Calreticulin Affects β-Catenin-associated Pathways

Abstract

Calreticulin, a Ca2؉ storage protein and chaperone in the endoplasmic reticulum, also modulates cell adhesiveness. Overexpression of calreticulin correlates with (i) increased cell adhesiveness, (ii) increased expression of N-cadherin and vinculin, and (iii) decreased protein phosphorylation on tyrosine. Among proteins that are dephosphorylated in cells that overexpress calreticulin is ␤-catenin, a structural component of cadherindependent adhesion complexes, a member of the armadillo family of proteins and a part of the Wnt signaling pathway. We postulate that the changes in cell adhesiveness may be due to calreticulin-mediated effects on a signaling pathway from the endoplasmic reticulum, which impinges on the Wnt signaling pathway via the cadherin/catenin protein system and involves changes in the activity of protein-tyrosine kinases and/or phosphatases. The endoplasmic reticulum (ER)1 plays a critical role in the synthesis and chaperoning of membrane-associated proteins and secreted proteins (1). It is also an important site for the storage and release of Ca 2ϩ (2). In fact, the ER is the major signal transducing organelle within the cell, continuously responding to extracellular stimuli by releasing Ca 2ϩ (3,4). The ER is also sensitive to intracellular signaling; in response to a variety of stimuli signals are transduced from the ER to the cytoplasm, the plasma membrane, and the nucleus, and they affect a variety of cellular activities including Ca 2ϩ influx, protein folding, and synthesis, cholesterol synthesis, and transcription (5-8). Signaling from the extracellular space to the cell interior has long been the subject of intense research. In contrast, little is known about intracellular signaling from the ER. The lumen of the ER is a unique environment, which contains a high concentration of Ca 2ϩ -binding chaperones (2). Calreticulin, a major Ca 2ϩ -binding protein resident in the ER, is ubiquitous in eukaryotic cells (9). It functions as a molecular chaperone (10, 11) and also participates in ER-dependent Ca 2ϩ homeostasis (2, 9). Calreticulin also affects processes external to the ER, such as steroid-sensitive gene expression and cell adhesiveness (12). Although the Ca 2ϩ storage and chaperone functions of calreticulin are consistent with its localization to the lumen of the ER and with its structure, it is not obvious how it can affect cell adhesiveness at the level of the plasma membrane. To investigate the mechanisms behind calreticulindependent modulation of cell adhesiveness, we used well characterized mouse L fibroblasts differentially expressing calreticulin (13-15). Here we show that stable overexpression of fulllength, ER-targeted calreticulin correlates with an increased adhesiveness in transformed fibroblasts, such that their cohesion resembles that of epithelial cells in culture. We also show that changes in the expression of calreticulin affect the tyrosine phosphorylation of cellular proteins, including ␤-catenin. ␤-Catenin is a structural component of cadherin-mediated adhesion comple...

Calreticulin Enhances Porcine Wound Repair by Diverse Biological Effects

“…19,20 Cells underexpressing CRT have increased expression of calmodulin, activated CAMKII (calmodulin-dependent kinase II), and c-src, causing weak adhesion and spreading via focal contacts. 18,20,82 Furthermore, CRT null mouse embryo fibroblasts demonstrate decreased ability to migrate on fibronectin and laminin.…”

“…19,20 Cells underexpressing CRT have increased expression of calmodulin, activated CAMKII (calmodulin-dependent kinase II), and c-src, causing weak adhesion and spreading via focal contacts. 18,20,82 Furthermore, CRT null mouse embryo fibroblasts demonstrate decreased ability to migrate on fibronectin and laminin. 17 Whereas GFP-CRT expression was not localized to focal contacts, one study shows that CRT and another ER-resident protein, RAP (low-density LRP receptor-associated protein) are associated with integrin-based cellular adhesion complexes that link to the cytoskeleton, containing such signaling proteins as vinculin, talin, paxillin, focal adhesion kinase, and ␣-actinin.…”

“…16 -18 An increase in vinculin and N-cadherin and a decrease in tyrosine phosphorylation of ␤-catenin are other adhesion related proteins/events that CRT is deemed to regulate from the ER in a calcium-dependent manner. 2,19,20 Apart from the recognized inside-out cellular signaling, CRT has been shown to engage in outside-in signaling through its presence on the cell surface. [21][22][23][24][25][26] As a cell surface protein without signaling capability, CRT has been shown to bind to the promiscuous low-density lipoprotein receptor-related protein (LRP/CD91/␣-2 macroglobulin receptor) and to thrombospondin-1 (TSP-1), a protein engaged in actin cytoskeletal and focal adhesion reorganization.…”

Overexpression of Calreticulin Modulates Protein Kinase B/Akt Signaling to Promote Apoptosis during Cardiac Differentiation of Cardiomyoblast H9c2 Cells

“…Recently, it was reported that CRT expression levels can modulate intracellular signaling, including the ␤-catenin pathway, by altering protein-tyrosine kinase or phosphatases (40). It was observed that overexpression of CRT in mouse L fibroblasts decreased protein phosphorylation at tyrosine and also that the dephosphorylated protein was ␤-catenin, although the mechanism of the CRT-dependent modulation of tyrosine dephosphorylation was not made clear.…”

Partial reversal of transformed fusiform phenotype by overexpression of calreticulin

“…Both types of adhesions universally share the cytoskeletal protein, vinculin [5], expression of which is affected by the level of calreticulin expression [6,7]. Changes in the level of calreticulin expression also affect expression of the intercellular junction protein, N-cadherin [8]. It is unclear whether the effects of calreticulin on cellular adhesiveness are mediated by its role in Ca 2+ -homeostasis or by its chaperone function.…”