Indole-3-carbinol (I3C), a naturally occurring component of Brassica vegetables, such as broccoli, cabbage, and Brussels sprouts, induces a G(1) cell-cycle arrest of human
breast cancer cells, although the direct cellular targets that mediate this process are unknown. Treatment of highly invasive MDA-MB-231
breast cancer cells with I3C shifted the stable accumulation of
cyclin E protein from the hyperactive lower-molecular-mass 35-kDa form that is associated with
cancer cell proliferation and poor clinical outcomes to the 50-kDa
cyclin E form that typically is expressed in normal mammary tissue. An in vitro
cyclin E processing assay, in combination with zymography, demonstrated that I3C, but not its natural dimer,
3,3'-diindolylmethane, disrupts proteolytic processing of the 50-kDa
cyclin E into the lower-molecular-mass forms by direct inhibition of human
neutrophil elastase enzymatic activity. Analysis of
elastase enzyme kinetics using either
cyclin E or N-methoxysuccinyl-
Ala-Ala-Pro-Val-p-nitroanalide as substrates demonstrated that I3C acts as a noncompetitive inhibitor of
elastase activity with an inhibitory constant of approximately 12 microM. Finally,
siRNA ablation of
neutrophil elastase protein production in MDA-MB-231 cells mimicked the I3C-disrupted processing of the 50-kDa
cyclin E protein and the
indole-induced cell-cycle arrest. Taken together, our results demonstrate that
elastase is the first identified specific target
protein for I3C and that the direct I3C inhibition of
elastase enzymatic activity implicates the potential use of this
indole, or related compounds, in targeted
therapies of human breast
cancers where high
elastase levels are correlated with poor prognosis.