Impact of myeloid RIPK1 gene deletion on atherogenesis in ApoE-deficient mice

Programmed death of macrophages in atherosclerosis: mechanisms and therapeutic targets

Atherosclerosis is a progressive inflammatory disorder of the arterial vessel wall characterized by substantial infiltration of macrophages, which exert both favourable and detrimental functions. Early in atherogenesis, macrophages can clear cytotoxic lipoproteins and dead cells, preventing cytotoxicity. Efferocytosis - the efficient clearance of dead cells by macrophages - is crucial for preventing secondary necrosis and stimulating the release of anti-inflammatory cytokines. In addition, macrophages can promote tissue repair and proliferation of vascular smooth muscle cells, thereby increasing plaque stability. However, advanced atherosclerotic plaques contain large numbers of pro-inflammatory macrophages that secrete matrix-degrading enzymes, induce death in surrounding cells and contribute to plaque destabilization and rupture. Importantly, macrophages in the plaque can undergo apoptosis and several forms of regulated necrosis, including necroptosis, pyroptosis and ferroptosis. Regulated necrosis has an important role in the formation and expansion of the necrotic core during plaque progression, and several triggers for necrosis are present within atherosclerotic plaques. This Review focuses on the various forms of programmed macrophage death in atherosclerosis and the pharmacological interventions that target them as a potential means of stabilizing vulnerable plaques and improving the efficacy of currently available anti-atherosclerotic therapies.

The dance of macrophage death: the interplay between the inevitable and the microenvironment

Macrophages are highly plastic cells ubiquitous in various tissues, where they perform diverse functions. They participate in the response to pathogen invasion and inflammation resolution following the immune response, as well as the maintenance of homeostasis and proper tissue functions. Macrophages are generally considered long-lived cells with relatively strong resistance to numerous cytotoxic factors. On the other hand, their death seems to be one of the principal mechanisms by which macrophages perform their physiological functions or can contribute to the development of certain diseases. In this review, we scrutinize three distinct pro-inflammatory programmed cell death pathways - pyroptosis, necroptosis, and ferroptosis - occurring in macrophages under specific circumstances, and explain how these cells appear to undergo dynamic yet not always final changes before ultimately dying. We achieve that by examining the interconnectivity of these cell death types, which in macrophages seem to create a coordinated and flexible system responding to the microenvironment. Finally, we discuss the complexity and consequences of pyroptotic, necroptotic, and ferroptotic pathway induction in macrophages under two pathological conditions - atherosclerosis and cancer. We summarize damage-associated molecular patterns (DAMPs) along with other microenvironmental factors, macrophage polarization states, associated mechanisms as well as general outcomes, as such a comprehensive look at these correlations may point out the proper methodologies and potential therapeutic approaches.

Cell death in atherosclerosis

A complex and evolutionary process that involves the buildup of lipids in the arterial wall and the invasion of inflammatory cells results in atherosclerosis. Cell death is a fundamental biological process that is essential to the growth and dynamic equilibrium of all living things. Serious cell damage can cause a number of metabolic processes to stop, cell structure to be destroyed, or other irreversible changes that result in cell death. It is important to note that studies have shown that the two types of programmed cell death, apoptosis and autophagy, influence the onset and progression of atherosclerosis by controlling these cells. This could serve as a foundation for the creation of fresh atherosclerosis prevention and treatment strategies. Therefore, in this review, we summarized the molecular mechanisms of cell death, including apoptosis, pyroptosis, autophagy, necroptosis, ferroptosis and necrosis, and discussed their effects on endothelial cells, vascular smooth muscle cells and macrophages in the process of atherosclerosis, so as to provide reference for the next step to reveal the mechanism of atherosclerosis.

New Dawn for Atherosclerosis: Vascular Endothelial Cell Senescence and Death

Endothelial cells (ECs) form the inner linings of blood vessels, and are directly exposed to endogenous hazard signals and metabolites in the circulatory system. The senescence and death of ECs are not only adverse outcomes, but also causal contributors to endothelial dysfunction, an early risk marker of atherosclerosis. The pathophysiological process of EC senescence involves both structural and functional changes and has been linked to various factors, including oxidative stress, dysregulated cell cycle, hyperuricemia, vascular inflammation, and aberrant metabolite sensing and signaling. Multiple forms of EC death have been documented in atherosclerosis, including autophagic cell death, apoptosis, pyroptosis, NETosis, necroptosis, and ferroptosis. Despite this, the molecular mechanisms underlying EC senescence or death in atherogenesis are not fully understood. To provide a comprehensive update on the subject, this review examines the historic and latest findings on the molecular mechanisms and functional alterations associated with EC senescence and death in different stages of atherosclerosis.

Regulated Necrosis in Atherosclerosis

During atherosclerosis, lipid-rich plaques are formed in large- and medium-sized arteries, which can reduce blood flow to tissues. This situation becomes particularly precarious when a plaque develops an unstable phenotype and becomes prone to rupture. Despite advances in identifying and treating vulnerable plaques, the mortality rate and disability caused by such lesions remains the number one health threat in developed countries. Vulnerable, unstable plaques are characterized by a large necrotic core, implying a prominent role for necrotic cell death in atherosclerosis and plaque destabilization. Necrosis can occur accidentally or can be induced by tightly regulated pathways. Over the past decades, different forms of regulated necrosis, including necroptosis, ferroptosis, pyroptosis, and secondary necrosis, have been identified, and these may play an important role during atherogenesis. In this review, we describe several forms of necrosis that may occur in atherosclerosis and how pharmacological modulation of these pathways can stabilize vulnerable plaques. Moreover, some challenges of targeting necrosis in atherosclerosis such as the presence of multiple death-inducing stimuli in plaques and extensive cross-talk between necrosis pathways are discussed. A better understanding of the role of (regulated) necrosis in atherosclerosis and the mechanisms contributing to plaque destabilization may open doors to novel pharmacological strategies and will enable clinicians to tackle the residual cardiovascular risk that remains in many atherosclerosis patients.

Current Progress on Neuroprotection Induced by Artemisia, Ginseng, Astragalus, and Ginkgo Traditional Chinese Medicines for the Therapy of Alzheimer's Disease

Aging is associated with the occurrence of diverse degenerative changes in various tissues and organs and with an increased incidence of neurological disorders, especially neurodegenerative diseases such as Alzheimer's disease (AD). In recent years, the search for effective components derived from medicinal plants in delaying aging and preventing and treating neurodegenerative diseases has been increasing and the number of related publications shows a rising trend. Here, we present a concise, updated review on the preclinical and clinical research progress in the assessment of the therapeutic potential of different traditional Chinese medicines and derived active ingredients and their effect on the signaling pathways involved in AD neuroprotection. Recognized by their multitargeting ability, these natural compounds hold great potential in developing novel drugs for AD.

Gasdermin D Deficiency Limits the Transition of Atherosclerotic Plaques to an Inflammatory Phenotype in ApoE Knock-Out Mice

Gasdermin D (GSDMD) is the key executor of pyroptotic cell death. Recent studies suggest that GSDMD-mediated pyroptosis is involved in atherosclerotic plaque destabilization. We report that cleaved GSDMD is expressed in macrophage- and smooth muscle cell-rich areas of human plaques. To determine the effects of GSDMD deficiency on atherogenesis, ApoE^−/− Gsdmd^−/− (n = 16) and ApoE^−/−Gsdmd^+/+ (n = 18) mice were fed a western-type diet for 16 weeks. Plaque initiation and formation of stable proximal aortic plaques were not altered. However, plaques in the brachiocephalic artery (representing more advanced lesions compared to aortic plaques) of ApoE^−/− Gsdmd^−/− mice were significantly smaller (115 ± 18 vs. 186 ± 16 × 10³ µm², p = 0.006) and showed features of increased stability, such as decreased necrotic core area (19 ± 4 vs. 37 ± 7 × 10³ µm², p = 0.03) and increased αSMA/MAC3 ratio (1.6 ± 0.3 vs. 0.7 ± 0.1, p = 0.01), which was also observed in proximal aortic plaques. Interestingly, a significant increase in TUNEL positive cells was observed in brachiocephalic artery plaques from ApoE^−/− Gsdmd^−/− mice (141 ± 25 vs. 62 ± 8 cells/mm², p = 0.005), indicating a switch to apoptosis. This switch from pyroptosis to apoptosis was also observed in vitro in Gsdmd^−/− macrophages. In conclusion, targeting GSDMD appears to be a promising approach for limiting the transition to an inflammatory, vulnerable plaque phenotype.

Foam Cells in Atherosclerosis: Novel Insights Into Its Origins, Consequences, and Molecular Mechanisms

Foam cells play a vital role in the initiation and development of atherosclerosis. This review aims to summarize the novel insights into the origins, consequences, and molecular mechanisms of foam cells in atherosclerotic plaques. Foam cells are originated from monocytes as well as from vascular smooth muscle cells (VSMC), stem/progenitor cells, and endothelium cells. Novel technologies including lineage tracing and single-cell RNA sequencing (scRNA-seq) have revolutionized our understanding of subtypes of monocyte- and VSMC-derived foam cells. By using scRNA-seq, three main clusters including resident-like, inflammatory, and triggering receptor expressed on myeloid cells-2 (Trem2 ^hi ) are identified as the major subtypes of monocyte-derived foam cells in atherosclerotic plaques. Foam cells undergo diverse pathways of programmed cell death including apoptosis, autophagy, necroptosis, and pyroptosis, contributing to the necrotic cores of atherosclerotic plaques. The formation of foam cells is affected by cholesterol uptake, efflux, and esterification. Novel mechanisms including nuclear receptors, non-coding RNAs, and gut microbiota have been discovered and investigated. Although the heterogeneity of monocytes and the complexity of non-coding RNAs make obstacles for targeting foam cells, further in-depth research and therapeutic exploration are needed for the better management of atherosclerosis.

The Impact of RIPK1 Kinase Inhibition on Atherogenesis: A Genetic and a Pharmacological Approach

RIPK1 (receptor-interacting serine/threonine-protein kinase 1) enzymatic activity drives both apoptosis and necroptosis, a regulated form of necrosis. Because necroptosis is involved in necrotic core development in atherosclerotic plaques, we investigated the effects of a RIPK1^S25D/S25D mutation, which prevents activation of RIPK1 kinase, on atherogenesis in ApoE^−/− mice. After 16 weeks of western-type diet (WD), atherosclerotic plaques from ApoE^−/− RIPK1^S25D/S25D mice were significantly larger compared to ApoE^−/− RIPK1^+/+ mice (167 ± 34 vs. 78 ± 18 × 10³ µm², p = 0.01). Cell numbers (350 ± 34 vs. 154 ± 33 nuclei) and deposition of glycosaminoglycans (Alcian blue: 31 ± 6 vs. 14 ± 4%, p = 0.023) were increased in plaques from ApoE^−/− RIPK1^S25D/S25D mice while macrophage content (Mac3: 2.3 ± 0.4 vs. 9.8 ± 2.4%, p = 0.012) was decreased. Plaque apoptosis was not different between both groups. In contrast, pharmacological inhibition of RIPK1 kinase with GSK’547 (10 mg/kg BW/day) in ApoE^−/− Fbn1^C1039G+/− mice, a model of advanced atherosclerosis, did not alter plaque size after 20 weeks WD, but induced apoptosis (TUNEL: 136 ± 20 vs. 62 ± 9 cells/mm², p = 0.004). In conclusion, inhibition of RIPK1 kinase activity accelerated plaque progression in ApoE^−/− RIPK1^S25D/S25D mice and induced apoptosis in GSK’547-treated ApoE^−/− Fbn1^C1039G+/− mice. Thus, without directly comparing the genetic and pharmacological studies, it can be concluded that targeting RIPK1 kinase activity does not limit atherogenesis.

Stage-Dependent Impact of RIPK1 Inhibition on Atherogenesis: Dual Effects on Inflammation and Foam Cell Dynamics

Objective: Atherosclerosis is an arterial occlusive disease with hypercholesterolemia and hypertension as common risk factors. Advanced-stage stenotic plaque, which features inflammation and necrotic core formation, is the major reason for clinical intervention. Receptor interacting serine/threonine-protein kinase 1 (RIPK1) mediates inflammation and cell death and is expressed in atherosclerotic lesions. The role of RIPK1 in advanced-stage atherosclerosis is unknown.

Approach and Results: To investigate the effect of RIPK1 inhibition in advanced atherosclerotic plaque formation, we used ApoE^SA/SA mice, which exhibit hypercholesterolemia, and develop angiotensin-II mediated hypertension upon administration of doxycycline in drinking water. These mice readily develop severe atherosclerosis, including that in coronary arteries. Eight-week-old ApoE^SA/SA mice were randomized to orally receive a highly selective RIPK1 inhibitor (RIPK1i, GSK547) mixed with a western diet, or control diet. RIPK1i administration reduced atherosclerotic plaque lesion area at 2 weeks of treatment, consistent with suppressed inflammation (MCP-1, IL-1β, TNF-α) and reduced monocyte infiltration. However, administration of RIPK1i unexpectedly exacerbated atherosclerosis at 4 weeks of treatment, concomitant with increased macrophages and lipid deposition in the plaques. Incubation of isolated macrophages with oxidized LDL resulted in foam cell formation in vitro. RIPK1i treatment promoted such foam cell formation while suppressing the death of these cells. Accordingly, RIPK1i upregulated the expression of lipid metabolism-related genes (Cd36, Ppara, Lxrα, Lxrb, Srebp1c) in macrophage foam cells with ABCA1/ABCG1 unaltered. Furthermore, RIPK1i treatment inhibited ApoA1 synthesis in the liver and reduced plasma HDL levels.

Conclusion: RIPK1 modulates the development of atherosclerosis in a stage-dependent manner, implicating both pro-atherosclerotic (monocyte infiltration and inflammation) and anti-atherosclerotic effects (suppressing foam cell accumulation and promoting ApoA1 synthesis). It is critical to identify an optimal therapeutic duration for potential clinical use of RIPK1 inhibitor in atherosclerosis or other related disease indications.

RIPK1 targeting protects against obesity and atherosclerosis

Unhealthy lifestyles and dietary habits often lead to diet-associated inflammatory diseases such as obesity and atherosclerosis. Recent studies have provided novel insight into the role of RIPK1 in inflammation and metabolism. RIPK1 silencing can reduce diet-induced obesity, nonalcoholic fatty liver disease (NAFLD), and atherosclerosis by reducing inflammation, lipid synthesis, and inflammasome activation. Targeting RIPK1 may therefore attenuate chronic metabolic disease and would likely be therapeutic.