Role of Gut Microbiome in Atherosclerosis: Molecular and Therapeutic Aspects

Abstract

Atherosclerosis is one of the most relevant and prevalent cardiovascular diseases of our time. It is one of the pathological entities that increases the morbidity and mortality index in the adult population. Pathophysiological connections have been observed between atherosclerosis and the gut microbiome (GM), represented by a group of microorganisms that are present in the gut. These microorganisms are vital for metabolic homeostasis in humans. Recently, direct and indirect mechanisms through which GM can affect the development of atherosclerosis have been studied. This has led to research into the possible modulation of GM and metabolites as a new target in the prevention and treatment of atherosclerosis. The goal of this review is to analyze the physiopathological mechanisms linking GM and atherosclerosis that have been described so far. We also aim to summarize the recent studies that propose GM as a potential target in atherosclerosis management.

Keywords: Atherosclerosis; dysbiosis; gut; inflammation; microbiome; treatment.

Fig. (1)

(A) Endotoxemia: inflammation and deregulation in cholesterol metabolism. High LPS concentrations can activate intracellular pathways that lead to the activation of transcription factors, such as NF-𝜅B and AP-1. These contribute to the induction of pro-inflammatory mediators and adhesion molecules through the TLR4 present in immune and non-immune cells. In parallel to the routes triggered by LPS, a fat-rich diet increases PCSK9 expression, which can also be promoted by the coupling of LPS to its receptor, favoring even more the pro-atherogenic inflammatory response. In addition, the cytokines involved in the inflammatory mechanism led by LPS also regulate PCSK9 expression. This adds another layer of metabolic interaction between both of them. (B) On the other hand, the union of the catalytic domain of PCSK9 with the EGF-A domain of the LFLR located in the cellular membranes of the hepatocytes trigger the endosomal process. This leads to the degradation of LDLR and impairs their recycling, which prevents the capture of circulating LDL, indirectly increasing the plasma levels of this molecule. This leads to higher cardiovascular risk for the individual. Abbreviations: TLR4: toll-like receptor 4; LBP: lipopolysaccharides binding protein; CD14: cluster of differentiation 14; MD2: myeloid differentiation protein 2; TIRAP: TIR domain-containing adaptor protein; TRAM: TRIF-related adaptor molecule; IRAK: IL-1R associated kinase; MyD88: myeloid differentiation primary response 88; TRAF6: tumor necrosis factor receptor-associated factor 6; TAB2/3: TAK2/3 binding protein; TAK1: transforming growth factor-β-activated kinase 1; MAPks: MAP kinase kinase; ERK1/2: Extracellular signal-regulated pro-tein kinases 1 and 2; JNK: c-Jun N-terminal kinases; AP-1 activator protein-1; IKKs: IkB kinase complex; NFkB: 𝜅B nuclear factor; RIP receptor-interacting protein 1; TRIF: TIR-domain-containing adapter-inducing interferon-β; TBK1: TANK-binding kinase 1; IRF3: interferon regulatory factor 3; PCSK9: proprotein convertase subtilisin/kexin type 9 serine protease; LDL: low-density lipoprotein; LDLR: LDL receptor.

Fig. (2)

Bile Acids metabolism and its role in atherosclerosis. Primary BA (1) are synthesized in the liver from cholesterol thanks to the action of the CYP7A1. BA conjugate (2) with glycine or taurine to form conjugated BA, which (3) are stored in the gallbladder. After a meal (4), they are released inTO the duodenum. Once in the gut, BA can be (5) directly absorbed in the ileum or (6) be deconjugated through the action of bacteria with BSH ac-tivity present in the colon (7) where they ARE absorbed. This way, 95% of the BA pool is preserved while (8) the remaining BA suffer a dihydroxylation process mediated by GM, forming secondary BA. These are hydrophobic components that facilitate fecal excretion of around 5% of BA. BA also serve as a FXR ligand that, when activated at the hepatic level (I), favors their secretion by promoting SHP transcription. This inhibits CYPA71, leading to a negative feedback loop. In the enterocyte (II), SHP transcrip-tion inhibits BA reabsorption, increasing the excretion of BA. Likewise, in the ileocyte, FXR activation promotes FGF15/19 tran-scription, which acts on the FGFR4/b-klotho in the hepatocyte. This leads to the repression of CYP7A1 expression and the maintenance of BA homeostasis. Lastly, BA can also act on the TGR5, which is expressed on immune and non-immune cells. More specifically, TGR5 activation (A) in macrophages appears to mitigate their activation and phagocytic activity, the produc-tion of pro-inflammatory cytokines, and the expression of CD36 and SR-A. Similarly, it promotes (B) the differentiation of mon-ocytes to DCs, and (C) it mediates eNOs phosphorylation and the consequent production of NO. In addition, it inhibits the mon-ocyte adhesion process in response to inflammatory stimuli. BA have an atheroprotective role through the metabolism of cho-lesterol and the regulation of inflammation and endothelial function. This role can be disrupted in a state of dysbiosis. Abbreviations: PXR: pregnane X receptor; LXR: liver X receptor; CYP7A1: cholesterol 7α-hydroxylase; FXR: farneosoid X receptor; SHP: small heterodimer partner; Β-Klotho: subfamily β of Klotho protein; FGFR4: fibroblast growth factor receptor 4; NTCP: sodium ion/bile acid cotransporter; ASBT: apical sodium Ddpendent bile acid transporter; FGF15: fibroblasts growth factor 15; OSTαβ: organic solute transporter αβ; BA: bile acids; HSB: bile salts hydrolase; TGR5: G-protein-coupled bile acid receptor; SRA: scaven-ger receptors class A; CD36: differentiation cluster 36; cAMP: Cyclic adenosine monophosphate; FNkB: 𝜅B nuclear factor; IL: in-terleukin; TNF: tumoral necrosis factor; DC: dendritic cells; eNOS: endothelial nitric oxide synthase; CA: cholic acid; CDCA: Chenodeoxycholic acid; GCA: glycocholic acid; TCA: taurocholic acid; GCDCA: glycochenodeoxycholic Acid; TCDCA: tau-rochenodeoxycholic acid; LCA: lithocholic acid; DCA: deoxycholic acid.

Fig. (3)

TMAO metabolism and its role in atherosclerosis. (1) Red meat and dairy ingestion are the main source of L-carnitine and choline, two metabolites that (2), once degraded in the colon through the action of the GM, are transformed into TMA. (3) Once TMA is formed, it enters the enterohepatic circulation (4) where it is metabolized by the FMO1/3 to generate TMAO at the hepatic level. The production of TMAO increases in the presence of dysbiosis. Increased TMAO levels can favor atherosclerosis through different mechanisms. Initially, they can (A) inhibit RTC by interfering with BA synthesis and CPY7A1 expression. At the same time, increased TMAO levels can down-regulate the ex-pression of cholesterol absorption targets, such as NPC1L1 and ABCG5/8. This leads to extremely low concentrations of the ef-flux of cholesterol from the liver to the bile and the feces. Likewise, high TMAO levels promote the (B) up-regulation of macro-phages and their transformation into foamy cells by increasing SR-A, CD36, ABCA1, and ABCG1 expression. Lastly, this metabo-lite is also able to act on (C) endothelial cells, increasing the production of pro-inflammatory cytokines, adhesion molecules, and oxidative stress. This generates an environment prime for the development of atherosclerosis. Abbreviations: CYP7A1: choles-terol 7α-hydroxylase; FNkB: 𝜅B nuclear factor; ABCA: ATP binding cassette transporter; ABGC: ATP Binding Cassette Subfamily G; LDL: low-density lipoprotein; VLDL: very low-density lipoprotein; HDL: high-density lipoprotein; NPC1L1: Niemann-Pick C1-Like 1; FMO: flavin-containing monooxygenase; TMAO: trimethylamine N-oxide; MAPK: mitogen-activated protein kinase; PKC: protein kinase C; ROS: reactive oxygen species; SIRT3: sirtuin 3; SRA: scavenger receptors class A; SRB1: scavenger recep-tors class B type 1.

Fig. (4)

Role of SCFA in atherosclerosis: Among the variety of SCFA, the one with the highest evidence regarding its protective role in atherosclerosis is butyrate. More specifically, butyrate appears to be involved in the different steps of cholesterol metabolism, including gut absorption and elimination of excess cholesterol as BA. It does this by favoring the up-regulation of CYP7A1, ABCA1, ABCG5/8, SRB1, and the down-regulation of NPC1L1. Similarly, SCFA can attenuate inflammation, oxidative stress, and VCAM expression induced by TNFα; and increase NO bioavailability, regulating endothelial function. Abbreviations: ABC ATP binding cassette transporter; NPC1L1: Niemann-Pick C1-Like 1; ABCG: ATP Binding Cassette Subfamily G; CPY7A1: cholesterol 7α-hydroxylase; SRB1: scavenger receptors class B type 1; BA: bile acids; SCFA: short-chain fatty acids; GPR: G protein-coupled receptors; HDL: high-density lipoprotein; HDAC: histone deacetylases; HAT: histone acetyltransferases; VCAM-1: vascular cell adhesion molecule 1; NFkB: 𝜅B nuclear factor; NOXs: nitrogen oxides.