Calreticulin Regulates VEGF-A in Neuroblastoma Cells

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.

Brain Cells Release Calreticulin That Attracts and Activates Microglia, and Inhibits Amyloid Beta Aggregation and Neurotoxicity

Calreticulin is a chaperone, normally found in the endoplasmic reticulum, but can be released by macrophages into the extracellular medium. It is also found in cerebrospinal fluid bound to amyloid beta (Aβ). We investigated whether brain cells release calreticulin, and whether extracellular calreticulin had any effects on microglia and neurons relevant to neuroinflammation and neurodegeneration. We found that microglia release nanomolar levels of calreticulin when inflammatory-activated with lipopolysaccharide, when endoplasmic reticulum stress was induced by tunicamycin, or when cell death was induced by staurosporine, and that neurons release calreticulin when crushed. Addition of nanomolar levels of extracellular calreticulin was found to chemoattract microglia, and activate microglia to release cytokines TNF-α, IL-6 and IL-1β, as well as chemokine (C-C motif) ligand 2. Calreticulin blocked Aβ fibrillization and modified Aβ oligomerization, as measured by thioflavin T fluorescence and transmission electron microscopy. Extracellular calreticulin also altered microglial morphology and proliferation, and prevented Aβ-induced neuronal loss in primary neuron-glial cultures. Thus, calreticulin is released by microglia and neurons, and acts: as an alarmin to recruit and activate microglia, as an extracellular chaperone to prevent Aβ aggregation, and as a neuroprotectant against Aβ neurotoxicity.

Calreticulin in phagocytosis and cancer: opposite roles in immune response outcomes

Calreticulin (CRT) is a pleiotropic and highly conserved molecule that is mainly localized in the endoplasmic reticulum. Recently, CRT has gained special interest for its functions outside the endoplasmic reticulum where it has immunomodulatory properties. CRT translocation to the cell membrane serves as an "eat me" signal and promotes efferocytosis of apoptotic cells and cancer cell removal with completely opposite outcomes. Efferocytosis results in a silenced immune response and homeostasis, while removal of dying cancer cells brought about by anthracycline treatment, ionizing-irradiation or photodynamic therapy results in immunogenic cell death with activation of the innate and adaptive immune responses. In addition, CRT impacts phagocyte activation and cytokine production. The effects of CRT on cytokine production depend on its conformation, species specificity, degree of oligomerization and/or glycosylation, as well as its cellular localization and the molecular partners involved. The controversial roles of CRT in cancer progression and the possible role of the CALR gene mutations in myeloproliferative neoplasms are also addressed. The release of CRT and its influence on the different cells involved during efferocytosis and immunogenic cell death points to additional roles of CRT besides merely acting as an "eat me" signal during apoptosis. Understanding the contribution of CRT in physiological and pathological processes could give us some insight into the potential of CRT as a therapeutic target.

Calreticulin regulates vascular endothelial growth factor-A mRNA stability in gastric cancer cells

Calreticulin (CRT) and vascular endothelial growth factor-A (VEGF-A) are crucial for angiogenesis, and mediate multiple malignant behaviors in gastric cancer. In this study, we report that CRT is positively correlated with VEGF-A in gastric cancer patients. Moreover, high expressions of both CRT and VEGF-A are markedly associated with the pathological stage, progression, and poor prognosis in the patients. Therefore, we sought to elucidate the mechanism by which CRT affects VEGF-A in gastric cancer. Firstly, we demonstrate the novel finding that knockdown of CRT reduced VEGF-A mRNA stability in two gastric cancer cell lines, AGS and MKN45. The AU-Rich element (ARE) is believed to play a crucial role in the maintenance of VEGF-A mRNA stability. Luciferase reporter assay shows that knockdown of CRT significantly decreased the activity of renilla luciferase with VEGF-A ARE sequence. Additionally, competition results from RNA-binding/electrophoretic mobility shift assay indicate that CRT forms an RNA-protein complex with the VEGF-A mRNA by binding to the ARE. In addition, the proliferation rate of human umbilical vein endothelial cells (HUVEC) was significantly reduced when treated with conditioned medium from CRT knockdown cells; this was rescued by exogenous VEGF-A recombinant protein. Our results demonstrate that CRT is involved in VEGF-A ARE binding protein complexes to stabilize VEGF-A mRNA, thereby promoting the angiogenesis, and progression of gastric cancer.

Calreticulin regulates MYCN expression to control neuronal differentiation and stemness of neuroblastoma

Oncogenic N-MYC (MYCN) is widely used as a biomarker in clinics for neuroblastoma (NB) patients; nevertheless, mechanism that underlines MYCN regulation remains elusive. In the present study, we identified calreticulin (CRT) as a novel MYCN suppressor that downregulated MYCN promoter activity and protein expression to modulate neuronal differentiation and stemness. Our data showed that CRT-mediated MYCN suppression led to increased neurite length and commensurate elevation in differentiation marker GAP-43. We examined effect of radiotherapy and discovered that ionizing radiation (IR) was able to augment CRT expression dose-dependently in NB. Interestingly, neuronal differentiation and neurosphere formation (NSF) of NB were not only co-modulated by IR and CRT but were also dependent on Ca²⁺-buffering domain (C-domain) of CRT. Mutagenesis analysis showed that C-domain was indispensable for CRT-mediated MYCN regulation in NB differentiation and NSF. Of note, IR-induced formation of neural stem-like neurospheres (NS) was significantly impaired in CRT-overexpressed NB cells. The occupancy of CRT on MYCN 5' proximal promoter was confirmed by chromatin immunoprecipitation assays, revealing potential CRT binding sites that coincided with transcription factor E2F1 binding elements. In addition, we identified a physical interaction between CRT and E2F1, and demonstrated that CRT occupancy on MYCN promoter prevented E2F1-mediated MYCN upregulation. In line with in vitro findings, hampered tumor latency and retarded tumor growth in xenograft model corroborated IR and CRT co-mediated neuronal differentiation of NB. Together, our data delineated a novel mechanism of CRT-mediated MYCN regulation and warranted further preclinical investigation towards new therapeutic strategy for NB. CRT suppresses MYCN expression and promotes neuronal differentiation in NB. CRT regulates MYCN via interaction with E2F1 and direct binding to MYCN promoter. Ca²⁺-buffering domain of CRT is critical in MYCN regulation and NB differentiation. CRT-MYCN axis impacts on NB stemness by modulating neurosphere formation. Xenograft model corroborates in vitro NB differentiation mediated by CRT and IR.

The family of 14-3-3 proteins and specifically 14-3-3σ are up-regulated during the development of renal pathologies

Chronic kidney disease, the end result of most renal and some systemic diseases, is a common condition where renal function is compromised due to fibrosis. During renal fibrosis, calreticulin, a multifunctional chaperone of the endoplasmic reticulum (ER) is up‐regulated in tubular epithelial cells (TECs) both in vitro and in vivo. Proteomic analysis of cultured TECs overexpressing calreticulin led to the identification of the family of 14‐3‐3 proteins as key proteins overexpressed as well. Furthermore, an increased expression in the majority of 14‐3‐3 family members was observed in 3 different animal models of renal pathologies: the unilateral ureteric obstruction, the nephrotoxic serum administration and the ischaemia‐reperfusion. In all these models, the 14‐3‐3σ isoform (also known as stratifin) was predominantly overexpressed. As in all these models ischaemia is a common denominator, we showed that the ischaemia‐induced transcription factor HIF1α is specifically associated with the promoter region of the 14‐3‐3σ gene. Finally, we evaluated the expression of the family of 14‐3‐3 proteins and specifically 14‐3‐3σ in biopsies from IgA nephropathy and membranous nephropathy patients. These results propose an involvement of 14‐3‐3σ in renal pathology and provide evidence for the first time that hypoxia may be responsible for its altered expression.

VEGF expression correlates with neuronal differentiation and predicts a favorable prognosis in patients with neuroblastoma

Neuroblastoma (NB) is a childhood cancer with a low survival rate and great metastatic potential. Vascular endothelial growth factor (VEGF), an angiogenesis factor, has been found to be involved in CRT-related neuronal differentiation of NB cells. In this study, we further confirmed the role VEGF in NB through mouse xenograft model and clinical analysis from NB patients. In xenograft experiments, CRT overexpression effectively inhibited the tumor growth. In addition, the mRNA and protein levels of VEGF and differentiation marker GAP-43 were upregulated by induced CRT expression. However, no significant correlation between the expression level of VEGF and microvessel density was observed in human NB tumors, suggesting a novel mechanism of VEGF participating in NB tumorigenesis through an angiogenesis-independent pathway. In NB patients' samples, mRNA expression levels of CRT and VEGF were positively correlated. Furthermore, positive VEGF expression by immunostaining of NB tumors was found to correlate well with histological grade of differentiation and predicted a favorable prognosis. In conclusion, our findings suggest that VEGF is a favorable prognostic factor of NB and might affect NB tumor behavior through CRT-driven neuronal differentiation rather than angiogenesis that might shed light on a novel therapeutic strategy to improve the outcome of NB.

A sketch of known and novel MYCN-associated miRNA networks in neuroblastoma

Neuroblastoma (NB) originates from neural crest-derived precursors and represents the most common childhood extracranial solid tumour. MicroRNAs (miRNAs), a class of small non-coding RNAs that participate in a wide variety of biological processes by regulating gene expression, appear to play an essential role within the NB context. High-throughput next generation sequencing (NGS) was applied to study the miRNA transcriptome in a cohort of NB tumours with and without MYCN-amplification (MNA and MNnA, respectively) and in dorsal root ganglia (DRG), as a control. Out of the 128 miRNAs differentially expressed in the NB vs. DRG comparison, 47 were expressed at higher levels, while 81 were expressed at lower levels in the NB tumours. We also found that 23 miRNAs were differentially expressed in NB with or without MYCN-amplification, with 17 miRNAs being upregulated and 6 being downregulated in the MNA subtypes. Functional annotation analysis of the target genes of these differentially expressed miRNAs demonstrated that many mRNAs were involved in cancer-related pathways, such as DNA-repair and apoptosis as well as FGFR and EGFR signalling. In particular, we found that miR-628-3p negatively affects MYCN gene expression. Furthermore, we identified a novel miRNA candidate with variable expression in MNA vs. MNnA tumours, whose putative target genes are implicated in the mTOR pathway. The present study provides further insight into the molecular mechanisms that correlate miRNA dysregulation to NB development and progression.

Identification of potential targets for differentiation in human leukemia cells induced by diallyl disulfide

Diallyl disulfide (DADS) is a primary component of garlic, which has chemopreventive potential. We previously found that moderate doses (15-120 µM) of DADS induced apoptosis and G2/M phase cell cycle arrest. In this study, we observed the effect of low doses (8 µM) of DADS on human leukemia HL-60 cells. We found that DADS could inhibit proliferation, migration and invasion in HL-60 cells, and arrested cells at G0/G1 stage. Then, cell differentiation was displayed by morphologic observation, NBT reduction activity and CD11b evaluation of cytometric flow. It showed that DADS induced differentiation, reduced the ability of NBT and increased CD11b expression. Likewise, DADS inhibited xenograft tumor growth and induced differentiation in vivo. In order to make sure how DADS induced differentiation, we compared the protein expression profile of DADS-treated cells with that of untreated control. Using high resolution mass spectrometry, we identified 18 differentially expressed proteins after treatment with DADS, including four upregulated and 14 downregulated proteins. RT-PCR and western blot assay showed that DJ-1, cofilin 1, RhoGDP dissociation inhibitor 2 (RhoGDI2), Calreticulin (CTR) and PCNA were decreased by DADS. These data suggest that the effects of DADS on leukemia may be due to multiple targets for intervention.

Myeloproliferative Neoplasms: Molecular Drivers and Therapeutics

Activating mutations in genes that drive neoplastic cell growth are numerous and widespread in cancer, and specific genetic alterations are associated with certain types of cancer. For example, classic myeloproliferative neoplasms (MPNs) are hematopoietic stem cell disorders that affect cells of the myeloid lineage, including erythrocytes, platelets, and granulocytes. An activating mutation in the JAK2 tyrosine kinase is prevalent in these diseases. In MPN patients that lack such a mutation, other genetic changes that lead to activation of the JAK2 signaling pathway are present, indicating deregulation of JAK2 signaling plays an etiological driving role in MPNs, a concept supported by significant evidence from in vivo experimental MPN systems. Thus, small molecules that inhibit JAK2 activity are ideal drugs to impede the progression of disease in MPN patients. However, even though JAK inhibitors provide significant symptomatic relief, they have failed as a remission-inducing therapy. Nonetheless, the progress made understanding the molecular etiology of MPNs since 2005 is significant and has provided insight for the development and testing of novel molecular targeted therapeutic approaches. The current understanding of driver mutations in MPNs and an overview of current and potential therapeutic strategies for MPN patients will be discussed.

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.