## 3 Lipid Signalling in Microgravity  
### Eicosanoids  
The possibility that metabolic enzymes of eicosanoids could be altered by microgravity was documented for the first time by us with a project performed under authentic microgravity during the 28 th parabolic flight campaign of ESA [21]. These experiments measured the catalytic constants  
Figure 3: Metabolism of endocannabinoids and related bioactive lipids. Targets that are up-regulated or down-regulated in microgravity are highlighted in green or red, respectively. 2-AG: 2-Arachidopolyglycerol; AEA: \(N\)-Arachidonylethanolamine; CB: Cannabinoid receptor; DAG: Diacylglycerolglycerol; DAGL: Diacylglycerolglycerol lipase; EtNH2: Ethanolamine; FAAH: Fatty acid amide hydrolase; HETE-EA: Hydroxyeicosatetraenoyl-ethanolamine; HETE-G: Hydroxyeicosatetraenoyl-glycerol; LOX: Lipoxygenase; LX: lipoxin; MAGL: Monoacylglycerolglycerol lipase; NAPE-PLD: \(N\)-Acyl phosphatidylcholineylethanolamine specific-phospholipase D; NAT: \(N\)-Acyl transferase; NArPE: \(N\)-Arachidonyl-phosphatidylylethanolamine; PA: Phosphatidic acid; PC: Phosphatidylcholine; PE: Phosphatidylethanolamine; PG: Prostaglandin; PI: Phosphatidylinositol; PLC: Phospholipase C; TX: Tromboxane. (Michaelis-Menten constant, \(K_{\rm m}\), and maximum velocity, \(V_{\rm max}\)) of a purified LOX-1, using the EMEC (Effect of Microgravity on Enzymatic Catalysis) module, a fiber optics spectrophotometer specifically developed to measure enzyme activity in weightlessness [21]. Microgravity ( \(\sim\)10\({}^{-2}\)\(g\)) was shown to reduce the \(K_{\rm m}\) value (i.e., to enhance the affinity) of LOX-1 for its fatty acid substrate without affecting \(V_{\rm max}\), thus pointing to LOX-1 as the first protein able to act as a direct \"gravity sensor\" [18]. Later on, we reported that simulated microgravity (\(\sim 10^{-3}\)\(g\))--achieved by means of clinorotation--is able to almost double COX-2 activity in human bone marrow K562 cells, whereas simulated hypergravity had the opposite effect [21], further supporting the observation that eicosanoid-metabolising enzymes do react to gravitational changes. Remarkably, COXs and LOXs represent the two main enzymes controlling eicosanoid levels during inflammation, therefore these data suggest that microgravity might have an impact on phlogistic processes by targeting AA-derived endogenous lipids. In addition, LOXs are well-known modulators of immune cell biology [49], and their alteration during spaceflight might represent one of the causes leading to the microgravity-related impairment of lymphocyte function. We were able to confirm this hypothesis in two additional projects. In the first study, we demonstrated that human peripheral blood mononuclear cells (PBMCs) undergo enhanced apoptosis when exposed to RPM-simulated microgravity ( \(\sim\)10\({}^{-3}\)\(g\)) and--most importantly--that programmed cell death was strictly due to enhanced 5-LOX activity. Indeed, microgravity-induced apoptosis was prevented by 5-LOX pharmacological inhibition, and human U937 immune cells devoid of 5-LOX were resistant to it [87]. Of note, the engagement of 5-LOX in this type of apoptosis was isozyme-specific, as pharmacological inhibition of other LOX isoforms had no effect [87]. More recently, we were also able to extend these data under authentic microgravity, by studying human PBMCs on board the ISS in the frame of the ROALD mission [68]. During this project, not only could we confirm that human PBMCs display enhanced apoptosis-associated markers (e.g., DNA fragmentation and poly-ADP ribose polymerase expression), but also that this feature was due to increased 5-LOX activity [12]. Additional studies corroborated and extended these findings, showing that 5-LOX inhibition reverts both apoptosis and altered production of pro-inflammatory cytokines (e.g., IL-2 and INF-\(\gamma\)) in human Jurkat T cells [22]. Besides the alterations of immune functions, bone loss is a major threat to astronauts' health during Space missions, and represents a limit to long-term manned flight.