Effect of nicotinamide-adenine dinucleotides on Ca2+ transport system in rat liver nuclei: stimulation of Ca2+ release by NAD+

Effect of graded hypoxia on high-affinity Ca2+-ATPase activity in cortical neuronal nuclei of newborn piglets

Previous studies have shown that nuclear calcium signals control a variety of nuclear functions including gene transcription, DNA synthesis, DNA repair and nuclear envelope breakdown. The present study tested the hypothesis that the activity of the neuronal nuclear high affinity Ca2+-ATPase increases as a function of decreased energy metabolism in the cerebral cortex. Studies were performed in 11 ventilated newborn piglets, age 3-5 days, divided into normoxic (Nx, n = 4) and hypoxic (Hx, n = 7) groups. The animals were exposed to a single FiO2 in the range from 0.21 to 0.05 for one hr. Cerebral tissue hypoxia was confirmed biochemically by determining brain tissue ATP and phosphocreatine levels. Neuronal nuclei were isolated and the high-affinity Ca2+-ATPase activity determined. During graded hypoxia, cerebral tissue ATP decreased from 4.80 +/- 0.58 (normoxic) to 1.03 +/- 0.38 (ranging from 0.61-1.63) micromol/g brain (p < 0.05) and PCr decreased from 3.94 +/- 0.75 (normoxic) to 0.99 +/- 0.27 (ranging from 0.50 to 1.31) micromol/g brain (p < 0.05). The total high affinity Ca2+-ATPase activity in the hypoxic nuclei increased and ranged from 541 to 662 nmol/mg protein/hr, compared to activity in normoxic group of 327 to 446 nmol/mg protein/hr. During graded hypoxia, the level of nuclear high affinity Ca2+-ATPase activity correlated inversely with ATP (r = 0.91) and PCr levels (r = 0.82), with activity increasing as tissue high energy phosphates decreased. The results demonstrate that the decrease in cerebral energy metabolism during hypoxia is linearly correlated with an increase in activity of high affinity Ca2+-ATPase in cerebral cortical nuclei from immature brain. We propose that increased nuclear membrane high affinity Ca2+-ATPase activity, leading to increased nuclear Ca2+, will result in altered expression of apoptotic genes that could initiate programmed neuronal death.

Localization of the cyclic ADP-ribose-dependent calcium signaling pathway in hepatocyte nucleus

CD38 is a type II transmembrane glycoprotein found on both hematopoietic and non-hematopoietic cells. It is known for its involvement in the metabolism of cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate, two nucleotides with calcium mobilizing activity independent of inositol trisphosphate. It is generally believed that CD38 is an integral protein with ectoenzymatic activities found mainly on the plasma membrane. Here we show that enzymatically active CD38 is present intracellularly on the nuclear envelope of rat hepatocytes. CD38 isolated from rat liver nuclei possessed both ADP-ribosyl cyclase and NADase activity. Immunofluorescence studies on rat liver cryosections and isolated nuclei localized CD38 to the nuclear envelope of hepatocytes. Subcellular localization via immunoelectron microscopy showed that CD38 is located on the inner nuclear envelope. The isolated nuclei sequestered calcium in an ATP-dependent manner. cADPR elicited a rapid calcium release from the loaded nuclei, which was independent of inositol trisphosphate and was inhibited by 8-amino-cADPR, a specific antagonist of cADPR, and ryanodine. However, nicotinic acid adenine dinucleotide phosphate failed to elicit any calcium release from the nuclear calcium stores. The nuclear localization of CD38 shown in this study suggests a novel role of CD38 in intracellular calcium signaling for non-hematopoietic cells.

Alteration in calcium content and Ca2+-ATPase activity in the liver nuclei of rats orally administered carbon tetrachloride

The alteration in calcium transport in the liver nuclei of rats orally administered carbon tetrachloride (CCl4) was investigated. Rats received a single oral administration of CCl4 (5, 10, and 25%, 1.0 ml/100 g body weight), and 5, 24 and 48 h later the animals were sacrificed. The administration of CCl4 (25%) caused a remarkable elevation of calcium content in the liver tissues and the nuclei of rats. Liver nuclear Ca2+-ATPase activity was markedly decreased by CCl4 (25%) administration. The presence of dibutyryl cyclic AMP(10(-4) and 10(-3) M) or inositol 1,4,5-trisphosphate (10(-6) and 10(-5) M) in the enzyme reaction mixture caused a significant decrease in Ca2+-ATPase activity in the liver nuclei obtained from normal rat, while the enzyme activity was significantly increased by calmodulin (1.0 and 2.0 microg/ml). These signaling factor's effects were completely impaired in the liver nuclei obtained from CCl4 (25%)-administered rats. DNA fragmentation in the liver nuclei obtained from CCl4-administered rats was significantly decreased by the presence of EGTA (2 mM) in the reaction mixture, suggesting that the endogenous calcium activates nuclear DNA fragmentation. The present study demonstrates that calcium transport system in the liver nuclei is impaired by liver injury with CCl4 administration in rats.

Does the nuclear envelope contain two types of ligand-gated Ca2+ release channels?

The nuclear envelope is composed of two membranes deliminating a perinuclear space which displays functional properties similar to those of a Ca2+-storing compartment. ATP-driven Ca2+ uptake and InsP3-induced Ca2+ release processes have been described in isolated nuclei. Recently, it was reported that cADP-ribose and InsP3 can trigger a nucleoplasmic Ca2+ increase. It was hypothesized that the inner nuclear membrane possesses Ca2+ channels that are regulated by ryanodine or InsP3. Radio-ligand binding assays and Western blot experiments were performed in order to investigate their presence in sheep cardiac and rat liver nuclear envelopes. Ryanodine receptors (RyR) were not detected in liver nuclear envelopes by either binding assay or Western blot analysis. However, cardiac nuclear envelopes were found to retain a very low level of specific ryanodine binding, which was not detected on immuno-blots obtained with three types of isoform-specific RyR antibodies. In contrast, nuclear InsP3-binding sites were consistently detected in both cardiac and liver nuclear envelopes. Altogether, these results provide evidence for the major contributor InsP3-gated Ca2+ channels in control of Ca2+ release from the perinuclear space in liver and cardiac cells.

Effect of apoptosis-related compounds on Ca2+ transport system in isolated rat liver nuclei

The effect of various inhibitors of DNA topoisomerase II, which has been shown to induce apoptotic cell death, on Ca2+ transport in isolated rat liver nuclei was investigated. Ca2+ uptake and release were determined with a Ca2+ electrode. The presence of aurintricarboxylic acid (ATA; 10(-6) to 10(-4) M), etoposide (10(-4) M), genistein (10(-5) and 10(-4) M) or amsacrine (10(-4) M) in the reaction mixture caused a significant increase in Ca2+ release from the nuclei. Also, these compounds (10(-4) M) significantly inhibited Ca2+ uptake by the nuclei. However, the presence of ATA (10(-5) and 10(-4) M) in the enzyme reaction mixture did not significantly inhibit Ca2+-ATPase activity, which is involved in the nuclear Ca2+ uptake, in the liver nuclei, while etoposide (10(-4) M), genistein (10(-4) M) and amsacrine (10(-4) M) appreciably decreased the enzyme activity. Meanwhile, addition of Ca2+ clearly activated DNA fragmentation in the liver nuclei. The Ca2+ activated DNA fragmentation was significantly prevented by the presence of etoposide, genistein and amsacrine with the concentrations of 10(-5) and 10(-4) M in the reaction mixture, although ATA (10(-5) and 10(-4) M) had no effect. The present study demonstrates that some apoptosis inducible compounds used can influence on Ca2+ transport system in isolated rat liver nuclei, suggesting a decrease of nuclear Ca2+ level involved in nuclear functions.

Inositol 1,4,5-trisphosphate receptor is located to the inner nuclear membrane vindicating regulation of nuclear calcium signaling by inositol 1,4,5-trisphosphate. Discrete distribution of inositol phosphate receptors to inner and outer nuclear membranes

Transient rise in nuclear calcium concentration is implicated in the regulation of events controlling gene expression. Mechanism by which calcium is transported to the nucleus is vehemently debated. Inositol 1,4,5-trisphosphate (InsP3) and inositol-1,3,4,5-tetrakisphosphate (InsP4) receptors have been located to the nucleus and their role in nuclear calcium signaling has been proposed. Outer nuclear membrane was separated from the inner membrane. The two membrane preparations were, as best as possible, devoid of cross contamination as attested by marker enzyme activity, Western blotting with antilamin antibody, and electron microscopy. InsP4 receptor and Ca(2+)-ATPase were located to the outer nuclear membrane. InsP3 receptor was located to the inner nuclear membrane. ATP or InsP4 induced nuclear calcium uptake. External free calcium concentration, in the medium bathing the nuclei, determined the choice for ATP or InsP4-mediated calcium transport. We present a mechanistic model for nuclear calcium transport. According to this model, calcium can reach the nucleus envelope either by the action of ATP or InsP4. However, the calcium release from the nucleus envelope to the nucleoplasm is mediated by InsP3 through the activation of InsP3 receptor, which is located to the inner nuclear membrane. The action of InsP3 in this process was instantaneous and transient and was sensitive to heparin.

Effect of nuclear Ca2+ uptake inhibitors on Ca(2+)-activated DNA fragmentation in rat liver nuclei

The effect of nuclear Ca2+ uptake inhibitors on the Ca(2+)-activated DNA fragmentation in rat liver nuclei was investigated. The addition of Ca2+ (40 microM) into the reaction mixture containing liver nuclei in the presence of 2.0 mM ATP caused a remarkable increase in nuclear DNA fragmentation. This Ca(2+)-activated DNA fragmentation was not seen in the absence of ATP, because nuclear Ca2+ uptake is not initiated without ATP addition. Moreover, the presence of various reagents (10 microM arachidonic acid, 2.0 mM NAD+, 10 microM zinc sulfate and 0.2 mM N-ethylmaleimide), which could inhibit Ca(2+)-ATPase activity and Ca2+ uptake in the nuclei, produced a significant inhibition of the Ca(2+)-activated DNA fragmentation in the nuclei. The results show that the Ca(2+)-activated DNA fragmentation is involved in the uptake of Ca2+ by the nuclei, suggesting a role of Ca2+ transport system in the regulation of liver nuclear functions.

Enhancement of nuclear Ca(2+)-ATPase activity in regenerating rat liver: involvement of nuclear DNA increase

The alteration of calcium content, Ca(2+)-ATPase activity, DNA content and DNA fragmentation in the nuclei of regenerating rat liver was investigated. Liver was surgically removed about 70% of that of sham-operated rats. The reduced liver weight by partial hepatectomy was completely restored at 3 days after the surgery. Regenerating liver significantly increased Ca(2+)-ATPase activity and DNA content in the nuclei between 1 and 5 days after hepatectomy. The nuclear calcium content was clearly increased from 2 days after hepatectomy. The increase of Ca(2+)-ATPase activity in regenerating liver was clearly inhibited by the presence of trifluoperazine (10 microM), staurosporine (2.5 microM) and dibucaine (10 microM), which are inhibitors of calmodulin and protein kinase, in the enzyme reaction mixture. However, the nuclear enzyme activity in normal rat liver was not significantly altered by these inhibitors. Meanwhile, the increase of nuclear DNA content in regenerating liver was completely blocked by the administration of trifluoperazine (2.5 mg/100 g body weight), suggesting an involvement of calmodulin. Now, the nuclear DNA fragmentation was significantly decreased in regenerating liver, suggesting that this decrease is partly contributed to the increase in nuclear DNA content. The present study clearly demonstrates that regenerating liver enhances nuclear Ca(2+)-ATPase activity and induces a corresponding elevation of nuclear calcium content. This Ca(2+)-signaling system may be involved in the regulation of nuclear DNA functions in regenerating rat liver.

Nuclear calcium transport and the role of calcium in apoptosis

The last decade has been the rapid development of research investigating the molecular mechanisms whereby hormones, peptide growth factors and cytokines regulate cell metabolism, differentiation and proliferation. One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid Ca2+ redistribution throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Ca2+ signals are required for a number of cellular functions, including the activation of nuclear processes such as gene transcription and cell cycle events. The latter requires that appropriate Ca2+ signals elicited in response to agonists be transduced across the nuclear envelope. It has generally been assumed that small molecules, metabolites and ions could diffuse freely across the nuclear envelope. Nevertheless, several findings during the past few years have suggested that nuclear pore permeability can be regulated and that ion transport systems and ion-selective channels may exist in the nuclear membranes and regulate intranuclear processes. Intranuclear Ca2+ fluctuations can affect chromatin organization, induce gene expression and also activate cleavage of nuclear DNA by nucleases during programmed cell death or apoptosis. The possible mechanisms involved in nuclear Ca2+ transport and the regulation of nuclear Ca(2+)-dependent enzymes in apoptosis are discussed in the following sections.

Nuclear Ca2+: physiological regulation and role in apoptosis

The last decade has seen the rapid development of research investigating the molecular mechanisms whereby hormones, peptide growth factors and cytokines regulate cell metabolism, differentiation and proliferation. One general signalling mechanism used to transfer the information delivered by agonists into appropriate intracellular compartments involves the rapid Ca2+ redistribution throughout the cell, which results in transient elevations of the cytosolic free Ca2+ concentration. Ca2+ signals are required for a number of cellular processes including the activation of nuclear processes such as gene transcription and cell cycle events. The latter require that appropriate Ca2+ signals elicited in response to agonists be transduced across the nuclear envelope. It has generally been assumed that small molecules, metabolites and ions could freely diffuse across the nuclear envelope. Nevertheless several findings during the past few years have suggested that nuclear pore permeability can be regulated and that ion transport systems and ion-selective channels may exist on the nuclear membranes and regulate intranuclear processes. Intranuclear Ca2+ fluctuations can affect chromatin organization, induce gene expression and also activate cleavage of nuclear DNA by nucleases during programmed cell death or apoptosis. The possible mechanisms involved in nuclear Ca2+ transport and the control of nuclear Ca(2+)-dependent enzymes in apoptosis is discussed below.

Effect of phorbol 12-myristate 13-acetate on Ca2+-ATPase activity in rat liver nuclei

The effect of phorbol 12-myristate 13-acetate (PMA) on Ca(2+)-ATPase activity in rat liver nuclei was investigated. Ca(2+)-ATPase activity was calculated by subtracting Mg(2+)-ATPase activity from (Ca(2+)-Mg(2+)-ATPase activity. The nuclear Ca(2+)-ATPase activity was significantly increased by the presence of PMA (2-20 microM) in the enzyme reaction mixture; the maximum effect was seen at 10 microM. The PMA (10 microM)-increased Ca(2+)-ATPase activity was not blocked by the presence of staurosporine (2 microM) or dibucaine (2 and 10 microM), an inhibitor of protein kinase. Meanwhile, vanadate (20 and 100 microM) caused a significant reduction in the nuclear Ca(2+)-ATPase activity increased by PMA (10 microM). The present finding suggests that PMA has an activating effect on liver nuclear Ca(2+)-ATPase independent of protein kinase.

Involvement of Ca(2+)-stimulated adenosine 5'-triphosphatase in the Ca2+ releasing mechanism of rat liver nuclei

The regulatory role of Ca(2+)-stimulated adenosine 5'-triphosphatase (Ca(2+)-ATPase) in Ca2+ transport system of rat liver nuclei was investigated. Ca2+ uptake and release were determined with a Ca2+ electrode. Ca(2+)-ATPase activity was calculated by subtracting Mg(2+)-ATPase activity from (Ca(2+)-Mg2+)-ATPase activity. The release of Ca2+ from the Ca(2+)-loaded nuclei was evoked progressively after Ca2+ uptake with 1.0 mM ATP addition, while it was only slightly in the case of 2.0 mM ATP addition, indicating that the consumption of ATP causes a leak of Ca2+ from the Ca(2+)-loaded nuclei. The presence of N-ethylmaleimide (NEM; 0.1 mM) caused an inhibition of nuclear Ca2+ uptake and induced a promotion of Ca2+ release from the Ca(2+)-loaded nuclei. NEM (0.1 and 0.2 mM) markedly inhibited nuclear Ca(2+)-ATPase activity. This inhibition was completely blocked by the presence of dithiothreitol (DTT; 0.1 and 0.5 mM). Also, DTT inhibited the effect of NEM (0.1 mM) on nuclear Ca2+ uptake and release. Meanwhile, verapamil and diltiazem (10 microM), a blocker of Ca2+ channels, did not prevent the NAD+ (1.0 and 2.0 mM), zinc sulfate (1.0 and 2.5 microM) and arachidonic acid (10 microM)-induced increase in nuclear Ca2+ release, suggesting that Ca2+ channels do not involve on Ca2+ release from the nuclei. These results indicates that an inhibition of nuclear Ca(2+)-ATPase activity causes the decrease in nuclear Ca2+ uptake and the release of Ca2+ from the Ca(2+)-loaded nuclei. The present finding suggests that Ca(2+)-ATPase plays a critical role in the regulatory mechanism of Ca2+ uptake and release in rat liver nuclei.

Characterization of Ca(2+)-stimulated adenosine 5'-triphosphatase and Ca2+ sequestering in rat liver nuclei

The role of Ca(2+)-stimulated adenosine 5'-triphosphatase (Ca(2+)-ATPase) in Ca2+ sequestering of rat liver nuclei was investigated. Ca(2+)-ATPase activity was calculated by subtracting Mg(2+)-ATPase activity from (Ca(2+)-Mg2+)-ATPase activity. Ca2+ uptake and release were determined with a Ca2+ electrode. Nuclear Ca(2+)-ATPase activity increased linearly in the range of 10-40 microM Ca2+ addition. With those concentrations, Ca2+ was completely taken up by the nuclei dependently on ATP (2 mM). Nuclear Ca(2+)-ATPase activity was decreased significantly by the presence of arachidonic acid (25 and 50 microM), nicotinamide-adenine dinucleotide (NAD+; 2 mM) and zinc sulfate (2.5 and 5.0 microM). These reagents caused a significant decrease in the nuclear Ca2+ uptake and a corresponding elevation in Ca2+ release from the nuclei. Moreover, calmodulin (10 micrograms/ml) increased significantly nuclear Ca(2+)-ATPase activity, and this increase was not seen in the presence of trifluoperazine (10 microM), an antagonist of calmodulin. The present findings suggest that Ca(2+)-ATPase plays a role in Ca2+ sequestering by rat liver nuclei, and that calmodulin is an activator. Moreover, the inhibition of Ca(2+)-ATPase may evoke Ca2+ release from the Ca(2+)-loaded nuclei.