Usnic acid is a novel Pim-1 inhibitor with the abilities of inhibiting growth and inducing apoptosis in human myeloid leukemia cells

Usnic acid (UA) is a secondary metabolite of lichens with a unique dibenzofuran scaffold. The growth inhibitory and apoptotic effects of UA as well as the potential mechanisms of action were determined in human acute myeloid leukemia HL-60 cells and chronic myeloid leukemia K562 cells. UA inhibits growth and induces apoptosis in both cell lines with HL-60 cells more responsive. The apoptotic effects are associated with a decrease in the levels of the anti-apoptotic protein Mcl-1. UA inhibits mitogen-activated protein kinase-interacting kinase 1/eukaryotic translation-initiation factor 4E (Mnk1/eIF4E) and the proviral integration site of moloney murine leukemia virus-1/eIF4E-binding protein 1 (Pim-1/4E-BP1) signallings in both cell lines. A kinase inhibition assay reveals that UA is a potent Pim-1 inhibitor with limited activity to inhibit Mnk1. Considering the important role of Pim-1 in myeloid leukemia, UA represents a lead compound for developing effective therapeutics for myeloid leukemia treatment.