AMPK activation by metformin promotes survival of dormant ER+ breast cancer cells
PurposeDespite adjuvant anti-estrogen therapy for patients with estrogen receptor alpha (ER)-positive breast cancer, dormant residual disease can persist for years and eventually cause tumor recurrence. We sought to deduce mechanisms underlying the persistence of dormant cancer cells to identify therapeutic strategies. Experimental DesignMimicking the aromatase inhibitor-induced depletion of estrogen levels used to treat patients, we developed preclinical models of dormancy in ER+ breast cancer induced by estrogen withdrawal in mice. We analyzed tumor xenografts and cultured cancer cells for molecular and cellular responses to estrogen withdrawal and drug treatments. Publicly available clinical breast tumor gene expression datasets were analyzed for responses to neoadjuvant anti-estrogen therapy. ResultsDormant breast cancer cells exhibited upregulated 5 adenosine monophosphate-activated protein kinase (AMPK) levels and activity, and upregulated fatty acid oxidation. While the anti-diabetes AMPK-activating drug metformin slowed the estrogen-driven growth of cells and tumors, metformin promoted the persistence of estrogen-deprived cells and tumors through increased mitochondrial respiration driven by fatty acid oxidation. Pharmacologic or genetic inhibition of AMPK or fatty acid oxidation promoted clearance of dormant residual disease, while dietary fat increased tumor cell survival. ConclusionsAMPK has context-dependent effects in cancer, cautioning against the widespread use of an AMPK activator across disease settings. The development of therapeutics targeting fat metabolism is warranted in ER+ breast cancer. Statement of Translational RelevanceDormant cancer cells that survive adjuvant therapy can ultimately give rise to recurrent/advanced tumors that frequently develop resistance to all approved therapies. Patients with early-stage estrogen receptor alpha (ER)-positive breast cancer are typically treated with surgical resection followed by [≥]5 years of adjuvant anti-estrogen therapy that neutralizes ER and suppresses, but often does not eliminate, tumor-initiating cells. Estrogen withdrawal, which mimics aromatase inhibitor therapy, induced activation of the metabolic sensor 5 adenosine monophosphate-activated protein kinase (AMPK) and upregulated fatty acid oxidation (FAO) in preclinical models. Treatment with the anti-diabetes AMPK-activating drug metformin or high dietary fat intake promoted survival of dormant ER+ breast cancer cells, while anti-anginal drugs that inhibit FAO induced clearance of dormant tumor cells. These findings caution against using AMPK modulators with anti-estrogens in patients with ER+ breast cancer, and warrant testing of FAO inhibitors as anti-cancer agents in combination with anti-estrogens.