Mutual protection of ribosomal proteins L5 and L11 from degradation is essential for p53 activation upon ribosomal biogenesis stress

Abstract

Impairment of ribosomal biogenesis can activate the p53 protein independently of DNA damage. The ability of ribosomal proteins L5, L11, L23, L26, or S7 to bind Mdm2 and inhibit its ubiquitin ligase activity has been suggested as a critical step in p53 activation under these conditions. Here, we report that L5 and L11 are particularly important for this response. Whereas several other newly synthesized ribosomal proteins are degraded by proteasomes upon inhibition of Pol I activity by actinomycin D, L5 and L11 accumulate in the ribosome-free fraction where they bind to Mdm2. This selective accumulation of free L5 and L11 is due to their mutual protection from proteasomal degradation. Furthermore, the endogenous, newly synthesized L5 and L11 continue to be imported into nucleoli even after nucleolar disruption and colocalize with Mdm2, p53, and promyelocytic leukemia protein. This suggests that the disrupted nucleoli may provide a platform for L5-and L11-dependent p53 activation, implying a role for the nucleolus in p53 activation by ribosomal biogenesis stress. These findings may have important implications with respect to understanding the pathogenesis of diseases caused by impaired ribosome biogenesis.proteasome | ribosomal stress

Molecular dynamics of the full-length p53 monomer

“…[151][152][153] This proteosomal degradation is indeed a powerful therapeutic target. [154][155][156][157][158] The full-length p53 tetramer, bound to DNA, acquires different quaternary conformations 82 where the C-terminal and DBD directly interact; the N-terminal seems to only weakly associate with DBD, and no direct interactions between N-terminal and C-terminal are observed. Additional conformations were detected at the monomeric level.…”

Loss of Tumor Suppressor RPL5/RPL11 Does Not Induce Cell Cycle Arrest but Impedes Proliferation Due to Reduced Ribosome Content and Translation Capacity

“…Recent studies have implicated three additional inhibitory cofactors in addition to ARF that directly bind to and suppress Hdm2-mediated p53 degradation. These include the tumor suppressor NUMB, a negative regulator of Notch 1 (4), and, most recently, two essential 60S ribosomal proteins (RPs), RPL5 and RPL11 (5), which play a central role in mediating p53 stabilization following impaired ribosome biogenesis (6,7).…”

“…These studies led to the finding that the upregulation of p53 upon impairment of the biogenesis of either subunit was mediated by the binding and inhibition of Hdm2 by RPL5 and RPL11 (7,15). The inhibitory effects of RPL5 and RPL11 on Hdm2 are mutually dependent on both proteins as depletion of either was sufficient to relieve p53 induction and cell cycle arrest (6,7). In addition to RPL5 and RPL11, RPS7 and RPL23 have been shown to bind and inhibit Hdm2 under conditions of acute inhibition of rRNA synthesis caused by low doses of actinomycin D (16)(17)(18)(19).…”

mTOR inhibitors blunt the p53 response to nucleolar stress by regulating RPL11 and MDM2 levels

“…It has been suggested that a degradation resistant pool of RPL11 and RPL5 remains stably bound to MDM2. 14 We observed that long term (18 hours) Act D treatment lowered the steady-state levels of detergent soluble RPL11 in U2OS and U343MGa Cl2:6 cell cultures (Figs. 6A and 7A, E, and F).…”

“…[9][10][11][12][13] Stabilization of p53 in response to nucleolar stress requires de novo synthesis of RPL11 and RPL5, and these 2 ribosomal proteins regulate p53 as part of the 5S ribonucleoprotein particle (5S RNP), in which also the 5S rRNA is a critical component. [14][15][16] A number of chemotherapeutic agents trigger nucleolar stress and one example is actinomycin D (Act D), a DNA intercalating molecule and inhibitor of cancer cell proliferation. Low concentrations of Act D (1-10 nM) preferentially inhibit Pol I-mediated transcription of the rRNA genes and activate p53 through the nucleolar stress pathway.…”