Abstract
Previous studies from our laboratory demonstrated native high density lipoproteins and apolipoproteins AI, AII, and CI, stimulate the release of human placental lactogen (hPL) from trophoblast cells in culture. To examine the mechanisms by which these apolipoproteins stimulate hPL release, we have studied hPL secretion in response to several synthetic peptide analogs of the amphipathic helical structure of the apolipoproteins. The magnitude of the stimulation of hPL release in response to the analog peptides correlated with the ability to displace apolipoproteins from high density lipoprotein and with other measures of phospholipid binding affinity such as the increase in alpha-helicity and the size of complexes formed between the peptide and phospholipid. The correlation of stimulatory ability and lipid affinity suggests that the action of the apolipoproteins on hPL release may be mediated through an interaction with plasma membrane phospholipids.
Highlights
Previous studies from our laboratory demonstrated native high density lipoproteins and apolipoproteins AI, AII,and CI, stimulate the release of humanplacental lactogen from trophoblastcells in culture
Wehave recently demonstrated that high density lipoprotein (HDL) at physiologic concentrations stimulatesthe release of the protein hormone humanplacental lactogen (hPL) from an enriched fraction of cultured trophoblast cells and that theactivity is dueto apolipoproteins AI, AII, and CI and not thelipid constituents on HDL (4)
Apolipoprotein A1 contains eight tandem 22-mer repeats at its carboxyl-terminal end, each of which has the properties of an amphipathic helical domain (9). To examine whether this characteristic structureof the apolipoproteins is important in the stimulation of hPL release, we have examined the effects on hPL release of several synthetic peptides that were designed to mimic the apolipoproteins in their structure andproperties
Summary
The synthetic peptides used in this study were designed to examine how the amphipathic a-helicalregions of apolipoproconc (ug/ml). Control (O),apoAl model peptide, peptide HA, is 18-amino acid residues long (O), 18A-Pro-18A (A),18A (A), (Glu'~8Leu''~'8)18A(B),reverse-18A and forms an a-helix with a polar and a nonpolar face and a specific distribution of charged residues. Peptide 18A-Pro-18A stimulated more release than the single helical peptide MA, with a maximal 18-fold increase in hPL release. This peptide has twomodel helixes joined by a proline, giving itthe potential for cooperativity between helixes as thetwo helical segments can pivot around proline (an amino acid which does not permithelix formation). Model amphipathic helix representing the charged residue positions (Gl~',~Leu",'~)18sAtimulating the most release, reverse-18A analogous to apolipoproteins. 18A-Pro-18A:repeating units of 18A joined by Pro. Phe-Tyr-Lys-Asp-Val-Ala-Lys-Glu-Leu-Glu-Lys-Ala-PhCeh.arged residue positions in 18A reversed. Peptide reverse-18A exhibits low percent a-helicity both in buffer and in the presence of DMPC, whereas peptide 18A
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