N-Alkoxy derivatization of indole-3-carbinol increases the efficacy of the G1 cell cycle arrest and of I3C-specific regulation of cell cycle gene transcription and activity in human breast cancer cells.
Indole-3-carbinol (I3C), a naturally occurring component of Brassica vegetables, such as cabbage, broccoli, and Brussels sprouts, induces a G1 cell cycle arrest of human breast cancer cells. Structure-activity relationships of I3C that mediate this anti-proliferative response were investigated using synthetic and natural I3C derivatives that contain substitutions at the indole nitrogen. Nitrogen substitutions included N-alkoxy substituents of one to four carbons in length, which inhibit dehydration and the formation of the reactive indolenine. Analysis of growth and cell cycle arrest of indole-treated human breast cancer cells revealed a striking increase in efficacy of the N-alkoxy I3C derivatives that is significantly enhanced by the presence of increasing carbon lengths of the N-alkoxy substituents. Compared to I3C, the half maximal growth arrest responses occurred at 23-fold lower indole concentration for N-methoxy I3C, 50-fold lower concentration for N-ethoxy I3C, 217-fold lower concentration for N-propoxy I3C, and 470-fold lower concentration for N-butoxy I3C. At these lower concentrations, each of the N-alkoxy substituted compounds induced the characteristic I3C response in that CDK6 gene expression, CDK6 promoter activity, and CDK2 specific enzymatic activity for its retinoblastoma protein substrate were strongly down-regulated. 3-Methoxymethylindole and 3-ethoxymethylindole were approximately as bioactive as I3C, whereas both tryptophol and melatonin failed to induce the cell cycle arrest, showing the importance of the C-3 hydroxy methyl substituent on the indole ring. Taken together, our study establishes the first I3C structure-activity relationship for cytostatic activities, and implicates I3C-based N-alkoxy derivatives as a novel class of potentially more potent experimental therapeutics for breast cancer.
Jump
Sarah M
SM
Department of Molecular and Cell Biology and The Cancer Research Laboratory, University of California at Berkeley, Berkeley, CA 94720-3200, USA.
Kung
Jenny
J
Staub
Richard
R
Kinseth
Matthew A
MA
Cram
Erin J
EJ
Yudina
Larisa N
LN
Preobrazhenskaya
Maria N
MN
Bjeldanes
Leonard F
LF
Firestone
Gary L
GL
D000970:Antineoplastic Agents / Q000494:pharmacology*; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000378:metabolism / Q000473:pathology; D045744:Cell Line, Tumor; D051357:Cyclin-Dependent Kinase 2 / Q000037:antagonists & inhibitors; D051361:Cyclin-Dependent Kinase 6 / Q000037:antagonists & inhibitors / Q000235:genetics; D005260:Female; D016193:G1 Phase / Q000187:drug effects*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects*; D006801:Humans; D007211:Indoles / Q000494:pharmacology*; D011401:Promoter Regions, Genetic; D013329:Structure-Activity Relationship
D000970:Antineoplastic Agents; D007211:Indoles; C016517:indole-3-carbinol; D051357:Cyclin-Dependent Kinase 2; D051361:Cyclin-Dependent Kinase 6