**3. Mechanism of action**

Specific receptor binding of OxPLs is the subject of an ongoing debate. Available evidence suggests that OxPLs interact with various signal transduction receptors and pattern recognition receptors present on the cell surface. Most commonly known receptors include CD36, SRB1, EP2, VEGFR2 and the PAF receptor (Bochkov et al 2007, Zimman et al 2007). It has been demonstrated that when present in vesicles, truncated oxidized fatty acids at the sn-2 position move from the hydrophobic interior to the aqueous exterior of the vesicle. this would allow their recognition by cell surface receptors Earlier models of isoprostanecontaining phospholipids have suggested that they are highly twisted and may distort membrane areas in which they are present (Morrow et al 1992). Moumtzi et al (2007) have shown that phospholipid oxidation products can integrate into lipid membranes of cells and lipoproteins; they can either act as ligands or may cause local membrane disruption. Besides, peroxidation of phospholipids leads to the accumulation of lysoforms as a result of both non-enzymatic decylation and enzymatic hydrolysis reactions catalyzed to a large extent by lipoprotein-as-associated phospholipase A2 (also known as PAF acetylhydrolase), which has high substrate selectivity toward polar phospholipids, including the oxidized forms (Zalewski et al 2005). Some lysophospholipids bind and activate G protein-coupled receptors (GPCR). Parhami et al (1993 & 1995) explained that oxidized phospholipids act by binding to a G protein-coupled receptor. These authors demonstrated that minimally modified LDL stimulated a putative Gs-coupled receptor, thus increasing cyclic AMP (cAMP) levels in endothelial cells. Lysophosphatidylcholine and lysophosphatidic acid triggered the activity of G2A and LPA1-LPA4 receptors respectively (Tomura et al 2005, Anliker et al 2004). In addition to GPCR, OxPLs also activate other classes of receptors such as peroxisome proliferator-activated receptors (PPAR). Thus, phospholipid peroxidation may induce the generation of lysophospholipids that are known to accumulate in LDL (OxLDL) and atherosclerotic lesions (Siess et al 2004, Tselepis et al 2002).

Prostaglandin receptors have been recently implicated into OxPLs-induced inflammation. OXPAPC and its component lipid PEIPC are able to stimulate prostaglandin E2 and D2 receptors (EP2 and DP respectively) and to compete with receptor binding of radio labeled prostaglandin E2 (Li et al 2006). Previously, it was observed that POVPC binds to human macrophages via the PAF receptor (PAF-R). Occupancy of the PAF-R by the OxPLs modifies the transcription levels of pro-inflammatory genes such as IL-8 (Pegorier et al 2006).

Some effects of OxPLs are probably not mediated by signal transducing receptors. Modulation of cellular cholesterol depots has been suggested as a non-receptor mediated mechanism of OxPLs sensing by cells. It is well illustrated that OxPAPC induces depletion and re-distribution of cellular cholesterol reserves finally leading to the activation of a transcription factor SREBP, a well recognized sensor for cellular cholesterol contents. In turn, SREBP activates IL-8 production (Yeh et al 2004).The human aortic EC gene expression was found to be stimulated by PAPC. Furthermore, OxPAPC may bind to a 37KDa glycosylphosphatidylinositol anchored protein, which interacts with TLR4 to induce interleukin-8 (IL-8) transcription (Walton et al 2003). Leitinger et al (2003) and Watson et al (1997) have described a possible role of toll-like receptors (TLRs) in OxPLs-induced inflammation. Studies have confirmed that Asp299Gly-TLR4 polymorphism plays a protective role in attenuation of atherosclerosis.

Mitogen activated protein kinase phosphatase-1 (MKP-1) was reported to be involved in OxPAPC-induced MCP-1 production. Also activation of eNOS by OxPAPC is regulated via a phosphatidylinositol-3-kinase/Akt-mediated mechanism, OxPAPC-induced SREBP activation is significantly reduced with eNOS inhibition (Berliner and Gharavi, 2008).

Chen et al (2007) reported that LDL-associated phosphatidylcholine esterified with *sn*-2 azelaic acid at the *sn*-2 position is readily taken up by cells. This compound, one of the main phospholipid oxidation products in LDL, induces apoptosis of HL60 cells at low micromolar concentrations. Since the intact phospholipid is required for signaling, this effect can be prevented by over-expression of PAF acetyl hydrolase known for oxidizing phospholipids with polar residues at the *sn*-2 position.

Another biologically active phospholipid described is platelet activating factor (PAF) having various inflammatory actions such as platelet aggregation, hypotension, anaphylactic shock and increased vascular permeability (Prescott et al 2000). PAF is structurally identified as 1- 0-alkyl-2-acetyl-*sn*-glycero-3-phosphocholine. Atherogenic effects are also induced by PAF by activating monocytes and stimulating smooth muscle cell growth. In contrast to the tightly regulated physiological generation of PAF, uncontrolled processes of free radical oxidation generate analogs of PAF *in vivo* and *in vitro*. As a result of this uncontrolled chemical reaction, fragmentation of the residue at sn-2 position occurs and these oxidatively generated PAF mimetics stimulate monocytes, leukocytes and platelets. They are found in atherosclerotic lesions and even in blood from individuals exposed to cigarette smoke (Heery et al 1995).

Other oxidized phospholipids such as POVPC and PGPC have also been shown to play major roles in activation of endothelial cells and induction of leukocyte binding. They are identified as abundant products in oxidized LDL. The effect of POVPC is protein kinase-A dependent leading to the stimulation of the cAMP-mediated pathway (Berliner and Gharavi, 2008).

OxPLs also induces autocrine mediators such as vascular endothelial growth factor (VEGF), which works through activation of transcription factor-4 (ATF4) (Oskolkova et al 2008).
