Calreticulin regulates insulin receptor expression and its downstream PI3 Kinase/Akt signalling pathway

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

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

Calreticulin and PDIA3, two markers of endoplasmic reticulum stress, are associated with metabolic alterations and insulin resistance in pediatric obesity: A pilot study

Our aim was to evaluate the markers of endoplasmic reticulum (ER) stress among children and adolescents with obesity in relation to metabolic alterations. Calreticulin (CALR) and PDIA3 circulating levels were assessed on 52 pediatric subjects-26 patients with obesity and 26 normal weight controls (4-18 years)-enrolled in a pilot study. Clinical and metabolic evaluations were performed (BMI-SDS, insulin, and glucose at fasting and during an oral glucose tolerance test, lipid profile, blood pressure), and metabolic syndrome was detected. PDIA3 was higher (p < 0.02) and CALR slightly higher in children with obesity than in controls. PDIA3 was related positively to the Tanner stages. Both PDIA3 and CALR were positively associated with insulin resistance, cholesterol, and triglycerides and the number of criteria identifying metabolic syndrome and negatively with fasting and post-challenge insulin sensitivity. Our preliminary findings suggest the existence of a link between ER stress and metabolic changes behind obesity complications even at the pediatric age. CALR and PDIA3 could be early markers of insulin resistance and dyslipidemia-related ER stress useful to stratify patients at high risk of further complications.

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 and cancer

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

Towards frailty biomarkers: Candidates from genes and pathways regulated in aging and age-related diseases

OBJECTIVE

Use of the frailty index to measure an accumulation of deficits has been proven a valuable method for identifying elderly people at risk for increased vulnerability, disease, injury, and mortality. However, complementary molecular frailty biomarkers or ideally biomarker panels have not yet been identified. We conducted a systematic search to identify biomarker candidates for a frailty biomarker panel.

METHODS

Gene expression databases were searched (http://genomics.senescence.info/genes including GenAge, AnAge, LongevityMap, CellAge, DrugAge, Digital Aging Atlas) to identify genes regulated in aging, longevity, and age-related diseases with a focus on secreted factors or molecules detectable in body fluids as potential frailty biomarkers. Factors broadly expressed, related to several "hallmark of aging" pathways as well as used or predicted as biomarkers in other disease settings, particularly age-related pathologies, were identified. This set of biomarkers was further expanded according to the expertise and experience of the authors. In the next step, biomarkers were assigned to six "hallmark of aging" pathways, namely (1) inflammation, (2) mitochondria and apoptosis, (3) calcium homeostasis, (4) fibrosis, (5) NMJ (neuromuscular junction) and neurons, (6) cytoskeleton and hormones, or (7) other principles and an extensive literature search was performed for each candidate to explore their potential and priority as frailty biomarkers.

RESULTS

A total of 44 markers were evaluated in the seven categories listed above, and 19 were awarded a high priority score, 22 identified as medium priority and three were low priority. In each category high and medium priority markers were identified.

CONCLUSION

Biomarker panels for frailty would be of high value and better than single markers. Based on our search we would propose a core panel of frailty biomarkers consisting of (1) CXCL10 (C-X-C motif chemokine ligand 10), IL-6 (interleukin 6), CX3CL1 (C-X3-C motif chemokine ligand 1), (2) GDF15 (growth differentiation factor 15), FNDC5 (fibronectin type III domain containing 5), vimentin (VIM), (3) regucalcin (RGN/SMP30), calreticulin, (4) PLAU (plasminogen activator, urokinase), AGT (angiotensinogen), (5) BDNF (brain derived neurotrophic factor), progranulin (PGRN), (6) α-klotho (KL), FGF23 (fibroblast growth factor 23), FGF21, leptin (LEP), (7) miRNA (micro Ribonucleic acid) panel (to be further defined), AHCY (adenosylhomocysteinase) and KRT18 (keratin 18). An expanded panel would also include (1) pentraxin (PTX3), sVCAM/ICAM (soluble vascular cell adhesion molecule 1/Intercellular adhesion molecule 1), defensin α, (2) APP (amyloid beta precursor protein), LDH (lactate dehydrogenase), (3) S100B (S100 calcium binding protein B), (4) TGFβ (transforming growth factor beta), PAI-1 (plasminogen activator inhibitor 1), TGM2 (transglutaminase 2), (5) sRAGE (soluble receptor for advanced glycosylation end products), HMGB1 (high mobility group box 1), C3/C1Q (complement factor 3/1Q), ST2 (Interleukin 1 receptor like 1), agrin (AGRN), (6) IGF-1 (insulin-like growth factor 1), resistin (RETN), adiponectin (ADIPOQ), ghrelin (GHRL), growth hormone (GH), (7) microparticle panel (to be further defined), GpnmB (glycoprotein nonmetastatic melanoma protein B) and lactoferrin (LTF). We believe that these predicted panels need to be experimentally explored in animal models and frail cohorts in order to ascertain their diagnostic, prognostic and therapeutic potential.

Upregulation of Early and Downregulation of Terminal Pathway Complement Genes in Subcutaneous Adipose Tissue and Adipocytes in Acquired Obesity

Inflammation is an important mediator of obesity-related complications such as the metabolic syndrome but its causes and mechanisms are unknown. As the complement system is a key mediator of inflammation, we studied whether it is activated in acquired obesity in subcutaneous adipose tissue (AT) and isolated adipocytes. We used a special study design of genetically matched controls of lean and heavy groups, rare monozygotic twin pairs discordant for body mass index (BMI) [n = 26, within-pair difference (Δ) in body mass index, BMI >3 kg/m²] with as much as 18 kg mean Δweight. Additionally, 14 BMI-concordant (BMI <3 kg/m²) served as a reference group. The detailed measurements included body composition (DEXA), fat distribution (MRI), glucose, insulin, adipokines, C3a and SC5b-9 levels, and the expression of complement and insulin signaling pathway-related genes in AT and adipocytes. In both AT and isolated adipocytes, the classical and alternative pathway genes were upregulated, and the terminal pathway genes downregulated in the heavier co-twins of the BMI-discordant pairs. The upregulated genes included C1q, C1s, C2, ficolin-1, factor H, receptors for C3a and C5a (C5aR1), and the iC3b receptor (CR3). While the terminal pathway components C5 and C6 were downregulated, its inhibitor clusterin was upregulated. Complement gene upregulation in AT and adipocytes correlated positively with adiposity and hyperinsulinemia and negatively with the expression of insulin signaling-related genes. Plasma C3a, but not SC5b-9, levels were elevated in the heavier co-twins. There were no differences between the co-twins in BMI-concordant pairs. Obesity is associated with increased expression of the early, but not late, complement pathway components and of key receptors. The twins with acquired obesity have therefore an inflated inflammatory activity in the AT. The results suggest that complement is likely involved in orchestrating clearance of apoptotic debris and inflammation in the AT.

Temporal characterization of β cell-adaptive and -maladaptive mechanisms during chronic high-fat feeding in C57BL/6NTac mice

The onset of type 2 diabetes is characterized by transition from successful to failed insulin secretory compensation to obesity-related insulin resistance and dysmetabolism. Energy-rich diets in rodents are commonly studied models of compensatory increases in both insulin secretion and β cell mass. However, the mechanisms of these adaptive responses are incompletely understood, and it is also unclear why these responses eventually fail. We measured the temporal trends of glucose homeostasis, insulin secretion, β cell morphometry, and islet gene expression in C57BL/6NTac mice fed a 60% high-fat diet (HFD) or control diet for up to 16 weeks. A 2-fold increased hyperinsulinemia was maintained for the first 4 weeks of HFD feeding and then further increased through 16 weeks. β cell mass increased progressively starting at 4 weeks, principally through nonproliferative growth. Insulin sensitivity was not significantly perturbed until 11 weeks of HFD feeding. Over the first 8 weeks, we observed two distinct waves of increased expression of β cell functional and prodifferentiation genes. This was followed by activation of the unfolded protein response at 8 weeks and overt β cell endoplasmic reticulum stress at 12-16 weeks. In summary, β cell adaptation to an HFD in C57BL/6NTac mice entails early insulin hypersecretion and a robust growth phase along with hyperexpression of related genes that begin well before the onset of observed insulin resistance. However, continued HFD exposure results in cessation of gene hyperexpression, β cell functional failure, and endoplasmic reticulum stress. These data point to a complex but not sustainable integration of β cell-adaptive responses to nutrient overabundance, obesity development, and insulin resistance.

Endoplasmic reticulum stress in insulin resistance and diabetes

The endoplasmic reticulum is the main intracellular Ca(2+) store for Ca(2+) release during cell signaling. There are different strategies to avoid ER Ca(2+) depletion. Release channels utilize first Ca(2+)-bound to proteins and this minimizes the reduction of the free luminal [Ca(2+)]. However, if release channels stay open after exhaustion of Ca(2+)-bound to proteins, then the reduction of the free luminal ER [Ca(2+)] (via STIM proteins) activates Ca(2+) entry at the plasma membrane to restore the ER Ca(2+) load, which will work provided that SERCA pump is active. Nevertheless, there are several noxious conditions that result in decreased activity of the SERCA pump such as oxidative stress, inflammatory cytokines, and saturated fatty acids, among others. These conditions result in a deficient restoration of the ER [Ca(2+)] and lead to the ER stress response that should facilitate recovery of the ER. However, if the stressful condition persists then ER stress ends up triggering cell death and the ensuing degenerative process leads to diverse pathologies; particularly insulin resistance, diabetes and several of the complications associated with diabetes. This scenario suggests that limiting ER stress should decrease the incidence of diabetes and the mobility and mortality associated with this illness.

Loss of calreticulin function decreases NFκB activity by stabilizing IκB protein

Transcription factor NFκB is activated by several processes including inflammation, endoplasmic-reticulum (ER) stress, increase in Akt signaling and enhanced proteasomal degradation. Calreticulin (CRT) is an ER Ca(2+)-binding chaperone that regulates many cellular processes. Gene-targeted deletion of CRT has been shown to induce ER stress that is accompanied with a significant increase in the proteasome activity. Loss of CRT function increases the resistance of CRT-deficient (crt-/-) cells to UV- and drug-induced apoptosis. Based on these reports we hypothesized that loss of CRT will activate NFκB signaling thus contributing to enhanced resistance to apoptosis. In contrast to our hypothesis, we observed a significant decrease in the basal transcriptional activity of NFκB in CRT-deficient cells. Treatment with lipopolysaccharide failed to increase the transcriptional activity of NFκB in the crt-/- cells to the same level as in the wt cells. Our data illustrate that the mechanism of decreased NFκB activity in CRT-deficient cells is mediated by a significant increase in IκB protein expression. Furthermore, we showed a significant increase in protein phosphatase 2A activity inhibition which resulted in decreased IκBα protein level in CRT-deficient cells. Based on our data we concluded that loss of CRT increases the stability of IκB protein thus reducing NFκB activity.

Altered gene and protein expression in liver of the obese spontaneously hypertensive/NDmcr-cp rat

BACKGROUND

It is difficult to study the mechanisms of the metabolic syndrome in humans due to the heterogeneous genetic background and lifestyle. The present study investigated changes in the gene and protein profiles in an animal model of the metabolic syndrome to identify the molecular targets associated with the pathogenesis and progression of obesity related to the metabolic syndrome.

METHODS

We extracted mRNAs and proteins from the liver tissues of 6- and 25-week-old spontaneously hypertensive/NIH -corpulent rat SHR/NDmcr-cp (CP), SHR/Lean (Lean) and Wistar Kyoto rats (WKY) and performed microarray analysis and two-dimensional difference in gel electrophoresis (2D-DIGE) linked to a matrix-assisted laser desorption ionization time-of-flight tandem mass spectrometry (MALDI-TOF/TOF MS).

RESULTS

The microarray analysis identified 25 significantly up-regulated genes (P < 0.01; log10 > 1) and 31 significantly down-regulated genes (P < 0.01; log10 < -1) in 6- and 25-week-old CP compared with WKY and Lean. Several of these genes are known to be involved in important biological processes such as electron transporter activity, electron transport, lipid metabolism, ion transport, transferase, and ion channel activity. MALDI-TOF/TOF MS identified 31 proteins with ±1.2 fold change (P < 0.05) in 6- and 25-week-old CP, compared with age-matched WKY and Lean. The up-regulated proteins are involved in metabolic processes, biological regulation, catalytic activity, and binding, while the down-regulated proteins are involved in endoplasmic reticulum stress-related unfolded protein response.

CONCLUSION

Genes with significant changes in their expression in transcriptomic analysis matched very few of the proteins identified in proteomics analysis. However, annotated functional classifications might provide an important reference resource to understand the pathogenesis of obesity associated with the metabolic syndrome.

Connexin 43 phosphorylation and degradation are required for adipogenesis

Connexin-43 (Cx43) is a membrane phosphoprotein that mediates direct inter-cellular communication by forming gap junctions. In this way Cx43 can influence gene expression, differentiation and growth. Its role in adipogenesis, however, is poorly understood. In this study, we established that Cx43 becomes highly phosphorylated in early adipocyte differentiation and translocates to the plasma membrane from the endoplasmic reticulum. As preadipocytes differentiate, Cx43 phosphorylation declines, the protein is displaced from the plasma membrane, and total cellular levels are reduced via proteosomal degradation. Notably, we show that inhibiting Cx43 degradation or constitutively over-expressing Cx43 blocks adipocyte differentiation. These data reveal that transient activation of Cx43 via phosphorylation followed by its degradation is vital for preadipocyte differentiation and maturation of functional adipocytes.

NO-1886 ameliorates glycogen metabolism in insulin-resistant HepG2 cells by GSK-3β signalling

OBJECTIVES

The aim of the study was to elucidate the possible role and mechanism of NO-1886 (ibrolipim, a lipoprotein lipase activator) in ameliorating insulin resistance induced by high palmitate.

METHODS

HepG2 cells were cultured in RPMI 1640 medium and were treated with palmitate to induce insulin resistance. Free fatty acids (FFAs), glucose, glycogen, cell viability and mRNA and protein levels were analysed separately.

KEY FINDINGS

We found that HepG2 cells treated with 0.5 mm palmitate for 48 h led to a significant decrease of insulin-induced glucose consumption (from 2.89 ± 0.85 mm in the control to 0.57 ± 0.44 mm in palmitate). Insulin resistance (IR) of HepG2 cells was induced by 0.5 mm palmitate for 48 h. NO-1886 stimulated glucose consumption, glycogen synthesis and FFA absorption in insulin-resistant HepG2 cells. Maximum stimulation effects were observed with 10 µm NO-1886 for 24 h. Compared with the dimethyl sulfoxide-treated group, 2.5 µm NO-1886 or higher could induce the mRNA expression of lipoprotein lipase. Meanwhile, NO-1886 increased the protein content of P-GSK-3βser(9) and decreased the protein level of GSK-3β in insulin-resistant HepG2 cells, but NO-1886 didn't change the protein levels of PI3-Kp85 and Akt2.

CONCLUSION

Lipoprotein lipase activator NO-1886 could increase glycogen synthesis in HepG2 cells and could ameliorate the insulin resistance, which was associated with GSK-3 signalling.

Shotgun proteomic analysis of the fat body during metamorphosis of domesticated silkworm (Bombyx mori)

Protein expression profiles in the fat bodies of larval, pupal, and moth stages of silkworm were determined using shotgun proteomics and MS sequencing. We identified 138, 217, and 86 proteins from the larval, pupal and moth stages, respectively, of which 12 were shared by the 3 stages. There were 92, 150, and 45 specific proteins identified in the larval, pupal and moth stages, respectively, of which 17, 68, and 9 had functional annotations. Among the specific proteins identified in moth fat body, sex-specific storage-protein 1 precursor and chorion protein B8 were unique to the moth stage, indicating that the moth stage fat body is more important for adult sexual characteristics. Many ribosomal proteins (L23, L4, L5, P2, S10, S11, S15A and S3) were found in pupal fat bodies, whereas only three (L14, S20, and S7) and none were identified in larval and moth fat bodies, respectively. Twenty-three metabolic enzymes were identified in the pupal stage, while only four and two were identified in the larval and moth stages, respectively. In addition, an important protein, gloverin2, was only identified in larval fat bodies. Gene ontology (GO) analysis of the proteins specific to the three stages linked them to the cellular component, molecular function, and biological process categories. The most diverse GO functional classes were involved by the relatively less specific proteins identified in larva. GO analysis of the proteins shared among the three stages showed that the pupa and moth stages shared the most similar protein functions in the fat body.

Novel mutations in the calreticulin gene core promoter and coding sequence in schizoaffective disorder

We have recently reported the first case of mutation in the core promoter sequence of the human calreticulin gene in a family case of schizoaffective disorder. Remarkably, this gene coincides with a region of suggested linkage at 19p13.2, identified in a whole genome scan [Hamshere et al. (2005); Arch Gen Psychiatry 62;1081-1088]. The identified mutation was located at the conserved position -48 from the transcription start site, and was shown to be of functional effect, resulting in the aberrant expression of the gene. Following screening of the gene in 60 independent cases of schizoaffective disorder, we report novel germ-line mutations at positions -205 C > T and the conserved exon 5 (c: 682 C > T, pro228ser) in two unrelated cases of schizoaffective disorder. These mutations were disease-specific, and as for the -48 G > C mutation, neither was detected in a control population of 370 individuals, indicating a contribution of 3.17% in this sample series. To our knowledge, this is the first instance of disease-specific mutations in schizoaffective disorder, which warrants systematic screening of the regulatory and coding regions of the calreticulin gene in this disorder.

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.