Among several additional regulators implicated in mediating stress signaling via RPs to p53, most data available to date concern MdmX, ARF, and B23/nucleophosmin.
(also called Mdm4) is a protein related to Mdm2 that regulates p53, in part by binding to Mdm2 via the RING domain and stimulating Mdm2’s E3 ubiquitin ligase activity.89,90
Treatment of cells with actinomycin D was reported to induce MdmX degradation, which was attributed to the ability of L11 to increase Mdm2 polyubiquitination of MdmX (), thereby augmenting p53 response.86
Interestingly, L5 and L23 had no effect on MdmX degradation.86
MdmX was also found to cooperate with the small subunit protein S7 in inhibiting Mdm2 activity (), including lower autoubiquitination in vitro
A possible role for MdmX may thus be to regulate activity and substrate specificity of Mdm2, thereby modulating the levels of both Mdm2 and p53. The story becomes more complicated as Mdm2 can also polyubiquitinate S7, targeting it for proteasomal degradation in the cytoplasm (),82
which suggests another potential feedback mechanism affecting p53 stability.
(alternative reading frame; p19ARF
in mouse and p14ARF
in human) is a tumor-suppressor protein that mediates p53 response to oncogene activation, viral infection, and other types of stress mainly by counteracting Mdm2 activity.91
ARF is normally found in the nucleolus, but many aspects of its nucleolar activities are not well understood. Initially, ARF was proposed to sequester Mdm2 to the nucleolus, thus inhibiting p53 degradation.92
Subsequent studies, however, suggested that ARF can interact with Mdm2 in the nucleoplasm and that the nucleolar relocalization of Mdm2 is not required for p53 activation.93,94
Interestingly, p53-independent functions of ARF have been linked to its ability to antagonize another nucleolar target, the SUMO protease Senp3.95
ARF associates with high molecular weight complexes containing RPs and other nucleolar proteins,96
binds to preribosomal particles,97
and was reported to affect rRNA processing.98,99
Although these properties seemingly put ARF in the right place to serve as a modulator of the p53 response to aberrant ribosome synthesis, p53 is activated by nucleolar stress in a variety of cell lines lacking ARF. Moreover, ARF did not compete with, or promote, binding of L5 and L11 to Mdm2, at least in immunoprecipitation experiments with ectopically expressed ARF, even though complexes between Mdm2, ARF, L11,75
can be detected in cells. In agreement with these data, binding sites for ARF, L5, and L11 on Mdm2 do not appear to overlap.75,80
Intriguingly, binding sites for ARF and L23 on Mdm2 do overlap, but the functional significance of this was not addressed.79
One explanation of these data is that ARF and L5/L11 act in separate pathways, which respond to different stimuli and converge only at the point of Mdm2 inactivation. Another nonexclusive possibility is that ARF provides nuances to the p53 response that are only relevant in certain cell types or in the context of an organism.
(also known as nucleophosmin or NPM) is an abundant cellular protein with diverse activities, implicated in the regulation of ribosome biogenesis, embryonic development, maintaining genome stability, and tumorigenesis.100
B23 is present in large quantities in the nucleolus, but it can also shuttle between different cellular compartments.101
B23 associates with many nucleolar proteins including RPs and rRNA maturation factors102
and promotes nuclear export of ribosomes.103
B23 also binds most of the cellular ARF96,104
and is crucial for its stability.105
For its part, overexpressed ARF was reported to facilitate degradation99
and inhibit nucleocytoplasmic shuttling of B23,106
suggesting that ARF may alter the activity of a certain fraction of B23 in a cell. It is tempting to speculate that impaired ribosome assembly increases association of L23 with Mdm2, and this diverts more ARF to B23 complexes, which in turn could affect export of new ribosomes and their function in translation ().