Inhibitory effect of regucalcin on Ca2+/calmodulin-dependent protein kinase activity in rat renal cortex cytosol

The diverse roles of calcium-binding protein regucalcin in cell biology: from tissue expression and signalling to disease

Regucalcin (RGN) is a calcium (Ca(2+))-binding protein widely expressed in vertebrate and invertebrate species, which is also known as senescence marker protein 30, due to its molecular weight (33 kDa) and a characteristically diminished expression with the aging process. RGN regulates intracellular Ca(2+) homeostasis and the activity of several proteins involved in intracellular signalling pathways, namely, kinases, phosphatases, phosphodiesterase, nitric oxide synthase and proteases, which highlights its importance in cell biology. In addition, RGN has cytoprotective effects reducing intracellular levels of oxidative stress, also playing a role in the control of cell survival and apoptosis. Multiple factors have been identified regulating the cell levels of RGN transcripts and protein, and an altered expression pattern of this interesting protein has been found in cases of reproductive disorders, neurodegenerative diseases and cancer. Moreover, RGN is a serum-secreted protein, and its levels have been correlated with the stage of disease, which strongly suggests the usefulness of this protein as a potential biomarker for monitoring disease onset and progression. The present review aims to discuss the available information concerning RGN expression and function in distinct cell types and tissues, integrating cellular and molecular mechanisms in the context of normal and pathological conditions. Insight into the cellular actions of RGN will be a key step towards deepening the knowledge of the biology of several human diseases.

Overexpression of regucalcin suppresses cell response for tumor necrosis factor-alpha or transforming growth factor-beta1 in cloned normal rat kidney proximal tubular epithelial NRK52E cells

The regulatory role of regucalcin on cell responses for tumor necrosis factor-alpha (TNF-alpha) or transforming growth factor-beta1 (TGF-beta1) was investigated using the cloned normal rat kidney proximal tubular epithelial NRK52E cells overexpressing regucalcin. NRK52E cells (wild type) and stable regucalcin (RC)/pCXN2-transfected cells (transfectant) were cultured for 72 h in a medium containing 5% bovine serum (BS) to obtain subconfluent monolayers. After culture, cells were further cultured for 24-72 h in medium without BS containing either vehicle, TNF-alpha (0.1 or 1.0 ng/ml of medium), or TGF-beta1 (1.0 or 5.0 ng/ml). Culture with TNF-alpha or TGF-beta1 caused a significant decrease in the number of wild-type cells. This decrease was significantly prevented in transfectants overexpressing regucalcin. Agarose gel electrophoresis showed the presence of low-molecular-weight deoxyribonucleic acid (DNA) fragments of adherent wild-type cells cultured with TNF-alpha (1.0 ng/ml) or TGF-beta1 (5.0 ng/ml). This DNA fragmentation was significantly suppressed in transfectants. TNF-alpha- or TGF-beta1-induced cell death was significantly prevented in culture with caspase-3 inhibitor (10(-8) M). Nitric oxide (NO) synthase activity in wild-type cells was significantly increased by addition of calcium chloride (10 microM) and calmodulin (5 microg/ml) into the enzyme reaction mixture. This increase was significantly suppressed in transfectants. Culture with TNF-alpha caused a significant increase in NO synthase activity in wild-type cells. The effect of TNF-alpha was not seen in transfectants. Culture with TGF-beta1 did not cause a significant increase in NO synthase activity in wild-type cells and transfectants. Culture with TNF-alpha or TGF-beta1 caused a remarkable increase in alpha-smooth muscle actin in wild-type cells. This increase was significantly prevented in transfectants. The expression of Smad 2 or NF-kappaB mRNAs was significantly increased in transfectants as compared with that of wild-type cells. Smad 3 or glyceroaldehyde-3-phosphate dehydrogenase (G3PDH) mRNA expression was not significantly changed in transfectants. NF-kappaB mRNA expression in wild-type cells was significantly increased with culture of TNF-alpha. Smad 2 mRNA expression was significantly enhanced in wild-type cells cultured with TGF-beta1. These effects of TNF-alpha or TGF-beta1 were not significantly enhanced in transfectants. This study demonstrates that overexpression of regucalcin has suppressive effects on cell responses which are mediated through intracellular signaling pathways of TNF-alpha or TGF-beta1 in kidney NRK52E cells.

Overexpression of regucalcin enhances its nuclear localization and suppresses L-type Ca2+ channel and calcium-sensing receptor mRNA expressions in cloned normal rat kidney proximal tubular epithelial NRK52E cells

The effect of regucalcin (RC), a regulatory protein in intracellular signaling pathway, on the gene expression of various mineral ion transport-related proteins was investigated using the cloned normal rat kidney proximal tubular epithelial NRK52E cells overexpressing RC. NRK52E cells (wild-type) and stable RC/pCXN2 transfectant were cultured for 72 h in medium containing 5% bovine serum (BS) to obtain subconfluent monolayers. After culture for 72 h, cells were further cultured 24-72 h in a medium containing either vehicle, aldosterone (10(-8) or 10(-7) M), or parathyroid hormone (PTH) (1-34) (10(-8) or 10(-7) M) without BS. RC was markedly localized in the nucleus of transfectants. Overexpression of RC caused a significant increase in rat outer medullary K(+) channel (ROMK) mRNA expression, while it caused a remarkable decrease in L-type Ca(2+) channel and calcium-sensing receptor (CaR) mRNA expressions. Overexpression of RC did not have an effect on epithelial sodium channel (ENaC), Na, K-ATPase (alpha-subunit), Type II Na-Pi cotransporter (NaPi-IIa), angiotensinogen, Na(+)-Ca(2+) exchanger, and glyceroaldehyde-3-phosphate dehydrogenase (G3PDH) mRNA expressions. Hormonal effect on gene expression, moreover, was examined. Culture with aldosterone (10(-8) or 10(-7) M) caused a significant increase in ENaC, Na, K-ATPase, and ROMK mRNA expressions in the wild-type cells. Those increases were weakened in the transfectants. Culture with PTH (10(-8) or 10(-7) M) significantly decreased NaPi-IIa mRNA expression in the wild-type cells. This effect was not altered in the transfectants. PTH significantly decreased angiotensinogen mRNA expression in the wild-type cells and the transfectants, while aldosterone had no effect. Culture with PTH (10(-8) or 10(-7) M) caused a significant decrease in L-type Ca(2+) channel and CaR mRNA expressions in the wild-type cells, while the hormone significantly increased Na(+)-Ca(2+) exchanger mRNA expression. The effects of PTH on L-type Ca(2+) channel, CaR, and Na(+)-Ca(2+) exchanger mRNA expressions were also seen in the transfectants. This study demonstrates that overexpression of RC caused a remarkable increase in its nuclear localization, and that it has suppressive effects on the gene expression of L-type Ca(2+) channel or CaR, which regulates intracellular Ca(2+) signaling, among various regulator proteins for mineral ions in NRK52E cells.

Hormonal regulation on regucalcin mRNA expression in cloned normal rat kidney proximal tubular epithelial NRK52E cells

Regucalcin is a regulatory protein in cell signaling. This study was undertaken to determine whether regucalcin mRNA expresses in the cloned normal rat kidney proximal tubular epithelial NRK52E cells and its expression regulates due to hormones and cell signaling-related factors. Cells with subconfluency were cultured for 24, 48, or 72 h in a Dulbecco's modified Eagle medium supplemented with non-essential amino acid without bovine serum (BS). The result of Western blot analysis showed that regucalcin protein was present in the NRK52E cells. The expression of regucalcin mRNA in the cells was determined using reverse transcription-polymerase chain reaction (RT-PCR). Regucalcin mRNA expression in the NRK52E cells was significantly increased by culture with parathyroid hormone (PTH, 10(-8) or 10(-7) M), aldosterone (10(-8) or 10(-7) M), or dexamethasone (10(-8) M). The presence of 1,25-dihydroxyvitamin D(3) (1,25(OH)2D3, 10(-8) or 10(-7) M) or calcitonin (10(-9) or 10(-8) M) did not have a significant effect on regucalcin mRNA levels in the cells. Culture with dibutyryl cyclic AMP (DcAMP, 10(-5) or 10(-4) M) or phorbol 12-myristate 13-acetate (PMA, 10(-6) M), an activator of protein kinase C, caused a significant increase in regucalcin mRNA expression. The presence of staurosporine (10(-8) M) caused a significant decrease in regucalcin mRNA expression. Dibucaine (10(-7) M), PD98059 (10(-7) M), or vanadate (10(-6) or 10(-5) M) did not have an effect on regucalcin mRNA levels. The present study demonstrates that regucalcin mRNA and its protein are expressed in the cloned normal rat kidney proximal tubular epithelial NRK52E cells, and that the expression is enhanced by hormones which regulate ion transport in the proximal tubule.

Overexpression of regucalcin suppresses apoptotic cell death in cloned normal rat kidney proximal tubular epithelial NRK52E cells: Change in apoptosis-related gene expression

The effect of regucalcin, a regulatory protein in intracellular signaling pathway, on cell death and apoptosis was investigated using the cloned normal rat kidney proximal tubular epithelial NRK52E cells overexpressing regucalcin. NRK52E cells (wild type) and stable regucalcin (RC)/pCXN2 transfectants were cultured for 72 h in a medium containing 5% bovine serum (BS) to obtain subconfluent monolayers. After culture for 72 h, cells were further cultured for 24-72 h in a medium without BS containing either vehicle, tumor necrosis factor-alpha (TNF-alpha; 0.1 or 1.0 ng/ml of medium), lipopolysaccharide (LPS; 0.1 or 1.0 microg/ml), Bay K 8644 (10(-9)-10(-7) M), or thapsigargin (10(-9)-10(-7) M). The number of wild-type cells was significantly decreased by culture for 42-72 h in the presence of TNF-alpha (0.1 or 1.0 ng/ml), LPS (0.1 or 1.0 microg/ml), Bay K 8644 (10(-7)-10(-5) M), or thapsigargin (10(-8) or 10(-7) M). The effect of TNF-alpha (0.1 or 1.0 ng/ml), LPS (0.1 or 1.0 microg/ml), Bay K 8644 (10(-7)-10(-6) M), or thapsigargin (10(-7) M) in decreasing the number of wild-type cells cultured for 24-72 h was significantly prevented in transfectants overexpressing regucalcin. Agarose gel electrophoresis showed the presence of low-molecular-weight deoxyribonucleic acid (DNA) fragments of adherent wild-type cells cultured with LPS (1.0 microg/ml), Bay K 8644 (10(-7) M), or thapsigargin (10(-8) M) for 24 h, and this DNA fragmentation was significantly suppressed in transfectants. DNA fragmentation in adherent cells was not seen by culture with TNF-alpha (1.0 ng/ml). TNF-alpha-induced decrease in the number of wild-type cells was significantly prevented by culture with caspase-3 inhibitor (10(-8) M), while LPS- or Bay K 8644-induced decrease in cell number was significantly prevented by caspase-3 inhibitor or N omega-nitro-L-arginine methylester (NAME) (10(-5) M), an inhibitor of nitric oxide (NO) synthase. Thapsigargin-induced decrease in cell number was not prevented in the presence of two inhibitors. Bcl-2 and Akt-1 mRNA levels were significantly increased in transfectants cultured for 24 h as compared with those of wild-type cells, while Apaf-1, caspase-3, or glyceroaldehyde-3-phosphate dehydrogenase (G3PDH) mRNA expressions were not significantly changed in transfectants. Culture with TNF-alpha (1.0 ng/ml), LPS (1.0 microg/ml), Bay K 8644 (l0(-7) M), or thapsigargin (10(-8) M) caused a significant increase in caspase-3 mRNA levels in wild-type cells. LPS (1.0 microg/ml) significantly decreased Bcl-2 mRNA expression in the cells. Their effects on the gene expression of apoptosis-related proteins were not significantly changed in transfectants. This study demonstrates that overexpression of regucalcin has a suppressive effect on cell death and apoptosis induced by various factors which their action are mediated through many intracellular signaling pathways, and that it modulates the gene expression of apoptosis-related proteins.

Inhibitory role of regucalcin in the regulation of nitric oxide synthase activity in rat brain cytosol: involvement of aging

Regucalcin is shown to play a regulatory role in intracellular signaling system in many cells. The protein has been reported to be present in rat brain neurons. The role of regucalcin in the regulation of nitric oxide (NO) synthase activity in the brain cytosol of female young and aged rats was investigated. The presence of regucalcin (10(-9) and 10(-8) M) in the enzyme reaction mixture caused a significant decrease in NO synthase activity in the absence or presence of both calcium chloride (10 micro M) and calmodulin (2.5 micro g/ml). The effect of regucalcin (10(-8) M) in decreasing brain cytosolic NO synthase activity was not seen in the presence of Nw-nitro-L-argine methyl ester (NAME) (10(-4) M), trifluoperazine (20 micro M) or EGTA (1 mM). Regucalcin protein levels were significantly lowered in the brain cytosol of aged (50 weeks old) rats as compared with that of young (5 weeks old) rats. Brain cytosolic NO synthase activity was significantly reduced with increasing age. In aged rat brain cytosol, regucalcin caused a significant decrease in NO synthase activity. The presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) caused a significant increase in NO synthase activity in the brain cytosol of young and aged rats. This elevation was completely blocked by the addition of regucalcin (10(-8) M). The present study demonstrates that endogenous regucalcin has an inhibitory effect on NO synthase activity in the brain cytosol of young and aged rats.

Endogenous regucalcin suppresses the enhancement of protein phosphatase activity in the cytosol and nucleus of kidney cortex in calcium-administered rats

The suppressive role of endogenous regucalcin (RC), which is a regulatory protein of calcium signaling, in the enhancement of protein phosphatase activity (PPA) in the cytosol and nucleus of kidney cortex in calcium-administered rats was investigated. Calcium content in the kidney cortex was significantly increased at 0.5-5 h after a single intraperitoneal administration of calcium chloride solution (10 mg Ca/100 g body weight) to rats. The analysis with Western blotting of RC protein showed that RC levels in the cytosol and nucleus were significantly increased 0.5-5 h after the administration of calcium (10 mg/100 g). PPA toward phosphotyrosine, phosphoserine, and phosphothreonine was found in the cytosol and nucleus of kidney cortex. PPA toward three phosphoamino acids in the cytosol and nucleus was significantly increased by the administration of calcium (10 mg/100 g). The presence of anti-RC monoclonal antibody (25 ng/ml) in the enzyme reaction caused a significant increase in PPA toward phosphotyrosine, phosphoserine, and phosphothreonine in the cytosol and nucleus of kidney cortex in normal rats. The effect of anti-RC monoclonal antibody (25 ng/ml) in increasing PPA toward three phosphoamino acids in the cytosol and nucleus was significantly enhanced in calcium-administered rats. The effect of anti-RC monoclonal antibody (25 ng/ml) in increasing PPA in the cytosol and nucleus of normal rats and calcium-administered rats was completely abolished by the addition of RC (10(- 6) M) in the enzyme reaction mixture. The present study suggests that endogenous RC suppresses the enhancement of PPA in the cytosol and nucleus of kidney cortex in calcium-administered rats.

Suppressive effect of endogenous regucalcin on deoxyribonuclic acid synthesis in the nuclei of rat renal cortex

The effect of regucalcin, a regulatory protein of Ca2+ signaling, on deoxyribonucleic acid (DNA) synthesis activity in the nuclei isolated from rat renal cortex was investigated. The addition of calcium chloride (10-100 microM) in the reaction mixture containing the nuclei caused a significant decrease in DNA synthesis activity. Nuclear DNA synthesis activity was significantly raised in the presence of EGTA (1 mM), a chelator of Ca2+, indicating that nuclear Ca2+ has an inhibitory effect. Regucalcin (0.1-0.5 microM) added in the reaction mixture in the presence of either EGTA (1 mM) or calcium chloride (50 microM) had a significant inhibitory effect on nuclear DNA synthesis activity. The presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) in the reaction mixture caused a significant increase in DNA synthesis activity. This increase was completely abolished by the addition of regucalcin (0.5 microM). The effect of anti-regucalcin monoclonal antibody in increasing DNA synthesis was enhanced in the presence of EGTA. Additionally, an inhibitory effect of calcium chloride (10 or 50 microM) was enhanced in the presence of anti-regucalcin monoclonal antibody (25 ng/ml). The present study demonstrates that endogenous regucalcin has a suppressive effect on DNA synthesis in the nuclei of rat renal cortex.

Involvement of nuclear factor-1 (NF1) binding motif in the regucalcin gene expression of rat kidney cortex: the expression is suppressed by cisplatin administration

The binding of nuclear factor on the promoter region of the regucalcin gene and the expression of regucalcin in the kidney cortex of rats was investigated. Nuclear extracts from kidney cortex were used for oligonucleotide competition gel mobility shift assay. An oligonucleotide between position -523 and -506 in the 5'-flanking region of the rat regucalcin gene, which contains a nuclear factor 1 (NF1) consensus motif TTGGC(N)6CC, competed with the probe for the binding of the nuclear protein from kidney cortex. The mutation of TTGGC in the consensus sequence caused an inhibition of the binding of nuclear factors. The binding of nuclear factor on the 5'-flanking region was clearly reduced in the kidney cortex obtained at 1, 2, and 3 days after a single intraperitoneal administration of cisplatin (1.0 mg/100 g body wt) to rats. Moreover, cisplatin administration caused a remarkable decrease in regucalcin mRNA levels and regucalcin concentration in the kidney cortex. Also, serum regucalcin concentration was significantly decreased by cisplatin administration. Meanwhile, serum urea nitrogen concentration was markedly elevated by cisplatin administration. The present study demonstrates that the specific nuclear factor binds to the NF1-like sequence in the promotor region of regucalcin gene in the kidney cortex of rats, and that the nuclear factor binding and regucalcin expression are suppressed by cisplatin administration.

Suppressive role of endogenous regucalcin in the regulation of protein phosphatase activity in rat renal cortex cytosol

The role of endogenous regucalcin, which is a regulatory protein of calcium signaling, in the regulation of protein phosphatase activity in the cytosol of rat renal cortex was investigated. Protein phosphatase activity toward phosphotyrosine, phosphoserine, and phosphothreonine was found in the cytosol of kidney cortex. The addition of regucalcin (50-250 nM) in the enzyme reaction mixture caused a significant decrease in protein phosphatase activity toward three phosphoamino acids. The effect of calcium (25 microM) and calmodulin (2.5 microg/ml) in increasing protein phosphatase activity toward three phosphoamino acids was significantly decreased by the addition of regucalcin (100 nM). Protein phosphatase activity toward three phosphoamino acids was significantly increased in the presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) in the enzyme reaction mixture. The effect of antibody (25 ng/ml) in increasing the enzyme activity was significantly inhibited by cyclosporin A (10(-5) M) or vanadate (10(-5) M). Regucalcin in the kidney cortex cytosol was clearly decreased by the administration of saline (0.9% NaCl) for seven days in rats. Protein phosphatase activity toward three phosphoamino acids was significantly decreased by saline administration. The effect of anti-regucalcin antibody (25 ng/ml) in increasing protein phosphatase activity toward three phosphoamino acids was not seen in the renal cortex cytocol of saline-administered rats. The present study demonstrates that endogenous regucalcin plays a suppressive role in the regulation of protein phosphatase activity in the cytoplasm of rat kidney cortex.

Inhibitory effect of regucalcin on protein phosphatase activity in the nuclei of rat kidney cortex

The role of regucalcin, which is a regulatory protein of calcium signaling, in the regulation of protein phosphatase activity in the nuclei of rat kidney cortex was investigated. Protein phosphatase activity towards phosphotyrosine, phosphoserine, and phosphothreonine was found in the nuclei. The enzyme activity towards three phosphoamino acids was significantly increased by the addition of calcium chloride (10-50 microM) in the enzyme reaction mixture. This increase was significantly inhibited by trifluoperazine (25 or 50 microM), an antagonist of calmodulin. The presence of regucalcin (50 or 100 nM) in the enzyme reaction mixture caused a significant decrease in protein phosphatase activity towards three phosphoamino acids. This effect was also seen in the presence of calcium (25 microM) and/or calmodulin (5 microg/ml). Protein phosphatase activity towards three phosphoamino acids was significantly increased in the presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) in the enzyme reaction mixture. This effect was completely blocked by the addition of regucalcin (100 nM). The effect of antibody (25 ng/ml) in increasing protein phosphatase activity towards phosphotyrosine was significantly inhibited by vanadate (10(-4) M). Also, the antibody's effect towards phosphoserine and phosphothreonine was significantly inhibited by cyclosporin A (10(-5) M). Endogenous regucalcin was found in the nuclei of rat kidney cortex using Western blot analysis. Nuclear regucalcin level was significantly reduced by the administration of saline (0.9% NaCl) for seven days in rats. Protein phosphatase activity towards three phosphoamino acids was significantly decreased by saline administration. The effect of anti-regucalcin monoclonal antibody (25 ng/ml) in increasing protein phosphatase activity towards three phosphoamino acids was weakened in the renal cortex nuclei of saline-administrated rats. The present study demonstrates that endogenous regucalcin plays a suppressive role in the regulation of protein phosphatase activity in the nuclei of rat kidney cortex cells.

Inhibitory role of regucalcin in the regulation of Ca2+ dependent protein kinases activity in rat brain neurons

The role of regucalcin in the regulation of protein kinase activity in rat brain neuronal cells obtained from primary culture was investigated. Protein kinase activity was assayed using the 5500 g supernatant fraction of the cell homogenate. Protein kinase activity was significantly raised by the addition of calmodulin (5 microg/ml) or dioctanoylglycerol (5 microg/ml) in the presence of CaCl2 (10(-4) M), indicating that Ca2+/calmodulin-dependent protein kinase and protein kinase C is present in the neuronal cells. The addition of regucalcin (10(-9)-10(-7) M) in the enzyme reaction mixture caused a significant decrease in protein kinase activity in the absence of calmodulin or dioctanoylglycerol without Ca(2+) addition. Moreover, regucalcin completely prevented the activation of protein kinase by the addition of calmodulin or dioctonoylglyceral in the presence of CaCl(2) (10(-4) M). The presence of anti-regucalcin monoclonal antibody (25 or 50 ng/ml) caused a significant elevation of protein kinase activity without CaCl2 addition. Such an effect was significantly inhibited by the addition of trifluoperazine (2x10(-5) M), an antagonist of calmodulin, or staurosporine (10(-6) M), an inhibitor of protein kinase C. The present study demonstrates that endogenous regucalcin in rat brain neuronal cells has an inhibitory effect on Ca2+ dependent protein kinase activity.

The role of regucalcin in nuclear regulation of regenerating liver

Regucalcin was discovered in 1978 as a Ca(2+)-binding protein that does not contain EF-hand motif of Ca(2+)-binding domain [Yamaguchi, M., and Yamamoto T., Chem. Pharm. Bull. 26, 1915-1918, 1978]. The name regucalcin was proposed for this Ca(2+)-binding protein, which can regulate liver cell functions related to Ca(2+). Regucalcin has been demonstrated to play a multifunctional role in liver and kidney cells, for which regucalcin mRNA expression and its protein content are pronounced. Hepatic regucalcin mRNA expression has been shown to be mediated through signaling pathway of Ca(2+)/calmodulin-dependent protein kinase, protein kinase C, and tyrosine kinase. AP-1- and NF-1-like factors can bind to the promotor region of the rat regucalcin gene to mediate the Ca(2+) response for transcriptional activation. Growing evidence supports the view, moreover, that regucalcin plays an important role in the regulation of Ca(2+) signaling from the cytoplasm to nuclei in the proliferative cells of regenerating rat liver. Also, regucalcin has been demonstrated to be transported to liver nucleus, and it can inhibit nuclear protein kinase, protein phosphatase, and DNA and RNA synthesis in regenerating liver. Regucalcin plays a physiologic role in the control for overexpression of proliferative cells. Regucalcin has been proposed to be an important regulatory protein in nuclear signaling system.

Expression of Ca(2+)-binding protein regucalcin in rat brain neurons: inhibitory effect on protein phosphatase activity

The expression of Ca(2+)-binding protein regucalcin and its role in the regulation of protein phosphatase activity in rat brain neuronal cells obtained with primary culture was investigated. The expression of regucalcin mRNA was demonstrated by reverse transcription-polymerase chain reaction (RT-PCR) analysis in brain neuronal cells using rat regucalcin-specific primers. Moreover, regucalcin protein in brain neuronal cells was detected by Western blot analysis using a polyclonal rabbit anti-regucalcin antibody. The presence of anti-regucalcin monoclonal antibody (20 or 50 ng/ml) in the enzyme reaction mixture caused a significant increase in protein phosphatase activity toward phosphotyrosine, phosphoserine and phosphothreonine in the reaction mixture containing the cytosol of neuronal cell homogenates. This increase was completely prevented by the addition of regucalcin (10(-8) M). Protein phosphatase activity toward three phosphoaminoacids was significantly elevated by the addition of Ca(2+) (100 microM) and calmodulin (5 microg/ml). This elevation was completely blocked by the addition of regucalcin (10(-8) M). The present study demonstrates that regucalcin is expressed in rat brain neuronal cells, and that it has an inhibitory effect on protein phosphatase activity in the cells.

Role of regucalcin in calcium signaling

Regucalcin was discovered in 1978 as a calcium-binding protein that does not contain EF-hand motif of Ca(2+)-binding domain [M. Yamaguchi and T. Yamamoto, Chem. Pharm. Bull. 26 1915-1918 (1978)]. In recent years, regucalcin has been demonstrated to play an important role as a regulatory protein in Ca2+ signaling in rat liver and kidney cells. The organization of the rat regucalcin gene consists of seven exons and six introns. The mRNA is mainly present in liver and kidney with a size of 1.8 kb. Hepatic regucalcin mRNA expression has been shown to be stimulated by various factors including calcium, calcitonin, insulin, and estrogen in rats. The mRNA is also expressed in hepatoma cells (Morris hepatoma, HepG2, and rat hepatoma H4-II-E cells). Regucalcin plays a role in the maintenance of intracellular Ca2+ homeostasis due to activating Ca2+ pump enzymes in the plasma membrane (basolateral membrane) and microsomes of liver and renal cortex cells. Moreover, regucalcin has an inhibitory effect on the activation of Ca2+/calmodulin-dependent enzymes and protein kinase C. Also, regucalcin has been demonstrated to regulate nuclear function in liver cells; it can inhibit Ca(2+)-activated DNA fragmentation, DNA and RNA synthesis, protein kinase and protein phosphatase activities in the nuclei. Such an effect is also seen in the nuclei of regenerating rat liver. Regucalcin may play a physiological role in the control for overexpression of proliferative cells. Regucalcin has been proposed to be an important regulatory protein in Ca2+ signaling system, and it plays a multifunctional role in liver and kidney cells.

Stimulatory effect of regucalcin on proteolytic activity is impaired in the kidney cortex cytosol of rats with saline ingestion

The effect of regucalcin (RC) on neutral proteolytic activity in the cytosol of rat kidney cortex was investigated. Proteolytic activity was significantly increased by the presence of RC (0.01-0.10 microM) in the enzyme reaction mixture. This increase was completely abolished by the addition of anti-RC monoclonal antibody (150 ng/ml). When the renal cortex cytosol was incubated without RC addition, the degradation of globin of substrate was demonstrated by sodium dodecylsulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. This degradation was clearly inhibited by the addition of anti-RC antibody (150 ng/ml), indicating that protein degradation results partly from the cytosolic endogenous RC. Meanwhile, proteolytic activity was significantly decreased in the renal cortex cytosol of rats with saline ingestion for 2, 7, and 14 days. The effect of RC (0.1 microM) in increasing proteolytic activity was weakened in the kidney cortex cytosol of saline-ingested rats. The present study suggests that endogenous RC plays a role in the activation of proteases in the renal cortex cytosol, and that the RC effect is impaired in saline-ingested rats.

Inhibitory effect of regucalcin on Ca(2+)-dependent protein kinase activity in rat brain cytosol: involvement of endogenous regucalcin

The effect of regucalcin, a Ca(2+)-binding protein, on Ca(2+)-dependent protein kinase activity in the brain cytosol of rats with different ages (5 and 50 weeks old) was investigated. The addition of calmodulin (10 microg/ml) or dioctanoylglycerol (5 microg/ml) in the enzyme reaction mixture caused a significant increase in protein kinase activity in the presence of CaCl2 (1 mM), indicating that Ca2+ calmodulin or protein kinase C is present in the cytosol. Such an increase was completely prevented by the addition of regucalcin (10(-7) M). Moreover, regucalcin (10(-7) M) significantly inhibited cytosolic protein kinase activity without Ca2+/calmodulin or dioctanoylglycerol addition. Meanwhile, the presence of anti-regucalcin monoclonal antibody (10-50 ng/ml) in the enzyme reaction mixture caused a significant elevation of protein kinase activity, suggesting an inhibitory effect of endogenous regucalcin. Brain cytosolic protein kinase activity was significantly elevated by increasing age (50-week-old rats). Also, regucalcin (10(-7) M) significantly decreased protein kinase activity without Ca(2+) addition in the brain cytosol of aged rats. However, the effect of anti-regucalcin monoclonal antibody (50 ng/ml) in elevating protein kinase activity was not seen in the brain cytosol of aged rats. These results suggest that regucalcin has an inhibitory effect on Ca(2+)-dependent protein kinase activity in rat brain cytosol, and that the effect of endogenous regucalcin may be weakened in the brain cytosol of aged rats.

Stimulatory effect of regucalcin on proteolytic activity in rat renal cortex cytosol: involvement of thiol proteases

The effect of regucalcin, a calcium-binding protein, on neutral proteolytic activity in the cytosol of rat kidney cortex was investigated. Proteolytic activity was significantly increased in the presence of regucalcin (0.01-0.25 microM) in the enzyme reaction mixture. This increase was not significantly altered by the addition of CaCl, (0.01 and 1.0 mM) or EGTA (1.0 mM), indicating that the effect of regucalcin was independent on Ca2+. The effect of regucalcin to increase proteolytic activity was completely prevented in the presence of N-ethylmaleimide (5 mM), a modifying reagent of thiol groups. Proteolytic activity was clearly elevated by dithiothreitol (5 mM). This elevation was further enhanced by regucalcin (0.1 microM). Meanwhile, the stimulatory effect of regucalcin on proteolytic activity was not significantly altered in the presence of diisopropylfluorophosphate (2.5 mM), an inhibitor of serine proteases. Also, the regucalcin effect was not appreciably changed by the addition of EDTA (2.5 mM), a chelator of metal ions, indicating that it is not involved in metal-related proteases. These results demonstrate that regucalcin can increase proteolytic activity in the cytosol of rat kidney cortex. Regucalcin may activate thiol proteases independent on Ca2+.

Stimulatory effect of calcium-binding protein regucalcin on phosphatase activity in the brain cytosol of rats with different ages

The effect of calcium-binding protein regucalcin on phosphatase activity in the brain cytosol of rats with different ages was investigated. The presence of regucalcin (10(-8) and 10(-7) M) in the enzyme reaction mixture caused a significant increase of neutral p-nitrophenylphosphatase activity in the brain cytosol obtained from 5- and 50-week-old rats. This increase was seen in the absence or presence of calmodulin (2 microg/mL) and calcium chloride (100 microM). Brain cytosolic phosphatase activity was not significantly altered by S-100A (10(-6) M) or calbindin (10(-7) M), which is a calcium-binding protein. Regucalcin-increased phosphatase activity was clearly decreased by N-ethylmaleimide, a modifying reagent of thiol(SH)-group, suggesting that regucalcin acts on the SH-group of the enzyme. Moreover, the presence of anti-regucalcin monoclonal antibody (50-200 ng/mL) in the reaction mixture caused a significant decrease of brain cytosolic phosphatase activity, suggesting that the endogenous regucalcin has a stimulatory effect on the enzyme activity. These results suggest that regucalcin plays a role in the regulation of protein phosphatase in rat brain cytosol.