title: 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.
abstract: 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.
authors.affiliation: Department of Molecular and Cell Biology and The Cancer Research Laboratory, University of California, Berkeley, CA 94720-3200, USA.
authors.forename: Crystal N
authors.initials: CN
authors.lastname: Marconett
authors.forename: Shyam N
authors.initials: SN
authors.lastname: Sundar
authors.forename: Min
authors.initials: M
authors.lastname: Tseng
authors.forename: Antony S
authors.initials: AS
authors.lastname: Tin
authors.forename: Kalvin Q
authors.initials: KQ
authors.lastname: Tran
authors.forename: Kelly M
authors.initials: KM
authors.lastname: Mahuron
authors.forename: Leonard F
authors.initials: LF
authors.lastname: Bjeldanes
authors.forename: Gary L
authors.initials: GL
authors.lastname: Firestone
mesh_terms: D016588:Anticarcinogenic Agents / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000473:pathology*; D045744:Cell Line, Tumor; D051361:Cyclin-Dependent Kinase 6 / Q000235:genetics; D015536:Down-Regulation; D047628:Estrogen Receptor alpha / Q000037:antagonists & inhibitors / Q000235:genetics / Q000502:physiology*; D005260:Female; D016193:G1 Phase / Q000187:drug effects*; D005786:Gene Expression Regulation; D006801:Humans; D007211:Indoles / Q000494:pharmacology*; D010766:Phosphorylation; D011401:Promoter Regions, Genetic; D016329:Sp1 Transcription Factor / Q000037:antagonists & inhibitors / Q000502:physiology*; D019098:Telomerase / Q000235:genetics*
chemical_list: D016588:Anticarcinogenic Agents; C506487:ESR1 protein, human; D047628:Estrogen Receptor alpha; D007211:Indoles; D016329:Sp1 Transcription Factor; C016517:indole-3-carbinol; C495911:CDK6 protein, human; D051361:Cyclin-Dependent Kinase 6; C509186:TERT protein, human; D019098:Telomerase