Calreticulin and cellular adhesion/migration-specific signalling pathways

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.

Dissecting focal adhesions in cells differentially expressing calreticulin: a microscopy study

BACKGROUND INFORMATION

Our previous studies have shown that calreticulin, a Ca2+-binding chaperone located in the endoplasmic reticulum, affects cell-substratum adhesions via the induction of vinculin and N-cadherin. Cells overexpressing calreticulin contain more vinculin than low expressers and make abundant contacts with the substratum. However, cells that express low levels of calreticulin exhibit a weak adhesive phenotype and make few, if any, focal adhesions. To date, the identity of the types of focal adhesions made by calreticulin overexpressing and low expressing cells has not been dissected.

RESULTS

The results of the present study show that calreticulin affects fibronectin matrix assembly in L fibroblast cell lines that differentially express the protein, and that these cells also differ profoundly in focal adhesion formation. Although the calreticulin overexpressing cells generate numerous interference-reflection-microscopy-dark, vinculin- and paxillin-containing classical focal contacts, as well as some fibrillar adhesions, the cells expressing low levels of calreticulin generate only a few weak focal adhesions. The fibronectin receptor was found to be clustered in calreticulin overexpressing cells, but diffusely distributed over the cell surface in low expressing cells. Plating L fibroblasts on fibronectin-coated substrata induced extensive spreading in all cell lines tested. However, although calreticulin overexpressing cells were induced to form classical vinculin-rich focal contacts, the low calreticulin expressing cells overcame their weak adhesive phenotype by induction of many tensin-rich fibrillar adhesions, thus compensating for the low level of vinculin in these cells.

CONCLUSIONS

We propose that calreticulin affects fibronectin production and, thereby, assembly, and it indirectly influences the formation and/or stability of focal contacts and fibrillar adhesions, both of which are instrumental in matrix assembly and remodelling.

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 affects fibronectin-based cell-substratum adhesion via the regulation of c-Src activity

Calreticulin is an endoplasmic reticulum Ca2+-storage protein, which influences gene expression and cell adhesion. In this study, we show that calreticulin induces fibronectin gene expression and matrix deposition, leading to differences in cell spreading and focal adhesion formation in cells differentially expressing calreticulin. We further show that these effects of calreticulin occur via a c-Src-regulated pathway and that c-Src activity is inversely related to calreticulin abundance. Since c-Src is an important regulator of focal contact turnover, we investigated the effect of c-Src inhibition on cells differentially expressing calreticulin. Inhibition of c-Src rescued the poorly adhesive phenotype of the calreticulin-underexpressing cells in that they became well spread, commenced formation of numerous focal contacts, and deposited a rich fibronectin matrix. Importantly, we show that c-Src activity is dependent on releasable Ca2+ from the endoplasmic reticulum, thus implicating Ca2+-sensitive pathways that are affected by calreticulin in cell-substratum adhesion. We propose that calreticulin affects fibronectin synthesis and matrix assembly via the regulation of fibronectin gene expression. In parallel, calcium-dependent effects of calreticulin on c-Src activity influence the formation and/or stability of focal contacts, which are instrumental in matrix assembly and remodeling.

Partial reversal of transformed fusiform phenotype by overexpression of calreticulin

Calreticulin, a Ca²⁺-storage and chaperone protein of the ER, has also been shown to affect cell adhesiveness. To examine the effects of differential expression of calreticulin on cellular adhesiveness, we used L fibroblast cell lines stably expressing either elevated or reduced amounts of full length, ER-targeted calreticulin. Overexpression of calreticulin correlates with an increase in adhesiveness of L fibroblasts such that these transformed cells acquire epithelioid morphology and form an epithelial-cell sheet when crowded. Functionally, the “reversal” of transformed phenotype in L fibroblasts differentially overexpressing calreticulin can be accounted for by changes in levels of expression of N-cadherin and vinculin. Structurally, however, although the form and extent of cell-cell contacts in L fibroblasts overexpressing calreticulin mimicked those in normal epithelia, electron microscopical examination revealed that cell-cell junctions formed by these transformed cells bore only superficial resemblance to those of normal epithelia in culture. Our data imply that overexpression of calreticulin, while partially reverses fusiform transformed phenotype is in itself insufficient to re-establish bona fide zonulae adherens in transformed fibroblasts.