Caveolin-1 down-regulation activates estrogen receptor alpha expression and leads to 17beta-estradiol-stimulated mammary tumorigenesis.

Constitutive activation of estrogen receptor alpha (ER-alpha) expression is an early event in breast cancer tumorigenesis. However, the mechanism whereby ER-alpha is constitutively activated during transformation of normal mammary cells has not been well established. Previously, we reported that haploinsufficiency of caveolin-1, a major structural protein that forms caveolae, resulted in anchorage-independent growth of a normal mammary epithelial cell line, MCF10A. Here, we further demonstrated that ER-alpha but not ER-beta expression was constitutively activated in these caveolin-1 haploinsufficient cells. Transient treatment of MCF10A cells with beta-methyl-cyclodextrin, a chemical that can displace caveolin-1 from the plasma membrane, also stimulated ER-alpha expression. We further found that the 17beta-estradiol (E2) accelerated anchorage-independent growth of these cells in vitro and promoted their tumorigenesis in nude mice. These results suggest that dysregulation of caveolin-1 is one of the mechanisms by which ER-alpha expression is activated during initiation of breast tumorigenesis.