ApoE and apoC-III-defined HDL subtypes: A descriptive study of their LCAT and CETP content and activity

Background High-density lipoproteins (HDL) functionality predicts cardiovascular risk better than HDL concentrations. The apolipoprotein composition of HDL may be a determinant of their function. Lecithin-cholesterol acyl transferase (LCAT) and cholesterol-ester transfer protein (CETP) are key enzymes for HDL-mediated reverse cholesterol transport. We assessed the distribution and activity of LCAT and CETP in HDL subspecies defined by their content of apolipoproteins E (apoE) and C-III (apoC-III) in humans. Methods We isolated in 18 adult humans of both sexes (mean age 55.6, BMI 26.9 Kg/m2, HbA1c 5.4%), four subspecies of HDL containing respectively: No apoE and no apoC-III (E-C-), apoE but not apoC-III (E+C-), apoC-III but no apoE (E-C+) and both apoE and apoC-III (E+C+). In each HDL subspecies, we measured LCAT and CETP concentration and activity using immunoenzymatic and fluorometric methods. Additionally, we determined the size distribution of HDL in each apolipoprotein-defined fraction using non-denaturing electrophoresis and anti-ApoA-I western blot. Results Similar to previous studies, HDL in the E-C- fraction was the predominant subtype. The size distribution of HDL was very similar across all four apolipoprotein-defined fractions. LCAT was most abundant in E-C- HDL (3.58 mg/mL, 59.6 % of plasma LCAT mass), while HDL with apoE or apoC-III had much less LCAT (19.8%, 12.2% and 8.37% of plasma LCAT respectively for E+C-, E-C+ and E+C+). LCAT mass was lower in E+C- HDL relative to E-C- HDL, but LCAT activity was similar in both fractions, signaling a greater activity-to-mass ratio associated with the presence of apoE. Both CETP mass and CETP activity showed only slight variations across HDL subspecies. There was an inverse correlation between plasma LCAT activity and both E-C+ pre-beta HDL (r=-0.55, p=0.017) and E-C- alpha 1 HDL (r=-0.49, p=0.041). Conversely, there was a direct correlation between E-C+ alpha 1 HDL and CETP activity in plasma (r=0.52, p=0.025). Conclusions Our results suggest that LCAT activity in humans is influenced by the presence of small interchangeable apolipoproteins. The presence of apoE in small HDL is correlated with increased LCAT activity and esterification of plasma cholesterol.