HACS————HACS|导读：
Krebs循环中间体传统上与氧化磷酸化有关，同时也制造关键的细胞成分。现在很清楚的是，这些代谢物中的一些也起到了信号的作用。琥珀酸在炎症、缺氧和代谢信号中起着重要作用，而衣康酸(Itaconate ,来自另一种TCA循环中间体顺式-乌头酸盐)则具有抗炎作用。
Krebs cycle intermediates traditionally link to oxidative phosphorylation whilst also making key cell components. It is now clear that some of these metabolites also act as signals. Succinate plays an important role in inflammatory, hypoxic, and metabolic signaling, while itaconate (from another Krebs cycle intermediate, cis-aconitate) has an anti-inflammatory role.
transducer[trænzˈduːsər,trænsˈduːsər]：any device, such as a microphone or electric motor, that converts one form of energy into another
|背景介绍：
Recently, the Krebs cycle metabolite succinate was shown to accumulate in macrophages during inflammation, acting as a signal to activate pro-inflammatory gene expression .
macrophage: a large phagocytic cell found in stationary form in the tissues or as a mobile white blood cell, especially at sites of infection.
Succinate was also found to increase in tissues during ischemia (Chouchani et al., 2014) and during reperfusionpromoted production of reactive oxygen species (ROS) by the mitochondrial respiratory chain, initiating the damage that underlies ischemia-reperfusion (IR) injury (Chouchani et al., 2014, 2016).
Subsequently, these two strands of work came together: during inflammatory macrophage activation succinate enhanced mitochondrial ROS production using the same process that led to IR injury, with the ROS acting as a pro-inflammatory redox signal to the transcription factor HIF1a.
Succinate oxidation by succinate dehydrogenase (SDH) induced production of pro-in- flammatory factors, notably IL-1b, and also suppressed production of anti-inflammatory factors (Mills et al., 2016).
`Succinate induces pro-inflammatory by it oxidation through SDH.
A pro-inflammatory signal needs to be switched off, and the next discovery was that the inflammatory effects of succinate were counteracted by the metabolite itaconate (Lampropoulou et al., 2016).
counteract[ˌkaʊntərˈækt] To counteract something means to reduce its effect by doing something that produces an opposite effect. act in opposition to.
This metabolite is derived from the Krebs cycle intermediate cis-aconitate (Bambouskova et al., 2018; Michelucci et al., 2013; Mills et al., 2018b).
Itaconate was therefore identified as an anti-inflammatory signal.
Later, itaconate was shown to upregulate cellular antioxidant defenses via induction of Nrf2 (Bambouskova et al., 2018; Mills et al., 2018b).
We therefore had an intriguing series of observations in which one Krebs cycle intermediate, succinate, acts as a pro-inflammatory signal, which is countered by itaconate, derived from another Krebs cycle intermediate, cis-aconitate.
Succinate is also released from cells and activates a receptor termed SUNCR1 to activate inflammatory pathways (Littlewood-Evans et al., 2016).
Succinate has also been proposed to activate type 2 immunity in the gut to certain infectious agents acting via so-called Tuft cells, which express high levels of SUCRN1 (Lei et al., 2018).
In a model of multiple sclerosis (MS), succinate acted as a pro-inflammatory mediator in the central nervous system upon its release from cells (PeruzzottiJametti et al., 2018).
Succinate-dependent mitochondrial metabolism was also central to how the carotid body senses oxygen and allows the organism to respond to hypoxia (Ferna´ ndezAgu¨ era et al., 2015).
Succinate and itaconate have therefore emerged in several areas of cell function, playing key, but opposing, signaling roles that link mitochondrial function to the rest of the cell.
Here, we discuss these recent studies and present novel therapeutic options for the injury that occurs in IR and in inflammatory diseases.
Succinate as a Signaling Molecule
Succinate occupies a pivotal position in metabolism as the only direct link between the Krebs cycle and the mitochondrial respiratory chain (Figure 1).
Signaling Pathways of Succinate and Itaconate
Succinate is generated within mitochondria from 2-oxoglutarate and by reversal of SDH during hypoxia/ischemia.
The succinate is then released into the cytosol, where it can act as a signal that stabilizes HIF1a.
Succinate can also generate a redox signal by driving mitochondrial ROS production at complex I by reverse electron transport (RET), which also promotes HIF1a.
Later in inflammation, the expression of the IRG1/CAD protein in mitochondria can lead to the generation of itaconate from cis-aconitate.
This inhibits SDH to limit ROS production in response to succinate.
Itaconate is also released into the cytosol, where it modifies KEAP1, leading to Nrf2 activation promoting expression of anti-inflammatory and antioxidant genes.
Itaconate also induces ATF3, an anti-inflammatory transcription factor whose activity in part depends on inhibition of IkBz.
Succinate-driven ROS can also promote adipose tissue thermogenesis and via SUCNR1 inflammation and type 2 immunity against infectious agents.
The other link is provided by NADH, produced by oxidation reactions in the mitochondrial matrix, which also provides electrons for the respiratory chain.
SDH links the succinate/fumarate couple to the CoenzymeQ (CoQ) pool, as succinate oxidation to fumarate by SDH is coupled to the reduction of ubiquinone (UQ) to ubiquinol (UQH2) (Figure 1).
As the mid-point potentials for the UQ/UQH2 and the fumarate/succinate couples are close, it enables electrons to flow in either direction across SDH between the Krebs cycle and the CoQ pool, depending on conditions.
The close links between the fumarate/succinate ratio and the redox state of the CoQ pool level enables the status of this key determinant of mitochondrial function to be transmitted to the rest of the cell.
As succinate is generated in the mitochondrial matrix, its export from the organelle to the cytosol will act as a signal of mitochondrial status.
The dicarboxylate carrier SLC25A10 facilitates this by catalyzing the rapid exchange of dianions across the inner membrane with succinate, malate, and phosphate as favored substrates, enabling the mitochondrial succinate pool to equilibrate rapidly with that in the cytosol (Figure 1).
Of all the mitochondrial respiratory substrates, succinate is the one most frequently supplied when investigating isolated mammalian mitochondria because it is rapidly taken up by the organelle and oxidized by the respiratory chain.
But this well-known fact has always been a puzzle: why should mitochondria rapidly take up and oxidize a respiratory substrate that is both generated and consumed within the matrix?
We now know why: the main role of the transporter that takes up succinate is actually to export it to the rest of the cell for succinate to act as a signal.
Succinate signaling in the cytosol involves the inhibition of 2-oxoglutarate (2OG)-dependent dioxygenases (Loenarz and Schofield, 2011).
These enzymes generate succinate as a product (Loenarz and Schofield, 2011).
Thus, high levels of succinate slow dioxygenase activity by product inhibition (Figure 1).
Among these enzymes are prolyl hydroxylases (PHDs), which regulate hypoxia inducible factor 1 (Hif1a).
During hypoxia, the lack of O2 prevents Hif1a degradation and enhances the expression of anti-hypoxia genes (Figure 1).
High levels of succinate in the cytosol also promote expression of Hif1a-dependent genes, even in the presence of O2.
There are a number of other 2OG-dependent dioxygenases such as ten eleven translocation (TET) DNA demethylases, which hydroxylate methylcytosines in DNA to erase this epigenetic mark, and the histone lysine demethylase Jumonji C domain-containing proteins, which remove methyl-lysine marks on histones.
还有许多其他2OG依赖的双加氧酶，如TET DNA去甲基化酶（在DNA中羟化甲基胞嘧啶并消除这一表观遗传标记），组蛋白赖氨酸去甲基化酶JumonjiC结构域蛋白（去除组蛋白上的甲基赖氨酸标记）。
Hence, elevating cytosolic succinate levels is likely to alter epigenetic marks, with long-term consequences for gene expression.
Another mode of signaling driven by elevated succinate is ROS production.
When the mitochondrial respiratory chain is oxidizing high levels of succinate under conditions of high protonmotive force (Dp)—for example, when mitochondria are not making much ATP—the CoQ pool becomes reduced, leading to reversal of the normal direction of electron flow through complex I, causing reverse electron transport (RET) at complex I (Figure 1).
As RET leads to the dramatic production of ROS, RET enables mitochondria to release a variable redox signal that can respond sensitively to mitochondrial status.
This may involve the efflux of ROS such as hydrogen peroxide or more likely by dithiol/sulphide switches on redox-sensitive proteins.
ROS will also activate HIF1a and may be the dominant mechanism whereby succinate activates HIF1a in inflammatory macrophages (Mills et al., 2016).
Succinate is also released from the cell by poorly understood processes (most likely cell death) where it activates the cell surface G-protein-coupled receptor, SUCNR1 (Peruzzotti-Jametti et al., 2018)
Therapeutic Opportunities
In IR injury, the simple SDH inhibitor malonate protected against IR in heart attack and stroke (Chouchani et al., 2014; Valls-Lacalle et al., 2016).
Similarly, this approach protected in an animal model of sepsis (Mills et al., 2016).
The activation of Nrf2 signaling by cell permeable forms of itaconate also suggests new therapeutic approaches to activate this cell-protective pathway.
To mimic the anti-inflammatory activity of itaconate within cells requires its delivery to the cytosol, but in most cells, the doubly charged itaconate would be poorly taken up across the plasma membrane (unless there is a transporter).
This can be overcome by the use of the simple, cell-permeable itaconate-derivative dimethylitaconate (DMI), which also activates the Nrf2 pathway.
DMI is however a far more reactive Michael acceptor than itaconate (ElAzzouny et al., 2017; Mills et al., 2018b).
To overcome these limitations, we developed the itaconate monoester 4-octyl itaconate (4-OI) (Mills et al., 2018b), whose hydrophobicity enabled far more rapid cell uptake than for itaconate itself, but whose thiol reactivity was the same as that of native itaconate.
4-OI was effective in an animal model of sepsis (Mills et al., 2018b). However, the hydrophobic octyl group may also affect the activity of this compound; therefore, the future development of itaconate esters with enhanced cell delivery and hydrolysis to itaconate within the cell is a promising area for future development as anti-inflammatory agents.
Another option would be to boost IRG1 activity pharmacologically to generate endogenous itaconate.
One further point is that itaconate is readily converted to various itaconyl CoA derivatives, which have reactivity distinct from itaconate itself (Shen et al., 2017), opening up other possibilities for drug development.
Implications and Conclusions ：
The repurposing of mitochondrial metabolism to use the mitochondrion as a signaling hub utilizing metabolites as indicators of mitochondrial status may be a general principle of metabolic organization.
If part of the normal dialog between mitochondria and the cell utilizes the exchange of metabolites, then this indicates new avenues of exploration to both better understand how cells respond to external challenges as well as potentially opening up new therapeutic opportunities.
线粒体代谢的重组利用线粒体作为信号枢纽，利用代谢产物作为线粒体状态的指标，可能是代谢组织的一般原则。如果线粒体和细胞之间正常对话的一部分利用代谢物的交换，那么这表明了新的探索途径，以更好地理解细胞如何应对外部挑战，以及开辟潜在的新的治疗机会。
