Inhibitors of renal chloride transport do not block toxicant-induced chloride influx in the proximal tubule

The role of glycine in regulated cell death

The cytoprotective effects of glycine against cell death have been recognized for over 28 years. They are expressed in multiple cell types and injury settings that lead to necrosis, but are still not widely appreciated or considered in the conceptualization of cell death pathways. In this paper, we review the available data on the expression of this phenomenon, its relationship to major pathophysiologic pathways that lead to cell death and immunomodulatory effects, the hypothesis that it involves suppression by glycine of the development of a hydrophilic death channel of molecular dimensions in the plasma membrane, and evidence for its impact on disease processes in vivo.

The role of calpain in oncotic cell death

Numerous lines of evidence demonstrate that calpains, a family of 14 Ca(2+)-activated neutral cysteine proteases, are involved in oncotic cell death in a variety of models. At this time, the biochemistry of most calpains and the specific roles of different calpains in physiology and pathology remain to be determined. A number of calpain substrates have been identified in cellular systems, including cytoskeletal proteins, and recent studies suggest that calpains mediate the increase in plasma membrane permeability to ions and the progressive breakdown of the plasma membrane observed in oncosis through the proteolysis of cystokeletal and plasma membrane proteins. Further, a number of reports provide evidence that the mitochondrial dysfunction observed in oncosis may be mediated by a mitochondrial calpain of unknown identity. Finally, a number of diverse calpain inhibitors have been developed that show cytoprotective properties in cellular systems and in vivo following diverse insults. It is suggested that future research be directed toward elucidation of the role(s) of specific calpain isozymes in physiological and pathological conditions; identifying and linking specific calpain substrates with altered cellular functions; and developing cell-permeable, potent, isozyme-selective calpain inhibitors.

Calpain mediates progressive plasma membrane permeability and proteolysis of cytoskeleton-associated paxillin, talin, and vinculin during renal cell death

The goal of the present study was to determine the role of calpain in changes in plasma membrane permeability and cytoskeleton-associated paxillin, vinculin, talin, and alpha-actinin levels during acute renal cell death. The mitochondrial inhibitor antimycin A or hypoxia produced graded plasma membrane permeability in renal proximal tubules (RPTs), first allowing propidium iodide (PI, molecular mass 668 Da) influx and then lactate dehydrogenase (LDH, molecular mass 130 kDa) release. Cytoskeleton-associated paxillin levels decreased concomitantly with PI influx and before LDH release, whereas cytoskeleton-associated talin and vinculin levels decreased concomitantly with LDH release. Cytoskeleton-associated alpha-actinin levels did not change during antimycin A exposure or hypoxia. Purified micro-calpain cleaved paxillin, talin, vinculin, but not alpha-actinin. The dissimilar calpain inhibitors 3-(4-iodophenyl)-2-mercapto-(Z)-2-propenoic acid (PD150606) or chloroacetic acid N'-[6,7-dichloro-4-phenyl)-3-oxo-3,4-dihydroquinoxalin-2-yl] hydrazide (SJA7029) preserved cytoskeleton-associated paxillin, talin, and vinculin levels and prevented PI influx and LDH release in antimycin A-exposed or hypoxic RPTs. These results suggest that calpain mediates increased plasma membrane permeability and hydrolysis of cytoskeleton-associated paxillin, vinculin, and talin during renal cell death.

Linoleic acid prevents chloride influx and cellular lysis in rabbit renal proximal tubules exposed to mitochondrial toxicants

Despite many studies elucidating the mechanisms of necrotic cell death, the role of fatty acids released during necrosis remains to be determined. The goals of this study were to determine whether linoleic acid could protect rabbit renal proximal tubules (RPT) from necrotic cell death associated with mitochondrial dysfunction and oxidative injury and to determine the mechanisms involved. Exposure to antimycin A (10 microM) for 1 h or hypoxia (perfusion with 95% N(2)/5% CO(2)) for 1 or 2 h induced approximately 70% cellular lysis, as measured by lactate dehyrogenase release, versus 10% in controls. Preincubation with linoleic acid (100 microM) fully protected RPT from cellular lysis. RPT were also protected from lysis if linoleic acid was added 15 min after the addition of antimycin A. Measurements of free intracellular Ca(2+) concentrations showed that linoleic acid did not prevent the rise in intracellular Ca(2+) associated with a 30-min exposure to antimycin A. However, the influx of extracellular (36)Cl(-) following a 30-min exposure to antimycin A was ameliorated in the presence of linoleic acid. Linoleic acid did not prevent cellular lysis after exposure to hypoxia/reoxygenation (1 h/1 h) or t-butyl hydroperoxide (500 microM, 3 h). These data suggest that linoleic acid protects RPT during the late phase of cell death associated with inhibition of the electron transport chain but not oxidative injury. Several other fatty acids also protected RPT from lysis, and structure-activity relationship studies suggest that a free carboxyl terminus and at least one double bond are required for this action.

Glycine blocks opening of a death channel in cultured hepatic sinusoidal endothelial cells during chemical hypoxia

Using confocal microscopy, we investigated mechanisms underlying loss of plasma membrane integrity during necrotic death of cultured hepatic sinusoidal endothelial cells exposed to 2.5 mM potassium cyanide (chemical hypoxia). After 2-3 h, the anionic fluorophore calcein abruptly began to enter the cytosol, and nuclei labeled with cationic propidium after another 2-5 min. As calcein permeated, growth of blebs on the plasma membrane accelerated. Lucifer yellow, another anionic fluorophore, entered identically to calcein, whereas high molecular weight dextrans (40-2000 kDa) entered like propidium. Glycine slowed, but did not prevent calcein entry, whereas permeation of propidium and high molecular weight dextrans was blocked completely by glycine. These findings suggest that opening of a glycine-sensitive organic anion channel, or death channel, precipitates a metastable state characterized by rapid cell swelling and bleb growth. This metastable state culminates in non-specific breakdown of the plasma membrane permeability barrier and irreversible cell death.

Progressive disruption of the plasma membrane during renal proximal tubule cellular injury

The goal of this study was to examine the progression of plasma membrane disruption during cell injury using rabbit renal proximal tubules (RPT). The results demonstrated that the plasma membrane became permeable to larger and larger molecules as anoxia proceeded. At least three distinctive phases of membrane disruption were differentiated during anoxia. In phases 1, 2, and 3, plasma membranes became permeable to propidium iodide (PI, molecular weight = 668), 3 kDa dextrans, and 70 kDa dextrans or lactate dehydrogenase (LDH, molecular weight = 140 kDa), respectively. Phase 1 was reversible by reoxygenation but not prevented by the glycine. Phase 2 was inhibited by glycine. Phase 3 was inhibited by several membrane-permeable homobifunctional crosslinkers, dimethyl-pimelimidate (DMP), ethylene-glycolbis(succinimidylsuccinate), and dithiobis(succinimidylpropionate), but not by the membrane-impermeable crosslinker dithiobis(sulfosuccinimidylpropionate). In addition, DMP decreased RPT LDH release produced by mitochondrial inhibition (antimycin A), an oxidant (t-butylhydroperoxide) and a nephrotoxicant that is metabolized to an electrophile (tetrafluoroethyl-l-cysteine). These results identify (1) different phases of plasma membrane damage with increasing permeability during cell injury, (2) the reversibility of phase 1, (3) the relative site of action of the cytoprotectant glycine (prevents phase 2), and (4) the protective effects of chemical crosslinkers in RPT cell death produced by different toxicants.

Protection by glycine against hypoxic injury of rat hepatocytes: inhibition of ion fluxes through nonspecific leaks

BACKGROUND/AIMS

Glycine has long been shown to exert strong protective effects against hypoxic injury of hepatocytes. Recently, it was suggested that glycine exerts this protection via inhibition of ligand-gated chloride channels, thereby secondarily inhibiting sodium influx. The purpose of this study was to examine this suggestion.

METHODS

Cultured rat hepatocytes were incubated under normoxic and hypoxic conditions. Loss of viability was determined by release of lactate dehydrogenase. Cytosolic ion concentrations were measured using digital fluorescence microscopy.

RESULTS

Glycine prevented the hypoxic increase in cytosolic sodium and strongly protected against hypoxic injury. The amino acid was not only protective in Krebs-Henseleit buffer but also in a chloride-free modification thereof and offered additional protection in a sodium-free medium (which already yielded substantial protection in its own right). Glycine also prevented the hypoxic release of the anionic fluorescent dye Newport Green and appeared to prevent the hypoxic entrance of the "nonphysiological" cations cobalt and nickel.

CONCLUSION

The results strongly argue against inhibition of ligand-gated chloride channels as being responsible for the potent protective effect of glycine against hypoxic injury of hepatocytes. Instead, they suggest that glycine prevents the formation of nonspecific leaks for small ions including sodium, thereby providing protection.

Examination of the mechanisms of action of diverse cytoprotectants in renal cell death

Glycine, strychnine, muscimol, allopregnanolone, and pregnenolone sulfate act in the late phase of renal cell injury, block Cl- influx and cell lysis induced by the mitochondrial inhibitor antimycin A, and promote the recovery of respiration and ion transport following hypoxia/reoxygenation. However, the mechanism of action of these compounds has not been completely elucidated. Recently, we have shown that calpains are critical mediators of renal cell death produced by diverse toxicants and that antimycin A exposure results in calpain translocation from the cytosol to the membrane fraction that is temporally associated with Cl- influx and precedes cell death/lysis. The current study examined the effects of a group of diverse cytoprotectants on calpain activity and determined if calpain inhibition plays a role in the cytoprotection produced by these compounds. The cytoprotection produced by glycine, strychnine, muscimol, allopregnanolone, and pregnenolone sulfate in rabbit renal proximal tubules exposed to antimycin A was associated with the inhibition of antimycin A-induced calpain translocation. None of the cytoprotectants had a direct effect on calpain activity. All of the cytoprotectants decreased calcium-ionophore-induced cell death. Glycine, strychnine, and muscimol also blocked antimycin A mediated extracellular Ca2+ influx. These data suggest that the cytoprotective mechanism of action of glycine, strychnine, and muscimol involves the inhibition of antimycin A mediated extracellular Ca2+ influx as well as calpain translocation and associated Cl- influx. In contrast, the mechanism of action of the neurosteroids results only from the blockade of calpain translocation and associated Cl- influx.

Mitochondrial permeability transition in pH-dependent reperfusion injury to rat hepatocytes

To simulate ischemia and reperfusion, cultured rat hepatocytes were incubated in anoxic buffer at pH 6.2 for 4 h and reoxygenated at pH 7.4. During anoxia, intracellular pH (pHi) decreased to 6.3, mitochondria depolarized, and ATP decreased to < 1% of basal values, but the mitochondrial permeability transition (MPT) did not occur as assessed by confocal microscopy from the redistribution of cytosolic calcein into mitochondria. Moreover, cell viability remained > 90%. After reperfusion at pH 7.4, pHi returned to pH 7.2, the MPT occurred, and most hepatocytes lost viability. In contrast, after reperfusion at pH 6.2 or with Na(+)-free buffer at pH 7.4, pHi did not rise and cell viability remained > 80%. After acidotic reperfusion, the MPT did not occur. When hepatocytes were reperfused with cyclosporin A (0.5-1 microM) at pH 7.4, the MPT was prevented and cell viability remained > 80%, although pHi increased to 7.2. Reperfusion with glycine (5 mM) also prevented cell killing but did not block recovery of pHi or the MPT. Retention of cell viability was associated with recovery of 30-40% of ATP. In conclusion, preventing the rise of pHi after reperfusion blocked the MPT, improved ATP recovery, and prevented cell death. Cyclosporin A also prevented cell killing by blocking the MPT without blocking recovery of pHi. Glycine prevented cell killing but did not inhibit recovery of pHi or the MPT.

Neurosteroid inhibition of cell death

Diverse gamma-aminobutyric acid (GABAA) receptor modulators exhibited novel cytoprotective effects and mechanisms of action in rabbit renal proximal tubules subjected to mitochondrial inhibition (antimycin A) or hypoxia. Cytoprotective potencies (50% effective concentration, EC50) were 0.3 nM allopregnanolone (AP) > 0.4 nM 17 alpha-OH-allopregnanolone (17 alpha-OH-AP) > 30 nM dehydroepiandrosterone sulfate (DHEAS) = 30 nM pregnenolone sulfate (PS) > 500 nM pregnenolone (PREG) > 30 microM muscimol > 10 mM GABA following antimycin A exposure. Maximal protection with AP and 17 alpha-OH-AP was 70%, whereas DHEAS, PS, PREG, and muscimol produced 100% cytoprotection. Experiments with AP, PS, and muscimol revealed the return of mitochondrial function and active Na+ transport following hypoxia/reoxygenation. Muscimol inhibited the antimycin A-induced influx of both extracellular Ca2+ and Cl- that occurs during the late phase of cell injury, whereas the neurosteroids only inhibited influx of Cl-. Radioligand binding studies with AP and PS did not reveal a specific binding site; however, structural requirements were observed for cytoprotective potency and efficacy. In conclusion, we suggest that the GABAA receptor modulators muscimol and neurosteroids are cytoprotective at different cellular sites in the late phase of cell injury; muscimol inhibits Ca2+ and subsequent Cl- influx, whereas the neurosteroids inhibit Cl- influx.

Depletion of endoplasmic reticulum calcium stores protects against hypoxia- and mitochondrial inhibitor-induced cellular injury and death

We have shown previously that intracellular Ca+2 chelation and calpain inhibitors block the influx of extracellular Ca+2 and Cl- during the late phase of cell injury in renal proximal tubules (RPT) exposed to the mitochondrial inhibitor antimycin A. Since the endoplasmic reticulum (ER) is the major intracellular Ca+2 storage site, ER Ca+2 release/depletion may mediate the Ca+2 influx and cell death. Treatment of RPT suspensions with thapsigargin, an ER Ca+2-ATPase inhibitor, increased cytosolic free Caf+2 (Ca+2) levels from 122 +/- 7 to 322 +/- 55 nM within 10 sec of addition followed by a return to control levels within 3 min. A 5-min pretreatment of RPT suspensions with thapsigargin blocked antimycin A- and hypoxia-induced influx of extracellular Ca+2 and Cl- and the resulting cell death/lysis. These data suggest that ER Ca+2 release/depletion during cell injury may trigger a signaling cascade that causes extracellular Ca+2 influx followed by Cl- influx, cell swelling, and ultimately cell death/ lysis.

Role of water and electrolyte influxes in anoxic plasma membrane disruption

The role of water and electrolyte influxes in anoxia-induced plasma membrane disruption was investigated using rabbit proximal tubule suspension. The results indicated that normal proximal tubule (PT) cells have a great capacity for expanding cell volume in response to water influx, whereas anoxia increases the susceptibility to water influx-induced disruption, and this was attenuated by glycine. However, resistance of anoxic plasma membranes to water influx-induced stress is not lost, although their mechanical strength was diminished, compared with normoxic membranes. Anoxic membranes did not disrupt under an intra-to-extracellular osmotic difference as great as 150 mosM. Potentiating or attenuating water influx by incubating PT cells in hypotonic or hypertonic medium, respectively, during anoxia, did not affect anoxia-induced membrane disruption. After the transmembrane electrolyte concentration gradient was eliminated by a "intracellular" buffer or by permeabilizing the plasma membrane to molecules <4 kDa using alpha-toxin, anoxia still caused further membrane disruption that was prevented by glycine or low pH. These results demonstrate that 1) water or net electrolyte influxes are probably not a primary cause for anoxia-induced membrane disruption and 2) glycine could prevent the plasma membrane disruption during anoxia independently from its effect on transmembrane electrolyte or water influxes. The present data support a biochemical rather than a mechanical alteration of the plasma membrane as the underlying cause of membrane disruption during anoxia.

Protection by Carolina rinse solution, acidotic pH, and glycine against lethal reperfusion injury to sinusoidal endothelial cells of rat livers stored for transplantation

The critical injury causing graft failure after prolonged liver storage involves reperfusion-induced killing of sinusoidal endothelial cells and activation of Kupffer cells. Treatment of stored livers with Carolina rinse solution (CRS) prevents endothelial cell killing, reduces Kupffer cell activation, and improves graft survival. Accordingly, our aim was to evaluate the components of CRS and other agents for protection against reperfusion injury to rat livers stored 24 hr in University of Wisconsin solution. CRS virtually abolished endothelial cell killing, prevented denudation of the sinusoidal lining, and decreased structural changes in Kupffer cells indicative of activation. The only component of CRS preventing endothelial cell killing was acidic pH of 6.5. However, when pH was subsequently increased to 7.4, antioxidants (allopurinol, deferoxamine mesylate, and glutathione), vasodilators (adenosine and nicardipine), and possibly energy substrates (fructose, glucose, and insulin) partially blocked pH-dependent cell killing (pH paradox). Na+/H+ exchange inhibition, protease inhibition, and Ca(2+)-free buffer did not decrease reperfusion injury, but the amino acid glycine protected strongly. Strychnine, which binds to glycine receptors in the central nervous system, protected equally well. Protection by glycine and CRS was synergistic, virtually.