Cobalt chloride-induced estrogen receptor α down-regulation involves hypoxia-inducible factor-1α in MCF-7 human breast cancer cells

The estrogen receptor ( ER) is down-regulated under hypoxia via a proteasome-dependent pathway. We studied the mechanism of ER alpha degradation under hypoxic mimetic conditions. Cobalt chloride-induced ER alpha down-regulation was dependent on the expression of newly synthesized protein(s), one possibility of which was hypoxia-inducible factor-1 alpha (HIF-1 alpha). To examine the role of HIF-1 alpha expression in ER alpha down-regulation under hypoxicmimetic conditions, we used a constitutively active form of HIF-1 alpha, HIF-1 alpha/herpes simplex viral protein 16 (VP16), constructed by replacing the transactivation domain of HIF-1 alpha with that of VP16. Western blot analysis revealed that HIF-1 alpha/VP16 down-regulated ER alpha 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 alpha interacted with HIF-1 alpha physically. These results suggest that ER alpha down-regulation under hypoxia involves protein-protein interactions between the ER alpha and HIF-1 alpha.