Abstract P2-05-22: Preferential Activation of BP1 and c-Myc in Breast Cancer of African American Women Compared with Caucasian American Women
Authors: Berg PE , Ghimbovschi S , Grizzle WE , Nguyen TH 
Published: August 2014

Introduction: BP1 is a member of the homeobox gene family of transcription factors. BP1 is activated in 80% of breast tumors, including 89% of the tumors of African American women (AAW) compared with 57% of the tumors of Caucasian American women (CAW). AAW with breast cancer have larger tumors and an almost 50% higher mortality rate than CAW. BP1 expression is also associated with larger and more aggressive tumors, suggesting BP1 may contribute to the aggressiveness of tumors of AAW. Our goal is to identify molecular pathways underlying the discrepancy of breast cancer aggressiveness in AAW and CAW with particular attention to BP1 regulated pathways.

Materials and Methods: Gene expression analysis using RNA from cell lines derived from tumors of AAW or CAW was performed using Illumina microarrays. Changes in c-Myc mRNA were measured by real-time PCR and levels of c-Myc protein by immunoblotting. Knock down of c-Myc and BP1 expression was performed using their respective siRNA. Chromatin immunoprecipation (ChIP) analysis was used to determine potential BP1 and c-Myc binding sites in vivo. Immunohistochemistry was performed using a commercial rabbit polyclonal antibody to c-Myc and a rabbit polyclonal antibody to BP1 developed by the laboratory of one of the authors (PB).

Results: To study differential gene expression related to racial disparities and BP1 expression, we used breast cancer cell lines derived from tumors of AAW and cell lines derived from tumors of CAW. Microarray analysis was performed on three cell lines from AAW and two cell lines from CAW. The oncogene c-Myc was overly represented in the cell lines derived from AAW compared to their CAW derived counterparts; these cell lines also overexpress BP1. The data was verified by real-time PCR and Western blot analysis. Knock down of BP1 and c-Myc in separate experiments using siRNA showed a significant decrease of BP1 protein and c-Myc protein, respectively. ChIP analysis revealed binding of BP1 protein to DNA upstream of the c-Myc gene and of BP1 DNA by c-Myc protein, suggesting reciprocal activation.

Clinical studies. Age and stage matched cases of ductal breast cancers from AAW (n = 15) and CAW (n = 15) were immunostained and evaluated for nuclear c-Myc and nuclear BP1. In CAW, there was a linear correlation between BP1 and c-Myc staining. However, in AAW there was not a linear correlation due to extensive variability in individual results.

Conclusions: c-Myc is a potent oncogene known to increase transformation and proliferation. We show here that c-Myc is upregulated by BP1, a gene which is preferentially activated in breast tumors of AAW. Furthermore, we have identified an interesting mechanism by which BP1 and c-Myc may co-activate transcription involving a positive feedback loop, an attractive therapeutic target. The proposed mechanism could partially explain the aggressiveness of tumors of AAW.

Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P2-05-22.