromolecular complexes involved in chromatin structure alterations and transcription of the globin genes.