In vivo tissue cholesterol efflux is reduced in carriers of a mutation in APOA1

Abstract

Atheroprotection by high density lipoprotein (HDL) is considered to be mediated through reverse cholesterol transport (RCT) from peripheral tissues. We investigated in vivo cholesterol fluxes through the RCT pathway in patients with low plasma high density lipoprotein cholesterol (HDL-c) due to mutations in APOA1. Seven carriers of the L202P mutation in APOA1 (mean HDL-c: 20 ± 19 mg/dl) and seven unaffected controls (mean HDL-c: 54 ± 11 mg/dl, P < 0.0001) received a 20 h infusion of (13)C2-cholesterol ((13)C-C). Enrichment of plasma and erythrocyte free cholesterol and plasma cholesterol esters was measured. With a three-compartment SAAM-II model, tissue cholesterol efflux (TCE) was calculated. TCE was reduced by 19% in carriers (4.6 ± 0.8 mg/kg/h versus 5.7 ± 0.7 mg/kg/h in controls, P = 0.02). Fecal (13)C recovery and sterol excretion 7 days postinfusion did not differ significantly between carriers and controls: 21.3 ± 20% versus 13.3 ± 6.3% (P = 0.33), and 2,015 ± 1,431 mg/day versus 1456 ± 404 mg/day (P = 0.43), respectively. TCE is reduced in carriers of mutations in APOA1, suggesting that HDL contributes to efflux of tissue cholesterol in humans. The residual TCE and unaffected fecal sterol excretion in our severely affected carriers suggest, however, that non-HDL pathways contribute to RCT significantly.

Keywords: fecal sterol excretion; genetics; high density lipoprotein; reverse cholesterol transport.

Fig. 1.

A: Three-compartment SAAM-II model. Parameters: Ex1, infusion rate (mg/kg/h); V1, pool size plasma FC and rapidly equilibrating liver pool (mg/kg body weight); V2, RBC FC pool size (mg/kg body weight); V3, plasma CE pool size (mg/kg body weight); k(0,1), rate constant for transfer of tracer from V1 to environment (h⁻¹); k(0,3), rate constant for transfer of tracer from plasma CE pool to environment (h⁻¹); k(3,1), rate constant for transfer of tracer from V1 to plasma CE pool (h⁻¹); k(1,2), rate constant for transfer of tracer from RBC FC pool to V1 (h⁻¹); k(2,1), rate constant transfer of tracer from V1 to RBC pool (h⁻¹); s1, s2, s3, sampling sites, corresponding with V1, V2, V3; Metabolic steady-state equations: flux 1 = k(0,1) × V1 = flux of V1 to the environment (mg/kg/h); flux 2 = k(2,1) × V1 = k(1,2) × V2 = exchange flux between V1 and RBC FC (mg/kg/h); flux 3 = k(0,3) × V3 = k(3,1) × V1 = flux of V1 to plasma CE pool (mg/kg/h); flux 1 + flux 3 equals TCE (mg/kg/h). B: Tracee model of cholesterol fluxes. Model indicating the traced fluxes: TCE, exchange flux of plasma FC with RBC FC (flux 2), and cholesterol esterification (flux 3).

Fig. 2.

¹³C-enrichment curves of plasma FC, CE, and RBCs as sampled during ¹³C-cholesterol infusion. ¹³C enrichment profiles of plasma FC, CEs, and RBCs as sampled during the ¹³C-cholesterol infusion of a representative carrier (closed symbols) and a representative control participant (open symbols). Data are presented as molar percent excess.

Fig. 3.

TCE in APOA1^L202P carriers and unaffected controls. TCE (mg/kg/h) was calculated as the sum of flux 1 and flux 3 (Figure 1A). **P value for univariate analysis (unpaired Student's t-test). The observed difference was statistically independent of age and BMI (P after adjustment for age and BMI: 0.017).