Nup155 is linked to the p53 pathway in HCC

Virtually all signaling cascades of (liver-)cancer relevant pathways have to pass the nuclear pore complex (NPC) as the only gate between the cytoplasm and the nucleus. The NPC is embedded in the nuclear envelope and consists of approximately 30 different nucleoporins (Nups). We could previously show that Nup98 is linked to the p53 pathway by preventing a subset of p53 target gene-mRNAs from exosomal degradation. Nup98 emerged from a Nup-siRNA screen that indicated pleiotropic effects of a variety of Nups on p53 target gene induction. Here, we investigated the role of Nup155 (that was not included in the aforementioned screen) in modulating the p53 response in HCC cell lines.

Depletion of Nup155 by using two different siRNAs in HepG2 cells (p53 wild-type) was followed by a significantly reduced p21 protein accumulation upon Nutlin treatment. Importantly, Mdm-2 and p53 were not altered by Nup155 knockdown. A selective impact of Nup155 depletion on p21 induction was also observed in Hep3B-4Bv cells harboring a temperature-sensitive mutant version of p53 that acquires wild-type function at 32 ° incubation temperature. Interestingly, p21 mRNA induction was not significantly affected in these cells. Similarly, in a cell line expressing p21 from a tet-sensitive cDNA construct (“tet-off”) Nup155 knockdown again led to a decrease in p21 protein accumulation without affecting p21 mRNA levels. Hypothesizing that Nup155 may be involved in p21 protein stabilization we performed cycloheximide chase experiments. However, these experiments did not indicate obvious changes of p21 protein half-life upon silencing of Nup155. We are currently investigating if Nup155 depletion affects p21 mRNA translation or p21 mRNA export as alternative mechanisms of regulation. To identify other p53 targets with Nup155-dependency we conducted shotgun proteomics in Nutlin-treated HepG2 in the presence or absence of Nup155. Based on this approach we could show that also protein levels of CYFIP2 (cytoplasmic FMR1 interacting protein 2), previously reported to play a role in p53-mediated apoptosis, also significantly decreased upon Nup155 knockdown.

In summary, the data indicate that Nup155 is required for full induction of a subset of p53 targets such as p21 and CYFIP2. The underlying mechanism analyzed so far is post-transcriptional but independent of mRNA stability and thereby different to the mechanism described before for Nup98-dependent p21 regulation. This study further supports the link between NPC components and the p53 pathway.