Tumor suppressor p53 mediates interleukin-6 expression to enable cancer cell evasion of genotoxic stress

The tumor suppressor p53 primarily functions as a mediator of DNA damage-induced cell death, thereby contributing to the efficacy of genotoxic anticancer therapeutics. Here, we show, on the contrary, that cancer cells can employ genotoxic stress-induced p53 to acquire treatment resistance through the production of the pleiotropic cytokine interleukin (IL)-6. Mechanistically, DNA damage, either repairable or irreparable, activates p53 and stimulates Caspase-2-mediated cleavage of its negative regulator mouse double minute 2 (MDM2) creating a positive feedback loop that leads to elevated p53 protein accumulation. p53 transcriptionally controls the major adenosine triphosphate (ATP) release channel pannexin 1 (Panx1), which directs IL-6 induction via a mechanism dependent on the extracellular ATP-activated purinergic P2 receptors as well as their downstream intracellular calcium (iCa2+)/PI3K/Akt/NF-ĸB signaling pathway. Thus, p53 silencing impairs Panx1 and IL-6 expression and renders cancer cells sensitive to genotoxic stress. Moreover, we confirm that IL-6 hampers the effectiveness of genotoxic anticancer agents by mitigating DNA damage, driving the expression of anti-apoptotic Bcl-2 family genes, and maintaining the migratory and invasive properties of cancer cells. Analysis of patient survival and relevant factors in lung cancer and pan-cancer cohorts supports the prognostic and clinical values of Panx1 and IL-6. Notably, IL-6 secreted by cancer cells during genotoxic treatments promotes the polarization of monocytic THP-1-derived macrophages into an alternative (M2-like) phenotype that exhibits impaired anti-survival activities but enhanced pro-metastatic effects on cancer cells as compared to nonpolarized macrophages. Our study reveals the precise mechanism for genotoxic-induced IL-6 and suggests that targeting p53-mediated IL-6 may improve the responsiveness of cancer cells to genotoxic anticancer therapy.

Action mechanism of snake venom l-amino acid oxidase and its double-edged sword effect on cancer treatment: Role of pannexin 1-mediated interleukin-6 expression

Snake venom l-amino acid oxidases (svLAAOs) have been recognized as promising candidates for anticancer therapeutics. However, multiple aspects of their catalytic mechanism and the overall responses of cancer cells to these redox enzymes remain ambiguous. Here, we present an analysis of the phylogenetic relationships and active site-related residues among svLAAOs and reveal that the previously proposed critical catalytic residue His 223 is highly conserved in the viperid but not the elapid svLAAO clade. To gain further insight into the action mechanism of the elapid svLAAOs, we purify and characterize the structural, biochemical, and anticancer therapeutic potentials of the Thailand elapid snake Naja kaouthia LAAO (NK-LAAO). We find that NK-LAAO, with Ser 223, exhibits high catalytic activity toward hydrophobic l-amino acid substrates. Moreover, NK-LAAO induces substantial oxidative stress-mediated cytotoxicity with the magnitude relying on both the levels of extracellular hydrogen peroxide (H₂O₂) and intracellular reactive oxygen species (ROS) generated during the enzymatic redox reactions, but not being influenced by the N-linked glycans on its surface. Unexpectedly, we discover a tolerant mechanism deployed by cancer cells to dampen the anticancer activities of NK-LAAO. NK-LAAO treatment amplifies interleukin (IL)-6 expression via the pannexin 1 (Panx1)-directed intracellular calcium (iCa²⁺) signaling pathway to confer adaptive and aggressive phenotypes on cancer cells. Accordingly, IL-6 silencing renders cancer cells vulnerable to NK-LAAO-induced oxidative stress together with abrogating NK-LAAO-stimulated metastatic acquisition. Collectively, our study urges caution when using svLAAOs in cancer treatment and identifies the Panx1/iCa²⁺/IL-6 axis as a therapeutic target for improving the effectiveness of svLAAOs-based anticancer therapies.

The evolution and structure of snake venom phosphodiesterase (svPDE) highlight its importance in venom actions

For decades, studies of snake venoms focused on the venom-ome-specific toxins (VSTs). VSTs are dominant soluble proteins believed to contribute to the main venomous effects and emerged into gene clusters for fast adaptation and diversification of snake venoms. However, the conserved minor venom components, such as snake venom phosphodiesterase (svPDE), remain largely unexplored. Here, we focus on svPDE by genomic and transcriptomic analysis across snake clades and demonstrate that soluble svPDE is co-opted from the ancestral membrane-attached ENPP3 (ectonucleotide pyrophosphatase/phosphodiesterase 3) gene by replacing the original 5' exon with the exon encoding a signal peptide. Notably, the exons, promoters and transcription/translation starts have been replaced multiple times during snake evolution, suggesting the evolutionary necessity of svPDE. The structural and biochemical analyses also show that svPDE shares the similar functions with ENPP family, suggesting its perturbation to the purinergic signaling and insulin transduction in venomous effects.

Snake Venomics and Antivenomics of Cape Cobra (Naja nivea) from South Africa: Insights into Venom Toxicity and Cross-Neutralization Activity

Naja nivea (Cape Cobra) is endemic to southern Africa. Envenoming by N. nivea is neurotoxic, resulting in fatal paralysis. Its venom composition, however, has not been studied in depth, and specific antivenoms against it remain limited in supply. Applying a protein decomplexation approach, this study unveiled the venom proteome of N. nivea from South Africa. The major components in the venom are cytotoxins/cardiotoxins (~75.6% of total venom proteins) and alpha-neurotoxins (~7.4%), which belong to the three-finger toxin family. Intriguingly, phospholipase A₂ (PLA₂) was undetected-this is a unique venom phenotype increasingly recognized in the African cobras of the Uraeus subgenus. The work further showed that VINS African Polyvalent Antivenom (VAPAV) exhibited cross-reactivity toward the venom and immunorecognized its toxin fractions. In mice, VAPAV was moderately efficacious in cross-neutralizing the venom lethality with a potency of 0.51 mg/mL (amount of venom completely neutralized per milliliter of antivenom). In the challenge-rescue model, VAPAV prevented death in 75% of experimentally envenomed mice, with slow recovery from neurotoxicity up to 24 h. The finding suggests the potential para-specific utility of VAPAV for N. nivea envenoming, although a higher dose or repeated administration of the antivenom may be required to fully reverse the neurotoxic effect of the venom.

Comparison of Protein Variation in Protobothrops mucrosquamatus Venom between Northern and Southeast Taiwan and Association with Human Envenoming Effects

Reports of bite from Protobothrops mucrosquamatus (Pmu) are frequent in Taiwan, and its wide-spread distribution and diverse habitats drove us to investigate its envenoming effects and relevant venom variations. We used reversed-phase high-performance liquid chromatography and mass spectrometry to analyze 163 Pmu venom samples collected from northern and southeastern Taiwan. Twenty-two major protein fractions were separated and analyzed, and their contents were determined semi-quantitatively. The results showed that despite the trivial differences in the protein family, there is an existing variation in acidic phospholipases A₂s, serine proteinases, metalloproteinases, C-type lectin-like proteins, and other less abundant components in the Pmu venoms. Moreover, clinical manifestations of 209 Pmu envenomed patients hospitalized in northern or southeastern Taiwan revealed significant differences in local symptoms, such as ecchymosis and blistering. The mechanism of these local effects and possibly relevant venom components were examined. Further analysis showed that certain venom components with inter-population variation might work alone or synergistically with others to aggravate the local effects. Therefore, our findings of the venom variation may help one to improve antivenom production and better understand and manage Pmu bites.

Improving immunogenicity of influenza virus H7N9 recombinant hemagglutinin for vaccine development

Human infections of novel avian influenza A virus (H7N9) emerged in early 2013 and caused about 40% case-fatality through 2017. Therefore, development of influenza H7N9 vaccines is critical for pandemic preparedness. Currently, there are three means of production of commercial influenza vaccines: egg-based, mammalian cell-based, and insect cell-based platforms. The insect cell-based platform has the advantage of high speed in producing recombinant protein. In this study, we evaluate the stability and immunogenicity of two different influenza H7 HA expression constructs generated using the baculovirus system, including membrane-based full-length HA (mH7) and secreted ectodomain-based H7 (sH7). The mH7 construct could form an oligomer-rosette structure and had a high hemagglutinin (HA) titer 8192. In contrast to mH7, the sH7 construct could not form an oligomer-rosette structure and did not have HA titer before cross-linking with anti-His antibody. Thermal stability tests showed that the sH7 and mH7 constructs were unstable at 43 °C and 52 °C, respectively. In a mice immunization study, the mH7 construct but not the sH7 construct could induce robust HI and neutralizing antibody titers. In conclusion, further development of the mH7 vaccine candidate is desirable.

Abstract 1797: Characterization of a novel androgen receptor coregulator RIPK1

Using bicalutamide-androgen receptor (AR) DNA binding domain-ligand binding domain as bait, we observed enrichment of FxxFY motif-containing peptides. Protein database searches revealed the presence of receptor-interacting protein kinase 1 (RIPK1) harboring one FxxFY motif. RIPK1 interacted directly with AR and suppressed AR transactivation in a dose-dependent manner. Domain mapping experiments showed that the FxxFY motif in RIPK1 is critical for interactions with AR and the death domain of RIPK1 plays a crucial role in its inhibitory effect on transactivation. In terms of tissue expression, RIPK1 levels were markedly higher in benign prostate hyperplasia and non-cancerous tissue regions relative to the tumor area. With the aid of computer modeling for screening of chemicals targeting activation function 2 (AF-2) of AR, we identified oxadiazole derivatives as good candidates and subsequently generated a small library of these compounds. A number of candidates could effectively suppress AR transactivation and AR-related functions in vitro and in vivo with tolerable toxicity via inhibiting AR-peptide, AR-coregulator and AR N-C interactions. Combination of these chemicals with antiandrogen had an additive suppressive effect on AR transcriptional activity. Our collective findings may pave the way in creating new strategies for the development and design of anti-AR drugs.

Citation Format: Cheng-Lung Hsu, Jai-Shin Liu, Ting-Wei Lin, Chun-Cheng Lin, Wen-Guey Wu. Characterization of a novel androgen receptor coregulator RIPK1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1797.

Three-dimensional cavity nanoantennas with resonant-enhanced surface plasmons as dynamic color-tuning reflectors

As plasmonic antennas for surface-plasmon-assisted control of optical fields at specific frequencies, metallic nanostructures have recently emerged as crucial optical components for fascinating plasmonic color engineering. Particularly, plasmonic resonant nanocavities can concentrate lightwave energy to strongly enhance light-matter interactions, making them ideal candidates as optical elements for fine-tuning color displays. Inspired by the color mixing effect found on butterfly wings, a new type of plasmonic, multiresonant, narrow-band (the minimum is about 45 nm), high-reflectance (the maximum is about 95%), and dynamic color-tuning reflector is developed. This is achieved from periodic patterns of plasmonic resonant nanocavities in free-standing capped-pillar nanostructure arrays. Such cavity-coupling structures exhibit multiple narrow-band selective and continuously tunable reflections via plasmon standing-wave resonances. Consequently, they can produce a variety of dark-field vibrant reflective colors with good quality, strong color signal and fine tonal variation at the optical diffraction limit. This proposed multicolor scheme provides an elegant strategy for realizing personalized and customized applications in ultracompact photonic data storage and steganography, colorimetric sensing, 3D holograms and other plasmon-assisted photonic devices.

Ghrelin to obestatin ratio in maternal serum in pregnancies complicated by intrauterine growth restriction

Ghrelin, an endogenous for the growth hormone secretagogue receptor, has been shown to participate in fetal growth. Obestatin, encoded by the same gene as ghrelin, is described as a physiological opponent of ghrelin. This study was designed to determine the changes of ghrelin/obestatin ratio in maternal serum in pregnancies with intrauterine growth restriction (IUGR). The authors found that the ghrelin levels in maternal serum were significent lower in IUGR group than in control group (236.34 ± 14.58 pg/ml vs. 321.49 ± 18.19 pg/ml, p = 0.003). However, the difference of obestatin levels in maternal serum in IUGR group than in control group was not significent (276.25, ±20.54 pg/ml vs. 256.34 ± 21.21 pg/ml, p = 0.308). The ratio of ghrelin to obestatin in maternal serum were significent lower in UGR group than in control group (1.05 ± 0.09 vs. 0.82 ± 0.08, p = 0.03). Meanwhile, the maternal serum growth hormone (GH) concentration in IUGR group was lower than that in control group (1.08 ± 0.08 pg/ml vs. 1.41 ± 0.09 pg/ml, p = 0.009), and the maternal serum pla- cental growth hormone (PGH) concentration in IUGR group was lower than that in control group (2.21 ± 1.24 pg/ml vs. 2.92 ± 0.27 pg/ml,p = 0.031). The ratio of ghrelin to obestatin in maternal serum were positively correlation with GH and PGH concentrations in IUGR group (r = 0.876, p = 0.52; r = 0.764, p = 0.64). The findings of this study suggest that the ratio of ghrelin to obestatin in maternal serum were low, and were positively correlated with GH and PGH concentration in IUGR group, which can been considered as evidencees of ghrelin/obestatin balance disorder role in pathogenesis of IUGR.

[Method and significance of specimens standardized pathological treatment in pancreaticoduodenectomy for pancreatic head cancer]

Pancreatic ductal adenocarcinoma is a highly aggressive disease with a grim prognosis. Surgical resection offers the best chance for long-term survival. Negative-margin resection still remains the goal, the influence of margin status on outcomes in pancreatic head carcinoma remains controversial, as conflicting data have been plagued by a lack of standardization in R0 resection and margin definitions, pathologic analysis, and reporting. In contrast to common belief, a high rate of R1 resections in pancreatic cancer is not a marker of low-quality surgery but rather of high-quality pathology. The international pathological consensus of pancreatic head carcinoma is still needed to fully understand the prognostic value of margin status in order to optimize treatment strategy for this disease.

Inhibitor of signal transducer and activator of transcription 3 (STAT3) suppresses ovarian cancer growth, migration and invasion and enhances the effect of cisplatin in vitro

The aim of the present study was to investigate the anti-ovarian cancer effect of the inhibitor of signal transducer and activator of transcription 3 (STAT3), WP1066. Western blot was used to detect the phosphorylation of STAT3 in ovarian cancer cell line SKOV3 and cisplatin-resistant ovarian cancer cell line SKOV3/DDP. MTT and colony-forming assays were performed to evaluate the viability and growth of ovarian cancer cells. The apoptosis of ovarian cancer cells was determined by flow cytometry. The wound healing assay and Transwell assay were performed to examine the migration and invasion of ovarian cancer cells. WP1066 significantly inhibited the phosphorylation of STAT3 in SKOV3 and SKOV3/DDP cells. WP1066 treatment inhibited the proliferation and clonogenicity of both SKOV3 and SKOV3/DDP cells. After WP1066 treatment for 24 h, the apoptosis rates of SKOV3 and SKOV3/DDP cells were significantly increased compared with the control cells. After treatment with WP1066, the reduction of the wound gaps was significantly less in both SKOV3 and SKOV3/DDP cells. WP1066 also significantly inhibited the invasion capacity of SKOV3 and SKOV3/DDP cells compared with the control group. Treatment with WP1066 combined with cisplatin significantly increased proliferation inhibition and apoptosis in SKOV3 and SKOV3/ DDP cells compared with treatment with cisplatin alone. A synergistic action between WP1066 and cisplatin on the proliferation and apoptosis of ovarian cancer cells was determined. In conclusion, inhibition of STAT3 may suppress the proliferation, migration and invasion, induce apoptosis and enhance the chemosensitivity of ovarian cancer cells, indicating that STAT3 is a new therapeutic target of ovarian cancer.

Minoxidil may suppress androgen receptor-related functions

Although minoxidil has been used for more than two decades to treat androgenetic alopecia (AGA), an androgen-androgen receptor (AR) pathway-dominant disease, its precise mechanism of action remains elusive. We hypothesized that minoxidil may influence the AR or its downstream signaling. These tests revealed that minoxidil suppressed AR-related functions, decreasing AR transcriptional activity in reporter assays, reducing expression of AR targets at the protein level, and suppressing AR-positive LNCaP cell growth. Dissecting the underlying mechanisms, we found that minoxidil interfered with AR-peptide, AR-coregulator, and AR N/C-terminal interactions, as well as AR protein stability. Furthermore, a crystallographic analysis using the AR ligand-binding domain (LBD) revealed direct binding of minoxidil to the AR in a minoxidil-AR-LBD co-crystal model, and surface plasmon resonance assays demonstrated that minoxidil directly bound the AR with a K_d value of 2.6 μM. Minoxidil also suppressed AR-responsive reporter activity and decreased AR protein stability in human hair dermal papilla cells. The current findings provide evidence that minoxidil could be used to treat both cancer and age-related disease, and open a new avenue for applications of minoxidil in treating androgen-AR pathway-related diseases.

Identification of a new androgen receptor (AR) co-regulator BUD31 and related peptides to suppress wild-type and mutated AR-mediated prostate cancer growth via peptide screening and X-ray structure analysis

Treatment with individual anti-androgens is associated with the development of hot-spot mutations in the androgen receptor (AR). Here, we found that anti-androgens-mt-ARs have similar binary structure to the 5α-dihydrotestosterone-wt-AR. Phage display revealed that these ARs bound to similar peptides, including BUD31, containing an Fxx(F/H/L/W/Y)Y motif cluster with Tyr in the +5 position. Structural analyses of the AR-LBD-BUD31 complex revealed formation of an extra hydrogen bond between the Tyr+5 residue of the peptide and the AR. Functional studies showed that BUD31-related peptides suppressed AR transactivation, interrupted AR N-C interaction, and suppressed AR-mediated cell growth. Combination of peptide screening and X-ray structure analysis may serve as a new strategy for developing anti-ARs that simultaneously suppress both wt and mutated AR function.

Localization of B4GALNT2 and its role in mouse embryo attachment

OBJECTIVE

To investigate the location of β-1,4-N-acetylgalactosaminyltransferase II (B4GALNT2) and the involvement of this protein and Sd(a) antigen in embryonic implantation.

DESIGN

Cell and animal study.

SETTING

University.

ANIMAL(S)

Adult outbred Institute for Cancer Research mice.

INTERVENTION(S)

B4GALNT2 antibody injected into the uteri of mice in early pregnancy; E3.5 blastocysts and pregnant uterine tissues were collected.

MAIN OUTCOME MEASURE(S)

Protein expression was detected by immunofluorescence staining and Western blotting. Embryo attachment was assayed via in vitro and in vivo embryo implantation models.

RESULT(S)

The b4galnt2 gene expression in the 293T cell line showed the protein localized in the plasma membrane. We confirmed that B4GALNT2 was localized on the surface of E3.5 blastocysts but was an intracellular component in uterine epithelia. Finally, anti-B4GALNT2 and lectins inhibition assays demonstrated the involvement of B4GALNT2 and Sd(a) antigen in embryonic attachment in vitro and in vivo via the mouse system and human endometrial cell line (Ishikawa).

CONCLUSION(S)

B4GALNT2 expressed in the blastocyst may interact with a ligand on the endometrial surface, perhaps via Sd(a) also, to permit embryo implantation. Our data suggest that B4GALNT2 and Sd(a) antigen are essential for embryo implantation.

The role of sulfatide lipid domains in the membrane pore-forming activity of cobra cardiotoxin

Cobra CTX A3, the major cardiotoxin (CTX) from Naja atra, is a cytotoxic, basic β-sheet polypeptide that is known to induce a transient membrane leakage of cardiomyocytes through a sulfatide-dependent CTX membrane pore formation and internalization mechanism. The molecular specificity of CTX A3-sulfatide interaction at atomic levels has also been shown by both nuclear magnetic resonance (NMR) and X-ray diffraction techniques to reveal a role of CTX-induced sulfatide conformational changes for CTX A3 binding and dimer formation. In this study, we investigate the role of sulfatide lipid domains in CTX pore formation by various biophysical methods, including fluorescence imaging and atomic force microscopy, and suggest an important role of liquid-disordered (ld) and solid-ordered (so) phase boundary in lipid domains to facilitate the process. Fluorescence spectroscopic studies on the kinetics of membrane leakage and CTX oligomerization further reveal that, although most CTXs can oligomerize on membranes, only a small fraction of CTXs oligomerizations form leakage pores. We therefore suggest that CTX binding at the boundary between the so and so/ld phase coexistence sulfatide lipid domains could form effective pores to significantly enhance the CTX-induced membrane leakage of sulfatide-containing phosphatidylcholine vesicles. The model is consistent with our earlier observations that CTX may penetrate and lyse the bilayers into small aggregates at a lipid/protein molar ratio of about 20 in the ripple P(β)' phase of phosphatidylcholine bilayers and suggest a novel mechanism for the synergistic action of cobra secretary phospholipase A2 and CTXs.

Cobra CRISP functions as an inflammatory modulator via a novel Zn2+- and heparan sulfate-dependent transcriptional regulation of endothelial cell adhesion molecules

Cysteine-rich secretory proteins (CRISPs) have been identified as a toxin family in most animal venoms with biological functions mainly associated with the ion channel activity of cysteine-rich domain (CRD). CRISPs also bind to Zn(2+) at their N-terminal pathogenesis-related (PR-1) domain, but their function remains unknown. Interestingly, similar the Zn(2+)-binding site exists in all CRISP family, including those identified in a wide range of organisms. Here, we report that the CRISP from Naja atra (natrin) could induce expression of vascular endothelial cell adhesion molecules, i.e. intercellular adhesion molecule-1, vascular adhesion molecule-1, and E-selectin, to promote monocytic cell adhesion in a heparan sulfate (HS)- and Zn(2+)-dependent manner. Using specific inhibitors and small interfering RNAs, the activation mechanisms are shown to involve both mitogen-activated protein kinases and nuclear factor-κB. Biophysical characterization of natrin by using fluorescence, circular dichroism, and x-ray crystallographic methods further reveals the presence of two Zn(2+)-binding sites for natrin. The strong binding site is located near the putative Ser-His-Glu catalytic triad of the N-terminal domain. The weak binding site remains to be characterized, but it may modulate HS binding by enhancing its interaction with long chain HS. Our results strongly suggest that natrin may serve as an inflammatory modulator that could perturb the wound-healing process of the bitten victim by regulating adhesion molecule expression in endothelial cells. Our finding uncovers a new aspect of the biological role of CRISP family in immune response and is expected to facilitate future development of new therapeutic strategy for the envenomed victims.

Crystal structures of Bacillus cereus NCTU2 chitinase complexes with chitooligomers reveal novel substrate binding for catalysis: a chitinase without chitin binding and insertion domains

Chitinases hydrolyze chitin, an insoluble linear polymer of N-acetyl-d-glucosamine (NAG)(n), into nutrient sources. Bacillus cereus NCTU2 chitinase (ChiNCTU2) predominantly produces chitobioses and belongs to glycoside hydrolase family 18. The crystal structure of wild-type ChiNCTU2 comprises only a catalytic domain, unlike other chitinases that are equipped with additional chitin binding and insertion domains to bind substrates into the active site. Lacking chitin binding and chitin insertion domains, ChiNCTU2 utilizes two dynamic loops (Gly-67-Thr-69 and Ile-106-Val-112) to interact with (NAG)(n), generating novel substrate binding and distortion for catalysis. Gln-109 is crucial for direct binding with substrates, leading to conformational changes of two loops with a maximum shift of ∼4.6 Å along the binding cleft. The structures of E145Q, E145Q/Y227F, and E145G/Y227F mutants complexed with (NAG)(n) reveal (NAG)(2), (NAG)(2), and (NAG)(4) in the active site, respectively, implying various stages of reaction: before hydrolysis, E145G/Y227F with (NAG)(4); in an intermediate state, E145Q/Y227F with a boat-form NAG at the -1 subsite, -1-(NAG); after hydrolysis, E145Q with a chair form -1-(NAG). Several residues were confirmed to play catalytic roles: Glu-145 in cleavage of the glycosidic bond between -1-(NAG) and +1-(NAG); Tyr-227 in the conformational change of -1-(NAG); Asp-143 and Gln-225 in stabilizing the conformation of -1-(NAG). Additionally, Glu-190 acts in the process of product release, and Tyr-193 coordinates with water for catalysis. Residues Asp-143, E145Q, Glu-190, and Tyr-193 exhibit multiple conformations for functions. The inhibitors zinc ions and cyclo-(l-His-l-Pro) are located at various positions and confirm the catalytic-site topology. Together with kinetics analyses of related mutants, the structures of ChiNCTU2 and its mutant complexes with (NAG)(n) provide new insights into its substrate binding and the mechanistic action.

Role of heparan sulfates and glycosphingolipids in the pore formation of basic polypeptides of cobra cardiotoxin

Cobra venom contains cardiotoxins (CTXs) that induce tissue necrosis and systolic heart arrest in bitten victims. CTX-induced membrane pore formation is one of the major mechanisms responsible for the venom's designated cytotoxicity. This chapter examines how glycoconjugates such as heparan sulfates (HS) and glycosphingolipids, located respectively in the extracellular matrix and lipid bilayers of the cell membranes, facilitate CTX pore formation. Evidences for HS-facilitated cell surface retention and glycosphingolipid-facilitated membrane bilayer insertion of CTX are reviewed. We suggest that similar physical steps could play a role in the mediation of other pore forming toxins (PFT). The membrane pores formed by PFT are expected to have limited lifetime on biological cell surface as a result of membrane dynamics during endocytosis and/or rearrangement of lipid rafts.

Complementary quantitative proteomics reveals that transcription factor AP-4 mediates E-box-dependent complex formation for transcriptional repression of HDM2

Transcription factor activating enhancer-binding protein 4 (AP-4) is a basic helix-loop-helix protein that binds to E-box elements. AP-4 has received increasing attention for its regulatory role in cell growth and development, including transcriptional repression of the human homolog of murine double minute 2 (HDM2), an important oncoprotein controlling cell growth and survival, by an unknown mechanism. Here we demonstrate that AP-4 binds to an E-box located in the HDM2-P2 promoter and represses HDM2 transcription in a p53-independent manner. Incremental truncations of AP-4 revealed that the C-terminal Gln/Pro-rich domain was essential for transcriptional repression of HDM2. To further delineate the molecular mechanism(s) of AP-4 transcriptional control and its potential implications, we used DNA-affinity purification followed by complementary quantitative proteomics, cICAT and iTRAQ labeling methods, to identify a previously unknown E-box-bound AP-4 protein complex containing 75 putative components. The two labeling methods complementarily quantified differentially AP-4-enriched proteins, including the most significant recruitment of DNA damage response proteins, followed by transcription factors, transcriptional repressors/corepressors, and histone-modifying proteins. Specific interaction of AP-4 with CCCTC binding factor, stimulatory protein 1, and histone deacetylase 1 (an AP-4 corepressor) was validated using AP-4 truncation mutants. Importantly, inclusion of trichostatin A did not alleviate AP-4-mediated repression of HDM2 transcription, suggesting a previously unidentified histone deacetylase-independent repression mechanism. In contrast, the complementary quantitative proteomics study suggested that transcription repression occurs via coordination of AP-4 with other transcription factors, histone methyltransferases, and/or a nucleosome remodeling SWI.SNF complex. In addition to previously known functions of AP-4, our data suggest that AP-4 participates in a transcriptional-regulating complex at the HDM2-P2 promoter in response to DNA damage.

Structures of heparin-derived tetrasaccharide bound to cobra cardiotoxins: heparin binding at a single protein site with diverse side chain interactions

Cobra cardiotoxins (CTXs) are three-fingered polypeptides with positively charged domains that have been shown to bind to anionic ligands of snake venom citrate, glycosaminoglycans, sulfoglycosphingolipid, and nucleotide triphosphate with various biochemical effects including toxin dimerization, cell surface retention, membrane pore formation, cell internalization and blocking of enzymatic activities of kinase and ATPase. The reported anionic binding sites, however, are found to be different among different CTX homologues for potentially different CTX activities. Herein, by NMR studies of the binding of inorganic phosphate, dATP (stable form of ATP), and heparin-derived tetrasaccharide to Naja atra CTX A1, a novel CTX molecule exhibiting in vivo necrotic activity on skeletal muscle, we demonstrate that diverse ligands binding to CTXs could also occur at a single protein site with flexible side chain interactions. The flexibility of such an interaction is also illustrated by the available heparin-CTX A3 complex structures with different heparin chain lengths binding at the same site. Our results provide a likely structural explanation on how the interaction between heparan sufate and proteins depends more on the overall charge cluster organization rather than on their fine structures. We also suggest that the ligand binding site of CTX homologues can be fine-tuned by nonconserved residues near the binding pocket because of their flexible side chain interaction and dimerization ability, even for the rigid CTX molecules tightened by four disulfide bonds.

Glycosphingolipid-facilitated membrane insertion and internalization of cobra cardiotoxin. The sulfatide.cardiotoxin complex structure in a membrane-like environment suggests a lipid-dependent cell-penetrating mechanism for membrane binding polypeptides

Cobra cardiotoxins, a family of basic polypeptides having lipid- and heparin-binding capacities similar to the cell-penetrating peptides, induce severe tissue necrosis and systolic heart arrest in snakebite victims. Whereas cardiotoxins are specifically retained on the cell surface via heparan sulfate-mediated processes, their lipid binding ability appears to be responsible, at least in part, for cardiotoxin-induced membrane leakage and cell death. Although the exact role of lipids involved in toxin-mediated cytotoxicity remains largely unknown, monoclonal anti-sulfatide antibody O4 has recently been shown to inhibit the action of CTX A3, the major cardiotoxin from Taiwan cobra venom, on cardiomyocytes by preventing cardiotoxin-induced membrane leakage and CTX A3 internalization into mitochondria. Here, we show that anti-sulfatide acts by blocking the binding of CTX A3 to the sulfatides in the plasma membrane to prevent sulfatide-dependent CTX A3 membrane pore formation and internalization. We also describe the crystal structure of a CTX A3-sulfatide complex in a membrane-like environment at 2.3 angstroms resolution. The unexpected orientation of the sulfatide fatty chains in the structure allows prediction of the mode of toxin insertion into the plasma membrane. CTX A3 recognizes both the headgroup and the ceramide interfacial region of sulfatide to induce a lipid conformational change that may play a key role in CTX A3 oligomerization and cellular internalization. This proposed lipid-mediated toxin translocation mechanism may also shed light on the cellular uptake mechanism of the amphiphilic cell-penetrating peptides known to involve multiple internalization pathways.

Structural Difference between Group I and Group II Cobra Cardiotoxins: X-ray, NMR, and CD Analysis of the Effect ofcis-Proline Conformation on Three-Fingered Toxins†

Natural homologues of cobra cardiotoxins (CTXs) were classified into two structural subclasses of group I and II based on the amino acid sequence and circular dichroism analysis, but the exact differences in their three-dimensional structures and biological significance remain elusive. We show by circular dichroism, NMR spectroscopic, and X-ray crystallographic analyses of a newly purified group I CTX A6 from eastern Taiwan cobra (Naja atra) venoms that its loop I conformation adopts a type VIa turn with a cis peptide bond located between two proline residues of PPxY. A similar "banana-twisted" conformation can be observed in other group I CTXs and also in cyclolinopeptide A and its analogues. By binding to the membrane environment, group I CTX undergoes a conformational change to adopt a more extended hydrophobic domain with beta-sheet twisting closer to the one adopted by group II CTX. This result resolves a discrepancy in the CTX structural difference reported previously between solution as well as crystal state and shows that, in addition to the hydrophobicity, the exact loop I conformation also plays an important role in CTX-membrane interaction. Potential protein targets of group I CTXs after cell internalization are also discussed on the basis of the determined loop I conformation.

Cobra cardiotoxin and phospholipase A2 as GAG-binding toxins: on the path from structure to cardiotoxicity and inflammation

Glycosaminoglycans (GAGs) represent the sulfated carbohydrate moieties of proteoglycans which occur abundantly in tissues of the cardiovascular system. Many proteins bind specifically to GAGs and perform an important role in inflammation, cell proliferation, and blood coagulation processes. Recently, in vitro GAG-binding studies of cardiotoxins (CTXs) and basic phospholipase A(2) (PLA(2)) from cobra venom established the toxins as two new families of GAG-binding proteins. In particular, discontinuous basic residues in beta-sheet CTXs may form a cationic cradle suitable for heparin binding, as in the case of fibronectin module III-13. The binding specificity of beta-sheet proteins to different GAGs can be further enhanced by involving other cationic clusters near the flexible loop of the molecule. Since the three-dimensional structures of many CTXs and PLA(2) are available, these two toxins may serve as models for the elucidation of the molecular recognition of GAG-binding proteins and also as polypeptide templates for further improvement of the binding specificity suitable for future biomedical application. Research along the line of GAG-guided toxicity of cobra venom components may help us to understand the functional role of GAGs and the action mechanism of cobra venom components in the cardiovascular system.

Effects of alterations of the amino-terminal glycine of influenza hemagglutinin fusion peptide on its structure, organization and membrane interactions

Mutations of the glycine residue at the amino terminus of HA2 have been shown to have a large effect on the fusion activity of HA2, the extent of which apparently correlates with the side chain bulkiness of the substituting amino acids. To investigate into the cause of abrogation in fusogenicity and virus-promoted fusion mechanism, we synthesized several peptides in which this glycine was substituted by serine, glutamic acid, or lysine. 1,2-Dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dimyristoyl sn-glycero-3-phosphoglycerol (DMPG) were used as model membranes in the fluorescence, circular dichroism (CD), and FTIR measurements while sodium dodecyl sulfate was used in NMR studies. We found that, for the less active variants, affinity to membrane, degree of solvent dehydration, lipid perturbation, depth of insertion, and helicity were less. Comparison of affinity to membrane bilayer among these analogs revealed that binding of the fusion peptide is determined largely by the hydrophobic effect. Additionally, the orientation is closer to the membrane normal for the wild-type fusion peptide in the helix form while the inactive analogs inserted more parallel to the membrane surface.

Activity of ALRT 1550, a new retinoid, with interferon-gamma on ovarian cancer cell lines

Retinoids have been shown to be effective regulators of cell proliferation and differentiation in many human cancers. The major biologic activity of the retinoids is mediated by two families of nuclear receptors: retinoic acid receptors (RARs) and retinoid X receptors (RXRs). ALRT 1550 is one of the most potent RAR selective retinoids discovered to date, with 10-100 times more activity than ATRA in competitive binding and cotransfection assays and 300 times more inhibiting activity against proliferation of cervical carcinoma cell. To evaluate the role of ALRT 1550 in ovarian cancer, the growth inhibitory activity of ALRT 1550 was determined in the ATRA-resistant ovarian cancer cell line SKOV-3 and ovarian cancer cell line 2774 after exposure to concentrations of 0.1, 1, 2.5, 5, and 10 microM for 7 days. SKOV-3 showed 51%, 53%, and 68% cell growth inhibition after treatment with ALRT 1550 at concentrations of 2.5, 5, and 10 microM, respectively, and the 2774 cell line showed 46% inhibition after treatment at 10 microM. Because interferon (IFN)-gamma was found to synergistically amplify the growth inhibition of retinoids in cultured breast cancer cells, we investigated the combination of ALRT 1550 with IFN-gamma in two ovarian cancer cell lines. ALRT 1550 (5 microM) in combination with IFN-gamma at a concentration of 500 U/ml inhibited cell growth of SKOV-3 by as much as 81% (CI = 1.88). This is a 28% greater effect than with ALRT alone. Cell line 2774 showed a 69% cell growth inhibitory effect with ALRT 1550 (5 microM) in combination with IFN-gamma at a concentration of 1000 U/ml (CI = 1.03). ALRT 1550 and IFN-gamma may act synergistically in the SKOV-3 ovarian cancer cell line and additively in the 2774 cell line. In conclusion, ALRT 1550 may be a promising drug with a high biologic modulating activity against ovarian cancer. In combination with IFN-gamma, additive and perhaps synergistic effects may be seen in some ovarian cancer cell lines. Combining these two biologic modifiers for the treatment of ovarian cancer may lower the effective dose of the retinoids, thus decreasing their side effects.

Dynamic characterization of the water binding loop in the P-type cardiotoxin: implication for the role of the bound water molecule

Recent studies of cobra P-type cardiotoxins (CTXs) have shown that the water-binding loop (loop II) plays a crucial role in toxin binding to biological membranes and in their cytotoxicity. To understand the role of bound water in the loop, the structure and dynamics of the major P-type CTX from Taiwan cobra, CTX A3, were determined by a comprehensive NMR analysis involving (1)H NOESY/ROESY, (13)C[1)H]NOE/T(1) relaxation, and (17)O triple-quantum filtered NMR. A single water molecule was found to be tightly hydrogen bonded to the NH of Met26 with a correlation time (5-7 ns) approaching the isotropic tumbling time (3.8-4.5 ns) of the CTX A3 molecule. Surprisingly, despite the relatively long residence time (ca. 5 ns to 100 micros), the bound water molecule of CTX A3 is located within a dynamic (order parameter S(2) approximately 0.7) and solvent accessible loop. Comparison among several P-type CTXs suggests that proline residues in the consensus sequence of MxAxPxVPV should play an important role in the formation of the water binding loop. It is proposed that the exchange rate of the bound water may play a role in regulating the lipid binding mode of amphiphilic CTX molecules near membrane surfaces.

TRAIL-R2 is not correlated with p53 status and is rarely mutated in non-small cell lung cancer

BACKGROUND

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptors play an important role in regulating apoptosis. Recently, it was shown that the expression of TRAIL-R2, also known as KILLER, Trick or DR5, can be induced by either DNA damage or overexpression of a wild-type p53 transgene, suggesting a role for p53 in the death-signaling pathway. Furthermore, mutations in the death domain of TRAIL-R2 were reported in 10.6% of non-small cell lung cancer (NSCLC) patients in a Korean population, suggesting a role for TRAIL-R2 in lung tumorigenesis.

MATERIALS AND METHODS

To determine the association between expression of TRAIL-R2 and p53 mutation status in lung cancers, we compared the two events in 20 small-cell lung cancer (SCLC) cell lines, 20 NSCLC cell lines, and 30 primary NSCLC tumors. We also sequenced the death domain of TRAIL-R2 in a total of 100 primary NSCLC.

RESULTS

Lack of TRAIL-R2 expression was found in eight of 20 (40%) SCLC cell lines and in eleven of 20 (55%) NSCLC cell lines. Interestingly, in primary NSCLC, TRAIL-R2 was overexpressed in seven (23%) of the 30 tumors tested, and all primary tumors expressed TRAIL-R2. No association was found between the expression status of TRAIL-R2 and p53 mutation status in primary NSCLC tumors, SCLC cell lines or NSCLC cell lines. Further analysis of the death domain of TRAIL-R2 failed to identify any mutation in 100 primary NSCLC tumors.

CONCLUSIONS

Our data indicate that the expression profile of TRAIL-R2 is significantly different in lung cancer cell lines and primary tumors, that the expression of TRAIL-R2 is independent from p53 mutation status and that mutations in the death domain of TRAIL-R2 play a minimal role in NSCLCs in white Americans.

C-CAM1, a candidate tumor suppressor gene, is abnormally expressed in primary lung cancers

Previous studies have shown that the expression of the cell-cell adhesion molecule (C-CAM1), located at chromosome 19, is down-regulated in several types of human cancers, including prostate and breast cancers. Two major isoforms of C-CAM1, the long or L-form C-CAM1 and the short or S-form C-CAM1, are derived from the C-CAM1 gene through alternative splicing. Tumor cells transfected with L-form C-CAM1, which contains a cytoplasmic domain, display significantly lower growth rates and less tumorigenicity in both in vitro and in vivo models compared with untransfected tumor cells, suggesting that L-form C-CAM1 may be a tumor suppressor. The transfection of the cytoplasmic domain of L-form C-CAM1 could also cause suppression of tumor growth, further supporting the role of L-form C-CAM1 in tumorigenesis. In contrast to reports of most of the tumor types tested, Ohwada et al. (Am. J. Respir. Cell Mol. Biol., 11: 214-220, 1994) reported that C-CAM1 was not down-regulated or even up-regulated in lung cancer. Because the cytoplasmic domain of L-form C-CAM1 is critical for the tumor suppressor function of C-CAM1, we hypothesized that switching of the isoform rather than down- regulation of C-CAM1 gene expression occurs during lung tumorigenesis. To test this hypothesis, we analyzed pairs of tumor tissue and corresponding normal-appearing lung tissue from 51 patients with non-small cell lung cancer (NSCLC) and 43 cell lines to determine expression profiles of L-form C-CAM1 and S-form C-CAM1 using reverse transcription-PCR. We found that L-form C-CAM1 was the predominant form (75%; 38 of 51) in normal-appearing lung tissue, whereas most (84%; 43 of 51) of the primary NSCLC tissue samples expressed predominantly S-form C-CAM1 (P < 0.0001). Similarly, 19 (79%) of the 24 NSCLC cell lines and 17 (85%) of the 20 small cell lung cancer cell lines expressed predominantly S-form C-CAM1. The frequent alteration of the C-CAM1 expression pattern suggests that C-CAM1 has an important role in lung tumorigenesis.

Heparin reduces the alpha-helical content of cobra basic phospholipase A(2) and promotes its complex formation

The interaction of phospholipase A(2) (PLA(2)) with glycosaminoglycans (GAGs) has recently attracted attention in view of its implication on inflammation and cell proliferation. By using Fourier Transformed Infrared (FTIR) spectroscopic measurements, we demonstrate here that binding of cobra basic phospholipase A(2) from Naja nigricollis (N-PLA(2)) to heparin may induce a significant conformational change observed in the amide I region of the enzyme's alpha-helical and beta-sheet structure. It is observed that notable conformational change of N-PLA(2) due to heparin binding occurs only when heparin's chain length is at least an octasaccharide as evidenced by circular dichroism and optical density measurements. This correlation may be an important factor in the aggregation of N-PLA(2) and N-PLA(2)-heparin complexes. Heparin induced change in conformation of PLA(2) is suggested to be a notable link in understanding the diversity in PLA(2) activity when rendered to the extracellular matrix of cell membranes that is full of GAG molecules.

Heparin binding to cobra basic phospholipase A2 depends on heparin chain length and amino acid specificity

Heparin is shown to bind specifically to the carboxy-terminal region of toxic type I phospholipase A2 from Naja nigricollis (N-PLA2) by competition assay using synthetic polypeptides and heparin affinity chromatography. The binding strength is seen to depend on heparin chain length and the presence of N-sulfate groups of heparin. It is observed that both electrostatic and non-electrostatic interactions are involved in the specific binding of heparin to the carboxy-terminus. When heparin's size is at least a decasaccharide, about two molecules of N-PLA2 bind to one molecule of heparin, as evidenced by the chemical estimate of protein to carbohydrate ratio in such N-PLA2/heparin complexes. Based on such a stoichiometric measurement and computer modeling of the N-PLA2/heparin complex, it is suggested that the binding sites of the two N-PLA2 molecules on one heparin molecule lie on the opposite sides of the heparin chain.

Membrane packing geometry of diphytanoylphosphatidylcholine is highly sensitive to hydration: phospholipid polymorphism induced by molecular rearrangement in the headgroup region

Diphytanoylphosphatidylcholine (DPhPC) has often been used in the study of protein-lipid interaction and membrane channel activity, because of the general belief that it has high bilayer stability, low ion leakage, and fatty acyl packing comparable to that of phospholipid bilayers in the liquid-crystalline state. In this solid-state 31P and 2H NMR study, we find that the membrane packing geometry and headgroup orientation of DPhPC are highly sensitive to the temperature studied and its water content. The phosphocholine headgroup of DPhPC starts to change its orientation at a water content as high as approximately 16 water molecules per lipid, as evidenced by hydration-dependent 2H NMR study at room temperature. In addition, a temperature-induced structural transition in the headgroup orientation is detected in the temperature range of approximately 20-60 degrees C for lipids with approximately 8-11 water molecules per DPhPC. Dehydration of the lipid by one more water molecule leads to a nonlamellar, presumably cubic, phase formation. The lipid packing becomes a hexagonal phase at approximately 6 water molecules per lipid. A phase diagram of DPhPC in the temperature range of -40 degrees C to 80 degrees C is thus constructed on the basis of NMR results. The newly observed hydration-dependent DPhPC lipid polymorphism emphasizes the importance of molecular packing in the headgroup region in modulating membrane structure and protein-induced pore formation of the DPhPC bilayer.

Crystal structure of cardiotoxin V from Taiwan cobra venom: pH-dependent conformational change and a novel membrane-binding motif identified in the three-finger loops of P-type cardiotoxin

The crystal structure of cardiotoxin V from Taiwan cobra venom (CTX A5) has been solved at pH 8.5 and refined to an R-factor of 20.7% for 7013 reflections [>2sigma(F)] between 8- and 2.19-A resolution. The refined model shows that CTX A5 exists as a dimer. The assembly consists of 974 non-hydrogen atoms from 124 residues and 73 water molecules. The global monomeric structure is similar to that determined by NMR at pH 3.7, characterized by a core formed by two beta-sheets connected with three-finger loops. However, local conformational differences are detected in two functionally important regions, loops I and II. A disparity between the NMR and X-ray structure of CTX A5 is detected near the tip of loop I and can be attributed to the difference in the protonation state of His4 at different pH, resulting in a reorientation of the His4 imidazole ring. A concerted motion of amino acid side chains located near His4 is detected and possibly contributes to the pH-dependent binding ability of CTX A5 to phospholipid model membranes. The second difference, detected at the tip of loop II, is due to the hydrophobic contact between CTX dimers in the crystal packing and the interaction of water molecules with amino acid residues in the loop II region of the CTX containing Pro31 (P-type CTX). This interaction forces loop II into a more rigid omega shape bridging the main chain at positions 27 and 34, contradictory to the flexible, tapering shape detected by NMR. Thus, a novel continuous hydrophobic column capable of binding to and possibly penetrating the membrane lipid bilayer is formed by the tips of the three-finger loops. In this respect, the X-ray crystal structure of CTX A5 may represent the CTX structure in the membrane-binding mode.

Structure and dynamics of primary hydration shell of phosphatidylcholine bilayers at subzero temperatures

Deuterium NMR relaxation and intensity measurements of the 2H-labeled H2O/dimyristoyl phosphatidylcholine bilayer were performed to understand the molecular origin of the freezing event of phospholipid headgroup and the structure and dynamics of unfrozen water molecules in the interbilayer space at subzero temperatures. The results suggest that about one to two water molecules associated with the phosphate group freeze during the freezing event of phospholipid headgroups, whereas about five to six waters near the trimethylammonium group behave as a water cluster and remain unfrozen at temperatures as low as -70 degrees C. In addition, temperature-dependent T1 and T2 relaxation times suggest that dynamic coupling occurs not only between the phosphate group and its bound water, but also between the methyl group and the adjacent water molecules. Based on these observations, the primary hydration shell of phosphatidylcholine headgroup at subzero temperatures is suggested to consist of two distinct regions: a clathrate-like water cluster, most likely a water pentamer, near the hydrophobic methyl group, and hydration water molecules associated with the phosphate group.

The role of acidic amino acid residues in the structural stability of snake cardiotoxins

We have recently shown that membrane-related activities of cardiotoxin V from Naja naja atra (CTX A5) are diminished at acidic pH although the overall beta-sheet structure of the molecule is maintained. In order to understand more about the mechanism of inactivation of CTX at acidic pH, we studied the effect of pH and denaturing reagents on the structural stability of CTX. We found, first, pH-induced structural transitions occurred in CTX A5 at two pH values as judged by the CD ellipticity around 195 nm: an increase in the beta-sheet content occurred around pH 4 and followed by a decrease, therein, around pH 2. The pKa of three acidic amino acid residues in CTX A5, i.e., Glu-17, Asp-42, and Asp-59, were determined to be 4.0, 3.2, and below 2.3, respectively, by NMR spectroscopy. The low pKa value of Asp-59 implies salt bridge formation between Lys-2 and Asp-59. Thus, electrostatic interaction may stabilize the three loop structure in addition to the hydrogen bonds between N- and C-termini of CTX molecule. Second, 2,2,2-trifluoroethanol (TFE) and guanidinium chloride (GdmHCI) were found to induce alpha-helical and random coil formation, respectively, in CTX A5 and eight other beta-sheet CTXs. Comparison of the relative potencies of TFE and GdmHCI to induce structural changes suggests that the amino acid residue located at position 17 plays a role in the structural stability. Specifically, CTXs containing negatively charged Glu-17 are least stable. It is suggested that Glu-17 may perturb the interaction between Lys-2 and Asp-59, and thus the overall stability of beta-sheet, in the presence of denaturing reagent. In conclusion, the perturbed structural stability of CTXs may partially explain the lower activity CTX exhibits at acidic pH. A structural model to account for the unfolding and refolding of CTX molecules without the breaking of disulfide bonds is also proposed.

Conformational change and inactivation of membrane phospholipid-related activity of cardiotoxin V from Taiwan cobra venom at acidic pH

The phospholipid binding activity of cardiotoxin V from Naja naja atra (CTX A5) was studied by use of Langmuir monolayers and found to exhibit pH-dependence in binding to phosphatidylcholine membrane with an apparent pKa around 6.0. Proton NMR investigation of the CTX A5 molecule in the presence of phosphatidylcholine micelles reveals a decrease in association of CTX A5 with membranes at low pH as a result of the protonation of His-4 near the membrane binding site of loop I region of CTX. The pH-dependent binding can be attributed mainly, but not solely, to the change in charge content of the CTX molecule upon His-4 protonation at the membrane/water interface. This is shown by analyzing the pH- and ionic strength dependence of binding of CTXs to phospholipid monolayers according to Gouy-Chapman theory. The protonation of the His-4 residue also results in a local conformational change in the loop I region since the chemical shifts of amide protons for the amino acid residues from Cys-3 to Thr-14 are all found to vary as a function of pH with an apparent pKa similar to that of His-4. Interestingly, the effect is relayed to other amino acid residues in the structural core of the protein such as those in C-terminal (Lys-60, Cys-61, and Asn-62) and triple-stranded antiparallel beta-sheet (Cys-22, Lys-24, Ala-25, Arg-38, and Ala-41) regions. An additional local conformational change in the molecule results around pH 5 as evidenced by circular dichroism spectroscopic studies, although this change does not affect the characteristic beta-sheet and three-finger loop structure of CTX molecule as revealed by two-dimensional NOESY 1H NMR study. The latter conformational change at acidic pH, however, completely inactivates CTX-induced aggregation/fusion activity of sphingomyelin vesicles. The results suggest that deciphering the functional sites of CTXs on the basis of structure and dynamics determined at low pH should be done with caution. Since 19 out of 44 CTX homologues with known amino acid sequence contain His-4, the effect of His-4 on the structure and function of CTX molecules is important and is discussed in terms of the diverse membrane targets of CTX subtypes. Also discussed is the pH-induced activation of snake venom proteins in the victim.

Molecular order and hydration property of amine group in phosphatidylethanolamine and its N-methyl derivatives at subzero temperatures

The molecular order and hydration properties of the amine group in phosphatidylethanolamine and its N-methyl derivatives were studied by 2H-NMR at subzero temperatures. Three coexisting signals with 2H-NMR quadrupolar splittings of 146, 106, and 28.8 KHz were detected from the fully hydrated phosphatidylethanolamine/D2O at the lowest studied temperature of -120 degrees C by using short recycle time in the applied NMR pulse sequence. These signals have been assigned to originate from frozen D2O in the interbilayer space and the deuterated amine group, i.e., -ND, with and without threefold symmetric motions. Comparative 2H-NMR studies of phosphatidylethanolamine/D2O with different degrees of methylation over a temperature range between -40 and -120 degrees C lead to the following conclusions. First, the bond angle of -D attached to the nitrogen atom of the amine group may be determined by the 2H-NMR quadrupolar splittings, i.e., 106 and 28.8 KHz, of the two coexisting signals of the deuterated amine group and found to be 112.9 for the gel-state phosphatidylethanolamine. Second, assuming the applicability of the empirical equation for the hydrogen bond distance of N+D--O with deuteron quadrupole coupling constants and using the intermolecular hydrogen bond distance of the amine group determined in single crystals of phosphatidylethanolamine bilayers, the largest measured quadrupolar splitting (delta nu Q) of N-D in this study, i.e., 106 KHz, is close to the static value. This interpretation is also consistent with the fact that the delta nu Q value determined remains constant in the temperature range between -70 and -120 degrees C. Third, the molecular order parameter of the amine group, as calculated from the ratio of the libration-averaged and static delta nu Q value for the lipid with different degrees of methylation, suggests that the perturbation of the headgroup interaction is most significant for the final methylation step. Finally, measurement of the spectral intensity of isotropic unfrozen D2O signals in D2O/phospholipid dispersions at temperatures below the homogeneous nucleation temperature of ice formation for D2O, i.e., below -34 degrees C, suggests that the first methylation step perturbs the neighboring water most significantly. Assuming that the molecular order of the amine group and the amount of unfrozen water detected under the present experimental condition can be taken as a measure of the hydrogen-bonding ability and the extent of perturbation caused by the methyl group, respectively, the gradual methylation of the amine group perturbs the interactions of the N-methylated headgroups in a nonlinear fashion. The results provide a molecular explanation for the phase behavior of phospholipids with different degrees of methylation.

A new method of superoxide free radical determination in ischemia and reperfusion injury of rat liver

The present study was designed to utilize the chemiluminescence (CL) method for O2- analysis in ischemia and reperfusion of the whole rat liver. The results indicated that the O2- level was lower after 30 and 60 min ischemia. After 40 min reperfusion, the level of O2- increased obviously, reaching about 8.3-fold (30 min ischemia) and 9.0-fold (60 min ischemia) (P < 0.001, P < 0.01) respectively. The results also show that Chinese Traditional Medicines ligustrazine and salvia compound can scavenge O2- effectively.

Solvent effect on phosphatidylcholine headgroup dynamics as revealed by the energetics and dynamics of two gel-state bilayer headgroup structures at subzero temperatures

The packing and dynamics of lipid bilayers at the phosphocholine headgroup region within the temperature range of -40 to -110 degrees C have been investigated by solid-state nuclear magnetic resonance (NMR) measurements of selectively deuterium-labeled H2O/dimyristoylphosphatidylcholine (DMPC) bilayers. Two coexisting signals with 2H NMR quadrupolar, splittings of 36.1 and 9.3 (or smaller) kHz were detected from the -CD3 of choline methyl group. These two signals have been assigned to two coexisting gel-state headgroup structures with fast rotational motion of -CD3 and -N(CD3)3 group, respectively, with a threefold symmetry. The largest quadrupolar splitting of the NMR signal detected from the -CD2 of C alpha and C beta methylene segment was found to be 115.2 kHz, which is 10% lower than its static value of 128.2 kHz. Thus, there are extensive motions of the entire choline group of gel-state phosphatidylcholine bilayers even at a subzero temperature of -110 degrees C. These results strongly support the previous suggestion (E. J. Dufourc, C. Mayer, J. Stohrer, G. Althoff, and G. Kothe, 1992, Biophys. J. 61:42-57) that 31P chemical shift tensor elements of DMPC determined under similar conditions are not the rigid static values. The free energy difference between the two gel-state headgroup structures was determined to be 26.3 +/- 0.9 kJ/mol for fully hydrated bilayers. Furthermore, two structures with similar free energy difference were also detected for "frozen" phosphorylcholine chloride solution in a control experiment, leading to the conclusion that the two structures may be governed solely by the energetics of fully hydrated phosphocholine headgroup. The intermolecular interactions among lipids, however, stabilize the static headgroup structure as evidenced by the apparently lower free energy difference between the two structures for partially hydrated lipid bilayers. Evidence is also presented to suggest that one of the headgroup structures with trimethylammonium group rotation, which is not compatible with the static headgroup structure in crystals, is due to the dielectric relaxation of the slowly reorienting inter bilayer water molecules near the physical edge of membrane surface. Finally, a molecular model of the hydration-induced conformational changes at the torsion angle a5 of the O-C-CN+ bond is proposed to explain the two detected coexisting headgroup structures. These results emphasize the important role of the trimethylammonium group in monitoring the structure and dynamics of the lipid headgroup.

Expression of glutathione S-transferase-cardiotoxin fusion protein in Escherichia coli

We report here the construction of cardiotoxin V gene, from cobra snake venom (Naja naja atra), by chemically synthesized oligonucleotides and its expression as a glutathione S-transferase-cardiotoxin fusion protein in the inclusion bodies of Escherichia coli. The expression of cardiotoxin fusion protein in protein with a yield of about 35 mg/liter culture was confirmed by highly specific anti-peptide antibodies generated against the unique amino acid residues located at the tip of loop II of cardiotoxin V. Since the fusion protein can be easily treated by CNBr to free the toxin moiety, as revealed by immunoblotting of the cleaved protein, the results provide an avenue for future structural and functional studies of cardiotoxin molecules.

Solution structure of cardiotoxin V from Naja naja atra

Cardiotoxins are small proteins that are found in the venoms of snakes from the Elapidae family. These toxins are known to bind to and disrupt the organization, integrity, and function of the cell membrane. Most of the well-studied cardiotoxins cause depolarization of membrane potentials and/or lysis of red cells. In contrast, CTX V from Naja naja atra displays poor hemolytic activity but is proficient at inducing aggregation and fusion of sphingomyelin vesicles [Chien et al. (1991) J. Biol. Chem. 266, 3252-3259]. To determine whether the unique activity of this CTX is attributable to its tertiary structure, the solution structure of CTX V was determined by NMR methods. On the basis of these studies, this cardiotoxin has the same general topology as other members of the family, and thus its unusual properties do not arise from any gross structural differences that are detectable by solution NMR methods. Molecular dynamics calculations indicate that residues 36-50 show concerted fluctuations. On the basis of sequence similarity, we postulate that residues 30-34 are important in determining the specificity of cardiotoxins for fusion versus lysis of vesicles.

Aspartyl residue 10 is essential for ATPase activity of rat hsc70

Three mutants of rat hsc70 were constructed, overexpressed in Escherichia coli, purified, and characterized. First, site-directed mutation was utilized to substitute Asn for Asp-10. The recombinant protein, hsc70(D10N), loses not only its peptide-stimulated ATPase activity but also its basal ATPase activity. The measured dissociation constants of ATP (0.3 microM) and S-peptide (5 microM) for hsc70(D10N), however, are virtually identical to those of hsc70. The intrinsic fluorescence spectra of hsc70(D10N) also remain largely unchanged. Therefore, the overall structure of the hsc70 protein is most likely intact after mutation. Second, the entire C-terminal peptide-binding domain was deleted and the resultant mutant contains only the N-terminal ATPase domain of hsc70. This recombinant protein, Nt-hsc70, is a peptide-independent ATPase. The ATPase activity at 37 degrees C of the Nt-hsc70, 270 pmol/h/micrograms of protein, is comparable to that of maximally peptide-activated hsc70. Third, the Asp-10 of Nt-hsc70 was replaced by Asn. Despite that this mutant, Nt-hsc70(D10N), is capable of binding ATP and it loses the capability to hydrolyze ATP. Taken together, these results indicate that aspartyl residue 10 of hsc70 is essential for ATP hydrolysis. Purified hsc70 and its mutants autophosphorylate in vitro at a substoichiometric level. On average, less than 1% of the hsc70 and Nt-hsc70 proteins are phosphorylated. Although the amount of phosphate incorporated into hsc70(D10N) and Nt-hsc70(D10) is reduced, a significant level of phosphorylation can still be achieved in these two site-directed mutants. Hence, autophosphorylation of hsc70 and its mutants is not correlated with their ability to hydrolyze ATP.

Freezing of phosphocholine headgroup in fully hydrated sphingomyelin bilayers and its effect on the dynamics of nonfreezable water at subzero temperatures

Differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spectroscopy are applied to characterize the nonfreezable water molecules in fully hydrated D2O/sphingomyelin at temperatures below 0 degrees C. Upon cooling, DSC thermogram displays two thermal transitions peaked at -11 and -34 degrees C. The high-temperature exothermic transition corresponds to the freezing of the bulk D2O, and the low-temperature transition, which has not previously been reported, can be ascribed to the freezing of the phosphocholine headgroup in the lipid bilayer. The dynamics of nonfreezable water are also studied by 2H NMR T1 (spin-lattice relaxation time) and T2e (spin-spin relaxation time obtained by two pulse echo) measurements at 30.7 MHz and at temperatures down to -110 degrees C. The temperature dependence of the T1 relaxation time is characterized by a distinct minimum value of 2.1 +/- 0.1 ms at -30 degrees C. T2e is discontinuous at temperature around -70 degrees C, indicating another freezing-like event for the bound water at this temperature. Analysis of the relaxation data suggest that nonfreezable water undergoes both fast and slow motions at characteristic NMR time scales. The slow motions are affected when the lipid headgroup freezes.

Preliminary crystallographic analysis of cardiotoxin V with major fusion activity from Taiwan cobra (Naja naja atra) venom

Crystals of a cardiotoxin from Taiwan cobra venom have been obtained by the vapor diffusion method using methyl pentanediol as precipitant. The crystals belong to the hexagonal space group P6(1)22 (or P6(5)22), with cell dimensions a = b = 47.5 A, c = 111.3 A, alpha = beta = 90 degrees and gamma = 120 degrees and diffract to a resolution of 2.2 A. There is one molecule per asymmetric unit and the solvent content is estimated to be 53%.