SRC Regulates Proliferation in ER+ Breast Cancer Cells by Stabilizing MYC mRNA

Abstract

Estrogen (E2) stimulation promotes proliferation in estrogen receptor‐positive breast cancer (ER+ BC), which accounts for 60–75% of all breast cancer. E2 signals through the ER via three pathways: the classical genomic pathway involving transcriptional activation of the ER, crosstalk between ER and other transcription factors, and non‐genomic signaling cascades, which do not involve the transcriptional activity of the ER. The non‐genomic pathway likely involves SRC family kinases (SFKs) and has been implicated in cell cycle downstream of the ER. While not typically amplified or mutationally cancer, SRC is often highly active in most breast cancers.To test whether SRC is required for cell cycle progression in ER+ breast cancer cells, we quiesced ER+ MCF7 or ZR75‐1 cells prior to stimulation with E2 and utilized either a selective SFK inhibitor (SU11333) or shRNA‐mediated knockdown. Inhibition of SFK activity blocked progression from G1 to S phase and proliferation in 2D and 3D assays. We additionally tested the clinically available SRC inhibitor, saracatinib, in an orthotopic xenograft mouse model of ER+ BC and observed decreased tumor growth upon treatment with saracatinib.As MYC is a known target of both SRC and ER and also is known to drive cell cycle progression, we tested whether MYC could be mediating SRC's proliferative effects in this model. Using MYC‐targeting shRNA constructs, we showed that MYC is required for E2‐dependent cell cycle progression and proliferation. Additionally, E2 stimulates MYC mRNA expression dependent upon SRC activity. We also showed that MYC mRNA expression is mediated primarily through SRC‐dependent stabilization of the transcript. We next aimed to understand the mechanism by which non‐genomic ER‐SRC signaling drives proliferation in ER+ BC cells. A known stabilizer of MYC mRNA, the RNA‐binding protein IMP1, has a truncated form (ΔN‐IMP1) expressed in ER+ BC cells. Our data suggests that ΔN‐IMP1 is required for the SRC‐mediated stabilization of MYC mRNA and proliferation. Taken together, these data suggest that SRC mediates cell cycle progression and proliferation through a non‐genomic E2 signaling pathway by stabilizing MYC mRNA.Support or Funding InformationNIH 1F31CA180740 to CANIH 1R21CA177382 to SAC