Abstract Objective: Cancer cells are characterized by a changed cellular metabolism, favoring glycolysis. AMPK, a serine/threonine kinase regulates cellular metabolism, and induces fatty acid oxidation and has in regard to this, been shown to suppress tumor cell growth. During physical activity, which is associated with a reduced cancer risk, working muscles secrete myokines, known to activate AMPK. This could indicate a connection between AMPK activity, myokine release from the working muscle, and an anti-cancer response. We hypothesize that exercise-induced myokine secretion causes an anti-cancer response via AMPK activation. Methods: Exercise-induced AMPK activation was studied in two models, 1) A cell study using the two breast cancer cell lines, MDA-MB-231 and MCF-7, incubated with exercise-conditioned serum, representing a myokine pool. Serum was collected from 9 females during a 2 hours cycling bout, and during a subsequent 3 hours rest. Following incubation, cell viability and AMPK phosphorylation was determined in the cells. 2) An In Vivo model, where the two cell lines were injected into nude mice. When the tumors reached 6 mm in diameter, the mice were subjected to 1 hour forced swimming bout, after which the tumors were dissected and analyzed for pAMPK content. Results: 1) Cell study: Two hours of cycling induced a 6-fold increase in serum IL-6, as expected. Incubating cancer cells with this post-exercise serum resulted in a significant reduction in the viability of MCF-7 cells, while no effect on the viability of MDA-MB-231 cells were observed. Preliminary data show increased AMPK phosphorylation in both MDA-MB-231 and MCF-7 cells after incubation with 5% exercise-conditioned serum. 2) In vivo study: 1 hour of swimming resulted in significantly increased AMPK phosphorylation in the MCF-7 tumors. Conclusion: Our preliminary data indicate that exercise-induced factors can cause decreased viability and increased AMPK activity in MCF-7 cells, while MDA-MB-231 cells need further investigation. Acknowledgement: The Centre of Inflammation and Metabolism (CIM) is supported by a grant from the Danish National research Foundation (#02-512-55). This study was further supported by the Danish Medical Research Council and the Lundbeck Foundation. Citation Format: Grith W. Højfeldt, Christine Dethlefsen, Bente K. Pedersen, Pernille Hojman. Exercise activates AMP-activated protein kinase in breast cancer cells. abstract. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 5400. doi:10.1158/1538-7445.AM2013-5400