Abstract P6-18-20: Targeting mitochondrial function for the treatment of triple negative breast cancer: Development of a small molecule inhibitor against mitochondrial STAT3

Abstract

Background: Patients with Triple Negative Breast Cancer (TNBC) can benefit significantly from earlier diagnosis/prognosis, targeted therapy, and predictive biomarker panels for optimal therapy. However, currently there are no clinically accepted markers for the prognosis of TNBC and to predict its potential to metastasize. It is well documented that numerous cancer subtypes with increased mitochondrial oxidative phosphorylation in which enhanced mitochondrial activity is linked to aggressiveness. Also, there is greater awareness of metabolic heterogeneity within tumors, with some cells using glycolysis as their main energy source, whereas others use oxidative phosphorylation. Interestingly, TNBC has been shown to adopt increased mitochondrial biogenesis to "fuel" enhanced growth and aggressiveness. Signal Transducers and Activators of Transcription family 3 (STAT3) has been studied extensively as a transcription factor, however the finding that STAT3 also localizes to mitochondria has opened a new area to discover non-classical functions. Methods: Targeting mitochondrial STAT3 functions challenge the current design of therapies that solely target STAT3 as a transcription factor and suggest the need for “design thinking,” to intervene the STAT3 pathway. With this in mind, we developed an in-house mitochondrial targeting - MitoTam. Data from in vitro cell-based assays, in vivo subcutaneous xenograft and patient-derived xenograft (PDX) models of TNBC will be reported. Results: Our data shows MitoTam robustly inhibited proliferation of TNBC cells at pharmacological doses and induced apoptosis. Mechanistically, we observed the MitoTam was able to target STAT3 leading to the downregulation of genes which is highly upregulated in most of the cancers. Furthermore, we show inhibition of STAT3 transcriptional activity hampers mitochondrial biogenesis, a prominent feature of cancer cell. Interestingly our in vivo and in vitro protein data showed the decreased phosphorylation of nuclear STAT3 and decreased mitochondria import of STAT3. We also found the decreased phosphorylation of STAT3 is associated with the interaction of GRIM-19 which is a cell death regulatory protein in complex1. Treatment of MitoTam was able to deplete the super complexes involved in OXPHOS and also in the regulation of mitochondrial transcription regulation. Our in vivo and PDX models show significant reduction of tumor size and tumor burden with treatment of MitoTam without effecting body mass. In addition we also found decrease in protein kinases associated with regulation of STAT3 for tumor survival. In addition, nuclear DNA encoded mitochondrial transcription factor A (TFAM), which enhances both transcription and replication of mitochondrial DNA is also shown to be downregulated with treatment, suggesting that MitoTam effectively inhibit TFAM binding to the mitochondrial DNA genes involved in OXPHOS regulation which was further validated by TFAM Chip-seq. Conclusion: Our results places MitoTam is a promising candidate drug against TNBC and establish mitochondrial STAT3 as its molecular target. Citation Format: Kanchi MM, Hirpara JL, Sachaphibulkij K, Tan TZ, Dietzel H, Lim LH, Huang RY-J, Pervaiz S, Neuzil J, Kumar AP. Targeting mitochondrial function for the treatment of triple negative breast cancer: Development of a small molecule inhibitor against mitochondrial STAT3 [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P6-18-20.

Pinpointing Cancer Sub-Type Specific Metabolic Tasks Facilitates Identification of Anti-cancer Targets

“…Basal subtype also enriches oxidative phosphorylation pathway. Clinical studies have shown that targeting oxidative phosphorylation provides an effective way to treat basal subtype ( 31 , 32 ). Glyoxylic acid and dicarboxylic acid pathways are highly expressed in basal breast cancer ( 33 ).…”

Mitochondrial STAT3 exacerbates LPS-induced sepsis by driving CPT1a-mediated fatty acid oxidation

“…MDC-1112 reduces the accumulation of mitochondrial STAT3 and induces apoptosis in pancreatic cancer cells [64]. In-house mitochondriatargeted tamoxifen (MitoTam) could suppress the phosphorylation of nuclear and mitochondrial STAT3, leading to proliferation inhibition and apoptosis in triple-negative breast cancer (TNBC) in vitro and in vivo [65]. Additionally, OPB-51602 results in mitochondrial dysfunction and cell death in human prostate cancer cells by interference with mitochondrial STAT3 [66].…”