Understanding the mechanisms supporting tumor-initiating cells (TIC) is vital to combat advanced stage recurrent cancers. Here we show that in advanced ovarian cancers NF-kB signaling via the RelB transcription factor supports TIC populations by directly regulating the cancer stem-like associated enzyme aldehyde dehydrogenase (ALDH). Loss of RelB significantly inhibited spheroid formation, ALDH expression and activity, chemoresistance, and tumorigenesis in subcutaneous and intrabursal mouse xenograft models of human ovarian cancer. RelB also affected expression of the ALDH gene ALDH1A2. Interestingly, classical NF-kB signaling through the RelA transcription factor was equally important for tumorigenesis in the intrabursal model, but had no effect on ALDH. In this case, classical signaling via RelA was essential for proliferating cells, whereas the alternative signaling pathway was not. Our results show how NF-kB sustains diverse cancer phenotypes via distinct classical and alternative signaling pathways, with implications for improved understanding of disease recurrence and therapeutic response.

I-κB kinases (IKKs) are key regulators of NF-κB signaling. Three IKK isoforms – α, β and ε – have been linked to oncogenesis, yet the precise components of NF-κB signaling in ovarian cancer have not yet been dissected. We surveyed 120 ovarian cancer specimens for IKKε expression. Notably, cytoplasmic expression was elevated in metastatic lesions relative to primary tumors (p=0.03). Therefore, we hypothesized that IKKε drives ovarian cancer metastasis. IKKε was identified previously as a breast cancer oncogene and was associated with poor clinical outcome in ovarian cancer. We now define an ovarian cancer-specific IKKε-regulated gene expression signature using stably expressed shRNA targeting IKKε. Pathway analysis of the signature indicated that IKKε regulates expression of genes involved in cell motility and inflammation. We further showed that IKKε depletion in metastatic ovarian cancer cell lines decreased growth, adhesion, and invasion. Consistently, human xenografts depleted of IKKε in mice demonstrated decreased aggressiveness, while overexpression of IKKε in a less invasive ovarian cancer cell line increased metastasis in vivo. Taken together, these data provide evidence that IKKε is a key coordinator of invasion and metastasis programs in ovarian cancer. Inhibition of IKKε signaling thus emerges as a viable therapeutic strategy in women whose ovarian cancer demonstrates aberrant activation of this pathway.

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