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We have recently demonstrated that activated nuclear factor-κB (NFκB) identifies a high-risk subset of hormone-dependent (ER-positive) primary breast cancers destined for earlier metastatic relapse despite adjuvant tamoxifen (TAM) therapy. To explore the endocrine-modulating role of NFκB and improve the endocrine responsiveness of such high-risk hormone-dependent breast cancers, we compared ER-positive, TAM-sensitive MCF7 with ER-positive, ErbB2 overexpressing and TAM-resistant MCF7/HER2 and BT474 breast cancer cell lines. Activated nuclear levels of NFκB were measured by an ELISA-based (TransAM, ActiveMotif) assay quantitating p50 and p65 subunit DNA-binding; MCF7/HER2 and BT474 cells showed 2-to-3 fold increased NFκB p50 levels relative to MCF7 cells. The specific IKK inhibitor, parthenolide (PA), and the proteasome inhibitor, bortezomib (PS-341), were used to treat MCF7/HER2 and BT474 cells (+/–500 nM TAM) at PA and PS-341 doses previously shown to inhibit intracellular NFκB DNA-binding. PA (25-50 μM x 24 hr) and PS-341 (10-25 nM x 72 hr) alone showed comparable growth inhibition of all three lines. While PA and PS-341 failed to enhance TAM induced growth inhibition of MCF7 within 72 hr, these same PA and PS-341 doses significantly enhanced TAM induced growth inhibition of the TAM-resistant MCF7/HER2 and BT474 cells. To better understand the endocrine-modulating effects of these NFκB inhibitors, PA and PS-341 (+/–TAM x 24 hr) treatments were evaluated in the MCF7/HER2 subline vs. parental MCF7 cells following transient transfection with luciferase (luc) reporters driven by ERE, AP-1 or NFκB promoter-containing response elements. PA and PS-341 showed minimally stronger ERE-luc inhibiting effects in MCF7/HER2 as compared to MCF7; in contrast, these NFκB inhibiting drugs showed substantially stronger suppressive effects on both AP-1-luc and NFκB-luc reporters in MCF7/HER2 vs. MCF7 cells. Altogether, these findings suggest that PA and PS-341 enhance tamoxifen induced growth suppression of ER-positive, ErbB2 overexpressing breast cancers by suppressing NFκB and AP-1 driven genes, whose induction is thought to contribute to the increased clinical aggressiveness of such breast cancers.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]