Ligand-independent activation of oestrogen receptor α by caveolin-1

Expression or caveolin-1 in the human mammary adenocarcinoma cell line MCF-7 causes ligand-independent concentration of oestrogen receptor alpha (ER alpha) in the nucleus. and potentiates ligand-independent and ligand-dependent transcription from an oestrogen response element-driven reporter gene. Furthermore. caveolin-1 co-immunoprecipitates with ER alpha [Schlegel, Wang, Katzenellenbogen, Pestell and Lisanti (1999) J. Biol. Chem. 274, 33551-33556]. In the present study we show that caveolin-1 binds directly to ER alpha. This interaction is mediated by residues 82-101 of caveolin-1 (i.e. the caveolin scaffolding domain) and residues 1-282 of ER alpha. The caveolin-binding domain of ER alpha includes the ligand-independent transactivation domain., activation function (AF)- 1, but lacks the hormone-binding domain and the ligand-gated transactivation domain, AF-2. In co-transfection studies. caveolin-1 potentiates the transcriptional activation of ER alpha (1-282), a truncation mutant that has intact AF-1 and DNA-binding domains. Since AF-1 activity is regulated largely by phosphorylation we determined that co-expression with caveolin-1 increased the basal phosphorylation of ER alpha (1-282). but blocked the epidermal growth factor-dependent increase in phosphorylation. Indeed. caveolin-1 interacted with and potentiated the transactivation of an ER alpha mutant that cannot be phosphorylated by extracellular signal-regulated kinase (ERK)1/2 [ER alpha (Ser(118) --> Ala)]. Thus caveolin-1 is a novel ER alpha regulator that drives ERK1/2-independent phosphorylation and activation of AF-1.