ELOVL5 expression is higher in GBM tumors compared to healthy brain tissue, according to GEPIA
[8] and Seifert et al.
[9]. However, previous results from the research team did not show significant differences in the expression of ELOVL5 in GBM tumor tissue versus the peritumoral area
[11]. Discrepancies between the results and the data from GEPIA and transcriptomics analysis performed by Seifert et al. may have resulted from studying different groups of patients. In addition, it was observed that ELOVL5 expression was lower in GBM tumors in women relative to both the peritumoral area and GBM tumors in men
[11]. Higher ELOVL5 expression does not affect the prognosis for GBM patients, according to GEPIA
[8]. ELOVL5 expression can be higher in a GBM tumor as a result of hypoxia, as shown by researchers' experiments with U87 MG line cells
[11]. This is very important because hypoxia in a GBM tumor also increases the expression of COX-2
[12], an enzyme that converts ARA into prostanoids. This means that hypoxia increases the production of ARA and, at the same time, its conversion into prostanoids.
2. Lipoxygenases and Arachidonic Acid in Glioblastoma Multiforme
2.1. Lipoxygenases Pathway
In addition to the COX pathway, PUFA can be transformed with LOX. These enzymes exhibit dioxygenase activity, catalyzing the insertion of a hydroperoxyl group into a PUFA, most commonly ARA 20:4n-6. Hydroperoxyeicosatetraenoic acids (HpETE) are then formed from ARA 20:4n-6, which are further processed in the lipoxygenase pathway. The names of LOX enzymes are related to their sites of formation and the configuration of the hydroperoxyl group in ARA 20:4n-6. In humans, there are six LOX:
-
epidermal lipoxygenase 3/arachidonate lipoxygenase 3 (eLOX3/ALOXE3),
-
5-lipoxygenase/arachidonate 5-lipoxygenase (5-LOX/ALOX5),
-
12S-lipoxygenase/arachidonate 12-lipoxygenase, 12S type (12S-LOX/ALOX12),
-
12R-lipoxygenase/arachidonate 12-lipoxygenase, 12R type (12R-LOX/ALOX12B),
-
15-lipoxygenase-1/arachidonate 15-lipoxygenase (15-LOX-1/ALOX15), also known as 12/15-LOX, and
-
15-lipoxygenase-2/arachidonate 15-lipoxygenase type B (15-LOX-2/ALOX15B).
The
ALOX5 gene is found on chromosome 10. The other LOX form a gene cluster on 17p13.1
[13][14]. There is also a mouse 8-LOX
[15], whose sequence is 78% identical to that of human 15-LOX-2/
ALOX15B [15][16]. It is likely that mouse 8-LOX and human 15-LOX-2/ALOX15B are derived from a common ancestor, which was indirectly confirmed by mutagenesis experiments on these two enzymes. Changing only two amino acids in either mouse 8-LOX or human 15-LOX-2/
ALOX15B alters the catalytic properties of these two enzymes in 15-LOX and 8-LOX, respectively
[15].
2.1.1. Epidermal Lipoxygenase 3
The
ALOXE3 gene forms a gene cluster on 17p13.1 together with other LOX
[14]. The highest expression of the
ALOXE3 gene is found in the skin
[14][17]; very low expression of this gene is found in the brain, placenta, pancreas, ovary, and testis.
eLOX3/
ALOXE3 shows no significant activity against ARA 20:4n-6 or linoleic acid C18:2n-6
[18], which is related to the low availability of molecular oxygen in the active center of this enzyme
[19]. For this reason, the processing of ARA 20:4n-6 by eLOX3/
ALOXE3 is very inefficient, but eLOX3/
ALOXE3 can exhibit dioxygenase activity to ARA 20:4n-6.
eLOX3/
ALOXE3 has hydroperoxide isomerase activity
[18]. eLOX3/
ALOXE3 converts HpETE into hydroxy-epoxyeicosatrienoic acid, which is the main product of eLOX3/
ALOXE3 activity. eLOX3/
ALOXE3 also converts HpETE into oxo-eicosatetraenoic acid (oxo-ETE)/ketoeicosatetraenoic acid (KETE)
[18][20]. 15S-HpETE is converted by eLOX3/ALOXE3 into either 13
R-hydroxy-14
S,15
S-epoxyeicosa-5
Z,8
Z,11
Z-trienoic acid or 15-oxo-ETE
[18].
eLOX3/
ALOXE3 also converts 12
S-HpETE into hepoxilin A
3 (HxA
3), HxB
3 [18][21], or 12-oxo-ETE
[22][23]. On the other hand, 12
R-HpETE is converted by eLOX3/
ALOXE3 into either 11,12-bis-epi-HxA
3 or 12-oxo-ETE
[18].
In addition, eLOX3/
ALOXE3 shows activity to 5-HpETE and other HpETEs
[20]. Because HETE and oxo-ETE
[24] as well as hepoxilins
[25] exhibit biological activity, eLOX3/
ALOXE3 affects biological and pathological processes, particularly in the skin, where expression of this enzyme is highest. For this reason, mutations in the
ALOXE3 gene lead to ichthyosis
[26][27][28].
2.1.2. 5-Lipoxygenase
The best-studied LOX is 5-LOX/
ALOX5. The highest expression of 5-LOX/
ALOX5 is found in the bone marrow, appendix, lung, urinary bladder, spleen, and lymph node
[17]. This enzyme converts ARA 20:4n-6 to 5
S-hydroperoxyeicosatetraenoic acid (5-HpETE) and then to leukotriene A
4 (LTA
4)
[29]. Importantly, 5-lipoxygenase-activating protein (FLAP)/
ALOX5AP is required for the activity of 5-LOX/
ALOX5. FLAP/
ALOX5AP is a substrate carrier
[30][31]. 5-HpETE is an activator of PPARα
[32]; for this reason, if it is not converted to other lipid mediators, then it will activate this nuclear receptor. Subsequently, LTA
4 is converted to other lipid mediators, in particular to other leukotrienes. LTA
4 can also undergo spontaneous conversion to 5,6-diHETE, 5,12-diHETE, and 5-oxo-ETE
[33]. In turn, 5-HpETE is converted to 5-hydroxyeicosatetraenoic acid (5-HETE) with glutathione peroxidase
[34]. The identified receptor for 5-HETE is G2A/GPR132
[35]; this receptor is also activated by other lipid mediators, such as various HETE and 9-HODE.
5-oxo-ETE can also be formed from 5-HETE with the participation of an enzyme with 5-hydroxyeicosanoid dehydrogenase (5-HEDH) activity
[36][37][38]. 5-oxo-ETE is an important lipid mediator with a receptor oxoeicosanoid receptor 1 (OXER1)/GPR99
[39][40][41].
LTA
4 is a precursor for the production of other leukotrienes and lipoxins; it is converted to lipoxins in a reaction catalyzed by 12-LOX or 15-LOX
[42]. LTA
4 can also be converted to LTB
4 by LTA
4 hydrolase (LTA
4H)
[43][44]. LTA
4H also has aminopeptidase activity unrelated to the production of leukotrienes
[44]; this activity is important in moderating the immune response
[45]. LTB
4 has its own membrane receptors: LTB
4R1/BLT1
[46] and LTB
4R2/BLT2
[47]. Inside the cell, LTA
4 and LTB
4 activate PPARα, by which these leukotrienes can exert anti-inflammatory effects
[32][48].
Glutathione can be attached to LTA
4 by LTC
4 synthase (LTC
4S) (
Figure 2)
[49][50]. LTC
4 is then formed. LTC
4S combines with 5-LOX and FLAP to increase the efficiency of LTC
4 production with ARA 20:4n-6
[51]. Subsequently, amino acids from the conjugated glutathione in LTC
4 can be removed. As a consequence of this, LTC
4 is converted into other leukotrienes, namely LTD
4, LTE
4, and LTF
4. All of these leukotrienes, together with LTC
4, form a group called cysteinyl leukotrienes. LTD
4 is then formed from LTC
4 with the involvement of γ-glutamyltransferase 1 (GGT1) and γ-glutamyltransferase 5 (GGT5)
[52]. Subsequently, LTD
4 can be converted to LTE
4 with the participation of dipeptidase 1 (DPEP1) and dipeptidase 2 (DPEP2)
[53][54]. LTC
4 can also be converted to LTF
4 with the participation of carboxypeptidase A
[55]. Amino acids can be attached back to cysteine in cysteinyl leukotriene, as exemplified by the conversion of LTE
4 to LTF
4 with the participation of an enzyme with γ-glutamyltranspeptidase activity
[56]. LTF
4, however, has a much weaker effect than LTE
4, and the latter reaction can be considered an inactivation of LTE
4.
Figure 2. 5-LOX pathway. ARA C20:4n-6 is converted to 5-HpETE with 5-LOX. This enzyme also catalyzes the next step in leukotriene biosynthesis. It converts 5-HpETE into LTA4, which can then be converted into LTB4 with LTA4H, into LTC4 with LTC4S, or into 5-oxo-ETE. 5-HpETE can also be converted to 5-oxo-ETE. LTC4 can be converted to other cysteinyl leukotrienes. LTC4 can be converted to LTF4 with the involvement of carboxypeptidase A or to LTD4 with the involvement of GGT1 and GGT5. Subsequently, LTD4 can be converted into LTE4 with the participation of DPEP1 and DPEP2, and then converted into LTF4 with γ-glutamyltranspeptidase. ↑—higher expression of given enzymes in GBM tumor relative to healthy tissue.
Once synthesized, leukotrienes are secreted from the cell. LTC
4 is secreted from cells by multidrug resistance-associated proteins (MRP)
[57]. In particular, MRP1/ABCC1
[58][59], MRP2/ABCC2
[60][61], MRP3/ABCC3
[62], MRP4/ABCC4
[63], MRP6/ABCC6
[64], MRP7/ABCC10
[65], and MRP8/ABCC11
[66] are responsible for this process. In contrast, OATP1/SLCO1C1 and OATP4 are responsible for the uptake of LTC
4, particularly into liver cells where leukotrienes are degraded
[67][68]. In contrast, LTB
4 transport is still poorly studied; it is known that efflux of LTB
4 occurs via MRP4/ABCC4
[63].
Once leukotrienes are secreted outside the cells, they can activate their membrane receptors. LTB
4 has two receptors: LTB
4R1/BLT1
[46] and LTB
4R2/BLT2
[47], the former of which has a 20 times better dissociation constant (Kd) than LTB
4R2 in binding LTB
4 [47]. With that said, LTB
4R2 can be activated by other ARA-derived lipid mediators. These include 12
S-HETE, 12
R-HETE, 15-HETE, 15-HpETE
[69], and 12-HHT
[70][71][72]. 12-HHT is formed together with malondialdehyde in a reaction catalyzed by TBXAS1, whose substrate is PGH
2 [73][74]. In addition, 12-HHT can be formed independently of TBXAS1 but in smaller amounts
[74].
The receptors for cysteinyl-leukotrienes are CysLTR
1 [75] and CysLTR
2 [76][77]. Both receptors show a 38% similarity in amino acid sequence
[76]. CysLTR
1 shows a high affinity for LTD
4 and low affinity for LTC
4 and LTE
4, and it shows no affinity at all for LTB
4 [75]. CysLTR
2 has the best affinity for LTC
4 and LTD
4 and a very low affinity for LTE
4, and it shows no affinity at all for LTB
4 [76][77]. A receptor specific for LTE
4 is 2-oxoglutarate receptor 1 (OXGR1)/GPR99
[78], which is also the receptor for 2-oxoglutarate. This receptor has a lower affinity for LTC
4 and LTD
4. Another identified receptor for cysteinyl-leukotrienes specifically for LTC
4 and LTD
4 is G protein-coupled receptor 17 (GPR17)
[79], which is also activated by uridine diphosphate (UDP), UDP-glucose, and UDP-galactose
[79]. Further studies have not confirmed that GPR17 is a receptor for UDP, LTC
4, and LTD
4 [80][81]. This receptor can, independently of its ligand, downregulate CysLTR
1 [82], which means it can reduce the action of cysteinyl leukotrienes.
Leukotrienes can be inactivated and excreted. LTB
4 is oxidized to 12-oxo-LTB
4 with 12-hydroxyeicosanoid dehydrogenase (12-HEDH)/PTGR1
[83][84][85]. This enzyme is also involved in prostaglandin degradation
[86]. Subsequently, 12-oxo-LTB
4 is reduced with the formation of 12-oxo-10,11-dihydro-LTB
4 with an enzyme with Δ
10-reductase activity
[87]. 12-oxo-10,11-dihydro-LTB
4 can then be converted to 10,11-dihydro-LTB
4 and 10,11-dihydro-12-epi-LTB
4, which undergo ω-oxidation, β-oxidation, or elongation
[83]; compounds formed after ω-oxidation and β-oxidation are excreted in the feces
[88] and urine
[89] as ω-carboxymetabolites of LTB
4. HETE are similarly degraded, such as 12-HETE with the formation of 10,11-dihydro-12-HETE and 10,11-dihydro-12-oxo-ETE
[90]. Cysteinyl-leukotrienes are first converted to LTE
4 [91]; this leukotriene then undergoes ω-oxidation with the formation of ω-carboxy-tetranor-dihydro-LTE4, which is eliminated in the feces and urine.
2.1.3. 12S-Lipoxygenase
ALOX12 gene expression is found in the esophagus and skin
[17]. 12S-LOX/
ALOX12 can participate in the conversion of LTA
4 into lipoxins
[42], but the best-described activity of 12S-LOX/
ALOX12 is to catalyze the insertion of a hydroperoxyl group into ARA 20:4n-6 at position 12—12
S-HpETE is then formed
[92]—the compound which can also be formed with 15-LOX-1/
ALOX15 [93].
12S-LOX can convert dihomo-γ-linolenic acid to 12
S-hydroxy-8
Z,10
E,14
Z-eicosatrienoic acid (12
S-HETrE)
[94][95]. In contrast, linoleic acid C18:2n-6 is not a substrate for 12S-LOX/
ALOX12 [94]. 12
S-HpETE can be converted to 12
S-HETE, whose receptors are G protein-coupled receptor 31 (GPR31)
[96] and G2A/GPR132
[35].
12
S-HETE also activates PPARγ
[97], as 12
S-HpETE
[18] and 12
S-HETE can be converted to 12-oxo-ETE
[87], a PPARγ ligand and activator
[22]. 12-oxo-ETE can be converted back to 12
S-HETE with an enzyme with 12-oxo-ETE reductase activity
[98].
12
S-HpETE can be converted to HxA
3 (8-hydroxy-11,12-epoxyeicosatrienoic acid) or HxB
3 (10-hydroxy-11,12-epoxyeicosatrienoic acid) with enzymes with hepoxilin synthase activity, for example, heme, as shown by experiments on hemoglobin and hemin
[99][100]. Hepoxilin synthase activity is also demonstrated by eLOX3/
ALOXE3, 12S-LOX/
ALOX12, and 15-LOX-1/
ALOX15, as shown by experiments on human, rat, and mouse models
[18][21][101][102].
Then, HxA
3 may bind glutathione via glutathione S-transferase at position 11
[103][104]. HxA
3 then gives rise to 11-glutathionyl-HxA
3, or otherwise HxA
3-C. HxB
3 is not subject to such modification
[105]. HxA
3-C can be produced in the brain and may be a neuromodulator
[106]. Like cysteinyl-leukotrienes, HxA
3-C can be converted to other cysteinyl-hepoxilins
[106]. HxA
3-C is converted to HxA
3-D by γ-glutamyltranspeptidase. HxA
3 and HxB
3 can also be converted into trioxilin A
3 (TrXA
3) (8,11,12-trihydroxyepoxyeicosatrienoic acid) and TrXB
3 (10,11,12-trihydroxyepoxyeicosatrienoic acid) with soluble epoxide hydrolase (sEH) (current name: epoxide hydrolase 2 (EPHX2))
[103][107]. HxA
3 receptors are TRPV1 and transient receptor potential ankyrin 1 (TRPA1)
[108][109]. HxA
3 and TrXA
3 are also antagonists of the TP receptor
[110], the receptor for TxA
2.
2.1.4. 12R-Lipoxygenase
In addition to 12S-LOX/
ALOX12, there is a second enzyme with 12-LOX activity
[13], namely 12R-LOX/
ALOX12B [111]. This enzyme shows activity towards ARA C20:4n-6 but not linoleic acid C18:2n-6
[111]. 12R-LOX/
ALOX12B transforms ARA C20:4n-6 into 12
R-HpETE, a stereoisomer of the product of 12S-LOX/
ALOX12’s enzyme activity. 12
R-HpETE is converted to 11,12-bis-epi-HxA
3 with eLOX3/
ALOXE3 [18]. 12
R-HpETE is a stereoisomer of 12
S-HpETE. Similar to this compound, 12
R-HpETE can also be converted to 12
R-HETE
[24], which is then converted to 12-oxo-ETE with an enzyme with 12-hydroxyeicosanoid dehydrogenase activity
[24][87], including eLOX3/
ALOXE3 [18].
The
ALOX12B gene is only 38% similar to the
ALOX12 gene. The highest expression of this enzyme is found in the skin, and it is much lower in the prostate and adrenal gland
[14][17][111]. 12R-LOX is important in skin function; mutations in the
ALOX12B gene lead to ichthyosis
[26][28][112], as do mutations in the
ALOXE3 gene. 12R-LOX/
ALOX12B and eLOX3/
ALOXE3 participate in a common pathway in lipid mediator production. 12R-LOX produces 12
R-HpETE, which is converted to 11,12-bis-epi-HxA
3 with eLOX3 (
Figure 3)
[18]. Under the influence of eLOX3/
ALOXE3, 12-oxo-ETE is also formed from 12
R-HpETE in small amounts
[18].
Figure 3. 12-LOX pathway. ARA C20:4n-6 is converted to 12S-HpETE and 12R-HpETE with 12S-LOX and 12R-LOX, respectively. Either 12-oxo-ETE or the corresponding 12-HETE can be formed from these compounds. 12S-HpETE can also be converted to HxA3 or HxB3 with hemin and lipoxygenases: eLOX3, 12S-LOX, or 15-LOX-1. 12R-HpETE can undergo a similar conversion to 11,12-bis-epi-HxA3. HxA3 may undergo further transformations. HxA3 can be conjugated to glutathione. HxA3-C is then formed, from which amino acids can be detached—HxA3-D is then formed in a reaction similar to the transformation of cysteinyl-leukotrienes. HxA3 can also be converted to TrXA3. Arrows next to enzymes: higher or lower expression of given enzymes in GBM tumor relative to healthy tissue. ↓—lower expression of given enzymes in GBM tumor relative to healthy tissue.
2.1.5. 15-Lipoxygenases
Like the previously described LOX, 15-LOX catalyzes the formation of 15
S-hydroperoxyeicosatetraenoic acids (15-HpETE) from ARA 20:4n-6
[113]. In humans, two 15-LOX isoforms are distinguished: 15-LOX-1/
ALOX15 [114] and 15-LOX-2/
ALOX15B [115]. The highest expression of 15-LOX-1/
ALOX15 is found in the lung, and the lower expressions are in the skin, intestine, heart, lymph node, and testis
[17]. The highest expression of 15-LOX-2/
ALOX15B is found in the prostate and skin. Expression of this enzyme is also observed in the lung, esophagus, and cornea
[14][17][115].
The enzymatic properties of the two isoforms differ. 15-LOX-1/
ALOX15 catalyzes the formation of 15-HpETE, but it also converts part of the substrate, ARA 20:4n-6, into 12-HpETE
[93]—for this reason, the enzyme owns its historical name: 12/15-LOX. 15-LOX-2/
ALOX15B has no such activity
[93][115].
15-LOX-1/
ALOX15 shows much higher activity with linoleic acid C18:2n-6 than 15-LOX-2/
ALOX15B (
Figure 4)
[93]. These enzymes convert linoleic acid C18:2n-6 into 13
S-hydroperoxyoctadecadienoic acid (13-HpODE), which converts to 13
S-hydroxyoctadecadienoic acid (13-HODE). The identified receptor for 13-HpODE is G2A/GPR132
[35]. 13-HODE also activates the TRPV1 receptor
[116]. 13-HODE undergoes the same transformations as HETE and can be oxidized to 13-oxo-ODE. 13-oxo-ODE
[117] and 13-HODE
[118] are PPARγ ligands.
Figure 4. 15-LOX pathway. (A). Linoleic acid C18:2n-6 can be converted by 15-LOX-1 and 15-LOX-2 into 13-HpODE. This compound can then be converted into 13-HODE and 13-oxo-ODE. (B) 15-LOX-1 and 15-LOX-2 can convert ARA C20:4n-6 into 15-HpETE. 15-LOX-1 can also convert this fatty acid into 12-HpETE. 15-HpETE can then be converted into EXA4 and into cysteinyl-eoxins EXC4, EXD4, and EXE4. 15-HpETE can also be converted into hepoxilins 14,15-HxA3 11S, and 14,15-HxB3 13R. 14,15-HxA3 11S can be converted to cysteinyl hepoxilins, such as 14,15-HxA3-C 11S.
15-HpETE is transformed into many lipid mediators. It can be transformed into 15-HETE, which is an activator of PPARγ
[97] and G2A/GPR132
[35]. 15-HpETE can be converted to 13
R-hydroxy-14
S,15
S-epoxyeicosa-5
Z,8
Z,11
Z-trienoic acid (14,15-HxB
3 13
R), 11
S-hydroxy-14
S,15
S-epoxy-5
Z,8
Z,12
E-eicosatrienoic acid (14,15-HxA
3 11
S), and 15-oxo-ETE
[18][119]. 14,15-HxA
3 11
S, analogous to HxA
3, can be conjugated with glutathione. This produces 14,15-HxA
3-C 11
S and cysteinyl-14,15-HxA
3 11
S, having conjugated glutathione without further amino acids, which is analogous to that of cysteinyl-leukotriene
[119].
15-HpETE can also be converted to eoxins
[120], which are isomers of leukotrienes.
15-HpETE can also be converted to lipoxins with 5-LOX
[42], resulting in the formation of 5
S,15
S-dihydroperoxyeicosatetraenoic acid (5,15-diHpETE), and then converted to LXA
4 or LXB
4 [121]. 5-HpETE can also be converted with 15-LOX-1/
ALOX15 into 5,15-diHpETE and, via the same pathway, be converted into LXA
4 or LXB
4 [121]. 15-HETE can be converted to LXA
4 with 5-LOX/
ALOX5 [122]. Lipoxins can also be formed from LTA
4, which is processed by 15-LOX-1/
ALOX15 or 12-LOX
[121][123].
LXA
4 is a lipid mediator with biological activity whose receptors are lipoxin A
4 receptor (ALX)/formyl peptide receptor type 2 (FPR2)
[124][125], aryl hydrocarbon receptor (AHR)
[126], and estrogen receptors subtypes alpha (ERα)
[127], the former of which is not a receptor for LXB
4 [124]. The ALX/FPR2 receptor is responsible for the anti-inflammatory properties of lipoxins.
There are also cysteinyl lipoxins, which, just like cysteinyl leukotrienes, are lipoxins with conjugated glutathione at carbon 6
[122]. They are synthesized from 15-HETE, from which, with the participation of 5-LOX/
ALOX5, 15-hydroxy-5,6-epoxy-eicosatetraenoic acid is formed, a compound similar in structure to LTA
4. The epoxy group from these two compounds is converted to a hydroxyl group and conjugated glutathione
[122]. However, it is not known whether cysteinyl lipoxins are essential lipid mediators or merely arise as a result of the nonspecificity of enzymes conjugating glutathione to various compounds.
2.2. Lipoxygenases in Glioblastoma Multiforme
In GBM tumors, ARA C20:4n-6 is mainly processed by COX, as shown by experiments on the C6 cell line
[128]. In contrast, in the healthy brain, this PUFA is mainly processed by the LOX pathway. This shows that in GBM tumors, the LOX pathway may not be as important as the COX pathway, although it is still important in tumor mechanisms in GBM tumors.
2.2.1. 5-Lipoxygenase Pathway in Glioblastoma Multiforme
The expression of 5-LOX/
ALOX5 in a GBM tumor is higher than in non-tumor brain tissue
[129][130][131]. This is also confirmed by data obtained from the GEPIA portal
[8] and from Seifert et al. transcriptomics analysis
[9].
Expression of 5-LOX/
ALOX5 in the GBM tumor is found in macrophage and microglial cells as well as in other cells, such as cancer cells
[130][131]. It is higher in GBM cancer stem cells than in other GBM cancer cells
[132]. According to GEPIA, higher expressions of FLAP/
ALOX5AP, LTC
4S, LTA
4H, GGT5, and DPEP1 but not DPEP2
[8], the enzymes that synthesize LTB
4 and LTE
4 from the product of 5-LOX/
ALOX5 activity, were also found in GBM tumors
[43][44][49][50][52][54]. Seifert et al. showed that there are higher expressions of FLAP/
ALOX5AP, LTA
4H, and GGT5 in GBM tumors than in healthy brain tissue
[9]. In contrast, LTC
4S, DPEP1, and DPEP2 are not affected. The higher expression of enzymes responsible for leukotriene biosynthesis increases the production
[133] and levels
[134] of these lipid mediators further in GBM tumors than in healthy brain tissue, particularly cysteinyl-leukotrienes.
The expression level of 5-LOX/
ALOX5 in GBM tumors does not affect prognosis
[8][135], although simultaneous high expression of COX-2 and 5-LOX/
ALOX5, two major ARA C20:4n-6 processing enzymes, is associated with a worse prognosis
[135]. This shows that the two pathways in cooperation can impinge on prognosis severity.
The expression levels of most enzymes involved in leukotriene production and metabolism do not affect prognosis
[8]. Only for GGT1, higher expression in GBM tumors is associated with a worse prognosis
[8]. GGT5 expression showed a positive trend (
p = 0.055) toward a worse prognosis. GGT1 and GGT5 are enzymes that catalyze the transformation of LTC
4 into LTD
4 [52], demonstrating that the transformation of cysteinyl leukotrienes may be important in tumorigenesis in GBM.
In addition, higher expression of 12-HEDH/PTGR1, an enzyme that degrades LTB
4, as well as prostaglandins, may be associated with worse prognoses for GBM patients
[86], although GEPIA did not confirm such a link
[8]. In addition, GEPIA and Seifert et al. did not show that 12-HEDH/PTGR1 expression differs between GBM tumors and healthy brain tissue
[8][9]. According to GEPIA
[8] and Seifert et al.
[9], expression levels of receptors for leukotrienes LTB
4R1, LTB
4R2, CysLTR
1, CysLTR
2, GPR17, and OXGR1/GPR99 do not differ between GBM tumors and healthy brain tissue. In addition, the expression levels of these receptors in GBM tumors do not affect prognosis
[8].
Leukotrienes as well as the entire 5-LOX pathway are important in tumorigenesis in GBM. They may also be important in the onset of GBM and in the first stages of tumorigenesis. The GA genotype of rs2291427 in the
ALOX5 gene is associated with a higher risk of GBM in men
[136].
Expression of 5-LOX/
ALOX5 is higher in GBM cancer stem cells than in other GBM cancer cells
[132]. The products of 5-LOX/
ALOX5 activity induce proliferation and self-renewal of GBM cancer stem cells. The effects of 5-LOX/
ALOX5 on GBM cancer stem cells are autocrine in nature.
LTB
4 also increases the proliferation of GBM cells
[137]. This is associated with an increase in Ca
2+ levels in the cytoplasm of GBM cells
[137]. Studies of various cell lines show that 5-LOX/
ALOX5 expression is present in only a portion of them
[138][139]. Expression of 5-LOX/
ALOX5 causes an autocrine increase in the proliferation of such a line and, thus, makes culture growth dependent on 5-LOX/
ALOX5 activity. All GBM lines express LTA
4H, LTB
4R1/BLT1, LTB
4R2/BLT2, and CysLTR
2, but only some lines express LTC
4S
[139], indicating heterogeneity in the production of cysteinyl-leukotrienes and 5-HETE by GBM cancer cells.
The dependence of the proliferation of some GBM cancer cell lines on the 5-LOX pathway may be a potential therapeutic target for GBM treatment in personalized therapy. For this reason, the pan-LOX inhibitor Nordy
[132][140], 5-LOX inhibitors such as caffeic acid
[137], A861
[141], AA-863, and U-60,257 (pyriprost)
[142], LTA
4H inhibitors such as bestatin
[141], and CysLTR
1 and CysLTR
2 receptor inhibitors such as montelukast and zafirlukast
[143] have anti-tumor properties against GBM and inhibit proliferation. This is associated with decreased ERK MAPK activation and induction of apoptosis as a result of decreased expression of anti-apoptotic Bcl-2 and increased expression of pro-apoptotic Bax
[138].
Cysteinyl leukotrienes may have anticancer properties by increasing the bioavailability of various chemotherapeutics. In the brain, as well as in GBM tumors, there is a blood-brain barrier (BBB) that is poorly permeable to many substances, including anticancer drugs
[144]. However, cysteinyl leukotrienes have BBB permeability, as shown by experiments on rat RG-2 glioma tumors
[145]. BBB permeability is highest for LTE
4 [145], with cysteinyl leukotrienes not causing BBB permeability in healthy brain tissue
[145][146]. For this reason, the administration of LTC
4 prior to the administration of chemotherapeutics that pass poorly through the BBB increases the bioavailability of drugs such as cisplatin
[147]. However, this method does not increase the bioavailability of all chemotherapeutics, as exemplified by paclitaxel
[148].
The receptor for cysteinyl leukotrienes is GPR17
[79]. According to GEPIA
[8] and Seifert et al.
[9], the expression level of this receptor does not differ between GBM tumors and healthy brain tissue. Higher GPR17 expression is associated with better prognosis in patients with low-grade gliomas, according to the Chinese Glioma Genome Atlas (CGGA)
[149] and GEPIA
[8], but the expression of this receptor is not associated with prognosis in a GBM patient
[8]. GPR17 expression is also higher in low-grade gliomas than in healthy brain tissue
[149]. Activation of this receptor by the ligand inhibits proliferation in the G
1 phase and induces apoptosis of GBM cell lines LN-229 and SNB-19
[149]. In addition, GPR17 ligands inhibit tumor growth, as shown by experiments using patient-derived xenograft mouse models. The action of GPR17 is associated with a decrease in the levels of cyclic adenosine monophosphate (cAMP) and Ca
2+ in the cytoplasm, which reduces the activation of the PI3K → Akt/PKB pathway
[149][150]. An increase in GPR17 expression can cause the proliferation and migration of GBM cells
[151], particularly with an increase in the expression of this receptor by long non-coding RNA (lncRNA) colorectal neoplasia differentially expressed (CRNDE) in low-grade glioma cells
[151].
The receptor for 5-HETE, and also other lipid mediators, is G2A/GPR132
[35]. Higher expression of this receptor, according to GEPIA, is associated with a worse prognosis for a GBM patient (
p = 0.052)
[8], yet there is no significant upregulation of this receptor expression in GBM tumors
[8][9].
5-oxo-ETE may also play an important role in tumorigenic mechanisms in GBM. The receptor for this lipid mediator is OXER1/GPR99
[39][40][41]. The expression of this receptor does not differ between GBM tumor and healthy brain tissue
[8][9]. According to GEPIA, higher expression of OXER1/GPR99, the receptor for 5-oxo-ETE, is associated with a worse prognosis for a GBM patient
[8]. OXER1/GPR99 is also a receptor for 2-oxoglutarate, LTC
4, and LTD
4 [78]. There is a lack of thorough research on the importance of 5-oxo-ETE in tumorigenesis in GBM tumors.
2.2.2. 12-Lipoxygenase Pathway in Glioblastoma Multiforme
In GBM tumors, expression of 12S-LOX/
ALOX12 and 12R-LOX/
ALOX12B is not different from healthy brain tissue
[8][9], nor is it associated with prognosis severity
[8], nor is the expression of the receptor for 12
S-HETE, i.e., GPR31, elevated and affecting prognosis
[8][9]. In contrast, the expression of eLOX3/
ALOXE3 in GBM tumors is lower than in other brain tissue
[8][23]. On the other hand, the transcriptomics analysis by Seifert et al. showed no differences between eLOX3/
ALOXE3 expression levels in GBM tumor and healthy brain tissue
[9]. Downregulation of eLOX3/
ALOXE3 expression in GBM tumor is associated with increased expression of miR-18a, which downregulates eLOX3/
ALOXE3 expression
[23]. At the same time, eLOX3/
ALOXE3 expression is also not related to the prognoses of GBM patients
[8].
12-LOX is involved in tumorigenesis in GBM. Studies on various cell lines have shown that 12-LOX expression is common in GBM cancer cells
[139]. For this reason, 12-LOX inhibitors inhibit proliferation and reduce the viability of GBM cells
[139][152]. 12-LOX inhibitors also inhibit the migration of GBM cells because they reduce the expression of matrix metalloproteinase 2 (MMP2) in these cells
[139]. However, the exact mechanism of 12-LOX action on tumorigenic processes in GBM is poorly studied. The fact that eLOX3/
ALOXE3 is anticancer in nature
[23] suggests that a lipid mediator not formed by eLOX3/
ALOXE3 is responsible for the pro-cancer properties of 12-LOX. Perhaps it is 12-HETE, a lipid mediator with proven pro-cancer properties in other cancers
[153][154]. In addition, higher expression of G2A/GPR132, a receptor for 5-HETE, 12-HETE, 15-HETE, and 9-HODE, is associated with a worse prognosis for a GBM patient (
p = 0.052)
[8]. The oncogenic properties of G2A/GPR132 were also demonstrated in a study on fibroblasts
[155], although there is no higher expression of G2A/GPR132 in GBM tumors than in healthy brain tissue
[8][9].
12-LOX may also have anti-cancer properties. It converts ARA 20:4n-6 into 12-HpETE, a lipid from the hydroperoxyl group, and for this reason, it can cause lipid peroxidation, which, when free ARA 20:4n-6 is in excess and this PUFA is over-processed, has a destructive effect on the cell
[156].
eLOX3/
ALOXE3 has anti-tumor properties in GBM. eLOX3/
ALOXE3 converts 12-HpETE into 12-oxo-ETE. In the absence of eLOX3/
ALOXE3, 12-HpETE is converted to 12-HETE
[23], meaning that eLOX3/
ALOXE3 decreases 12-HETE production. This lipid mediator increases GBM cell migration. When 12-HETE production is decreased, GBM cell migration is reduced.
The lipid mediators produced by eLOX3/
ALOXE3, including 12-oxo-ETE, have anti-tumor effects, particularly 12-oxo-ETE, which is a ligand for PPARγ
[22][23]. Activation of this nuclear receptor inhibits proliferation and induces apoptosis of GBM cancer cells
[157][158][159].
The products of eLOX3/
ALOXE3 activity are hepoxilins and trioxilins
[18][21], lipid mediators of physiological importance. However, there is a lack of studies on the importance of these lipid mediators in tumorigenesis in GBM.
Analysis on the GEPIA portal
[8] and the transcriptomics analysis by Seifert et al.
[9] showed no differences in the expression of EPHX2, the enzyme responsible for converting hepoxilins into trioxilins, between GBM tumors and healthy brain tissue
[103][107]. At the same time, according to GEPIA, higher EPHX2 expression in GBM tumors is associated with a tendency toward a worse prognosis (
p = 0.072), which may indicate that hepoxilins and trioxilins may have some role in neoplastic processes in GBM.
2.2.3. 15-Lipoxygenase Pathway in Glioblastoma Multiforme
GEPIA
[8] and Seifert et al.
[9] showed no differences in the expression of 15-LOX-1/
ALOX15 and 15-LOX-2/
ALOX15B between GBM tumors and healthy brain tissue. According to GEPIA, the expression level of these enzymes does not affect the prognosis for patients
[8]. Studies on various GBM lines have shown differences in the expression of 15-LOX-1/
ALOX15 and 15-LOX-2/
ALOX15B in GBM cancer cells
[139]. 15-LOX is important in the function of GBM cancer cells, and 15-LOX inhibitors reduce the viability and migration of GBM cancer cells
[139]. On the other hand, increasing the expression and activity of 15-LOX-1/
ALOX15 throughout the body may have an anti-tumor effect against GBM, as shown by gene therapy using an adenovirus transducing the
ALOX15 gene
[160]. This effect may depend on 13-HODE and 15-HETE.
All GBM lineages secrete 13-HODE, a product of the linoleic acid C18:2n-6 conversion with 15-LOX-1/
ALOX15 and 15-LOX-2/
ALOX15B [93]. 13-HODE increases MMP2 expression in GBM cells, which causes migration
[139]. At the same time, 13-HODE also decreases the viability of GBM cells
[139], which may depend on the activation of PPARγ via this lipid mediator
[118]. This mechanism was confirmed in other cancers, including non-small cell lung cancer
[161].
15-HETE can activate G2A/GPR132
[35]. Higher expression of this receptor. according to GEPIA. is associated with a worse prognosis for a GBM patient (
p = 0.052)
[8]. At the same time, the importance of this receptor in GBM has not been thoroughly investigated. Studies in other models have shown that G2A/GPR132 is an oncogene
[155]; that is, 15-HETE through activation of G2A/GPR132 has a pro-cancer effect. At the same time, there is no significant upregulation of this receptor expression in GBM tumors
[8][9].
The significance of lipoxins in GBM tumors has not been thoroughly investigated. The expression level of the LXA
4 receptor ALX/FPR2 does not differ between GBM tumors and healthy brain tissue (
Table 1)
[8][9]. The expression level of this receptor in GBM tumors does not affect prognosis. However, it may be important in tumorigenesis in GBM tumors. Studies on U-87 MG cells have shown that silencing ALX/FPR2 reduces the proliferation and migration of the cells tested
[162]. In addition, cells with silenced ALX/FPR2 showed lower expressions of VEGF, a major pro-angiogenic factor. However, this receptor is activated not only by LXA
4 but also by other factors
[163]—for this reason, the importance of LXA
4 in tumorigenic processes in GBM cannot be determined.
Table 1. Description of the various enzymes involved in the synthesis, action, and degradation of lipoxygenases along with their involvement in tumorigenesis in GBM.