A major challenge to broadening oncology applications for inhibitors of the
ubiquitin-
proteasome system (UPS) is the identification of UPS-dependent
cancer pathways predictive of
tumors responsive to
peptidomimetic inhibitors of its 20S core
protease activity. To inform clinical studies evaluating UPS inhibitors as
breast cancer therapeutics, seven phenotypically diverse human
breast cancer cell line models were characterized for their cellular and molecular responses to the clinically approved 20S inhibitor
bortezomib (
PS341;
Velcade), focusing on those overexpressing
estrogen receptor (ER) or ERBB2/HER2, because these oncogenic receptor pathways are constitutively activated in approximately 80% of all breast
cancers. All models demonstrated dose-dependent
bortezomib reduction in intracellular 20S activity correlating with cell growth inhibition, and
bortezomib IC(50) values (concentrations producing 50% growth inhibition) varied directly with pretreatment 20S activities (r = 0.74; *, p < 0.05), suggesting that basal 20S activity may serve as a clinical predictor of
tumor responsiveness to UPS inhibition. Reduction in 20S activity (> 60%) was associated with early (24 h) intracellular relocalization of ER (nucleus to cytoplasm) and ERBB2 (plasma membrane to perinuclear lysosomes), buildup of ubiquitinated and Hsp70-associated receptor, degradation and loss of ER and ERBB2 function, and induction of cellular apoptosis. These models were also used to screen a pharmacologic panel of pathway-targeted
anticancer agents [4-hydroxy-3-methoxy-5-(benzothiazolylthiomethyl)benzylidenecyanoacetamide (
AG825), 6-(4-bromo-2-chloro-phenylamino)-7-fluoro-3-methyl-3H-benzoimidazole-5-carboxylic
acid (2-hydroxy-ethoxy)-amide (
AZD6244/
ARRY142886),
2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one hydrochloride (
LY294002), 17-N-allylamino-17-demethoxy
geldanamycin (
17AAG), and (2E)-N-hydroxy-3-[4-[[(2-hydroxyethyl)[2-(1H-indol-3-yl)ethyl]amino]methyl]phenyl]-2-propenamide (
LAQ824)] for those capable of sensitizing to
bortezomib. In keeping with the observation that 20S reduction has little effect on
mitogen-activated protein kinase kinase 1/2 (MEK1/2) signaling in either ER-positive or ERBB2-positive models, only the
MEK-1/2 inhibitor
AZD6244 consistently improved the antitumor activity of
bortezomib.