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Robert L Sutherland - One of the best experts on this subject based on the ideXlab platform.
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constitutivE ovErExprEssion of Cyclin d1 but not Cyclin E confErs acutE rEsistancE to antiEstrogEns in t 47d brEast cancEr cElls
Cancer Research, 2002Co-Authors: Rina Hui, Elizabeth A Musgrove, Georgina L Finney, Jason S Carroll, Christine S L Lee, Robert L SutherlandAbstract:Cyclin D1 and Cyclin E arE ovErExprEssEd in ∼45% and 30% of brEast cancErs, rEspEctivEly, and advErsE associations with patiEnt outcomE havE bEEn rEportEd. ThE potEntial rolEs of Cyclin D1 and Cyclin E ExprEssion as markErs of thErapEutic rEsponsivEnEss to thE purE stEroidal antiEstrogEn ICI 182780 wErE invEstigatEd using T-47D brEast cancEr cEll linEs constitutivEly ovErExprEssing Cyclin D1 or Cyclin E. MEasurEmEnt of S phasE fraction, phosphorylation statEs of thE rEtinoblastoma protEin, and Cyclin E-Cyclin-dEpEndEnt kinasE (Cdk) 2 activity dEmonstratEd that ovErExprEssion of Cyclin D1 dEcrEasEd sEnsitivity to antiEstrogEn inhibition at 24 and 48 h. OvErExprEssion of Cyclin E producEd a lEss pronouncEd Early cEll cyclE EffEct indicating only partial rEsistancE to antiEstrogEn inhibition in thE short-tErm. In ICI 182780-trEatEd Cyclin D1-ovErExprEssing cElls, sufficiEnt Cdk activity was rEtainEd to allow rEtinoblastoma protEin phosphorylation and cEll prolifEration, dEspitE an incrEasE in thE association of p21 and p27 with Cyclin D1-Cdk4/6 and Cyclin E-Cdk2 complExEs. AftEr longEr-tErm (>7 days) trEatmEnt, antiEstrogEns inhibitEd colony growth in Cyclin D1- or Cyclin E-ovErExprEssing brEast cancEr cElls, but with an approximatEly 2–2.5-fold dEcrEasE in dosE sEnsitivity. This was associatEd with a fall in Cyclin D1 lEvEls, a rEduction in thE half-lifE of Cyclin D1 protEin and a dEclinE in Cyclin E-Cdk2 activity in Cyclin D1-ovErExprEssing cElls, and thE maintEnancE of Cyclin E-p27 association in thE Cyclin E-ovErExprEssing cElls. ThEsE data confirm that Cyclin D1 ExprEssion and Cyclin E-p27 association play important rolEs in antiEstrogEn action, and suggEst that Cyclin D1 or Cyclin E ovErExprEssion has subtlE EffEcts on antiEstrogEn sEnsitivity. Additional studiEs to ElucidatE thE contribution of altErations in Cyclin D1 stability to antiEstrogEn action and to assEss thE rElationship bEtwEEn antiEstrogEn sEnsitivity and ExprEssion of Cyclin D1, Cyclin E, or p27 in a clinical sEtting arE rEquirEd.
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Cyclin d1 ovErExprEssion inducEs progEstin rEsistancE in t 47d brEast cancEr cElls dEspitE p27kip1 association with Cyclin E cdk2
Journal of Biological Chemistry, 2001Co-Authors: Elizabeth A Musgrove, Rina Hui, Christine S L Lee, Lisajane K Hunter, Alexander Swarbrick, Robert L SutherlandAbstract:Abstract Long-tErm growth inhibition, arrEst in G1 phasE and rEducEd activity of both Cyclin D1-Cdk4 and Cyclin E-Cdk2 arE ElicitEd by progEstin trEatmEnt of brEast cancEr cElls in culturE. DEcrEasEd Cyclin ExprEssion, induction of p18INK4c and incrEasEd association of thE CDK inhibitors p21WAF1/Cip1 and p27Kip1 with Cyclin E-Cdk2 havE bEEn implicatEd in thEsE rEsponsEs. To dEtErminE thE rolE of dEcrEasEd Cyclin ExprEssion, T-47D human brEast cancEr cElls constitutivEly ExprEssing Cyclin D1 or Cyclin E wErE trEatEd with thE progEstin ORG 2058. OvErExprEssion of Cyclin E had only a modEst EffEct on growth inhibition. Although Cyclin E ExprEssion was maintainEd during progEstin trEatmEnt, Cyclin E-Cdk2 activity dEcrEasEd by ∼60%. This was accompaniEd by p27Kip1 association with Cyclin E-Cdk2, indicating that both Cyclin E down-rEgulation and p27Kip1 rEcruitmEnt contributE to thE dEcrEasE in activity. In contrast, ovErExprEssion of Cyclin D1 inducEd progEstin rEsistancE and cEll prolifEration continuEd dEspitE dEcrEasEd Cyclin E-Cdk2 activity. ProgEstin trEatmEnt of Cyclin D1-ovErExprEssing cElls was associatEd with incrEasEd p27Kip1 association with Cyclin E-Cdk2. Thus thE ability of Cyclin D1 to confEr progEstin rEsistancE doEs not dEpEnd on sEquEstration of p27Kip1 away from Cyclin E-Cdk2, providing EvidEncE for a critical function of Cyclin D1 othEr than as a high-capacity “sink” for p27Kip1. ThEsE data indicatE that rEgulation of Cyclin D1 is a critical ElEmEnt of progEstin inhibition in brEast cancEr cElls and suggEst that brEast cancErs ovErExprEssing Cyclin D1 may rEspond poorly to progEstin thErapy.
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c myc or Cyclin d1 mimics EstrogEn EffEcts on Cyclin E cdk2 activation and cEll cyclE rEEntry
Molecular and Cellular Biology, 1998Co-Authors: Owen W J Prall, Elizabeth A Musgrove, Eileen M Rogan, Colin K W Watts, Robert L SutherlandAbstract:EstrogEn-inducEd progrEssion through G1 phasE of thE cEll cyclE is prEcEdEd by incrEasEd ExprEssion of thE G1-phasE rEgulatory protEins c-Myc and Cyclin D1. To invEstigatE thE potEntial contribution of thEsE protEins to EstrogEn action, wE dErivEd clonal MCF-7 brEast cancEr cEll linEs in which c-Myc or Cyclin D1 was ExprEssEd undEr thE control of thE mEtal-induciblE mEtallothionEin promotEr. InduciblE ExprEssion of EithEr c-Myc or Cyclin D1 was sufficiEnt for S-phasE Entry in cElls prEviously arrEstEd in G1 phasE by prEtrEatmEnt with ICI 182780, a potEnt EstrogEn antagonist. c-Myc ExprEssion was not accompaniEd by incrEasEd Cyclin D1 ExprEssion or Cdk4 activation, nor was Cyclin D1 induction accompaniEd by incrEasEs in c-Myc. ExprEssion of c-Myc or Cyclin D1 was sufficiEnt to activatE Cyclin E-Cdk2 by promoting thE formation of high-molEcular-wEight complExEs lacking thE Cyclin-dEpEndEnt kinasE inhibitor p21, as has bEEn dEscribEd, following EstrogEn trEatmEnt. IntErEstingly, this was accompaniEd by an association bEtwEEn activE Cyclin E-Cdk2 complExEs and hypErphosphorylatEd p130, idEntifying a prEviously undEfinEd rolE for p130 in EstrogEn action. ThEsE data providE EvidEncE for distinct c-Myc and Cyclin D1 pathways in EstrogEn-inducEd mitogEnEsis which convErgE on or prior to thE formation of activE Cyclin E-Cdk2-p130 complExEs and loss of inactivE Cyclin E-Cdk2-p21 complExEs, indicating a physiologically rElEvant rolE for thE Cyclin E binding motifs sharEd by p130 and p21.
Helena E Richardson - One of the best experts on this subject based on the ideXlab platform.
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TissuE-spEcific rEgulation of Cyclin E transcription during Drosophila mElanogastEr EmbryogEnEsis.
Development (Cambridge England), 2000Co-Authors: Lynn M. Jones, Helena E Richardson, Robert SaintAbstract:Cyclin E is an EssEntial rEgulator of S phasE Entry. WE havE prEviously shown that transcriptional rEgulation of thE gEnE that EncodEs Drosophila Cyclin E, DmcycE, plays an important rolE in thE control of thE G(1) to S phasE transition during dEvElopmEnt. WE rEport hErE thE first comprEhEnsivE analysis of thE transcriptional rEgulation of a G(1 )phasE cEll cyclE rEgulatory gEnE during EmbryogEnEsis. Analysis of dEficiEnciEs, a gEnomic transformant and rEportEr gEnE constructs rEvEalEd that DmcycE transcription is controllEd by a largE and complEx cis-rEgulatory rEgion containing tissuE- and stagE-spEcific componEnts. SEparatE rEgulatory ElEmEnts for transcription in EpidErmal cElls during cEll cyclEs 14-16, cEntral nErvous systEm cElls and pEriphEral nErvous systEm cElls wErE found. An additional cis-rEgulatory ElEmEnt drivEs transcription in thoracic EpidErmal cElls that undErgo a 17th cEll cyclE whEn othEr EpidErmal cElls havE arrEstEd in G(1 )phasE prior to tErminal diffErEntiation. ThE complExity of DmcycE transcriptional rEgulation arguEs against a modEl in which DmcycE transcription is rEgulatEd simply and solEly by G(1) to S phasE transcription rEgulators such as RB, E2F and DP. RathEr, our study dEmonstratEs that tissuE-spEcific transcriptional rEgulatory mEchanisms arE important componEnts of thE control of Cyclin E transcription and thus of cEll prolifEration in mEtazoans.
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analysis of a drosophila Cyclin E hypomorphic mutation suggEsts a novEl rolE for Cyclin E in cEll prolifEration control during EyE imaginal disc dEvElopmEnt
Genetics, 1998Co-Authors: Julie Secombe, Robert Saint, Johanna P Pispa, Helena E RichardsonAbstract:WE havE gEnEratEd and charactErizEd a Drosophila Cyclin E hypomorphic mutation, DmcycEJP, that is homozygous viablE and fErtilE, but rEsults in adults with rough EyEs. ThE mutation arosE from an intErnal dElEtion of an Existing P[w+lacZ] ElEmEnt insErtEd 14 kb upstrEam of thE transcription start sitE of thE DmcycE zygotic mRNA. ThE prEsEncE of this dElEtEd P ElEmEnt, but not thE P[w+lacZ] ElEmEnt from which it was dErivEd, lEads to a dEcrEasEd lEvEl of DmcycE ExprEssion during EyE imaginal disc dEvElopmEnt. EyE imaginal discs from DmcycEJP larvaE contain fEwEr S phasE cElls, both antErior and postErior to thE morphogEnEtic furrow. This rEsults in adults with small rough EyEs, largEly duE to insufficiEnt numbErs of pigmEnt cElls. AltEring thE dosagE of thE Drosophila cdk2 homolog, cdc2c, rEtinoblastoma, or p21(CIP1) homolog dacapo, which EncodE protEins known to physically intEract with Cyclin E, modifiEd thE DmcycEJP rough EyE phEnotypE as ExpEctEd. DEcrEasing thE dosagE of thE S phasE transcription factor gEnE, dE2F, EnhancEd thE DmcycEJP rough EyE phEnotypE. Surprisingly, mutations in G2/M phasE rEgulators Cyclin A and string (cdc25), but not Cyclin B1, B3, or cdc2, EnhancEd thE DmcycEJP phEnotypE without affEcting thE numbEr of cElls EntEring S phasE, but by dEcrEasing thE numbEr of cElls EntEring mitosis. Our analysis EstablishEs thE DmcycEJP allElE as an ExcEllEnt rEsourcE for sEarching for novEl Cyclin E gEnEtic intEractors. In addition, this analysis has idEntifiEd Cyclin A and string as DmcycEJP intEractors, suggEsting a novEl rolE for Cyclin E in thE rEgulation of Cyclin A and String function during EyE dEvElopmEnt.
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Ectopic Cyclin E ExprEssion inducEs prEmaturE Entry into s phasE and disrupts pattErn formation in thE drosophila EyE imaginal disc
Development, 1995Co-Authors: Helena E Richardson, Louise V Okeefe, Thomas Marty, Robert SaintAbstract:During animal dEvElopmEnt, cEll prolifEration is controllEd in many casEs by rEgulation of thE G1 to S phasE transition. StudiEs of mammalian tissuE culturE cElls havE shown that thE G1-spEcific Cyclin, Cyclin E, can bE ratE limiting for progrEssion from G1 to S phasE. During Drosophila dEvElopmEnt, down-rEgulation of Cyclin E is rEquirEd for G1 arrEst in tErminally diffErEntiating Embryonic EpidErmal cElls. WhEthEr Cyclin E ExprEssion limits progrEssion into S phasE in prolifErating, as opposEd to diffErEntiating, cElls during dEvElopmEnt has not bEEn invEstigatEd. HErE wE show that Drosophila Cyclin E (DmcycE) protEin is absEnt in G1 phasE cElls but appEars at thE onsEt of S phasE in prolifErating cElls of thE larval optic lobE and EyE imaginal disc. WE havE ExaminEd cElls in thE EyE imaginal EpithElium, whErE a clEarly dEfinEd dEvElopmEntally rEgulatEd G1 to S phasE transition occurs. Ectopic ExprEssion of DmcycE inducEs prEmaturE Entry of most of thEsE G1 cElls into S phasE. Thus in thEsE cElls, control of DmcycE ExprEssion is rEquirEd for rEgulatEd Entry into S phasE. Significantly, a band of EyE imaginal disc cElls in G1 phasE was not inducEd to EntEr S phasE by Ectopic ExprEssion of DmcycE. This providEs EvidEncE for additional rEgulatory mEchanisms that opEratE during G1 phasE to limit cEll prolifEration during dEvElopmEnt. ThEsE rEsults dEmonstratE that thE rolE of Cyclin E in rEgulating progrEssion into S phasE in mammalian tissuE culturE cElls appliEs to somE, but not all, cElls during Drosophila dEvElopmEnt.(ABSTRACT TRUNCATED AT 250 WORDS)
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distinct modEs of Cyclin E cdc2c kinasE rEgulation and s phasE control in mitotic and EndorEduplication cyclEs of drosophila EmbryogEnEsis
Genes & Development, 1995Co-Authors: Karsten Sauer, Helena E Richardson, Jiirgen A Knoblich, Christian F LehnerAbstract:Drosophila Cyclin E (DmcycE) is rEquirEd in Embryos for S phasE of mitotic and EndorEduplication cyclEs. HErE, wE dEscribE rEgulatory diffErEncEs charactEristic for thEsE two cEll cyclE typEs. WhilE DmcycE transcript lEvEls dEclinE in DmcycE mutant cElls programmEd for mitotic prolifEration, thEy arE maintainEd and no longEr rEstrictEd to transiEnt pulsEs in DmcycE mutant cElls programmEd for EndorEduplication. MorEovEr, DmcycE ExprEssion in EndorEduplicating cElls is down-rEgulatEd by Ectopic ExprEssion of a hEat-induciblE Cyclin E transgEnE. DmcycE ExprEssion in EndorEduplicating tissuEs, thErEforE, is rEstrictEd by a nEgativE fEEdback to thE transiEnt pulsE triggEring Entry into S-phasE. ConvErsEly, during mitotic cyclEs, whErE S phasE Entry is not only dEpEndEnt on Cyclin E but also on progrEssion through M phasE, Cyclin E and associatEd Dmcdc2c kinasE activity arE prEsEnt throughout thE cEll cyclE. REinitiation of DNA rEplication during thE G2 phasE of thE mitotic cEll cyclE, thErEforE, is prEvEntEd by Cyclin E/Dmcdc2c kinasE-indEpEndEnt rEgulation. ObsErvations in Cyclin A mutants implicatE G2 Cyclins in this rEgulation. Our rEsults suggEst molEcular Explanations for thE diffErEnt rulEs govErning S phasE during mitotic and EndorEduplication cyclEs.
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a drosophila g1 spEcific Cyclin E homolog Exhibits diffErEnt modEs of ExprEssion during EmbryogEnEsis
Development, 1993Co-Authors: Helena E Richardson, Steven I Reed, Louise V Okeefe, Robert SaintAbstract:WE havE isolatEd a Drosophila homolog of thE human G1-spEcific Cyclin E gEnE. Cyclin E protEins thus constitutE an Evolutionarily consErvEd subfamily of mEtazoan Cyclins. ThE Drosophila Cyclin E gEnE, DmcycE, EncodEs two protEins with a common C-tErminal rEgion and uniquE N-tErminal rEgions. UnlikE othEr Drosophila Cyclins, DmcycE Exhibits a dynamic pattErn of ExprEssion during dEvElopmEnt. DmcycE is suppliEd matErnally, but at thE complEtion of thE clEavagE divisions and prior to mitosis 14, thE matErnal transcripts arE rapidly dEgradEd in all cElls ExcEpt thE polE (gErm) cElls. Two modEs of DmcycE ExprEssion arE obsErvEd in thE subsEquEnt divisions. During cyclEs 14, 15 and 16 in non-nEural cElls, DmcycE mRNA lEvEls show no cEll-cyclE-associatEd variation. DmcycE ExprEssion in thEsE cElls is thErEforE indEpEndEnt of thE cEll cyclE phasE. In contrast, ExprEssion in prolifErating Embryonic pEriphEral nErvous systEm cElls occurs during intErphasE as a briEf pulsE that initiatEs bEforE and ovErlaps with S phasE, dEmonstrating thE prEsEncE of a G1 phasE in thEsE Embryonic nEural cEll cyclEs. DmcycE appEars not to bE ExprEssEd in cElls that undErgo EndorEplication cyclEs during polytEnization. ThE structural homology to human Cyclin E, thE ability of DmcycE to rEscuE a G1 Cyclin-dEficiEnt yEast strain, thE prEsEncE of multiplE PEST sEquEncEs charactEristic of G1-spEcific Cyclins and ExprEssion during G1 phasE in prolifErating pEriphEral nErvous systEm cElls all arguE that Drosophila Cyclin E is a G1 Cyclin. ConstitutivE DmcycE ExprEssion in Embryonic cyclEs lacking a G1 phasE, in contrast to ExprEssion during thE G1-S phasE transition in cyclEs Exhibiting a G1 phasE, implicatEs DmcycE ExprEssion in thE rEgulation of thE G1 to S phasE transition during Drosophila EmbryogEnEsis.
Robert Saint - One of the best experts on this subject based on the ideXlab platform.
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TissuE-spEcific rEgulation of Cyclin E transcription during Drosophila mElanogastEr EmbryogEnEsis.
Development (Cambridge England), 2000Co-Authors: Lynn M. Jones, Helena E Richardson, Robert SaintAbstract:Cyclin E is an EssEntial rEgulator of S phasE Entry. WE havE prEviously shown that transcriptional rEgulation of thE gEnE that EncodEs Drosophila Cyclin E, DmcycE, plays an important rolE in thE control of thE G(1) to S phasE transition during dEvElopmEnt. WE rEport hErE thE first comprEhEnsivE analysis of thE transcriptional rEgulation of a G(1 )phasE cEll cyclE rEgulatory gEnE during EmbryogEnEsis. Analysis of dEficiEnciEs, a gEnomic transformant and rEportEr gEnE constructs rEvEalEd that DmcycE transcription is controllEd by a largE and complEx cis-rEgulatory rEgion containing tissuE- and stagE-spEcific componEnts. SEparatE rEgulatory ElEmEnts for transcription in EpidErmal cElls during cEll cyclEs 14-16, cEntral nErvous systEm cElls and pEriphEral nErvous systEm cElls wErE found. An additional cis-rEgulatory ElEmEnt drivEs transcription in thoracic EpidErmal cElls that undErgo a 17th cEll cyclE whEn othEr EpidErmal cElls havE arrEstEd in G(1 )phasE prior to tErminal diffErEntiation. ThE complExity of DmcycE transcriptional rEgulation arguEs against a modEl in which DmcycE transcription is rEgulatEd simply and solEly by G(1) to S phasE transcription rEgulators such as RB, E2F and DP. RathEr, our study dEmonstratEs that tissuE-spEcific transcriptional rEgulatory mEchanisms arE important componEnts of thE control of Cyclin E transcription and thus of cEll prolifEration in mEtazoans.
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analysis of a drosophila Cyclin E hypomorphic mutation suggEsts a novEl rolE for Cyclin E in cEll prolifEration control during EyE imaginal disc dEvElopmEnt
Genetics, 1998Co-Authors: Julie Secombe, Robert Saint, Johanna P Pispa, Helena E RichardsonAbstract:WE havE gEnEratEd and charactErizEd a Drosophila Cyclin E hypomorphic mutation, DmcycEJP, that is homozygous viablE and fErtilE, but rEsults in adults with rough EyEs. ThE mutation arosE from an intErnal dElEtion of an Existing P[w+lacZ] ElEmEnt insErtEd 14 kb upstrEam of thE transcription start sitE of thE DmcycE zygotic mRNA. ThE prEsEncE of this dElEtEd P ElEmEnt, but not thE P[w+lacZ] ElEmEnt from which it was dErivEd, lEads to a dEcrEasEd lEvEl of DmcycE ExprEssion during EyE imaginal disc dEvElopmEnt. EyE imaginal discs from DmcycEJP larvaE contain fEwEr S phasE cElls, both antErior and postErior to thE morphogEnEtic furrow. This rEsults in adults with small rough EyEs, largEly duE to insufficiEnt numbErs of pigmEnt cElls. AltEring thE dosagE of thE Drosophila cdk2 homolog, cdc2c, rEtinoblastoma, or p21(CIP1) homolog dacapo, which EncodE protEins known to physically intEract with Cyclin E, modifiEd thE DmcycEJP rough EyE phEnotypE as ExpEctEd. DEcrEasing thE dosagE of thE S phasE transcription factor gEnE, dE2F, EnhancEd thE DmcycEJP rough EyE phEnotypE. Surprisingly, mutations in G2/M phasE rEgulators Cyclin A and string (cdc25), but not Cyclin B1, B3, or cdc2, EnhancEd thE DmcycEJP phEnotypE without affEcting thE numbEr of cElls EntEring S phasE, but by dEcrEasing thE numbEr of cElls EntEring mitosis. Our analysis EstablishEs thE DmcycEJP allElE as an ExcEllEnt rEsourcE for sEarching for novEl Cyclin E gEnEtic intEractors. In addition, this analysis has idEntifiEd Cyclin A and string as DmcycEJP intEractors, suggEsting a novEl rolE for Cyclin E in thE rEgulation of Cyclin A and String function during EyE dEvElopmEnt.
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Ectopic Cyclin E ExprEssion inducEs prEmaturE Entry into s phasE and disrupts pattErn formation in thE drosophila EyE imaginal disc
Development, 1995Co-Authors: Helena E Richardson, Louise V Okeefe, Thomas Marty, Robert SaintAbstract:During animal dEvElopmEnt, cEll prolifEration is controllEd in many casEs by rEgulation of thE G1 to S phasE transition. StudiEs of mammalian tissuE culturE cElls havE shown that thE G1-spEcific Cyclin, Cyclin E, can bE ratE limiting for progrEssion from G1 to S phasE. During Drosophila dEvElopmEnt, down-rEgulation of Cyclin E is rEquirEd for G1 arrEst in tErminally diffErEntiating Embryonic EpidErmal cElls. WhEthEr Cyclin E ExprEssion limits progrEssion into S phasE in prolifErating, as opposEd to diffErEntiating, cElls during dEvElopmEnt has not bEEn invEstigatEd. HErE wE show that Drosophila Cyclin E (DmcycE) protEin is absEnt in G1 phasE cElls but appEars at thE onsEt of S phasE in prolifErating cElls of thE larval optic lobE and EyE imaginal disc. WE havE ExaminEd cElls in thE EyE imaginal EpithElium, whErE a clEarly dEfinEd dEvElopmEntally rEgulatEd G1 to S phasE transition occurs. Ectopic ExprEssion of DmcycE inducEs prEmaturE Entry of most of thEsE G1 cElls into S phasE. Thus in thEsE cElls, control of DmcycE ExprEssion is rEquirEd for rEgulatEd Entry into S phasE. Significantly, a band of EyE imaginal disc cElls in G1 phasE was not inducEd to EntEr S phasE by Ectopic ExprEssion of DmcycE. This providEs EvidEncE for additional rEgulatory mEchanisms that opEratE during G1 phasE to limit cEll prolifEration during dEvElopmEnt. ThEsE rEsults dEmonstratE that thE rolE of Cyclin E in rEgulating progrEssion into S phasE in mammalian tissuE culturE cElls appliEs to somE, but not all, cElls during Drosophila dEvElopmEnt.(ABSTRACT TRUNCATED AT 250 WORDS)
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a drosophila g1 spEcific Cyclin E homolog Exhibits diffErEnt modEs of ExprEssion during EmbryogEnEsis
Development, 1993Co-Authors: Helena E Richardson, Steven I Reed, Louise V Okeefe, Robert SaintAbstract:WE havE isolatEd a Drosophila homolog of thE human G1-spEcific Cyclin E gEnE. Cyclin E protEins thus constitutE an Evolutionarily consErvEd subfamily of mEtazoan Cyclins. ThE Drosophila Cyclin E gEnE, DmcycE, EncodEs two protEins with a common C-tErminal rEgion and uniquE N-tErminal rEgions. UnlikE othEr Drosophila Cyclins, DmcycE Exhibits a dynamic pattErn of ExprEssion during dEvElopmEnt. DmcycE is suppliEd matErnally, but at thE complEtion of thE clEavagE divisions and prior to mitosis 14, thE matErnal transcripts arE rapidly dEgradEd in all cElls ExcEpt thE polE (gErm) cElls. Two modEs of DmcycE ExprEssion arE obsErvEd in thE subsEquEnt divisions. During cyclEs 14, 15 and 16 in non-nEural cElls, DmcycE mRNA lEvEls show no cEll-cyclE-associatEd variation. DmcycE ExprEssion in thEsE cElls is thErEforE indEpEndEnt of thE cEll cyclE phasE. In contrast, ExprEssion in prolifErating Embryonic pEriphEral nErvous systEm cElls occurs during intErphasE as a briEf pulsE that initiatEs bEforE and ovErlaps with S phasE, dEmonstrating thE prEsEncE of a G1 phasE in thEsE Embryonic nEural cEll cyclEs. DmcycE appEars not to bE ExprEssEd in cElls that undErgo EndorEplication cyclEs during polytEnization. ThE structural homology to human Cyclin E, thE ability of DmcycE to rEscuE a G1 Cyclin-dEficiEnt yEast strain, thE prEsEncE of multiplE PEST sEquEncEs charactEristic of G1-spEcific Cyclins and ExprEssion during G1 phasE in prolifErating pEriphEral nErvous systEm cElls all arguE that Drosophila Cyclin E is a G1 Cyclin. ConstitutivE DmcycE ExprEssion in Embryonic cyclEs lacking a G1 phasE, in contrast to ExprEssion during thE G1-S phasE transition in cyclEs Exhibiting a G1 phasE, implicatEs DmcycE ExprEssion in thE rEgulation of thE G1 to S phasE transition during Drosophila EmbryogEnEsis.
Elizabeth A Musgrove - One of the best experts on this subject based on the ideXlab platform.
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constitutivE ovErExprEssion of Cyclin d1 but not Cyclin E confErs acutE rEsistancE to antiEstrogEns in t 47d brEast cancEr cElls
Cancer Research, 2002Co-Authors: Rina Hui, Elizabeth A Musgrove, Georgina L Finney, Jason S Carroll, Christine S L Lee, Robert L SutherlandAbstract:Cyclin D1 and Cyclin E arE ovErExprEssEd in ∼45% and 30% of brEast cancErs, rEspEctivEly, and advErsE associations with patiEnt outcomE havE bEEn rEportEd. ThE potEntial rolEs of Cyclin D1 and Cyclin E ExprEssion as markErs of thErapEutic rEsponsivEnEss to thE purE stEroidal antiEstrogEn ICI 182780 wErE invEstigatEd using T-47D brEast cancEr cEll linEs constitutivEly ovErExprEssing Cyclin D1 or Cyclin E. MEasurEmEnt of S phasE fraction, phosphorylation statEs of thE rEtinoblastoma protEin, and Cyclin E-Cyclin-dEpEndEnt kinasE (Cdk) 2 activity dEmonstratEd that ovErExprEssion of Cyclin D1 dEcrEasEd sEnsitivity to antiEstrogEn inhibition at 24 and 48 h. OvErExprEssion of Cyclin E producEd a lEss pronouncEd Early cEll cyclE EffEct indicating only partial rEsistancE to antiEstrogEn inhibition in thE short-tErm. In ICI 182780-trEatEd Cyclin D1-ovErExprEssing cElls, sufficiEnt Cdk activity was rEtainEd to allow rEtinoblastoma protEin phosphorylation and cEll prolifEration, dEspitE an incrEasE in thE association of p21 and p27 with Cyclin D1-Cdk4/6 and Cyclin E-Cdk2 complExEs. AftEr longEr-tErm (>7 days) trEatmEnt, antiEstrogEns inhibitEd colony growth in Cyclin D1- or Cyclin E-ovErExprEssing brEast cancEr cElls, but with an approximatEly 2–2.5-fold dEcrEasE in dosE sEnsitivity. This was associatEd with a fall in Cyclin D1 lEvEls, a rEduction in thE half-lifE of Cyclin D1 protEin and a dEclinE in Cyclin E-Cdk2 activity in Cyclin D1-ovErExprEssing cElls, and thE maintEnancE of Cyclin E-p27 association in thE Cyclin E-ovErExprEssing cElls. ThEsE data confirm that Cyclin D1 ExprEssion and Cyclin E-p27 association play important rolEs in antiEstrogEn action, and suggEst that Cyclin D1 or Cyclin E ovErExprEssion has subtlE EffEcts on antiEstrogEn sEnsitivity. Additional studiEs to ElucidatE thE contribution of altErations in Cyclin D1 stability to antiEstrogEn action and to assEss thE rElationship bEtwEEn antiEstrogEn sEnsitivity and ExprEssion of Cyclin D1, Cyclin E, or p27 in a clinical sEtting arE rEquirEd.
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Cyclin d1 ovErExprEssion inducEs progEstin rEsistancE in t 47d brEast cancEr cElls dEspitE p27kip1 association with Cyclin E cdk2
Journal of Biological Chemistry, 2001Co-Authors: Elizabeth A Musgrove, Rina Hui, Christine S L Lee, Lisajane K Hunter, Alexander Swarbrick, Robert L SutherlandAbstract:Abstract Long-tErm growth inhibition, arrEst in G1 phasE and rEducEd activity of both Cyclin D1-Cdk4 and Cyclin E-Cdk2 arE ElicitEd by progEstin trEatmEnt of brEast cancEr cElls in culturE. DEcrEasEd Cyclin ExprEssion, induction of p18INK4c and incrEasEd association of thE CDK inhibitors p21WAF1/Cip1 and p27Kip1 with Cyclin E-Cdk2 havE bEEn implicatEd in thEsE rEsponsEs. To dEtErminE thE rolE of dEcrEasEd Cyclin ExprEssion, T-47D human brEast cancEr cElls constitutivEly ExprEssing Cyclin D1 or Cyclin E wErE trEatEd with thE progEstin ORG 2058. OvErExprEssion of Cyclin E had only a modEst EffEct on growth inhibition. Although Cyclin E ExprEssion was maintainEd during progEstin trEatmEnt, Cyclin E-Cdk2 activity dEcrEasEd by ∼60%. This was accompaniEd by p27Kip1 association with Cyclin E-Cdk2, indicating that both Cyclin E down-rEgulation and p27Kip1 rEcruitmEnt contributE to thE dEcrEasE in activity. In contrast, ovErExprEssion of Cyclin D1 inducEd progEstin rEsistancE and cEll prolifEration continuEd dEspitE dEcrEasEd Cyclin E-Cdk2 activity. ProgEstin trEatmEnt of Cyclin D1-ovErExprEssing cElls was associatEd with incrEasEd p27Kip1 association with Cyclin E-Cdk2. Thus thE ability of Cyclin D1 to confEr progEstin rEsistancE doEs not dEpEnd on sEquEstration of p27Kip1 away from Cyclin E-Cdk2, providing EvidEncE for a critical function of Cyclin D1 othEr than as a high-capacity “sink” for p27Kip1. ThEsE data indicatE that rEgulation of Cyclin D1 is a critical ElEmEnt of progEstin inhibition in brEast cancEr cElls and suggEst that brEast cancErs ovErExprEssing Cyclin D1 may rEspond poorly to progEstin thErapy.
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c myc or Cyclin d1 mimics EstrogEn EffEcts on Cyclin E cdk2 activation and cEll cyclE rEEntry
Molecular and Cellular Biology, 1998Co-Authors: Owen W J Prall, Elizabeth A Musgrove, Eileen M Rogan, Colin K W Watts, Robert L SutherlandAbstract:EstrogEn-inducEd progrEssion through G1 phasE of thE cEll cyclE is prEcEdEd by incrEasEd ExprEssion of thE G1-phasE rEgulatory protEins c-Myc and Cyclin D1. To invEstigatE thE potEntial contribution of thEsE protEins to EstrogEn action, wE dErivEd clonal MCF-7 brEast cancEr cEll linEs in which c-Myc or Cyclin D1 was ExprEssEd undEr thE control of thE mEtal-induciblE mEtallothionEin promotEr. InduciblE ExprEssion of EithEr c-Myc or Cyclin D1 was sufficiEnt for S-phasE Entry in cElls prEviously arrEstEd in G1 phasE by prEtrEatmEnt with ICI 182780, a potEnt EstrogEn antagonist. c-Myc ExprEssion was not accompaniEd by incrEasEd Cyclin D1 ExprEssion or Cdk4 activation, nor was Cyclin D1 induction accompaniEd by incrEasEs in c-Myc. ExprEssion of c-Myc or Cyclin D1 was sufficiEnt to activatE Cyclin E-Cdk2 by promoting thE formation of high-molEcular-wEight complExEs lacking thE Cyclin-dEpEndEnt kinasE inhibitor p21, as has bEEn dEscribEd, following EstrogEn trEatmEnt. IntErEstingly, this was accompaniEd by an association bEtwEEn activE Cyclin E-Cdk2 complExEs and hypErphosphorylatEd p130, idEntifying a prEviously undEfinEd rolE for p130 in EstrogEn action. ThEsE data providE EvidEncE for distinct c-Myc and Cyclin D1 pathways in EstrogEn-inducEd mitogEnEsis which convErgE on or prior to thE formation of activE Cyclin E-Cdk2-p130 complExEs and loss of inactivE Cyclin E-Cdk2-p21 complExEs, indicating a physiologically rElEvant rolE for thE Cyclin E binding motifs sharEd by p130 and p21.
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human f box protEin hcdc4 targEts Cyclin E for protEolysis and is mutatEd in a brEast cancEr cEll linE
Nature, 2001Co-Authors: Heimo Strohmaier, Peter K Kaiser, Charles H Spruck, Olle Sangfelt, Steven I ReedAbstract:Cyclin E, onE of thE activators of thE Cyclin-dEpEndEnt kinasE Cdk2, is ExprEssEd nEar thE G1–S phasE transition and is thought to bE critical for thE initiation of DNA rEplication and othEr S-phasE functions1,2,3. Accumulation of Cyclin E at thE G1–S boundary is achiEvEd by pEriodic transcription couplEd with rEgulatEd protEolysis linkEd to autophosphorylation of Cyclin E4. ThE propEr timing and amplitudE of Cyclin E ExprEssion sEEm to bE important, bEcausE ElEvatEd lEvEls of Cyclin E havE bEEn associatEd with a variEty of malignanciEs5,6 and constitutivE ExprEssion of Cyclin E lEads to gEnomic instability7. HErE wE show that turnovEr of phosphorylatEd Cyclin E dEpEnds on an SCF-typE protEin-ubiquitin ligasE that contains thE human homologuE of yEast Cdc4, which is an F-box protEin containing rEpEatEd sEquEncEs of WD40 (a unit containing about 40 rEsiduEs with tryptophan (W) and aspartic acid (D) at dEfinEd positions). ThE gEnE Encoding hCdc4 was found to bE mutatEd in a cEll linE dErivEd from brEast cancEr that ExprEssEd ExtrEmEly high lEvEls of Cyclin E.
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accumulation of Cyclin E is not a prErEquisitE for passagE through thE rEstriction point
Molecular and Cellular Biology, 2001Co-Authors: Susanna V Ekholm, Peter Zickert, Steven I Reed, Anders ZetterbergAbstract:In thE Eukaryotic cEll cyclE, a rEvErsiblE growth arrEst can bE inducEd in thE G1 phasE if cElls arE dEprivEd of growth factors or allowEd to grow to confluEncy (10, 49, 62, 73, 74). TEmin (61) showEd that chickEn cElls bEcomE indEpEndEnt of ExtErnal mitogEnic growth factors during G1 sEvEral hours bEforE Entry into S phasE. ThE tErm rEstriction point (R) was introducEd by PardEE (48) to dEfinE thE point in G1 aftEr which cElls can complEtE a division cyclE indEpEndEntly of mitogEnic signals (49). WE havE prEviously dEtErminEd thE Exact position of R in G1 and its rElationship to thE prEvious mitosis and subsEquEnt S phasE with timE-lapsE cinEmatography (TLC) analysis of mousE and human cElls (33, 74, 75, 76, 77). TimE-lapsE rEcordings of cElls in culturE EnablE analysis of individual cElls of an unpErturbEd, asynchronously growing population. This mEthod is a powErful tool for dEtailEd kinEtic analysis of transition EvEnts in thE cEll cyclE bEcausE, unlikE synchronization procEdurEs, it addrEssEs thE problEm of intErcEllular variability in cEll cyclE timEs, particularly G1 variability. PrEvious studiEs using TLC analysis rEvEalEd that thE G1 phasE in Cycling cElls is sEparatEd into two functionally diffErEnt intErvals. During thE first part of G1, thE G1-pm (postmitosis) pEriod, cEll cyclE progrEssion is highly dEpEndEnt on thE continuous prEsEncE of sErum growth factors and on a high ratE of protEin synthEsis. If growth factors arE rEmovEd from thE mEdium or if protEin synthEsis is EvEn only modEratEly inhibitEd during this pEriod, cElls will rapidly (within 30 to 60 min) lEavE thE cEll cyclE and EntEr a quiEscEnt statE (G0). ThE G1-pm pEriod has a constant duration of 3 to 4 h in all of thE cElls studiEd so far (74, 75, 76). ThE transition from growth factor-dEpEndEnt progrEssion to growth factor-indEpEndEnt progrEssion rEprEsEnts passagE through R. ThE part of G1 that follows R, known as thE G1-ps (prE-DNA-synthEtic) pEriod, is highly variablE in duration. SomE G1-ps cElls initiatE DNA rEplication immEdiatEly aftEr passagE through R, whilE othErs may spEnd up to 20 h in G1-ps bEforE EntEring S. This variability in thE lEngth of timE bEtwEEn R and S impliEs that EvEn though passagE through R is nEcEssary for furthEr progrEssion through thE cEll cyclE, othEr rEgulatory EvEnts must bE complEtEd during G1-ps in ordEr for cElls to EntEr S phasE. ThE Cyclins and thEir catalytic subunits, thE Cyclin-dEpEndEnt kinasEs (Cdks), control cEll cyclE progrEssion by rEgulating EvEnts that drivE thE transitions bEtwEEn cEll cyclE phasEs. Cyclins wErE first idEntifiEd in clam and sEa urchin Embryos, whErE thEy wErE obsErvEd to accumulatE during intErphasE and to bE dEgradEd during mitosis (16). BasEd on homology to invErtEbratE and frog Embryonic Cyclins, human A- and B-typE Cyclins, EssEntial for progrEssion through S, G2, and M phasE, wErE thE first human Cyclins to bE idEntifiEd (50, 64). SubsEquEntly, thE human G1 Cyclins, thE D-typE Cyclins and Cyclin E, wErE idEntifiEd functionally by scrEEning of human cDNA librariEs for sEquEncEs that could complEmEnt G1 Cyclin mutations in SaccharomycEs cErEvisiaE (30, 36, 69). ThE gEnE for Cyclin D1 is inducEd in rEsponsE to mitogEnic signals as an Early-rEsponsE gEnE during thE transition from G0 to G1 phasE and is associatEd with thE catalytic partnEr Cdk4 or Cdk6. Cyclin E shows a pEriodic pattErn of ExprEssion with accumulation in latE G1 and downrEgulation in S (11, 31, 36; rEviEwEd in rEfErEncE 54). Cyclin E transcription is activatEd whEn thE rEtinoblastoma tumor suprEssor protEin (pRb) is hypErphosphorylatEd and no longEr ExErts rEprEssion of thE Cyclin E promotEr via E2F-DP transcription factor complExEs (sEE bElow). ConsistEnt with this, a numbEr of putativE E2F binding sitEs havE bEEn idEntifiEd in thE Cyclin E promotEr (19). E2F-mEdiatEd rEprEssion was first suggEstEd by ExpErimEnts showing that thE combinEd mutation of two diffErEnt E2F sitEs in thE human Cyclin E promotEr lEads to partial dErEprEssion of thE promotEr in G1 (45). REcEntly, a variant E2F-binding sitE was found to mEdiatE transcriptional rEprEssion by binding of a largE E2F4-pRb-containing rEprEssor complEx (34, 78). Cyclin E associatEs spEcifically with Cdk2, and a numbEr of invEstigations havE dEmonstratEd a rEquirEmEnt for Cyclin E-cdk2 activity for thE initiation of DNA rEplication (24, 32, 47). Cyclin E is subjEctEd to ubiquitin-dEpEndEnt dEgradation during S phasE (7, 58, 68). Many of thE molEcular componEnts that arE involvEd in passagE through G1 havE bEEn idEntifiEd, but thE molEcular mEchanism undErlying R point control still rEmains to bE ElucidatEd. PassagE through thE R point and phosphorylation-inactivation of pRb havE bEEn obsErvEd to occur roughly during thE samE timE pEriod in cElls EntEring thE cEll cyclE from G0, but thE Exact functional rElationship bEtwEEn thE two EvEnts is still unknown. In Early G1 phasE, pRb is prEsEnt in an activE, hypophosphorylatEd form, whErE it is bEliEvEd to inhibit cEll cyclE progrEssion by binding to rEgulatory protEins, including mEmbErs of thE E2F family of transcription factors. Binding of pRb to E2F has bEEn shown to inhibit thE transactivation of E2F-dEpEndEnt gEnEs that arE rEquirEd for cEll cyclE progrEssion (5, 21, 42). ThE association bEtwEEn pRb and E2F, as wEll as othEr rEgulatory targEts, has bEEn shown to bE govErnEd by phosphorylation. pRb is phosphorylatEd at multiplE sitEs as Cdk activity incrEasEs during G1 phasE. HypErphosphorylatEd pRb first appEars during latE G1 phasE. Although sEvEral Cdks havE bEEn implicatEd in pRb phosphorylation in vitro (1, 17, 40, 41), thE prEcisE mEchanism by which pRb is phosphorylatEd in vivo is still unclEar. By Ectopically ExprEssing Cyclin D1 or E during Early G1, it was dEmonstratEd that ExprEssion of EithEr Cyclin shortEns thE G1 phasE in rat Embryonic fibroblasts but only Cyclin D1 ExprEssion lEads to prEmaturE pRb phosphorylation (55). OthEr EvidEncE suggEsts that pRb is phosphorylatEd by both Cyclin D- and E-dEpEndEnt kinasEs in a sEquEntial mannEr to achiEvE hypErphosphorylation (8, 20, 39, 70). ThErEforE, it has bEEn proposEd that thE accumulation of EithEr D-typE Cyclins or Cyclin E and thE concomitant hypErphosphorylation/inactivation of pRb constitutE progrEssion through thE R point (9, 53, 77). ConsistEnt with this idEa, it has bEEn dEmonstratEd that passagE through thE R point is dEpEndEnt on thE accumulation of a labilE protEin (48), a charactEristic of both D-typE Cyclins and Cyclin E (11, 36, 37, 47). ThE aim of thE prEsEnt study was to pErform a dEtailEd analysis of Cyclin E ExprEssion in rElation to passagE through R and Entry into S phasE, in ordEr to dEtErminE whEthEr Cyclin E could bE thE labilE R-associatEd protEin. In ordEr to dEtErminE thE Exact timing of Cyclin E accumulation and downrEgulation, wE carriEd out an analysis of individual cElls which allowEd us to considEr thE variability of cEll bEhavior within thE population. WE found that cElls youngEr than 3.5 h aftEr mitosis, i.E., cElls that had not yEt passEd R, wErE nEgativE for Cyclin E accumulation. AftEr passagE through R, Cyclin E bEgins to accumulatE as a cEll approachEs S, howEvEr, with a high dEgrEE of tEmporal variability. ThEsE data indicatE that passagE through R cannot bE dEpEndEnt on thE accumulation of Cyclin E, as has bEEn proposEd, and suggEst that sincE R occurs prior to thE accumulation of Cyclin E, passagE through R may bE a prErEquisitE for Cyclin E accumulation. FurthErmorE, thE tEmporal variability of Cyclin E accumulation aftEr passagE through R suggEsts that anothEr latE-G1 EvEnt(s) controls thE prEcisE timing of Cyclin E accumulation.
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dErEgulatEd Cyclin E inducEs chromosomE instability
Nature, 1999Co-Authors: Charles H Spruck, Steven I ReedAbstract:Cyclin E, a rEgulatory subunit of Cyclin-dEpEndEnt kinasE 2 (Cdk2), is an important rEgulator of Entry into S phasE in thE mammalian cEll cyclE. In normal dividing cElls, Cyclin E accumulatEs at thE G1/S-phasE boundary and is dEgradEd as cElls progrEss through S phasE1,2. HowEvEr, in many human tumours Cyclin E is ovErExprEssEd3 and thE lEvEls of protEin and kinasE activity arE oftEn dErEgulatEd rElativE to thE cEll cyclE4,5,6,7. It is not undErstood how altErations in ExprEssion of Cyclin E contributE to tumorigEnEsis. HErE wE show that constitutivE Cyclin-E ovErExprEssion in both immortalizEd rat Embryo fibroblasts and human brEast EpithElial cElls rEsults in chromosomE instability (CIN). In contrast, analogous ExprEssion of Cyclin D1 or A doEs not incrEasE thE frEquEncy of CIN. Cyclin-E-ExprEssing cElls that Exhibit CIN havE normal cEntrosomE numbErs. HowEvEr, constitutivE ovErExprEssion of Cyclin E impairs S-phasE progrEssion, indicating that abErrant rEgulation of this procEss may bE rEsponsiblE for thE CIN obsErvEd. ThEsE rEsults indicatE that downrEgulation of Cyclin-E/Cdk2 kinasE activity following thE G1/S-phasE transition may bE nEcEssary for thE maintEnancE of karyotypic stability.
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Cyclin E inducEd s phasE without activation of thE prb E2f pathway
Genes & Development, 1997Co-Authors: Jiri Lukas, Steven I Reed, Kristian Helin, Thomas Herzinger, Klaus Hansen, Maria Cristina Moroni, Dalia Resnitzky, Jiri BartekAbstract:In cElls of highEr EukaryotEs, Cyclin D-dEpEndEnt kinasEs Cdk4 and Cdk6 and, possibly, Cyclin E-dEpEndEnt Cdk2 positivEly rEgulatE thE G1- to S-phasE transition, by phosphorylating thE rEtinoblastoma protEin (pRb), thErEby rElEasing E2F transcription factors that control S-phasE gEnEs. HErE wE pErformEd microinjEction and transfEction ExpErimEnts using rat R12 fibroblasts, thEir dErivativEs conditionally ovErExprEssing Cyclins D1 or E, and human U-2-OS cElls, to ExplorE thE action of G1 Cyclins and thE rElationship of E2F and Cyclin E in S-phasE induction. WE dEmonstratE that Ectopic ExprEssion of Cyclin E, but not Cyclin D1, can ovErridE G1 arrEst imposEd by EithEr thE p16INK4a Cdk inhibitor spEcific for Cdk4 and Cdk6 or a novEl phosphorylation-dEficiEnt mutant pRb. SEvEral complEmEntary approachEs to assEss E2F activation, including quantitativE rEportEr assays in livE cElls, showEd that thE Cyclin E-inducEd S phasE and complEtion of thE cEll division cyclE can occur in thE absEncE of E2F-mEdiatEd transactivation. TogEthEr with thE ability of Cyclin E to ovErcomE a G1 block inducEd by ExprEssion of dominant-nEgativE mutant DP-1, a hEtErodimEric partnEr of E2Fs, thEsE rEsults providE EvidEncE for a Cyclin E-controllEd S phasE-promoting EvEnt in somatic cElls downstrEam of or parallEl to phosphorylation of pRb and indEpEndEnt of E2F activation. ThEy furthErmorE indicatE that a lack of E2F-mEdiatEd transactivation can bE compEnsatEd by hypEractivation of this Cyclin E-controllEd EvEnt.
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a drosophila g1 spEcific Cyclin E homolog Exhibits diffErEnt modEs of ExprEssion during EmbryogEnEsis
Development, 1993Co-Authors: Helena E Richardson, Steven I Reed, Louise V Okeefe, Robert SaintAbstract:WE havE isolatEd a Drosophila homolog of thE human G1-spEcific Cyclin E gEnE. Cyclin E protEins thus constitutE an Evolutionarily consErvEd subfamily of mEtazoan Cyclins. ThE Drosophila Cyclin E gEnE, DmcycE, EncodEs two protEins with a common C-tErminal rEgion and uniquE N-tErminal rEgions. UnlikE othEr Drosophila Cyclins, DmcycE Exhibits a dynamic pattErn of ExprEssion during dEvElopmEnt. DmcycE is suppliEd matErnally, but at thE complEtion of thE clEavagE divisions and prior to mitosis 14, thE matErnal transcripts arE rapidly dEgradEd in all cElls ExcEpt thE polE (gErm) cElls. Two modEs of DmcycE ExprEssion arE obsErvEd in thE subsEquEnt divisions. During cyclEs 14, 15 and 16 in non-nEural cElls, DmcycE mRNA lEvEls show no cEll-cyclE-associatEd variation. DmcycE ExprEssion in thEsE cElls is thErEforE indEpEndEnt of thE cEll cyclE phasE. In contrast, ExprEssion in prolifErating Embryonic pEriphEral nErvous systEm cElls occurs during intErphasE as a briEf pulsE that initiatEs bEforE and ovErlaps with S phasE, dEmonstrating thE prEsEncE of a G1 phasE in thEsE Embryonic nEural cEll cyclEs. DmcycE appEars not to bE ExprEssEd in cElls that undErgo EndorEplication cyclEs during polytEnization. ThE structural homology to human Cyclin E, thE ability of DmcycE to rEscuE a G1 Cyclin-dEficiEnt yEast strain, thE prEsEncE of multiplE PEST sEquEncEs charactEristic of G1-spEcific Cyclins and ExprEssion during G1 phasE in prolifErating pEriphEral nErvous systEm cElls all arguE that Drosophila Cyclin E is a G1 Cyclin. ConstitutivE DmcycE ExprEssion in Embryonic cyclEs lacking a G1 phasE, in contrast to ExprEssion during thE G1-S phasE transition in cyclEs Exhibiting a G1 phasE, implicatEs DmcycE ExprEssion in thE rEgulation of thE G1 to S phasE transition during Drosophila EmbryogEnEsis.
