Because signal transducer and activator of transcription (STAT) 3 is constitutively phosphorylated in several malignant disorders and plays a role in the pathophysiology of lymphoid malignancies we sought to investigate the activity of STAT3 in CLL. Using Western immunoblotting we studied peripheral blood (PB) cells from 40 patients with CLL and found that in all patients, regardless of disease stage or PB cell count, STAT3 is constitutively phosphorylated (p) on serine 727, but not on tyrosine, residues. Because it is thought that phosphorylation of tyrosine residues is requires for activation of STAT3 we asked whether the serine phosphoryalted form of STAT3 is biologically active in CLL. First we explored whether serine pSTAT3 translocates to the nucleus. Using confocal microscopy and Western blot analysis of cytoplasmic and nuclear cellular fractions we demonstrated that serine pSTAT3 is present both in the cytoplasm and nucleus of CLL cells. Then we asked whether serine pSTAT3 binds to DNA. Using the electrophoretic mobility shift assay (EMSA) we identified both a ‘shift’ and a ‘super-shift’ with anti-STAT3 and anti-serine 727 pSTAT3 antibodies, indicating that serine pSTAT3 binds to DNA. To further elucidate the role of serine pSTAT3 in CLL we inhibited its activity. We used PM, an analog of the STAT3 inhibitor Ac-Tyr(PO3H2)-Leu-Pro-Gly-Thr-Val-NH2 (

Coleman DR et al.,
J Med Chem
). Previous studies demonstrated that PM binds to the SH2 domain of STAT3 (but not to that of STAT5) and specifically inhibits STAT3 activity. Therefore, we incubated CLL cells with increasing concentrations of PM and found that PM inhibited the phosphorylation of STAT3 on serine 727 residues and downregulated Bcl-2 protein levels in a dose-dependent fashion. Because the Bcl-2 gene is activated by STAT3 it is conceivable that serine pSTAT3 activates its transcription in CLL cells. Furthermore, PM suppressed CLL colony-forming cell proliferation and induced apoptosis of CLL cells as determined by Annexin V, suggesting that constitutive serine phosphorylation of STAT3 induces both proliferative and antiapoptotic effects in CLL cells. Taken together, our data suggest that STAT3 is constitutively phosphorylated on serine 727 residues and is biologically active in CLL. Therefore, the activity of STAT3 inhibitors should be investigated in this disease.

Author notes

Disclosure: No relevant conflicts of interest to declare.