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Low Concentrations of Diindolylmethane, a Metabolite of Indole-3-Carbinol, Protect against Oxidative Stress in a BRCA1-Dependent Manner

Departments of ¹ Oncology, ² Biochemistry, and ³ Radiation Medicine, Georgetown University, Washington, District of Columbia and ⁴ Department of Pathology, Barbara Ann Karmanos Cancer Institute, Wayne State University School of Medicine, Detroit, Michigan

Requests for reprints: Eliot M. Rosen, Lombardi Comprehensive Cancer Center, Georgetown University, Preclinical Sciences Building, Room GM12B, 3970 Reservoir Road, Northwest, Washington, DC 20057. Phone: 202-687-7695; Fax: 202-687-7256; E-mail: emr36{at}georgetown.edu.

Key Words: indole-3-carbinol (I3C) • diindolylmethane (DIM) • oxidative stress • BRCA1 • autophagy

The indole-3-carbinol (I3C) metabolite 3,3'-diindolylmethane (DIM) is a proposed cancer prevention agent for various tumor types, including breast cancer. Here, we show that DIM up-regulates expression of the tumor suppressor protein BRCA1 in carcinoma and normal cell types. Up-regulation of BRCA1 was dose and time dependent, and it was observed at physiologically relevant micromolar and submicromolar DIM concentrations when cells were exposed for 72 hours. Treatment with the parent compound (I3C) or DIM (1 µmol/L) protected against cell killing due to H₂O₂ and other oxidants, and the protection was abrogated by knockdown of BRCA1. DIM stimulated signaling by the antioxidant transcription factor NFE2L2 (NRF2) through the antioxidant response element in a BRCA1-dependent manner. We further showed that DIM rapidly stimulated phosphorylation of BRCA1 on Ser ¹³⁸⁷ and Ser ¹⁵²⁴ and that these phosphorylations are required for protection against oxidative stress. DIM-induced phosphorylation of BRCA1 on Ser ¹³⁸⁷ was dependent on ataxia-telangiectasia mutated. Finally, in our assay systems, H₂O₂-induced cell death was not due to apoptosis. However, a significant component of cell death was attributable to autophagy, and both DIM and BRCA1 inhibited H₂O₂-induced autophagy. Our findings suggest that low concentrations of DIM protect cells against oxidative stress via the tumor suppressor BRCA1 by several distinct mechanisms. [Cancer Res 2009;69(15):6083–91]