Roles of endogenous ether lipids and associated PUFA in the regulation of ion channels and their relevance for disease.

Ether lipids (ELs) are lipids characterized by the presence of either an ether linkage (alkyl lipids) or a vinyl ether linkage (i.e. plasmalogens [Pls]) at the sn1 position of the glycerol backbone and they are enriched in PUFAs at the sn2 position. In this review, we highlight that ELs have various biological functions, act as a reservoir for second messengers (such as PUFAs), and have roles in many diseases. Some of the biological effects of ELs may be associated with their ability to regulate ion channels that control excitation-contraction/secretion/mobility coupling and therefore cell physiology. These channels are embedded in lipid membranes, and lipids can regulate their activities directly or indirectly as second messengers or by incorporating into membranes. Interestingly, ELs and EL-derived PUFAs have been reported to play a key role in several pathologies, including neurological disorders, cardiovascular diseases, and cancers. Investigations leading to a better understanding of their mechanisms of action in pathologies have opened a new field in cancer research. In summary, newly identified lipid regulators of ion channels, such as ELs and PUFAs, may represent valuable targets to improve disease diagnosis and advance the development of new therapeutic strategies for managing a range of diseases and conditions.


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This is supported in a study by Caldwell on rabbit cardiomyocytes at early stages of infarction (78). The importance of Pls in heart is illustrated by supplementation experiments with chimyl alcohol (ether lipid precursor), which decreases the effects of ischemia-reperfusion by enhancing ventricular function and decreasing lipid peroxidation (79). In addition, it has been reported that cytochrome c released from mitochondria can cleave the ether link of Pls in the ischemia-reperfusion syndrome (80). In the plasma of hypertensive patients and in the aorta of patients with atherosclerosis, Pls are decreased (81, 82). In a model of atherosclerosis in mice, supplementation with batyl alcohol (an ether lipid precursor) led to a decrease of atherosclerosis (83). In contrast, an increase of Pls has been observed in diabetic heart rats with cardiomyopathy, and this increase is partly corrected with insulin treatment (84).
It has been observed that ELs could regulate excitation-contraction coupling and ion channels in cardiac diseases (see Figure 2 for the potent mechanisms of action of ether lipids on ion channels). LysoPls, whose synthesis is increased in several heart diseases, can affect cardiac electrophysiology (85,86) and thus cause arrhythmias. For example, lysoPls-Cho can induce spontaneous contractions faster than LPC. In fact, lysoPls-Cho application leads to a depolarization, which can be reversed by a decrease of extracellular sodium concentration (78).
Thus, we can hypothesize that lysoPls-Cho activates a sodium conductance or inhibits potassium channels. Moreover, in rabbit cardiomyocytes, lysoPls-Cho has been found to activate PKA (87). This data is interesting, since it is well-known that the activity of some ion channels are regulated by this kinase, such as the SK3 channel for example (88), which participates in atrial action potential repolarization (89)(90)(91). Pls can also modulate the activity of the sodium-calcium exchanger (NCX), which is crucial for heart activity (92). In fact, in synthetic vesicles containing Pls and negative phospholipids (such as phosphatidyl serine), NCX conformation is modified, leading to an increase of its activity. These lipids interact with by guest, on May 7, 2020 www.jlr.org Downloaded from the cationic exchanger inhibitory peptide (XIP) site of NCX, leading to a change of the threedimensional structure (71). Furthermore, NCX can be activated by Pls with a phosphatidic acid as head group (Pls-PA). Pls-PA can also directly modulate NCX activity via this XIP domain (93). As evoked in the previous part of the review, the effect of PAF remains better documented than other ELs in heart pathophysiology. This EL is known to induce coronary vasoconstriction and has negative cardiac inotropic effect (94). Moreover, it appears to be implied in the ischemia-reperfusion syndrome by causing arrhythmias. In an animal model of infarction, an increase in the amount of PAF t has been observed, as in the blood of patients with acute myocardial infarction. This increase is even more important in patients with acute myocardial infarction and arrhythmia (95). Some studies found a decrease of action potential duration in the papillary muscles of guinea pigs treated with PAF. These results were also found in guinea pigs models of myocardial ischemia (95). Interestingly, in this model, the effect of PAF is time dependent with an increase in the action potential duration at the beginning of reperfusion, which follows a period of ischemia of the papillary muscles (95). Another study reported a decrease of cytosolic calcium concentration in cardiomyocytes after treatment with PAF (96).
In a model of atrial frog, a change in the equilibrium of potassium and calcium at the myocardial sarcolemma after treatment with PAF has been described, with PAF increasing the activity of the delayed outward potassium current (Kv) and decreasing the slow inward calcium current (CaV) (97). In a model of guinea pig ventricles, this lipid was also found to increase the duration of action potential (without affecting the resting membrane potential) by decreasing both the cardiac delayed rectifier and the cardiac inward rectifier potassium currents, which control the repolarization phase of the action potential (98). However, in the absence of ATP, PAF activates KATP channels, which decreases action potential duration (98).
In contrast, another study shows a decrease of atrial and ventricular action potential duration by guest, on May 7, 2020 www.jlr.org Downloaded from after treatment with an antagonist of the PAF receptor (PAF-R). This effect was prevented by a treatment with glibenclamide, a blocker of KATP (99).
Altogether, these results show that PAF is a bivalent actor, which can modulate action potential duration, depending on cell states concerning ischemia (pre-ischemia or ischemia reperfusion syndrome), leading to arrhythmias. PAF arrhythmogenic effects are also attributed to its ability to inhibit the potassium TASK-1 channel activity via the PAF-R and PKC, leading to an automaticity with a maintained depolarization state in mouse ventricular myocytes (100, 101).
Interestingly, PAF is also described as a cardioprotective lipid via activation of mitochondrial KATP channels and redox signaling. Pre-treatment with low concentrations of PAF can decrease the infarct size (102) and exerts positive ionotropic effects (97).

Indirect effect through fatty acids release
As previously mentioned, ELs are also known to be a reservoir of second messengers, such as FAs. Nutritional intervention concerning PUFAs has been shown to decrease the risk of developing cardiac diseases. Studies analyzing the FA effect on cardiovascular disease first appeared in a study of Eskimos from Greenland. This population has a diet rich in fish and marine mammals (and therefore rich in n-3 PUFAs) and they develop few cases of coronary artery disease (103). Many other epidemiological studies have confirmed this association between the low risk of cardiovascular diseases and a diet rich in n-3 PUFAs (104). Intake of n-3 PUFAs prevents arrhythmias (tachycardia and ventricular fibrillation), decreases heart rate and blood pressure and improves efficiency of the left ventricle. They also have antithrombotic actions by reducing plasma cholesterol levels, thus protecting against atherosclerosis. N-3 PUFAs also decrease mortality due to cardiac arrest or cardiovascular disease (105-108). In contrast, for n-6 PUFAs, few studies have investigated the roles of LA and AA in cardiovascular disease, and the results are contradictory (109). For example, Kark et al. showed a positive association between AA quantities in the adipose tissue of patients and the incidence of myocardial infarction, whereas there was no association for LA rates (110). Conversely, Cho et al. suggest a protective effect of LA and AA on cardiovascular disease by decreasing serum low-density lipoprotein (LDL) and increasing serum high-density lipoprotein (HDL) (111).
Part of these discrepancies may be resolved when studies are focused on the importance of the n-6:n-3 ratio rather than on the individual role of each lipid in cardiovascular disease. Indeed, as in several pathologies, a high n-6:n-3 ratio (equal or superior to 10) is considered unfavorable, while a n-6:n-3 ratio close to 1 is considered to be protective. However, the use of this ratio as a marker (risk factor or predictor) has been discussed given not only the contradictory effects of n-6 PUFAs (including LA and AA) but also because this ratio does not take into account the different interactions between foods (108, 112, 113). Thus, Von Schacky and Harris have proposed the "Omega-3 index" as a new marker for cardiovascular diseases.
This index is the percentage (of total serum FA) of EPA+DHA, representing the n-3 PUFAs rate (114). Concerning the saturated fatty acids (SFAs), most studies do not determine the individual effect of SFAs but the effect of their replacement by monounsaturated fatty acids (MUFAs) or PUFAs (112, 113, 115). Thus, despite few contradictory studies, the World Health Organization, the American Dietetic Association, the dietitians of Canada, the American Heart Association and the American College of Cardiology recommend reducing the intake of SFAs for a healthier cardiovascular system. SFAs should be limited to at least 10% of total energy and less than 7% for high-risk groups. Indeed, studies in primates, human prospective observational studies and randomized clinical trials have shown that lower consumption of SFAs and its replacement by unsaturated fats, in particular PUFAs, decreases the incidence of cardiovascular disease and reduces atherosclerosis by lowering LDL levels (112, 113, 115).
Heart energy comes mainly from oxidative phosphorylation (95%) and from glycolysis (5%) (116). However, these processes are altered during ischemia-reperfusion injury and FA accumulation increasing the beta-oxidation is observed (117). Moreover, the presence of PUFAs in phospholipids makes them more sensitive to oxidation and leads to the oxidized phospholipids formation involved in several cardiovascular diseases (105). Currently, there is a pharmacological approach aiming to inhibit FA oxidation (trimetazidine) and to improve cardiac efficiency with a decrease of ischemic heart disease (118).
There are different mechanisms by which FAs, in particular n-3 PUFAs, can prevent arrhythmias. Indeed, FAs can modulate ion channel activities by a direct interaction or by their incorporation into the myocyte membrane (see Figure 2 for the potent mechanisms of action of ether lipids on ion channels). Indeed, it has been demonstrated that n-3 PUFAs decrease the activity of NaV in cardiomyocytes, increasing the threshold of depolarization of the membrane potential and reducing the heart frequency (119, 120). The n-3 PUFAs modulate the activity of L-type calcium channels (from CaV) and NCX, reducing the cytosolic free calcium concentration and the excitability of myocytes, permitting them to prevent arrhythmias (121, 122). Several studies suggest that n-3 PUFAs also modulate the activity of the Kv11.1 channel, whose mutations can cause long QT syndrome, and Kv7 channels, which are potent vasodilators (123).
Same as observed in neurological disorders, cardiac diseases are the cause or the consequence of the changes of ELs and associated PUFAs, and all these data lead us to propose that these changes would have profound effects on ion channels that control cardiac excitationcontraction coupling leading to or exacerbating cardiac disorders ( Figure 3).

History
At the end of the 1960s, an association between endogenous ELs and cancer had been described in many studies, first in order to characterize these lipids in tumor tissues and then to identify their lipid chain composition in the sn1, sn2 and sn3 positions of the glycerol. Snyder and Wood (8, 9, 124) described a higher amount of ELs in both rat and mouse tumor tissues compared to normal tissues. These results have been confirmed in a huge range of human tumors (125), such as in the lungs, liver and brain. For example, glioblastoma contained high quantities of Pls-Cho when compared to normal brain tissue (126, 127). Soodsma, Piantadosi and Snyder (128) observed that the higher content of ELs in tumors of rat liver compared to normal rat liver could be explained by the suppression of the activity of the ether cleavage system of ELs. Later, Howard et al. (10) showed a correlation between EL content and growth rate of rats bearing hepatomas in vivo and also in cell lines in vitro. In membranes of cancer cells, ELs can be metabolized into free FAs, for example, by the plasmalogen-selective PLA2, leading to FA release with biological activity. These pioneer studies led to an increased interest for this family of lipids in cancer, more precisely for Pls, that could be potential markers of carcinogenesis (129).

Quantification and composition of ether lipids and associated fatty acids in human cancer samples
Ether lipids by guest, on May 7, 2020 www.jlr.org

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Several studies described changes in the content of ELs in the plasma of cancer patients compared to healthy subjects (Table 3). An increase in the content of Pls was observed in several types of tumors, such as lung, breast, gastric and prostate cancer (11, 130, 131).
Interestingly, it was proposed that some specific lipids in the plasma could be used as specific biomarkers 12 of prostate cancer, such as alkyl-glycerophosphocholine 10 . Similar studies have been obtained for breast cancer (132). In some cases, a switch inside EL species composition was observed, and in pancreatic cancer, blood alkyl-glycerophosphocholine species decreased (compared to control patient blood) in benefit for Pls-Etn (133).
Surprisingly, in other types of cancers, such as oral squamous cell carcinoma and acute leukemia, the plasma content of some EL species appeared to decrease in advanced stages (134, 135). Table 3  Few studies have quantified PAF in tumors. One of them reported increased PAF in human breast tumors compared to normal tissues, but this increase was lost at advanced stages of the disease (139). Others available studies described PAF-R expression, which could be considered as a reflection of PAF-induced activity. In gastric adenocarcinoma, the PAF-R is mainly expressed in tumors with good prognosis (well-differentiated, small size and without metastases) (140). Such observations have also been observed in hepatocellular carcinoma (141). It is important to consider that PAF is a second messenger involved in the immune system, and a loss of this lipid and its receptor in high-grade and invasive tumors could be the result of disturbed immunity in the context of cancer.

Associated fatty acids
FAs from the degradation of Pls, especially in blood samples, represent an interesting biomarker of breast, prostate and lung cancers, and their presence correlates with cancer aggressiveness (11).
In breast carcinogenesis, studies using a dietary questionnaire show an association between LA and reduction of breast cancer risk, as already observed in serum dosage. However, no association between LA content in breast adipose tissue and breast cancer risk has been In summary, currently, epidemiological studies and their contradictory results do not allow to answer conclusively whether FAs represent a risk (or a protective) factor for breast, prostate or colon cancer (142). These contradictory results can be explained by the use of different methods of analysis (food questionnaire or biological samples) and by the heterogeneity of the population, without stratification with some factors, such as age or ethno-geographical origin.
Moreover, the FA composition of both adipose tissue and serum samples reflects dietary profiles over different time periods. Indeed, the serum FA composition reflects the last weeks (or months) diet, while the adipose tissue FA composition determines the long-term diet, due to its slow turnover. Therefore, adipose tissue better reflects dietary intake than the blood composition, especially for essential FAs (150). Thus, an association between alimentation and cancer development and progression could constitute a new tool for cancer prevention and/or adjuvant treatment. However, this remains quite unexplored.

In vitro studies
ELs, Pls-Etn in particular, are present in a larger amount in some breast cancer cell lines than in non-cancer cell lines (137, 138), as well as in melanoma cells (13). The role of ELs in the biology of cancer cells has been mainly studied by modulating the enzymes of EL biosynthesis.
AGPS represents one of the most studied enzymes in EL biology, and more precisely, in the cancer field. Recently, Benjamin et al., (13) showed that AGPS is overexpressed in breast tumors compared to normal tissues as well as in aggressive cancer cell lines compared to less aggressive ones in prostate, breast and melanoma models. Moreover, this enzyme participates in cell migration, invasion and proliferation, and some ELs can rescue cell migration in AGPS KO cells. This enzyme was found to be implied in epithelial to mesenchymal transition (EMT) of glioma and hepatocarcinoma cell lines, and its suppression leads to a decrease of key transcription factors implied in EMT, such as Snail or Twist (151). Moreover, this enzyme, through alkyl LPA and PGE2 pathways, increases cancer cell resistance to chemotherapy (14).
Some chemical inhibitors of AGPS have been developed (152, 153), and they decrease cell migration and expression of EMT transcription factors. Moreover, GNPAT expression, which catalyzes the transformation of DHAP into acyl-DHAP, has been described as amplified in hepatocarcinoma and its suppression, in vivo, decreases tumor growth (154). Among ELs, PAF has been reported to increase cancer cell proliferation, migration and metastasis through its receptor in several cancers, such as prostate, breast, ovarian or skin cancers (155-162).
Contrary to ELs, FAs have been widely studied in several cancer cell lines and animal models.
Thus, several FAs and their mechanisms of action have been described and elucidated in breast, prostate and colon cancers. In breast carcinogenesis, it has been shown that saturated FAs inhibit cell proliferation by inducing apoptosis (163, 164). Concerning OA, the results are more contradictory, but this FA seems to stimulate cell proliferation by activating the PI3K pathway In prostate carcinogenesis, there are few in vitro studies. A recent one described a downregulation of the EMT pathway, mediated by calcium signaling (178), by EPA and LA, two PUFAs found in peri-prostatic adipose tissue, inversely correlated with the disease progression (149). Treatment with these two lipids resulted in a decrease of cancer cell migration, invasion and store-operated calcium entry, with a decrease of Zeb-1 expression, a key EMT transcription factor implied in prostate cancer progression (179). Some studies have reported the anti-proliferative, anti-migratory and pro-apoptotic effects of EPA and DHA (180-185) by inhibition of the Akt signaling pathway (186-188). It has also been shown that DHA improves the effectiveness of some treatments (celecoxib and docetaxel) (189, 190). In contrast, AA appears to promote the migration and invasion of PC3 prostate cancer cells (191,192).
In colonic carcinogenesis, EPA and DHA have been shown to inhibit cell proliferation, in particular by arresting the G1 phase cell cycle. These lipids also induce cell apoptosis by inhibiting COX2/PGE2, PI3K/Akt and p38 pathways. ALA was found to have the same effects as its metabolites (EPA and DHA). For n-6 PUFAs, the results are more discordant. Indeed, some studies do not show any in vitro effect of n-6 PUFAs, while others studies show a protective effect of AA and LA by inducing cell apoptosis (for review Brandão and Ribeiro, 2018) (193).

In vivo studies
Studies on the role of ELs in tumor growth and metastasis in animal models are based on the knock-down of EL biosynthesis enzymes. In a rodent model of breast cancer, AGPS suppression in cancer cells led to a drastic decrease in tumor growth (13). The same conclusions were found after GNPAT suppression (154). Interestingly, the suppression of IIF-secreted PLA2, which degrades ELs, in fibroblasts injected into mice reduces the number of large skin tumors and decreases the quantity of LysoPls-Etn, which suggests an important role of Pls metabolites in skin carcinogenesis (194).
Studies on the role of FAs in animal models are mainly through FA-enriched oil diets, despite the difficulty to determine the most relevant control diet (isocaloric or isolipidic). In fact, this control diet could also affect the FA composition of animals, which could include some bias.
In breast carcinogenesis, LA stimulates tumor growth and increases the frequency of murine or human mammary tumor metastases in mice. N-3 PUFAs, such as ALA, have been poorly studied but appear to inhibit mammary carcinogenesis. Long-chain n-3 PUFAs (EPA and DHA) are mainly provided by supplementation with fish oils. Compared with n-6 PUFAs, EPA and DHA not only inhibit tumor growth but also lung metastases (for review, Bougnoux and Menanteau, 2005) (195). They can also increase the effectiveness of several anti-cancer drugs (doxorubicin, epirubicin, and docetaxel) and have anti-angiogenic properties (196,197). The quantities of n-3 and n-6 PUFAs need to be close to get the anti-tumor effects of n-3 PUFAs.
In prostate carcinogenesis, studies focus on the comparison between n-6 PUFAs and n-3 PUFAs. As described in mammary tumors, n-6 PUFAs were found to stimulate tumor growth in tumors transplanted from human prostate cells (198)(199)(200)(201) or in spontaneous tumors from murine cells (202)(203)(204)(205). In contrast, long-chain n-3 PUFAs inhibit tumor growth. The most convincing example is the Kelavkar study, which showed a regime switch from n-6 to n-3 PUFAs induced a decrease in tumor growth previously induced by LA. On the contrary, the tumors grew more rapidly when the mice switched from an n-3 PUFAs enriched diet to an n-6 PUFAs enriched diet (201). The n-6:n-3 ratio of PUFAs is also important, since it leads to a reduction in tumor volume and an increase in cell apoptosis (206).
In colonic carcinogenesis, similar results were found to what has been observed in prostate and breast cancer studies. Indeed, compared to n-6 PUFAs, n-3 PUFAs inhibit tumor growth of both chemotherapy-induced and transplanted scenarios and also inhibit aberrant crypt formation (the most frequent risk marker used in colon cancer) and the formation of liver metastases. The protective effect of n-3 PUFAs is not observed when tumours are implanted at other sites, which shows the importance of the microenvironment of colon tumors. ALA also appears to have a protective effect, whereas OA has no effect (for review, Bougnoux and Menanteau, 2005) (195). In summary, in vivo studies show that n-6 PUFAs promote while n-3 PUFAs reduce breast, colon and prostate cancer development (195).

Ether lipids and associated PUFAs as regulators of ion channels implied in cancer cell migration and metastatic development
Ion channels regulating calcium signaling participate not only in several mechanisms implied in tumor development and progression but also in cancer cell migration and metastatic development. Thus, some CaVs, such as CaV1.3, are abnormally expressed in several cancers, such as prostate, ovarian, colon (207) and breast cancers (1) and participate in prostate and colon cell proliferation, migration and invasion (208). The same observations have been made for some TRP and SOC channels (Orai and TRP families) (209). Several studies described that associations between both potassium and calcium channels can also fuel these processes with by guest, on May 7, 2020 www.jlr.org Downloaded from potassium channels acting as amplifiers of calcium entry. Gueguinou et al. described associations between calcium activated potassium channels and calcium channels, which control proliferation and migration of breast and prostate cancer cells (210). More precisely, we demonstrated that an association of the SK3 channel with the calcium channel Orai1 within cholesterol-rich nanodomains (also called lipid rafts) promotes constitutive calcium entry and breast cancer cell migration and metastasis in a metastatic rodent model (2). This association in cholesterol-rich nanodomains appears to be necessary, since channel delocalization outside these nanodomains decrease SK3-dependent constitutive calcium entry, cancer cell migration and metastatic development.
Interestingly, a synthetic EL we called Ohmline was found to decrease SK3 current, breast SK3-dependent constitutive calcium entry, cell migration and bone metastasis development (2). We demonstrated that this synthetic EL, by interacting with the carbonyl and phosphate groups of stearoylphosphatidylcholine, sphingomyelin and cholesterol can induce a membrane disorder (3). More precisely, it seems that Ohmline can change membrane lipid arrangement by competing with cholesterol, inhibiting its interactions with its binding sites. These observations could explain the observed delocalization of SK3 and Orai1 channels outside cholesterol-enriched nanodomains leading to the decrease of SK3 activity (2). These results can lean on the fact that SK3 activity is sensitive not only to cholesterol content in pig and rat arteries but also in breast and colorectal cancer cells, where its activity is decreased by MβCD (34, 211) and strongly associated with caveolin-rich domains (2,211). We hypothesize that the presence of many cholesterol recognition/interaction amino acid consensus sequence (CRAC) domains, allowing tight interactions with cholesterol, on SK3 protein sequences could explain its sensitivity to cholesterol.
These observations are especially interesting, since endogenous ELs and cholesterol homeostasis appear to be tightly linked. In fact, Jiménez-Rojo and Riezman (212) reviewed that a decrease of EL content decreased esterified cholesterol content, whereas an increase of Pls decreases the stability of squalene monooxygenase, a key enzyme of steroid biosynthesis.
Moreover, the effects of ELs on cell membranes are closely linked to the concentration of sterols, which allow a better incorporation of high concentrations of ELs, leading to an increase of lipid interactions and membrane packing (213). We hypothesize that the presence of several ELs in cancer cell membranes can increase membrane packing, stabilizing SK3 and Orai1 channels within nanodomains enriched in cholesterol (see Figure 2 for the potent mechanisms of action of ether lipids on ion channels). In fact, we observed that in EL enriched breast cancer cells, SK3-dependent constitutive calcium entry and cell migration were enhanced (unpublished data). Thus, ELs could increase SK3 activity by stabilizing interactions between cholesterol and the SK3 channel. Moreover, we showed that PAF increases SK3 current by 30% (214), as well as several others channels we previously described. We can also consider that if SK3 has a XIP domain (as we discussed with the NCX exchanger), direct interactions between some ELs and SK3 should be possible, leading to a modulation of SK3 activity. SK3 has also been described as associated with TRPC1 and Orai1 in colon cancer. This association led to an increase of SOCE, which mediates colon cancer cell migration.
Interestingly, treatment with Ohmline decreased SK3 current and associated cell migration, showing that Ohmline's effect is not limited to breast cancer cells (215).
In prostate cancer, SOCE mediated by SK3 after treatment with TGFβ is also sensitive to Ohmline, leading to a decrease of calcium entry and cancer cell migration. Interestingly, this pathway is also sensitive to EPA and LA: in fact, these lipids repress SK3 expression and calcium entry and cancer cell migration as a consequence. The main hypothesis is that EPA and LA can regulate SK3 and associated calcium channels at the plasma membrane, probably by dissociation of these complexes outside nanodomains enriched in cholesterol, where they are supposed to complex themselves (178). The effect of PUFAs is also found in breast cancer by guest, on May 7, 2020 www.jlr.org Downloaded from cells, where AA and LA can reduce TRPC3 SOCE and associated cell proliferation and migration (216).
Same as observed in excitable cells pathologies, cancer could be the cause or the consequence of the changes of ELs and associated PUFAs. We propose that tumors changes of ELs would have profound effects on ion channels that control excitation-mobility coupling leading to exacerbation of cancer ( Figure 4).
To conclude this review shows that EL and associated PUFAs are lipids that regulate ion channels in neurological, cardiac and cancer physiology. Interestingly, in pathologies such as Alzheimer's and Parkinson's diseases or myocardial infarction, EL homeostasis is dysregulated, which impairs the ion transportome. Thus, ELs and associated PUFAs start to be proposed and used as diagnostic tools and markers to follow disease progression, such as in Alzheimer's disease. ELs are even suggested as therapeutic tools, especially via nutritional intervention in order to increase the EL pool in neurological disorders.
In cancer, endogenous EL rediscovery in these last years has lead to the development of new therapeutic tools and new diagnostic tools through lipidomic analysis of patient biopsies and blood samples. We can speculate that ELs and associated PUFAs may be used as predictive markers of activity or expression of ion channels and thus of cancer progression. We can propose that ELs and associated PUFAs could be used as supplemental interventions with potential EL inhibitors of some ion channels, such as the SK3 channel in breast cancer.

Figure 2. Potential mechanisms of action of ether-lipids for ion channels / transporters regulation.
Ether-lipids exert different functions within cells and regulate numerous proteins such as ion channels or transporters. (A) Ether-lipids are known to be implied in many fusion processes of cells as endo and exocytosis or vesicles trafficking into cells. This property can lead to a modulation of ion channels/transporters translocation to plasma membrane or membranes of intracellular organelles. (B) Ether-lipids, and more precisely those containing PUFA participate to the structuration of nanodomains also named lipid rafts which which consist in platforms for cell signalling regulating ion channel/transporters activities. (C) By their incorporation into plasma membrane, ether-lipids can promote interaction between ion channels and their accessory proteins or (D) interact directly with ion channels/transporters and regulate their gating properties for example. (E) Several ether-lipids as PAF or LPAe can be synthesized by cells and secreted in cell microenvironment. These lipids have the particularity to bind some receptors, which are coupled, to some kinases as PKA or PLC, which can regulate directly ion channels/transporters activity, or indirectly through their genic expression via second messengers. (F) Moreover, receptor binding can lead to the activation of PLA2, which cleaves fatty acids at the sn2 position of the glycerol, leading to a production of PUFA and lyso etherlipids. These lipids metabolites can directly interact with ion channels/transporters or modulate their genic expression. PUFA: Polyunsaturated Fatty Acid, PAF: Platelet-Activating Factor, LPAe: LysoPhosphatidic Acid ether, PLA2: PhosphoLipase A2. Cardiac diseases and neurological disorders can be the cause or the consequence of a dysregulation of ether-lipid metabolism and thus of ether-lipids content. We hypothesize that this dysregulation can lead to a modification of ion channels expression and/or activity leading to a modification of excitation-response couplings. Several consequences can be observed as an increase of potassium conductance and/or a decrease of sodium conductance, which leads to a membrane hyperpolarization, leading to a decrease of secretion/contraction. At the opposite, a decrease of potassium conductance and/or an increase of sodium conductance can lead to a membrane depolarization responsible for an increase of secretion/contraction. In these two cases, ion homeostasis is disturbed which results in a pathology development or an increase of the pathology phenotype.

Figure 4. Involvement of ether-lipids in the modification of excitation-response couplings in non-excitable tumor cells.
Cancer can be the cause or the consequence of a dysregulation of ether-lipid metabolism and thus of ether-lipids content. In several cancers, a dysregulation of ether-lipid metabolism and of ether-lipids content has been observed. An increase of ether-lipids content in breast cancer cells leads to an increase of SK3 expression, a potassium channel which hyperpolarizes plasma membrane, promotes calcium entry leading to an increase of cancer cell migration (unpublished data). We previously found that SK3 channel increases calcium entry and calcium mediated breast cancer cell migration and bone metastasis development (2).