Differentiation-inducing factors (DIFs), produced by Dictyostelium discoideum, show anti-tumor activity via several different signaling molecules, including glycogen synthase kinase-3 (GSK-3) and signal transducer and activator of transcription 3 (STAT3). However, the exact mechanism of action of DIFs is still poorly understood. Therefore, to better understand the action of DIF, we performed DNA microarray analysis and found that the activity of the Hippo signaling pathway may be modified. Therefore, we analyzed mRNA expressions of Hippo signaling pathway target genes. Although the mRNA expression of the target genes including CTGF and Cyr61were elevated by DIF-1, surprisingly the protein expression themselves were reduced. In an attempt to elucidate the mechanism, we found that DIF promoted proteolysis of Yes-associated protein (YAP), a transcriptional co-activator of the Hippo signaling pathway, in the human cervical cell line HeLa and the human colon cancer cell line HCT-116. YAP has also been reported to promote epithelial-to-mesenchymal transition (EMT). Indeed, cell migration, cell invasion and expressions of EMT-related proteins (fibronectin, vimentin and N-cadherin) were reduced by DIF-1. These results suggest that DIF-1 suppresses EMT via degradation of YAP in human cancer cell lines.