IUBMB Life,49:177–180,2000
Copyright c2000IUBMB
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Hypothesis Paper
The p66shc Protein:A Mediator of the Programmed Death of an Organism?
Vladimir P.Skulachev
Department of Bioenergetics,Belozersky Institute of Physico-Chemical Biology,Moscow State University, Moscow119899,Russia
Summary
Recently knockout of the gene encoding an adaptor protein (p66shc)was shown both to prolong the life span of an animal and to prevent apoptosis of cells in response to added H2O2(Migliaccio et al.[1999]Nature402,309–313).A hypothesis is put forward in which p66shc is assumed to be involved in phe
,pro-grammed death of an organism,mediated by the reactive oxygen species–dependent massive apoptosis in an organ of vital impor-tance.The reactive oxygen species are suggested to oxidize phos-phatidyl serine in the inner lea et of the cell plasma membrane,re-sulting in appearance of this phospholipid in the outer membrane lea et,an effect recognized by a special receptor and causing the p66shc phosphorylation at a serine residue.Serine-phosphorylated p66shc there is proposed to block mitosis and initiate apoptosis.The large-scale apoptosis leads to phenoptosis and,hence,shortens the life span of the organism.
IUBMB Life,49:177–180,2000
Keywords Apoptosis;phenoptosis;programmed death of organism;
p66shc;reactive oxygen species.
INTRODUCTION
A highly interesting paper by Migliaccio et al.(1)with com-ment by Guarente(2)appeared in a November1999issue of Nature.Migliaccio ported that mice lacking a particular 66-kDa protein(p66shc)live30%longer than control animals and that cells derived from these mice do not respond to reac-tive oxygen species(ROS)by initiating the cell suicide program (apoptosis).
The protein in question was found to belong to the family of so-called adaptor proteins,which transmit the mitogenic signal of extracellular growth factors to the cell interior.p66shc contains
Received and accepted15February2000.
Address correspondence to Vladimir P.Skulachev.Fax:7(095)939 0338.E-mail:skulach@genebee.msu.su the same domains as its shorter relatives(p52shc and p46shc)as well as a domain of unique sequence,including a serine residue (S36)that is phosphorylated in response to ROS.Replacement of this serine by alanine prevented ROS-induced apoptosis,just as knockout of the entire gene coding for p66shc did.Like p52shc and p64shc,p66shc can be phosphorylated at three tyrosines when a growth factor is added,but this event fails to initiate mitosis.
The discovery described above raises two crucial questions, namely,why does an apoptosis-mediating protein shorten the life span of an organism,and why is this protein related to those involved in mitotic signal transmission?
Unfortunately,the authors of both the article and the comment paid little attention to these questions.However,the longevity problem is so important for biology and medicine that any chance to break through should be carefully considered.Here I present a possible explanation of the p66shc phe
nomenon.
With regard to the second question above,the key may be found in an observation made by the same group(3)that p66shc, being tyrosine-phosphorylated,forms a nonproductive complex with the Grb2protein,the next component of the mitogenic cas-cade(Fig.1A).If S36phosphorylation of the tyrosine-phospho-rylated p66shc is assumed to be favourable for its complex formation with Grb2,then ROS would arrest division of the apoptosis-subjected cells because S36-phosphorylated p66shc would effectively compete with p52shc and p46shc for the growth factor receptor.
A possible mechanism of the ROS-induced p66shc-mediated apoptosis is shown in Fig.1B.Presumably a receptor in the outer cell membrane is activated by phosphatidylserine appearing in the outer lea et of this membrane in a ROS-dependent fashion (4).Such a receptor should operate in phagocytes,recognizing in this way cells“sentenced”to apoptosis(5).The receptor in question seems to hardly be inherent in phagocytes only.Ac-cording to Uchida et al.(6),treatment of Chinese hamster ovary cells with phosphatidylserine caused apoptosis.The synthetic stereoisomer of this phospholipid,which contains D-serine in-stead of the natural L-serine,was ineffective.
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Figure1.Possible functions of p66shc:Arrest of mitosis(A)and induction of apoptosis(B).(A)Phosphorylation of three tyrosine residues of p66shc initiated by attachment of a growth factor(GF)to the outer membrane GF receptor(GFR)results in formation of a complex of p66shc with Grb2protein.In contrast to a productive complex of Grb2with p52shc and p46shc,which would be comp
etent in actuating the mitogenic cascade,the Grb2p66shc complex is inactive(3).(B)The same membrane is assumed to contain a receptor(OPSR)that recognizes phosphatidylserine(PS)in the outer membrane lea et.The outer PS is shown to appear in the outer lea et when it is oxidized by ROS because a PS pump that translocates PS from the outer to inner lea et does not deal with the oxidized phospholipid(4).The PS OPSR complex binds the p66shc domain containing S36,which gives rise to phosphorylation of S36.The S36-phosphorylated p66shc combines with protein X,initiating the p53-mediated apoptosis. Phosphorylation of S36might also stimulate formation of an inactive complex of p66shc with Grb2(not shown).
p66shc PROTEIN AND PROGRAMMED DEATH179
I suggest(Fig.1B)that the hypothetical outer phosphatidyl-serine receptor,when combined with phosphatidylserine,binds p66shc and initiates its serine phosphorylation.Then S36-phos-phorylated p66shc is assumed to actuate apoptosis by interacting with a protein(designated X in Fig.1B)responsible for the further transmission of the apoptotic signal(in this cascade,p53 seems to be involved[1]).
Thus p66shc can be assumed to operate as an adaptor pro-tein for two different receptors,growth fact
or receptor or outer phosphatidylserine receptor.In this way,p66shc can both arrest mitosis and initiate apoptosis in response to ROS.Such a dual personality of p66shc looks quite reasonable if one takes into account that both the ROS-induced arrest of cell division and actuation of apoptosis may well be two lines of antioxidant de-fence of a tissue.This allows the organism, rst,to stop multi-plication of the ROS-overproducing cells and,second,to purify the tissue of such cells.
The increase in ROS may be pathological in origin when something is wrong with ROS metabolism or,alternatively,may be physiological in response to a signal that stimulates ROS formation or inhibits their decomposition and thereby actuates apoptosis of cells that have become unwanted for the organ-ism(7–9).In both cases,the strategy is to arrest and remove not only the ROS-overproducing cell but also the neighbouring cells (“bystanders”),an effect that is ,to localize a pathol-ogy such as a viral infection(8,9).This means that the majority of cells arrested or committing suicide in a ROS-dependent way are therefore dealing with extracellular,rather with intracellular, ROS.This may explain why an outer phosphatidylserine recep-tor controlling both mitosis and apoptosis seems to be inherent in the outer lea et of the cell membrane.
The most paradoxical result of Migliaccio et al.(1)is that knockout of the p66shc-encoding gene entails a marked increase in life span.On the face of it,the disappearance of a protein in-volved in puri cation
of tissues from ROS-overproducing,ROS-damaged,or some other unwanted cells should shorten,rather than prolong,the life of an organism.However,the situation may be not so simple.
More than a century ago,August Weismann hypothesized that death because of ageing was invented by biological evolution as a kind of adaptation.Weismann wrote(10):
Worn-out individuals are not only valueless to the species,but they are even harmful,for they take the place of those which are
has been secondarily acquired as an adaptation.I believe that life is
endowed with a xed duration,not because it is contrary to its nature
to be unlimited,but because the unlimited existence of individuals
would be a luxury without any corresponding advantage.
Weismann’s idea was criticized by Medawar(11),who ar-gued that in wild nature,animals hardly live suf
ciently long to die from ageing.Nevertheless,some cases of the programmed death of bacteria(Escherichia coli),plants(bamboo),or ani-mals(salmon,some species of squids)are already documented (for references,see[9]).Recently,the obvious success in stud-ies of apoptosis has stimulated interest in the old suggestion of Weismann.The concept of phenoptosis has been put forward, in the assumption that massive apoptosis in organs of vital im-portance(septic shock,ischemic diseases of the heart and brain) may represent mechanisms of programmed death of organisms (8,9,12).
It was suggested that phenoptoses purify kins,communi-ties of organisms,or populations from individuals that had be-come dangerous.These would include organisms badly infected by a pathogen or those whose essential(genetic,behavioural, etc.)programs are damaged so strongly that their coexistence with normal individuals and reproduction of defective proge-nies could ruin kin,community,or population.
In the case of septic shock,bacterial eipopolysaccharide is recognized by special blood proteins.The resulting complex formed affects the eipopolysaccharide–protein receptor com-plex in the outer cell membrane,which initiates massive pro-duction of cytokines and causes ROS overproduction and apop-tosis(13,14).As a result,so many cells commit suicide that the life of the organism becomes impo
ssible.On ageing,repeated stresses result in ischemic diseases of heart or brain,causing the appearance of small,necrotic regions surrounded by large, apoptotic areas,an effect also mediated by cytokine ef ux and ROS overproduction(9).
It is rather easy to nd a probable place for p66shc in the above chain of events.It should t between ROS overproduction and massive apoptosis:
Phenoptotic stimulus cytokines ROS overproduction p66shc massive apoptosis programmed death of
organism(phenoptosis)
In young organisms,p66shc may well be involved in the pu-ri cation of tissues from ROS-overproducing cells by means of limited apoptosis.This is good for the organism and might,in principle,prolong its life.However,in old organisms,the role of p66shc in the phenoptotic cascade becomes dominating and shortens life.
Interestingly,the p66shc-de cient mice studied by Migliaccio et al.(1)did not show any changes in food uptake,body weight, or tissue morphology except for the lungs.Here there was“fo-cal accumulation of large numbers of eosinophilic macrophages within alveoles,generally associated with variable amount
s of other in ammatory cells”(1;see Supplementary information). In lungs the ROS danger is especially strong.In this tissue, in contrast to others,the oxygen level cannot be lowered;other-wise,lungs could not perform their function.For lung tissue,the purifying function of p66shc should be more important than for other tissues,where the oxygen concentration is usually much lower(7).
In conclusion,the very fact that a mutation in the gene en-coding a single protein substantially prolongs the life of an or-ganism could be predicted by the concept that considers ageing as a mechanism of the programmed death of individuals.
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