Homocysteine Modulates the CD40/CD40L System

The Controversial Role of HCY and Vitamin B Deficiency in Cardiovascular Diseases

Plasma homocysteine (HCY) is an established risk factor for cardiovascular disease CVD and stroke. However, more than two decades of intensive research activities has failed to demonstrate that Hcy lowering through B-vitamin supplementation results in a reduction in CVD risk. Therefore, doubts about a causal involvement of hyperhomocysteinemia (HHcy) and B-vitamin deficiencies in atherosclerosis persist. Existing evidence indicates that HHcy increases oxidative stress, causes endoplasmatic reticulum (ER) stress, alters DNA methylation and, thus, modulates the expression of numerous pathogenic and protective genes. Moreover, Hcy can bind directly to proteins, which can change protein function and impact the intracellular redox state. As most mechanistic evidence is derived from experimental studies with rather artificial settings, the relevance of these results in humans remains a matter of debate. Recently, it has also been proposed that HHcy and B-vitamin deficiencies may promote CVD through accelerated telomere shortening and telomere dysfunction. This review provides a critical overview of the existing literature regarding the role of HHcy and B-vitamin deficiencies in CVD. At present, the CVD risk associated with HHcy and B vitamins is not effectively actionable. Therefore, routine screening for HHcy in CVD patients is of limited value. However, B-vitamin depletion is rather common among the elderly, and in such cases existing deficiencies should be corrected. While Hcy-lowering with high doses of B vitamins has no beneficial effects in secondary CVD prevention, the role of Hcy in primary disease prevention is insufficiently studied. Therefore, more intervention and experimental studies are needed to address existing gaps in knowledge.

Peptides Regulating Proliferative Activity and Inflammatory Pathways in the Monocyte/Macrophage THP-1 Cell Line

This study evaluates the effects of five different peptides, the Epitalon^® tetrapeptide, the Vilon^® dipeptide, the Thymogen^® dipeptide, the Thymalin^® peptide complex, and the Chonluten^® tripeptide, as regulators of inflammatory and proliferative processes in the human monocytic THP-1, which is a human leukemia monocytic cell line capable of differentiating into macrophages by PMA in vitro. These peptides (Khavinson Peptides^®), characterized by Prof. Khavinson from 1973 onwards, were initially isolated from animal tissues and found to be organ specific. We tested the capacity of the five peptides to influence cell cultures in vitro by incubating THP-1 cells with peptides at certain concentrations known for being effective on recipient cells in culture. We found that all five peptides can modulate key proliferative patterns, increasing tyrosine phosphorylation of mitogen-activated cytoplasmic kinases. In addition, the Chonluten tripeptide, derived from bronchial epithelial cells, inhibited in vitro tumor necrosis factor (TNF) production of monocytes exposed to pro-inflammatory bacterial lipopolysaccharide (LPS). The low TNF release by monocytes is linked to a documented mechanism of TNF tolerance, promoting attenuation of inflammatory action. Therefore, all peptides inhibited the expression of TNF and pro-inflammatory IL-6 cytokine stimulated by LPS on terminally differentiated THP-1 cells. Lastly, by incubating the THP1 cells, treated with the peptides, on a layer of activated endothelial cells (HUVECs activated by LPS), we observed a reduction in cell adhesion, a typical pro-inflammatory mechanism. Overall, the results suggest that the Khavinson Peptides^® cooperate as natural inducers of TNF tolerance in monocyte, and act on macrophages as anti-inflammatory molecules during inflammatory and microbial-mediated activity.

CD40/CD40L Signaling as a Promising Therapeutic Target for the Treatment of Renal Disease

The cluster of differentiation 40 (CD40) is activated by the CD40 ligand (CD40L) in a variety of diverse cells types and regulates important processes associated with kidney disease. The CD40/CD40L signaling cascade has been comprehensively studied for its roles in immune functions, whereas the signaling axis involved in local kidney injury has only drawn attention in recent years. Clinical studies have revealed that circulating levels of soluble CD40L (sCD40L) are associated with renal function in the setting of kidney disease. Levels of the circulating CD40 receptor (sCD40), sCD40L, and local CD40 expression are tightly related to renal injury in different types of kidney disease. Additionally, various kidney cell types have been identified as non-professional antigen-presenting cells (APCs) that express CD40 on the cell membrane, which contributes to the interactions between immune cells and local kidney cells during the development of kidney injury. Although the potential for adverse CD40 signaling in kidney cells has been reported in several studies, a summary of those studies focusing on the role of CD40 signaling in the development of kidney disease is lacking. In this review, we describe the outcomes of recent studies and summarize the potential therapeutic methods for kidney disease which target CD40.

Molecular basis and therapeutic implications of CD40/CD40L immune checkpoint

The CD40 receptor and its ligand CD40L is one of the most critical molecular pairs of the stimulatory immune checkpoints. Both CD40 and CD40L have a membrane form and a soluble form generated by proteolytic cleavage or alternative splicing. CD40 and CD40L are widely expressed in various types of cells, among which B cells and myeloid cells constitutively express high levels of CD40, and T cells and platelets express high levels of CD40L upon activation. CD40L self-assembles into functional trimers which induce CD40 trimerization and downstream signaling. The canonical CD40/CD40L signaling is mediated by recruitment of TRAFs and NF-κB activation, which is supplemented by signal pathways such as PI3K/AKT, MAPKs and JAK3/STATs. CD40/CD40L immune checkpoint leads to activation of both innate and adaptive immune cells via two-way signaling. CD40/CD40L interaction also participates in regulating thrombosis, tissue inflammation, hematopoiesis and tumor cell fate. Because of its essential role in immune activation, CD40/CD40L interaction has been regarded as an attractive immunotherapy target. In recent years, significant advance has been made in CD40/CD40L-targeted therapy. Various types of agents, including agonistic/antagonistic monoclonal antibodies, cellular vaccines, adenoviral vectors and protein antagonist, have been developed and evaluated in early-stage clinical trials for treating malignancies, autoimmune diseases and allograft rejection. In general, these agents have demonstrated favorable safety and some of them show promising clinical efficacy. The mechanisms of benefits include immune cell activation and tumor cell lysis/apoptosis in malignancies, or immune cell inactivation in autoimmune diseases and allograft rejection. This review provides a comprehensive overview of the structure, processing, cellular expression pattern, signaling and effector function of CD40/CD40L checkpoint molecules. In addition, we summarize the progress, targeted diseases and outcomes of current ongoing and completed clinical trials of CD40/CD40L-targeted therapy.

Co-signaling receptors regulate T-cell plasticity and immune tolerance

We took an experimental database mining analysis to determine the expression of 28 co-signaling receptors in 32 human tissues in physiological/pathological conditions. We made the following significant findings: 1) co-signaling receptors are differentially expressed in tissues; 2) heart, trachea, kidney, mammary gland and muscle express co-signaling receptors that mediate CD4⁺T cell functions such as priming, differentiation, effector, and memory; 3) urinary tumor, germ cell tumor, leukemia and chondrosarcoma express high levels of co-signaling receptors for T cell activation; 4) expression of inflammasome components are correlated with the expression of co-signaling receptors; 5) CD40, SLAM, CD80 are differentially expressed in leukocytes from patients with trauma, bacterial infections, polarized macrophages and in activated endothelial cells; 6) forward and reverse signaling of 50% co-inhibition receptors are upregulated in endothelial cells during inflammation; and 7) STAT1 deficiency in T cells upregulates MHC class II and co-stimulation receptors. Our results have provided novel insights into co-signaling receptors as physiological regulators and potentiate identification of new therapeutic targets for the treatment of sterile inflammatory disorders.

Chronic Kidney Disease Induces Inflammatory CD40+ Monocyte Differentiation via Homocysteine Elevation and DNA Hypomethylation

RATIONALE

Patients with chronic kidney disease (CKD) develop hyperhomocysteinemia and have a higher cardiovascular mortality than those without hyperhomocysteinemia by 10-fold.

OBJECTIVE

We investigated monocyte differentiation in human CKD and cardiovascular disease (CVD).

METHODS AND RESULTS

We identified CD40 as a CKD-related monocyte activation gene using CKD-monocyte -mRNA array analysis and classified CD40 monocyte (CD40⁺CD14⁺) as a stronger inflammatory subset than the intermediate monocyte (CD14⁺⁺CD16⁺) subset. We recruited 27 patients with CVD/CKD and 14 healthy subjects and found that CD40/CD40 classical/CD40 intermediate monocyte (CD40⁺CD14⁺/CD40⁺CD14⁺⁺CD16^-/CD40⁺CD14⁺⁺CD16⁺), plasma homocysteine, S-adenosylhomocysteine, and S-adenosylmethionine levels were higher in CVD and further elevated in CVD+CKD. CD40 and CD40 intermediate subsets were positively correlated with plasma/cellular homocysteine levels, S-adenosylhomocysteine and S-adenosylmethionine but negatively correlated with estimated glomerular filtration rate. Hyperhomocysteinemia was established as a likely mediator for CKD-induced CD40 intermediate monocyte, and reduced S-adenosylhomocysteine/S-adenosylmethionine was established for CKD-induced CD40/CD40 intermediate monocyte. Soluble CD40 ligand, tumor necrosis factor (TNF)-α/interleukin (IL)-6/interferon (IFN)-γ levels were elevated in CVD/CKD. CKD serum/homocysteine/CD40L/increased TNF-α/IL-6/IFN-γ-induced CD40/CD40 intermediate monocyte in peripheral blood monocyte. Homocysteine and CKD serum-induced CD40 monocyte were prevented by neutralizing antibodies against CD40L/TNF-α/IL-6. DNA hypomethylation was found on nuclear factor-κB consensus element in CD40 promoter in white blood cells from patients with CKD with lower S-adenosylmethionine / S-adenosylhomocysteine ratios. Finally, homocysteine inhibited DNA methyltransferase-1 activity and promoted CD40 intermediate monocyte differentiation, which was reversed by folic acid in peripheral blood monocyte.

CONCLUSIONS

CD40 monocyte is a novel inflammatory monocyte subset that appears to be a biomarker for CKD severity. Hyperhomocysteinemia mediates CD40 monocyte differentiation via soluble CD40 ligand induction and CD40 DNA hypomethylation in CKD.

Atherogenesis: hyperhomocysteinemia interactions with LDL, macrophage function, paraoxonase 1, and exercise

Despite great strides in understanding the atherogenesis process, the mechanisms are not entirely known. In addition to diet, cigarette smoking, genetic predisposition, and hypertension, hyperhomocysteinemia (HHcy), an accumulation of the noncoding sulfur-containing amino acid homocysteine (Hcy), is a significant contributor to atherogenesis. Although exercise decreases HHcy and increases longevity, the complete mechanism is unclear. In light of recent evidence, in this review, we focus on the effects of HHcy on macrophage function, differentiation, and polarization. Though there is need for further evidence, it is most likely that HHcy-mediated alterations in macrophage function are important contributors to atherogenesis, and HHcy-countering strategies, such as nutrition and exercise, should be included in the combinatorial regimens for effective prevention and regression of atherosclerotic plaques. Therefore, we also included a discussion on the effects of exercise on the HHcy-mediated atherogenic process.

A novel flow cytometric approach to distinguish circulating endothelial cells from endothelial microparticles: relevance for the evaluation of endothelial dysfunction

Circulating endothelial cells (CEC) and endothelial microparticles (EMP) are emerging as markers of endothelial repair and activation/apoptosis. Although significant changes in the number of CEC and EMP in pathological conditions have been reported, their reliable identification and quantification still remain a technical challenge. Here, we present a novel methodology for the identification and quantitation of CEC and EMP based on multicolor flow cytometry. Using a lyse/no wash protocol, we observed that in 50 μl of peripheral blood, the large majority of events expressing an endothelial phenotype (CD45-/CD146+/CD34+) are due to non-nucleated particles (DRAQ5-) carrying mitochondrial activity (MitoTracker+) and, therefore, classified as EMP. We enumerated circulating EMP by single platform absolute count in a lyse/no wash four-color flow-cytometric procedure, which allowed the distinction, within the whole endothelial compartment, of EMP derived from endothelial progenitors (CD45-/CD146+/CD34+/CD117+) and from mature endothelial cells (CD45-/CD146+/CD34+/CD117-). A significant increase in both subsets was observed in patients with diabetes mellitus. Thus, this simple and highly reproducible method may be useful for monitoring endothelial dysfunction in clinical settings.

The effect of hormone therapy and tibolone on serum CD40L and ADAM-8 in healthy post-menopausal women

BACKGROUND/AIM

The role of neutrophils and platelets in atherothrombotic disease is well established. The aim of our study was to investigate the effect of HT and tibolone on the soluble markers of neutrophil and platelet activation, "a disentigrin and metalloproteinase domain" (ADAM-8) and CD40 ligand (CD40L) respectively, in healthy post-menopausal women.

SUBJECTS AND METHODS

One hundred and six healthy post-menopausal women were randomly allocated to: estradiol plus drospirenone (E₂/DSP), E₂ hemihydrate 1 mg plus norethisterone acetate (E₂/NETA) 0.5 mg, and tibolone 2.5 mg. Serum ADAM-8 and CD40L were measured at baseline and at 6 months.

RESULTS

Baseline values of ADAM-8 and CD40L were similar between groups. No significant correlation was revealed between ADAM-8 or CD40L and parameters related to cardiovascular risk factors in each group. No significant changes were observed between baseline values and values at 6 months (E₂/DSP group: ADAM-8: 267.4±71.3 pg/ml vs 270.7±42.8 pg/ml, p=0.86, CD40L: 6.43±3.13 vs 6.79±2.70 ng/ml, p=0.67), (E₂/NETA group: ADAM-8: 308.3±64.3 vs 294.7±57.7 pg/ml, p=0.40, CD40L: 9.68±2.81 vs 8.59±5.13 ng/ml, p=0.51), (tibolone group: ADAM-8: 307.5±87.5 vs 289±48.1 pg/ml, p=0.48, CD40L: 9.46±4.30 vs 9.26±4.60 ng/ml, p=0.99).

CONCLUSIONS

Our study has not revealed an association between estrogen plus progestin treatment or tibolone on serum ADAM-8 and CD40L levels in healthy post-menopausal women. Larger prospective studies are needed to further investigate the effect of low-dose HT or tibolone on serum markers of neutrophil and platelet activation.

From atherosclerosis to acute coronary syndromes: the role of soluble CD40 ligand

The binding of CD40 ligand (CD40L) to CD40 stimulates inflammatory processes including the release of proinflammatory cytokines and the expression of adhesion molecules implying a role in atherosclerosis. Patients exhibiting hypercholesterolemia, unstable angina, or acute myocardial infarction present with increased CD40L levels. Novel data suggest that elevated soluble CD40L levels not only represent a risk factor for cardiovascular disease but also predict future adverse events, especially in patients with acute coronary syndromes (ACS). Examination of the potential role of the genetic variability on CD40/CD40L genes in ACS, as regards the regulation of CD40L, appears to be of great interest. Moreover, several therapeutic approaches such as statins, antihypertensive agents, and antiplatelet agents have been suggested as potential modulators of CD40L levels anticipating a positive impact on the outcomes of patients with ACS. Whether specific agents target the CD40/CD40L system as well as its pathogenic role in ACS remains to be elucidated by large-scale studies in the future.

The CD40/CD40 ligand system: linking inflammation with atherothrombosis

The role of CD40/CD40 ligand (CD40L) in atherothrombosis is now widely accepted. However, the exact mechanisms linking the CD40/CD40L system and the soluble form of CD40 ligand (sCD40L) with atherothrombosis are currently a topic of intensive research. CD40L and sCD40L belong to the tumor necrosis factor superfamily, and they are molecules with a dual prothrombotic and proinflammatory role. They are expressed in a variety of tissues such as the immune system (in both B and T cells), the vascular wall, and activated platelets. Soluble CD40L has multiple autocrine, paracrine, and endocrine actions, and it may trigger key mechanisms participating in atherothrombosis. CD40/CD40L may participate in the development of coronary atherosclerosis and the triggering of acute coronary syndromes, while sCD40L seems to have a prognostic role not only in subjects with advanced atherosclerosis but also in the general population. Although conventional cardiovascular medication such as antiplatelet therapy, statins, angiotensin-converting enzyme inhibitors, and many others have been shown to reduce both sCD40L and cardiovascular risk, it is still unclear whether specific treatments targeting the CD40/CD40L system will prove to be beneficial against atherothrombosis in the near future.

Comparative studies on homocysteine and its metabolite-homocysteine thiolactone action in blood platelets in vitro

Homocysteine (Hcy), an intermediate formed during the catabolism of the essential dietary amino acid methionine, and its cyclic thioester, homocysteine thiolactone (TL) formed from Hcy in plasma, may be implicated in pathological haemostasis and atherosclerosis. The mechanism by which TL exerts the prothrombotic effect and influences blood platelets remains unclear. Activation of blood platelets plays an important role in prothrombotic events. The aim of our study was to establish and compare the influence of a reduced form of homocysteine (at final doses of 10-100 microM) and its cyclic thioester, homocysteine thiolactone (0.1-1 microM), on platelet activation induced by thrombin (platelet aggregation), on platelet protein modifications (determined by parameters such as level of protein carbonyl groups, 3-nitrotyrosine residues in proteins) and on superoxide anion radicals ( O2-*) generation using the model system in vitro. We have observed that TL, like its precursor, Hcy, stimulates the generation of O2* in platelets and causes an augmentation of platelet aggregation induced by thrombin. Our present results in vitro also demonstrate that Hcy (10-100 microM) and TL at lower doses than Hcy (0.1-1 microM) cause modification of platelet proteins: diminished formation of carbonyl groups and distinctly decreased tyrosine nitration in platelet proteins after thrombin stimulation, but increased platelet aggregation induced by thrombin. TL like Hcy (at concentrations corresponding to concentrations in blood during hyperhomocysteinemia) modifies platelet responses to an important physiological agonist--thrombin.

Homocysteine and coronary atherosclerosis: from folate fortification to the recent clinical trials

Plasma total homocysteine (Hcy) has been associated with cardiovascular risk in multiple large-scale epidemiological studies, and it has been considered as an independent risk factor for atherosclerosis. Homocysteine lowering, achieved after the introduction of the folate food fortification programme in North America, was accompanied by an accelerated decline of cardiovascular risk and especially of stroke. Although the initial clinical trials suggested that homocysteine-lowering treatment with folates and B vitamins induces coronary plaque regression, this finding was not confirmed by more recent clinical studies. Under the light of the findings from the recent large randomized clinical trials that failed to document a benefit of Hcy lowering on clinical outcome of patients with atherosclerosis, the role of Hcy as a risk factor and the efficacy of Hcy lowering against atherosclerosis have been questioned. Therefore, better understanding of the mechanisms relating Hcy and Hcy-lowering treatment with vascular function and atherogenesis is crucial, to help us understand why clinical trials failed to show a benefit from Hcy-lowering treatment. Are these therapeutic strategies ineffective because they fail to reduce intracellular Hcy levels and vascular redox state or should Hcy stop being considered as an independent risk factor for atherosclerosis from now on? In this review article, we provide a global approach of the molecular mechanisms relating Hcy with cardiovascular risk and introduce possible mechanistic explanations regarding the inability of clinical trials to detect any clinical benefit from Hcy-lowering treatment in secondary prevention. Finally, we provide clinical recommendations regarding the therapeutic strategies targeting homocysteine in the general population.