Activation of phospholipase A(2) by amyloid beta-peptides in vitro

Amyloid beta-peptides (A beta) are centrally involved in the pathogenesis of Alzheimer's disease, Using secretory phospholipase A(2) (PLA(2)) from porcine pancreas as a model and in the presence of a limiting Ca2+ concentration of approximately 50 nM, the synthetic peptide A beta(1-42) activates the hydrolysis of the pyrene-labeled acidic phospholipid analog 1-palmitoyl-2-[(pyren-1-yl)]hexanoyl-sn-glycero-3-phosphoglycerol (PPHPG) maximally 2.3-fold, whereas an inhibition of PLA(2) action by 50% on the corresponding phosphatidylcholine derivative (PPHPC) was observed. The above effects were evident at 0.24 nM A beta(1-42) corresponding to A beta(1-42):phospholipid and A beta(1-42):PLA(2) molar ratios of 1:10 650 and 1:7.6, respectively. The presence of 10 mol % 1-palmitoyl-2-oleqyl-sn-glycero-3-phosphoglycerol (POPG) in PPHPC reversed the inhibitory effect of A beta(1-42) peptide and for these vesicles the hydrolytic activity of PLA(2) toward the fluorescent phosphatidylcholine was enhanced similar to 1.8-fold by A beta(1-42). In contrast, inclusion of 10 mol % POPG into PPHPG did not influence either the hydrolytic rate toward the latter lipid or the activating effect of A beta(1-42). Ca2+ concentrations exceeding 15 mu M abolished the enhancing effect of A beta(1-42) on the hydrolysis of PPHPG whereas a slight activation of PPHPC hydrolysis now became evident. With limiting [Ca2+] preaggregated A beta(1-42) enhanced the hydrolysis of both PPHPG as well as PPHPC but the peptide concentrations required were higher by 3-4 orders of magnitude. The synthetic peptide A beta(25-35) corresponding to the hydrophobic membrane-spanning segment of the beta amyloid precursor protein activated PLA(2) when using PPHPG as a substrate; however, compared to A beta(1-42) the extent of activation was less (similar to 2-fold) and required higher (1 nM) peptide. A beta(25-35) did not affect the hydrolysis of the phosphatidylcholine derivative. The hydrophilic peptide A beta(1-28) had no effect on PLA(2)-catalyzed hydrolysis of either PPHPG or PPHPC under the conditions used in the present study. Interestingly, the above activating effects of A beta(1-42) and A beta(25-35) on PLA(2)-catalyzed hydrolysis of the acidic phospholipid substrate parallel their toxicity on cultured neurons whereas A beta(1-28) had no influence either on cultured cells or on PLA(2) activity.