Breast cancers are now subdivided into distinct tumor types based upon molecular classification of tumor gene expression. While most breast cancers are driven by estrogen-mediated signaling (estrogen receptor-positive, (ER+) or Luminal-type breast cancers) or by addiction to the HER2 oncogene (HER2+ breast cancer), the essential cellular signals that control the growth of ER-negative and HER2-negative (or basal-type) breast cancers (approximately 15% of all breast cancers), are incompletely understood. Patients with these ER− and HER2− breast cancers have poor outcomes and few available treatment options other than non-specific chemotherapy. To identify novel targets for the treatment and prevention of basal breast cancers, we investigated the activity of inflammatory genes in these cancers, previously linked to the progression of several other types of cancer. To identify critical inflammatory genes in basal breast cancer, we performed univariate t-tests (FDR<.05) on RNA profiles from the Baylor College of Medicine tumor bank (N=106) to identify inflammatory genes (from 226 genes in the KEGG Cytokine-Cytokine Receptor set) that are differentially expressed in ER-negative breast cancers as compared to ER+ or luminal breast cancers. Combining this analysis with analyses of KEGG cytokine pathway genes in the Richardson (GSE5460, N=129), Bild (GSE3143, N=158), Wang (GSE2034, N=287), Børrensen-Dale (GSE19783, N=115), and Chin Datasets (E-TABM-158, N=119), obtained from the Gene Expression Omnibus and ArrayExpress Repositories, we identified a consensus set of 58 inflammatory genes that are differentially expressed in ER-negative breast cancers as compared to ER+ or luminal breast cancers (genes were considered statistically significant if their p-value was less than 0.05 and displayed fold-differences of >1.5 or < 0.7 in at least two of the datasets analyzed). Further analysis of inflammatory gene expression in breast cancer cell lines revealed that 24 of these genes were highly expressed in basal-like cell lines at the RNA and protein level, as compared to luminal-like ER+ breast cancer lines (showing fold-differences of >1.5 or <0.7). To investigate the role these differentially-expressed genes play in regulating cell growth and transformation, we performed shRNA knock-down studies for 18 of these genes. These studies demonstrated that knockdown of many of these inflammatory genes (including IL-6, IL-8, CCL5, BMP2, TNFRSF21) reduced anchorage-independent growth of basal-like breast cancer cell lines. Further investigation of two cytokine genes, IL-6 and IL-8, revealed that autocrine tumor cell production of these cytokines was critical for maintenance of the transformed state of multiple basal-like cancer lines. We also found that combined inhibition of IL-6 and IL-8 using Tet-induced shRNA suppressed anchorage-independent growth more than either alone, suggesting that these cytokines have non-redundant roles in regulating autocrine-mediated cell growth. These findings suggest that these specific inflammatory pathways regulate the growth and maintenance of the transformed phenotype of basal breast cancers. These results suggest that combined inhibition of both IL-6 and IL-8 inflammatory pathways represents a promising strategy to treat or prevent basal breast cancers. These studies were supported by a Susan G. Komen Promise Grant (to PHB and GM).
Citation Information: Cancer Prev Res 2011;4(10 Suppl):A42.