Metabolic Control of Breast Cancer Metastasis
Mitochondrial oxidative phosphorylation is critical for seeding and micrometastasis
Although metastasis remains the cause of most cancer-related mortality, mechanisms governing seeding in distal tissues are poorly understood. In Davis, et. al. 2020, we established a robust method for the identification of global gene expression changes in rare metastatic cells during seeding using single-cell RNA sequencing and patient-derived-xenograft models of breast cancer. We found micrometastatic cells undergo a bioenergetic switch in favor or mitochondrial oxidative phosphorylation (OXPHOS). We demonstrated that pharmacological inhibition of OXPHOS dramatically reduced metastatic seeding in the lungs, thus demonstrating the functional importance of OXPHOS in metastasis and highlights its potential as a therapeutic target to prevent metastatic spread in patients with breast cancer.
Singe-cell RNS sequencing to compare cellular diversity in micrometastases and primary tumors
To identify fundamental cellular programs important for seeding in metastatic tissues, we investigated gene expression programs uniquely expressed by cancer cells during the seeding and establishment of micrometastatic lesions. We analyzed three previously established PDX models of triple-negative breast cancer. As in many patients with breast cancer, metastatic progression is slow and sporadic in these models, where most animals display dispersed micrometastases in the lung and lymph nodes and very low metastatic burden at the endpoint. This enabled us to investigate the gene expression changes associated with early events in the seeding and establishment of micrometastasis.
Micrometastatic cells display increased mitochondrial oxidative phosphorylation
Using cutting-edge bioinformatic approaches, we identified OXPHOS as one of the most significantly upregulated gene expression programs in micrometastatic cells. We evaluated the expression of 1,402 genes associated with 37 metabolic pathways—such as the pentose phosphate pathway, the citric acid cycle and fatty-acid metabolism—to further investigate the metabolic differences between primary tumor cells and micrometastases. Gene scoring for each pathway showed that glycolysis and OXPHOS were the most significantly differentially expressed of all 37 pathways, indicating this phenotype was highly specific.
Oxidative phosphorylation is critical for lung metastasis
To test whether increased OXPHOS is functionally important for metastasis, we inhibited OXPHOS pharmacologically with oligomycin in the human MDA-231 cell line and the mouse 4T1 cell line. Flow cytometry analysis showed an almost threefold decrease in the frequency of metastatic MDA-231 cells and a sevenfold decrease in the frequency of metastatic 4T1 cells in the lungs of the treatment group, showing that OXPHOS inhibition dramatically attenuates the metastatic capacity.
OXPHOS may promote metastatic seeding in several ways. Increased ATP generation through OXPHOS may provide energy for cytoskeleton remodeling for motility or to survive anoikis during cell detachment and migration. Increased ROS production through OXPHOS may promote cell motility by activating oncogenic signaling, as mitochondrial ROS‐inducing mutations are sufficient to trigger metastasis. Epidemiological data in humans also support a role for OXPHOS in cancer progression, showing that treatment with the diabetes drug metformin (complex I inhibitor) is protective against the relapse and metastasis of breast cancer. We are excited to explore these possibilities in future projects in the lab, including studies that define which specific steps of the metastatic cascade OXPHOS is critical for to determine when it might be applied for clinical benefit against metastasis.