The nuclear factor κB inhibitor parthenolide restores ICI 182,780 (Faslodex; fulvestrant)-induced apoptosis in antiestrogen-resistant breast cancer cells

The molecular mechanisms underlying the acquisition of resistance to the antiestrogen Faslodex are poorly understood, although enhanced expression and activity of nuclear factor κB (NFκB) have been implicated as a critical element of this phenotype. The purpose of this study was to elucidate the mechanism by which NFκB up-regulation contributes to Faslodex resistance and to determine whether pharmacologic inhibition of NFκB by the small molecule parthenolide could restore Faslodex-mediated suppression of cell growth. Basal expression of multiple NFκB-related molecules in MCF7-derived LCC1 (antiestrogen-sensitive) and LCC9 (antiestrogen-resistant) breast cancer cells was determined, and cells were treated with Faslodex or parthenolide. The effect of these drugs either singly or in combination was assessed by cell proliferation, estrogen receptor (ER)-dependent transcriptional activation, cell cycle analysis, and apoptosis assays. Expression of the p65 NFκB subunit and the upstream NFκB regulator IκB kinase γ/NFκB essential modulator were increased in the resistant MCF7/LCC9 cells (P = 0.001 and 0.04, respectively). Whereas MCF7/LCC9 cells were unresponsive to Faslodex alone, parthenolide effectively inhibited MCF7/LCC9 cell proliferation and the combination of Faslodex and parthenolide resulted in a 4-fold synergistic reduction in cell growth (P = 0.03). This corresponded to a restoration of Faslodex-induced apoptosis (P = 0.001), with no observable changes in ER-dependent transcription or cell cycle phase distribution. Because parthenolide has shown safety in Phase I clinical trials, these findings have direct clinical relevance and provide support for the design of clinical studies combining antiestrogens and parthenolide in ER-positive breast cancer.