Obesity is a well-established risk factor for postmenopausal breast cancer and is associated with poor outcomes in both premenopausal and postmenopausal women. Given that approximately one third of the US population is obese, interventions to reverse the detrimental effects of obesity on breast cancer progression are urgently needed. The molecular target of Rapamycin (mTOR) modulates pathways regulating energy balance and cell growth. Therefore, agents that target the mTOR pathway are promising for breaking the obesity-breast cancer link. Here, we directly compared the two most recommended strategies for reversing obesity, calorie restriction (CR) and exercise (EX), and a well-established mTOR inhibitor (rapamycin) on mammary tumor growth and metabolic profiles in obese mice.

Ovariectomized C57BL/6 mice were administered a diet-induced obesity regimen for 8 weeks, then randomized into 3 diet or exercise treatment groups: Control (AIN-76A diet fed ad libitum to maintain the obese state, n=30); 30% CR (70% of the carbohydrate calories but otherwise isonutrient relative to control, n=15); EX (control diet fed ad libitum plus treadmill exercise regimen, n=15). Mice were orthotopically implanted (4th mammary gland) with syngeneic MMTV-Wnt-1 mammary tumor cells at week 12. Of the 30 mice in the control group, 15 received rapamycin (5mg/kg, administered ip every 48hrs) starting at week 14. All other mice received vehicle treatment (saline, administered ip every 48 hrs) starting at week 14. Tumors were excised at week 18. Final tumor weights from mice in the CR (0.043 ± 0.007 g) and rapamycin (0.072 ± 0.010 g) groups, but not EX (0.380 ± 0.072 g) group, were significantly reduced compared to control (0.390 ± 0.066 g).

The crucial role of mTOR in the link between obesity and breast cancer progression was confirmed in a follow up study where constitutive activation of mTOR (mTOR) ablated the inhibitory effects of CR on mammary tumor growth. CR failed to reduce tumor weight in mice injected with cells transfected with mTOR (0.38 ± 0.05 g), compared to control (0.55 ± 0.08 g). Transfection effect bias was discarded since CR significantly reduced weight of tumors (0.29 ± 0.03 g) in mice injected with cells transfected with WT mTOR, compared to control (0.62 ± 0.12 g).

We conclude that mTOR inhibition may be a pharmacologic strategy to mimic the anticancer effects of CR and break the obesity-breast cancer progression link.

Citation Information: Cancer Prev Res 2011;4(10 Suppl):A75.