IL-32θ inhibits monocytic differentiation of leukemia cells by attenuating expression of transcription factor PU.1.

PU.1 is a key transcription factor regulating the myeloid differentiation. PU.1-induced monocytic differentiation into macrophage is also important for blood cancer development. Therefore, we chose THP-1 monocytic leukemia cells to investigate the function of a recently discovered IL-32 theta. Genetic analyses identified differences in the sequences of IL-32 theta and IL-32 beta. Using previously established cell lines that stably express IL-32 theta and IL-32 beta and cell lines transiently expressing IL-32 theta, we observed that expression of IL-32 theta inhibited phorbol 12-myristate 13-acetate (PMA)-induced monocytic differentiation in both THP-1 and HL-60 cells. IL-32 theta also suppressed expression of the macrophage cell surface markers, CD11b, CD18, and CD36. Interestingly, expression of IL-32 beta or IL-32 theta had no effect on the expression levels of cell cycle related factors. As a result, we concluded that these isoforms did not contribute to PMA-induced cell cycle arrest. IL-32 theta was found to modulate expression of PU.1, a transcription factor necessary for myeloid lineage commitment. Transient expression of PU.1 in THP-1/IL-32 theta cells rescued the observed differentiation defect. Additionally, transient expression of both CCAAT-enhancer-binding protein a (C/EBP alpha) and PU.1 in THP-1/IL-32 theta cells exhibited synergistic effects in rescuing the differentiation defect. These observations indicate that intracellular IL-32 theta inhibits the differentiation of monocytes into macrophages by attenuating PU.1 expression.