Trastuzumab-Resistant Cells Rely on a HER2-PI3K-FoxO-Survivin Axis and Are Sensitive to PI3K Inhibitors

Abstract

The antibody trastuzumab is approved for treatment of patients with HER2 (ERBB2)-overexpressing breast cancer. A significant fraction of these tumors are either intrinsically resistant or acquire resistance rendering the drug ineffective. The development of resistance has been attributed to failure of the antibody to inhibit phosphatidylinositol-3 kinase (PI3K), which is activated by the HER2 network. Herein, we examined the effects of PI3K blockade in trastuzumab-resistant breast cancer cell lines. Treatment with the pan-PI3K inhibitor XL147 and trastuzumab reduced proliferation and pAKT levels, triggering apoptosis of trastuzumab-resistant cells. Compared to XL147 alone, the combination exhibited a superior antitumor effect against trastuzumab-resistant tumor xenografts. Further, treatment with XL147 and trastuzumab reduced the cancer stem cell (CSC) fraction within trastuzumab-resistant cells both in vitro and in vivo. These effects were associated with FoxO-mediated inhibition of transcription of the anti-apoptosis gene survivin (BIRC5) and the CSC-associated cytokine IL-8. RNAi-mediated or pharmacological inhibition of survivin restored sensitivity to trastuzumab in resistant cells. In a cohort of patients with HER2-overexpressing breast cancer treated with trastuzumab, higher pre-treatment tumor levels of survivin RNA correlated with poor response to therapy. Together, our results suggest that survivin blockade is required for therapeutic responses to trastuzumab and that by combining trastuzumab and PI3K inhibitors CSCs can be reduced within HER2+ tumors, potentially preventing acquired resistance to anti-HER2 therapy.

Artemin, a Member of the Glial Cell Line-derived Neurotrophic Factor Family of Ligands, Is HER2-regulated and Mediates Acquired Trastuzumab Resistance by Promoting Cancer Stem Cell-like Behavior in Mammary Carcinoma Cells

“…Both BT474 and SKBR3 trastuzumab-resistant cells exhibited increased growth of mammospheres compared with the respective parental cells (Fig. 7A), consistent with previous literature (29). Conversely, depletion of ARTN by siRNA significantly decreased mammospheric growth in both BT474 and SKBR3 parental cells (Fig.…”

“…6C). It has been reported previously that the mechanisms contributing to acquired resistance to trastuzumab are heterogeneous (3), and even the trastuzumab doses at which resistance is acquired can produce different cell phenotypes and transcriptome profiles (29). We chose to use pool 2 from each of the cell lines because pool 2 from each of the lines consistently exhibited increased mRNA and protein for ARTN.…”

The Selective PI3K Inhibitor XL147 (SAR245408) Inhibits Tumor Growth and Survival and Potentiates the Activity of Chemotherapeutic Agents in Preclinical Tumor Models

“…26; NCT00756847). Moreover, the enhanced efficacy evident when XL147 is combined with trastuzumab or lapatinib in mouse models (27,28) provides strong support for exploring combinations of XL147 with RTK inhibitors. These and similar data provide a rationale supporting clinical studies where XL147 is combined with other targeted or cytotoxic agents (e.g., NCT01042925, NCT01082068, and NCT00692640).…”

Biomarker-guided sequential targeted therapies to overcome therapy resistance in rapidly evolving highly aggressive mammary tumors

“…Additionally, we demonstrated that in two different genetically-engineered mouse models of trastuzumab resistance (mammary gland-specific ErbB2/ neu transgenic and PTEN knockout), an Akt inhibitor (triciribine) synergizes with trastuzumab in tumor inhibition [13]. Targeting the PI3K pathway in combination with trastuzumab has also been reported to overcome trastuzumab resistance in other models [15]. Although initially successful, these combinatorial regimens rapidly develop resistance and are often too toxic for clinical usage [16].…”