• Function of TNF Death Receptors in Apoptosis and Cancer Immune Surveillance

      Hu, Xiaolin; Department of Biochemistry and Molecular Biology (2012)
      Tumor necrosis factor receptor (TNFR) is a group of death receptors that can mediate extrinsic apoptosis in target cells upon specific ligand engagement. Members of the TNFR super family play a pivotal role in cytotoxic T cell-mediated immune surveillance and immune regulation. However, cancer cells can acquire apoptosis resistance through down-regulating surface TNFR level or alteration of key mediators in the TNFR signaling pathway so that cancer cells can evade TNFR-mediated and immune cell-based cytotoxicity. Therefore, understanding the underlying mechanism of cancer resistance to TNFR-mediated apoptosis will provide the basis for identifying specific molecular targets and effecient cancer therapy. In this study, we discovered that chronic myelogeneous leukemia (CML) cells use DNA methylation to down-regulate IRF8, a tumor suppressor gene, to acquire apoptosis resistance to Fas-mediated apoptosis. More importantly, we showed that acid ceramidase is a transcription target of IRF8. Restoration of IRF8 or inhibition of acid ceramidase can re-sensitize CML cells to FasL-induced apoptosis in vitro. Furthermore, such manipulation can prolong mouse survival in vivo. While targeting Fas-mediated apoptosis is one strategy to restore immune cell-mediated surveillance, targeting other TNFR members sheds light on monoclonal antibody (mAb) based therapy. We demonstrated that LTβR, as a member of the death receptor super family, could function in cancer immune surveillance through mediating tumor cell apoptosis in sarcoma, colon and mammary carcinoma in a caspase dependent way. Because LTR is selectively up-regulated on cancer cells, it could potentially be a good target for cancer-selective killing. Targeting the TNFR pathway might not only eliminate tumors, but it may also emerge as a promising target to eliminate myeloid derived suppressor cells that accumulate in the peripheral blood under neoplastic conditions. We demonstrated that tumor induced myeloid derived suppressor cells (MDSC) were more resistant to apoptosis than the cells with the same phenotype in tumor free mice. We also identified that up-regulation of Bcl-xL is one of the mechanisms responsible for apoptosis resistance in tumor induced MDSC and that inhibiting Bcl-xL by BH3 mimetics could greately sensitize MDSC to FasL induced apoptosis.
    • Functional Characterization of the P53 Family Protein P63 and the EPHA2 Receptor Tyrosine Kinase, a Novel P53 Family Target Gene

      Dohn, Michael Robert; Department of Biochemistry and Molecular Biology (2001-10)
      A cell’s ability to survive as part of a multicellular organism often depends on how well it communicates with other cells in its surrounding environment. The need for coordinated and complementary cellular activities requires a means o f sending and receiving signals to and from neighboring cells. This is achieved by the presence of an assortment of proteins both linked to and within the plasma membrane. The plasma membrane acts as a barrier between the intracellular and extracellular milieu, and plasma membrane proteins allow for much o f the communication between these two environments. An important class o f proteins found on the plasma membrane is the superfamily of receptor protein tyrosine kinases (RPTKs). RPTKs are membrane-spanning proteins, that contain domains on both sides of the plasma membrane, joined by a single transmembrane domain (1). Binding of a growth factor or cytokine to the extracellular domain of an RPTK results in receptor dimerization and autophosphorylation o f specific tyrosine residues on the cytosolic domains (1). These phosphotyrosine residues then act as docking sites for cytosolic proteins containing modular structures such as Srchomology- 2 (SH2) and phosphotyrosine binding (PTB) domains (2). Binding of SH2- and PTB-containing proteins initiates an intracellular signaling cascade that often results in the regulation of transcription factors in the nucleus (3-5). It is through these RPTKmediated cascades that many extracellular signals are received by a cell.