Research ArticleCancer therapy

Skp2-dependent reactivation of AKT drives resistance to PI3K inhibitors

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Sci. Signal.  13 Mar 2018:
Vol. 11, Issue 521, eaao3810
DOI: 10.1126/scisignal.aao3810
  • Fig. 1 PI3K inhibition and depletion lead to AKT reactivation.

    (A) MDA-MB-468 cells were treated with BKM-120 (1 or 5 μM) and MK-2206 (10 μM) for 3 or 48 hours, alone or with 1 μM of BKM-120, BYL-719, or MK-2206 for an additional 3 hours, as indicated. Cells were then harvested, and lysates were immunoblotted for the indicated total and phosphorylated proteins. (B) MDA-MB-468 cells were infected with shRNA lentivirus targeting PIK3CA or control vector (pLKO). After selection, cells were harvested, and lysates were immunoblotted for the indicated total and phosphorylated proteins. (C) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+) and harvested, and lysates were immunoblotted with the indicated antibodies. (D) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA, PIK3CB, PIK3CD, PIK3CG, or control pLKO. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+) and harvested, and lysates were immunoblotted with the indicated antibodies. Blots are representative of at least three independent experiments. See also fig. S1 (A to C) for additional experiments pertaining to BKM-120 time course, depletion of PIK3CA with multiple shRNA hairpins, and analysis of pAKT1 pSer473 and pSer474 with PIK3CA depletion.

  • Fig. 2 PI3K depletion does not lead to AKT reactivation in all breast cancer cell lines.

    (A) SUM149PT, HCC1937, MDA-MB-468, and MCF7 cells were infected with shRNA lentivirus targeting PIK3CA, control pLKO, or shGFP as additional negative control. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+) and harvested, and lysates were immunoblotted with the indicated antibodies. Blots are representative of at least three independent experiments. (B) Summary table showing the different breast cancer cell lines tested in this study for AKT reactivation. TNBC classification according to Lehmann et al. (55): BL1, basal-like 1; BL2, basal-like 2; M, mesenchymal; MSL, mesenchymal stem–like; LAR, luminal androgen receptor. Note that BT20 and MDA-MB-435 are only classified as TNBC.

  • Fig. 3 AKT reactivation is dependent on upstream regulators.

    (A) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO. Cells were serum-starved overnight (−) and treated with the IGF-1R inhibitor NVP-AEW541 for 15 min before stimulation with IGF-1 for 20 min (+). Cells were harvested, and lysates were immunoblotted with the indicated antibodies. (B) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO alone or with the indicated combinations of shRNA specific for Raptor or Rictor, respectively. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+). Cells were harvested, and lysates were immunoblotted with the indicated antibodies. (C) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO. Cells were serum-starved overnight (−) and treated with the PDK-1 inhibitor GSK2334470 for 15 min before stimulation with IGF-1 for 20 min (+). Cells were harvested, and lysates were immunoblotted with the indicated antibodies. Data are representative of at least three independent experiments. (D) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO in the absence or presence of complementary DNA (cDNA) directing expression of p110α. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+). Cells were harvested, and lysates were immunoblotted with the indicated antibodies. Data are representative of at least three independent experiments. WT, wild type.

  • Fig. 4 AKT reactivation is PI3K-independent.

    (A) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO. Cells were serum-starved overnight (−) and treated with indicated inhibitors for 15 min before stimulation with IGF-1 for 20 min (+). Cells were harvested, and lysates were immunoblotted with the indicated antibodies. DMSO, dimethyl sulfoxide. (B) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO and then transfected with PTEN WT or control pcDNA vector. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+). Cells were harvested, and lysates were immunoblotted with the indicated antibodies. (C and D) MDA-MB-231 cells were infected with shRNA lentivirus targeting PIK3CA or control pLKO. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+). Phospholipids were isolated, PIP3 (C) or PI(3,4)P2 (D) was quantified by enzyme-linked immunosorbent assay (ELISA), or cell lysates were immunoblotted with the indicated antibodies. Data are means ± SEM from three independent experiments, each carried out in triplicate. **P < 0.01 by a two-sided Student’s t test.

  • Fig. 5 AKT reactivation is Skp2-dependent.

    (A) The indicated cell lines were infected with shRNA lentivirus targeting PIK3CA or control pLKO. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+) and harvested, and lysates were immunoblotted with the indicated antibodies. (B) MDA-MB-231 cells infected with PIK3CA and/or SKP2 shRNA lentiviral vectors or control pLKO. SKP2 mRNA expression was measured by real-time reverse transcription polymerase chain reaction (RT-PCR), and corresponding lysates were immunoblotted with indicated antibodies. Data are means ± SEM from three independent experiments. **P < 0.01 by a two-sided Student’s t test. (C) Immunoblotting in lysates from MDA-MB-468 cells treated with BKM-120 (1 μM) for up to 144 hours and then treated again with BKM-120 (1 μM) for an additional 30 min. (D) Immunoblotting in lysates from MDA-MB-231 cells infected with PIK3CA, SKP2, or PIK3CA in combination with SKP2 shRNA lentiviral vectors or control vector (pLKO) and serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+). (E) Immunoblotting in lysates from MDA-MB-468 cells infected with SKP2 shRNA lentiviral vector or control pLKO and then treated with BKM-120 (1 μM) for 3 or 48 hours. (F) Immunoblotting in lysates from MDA-MB-231 cells, parental and resistant to 1 μM BKM-120 (BKM-120), that were infected with SKP2 shRNA lentiviral vector or control pLKO vector and serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+). (G) MDA-MB-231 cells, parental and BKM-120–resistant, were infected with SKP2 shRNA lentiviral vector or control pLKO. After transfection with pcDNA3 or His-ubiquitin (His-Ub), His-ubiquitin complexes were isolated, followed by immunoblotting. Blots are representative of at least three independent experiments. See also fig. S1D for additional time course with BKM-120 in cells expressing cDNA for SKP2.

  • Fig. 6 Skp2 promotes 3D culture and anchorage independence of growth in BKM-120–resistant cells.

    (A) Representative images of MDA-MB-231 spheroids from parental or BKM-120–resistant cells that were infected with SKP2 shRNA lentiviral vector or control pLKO and grown for 14 days in a 3D Matrigel/growth medium mixture and treated with BKM-120 and/or MK-2206 (each 1 μM). Scale bar, 500 μm (magnification, ×4). (B) Quantitation of spheroid sizes from (A) was performed using ImageJ software. Data are means ± SEM. ***P < 0.001 by a two-sided Student’s t test. (C) Representative images of MDA-MB-231 colony growth from parental or BKM-120–resistant cells that were infected with SKP2 shRNA lentiviral vector or control pLKO and grown in an agar/growth medium mixture for 28 days in the presence of DMSO, BKM-120, and/or MK-2206 (each 1 μM). (D) Number of colonies in soft agar obtained in (C) was quantitated using MATLAB software. Data are means ± SEM. **P < 0.01 by a two-sided Student’s t test. All data and blots are representative of at least three independent experiments.

  • Fig. 7 Skp2 depletion attenuates tumor growth in vivo.

    (A) Immunoblotting of lysates from MDA-MB-231, parental and BKM-120–resistant, cells that were infected with SKP2 shRNA lentiviral vector or control pLKO. Cells were serum-starved overnight (−) or stimulated with IGF-1 for 20 min (+). (B and C) Tumor weight (B) and volume (C) of xenografts formed from cells described in (A) inoculated subcutaneously into nude mice (n = 10). Cells (1 × 106) were injected in each flank. Tumor measurements were taken 22 days after injection. Data are means ± SEM from 10 mice for each condition. **P < 0.01, ***P < 0.001 by a two-sided Student’s t test. (C) Tumor xenograft volumes taken 22 days after injection. (D) Growth curves of tumors described in (B). Tumor volume was measured starting day 1 after injection, then on days 5 and 8, and every 2 days up to day 22. (E) Representative pictures of tumors described in (B), surgically removed at day 22.

Supplementary Materials

  • Supplementary Materials for:

    Skp2-dependent reactivation of AKT drives resistance to PI3K inhibitors

    Emilie Clement, Hiroyuki Inuzuka, Naoe T. Nihira, Wenyi Wei, Alex Toker*

    *Corresponding author. Email: atoker{at}bidmc.harvard.edu

    This PDF file includes:

    • Fig. S1. Reactivation of AKT in response to PIK3CA depletion and inhibition.

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    © 2018 American Association for the Advancement of Science

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