Low-Density Lipoprotein Cholesterol, Structural Atherosclerosis, and Functional Atherosclerosis in Older Japanese

Aggressive endothelial repair results in the progression of both structural and functional atherosclerosis, while insufficient endothelial repair worsens functional but not structural atherosclerosis. Aging increases the risk of inadequate endothelial repair. Since low-density lipoprotein cholesterol (LDLc) activates endothelial repair, LDLc may be positively associated with structural atherosclerosis but inversely associated with functional atherosclerosis in older individuals. This cross-sectional study analyzed 1458 participants aged 60 to 79 years. We defined structural atherosclerosis as a carotid intima-media thickness (CIMT) of at least 1.1 mm and functional atherosclerosis as a cardio-ankle vascular index (CAVI) of at least 9.0. LDLc was significantly positively associated with structural atherosclerosis and significantly inversely associated with functional atherosclerosis, independently of known cardiovascular risk factors. For 1 standard increment of LDLc (28 mg/dL for men and 29 mg/dL for women), the odds ratios and 95% confidence intervals after adjustment for known cardiovascular risk factors were 1.28 (1.10, 1.50) for structural atherosclerosis and 0.85 (0.75, 0.96) for functional atherosclerosis. LDLc activates endothelial repair, which results in the development of structural atherosclerosis but maintains endothelial function in older individuals. To evaluate atherosclerosis in clinical practice, the combination of structural and functional assessment of atherosclerosis could be informative.

Height and Active Arterial Wall Thickening in Relation to Thyroid Cysts Status among Elderly Japanese: A Prospective Study

Height is inversely associated with inflammation that stimulates endothelial repair. In our previous study involving elderly men aged 60-69 years, we found that active arterial wall thickening, which is known as the process of endothelial repair, requires CD34-positive cells. As thyroid hormone regulates CD34-positive cell production and as the absence of thyroid cysts might indicate latent damage in the thyroid, the status of thyroid cysts possibly influences the association between height and active arterial wall thickening. We conducted a 2-year follow-up study of Japanese aged 60-69 years. For participants with thyroid cysts, height was significantly inversely associated with active arterial wall thickening (thyroid function and baseline CIMT adjusted odds ratio of active arterial wall thickening for one increment of standard deviation of height (5.7 cm for men and 4.8 cm for women), 0.66 [0.49, 0.89]), while for those without thyroid cysts, a positive tendency between the two parameters was observed (1.19 [0.96, 1.50]). An inverse association between height and active arterial wall thickening was observed only for elderly participants with thyroid cysts possibly because of a supportive role of thyroid hormone, as the absence of thyroid cysts might indicate latent damage in the thyroid.

Air Pollution Exposure Induces Vascular Injury and Hampers Endothelial Repair by Altering Progenitor and Stem Cells Functionality

Extensive evidence indicates an association of air pollution exposure with an increased risk of cardiovascular disease (CVD) development. Fine particulate matter (PM) represents one of the main components of urban pollution, but the mechanisms by which it exerts adverse effects on cardiovascular system remain partially unknown and under investigation. The alteration of endothelial functions and inflammation are among the earliest pathophysiological impacts of environmental exposure on the cardiovascular system and represent critical mediators of PM-induced injury. In this context, endothelial stem/progenitor cells (EPCs) play an important role in vascular homeostasis, endothelial reparative capacity, and vasomotor functionality modulation. Several studies indicate the impairment of EPCs' vascular reparative capacity due to PM exposure. Since a central source of EPCs is bone marrow (BM), their number and function could be related to the population and functional status of stem cells (SCs) of this district. In this review, we provide an overview of the potential mechanisms by which PM exposure hinders vascular repair by the alteration of progenitor and stem cells' functionality.

Pathophysiology of Spontaneous Coronary Artery Dissection Determines Anticoagulation Strategy

Spontaneous coronary dissection is an uncommon disorder, lacking convincing pathophysiologic evidence. Scientific statements and state-of-the-art articles suggest intramural hematoma from bleeding vasa vasorum is the cause. Evidence is based on limited invasive evaluation with optical coherence tomography. This assumption, therefore, suggests anti-coagulation be discontinued. Mechanical shear forces, intraluminal pressures do not support bleeding vasa vasorum closing a higher luminal pressure vessel.

The endothelium’s role in inflammation, thrombosis, and repair suggests the pathophysiology is failure to repair endothelium with the lack of repair as the nidus of disruption. A tear ensues and can spontaneously reseal. The lack of inflammatory cells in pathological specimens and association with another poorly understood disease fibromuscular dysplasia supports the etiology of both entities as failure to replace endothelium. The endothelium is the fulcrum of both inflammation and thrombosis. The ability to heal the rift supports conservative therapy.

Anticoagulants and antiplatelet reduce thrombosis and inflammation which will ensue when the endothelium is disrupted. These agents will substitute for the failed endothelium allowing thrombosis to be kept in check, reduce inflammation, and promote healing. This thesis and the state-of-the-art articles do not present clinical outcome data. Both support conservative interventions. Anticoagulation recommendations are however in opposite realms. Failure to repair endothelium suggest additional therapies of statins, exercise, smoking cessation will increase circulating stem cells may reduce future events and slow the progression of fibromuscular dysplasia. Future directions in understanding this disease and new therapies requires measurement of repair mechanisms such as the quantity of circulating endothelial progenitor cells.

Different Roles of Stem/Progenitor Cells in Vascular Remodeling

Significance: Since the discovery of vascular stem cells, there has been considerable advancement in comprehending the nature and functions of these cells. Due to their differentiation potential to repair endothelial cells and to participate in lesion formation during vascular remodeling, it is crucial to elucidate vascular stem cell behaviors and the mechanisms underlying this process, which could provide new chances for the design of clinical therapeutic application of stem cells. Recent Advances: Over the past decades, major progress has been made on progenitor/vascular stem cells in the field of cardiovascular research. Vascular stem cells are mostly latent in their niches and can be bioactivated in response to damage and get involved in endothelial repair and smooth muscle cell aggregation to generate neointima. Accumulating evidence has been shown recently, using genetic lineage tracing mouse models, to particularly provide solutions to the nature of vascular stem cells and to monitor both cell migration and the process of differentiation during physiological angiogenesis and in vascular diseases. Critical Issues: This article reviews and summarizes the current research progress of vascular stem cells in this field and highlights future prospects for stem cell research in regenerative medicine. Future Directions: Despite recent advances and achievements of stem cells in cardiovascular research, the nature and cell fate of vascular stem cells remain elusive. Further comprehensive studies using new techniques including genetic cell lineage tracing and single-cell RNA sequencing are essential to fully illuminate the role of stem cells in vascular development and diseases. Antioxid. Redox Signal. 35, 192-203.

The Role of Inflammation and Immune Activation on Circulating Endothelial Progenitor Cells in Chronic HIV Infection

Endothelial inflammation and damage are the main drivers of cardiovascular risk/disease. Endothelial repair is mediated in part by recruitment of bone marrow endothelial progenitor/endothelial colony forming cells (EPC/ECFC). People with HIV (PWH) have increased cardiovascular risk and the impact of infection in endothelial repair is not well defined. The low frequencies and challenges to in vitro isolation and differentiation of EPC/ECFC from PBMCs had made it difficult to study their role in this context. We hypothesized that HIV driven inflammation induces phenotypic changes that reflects the impact of infection. To test this hypothesis, we evaluated expression of markers of trafficking, endothelial differentiation, and angiogenesis, and study their association with biomarkers of inflammation in a cohort of PWH. In addition, we investigated the relationship of circulating endothelial progenitors and angiogenic T cells, a T cell subset with angiogenic function. Using a flow cytometry approach, we identified two subsets of circulating progenitors LIN4^-CD45^-CD34⁺ and LIN4^-CD45^dimCD34⁺ in PWH. We found that the phenotype but not frequencies were associated with biomarkers of inflammation. In addition, the percentage of LIN4^-CD45^dimCD34⁺ was associated with serum levels of lipids. This data may provide a new tool to better address the impact of HIV infection in endothelial inflammation and repair.

IGFBP-1 in Cardiometabolic Pathophysiology-Insights From Loss-of-Function and Gain-of-Function Studies in Male Mice

We have previously reported that overexpression of human insulin-like growth factor binding protein (IGFBP)-1 in mice leads to vascular insulin sensitization, increased nitric oxide bioavailability, reduced atherosclerosis, and enhanced vascular repair, and in the setting of obesity improves glucose tolerance. Human studies suggest that low levels of IGFBP-1 are permissive for the development of diabetes and cardiovascular disease. Here we seek to determine whether loss of IGFBP-1 plays a causal role in the predisposition to cardiometabolic disease. Metabolic phenotyping was performed in transgenic mice with homozygous knockout of IGFBP-1. This included glucose, insulin, and insulin-like growth factor I tolerance testing under normal diet and high-fat feeding conditions. Vascular phenotyping was then performed in the same mice using vasomotor aortic ring studies, flow cytometry, vascular wire injury, and angiogenesis assays. These were complemented with vascular phenotyping of IGFBP-1 overexpressing mice. Metabolic phenotype was similar in IGFBP-1 knockout and wild-type mice subjected to obesity. Deletion of IGFBP-1 inhibited endothelial regeneration following injury, suggesting that IGFBP-1 is required for effective vascular repair. Developmental angiogenesis was unaltered by deletion or overexpression of IGFBP-1. Recovery of perfusion following hind limb ischemia was unchanged in mice lacking or overexpressing IGFBP-1; however, overexpression of IGFBP-1 stimulated hindlimb perfusion and angiogenesis in insulin-resistant mice. These findings provide new insights into the role of IGFBP-1 in metabolic and vascular pathophysiology. Irrespective of whether loss of IGFBP-1 plays a causal role in the development of cardiometabolic disorders, increasing IGFBP-1 levels appears effective in promoting neovascularization in response to ischemia.

Cardio-ankle vascular index and circulating CD34-positive cell levels as indicators of endothelial repair activity in older Japanese men

AIM

The cardio-ankle vascular index (CAVI) reflects functional arterial stiffness, which is related to endothelial dysfunction. CD34-positive cells carry out an important function in endothelial repair. However, there have been no reports assessing the association between CAVI and the number of circulating CD34-positive cells.

METHODS

We carried out a cross-sectional study of 249 Japanese men, aged 60-69 years, who underwent annual health checkups between 2013 and 2015. As individuals with high levels of circulating CD34-positive cells might indicate the influence of consumptive reduction of circulating CD34-positive cells as a result of aggressive endothelial repair, participants were stratified by circulating CD34-positive cell levels, using the median value in this population (0.95 cells/μL) as the cut-off.

RESULTS

For participants with low circulating CD34-positive cell levels, logarithmic values of circulating CD34-positive cells were inversely associated with CAVI (multivariable standardized parameter estimate [β] = -0.22, P = 0.014), but not for participants with high levels (β = -0.04, P = 0.638). In addition, even when no significant associations between CAVI and carotid intima-media thickness were detected for participants with low circulating CD34-positive cell levels (β = -0.02, P = 0.865), significant positive associations were identified for participants with high levels (β = 0.22, P = 0.028).

CONCLUSIONS

As circulating CD34-positive cell count might indicate endothelial repair activity, the present results show that CAVI is affected by insufficient endothelial repair in individuals with low circulating CD34-positive cell counts. Our results also show that a positive association between CAVI and carotid intima-media thickness exists only in individuals with aggressive endothelial repair, which indicates the presence of organic arterial disease, such as atherosclerosis. Geriatr Gerontol Int 2019; 19: 557-562.

Sulodexide recovers endothelial function through reconstructing glycocalyx in the balloon-injury rat carotid artery model

Disruption of endothelial cell function is a principle event in cardiovascular disease. Accordingly, therapies have mostly focused on repairing the endothelium, but little attention has been paid to the reconstruction of glycocalyx, which covers the endothelium and protects the function of endothelial cells. Sulodexide has a similar glycosaminoglycan structure to glycocalyx, so it is assumed to be effective in remodeling the glycocalyx following damage. We assessed the effect of sulodexide on glycocalyx remodeling and endothelial function in the balloon-injury rat carotid artery model. Electron micrographs showed that sulodexide (2mg/kg, administered by intraperitoneal injection for seven days after injury) could reconstruct the endothelial glycocalyx and recover the clear cytoarchitecture. With regard to endothelial function, sulodexide increased endothelial nitric oxide synthase level, attenuated endothelial hyperplasia, and inhibited platelet aggregation that benefitted from glycocalyx reforming. Sulodexide decreased the glycocalyx damage related expression of CD31 and intercellular cell adhesion molecule-1 in endothelium, accompanying by the downregulation of leukocyte counts and C-reactive protein levels. The levels of the atherosclerosis-related factors, osteopontin and vascular cell adhesion molecule-1, which increased in activated endothelial cells lacking glycocalyx, were normalized by sulodexide. Along with the benefit of glycocalyx reconstruction, sulodexide reversed the dyslipidemia. Moreover, sulodexide prevented CD68-positive inflammatory cells infiltration into the vascular wall, presumably as a result of glycocalyx reconstruction. In summary, sulodexide treatment reconstructed glycocalyx which therefore preserved endothelial function and attenuated the expression of inflammatory factors, and decreased the blood coagulation and lipid metabolism, all of which are important for vascular healing.

Increased carbamylation level of HDL in end-stage renal disease: carbamylated-HDL attenuated endothelial cell function

It is thought that carbamylated modification plays a crucial role in the development and progression of cardiovascular disease (CVD) in patients with end-stage renal disease (ESRD). However, information on the biological effects of carbamylated high-density lipoprotein (C-HDL) in ESRD is poor. The present study investigated the carbamylation level of HDL in ESRD and the effects of C-HDL on endothelial repair properties. HDL was isolated from healthy control subjects (n = 22) and patients with ESRD (n = 30). The carbamylation level of HDL was detected using ELISA. Isolated C-HDL for use in tissue culture experiments was carbamylated in vitro to a similar extent to that observed in ESRD. Human arterial endothelial cells were treated with C-HDL or native HDL to assess their migration, proliferation, and angiogenesis properties. HDL-associated paraoxonase 1 activity was also determined by spectrophotometry assay. Compared with healthy control subjects, the carbamylation level of HDL in ESRD patients was increased and positively correlated with blood urea concentration. In vitro, C-HDL significantly inhibited migration, angiogenesis, and proliferation in endothelial cells. Mechanistic studies revealed that HDL-associated paraoxonase 1 activity was decreased and negatively correlated with the carbamylation level of HDL in ESRD patients. In addition, C-HDL suppressed the expression of VEGF receptor 2 and scavenger receptor class B type I signaling pathways in endothelial cells. In conclusion, the present study identified a significantly increased carbamylation level of HDL in ESRD. Furthermore, C-HDL inhibited endothelial cell repair functions.

Vascular repair strategies in type 2 diabetes: novel insights

Impaired functions of vascular cells are responsible for the majority of complications in patients with type 2 diabetes (T2D). Recently a better understanding of mechanisms contributing to development of vascular dysfunction and the role of systemic inflammatory activation and functional alterations of several secretory organs, of which adipose tissue has more recently been investigated, has been achieved. Notably, the progression of vascular disease within the context of T2D appears to be driven by a multitude of incremental signaling shifts. Hence, successful therapies need to target several mechanisms in parallel, and over a long time period. This review will summarize the latest molecular strategies and translational developments of cardiovascular therapy in patients with T2D.

Extrarenal Progenitor Cells Do Not Contribute to Renal Endothelial Repair

Endothelial progenitor cells (EPCs) may be relevant contributors to endothelial cell (EC) repair in various organ systems. In this study, we investigated the potential role of EPCs in renal EC repair. We analyzed the major EPC subtypes in murine kidneys, blood, and spleens after induction of selective EC injury using the concanavalin A/anti-concanavalin A model and after ischemia/reperfusion (I/R) injury as well as the potential of extrarenal cells to substitute for injured local EC. Bone marrow transplantation (BMTx), kidney transplantation, or a combination of both were performed before EC injury to allow distinction of extrarenal or BM-derived cells from intrinsic renal cells. During endothelial regeneration, cells expressing markers of endothelial colony-forming cells (ECFCs) were the most abundant EPC subtype in kidneys, but were not detected in blood or spleen. Few cells expressing markers of EC colony-forming units (EC-CFUs) were detected. In BM chimeric mice (C57BL/6 with tandem dimer Tomato-positive [tdT+] BM cells), circulating and splenic EC-CFUs were BM-derived (tdT+), whereas cells positive for ECFC markers in kidneys were not. Indeed, most BM-derived tdT+ cells in injured kidneys were inflammatory cells. Kidneys from C57BL/6 donors transplanted into tdT+ recipients with or without prior BMTx from C57BL/6 mice were negative for BM-derived or extrarenal ECFCs. Overall, extrarenal cells did not substitute for any intrinsic ECs. These results demonstrate that endothelial repair in mouse kidneys with acute endothelial lesions depends exclusively on local mechanisms.

Vascular Repair by Tissue-Resident Endothelial Progenitor Cells in Endotoxin-Induced Lung Injury

Vascular disruption is one of the pathological hallmarks in acute respiratory distress syndrome. Bone marrow (BM)-derived circulating endothelial progenitor cells (EPCs) and lung tissue-resident EPCs have been considered to play a pivotal role in pulmonary vascular repair; however, which population is predominant in local pulmonary vasculogenesis remains to be clarified. We therefore examined the origin of EPCs participating in the regenerative process of pulmonary vascular endothelial cells (PVECs) in experimental acute respiratory distress syndrome. Lung samples from mice administered LPS intratracheally were investigated for cell dynamics and EPC functions. Quantitative flow cytometric analysis demonstrated that the number of PVECs decreased by roughly 20% on Day 1 and then recovered on Day 7 of LPS challenge. Bromodeoxyuridine-incorporation assays and immunofluorescence microscopy demonstrated that proliferating PVECs preferentially located in the capillary vessels. Experiments using BM chimera mice revealed that most of the regenerating PVECs were tissue-resident cells, and BM-derived cells hardly engrafted as PVECs. The population of circulating putative phenotypical EPCs decreased during the first week after LPS challenge. The regenerating PVECs were characterized by high colony-forming and vasculogenic capacities, intracellular reactive oxygen species scavenging and aldehyde dehydrogenase activites, and enhanced gene expression of Abcb1b (a drug-resistant gene), suggesting that the population of PVECs included tissue-resident EPCs activated during regenerative process of PVECs. The proliferating PVECs expressed CD34, Flk-1/KDR, and c-kit more strongly and Prom1/CD133 less strongly on the surface than nonproliferating PVECs. Our findings indicated that lung tissue-resident EPCs predominantly contribute to pulmonary vascular repair after endotoxin-induced injury.

Hyperhomocysteinemia suppresses bone marrow CD34+/VEGF receptor 2+ cells and inhibits progenitor cell mobilization and homing to injured vasculature-a role of β1-integrin in progenitor cell migration and adhesion

Hyperhomocysteinemia (HHcy) impairs re-endothelialization and accelerates vascular remodeling. The role of CD34(+)/VEGF receptor (VEGFR) 2(+) progenitor cells (PCs) in vascular repair in HHcy is unknown. We studied the effect of HHcy on PCs and its role in vascular repair in severe HHcy (∼150 μM), which was induced in cystathionine-β synthase heterozygous mice fed a high-methionine diet for 8 weeks. Vascular injury was introduced by carotid air-dry endothelium denudation. CD34(+)/VEGFR2(+) cells were examined by flow cytometry. HHcy reduced bone marrow (BM) CD34(+)/VEGFR2(+) cells and suppressed replenishment of postinjury CD34(+)/VEGFR2(+) cells in peripheral blood (PB). Donor green fluorescent protein-positive PC homing to the injured vessel was reduced in HHcy after CD34(+) PCs from enhanced green fluorescent protein mice were adoptively transferred following carotid injury. CD34(+) PC transfusion partially reversed HHcy-suppressed re-endothelialization and HHcy-induced neointimal formation. Furthermore, homocysteine (Hcy) inhibited proliferation, adhesion, and migration and suppressed β1-integrin expression and activity in human CD34(+) endothelial colony-forming cells (ECFCs) isolated from PBs in a dose-dependent manner. A functional-activating β1-integrin antibody rescued Hcy-suppressed adhesion and migration in CD34(+) ECFCs. In conclusion, HHcy reduces BM CD34(+)/VEGFR2(+) generation and suppresses CD34(+)/VEGFR2(+) cell mobilization and homing to the injured vessel via β1-integrin inhibition, which partially contributes to impaired re-endothelialization and vascular remodeling.

Altered in vitro endothelial repair and monocyte migration in obstructive sleep apnea: implication of VEGF and CRP

STUDY OBJECTIVES

Although obstructive sleep apnea (OSA) causes cardiovascular morbidities through atherosclerosis induced by inflammation and endothelial dysfunction, OSA patients exhibit elevated plasma vascular endothelial growth factor (VEGF), which may represent an adaptive response to intermittent hypoxia. The aims of this study were to investigate whether in vitro endothelial wound healing and monocyte migration are affected by patient serum, and to determine the implication of circulating factors (VEGF and C-reactive protein).

PATIENTS

Serum was collected from healthy controls (HC), "healthy" OSA, and metabolic syndrome (MS) patients with or without OSA.

MEASUREMENTS AND RESULTS

Along with the presence of OSA and/or MS, both VEGF and hsCRP were significantly elevated in patient serum. Their specific role was tested with blocking antibodies on primary endothelial cells for wound healing assay and on human monocytes for migration assay. Endothelial wound healing was reduced with OSA compared to HC serum, and even more significantly using MS+OSA patient serum. Altered wound healing with OSA serum was unmasked when blocking VEGF and restored when blocking CRP. Monocyte migration was activated with OSA serum, and further enhanced by MS+OSA patient serum. Blocking CRP in serum inhibited this migration.

CONCLUSIONS

Serum from OSA patient alters in vitro endothelial cell repair function and activates monocyte migration; this is further aggravated with the presence of metabolic syndrome. These effects are partly driven by VEGF and CRP, suggesting an unfavorable balance between the pro healing (VEGF) and pro injury (CRP) factors that may promote vascular injury in OSA with and without metabolic syndrome.

CXCR7 upregulation is required for early endothelial progenitor cell-mediated endothelial repair in patients with hypertension

Dysfunction of early endothelial progenitor cells (EPCs) is responsible for impaired endothelial repair capacity after arterial injury in patients with hypertension. Here, we hypothesized that diminished signaling of CXC chemokine receptor 7 (CXCR7) contributes to the reduced EPC functions, and enhanced CXCR7 expression restores the capacities of EPCs from hypertensive patients. CXCR7 expression of EPCs from hypertensive patients was significantly reduced when compared with that from healthy subjects. Meanwhile, the phosphorylation of p38 mitogen-activated protein kinase, a downstream signaling of CXCR7, was elevated, which increased cleaved caspase-3 level of EPCs. CXCR7 gene transfer augmented CXCR7 expression and decreased the phosphorylation of p38 mitogen-activated protein kinase, which was paralleled to EPC functional upregulation of in vitro adhesion, antiapoptosis activities, and in vivo re-endothelialization capacity in a nude mouse model of carotid artery injury. The enhanced in vitro and in vivo functions of EPCs were markedly inhibited by neutralizing monoclonal antibody against CXCR7, which was blocked by p38 mitogen-activated protein kinase inhibitor SB203580. Downregulation of cleaved caspase-3 level induced by CXCR7 gene transfer or SB203580 pretreatment improved EPC functions. Furthermore, we found that lercanidipine, a dihydropyridine calcium channel antagonist, enhanced CXCR7 expression and facilitated in vitro and in vivo functions of EPCs. Our study demonstrated for the first time that diminished CXCR7 signal at least partially contributes to the reduced in vitro functions and in vivo re-endothelialization capacity of EPCs from hypertensive patients. Upregulation of CXCR7 expression induced by gene transfer or lercanidipine treatment may be a novel therapeutic target for increased endothelial repair capacity in hypertension.

Cardioprotective functions of HDLs

Multiple human population studies have established the concentration of high density lipoprotein (HDL) cholesterol as an independent, inverse predictor of the risk of having a cardiovascular event. Furthermore, HDLs have several well-documented functions with the potential to protect against cardiovascular disease. These include an ability to promote the efflux of cholesterol from macrophages in the artery wall, inhibit the oxidative modification of low density lipoproteins (LDLs), inhibit vascular inflammation, inhibit thrombosis, promote endothelial repair, promote angiogenesis, enhance endothelial function, improve diabetic control, and inhibit hematopoietic stem cell proliferation. There are undoubtedly other beneficial functions of HDLs yet to be identified. The HDL fraction in human plasma is heterogeneous, consisting of several subpopulations of particles of varying size, density, and composition. The functions of the different HDL subpopulations remain largely unknown. Given that therapies that increase the concentration of HDL cholesterol have varying effects on the levels of specific HDL subpopulations, it is of great importance to understand how distribution of different HDL subpopulations contribute to the potentially cardioprotective functions of this lipoprotein fraction. This review summarizes current understanding of the relationship of HDL subpopulations to their cardioprotective properties and highlights the gaps in current knowledge regarding this important aspect of HDL biology.