The tumor suppressor NDRG2 recruits protein phosphatase 2A to suppress STAT5 phosphorylation in adult T-cell leukemia/lymphoma.

Adult T-cell leukemia/lymphoma (ATL) is an aggressive T-cell malignancy with a poor prognosis that is caused by human T-cell leukemia virus type 1 infection. We previously demonstrated that N-myc downstream-regulated gene 2 (NDRG2) is significantly downregulated in ATL, resulting in aberrant activation of the signal transduction pathways through the dissociation of serine/threonine protein phosphatase 2A. To identify potential targets of NDRG2, we performed comprehensive mass spectrometry of differentially phosphorylated peptides in ATL cells with overexpression of NDRG2 using a TiO2-based enrichment method. Kyoto Encyclopedia of Genes and Genomes and gene ontology analysis revealed that the downregulated phosphopeptides correlated with signaling pathways, T-cell differentiation, and proliferation. Our results identified signal transducer and activator of transcription 5B as a novel NDRG2-regulated protein that is dephosphorylated at serine 193 and tyrosine 699. Although enforced expression of NDRG2 in ATL cell lines does not change the phosphorylation of Janus kinase 3, an upstream regulator of STAT5, phosphorylated STAT5 at tyrosine and serine is significantly suppressed by the direct binding to STAT5 with NDRG2 leading to the inhibition of STAT5 downstream gene expression. Furthermore, NDRG2 binds to STAT5B with alanine replacement of Y699 (Y699A), but only weakly associates with S193A, suggesting that NDRG2 is directly involved in serine phosphorylation through the recruitment of serine/threonine protein phosphatase 2A to STAT5. Because S193 A remarkably induces reduced phosphorylation of Y699 and subsequent transcriptional activity, the induction of serine phosphorylation through the loss of NDRG2 expression is dispensable for STAT5 tyrosine phosphorylation and activity. Since the loss of NDRG2 expression is essential factor to maintenance of ATL cells by STAT5 activity through phosphorylation of serine and tyrosine, targeting STAT5 becomes a feasible and effective strategy in NDRG2-deficient ATL.