The estrogen receptor (ER) is downregulated under hypoxia via a proteasome-dependent pathway. We studied the mechanism of ER degradation under hypoxic mimetic conditions. Cobalt chloride-induced ER down-regulation was dependent on the expression of newly synthesized protein(s), one possibility of which was hypoxia-inducible factor-1 (HIF-1). To examine the role of HIF-1 expression in ER down-regulation under hypoxic-mimetic conditions, we used a constitutively active form of HIF-1, HIF-1/VP16, constructed by replacing the transactivation domain of HIF-1 with that of VP16. Western blot analysis revealed that HIF-1/VP16 downregulated ER in a dose-dependent manner via a proteasome-dependent pathway. The kinase pathway inhibitors PD98059, U0126, wortmannin, and SB203580 did not affect the down-regulation. A mammalian two-hybrid screen and immunoprecipitation assays indicated that ER interacted with HIF-1 physically. These results suggest that ER down-regulation under hypoxia involves protein-protein interactions between the ER and HIF-1.