The Sitcheran Lab
at Texas A&M Health Science Center
Noncanonical NF-κB signaling in brain tumor pathogenesis
The ability of cancer cells to migrate and invade normal, healty tissue is a major contributing factor in patient mortality. We are broadly interested in understanding the molecular regulatory pathways that govern the invasive potential of cancer cells, with a focus on the role of NF-κB signaling and transcription networks.
We have shown that noncanonical NF-κB signaling, mediated by the transcription factors RelB/p52 and the upstream regulatory kinase NIK (also known as MAP3K14), predominates in aggressive, invasive mesenchymal glioma where it promotes migration and invasion through regulation of epithelial-to-mesenchymal transition (EMT) and degradation of the tumor extracellular matrix through regulation of MMP14 activity. Although NIK is thought to be primarily regulated at the post-translational level, we have discovered signal-specific regulation of NIK mRNA levels in response to pro-invasive signals, as well as in actively invading cells. Current studies address the mechanisms by which NIK is regulated at both the transcriptional and post-transcriptional levels.
Our findings suggest that targeting noncanonical NF-κB activation must be considered as part of efficacious for attenuation of tumor cell invasion in a broad range of glioma subtypes.
This work is supported by an NIH R01 award.
Glioma cells with knockdown of RelB by shRNA (shRelB-1, -3) exhibit diminished orthotopic tumor growth compared with shControl cells. Tumors were imaged in vivo using an IVIS Spectrum.
Lower panels depict side-view images of cells invading a 3D collagen matrix. NIK-KO cells (generated by Crispr-Cas9 genome editing) are significantly less invasive compared with wild type (WT) controls.
Super-resolution microscopy was used to visualize NIK protein (green) and mitochondria (red) in WT cells, cells lacking NIK (NIK KO) and NIK KO cells re-expressing NIK (NIK KO + NIK rescue). Overlapping signal is yellow.
Lower panel depicts an immunoblot of mitochondria-enriched (M) and cytosolic (C) cell fractions to examine expression of NIK, mitochondrial Tom20, and cytosolic GAPDH.
Novel functions for NIK in mitochondria
Although the importance of NF-κB-inducing kinase (NIK/MAP3K14) in immunity is well-established, its role in cancer is still emerging. We have identified a novel pool of NIK protein that is localized to mitochondria in several different human cancer cell lines, mouse embryonic fibroblasts, and ex vivo human tumor tissue. Loss of NIK not only abrogates cell invasion, but is also accompanied by diminished invadopodia formation and decreased cell size. Moreover, cells lacking NIK exhibit fused, tangled mitochondria, and the ability of NIK to regulate cell size, invasion and mitochondrial fission does not require its established downstream targets IKKa/b (and consequently, downstream NF-κB activation). These results significantly expand the current dogma that NIK is solely cytosolic, establishing a new paradigm for IKK/NF-κB-independent NIK signaling in mitochondria. Current studies are aimed at identifying novel NIK downstream targets that regulate mitochondrial function.