Oxidative Stress Signaling

Oxidative stress signaling describes how cells sense and respond to reactive oxygen species (ROS). At low, controlled levels ROS act as signaling messengers that reversibly oxidize specific protein cysteines; when ROS production outpaces antioxidant defences, the resulting oxidative stress damages lipids, proteins, and DNA and engages protective transcriptional programs, most prominently the KEAP1-Nrf2 antioxidant response.

Definition

Oxidative stress signaling is the redox-based signal transduction in which reactive oxygen species reversibly oxidize sensor cysteines to transmit signals, and in which an imbalance favouring oxidants over antioxidants activates protective programs — chiefly the KEAP1-Nrf2 pathway — to restore redox homeostasis or, if overwhelmed, contributes to macromolecular damage.

Scope

This entry covers the dual role of ROS as both signaling molecules and damaging agents, the redox-sensitive cysteine chemistry that underlies redox signaling, and the KEAP1-Nrf2 antioxidant response that defends against oxidative stress. It is a mechanistic reference within cellular stress response signaling and does not provide clinical guidance.

Core questions

• How can reactive oxygen species act as specific signals rather than only as indiscriminate damage?
• What distinguishes physiological redox signaling from pathological oxidative stress?
• How does the KEAP1-Nrf2 system sense oxidants and mount an antioxidant transcriptional response?

Key concepts

• Reactive oxygen species (ROS)
• Redox homeostasis and antioxidant defence
• Reversible cysteine oxidation
• KEAP1-Nrf2-ARE pathway
• Hydrogen peroxide as signaling molecule
• Mitochondrial ROS
• Oxidative damage to macromolecules

Key theories

Redox signaling via reversible cysteine oxidation

The view that hydrogen peroxide and related oxidants transmit specific signals by reversibly oxidizing reactive cysteine residues on target proteins, altering their activity, so that ROS function as second messengers within a controlled physiological range.

KEAP1-Nrf2 antioxidant response

The model in which oxidant or electrophile modification of sensor cysteines on KEAP1 releases the transcription factor Nrf2 from degradation, allowing it to activate cytoprotective antioxidant and detoxification genes.

Mechanisms

Reactive oxygen species, generated largely by mitochondrial respiration and NADPH oxidases, can react with low-pKa cysteine thiols on target proteins, reversibly forming sulfenic acids and disulfides that change protein activity; this is the chemical basis of redox signaling. The principal protective sensor is KEAP1, an adaptor for an E3 ubiquitin ligase that normally targets the transcription factor Nrf2 for degradation. Oxidant or electrophile modification of KEAP1 sensor cysteines impairs this degradation, so Nrf2 accumulates, enters the nucleus, and activates antioxidant-response-element genes encoding antioxidant enzymes and detoxifying proteins. When ROS exceed the buffering capacity of these systems, oxidative damage accumulates in lipids, proteins, and DNA.

Clinical relevance

Oxidative stress is implicated in ageing, neurodegeneration, cardiovascular and metabolic disease, and cancer, where mitochondrial dysfunction or chronic inflammation raises ROS and the antioxidant response is engaged. This entry explains the signaling mechanism to clarify that biology; it is not a basis for individual diagnostic or treatment decisions, and does not endorse antioxidant supplementation.

History

The concept of oxidative stress as an imbalance between oxidants and antioxidants was articulated in the 1980s. Subsequent work recognized that reactive oxygen species are not only damaging by-products but also physiological signaling molecules acting through reversible cysteine oxidation, and the discovery of the KEAP1-Nrf2 system provided the central inducible defence against oxidative and electrophilic stress.

Key figures

• Helmut Sies
• Navdeep S. Chandel
• Masayuki Yamamoto
• John D. Hayes

Seminal works

• schieber-2014
• tebay-2015

Frequently asked questions

Are reactive oxygen species always harmful?

No. At low, regulated levels ROS such as hydrogen peroxide act as signaling molecules by reversibly oxidizing specific protein cysteines; harm arises mainly when ROS production overwhelms antioxidant defences.

How do cells defend against oxidative stress?

A major inducible defence is the KEAP1-Nrf2 pathway: oxidants modify KEAP1, releasing the transcription factor Nrf2 to switch on genes encoding antioxidant and detoxifying enzymes.