48 P. Csermely et al. / Seminars in Cancer Biology 30 (2015) 42–51

Sem in ars in Can cer Biology 30 (2015) 42–51

Cancer stem cells display extremely large evolvability:

alternating plastic and rigid networks as a potential mechanism.

Network models, novel therapeutic target strategies,

and the contributions of hypoxia, infl ammation and cellular senescence

Peter Cserm ely

, János H ód sági

, Tam ás Korcsm áros

, Dezso˝ Mód os

, Áron R. Perez-Lop ez

, Kristóf Szalay

, Dániel V. Veres

, Katalin Len ti

, Ling-Yu n W u

, Xiang-Su n Zh an g

a Department of Medical Chemistry, Semmelweis University, P.O. Box 260, H-1444 Budapest 8, Hungary b Depa rtment of Genetics, Eötvös Loránd University, Pázmány P. s. 1C, H-1117 Budapest, Hungary

c Semmelweis University, Department of Morphology and Physiology, Faculty of Health Sciences, Vas u. 17, H-1088 Budapest, Hungary

d Institute of Applied Ma thematics, Academy of Ma thematics and Systems Science, Chinese Academy of Sciences, No. 55, Zhongguancun Ea st Roa d, Beijing 100190, China

Keywords:

Anti-cancer therap ies Cancer attractors Cancer stem cells H ypoxia In flammation Interactom e Metabolic mod eling N etw orks Senescence Signaling

Cancer is increasingly perceived as a system s-level, netw ork phenom enon. The m ajor trend of malignant transform ation can be d escribed as a tw o-phase process, w here an initial increase of netw ork plasticity is follow ed by a d ecrease of plasticity at late stages of tu mor d evelop m ent. The fluctu ating intensity of stress factors, like hypoxia, inflam mation and the either cooperative or hostile interactions of tum or inter-cellular netw orks, all increase the ad aptation potential of cancer cells. This m ay lead to the bypass of cellu lar senescence, and to the d evelop m ent of cancer stem cells. We propose that the central tenet of cancer stem cell d efinition lies exactly in the ind efinability of cancer stem cells. Actual prop erties of cancer stem cells d epend on the individu al “stress-history” of the given tumor. Cancer stem cells are characterized by an extrem ely large evolvability (i.e. a capacity to generate heritable phenotypic variation), w hich correspond s w ell w ith the d efining hallmarks of cancer stem cells: the possession of the capacity to self-renew and to repeatedly re-bu ild the heterogeneous lineages of cancer cells that com prise a tu mor in new environm ents.

Cancer stem cells represent a cell popu lation, w hich is ad apted to ad ap t.

We argu e that the high evolvability of cancer stem cells is helped by

their repeated transitions betw een plastic (proliferative, sym metrically

d ivid ing) and rigid (quiescent, asymm etrically d ividing, often m ore

invasive) phenotyp es having plastic and rigid netw orks. Thus, cancer

stem cells reverse and replay cancer develop ment m ultiple tim es. We

d escribe netw ork m od els potentially explaining cancer stem cell-like

behavior. Finally, w e propose novel strategies inclu ding com bination

1 . Ca n ce ra san e t w o r kd e ve lo p m e n t d ise a se

Malign an t t ran sform at ion is in creasin gly d escribe d as a syst em s-level, n e tw ork p h en om en on . Bot h h ealt h y an d tu m or cells can be p erce ived as n e tw orks. Nod es m ay be t h e am in o acid s ofcan cer-re lat edp rot ein s,w h e re ed ges arerelate d t o sec- on d arych em icalbon d s.Nod e sm ayalsobed efin edasp rot ein /RNA m ole cu les or DNA-segm e n ts,w he re ed ges are t h eir p h ysical or sign alin gcon t act s.In m et abolicn et w orks,n od e s arem et abolit e s an d ed gesaret h een zym e s,w h ichcat alyze th ere action st ocon - vertt h emt oe achot h e r[1–3].Mostoft h estat em en t soft h isre view m aych aract erizeallth e se m olecu larn e t w orksoft u m orcellsan d can ce rst emcells.

As st at ed alread yby Virch ow in 1859 [4],can cer is a d e vel- op m en t alp roce ss.Can cer cellsare p rod u ct sofa com p le x serie s ofcellt ran sform at ion e ven t s.Th e startin gste p sare oft e n m u t a- t ion sorDNA-re arran ge m en t s,w h ichd est abilizet h eform e rce llu lar p h en ot yp e s.Asaresu lt ,ace llp opu lat ionw it halargevariabilit yin ch rom at inorgan ization ,ge n ee xpressionp at t ern san din te ract om e com p osit ion isform ed [5–9].Inth isp rocess,ch an gesinn et w ork st ru ct u rean dd yn am icsp layacrucialrole .

Th is revie w w ill focu s on t h e large-scale n e t w ork rear- ran gem e n ts d u rin g can cer d evelop m en t —an d t h e em erge n t, syst em s-le velch an gest h eyd evelop .W ew illsh owt h atch an gesin n e tw orkp last icit y(an ditsop p osit e:n et w orkrigid it y)m aye xp lain cen t ralte n et s in bot h can cer d evelop m e n t an d can ce r st em cell be h avior.Net w orkp last icit y(orinoth e rw ord sn e tw orkflexibilit y) canbed efin e datt h eleve lofbot hn et w orkre sp on sesan dstru ct u re [7,10]asitw illbed et aile dinSe ction2.

2 . Ma lign a n tt r a n sfo r m a t io np r o ce e d svi ast a t e s

ch a r a ct e r ize db yin cr e a se da n dd e cr e a se dn e t w o r k p la st icit y 2.1. Initia lincreaseofnetw orkplasticityisfollow edbyadecrease ofnetw orkpla sticitya tla testa gesofca rcinogenesis

Inou rearlierw orks[3,11,12],su m m arizin gseveralp iecesofe vi- d en cew ep rop osedt h atm align an tt ran sform at ionisat w o-p h ase p roce ss,w h e reanin it ialin creaseofn et w orkp last icit yisfollow ed byit sd ecreaseatlat e stagesofcarcin ogen e sis.Th ep h e n ot yp e of t h ealre ad yest ablish ed ,late -st agecan ce rce llsisst illm orep last ic an d im m atu re t h an t h at ofn orm alcells,bu t m ay oft en be m ore rigidt h anth ep h en ot yp eoft h ecellsint h ein t erm ed iatest agesof carcin ogen e sis.

Inth iscon ce p t ,n et w orkp lasticit y(orinot h erw ord sfu n ct ion al n e tw orkfle xibilit y)can be d et e rm in ed eit h erat le ve lofn et w ork resp on se s(n et w orkd yn am ics,at tract orst ru ct u re )an datt h eleve l ofn et w orkst ru ctu re .Th en et w orkh asah ighp last icityatt h eleve l of its resp on se s,if sm all p e rt u rbation s in d u ce large ch an ge s in n e tw ork st ru ctu re an d d yn am ics[7,10].At th e le ve lofn et w ork, st ru ct u re n et w ork p last icit yd e pen d s on t h e in t ern ald egree s of

ofn et w orkd isord erd u rin gt u m ord evelop m e n t.Se lf-am p lification occu rs,w h e n th e in cre ased d isord e r of n od es cau se s a d isord er oft h eir n e tw orks,w h icham p lifie st h ed isorderofth e n od esfu r- t h er.Th e se syn ergist ic p rocesses cau se an accelerat e d d ecrease ofsyst em -con strain t s,w it hap arallelin creaseinen t rop yan dt h e d e gre esoffree d om bot hatt h eleveloft h ein d ivid u aln od es an d t h eirn et w orks.Allt h esech an gesle adtoth ed eve lop m e n tofm ore p last iccellu larn et w orksint h eearlyp h aseofcan cer.Th eearlyst age ofcan ce rd e velop m en t ,ch aract erized by an in crease ofn et w ork p last icit y,m aycorresp on dt ot h e“clon alexp an sion ”p h asean dt h e ap p earan ceoft u m orin it iatin gce lls.Su ch p last icit yin cre asem ay ch aract erizem u lt ip leclon alexp an sion soccu rrin ginsom e can cer t yp e s.

Lat e st age carcin oge n esis is ch aract erized by a d ecrease of n et w orkp last icityreflect e dbyd ecreasin gen t rop ybot hatth ein t er- act om ean dsign alin gn et w orkle ve l(su chasincaseofcom p arin g colon carcin om as t o ad en om as; Hód ságiet al.an d Mód ose t al., u n p u blish ed obse rvat ion s).Lat est aget u m orcellsm ayrep rese n t e it h er lat e stage p rim ary t u m orce lls,or m et ast at ic ce lls,w h ich alread yset t le dinth e irn oveltissu ee n viron m e n t.Th esefin d in gsare inagree m en tw it ht h erece n td at aofAih araan dco-w orke rs[31,32]

sh ow in gat ran sien t d ecreaseofen t rop yofh u m an bio-m ole cu lar in t eract ionn e t w orkd u rin gBcelllym p h om a,h e p at ocellu larcarci- n om aan dch ron ich ep at it isBlivercan ce rd evelop m en t .Itisyett o besh ow n ,w h et h ert h eot h ert yp esofp lasticit yin cre ase sliste din Table1forearlyst agecan ce rcellsarealsoreverse d inlat est age can cercells.

Th e d u al ch an ges d escribed above corresp on d w ell t o vari- ou s st ep s in t h e t ran sit ion t o t h e can cer-sp ecific st ate s,t erm ed as “can ce r att ractors” by St u art Kau ffm an in 1971 [33]. Can cer ce lls h ave t o first cross a barrie r in t h e qu asi-p ot en t ial(ep ige - n et ic)lan d scap e.Th isbarrier m igh t be low e red by m u t at ion sor e p igen e ticch an ges[5],bu tit s byp assreq u ires at ran sien t d est a- bilization oft h etran sform in gcell.Th isd e stabilizat ion le ad st oa m ore p last ic p h e n otyp e.Th is is follow ed by t h e st abilizat ion of t h ecan ce rcellinth e can cerat t ract orin vokin ga m orerigid p h e - n ot yp e .Im p ortan t ly, t h e at t ract or st ru ct u re it self m ay u n d ergo gross ch an ge s d u rin g can cer d evelop m e n t, d u e t o ch an ge s in n et w orkst ru ct u re,d yn am ics an d in t eract ion s w it h t h e en viron - m e n t .

Th e in crease an d d ecrease of n e t w ork en t rop y re sem bles t o t h at obse rved in celld ifferen t iat ion p rocesses, w h e re an in it ial in cre ase ofe n t rop y ofco-regu lat e d gen ee xpression p at t ernw as follow e d by a lat er d e cre ase [34]. An an alogou s se t of e ve n ts h ap p en sin cellu larre p rogram m in g,w h erean e arly,veryh et ero- gen eou s, st och astic p h ase is follow e d by a lat e p h ase, w h ich is p rogram m edbyah ierarch icalsetoft ran scrip t ionfact ors[35].Plas- t ic/rigid p h en ot yp e s of early/lat e p h ases m ay corre sp on d t ot h e p rolife rat ive/rem od elin g p h e n ot yp es of can ce r ce lls obt ain ed by gen ee xp ression sign atu re an alysis[36].Im p ort an t ly,t h ep rolife - rat ive /re m od e lin gp h e n ot yp ed u alit yisverysim ilartoth ed u ality ofp roliferat ive/qu iescen tstat esofcan cerst e mcells,w h ichw ew ill

44 P.Cserm elyeta l./Sem inarsinCa ncerBiology30(2015)42–51

Ta b le1

Sou rcesan dsign sofin creasedn e tw orkp last icityincan cerd evelop m en t.

Sou rce/signofin creasedn etw orkp last icity Referen ces Ration ale

Morein trin sicallyu n st ru ct u redp rotein s [13,16,27] Th ein cre ased“con form at ion aln oise”ofin d ivid u alp rot ein sm akes p rot ein –p rot einin teract ion s,sign alin gan dm et abolismfu zzierincan cercells Gen om ein st abilit y,ch rom osom alan om alies [7] Dest abilizationofDNAan dch rom atinst ru ct u re,asw ellasch rom osom al

an om aliesarebothcon sequ en cesan dsou rcesofin creasedsyste md isord er Largern oiseofn et w orkd yn am ics(in clu d ingth atofsign alin g

an dm e tabolicn et w orks,asw ellaslargerflu ctu at ion sof st ead y-st at evalu esan dsen sit ivit yth resh old sin d u cin g m orest och asticsw itch -t yp eresp on ses)

[5,14,21] Th elargern oiseofsyst emd yn am icsisbothasignofin creasedn et w ork p last icit yatt h e“bot t omn e tw ork”le velan dasou rceoffu rth erin creasein n e tw orkp last icityatt h e“t opn etw ork”level,t h u sw orksasaself-am p lifier (“bott omn etw orks”d escriben od esoft h e“t opn et w orks”likep rot ein stru ctu ren etw orksd escriben od esofin teract om es)

In creaseden trop yofp rot ein –p rot einin t eractionan d sign alin gn etw orks

[18–20,23] Th ein cre aseden t rop yofvariou sn et w orksexten d st h ein creasedd isord erof t h eelem en taryp rocessest ot h esyste mlevel

Largerp h ysicald eform abilit yofcan cercellsan dlargersh ap e h et erogen eit y

[15,25,30] Largercellu lard eform abilit ysh ow sanin creasedd isord eratth eleve loft h e cytoskelet aln et w ork,an dcon t ribu t est och rom osom ald am agein creasin gt h e ce llu lardisord erfu rth er.Act ivem ech an ism sch an gin gcellsfromrou n d edt o elon gat edsh ap eorviceversain cre asest ru ct u rald ive rsityfu rth er Alt ern atin gsym m etrican dasym m et riccelld ivisionofcan cer

st emcells

[17,24,29] Asym m et riccelld ivisionin crease scellu larh et erogen eit y.It sen viron m en tally regu lat edalte rn at in gch aract erisbot hasignan dsou rceofin creasedd iversit y an dp lasticit yofbot hin t ra-an din t er-cellu larn etw orks

Het erogen eou sce llu larresp on sest ot h esam est im u li, in creasedcellu larh et erogen eit y(oftu m orcellsan d in filt rat in glym p h ocyte s,m acrop h ages,m astcells, ep it h elialcells,e n d ot h elialcells,fibroblast san dst rom al cells)

[7,22,26,28] Cellu larh et erogen eit yisbot hasignofin creasedp last icityofin t ra-cellu lar n e tw orksoftu m orcells,an d ,viaanin creaseoft h eh et erogen eit yof in t er-cellu larsign alsofth etu m orm icroen viron m en t,act sasasou rceof fu rt h e rin creaseinsyst emp last icit yw orkin gasase lf-am plifier

2.3. Fluctua tingcha ngesoftum orm icroenvironm entm ayinduce a nincrea sedada ptationpotentia lofca ncercellsviatheir a lterna tingpla stica ndrigidnetw orkstructures

Plast icit y an d rigid it y ch an ges of tu m or ce ll n et w orks are oft e n p rovoke d by ch an ges in t h e en viron m en t of tu m or cells.

W e su m m arizet h em ajore n viron m e n talfactorse n h an cin gp las- t icity/rigid it y t ran sit ion s d u rin g can cer d e velop m en t in Table 2 [22,26,28,37–64]. As sh ow n in Table 2, in t er-cellu lar n et w orks oft e n d e velop p ro-on cogen iccoop eration ,bu talsogivea con tin - u ou slych an gin ge n viron m e n tin cre asin gt h ead ap t at ion p ot en t ial ofcan cer ce lls.Moreover,t u m orsgrow in a h ost ile e n viron m e n t ch aract erized by h yp oxia,in flam m at ory resp on ses,low p H,low n u trien t s,exte n siven e crosis an d t arget ed byim m u n e an d th e r- ap e u t icat tacks.Th econ t in u ou sflu ct u at ionoft h esest ressfact ors in cre ase st h ead ap t at ionp ot en t ialofcan cerce llsfu rt h er.Alt ern at - in gch an ge sinn et w orkp lasticit yan drigid it ym ayp layakeyrole in t h is“m at u ration ”p rocess.En viron m e n t-in d u ced ch an ges m ay leadto t h ebyp assofcellu larse n e scen cean dt ot h ed evelop m e n t ofcan cerst e mcells(Table2).

3. Ne t w o r km o d e lin go fca n ce rst em ce lls 3.1. Definition(s)andpropertiesofcancerstemcells

Can ce r st em ce llsh ave been d e fin edin an Am e rican Associa- t ion forCan ce r Research w orksh oph e ld in 2006as“cells w it h in

t h et erm“can cerst emcell”in st eadoft h elargevariabilityofn am es int h elite rat u re.

Can cerst em cellsh aveane xt re m elyh ighad ap tat ionp ot en t ial t od ifferen t en viron m en t s.Th u s,t h e e xt re m elyh igh p lasticit yof can cerste mcellscanbee xp ressedm orep reciselyasanextrem ely h ighabilit yt och an get h ep last icit y/rigid ityoft h eirn et w orks.Su ch ap rop ertyiscalled m et ap last icit yinn eu roscien ce[79 ]an d evol- vabilit yin ge n e t ics [80].Evolvabilit y(i.e .“a n orga nism ’sca pacity togenera teherita blephenotypicva ria tion”[80])isase le ct abletrait [81],w h ich ism ed iat ed bycom p lex n e t w orks[82].Th isgives u s h op et h atth en et w orkreq u ire m en t softh eextrem elyh ighevolva- bilit yofcan ce rst e mce llscanbeelu cid ate dan du se dinan ti-can ce r t h erap ie sint h efu t u re .W ew illsu m m arizet h ecu rren tviewoncan - cerst e mcelln et w orksinSect ion3.2an dsu ggestn ove lt h erap e u t ic st rat egie sinSect ion4.

Th re e p rop ert ies em erge as d iffere n t iat in g h allm arks of can - cerste m cellsfrom n orm alst em cells:(A)in crease d p rolife rative p ote n tial w it h a loss of n orm al t erm in al d iffe ren t iat ion p ro- gram s;(B)in creasedin d ivid u alit y(in crease dn ich e-in d e p en d e n ce orn ich e-p arasit ebe h avior); an d (C)large efficien cyinre sp on ses ofen viron m en t alch an ges.Th eh ighself-ren ew alp ot en t ialofn or- m alst emce llsgivesyetan ot h erp ow erfu lt oolofcan cerste m ce ll su rvivalan de volvability[75,78].

St em n e ss,e sp eciallyint u m ors,isch aract e rizedbylargeu n p re - d ict abilit y, w h e re t h e ou t com e is h e avily d ep en d en t on t h e in d ivid u al“st re ss-h ist ory”(p astch an ge sofit sen viron m en t )oft h e givencan cer ste m cell.In d eed ,m ore an d m ore d at asu p p ort t h e

Ta b le2

En viron m en t alch alle n gesp rovokin gch an gesinn et w orkp lasticit yan drigidit yincan cerd evelop m en t .

En viron m en t alch allen ge Effectsoncan cerd evelop m e n tan donn etw orkp last icity/rigid it y Referen ce s Alt ern atin gcoop erat ionan d

com p etit ionofin ter-cellu lar n et w orksoft h et u m or m icroen viron m en t

Tu m orsh arboranext rem elyh et erogen eou scellp op u lat ionoftu m orcells, im m u n ecells,ep ith elialce lls,en d ot h elialcells,fibroblastsan dstrom alcells.

Th esecellsm ayformacoop erat in gin ter-cellu larn etw ork,an dm ayd eve lopa p ro-on cogen icm icroe n viron m en t .St abilizationoft u m orm icroen viron m en t sh ift scellu larn et w orkstow ard sam orerigidst at e.Onth econ t rary,a flu ct u atin gen viron m en tp rovokesth ed evelop m en tofm oreflexiblen etw orks.

[22,26,28,38,49,55,58,64]

Ch an gesinth e“e m bed d ed n es”of t u m orcellsinth eext racellu lar m at rixoft u m orm icroen viron m en t

Can cerst emce llsoft enoccu p yasp ecialn ich e,w h eren ich ecellsan dcan cer stemcellsm u t u allyh elpeachot h er’ssu rvivalbycellad h esionbeyon dsolu ble factors.Can cer-associat edfibroblast sh e lpt h ed evelop m en tofap ro-on cogen ic m icroen viron m en tbyacon t actd ep en d en tm ech an ism .Highm olecu larw eigh t h yalu ron icacidem ergesasakeyregu latoroftu m orm icroen viron m en t.

In creased“em bed d ed n ess”oft u m orce llsint h eirext racellu laren viron m en t m ayin creaseth erigidit yoft h eirn et w orks.(Th isisth ecaseint h ein vasive p h en otyp eu sin gext racellu larcon t actst om igrat e,an din vad en ewn ich es,as d iscu ssedinSection3.2).Onth econ t rary,in creasedin d ep en d en cefromth e extracellu laren viron m en tallow sin creasedn e tw orkp last icit y.

[39,42,44,49,52,53,56,61,63]

Flu ctu at ion sinoxygent en sionlead in g t ovariou sd egreesofh yp oxiaan dt o alargevariabilit yofh yp oxia-relat ed resp on ses

Inagreem en tw it ht h eh yp oxia-in d u cedin h ibitionofsen e scen ce,st emcells u su allyresid einth em osth yp oxicregionoft h eresp ect ivet issu e,an dare resistan tt ooxid at ivestress.Hyp oxia-relat edn et w orksareh igh lysen sit iveto m in orch an gesinoxygencon cen t rat ion ,w h ichin d u cesalargevariabilit yof h yp oxia-relat edresp on ses.Th u s,flu ct u at ion sofoxygent en sion(w h ichare m agn ifiedint u m orm icroen viron m en t )m ayp layakeyroleint h e

“m atu ration ”ofp last icit y/rigid it yn et w orkt ran sition s,an dt h u st h e d evelop m en tofcan cerst emcells.

[40,41,46,51]

Flu ctu at in gin te n sit yofch ron ic in flam m ation

In flam m ationh asakeyroleinallp h asesoft u m ord evelop m e n t .In flam m at ion m ayt ran sformst emcellst oam orep lu rip ot en t,m oreaggressivep h en ot yp e rese m blin gt h atofcan cerst emcells.Th eToll-recep t orrelatedin flam m at ory sign alin gn et w orkan ditskeyp layers,su chasMYD88,p layanim p ort an trole incan cerst emcellin d u ct ion .Th eflu ct u at in gin ten sit yofch ron ic

in flam m at ion sm ayin creasefu rth ert h ead ap tat ionp ot en tialofcan cercells m ed iatedbyp last icit y/rigid it y-ch an gesoft h eirn et w orks.

[22,43,57,59,60]

Cellu larsen escen cean descap efrom t h esen escen tstat e

Alt h ou ght h erap y-in d u ce dcellu larsen escen cem aybeben eficial,sen escen t t u m orce llsh avean u m berofh arm fu lp rop ert iessu chas:p rod u ctionof in flam m at orycytokin esan dp aracrin eact ivat orsofcan cerstemcells, d egrad at ionoft u m orm icroen viron m en t ,asw ellasth eirp ot en tialre -en tryt o t h ecellcycle.Escap efromth esen escen tst at eisam ajort h reatint u m or p rogression .On eoft h em e diators,su rvivin ,p rote ct ssen e scen tcells,m ay reversesen escen ce,an dch aract erize scan cergrow t hsh ow in gaco-exp ression w it hcan cerst emcellm arkers.Re cen tstu d iesu n coveredasetofh igh ly p roliferat ivem icroRNAs(su chasm em be rsoft h em iR302/367an dm iR520 clu st ers)p layin gam ajorroleinsen escen cebyp ass.Man yofth esech an ges ch aract erizecan cerst emcells.Cellsescap in gt h erap y-in d u ce dsen escen ce d isp layan u m berofp rot ein sch aract eristicofcan cerst emcellssu chasCD133 orOCT4,an darech aract erizedbyanin creasedam ou n tofan t ioxid an t en zym es.Se n escen tcellsm ayh arborm orerigidn etw orks,w h ileescap efrom sen escen cem aybech aract erizedbyre-gain edn et w orkp lasticit y.

[37,45,47,48,50,51,54,62]

a syst e m icyou n g e n viron m e n t [85],w h ich raisest h e p ossibil- it y t h at in t eract ion s bet w e en qu iesce n t can cer st em cellsan d t h eirrap id lyp roliferat in ge n viron m e n tm ayh e lpt h ep rolon ged su rvivalofcan cerste mcells.

• Bioact ivefoodcom p on e n t sm ayfoste rt h eaberran tse lf-ren e w al ofcan ce rst emce llsin flu en cin gt h ebalan cebe t w ee nth e irp roli- ferativean dq u ie sce n tp h en ot ype s[86].

• An ti-can ce r th e rap y oft en in d u ces a w ou n d -h ealin g re sp on se, an dcon t ribu te stoth ein creaseoft u m origen icp ot en t ialofresid -

p h e n ot yp es[90].Th e sep h e n ot yp etran sit ion s,oft en m ed iate dby gen et iclesion s,offeran in cre ase d ch an cet oad op tacan cer st em ce ll-likeid en t it y[91].Dru gresist an ceisalsoap lasticp rop ertyof som ecan ce rcells.Mu lt i-re sist an tcan cerce lllin esre versiblyform sen sit iveorresist an tp rogen yd ep e n d in gonw h et h ert h ecellsare p assage d w it h or w it h ou t t h e d ru g [92]. How ever,m an y oft h e e xist in ge vid en cefor re versible t ran sition sbet w e en t u m origen ic an dn on -t u m origen icst at escom esfrom st u d iesofcellsincu lt u re, an dt h eirsign ifican ceh asyett ob ee st ablish edinvivo[28].

46 P.Cserm elyeta l./Sem inarsinCa ncerBiology30(2015)42–51

Ta b le3

Com m on lybelievedcan cerstemcelld efin in gh allm arks—an dt h eirqu est ionm arks.

Com m on lybelievedcan cerst emcell d efin in gh allm ark

Qu est ionm arksofh allm ark Referen ces

Isabletoformt u m orsind ifferen t en viron m en ts(e sp eciallyw h en u sin gserialt ran sp lan t at ionassays)

Th isd efin itionisth eorigin alan dm ostap p licabled efin itionofcan cerst em cells.Im m u n o-com p rom isedm ice,w h ichareoftenu sedint ran sp lan t at ion assay,st illm aysu p p resscertaintu m origen iccellsbyaw ou n d -h ealin gtyp e re sp on se,an dm aym issseveralh u m ancellt yp es(form in gt h ecan cerst emcell

“n ich e”)n eed edfort u m ord evelop m en t .Mou se -relat edtesten viron m en t sare n otfu llyin form at iveoft u m orsinh u m anp at ien t s.

[28,65–67,77]

Isth ep recu rsorofn on -t u m origen ic cellsoft h eorigin altu m or

Th isw ascon side redasacon sequ en ceoft h eaboved efin ition .How ever se verallin esofevid en cesu ggestth att h et w ost at em e n t sm aybeu n relate dt o eachoth er,an dt h eh ierarch icallin eagetyp icalt om ostn orm alst emcellsd oes n otch aracterizem an ycan cerst emcells.Man yt yp esoft u m orsh avea su rp risin glylargegen e tich eteroge n eitysu ggest in gd istin ctcan cerst emcell p op u lation sw it h int h esam et u m or.How ever,d etailedm easu rem en tofth e tu m origen icp ot en tialofth evariou sin tra-t u m orgen ot yp esislargelym issin g.

Resu ltsoft h efewlin eaget racin gan dselectivecell-ablat ionexp erim en t s cou ldn otbege n eralizedsofar.

[26,28,68,74]

Form son lyam in orcellsu bp op u lation oft u m ors

Can cerst emcellsm ayformararesu bp op u lationoft u m orcells.How ever,in som eexp erim e n t sah igham ou n tofcan cercells(e.g.50%in st eadoft h e origin al0.0004%)w erefou n dt u m origen ic.

[28,69,70,72–74,77]

Isd erivedfromn orm alste mcells Th ou ghseveralexp e rim en tssh ow edth atn orm alst emcellscanbe

tran sform edtocan cerst emcells,m an ycan cerst emcellsseemt obegen erat ed fromm oredifferen t iat edcells.

[47,71,75– 78]

Isresist an ttoth erap y Th erap iesin d u cin gcelld ifferen tiat ionsu ccessfu llytarge tsom ecan cerstem cells.Th erap ysu rvivalseem stob eastoch ast icp rocessoft end ep en d in gonde novom u tat ion s.

[28]

Canbech aracterizedbyvariou scan ce r st emcellm arkers

An u m berofp rom isin gcan cerst emcellm arkers(su chasCD34,CD38,CD133, CD271,Lgr5)w eren otgen erallyap p licablet ot h eresp ect ivet u m ort yp e.

[28]

Un d ergoesasym m et riccelld ivision Can cerst emcellsseemt oalt ern at ebet w eensym m et rican dasym m etriccell d ivision s.Th isp rop ert y,w h ichd ep en d sonth een viron m en tofcan cerste m cells,m aybeanim p ort an tsou rceoft h ein creasedp last icit yan devolvabilit yof can cerst emce lls.

[17]

w it h t h e fin d in gs t h at:(A)t h et u m or su p p ressor,p 53 p rom ot ed asym m et riccelld ivisionofbreastcan cerste m cells[93]p ossibly in h ib it in gt h eirabilitytocyclebe tw eenasym m e trican dsym m et - riccelld ivisionform s;(B)blockofasym m et riccelld ivisionle dt o abn orm alp roliferat ionan d ge n om icin stabilit yin Drosophila[94]

an d (C)rat e sofasym m et riccelld ivision w ered iffere n t be tw een t u m orsofasin glet u m ortyp e[95],an dm ayh avet obed e te rm in ed in d ivid u allyforeacht u m or(ort u m orce ll).

In crease inasym m et ricce lld ivision issim ilart ot h ed e ve lop - m e n t of t h e qu iescen t , coop e rat ive stat e afte r qu oru m sen sin g, w h ile in crease in sym m et ric cell division is sim ilar t o q u oru m q u e n ch in g[96].Th u s,can cerste mcellsm ayh aved e d iffere n tiat ed t ot h elevelt h atu sescom m u n ityregu lationm e ch an ism ssim ilart o t h oseofu n ice llu larorgan ism s,su chasbact eria.How e ve r,it isan op en qu est ion ,w h et h er in t er-cellu lar coop erat ion oft u m orcells in cre ase sw it h in creased asym m e tricce lld ivisionofcan cerst em ce lls.Th erecen tp ap erofDejosezetal.[97]sh ow edt h eexist en ce ofap 53,top oisom erase1an dolfactoryrece p t or-cen t ered m ole c- u larn e t w ork,w h ichre gu lat est h ecoop e rationofm u rin ein d u ced p lu rip ot e n tst em ce lls.It w illbe aqu est ion oflate rexcitin gst u d - iest oe xam in ew h et h ert h ism olecu larn et w orkisin volve dint h e

In con clu sion , as t h e m ajor h yp ot h esis of t h is re view w e p rop ose t h at t h e ad ap t at ion p ot en t ial of can ce r st e m ce lls is in creased by re p eat ed t ran sit ion sbet w een t h eir p last ic(p rolife - rat ive )an drigid(q u ie scen t /oft enm orein vasive )p h en ot yp e s.Th e m ajord rivin gforcebe h in dt h eset ran sit ion sist h ech an gesint h e m icroe n viron m e n tofcan cer st em cells(exem p lified byt h eserial t ran sp lan t at ion sint h ed e fin in ge xp erim en tofcan cerst em cells).

Th u s,can ce rst e m ce llsse emtobeablet ore versean dre p laycan - cer d e ve lop m en t m u ltip le t im es. In can ce r d e ve lop m en t can ce r st emcellsarerep eat ed lyselect e dforh ighe volvabilit y,an dbe cam e

“ad ap t edt oad ap t ”(Fig.1).

3.2. Netw orkm odelsofca ncerstemcells

In t h elast ye ars,several n e t w orkm od els ofvariou s t yp e s of st em ce llsh avebe en p u blish e d .W e qu ot eh ere on lyt h ew orkof Mu lle re tal.[101],w h ere gen eexp ression d at aofap p roxim at ely 150ofd ifferen th u m anste mcelllin esw e recolle cte dan dan alyze d . Tran scrip t om es ofp lu rip ot en t ste m cells(e m bryon ic st em ce lls, st emem bryon alcarcin om asan din d u cedp lu rip ot en tcells)form ed at igh tclu st er,w h ilet h ose ofot h erst e m cellsw erevery d iverse .

Fig.1.“W h atdoesn otkillm em ake sm estron ger”:En viron m en t alst ress-in d u cedaltern at ionofp last ican drigidn etw orksm ayexp laint h eextrem elyh ighevolvabilityof can cerstemcells.Th efigu reillu st rat est h em ajorh yp oth esisoft h ecu rren treviewsh ow in gt h atalargevariet yofen viron m en t alst resses(su chash yp oxia,in flam m at ionan d an ti-can cert h erap yit self)an dst ress-respon ses(su chasin flam m at oryresp on sesan dsen escen ceofsu rrou n d in gce lls)m ayin d u cet h ealt ern ationofp lastican drigidst ates ofm olecu larn et w orksofcan cerst emcells.Int h esen etw orksn od esm aybep rot ein sorRNAm olecu les,w h ilee dge sm ayrep resen tth eirp h ysicalorsign alin gin te ract ion s.

Sim ilarp last icity/rigid it yalt erat ion sm ayoccu rinm e tabolicn et w orks(w h eren od esaresm allm et abolit esan ded gesareen zym escon vertin gth emt oeachot h er),orin n et w orksoflargercellu larcom p lexes,w heren od esrep rese n tvariou sm icrofilam en t sorcellu larorgan e lles(su chasm itoch on d ria)an ded gesst an dfort h eirin te ract ion s.

Alt ern at in gp last ican drigidn et w orkst at esm ayh elpt h em atu rationofcan cerst emcellsd evelop in gth eirlargerad ap tability,evolvabilit yan dlon g-t ermsu rvival.Th u s t h ep roverbialsayin gofNiet zsch e:“W h atd oesn otkillm em akesm estron ger”[7,125]isesp eciallyt ru eforcan cerstemce lls,an din volvesth ecyclin gofth eirm ole cu lar n et w orks(su chasin t eractom es,m et abolican dsign alin gn et w orks)betw eenm orep last ican dm orerigidst at es.Ch an gesofrigidan dp last icn et w orkst ru ct u re sm ayin volve t h ecreat ionofn ewn od esan ded ges,asw ellasch an gesofth ew eigh t sofexist in ged gesasde scribedinTable4ind et ail.

su ggestt h atcan ce rst emce llsh arborsign alin gcircu it s,w h ichare se riallysw itch e donan doffinan alt e rn atin gan dcross-re gu lat ed m an n er[76,78,86,103–108].

An u m berofcom p lexm od elan dre alw orldn e t w orksu n d ergo abru p t an d e xt en sive ch an ge s in t h eir t op ology as a resp on se t o a sh ort age of resou rce s n ee d ed t o m ain t ain in te r-n od alcon - n e ct ion s an d /or u p on en viron m en t al stress. Th ese ch an ge s are called n e tw orkt op ologicalp h ase t ran sit ion san dre su lt inam as- sive re -organ izat ionofn et w orkst ru ctu re ,d yn am icsan d fu n ct ion [109–112].Sin cet h ee n viron m e n tofcan cersp rovid esanextrem e variet y ofre sou rce/st re ss levels,w h ich are m agn ified fu rth e r by an t i-can cert h erap ie s,w eh aveallreason st oassu m et h atn e tw orks ofcan ce rcellsre sp on d t ot h isen viron m en t alvariabilit y bytop o- logicalp h aset ran sit ion srat h eroft en .

Table 4 d escribes se ve ral h yp ot h et ical n et w ork beh aviors, w h ichm ayexp laint h ee xt rem e lyh ighe volvabilityofcan ce rst em cells.Th e sem ech an ism sarein te rre lat ed ,an dm ayin volvet h ed ele- t ionofe xistin gn od e san de d ge s,creat ionofn ewn od esan ded ges, asw e llasch an gesofe d gew eigh ts.High lyd yn am ic,in t er-m od u lar creat ive n od e s m ay break rigid n et w ork st ru ctu re s sim ilarly t o lat t iced e fect s.In creasin gn e tw orkcoresizeorfu zzin e ss,overlap -

t op ologicaln et w orkp h ase t ran sit ion sm en t ion ed before.Im p or- t an tly,t h eq u ie scen t can cer st em cellp h en otyp e oft en coin cid es w it h t h ein vasive p h e n ot yp e [98–100].It is rath e r p lau siblet h at h ighm obilit yan din vasiven essrequ iresam orerigidn e tw orkst ru c- t u reu p to t h elevelofcytoskelet aln e tw orks,sin cep h ysicalforce re qu iredforbot hm igrat ionan din vasioncanon lybeexercise de ffi- cien t ly,ift h eu n d erlyin gn et w orkisn ot p lastic(Fig.1).How eve r, m orest u d ie saren eed e dt oassesst h ege n e ralit yoft h eassociation oft h equ iescen tan dt h em orein vasivecan ce rste mcellp h e n ot yp es.

4. Ne t w o r k -r e la t e dd r u gt a r ge t in go fca n ce rst e m ce lls Can ce r st em cells follow Nie tzsch e’s p roverbialsayin g “w h at d oes n ot kill m e m akes m e st ron ger”[125].Th u s,con ven t ion al an t i-can cert h e rap iesm ayact u allyp rovokecan cerst e mcelld evel- op m en t [7,28,87].How ever,in t h elastyears a n u m berofcan cer st emce ll-sp ecifict h erap iesh avebeend e ve loped .Man yoft h ekey n od e sofcan cer st e m cellsign alin gn et w ork,su chasm em be rsof Hed geh og,W NTan dNot ch p at h w ays,asw ellas m icroRNAs,are alread yu sedast arge t softh e rap eu t icin te rven t ion sagain stcan cer st emcells.Th isrep ert oireisexte n d edbyan t ibod iesagain stp u t a-

48P.Csermelyetal./SeminarsinCancerBiology30(2015)42–51

Ta b le4

Hyp ot h et icaln et w ork-levelexp lan at ion san dp ossiblen et w orkm od elin gap p roach esofcan cerst emcell-likebeh avior.

Alt ern at in gn et w orkp rop ert yin d u cin g m ore/lesssyst emp lasticit y

It sp ossiblecon t ribu t iont ocan cerst emce llbe h avior Possiblen etw orkm od elin gap p roach Referen ces

(A)Ch an gesinn et w orktop ologyan dd yn am ics Su ccessivelarger/sm allerexp r essionof

cap acit orsofevolvabilit yin clu d in g creat iven e tw orkn od es

High lyd yn am ic,in t er-m od u larcreativen od es(an d/orot h ercap acitor p rot ein sofevolvabilit yin clu d in gm olecu larch ap eron es,p rion s,an d p rion -likeQ/N-richp rot ein s)m aybreakrigidn etw orkst ru ct u re s sim ilarlyt olat t iced efects.Rigid it y-seedn od esm ayestablisharigid clu ster,an drigid it y-p rom ot in gn od esm ayh elpitsgrow t hcau sin ga rigid it yp h asetran sit ion .Decrease dcreat iven od ed yn am icsm ay con t ribu t etoth isp rocess.

Cre at iven od escanbed eterm in edbyt h eir in t e r-m od u larn et w orkp ositioncon n ect in g m u lt ip lem odu lesatth esam et im ean dbyth eir ext rem elylarged yn am ics.

[11,82,110,113,116,119–122]

Pu lsat ionoflarge /sm all(or fu zzy/com p act )n et w orkcore s

Larger/fu zzyn et w orkcores(i.e.alargercen tralan dd en sen etw ork segm en t s)in creasesyst emrobu stn ess,evolvabilit yan dp lasticit y.

De creasedn et w orkcoresizean d /orin creasedcorecom p actn essm ay in creaset h econ trollabilityoft h esystem .

Net w orkcoresizecanbecalcu latedbyseveral m e t h od sbothforu n d irect edan dd irect ed (bow -t ie )core-p erip h eryn et w orks.

[112]

Alt ern at in g‘strat u s’/‘cu m u lu s’

(fu zzy/w ell-sep arat ed )n e tw ork m od u les

Fu zzyn et w orkm od u lesh avealargeoverlap ,an dformast ru ct u re re sem blin gt oth atofflat ,d en se,d ark,low -lyin g,stratu sclou d s.Th is p lasticst ru ct u red issip at esp ert u rbation sw ell.W ell-d efin ed , sep aratedn etw orkm od u lesh aveast ru ct u reresem blin gtoth atof p u ffy,w h it e,cu m u lu sclou d s.Th islocallyrigidst ru ct u red evelop safter stress,an dd isp laysred u cedp ert u rbat iond issip at ion .

Net w orkm od u larizat ionm et h od sd et ect in g ove rlap p in gm od u lesm aybeu sedt oassesst h e sep arat ionofn et w orkm od u les.Meth od sd et ect in g ext e n sive(p ervasive )overlap sareesp eciallygood fort h isp u rp ose.

[18,116–118,124]

Alt ern at in gp lastic/rigidn et w ork t op ologies

Allt h eabovech an gesm ayleadtoaltern at in gp lastican drigid n et w orkstru ctu res.Ch an gin gsu b-cellu larlocalizat ion sm ayoft en con t ribu t etoch an gesinn etw orkp last icity/rigid it y.

Net w orkrigid it yan drigidn etw orkclu st ersm ay becalcu latedbygen eralizedp ebblegam em od els.

[3,10–12]

(B)Ch an gesinn et w orken viro n m e n t Alt ern at in glarger/sm allerin t racellu lar

w at ercon ten t

Th em oret h anad ozenkn ow nh u m anaqu ap orinw at ert ran sp orters h aved iffere n t ialeffect sont u m ord evelop m en t.In cre asedin t racellu lar w at erm ayactas“lu brican t ”in cre asin gsyst emp last icit y,w h ile d ecreasedin t racellu larw atercon t en tm ayin creasem olecu lar crow d in gan dt h u sn etw orkrigid ity.

In tr acellu larw at ercon te n tm aybem easu redby severalexp e rim en talm e th od ssu chasn u clear m agn eticreson an cesp ect roscop y.W ater-in d u ced ch a n gesinm olecu larn et w orkdyn am icsm aybe calcu latedu sin gp rot eind yn am icm od elsan dth eir con seq u en cesofin t eractom e,sign alin gn et w ork an dm et abolicn et w orkd yn am ics.

[114,115]

(C)Con sequ en tch an ge sint h equ asi-p oten tial(ep igen etic)lan d scap eofcan cerst emcells Alt ern at in gsm ooth /rou gh

qu asi-p oten t ial(ep igen et ic) lan d scap eofcan cercelln etw orks

Allt h eabovech an gesm ayh elpth ealt ern at ionofasm oot h qu asi-p ot en tial(ep igen etic)lan d scap ech aract erizedby“sh allow ” can cerat tract orsan darou ghq u asi-p oten t ial(ep igen et ic)lan dscap e ch aract erizedby“d eep ”can cerat tract ors.

Th est at esp acelan d scap ean dn et w orkatt ractors canbecalcu lat edfroman alyticalm od elsan d sim u lat ion sofn etw orkd yn am ics.

[5,7,118,123,124]