data suggest estradiol induced resistance is just a shared characteristic across all three classes of PI3K pathway inhibitors tested, but there’s marked heterogeneity within the inhibitory effect of estradiol across ER positive breast cancer cell lines. BGT226, ubiquitin-conjugating BKM120 and RAD001 restrict PI3K pathway signaling despite long-term estrogen deprivation To model the effects of PI3K pathway inhibition in aromatase inhibitor resistant breast cancer cells, variations of the MCF7 and T47D lines were developed through LTED by more than 9 months of culture in low estrogen conditions. ER up-regulation and increased phosphorylation of Akt, S6 and the MAPK/ERKs was noticed in MCF7 LTED cells compared with the line. Within the T47D LTED point, S6 and ERK phosphorylation, but not p Akt, was greater than in parental T47D cells, and ER expression was downregulated to undetectable levels. Both LTED lines were therefore retreated with estradiol for a minimum of 4 months Chromoblastomycosis to determine whether estradiol re publicity can reverse the signaling effects associated with LTED. . In the ensuing MCF7 revertant subline, ER expression and levels of p Akt, p S6 and p ERKs were downregulated to similar levels seen in the parental MCF7 cells, indicating that prolonged estradiol re coverage reversed the results of LTED on these proteins. In contrast, while S6 and ERK phosphorylation were down-regulated by estradiol in T47D LTED Page1=46 cells, ER expression levels were not repaired at least not to an even detectable by western blot. The effect of the three PI3K pathway inhibitors on signal transduction demonstrated that the dose response relationships for all three agents were much like those seen in the e3 ubiquitin adult MCF7 and T47D cell lines. . The awareness of the lines to estradiol and fulvestrant was also determined. Expansion of MCF7 LTED and T47D LTED cells was not increased by increasing concentrations of estradiol, as expected. Certainly the MCF7 LTED model was paradoxically inhibited by estradiol because 10 nmol/l treatment for 10 days inhibited development and induced cell death. Treatment of estrogen deprived MCF7 LTED with all the ER particular inhibitor fulvestrant inhibited the growth of cells, demonstrating that ER remains functionally important for the growth of these cells despite the lack of supplemental estradiol. In contrast, therapy with estradiol or fulvestrant did not have major effects on the progress of ERnegative T47D LTED cells. Longterm estrogen deprived cells are resistant to the induction of apoptosis by low-dose PI3K pathway inhibitors To determine the effect of LTED on PI3K drug sensitivity, we compared the power of BGT226 and BKM120 to induce apoptosis in STED and LTED cell line pairs. When compared with MCF7 and T47D STED cells, higher drug levels were required for both BGT226 and BKM120 to induce apoptosis under LTED conditions.