Essential role of ribosomal protein L11 in mediating growth inhibition-induced p53 activation

Abstract

The ribosomal protein L11 binds to and suppresses the E3 ligase function of HDM2, thus activating p53. Despite being abundant as a component of the 60S large ribosomal subunit, L11 does not induce p53 under normal growth conditions. In search of mechanisms controlling L11-HDM2 interaction, we found that the induction of p53 under growth inhibitory conditions, such as low dose of actinomycin D or serum depletion, can be significantly attenuated by knocking down L11, indicating the importance of L11 in mediating these growth inhibitory signals to p53. We show that L11 is not regulated by transcription or protein stability and its level remains relatively constant during serum starvation. However, serum starvation induces translocation of L11 from the nucleolus to the nucleoplasm, where it participates in a complex with HDM2. We propose that the nucleolus acts as a barrier to prevent L11 interacting with HDM2 during normal growth. Growth inhibition, presumably through suppression of rRNA production in the nucleolus, facilitates translocation of L11 to the nucleoplasm, thus activating p53 through inhibiting HDM2.

The balance between rRNA and ribosomal protein synthesis up- and downregulates the tumour suppressor p53 in mammalian cells

“…Analysis of polysomal profiles of both starved and rapamycintreated cells (Figures 3c and g) indicated that the global rRNA transcription modulates p53 G Donati et al level of protein synthesis also was reduced in these conditions, in comparison with control cells. These data were consistent with the absence of p53 stabilisation after serum deprivation observed in TGR-1 cells (Ho¨lzel et al, 2005), but not with the reported activation of p53 in WI-38 cells after serum starvation (Bhat et al, 2004), thus suggesting that serum starvation may induce a cell-specific p53 activation.…”

“…In fact, the large subunit ribosome proteins rpL5, rpL11 and rpL23 were all reported to bind MDM2, thus inducing p53 stabilisation by inhibiting its E3 ubiquitin ligase function (Lohrum et al, 2003;Zhang et al, 2003;Bhat et al, 2004;Jin et al, 2004). The impairment of the nucleolar function has been proposed as a common denominator of many signalling pathways leading both to the suppression of MDM2 function and to p53 stabilisation and activation (Rubbi and Milner, 2003).…”

Ribosome biogenesis surveillance: probing the ribosomal protein-Mdm2-p53 pathway

“…Given the presumed importance of specific RPs to inhibit Mdm2, it would seem logical that depletion of these Mdm2-binding partners would mitigate a p53-dependent response. This seems to be the case for Rpl11 (Bhat et al, 2004), Rpl5 , Rps7 (Zhu et al, 2009) and Rps3 (Yadavilli et al, 2009) where, in the absence of stress, small interfering RNA knockdown had no effect on p53 stability. However, this was not the case for Rpl23, which was not only found to bind to and regulate the Mdm2-p53 interaction, but was also found to cause p53-dependent cell cycle arrest when depleted in cell culture models .…”

“…Nearly a decade later, in screens seeking out novel Mdm2 modulating proteins, the large subunit RPs RPL5, RPL11 and RPL23 were all reported to bind to Mdm2, block the E3 ubiquitin ligase function of Mdm2, and promote p53 accumulation (Lohrum et al, 2003;Zhang et al, 2003;Bhat et al, 2004;Jin et al, 2004). Following these initial reports, additional evidence subsequently was produced to support the roles of RPS7 (Chen et al, 2007;Zhu et al, 2009), RPL26 (Ofir-Rosenfeld et al, 2008 and RPS3 (Yadavilli et al, 2009) as Mdm2-binding partners.…”

“…In part, these observations have led to the hypothesis of the nucleolus as a central stress response regulator for p53 activation (Rubbi and Milner, 2003). One technique that has been used to induce nucleolar stress is inhibition of precursor rRNA synthesis using low doses of actinomycin D (Bhat et al, 2004;Jin et al, 2004), 5-flourouracil (Sun et al, 2007) and mycophenolic acid (Sun et al, 2008). However, with regards to an RP-Mdm2-p53 pathway, it is imperative to identify the natural biological mechanisms of nucleolar stress, particularly as they pertain to ribosome biogenesis.…”