Evasion of Cell Death

Slide 1 · Evasion of Cell Death

Mitochondrial apoptosis, oncogenic survival, and therapeutic vulnerabilities

Cell stress

Apoptotic gate

Cell fate

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Explain how apoptosis limits survival of damaged or stressed cells.

Trace mitochondrial signaling from BH3-only proteins to executioner caspases.

Distinguish p53 loss, BCL2 upregulation, and inhibitor-of-apoptosis activity.

Apply these mechanisms to follicular lymphoma, CLL, and targeted therapy.

Slide 3 · Why evasion of apoptosis matters in cancer

Tumors often acquire lesions that make cells resistant to regulated apoptotic death.

This survival advantage operates even when growth signals are abnormal or DNA is damaged.

The dominant tumor-associated lesions affect the intrinsic, mitochondrial pathway.

Anticancer therapy may select additional apoptosis-resistant subclones.

Tumor burden reflects survival as well as proliferation.

Slide 4 · Population balance: Birth and death

Cell production / proliferation

Cell elimination / apoptosis

−

Net cellular accumulation

Net expansion increases when production rises or apoptotic elimination falls.

Slide 5 · Apoptosis: Orderly cellular dismantling

A tightly regulated death program activates enzymes that degrade cellular proteins and DNA.

The dying cell is partitioned into membrane-bound apoptotic bodies.

Phagocytes remove fragments efficiently, usually without provoking inflammation.

Failure to engage this program preserves cells carrying oncogenic damage.

Apoptosis removes individual cells while limiting collateral injury.

Slide 6 · Morphologic sequence of apoptosis

Cell shrinkage

Chromatin condensation

Membrane blebbing

Apoptotic bodies

Phagocytic clearance

Conceptual sequence: No photomicrograph or diagnostic specimen is depicted.

Slide 7 · Apoptosis is not synonymous with necrosis

Apoptosis

Regulated enzyme cascade

Membranes retained around fragments

Usually rapid phagocytic clearance

Little or no associated inflammation

Necrosis

Cell death with membrane disruption

Cell contents escape into tissue

Degradative digestion and debris

Often elicits local inflammation

A tumor can resist apoptotic signaling yet still contain necrotic regions.

Slide 8 · Why apoptosis is usually noninflammatory

The plasma membrane remains intact during formation of apoptotic bodies.

Phosphatidylserine becomes exposed on the outer membrane leaflet.

Macrophages recognize and engulf dying cells: Efferocytosis.

Rapid engulfment limits release of cellular contents into surrounding tissue.

Morphology reflects an actively regulated clearance process.

Slide 9 · Two initiation pathways converge on caspases

Extrinsic / receptor pathway

Triggered at the cell surface

Death receptors such as FAS

Adaptor FADD; Initiator caspase-8

Can be activated by immune-cell ligands

Intrinsic / mitochondrial pathway

Triggered by intracellular stress

Controlled by BCL2-family balance

Cytochrome c–APAF-1; Caspase-9

Dominant route dysregulated in many cancers

Both pathways activate executioner caspases.

Slide 10 · Extrinsic signaling: A reference pathway

FAS ligand

FAS receptor

FADD adaptor

Caspase-8

Executioner caspases

Relevant contrast: Tumors most often evade the mitochondrial rather than receptor arm.

Slide 11 · How the extrinsic pathway amplifies death signals

Ligation of FAS recruits the adaptor FADD via death-domain interactions.

Recruitment and activation of procaspase-8 initiates the downstream caspase cascade.

Caspase-8 may cleave BID, linking death-receptor signaling to mitochondrial apoptosis.

This cross-talk explains why the two pathways are not fully independent.

Slide 12 · The intrinsic pathway is cancer’s key survival checkpoint

Intracellular injury normally destabilizes mitochondrial outer-membrane integrity.

Permeabilization releases death-initiating proteins into the cytosol.

Cancer cells alter the sensor–inhibitor balance to preserve the membrane.

The result is persistence despite stresses that would normally be lethal.

Mitochondrial outer-membrane permeabilization is a decisive control point.

Slide 13 · DNA damage activates an apoptotic checkpoint

Mutagenic injury threatens genomic integrity and normally activates p53 responses.

With sustained or irreparable damage, pro-apoptotic signaling may be induced.

This removes cells that might otherwise propagate harmful mutations.

Cancers with impaired p53 responses can retain damaged clones.

DNA damage is both a carcinogenic pressure and an apoptotic trigger.

Slide 14 · Growth-factor withdrawal threatens cell survival

Normal survival depends partly on growth factors and microenvironmental inputs.

Loss of survival signals reduces anti-apoptotic protection and favors BH3 activity.

The mitochondrial pathway translates this imbalance into cell death.

Cancer survival programs can uncouple viability from normal external support.

Slide 15 · Hypoxia challenges expanding tumors

Rapid tumor expansion may outstrip oxygen delivery from the local vasculature.

Inadequate oxygenation is one of the intrinsic stresses emphasized.

Persistent hypoxic stress can favor mitochondrial apoptotic signaling.

Surviving tumor cells have acquired or retained anti-apoptotic adaptations.

Slide 16 · Metabolic imbalance adds apoptotic pressure

Dysregulated growth raises metabolic and biosynthetic demands.

Metabolic disturbances can initiate intrinsic-pathway stress responses.

Apoptotic resistance helps proliferating tumor cells tolerate this strain.

This is distinct from metabolic reprogramming itself, discussed in the preceding topic.

Slide 17 · Misfolded proteins can become a death signal

Some tumors accumulate increased quantities of misfolded proteins.

Protein-folding disturbance stresses the endoplasmic reticulum.

Stress-sensitive BH3-only proteins may be activated or upregulated.

An apoptosis-resistant tumor can remain viable under the same conditions.

Slide 18 · Therapy increases pressure on the apoptotic gate

Genotoxic treatment

Cellular / DNA injury

Mitochondrial signaling

Caspases

Tumor-cell loss

A defective death pathway can diminish the cytotoxic effect of treatment.

Slide 19 · The mitochondrial outer membrane acts as a gate

An intact outer membrane retains cytochrome c inside mitochondria.

BAX and BAK mediate stress-dependent membrane permeabilization.

BCL2-family inhibitors oppose this loss of mitochondrial integrity.

Once cytochrome c is released, the downstream caspase cascade can begin.

Cancer cells preferentially maintain the anti-apoptotic state.

Slide 20 · Three functional groups of BCL2-family proteins

Anti-apoptotic

BCL2

BCL-XL

MCL1

Prevent cytochrome c release

Effectors

BAX

BAK

Promote mitochondrial permeabilization

BH3-only sensors

BIM, BAD, BID

PUMA, NOXA

Shift balance toward death

Apoptotic fate depends on functional balance, not one protein in isolation.

Slide 21 · BCL2 proteins preserve mitochondrial integrity

BCL2 and BCL-XL are representative anti-apoptotic family members.

They restrain BAX/BAK-mediated permeabilization of the outer membrane.

Their protective action prevents cytochrome c leakage.

Overexpression moves the threshold for apoptosis upward.

Slide 22 · BAX and BAK: The essential effectors

BH3 pro-death input

BAX / BAK activation

Outer-membrane pores

Cytochrome c release

When the mitochondrial gate opens, the execution program becomes accessible.

Slide 23 · BAX and BAK promote cell death rather than survival

BAX and BAK are pro-apoptotic family members, distinct from BCL2.

Upon activation, they oligomerize at the outer mitochondrial membrane.

This causes membrane permeabilization and efflux of apoptotic mediators.

BCL2/BCL-XL counteract their effect.

Slide 24 · BH3-only proteins report cellular stress

BIM, BAD, BID, and PUMA are examples specified in the cancer chapter.

NOXA is additionally listed in the general apoptosis chapter.

Their activities respond to loss of survival signals or cellular injury.

They favor activation of mitochondrial apoptosis.

Slide 25 · How BH3-only proteins tip the balance

Intracellular stress

BH3-only induction

BCL2 / BCL-XL neutralization

BAX / BAK available

Some BH3-only proteins also contribute directly to effector activation.

Slide 26 · Survival and death reflect competing forces

Survival-favored state

Abundant BCL2 or BCL-XL

BAX / BAK restrained

Mitochondrial membrane intact

Cytochrome c stays compartmentalized

Apoptosis-favored state

BH3-only signaling rises

Anti-apoptotic reserve is overwhelmed

BAX / BAK oligomerize

Cytochrome c enters the cytosol

The BCL2-family network acts as a molecular threshold.

Slide 27 · Cytochrome c links mitochondria to caspases

Outer membrane permeabilized

Cytochrome c exits

Cytosolic APAF-1 binds

Apoptosome forms

Cytochrome c is necessary for respiration inside mitochondria but signals death when released.

Slide 28 · APAF-1 assembles the apoptosome

APAF-1 is the cytosolic adaptor for released cytochrome c.

Cytochrome c binding favors multimeric apoptosome assembly.

The apoptosome recruits initiator procaspase-9.

Caspase-9 activation propagates the apoptotic program.

Slide 29 · Caspase-9 initiates the execution cascade

Cytochrome c + APAF-1

Apoptosome

Caspase-9

Caspase-3 and -6

Cell dismantling

Initiator and executioner caspases have distinct positions in the pathway.

Slide 30 · Executioner caspases dismantle the cell

Executioner caspases cleave many nuclear and cytoplasmic proteins.

Proteolysis activates a DNase by removing its inhibitor.

DNA fragmentation, nuclear breakup, and apoptotic-body formation follow.

Intact fragments are then removed by phagocytes.

Slide 31 · Inhibitor of apoptosis proteins provide another brake

IAPs physiologically suppress inappropriate activation of caspases.

The cancer chapter specifically highlights inhibition of caspase-9.

Some cancers increase IAP expression, adding protection against apoptosis.

This route as less common than p53 loss or BCL2 upregulation.

Slide 32 · Mitochondrial proteins can counter IAPs

Mitochondrial permeabilization

Smac / DIABLO release

IAP neutralization

Caspase activity permitted

An additional regulatory layer acts downstream of the BCL2-family gate.

Slide 33 · The intrinsic death pathway — integrated

BH3-only input

BAX / BAK

Cytochrome c

APAF-1 / caspase-9

Executioners

Interference anywhere along this chain can change how a cell responds to injury.

Slide 34 · What cancer must overcome to stay alive

Repeated DNA injury would normally provoke checkpoint or apoptotic responses.

Hypoxia and metabolic stress intensify during unregulated expansion.

Chemotherapy and radiation impose additional pro-death pressure.

Selected tumor clones evade mitochondrial apoptosis through genomic or expression changes.

Apoptotic evasion is a selected phenotype, not merely absence of injury.

Slide 35 · Major tumor-associated escape routes

Loss of p53 signaling

TP53 mutation

MDM2 overexpression

Reduced PUMA induction

BCL2-family shift

BCL2 upregulation

BAX / BAK function opposed

Mitochondria remain intact

Downstream blockade

Increased IAP activity

Caspase activation inhibited

Less common escape route

Multiple lesions converge on the same phenotype: Resistance to apoptotic death.

Slide 36 · P53 is a stress-responsive tumor suppressor

TP53 encodes p53, which coordinates cell-cycle arrest, repair, senescence, and apoptosis.

DNA injury can stabilize p53 and activate transcriptional responses.

Repairable damage may be followed by recovery and cell-cycle re-entry.

Persistent severe injury can direct the cell toward apoptosis or senescence.

Slide 37 · Functional p53: From DNA damage to cell fate

DNA damage

P53 accumulation

Damage response

Repair or apoptosis

P53 couples detection of injury to a biologically appropriate response.

Slide 38 · PUMA links p53 activity to mitochondria

PUMA is a BH3-only, pro-apoptotic BCL2-family member.

DNA damage can induce PUMA as part of the p53 response.

PUMA shifts mitochondrial regulation toward BAX / BAK action.

This creates an executable death signal downstream of genomic injury.

Slide 39 · The p53–PUMA mitochondrial axis

DNA damage

Functional p53

PUMA upregulation

BAX / BAK activation

Apoptosis

Loss of p53 function interrupts a major stress-to-apoptosis link.

Slide 40 · TP53 loss uncouples damage from apoptosis

DNA injury is present, but p53-dependent transcription is impaired

PUMA is not adequately induced; Damaged cells can survive

The defect affects signaling from stress, not necessarily the stress itself.

Slide 41 · TP53 mutation is a prevalent resistance mechanism

Loss of TP53 function as a major route to apoptotic evasion.

It is common in cancers at diagnosis and more frequent in some relapsed tumors.

A plausible selection advantage is resistance to genotoxic therapy.

This interpretation is mechanistic; Do not infer resistance from mutation alone in every clinical setting.

Slide 42 · MDM2 can suppress otherwise intact p53

MDM2 amplification

More p53 inhibition / turnover

Less PUMA signaling

Survival after injury

Functionally disabling p53 does not always require a TP53 mutation.

Slide 43 · Two routes to deficient p53 signaling

TP53-mutant tumor

Defective p53 gene or protein

P53-dependent apoptotic response compromised

Restoring a defective gene is difficult

May resist DNA-damaging therapy

MDM2-amplified tumor

TP53 may remain genetically intact

Excess MDM2 limits p53 abundance

Blocking MDM2 could restore p53 activity

Therapeutic approach described as under study

Distinguish genetic loss from negative regulation of functional p53.

Slide 44 · Applied mechanism: Surviving DNA damage

Clinical / molecular observation

A malignant clone sustains substantial DNA injury after cytotoxic treatment.

Molecular analysis identifies inactivating TP53 abnormalities.

The clone persists despite cellular damage.

Why may the clone survive?

P53-dependent induction of PUMA is impaired.

The intrinsic pathway is less efficiently activated.

This provides a plausible resistance mechanism—not a patient-specific prediction.

Slide 45 · BCL2 overexpression raises the apoptotic threshold

Abundant anti-apoptotic BCL2 restrains mitochondrial permeabilization.

Cells require stronger BH3-only input to activate BAX / BAK.

Pro-death injury can therefore coexist with continued viability.

The growth advantage can arise primarily from reduced cell loss.

BCL2 acts on the mitochondrial gate rather than initiating proliferation.

Slide 46 · Follicular lymphoma: Translocation to survival

T(14;18)(q32;q21)

BCL2 near IGH locus

BCL2 overexpression

Less B-cell apoptosis

The hallmark biological advantage is survival of transformed B lymphocytes.

Slide 47 · T(14;18) alters BCL2 expression

The translocation involves the immunoglobulin heavy-chain locus and BCL2.

The transcriptionally active IGH regulatory environment increases BCL2 expression.

Excess BCL2 protects lymphocytes from intrinsic apoptotic cues.

The lesion promotes survival rather than directly acting as a growth-factor receptor.

Slide 48 · Why follicular lymphoma can be indolent

A tumor mass reflects the net balance of proliferation and cell death.

In BCL2-overexpressing follicular lymphoma, reduced apoptosis is particularly important.

Accumulation may therefore be gradual rather than driven by explosive proliferation.

This survival-dominant mechanism with typically indolent behavior.

Slow growth does not imply absence of malignant survival signaling.

Slide 49 · Two distinct routes to larger tumor mass

Proliferation-dominant expansion

More cells enter and complete the cell cycle

Division rate rises

Net cell numbers can increase rapidly

Not the primary mechanism emphasized for BCL2 in follicular lymphoma

Survival-dominant accumulation

Cells are eliminated less efficiently

Ordinary divisions yield greater net gain

Tumor persists despite stresses

Central to the follicular lymphoma illustration

Apoptosis resistance is an independent driver of tumor accumulation.

Slide 50 · CLL illustrates post-transcriptional survival control

Increased BCL2 expression in chronic lymphocytic leukemia.

Loss of microRNAs that normally restrain BCL2 levels is a proposed mechanism.

Reduced microRNA-mediated repression permits additional BCL2 production.

This differs from the IGH–BCL2 translocation mechanism in follicular lymphoma.

Slide 51 · Follicular lymphoma versus CLL: BCL2 control

Follicular lymphoma

Canonical example: T(14;18)(q32;q21)

IGH-associated transcriptional drive

Survival advantage for transformed B cells

Clinically often indolent

Chronic lymphocytic leukemia

BCL2 upregulation is emphasized

Loss of restraining microRNAs is implicated

Post-transcriptional regulation is altered

BCL2-directed therapy is a relevant concept

Different molecular defects converge on BCL2-dependent survival.

Slide 52 · Anti-apoptotic signaling is not the same as mitogenic signaling

A mitogenic lesion promotes cell-cycle entry and division.

An anti-apoptotic lesion prevents elimination of stressed or aged cells.

Both may increase tumor-cell abundance but act through different mechanisms.

A tumor commonly combines proliferative drive with survival adaptation.

Slide 53 · BCL2-family changes can limit treatment response

Many cytotoxic therapies induce DNA injury and related apoptotic stress.

An intact apoptotic apparatus converts injury into executioner caspase activation.

Excess BCL2 opposes mitochondrial permeabilization upstream of caspases.

Cancer-cell killing can be reduced even when treatment-induced injury occurs.

Slide 54 · IAP overexpression is an additional escape route

Cancers can increase expression of inhibitors of apoptosis proteins.

IAP activity blocks steps in the caspase-dependent killing program.

This as less frequent than p53 or BCL2 abnormalities.

It is mechanistically distinct from directly maintaining mitochondrial membrane integrity.

Slide 55 · Compare the molecular points of resistance

Lesion

Immediate effect

Downstream consequence

TP53 loss

Less p53-driven PUMA induction

Insufficient pro-death signaling

MDM2 excess

p53 activity suppressed

Reduced stress response

BCL2 excess

BAX / BAK restrained

Cytochrome c remains in mitochondria

IAP excess

Caspase activity suppressed

Execution program obstructed

Different upstream lesions converge on failure of cellular execution.

Slide 56 · Target concept: Relieve MDM2 suppression

MDM2-amplified tumor

MDM2 inhibitor

Functional p53 restored

Apoptotic signaling

MDM2 inhibition as a clinical-trial strategy, not a universal standard.

Slide 57 · Why restoring p53 is therapeutically challenging

Loss-of-function TP53 mutations damage the molecular effector itself.

Reversing a defective tumor suppressor gene is technically difficult.

In contrast, pharmacologic MDM2 inhibition may release an otherwise intact p53 protein.

The distinction determines the mechanistic plausibility of the intervention.

Slide 58 · Target concept: Directly inhibit BCL2

BCL2-dependent tumor

BCL2 inhibition

Pro-death proteins freed

Mitochondrial apoptosis

The source reports potent activity in selected BCL2-overexpressing cancers, including CLL.

Slide 59 · Clinical significance of BCL2-targeted drugs

Pharmacologic blockade can counteract an acquired survival dependency.

Selected cancers with excess BCL2 are particularly responsive.

Chronic lymphocytic leukemia as a prominent example.

Therapeutic implications here are mechanistic; Dosing and eligibility are not addressed.

Slide 60 · Two intervention strategies act at different levels

MDM2-directed strategy

Removes inhibition of p53

Requires potentially functional p53 signaling

Attempts to re-create upstream stress response

Robbins: Agents being tested in trials

BCL2-directed strategy

Removes mitochondrial anti-apoptotic brake

Targets survival dependency downstream of p53

Enables BAX / BAK-mediated permeabilization

Robbins: Clinical activity in selected cancers

Drug target selection should follow the actual molecular defect.

Slide 61 · Selection occurs before and after treatment

Spontaneous DNA damage, metabolic stress, and hypoxia favor apoptosis resistance.

Treatment adds strong stress, especially for the intrinsic pathway.

Cells with defective p53 signaling or excess BCL2 may preferentially survive.

Relapsed disease can therefore be enriched for anti-apoptotic phenotypes.

Slide 62 · Mechanistic case: Follicular lymphoma

Clinical / molecular observation

A B-cell neoplasm exhibits t(14;18)(q32;q21).

BCL2 expression is abnormally increased.

Tumor-cell accumulation is relatively slow.

What is the primary advantage?

IGH-associated dysregulation elevates BCL2.

Intrinsic apoptosis is suppressed.

Reduced cell loss can explain indolent growth.

Slide 63 · Mechanistic case: Relapse after genotoxic therapy

Clinical / molecular observation

A tumor recurs after a DNA-damaging treatment regimen.

Molecular profiling now identifies TP53 loss of function.

The tumor persists despite treatment-induced damage.

Which death signal is reduced?

P53–PUMA signaling may be impaired.

The apoptotic threshold is increased.

Selection is plausible; Causal attribution needs more than a single genomic observation.

Slide 64 · Mechanistic case: BCL2 dysregulation in CLL

Clinical / molecular observation

A chronic B-cell malignancy is associated with increased BCL2.

Regulatory microRNAs that ordinarily repress BCL2 are reduced.

Cellular survival is enhanced.

Where is control lost?

Loss of microRNA repression permits BCL2 elevation.

BAX / BAK activation is opposed.

This is a different regulatory mechanism from t(14;18).

Slide 65 · Integrated resistance map: Points of intervention

DNA damage → p53

BH3-only proteins

BCL2 / BAX–BAK gate

Cytochrome c

Caspases

Failures can arise upstream of mitochondria, at the membrane, or downstream.

Slide 66 · Check understanding: Identify the blocked step

If MDM2 is amplified, ask whether the p53 response can still be induced.

If BCL2 is overexpressed, identify the inhibited mitochondrial effectors.

If IAPs are increased, identify the caspase-stage restriction.

If PUMA induction fails, trace the defect back to p53 signaling.

Slide 67 · Correct four common misconceptions

Incorrect claim

Mechanistic correction

“BCL2 is a mitogen.”

BCL2 chiefly protects cells from mitochondrial apoptosis.

“TP53 mutation is the only way to lose p53.”

MDM2 overexpression can suppress p53 without TP53 mutation.

“Cytochrome c is always a death signal.”

Its release into the cytosol enables apoptosis; within mitochondria it supports respiration.

“All apoptosis uses caspase-9.”

Intrinsic uses caspase-9; extrinsic begins through caspase-8/10.

Slide 68 · Mechanistic synthesis

Intrinsic stress normally activates BH3-only proteins and BAX / BAK.

Outer-membrane permeabilization releases cytochrome c and recruits APAF-1.

Caspase-9 then activates executioner caspases.

Cancer cells commonly evade death through p53 loss or excessive BCL2.

Molecular escape pathways also create selectively targetable vulnerabilities.