Review
Transcriptional control of the calreticulin gene in health and disease

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Abstract

Calreticulin is a multifunctional Ca2+ binding chaperone in the endoplasmic reticulum and expression of the protein is tightly regulated at the transcriptional level. There are two calreticulin genes, named calreticulin-1 and calreticulin-2 gene. The calreticulin-1 promoter contains a number of putative binding sites for transcription factors including tissue specific factors. Direct regulation of the calreticulin-1 promoter by several of these factors has been confirmed experimentally including Nkx2.5, MEF2C, GATA6, PPAR, COUP-TF1 and Evi-1 factors. Studies on calreticulin-deficient mice and transgenic animal models indicate that calreticulin is critical for cardiac development and that expression of the protein must be tightly regulated during cardiogenesis. Moreover, differential expression of calreticulin has been associated with several diseases, including neurodegenerative problems, cancers, autoimmune diseases and wound healing. Understanding the mechanisms responsible for the regulation of expression of calreticulin may contribute to the treatment of many diverse diseases.

Introduction

Calreticulin is a multifunctional Ca2+ buffering chaperone in the endoplasmic reticulum (ER) (Table 1). As a molecular chaperone, calreticulin binds to the newly synthesized (glyco)proteins, preventing their aggregation and assisting in their correct folding (Hebert and Molinari, 2007, Michalak et al., 2002, Williams, 2006). Together with calnexin and ERp57 the protein forms a folding cycle to control protein folding (Hebert and Molinari, 2007, Michalak et al., 2002). As a modulator of Ca2+ homeostasis, calreticulin binds large quantities of Ca2+ and consequently directly impacts on Ca2+ capacity of the ER (Michalak, 1995). This has important functional implications since Ca2+ is critical for many ER functions including protein folding and posttranslational modification, activation of store-operated Ca2+ influx, apoptosis, lipid and steroid synthesis, to name a few (Groenendyk and Michalak, 2005). Increased expression of calreticulin results in an elevated Ca2+ level in the ER (Bastianutto et al., 1995, Mery et al., 1996). In turn, calreticulin-deficient cells have significantly reduced Ca2+ storage capacity in the ER, which implies the importance of calreticulin in ER Ca2+ storage and buffering (Nakamura et al., 2001b).

Calreticulin is differentially expressed under variety of physiological and pathological conditions (Table 1, Table 2). For example, reduced levels of calreticulin are found in differentiated tissues such as heart and the brain (Abe et al., 1992, Guo et al., 2001, Imanaka-Yoshida et al., 1996, Langdown et al., 2003, Mesaeli et al., 1999a). In contrast calreticulin is up-regulated in highly differentiated tissues or upon induction of ER stress (Luczynski et al., 2007, Waser et al., 1997, Zhang et al., 2007, Zhu and Wang, 1999). Considering a differential expression of calreticulin in many tissues and cell types it is not surprising that the calreticulin gene is under a tight control of several specific transcription factors. It is, therefore, of a great interest to understand the molecular mechanisms responsible for the regulation of calreticulin expression at transcriptional level. In this review we focused on transcriptional regulation of the calreticulin-1 gene (referred to throughout the manuscript as the calreticulin gene) with a major emphasis on its relevance to health and disease.

Section snippets

The calreticulin gene and promoter

Calreticulin is a highly conserved protein found across a diverse species except for yeast and prokaryotes. To date, two calreticulin genes, named calreticulin-1 and calreticulin-2, have been identified in human, pig, rat and mouse and these genes share approximately 50% identity (Persson et al., 2002). The function of calreticulin-2 and regulation of the calreticulin-2 promoter remain to be elucidated. The product of the calreticulin-2 gene has only been detected in testis suggesting that the

Transcriptional regulation of the calreticulin gene in the heart

Cardiac development is a well controlled molecular and morphogenetic event and even small perturbation of this process can have devastating consequences in the form of congenital heart disease. At the molecular level there are a series of transcription factors involved in the regulation of expression of cardiac specific genes. The calreticulin promoter contains several regulatory sites for muscle specific transcription factors including GATA, Nkx2.5, MEF2C and MyoD (Fig. 1) suggesting that the

Calreticulin in the brain and neurodegenerative diseases

Proteomic analysis identified expression of calreticulin in the rat brain (Decca et al., 2006). In situ hybridization of adult mice brain shows a broad and heterogeneous expression of calreticulin mRNA in the discrete neurons throughout the entire brain, including cerebrum, mesencephalon and cerebellum (Abe et al., 1992). Interestingly, calreticulin is more abundant in the mouse embryonic brain than in adult (Zhang et al., 2007). The protein may be essential during early stages of brain

Calreticulin in the liver

Calreticulin is expressed in a very high level in the liver of many species (Fairburn et al., 2006, Khanna et al., 1987, Milner et al., 1991). Interestingly, mice fed with energy restricted diet show significantly reduced expression of ER chaperone proteins including calreticulin in the liver (Dhahbi et al., 1997) suggesting that calreticulin might be involved in the regulation of energy metabolism in the liver. Lipid metabolism is regulated by several transcription factor families, including

Calreticulin in adipose tissue

Adipose tissue, a loose connective tissue composed of adipocytes, is a major energy reserve for the triglyceride storage and also involved in the regulation of energy homeostasis (Rosen and Spiegelman, 2006). Adipogenesis is a process in which non-adipocyte cells accumulate lipid and develop into mature adipocytes. Ca2+ plays important role in the regulation of adipogenesis and obesity (Draznin et al., 1988, Ntambi and Takova, 1996, Shi et al., 2000, Szabo et al., 2008). DNA microarray analysis

Calreticulin and cancer

The role of calreticulin in cancer has now been established for several years (Bini et al., 1997, Pike et al., 1998). The protein is highly expressed in the human breast carcinoma and hepatoma cells (Bini et al., 1997, Yu et al., 2000). In human prostate cancer cells, androgen induction increases the mRNA level of calreticulin indicating that the protein might be an androgen-response gene in prostate cancer (Zhu and Wang, 1999). Furthermore, calreticulin has also been identified as a marker for

Calreticulin and autoimmune diseases

Calreticulin has been identified as an antigen in sera from patients suffering from several autoimmune diseases including systemic lupus erythematosus (SLE) (Kishore et al., 1997, van den Berg et al., 1998), celiac disease (Alaedini and Green, 2008), rheumatic disease (Jorgensen et al., 2005) and various parasitic diseases (Nakhasi et al., 1998), which implies a pathological role of calreticulin in autoimmune diseases. Parasite calreticulin also impacts on parasite infectivity by modulating the

Calreticulin and wound healing

Calreticulin-deficient embryonic stem cell-derived neural cells have been shown slower cell migration using the wound healing assay (Rauch et al., 2000). Recent studies indicate that calreticulin induced cell migration, cell proliferation and wound closure in human keratinocytes and fibroblasts (Gold et al., 2006). In mouse and porcine models of skin injury, calreticulin enhances wound healing through a dose-dependent increase in epithelial migration and the formation of granulation tissue (

Crosstalk between calreticulin and transcription factors

Calreticulin is an important modulator for Ca2+ homeostasis in many cells. Since Ca2+ signaling from the ER impacts on many cellular functions through Ca2+-dependent signaling pathway such as Ca2+/calmodulin/calcineurin/NF-AT and Ca2+/Ras/MAPK pathway (Michalak et al., 2002), it is conceivable that calreticulin may modulate the transcriptional regulation. Calreticulin inhibits the steroid-dependent gene expression by direct interaction with the DNA-binding domain of steroid receptors (Burns et

Conclusion

As a multifunctional and multi-compartmental protein, calreticulin is involved in many biological functions. Studies on calreticulin-deficient mice reveal that the protein is important for heart development and embryogenesis. Several transcription factors control expression of calreticulin in many different tissues. Extensive studies on the function of calreticulin reveal that a crosstalk exists between the protein and transcription factors. Not only transcription factors regulate calreticulin

Acknowledgments

We are grateful to Dr. Opas (University of Toronto) for helpful comments and suggestions. Work in our laboratory is supported by grants from CHIR (MOP-53050, MOP-15415, MOP-15291), AHFMR and HSFA. Y.Q. is supported by the Faculty of Medicine and Dentistry 75th Anniversary Scholarship.

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