Metformin prolongs lifespan through remodeling the energy distribution strategy in silkworm, Bombyx mori

Abstract

Metformin is a hypoglycemic agent used clinically in the treatment of type 2 diabetics. In addition, metformin is being investigated as a potential geroprotector. Here, we investigated the effects of metformin silkworm lifespan and the underlying molecular pathways involved. We found that metformin prolonged the lifespan of the male silkworm without reducing body weight, which suggests metformin can increase lifespan through remodeling of the animal's energy distribution strategy. Consistent with that idea, metformin reduced silk production and thus the energy devoted to that process. Metformin also increased fasting tolerance and levels of the antioxidant glutathione, and also activated an adenosine monophosphate-activated protein kinase-p53-forkhead box class O signaling pathway in silkworm. These results suggest that activity in this pathway may contribute to metformin-induced lifespan extension in silkworm by increasing stress resistance and antioxidative capacity while reducing energy output for silk product. The results also show that the silkworm is a potential useful animal model for evaluating the effects of small molecules with potential clinical utility.

Keywords: AMPK-p53-FoxO pathway; Bombyx mori; energy distribution; lifespan; metformin.

Figure 1

Metformin increases the adult and total mean lifespan in the male silkworm. (A) Adult stage (n=27), (B) mean total lifespan (n=30), and (C) maximum lifespan (n=3) of unmated female silkworms administered metformin (Treatment) or deionized water (Control). (D) Adult stage (n=24), (E) mean total lifespan (n=54) and (F) maximum lifespan of unmated male silkworms administered metformin (Treatment) of deionized water (Control). Bars depict the mean + SEM, *P < 0.05, **P < 0.01, ***P < 0.001.

Figure 2

Effects of metformin on larval weight, fecundity, pupal weight, cocoon weight, and cocoon-shell ratio. (A-A’) Larval weights measured daily from L3D1 to L5D6. Bars and symbols depict the mean + SEM, n=5. (B) Fecundity. Bars depict the mean + SEM, n=6. (C) Female pupal weight, (D) cocoon weight, and (E) cocoon-shell ratio. Horizontal bars depict the mean ± SEM, n=61. (F) Male pupal weight, (G) cocoon weight, and (H) cocoon-shell ratio. Horizontal bars depict the mean ± SEM, n=75.

Figure 3

Metformin increases starvation tolerance in the silkworm. Survival curves showing percent survival over time among silkworms administered metformin (Treatment) or deionized water (Control) and exposed to (A) starvation or (B) a heated environment (37°C) (n=30).

Figure 4

Metformin increases the antioxidant content in silkworms at the P7 stage. Glutathione (GSH) content measured at the indicated developmental stages in silkworms administered metformin (Treatment) or deionized water (Control). Bars depict the mean + SEM, n=9. *P < 0.05, **P < 0.01.

Figure 5

Effects of metformin on the expression of BmAMPK, Bmp53 and BmFoxO. Expression levels of BmAMPK, Bmp53 and BmFoxO at the indicated developmental stages in silkworms administered metformin (Treatment) or deionized water (Control) determined using real-time PCR. Bars depict the mean + SEM, n=9. *P < 0.05, **P < 0.01.