Elastase is the only currently identified target
protein for
indole-3-carbinol (I3C), a naturally occurring hydrolysis product of
glucobrassicin in cruciferous vegetables such as broccoli, cabbage, and Brussels sprouts that induces a cell cycle arrest and apoptosis of human
breast cancer cells. In vitro
elastase enzymatic assays demonstrated that I3C and at lower concentrations its more potent derivative 1-benzyl-indole-3-carbinol (1-benzyl-I3C) act as non-competitive allosteric inhibitors of
elastase activity. Consistent with these results, in silico computational simulations have revealed the first predicted interactions of I3C and 1-benzyl-I3C with the crystal structure of human
neutrophil elastase, and identified a potential binding cluster on an external surface of the
protease outside of the catalytic site that implicates
elastase as a target
protein for both indolecarbinol compounds. The Δ205 carboxyterminal truncation of
elastase, which disrupts the predicted indolecarbinol binding site, is enzymatically active and generates a novel I3C resistant
enzyme. Expression of the wild type and Δ205
elastase in MDA-MB-231 human
breast cancer cells demonstrated that the carboxyterminal domain of
elastase is required for the I3C and 1-benzyl-I3C inhibition of enzymatic activity, accumulation of the unprocessed form of the CD40
elastase substrate (a
tumor necrosis factor receptor family member), disruption of NFκB nuclear localization and transcriptional activity, and induction of a G1 cell cycle arrest. Surprisingly, expression of the Δ205
elastase molecule failed to reverse indolecarbinol stimulated apoptosis, establishing an
elastase-dependent bifurcation point in anti-proliferative signaling that uncouples the cell cycle and apoptotic responses in human
breast cancer cells.