Tumor necrosis factor alpha (TNFalpha) is a proinflammatory cytokine produced by activated macrophages as well as by several other cell types. TNFalpha is required for protection against bacterial infection but is also strongly implicated in acute and chronic inflammatory diseases such as septic shock and arthritis. TNFalpha also functions in T cells to elicit death signals for activated mature T cells and to exert costimulatory signals for proliferation and differentiation. The effects of TNFalpha are mediated by two distinct cell surface receptors, p55 TNF-R1 and p75 TNF-R2, that are expressed on almost all cells. Both receptors lack kinase domains and are dependent on their ability to interact with regulatory molecules to transduce intracellular signals. For example, TRADD has been identified as a TNF-R1 associated protein that contacts the TNF-R1 death domain in a TNF-dependent manner. TRADD functions as an adapter to recruit to the receptor other important regulatory molecules such as FADD and TRAF2. TNF signaling leads to the activation of several important regulatory pathways, including: (1) the activation of the transcription factor NF-kappaB, (2) the potential initiation of apoptosis, and (3) the activation of the jun kinase/stress activated protein kinase (JNK/SAPK) pathway. Evidence has been presented that TRAF2 can signal to activate both NF-kappaB and JNK. Although the activation of NF-kappaB has been studied primarily at the level of phosphorylation and degradation of IkappaB, our data and that of others indicate that modification of NF-kappaB subunits are required for transcriptional function. In this regard, we have identified a TNFalpha-induced phosphorylation site on the p65 subunit for which the functional role is not understood. We have also found that TNFalpha and IFNgamma synergize at the level of NF-kappaB activation through the enhanced degradation of IkappaBalpha and the de novo degradation of IkappaBbeta which leads to a persistent activtion of NF-kappaB. The mechanisms involved in this important synergistic pathway are not understood. We and others have found that the activation of NF-kappaB blocks the initiation of apoptosis controlled by TNF. We have identified different potential mechanisms whereby this occurs: the activation of genes which suppress apoptosis and the suppression of persistent activation of the JNK pathway. In this application, we propose to (1) determine the role of the TNFalpha-induced phosphorylation of the p65 subunit relative to control of NF-kappaB activity, (2) determine the mechanism(s) controlling the synergy between TNF and IFNs relative to the activation of NF-kappaB, (3) determine the mechanisms whereby NF-kappaB suppresses TNFalpha-induced apoptosis, and (4) determine how NF-kappaB suppresses persistent, TNFalpha-induced JNK activity. These experiments will provide new insight into mechanisms whereby TNFalpha initiates and controls critical pathways involved in mediating inflammatory, proliferative and apoptotic functions.
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