Kinase-dependent adhesion to fibronectin: Regulation by calreticulin

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.

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.

LPA1/3 signaling mediates tumor lymphangiogenesis through promoting CRT expression in prostate cancer

Lysophosphatidic acid (LPA) is a bioactive lipid growth factor which is present in high levels in serum and platelets. LPA binds to its specific G-protein-coupled receptors, including LPA₁ to LPA₆, thereby regulating various physiological functions, including cancer growth, angiogenesis, and lymphangiogenesis. Our previous study showed that LPA promotes the expression of the lymphangiogenic factor vascular endothelial growth factor (VEGF)-C in prostate cancer (PCa) cells. Interestingly, LPA has been shown to regulate the expression of calreticulin (CRT), a multifunctional chaperone protein, but the roles of CRT in PCa progression remain unclear. Here we investigated the involvement of CRT in LPA-mediated VEGF-C expression and lymphangiogenesis in PCa. Knockdown of CRT significantly reduced LPA-induced VEGF-C expression in PC-3 cells. Moreover, LPA promoted CRT expression through LPA receptors LPA₁ and LPA₃, reactive oxygen species (ROS) production, and phosphorylation of eukaryotic translation initiation factor 2α (eIF2α). Tumor-xenografted mouse experiments further showed that CRT knockdown suppressed tumor growth and lymphangiogenesis. Notably, clinical evidence indicated that the LPA-producing enzyme autotaxin (ATX) is related to CRT and that CRT level is highly associated with lymphatic vessel density and VEGF-C expression. Interestingly, the pharmacological antagonist of LPA receptors significantly reduced the lymphatic vessel density in tumor and lymph node metastasis in tumor-bearing nude mice. Together, our results demonstrated that CRT is critical in PCa progression through the mediation of LPA-induced VEGF-C expression, implying that targeting the LPA signaling axis is a potential therapeutic strategy for PCa.

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.

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.

Epithelial calreticulin up-regulation promotes profibrotic responses and tubulointerstitial fibrosis development

Renal fibrosis is the common anatomical feature underlying the progression of chronic kidney disease, a leading cause of morbidity and mortality worldwide. In a previous study, we demonstrated that during development of renal fibrosis in a rat model of unilateral ureteric obstruction, calreticulin (CRT) is up-regulated in tubular epithelial cells (TECs). In the present study, we used in vitro and in vivo approaches to examine the role of CRT in TECs and its contribution to the progression of fibrosis. In cultured renal TECs, CRT overexpression induced acquisition of an altered, profibrotic cellular phenotype. Consistently, the opposite effects were observed for CRT knockdown. Subsequently, we confirmed that critical changes observed in vitro were also apparent in tubular cells in vivo in the animal model of unilateral ureteric obstruction. In agreement with these results, we demonstrate that substantial (50%) reduction in the expression of CRT reduced the development of tubulointerstitial fibrosis at a comparable level through regulation of inflammation, transcriptional activation, transforming growth factor β1-associated effects, and apoptosis. In summary, our findings establish that CRT is critically involved in the molecular mechanisms that drive renal fibrosis progression and indicate that inhibition of CRT expression might be a therapeutic target for reduction of fibrosis and chronic kidney disease development.

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.

Exogenous calreticulin improves diabetic wound healing

A serious consequence of diabetes mellitus is impaired wound healing, which largely resists treatment. We previously reported that topical application of calreticulin (CRT), an endoplasmic reticulum chaperone protein, markedly enhanced the rate and quality of wound healing in an experimental porcine model of cutaneous repair. Consistent with these in vivo effects, in vitro CRT induced the migration and proliferation of normal human cells critical to the wound healing process. These functions are particularly deficient in poor healing diabetic wounds. Using a genetically engineered diabetic mouse (db/db) in a full-thickness excisional wound healing model, we now show that topical application of CRT induces a statistically significant decrease in the time to complete wound closure compared with untreated wounds by 5.6 days (17.6 vs. 23.2). Quantitative analysis of the wounds shows that CRT increases the rate of reepithelialization at days 7 and 10 and increases the amount of granulation tissue at day 7 persisting to day 14. Furthermore, CRT treatment induces the regrowth of pigmented hair follicles observed on day 28. In vitro, fibroblasts isolated from diabetic compared with wild-type mouse skin and human fibroblasts cultured under hyperglycemic compared with normal glucose conditions proliferate and strongly migrate in response to CRT compared with untreated controls. The in vitro effects of CRT on these functions are consistent with CRT's potent effects on wound healing in the diabetic mouse. These studies implicate CRT as a potential powerful topical therapeutic agent for the treatment of diabetic and other chronic wounds.

EGFR dependent subcellular communication was responsible for morphine mediated AC superactivation

Compensatory adenylyl cyclase (AC) superactivation has been postulated to be responsible for the development of morphine tolerance and dependence, the underlying mechanism was demonstrated to comprise c-Src-dependent upregulation of AC5 within the lipid rafts. In the present study, we demonstrated that chronic morphine treatment sensitized EGFR signaling by augmenting EGFR phosphorylation and translocation into ER, which was essential for CRT-MOR tethering within the lipid rafts and AC5 superactivation. Intriguingly, synaptic clustering of CRT-MOR was dependent on EGFR phosphorylation and presumed to implicate in alignment and organization of synaptic compartments. Taken together, our data raised the possibility that an adaptive change in MOR and EGFR signal systems might establish CRT related subcellular communication, the signaling network within brain synaptic zone was proposed to implicate in morphine tolerance and dependence.

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.

How and why does the endoplasmic reticulum move?

The ER (endoplasmic reticulum) is a fascinating organelle that is highly dynamic, undergoing constant movement and reorganization. It has many key roles, including protein synthesis, folding and trafficking, calcium homoeostasis and lipid synthesis. It can expand in size when needed, and the balance between tubular and lamellar regions can be altered. The distribution and organization of the ER depends on both motile and static interactions with microtubules and the actin cytoskeleton. In the present paper, we review how the ER moves, and consider why this movement may be important for ER and cellular function.

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.

A novel non-transcriptional pathway mediates the proconvulsive effects of interleukin-1beta

Interleukin-1beta (IL-1beta) is overproduced in human and rodent epileptogenic tissue and it exacerbates seizures upon brain application in rodents. Moreover, pharmacological prevention of IL-1beta endogenous synthesis, or IL-1 receptor blockade, mediates powerful anticonvulsive actions indicating a significant role of this cytokine in ictogenesis. The molecular mechanisms of the proconvulsive actions of IL-1beta are not known. We show here that EEG seizures induced by intrahippocampal injection of kainic acid in C57BL6 adult mice were increased by 2-fold on average by pre-exposure to IL-1beta and this effect was blocked by 3-O-methylsphingomyelin (3-O-MS), a selective inhibitor of the ceramide-producing enzyme sphingomyelinase. C2-ceramide, a cell permeable analog of ceramide, mimicked IL-1beta action suggesting that ceramide may be the second messenger of the proconvulsive effect of IL-1beta. The seizure exacerbating effects of either IL-1beta or C2-ceramide were dependent on activation of the Src family of tyrosine kinases since they were prevented by CGP76030, an inhibitor of this enzyme family. The proconvulsive IL-1beta effect was associated with increased Tyr(418) phosphorylation of Src-family of kinases indicative of its activation, and Tyr(1472) phosphorylation of one of its substrate, the NR2B subunit of the N-methyl-d-aspartate receptor, which were prevented by 3-O-MS and CGP76030. Finally, the proconvulsive effect of IL-1beta was blocked by ifenprodil, a selective NR2B receptor antagonist. These results indicate that the proconvulsive actions of IL-1beta depend on the activation of a sphingomyelinase- and Src-family of kinases-dependent pathway in the hippocampus which leads to the phosphorylation of the NR2B subunit, thus highlighting a novel, non-transcriptional mechanism underlying seizure exacerbation in inflammatory conditions.