Impaired ribosome biogenesis checkpoint activation induces p53-dependent MCL-1 degradation and MYC-driven lymphoma death.

MYC-driven B-cell lymphomas are addicted to increased levels of ribosome biogenesis (RiBi), offering the potential for therapeutic intervention. However, it is unclear whether inhibition of RiBi suppresses lymphomagenesis by decreasing translational capacity and/or by p53 activation mediated by the impaired RiBi checkpoint (IRBC). Here we generated Eµ-Myc lymphoma cells expressing inducible short hairpin RNAs to either ribosomal protein L7a (RPL7a) or RPL11, the latter an essential component of the IRBC. The loss of either protein reduced RiBi, protein synthesis, and cell proliferation to similar extents. However, only RPL7a depletion induced p53-mediated apoptosis through the selective proteasomal degradation of antiapoptotic MCL-1, indicating the critical role of the IRBC in this mechanism. Strikingly, low concentrations of the US Food and Drug Administration-approved anticancer RNA polymerase I inhibitor Actinomycin D (ActD) dramatically prolonged the survival of mice harboring Trp53+/+;Eµ-Myc but not Trp53-/-;Eµ-Myc lymphomas, which provides a rationale for treating MYC-driven B-cell lymphomas with ActD. Importantly, the molecular effects of ActD on Eµ-Myc cells were recapitulated in human B-cell lymphoma cell lines, highlighting the potential for ActD as a therapeutic avenue for p53 wild-type lymphoma.

5S ribosomal RNA is an essential component of a nascent ribosomal precursor complex that regulates the Hdm2-p53 checkpoint.

Recently, we demonstrated that RPL5 and RPL11 act in a mutually dependent manner to inhibit Hdm2 and stabilize p53 following impaired ribosome biogenesis. Given that RPL5 and RPL11 form a preribosomal complex with noncoding 5S ribosomal RNA (rRNA) and the three have been implicated in the p53 response, we reasoned they may be part of an Hdm2-inhibitory complex. Here, we show that small interfering RNAs directed against 5S rRNA have no effect on total or nascent levels of the noncoding rRNA, though they prevent the reported Hdm4 inhibition of p53. To achieve efficient inhibition of 5S rRNA synthesis, we targeted TFIIIA, a specific RNA polymerase III cofactor, which, like depletion of either RPL5 or RPL11, did not induce p53. Instead, 5S rRNA acts in a dependent manner with RPL5 and RPL11 to inhibit Hdm2 and stabilize p53. Moreover, depletion of any one of the three components abolished the binding of the other two to Hdm2, explaining their common dependence. Finally, we demonstrate that the RPL5/RPL11/5S rRNA preribosomal complex is redirected from assembly into nascent 60S ribosomes to Hdm2 inhibition as a consequence of impaired ribosome biogenesis. Thus, the activation of the Hdm2-inhibitory complex is not a passive but a regulated event, whose potential role in tumor suppression has been recently noted.

A copia-like retrotransposon gene encoding gypsy-like integrase in a red alga, Porphyra yezoensis.

A genomic PCR fragment of 4581 bp, referred to as PyRE10G, was isolated from the red alga Porphyra yezoensis. PyRE10G contained a putative open reading frame encoding gag, protease, integrase, reverse transcriptase (RT), and RNase H, but one stop codon was present in the integrase region. Southern blot analysis revealed that PyRE10G exists as a single copy in the genome. From the order of gene arrangement of polyproteins, PyRE10G appears to be a copia-like retrotransposon. Amino acid sequences of PyRE10G RT and RNase H were closely related to those of copia-like retrotransposons. In contrast, the phylogenetic tree suggested that PyRE10G integrase stands within the gypsy elements and outside the copia group. PyRE10G is the first example of a chimeric composition of copia- and gypsy-like polyprotein genes in a single element, supporting the hypothesis that long terminal repeat-containing retrotransposons have evolved by fusion of ancestral RT/RNase H and other polyprotein genes.