Nucleocytoplasmic shuttling contributes to the oncogenic role of NPM/B23, a novel, p53-independent ARF binding partner

The a&barbelow;lternative r&barbelow;eading f&barbelow;rame (ARF) tumor suppressor protein plays an important role in the surveillance of human and mouse cancers. While ARF expression is nearly undetectable in normal cells, it is upregulated by tumorigenic stimuli that activate oncogenic signaling pathways, such as those originating from the Myc and Ras oncoproteins. ARF then diverts hyperproliferating cells to undergo p53-dependent cell cycle arrest or apoptosis. This is accomplished through ARF's interaction and nucleolar sequestration of the p53-negative regulator Mdm2. While activation of the ARF-Mdm2-p53 tumor suppressor pathway is one of the cell's major defense mechanisms against cancer induced by oncogenes, tumor profiles of mice lacking ARF, p53, or ARF/p53 provide genetic evidence that ARF can also prevent cell cycle progression via an additional p53-independent pathway.; We identified nucleophosmin (NPM) as a new protein involved in such p53-independent ARF tumor suppressor pathways. NPM is a 38 kDa nucleolar phosphoprotein that shuttles between the nucleolus and the cytoplasm and serves as a marker of proliferation. In response to hyperproliferative signals, ARF is upregulated, resulting in nucleolar retention and subsequent prevention of NPM's transit out of the nucleolus/nucleus. NPM is a positive sensor of mitogenic signals and its interaction with ARF constitutes a novel cell cycle checkpoint. Furthermore, the intrinsic growth-promoting potential of NPM may be regulated by its nucleocytoplasmic shuttling.; Here, I have uncovered a characteristic leucine-rich nuclear export signal (NES) within the amino-terminus of NPM that is critical for its CRM1-dependent nuclear export. Deletion or mutation of this signal blocks not only NPM's own nuclear export, but also the nucleolar-cytoplasmic trafficking of the 5S RNA-binding protein, L5. In addition, transduction of NPM shuttling mutants leads to attenuation of cytoplasmic trafficking of mature 60S ribosome subunits, as well as S phase entry inhibition in both normal and breast carcinoma cells. Furthermore, mutation of the NES within the NPM-ALK fusion protein, the translocation product of anaplastic large cell lymphomas, affects its potential for cell growth and transformation. This thesis project aims to explore the mechanism and significance of NPM trafficking in cellular proliferation and cell cycle progression, processes that are derailed in many cancers.