Calmodulin binding to the Fas-mediated death-inducing signaling complex in cholangiocarcinoma cells

TNF-Related Apoptosis-Inducing Ligand: Non-Apoptotic Signalling

TNF-related apoptosis-inducing ligand (TRAIL or Apo2 or TNFSF10) belongs to the TNF superfamily. When bound to its agonistic receptors, TRAIL can induce apoptosis in tumour cells, while sparing healthy cells. Over the last three decades, this tumour selectivity has prompted many studies aiming at evaluating the anti-tumoral potential of TRAIL or its derivatives. Although most of these attempts have failed, so far, novel formulations are still being evaluated. However, emerging evidence indicates that TRAIL can also trigger a non-canonical signal transduction pathway that is likely to be detrimental for its use in oncology. Likewise, an increasing number of studies suggest that in some circumstances TRAIL can induce, via Death receptor 5 (DR5), tumour cell motility, potentially leading to and contributing to tumour metastasis. While the pro-apoptotic signal transduction machinery of TRAIL is well known from a mechanistic point of view, that of the non-canonical pathway is less understood. In this study, we the current state of knowledge of TRAIL non-canonical signalling.

Screening and identification of potential biomarkers and therapeutic drugs in melanoma via integrated bioinformatics analysis

Melanoma is a highly aggressive malignant skin tumor with a high rate of metastasis and mortality. In this study, a comprehensive bioinformatics analysis was used to clarify the hub genes and potential drugs. Download the GSE3189, GSE22301, and GSE35388 microarray datasets from the Gene Expression Omnibus (GEO), which contains a total of 33 normal samples and 67 melanoma samples. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) approach analyze DEGs based on the DAVID. Use STRING to construct protein-protein interaction network, and use MCODE and cytoHubba plug-ins in Cytoscape to perform module analysis and identified hub genes. Use Gene Expression Profile Interactive Analysis (GEPIA) to assess the prognosis of genes in tumors. Finally, use the Drug-Gene Interaction Database (DGIdb) to screen targeted drugs related to hub genes. A total of 140 overlapping DEGs were identified from the three microarray datasets, including 59 up-regulated DEGs and 81 down-regulated DEGs. GO enrichment analysis showed that these DEGs are mainly involved in the biological process such as positive regulation of gene expression, positive regulation of cell proliferation, positive regulation of MAP kinase activity, cell migration, and negative regulation of the apoptotic process. The cellular components are concentrated in the membrane, dendritic spine, the perinuclear region of cytoplasm, extracellular exosome, and membrane raft. Molecular functions include protein homodimerization activity, calmodulin-binding, transcription factor binding, protein binding, and cytoskeletal protein binding. KEGG pathway analysis shows that these DEGs are mainly related to protein digestion and absorption, PPAR signaling pathway, signaling pathways regulating stem cells' pluripotency, and Retinol metabolism. The 23 most closely related DEGs were identified from the PPI network and combined with the GEPIA prognostic analysis, CDH3, ESRP1, FGF2, GBP2, KCNN4, KIT, SEMA4D, and ZEB1 were selected as hub genes, which are considered to be associated with poor prognosis of melanoma closely related. Besides, ten related drugs that may have therapeutic effects on melanoma were also screened. These newly discovered genes and drugs provide new ideas for further research on melanoma.

ADAM15 in Apoptosis Resistance of Synovial Fibroblasts: Converting Fas/CD95 Death Signals Into the Activation of Prosurvival Pathways by Calmodulin Recruitment

OBJECTIVE

To investigate mechanisms underlying the capability of ADAM15 to transform FasL-mediated death-inducing signals into prosurvival activation of Src and focal adhesion kinase (FAK) in rheumatoid arthritis synovial fibroblasts (RASFs).

METHODS

Caspase 3/7 activity and apoptosis rate were determined in RASFs and ADAM15-transfected T/C28a4 cells upon Fas/CD95 triggering using enzyme assays and annexin V staining. Phosphorylated Src and FAK were analyzed by immunoblotting. Interactions of ADAM15 and CD95 with calmodulin (CaM), Src, or FAK were analyzed by pull-downs using CaM-Sepharose and coimmunoprecipitations with specific antibodies. Protein binding assays were performed using recombinant CaM and ADAM15. Immunofluorescence was performed to investigate subcellular colocalization of ADAM15, Fas/CD95, and CaM.

RESULTS

The antiapoptotic effect of ADAM15 in FasL-stimulated cells was demonstrated either by increased apoptosis of cells transfected with an ADAM15 construct lacking the cytoplasmic domain compared to cells transfected with full-length ADAM15 or by reduced apoptosis resistance of RASFs upon RNA interference silencing of ADAM15. Fas ligation triggered a Ca²⁺  release-activated Ca²⁺ /calcium release-activated calcium channel protein 1 (CRAC/Orai1) channel-dependent CaM recruitment to Fas/CD95 and ADAM15 in the cell membrane. Simultaneously, Src associated with CaM was shown to become engaged in the ADAM15 complex also containing cytoplasmic-bound FAK. Accordingly, Fas ligation in RASFs led to ADAM15-dependent phosphorylation of Src and FAK, which was associated with increased survival. Pharmacologic interference with either the CaM inhibitor trifluoperazine or the CRAC/Orai inhibitor BTP-2 simultaneously applied with FasL synergistically enhanced Fas-mediated apoptosis in RASFs.

CONCLUSION

ADAM15 provides a scaffold for formation of CaM-dependent prosurvival signaling complexes upon CRAC/Orai coactivation by FasL-induced death signals and a potential therapeutic target to break apoptosis resistance in RASFs.

Calmodulin as a protein linker and a regulator of adaptor/scaffold proteins

Calmodulin (CaM) is a universal regulator for a huge number of proteins in all eukaryotic cells. Best known is its function as a calcium-dependent modulator of the activity of enzymes, such as protein kinases and phosphatases, as well as other signaling proteins including membrane receptors, channels and structural proteins. However, less well known is the fact that CaM can also function as a Ca²⁺-dependent adaptor protein, either by bridging between different domains of the same protein or by linking two identical or different target proteins together. These activities are possible due to the fact that CaM contains two independently-folded Ca²⁺ binding lobes that are able to interact differentially and to some degree separately with targets proteins. In addition, CaM can interact with and regulates several proteins that function exclusively as adaptors. This review provides an overview over our present knowledge concerning the structural and functional aspects of the role of CaM as an adaptor protein and as a regulator of known adaptor/scaffold proteins.

Cell death-independent activities of the death receptors CD95, TRAILR1, and TRAILR2

Since their identification more than 20 years ago, the death receptors CD95, TRAILR1, and TRAILR2 have been intensively studied with respect to their cell death-inducing activities. These receptors, however, can also trigger a variety of cell death-independent cellular responses reaching from the activation of proinflammatory gene transcription programs over the stimulation of proliferation and differentiation to induction of cell migration. The cell death-inducing signaling mechanisms of CD95 and the TRAIL death receptors are well understood. In contrast, despite the increasing recognition of the biological and pathophysiological relevance of the cell death-independent activities of CD95, TRAILR1, and TRAILR2, the corresponding signaling mechanisms are less understood and give no fully coherent picture. This review is focused on the cell death-independent activities of CD95 and the TRAIL death receptors and addresses mainly three questions: (a) how are these receptors linked to noncell death pathways at the molecular level, (b) which factors determine the balance of cell death and cell death-independent activities of CD95 and the TRAIL death receptors at the cellular level, and (c) what are the consequences of the cell death-independent functions of these receptors for their role in cancer and inflammatory diseases.

Characterization of the Interactions between Calmodulin and Death Receptor 5 in Triple-negative and Estrogen Receptor-positive Breast Cancer Cells: AN INTEGRATED EXPERIMENTAL AND COMPUTATIONAL STUDY

Activation of death receptor-5 (DR5) leads to the formation of death inducing signaling complex (DISC) for apoptotic signaling. Targeting DR5 to induce breast cancer apoptosis is a promising strategy to circumvent drug resistance and present a target for breast cancer treatment. Calmodulin (CaM) has been shown to regulate DR5-mediated apoptotic signaling, however, its mechanism remains unknown. In this study, we characterized CaM and DR5 interactions in breast cancer cells with integrated experimental and computational approaches. Results show that CaM directly binds to DR5 in a calcium dependent manner in breast cancer cells. The direct interaction of CaM with DR5 is localized at DR5 death domain. We have predicted and verified the CaM-binding site in DR5 being (354)WEPLMRKLGL(363) that is located at the α2 helix and the loop between α2 helix and α3 helix of DR5 DD. The residues of Trp-354, Arg-359, Glu-355, Leu-363, and Glu-367 in DR5 death domain that are important for DR5 recruitment of FADD and caspase-8 for DISC formation to signal apoptosis also play an important role for CaM-DR5 binding. The changed electrostatic potential distribution in the CaM-binding site in DR5 DD by the point mutations of W354A, E355K, R359A, L363N, or E367K in DR5 DD could directly contribute to the experimentally observed decreased CaM-DR5 binding by the point mutations of the key residues in DR5 DD. Results from this study provide a key step for the further investigation of the role of CaM-DR5 binding in DR5-mediated DISC formation for apoptosis in breast cancer cells.

Screening of Pre-miRNA-155 Binding Peptides for Apoptosis Inducing Activity Using Peptide Microarrays

MicroRNA-155, one of the most potent miRNAs that suppress apoptosis in human cancer, is overexpressed in numerous cancers, and it displays oncogenic activity. Peptide microarrays, constructed by immobilizing 185 peptides containing the C-terminal hydrazide onto epoxide-derivatized glass slides, were employed to evaluate peptide binding properties of pre-miRNA-155 and to identify its binding peptides. Two peptides, which were identified based on the results of peptide microarray and in vitro Dicer inhibition studies, were found to inhibit generation of mature miRNA-155 catalyzed by Dicer and to enhance expression of miRNA-155 target genes in cells. In addition, the results of cell experiments indicate that peptide inhibitors promote apoptotic cell death via a caspase-dependent pathway. Finally, observations made in NMR and molecular modeling studies suggest that a peptide inhibitor preferentially binds to the upper bulge and apical stem-loop region of pre-miRNA-155, thereby suppressing Dicer-mediated miRNA-155 processing.

Identification of the Calmodulin-Binding Domains of Fas Death Receptor

The extrinsic apoptotic pathway is initiated by binding of a Fas ligand to the ectodomain of the surface death receptor Fas protein. Subsequently, the intracellular death domain of Fas (FasDD) and that of the Fas-associated protein (FADD) interact to form the core of the death-inducing signaling complex (DISC), a crucial step for activation of caspases that induce cell death. Previous studies have shown that calmodulin (CaM) is recruited into the DISC in cholangiocarcinoma cells and specifically interacts with FasDD to regulate the apoptotic/survival signaling pathway. Inhibition of CaM activity in DISC stimulates apoptosis significantly. We have recently shown that CaM forms a ternary complex with FasDD (2:1 CaM:FasDD). However, the molecular mechanism by which CaM binds to two distinct FasDD motifs is not fully understood. Here, we employed mass spectrometry, nuclear magnetic resonance (NMR), biophysical, and biochemical methods to identify the binding regions of FasDD and provide a molecular basis for the role of CaM in Fas-mediated apoptosis. Proteolytic digestion and mass spectrometry data revealed that peptides spanning residues 209-239 (Fas-Pep1) and 251-288 (Fas-Pep2) constitute the two CaM-binding regions of FasDD. To determine the molecular mechanism of interaction, we have characterized the binding of recombinant/synthetic Fas-Pep1 and Fas-Pep2 peptides with CaM. Our data show that both peptides engage the N- and C-terminal lobes of CaM simultaneously. Binding of Fas-Pep1 to CaM is entropically driven while that of Fas-Pep2 to CaM is enthalpically driven, indicating that a combination of electrostatic and hydrophobic forces contribute to the stabilization of the FasDD-CaM complex. Our data suggest that because Fas-Pep1 and Fas-Pep2 are involved in extensive intermolecular contacts with the death domain of FADD, binding of CaM to these regions may hinder its ability to bind to FADD, thus greatly inhibiting the initiation of apoptotic signaling pathway.

Identification and Characterization of the Interaction Site between cFLIPL and Calmodulin

Overexpression of the cellular FLICE-like inhibitory protein (cFLIP) has been reported in a number of tumor types. As an inactive procaspase-8 homologue, cFLIP is recruited to the intracellular assembly known as the Death Inducing Signaling Complex (DISC) where it inhibits apoptosis, leading to cancer cell proliferation. Here we characterize the molecular details of the interaction between cFLIP_L and calmodulin, a ubiquitous calcium sensing protein. By expressing the individual domains of cFLIP_L, we demonstrate that the interaction with calmodulin is mediated by the N-terminal death effector domain (DED1) of cFLIP_L. Additionally, we mapped the interaction to a specific region of the C-terminus of DED1, referred to as DED1 R4. By designing DED1/DED2 chimeric constructs in which the homologous R4 regions of the two domains were swapped, calmodulin binding properties were transferred to DED2 and removed from DED1. Furthermore, we show that the isolated DED1 R4 peptide binds to calmodulin and solve the structure of the peptide-protein complex using NMR and computational refinement. Finally, we demonstrate an interaction between cFLIP_L and calmodulin in cancer cell lysates. In summary, our data implicate calmodulin as a potential player in DISC-mediated apoptosis and provide evidence for a specific interaction with the DED1 of cFLIP_L.

Calmodulin antagonists promote TRA-8 therapy of resistant pancreatic cancer

Pancreatic cancer is highly malignant with limited therapy and a poor prognosis. TRAIL-activating therapy has been promising, however, clinical trials have shown resistance and limited responses of pancreatic cancers. We investigated the effects of calmodulin(CaM) antagonists, trifluoperazine(TFP) and tamoxifen(TMX), on TRA-8-induced apoptosis and tumorigenesis of TRA-8-resistant pancreatic cancer cells, and underlying mechanisms. TFP or TMX alone did not induce apoptosis of resistant PANC-1 cells, while they dose-dependently enhanced TRA-8-induced apoptosis. TMX treatment enhanced efficacy of TRA-8 therapy on tumorigenesis in vivo. Analysis of TRA-8-induced death-inducing-signaling-complex (DISC) identified recruitment of survival signals, CaM/Src, into DR5-associated DISC, which was inhibited by TMX/TFP. In contrast, TMX/TFP increased TRA-8-induced DISC recruitment/activation of caspase-8. Consistently, caspase-8 inhibition blocked the effects of TFP/TMX on TRA-8-induced apoptosis. Moreover, TFP/TMX induced DR5 expression. With a series of deletion/point mutants, we identified CaM antagonist-responsive region in the putative Sp1-binding domain between -295 to -300 base pairs of DR5 gene. Altogether, we have demonstrated that CaM antagonists enhance TRA-8-induced apoptosis of TRA-8-resistant pancreatic cancer cells by increasing DR5 expression and enhancing recruitment of apoptotic signal while decreasing survival signals in DR5-associated DISC. Our studies support the use of these readily available CaM antagonists combined with TRAIL-activating agents for pancreatic cancer therapy.

Keratoacanthoma and squamous cell carcinoma are distinct from a molecular perspective

Keratoacanthoma is a controversial entity. Some consider keratoacanthoma as a variant of squamous cell carcinoma, whereas others see it as a distinct self-resolving squamoproliferative lesion. Our objective is to examine the relationship of keratoacanthoma with squamous cell carcinoma and normal skin by using DNA microarrays. DNA microarray studies were performed on formalin-fixed and paraffin-embedded blocks from ten cases of actinic keratoacanthoma utilizing the U133plus2.0 array. These results were compared with our previously developed microarray database of ten squamous cell carcinoma and ten normal skin samples. Keratoacanthoma demonstrated 1449 differentially expressed genes in comparison with squamous cell carcinoma (>5-fold change: P<0.01) with 908 genes upregulated and 541 genes downregulated. Keratoacanthoma showed 2435 differentially expressed genes in comparison with normal skin (>5-fold change: P<0.01) with 1085 genes upregulated and 1350 genes downregulated. The most upregulated genes, comparing keratoacanthoma with normal skin included MALAT1, S100A8, CDR1, TPM4, and CALM1. The most downregulated genes included SCGB2A2, DCD, THRSP, ADIPOQ, adiponectin, and ADH1B. The molecular biological pathway analysis comparing keratoacanthoma with normal skin showed that cellular development, cellular growth and proliferation, cell death/apoptosis, and cell cycle pathways are prominently involved in the pathogenesis of keratoacanthoma. The most enriched canonical pathways were clathrin-mediated endocytosis signaling, molecular mechanisms of cancer and integrin signaling. The distinctive gene expression profile of keratoacanthoma reveals that it is molecularly distinct from squamous cell carcinoma. The molecular pathways and genes differentially expressed in comparing keratoacanthoma with normal skin suggest that keratoacanthoma is a neoplasm that can regress due to upregulation of the cell death/apoptosis pathway.

Structural insights into the mechanism of calmodulin binding to death receptors

The death receptors Fas, p75NTRand DR6 are key components of extrinsically activated apoptosis. Characterization of how they interact with the adaptors is crucial in order to unravel the signalling mechanisms. However, the exact conformation that their intracellular death domain adopts upon binding downstream partners remains unclear. One model suggests that it adopts a typical compact fold, whilst a second model proposed an open conformation. Calmodulin (CaM), a major calcium sensor, has previously been reported to be one of the Fas adaptors that modulate apoptosis. This work reports that CaM also binds directly to the death domains of p75NTRand DR6, indicating that it serves as a common modulator of the death receptors. Two crystal structures of CaM in complexes with the corresponding binding regions of Fas and p75NTRare also reported. Interestingly, the precise CaM-binding sites were mapped to different regions: helix 1 in Fas and helix 5 in p75NTRand DR6. A novel 1–11 motif for CaM binding was observed in p75NTR. Modelling the complexes of CaM with full-length receptors reveals that the opening of the death domains would be essential in order to expose their binding sites for CaM. These results may facilitate understanding of the diverse functional repertoire of death receptors and CaM and provide further insights necessary for the design of potential therapeutic peptide agents.

Characterization of calmodulin-Fas death domain interaction: an integrated experimental and computational study

The Fas death receptor-activated death-inducing signaling complex (DISC) regulates apoptosis in many normal and cancer cells. Qualitative biochemical experiments demonstrate that calmodulin (CaM) binds to the death domain of Fas. The interaction between CaM and Fas regulates Fas-mediated DISC formation. A quantitative understanding of the interaction between CaM and Fas is important for the optimal design of antagonists for CaM or Fas to regulate the CaM-Fas interaction, thus modulating Fas-mediated DISC formation and apoptosis. The V254N mutation of the Fas death domain (Fas DD) is analogous to an identified mutant allele of Fas in lpr-cg mice that have a deficiency in Fas-mediated apoptosis. In this study, the interactions of CaM with the Fas DD wild type (Fas DD WT) and with the Fas DD V254N mutant were characterized using isothermal titration calorimetry (ITC), circular dichroism spectroscopy (CD), and molecular dynamics (MD) simulations. ITC results reveal an endothermic binding characteristic and an entropy-driven interaction of CaM with Fas DD WT or with Fas DD V254N. The Fas DD V254N mutation decreased the association constant (Ka) for CaM-Fas DD binding from (1.79 ± 0.20) × 10(6) to (0.88 ± 0.14) × 10(6) M(-1) and slightly increased a standard state Gibbs free energy (ΔG°) for CaM-Fas DD binding from -8.87 ± 0.07 to -8.43 ± 0.10 kcal/mol. CD secondary structure analysis and MD simulation results did not show significant secondary structural changes of the Fas DD caused by the V254N mutation. The conformational and dynamical motion analyses, the analyses of hydrogen bond formation within the CaM binding region, the contact numbers of each residue, and the electrostatic potential for the CaM binding region based on MD simulations demonstrated changes caused by the Fas DD V254N mutation. These changes caused by the Fas DD V254N mutation could affect the van der Waals interactions and electrostatic interactions between CaM and Fas DD, thereby affecting CaM-Fas DD interactions. Results from this study characterize CaM-Fas DD interactions in a quantitative way, providing structural and thermodynamic evidence of the role of the Fas DD V254N mutation in the CaM-Fas DD interaction. Furthermore, the results could help to identify novel strategies for regulating CaM-Fas DD interactions and Fas DD conformation and thus to modulate Fas-mediated DISC formation and thus Fas-mediated apoptosis.

Interplay of calcium and cadmium in mediating cadmium toxicity

The environmentally important toxic metal, cadmium, exists as the Cd(2+) ion in biological systems, and in this state structurally resembles Ca(2+). Thus, although cadmium exerts a broad range of adverse actions on cells by virtue of its propensity to bind to protein thiol groups, it is now well appreciated that Cd(2+) participates in a number of Ca(2+)-dependent pathways, attributable to its actions as a Ca(2+) mimetic, with a central role for calmodulin, and the Ca(2+)/calmodlin-dependent protein kinase II (CaMK-II) that mediates effects on cytoskeletal dynamics and apoptotic cell death. Cadmium interacts with receptors and ion channels on the cell surface, and with the intracellular estrogen receptor where it binds competitively to residues shared by Ca(2+). It increases cytosolic [Ca(2+)] through several mechanisms, but also decreases transcript levels of some Ca(2+)-transporter genes. It initiates mitochondrial apoptotic pathways, and activates calpains, contributing to mitochondria-independent apoptosis. However, the recent discovery of the role CaMK-II plays in Cd(2+)-induced cell death, and subsequent implication of CaMK-II in Cd(2+)-dependent alterations of cytoskeletal dynamics, has opened a new area of mechanistic cadmium toxicology that is a focus of this review. Calmodulin is necessary for induction of apoptosis by several agents, yet induction of apoptosis by Cd(2+) is prevented by CaMK-II block, and Ca(2+)-dependent phosphorylation of CaMK-II has been linked to increased Cd(2+)-dependent apoptosis. Calmodulin antagonism suppresses Cd(2+)-induced phosphorylation of Erk1/2 and the Akt survival pathway. The involvement of CaMK-II in the effects of Cd(2+) on cell morphology, and particularly the actin cytoskeleton, is profound, favouring actin depolymerization, disrupting focal adhesions, and directing phosphorylated FAK into a cellular membrane. CaMK-II is also implicated in effects of Cd(2+) on microtubules and cadherin junctions. A key question for future cadmium research is whether cytoskeletal disruption leads to apoptosis, or rather if apoptosis initiates cytoskeletal disruption in the context of Cd(2+).

The many faces of calmodulin in cell proliferation, programmed cell death, autophagy, and cancer

Calmodulin (CaM) is a ubiquitous Ca(2+) receptor protein mediating a large number of signaling processes in all eukaryotic cells. CaM plays a central role in regulating a myriad of cellular functions via interaction with multiple target proteins. This review focuses on the action of CaM and CaM-dependent signaling systems in the control of vertebrate cell proliferation, programmed cell death and autophagy. The significance of CaM and interconnected CaM-regulated systems for the physiology of cancer cells including tumor stem cells, and processes required for tumor progression such as growth, tumor-associated angiogenesis and metastasis are highlighted. Furthermore, the potential targeting of CaM-dependent signaling processes for therapeutic use is discussed.

Structural and biophysical characterization of the interactions between the death domain of Fas receptor and calmodulin

The extrinsic apoptotic pathway is initiated by cell surface death receptors such as Fas. Engagement of Fas by Fas ligand triggers a conformational change that allows Fas to interact with adaptor protein Fas-associated death domain (FADD) via the death domain, which recruits downstream signaling proteins to form the death-inducing signaling complex (DISC). Previous studies have shown that calmodulin (CaM) is recruited into the DISC in cholangiocarcinoma cells, suggesting a novel role of CaM in Fas-mediated signaling. CaM antagonists induce apoptosis through a Fas-related mechanism in cholangiocarcinoma and other cancer cell lines possibly by inhibiting Fas-CaM interactions. The structural determinants of Fas-CaM interaction and the underlying molecular mechanisms of inhibition, however, are unknown. Here we employed NMR and biophysical techniques to elucidate these mechanisms. Our data show that CaM binds to the death domain of Fas (FasDD) with an apparent dissociation constant (Kd) of ~2 μM and 2:1 CaM:FasDD stoichiometry. The interactions between FasDD and CaM are endothermic and entropically driven, suggesting that hydrophobic contacts are critical for binding. We also show that both the N- and C-terminal lobes of CaM are important for binding. NMR and surface plasmon resonance data show that three CaM antagonists (N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide, tamoxifen, and trifluoperazine) greatly inhibit Fas-CaM interactions by blocking the Fas-binding site on CaM. Our findings provide the first structural evidence for Fas-CaM interactions and mechanism of inhibition and provide new insight into the molecular basis for a novel role of CaM in regulating Fas-mediated apoptosis.

Structural insight for the roles of fas death domain binding to FADD and oligomerization degree of the Fas-FADD complex in the death-inducing signaling complex formation: a computational study

Fas binding to Fas-associated death domain (FADD) activates FADD-caspase-8 binding to form death-inducing signaling complex (DISC) that triggers apoptosis. The Fas-Fas association exists primarily as dimer in the Fas-FADD complex, and the Fas-FADD tetramer complexes have the tendency to form higher order oligomer. The importance of the oligomerized Fas-FADD complex in DISC formation has been confirmed. This study sought to provide structural insight for the roles of Fas death domain (Fas DD) binding to FADD and the oligomerization of Fas DD-FADD complex in activating FADD-procaspase-8 binding. Results show Fas DD binding to FADD stabilized the FADD conformation, including the increased stability of the critical residues in FADD death effector domain (FADD DED) for FADD-procaspase-8 binding. Fas DD binding to FADD resulted in the decreased degree of both correlated and anticorrelated motion of the residues in FADD and caused the reversed correlated motion between FADD DED and FADD death domain (FADD DD). The exposure of procaspase-8 binding residues in FADD that allows FADD to interact with procaspase-8 was observed with Fas DD binding to FADD. We also observed different degrees of conformational and motion changes of FADD in the Fas DD-FADD complex with different degrees of oligomerization. The increased conformational stability and the decreased degree of correlated motion of the residues in FADD in Fas DD-FADD tetramer complex were observed compared to those in Fas DD-FADD dimer complex. This study provides structural evidence for the roles of Fas DD binding to FADD and the oligomerization degree of Fas DD-FADD complex in DISC formation to signal apoptosis.

Expression of calmodulin in germ cells is associated with fenvalerate-induced male reproductive toxicity

Exposure to fenvalerate was demonstrated to be toxic to the male reproductive system. Our previous data revealed that intracellular calcium plays an important role in regulating the above toxicity, through actions on both T-type calcium channels and endoplasmic reticulum calcium signals. The present study explored the effects of fenvalerate on the expression of calmodulin in mouse testis and GC-2spd(ts) cells, and its association with fenvalerate-induced male reproductive toxicity. Male mice were subjected to different doses (3.71, 18.56, 37.12, 92.81 mg/kg bw) of fenvalerate or vehicle control for 4 weeks. Expression of calmodulin was determined by real-time polymerase chain reaction (PCR) and Western blot analysis in mouse testis. Similar approaches were utilized in GC-2spd(ts) cells cultured with 5 μM fenvalerate at different time points. In the in vivo study, all mice survived through the entire 4 weeks. Administration of fenvalerate resulted in a dose-dependent reduction in testis weight/body weight, sperm motility, and increased head abnormality rate. By histological staining, mice treated with fenvalerate at higher doses showed dilated seminiferous tubules and disturbed arrangement of spermatogenic cells. Meanwhile, both mRNA and protein expression of calmodulin were significantly increased in the testes of mice exposed to fenvalerate compared to control mice. Moreover, in the in vitro study, 5 μM fenvalerate significantly increased the expression of calmodulin at the mRNA and protein levels in GC-2spd(ts) cells after 8 h of incubation and sustained these levels for at least 24 h. Collectively, these data suggested that enhanced expression of calmodulin correlates with male reproductive damage induced by fenvalerate.

Reduced CaM/FLIP binding by a single point mutation in c-FLIP(L) modulates Fas-mediated apoptosis and decreases tumorigenesis

We have previously demonstrated that calmodulin (CaM) binds directly to c-FLIP(L) in a Ca(2+)-dependent manner. Deletion of the CaM-binding region (amino acid 197-213) results in reduced CaM binding, and increased Fas-mediated apoptosis and decreased tumorigenesis of cholangiocarcinoma cells. The present studies were designed to identify the precise amino acids between 197 and 213 that are responsible for CaM/FLIP binding, and their roles in mediating the anti-apoptotic function of c-FLIP(L). Sequence analysis of the CaM-binding region at 197-213 predicted three unique positively charged residues at 204, 207 and 209, which might be responsible for the CaM/FLIP binding. A point mutation at H204 of c-FLIP(L) was found to markedly reduce CaM binding, whereas point mutation at R207 or K209 did not affect c-FLIP(L) binding to CaM. Decreased CaM/FLIP binding was confirmed in cholangiocarcinoma cells overexpressing the H204 c-FLIP(L) mutant. Reduced CaM binding by the H204 mutant resulted in increased sensitivity to Fas-mediated apoptosis and inhibited tumor growth in mice compared with wild-type c-FLIP(L). Death-inducing signaling complex (DISC) analysis showed that the reduced CaM binding to H204 mutant resulted in less c-FLIP(L) recruited into the DISC. Concurrently, increased caspase 8 was recruited to the DISC, which resulted in increased cleavage and activation of caspase 8, activation of downstream caspase 3 and increased apoptosis. Therefore, these results demonstrate that the H204 residue is responsible for c-FLIP(L) binding to CaM, which mediates the anti-apoptotic function of c-FLIP(L), most likely through affecting recruitment of caspase 8 into the DISC and thus caspase 8 activation. These studies further characterized CaM/FLIP interaction and its function in regulating Fas-mediated apoptosis and tumorigenesis, which may provide new therapeutic targets for cancer therapy.

Trifluoperazine regulation of calmodulin binding to Fas: a computational study

Death-inducing signaling complex (DISC) formation is a critical step in Fas-mediated signaling for apoptosis. Previous experiments have demonstrated that the calmodulin (CaM) antagonist, trifluoperazine (TFP) regulates CaM-Fas binding and affects Fas-mediated DISC formation. In this study, we investigated the anti-cooperative characteristics of TFP binding to CaM and the effect of TFP on the CaM-Fas interaction from both structural and thermodynamic perspectives using combined molecular dynamics simulations and binding free energy analyses. We studied the interactions of different numbers of TFP molecules with CaM and explored the effects of the resulting conformational changes in CaM on CaM-Fas binding. Results from these analyses showed that the number of TFP molecules bound to CaM directly influenced α-helix formation and hydrogen bond occupancy within the α-helices of CaM, contributing to the conformational and motion changes in CaM. These changes affected CaM binding to Fas, resulting in secondary structural changes in Fas and conformational and motion changes of Fas in CaM-Fas complexes, potentially perturbing the recruitment of Fas-associated death domain for DISC formation. The computational results from this study reveal the structural and molecular mechanisms that underlie the role of the CaM antagonist, TFP, in regulation of CaM-Fas binding and Fas-mediated DISC formation in a concentration-dependent manner.

Calmodulin mediates Fas-induced FADD-independent survival signaling in pancreatic cancer cells via activation of Src-extracellular signal-regulated kinase (ERK)

Pancreatic cancer remains a devastating malignancy with a poor prognosis and is largely resistant to current therapies. To understand the resistance of pancreatic tumors to Fas death receptor-induced apoptosis, we investigated the molecular mechanisms of Fas-activated survival signaling in pancreatic cancer cells. We found that knockdown of the Fas-associated protein with death domain (FADD), the adaptor that mediates downstream signaling upon Fas activation, rendered Fas-sensitive MiaPaCa-2 and BxPC-3 pancreatic cells resistant to Fas-induced apoptosis. By contrast, Fas activation promoted the survival of the FADD knockdown MiaPaCa-2 and BxPC-3 cells in a concentration-dependent manner. The pharmacological inhibitor of ERK, PD98059, abrogated Fas-promoted cell survival in FADD knockdown MiaPaCa-2 and BxPC-3 cells. Furthermore, increased phosphorylation of Src was demonstrated to mediate Fas-induced ERK activation and cell survival. Immunoprecipitation of Fas in the FADD knockdown cells identified the presence of increased calmodulin, Src, and phosphorylated Src in the Fas-associated protein complex upon Fas activation. Trifluoperazine, a calmodulin antagonist, inhibited Fas-induced recruitment of calmodulin, Src, and phosphorylated Src. Consistently, trifluoperazine blocked Fas-promoted cell survival. A direct interaction of calmodulin and Src and their binding site were identified with recombinant proteins. These results support an essential role of calmodulin in mediating Fas-induced FADD-independent activation of Src-ERK signaling pathways, which promote survival signaling in pancreatic cancer cells. Understanding the molecular mechanisms responsible for the resistance of pancreatic cells to apoptosis induced by Fas-death receptor signaling may provide molecular insights into designing novel therapies to treat pancreatic tumors.

Allosteric effects of the antipsychotic drug trifluoperazine on the energetics of calcium binding by calmodulin

Trifluoperazine (TFP; Stelazine) is an antagonist of calmodulin (CaM), an essential regulator of calcium-dependent signal transduction. Reports differ regarding whether, or where, TFP binds to apo CaM. Three crystallographic structures (1CTR, 1A29, and 1LIN) show TFP bound to (Ca(2+))(4)-CaM in ratios of 1, 2, or 4 TFP per CaM. In all of these, CaM domains adopt the "open" conformation seen in CaM-kinase complexes having increased calcium affinity. Most reports suggest TFP also increases calcium affinity of CaM. To compare TFP binding to apo CaM and (Ca(2+))(4)-CaM and explore differential effects on the N- and C-domains of CaM, stoichiometric TFP titrations of CaM were monitored by (15)N-HSQC NMR. Two TFP bound to apo CaM, whereas four bound to (Ca(2+))(4)-CaM. In both cases, the preferred site was in the C-domain. During the titrations, biphasic responses for some resonances suggested intersite interactions. TFP-binding sites in apo CaM appeared distinct from those in (Ca(2+))(4)-CaM. In equilibrium calcium titrations at defined ratios of TFP:CaM, TFP reduced calcium affinity at most levels tested; this is similar to the effect of many IQ-motifs on CaM. However, at the highest level tested, TFP raised the calcium affinity of the N-domain of CaM. A model of conformational switching is proposed to explain how TFP can exert opposing allosteric effects on calcium affinity by binding to different sites in the "closed," "semi-open," and "open" domains of CaM. In physiological processes, apo CaM, as well as (Ca(2+))(4)-CaM, needs to be considered a potential target of drug action.

Calmodulin inhibition contributes to sensitize TRAIL-induced apoptosis in human lung cancer H1299 cells

Tumor necrosis factor related apoptosis-inducing ligand (TRAIL) preferentially triggers apoptosis in tumor cells versus normal cells. However, TRAIL alone is not effective in treating TRAIL-resistant tumors. We evaluated the effect of 180 enzyme inhibitors on TRAIL-induced apoptosis in human lung cancer H1299 cells, and found fluphenazine-N-2-chloroethane (a calmodulin (CaM) antagonist) sensitized TRAIL-induced apoptosis. Interestingly, in the presence of TRAIL, it increased caspase-8 binding to the Fas-associated death domain (FADD), but decreased binding of FADD-like interleukin-1β-converting enzyme inhibitory proteins (FLIPs). Additionally, its combination with TRAIL inhibited Akt phosphorylation. These results were consistently observed in cells treated with CaM siRNA. We suggested the blockade of CaM could sensitize lung cancer cells to TRAIL-induced apoptosis in at least 2 ways: (i) it can activate death-inducing signaling complex mediated apoptosis by inhibiting TRAIL-induced binding of FLIP and TRAIL-enhanced binding of caspase-8 to FADD; (ii) it can inhibit Akt phosphorylation, consequently leading to decreased expression of anti-apoptotic molecules such as FLIP and members of the inhibitor of apoptosis protein family. This study suggests the combination of CaM antagonists with TRAIL may have the therapeutic potential to overcome the resistance of lung cancers to apoptosis.

Conformation and free energy analyses of the complex of calcium-bound calmodulin and the Fas death domain

Previous studies have demonstrated a calcium-dependent interaction of calmodulin (CaM) and Fas that is regulated during Fas-induced apoptosis in several cell lines, including cholangiocarcinoma, Jurkat cells, and osteoclasts. The binding of CaM and Fas has been identified on residues 231-254 of Fas; the V254N point mutation decreases the CaM/Fas binding, and the C-terminal deletion mutation increases the CaM/Fas binding. Recent studies have shown that CaM is recruited into the Fas-mediated death-inducing signaling complex (DISC) in a calcium-dependent manner. However, the molecular mechanisms whereby Fas mutations and CaM/Fas binding might regulate Fas-mediated DISC formation are unknown. In this study we investigated the binding thermodynamics and conformation of the CaM/Fas complexes with combined explicit solvent molecular-dynamics simulations and implicit solvent binding free-energy calculations. The binding free-energy analysis demonstrated that the Fas V254N point mutation reduced its binding affinity with CaM. In contrast, the Fas mutant with the deletion of the 15 amino acid at the C-terminus increased its binding to CaM. These observations are consistent with previous findings from biochemical studies. Conformational analyses further showed that the Fas V254N mutation resulted in an unstable conformation, whereas the C-terminal deletion mutation stabilized the Fas conformation, and both mutations resulted in changes of the degree of correlation between the motions of the residues in Fas. Analysis of the CaM/Fas complex revealed that CaM/Fas binding stabilized the conformation of both CaM and Fas and changed the degree of correlated motion of the residues of CaM and Fas. The results presented here provide structural evidence for the roles of Fas mutations and CaM/Fas binding in Fas-induced DISC formation. Understanding the molecular mechanisms of CaM/Fas binding in Fas-mediated DISC formation should provide important insights into the function of Fas mutations and CaM in regulating Fas-mediated apoptosis.

Calmodulin binding to cellular FLICE-like inhibitory protein modulates Fas-induced signalling

We and others have demonstrated that Fas-mediated apoptosis is a potential therapeutic target for cholangiocarcinoma. Previously, we reported that CaM (calmodulin) antagonists induced apoptosis in cholangiocarcinoma cells through Fas-related mechanisms. Further, we identified a direct interaction between CaM and Fas with recruitment of CaM into the Fas-mediated DISC (death-inducing signalling complex), suggesting a novel role for CaM in Fas signalling. Therefore we characterized the interaction of CaM with proteins recruited into the Fas-mediated DISC, including FADD (Fas-associated death domain)-containing protein, caspase 8 and c-FLIP {cellular FLICE [FADD (Fas-associated death domain)-like interleukin 1beta-converting enzyme]-like inhibitory protein}. A Ca(2+)-dependent direct interaction between CaM and FLIP(L), but not FADD or caspase 8, was demonstrated. Furthermore, a 37.3+/-5.7% increase (n=6, P=0.001) in CaM-FLIP binding was observed at 30 min after Fas stimulation, which returned to the baseline after 60 min and correlated with a Fas-induced increase in intracellular Ca(2+) that reached a peak at 30 min and decreased gradually over 60 min in cholangiocarcinoma cells. A CaM antagonist, TFP (trifluoperazine), inhibited the Fas-induced increase in CaM-FLIP binding concurrent with inhibition of ERK (extracellular-signal-regulated kinase) phosphorylation, a downstream signal of FLIP. Direct binding between CaM and FLIP(L) was demonstrated using recombinant proteins, and a CaM-binding region was identified in amino acids 197-213 of FLIP(L). Compared with overexpression of wild-type FLIP(L) that resulted in decreased spontaneous as well as Fas-induced apoptosis, mutant FLIP(L) with deletion of the CaM-binding region resulted in increased spontaneous and Fas-induced apoptosis in cholangiocarcinoma cells. Understanding the biology of CaM-FLIP binding may provide new therapeutic targets for cholangiocarcinoma and possibly other cancers.