Acidic pH inhibits ATP depletion-induced tubular cell apoptosis by blocking caspase-9 activation in apoptosome

Muscle proteolysis is differentially influenced by mitochondrial intactness

This study examined the potential influence of mitochondrial calcium sequestering ability on calpain-1 autolysis and proteolysis in vitro. We first tested whether mitochondria can sequester calcium in an in vitro setting. Isolated bovine mitochondria (0, 0.5, or 2 mg/mL) were incubated in a buffer containing varying calcium levels (0, 50, or 100 μM). An inverse relationship between mitochondrial content and measured free calcium was observed (P < 0.05), confirming that mitochondria can sequester calcium within the concentration range tested. In the first in vitro experiment, intact mitochondria (0, 0.5, or 2 mg/mL) were incorporated into an in vitro model simulating postmortem muscle conditions, and calpain-1 autolysis and proteolysis were evaluated over a 168-h period. Adding intact mitochondria to the in vitro model decreased calpain-1 autolysis and proteolysis during the first 4 h of incubation (P < 0.05), likely through reducing calcium availability. However, accentuated calpain-1 autolysis and proteolysis were observed at 24 h. To further explore these effects, mitochondrial integrity was evaluated at varying pH and calcium levels. Mitochondrial integrity decreased as pH declined (P < 0.05), especially in the presence of calcium. Based on these results, we conducted a second in vitro experiment involving disrupted mitochondria. Unlike intact mitochondria, which exerted a suppressive effect on calpain-1 autolysis and proteolysis early on, disrupted mitochondria increased both parameters at most time points (P < 0.05). Overall, it appears that intact mitochondria initially cause a delay in calpain-1 autolysis and proteolysis, but as their integrity diminishes, both processes are enhanced.

The potential mediation of hypoxia-inducible factor-1α in heat shock protein 27 translocations, caspase-3 and calpain activities and yak meat tenderness during postmortem aging

The present study aimed to characterize the influence of hypoxia-inducible factor-1α on heat shock protein 27 and cytochrome c translocation, yak meat microstructure destruction, and endogenous enzymes activities, refining the understanding of the tenderization process after slaughter. Postmortem yak longissimus thoracis et lumborum muscles were incubated with 0.9% saline or hypoxia-inducible factor-1α stabilizer dimethyloxaloylglycine at 4 °C for 6, 12, 24, 72, and 120 h. Results showed that hypoxia-inducible factor-1α activation promoted heat shock protein 27 migration and cytochrome c release, facilitating (P < 0.05) caspase-3 activity by mediating the heat shock protein 27/caspase-3 interaction but did not exert (P > 0.05) significant effects on the calpain-1 activity. Additionally, hypoxia-inducible factor-1α activation contributed to the mitochondrial apoptosis cascade, leading to a higher (P < 0.01) apoptosis rate. Therefore, these observations indicate that hypoxia-inducible factor-1α affects caspase-3 activity and tenderness of postmortem muscle through distinct regulatory mechanisms, possibly, in part, with heat shock protein 27 and cytochrome c mediation.

Smad5 acts as an intracellular pH messenger and maintains bioenergetic homeostasis

Both environmental cues and intracellular bioenergetic states profoundly affect intracellular pH (pHi). How a cell responds to pHi changes to maintain bioenergetic homeostasis remains elusive. Here we show that Smad5, a well-characterized downstream component of bone morphogenetic protein (BMP) signaling responds to pHi changes. Cold, basic or hypertonic conditions increase pHi, which in turn dissociates protons from the charged amino acid clusters within the MH1 domain of Smad5, prompting its relocation from the nucleus to the cytoplasm. On the other hand, heat, acidic or hypotonic conditions decrease pHi, blocking the nuclear export of Smad5, and thus causing its nuclear accumulation. Active nucleocytoplasmic shuttling of Smad5 induced by environmental changes and pHi fluctuation is independent of BMP signaling, carboxyl terminus phosphorylation and Smad4. In addition, ablation of Smad5 causes chronic and irreversible dysregulation of cellular bioenergetic homeostasis and disrupted normal neural developmental processes as identified in a differentiation model of human pluripotent stem cells. Importantly, these metabolic and developmental deficits in Smad5-deficient cells could be rescued only by cytoplasmic Smad5. Cytoplasmic Smad5 physically interacts with hexokinase 1 and accelerates glycolysis. Together, our findings indicate that Smad5 acts as a pHi messenger and maintains the bioenergetic homeostasis of cells by regulating cytoplasmic metabolic machinery.

Intracellular pH Regulates TRAIL-Induced Apoptosis and Necroptosis in Endothelial Cells

During ischemia or inflammation of organs, intracellular pH can decrease if acid production exceeds buffering capacity. Thus, the microenvironment can expose parenchymal cells to a reduced extracellular pH which can alter pH-dependent intracellular functions. We have previously shown that while silencing caspase-8 in an in vivo ischemia reperfusion injury (IRI) model results in improved organ function and survival, removal of caspase-8 function in a donor organ can paradoxically result in enhanced receptor-interacting protein kinase 1/3- (RIPK1/3-) regulated necroptosis and accelerated graft loss following transplantation. In our current study, TRAIL- (TNF-related apoptosis-inducing ligand-) induced cell death in vitro at neutral pH and caspase-8 inhibition-enhanced RIPK1-dependent necroptotic death were confirmed. In contrast, both caspase-8 inhibition and RIPK1 inhibition attenuated cell death at a cell pH of 6.7. Cell death was attenuated with mixed lineage kinase domain-like (MLKL) silencing, indicating that MLKL membrane rupture, a distinctive feature of necroptosis, occurs regardless of pH. In summary, there is a distinct regulatory control of apoptosis and necroptosis in endothelial cells at different intracellular pH. These results highlight the complexity of modulating cell death and therapeutic strategies that may need to consider different consequences on cell death dependent on the model.

Focal photodynamic intracellular acidification as a cancer therapeutic

Cancer cells utilize an array of proton transporters to regulate intra- and extracellular pH to thrive in hypoxic conditions, and to increase tumor growth and metastasis. Efforts to target many of the transporters involved in cancer cell pH regulation have yielded promising results, however, many productive attempts to disrupt pH regulation appear to be non-specific to cancer cells, and more effective in some cancer cells than others. Following a review of the status of photodynamic cancer therapy, a novel light-activated process is presented which creates very focal, rapid, and significant decreases in only intracellular pH (pHi), leading to cell death. The light-activation of the H⁺ carrier, nitrobenzaldehyde, has been effective at initiating pH-induced apoptosis in non-cancerous and numerous cancerous cell lines in vitro, to include breast, prostate, and pancreatic cancers. Also, this intracellular acidification technique caused significant reductions in tumor growth rate and enhanced survival in mice bearing triple negative breast cancer tumors. The efficacy of an NBA-upconverting nanoparticle to kill breast cancer cells in vitro is described, as well as a discussion of the potential intracellular mechanisms underlying the pH-induced apoptosis.

Acidic cellular microenvironment modifies carcinogen-induced DNA damage and repair

Chronic inflammation creates an acidic microenvironment, which plays an important role in cancer development. To investigate how low pH changes the cellular response to the carcinogen benzo[a]pyrene (B[a]P), we incubated human pulmonary epithelial cells (A549 and BEAS-2B) with nontoxic doses of B[a]P using culturing media of various pH’s (extracellular pH (pH_e) of 7.8, 7.0, 6.5, 6.0 and 5.5) for 6, 24 and 48 h. In most incubations (pH_e 7.0–6.5), the pH in the medium returned to the physiological pH 7.8 after 48 h, but at the lowest pH (pH_e < 6.0), this recovery was incomplete. Similar changes were observed for the intracellular pH (pH_i). We observed that acidic conditions delayed B[a]P metabolism and at t = 48 h, and the concentration of unmetabolized extracellular B[a]P and B[a]P-7,8-diol was significantly higher in acidic samples than under normal physiological conditions (pH_e 7.8) for both cell lines. Cytochrome P450 (CYP1A1/CYP1B1) expression and its activity (ethoxyresorufin-O-deethylase activity) were repressed at low pH_e after 6 and 24 h, but were significantly higher at t = 48 h. In addition, a DNA repair assay showed that the incision activity was ~80% inhibited for 6 h at low pH_e and concomitant exposure to B[a]P. However, at t = 48 h, the incision activity recovered to more than 100% of the initial activity observed at neutral pH_e. After 48 h, higher B[a]P-DNA adduct levels and γ-H2AX foci were observed at low pH samples than at pH_e 7.8. In conclusion, acidic pH delayed the metabolism of B[a]P and inhibited DNA repair, ultimately leading to increased B[a]P-induced DNA damage.

Necroptosis, a novel form of caspase-independent cell death, contributes to renal epithelial cell damage in an ATP-depleted renal ischemia model

Acute kidney injury (AKI) induced by renal ischemia is a common clinical problem associated with a high morbidity and mortality. The present study investigated whether necroptosis was present in an in vitro renal ischemia model and whether the addition of necrostatin-1 (Nec-1) has a protective effect. In addition, whether autophagy was inhibited following the use of Nec-1 was also examined. When apoptosis was inhibited by z-VAD‑fmk and energy was depleted with antimycin A for 1 h, the morphological abnormalities of human proximal tubular epithelial (HK-2) cells were markedly attenuated, and the cell viability was significantly improved following incubation with Nec-1. LC3-II/I ratios and LC3-II/GAPDH ratios demonstrated a statistically significant decrease in the Nec-1 + tumor necrosis factor (TNF)-α + z-VAD-fmk + antimycin A (1 h) group compared with the control group. In conclusion, the present study suggested that necroptosis was present in HK-2 cells subjected to TNF-α stimulation and energy depletion. Nec-1 inhibits a caspase‑independent necroptotic pathway involving autophagy and may have therapeutic potential to prevent and treat renal ischemic injury.

Mitochondrial and postmitochondrial survival signaling in cancer

Cancer cells are resistant to conventional chemotherapy and radiotherapy, however, the molecular mechanisms of resistance to therapy remain unclear. Cellular survival machinery protects mitochondrial integrity against endogenous or exogenous stresses. Prodeath molecules orchestrate around mitochondria to initiate and execute cell death in cancer, and also play an underappreciated role in survival of cancer cells. Prosurvival mechanisms can operate at mitochondrial and postmitochondrial levels to attenuate core apoptotic death program. It is intriguing to explore how prosurvival and prodeath molecules crosstalk to regulate mitochondrial functions leading to increased cancer cell survival. This review describes some putative survival mechanisms at mitochondria, which may play a role in designing effective agents for cancer prevention and therapy. These survival pathways may also have significance in understanding other human pathophysiological conditions including diabetes, cardiovascular, autoimmune, and neurodegenerative diseases.

Electrostatics control actin filament nucleation and elongation kinetics

The actin cytoskeleton is a central mediator of cellular morphogenesis, and rapid actin reorganization drives essential processes such as cell migration and cell division. Whereas several actin-binding proteins are known to be regulated by changes in intracellular pH, detailed information regarding the effect of pH on the actin dynamics itself is still lacking. Here, we combine bulk assays, total internal reflection fluorescence microscopy, fluorescence fluctuation spectroscopy techniques, and theory to comprehensively characterize the effect of pH on actin polymerization. We show that both nucleation and elongation are strongly enhanced at acidic pH, with a maximum close to the pI of actin. Monomer association rates are similarly affected by pH at both ends, although dissociation rates are differentially affected. This indicates that electrostatics control the diffusional encounter but not the dissociation rate, which is critical for the establishment of actin filament asymmetry. A generic model of protein-protein interaction, including electrostatics, explains the observed pH sensitivity as a consequence of charge repulsion. The observed pH effect on actin in vitro agrees with measurements of Listeria propulsion in pH-controlled cells. pH regulation should therefore be considered as a modulator of actin dynamics in a cellular environment.

pH control mechanisms of tumor survival and growth

A distinguishing phenotype of solid tumors is the presence of an alkaline cellular feature despite the surrounding acidic microenvironment. This phenotypic characteristic of tumors, originally described by Otto Warburg, arises due to alterations in metabolism of solid tumors. Hypoxic regions of solid tumors develop due to poor vascularization and in turn regulate the expression of numerous genes via the transcription factor HIF-1. Ultimately, the tumor microenvironment directs the development of tumor cells adapted to survive in an acidic surrounding where normal cells perish. The provision of unique pH characteristics in tumor cells provides a defining trait that has led to the pursuit of treatments that target metabolism, hypoxia, and pH-related mechanisms to selectively kill cancer cells. Numerous studies over the past decade involving the cancer-specific carbonic anhydrase IX have re-kindled an interest in pH disruption-based therapies. Although an acidification of the intracellular compartment is established as a means to induce normal cell death, the defining role of acid-base disturbances in tumor physiology and survival remains unclear. The aim of this review is to summarize recent data relating to the specific role of pH regulation in tumor cell survival. We focus on membrane transport and enzyme studies in an attempt to elucidate their respective functions regarding tumor cell pH regulation. These data are discussed in the context of future directions for the field of tumor cell acid-base-related research.

Raf-1 activation prevents caspase 9 processing downstream of apoptosome formation

In many cell types, growth factor removal induces the release of cytochrome-c from mitochondria that leads to activation of caspase-9 in the apoptosome complex. Here, we show that sustained stimulation of the Raf-1/MAPK1,3 pathway prevents caspase-9 activation induced by serum depletion in CCL39/ΔRaf-1:ER fibroblasts. The protective effect mediated by Raf-1 is sensitive to MEK inhibition that is sufficient to induce caspase-9 cleavage in exponentially growing cells. Raf-1 activation does not inhibit the release of cytochrome-c from mitochondria while preventing caspase-9 activation. Gel filtration chromatography analysis of apoptosome formation in cells shows that Raf-1/MAPK1,3 activation does not interfere with APAF-1 oligomerization and recruitment of caspase 9. Raf-1-mediated caspase-9 inhibition is sensitive to emetine, indicating that the protective mechanism requires protein synthesis. However, the Raf/MAPK1,3 pathway does not regulate XIAP. Taken together, these results indicate that the Raf-1/MAPK1,3 pathway controls an apoptosis regulator that prevents caspase-9 activation in the apoptosome complex.

Drp1 dephosphorylation in ATP depletion-induced mitochondrial injury and tubular cell apoptosis

Recent studies revealed a striking morphological change of mitochondria during apoptosis. Mitochondria become fragmented and notably, the fragmentation contributes to mitochondrial outer membrane permeabilization and consequent release of apoptotic factors. In renal tubular cells, mitochondrial fragmentation involves the activation of Drp1, a key mitochondrial fission protein. However, it is unclear how Drp1 is regulated during tubular cell apoptosis. In this study, we examined Drp1 regulation during tubular cell apoptosis following ATP depletion. Rat kidney proximal tubular cells (RPTC) were subjected to azide treatment or severe hypoxia in glucose-free medium to induce ATP depletion. During ATP depletion, Drp1 was shown to be dephosphorylated at serine-637. Drp1 dephosphorylation could be suppressed by cyclosporine A and FK506, two calcineurin inhibitors. Importantly, cyclosporine A and FK506 could also prevent mitochondrial fragmentation, Bax accumulation, cytochrome c release, and apoptosis following ATP depletion in RPTC. The results suggest that calcineurin-mediated serine-637 dephosphorylation is involved in Drp1 activation during ATP depletion in renal tubular cells. Upon activation, Drp1 contributes to mitochondrial fragmentation and outer membrane permeabilization, resulting in the release of apoptogenic factors and apoptosis.

Lidocaine increases intracellular sodium concentration through a Na+-H+ exchanger in an identified Lymnaea neuron

BACKGROUND

The intracellular sodium concentration ([Na(+)]in) is related to neuron excitability. For [Na(+)]in, a Na(+)-H(+) exchanger plays an important role, which is affected by intracellular pH ([pH]in). However, the effect of lidocaine on [pH]in and a Na(+)-H(+) exchanger is unclear. We used neuron from Lymnaea stagnalis to determine how lidocaine affects [pH]in, Na(+)-H(+) exchanger, and [Na(+)]in.

METHODS

Intracellular sodium imaging by sodium-binding benzofuran isophthalate and intracellular pH imaging by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein were used to measure [Na(+)]in and [pH]in. Measurements for [Na(+)]in were made in normal, Na(+) free saline, with modified extracellular pH, and a Na(+)-H(+) exchanger antagonist [(5-N-ethyl-N-isopropyl amiloride, N-methylisopropylamiloride, and 5-(N,N-hexamethylene)-amiloride) pretreatment trials. Furthermore, [Na(+)]in and [pH]in were recorded simultaneously. From 0.1 to 10 mM, lidocaine, mepivacaine, bupivacaine, prilocaine, and QX-314 were evaluated.

RESULTS

Lidocaine, mepivacaine, and prilocaine increased the [Na(+)]in in a dose-dependent manner. In contrast, QX-314 did not change the [Na(+)]in at each dose. In the Na(+) free saline or in the presence of each Na(+)-H(+) exchanger antagonist, lidocaine failed to increase [Na(+)]in. Lidocaine, mepivacaine, and prilocaine induced a significant decrease in [pH]in below baseline with an increase in [Na(+)]in. In contrast, QX-314 did not change the [pH]in. These results demonstrated that lidocaine increases [Na(+)]in through Na(+)-H(+) exchanger activated by intracellular acidification, which is induced by the proton trapping of lidocaine. This [Na(+)]in increase and [pH]in change induces cell toxicity.

CONCLUSION

Lidocaine increases the [Na(+)] through a Na(+)-H(+) exchanger by proton trapping.

STAT3 inhibits apoptosis of human renal tubular epithelial cells induced by ATP depletion/recovery

BACKGROUND/AIMS

Apoptosis has been implicated in renal ischemic injury, the regulating mechanism of which is still unclear. Signal transducers and activators of transcription (STAT) participate in inflammation, apoptosis, and tumorigenesis. In the in vitro model of renal ischemic injury, we explored the role of the STAT3, a major component of the STAT family, in apoptosis of human proximal tubular epithelial cell (HKC) induced by ATP depletion/recovery.

METHODS

Apoptosis of HKC was induced in an in vitro model of acute renal failure. STAT3 activation was analyzed by Western blotting. RNA interference was used to knock down STAT3 expression. The effect of STAT3 knockdown or STAT3C overexpression on apoptosis was assessed by annexin V binding and propidium iodide uptake.

RESULTS

STAT3 was phosphorylated during the course of ATP depletion-induced HKC cell apoptosis. STAT3 knockdown suppressed STAT3 phosphorylation, and promoted apoptosis of HKC subjected to ATP depletion/recovery, while STAT3C overexpression conferred resistance of HKC to ATP depletion-induced apoptosis.

CONCLUSION

Our results demonstrated that STAT3 mediates resistance to ATP depletion-induced apoptosis of HKC, which may be a potential target in treatment of renal ischemic injury.

Acute acidification or amiloride treatment suppresses the ability of Hsp70 to inhibit heat-induced apoptosis

Inhibition of stress-induced apoptosis by the molecular chaperone protein Hsp70 is a contributing factor in tumorigenesis and suppression of this ability could increase the effectiveness of anti-tumor therapy. Tumor cells exist in an acidic environment and acute acidification can sensitize tumor cells to heat-induced cell death. However, the ability of Hsp70 to prevent apoptosis under these conditions has not been examined. The effect of acute acidification on heat-induced apoptosis was examined in a human T-cell line with tetracycline-regulated Hsp70 expression. Apoptosis was inhibited in cells exposed to hyperthermia in acidic media when examined 6 h after the heat stress, but resumed if cells were returned to physiological pH during this recovery period. Long-term proliferation assays showed that acute acidification sensitized cells to heat-induced apoptosis. Hsp70 expressing cells were also sensitized and this was correlated with a reduced ability to suppress the activation of JNK (c-jun N-terminal kinase), Bax and caspase-3. Further sensitization could be achieved with the NHE1 (Na(+)/H(+) exchanger) inhibitor HMA (5-(N, N-hexamethylene) amiloride), which potentiated JNK activation in heat-shocked cells. These results demonstrate that the ability of Hsp70 to suppress apoptosis is compromised when cells are exposed to hyperthermia in an acidic environment, which is correlated with an impaired ability to inhibit JNK activation.

Mitochondrial membrane permeabilization in cell death

Irrespective of the morphological features of end-stage cell death (that may be apoptotic, necrotic, autophagic, or mitotic), mitochondrial membrane permeabilization (MMP) is frequently the decisive event that delimits the frontier between survival and death. Thus mitochondrial membranes constitute the battleground on which opposing signals combat to seal the cell's fate. Local players that determine the propensity to MMP include the pro- and antiapoptotic members of the Bcl-2 family, proteins from the mitochondrialpermeability transition pore complex, as well as a plethora of interacting partners including mitochondrial lipids. Intermediate metabolites, redox processes, sphingolipids, ion gradients, transcription factors, as well as kinases and phosphatases link lethal and vital signals emanating from distinct subcellular compartments to mitochondria. Thus mitochondria integrate a variety of proapoptotic signals. Once MMP has been induced, it causes the release of catabolic hydrolases and activators of such enzymes (including those of caspases) from mitochondria. These catabolic enzymes as well as the cessation of the bioenergetic and redox functions of mitochondria finally lead to cell death, meaning that mitochondria coordinate the late stage of cellular demise. Pathological cell death induced by ischemia/reperfusion, intoxication with xenobiotics, neurodegenerative diseases, or viral infection also relies on MMP as a critical event. The inhibition of MMP constitutes an important strategy for the pharmaceutical prevention of unwarranted cell death. Conversely, induction of MMP in tumor cells constitutes the goal of anticancer chemotherapy.

Acidic extracellular pH shifts colorectal cancer cell death from apoptosis to necrosis upon exposure to propionate and acetate, major end-products of the human probiotic propionibacteria

The human probiotic Propionibacterium freudenreichii kills colorectal adenocarcinoma cells through apoptosis in vitro via its metabolites, the short chain fatty acids (SCFA), acetate and propionate. However, the precise mechanisms, the kinetics of cellular events and the impact of environmental factors such as pH remained to be specified. For the first time, this study demonstrates a major impact of a shift in extracellular pH on the mode of propionibacterial SCFA-induced cell death of HT-29 cells, in the pH range 5.5 to 7.5 prevailing within the colon. Propionibacterial SCFA triggered apoptosis in the pH range 6.0 to 7.5, a lethal process lasting more than 96 h. Indeed at pH 7.5, SCFA induced cell cycle arrest in the G2/M phase, followed by a sequence of cellular events characteristic of apoptosis. By contrast, at pH 5.5, the same SCFA triggered a more rapid and drastic lethal process in less than 24 h. This was characterised by sudden mitochondrial depolarisation, inner membrane permeabilisation, drastic depletion in ATP levels and ROS accumulation, suggesting death by necrosis. Thus, in digestive cancer prophylaxis, the observed pH-mediated switch between apoptosis and necrosis has to be taken into account in strategies involving SCFA production by propionibacteria to kill colon cancer cells.

Characterization of cell clones isolated from hypoxia-selected renal proximal tubular cells

Under hypoxia, some cells survive and others are irreversibly injured and die. The factors that determine cell fate under stress remain largely unknown. We recently selected death-resistant cells via repeated episodes of hypoxia. In the present study, 80 clones were isolated from the selected cells and their response to apoptotic injury was characterized. Compared with the wild-type cells, the isolated clones showed a general resistance to apoptosis: 13 were extremely resistant to azide-induced apoptosis, 10 to staurosporine, and 9 to cisplatin. The cell clones that most consistently demonstrated resistance or sensitivity to injury were further studied for their response to azide treatment. Azide induced comparable ATP depletion in these clones and wild-type cells. Hypoxia inducible factor-1 (HIF-1) was upregulated in several clones, but the upregulation did not correlate with cell death resistance. The selected clones maintained an epithelial phenotype, showing typical epithelial morphology, forming "domes" at high density, and expressing E-cadherin. Azide-induced Bax translocation and cytochrome c release, two critical mitochondrial events of apoptosis, were abrogated in death-resistant clones. In addition, cell lysates isolated from these clones showed lower caspase activation on addition of exogenous cytochrome c. Bax, Bak, and Bid expression in these clones was similar to that in wild-type cells, whereas Bcl-2 expression was higher in all the selected clones and, interestingly, Bcl-xL was markedly upregulated in the most death-resistant clones. The results suggest that apoptotic resistance of the selected clones is not determined by a single factor or molecule but, rather, by various alterations at the core apoptotic pathway.

Mechanisms of N-acetylcysteine-driven enhancement of MK886-induced apoptosis

N-Acetylcysteine (NAC), besides being a precursor of glutathione, has an array of other effects including an ability to scavenge free radicals, modulate gene expression and signal transduction pathways, and regulate cell survival and apoptosis. At concentrations lower than 20 mmol/L, NAC is nontoxic to cultured cells and can protect against apoptosis induced by a number of agents. A few recent reports, however, have indicated that NAC can also increase apoptosis. MK886, a 5-lipoxygenase activating protein (FLAP) inhibitor, induces apoptosis in many cell lines by an unknown mechanism that is independent of FLAP and lipoxygenase activity but is possibly related to effects on kinases such as Akt. In Jurkat T lymphocytes, NAC pretreatment (10 mmol/L) enhanced MK886-induced apoptosis by 2.4-fold. Following NAC-MK886 treatment, there was a significant increase in caspase-3 activity, and a decrease in mitochondrial transmembrane potential compared to MK886 alone. However, the extent of cytochrome c release was comparable between MK886 alone and MK886-NAC treatments. The enhancement of MK886-induced apoptosis by 10 mmol/L NAC appears to be partly related to a decrease in pH caused by this concentration of NAC, because an acidic environment favors activation of effector caspases and triggering of mitochondrial apoptosis. However, because neutralized NAC also enhanced apoptosis (1.6-fold), a direct role for NAC in augmenting the apoptotic pathways initiated by MK886 is suggested.

The apoptosome: physiological, developmental, and pathological modes of regulation

Apoptosis, a form of programmed cell death, is executed by a family of zymogenic proteases known as caspases, which cleave an array of intracellular substrates in the dying cell. Many proapoptotic stimuli trigger cytochrome c release from mitochondria, promoting the formation of a complex between Apaf-1 and caspase-9 in a caspase-activating structure known as the apoptosome. In this review, we describe knockout and knockin studies of apoptosome components, elegant structural and biochemical experiments, and analyses of the apoptosome in various cancers and other disease states, all of which have provided new insight into this critical locus of apoptotic control.

Cytoprotective effects of hypoxia against cisplatin-induced tubular cell apoptosis: involvement of mitochondrial inhibition and p53 suppression

Hypoxia that is caused by vascular defects or disruption is commonly associated with renal diseases. During cisplatin nephrotoxicity, hypoxic regions are identified in the outer medulla and the renal cortex. However, the regulation of cisplatin injury by hypoxia is unclear. Previous work has demonstrated the cytoprotective effects of hypoxia against apoptotic injury. This study further examines the cytoprotective mechanisms in models of cisplatin-induced tubular cell apoptosis. In cultured renal tubular cells, 20 microM cisplatin induced approximately 60% apoptosis within 16 h. The rate of apoptosis was suppressed to < 20%, when the incubation was conducted under hypoxia (2% O2). Mitochondrial events of apoptosis, namely Bax accumulation and cytochrome c release, also were ameliorated. During cisplatin treatment, cell ATP was maintained in both normoxic and hypoxic cells. Hypoxic incubation lowered extracellular pH, but prevention of the pH decrease did not restore cisplatin-induced apoptosis. The cytoprotective effects of hypoxia also were independent of hypoxia-inducible factor 1 (HIF-1). Cobalt, as hypoxia, activated HIF-1 yet did not suppress cisplatin-induced apoptosis. Moreover, hypoxia suppressed cisplatin-induced apoptosis in HIF-1-deficient mouse embryonic stem cells and renal proximal tubular cells. Conversely, mitochondrial inhibitors, particularly inhibitors of respiration complex III (antimycin A and myxothiazol), mimicked hypoxia in apoptosis suppression. The effects of hypoxia and mitochondrial inhibitors were not additive. It is interesting that both hypoxia and complex III inhibitors ameliorated cisplatin-induced p53 activation. Therefore, the cytoprotective effects of hypoxia are independent of changes in cell ATP, pH, or HIF but may involve mitochondrial inhibition and the suppression of p53.