NF-κB Activation in Breast Cancer Occurs Via Different Mechanisms in Triple Negative and HER2+ Tumors with Lymphocytic Infiltrate.

Abstract

Abstract NF-κB transcription factors regulate expression of genes involved in immune response, inflammation, apoptosis and cancer. NF-κB is known to be regulated via two main mechanisms: (i) through the IκB kinase complex (via the canonical pathway), and (ii) through the IKKα and NIK kinases (via the alternative pathway). Recently, NF-κB has also been shown to be activated by the IKBKE kinase. In breast cancer the level of NF-κB was found elevated only in triple negative (basal) and in HER2+ tumors. Inhibition of NF-κB has been shown to block cell proliferation and tumor formation, and therefore, genes that activate NF-κB might be potential drug targets in the treatment of cancer. Our study aims to identify the mechanisms of activation of NF-κB in various breast cancer subtypes.We analyzed several breast cancer gene expression microarray datasets from public repositories (GSE 2034, 4922, 7390) and used our molecular classification of breast cancer proposed in (Can Res 2007 67: 10669-76) to identify the subtypes of breast cancer in which the NF-κB pathway is altered. The NF-κB activity was evaluated based on upstream and downstream events for the canonical and the alternative pathways, and quantified through a consensus of geneset enrichment scores. We found that the NF-κB downstream genes are upregulated only in triple negative (subtype basal BA) and Her2+ tumors having an immune signature provided by the presence of a lymphocytic infiltrate (subtype Her2I).In order to identify the pathways involved in NF-κB activation, we analyzed the expression of the upstream genes in the canonical, alternative, IKBKE and PI3K/AKT pathways. We found that AKT1 -- an AKT isoform that activates IKKα -- is overexpressed in Her2I, while other AKT isoforms, AKT2 and AKT3, are overexpressed in BA. PI3K is overexpressed in BA, but not in Her2I. We also found that PDK1, which phosphorylates and activates AKT, is overexpressed in Her2I but not in BA. TNFα -- an activator of the NF-κB canonical pathway -- was found overexpressed in BA. However, in BA the components of the IKK complex have low expression apart from IKKγ. IKBKE was found upregulated in both BA and Her2I subtypes.In Her2I, the absence of high expression of TRAF2/5, which recruits the IKK complex, and also the low expression of IKKβ, indicates that NF-κB is not activated by the canonical pathway. Combined with the overexpression of the NIK gene in Her2I, this leads to the hypothesis that in Her2I, NF-κB is being activated by IKBKE and the alternative pathway.In BA, the overexpression of TRAF3 -- an inhibitor of the alternative pathway, suggests that the activation of NF-κB may also occur via the canonical pathway.In summary, we hypothesize that the activation of NF-κB in Her2I tumors is initiated by the IKBKE oncogene and the alternative pathway, while the activation of NF-κB in BA tumors is initiated by IKBKE and the canonical pathway. Boehm et al (Cell, 2007) shown that NF-κB can be inhibited by simultaneously targeting AKT and IKBKE. Combined with our findings, this raises the interesting possibility of developing a targeted therapy for the BA and Her2I breast tumors. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 1164.