Energy balance and breast cancer : mechanistic studies
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Obesity is one the few modifiable risk factors for breast cancer. Hence, an evaluation of the metabolic and cancer inhibitory effects of the obesity reversing strategies, calorie restriction (CR) and exercise, is important for breast cancer prevention. Additionally, a better understanding of the mechanisms underlying the effects of these interventions on cancer will provide scientific basis for therapeutic recommendations, and facilitate the identification of therapeutic agents for breast cancer treatment in obese patients. We found that CR is more effective than exercise in reversing the metabolic and cancer enhancing effects of obesity. Even at comparable levels of adiposity, CR effects on insulin resistance, energy balance related hormones levels, and metabolic genes expression in adipose tissue were more profound than those of exercise. The mechanism by which CR influences tumor progression is thought to involve molecules that respond to energy balance changes and control cell growth, such as the insulin-like growth factor-1 (IGF-1) and the mammalian target of rapamycin (mTOR). The mTOR inhibitor rapamycin decreased mammary tumor burden to levels comparable to CR. While established tumors did not display decreased mTOR activity, constitutively active mTOR was capable of overcoming some of the inhibitory effects of CR on tumor cells invasion and migration. Effects of increasing levels of CR on gene expression indicate that 30% and 40% CR, but not 20% CR, induce beneficial metabolic changes in the liver. However, 40% CR also increases apoptosis of hepatic cells which appears to be detrimental for the liver. IGF-1 infusion partially overcame the beneficial effects of CR on expression of tumor-related genes in the mammary fat pad and on mammary tumor growth. Taken together, our data show that CR, but not exercise, is able to reverse the metabolic and tumorigenic effects of obesity. Furthermore, the IGF-1 and mTOR pathways may mediate, at least in part, many of the beneficial effects of CR on metabolism and tumor progression.