The Effects of Indirubin-3′-Monoxime, A Novel AHR Ligand, on Stress and Toxicity-Related Gene/Protein Expression in Human U937 Cells Undergoing Differentiation and Activation

Indirubin-3-monoxime is an aryl hydrocarbon receptor (AhR)-binding, idole-derived compound known to be a potent inducer of CYP1A1, at least in hepatic cell lines. To date, very little is known about the effects of indirubin-3-monoxime on cells relevant to the immune system. We treated human U937 histiocytic lymphoma cells with indirubin-3′-monoxime in four different regimens, representing various stages of PMA-induced differentiation and further activation with LPS. Gene expression profiles were monitored using commercial macroarrays for 96 select stress and toxicity genes. The four treatment regimens were (A) the monocyte model: U937 cells were treated with 1 μM indirubin-3′-monoxime or carrier control for 24 hrs; (B) the differentiated model: PMA-differentiated U937 cells were treated with 1 μM indirubin-3′-monoxime or carrier control for 24 hrs; (C) the differentiation process model: U937 cells were treated for 24 hours with 1 μM indirubin-3′-monoxime or carrier control, followed by PMA-induced differentiation over a 24-hr period; and (D) the LPS-activated model: PMA-differentiated and LPS-activated U937 cells were treated with 1 μM indirubin-3′-monoxime or carrier control for 24 hrs. Indirubin-3′-monoxime treatment caused a 12.7-, 2.1-, 3.2-, and 4.5-fold increases in CYP1A1 in regimes A, B, C, and D, respectively. Indirubin-3′-monoxime treatment also enhanced cyclo-oxygenase 2 expression in PMA-differentiated cells, but reduced the expression of macrophage inflammatory protein, catalase, heme oxygenase-1, glutathione peroxidase, and manganese superoxide dismutase; a scenario consistent with oxidative stress. Macroarray validity was confirmed by RT-PCR using CYP1A1, COX-2, MIP-β, and GADPH as target genes, and at the protein level for CYP1A1, COX-2, and β-actin. To our knowledge, this is the first study to examine the effects of indirubin-3-monoxime in a human cell line relevant to the immune system. Overall, our study suggests that indirubin-3′-monoxime is either a classical AhR ligand, or that it modulates endogenous AhR-regulated activity.