**2. Hypercholesterolemia and chronic inflammation**

Hypercholesterolemia is the dominant risk factor for atherosclerosis and its downstream complications (e.g., heart attack, stroke, etc.). Over the past two decades, a wealth of insight has pointed to the development of atherosclerotic lesions in the large vessels (e.g., aorta, carotid, femoral artery, etc.) as occurring at the interface between hypercholesterolemia and inflammation (see reviews[4, 5]). According to the current paradigm, at atheroma-prone sites, inflamed endothelial cells (due to damage or dysregulation) initiate the invasion of blood leukocytes (predominantly, monocytes) and smooth muscle cells (SMCs) into the subendothelial (e.g., the intimal) layer of the vascular wall contributing to atherosclerotic tissue remodeling, thrombosis, and finally embolus formation. The main lipid species that appear to dominate this inflammatory process are modified low-density lipoprotein (LDL) particles, particularly oxidized LDL (oxLDL), which act as potent proatherogenic and proinflammatory factors responsible for not only loading monocyte-derived macrophages with cholesterol but also directly stimulating leukocytes and other vascular wall cells (for a more complete explanation, see reviews[5, 6]).

Hypercholesterolemia also induces chronic inflammation in the microcirculation[7]. Phenotypic changes in the microvasculature are observed long before the appearance of fatty streak lesions in the large arteries of animals placed on high fat (HFD), i.e. proatherogenic, diet[8, 9]. The inflammatory phenotype of the microvessels in hypercholesterolemic animals results in increased basal levels of rolling, adherent, and emigrating leukocytes in the postcapillary venules, predominantly neutrophils, as well as enhanced production of reactive oxygen species (ROS). Hypercholesterolemia also exaggerates microvascular responses to a range of proinflammatory stimuli. For example, the postcapillary venules of LDL receptor deficient (LDLr-/-) mice, a murine model of modest hypercholesterolemia (with 3-fold higher levels of plasma cholesterol compared to their wild-type (WT) counterparts), exhibit enhanced leukocyte adhesion and albumin leakage in response to experimental ischemia/reperfusion injury as compared to those of WT mice[10]. Interestingly, similar phenotypic changes can be observed in the microvasculature of normocholesterolemic animals administered oxLDL[11, 12], suggesting that oxLDL participates in hypercholesterolemia-related microvascular dysfunction.

Although the mechanisms responsible for the induction of inflammation by hypercholesterolemia in both microvessels and larger arteries remain unclear, it appears that they both begin with endothelial dysfunction characterized by reduced vasodilation, a proinflammatory state, and enhanced permeability to macromolecules (e.g., lipids). However, the original triggers for this endothelial dysfunction are still controversial. In this regard, oxLDL, based on its potent proinflammatory effects, has been considered as a candidate that initiates the inflammatory responses. In fact, production and release of ROS and myeloperoxidase (MPO)[13], which play critical roles in the oxidation of LDLcholesterol conjugates and are tightly controlled under the physiological non-inflamed conditions, increase in response to hypercholesterolemia. The cellular basis for the causality between oxLDL formation and chronic inflammation, however, remains elusive.

224 Lipoproteins – Role in Health and Diseases

pathologies.

will first discuss the link between chronic inflammation and hypercholesterolemia and then highlight the neutrophil involvement in the pathophysiology of related cardiovascular diseases, e.g. atherosclerosis and microvascular dysfunction. To further exemplify this link, we will explain the potential mechanism(s) by which cholesterol in blood may impact the biochemical regulation of neutrophil activity at the cellular level. Finally, we will introduce our own evidence as well as those of others pointing to dysregulated neutrophil mechanotransduction as an important component of hypercholesterolemic

Hypercholesterolemia is the dominant risk factor for atherosclerosis and its downstream complications (e.g., heart attack, stroke, etc.). Over the past two decades, a wealth of insight has pointed to the development of atherosclerotic lesions in the large vessels (e.g., aorta, carotid, femoral artery, etc.) as occurring at the interface between hypercholesterolemia and inflammation (see reviews[4, 5]). According to the current paradigm, at atheroma-prone sites, inflamed endothelial cells (due to damage or dysregulation) initiate the invasion of blood leukocytes (predominantly, monocytes) and smooth muscle cells (SMCs) into the subendothelial (e.g., the intimal) layer of the vascular wall contributing to atherosclerotic tissue remodeling, thrombosis, and finally embolus formation. The main lipid species that appear to dominate this inflammatory process are modified low-density lipoprotein (LDL) particles, particularly oxidized LDL (oxLDL), which act as potent proatherogenic and proinflammatory factors responsible for not only loading monocyte-derived macrophages with cholesterol but also directly stimulating leukocytes and other vascular wall cells (for a

Hypercholesterolemia also induces chronic inflammation in the microcirculation[7]. Phenotypic changes in the microvasculature are observed long before the appearance of fatty streak lesions in the large arteries of animals placed on high fat (HFD), i.e. proatherogenic, diet[8, 9]. The inflammatory phenotype of the microvessels in hypercholesterolemic animals results in increased basal levels of rolling, adherent, and emigrating leukocytes in the postcapillary venules, predominantly neutrophils, as well as enhanced production of reactive oxygen species (ROS). Hypercholesterolemia also exaggerates microvascular responses to a range of proinflammatory stimuli. For example, the postcapillary venules of LDL receptor deficient (LDLr-/-) mice, a murine model of modest hypercholesterolemia (with 3-fold higher levels of plasma cholesterol compared to their wild-type (WT) counterparts), exhibit enhanced leukocyte adhesion and albumin leakage in response to experimental ischemia/reperfusion injury as compared to those of WT mice[10]. Interestingly, similar phenotypic changes can be observed in the microvasculature of normocholesterolemic animals administered oxLDL[11, 12], suggesting

that oxLDL participates in hypercholesterolemia-related microvascular dysfunction.

Although the mechanisms responsible for the induction of inflammation by hypercholesterolemia in both microvessels and larger arteries remain unclear, it appears

**2. Hypercholesterolemia and chronic inflammation** 

more complete explanation, see reviews[5, 6]).

Interestingly, the preferential formation of atherosclerotic lesions at bifurcations, severe curvatures, and stenoses in the arterial circulation strongly suggests that the hemodynamic flow environment is an important determinant in atherogenesis. Fluid flow-derived frictional (i.e., shear) stresses imposed on the surfaces of endothelium lining the vascular wall have been shown to serve an atheroprotective function when blood flow is laminar (i.e., smooth and ordered; for a more comprehensive discussion, see review[14]). For example, laminar fluid flow stimulates endothelial production of nitric oxide (NO), with vasodilatory and anti-inflammatory actions[15]. In contrast, oscillatory shear stresses enhance production and release of ROS[16]. In addition, disturbed flows lead to the upregulation of adhesion molecules on the endothelial surface (e.g., intercellular adhesion molecule-1 or ICAM-1) responsible for recruiting leukocytes to the vascular wall[17]. In effect, generation of complex distributions of fluid shear stresses on the vascular wall, such as at sites of bifurcations and branch points, appears to shift the endothelial phenotype from atheroprotective to proatherogenic.

However, complex flow fields are not sufficient for the onset/progression of hypercholesterolemia-related atherosclerosis since, for example, we are born with bifurcations and curvatures but do not develop atherosclerosis from birth. It is, therefore, clear that cardiovascular pathobiology due to hypercholesterolemia occurs at the intersection between vascular cell biology and the surrounding fluid stress environment. In this regard, it may be the sensitivity (i.e., responsiveness) of vascular cells to fluid shear stress that is altered in the face of hypercholesterolemia leading to a proinflammatory and a proatherogenic phenotype. Moreover, the endothelial cells are not the only cells in the vasculature. Neutrophils also exist in the cholesterol-enriched, fluid flow environment of the circulation and are critical for initiating acute inflammation. Recently, a growing body of evidence supporting the involvement of neutrophils has emerged.
