Signal therapy for RAS-induced cancers in combination of AG 879 and PP1, specific inhibitors for ErbB2 and Src family kinases, that block PAK activation

BACKGROUND Both EGF family ligands and ErbB family receptor kinases act upstream of RA S to induce mitogenesis of normal cells, such as NIH 3T3 fibroblasts, Howev er, oncogenically mutated RAS, such as v-Ha-RAS is constitutively activated and therefore no longer requires these ligands or receptors for its activa tion. Nevertheless, it up-regulates the expression of these EGF family liga nds, To understand the biologic significance of RAS-induced up-regulation o f these ligands in both RAS-induced PAK activation and malignant transforma tion, we have conducted the following studies, based on the previous observ ations that (1) the N-terminal SH3 domain of PIX selectively binds a Pro-ri ch domain of 18 amino acids of PAKs, CDC42/Rac-dependent Ser/Thr kinase fam ily, and (2) this specific interaction is essential for both PAK activation and membrane ruffling RESULTS Using four distinct, cell-permeable, and highly specific inhibitors, namely WR-PAK18, which blocks the PAK-PIX interaction; AG 1478, which inhibits Er bB1 kinase activity; and AG 825 or AG 879, which inhibits ErbB2 kinase acti vity, we demonstrate that (1) the PAK-PIX interaction is essential for v-Ha -RAS-induced malignant transformation; (2) v-Ha-RAS requires not only ErbB1 but also ErbB2, which are activated through two independent autocrine path ways to induce both the PIX/Rac/CDC42-dependent PAK activation and malignan t transformation in vitro; and (3) a combination of AG 879 and the Src fami ly kinase-specific inhibitor PP1 suppresses almost complete ly the growth o f RAS-induced sarcomas in nude mice. CONCLUSION These findings not only change our conventional view on the role of these R AS-inducible ligands and ErbB family receptors (serving as RAS activators) but also suggest a new avenue for the treatment of RAS-associated cancers b y a combination of inhibitors specific for ErbB, Src, or PAK family kinases .