Cellular Functional, Protective or Damaging Responses Associated with Different Redox Imbalance Intensities: A Comprehensive Review

Reactive species (RS) are produced in aerobic and anaerobic cells at different concentrations and exposure times, which may trigger diverse responses depending on the cellular antioxidant potential and defensive devices. Study searches were carried out using the PubMed database of the National Library of Medicine-National Institutes of Health. Cellular RS include reactive oxygen (ROS), nitrogen (RNS), lipid (RLS) and electrophilic species that determine either cell homeostasis or dysfunctional biomolecules. The complexity of redox signalling is associated with the variety of RS produced, the reactivity of the target biomolecules with RS, the multiplicity of the counteracting processes available, and the exposure time. The continuous distortion in the prooxidant/ antioxidant balance favoring the former is defined as oxidative stress, whose intensity determines (i) the basal not harmful unbalance (oxidative eustress) at RS levels in the pM to nM range that supports physiological processes (e.g., immune function, thyroid function, insulin action) and beneficial responses to external interventions via redox signalling; or (ii) the excessive, toxic distortion (oxidative distress) at RS levels exceeding those in the oxidative eustress zone, leading to the unspecific oxidation of biomolecules and loss of their functions causing cell death with associated pathological states. The cellular redox imbalance is a complex phenomenon whose underlying mechanisms are beginning to be understood, although how RS initiates cell signalling is a matter of debate. Knowledge of this aspect will provide a better understanding of how RS triggers the pathogenesis and progression of the disease and uncover future therapeutic measures.