Src promotes estrogen-dependent estrogen receptor α proteolysis in human breast cancer

Estrogen drives both transcriptional activation and proteolysis of estrogen receptor alpha (ER alpha; encoded by ESR1). Here we observed variable and overlapping ESR1 mRNA levels in 200 ER alpha-negative and 50 ER alpha-positive primary breast cancers examined, which suggests important posttranscriptional ERa regulation. Our results indicate that Src cooperates with estrogen to activate ER alpha. proteolysis. Inducible Src stimulated ligand-activated ER alpha transcriptional activity and reduced ER alpha t(1/2). Src and ER alpha levels were inversely correlated in primary breast cancers. ER alpha-negative primary breast cancers and cell lines showed increased Src levels and/or activity compared with ERa-positive cancers and cells. ER alpha t(1/2) was reduced in ER alpha-negative cell lines. In both ER alpha-positive and -negative cell tines, both proteasome and Src inhibitors increased ERa levels. Src inhibition impaired ligand-activated ER alpha ubiquitylation and increased ERa levels. Src siRNA impaired ligand-activated ER alpha. loss in BT-20 cells. Pretreatment with Src increased ER alpha ubiquitylation and degradation in vitro. These findings provide what we believe to be a novel link between Src activation and ERa proteolysis and support a model whereby crosstalk between liganded ERa and Src drives ER alpha. transcriptional activity and targets ER alpha. for ubiquitin-dependent proteolysis. Oncogenic Src activation may promote not only proliferation, but also estrogen-activated ER alpha loss in a subset of ERa-negative breast cancers, altering prognosis and response to therapy.