Measurement of PIP3 levels reveals an unexpected role for p110β in early adaptive responses to p110α-specific inhibitors in luminal breast cancer

Abstract

BYL719, which selectively inhibits the alpha isoform of the phosphatidylinositol 3-kinase (PI3K) catalytic subunit (p110a), is currently in clinical trials for the treatment of solid tumors, especially luminal breast cancers with PIK3CA mutations and/or HER2 amplification. This study reveals that, even among these sensitive cancers, the initial efficacy of p110α inhibition is mitigated by rapid re-accumulation of the PI3K product PIP3 produced by the p110β isoform. Importantly, the reactivation of PI3K mediated by p110β does not invariably restore AKT phosphorylation, demonstrating the limitations of using phospho-AKT as a surrogate to measure PI3K activation. Consistently, we show that the addition of the p110β inhibitor to BYL719 prevents the PIP3 rebound and induces greater antitumor efficacy in HER2-amplified and PIK3CA mutant cancers.

Figure 1. Rebound of Phospho-AKT Occurs after 24 hr Treatment with Selective p110α Inhibitor in HER2-Amplified Cell Lines

(A) Effects of BYL719 on proliferation in a panel of 36 breast cancer cell lines. HER2-amplified versus HER2 WT, Fishers test p = 0.0363; PIK3CA mutant versus PIK3CA WT, Fishers test p = 0.0188. (B) HER2-amplified (upper) and PIK3CA hot-spot mutant (lower) cell lines were treated with 1 µM BYL719 for the indicated period of time. Lysates were immunoblotted to detect the indicated proteins. (C) The indicated cell lines were treated with 1 µM BYL719 for indicated period of time (fresh drug was added after 22 hr). Lysates were immunoblotted to detect the indicated proteins. See also Figure S1 and Tables S1 and S2.

Figure 2. Validation of ELISA Kit to Measure the Ratio of PIP₃/PI(4,5)P₂

(A) Phospholipids were isolated from cells treated with DMSO or 1 µM GDC-0941 for 1 hr and relative PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. Each data point represents the average of three independent experiments, each carried out in triplicate. For normalization, the PIP₃/PI(4,5)P₂ ratio was set to 100% for DMSO treated cells. (B) Phospholipids were isolated from cells treated as in (A) and measured with HPLC analysis after metabolic labeling of the cells for 48 hr. (C and D) T47D cells were treated with indicated concentrations of BYL719 for 1 hr (C) or AZD8055 for 24 hr (D). Phospholipids were isolated and PIP₃ and PI(4,5) P₂ levels were quantified by ELISA (left) or cell lysates were immunoblotted to detect the indicated proteins (right). (E) Cells were treated with 1 µM BYL719 or 1 µM GDC-0941 for 1 hr and relative PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. (F) Phospholipds were isolated from cells treated with 1 µM BYL719 for indicated time and relative PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. p < 0.05 by Student’s t test. All error bars in this figure represent ± SEM. See also Figure S2.

Figure 3. Inhibition of p110β Blocks the Rebound of AKT Phosphorylation and PIP₃ Levels in HER2-Amplified Cell Lines

(A) The indicated HER2-amplified cell lines were treated with p110α inhibitor BYL719 (1 µM), p110β inhibitor TGX-221 (1 µM), and p110δ inhibitor IC87114 (1 µM) or combinations of these inhibitors for the indicated times. Lysates were immunoblotted to detect the indicated proteins. (B) Cells were treated with BYL719 (1 µM), or TGX-221 (1 µM) or both for 24 hr, phospholipids were isolated from cell lysates and relative PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. Each data point is the average ± SEM (p < 0.05 by Student’s t test). (C) Cells were transfected with control (Scr) or p110β-targeted siRNA for 48 hr, followed by treatment with BYL719 (1 µM) for an additional 24 hr. Lysates were immunoblotted to detect the indicated proteins. (D and E) BT474 cells were treated with 1 µM BYL719 alone for different durations of time (D) or with 1 µM Lapatinib (LAP) for 24 hr (E) and lysates were immunoprecipitated with ERBB3 antibody. Precipitates were analyzed by western blot with the indicated antibodies. (F) Cells were treated with indicated drugs at 1 µM for 24 hr. Lysates were immunoblotted to detect the indicated proteins. (G) Cells were treated with 1 µM BYL719 alone or in combination with 10 µg/ml LJM716 (LJM) for 24 hr. Lysates were immunoblotted to detect the indicated proteins. (H) Cells were treated or not with pertussis toxin (PTX) 100 ng/ml and the indicated drug(s) for 24hr. Lysates were immunoblotted to detect the indicated proteins. See also Figure S3.

Figure 4. p110β Contributes to Cell Growth and Survival

(A) Cells were treated with 1 µM BYL719, 1 µM TGX-221, or both for 14 days and cell viability was determined by Syto60 staining (left) or for 72 hr and the percentage of cells undergoing apoptosis was measured by annexin V positivity (right). Because BT-20 cells did not survive when seeded at low density, they were not assayed for viability. (B) Cells were treated with DMSO, 1 µM BYL719, 1 µM TGX-221, or the combination for 24 hr and lysates were immunoblotted to detect the indicated proteins. (C) Cells were treated as (B) and PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. p < 0.05 by Student’s t test. (D) Cells were treated with 1 µM BYL719 for 24 hr and lysates were immunoblotted to detect the indicated proteins. (E) Cells were treated with single agent or combination of BYL719 (1 µM), LJM716 (LJM) (10 µg/ml), NVP-AEW541 (AEW) (1 µM), or NVP-BGJ398 (BGJ) (250 nM) or the indicated combination for 24 hr and PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. p < 0.05 by Student’s t test. (F) Cells were treated with BYL719 (1 µM), TGX-221 (1 µM), or pertussis toxin (PTX) (100 ng/ml) or the indicated combination for 24 hr and PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. p < 0.05 by Student’s t test. All error bars in this figure represent ± SEM. See also Figure S4.

Figure 5. Combined p110α and p110β Inhibition Leads to Tumor Regression In Vivo

(A) Waterfall plot showing the percent change in tumor volume (relative to initial volume) for individual BT474 tumors in tumor-bearing mice treated with the vehicle (VHL), BYL719 (BYL) 25 mg/kg per oral gavage daily, KIN-193 (KIN) 20 mg/kg per IP injection twice per day, and combination (BYL/KIN) groups following 21 days of treatment. (B) Mice harboring BT474 tumors were administered with vehicle (VHL), BYL719 (BYL) 25 mg/kg, KIN-193 (KIN) 20 mg/kg, or the combination of BYL/KIN for 3 days. Mice were killed 12 hr after the last treatment with BYL719 and 2 hr after treatment with KIN-193. For the combination-treated mice, tumors were also collected 12 hr after treatment with BYL719 and 2 hr after KIN-193. Lysates were prepared from tumors and blotted with the indicated antibodies. (C) Tumor growth assay showing the antitumor effect of BYL719 25 mg/kg per oral gavage daily and GSK2636771 30 mg/kg per oral gavage daily alone or in combination on endogenous NeuT tumors. p < 0.05 by two-way ANOVA. The average tumor volume ± SEM for each cohort is displayed. (D) Waterfall plot showing the percent change in tumor volume of MCF7 xenografts (relative to initial volume) for individual tumors treated as in (A). (E) Mice harboring MCF7 were administered as in (B). Mice were killed 2 hr after the last treatment with BYL719 and/or KIN-193. Lysates were prepared and blotted with the indicated antibodies. (F) Phospholipids were isolated from the same tumors as (E) and PIP₃ and PI(4,5)P₂ levels were quantified by ELISA. Each data point is the average ± SEM (p < 0.05 by Student’s t test). See also Figure S5.