Vascular manifestations associated with systemic lupus erythematosus (SLE) span a broad range, including vasculopathy. An understudied pathway of this morbidity is a repair component. Recent studies have elevated the anti-injury biomarkers, adiponectin and membrane endothelial protein C receptor (EPCR), for consideration with roles to antagonize premature atherosclerosis and SLE nephritis, respectively. For example, adiponectin was found to serve as an independent predictor of carotid plaque, and its elevations were persistent over more than one visit. Unexpectedly, this biomarker was present despite clinical quiescence. In vasculopathy as a comorbidity to SLE nephritis, the persistent expression of membrane EPCR at peritubular capillaries may represent a response to the local cues of a deficit of active protein C. Under conditions of unresolved morbidity, higher levels of adiponectin and membrane EPCR may represent a physiologic attempt to limit further endothelial damage, and the observed increase in plaque and progression of SLE nephritis represent an overwhelming of this reparative process by disease-provoking stimuli.
Systemic lupus erythematosus (SLE) is a disease state posing several challenges to clinicians, including heterogeneity of presentation, undulating course, and an extraordinary risk for vascular injury, including premature atherosclerosis and endothelial injury related to renal disease. Central to this concept is a focus on the endothelium, because it provides the physiologic boundary that limits extravasation and diapedesis of inflammatory cells. This article provides a clinical overview and outlines the putative pathogenic events that occur in SLE autoimmune-associated vasculopathy for atherosclerosis and renal disease.
Regarding premature cardiovascular disease, more than 3 decades ago investigators noted that most deaths in patients who had SLE with longer disease duration were attributed to atherosclerosis. The rate of myocardial infarction in women aged 35 to 44 years is 50 times greater than expected. Patients who have SLE have an increased atherosclerotic risk despite adjustment for traditional Framingham risk factors. Risk factors among these patients are somewhat controversial but may include longer duration of disease and lower likelihood of treatment with prednisone, cyclophosphamide, or hydroxychloroquine. Thus, inflammation related to underlying disease is likely to be contributory.
McMahon and coworkers recently showed that plasma from patients who have SLE with premature atherosclerosis is enriched in proinflammatory high-density lipoprotein (HDL). However, the inflammation may be clinically subtle because detectable cardiovascular events have been unexpectedly reported in patients who have SLE with extended periods of quiescence, and subclinical atherosclerosis has not correlated with disease activity index scores.
Functional impairment of the endothelium is reflected by the pattern of proinjury mediators, such as circulating endothelial cells (CECs), apoptotic circulating endothelial cells, and soluble E-selectin (sE-selectin). For example, generation of nitric oxide (NO) by the endothelium promotes relaxation of the contractile elements of the smooth muscle of the arterial blood vessels. In atherogenic disease, endothelial protection may be subverted because of a loss of boundary function through detachment of endothelial cells into the circulation or a change in the endothelial cell phenotype. Increased levels of CECs have been observed in patients who have active disease, and apoptotic CECs have been reported in patients who have SLE with diminished flow-mediated dilatation. sE-selectin, likely shed from an abnormally activated endothelium, was recently associated with atherosclerosis through an abnormal coronary artery calcium detected with electron beam CT.
Blood vessel homeostasis involves a complex interplay between inflammatory signals, coagulation signals, and other mediators. T-cell recruitment of monocytes into artery walls may be a critical step in the effective handling of cholesterol. Cholesterol in the bloodstream is scavenged by a low-density lipoprotein molecule and deposited in the arterial wall, where monocytes, which have been recruited by activated T cells, are a part of normal homeostasis ( Fig. 1 ). The monocyte should optimally serve as a temporary depot for fats, differentiating into an efficient cholesterol-metabolizing macrophage until the excess lipid can be picked up by HDL.
The scenario is dramatically altered when complement is activated. Complement activation, T-cell cytokine interferon -γ (IFN-γ), or T cell–stimulated immune complexes promote the activation of endothelial cells to express surface metalloproteinases; an impaired lipid handling with sequential arterial narrowing occurs concomitantly. Once this system becomes overwhelmed, cholesterol plaques develop, which induce both inflammatory and fibrotic reactions.
Lupus nephritis (LN) predominantly affects African American and Hispanic women of reproductive age. A higher incidence of progression to end-stage renal disease (ESRD) in these ethnic subsets has been shown, compared with other predominantly Caucasian populations. Although the overall 10-year survival has improved to more than 90% in patients who have SLE, the incidence of LN progressing to ESRD has remained constant.
A literature review has shown a difference in response to treatment modalities for LN based on ethnic/racial differences. For patients who have proliferative forms of LN, renal survival is worse with progression to ESRD in African American and Hispanic patients despite treatment with intravenous cyclophosphamide (IVC), even after controlling for hypertension, initial renal functional impairment, and quantity of corticosteroid therapy. Socioeconomic features (income, educational level, access to health care) may contribute to the poorer prognosis in these populations. In one study, however, the relative risk for progression to ESRD remained higher in the Hispanic population. Several recent studies have found that African American and Hispanic populations had a better response to mycophenolate mofetil than to IVC.
No new medications have been FDA approved specifically for treating SLE or LN in more than 50 years. Most drugs used in the treatment regimen are commercially available and prescribed off-label, or available only as part of an investigational protocol. Recent clinical trials for newer agents in the treatment of LN may not have achieved predetermined end points because of either an actual absence of efficacy or shortcomings in the design of the study protocol. Furthermore, pharmacogenomics may have a role in determining the efficacy and safety of a medication. Therefore, the effect a medication may have in improving the renal survival and overall outcome in select subgroups of patients or even individual patients is important to know before subjecting them to aggressive immunosuppression and its potential side effects.
Nephritis, a life-threatening manifestation of SLE, is strongly influenced by blood vessels partly because the vasculature plays a central role in supporting homeostasis. The contribution of the vascular endothelium to the pathogenesis of renal injury has not been emphasized in LN. Despite potential biologic insights and treatment strategies to be gained by studying the endothelium in LN, historic World Health Organization (WHO) classification, National Institutes of Health (NIH) chronicity (CI) and activity (AI) indices, and recent International Society of Nephrology/Renal Pathology Society (ISN/RPS) 2003 pathologic classifications of LN do not specifically address the state of the microvasculature in their definitions. However, recent murine data based on microarray analysis suggest that endothelial activation is a feature shared by progressive glomerulosclerosis compared with nonprogressive glomerulosclerosis.
For one scenario of putative pathogenic events, exposure of healthy endothelial cells to potential stimuli such as circulating IFN-α, tumor necrosis factor α (TNF-α), or immune complexes present in patients who have active SLE results in the expression of NO synthase 2 (NOS2) and generation of NO and adhesion molecules. As shown in Fig. 2 , this activated endothelium has now lost its function to serve as a physiologic brake, which normally prevents the infiltration of inflammatory cells that produce IL-18, a potent chemoattractant for plasmacytoid dendritic cells.
Endothelial cells may also be activated by IL-18. Plasmacytoid dendritic cells release IFNs, which have a paracrine effect on other cell types to express NOS2. In addition to the local inflammatory consequences of activation, endothelial cells are shed into the circulation and membrane endothelial protein C receptor (EPCR) is lost so that EPCR circulates as a soluble form (sEPCR), with a procoagulant effect. Prothrombin F1+2 (a marker of thrombin generation) are generated, a consequence of thrombin that is indirectly responsible for release of sEPCR. The increased release of sEPCR coupled with higher thrombin generation suggests that less membrane-bound EPCR will be available in these individuals for efficient protein C activation.
Although prior studies on premature atherosclerosis and vascular injury in nephritis have highlighted the potential association of markers directly reflecting injury, absent is a focus on protective (anti-injury) molecules such as the adipocyte-derived protein, adiponectin, and membrane EPCR (mEPCR).
This article describes how adiponectin and mEPCR serve as a pathway that focuses on anti-injury. In addition, the authors propose that mEPCR and adiponectin may herald a thwarted attempt at protection in patients at risk for progression of atherosclerotic and renal disease. Moreover, the identification of these molecules as biomarkers may represent an incremental advance that offers insight into pathogenesis and therapy.
Profile of adiponectin in health and disease
Adiponectin (also known as 30-kd adipocyte complement-related protein [Acrp30]) is a secreted protein that is constitutively produced by adipocytes. Adiponectin, a trimer in serum, is a 30-kd protein consisting of four domains, including signal peptide at the N-terminus, a variable domain, a collagenous domain, and a C-terminal globular domain homologous to C1q. The protein is well characterized regarding its capacity to improve insulin sensitivity. During the early 1990s the best-characterized biologic property was enhancing glycogen accumulation and fatty acid oxidation in C2C12 myotubes. The in vivo action to regulate serum levels of fatty acids and glucose was linked to adiponectin’s effect on the liver to suppress glucose output while acting on muscle to increase glucose uptake and fatty acid oxidation.
However, adiponectin’s actions are not restricted to controlling glucose and lipid metabolism. Properties involving anti-inflammatory and anti-atherosclerotic functions have also been reported. Adiponectin accumulates in the subendothelium of injured human arteries, where it inhibits monocyte adhesion to endothelial cells and ultimately inhibits the migration and proliferation of vascular smooth muscle, which contribute to the atherosclerotic process through a mechanism that partly involves the down-regulation of adhesion molecules through attenuating the nuclear factor κB pathway.
Adiponectin also has been found in blood vessel walls after experimental endothelial injury, and is strongly expressed around infarcted but not normal myocardium, supporting a role in vascular and endothelial remodeling. Specifically, adiponectin was recruited to the affected area of an arterial injury site in balloon-injured rat carotid arteries. Adiponectin knockout (KO) mice were used to further study the relationship between adiponectin and the properties of the vasculature. The mice displayed an impaired endothelium-dependent vasodilation. A series of experiments were performed that suggested that adiponectin serves a role as a proangiogenic regulator. Angiogenic repair of ischemic hind limbs was impaired in adiponectin KO mice compared with wild-type. These data suggested that the exogenous supplementation of adiponectin could be a beneficial treatment for obesity-related vascular disorders.
Membrane endothelial protein C receptor: protection of the endothelium
EPCR, which has been cloned in mice and human tissues, is constitutively expressed by endothelial cells, particularly in large blood vessels and monocyte/macrophages. EPCR is a 46-kd type 1 transmembrane glycoprotein with structural features consistent with an antigen-presenting groove analogous to major histocompatibility complex (MHC) class 1 and the CD1 family of proteins. Its structure consists of a large extracellular domain (221 amino acids), a transmembrane domain (25 amino acids), and a short highly conserved cytoplasmic sequence (3 amino acids).
The 5′ flanking region of the murine EPCR gene was recently examined and shown to contain elements that reflect involvement in cell growth and development and a thrombin response element. Endotoxin and thrombin elevate rodent EPCR mRNA levels and increase receptor shedding in vivo. A phospholipid is tightly bound in the position of the antigen-presenting groove, suggesting that EPCR recycles to membrane endosomes and participates in antigen presentation.
A well-characterized biologic property of membrane EPCR is its role as an accessory factor to thrombin–thrombomodulin complexes causing a dramatic augmentation of the formation of activated protein C (APC). For example, previous studies showed that APC generation was dependent on EPCR and that blocking protein C–EPCR interaction decreased APC formation approximately 10-fold. In baboons, EPCR blocking antibodies were found to decrease protein C activation and increase susceptibility to bacterial sepsis.
These studies have advanced the notion that high levels of mEPCR are a beneficial property and that lower levels of are deleterious, which was supported by several studies focusing on genetic manipulation of EPCR in mice. Overexpression of EPCR under the control of an endothelium-specific Tie2 promoter was shown to dramatically alter patterns of EPCR expression, although the mice did not exhibit any gross hemorrhagic abnormalities. They did, however, exhibit an eightfold increase in APC generation in response to infusion of thrombin, and were partially resistant to a lethal dose of bacterial lipopolysaccharide. These findings confirm and extend the results of previous studies, which used blocking antibodies.
Cleavage of EPCR from the cell surface by matrix metalloproteinases has been shown. This action is initiated when endothelial cells are treated with lipopolysaccharide, inflammatory cytokines, and thrombin. A single nucleotide polymorphism (SNP) in exon 4 of the EPCR gene at 20q11, which converts serine 219 to glycine (219 Gly) in a region of the molecule close to the plasma membrane, is associated with increased basal and stimulated shedding of EPCR from endothelial cells. The absence of membrane EPCR resulted in an attenuation of the thrombin–thrombomodulin complex–dependent protein C activation. Because sEPCR also binds to protein C and APC, functionally it results in a loss of a brake to thrombosis and inflammation.
In a murine model using heterozygotes of EPCR knockout mice, administration of endotoxin-induced disseminated intravascular coagulation, which was aggravated in heterozygous protein C–deficient mice compared with wild-type. It is tempting to speculate that low levels of membrane EPCR secondary to high shedding of EPCR may also have deleterious consequences to coagulation and inflammation. However, studies in humans, which initially focused on deep vein thrombosis, have not shown uniform results of the risk for venous thrombosis for the genotype in separate cohorts.
mEPCR may also have direct effects on endothelial cell phenotype. For example, the shedding of EPCR is associated with the activation of protease-activated receptors (PARs), a novel family of G-protein–coupled receptors that are constitutively expressed on endothelial cells and are involved in the early recruitment of leukocytes.
Membrane endothelial protein C receptor: protection of the endothelium
EPCR, which has been cloned in mice and human tissues, is constitutively expressed by endothelial cells, particularly in large blood vessels and monocyte/macrophages. EPCR is a 46-kd type 1 transmembrane glycoprotein with structural features consistent with an antigen-presenting groove analogous to major histocompatibility complex (MHC) class 1 and the CD1 family of proteins. Its structure consists of a large extracellular domain (221 amino acids), a transmembrane domain (25 amino acids), and a short highly conserved cytoplasmic sequence (3 amino acids).
The 5′ flanking region of the murine EPCR gene was recently examined and shown to contain elements that reflect involvement in cell growth and development and a thrombin response element. Endotoxin and thrombin elevate rodent EPCR mRNA levels and increase receptor shedding in vivo. A phospholipid is tightly bound in the position of the antigen-presenting groove, suggesting that EPCR recycles to membrane endosomes and participates in antigen presentation.
A well-characterized biologic property of membrane EPCR is its role as an accessory factor to thrombin–thrombomodulin complexes causing a dramatic augmentation of the formation of activated protein C (APC). For example, previous studies showed that APC generation was dependent on EPCR and that blocking protein C–EPCR interaction decreased APC formation approximately 10-fold. In baboons, EPCR blocking antibodies were found to decrease protein C activation and increase susceptibility to bacterial sepsis.
These studies have advanced the notion that high levels of mEPCR are a beneficial property and that lower levels of are deleterious, which was supported by several studies focusing on genetic manipulation of EPCR in mice. Overexpression of EPCR under the control of an endothelium-specific Tie2 promoter was shown to dramatically alter patterns of EPCR expression, although the mice did not exhibit any gross hemorrhagic abnormalities. They did, however, exhibit an eightfold increase in APC generation in response to infusion of thrombin, and were partially resistant to a lethal dose of bacterial lipopolysaccharide. These findings confirm and extend the results of previous studies, which used blocking antibodies.
Cleavage of EPCR from the cell surface by matrix metalloproteinases has been shown. This action is initiated when endothelial cells are treated with lipopolysaccharide, inflammatory cytokines, and thrombin. A single nucleotide polymorphism (SNP) in exon 4 of the EPCR gene at 20q11, which converts serine 219 to glycine (219 Gly) in a region of the molecule close to the plasma membrane, is associated with increased basal and stimulated shedding of EPCR from endothelial cells. The absence of membrane EPCR resulted in an attenuation of the thrombin–thrombomodulin complex–dependent protein C activation. Because sEPCR also binds to protein C and APC, functionally it results in a loss of a brake to thrombosis and inflammation.
In a murine model using heterozygotes of EPCR knockout mice, administration of endotoxin-induced disseminated intravascular coagulation, which was aggravated in heterozygous protein C–deficient mice compared with wild-type. It is tempting to speculate that low levels of membrane EPCR secondary to high shedding of EPCR may also have deleterious consequences to coagulation and inflammation. However, studies in humans, which initially focused on deep vein thrombosis, have not shown uniform results of the risk for venous thrombosis for the genotype in separate cohorts.
mEPCR may also have direct effects on endothelial cell phenotype. For example, the shedding of EPCR is associated with the activation of protease-activated receptors (PARs), a novel family of G-protein–coupled receptors that are constitutively expressed on endothelial cells and are involved in the early recruitment of leukocytes.
Endothelial dysfunction and progression of atherosclerosis and renal disease
The usefulness of biomarkers, which identify patients who have endothelial dysfunction and progression of atherosclerosis and renal disease, has fallen short of the mark. This section reviews the biomarker discovery initiative in the context of inflammation hypothesis and Schwartzman phenomenon. Recent studies are then reviewed that offer a new direction for risk stratification. New mechanistic insights into the pathophysiology underlying accelerated atherogenesis and renal disease are then highlighted.
Many have speculated that inflammation, the central feature of SLE pathogenesis and clinical flares, is linked to the increased atherosclerotic risk among these patients, in addition to higher rates of traditional risk factors. However, the absence of an association between carotid plaque and overt disease activity in this study and others contradicts the inflammation hypothesis. One simple explanation is that the SELENA-SLEDAI (SLE Disease Activity Index) instrument may underestimate disease activity. However, this instrument does capture current inflammation in the cutaneous, renal, serosal, hematologic, neurologic, and serologic systems. In fact, these clinical settings of activity are where the traditional markers of endothelial (luminal) injury have been identified.
However, the scope of the vasculopathy may include normal-appearing blood vessels. For example, in a skin biopsy obtained from nonlesional, non–sun exposed skin of the buttocks, immunohistochemistry found levels of adhesion molecules (VCAM1, ICAM1 and E selectin) to be significantly elevated in patients versus normal controls. Because these phenotypic changes occurred in normal-appearing tissue, the widespread endothelial activation was portrayed in the context of the Schwartzman phenomenon. However, expression of the anti-injury molecules (eg, adiponectin, mEPCR) was not evaluated.
As the authors reported at the 2009 American College of Rheumatology meeting, in a cohort of 131 patients and 73 race/ethnicity-matched healthy controls, carotid plaque was observed in more than twice the proportion of patients who had lupus compared with age-and sex-matched controls (43% vs 17%, P = .0002). Excess prevalence of plaque was seen beginning in the fifth decade of life. On multivariate analysis, age, SLE disease duration, sE-selectin, and adiponectin were the only independent predictors of plaque. Among patients who had lupus who showed plaque, elevations in these biomarkers were persistent over more than one visit in those who had multiple measurements. Elevation of sE-selectin was anticipated, because this biomarker reflects activation of the endothelium and has been previously associated with atherosclerosis and cardiovascular risk in both SLE and non-SLE cohorts. The association with elevated adiponectin was unexpected because adiponectin is generally considered to be vasoprotective. In fact, the authors’ hypothesis was that adiponectin would be decreased.
This finding led the authors to speculate that perhaps elevated adiponectin represents a continued but unsuccessful attempt at vascular repair. Table 1 reports percentage of plaque for different groups defined according to adiponectin and E-selectin levels. Given the study design, these percentages can be directly interpreted as predictive values or, more accurately, predicted probabilities of plaque. For example, the probability of plaque is predicted to be 75% if an individual has high levels of both adiponectin and E-selectin, whereas the probability of plaque is 19% for individuals who have low levels of both biomarkers (see Table 1 ). The two biomarkers must be considered simultaneously when evaluating predictive values because each was found to be independently associated with plaque. Therefore, the predicted probability of plaque for adiponectin depends on whether the E-selectin level is high or low, and vice versa.
Related posts:
Cutaneous Lupus and the Cutaneous Lupus Erythematosus Disease Area and Severity Index Instrument
Pediatric Lupus—Are There Differences in Presentation, Genetics, Response to Therapy, and Damage Accrual Compared with Adult Lupus?
The Metabolic Syndrome in Systemic Lupus Erythematosus
Interferon-alpha: A Therapeutic Target in Systemic Lupus Erythematosus
Cell-Bound Complement Biomarkers for Systemic Lupus Erythematosus: From Benchtop to Bedside
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