Inflammation, not Cholesterol, Is a Cause of Chronic Disease

Abstract

Since the Seven Countries Study, dietary cholesterol and the levels of serum cholesterol in relation to the development of chronic diseases have been somewhat demonised. However, the principles of the Mediterranean diet and relevant data linked to the examples of people living in the five blue zones demonstrate that the key to longevity and the prevention of chronic disease development is not the reduction of dietary or serum cholesterol but the control of systemic inflammation. In this review, we present all the relevant data that supports the view that it is inflammation induced by several factors, such as platelet-activating factor (PAF), that leads to the onset of cardiovascular diseases (CVD) rather than serum cholesterol. The key to reducing the incidence of CVD is to control the activities of PAF and other inflammatory mediators via diet, exercise, and healthy lifestyle choices. The relevant studies and data supporting these views are discussed in this review.

Keywords: atherosclerosis; cardiovascular disease; cholesterol; chronic diseases; inflammation; oxidised lipoproteins; platelet-activating factor.

Figure 1

(A) Typical structure of classic platelet-activating factor (PAF) molecule [86]. (B) Representative structures of bioactive polar lipids (PL) towards the PAF inflammatory pathways (B), which have been identified in several foods of the Mediterranean diet [56].

Figure 2

Role of PAF, PAF-R, and its related pathways in the inflammatory cascades and in the pathogenesis of inflammation-related chronic disorders; increased PAF levels by pro-inflammatory stimuli and binding of PAF on its receptor, PAF-R, on the membranes of several cell types can lead to intracellular cascades and a PAF cycle-related amplification of the initial stimuli (A) and in numerous cell responses according to each cell type (B), which can lead to endothelial dysfunction and the onset and progression of inflammation-related chronic diseases. A1. Several risk factors and related upstream pro-inflammatory stimuli trigger formation of PAF and PAF-like molecules (i.e., oxidised phospholipids) and expression of PAF-R. A2. Binding of PAF/PAF-like molecules on PAF-R promote several inflammation-related intracellular pathways; activation of the PAF-R signalling initiates (through a Gq-linked mechanism) PLCβ-mediated hydrolysis of PIP₂ to produce IP₃ and DAG, leading to transient elevation of cytosolic Ca²⁺ released from intracellular stores and activation of PKC. The rise in Ca²⁺ also activates cPLA_2α, leading to the release of AA and lysophosphatides, which can serve as substrates for further synthesis of eicosanoids and PAF, respectively. Signalling through Gi-linked PAF-R inhibits the conversion of ATP to cAMP by adenylate cyclase, in this way preventing the activation of PKA and related anti-inflammatory signalling events. A3. Activation of the PAF/PAF-R intracellular pathways leads to the activation of cPLA₂ and PAF biosynthetic enzymes (LPCAT) for further formation of PAF and other lipid second messengers, thus creating a PAF cycle and further amplification of the initial inflammatory stimuli, while expression of genes involved in inflammatory manifestations (such as genes of several cytokines, integrins, selectins, metalloproteinase, several enzymes for eicosanoids, and ROS, etc.) is also induced. The pathways inducing the PAF-CPT-related synthesis of PAF are not fully elucidated. B. Increased PAF levels at the site of inflammation and ligand binding (PAF and/or oxidised phospholipids binding) on PAF-R can promote a broad spectrum of PAF effects depending on the cell type and tissue, which is achieved through the production and release of various downstream mediators, such as PAF itself and several other mediators of inflammation such as eicosanoids, cytokines (i.e., TNF-α, IL-1α, IL-6, IL-8, INF-γ, etc.), growth factors (i.e., VEGF, IGF, TGF), ROS, and RNS, but also through the expression of selectins and integrins (i.e., ICAM, VCAM, P-Selectin, E-Selectin) in the membranes of activated cells. Thus, increased downstream mediators, PAF levels, and the subsequent further activation of the PAF/PAF-R pathways promotes the activation and aggregation of platelets and leukocytes, activation of endothelial cells, leukocyte adherence, motility, chemotaxis, invasion, migration, and subsequent endothelial dysfunction, thus stimulating the onset and development of inflammation-related chronic diseases and disorders. C. Microconstituents of several foods of the Mediterranean diet have been found to beneficially inhibit the PAF/PAF-R pathways and PAF synthesis towards homeostatic re-equilibration of PAF levels and activities [57]. PAF: platelet-activating factor; PAF-R: G-protein-coupled PAF-receptor; AC: adenylate cyclase; NF-kB: nuclear factor-kappa light-chain-enhancer of activated B cells; MAPK: mitogen activated protein kinase; ERK: extracellular signal-regulated kinases; Akt: protein kinase B; PI3K: phosphatidylinositol 3-kinase; mTOR: mechanistic target of rapamycin; DAG: diacylglycerol; AA: arachidonic acid; cPLA₂: cytosolic phospholipase A₂; PKC: protein kinase C; PKA: protein kinase A; LPCAT: acetyl-CoA: lyso-PAF acetyltransferases; PAF-CPT: dithiothreitol l-insensitive CDP-choline: 1-alkyl-2-acetyl-sn-glycerol cholinephosphotransferase; ATP: adenosine triphosphoric acid; cAMP: cyclic adenosine monophosphate; PLC: phospholipase C; MMP: metalloproteinase; COX: cyclooxygenase; iNOS: nitric oxide synthase; eNOS: endothelial nitric oxide synthase; ROS: reactive oxygen species; RNS: reactive nitrogen species; NADPO: nicotinamide-adenine dinucleotide phosphate oxidase; XO: xanthine oxidase; IL-6: interleukin-6; IL-1: interleukin-1; TNFα: tumour necrosis factor-α; ACS: acute coronary syndrome; VEGF: vascular endothelial growth factor; PL: phospholipids; CVD: cardiovascular diseases; CNS: central nervous system.

Figure 3

PAF levels result from enzymatic biosynthesis, non-enzymatic oxidative synthesis, and enzymatic catabolism, while bioactive microconstituents of the Med-diet beneficially affect these pathways. (A1) The enzymatic biosynthesis of PAF contributes to basal PAF levels or a periodic increase of PAF levels during normal inflammatory responses, while during unresolved and chronic inflammatory manifestations, the enzymatic biosynthesis of PAF is responsible for pathologically increased PAF levels through a continuous induction of the PAF cycle; (A2) Non-enzymatic synthesis of PAF occurs during oxidative stress, increasing ROS and RNS and inducing the synthesis of PAF and PAF-like molecules. When Ox-LDL is produced, PAF-like molecules mimic the activities of PAF. These pathways are not regulated enzymatically; (B) Catabolism of PAF is enzymatically regulated by PAF-AH. PAF catabolism is activated during both acute and chronic inflammatory manifestations and inactivates both PAF and PAF-like molecules; (C) Bioactive microconstituents present in foods of the Med-diet (i.e., polar lipids) have demonstrated beneficial outcomes by inducing homeostatic equilibration of PAF levels and activities through the Inhibition of the PAF/PAF-R pathways and modulation of the PAF anabolic and catabolic enzymes. PAF: platelet-activating factor; PAF-R: G-protein coupled PAF-receptor; PAF-CPT: dithiothreitol l-insensitive CDP-choline: 1-alkyl-2-acetyl-sn-glycerol cholinephosphotransferase; Lyso-PAF-ATs (LPCAT1, LPCAT2): acetyl-CoA: lyso-PAF acetyltransferases; cPLA₂: cytoplasmic phospholipase A₂; PAF-AH: PAF-acetylhydrolase; PC: Phosphatidylcholine; ROS: reactive oxygen species; RNS: reactive nitrogen species; LDL: low-density lipoprotein; Ox-LDL: oxidised-LDL; Med-diet: Mediterranean diet.

Figure 4

A schematic of the key role of PAF in the onset, progression, and expansion of atherosclerotic plaques and their subsequent cardiovascular disorders. Atherosclerotic events take place in four discrete stages (IIa–IV) as follows: (I) Under normal conditions, blood cells roll within the blood stream during physiological blood circulation. Leukocytes scavenge the endothelium by weak adhesion on it and after rolling, return to the blood stream. (IIa) Upstream pro-inflammatory stimuli (cytokines, PAF, etc.) induce PAF synthesis and expression of the PAF-R on the membranes of endothelial and blood cells. (IIb) Binding of PAF to its receptor on the membranes of these cells further induces the PAF cycle-related amplification of the initial inflammatory stimuli, which is achieved through the expression of inflammation-related genes and the subsequent production and release of various downstream mediators, such as PAF itself and several other mediators of inflammation including eicosanoids, cytokines, growth factors, further oxidative stress (ROS, RNS, Ox-LDL, and Ox-PL), and selectins and integrins in the membranes of activated endothelial cells and leukocytes. (III) If unresolved, the PAF cycle-related inflammatory activation of endothelial cells leads to tight adhesion of leukocytes on the activated endothelium and subsequent migration of these leukocytes and Ox-LDL to the subendothelium. There, the crosstalk of key-junction inflammatory mediators such as PAF within the developing plaque microenvironment, with a panel of inflammatory cells of both the innate and adaptive immune system, favours inflammatory phenotypes in these cells and perpetuates a continuous inflammatory milieu, leading to the differentiation of monocytes to macrophages, which engulf Ox-LDL and further transform to foam cells; thus, facilitating the onset, increase, and expansion of atherosclerotic plaque. (IV) Although plaques can grow to a sufficiently large size to compromise blood flow, most of their clinical complications are attributable to arterial occlusion due to plaque erosion or rupture. Vulnerable plaques are typically large, with a necrotic core covered by a thin fibrous cap, and they contain high levels of inflammatory immune cells. Gradually accumulating foam cells die in the intima due to inflammation-induced apoptosis, and when not promptly disposed of, become necrotic, progressively leading to the formation of a thrombogenic and pro-inflammatory necrotic core with cholesterol crystals. In addition, the thin layer of the fibrous cap easily ruptures due to PAF-related inflammatory and atherothrombotic stimuli. Thus, as the plaque continues to develop, it can become unstable and rupture, leading to major cardiovascular event. PAF: platelet-activating factor; PAF-R: G-protein coupled PAF-receptor; ROS: reactive oxygen species; RNS: reactive nitrogen species; Ox-LDL: oxidised LDL; Ox-PL: oxidised phospholipids; IL-6: interleukin-6; IL-1: interleukin-1; TNFα: tumor necrosis factor-α; VEGF: vascular endothelial growth factor.