HEALTH NEWS

Study Title:

Indole-3-Carbinol Blocks Cancer Growth

Study Abstract

Indole-3-carbinol (I3C), a naturally occurring component of Brassica vegetables, such as broccoli, cabbage, and Brussels sprouts, induces a G1 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 ≈12 μM. 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.

From press release:

An anti-cancer compound found in broccoli and cabbage works by lowering the activity of an enzyme associated with rapidly advancing breast cancer, according to a University of California, Berkeley, study appearing Dec 3 in the online early edition of the journal Proceedings of the National Academy of Sciences.

The compound, indole-3-carbinol, is already undergoing clinical trials in humans because it was found to stop the growth of breast and prostate cancer cells in mice.
The new findings are the first to explain how indole-3-carbinol (I3C) stops cell growth, and thus provides the basis for designing improved versions of the chemical that would be more effective as a drug and could work against a broader range of breast as well as prostate tumors.

"I think one of the real uses of this compound and its derivatives is combining it with other kinds of therapies, such as tamoxifen for breast cancer and anti-androgens for prostate cancer," said coauthor Gary Firestone, UC Berkeley professor of molecular and cell biology. "Humans have co-evolved with cruciferous vegetables like broccoli and Brussels sprouts, so this natural source has a lot fewer side effects."

"This is a major breakthrough in trying to understand what the specific targets of these natural products are," said coauthor Leonard Bjeldanes, UC Berkeley professor of toxicology. "The field is awash with different results in various cells, but no real identification of a specific molecular target for these substances. The beauty of identifying the target like this is that it suggests further studies that could augment the activity of this type of molecule and really specify uses for specific cancers."

Firestone, Bjeldanes and their colleagues showed that I3C inhibits the enzyme elastase, which at high levels in breast cancer cells heralds a poor prognosis: decreased response to chemotherapy, reduced response to endocrine treatment and reduced survival rates.

Elastase is an enzyme that shortens a cellular chemical, cyclin E, that is involved in controlling the cell cycle. The shortened version of cyclin E accelerates the cell cycle, making cancer cells proliferate faster. Firestone showed that I3C prevents the elastase shortening of cyclin E, thereby arresting development of breast cancer cells.

For more than 15 years, Firestone, Bjeldanes and their colleagues have studied the anti-cancer benefits of vegetables in the cabbage family that are lumped together in the genus Brassica and, because of their cross-shaped flowers, are often referred to as cruciferous vegetables.

Though the anti-cancer benefits have been recognized since the 1970s, the mechanism is only now being discovered, in part through the work of Firestone, Bjeldanes and their UC Berkeley colleagues.

"We have connected the dots on one extremely important pathway" by which indole-3-carbinol works, Firestone said.

In previous work, they found that indole-3-carbinol interferes with more than cell proliferation. It also disrupts the migration and alters adhesion properties of cancer cells, as well as counteracts the survival ability of cancer cells, all of which are implicated in cancer cell growth. To have such broad downstream effects, I3C must act at the beginning of a major cellular pathway, Firestone said. The newly reported research pins this activity to elastase and its effect on cyclin E.

Bjeldanes noted that I3C is available as a supplement and is a preferred preventative treatment for recurrent respiratory papillomatosis, a condition involving non-malignant tumors of the larynx. Improved versions of the chemical could thus help treat cancers other than those of the breast and prostate.

Graduate student Ida Aronchik and recent Ph.D. recipient Hanh H. Nguyen, along with colleagues in the Firestone and Bjeldanes labs, have already chemically modified I3C and boosted its activity in cell culture by at least a factor of 100. The lab teams currently are probing the elastase structure and how I3C interacts with it to identify the best parts of the I3C molecule to modify.

I3C is only one of many plant-derived chemicals, called phytochemicals, that Firestone is investigating in his laboratory as potential anti-cancer agents. Among them is the anti-malarial drug artemisinin. Last month, the Journal of Biological Chemistry accepted a paper by Firestone and his colleagues showing that artemisinin blocks prostate cancer cell growth by interfering with the same intracellular pathway as does I3C. This pathway involves the transcription factor SP1, which latches onto other genes to boost their activity.

"SP1 could be a generalized target of phytochemicals," Firestone said. "Phytochemicals work because they interact with and inhibit enzymes that control a host of cellular processes, including migration and adhesion."

Study Information

Hanh H. Nguyen, Ida Aronchik, Gloria A. Brar, David H. H. Nguyen, Leonard F. Bjeldanes, and Gary L. Firestone.
The dietary phytochemical indole-3-carbinol is a natural elastase enzymatic inhibitor that disrupts cyclin E protein processing.

2008 December
UC Berkeley.