Hypoxia-inducible factor 1 (HIF-1) has been associated with distant tumor metastasis; however, its function in the complex and multiple metastatic processes have not been fully elucidated yet.Purpose: Examine whether HIF-l-mediated metabolic switch is responsible for the colony formation in lung.Methods and results: We established a stable transfectant of a mouse breast cancer cell line, EMT6,expressing both a secretory luciferase (CLuc from Cypridina noctiluca) and a luciferase (Lue from firefly) under the control of a constitutive active promoter, CMVp, and a HIF-l-dependent promoter, 5HREp, respectively.Using tumor-bearing mice with a subcutaneous xenograft of the stable transfectant, we confirmed that CLuc was secreted from the xenografls into the serum and the activity showed a good positive correlation with the real tumor volume quantified with a digital caliper.Applying such an in vivo CLuc assay, we confirmed that, after tail vein injection of the stable transfectant, the volume of metastatic lung tumors increased aggressively over time.On the other hand, we imaged the intratumoral HIF-1 activity as luciferase bioluminescence with IVIS SPECTRUMTM optical imaging system.Combination of the in vivo CLuc assay and the optical imaging experiment, we found that metastatic cancer cells transiently obtained high HIF-1 activity around 5 days after gaining access to lungs in reactive oxygen species-dependent and hypoxia-independent manners.The transient activation induced the expression of lactate dehydrogenase A (LDHA) and phosphorylation of E1 subunit of pyruvate dehydrogenase (PDH-E1), indicating a conversion of cellular metabolic pathways from mitochondrial oxidative phosphorylation into lactic acid fermentation.Administration of a HIF-1 inhibitor, YC-1, inhibited the conversion and significantly suppressed the formation of metastatic lung tumors.Conclusions: These results indicate that HIF-l-mediated metabolic switch is responsible for metastatic colony formation in lungs and its blockade by HIF-1-inhibitors is a rational strategy to prevent tumor metastasis.