Overexpression of IL-9 induced by STAT3 phosphorylation is mediated by miR-155 and miR-21 in chronic lymphocytic leukemia

Interleukin-9 (IL-9) can function as both a positive and negative regulator of immune response, however the role of IL-9 in tumor immunity is poorly understood. Chronic lymphocytic leukemia (CLL) is the most common chronic lympho-proliferative disorder. Twenty CLL patients from 2010 to 2011 were recruited in the study. Expression and phosphorylation of transcription factor STAT3 and differential microRNAs (miRs) in peripheral blood mononuclear cells (PBMCs) from CLL patient samples were analyzed. In a previous study, we found a high level of IL-9 in CLL patients. Concomitantly, overexpression of pSTAT3, miR-155, and miR-21 were observed in PBMCs from CLL patients in the present study. To elucidate whether there was interaction among IL-9, STAT3, miR-155, and miR-21, MEC-1 cells were used for further study. Our results revealed that there was no detectable IL-9 in the culture medium of MEC-1 cells. However, the IL-9 protein could be detected using western blotting in MEC-1 cells. Notably, when recombinant human IL-9 (rIL-9) was added to the medium of culturing MEC-1 cells, the expression levels of pSTAT3 and IL-9 in MEC-1 cells were increased in a time-dependent manner, which could be blocked by STAT3 inhibitor. Both miR-155 and miR-21 could increase IL-9 expression, which could also be suppressed by the inhibitor of STAT3. Our data indicated that the existence of the novel 'extracellular IL-9/pSTAT3/miR-155/miR-21/intracellular IL-9' positive feedback system in CLL cells, provides a novel insight in the pathogenesis and possible therapeutic strategy of CLL.