The Role of IκB kinase β in Redox Modulation

The IκB kinase β (IKKβ) is a critical component of the classical inflammatory signaling pathway that activates nuclear factor κB (NF-κB) and propagates inflammatory responses. Recent genetic studies in mice suggest that IKKβ also has physiological roles in the prevention of oxidative stress. The central hypothesis of this thesis is that the IKKβ-NF-κB signal transduction pathway is crucial for maintaining redox homeostasis and preventing excessive reactive oxygen species (ROS). We test this hypothesis by addressing the following pertinent questions. 1. Does the IKKβ-NF-κB pathway regulate redox homeostasis? Redox homeostasis is largely maintained by a group of antioxidant enzymes and small scavenging molecules, such as reduced glutathione (GSH) and metallothionein (MT). Our results show that genetic and pharmacological inactivation of the IKKβ-NF-κB pathway reduces the cellular GSH and MT1 levels. 2. What are the consequences of IKKβ inactivation? We show that the IKKβ-NF-κB pathway is required for protection against oxidative injury caused by a wide variety of oxidative stress inducers, including environmental toxic compounds, such as arsenic, and therapeutic anti-cancer drugs, such as cisplatin and taxol. Under stress conditions, cells deficient in IKKβ display a marked increase in ROS accumulation, which leads to activation of the MKK4-c-JUN N-terminal kinase (JNK) pathway, c-JUN phosphorylation and apoptosis. Hence, the IKKβ-NF-κB pathway maintains ROS scavenging capacity and serves as a critical biological checkpoint for cell fate determination in stress environment. 3. What are the mechanisms for the IKKβ-NF-κB pathway in GSH regulation? It has long been established that GSH is an important defense against the toxicity of oxidants, of which exposure is an everyday occurrence. Our results show that at least two mechanisms are involved in GSH regulation by IKKβ. First, IKKβ is needed for the optimal activities of the transcription factors NF-κB and aryl hydrocarbon receptor (AHR), which coordinately activate the expression of genes encoding the catalytic (GCLC) and modifier (GCLM) subunits of glutathione-cysteine ligase (GCL). Because GCL is the rate limiting enzyme for GSH biosynthesis, its reduced expression in the IKKβ inactive cells leads to severe GSH deficiency. Second, IKKβ acts as a suppressor of the transforming growth factor β (TGFβ) pathway preventing TGFβ induced GSH depletion. 4. What is the implication of the IKKβ-NF-κB pathway in diseases? Many environmental toxic agents, such as arsenic, exert deleterious effects by directly or indirectly altering the cellular redox status. Activation of the IKKβ-NF-κB pathway, essentially required for maintaining redox capacity, may be useful for developing preventive antidotes for environmental toxicants. On the other hand, a relatively high NFκB activity in cancer cells may lead to high GSH, thereby conferring resistance to chemotherapy. Therefore, inactivation of the IKKβ-NF-κB pathway may potentiate the efficacy of oxidative stress-based anti-cancer therapy. Furthermore, modulation of IKKβ could affect other signaling networks. Studies presented in this thesis describe comprehensive crosstalk of IKKβ with the TGFβ and JNK pathways. Such crosstalk may constitute a decision-making process that determines the cell fate that needs to be addressed in the context of specific biological settings.