3-Methyladenine (3-MA) inhibits class III phosphoinositide 3-kinase (PI3K) and is widely used as an inhibitor of autophagy. 3-MA has also been shown to stimulate cell death of tumor cells under nutrient-starved conditions by inhibiting autophagy. To explore the possibility of this type of autophagy inhibitors as anticancer drugs, we examined the effects of 3-MA on the phenotypes of highly metastatic human fibrosarcoma HT1080 cells. We report here that although 3-MA did not markedly affect cell survival of the cells under either normal or amino acid-starved conditions, it strongly inhibited the invasiveness of the cells. 3-MA rapidly suppressed actin rich membrane ruffle and/or lamellipodia formation under normal conditions, leading to inhibition of cell migration and invasion of the cells without substantial inhibitions of small GTPase Rac activity and the production of matrix metalloproteinases MMP-2 and MMP-9. 3-MA abolished class I and class II PI3Ks in in vitro lipid kinase assays, and suppressed cell motility of the cells more strongly than the other PI3K inhibitors wortmannin and LY294002. Downregulation of Beclin 1, a protein required for autophagic body formation, by transfection of Beclin 1 siRNA did not inhibit membrane ruffle formation and cell migration. These results suggest that 3-MA suppresses the invasion of HT1080 cells, independently of autophagy inhibition, through inhibition of type I and II PI3Ks and possibly other molecules.

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