Cyclins and Cyclin Dependent Kinases

Slide 1 · Cyclins and Cyclin-Dependent Kinases

Cell-cycle control, checkpoints, and neoplastic deregulation

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Describe the sequential cyclin–CDK complexes that govern cell-cycle transitions.

Explain how RB, E2F, CDK inhibitors, and DNA-damage checkpoints regulate G₁/S progression.

Relate cyclin/CDK dysregulation to oncogenesis and selected clinicopathologic examples.

Slide 3 · Why proliferating cells need a cell-cycle engine

Division requires faithful duplication of DNA and coordinated synthesis of cellular constituents.

Each stage depends on completion of the preceding stage.

Cyclin–CDK complexes provide an ordered phosphorylation-based mechanism for progression.

Slide 4 · Cycling versus quiescent cells

Actively cycling

Traverse G₁, S, G₂, and M.

Require phase-specific cyclin–CDK activity.

Produce daughter cells after mitosis.

Quiescent (G₀)

Not actively traversing the cycle.

Some cell populations can re-enter G₁.

Quiescence is distinct from an active DNA-synthesis phase.

Slide 5 · The cell cycle has four main phases

G₁

S

G₂

M

G₁: Growth and preparation

S: DNA synthesis

G₂: Preparation for mitosis

M: Chromosome segregation and division

Slide 6 · G₁ prepares the cell for genome duplication

G₁

S

G₂

M

Follows mitosis or exit from G₀

Integrates extracellular growth cues

Contains the restriction point and G₁/S surveillance

Slide 7 · S phase replicates the DNA

G₁

S

G₂

M

Genome replication proceeds after G₁ commitment

Cyclin A complexes support S-phase events

Replication integrity is essential before division

Slide 8 · G₂ checks readiness for mitosis

G₁

S

G₂

M

Follows DNA replication

G₂/M surveillance checks replication integrity

Mitotic entry depends on cyclin B–CDK1

Slide 9 · M phase partitions duplicated chromosomes

G₁

S

G₂

M

Chromosomes are segregated

Mitosis and cytokinesis yield daughter cells

Cells may enter another G₁ or leave active cycling

Slide 10 · G₀ provides a quiescent starting pool

G₀ denotes cells outside active cycling.

Appropriate stimulation can recruit certain quiescent cells into G₁.

Other differentiated cells have limited or absent proliferative capacity.

G₀: Quiescence

Growth stimulus

G₁ entry

Slide 11 · The restriction point creates a commitment decision

The restriction point occurs within G₁.

After passing it, progression no longer requires the initiating growth signal.

Its principal molecular control involves cyclin D/CDKs, RB, and E2F.

Mitogenic signal

Cyclin D–CDK4/6

RB inhibition

G₁ restriction point

Slide 12 · The two major checkpoint transitions

G₁/S checkpoint

Assesses DNA integrity before DNA synthesis.

Prevents damaged templates from entering replication.

Especially consequential in oncogenesis.

G₂/M checkpoint

Assesses whether DNA replication is complete and accurate.

Restrains entry into mitosis after damage.

Defects may permit division with damaged DNA.

Slide 13 · G₁/S surveillance protects DNA fidelity

DNA damage can activate checkpoint signaling in late G₁.

Checkpoint proteins reduce cyclin–CDK drive.

The resulting delay permits DNA repair before synthesis.

DNA abnormality

Checkpoint signaling

CDK inhibition

G₁/S arrest

Slide 14 · G₂/M surveillance protects chromosome segregation

Cells should not enter mitosis with incompletely or inaccurately replicated DNA.

A G₂/M checkpoint delays mitotic progression when replication integrity is impaired.

Cyclin B–CDK1 normally drives the G₂-to-M transition.

Replicated DNA

G₂/M checkpoint

Cyclin B–CDK1

Mitosis

Slide 15 · DNA damage can pause progression for repair

Checkpoint activation transiently delays the cell cycle.

Repair can restore conditions for controlled progression.

Unrepairable damage can culminate in apoptosis or senescence.

Damage sensed

Cell-cycle arrest

DNA repair

Resume or exit

Slide 16 · Cyclins are dynamic regulatory proteins

Cyclin abundance rises and falls during successive phases.

Synthesis of a cyclin permits activation of its partner CDK.

Cyclin destruction terminates the corresponding burst of kinase activity.

Cyclin synthesis

Cyclin binds CDK

Substrate phosphorylation

Cyclin degradation

Slide 17 · CDKs are the catalytic components

CDKs phosphorylate target proteins when activated.

CDKs are synthesized relatively constitutively, unlike their cycling partners.

Their phase-specific activity is largely imposed by cyclin availability.

CDK protein

Cyclin association

Active kinase

Target phosphorylation

Slide 18 · A cyclin–CDK complex is a functional unit

Cyclin availability alone is insufficient without a partnering CDK.

CDK abundance alone does not imply activity.

The complex drives key transitions through phosphorylation of substrate proteins.

Cyclin

CDK

Active complex

Cell-cycle targets

Slide 19 · Oscillating cyclin levels generate sequential activity

New cyclins appear as earlier cyclins disappear.

Partnered CDK activity rises and declines with cyclin abundance.

This ordered relay limits inappropriate activation of later phases.

Cyclin D

Cyclin E

Cyclin A

Cyclin B

Cyclin sequence is schematic; Exact timing overlaps.

Slide 20 · Cyclin families form an ordered relay

G₁

S

G₂

M

Cyclin D predominates in G₁ control

Cyclin E contributes at G₁/S

Cyclin A acts through S phase

Cyclin B drives G₂/M

Slide 21 · Core cyclin–CDK partnerships

Cyclin

Partner CDK

Main interval / function

D

CDK4, CDK6

G₁ progression; RB phosphorylation

E

CDK2

G₁/S progression; RB phosphorylation

A

CDK2, CDK1

S-phase events

B

CDK1

G₂-to-M transition

Slide 22 · Cyclin D–CDK4 initiates a pivotal G₁ signal

D-type cyclins form active complexes with CDK4.

These complexes phosphorylate RB.

The RB change supports passage through the G₁ restriction point.

Cyclin D + CDK4

RB phosphorylation

Reduced RB restraint

G₁ progression

Slide 23 · Cyclin D–CDK6 provides related G₁ activity

Cyclin D also partners with CDK6.

Cyclin D–CDK6 contributes to RB phosphorylation.

CDK4 and CDK6 are therefore related targets of cell-cycle-directed therapy.

Cyclin D + CDK6

RB phosphorylation

E2F availability

G₁/S progression

Slide 24 · Cyclin E–CDK2 reinforces G₁/S progression

Cyclin E–CDK2 contributes to RB phosphorylation.

Together with cyclin D complexes, it promotes the G₁-to-S switch.

Cyclin D/CDK4 as the major frequently altered oncogenic node.

Cyclin E + CDK2

RB phosphorylation

E2F-dependent genes

S-phase entry

Slide 25 · Cyclin A complexes act in S phase

G₁

S

G₂

M

Cyclin A partners with CDK2 and CDK1

Their activity is associated with DNA-synthesis phase

This role differs from cyclin D–CDK4/6 control of G₁

Slide 26 · Cyclin B–CDK1 controls mitotic entry

Cyclin B partners with CDK1.

The complex is essential for the G₂-to-M transition.

Its action represents a later checkpoint-controlled step than the G₁/S gate.

G₂ completion

Cyclin B + CDK1

Mitotic entry

M phase

Slide 27 · RB is the central G₁/S integrator

Active, hypophosphorylated RB binds E2F transcription factors.

E2F sequestration suppresses genes needed for S phase.

Mitogenic cyclin–CDK activity inhibits RB by phosphorylation.

Hypophosphorylated RB

E2F restrained

S-phase genes off

G₁ arrest

Slide 28 · RB changes state across the G₁/S switch

Active RB

Predominantly hypophosphorylated.

Binds E2F transcription factors.

Restrains G₁/S progression.

Inactive RB

Hyperphosphorylated by cyclin–CDK drive.

E2F factors are released.

S-phase gene transcription increases.

Slide 29 · RB phosphorylation releases E2F

Cyclin D–CDK4/6 and cyclin E–CDK2 phosphorylate RB.

Hyperphosphorylated RB no longer restrains E2F effectively.

E2F drives transcription of genes required for S-phase entry.

Active cyclin–CDKs

RB hyperphosphorylation

E2F release

S-phase genes

Slide 30 · The restriction point converts signals into commitment

Upstream growth signals increase cyclin–CDK activity.

RB integrates these stimulatory and inhibitory influences.

Release of E2F commits cells to a program supporting DNA synthesis.

Growth signals

Cyclin D/E–CDKs

RB–E2F switch

Commitment to S

Slide 31 · Mitogenic signaling pushes the RB switch forward

Growth factor receptors activate intracellular signaling pathways.

These signals increase activity of cyclin–CDK complexes.

Mitogenic signaling can also downregulate CDK inhibitors.

Growth factor

Intracellular signaling

Cyclin–CDK gain

G₁/S progression

Slide 32 · Growth-suppressive signals reinforce the brake

Negative growth cues promote CDK inhibitor expression.

Reduced cyclin–CDK activity preserves hypophosphorylated RB.

RB retains E2F and suppresses S-phase entry.

Growth inhibition

CDK inhibitors

RB remains active

E2F restrained

Slide 33 · Proliferation reflects opposing molecular forces

Pro-proliferative arm

Growth factors induce cyclin–CDK activity.

RB becomes phosphorylated.

E2F-dependent transcription promotes S phase.

Antiproliferative arm

Growth-suppressive signals induce CDK inhibitors.

RB remains hypophosphorylated.

E2F-dependent transcription is restrained.

Slide 34 · CDK inhibitors provide a molecular brake

CDK inhibitors (CDKIs) reduce kinase activity of cyclin–CDK complexes.

CDKI expression is regulated by inhibitory signals and DNA-damage responses.

Their dysfunction can remove checkpoint restraint on abnormal cells.

Slide 35 · The CIP/KIP inhibitor family

Protein

Gene

Principal role

P21

CDKN1A

Broad inhibition of CDK complexes; P53-responsive

P27

CDKN1B

Broad CDK inhibition; Responsive to growth suppression

P57

CDKN1C

Broad inhibitory CDKI family member

Slide 36 · P21 links DNA damage to CDK inhibition

Activation of p53 stimulates p21 expression.

P21 binds and inhibits cyclin–CDK complexes.

This produces G₁/S arrest and allows time for DNA repair.

DNA damage

P53 activation

P21 induction

CDK inhibition

Slide 37 · P27 responds to growth-inhibitory cues

P27 is a CIP/KIP-family CDK inhibitor.

P27 regulation to signals such as TGF-β.

Its inhibitory action counters excessive cyclin–CDK activity.

Growth suppression

P27 expression

CDK complex inhibition

Reduced G₁/S drive

Slide 38 · P57 completes the broad-inhibitor family

P57 is encoded by CDKN1C.

Alongside p21 and p27, it belongs to the CIP/KIP class.

The class broadly restrains CDK complexes rather than selectively targeting only CDK4/6.

Slide 39 · The INK4 inhibitor family

Protein

Gene

Principal target

P15

CDKN2B

Cyclin D–CDK4/6

P16

CDKN2A

Cyclin D–CDK4/6

P18

CDKN2C

Cyclin D–CDK4/6

P19

CDKN2D

Cyclin D–CDK4/6

Slide 40 · P15 and p16 selectively oppose D cyclin complexes

P15 (CDKN2B)

INK4-family CDK inhibitor.

Restrains D cyclin–CDK4/6 activity.

Contributes to negative G₁ regulation.

P16 (CDKN2A)

INK4-family CDK inhibitor.

Blocks CDK4/cyclin D–mediated RB phosphorylation.

Acts as an important tumor suppressor.

Slide 41 · P18 and p19 share the INK4 target preference

P18 (CDKN2C) and p19 (CDKN2D) are members of the INK4 family.

Both predominantly inhibit CDK4/6 complexes associated with cyclin D.

The shared target distinguishes INK4 proteins from broad CIP/KIP inhibitors.

Slide 42 · CIP/KIP and INK4 inhibitors are not interchangeable

Feature

CIP/KIP

INK4

Members

P21, p27, p57

P15, p16, p18, p19

CDK scope

Broad inhibition

Selective CDK4/6 inhibition

Link to cancer

Checkpoint loss

Loss of G₁ restraint / p16 alteration

Representative input

P53 → p21

P16 → CDK4/6

Slide 43 · The p53–p21 arm stops damaged cells

DNA damage sensors can activate p53.

P53 raises the abundance of p21.

P21 restrains cyclin–CDK activity and delays the G₁/S transition.

DNA damage sensing

P53

P21

G₁/S arrest

Slide 44 · Severe damage can terminate proliferation

Checkpoint arrest first gives time to correct DNA abnormalities.

If damage cannot be repaired, cells may undergo apoptosis.

Alternatively, cells may enter a stable nonreplicative senescent state.

Checkpoint arrest

Repair fails

Apoptosis or senescence

Slide 45 · Checkpoint failure can create a mutator phenotype

Defective G₁/S control permits replication despite DNA damage.

Resulting daughter cells may inherit mutations.

This genomic instability can facilitate tumor development and progression.

Faulty G₁/S checkpoint

Damaged DNA copied

Mutated progeny

Cancer evolution

Slide 46 · Two broad routes to inappropriate G₁/S entry

Oncogene activation

Gain of cyclin D or CDK4 function.

Excessive phosphorylation of RB.

Increased G₁/S passage.

Tumor-suppressor loss

Inactivation of CDK inhibitor genes.

Insufficient restraint on cyclin–CDKs.

Loss of checkpoint control.

Slide 47 · The three D-type cyclins have overlapping functions

Cyclin D1, D2, and D3 are the three D cyclin proteins.

The three as functionally interchangeable in the G₁-control setting.

Acquired changes can dysregulate these proteins in human malignancies.

Slide 48 · Cyclin D amplification produces excessive drive

Gene amplification can increase D cyclin abundance.

More D cyclin supports greater CDK4/6 complex activity.

Excessive RB phosphorylation favors G₁/S progression.

Cyclin D gene amplification

D cyclin excess

CDK4/6 drive

RB inhibition

Slide 49 · Cyclin D translocation can deregulate expression

Chromosomal translocations can place D cyclin genes under abnormal regulatory control.

This mechanism is particularly associated with some lymphoid tumors.

The downstream effect is heightened G₁/S progression.

Chromosomal translocation

Cyclin D overexpression

CDK4/6 activation

Cell-cycle entry

Slide 50 · CCND1 rearrangement is central to mantle cell lymphoma

Mantle cell lymphoma typically harbors t(11;14) involving CCND1 and IGH.

The rearrangement elevates cyclin D1 expression.

Excess cyclin D1 promotes progression from G₁ toward S phase.

T(11;14): CCND1 and IGH

Cyclin D1 increase

CDK4/6 activity

G₁/S progression

Slide 51 · Cyclin D1 amplification also occurs in carcinomas

CCND1 amplification in breast and esophageal cancers.

Amplification and translocation are mechanistically distinct genetic alterations.

Both can converge on elevated cyclin D1 activity and altered G₁ control.

Slide 52 · CDK4 amplification increases catalytic capacity

CDK4 itself may be amplified or otherwise activated.

When partnered with D cyclins, excess CDK4 reinforces RB phosphorylation.

This provides an oncogenic route separate from cyclin gene alteration.

CDK4 amplification

Cyclin D–CDK4 increase

RB phosphorylation

G₁/S deregulation

Slide 53 · CDK4 genetic alteration spans several cancers

CDK4 amplification in melanomas and sarcomas.

Aggressive glial tumors are another associated setting.

Tumor type alone does not establish CDK4 amplification; The finding is a molecular alteration.

Slide 54 · Other CDK and cyclin E alterations are less frequent

Alterations involving cyclin E and CDKs other than CDK4 are described in cancer.

That these occur less frequently than major D cyclin/CDK4 events.

This pattern highlights the dominant regulatory importance of the G₁/S gate.

Slide 55 · Loss of inhibitors removes a negative regulator

CDKI genes may undergo inactivating mutation or silencing.

Loss of inhibitory proteins permits stronger cyclin–CDK activity.

The consequence converges on inappropriate G₁/S progression.

CDKI gene loss

Reduced CDK inhibition

RB phosphorylation

G₁/S advance

Slide 56 · CDKN2A produces two distinct tumor suppressors

P16 / INK4a — RB axis

Encoded by the CDKN2A locus.

Inhibits cyclin D–CDK4/6 activity.

Helps maintain active RB and E2F restraint.

P14 / ARF — p53 axis

A separate product of CDKN2A.

Inhibits MDM2 and raises p53 stability.

Supports p53-linked checkpoint activity.

Slide 57 · The p16 arm restrains RB phosphorylation

P16 binds to and inhibits CDK4-associated cyclin D complexes.

Reduced CDK activity helps RB remain hypophosphorylated.

E2F repression is thereby preserved.

P16

CDK4/6 inhibition

Active RB

E2F restrained

Slide 58 · The p14/ARF arm supports the p53 checkpoint

P14/ARF inhibits MDM2 activity.

Lower MDM2-mediated inhibition permits higher p53 levels.

The resulting p53 response can upregulate p21 and enforce arrest.

P14 / ARF

MDM2 inhibition

P53 stabilization

P21 induction

Slide 59 · Four molecular nodes form one vulnerable circuit

Cyclin D and CDK4 promote RB phosphorylation.

P16 normally inhibits the cyclin D–CDK4 axis.

RB opposes E2F-driven S-phase gene expression.

Alteration of any of these nodes can disrupt G₁/S control.

Cyclin D–CDK4/6 increase

RB hyperphosphorylation

E2F release

S-phase transcription

P16 inhibits the first step; Loss of p16 removes this brake.

Slide 60 · Wild-type RB can be functionally inactivated

RB need not carry a mutation to become ineffective.

Excessive D cyclin/CDK4 activity can hyperphosphorylate RB.

Loss of p16 can likewise disable RB-mediated growth restraint.

Excess CDK4/6 or lost p16

RB hyperphosphorylation

E2F release

S-phase entry

Slide 61 · Upstream oncogenic signals can feed the G₁ engine

Growth factor–receptor pathways provide mitogenic input.

Deregulated signaling enhances cyclin–CDK activity.

The downstream result may resemble a direct cell-cycle-gene alteration.

Oncogenic growth signal

Cyclin–CDK activity

RB inhibition

E2F-driven S phase

Slide 62 · Representative cell-cycle alterations in cancer

Molecular alteration

Cancer example

Pathogenic effect

CCND1 translocation

Mantle cell lymphoma

Cyclin D1 overexpression

CCND1 amplification

Breast, esophageal carcinomas

Increased cyclin D drive

CDK4 amplification

Melanoma, sarcoma, glial tumors

Greater CDK4 activity

CDKN2A inactivation

Melanoma and several carcinomas

Loss of p16 restraint

Slide 63 · Application: Cyclin D1 in a B-cell lymphoma

Scenario

A lymphoid tumor is found to have an IGH-associated CCND1 rearrangement.

Question

How does this rearrangement promote proliferation?

Suggested interpretation is provided in presenter notes.

Slide 64 · CDK4/6 inhibition is a therapeutic application

CDK4/6 inhibitors target kinase activity in the cyclin D–CDK4/6 pathway.

Clinical efficacy for certain forms of breast cancer.

Pathway biology supplies the rationale; Individual drug indications require independent current guideline review.

Slide 65 · Molecular mechanism and clinical prediction are different

Mechanistic statement

CDK4/6 activity promotes RB phosphorylation.

Targeting CDK4/6 can suppress this drive.

This is a pathway-level explanation.

Clinical interpretation

Not every tumor with pathway activation responds identically.

Therapy choices depend on disease context.

Do not infer treatment eligibility from a generic schematic.

Slide 66 · Application: Loss of p16 in a carcinoma

Scenario

A carcinoma retains an apparently intact RB gene, but CDKN2A is inactivated.

Question

Why can G₁/S control nevertheless fail?

Suggested interpretation is provided in presenter notes.

Slide 67 · Application: DNA damage before S phase

Scenario

A proliferating cell detects major DNA damage while still in G₁.

Question

Trace the protective response from p53 to the cell-cycle machinery.

Suggested interpretation is provided in presenter notes.

Slide 68 · Normal control versus neoplastic deregulation

Physiologic cycling

Mitogenic input is counterbalanced by inhibitors.

RB restrains E2F until appropriate phosphorylation.

Checkpoints halt damaged cells.

Neoplastic deregulation

Cyclin/CDK activation or CDKI loss biases the switch.

RB restraint can be overcome.

Checkpoint failure favors mutation accumulation.

Slide 69 · Integrated interpretation of the G₁/S pathway

Cyclin abundance controls when partner CDKs can become active.

RB/E2F converts CDK phosphorylation into gene-expression commitment.

CDKI and damage responses oppose inappropriate progression.

Cancer exploits both increased accelerator activity and weakened brakes.

Slide 70 · Three distinctions to retain

Cyclins regulate CDK activity; CDKs are the enzymes that phosphorylate targets.

The restriction point and DNA-damage checkpoints are related but distinct concepts.

RB can be genetically lost or functionally inhibited by upstream alterations.

Slide 71 · Recommended reading

Robbins, Cotran & Kumar: Pathologic Basis of Disease, 11th edition.

Cell-cycle regulation in Chapter 1 and cell-cycle oncogenes in Chapter 7.

Related mechanisms: RB, p53, and CDKN2A tumor-suppressor pathways.