The proposed research focuses on a novel regulatory mechanism and survival function for human Noxa, a pro- apoptotic 'BH3-only'Bcl-2 family protein, fast emerging as an important therapeutic target in hematological malignancies. Activated Noxa interacts with its binding partner, pro-survival Bcl-2 family member Mcl-1L, to facilitate the release of cytochrome c into the cytosol. Noxa/Mcl-1L interactions play a crucial role in the survival and death of cells of the immune system and Noxa is required for glucose deprivation-induced apoptosis of proliferating human lymphocytes and hematopoietic cancer cell lines. This BH3-only protein is stably and constitutively expressed in a majority of leukemias. Preliminary observations have determined that Noxa is phosphorylated on a serine residue by the kinase, Cdk5, in the presence of nutrients and glucose and have identified two cytosolic multi-protein particles that sequester the modified protein. Studies also suggest that modified, sequestered Noxa regulates glucose metabolism and has a pro-survival function. The hypothesis that human Noxa is post-translationally regulated and that it plays both pro-survival and pro-apoptotic roles in leukemia cells will be tested through three specific aims.
The first aim i s directed at determining how post-translationally modified human Noxa plays a role in aerobic glycolysis and cell survival. The contribution of Cdk5 and the glucose-sensitive PI3K/Akt signaling pathway to this novel metabolic function of Noxa will also be investigated. The cytosolic multi-protein particles containing Noxa will be purified and characterized;their functional relevance and role in regulating Noxa and aerobic glycolysis will be determined.
The second aim i s directed at investigating apoptotic stimuli and signaling pathway/s that lead to the activation of Noxa pro-apoptotic function in leukemia cells. The role of the tumor suppressor phosphatase, PHLPP2, in activating both Noxa and apoptosis in leukemia cells will be determined. Novel strategies that combine specific inhibitors of Cdk5 with inhibitors of glycolysis will be tested on leukemia cell lines and patient samples and later validated in murine xenograft assays. The ability of non- phosphorylatable mutants and peptides of human Noxa to promote apoptosis will also be determined.
The third aim i s directed at developing a conditional transgenic mouse model to investigate the role of human Noxa in leukemogenesis and glucose metabolism. The human BH3-only protein will be expressed in hematopoietic lineage cells, employing the Vav promoter-tetracyline transactivator (Vav-tTA)-driven doxycycline repressible system. The transgenic model will be utilized for determining whether expression of human Noxa promotes leukemogenesis, whether sustained expression of the protein is required for maintaining the leukemic phenotype, and whether constitutive expression of Noxa in lymphocytes alters their ability to metabolize glucose for energy generation. Preliminary studies show that Noxa, a canonical tumor suppressor BH3-only protein, also has a survival function in hematopoietic cancers. The proposed research will identify novel signaling pathways and intermediates involved in regulating Noxa in these dual and opposing roles, and should lead to the development of new therapeutic strategies for targeting this protein in lymphoid and myeloid leukemias.
The proposed research is aimed at investigating a putative pro-survival function for a small pro-apoptotic Bcl-2 family protein, Noxa, in human lymphoid and myeloid malignancies. Understanding the regulatory mechanisms that keep this tumor suppressor protein in check, as well as delineating the signaling pathways that activate its pro- apoptotic function, will aid in the design of novel and rational therapeutic strategies that target this multifunctional protein. The proposed research will use both established cell lines as well as patient cancer cells to carry out the studies. Additionally, a conditional transgenic mouse model will be established to investigate the role of human Noxa in leukemogenesis and glucose consumption. Once established, this mouse model will also be useful in designing and testing of novel therapies aimed at activating the tumor suppressor function of Noxa in leukemias.
