Tumor necrosis factor-a (TNFa), a pleiotropic cytokine derived from activated macrophages and other cells, plays pivotal roles in some inflammatory, cardiovascular, and systemic diseases. TNFa is a potential anticancer agent. However, TNFa has dual but opposing effects on cell growth/survival: it inhibits growth and induces apoptosis in some cancer cells but stimulates growth and maintains survival in others. By binding to its cell surface receptors (TNFR1/TNFR2), TNJFa elicits several signaling events including activation of the Caspase cascade and NF-KB. Caspase activation or over-expression of a component(s) of this pathway often leads to apoptosis. In contrast, NF-KB activation is critical for promoting cell survival and suppressing apoptosis. Thus, TNFa's output on cell is decided by the balance between NF-KB and Caspase activation. To trigger apoptosis, TNFa needs not only to activate Caspases but also to minimize or even shut down NF-KB activation. Little is known about how TNFa suppresses NF-xB activation while triggering Caspase activation. The role of protein tyrosine phosphorylation in signal transduction has been confirmed for many cytokines but remains unclear for TNFa. The JAK (Janus kinase)-STAT (signal transducer and activator of transcription) pathway is a tyrosine phosphorylation-signaling route used by interferon and other cytokines. Several lines of evidence suggest that JAK/STAT may play an important role in TNFa's signaling events. First, TNF receptors recruit JAK/Stati especially in TNFa-sensitive cells. Second, both Jaki- and Stati-deficient cells become TNFa resistant. Third, TNFR1 signaling adapter TRADD transfection-induced apoptosis is sensitized by co-transfection with Stati. These findings lead to the following working hypothesis: phosphoStati is a signal adapter associated with TNFR1/TRADD complex. It favors TNFa-induced cell death by stabilizing the death signaling complex formation and inhibiting NF-kB activation and its subsequent gene regulation. In the proposed study, the specific aims are designed to exploit this new information as follows: (1) TNFR-mediated JAK/Stat 1 activation and the role of Stati as a signal adapter in death signal transduction will be examined. (2) The interaction between Stati and TRADD in stabilizing death-signaling complex (TNFR1-TRADD-FADD) formation will be analyzed. (3) The inhibitory role of Stati on NF-KB-mediated gene regulation will be further explored. These complimentary approaches will elucidate the fundamental role of Stati in apoptosis induction by TNa and might eventually provide a boost to cancer therapies.
