Differential calreticulin expression affects focal contacts via the calmodulin/CaMK II pathway

Intensive stretch-activated CRT-PMCA1 feedback loop promoted apoptosis of myoblasts through Ca2+ overloading

Mechanical stretch exerted pro-apoptotic effect on myoblasts, the mechanism of which is currently unknown. Intracellular Ca²⁺ accumulation has been implicated in stretch-induced apoptosis. calreticulin (CRT) and plasma membrane Ca²⁺ transporting ATPase 1 (PMCA1) are two critical components of Ca²⁺ signaling system participating in intracellular Ca²⁺ homeostasis. In this study, we explored the contribution of CRT and PMCA1 in mediating stretch-induced Ca²⁺ accumulation and apoptosis of myoblasts. Stretching stimuli elevated level of CRT while inhibited activity of PMCA1. Moreover, there were bidirectional regulations between CRT and PMCA1, which formed the positive feedback loop leading to continuous increment of CRT level and repression of PMCA1 activity, in stretched myoblasts. Specifically, increased CRT level inhibited PMCA1 activity via suppressing Calmodulin (CaM), while reduced PMCA1 activity promoted CRT expression through activating p38MAPK pathway. Thus, the CRT-CaM-PMCA1 and PMCA1-p38MAPK-CRT pathways constituted a close cycle comprising CRT, PMCA1, CaM and p38MAPK. Inhibition of both CaM and p38MAPK affected the other three factors in stretched myoblasts. Circulation of the vicious cycle resulted in escalated Ca²⁺ overloading in myoblasts under continuous stretching stimuli. CRT knock-down, PMCA1 overexpression, and p38MAPK inhibition all attenuated the raised intracellular Ca²⁺ level and ameliorated myoblast apoptosis in the stretching environment. Conversely, CRT overexpression, PMCA1 knock-down, and CaM inhibition all aggravated stretch-induced Ca²⁺ overloading and myoblast apoptosis. A positive feedback loop between CRT and PMCA1 was activated in stretched myoblasts, which contributed to intracellular Ca²⁺ accumulation and resultant myoblast apoptosis.

Calreticulin nuclear translocalization alleviates CaM/CaMKII/CREB signaling pathway to enhance chemosensitivity in HDAC inhibitor-resistant hepatocellular carcinoma cells

Calreticulin (CRT) is located in the endoplasmic reticulum (ER), it helps proteins fold correctly inside the ER, and acts as a modulator of Ca²⁺ homeostasis. Aberrant expression of CRT is implicated in several cancer types, qualifying CRT as a potential therapeutic target. However, it remains unclear how CRT affects specific oncogenic pathways. In this study, we used histone deacetylase inhibitors (HDACis) to establish drug-resistant liver cancer cells and further analyzed the molecular mechanism of development of drug resistance in those cells. The 2D gel electrophoresis and RT-PCR data showed that CRT was downregulated in HDACis-resistant cells by comparing with HA22T parental cells. We previously elucidated the development of drug-resistance in HCC cells via activation of PP1-eIF2α pathway, but not via ER stress pathway. Here, we show that thapsigargin induced ER stress through mechanism other than ER stress downstream protein GRP78-PERK to regulate CRT expression in HDACis-R cells. Moreover, the expression level of CRT was not the main cause of apoptosis in HDACis-resistant cells. Mechanistic studies identified the apoptosis factors in the nucleus-the HDACis-mediated overexpression of CRT, CRT translocation to the cell nucleus, and reduced CaM/CaMKII/CREB pathway-that led to chemosensitivity in HDACis-R HCC cells.

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 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 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.

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.

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.

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.

Decoded calreticulin-deficient embryonic stem cell transcriptome resolves latent cardiophenotype

Genomic perturbations that challenge normal signaling at the pluripotent stage may trigger unforeseen ontogenic aberrancies. Anticipatory systems biology identification of transcriptome landscapes that underlie latent phenotypes would offer molecular diagnosis before the onset of symptoms. The purpose of this study was to assess the impact of calreticulin-deficient embryonic stem cell transcriptomes on molecular functions and physiological systems. Bioinformatic surveillance of calreticulin-null stem cells, a monogenic insult model, diagnosed a disruption in transcriptome dynamics, which re-prioritized essential cellular functions. Calreticulin-calibrated signaling axes were uncovered, and network-wide cartography of undifferentiated stem cell transcripts suggested cardiac manifestations. Calreticulin-deficient stem cell-derived cardiac cells verified disorganized sarcomerogenesis, mitochondrial paucity, and cytoarchitectural aberrations to validate calreticulin-dependent network forecasts. Furthermore, magnetic resonance imaging and histopathology detected a ventricular septal defect, revealing organogenic manifestation of calreticulin deletion. Thus, bioinformatic deciphering of a primordial calreticulin-deficient transcriptome decoded at the pluripotent stem cell stage a reconfigured multifunctional molecular registry to anticipate predifferentiation susceptibility toward abnormal cardiophenotype.

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.

Genomics and proteomics approaches to the study of cancer-stroma interactions

Background

The development and progression of cancer depend on its genetic characteristics as well as on the interactions with its microenvironment. Understanding these interactions may contribute to diagnostic and prognostic evaluations and to the development of new cancer therapies. Aiming to investigate potential mechanisms by which the tumor microenvironment might contribute to a cancer phenotype, we evaluated soluble paracrine factors produced by stromal and neoplastic cells which may influence proliferation and gene and protein expression.

Methods

The study was carried out on the epithelial cancer cell line (Hep-2) and fibroblasts isolated from a primary oral cancer. We combined a conditioned-medium technique with subtraction hybridization approach, quantitative PCR and proteomics, in order to evaluate gene and protein expression influenced by soluble paracrine factors produced by stromal and neoplastic cells.

Results

We observed that conditioned medium from fibroblast cultures (FCM) inhibited proliferation and induced apoptosis in Hep-2 cells. In neoplastic cells, 41 genes and 5 proteins exhibited changes in expression levels in response to FCM and, in fibroblasts, 17 genes and 2 proteins showed down-regulation in response to conditioned medium from Hep-2 cells (HCM). Nine genes were selected and the expression results of 6 down-regulated genes (ARID4A, CALR, GNB2L1, RNF10, SQSTM1, USP9X) were validated by real time PCR.

Conclusions

A significant and common denominator in the results was the potential induction of signaling changes associated with immune or inflammatory response in the absence of a specific protein.

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.

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.

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.

Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis

Anoikis, apoptotic cell death due to loss of cell adhesion, is critical for regulation of tissue homeostasis in tissue remodeling. Fibrogenesis is associated with reduced fibroblast apoptosis. The matricellular protein thrombospondin 1 (TSP1) regulates cell adhesion and motility during tissue remodeling and in fibrogenesis. The N-terminal domain of TSP1 binds to the calreticulin-LRP1 receptor co-complex to signal down-regulation of cell adhesion and increased cell motility through focal adhesion disassembly. TSP1 signaling through calreticulin-LRP1 activates cell survival signals such as PI3-kinase. Therefore, we tested the hypothesis that TSP1 supports cell survival under adhesion-independent conditions to facilitate tissue remodeling. Here, we show that platelet TSP1, its N-terminal domain (NoC1) as a recombinant protein, or a peptide comprising the calreticulin-LRP1 binding site [amino acids 17-35 (hep I)] in the N-terminal domain promotes fibroblast survival under anchorage-independent conditions. TSP1 activates Akt and decreases apoptotic signaling through caspase 3 and PARP1 in suspended fibroblasts. Inhibition of PI3K/Akt activity blocks TSP1-mediated anchorage-independent survival. Fibroblasts lacking LRP1 or expressing calreticulin lacking the TSP1 binding site do not respond to TSP1 with anchorage-independent survival. These data define a novel role for TSP1 signaling through the calreticulin/LRP1 co-complex in tissue remodeling and fibrotic responses through stimulation of anoikis resistance.-Pallero, M. A., Elzie, C. A., Chen, J., Mosher, D. F., Murphy-Ullrich, J. E. Thrombospondin 1 binding to calreticulin-LRP1 signals resistance to anoikis.

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.