1. Introduction to Stress Response

1.1 Concept of Stress

Stress is a condition in which a cell, tissue, or organism experiences a disturbance that challenges its normal physiological or biochemical state.

Stress may arise from external environmental conditions or internal changes. At the cellular level, stress can interfere with protein folding, DNA integrity, membrane stability, energy production, redox balance, and metabolic homeostasis.

Examples include:

High temperature
Low temperature
Oxidative damage
DNA damage
Nutrient deprivation
Oxygen deficiency
Toxic chemicals
Osmotic imbalance
Infection
Endoplasmic reticulum dysfunction

The ability of biological systems to detect and respond to these challenges is essential for survival.

1.2 Definition of Stress Response

The stress response is the coordinated set of molecular, cellular, physiological, and behavioral changes activated when an organism or cell encounters a stressful condition.

At the molecular level, a typical stress response involves:

Stress → Detection → Signal transduction → Gene regulation → Cellular adaptation → Recovery or cell death

1.3 Homeostasis and Stress

Cells normally maintain a relatively stable internal environment called homeostasis.

Stress disturbs this balance. The stress response attempts to restore homeostasis by modifying:

Gene expression
Protein synthesis
Metabolism
Energy production
Antioxidant defenses
DNA repair
Protein degradation
Autophagy
Cell-cycle progression

If the stress is mild or temporary, cells can generally recover. If stress is prolonged or severe, irreversible damage may occur.

1.4 Biological Importance

Stress responses allow organisms to:

Detect harmful changes.
Protect important cellular structures.
Repair damaged molecules.
Remove defective proteins.
Maintain energy balance.
Restore cellular homeostasis.
Adapt to environmental changes.
Prevent damaged cells from harming the organism.

2. Types of Stress

Stress can be classified according to its source and molecular consequences.

2.1 Physical Stress

Physical stress results from changes in physical environmental conditions.

Examples include:

Heat
Cold
Radiation
Mechanical pressure
Excessive osmotic pressure

Physical stress can alter membrane properties, protein stability, and cellular metabolism.

2.2 Chemical Stress

Chemical stress occurs when cells are exposed to harmful chemicals.

Examples include:

Heavy metals
Reactive oxygen species
Drugs
Toxins
Pollutants

Chemical stress may damage proteins, lipids, DNA, and cellular membranes.

2.3 Biological Stress

Biological stress results from living organisms or biological agents.

Examples include:

Bacterial infection
Viral infection
Parasitic infection
Inflammation
Competition for nutrients

Cells activate immune and antimicrobial pathways in response.

2.4 Metabolic Stress

Metabolic stress occurs when cellular energy or nutrient availability becomes insufficient.

Examples include:

Glucose deprivation
Amino-acid deficiency
ATP depletion
Excessive metabolic demand

AMP-activated protein kinase (AMPK) is an important sensor of cellular energy status.

2.5 Oxidative Stress

Oxidative stress develops when production of reactive oxygen species exceeds the ability of antioxidant systems to neutralize them.

ROS production > Antioxidant capacity → Oxidative stress

2.6 Genotoxic Stress

Genotoxic stress results from damage to genetic material.

Major causes include:

UV radiation
Ionizing radiation
Chemical mutagens
ROS
Replication errors

DNA damage activates repair pathways and checkpoint mechanisms.

2.7 ER Stress

Endoplasmic reticulum stress occurs when the ER cannot properly fold or process newly synthesized proteins.

Accumulation of misfolded proteins activates the unfolded protein response (UPR).

3. Stress-Sensing Mechanisms

3.1 Stress Sensors

Cells possess molecular systems capable of detecting disturbances.

Sensors may detect:

Changes in temperature
ROS
ATP levels
Calcium concentration
Protein misfolding
DNA damage
Oxygen concentration
Nutrient availability

3.2 Signal Transduction

After detecting stress, sensors activate signaling pathways.

General mechanism:

Stress → Sensor → Signaling protein → Transcription factor → Gene expression → Cellular response

3.3 Protein Kinases

Protein kinases regulate many stress responses by phosphorylating target proteins.

Important kinase pathways include:

MAPKs
AMPK
ATM
ATR
PERK

Phosphorylation can alter the activity, localization, stability, or interactions of proteins.

3.4 Transcription Factors

Transcription factors change gene expression during stress.

Important examples include:

HSF1
NRF2
HIF
p53
ATF4
FOXO

These factors activate genes involved in survival, repair, metabolism, and adaptation.

4. Cellular Stress Response

4.1 General Cellular Response

When a cell experiences stress, it does not respond randomly. Instead, it activates coordinated protective mechanisms.

Major responses include:

Reduction of unnecessary protein synthesis
Activation of protective genes
Increased antioxidant activity
DNA repair
Protein refolding
Protein degradation
Autophagy
Metabolic adjustment
Cell-cycle arrest

4.2 Stress-Induced Gene Expression

Stress can rapidly alter transcription.

Some genes become strongly activated because their products protect the cell.

These include genes encoding:

Heat shock proteins
Antioxidant enzymes
DNA repair proteins
Chaperones
Detoxification enzymes

4.3 Translational Control

During severe stress, cells often reduce general protein synthesis.

This conserves:

ATP
Amino acids
Ribosomes
Cellular resources

At the same time, selective translation of stress-response proteins may continue.

4.4 Cellular Adaptation

If stress persists but remains manageable, cells adapt by changing their metabolism and gene expression.

This adaptation can improve resistance to subsequent stress.

5. Heat Shock Response

5.1 Definition

The heat shock response is a cellular defense mechanism activated primarily by elevated temperature and other conditions that cause protein misfolding.

Heat stress can cause:

Protein unfolding
Protein aggregation
Membrane alterations
Metabolic disturbances

5.2 Heat Shock Factor 1

Heat Shock Factor 1 (HSF1) is a major transcriptional regulator of the heat shock response.

Under normal conditions, HSF1 is maintained in an inactive state.

Stress promotes HSF1 activation.

5.3 Mechanism

Heat stress → Protein misfolding → HSF1 activation → HSF1 binds heat shock elements → HSP gene transcription → Increased molecular chaperones

5.4 Heat Shock Proteins

Heat shock proteins are molecular chaperones that help maintain protein homeostasis.

Major groups include:

HSP70
HSP90
HSP60
Small HSPs

5.5 Functions of Heat Shock Proteins

HSPs can:

Prevent protein aggregation
Assist protein folding
Refold damaged proteins
Transport proteins
Direct severely damaged proteins toward degradation

6. Heat Shock Proteins

6.1 HSP70

HSP70 is one of the best-characterized molecular chaperones.

It binds exposed hydrophobic regions of unfolded proteins and helps prevent inappropriate aggregation.

6.2 HSP90

HSP90 assists in the folding and stabilization of many signaling proteins.

Its clients include several:

Protein kinases
Steroid receptors
Regulatory proteins

6.3 HSP60

HSP60 belongs to the chaperonin family.

It assists protein folding within a specialized cellular environment.

6.4 Small Heat Shock Proteins

Small HSPs can bind partially unfolded proteins and prevent their aggregation.

They are particularly important during cellular stress.

7. Oxidative Stress Response

7.1 Reactive Oxygen Species

Reactive oxygen species are chemically reactive molecules derived from oxygen.

Examples include:

Superoxide
Hydrogen peroxide
Hydroxyl radical

ROS can be produced during normal metabolism, particularly in mitochondria.

7.2 Oxidative Stress

Oxidative stress occurs when ROS generation exceeds antioxidant defenses.

Excess ROS → Molecular damage → Cellular stress

7.3 Molecular Damage

ROS can damage:

Proteins

Oxidation can modify amino-acid residues and alter protein structure.

Lipids

Lipid peroxidation damages cellular membranes.

DNA

ROS can produce modified bases and DNA strand damage.

7.4 Antioxidant Defense

Cells contain enzymatic and non-enzymatic antioxidants.

Important enzymes include:

Superoxide dismutase
Catalase
Glutathione peroxidase
Glutathione reductase

7.5 NRF2 Pathway

NRF2 is a major transcription factor controlling antioxidant and detoxification genes.

Under oxidative stress:

Oxidative stress → NRF2 stabilization → Nuclear localization → Antioxidant gene expression → Increased cellular protection

8. DNA Damage Response

8.1 Definition

The DNA damage response is a network of mechanisms that detect DNA damage, stop cell-cycle progression, repair DNA, or eliminate severely damaged cells.

8.2 Causes of DNA Damage

DNA damage can result from:

UV radiation
Ionizing radiation
ROS
Chemical agents
Replication errors

8.3 DNA Damage Sensors

Two important protein kinases are:

ATM is particularly important in responding to DNA double-strand breaks, whereas ATR has a major role in replication stress and certain forms of DNA damage.

8.4 Cell-Cycle Checkpoints

DNA damage can activate checkpoints that temporarily stop cell-cycle progression.

This provides time for DNA repair.

8.5 p53

p53 is an important regulator of the cellular response to DNA damage.

Depending on the extent of damage, p53 can promote:

Cell-cycle arrest
DNA repair
Senescence
Apoptosis

8.6 DNA Repair

Major DNA repair mechanisms include:

Base excision repair
Nucleotide excision repair
Mismatch repair
Homologous recombination
Non-homologous end joining

9. Unfolded Protein Response

9.1 Protein Folding Stress

Proteins must fold into correct three-dimensional structures to function properly.

Environmental or cellular disturbances can cause proteins to misfold.

9.2 ER Stress

The endoplasmic reticulum is an important site of protein folding.

Accumulation of misfolded proteins creates ER stress.

9.3 Purpose of UPR

The unfolded protein response attempts to restore protein homeostasis by:

Reducing protein synthesis.
Increasing protein-folding capacity.
Enhancing degradation of misfolded proteins.

9.4 Major UPR Sensors

Three major ER stress sensors are:

IRE1
PERK
ATF6

9.5 IRE1 Pathway

IRE1 activates signaling that promotes expression of genes involved in protein folding and ER adaptation.

One important downstream factor is XBP1.

9.6 PERK Pathway

PERK phosphorylates eIF2α.

This reduces general protein translation while allowing selective translation of certain stress-response proteins, including ATF4.

9.7 ATF6 Pathway

ATF6 moves from the ER toward the Golgi under ER stress, where it is processed and contributes to activation of stress-response genes.

9.8 UPR and Cell Death

If ER stress cannot be resolved, prolonged UPR signaling can contribute to apoptosis and other forms of cell injury.

10. Hypoxia Response

10.1 Definition

Hypoxia is a condition in which oxygen availability is insufficient for cellular requirements.

10.2 HIF Proteins

Hypoxia-inducible factors are major regulators of the cellular response to low oxygen.

HIF signaling changes expression of genes involved in:

Glycolysis
Angiogenesis
Oxygen transport
Metabolism
Cell survival

10.3 HIF-1α Regulation

Under normal oxygen conditions, HIF-1α is continuously targeted for degradation.

During hypoxia, this degradation is reduced, allowing HIF-1α to accumulate.

10.4 Hypoxia Response Mechanism

Low oxygen → HIF stabilization → Nuclear localization → DNA binding → Hypoxia-responsive gene expression

10.5 Biological Importance

Hypoxia responses help cells adapt to limited oxygen by modifying energy metabolism and promoting mechanisms that improve oxygen delivery.

11. Osmotic Stress Response

11.1 Definition

Osmotic stress occurs when changes in external solute concentration disturb cellular water balance.

11.2 Hyperosmotic Stress

When the external environment has higher solute concentration, water tends to leave the cell.

This may cause:

Cell shrinkage
Increased ionic concentration
Protein stress

11.3 Hypoosmotic Stress

When the external environment has lower solute concentration, water enters cells.

Excessive water entry may cause swelling and, in some cells, membrane rupture.

11.4 Cellular Adaptation

Cells regulate:

Ion transport
Water movement
Compatible solutes
Gene expression

to restore volume and osmotic balance.

12. Nutrient and Metabolic Stress

12.1 Nutrient Stress

Cells require nutrients for:

ATP production
Protein synthesis
DNA synthesis
Membrane synthesis
Growth

Nutrient deprivation therefore activates adaptive pathways.

12.2 Energy Stress

When ATP levels decrease and AMP/ADP levels increase, cells activate energy-sensing pathways.

12.3 AMPK

AMPK is a major cellular energy sensor.

AMPK activation promotes energy-producing pathways while reducing energy-consuming processes.

12.4 Metabolic Adaptation

During energy stress, cells may:

Increase glucose uptake
Increase fatty-acid oxidation
Reduce protein synthesis
Inhibit unnecessary biosynthetic pathways
Activate autophagy

13. Antioxidant Defense Mechanisms

13.1 Enzymatic Antioxidants

Important antioxidant enzymes include:

Superoxide Dismutase

Converts superoxide into hydrogen peroxide and oxygen.

Catalase

Converts hydrogen peroxide into water and oxygen.

Glutathione Peroxidase

Reduces hydrogen peroxide and lipid peroxides using reducing equivalents.

13.2 Glutathione System

Glutathione exists mainly in reduced and oxidized forms.

Reduced glutathione helps maintain the cellular redox environment.

13.3 Non-Enzymatic Antioxidants

Examples include:

Glutathione
Vitamin C
Vitamin E
Uric acid

These compounds help limit oxidative damage.

14. Transcription Factors in Stress Response

14.1 HSF1

Regulates heat shock protein expression.

14.2 NRF2

Controls many antioxidant and detoxification genes.

14.3 HIF

Regulates adaptation to low oxygen.

14.4 p53

Coordinates responses to DNA damage and other cellular stresses.

14.5 ATF4

Important in integrated stress and ER stress responses.

14.6 FOXO

FOXO transcription factors regulate genes involved in:

Stress resistance
Metabolism
Autophagy
Cell survival

15. MAPK Signaling in Stress

15.1 MAPK Pathway

Mitogen-activated protein kinase pathways transmit extracellular and intracellular stress signals.

Important stress-associated MAPKs include:

15.2 JNK

JNK can regulate stress responses, inflammation, survival, and apoptosis.

15.3 p38 MAPK

p38 is strongly associated with cellular responses to:

Oxidative stress
Inflammatory signals
Osmotic stress
DNA damage

15.4 ERK

ERK is commonly associated with growth and proliferation but can also participate in stress-dependent responses.

16. p53-Mediated Stress Response

16.1 p53 as a Stress Sensor

p53 is activated by several types of cellular stress, especially DNA damage.

16.2 Cell-Cycle Arrest

p53 can induce expression of p21.

p21 inhibits cyclin-dependent kinases and contributes to cell-cycle arrest.

16.3 DNA Repair

p53 promotes expression of genes that support DNA repair and genome stability.

16.4 Apoptosis

When damage is severe, p53 can activate pro-apoptotic pathways.

Thus:

Stress → p53 activation → Arrest/repair OR apoptosis

17. HPA Axis and Systemic Stress Response

17.1 Overview

In animals, particularly mammals, stress is not limited to individual cells. The nervous and endocrine systems coordinate whole-body responses.

The hypothalamic-pituitary-adrenal (HPA) axis is a major component of systemic stress regulation.

17.2 HPA Axis

The basic pathway is:

Stress → Hypothalamus → CRH → Pituitary → ACTH → Adrenal cortex → Cortisol

17.3 Cortisol

Cortisol is a glucocorticoid hormone that helps regulate:

Energy metabolism
Glucose availability
Immune activity
Cardiovascular function
Adaptation to stress

17.4 Acute Stress

Acute stress produces rapid physiological changes that help an organism respond to an immediate challenge.

17.5 Chronic Stress

Long-term stress can alter endocrine, metabolic, immune, and neurological functions.

18. Cell Survival and Adaptation

18.1 Protective Adaptation

The primary goal of most stress responses is survival.

Cells can increase:

Chaperone production
Antioxidant defenses
DNA repair
Autophagy
Metabolic flexibility

18.2 Cellular Senescence

Persistent stress can cause cells to enter a stable state of growth arrest called cellular senescence.

Senescent cells remain metabolically active but generally stop proliferating.

18.3 Stress Memory

Some cells can retain altered physiological states after an initial stress exposure.

This phenomenon is called stress memory and is particularly important in plants and microorganisms, although stress-induced memory mechanisms also occur in other biological systems.

19. Stress-Induced Apoptosis

19.1 Apoptosis

Apoptosis is a regulated form of cell death.

It can be activated when cellular damage becomes too severe for effective recovery.

19.2 Intrinsic Pathway

Severe stress can damage mitochondria and promote activation of the intrinsic apoptotic pathway.

General sequence:

Stress → Mitochondrial damage → Cytochrome c release → Apoptosome → Caspase-9 → Caspase-3/7 → Apoptosis

19.3 Extrinsic Pathway

Stress-related signaling can also influence death-receptor pathways.

These can activate initiator caspases such as caspase-8.

19.4 Biological Importance

Stress-induced apoptosis prevents severely damaged cells from continuing to function abnormally or transmitting dangerous genetic defects.

20. Stress Response and Autophagy

20.1 Definition

Autophagy is a cellular degradation and recycling process.

It allows cells to remove:

Damaged proteins
Damaged organelles
Protein aggregates

20.2 Autophagy During Stress

Stress can activate autophagy to recycle cellular components and generate useful metabolites.

20.3 Relationship with Survival

Moderate autophagy generally supports adaptation and survival.

However, excessive or dysregulated autophagy can be associated with cellular dysfunction and disease.

20.4 AMPK and mTOR

Two important regulators are:

AMPK
mTOR

Energy stress often activates AMPK, whereas nutrient abundance generally supports mTOR activity.

21. Stress Response in Plants

21.1 Abiotic Stress

Plants experience:

Drought
Salinity
Heat
Cold
Flooding
Oxidative stress

21.2 Biotic Stress

Plants also respond to:

Pathogens
Herbivores
Insects

21.3 Plant Hormones

Important signaling molecules include:

Abscisic acid
Salicylic acid
Jasmonic acid
Ethylene

21.4 Drought Response

During drought, plants increase abscisic acid signaling, which contributes to stomatal closure and water conservation.

21.5 Heat Stress

Plants increase heat shock proteins and other protective mechanisms during elevated temperatures.

22. Stress Response in Microorganisms

22.1 Environmental Stress

Microorganisms experience:

Temperature changes
pH changes
Osmotic stress
Oxidative stress
Nutrient limitation

22.2 Bacterial Stress Responses

Bacteria use regulatory systems to modify gene expression during unfavorable conditions.

22.3 General Stress Response

Some bacteria activate broad stress-response programs that increase resistance to multiple environmental challenges.

22.4 Importance

Stress adaptation allows microorganisms to survive rapidly changing environments and contributes to their ecological success.

23. Stress Response in Animals

23.1 Cellular Response

Animal cells use:

Heat shock proteins
Antioxidant systems
DNA repair
Autophagy
Apoptosis

23.2 Nervous System

The nervous system detects environmental threats and coordinates rapid responses.

23.3 Endocrine Response

Hormones such as adrenaline and cortisol help coordinate systemic responses.

23.4 Immune Response

Stress can influence immune cell activity and inflammatory signaling.

24. Acute and Chronic Stress

24.1 Acute Stress

Acute stress is short-term.

Examples include:

Sudden environmental changes
Temporary nutrient deficiency
Short-term heat exposure

The response is generally rapid and adaptive.

24.2 Chronic Stress

Chronic stress persists for a prolonged period.

Continuous activation of stress pathways can interfere with:

Metabolism
Growth
Immune regulation
Cellular repair
Tissue homeostasis

24.3 Comparison

Feature	Acute Stress	Chronic Stress
Duration	Short	Long
Main purpose	Immediate adaptation	Long-term adaptation
Response	Rapid	Persistent
Recovery	Usually rapid	May be incomplete
Cellular damage	Usually limited	Can accumulate

25. Stress Response and Disease

25.1 Cancer

Cancer cells frequently experience:

Oxidative stress
ER stress
DNA damage
Metabolic stress
Hypoxia

Cancer cells may modify stress responses to survive unfavorable conditions.

25.2 Neurodegenerative Disorders

Accumulation of misfolded proteins, oxidative stress, mitochondrial dysfunction, and impaired protein quality control can contribute to neurodegenerative processes.

25.3 Metabolic Disorders

Altered ER stress, oxidative stress, and metabolic signaling can contribute to metabolic dysfunction.

25.4 Inflammation

Stress pathways can interact with inflammatory signaling networks.

Excessive or prolonged activation may contribute to chronic tissue injury.

26. Integrated Stress Response

26.1 Concept

The integrated stress response (ISR) is a conserved cellular program that coordinates responses to diverse types of stress.

26.2 Major Features

The ISR commonly involves:

eIF2α phosphorylation
Reduced global translation
Selective translation of stress-response proteins
ATF4 activation
Metabolic adaptation

26.3 General Mechanism

Stress → Stress sensor kinase → eIF2α phosphorylation → Reduced general translation → Selective ATF4 translation → Stress-adaptive gene expression

26.4 Biological Importance

The ISR helps cells conserve resources while selectively producing proteins required for adaptation.

27. Molecular Regulation of Stress Response

Stress responses must be carefully regulated.

Important regulatory mechanisms include:

27.1 Phosphorylation

Changes protein activity rapidly.

27.2 Ubiquitination

Marks proteins for degradation or changes their signaling functions.

27.3 Transcriptional Regulation

Changes the production of stress-response proteins.

27.4 Epigenetic Regulation

Stress can influence:

DNA methylation
Histone modifications
Chromatin structure
Non-coding RNAs

27.5 Protein Degradation

Proteasomes and lysosomes remove damaged or unnecessary proteins.

28. Stress Response and Protein Homeostasis

28.1 Proteostasis

Proteostasis refers to maintenance of a functional protein population within cells.

It includes:

Protein synthesis
Protein folding
Protein trafficking
Protein refolding
Protein degradation

28.2 Molecular Chaperones

Chaperones prevent inappropriate protein aggregation and assist correct folding.

28.3 Ubiquitin-Proteasome System

Damaged proteins can be tagged with ubiquitin and directed to the proteasome for degradation.

28.4 Autophagic Degradation

Large protein aggregates and damaged organelles can be removed through autophagy.

29. Stress Response and Cell-Cycle Regulation

Stress can temporarily stop cell division.

29.1 Checkpoint Activation

DNA damage and metabolic stress can activate checkpoints.

29.2 Cell-Cycle Arrest

Arrest gives the cell time to:

Repair DNA
Restore energy
Remove damaged proteins

29.3 Recovery

If stress is successfully resolved, the cell may resume proliferation.

29.4 Irreversible Outcomes

If damage remains severe, the cell may enter:

Senescence
Apoptosis

30. Stress Response and Epigenetic Regulation

30.1 DNA Methylation

Stress can alter DNA methylation patterns and thereby influence gene expression.

30.2 Histone Modification

Changes in histone acetylation and methylation can modify chromatin accessibility.

30.3 Chromatin Remodeling

Stress-responsive transcription factors may require chromatin remodeling to access target genes.

30.4 Non-Coding RNAs

MicroRNAs and other non-coding RNAs can regulate stress-response genes at the post-transcriptional level.

31. Experimental Study of Stress Response

31.1 Gene Expression Analysis

Stress-responsive genes can be studied using:

RT-qPCR
RNA sequencing

31.2 Protein Analysis

Protein responses can be examined using:

Western blotting
Immunofluorescence
ELISA

31.3 ROS Measurement

Fluorescent probes can be used to estimate intracellular ROS.

31.4 Apoptosis Detection

Common methods include:

Annexin V staining
Caspase assays
TUNEL assay
Flow cytometry

31.5 Cell Viability Assays

Stress-induced changes in cell survival can be examined using metabolic or membrane-integrity assays.

32. Major Stress-Response Pathways

Stress	Major Sensor/Regulator	Important Response
Heat stress	HSF1	Heat shock proteins
Oxidative stress	NRF2	Antioxidant defense
DNA damage	ATM/ATR, p53	Repair/checkpoint
ER stress	IRE1, PERK, ATF6	UPR
Hypoxia	HIF	Metabolic adaptation
Energy stress	AMPK	Energy conservation
Nutrient stress	AMPK/mTOR-related pathways	Metabolic adaptation
Osmotic stress	Osmosensors/signaling pathways	Volume regulation
Protein misfolding	Chaperone/UPR systems	Proteostasis

33. Integrated Stress Response Flowchart

Environmental or intracellular stress

↓

Cellular stress detection

↓

Activation of molecular sensors

↓

Signal transduction

↓

Activation of transcription factors

↓

Alteration of gene expression

↓

Protein-quality control + DNA repair + antioxidant defense + metabolic adaptation

↓

Restoration of homeostasis

↓

Cell survival and adaptation

↓

Persistent/severe damage

↓

Senescence or apoptosis

34. Relationship Between Major Stress Pathways

Stress responses do not work independently.

For example:

Oxidative stress → DNA damage → ATM/p53 activation → Cell-cycle arrest

Similarly:

Protein misfolding → ER stress → UPR → Autophagy

And:

Energy depletion → AMPK activation → Reduced anabolic activity + Increased catabolism

Therefore, cellular stress response is best understood as an interconnected regulatory network.

35. Important Stress-Response Molecules

Molecule	Major Function
HSF1	Heat shock response
HSP70	Protein folding/protection
HSP90	Protein stabilization
NRF2	Antioxidant defense
p53	DNA damage response
ATM	DNA damage signaling
ATR	Replication stress/DNA damage
HIF	Hypoxia response
AMPK	Energy sensing
mTOR	Nutrient/growth signaling
PERK	ER stress signaling
IRE1	ER stress signaling
ATF6	ER stress transcriptional response
ATF4	Integrated stress response
JNK	Stress signaling
p38	Stress/inflammatory signaling
FOXO	Stress resistance and metabolism

36. Stress Response: Survival Versus Cell Death

The final outcome of stress depends on several factors:

Stress intensity
Stress duration
Type of stress
Cell type
Cellular energy status
DNA damage
Ability to repair damage
Antioxidant capacity

Mild Stress

Stress → Adaptation → Recovery

Moderate Persistent Stress

Stress → Adaptation → Senescence/dysfunction

Severe Stress

Stress → Irreversible damage → Apoptosis or other forms of cell death