PKC (Protein Kinase C)-δ Modulates AT (Antithrombin) Signaling in Vascular Endothelial Cells

SIRT3 suppresses vascular endothelial senescence via DHRS2 and contributes to the anti-vascular aging effect of Bazi Bushen capsule

AIMS

Vascular and endothelial aging are significant causes of chronic diseases among the elderly. This study investigated the specific mechanism by which sirtuin 3 (SIRT3) regulates vascular endothelial senescence and the beneficial role of Bazi Bushen capsule (BZBS) in preventing vascular aging.

METHODS

Human umbilical vein endothelial cells and mouse aortic endothelial cells were cultured with D-galactose (D-gal) to induce aging and evaluate the beneficial effects of the SIRT3-dehydrogenase/reductase member 2 (DHRS2) axis on the inhibition of vascular endothelial aging. d-Gal was injected intraperitoneally into wild-type and Sirt3 knockout mice, while BZBS was administered orally. Histochemical staining, immunohistochemistry, and western blotting assays were used to explore the beneficial effects of BZBS against aging-associated vascular remodelling. Endothelial cell function assays were used to evaluate the role of BZBS in suppressing endothelial aging in vitro.

RESULTS

SIRT3 deacetylated DHRS2 and modulated the translation of DHRS2. The SIRT3-DHRS2 axis played an important role in preserving mitochondrial homeostasis and reducing reactive oxygen species generation through suppressing endothelial nitric oxide synthase (eNOS) translocating to mitochondria and eNOS-Thr495 phosphorylation mediated by protein kinase C δ (PKCδ). BZBS mitigated vascular remodelling and relieved endothelial oxidative stress via the SIRT3-DHRS2 axis.

CONCLUSION

SIRT3 activates DHRS2-PKCδ to stop aging in endothelial cells by inhibiting uncoupled eNOS translocating to mitochondria. BZBS rescued vascular aging and endothelial dysfunction via the SIRT3-DHRS2 axis. Revealing a protective mechanism by which SIRT3 inhibits endothelial senescence, this study provides evidence for BZBS in delaying vascular aging.

Physiological significance of antithrombin D-helix interaction with vascular GAGs

ABSTRACT

Antithrombin (AT) is an anticoagulant serpin involved in the regulation of proteolytic activities of coagulation proteases. AT also possesses a direct anti-inflammatory function. The anticoagulant function of AT is mediated through its reactive center loop-dependent inhibition of coagulation proteases, but anti-inflammatory function of AT is mediated via its D-helix-dependent interaction with vascular glycosaminoglycans (GAGs). In vitro assays have established that therapeutic heparins promote the anticoagulant function of AT by binding D-helix and activating the serpin, however, the contribution of vascular GAGs to D-helix-dependent anticoagulant function of AT has remained poorly understood in vivo. Here, we explored this question by using 2 AT mutants, (AT-4Mut), which exhibits neither affinity for heparin nor D-helix-dependent anti-inflammatory signaling but possesses normal protease-inhibitory function and an inactive signaling-selective AT mutant in which its P1-Arg425 is deleted (AT-R425del). In vivo properties of mutants were compared with wild-type AT (AT-WT) in a small interfering RNA (siRNA)-mediated AT-deficient mouse model. The siRNA knockdown efficiently reduced expression of AT and induced robust procoagulant and proinflammatory phenotypes in mice. Infusion of both AT-WT and AT-4Mut rescued the procoagulant phenotype of AT-deficient mice as evidenced by restoration of the plasma clotting time and inhibition of fibrin deposition. AT-WT also attenuated inflammation as evidenced by reduced VCAM-1 expression and leukocyte infiltration in the liver and lungs; however, AT-4Mut failed to attenuate inflammation. Interestingly, AT-R425del also effectively attenuated inflammation in AT-depleted mice. These results suggest that interaction of AT D-helix with vascular GAGs may primarily be responsible for anti-inflammatory signaling rather than protease-inhibitory function of the serpin.

Mutual Inhibition of Antithrombin III and SARS-CoV-2 Cellular Attachment to Syndecans: Implications for COVID-19 Treatment and Vaccination

Antithrombin III (ATIII) is a potent endogenous anticoagulant that binds to heparan sulfate proteoglycans (HSPGs) on endothelial cells' surfaces. Among these HSPGs, syndecans (SDCs) are crucial as transmembrane receptors bridging extracellular ligands with intracellular signaling pathways. Specifically, syndecan-4 (SDC4) has been identified as a key receptor on endothelial cells for transmitting the signaling effects of ATIII. Meanwhile, SDCs have been implicated in facilitating the cellular internalization of SARS-CoV-2. Given the complex interactions between ATIII and SDC4, our study analyzed the impact of ATIII on the virus entry into host cells. While ATIII binds to all SDC isoforms, it shows the strongest affinity for SDC4. SDCs' heparan sulfate chains primarily influence ATIII's SDC attachment, although other parts might also play a role in ATIII's dominant affinity toward SDC4. ATIII significantly reduces SARS-CoV-2's cellular entry into cell lines expressing SDCs, suggesting a competitive inhibition mechanism at the SDC binding sites, particularly SDC4. Conversely, the virus or its spike protein decreases the availability of SDCs on the cell surface, reducing ATIII's cellular attachment and hence contributing to a procoagulant environment characteristic of COVID-19.

Hemostasis and endothelial functionality: the double face of coagulation factors

Hemostasis is a sophisticated sequence of events aimed at repairing vessel injury. This process occurs in combination with angiogenesis, which leads to new blood vessel formation, helping in wound repair and facilitating tissue healing. The fine mechanisms that regulate hemostasis and angiogenesis are well described, but for a long time, coagulation factors (CF) have been considered merely players in the coagulation cascade. However, evidence from several experiments highlights the crucial functions of these CF in regulating endothelial functionality, especially in the angiogenic process. Some of these CF (e.g., thrombin and tissue factor) have been widely investigated and have been described as triggering intracellular signaling related to endothelial cell (EC) functionality. For others (e.g., factor VIII and thrombomodulin), potential receptors and molecular mechanisms have not been fully elucidated but some data show their potential to induce EC response. This review focuses on the emerging roles of selected CF in regulating EC functions, highlighting in particular their ability to activate signaling pathways involved in angiogenesis, migration, proliferation and endothelial barrier stability.

Thrombomodulin Switches Signaling and Protease-Activated Receptor 1 Cleavage Specificity of Thrombin

BACKGROUND

Cleavage of the extracellular domain of PAR1 (protease-activated receptor 1) by thrombin at Arg41 and by APC (activated protein C) at Arg46 initiates paradoxical cytopathic and cytoprotective signaling in endothelial cells. In the latter case, the ligand-dependent coreceptor signaling by EPCR (endothelial protein C receptor) is required for the protective PAR1 signaling by APC. Here, we investigated the role of thrombomodulin in determining the specificity of PAR1 signaling by thrombin.

METHODS

We prepared a PAR1 knockout (PAR1-/-) EA.hy926 endothelial cell line by CRISPR/Cas9 and transduced PAR1-/- cells with lentivirus vectors expressing PAR1 mutants in which either Arg41 or Arg46 was replaced with an Ala. Furthermore, human embryonic kidney 293 cells were transfected with wild-type or mutant PAR1 cleavage reporter constructs carrying N-terminal Nluc (NanoLuc luciferase) and C-terminal enhanced yellow fluorescent protein tags.

RESULTS

Characterization of transfected cells in signaling and receptor cleavage assays revealed that, upon interaction with thrombomodulin, thrombin cleaves Arg46 to elicit cytoprotective effects by a β-arrestin-2 biased signaling mechanism. Analysis of functional data and cleavage rates indicated that thrombin-thrombomodulin cleaves Arg46>10-fold faster than APC. Upon interaction with thrombin, the cytoplasmic domain of thrombomodulin recruited both β-arrestin-1 and -2 to the plasma membrane. Thus, the thrombin cleavage of Arg41 was also cytoprotective in thrombomodulin-expressing cells by β-arrestin-1-biased signaling. APC in the absence of EPCR cleaved Arg41 to initiate disruptive signaling responses like thrombin.

CONCLUSIONS

These results suggest that coreceptor signaling by thrombomodulin and EPCR determines the PAR1 cleavage and signaling specificity of thrombin and APC, respectively.

Thrombomodulin Regulates PTEN/AKT Signaling Axis in Endothelial Cells

BACKGROUND

We recently demonstrated that deletion of thrombomodulin gene from endothelial cells results in upregulation of proinflammatory phenotype. In this study, we investigated the molecular basis for the altered phenotype in thrombomodulin-deficient (TM-/-) cells.

METHODS

Different constructs containing deletions or mutations in the cytoplasmic domain of thrombomodulin were prepared and introduced to TM-/- cells. The phenotype of cells expressing different derivatives of thrombomodulin and tissue samples of thrombomodulin-knockout mice were analyzed for expression of distinct regulatory genes in established signaling assays.

RESULTS

The phosphatase and tensin homolog were phosphorylated and its recruitment to the plasma membrane was impaired in TM-/- cells, leading to hyperactivation of AKT (protein kinase B) and phosphorylation-dependent nuclear exclusion of the transcription factor, forkhead box O1. The proliferative/migratory properties of TM-/- cells were enhanced, and cells exhibited hypersensitivity to stimulation by angiopoietin 1 and vascular endothelial growth factor. Reexpression of wild-type thrombomodulin in TM-/- cells normalized the cellular phenotype; however, thrombomodulin lacking its cytoplasmic domain failed to restore the normal phenotype in TM-/- cells. Increased basal permeability and loss of VE-cadherin were restored to normal levels by reexpression of wild-type thrombomodulin but not by a thrombomodulin construct lacking its cytoplasmic domain. A thrombomodulin cytoplasmic domain deletion mutant containing 3-membrane-proximal Arg-Lys-Lys residues restored the barrier-permeability function of TM-/- cells. Enhanced phosphatase and tensin homolog phosphorylation and activation of AKT and mTORC1 (mammalian target of rapamycin complex 1) were also observed in the liver of thrombomodulin-KO mice.

CONCLUSIONS

These results suggest that the cytoplasmic domain of thrombomodulin interacts with the actin cytoskeleton and plays a crucial role in regulation of phosphatase and tensin homolog/AKT signaling in endothelial cells.

PKCδ regulates the vascular biology in diabetic atherosclerosis

Diabetes mellitus, known for its complications, especially vascular complications, is becoming a globally serious social problem. Atherosclerosis has been recognized as a common vascular complication mechanism in diabetes. The diacylglycerol (DAG)-protein kinase C (PKC) pathway plays an important role in atherosclerosis. PKCs can be divided into three subgroups: conventional PKCs (cPKCs), novel PKCs (nPKCs), and atypical PKCs (aPKCs). The aim of this review is to provide a comprehensive overview of the role of the PKCδ pathway, an isoform of nPKC, in regulating the function of endothelial cells, vascular smooth muscle cells, and macrophages in diabetic atherosclerosis. In addition, potential therapeutic targets regarding the PKCδ pathway are summarized. Video Abstract.

A reduction of Syndecan-4 in macrophages promotes atherosclerosis by aggravating the proinflammatory capacity of macrophages

BACKGROUND

Cardiovascular diseases (CVDs) are a significant cause of mortality worldwide and are characterized by severe atherosclerosis (AS) in patients. However, the molecular mechanism of AS formation remains elusive. In the present study, we investigated the role of syndecan-4 (SDC4), a member of the syndecan family, in atherogenesis.

METHODS AND RESULTS

The expression of SDC4 decreased in mouse severe AS models. Moreover, knockout of SDC4 accelerated high-cholesterol diets (HCD)-induced AS in ApoE^-/- mice. Mechanistically, the decrease of SDC4 increased macrophage proinflammatory capacity may be through the PKCα-ABCA1/ABCG1 signaling pathway.

CONCLUSION

These findings provide evidence that SDC4 reduction links macrophages and inflammation to AS and that SDC4 in macrophages provides a therapeutic target for preventing AS formation.

Antithrombin protects against Plasmodium falciparum histidine-rich protein II-mediated inflammation and coagulation

Plasmodium falciparum (Pf)-derived histidine-rich protein II (HRPII) has been shown to inhibit heparin-dependent anticoagulant activity of antithrombin (AT) and induce inflammation in vitro and in vivo. In a recent study, we showed that HRPII interacts with the AT-binding vascular glycosaminoglycans (GAGs) to not only disrupt the barrier-permeability function of endothelial cells but also inhibit the anti-inflammatory signaling function of AT. Here we investigated the mechanisms of the pro-inflammatory function of HRPII and the protective activity of AT in cellular and animal models. We found that AT competitively inhibits the GAG-dependent HRPII-mediated activation of NF-κB and expression of intercellular cell adhesion molecule 1 (ICAM1) in endothelial cells. Furthermore, AT inhibits HRPII-mediated histone H3 citrullination and neutrophil extracellular trap (NET) formation in HL60 cells and freshly isolated human neutrophils. In vivo, HRPII induced Mac1 expression on blood neutrophils, MPO release in plasma, neutrophil infiltration and histone H3 citrullination in the lung tissues. HRPII also induced endothelial cell activation as measured by increased ICAM1 expression and elevated vascular permeability in the lungs. AT effectively inhibited HRPII-mediated neutrophil infiltration, NET formation and endothelial cell activation in vivo. AT also inhibited HRPII-meditated deposition of platelets and fibrin(ogen) in the lungs and circulating level of von Willebrand factor in the plasma. We conclude that AT exerts protective effects against pathogenic effects of Pf-derived HRPII in both cellular and animal models.

Extracellular Histones Bind Vascular Glycosaminoglycans and Inhibit the Anti-Inflammatory Function of Antithrombin

BACKGROUND/AIMS

Binding of histones to molecular pattern recognition receptors on endothelial cells and leukocytes provokes proinflammatory responses and promotes activation of coagulation. Histones also bind therapeutic heparins, thereby neutralizing their anticoagulant functions. The aim of this study was to test the hypothesis that histones can interact with the antithrombin (AT)-binding vascular glycosaminoglycans (GAGs) to induce inflammation and inhibit the anti-inflammatory function of AT.

METHODS

We evaluated the heparin-binding function of histones by an AT-dependent protease-inhibition assay. Furthermore, we treated endothelial cells with histones in the absence and presence of AT and monitored cellular phenotypes employing established signaling assays.

RESULTS

Histones neutralized AT-dependent anticoagulant function of heparin in both purified protease-inhibition and plasma-based assays. Histones also disrupted endothelial cell barrier-permeability function by a GAG-dependent mechanism as evidenced by the GAG-antagonist, surfen, abrogating their disruptive effects. Further studies revealed histones and AT compete for overlapping binding-sites on GAGs, thus increasing concentrations of one protein abrogated effects of the other. Histones elicited proapoptotic effects by inducing nuclear localization of PKC-δ in endothelial cells and barrier-disruptive effects by destabilizing VE-cadherin, which were inhibited by AT, but not by a D-helix mutant of AT incapable of interacting with GAGs. Finally, histones induced release of Weibel-Palade body contents, VWF and angiopoietin-2, and promoted expression of cell adhesion molecules on endothelial cells, which were all downregulated by AT but not by D-helix mutant of AT.

CONCLUSION

We conclude that histones and AT compete for overlapping binding sites on vascular GAGs to modulate coagulation and inflammation.

Anticoagulant and signaling functions of antithrombin

Antithrombin (AT) is a major plasma glycoprotein of the serpin superfamily that regulates the proteolytic activity of the procoagulant proteases of both intrinsic and extrinsic pathways. Two important structural features that participate in the regulatory function of AT include a mobile reactive center loop that binds to active site of coagulation proteases, trapping them in the form of inactive covalent complexes, and a basic D-helix that binds to therapeutic heparins and heparan sulfate proteoglycans (HSPGs) on vascular endothelial cells. The binding of D-helix of AT by therapeutic heparins promotes the reactivity of the serpin with coagulation proteases by several orders of magnitude by both a conformational activation of the serpin and a template (bridging) mechanism. In addition to its essential anticoagulant function, AT elicits a potent anti-inflammatory signaling response when it binds to distinct vascular endothelial cell HSPGs, thereby inducing prostacyclin synthesis. Syndecans-4 has been found as a specific membrane-bound HSPG receptor on endothelial cells that relays the signaling effect of AT to the relevant second messenger molecules in the signal transduction pathways inside the cell. However, following cleavage by coagulation proteases and/or by spontaneous conversion to a latent form, AT loses both its anti-inflammatory activity and high-affinity interaction with heparin and HSPGs. Interestingly, these low-affinity heparin conformers of AT elicit potent proapoptotic and antiangiogenic activities by also binding to specific HSPGs by unknown mechanisms. This review article will summarize current knowledge about mechanisms through which different conformers of AT exert their serine protease inhibitory and intracellular signaling functions in these biological pathways.