Phospholipid transfer protein (PLTP): Secretion, gene expression, and functions in cultured cells

The importance of phospholipid transfer protein (PLTP) in HDL metabolism and reverse cholesterol transfer has been well recognized. Previous study of the formation of apoA-I containing particles in fibroblast cell-conditioned media was extended. The apoA-I containing particles and fibroblast-secreted PLTP were detected by non-denaturing gradient gel electrophoresis and immunoblotting. Incubation of fibroblasts with lipid-free apoA-I resulted in the formation of large apoA-I containing particles in the cell-conditioned media. A major fraction of fibroblast-secreted PLTP showed the particle size indicative of PLTP-monomers having large amounts of bound lipids. PLTP dimer and monomer particles were enlarged when purified PLTP was incubated with phosphatidylcholine vesicles. Addition of apoA-I to purified PLTP facilitated the conversion of PLTP dimers into the monomers. This conversion occurred through the interaction of PLTP with apoA-I, which was demonstrated by means of protein cross-linking. The treatment of fibroblasts with brefeldin A prevented not only the formation of apoA-I containing particles but also the secretion of PLTP. In contrast, the treatment of fibroblasts with cAMP resulted in an enhancement of apoA-I containing particle formation in the media and PLTP secretion. When fibroblasts were treated with dithiothreitol, an agent known to inhibit PLTP secretion, the formation of apoA-I containing particles was substantially inhibited. This inhibition, however, was greatly reduced by the cAMP treatment. In addition, newly secreted PLTP was shown to promote apoA-I lipidation to a greater extent than the PLTP accumulated in the media.; The mechanism of butyrate-enhanced PLTP gene transcription in Hep G2 was investigated. Upon treatment of Hep G2 cells with genistein, a tyrosine kinase inhibitor, the enhancing effects of butyrate on PLTP secretion and PLTP mRNA levels were nullified. In contrast, PLTP secretion and mRNA levels were not changed by a protein kinase C activator, PMA. Thus, the up-regulatory effect of butyrate on gene transcription may involve tyrosine kinase pathways.; PLTP secretion was also investigated by using a human small intestine cell model, Caco-2. Properties of Caco-2 secreted-PLTP were similar to those of purified human plasma PLTP. Treatment of the cells with butyrate and propionate enhanced PLTP secretion and PLTP gene expression.