Ferroptosis (1) is an iron-dependent form of cell death associated with the accumulation
of lipid peroxidation products, which eventually results in membrane permeabilization
and osmolysis. It is believed to link the long-observed increase in lipid peroxidation
products to the tissue damage observed in many acute and chronic pathological contexts,
including neurodegeneration, ischemic stroke, and traumatic brain injury. As such,
ferroptosis has emerged as an important target for therapeutic development. Distinct
from apoptosis and other forms of cell death, ferroptosis is triggered when the capacity
of the cell to detoxify (phospho)lipid hydroperoxides is overcome (2, 3). Lipid hydroperoxides
arise due to autoxidation, a chain reaction propagated by reactions of lipid-derived
free radicals (4), as well as by enzyme-catalyzed processes (Fig. 1, Middle) (5).
Hydroperoxides derived from polyunsaturated lipids esterified to phosphatidyl-ethanolamine
(PE) are believed to be particularly important in ferroptosis execution (6). In PNAS,
Dar et al. (7) report the first attempt to develop inhibitors designed to target an
enzyme-protein complex that oxidizes PE lipids.
Fig. 1.
Unrestrained peroxidation of membrane lipids drives ferroptotic cell death. Inhibition
of both nonenzymatic lipid peroxidation (autoxidation) and enzyme-catalyzed lipid
peroxidation may suppress ferroptosis. Newly reported FerroLOXins have been designed
to inhibit oxidation of phosphatidylethanolamine lipids by a 15-LOX2/PEBP1 complex.
Since FerroLOXins are aromatic amines, such as the canonical RTA inhibitors ferrostatin-1,
liproxstatin-1, and PTG-00, they may also prevent nonenzymatic lipid peroxidation.
Of the limited number of enzymes that directly catalyze lipid peroxidation, the 15-lipoxygenases
(15-LOX1 and 15-LOX2) have been most often implicated in ferroptosis execution (8
–11). Lipoxygenases (LOXs) are nonheme iron-containing enzymes that catalyze oxygenation
of free arachidonic acid and some related polyunsaturated fatty acids in a largely
regioselective and stereoselective manner (5). Although LOXs have been found to be
dispensable for ferroptosis both in vitro (12) and in vivo (2, 13, 14), cells which
express substantial levels may be sensitized to ferroptosis induction (12). To date,
only LOX inhibitors with off-target radical-trapping antioxidant (RTA) activity have
been reported to suppress ferroptosis (6, 15). RTAs suppress ferroptosis by trapping
the lipid-derived free radicals that propagate nonenzymatic lipid peroxidation (16,
17) and comprise the overwhelming majority of ferroptosis inhibitors yet identified,
including the archetype ferroptosis inhibitors ferrostatin-1 (1) and liproxstatin-1
(2), and the more potent phenoxazine PTG-00 (Fig. 1, Left) (18). LOX inhibitors lacking
RTA activity have been shown only to desensitize cells to ferroptosis (12).
Based on the previous observation that 15-lipoxygenase-2 (15-LOX2), when complexed
with PE binding protein-1 (PEBP1) (19), can catalyze the oxygenation of polyunsaturated
lipids esterified to PE (Fig. 1, Right), Dar et al. sought to develop inhibitors with
specificity for the complex. Starting from scaffolds of known 15-LOX2 inhibitors,
and the results of computations suggesting that association with PEBP1 increases the
volume of the catalytic site of 15-LOX2, they added substituents to increase affinity
for the complex over 15-LOX2 alone. Cell-based assays (in human bronchial epithelial
cells) revealed that derivatives which featured an N–H bond adjacent to the imidazole
substituent were good ferroptosis suppressors. Increasing the lipophilicity of the
compounds, particularly via inclusion of fluoroalkyl substituents, afforded the most
potent inhibitors (EC50~ 100 nM).
In PNAS, Dar et al. report the first attempt to develop inhibitors designed to target
an enzyme-protein complex that oxidizes PE lipids.
Consistent with the premise, these inhibitors suppressed oxygenation of arachidonic
acid esterified to PE by the combination of 15-LOX2 and PEBP1 in a liposome model
(by ~20 to 25%), but had no measurable effect on 15-LOX2 alone. Computational studies
suggest that the enhanced inhibition of the 15-LOX2/PEBP1 complex over 15-LOX2 may
result from blocking O2 access to the active site, dislodging the oxidizable sidechain
of the phospholipid from the active site, and favoring binding of the nonoxidizable
sidechain. Further studies demonstrated that two of the compounds, coined FerroLOXIN-1
and FerroLOXIN-2, were unable to rescue cells from necroptosis, pyroptosis, or apoptosis—suggesting
specificity for ferroptosis, similarly to the archetype inhibitors ferrostatin-1 and
liproxstatin-1. Similar experiments in four additional cell lines (intestinal epithelial
cells Caco2 and FHs 74 Int) and cancer cells (HT-1080 and A375) suggest that ferroptosis
suppression by FerroLOXINs is general.
Since ferroptosis suppression by the FerroLOXINs was observed in cells that have been
reported to not express 15-LOX2 (e.g., HT-1080s) (20), it is possible that they possess
off-target RTA activity in addition to their ability to target the 15-LOX2/PEBP1 complex.
It is perhaps not a coincidence that the FerroLOXINs possess aromatic amine moieties—the
key chemical functionality that features in ferrostatin-1, liproxstatin-1, and PTG-00.
Future work should address this point, by carrying out studies to assess the ability
of FerroLOXINs to prevent nonenzymatic lipid peroxidation in phospholipid bilayers,
as well as to validate 15-LOX2/PEBP1 as the target in cells which do express 15-LOX2
by demonstrating the loss of antiferroptotic activity when the genes encoding 15-LOX2
and/or PEBP1 are knocked down/out.
Regardless of the precise mechanism of ferroptosis suppression, the FerroLOXINs were
found to be active in vivo as demonstrated by the suppression of radiation damage
in mice. Radiation damage has long been associated with augmented levels of lipid
peroxidation, suggesting a role of ferroptosis in the associated tissue damage. Indeed,
the most common ferroptosis inducers are known to synergize with ionizing radiation,
and genetic and biochemical hallmarks of ferroptosis are observed in radiation-treated
cells (21, 22). Moreover, RTAs such as ferrostatin-1 are good radioprotectants (21,
22). Similarly, FerroLOXIN treatment (25 mg/kg 24 h after irradiation) significantly
prolonged the survival of both female and male mice. This was associated with the
protection of the intestinal epithelium, a particularly radiosensitive tissue. The
levels of PE-derived hydroperoxides were correspondingly reduced, linking the protective
effect with suppression of lipid peroxidation. In contrast, markers of apoptotic and
necroptotic cell death were unaffected by FerroLOXIN treatment—again implicating ferroptosis
specifically in radiation damage.
Of the many individual proteins and metabolic pathways which have been found to modulate
a cell’s sensitivity to ferroptosis (23), LOXs are unique because they directly contribute
to the cellular pool of (phospho)lipid hydroperoxides. Accordingly, they can seed
nonenzymatic lipid peroxidation when their reaction products undergo one-electron
reduction by labile iron in the same manner as nonenzymatically produced (phospho)lipid
hydroperoxides. Since ferroptosis occurs once lipid peroxidation can no longer be
managed by the cell, preventing the accumulation of hydroperoxides of any origin—enzymatic
or not—should contribute to ferroptosis suppression. Thus, LOXs and/or their complexes
with scaffolding proteins such as PEBP1 present a unique point of intervention upstream
of, and/or complementary to, RTAs. Moreover, they may afford a means to achieve tissue
specificity in the fight against ferroptosis, given that LOXs are not expressed at
meaningful levels in all cells and tissues (24). It will be exciting to see whether
further research establishes the FerroLOXINs as first-in-class inhibitors that can
provide such a proof of principle and whether they can be developed into therapeutics
for ferroptosis-related disease.