Ca2+-mobilizing agonists increase mitochondrial ATP production to accelerate cytosolic Ca2+ removal: aberrations in human complex I deficiency

Previously, we reported that both the bradykinin (Bk)-induced increase in mitochondrial ATP concentration ([ATP]_M) and the rate of cytosolic Ca²⁺ removal are significantly decreased in skin fibroblasts from a patient with an isolated complex I deficiency. Here we demonstrate that the mitochondrial Ca²⁺ indicator rhod-2 can be used to selectively buffer the Bk-induced increase in mitochondrial Ca²⁺ concentration ([Ca²⁺]_M) and, consequently, the Ca²⁺-stimulated increase in [ATP]_M, thus allowing studies of how the increase in [ATP]_M and the cytosolic Ca²⁺ removal rate are related. Luminometry of healthy fibroblasts expressing either aequorin or luciferase in the mitochondrial matrix showed that rhod-2 dose dependently decreased the Bk-induced increase in [Ca²⁺]_M and [ATP]_M by maximally 80 and 90%, respectively. Digital imaging microscopy of cells coloaded with the cytosolic Ca²⁺ indicator fura-2 revealed that, in parallel, rhod-2 maximally decreased the cytosolic Ca²⁺ removal rate by 20%. These findings demonstrate that increased mitochondrial ATP production is required for accelerating cytosolic Ca²⁺ removal during stimulation with a Ca²⁺-mobilizing agonist. In contrast, complex I-deficient patient fibroblasts displayed a cytosolic Ca²⁺ removal rate that was already decreased by 40% compared with healthy fibroblasts. Rhod-2 did not further decrease this rate, indicating the absence of mitochondrial ATP supply to the cytosolic Ca²⁺ pumps. This work reveals the usefulness of rhodamine-based Ca²⁺ indicators in examining the role of intramitochondrial Ca²⁺ in mitochondrial (patho) physiology. human skin fibroblast; OXPHOS disease; calcium ion extrusion; rhod-2; CGP-37157     

Intramitochondrial K+ as activator of carboxyatractyloside-induced Ca2+ release.

The role of intramitochondrial K+ content on the increase in membrane permeability to Ca2+, as induced by carboxyatractyloside was studied. In mitochondria containing a high K+ concentration (83 nmol/mg), carboxyatractyloside induced a fast and extensive mitochondrial Ca2+ release, membrane de-energization, and swelling. Conversely, in K(+)-depleted mitochondria (11 nmol/mg), carboxyatractyloside was ineffective. The addition of 40 mM K+ to K(+)-depleted mitochondria restored the capability of atractyloside to induce an increase in membrane permeability to Ca2+ release. The determination of matrix free Ca2+ concentration showed that, at an external free-Ca2+ concentration of 0.8 microM, control mitochondria contained 3.9 microM of free Ca2+ whereas K(+)-depleted mitochondria contained 0.9 microM free Ca2+. It is proposed that intramitochondrial K+ affects the matrix free Ca2+ concentration required to induce a state of high membrane permeability.     

Calmodulin an activator of human erythrocyte (Ca2+ + Mg2+)-ATPase phosphorylation

The effect of purified calmodulin on the calcium-dependent phosphorylation of human erythrocyte membranes was studied. Under the conditions employed, only one major peak of phosphorylation was observed when solubilized membrane proteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of this phosphorylated protein band was estimated to be 130 000 and in the presence of purified red blood cell calmodulin, the rate of phosphorylation of this band was increased. These data suggest that calmodulin activation of (Ca²⁺ + Mg²⁺)-ATPase could be a partial reflection of an increased rate of phosphorylation of the (Ca²⁺ + Mg²⁺)-ATPase of human erythrocyte membranes.     

Synergistic stimulation of Ca2+ uptake by glucagon and Ca2+-mobilizing hormones in the perfused rat liver. A role for mitochondria in long-term Ca2+ homoeostasis.

A perfused liver system incorporating a Ca2+-sensitive electrode was used to study the long-term effects of glucagon and cyclic AMP on the mobilization of Ca2+ induced by phenylephrine, vasopressin and angiotensin. At 1.3 mM extracellular Ca2+ the co-administration of glucagon (10 nM) or cyclic AMP (0.2 mM) and a Ca2+-mobilizing hormone led to a synergistic potentiation of Ca2+ uptake by the liver, to a degree which was dependent on the order of hormone administration. A maximum net amount of Ca2+ influx, corresponding to approx. 3800 nmol/g of liver (the maximum rate of influx was 400 nmol/min per g of liver), was induced when cyclic AMP or glucagon was administered about 4 min before vasopressin and angiotensin. These changes are over an order of magnitude greater than those induced by Ca2+-mobilizing hormones alone [Altin & Bygrave (1985) Biochem. J. 232, 911-917]. For a maximal response the influx of Ca2+ was transient and was essentially complete after about 20 min. Removal of the hormones was followed by a gradual efflux of Ca2+ from the liver over a period of 30-50 min; thereafter, a similar response could be obtained by a second administration of hormones. Dose-response measurements indicate that the potentiation of Ca2+ influx by glucagon occurs even at low (physiological) concentrations of the hormone. By comparison with phenylephrine, the stimulation of Ca2+ influx by vasopressin and angiotensin is more sensitive to low concentrations of glucagon and cyclic AMP, and can be correlated with a 20-50-fold increase in the calcium content of mitochondria. The reversible uptake of such large quantities of Ca2+ implicates the mitochondria in long-term cellular Ca2+ regulation.     

Desensitization of the Ca2+-mobilizing system to serum growth factors by Ha-ras and v-mos.

An elevation of the intracellular pH and a rise in the cytoplasmic Ca2+ concentration are considered important mitogenic signals which are observed after stimulation by various growth factors. In a preceding report it was demonstrated that the expression of Ha-ras or v-mos in cells transfected with Ha-ras or v-mos, respectively, leads to an activation of the Na+/H+ antiporter and a concomitant rise in intracellular pH (W. Doppler, R. Jaggi, and B. Groner, Gene 54:145-151, 1987). This report describes the effect of the Ha-ras and v-mos oncogenes on intracellular Ca2+ release. The expression of Ha-ras in NIH 3T3 cells carrying a glucocorticoid-inducible transforming Ha-ras gene caused a desensitization of the Ca2+-mobilizing system to serum growth factors. The induction of p21ras in cells carrying the corresponding glucocorticoid-inducible proto-oncogene did not affect the Ca2+ response to growth factors. Conditions leading to the expression of the transforming Ha-ras gene but not those causing the induction of the normal Ha-ras gene yielded an increase in phosphatidylinositol turnover and a concomitant rise in inositol phosphates. Results similar to those obtained with the transforming Ha-ras gene were seen after the expression of v-mos. The data are consistent with a mechanism in which expression of the transforming Ha-ras gene leads to a release of Ca2+ from intracellular stores via elevated levels of inositol trisphosphate.     

Intracellular pH-regulatory mechanisms in pancreatic acinar cells. II. Regulation of H+ and HCO3- transporters by Ca2(+)-mobilizing agonists

Pancreatic acini loaded with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein were used to examine the effect of Ca2(+)-mobilizing agonists on the activity of acid-base transporters in these cells. In the accompanying article (Muallen, S., and Loessberg, P. A. (1990) J. Biol. Chem. 265, 12813-12819) we showed that in 4-(2-hydroxyethyl)-1-piperazine-ethanesulfonic acid (HEPES)-buffered medium the main pHi regulatory mechanism is the Na+/H+ exchanger, a while in HCO3(-)-buffered medium pHi is determined by the combined activities of a Na+/H+ exchanger, a Na(+)-HCO3- cotransporter and a Cl-/HCO3- exchanger. In this study we found that stimulation of acini with Ca2(+)-mobilizing agonists in HEPES or HCO3(-)-buffered media is followed by an initial acidification which is independent of any identified plasma membrane-located acid-base transporting mechanism, and thus may represent intracellularly produced acid. In HEPES-buffered medium there was a subsequent large alkalinization to pHi above that in resting cells, which could be attributed to the Na+/H+ exchanger. Measurements of the rate of recovery from acid load indicated that the Na+/H+ exchanger was stimulated by the agonists. In HCO3(-)-buffered medium the alkalinization observed after the initial acidification was greatly attenuated. Examination of the activity of each acid-base transporting mechanism in stimulated acini showed that in HCO3(-)-buffered medium: (a) recovery from acid load in the presence of H2-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (H2DIDS) (Na+/H+ exchange) was stimulated similar to that found in HEPES-buffered medium; (b) recovery from acid load in the presence of amiloride and acidification due to removal of external Na+ in the presence of amiloride (HCO3- influx and efflux, respectively, by Na(+)-HCO3- cotransport) were inhibited; and (c) HCO3- influx and efflux due to Cl-/HCO3- exchange, which was measured by changing the Cl- or HCO3- gradients across the plasma membrane, were stimulated. Furthermore, the rate of Cl-/HCO3- exchange in stimulated acini was higher than the sum of H+ efflux due to Na+/H+ exchange and HCO3- influx due to Na(+)-HCO3- cotransport. Use of H2DIDS showed that the latter accounted for the attenuated changes in pHi in HCO3(-)-buffered medium, as much as treating the acini with H2DIDS resulted in similar agonist-mediated pHi changes in HEPES- and HCO3(-)-buffered media. The effect of agonists on the various acid-base transporting mechanisms is discussed in terms of their possible role in transcellular NaCl transport, cell volume regulation, and cell proliferation in pancreatic acini.     

Tumor necrosis factor-alpha inhibits store-mediated Ca2+ entry in the human hepatocellular carcinoma cell line HepG2

Tumor necrosis factor-alpha (TNF-alpha) is an important component of the early signaling pathways leading to liver regeneration and proliferation, but it is also responsible for several hepatotoxic effects. We have investigated the effect of TNF-alpha on thapsigargin (TG)-induced store-mediated Ca2+ entry (SMCE) in the human hepatocellular carcinoma cell line HepG2. In these cells, short-term (10 min) exposure to TNF-alpha slightly increased SMCE. In contrast, long-term (12 h) exposure to TNF-alpha significantly reduced SMCE. This effect was reversed by coincubation with atrial natriuretic peptide (ANP), which itself had no effect on SMCE. Cytochalasin D and latrunculin A, inhibitors of actin polymerization, abolished SMCE. Long-term exposure of HepG2 cells to TNF-alpha abolished TG-induced actin polymerization and membrane association of Ras proteins. When TNF-alpha was added in combination with ANP, these effects were reduced. These findings suggest that in HepG2 cells, TNF-alpha inhibits SMCE by affecting reorganization of the actin cytoskeleton, probably by interfering with the activation of Ras proteins, and that ANP protects against these inhibitory effects of TNF-alpha.     

Role of different Ca2+ sources in the superoxide production of human neutrophil granulocytes.

The role of different Ca2+ sources in the activation of the NADPH oxidase was investigated in human neutrophil granulocytes. Selective depletion of the stimulus-responsive intracellular Ca2+ -pool and the consequent opening of the store-dependent Ca2+ channel of the plasma membrane was achieved with thapsigargin, an inhibitor of microsomal Ca2+ -ATPase. Low concentration (10-100 nM) of thapsigargin did not induce any O2*- -production, indicating that elevation of [Ca2+]ic to similar level and probably via similar route as following stimulation of chemotactic receptors, by itself is not sufficient to activate the NADPH oxidase. In significantly higher concentration (1-10 microM) thapsigargin did induce O2*- -generation but this effect was not the result of elevation of [Ca2+]ic. In the absence of external Ca2+ a gradual decrease of the responsive Ca2+ pool was accompanied by a gradual decrease of the rate and duration of the respiratory response stimulated by formyl-methionyl-leucyl-phenylalanin. Maximal extent of receptor-initiated O2*- -production could only be obtained when the intracellular [Ca2+] was higher than the resting level. Under this condition Ca2+ originating from intracellular or external source was equally effective in supporting the biological response.     

Reversible shift between two states of Ca2+-ATPase in human erythrocytes mediated by Ca2+ and a membrane-bound activator.

The (Ca2+ + Mg2+)-dependent ATPase (ATP phosphohydrolase, EC 3.6.1.3) from human erythrocytes occurred in two different states, A-state and B-state, depending on the membrane preparation. The A-state showed low maximum activity (V) and the Ca2+ activation was characterized by a Hill coefficient, nH, of about 1 and a Michaelis constant, KCa, about 30 micron. The B-state showed high V, a nH above 1, which indicates positive cooperativity of Ca2+ activation, and KCa of about 1 micron. With varying ATP concentrations, both the A-state and B-state showed negative cooperativity and slightly different values of Km. The B-state was shifted to A-state when the membranes were exposed to low Ca2+ concentration. The shift reached 50% at approx. 0.5 micron Ca2+. At the low Ca2+ concentrations an activator was released from the membranes. The A-state was shifted to the B-state when the membranes were exposed to Ca2+ in the presence of the activator. The shift reached 50% at about 30 micron Ca2+. The recovery of high V was time dependent and lasted several minutes. Increasing concentrations of Ca2+ and activator accelerated the recovery. It is suggested that the A-state and the B-state correspond to enzyme free of activator and enzyme associated with activator, respectively. Furthermore, the two states may respresent a resting and an active state, respectively, of the calcium pump.     

The interaction of Ca2+/Mg2+ ATPase activator protein and Ca2+ with human erythrocyte membranes.

This paper presents the first unambiguous demonstration that a unique protein isolated from the hemolysate of human erythrocytes is responsible for increasing both the apparent Ca²⁺ ion affinity and maximum rate of ATP hydrolysis of the membrane-bound $\text{Ca}{}^{\mathrm{2+}}\text{Mg}{}^{\mathrm{2+}}$ ATPase. Unlike previous reports where an unpurified extract from red blood cells was used to activate the ATPase, our results clearly demonstrate that a single protein species, whether initially associated with or added back to the membrane is responsible for the observed changes in ATPase activity.     

Studies on an activator of the (Ca2+ plus Mg2+)-ATPase of human erythrocyte membranes.

1. An activator of the (Ca2+ plus Mg2+)-stimulated ATPase present in the human erythrocytes (membrane) has been isolated in soluble form from hemolysates of these cells. Partial purification has been achieved through use of carboxymethyl-Sephadex chromatography. The resulting activator fraction contained no hemoglobin and only 0.3% of the total adenylate kinase activity of the cell. 2. Whereas the activator was released from erythrocytes subjected to hemolysis in 20 miosM buffer at pH 7.6 or at pH 5.8, only the membranes prepared at pH 7.6 were affected by it. 2. Whereas the activator was released from erythrocytes subjected to hemolysis in 20 miosM buffer at pH 7.6 or at pH 5.8, only the membranes prepared at pH 7.6 were affected by it. 3. When (Ca2+ plus Mg2+)-ATPase activity was measured by 32Pi release from (gamma-32P)ATP, freeze-thawed erythrocytes, as well as membranes prepared at pH 5.8 and at pH 7.6, expressed lower values than noted by assay for total Pi release. When ADP instead of ATP was used as substrate, significant amount of Pi were released by these erythrocyte preparations. Further study revealed (a) production of ATP and AMP from ADP with membranes and hemolysate alone, and (b) exchange of the gamma-and B-position phosphate on (gama-32P)ATP in the presence of membranes plus hemolysates. These observations established the presence of adenylate kinase activity in the (membrane-free) hemolysates and in membranes. It further supports the conclusion that Pi release from ADP by human erythrocytes (freeze-thawed) and by their isolated membranes is due to formation of ATP by adenylate kinase and hydrolysis of this generated ATP by (Ca2+ plus Mg2+)-ATPase. 4. The following points were also established: (a) absence of an ADPase in human erythrocytes; (b) the (Ca2+ plus Mg2+)-ATPase activator enhanced cleavage only of the gama-position of ATP and (c) the (Ca2+ plus Mg2+)-ATPase activator is neither adenylate kinase nor hemoglobin.     

Calmodulin. An activator of human erythrocyte (Ca2+ + Mg2+)ATPase phosphorylation

The effect of purified calmodulin on the calcium-dependent phosphorylation of human erythrocyte membranes was studied. Under the conditions employed, only one major peak of phosphorylation was observed when solubilized membrane proteins were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of this phosphorylated protein band was estimated to be 130000 and in the presence of purified red blood cell calmodulin, the rate of phosphorylation of this band was increased. These data suggest that calmodulin activation of (Ca2+ + Mg2+)-ATPase could be a partial reflection of an increased rate of phosphorylation of the (Ca2+ + Mg2+)-ATPase of human erythrocyte membranes.     

Substitution at the 8-position of 3'-deoxy-cyclic ADP-carbocyclic-ribose, a highly potent Ca2+-mobilizing agent, provides partial agonists

We previously showed that 3'-deoxy-cyclic ADP-carbocyclic-ribose (3'-deoxy-cADPcR, 4) is a stable and highly potent analogue of cyclic ADP-ribose (cADPR, 1), a Ca(2+)-mobilizing second messenger. From these results, we designed and synthesized other 3'-modified analogues of cADPcR having a substituent at the 8-position and found that this modification at the 8-position made them partial agonists. Among these compounds, 8-NH(2)-3'-deoxy-cADPcR (10) was identified as a potent partial agonist with an EC(50) value of 17 nM.     

Dual mechanisms in priming of the chemoattractant-induced respiratory burst in human granulocytes. A Ca2+-dependent and a Ca2+-independent route

After interaction with so-called priming agents, the respiratory burst in human granulocytes does not become activated, but is enhanced upon subsequent stimulation with the chemoattractant FMLP. Investigating the mechanism of the priming reaction, we found that a transient rise in the cytosolic free calcium concentration [( Ca2+]i) suffices to irreversibly prime human granulocytes. Thus, platelet-activating factor (PAF) induced a transient increase in [Ca2+]i and primed the cells to an enhanced respiratory burst upon subsequent interaction with FMLP. Artificially, the transient rise in [Ca2+]i was mimicked by addition and subsequent removal of the Ca2+ ionophore ionomycin; this treatment too, primed the respiratory burst of the granulocytes. The priming induced by ionomycin was completely abolished when [Ca2+]i changes were buffered during exposure of the cells to the ionophore. The priming induced by PAF was only partially inhibited under [Ca2+]i-buffering conditions during priming, indicating that multiple pathways exist in the priming of granulocytes by PAF.     

Purification of the Ca2+-stimulated ATPase activator from human erythrocytes. Its membership in the class of Ca2+-binding modulator proteins.

The activator of the Ca2+-stimulated ATPase of erythrocyte membranes was purified 13,000-fold to homogeneity from human erythrocytes. The protein gave a single band upon electrophoresis both with and without detergent, and upon isoelectric focusing. This protein was compared with Ca2+-binding modulator proteins from bovine brain and rat testis. All three proteins were homogeneous and co-migrated on electrophoresis both in the presence of detergent and without detergent at pH values on both sides of the isoelectric point of the protein. The amino acid compositions of the three proteins were nearly indistinguishable, and all three proteins contained 1 residue of the unusual amino acid, trimethyllysine. All three were also indistinguishable as measured by their ability to further stimulate the Ca2+-stimulated ATPase of human erythrocyte membranes. Thus, we conclude that they represent functionally the same protein. Upon storage of all three proteins, a second band was detectable by detergent gel electrophoresis; the biochemical activity and the behavior on nondetergent gels were not changed. The presence of this second band is probably responsible for previous reports of differences between the rat testis and bovine brain modulator protein. The possibility is discussed that this protein is a general intracellular Ca2+ receptor, which mediates the activities of Ca2+ as an intracellular messenger.     

Tumor necrosis factor is an important mediator of tumor cell killing by human monocytes

The mechanism of human peripheral blood monocyte-mediated cytotoxicity for tumor cells was investigated, using the A673 human rhabdomyosarcoma and HT-29 human colon adenocarcinoma lines as target cells. A673 cells were shown to be susceptible to the cytotoxic action of purified recombinant human tumor necrosis factor (TNF). A673 cells were also highly sensitive to the cytotoxic action of peripheral blood monocytes. Clones of A673 cells sensitive and resistant to TNF were isolated and characterized for their sensitivity to monocyte killing. A good correlation was found between the sensitivity of these clones to the cytotoxicity of TNF and their susceptibility to killing by monocytes. A TNF-specific neutralizing monoclonal antibody (MAb) reduced monocyte killing of parental A673 cells and of a TNF-sensitive clone of A673 cells. Inhibition of monocyte killing by this MAb was particularly pronounced at a low effector to target cell ratio. HT-29 cells were relatively resistant to the cytotoxic action of recombinant TNF and to monocyte killing. Treatment of HT-29 cells with recombinant human IFN-gamma increased their susceptibility to both TNF cytotoxicity and monocyte killing. In addition, MAb to TNF inhibited monocyte killing in HT-29 cells sensitized by incubation with IFN-gamma. Our data show that TNF is an important mediator of the cytotoxicity of human monocytes for tumor cells and that IFN-gamma can increase monocyte cytotoxicity by sensitizing target cells to the lytic action of TNF.     

Regulation of Ca2+ release-activated Ca2+ channels by INAD and Ca2+ influx factor

Thecoupling mechanism between depletion of Ca²⁺ stores in theendoplasmic reticulum and plasma membrane store-operated ion channelsis fundamental to Ca²⁺ signaling in many cell types and hasyet to be completely elucidated. Using Ca²⁺release-activated Ca²⁺ (CRAC) channels in RBL-2H3 cells asa model system, we have shown that CRAC channels are maintained in theclosed state by an inhibitory factor rather than being opened by theinositol 1,4,5-trisphosphate receptor. This inhibitory role can befulfilled by the Drosophila protein INAD (inactivation-noafter potential D). The action of INAD requires Ca²⁺ andcan be reversed by a diffusible Ca²⁺ influx factor. Thusthe coupling between the depletion of Ca²⁺ stores and theactivation of CRAC channels may involve a mammalian homologue of INADand a low-molecular-weight, diffusible store-depletion signal.

     

Opioid receptor agonists and Ca2+ modulation in human B cell lines.

Opiates and opioid peptides have been shown to modulate lymphocyte functions; however, little attention has been given to the type of receptors or receptor signaling mechanisms that are involved. Receptor-mediated signaling via ionized free Ca2+ is an event thought to be important in the triggering of lymphocyte activities. We report use of the calcium indicator dye, indo-1, and flow cytometry to identify B lymphocyte calcium responses to physiologic concentrations of opioid peptides. The human B cell lines Nalm 6 and JY responded to the naturally occurring opioid pentapeptide methionine-enkephalin or other opiate receptor agonists with a rapid, dose-dependent rise in free cytoplasmic Ca2+. This opioid peptide effect on Ca2+ modulation was inhibited by the opiate receptor antagonist naloxone. The synthetic enkephalin analogue DAMGO with specificity for mu-type opiate receptors and the synthetic opiate receptor agonists U50,488H and U69,593 with selectivity for kappa-type sites also stimulated calcium responses when applied to the B cell lines. These studies provide evidence that human B cell lines express functional opiate receptors of the mu- and kappa-types and suggest that such receptors, coupled with Ca2+ modulation, are instrumental in the B cell response to opiates and endogenous opioid neuropeptides.     

Ca2+ -regulated secretion of tissue-type plasminogen activator and von Willebrand factor in human endothelial cells

von Willebrand factor (vWF) and tissue-type plasminogen activator (tPA) are products of endothelial cells which are secreted into the bloodstream upon a stimulus-induced rise in intracellular Ca(2+). Although the release of both factors appears to be regulated similarly, they exhibit opposing physiological effects in the vasculature with vWF inducing coagulation and platelet aggregation and tPA triggering fibrinolysis and thrombolysis. To analyze possible differences in the regulated secretion of vWF and tPA in more detail, we recorded the Ca(2+)-triggered exocytosis of both factors in cultured human endothelial cells. We demonstrate that vWF and tPA which are stored in different granules within endothelial cells are released with different kinetics following endothelial stimulation with histamine or the Ca(2+) ionophore A23187. While the stimulus-induced release of vWF increases with time over a course of 30 min, maximal acute secretion of tPA is observed 5 min following stimulation and subsequently drops to background levels. In the case of vWF, secretion can also be monitored indirectly through an antibody-reinternalization assay which indicates an incomplete release of vWF during single exocytotic fusion events. Our data thus point to differences in the Ca(2+)-triggered secretion of vWF and tPA which could allow a fine-tuning of their release thereby ensuring a balanced physiological action.