Introduction: Overexpression of oncomir-21 promotes proliferation of breast cancer cells. This study aimed to assess the effect of endurance training on the expression of miR-21 and its downstream, Bcl2 and upstream targets, STAT3 in breast cancer bearing mice.
Materials and methods: After orientation to the environment, breast cancer cells, MC4-L2 were injected to mice and they randomly were categorized into two groups, control (n=10) and training (n=10) groups. Training group performed progressive endurance training 5 days per week for 6 weeks and control group did not perform any exercise. Tumor volume was measured by a digital caliper every week. Finally, the mice were sacrificed; tumor tissue was removed and immediately frozen and kept in -70°C. RNA extraction and cDNA synthesis were carried out by trizol reagent and specific kits and level of genes were measured by quantitative real-time PCR.  
Results: Endurance training decreased significantly expression of miR-21, STAT3 and Bcl2 (P<0.05). In addition, Tumor volume developed further in control group compared to training group (P<0.05). There was significantly positive correlation (P<0.001) between miR-21 with STAT3(R=0.66) and miR-21 with Bcl2 (R=0.61)
Conclusion: Endurance training leads to suppress expression of STAT3/miR-21/Bcl2 signaling pathway, thereby involved in slow tumor growth. Therefore, one of the beneficial effects of endurance training on tumor progression in estrogen dependent mouse model of breast cancer is reducing intrtumor anti-apoptotic genes.