Synthesis and Significance of Arachidonic Acid, a Substrate for Cyclooxygenases, Lipoxygenases, and Cytochrome P450 Pathways in the Tumorigenesis of Glioblastoma Multiforme, Including a Pan-Cancer Comparative Analysis

Abstract

Glioblastoma multiforme (GBM) is one of the most aggressive gliomas. New and more effective therapeutic approaches are being sought based on studies of the various mechanisms of GBM tumorigenesis, including the synthesis and metabolism of arachidonic acid (ARA), an omega-6 polyunsaturated fatty acid (PUFA). PubMed, GEPIA, and the transcriptomics analysis carried out by Seifert et al. were used in writing this paper. In this paper, we discuss in detail the biosynthesis of this acid in GBM tumors, with a special focus on certain enzymes: fatty acid desaturase (FADS)1, FADS2, and elongation of long-chain fatty acids family member 5 (ELOVL5). We also discuss ARA metabolism, particularly its release from cell membrane phospholipids by phospholipase A₂ (cPLA₂, iPLA₂, and sPLA₂) and its processing by cyclooxygenases (COX-1 and COX-2), lipoxygenases (5-LOX, 12-LOX, 15-LOX-1, and 15-LOX-2), and cytochrome P450. Next, we discuss the significance of lipid mediators synthesized from ARA in GBM cancer processes, including prostaglandins (PGE₂, PGD₂, and 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂)), thromboxane A₂ (TxA₂), oxo-eicosatetraenoic acids, leukotrienes (LTB₄, LTC₄, LTD₄, and LTE₄), lipoxins, and many others. These lipid mediators can increase the proliferation of GBM cancer cells, cause angiogenesis, inhibit the anti-tumor response of the immune system, and be responsible for resistance to treatment.

Keywords: 5-HETE; 5-lipoxygenase; PUFA; arachidonic acid; cyclooxygenase-2; fatty acid; glioblastoma multiforme; leukotriene; prostaglandin.

Figure 1

ARA biosynthesis. ARA C20:4n-6 in humans is not synthesized de novo but from linoleic acid C18:2n-6. As linoleoyl-CoA C18:2n-6, this PUFA undergoes desaturation to γ-linolenoyl-CoA C18:3n-6 with FADS2/D6D. This fatty acyl-CoA is then converted to dihomo-γ-linolenoyl-CoA C20:3n-6 with ELOVL5 and, finally, to arachidonyl-CoA C20:4n-6 with FADS1/D5D. Dihomo-γ-linolenoyl-CoA C20:3n-6 can also be formed from linoleoyl-CoA via an alternative pathway. Linoleoyl-CoA C18:2n-6 first undergoes elongation with ELOVL5 and then desaturation with FADS2. The latter enzyme in this pathway exhibits Δ⁸-desaturase activity. ↑—higher expression of given enzymes in GBM tumor relative to healthy tissue.

Figure 2

Importance of PLA₂ in metabolism of ARA and production of lipids mediators from ARA. ARA C20:4n-6 is cleaved from PC by PLA₂. This reaction also produces LPC, which can be converted in the intercellular space to LPA by ATX. LPA can be considered a lipid mediator because its biological activity is related to the activation of its specific receptors: LPAR₁-LPAR₆. Free ARA C20:4n-6, on the other hand, can be used for eicosanoid production in either the COX pathway or the LOX pathway. ↑—higher expression of given enzymes in GBM tumor relative to healthy tissue; ↓—lower expression of given enzymes in GBM tumor relative to healthy tissue.

Figure 3

COX pathway. After release by PLA₂, ARA C20:4n-6 is converted into prostanoids with COX. It is transformed into PGH₂ with either COX-1 or COX-2. Then, this prostaglandin is transformed into other prostaglandins (PGE₂, PGD₂, PGI₂, and PGF_2α) or TxA₂ by the respective synthases. These lipid mediators undergo further transformations. TxA₂ is unstable and undergoes a spontaneous transformation into TxB₂. Similarly, PGD₂ undergoes spontaneous transformation to PGJ₂—this prostaglandin can then be transformed into 15-deoxy-Δ^12,14-PGJ₂ (15d-PGJ₂) or Δ¹²-PGJ₂. PGE₂ can be transformed into PGA₂, and then into PGC₂ and PGB₂. Prostanoids also undergo degradation. The figure shows an example of PGE₂, which undergoes inactivation by oxidation with 15-PGDH and reduction with PTGR1/2. PGE₂ can also undergo degradation by β-oxidation and ω-oxidation, followed by the action of 15-PGDH and PTGR1/2. The resulting degradation product is PGE₂, which is removed from the body. ↑—higher expression of given enzymes in GBM tumor relative to healthy tissue; ↓—lower expression of given enzymes in GBM tumor relative to healthy tissue.

Figure 4

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.

Figure 5

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.

Figure 6

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.

Figure 7

Cytochrome P450 pathway. ARA 20:4n-6 can be converted in the cytochrome P450 pathway, resulting in the formation of various ETT and HETE. ETT can undergo further transformations where they are incorporated into glycerophospholipids in the sn-2 position; in this form, they build the cell membrane and intracellular membranes. In addition, the epoxide bond in ETT can be transformed by EPHX1 and EPHX2 into two hydroxyl groups, resulting in the formation of various DHET. ETT can also undergo ω-hydroxylation, which results in the formation of various HEET. ETT can be converted with COX. 5,6-EET then produces 5,6-epoxy-PGH₂, whereas 8,9-EET produces either 8,9,11-EHET or 8,9,15-EHET. ↑—higher expression of given enzymes in GBM tumor relative to healthy tissue; ↓—lower expression of given enzymes in GBM tumor relative to healthy tissue.