Induction of alternative NF-κB within TAg-induced basal mammary tumors in activation-resistant inhibitor of κ-B kinase (IKKα) mutant mice.

Background: The alternative NF-κB pathway is activated by the NF-κB-inducing kinase (NIK) mediated phosphorylation of the inhibitor of κ-B kinase α (IKKα). IKKα then phosphorylates p100/NFKB2 to result in its processing to the active p52 subunit. Evidence suggests that basal breast cancers originate within a subpopulation of luminal progenitor cells which is expanded by signaling to IKKα.

Objective: To determine the role of IKKα in the development of basal tumors.

Methods: Kinase dead IkkαAA/AA mice were crossed with the C3(1)-TAg mouse model of basal mammary cancer. Tumor growth and tumor numbers in WT and IkkαAA/AA mice were assessed and immunopathology, p52 expression and stem/progenitor 3D colony forming assays were performed. Nik-/- mammary glands were isolated and mammary colonies were characterized.

Results: While tumor growth was slower than in WT mice, IkkαAA/AA tumor numbers and pathology were indistinguishable from WT tumors. Both WT and IkkαAA/AA tumors expressed p52 except those IkkαAA/AA tumors where NIK, IKKαAA/AA and ErbB2 were undetectable. Colonies formed by WT and IkkαAA/AA mammary cells were nearly all luminal/acinar however, colony numbers and sizes derived from IkkαAA/AA cells were reduced. In contrast to IkkαAA/AA mice, virgin Nik-/- mammary glands were poorly developed and colonies were primarily derived from undifferentiated bipotent progenitor cells.

Conclusions: C3(1)-TAg induced mammary tumors express p100/p52 even without functional IKKα. Therefore the development of basal-like mammary cancer does not strictly rely on IKKα activation. Signal-induced stabilization of NIK may be sufficient to mediate processing of p100NFKB2 which can then support basal-like mammary tumor formation. Lastly, in contrast to the pregnancy specific role of IKKα in lobuloalveogenesis, NIK is obligatory for normal mammary gland development.