Inhibition of breast cancer cell growth and induction of cell death by 1,1-bis(3′-indolyl)methane (DIM) and 5,5′-dibromoDIM

1,1-Bis(3′-indolyl)methane (DIM) and the 5,5′-dibromo ring substituted DIM (5,5′-diBrDIM) inhibited growth of MCF-7 and MDA-MB-231 breast cancer cells, and IC₅₀ values were 10-20 and 1-5 μM, respectively, in both cell lines. DIM and 5,5′-diBrDIM did not induce p21 or p27 protein levels or alter expression of Sp1 or Sp3 proteins in either cell line. In contrast, 10 μM 5,5′-diBrDIM downregulated cyclin D1 protein in MCF-7 and MDA-MB-231 cells 12 and 24 h after treatment. DIM (20 μM) also decreased cyclin D1 in MCF-7 (24 h) and MDA-MB-231 (12 h), and the DIM/5,5′-diBrDIM-induced degradation of cyclin D1 was blocked by the proteasome inhibitor MG132. Both DIM and 5,5′-diBrDIM induced apoptosis in MCF-7 cells and this was accompanied by decreased Bcl-2, release of mitochondrial cytochrome c, and decreased mitochondrial membrane potential as determined by the red/green fluorescence of JC-1. DIM and 5,5′-diBrDIM induced extensive necrosis in MDA-MB-231 cells; however, this was accompanied by decreased mitochondrial membrane potential primarily in cells treated with 5,5′-diBrDIM but not DIM. Thus, DIM and 5,5′-diBrDIM induce cell death in MCF-7 and MDA-MB-231 cells by overlapping and different pathways, and the ring-substituted DIM represents a novel class of uncharged mitochondrial poisons that inhibit breast cancer cell and tumor growth.