Kuliah Cell Cycle

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CELL CYCLE

Dr. JUSUF FANTONI, SpPA, MSc.

PATH

(2)

CELL CYCLE

CELL CYCLE = “ PROGRAM “ FOR CELL GROWTH

AND CELL DIVISION ( PROLIFERATION)

4 BROAD PHASES OF CELL CYCLE :

4 BROAD PHASES OF CELL CYCLE : G1, S, G2, G1, S, G2, AND AND

M M

(3)

G1

Cell Cycle

Eric Eric Niederho Niederho ffer ffer

G0

6-8 h

DNA, RNA,

Protein

3-4 h

RNA, Protein

1 h

Mitosis, Cytokinesis

S

G2

Cyc D’s

CDK4,6

Cyc B/A

CDK1

Cyc A

CDK2

M

Cyc E

CDK2

6-12 h

RNA, Protein

p53 pRb Lamin H1 Abl

(4)

THE

G1

( GAP 1 ) PHASE IS CHARACTERIZED BY

GENE

EXPRESSION AND PROTEIN SYNTHESIS.

REGULATED PRIMARILY BY

EXTRACELLULAR

STIMULI.

THE CELL GROWS AND PRODUCES ALL

THE

NECESSARY PROTEINS FOR DNA

SYNTHESIS.

THE

S PHASE :

THE CELL REPLICATES ITS DNA,

SO IT

NOW HAS 2 COMPLETE SETS OF DNA.

THIS ALLOWS THE CELL TO DIVIDE INTO

(5)

DURING THE

DURING THE G2G2 PHASE , THE CELL UNDERGOES PHASE , THE CELL UNDERGOES

GROWTH AND PROTEIN SYNTHESIS --- GROWTH AND PROTEIN SYNTHESIS --- PRIMING

PRIMING

IT TO BE ABLE TO DIVIDE.IT TO BE ABLE TO DIVIDE.

ONCE THIS IS COMPLETE , THE CELL ONCE THIS IS COMPLETE , THE CELL FINALLY

FINALLY

ENTERS THE FOURTH & FINAL PHASE, M ENTERS THE FOURTH & FINAL PHASE, M PHASE

PHASE

DURING THE

DURING THE M PHASE M PHASE, THE CELL SPLITS APART, THE CELL SPLITS APART

INTO 2 DAUGHTER CELLS.INTO 2 DAUGHTER CELLS.

NOW THE CYCLE HAS BEEN COMPLETED.NOW THE CYCLE HAS BEEN COMPLETED.

WHAT DO THE CELLS DO NOW ?

THERE ARE 2 CHOICES : EITHER……… START

THE CYCLE AGAIN BY ENTERING

THE CYCLE AGAIN BY ENTERING G1G1, OR IT CAN, OR IT CAN BE QUIESCENT BY ENTERING

(6)

THE INHERRENT PROBLEM WITH THIS CYCLE ;

IF IT WASN’T CONTROLLED, THE CELLS WOULD

CONTINUE TO GROW AND DIVIDE…OVER AND

OVER AGAIN, SO THERE ARE A NUMBER OF

PROTEINS THAT REGULATE AND CONTROL THE

CELL CYCLE.

THESE CONTROL MECHANISMS ARE DEFECTIVE

IN MALIGNANT CANCER CELLS.

THE CONTROL MECHANISMS ARE THE

THE CONTROL MECHANISMS ARE THE CHECK- CHECK-POINTS

POINTS..

THERE 2 CHECKPOINTS FOR THE CELL CYCLE :

* AT THE END OF THE

* AT THE END OF THE G1 PHASEG1 PHASE,, * AT THE END OF THE

* AT THE END OF THE G2 PHASEG2 PHASE

WHEN THE DIVIDING CELLS ARE IN CONTACT

WITH OTHER CELLS --- STOP DIVIDING

(7)

IF THE CELLS ARE SURROUNDED BY LOTS OF

IF THE CELLS ARE SURROUNDED BY LOTS OF OTHER OTHER CELLS, IT MAY FAIL TO PASS THE CHECKPOINT

CELLS, IT MAY FAIL TO PASS THE CHECKPOINT

BECAUSE IT COULDN’T DIVIDE IT IS ALREADY

CROWDED.

THE CHECKPOINTS ARE OUR DEFENSE AGAINST

TUMORS.

CONTROL OF THE CELL CYCLE

AMONG THE MAIN PLAYERS ARE :

*

CYCLINS

. THERE ARE 3 GROUPS :

- G1 CYCLINS

- S-PHASE CYCLINS

- M-PHASE CYCLINS

(8)

THEIR LEVELS IN THE CELL RISE AND FALL

WITH THE STAGES OF THE CELL CYCLE.

*

* CYCLIN-DEPENDENT KINASESCYCLIN-DEPENDENT KINASES ( ( CDKs CDKs ).).

HERE ARE 3 GROUPS : - G1 CDKsHERE ARE 3 GROUPS : - G1 CDKs - S-PHASE - S-PHASE CDKs CDKs - M-PHASE - M-PHASE CDKs CDKs

THEIR LEVELS IN THE CELL REMAIN STABILE, THEIR LEVELS IN THE CELL REMAIN STABILE, BUT EACH MUST BEHIND THE APPROPRIATE

BUT EACH MUST BEHIND THE APPROPRIATE

CYCLIN IN ORDER TO BE ACTIVATED.CYCLIN IN ORDER TO BE ACTIVATED. *

* THE ANAPHASE-PROMOTING COMPLEX THE ANAPHASE-PROMOTING COMPLEX ( ( APC APC ))

(9)

Variation in Cell Cycle Cyclins

M

G

₁

S

G

₂

M G

₁

Start

Cell cycle phases

Cyclin-dependent kinases

D

E

A

B(A)

cyclins

(10)

pRb

Cell Cycle Regulation

CDK2 CE

E2F Enzymes for _{DNA synthesis} Passage from _{G1 to S} DNA damage 1. CDK phosphorylation 2. C degradation 3. C & CDK synthesis 4. CDK inhibition pRb P Active p53 p21 pRb

(11)

THE APC :

- TRIGGERS THE EVENTS LEADING

TO

DESTRUCTION OF THE COHESINS DESTRUCTION OF THE COHESINS AND

AND

THUS ALLOWING THE SISTER THUS ALLOWING THE SISTER

TIDS TO SEPARATETIDS TO SEPARATE

- DEGRADES THE MITOTIC (

- DEGRADES THE MITOTIC ( M-

M-PHASE)

PHASE)

CYCLINS.CYCLINS.

CYCLINS & CYCLIN-DEPENDENT KINASES ( CDK )

THE ORDERLY PROGRESSION OF CELLS

THROUGH THE CELL CYCLE IS ORCHESTRATED BY

CYCLINS AND CDKs AND THEIR INHIBITORS.

CDKs DRIVE THE CELL CYCLE BY

PHOSPHORYLATING CRITICAL TARGET PROTEINS.

CDKs ARE EXPRESSED DURING THE CELL

CYCLE IN AN INACTIVE FORM. THEY ARE

ACTIVATED BY PHOSPHORYLATION AFTER

(12)

CYCLINS ARE SYNTHESIZED DURING SPECIFIC PHASES

OF THE CELL CYCLE, AND THEIR FUNCTION IS TO

ACTIVATE THE CDKs.

THE TRANSITION FROM G1 TO S IS AN EXTREMELY

IMPORTANT CHECKPOINT IN THE CELL CYCLE

BECAUSE ONCE CELLS CROSS THIS BARRIER THEY

ARE COMMITED TO PROGRESS INTO S PHASE.

WHEN A CELL RECEIVES GROWTH-PROMOTING

SIGNALS, THE SYNTHESIS OF D TYPE CYCLINS THAT

BIND TO CDK4 AND CDK6 IS STIMULATED IN THE

EARLY PART OF G1.

(13)

PROGRESSION FROM S PHASE INTO G2 PHASE IS

FASCILITATED BY CYCLINS WHICH BINDS TO

CDK2 AND TO CDK1.

EARLY IN THE G2 PHASE, B CYCLIN TAKES

OVER FORMING COMPLEXES WITH CDK1,

WHICH

HELPS THE CELL MOVE FROM G2 TO M.

THE ACTIVITY OF CDKs IS REGULATED BY 2

FAMILIES OF CDK INHIBITORS ( CDKIS ).

ONE FAMILY OF CDKIS, COMPOSED OF

ONE FAMILY OF CDKIS, COMPOSED OF P21, P21, P27,

P27, AND P 57, AND P 57, INHIBITS THE CDKs BROADLY.INHIBITS THE CDKs BROADLY.

THE OTHER FAMILY OF CDKIS HAS

SELECTIVE

EFFECTS ON CYCLIN D / CDK4 AND CYCLIN D

/

(14)

THE FOUR MEMBERS OF THIS FAMILY (

THE FOUR MEMBERS OF THIS FAMILY ( P15, P16,P15, P16, P18, P19 )

P18, P19 ) ARE SOMETIMES CALLED INK4 ARE SOMETIMES CALLED INK4

BECAUSE THEY ARE INHIBITOR OF CDK4 AND

CDK6.

WITH THIS BACKGROUND, IT COULD BE

UNDER-STOOD THAT MUTATIONS THAT DYSREGULATE

STOOD THAT MUTATIONS THAT DYSREGULATE

THE ACTIVITY OF CYCLINS AND CDKs WOULD

FAVOR CELL RPOLIFERATION.

MISHAPS AFFECTING THE EXPRESSION OF

CYCLIN D OR CDK4 SEEM TO BE A COMMON

EVENT IN NEOPLASTIC TRANSFORMATION.

(15)

THE CYCLIN D GENES ARE EXPRESSED IN

MANY CANCERS, INCLUDING THOSE

AFFECTING THE BREAST, ESOPHAGUS AND

LIVER AND IN A SUBSET OF LYMPHOMAS.

AMPLIFICATION OF THE CDK4 GENE

OCCURS IN

MELANOMAS, SARCOMAS AND

GLIOBLASTOMAS.

(16)

p53

PATHWAY

p53

p53 IS THE TUMOR SUPPRESSOR PROTEIN IS THE TUMOR SUPPRESSOR PROTEIN THAT REGULATES THE CELL CYCLE.

THAT REGULATES THE CELL CYCLE.

p

p53 IS THE MOST FREQUENTLY DISRUPTED 53 IS THE MOST FREQUENTLY DISRUPTED

GENE IN CANCER, ILLUSTRATING ITS IMPORTANCE.

P

P53 IS A DNA-BINDING PROTEIN INVOLVED IN 53 IS A DNA-BINDING PROTEIN INVOLVED IN REGULATING THE EXPRESSION OF GENES

REGULATING THE EXPRESSION OF GENES

INVOLVED IN CELL CYCLE ARREST AND

APOPTOSIS.

p

p53 RECOGNIZES WHEN SOMETHING IN THE 53 RECOGNIZES WHEN SOMETHING IN THE CELL HAS GONE WRONG AND EITHER TELLS THE

(17)

p

p53 PROTEIN LEVELS ARE NORMALLY KEPT VERY 53 PROTEIN LEVELS ARE NORMALLY KEPT VERY LOW WITHIN THE CELL; ONCE STIMULATED, THE

LOW WITHIN THE CELL; ONCE STIMULATED, THE

LEVELS ARE RAPIDLY INCREASED ALONG WITH ITS

HALF-LIFE.

THE NEGATIVE REGULATOR OF

THE NEGATIVE REGULATOR OF pp53 IS Mdm2 53 IS Mdm2 WHICH IS ACTUALLY A

WHICH IS ACTUALLY A pp53 RESPONSIVE GENE. 53 RESPONSIVE GENE.

p

p53 ACTIVATED, THEN INCREASED THE Mdm2 53 ACTIVATED, THEN INCREASED THE Mdm2 LEVELS WHICH INACTIVATES

LEVELS WHICH INACTIVATES pp53, TURNING IT OFF.53, TURNING IT OFF.

ONE MECHANISM THAT INHIBITS Mdm2 IS BY

ONCOGENES BY INDUCING THE EXPRESSION OF A

TUMOR SUPPRESOR PROTEIN CALLED p19ARF.

(18)

TWO CDKs ARE ACTIVATED NEAR THE END OF G1

AND S PHASE : CDK2 AND cdc2 ( CDK1 ).

ONCE THE CELL HAS BEGUN TO CYCLE THESE 2

CDKs CAN ALSO INHIBIT

CDKs CAN ALSO INHIBIT pp53.53.

CDK2 AND cdc2 ARE THOUGHT TO KEEP

CDK2 AND cdc2 ARE THOUGHT TO KEEP pp53 IN 53 IN THE CYTOPLASM.

THE CYTOPLASM.

WHEN THE CELL IS NOT IN G1, CDK2 ACTIVITY (

BY BINDING TO CYCLIN E OR CYCLIN A ) IS

INCREASED AT THE END OF G1 WHICH CAN

PHOSPHORYLATE

(19)

CELL DEATH

IN 1972 KERR, WYLLIE AND CURRIE DISTINGUISHED BETWEEN TWO FORMS OF CELL DEATH, NECROSIS AND APOPTOSIS. NECROSIS IS THE FORM OF DEATH WHICH RESULTS FROM CELLULAR METABOLIC COLLAPSE, WHEN A CELL CAN NO LONGER MAINTAIN IONIC HOMEOSTASIS. NECROSIS IS NOT SEEN IN NORMAL DEVELOPMENT, BUT IS INVARIABLY THE RESPONSE TO INURY OR TOXIC DAMAGE.

APOPTOSIS, OR PROGRAMMED CELL DEATH , IS A CENTRAL PART OF NORMAL DEVELOPMENT. IT IS A GENETICALLY MEDIATED FORM OF CELL DEATH, WHICH IS INVOLVED IN ORGANOGENESIS, TISSUE HOMEOSTASIS AND THE EDITING OF THE IMMUNE SYSTEM TO REMOVE AUTOREACTIVE CLONES.

IN TISSUE, APOPTOSIS CHARACTERISTICALLY AFFECTS SCATTERED INDIVIDUAL CELLS IN CONTRAST TO NECROSIS WHICH USUALLY INVOLVES TRACTS OF CONTIGUOUS CELLS. INFLAMMATION IS TYPICLLY ABSENT IN AREAS OF TISSUE UNDERGOING APOPTOSIS.

APOPTOTIC CELLS LOSE CONTACT WITH THEIR NEIGHBORS, DECREASE IN SIZE AND SHOW CONDENSATION OF CHROMATIN. THE CELLULAR DNA IS DEGRADED .

THE CELL MEMBRANE PROTIENS ARE CROSS LINKED MAKING THE MEMBRANE MORE RIGID AND THE APOPTOTIC CELLS ARE TYPICALLY PHAGOCYTOSED BY ADJACENT CELLS OR MACROPHAGES.

UNDER SOME CIRCUMSTANCES, HOWEVER, APOPTOSIS CAN ALSO BE SEEN IN RESPONSE TO A NUMBER OF ABNORMAL STIMULI WHICH INCLUDE HORMONAL OR GROWTH FACTOR MANIPULATIONS, ABERRANT GENE EXPRESSION, PARTICULARLY OF ONCOGENES, OR ANTI-ONCOGENES, AND IN RESPONSE TO A NUMBER OF TOXIC AGENTS INCLUDING

(20)

THEREFORE, APOPTOSIS IS MAINLY THE RESULT OF A PHYSIOLOGICAL PROCESS BY WHICH CELLS ARE ELIMINATED WHEN THEY ARE NO LONGER REQUIRED BY THE BODY.

APOPTOSIS ( LITERAALY “ FALLING OFF” AS OF LEAVES IN AUTUMN ) IS CHARACTERIZED BY SHRINKAGE AND COMPACTION OF THE DYING CELL. IT RAPIDLY BREAKS UP TO FORM “ APOPTOTIC BODIES “ . CELLS APPEAR AS INCONSPICUOUS ROUND OR OVAL EOSINOPHILIC STRUCTURES WITH DENSE CHROMATIN INCLUSIONS.

APOPTOSIS REQUIRES CONTINUING SYNTHESIS OF RNA AND PROTEIN AND A SUPPLY OF ATP, FEATURES SUGGESTING THAT THE PROCESS IS ONE OF THE ACTIVE SELF DESTRUCTION.

REGULATION OF CELL PROLIFERATION AND THE CONTROL OF APOPTOSIS

THERE ARE SEVERAL INSTANCES WHERE CONTROL OF APOPTOSIS SEEMS TO BE LINKED TO SOME OF THE GENES WHICH ALSO REGULATE CELL CYCLE PROGRESSION.

THYMOCYTES UNDERGO APOPTOSIS AT THE G0- TRANSITION IN RESPONSE TO GLUCOCORTICOIDS OR XRAYS.

p53 GENE WHICH IS STRONGLY IMPLICATED IN HUMAN AND ANIMAL CARCINOGENESIS CAN ALSO BE A SIGNIFICANT REGU;ATOR OF THE PROCESS OF APOPTOSIS.

ALTERATIONS IN CELL PROLIFERATIVE ACTIVITY ARE ALSO INTIMATELY LINKED TO THE CONTROL OF APOPTOSIS. THIS LINKAGE HAS BEEN RECOGNIZED BOTH FOR THE TUMOR SUPPRESSOR GENES p53 AND RETINOBLSTOMA AND FOR THE DOMINANT ONCOGENE Myc.

IN VIRUSES WITH ANTI-APOPTOTIC FUNCTIUONS PRESUMABLY EVOLVED TO PERMIT VIRAL REPLICATION, PROMOTE THE UNCONTROLLED CELLULAR

(21)

p53 AND APOPTOSIS

p53 MUTATIONS ARE NOW RECOGNIZED TO BE THE MOST COMMON GENETIC CHANGES IN HUMAN CANCER. P53 IS NOW MORE PROPERLY THOUGHT OF AS A TUMOR SUPPRESSOR GENE.

THE ACTION OF p53 IS COMPLEX. IT BINDS TO MANY IMPORTANT CELLULAR PROTEINS AND IS INVOLVED IN THE CONTROL OF GENE EXPRESSION. NOW MORE FOCUSING ON ITS ROLES IN CEL CYCLE DELAY IN G1 PHASE AND IN APOPTOSIS.

IT IS NOW RECOGNIZED THAT p53 CAN REGULATE BOTH CELL PROLIFERATION AND INDUCES APOPTOSIS DEPENDING ON THE CIRCUMSTANCES AND CELLULAR BACKGROUND.

RECENT STUDIES DEMOSTRATED THAT, IN THE ABSENCE OF RETINOBLASTOMA GENE FUNCTION, THE APOPTOTIC ACTION OF p53 CAN COMPENSATE FOR THIS LOSS AND THUS PREVENT MALIGNANT TRANSFORMATION.

THE Rb PROTEIN PREVENTS CELL CYCLE PROGRESSION AT G1 / S BY INHIBITING MEMBERS OF THE E2F FAMILY OF TRANSRIPTIONAL FACTORS THEREBY INHIBITING THE EXPRESSION OF MANY GENES IMPLICATED IN S PHASE, INCLUDING TK, Myc, DHFR AND DNA POLYMERASE. THUS , THE Rb GENE PRODUCT IN THE DEVELOPING LENS CAUSES THE CELLS TO UNDERGO GROWTH ARREST AND TO TERMINALLY DIFFERENTIATE.

(22)

THE FUNCTIONS OF p53 AND Rb GENE PRODUCTS ARE TIGHTLY LINKED TO CELL CYCLE EVENTS IN THE G1 PHASE OF THE CELL CYCLE. p53 IS KNOWN TO INDUCE G1 ARREST AS WELL AS APOPTOSIS.

XRAYS OR OTHER DNA-DAMAGING AGENTS HAVE BEEN SHOWN TO INCREASE THE LEVELS OF p53 BY A POST-TRANSLATIONAL MECHANISM. INCREASED p53 IS KNOWN TO RESULT IN A G1 ARREST WHICH IS MEDIATED BY INDUCTION OF TWO IDENTIFIED GENES WAF1 / CIP 1, A POTENT INHIBITOR OF SEVERAL CYCLIN DEPENDENT KINASES ( cdks ), PARTICULARLY OF G1 Cdks WHICH ARE ESSENTIAL FOR THE G1/S TRANSITON

Bcl-2 and APOPTOSIS

Bcl-2 WAS DISCOVERED AS A GENE WHOSE EXPRESSION WAS INCREASED BY CHROMOSOMAL TRANSLOCATIONS IN B-CELL MALIGNANCIES. IT IS FOUND TO BE ACTIVATED IN THE M AJORITY OF FOLLICULAR NON-HODGKIN’S LYMPHOMAS AND LESS COMMONLY OIN PROSTATE CANCER.

ITS ACTIVATION HAS ALSO BEEN SEEN IN SOME BENIGN CONDITIONS SUCH AS FOLLICULAR HYPERTROPHY OF LYMPH NODES AND TONSILS.

RECENT YEARS Bcl-2 HAS BEENNFOCUSED ON ITS FUNCTION AS OPPONENT OF APOPTOSIS.

IN MULTIPLE SYSTEMS, LYMPHOCYTES, FIBROBLASTS, NEURONS, HEMATOPOIETIC CELLS ETC, EXPRESSION OF Bcl-2 SHOWN TO ABLE TO DELAY OR EVEN PREVENT APOPTOSIS.