Down-regulation of parathyroid (PTH)/PTH-related peptide receptor immunoreactivity and PTH binding in opossum kidney cells by PTH and dexamethasone

Ubiquitination-deubiquitination balance dictates ligand-stimulated PTHR sorting

Parathyroid hormone receptors (PTHR) are promptly internalized upon stimulation by activating (PTH[1-84], PTH[1-34]) and non-activating (PTH[7-84], PTH[7-34]) ligands. Here, we characterized the mechanism regulating the sorting of internalized receptors between recycling and degradative pathways. PTHR recycles faster after challenge with PTH(1-34) than with PTH(7-34). PTHR recycling is complete by 2 h after PTH(1-34) stimulation, but incomplete at this time in cells treated with PTH(7-34). The slower and incomplete recycling induced by PTH(7-34) is due to proteasomal degradation. Both PTH(1-34) and PTH(7-34) induced PTHR polyubiquitination. Ubiquitination by PTH(1-34) was transient, whereas receptor ubiquitination after PTH(7-34) was sustained. PTH(1-34), but not PTH(7-34), induced expression of the PTHR-specific deubiquitinating enzyme USP2. Overexpression of USP2 prevented PTH(7-34)-induced PTHR degradation. We conclude that PTH(1-34) promotes coupled PTHR ubiquitination and deubiquitination, whereas PTH(7-34) activates only ubiquitination, thereby leading to PTHR downregulation. These findings may explain PTH resistance in diseases associated with elevated PTH(7-84) levels.

The N- and C-terminal domains of parathyroid hormone-related protein affect differently the osteogenic and adipogenic potential of human mesenchymal stem cells

Parathyroid hormone-related protein (PTHrP) is synthesized by diverse tissues, and its processing produces several fragments, each with apparently distinct autocrine and paracrine bioactivities. In bone, PTHrP appears to modulate bone formation in part through promoting osteoblast differentiation. The putative effect of PTH-like and PTH-unrelated fragments of PTHrP on human mesenchymal stem cell (MSCs) is not well known. Human MSCs were treated with PTHrP (1-36) or PTHrP (107-139) or both (each at 10 nM) in osteogenic or adipogenic medium, from the start or after 6 days of exposure to the corresponding medium, and the expression of several osteoblastogenic and adipogenic markers was analyzed. PTHrP (1-36) inhibited adipogenesis in MSCs and favoured the expression of osteogenic early markers. The opposite was observed with treatment of MSCs with PTHrP (107-139). Moreover, inhibition of the adipogenic differentiation by PTHrP (1-36) prevailed in the presence of PTHrP (107-139). The PTH/PTHrP type 1 receptor (PTH1R) gene expression was maximum in the earlier and later stages of osteogenesis and adipogenesis, respectively. While PTHrP (107-139) did not modify the PTH1R overexpression during adipogenesis, PTHrP (1-36) did inhibit it; an effect which was partially affected by PTHrP (7-34), a PTH1R antagonist, at 1 microM. These findings demonstrate that both PTHrP domains can exert varying effects on human MSCs differentiation. PTHrP (107-139) showed a tendency to favor adipogenesis, while PTHrP (1-36) induced a mild osteogenic effect in these cells, and inhibited their adipocytic commitment. This further supports the potential anabolic action of the latter peptide in humans.

Normal human osteoclasts formed from peripheral blood monocytes express PTH type 1 receptors and are stimulated by PTH in the absence of osteoblasts

The prevailing view for many years has been that osteoclasts do not express parathyroid hormone (PTH) receptors and that PTH's effects on osteoclasts are mediated indirectly via osteoblasts. However, several recent reports suggest that osteoclasts express PTH receptors. In this study, we tested the hypothesis that human osteoclasts formed in vitro express functional PTH type 1 receptors (PTH1R). Peripheral blood monocytes (PBMC) were cultured on bone slices or plastic culture dishes with human recombinant RANK ligand (RANKL) and recombinant human macrophage colony-stimulating factor (M-CSF) for 16-21 days. This resulted in a mixed population of mono- and multi-nucleated cells, all of which stained positively for the human calcitonin receptor. The cells actively resorbed bone, as assessed by release of C-terminal telopeptide of type I collagen and the formation of abundant resorption pits. We obtained evidence for the presence of PTH1R in these cells by four independent techniques. First, using immunocytochemistry, positive staining for PTH1R was observed in both mono- and multi-nucleated cells intimately associated with resorption cavities. Second, PTH1R protein expression was demonstrated by Western blot analysis. Third, the cells expressed PTH1R mRNA at 21 days and treatment with 10(-7) M hPTH (1-34) reduced PTH1R mRNA expression by 35%. Finally, bone resorption was reproducibly increased by two to threefold when PTH (1-34) was added to the cultures. These findings provide strong support for a direct stimulatory action of PTH on human osteoclasts mediated by PTH1R. This suggests a dual regulatory mechanism, whereby PTH acts both directly on osteoclasts and also, indirectly, via osteoblasts.

Angiotensin II increases parathyroid hormone-related protein (PTHrP) and the type 1 PTH/PTHrP receptor in the kidney

Angiotensin II (AngII) participates in the pathogenesis of kidney damage. Parathyroid hormone (PTH)-related protein (PTHrP), a vasodilator and mitogenic agent, is upregulated during renal injury. The aim of this study was to investigate the potential relation between AngII and PTHrP system in the kidney. Different methods were used to find that both rat mesangial and mouse tubuloepithelial cells express PTHrP and the type 1 PTH/PTHrP receptor (PTH1R). In these cells, AngII increased PTHrP mRNA and protein production. In contrast, PTH1R mRNA was increased in mesangial cells and downregulated in tubular cells, but its protein levels were unmodified in both cells. AT(1) antagonist, but not AT(2), abolished AngII effects on PTHrP/PTH1R. The in vivo effect of AngII was further investigated by systemic infusion (a low dose of 50 ng/kg per min) into normal rats. In controls, PTHrP immunostaining was mainly detected in renal tubules. In AngII-infused rats, PTHrP staining increased in renal tubules and appeared in the glomerulus and the renal vessels. After AngII infusion, PTHR1 staining was markedly increased in all these renal structures at day 3 but remained elevated only in tubules at day 7. The AT(1) antagonist, but not the AT(2), significantly diminished AngII-induced PTHrP and PTHR1 overexpression in the renal tissue, associated with a decrease in tubular damage and fibrosis. The results indicate that AngII regulates renal PTHrP/PTH1R system via AT(1) receptors. These findings demonstrate that PTHrP upregulation occurs in association with the mechanisms of AngII-induced kidney injury.

Up-regulation of parathyroid hormone-related protein in folic acid-induced acute renal failure

BACKGROUND

Parathyroid hormone (PTH)-related protein (PTHrP) is present in many normal tissues, including the kidney. Current evidence supports that PTHrP is involved in renal pathophysiology, although its role on the mechanisms of renal damage and/or repair is unclear. Our present study examined the changes in PTHrP and the PTH/PTHrP receptor (type 1) in folic acid-induced acute renal failure in rats. The possible role of PTHrP on the process of renal regeneration following folic acid administration, and potential interaction between angiotensin II (Ang II) and endothelin-1, and PTHrP, were examined in this animal model.

METHODS

PTHrP, PTH/PTHrP receptor, ACE, and preproendothelin-1 (preproET-1) mRNA levels in the rat kidney were analyzed by reverse transcription-polymerase chain reaction (RT-PCR) and/or RNase protection assay. Immunohistochemistry also was performed for PTHrP, the PTH/PTHrP receptor, and Ang II in the renal tissue of folic acid-injected rats. The role of PTHrP on tubular cell proliferation following folic acid injury was investigated in vitro in rat renal epithelial cells (NRK 52E). PTHrP secretion in the medium conditioned by these cells was measured by an immunoradiometric assay specific for the 1-36 sequence.

RESULTS

Using RT-PCR, PTHrP mRNA was rapidly (1 hour) and maximally increased (3-fold) in the rat kidney after folic acid, decreasing after six hours. At 72 hours, renal function was maximally decreased in these rats, associated with an increased PTHrP immunostaining in both renal tubules and glomeruli. In contrast, the PTH/PTHrP receptor mRNA (RNase protection assay) decreased shortly after folic acid administration. Moreover, PTH/PTHrP receptor immunostaining dramatically decreased in renal tubular cell membranes after folic acid. A single subcutaneous administration of PTHrP (1-36), 3 or 50 microg/kg body weight, shortly after folic acid injection increased the number of tubular cells staining for proliferating cell nuclear antigen by 30% (P < 0.05) or 50% (P < 0.01), respectively, in these rats at 24 hours, without significant changes in either renal function or calcemia. On the other hand, this peptide failed to modify the increase (2-fold over control) in ACE mRNA, associated with a prominent Ang II staining into tubular cell nuclei, in the kidney of folic acid-treated rats at this time period. The addition of 10 mmol/L folic acid to NRK 52E cells caused a twofold increase in PTHrP mRNA at six hours, without significant changes in the PTH/PTHrP receptor mRNA. The presence of two anti-PTHrP antibodies, with or without folic acid, in the cell-conditioned medium decreased (40%, P < 0.01) cell growth.

CONCLUSIONS

Renal PTHrP was rapidly and transiently increased in rats with folic acid-induced acute renal failure, featuring as an early response gene. In addition, changes in ACE and Ang II expression were also found in these animals. PTHrP induces a mitogenic response in folic acid-damaged renal tubular cells both in vivo and in vitro. Our results support the notion that PTHrP up-regulation participates in the regenerative process in this model of acute renal failure and is a common event associated with the mechanisms of renal injury and repair.

Parathyroid hormone receptor internalization is independent of protein kinase A and phospholipase C activation

Parathyroid hormone (PTH) and PTH-related peptide (PTHrP) binding to their common receptor stimulates second messenger accumulation, receptor phosphorylation, and internalization. LLC-PK(1) cells expressing a green fluorescent protein-tagged PTH/PTHrP receptor show time- and dose-dependent receptor internalization. The internalized receptors colocalize with clathrin-coated pits. Internalization is stimulated by PTH analogs that bind to and activate the PTH/PTHrP receptor. Cell lines expressing a mutant protein kinase A regulatory subunit that is resistant to cAMP and/or a mutant receptor (DSEL mutant) that does not activate phospholipase C internalize their receptors normally. In addition, internalization of the wild-type receptor and the DSEL mutant is stimulated by the PTH analog [Gly(1),Arg(19)]hPTH-(1-28), which does not stimulate phospholipase C. Forskolin, IBMX, and the active phorbol ester, phorbol-12-myristate-13-acetate, did not promote receptor internalization or increase PTH-induced internalization. These data indicate that ligand-induced internalization of the PTH/PTHrP receptor requires both ligand binding and receptor activation but does not involve stimulation of adenylate cyclase/protein kinase A or phospholipase C/protein kinase C.

Dual signaling and ligand selectivity of the human PTH/PTHrP receptor

Parathyroid hormone (PTH) activates PTH/PTH-related peptide-related receptors (PTHRs) to stimulate both adenylyl cyclase (AC) and phospholipase C (PLC). How these parallel signals mediate specific cellular and tissue responses to PTH, such as the complex anabolic versus catabolic actions of PTH on bone, remains unsettled. Previous studies of PTHR signaling and function employed mainly rodent or other cell lines that express endogenous PTHRs and, possibly, alternate species of PTH receptors. To preclude confounding effects of such receptors, we stably expressed recombinant human PTHRs (hPTHRs) at different levels of surface density in LLC-PK1 porcine renal epithelial cells that lack endogenous PTH responsiveness. hPTH(1-34) induced concentration-dependent activation of both AC and PLC via transfected hPTHRs. Maximal intensity of each signal increased with receptor density, but more hPTHRs were required for PLC than for AC activation. Coupling to AC was saturated at receptor densities too low to detect sustained PLC activation. hPTH(3-34), found by others to be a PLC/protein kinase C (PKC)-selective peptide in rat cells, did not activate PLC via human (or rat) PTHRs under conditions (1 microM peptide, 106 hPTHRs/cell) where hPTH(1-34) stimulated PLC severalfold. Other cellular responses that require PKC activation in these cells, such as sodium-dependent phosphate transport and cAMP-independent secretion of plasminogen activator, were induced by PTH(1-34) but not by hPTH(3-34) or hPTH(7-34). We conclude that amino-truncated PTH analogs reported to activate PKC cannot directly activate phosphatidylinositol-specific PLC via the human or rat PTHR and therefore that PTH receptors may access alternate, PLC-independent pathways of PKC activation in some target cells. The relative intensity of AC and PLC signaling via the hPTHR may be strongly regulated by changes in its surface expression.

Epitope tag mapping of the extracellular and cytoplasmic domains of the rat parathyroid hormone (PTH)/PTH-related peptide receptor

The PTH/PTH-related peptide (PTHrP) receptor is predicted to span the plasma membrane seven times with an amino-terminal extracellular extension and a cytoplasmic carboxyl-terminal tail. To assess this prediction, we inserted 10- or 9-amino acid epitope tags from c-myc or hemophilus influenza hemaglutinin (HA), which are recognized by the monoclonal antibodies 9E10 and 12Ca5, respectively, in different extracellular and cytoplasmic regions of the receptor and examined the immunoreactivity of the epitopes in intact and permeabilized cells. The data show that the epitopes were well tolerated when introduced into the E2 region of the extracellular amino-terminus (E2-myc and E2-HA), in the first extracellular loop (EL1), in the second and third cytoplasmic loops (CL2c and CL3), or in the carboxyl-terminal tail (T-myc). Receptors tagged at these locations were well expressed, bound PTH with high affinity, and increased cAMP accumulation with a good efficiency. Receptors tagged in the second and third extracellular loops (EL2c and EL3c) or the first cytoplasmic loop (CL1c) bound the PTH radioligand with a low affinity, stimulated cAMP accumulation with a low efficiency, and had low expression levels. The receptors tagged on presumed extracellular regions, E2-myc, E2-HA, EL1, EL2c, and EL3c, were readily detected on the surface of intact cells with the monoclonal antibody against the epitope tag. In contrast, receptors tagged with the c-myc epitope in the cytoplasmic loops (CL1c, CL2c, and CL3) or in the carboxyl-terminal tail (T-myc) did not show any 9E10 binding in intact cells. These receptors, however, were well expressed on the cell surface, as detected by the binding of the monoclonal antibody, 12Ca5, to the HA tag that was introduced into the E2 region of these constructs. The c-myc epitopes, however, became accessible after permeabilization of the cell membrane. In conclusion, these data provide experimental evidence for the sidedness of the extracellular and cytoplasmic domains of the PTH/PTHrP receptor.

A new action of parathyroid hormone. receptor-mediated stimulation of extracellular acidification in human osteoblast-like SaOS-2 cells

The major physiological function of parathyroid hormone (PTH) is the maintenance of Ca2+/Pi homeostasis via the parathyroid hormone/parathyroid hormone-related protein receptor (PTHR) in kidney and bone. An important consequence of PTHR activation in bone is enhanced local acidification of the extracellular space. Agonist activation of some seven transmembrane-domain receptors increases the extracellular acidification rate (ECAR). We utilized microphysiometry to investigate PTH-stimulated, receptor-mediated increases in ECAR in human osteoblast-like SaOS-2 cells. PTH-(1-34) elicited a large, acute, dose-dependent increase in ECAR with an EC50 of about 2 nM. The PTH-induced increase in ECAR was specific to cells expressing the PTHR and was inhibited by PTHR antagonists. Rapid, partial, homologous desensitization of the PTH-induced increase in ECAR was observed. Incubation of SaOS-2 cells with 8-bromo-cyclic AMP neither mimicked nor abrogated the PTH effect, and PTH stimulated an acute increase in ECAR in cAMP-resistant SaOS-2 Ca#4A cells. Stimulation of ECAR by PTH was independent of transient increases in cytosolic free calcium. Both inhibition and down-regulation of PKC reduced the PTH-induced increase in ECAR. Inhibition of Na+/H+ exchange did not affect the PTH-induced ECAR response. We conclude that PTH caused a receptor-mediated, concentration-dependent, increase in ECAR, which was not dependent on the cAMP/PKA signaling pathway or the Na+/H+ exchanger but involved the action of PKC. Thus, acid production in bone, a physiologically important action of PTH, is not confined to osteoclasts as previously considered but is also mediated by osteoblasts.

Beta-adrenergic receptor kinase-1 acutely regulates PTH/PTHrP receptor signalling in human osteoblastlike cells

To investigate whether G protein-coupled receptor kinases (GRKs) are involved in the regulation of the PTH/PTHrPR, we have established mutant SaOS-2 cells which stably overexpress (> 10-20-fold) a dominant negative form of the beta-adrenergic receptor kinase-1 (beta ARK-1). Acute (< or = 2 h) incubation with hPTH (1-34) induced significantly less (by up to 50%) downregulation of the PTH/PTHrPR in beta ARK-1 mutant SaOS-2 cells than observed in wild-type cells. Pretreatment of wild-type cells with PTH for 2 h induced homologous cAMP desensitisation to a second challenge with PTH, while the effect was blunted by up to 60% in beta ARK-1 mutant cells. We conclude that activation of beta ARK-1 (or a closely related GRK) is a critical component of the acute phase (< or = 2 h) of PTH-induced receptor downregulation and homologous cAMP desensitisation of the PTH/PTHrPR.

Down-regulation of protein kinase C by parathyroid hormone and mezerein differentially modulates cAMP production and phosphate transport in opossum kidney cells

We examined the effects of prolonged exposure to parathyroid hormone (PTH) and the protein kinase C (PKC) activator mezerein (MEZ) on cyclic adenosine monophosphate (cAMP) production, PKC activity, and Na(+)-dependent phosphate (Na/Pi) transport in an opossum kidney cell line (OK/E). A 5 minute exposure to PTH stimulated, while a 6 h incubation reduced, cAMP production, Na/Pi transport was maximally inhibited under desensitizing conditions and was not affected by reintroduction of the hormone. MEZ pretreatment (6 h) enhanced PTH-, cholera toxin (CTX)-, and forskolin (FSK)-stimulated cAMP production, suggesting enhanced Gs alpha coupling and increased adenylyl cyclase activity. However, PKA- and PKC-dependent regulation of Na/Pi were blocked in MEZ-treated cells. The PTH-induced decrease in cAMP production was associated with a reduction in membrane-associated PKC activity while MEZ-induced increases in cAMP production were accompanied by decreases in membrane and cytosolic PKC activity. Enhanced cAMP production was not accompanied by significant changes in PTH/PTH related peptide (PTHrP) receptor affinity or number, nor was the loss of Na/Pi transport regulation associated with changes in PKA activity. The results indicate that down-regulation of PKC by PTH or MEZ differentially modulates cAMP production and regulation of Na/Pi transport. The distinct effects of PTH and MEZ on PKC activity suggest that agonist-specific activation and/or down-regulation of PKC isozyme(s) may be involved in the observed changes in cAMP production and Na/Pi transport.

Mechanisms of homologous and heterologous desensitization of PTH/PTHrP receptor signaling in LLC-PK1 cells

Parathyroid hormone (PTH) activates multiple intracellular effectors, including adenylyl cyclase (AC) and phospholipase C (PLC), via a single receptor [PTH/parathyroid hormone-related protein receptor (PTHR)] expressed in bone and kidney. Homologous desensitization of PTHR signaling occurs, but the relative importance of reduced receptor expression vs. impaired receptor-effector coupling in this process remains unclear. It also is not known if AC and PLC responses to PTH are desensitized independently or interdependently. In LLC-PK1 cells that expressed transfected wild-type PTHRs, PTH caused dose- and time-dependent desensitization of both the AC and PLC-responses to PTH without altering PTHR expression. Desensitization of AC was blocked in mutant cells resistant to adenosine 3',5'-cyclic monophosphate but not when cells expressed mutant PTHRs with defective PLC coupling. Desensitization of PLC was unaffected by PKA blockade, partially mimicked by phorbol ester, and not reproduced by agents that selectively activated AC. The finding that homologous PTHR desensitization in LLC-PK1 cells is signal specific suggests that prior exposure of other cells to PTH also may induce discordant regulation of subsequent PTHR signaling, altering the character as well as the intensity of the hormonal response.

Activity-dependent expression of parathyroid hormone-related protein (PTHrP) in rat cerebellar granule neurons. Requirement of PTHrP for the activity-dependent survival of granule neurons

To identify genes whose expression is neuronal activity-dependent, we used an mRNA differential display technique and discovered that parathyroid hormone-related protein (PTHrP) is expressed in an activity-dependent manner in primary cultures of rat cerebellar granule neurons. PTHrP mRNA was expressed as early as 1 h by the addition of KCl to a final concentration of 25 mM to the culture medium. This expression was induced by Ca2+ influx through voltage-dependent L-type Ca2+ channels and regulated at the transcriptional step. PTHrP mRNA was persistently expressed before and after the time of commitment of granule neurons to apoptosis when they are cultured in the presence of 25 mM KCl or both 150 microM N-methyl-D-aspartic acid and 15 mM KCl, both of which promote the survival of these neurons. PTHrP was rapidly secreted into the culture medium in a depolarization-dependent manner. Parathyroid hormone/PTHrP receptor mRNA was also expressed in the primary cultures, and its expression was up-regulated by KCl and/or N-methyl-D-aspartic acid. The addition of anti-PTHrP antiserum to the culture medium resulted in a reduction of the activity-dependent survival of the granule neurons. These results suggest that PTHrP is involved in an autocrine loop and required for the survival of granule neurons.

Tissue-specific transcription start sites and alternative splicing of the parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor gene: a new PTH/PTHrP receptor splice variant that lacks the signal peptide

The PTH/PTHrP receptor gene is expressed in bone and kidney as well as in many other tissues. Using primer extension followed by rapid cloning of amplified complementary DNA ends, we have isolated new PTH/PTHrP receptor complementary DNAs with different splicing patterns and have characterized a new upstream transcription start site. Three 5' nontranslated exons, U3, U2 and U1, located 4.8, 2.5, and 1.2 kb upstream of the exon that encodes the putative signal peptide of the classical receptor (exon S), have been characterized. Four types of splicing patterns were recognized. Type I splicing pattern is transcribed from exon U1 and is spliced to exons S and E1; this pattern was found in most tissues tested. Types II, III, and IV splicing patterns are transcribed from exon U3 and have a restricted tissue distribution. Type II splice pattern, containing exons U3, U2, and S and type III splicing pattern, containing exon U3, U2, and E1 (skipping exon S), was found only in kidney. Type IV splice pattern, containing exon U3 and S was found both in kidney and ovary. Because the type III splice variant skips exon S, translation of this splice variant initiates at a different AUG codon. The type III splice variant was weakly expressed on the cell surface of COS-7 cells, as assessed by double antibody binding assay, and no detectable ligand binding was observed on intact cells. The type III splice variant, however, increased cAMP accumulation in COS-7 cells when challenged with PTH(1-34), PTH(1-84) and hPTHrP(1-36) with EC50s that are similar to those observed in COS-7 cells expressing the type I variant but with a maximum stimulation that was lower than that observed in COS-7 cells expressing the type I variant. These data indicate low levels of cell surface expression of the type III splice variant. Treatment of COS-7 cells with tunicamycin decreased the size of the type I splice variant from a broad band of 85 kDa to a compact band of about 60 kDa. The type III splice variant did not change in size in COS-7 cells treated with tunicamycin, indicating that the type III splice variant did not undergo any glycosylation step. In conclusion, the PTH/PTHrP receptor gene uses alternate promoters in a tissue-specific manner that results in several tissue-specific alternatively spliced transcripts. One of these transcripts, the type III splice variant, is expressed in kidney and lacks the signal peptide.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.