Nrf2 mediates cancer protection but not prolongevity induced by caloric restriction

Abstract

Caloric restriction (CR) is the most potent intervention known to both protect against carcinogenesis and extend lifespan in laboratory animals. A variety of anticarcinogens and CR mimetics induce and activate the NF-E2-related factor 2 (Nrf2) pathway. Nrf2, in turn, induces a number of antioxidative and carcinogen-detoxifying enzymes. Thus, Nrf2 offers a promising target for anticarcinogenesis and antiaging interventions. We used Nrf2-disrupted (KO) mice to examine its role on the biological effects of CR. Here, we show that Nrf2 is responsible for most of the anticarcinogenic effects of CR, but is dispensable for increased insulin sensitivity and lifespan extension. Nrf2-deficient mice developed tumors more readily in response to carcinogen exposure than did WT mice, and CR was ineffective in suppressing tumors in the KO mice. However, CR extended lifespan and increased insulin sensitivity similarly in KO and WT mice. These findings identify a molecular pathway that dissociates the prolongevity and anticarcinogenic effects of CR.

Fig. 1.

NQO1 levels are increased in response to CR. (A) Relative RNA levels of NQO1 were determined by real-time PCR. Liver homogenates from AL and 40% CR-fed WT and KO mice were tested. CR significantly increased NQO1 levels in WT mice (P < 0.05), and there was a trend toward an increase in CR-fed KO livers compared with AL-fed KO mice (P < 0.07). n = 3 for all groups. (B) NQO1 activity was determined and CR-fed WT liver homogenates showed increased activity compared with AL (P < 0.001). Also, there was a significant increase in the CR-fed KO mice compared with the AL-fed KO (P < 0.001). n = 5 for all groups. Black, AL WT; red, CR WT; green, AL KO; blue, CR KO.

Fig. 2.

Lack of protection against induced tumors in CR KO mice. (A) Forty percent CR WT, 40% CR KO. (B) Thirty percent CR WT, 30% CR KO. (C) Twenty percent CR WT, 20% CR KO. Shown are the percentages of mice with at least one papilloma for AL and 40% CR-fed WT and KO (A) AL and 30% CR-fed WT and KO (B) and AL and 20% CR-fed WT and KO (C) mice presenting tumors after DMBA treatment at time 0. Black, AL WT; red, CR WT (20, 30, or 40% restriction); green, AL KO; blue, CR KO (20, 30, or 40% restriction).

Fig. 3.

Caloric restriction improves insulin sensitivity in WT and KO mice. (A) WT and KO mice were fed AL or 40% CR for 4 weeks. They were fasted overnight (16 h), and glucose levels were determined. There was a trend toward a decrease in the CR-fed WT mice (P < 0.09) and a significant decrease in the CR-fed KO mice (P < 0.01) compared with their respective AL-fed control mice. n = 10 for all groups except CR-fed WT, for which n = 9. (B) Fasting insulin values were determined by ELISA. There was a trend toward a decrease in the CR-fed WT mice (P < 0.10) and a significant decrease in the CR-fed KO mice (P < 0.05) compared with their respective AL-fed control mice. n = 5 for all groups except CR-fed WT, for which n = 4. (C) HOMA values were calculated from fasted glucose and insulin values, using the HOMA2 software available from the Oxford Centre for Diabetes, Endocrinology, and Metabolism Diabetes Trials Unit (www.dtu.ox.ac.uk). The values were from five mice per group (except CR-fed WT, n = 4) after a 16-h fast. There was a significant decrease in the HOMA values for CR vs. AL-fed KO mice (P < 0.01). (D) Fasting corticosterone levels. Corticosterone levels were significantly increased in CR-fed WT and KO mice compared with AL-fed WT and KO mice respectively (P < 0.01 in both cases). These mice were fed CR for 8 weeks. n = 9 for all groups except AL-fed WT, for which n = 7. Black, AL WT; red, CR WT; green, AL KO; blue, CR KO.

Fig. 4.

Kaplan–Meier survival analyses for KO mice. (A) Body weight of male mice in grams. KO mice were separated into two groups, AL and CR. The food was gradually restricted to 40% less than that of the AL-fed KO mice. (B) Kaplan–Meier survival analyses of AL and CR-fed KO mice were performed. n = 30 and n = 32 for AL and CR-fed mice, respectively. The survival curves for the AL and CR-fed KO mice differ significantly by the logrank test; (χ ² = 28.9, P < 0.0001). Green, AL KO; blue, CR KO.