A comprehensive lipid binding and activity validation of a cancer-specific peptide-peptoid hybrid PPS1.

We recently identified a peptide-peptoid hybrid, PPS1, which recognizes lipids that have an overall negative charge, such as phosphatidylserine (PS), phosphatidylglycerol (PG), phosphatidic acid (PA), and phosphatidylinositol (PI), but that does not bind to neutral lipids, such as phosphatidylcholine (PC), phosphatidylethanolamine (PE), and sphingomyelin (SM). The simple dimeric version of PPS1, PPS1D1, displayed strong cytotoxicity to cancer cells over normal cells in vitro and tumor burden in vivo. In this study, we comprehensively characterized the direct binding and activity of PPS1 on PS, PG, and PA using liposome-based assays and lung cancer cell lines that express these negatively charged lipids. First, the fluorescence polarization (FP) binding studies of fluoresceinated-PPS1 (PPS1-FITC) to PS-, PG-, and PA-containing PC-liposomes showed that the binding of PPS1 to PC-liposomes increased as concentrations of these lipids increased. In terms of activity, PPS1D1 induced the release of calcein from large, unilamellar PC-liposomes containing 15-30% PS, PG, and PA. PPS1D1 had no activity when the liposomes were composed of 100% PC. This effect was higher at 30% lipids than 15%, and the EC50 for PG and PA were higher than that of PS, indicating that PPS1D1 is more specific towards PS. PPS1D1 binds to and induces significant cytotoxicity in lung cancer cell lines H1693, HCC95, and H1395, which express negatively charged lipids, but had no effect on normal HBEC30KT cells, which has mostly PC in the outer layer. In addition, a series of previously developed PPS1D1 derivatives, which retain or lose activity, were tested with these liposome-based assays, and the data were equivalent to previous observations. This study provides comprehensive binding and activity validations of a unique peptide-peptoid hybrid, PPS1, on negatively charged lipids PS, PA, and PG that are elevated on cancer cell surfaces relative to normal human cell surfaces.