Abstract
Blockade of epidermal growth factor receptor (EGFR) by EGFR tyrosine kinase inhibitors is insufficient for effective antitumor activity because of independently activated survival pathways. A multitargeted approach may therefore improve the outcome of anti-EGFR therapies. In the present study, we determined the effects of 3,3'-diindolylmethane (Bioresponse BR-DIM referred to as B-DIM), a formulated DIM with greater bioavailability on cell viability and apoptosis with erlotinib in vitro and in vivo using an orthotopic animal tumor model. BxPC-3 and MIAPaCa cells with varying levels of EGFR and nuclear factor-kappaB (NF-kappaB) DNA-binding activity were treated with B-DIM (20 micromol/L), erlotinib (2 micromol/L), and the combination. Cell survival and apoptosis was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and histone-DNA ELISA. Electrophoretic mobility shift assay was used to evaluate NF-kappaB DNA-binding activity. We found significant reduction in cell viability by both 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and clonogenic assays, induction of apoptosis, down-regulation of EGFR phosphorylation, NF-kappaB DNA-binding activity, and expression of antiapoptotic genes in BxPC-3 cells when treated with the combination of erlotinib and B-DIM compared with either agent alone. In contrast, no such effect was observed in MIAPaCa cells by similar treatment. Most importantly, these in vitro results were recapitulated in animal model showing that B-DIM in combination with erlotinib was much more effective as an antitumor agent compared with either agent alone. These results suggest that the utilization of B-DIM could be a useful strategy for achieving better treatment outcome in patients with activated status of EGFR and NF-kappaB in their tumors.