Oxidative stress: Mechanism of cell death clarified

Dr Marcus Conrad of the Institute of Clinical Molecular Biology and Tumour Genetics at the Helmholtz Zentrum Muenchen has decrypted the molecular mechanism through which the death of cells is caused by oxidative stress.

This knowledge opens novel perspectives to systematically explore the benefit of targeted therapeutic interventions in the cure of ageing and stress-related degenerative diseases.

Life processes in cells require a reducing environment that needs to be sustained with the help of a large number of antioxidative enzymes. This may sound abstract and incomprehensible, but everyone knows the phenomenon that a piece of cut apple or a piece of cut meat changes colour quickly and deteriorates, because the oxygen in the air produces chemical reactions in the tissues (oxidation of biomolecules).

If the equilibrium in the organism moves towards oxidative processes, then this is known as oxidative stress. Oxidative stress, for instance, is associated with the aging of body cells. Furthermore, a strong accumulation of reactive oxygen species (ROS) along with drops in cellular concentrations of glutathione, (GSH), the major antioxidant produced by the body, is well known as a common cause of acute and chronic degenerative diseases, such as, arteriosclerosis, diabetes, stroke, Alzheimer's and Parkinson's diseases.

'To investigate the molecular function of the cellular reducing agent GSH in the metabolic pathway of cell death triggered by oxidative stress, mice and cells were generated that specifically lack glutathione peroxidase 4 (GPx4), which is emerging as one of the most important GSH dependent enzymes,' explains Marcus Conrad. The induced inactivation of GPx4 caused massive oxidation of lipids and eventually cell death. A similar phenotype could be observed when intracellular GSH was removed from wild-type cells by a chemical inhibitor of GSH biosynthesis.

Interestingly enough, this cell death could be completely prevented by Vitamin E, but not by water-soluble antioxidants. Since the oxidation of fatty acids in this cell death pathway, was of paramount importance, multiple studies were performed to describe, in greater detail, the source and nature of lipid peroxides.

Pharmacological and reverse genetic analyses showed that lipid peroxides in GPx4-depleted cells do not appear by coincidence, but accumulate due to increased activity of a specific enzyme of the arachidonic acid metabolism, the 12/15-lipoxygenase. Activation of apoptosis inducing factor (AIF), evidenced by its relocation from mitochondria to the cell nucleus, was identified as another important event in this signalling cascade.

The fact that oxidative stress is a major inducer of cell death is a well accepted current model. Until now however, the source and nature of the reactive oxygen species has remained obscure, as have questions concerning the way they act. Marcus Conrad: 'So far, it was assumed that oxidative stress is detrimental to cells by unspecific oxidation of many essential biomolecules, such as proteins and lipids. That is why we were amazed to find that in cells lacking either glutathione or glutathione peroxidase 4, a distinctive signalling pathway is engaged, which causes cell death. The data represent the first molecular analyses of a redox-regulated signalling pathway, describing how oxidative stress is recognised in the body and translated into cell death.'

Since this cell death cascade can be interrupted at any single stage with the help of drugs, this pathway harbours promising targets for therapeutic intervention to mitigate the deleterious effects of oxidative stress in complex degenerative human diseases.