Abstract

Background: Several phosphoinositide-3-kinase (PI3K) catalytic subunit inhibitors are currently in clinical development. However, targeting strategies for PI3K catalytic subunits for treatment in estrogen receptor positive (ER+) disease have not been fully developed. We have recently reported that targeted inhibition of the two major PI3K catalytic subunits, p110α and p110β, using RNA interference (RNAi) induces apoptosis in short term estrogen-deprived ER+ breast cancer cells. Similarly, treatment with BEZ235, a clinical grade p110α/mammalian target of rapamycin (mTOR)-selective inhibitor, also promoted apoptosis in estrogen-deprived ER+ breast cancer cells. Importantly, we found that estradiol significantly suppressed death in ER+ cells induced by inhibition of PI3K catalytic subunits through both RNA interference and BEZ235. In contrast estrogen deprivation or PI3K inhibition individually caused little or no increase in apoptosis, indicating that there is a synthetic lethal interaction with estrogen deprivation/PI3K inhibitor combination treatment in ER+ breast cancer. These data suggest that combining estrogen deprivation with PI3K inhibition may improve clinical outcome by eradicating ER+ cells through increased tumor cell kill. We have extended these studies in order to determine the effect of PI3K catalytic subunit inhibition in long-term estrogen deprived (LTED) cells, a model for acquired resistance to endocrine therapy.Methods: LTED sub-lines of the ER+ MCF7 and T47D breast cancer cell lines were derived by depriving cells of estrogen for 6–9 months in phenol red-free medium containing charcoal-stripped serum after which time cells grow readily without supplemental estrogen. The effects of PI3K inhibition using RNAi and treatment with BEZ235 or BGT226, a p110α-selective inhibitor, were compared in short-term estrogen deprived MCF7 and T47D cells and their LTED derivatives by measuring effects on cell signaling, growth and apoptosis. Cell signaling was evaluated through western blotting, cell growth was measured using resazurin reduction and apoptosis was quantified by flow cytometry using the TUNEL assay.Results: PI3K inhibition using p110α RNAi or treatment with BEZ235 inhibited the growth of MCF7 LTED cells in the absence of supplemental estrogen. However, p110α RNAi or treatment with BEZ235 or the clinical grade p110α-selective inhibitor BGT226 failed to significantly induce apoptosis in MCF7 LTED cells. The effects of PI3K inhibition in T47D LTED cells are under investigation and will be presented.Conclusions: These results suggest that that the efficacy of PI3K inhibitors will be blunted after the onset of acquired resistance to estrogen deprivation through treatment with aromatase inhibitors and that a combined ER/PI3K targeting strategy in ER+ breast cancer treatment should be employed in the endocrine therapy sensitive phase of the disease.