Calreticulin affects fibronectin-based cell-substratum adhesion via the regulation of c-Src activity

Acute promyelocytic leukemia with additional chromosome abnormalities in a patient positive for HIV: A case report and literature review

Acute promyelocytic leukemia (APL), especially cases of high-risk with complex chromosomes (CK), is rare in individuals infected with human immunodeficiency virus (HIV), making the establishment of therapeutic approaches challenging; often the treatment is individualized. This report describes a 49-year-old female patient with HIV who was diagnosed with high-risk APL with a new CK translocation and presents a literature review. At diagnosis, the patient presented with typical t(15;17)(q24;q21) with additional abnormalities, including add(5)(q15), add(5)(q31), add(7)(q11.2) and add(12) (p13). The results of acute myeloid leukemia mutation analysis suggested positivity for calreticulin and lysine methyltransferase 2C genes. The patient received all-trans retinoic acid combined with arsenic trioxide and chemotherapy, with morphologically complete remission after the first cycle of chemotherapy. The present report provided preliminary data for future clinical research.

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.

Bioinformatics analysis for the role of CALR in human cancers

Cancer is one of the most important public health problems in the world. The curative effect of traditional surgery, radiotherapy and chemotherapy is limited and has inevitable side effects. As a potential target for tumor therapy, few studies have comprehensively analyzed the role of CALR in cancers. Therefore, by using GeneCards, UALCAN, GEPIA, Kaplan-Meier Plotter, COSMIC, Regulome Explorer, String, GeneMANIA and TIMER databases, we collected and analyzed relevant data to conduct in-depth bioinformatics research on the CALR expression in Pan-cancer to assess the possibility of CALR as a potential therapeutic target and survival biomarker. We studied the CALR expression in normal human tissues and various tumors of different stages, and found that CALR expression was associated with relapse free survival (RFS). We verified the expression of CALR in breast cancer cell lines by vitro experiments. Mutations of CALR were widely present in tumors. CALR interacted with different genes and various proteins. In tumors, a variety of immune cells are closely related to CALR. In conclusion, CALR can be used as a biomarker for predicting prognosis and a potential target for tumor molecular and immunotherapy.

Calreticulin – a multifaced protein

Calreticulin (CALR) is a highly conserved multi-function protein that primarily localizes within the lumen of the endoplasmic reticulum (ER). It participates in various processes in the cells, including glycoprotein chaperoning, regulation of Ca2+ homeostasis, antigen processing and presentation for adaptive immune response, cell adhesion/migration, cell proliferation, immunogenic cell death, gene expression and RNA stability. The role of CALR in the assembly, retrieval and cell surface expression of MHC class I molecules is well known. A fraction of the total cellular CALR is localized in the cytosol, following its retro-translocation from the ER. In the cell stress conditions, CALR is also expressed on the cell surface via an interaction with phosphatidylserine localized on the inner leaflet of the plasma membrane. The abovementioned mechanism is relevant for the recognition of the cells, as well as immunogenicity and phagocytic uptake of proapoptotic and apoptotic cells. Lastly, the presence of CALR exon 9 gene mutations was confirmed in patients with myeloproliferative neoplasms. Their presence results in an abnormal CALR structure due to the loss of its ER-retention sequence, CALR extra-ER localisation, the formation of a complex with thrombopoietin receptor, and oncogenic transformation of hematopoietic stem cells. It is also known that CALR exon 9 mutants are highly immunogenic and induce T cell response. Despite this fact, CALR mutant positive hematopoietic cells emerge. The last phenomenon is probably the result of the inhibition of phagocytosis of the cancer cells exposing CALR mutant protein by dendritic cells.

c-Src kinase inhibits osteogenic differentiation via enhancing STAT1 stability

The proto-oncogene Src is ubiquitously expressed and is involved in cellular differentiation. However, the role of Src in embryonic stem (ES) cell osteogenic differentiation is largely unknown. Using the small molecule inhibitor PP2, c-Src specific siRNAs, and tet-inducible lentiviral vectors overexpressing active c-Src, we delineated an inhibitory role of c-Src in osteogenic differentiation of mouse embryonic stem cells (mESCs) and mouse MC3T3-E1s preosteoblasts. Active c-Src was shown to restrict the nuclear residency of Runt-related transcription factor 2 (Runx2) and its transcriptional activity with no detectable effect on Runx2 expression level. Furthermore, we showed Signal Transducer and Activator of Transcription 1 (STAT1) was indispensable to the inhibitory role of c-Src on Runx2 nuclear localization. Specifically, higher levels of active c-Src increased STAT1 half-life by inhibiting its proteasomal degradation, thereby increasing the cytoplasmic abundance of STAT1. More abundant cytoplasmic STAT1 bound and anchored Runx2, which restricted its nucleocytoplasmic shuttling and ultimately reduced Runx2 transcriptional activity. Collectively, this study has defined a new mechanism by which c-Src inhibits the transcriptional regulation of osteogenesis from mESCs in vitro.

Calreticulin is important for the development of renal fibrosis and dysfunction in diabetic nephropathy

Previously, our lab showed that the endoplasmic reticulum (ER) and calcium regulatory protein, calreticulin (CRT), is important for collagen transcription, secretion, and assembly into the extracellular matrix (ECM) and that ER CRT is critical for TGF-β stimulation of type I collagen transcription through stimulation of ER calcium release and NFAT activation. Diabetes is the leading cause of end stage renal disease. TGF-β is a key factor in the pathogenesis of diabetic nephropathy. However, the role of calreticulin (Calr) in fibrosis of diabetic nephropathy has not been investigated. In current work, we used both in vitro and in vivo approaches to assess the role of ER CRT in TGF-β and glucose stimulated ECM production by renal tubule cells and in diabetic mice. Knockdown of CALR by siRNA in a human proximal tubular cell line (HK-2) showed reduced induction of soluble collagen when stimulated by TGF-β or high glucose as compared to control cells, as well as a reduction in fibronectin and collagen IV transcript levels. CRT protein is increased in kidneys of mice made diabetic with streptozotocin and subjected to uninephrectomy to accelerate renal tubular injury as compared to controls. We used renal-targeted ultrasound delivery of Cre-recombinase plasmid to knockdown specifically CRT expression in the remaining kidney of uninephrectomized Calr fl/fl mice with streptozotocin-induced diabetes. This approach reduced CRT expression in the kidney, primarily in the tubular epithelium, by 30-55%, which persisted over the course of the studies. Renal function as measured by the urinary albumin/creatinine ratio was improved in the mice with knockdown of CRT as compared to diabetic mice injected with saline or subjected to ultrasound and injected with control GFP plasmid. PAS staining of kidneys and immunohistochemical analyses of collagen types I and IV show reduced glomerular and tubulointerstitial fibrosis. Renal sections from diabetic mice with CRT knockdown showed reduced nuclear NFAT in renal tubules and treatment of diabetic mice with 11R-VIVIT, an NFAT inhibitor, reduced proteinuria and renal fibrosis. These studies identify ER CRT as an important regulator of TGF-β stimulated ECM production in the diabetic kidney, potentially through regulation of NFAT-dependent ECM transcription.

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 regulates Src kinase in osteogenic differentiation from embryonic stem cells

Calreticulin, the major Ca²⁺ buffer of the endoplasmic reticulum plays an important role in the choice of fate by embryonic stem cells. Using the embryoid body method of organogenesis, we showed impaired osteogenesis in crt^-/- cells vis-à-vis calreticulin-containing osteogenic WT cells. In the non-osteogenic crt^-/- cells, c-Src- a non-receptor tyrosine kinase- was activated and its inhibition rescued osteogenesis. Most importantly, we demonstrated that calreticulin-containing cells had lower c-Src kinase activity, and this was accomplished via the Ca²⁺-homeostatic function of calreticulin. Specifically, lowering cytosolic [Ca²⁺] in calreticulin-containing osteogenic WT cells with BAPTA-AM, activated c-Src and impaired osteogenic differentiation. Conversely, increasing cytosolic [Ca²⁺] in crt^-/- cells with ionomycin deactivated c-Src kinase and restored osteogenesis. The immediate effector of calreticulin, the Ser/Thr phosphatase calcineurin, was less active in crt^-/- cells, however, its activity was rescued upon inhibition of c-Src activity by small molecule inhibitors. Finally, we showed that higher activity of calcineurin correlated with increased level of nuclear Runx2, a transcription factor that is the master regulator of osteogenesis. Collectively, our work has identified a novel pathway involving calreticulin regulated Ca²⁺ signalling via c-Src in osteogenic differentiation of embryonic stem cells.

Calreticulin is a Critical Cell Survival Factor in Malignant Neoplasms

Calreticulin (CRT) is a high-capacity Ca²⁺ protein whose expression is up-regulated during cellular transformation and is associated with disease progression in multiple types of malignancies. At the same time, CRT has been characterized as an important stress-response protein capable of inducing immunogenic cell death (ICD) when translocated to the cell surface. It remains unclear why CRT expression is preserved by malignant cells during the course of transformation despite its immunogenic properties. In this study, we identify a novel, critical function of CRT as a cell survival factor in multiple types of human solid-tissue malignancies. CRT knockdown activates p53, which mediates cell-death response independent of executioner caspase activity and accompanied full-length poly ADP ribose polymerase (PARP) cleavage. Mechanistically, we show that down-regulation of CRT results in mitochondrial Ca²⁺ overload and induction of mitochondria permeability transition pore (mPTP)-dependent cell death, which can be significantly rescued by the mPTP inhibitor, Cyclosporin A (CsA). The clinical importance of CRT expression was revealed in the analysis of the large cohort of cancer patients (N = 2,058) to demonstrate that high levels of CRT inversely correlates with patient survival. Our study identifies intracellular CRT as an important therapeutic target for tumors whose survival relies on its expression.

This study reveals a novel role for the calcium-binding protein calreticulin in the survival of cancer cells; downregulation of calreticulin leads to mitochondrial calcium overload and an induction of non-apoptotic cell death. Calreticulin levels inversely correlate with the survival of patients diagnosed with various types of solid cancers.

Calreticulin as A Novel Potential Metastasis-Associated Protein in Myxoid Liposarcoma, as Revealed by Two-Dimensional Difference Gel Electrophoresis

Myxoid liposarcoma (MLS) is a mesenchymal malignancy. To identify innovate seeds for clinical applications, we examined the proteomes of primary tumor tissues from 10 patients with MLS with different statuses of postoperative metastasis. The protein expression profiles of tumor tissues were created, and proteins with differential expression associated with postoperative metastasis were identified by two-dimensional difference gel electrophoresis (2D-DIGE) and mass spectrometry. The validation was performed using specific antibodies and in vitro analyses. Using 2D-DIGE, we observed 1726 protein species and identified proteins with unique expression levels in metastatic MLS. We focused on the overexpression of calreticulin in metastatic MLS. The higher expression of calreticulin was confirmed by Western blotting, and gene silencing assays demonstrated that reduced expression of calreticulin inhibited cell growth and invasion. Our findings suggested the important roles of calreticulin in MLS metastasis and supported its potential utility as a prognostic biomarker in MLS. Further investigations of the functional properties of calreticulin and other proteins identified in this study will improve our understanding of the biology of MLS and facilitate novel clinical applications.

Dexamethasone-Mediated Upregulation of Calreticulin Inhibits Primary Human Glioblastoma Dispersal Ex Vivo

Dispersal of Glioblastoma (GBM) renders localized therapy ineffective and is a major cause of recurrence. Previous studies have demonstrated that Dexamethasone (Dex), a drug currently used to treat brain tumor-related edema, can also significantly reduce dispersal of human primary GBM cells from neurospheres. It does so by triggering α5 integrin activity, leading to restoration of fibronectin matrix assembly (FNMA), increased neurosphere cohesion, and reduction of neurosphere dispersal velocity (DV). How Dex specifically activates α5 integrin in these GBM lines is unknown. Several chaperone proteins are known to activate integrins, including calreticulin (CALR). We explore the role of CALR as a potential mediator of Dex-dependent induction of α5 integrin activity in primary human GBM cells. We use CALR knock-down and knock-in strategies to explore the effects on FNMA, aggregate compaction, and dispersal velocity in vitro, as well as dispersal ex vivo on extirpated mouse retina and brain slices. We show that Dex increases CALR expression and that siRNA knockdown suppresses Dex-mediated FNMA. Overexpression of CALR in GBM cells activates FNMA, increases compaction, and decreases DV in vitro and on explants of mouse retina and brain slices. Our results define a novel interaction between Dex, CALR, and FNMA as inhibitors of GBM dispersal.

Non-Endoplasmic Reticulum-Based Calr (Calreticulin) Can Coordinate Heterocellular Calcium Signaling and Vascular Function

OBJECTIVE

In resistance arteries, endothelial cell (EC) extensions can make contact with smooth muscle cells, forming myoendothelial junction at holes in the internal elastic lamina (HIEL). At these HIEL, calcium signaling is tightly regulated. Because Calr (calreticulin) can buffer ≈50% of endoplasmic reticulum calcium and is expressed throughout IEL holes in small arteries, the only place where myoendothelial junctions form, we investigated the effect of EC-specific Calr deletion on calcium signaling and vascular function.

APPROACH AND RESULTS

We found Calr expressed in nearly every IEL hole in third-order mesenteric arteries, but not other ER markers. Because of this, we generated an EC-specific, tamoxifen inducible, Calr knockout mouse (EC Calr Δ/Δ). Using this mouse, we tested third-order mesenteric arteries for changes in calcium events at HIEL and vascular reactivity after application of CCh (carbachol) or PE (phenylephrine). We found that arteries from EC Calr Δ/Δ mice stimulated with CCh had unchanged activity of calcium signals and vasodilation; however, the same arteries were unable to increase calcium events at HIEL in response to PE. This resulted in significantly increased vasoconstriction to PE, presumably because of inhibited negative feedback. In line with these observations, the EC Calr Δ/Δ had increased blood pressure. Comparison of ER calcium in arteries and use of an ER-specific GCaMP indicator in vitro revealed no observable difference in ER calcium with Calr knockout. Using selective detergent permeabilization of the artery and inhibition of Calr translocation, we found that the observed Calr at HIEL may not be within the ER.

CONCLUSIONS

Our data suggest that Calr specifically at HIEL may act in a non-ER dependent manner to regulate arteriolar heterocellular communication and blood pressure.

Calreticulin Is Involved in Invasion of Human Extravillous Trophoblasts Through Functional Regulation of Integrin β1

Calreticulin (CRT), a molecular chaperone in the endoplasmic reticulum (ER), plays a variety of roles in cell growth, differentiation, apoptosis, immunity, and cancer biology. It has been reported that CRT is expressed in the human placenta, although its function in placental development is poorly understood. Appropriate invasion of extravillous trophoblasts (EVTs) into the maternal decidua is necessary for successful pregnancy. The objective of the present study was to investigate the expression and functional role of CRT in EVTs using the human EVT cell line HTR8/SVneo, in which CRT gene expression was knocked down. We found that CRT was highly expressed in the human placenta in the early stage of pregnancy and localized to the EVTs. CRT knockdown markedly suppressed the invasion ability of HTR8/SVneo cells. Furthermore, the adhesion to fibronectin was suppressed in the CRT-knockdown cells via the dysfunction of integrin α5β1. In the CRT-knockdown cells, terminal sialylation and fucosylation were decreased, and the core galactose-containing structure was increased in the N-glycans of integrin β1. In addition, the expression levels of several critical glycosyltransferases were changed in the CRT-knockdown cells, consistent with the changes in the N-glycans. These results showed that CRT regulates the function of integrin β1 by affecting the synthesis of N-glycans in HTR8/SVneo cells. Collectively, the results of the present study demonstrate that the ER chaperone CRT plays a regulatory role in the invasion of EVTs, suggesting the importance of CRT expression in placental development during early pregnancy.

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.

Amniotic fluid calreticulin in pregnancies complicated by the preterm prelabor rupture of membranes

OBJECTIVE

This study aimed to determine the amniotic fluid calreticulin concentrations in women with the preterm prelabor rupture of membranes (PPROM) based on the microbial invasion of the amniotic cavity (MIAC), intraamniotic inflammation (IAI) and microbial-associated IAI.

METHODS

One hundred sixty-eight women with singleton pregnancies were included in this study. Amniotic fluid samples were obtained by transabdominal amniocentesis and were assayed for calreticulin concentrations by ELISA. IAI was defined as an amniotic fluid interleukin-6 concentration > 745 pg/ml. Microbial-associated IAI was defined as the presence of both MIAC and IAI.

RESULT

Women with MIAC (with MIAC: median 54.4 ng/ml, versus without MIAC: median 32.6 ng/ml; p < 0.0001), IAI (with IAI: median 66.8 ng/ml, versus without IAI: median 33.0 ng/ml; p < 0.0001) and microbial-associated IAI (with microbial-associated IAI: median 82.5 ng/ml, versus without microbial-associated IAI: median 33.7 ng/ml; p < 0.0001) had higher concentrations of calreticulin than women without these complications. An amniotic fluid calreticulin concentration of 81.4 ng/ml was found to be the best cutoff point for identifying women with microbial-associated IAI.

CONCLUSIONS

The presence of microbial-associated IAI is associated with increased amniotic fluid calreticulin concentrations. Calreticulin seems to be a promising marker for the early identification of PPROM complicated by microbial-associated IAI.

Functional roles of calreticulin in cancer biology

Calreticulin is a highly conserved endoplasmic reticulum chaperone protein which participates in various cellular processes. It was first identified as a Ca²⁺-binding protein in 1974. Accumulated evidences indicate that calreticulin has great impacts for the development of different cancers and the effect of calreticulin on tumor formation and progression may depend on cell types and clinical stages. Cell surface calreticulin is considered as an “eat-me” signal and promotes phagocytic uptake of cancer cells by immune system. Moreover, several reports reveal that manipulation of calreticulin levels profoundly affects cancer cell proliferation and angiogenesis as well as differentiation. In addition to immunogenicity and tumorigenesis, interactions between calreticulin and integrins have been described during cell adhesion, which is an essential process for cancer metastasis. Integrins are heterodimeric transmembrane receptors which connect extracellular matrix and intracellular cytoskeleton and trigger inside-out or outside-in signaling transduction. More and more evidences reveal that proteins binding to integrins might affect integrin-cytoskeleton interaction and therefore influence ability of cell adhesion. Here, we reviewed the biological roles of calreticulin and summarized the potential mechanisms of calreticulin in regulating mRNA stability and therefore contributed to cancer metastasis.

Calreticulin affects cell adhesiveness through differential phosphorylation of insulin receptor substrate-1

Cellular adhesion to the underlying substratum is regulated through numerous signaling pathways. It has been suggested that insulin receptor substrate 1 (IRS-1) is involved in some of these pathways, via association with and activation of transmembrane integrins. Calreticulin, as an important endoplasmic reticulum-resident, calcium-binding protein with a chaperone function, plays an obvious role in proteomic expression. Our previous work showed that calreticulin mediates cell adhesion not only by affecting protein expression but also by affecting the state of regulatory protein phosphorylation, such as that of c-src. Here, we demonstrate that calreticulin affects the abundance of IRS-1 such that the absence of calreticulin is paralleled by a decrease in IRS-1 levels and the unregulated overexpression of calreticulin is accompanied by an increase in IRS-1 levels. These changes in the abundance of calreticulin and IRS-1 are accompanied by changes in cell-substratum adhesiveness and phosphorylation, such that increases in the expression of calreticulin and IRS-1 are paralleled by an increase in focal contact-based cell-substratum adhesiveness, and a decrease in the expression of these proteins brings about a decrease in cell-substratum adhesiveness. Wild type and calreticulin-null mouse embryonic fibroblasts (MEFs) were cultured and the IRS-1 isoform profile was assessed. Differences in morphology and motility were also quantified. While no substantial differences in the speed of locomotion were found, the directionality of cell movement was greatly promoted by the presence of calreticulin. Calreticulin expression was also found to have a dramatic effect on the phosphorylation state of serine 636 of IRS-1, such that phosphorylation of IRS-1 on serine 636 increased radically in the absence of calreticulin. Most importantly, treatment of cells with the RhoA/ROCK inhibitor, Y-27632, which among its many effects also inhibited serine 636 phosphorylation of IRS-1, had profound effects on cell-substratum adhesion, in that it suppressed focal contacts, induced extensive close contacts, and increased the strength of adhesion. The latter effect, while counterintuitive, can be explained by the close contacts comprising labile bonds but in large numbers. In addition, the lability of bonds in close contacts would permit fast locomotion. An interesting and novel finding is that Y-27632 treatment of MEFs releases them from contact inhibition of locomotion, as evidenced by the invasion of a cell's underside by the thin lamellae and filopodia of a cell in close apposition.

Calreticulin regulates transforming growth factor-β-stimulated extracellular matrix production

Endoplasmic reticulum (ER) stress is an emerging factor in fibrotic disease, although precise mechanisms are not clear. Calreticulin (CRT) is an ER chaperone and regulator of Ca(2+) signaling up-regulated by ER stress and in fibrotic tissues. Previously, we showed that ER CRT regulates type I collagen transcript, trafficking, secretion, and processing into the extracellular matrix (ECM). To determine the role of CRT in ECM regulation under fibrotic conditions, we asked whether CRT modified cellular responses to the pro-fibrotic cytokine, TGF-β. These studies show that CRT-/- mouse embryonic fibroblasts (MEFs) and rat and human idiopathic pulmonary fibrosis lung fibroblasts with siRNA CRT knockdown had impaired TGF-β stimulation of type I collagen and fibronectin. In contrast, fibroblasts with increased CRT expression had enhanced responses to TGF-β. The lack of CRT does not impact canonical TGF-β signaling as TGF-β was able to stimulate Smad reporter activity in CRT-/- MEFs. CRT regulation of TGF-β-stimulated Ca(2+) signaling is important for induction of ECM. CRT-/- MEFs failed to increase intracellular Ca(2+) levels in response to TGF-β. NFAT activity is required for ECM stimulation by TGF-β. In CRT-/- MEFs, TGF-β stimulation of NFAT nuclear translocation and reporter activity is impaired. Importantly, CRT is required for TGF-β stimulation of ECM under conditions of ER stress, as tunicamycin-induced ER stress was insufficient to induce ECM production in TGF-β stimulated CRT-/- MEFs. Together, these data identify CRT-regulated Ca(2+)-dependent pathways as a critical molecular link between ER stress and TGF-β fibrotic signaling.

The Drosophila TRPP cation channel, PKD2 and Dmel/Ced-12 act in genetically distinct pathways during apoptotic cell clearance

Apoptosis, a genetically programmed cell death, allows for homeostasis and tissue remodelling during development of all multi-cellular organisms. Phagocytes swiftly recognize, engulf and digest apoptotic cells. Yet, to date the molecular mechanisms underlying this phagocytic process are still poorly understood. To delineate the molecular mechanisms of apoptotic cell clearance in Drosophila, we have carried out a deficiency screen and have identified three overlapping phagocytosis-defective mutants, which all delete the fly homologue of the ced-12 gene, known as Dmel\ced12. As anticipated, we have found that Dmel\ced-12 is required for apoptotic cell clearance, as for its C. elegans and mammalian homologues, ced-12 and elmo, respectively. However, the loss of Dmel\ced-12 did not solely account for the phenotypes of all three deficiencies, as zygotic mutations and germ line clones of Dmel\ced-12 exhibited weaker phenotypes. Using a nearby genetically interacting deficiency, we have found that the polycystic kidney disease 2 gene, pkd2, which encodes a member of the TRPP channel family, is also required for phagocytosis of apoptotic cells, thereby demonstrating a novel role for PKD2 in this process. We have also observed genetic interactions between pkd2, simu, drpr, rya-r44F, and retinophilin (rtp), also known as undertaker (uta), a gene encoding a MORN-repeat containing molecule, which we have recently found to be implicated in calcium homeostasis during phagocytosis. However, we have not found any genetic interaction between Dmel\ced-12 and simu. Based on these genetic interactions and recent reports demonstrating a role for the mammalian pkd-2 gene product in ER calcium release during store-operated calcium entry, we propose that PKD2 functions in the DRPR/RTP pathway to regulate calcium homeostasis during this process. Similarly to its C. elegans homologue, Dmel\Ced-12 appears to function in a genetically distinct pathway.

Alteration of integrin-dependent adhesion and signaling in EMT-like MDCK cells established through overexpression of calreticulin

Calreticulin (CRT) is a multi-functional Ca(2+) -binding molecular chaperone in the endoplasmic reticulum. We previously reported that kidney epithelial cell-derived Madin-Darby Canine Kidney cells were transformed into mesenchymal-like cells by gene transfection of CRT. In this study, we investigated the altered characteristics of cell adhesion in these epithelial-mesenchymal transition (EMT)-like cells. Several extracellular matrix substrata were tested, and cell adhesion to fibronectin was found to be specifically increased in the CRT-overexpressing cells compared to controls. The expression of integrins was significantly up-regulated in subunits α5 and αV, resulting in an increase in the formation of complexes such as α5β1 and αVβ3. These integrins also contributed to the enhanced binding of fibronectin. In the CRT-overexpressing cells, the phosphorylation of Akt, a downstream target of integrin-linked kinase (ILK), was up-regulated on attachment to fibronectin or collagen IV. Integrin-associated signaling through ILK was also promoted on attachment to fibronectin, suggesting some of the correlation between ILK and Akt in the CRT-overexpressing cells. Furthermore, on treatment with 1,2-bis (2-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid tetra (acetoxymethyl) ester, a membrane-permeable Ca(2+) chelator, the enhanced Akt signaling was suppressed with a concomitant decrease in the formation of complexes between integrins and ILK in the CRT-overexpressing cells. In conclusion, these findings demonstrate that CRT regulates cell-substratum adhesion by modulating integrin-associated signaling through altered Ca(2+) homeostasis in the CRT-overexpressing EMT-like cells, suggesting a novel regulatory role for CRT in EMT.

Changes in tumor growth and metastatic capacities of J82 human bladder cancer cells suppressed by down-regulation of calreticulin expression

Bladder cancer is a common urothelial cancer. Through proteomic approaches, calreticulin (CRT) was identified and proposed as a urinary marker for bladder cancer. CRT is a multifunctional molecular chaperone that regulates various cellular functions such as Ca(2+) homeostasis and cell adhesion. CRT is overexpressed in various cancers, but its mechanism of action in the development of bladder tumors remains unclear. We generated J82 bladder cancer cells lines that either stably overexpressed or knocked down CRT to investigate the physiological effects of CRT on bladder tumors. Compared with the transfected control vector cells, the knockdown of CRT suppressed cell proliferation, migration, and attachment, whereas overexpression of CRT enhanced cell migration and attachment. We further demonstrated that the phosphorylation status of focal adhesion kinase and paxillin, important regulators of the focal adhesion complex, was also regulated in these cells. In contrast, phosphorylation of Src, a protein tyrosine kinase reported to be affected by CRT, was not significantly different between the control and CRT-RNAi groups. Most importantly, we observed that tumors derived from J82 CRT-RNAi cells were significantly smaller and had fewer metastatic sites in the lung and liver in vivo than did transfected control vector cells. In conclusion, our results suggest that alteration of CRT expression levels might affect bladder cancer progression in vitro and in vivo.

Higher plant calreticulins have acquired specialized functions in Arabidopsis

Background

Calreticulin (CRT) is a ubiquitous ER protein involved in multiple cellular processes in animals, such as protein folding and calcium homeostasis. Like in animals, plants have evolved divergent CRTs, but their physiological functions are less understood. Arabidopsis contains three CRT proteins, where the two CRTs AtCRT1a and CRT1b represent one subgroup, and AtCRT3 a divergent member.

Methodology/Principal Findings

Through expression of single Arabidopsis family members in CRT-deficient mouse fibroblasts we show that both subgroups have retained basic CRT functions, including ER Ca²⁺-holding potential and putative chaperone capabilities. However, other more general cellular defects due to the absence of CRT in the fibroblasts, such as cell adhesion deficiencies, were not fully restored. Furthermore, in planta expression, protein localization and mutant analyses revealed that the three Arabidopsis CRTs have acquired specialized functions. The AtCRT1a and CRT1b family members appear to be components of a general ER chaperone network. In contrast, and as recently shown, AtCRT3 is associated with immune responses, and is essential for responsiveness to the bacterial Pathogen-Associated Molecular Pattern (PAMP) elf18, derived from elongation factor (EF)-Tu. Whereas constitutively expressed AtCRT1a fully complemented Atcrt1b mutants, AtCRT3 did not.

Conclusions/Significance

We conclude that the physiological functions of the two CRT subgroups in Arabidopsis have diverged, resulting in a role for AtCRT3 in PAMP associated responses, and possibly more general chaperone functions for AtCRT1a and CRT1b.

Cytoskeletal disassembly and cell rounding promotes adipogenesis from ES cells

Biomechanical signals such as cell shape and spreading play an important role in controlling stem cell commitment. Cell shape, adhesion and spreading are also affected by calreticulin, a multifunctional calcium-binding protein, which influences several cellular processes, including adipogenesis. Here we show that cytoskeletal disruption in mouse embryonic stem cells using cytochalasin D or nocodazole promotes adipogenesis. While cytochalasin D disrupts stress fibres and inhibits focal adhesion formation, nocodazole depolymerises microtubules and promotes focal adhesion formation. Furthermore, cytochalasin D increases the levels of both total and activated calcium/calmodulin-dependent protein kinase II, whereas nocodazole decreases it. Nevertheless, both treatments significantly increase the adipogenic potential of embryonic stem cells in vitro. Both cytochalasin D and nocodazole exposure caused cell rounding suggesting that it is cell shape that causes the switch towards the adipogenic programme. Calreticulin-containing embryonic stem cells, under baseline conditions, show low adipogenic potential, have low activity of signalling via calcium/calmodulin-dependent protein kinase II and display normal adhesive properties and cellular spreading in comparison to the highly adipogenic but poorly spread calreticulin-deficient ES cells. We conclude that forced cell rounding via cytoskeletal disruption overrides the effects of calreticulin, an ER chaperone, thus negatively regulating adipogenesis via focal adhesion-mediated cell spreading.

Intracellular calreticulin regulates multiple steps in fibrillar collagen expression, trafficking, and processing into the extracellular matrix

Calreticulin (CRT), a chaperone and Ca(2+) regulator, enhances wound healing, and its expression correlates with fibrosis in animal models, suggesting that CRT regulates production of the extracellular matrix. However, direct regulation of collagen matrix by CRT has not been previously demonstrated. We investigated the role of CRT in the regulation of fibrillar collagen expression, secretion, processing, and deposition in the extracellular matrix by fibroblasts. Mouse embryonic fibroblasts deficient in CRT (CRT(-/-) MEFs) have reduced transcript levels of fibrillar collagen I and III and less soluble collagen as compared with wild type MEFs. Correspondingly, fibroblasts engineered to overexpress CRT have increased collagen type I transcript and protein. Collagen expression appears to be regulated by endoplasmic reticulum (ER) calcium levels and intracellular CRT, because thapsigargin treatment reduced collagen expression, whereas addition of exogenous recombinant CRT had no effect. CRT(-/-) MEFs exhibited increased ER retention of collagen, and collagen and CRT were co-immunoprecipitated from isolated cell lysates, suggesting that CRT is important for trafficking of collagen through the ER. CRT(-/-) MEFs also have reduced type I procollagen processing and deposition into the extracellular matrix. The reduced collagen matrix deposition is partly a consequence of reduced fibronectin matrix formation in the CRT-deficient cells. Together, these data show that CRT complexes with collagen in cells and that CRT plays critical roles at multiple stages of collagen expression and processing. These data identify CRT as an important regulator of collagen and suggest that intracellular CRT signaling plays an important role in tissue remodeling and fibrosis.

Evidence for calreticulin attenuation of cardiac hypertrophy induced by pressure overload and soluble agonists

While calreticulin has been shown to be critical for cardiac development, its role in cardiac pathology is unclear. Previous studies have shown the detrimental effects on the heart of sustained germline calreticulin overexpression, yet without calreticulin, the heart does not develop normally. Thus, carefully balanced calreticulin levels are required for the heart to develop and to function properly into adulthood. But what happens to calreticulin levels, and how is this regulated, during cardiac hypertrophy, during which the fetal gene program is reactivated, at least partially? Our working hypothesis was that c-Src, a kinase whose activity we previously found to be correlated with calreticulin expression, was involved with calreticulin in regulating the response to hypertrophic signals. Thus, we subjected adult mice to transverse aortic constriction to induce left ventricular hypertrophy. We found that aortic constriction caused calreticulin levels to increase, whereas those of c-Src fell with longer constriction time. We also examined the ability of embryonic stem cell-derived cardiomyocytes to respond to soluble hypertrophic agonists. Endothelin-1 treatment caused a significantly greater cell area increase of calreticulin-null cardiomyocytes, which had higher c-Src activity, compared with wild-type cells. c-Src inhibition abolished this difference. Greater c-Src activity may explain the efficacy with which calreticulin-null cells are able to induce the hypertrophic program, while cells containing calreticulin may be able to attenuate the hypertrophic response as a result of decreased c-Src activity. Thus, calreticulin may have a protective effect on the heart in the face of cardiac hypertrophy.

Cell adhesion and spreading affect adipogenesis from embryonic stem cells: the role of calreticulin

Calreticulin is an endoplasmic reticulum-resident multifunctional protein, which has been shown to influence numerous cellular processes, including cell adhesion. In this study, we characterized the adhesive properties of embryonic stem cells (ESCs) lacking calreticulin and showed that adipogenesis from ESCs is directly and reciprocally controlled by the adhesive status of a cell, which in turn is modulated by calreticulin. Calreticulin-deficient ESCs are not only highly adipogenic but also show elevated calmodulin/CaMKII signaling and poor adhesiveness compared with the wild-type ESCs. Calreticulin deficiency leads to a disorganized cytoskeleton and low levels of focal adhesion-related proteins, such as vinculin, paxillin, and phosphorylated focal adhesion kinase, which cause limited focal adhesion formation and limited fibronectin deposition. Moreover, differentiation on nonadhesive substrata, which hinder cell spreading, promoted adipogenesis in the wild-type ESCs that normally have low adipogenic potential, causing a decrease in focal adhesion protein expression and an increase in calmodulin/CaMKII signaling. In contrast, inhibition of CaMKII effectively increased focal adhesion protein levels and inhibited adipogenesis in calreticulin-deficient ESCs, causing them to behave like the low adipogenic, wild-type ESCs. Thus, the adipogenic potential of ESCs is proportional to their calmodulin/CaMKII activity but is inversely related to their focal adhesion protein levels and degree of adhesiveness/spreading.

Calreticulin and focal-contact-dependent adhesion

Cell adhesion is regulated by a variety of Ca2+-regulated pathways that depend on Ca2+-binding proteins. One such protein is calreticulin, an ER-resident protein. Calreticulin signalling from within the ER can affect processes outside the ER, such as expression of several adhesion-related genes, most notably vinculin and fibronectin. In addition, changes in the expression level of calreticulin strongly affect tyrosine phosphorylation of cellular proteins, which is known to affect many adhesion-related functions. While calreticulin has been localized to cellular compartments other than the ER, it appears that only the ER-resident calreticulin affects focal-contact-dependent adhesion. In contrast, calreticulin residing outside the ER may be involved in contact disassembly and other adhesion phenomena. Here, we review the role of calreticulin in focal contact initiation, stabilization, and turnover. We propose that calreticulin may regulate cell-substratum adhesion by participating in an "ER-to-nucleus" signalling and in parallel "ER-to-cell surface" signalling based on posttranslational events.

Calreticulin: non-endoplasmic reticulum functions in physiology and disease

Calreticulin (CRT), when localized to the endoplasmic reticulum (ER), has important functions in directing proper conformation of proteins and glycoproteins, as well as in homeostatic control of cytosolic and ER calcium levels. There is also steadily accumulating evidence for diverse roles for CRT localized outside the ER, including data suggesting important roles for CRT localized to the outer cell surface of a variety of cell types, in the cytosol, and in the extracellular matrix (ECM). Furthermore, the addition of exogenous CRT rescues numerous CRT-driven functions, such as adhesion, migration, phagocytosis, and immunoregulatory functions of CRT-null cells. Recent studies show that topically applied CRT has diverse and profound biological effects that enhance cutaneous wound healing in animal models. This evidence for extracellular bioactivities of CRT has provided new insights into this classically ER-resident protein, despite a lack of knowledge of how CRT exits from the ER to the cell surface or how it is released into the extracellular milieu. Nonetheless, it has become clear that CRT is a multicompartmental protein that regulates a wide array of cellular responses important in physiological and pathological processes, such as wound healing, the immune response, fibrosis, and cancer.-Gold, L. I., Eggleton, P., Sweetwyne, M. T., Van Duyn, L. B., Greives, M. R., Naylor, S.-M., Michalak, M., Murphy-Ullrich, J. E. Calreticulin: non-endoplamic reticulum functions in physiology and disease.

Embryonic stem cell-derived cardiomyogenesis: a novel role for calreticulin as a regulator

A role for calreticulin, an endoplasmic reticulum (ER)-resident, Ca(2+)-binding chaperone, has recently emerged in the context of cardiomyogenesis. We previously proposed calreticulin to be a novel cardiac fetal gene, because calreticulin knockout causes embryonic lethality in mice as a result of cardiac defects, it is transiently activated during heart development, and heart-targeted overexpression of constitutively active calcineurin in calreticulin-null mice rescues the lethal phenotype. Calreticulin affects Ca(2+) homeostasis and expression of adhesion-related genes. Using cardiomyocytes derived from both calreticulin-null and wild-type embryonic stem (ES) cells, we show here that cardiomyogenesis from calreticulin-null ES cells is accelerated but deregulated, such that the myofibrils of calreticulin-null cardiomyocytes become disorganized and disintegrate with time in culture. We have previously shown that the disorganization of the actin cytoskeleton in calreticulin-null cells may be explained, at least in part, by the downregulation of adhesion proteins, implying that calreticulin ablation causes adhesion-related defects. Here, upon examination of adhesion proteins, we found that vinculin is downregulated in calreticulin-null cardiomyocytes. We also found c-Src activity to be higher in calreticulin-null cardiomyocytes than in wild-type cardiomyocytes, and c-Src activity is affected by both calreticulin and [Ca(2+)]. Finally, we show that calreticulin and calsequestrin, the major Ca(2+) storage proteins of the ER and sarcoplasmic reticulum, respectively, exhibit alternate distributions. This suggests that calreticulin may have a housekeeping role to play in mature cardiomyocytes as well as during cardiomyogenesis. We propose here that calreticulin, an ER Ca(2+) storage protein, is a crucial regulator of cardiomyogenesis whose presence is required for controlled cardiomyocyte development from ES cells.

Suppressive roles of calreticulin in prostate cancer growth and metastasis

Calreticulin is an essential, multifunctional Ca(2+)-binding protein that participates in the regulation of intracellular Ca(2+) homeostasis, cell adhesion, and chaperoning. Calreticulin is abundantly expressed and regulated by androgens in prostate epithelial cells. Given the importance of both calreticulin in multiple essential cellular activities and androgens in prostate cancer, we investigated the possibility of a role for calreticulin in prostate cancer progression. Immunohistochemistry revealed the down-regulation of calreticulin in a subset of human prostate cancer specimens. Prostate cancer cells overexpressing exogenous calreticulin produced fewer colonies in both monolayer culture and soft agar. Furthermore, calreticulin overexpression also inhibited tumor growth in the orthotopic PC3 xenograft tumor model and macroscopic lung metastasis in the rat Dunning AT3.1 prostate tumor model. To address the potential mechanism of calreticulin suppression of prostate cancer, we generated calreticulin mutants with different functional domains deleted. The calreticulin mutants containing the P-domain, which binds to other endoplasmic reticulum chaperone proteins, were sufficient for the suppression of PC3 growth in colony formation assays. Overall, our data support the hypothesis that calreticulin inhibits growth and/or metastasis of prostate cancer cells and that this suppression requires the P-domain.

Disruption of the novel gene fad104 causes rapid postnatal death and attenuation of cell proliferation, adhesion, spreading and migration

The molecular mechanisms at the beginning of adipogenesis remain unknown. Previously, we identified a novel gene, fad104 (factor for adipocyte differentiation 104), transiently expressed at the early stage of adipocyte differentiation. Since the knockdown of the expression of fad104 dramatically repressed adipogenesis, it is clear that fad104 plays important roles in adipocyte differentiation. However, the physiological roles of fad104 are still unknown. In this study, we generated fad104-deficient mice by gene targeting. Although the mice were born in the expected Mendelian ratios, all died within 1 day of birth, suggesting fad104 to be crucial for survival after birth. Furthermore, analyses of mouse embryonic fibroblasts (MEFs) prepared from fad104-deficient mice provided new insights into the functions of fad104. Disruption of fad104 inhibited adipocyte differentiation and cell proliferation. In addition, cell adhesion and wound healing assays using fad104-deficient MEFs revealed that loss of fad104 expression caused a reduction in stress fiber formation, and notably delayed cell adhesion, spreading and migration. These results indicate that fad104 is essential for the survival of newborns just after birth and important for cell proliferation, adhesion, spreading and migration.

Calreticulin, a multi-process calcium-buffering chaperone of the endoplasmic reticulum

Calreticulin is an ER (endoplasmic reticulum) luminal Ca2+-buffering chaperone. The protein is involved in regulation of intracellular Ca2+ homoeostasis and ER Ca2+ capacity. The protein impacts on store-operated Ca2+ influx and influences Ca2+-dependent transcriptional pathways during embryonic development. Calreticulin is also involved in the folding of newly synthesized proteins and glycoproteins and, together with calnexin (an integral ER membrane chaperone similar to calreticulin) and ERp57 [ER protein of 57 kDa; a PDI (protein disulfide-isomerase)-like ER-resident protein], constitutes the ‘calreticulin/calnexin cycle’ that is responsible for folding and quality control of newly synthesized glycoproteins. In recent years, calreticulin has been implicated to play a role in many biological systems, including functions inside and outside the ER, indicating that the protein is a multi-process molecule. Regulation of Ca2+ homoeostasis and ER Ca2+ buffering by calreticulin might be the key to explain its multi-process property.

Undertaker, a Drosophila Junctophilin, links Draper-mediated phagocytosis and calcium homeostasis

Phagocytosis is important during development and in the immune response for the removal of apoptotic cells and pathogens, yet its molecular mechanisms are poorly understood. In Caenorhabditis elegans, the CED2/5/10/12 pathway regulates actin during phagocytosis of apoptotic cells, whereas the role of the CED1/6/7 pathway in phagocytosis is unclear. We report that Undertaker (UTA), a Drosophila Junctophilin protein, is required for Draper (CED-1 homolog)-mediated phagocytosis. Junctophilins couple Ca2+ channels at the plasma membrane to those of the endoplasmic reticulum (ER), the Ryanodine receptors. We place Draper, its adaptor drCed-6, UTA, the Ryanodine receptor Rya-r44F, the ER Ca2+ sensor dSTIM, and the Ca2+-release-activated Ca2+ channel dOrai in the same pathway that promotes calcium homeostasis and phagocytosis. Thus, our results implicate a Junctophilin in phagocytosis and link Draper-mediated phagocytosis to Ca2+ homeostasis, highlighting a previously uncharacterized role for the CED1/6/7 pathway.

Influence of Helicobacter pylori CagA on Ca2+-associated phosphorylated proteins of AGS cells

AIM: To analyze the differential expression of Ca²⁺-associated protein (CagA) in human gastric adenocarcinoma epithelial cells (AGS) infected with H. pylori CagA and to move forward a single step for revealing CagA pathogenic mechanism.

METHODS: Proteins of AGS cells infected with H. pylori 26695 for 4 h, AGS cells infected withH. pylori 26695 △CagA for 4 h, and the uninfected AGS cells as a control group were extracted. Phosphorylated proteins were enriched by metal ion affinity adsorption enrichment techniques. The samples were separated by 2-dimensional polyacrylamide gel electrophoresis (2-DE) technique. Computer assisted image analysis was used to analyze the differential proteomic expression. The significantly differentially expressed proteins were unambiguously assigned identities by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF/TOF).

RESULTS: AGS cells treated with H. pylori △CagA showed the same expression as those cultured with the same period of AGS cells. However, expression of AGS cells treated with H. pylori △CagA had significant changes, indicating such a change was simply caused by CagA. Nineteen such protein spots were identified, of which, 3 protein spots were Ca²⁺-associated. Calcium-binding protein (nucleobindin-2 precursor, CALNUC) had almost the same expression either in AGS cells or 2-D gel under interaction of H. pylori △CagA with the AGS. However, such protein expression was markedly decreased following interaction between H. pylori and AGS.

CONCLUSION: CRT and CALNUC may affect the calcium homeostasis of endoplasmic reticulum, mitochondria and golgi, and then induce the apoptosis or proliferation pathway, which thus serves as a predisposing factor for gastritis, gastric ulcer and gastric cancer.

Fibronectin stimulates TRPV1 translocation in primary sensory neurons

Extracellular matrix (ECM) molecules are highly variable in their composition and receptor recognition. Their ubiquitous expression profile has been linked to roles in cell growth, differentiation, and survival. Recent work has identified certain ECM molecules that serve as dynamic signal modulators, versus the more-recognized role of chronic modulation of signal transduction. In this study, we investigated the role that fibronectin (FN) plays in the dynamic modulation of transient receptor potential family V type 1 receptor (TRPV1) translocation to the plasma membrane in trigeminal ganglia (TG) sensory neurons. Confocal immunofluorescence analyses identify co-expression of the TRPV1 receptor with integrin subunits that bind FN. TG neurons cultured upon or treated with FN experienced a leftward shift in the EC(50) of capsaicin-stimulated neuropeptide release. This FN-induced increase in TRPV1 sensitivity to activation is coupled by an increase in plasma membrane expression of TRPV1, as well as an increase in tyrosine phosphorylation of TRPV1 in TG neurons. Furthermore, TG neurons cultured on FN demonstrated an increase in capsaicin-mediated Ca(2+) accumulation relative to neurons cultured on poly-D-lysine. Data presented from these studies indicate that FN stimulates tyrosine-phosphorylation-dependent translocation of the TRPV1 receptor to the plasma membrane, identifying FN as a critical component of the ECM capable of sensory neuron sensitization.

Microarray and pathway analysis reveals decreased CDC25A and increased CDC42 associated with slow growth of BCL2 overexpressing immortalized breast cell line

Bcl-2 is an anti-apoptotic protein that is frequently overexpressed in cancer cells but its role in carcinogenesis is not clear. We are interested in how Bcl-2 expression affects non-cancerous breast cells and its role in the cell cycle. We prepared an MCF10A breast epithelial cell line that stably overexpressed Bcl-2. We analyzed the cells by flow cytometry after synchronization, and used cDNA microarrays with quantitative reverse-transcription PCR (qRT-PCR) to determine differences in gene expression. The microarray data was subjected to two pathway analysis tools, parametric analysis of gene set enrichment (PAGE) and ingenuity pathway analysis (IPA), and western analysis was carried out to determine the correlation between mRNA and protein levels. The MCF10A/Bcl-2 cells exhibited a slow-growth phenotype compared to control MCF10A/Neo cells that we attributed to a slowing of the G(1)-S cell cycle transition. A total of 363 genes were differentially expressed by at least two-fold, 307 upregulated and 56 downregulated. PAGE identified 22 significantly changed gene sets. The highest ranked network of genes identified by IPA contained 24 genes. Genes that were chosen for further analysis were confirmed by qRT-PCR, however, the western analysis did not always confirm differential expression of the proteins. Downregulation of the phosphatase CDC25A could solely be responsible for the slow growth phenotype in MCF10A/Bcl-2 cells. Increased levels of GTPase Cdc42 could be adding to this effect. PAGE and IPA are valuable tools for microarray analysis, but protein expression results do not always follow mRNA expression results.

Calreticulin enhances porcine wound repair by diverse biological effects

Extracellular functions of the endoplasmic reticulum chaperone protein calreticulin (CRT) are emerging. Here we show novel roles for exogenous CRT in both cutaneous wound healing and diverse processes associated with repair. Compared with platelet-derived growth factor-BB-treated controls, topical application of CRT to porcine excisional wounds enhanced the rate of wound re-epithelialization. In both normal and steroid-impaired pigs, CRT increased granulation tissue formation. Immunohistochemical analyses of the wounds 5 and 10 days after injury revealed marked up-regulation of transforming growth factor-beta3 (a key regulator of wound healing), a threefold increase in macrophage influx, and an increase in the cellular proliferation of basal keratinocytes of the new epidermis and of cells of the neodermis. In vitro studies confirmed that CRT induced a greater than twofold increase in the cellular proliferation of primary human keratinocytes, fibroblasts, and microvascular endothelial cells (with 100 pg/ml, 100 ng/ml, and 1.0 pg/ml, respectively). Moreover, using a scratch plate assay, CRT maximally induced the cellular migration of keratinocytes and fibroblasts (with 10 pg/ml and 1 ng/ml, respectively). In addition, CRT induced concentration-dependent migration of keratinocytes, fibroblasts macrophages, and monocytes in chamber assays. These in vitro bioactivities provide mechanistic support for the positive biological effects of CRT observed on both the epidermis and dermis of wounds in vivo, underscoring a significant role for CRT in the repair of cutaneous wounds.

Calreticulin inhibits commitment to adipocyte differentiation

Calreticulin, an endoplasmic reticulum (ER) resident protein, affects many critical cellular functions, including protein folding and calcium homeostasis. Using embryonic stem cells and 3T3-L1 preadipocytes, we show that calreticulin modulates adipogenesis. We find that calreticulin-deficient cells show increased potency for adipogenesis when compared with wild-type or calreticulin-overexpressing cells. In the highly adipogenic crt^−/− cells, the ER lumenal calcium concentration was reduced. Increasing the ER lumenal calcium concentration led to a decrease in adipogenesis. In calreticulin-deficient cells, the calmodulin–Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) pathway was up-regulated, and inhibition of CaMKII reduced adipogenesis. Calreticulin inhibits adipogenesis via a negative feedback mechanism whereby the expression of calreticulin is initially up-regulated by peroxisome proliferator–activated receptor γ (PPARγ). This abundance of calreticulin subsequently negatively regulates the expression of PPARγ, lipoprotein lipase, CCAAT enhancer–binding protein α, and aP2. Thus, calreticulin appears to function as a Ca²⁺-dependent molecular switch that regulates commitment to adipocyte differentiation by preventing the expression and transcriptional activation of critical proadipogenic transcription factors.

Analysis of the suitability of calreticulin inducible HEK cells for adhesion studies: microscopical and biochemical comparisons

Calreticulin is a Ca(2+)-buffering ER chaperone that also modulates cell adhesiveness. In order to study the effect of calreticulin on the expression of adhesion-related genes, we created a calreticulin inducible Human Embryonic Kidney (HEK) 293 cell line. We found that fibronectin mRNA and both intra- and extra-cellular fibronectin protein levels increased following calreticulin induction. However, despite this increase in fibronectin, HEK293 cells did not assemble an extracellular fibrillar fibronectin matrix regardless of the level of calreticulin expression. Furthermore, HEK293 cells exhibited a poorly organized actin cytoskeleton, did not have clustered fibronectin receptors at the cell surface, and did not form focal contacts. This likely accounts for the lack of fibronectin matrix deposition by these cells regardless of calreticulin expression level. Vinculin abundance did not appreciably increase upon calreticulin induction and the level of active c-Src, a regulatory kinase of focal contacts, was found to be abundant and unregulated by calreticulin induction in these cells. The inability to form stable focal contacts and to commence fibronectin fibrillogenesis due to high c-Src activity may be responsible for the poor adhesive phenotype of HEK 293 cells. Thus, we show here that HEK293 cells are not suitable for microscopical studies of cell-substratum adhesions, but are best suited for biochemical studies.