Abstract #823: NFkB inhibition restores 4-Hydroxy Tamoxifen- induced cell death and sensitivity through a caspase dependent intrinsic apoptotic cell death pathway involving Caspase 8 and Bcl-2 In antiestrogen-resistant MCF-7 breast cancer cells

Breast tumors are initially dependent on estrogen for growth. Both steroidal (e.g., ICI 182,780; Faslodex) and nonsteroidal (e.g., Tamoxifen) antiestrogens can inhibit estrogenic effects in breast tissues. However, acquired drug resistance is a major limitation to their effectiveness in breast cancer. Precise mechanisms that contribute to acquired resistance remain undefined. NF\#954;B, a transcription factor, may play an important role in breast cancer and specifically in antiestrogen resistance. Thus, it is important to establish the functional role of increased NF\#954;B activity in the antiestrogen resistant phenotype, and whether interfering with NF\#954;B activity might provide a means to improve existing antiestrogen therapy. We have previously reported that p65 NF\#954;B expression and activity is elevated in MCF7/LCC9 breast cancer cells that have acquired resistance to 4-hydroxy tamoxifen (4HT) and ICI 182,780 (ICI; FAS). We hypothesize that upregulation of NF\#954;B\#8217;s transcriptional activity contributes to an antiestrogen resistant phenotype, whereas a reduced activation would provide a means to overcome resistance and re-establish sensitivity to antiestrogens. We have previously reported that NF\#954;B inhibition by the small molecule inhibitor parthenolide restores sensitivity to the antiestrogen FAS. Most recently, we have found that parthenolide as well as a dominant negative NFkB inhibitor (IkBSR) are both very effective in downregulating NFkB\#8217;s activity and restoring sensitivity to the antiestrogen Tamoxifen. Specifically, while LCC9 breast cancer cells are unresponsive to 4HT alone they are sensitive to parthenolide, and a combination of 4HT and parthenolide results in significant sensitization of LCC9 cells to 4HT. Restoration of sensitivity to 4HT is at least in part achieved by activating the intrinsic apoptotic pathway leading to permeabilization of the outer mitochondrial membrane in the LCC9 cells. Interestingly, parthenolide and 4HT do synergize to decrease expression of the anti-apoptotic protein Bcl-2, thereby restoring 4HT-induced apoptosis in LCC9 cells. Our data further strongly implicate caspases acting upstream of the mitochondria because the effects of treatment on mitochondrial membrane permeability (MMP) were fully reversed with the pancaspase inhibitor Taken together our findings suggest that NFkB plays a critical role in development of antiestrogen resistant phenotype and NFkB inhibition by both pharmacological (parthenolide) and molecular (IkBSR) approaches effectively restore 4HT induced cell death and sensitivity to 4HT in previously resistant LCC9 cells through a caspase dependent intrinsic apoptotic pathway. These findings provide support for designing clinical trials to combine parthenolide and antiestrogens in patients. Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 823.