Chu, I; Arnaout, A; Loiseau, S; Sun, J; Seth, A; Mcmahon, C; Chun, K; Hennessy, B; Mills, GB; Nawaz, Z; Slingerland, JM
2205-15. [The Journal of clinical investigation]
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 ER alpha 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 ER alpha-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 lines, both proteasome and Src inhibitors increased ER alpha levels. Src inhibition impaired ligand-activated ER alpha ubiquitylation and increased ER alpha 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 ER alpha proteolysis and support a model whereby crosstalk between liganded ER alpha 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 ER alpha-negative breast cancers, altering prognosis and response to therapy.
Breast Neoplasms/diagnosis/metabolism/pathology/therapy; Cell Line, Tumor; Enzyme Activation/drug effects; Estrogen Receptor alpha/biosynthesis; Estrogens/metabolism/pharmacology; Gene Expression Regulation, Neoplastic/drug effects; Neoplasm Proteins/biosynthesis; Prognosis; Proteasome Endopeptidase Complex/metabolism; Protein Kinase Inhibitors/pharmacology; Protein Processing, Post-Translational/drug effects; RNA, Messenger/biosynthesis; RNA, Neoplasm/biosynthesis; RNA, Small Interfering/pharmacology; Transcription, Genetic/drug effects; Ubiquitin/metabolism; src-Family Kinases/antagonists & inhibitors/metabolism