Phosphatase of regenerating liver 3 (PRL3, also known as PTP4A3) is a p53 target gene that can induce cell cycle arrest, which explains the observed downreglation of PRL3 in invasive tumors and metastasis.
DIGITAL VISION
Laura Attardi and colleagues used wild-type and Trp53-null mouse embryonic fibroblasts (MEFs) treated with doxorubicin to identify p53-dependent genes. The microarray data showed that Prl3 was one of several genes induced in an apparently p53-dependent manner, and because of its known association with metastasis the authors chose to study it further. After establishing that Prl3 was induced in response to a number of different p53 stimuli, they examined the sequence of Prl3 and identified two putative p53 binding sites, with one apparently conserved site evident in human PRL3. Electromobility shift assays and chromatin immunoprecipitation were used to establish that p53 is able to bind to this site in mouse Prl3, and luciferase assays confirmed that p53 can activate the transcription of Prl3.
Apoptosis and cell cycle arrest are two possible outcomes of p53 stabilization and activation, and expression of Prl3 in both wild-type and Trp53-null MEFs resulted in G1 arrest rather than cell death. Therefore, PRL3 seems to function downstream of p53, but how does it trigger cell cycle arrest? PRL3 is known to activate the phosphatidylinositol 3-kinase–Akt pathway and the authors found that, although low levels of PRL3 resulted in activation (phosphorylation) of Akt, high levels resulted in reduced levels of phospho-Akt. In agreement with this, PRL3 was unable to induce arrest in Akt1-/-;Akt2-/- MEFs. Moreover, when PRL3 was expressed in a fibrosarcoma cell line, members of the Foxo family of transcription factors that are regulated by Akt were present in the nucleus, where they are able to induce growth arrest by induction of the cyclin-dependent kinase inhibitors p21 and p27. Further analyses indicated that PRL3 was able to induce arrest downstream of the RB-mediated G1 restriction point through a mechanism that requires cyclin-dependent kinase 2.
Paradoxically, when the authors used short hairpin RNAs to knock down expression of Prl3, they did not find cells with increased proliferation; instead these cells underwent cell cycle arrest in a p53-dependent manner. The tumour suppressor ARF is a regulator of the p53 pathway, and reduced expression of PRL3 triggered ARF expression (possibly through a p38 mitogen-activated protein kinase pathway). Interestingly, cell cycle arrest triggered by knockdown of Prl3 required, in part, expression of p21, leading the authors to conclude that basal expression levels of PRL3 are required for cell cycle progression and its absence triggers an ARF–p53–p21-dependent arrest.
But why does PRL3 expression promote metastasis in some tumour cells? The authors argue that tumour cells are likely to have deregulated signalling and cell cycle pathways, and in these cells expression of PRL3 fails to mediate arrest, as the authors found in RKO and U2OS tumour cell lines. How PRL3 then mediates its oncogenic-like functions needs to be clarified.
Nicola McCarthy
References
Basak, S. et al. The metastasis-associated gene Prl-3 is a p53 target involved in cell-cycle regulation. Mol. Cell30, 303–314 (2008) | Article | PubMed |
