Calreticulin affects beta-catenin-associated pathways.

Calreticulin, a Ca(2+) 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 beta-catenin, a structural component of cadherin-dependent 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.

Accumulation of unsaturated lipids in monocytes during early phase pyrogen tolerance.

This paper presents data that inspired a new explanation for the mechanism of early phase endotoxin tolerance. Rabbits injected intravenously with LPS from Salmonella abortus developed a two-phase fever (6 h) and monophasic hyperlipidemia of very low density lipoproteins (two consecutive days). If during these days rabbits were injected with the same dose of LPS at 24-h intervals, the second phase of fever disappeared, i.e. early phase pyrogenic tolerance was obtained. This was correlated with a decrease of lipoprotein hyperlipidemia (measured 1.5 h after LPS injection) and an accumulation of lipids rich in double bonds in monocytes (measured 3.5 h after LPS injection). Results showed that the degree of unsaturation of acyl chains (AC) in monocytes (AC/DB, DB=double bonds) is negatively correlated (r=-0.72) with fever response (fever index). The authors maintain that a gradual increase in monocyte membrane fluidity is an adaptation to repeated exposure of monocytes to lipid A and is responsible for the progressive desensitization of monocytes to endotoxin. It is suggested that disorders of this mechanism lead to an accumulation of abnormal quantities of saturated lipids and cholesterol within macrophages, which, as foam cells, are the starting point for atherosclerosis pathology.

Heat shock-regulated expression of calreticulin in retinal pigment epithelium.

Calreticulin is a major Ca2+ binding protein in the endoplasmic reticulum of non-muscle cells. In this report we show that calreticulin protein is strongly induced by heat shock. Activation and attenuation of the heat shock transcriptional response is caused by heat shock factor that binds to 5'-flanking sequences of heat shock responsive genes, the heat shock element. The smallest stretch of DNA that shows detectable binding of heat shock factor in vitro contains a two-sequence unit nGAAnnTTCn which exists in the 5'-flanking region of calreticulin DNA (5'-gGAAccCAGcgTTC-3'). The present data provide direct evidence that calreticulin expression can be modulated by heat shock. Thus, our results strengthen the hypothesis that calreticulin, in addition to its function as a cellular Ca2+ store, is a multifunctional protein which performs at least some of its functions from the lumen of the ER.

Calreticulin modulates cell adhesiveness via regulation of vinculin expression.

Calreticulin is an ubiquitous and highly conserved high capacity Ca(2+)-binding protein that plays a major role in Ca2+ storage within the lumen of the ER. Here, using L fibroblast cell lines expressing different levels of calreticulin, we show that calreticulin plays a role in the control of cell adhesiveness via regulation of expression of vinculin, a cytoskeletal protein essential for cell-substratum and cell-cell attachments. Both vinculin protein and mRNA levels are increased in cells overexpressing calreticulin and are downregulated in cells expressing reduced level of calreticulin. Abundance of actin, talin, alpha 5 and beta 1 integrins, pp125 focal adhesion kinase, and alpha-catenin is not affected by the differential calreticulin expression. Overexpression of calreticulin increases both cell-substratum and cell-cell adhesiveness of L fibroblasts that, most surprisingly, establish vinculin-rich cell-cell junctions. Upregulation of calreticulin also affects adhesion-dependent phenomena such as cell motility (which decreases) and cell spreading (which increases). Downregulation of calreticulin brings about inverse effects. Cell adhesiveness is Ca2+ regulated. The level of calreticulin expression, however, has no effect on either the resting cytoplasmic Ca2+ concentration or the magnitude of FGF-induced Ca2+ transients. Calreticulin, however, participates in Ca2+ homeostasis as its level of expression affects cell viability at low concentrations of extracellular Ca2+. Consequently, we infer that it is not the Ca2+ storage function of calreticulin that affects cell adhesiveness. Neither endogenous calreticulin nor overexpressed green fluorescent protein-calreticulin construct can be detected outside of the ER. Since all of the adhesion-related effects of differential calreticulin expression can be explained by its regulation of vinculin expression, we conclude that it is the ER-resident calreticulin that affects cellular adhesiveness.