Secreted phospholipase A2 inhibitors are also potent blockers of binding to the M-type receptor

Abstract

Mammalian secreted phospholipases A(2) (sPLA(2)s) constitute a family of structurally related enzymes that are likely to play numerous biological roles because of their phospholipid hydrolyzing activity and binding to soluble and membrane-bound proteins, including the M-type receptor. Over the past decade, a number of competitive inhibitors have been developed against the inflammatory-type human group IIA (hGIIA) sPLA(2) with the aim of specifically blocking its catalytic activity and pathophysiological functions. The fact that many of these inhibitors, including the indole analogue Me-Indoxam, inhibit several other sPLA(2)s that bind to the M-type receptor prompted us to investigate the impact of Me-Indoxam and other inhibitors on the sPLA(2)-receptor interaction. By using a Ca(2+) loop mutant derived from a venom sPLA(2) which is insensitive to hGIIA inhibitors but still binds to the M-type receptor, we demonstrate that Me-Indoxam dramatically decreases the affinity of various sPLA(2)s for the receptor, yet an sPLA(2)-Me-Indoxam-receptor complex can form at very high sPLA(2) concentrations. Me-Indoxam inhibits the binding of iodinated mouse sPLA(2)s to the mouse M-type receptor expressed on live cells but also enhances binding of sPLA(2) to phospholipids. Because Me-Indoxam and other competitive inhibitors protrude out of the sPLA(2) catalytic groove, it is likely that the inhibitors interfere with the sPLA(2)-receptor interaction by steric hindrance and to different extents that depend on the type of sPLA(2) and inhibitor. Our finding suggests that the various anti-inflammatory therapeutic effects of sPLA(2) inhibitors may be due not only to inhibition of enzymatic activity but also to modulation of binding of sPLA(2) to the M-type receptor or other as yet unknown protein targets.