Interdependent regulation of intracellular acidification and SHP-1 in apoptosis.
The G protein-coupled receptor agonist somatostatin (SST)-induces apoptosis in MCF-7 human breast cancer cells. This is associated with induction of wild-type p53, Bax, and an acidic endonuclease. We have shown recently that its cytotoxic signaling is mediated via membrane-associated SHP-1 and is dependent on decrease in intracellular pH (pHi) to 6.5. Here we investigated the relationship between intracellular acidification and SHP-1 in cytotoxic signaling. Clamping of pHi at 7.25 by the proton-ionophore nigericin abolished SST-signaled apoptosis without affecting its ability to regulate SHP-1, p53, and Bax. Apoptosis could be induced by nigericin clamping of pHi to 6.5. Such acidification-induced apoptosis was not observed at pHi <6.0 or >6.7. pHi-dependent apoptosis was associated with the translocation of SHP-1 to the membrane, enhanced in cells overexpressing SHP-1, and was abolished by its inactive mutant SHP-1C455S. Acidification caused by inhibition of Na+/H+ exchanger and H+ ATPase (pHi = 6.55 and 6.65, respectively) also triggered apoptosis. The effect of concurrent inhibition of Na+/H+ exchanger and H(+)-ATPase on pHi and apoptosis was comparable with that of SST. Acidification-induced, SHP-1-dependent apoptosis occurred in breast cancer cell lines in which SST was cytotoxic (MCF-7 and T47D) or not (MDA-MB-231). We conclude that: (a) SST-induced SHP-1-dependent acidification occurs subsequent to or independent of the induction of p53 and Bax; (b) SST-induced intracellular acidification may arise due to inhibition of Na+/H+ exchanger and H(+)-ATPase; and (c) SHP-1 is necessary not only for agonist-induced acidification but also for the execution of acidification-dependent apoptosis. We suggest that combined targeting of SHP-1 and intracellular acidification may lead to a novel strategy of anticancer therapy bypassing the need for receptor-mediated signaling. (+info)
Mobilization of intracellular Ca2+ modulates activation of Na+/H+ exchange in thrombin-stimulated platelets.
AIM: To study the relationship between intracellular calcium translocation and activation of Na+/H+ exchange in thrombin-stimulated platelets. METHODS: Intracellular Ca2+ ([Ca2+]i) and pH (pHi) were measured by a dual wavelength fluorophotometer with Fura-2 and pH-sensitive probe BCECF. RESULTS: Thrombin 0.1 IU.L-1 elicited an increase in platelet [Ca2+]i and pHi, the maximal increase in [Ca2+]i occurred earlier than the rise in pHi. In Na(+)-free buffers, the Na+/H+ exchange was markedly suppressed without affecting the elevation of [Ca2+]i; while intracellular acidification with nigericin 1 mg.L-1 inhibited the increment of [Ca2+]i. Blockade of Ca(2+)-influx with egtazic acid (EGTA) did not affect cytosolic alkalinization. Depletion of intracellular Ca2+ store with ionomycin in the presence of EGTA, no increment in pHi was observed, the basal value of pHi was even more acidic, this response of pHi to thrombin was rehabilitated after refilling of intracellular Ca2+ store with extracellular Ca2+ 1 mmol.L-1. CONCLUSION: Intracellular Ca2+ mobilization modulated activation of Na+/H+ exchange, which required an effective increment of [Ca2+]i. (+info)
Phosphorylation-dependent stimulation of prostanoid synthesis by nigericin in cerebral endothelial cells.
Nigericin decreases intracellular pH (pH(i)) and stimulates prostanoid (PG) synthesis in endothelial cells from cerebral microvessels of newborn pigs. Nigericin-induced PG production was abolished by protein tyrosine kinase (PTK) inhibitors and amplified by phorbol 12-myristate 13-acetate (PMA) or protein tyrosine phosphatase (PTP) inhibitors. Nigericin-induced PG production in PMA-primed cells was potentiated by PTP inhibitors and abrogated by PTK inhibitors. Phospholipase A(2) (PLA(2)) activity was stimulated by nigericin in a phosphorylation-dependent manner. Nigericin's effects on PG production and PLA(2) activity were reproduced by ionomycin, which activates cytosolic PLA(2) (cPLA(2)). cPLA(2) was immunodetected in endothelial cell lysates. We found no evidence that nigericin's effects are mediated via mitogen-activated protein (MAP) kinase [extracellularly regulated kinase 1 (ERK1) and ERK2] activation: although nigericin stimulated detergent-soluble MAP kinase, its effects were not amplified by PMA or PTP inhibitors. Phosphorylation-dependent stimulation of PG synthesis was also observed when pH(i) was decreased by sodium propionate or a high level of CO(2). Altogether, our data indicate that nigericin and decreased pH(i) stimulate PG synthesis by a protein phosphorylation-dependent mechanism involving cross talk between pathways mediated by PTK and PTP and by protein kinase C; cPLA(2) appears to be a key enzyme affected by nigericin and decreased pH(i). (+info)
LLC-PK(1) cells stably expressing the human norepinephrine transporter: A functional model of carrier-mediated norepinephrine release in protracted myocardial ischemia.
In myocardial ischemia, adrenergic terminals undergo ATP depletion, hypoxia, and intracellular pH reduction, causing the accumulation of axoplasmic norepinephrine (NE) and intracellular Na(+) [via the Na(+)-H(+) exchanger (NHE)]. This forces the reversal of the Na(+)- and Cl(-)-dependent NE transporter (NET), triggering massive carrier-mediated NE release and, thus, arrhythmias. We have now developed a cellular model of carrier-mediated NE release using an LLC-PK(1) cell line stably transfected with human NET cDNA (LLC-NET). LLC-NET cells transported [(3)H]NE and [(3)H]N-methyl-4-phenylpyridinium ([(3)H]MPP(+)) in an inward direction. This uptake was abolished by the NET inhibitors desipramine (100 nM) and mazindol (300 nM) and by extracellular Na(+) removal. Na(+)-gradient reversal induced an efflux of (3)H-substrate from preloaded LLC-NET cells. Desipramine and mazindol blocked this efflux. Because of its greater intracellular stability and higher sensitivity to Na(+)-gradient reversal, [(3)H]MPP(+) proved preferable to [(3)H]NE as an NET substrate; therefore, only [(3)H]MPP(+) was used for subsequent studies. The K(+)/H(+) ionophore nigericin (10 microM) evoked a large efflux of [(3)H]MPP(+). This efflux was potentiated by the Na(+),K(+)-ATPase inhibitor ouabain (100 microM), was sensitive to desipramine, and was blocked by the NHE inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (EIPA; 10 microM). In contrast, EIPA failed to inhibit the [(3)H]MPP(+) efflux elicited by the Na(+) ionophore gramicidin (10 microM). Furthermore, [(3)H]MPP(+) efflux induced by the NHE-stimulant proprionate (25 mM) was negatively modulated by imidazoline receptor activation. Our findings suggest that LLC-NET cells are a sensitive model for studying transductional processes of carrier-mediated NE release associated with myocardial ischemia. (+info)
Use of a lipophilic cation to monitor electrical membrane potential in the intact rat lens.
PURPOSE: Tetraphenylphosphonium (TPP+) is a permeant lipophilic cation that accumulates in cultured cells and tissues as a function of the electrical membrane potential across the plasma membrane. This study was undertaken to determine whether TPP+ can be used for assessing membrane potential in intact lenses in organ culture. METHODS: Rat lenses were cultured in media containing 10 microM TPP+ and a tracer level of 3H-TPP+ for various times. 3H-TPP+ levels in whole lenses or dissected portions of lenses were determined by liquid scintillation counting. Ionophores, transport inhibitors, and neurotransmitters were also added to investigate their effects on TPP+ uptake. RESULTS. Incubation of lenses in low-K+ balanced salt solution and TC-199 medium, containing physiological concentrations of Na+ and K+, led to a biphasic accumulation of TPP+ in the lens that approached equilibrium by 12 to 16 hours of culture. The TPP+ equilibrated within 1 hour in the epithelium but penetrated more slowly into the fiber mass. The steady state level of TPP+ accumulation in the lens was depressed by 90% when the lenses were cultured in a medium containing high K+. The calculated membrane potential for the normal rat lens in TC-199 was -75 +/- 3 mV. Monensin (1 microM) and nigericin (1 microM), Na+H+ and K+H+ exchangers respectively, as well as the protonophore carbonylcyanide-m-chlorophenylhydrazone (CCCP, 10 microM) and the calcium ionophore A23187 (10 microM), abolished TPP+ accumulation and caused cloudiness of the lenses. The neurotransmitter acetylcholine at 50 microM decreased TPP+ accumulation in the lens, but this effect could be prevented by simultaneous application of 1 mM atropine. CONCLUSIONS: TPP+ accumulation can be used as an indicator of changes in membrane potential in intact lenses, but because of the long time required to reach steady state, its utility is limited. The slow accumulation of TPP+ and its slow efflux from the lens under conditions known to depolarize membranes are consistent with a diffusion barrier in the deep cortex and nucleus of the lens. (+info)
Acidocalcisomes and a vacuolar H+-pyrophosphatase in malaria parasites.
Plasmodium berghei trophozoites were loaded with the fluorescent calcium indicator, fura-2 acetoxymethyl ester, to measure their intracellular Ca(2+) concentration ([Ca(2+)](i)). [Ca(2+)](i) was increased in the presence of the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase inhibitor, thapsigargin. Trophozoites also possess a significant amount of Ca(2+) stored in an acidic compartment. This was indicated by: (1) the increase in [Ca(2+)](i) induced by bafilomycin A(1), nigericin, monensin, or the weak base, NH(4)Cl, in the nominal absence of extracellular Ca(2+), and (2) the effect of ionomycin, which cannot take Ca(2+) out of acidic organelles and was more effective after alkalinization of this compartment by addition of bafilomycin A(1), nigericin, monensin, or NH(4)Cl. Inorganic PP(i) promoted the acidification of a subcellular compartment in cell homogenates of trophozoites. The proton gradient driven by PP(i) collapsed by addition of the K(+)/H(+) exchanger, nigericin, and eliminated by the PP(i) analogue, aminomethylenediphosphonate (AMDP). Both PP(i) hydrolysis and proton transport were dependent upon K(+), and Na(+) caused partial inhibition of these activities. PP(i) hydrolysis was sensitive in a dose-dependent manner to AMDP, imidodiphosphate, sodium fluoride, dicyclohexylcarbodi-imide and to the thiol reagent, N-ethylmaleimide. Immunofluorescence microscopy using antibodies raised against conserved peptide sequences of a plant vacuolar pyrophosphatase (V-H(+)-PPase) suggested that the proton pyrophosphatase is located in intracellular vacuoles and the plasma membrane of trophozoites. AMDP caused an increase in [Ca(2+)](i) in the nominal absence of extracellular Ca(2+). Ionomycin was more effective in releasing Ca(2+) from this acidic intracellular compartment after treatment of the cells with AMDP. Taken together, these results suggest the presence in malaria parasites of acidocalcisomes with similar characteristics to those described in trypanosomatids and Toxoplasma gondii, and the colocalization of the V-H(+)-PPase and V-H(+)-ATPase in these organelles. (+info)
Light-induced stimulation of carbonic anhydrase activity in pea thylakoids.
Stimulation of the bicarbonate dehydration reaction in thylakoid suspension under conditions of saturating light at pH 7.6-8.0 was discovered. This effect was inhibited by nigericin or the lipophilic carbonic anhydrase (CA) inhibitor ethoxyzolamide (EZ), but not by the hydrophilic CA inhibitor, acetazolamide. It was shown that the action of EZ is not caused by an uncoupling effect. It was concluded that thylakoid CA is the enzyme utilizing the light-generated proton gradient across the thylakoid membrane thus facilitating the production of CO(2) from HCO(3)(-) and that this enzyme is covered from the stroma side of thylakoids by a lipid barrier. (+info)
A role for ectophosphatase in xenobiotic resistance.
Xenobiotic resistance in animals, plants, yeast, and bacteria is known to involve ATP binding cassette transporters that efflux invading toxins. We present data from yeast and a higher plant indicating that xenobiotic resistance also involves extracellular ATP degradation. Transgenic upregulation of ecto-ATPase alone confers resistance to organisms that have had no previous exposure to toxins. Similarly, cells that are deficient in extracellular ATPase activity are more sensitive to xenobiotics. On the basis of these and other supporting data, we hypothesize that the hydrolysis of extracellular ATP by phosphatases and ATPases may be necessary for the resistance conferred by P-glycoprotein. (+info)