FSP1 is a glutathione-independent ferroptosis suppressor

Ferroptosis is an iron-dependent form of necrotic cell death marked by oxidative damage to phospholipids1,2. To date, ferroptosis has been thought to be controlled only by the phospholipid hydroperoxide-reducing enzyme glutathione peroxidase 4 (GPX4)3,4 and radical-trapping antioxidants5,6. However, elucidation of the factors that underlie the sensitivity of a given cell type to ferroptosis7 is crucial to understand the pathophysiological role of ferroptosis and how it may be exploited for the treatment of cancer. Although metabolic constraints8 and phospholipid composition9,10 contribute to ferroptosis sensitivity, no cell-autonomous mechanisms have been identified that account for the resistance of cells to ferroptosis. Here we used an expression cloning approach to identify genes in human cancer cells that are able to complement the loss of GPX4. We found that the flavoprotein apoptosis-inducing factor mitochondria-associated 2 (AIFM2) is a previously unrecognized anti-ferroptotic gene. AIFM2, which we renamed ferroptosis suppressor protein 1 (FSP1) and which was initially described as a pro-apoptotic gene11, confers protection against ferroptosis elicited by GPX4 deletion. We further demonstrate that the suppression of ferroptosis by FSP1 is mediated by ubiquinone (also known as coenzyme Q10, CoQ10): the reduced form, ubiquinol, traps lipid peroxyl radicals that mediate lipid peroxidation, whereas FSP1 catalyses the regeneration of CoQ10 using NAD(P)H. Pharmacological targeting of FSP1 strongly synergizes with GPX4 inhibitors to trigger ferroptosis in a number of cancer entities. In conclusion, the FSP1-CoQ10-NAD(P)H pathway exists as a stand-alone parallel system, which co-operates with GPX4 and glutathione to suppress phospholipid peroxidation and ferroptosis.

On the Mechanism of Cytoprotection by Ferrostatin-1 and Liproxstatin-1 and the Role of Lipid Peroxidation in Ferroptotic Cell Death

Ferroptosis is a form of regulated necrosis associated with the iron-dependent accumulation of lipid hydroperoxides that may play a key role in the pathogenesis of degenerative diseases in which lipid peroxidation has been implicated. High-throughput screening efforts have identified ferrostatin-1 (Fer-1) and liproxstatin-1 (Lip-1) as potent inhibitors of ferroptosis - an activity that has been ascribed to their ability to slow the accumulation of lipid hydroperoxides. Herein we demonstrate that this activity likely derives from their reactivity as radical-trapping antioxidants (RTAs) rather than their potency as inhibitors of lipoxygenases. Although inhibited autoxidations of styrene revealed that Fer-1 and Lip-1 react roughly 10-fold more slowly with peroxyl radicals than reactions of α-tocopherol (α-TOH), they were significantly more reactive than α-TOH in phosphatidylcholine lipid bilayers - consistent with the greater potency of Fer-1 and Lip-1 relative to α-TOH as inhibitors of ferroptosis. None of Fer-1, Lip-1, and α-TOH inhibited human 15-lipoxygenase-1 (15-LOX-1) overexpressed in HEK-293 cells when assayed at concentrations where they inhibited ferroptosis. These results stand in stark contrast to those obtained with a known 15-LOX-1 inhibitor (PD146176), which was able to inhibit the enzyme at concentrations where it was effective in inhibiting ferroptosis. Given the likelihood that Fer-1 and Lip-1 subvert ferroptosis by inhibiting lipid peroxidation as RTAs, we evaluated the antiferroptotic potential of 1,8-tetrahydronaphthyridinols (hereafter THNs): rationally designed radical-trapping antioxidants of unparalleled reactivity. We show for the first time that the inherent reactivity of the THNs translates to cell culture, where lipophilic THNs were similarly effective to Fer-1 and Lip-1 at subverting ferroptosis induced by either pharmacological or genetic inhibition of the hydroperoxide-detoxifying enzyme Gpx4 in mouse fibroblasts, and glutamate-induced death of mouse hippocampal cells. These results demonstrate that potent RTAs subvert ferroptosis and suggest that lipid peroxidation (autoxidation) may play a central role in the process.

Resolving the Role of Lipoxygenases in the Initiation and Execution of Ferroptosis

Lipoxygenases (LOXs) have been implicated as central players in ferroptosis, a recently characterized cell death modality associated with the accumulation of lipid hydroperoxides: the products of LOX catalysis. To provide insight on their role, human embryonic kidney cells were transfected to overexpress each of the human isoforms associated with disease, 5-LOX, p12-LOX, and 15-LOX-1, which yielded stable cell lines that were demonstrably sensitized to ferroptosis. Interestingly, the cells could be rescued by less than half of a diverse collection of known LOX inhibitors. Furthermore, the cytoprotective compounds were similarly potent in each of the cell lines even though some were clearly isoform-selective LOX inhibitors. The cytoprotective compounds were subsequently demonstrated to be effective radical-trapping antioxidants, which protect lipids from autoxidation, the autocatalytic radical chain reaction that produces lipid hydroperoxides. From these data (and others reported herein), a picture emerges wherein LOX activity may contribute to the cellular pool of lipid hydroperoxides that initiate ferroptosis, but lipid autoxidation drives the cell death process.

Metabolic determinants of cancer cell sensitivity to canonical ferroptosis inducers

Cancer cells rewire their metabolism and rely on endogenous antioxidants to mitigate lethal oxidative damage to lipids. However, the metabolic processes that modulate the response to lipid peroxidation are poorly defined. Using genetic screens, we compared metabolic genes essential for proliferation upon inhibition of cystine uptake or glutathione peroxidase-4 (GPX4). Interestingly, very few genes were commonly required under both conditions, suggesting that cystine limitation and GPX4 inhibition may impair proliferation via distinct mechanisms. Our screens also identify tetrahydrobiopterin (BH4) biosynthesis as an essential metabolic pathway upon GPX4 inhibition. Mechanistically, BH4 is a potent radical-trapping antioxidant that protects lipid membranes from autoxidation, alone and in synergy with vitamin E. Dihydrofolate reductase catalyzes the regeneration of BH4, and its inhibition by methotrexate synergizes with GPX4 inhibition. Altogether, our work identifies the mechanism by which BH4 acts as an endogenous antioxidant and provides a compendium of metabolic modifiers of lipid peroxidation.

A non-canonical vitamin K cycle is a potent ferroptosis suppressor

Ferroptosis, a non-apoptotic form of cell death marked by iron-dependent lipid peroxidation¹, has a key role in organ injury, degenerative disease and vulnerability of therapy-resistant cancers². Although substantial progress has been made in understanding the molecular processes relevant to ferroptosis, additional cell-extrinsic and cell-intrinsic processes that determine cell sensitivity toward ferroptosis remain unknown. Here we show that the fully reduced forms of vitamin K—a group of naphthoquinones that includes menaquinone and phylloquinone³—confer a strong anti-ferroptotic function, in addition to the conventional function linked to blood clotting by acting as a cofactor for γ-glutamyl carboxylase. Ferroptosis suppressor protein 1 (FSP1), a NAD(P)H-ubiquinone reductase and the second mainstay of ferroptosis control after glutathione peroxidase-4^4,5, was found to efficiently reduce vitamin K to its hydroquinone, a potent radical-trapping antioxidant and inhibitor of (phospho)lipid peroxidation. The FSP1-mediated reduction of vitamin K was also responsible for the antidotal effect of vitamin K against warfarin poisoning. It follows that FSP1 is the enzyme mediating warfarin-resistant vitamin K reduction in the canonical vitamin K cycle⁶. The FSP1-dependent non-canonical vitamin K cycle can act to protect cells against detrimental lipid peroxidation and ferroptosis.

Biochemical and lipidomic analyses identify an anti-ferroptotic function of vitamin K and reveal ferroptosis suppressor protein 1 (FSP1) as the enzyme mediating warfarin-resistant vitamin K reduction in the canonical vitamin K cycle.

Control of oxygenation in lipoxygenase and cyclooxygenase catalysis

Lipoxygenases (LOX) and cyclooxygenases (COX) react an achiral polyunsaturated fatty acid with oxygen to form a chiral peroxide product of high regio- and stereochemical purity. Both enzymes employ free radical chemistry reminiscent of hydrocarbon autoxidation but execute efficient control during catalysis to form a specific product over the multitude of isomers found in the nonenzymatic reaction. Exactly how both dioxygenases achieve this positional and stereo control is far from clear. We present four mechanistic models, not mutually exclusive, that could account for the specific reactions of molecular oxygen with a fatty acid in the LOX or COX active site.

Free radical oxidation of polyunsaturated lipids: New mechanistic insights and the development of peroxyl radical clocks

The peroxidation of lipids in biological membranes has been implicated in both the onset and development of most degenerative diseases. The primary products of this autoxidation process are usually lipid hydroperoxides. They form as a consequence of a free radical chain reaction: lipid peroxyl radicals propagate the chain by rate-limiting H-atom abstraction from another lipid. Studies of the mechanism of lipid peroxidation are a specific part of a wider effort to understand the more general phenomenon of hydrocarbon autoxidation, which dates back some 70 years. However, the autoxidation of lipids is generally much more complicated than that of other hydrocarbons because of additional reaction pathways afforded by a variety of uniquely positioned unsaturated bonds. Indeed, polyunsaturation is an important aspect of many of the most relevant of physiological lipids, such as linoleate and arachidonate. In this Account, we present our current understanding of the mechanism of unsaturated lipid peroxidation, effectively updating our Account on the same topic published 25 years ago. Our more recent work has, in large part, been stimulated by the discovery of the nonconjugated linoleate hydroperoxide as a product under certain autoxidation conditions. The identification of this long-elusive bis-allylic hydroperoxide prompted our kinetic characterization of the reaction leading to its formation. The product distributions obtained from autoxidations of newly synthesized model compounds, which vary in either the substitution of the bis-allylic moiety or the configuration of the double bonds, have provided key insights into the overall mechanism. These insights have in turn been reinforced by the results of theoretical calculations. The picture that emerges is one wherein the delocalized carbon-centered radicals, which arise as intermediates in these reactions, first associate with dioxygen to form pre-reaction complexes. These complexes then collapse through transition state structures that maximize the orbital interactions between the delocalized radical SOMO and dioxygen. The energies of these transition states are influenced by steric effects; thus, there are distinct changes in product distribution in the autoxidation of dienes having different substitution patterns. The radical-dioxygen complexes are also intermediates in the isomerization of allylperoxyl and pentadienylperoxyls, helping explain the high regio- and stereochemical fidelity of these processes. We have taken advantage of the rapid fragmentation of nonconjugated peroxyl radicals to develop a powerful peroxyl radical clock methodology, which can be used to determine rate constants for reactions of peroxyl radicals with molecules having rate constants ranging from 1 to 10(7) M(-1) s(-1). We can make use of this methodology to address various questions, both fundamental and applied, relating to lipid peroxidation and its inhibition by radical-trapping antioxidants.

Critical Re-evaluation of the O−H Bond Dissociation Enthalpy in Phenol

The gas-phase O-H bond dissociation enthalpy, BDE, in phenol provides an essential benchmark for calibrating the O-H BDEs of other phenols, data which aids our understanding of the reactivities of phenols, such as their relevant antioxidant activities. In a recent review, the O-H BDE for phenol was presented as 90 +/- 3 kcal mol(-1) (Acc. Chem. Res. 2003, 36, 255-263). Due to the large margin of error, such a parameter cannot be used for dynamic interpretations nor can it be used as an anchor point in the development of more advanced computational models. We have reevaluated the existing experimental gas-phase data (thermolyses and ion chemistry). The large errors and variations in thermodynamic parameters associated with the gas-phase ion chemistry methods produce inconsistent results, but the thermolytic data has afforded a value of 87.0 +/- 0.5 kcal mol(-1). Next, the effect of solvent has been carefully scrutinized in four liquid-phase methods for measuring the O-H BDE in phenol: photoacoustic calorimetry, one-electron potential measurements, an electrochemical cycle, and radical equilibrium electron paramagnetic resonance (REqEPR). The enthalpic effect due to solvation, by, e.g., water, could be rigorously accounted for by means of an empirical model and the difference in hydrogen bond interactions of the solvent with phenol and the phenoxyl radical. For the REqEPR method, a second correction is required since the calibration standard, the O-H BDE in 2,4,6-tri-tert-butylphenol, had to be revised. From the gas-phase thermolysis data and three liquid-phase techniques (excluding the electrochemical cycle method), the present analysis yields a gas-phase BDE of 86.7 +/- 0.7 kcal mol(-1). The O-H BDE was also estimated by state-of-the-art computational approaches (G3, CBS-APNO, and CBS-QB3) providing a range from 86.4 to 87.7 kcal mol(-1). We therefore recommend that in the future, and until further refinement is possible, the gas-phase O-H BDE in phenol should be presented as 86.7 +/- 0.7 kcal mol(-1).

Radicals in natural product synthesis

Free radical intermediates have intrigued chemists since their discovery, and an ever-increasing appreciation for their unique reactivity has resulted in the widespread utilization of these species throughout the field of chemical synthesis. This is most evident from the recent surge in the application of intermolecular radical reactions that feature in complex molecule syntheses. This tutorial review will discuss the diverse methods utilized for radical generation and reactivity to form critical bonds in natural product total synthesis. In particular, stabilized (e.g. benzyl) and persistent (e.g. TEMPO) radicals will be the primary focus.

Dissecting the mechanisms of a class of chemical glycosylation using primary ¹³C kinetic isotope effects

Although arguably the most important reaction in glycoscience, chemical glycosylations are among the least well understood of organic chemical reactions, resulting in an unnecessarily high degree of empiricism and a brake on rational development in this critical area. To address this problem, primary (13)C kinetic isotope effects have now been determined for the formation of β- and α-manno- and glucopyranosides using a natural abundance NMR method. In contrast to the common current assumption, for three of the four cases studied the experimental and computed values are indicative of associative displacement of the intermediate covalent glycosyl trifluoromethanesulfonates. For the formation of the α-mannopyranosides, the experimentally determined KIE differs significantly from that computed for an associative displacement, which is strongly suggestive of a dissociative mechanism that approaches the intermediacy of a glycosyl oxocarbenium ion. The application of analogous experiments to other glycosylation systems should shed further light on their mechanisms and thus assist in the design of better reactions conditions with improved stereoselectivity.

Theoretical calculations of carbon-oxygen bond dissociation enthalpies of peroxyl radicals formed in the autoxidation of lipids

Theoretical calculations were carried out to provide a framework for understanding the free radical oxidation of unsaturated lipids. The carbon[bond]hydrogen bond dissociation enthalpies (BDEs) of organic model compounds and oxidizable lipids (R[bond]H) and the carbon[bond]oxygen bond dissociation enthalpies of peroxyl radical intermediates (R[bond]OO*) have been calculated. The carbon[bond]hydrogen BDEs correlate with the rate constant for propagation of free radical autoxidation, and the carbon[bond]oxygen BDEs of peroxyl radicals correlate with rate constants for beta-fragmentation of these intermediates. Oxygen addition to intermediate carbon radicals apparently occurs preferentially at centers having the highest spin density. The calculated spin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals. Where the C[bond]H BDEs are a function of the extent of conjugation in the parent lipid and the stability of the carbon radical derived therefrom, C[bond]OO* BDEs are also affected by hyperconjugation. This gives way to different rates of beta-fragmentation of peroxyl radicals formed from oxygen addition at different sites along the same delocalized radical. We have also studied by both theory and experiment the propensity for benzylic radicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalent unpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicals in these cases have negative C[bond]OO* BDEs and, thus, have rate constants for beta-fragmentation that exceed the diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.

Bond strengths of toluenes, anilines, and phenols: to hammett or not

The Hammett equation correlates the effects of Y on many different chemical properties of YC(6)H(4)ZX families of compounds. One of the most surprising is that the Z-X bond dissociation enthalpy (BDE), a homolytic property, can be correlated for some 4-YC(6)H(4)ZX families with electrophilic substituent constants, sigma(p)(+)(Y), which were largely derived from the rates of the heterolytic S(N)1 solvolyses of para-substituted cumyl chlorides. Although there is no Hammett correlation of the C-X BDEs in 4-YC(6)H(4)CH(2)X (X = H, halide, OPh) families, there are good correlations of N-X BDEs with sigma(p)(+)(Y) in 4-YC(6)H(4)NHX (X = H, CH(3), OH, F) and excellent correlations of O-X BDEs with sigma(p)(+)(Y) in 4-YC(6)H(4)OX (X = H, CH(3), CH(2)Ph) families. The reasons for this varied behavior are discussed.

The Potency of Diarylamine Radical-Trapping Antioxidants as Inhibitors of Ferroptosis Underscores the Role of Autoxidation in the Mechanism of Cell Death

Two aromatic amines (ferrostatin-1 and liproxstatin-1) were recently identified from high-throughput screening efforts to uncover potent inhibitors of ferroptosis, the necrotic-like cell death induced by inhibition of glutathione peroxidase 4 (GPX4), deletion of the corresponding gpx4 gene, or starvation of GPX4 of its reducing cosubstrate, glutathione (GSH). We have since demonstrated that these two aromatic amines are highly effective radical-trapping antioxidants (RTAs) in lipid bilayers, suggesting that they subvert ferroptosis by inhibiting lipid peroxidation (autoxidation) and, thus, that this process drives the execution of ferroptosis. Herein, we show that diarylamine RTAs used to protect petroleum-derived products from autoxidation can be potent inhibitors of ferroptosis. The diarylamines investigated include representative examples of additives to engine oils, greases and rubber (4,4'-dialkyldiphenylamines), core structures of dyes and pharmaceuticals (phenoxazines and phenothiazines), and aza-analogues of these three classes of compounds that we have recently shown can be modified to achieve much greater reactivity. We find that regardless of how ferroptosis is induced (GPX4 inhibition, gpx4 deletion or GSH depletion), compounds which possess good RTA activity in organic solution (k_inh > 10⁵ M^-1 s^-1) and lipid bilayers (k_inh > 10⁴ M^-1 s^-1) are generally potent inhibitors of ferroptosis (in mouse embryonic fibroblasts). Likewise, structural analogs that do not possess RTA activity are devoid of antiferroptotic activity. These results further support the argument that lipid peroxidation (autoxidation) plays a major role in the mechanism of cell death induced by either GPX4 inhibition, gpx4 deletion, or GSH depletion. Moreover, it offers clear direction that ongoing medicinal chemistry efforts on liproxstatin and ferrostatin derivatives, which have been proposed as lead compounds for the treatment and/or prevention of ischemia/reperfusion injury, renal failure, and neurodegeneration, can be widened to include other aminic RTAs. To aid in these efforts, some relevant structure-reactivity relationships are discussed.

Substituent effects on the bond dissociation enthalpies of aromatic amines

Bond dissociation enthalpy differences, Z-X DeltaBDE = BDE(4-YC(6)H(4)Z-X) - BDE(C(6)H(5)Z-X), for Z = CH(2) and O are largely independent of X and are determined mainly by the stabilization/destabilization effect of Y on the 4-YC(6)H(4)Z(*) radicals. The effects of Y are small (< or =2 kcal/mol for all Y) for Z = CH(2), but they are large for Z = O, where good correlations with sigma(p)(+)(Y) yield rho(+) = 6.5 kcal/mol. For Z = NH, two sets of electrochemically measured N-H DeltaBDEs correlate with sigma(p)(+)(Y), yielding rho(+) = 3.9 and 3.0 kcal/mol. However, in contrast to the situation with phenols, these data indicate that the strengthening effect on N-H BDEs of electron-withdrawing (EW) Y's is greater than the weakening effect of electron-donating (ED) Y's. Attempts to measure N-H DeltaBDEs in anilines using two nonelectrochemical techniques were unsuccessful; therefore, we turned to density functional theory. Calculations on 15 4-YC(6)H(4)NH(2) gave N-H DeltaBDEs correlating with sigma(p)(+) (rho(+) = 4.6 kcal/mol) and indicated that EW and ED Y's had comparable strengthening and weakening effects, respectively, on the N-H bonds. To validate theory by connecting it to experiment, the N-H DeltaBDEs of four 4,4'-disubstituted diphenylamines and five 3,7-disubstituted phenothiazines were both calculated and measured by the radical equilibration EPR technique. For all compounds, theory and experiment agreed to better than 1 kcal/mol. Dissection of N-H DeltaBDEs in 4-substituted anilines and O-H DeltaBDEs in 4-substituted phenols into interaction enthalpies between Y and NH(2)/OH (molecule stabilization/destabilization enthalpy, MSE) and NH*/O* (radical stabilization/destabilization enthalpy, RSE) reveals that for both groups of compounds, ED Y's destabilize the molecule and stabilize the radical, while the opposite holds true for EW Y's. However, in the phenols the effects of substituents on the radical are roughly 3 times as great as those in the molecule, whereas in the anilines the two effects are of comparable magnitudes. These differences arise from the stronger ED character of NH(2) vs OH and the weaker EW character of NH* vs O*. The relatively large contributions to N-H BDEs in anilines arising from interactions in the molecules suggested that N-X DeltaBDEs in 4-YC(6)H(4)NH-X would depend on X, in contrast to the lack of effect of X on O-X and CH(2)-X DeltaBDEs in 4-YC(6)H(4)O-X and 4-YC(6)H(4)CH(2)-X. This suggestion was confirmed for X = CH(3), H, OH, and F, for which the calculated NH-X DeltaBDEs yielded rho(+) = 5.0, 4.6, 4.0, and 3.0 kcal/mol, respectively.

Automated analysis of the pyridinium crosslinks of collagen in tissue and urine using solid-phase extraction and reversed-phase high-performance liquid chromatography

A fully automated method for assaying the collagen crosslinking amino acids, pyridinoline and deoxypyridinoline, in human urine samples or tissue hydrolysates is described. Samples were processed using a Gilson ASPEC system with solid-phase extraction of the crosslinks on columns containing 100 mg of microgranular cellulose. Introduction of an additional solvent step during sample preparation allowed direct analysis by reversed-phase HPLC and elimination of the drying step used previously in a manual method. Use of a synthetic pyridinoline derivative as internal standard enabled accurate quantification of the crosslinks by correcting for recoveries through the whole assay. Samples were analyzed in sequential mode with a total assay time of 30 min. The automated assay showed close correlation with the manual method for both free and total crosslink determinations in human urine (r > 0.97). Reproducibility was improved, as seen from replicate analyses of human urine (CV < 3% for automated pyridinoline measurement compared with 8-12% previously observed for the manual method). Crosslink excretion is the most useful marker of collagen degradation in metabolic bone diseases and arthritic disorders. The automated assay which has been developed is rapid, convenient, and reliable and will greatly facilitate the monitoring of urinary collagen crosslinks and their tissue levels in clinical investigations.

Autoxidative and cyclooxygenase-2 catalyzed transformation of the dietary chemopreventive agent curcumin

The efficacy of the diphenol curcumin as a cancer chemopreventive agent is limited by its chemical and metabolic instability. Non-enzymatic degradation has been described to yield vanillin, ferulic acid, and feruloylmethane through cleavage of the heptadienone chain connecting the phenolic rings. Here we provide evidence for an alternative mechanism, resulting in autoxidative cyclization of the heptadienone moiety as a major pathway of degradation. Autoxidative transformation of curcumin was pH-dependent with the highest rate at pH 8 (2.2 μM/min) and associated with stoichiometric uptake of O(2). Oxidation was also catalyzed by recombinant cyclooxygenase-2 (COX-2) (50 nm; 7.5 μM/min), and the rate was increased ≈10-fold by the addition of 300 μM H(2)O(2). The COX-2 catalyzed transformation was inhibited by acetaminophen but not indomethacin, suggesting catalysis occurred by the peroxidase activity. We propose a mechanism of enzymatic or autoxidative hydrogen abstraction from a phenolic hydroxyl to give a quinone methide and a delocalized radical in the heptadienone chain that undergoes 5-exo cyclization and oxygenation. Hydration of the quinone methide (measured by the incorporation of O-18 from H(2)(18)O) and rearrangement under loss of water gives the final dioxygenated bicyclopentadione product. When curcumin was added to RAW264.7 cells, the bicyclopentadione was increased 1.8-fold in cells activated by LPS; vanillin and other putative cleavage products were negligible. Oxidation to a reactive quinone methide is the mechanistic basis of many phenolic anti-cancer drugs. It is possible, therefore, that oxidative transformation of curcumin, a prominent but previously unrecognized reaction, contributes to its cancer chemopreventive activity.

Molecular characterization of replication-competent variants of adenovirus vectors and genome modifications to prevent their occurrence

Adenovirus (Ad) vectors for gene therapy are made replication defective by deletion of E1 region genes. For isolation, propagation, and large-scale production of such vectors, E1 functions are supplied in trans from a stable cell line. Virtually all Ad vectors used for clinical studies are produced in the 293 cell, a human embryonic kidney cell line expressing E1 functions from an integrated segment of the left end of the Ad type 5 (Ad5) genome. Replication-competent vector variants that have regained E1 sequences have been observed within populations of Ad vectors grown on 293 cells. These replication-competent variants presumably result from recombination between vector and 293 cell Ad5 sequences. We have developed Ad2-based vectors and have characterized at the molecular level examples of replication-competent variants. All such variants analyzed are Ad2-Ad5 chimeras in which the 293 cell Ad5 E1 sequences have become incorporated into the viral genome by legitimate recombination events. A map of Ad5 sequences within the 293 cell genome developed in parallel is consistent with the proposed recombination events. To provide a convenient vector production system that circumvents the generation of replication-competent variants, we have modified the Ad2 vector backbone by deleting or rearranging the protein IX coding region normally present downstream from the E1 region such that the frequency of recombination between vector and 293 cell Ad5 sequences is greatly reduced. Twelve serial passages of an Ad2 vector lacking the protein IX gene were carried out without generating replication-competent variants. In the course of producing and testing more than 30 large-scale preparations of vectors lacking the protein IX gene or having a rearranged protein IX gene, only three examples of replication-competent variants were observed. Use of these genome modifications allows use of conventional 293 cells for production of large-scale preparations of Ad-based vectors lacking replication-competent variants.

The unusual reaction of semiquinone radicals with molecular oxygen

Hydroquinones (benzene-1,4-diols) are naturally occurring chain-breaking antioxidants, whose reactions with peroxyl radicals yield 1,4-semiquinone radicals. Unlike the 1,2-semiquinone radicals derived from catechols (benzene-1,2-diols), the 1,4-semiquinone radicals do not always trap another peroxyl radical, and instead the stoichiometric factor of hydroquinones varies widely between 0 and 2 as a function of ring-substitution and reaction conditions. This variable antioxidant behavior has been attributed to the competing reaction of the 1,4-semiquinone radical with molecular oxygen. Herein we report the results of experiments and theoretical calculations focused on understanding this key reaction. Our experiments, which include detailed kinetic and mechanistic investigations by laser flash photolysis and inhibited autoxidation studies, and our theoretical calculations, which include detailed studies of the reactions of both 1,4-semiquinones and 1,2-semiquinones with O2, provide many important insights. They show that the reaction of O2 with 2,5-di-tert-butyl-1,4-semiquinone radical (used as model compound) has a rate constant of 2.4 +/- 0.9 x 10(5) M-1 s-1 in acetonitrile and as high as 2.0 +/- 0.9 x 10(6) M-1 s-1 in chlorobenzene, i.e., similar to that previously reported in water at pH approximately 7. These results, considered alongside our theoretical calculations, suggest that the reaction occurs by an unusual hydrogen atom abstraction mechanism, taking place in a two-step process consisting first of addition of O2 to the semiquinone radical and second an intramolecular H-atom transfer concerted with elimination of hydroperoxyl to yield the quinone. This reaction appears to be much more facile for 1,4-semiquinones than for their 1,2-isomers.

Lipid Peroxidation: Kinetics, Mechanisms, and Products

Recent advances in our understanding of lipid peroxidation, a degenerative process that is believed to play a key role in the pathogenesis of many diseases, are highlighted. In particular, the factors that control the kinetics and regio-/stereochemical outcomes of the autoxidation of both polyunsaturated fatty acids and sterols and the subsequent decomposition of the hydroperoxide products to cytotoxic derivatives are discussed. These advances promise to help clarify the role of lipid peroxidation in cell death and human disease.

Ferroptosis: A flexible constellation of related biochemical mechanisms

It is common to think about and depict biological processes as being governed by fixed pathways with specific components interconnected by concrete positive and negative interactions. However, these models may fail to effectively capture the regulation of cell biological processes that are driven by chemical mechanisms that do not rely absolutely on specific metabolites or proteins. Here, we discuss how ferroptosis, a non-apoptotic cell death mechanism with emerging links to disease, may be best understood as a highly flexible mechanism that can be executed and regulated by many functionally related metabolites and proteins. The inherent plasticity of ferroptosis has implications for how to define and study this mechanism in healthy and diseased cells and organisms.

7-Dehydrocholesterol is an endogenous suppressor of ferroptosis

Ferroptosis is a form of cell death that has received considerable attention not only as a means to eradicate defined tumour entities but also because it provides unforeseen insights into the metabolic adaptation that tumours exploit to counteract phospholipid oxidation1,2. Here, we identify proferroptotic activity of 7-dehydrocholesterol reductase (DHCR7) and an unexpected prosurvival function of its substrate, 7-dehydrocholesterol (7-DHC). Although previous studies suggested that high concentrations of 7-DHC are cytotoxic to developing neurons by favouring lipid peroxidation3, we now show that 7-DHC accumulation confers a robust prosurvival function in cancer cells. Because of its far superior reactivity towards peroxyl radicals, 7-DHC effectively shields (phospho)lipids from autoxidation and subsequent fragmentation. We provide validation in neuroblastoma and Burkitt's lymphoma xenografts where we demonstrate that the accumulation of 7-DHC is capable of inducing a shift towards a ferroptosis-resistant state in these tumours ultimately resulting in a more aggressive phenotype. Conclusively, our findings provide compelling evidence of a yet-unrecognized antiferroptotic activity of 7-DHC as a cell-intrinsic mechanism that could be exploited by cancer cells to escape ferroptosis.