We reported that Dictyostelium-derived differentiation-inducing factor-1 (DIF-1) inhibits proliferation and motility (migration and infiltration) of several cancer cells, although the mechanism is not sufficiently clarified. In our recent study, we investigated whether DIF-1 can inhibit the tumor growth and metastasis of triple-negative breast cancer (TNBC) with regard to the mechanism of action. In in-vivo experiments, orally administered DIF-1 significantly suppressed the tumor growth and spontaneous lung metastasis of 4T1-Luc cells (mouse TNBC cell line) injected into the mammary fat pad of BALB/c mice. In in-vitro experiments using 4T1-Luc cells, DIF-1 inhibited cell proliferation by suppressing signal transducer and activator of transcription 3 (STAT3)-mediated expression of cyclin D1 and inhibited cell motility by suppressing the expression of Snail. DIF-1 phosphorylated (activated) AMP-activated protein kinase (AMPK) rapidly (within 1 min), subsequently phosphorylated (inhibited) its substrate Raptor, a component of mechanistic target of rapamycin complex 1 (mTORC1), and then inhibited the phosphorylation (activation) of 70 kDa ribosomal protein S6 kinase (p70^S6K). These results suggested that DIF-1 suppresses STAT3-cyclin D1-mediated cell proliferation and Snail-mediated cell motility through AMPK-mediated inhibition of mTORC1-p70^S6K signaling pathway in 4T1-Luc cells. In this talk, we would also like to mention the latest findings and the potential for clinical application of DIF-1.