Role of PHD2 in cancer: B55alpha as new hub in the understanding of hypoxia driven-tumorigenesis.

In cancer, the response to hypoxia induces dysfunctional and abnormal blood vessels and promotes tumor cell invasiveness and metastasis. The main executors of the hypoxia signaling are the hypoxia-inducible factors HIF-1 and HIF-2, whose stability is regulated by prolyl hydroxylase domain proteins PHD1, PHD2, and PHD3. In line with the important contribution of hypoxia to cancer progression, the expression and function of PHDs have been correlated to disease outcome. It has been demonstrated that knockdown of PHD2 in tumor cells favors growth by inducing NF-κB mediated angiogenesis. Conversely, others showed that PHD2 silencing reduces tumor growth mainly by enhancing the antiproliferative activity of TGFβ. Through Mammalian Protein-Protein Interaction Trap (MAPPIT) we have identified a new interactor of PHD2, B55α, the B subunit of the phosphatase PP2A. B55α positively regulates TGFβ signaling, promoting the activation of Smad proteins, but it is also involved in the control of proliferation, mitotic exit, and survival pathways. Here we show that PHD2 binds B55α, inducing its proteasomal degradation. We demonstrate that this degradation is rescued in a dose dependent manner by increasing doses of DMOG (a chemical inhibitor of prolyl hydroxylases). These data indicate that the degradation of B55α is dependent on PHD2 mediated hydroxylation. On the other hand, B55α is able to decrease the prolyl hydroxylation activity of PHD2. In fact, in vitro silencing of B55α in several tumor cell lines reduces HIF-1 protein levels and activity in a PHD2 dependent-manner. Accordingly, B55α silencing in DLD1 colon carcinoma cells inhibited tumor growth in athymic mice. This oncosuppressive effect was completely rescued by combined silencing of B55α and PHD2. Since DLD1 colon carcinoma cells are resistant to TGFβ stimulation, our findings can be mainly ascribed to negative regulation of PHD2 activity by B55α. This study shows an unprecedented regulation of the prolyl hydroxylase PHD2 through phosphorylation/dephosphorylation pathways, and thus highlights hypoxia-independent mechanisms of control of HIF-1 levels. Overall, B55α represents a new hub to understand the complex role of PHD2 in cancer