gCTRP3 inhibits oophorectomy-induced osteoporosis by activating the AMPK/SIRT1/Nrf2 signaling pathway in mice

C1q/tumor necrosis factor-related protein 3 (CTRP3) expression is markedly reduced in the serum of patients with osteoporosis. The present study aimed to investigate whether CTRP3 reduces bone loss in oophorectomy (OVX)-induced mice via the AMP-activated protein kinase (AMPK)/sirtuin 1 (SIRT1)/nuclear factor E2-related factor 2 (Nrf2) signaling pathway. Female C57BL/6J mice and MC3T3-E1 cells were used to construct in vivo and in vitro models of osteoporosis, respectively. The left femurs of mice were examined using micro-computed tomography scans and bone-related quantitative morphological evaluation was performed. Pathological changes and the number of osteoclasts in the left femurs of mice were detected using hematoxylin and eosin, and tartrate-resistant acid phosphatase (TRAP) staining. Runt-related transcription factor-2 (RUNX2) expression in the left femurs was detected using immunofluorescence analysis, and the serum levels of bone resorption markers (C-telopeptide of type I collagen and TRAP) and bone formation markers [osteocalcin (OCN) and procollagen type 1 N-terminal propeptide] were detected. In addition, osteoblast differentiation and calcium deposits were examined in MC3T3-E1 cells using alkaline phosphatase (ALP) and Alizarin red staining, respectively. Moreover, RUNX2, ALP and OCN expression levels were detected using reverse transcription-quantitative PCR, and the expression levels of proteins associated with the AMPK/SIRT1/Nrf2 signaling pathway were detected using western blot analysis. The results revealed that globular CTRP3 (gCTRP3) alleviated bone loss and promoted bone formation in OVX-induced mice. gCTRP3 also facilitated the osteogenic differentiation of MC3T3-E1 cells through the AMPK/SIRT1/Nrf2 signaling pathway. The addition of an AMPK inhibitor (Compound C), SIRT1 inhibitor (EX527) or Nrf2 inhibitor (ML385) reduced the osteogenic differentiation of MC3T3-E1 cells via inhibition of gCTRP3. In conclusion, gCTRP3 inhibits OVX-induced osteoporosis by activating the AMPK/SIRT1/Nrf2 signaling pathway.