Abstract 1866: Constitutively active NF-κB signaling switches estrogen receptor positive breast cancer cells from a proliferative to an invasive phenotype through luminal/basal plasticity

Estrogen receptor α (ER) is a good prognostic marker expressed in ≈75% of breast tumors. Women with ER+ tumors will receive endocrine therapy, yet ≈50% will experience relapse, often with more aggressive and metastatic tumors. A growing body of evidence indicates that an inflammatory microenvironment and activation of the NF-κB pathway are highly associated with progression of ER+ tumors to more aggressive stages. However, it is not known whether activation of the canonical arm of the NF-κB pathway is sufficient to drive progression to a more aggressive phenotype. In order to study this, we developed MCF-7 and T47D breast cancer cell lines expressing a DOX-inducible, constitutively active form of IκB kinase β (CA-IKKβ), a key kinase in the canonical arm of the NF-κB pathway. Unexpectedly, we found that DOX induced CA-IKKβ inhibited 17β-estradiol (E2)-dependent proliferation. However, the combination of CA-IKKβ and E2 induced a highly migratory and invasive phenotype. Although this phenotype was accompanied by a change in cell morphology, typical markers of epithelial-mesenchymal transition were not observed. Rather, a decrease in luminal markers (e.g. Gata3, cytokeratin 8 and 18) and an increase in basal markers (e.g. p63, cytokeratin 5 and 14) were observed, suggesting luminal to basal plasticity as an underlying mechanism in the phenotype switch. In addition, we find that the invasive phenotype coincides with significantly lower ER expression, which is similar to that observed in aggressive, Luminal B, ER+ breast tumors. However, ER was still transcriptionally active since the combination of CA-IKKβ and E2 recapitulated the regulation of genes from an ER-NF-κB crosstalk signature, which we have previously reported as enriched in Luminal B tumors. In fact, both transcription factors, ER and NF-κB, proved to be necessary for the observed invasive phenotype. In order to explore the clinical relevance of our finding, we examined primary invasive breast tumors for the presence of phosphorylated IκB kinases α/β (IKKα/β), as an indicator of IKK activity. IKKα/β phosphorylation was observed at the invasive edge of the tumors whereas total IKKβ was observed throughout the tumor, further supporting a role for IKKβ activity in breast tumor invasiveness. Taken together, our findings indicate that activation of the canonical arm of the NF-κB pathway in the presence of E2 switches ER+ breast cancer cells from a highly proliferative to a highly migratory and invasive phenotype, potentially through luminal to basal plasticity. Since the most deadly aspect of breast cancer is metastasis and ≈70% of metastatic tumors are ER+, understanding the effect of the canonical NF-κB pathway can help define new specific drug targets and therapeutic strategies for the treatment of ER+ breast cancers. Citation Format: Lamiaa El-Shennawy, Jonna Frasor. Constitutively active NF-κB signaling switches estrogen receptor positive breast cancer cells from a proliferative to an invasive phenotype through luminal/basal plasticity. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1866. doi:10.1158/1538-7445.AM2015-1866