Jos arakidonihappo ei metabolisoidu, se aktivoi neutraalin sfingomyelinaasin . CytP450:n osuus.

 http://mcb.asm.org/content/21/18/6322/F7.expansion.html
 JOS ARAKIDONIHAPPO ei  tapaa tavallisia metabolisoivia entsyymeitään ja jää vapaaseen esteröitymättömään muotoon, se tarvitsee kuitenkin jotain detoksikaatiota tai aineenvaihduntaa.  Vapaa  arakidonihappo aktivoi nimittäin  neutraalia  sifingomyelinaasia, jolloin kalvon sfingomyeliinista  hajoaa esiin keramidia. Keramidi sinänsä  aktivoi kaspaasia ja apoptoositietä.   Munuaisessa on  COX ja LOX entsyymiejä  havaitsemattomia määriä, muta siellä toimii sytokromi  P450 Cytokromi P450   muodostaa ylimääräisestä arakidonihaposta  epoksituotteita   EPOXY-EIKOSA-TRIENOIC  acid.  (EET). 
EET taas kykenee  aktivoimaan PI3K ja AKT, siis apoptoosia estävän tien.


 Arakidonihappo-Keramidi- Apoptosis signalointitie 

Fig. 7.

Arachidonic acid-ceramide-apoptosis-signaling pathway 

 is regulated by cytochrome P450 in LLCPKcl4 cells.

 

Arachidonic acid, an important constituent of cell membrane, is released by activation of specific phospholipases (PLA₂) and further metabolized by cyclooxygenases (COXs) , lipoxygenases (LOXs) , and cytochrome P450 pathways.

 If unesterified arachidonic acid (free arachidonic acid)   is not metabolized, it activates neutral sphingomyelinase (N-SMase), which converts sphingomyelin to the second messenger, ceramide.

Ceramide induces caspase activation, which leads to apoptosis.

 In cells such as the renal proximal tubule, in which cyclooxygenase (COX)  and lipoxygenase (LOX)  are expressed at nearly undetectable levels, arachidonic acid (ETE)  metabolism is shunted to the cytochrome P450 pathway.
This metabolism not only metabolizes and detoxifies excess unesterified arachidonic acid to prevent proapoptotic ceramide formation but also produces a metabolite, 14,15-EET, which activates a PI-3 kinase–Akt-signaling pathway.

 Thus, cytochrome P450 mediates cell survival by two complementary mechanisms.

Otan tähän sitaatin koko artikkelin alusta 2001:

Cytochrome P450 Epoxygenase Metabolism of Arachidonic Acid Inhibits Apoptosis

1. Jian-Kang Chen1,
2. Jorge Capdevila1,2, and
3. Raymond C. Harris1,^*

+ Author Affiliations

1. Departments of Medicine¹ and
2. Biochemistry,² Vanderbilt University, Nashville, Tennessee

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ABSTRACT

The ubiquitous cytochrome P450 hemoproteins play important functional roles in the metabolism and detoxification of foreign chemicals. However, other than established roles in cholesterol catabolism and steroid hormone biosynthesis, their cellular and/or organ physiological functions remain to be fully characterized. Here we show that the cytochrome P450 epoxygenase arachidonic acid metabolite 14,15-epoxyeicosatrienoic acid (14,15-EET) inhibits apoptosis induced by serum withdrawal, H₂O₂, etoposide, or excess free arachidonic acid (AA), as determined by DNA laddering, Hoechst staining, and fluorescein isothiocyanate-labeled annexin V binding. In the stable transfectants (BM3 cells) expressing a mutant bacterial P450 AA epoxygenase, F87V BM3, which was genetically engineered to metabolize arachidonic acid only to 14,15-EET, AA did not induce apoptosis and protected against agonist-induced apoptosis. Ceramide assays demonstrated increased AA-induced ceramide production within 1 h and elevated ceramide levels for up to 48 h, the longest time tested, in empty-vector-transfected cells (Vector cells) but not in BM3 cells. 

 Inhibition of cytochrome P450 activity by 17-octadecynoic acid restored AA-induced ceramide production in BM3 cells. Exogenous C2-ceramide markedly increased apoptosis in quiescent Vector cells as well as BM3 cells, and apoptosis was prevented by pretreatment of Vector cells with exogenous 14,15-EET and by pretreatment of BM3 cells with AA. 

The ceramide synthase inhibitor fumonisin B1 did not affect AA-induced ceramide production and apoptosis; in contrast, these effects of AA were blocked by the neutral sphingomyelinase inhibitor scyphostatin. The pan-caspase inhibitor Z-VAD-fmk had no effect on AA-induced ceramide generation but abolished AA-induced apoptosis. 

The antiapoptotic effects of 14,15-EET were blocked by two mechanistically and structurally distinct phosphatidylinositol-3 (PI-3) kinase inhibitors, wortmannin and LY294002, but not by the specific mitogen-activated protein kinase kinase inhibitor PD98059. 

Immunoprecipitation followed by an in vitro kinase assay revealed activation of Akt kinase within 10 min after 14,15-EET addition, which was completely abolished by either wortmannin or LY294002 pretreatment. In summary, the present studies demonstrated that 14,15-EET inhibits apoptosis by activation of a PI-3 kinase–Akt signaling pathway. Furthermore, cytochrome P450 epoxygenase promotes cell survival both by production of 14,15-EET and by metabolism of unesterified AA, thereby preventing activation of the neutral sphingomyelinase pathway and proapoptotic ceramide formation. 

 

Arachidonic acid is an important constituent of cellular membranes that is esterified to the sn-2 position of glycerophospholipids. 

Under normal conditions, the concentration of free, nonesterified arachidonic acid is nearly undetectable, and its release is under tight metabolic and physiologic control. 

As an important component of the signaling pathways of many receptor-mediated processes, specific phospholipases are activated, and arachidonic acid is released from selected lipid stores and metabolized by cyclooxygenases (COX) and/or lipoxygenases (LOX)  to potent bioactive lipid mediators such as prostanoids, thromboxanes, leukotrienes, lipoxins, or hydroxyeicosatetraenoic acids (HETEs) (38, 49). Numerous cellular responses have been attributed to cyclooxygenase- and lipoxygenase-dependent pathways, including regulation of cell growth and induction or inhibition of apoptosis (4, 26, 45, 54). 

In addition to cyclooxygenase and lipoxygenase pathways, cytochrome P450 also catalyzes the in vivo metabolism of arachidonic acid to biologically active compounds by three types of NADPH-dependent oxidative reactions (12):

 (i) olefin epoxidation produces 5,6-, 8,9-, 11,12-, and 14,15-epoxyeicosatrienoic acids (EETs), in a regio- and stereo-selective manner;

 (ii) allylic oxidation generates 5,8,9,11,12,15 HETEs; and 

(iii) ω- and ω-1-hydroxylation results in the formation of 19- and 20-HETEs. 

 Endogenous EETs are biosynthesized in liver, kidney, and many other organs (12, 38) and are present in human plasma and urine (13, 56). Previous studies have demonstrated that EETs have potent biological activities, including modulation of vascular tone (31), glomerular hemodynamics (53), and regulation of mitogenesis (17, 32). EETs have been suggested to be an endothelium-derived hyperpolarizing factor (7). In addition, recent studies have also suggested that EETs serve as intracellular second messengers in vasculature (29) and in epithelia (6, 18). Cytochrome P450 is the predominant arachidonic acid metabolic pathway in cells such as the renal proximal tubule, in which cyclooxygenase and lipoxygenase are expressed at nearly undetectable levels (3, 21). 

In certain cells, free arachidonic acid itself serves as a regulator of specific cellular processes, including the activation of intracellular kinases and lipases and modulation of Ca²⁺ transients (19, 33, 40). Intracellular concentrations of free arachidonic acid can be increased in response to oxidant stress and other stimuli that may induce apoptosis, and increasing evidence shows that high intracellular concentrations of free arachidonic acid may be proapoptotic in many cell types (9, 22, 52, 59, 60). Since cytochrome P450 epoxygenase is a major pathway for metabolism of arachidonic acid, the present studies were designed to explore the potential roles and mechanisms of P450-mediated arachidonic acid metabolism in cell survival.