Keywords: apolipoprotein A-I, high-density lipoprotein, mitochondrial function, skeletal muscle
Abstract
Prior animal and cell studies have demonstrated a direct role of high-density lipoprotein (HDL) and apolipoprotein A-I (ApoA-I) in enhancing skeletal muscle mitochondrial function and exercise capacity. However, the relevance of these animal and cell investigations in humans remains unknown. Therefore, a cross-sectional study was conducted in 48 adults (67% female, 8% Black participants, age 39 ± 15.4 yr old) to characterize the associations between HDL measures, ApoA-I, and muscle mitochondrial function. Forearm muscle oxygen recovery time (tau) from postexercise recovery kinetics was used to assess skeletal muscle mitochondrial function. Lipoprotein measures were assessed by nuclear magnetic resonance. HDL efflux capacity was assessed using J774 macrophages, radiolabeled cholesterol, and apolipoprotein B-depleted plasma both with and without added cyclic adenosine monophosphate. In univariate analyses, faster skeletal muscle oxygen recovery time (lower tau) was significantly associated with higher levels of HDL cholesterol (HDL-C), ApoA-I, and larger mean HDL size, but not HDL cholesterol efflux capacity. Slower recovery time (higher tau) was positively associated with body mass index (BMI) and fasting plasma glucose (FPG). In multivariable linear regression analyses, higher levels of HDL-C and ApoA-I, as well as larger HDL size, were independently associated with faster skeletal muscle oxygen recovery times that persisted after adjusting for BMI and FPG (all P < 0.05). In conclusion, higher levels of HDL-C, ApoA-I, and larger mean HDL size were independently associated with enhanced skeletal muscle mitochondrial function in healthy humans.
NEW & NOTEWORTHY Our study provides the first direct evidence supporting the beneficial role of HDL-C and ApoA-I on enhanced skeletal muscle mitochondrial function in healthy young to middle-aged humans without cardiometabolic disease.
INTRODUCTION
Low circulating levels of high-density lipoprotein cholesterol (HDL-C) has long been recognized as an independent risk factor for atherosclerotic cardiovascular disease (CVD) because of its antiatherogenic role in reverse cholesterol transport (1, 2). Along with HDL-C, HDL cholesterol-efflux capacity (HDL-CEC) (3), a measure of HDL functionality, has likewise been shown to be associated with reduced CVD risk, regardless of HDL-C levels (4–7).
In addition to its antiatherogenic property, there is growing evidence from several animal and cell studies suggesting that higher HDL levels play a critical role in the maintenance of skeletal muscle glucose metabolism and skeletal muscle mitochondrial bioenergetics (8, 9). Specifically, a prior study demonstrated that mice deficient in apolipoprotein A-I (ApoA-I), the main protein component of HDL particles, had decreased exercise capacity (8). Moreover, both HDL and ApoA-I were shown to directly enhance glucose oxidation and mitochondrial respiration rate in mitochondria isolated from mouse skeletal muscle (8). In humans, disruptions in skeletal muscle metabolism and mitochondrial function have similarly been associated with exercise intolerance and cardiometabolic diseases (10, 11). However, the associations between HDL measures with skeletal muscle mitochondrial function have not been examined in humans.
Accordingly, we performed a cross-sectional study in a cohort of young to middle-aged adults free of cardiometabolic disease aimed at characterizing the associations between HDL composition, HDL function (HDL-CEC), and skeletal muscle mitochondrial function.
MATERIALS AND METHODS
Study Overview
All studies performed were approved by the Institutional Review Board at the University of Texas Southwestern Medical Center. Furthermore, all studies were conducted in accordance with current guidelines and regulations, as appropriate. Written informed consent was obtained from all subjects. All studies were conducted with participants who met inclusion criteria from a pool of subjects who were recruited for another study that assessed mitochondrial function. In a subset of participants with mitochondrial function data and adequate blood sample volume stored at −80°C, we also evaluated HDL structure and function. Inclusion criteria were age 18–80 yr old, without previous history of CVD, chronic kidney disease, diabetes mellitus, use of antihyperlipidemic medications, an estimated glomerular filtration rate (eGFR) < 60 mL/min/1.73 m2, history of substance abuse, pregnancy, or current smoker.
Experimental Measures
Assessment of participant demographics and biochemical measures.
All studies were performed in a single visit, using cross-sectional design. Participants underwent a physical exam and medical history that included basic self-reported demographics such as age, race, ethnicity, and comorbidities. Participants had systolic and diastolic blood pressure (SBP and DBP, respectively) measured in triplicate with the last two measurements averaged, using an oscillometric device (Welch Allyn, Vital Signs, NC), after the participant had been resting quietly for 5 min according to the most recent clinical guidelines (12). Height and weight were obtained by research staff, and body mass index (BMI) was calculated as weight in kilograms divided by height in meters squared. Fasting blood was collected via venipuncture in two different types of tubes. Serum was collected in serum separator tubes for quantification of serum electrolytes and fasting glucose. Serum was sent to Quest Diagnostics and electrolytes were assessed via autoanalyzer and eGFR was calculated according to the Chronic Kidney Disease Epidemiology Collaboration equation (Quest Diagnostics). Plasma was collected in tubes with ethylenediaminetetraacetic acid (EDTA) for lipoprotein analyses by LabCorp. Physical activity was assessed from a questionnaire (based on the Mifflin-St. Jeor Equation) as previously described (13, 14). The time spent in light versus moderate-to-vigorous physical activity per week was estimated from the questionnaire.
High-density lipoprotein cholesterol efflux capacity.
High-density lipoprotein cholesterol efflux capacity (HDL-CEC) assays were performed to quantify the ability of HDL to remove cholesterol from J774 mouse macrophages as an index of HDL’s endogenous reverse cholesterol transport function (5). Blood was collected from participants via venipuncture and stored at −70°C in tubes containing EDTA. For HDL-CEC quantification, the plasma samples of participants were incubated with J774 mouse macrophages containing radiolabeled (3H) cholesterol in apolipoprotein B (ApoB)-depleted plasma, as previously described (15). Incubations occurred in the absence and presence of added cyclic adenosine monophosphate (cAMP) to amplify ATP-binding cassette transporter A1 (ABCA1) expression (5). Therefore, the assay quantified the total efflux mediated by pathways from macrophages that included ABCA1 and G1 (ABCG1) transporters, scavenger receptor B1 (SRB1), and aqueous diffusion (16). ABCA1-specific efflux was calculated as efflux from cAMP-stimulated cells minus efflux from unstimulated cells. All samples were completed in duplicate and normalized to a pooled sample. HDL-CEC was expressed in arbitrary units (AUs).
Lipoprotein characterization.
HDL-C, low-density lipoprotein cholesterol (LDL-C) concentration, HDL size, HDL subspecies, total HDL particle (HDL-P) number, ApoA-I, and apolipoprotein B (ApoB) concentrations were measured from plasma using the Vantera nuclear magnetic resonance (NMR) analyzer, as previously described (Labcorp, Morrisville, NC) (17, 18). The ranges of particle diameter for HDL sizes were small HDL, 7.4–7.8 nm; medium HDL, 8.7–9.5 nm; and large HDL, 10.3–12 nm, whereas the concentrations of mean-weighted HDL size spans were 7.4–12 nm in diameter. The particle diameters for the concentrations of HDL subspecies were H1P, 7.4 nm; H2P, 7.8 nm; H3P, 8.7 nm; H4P, 9.5 nm; H5P, 10.3 nm; H6P, 10.8 nm; and H7P, 12.0 nm. Small HDL comprises H1P and H2P, medium HDL comprises H3P and H4P, and large HDL comprises H5P to H7P. HDL-P, small HDL, medium HDL, large HDL, and all subspecies concentration(s) are reported in micromoles per liter; HDL-C, LDL-C, ApoA-I, and ApoB concentrations, in milligrams per deciliter; and mean HDL size, in nanometers.
Skeletal muscle mitochondrial function protocol.
Skeletal muscle mitochondrial function was assessed from postexercise recovery kinetics using near-infrared spectroscopy (NIRS) (19, 20), a noninvasive technique used to assess skeletal muscle oxidative capacity that has been cross-validated with phosphorus (31P) magnetic resonance spectroscopy (21). Tissue oxygen saturation (%Sto2) was measured during the exercise protocol and calculated as follows: (O2Hb)/(O2Hb + H Hb × 100), where O2Hb is oxygenated hemoglobin + myoglobin and H Hb is deoxygenated hemoglobin + myoglobin. Briefly, each participant was placed in the supine position and had a continuous-wave moorVMS-NIRS probe (Moor Instruments, Inc.) placed over a prominent muscle belly of the anterior forearm compartment while avoiding tattoos, scars, or superficial veins. The NIRS probe was placed against the skin with adhesive medical tape and covered with a blue cloth to block light near the optical sensor and an inflatable cuff was placed at the top of the arm just below the axilla. Each participant then underwent a 3-min baseline period. Once stabilization of Sto2 occurred, participants were guided in performing 3 min of rhythmic hand grip at 30% maximal voluntary contraction. At the 3-min time point, the participant was asked to stop, and we immediately performed a series of 15 suprasystolic (220 mmHg) arterial cuff occlusions (10 s on/off) following the cessation of exercise. Sto2 consumption was calculated as the slope of change in forearm tissue oxygen saturation during these occlusions (%Sto2) (22). Then, the slopes of %Sto2 consumption were fit to a monoexponential curve, and the skeletal muscle oxygen recovery time constant (tau) was derived as previously described (21), using GraphPad Prism 9.4.1 Software (Boston, MA).
Statistical Analyses
Statistical analyses were performed using SAS version 9.4 (Carey, NC). Participant characteristics are reported as means ± SD for continuous data, medians with interquartile ranges (IQR), or means with 95% confidence intervals of SD for skewed continuous variables, and frequency with percentage for categorical data. Univariate relationships between exposures and skeletal muscle oxygen recovery time were assessed by Pearson or Spearman rank correlation coefficients, as appropriate. Nonnormally distributed continuous variables were log transformed before use in regression analyses. Multivariable linear regression analyses were conducted for exposures found to be significant in univariate analyses, and standardized β-estimates with 95% confidence intervals were reported. Hierarchical variable selection was performed to reduce collinearity. The standardized β predicted the change in the response variable for 1SD of change in the explanatory variable (while controlling for the other variables).
Specifically, multivariable linear regression analyses were performed to assess the associations between HDL-C and functionality, ApoA-I, and mean HDL size with skeletal muscle mitochondrial function after adjustment of covariates. HDL-CEC with cAMP (stimulated) was used in the multivariable linear regression analysis. For each analysis, three multivariable models were assembled: model 1, adjusted for age; model 2, adjusted for age and BMI; and model 3, adjusted for age, body mass index (BMI), fasting plasma glucose, and physical activity. All modeling assumptions were verified using diagnostic tools (i.e., residual vs. predicted value plot, Q-Q plot, and Cook’s distance). All P values were two tailed, and significance was set a priori as P < 0.05.
RESULTS
Participant Characteristics
The characteristics of study participants are shown in Table 1. The mean age was 39 ± 15.4 yr old, with 67% female, 84% White adults, 8% Black/African-American adults, and 8% Asian adults. There were 29% Hispanic/Latino adults (of those, 100% self-identified as Hispanic White adults). Overall, mean HDL-C was 66.4 ± 17.2 mg/dL, mean HDL size was 9.3 ± 0.5 nm, mean stimulated HDL-CEC was 1.25 ± 0.40 AU, mean fasting plasma glucose was 91.1 ± 7.0 mg/dL, mean SBP and DBP was 118.5 ± 7.7 and 74.4 ± 6.4 mmHg, respectively, and mean skeletal muscle oxygen recovery time was 49.8 ± 13.7 s. The mean time spent engaging in light and moderate-to-vigorous physical activity was 64 (95% CI of SD, 42.5, 85.5) and 73 (95% CI of SD, 41.9, 104) min/wk, respectively. The average R2 of the post-exercise recovery kinetic curves was 0.98.
Table 1.
Clinical, laboratory, and demographic characteristics of participants
| Variables | Total Cohort |
|---|---|
| Physiological metrics | |
| n | 48 |
| Age, yr | 39 ± 15.4 |
| Females, n (%) | 32 (67) |
| Black participants, n (%) | 4 (8) |
| Hispanic/Latino participants, n (%) | 14 (29) |
| Body mass index, kg/m2 | 24.0 ± 3.6 |
| eGFR, mL/min/1.73 m2 | 102.6 ± 18.3 |
| Systolic blood pressure, mmHg | 118.5 ± 7.7 |
| Diastolic blood pressure, mmHg | 74.4 ± 6.4 |
| Physical activity, min/wk (95% CI)* | |
| Light | 64 (42.5, 85.5) |
| Moderate to vigorous | 73 (41.9, 104) |
| Mitochondrial function measure | |
| Tau, s | 49.8 ± 13.7 |
| Serum measures | |
| Fasting plasma glucose, mg/dL | 91.1 ± 7.0 |
| Creatinine, mg/dL | 0.81 ± 0.15 |
| Sodium, mmol/L | 138.9 ± 1.9 |
| Phosphate, mg/dL | 3.7 ± 0.5 |
| Potassium, mmol/L | 4.1 ± 0.2 |
| Calcium, mg/dL | 9.4 ± 0.39 |
| Lipoprotein measures | |
| HDL-CEC (stimulated), AU | 1.25 ± 0.40 |
| HDL-CEC (unstimulated), AU | 0.98 ± 0.31 |
| ΔCEC, AU | 0.27 ± 0.23 |
| Total cholesterol, mg/dL | 201.7 ± 40.6 |
| LDL-C, mg/dL | 121.7 ± 36.4 |
| HDL-C, mg/dL | 66.4 ± 17.2 |
| Mean HDL size, nm | 9.3 ± 0.5 |
| Triglycerides, mg/dL [IQR]* | 64.0 [45.0, 88.5] |
| Apolipoprotein B, mg/dL | 99.5 ± 22.2 |
| Apolipoprotein AI, mg/dL | 148.2 ± 29.6 |
Values are means ± SD, means with 95% confidence intervals (CI) of SD, medians [interquartile ranges, IQR], or n, number of participants (%). *Not normally distributed. AU, arbitrary units; eGFR, estimated glomerular filtration rate; HDL-C, high-density lipoprotein cholesterol; HDL-CEC, high-density lipoprotein cholesterol efflux capacity; LDL-C, low-density lipoprotein cholesterol.
Univariate Associations with Skeletal Muscle Mitochondrial Function
In univariate analyses, a faster skeletal muscle oxygen recovery time was significantly associated with higher levels of HDL-C, ApoA-I, and larger mean HDL size (r = −0.49, P value < 0.001, r = −0.38, P value < 0.01, and r = −0.48, P value = 0.001, respectively, Table 2 and Fig. 1). Conversely, a prolonged skeletal muscle oxygen recovery time was associated with higher BMI, fasting plasma glucose levels, and H7P subspecies concentration (r = 0.34, P value = 0.02, r = 0.30, P value = 0.04, and r = 0.35, P value = 0.02, respectively, Table 2 and Fig. 1). Skeletal muscle oxygen recovery time was not significantly related to age, minutes of light or moderate-to-vigorous physical activity per week, HDL-CEC (stimulated, unstimulated, or ABCA1-specific), HDL-P, small HDL, medium HDL, large HDL, H6-H1P subspecies, LDL-C, total cholesterol, log triglycerides, or ApoB on univariate analyses.
Table 2.
Univariate correlation coefficients of skeletal muscle mitochondrial function with anthropometric measures and markers of HDL function and lipid composition
| Variable | Coefficient | R 2 | P Value |
|---|---|---|---|
| Age, yr | 0.11 | 0.01 | 0.44 |
| Body mass index | 0.34 | 0.12 | 0.02 |
| Fasting plasma glucose | 0.30 | 0.09 | 0.04 |
| Physical activityb | |||
| Light | 0.04 | <0.01 | 0.81 |
| Moderate to vigorous | 0.01 | <0.01 | 0.93 |
| Total cholesterol | 0.07 | <0.01 | 0.65 |
| LDL cholesterol | 0.27 | 0.07 | 0.07 |
| Apolipoprotein B | 0.24 | 0.06 | 0.09 |
| HDL cholesterol | −0.49 | 0.24 | <0.001 |
| Apolipoprotein A-I | −0.38 | 0.14 | <0.01 |
| HDL-CEC | |||
| Stimulated | −0.08 | <0.01 | 0.61 |
| Unstimulated | −0.24 | 0.06 | 0.12 |
| ABCA1-specific CEC | 0.19 | 0.04 | 0.19 |
| HDL particle concentration* | −0.25 | 0.06 | 0.09 |
| Mean HDL size | −0.48 | 0.23 | 0.001 |
| Particle Concentration* | |||
| Large HDL | −0.11 | 0.01 | 0.44 |
| Medium HDL | −0.08 | <0.01 | 0.60 |
| Small HDL | 0.04 | <0.01 | 0.77 |
| Subspecies* | |||
| H7P | 0.35 | 0.12 | 0.02 |
| H6P | −0.10 | 0.01 | 0.52 |
| H5P | 0.23 | 0.05 | 0.14 |
| H4P | −0.30 | 0.09 | 0.05 |
| H3P | 0.17 | 0.03 | 0.28 |
| H2P | 0.12 | 0.01 | 0.41 |
| H1P | −0.05 | <0.01 | 0.78 |
| Triglycerides* | 0.23 | 0.05 | 0.12 |
Values are Pearson correlation coefficient and Spearman rank coefficient. *Log transformed. Negative correlation coefficient indicates the exposure is associated with lower skeletal muscle oxygen recovery time (higher mitochondrial function), whereas positive coefficient is associated with longer skeletal muscle oxygen recovery time (lower mitochondrial function). ABCA1, ATP-binding cassette transporter A1; HDL, high-density lipoprotein; HDL-CEC, high-density lipoprotein cholesterol efflux capacity; LDL, low-density lipoprotein. Boldface indicates significant values. Statistical significance is indicated by boldface for P < 0.05.
Figure 1.
Scatterplot of skeletal muscle mitochondrial function with significant high-density lipoprotein (HDL) markers. Scatterplot displaying simple linear regression analyses of tau with HDL-cholesterol (HDL-C; A), apolipoprotein A-I (ApoA-I, B), mean HDL size (C), body mass index (BMI; D), and fasting plasma glucose (FPG; E). Pearson correlation coefficient denoted as “r” and coefficient of determination denoted as “R-squared.”
Multivariable Analyses of HDL-C with Skeletal Muscle Mitochondrial Function
In an age-adjusted analysis, higher HDL-C was independently associated with faster skeletal muscle oxygen recovery time [β = −0.50 (−0.75, −0.25), P value < 0.001, Table 3, model 1]. Moreover, the inverse association between HDL-C and skeletal muscle oxygen recovery time remained after additional adjustment for fasting plasma glucose [β = −0.45 (−0.75, −0.15), P value < 0.01, Table 3, model 2] and BMI and physical activity [β = −0.42 (−0.71, −0.12), P value = < 0.01, Table 3, model 3]. Conversely, neither stimulated HDL-CEC or HDL-P concentration was associated with skeletal muscle mitochondrial function (Table 3).
Table 3.
Adjusted multivariable linear regression analyses of skeletal muscle mitochondrial function with markers of HDL function and composition
|
Model 1* |
Model 2* |
Model 3* |
||||
|---|---|---|---|---|---|---|
| Variable | β (95% CI) | P value | β (95% CI) | P value | β (95% CI) | P value |
| HDL-C | −0.50 (−0.75, −0.25) | <0.001 | −0.45 (−0.75, −0.15) | <0.01 | −0.42 (−0.71, −0.12) | <0.01 |
| ApoA-I | −0.39 (−0.65, −0.12) | 0.01 | −0.31 (−0.61, −0.01) | 0.048 | −0.30 (−0.60, −0.01) | 0.048 |
| HDL-Pa | −0.26 (−0.54, 0.02) | 0.08 | −0.14 (−0.45, 0.16) | 0.37 | −0.21 (−0.51, 0.10) | 0.18 |
| HDL size | −0.48 (−0.73, −0.22) | 0.001 | −0.41 (−0.69, −0.13) | 0.01 | −0.36 (−0.67, −0.10) | 0.02 |
| HDLa | ||||||
| Large | −0.11 (−0.40, 0.18) | 0.45 | −0.12 (−0.40, 0.16) | 0.41 | −0.08 (−0.3, 0.24) | 0.57 |
| Medium | −0.08 (−0.37, 0.21) | 0.60 | −0.14 (−0.42, 0.14) | 0.33 | −0.10 (−0.37, 0.19) | 0.52 |
| Small | 0.04 (−0.25, 0.33) | 0.79 | −0.01 (−0.30, 0.28) | 0.95 | 0.01 (−0.27, 0.30) | 0.96 |
| HDL-CEC** | −0.08 (−0.33, 0.17) | 0.54 | −0.08 (−0.34, 0.17) | 0.54 | −0.06 (−0.32, 0.20) | 0.67 |
Values are standardized regression coefficients (β) and 95% confidence intervals (CI). aLog transformed. Small, medium, and large high-density lipoprotein (HDL) refers to particle concentrations. Boldface indicates significant values. *Model 1 was adjusted for age, model 2 was adjusted for model 1 + body mass index (BMI), and model 3 was adjusted for model 2 + fasting plasma glucose. **Model 1 was adjusted for HDL-cholesterol (HDL-C) and age, model 2 was adjusted for model 1 + BMI, and model 3 was adjusted for model 2 + fasting plasma glucose and physical activity. ApoA-I, apolipoprotein A-I; HDL-CEC, HDL-cholesterol efflux capacity.
Multivariable Analyses of HDL Size with Skeletal Muscle Mitochondrial Function
In the age-adjusted analysis, larger mean HDL size was independently associated with faster skeletal muscle oxygen recovery time [β = −0.48 (−0.73, −0.22), P value = 0.001, Table 3, model 1]. Similarly, the inverse association between mean HDL size and skeletal muscle oxygen recovery time remained after additional adjustment for fasting plasma glucose [β = −0.41 (−0.69, −0.13), P value = 0.01, Table 3, model 2] and BMI and physical activity [β = −0.36 (−0.67, −0.10), P value = 0.02, Table 3, model 3]. However, neither the concentrations of small HDL, medium HDL, or large HDL (Table 3), or the H1P–H7P subspecies (data not shown), were associated with skeletal muscle mitochondrial function.
Multivariable Analyses of ApoA-I with Skeletal Muscle Mitochondrial Function
In an age-adjusted analysis, higher ApoA-I was independently associated with faster skeletal muscle oxygen recovery time [β = −0.39 (−0.65, −0.12), P value = 0.01, Table 3, model 1]. Moreover, the inverse association between ApoA-I and skeletal muscle oxygen recovery time remained after additional adjustment for fasting plasma glucose [β = −0.31 (−0.61, −0.01), P value = 0.048, Table 3, model 2] and BMI and physical activity [β = −0.30 (−0.60, −0.01), P value = 0.048, Table 3, model 3].
DISCUSSION
The new major findings in the present study were threefold. First, higher HDL-C was independently associated with faster skeletal muscle oxygen recovery time in young to middle-aged healthy humans, which persisted after adjustment of relevant covariates. Second, higher ApoA-I was associated with enhanced mitochondrial function after adjustment. Third, larger mean HDL size was likewise associated with faster skeletal muscle oxygen recovery.
Mechanisms underlying the protective effects of HDL and/or ApoA-I on skeletal muscle mitochondrial function largely come from prior animal and in vitro investigations. In this regard, Lehti et al. (8) conducted a study using two genetic mouse models, one with increased HDL levels (apolipoprotein A-I transgenic [ApoA-I TG]) and one with reduced HDL levels (apoA-I-deficient [ApoA-I KO]), alongside wild-type mice. This study demonstrated that exercise capacity was reduced in ApoA-I KO mice, that mitochondria isolated from the ApoA-I KO mice displayed diminished ATP synthesis, and that ApoA-I KO mice had higher fasting glucose and HbA1c levels. Moreover, HDL and ApoA-I both directly enhanced glucose oxidation, intramuscular glycogen levels postexercise, and mitochondrial respiration rate in mitochondria isolated from skeletal muscle (8). These findings suggest that HDL/ApoA-I is needed for normal glucose regulation and glycogen replenishment postexercise in addition to adequate ATP synthesis. Importantly, the ApoA-I TG mice exhibited features that are associated with protection against age-induced decline of endurance capacity, such as lower fasting glucose levels and reduced fat mass, when compared with wild-type mice (8). Our finding of an association between ApoA-I and mitochondrial function in humans is consistent with these animal and in vitro data, highlighting the important role of ApoA-I on regulating muscle mitochondrial function in protecting against age-related decline in exercise capacity. Although physical inactivity may lead to muscle mitochondrial dysfunction, which is independent of HDL or ApoA-I, there was no significant correlation between time spent in physical activity to tau in our group of otherwise healthy adults. Thus, our data provide support for the beneficial role of HDL and ApoA-I in muscle mitochondrial function.
Aside from the present study, there are very few prior investigations into the relationship between HDL measures and mitochondrial function in humans. Analysis from the Hawaii aging with HIV cardiovascular study cohort demonstrated that low levels of HDL-C and smaller HDL size were related to increased peripheral blood mononuclear cell (PBMC) mitochondrial-specific oxidative stress (measured by 8-oxo-deoxyguanine) (23). Moreover, they found that HDL-CEC was positively associated with PBMC mitochondrial oxidative phosphorylation (measured by Complex IV activity) (23). However, PBMC mitochondrial respiration may not accurately reflect skeletal muscle mitochondrial respiration (24). Our study provides the first direct evidence linking HDL-C, ApoA-I, and larger HDL particles to enhanced skeletal muscle mitochondrial function in healthy young to middle-aged humans. Importantly, our data also demonstrated these associations were independent of age and indices of insulin resistance including fasting plasma glucose and obesity. These findings are important because disruptions in skeletal muscle metabolism have been associated with an accelerated age-related decline in exercise capacity (25, 26) and have been observed among those with insulin resistance and disturbed HDL metabolism, commonly identified in the metabolic syndrome (27). Taken into context, our data highlight a direct link between HDL and ApoA-I in muscle mitochondrial function that is independent of relevant factors such as age and indices of insulin resistance.
To our best knowledge, the present study represents the first investigation in healthy humans to characterize the associations between HDL measures and skeletal muscle mitochondrial function. Study limitations include a relatively small sample size, the cross-sectional design that limits the ability to derive causal effects from the results, knowledge of family history for dyslipidemia, and lack of formal cardiopulmonary testing that would more directly assess exercise capacity. Nevertheless, when physical activity is included in the model, the results remain unchanged, which suggests that the observed relationship between HDL as well as ApoA-I and mitochondrial function cannot explained by physical activity alone in our study.
In conclusion, our study provides novel insight into the associations between HDL composition and function with skeletal muscle mitochondrial function, in young to middle-aged healthy humans. We demonstrate that higher circulating levels of HDL-C, ApoA-I, and larger-size HDL are independently associated with enhanced skeletal muscle mitochondrial function. Future prospective and longitudinal studies in a larger number of participants are required to replicate our findings and further establish the causal relationship between HDL structure/function and age-related decline in exercise endurance.
DATA AVAILABILITY
Data sets generated and analyzed during the current study are available from the corresponding author upon reasonable request.
GRANTS
This work was supported by National Institutes of Health Grants R01 HL15999 (to W.V.), R01 HL133179 (to W.V. and S.A.S.), and T32 DK007257 (to G.K.).
DISCLOSURES
No conflicts of interest, financial or otherwise, are declared by the authors.
AUTHOR CONTRIBUTIONS
J.M.G., A.R., W.K., and W.V. conceived and designed research; J.M.G., U.B.B., W.K., S.S., and G.K. performed experiments; J.M.G., S.S., A.R., and W.V. analyzed data; J.M.G., W.K., A.R., and W.V. interpreted results of experiments; J.M.G. prepared figures; J.M.G. drafted manuscript; J.M.G., B.E.Y., S.A.S., and W.V. edited and revised manuscript; J.M.G., U.B.B., W.K., J.W., S.S., G.K., M.A.C., S.A.S., A.R., and W.V. approved final version of manuscript.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
Data sets generated and analyzed during the current study are available from the corresponding author upon reasonable request.
