**4. OxPLs receptors**

414 Lipoproteins – Role in Health and Diseases

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

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

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

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

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

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

activation is significantly reduced with eNOS inhibition (Berliner and Gharavi, 2008).

the transcription levels of pro-inflammatory genes such as IL-8 (Pegorier et al 2006).

(OxLDL) and atherosclerotic lesions (Siess et al 2004, Tselepis et al 2002).

protective role in attenuation of atherosclerosis.

with polar residues at the *sn*-2 position.

It has been shown that OxPLs stimulate a number of signal-transducing receptors located on the cell surface or in the nucleus, including G protein-coupled receptors, receptor tyrosine kinases, Toll-like receptors, receptors coupled to endocytosis, and nuclear ligand-activated transcription factors such as PPARs.

#### **4.1. Prostaglandin receptors**

OxPCs containing esterified PEIPC activate receptors recognizing prostaglandins E2 and D respectively (Li et al 2006). Activation of EP2 receptor on ECs results in activation of integrins and increased binding of monocytes.

#### **4.2. Scavenger receptors**

OxPLs comprise a major group of ligands for scavenger receptors. Different classes of Scavenger receptors range from Class A, B, D, E and F depending upon the nature and type of ligand (FIGURE-2). CD36 have been described as the major receptor expressed on macrophages and involved in the process of atherogenesis and apoptosis. The role of CD36 has been shown to be responsible for recognition of free oxidized phospholipids (Boullier et al 2000, Podrez et al 2000). Also Boullier et al (2000) and Watson et al (1997) have pointed out that oxidized phospholipid is covalently linked to apolipoprotein B-100 in extensively oxidized LDL (e.g. Cu2+ -oxLDL) and serve as ligand for CD36. Scavenger receptor- ligand interaction initiates signaling cascades that regulate macrophage activation, lipid metabolism and inflammatory pathways which may influence the development and stability of atherosclerotic plaque. Recent studies have demonstrated the expression of scavenger receptors especially CD36 and SR-BI on platelets suggesting their critical role in platelet hyper-reactivity in dyslipidemia and atheroprogression.

Ashraf & Srivastava, 2012

**Figure 2.** Schematic representations of different class of scavenger receptors involved in OxPLs binding.

#### **4.3. PAF receptors**

OxPLs initiate activation of receptor specific for PAF, which act as an important lipid mediator of inflammation and platelet aggregation. It recognizes alkyl-acylphosphatidylcholines specifically and contains an ether bond at the sn-1 position in combination with unusually short sn-2 acetyl residue. Oxidative fragmentation of sn-2 PUFAs in alkyl-PCs generates products such as 1-alkyl-2-butenoyl and 1-alkyl-2-butanoyl that are recognized by PAF receptor (Androulakis et al 2005, Marathe et al 1999). However, the role of the PAF receptor in the overall biological activity of OxPCs is not characterized.

#### **4.4. VEGF receptors**

It has been demonstrated that phosphorylation (activation) of VEGFR2 is enhanced within the first minutes of incubation with OxPAPC (Zimman et al 2007). They hypothesized that trans-activation of VEGFR2 in OxPAPC-treated cells was mediated by c-SRC.

#### **4.5. Sphingosine-1-phosphate (S1P) receptor 1**

It has shown that OxPAPC stimulates the recruitment of S1P1 to caveolin-enriched membrane microdomains, and induces its phosphorylation (activation) by AKT. Transactivation of S1P1 by OxPAPC plays a role in barrier-protective function of OxPLs.

#### **4.6. Toll-like receptor 4**

416 Lipoproteins – Role in Health and Diseases

Ashraf & Srivastava, 2012

**4.3. PAF receptors** 

**4.4. VEGF receptors** 

binding.

**Figure 2.** Schematic representations of different class of scavenger receptors involved in OxPLs

OxPLs initiate activation of receptor specific for PAF, which act as an important lipid mediator of inflammation and platelet aggregation. It recognizes alkyl-acylphosphatidylcholines specifically and contains an ether bond at the sn-1 position in combination with unusually short sn-2 acetyl residue. Oxidative fragmentation of sn-2 PUFAs in alkyl-PCs generates products such as 1-alkyl-2-butenoyl and 1-alkyl-2-butanoyl that are recognized by PAF receptor (Androulakis et al 2005, Marathe et al 1999). However, the role of the PAF receptor in the overall biological activity of OxPCs is not characterized.

It has been demonstrated that phosphorylation (activation) of VEGFR2 is enhanced within the first minutes of incubation with OxPAPC (Zimman et al 2007). They hypothesized that

It has shown that OxPAPC stimulates the recruitment of S1P1 to caveolin-enriched membrane microdomains, and induces its phosphorylation (activation) by AKT. Transactivation of S1P1 by OxPAPC plays a role in barrier-protective function of OxPLs.

trans-activation of VEGFR2 in OxPAPC-treated cells was mediated by c-SRC.

**4.5. Sphingosine-1-phosphate (S1P) receptor 1** 

TLR4 plays a role in OxPAPC-mediated induction of IL-8 in HeLa cells. OxPAPC also induces lung injury and IL-6 production by mouse lung macrophages via the TLR4–TRIF– TRAF6 pathway (Imai et al 2008). On the other hand various classes of OxPLs do not influence the basal levels of E-selectin, ICAM-1, VCAM-1, TNFa, IL-6, IL-1a, IL-1b, and COX-2 in whole blood or individual cell types, including human umbilical vein ECs, blood monocytes, macrophage cell line, or fibroblasts (Bochkov et al 2002, Erridge et al 2008).

#### **4.7. PPARα and PPARγ**

Peroxisome proliferator-activated receptors (PPARs) are intracellular ligand-activated transcription factors. Diacyl-OxPLs stimulated a PPAR response element-driven reporter construct in transfected HAECs and the effect of OxPAPC, POVPC, and PGPC was mediated by PPARα as indicated by the activation of the ligand binding domain of PPARα, but not PPARγ or PPARδ (Lee et al 2000).

Second messengers up-regulated by OxPLs: Apart from the above described receptors , minimally modified Low Density Lipoproteins (MM-LDL) also induces elevation of Ca2+ in ECs (Honda et al 1999) and also OxPAPC was shown to induce rapid and reversible Ca2þresponses in ECs (Bochkov et al 2002). MM-LDL causes a saturable dose-dependent increase in cAMP levels in aortic ECs that may arise due to activation of Gs and inhibition of Gi heterotrimeric G-protein complexes (Parhami et al 1995).
