| The IkappaB kinase beta (IKKbeta) is a critical component of the classical inflammatory signaling pathway that activates nuclear factor kappaB (NF-kappaB) and propagates inflammatory responses. Recent genetic studies in mice suggest that IKKbeta also has physiological roles in the prevention of oxidative stress. The central hypothesis of this thesis is that the IKKbeta-NF-kappaB 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 IKKbeta-NF-kappaB 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 IKKbeta-NF-kappaB pathway reduces the cellular GSH and MT1 levels.;2. What are the consequences of IKKbeta inactivation? We show that the IKKbeta-NF-kappaB 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 IKKbeta 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 IKKbeta-NF-kappaB 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 IKKbeta-NF-kappaB 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 IKKbeta. First, IKKbeta is needed for the optimal activities of the transcription factors NF-kappaB 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 IKKbeta inactive cells leads to severe GSH deficiency. Second, IKKbeta acts as a suppressor of the transforming growth factor beta (TGFbeta) pathway preventing TGFbeta induced GSH depletion.;4. What is the implication of the IKKbeta-NF-kappaB 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 IKKbeta-NF-kappaB pathway, essentially required for maintaining redox capacity, may be useful for developing preventive antidotes for environmental toxicants. On the other hand, a relatively high NF-kappaB activity in cancer cells may lead to high GSH, thereby conferring resistance to chemotherapy. Therefore, inactivation of the IKKbeta-NF-kappaB pathway may potentiate the efficacy of oxidative stress-based anti-cancer therapy. Furthermore, modulation of IKKbeta could affect other signaling networks. Studies presented in this thesis describe comprehensive crosstalk of IKKbeta with the TGFbeta 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... |
