1,1-Bis(3'-indolyl)-1-(p-substituted phenyl)methanes containing para-trifluoromethyl (DIM-C-pPhCF(3)), t-butyl (DIM-C-pPhtBu), and phenyl (DIM-C-pPhC(6)H(5)) groups are methylene-substituted diindolylmetyhanes (C-DIMs) that activate peroxisome proliferator-activated receptor gamma (PPARgamma) in estrogen receptor alpha-negative MDA-MB-231 and MDA-MB-453 breast cancer cells. C-DIMs inhibit breast cancer cell proliferation; however, inhibition of G(0)/G(1) to S phase progression and cyclin D1 downregulation was observed in MDA-MB-231 but not MDA-MB-453 cells. Nonsteroidal anti-inflammatory drug-activated gene 1 (NAG-1), a transforming growth factor beta-like peptide, was also induced by these compounds, and the response was dependent on cell-context dependent activation of kinase pathways. However, inhibition of cell growth, induction of NAG-1 and activation of kinases by C-DIMs were not inhibited by PPARgamma antagonists. Despite the induction of NAG-1 and downregulation of the antiapoptotic protein survivin by C-DIMs in both MDA-MB-231 and MDA-MB-453 cells, apoptotic cell death was not observed. Nevertheless, the cytotoxicity of C-DIMs in vitro was complemented by inhibition of tumor growth in athymic nude mice bearing MDA-MB-231 cells as xenografts and treated with DIM-C-pPhC(6)H(5) (40 mg/kg/day). The growth inhibition of tumors derived from highly aggressive MDA-MB-231 cells suggests a potential role for the C-DIM compounds in the clinical treatment of ER-negative breast cancer.