Nucleophosmin Sets a Threshold for p53 Response to UV Radiation

Abstract

Because activation of p53 can trigger cell cycle arrest and apoptosis, it is necessary for a cell to suppress this activation until it is absolutely required for survival. The mechanisms underlying this important regulatory event are poorly understood. Here we show that nucleophosmin (NPM) acts as a natural repressor of p53 by setting a threshold for p53 activation in response to UV radiation. NPM binds to the p53 N terminus and inhibits p53 transcriptional activity by more than 70%. Our data indicate that the levels of NPM in a cell determine the UV dose at which the tumor suppressor p53 can be phosphorylated on Ser15. Moreover, we show that NPM is a substrate for the UV-activated protein kinase ATR and inhibits the UV-induced p53 phosphorylation at Ser15. In addition, NPM forms a complex with p53 and ATR in vivo. These data suggest that NPM is an early responder to DNA damage that prevents premature activation of p53. In normal cells, NPM could contribute to suppressing p53 activation until its functions are absolutely required while in cancer cells overexpression of NPM could contribute to p53 inactivation and tumor progression.Activation of the tumor suppressor p53 in response to DNA damage is an important event that prevents a normal cell from undergoing cellular aberrations that can lead to cancer progression. Because p53 can trigger cell cycle arrest or apoptosis, it is fundamental that the activation of p53 remains in check until it is absolutely required for cellular homeostasis. The restraint on p53 must be sufficient to allow normal growth and development while allowing it to retain the capacity for rapid induction in response to stress associated with tumor progression (28). In recent years, much emphasis has been put on the mechanisms that activate p53 in response to genotoxic stress, but little is known about how p53 functions are kept on hold under normal or low-stress conditions. Here we show data indicating that nucleophosmin (NPM) is a natural repressor of p53 that sets a threshold for p53 response to UV radiation.NPM, also known as B23, NO38, and numatrin (30), is a nucleolar protein that was initially identified as an important player in ribosome biogenesis (5). Since then a number of cellular activities associated with NPM indicate that the protein has multiple functions, especially in cell proliferation. For example, in anaplastic large-cell lymphoma NPM is fused to a receptor tyrosine kinase (anaplastic lymphoma kinase [ALK]) and works as an oncogene (10). NPM protein levels are 20 times higher in Novikoff hepatoma and 5 times higher in hypertrophic rat liver than in normal rat liver (5). NPM binds to pRb and synergistically stimulates DNA polymerase ␣ (25). NPM also binds to interferon regulatory factor 1 (IRF-1) and inhibits its tumor suppression function, probably by preventing expression of p21 (19). Another indication of NPM's role in cell proliferation is its association with the nucleolar organizer regions. The nucleolar organizer regions correlate with cell proliferation and tumor ...

NSC348884, a nucleophosmin inhibitor disrupts oligomer formation and induces apoptosis in human cancer cells

“…Mounting evidence indicate that NPM negatively regulates tumor suppressor p53 through interaction with p53 directly to prevent its phosphorylation on Ser15 (Maiguel et al, 2004) or through binding with p14 ARF to impair the ARF-Hdm2 association and increase Hdm2/p53 complex formation (Gjerset, 2006). NPM knockdown by small interfering RNA enhances p53 phosphorylation and transcriptional activity.…”

“…The structural features of NPM consist of an oligomerization domain, a metal-binding motif, a bipartite nuclear localization signal, phosphorylation sites and a nucleolar localization signal (Wang et al, 1993;Wang et al, 2005). Mounting evidence supports that NPM interacts with a variety of proteins including nucleolin (Li et al, 1996), cell cycle-related protein p120 (Valdez et al, 1994), human immunodeficiency virus (HIV)-1 Rev protein (Fankhauser et al, 1991), Human MDM2 (HDM2) (Kurki et al, 2004), tumor suppressor ARF (Itahana et al, 2003;Bertwistle et al, 2004;Korgaonkar et al, 2005), p53 (Colombo et al, 2002;Li et al, 2004;Maiguel et al, 2004) and pRb (Takemura et al, 1999).…”

“…In addition, NPM is frequently found in chromosomal translocations associated with hematological malignancies (Naoe et al, 2006). The role of NPM as an oncogene is further enhanced as it binds several tumor suppressor genes, including pRb (Takemura et al, 1999), p14 ARF (Brady et al, 2004;Zhang, 2004;Lee et al, 2005;Gjerset, 2006) and p53 (Colombo et al, 2002;Li et al, 2004;Maiguel et al, 2004). The p53/p14 ARF /Mdm2 (Hdm2) stress response pathway plays a central role in mediating cellular responses to oncogene activation, genome instability and therapy-induced DNA damage.…”

“…NPM knockdown markedly enhances ARF-Mdm2 association and diminishes ARF nucleolar localization. In addition, it has been shown that NPM directly interacts with p53 and downregulates its transcriptional activity (Maiguel et al, 2004). NPM binds to the p53 N-terminal end and prevents p53 phosphorylation at Ser15 in response to low doses of UV radiation.…”

Proteome analysis of proliferative response of bystander cells adjacent to cells exposed to ionizing radiation

“…While oxidative stress is well known to be detrimental for normal cell function and can even lead to cell death, very low levels of ROS have been reported to stimulate cell proliferation [35,36]. Notably, NPM/B23 cooperates with p53 in response to DNA damage [37][38][39]. Activation of p53 blocks cellcycle progression to allow time for the repair of DNA damage [40].…”