Expression of caveolin-1 in the human mammary adenocarcinoma cell line MCF-7 causes ligand-independent concentration of oestrogen receptor α (ERα) 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 ERα [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 ERα. This interaction is mediated by residues 82–101 of caveolin-1 (i.e. the caveolin scaffolding domain) and residues 1–282 of ERα. The caveolin-binding domain of ERα 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 ERα(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 ERα(1–282), but blocked the epidermal growth factor-dependent increase in phosphorylation. Indeed, caveolin-1 interacted with and potentiated the transactivation of an ERα mutant that cannot be phosphorylated by extracellular signal-regulated kinase (ERK)1/2 [ERα(Ser118 → Ala)]. Thus caveolin-1 is a novel ERα regulator that drives ERK1/2-independent phosphorylation and activation of AF-1.
Abbreviations used: AF, activation function; AR, androgen receptor; ERα, oestrogen receptor α; C/D FBS, charcoal/dextran-stripped fetal bovine serum; GFP, green fluorescent protein; GST, glutathione S-transferase; ERK, extracellular signal-regulated kinase; MAP, mitogen-activated protein; EGF, epidermal growth factor; DMEM, Dulbecco's modified Eagle's medium; mAb, monoclonal antibody.