INHIBITION OF PHOSPHOINOSITIDE HYDROLYSIS AND CELL-GROWTH OF SWISS 3T3 CELLS BY MYRISTOYLATED PHOSPHOLIPASE-C INHIBITOR PEPTIDES

It has been demonstrated that the phospholipase C-gamma (PLC-gamma) mo lecule contains within it a phospholipase C inhibitor (PCI) region and that synthetic peptides based on the sequence of this region (PCI pep tides) suppress the enzymatic activity of PLC isoforms [Y, Homma and T , Takenawa (1992) J. Biol. Chem. 267, 21884-21889], In order to improv e the permeability of the plasma membrane to PCI peptides, we synthesi zed myristoylated PCI peptides, myr-GLYRKAMRLRYPV [myr-PCI(Y)] and myr -GLFRKAMRLRFPV [myr-PCI(F)], which are identical except for the replac ement of the two tyrosine residues in myr-PCI(Y) by phenylalanines in myr-PCI(F), and examined their inhibitory activity on PLC enzymes in v itro and in vivo. This fatty acid modification potentiated the inhibit ory activity of the original PCI peptides and both myr-PCI(Y) and myr- PCI(F) suppressed the PIP2-hydrolyzing activity of purified PLC isofor ms in vitro. The K-i values of myr-PCI(Y) and myr-PCI(F) for purified PLC-gamma 1 were 3.5 and 55 mu M, respectively, Myr-PCI(Y) at concentr ations in the sub-micromolar range significantly suppressed IP3 format ion induced by EGF, PDGF, bombesin, or serum in Swiss 3T3 cells. Furth ermore, myr-PCI(Y) also strongly inhibited cell proliferation induced by these stimuli. The inhibitory effect on IP3 formation and prolifera tion of myr-PCI(F) was much less potent than that of myr-PCI(Y). These results suggest that myristoylated PCI peptides could be applied to l iving cells as specific inhibitors of PLC signaling pathways and that PLC pathways are at least in part required for growth in Swiss 3T3 cel ls.