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CELL CYCLE CONTROL

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					    CELL CYCLE CONTROL
• Introduction
• Cell Cycle Progression Control Points
• Cell Cycle Control Mechanisms
Cyclin-Dependent Kinases (CDKs)
The Role of Cyclins
    CELL CYCLE CONTROL
Regulatory    Roles   of   CDK-Cyclin
 complexes
The Role of Phosphorylation and
  Dephosphorylation
Cyclin-Dependent Kinase Inhibitors
 (CDKIs)

• Cancer Cell Cycles
          INTRODUCTION
• An     intricate balance between cell
  proliferation, growth arrest, and cell death
  is essential:
Embryonic development
Maintenance of tissue homeostasis in the
   adult
• Molecular processes that regulate these
  cellular processes have started to be
  investigated
• The discovery of cellular oncogenes has
  helped to focus research on cell
  proliferation
            INTRODUCTION
• The first tumor suppressor genes were
 identified, and boosted research in the
 molecular      biology    of    growth
 suppression

• Lately,the study of programmed cell
 death or apoptosis caught the interest
 of scientists and turned it into one of
 the hottest fields in biomedical
 research
              CELL CYCLE
• The cell cycle consists of four phases:
    G1, S, G2, and M
•   The period between two mitotic
    divisions defines the somatic cell cycle
•   The time from the end of one mitosis to
    the start of the next is called interphase
•   The     period     of    actual    division
    corresponding to mitosis is called M
    phase
            CELL CYCLE
• Interphase is divided into periods in
 reference to the timing of DNA
 synthesis: G1, S, and G2

• Synthesis of RNA and proteins occurs
 continuously, but DNA synthesis
 occurs only in the S phase period

• Somaticcell cycles differ from early
 embryonic cell cycles
Stages of the Cell Cycle
Interphase is
divided into the
G1, S, and G2
periods.
Demarcation
between one cell
cycle and the next
is provided by
mitosis (M).
Cells may
withdraw from the
cycle into G0 or
reenter from it.
Synthesis of RNA
and proteins occurs
continuously, but
DNA synthesis
occurs only in the
discrete period of S
phase. The units of
mass are abitrary.
   Somatic Cell         Early Embryonic
     Cycle                   Cycles

• Non-synchronized • Usually synchronized
• 24 hr average      • 20 × faster
• Constant cell size • Division w/o growth
                      G1 and G2 suppressed
Early embryonic and
somatic cell cycles in
frogs.
Somatic cell cycles in
frogs typically last
about 1 day and have a
G1 and a G2.
Twelve rapid divisions
occur after fertilization,
after which the
synchrony between
the cell cycles of
neighboring cells
breaks down.
     CELL CYCLE PROGRESSION
         CONTROL POINTS
    Critical points in cell cycle progression:

• Commitment       to chromosome replication
    occurs in G1 (START in yeast and
    restriction point in animal cells)
•   The cell makes the decision whether to
    start dividing or not
•   This decision is affected by external
    stimuli and appropriate cell mass
    CELL CYCLE PROGRESSION
        CONTROL POINTS
• The beginning of S phase is identified
    by the beginning of DNA synthesis
•   Commitment to mitotic division occurs
    at the end of G2
•   Checkpoints ensure the readiness of
    the cell to proceed:
Beginning of cell division
DNA integrity
Unreplicated DNA
Mitosis
   CELL CYCLE PROGRESSION
       CONTROL POINTS

• The molecular basis for the regulatory
 events that control the phases of the cell
 cycle have been investigated

• Some   of these events require the
 synthesis of new proteins or degradation
 of pre-existing components
Checkpoints control the ability of the cell to progress through the cycle by
 determining whether earlier stages have been completed successfully
CELL CYCLE CONTROL
    MECHANISMS
  Cyclin-Dependent Kinases
•A   family of protein kinases termed
 cyclin-dependent     kinases   (CDKs)
 controls the transitions between
 successive phases of the cell cycle in
 all eukaryotic cells
• AllCDKs are structurally related to
 each other and require associated
 cyclin proteins for activity
• The cell cycle control mechanism is
 universal
  Cyclin-Dependent Kinases
• Founding members of the CDK family
 are encoded by the homologous 34 kDa
 product of the yeast CDC (Cell Division
 Cycle) genes (CDC28 or cdc2)  CDK1

• In yeast cells, cell cycle progression is
 mostly controlled by this single CDK

• In metazoans different cell cycle
 transitions require distinct CDKs
    Cyclin-Dependent Kinases
• The first vertebrate CDK p34CDC2 (CDK1)
    was shown to be the catalytic subunit
    of M-phase promoting factor (MPF), a
    universal inducer of mitosis
•   Later, additional vertebrate CDKs (at
    least 6) have been identified
•   The positive regulatory subunits of
    CDKs, the cyclins, also constitute a
    large family
          The Role of Cyclins
• So   far, twelve different cyclins have been
    cloned in vertebrates
•   Synthesis and destruction of individual
    cyclins:

Regulates the activation of CDKs
Maintains an appropriate order           of cell
    cycle events

• D-type cyclins (D1, D2, and D3) are the key
    regulators for G1 phase progression
       The Role of Cyclins
• Cyclin  E is required for the G1/S
 transition

• CyclinA is required for progression
 through S phase

• Cyclin A and B are required for entry
 into mitosis
Regulatory Roles of CDK-Cyclin
          Complexes

• CDKs  are cell cycle regulators into S
 (+G1/S cyclins) and M phases (+G2/M
 cyclins)

• Entry into the different phases of the
 cell cycle is due to the following CDKs
 and cyclin complexes:
Regulatory Roles of CDK-Cyclin
          Complexes
• G1- progression: CDK 2, CDK4, CDK5,
                  CDK6 + Cyclin D (D1,
                  D 2, D 3)
• S- phase entry: CDK2 + Cyclin E
• S- progression: CDK2 + Cyclin A
• M- phase entry: CDK1 + Cyclins A or B
     MPF (Maturation Promoting Factor)
  Regulatory Roles of CDK-Cyclin
            Complexes
• At G2 phase:     CDK1 activity is kept
 relatively low. At onset of mitosis,
 activation of CDK1 activity
• Exit from mitosis:         A & B-cyclin
  degradation and CDK1 inactivation
• Changes in CDK activity ensure the
  presence of only one S-phase /cell cycle
• Deregulation of cell cycle control leads to
 incomplete development, inappropriate
 tissue differentiation, genomic instability,
 cancer...
Regulatory Roles of Cdk-Cyclin
         Complexes
The Role of Phosphorylation and
      Dephosphorylation
• Protein      phosphorylation        and
  dephosphorylation are key players that
  control the activities of regulators in
  the interphase cycle and at mitosis

• Activation  of M phase kinase is the
  event that triggers onset of M phase

• Itsinactivation is necessary to exit M
  phase
The Role of Phosphorylation and
      Dephosphorylation
•A     series of kinases and phosphatases
    control the activities of the M phase
    kinase and other regulators related to it
•   Multiple      phosphorylation        and
    dephosphorylation events occur on
    both CDK and cyclin subunits
•   In CDK1 (p34CDC2), phosphorylation
    regulates its activity both negatively
    and positively, depending on the
    phosphorylation sites
                                                      G2/M




                                        A
                                        B




The activity of M phase kinase is regulated by phosphorylation,
dephosphorylation, and protein proteolysis. The 3 phosphorylated amino acids
are: Thr-14, Tyr-15, Thr-161. The first two are in the ATP-binding site.
The Role of Phosphorylation and
      Dephosphorylation
• Conserved    phosphorytable residues
  corresponding to those identified in
  p34CDC2 are present in other CDKs

• The  level of CDK proteins remains
  constant throughout the cell cycle

• It is the activity that is regulated
  throughout the cell cycle
Major regulatory phosphorylation
         sites in p34CDC2
                         Kinases
                         Phosphatases




                                   = CDK1
The Role of Phosphorylation and
      Dephosphorylation
• For M phase kinase activation, phosphate
 groups must be absent at some positions,
 but present at another position

• The phosphorylation of threonine 14 and
 tyrosine 15 (two-neighboring residues in
 the ATP-binding site) causes the
 CDC2/cyclin B complex to be inactive
 until the G2/M transition
The Role of Phosphorylation and
      Dephosphorylation
• Another  phosphorylation occurs on
 threonine 161 (phosphorylated by CDK-
 Activating Kinase "CAK")

• Thisphosphate group is added in G2
 and is essential for the kinase activity

• The phosphate group on threonine 161
 is subsequently removed at the end of
 mitosis
The Role of Phosphorylation and
      Dephosphorylation
• Inactivation
             of mitosis is achieved by
 the physical destruction of cyclins A
 and B and dephosphorylation of
 Threonine-161

• Cyclins are degraded by a common
 proteolytic system, the proteasome
 (complex      containing  proteolytic
 enzymes) that recognizes its targets
 when ubiquitin (76 aa polypeptide) is
 added to cyclins
Progress through
mitosis requires
destruction of
cyclins and other
targets.
    Cyclin-Dependent Kinase
       Inhibitors (CDKIs)
 Inhibitors of CDKs and CDK/cyclin
 complex are powerful regulators of
 G0/G1 and G1/S transitions
 Functions of CDKIs:
Mediate cell cycle arrest in response to
 antimitogenic factors
Ensure that particular cell cycle events
 do not initiate before others are
 completed
       Cyclin-Dependent Kinase
          Inhibitors (CDKIs)
The following represents some of the
major mammalian CDKIs
Inhibitor                      Target CDK
Cip/Kip Family
(CDK Interacting Proteins/
 Kinase Inhibitory Proteins)
p21, p27, p57                  Multiple CDKs


INK 4 Family
(Inhibitors of CDK4)

p15, p16, p18, p19             CDK4/6
                    p21
• Upregulated in senescent cells
• Induced by the tumor suppressor gene p53
• Inhibits multiple CDKs and CDK/Cyclin
 complexes (universal CDK inhibitor 2, 4, 6)

• Interacts    with the DNA polymerase 
 subunit      resulting in DNA synthesis
 inhibition
                     p27
• Structurally related to p21
• Mediates growth-inhibition by Transforming
 Growth Factor  (TGF)

• Mediates    contact-induced   inhibition   of
 proliferation

• A universal CDK inhibitor (CDK 2, 4, 6)
            p16 and p15
• Closely related proteins define a second
 class of CDKIs

• Structurally and functionally distinct from
 p21/p27

• Target  exclusively CDK4/6 and prevent
 their binding to cyclins

• P15 is strongly induced by TGF
           p16 and p15
• p16  and p15 map to human
 chromosome 9p21, a region which is
 frequently deleted   in   hereditary
 melanoma and other human tumors

• Tumor suppressor genes (genes which
 loss of function causes tumor
 formation) code for products that
 interact with CDK/Cyclin complexes or
 with other cell cycle regulatory proteins
    Retinoblastoma Protein
• The product of the tumor suppressor
 retinoblastoma (RB) is a substrate for
 CDK/Cyclin D complexes, and exerts its
 effect during the part of G1 phase that
 precedes the restriction point

• During  the first part of G1 (quiescent
 cells), RB is bound to the transcription
 factor E2F resulting in:
Cyclin A or B + CDK1 (for entry into mitosis)
      A block to the cell cycle
      is released when RB is
      phosphorylated by cdk-
      cyclin.



CDK2
+
Cyclin E
    Retinoblastoma Protein
Some S phase products whose genes are
 dependent on E2F are not active, so S
 phase can not start
E2F-RB complex represses other genes
At the restriction point, RB is
 phosphorylated by CDK4,6-Cyclin D
The phosphorylation causes RB to
 release E2F
Dephosphorylated RB again binds E2F
 and blocks G1/S transition
    Retinoblastoma Protein
• RB is also a substrate for CDK2-Cyclin
 E, and therefore affects G1/S transition

 Retinoblastoma is a target for several
 pathways that inhibit growth, and may
 be the mean by which growth inhibitory
 signals such as TGF maintains cells in
 G0 or G1
       Retinoblastoma Protein

• Severalof these inhibitory signals act
 through CDKIs

• In turn, these CDKIs are found in
 inactive complexes bound to CDK-
 cyclin complexes
p16 binds to cdk4
and cdk6 and to
cdk4,6-cyclin
dimers.
By inhibiting cdk-
cyclin D activity,
p16 prevents
phosphorylation of
RB and keeps E2F
sequestered so that
it is unable to
initiate S phase.
    Stages of CDKIs’ Function
• By    binding to the CDK subunits, p16
    inhibits assembly of the CDK4,6-
    CyclinD complexes
•   p21 and p27 are universal CDK
    inhibitors, binding to all complexes of
    CDK2, 4, 6
•   The way p21 and p27 inhibit the cell
    cycle progression is not entirely clear,
    but it does not only depend on
    controlling RB, because they can
    inhibit proliferation in cells lacking RB
Importance of cell cycle regulatory
proteins in the control of cell cycle

• Their   deregulation   contributes   to
  cancer cell cycles

• Several of these regulatory proteins
  may work as tumor suppressor genes
  and oncogenes
       CANCER CELL CYCLES
• Oncogenic processes exert their greatest
 effect by targeting particular regulators
 of G1 phase progression

• An understanding of restriction point
 control and entry into S phase is central
 to our understanding of how and why
 cancer cells continuously cycle
      CANCER CELL CYCLES
• D-type    cyclins act as growth factor
    sensors,     with    their    expression
    depending more on extracellular cues
    than on the cell’s position in the cell
    cycle
•   Cyclin D1 is overexpressed in many
    human cancers as a result of gene
    amplification     or       translocations
    targeting the D1 locus on human
    chromosome 11q13
    CANCER CELL CYCLES
• Amplification of chromosome 11q13 region
 is frequent in a broad spectrum of common
 adult cancers:

Squamous     cell carcinomas of the head
 and neck (45%)
Esophageal carcinomas (35%)
Bladder and primary breast carcinomas
 (15%)
Small-cell lung tumors and hepatocellular
 carcinomas (10%)
   CANCER CELL CYCLES

• The gene encoding CDK4 is also
 amplified in sarcomas and gliomas

• Mutations that inactivate p16 are
 associated with familial melanoma and
 are frequent in biliary tract and
 esophageal carcinomas
   CANCER CELL CYCLES
• Homozygous  deletions of the 9p
 chromosomal area containing the p16
 gene are common:

Gliomas (55%)
Nasopharyngeal carcinomas (40%)
Acute lymphocytic leukemias (30%)
Sarcomas, bladder and ovarian tumors
      CANCER CELL CYCLES
• P15 is also included in the deletion but
    no mutations have been reported yet in
    this protein
•   Reduction in p27 levels in a subset of
    colon and breast cancers correlates
    with poor prognosis
•   P53, followed by RB and p16 pathways,
    are the most frequently disrupted in
    human cancers
•   As yet, no mutations in p21 have been
    implicated in human tumors
    CANCER CELL CYCLES

   In the future, a better understanding of
the cell cycle control machinery and its
deregulation during oncogenesis may
provide novel opportunities for the
diagnostic and therapeutic management
of cancer and other proliferation-related
diseases
THE END

				
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posted:8/28/2012
language:English
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