Desialylation of human apolipoprotein E decreases its binding to human high-density lipoprotein and its ability to deliver esterified cholesterol to the liver.

Apolipoprotein E (apoE) plays a significant role in the delivery of high-density lipoprotein (HDL) cholesterol to the liver via the apoB/E receptor. The roles of the apoE sialylation status in its association with HDL and in the reverse cholesterol transport (RCT) function of HDL have not been well defined. Furthermore, long-term ethanol treatment impairs apoE sialylation and leads to its decreased content in HDL. Therefore, we investigated the association of either sialo apoE (SapoE) or desialo apoE (DSapoE) with HDL and its effect on the RCT function of HDL. The dextran sulfate precipitation method showed that [125I]DSapoE binding to HDL was 27.3% (P < .02) to 35.5% (P < .001) lower versus [125I]SapoE. Scatchard analysis of the specific binding data showed that [125I]SapoE had 11.2 times more affinity for HDL than [125I]DSapoE based on size-exclusion chromatography (Kd = 89.7 v 1,010 nmol/L). Similarly, [1251]HDL had 4.5 times more affinity for SapoE compared with DSapoE based on solid-phase binding (Kd = 21.9 v 104.4 nmol/L). Furthermore, esterified cholesterol uptake from reconstituted HDL particles (rHDLs) by HepG2 cells increased over basal uptake up to 153% when rHDLs contained SapoE, versus only 37% with DSapoE. Enzymatic resialylation of DSapoE completely restored its HDL-binding and RCT properties, identical to those of SapoE. It is therefore concluded that desialylation of apoE decreases its binding to plasma HDL, leading to an impaired RCT function.