A Novel Ring-Substituted Diindolylmethane 1,1-bis [3′-(5-methoxyindolyl)]-1-(p-t-butylphenyl) Methane Abrogates ERK Activation and Induces Apoptosis in Acute Myeloid Leukemia (AML).

We investigated the antileukemic activity and molecular mechanisms of action of a newly synthesized ring-substituted diindolylmethane (DIM) derivative, named, 1,1-bis [3′-(5-methoxyindolyl)]-1-(p-t-butylphenyl) methane (DIM #34), in myeloid leukemic cells. DIM #34 inhibited leukemic cell growth via induction of apoptosis. DIM #34 inhibited clonogenic growth and induced apoptosis of AML CD34+ progenitor cells but spared normal progenitors. DIM #34 induced loss of mitochondrial membrane potential, which was accompanied by the release of cytochrome c into the cytosol and early cleavage of caspase-9 followed by the cleavage of caspases -8, and -3. Bcl-2 overexpression and caspase-9-deficient cells were partially protected against DIM #34-induced apoptosis, suggesting activation of the intrinsic apoptotic pathway. DIM #34 induced Bax cleavage, and Bax knockout cells were partially resistant to cell death. Furthermore, DIM #34 transiently inhibited the phosphorylation and the activity of the extracellular-regulated kinase (ERK) and abrogated Bcl-2 phosphorylation. Because other methylene substituted DIM analogs transactivate the nuclear receptor PPARγ, we studied the role of PPARγ in apoptosis induction. Although the co-treatment of cells with a selective PPARγ antagonist T007, and a low dose of DIM #34 partially diminished apoptosis, apoptosis was not inhibited at higher concentrations of DIM #34, suggesting the involvement of both, receptor-dependent and independent mechanisms. Co-treatment with RXR- and RAR-ligands enhanced DIM #34-induced cell death. Together, these findings showed that substituted DIMs represent a new class of compounds that selectively induce apoptosis in AML cells through interference with ERK and activation of PPARγ signaling pathways.