Involvement of STAT3 Transcription Factor in Disseminated Nasal-Type Natural Killer Cell Lymphomas

Disseminated nasal-type natural killer (NK) cell lymphoma is an aggressive disease with very poor prognosis. The usual chemoresistance of this disease led us to explore the possible role of the transcription factor STAT3 in oncogenesis. For this, we established and characterized a continuous interleukin (IL)2-dependent NK cell line (MEC04) from a patient with a fatal nasal-type NK cell lymphoma. MEC04 cells phenotype was CD3−, CD7+, TCR−, CD16−, CD56+bright, p58−, p70−, NKP−, and NKG2A+, they harbored poor cytotoxic activity against K562 cells, and spontaneously secreted interferon-γ, IL-10 and vascular-endothelium growth factor in vitro, suggesting that malignant cells arise from the CD56+bright regulatory NK cell population. Injection of MEC04 cells to NOD/SCID mice led to a fatal multiorgan infiltration by malignant cells. We show by immunohistochemistry that STAT3 is phosphorylated in Y705 dimerization residue in MEC04 and YT cell lines, and on biopsies from 6/6 patients with nasal-type NK cell lymphoma, suggesting that STAT3 may be oncogenic in this disease. By contrast, Y705 residue was not phosphorylated on 2 patients with hepatosplenic γδT-cell lymphoma. By confocal microscopy, we showed that STAT3 was located in nucleus in MEC04 cells. Y705 STAT3 phosphorylation involved JAK2, since exposure of cells to AG490 inhibitor inhibited Y705 STAT3 phosphorylation, and correlated in a dose-dependent growth inhibition at 24 hours and 48 hours. By using transducible TAT-STAT3b or TAT-STAT3Y705F recombinant proteins (a dominant-negative form of STAT3 [STAT3b isoform], and a STAT3 protein mutated on Y705 residue which prevents STAT3 dimerization [STAT3Y705F]), we were able to demonstrate that inhibition of STAT3 activation increased mortality and decreased proliferation by more than 50 p. cent at 24 hours and 48 hours, which correlated with decreased expression of 2 STAT3 target genes (Bcl-XL and c-Myc). These results suggest that STAT3 protein has a major role in the oncogenic process of nasal-type NK cell lymphomas, and may thus represent a promising therapeutical target.