Abstract P4-01-08: Morphological and functional plasticity of mitochondria in chemoresistant triple negative breast cancer
Authors: Baek Lily Mokryun , Lee Junegoo , Barrish James P. , Lim Bora , Chang Jeffrey T. , Lorenzi Phillip L. , Porter Weston , Echeverria Gloria V. 
Published: March 2022

Abstract
BACKGROUND: Neoadjuvant chemotherapy (NACT) used for triple negative breast cancer (TNBC) eradicates tumors in only 45% of patients. TNBC patients with substantial residual cancer burden have poor metastasis free and overall survival rates. Therefore, understanding the mechanisms of resistance to standard chemotherapy is essential. Our previous studies found mitochondrial oxidative phosphorylation (OXPHOS) was elevated and was a unique therapeutic dependency of residual tumor cells that survived after NACT. Mitochondria are morphologically plastic and dynamic organelles that continuously cycle between fission and fusion to maintain mitochondrial integrity and metabolic homeostasis. Mitochondrial fusion is thought to support OXPHOS through maintenance of the mitochondrial genome (mtDNA) which encodes 13 subunits of electron transport chain complexes. On the other hand, mitochondrial fission can be the precursor to mitophagy, the selective degradation of damaged mitochondria. We are investigating how mitochondrial structure dynamics impact metabolic states driving chemoresistance in TNBC. Methods We used a previously characterized orthotopic patient-derived xenograft (PDX) mouse model derived from the primary tumor of a treatment-naïve metastatic TNBC patient. After treatment with a standard NACT regimen, combined Adriamycin (aka doxorubicin) and cyclophosphamide (AC), we observed initial partial response followed by tumor regrowth. We collected PDX tumors at three time points: pre-treatment, post-AC residual (when tumors reached the volume nadir), and post-AC tumors that later regrew to the starting tumor volume. We used transmission electron microscopy (TEM) and immunohistochemical (IHC) analysis of human mitochondria to assess mitochondrial morphology and mass, respectively. For in vitro assays, we assessed mitochondria morphology, mtDNA content, and OXPHOS activity in TNBC cells treated with two different types of chemotherapeutic drugs, DNA-damaging agents (doxorubicin and carboplatin) and a microtubule-stabilizing agent (paclitaxel). To examine metabolic adaptations upon chemotherapy, we conducted 13C-glucose and glutamine flux metabolomics. To pharmaceutically perturb mitochondrial morphology, we used Mdivi-1 and Silibinin, known to induce mitochondrial fusion and fission, respectively. We assessed cell growth by Incucyte real-time imaging. Results: Analyses of residual PDX tumors after in vivo AC treatment revealed increased mitochondrial content, and increased average mitochondrial length in residual tumor cells. Metabolomic analysis of TNBC cells revealed all three chemotherapies induced glucose, but not glutamine, flux through the TCA cycle. Flux through glycolysis was not affected by chemotherapies. Treatment with doxorubicin and carboplatin, increased mitochondrial elongation, mtDNA content, and OXPHOS activity. Conversely, paclitaxel treatment reduced mitochondrial length and OXPHOS activity. We observed Mdivi-1 induced fusion concomitant with increased OXPHOS activity, while treatment with Silibinin reduced fusion and decreased OXPHOS. We next tested if induction of mitochondrial fission enhanced chemosensitivity. Sequential treatment of TNBC cells with DNA-damaging chemotherapeutics followed by Silibinin perturbed OXPHOS and resulted in enhanced cell killing. Conclusion: These findings establish a functional role for mitochondrial structure in chemotherapeutic response and metabolic reprogramming, which may confer a survival advantage to TNBC cells. Given the increased chemosensitivity we observed after pharmacologic inhibition of mitochondrial fusion, our study suggests perturbing mitochondrial structure adaptations may provide an opportunity to overcome TNBC chemoresistance in the neoadjuvant setting.
Citation Format: Lily Mokryun Baek, Junegoo Lee, James P. Barrish, Bora Lim, Jeffrey T. Chang, Phillip L. Lorenzi, Weston Porter, Gloria V. Echeverria. Morphological and functional plasticity of mitochondria in chemoresistant triple negative breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-01-08.