PP1A-Mediated Dephosphorylation Positively Regulates YAP2 Activity

Background: The Hippo/MST1 signaling pathway plays an important role in the regulation of cell proliferation and apoptosis. As a major downstream target of the Hippo/MST1 pathway, YAP2 (Yes-associated protein 2) functions as a transcriptional cofactor that has been implicated in many biological processes, including organ size control and cancer development. MST1/Lats kinase inhibits YAP2’s nuclear accumulation and transcriptional activity through inducing the phosphorylation at serine 127 and the sequential association with 14-3-3 proteins. However, the dephosphorylation of YAP2 is not fully appreciated. Methodology/Principal Findings: In the present study, we demonstrate that PP1A (catalytic subunit of protein phosphatase-1) interacts with and dephosphorylates YAP2 in vitro and in vivo, and PP1A-mediated dephosphorylation induces the nuclear accumulation and transcriptional activation of YAP2. Inhibition of PP1 by okadiac acid (OA) increases the phosphorylation at serine 127 and cytoplasmic translocation of YAP2 proteins, thereby mitigating its transcription activity. PP1A expression enhances YAP2’s pro-survival capability and YAP2 knockdown sensitizes ovarian cancer cells to cisplatin treatment. Conclusions/Significance: Our findings define a novel molecular mechanism that YAP2 is positively regulated by PP1mediate

Activation of Cell Cycle Arrest and Apoptosis by the Proto-Oncogene Pim-2

Potent survival effects have been ascribed to the serine/threonine kinase proto-oncogene PIM-2. Elevated levels of PIM-2 are associated with various malignancies. In human cells, a single Pim-2 transcript gives rise mainly to two protein isoforms (34, 41 kDa) that share an identical catalytic site but differ at their N-terminus, due to in-frame alternative translation initiation sites. In this study we observed that the 34 kDa PIM-2 isoform has differential nuclear and cytoplasmic forms in all tested cell lines, suggesting a possible role for the balance between these forms for PIM-2's function. To further study the cellular role of the 34 kDa isoform of PIM-2, an N-terminally HA-tagged form of this isoform was transiently expressed in HeLa cells. Surprisingly, this resulted in increased level of G1 arrested cells, as well as of apoptotic cells. These effects could not be obtained by a Flag-tagged form of the 41 kDa isoform. The G1 arrest and apoptotic effects were associated with an increase in T14/Y15 phosphorylation of CDK2 and proteasom-dependent down-regulation of CDC25A, as well as with up-regulation of p57, E2F-1, and p73. No such effects were obtained upon over-expression of a kinase-dead form of the HA-tagged 34 kDa PIM-2. By either using a dominant negative form of p73, or by over-expressing the 34 kDa PIM-2 in p73-silenced cells, we demonstrated that these effects were p73-dependent. These results demonstrate that while PIM-2 can function as a potent survival factor, it can, under certain circumstances, exhibit pro-apoptotic effects as well

WW domain interactions regulate the Hippo tumor suppressor pathway

The Hippo kinase pathway is emerging as a conserved signaling pathway that is essential for organ growth and tumorigenesis in Drosophila and mammalians. Although the signaling of the core kinases is relatively well understood, less is known about the upstream inputs, downstream outputs and regulation of the whole cascade. Enrichment of the Hippo pathway components with WW domains and their cognate proline-rich interacting motifs provides a versatile platform for further understanding the mechanisms that regulate organ growth and tumorigenesis. Here, we review recently discovered mechanisms of WW domain-mediated interactions that contribute to the regulation of the Hippo signaling pathway in tumorigenesis. We further discuss new insights and future directions on the emerging role of such regulation