Rapamycin Increases Mortality in db/db Mice, a Mouse Model of Type 2 Diabetes

Abstract

We examined the effect of rapamycin on the life span of a mouse model of type 2 diabetes, db/db mice. At 4 months of age, male and female C57BLKSJ-lepr (db/db) mice (db/db) were placed on either a control diet, lacking rapamycin or a diet containing rapamycin and maintained on these diets over their life span. Rapamycin was found to reduce the life span of the db/db mice. The median survival of male db/db mice fed the control and rapamycin diets was 349 and 302 days, respectively, and the median survival of female db/db mice fed the control and rapamycin diets was 487 and 411 days, respectively. Adjusting for gender differences, rapamycin increased the mortality risk 1.7-fold in both male and female db/db mice. End-of-life pathological data showed that suppurative inflammation was the main cause of death in the db/db mice, which is enhanced slightly by rapamycin treatment.

Keywords: Life span; Obesity; Toxicity; mTOR Inhibition.

Figure 1.

Survival of diabetic mice fed control diet or rapamycin-containing diet. Male and female db/db mice were randomized to control diet (black) or diet containing 14 ppm of rapamycin (red) at the age of 4 months and followed until natural death (n = 35 mice for rapamycin-treated males and 40 mice in the three other groups). After adjusting for gender difference, rapamycin was associated with higher rate of death by 1.7-fold (p = .0496) in male and female db/db mice.

Figure 2.

Body weight of diabetic mice fed control diet or rapamycin-containing diet. The body weights of the male and female db/db mice in Figure 1 were followed over their life spans. The graphs on the top show the raw body weights with a separate line for each mouse maintained on either the control (black) or rapamycin-containing (red) diet. The graphs on the bottom show the fitted body weights with 95% confidence intervals (dotted lines) for the male and female mice fed the two diets. In both genders, the rate of body weight loss for mice fed the rapamycin-containing diet was significantly (p < .0001) greater than that of the mice fed the control diet. In female mice, the gap between the two groups diminished significantly over time but nevertheless persisted throughout the 400-day follow-up period.

Figure 3.

Major Pathological Lesions in db/db mice. (A, C) Slides of sections (H.E.) from the kidneys of db/db mice on the control diet: (A) an example of normal kidney tissue (×200) and (C) an example of suppurative inflammation in kidney (×100). (B, D) Slides of sections (×100) from the livers of db/db mice on the control diet: (B) an example of normal liver tissue and (D) an example of hepatocellular carcinoma in liver.

Figure 4.

Severity of pathological lesions. The severity of renal lesions (A, D), tumor burden (B, E), and disease burden (C, F) of male (A–C) and female (D–F) db/db mice fed a control diet (white bars) or rapamycin-containing diet (black bars) are shown. These data were obtained from the mice listed in Table 3. The severity of the renal lesions was scored using the criteria as follows: Grade 0: no lesions; Grade 1: minimal change in glomeruli (minimal glomerulosclerosis); Grade 2: Grade 1 with a few (less than 10) casts in renal tubules; Grade 3: Grade 1 with more than 10 casts in renal tubules; and Grade 4: Grade 3 with interstitial fibrosis. Tumor burden was calculated as the average number of different types of tumors in the group. The disease burden is the average of total number of pathological changes of any type found in individual mice from each group. The data were analyzed using ANOVA, and tumor burden was shown to be significantly different in the mice fed the control diet and the rapamycin-containing diet.