Role of signal transducer and activator of transcription 5 in nucleophosmin/anaplastic lymphoma kinase-mediated malignant transformation of lymphoid cells

The NPM/ALK fusion gene, formed by the t(2;5) translocation in anaplastic l arge-cell lymphoma, encodes a M-r 75,000 hybrid protein that contains the a mino-terminal portion of the nucleolar phosphoprotein nucleophosmin (NPM) j oined to the entire cytoplasmic portion of the receptor tyrosine kinase ana plastic lymphoma kinase (ALK). NPM/ALK encodes a constitutively activated t yrosine kinase that belongs to the family of tyrosine kinases activated by chromosomal translocation. Our studies show that NPM/ALK, similar to other members of this family, activates signal transducer and activator of transc ription 5 (STAT5) and that this activation is essential for lymphomagenesis . NPM/WALK-mediated activation of STAT5 was demonstrated by detection of: ( a) constitutive tyrosine phosphorylation and enhanced DNA binding ability o f STAT5 in NPIWALK-transformed cells; and (b) NPM/WALK-dependent stimulatio n of STAT5-mediated transactivation of the beta -casein promoter. Retrovira l infection of NPM/ALK+ cells with a dominant-negative STAT5B mutant (STAT5 -DNM) inhibited the antiapoptotic activity of NPM/WALK in growth factor and serum-free medium. In addition, STAT5-DNM inhibited proliferation and dimi nished the clonogenic properties of NPM/WALK-positive cells. Finally, SCID mice injected with NPM/ALK+ cells infected with a virus carrying STAT5-DNM survived significantly longer than mice inoculated with NPM/WALK+ cells inf ected with the empty virus. Necropsy identified a widespread ALK+ lymphoma in lymph nodes and liver of the affected animals. Together, our data indica te that NPM/WALK-induced activation of STAT5 may play an important role in NPM/WALK-mediated lymphomagenesis.