Two transcription factors, NF-κB and AP-1, have emerged in recent years as major targets for signaling pathways that are activated by a variety of proinflammatory stimuli. The same transcription factors were also found to be regulated by oxidative stress, although in this case, we find evidence for both positive and negative regulation (especially for NF-κB). After discussing how each of these transcription factors is regulated by proinflammatory stimuli, their regulation by oxidative stress and pro-oxidants will be addressed. Transcription factor NF-κB is regulated through interaction with inhibitory proteins, the IκBs. In response to cell stimulation with proinflammatory stimuli, the IκBs are rapidly phosphorylated at two conserved serines, followed by subsequent polyubiquitination and degradation through the 26S proteasome. Although many diverse stimuli including TNFα, IL-1, lipopolysaccharide (LPS) and other components of bacterial cell walls, double stranded (ds) RNA, viruses and ionizing radiation activate NF-κB through induction of IκB phosphorylation, they all operate through a common protein kinase-the IκB kinase (IKK). Purification of IKK indicates it is a large complex composed of at least three subunits, IKKα, IKKβ and IKKγ. The first two are closely related catalytic subunits (kinases) and the third is a regulatory subunit.

The integrity of IKKγ is required for the ability of IKK to respond to all the stimuli listed above, IKKα and IKKβ, however, are functionally different despite their close similarity. Only IKKβ is essential for activation of IKK and NF-κB by proinflammatory stimuli. IKKα, on the other hand is dispensable for proinflammatory signaling but is required for control of cell differentiation, especially in the epidermis. Proinflammatory stimuli activate IKK by stimulating the posphorylation of the IKKβ subunit at two serines within its activation loop. In addition to proinflammatory stimuli, NF-κB is believed to be activated by oxidants and pro-oxidants. However, in most cells that we (and others) have examined oxidants and pro-oxidants, such as H2O2 or arsenite (As3+), are not potent NF-κB activators and in fact can prevent its activation by proinflammatory stimuli, such as TNFα or IL-1. In a few cell lines, however, H2O2 exposure does lead to NF-κB activation but two different types of responses were identified. In Rat1 cells, for instance, H2O2 acts via IKK activation, but in Jurkat cells, as shown by others, the activation of NF-κB by H2O2 is IKK independent and seems to be mediated via tyrosine phosphorylation of IκBα. In all cell lines, exposure to As3+ inhibits NF-κB activation. We found that As3+ treatment of cells or purified IKK directly inhibits its kinase activity towards IκBα. We identified a specific cyteine residue in IKKβ that if converted to alanine protects the enzyme from inhibition by As3+. When this mutant is expressed in cells, it prevents the inhibition of NF-κB activity. This cysteine