Ceramide and the Regulation of Apoptosis and the Stress Response

Ceramide and the Regulation of Apoptosis and the Stress Response David K. Perry, Lina M. Obeid, and Yusuf A. Hannun

Theprocessof apoptosisorprogramrned celldeuthhasbeenthesubjectof intensestudydue to the realizationof its impotiancein a widevarietyof biologicalandpathologicalconditions,includingdevelopment,ischemia, cancer, and neurodegenerative disorders.Althougha vast numberof inducersof thisprocessandseveralcriticalcomponentshave been identified,the signaltransductionpathwaysregulatingapoptosisarepoorly understood.Recently,a pathwayinvolvingsphingolipidturnoverandthe productionof the lipiddiator, ceramide,has emergedas a candidate regukztorof apoptosis.Thisreviewprovidesa summmyof the evidence implicatingceramideas a mediatorof apoptosisandthe stressresponse. (Trends Cardiovasc Med 1996;6:158-162).

development, apoptosis is responsible for removing unwanted cells and may Apoptosis (or pmgmrnmed cell death) is a play an important role in the postnatal term given by Kerr et al. (1972) to repre- morphogenesis of the sinus node, atriosent a cellular process marked by charac- ventricular node, and His bundle (James teristic morphological changes and culmin- 1994). Apoptosis also appears to have a ating in cell death. It is conserved across critical role in the most important species ranging fmm the nematode, Cae- pathological occurrences in the cardionorhabditiselegans,to humans. Further- vascular system: ischemia, infarction, more, apoptosis is thought to play a vital and reperfusion injury. These conditions role in a vast number of biological and may result in significant stress and dampathological conditions, including devel- age to the affected cardiac muscle, and opment and tissue remodeling, cancer, in analogy with better studied systems ischemia and infarction, immune disor- (for example, liver), these events may ders, and neurodegenerativediseases (for result in significant cell death through example, Alzheimer’sand Parkinson’sdis- apoptosis. eases). Thus, defective apoptosis may conA major advance in cell biology in the tribute to cancer pathogenesis and au- last decade has come from the realization toimmune disordem, whereas increased that apoptosis is not simply a failure of apoptotic activity may contribute to neu- cell processes leadingto passivedeath,but mdegenemtive disordem and immune de- rather a biochemically regulated process. ficiency states [for example, human im- Many cellular components-some inducing and others protecting from cell munodeficiency virus (HIV) infection]. death—havebeen found to regulateapop In the cardiovascular system, apoptosis has a role in developmental and tosis. Therefore, determining the mechapathological conditions. During cardiac nisms involvedin regulatingapoptosis has become a major pursuit of basic biomedical investigation. This promises important insight into these pathways and DavidK. Perry,LinaM. Obeid,andYusufA. should eventuallyresult in the ability to Hannunare at the Departmentof Medicine, modulate cell death in order to combat Duke UniversityMedical Center,Durham,NC pathological conditions in which apopto27710, USA. .

Introduction

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sis may play a detrimental role. This review summarizes the current state of knowledge about the regulation of apop tosis, with special emphasis on sphingolipid metabolizesas possible key regulators of apoptosis.

.

Known Components in the Apoptotic Response

Much of what we know concerning the apoptotic process and the components involved therein has been elucidated from studies on Caenorhabditkelegans (Hengartner and Horvitz 1994a). Genetic studies have identified two genes, ced-3 and ced-4, that are required for apoptosis. A third gene, ced-9, antagonizes the effects of cez+3 and ced-4, in that mutations in ced-9 cause cells that would normally live to undergo apoptosis. The mammalian homologies of ced-3 and ced-9 have recently been identified and are proteases of the interleuldn (IL)-1~ converting enzyme (ICE) family (Yuan et al 1993) and bcl-2 (Hengartner and Horvitz 1994b), respectively. The original fimction of ICE was reported to be the processing of pr&L.-l~ to IL-1(3,a mediator of inflammation (Black et al. 1989,Kostura et al. 1989).Transection of ICE into mammalian cells, however, induces apoptosis independent of its pr~ IL-1P processing activity (Miura et d. 1993). The significance of ICE in apop tosis was attenuated by further studies, demonstrating that mice that contain knockouts in the ICE gene unde~o development in a normal manner and remain sensitiveto most inducers of apoptosis (Li et al. 1995, Kuda et al. 1995). Recently, a plethora of ICE-like proteases has been identified (Kumar 1995), and it now ap pears that other members of this family, such as CPP32 (Fernandes-Afnernriet al. 1994) and ICE-LAP3 (Duan et al. 1996), are more fundamentallyinvolvedin carrying out the apoptotic process. Bc[-2, on the other hand, protects celIs that are overexpressing ICE, an event that would normally result in apoptosis (Miura et al. 1993). BcZ-2 was initially discovered (as a result of a translocation) as an overexpressed protein in human lymphoma (Pegoraro et al. 1984). It was later found to act by promoting cell survival and protecting from apoptotic death in response to a wide variety of agents and insults. The manner in which this protection is accomplished is largely

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unknown. One way in which it may occur, however, is by the heterodimerization of bcl-2 with bax, a member of the bcl-2 gene family and a positive regulator of cell death (Oltvai et al. 1993). Therefore, bax function is abrogated upon formation of heterodimers with bcl-2 resulting in cell survival. Aside from what has been learned from the studies in C. elegans,a number of other players in the apoptotic process have been identified, although their role is less clear. These include the stressactivated protein kinases (SAPKS) and the tumor suppressor gene p53. SAPK (or jun kinase) is a member of the extended family of mitogen-activated protein (MAP) kinases (Kyriakis et al. 1995, Davis 1994) and is rapidly activated following treatment of cells with tumor necrosis factor (TNF)-ci (Westwick et al. 1994) or other stresses, such as reperfusion and ischemia (Pombo et al. 1994). This leads to enhanced transcription of c-jun. Overexpression of SAPK induces apoptosis, and interference with activation of SAPK protects from apoptosis (Xia et al. 1995). p53 is now recognized to participate in DNA-damage–induced injury and apoptosis. Mice that are homozygous knockouts in p53 become resistant to some agents that would normally result in cell death (Clarke et al. 1993). Moreover, transection of cells with the gene for adenovirus EIA protein stabilizes p53 and sensitizes the cells to drug-induced apoptosis (Lowe et al. 1993). Both lines of evidence implicate the tumor suppressor gene p53 as a mediator of apoptosis, and it has been labeled as the guardian of the genome. Although the genetic studies in C. eleganshave been instrumental in identifying the necessary components for cell death and survival, there has been a paucity of data in determining the signal transduction pathways leading to the activation or inhibition of these components. It is known that activation of protein kinase C (PKC) by diacylglycerol or phorbol esters reverses the degree of apoptosis induced by many agents (Obeid et al. 1993). PKC may also “activate” bcl-2 (May et al. 1994); therefore, PKC maybe a regulator of the cell preservation effect of bcZ-2. While classic pathways of signal transduction, including those of glycerolipid metabolism and tyrosine kinase cascades, have shed little

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Death or Stress Signal

?

SA-PK-CAPP

1

c-jun

PKC

+ prICE

\

1 death

growth arrest

Figure 1. Proposed scheme for ceramide-mediatedsignal transduction.Generation of ceramide via a deathor stresssignal,suchas tumor necrosisfactor (TNF)-a or Fas,can lead to growth arrest or cell death. The growth arrest pathway is mediated by the retinoblastoma (Rb) gene product. Alternatively,ceramide can act through a protein phosphatase(CAPP) or the stressactivatedprotein kinases(SAPKS) to induce cell death (see discussionin the text). The SAPKS induce cell death through the activation of jun, and CAPP may act as a mediator of cell death by acting upstream of the interleukin-1~ converting enzyme (ICE) -like proteases [protease resembling ICE (prICE)/CPP32, for example]. Additionally, activation of prICE/CPP32 via ceramide can be inhibited by both PKC and bcZ-2.

light on mechanisms involved in transducing an apoptotic signal, a signal transduction pathway involving sphingolipids is emerging as a strong candidate.

●

Overviewof Lipid Signal Transduction

Whereas signaling through glycerophospholipids is well established, the observation of signaling through sphingolipids is relatively recent. Sphingolipids are much more diverse than glycerolipids in their structures, particularly in the headgroup moiety. However, they have been considered largely as structural components of cell membranes. The discovery of the inhibition of PKC by sphingosine led to the hypothesis that sphingolipid turnover may be involved in cellular signaling events (Hannun et al. 1986). This hypothesis was confirmed by the dem-

onstration that l,25-dihydroxyvitamin D3 stimulated sphingomyelin turnover in HL-60 cells (Okazaki et al. 1989) and as such, formed the foundation for the elucidation of the sphingomyelin cycle (Hannun and Bell 1989).

.

The Sphingomyelin Cycle and Ceramide

The sphingomyelin cycle is initiated by agents that cause a transient decrease in plasma membrane sphingomyelin content. This reaction is catalyzed by a specific phospholipase C, sphingomyelinase, resulting in the liberation of both ceramide and phosphocholine. The cycle is completed by the resynthesis of sphingomyelin via the transfer of phosphatidylcholine-derived phosphocholine to ceramide, catalyzed by a hitherto poorly characterized sphingomyelin synthase.

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et al. 1996). This protease was originally discovered in extracts from chicken cells that had been committed to apoptosis, and it causes the proteolysis of the nuclear enzyme poly(ADP-ribose) polymerase (PARP) (Lazebnik et al. 1994). PARP has since been termed the “death substrate” and is now a widely recognized and relatively early event in the apoptotic process. Recently, it has been determined that ceramide-induced apoptosis can be reversed in cells overexpressing bcl-2 (Martinet al. 1995, Zhang et al. 1996). Further studies demonstrated that bcl-2 prevents the activation of prICE/CPP32 (D.K. Perry, M.J. Smyth, H.-G. Wang, J.C. Reed, G.G. Poirier, L.M. Obeid, and Y.A. Hannun submitted for publication) and, hence, suggest a new mechanism whereby bcl-2 inhibits cell death (Figure 1).

OH CH20H R Dihydroceramide

NH Y

o

OH CH20H R

NH l’

Ceramide

o

OH o–~—.

,holine ●

l– o. R

Sphingomyelin

NH

Y

0

Figure 2. Structuresof dihydroceramide, ceramide, and sphingomyelin.

The inducers of sphingomyelin turnoverare numerous, diverse, and often cell specific. These include various members of the cytokine family (TNF-w, IL-1, and ~-interfenm), 1,25-dihydroxyvitamin Da, the Fas ligand, and neurotrophins. Furthermore, conditions that induce environmentalstressam widely known to activate the cycle as well, such as osmotic or heat shock, serum withdrawal, ultmvioletirradiation, chemotherapeutic agents, and ischemia (Hannun 1994, Kolesnick and Fuks 1995). .

Role of Ceramide in Apoptosis/Stress

Early studies conducted to determine the role of ceramide in cellular biology were complicated by the toxicity of ceramide on cells. Further observation in u937 cells, however, revealed that ceramide treatment resulted in DNA fragmentation, and that this effect was specific for ceramide in that the closely related dihydroceramide and diacylglycerol were without effect (Obeid et al. 1993). Similar results were observed with TNF-ci treatment and suggested

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that ceramide may be an intracellular mediator of agents that induce cell death (Figure 1). Further studies have also demonstrated that ceramide may be partially responsible in mediating growth arrest and the senescent phenotype. In this respect, ceramide will cause the dephosphorylation of the retinoblastoma (Rb) gene product (Dbaibo et al. 1995), which correlates with the mediation of G1 arrest by Rb (Figure 1). In addition, ceramide levels were elevated in senescent cells (Venable et al. 1994). This elevation was paralleled by an inhibition of diacylglycerol accumulation via a phospholipase D pathway and a lack of PKC trzurdocation. It was then shown that the addition of cell-permeable ceramides to intact cells results in an inhibition of the phospholipase D activity. Thus, ceramide may induce a senescent phenotype by inhibiting the proliferative effects of PKC activation and the phospholipase D pathway. We have also shown that ceramide treatment of cells results in activation of an ICE-like protease, termed prICE (protease resembling ICE) or CPP32 (Smyth

Regulation of the Sphingomyelinase/Ceramide Pathway

The sphingomyelinases, which catalyze the formation of ceramide from sphin,Yomyelin, are a diverse group of en0 zymes and are characterized based on their cellular localization, cation dependencies, and pH optima. The most widely studied member of this group is an acid sphingomyelinase that is localized to the lysosomes. It has been postulated that this enzyme is activated by TNF-cs secondary to phospholipase CCatalyzed diacylglycerol (DAG) production and transmits at least some aspects of TNF-a-mediated biology (Schutze et al. 1992). Mutations in the human acid sphingomyelinase gene have been found that cause either the neurovisceral (type A) or visceral (type B) forms of Niemann-Pick disease. Cells derived from Niemann-Pick patients retain their responsiveness to TNF-a (Kuno and Matsushita 1996), suggesting that the acid sphingomyelinase may not be a critical component in these sigmling pathways. There also exist two neutral pH optima enzymes, one that is Mg2+ independent and one that is Mg2+ dependent. The Mg2+-independent enzyme resides in the cytosol and was originally reported to be activated after l,25-dihydroxyvitamin Dq treatment of HL-60 cells (Okazaki et al. 1994). The Mg2+dependent enzyme is membrane bound, and its level of activity is increased in

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response to serum withdrawal, TNF-ci, and the Fas ligand. The validityof a signalingrole for ceramide has been substantiatedby structurefunction studies. Ceramide can exist in four different isomeric forms (D- and Lerythroand D- and L-three), and the naturally occurring D-erythroisomer is generally the most effective in transmittingthe cerarnide-mediated effects (Bielawska et al. 1993). Moreover, the naturally occurring dihydroceramide, which lacks the 45 tram double bond, is completely inactive (Figure 2). These studiesdemonstratethat cerarnide is very specific in its role as a lipid second messenger. As with all second messengers, there has been intense interest in determining the cellular target(s) of ceramide. This search has led to the identification of a ceramide-activated protein phosphatase (CAPP) (Dobrowsky and Hannun 1992). CAPP is a cytosolic enzyme and, based on pharmacologic and chromatographic data, is a member of the heterotrimeric protein phosphatase class 2A. It mediates ceramide-induced c-myc downregulation in mammalian cells (Wolff et al. 1994) and is consewed in yeast, as it appears to be responsible for the growth suppression effects of ceramide (Fishbein et al. 1993). Furthermore, a ceramide-activated protein (CAP) kinase has been reported (Mathias et al. 1991). This kinase is membrane bound and is proline directed. Recently, it has been determined that an activated CAP kinase from

TNF-a or ceramide-stimulated cells will phosphorylate and activate raf kinase, and as such, CAP kinase may form a link between the TNF receptor and the MAP kinase cascade (Yao et al. 1995). It is yet unclear, however, whether the purified form of this enzyme is activated directly by ceramide. In addition, a member of the protein kinase C family, (~), can be stimulatedin vitro by ceramide (Lozano et al. 1994). Cerarnideactivationof this kinasealso enhances its I-KB phosphorylating activity and, therefore, the activation of NFwB. The diversity of cellular targetsthat exist for ceramide and the wide range of biological outcomes observed upon ceramide generation indicate that responses are likely to be cell specific. In addition, signaling through the sphingomyelin cycle may be greatlyinfluenced by cross-talk with other signalingpathways.

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Conclusions

of cellular stimuli appear to converge on This reviewhas provided a short summary the activation of this protease. Apoptotic of studies implicating cerarnide as a sec- extracts, in which prICE/CPP32 has ond messengerinvolvedin regulatingapo- been depleted, lose their ability to incharacteristic morphological ptosis and stress responses. From prelimi- duce changes in nuclei. However, prICE/ nary experimentsidentifyingceramide as CPP32 alone does not induce these an inducer of cell death to the determinachanges. Therefore, prICE/CPP32 function of both upstream and downstream tion is a necessary, though not suffieffecter mechanisms of ceramide, much cient, component of the apoptotic mahas been learnedof the role of cemmide in chinery (Nicholson et al. 1995). In apoptosis. Wide gaps exist, however, in addition, the studies implicating PKC as the understandingof the regulatoryevents an antagonist of apoptosis suggest that in this pathway. stimulators of this pathway would proThe mechanisms determining the acvide an additional means of attenuating tivation of sphingomyelinases by extrathe degree of apoptosis. celhdar stimuli are not well understood. In conclusion, the transformation of For instance, what are the determinants studies on apoptosis to the biochemical of activation of lysosomal acid sphingolevel promises great insight into this funmyelinase versus plasma membrane damental area of cell biology and may neutral sphingomyelinase? Evidence ex- provide novel targets for modulating disists to suggest that the effecter mecha- orders in which apoptosis plays a crucial nisms activated by ceramide are in part role. The study of ceramide signaling determined by the microenvironment in may emerge as a critical component of which the ceramide is produced. There- these pathways regulating apoptosis. fore, it can be inferred that understanding the regulatory events involved in the . Acknowledgment activation of the respective sphingomyelinases will greatly enhance the deter- We thank Dr. Supriya Jayadev for the mination of the ceramide effecters acti- preparation of figures. vated under specific conditions. Furthermore, it is worth reiterating the role of the invaluable studies in C. References elegans,demonstrating the vitaJfunction of ced-3 and ced-9 in regulating apopto- BielawskaA, Crane HM, Liotta D, Obeid LM, Harmun YA: 1993.Selectivity of ceramidesis in this organism. One would expect mediated biolo~. J Biol Chem 268:26,226that critical parameters in understand26,232. ing the role of ceramide in apoptosis Black RA, Kronheim SR, SleathPR: 1989.Accould arise with the identification of the tivation of interleukin-lf3by a co-induced intervening links between immediate ceprotease.FEBS Lett 247:386-390. ramide effecters, such as CAPP and the CfarkeAR, Purdie CA,HarrisonDJ,et al.: 1993. mammalian homologies of ced-.3 and Thymocyteapoptosisinducedby p53-depenced-9, the ICE-like proteases, and bcl-2, dentand independentpathways.Nature362: respectively. 849-852. Understanding these mechanisms of Davis RJ: 1994.MAPKs: new JNK expandsthe apoptosis and the stress response should group. Trends Biochem Sci 19:470473. have important application to those clin- Dbaibo GS, PushkarevaMY, JayadevS, et al.: ical conditions in which it would be ad1995. Retinoblastoma gene product as a vantageous to prevent apoptosis, such as downstreamtargetfora ceramidedependent pathwayof growtharrest.Pmc Natl Acad Sci ischemia and degenerative disorders. USA 92:1347-1351. The current results from basic research indicate that one group of candidates for Dobrowsky RT, Hannun YA: 1992.Cerarnide stimulatesa cytosolicprotein phosphatase.J pharmacologic intervention would be Biol Chem 267:5048-5051. those enzymes that directly regulate ceramide levels, for example, the sphingo- DuanJ, ChinnaiyanAM, Hudson PL, Wing JP, He W-W, Dixit VM: 1996.ICE-LAP3,a novel myelinases involved in cerarnide genermammalian homolog of the Caenorhbddis ation and the ceramidases involved in elegarzscelf death protein CED-3is activated ceramide catabolism. An additional tarduring Fas- and tumor necrosis factor-inget for apoptotic inhibition at this time duced apoptosis. J Biol Chem 271:1621would be prICE/CPP32, as a wide variety 1625. ●

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inflammato~ cytokines.Ann NY Acad Sci Fernandes-AfnemriT, Litwack G, AlnernriES: 766:303-319. 1994.CPP32, a novel hutnanapoptotic proteinwith homology to Caenorhabditisefegans Lazebnik YA, Kaufinann SH, Desnoyers S, cell deathprotein Ced-3and mammafiattinPoirier GG,EamshawWC: 1994.Cleavageof terleukiml(konverting enzyme.J Biol Chem poly(ADP-ribose)polymerasebya proteinase 269:30,761-30,764. with properties like ICE. Nature 371:346347. FishbeinJD,DobrowskyRT, BiefawskaA, GarrettS, HannunYA: 1993.Ceramide-mediated Li P, Allen H, Banerjee S, et al.: 1995.Mice growthinhibitionand CAPP are conservedin deficientin IL-1 beta-convertingenzyme are .%ccharomycescerevisiae.J Biol Chem 268: defective in productionof mature IL-1 beta 9255-9261. andresistantto endotoxicshock.Celf80:401411. Hannun YA: 1994. The sphingomyelin cycle and the second messengerfunction of cera- Lowe SW, Ruley HE, JacksT, Housman DE: mide. J Biol Chem 269:3125-3128. 1993. p53-cfependentapoptosis modulates

HannunYA, Befl RM: 1989. Functions of sphingolipids and sphingolipid breakdown productsin celhdarregulation.Science 243: 500-507. Hanmm YA, Loomis CR, Merrill AH, Bell RM: 1986.Sphingosineinhibition of protein kinase C activity and of phorbol dibutyrate binding in vitro and in human pfatelets.J Biol Chem 261:12,604-12,609.

the cytotoxicityof anticanceragents.Cell 74: 957-967. Lma.noJ, Berra E, Municio MM, et al.: 1994. Protein kinaseC ~ isoform is ctiticaf for KBdependentpromoter activation by sphingomyelinase.J Biol Chem 269:19,200-19,202.

Oltvai ZN, MillirnanCL, Korsmeyer SJ: 1993. Bc1-2heterodimerizes in vivo with a conserved homolog, Bax, that acceleratesprogrammedcelf death.Celf 74:609419. Pegoraro L, PalumboA, EriksonJ, et al.: 1984. A 14;18and a 8;14 chromosome translocation in a celfline derivedfrom an acuteB-cell leukemia.Proc Natl Acad Sci USA 81:71667170. Pombn CM, BonventreJV,AvruchJ, Woodgett JR, Kyriakis JM, Force T: 1994.The stressactivated protein kinases are major c-Jun amino-temninafkinases activated by ischemia and reperfusion. J Biol Chem 269:26,546-26,551. SchutzeS, PotthoffK, MachleidtT, BerkovicD, WiegmannK, KronkeM: 1992.TNFactivates NF-KBby phosphatidylcholine-specific phospholipase C-induced“acidic” sphingomyelin breakdown.Cell 71:765-776.

Martin SJ, Takayama S, McGahon AJ, et al.: 1995.Inhibition of ceramide-inducedapoptosisby Bc1-2.Cell DeathDiffer 2:253-257.

Smyth MJ, Perry DK, Zhang J, Poirier GG, HanmmYA, ObeidLM: 1996.prICE:a downstreamtarget for ceramide-inducecfapoptoHengartnerMO, Horvitz HR: 1994a.The ins Mathias S, DressierK4, Kolesnick RN: 1991. sisand for the inhibitoryactionof bcl-2. Biom-doutsof programmedcelf deathduringC. chem J (in press). Characterization of a ceramide-activated efegans development. Philos Trans R Soc protein kinase:stimulationby tumor necro- VenableME, Blobe GC, Obeid LM: 1994.IdenLend 345:243-246. sis factor alpha.Proc Natl Acad Sci USA 88: tification of a defect in the phospholipase HengartnerMO, Horvitz HR: 1994b.C.efegans 10,009-10,013. D/diacylgfycerolpathway in cellular senescell survivalgene ced-9encodesa functional May WS, Tyler PG, Ito T, ArmstrongDK, Qatcence.J Biol Chem 269:26,040-26,044. homolog of the mammalianproto-oncogene shaA, DavidsonNE: 1994.Interleukirt-3and WestWickJK, Weitzel C, Minden A, Karin M, bcl-2. Celf 76:665476. bryostatin-1mediate hyperphosphorylation BrennerDA: 1994.Tumor necrosisfactor alJamesTN: 1994.Normal and abnormafconseof BCL2ciin associationwith suppressionof pha stimulatesAP-1 activity through pro quences of apoptosis in the human heart. apoptosis.J Biol Chem269:26,865-26,870. longed activationof tlie c-jun kinase.J Biol Circulation90:556-573. MiuraM, ZhuH, Rotello R, HartwiegEA, Yuan Chem 269:26,396-26,401. KerrJF,Wyllie AH, CurneAR: 1972.Apoptosis: J: 1993.Inductionof apoptosisin fibroblasts Wolff RA, DobrowskyRT, BielawskaA, Obeid a basic biological phenomenon with wideby IL-l&converting enzyme, a mammalian LM, Harmun YA: 1994.Role of ceramidehomolog of the C. eleganscell death gene ranging imPli~tions in tissuekinetics.Br J activated protein phosphatasein ceramideCancer26:239-257. ce&3. Cell 75:653-660. mediated signal transduction.J Biol Chem KolesnickR, Fuks Z: 1995.Ceramide:a signaf Nicholson DW, Ali A, Thornberry NA, et al.: 269:19,605-19,609. forapoptosis or mitogenesis?J Exp Med 181: 199s. Identification and inhibition of the Xia Z, Dickens M, Raingeaud J, Davis RJ, 1949-1952. ICE/CED-3proteasenecessaryfor mammaGreenberg ME: 1995. Opposing effects of lian apoptosis.Nature 376:37-43. KosturaMJ, Tocci MJ, Limjuco G, et aL: 1989. ERK and JNK-P38 MAP kinaseson apoptoIdentificationof a monocyte specific pre-in- Obeid LM, Linardic CM, Karolak LA, Hannun sis.Science270:1326-1331. tedeukin 1P convertase activity. Proc Natl YA: 1993.Programmedcefldeathinducedby Yao B, ZhangY, DelikatS, MathiasS, Basu S, Acad Sci USA 86:5227-5231. ceramide.Science259:1769–1771. KolesnickR: 1995.Phosphorylationof raf by Kuda K, Lippke JA, Ku G, et al.: 1995.Aftered OkazakiT, Belf RM, HarmunYA: 1989.Sphinceramide-activatedprotein kinase. Nature cytokine export and apoptosis in mice defigomyelinturnoverinducedby vitamin D3 in 378:307-310. cient in interleukin-1beta converting enHL-60 cells:role in celf differentiation.J Biol YuanJ,ShahamS,Ledoux S, EllisHM, Horvitz zyme. Science267:2000-2003. Chem 264:19,076-19,080. HR: 1993.The C.eleganscell deathgeneced-3 KumarS: 1995.ICE-likeproteasesin apoptosis. Okazaki T, BielawskaA, Domae N, Belf RM, encodesa protein similar to mammalianinTrends Biochem Sci 20:198-202. HannunYA: 1994.Characteristicsandpartial terleukin-1~-converting enzyme.Cell 75:641Kuno K, Matsushita K: 1996.The IL-1 receppurificationof a novel cytosolic,magnesium652. tor signalingpathway.J Leukoc Biol 56:542– mdependent,neutralsphingomyeha.seactiZhangJ,AlterN, Reed JC,Bomer C, Ofxid LM, 547. vated in the early signal transduction of HannunYH: 1996.Bc2-2interruptsthe cerala,25-dihydrox@amin D@duced HL-60 KyriakisJM,Woodgett JR,AvruchJ: 1995.The mide-mediatedpathway of celf death. Proc cefl differentiation.J Biol Chem 269:4070stress-activated proteinkinases:a novel ERK TCM Natf Acad Sci USA (in press). 4077. subfamily responsive to cellular stress and

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