Controlled proteolysis of Ca(2+)-ATPases in human platelet and non-muscle cell membrane vesicles. Evidence for a multi-sarco/endoplasmic reticulum Ca(2+)-ATPase system

How many Ca2+ATPase isoforms are expressed in a cell type? A growing family of membrane proteins illustrated by studies in platelets

Ca(2+) signaling plays a key role in normal and abnormal platelet functions. Understanding platelet Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are Ca(2+)ATPases or Ca(2+) pumps that deplete the cytosol of Ca(2+) ions. Here, we will particularly focus on two Ca(2+) pump families: the plasma membrane Ca(2+)ATPases (PMCAs) that extrude cytosolic Ca(2+) towards the extracellular medium and the sarco/endoplasmic reticulum Ca(2+)ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). In the present review, we will summarize data on platelet Ca(2+)ATPases including their identification and biogenesis. First of all, we will present the Ca(2+)ATPase genes and their isoforms expressed in platelets. We will especially focus on a member of the SERCA family, SERCA3, recently found to give rise to a number of species-specific isoforms. Next, we will describe the differences in Ca(2+)ATPase patterns observed in human and rat platelets. Last, we will analyze how the expression of Ca(2+)ATPase isoforms changes during megakaryocytic maturation and show that megakaryocytopoiesis is associated with a profound reorganization of the expression and/or activity of Ca(2+)ATPases. Taken together, these data provide new aspects of investigations to better understand normal and abnormal platelet Ca(2+) signaling.

Expression des pompes calciques de type SERCA au cours de la différenciation cellulaire et de la tumorigenèse: application à la carcinogenèse colique

Calcium homeostasis of the endoplasmic reticulum (ER) is involved in intracellular signaling pathways and is implicated in major cell functions such as cell growth, differentiation, protein synthesis and apoptosis. The accumulation of calcium in the ER is performed by specific sarco/endoplasmic reticulum calcium transport ATPases (SERCA iso-enzymes). The expression of biochemically distinct SERCA isoforms is cell type dependent and developmentally regulated. This review summarizes pertinent data about the modulation of the expression of SERCA enzymes during the differentiation of normal and tumor cells. These data support the implication of SERCA pumps and especially SERCA3 in the differentiation program of cancer and leukemia cells. During the multi-step process of colon carcinogenesis, the decrease of SERCA3 expression seems to be linked to enhanced APC/ss-catenin/TCF4 signaling and deficient Sp1-like factor-dependent transcription.

Identification, Expression, Function, and Localization of a Novel (Sixth) Isoform of the Human Sarco/Endoplasmic Reticulum Ca2+ATPase 3 Gene

Understanding of Ca(2+) signaling requires the knowledge of proteins involved in this process. Among these proteins are sarco/endoplasmic reticulum Ca(2+)-ATPases (SERCAs) that pump Ca(2+) into the endoplasmic reticulum (ER). Recently, the human SERCA3 gene was shown to give rise to five isoforms (SERCA3a-e (h3a-h3e)). Here we demonstrate the existence of an additional new member, termed SERCA3f (h3f). By reverse transcriptase-PCR using monocytic U937 cell RNA, h3f mRNA was found to exclude the antepenultimate exon 21. h3f mRNA expression appeared as a human-specific splice variant. It was not found in rats or mice. h3f mRNA gave rise to an h3f protein differing in its C terminus from h3a-h3e. Of particular interest, h3f diverged in the first amino acids after the first splice site but presented the same last 21 amino acids as h3b. Consequently, we further investigated the structure-function-location relationships of the h3b and h3f isoforms. Comparative functional study of h3b and h3f recombinant proteins in intact HEK-293 cells and in fractionated membranes showed the following distinct characteristics: (i) resting cytosolic Ca(2+) concentration ([Ca(2+)](c)) and (ii) ER Ca(2+) content ([Ca(2+)](er)); similar characteristics were shown for the following: (i) the effects of the SERCA inhibitor, thapsigargin, on Ca(2+) release ([Ca(2+)](Tg)) and subsequent Ca(2+) entry ([Ca(2+)](e)) and (ii) the low apparent Ca(2+) affinity and the enhanced rate of dephosphorylation of the E(2)P phosphoenzyme intermediate. Subcellular location of h3b and h3f by immunofluorescence and/or confocal microscopy using the h3b- and h3f-specific polyclonal and the pan-h3 monoclonal (PL/IM430) antibodies suggested overlapping but distinct ER location. The endogenous expression of h3f protein was also proved in U937 cells. Altogether these data suggest that the SERCA3 isoforms have a more widespread role in cellular Ca(2+) signaling than previously appreciated.

Three novel sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) 3 isoforms. Expression, regulation, and function of the membranes of the SERCA3 family

Sarco/endoplasmic reticulum Ca2+-ATPases (SERCAs) pump Ca2+ into the endoplasmic reticulum. Recently, three human SERCA3 (h3a-c) proteins and a previously unknown rat SERCA3 (r3b/c) mRNA have been described. Here, we (i) document two novel human SERCA3 splice variants h3d and h3e, (ii) provide data for the expression and mechanisms regulating the expression of all known SERCA3 variants (r3a, r3b/c, and h3a-e), and (iii) show functional characteristics of the SERCA3 isoforms. h3d and h3e are issued from the insertion of an additional penultimate exon 22 resulting in different carboxyl termini for these variants. Distinct distribution patterns of the SERCA3 gene products were observed in a series of cell lines of hematopoietic, epithelial, embryonic origin, and several cancerous types, as well as in panels of rat and human tissues. Hypertension and protein kinase C, calcineurin, or retinoic acid receptor signaling pathways were found to differently control rat and human splice variant expression, respectively. Stable overexpression of each variant was performed in human embryonic kidney 293 cells, and the SERCA3 isoforms were fully characterized. All SERCA3 isoforms were found to pump Ca2+ with similar affinities. However, they modulated the cytosolic Ca2+ concentration ([Ca2+]c) and the endoplasmic reticulum Ca2+ content ([Ca2+]er) in different manners. A newly generated polyclonal antibody and a pan-SERCA3 antibody proved the endogenous expression of the three novel SERCA3 proteins, h3d, h3e, and r3b/c. All these data suggest that the SERCA3 gene products have a more widespread role in cellular Ca2+ signaling than previously appreciated.

Platelet Ca(2+)ATPases : a plural, species-specific, and multiple hypertension-regulated expression system

Gaining insight into nonmuscle Ca(2+) signaling requires basic knowledge of the major structures involved. We investigated the expression of platelet Ca(2+)ATPases in normal and hypertension-associated abnormal Ca(2+) signaling. First, overall identification of normotensive Wistar-Kyoto rat Ca(2+)ATPases was attempted by looking for newly described human platelet 3'-end alternatively spliced sarco/endoplasmic reticulum Ca(2+)ATPases (SERCA) 3b mRNA and plasma membrane Ca(2+)ATPase (PMCA) 1b and 4b proteins, in addition to SERCA2b and SERCA3a isoforms. For SERCAs, comparative analyses of human and Wistar-Kyoto rat SERCA3 platelet mRNA by reverse transcription-polymerase chain reaction (RT-PCR) followed by sequencing established that human platelets coexpressed SERCA3b and a third SERCA3c, while rat cells were devoid of them but expressed a still unknown splice variant that we termed rSERCA3b/3c. Its identification using 3'-end SERCA3 gene and rapid amplification of cDNA ends (RACE)-PCR studies showed that it results from an additional SERCA3 alternative splicing process, which uses a second alternative polyadenylation site located in the last intron. For PMCAs, with the use of gene-specific RT-PCR followed by sequencing and Western blotting using 5F10 monoclonal antibody, expression of human and rat platelet PMCA1b and PMCA4b was similar. Second, comparative analysis of these newly identified Ca(2+)ATPases and SERCA3a in age-matched spontaneously hypertensive rat platelets demonstrated (1) a marked downregulation of rSERCA3b/3c, which became null, and a 1.71-fold increase in SERCA3a and (2) an opposite regulation of the 2 PMCAs, namely, a 3.3-fold decrease in PMCA1b mRNA and a 3.7-fold increase in PMCA4b mRNA. Hence, platelets coexpress multiple, diverse, and species-specific Ca(2+)ATPases, including a novel fourth SERCA3. Moreover, expression of PMCA (1b and 4b), SERCA3a, and rSERCA3b/3c was modulated in rat hypertension. Hence, Ca(2+)ATPases should be regarded as constituting a new rational basis for the understanding of nonmuscle cell Ca(2+) signaling.

Functional characterization of alternatively spliced human SERCA3 transcripts

The sarcoplasmic (or endoplasmic) reticulum Ca2+-ATPase (SERCA)-3 has been implicated in the possible dysregulation of Ca2+ homeostasis that accompanies the pathology of hypertension and diabetes. We report the molecular cloning of two alternatively spliced transcripts from the human SERCA3 gene, ATP2A3, that encode proteins that differ at their carboxy termini by 36 amino acids. SERCA3 transcripts were most abundantly expressed in lymphoid tissues, intestine, pancreas, and prostate. The two human SERCA3 proteins encoded by alternatively spliced transcripts were recognized by the monoclonal antibody PL/IM430 and demonstrated Ca2+ uptake and ATPase activity with an apparent Ca2+ affinity 0.5 pCa unit lower than that of other SERCA gene products. The subcellular distribution of SERCA3 protein was indistinguishable from that of SERCA2b, with expression in the nuclear envelope and in the endoplasmic reticulum throughout the cell. Two variant SERCA3 constructs, huS3-I and huS3-II, were isolated that encode proteins with three amino acid differences: Ala-673 (in huS3-I) substituted for Thr (in huS3-II), Ile-817 substituted for Met, and an insertion of Glu-994. huS3-I displayed a 10-fold lower capacity to transport Ca2+ than huS3-II.

Subplasmalemmal Ca-stores in Paramecium tetraurelia. Identification and characterisation of a sarco(endo)plasmic reticulum-like Ca(2+)-ATPase by phosphoenzyme intermediate formation and its inhibition by caffeine

Considering increasing interest in calcium stores in protozoa, including parasitic forms, and specifically in subplasmalemmal stores in higher eukaryotes, we have isolated subplasmalemmal calcium stores (alveolar sacs) from the ciliated protozoan, Paramecium tetraurelia. Using antibodies against established sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCAs) we detected in Western blots of subcellular fractions a band of approximately 106 kDa size selectively in alveolar sacs--but not, for example, in plasma membranes--and concomitant restriction of immunofluorecence labelling to the cell cortex of permeabilised cells. These results are the same as with ABs against a peptide derived from a cloned SERCA-like gene from Paramecium [Hauser K., Pavlovic N., Kissmehl R., Plattner H. Molecular characterization of a sarco(endo)plasmic reticulum Ca(2+)-ATPase gene from Paramecium tetraurelia and localisation of its gene product to subplasmalemmal calcium stores. Biochem J 1998; 334: 31-38]. When such isolated alveolar sacs were now tested for phosphoenzyme intermediate (EP) formation, a phosphoprotein of the same apparent molecular mass (approximately 106 kDa) as in blots could be identified in gel autoradiograms. This EP corresponds to that formed in the reaction cycle of different SERCA-types, with dependency on Ca2+ and Mg2+, sensitivity to La3+ or insensitivity towards calmodulin, calmodulin antagonists and vanadate. However, EP formation in alveolar sacs is not inhibited by established SERCA inhibitors (e.g. thapsigargi[ci]n tested up to 100 microM). Surprisingly, caffeine, which is frequently used to mobilise Ca2+ from intracellular stores, strongly inhibits EP formation. In parallel experiments, we did not find any similar effect with sarcoplasmic reticulum isolated from skeletal muscle. We conclude that the approximately 106 kDa protein of alveolar sacs in Paramecium may represent a SERCA-like Ca(2+)-ATPase with some unorthodox features, which might be relevant also for some other protozoan systems. In this case, the established Ca(2+)-mobilizing effect of caffeine may be amplified by inhibiting store refilling.

Expression of hPMCA4b, the major form of the plasma membrane calcium pump in megakaryoblastoid cells is greatly reduced in mature human platelets

Antibodies 5F10 and JA3 (raised against the erythrocyte Ca2+ pump) were used to identify hPMCA4b as the major form of the plasma membrane Ca2+ pump in human platelets and in three human megakaryoblastoid cell lines, MEG 01, DAMI and CHRF 288-11. 5F10 was used because it has been shown to recognize all known isoforms of the hPMCA and JA3 because it reacts exclusively with hPMCA4b [Caride A.J., Filoteo A.G., Enyedi A., Verma A.K., Penniston J.T. Detection of isoform 4 of the plasma membrane calcium pump in human tissues by using isoform-specific monoclonal antibodies. Biochem J 1996; 316: 353-359]. In addition to hPMCA4b, hPMCA1b was also detected in the megakaryoblastoid cells by using isoform-specific polyclonal antibodies. The apparent size of this isoform, however, was smaller than that seen in HeLa and COS-7 cell membranes indicating the presence of a modified form of hPMCA1b. In platelets, no evidence of the expression of hPMCA1b could be found. The amount of PMCA in these cells was compared with that of the constitutive form of the sarco/endoplasmic reticulum Ca2+ pump in non-muscle cells (SERCA2b) and also with the amount of PMCA in human erythrocytes. A very low level of the plasma membrane Ca2+ pump was found in platelets while in their precursor cells the expression of this Ca2+ pump was much more abundant. Whereas the expression level of PMCA decreased dramatically in mature human platelets, the expression of SERCA2b did not change substantially upon megakaryocytic differentiation.

Inter-individual variability in Ca2+ signalling in platelets from healthy volunteers: effects of aspirin and relationship with expression of endomembrane Ca2+-ATPases

Increased Ca2+ signal generation may lead to hyperactivity of platelets and contribute to thrombotic complications. Using fura-2-loaded platelets from 51 healthy volunteers, high variability was detected in the Ca2+ responses evoked by the receptor agonists, thrombin and collagen, and the inhibitor of sarco/endoplasmic reticulum Ca2+-ATPases (SERCA), thapsigargin (Tg). Oral intake of 500mg aspirin reduced the magnitude of the Ca2+ responses, and lowered the intra-individual coefficients of variance of the responses by 50%. However, the corresponding inter-individual variance coefficients were only a little influenced by aspirin intake, pointing to subject-dependent factors in Ca2+ handling that are unrelated to thromboxane formation. With each agonist, 6-9% of the subjects had platelets with relatively high Ca2+ responses (> mean + SD) both before and after aspirin intake. In 90% (9/10) of these cases the high responsiveness was confirmed in platelets obtained 6-12 months later. The Tg- but not thrombin-induced Ca2+ responses correlated inversely with the expression levels of SERCA PL/IM 430 (SERCA-3b) in platelets. After aspirin intake, the Ca2+ responses with collagen but not thrombin correlated inversely with SERCA-2b expression. These results suggest that, in the absence of potentiating effects of thromboxane, (i) the amount of PL/IM 430-recognizable SERCA may control the Ca2+ signal when SERCA-2b is specifically inhibited (with Tg), and (ii) the expression of SERCA-2b determine the collagen- but not the thrombin-evoked Ca2+ signal. Accordingly, limited Ca2+-pumping activity by low expression of one of the SERCA isoforms is likely to be one of the factors resulting in increased platelet activity towards collagen or thapsigargin but not thrombin.

A tool coming of age: thapsigargin as an inhibitor of sarco-endoplasmic reticulum Ca(2+)-ATPases

Thapsigargin is the most widely used inhibitor of the ubiquitous sarco-endoplasmic reticulum Ca(2+)-ATPases in mammalian cells. Over the past ten years, this guaianolide compound of plant origin has become a popular tool in a host of studies directed at elucidating the mechanisms of intracellular Ca2+ signalling. Its remarkable potency and selectivity have been instrumental in widening our view of the function of intracellular Ca2+ stores to include such key aspects as store-operated Ca2+ entry or the involvement of the stores in protein synthesis or cell growth. In this article Marek Treiman, Casper Caspersen and Søren Brøgger Christensen review the key pharmacological features of thapsigargin action; they also discuss some of the ways in which its unique properties have shown to be important for obtaining new insights into the biology of Ca2+ stores, and how these properties might encompass a therapeutic potential. In parallel, attention is drawn to some of the limitations and pitfalls encountered when working with thapsigargin.

Expression of two isoforms of the third sarco/endoplasmic reticulum Ca2+ ATPase (SERCA3) in platelets. Possible recognition of the SERCA3b isoform by the PL/IM430 monoclonal antibody

Human platelets express several sarco/endoplasmic reticulum Ca2+ ATPase (SERCA) isoenzymes: SERCA2b of 100 kDa apparent molecular mass and two distinct enzymes of 97 kDa, one of them identified as being the SERCA3a isoform. The molecular identity of the third enzyme specifically recognized by the PL/IM430 monoclonal antibody has remained elusive. First, the study of the 3'-end part of platelet SERCA3 mRNA, by means of RT-PCR amplification using sets of primers covering the N-3 to N (ultimate) exons of the human SERCA3 sequence, revealed the presence of two distinct mRNA sequences, SERCA3a and a longer variant. Second, this additional sequence was identified as SERCA3b and found to refer to the insertion of a new exon of 73 bp, located at bp 349 from the beginning of the intronic sequence, linking the penultimate (N-1) exon to the last exon (N) of the human SERCA3 gene. Third, a relationship between the expression of this SERCA3b mRNA and the PL/ IM430 recognizable SERCA protein was observed. SERCA3b mRNA was found to be absent in epithelial HeLa cells not recognized by the PL/IM430 antibody and the expression of this SERCA3b RNA species correlated with that of the SERCA protein recognized by PL/IM430 which was down-modulated in the platelet precursor megakaryocytic CHRF 288-11 cell line as well as upon in vitro lymphocyte activation. Taken together, these results strongly support the notion of the presence of the SERCA3b protein in human cells by showing SERCA3b mRNA in platelets and the fact that the protein corresponding to this mRNA species is very likely the 97 kDa protein recognized by the PL/IM430 antibody.

Ca2+ in the dense tubules: a model of platelet Ca2+ load

In this work, we explored the relationship between the freely exchangeable Ca2+ (FECa2+) in the dense tubules (DT) and the sarco(endo)plasmic reticulum (SER) Ca2+-ATPase (SERCA) in circulating human platelets and examined the relationship between blood pressure (BP) and these platelet parameters. Studying platelets from 32 healthy men, we showed that the maximal reaction velocity (Vmax) of the SERCA significantly correlated with FECa2+ in the DT and with the protein expressions of SERCA 2 and 3. BP positively correlated with both the Vmax of the SERCA (r=.462, P=.010) and the FECa2+ sequestered in the DT (r=.492, P=.005). The relationships between these platelet Ca2+ parameters and BP were in part confounded by increased levels of serum triglycerides and diminished HDL cholesterol with a higher BP. No correlation was observed between the resting cytosolic Ca2+ and BP. Collectively, these findings indicate that (1) an increase in the cellular Ca2+ load in platelets is expressed by a higher activity of the SERCA and an increase in the expressions of SERCA 2 and 3 proteins, coupled with an increase in the FECa2+ in the DT, and (2) a higher BP is associated with an increase in platelet Ca2+ load in human beings, expressed by a rise in the FECa2+ in the DT and the upregulation of SERCA activity.

Modulation of endoplasmic reticulum calcium pump expression during T lymphocyte activation

Calcium mobilization from intracellular storage organelles is a key component of the second messenger system inducing cell activation. Calcium transport ATPases associated with intracellular calcium storage organelles play a major role in controlling this process by accumulating calcium from the cytosol into intracellular calcium pools. In this study the modulation of the expression of the sarco-endoplasmic reticulum calcium transport ATPase (SERCA) isoenzymes has been studied in lymphocytes undergoing phorbol myristate acetate and ionomycin-induced activation. In several T lymphocyte cell lines a combined treatment by the two drugs resulted in an approximately 90% decrease of the expression of the calcium pump isoform recognized by the PLIM430 isoform-specific antibody, whereas the expression of the SERCA 2b isoform was increased approximately 2-fold. Phorbol ester or ionomycin applied separately was ineffective. In Jurkat T cells the down-modulation of expression of the SERCA isoform recognized by the PLIM430 antibody appeared concomitantly with the induction of interleukin-2 expression and could be inhibited by the immunosuppressant drug cyclosporine-A. These data indicate that T cell activation induces a selective and cyclosporine-A-sensitive modulation of the expression of the SERCA calcium pump isoforms. This reflects a profound reorganization of the calcium homeostasis of T cells undergoing activation and may open new avenues in the understanding of the plasticity of the calcium homeostasis of differentiating cells and in the pharmacological modulation of lymphocyte function.

The endoplasmic reticulum in PC12 cells. Evidence for a mosaic of domains differently specialized in Ca2+ handling

Velocity and isopycnic gradient centrifugation were employed to fractionate post-nuclear supernatants rapidly prepared from PC12 cells in order to characterize areas of the endoplasmic reticulum involved in various aspects of intracellular Ca2+ homeostasis. The endoplasmic reticulum Ca2+ pumping activity, defined by three properties studied in parallel in the isolated fractions; thapsigargin-sensitive uptake of 45Ca2+, Ca2+-dependent, thapsigargin-sensitive protein phosphorylation and Western blotting of sarcoplasmic reticulum calcium ATPase (SERCA) 2b and putative SERCA3 ATPases, was concentrated primarily in a few fractions located at the top and toward the bottom of velocity and isopycnic gradients, respectively. The endoplasmic reticulum Ca2+ release channel, the inositol 1,4,5-trisphosphate receptor, was concentrated in the same fractions as the Ca2+ pumps, and additionally in a few fractions distinctly poor in SERCAs. In contrast, two lumenal markers (protein disulfide isomerase and calreticulin, the major Ca2+ storage protein of non-muscle endoplasmic reticulum) were enriched in the middle fractions of the velocity gradients while calnexin, a Ca2+-binding membrane protein, was more widely distributed throughout the gradients. These results document a considerable degree of functional and compositional heterogeneity in the endoplasmic reticulum of neurosecretory PC12 cells. Even in the limited areas that appear specialized for rapid Ca2+ uptake and release the ratio between pumps and channels varies considerably. Within the rest of the system, insulated from short-term fluctuations of Ca2+ concentration, Ca2+-binding proteins appear to be extensively distributed, in agreement with the idea that the Ca2+ content of the endoplasmic reticulum serves multiple functions.

Thapsigargin discriminates strongly between Ca(2+)-ATPase phosphorylated intermediates with different subcellular distributions in bovine adrenal chromaffin cells

We studied the effects of thapsigargin on the formation of the phosphorylated intermediates (E approximately Ps) of endoplasmic reticulum Ca(2+)-ATPases in microsomes from bovine adrenal medulla. When submicrosomal fractions were separated on a sucrose gradient, two components of 100 kDa Ca(2+)-ATPase E approximately P displaying distinct subcellular distributions were resolved. The first component was defined by Ca(2+)-induced protection against thapsigargin inhibition. The second component did not display such protection, with a 3 orders of magnitude difference in thapsigargin inhibitory potency towards the 2 components. In the absence of Ca2+, both E approximately P components were highly sensitive to thapsigargin inhibition, revealing the presence of high-affinity thapsigargin-binding sites characteristic of SERCA ATPases. These data demonstrate a new level of molecular heterogeneity among Ca(2+)-ATPases of endoplasmic reticulum, and provide the first evidence of differential subcellular localization of individual Ca2+ pump subtypes in cells of neural origin.

The SERCA3-type of organellar Ca2+ pumps

Of all the SERCA pumps, SERCA3 was the latest to be described and the least well known. Its primary structure deviates more than usual from the other members of the SERCA family. It is not known whether its remarkably low affinity for Ca2+ (K0.5 > 1 microM) observed upon expression in the COS cell system occurs also in its normal cellular context. SERCA3 is particularly expressed at high levels in different types of blood cells and related cells like platelets, lymphocytes, mast cells and arterial endothelial cells. It is also found in cerebellar Purkinje neurons. The physiological significance of this expression pattern remains unknown.

Localization and quantification of endoplasmic reticulum Ca(2+)-ATPase isoform transcripts

The Ca(2+)-adenosinetriphosphatase pump of the sarcoplasmic or endoplasmic reticulum (SERCA) plays a critical role in Ca2+ signaling and homeostasis in all cells and is encoded by a family of homologous and alternatively spliced genes. To understand more clearly the role the different isoforms play in cell physiology, we have undertaken a quantitative and qualitative assessment of the tissue distribution of transcripts encoding each SERCA isoform. SERCA1 expression is restricted to fast-twitch striated muscles, SERCA2a to cardiac and slow-twitch striated muscles, whereas SERCA2b is ubiquitously expressed. SERCA3 is expressed most abundantly in large and small intestine, thymus, and cerebellum and at lower levels in spleen, lymph node, and lung. In situ hybridization analyses revealed SERCA3 transcripts in cells of the intestinal crypt, the thymic cortex, and Purkinje cells in cerebellum. In addition, SERCA3 was expressed abundantly in isolated rat spleen lymphocytes, in various murine lymphoid cell lines, and in primary cultured microvascular endothelial cells. This analysis demonstrates that SERCA3 is expressed selectively in cells in which Ca2+ signaling plays a critical and sensitive role in regulating physiological processes.

Thapsigargin-resistant intracellular calcium pumps. Role in calcium pool function and growth of thapsigargin-resistant cells

Exposure of cells to the intracellular Ca2+ pump blocker, thapsigargin (TG), results in emptying of Ca2+ pools and termination of cell proliferation (Short, A. D., Bian, J., Ghosh, T. K., Waldron, R. T., Rybak, S. L., and Gill, D. L. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 4986-4990). DC-3F Chinese hamster lung cells were made resistant to TG by long-term stepwise exposure to increasing TG concentrations in culture (Gutheil, J. C., Hart, S. R., Belani, C. P., Melera, P. W., and Hussain, A. (1994) J. Biol. Chem. 269, 7976-7981). Since these cells (DC-3F/TG2) grow in the presence of TG, it was important to ascertain what Ca2+ pool function they retain. TG-resistant DC-3F/TG2 cells cultured with 2 microM TG had a doubling time (24 h) not significantly different from the parent DC-3F cells without TG. Analysis of TG-induced inhibition of 45Ca2+ uptake into permeabilized parent DC-3F cells revealed two distinct Ca2+ pump activities with 20,000-fold different sensitivities to TG; the IC50 values for TG were 200 pM and 4 microM, representing 80% and 20% of total pumping activity, respectively. Total pump activity in parent DC-3F and resistant DC-3F/TG2 cells was similar (0.23 +/- 0.10 and 0.18 +/- 0.08 nmol of Ca2+/10(6) cells, respectively). In DC-3F/TG2 cells, up to 100 nM TG had no effect on Ca2+ pumping; however, almost all pumping was blocked at higher TG concentrations with an IC50 of 5 microM. In both cell types, each Ca2+ pump activity (regardless of TG sensitivity) had high Ca2+ affinity (Km values congruent to 0.1 microM) and similar ATP dependence and vanadate sensitivity. In DC-3F cells, the TG-sensitive Ca2+ pool was releasable with inositol 1,4,5-trisphosphate (InsP3) or GTP and was oxalate-permeable; the TG-insensitive pool in these cells was not InsP3-releasable. GTP-induced Ca2+ uptake in the presence of oxalate indicated Ca2+ transfer between distinct pools in the DC-3F cells. In resistant DC-3F/TG2 cells, almost 50% of total TG-insensitive Ca2+ accumulation was releasable with InsP3; unlike the parent cells, this pool was not oxalate-permeable, and GTP induced no Ca2+ transfer between pools in the presence of oxalate. Thus, whereas InsP3 releases Ca2+ only from the high TG sensitivity Ca2+ pumping pool in parent DC-3F cells, in resistant DC-3F/TG2 cells the TG-resistant Ca2+ pumping pool now contains functional InsP3 receptors.(ABSTRACT TRUNCATED AT 400 WORDS)