SIRT1 is a positive regulator of in vivo bone mass and a therapeutic target for osteoporosis

Abstract

Overexpression or pharmacological activation of SIRT1 has been shown to extend the lifespan of mice and protect against aging-related diseases. Here we show that pharmacological activation of SIRT1 protects in two models of osteoporosis. Ovariectomized female mice and aged male mice, models for post-menopausal and aging-related osteoporosis, respectively, show significant improvements in bone mass upon treatment with SIRT1 agonist, SRT1720. Further, we find that calorie restriction (CR) results in a two-fold upregulation of sirt1 mRNA expression in bone tissue that is associated with increased bone mass in CR mice. Reciprocally, SIRT1 whole-body knockout (KO) mice, as well as osteoblast and osteoclast specific KOs, show a low bone mass phenotype; though double knockout mice (containing SIRT1 deleted in both osteoblasts and osteoclasts) do not show a more severe phenotype. Altogether, these findings provide strong evidence that SIRT1 is a positive regulator of bone mass and a promising target for the development of novel therapeutics for osteoporosis.

Fig 1. Deletion of SIRT1 leads to a low bone mass phenotype.

SIRT1 whole-body knockout mice show decreased bone volume/total volume (BV/TV) at both 1 month and 4 months of age as assessed by microcomputed tomography (μCT). (n ≥ 4 for each group; ** p < .01; *** p < .005).

Fig 2. Deletion of SIRT1 in osteoclasts or osteoblasts results in a low bone mass phenotype.

(A) Osteoclast tissue specific SIRT1 knockout mice (OcKO) show excision of SIRT1 in osteoclasts, but not liver, as indicated by a smaller PCR product obtained with primers flanking SIRT1 catalytic exon 4 (T1Δ4). OcKOs show decreased bone mass at both 1 month and 4 months of age. (B) Osteoblast tissue specific SIRT1 knockout mice (ObKO) mice show excision of SIRT1 in calvaria, but not liver. ObKOs show decreased bone mass only at 4 months of age. (n ≥ 5 for each group; * p < .05; ** p < .01).

Fig 3. Deletion of SIRT1 in both osteoblasts and osteoclasts does not lead to a more severe phenotype.

Adult double knockout mice (DKO) containing SIRT1 deleted in both osteoblasts and osteoclasts show similar bone deficits as compared to individual osteoblast (ObKO) or osteoclasts (OcKO) knockout mice. (n ≥ 6 for each group; * p < .05).

Fig 4. SIRT1 agonist, SRT1720, increases bone mass in aged male mice and ovariectomized female mice.

(A) 12 month old male mice treated for 5 months with 100mg/kg/day of SIRT1 agonist, SRT1720, show increased bone mass compared to mice treated with vehicle control (final age 17 months). (B) Ovariectomized (OVX) female mice treated with SRT1720 for 1 month show a modest but significant increase in bone mass compared to vehicle treated controls (final age 4 months). (n ≥ 9 for each group; * p < .05; ** p < .01; *** p < .005).

Fig 5. Calorie restriction (CR) leads to upregulation of SIRT1 expression and increased bone mass.

(A) Calorie restricted (CR) mice show increased expression of SIRT1 in whole calvaria as assessed by reverse transcriptase polymerase chain reaction (RT-PCR). (B) Quantitative RT-PCR (qRT-PCR) confirms a two-fold increase in expression of SIRT1 (after normalization to RPL19) in the calvaria of CR mice. (C) 8 month old CR mice (placed on a CR diet for 4 months at 4 months of age) show marked increases in bone mass in all bones examined as compared to ad libitum (AL) fed controls. (n ≥ 4 for each group; *** p < .005).

Fig 6. SIRT1 expression in bone tissue does not change with age.

Expression of SIRT1 mRNA in the calvaria of mice remains constant with age. (n = 4 for each group).