Regulation of Apoptosis Signal-regulating Kinase 1 (ASK1) by PolyamineLevels via Protein Phosphatase5

Abstract

Recent evidence has implicated the protein phosphatase PP5 in a variety ofsignaling pathways. Whereas several proteins have been identified thatinteract with PP5 and regulate its activity, a possibility of its regulationby second messengers remains speculative. Activation of PP5 in vitroby polyunsaturated fatty acids (e.g. arachidonic acid) and fattyacyl-CoA esters (e.g. arachidonoyl-CoA) has been reported. We reporthere that PP5 is strongly inhibited by micromolar concentrations of a naturalpolyamine spermine. This inhibition was observed both in assays with a lowmolecular weight substrate p-nitrophenyl phosphate as well asphosphocasein and apoptosis signal-regulating kinase 1 (ASK1), thought to be aphysiological substrate of PP5. Furthermore, a decrease in polyamine levels inCOS-7 cells induced by α-difluoromethylornithine (DFMO), an inhibitor ofornithine decarboxylase, led to accelerated dephosphorylation of oxidativestress-activated ASK1. This effect was suppressed by okadaic acid and bysiRNA-mediated PP5 depletion, indicating that the effect of polyamine levelson ASK1 dephosphorylation was mediated by PP5. In line with the decreased ASK1activation, polyamine depletion in COS-7 cells abrogated oxidativestress-induced activation of caspase-3, which executes ASK1-induced apoptosis,as well as caspase-3 activation induced by ASK1 overexpression, but had noeffect on basal caspase-3 activity. These results implicate polyamines,emerging intracellular signaling molecules, as potential physiologicalregulators of PP5. Our findings also suggest a novel mechanism of theanti-apoptotic action of a decrease in polyamine levels via de-inhibition ofPP5 and accelerated dephosphorylation and deactivation of ASK1. Recent evidence has implicated the protein phosphatase PP5 in a variety ofsignaling pathways. Whereas several proteins have been identified thatinteract with PP5 and regulate its activity, a possibility of its regulationby second messengers remains speculative. Activation of PP5 in vitroby polyunsaturated fatty acids (e.g. arachidonic acid) and fattyacyl-CoA esters (e.g. arachidonoyl-CoA) has been reported. We reporthere that PP5 is strongly inhibited by micromolar concentrations of a naturalpolyamine spermine. This inhibition was observed both in assays with a lowmolecular weight substrate p-nitrophenyl phosphate as well asphosphocasein and apoptosis signal-regulating kinase 1 (ASK1), thought to be aphysiological substrate of PP5. Furthermore, a decrease in polyamine levels inCOS-7 cells induced by α-difluoromethylornithine (DFMO), an inhibitor ofornithine decarboxylase, led to accelerated dephosphorylation of oxidativestress-activated ASK1. This effect was suppressed by okadaic acid and bysiRNA-mediated PP5 depletion, indicating that the effect of polyamine levelson ASK1 dephosphorylation was mediated by PP5. In line with the decreased ASK1activation, polyamine depletion in COS-7 cells abrogated oxidativestress-induced activation of caspase-3, which executes ASK1-induced apoptosis,as well as caspase-3 activation induced by ASK1 overexpression, but had noeffect on basal caspase-3 activity. These results implicate polyamines,emerging intracellular signaling molecules, as potential physiologicalregulators of PP5. Our findings also suggest a novel mechanism of theanti-apoptotic action of a decrease in polyamine levels via de-inhibition ofPP5 and accelerated dephosphorylation and deactivation of ASK1. The protein Ser/Thr phosphatase PP5 is a member of thePPP 1The abbreviations used are: PPP, phosphoprotein phosphatases of P family;DFMO, α-difluoromethylornithine; siRNA, short interfering RNA; ASK1,apoptosis signal-regulating kinase 1; Ni-NTA, Ni2+-nitrilotriaceticacid; pNPP, 4-nitrophenyl phosphate; TPR, tetratricopeptide repeat;DMEM, Dulbecco's modified Eagle's medium.1The abbreviations used are: PPP, phosphoprotein phosphatases of P family;DFMO, α-difluoromethylornithine; siRNA, short interfering RNA; ASK1,apoptosis signal-regulating kinase 1; Ni-NTA, Ni2+-nitrilotriaceticacid; pNPP, 4-nitrophenyl phosphate; TPR, tetratricopeptide repeat;DMEM, Dulbecco's modified Eagle's medium. family, one ofthe two known families of Ser/Thr-specific protein phosphatases(1Cohen P.T.W. Trends Biochem.Sci. 1997; 22: 245-251Abstract Full Text PDF PubMed Scopus (448) Google Scholar). PP5 (also termed PPT infungi) was identified in 1994 by three independent groups(2Becker W. Kentrup H. Klumpp S. Schultz J.E. Joost H.G. J. Biol. Chem. 1994; 269: 22586-22592Abstract Full Text PDF PubMed Google Scholar, 3Chen M.X. McPartlin A.E. Brown L. Chen Y.H. Barker H.M. Cohen P.T.W. EMBO J. 1994; 13: 4278-4290Crossref PubMed Scopus (250) Google Scholar, 4Chinkers M. Proc. Natl. Acad. Sci.U. S. A. 1994; 91: 11075-11079Crossref PubMed Scopus (135) Google Scholar)as a phosphatase with a tetratricopeptide repeat (TPR) domain N-terminallyfused to a typical PPP catalytic domain. PP5 appears to be ubiquitous ineukaryotes (5Andreeva A.V. Kutuzov M.A. Mol. Biol. Evol. 2001; 18: 448-452Crossref PubMed Scopus (34) Google Scholar). Recent yearshave seen a steadily increasing number of publications addressing the ways ofPP5 regulation and its roles in intracellular signaling. A variety ofPP5-interacting proteins has been reported; interestingly, all knowninteractions require the TPR domain of PP5. A significant proportion of PP5 in the cell is bound to a molecularchaperone Hsp90 and is a component of glucocorticoid receptor-Hsp90heterocomplexes (6Chen M.X. Cohen P.T.W. FEBS Lett. 1997; 400: 136-140Crossref PubMed Scopus (116) Google Scholar,7Silverstein A.M. Galigniana M.D. Chen M.S. Owens-Grillo J.K. Chinkers M. Pratt W.B. J. Biol.Chem. 1999; 272: 16224-16230Abstract Full Text Full Text PDF Scopus (215) Google Scholar). Hsp90-associated PP5 wasfound to regulate nucleocytoplasmic shuttling of glucocorticoid receptors(8Dean D.A. Urban G. Aragon I.V. Swingle M. Miller B. Rusconi S. Bueno M. Dean N.M. Honkanen R.E. BMC Cell Biol. 2001; 2: 6Crossref PubMed Scopus (63) Google Scholar). Another function ofHsp90-associated PP5 is suggested by the finding that it negatively modulatesmaturation of the Hsp90-dependent heme-regulated eIF2α kinase(9Shao J. Hartson S.D. Matts R.L. Biochemistry. 2002; 41: 6770-6779Crossref PubMed Scopus (59) Google Scholar). Whereas PP5 was initially reported to be predominantly nuclear, laterstudies established that it is localized in both the nucleus and the cytoplasm(reviewed in Ref. 4Chinkers M. Proc. Natl. Acad. Sci.U. S. A. 1994; 91: 11075-11079Crossref PubMed Scopus (135) Google Scholar). Recently,the ability of PP5 to translocate to the nucleus during apoptosis has beenreported (10Shinoda S. Skradski S.L. Araki T. Schindler C.K. Meller R. Lan J.Q. Taki W. Simon R.P. Henshall D.C. Eur. J. Neurosci. 2003; 17: 2065-2076Crossref PubMed Scopus (80) Google Scholar). PP5 has anatypical nuclear localization signal in its C-terminal extension(11Borthwick E.B. Zeke T. Prescott A.R. Cohen P.T.W. FEBS Lett. 2001; 491: 279-284Crossref PubMed Scopus (49) Google Scholar). Cytoplasmic PP5 has beenfound to interact with the intermediate chain of cytoplasmic dynein and tocolocalize with both cytoplasmic dynein and microtubules(12Galigniana M.D. Harrell J.M. Murphy P.J. Chinkers M. Radanyi C. Renoir J.M. Zhang M. Pratt W.B. Biochemistry. 2002; 41: 13602-13610Crossref PubMed Scopus (90) Google Scholar). Copurification of PP5with microtubules has also been documented, as well as its ability todephosphorylate a microtubule-associated protein tau in vitro(13Gong C.X. Liu F. Wu G. Rossie S. Wegiel J. Li L. Grundke-Iqbal I. Iqbal K. J.Neurochem. 2004; 88: 298-310Crossref PubMed Scopus (60) Google Scholar); physiological relevanceof the latter observation has not yet been demonstrated. PP5 participates in the regulation of the cell cycle in a dual way. (i) Itbinds to CDC16 and CDC27, two subunits of the anaphase-promoting complex, andis thought to maintain it in an inactive dephosphorylated state(14Ollendorff V. Donoghue D.J. J. Biol. Chem. 1997; 272: 32011-32018Abstract Full Text Full Text PDF PubMed Scopus (65) Google Scholar). (ii) It regulatestranscriptional induction of p21WAF1/Cip1 (a universal inhibitor ofcyclin-dependent protein kinases), presumably by directly controllingphosphorylation status of p53(15Zuo Z. Dean N.M. Honkanen R.E. J. Biol. Chem. 1998; 273: 12250-12258Abstract Full Text Full Text PDF PubMed Scopus (106) Google Scholar). Furthermore, a recentstudy (16Urban G. Golden T. Aragon I.V. Cowsert L. Cooper S.R. Dean N.M. Honkanen R.E. J. Biol.Chem. 2003; 278: 9747-9753Abstract Full Text Full Text PDF PubMed Scopus (49) Google Scholar) has suggested alink between glucocorticoid receptor activation and the phosphorylation statusof p53 that regulates expression of p21WAF1/Cip1. Therefore, PP5appears to control two distinct steps of the signaling pathway that linksglucocorticoid receptor activation and cell growth arrest. A link between PP5 and G protein-coupled signaling has recently been found.PP5 directly interacts with and is stimulated byGα12/Gα13(17Yamaguchi Y. Katoh H. Mori K. Negishi M. Curr. Biol. 2002; 12: 1353-1358Abstract Full Text Full Text PDF PubMed Scopus (94) Google Scholar). On the other hand, underredox stress PP5 associates with and inhibits apoptosis signal-regulatingkinase 1 (ASK1) (18Morita K. Saitoh M. Tobiume K. Matsuura H. Enomoto S. Nishitoh H. Ichijo H. EMBOJ. 2001; 20: 6028-6036Crossref PubMed Scopus (245) Google Scholar,19Zhou G. Golden T. Aragon I.V. Honkanen R.E. J. Biol. Chem. 2004; 279: 46595-46605Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar), which in its turn isstimulated by Gα12/Gα13(20Berestetskaya Yu V. Faure M.P. Ichijo H. Voyno-Yasenetskaya T.A. J. Biol.Chem. 1998; 273: 27816-27823Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar). Potential interplaybetween PP5, Gα12/Gα13, and ASK1 is furthercomplicated by the fact that Gα12(21Vaiskunaite R. Kozasa T. Voyno-Yasenetskaya T.A. J. Biol. Chem. 2001; 276: 46088-46093Abstract Full Text Full Text PDF PubMed Scopus (51) Google Scholar) and PP5 (see above) bothinteract with Hsp90. In addition to the above findings, which already suggest PP5 involvement inelaborate signaling pathways, interaction of this phosphatase with a number ofother proteins has been reported although functional significance of theseinteractions is less clear. Interaction of PP5 with atrial natriuretic factorreceptor (ANP) has been detected using yeast two-hybrid screening(4Chinkers M. Proc. Natl. Acad. Sci.U. S. A. 1994; 91: 11075-11079Crossref PubMed Scopus (135) Google Scholar). There is extensive butindirect evidence for the involvement of PP5 in regulation of K+channels (reviewed in Ref. 4Chinkers M. Proc. Natl. Acad. Sci.U. S. A. 1994; 91: 11075-11079Crossref PubMed Scopus (135) Google Scholar).PP5 has also been reported to associate with both isoforms of chryptochromes(blue light photoreceptors) and to be stimulated by at least one of them(22Zhao S. Sancar A. Photochem.Photobiol. 1997; 66: 727-731Crossref PubMed Scopus (80) Google Scholar). Intriguingly, adistantly related plant PPP phosphatase PP7, which has no TPR domain butshares with PP5 a common atypical nuclear targeting signal(23Andreeva A.V. Kutuzov M.A. Mol. Cell. Biol. Res. Commun. 2001; 4: 345-352Crossref PubMed Scopus (24) Google Scholar), has recently beenimplicated in signaling downstream of cryptochromes(24Møller S.G. Kim Y.S. Kunkel T. Chua N.H. Plant Cell. 2003; 15: 1111-1119Crossref PubMed Scopus (77) Google Scholar). Because animal and plantchryptochromes are thought to have evolved from DNA photolyases independently(25Cashmore A.R. Jarillo J.A. Wu Y.J. Liu D.M. Science. 1999; 284: PubMed Scopus Google Scholar), this not of but also of PP5 has also been to interact as S.L. J. Biol. Chem. 2003; 278: Full Text Full Text PDF PubMed Scopus Google Scholar) and the A the protein phosphatase Y. J. Biol. Chem. 2001; 276: Full Text Full Text PDF PubMed Scopus Google Scholar). Another protein with both PP5 and is to Y. K. K. H. K. K. 2002; PubMed Scopus Google Scholar). of remains to be In to the of of is known its potential regulation by second acid and fatty as well as to via its TPR domain (6Chen M.X. Cohen P.T.W. FEBS Lett. 1997; 400: 136-140Crossref PubMed Scopus (116) Google J. C. C. H. Rossie S. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google C. C. C. K. H. Rossie S. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar). Whereas acids are known to for PP5 in are thought to in and Chinkers M. Biochemistry. 2002; 41: PubMed Scopus Google Scholar) have recently chain fatty esters chain of are of PP5 fatty acids and suggested that of this and are in which are to to and to in the H.M. A.V. A. J. 2003; PubMed Scopus Google Scholar). are as control of cell cycle and and regulation of and protein H.M. A.V. A. J. 2003; PubMed Scopus Google Scholar, K. 1997; PubMed Scopus Google Scholar, T. 2001; Scopus Google Scholar, D.J. Cell Mol. 2003; PubMed Scopus Google of the of in regulation of cell have been as T. S. M.A. Res. 2002; 13: Google Curr. 2003; 4: PubMed Scopus Google Scholar). a of proteins are known that are by on one have been to as signaling G. 2002; Full Text Full Text PDF Scopus Google Scholar). We that PP5 is strongly inhibited by micromolar a polyamine spermine. We that polyamine levels of thought to be a physiological substrate of PP5 and signaling in This polyamines,emerging intracellular signaling molecules, as potential physiologicalregulators of PP5. Our findings also suggest a novel mechanism of theanti-apoptotic action of a decrease in polyamine levels via de-inhibition ofPP5 and accelerated dephosphorylation and deactivation of ASK1. in and of was by M. of PP5 was in cells to the that expression was induced with at from 1 of by and a and a of was in an at for with a of of and on a for 1 at was a with the proteins by a of of to by and and on from to to and on a by a in 1 PP5 and by in to protein PP5 in at PP5 and ASK1 from COS-7 cells on a with of by M. Yu V. Faure M.P. Ichijo H. Voyno-Yasenetskaya T.A. J. Biol.Chem. 1998; 273: 27816-27823Abstract Full Text Full Text PDF PubMed Scopus (63) Google Scholar). 1 was to to cell in of inhibitor by a was of and of protein and for at and three with 1 of and PP5 was with in for at PP5 at in a and assays phosphate as was to with concentrations of spermine. by addition to from a with to by at from the of the using a of phosphatase assays with using the of and PubMed Scopus Google Scholar). In ASK1 dephosphorylation ASK1 cells as above was in a and for at three in the at in ASK1 at with PP5 from COS-7 cells in the by addition of and by addition acid and to and by and was by PP5 using PP5 PP5 was to G. Golden T. Aragon I.V. Honkanen R.E. J. Biol. Chem. 2004; 279: 46595-46605Abstract Full Text Full Text PDF PubMed Scopus (101) Google for PP5 and control by In ASK1 and to stress by addition of cells with in phosphate 1 addition of a and for on a with and using of ASK1 was on to ASK1 with of its phosphorylation ASK1 dephosphorylation for as the between the in and the of PP5 inhibition on the effect of PP5 ASK1 dephosphorylation was at in the cells with okadaic acid to dephosphorylation in the the of was the in it was as The as a of in the control was used to In the PP5 depletion by siRNA, the depletion was using PP5 and was with Because the with the signal and with the of from one for to of the activation was in a dual using by of caspase-3 by from was caspase-3 was detected using an the ofthe PP5 to by of evidence to the ability of several phosphatases ofthe PPP to be by is thought to be polyamine levels in S. Cohen Eur. J. PubMed Scopus Google Scholar, K. M.D. I. J. 269: PubMed Google Scholar, Honkanen R.E. 20: PubMed Scopus Google Scholar, K. 2002; PubMed Scopus Google regulation by in was S. Cohen Eur. J. PubMed Scopus Google Scholar), although in this polyamine concentrations be above the physiological although not with was to be concentrations of and was suggested to M.A. A.V. FEBS Lett. 1998; PubMed Scopus Google Scholar). Therefore, the of on PP5 activity. PP5, with pNPP, was to be inhibition by an for the of Another was PP5 by at 1 not that the observed effect was not an expression of PP5, PP5 in COS-7 it by with and the on its activity. to PP5, by was observed for PP5 in of by PP5 is it the in with this lowmolecular weight substrate not regulation protein M.A. A.V. FEBS Lett. 1998; PubMed Scopus Google F. PubMed Scopus Google Scholar). Therefore, dephosphorylation of a protein substrate by PP5 also be spermine. PP5 with was also by with was that observed with was the physiological of (see and is to that S. Cohen Eur. J. PubMed Scopus Google Scholar). These results PP5 be by physiological concentrations of In of a of by the to of PP5 regulation by its dephosphorylation of a which is thought to be aphysiological substrate of K. Saitoh M. Tobiume K. Matsuura H. Enomoto S. Nishitoh H. Ichijo H. EMBOJ. 2001; 20: 6028-6036Crossref PubMed Scopus (245) Google Scholar,19Zhou G. Golden T. Aragon I.V. Honkanen R.E. J. Biol. Chem. 2004; 279: 46595-46605Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar). ASK1 is on and by this by K. Saitoh M. Tobiume K. Matsuura H. Enomoto S. Nishitoh H. Ichijo H. EMBOJ. 2001; 20: 6028-6036Crossref PubMed Scopus (245) Google Scholar). PP5 from was to be todephosphorylate that its from that ofthe that an in for dephosphorylation of ASK1. This is in line by K. Saitoh M. Tobiume K. Matsuura H. Enomoto S. Nishitoh H. Ichijo H. EMBOJ. 2001; 20: 6028-6036Crossref PubMed Scopus (245) Google Scholar) that the in was to with arachidonic using polyunsaturated fatty are of PP5, Chinkers M. Biochemistry. 2002; 41: PubMed Scopus Google Scholar) not be presumably Therefore, PP5 in COS-7 cells was todephosphorylate ASK1. PP5 was from COS-7 cells (see above and and with These PP5 to by in In the of the of phosphate from ASK1 was at least to of In of an of was observed during the of by inhibition This of ASK1 dephosphorylation with three independent PP5 although the of The by was also observed (i) ASK1 in (ii) was dephosphorylated in by PP5, ASK1 dephosphorylation was in a of cells ASK1 and PP5 not Because in ASK1 dephosphorylation was of the by be dephosphorylation at a distinct from ASK1 dephosphorylation by PP5 was at of dephosphorylation in of not an inhibition of ASK1 dephosphorylation physiological of ASK1 Activation by by above led to a that of PP5 by in the of activation of ASK1 by the of by PP5. Therefore, the of and deactivation in COS-7 cells to stress in the of an inhibitor of the in polyamine In control ASK1 was in by and the second activation was at A and and Whereas the of the second phosphorylation and the of by between the the two of phosphorylation ASK1 dephosphorylation is decreased by COS-7 cells in a with and for with for and was at cell of phosphorylation and and as in to and cells in the of in of PP5 and is as control in and the used to the of of in cells as and in a of the from the and two in a In the ASK1 activation was not was which the second between the control and cells The effect of was it by addition of of other cells to the of polyamine These results that a kinase for phosphorylation was inhibited by polyamine depletion, was the effect of polyamine depletion PP5, have the effect PP5 Whereas no of PP5 are PP5 inhibited by okadaic a that other PPP phosphatases but not dual of the cells with okadaic acid led to a in the basal of phosphorylation its activation In the of okadaic not and of the effect of in two using concentrations of okadaic acid of ASK1 These findings that the effect of is mediated by protein as be for the of okadaic an ASK1 the phosphorylation was effect of an okadaic was to be by has as an T. Ichijo H. H. Mol. 2001; PubMed Scopus Google Scholar), it was by a inhibitor of We no to the okadaic that the effect of polyamine depletion PP5, PP5 by and ASK1 of with PP5 in a decrease in the levels of with control as by with okadaic depletion of PP5 led to basal of ASK1 phosphorylation and the effect of was but was and the between cells by of the from three a significant decrease in in cells with cells control that PP5 was one the effect of to be but These are with to de-inhibition of PP5, which in turn results in a dephosphorylation of ASK1. in a in the by K. A. Nishitoh H. Ichijo H. Cell 2003; PubMed Scopus Google Scholar). the have been implicated in both on stress and Biol. PubMed Scopus Google Scholar). to be for ASK1 activation in COS-7 in polyamine levels be to COS-7 cells ASK1 signaling is known to apoptosis via activation of T. A. S. Y. T. K. K. S. Ichijo H. K. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). Therefore, activation of this in COS-7 cells in the of polyamine caspase-3 activation both by the caspase-3 and with of that polyamine depletion inhibits the ability of ASK1 to activation in the in the of In the ASK1 induced an in caspase-3 in the cells the of caspase-3 to that in control of ASK1 had no significant effect on basal caspase-3 at and ASK1 in These results that a decrease of in inhibits ASK1-induced Recent evidence has implicated the protein phosphatase PP5 as a several signaling pathways, and a number of potential proteins have been to interact with PP5 (reviewed in A.V. Kutuzov M.A. Cell. 1999; PubMed Scopus Google M. Trends 2001; 12: Full Text Full Text PDF PubMed Scopus Google Scholar). A possibility of by second messengers acid and A has been M.X. Cohen P.T.W. FEBS Lett. 1997; 400: 136-140Crossref PubMed Scopus (116) Google J. C. C. H. Rossie S. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Chinkers M. Biochemistry. 2002; 41: PubMed Scopus Google H. S.L. Chinkers M. Biochemistry. 2001; PubMed Scopus Google Scholar), although it is on in In this have evidence for in regulation of PP5 by one of have that depletion of in to a of physiological substrate of PP5, and that this effect These findings of PP5 and suggest a novel of of via polyamine as well as a novel mechanism of signaling. and are as control of cell cycle and and regulation of and protein C. C. C. K. H. Rossie S. J. Biol. Chem. 1999; Full Text Full Text PDF PubMed Scopus Google Scholar, Chinkers M. Biochemistry. 2002; 41: PubMed Scopus Google Scholar, H.M. A.V. A. J. 2003; PubMed Scopus Google of the of in regulation of cell have been as T. S. M.A. Res. 2002; 13: Google Curr. 2003; 4: PubMed Scopus Google Scholar). of in the cell is in the are bound to that polyamine concentrations are in the S. K. H. K. J. Biol. Chem. Full Text PDF PubMed Google 15: Full Text PDF PubMed Scopus Google Scholar). for by is In regulation has been reported for a PPP phosphatase S. Cohen Eur. J. PubMed Scopus Google T. S. J. Res. Google Scholar) and A. H. PubMed Scopus Google Scholar); used in are above the In several other reported polyamine concentrations the physiological reported inhibition of by with a of K. K. A. T. Res. Commun. PubMed Scopus Google Scholar). of the PPP family, is in by S. Cohen Eur. J. PubMed Scopus Google Scholar), and the of this regulation has been by K. M.D. I. J. 269: PubMed Google Scholar, Honkanen R.E. 20: PubMed Scopus Google Scholar, K. 2002; PubMed Scopus Google identified signaling proteins D.J. Cell Mol. 2003; PubMed Scopus Google J.K. C. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus (90) Google Scholar) and several K. 1997; PubMed Scopus Google 15: Full Text PDF PubMed Scopus Google Trends 17: Full Text PDF PubMed Scopus Google Z. J. 2003; PubMed Scopus (59) Google Scholar). for regulation by in J.K. C. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus (90) Google Scholar) 15: Full Text PDF PubMed Scopus Google Scholar), regulation of has been reported G. 2002; Full Text Full Text PDF Scopus Google that as in levels are to regulation cell cycle as well as by H.M. A.V. A. J. 2003; PubMed Scopus Google Scholar). concentrations in this to regulate PP5 for of and protein The concentrations of with protein are to that reported for Ref. S. Cohen Eur. J. PubMed Scopus Google results suggest that PP5 be by inhibition of PP5 by that PP5 have as for PPP protein and plant in M.A. A.V. FEBS Lett. 1998; PubMed Scopus Google Scholar). Another possibility with PP5 interaction with its via with the phosphate PP5 by as arachidonic acid and A fatty acid be in PP5 and also be with of PP5 in the is to are known to both and on the and the Biol. PubMed Scopus Google Scholar); the of are A of publications S. J. 2004; PubMed Scopus Google and a effect of polyamine depletion in cells which is the activation of is known to which to ASK1 activation K. A. Nishitoh H. Ichijo H. Cell 2003; PubMed Scopus Google Our polyamine depletion ASK1 activation of its PP5 for the of polyamine depletion and for a for a We M. Chinkers of for the