The anti-adhesive activity of thrombospondin is mediated by the N-terminal domain of cell surface calreticulin

Identification and characterization of calreticulin as a novel plasminogen receptor

Calreticulin (CRT) was originally identified as a key calcium-binding protein of the endoplasmic reticulum. Subsequently, CRT was shown to possess multiple intracellular functions, including roles in calcium homeostasis and protein folding. Recently, several extracellular functions have been identified for CRT, including roles in cancer cell invasion and phagocytosis of apoptotic and cancer cells by macrophages. In the current report, we uncover a novel function for extracellular CRT and report that CRT functions as a plasminogen-binding receptor that regulates the conversion of plasminogen to plasmin. We show that human recombinant or bovine tissue-derived CRT dramatically stimulated the conversion of plasminogen to plasmin by tissue plasminogen activator or urokinase-type plasminogen activator. Surface plasmon resonance analysis revealed that CRT-bound plasminogen (KD = 1.8 μM) with moderate affinity. Plasminogen binding and activation by CRT were inhibited by ε-aminocaproic acid, suggesting that an internal lysine residue of CRT interacts with plasminogen. We subsequently show that clinically relevant CRT variants (lacking four or eight lysines in carboxyl-terminal region) exhibited decreased plasminogen activation. Furthermore, CRT-deficient fibroblasts generated 90% less plasmin and CRT-depleted MDA MB 231 cells also demonstrated a significant reduction in plasmin generation. Moreover, treatment of fibroblasts with mitoxantrone dramatically stimulated plasmin generation by WT but not CRT-deficient fibroblasts. Our results suggest that CRT is an important cellular plasminogen regulatory protein. Given that CRT can empower cells with plasmin proteolytic activity, this discovery may provide new mechanistic insight into the established role of CRT in cancer.

Dynamic immuno-nanomedicines in oncology

Anti-cancer therapeutics have achieved significant advances due to the emergence of immunotherapies that rely on the identification of tumors by the patients' immune system and subsequent tumor eradication. However, tumor cells often escape immunity, leading to poor responsiveness and easy tolerance to immunotherapy. Thus, the potentiated anti-tumor immunity in patients resistant to immunotherapies remains a challenge. Reactive oxygen species-based dynamic nanotherapeutics are not new in the anti-tumor field, but their potential as immunomodulators has only been demonstrated in recent years. Dynamic nanotherapeutics can distinctly enhance anti-tumor immune response, which derives the concept of the dynamic immuno-nanomedicines (DINMs). This review describes the pivotal role of DINMs in cancer immunotherapy and provides an overview of the clinical realities of DINMs. The preclinical development of emerging DINMs is also outlined. Moreover, strategies to synergize the antitumor immunity by DINMs in combination with other immunologic agents are summarized. Last but not least, the challenges and opportunities related to DINMs-mediated immune responses are also discussed.

Calreticulin as a marker and therapeutic target for cancer

Calreticulin (CRT) is a multifunctional protein found within the endoplasmic reticulum (ER). In addition, CRT participates in the formation and development of tumors and promotes the proliferation and migration of tumor cells. When a malignant tumor occurs in the human body, cancer cells that die from immunogenic cell death (ICD) expose CRT on their surface, and CRT that is transferred to the cell surface represents an "eat me" signal, which promotes dendritic cells to phagocytose the tumor cells, thereby increasing the sensitivity of tumors to anticancer immunotherapy. Expression of CRT in tumor tissues is higher than in normal tissues and is associated with disease progression in many malignant tumors. Thus, the dysfunctional production of CRT can promote tumorigenesis because it disturbs not only the balance of healthy cells but also the body's immune surveillance. CRT may be a diagnostic marker and a therapeutic target for cancer, which is discussed extensively in this review.

Calreticulin: a multifunctional protein with potential therapeutic applications for chronic wounds

Calreticulin is recognized as a multifunctional protein that serves an essential role in diverse biological processes that include wound healing, modification and folding of proteins, regulation of the secretory pathway, cell motility, cellular metabolism, protein synthesis, regulation of gene expression, cell cycle regulation and apoptosis. Although the role of calreticulin as an endoplasmic reticulum-chaperone protein has been well described, several studies have demonstrated calreticulin to be a highly versatile protein with an essential role during wound healing. These features make it an ideal molecule for treating a complex, multifactorial diseases that require fine tuning, such as chronic wounds. Indeed, topical application of recombinant calreticulin to wounds in multiple models of wound healing has demonstrated remarkable pro-healing effects. Among them include enhanced keratinocyte and fibroblast migration and proliferation, induction of extracellular matrix proteins, recruitment of macrophages along with increased granulation tissue formation, all of which are important functions in promoting wound healing that are deregulated in chronic wounds. Given the high degree of diverse functions and pro-healing effects, application of exogenous calreticulin warrants further investigation as a potential novel therapeutic option for chronic wound patients. Here, we review and highlight the significant effects of topical application of calreticulin on enhancing wound healing and its potential as a novel therapeutic option to shift chronic wounds into healing, acute-like wounds.

Calreticulin: Endoplasmic reticulum Ca2+ gatekeeper

Endoplasmic reticulum (ER) luminal Ca2+ is vital for the function of the ER and regulates many cellular processes. Calreticulin is a highly conserved, ER-resident Ca2+ binding protein and lectin-like chaperone. Over four decades of studying calreticulin demonstrate that this protein plays a crucial role in maintaining Ca2+ supply under different physiological conditions, in managing access to Ca2+ and how Ca2+ is used depending on the environmental events and in making sure that Ca2+ is not misused. Calreticulin plays a role of ER luminal Ca2+ sensor to manage Ca2+ -dependent ER luminal events including maintaining interaction with its partners, Ca2+ handling molecules, substrates and stress sensors. The protein is strategically positioned in the lumen of the ER from where the protein manages access to and distribution of Ca2+ for many cellular Ca2+ -signalling events. The importance of calreticulin Ca2+ pool extends beyond the ER and includes influence of cellular processes involved in many aspects of cellular pathophysiology. Abnormal handling of the ER Ca2+ contributes to many pathologies from heart failure to neurodegeneration and metabolic diseases.

Controlled extracellular proteolysis of thrombospondins

Limited proteolysis of thrombospondins is a powerful mechanism to ensure dynamic tuning of their activities in the extracellular space. Thrombospondins are multifunctional matricellular proteins composed of multiple domains, each with a specific pattern of interactions with cell receptors, matrix components and soluble factors (growth factors, cytokines and proteases), thus with different effects on cell behavior and responses to changes in the microenvironment. Therefore, the proteolytic degradation of thrombospondins has multiple functional consequences, reflecting the local release of active fragments and isolated domains, exposure or disruption of active sequences, altered protein location, and changes in the composition and function of TSP-based pericellular interaction networks. In this review current data from the literature and databases is employed to provide an overview of cleavage of mammalian thrombospondins by different proteases. The roles of the fragments generated in specific pathological settings, with particular focus on cancer and the tumor microenvironment, are discussed.

Thrombospondin-1 Signaling Through the Calreticulin/LDL Receptor Related Protein 1 Axis: Functions and Possible Roles in Glaucoma

Thrombospondin-1 (TSP-1) is a matricellular extracellular matrix protein. Matricellular proteins are components of the extracellular matrix (ECM) that regulate key cellular functions and impact ECM organization, but which lack direct primary structural roles in the ECM. TSP-1 expression is upregulated in response to injury, hypoxia, growth factor stimulation, inflammation, glucose, and by reactive oxygen species. Relevant to glaucoma, TSP-1 is also a mechanosensitive molecule upregulated by mechanical stretch. TSP-1 expression is increased in ocular remodeling in glaucoma in both the trabecular meshwork and in the optic nerve head. The exact roles of TSP-1 in glaucoma remain to be defined, however. It plays important roles in cell behavior and in ECM remodeling during wound healing, fibrosis, angiogenesis, and in tumorigenesis and metastasis. At the cellular level, TSP-1 can modulate cell adhesion and migration, protease activity, growth factor activity, anoikis resistance, apoptosis, and collagen secretion and matrix assembly and cross-linking. These multiple functions and macromolecular and receptor interactions have been ascribed to specific domains of the TSP-1 molecule. In this review, we will focus on the cell regulatory activities of the TSP-1 N-terminal domain (NTD) sequence that binds to cell surface calreticulin (Calr) and which regulates cell functions via signaling through Calr complexed with LDL receptor related protein 1 (LRP1). We will describe TSP-1 actions mediated through the Calr/LRP1 complex in regulating focal adhesion disassembly and cytoskeletal reorganization, cell motility, anoikis resistance, and induction of collagen secretion and matrix deposition. Finally, we will consider the relevance of these TSP-1 functions to the pathologic remodeling of the ECM in glaucoma.

Immunogenic Cell Death in Cancer Therapy

Apoptosis plays a crucial role in chemotherapy-induced cell death. The conventional theory holding that apoptosis needs to be immunologically silent has recently been revised, and the concept of immunogenic cell death (ICD) has been proposed. This review describes the main features of ICD induction. These ICD markers are important for the effectiveness of anticancer therapy, as well as for basic research into cell death regulation. The mechanism of the "vaccination effect" of dying cancer cells undergoing ICD has been fully described, including the activation of specific antitumor response after re-challenge by the same living tumor cells. This review also discusses the whole set of molecular events attributing cell death to immunogenic type: the exposure of calreticulin and the heat shock protein HSP70 to the outer surface of the cell membrane and the release of the nuclear protein HMGB1 and ATP into the extracellular space. ICD inducers of various nature (chemotherapy drugs, cytotoxic proteins, and oncolytic viruses), as well as physical methods, are classified in the current review.

Endoplasmic reticulum stress-induced release and binding of calreticulin from human ovarian cancer cells

BACKGROUND

Calreticulin (CRT) is an endoplasmic reticulum (ER) chaperone, but can appear surface bound on cancers cells, including ovarian cancers (OC). We investigated at what stage of cell viability, CRT appeared associated with surface of human OC cells. CRT on pre-apoptotic tumour cells is thought to initiate their eradication via a process termed immunogenic cell death (ICD).

METHODS

We treated OC cells with the chemotherapeutic-doxorubicin (DX) known to induce translocation of CRT to some tumour cell surfaces, with and without the ER stressor-thapsigargin (TG)-and/or an ER stress inhibitor-TUDCA. We monitored translocation/release of CRT in pre-apoptotic cells by flow cytometry, immunoblotting and ELISA. We investigated the difference in binding of FITC-CRT to pre-apoptotic, apoptotic and necrotic cells and the ability of extracellular CRT to generate immature dendritic cells from THP-1 monocytes.

RESULTS

Dx-treatment increased endogenously released CRT and extracellular FITC_CRT binding to human pre-apoptotic OC cells. DX and TG also promoted cell death in OC cells which also increased CRT release. These cellular responses were significantly inhibited by TUDCA, suggesting that ER stress is partially responsible for the changes in CRT cellular distribution. Extracellular CRT induces maturation of THP-1 towards a imDC phenotype, an important component of ICD.

CONCLUSION

Collectively, these cellular responses suggest that ER stress is partially responsible for the changes in CRT cellular distribution. ER-stress regulates in part the release and binding of CRT to human OC cells where it may play a role in ICD.

Advances in Intracellular Calcium Signaling Reveal Untapped Targets for Cancer Therapy

Intracellular Ca2+ distribution is a tightly regulated process. Numerous Ca2+ chelating, storage, and transport mechanisms are required to maintain normal cellular physiology. Ca2+-binding proteins, mainly calmodulin and calbindins, sequester free intracellular Ca2+ ions and apportion or transport them to signaling hubs needing the cations. Ca2+ channels, ATP-driven pumps, and exchangers assist the binding proteins in transferring the ions to and from appropriate cellular compartments. Some, such as the endoplasmic reticulum, mitochondria, and lysosomes, act as Ca2+ repositories. Cellular Ca2+ homeostasis is inefficient without the active contribution of these organelles. Moreover, certain key cellular processes also rely on inter-organellar Ca2+ signaling. This review attempts to encapsulate the structure, function, and regulation of major intracellular Ca2+ buffers, sensors, channels, and signaling molecules before highlighting how cancer cells manipulate them to survive and thrive. The spotlight is then shifted to the slow pace of translating such research findings into anticancer therapeutics. We use the PubMed database to highlight current clinical studies that target intracellular Ca2+ signaling. Drug repurposing and improving the delivery of small molecule therapeutics are further discussed as promising strategies for speeding therapeutic development in this area.

A View of the Endoplasmic Reticulum Through the Calreticulin Lens

Calreticulin is well known as an ER-resident protein that serves as the major endoplasmic reticulum (ER) Ca²⁺ binding protein. This protein has been the major topic of discussion in an international workshop that has been meeting for a quarter of a century. In sharing information about this protein, the field also witnessed remarkable insights into the importance of the ER as an organelle and the role of ER Ca²⁺ in coordinating ER and cellular functions. Recent technological advances have helped to uncover the contributions of calreticulin in maintaining Ca²⁺ homeostasis in the ER and to unravel its involvement in a multitude of cellular processes as highlighted in this collection of articles. The continuing revelations of unexpected involvement of calreticulin and Ca²⁺ in many critical aspects of cellular function promises to further improve insights into the significance of this protein in the promotion of physiology as well as prevention of pathology.

Breast Cancer-Derived Microvesicles Are the Source of Functional Metabolic Enzymes as Potential Targets for Cancer Therapy

Membrane-derived extracellular vesicles, referred to as microvesicles (MVs), have been proposed to participate in several cancer diseases. In this study, MV fractions were isolated by differential ultracentrifugation from a metastatic breast cancer (BC) cell line MDA-MB-231 and a non-cancerous breast cell line MCF10A, then analyzed by nano-liquid chromatography coupled to tandem mass spectrometry. A total of 1519 MV proteins were identified from both cell lines. The data obtained were compared to previously analyzed proteins from small extracellular vesicles (sEVs), revealing 1272 proteins present in both MVs and sEVs derived from the MDA-MB-231 cell line. Among the 89 proteins unique to MDA-MB-231 MVs, three enzymes: ornithine aminotransferase (OAT), transaldolase (TALDO1) and bleomycin hydrolase (BLMH) were previously proposed as cancer therapy targets. These proteins were enzymatically validated in cells, sEVs, and MVs derived from both cell lines. The specific activity of OAT and TALDO1 was significantly higher in MDA-MB-231-derived MVs than in MCF10A MVs. BLMH was highly expressed in MDA-MB-231-derived MVs, compared to MCF10A MVs. This study shows that MVs carry functional metabolic enzymes and provides a framework for future studies of their biological role in BC and potential in therapeutic applications.

Calreticulin-Multifunctional Chaperone in Immunogenic Cell Death: Potential Significance as a Prognostic Biomarker in Ovarian Cancer Patients

Immunogenic cell death (ICD) is a type of death, which has the hallmarks of necroptosis and apoptosis, and is best characterized in malignant diseases. Chemotherapeutics, radiotherapy and photodynamic therapy induce intracellular stress response pathways in tumor cells, leading to a secretion of various factors belonging to a family of damage-associated molecular patterns molecules, capable of inducing the adaptive immune response. One of them is calreticulin (CRT), an endoplasmic reticulum-associated chaperone. Its presence on the surface of dying tumor cells serves as an "eat me" signal for antigen presenting cells (APC). Engulfment of tumor cells by APCs results in the presentation of tumor's antigens to cytotoxic T-cells and production of cytokines/chemokines, which activate immune cells responsible for tumor cells killing. Thus, the development of ICD and the expression of CRT can help standard therapy to eradicate tumor cells. Here, we review the physiological functions of CRT and its involvement in the ICD appearance in malignant disease. Moreover, we also focus on the ability of various anti-cancer drugs to induce expression of surface CRT on ovarian cancer cells. The second aim of this work is to discuss and summarize the prognostic/predictive value of CRT in ovarian cancer patients.

Calreticulin exploits TGF-β for extracellular matrix induction engineering a tissue regenerative process

Topical application of extracellular calreticulin (eCRT), an ER chaperone protein, in animal models enhances wound healing and induces tissue regeneration evidenced by epidermal appendage neogenesis and lack of scarring. In addition to chemoattraction of cells critical to the wound healing process, eCRT induces abundant neo-dermal extracellular matrix (ECM) formation by 3 days post-wounding. The purpose of this study was to determine the mechanisms involved in eCRT induction of ECM. In vitro, eCRT strongly induces collagen I, fibronectin, elastin, α-smooth muscle actin in human adult dermal (HDFs) and neonatal fibroblasts (HFFs) mainly via TGF-β canonical signaling and Smad2/3 activation; RAP, an inhibitor of LRP1 blocked eCRT ECM induction. Conversely, eCRT induction of α5 and β1 integrins was not mediated by TGF-β signaling nor inhibited by RAP. Whereas eCRT strongly induces ECM and integrin α5 proteins in K41 wild-type mouse embryo fibroblasts (MEFs), CRT null MEFs were unresponsive. The data show that eCRT induces the synthesis and release of TGF-β3 first via LRP1 or other receptor signaling and later induces ECM proteins via LRP1 signaling subsequently initiating TGF-β receptor signaling for intracellular CRT (iCRT)-dependent induction of TGF-β1 and ECM proteins. In addition, TGF-β1 induces 2-3-fold higher level of ECM proteins than eCRT. Whereas eCRT and iCRT converge for ECM induction, we propose that eCRT attenuates TGF-β-mediated fibrosis/scarring to achieve tissue regeneration.

Calreticulin and cancer

Calreticulin (CALR) is an endoplasmic reticulum (ER)-resident protein involved in a spectrum of cellular processes. In healthy cells, CALR operates as a chaperone and Ca²⁺ buffer to assist correct protein folding within the ER. Besides favoring the maintenance of cellular proteostasis, these cell-intrinsic CALR functions support Ca²⁺-dependent processes, such as adhesion and integrin signaling, and ensure normal antigen presentation on MHC Class I molecules. Moreover, cancer cells succumbing to immunogenic cell death (ICD) expose CALR on their surface, which promotes the uptake of cell corpses by professional phagocytes and ultimately supports the initiation of anticancer immunity. Thus, loss-of-function CALR mutations promote oncogenesis not only as they impair cellular homeostasis in healthy cells, but also as they compromise natural and therapy-driven immunosurveillance. However, the prognostic impact of total or membrane-exposed CALR levels appears to vary considerably with cancer type. For instance, while genetic CALR defects promote pre-neoplastic myeloproliferation, patients with myeloproliferative neoplasms bearing CALR mutations often experience improved overall survival as compared to patients bearing wild-type CALR. Here, we discuss the context-dependent impact of CALR on malignant transformation, tumor progression and response to cancer therapy.

In Vitro Employment of Recombinant Taenia solium Calreticulin as a Novel Strategy Against Breast and Ovarian Cancer Stem-like Cells

BACKGROUND AND AIMS

Calreticulin is a chaperone and master regulator of intracellular calcium homeostasis. Several additional functions have been discovered. Human and parasite calreticulin have been shown to suppress mammary tumor growth in vivo. Here, we explored the capacity of recombinant Taenia solium calreticulin (rTsCRT) to modulate cancer cell growth in vitro.

METHODS

We used different concentrations of rTsCRT to treat cancer cell lines and analyzed viability and colony formation capacity. We also tested the combination of the IC₂₀ or IC₅₀ doses of rTsCRT and of the chemotherapeutic drug 5-fluorouracil on MCF7 and SKOV3 cell lines. As a control, the non-tumorigenic cell line MCF10-A was employed. The effect of the drug combinations was also assessed in cancer stem-like cells. Additionally, scavenger receptor ligands were employed to identify the role of this receptor in the rTsCRT anti-tumoral effect.

RESULTS

rTsCRT has a dose-dependent in vitro anti-tumoral effect, being SKOV3 the most sensitive cell line followed by MCF7. When rTsCRT/5-fluorouracil were used, MCF7 and SKOV3 showed a 60% reduction in cell viability; colony formation capacity was also diminished. Treatment of cancer stem-like cells from MCF7 showed a higher reduction in cell viability, while those from SKOV3 were more sensitive to colony disaggregation. Finally, pharmacological inhibition of the scavenger receptor, abrogated the reduction in viability induced by rTsCRT in both the parental and stem-like cells.

CONCLUSION

Our data suggest that rTsCRT alone or in combination with 5-fluorouracil inhibits the growth of breast and ovarian cancer cell lines through its interaction with scavenger receptors.

Molecular insights into the effect of an apoptotic raft-like bilayer on the conformation and dynamics of calreticulin

Cell surface calreticulin (CRT) can mediate apoptotic cells removal by binding and activating LDL receptor-related protein (LRP1). Phosphatidylserine (PS) lipids in the inner leaflet of the cell membrane are externalized and become exposed in cholesterol (CHOL)-rich membrane raft-like microdomain during apoptosis and co-localized with cell surface CRT. How the apoptotic raft-like membrane microdomain affects the structure and dynamics of CRT, further affecting CRT binding with LRP1 to signal apoptotic-cell clearance, remains unknown. In this study, we investigate the interactions between CRT and raft-like bilayers with or without POPS lipids with molecular dynamics simulations. In addition, the effect of an apoptotic raft bilayer on the binding between CRT and thrombospondin-1 (TSP1), a ligand of CRT on the cell surface to signal focal adhesion disassembly, was also investigated. Results of single CRT interactions with raft-like bilayers show that PS lipids in apoptotic raft-like bilayer increased the interactions between CRT and lipid bilayer, which enhanced the conformational stability and increased dynamical motion of CRT. The microscopic and mesoscopic properties of apoptotic raft-like bilayer were altered by the binding of CRT with lipid bilayer. Results of CRT-TSP1 complex interactions with raft-like bilayers show that the binding free energy between TSP1 and CRT was reduced in apoptotic raft-like bilayer environment. This study provides molecular and structural insight into the effect of an apoptotic raft-like bilayer on the conformation and dynamics of CRT, which could enrich our understanding of CRT-mediated apoptotic-cell clearance and focal adhesion disassembly.

Thrombospondin 1 and Its Diverse Roles as a Regulator of Extracellular Matrix in Fibrotic Disease

Thrombospondin 1 (TSP1) is a matricellular extracellular matrix protein that has diverse roles in regulating cellular processes important for the pathogenesis of fibrotic diseases. We will present evidence for the importance of TSP1 control of latent transforming growth factor beta activation in renal fibrosis with an emphasis on diabetic nephropathy. Other functions of TSP1 that affect renal fibrosis, including regulation of inflammation and capillary density, will be addressed. Emerging roles for TSP1 N-terminal domain regulation of collagen matrix assembly, direct effects of TSP1-collagen binding, and intracellular functions of TSP1 in mediating endoplasmic reticulum stress responses in extracellular matrix remodeling and fibrosis, which could potentially affect renal fibrogenesis, will also be discussed. Finally, we will address possible strategies for targeting TSP1 functions to treat fibrotic renal disease.

Unfolding the Role of Calreticulin in Myeloproliferative Neoplasm Pathogenesis

In 2013, two seminal studies identified gain-of-function mutations in the Calreticulin (CALR) gene in a subset of JAK2/MPL-negative myeloproliferative neoplasm (MPN) patients. CALR is an endoplasmic reticulum (ER) chaperone protein that normally binds misfolded proteins in the ER and prevents their export to the Golgi and had never previously been reported mutated in cancer or to be associated with hematologic disorders. Further investigation determined that mutated CALR is able to achieve oncogenic transformation primarily through constitutive activation of the MPL-JAK-STAT signaling axis. Here we review our current understanding of the role of CALR mutations in MPN pathogenesis and how these insights can lead to innovative therapeutics approaches.

Multiscale simulation of the interaction of calreticulin-thrombospondin-1 complex with a model membrane microdomain

Cell surface calreticulin (CRT) binding to thrombospondin-1 (TSP1), regulates cell adhesion, migration, anoikis resistance, and collagen production. Due to the essential role of membrane microdomains in CRT-mediated focal adhesion disassembly, we previously studied the effect of raft-like bilayers on TSP1-CRT interactions with all-atom molecular dynamics (AAMD) simulations. However, the simulated systems of protein on the surface of the bilayer(s) in the explicit solvent are too large for long timescale AAMD simulations due to computational expense. In this study, we adopted a multiscale modeling approach of combining AAMD, coarse-grained molecule dynamics (CGMD), and reversed AAMD (REV AAMD) simulations to investigate the interactions of single CRT or of the TSP1-CRT complex with a membrane microdomain at microsecond timescale. Results showed that CRT conformational stabilization by binding of TSP1 in AAMD simulation was undetectable in CGMD simulation, but it was recovered in REV AAMD simulation. Similarly, interactions of the CRT N-domain and TSP1 with the membrane microdomain were lost in CGMD simulations but they were re-gained in the REV AAMD simulations. There was the higher coordination of the CRT P-domain in the TSP1-CRT complex with the lipid components of membrane microdomain compared to that of single CRT, which could directly affect the conformation of CRT and further mediate CRT recruitment of LDL receptor-related protein for signaling events. This study provides structural and molecular insights into TSP1-CRT interactions in a membrane microdomain environment and demonstrates the feasibility of using multiscale simulations to investigate the interactions between protein and membrane microdomains at a long timescale.

Calreticulin: Challenges Posed by the Intrinsically Disordered Nature of Calreticulin to the Study of Its Function

Calreticulin is a Ca²⁺-binding chaperone protein, which resides mainly in the endoplasmic reticulum but also found in other cellular compartments including the plasma membrane. In addition to Ca²⁺, calreticulin binds and regulates almost all proteins and most of the mRNAs deciding their intracellular fate. The potential functions of calreticulin are so numerous that identification of all of them is becoming a nightmare. Still the recent discovery that patients affected by the Philadelphia-negative myeloproliferative disorders essential thrombocytemia or primary myelofibrosis not harboring JAK2 mutations carry instead calreticulin mutations disrupting its C-terminal domain has highlighted the clinical need to gain a deeper understanding of the biological activity of this protein. However, by contrast with other proteins, such as enzymes or transcription factors, the biological functions of which are strictly defined by a stable spatial structure imprinted by their amino acid sequence, calreticulin contains intrinsically disordered regions, the structure of which represents a highly dynamic conformational ensemble characterized by constant changes between several metastable conformations in response to a variety of environmental cues. This article will illustrate the Theory of calreticulin as an intrinsically disordered protein and discuss the Hypothesis that the dynamic conformational changes to which calreticulin may be subjected by environmental cues, by promoting or restricting the exposure of its active sites, may affect its function under normal and pathological conditions.

The role of the endoplasmic reticulum protein calreticulin in mediating TGF-β-stimulated extracellular matrix production in fibrotic disease

Endoplasmic reticulum (ER) stress is a key factor contributing to fibrotic disease. Although ER stress is a short-term adaptive response, with chronic stimulation, it can activate pathways leading to fibrosis. ER stress can induce TGF-β signaling, a central driver of extracellular matrix production in fibrosis. This review will discuss the role of an ER protein, calreticulin (CRT), which has both chaperone and calcium regulatory functions, in fibrosis. CRT expression is upregulated in multiple different fibrotic diseases. The roles of CRT in regulation of fibronectin extracellular matrix assembly, extracellular matrix transcription, and collagen secretion and processing into the extracellular matrix will be discussed. Evidence for the importance of CRT in ER calcium release and NFAT activation downstream of TGF-β signaling will be presented. Finally, we will summarize evidence from animal models in which CRT expression is genetically reduced or experimentally downregulated in targeted tissues of adult animals and discuss how these models define a key role for CRT in fibrotic diseases.

T-cell regulation through a basic suppressive mechanism targeting low-density lipoprotein receptor-related protein 1

Cell adhesion is generally considered to depend on positive regulation through ligation of integrins and cytokine receptors. However, here we show that T-cell adhesion, and notably also T-cell receptor (TCR) -induced activation, are subject to constant suppression through shedding of low-density lipoprotein receptor-related protein 1 (LRP1). The broad-spectrum metalloprotease inhibitor GM6001 abrogated shedding, so inducing prominent cell surface expression of LRP1 while enhancing TCR-induced activation and adhesion to β₁ and β₂ integrin ligands, hence arresting the cells. Integrin ligands also inhibited shedding but the effect was less potent than that of GM6001. Unlike GM6001, integrin ligands also induced cell surface expression of full-length thrombospondin-1 (TSP170) and TSP130, which associated with LRP1, and TSP110, which did not associate with LRP1. Cell surface expression of LRP1 and TSP130 were induced exclusively in adhering cells, expression of TSP110 preferentially in non-adhering cells and expression of TSP170 correlated with T-cell motility. The pro-adhesive chemokine CXCL12 also inhibited LRP1 shedding and induced surface expression of TSP170 and TSP130 while inhibiting TSP110. Exogenous TSP-1 and ligation of CD28 inhibited shedding although less effectively than GM6001, and the inhibition through CD28 was independent of TSP-1. Small interfering RNA silencing experiments confirmed involvement of LRP1 and TSP-1 in integrin-dependent adhesion and TCR-induced activation. Hence, the poor LRP1 expression in T cells depends on shedding. Integrin ligands and CXCL12 antagonize shedding through a TSP-1-dependent pathway and ligation of CD28 antagonizes shedding independent of TSP-1. The disappearance of LRP1 from the cell surface may provide basic immunosuppression at the T-cell level.

Dissecting physical structure of calreticulin, an intrinsically disordered Ca2+-buffering chaperone from endoplasmic reticulum

Calreticulin (CALR) is a Ca²⁺ binding multifunctional protein that mostly resides in the endoplasmic reticulum (ER) and plays a number of important roles in various physiological and pathological processes. Although the major functions ascribed to CALR are controlling the Ca²⁺ homeostasis in ER and acting as a lectin-like ER chaperon for many glycoproteins, this moonlighting protein can be found in various cellular compartments where it has many non-ER functions. To shed more light on the mechanisms underlying polyfunctionality of this moonlighting protein that can be found in different cellular compartments and that possesses a wide spectrum of unrelated biological activities, being able to interact with Ca²⁺ (and potentially other metal ions), RNA, oligosaccharides, and numerous proteins, we used a set of experimental and computational tools to evaluate the intrinsic disorder status of CALR and the role of calcium binding on structural properties and conformational stability of the full-length CALR and its isolated P- and C-domains.

The Calreticulin control of human stress erythropoiesis is impaired by JAK2V617F in polycythemia vera

Calreticulin (CALR) is a Ca²⁺-binding protein that shuttles among cellular compartments with proteins bound to its N/P domains. The knowledge that activation of the human erythropoietin receptor induces Ca²⁺ fluxes prompted us to investigate the role of CALR in human erythropoiesis. As shown by Western blot analysis, erythroblasts generated in vitro from normal sources and JAK2V617F polycythemia vera (PV) patients expressed robust levels of CALR. However, Ca²⁺ regulated CALR conformation only in normal cells. Normal erythroblasts expressed mostly the N-terminal domain of CALR (N-CALR) on their cell surface (as shown by flow cytometry) and C-terminal domain (C-CALR) in their cytoplasm (as shown by confocal microscopy) and expression of both epitopes decreased with maturation. In the proerythroblast (proEry) cytoplasm, C-CALR was associated with the glucocorticoid receptor (GR), which initiated the stress response. In these cells, Ca²⁺ deprivation and inhibition of nuclear export increased GR nuclear localization while decreasing cytoplasmic detection of C-CALR and C-CALR/GR association and proliferation in response to the GR agonist dexamethasone (Dex). C-CALR/GR association and Dex responsiveness were instead increased by Ca²⁺ and erythropoietin. In contrast, JAK2V617F proErys expressed normal cell-surface levels of N-CALR but barely detectable cytoplasmic levels of C-CALR. These cells contained GR mainly in the nucleus and were Dex unresponsive. Ruxolitinib rescued cytoplasmic detection of C-CALR, C-CALR/GR association, and Dex responsiveness in JAK2V617F proErys and its effects were antagonized by nuclear export and Ca²⁺ flux inhibitors. These results indicates that Ca²⁺-induced conformational changes of CALR regulate nuclear export of GR in normal erythroblasts and that JAK2V617F deregulates this function in PV.

Calreticulin mediates an invasive breast cancer phenotype through the transcriptional dysregulation of p53 and MAPK pathways

BACKGROUND

The introduction of effective novel biomarkers of invasion and metastasis is integral for the advancement of breast cancer management. The present study focused on the identification and evaluation of calreticulin (CRT) as a potential biomarker for breast cancer invasion.

METHODS

Two-dimensional gel protein electrophoresis and MALDI-TOF were utilized in the analysis of fresh-frozen invasive intra-ductal carcinoma specimens. Calreticulin-associated expression was analyzed using immunohistochemistry of FFPE non-malignant/malignant breast specimens. A CRT-knockdown model of MCF7 cell line was developed using siRNA and the CRT genotype/phenotype correlations based on migration and trans-well invasion assays were determined. Finally, microarray-based global gene expression profiling was conducted to elucidate the possible calreticulin pro-invasive regulatory pathways.

RESULTS

Two-dimensional gel protein electrophoresis and MALDI-TOF analysis showed upregulation of calreticulin expression in tumor tissues as compared to the normal adjacent tissues. Meta-analysis of the immunohistochemical results confirmed significantly higher expression of calreticulin (p < 0.05) in the stromal compartments of malignant tissues as compared to non-malignant tissues. Migration and transwell invasion assays showed significant loss in the migratory and invasive potential of CRT-knockdown cells (p < 0.05). Global gene expression profiling successfully identified various putative gene networks such as p53 and MAPK pathways that are involved in calreticulin breast cancer signaling.

CONCLUSION

Besides confirming calreticulin overexpression in invasive breast cancer tissues, this study reveals a calreticulin-dependent pro-invasive potential and suggests possible contributing pathways. Defining the mechanistic role of invasion and characterizing the possible calreticulin-dependent molecular targets will be the focus of future work.

Contributions of the Lectin and Polypeptide Binding Sites of Calreticulin to Its Chaperone Functions in Vitro and in Cells

Calreticulin is a lectin chaperone of the endoplasmic reticulum that interacts with newly synthesized glycoproteins by binding to Glc1Man9GlcNAc2 oligosaccharides as well as to the polypeptide chain. In vitro, the latter interaction potently suppresses the aggregation of various non-glycosylated proteins. Although the lectin-oligosaccharide association is well understood, the polypeptide-based interaction is more controversial because the binding site on calreticulin has not been identified, and its significance in the biogenesis of glycoproteins in cells remains unknown. In this study, we identified the polypeptide binding site responsible for the in vitro aggregation suppression function by mutating four candidate hydrophobic surface patches. Mutations in only one patch, P19K/I21E and Y22K/F84E, impaired the ability of calreticulin to suppress the thermally induced aggregation of non-glycosylated firefly luciferase. These mutants also failed to bind several hydrophobic peptides that act as substrate mimetics and compete in the luciferase aggregation suppression assay. To assess the relative contributions of the glycan-dependent and -independent interactions in living cells, we expressed lectin-deficient, polypeptide binding-deficient, and doubly deficient calreticulin constructs in calreticulin-negative cells and monitored the effects on the biogenesis of MHC class I molecules, the solubility of mutant forms of α1-antitrypsin, and interactions with newly synthesized glycoproteins. In all cases, we observed a profound impairment in calreticulin function when its lectin site was inactivated. Remarkably, inactivation of the polypeptide binding site had little impact. These findings indicate that the lectin-based mode of client interaction is the predominant contributor to the chaperone functions of calreticulin within the endoplasmic reticulum.

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.

Calreticulin gene mutations in myeloproliferative neoplasms without Janus kinase 2 mutations

Calreticulin, an endoplasmic reticulum protein with multiple functions involving chaperone activity and calcium homeostasis, plays an important role in cellular proliferation and differentiation. Calreticulin dysfunction is known to be associated with different cancers. Very recently, calreticulin mutations have been identified in myeloproliferative neoplasms (MPNs), with a particularly high frequency in MPNs without Janus kinase 2 (JAK2) mutations, which exhibit clinical characteristics different from those with mutant JAK2. Here, we focus on the structure, function and carcinogenicity of calreticulin, as well as its relationship with MPNs not involving JAK2 mutations.

Molecular insight into the effect of lipid bilayer environments on thrombospondin-1 and calreticulin interactions

Thrombospondin-1 (TSP1) binding to cell surface calreticulin (CRT) stimulates the association of CRT with low-density lipoprotein (LDL) receptor-related protein (LRP1) to signal focal adhesion disassembly and engagement of cellular activities. A recent study demonstrated that membrane rafts are necessary for TSP1-mediated focal adhesion disassembly, but the molecular role of membrane rafts in mediating TSP1-CRT-LRP1 signaling is unknown. In this study, we investigated the effect of lipid bilayer environments on TSP1 and CRT interactions via atomically detailed molecular dynamics simulations. Results showed that the microscopic structural properties of lipid molecules and mesoscopic mechanical properties and electrostatic potential of the bilayer were significantly different between a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer and a raftlike lipid bilayer [a POPC/cholesterol (CHOL) raftlike lipid bilayer or a POPC/CHOL/sphingomyelin (SM) raftlike lipid bilayer], and the difference was enhanced by SM lipids in a raftlike lipid bilayer. These bilayer property differences affect the interactions of CRT with the bilayer, further influencing CRT conformation and TSP1-CRT interactions. A raftlike lipid bilayer stabilized CRT conformation as compared to a POPC bilayer environment. TSP1 binding to CRT resulted in a conformation for the CRT N-domain more "open" than that of the CRT P-domain in a raftlike lipid bilayer environment, which could facilitate binding of CRT to LRP1 to engage downstream signaling. The open conformational changes of CRT by binding to TSP1 in a raftlike lipid bilayer were enhanced by SM lipids in a lipid bilayer. The direct interactions of both the N- and P-domains of CRT with the bilayer contribute to the more open conformation of CRT in the TSP1-CRT complex on a raftlike lipid bilayer as compared to that on a POPC bilayer. The interactions of CRT or the TSP1-CRT complex with the lipid bilayer also caused CHOL molecules and/or lipids to be more coordinated and to aggregate into patchlike regions in the raftlike lipid bilayers. The lipid and CHOL molecule coordination and aggregation could in turn affect the interactions of CRT with the membrane raft, thereby altering TSP1-CRT interactions and CRT conformational changes that potentially regulate its interactions with LRP1. This study provides molecular insights into the role of lipid bilayer environments in TSP1-CRT interactions and in the CRT conformational changes that are predicted to facilitate binding of CRT to LRP1 to engage downstream signaling events.

Characterization of the reniform nematode genome by shotgun sequencing

The reniform nematode (RN), a major agricultural pest particularly on cotton in the United States, is among the major plant-parasitic nematodes for which limited genomic information exists. In this study, over 380 Mb of sequence data were generated from pooled DNA of four adult female RNs and assembled into 67,317 contigs, including 25,904 (38.5%) predicted coding contigs and 41,413 (61.5%) noncoding contigs. Most of the characterized repeats were of low complexity (88.9%), and 0.9% of the contigs matched with 53.2% of GenBank ESTs. The most frequent Gene Ontology (GO) terms for molecular function and biological process were protein binding (32%) and embryonic development (20%). Further analysis showed that 741 (1.1%), 94 (0.1%), and 169 (0.25%) RN genomic contigs matched with 1328 (13.9%), 1480 (5.4%), and 1330 (7.4%) supercontigs of Meloidogyne incognita, Brugia malayi, and Pristionchus pacificus, respectively. Chromosome 5 of Caenorhabditis elegans had the highest number of hits to the RN contigs. Seven putative detoxification genes and three carbohydrate-active enzymes (CAZymes) involved in cell wall degradation were studied in more detail. Additionally, kinases, G protein-coupled receptors, and neuropeptides functioning in physiological, developmental, and regulatory processes were identified in the RN genome.

A cytokine-controlled mechanism for integrated regulation of T-lymphocyte motility, adhesion and activation

The co-ordination of T-cell motility, adhesion and activation remains poorly understood. It is also unclear how these functions are co-ordinated with external stimuli. Here we unveil a series of molecular interactions in cis at the surface of T lymphocytes with potent effects on motility and adhesion in these cells, and communicating with proliferative responses. These interactions were controlled by the signature cytokines of T helper subsets interleukin-2 (IL-2) and IL-4. Low-density lipoprotein receptor-related protein 1 (LRP1) was found to play a key role for T-cell motility by promoting development of polarized cell shape and cell movement. Endogenous thrombospondin-1 (TSP-1) enhanced cell surface expression of LRP1 through CD47. Cell surface expressed LRP1 induced motility and processing of TSP-1 while inhibiting adhesion to intercellular adhesion molecule 1 and fibronectin. Interleukin-2, but not IL-4, stimulated synthesis of TSP-1 and motility through TSP-1 and LRP1. Stimulation of the T-cell receptor (TCR)/CD3 complex inhibited TSP-1 expression. Inhibitor studies indicated that LRP1 regulated TSP-1 expression and promoted motility through JAK signalling. This LRP1-mediated motogenic signalling was connected to CD47/Gi protein signalling and IL-2-induced signalling through TSP-1. The motogenic TSP-1/LRP1 mechanism antagonized TCR/CD3-induced T-cell proliferation. These results indicate that LRP1 in collaboration with TSP-1 directs a counter-adhesive and counter-proliferative motogenic cascade. T cells seem programmed to prioritize movement before adhesion through this cascade. In conclusion, vital decision-making in T lymphocytes regulating motility, adhesive interactions and proliferation, are integrated through a molecular mechanism connecting different cell surface receptors and their signalling pathways.

Citrullinated calreticulin potentiates rheumatoid arthritis shared epitope signaling

OBJECTIVE

Citrullinated proteins are immunogenic in rheumatoid arthritis (RA), particularly in patients who carry shared epitope (SE)-coding HLA-DRB1 alleles. The mechanism underlying this association is unknown. We have previously identified the SE as a ligand that interacts with cell surface calreticulin (CRT) and activates immune dysregulation. This study was undertaken to determine the effect of CRT citrullination on SE signaling.

METHODS

CRT-SE binding affinity was measured by surface plasmon resonance. The role of individual CRT arginine residues was determined by site-directed mutagenesis, and nitric oxide levels were measured using a fluorochrome-based assay. CRT citrullination in synovial tissue samples and cell cultures was determined by 2-dimensional gel electrophoresis, immunoblotting, and mass spectrometry techniques.

RESULTS

Synovial tissue and fibroblast-like synoviocytes from RA patients were found to express a higher abundance of citrullinated CRT than samples from osteoarthritis patients. Citrullinated CRT showed more robust interaction with the SE ligand, and transduced SE signaling at a 10,000-fold higher potency, compared to noncitrullinated CRT. Site-directed mutation analysis identified Arg(205), which is spatially adjacent to the SE binding site in the CRT P-domain, as a dominant inhibitor of SE-CRT interaction and signaling, while a more remote arginine residue, Arg(261), was found to enhance these SE functions.

CONCLUSION

Our findings indicate that citrullinated CRT is overabundant in the RA synovium and potentiates SE-activated signaling in vitro. These findings could introduce a new mechanistic model of gene-environment interaction in RA.

Calreticulin and cancer

Calreticulin (CRT) as a multi-functional endoplasmic reticulum protein is involved in a spectrum of cellular processes which ranges from calcium homeostasis and chaperoning to cell adhesion and finally malignant formation and progression. Previous studies have shown a contributing role for CRT in a range of different cancers. This present review will focus on the possible roles of CRT in the progression of malignant proliferation and the mechanisms involved in its contribution to cancer invasion.

Thrombospondin-1 interacts with Trypanosoma cruzi surface calreticulin to enhance cellular infection

Trypanosoma cruzi causes Chagas disease, which is a neglected tropical disease that produces severe pathology and mortality. The mechanisms by which the parasite invades cells are not well elucidated. We recently reported that T. cruzi up-regulates the expression of thrombospondin-1 (TSP-1) to enhance the process of cellular invasion. Here we characterize a novel TSP-1 interaction with T. cruzi that enhances cellular infection. We show that labeled TSP-1 interacts specifically with the surface of T. cruzi trypomastigotes. We used TSP-1 to pull down interacting parasite surface proteins that were identified by mass spectrometry. We also show that full length TSP-1 and the N-terminal domain of TSP-1 (NTSP) interact with T. cruzi surface calreticulin (TcCRT) and other surface proteins. Pre-exposure of recombinant NTSP or TSP-1 to T. cruzi significantly enhances cellular infection of wild type mouse embryo fibroblasts (MEF) compared to the C-terminal domain of TSP-1, E3T3C1. In addition, blocking TcCRT with antibodies significantly inhibits the enhancement of cellular infection mediated by the TcCRT-TSP-1 interaction. Taken together, our findings indicate that TSP-1 interacts with TcCRT on the surface of T. cruzi through the NTSP domain and that this interaction enhances cellular infection. Thus surface TcCRT is a virulent factor that enhances the pathogenesis of T. cruzi infection through TSP-1, which is up-regulated by the parasite.

Arginylated calreticulin at plasma membrane increases susceptibility of cells to apoptosis

Post-translational modifications of proteins are important for the regulation of cell fate and functions; one of these post-translational modifications is arginylation. We have previously established that calreticulin (CRT), an endoplasmic reticulum resident, is also one of the arginylated substrates found in the cytoplasm. In the present study, we describe that arginylated CRT (R-CRT) binds to the cell membrane and identified its role as a preapoptotic signal. We also show that cells lacking arginyl-tRNA protein transferase are less susceptible to apoptosis than wild type cells. Under these conditions R-CRT is present on the cell membrane but at early stages is differently localized in stress granules. Moreover, cells induced to undergo apoptosis by arsenite show increased R-CRT on their cell surface. Exogenously applied R-CRT binds to the cell membrane and is able to both increase the number of cells undergoing apoptosis in wild type cells and overcome apoptosis resistance in cells lacking arginyl-tRNA protein transferase that express R-CRT on the cell surface. Thus, these results demonstrate the importance of surface R-CRT in the apoptotic response of cells, implying that post-translational arginylation of CRT can regulate its intracellular localization, cell function, and survival.

Gene expression profiles of the NCI-60 human tumor cell lines define molecular interaction networks governing cell migration processes

Although there is extensive information on gene expression and molecular interactions in various cell types, integrating those data in a functionally coherent manner remains challenging. This study explores the premise that genes whose expression at the mRNA level is correlated over diverse cell lines are likely to function together in a network of molecular interactions. We previously derived expression-correlated gene clusters from the database of the NCI-60 human tumor cell lines and associated each cluster with function categories of the Gene Ontology (GO) database. From a cluster rich in genes associated with GO categories related to cell migration, we extracted 15 genes that were highly cross-correlated; prominent among them were RRAS, AXL, ADAM9, FN14, and integrin-beta1. We then used those 15 genes as bait to identify other correlated genes in the NCI-60 database. A survey of current literature disclosed, not only that many of the expression-correlated genes engaged in molecular interactions related to migration, invasion, and metastasis, but that highly cross-correlated subsets of those genes engaged in specific cell migration processes. We assembled this information in molecular interaction maps (MIMs) that depict networks governing 3 cell migration processes: degradation of extracellular matrix, production of transient focal complexes at the leading edge of the cell, and retraction of the rear part of the cell. Also depicted are interactions controlling the release and effects of calcium ions, which may regulate migration in a spaciotemporal manner in the cell. The MIMs and associated text comprise a detailed and integrated summary of what is currently known or surmised about the role of the expression cross-correlated genes in molecular networks governing those processes.

Calreticulin has opposing effects on the migration of human trophoblast and myometrial endothelial cells

Calreticulin is a calcium binding, endoplasmic reticulum resident protein best known for its roles in intracellular calcium homeostasis and the quality control processes of the endoplasmic reticulum. There is evidence for a range of activities for calreticulin outside the endoplasmic reticulum, including in the cytosol, on the surface of different cells types and in the extracellular matrix. Recent evidence indicates that human pregnancy is a condition of elevated circulating calreticulin. Calreticulin was increased in the plasma of women throughout pregnancy compared to the non-pregnant state. Calreticulin was also further increased during the hypertensive disorder of human pregnancy, pre-eclampsia. To clarify the roles of circulating calreticulin in pregnancy and pre-eclampsia, the aim of this study was to determine the effects of exogenous calreticulin on two cell types that are relevant to normal human pregnancy and to pre-eclampsia. Human primary myometrial microvascular endothelial cells (UtMVEC-Myo) and the human trophoblast cell line, HTR8/SVneo, were cultured with exogenous calreticulin at concentrations (2 μg/ml and 5 μg/ml) comparable to that measured in maternal blood. The higher concentration of calreticulin significantly increased the migration of the UtMVEC-Myo cells, but significantly reduced the migration of the HTR8/SVneo cells. In the presence of only FGF, FBS and antibiotics calreticulin at 5 μg/ml significantly reduced the number of UtMVEC-Myo cells during in vitro culture for 120 h. These results demonstrate that exogenous calreticulin can alter both HTR8/SVneo and UtMVEC-Myo cell functions in vitro at a (patho-) physiologically relevant concentration. Increased calreticulin may also contribute to altered functions of both cell types during pre-eclampsia.

A novel pathway for human endothelial cell activation by antiphospholipid/anti-β2 glycoprotein I antibodies

Antiphospholipid Abs (APLAs) are associated with thrombosis and recurrent fetal loss. These Abs are primarily directed against phospholipid-binding proteins, particularly β(2)GPI, and activate endothelial cells (ECs) in a β(2)GPI-dependent manner after binding of β(2)GPI to EC annexin A2. Because annexin A2 is not a transmembrane protein, the mechanisms of APLA/anti-β(2)GPI Ab-mediated EC activation are uncertain, although a role for a TLR4/myeloid differentiation factor 88-dependent pathway leading to activation of NF-κB has been proposed. In the present study, we confirm a critical role for TLR4 in anti-β(2)GPI Ab-mediated EC activation and demonstrate that signaling through TLR4 is mediated through the assembly of a multiprotein signaling complex on the EC surface that includes annexin A2, TLR4, calreticulin, and nucleolin. An essential role for each of these proteins in cell activation is suggested by the fact that inhibiting the expression of each using specific siRNAs blocked EC activation mediated by APLAs/anti-β(2)GPI Abs. These results provide new evidence for novel protein-protein interactions on ECs that may contribute to EC activation and the pathogenesis of APLA/anti-β(2)GPI-associated thrombosis and suggest potential new targets for therapeutic intervention in antiphospholipid syndrome.

Molecular recognition theory and sense-antisense interaction: therapeutic applications in autoimmunity

Perhaps behind only the understanding of the genetic code in importance is the comprehension of protein sequence and structure in its effect on modern scientific investigation. How proteins are structured and interact dictates a considerable amount of the body's processes in maintaining homeostasis. Unfortunately, in diseases of autoimmunity, these processes are directed against the body itself and most of the current clinical responses are severely lacking. This review addresses current therapeutics involved in the treatment of various autoimmune diseases and details potential future therapeutics designed with a more targeted approach. Detailed in this manuscript is the concept of utilizing peptides possessing an inverse hydropathy to the immunogenic region of proteins to generate anti-idiotypic (anti-Id) and anti-clonotypic T cell receptor (TCR) antibodies (Abs). Theoretically, the anti-Id Abs cross react with Id Abs and negate the powerful machinery of the adaptive immune response with little to no side effects. A series of studies by a number of groups have shown this to be an exciting and intriguing concept that will likely play a role in the future treatment of autoimmune diseases.

Calreticulin promotes tumor lymphocyte infiltration and enhances the antitumor effects of immunotherapy by up-regulating the endothelial expression of adhesion molecules

Tumor-induced angiogenesis has been shown to suppress immune responses. One mechanism is to suppress leukocyte-endothelial cell interaction by down-regulating the expression of adhesion molecules, such as intercellular adhesion molecule (ICAM)-1, vascular cell adhesion molecule (VCAM)-1 and E-selectin on the tumor endothelium, which enables tumor cells to escape immune surveillance. Calreticulin (CRT), a chaperone protein mainly located in the endoplasmic reticulum, has been shown to exert anti-angiogenic activity and inhibit tumor growth. Here, we demonstrate that in addition to inhibiting angiogenesis, CRT also enhances the expression of both ICAM-1 and VCAM-1 on tumor endothelial cells. This expression results in enhanced leukocyte-endothelial cell interactions and increased lymphocyte infiltration into tumors. Therefore, combining intramuscular CRT gene transfer with intratumoral cytokine gene therapies significantly improves the antitumor effects of immunotherapy by markedly increasing the levels of tumor-infiltrating lymphocytes. This combined treatment increased the levels of infiltrating lymphocytes to those achieved using four times the cytokine dosage. The combined therapy also resulted in lower levels of immunosuppressive molecules and higher levels of activated T-cells in the tumor microenvironment than immunotherapy alone. In conclusion, this study describes a new antitumor mechanism of CRT that involves the up-regulation of tumor endothelial adhesion molecules and the enhanced infiltration of tumor-specific lymphocytes. Thus, CRT treatment can make tumor cells more vulnerable to immunotherapy and improve the therapeutic efficacy of immunotherapy.

Structural and functional relationships between the lectin and arm domains of calreticulin

Calreticulin and calnexin are key components in maintaining the quality control of glycoprotein folding within the endoplasmic reticulum. Although their lectin function of binding monoglucosylated sugar moieties of glycoproteins is well documented, their chaperone activity in suppressing protein aggregation is less well understood. Here, we use a series of deletion mutants of calreticulin to demonstrate that its aggregation suppression function resides primarily within its lectin domain. Using hydrophobic peptides as substrate mimetics, we show that aggregation suppression is mediated through a single polypeptide binding site that exhibits a K(d) for peptides of 0.5-1 μM. This site is distinct from the oligosaccharide binding site and differs from previously identified sites of binding to thrombospondin and GABARAP (4-aminobutyrate type A receptor-associated protein). Although the arm domain of calreticulin was incapable of suppressing aggregation or binding hydrophobic peptides on its own, it did contribute to aggregation suppression in the context of the whole molecule. The high resolution x-ray crystal structure of calreticulin with a partially truncated arm domain reveals a marked difference in the relative orientations of the arm and lectin domains when compared with calnexin. Furthermore, a hydrophobic patch was detected on the arm domain that mediates crystal packing and may contribute to calreticulin chaperone function.

Molecular and structural insight into the role of key residues of thrombospondin-1 and calreticulin in thrombospondin-1-calreticulin binding

Thrombospondin-1 (TSP1) binding to calreticulin (CRT) on the cell surface signals focal adhesion disassembly, leading to the intermediate adhesive phenotype, cell migration, anoikis resistance, and collagen stimulation. Residues Lys 24 and 32 in TSP1 and amino acids 24-26 and 32-34 in CRT have been shown through biochemical and cell-based approaches to be critical for TSP1-CRT binding and signaling. This study investigated the molecular and structural basis for these key TSP1 and CRT residues in TSP1-CRT binding. On the basis of a validated TSP1-CRT complex structure, we adopted steered molecular dynamics simulations to determine the effect of mutation of these key residues on TSP1-CRT binding and validated the simulation results with experimental observations. We further performed 30 ns molecular dynamics simulations for wild-type TSP1, CRT, K24A/K32A mutant TSP1, and mutant CRT (residues 24-26 and 32-34 mutated to Ala) and studied the conformational and structural changes in TSP1 and CRT as the result of mutation of these critical residues. Results showed that mutation of residues 24 and 32 to Ala in TSP1 and of amino acids 24-26 and 32-34 to Ala in CRT results in a shortened β-strand in the binding site, decreased hydrogen bond occupancy for β-strand pairs that are located within or near the binding site, increased conformational flexibility of the binding site, a changed degree of dynamically correlated motion between the residues in the binding site and the other residues in protein, and a changed degree of overall correlated motion between the residues in the protein. These changes could directly contribute to the loss or weakened binding between TSP1 and CRT and the resultant effects on TSP1-CRT binding-induced cellular activities. Results from this study provide a molecular and structural insight into the role of these critical residues of TSP1 and CRT in TSP1-CRT binding.

Extracellular calreticulin is present in the joints of patients with rheumatoid arthritis and inhibits FasL (CD95L)-mediated apoptosis of T cells

OBJECTIVE

The binding of FasL (CD95L) to its receptor, Fas (CD95), induces apoptosis. Studies have shown that in patients with rheumatoid arthritis (RA), T lymphocytes are resistant to FasL-induced apoptosis in vivo but are susceptible to FasL-induced apoptosis in vitro. Dysfunction in this mechanism may be an important contributor to the pathophysiology of RA. Thus, the present study was undertaken to determine which factors might inhibit FasL-Fas binding in vivo and those that would inhibit apoptosis of T lymphocytes in an in vitro model system.

METHODS

Human Jurkat T cells rendered apoptotic by FasL exposure were analyzed by flow cytometry. Necrosis was determined according to measurement of lactate dehydrogenase release. Quantification of calreticulin in plasma and synovial fluid and of calreticulin-FasL binding was performed by enzyme-linked immunosorbent assay. Measurement of nitrite/nitrate in the plasma and synovial fluid was carried out by chemiluminescence assay.

RESULTS

Extracellular calreticulin was present at a significantly higher concentration in the plasma (median 10.3 ng/ml, interquartile range [IQR] 14.8 ng/ml) and synovial fluid (median 10.3 ng/ml, IQR 12.0 ng/ml) of RA patients (each P < 0.05) compared with the plasma (median 3.1 ng/ml, IQR 1.3 ng/ml) and synovial fluid (median 2.9 ng/ml, IQR 0.9 ng/ml) of patients with psoriatic arthritis and the plasma of healthy control subjects (median 2.9 ng/ml, IQR 0.9 ng/ml). Calreticulin concentrations in the synovial fluid correlated with the tender and swollen joint counts and the activity scores on the 28-joint Disease Activity Score assessment. Calreticulin also bound directly to FasL. In vitro, calreticulin (2-16 ng/ml) inhibited FasL-induced apoptosis of Jurkat T cells.

CONCLUSION

Calreticulin was present at higher concentrations in the plasma and synovial fluid of RA patients. Calreticulin had the capacity to bind directly to FasL and to inhibit FasL-mediated apoptosis of Jurkat T cells, and thus might play a role in inhibiting apoptosis of inflammatory T cells in RA.