| Abstract |
ATL is an aggressive T-cell malignancy with a poor prognosis that is caused by HTLV-1 infection. We previously demonstrated that NDRG2 is significantly downregulated in ATL, resulting in aberrant activation of the signal transduction pathways through the dissociation of serine/threonine phosphatase PP2A. 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. KEGG and GO analysis revealed that the downregulated phosphopeptides correlated with signaling pathways, T-cell differentiation, and proliferation. Our results identified STAT5B 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 JAK3, 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 is associated with S193A, suggesting that NDRG2 is directly involved in serine phosphorylation through the recruitment of PP2A to STAT5. Because S193A 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.
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