GPx4, Lipid Peroxidation, and Cell Death: Discoveries, Rediscoveries, and Open Issues

SIGNIFICANCE

Iron-dependent lipid peroxidation is a complex oxidative process where phospholipid hydroperoxides (PLOOH) are produced in membranes and finally transformed into a series of decomposition products, some of which are endowed with biological activity. It is specifically prevented by glutathione peroxidase 4 (GPx4), the selenoenzyme that reduces PLOOH by glutathione (GSH). PLOOH is both a product and the major initiator of peroxidative chain reactions, as well as an activator of lipoxygenases. α-Tocopherol both specifically breaks peroxidative chain propagation and inhibits lipoxygenases. Thus, GPx4, GSH, and α-tocopherol are integrated in a concerted anti-peroxidant mechanism. Recent Advances: Ferroptosis has been recently identified as a cell death subroutine that is specifically activated by missing GPx4 activity and inhibited by iron chelation or α-tocopherol supplementation. Ferroptosis induction may underlie spontaneous human diseases, such as major neurodegeneration and neuroinflammation, causing an excessive cell death. The basic mechanism of ferroptosis, therefore, fits the features of activation of lipid peroxidation.

CRITICAL ISSUES

Still lacking are convincing proofs that lipoxygenases are involved in ferroptosis. Also, unknown are the molecules eventually killing cells and the mechanisms underlying the drop of the cellular anti-peroxidant capacity.

FUTURE DIRECTIONS

Molecular events and mechanisms of ferroptosis to be unraveled and validated on animal models are GPx4 inactivation, role of GSH concentration, increased iron availability, and membrane structure and composition. This is expected to drive drug discovery that is aimed at halting cell death in degenerative diseases or boosting it in cancer cells. Antioxid. Redox Signal. 29, 61-74.

Cloning and characterization of PHGPx and its synergistic role with p53 in mediating stress in Penaeus monodon

Phospholipid-hydroperoxide glutathione peroxidase (PHGPx), a ubiquitous antioxidant enzyme in the glutathione peroxidase (GPx) family, plays multiple roles in different organisms. Here, a novel PHGPx (PmPHGPx) was identified from Penaeus monodon. The full-length PmPHGPx cDNA was 1885 bp in length with a 489-bp open reading frame (ORF) containing a selenocysteine codon, TGA^177-179, and a selenocysteine insertion sequence in the 3'-UTR. The typical signature motifs of the GPx family were also detected in the PmPHGPx amino acid sequence. The PmPHGPx expression pattern showed tissue-specific variations, with the highest expression level in the heart and the lowest expression level in the muscle. To examine the relationship between Pmp53 and PmPHGPx, Pmp53 was successfully silenced with a dsRNA-p53 injection, and an obvious down-regulation in PmPHGPx expression was apparent. To clarify the functional roles of Pmp53 and PmPHGPx, their expression patterns were also assessed after pH-induced stress, salinity stress and heavy metal (Cu, Zn, and Cd) challenges. Similar trends in the expression profiles for PmPHGPx and Pmp53 were detected in both the gills and hepatopancreas in response to all stressors. Therefore, we conclude from the results that PmPHGPx acts synergistically and subsequently works cooperatively with Pmp53 toward mediating cell stress. This study improves our understanding of PmPHGPx and its synergistic role with Pmp53 in counteracting stressors in P. monodon.

Glutathione peroxidase 4-catalyzed reduction of lipid hydroperoxides in membranes: The polar head of membrane phospholipids binds the enzyme and addresses the fatty acid hydroperoxide group toward the redox center

GPx4 is a monomeric glutathione peroxidase, unique in reducing the hydroperoxide group (-OOH) of fatty acids esterified in membrane phospholipids. This reaction inhibits lipid peroxidation and accounts for enzyme's vital role. Here we investigated the interaction of GPx4 with membrane phospholipids. A cationic surface near the GPx4 catalytic center interacts with phospholipid polar heads. Accordingly, SPR analysis indicates cardiolipin as the phospholipid with maximal affinity to GPx4. Consistent with the electrostatic nature of the interaction, KCl increases the K_D. Molecular dynamic (MD) simulation shows that a -OOH posed in the core of the membrane as 13 - or 9 -OOH of tetra-linoleoyl cardiolipin or 15 -OOH stearoyl-arachidonoyl-phosphaphatidylcholine moves to the lipid-water interface. Thereby, the -OOH groups are addressed toward the GPx4 redox center. In this pose, however, the catalytic site facing the membrane would be inaccessible to GSH, but the consecutive redox processes facilitate access of GSH, which further primes undocking of the enzyme, because GSH competes for the binding residues implicated in docking. During the final phase of the catalytic cycle, while GSSG is produced, GPx4 is disconnected from the membrane. The observation that GSH depletion in cells induces GPx4 translocation to the membrane, is in agreement with this concept.

p,p'-DDE damages spermatogenesis via phospholipid hydroperoxide glutathione peroxidase depletion and mitochondria apoptosis pathway

One, 1-dichloro-2,2-bis(p-chlorophenyl) ethylene (p,p'-DDE), the major metabolite of 2,2-bis(4-chlorophenyl)-1,1,1-trichloroethane (DDT), is a known persistent organic pollutant (POPs) and male reproductive toxicant. However, the mechanism by which p,p'-DDE exposure causes male reproductive toxicity remains unknown. The objective of this study was to elucidate some mechanisms involved in this process, including the mitochondria apoptosis pathway and the role of phospholipid hydroperoxide glutathione peroxidase (PHGPx). Puberty male SD rats were given different doses of p,p'-DDE (0, 20, 60, 100 mg/kg body weight), after the treatment, the semen quality was evaluated. Western blotting was used to detect the PHGPx protein expression. Furthermore, real-time PCR was used to analyze the genetic expression of PHGPx, Bax, Cytochrom C (Cyt C), Apaf-1, and caspase-3 in the testis. Results indicated that after the exposure, sperm malformation rate showed a significant rise compared with the control group, and meanwhile, the sperm density and sperm motility parameters were reduced to some extent in different treated groups. The mitochondria apoptosis pathway was activated. And remarkably, the expression of PHGPx protein was greatly reduced by the exposure. We conclude that p,p'-DDE can damage spermatogenesis via PHGPx depletion and mitochondria apoptosis pathway.