PTP boosts the cholesteryl ester transfer activity of CETP even though the two proteins move phospholipids by distinct mechanisms (J Lipid Res 1994)

In the present study, a sequential procedure was set up to separate simultaneously cholesteryl ester transfer protein (CETP), phospholipid transfer protein (PTP), and lecithin:cholesterol acyltransferase (LCAT) from human plasma. Subsequently, phospholipid transfer activities of purified lipid transfer proteins, deprived of LCAT activity, were compared and potential interactions between the two proteins were studied. Phospholipid transfer (PT) activity was determined by using three independent assays that measured the transfer of radiolabeled phosphatidylcholine ([14C]PC) either from phospholipid liposomes to high density lipoproteins-3 (PTliposome-->HDL3), from high density lipoproteins-3 to phospholipid liposomes (PTHDL3-->liposome), or from HDL3 to low density lipoproteins (PTHDL3-->LDL). Comparative study of CETP and PTP pointed out several differences in the ability of the two proteins to transfer phospholipids. i) Whereas both CETP and PTP were able to mediate phospholipid transfers from [14C]PC-HDL3 to LDL, only PTP facilitated phospholipid transfers from [14C]PC-liposomes to HDL3. ii) As PTP did not promote the transfer of phospholipids from [14C]PC-HDL3 to liposomes, it was concluded that it functions as a phospholipid transfer protein rather than a phospholipid exchange protein. This latter point was confirmed by the ability of purified PTP to induce the net mass transfer of phospholipids from PC-liposomes to HDL3. iii) While PTP presented no intrinsic cholesteryl ester transfer activity, it was able to significantly increase CETP-mediated cholesteryl ester transfers from HDL3 to LDL. iv) CETP did not influence the PTliposome-->HDL3 activity induced by PTP. v) Oleic acid was able to significantly increase the cholesteryl ester transfer activity of CETP, but not the PTliposome-->HDL3 activity of PTP. vi) PTHDL3-->LDL activity of purified CETP was explained, for a large part, by the copurification of nonesterified fatty acids. Taken together, data of the present report suggest that phospholipid transfer activity of CETP and PTP could occur through distinct processes. Since, in human plasma, PTP is not only responsible for the major part of phospholipid net mass transfer but is also able in vitro to modulate the CETP-mediated transfer of cholesteryl esters between various plasma lipoprotein fractions, it could play a determinant role in lipoprotein remodeling in vivo.