Nup155 is involved in the posttranscriptional regulation of p53 target genes in HCC

The nuclear pore complex (NPC) consists of different subunits (nuclear basket and ring, spoke and cytoplasmic rings and the cytoplasmic filaments) the components of which are 30 specialized proteins called the nucleoporins (Nups). The NPC is embedded in the nuclear envelope and represents the only transport gate between the cytoplasm and the nucleoplasm. Therefore, it obtains a central role with almost all oncogenic and tumorsuppressive signal transduction cascades passing through it. An increasing body of evidence suggests that in addition to their role in transport some Nups can also modulate cancer-relevant pathways on different levels (e.g. transport of transcriptions factors or the export/stability of gene products). In this context we could recently show that Nup98 stabilizes the mRNA of a subset of p53 target genes such as CDKN1A/p21 and thereby shapes the p53-response. A previously performed RNAi screen suggested that also other Nups are linked to the p53 pathway.

In HepG2 cells (p53 wild-type) we investigated the impact of Nup155 knockdown on p53 target gene induction upon drug treatment by using quantitative mass spectrometry (qMS). The depletion of Nup155 resulted in a significantly reduced p21 protein accumulation while the induction of other p53 targets such as bax and 14 – 3-3 sigma remained unaffected. A selective impact of Nup155 knockdown on p21 induction was also observed in Hep3B-4Bv cells and in a cell line expressing p21 from a tet-sensitive cDNA construct (“tet off”) by westernblot analysis. Interestingly, p21 mRNA levels were not significantly affected in the aforementioned cell lines. In addition, cycloheximide chase experiments did not point towards a role of Nup155 in p21 protein stability neither was the p21 mRNA transport affected by Nup155 depletion (tested by subcellular fractionationation followed by qRT-PCR). We hypothesized that Nup155 may play a role in p21 mRNA translation and performed polysome fractionation experiments. The resulting data, indeed, indicate an increased subpolysome/polysome p21 mRNA-ratio in the absence of Nup155. We are currently testing proteins involved in mRNA translation and altered in the above mentioned qMS data set that may mediate the observed effect.

We conclude that Nup155 regulates p21 on a post-transcriptional level. The underlying regulatory mechanism most likely involves mRNA translation and may represent a novel aspect in p53 target gene regulation by NPC components. Since p21 is a key mediator of several p53 responses such as cell cycle arrest and senescence it will also be of considerable interest to investigate how Nup155-dependent p21 regulation affects these outcomes.

References:

[1] Singer, S., Zhao, R. et al. Mol Cell (2012)

Corresponding author: Holzer, Kerstin

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