Molecular cloning and expression of lipid transfer inhibitor protein reveals its identity with apolipoprotein F

Published studies demonstrate that lipid transfer inhibitor protein (LTIP) is an important regulator of cholesteryl ester transfer protein (CETP) activity. Although LTIP inhibits CETP activity among different lipoprotein classes, it preferentially suppresses transfer events involving low density lipoprotein (LDL), whereas transfers involving high density lipoprotein as donor are less affected. In this study, we report the purification of LTIP and the expression of its cDNA in cultured cells. Purification of LTIP, in contrast to other published protocols, took advantage of the tight association of this protein with LDL. Ultracentrifugally isolated LDL was further purified on anti-apoE and apoA-I affinity columns. Affinity purified LDL was delipidated by tetramethylurea, and the tetramethylurea-soluble proteins were separated by SDS-polyacrylamide gel electrophoresis. The protein migrating at a molecular mass of approximately 33 kDa was excised from the gel and its N-terminal amino acid sequence determined. The 14-amino acid sequence obtained showed complete homology with the sequence deduced for apolipoprotein F (apoF) cDNA isolated from Hep G2 cells. On Western blots, peptide-specific antibodies raised against synthetic fragments of apoF reacted with the same 33-kDa protein in LTIP-containing fractions purified from LDL and from lipoprotein-deficient plasma. In contrast to that previously reported, apoF was shown to be associated almost exclusively with LDL, identical to the distribution of LTIP activity. The cDNA for apoF was cloned from a human liver cDNA library, ligated into a mammalian expression vector, and transiently transfected into COS-7 cells. Conditioned media containing secreted apoF demonstrated CETP inhibitor activity, whereas cells transfected with vector alone did not. This CETP inhibitor activity was efficiently removed from the media by nickel-Sepharose, consistent with the 6-His tag incorporated into recombinant apoF. By Western blot, the 6-His-tagged protein had a molecular weight slightly larger than native apoF. The CETP inhibitor activity of recombinant apoF possessed the same LDL specificity, oleate sensitivity, and dependence on lipoprotein concentration as previously noted for LTIP. We conclude that LTIP and apoF are identical.