Ligand-independent activation of oestrogen receptor alpha 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 a lpha) in the nucleus. and potentiates ligand-independent and ligand-depende nt transcription from an oestrogen response element-driven reporter gene. F urthermore. caveolin-1 co-immunoprecipitates with ER alpha [Schlegel, Wang, Katzenellenbogen, Pestell and Lisanti (1999) J. Biol. Chem. 274, 33551-335 56]. In the present study we show that caveolin-1 binds directly to ER alph a. 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-b inding domain of ER alpha includes the ligand-independent transactivation d omain., activation function (AF)- 1, but lacks the hormone-binding domain a nd the ligand-gated transactivation domain, AF-2. In co-transfection studie s. 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 pho sphorylation. Indeed. caveolin-1 interacted with and potentiated the transa ctivation of an ER alpha mutant that cannot be phosphorylated by extracellu lar signal-regulated kinase (ERK)1/2 [ER alpha (Ser(118) --> Ala)]. Thus ca veolin-1 is a novel ER alpha regulator that drives ERK1/2-independent phosp horylation and activation of AF-1.