Indole-3-carbinol (I3C), a naturally occurring hydrolysis product of glucobrassicin from cruciferous vegetables such as broccoli, cabbage and Brussels sprouts, is an anticancer phytochemical that triggers complementary sets of antiproliferative pathways to induce a cell cycle arrest of estrogen-responsive MCF7 breast cancer cells. I3C strongly downregulated transcript expression of the catalytic subunit of the human telomerase (hTERT) gene, which correlated with the dose-dependent indole-mediated G(1) cell cycle arrest without altering the transcript levels of the RNA template (hTR) for telomerase elongation. Exogenous expression of hTERT driven by a constitutive promoter prevented the I3C-induced cell cycle arrest and rescued the I3C inhibition of telomerase enzymatic activity and activation of cellular senescence. Time course studies showed that I3C downregulated expression of estrogen receptor-alpha (ERα) and cyclin-dependent kinase-6 transcripts levels (which is regulated through the Sp1 transcription factor) prior to the downregulation of hTERT suggesting a mechanistic link. Chromatin immunoprecipitation assays demonstrated that I3C disrupted endogenous interactions of both ERα and Sp1 with an estrogen response element-Sp1 composite element within the hTERT promoter. I3C inhibited 17β-estradiol stimulated hTERT expression and stimulated the production of threonine-phosphorylated Sp1, which inhibits Sp1-DNA interactions. Exogenous expression of both ERα and Sp1, but not either alone, in MCF7 cells blocked the I3C-mediated downregulation of hTERT expression. These results demonstrate that I3C disrupts the combined ERα- and Sp1-driven transcription of hTERT gene expression, which plays a significant role in the I3C-induced cell cycle arrest of human breast cancer cells.
Marconett CN, Sundar SN, Tseng M, Tin AS, Tran KQ, Mahuron KM, Bjeldanes LF, Firestone GL. Indole-3-carbinol downregulation of telomerase gene expression requires the inhibition of estrogen receptor-alpha and Sp1 transcription factor interactions within the hTERT promoter and mediates the G1 cell cycle arrest of human breast cancer cells. Carcinogenesis. 2011 September Department of Molecular and Cell Biology and The Cancer Research Laboratory, University of California, Berkeley, CA 94720-3200, USA.