ALK7 Acts as a Positive Regulator of Macrophage Activation through Down-Regulation of PPARγ Expression

PPARs in atherosclerosis: The spatial and temporal features from mechanism to druggable targets

BACKGROUND

Atherosclerosis is a chronic and complex disease caused by lipid disorder, inflammation, and other factors. It is closely related to cardiovascular diseases, the chief cause of death globally. Peroxisome proliferator-activated receptors (PPARs) are valuable anti-atherosclerosis targets that showcase multiple roles at different pathological stages of atherosclerosis and for cell types at different tissue sites.

AIM OF REVIEW

Considering the spatial and temporal characteristics of the pathological evolution of atherosclerosis, the roles and pharmacological and clinical studies of PPARs were summarized systematically and updated under different pathological stages and in different vascular cells of atherosclerosis. Moreover, selective PPAR modulators and PPAR-pan agonists can exert their synergistic effects meanwhile reducing the side effects, thereby providing novel insight into future drug development for precise spatial-temporal therapeutic strategy of anti-atherosclerosis targeting PPARs.

KEY SCIENTIFIC

Concepts of Review: Based on the spatial and temporal characteristics of atherosclerosis, we have proposed the importance of stage- and cell type-dependent precision therapy. Initially, PPARs improve endothelial cells' dysfunction by inhibiting inflammation and oxidative stress and then regulate macrophages' lipid metabolism and polarization to improve fatty streak. Finally, PPARs reduce fibrous cap formation by suppressing the proliferation and migration of vascular smooth muscle cells (VSMCs). Therefore, research on the cell type-specific mechanisms of PPARs can provide the foundation for space-time drug treatment. Moreover, pharmacological studies have demonstrated that several drugs or compounds can exert their effects by the activation of PPARs. Selective PPAR modulators (that specifically activate gene subsets of PPARs) can exert tissue and cell-specific effects. Furthermore, the dual- or pan-PPAR agonist could perform a better role in balancing efficacy and side effects. Therefore, research on cells/tissue-specific activation of PPARs and PPAR-pan agonists can provide the basis for precision therapy and drug development of PPARs.

Activin A alleviates neuronal injury through inhibiting cGAS-STING-mediated autophagy in mice with ischemic stroke

Activin A plays an essential role in ischemic stroke as a well-known neuroprotective factor. We previously reported that Activin A could promote white matter remyelination. However, the exact molecular mechanism of Activin A in neuronal protection post-stroke is still unclear. In this study, the middle cerebral artery occlusion/reperfusion (MCAO/R)-induced ischemic stroke mouse model and oxygen-glucose deprivation/reoxygenation (OGD/R)-treated primary neurons were used to explore the molecular mechanism of Activin A-mediated neuroprotection against ischemic injuries. We found that Activin A significantly inhibits cGAS-STING-mediated excessive autophagy through the PI3K-PKB pathway, but not mTOR-dependent autophagy. Consequently, Activin A protected neurons against OGD/R-induced ischemic injury and improved cell survival in a dose-dependent manner. In addition, Activin A improved neurological functions and reduced infarct size of mice with MCAO/R-induced ischemic stroke by inhibiting autophagy. Furthermore, Activin A depended on ACVR1C receptor to exert neuroprotective effects in 1 h MCAO/R treated mice. Our findings showed that Activin A alleviated neuronal ischemic injury through inhibiting cGAS-STING-mediated excessive autophagy in mice with ischemic stroke, which may suggest a potential therapeutic target for ischemic stroke.

Activin receptor-like kinase 7 promotes apoptosis of vascular smooth muscle cells via activating Smad2/3 signaling in diabetic atherosclerosis

Vascular smooth muscle cells (VSMCs) is a vital accelerator in the late phase of diabetic atherosclerosis, but the underlying mechanism remains unclear. The aim of our study was to investigate whether activin receptor-like kinase 7 (ALK7)-Smad2/3 pathway plays an important role in VSMC apoptosis of diabetic atherosclerosis. It was shown that ALK7 expression was obviously elevated in the aorta of ApoE−/− mice with type 2 diabetes mellitus. Inhibition of ALK7 expression significantly improved the stability of atherosclerotic plaques and reduced cell apoptosis. Further experiments showed that ALK7 knockdown stabilized atherosclerotic plaques by reducing VSMC apoptosis via activating Smad2/3. Our study uncovered the important role of ALK7-Smad2/3 signaling in VSMCs apoptosis, which might be a potential therapeutic target in diabetic atherosclerosis.

Activin receptor-like kinase 7 silencing alleviates cardiomyocyte apoptosis, cardiac fibrosis, and dysfunction in diabetic rats

Activin receptor-like kinase 7 (ALK7) is associated with lipometabolism and insulin sensitivity. Our previous study demonstrated that ALK7 participated in high glucose-induced cardiomyocyte apoptosis. The aim of our study was to investigate whether ALK7 plays an important role in modulating diabetic cardiomyopathy (DCM) and the mechanisms involved. The model of diabetes was induced in male Sprague-Dawley rats (120-140 g) by high-fat diet and intraperitoneal injections of low-dose streptozotocin (30 mg/kg). Animals were separated into four groups: control, DCM, DCM with ALK7 silencing, and DCM with vehicle control. The cardiac function was assessed by catheterization. Histopathologic analyses of collagen content and apoptosis rate, and protein analyses of ALK7, Smad2/3, Akt, Caspase3, and Bax/Bcl2 were performed. This study showed a rat model of DCM with hyperglycemia, severe insulin resistance, left ventricular dysfunction, and structural remodeling. With ALK7 silencing, the apoptotic cell death (apoptosis rate assessed by TUNEL, ratio of Bax/Bcl2 and expression of cleaved Caspase3), fibrosis areas, and Collagen I-to-III ratio decreased significantly. The insulin resistance and diastolic dysfunction were also ameliorated by ALK7 silencing. Furthermore, the depressed phosphorylation of Akt was restored while elevated phosphorylation of Smad2/3 decreased after the silencing of ALK7. The results suggest ALK7 silencing plays a protective role in DCM and may serve as a potential target for the treatment of human DCM.

Inclisiran inhibits oxidized low-density lipoprotein-induced foam cell formation in Raw264.7 macrophages via activating the PPARγ pathway

Proprotein convertase subtilisin kexin type 9 (PCSK9) is a well-known proprotein convertase that influences foam cell formation and modulates atherosclerosis. Inclisiran is a novel chemosynthetic small interfering RNA that inhibits PCSK9 synthesis. This study aimed to explore the effect of inclisiran on oxidized low-density lipoprotein (ox-LDL)-induced foam cell formation in Raw264.7 macrophages and to investigate the underlying mechanisms. Raw264.7 cells were treated with ox-LDL to induce the formation of macrophage-derived foam cells. Oil Red O staining and high-performance liquid chromatography were performed to detect lipid accumulation and cholesterol levels. Dil-ox-LDL uptake assay, CCK-8, RT-qPCR, and Western blotting analysis were performed to examine ox-LDL uptake, cell viability, and expression of scavenger receptor-related factors. Inclisiran reduced lipid accumulation in ox-LDL-treated macrophages in a dose-dependent manner. Inclisiran significantly inhibited the levels of total cholesterol, free cholesterol, and cholesterol ester in the supernatant of Raw264.7 cells. Inclisiran reduced ox-LDL uptake and increased Raw264.7 cell viability. Meanwhile, inclisiran downregulated the expression of SR-A, LOX-1, and CD36 and upregulated SR-BI, ApoE, and ABCA1. Furthermore, inclisiran increased PPARγ activity and decreased NF-κB activity. An inhibitor of PPARγ (T0070907) reversed the beneficial effects of inclisiran on ox-LDL uptake, NF-κB inactivation, and cytokine expression. In conclusion, these data suggested that inclisiran inhibited the formation of macrophage-derived foam cells by activating the PPARγ pathway.HighlightsInclisiran reduces lipid accumulation in Raw264.7 cells;Inclisiran reduces ox-LDL uptake and increases Raw264.7 cell viability;Inclisiran inhibits foam cell formation by activating the PPARγ pathway.

PPARα/γ signaling pathways are involved in Chlamydia pneumoniae-induced foam cell formation via upregulation of SR-A1 and ACAT1 and downregulation of ABCA1/G1

Chlamydia pneumoniae (Cpn) has been reported to be involved in the pathogenesis of early atherosclerosis by inducing macrophage-derived foam cell formation in the presence of low-density lipoprotein (LDL). However, the biochemical mechanisms underlying Cpn-induced foam cell formation are still not fully elucidated. The present study showed that in LDL-treated THP-1-derived macrophages, Cpn not only upregulated the expression of scavenger receptor A1 (SR-A1) and acyl-coenzyme A: cholesterol acyltransferase 1 (ACAT1), but it also downregulated the expression of ATP binding cassette transporters (ABCA1 and ABCG1) at both the mRNA and protein levels. These processes facilitated cholesterol accumulation and promoted macrophage-derived foam cell formation. Treatment with the peroxisome proliferator-activated receptor (PPAR)-γ agonist rosiglitazone or the PPARα agonist fenofibrate decreased the number of foam cells induced by Cpn, while the PPARγ antagonist GW9662, the PPARα antagonist MK886, or PPARα/γ siRNAs enhanced the effect of Cpn on foam cell formation and gene expression of SR-A1, ACAT1, and ABCA1/G1. Moreover, the PPARγ agonist rosiglitazone reversed the downregulation of CD36 by Cpn, while PPARγ siRNA and the PPARγ inhibitor GW9662 further suppressed CD36 expression. However, the PPARα agonist, inhibitor, and siRNA all showed no effect on CD36 expression. In conclusion, the PPARα and PPARγ pathways are both involved in Cpn-induced macrophage-derived foam cell formation by upregulating SR-A1 and ACAT1 and downregulating ABCA1/G1 expression.

[Exenatide promotes cholesterol efflux in pancreatic tissue of obese diabetic rats]

OBJECTIVE

To study the effect of exenatide on the expression of ABCA1 and cholesterol metabolism in the pancreas of obese diabetic rats.

OBJECTIVE

Twenty-four normal male SD rats and 18 obese diabetic rats (induced by high-fat feeding and STZ injection) were both divided equally into 2 groups for injections of saline or exenatide. After treatment for a week, the expression of ABCA1, cholesterol metabolism, and islet function of the rats were examined using real-time PCR, Western blotting, oil red O staining, cholesterol content determination, and HE staining.

OBJECTIVE

The expressions of ABCA1 at both mRNA and protein levels in pancreatic tissue were significantly lower in obese diabetic rats than in normal SD rats. The obese diabetic rats showed obvious lipid deposition and increased cholesterol content in the pancreatic tissue with significantly reduced islet volume and structural changes (P < 0.05); exenatide treatment of the diabetic rats significantly up-regulated ABCA1 expression, reduced lipid deposition and cholesterol content in pancreatic tissue, and increased number and volume of the islets, which presented with more orderly alignment (P < 0.05).

OBJECTIVE

Obese diabetic rats have lowered ABCA1 expression, cholesterol efflux block, and cholesterol accumulation in the pancreatic tissue. Exenatide can up-regulate ABCA1 expression and promote cholesterol efflux to reduce cholesterol content in the pancreatic tissue and improve islet function in obese diabetic rats.