ERK signaling promotes IKKε expression and oncogenic functions in pancreatic cancer cells in association with TBK1.

A variety of cancers utilize RAS-regulated signaling pathways to promote oncogenic phenotypes. A widely studied example is pancreatic cancer, where mutant KRAS signaling leads to activation of MEK (mitogen-activated protein kinase/extracellular signal-regulated kinase [ERK])-ERK and downstream signaling, which promotes oncogenic mechanisms, including cell proliferation. Importantly, ERK inhibitors have shown efficacy in some cancer clinical trials. Previously, others have studied the effects of the related kinases TANK-binding kinase 1 (TBK1) and inhibitor of nuclear factor kappa-B kinase epsilon (IKKε) in pancreatic cancer where they have been shown to promote cell survival. Here, we show that RAS-mitogen-activated protein kinase signaling promotes expression of IKKε through control of protein stability and not through control of RNA levels. RNA-Seq analysis indicates that TBK1 and IKKε contribute to the expression of a subset of ERK-regulated genes. Potentially related to the effects on IKKε, proteomic analysis reveals that ERK functions to stabilize a relatively large set of proteins independent of RNA regulation. Knockdown of IKKε and TBK1 individually does not affect growth of MIA PaCa-2 pancreatic cancer cells, but dual knockdown significantly inhibits MIA PaCa-2 growth, which is mediated through cell death. Concurrent silencing or inhibition of both TBK1 and IKKε also reduces tumor sphere growth in MIA PaCa-2 cells, correlating with a loss of stemness pathways found with RNA-Seq. The data suggest the importance of regulation of IKKε by ERK in pancreatic cancer cells and of the combined oncogenic activity of TBK1 and IKKε.