Abstract 3880: The natural dietary phytochemical indole-3-carbinole (I3C) sensitizes ovarian cancer cells to the proteasome inhibitor bortezomib through inhibition of cyclin E activity
Authors: Taylor-Harding Barbie , Orsulic Sandra , Miller Carl , Berenson James R. , Karlan Beth , Walsh Christine 
Published: February 2011

Introduction: Cyclin E deregulation appears to be an important event in the pathogenesis of a subset of epithelial ovarian cancers associated with poor outcome. One mechanism of cyclin E deregulation is through enhanced p27 degradation, which eliminates a powerful negative regulator of cyclin E. The proteasome inhibitor bortezomib has been shown to inhibit the growth of both ovarian and colorectal tumor cell lines through upregulation of p27 and induction of apoptosis, giving it a potential therapeutic role in the subset of ovarian cancers that overexpress cyclin E. Studies demonstrate that as many as five low molecular weight (LMW) isoforms of cyclin E exist, while only the 50-kDa cyclin E form is typically expressed in normal tissues. These LMW isoforms are tumor-specific and cause increased cell proliferation, elevated kinase activity and increased clonogenicity. LMW cyclin E isoforms are generated via proteolysis of the normal 50-kDa cyclin E form by elastase, which itself can be selectively inhibited by indole-3-carbinol (I3C), a natural component of Brassica vegetables. I3C exhibits potent anticarcinogenic properties and has recently been shown to shift the stable accumulation of cyclin E from the LMW to 50-kDa cyclin E form. By taking advantage of the specific inhibitory properties of I3C and bortezomib in the processing and potential expression of cyclin E, respectively, we hypothesize that ovarian cancers overexpressing cyclin E may demonstrate an enhanced response to targeted combination therapy with I3C and bortezomib.

Methods: A panel of ovarian cancer cell lines was screened for cyclin E and p27 protein levels through western blotting. Representative cell lines with high and low cyclin E expression were treated with I3C and bortezomib and evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) cell proliferation assay and western blotting analysis.

Results: OVCAR3 was found to express high levels of cyclin E and moderate levels of p27. In contrast, OVCAR5 was found to express low levels of cyclin E and high levels of p27. Treatment of OVCAR3 cells with I3C stabilized the expression of the normal 50-kDa cyclin E form in a dose-dependent manner. The combination of I3C and bortezomib treatment in OVCAR3 and OVCAR5 cells caused a significantly greater degree of cytotoxicity compared to either drug alone. While the effect was seen in both cell lines, inhibition of cell proliferation was more enhanced in the cyclin E overexpressing cell line OVCAR3.

Conclusions: Indole-3-carbinol is a natural dietary substance that sensitizes ovarian cancer cells to the proteasome inhibitor bortezomib through stabilization and inhibition of cyclin E activity.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3880.