High ATP Production Fuels Cancer Drug Resistance and Metastasis: Implications for Mitochondrial ATP Depletion Therapy

Abstract

Recently, we presented evidence that high mitochondrial ATP production is a new therapeutic target for cancer treatment. Using ATP as a biomarker, we isolated the "metabolically fittest" cancer cells from the total cell population. Importantly, ATP-high cancer cells were phenotypically the most aggressive, with enhanced stem-like properties, showing multi-drug resistance and an increased capacity for cell migration, invasion and spontaneous metastasis. In support of these observations, ATP-high cells demonstrated the up-regulation of both mitochondrial proteins and other protein biomarkers, specifically associated with stemness and metastasis. Therefore, we propose that the "energetically fittest" cancer cells would be better able to resist the selection pressure provided by i) a hostile micro-environment and/or ii) conventional chemotherapy, allowing them to be naturally-selected for survival, based on their high ATP content, ultimately driving tumor recurrence and distant metastasis. In accordance with this energetic hypothesis, ATP-high MDA-MB-231 breast cancer cells showed a dramatic increase in their ability to metastasize in a pre-clinical model in vivo. Conversely, metastasis was largely prevented by treatment with an FDA-approved drug (Bedaquiline), which binds to and inhibits the mitochondrial ATP-synthase, leading to ATP depletion. Clinically, these new therapeutic approaches could have important implications for preventing treatment failure and avoiding cancer cell dormancy, by employing ATP-depletion therapy, to target even the fittest cancer cells.

Keywords: ATP; anti-oxidant capacity; bedaquiline; cancer stem cells (CSCs); dormancy; metastasis; mitochondria; multi-drug resistance.

Figure 1

A brief history of the discovery of ATP and its energetic function. This timeline highlights the key scientists and the events that contributed to our deeper understanding the structure, function and molecular machinery responsible for the synthesis of ATP, especially within mitochondrial organelles.

Figure 2

Mitochondrial complexes I to IV can be safely targeted with FDA-approved drugs. This diagram illustrates that ATP-depletion can be induced in cancer cells by employing FDA-approved mitochondrial inhibitors that either i) block OXPHOS directly or ii) block OXPHOS indirectly, by halting mitochondrial protein translation. Inhibition of mitochondrial ATP production is a manageable side-effect that can re-purposed as a therapeutic effect to target and halt the propagation of CSCs. Inhibitors of mitochondrial protein translation (Doxycycline, Tigecycline and Azithromycin) prevent the production of the 13 proteins encoded by mitochondrial DNA (mt-DNA), including key subunits of complex I, III, and IV, as well as complex V (MT-ATP6, MT-ATP8) and Humanin (MT-RNR2).

Figure 3

High ATP levels are a major driver of aggressive cancer cell phenotypes. ATP-high cancer cells show increases in many aggressive properties or behaviors, including cell proliferation, stemness, anchorage-independence, migration, invasion, metastasis, anti-oxidant capacity and drug-resistance. In contrast, more “dormant” CSCs show low ATP levels. High mitochondrial ATP production may be related to increases in mitochondrial mass in ATP-high cancer cells.

Figure 4

Using several independent data sets to identify ATP5F1C as a key biomarker and therapeutic target for metastasis prevention. In order to define an ATP-related metastasis gene signature we first intersected two GEO DataSets focused on breast cancer metastasis (namely, GSE2034 and GSE59000), resulting in 5 common genes. The positive co-expression of ATP5F1C, with 3 other members of this gene signature (UQCRB, COX20, NDUFA2), was indeed confirmed by analyzing data from The Metastatic Breast Cancer Project (Provisional, February 2020; DataSet 3; https://mbcproject.org). Finally, 2 of these 4 gene transcripts (ATP5F1C and UQCRB) were independently found to be specifically-associated with i) maximal oxygen uptake (VO_2max) and ii) a higher percentage of mitochondrial-rich (type 1) fibers, in human skeletal muscle (DataSets 4/5), especially during exercise training. Therefore, ATP5F1C and UQCRB are likely to be key biomarkers of high OXPHOS and high mitochondrial ATP production in cancer cells. Modified from Reference 33 and reproduced with permission, under a Creative Commons License.

Figure 5

Targeting the human mitochondrial ATP synthase with Bedaquiline, an FDA-approved drug, prevents spontaneous metastasis. Mitochondrial ATP-synthase is a nano-scale rotary molecular motor that uses the transport of hydrogen ions to generate physical energy in the form of torque that is then converted into chemical energy in the form of ATP. Rotation of the gamma-subunit (ATP5F1C) helps to convert physical energy into chemical energy. Note that Bedaquiline treatment induces the degradation or down-regulation of the gamma-subunit (ATP5F1C), resulting in ATP-depletion and the prevention of metastasis.