Chronic calorie restriction (CCR) is an effective experimental intervention to prevent spontaneous and carcinogen-induced mammary tumor (MT) development in rodents. The prevailing wisdom has been that protection is proportional to the degree of restriction, however, we have reported that intermittent calorie restriction (ICR) provides greater protection in breast and prostate cancer models than CCR despite the same overall amount of restriction. One pathway of interest with respect to the impact of calorie restriction and cancer development is the mammalian target of rapamycin (mTOR) signaling pathway. Modulation of mTOR signaling proteins by calorie restriction has been reported in various tissues and cancer models. Here the impact of ICR versus CCR on MT development and proteins associated with mTOR and IGF-I signaling pathways in mammary fat pads (MFP) and MTs from MMTV-TGF-α mice were determined. Mice were enrolled at 10 wks of age into ad-libitum-fed (AL), CCR (25% reduction) and ICR (cycles of 3 weeks of 50% reduced intake and 3 weeks of AL intake resulting in overall reduction of 25%). Groups were followed until 37/38 or 73/74 wks of age. Time points 37 and 73 followed three wks of 50% calorie restriction for ICR mice (ICR-Restricted), while 38 and 74 followed one wk of refeeding of ICR mice (ICR-Refed).At 37/38 wks, 1/11, 2/10 and 0/15, AL, CCR and ICR mice had histopathologically confirmed MTs and at 73/74 wks the values were 9/17, 5/15 and 5/22, respectively. Both protocols significantly reduced serum IGF-I levels compared to AL mice. At 37/38 wks, IGF-I receptor (IGF-IR) expression levels from MFP of ICR-Restricted and CCR mice were significantly (∼20%) lower than AL mice, while .there were no differences for IGF-IR levels between ICR-Refed and AL mice. Further at 37/38 IGFBP3 protein expression levels in MFP were significantly higher in ICR-Restricted and CCR mice compared to AL mice while for ICR-Refed mice expression was similar to AL mice. Both types of calorie restriction decreased p70S6K, HIF-1, EGFR protein expression levels and Erk and Akt protein activation, although it increased p4EBP1 and VEGF protein expression levels. At wk 73/74 there were no differences for IGF-IR levels in MFP tissue and AMPK activation was similar in AL, ICR-Restricted and ICR-Refed mice. Erk1/2 protein activation level was significantly higher only in ICR-Restricted compared to the AL group. At 73/74 wks sufficient MT tissue was available for analyses. Both modes of calorie restriction lowered IGF-I protein expression levels and Akt activation in MTs and IGF-IR expression levels were significantly lower in all calorie restricted groups compared to the AL mice. In contrast only CCR increased mTOR, p70S6K, p4EBP1, and HIF-1 protein expression levels in MTs. On the other hand, ICR had inconsistent effects on these proteins in MTs at the 73/74 time point. In MTs from ICR-Restricted mice AMPK activation was significantly lower compared to the AL mice while the value for ICR-Refed mice was similar to AL mice. These findings confirm that calorie restriction, particularly intermittent calorie restriction delays the early development of MTs and that mTOR signaling proteins are modulated by tissue, age and type of calorie restriction. (Support CA101858 (MPC), DK16105 (JPG), The Breast Cancer Research Foundation and The Hormel Foundation).
Citation Information: Cancer Prev Res 2010;3(12 Suppl):A94.