HDL flux is higher in patients with nonalcoholic fatty liver disease

Abstract

Altered lipid metabolism and inflammation are involved in the pathogenesis of both nonalcoholic fatty liver disease (NAFLD) and cardiovascular disease (CVD). Even though high-density lipoprotein (HDL), a CVD protective marker, is decreased, whether HDL metabolism and function are perturbed in NAFLD are currently unknown. We examined the effect of NAFLD and disease severity on HDL metabolism and function in patients with biopsy-proven simple steatosis (SS), nonalcoholic steatohepatitis (NASH), and healthy controls. HDL turnover and HDL protein dynamics in SS (n = 7), NASH (n = 8), and healthy controls (n = 9) were studied in vivo. HDL maturation and remodeling, antioxidant, cholesterol efflux properties, and activities of lecithin-cholesterol ester acyltransferase and cholesterol ester transfer protein (CETP) were quantified using in vitro assays. All patients with NAFLD had increased turnover of both HDL cholesterol (HDLc; 0.16 ± 0.09 vs. 0.34 ± 0.18 days, P < 0.05) and apolipoprotein A1 (ApoAI) (0.26 ± 0.04 vs. 0.34 ± 0.06 days, P < 0.005) compared with healthy controls. The fractional catabolic rates of other HDL proteins, including ApoAII (and ApoAIV) were higher (P < 0.05) in patients with NAFLD who also had higher CETP activity, ApoAI/HDLc ratio (P < 0.05). NAFLD-induced alterations were associated with lower antioxidant (114.2 ± 46.6 vs. 220.5 ± 48.2 nmol·mL^-1·min^-1) but higher total efflux properties of HDL (23.4 ± 1.3% vs. 25.5 ± 2.3%) (both P < 0.05), which was more pronounced in individuals with NASH. However, no differences were observed in either HDL turnover, antioxidant, and cholesterol efflux functions of HDL or HDL proteins' turnover between subjects with SS and subjects with NASH. Thus, HDL metabolism and function are altered in NAFLD without any significant differences between SS and NASH.

Keywords: HDL; NASH; atherosclerosis; heavy water; mass spectrometry; proteomics; turnover.

Fig. 1.

HDL turnover in patients with nonalcoholic fatty liver disease (NAFLD; n = 14) and healthy controls (n = 8) was determined using a ²H₂O-metabolic labeling approach. Administration of a bolus dose (4 mL/kg) followed by maintenance doses (10% of bolus/day) of ²H₂O in drinking water resulted in a steady-state body water enrichment of ~0.8%–0.9%. Time course labeling of total HDL cholesterol (HDLc) (A) and tryptic ApoAI peptide VSFLALEEYTK (B). Data represent mean ± SD.

Fig. 2.

Effect of nonalcoholic fatty liver disease (NAFLD) on HDL metabolism and functions. Total cholesterol efflux and oxidative stress are increased, but antioxidant activity of HDL is reduced in patients with NAFLD. A: ApoB-depleted sera from patients with NAFLD and healthy controls were investigated for their ability to promote [³H]cholesterol efflux from RAW264.7 macrophages. B: serum paraoxonase 1 (PON1) activity was measured spectrophotometrically using paraoxon as a substrate. C: thiobarbituric acid reactive substances (TBARS) in EDTA plasma and ApoB-depleted plasma were measured to quantify lipid peroxidation in total plasma and HDL. D: cholesterol ester transfer protein (CETP) activity was measured in ApoB-depleted plasma. Data represent mean ± SD (n = 8 for control group; n = 14 for NAFLD group). *P < 0.05 and **P < 0.005.

Fig. 3.

Impact of the disease severity in nonalcoholic fatty liver disease (NAFLD) on HDL metabolism and functions. The data from subjects with NAFLD presented in Fig. 2 was divided into steatosis and nonalcoholic steatohepatitis (NASH) groups based on the liver histology data. A: cholesterol efflux. B: paraoxonase 1 (PON1) activity. C: thiobarbituric acid reactive substances (TBARS) measured to quantify lipid peroxidation in total plasma and HDL. D: cholesterol ester transfer protein (CETP) activity. Data represent mean ± SD (n = 8 for control group; n = 14 for NAFLD group). *P < 0.05, **P < 0.005 vs. controls. $$P < 0.005 vs. controls and steatosis.