Serum IF1 concentration is independently associated to HDL levels and to coronary heart disease: the GENES study

Pathogenesis of cardiovascular diseases: effects of mitochondrial CF6 on endothelial cell function

Cardiovascular disease (CVD) stands as a predominant global cause of morbidity and mortality, necessitating effective and cost-efficient therapies for cardiovascular risk reduction. Mitochondrial coupling factor 6 (CF6), identified as a novel proatherogenic peptide, emerges as a significant risk factor in endothelial dysfunction development, correlating with CVD severity. CF6 expression can be heightened by CVD risk factors like mechanical force, hypoxia, or high glucose stimuli through the NF-κB pathway. Many studies have explored the CF6-CVD relationship, revealing elevated plasma CF6 levels in essential hypertension, atherosclerotic cardiovascular disease (ASCVD), stroke, and preeclampsia patients. CF6 acts as a vasoactive and proatherogenic peptide in CVD, inducing intracellular acidosis in vascular endothelial cells, inhibiting nitric oxide (NO) and prostacyclin generation, increasing blood pressure, and producing proatherogenic molecules, significantly contributing to CVD development. CF6 induces an imbalance in endothelium-dependent factors, including NO, prostacyclin, and asymmetric dimethylarginine (ADMA), promoting vasoconstriction, vascular remodeling, thrombosis, and insulin resistance, possibly via C-src Ca2+ and PRMT-1/DDAH-2-ADMA-NO pathways. This review offers a comprehensive exploration of CF6 in the context of CVD, providing mechanistic insights into its role in processes impacting CVD, with a focus on CF6 functions, intracellular signaling, and regulatory mechanisms in vascular endothelial cells.

Identification of circulating apolipoprotein M as a new determinant of insulin sensitivity and relationship with adiponectin

BACKGROUND

The adiponectin is one of the rare adipokines down-regulated with obesity and protects against obesity-related disorders. Similarly, the apolipoprotein M (apoM) is expressed in adipocytes and its expression in adipose tissue is associated with metabolic health. We compared circulating apoM with adiponectin regarding their relationship with metabolic parameters and insulin sensitivity and examined their gene expression patterns in adipocytes and in the adipose tissue.

METHODS

Circulating apoM and adiponectin were examined in 169 men with overweight in a cross-sectional study, and 13 patients with obesity during a surgery-induced slimming program. Correlations with clinical parameters including the insulin resistance index (HOMA-IR) were analyzed. Multiple regression analyses were performed on HOMA-IR. The APOM and ADIPOQ gene expression were measured in the adipose tissue from 267 individuals with obesity and a human adipocyte cell line.

RESULTS

Participants with type 2 diabetes had lower circulating adiponectin and apoM, while apoM was higher in individuals with dyslipidemia. Similar to adiponectin, apoM showed negative associations with HOMA-IR and hs-CRP (r < -0.2), and positive correlations with HDL markers (HDL-C and apoA-I, r > 0.3). Unlike adiponectin, apoM was positively associated with LDL markers (LDL-C and apoB100, r < 0.20) and negatively correlated with insulin and age (r < -0.2). The apoM was the sole negative determinant of HOMA-IR in multiple regression models, while adiponectin not contributing significantly. After surgery, the change in HOMA-IR was negatively associated with the change in circulating apoM (r = -0.71), but not with the change in adiponectin. The APOM and ADIPOQ gene expression positively correlated in adipose tissue (r > 0.44) as well as in adipocytes (r > 0.81). In adipocytes, APOM was downregulated by inflammatory factors and upregulated by adiponectin.

CONCLUSIONS

The apoM rises as a new partner of adiponectin regarding insulin sensitivity. At the adipose tissue level, the adiponectin may be supported by apoM to promote a healthy adipose tissue.

TRIAL REGISTRATION

NCT01277068, registered 13 January 2011; NCT02332434, registered 5 January 2015; and NCT00390637, registered 20 October 2006.

Associations between physical activity levels and ATPase inhibitory factor 1 concentrations in older adults

BACKGROUND

Adenosine triphosphatase inhibitory factor 1 (IF1) is a key protein involved in energy metabolism. IF1 has been linked to various age-related diseases, although its relationship with physical activity (PA) remains unclear. Additionally, the apolipoprotein A-I (apoA-I), a PA-modulated lipoprotein, could play a role in this relationship because it shares a binding site with IF1 on the cell-surface ATP synthase. We examined here the associations between chronic PA and plasma IF1 concentrations among older adults, and we investigated whether apoA-I mediated these associations.

METHODS

In the present work, 1096 healthy adults (63.8% females) aged 70 years and over who were involved in the Multidomain Alzheimer Prevention Trial study were included. IF1 plasma concentrations (square root of ng/mL) were measured at the 1-year visit of the Multidomain Alzheimer Prevention Trial, while PA levels (square root of metabolic equivalent task min/week) were assessed using questionnaires administered each year from baseline to the 3-year visit. Multiple linear regressions were performed to investigate the associations between the first-year mean PA levels and IF1 concentrations. Mediation analyses were conducted to examine whether apoA-I mediated these associations. Mixed-effect linear regressions were carried out to investigate whether the 1-year visit IF1 concentrations predicted subsequent changes in PA.

RESULTS

Multiple linear regressions indicated that first-year mean PA levels were positively associated with IF1 concentrations (B = 0.021; SE = 0.010; p = 0.043). Mediation analyses revealed that about 37.7% of this relationship was mediated by apoA-I (Bab = 0.008; SE = 0.004; p = 0.023). Longitudinal investigations demonstrated that higher concentrations of IF1 at the 1-year visit predicted a faster decline in PA levels over the subsequent 2 years (time × IF1: B = -0.148; SE = 0.066; p = 0.025).

CONCLUSION

This study demonstrates that regular PA is associated with plasma IF1 concentrations, and it suggests that apoA-I partly mediates this association. Additionally, this study finds that baseline concentrations of IF1 can predict future changes in PA. However, further research is needed to fully understand the mechanisms underlying these observations.

Plasma Level of ATPase Inhibitory Factor 1 and Intrinsic Capacity in Community-Dwelling Older Adults: Prospective Data From the MAPT Study

BACKGROUND

Intrinsic capacity (IC) is a concept related to functionality that reflects healthy aging. ATPase inhibitory factor 1 (IF1) is a multifaceted protein that regulates mitochondrial oxidative phosphorylation (OXPHOS), and may be involved in IC. The objective of this study is to investigate the association between plasma levels of IF1 and IC changes in community-dwelling older adults.

METHODS

Community-dwelling older adults from the Multidomain Alzheimer Preventive Trial (MAPT Study) were enrolled in this study. A composite IC score was calculated based on 4 IC domains: locomotion, psychological dimension, cognition, and vitality (with data available annually over 4 years of follow-up). Secondary analyses were conducted on the sensory domain (with data available only for 1 year of follow-up). Mixed-model linear regression adjusted for confounders was conducted.

RESULTS

A total of 1 090 participants with usable IF1 values were included in the study (75.3 ± 4.4 years; 64% females). Compared to the lowest quartile, both the low- and high-intermediate IF1 quartiles were found to be cross-sectionally associated with greater composite IC scores across 4 domains (βlow-intermediate, 1.33; 95% confidence interval [CI] 0.06-2.60 and βhigh-intermediate, 1.78; 95% CI 0.49-3.06). In the secondary analyses, the highest quartile was found to be associated with a slower decline in composite IC scores across 5 domains over 1 year (βhigh 1.60; 95% CI 0.06-3.15). The low- and high-intermediate IF1 quartiles were also found to be cross-sectionally associated with greater locomotion (βlow-intermediate, 2.72; 95% CI 0.36-5.08) and vitality scores (βhigh-intermediate, 1.59; 95% CI 0.06-3.12), respectively.

CONCLUSIONS

This study is the first to demonstrate that levels of circulating IF1, a mitochondrial-related biomarker, are associated with IC composite scores in both cross-sectional and prospective analyses among community-dwelling older adults. However, further research is needed to confirm these findings and elucidate the potential underlying mechanisms that may explain these associations.

Plasma level of ATPase inhibitory factor 1 (IF1) is associated with type 2 diabetes risk in humans: A prospective cohort study

AIM

Mitochondrial dysfunction is associated with the development of type 2 diabetes mellitus (T2DM). It is thus of clinical relevance to identify plasma biomarkers of mitochondrial dysfunction associated with the risk of T2DM. ATPase inhibitory factor 1 (IF1) endogenously inhibits mitochondrial ATP synthase activity. Here, we analyzed association of the plasma IF1 level with markers of glucose homeostasis and with the conversion to new-onset diabetes (NOD) in individuals with prediabetes.

METHODS

In the IT-DIAB prospective study, the baseline plasma level of IF1 was measured in 307 participants with prediabetes. The primary outcome was the incidence of NOD within five years of follow-up. Cross-sectional analysis of the IF1 level was also done in two independent interventional studies. Correlations between plasma IF1 and metabolic parameters at baseline were assessed by Spearman's correlation coefficients, and the association with the risk of NOD was determined using Cox proportional-hazards models.

RESULTS

In IT-DIAB, the mean IF1 plasma level was lower in participants who developed NOD than in those who did not (537 ± 248 versus 621 ± 313 ng/mL, P   = 0.01). The plasma IF1 level negatively correlated with clinical variables associated with obesity and insulin resistance, including the body mass index (r = -0.20, P  = 0.0005) and homeostasis model assessment of insulin resistance (HOMA-IR). (r = -0.37, P < 0.0001). Conversely, IF1 was positively associated with plasma markers of cardiometabolic health, such as HDL-C (r = 0.63, P  <  0.0001) and apoA-I (r = 0.33, P  <  0.0001). These correlations were confirmed in cross-sectional analyses. In IT-DIAB, the IF1 level was significantly associated with a lower risk of T2DM after adjustment for age, sex, and fasting plasma glucose (HR [95% CI] per 1 SD = 0.76 [0.62; 0.94], P   = 0.012).

CONCLUSION

We identified for the first time the mitochondrial-related biomarker IF1 as being associated with the risk of T2DM.

The Multifaceted ATPase Inhibitory Factor 1 (IF1) in Energy Metabolism Reprogramming and Mitochondrial Dysfunction: A New Player in Age-Associated Disorders?

Significance: The mitochondrial oxidative phosphorylation (OXPHOS) system, comprising the electron transport chain and ATP synthase, generates membrane potential, drives ATP synthesis, governs energy metabolism, and maintains redox balance. OXPHOS dysfunction is associated with a plethora of diseases ranging from rare inherited disorders to common conditions, including diabetes, cancer, neurodegenerative diseases, as well as aging. There has been great interest in studying regulators of OXPHOS. Among these, ATPase inhibitory factor 1 (IF1) is an endogenous inhibitor of ATP synthase that has long been thought to avoid the consumption of cellular ATP when ATP synthase acts as an ATP hydrolysis enzyme. Recent Advances: Recent data indicate that IF1 inhibits ATP synthesis and is involved in a multitude of mitochondrial-related functions, such as mitochondrial quality control, energy metabolism, redox balance, and cell fate. IF1 also inhibits the ATPase activity of cell-surface ATP synthase, and it is used as a cardiovascular disease biomarker. Critical Issues: Although recent data have led to a paradigm shift regarding IF1 functions, these have been poorly studied in entire organisms and in different organs. The understanding of the cellular biology of IF1 is, therefore, still limited. The aim of this review was to provide an overview of the current understanding of the role of IF1 in mitochondrial functions, health, and diseases. Future Directions: Further investigations of IF1 functions at the cell, organ, and whole-organism levels and in different pathophysiological conditions will help decipher the controversies surrounding its involvement in mitochondrial function and could unveil therapeutic strategies in human pathology. Antioxid. Redox Signal. 37, 370-393.

Prediction of coronary heart disease incidence in a general male population by circulating non-coding small RNA sRNY1-5p in a nested case-control study

During the development of atherosclerotic lesion, s-RNYs (small RNAs of about 24/34 nucleotides) are derived by the processing of long Ro-associated non-coding RNAs (RNYs) in macrophages. The levels of serum s-RNYs have been found significantly upregulated in patients with coronary heart disease (CHD) compared to age-matched CHD-free individuals. The present study aimed to examine the predictive value of serum s-RNYs for CHD events in the general male population. Within the frame of nested-case–control study, the GENES study, we measured the absolute expression of a RNY-derived small RNA, the s-RNY1-5p, in the serum of individuals (without CHD at baseline) who encountered a CHD event within 12 years of follow-up (n = 30) (Cases) and compared them to individuals who remained event-free (Controls) (n = 30). The expression of s-RNY1-5p in serum was significantly upregulated in Cases compared to Controls (p = 0.027). The proportion of CHD event-free was significantly higher among individuals with serum s-RNY1-5p below the median value (631 molecules/mL). In a multivariable model adjusted for age, smoking, hypertension, diabetes and dyslipidemia, the risk of CHD events increased more than fourfold in individuals with serum s-RNY1-5p above the median value (HR, 4.36; 95% CI 1.22–15.60). A positive association with CHD events was also observed when considering s-RNY1-5p as a continuous variable (p = 0.022). Based on our results, we conclude that serum s-RNY1-5p is an independent predictor of CHD events in a general male population and might be a relevant biomarker for early detection of cardiovascular diseases.

Serum level of HDL particles are independently associated with long-term prognosis in patients with coronary artery disease: The GENES study

HDL-Cholesterol (HDL-C) is not an accurate surrogate marker to measure the cardioprotective functions of HDL in coronary artery diseases (CAD) patients. Hence, measurement of other HDL-related parameters may have prognostic superiority over HDL-C. In this work, we examined the predictive value of HDL particles profile for long-term mortality in CAD patients and to compare its informative value to that of HDL-C and apoA-I. HDL particles profiles were measured by nuclear magnetic resonance (NMR) spectroscopy in 214 male participants with stable CAD (45-74 years). Median follow up was 12.5 years with a 36.4% mortality rate. Cardiovascular mortality accounted for 64.5%. Mean concentrations of total HDL particles (HDL-P), small-sized HDL (SHDL-P) and apoA-I were lower in deceased than in surviving patients whereas no difference was observed according to HDL-C and large HDL particles. All NMR-HDL measures were correlated between themselves and with other HDL markers (HDL-C, apoA-I and LpA-I). In a multivariate model adjusted for cardiovascular risk factors and bioclinical variables, HDL-P and SHDL-P displayed the strongest inverse association with all-cause and cardiovascular mortality. Weaker associations were recorded for apoA-I. Based on our results, we conclude that HDL particle profile measured by NMR spectroscopy should be considered to better stratify risk in population at high risk or in the setting of pharmacotherapy.

Long-term Prognostic Impact of Physical Activity in Patients With Stable Coronary Heart Disease

Stable coronary heart disease (CHD) patients are advised to practice regular physical activity (PA). However, data on very long-term prognosis impact of regular exercise remain scarce. We aimed to evaluate the impact of physical activity level on mortality at long term in stable CHD patients. We analyzed 822 patients with stable CHD. They answered questionnaires on medical history, underwent a standardized clinical examination, and provided a fasting blood sample. PA was evaluated by the MOSPA questionnaire. Three tertiles of patients were individualized according to PA level: 0.0-9 Metabolic Equivalent of Task (METs) hour per week (n = 267); 10-39.9 METs hour per week (n = 279); and ≥40 METs hour per week (n = 276). After a median follow-up of 14.6 years, 324 patients had died. In a multivariate analysis adjusted for age, dyslipidemia, smoking status, diabetes, high blood pressure, waist circumference, left ventricular ejection fraction, Gensini score, heart rate, ankle-brachial index and duration of disease, physical activity was significantly and independently associated with all-cause mortality. Compared to the lowest PA tertile, both the median and the highest PA tertiles, were associated to a reduction of all-cause mortality risk with hazard ratios at 0.79 (95%confidence interval [0.61:1.03], P = 0.08) and 0.71 ([0.53:0.96], P = 0.025) respectively; P for trend = 0.02. Adjusted hazard ratios for an increase of 10 METs hour per week was 0.95 [0.92 to 0.98], (P <0.002). In conclusion, our study demonstrates an independent association between PA and long term vital prognosis with a 5% total mortality decrease for an increase of 10 METs hour per week.

Common p2y13 polymorphisms are associated with plasma inhibitory factor 1 and lipoprotein(a) concentrations, heart rate and body fat mass: The GENES study

BACKGROUND

The P2Y13 purinergic receptor regulates hepatic high-density lipoprotein uptake and biliary sterol secretion; it acts downstream of the membrane ecto-F1-adenosine triphosphatase, which generates extracellular adenosine diphosphate that selectively activates P2Y13, resulting in high-density lipoprotein endocytosis. Previous studies have shown that the serum concentration of the F1-adenosine triphosphatase inhibitor inhibitory factor 1 is negatively associated with cardiovascular risk.

AIM

To evaluate whether p2y13 genetic variants affect cardiovascular risk.

METHODS

Direct sequencing of the p2y13 coding and flanking regions was performed in a subcohort of 168 men aged 45-74 years with stable coronary artery disease and 173 control subjects from the GENES study. The two most frequent mutations, rs3732757 and rs1466684, were genotyped in 767 patients with coronary artery disease and 789 control subjects, and their association with cardiovascular risk markers was analysed.

RESULTS

Carriers of the rs3732757 261T and rs1466684 557G alleles represented 9% and 27.5% of the entire population, respectively. The allele frequencies were identical in patients with coronary artery disease and control subjects. The presence of 261T was associated with higher concentrations of plasma lipoprotein A-I and inhibitory factor 1, increased fat mass and a lower heart rate. Moreover, the proportion of patients with coronary artery disease with a pejorative systolic ankle-brachial index was lower in carriers of the 261T allele. In both populations, the 557G allele was associated with increased concentrations of lipoprotein(a), and an allele dose effect was observed.

CONCLUSIONS

Two frequent p2y13 variants are associated with specific bioclinical markers of cardiovascular risk. Although neither one of these variants appears to be related to the development of atherosclerotic disease, they may modulate the risk of additional cardiovascular complications.

Serum inhibitory factor 1, high-density lipoprotein and cardiovascular diseases

PURPOSE OF REVIEW

The atheroprotective properties of HDL are supported by epidemiological and preclinical research. However, the results of interventional trials paradoxically indicate that drugs increasing HDL-cholesterol (HDL-C) do not reduce coronary artery disease (CAD) risk. Moreover, Mendelian randomization studies have shown no effect of HDL-C-modifying variants on CAD outcome. Thus, the protective effects of HDL particles are more governed by their functional status than their cholesterol content. In this context, any successful clinical exploitation of HDL will depend on the identification of HDL-related biomarkers, better than HDL-C level, for assessing cardiovascular risk and monitoring responses to treatment.

RECENT FINDINGS

Recent studies have enlightened the role of ecto-F1-ATPase as a cell surface receptor for apoA-I, the major apolipoprotein of HDL, involved in the important metabolic and vascular atheroprotective functions of HDL. In the light of these findings, the clinical relevance of ecto-F1-ATPase in humans has recently been supported by the identification of serum F1-ATPase inhibitor (IF1) as an independent determinant of HDL-C, CAD risk and cardiovascular mortality in CAD patients.

SUMMARY

Serum IF1 measurement might be used as a novel HDL-related biomarker to better stratify risk in high-risk populations or to determine pharmacotherapy.

The potential role of mitochondrial ATP synthase inhibitory factor 1 (IF1) in coronary heart disease: a literature review

Cardiovascular disease (CVD) is the leading cause of death worldwide, and so the search for innovative and accurate biomarkers for guiding prevention, diagnosis, and treatment is a valuable clinical and economic endeavor. Due to a recent findings that the serum concentration of mitochondrial ATP synthase inhibitory factor 1 (IF1) is an independent prognostic factor in patients with coronary heart disease (CHD), we reviewed the role of this protein in myocardial ischemic preconditioning, its correlation to plasma high density lipoprotein (HDL), the predictive potential in patients with CHD, and its interplay with angiogenesis. IF1 has been positively correlated with plasma HDL-cholesterol, and is independently negatively associated with all-cause and CV mortality in patients with CHD. However, this conclusion is prevalently based on limited data, and more research is needed to draw definitive conclusions. IF1 seems to play an additional role in increasing cell vulnerability in oncologic diseases but may also function as modest inhibitor of angiogenesis in physiological conditions. It has been also explored that IF1 may rather act as a modulator of other molecules more significantly involved in angiogenesis, especially apolipoprotein A1 on which the largest effect could be observed. In conclusion, more research is needed to characterize the role of IF1 in patients with CHD.

Serum levels of mitochondrial inhibitory factor 1 are independently associated with long-term prognosis in coronary artery disease: the GENES Study

BACKGROUND

Epidemiological and observational studies have established that high-density lipoprotein cholesterol (HDL-C) is an independent negative cardiovascular risk factor. However, simple measurement of HDL-C levels is no longer sufficient for cardiovascular risk assessment. Therefore, there is a critical need for novel non-invasive biomarkers that would display prognostic superiority over HDL-C. Cell surface ecto-F1-ATPase contributes to several athero-protective properties of HDL, including reverse cholesterol transport and vascular endothelial protection. Serum inhibitory factor 1 (IF1), an endogenous inhibitor of ecto-F1-ATPase, is an independent determinant of HDL-C associated with low risk of coronary artery disease (CAD). This work aimed to examine the predictive value of serum IF1 for long-term mortality in CAD patients. Its informative value was compared to that of HDL-C.

METHOD

Serum IF1 levels were measured in 577 male participants with stable CAD (age 45-74 years) from the GENES (Genetique et ENvironnement en Europe du Sud) study. Vital status was yearly assessed, with a median follow-up of 11 years and a 29.5 % mortality rate. Cardiovascular mortality accounted for the majority (62.4 %) of deaths.

RESULTS

IF1 levels were positively correlated with HDL-C (r s = 0.40; P < 0.001) and negatively with triglycerides (r s = -0.21, P < 0.001) and CAD severity documented by the Gensini score (r s = -0.13; P < 0.01). Total and cardiovascular mortality were lower at the highest quartiles of IF1 (HR = 0.55; 95 % CI, 0.38-0.89 and 0.50 (0.28-0.89), respectively) but not according to HDL-C. Inverse associations of IF1 with mortality remained significant, after multivariate adjustments for classical cardiovascular risk factors (age, smoking, physical activity, waist circumference, HDL-C, dyslipidemia, hypertension, and diabetes) and for powerful biological and clinical variables of prognosis, including heart rate, ankle-brachial index and biomarkers of cardiac diseases. The 10-year mortality was 28.5 % in patients with low IF1 (<0.42 mg/L) and 21.4 % in those with high IF1 (≥0.42 mg/L, P < 0.02).

CONCLUSIONS

We investigated for the first time the relation between IF1 levels and long-term prognosis in CAD patients, and found an independent negative association. IF1 measurement might be used as a novel HDL-related biomarker to better stratify risk in populations at high risk or in the setting of pharmacotherapy.

Nutritional regulation of coupling factor 6, a novel vasoactive and proatherogenic peptide

High sodium, high glucose, and obesity are important risk factors for age-related diseases such as cardiovascular disease (CVDs), stroke, and cancer. Coupling factor 6 (CF6) is released from vascular endothelial cells and functions as a circulating peptide that inhibits prostacyclin and nitric oxide generation by intracellular acidosis. High glucose elevates CF6 by activation of protein kinase C and p38 mitogen-activated protein kinase, whereas CF6 causes type 2 diabetes mellitus, resulting in a high glucose vicious cycle. Low glucose increases inhibitory factor peptide 1, an endogenous inhibitor of CF6. High salt intake increases CF6 through nuclear factor κB signaling, whereas CF6 induces salt-sensitive hypertension and salt-induced congestive heart failure. Oral administration of vitamin C cancels salt-induced increase in CF6, and estrogen replacement leads to the delayed onset of CF6-induced salt-sensitive hypertension and the rescue from cardiac systolic dysfunction. Because CF6 contributes to the onset of CVDs, nutritional regulation of CF6 will shed light on the understanding of preventive strategy and mechanisms for CVDs and a target for therapy.

HDLs in crises

PURPOSE OF REVIEW

The clinical utility of HDLs has been scrutinized upon the publication of Mendelian randomization studies showing no effect of HDL-cholesterol (HDL-C) modifying variants on cardiovascular disease (CVD) outcome. The failures of randomized controlled HDL-C-directed intervention trials have further fueled this skepticism. This general criticism originates from oversimplification that has equated 'HDL-C' with 'HDL' and misconceived both as the 'good cholesterol'.

RECENT FINDINGS

HDL particles are heterogeneous and carry hundreds of different lipids, proteins, and microRNAs. Many of them but not cholesterol, that is, HDL-C, contributes to the multiple protective functions of HDLs that probably evolved to manage potentially life-threatening crises. Inflammatory processes modify the composition of HDL particles as well as their individual protein and lipid components, and, as a consequence, also their functionality. Gain of dominant-negative functions makes dysfunctional HDL a part rather than a solution of the endangering situation. Quantification of HDL particle numbers, distinct proteins or lipids, and modifications thereof as well as bioassays of HDL functionality are currently explored toward their diagnostic performance in risk prediction and monitoring of treatment response.

SUMMARY

Any successful clinical exploitation of HDLs will depend on the identification of the most relevant (dys)functions and their structural correlates. Stringent or prioritized structure-(dys)function relationships may provide biomarkers for better risk assessment and monitoring of treatment response. The most relevant agonists carried by either functional or dysfunctional HDLs as well as their cellular responders are interesting targets for drug development.

HDL biogenesis, remodeling, and catabolism

In this chapter, we review how HDL is generated, remodeled, and catabolized in plasma. We describe key features of the proteins that participate in these processes, emphasizing how mutations in apolipoprotein A-I (apoA-I) and the other proteins affect HDL metabolism. The biogenesis of HDL initially requires functional interaction of apoA-I with the ATP-binding cassette transporter A1 (ABCA1) and subsequently interactions of the lipidated apoA-I forms with lecithin/cholesterol acyltransferase (LCAT). Mutations in these proteins either prevent or impair the formation and possibly the functionality of HDL. Remodeling and catabolism of HDL is the result of interactions of HDL with cell receptors and other membrane and plasma proteins including hepatic lipase (HL), endothelial lipase (EL), phospholipid transfer protein (PLTP), cholesteryl ester transfer protein (CETP), apolipoprotein M (apoM), scavenger receptor class B type I (SR-BI), ATP-binding cassette transporter G1 (ABCG1), the F1 subunit of ATPase (Ecto F1-ATPase), and the cubulin/megalin receptor. Similarly to apoA-I, apolipoprotein E and apolipoprotein A-IV were shown to form discrete HDL particles containing these apolipoproteins which may have important but still unexplored functions. Furthermore, several plasma proteins were found associated with HDL and may modulate its biological functions. The effect of these proteins on the functionality of HDL is the topic of ongoing research.

Beyond the genetics of HDL: why is HDL cholesterol inversely related to cardiovascular disease?

There is unequivocal evidence that high-density lipoprotein (HDL) cholesterol levels in plasma are inversely associated with the risk of cardiovascular disease (CVD). Studies of families with inherited HDL disorders and genetic association studies in general (and patient) population samples have identified a large number of factors that control HDL cholesterol levels. However, they have not resolved why HDL cholesterol and CVD are inversely related. A growing body of evidence from nongenetic studies shows that HDL in patients at increased risk of CVD has lost its protective properties and that increasing the cholesterol content of HDL does not result in the desired effects. Hopefully, these insights can help improve strategies to successfully intervene in HDL metabolism. It is clear that there is a need to revisit the HDL hypothesis in an unbiased manner. True insights into the molecular mechanisms that regulate plasma HDL cholesterol and triglycerides or control HDL function could provide the handholds that are needed to develop treatment for, e.g., type 2 diabetes and the metabolic syndrome. Especially genome-wide association studies have provided many candidate genes for such studies. In this review we have tried to cover the main molecular studies that have been produced over the past few years. It is clear that we are only at the very start of understanding how the newly identified factors may control HDL metabolism. In addition, the most recent findings underscore the intricate relations between HDL, triglyceride, and glucose metabolism indicating that these parameters need to be studied simultaneously.