Crocin prevents the death of PC-12 cells through sphingomyelinase-ceramide signaling by increasing glutathione synthesis

Crocin is a pharmacologically active component of Crocus sativus L. (saffron) that has been used in traditional Chinese medicine. In a previous study, we demonstrated that crocin inhibits apoptosis in PC-12 cells by affecting the function of tumor necrosis factor-alpha. In this study, we found that depriving cultured PC-12 cells of serum/glucose causes a rapid increase in cellular ceramide levels, followed by an increase in the phosphorylation of c-jun kinase (JNK). The accumulation of ceramide was found to depend on the activation of magnesium-dependent neutral sphingomyelinase (N-SMase), but not on de novo synthesis. The serum/glucose-deprived PC-12 cells also decreased the cellular levels of glutathione (GSH), which is the potent inhibitor of N-SMase. Treating the PC-12 cells with crocin prevented N-SMase activation, ceramide production, and JNK phosphorylation. We also found that the chemical can enhance the activities of GSH reductase and gamma-glutamylcysteinyl synthase (gamma-GCS), contributing to a stable GSH supply that blocks the activation of N-SMase. Thus our data suggest that crocin combats the serum/glucose deprivation-induced ceramide formation in PC-12 cells by increasing GSH levels and prevents the activation of JNK pathway, which is reported to have a role of the signaling cascade downstream ceramide for neuronal cell death.     

Lack of correlation between 3-hydroxy-3-methylglutaryl coenzyme A reductase activity and lovastatin resistance in nerve growth factor treated PC-12 cells

Summary 1. The relationships among the mevalonic acid (MVA) forming enzyme, 3-hydroxy-3-methylglutaryl coenzyme A (CoA) reductase, cell growth and differentiation, and the cytotoxic effects of the reductase inhibitor lovastatin were studied in PC-12 cells, exposed to growth factors.2. When added individually, nerve growth factor (NGF), basic fibroblast growth factor, and epidermal growth factor induce an increase in HMG-CoA reductase activity in cells grown in serum-containing medium. In the presence of serum, the effect of NGF on HMG-CoA reductase is persistent.3. Short-term serum starvation and long-term NGF treatment, in combination, have an additive effect, resulting in a high reductase activity.4. Unlike serum and MVA, which downregulate levels of HMG-CoA reductase by accelerating its degradation, NGF upregulates reductase by slowing the rate of its degradation. This mechanism, however, appears to operate only in the presence of serum, as after prolonged growth with NGF in serum-free medium, cells have a low reductase activity.5. PC-12 cells grown in the absence of NGF are highly sensitive to lovastatin (25 µM) and more than 70% of the cells die after 48 hr. NGF confers lovastatin resistance on cells grown in the presence or in the absence of serum (only 30–40% cell death after 48 hr with lovastatin).6. NGF-induced resistance on lovastatin develops with time and is apparent only in the well-differentiated PC-12 cells whether or not the cells express a high reductase activity.7. Thus, levels of HMG-CoA reductase activity and lovastatin resistance in PC-12 cells are not directly correlated, though clearly inversed lovastatin cytotoxicity and elevated reductase activities are expressed during the period of cell proliferation.8. These data suggest that fully differentiated neuronal cells may not be affected by prolonged high doses of lovastatin.     

Identification of Heat Shock Protein 60 as a Regulator of Neutral Sphingomyelinase 2 and Its Role in Dopamine Uptake

Activation of sphingomyelinase (SMase) by extracellular stimuli is the major pathway for cellular production of ceramide, a bioactive lipid mediator acting through sphingomyelin (SM) hydrolysis. Previously, we reported the existence of six forms of neutral pH–optimum and Mg²⁺-dependent SMase (N-SMase) in the membrane fractions of bovine brain. Here, we focus on N-SMase ε from salt-extracted membranes. After extensive purification by 12,780-fold with a yield of 1.3%, this enzyme was eventually characterized as N-SMase2. The major single band of 60-kDa molecular mass in the active fractions of the final purification step was identified as heat shock protein 60 (Hsp60) by matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis. Proximity ligation assay and immunoprecipitation study showed that Hsp60 interacted with N-SMase2, prompting us to examine the effect of Hsp60 on N-SMase2 and ceramide production. Interestingly, Hsp60 siRNA treatment significantly increased the protein level of N-SMase2 in N-SMase2-overexpressed HEK293 cells. Furthermore, transfection of Hsp60 siRNA into PC12 cells effectively increased both N-SMase activity and ceramide production and increased dopamine re-uptake with paralleled increase. Taken together, these results show that Hsp60 may serve as a negative regulator in N-SMase2-induced dopamine re-uptake by decreasing the protein level of N-SMase2.     

Crocin suppresses tumor necrosis factor-alpha-induced cell death of neuronally differentiated PC-12 cells.

Crocus sativus L. is used in Chinese traditional medicine to treat some disorders of the central nervous system. Crocin is an ethanol-extractable component of Crocus sativus L.; it is reported to prevent ethanol-induced impairment of learning and memory in mice. In this study, we demonstrate that crocin suppresses the effect of tumor necrosis factor (TNF)-alpha on neuronally differentiated PC-12 cells. PC-12 cells dead from exposure to TNF-alpha show apoptotic morphological changes and DNA fragmentation. These hallmark features of cell death did not appear in cells treated in the co-presence of 10 microM crocin. Moreover, crocin suppressed the TNF-alpha-induced expression of Bcl-Xs and LICE mRNAs and simultaneously restored the cytokine-induced reduction of Bcl-X(L) mRNA expression. The modulating effects of crocin on the expression of Bcl-2 family proteins led to a marked reduction of a TNF-alpha-induced release of cytochrome c from the mitochondria. Crocin also blocked the cytochrome c-induced activation of caspase-3. To learn how crocin exhibits these anti-apoptotic actions in PC-12 cells, we tested the effect of crocin on PC-12 cell death induced by daunorubicin. We found that crocin inhibited the effect of daunorubicin as well. Our findings suggest that crocin inhibits neuronal cell death induced by both internal and external apoptotic stimuli.     

Ceramide generation in nitric oxide-induced apoptosis. Activation of magnesium-dependent neutral sphingomyelinase via caspase-3

Sodium nitroprusside (SNP), a NO donor, has been recognized as an inducer of apoptosis in various cell lines. Here, we demonstrated the intracellular formation of ceramide, a lipid signal mediator, in SNP-induced apoptosis in human leukemia HL-60 cells and investigated the mechanisms of ceramide generation. The levels of intracellular ceramide increased to, at most, 160% of the control level in a time- and dose-dependent manner when the cells were treated with 1 mM SNP. SNP also decreased the sphingomyelin level to approximately 70% of the control level and increased magnesium-dependent neutral sphingomyelinase (N-SMase) activity to 160% of the control activity 2 h after treatment. Neither acid SMase nor magnesium-independent N-SMase was affected by SNP. Caspases are thought to be key enzymes in apoptotic cell death. Acetyl-Asp-Glu-Val-Asp-aldehyde, a synthetic tetrapeptide inhibitor of caspases, inhibited magnesiumdependent N-SMase, ceramide generation, and apoptosis. Moreover, recombinant purified caspase-3 increased magnesium-dependent N-SMase in a cell-free system. These results suggest that the findings that SNP increased ceramide generation and magnesium-dependent N-SMase activity via caspase-3 are interesting to future study to determine the relation between caspases and sphingolipid metabolites in NO-mediated signaling.     

Inflammatory Cytokines and Survival Factors from Serum Modulate Tweak-Induced Apoptosis in PC-3 Prostate Cancer Cells

Tumor necrosis factor-like weak inducer of apoptosis (TWEAK, TNFSF12) is a member of the tumor necrosis factor superfamily. TWEAK activates the Fn14 receptor, and may regulate cell death, survival and proliferation in tumor cells. However, there is little information on the function and regulation of this system in prostate cancer. Fn14 expression and TWEAK actions were studied in two human prostate cancer cell lines, the androgen-independent PC-3 cell line and androgen-sensitive LNCaP cells. Additionally, the expression of Fn14 was analyzed in human biopsies of prostate cancer. Fn14 expression is increased in histological sections of human prostate adenocarcinoma. Both prostate cancer cell lines express constitutively Fn14, but, the androgen-independent cell line PC-3 showed higher levels of Fn14 that the LNCaP cells. Fn14 expression was up-regulated in PC-3 human prostate cancer cells in presence of inflammatory cytokines (TNFα/IFNγ) as well as in presence of bovine fetal serum. TWEAK induced apoptotic cell death in PC-3 cells, but not in LNCaP cells. Moreover, in PC-3 cells, co-stimulation with TNFα/IFNγ/TWEAK induced a higher rate of apoptosis. However, TWEAK or TWEAK/TNFα/IFNγ did not induce apoptosis in presence of bovine fetal serum. TWEAK induced cell death through activation of the Fn14 receptor. Apoptosis was associated with activation of caspase-3, release of mitochondrial cytochrome C and an increased Bax/BclxL ratio. TWEAK/Fn14 pathway activation promotes apoptosis in androgen-independent PC-3 cells under certain culture conditions. Further characterization of the therapeutic target potential of TWEAK/Fn14 for human prostate cancer is warranted.     

Ulk1/FUNDC1 Prevents Nerve Cells from Hypoxia-Induced Apoptosis by Promoting Cell Autophagy

Cell autophagy and cell apoptosis are both observed in the process of hypoxia-induced ischemic cerebral infarction (ICI). Unc-51 like autophagy activating kinase 1 (Ulk1) and FUN14 Domain-containing Protein 1 (FUNDC1) are both involved in the regulation of cell autophagy. This study aimed to investigate the regulatory effects of Ulk1 and FUNDC1 on hypoxia-induced nerve cell autophagy and apoptosis. Cell viability was measured using cell counting kit-8 (CCK-8) assay. Cell apoptosis was detected using Annexin V-PE/7-ADD staining assay. qRT-PCR was used to quantify the mRNA levels of Ulk1 and FUNDC1 in PC-12 cells. Cell transfection was performed to up-regulate the expression of Ulk1. 3-Methyladenine (3-MA) was used as autophagy inhibitor and rapamycin was used as autophagy activator in our experiments. SP600125 was used as c-Jun N-terminal kinase (JNK) inhibitor. Western blotting was performed to analyze the expression levels of key factors that are related to cell autophagy, apoptosis and JNK pathway. We found that hypoxia simultaneously induced apoptosis and autophagy of PC-12 cells. The activation of Ulk1 and FUNDC1 were also found in PC-12 cells after hypoxia induction. Overexpression of Ulk1 promoted the activation of FUNDC1 and prevented PC-12 cells from hypoxia-induced apoptosis. Suppression of Ulk1 had opposite effects. Furthermore, we also found that JNK pathway participated in the effects of Ulk1 overexpression on PC-12 cell apoptosis reduction. To conclude, Ulk1/FUNDC1 played critical regulatory roles in hypoxia-induced nerve cell autophagy and apoptosis. Overexpression of Ulk1 prevented nerve cells from hypoxia-induced apoptosis by promoting cell autophagy.     

Sphingomyelinase and ceramide analogs induce contraction and rises in [Ca(2+)](i) in canine cerebral vascular muscle

Studies were designed to investigate effects of neutral sphingomyelinase (N-SMase) and ceramide analogs as well as phosphorylcholine on vascular tone and Ca(2+) mobilization in isolated canine cerebral arterial smooth muscle. N-SMase (0.001-0.1 U/ml) provoked a gradual but sustained vasoconstriction of arterial rings in a concentration-related manner that was endothelium independent. Incubation of denuded arterial rings in Ca(2+)-free medium or pretreatment with verapamil in extracellular Ca(2+) resulted in a reduction of the N-SMase-evoked constriction. Exposure of arterial rings to 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA)-AM did not, however, result in a reduction of N-SMase-induced constriction. Both staurosporine and bisindolymaleimide I attenuated N-SMase-induced contractions to 66% and 72% of control, respectively. N-SMase caused gradual and sustained rises in intracellular Ca(2+) concentration ([Ca(2+)](i)) in primary cultured cerebral vascular smooth muscle cells. Pretreatment of these cultured cells with nimodipine and verapamil caused a steady decline in N-SMase-induced rises in [Ca(2+)](i). Exposure of the cells to Ca(2+)-free solution reversed the [Ca(2+)](i)-induced rise triggered by N-SMase to the resting baseline. Both C(8) and C(16) ceramide (10(-9)-10(-6) M), but not phosphorylcholine, constricted denuded canine arterial rings in a concentration-related manner and elevated [Ca(2+)](i). Our results suggest that the sphingomyelin-signaling pathway, via a probable release of ceramide molecules, may play an important role in regulation of cerebral arterial wall tone.     

Glaucocalyxin B protects against oxygen-glucose-deprivation/reperfusion-induced neuronal injury in PC-12 cells

Oxidative stress has been implicated in the development of cerebral ischemia/reperfusion (I/R) injury. Glaucocalyxin B (GLB), one of five ent-kauranoid diterpenoids, was reported to possess neuroprotective activity. However, the effect of GLB on oxygen-glucose-deprivation/reperfusion (OGD/R)-induced cell injury in PC-12 cells has not been explored. PC-12 cells was treated with various concentrations of GLB (0, 2.5, 5 and 10 μM), and cell viability was detected using the MTT assay. PC-12 cells were pretreated with the indicated concentration of GLB (2.5-10 μM, 2 hours pretreatment), and were maintained under OGD for 3 hours, followed by 24 hours of reoxygenation. Cell viability was assessed using the MTT assay. The levels of superoxide dismutase, malondialdehyde, and glutathione peroxidase were detected using commercially available ELISA Kits. Intracellular reactive oxygen species level was measured using the fluorescent probe 2′,7′-dichlorofluorescein diacetate. The levels of Bcl-2, Bax, p-Akt, Akt, p-mTOR, mTOR were detected using Western blot. Our results revealed that GLB significantly protected PC12 cells against OGD/R-induced cell injury. In addition, GLB efficiently inhibited oxidative stress and cell apoptosis in OGD/R-stimulated PC-12 cells. Mechanistic studies revealed that pretreatment with GLB could induce the activation of Akt/mTOR signaling pathway resulting in protection of OGD-treated PC12 cells. In conclusion, our data indicate for the first time that GLB protects against OGD/R-induced neuronal injury in PC-12 cells. The mechanism of the protective effect of GLB is partially associated with activation of the Akt/mTOR signaling pathway. Thus, GLB may be a potential agent for protection against cerebral I/R injury.     

Phorbol ester-induced cell death in PC-12 cells overexpressing Bcl-2 is dependent on the time at which cells are treated

The aim of the present study was to investigate the involvement of PKC in Bcl-2 protection against serum withdrawal-induced apoptosis in PC-12 cells. Human Bcl-2 was overexpressed in PC-12 cells and was found to totally inhibit serum withdrawal-induced apoptosis. 12-O-tetradecanoylphorbol-13-acetate (TPA) could induce cell death in PC-12 cells that overexpressed Bcl-2, implicating protein kinase C (PKC) in Bcl-2 protection. However, TPA-induced cell death did not involve caspase-3 activation or DNA degradation, suggesting that Bcl-2 was still inhibiting these processes and that TPA was mediating cell death either downstream of Bcl-2 or via independent signalling pathways. High cytosolic and particulate protein levels of PKC delta were correlated with TPA-induced cell death suggesting that PKC delta positively regulated this cell death. However, substantial down-regulation of PKC by prolonged exposure to TPA did not reduce the frequency of TPA-induced cell death, raising the possibility that PKC delta did not regulate cell death alone. Surprisingly, TPA-induced cell death was dependent on the time at which cells were treated, suggesting that cells were changing with time. Supporting this idea, the cytosolic and particulate protein levels of PKC delta and were found to change with time, and may account for the time-dependent manner in which TPA induced cell death. This is the first report to show that sensitivity to drug induced cell death in a cultured cell line changes with time. Experimental and theoretical evidence suggests that many cellular constituents exhibit temporal variations, oscillations or rhythms due to feedback regulation. We believe that investigation of these temporal changes, how they alter cell function with time and how they might be manipulated in single cells as well as across cellular populations is paramount in furthering our understanding of cellular behaviour.     

TNF-receptor I defective in internalization allows for cell death through activation of neutral sphingomyelinase

The cytoplasmic tail of the tumor necrosis factor receptor I (TNF-RI) contains several functionally distinct domains involved in apoptotic signaling. Mutants of TNF-RI carrying deletions of the death domain (DD), internalization domain (TRID), and neutral sphingomyelinase domain (NSD), respectively, retransfected in cells devoid of TNF-RI and TNF-RII, constituted distinct tools to evaluate the specific role of each domain in downstream apoptotic signaling events. Deletion of DD abolishes activation of caspase-3 and -9 and apoptosis following treatment with TNF because of blocked assembly of the DISC. Nevertheless, TNF-RI DeltaTRID, though lacking a DISC, still allows for residual activation of caspase-3 followed by cell death, although caspase-9 activation was not detected. This activity of caspase-3 is probably due to activation of neutral sphingomyelinase (N-SMase). Increased activity of this enzyme was detected in cells expressing TNF-RI DeltaTRID following treatment with TNF, but not in any other cell line investigated. N-SMase is activated by factor associated with N-SMase (FAN). Because TNF-RI DeltaTRID is retained at the cell surface, FAN may interact with the mutated receptor for a prolonged amount of time, thereby overactivating N-SMase. Double deletion of TRID and NSD abolished caspase-3 activation and apoptosis, confirming this hypothesis.     

Phytic acid exerts protective effects in cerebral ischemia-reperfusion injury by activating the anti-oxidative protein sestrin2

ABSTRACT

Cerebral ischemia reperfusion (I/R) is a therapeutic strategy for ischemia; however, it usually causes injury by the aspect of inflammation and neuron apoptosis. This investigation aims to investigate the protective effects of phytic acid (IP6) for cerebral I/R injury in vitro. PC-12 cells under Oxygen and glucose deprivation/reperfusion (OGD/R) were performed to mimic cerebral I/R. IP6 was pretreated before PC-12 cells under OGD/R treatment. The data showed that IP6 activated the expression of sestrin2 in OGD/R injured PC-12 cells. IP6 inhibited OGD/R induced inflammation, oxidative stress, and apoptosis by activating sestrin2. Besides, p38 MAPK may mediate the effects of sestrin2 activated by IP6. Therefore, IP6 can be a potential drug to prevent neurological damage in cerebral I/R injury.     

Amino acid starvation induced autophagic cell death in PC-12 cells: Evidence for activation of caspase-3 but not calpain-1

While the apoptotic and necrotic cell death pathways have been well studied, there lacks a comprehensive understanding of the molecular events involving autophagic cell death. We examined the potential roles of the apoptosis-linked caspase-3 and the necrosis/apoptosis-linked calpain-1 after autophagy induction under prolonged amino acid (AA) starvation conditions in PC-12 cells. Autophagy induction was observed as early as three hours following amino acid withdrawal. Cell death, measured by lactate dehydrogenase (LDH) and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays occurred within 24 h following starvation and was accompanied by an upregulation in caspase-3 activity but not calpain-1. The cell death that occurred following AA starvation was significantly alleviated by treatment with the autophagy inhibitor 3-methyl adenine but not with the broad spectrum caspase inhibitors. Thus, this study demonstrates that 3-methyladenine-sensitive autophagic cell death due to AA starvation in PC-12 cells is mechanistically and biochemically similar to, yet distinct from, classic caspase dependent apoptosis. Shankar Sadasivan and Anu Waghray have contributed equally to this work.     

Trefoil Factor 3 as an Endocrine Neuroprotective Factor from the Liver in Experimental Cerebral Ischemia/Reperfusion Injury

Cerebral ischemia, while causing neuronal injury, can activate innate neuroprotective mechanisms, minimizing neuronal death. In this report, we demonstrate that experimental cerebral ischemia/reperfusion injury in the mouse causes upregulation of the secretory protein trefoil factor 3 (TFF3) in the hepatocyte in association with an increase in serum TFF3. Partial hepatectomy (~60% liver resection) immediately following cerebral injury significantly lowered the serum level of TFF3, suggesting a contribution of the liver to the elevation of serum TFF3. Compared to wild-type mice, TFF3^-/- mice exhibited a significantly higher activity of caspase 3 and level of cell death in the ischemic cerebral lesion, a larger fraction of cerebral infarcts, and a smaller fraction of the injured cerebral hemisphere, accompanied by severer forelimb motor deficits. Intravenous administration of recombinant TFF3 reversed changes in cerebral injury and forelimb motor function due to TFF3 deficiency. These observations suggest an endocrine neuroprotective mechanism involving TFF3 from the liver in experimental cerebral ischemia/reperfusion injury.     

Neutral sphingomyelinase from human urine. Purification and preparation of monospecific antibodies

A neutral sphingomyelinase which cleaves phosphorylcholine from sphingomyelin at a pH optima of 7.4 was purified 440-fold to apparent homogeneity from normal human urine concentrate employing Sephadex G-75 column chromatography, preparative isoelectric focusing, and sphingosylphospholcholine CH-Sepharose column chromatography. The enzyme is composed of a single polypeptide whose apparent molecular weight is 92,000. Analytical isoelectric focusing revealed that the pI of this enzyme is 6.5. Purified neutral sphingomyelinase was devoid of beta-galactosidase and beta-N-acetylglucosaminidase activity originally present in the urine concentrate. The purified neutral sphingomyelinase (N-SMase) had low levels of phospholipase A1 and A2 activity when phosphatidylcholine was used as a substrate and detergents were included in the assay mixture. However, it had no phospholipase activity toward phosphatidylglycerol and sphingomyelin at pH 4.5 irrespective of the presence or absence of detergents. Monospecific polyclonal antibodies raised against N-SMase immunoprecipitated approximately 70% of N-SMase activity from urine, human kidney proximal tubular cells, and partially purified membrane-bound N-SMase from these cells. Western immunoblot assays revealed that the monospecific polyclonal antibody against urinary N-SMase recognized both the urinary N-SMase and the membrane-bound N-SMase. Because this enzyme is distinct biochemically and immunologically as compared to acid sphingomyelinase (EC 3.1.4.12), we would like to assign it an enzyme catalog number of EC 3.1.4.13. The availability of N-SMase and corresponding antibody will be useful in studying various aspects of this enzyme in biological systems.     

Neutral sphingomyelinase: past, present and future

Sphingomyelin and its metabolic products are now known to have second messenger functions in a variety of cellular signaling pathways. At the epicenter of the sphingomyelin--cell signaling pathway is a family of phospholipases called sphingomyelinases. These enzymes cleave sphingomyelin to produce ceramide and phosphocholine. Ceramide in turn serves as a lipid second messenger that induces a variety of cell regulatory phenomenon such as programmed cell death (apoptosis), cell differentiation, cell proliferation, and sterol homeostasis. Neutral sphingomyelinase (N-SMase) is a Mg2+ sensitive enzyme that can be activated by a host of physiologically relevant and structurally diverse molecules like tumor necrosis factor-alpha (TNF-alpha), oxidized human low density lipoproteins (Ox-LDL), and several growth factors. Large amounts of ceramide accumulate in human fatty streaks and plaques along with Ox-LDL, growth factors, and proinflammatory cytokines in human atherosclerosis. A further role of ceramide and N-SMase in atherosclerosis was uncovered by the finding that Ox-LDL and TNF-alpha stimulated N-SMase activity. In turn, ceramide and/or a homolog serves as an important stress signaling molecule in signal transduction, which leads to apoptosis. Interestingly, an antibody against N-SMase can abrogate Ox-LDL and TNF-alpha induced apoptosis, and therefore may be useful for additional studies of apoptosis in experimental animals. Overexpression of recombinant human N-SMase in human aortic smooth muscle cells markedly stimulate apoptosis, presumably via the multioligomerization of the 'death domain'. Since plaque stability is an integral aspect of atherosclerosis management, activation of N-SMase and subsequent apoptosis may be vital events in the onset of plaque rupture, stroke and heart failure. In contrast to these observations in human hepatocytes, TNF-alpha mediated N-SMase activation did not induce apoptosis. Rather it stimulated the maturation of sterol regulatory element (SRE) binding protein (SREBP-1). Moreover, a cell permeable ceramide was found to reconstitute the phenomenon above in a sterol-independent fashion. These findings provide alternate avenues for therapy of patients with hypercholesterolemia and atherosclerosis. The findings reported here suggests that N-SMase plays important cell regulatory roles and provide an exciting opportunity to further these findings to understand the pathophysiology of human disease states.     

Effects of gentamicin on sphingomyelinase activity in cultured human renal proximal tubular cells

We have previously shown that cultured human proximal tubular cells (PT) incubated with gentamicin contain numerous "myeloid bodies." This morphological change was accompanied by the storage of phosphatidylcholine and sphingomyelin. In order to delineate the biochemical mechanisms responsible for the accumulation of sphingomyelin in cells incubated with gentamicin, we pursued detailed studies on the activity of sphingomyelinase. Characterization studies on sphingomyelinase revealed that this enzyme has a bimodal pH optima in PT cells. Optimum activity was observed at pH 5.6 (designated as acid sphingomyelinase, A-SMase) and at pH 7.4 (designated as neutral sphingomyelinase, N-SMase). The activity of both the enzymes increased proportionately in control cells as a function of days of incubation. The activity of A-SMase was 16% lower in cells incubated with gentamicin as compared to control. The most striking observation was a gradual decline in the activity of N-SMase in cells incubated with gentamicin. Thus, following 21 days of incubation of cells with 0.3 mM gentamicin, the N-SMase was 2.7-fold lower than control cells. Mg2+ stimulated and Triton X-100 inhibited the activity of N-SMase. Whereas Mg2+ had no effects, Triton X-100 stimulated the activity of the A-SMase in PT cells. Moreover, A-SMase was relatively more heat-resistant than the N-SMase. The Km values for sphingomyelin using A-SMase in control cells and cells incubated with gentamicin were 0.07 X and 0.016 X 10(-7) M, respectively, whereas the Km values for sphingomyelin using N-SMase in control cells and cells incubated with gentamicin were 1.8 X and 1.5 X 10(-7) M, respectively. These findings suggest that gentamicin exerts a competitive inhibition of the A-SMase in PT cells. In contrast, gentamicin exerts a noncompetitive inhibition of the N-SMase in PT cells. Subcellular fractionation studies revealed that A-SMase was exclusively localized in the "lysosome-rich" fraction, whereas most, if not all, the N-SMase was localized in the microsomal fraction and "plasma-membrane"-rich fraction in cultured PT cells. Cells incubated with gentamicin for 21 days contained 25% lower activity of A-SMase associated with the lysosomal fraction as compared to control. In contrast, N-SMase activity in the microsomal and plasma membrane fraction was one-half as compared to control. We conclude that gentamicin-mediated decrease in sphingomyelinase activity may be responsible for the storage of sphingomyelin in cultured human PT cells.(ABSTRACT TRUNCATED AT 400 WORDS)