Abstract 2613: Integrin beta-3 signaling links chemoresistance and mitochondrial metabolism in breast cancer bone metastases
Authors: Fox Gregory C. , Ross Michael H. , Su Xinming , Xu Yalin , Esser Alison , Cordell Elizabeth , Wilson Elizabeth , Muz Barbara , Dang Ha , Maher Christopher A. , Azab Abdel Kareem , Veis Deborah , Achilefu Samuel , Lanza Gregory M. , Weilbaecher Katherine N. 
Published: September 2020

Abstract
We have previously demonstrated that expression of the integrin β3 (β3) subunit is consistently increased in breast cancer (BC) bone metastases across clinical samples and preclinical models. Bone metastases frequently exhibit resistance to chemotherapy. Based on evidence that integrin β3 can function as a promoter of tumor survival and metastasis, we asked whether increased β3 signaling in bone metastases promotes chemoresistance. In established murine bone metastases, the proportion of integrin β3+ tumor cells was increased after docetaxel administration. Genetic deletion of β3 in either MMTV-PyMT-derived BC cells (BO1-FL-GFP, modeling luminal B disease) or 4T1 BC cells (4T1-FL-GFP, modeling triple-negative disease) yielded bone metastases with increased sensitivity to docetaxel treatment in vivo. Retroviral rescue of β3 expression in β3-/- BO1-FL-GFP cells by a signaling-competent human integrin β3 construct (hβ3) restored relative docetaxel chemoresistance in bone metastases. By contrast, expression of a signaling-deficient mutant β3 (Δβ3) failed to restore chemoresistance, suggesting that signaling through β3 is critical to its capacity to promote docetaxel chemoresistance in bone-residing BC cells. Mechanistically, RNAseq analysis of docetaxel response in β3-/- and hβ3-rescued cells revealed β3-mediated enrichment in transcriptional pathways associated with oxidative phosphorylation and metabolism. Direct imaging of mitochondria by super-resolution microscopy and extracellular flux analysis of oxygen consumption rate further confirmed an alternative metabolic response to docetaxel in resistant, β3-expressing tumor cells. mTORC1 plays an important role in mitochondrial biogenesis and cellular metabolism, and can be activated downstream of integrin signaling in certain contexts. Based on transcriptomic data identifying enhanced mTORC1 activity as a possible mediator of β3-dependent metabolic changes, we asked whether mTOR inhibition could restore chemosensitivity in BC bone metastases. In mice bearing chemoresistant, β3-WT metastases, combination of the mTORC1 inhibitor rapamycin with docetaxel yielded synergistic, site-specific attenuation of bone tumor burden. Taken together, our data 1) establish integrin β3 as a mediator of chemoresistance in breast cancer bone metastases, 2) demonstrate a mechanistic link between integrin β3 expression and an alternative metabolic response to docetaxel in resistant cells, and 3) suggest mTORC1 inhibition as a candidate for rational combination with chemotherapy to interrupt treatment resistance in breast cancer bone metastases.
Citation Format: Gregory C. Fox, Michael H. Ross, Xinming Su, Yalin Xu, Alison Esser, Elizabeth Cordell, Elizabeth Wilson, Barbara Muz, Ha Dang, Christopher A. Maher, Abdel Kareem Azab, Deborah Veis, Samuel Achilefu, Gregory M. Lanza, Katherine N. Weilbaecher. Integrin beta-3 signaling links chemoresistance and mitochondrial metabolism in breast cancer bone metastases [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2613.