Cholesteryl ester transfer protein (CETP) plays a controversial role in atherogenesis by contributing to the net transfer of high density lipoprotein (HDL) cholesteryl ester (CE) to the liver via apolipoprotein-B-containing lipoproteins (apoB-LP). We evaluated in vitro the CETP-mediated bidirectional transfer of CE from HDL to the chemically modified pro-atherogenic low density lipoprotein (LDL) particles. Acetylated or oxidized (ox) LDL, either unlabeled or [3H]-CE labeled, were incubated with [14C]-CE-HDL in the presence of the lipoprotein-deficient plasma fraction (d>1.21 g/ml) as the source of CETP. The amount of radioactive CE transferred was determined after dextran sulfate/MgCl(2) precipitation of LDL. The results showed a 1.4-2.8-fold lower HDL-CE transfer to acetylated LDL while no effect was observed on the CE transfer to oxidized LDL. However, the reverse transfer rate of [3H]CE-LDL to HDL was 1.4-3.6 times greater when LDL was oxidized than when it was intact. Overall, HDL(2) was better than HDL(3) as donor of CE to native LDL, probably reflecting the relatively greater CE content of HDL(2). Oxidation of LDL enhanced the CETP-mediated cholesteryl ester transfer rate to HDL, bringing on a reduced net transfer rate of cholesteryl ester from HDL to ox LDL. This may diminish the oxLDL particle's atherogenic effect.