Mitochondria, the energy-producing organelles of the cell, play a central role in metabolism but also generate reactive oxygen species (ROS) as a byproduct of respiration. Excessive ROS can damage cellular structures and initiate lipid peroxidation, which may ultimately trigger ferroptosis, a form of regulated cell death dependent on iron. Cells possess several antioxidant defense systems to repair oxidized membrane lipids and prevent this lethal process. Recent research from scientists at the University of Texas MD Anderson Cancer Center has identified a mitochondrial enzyme, dihydroorotate dehydrogenase (DHODH), that helps protect mitochondrial membranes from oxidative damage by regenerating the antioxidant ubiquinol. Published in Nature in 2021, the study reveals that DHODH functions as a mitochondrial defense system against ferroptosis, particularly in rapidly proliferating cancer cells. Importantly, inhibition of DHODH can trigger ferroptosis in certain tumor cells and impair tumor growth, suggesting a promising therapeutic vulnerability. These findings provide new insights into mitochondrial antioxidant mechanisms and highlight DHODH as a potential target for novel anticancer therapies.
A Mitochondrial Defense System Against Lipid Peroxidation
As organelles that generate energy for our cells, mitochondria are believed to have evolved from formerly free-living, oxygen-dependent microorganisms. However, producing oxygen-dependent energy within organelles surrounded by lipid membranes comes at a cost. This energy-generating process, known as respiration, frequently leads to the production of reactive oxygen species (ROS). The ROS produced can damage cellular structures and impair their function.
For example, in a process known as lipid peroxidation, ROS react with membrane lipids. The abnormal lipid peroxides produced in this process can ultimately trigger an iron-dependent form of regulated cell death called ferroptosis. Cells employ multiple protective and repair systems to counteract the toxic effects of these modified membrane lipids.
In a new study, researchers from the University of Texas MD Anderson Cancer Center in the United States discovered a system that protects mitochondrial lipids from oxidative damage. The findings were published online on May 12, 2021, in the journal Nature, in a paper titled “DHODH-mediated ferroptosis defence is a targetable vulnerability in cancer.”
Mammalian cells rely on three major systems to repair lipid peroxidation. The key proteins supporting these systems are GPX4, FSP1, and DHFR2. Each of these antioxidant repair nodes involves a metabolite molecule that exists in chemically reduced and oxidized states. Among these metabolites, ubiquinone (also known as coenzyme Q10) is a lipid that functions in both mitochondrial and cellular membranes. The reduced form of ubiquinone, called ubiquinol, possesses antioxidant properties and can repair lipid peroxidation.
Cells must continuously replenish ubiquinol to maintain this protective function of membrane lipids. The enzyme FSP1 counteracts ferroptosis by converting ubiquinone into ubiquinol, but the activity of FSP1 is limited to the plasma membrane. This observation raised an important question: do mitochondria use a similar mechanism to generate ubiquinol and thereby repair oxidative damage to mitochondrial membrane lipids?
The authors hypothesized that a system also exists in mitochondria to alleviate lipid peroxidation. Given the close relationship between cellular metabolism and ferroptosis, they focused on metabolites that change during lipid peroxidation in cancer cells. Surprisingly, they observed that peroxidation was associated with substantial changes in the abundance of metabolites in the pathway that synthesizes pyrimidine bases, which are components of DNA and RNA. Based on this observation, they investigated whether a component of this synthesis pathway might participate in preventing ferroptosis.
Most components of this pathway are located in the cytoplasm, but one enzyme called DHODH resides in mitochondria. DHODH catalyzes the conversion of dihydroorotate into orotate through an oxidation reaction that uses ubiquinone, thereby generating ubiquinol. Further experiments by the authors showed that DHODH protects cells from lipid peroxidation by regenerating ubiquinol, enabling ubiquinol-mediated repair of oxidative damage to mitochondrial membrane lipids. Supplementing cells with the final products of the pyrimidine synthesis pathway did not affect lipid peroxidation, indicating that the anti-ferroptotic function of DHODH is independent of its role in pyrimidine synthesis.
Figure: A system that repairs mitochondrial lipids. Image from Nature, 2021, doi:10.1038/d41586-021-01203-8.
These findings establish that DHODH-mediated regulation of ubiquinol production is an effective system dedicated to alleviating lipid peroxidation specifically in mitochondria, with a mechanism similar to that of the FSP1 system. Interestingly, a version of the GPX4 protein has been found in the mitochondria of some cells, and its expression levels vary across different types of cancer. This mitochondrial form of GPX4 is not essential for survival in mice, suggesting that it plays a redundant role. In contrast, DHODH is universally expressed and, due to its function in nucleotide synthesis, plays an important role in cell proliferation. Therefore, rapidly dividing cells, such as cancer cells, may exploit this active pathway as a means to suppress lipid peroxidation.
Indeed, the authors found that when human tumor cells expressing low levels of GPX4 were transplanted into mice treated with a DHODH inhibitor, the resulting loss of DHODH function caused ferroptosis in these cells and impaired tumor growth. This effect was independent of DHODH’s role in pyrimidine synthesis. Whether blocking this mitochondrial antioxidant system that prevents ferroptosis contributes to cancer cell metastasis or influences tumor responses to radiotherapy remains to be determined. In these processes, inducing ferroptosis may help halt tumor progression. Potent DHODH inhibitors are currently being developed as anticancer drugs and are undergoing clinical trials. They may be particularly effective against cancer cells with low GPX4 expression.
Implications for Cellular Antioxidant Systems
The authors discovered a system dedicated to protecting mitochondrial membranes, suggesting that other subcellular compartments may also possess specialized mechanisms to counteract lipid peroxidation. Interestingly, squalene, an intermediate molecule in the biosynthetic pathway that produces cholesterol, can protect cancers known as lymphomas from lipid peroxidation. High concentrations of squalene have been found in lipid droplets. However, it is currently unclear whether the protective function of squalene arises from its specific localization within these droplets.
Organelles such as the endoplasmic reticulum and peroxisomes also undergo reactions that generate ROS. Glutathione and tetrahydrobiopterin are molecules with redox activity (they can alter the oxidation state of other molecules) and may therefore provide alternative means of protection against lipid peroxidation in these contexts. However, the precise mechanisms and components that enable these molecules to be transported into organelles remain poorly understood. Advances in techniques for assessing the molecular and protein composition of organelles—such as metabolomics and proteomics—should provide new insights into this fundamental question and improve our understanding of the role of antioxidants in tumor progression.