IKBKE inhibits TSC1 to activate the mTOR/S6K pathway for oncogenic transformation

Abstract

IKBKE (IKKε) has emerged as a key modulator of multiple substrates, controlling oncogenic pathways in various malignancies.mTOR signaling, required for cellular growth, proliferation, and vascular angiogenesis in cancer, is potentially one of the pathwaysregulated by IKKε. Upon activation by various stimuli, PI3K/AKT or similar effectors can relieve the inhibitory effect of the TSC1/TSC2complex through their phosphorylation to favor mTOR/S6K activation in the downstream. Therefore, any activity that interferes withPI3K/AKT or their downstream targets, such as TSC1/2 or GSK3α/β, may activate the mTOR/S6K pathway for oncogenic transformationin normal cells. Previous studies have shown that PI3K/AKT can be directly phosphoregulated by IKKε. Here, we propose a newregulatory function for IKKε in the mTOR/S6K pathway through its direct interaction with TSC1, leading to TSC1 phosphorylation,which is vital to suppress its inhibitory role in mTOR activation. Experimentally, upon IKKε deficiency in colorectal cancer cells, weobserved that S6K activity was diminished while TSC1 levels were found to be stabilized. We hypothesized that these observations mayresult from direct interaction between IKKε and TSC1. Indeed, the interaction of these two proteins involves the phosphoregulation ofTSC1 in various cell lines. Therefore, we propose a mechanism where IKKε, through regulating TSC1 stability in cancer cells, may createan alternative regulatory loop for the activation of mTOR signaling. These results can potentially be important for the development ofnovel therapeutic strategies targeting mTOR signaling.