Multiple mouse models of pancreatic, colon, prostate, skin and breast cancer have shown that calorie restriction (CR) prevents obesity and suppresses tumor development and growth. In normal muscle, studies have shown that mild CR attenuates the age-related impairment of autophagy, cellular damage, and cell death. Autophagy is a survival response in solid tumors that co-localizes with hypoxic regions, allowing tolerance to metabolic stress. Autophagy also protects cells from genome damage and limits both inflammation and cell death as possible means to tumor suppression. Thus, we hypothesized that autophagy plays a major role in CR-mediated tumor inhibition. To test this hypothesis, we utilized immortalized baby mouse kidney epithelium (iBMK) tumor cell lines. iBMK cells were first immortalized by E1A and dominant negative p53 (p54DD) transfection, and rendered apoptosis-deficient by Bcl-2 overexpression. They were then transfected with oncogenic Ras and EGFP-LC3 plasmids. Cell lines were either autophagy deficient through knockout of the essential autophagy gene Atg5 (Atg5 -/-) or were autophagy competent (Atg5 +/+). Female nude mice were fed either control AIN-76A diet ad-libitum or a CR regimen (isonutrient; 30% reduction in calories relative to control group) for 8 weeks prior to subcutaneous flank injection of cell lines (n=15 per group). CR mice were significantly lighter, had decreased body fat percentage, and had lower fasting glucose levels (p<0.001 for all). Additionally, CR mice had lower serum levels of IGF-1 (p<0.001), insulin (p<0.001), leptin (p<0.001), and resistin (p=0.01), and higher serum adiponectin (p<0.001). We found that autophagy competent tumors, transplanted into female nude mice (5x10^4 cells per mouse), grew at significantly different growth rates depending on the diet treatment (Control > CR; p<0.001), while autophagy deficient tumors were unaffected by CR (no difference between control and CR tumors). In order to determine the effect of CR on LC3 localization and activation of autophagy, we repeated the injections in a separate cohort of control and CR mice (n=2 per timepoint). Tumors were collected at 1, 3, 8, and 15 days post-injection. In CR mice injected with autophagy competent cells, we saw LC3 translocate to the autophagosomes, demonstrated by punctate perinuclear localization, as early as 1d post-injection. In contrast, in control mice injected with autophagy competent cells, and in all mice injected with autophagy deficient cells, we saw a diffuse cytoplasmic LC3 localization. In conclusion, we found that CR is capable of activating autophagy as a mechanism of tumor growth suppression and increases LC3 autophagosome localization in tumors. Taken together, these findings suggest that autophagy may play a key role in the anticancer effects of CR and may be an important intervention target for the prevention and treatment of obesity-related cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4104. doi:1538-7445.AM2012-4104

©2012 American Association for Cancer Research
2012