La dimensión existencial o formal

As a heterogeneous population, oxLDL is able to promote a range of atherogenic effects. All contribute further to the development of atherosclerosis but the exact biological activity differs depending on the extent of oxidation. Thus, the properties of mmLDL differ from those of the more heavily oxLDL, with the former still being recognized by the LDL receptor but also playing a critical role in the induction of various pro-inflammatory cytokines. Continued oxidation reduces this activity and ultimately produces a particle that is both rapidly taken up by scavenger receptors and is cytotoxic to cells (Berliner et al., 1995).

OxLDL-mediated cytotoxicity is directed not only towards oxidizing macrophage cells themselves (Marchant et al., 1996) but also towards other cells subsequently exposed to the oxLDL. Cell types tested under these conditions include SMCs (Nishio et al., 1996), endothelial cells (Li et al., 1998a), macrophages (Kinscherf et al., 1998), T cells (Alcouffe et al., 1999) and fibroblasts (Chisolm et al., 1994). Depending on the cell type and extent of damage, this cell death may occur via caspase-dependent or -independent apoptosis or via necrosis (Vicca et al., 2000; Yuan et al., 2000; Asmis & Begley, 2003; Baird et al., 2004). While the cytotoxic activity of oxLDL is predominantly attributed to 7β-hydroperoxy-cholesterol (Chisolm et al., 1994; Colles et al., 1996), other oxysterols like 7-ketocholesterol and 7β-hydroxycholesterol can also contribute to cell death (Colles et al., 1996; Nishio et al., 1996). These cholesterol oxidation products are reported to destabilize macrophage lysosomes, leading to

leakage of lysosomal contents and subsequent induction of apoptosis or necrosis (Yuan et al., 2000). 7β-hydroperoxy-cholesterol decomposition further contributes to oxysterol toxicity due to the formation of reactive alkoxyl, peroxyl and lipid radicals that can propagate peroxidation of cellular lipids (Coffey et al., 1995). Other hydroperoxides and their aldehyde break-down products have also been suggested to contribute to the cytotoxicity of oxLDL (Siow et al., 1999; Uchida, 2000; Choudhary et al., 2002), as have phospholipid hydrolysis products like lysophosphatidylcholine (Carpenter et al., 2001).

Components of oxLDL and ceroid are known to accumulate in macrophage lysosomes (Mander et al., 1994; Brown et al., 2000; Jessup & Kritharides, 2000) and this is caused, at least in part, by inactivation of some of the lysosomal proteases and hydrolases (Kritharides et al., 1998; Li et al., 1998b). This accumulation clearly contributes to cytotoxicity but it also has the potential to impair other cellular activities, including endocytic and secretory macrophage functions (Bolton et al., 1997).

Several pro-atherogenic consequences arise from oxLDL-mediated cytotoxicity. In particular, damaged endothelial cells may enhance entry of LDL and/or monocytes into the arterial intima (Steinberg et al., 1989) while macrophage death may contribute to growth of the necrotic core (Bjorkerud & Bjorkerud, 1996). Plaque instability may be promoted at later stages of atherosclerosis due to endothelial denudation (Steinberg et al., 1989) or macrophage and SMC death (section 1.2.1.3).

OxLDL promotes atherogenesis via additional non-cytotoxic pathways, particularly when present as mmLDL. It is known to act as a chemoattractant towards both T cells and monocytes (Young & McEneny, 2001) and, while some of this activity is directly attributable to mmLDL, it is further aided by the mmLDL-mediated induction of chemoattractant cytokines like P-selectin, GRO, VCAM-1 and MCP-1 (Berliner et al., 1995; Berliner & Heinecke, 1996). Once recruited to the intima, monocytes are then subject to an additional array of actions mediated by oxidized components of LDL. In particular, monocyte differentiation is promoted by the oxLDL-induced release of endothelial-derived M-CSF (Rajavashisth et al., 1990). Subsequent macrophage proliferation (Martens et al., 1999) and retention in the intima (Quinn et al., 1987) are also enhanced by this modified lipoprotein. Likewise, macrophage cholesterol metabolism is influenced by oxLDL at all stages, from uptake to degradation (as described above) and even efflux. While evidence for uptake is provided by increased

SRA and CD36 expression (Nakagawa et al., 1998; Glass & Witztum, 2001), cholesterol efflux studies have proved more contradictory. One group observed increased cholesterol export due to the oxLDL-mediated upregulation of ATP binding cassette transporter-1 (ABC1) (Tang et al., 2004) but others have noted inhibition of efflux (Kritharides et al., 1995b; Gelissen et al., 1996). Such disparity may be due to cell-specific effects or to differences in the nature of the oxLDL being studied.

The pro-atherogenic activity of oxLDL also extends to other cells, ranging from enhanced SMC proliferation (Auge et al., 2002) and SMC migration (Gorog & Kovacs, 1998) to activation of T lymphocytes (Frostegard et al., 1992) but inhibition of endothelial cell migration (Murugesan & Fox, 1996). Conversely, oxLDL may promote endothelial proliferation via upregulation of vascular endothelial growth factor (VEGF) expression (Inoue et al., 2001). This proliferation exerts an anti-atherogenic effect early in atherosclerosis but is pro-atherogenic at later stages due to microvessel formation. OxLDL further influences endothelial cell activity by inhibiting nitric oxide production (Huang et al., 1999) but stimulating endothelin synthesis (Berliner & Heinecke, 1996). The net result is a loss of vasodilatory activity in the artery wall.

Likewise, oxLDL adversely affects plaque stability. It promotes a net increase in macrophage-mediated matrix degradation by down-regulating the tissue inhibitor of metalloproteinase-1 (TIMP-1) but upregulating matrix metalloproteinase-9 (MMP-9) (Xu et al., 1998). OxLDL also stimulates the production of tissue factor (Berliner et al., 1995) and promotes platelet aggregation (Volf et al., 2000), further raising the likelihood of thrombus formation.

It is therefore clear that oxLDL induces a wide range of biological activities. While most of these activities are attributed to its oxidized phospholipids, including lysophosphatidylcholine (Chisolm & Chai, 2000; Subbanagounder et al., 2000), other components of oxLDL are also postulated to play a contributory role. For instance, oxLDL’s protein moiety appears to stimulate interleukin-1 (IL-1) production (Lipton et al., 1995) and respiratory burst activity in macrophages (Nguyen-Khoa et al., 1999) while lipid hydroperoxides inhibit nitric oxide synthesis (Huang et al., 1999). Furthermore, inhibition of cholesterol efflux is primarily ascribed to 7-ketocholesterol and the accumulation of undegradable cholesteryl esters in lysosomes (Jessup et al., 2004).

OxLDL’s capacity to influence so many seemingly disparate biological functions is ultimately attributed to an increase in secondary messengers, like cyclic adenosine

monophosphate (cAMP) (Parhami et al., 1993), and to the induction of various cytokines and signal transduction pathways (Berliner & Heinecke, 1996). Signal transduction occurs through the oxLDL-mediated activation of protein kinase C (PKC), nuclear factor-κβ (NF-κβ), activator protein-1 (AP-1) and peroxisome proliferator- activated receptor-γ (PPARγ) (Parhami et al., 1993; Berliner & Heinecke, 1996; Li et al., 1998a; Nagy et al., 1998; Tontonoz et al., 1998; Inoue et al., 2001; Whatling et al., 2004).