Dynamics of Calcium Fluxes in Nonexcitable Cells: Mathematical Modeling

Endoplasmic Reticulum Calcium Pumps and Tumor Cell Differentiation

Endoplasmic reticulum (ER) calcium homeostasis plays an essential role in cellular calcium signaling, intra-ER protein chaperoning and maturation, as well as in the interaction of the ER with other organelles. Calcium is accumulated in the ER by sarco/endoplasmic reticulum calcium ATPases (SERCA enzymes) that generate by active, ATP-dependent transport, a several thousand-fold calcium ion concentration gradient between the cytosol (low nanomolar) and the ER lumen (high micromolar). SERCA enzymes are coded by three genes that by alternative splicing give rise to several isoforms, which can display isoform-specific calcium transport characteristics. SERCA expression levels and isoenzyme composition vary according to cell type, and this constitutes a mechanism whereby ER calcium homeostasis is adapted to the signaling and metabolic needs of the cell, depending on its phenotype, its state of activation and differentiation. As reviewed here, in several normal epithelial cell types including bronchial, mammary, gastric, colonic and choroid plexus epithelium, as well as in mature cells of hematopoietic origin such as pumps are simultaneously expressed, whereas in corresponding tumors and leukemias SERCA3 expression is selectively down-regulated. SERCA3 expression is restored during the pharmacologically induced differentiation of various cancer and leukemia cell types. SERCA3 is a useful marker for the study of cell differentiation, and the loss of SERCA3 expression constitutes a previously unrecognized example of the remodeling of calcium homeostasis in tumors.

Sodium is not required for chloride efflux via chloride/bicarbonate exchanger from rat thymic lymphocytes

Sodium-dependent Cl⁻/HCO₃⁻ exchanger acts as a chloride (Cl⁻) efflux in lymphocytes. Its functional characterization had been described when Cl⁻ efflux was measured upon substituting extracellular sodium (Na⁺) by N-methyl-D-glucamine (NMDG). For Na⁺ and Cl⁻ substitution, we have used D-mannitol or NMDG. Thymocytes of male Wistar rats aged 7–9 weeks were used and intracellular Cl⁻ was measured by spectrofluorimetry using MQAE dye in bicarbonate buffers. Chloride efflux was measured in a Cl⁻-free buffer (Cl⁻ substituted with isethionate acid) and in Na⁺ and Cl⁻-free buffer with D-mannitol or with NMDG. The data have shown that Cl⁻ efflux is mediated in the absence of Na⁺ in a solution containing D-mannitol and is inhibited by H₂DIDS. Mathematical modelling has shown that Cl⁻ efflux mathematical model parameters (relative membrane permeability, relative rate of exchanger transition, and exchanger efficacy) were the same in control and in the medium in which Na⁺ had been substituted by D-mannitol. The net Cl⁻ efflux was completely blocked in the NMDG buffer. The same blockage of Cl⁻ efflux was caused by H₂DIDS. The study results allow concluding that Na⁺ is not required for Cl⁻ efflux via Cl⁻/HCO₃⁻ exchanger. NMDG in buffers cannot be used for substituting Na⁺ because NMDG inhibits the exchanger.

Thrombin-stimulated discharge of calcium stores in human platelets: analysis of experimental data

The purpose of this research was to analyze experimental data concerning thrombin-stimulated discharge of calcium stores in human platelets contained in calcium-free medium in view of better understanding the mechanisms involved in calcium fluxes. The model curves are reasonably close to experimental data; the parameters of the models are related to the properties of the entities responsible for control or maintenance of cytosolic calcium concentration. It has been shown that: (a) time-course of calcium concentration in cytosol of human platelets can be acceptably modeled on the basis of reasonable assumptions concerning agonist stimulated calcium redistribution in cellular compartments; (b) those assumptions are of fundamental importance for the model (c) some parameters of the model (taken arbitrarily) cannot be estimated independently of others from fitting the model to experimental data available; (d) special experiments are necessary to determine the unknown parameters; (e) agonist-stimulated change of the permeability of endomembrane of calcium stores can be regarded as a pulse of the permeability; it can be modeled as a sequence of transitions of the system from inactive to active and to inactive state again.

Endoplasmic reticulum calcium pumps and cancer cell differentiation

The endoplasmic reticulum (ER) is a major intracellular calcium storage pool and a multifunctional organelle that accomplishes several calcium-dependent functions involved in many homeostatic and signaling mechanisms. Calcium is accumulated in the ER by Sarco/Endoplasmic Reticulum Calcium ATPase (SERCA)-type calcium pumps. SERCA activity can determine ER calcium content available for intra-ER functions and for calcium release into the cytosol, and can shape the spatiotemporal characteristics of calcium signals. SERCA function therefore constitutes an important nodal point in the regulation of cellular calcium homeostasis and signaling, and can exert important effects on cell growth, differentiation and survival. In several cell types such as cells of hematopoietic origin, mammary, gastric and colonic epithelium, SERCA2 and SERCA3-type calcium pumps are simultaneously expressed, and SERCA3 expression levels undergo significant changes during cell differentiation, activation or immortalization. In addition, SERCA3 expression is decreased or lost in several tumor types when compared to the corresponding normal tissue. These observations indicate that ER calcium homeostasis is remodeled during cell differentiation, and may present defects due to decreased SERCA3 expression in tumors. Modulation of the state of differentiation of the ER reflected by SERCA3 expression constitutes an interesting new aspect of cell differentiation and tumor biology.

Calcium fluxes into and out of cytosol in human platelets: analysis of experimental data

The purpose of this research was to analyse experimental data concerning cytosolic calcium concentration in view of the mechanisms involved in calcium fluxes in human platelets. The parameters of model curves are related to the properties of the entities responsible for control or maintenance of cytosolic calcium concentration. It has been shown that: (a) biphasicity of increase in cytosolic calcium concentration caused by inhibition of SERCAs either by TBHQ and TG or by TG alone is related to fast and slow discharge of acidic calcium stores and DTS; (b) biphasicity of decline in cytosolic calcium concentration after its rise caused by stimulation of platelets by the agonists is related to non-synchronous extrusion of calcium by PMCA and NCX; (c) NCX is active only in calcium containing medium: calcium ion(s) are necessary to be bound to the site(s) located on the medium-facing side of the (macro)molecule; (d) PMCA is likely to be activated either by binding calcium ion(s) to the site(s) located on its cytosol-facing side or by unbinding identical ion(s) from the site(s) on its medium-facing side.

Altered Endoplasmic Reticulum Calcium Pump Expression during Breast Tumorigenesis

Endoplasmic reticulum calcium homeostasis is involved in several essential cell functions including cell proliferation, protein synthesis, stress responses or secretion. Calcium uptake into the endoplasmic reticulum is performed by Sarco/Endoplasmic Reticulum Calcium ATPases (SERCA enzymes). In order to study endoplasmic reticulum calcium homeostasis in situ in mammary tissue, in this work SERCA3 expression was investigated in normal breast and in its benign and malignant lesions in function of the cell type, degree of malignancy, and histological and molecular parameters of the tumors. Our data indicate, that although normal breast acinar epithelial cells express SERCA3 abundantly, its expression is strongly decreased already in very early non-malignant epithelial lesions such as adenosis, and remains low in lobular carcinomas. Whereas normal duct epithelium expresses significant amounts of SERCA3, its expression is decreased in several benign ductal lesions, as well as in ductal adenocarcinoma. The loss of SERCA3 expression is correlated with Elston-Ellis grade, negative hormone receptor expression or triple negative status in ductal carcinomas. The concordance between decreased SERCA3 expression and several histological, as well as molecular markers of ductal carcinogenesis indicates that endoplasmic reticulum calcium homeostasis is remodeled during tumorigenesis in the breast epithelium.

Endoplasmic reticulum calcium pumps and cancer

Endoplasmic reticulum calcium homeostasis is involved in a multitude of signaling, as well as "house-keeping" functions that control cell growth, differentiation or apoptosis in every human/eukaryotic cell. Calcium is actively accumulated in the endoplasmic reticulum by Sarco/Endoplasmic Reticulum Calcium transport ATPases (SERCA enzymes). SERCA-dependent calcium transport is the only calcium uptake mechanism in this organelle, and therefore the regulation of SERCA function by the cell constitutes a key mechanism to adjust calcium homeostasis in the endoplasmic reticulum depending on the cell type and its state of differentiation. The direct pharmacological modulation of SERCA activity affects cell differentiation and survival. SERCA expression levels can undergo significant changes during cell differentiation or tumorigenesis, leading to modified endoplasmic reticulum calcium storage. In several cell types such as cells of hematopoietic origin or various epithelial cells, two SERCA genes (SERCA2 and SERCA3) are simultaneously expressed. Expression levels of SERCA3, a lower calcium affinity calcium pump are highly variable. In several cell systems SERCA3 expression is selectively induced during differentiation, whereas during tumorigenesis and blastic transformation SERCA3 expression is decreased. These observations point at the existence of a cross-talk, via the regulation of SERCA3 levels, between endoplasmic reticulum calcium homeostasis and the control of cell differentiation, and show that endoplasmic reticulum calcium homeostasis itself can undergo remodeling during differentiation. The investigation of the anomalies of endoplasmic reticulum differentiation in tumor and leukemia cells may be useful for a better understanding of the contribution of calcium signaling to the establishment of malignant phenotypes.

Modulation of B-cell endoplasmic reticulum calcium homeostasis by Epstein-Barr virus latent membrane protein-1

Background

Calcium signaling plays an important role in B lymphocyte survival and activation, and is critically dependent on the inositol-1,4,5-tris-phosphate-induced release of calcium stored in the endoplasmic reticulum (ER). Calcium is accumulated in the ER by Sarco/Endoplasmic Reticulum Calcium ATPases (SERCA enzymes), and therefore these enzymes play an important role in ER calcium homeostasis and in the control of B of cell activation. Because Epstein-Barr virus (EBV) can immortalize B cells and contributes to lymphomagenesis, in this work the effects of the virus on SERCA-type calcium pump expression and calcium accumulation in the endoplasmic reticulum of B cells was investigated.

Results

Two Sarco-Endoplasmic Reticulum Calcium transport ATPase isoforms, the low Ca²⁺-affinity SERCA3, and the high Ca²⁺-affinity SERCA2 enzymes are simultaneously expressed in B cells. Latency type III infection of Burkitt's lymphoma cell lines with immortalization-competent virus expressing the full set of latency genes selectively decreased the expression of SERCA3 protein, whereas infection with immortalization-deficient virus that does not express the EBNA2 or LMP-1 viral genes was without effect. Down-modulation of SERCA3 expression could be observed upon LMP-1, but not EBNA2 expression in cells carrying inducible transgenes, and LMP-1 expression was associated with enhanced resting cytosolic calcium levels and increased calcium storage in the endoplasmic reticulum. Similarly to virus-induced B cell immortalisation, SERCA3 expression was also decreased in normal B cells undergoing activation and blastic transformation in germinal centers of lymph node follicles.

Conclusion

The data presented in this work indicate that EBV-induced immortalization leads to the remodelling of ER calcium homeostasis of B cells by LMP-1 that copies a previously unknown normal phenomenon taking place during antigen driven B cell activation. The functional remodelling of ER calcium homeostasis by down-regulation of SERCA3 expression constitutes a previously unknown mechanism involved in EBV-induced B cell immortalisation.