Obesity increases postmenopausal breast cancer risk and increases mortality in pre- and postmenopausal women. While the majority of breast cancers in obese women are estrogen receptor (ERα)-positive, ERα-negative tumors confer a much worse prognosis. The mechanism by which obesity affects ER-negative breast cancer risk and prognosis is not clear, and strategies for offsetting the negative effects of obesity are urgently needed. In this study, we utilized the MMTV-neu mouse model of luminal-type breast cancer to test the hypothesis that energy balance modulation, through diet-induced obesity (DIO) or calorie restriction (CR) regimens, alters mammary tumor development and progression through regulation of ER in the mammary epithelium. MMTV-neu mice form spontaneous mammary tumors that progress from an ERα-positive hyperplasia to aggressive ERα-negative ductal adenocarcinomas. Female MMTV-neu transgenic mice and non-transgenic host strain (FVB) mice (6-8 weeks old; n=90/genotype) were randomized (30/group) to receive: control diet (modified AIN-76A); a 30% CR regimen (isonutrient); or a DIO regimen. A subset of mice (n=4 per group) was killed at 1, 3, and 5 months following diet initiation, and tissues were collected for analysis; remaining animals were followed for a 60-week survival study. We found that, relative to control diet, the DIO regimen significantly increased body weight, percent body fat (p<0.0001), and obesity-associated serum hormones and growth factors (IGF-I, insulin, leptin, and estradiol; p<0.01 for all) in both MMTV-neu and FVB controls. Conversely, CR significantly decreased body weight, percent body fat (p<0.0001) and decreased serum hormones/ growth factors, while increasing adiponectin (each at p<0.01). Gene expression (qRT-PCR) and protein expression (immunohistochemistry) analysis revealed that loss of mammary ERα expression, known to occur by 8 weeks of age in control MMTV-neu mice, was accelerated by DIO and delayed by CR. Additionally, we found that CR (relative to control diet) significantly increased mammary ERβ expression (p<0.0001) and delayed the onset of hyperplasia (p<0.001) in both MMTV-neu and FVB mice. Importantly, we found that after 60 weeks of feeding, CR significantly increased tumor-free survival of MMTV-neu mice (p=0.01). CR tumor samples showed a decrease in vascularity and presence of mitotic figures. In conclusion, dietary energy balance modulation impacts spontaneous MMTV-neu mammary tumor development and ERα and ERβ levels in normal and tumor tissue. In addition, increased mammary ERβ expression may represent a novel mechanism underlying the anticancer effects of CR.

Citation Format: Sarah M. Dunlap, Nikki A. Ford, Laura W. Bowers, Emily L. Rossi, Samantha Miller, Lucia J. Chiao, Powel H. Brown, Stephen D. Hursting. Calorie restriction decreases spontaneous mammary tumorigenesis and upregulates ER-β in MMTV-neu transgenic mice. [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 174. doi:10.1158/1538-7445.AM2013-174