BRCA1 negatively regulates IGF-1 expression through an estrogen-responsive element-like site

The insulin-like growth factor-1 receptor (IGF-1R) signaling pathway is critical for both normal mammary gland development and malignant transformation. It has been reported that the IGF-1 stimulates breast cancer cell proliferation and is upregulated in tumors with BRCA1/2 mutations. We report here that IGF-1 is negatively regulated by BRCA1 at the transcriptional level in human breast cancer cells. BRCA1 knockdown (BRCA1-KD) induces the expression of IGF-1 mRNA in MCF7 cells in an estrogen receptor α (ERα)-dependent manner. We found that both BRCA1 and ERα bind to the endogenous IGF-1 promoter region containing an estrogen-responsive element-like (EREL) site. BRCA1-KD does not significantly affect ERα binding on the IGF-1 promoter. Reporter analysis demonstrates that BRCA1 could regulate IGF-1 transcripts via this EREL site. In addition, enzyme-linked immunosorbent assay revealed that de-repression of IGF-1 transcription by BRCA1-KD increases the level of extracellular IGF-1 protein, and secreted IGF-1 seems to increase the phospho-IGF-1Rβ and activate its downstream signaling pathway. Blocking the IGF-1/IGF-1R/phosphoinositide 3-kinase (PI3K)/AKT pathway either by a neutralizing antibody or by small-molecule inhibitors preferentially reduces the proliferation of BRCA1-KD cells. Furthermore, the IGF-1-EREL-Luc reporter assay demonstrates that various inhibitors, which can inhibit the IGF-1R pathway, can suppress this reporter activity. These findings suggest that BRCA1 defectiveness keeps turning on IGF-1/PI3K/AKT signaling, which significantly contributes to increase cell survival and proliferation.

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