ER Stress Activates NF-κB by Integrating Functions of Basal IKK Activity, IRE1 and PERK

NF-kappa B, a transcription factor, becomes activated during the Unfolded Protein Response (UPR), an endoplasmic reticulum (ER) stress response pathway. NF-kappa B is normally held inactive by its inhibitor, I kappa B alpha. Multiple cellular pathways activate IKK (I kappa B alpha Kinase) which phosphorylate I kappa B alpha leading to its degradation and NF-kappa B activation. Here, we find that IKK is required for maximum activation of NF-kappa B in response to ER stress. However, unlike canonical NF kappa B activation, IKK activity does not increase during ER stress, but rather the level of basal IKK activity is critical for determining the extent of NF-kappa B activation. Furthermore, a key UPR initiator, IRE1, acts to maintain IKK basal activity through IRE1's kinase, but not RNase, activity. Inputs from IRE1 and IKK, in combination with translation repression by PERK, another UPR initiator, lead to maximal NF-kappa B activation during the UPR. These interdependencies have a significant impact in cancer cells with elevated IKK/NF-kappa B activity such as renal cell carcinoma cells (786-0). Inhibition of IKK by an IKK inhibitor, which significantly decreases NF-kappa B activity, is overridden by UPR induction, arguing for the importance of considering UPR activation in cancer treatment.