Functional Crosstalk between PCSK9 Internalization and Pro-Inflammatory Activation in Human Macrophages: Role of Reactive Oxygen Species Release

PCSK9 Enhances Cardiac Fibrogenesis via the Activation of Toll-like Receptor and NLRP3 Inflammasome Signaling

Proprotein convertase subtilisin/kexin type 9 (PCSK9) has emerged as a novel target for reducing low-density lipoprotein cholesterol. PCSK9 activates the atherosclerosis process through pro-inflammation signaling. Furthermore, the serum level of PCSK9 is positively correlated with mortality in patients with heart failure (HF). Cardiac fibrosis plays a crucial role in the pathophysiology of HF. In this study, we intended to examine whether PCSK9 can increase fibroblast activities and explore what its underlying mechanisms are. Migration, proliferation analyses, and Western blotting were used on human cardiac fibroblasts with and without PCSK9. Alirocumab (a PCSK9 inhibitor, 10 mg/kg/week intra-peritoneally for 28 consecutive days) was treated in isoproterenol (100 mg/kg, subcutaneous injection)-induced HF rats. PCSK9 (50, 100 ng/mL) increased proliferation, myofibroblast differentiation capability, and collagen type I production. Compared with control cells, PCSK9 (100 ng/mL)-treated cardiac fibroblasts showed higher nucleotide-binding domain (NOD)-like receptor protein 3 (NLRP3), interleukin (IL)-1, myofibroblast differentiation, and collagen production capabilities, which were attenuated by MCC950 (an NLRP3 inhibitor, 100 μmol/L). PCSK9 upregulated Myd88 and NF-κB signaling, which were reduced by TAK242 (a toll-like receptor (TLR) 4 inhibitor, 10 μmol/L). Moreover, alirocumab significantly improved left ventricular systolic function and attenuated fibrosis in HF rats. In conclusion, PCSK9 upregulates NLRP3 signaling and the profibrotic activities of cardiac fibroblasts through the activation of TLR4/Myd88/NF-κB signaling.

The Cross-Talk Between the Peripheral and Brain Cholesterol Metabolisms

Cholesterol is an essential element for the development and normal function of the central nervous system. While peripheral cholesterol is influenced by liver metabolism and diet, brain cholesterol metabolism takes place in an isolated system due to the impermeability of the blood-brain barrier (BBB). However, cross-talk occurs between the brain and periphery, specifically through metabolites such as oxysterols that play key roles in regulating cholesterol balance. Several neurodegenerative conditions such as Alzheimer's disease or Parkinson's disease are considered to be affected by the loss of this balance. Also, the treatment of hypercholesterolemia needs to consider these discrete interferences between brain and peripheral cholesterol and the possible implications of each therapeutic approach. This is particularly important because of 27-hydroxycholesterol and 24-hydroxycholesterol, which can cross the BBB and are involved in cholesterol metabolism. This paper examines the metabolic pathways of cholesterol metabolism in the brain and periphery and focuses on the complex cross-talk between these metabolisms. Also, we emphasize the regulatory role of the BBB and the need for an integrated approach to cholesterol management.

Potential Beneficial Role of Nitric Oxide in SARS-CoV-2 Infection: Beyond Spike-Binding Inhibition

SARS-CoV-2, the causative virus for the COVID-19 disease, uses its spike glycoprotein to bind to human ACE2 as a first step for viral entry into the cell. For this reason, great efforts have been made to find mechanisms that disrupt this interaction, avoiding the infection. Nitric oxide (NO) is a soluble endogenous gas with known antiviral and immunomodulatory properties. In this study, we aimed to test whether NO could inhibit the binding of the viral spike to ACE2 in human cells and its effects on ACE2 enzymatic activity. Our results show that ACE2 activity was decreased by the NO donors DETA-NONOate and GSNO and by the NO byproduct peroxynitrite. Furthermore, we found that DETA-NONOate could break the spike-ACE2 interaction using the spike from two different variants (Alpha and Gamma) and in two different human cell types. Moreover, the same result was obtained when using NO-producing murine macrophages, while no significant changes were observed in ACE2 expression or distribution within the cell. These results support that it is worth considering NO as a therapeutic agent for COVID-19, as previous reports have suggested.

Alirocumab boosts antioxidant status and halts inflammation in rat model of sepsis-induced nephrotoxicity via modulation of Nrf2/HO-1, PCSK9/HMGB1/NF-ᴋB/NLRP3 and Fractalkine/CX3CR1 hubs

Acute kidney injury (AKI) is a devastating consequence of sepsis, accompanied by high mortality rates. It was suggested that inflammatory pathways are closely linked to the pathogenesis of lipopolysaccharide (LPS)-induced AKI. Inflammatory signaling, including PCSK9, HMGB1/RAGE/TLR4/MYD88/NF-κB, NLRP3/caspase-1 and Fractalkine/CX3CR1 are considered major forerunners in this link. Alirocumab, PCSK9 inhibitor, with remarkable anti-inflammatory features. Accordingly, this study aimed to elucidate the antibacterial effect of alirocumab against E. coli in vitro. Additionally, evaluation of the potential nephroprotective effects of alirocumab against LPS-induced AKI in rats, highlighting the potential underlying mechanisms involved in these beneficial actions. Thirty-six adult male Wistar rats were assorted into three groups (n=12). Group I; was a normal control group, whereas sepsis-mediated AKI was induced in groups II and III through single-dose intraperitoneal injection of LPS on day 16. In group III, animals were given alirocumab. The results revealed that LPS-induced AKI was mitigated by alirocumab, evidenced by amelioration in renal function tests (creatinine, cystatin C, KIM-1, and NGAL); oxidative stress biomarkers (Nrf2, HO-1, TAC, and MDA); apoptotic markers and renal histopathological findings. Besides, alirocumab pronouncedly hindered LPS-mediated inflammatory response, confirmed by diminishing HMGB1, TNF-α, IL-1β, and caspase-1 contents; the gene expression of PCSK9, RAGE, NF-ᴋB and Fractalkine/CX3CR1, along with mRNA expression of TLR4, MYD88, and NLRP3. Regarding the antibacterial actions, results showed that alirocumab displayed potential anti-bacterial activity against pathogenic gram-negative E. coli. In conclusion, alirocumab elicited nephroprotective activities against LPS-induced AKI via modulation of Nrf2/HO-1, PCSK9, HMGB1/RAGE/TLR4/MYD88/NF-ᴋB/NLRP3/Caspase-1, Fractalkine/CX3R1 and apoptotic axes.

PCSK9 ablation attenuates Aβ pathology, neuroinflammation and cognitive dysfunctions in 5XFAD mice

BACKGROUND

Increasing evidence highlights the importance of novel players in Alzheimer's disease (AD) pathophysiology, including alterations of lipid metabolism and neuroinflammation. Indeed, a potential involvement of Proprotein convertase subtilisin/kexin type 9 (PCSK9) in AD has been recently postulated. Here, we first investigated the effects of PCSK9 on neuroinflammation in vitro. Then, we examined the impact of a genetic ablation of PCSK9 on cognitive performance in a severe mouse model of AD. Finally, in the same animals we evaluated the effect of PCSK9 loss on Aβ pathology, neuroinflammation, and brain lipids.

METHODS

For in vitro studies, U373 human astrocytoma cells were treated with Aβ fibrils and human recombinant PCSK9. mRNA expression of the proinflammatory cytokines and inflammasome-related genes were evaluated by q-PCR, while MCP-1 secretion was measured by ELISA. For in vivo studies, the cognitive performance of a newly generated mouse line - obtained by crossing 5XFADHet with PCSK9KO mice - was tested by the Morris water maze test. After sacrifice, immunohistochemical analyses were performed to evaluate Aβ plaque deposition, distribution and composition, BACE1 immunoreactivity, as well as microglia and astrocyte reactivity. Cholesterol and hydroxysterols levels in mouse brains were quantified by fluorometric and LC-MS/MS analyses, respectively. Statistical comparisons were performed according to one- or two-way ANOVA, two-way repeated measure ANOVA or Chi-square test.

RESULTS

In vitro, PCSK9 significantly increased IL6, IL1B and TNFΑ mRNA levels in Aβ fibrils-treated U373 cells, without influencing inflammasome gene expression, except for an increase in NLRC4 mRNA levels. In vivo, PCSK9 ablation in 5XFAD mice significantly improved the performance at the Morris water maze test; these changes were accompanied by a reduced corticohippocampal Aβ burden without affecting plaque spatial/regional distribution and composition or global BACE1 expression. Furthermore, PCSK9 loss in 5XFAD mice induced decreased microgliosis and astrocyte reactivity in several brain regions. Conversely, knocking out PCSK9 had minimal impact on brain cholesterol and hydroxysterol levels.

CONCLUSIONS

In vitro studies showed a pro-inflammatory effect of PCSK9. Consistently, in vivo data indicated a protective role of PCSK9 ablation against cognitive impairments, associated with improved Aβ pathology and attenuated neuroinflammation in a severe mouse model of AD. PCSK9 may thus be considered a novel pharmacological target for the treatment of AD.

G-Protein-Coupled Receptor 91-Dependent Signalling Does Not Influence Vascular Inflammation and Atherosclerosis in Hyperlipidaemic Mice

The TCA cycle intermediate metabolite 'succinate' has been proposed as an inflammatory mediator, influencing autoimmunity and allergic reactions, through ligation to its sensing receptor SUCNR1/GPR91. Whether GPR91-mediated signalling influences the chronic inflammatory process of atherosclerosis has never been investigated. The examination of publicly available datasets revealed that the SUCNR1 gene is expressed in human atherosclerotic plaques, especially in vascular smooth muscle cells. Using GPR91 knockout (Gpr91-/-) and wildtype (WT) littermates, made hyperlipidaemic with the overexpression of the gain-of-function mutated Pcsk9 and Western diet feeding, we showed that the full ablation of GPR91 did not accelerate atherosclerosis-lesions in the aortic arch 2.18 ± 0.48% vs. 1.64 ± 0.31%, and in the aortic roots 10.06 ± 0.91% vs. 10.67 ± 1.53% for Gpr91-/- and WT mice, respectively. In line with this, no differences between groups were observed for macrophage and T-cell infiltration in the plaque, as well as the polarization towards M1- or M2-like macrophages in the aorta, spleen and liver of Gpr91-/- and WT control mice. In conclusion, our study indicates that the global ablation of GPR91 signalling does not influence vascular inflammation or atherogenesis.

Membrane contact sites orchestrate cholesterol homeostasis that is central to vascular aging

Chronological age causes structural and functional vascular deterioration and is a well-established risk factor for the development of cardiovascular diseases, leading to more than 40% of all deaths in the elderly. The etiology of vascular aging is complex; a significant impact arises from impaired cholesterol homeostasis. Cholesterol level is balanced through synthesis, uptake, transport, and esterification, the processes executed by multiple organelles. Moreover, organelles responsible for cholesterol homeostasis are spatially and functionally coordinated instead of isolated by forming the membrane contact sites. Membrane contact, mediated by specific protein-protein interaction, pulls opposing organelles together and creates the hybrid place for cholesterol transfer and further signaling. The membrane contact-dependent cholesterol transfer, together with the vesicular transport, maintains cholesterol homeostasis and has intimate implications in a growing list of diseases, including vascular aging-related diseases. Here, we summarized the latest advances regarding cholesterol homeostasis by highlighting the membrane contact-based regulatory mechanism. We also describe the downstream signaling under cholesterol homeostasis perturbations, prominently in cholesterol-rich conditions, stimulating age-dependent organelle dysfunction and vascular aging. Finally, we discuss potential cholesterol-targeting strategies for therapists regarding vascular aging-related diseases. This article is categorized under: Cardiovascular Diseases > Molecular and Cellular Physiology.

Cholesterol-autoxidation metabolites in host defense against infectious diseases

Cholesterol plays essential roles in biological processes, including cell membrane stability and myelin formation. Cholesterol can be metabolized to oxysterols by enzymatic or nonenzymatic ways. Nonenzymatic cholesterol metabolites, also called cholesterol-autoxidation metabolites, are formed dependent on the oxidation of reactive oxygen species (ROS) such as OH• or reactive nitrogen species, such as ONOO- . Cholesterol-autoxidation metabolites are abundantly produced in diseases such as inflammatory bowel disease and atherosclerosis, which are associated with oxidative stress. Recent studies have shown that cholesterol-autoxidation metabolites can further regulate the immune system. Here, we review the literature and summarize how cholesterol-autoxidation metabolites, such as 25-hydroxycholesterol (25-OHC), 7α/β-OHC, and 7-ketocholesterol, deal with the occurrence and development of infectious diseases through pattern recognition receptors, inflammasomes, ROS production, nuclear receptors, G-protein-coupled receptor 183, and lipid availability. In addition, we include the research regarding the roles of these metabolites in COVID-19 infection and discuss our viewpoints on the future research directions.

PCSK9 Confers Inflammatory Properties to Extracellular Vesicles Released by Vascular Smooth Muscle Cells

Vascular smooth muscle cells (VSMCs) are key participants in both early- and late-stage atherosclerosis and influence neighbouring cells possibly by means of bioactive molecules, some of which are packed into extracellular vesicles (EVs). Proprotein convertase subtilisin/kexin type 9 (PCSK9) is expressed and secreted by VSMCs. This study aimed to unravel the role of PCSK9 on VSMCs-derived EVs in terms of content and functionality. EVs were isolated from human VSMCs overexpressing human PCSK9 (VSMCPCSK9-EVs) and tested on endothelial cells, monocytes, macrophages and in a model of zebrafish embryos. Compared to EVs released from wild-type VSMCs, VSMCPCSK9-EVs caused a rise in the expression of adhesion molecules in endothelial cells and of pro-inflammatory cytokines in monocytes. These acquired an increased migratory capacity, a reduced oxidative phosphorylation and secreted proteins involved in immune response and immune effector processes. Concerning macrophages, VSMCPCSK9-EVs enhanced inflammatory milieu and uptake of oxidized low-density lipoproteins, whereas the migratory capacity was reduced. When injected into zebrafish embryos, VSMCPCSK9-EVs favoured the recruitment of macrophages toward the site of injection. The results of the present study provide evidence that PCSK9 plays an inflammatory role by means of EVs, at least by those derived from smooth muscle cells of vascular origin.